Applied Science, Faculty of
Materials Engineering, Department of
DSpace
UBCV
Le, Katherine
Doctor of Philosophy - PhD
University of British Columbia
Developments in electronic textiles (e-textiles) have focused on integration into remote physiological monitoring systems. This has encompassed functionalized textile sensors for continuous biopotential signal collection. This research examines structural hierarchical effects in e-textiles to understand properties influencing the performance of e-textile sensors. Electrocardiography (ECG), a bioelectric signal associated with electrical activity of the heart muscle, was selected for evaluating e-textile electrode performance. The structure-property relationships of fibre and yarn to fabric systems were investigated for silver (Ag)-nylon textiles. Fibre geometry and yarn twist were found to impact yarn linear resistance, and electrode contact impedance at the fabric level up to limiting values. Influencing factors at the yarn level were explained by contact between fibres in the yarn structure, yarn specific volume, fibre volume fraction, and orientation angles. At the fabric level, influencing factors included yarn diameter, fibre packing, and apparent contact area of the electrode. Skin-electrode interfacial interaction, and relationship to biopotential signal quality were characterized through surface roughness and skin contact impedance measurements of knitted electrodes. Compatibility at the skin-electrode interface was impacted by skin hydration, roughness, and applied pressure. A high throughput roll-to-roll system was developed to produce electrochemically coated silver/silver-chloride (Ag/AgCl) yarns, and the process was characterized. Electrodes were fabricated through embroidery of Ag/AgCl yarns. The performance of Ag/AgCl e-textile electrodes was compared against embroidered Ag e-textiles, and standard Ag/AgCl electrodes. The Ag/AgCl e-textile electrodes demonstrated excellent ECG signal quality, and high stability based on polarization potential measurements when compared to Ag e-textiles. Durability of e-textiles for long-term use was addressed through the development of a stretchable, self-healing, biocompatible and recyclable chloride-based ionogel coating. The material was employed as a cast film, and sprayable e-textile coating, applied to strain sensing and biosignal recording, respectively. The ionogel film demonstrated high recovery of material properties after a 30-minute healing period at room temperature and reformed after abrasion to a coated e-textile surface by the addition of ethanol. Electrical properties of ionogel coated electrodes met specified performance ranges for e-textile electrodes. The methods and findings presented in this dissertation can be used to guide e-textile electrode materials selection and design.
DESIGN AND EVALUATION OF TEXTILE ELECTRODES FOR BIOLOGICAL SIGNAL MONITORING by Katherine Le B.A.Sc., University of Toronto, 2014 M.A.Sc., University of Toronto, 2016 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Materials Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April 2023 © Katherine Le, 2023 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: Design and Evaluation of Textile Electrodes for Biological Signal Monitoring submitted by Katherine Le in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Materials Engineering Examining Committee: Frank Ko, Professor, Materials Engineering, UBC Supervisor Peyman Servati, Professor, Electrical and Computer Engineering, UBC Co-supervisor Elod Gyenge, Professor, Chemical and Biological Engineering, UBC University Examiner Urs Hafeli, Professor, Pharmaceutical Sciences, UBC University Examiner Jinlian Hu, Professor, Biomedical Engineering, City University of Hong Kong External Examiner Additional Supervisory Committee Members: John Madden, Professor, Electrical and Computer Engineering, UBC Supervisory Committee Member Rizhi Wang, Professor, Materials Engineering, UBC Supervisory Committee Member iii Abstract Developments in electronic textiles (e-textiles) have focused on integration into remote physiological monitoring systems. This has encompassed functionalized textile sensors for continuous biopotential signal collection. This research examines structural hierarchical effects in e-textiles to understand properties influencing the performance of e-textile sensors. Electrocardiography (ECG), a bioelectric signal associated with electrical activity of the heart muscle, was selected for evaluating e-textile electrode performance. The structure-property relationships of fibre and yarn to fabric systems were investigated for silver (Ag)-nylon textiles. Fibre geometry and yarn twist were found to impact yarn linear resistance, and electrode contact impedance at the fabric level up to limiting values. Influencing factors at the yarn level were explained by contact between fibres in the yarn structure, yarn specific volume, fibre volume fraction, and orientation angles. At the fabric level, influencing factors included yarn diameter, fibre packing, and apparent contact area of the electrode. Skin-electrode interfacial interaction, and relationship to biopotential signal quality were characterized through surface roughness and skin contact impedance measurements of knitted electrodes. Compatibility at the skin-electrode interface was impacted by skin hydration, roughness, and applied pressure. A high throughput roll-to-roll system was developed to produce electrochemically coated silver/silver-chloride (Ag/AgCl) yarns, and the process was characterized. Electrodes were fabricated through embroidery of Ag/AgCl yarns. The performance of Ag/AgCl e-textile electrodes was compared against embroidered Ag e-textiles, and standard Ag/AgCl electrodes. The Ag/AgCl e-textile electrodes demonstrated excellent ECG signal quality, and high stability based on polarization potential measurements when compared to Ag e-textiles. iv Durability of e-textiles for long-term use was addressed through the development of a stretchable, self-healing, biocompatible and recyclable chloride-based ionogel coating. The material was employed as a cast film, and sprayable e-textile coating, applied to strain sensing and biosignal recording, respectively. The ionogel film demonstrated high recovery of material properties after a 30-minute healing period at room temperature and reformed after abrasion to a coated e-textile surface by the addition of ethanol. Electrical properties of ionogel coated electrodes met specified performance ranges for e-textile electrodes. The methods and findings presented in this dissertation can be used to guide e-textile electrode materials selection and design. v Lay Summary Electronic textiles (e-textiles) are flexible fabrics with embedded functional properties typically associated with rigid electronics such as electrical conductivity. Applications of e-textiles include sensors for remote biological signal monitoring systems. Electrocardiogram (ECG) is a bioelectrical signal from the body that is associated with heart muscle activity. In clinical healthcare settings, ECG is measured by rigid disposable electrodes placed on the skin surface. In this work, the material and signal quality performance of fabricated flexible e-textile electrodes for ECG monitoring were characterized and compared to standard rigid electrodes. To understand material properties that impact e-textile electrode performance, a bottom-up approach was employed, by studying effects of fibre and yarn, to fabric levels. Influencing factors such as skin hydration, surface roughness, and applied pressure at the skin-electrode interface were examined. A functional, biocompatible, and self-healable coating material for electrodes was developed to improve the durability of e-textiles. vi Preface Unless otherwise stated below, the author of this dissertation structured and organized the design of the research, conducted the literature review, prepared experimental setups, test protocols, data collection, data analysis, prepared conference presentations and papers, and wrote the manuscripts. Supervision of this work was provided by Prof. F. Ko and Prof. P. Servati. Additional feedback was provided by Dr. H. Narayana, Dr. A. Servati, Dr. S. Soltanian, and Dr. A. Bahi. Unless otherwise stated, all other individuals listed in presentations, publications, and manuscripts contributed by providing guidance and feedback on design or analysis of the work, or revisions to the writing. Aspects of chapter 2 was presented at 2019 IEEE International Flexible Electronics Technology Conference. Le, K., Servati, A., Servati, P. and Ko, F. (2019) “Signal Quality Analysis of Electrocardiogram Textile Electrodes for Smart Apparel Applications,” IFETC, Vancouver, Canada, 2019. A version of chapter 2 has been published. Le, K., Narayana, H., Servati, A., Bahi, A., Soltanian, S., Servati, P., Ko, F. (2022) “Electronic textiles for electrocardiogram monitoring: a review on the structure–property and performance evaluation from fiber to fabric,” Textile Research Journal. 2022; doi:10.1177/00405175221108208. Geometric roughness measurement data presented in chapter 2 were performed by Ms. S. Tamura at Kyoto Institute of Technology (Japan). A version of chapter 3 has been published. Le, K., Servati, A., Soltanian, S., Servati, P. and Ko, F. Performance and Signal Quality Analysis of Electrocardiogram Textile Electrodes for vii Smart Apparel Applications. Front. Electron. 2:685264. doi: 10.3389/felec.2021.685264. This work was a MITACS Accelerate project with VITALI Wear Inc. A manuscript is in preparation for Chapter 4. Le, K., Narayana, H., Servati, A., Soltanian, S., Servati, P., Ko, F. “Influence of Yarn Geometry on Electrical Properties of Conductive Silver-coated Nylon Filaments for E-Textiles: A Fundamental Study.” Dr. H. Narayana assisted with fabrication of the embroidered electrodes characterized and tested in this work. Biopotential signal monitoring tests reported in chapters 4 to 7 were carried out under the approval of UBC Clinical Research Ethics Board, UBC CREB: H21-03312. Ms. Patricia Forcier from Groupe CTT (Quebec, Canada) fabricated the knitted materials studied in Chapter 5. A manuscript is in preparation for chapter 6. Le, K., Narayana, H., Servati, A., Soltanian, S., Servati, P., Ko, F. “Roll-to-Roll Fabrication of Silver/Silver Chloride Coated Yarns for Dry Electrodes and Applications in Biopotential Signal Monitoring.” Dr. S. Soltanian built the coating system based on configurations specified by K. Le. Chapter 7 is based on work conducted in collaboration with UBC Sustainable Functional Biomaterials Laboratory: Prof. F. Jiang, and X. Sun, and Terasaki Institute for Biomedical Innovation: Dr. A. Khademhosseini, Dr. J. V. John, J. Chen, and Dr. H. Heidari. A manuscript based on work from chapter 7 is in preparation. Le, K., Sun, X., Chen, J., John, J. V., Servati, A., Heidari, H., Khademhosseini, A., Ko, F., Jiang, F., Servati, P. “Stretchable, self-healing, biocompatible, and durable chloride-based ionogel for continuous wearable strain and physiological monitoring.” K. Le and X. Sun share equal authorship. Both authors worked together on the study design, fabricated the materials, performed testing, and wrote portions of the manuscript. Biocompatibility tests were carried out by collaborators at Terasaki Institute for Biomedical Innovation (California, USA). viii Table of Contents Abstract ............................................................................................................................. iii Lay Summary .....................................................................................................................v Preface ............................................................................................................................... vi Table of Contents ........................................................................................................... viii List of Tables ....................................................................................................................xv List of Figures ................................................................................................................. xix List of Symbols ............................................................................................................ xxvii List of Abbreviations ................................................................................................. xxviii Acknowledgements ........................................................................................................xxx Chapter 1: Introduction ....................................................................................................1 1.1 Research Questions ............................................................................................. 3 1.2 Objectives ........................................................................................................... 4 1.3 Methodology ....................................................................................................... 5 1.4 Scope and Thesis Organization ........................................................................... 6 Chapter 2: E-Textiles for Biological Signal Monitoring: Review on the Structure-Property and Performance Evaluation from Fibre to Fabric .......................................8 2.1 Introduction ......................................................................................................... 9 2.2 Textile Sensors for Electrocardiogram Monitoring .......................................... 11 2.3 Interfacial Properties – Skin-Electrode Interface .............................................. 14 2.4 Textile Structural Properties – Fibres to Fabrics .............................................. 15 2.4.1 Fibres and Yarns ........................................................................................... 17 ix 2.5 Fabric Structures ............................................................................................... 20 2.5.1 Knitted Fabric Structure ................................................................................ 20 2.5.2 Woven Fabric Structure ................................................................................ 24 2.5.3 Nonwoven Fabric Structure .......................................................................... 27 2.5.4 Embroidery ................................................................................................... 28 2.6 Performance Properties of Textile ECG Electrodes ......................................... 31 2.6.1 Electrical Properties ...................................................................................... 32 2.6.2 Textile Material Properties – Stability and Durability of Sensors ................ 36 2.6.3 Signal Quality Analysis ................................................................................ 44 2.6.3.1 Point Scale Weighing Criteria and Weighting Coefficients ............................. 45 2.7 Influence of E-Textile Design on Performance Properties ............................... 49 2.7.1 Electrode Size, Geometry, and Construction ................................................ 49 2.7.2 Applied Pressure ........................................................................................... 51 2.8 Fabric Handle and Physiological Comfort Evaluation of E-Textiles ............... 52 2.9 Relevant and Upcoming Standards for E-Textiles Testing............................... 55 2.10 Conclusions ....................................................................................................... 57 Chapter 3: Performance and Signal Quality Analysis of ECG Textile Electrodes for Smart Apparel Applications ...........................................................................................59 3.1 Introduction ....................................................................................................... 60 3.2 Materials and Methods ...................................................................................... 62 3.2.1 Electrodes ...................................................................................................... 62 3.2.2 Geometric Roughness Measurement ............................................................ 63 3.2.3 Sheet Resistance Measurement ..................................................................... 64 x 3.2.4 Electrode Pair Impedance ............................................................................. 66 3.2.5 Electrocardiogram Measurement .................................................................. 67 3.2.6 Electrocardiogram Signal Performance Comparison.................................... 68 3.3 Results and Discussion ..................................................................................... 74 3.3.1 Coefficient of Variation (C.V.) in R-R Intervals .......................................... 74 3.3.2 Kurtosis and Skewness ................................................................................. 75 3.3.3 Template Matching – Average Waveform ................................................... 77 3.3.4 Power Spectral Density Comparison ............................................................ 79 3.4 Conclusions ....................................................................................................... 79 Chapter 4: Influence of Yarn Geometry and Twist on the Electrical Properties of Conductive Silver-Nylon Yarns for E-Textiles..............................................................81 4.1 Introduction ....................................................................................................... 82 4.2 Experimental Materials and Procedures ........................................................... 85 4.2.1 Twisted Yarn Preparation ............................................................................. 85 4.2.2 Yarn Linear Resistance Measurement .......................................................... 87 4.2.3 Imaging and Cross-Sectional and Longitudinal Section Measurements ...... 87 4.2.4 E-Textile Electrode Fabrication .................................................................... 88 4.2.5 Electrical Impedance Spectroscopy Measurement ....................................... 89 4.3 Results and Discussion ..................................................................................... 90 4.3.1 Yarn Twist Properties ................................................................................... 90 4.3.2 Twisted Yarns Employed in Embroidered Fabric Structures ....................... 95 4.4 Conclusions ....................................................................................................... 98 xi Chapter 5: Structure-Property Evaluation and ECG Signal Quality Investigation of Knit Textile Electrodes ....................................................................................................99 5.1 Introduction ....................................................................................................... 99 5.2 Materials and Methods .................................................................................... 100 5.2.1 Selection and Fabrication of Knitted Silver-Nylon E-Textiles ................... 100 5.2.2 Characterization of Knitted E-Textile Electrodes ....................................... 103 5.2.3 Skin Replicas and Surface Roughness Measurements................................ 105 5.2.4 Effective Skin Contact Length and Area based on Plain Knitted Fabric Model……………………………………………………………………………...106 5.2.5 Electrocardiogram Testing and Signal Performance Evaluation ................ 108 5.3 Results and Discussion ................................................................................... 111 5.3.1 Textile Electrode Properties and Skin-Electrode Interface ......................... 111 5.3.2 Analysis of ECG Signal Quality based on Electrode Structure-Properties 121 5.4 Conclusions ..................................................................................................... 128 Chapter 6: Roll-to-Roll Electrochemical Fabrication of Silver/Silver Chloride Coated Yarns for E-Textile Electrodes ........................................................................129 6.1 Introduction ..................................................................................................... 130 6.2 Materials and Methods .................................................................................... 132 6.2.1 Fabrication of Ag/AgCl E-Textile Yarns - Roll-to-roll coating system ..... 132 6.2.2 Characterization of E-Textile Yarns and Electrodes .................................. 134 6.2.3 Electrode Fabrication by Embroidery ......................................................... 136 6.2.4 Electrochemical Characterization of E-Textile Electrodes ......................... 137 6.2.5 Electrocardiogram Measurements and Analysis Methods .......................... 139 xii 6.3 Results and Discussion ................................................................................... 141 6.3.1 Roll-to-Roll System Process Parameters and Yarn Electrical Properties ... 141 6.3.2 Electrochemical characterization ................................................................ 143 6.3.3 Yarn electrical and material characterization ............................................. 149 6.3.4 Electrode Characterization .......................................................................... 153 6.3.5 Electrocardiogram of E-Textile Electrodes – Signal Quality Evaluation ... 159 6.4 Conclusions ..................................................................................................... 165 Chapter 7: Stretchable, Self-Healing, Biocompatible and Durable Ionogel for Continuous Wearable Strain and Physiological Monitoring .....................................167 7.1 Introduction ..................................................................................................... 168 7.2 Materials and Methods .................................................................................... 171 7.2.1 Materials ..................................................................................................... 171 7.2.2 Preparation of pTA Ionogel ........................................................................ 171 7.2.3 Preparation of Skin Phantom ...................................................................... 172 7.2.4 Mechanical Properties ................................................................................. 173 7.2.5 Conductivity and Strain Sensitivity Tests ................................................... 173 7.2.6 Thickness .................................................................................................... 174 7.2.7 Bending Characterization............................................................................ 174 7.2.8 Sheet Resistance.......................................................................................... 175 7.2.9 Self-Healing Properties ............................................................................... 175 7.2.10 Electrochemical Impedance Spectroscopy ................................................ 175 7.2.11 X-Ray Diffraction (XRD) ......................................................................... 176 7.2.12 Field Emission Scanning Electron Microscopy (FE-SEM) ...................... 176 xiii 7.2.13 Biopotential Signal Measurement ............................................................. 177 7.2.14 Biocompatibility Testing ........................................................................... 178 7.3 Results and Discussion ................................................................................... 180 7.3.1 Materials Characterization .......................................................................... 180 7.3.2 Mechanical Properties and Self-Healing .................................................... 181 7.3.3 Bending Properties of E-Textiles ................................................................ 184 7.3.4 Electrical Characterization .......................................................................... 185 7.3.5 Mechanical Abrasion, Surface Reformation, and Recyclability ................. 191 7.3.6 Biocompatibility ......................................................................................... 192 7.3.7 Application as Strain Sensors and Biopotential Electrodes ........................ 193 7.4 Conclusions ..................................................................................................... 197 Chapter 8: Conclusions .................................................................................................199 8.1 Summary ......................................................................................................... 199 8.2 Contributions to Knowledge ........................................................................... 203 8.2.1 General framework for characterization and development of textile electrodes for ECG monitoring ............................................................................... 203 8.2.2 Structure-property effects from fibre to fabric levels for selection and design Ag e-textiles ............................................................................................................ 204 8.2.3 Electrochemical fabrication, characterization, and integration of Ag/AgCl e-textiles ..................................................................................................................... 205 8.2.4 Factors influencing skin-electrode interaction, electrode performance and biopotential signal quality ....................................................................................... 206 xiv 8.2.5 Development of a functional, biocompatible, self-healable, and adaptive chloride-based ionogel coating for e-textiles .......................................................... 207 8.2.6 Application of performance evaluation methods to newly developed e-textile and wearable systems .............................................................................................. 208 8.3 Recommendations and Future Work .............................................................. 210 8.4 Publications ..................................................................................................... 212 8.4.1 Journal Articles ........................................................................................... 212 8.4.2 Conference Papers & Presentations ............................................................ 213 Bibliography ...................................................................................................................214 Appendices ......................................................................................................................231 Appendix A .....................................................................................................................231 A.1 General framework for materials testing and signal quality analysis ............. 231 A.2 Contact pressure measurement from e-textile electrodes placed at underband...232 A.3 Surface roughness KES-SMD geometrical roughness data ............................ 233 A.4 Twisted Yarn Diameters ................................................................................. 234 A.5 Rib knit textile electrode properties & signal quality results .......................... 235 A.6 Skin surface replicas for surface roughness observation ................................ 238 A.7 Roll-to-roll fabrication of Ag/AgCl coated yarns – additional figures ........... 239 A.8 Chloride-based ionogel – Additional results................................................... 242 A.9 Modelling Approaches in Experimental Curve Fitting and Equivalent Circuit Parameter Identification Using the Simplified Randle’s Circuit as a Study ............... 246 A.10 Embroidered Ag and Ag/AgCl e-textiles for EEG monitoring ...................... 256 xv List of Tables Table 2.1 Conductive fibres/filaments and yarns used for e-textile applications ........................ 19 Table 2.2 Knit textile electrodes for ECG monitoring ................................................................. 22 Table 2.3 Woven textile electrodes for ECG monitoring............................................................. 25 Table 2.4 Nonwoven textile electrodes for ECG monitoring....................................................... 27 Table 2.5 Embroidered textile electrodes for ECG monitoring ................................................... 30 Table 2.6 Some Performance Requirements for Disposable ECG Electrodes. ............................ 32 Table 2.7 Studies investigating effects of mechanical deformation of textile electrodes on electrical performance properties .......................................................................................... 38 Table 2.8 Washing-laundering standards used or modified for e-textiles .................................... 40 Table 2.9 Studies investigating textile electrode property evaluation before and after launder/wash cycles .............................................................................................................. 42 Table 2.10 Assessment methods applied to ECG signals from textile electrodes and electrodes integrated in wearable devices/systems ................................................................................ 46 Table 2.11 Studies investigating effect of electrode size on performance and signal quality ..... 50 Table 2.12 Studies investigating the effect of applied pressure on electrode performance/comfort............................................................................................................................................... 52 Table 2.13 Standards development technical committees/working groups for e-textiles, existing and upcoming standards for e-textiles .................................................................................. 56 Table 3.1 Conductive fabric (commercially sourced) information .............................................. 63 Table 3.2 Average surface roughness results ............................................................................... 63 Table 3.3 Sheet resistance measurement ranking – lowest to highest ......................................... 65 xvi Table 3.4 Average coefficient of variation (%) from three trials, stationary and movement conditions. ............................................................................................................................. 75 Table 3.5 Average kurtosis values, and standard deviation across 10s ECG segments from 6-minute trials (stationary condition) ranked in descending order of results. .......................... 75 Table 3.6 Average skewness values from three 6-minute ECG trials ranked in descending order of results. ............................................................................................................................... 75 Table 3.7 Summary of PCC calculated from textile electrodes compared with standard gel electrode. ............................................................................................................................... 77 Table 4.1 Yarn twist parameters selected for 1- to 4-plied yarns investigated. ........................... 85 Table 4.2 Calculated theoretical yarn diameter, and measured longitudinal cross-section diameter................................................................................................................................. 92 Table 4.3 Measured impedance values at selected frequencies. .................................................. 96 Table 5.1 Ag-coated nylon yarn properties ................................................................................ 100 Table 5.2 Summary of loop length measurements for each yarn ply group (jersey and rib knit structures) ............................................................................................................................ 102 Table 5.3 Surface roughness profiles of dry skin, wet skin, and the gel electrode .................... 114 Table 5.4 Jersey knit e-textile material properties ..................................................................... 115 Table 5.5 Jersey knit e-textile surface resistance properties ...................................................... 115 Table 5.6 Calculated effective contact length (cm) for single loop ........................................... 116 Table 5.7 Calculated effective contact area per unit cm (considering circular yarn cross-section)............................................................................................................................................. 116 Table 5.8 Estimated circuit element parameters, and selected contact impedance results ........ 117 xvii Table 5.9 Mean surface peak height measurement results, with comparison to skin-electrode resistance estimations.......................................................................................................... 119 Table 5.10 Average IBI coefficient of variation (%) over three ECG trials .............................. 122 Table 5.11 Summary of PCC results calculated from e-textiles compared with standard gel electrodes ............................................................................................................................ 123 Table 5.12 Summary of frequency domain signal quality features, organized with respect to loop length groups ....................................................................................................................... 125 Table 6.1 Reaction time of 10 cm yarn segment in coating bath with respect to yarn winding speed ................................................................................................................................... 143 Table 6.2 Mean potential drift results calculated from 10-minute measurement period ........... 155 Table 6.3 Summary of average impedance results at 1 Hz, 10 Hz, 145 Hz, and estimates of circuit model parameters for electrode materials investigated ........................................... 159 Table 6.4 HR (beats/min) results of ECG trials for Ag and Ag/AgCl e-textile electrodes, measured simultaneously with standard gel electrodes, calculated standard deviation, and coefficient of variation (%) results. .................................................................................... 162 Table 6.5 PCC results for e-textile trials compared with respect to standard gel electrodes ..... 164 Table 6.6 Example results for occupied bandwidth calculations for e-textile and gel electrode ECG signals ........................................................................................................................ 164 Table 7.1 Summary of coating layer thicknesses, ionogel spray coated e-textile ...................... 174 Table 7.2 Summary of average bending rigidity and bending hysteresis results and comparative nylon textile substrate ......................................................................................................... 184 Table 7.3 Sheet resistance results for coated e-textile layers, and ionogel film ........................ 185 xviii Table 7.4 Summary of average impedance results at 1 Hz, 10 Hz, 145 Hz, and estimates of circuit model parameters for electrode materials investigated. .......................................... 190 Table 7.5 Comparison of electrode impedance values for studies investigating IL coatings .... 190 xix List of Figures Figure 2.1 a. Electrocardiogram b. Depiction of depolarization, and repolarization of the heart with resultant waves. ............................................................................................................. 12 Figure 2.2 Equivalent circuits for a) dry and b) wet electrodes, depicting skin-electrode interface [25] ........................................................................................................................................ 14 Figure 2.3 Textile construction platform and structural hierarchy. ............................................. 16 Figure 2.4 Schematic of cross sections of various conductive fibre and yarn structures. ........... 17 Figure 2.5 Schematics of types of (a) warp and weft knitted textile structures [50], and (b) examples of possible integration of e-textiles/conductive yarns/wires into knitted structures................................................................................................................................................ 20 Figure 2.6 Schematic of possible single-layer woven structures for e-textile assemblies (left to right) (a) plain (b) twill (c) satin (d) basket, and (e) an example of possible integration of e-textiles/conductive yarns/wires into woven fabric structures (light grey). ........................... 24 Figure 2.7 Example of nonwoven nanofibre mat. ........................................................................ 27 Figure 2.8 Examples of embroidered structures: (a) Moss embroidered electrodes (embroidery machine). (b) Silver-nylon and (c) Silver-silver chloride-nylon yarns embroidered by sewing machine ..................................................................................................................... 29 Figure 2.9 (a) SEM of platinum thin film strain sensor with microcrack junctions at different strain. (b) SEM of polypyrrole-coated elastane fibre............................................................ 38 Figure 3.1 Sheet resistance measurement results for conductive fabric. Input source current: 1 mA......................................................................................................................................... 65 Figure 3.2 Average face-to-face impedance from three measurement trials of electrode pairs. . 66 Figure 3.3 Schematic of clinical ECG system setup. ................................................................... 67 xx Figure 3.4 a. Underband electrode placement locations (not to scale). b. ECG waveform. c. Overlaid signals for visual comparison -recording of conductive textile and gel electrodes (not performed simultaneously). ........................................................................................... 68 Figure 3.6 Data windows for reach waveform in the ECG signal collected was obtained by centering the R-peak around the medial R-R interval duration, from which the average waveform was obtained. ....................................................................................................... 73 Figure 3.7 Example plot of calculated kurtosis values from 10-second segments of a selected test trial for Fabric A, compared to the gel electrode measurement (performed in the same-day session). ................................................................................................................................. 76 Figure 3.8 a-g Example average waveform for each electrode material compared with gel electrode (Fabrics A-G) ........................................................................................................ 78 Figure 3.9 Spectral plots for the different electrode materials compared demonstrate that the textiles follow the same trend as the gel electrode, with higher overall magnitudes observed................................................................................................................................................ 79 Figure 4.1 a. Nylon yarn specific volume with twist factor [165]. b. Relationship of fibre volume fraction to fibre orientation at various twist levels [79]. ....................................................... 83 Figure 4.2 Selected optical images of twisted yarns. 1-ply a. 30 TPM b. 300 TPM c. 600 TPM; 2-ply d. 30 TPM e. 300 TPM f. 600 TPM; 3-ply g. 30 TPM................................................ 86 h. 300 TPM i. 600 TPM; 4-ply j. 30 TPM k. 300 TPM l. 600 TPM ............................................ 86 Figure 4.3 Yarns wrapped between metal wires, cast in epoxy for subsequent cutting and surface preparation. ........................................................................................................................... 87 xxi Figure 4.4 Cross-sectional images of compressed yarns. a. SEM micrograph of compressed 2-ply yarn b. Optical image of 1-ply cross-section (approx. 218 ±5 µm) c. SEM micrograph of fibre cross-section (approx. 19.5 ±0.8 µm) ........................................................................... 88 Figure 4.5 a. Embroidered electrode from 2-ply twisted yarn b. Schematic of electrode layers c. Schematic of EIS experimental set-up on planar forearm. ................................................... 89 Figure 4.6 Effect of twist level on plied yarn linear resistance and yarn diameter. a. & b. Linear resistance as a function of TPM; c Yarn diameter (longitudinal section) as a function of TPM. ..................................................................................................................................... 91 Figure 4.7 Effect of twist and fibre orientation angle on geometrical properties. a. Calculated yarn specific volume with twist factor b. Calculated fibre volume fraction with orientation angle c. Yarn packing fraction with respect to twist (TPM). ................................................ 94 Figure 4.8 Relationship between yarn twist level and electrical conductance properties. Observable changes in fibre packing and orientation in yarn bundles visible from optical images shown. ....................................................................................................................... 95 Figure 4.9 a-b Contact impedance results from twisted 2-ply embroidered electrodes. c-d Contact impedance results at selected frequencies. .............................................................. 97 Figure 5.1 Microscope images (optical and SEM) of Ag-coated nylon yarns for prototype fabrication. a. Optical images of three yarns selected for prototype fabrication, b. 1-ply, c. 2-ply, d. 3-ply. ........................................................................................................................ 101 Figure 5.2 Summary of knit fabric structures investigated, organized into ply groups and machine loop lengths .......................................................................................................... 101 (higher loop length number corresponds to larger length) .......................................................... 101 Figure 5.3 Schematic of electrode structure, and images of top layer knit fabrics. ................... 102 xxii Figure 5.4 Schematic of knit structure elements ........................................................................ 103 Figure 5.5 a. Schematic of EIS experimental set-up on planar forearm b. Skin-electrode circuit model................................................................................................................................... 104 Figure 5.6 Single knitted loop based on Munden loop model [179]. Reproduced under terms of the CC-BY 4.0. ................................................................................................................... 106 Figure 5.7 Schematic of ECG collection setup .......................................................................... 110 Figure 5.8 Visual comparison of jersey knit e-textile electrode structures. ............................... 118 Figure 5.9 Surface roughness 3D profiles of knitted electrode samples a) 1-ply, LL: 0.46 mm b) 2-ply, LL: 0.46 mm, c) 3-ply, LL: 0.37 mm ....................................................................... 120 Figure 5.10 Plot of average inter-beat intervals (ms) from simultaneous e-textile and gel electrode signal recordings. ................................................................................................ 122 Figure 5.11 Example average waveform plots for knitted e-textiles compared with standard gel electrodes. ........................................................................................................................... 123 a. 3-ply MLL 20, b. 1-ply MLL 25, c. 2-ply MLL 30 ................................................................ 123 Figure 5.12 Example periodograms of e-textile and gel electrodes overlaid to demonstrate differences frequency spread and power. ............................................................................ 126 Figure 5.13 Impedance plots from EIS measurements displaying low frequency instabilities, a) Nyquist plots, b) Bode plots................................................................................................ 127 Figure 6.1 Roll-to-roll Ag/AgCl electrochemical coating system. ............................................ 134 Figure 6.2 Embroidered e-textile electrodes: a. Schematic of structure, b. Ag-nylon (left), Ag/AgCl-nylon (right), ....................................................................................................... 136 Figure 6.3 a. Electrode configuration for EIS measurement on planar forearm, b. Simplified Randle’s Circuit representing skin-electrode impedance [174], c. Electrode configuration xxiii for polarization potential measurement with two sets of electrodes (Ag or Ag/AgCl e-textile and standard Ag/AgCl gel) ................................................................................................. 138 Figure 6.4 ECG electrode configuration and measurement system schematic .......................... 139 Figure 6.5 a. Yarn winding speed (cm/s) calculated from motor speed (V) b. Linear resistance of the original Ag coated nylon yarn measured across a 25cm segment. Average resistance 1.8 ±0.2 Ω/cm. c. Linear resistance was measured for the Ag/AgCl coated nylon yarns at different applied currents between 0.5 mA to 2.0 mA, and winding speeds (0.022 cm/s to 0.272 cm/s) d. Fibre cross-section, showing Ag coating thickness (7.5K x) ...................... 142 Figure 6.6 a. CV scans (10 cycles) under static condition in 0.9wt.% NaCl solution. b. CV scans (10 cycles) under dynamic condition, with yarn winding speed of 0.13 cm/s in 0.9 wt.% NaCl solution. ..................................................................................................................... 146 Figure 6.7 a. PDP measurement for AgCl formation on Ag nylon yarn in 0.9 wt.% NaCl b. Open circuit potential measurement to assess stability of AgCl coating process over a 3-minute period. ................................................................................................................................. 148 Figure 6.8 Resistance of Ag-coated nylon (original), and Ag/AgCl coated nylon across 25 cm yarn length. ......................................................................................................................... 149 Figure 6.9 a. Ag-coated nylon yarn, with EDX map of Ag. b. Ag/AgCl-coated nylon yarn, with EDX map of Ag and Cl. ...................................................................................................... 151 Figure 6.10 XRD patterns for yarn surfaces of Ag-coated nylon yarn, and Ag/AgCl-coated nylon yarns, with annotated peaks corresponding to silver and silver chloride. .......................... 152 Figure 6.11 SEM micrographs of Ag-coated nylon yarn surfaces at a) 3.5k and b) 8.5k magnifications; SEM micrographs of Ag/AgCl-coated nylon yarn surfaces (2.0 mA applied current) at c) 4k and d) 12k magnifications. ....................................................................... 153 xxiv Figure 6.12 Measured surface resistance for a) Ag-nylon embroidered electrodes, and b) Ag/AgCl-nylon embroidered electrodes. ............................................................................ 154 Figure 6.13 a. Mean polarization potential across 10-minute measurement period. b. Boxplot displaying range/spread of potentials measured across 10-minute period.......................... 156 Figure 6.14 a. Example Nyquist plot with measured and fitted data, b. Bode plot displaying impedance vs. frequency for e-textile and standard gel Ag/AgCl electrodes ..................... 159 Figure 6.15 a. Characteristic ECG waveform; Sample visual comparison of ECG recording collected for b. Ag- and c. Ag/AgCl e-textiles simultaneously with standard gel Ag/AgCl electrodes. ........................................................................................................................... 160 Figure 6.16.Example average ECG waveform (PQRST) for Ag and Ag/AgCl coated nylon e-textiles. ................................................................................................................................ 163 Figure 6.17 Example spectral plots for a) Ag coated nylon and b) Ag/AgCl coated nylon embroidered e-textile electrodes compared to standard Ag/AgCl gel electrodes. .............. 165 Figure 7.1 a. IL:pTA solution b. IL:pTA cast film c. IL:pTA spray coated film (left) on conductive fabric ................................................................................................................. 172 Figure 7.2 a. Gelatin dissolution b. Cast skin phantom ............................................................. 172 Figure 7.3 a. Schematic of ionogel coated Ag-woven fabric electrode. b. Schematic of EMG setup for simultaneous recording using standard gel electrodes and ionogel coated textile electrode .............................................................................................................................. 178 Figure 7.4 a. Schematic illustration of TA and [BMIM]Cl polymerization b. Structure of ionogel c. XRD results for ionogel film (1:5, IL:TA) compared with TA powder, d. SEM of ionogel film spray coated on Ag-nylon plain weave fabric 55x, e. 300x ........................................ 180 xxv Figure 7.5 Mechanical properties and self-healing properties. (a) Stress-strain curve of pTA ionogel. (b) Photo of pTA ionogel under stretching. (c) Cyclic stretching-releasing stress-strain curves under different strains and (d) the corresponding dissipated energy. (e) Stress-strain curves of 85 cycles under 50% strain and (f) the corresponding maximum tensile stress. (g) Stress-strain curves of pTA ionogel after different healing periods. ................. 183 Figure 7.6 Bending characterization (a) Fabric being tested in bending tester, KES-FB2-S (b) Example bending moment vs curvature plot for uncoated and coated e-textiles ............... 185 Figure 7.7 Nyquist plot and calculated ionic conductivity. ....................................................... 186 Figure 7.8 a) Example Bode plots displaying impedance vs. frequency for uncoated, coated ionogel Ag-nylon fabrics, and sintered Ag/AgCl electrodes b) Example Nyquist plots with measured and fitted data for uncoated, coated ionogel Ag-nylon fabrics, and sintered Ag/AgCl (reference electrode) ............................................................................................ 189 Figure 7.9 SEM micrographs of ionogel-coated fabric a. Coated fabric b. Abraded fabric c. Abraded fabric after ethanol addition (55x) d. Regenerated film on abraded region after ethanol addition (300x). ...................................................................................................... 191 Figure 7.10 Biocompatibility results a. Cell proliferation (relative fluorescence units, optical density, ex 560 / em 590) b. Cell viability (%) ................................................................... 192 Figure 7.11 Strain sensitivity of pTA ionogel. (a) ΔR/R0 versus strain (0-500%) and (b) the corresponding GF. (c) ΔR/R0 under 100% strain for 300 cycles. ΔR/R0 under different strains: (d) 1%, 2% and 5%; (e) 10%, 20% and 50%; and (f) 100%, 200% and 500%. ..... 194 Figure 7.12 a. Schematic of experimental design to measure pre-recorded ECG signal from electrodes. b. ECG signal obtained from Ag/AgCl-gel and Ag/AgCl-ionogel electrodes. 195 xxvi Figure 7.13 a. Example simultaneous ECG signal recordings from standard reference electrodes and ionogel-coated Ag-nylon e-textile electrodes b. Example simultaneous EMG recordings from standard reference electrodes and ionogel-coated Ag-nylon e-textile electrodes ...... 196 xxvii List of Symbols ° Degree °C Degrees Celsius α Alpha A Ampere C Capacitance den or D Denier – linear mass density of fibres/yarns, grams per 9,000 metres of yarn dtex Deci-tex – leinear mass density of fibres/yarns, grams per 10,000 metres of yarn F Farad g Gram h Hour Hz Hertz I Current k- Kilo- l Length µ Micro- M Mega- m Metre min Minute n Number of trials N Newton Pa Pascal R Resistance S Siemen s or sec Second tex Linear mass density of fibres/yarn – grams per 1000 metres V Voltage X Magnification Z Impedance Ω Ohm xxviii List of Abbreviations AATCC American Association of Textile Chemists and Colorists AC Alternating current Ag Silver AgCl (& Ag/AgCl) Silver chloride (& Silver/Silver chloride) ANSI American National Standards Institute AS Australian Standard (developed by Standards Australia) Biosignal (or bio-signal) Biological signal [BMIM]Cl 1-Butyl-3-methylimidazolium chloride bpm or beats/min Beats per minute CAD Canadian Dollar CAGR Compound annual growth rate CE Counter electrode CNT Carbon nanotube CV Cyclic voltammetry DC DI Direct current DI Deionized MF Dimethylformamide e- Electron e-textile Electronic textile ECG Electrocardiogram EDA Electrodermal activity EDX (or EDS) Energy-dispersive X-ray spectroscopy EEG Electroencephalogram EIS Electrochemical impedance spectroscopy EMG Electromyography g/m2 or gsm Grams per square metre HR Heart rate HRV Heart rate variability IBI Inter-beat interval IEC International Electrotechnical Commission IL Ionic liquid IPC Association Connecting Electronics Industries (Accredited by the American National Standards Institute) ISO International Organization for Standardization JSA Japanese Standards Association MLL Machine Loop Length NaCl Sodium chloride PANI Polyaniline PDMS Polydimethylsiloxane xxix PDP Potentiodynamic polarization PEDOT:PSS Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate PPG Photoplethysmography PSD Power spectral density pTA Poly-thioctic acid (also known as poly-alpha-lipoic acid) RE Reference electrode RPM, rpm Revolutions per minute SCE Saturated calomel electrode SEM Scanning electron microscope (FE-SEM: field emission SEM) TPI Twists per inch SNR Signal-to-noise ratio TPU Thermoplastic polyurethane SERS Surface-enhanced Raman spectroscopy/scattering SQI Signal quality index/indices TPM Twists per metre TRL Technology readiness level USD United States Dollar WE Working electrode XRD X-ray diffraction . xxx Acknowledgements My sincerest thanks to my supervisors Prof. Frank Ko and Prof. Peyman Servati for the opportunity to work in this interdisciplinary research area, and for the diverse and collaborative research experiences they encouraged me to explore throughout the course of my degree program. Prof. Ko, your wisdom, expertise in many topic areas, vision, and curiosities in how we can better understand and contribute to the world of e-textiles and wearables through the lens of textile and materials engineering have guided my direction, and ability to tackle new and intriguing research questions from multiple perspectives. Prof. Servati, your methodical approach to well-justified decision making in experimentation, and in challenging the relevancy and applicable bearing of research directions, carries into your continued focus on the impact of tools and technology we develop for the world as engineers and scientists, and this has been inspiring to learn from. Many thanks to my PhD committee, Prof. John Madden, and Prof. Rizhi Wang, for their constructive suggestions, knowledge, challenging and curious questions that have helped to improve this work. I would also like to thank my examination committee, Prof. Elod Gyenge, Prof. Urs Hafeli, and Prof. Jinlian Hu for their meticulous review and feedback on this thesis. Financial support from the Department of Materials Engineering, University of British Columbia Four-Year Fellowship, MITACS Accelerate, VITALI Wear, Natural Sciences and Engineering Research Council of Canada, and Canada Foundation for Innovation, is gratefully acknowledged. I thank Dr. Saeid Soltanian, Dr. Amir Servati, Dr. Harishkumar Narayana, and Dr. Addie Bahi for their valuable insights, help and support on experimental work, tools, and analysis. Acknowledgements to Dr. Kevin Cole for the impromptu discussions, suggestions, insights and feedback on electrochemical process characterization. Thank you to Prof. Feng Jiang, and Xia Sun xxxi for enthusiastically agreeing to collaborate on my final study that spun out of a curiosity to leverage properties of ionic liquids. Thank you to Heli Eunike (SEM, XRD) from the Materials Engineering Department, and Dr. Guobin Sun (confocal microscopy) from the Centre for High-Throughput Phenogenomics for their technical support. I would also like to thank the past and present members of the AFML and FEEL laboratories for their helpful discussions, encouragement, and friendship. I am grateful for my wonderful friends who I can always count on to ensure I get enough fresh air, laughs, elevation gain, stair reps, disco, and Type 2 fun. Special thanks to Bart Scicchitano, Dr. Sridevi Thomas, and Dr. Sherry Esfahani for their friendships over the many years, shared research successes and battles, and for reviewing this thesis. Finally, I would like to thank my parents, pillars of strength and support. Their compassion, hard-work, and dedication to lifelong learning are admirable. From my mom, I have learned determination, the power of the human spirit as a guide to living. From my dad, I have learned the inextricable link between philosophy and research, and what it means to dedicate time to deeply ponder and explore research curiosities to uncover new knowledge, which is the true underpinning of a doctorate. xxxii “[...] study hard what interests you the most in the most undisciplined, irreverent, and original manner possible.” -Richard P. Feynman, 1965 R. P. Feynman, T. Ferris, and M. Feynman, Perfectly reasonable deviations: From the beaten track: The letters of Richard P. Feynman. New York: Basic Books, 2005. 1 Chapter 1: Introduction With technology developments and innovations continuously transforming healthcare, the industry is constantly improving to provide high quality of care and efficient delivery models. The global pandemic has brought about accelerated implementation of telehealth services, which forms part of the digital health delivery model that is expected to continuously evolve in care delivery offerings. Insights into the future of care delivery include a patient-centric approach with virtual or in-home care, that will be driven by data and analytics, and technology development [1]. Medical technologies and devices play a crucial role in the development of the digital health delivery model. Smart wearable systems for continuous and long-term physiological health monitoring of patients offer the potential to improve care delivery in an efficient and cost-effective manner beyond clinical settings [2]. Continuous data collected from these devices can provide clinicians with a comprehensive understanding of a patient’s health condition, which can help guide treatment direction, significantly improving overall patient care [3]. Holter monitors to observe cardiac activity are an excellent example of wearable systems that have been used for decades and have provided a greater understanding of heart disease [3]. In more recent times, progress in the development of wearables has focused on developing unobtrusive systems to monitor a variety of physiological signals. This has been approached by utilizing different form factors such as flexible bands, patches, soft and conformable thin films that can be attached to the skin or body, or textiles and clothing that can be worn. However, challenges exist in the durability and reliability of developed sensors and collection systems. Wearable sensors and systems must be able to record high-quality signals under stationary and dynamic use cases. In addition, advancements in powering these systems through battery or energy storage technologies must be developed to enable long-term and continuous monitoring. Privacy and security of collected data 2 have also been identified as areas requiring more standards and regulations. Addressing these challenges through an interdisciplinary approach will enable the adoption of wearable systems within the digital health delivery platform, which has potential to greatly improve quality of care and life, while also reducing costs for both the industry and patients in the long-term. Electronic textiles (e-textiles) refer to textiles with integrated capabilities, such as conductive materials with embedded electronic functionality including passive or actuating sensors, communication, and data processing [4]–[6]. Developments stemmed from early work in the 1990’s on wearable technology by Park and Jayaraman, from Georgia Institute of Technology (USA), who presented a vision for combining the areas of computing and textiles into a platform called the Wearable Motherboard™ - “Smart Shirt,” comprising various sensors for fully monitoring an individual’s condition unobtrusively, while also providing feedback mechanisms in real-time [7]. Subsequently, smart wearable systems, and e-textiles have developed at a rapid pace, with many advancements in textile-based sensors for biological signal and vital signs monitoring. While a multitude of materials have been developed and applied to various biological signal monitoring applications, a systematic approach to the understanding of e-textile structural properties, from fibre and yarn to fabric levels, with respect to performance properties, has not been organized to inform the design of sensing electrodes. The focus of this dissertation is to organize the understanding of structural hierarchy in e-textile materials, to properties related to function and performance of textile electrode sensors that may form part of a smart wearable system. This is subsequently used to assess how the electrode properties impact biopotential signal measurement and quality, specifically applied to electrocardiography (ECG). 3 1.1 Research Questions Initial work at the start of this degree program involved an industry research project with a Canadian company, VitaliWear, entitled: Mechanisms underlying stability and accuracy of electrocardiogram textile electrodes. The work focused on textile electrodes used for an everyday women’s smart bra. Research outcomes from this project included: the investigation of differences in ECG signal quality between textile materials compared to standard Ag/AgCl gel electrodes, development of testing methodology to compare data collected from the company’s test board compared with a clinical ECG system, and recommendations for textile-electrode structural requirements and properties from company-sourced fabrics. From this research, knowledge gaps identified included: standard methods or frameworks to compare signal quality between textile electrode materials, design criteria for e-textile sensors, characterization of electrode and skin interfacial contact area on signal quality. The following research questions posed from this work became the basis for the development of the thesis objectives: 1. How can we define and describe ECG signal quality, specific to developments for smart apparel products? - Parameters to investigate: time and frequency characteristics, statistical/trend-based analysis, signal quality indices 2. What influences signal quality, and what role do e-textiles play? - Parameters to investigate: material, structure, environment, skin-electrode interface 3. How can we apply these findings for use in materials/technology development? - Quantify and compare textile electrode material performance within a wearable product/system; test and improve textile electrode performance and design 4 1.2 Objectives The objective of this research was focused on the structure-property evaluation and design of e-textile dry electrodes used for biological signal monitoring applications. A portion of the work involved the organization of the existing knowledge base on biopotential textile electrode development and characterization in the last two decades. Subsequently, application of findings was made towards the materials selection, evaluation, and design of ECG textile electrodes from the yarn to fabric levels. The following objectives were explored: 1. Investigate textile electrode structural and performance requirements for reliable biopotential signal collection, with considerations into long-term functionality and durability. 2. Develop an improved method for electrochemical silver/silver chloride (Ag/AgCl) yarn coating to be used in e-textile electrode fabrication; utilize electrochemical characterization to understand process. 3. Identify and characterize factors that impact skin-electrode interfacial interaction and examine their relationship to material and signal properties. 4. Application of material property understanding, and performance evaluation methods from Objectives 1-3 to various e-textile and wearable systems, to assess and quantify performance. 5 1.3 Methodology Methods to investigate the objectives identified are described: 1. Investigate textile electrode structural and performance requirements for reliable biopotential signal collection (electrocardiogram), with considerations into long-term function and durability (Chapters 2-5,7). - Literature survey, organization of materials and signal performance metrics - Identify properties of interest (fibre, yarn, fabric levels), and optimal structures for textile electrode design - Considerations into improving e-textile electrode durability and lifetime, using ionic liquid-based (ionogel) electrolyte coating 2. Develop an improved method for electrochemical Ag/AgCl yarn coating to be used in e-textile electrode fabrication (Chapters 6). - Improve existing roll-to-roll yarn coating system developed in research group, and characterization of electrochemical coating process to ensure reproducibility and consistency of optimal yarn coating parameters - Characterize electrode performance properties from fabric structures constructed using Ag/AgCl coated yarn 3. Identify and characterize factors that impact skin-electrode interfacial interaction and examine their relationship to material and signal properties (Chapters 4-6). - Characterize skin-electrode interface and interaction through electrochemical impedance spectroscopy and polarization potential measurements 6 - Examine and identify influencing factors affecting skin-electrode interface and electrode performance: surface roughness, effective contact area, applied pressure, moisture, and hydration 4. Application of material property understanding, and performance evaluation methods from Objectives 1-3 to various e-textile and wearable systems. - Other studies: 1. Signal quality evaluation of commercially sourced silver-nylon fabrics for ECG textile electrodes (industry project, Chapter 3, Appendix A.1); 2. Incorporation of e-textile electrodes and sensors into textile wristband for multimodal monitoring of physiological parameters (Conference publication, reference provided in Section 8.4.2); 3. Embroidered silver and silver/silver chloride electrodes for electroencephalogram (EEG) monitoring (Appendix A.10); 4. Application of ECG signal quality comparison methods for investigating performance of liquid-free ionic conducting gel made from deep eutectic solvent hydrolyzed cellulose fibers (Journal publication, reference provided in Section 8.4.1) 1.4 Scope and Thesis Organization Chapter 1 is an overview of remote health monitoring and the role of wearable systems and e-textiles, and identifies the objectives, methodology and scope of the research. Chapter 2 is a literature review that has been published in Textile Research Journal and reproduced in this thesis with minor edits. The review focuses on the organization of structure-property and performance of e-textiles used for electrocardiogram monitoring from fibre to fabric levels. Chapter 3 details work carried out in applying performance and signal quality evaluation metrics for silver-coated nylon e-textile fabrics to be used as electrodes in a smart bra. This work has been reproduced with minor edits and has been published in Frontiers in Electronics. Chapters 4, 6, and 7 form part of 7 manuscripts reproduced with minor edits, and as a result, some information in the introduction and methods sections have been repeated. The focus of Chapter 4 is on the relationship of silver-coated nylon yarn geometry and twist on electrical properties of conductive yarns, with subsequent examination of yarn twist selection on the impedance properties of embroidered e-textile structures. Chapter 5 investigates yarn and fabric structure selection and design on electrode and skin-electrode interfacial properties of weft knitted e-textile electrodes applied to electrocardiogram signal quality. Chapter 6 describes the electrochemical fabrication of Ag/AgCl yarns, process and material characterization, application to embroidered e-textile electrodes, and evaluation of biopotential signal monitoring capabilities. As an exploration into durability aspects for e-textiles, Chapter 7 presents a newly developed biocompatible chloride-based ionogel material that can serve as a self-healable, sprayable, or castable conductive coating for e-textile electrodes, as well as a standalone strain sensing film. Chapter 8 summarizes the major contributions of this research work, specifically to the field of e-textiles for health monitoring applications and provides recommendations for future work in the topic area. 8 Chapter 2: E-Textiles for Biological Signal Monitoring: Review on the Structure-Property and Performance Evaluation from Fibre to Fabric The following chapter has been reproduced from the following publication with edits: Le K, Narayana H, Servati A, Ko F, Servati P. “Electronic textiles for electrocardiogram monitoring: a review on the structure–property and performance evaluation from fiber to fabric,” Textile Research Journal. 2022. doi:10.1177/00405175221108208 The development of electronic textile used for wearable devices and systems for healthcare monitoring applications has experienced rapid growth in the last decade. Knowledge and understanding of textile structural hierarchy, as well as the ability to define properties from fibre and yarn to fabric level are crucial to the selection of materials, and design and performance of wearable systems. However, few studies have applied a bottom-up approach to the selection of optimal e-textile structures with respect to material, electrical, and signal performance properties of sensors used for long-term biological signal monitoring. In this work, a review of e-textile structural properties (fibre, yarn, and fabric) for electrocardiogram (ECG) electrodes is presented. The relationship of structural properties to performance properties including electrical, material, ECG signal quality, fabric hand (sensory perception and quality), and physiological comfort are examined. Considerations and insights into textile fibre and yarn morphology, electrode structure, design and construction are outlined. In addition, relevant and upcoming standards for e-textiles testing and performance evaluation are summarized. This work serves to organize requirements for ECG textile electrodes into a general reference framework from a bottom-up approach, which can better guide the materials selection and design of ECG textile electrodes for wearable apparel applications. 9 2.1 Introduction Smart wearable systems have been developed for a variety of functions including sensing and monitoring of physiological signals, user activity and environment, reacting to stimuli, or providing useful feedback from collected signals and data. It is expected that the wearable devices market will reach $25 billion (USD) by 2022 with an estimated compound annual growth rate (CAGR) of 25.8% [8]. These systems have various applications in areas including healthcare, fitness and sport, security, military, and lifestyle monitoring [9]. Recent progress in the development of these systems has included continuous miniaturization, advances in sensor and interconnect materials, communication technologies, data analysis and classification. Electronic textiles (e-textiles) are a category of materials in smart wearable systems which are of great interest to the healthcare industry, with developments applied to physiological signal monitoring. The global market for e-textiles is expected to reach $5.55 billion (USD) by 2025 (CAGR: 30.4%)[8], with medicine and healthcare representing one of the largest investment sectors. E-textiles are advanced conductive fibre-based materials with either embedded or integrated electronic functionality that includes passive sensing to actuating sensors, communications, and data processing [10]. The development of textile-based sensors integrated into flexible wearable systems is significant in providing continuous and long-term monitoring of patients, which can guide treatment and enable earlier diagnostics by practitioners, thereby reducing costs for both the industry and patients. This could also revolutionize mode of treatment via telemedicine platforms. In their form, the textile sensors may be imparted with a wide range of functions conventionally employed for rigid electronic products[10], [11]. Developments in textile sensors as part of smart wearable systems for healthcare applications present vast opportunities for non-invasive, continuous, and long-term monitoring in daily life, or rehabilitation purposes outside 10 clinical environments. These developments can improve patient care, support disease detection and diagnosis, offering advancements to the healthcare industry. E-textiles are advantageous owing to their inherent flexibility, breathability, and conformability to the human body. As such, they can be integrated as a combined network of sensors for multimodal monitoring. Smart textile systems refer to textile-based systems that exhibit an intended or exploitable response to changes in the surroundings/environment, or external signal/input [12]. Textile-based sensors within a smart textile system may take the form of fibres, filaments, yarns, assembled into woven, knitted, or non-woven structures, with functionalized capabilities. E-textile-based systems for health monitoring have focused on sensor materials embedded in clothing to record biological signals or vital signs (electrocardiogram, electromyogram, heart rate, body temperature, breathing, etc.). The sensor materials are interconnected to other electronic components (cables/connectors, control board, power supply) within the system for data transmission to be observed or analyzed. The unique characteristics derived from the hierarchical structure of textiles, and properties of silicon/metal-based electronic materials present both challenges and opportunities for e-textile development [13]. Fundamental challenges relating to e-textile developments include mechanical compatibility between rigid semi-conductor/metal-based electronic components and soft compliant textiles, especially in terms of routing circuitry with compatible interconnects to system components[13], material property considerations (mechanical, chemical, thermal, electrical properties) [14], with respect to geometry, flexibility, and durability of the overall system assembly. The combination of functional textile sensor material requirements with anthropometric considerations influence performance and compatibility of the materials during use [15]. 11 Consequently, the interplay of bio-mechanical and -chemical surface properties next to human skin is another significant challenge [16], [17]. Ideally, an e-textile based wearable system should consist of a flexible and durable form factor that can interface well next to the human body, while also integrating optimal textile structures for the intended sensor material to provide reliable and continuous monitoring of biological signals. Few studies have been performed to evaluate and select ideal materials and structures for e-textile sensors applied to biological signal applications, specifically in considering a combination of material and electrical properties, signal fidelity, and long-term durability and comfort. There is still a need to optimize e-textile sensor assemblies through detailed understanding of textile engineering, to uncover the underlying relationship between structure parameters, material behaviour, and electrical properties and signals collected. This review focuses on research into textile structural properties (fibre to fabric) for the design of textile electrodes used in health monitoring, specifically in electrocardiogram (ECG) applications. From the understanding of the textile structural hierarchy, material performance properties and assessment methods were reviewed. In doing so, a more complete understanding of appropriate considerations for the materials selection for ECG textile electrodes used in smart apparel systems was achieved. 2.2 Textile Sensors for Electrocardiogram Monitoring Electrocardiography is a widely studied bioelectric signal, in which the electrical activity of the heart muscle, or myocardium, is recorded. It is the electrical potential (differential measurement) across the skin surface of the human body. The flow of electric current associated with contractions of the heart muscle, produces voltages (millivolts) detectable on the skin surface, which reflects the depolarization and repolarization of cardiac cells [18]. The ECG provides information and assessment of normal cardiac (or “sinus”) rhythm, heart health, arrythmias, other 12 cardiovascular diseases, and information relevant for the prediction and treatment of coronary events [18]. Electrical currents associated with the contractions produced by the heart muscle can be detected on the skin surface (millivolt scale) by electrodes [11], and the signal measured is represented as the ECG waveform, comprising four main segments. They reflect the atrial depolarization (P wave, PR segment – atrial ventricular nodal delay), depolarization (QRS complex, with atrial repolarization simultaneously) and repolarization of the ventricles (ST segment – ventricles contract and empty, T wave) of the heart (Figure 2.1): Figure 2.1 a. Electrocardiogram b. Depiction of depolarization, and repolarization of the heart with resultant waves Reproduced under terms of the CC-BY license 4.0 [19]. While a clinical 12-lead ECG system offers advantages in providing a full picture of the heart condition and function, single lead ECG measurement has become more widely employed in the development of portable and wearable biopotential monitoring systems. The single lead measurement system requires the application of three electrodes, usually with two active (positive and negative) and one reference/ground electrode, placed on left and right sides of the body (between arms or across chest); however, it is also possible to gather six leads of ECG with these three electrodes, by alternating the active and passive electrode positions. The single chest lead configuration offers heart rate and rhythm monitoring but does not reveal sufficient detail for diagnostic signal quality measures. a. b. 13 Electrodes are used to measure surface potentials from the body, when placed at specific locations on the surface of the body (skin), thereby serving as signal acquisition apparatus. Contact-based or resistive electrodes, which are the focus of this review, require direct and stable electrical contact with the skin, and can be categorized as wet and dry. Ionic current from the body can be transduced into electron current, from electrode to wires, respectively [20]. Disposable silver/silver chloride (Ag/AgCl) gel electrodes are the gold standard material for the collection of biopotential signals from the body, as a result of the high conductivity of silver, the weakly polarizing nature of Ag/AgCl, which enables low and stable skin-electrode contact impedance, translating to high quality signals [18], [21]. However, Ag/AgCl gel electrodes are not ideal for long-term monitoring since the gel electrolyte dries out over time, and can cause discomfort from the wet gel, adhesive and rigid supporting structure. Alternatives to wet gel electrodes for long-term monitoring have been explored, which have largely included dry electrodes that can be either rigid metal plates such as stainless steel discs, or microfabricated silicon structures, and flexible materials (polymer films or textile and foam based) which can better conform with the skin and provide more comfort and contact area [22]. Drawbacks of dry electrodes include higher sensitivity to motion artifacts, due to increased skin impedance in the absence of an electrolyte component; however, sweat and moisture on the skin surface over time is thought to act as an electrolyte layer. Marquez et al. prepared textile-based electrodes, and humidified the surfaces with water prior to testing, to improve electrical interface), and compared their performance with wet electrodes, observing no significant differences of collected signals [23]. The development of textile structured materials as dry electrodes has been explored and applied to practical applications in the last decade. 14 2.3 Interfacial Properties – Skin-Electrode Interface The skin is comprised of three layers: the epidermis layer, the dermis layer, and subcutaneous layer [24]. The epidermis, being the outermost of the skin interfaces the electrode, and plays the most important role in the interfacial properties, while the other two layers contain sweat glands. The epidermis is divided into three sublayers: the stratum corneum, which is the outermost layer that serves as a semi-permeable membrane to ions, stratum granulosum, and stratum germinativum [24]. There are several variables involved in understanding the interaction between skin and electrode. For a reaction system containing an electrode in contact with an electrolyte or skin, the ion-electron exchange or passing current, results in the formation of a double charge layer which generates a half cell potential. The double layer of charge consists of the redistribution of cations and anions from the metal electrode and electrolyte, respectively [25]. Half-cell potentials exist between all metals and electrolytes to various extents, and can be measured using a reference hydrogen electrode, with zero potential [21]. Figure 2.2 displays a general model representation of the electrical relationship between skin and electrodes -distinguished into dry and wet electrode types [26]. The model depicts a series connection of parallel resistor-capacitor circuits, with each element representing layers within the interface. The difference between the two electrode types is the layer present between the stratum corneum and electrode interface. Wet electrodes (standard gel reference electrodes) will have a resistive electrolytic gel layer, while Figure 2.2 Equivalent circuits for a) dry and b) wet electrodes, depicting skin-electrode interface [25]. © [2016] IEEE 15 dry electrodes do not, but will experience a greater effect from an air layer, modelled as a parallel resistor-capacitor circuit. In general, one high-impedance layer dominates the noise properties of the electrode (dry skin, fabric), and influences the overall electrode properties (electrode offset, long-term stability, or drift). Knowledge of skin-electrode impedance for a given electrode material is important for the acquisition of biopotential signals, with low impedance being ideal. High skin-electrode impedance results in a decreased signal amplitude and lower signal-to-noise ratios. Skin-electrode interface impedance has been cited to range from 1 Ω/cm2 to 5 Ω/cm2 [27]. As a reference, commonly used Ag/AgCl electrodes have an impedance of 3.5 Ω/cm2 [27][28]. 2.4 Textile Structural Properties – Fibres to Fabrics E-textiles are a promising platform for the development of flexible, unobtrusive, and multifunctional electronic systems for health monitoring applications. The inherent properties of textile structures allow for stretch, breathability, and the ability to conform and contour well next to the human body, when integrated as electrode sensors. These characteristics of textiles are derived from their hierarchical structure, which is a primary aspect to be considered in designing textile electrodes [13], [18]. Textile structural hierarchy (Figure 2.3) encompasses fibre and yarn to fabric levels (woven, knitted, non-woven, embroidered, braided), and their assembly into multi-layers, to composite structures. Also described are possible e-textile formations from the structures. 16 Figure 2.3 Textile construction platform and structural hierarchy. Chemical and morphological characteristic of fibres, as well as geometric structure of the fibrous assemblies in fabric structure influence textile properties [13]. Fibres are the basic level of textile structure, characterized by assemblies of highly oriented molecular chains. Fibres can be continuous or discrete (staple) lengths and can be assembled into yarns by twisting mechanism. Subsequently, yarns can be interlaced into fabric structures by weaving, knitting, or braiding. Key requirements in textile electrode development include (i) the unobtrusive integration of electronic functionalities into textile form required for a given electrode material, and (ii) preserving the desirable characteristics of textiles [13],[18]. Textile structure properties include sufficient mechanical strength, durability, and flexibility to stresses such as stretching, bending, twisting, and abrasion during fabrication and end-use [18]. Textile electrode fabrication involves the integration of conductive materials at any levels of the material structural hierarchy. Electrical conductivity can be imparted through various means including the use of metals (thin twisted wires, plating), conducting particle/polymer nanocomposite dispersions, conductive coatings of yarns and fabrics electroplating, electroless plating, physical vapour deposition, dip coating, 17 chemical polymerization, and patterning/printed layers on the fabric surface (out-of-scope for this review) [6], [29]–[33]. Tables 1-4 list various conductive fibres/yarns and fabrics (knits, wovens, non-wovens, embroidery, and stitching) used in e-textile development. 2.4.1 Fibres and Yarns Figure 2.4 Schematic of cross sections of various conductive fibre and yarn structures. Reproduced with permission [34]. Copyright 2018, John Wiley and Sons. Conductive fibres and yarns can be fabricated in several ways, using metals and conductive polymers (Figure 2.4). Metallic monofilament fibres possess high conductivities (upwards of 105 S/cm), however their mechanical properties present challenges for integration into mass production and conventional textile manufacturing processes, due to their stiffness and low extension properties. Consequently, metal fibres may be blended or twisted with non-conductive fibres as hybrid threads, which exhibit better mechanical properties closer to conventional yarns [34]. Metallized yarns are produced by applying thin metal coatings to non-conductive filaments (cotton, nylon, polyester). They are generally suitable for textile fabrication processes such as twisting, knitting, weaving, and embroidery owing to their good mechanical properties. However, they are often unreliable under long-term use and wash, owing to degradation and damage of the metallic coating [34]. Conductive polymers may either be drawn into filaments (intrinsically conductive polymers, ICPs), blending insulated polymers with conductive fillers (extrinsically conductive polymers, ECPs) to form fibres, electrospun into nanofibre mats, or applied as a conductive paste/coating for fabrics[13]. While they have been shown to exhibit good mechanical performance, with demonstrated capacity to be processed into woven, knit, and embroidered 18 structures, they often have lower electrical properties than metallic yarn counterparts. Table 2.1 lists examples of various conductive fibres and yarns and corresponding conductivities, that are commercially available and developed on the research scale used in e-textile development. General insights and observations for fibre and yarn selection in e-textile applications have been identified [18] [35]: - Fibre should be continuous rather than staple length, to decrease signal noise and contact resistance - Coated layer of fibre/yarn should be thick and a good electrical conductor, with large surface area-to-volume ratio, to decrease resistive effects of core - Yarn should be compacted instead of textured to provide more conductive fibres for a given diameter - Yarn should be elliptical or flat (cross-section) instead of round, to increase capacitance - Higher yarn filament number and stitch density can result in slightly higher conductivity and lower skin-electrode impedance, and increase in effective skin-electrode contact 19 Table 2.1 Conductive fibres/filaments and yarns used for e-textile applications Yarn Type[Ref] Conductivity S/cm Spinnability METALLIC FILAMENTS Silver[36] Copper/Silver[37] Steel 304[37] Bekintex – Bekaert fibre[38] (continuous cold drawn stainless steel fibres) HYBRID THREADS Bekintex 50/2 – Bekaert fibre (steel/polyester spun yarn)[38] VN 140 nyl/35 x 3 (nylon core wrapped with continuous SS wires)[38] METAL COATED YARNS Statex – Shieldex silver plated nylon thread, 117/17 dtex, 2ply Noble Biomaterials – Circuitex™ silver plated nylon yarn, 100/34 den, 2-ply Silver/Single-walled CNT on polypropylene [39] Aracon – DuPont, metal clad aramid fibre (core: Kevlar; cladding materials: Ag, Ni, Cu, Au, Sn)[38] Copper/Nickel on carbon fibre[40] CONDUCTIVE POLYMER COMPOSITE COATED YARNS PEDOT (poly(3,4-ethylenedioxythiopene) on polyester[41] PEDOT on viscose[42] Polypyrrole/Multi-walled CNT on polyester[41] Polyaniline (PANI) on polyester[43] INTRINSICALLY CONDUCTIVE POLYMERIC – FIBRES PEDOT:PSS microfibres (PSS: polystyrene) [44], [45] High molecular weight PANI[46], [47] PANI:AMPSA (2-acrylamido-2-methyl-1-propane sulfonic acid)[48] PEDOT:PSS/PVA multifilament (PVA: polyvinyl alcohol)[49] 6.25x105 5.75x105 1.4x105 1 50 10 0.5 0.4 4x10-2 1x10-3 5.9x10-6 1000 14.2 4x10-3 1.7x10-3 2804, 0.01 72 (unstretched), 725 (stretched) 70-150 7±1 Extrudable/drawn into wires/filaments & twistable Spun polyester & drawn filaments are twistable Staple nylon fibres wrap able on drawn SS wires/filaments. Coated filaments/yarns are twistable only Coated filaments/yarns are twistable only Staple fibres spinnable into yarn. Multi-filaments are twistable 20 2.5 Fabric Structures 2.5.1 Knitted Fabric Structure Knitting is a textile construction method in which yarns are interlooped using a basic assembly of knitting needles, stitch cams, and sinkers (Figure 2.5a). An assembly of interconnected loops are referred to as courses or wales, where courses consist of loops that run horizontally (across the length of the fabric), and wales/warp are columns of loops that run vertically across the fabric (along/down the length of the fabric) [50]. Figure 2.5 Schematics of types of (a) warp and weft knitted textile structures [50], and (b) examples of possible integration of e-textiles/conductive yarns/wires into knitted structures. Knitted fabrics can be constructed from a single yarn/thread [34], which is ideal for stretchable conductive electrode fabrication [34]. Circuit tracks or structures of conductive blocks can be integrated into knitted structures by utilizing different stitch types, such as an intarsia stitch/structure (Figure 2.5b). An additional mode of conductive fibre integration into knit structures is the weaving of conductive threads into a knit structure (Figure 2.5b). In conductive knit structures, current passes through yarn-yarn contact to reach the electrode sink. The conductive yarns that form part of the knitted structures must thereby be deformable in order to a. b. 21 perform reliably [34]. The inherent stretchability of the structure allows for greater comfort next to body/skin when worn. Table 2.2 summarizes knit structures employed as textile electrodes, with a focus on ECG sensing applications. Insights into structure-property relationships of knitted fabrics used in e-textile applications are listed: - Stitch density should be large enough to provide large current transfer through yarn-yarn contact, increased stitch density can reduce electrode-skin impedance [51]–[53]. - Stretching may result in better contact between stitch points and increased contact conductivity, but may increase motion artifacts and noise in signal, and cause variations in contact impedance due to loop deformation during use/wear [51]–[53]. - Float stitches can decrease overall resistance of conductive knitted fabric structure, compared to fabrics where only knit stitches (number of wales are fixed) are used, due to reduced yarn length per course [54]. - Full Milan double, Intarsia, jersey, and tubular stitches were found to reduce the electrical resistance of knitted fabrics for a given length of yarn (160 mm) [55]. 22 Table 2.2 Knit textile electrodes for ECG monitoring Fabric Composition Structural Properties and Geometry Performance Properties Reference Stainless steel metallic fibres (Bekintex) twisted around viscose yarn -Double-face weft knit -Signal quality (power spectral density-PSD, QRS detection); 30 min. tests over 1-week period [56] Ag- nylon, elastane -Electrode size: 50x50mm bipolar electrodes; 80x80mm ground -Signal-to-noise ratio (SNR) with increasing contact pressure (1-6 kPa) and moisture content; highest SNR ~28 dB [57] 50/50% Stainless steel/cotton -Single jersey; Warp x Weft/5x5 cm -Thickness: 0.44 mm, -Weight: 155 g/m2 -Electrode size: 15x30 mm -Surface resistance: 1.16x106 Ω/sq. -Signal quality (R-peak detection) [30] 92% Ag-nylon-elastane -Thickness: 0.5 mm -Surface resistance: <1 Ω/sq. -Signal quality (visual comparison, signal stability with applied pressure) [58] 78% Ag-nylon, 22% elastane -Weft knit -Thickness: 0.5 mm -Surface resistance: <1 Ω/sq. -Signal quality (R-peaks, max., min., mean) [59] Medtex P130 (Shieldex/Statex), 78% silver plated nylon, 22% elastane (99% pure silver) -Weft knit, double stretch -Thickness: 0.45 mm+/-10% -Weight: 130 g/m2+/-10% -Signal quality (motion artifacts RMS, power spectral density) -Skin-electrode impedance change with applied pressure [60] Nylon, elastane, graphene coated -Electrode size: 60x30 mm -Electrical conductivity of coating: 4.5 S/cm -Skin-electrode impedance (3 electrode, 1 Hz-1 kHz) -Signal quality (PSD, magnitude squared coherence) [61] X-static® (Noble Biomaterials, USA) Ag-nylon -Knit structure not specified -Electrode size: 80x120 mm -Electrical resistance of yarn: 4.8 kΩ/m -Signal quality (visual) [62] Ag-nylon -Thickness: 0.3 mm -Electrode diameter: 20 mm -Surface Resistance: Unwashed- 0.7-0.9 Ω; After 50 washes- 2.1-2.7 Ω -AC impedance, electrodes face-to-face (1 Hz-1 MHz)- approx.1.5 Ω (absolute) change in impedance -Weight (g) before and after 50 washes [63] A. 100% cotton B. 95% cotton-elastane fabric substrate PEDOT:PSS screen-printed -Thickness (before coating): A: 831 ±15.6 µm; B: 725 ±4.9 µm -Electrode size: 50x20 mm -Surface resistance: A. 117.3 kΩ/sq. B. 90.5 kΩ/sq. -Signal quality (SNR, PSD) -Impact of washing on properties [64] A. 78% Ag-nylon-elastane; B. 100% Ag nylon; C. 100% Ag nylon; D. 94% Ag-nylon-elastane -All weft knitted -Thickness (mm): A: 0.45; B: 1.25; C: 0.70; D: 2.5 -Linear resistance of yarns used in structures: ~ 2 Ω/cm -Skin-electrode impedance (20 Hz-20 kHz) -Signal quality (visual) [35] 23 Fabric Composition Structural Properties and Geometry Performance Properties Reference -Density (W/C, cm): A: 28/30; B: 5/6; C: 8/12; D: 17/28 -Electrode size: 15x15 mm PEDOT:PSS treated A. Cotton-Polyester; B. Polyester-Lycra; C. Medtex P130 (Statex), 78% Ag-nylon - elastane -Knit structure N/A for A and B; C: Weft knit, double stretch -Thickness: 0.45 mm±10% -Weight: 130 g/m2±10% -Electrode size: 20x20 mm -Electrical conductivity: A: 337 mS/cm; B: 393 mS/cm -Skin-electrode impedance, Polarization potential, noise -Signal quality (cross correlation between standard gel electrode, % error in signal detection) [65] A. Cotton/45% Ag-nylon, 70den; B. Cotton/30% Ag-nylon, 30den; C. 100% Cu-nylon, 70den; D: Cotton/70% Ag-nylon, 70den -Knit structures: A. Jacquard; B. Alan; C. Full Milano; D. Single Jersey -Thickness (mm): A. 0.66; B: 0.32; C: 0.64; D: 0.57 -Density (W/C, cm): A. 28/30; B. 5/8; C. 8/12; D. 17/28 -Electrode size: 45x45 mm -Linear resistance: Ag-nylon yarn - 0.2 Ω/cm Cu-nylon yarn - 1 Ω/cm -Skin-electrode impedance at 20 Hz -Signal quality (P, QRS, T detection, stabilization to baseline, R peak amplitude variation. SNR) -Subjective comfort evaluation [66] A. Medtex P130 (Shieldex/Statex), containing 78% silver plated nylon, 22% elastane (99% pure silver) B. Medtex P130 fabric with laminated polypropylene (PP) film on back of electrode (humidity barrier) C. Medtex P130 with laminated PP backing, coated with elastic conductive paste on surface of electrode -next to skin (polyurethane, 30wt.% silver plated copper flakes) -Knit structures: Weft knit, double stretch -Thickness: 0.45 mm+/-10% -Weight: 130 g/m2+/-10% -Electrode size: 40x60 mm -Skin-electrode impedance with applied pressure (3-28 mmHg) -Water absorption (immersed samples in water, weight change after 2, 30 min.) -Signal quality (visual) [67] A. Silver-coated nylon fabric (70%), with nickel (16%) and copper (14%) content B. Knit fabric with silver yarn (35%), cotton (63%), spandex (2%) content -Knit structures: A. Unspecified fabric type; B. Jersey knit -Electrode size: 30x25 mm (active skin-contact surface area) -8 mm support foam backing -Surface resistance (linear): A. <1 Ω/cm (both directions); B. 1.5 Ω/cm in wales direction (across rows/vertical) 15 Ω/cm in courses direction (across columns/horizontal) -Two-electrode skin-contact impedance (20 Hz, 100 Hz, 1000 Hz) – found to be <1 MΩ/cm2 -Signal quality (visual, PSD, kurtosis, baseline wander SNR) [68] Silver-plated knit fabrics, labelled as F1 (Faraday Defense LLC); F2 (Innovative Textile) -Knit structure not specified -Electrode size: 30x60 mm -Surface resistance: F1: 0.33 Ω/sq.; F2: 1.45 Ω/sq. -Signal quality (visual), compared before and after wash [69] 24 Fabric Composition Structural Properties and Geometry Performance Properties Reference -Power spectral density (0-140 Hz), compared before and after wash -Abrasion, 9 kPa load on samples up to 10,000 cycles, 0.8 s/cycle; compared resistance before and after -Pilling, 60 RPM box speed, up to 10,000 cycles; compared resistance before and after -Washing cycles – up to 50 cycles; compared resistance before and after -Immersion in water and water + detergent solutions, compared resistance before and after 2.5.2 Woven Fabric Structure Figure 2.6 Schematic of possible single-layer woven structures for e-textile assemblies (left to right) (a) plain (b) twill (c) satin (d) basket, and (e) an example of possible integration of e-textiles/conductive yarns/wires into woven fabric structures (light grey). Woven fabrics are constructed on a loom of a weaving machine by interlacing two orthogonal sets of yarns (warp and weft directions) in a repeating pattern (Figure 2.6). The plain weave is the simplest possible woven structure, which consists of the highest interlacing density among woven structures. Conductive yarns may be integrated as a regular grid pattern, integrated in one direction (warp or weft), or in localized positions [34]. While woven fabric structures are not inherently stretchable, some weaving patterns may allow for some tensile strain from yarns within the structure [34]. Some woven structures can be stretchable to some degree by altering the weave pattern such as employing a twill weave, using elastane yarns. Cherenack et al. reported up c. d. e. a. b. 25 to 11.1% tensile strain in an atlas pattern (one weft fibres under one and over seven warp fibres), while the plain weave fabric experience only 2.2% maximum strain [70]. The addition of stretchable yarns (elastomer) within woven structures can impart some stretch to the fabric. The conductive pathway in woven structures consists of current flow through both conductive yarns and cross-over points between conductive yarns to reach the electrode sink. In e-textile applications woven structures have demonstrated improved contact impedance (lower), owing to direct current transfer to reach the electrode sink [65]. Owing to fewer strain properties than knitted structures, woven fabrics have more uniform and consistent electrical properties [71]. Higher conductive yarn densities used in the structure in both warp and weft directions result in improved electrical properties [26]. Table 2.3 summarizes woven structures employed as textile electrodes, with a focus on ECG sensing applications. Table 2.3 Woven textile electrodes for ECG monitoring Fabric Composition Structural Properties Performance Properties Reference Weft yarns: Silver (Ag) covering yarn 150D/48f, wrapped/spun around polyester (PET) 150D/48f Warp yarns: PET 75D/36f Group A. Regular plain weave (100%) Group B. Half of warp ends removed (50%) -Yarn twist: 600 turns/m -Weft yarns: 150 picks per inch (PPI) -Warp yarns: 168 ends per inch (EPI) -Geometry/structure (convex, concave shapes) -Electrode size: 10x10 mm -Skin-electrode impedance (3 electrode, 20 Hz-5 kHz, constant pressure 40 gf/cm2) -SNR from 30 s ECG recording -Signal quality (visual) [71] Cu/Ni-plated nylon (2 µm) A. Warp: 50D/36f, Weft: 75D/54f; B. monofilament 50 µm) -Woven structures: A. Ripstop; B. Mesh -Thickness (mm): A. 0.12; B. 0.08 -Weight (g/m2): A. 111.8; B. 30 -Electrode size (mm): 15x30 -Surface resistance: A. 0.23 Ω/sq.; B. 0.29 Ω/sq. -Signal quality (R-peak detection rate from 60 s measurement, %) [30] Au-coated nylon, Ni/Cu foam -Plain weave, taffeta fabric (Au layer, 0.2 mm thick); -Electrode size: 14x8 mm (+polymer foam height, 8 mm) -Surface resistance: 0.07 Ω/sq. -EIS (1 V, 1 Hz-10 kHz, 10 h, 14-24 kΩ) -Signal quality: Correlation, % between pre-recorded ECG and dry electrode, wet and dry electrodes; visual comparison of motion artifacts between dry and wet electrodes; accuracy of HR detection for atrial fibrillation cases [72] 26 Fabric Composition Structural Properties Performance Properties Reference PEDOT:PSS with sorbitol, ethylene glycol or glycerol, coated on 100% cotton, and 100% polyester -Thickness: Cotton: 250 µm; Polyester: 400 µm -Electrode size: 30x30 mm (+4 mm foam backing) -Electrical conductivity: Cotton: 5±1 (sorbitol), 266±24 (ethylene glycol), 424±83 (glycerol) mS/cm; PET: 220±59 (sorbitol), 531±62 (ethylene glycol), 575±70 (glycerol) mS/cm -Electrode-skin contact impedance for dry textile electrodes, 20-250 Hz; median impedance at 20 Hz, 40 kΩ -Electrode-skin contact impedance at 10 Hz comparing dry and wet electrodes -Signal quality (PSD, baseline wander, QRS detection, noise) [73] Ni-P nano-electroless coating on 100% polyester -Plain weave -Warp: 49 ends/cm (8.4 tex); Weft: 38 picks/cm (8.7 tex) -Weight: 77 g/m2 -Electrode sizes: 30x30 mm and 40x40 mm -Surface resistance: 0.1 Ω/sq. -Noise in low frequency band (0-0.67 Hz) -Signal quality (P, QRS, T, PR, QT, R-R) [74] Ag-nylon filaments (70den, 100-200 nm coating); Cotton fibres (70/30 den) -2 groups with 2 structures A.Plain (1.20x64 yarns/cm; 2. 37.35x42.71 yarns/cm); B.Honeycomb (1. 27.03x38.57 yarns/cm; 2. 30x30 yarns/cm) -Electrode size: 40x40 mm -Filament electrical resistance: 0.02-0.03 Ω/cm -Signal quality (identification of waveform components, peaks, SNR, overall weighing coefficients for performance evaluation of signal features) -Skin-electrode impedance -Comfort evaluation: air resistance, thermal conductivity, subjective evaluation panel to rate tactile comfort [75] Ni, Cu, Au, plated layers on conductive polyester woven fabric (DOOSUNG, Korea) -Thickness : 0.05 mm -Conductive wristband size : 210x35 mm -Surface resistance of commercial conductive fabric: 0.0633 Ω/sq. -Resistance of additional plated layers: Ni:0.15 Ω/sq.; Cu:0.15 mΩ/sq.; Au:15 Ω/sq. -Skin-electrode impedance (4 Hz-1 kHz), 250 kΩ at 20 Hz -Signal quality (visual, PSD) [76] Hitoe® (Toray) Polyester nanofibre yarn (700 nm diameter), woven fabric coated with PEDOT:PSS -Electrode sizes: Men’s t-shirt- 80x40 mm; Women’s brasserie- 100x28 mm; 200 mm distance between centre of each electrode (2-electrode configuration) -Electrode properties based on ANSI standard (DC resistance: <0.1 kΩ; AC impedance: 1.26 kΩ; internal noise: 1-3 µV; defibrillation discharge at 200 V) -Signal quality in supine, sitting, twisting stepping positions tested (visual, P, QRS, T detection, baseline shift, SNR compared to standard gel electrodes) -Sheet resistance after washing up to 150 times (Before: <200 Ω/sq.; After: >500 Ω/sq.) [77] 27 2.5.3 Nonwoven Fabric Structure Nonwoven structures may comprise discrete fibres, continuous filaments, nanofibres (Figure 2.7) that are randomly distributed, preferentially oriented, or deposited into a mat, and interlocked by mechanical means (i.e., dynamic combing or hydrodynamic methods) [78]–[80]. Nonwoven fabrics can be produced by melt-blowing, melt spinning, electrospinning, or solution spinning [79]. Nonwoven fabrics have been used for textile electrode applications due to their ease of production, lightweight properties, strength, stability, and high surface area for functionalization, either by integration of functional fibres, or coated particles. However, a drawback of this structure in e-textile applications includes the fact that their electrical properties may be unstable or variable due to the random distribution of fibre-to-fibre contacts [18]. Table 2.4 summarizes nonwoven textile electrodes that have been used for ECG applications. Figure 2.7 Example of nonwoven nanofibre mat. Table 2.4 Nonwoven textile electrodes for ECG monitoring Nonwoven Fabric Properties Performance Properties Reference -Evolon microfibre fabric, 100 g/m2 (commercially available), 2µm diameter fibres; bicomponent segmented fibres of alternating polyester and nylon; mechanically bonded structure by hydroentangling -Coated with Ag/AgCl ink, or silver-filled polymer ink, by hand-printing (3x3 cm electrodes) and screen-printing (2.5x2.5 cm electrodes) -Visual comparison of electrodes during sitting and jogging conditions for hand-printed and screen-printed electrodes placed on test subject chest (stretchable belt) -Fast Fourier transform (FFT) of ECG signal of screen-printed electrode to analyze noise content, found to be free of 60 Hz powerline noise -Both hand- and screen-printed electrodes were both able to capture ECG signals under various daily activity conditions [81] 28 Nonwoven Fabric Properties Performance Properties Reference -Screen-printed electrodes found to be more susceptible to bending, mechanical damage from pressure and stretch on body -Nonwoven polyester fabric comprising silica finishing as delustering agent, screen-printed with PEDOT:PSS conducting polymer -Electrodes printed on t-shirt, secured with chest strap, electrode size approx. 19.5 cm2 -Visual comparison of ECG waveform and amplitude at different heart rates while exercising on treadmill, compared to Ag/AgCl gel electrodes -Comparison of sheet resistance (Ω/sq.) with increased screen-printed layers of PEDOT:PSS (1-5 layers); decreased sheet resistance with increased layers (5.6 Ω/sq. with five layers, combined mass of 45 mg) -Comparison of skin contact impedance between dry fabric electrode, standard Ag/AgCl electrode (0.35±0.04 MΩ, at 10Hz), fabric electrode with lotion applied to skin (0.27±0.6 MΩ, at 10 Hz) [82] -Blow-spun nonwoven fabric from poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), blended with conductive single-walled carbon nanotubes (SWCNTs) dissolved in acetone. -Detection and visual comparison of ECG signal, and corresponding heart rates under standing, running, relaxed walking (after running) condition [79] 2.5.4 Embroidery Embroidery is a patterning method by which a series of overlapping stitches is formed from a single continuous thread, or multiple threads, on a finished fabric substrate (Figure 2.8). It is one of the earliest methods employed for circuit routing and interconnections in e-textile fabric systems, and in more recent times for electrode structures [13], [34]. By creating several intersections of a continuous thread, this can enable greater conductivity for the embroidered structure compared to a single conductive thread [13]. Embroidery can be performed by hand, using a sewing machine, or commercial embroidery machine in combination with computer-aided design (CAD). While embroidered structures enable the creation of custom designs, the flexibility or stretch of the pattern/structure is dependent on whether the conductive thread is stretchable. In 29 addition, strains which the conductive thread is subjected to during machine embroidery processes must be considered. General design parameters and structures for stitched and embroidered e-textiles identified by researchers are listed: - Lock and zig-zag stitches (sewn on both sides, bobbin and spool), and chain stitch (more contact points) decrease resistance of base yarn value [83]. - Sewing conductive yarns into structures decreases resistance due to increased contact points from sustained tension; but may also be dependent on stitch class [83]. - Embroidered terry structure can offer good quality signal (i.e. ECG) and contact by easily penetrating hair [84]. - Embroidered structures can provide better signal quality due to yarn being on the surface of the electrode (raised structure), thereby providing good skin surface contact; also found to be more resistant to motion artifacts in being less prone to slipping on skin surface [62], [85]. Table 2.5 summarizes applications in which embroidery was employed for ECG textile electrodes. Figure 2.8 Examples of embroidered structures: (a) Moss embroidered electrodes (embroidery machine). Reproduced under terms of the CC-BY 4.0 license [86]. Copyright 2020, ZSK Stickmaschinen GmbH. (b) Silver-nylon and (c) Silver-silver chloride-nylon yarns embroidered by sewing machine c. a. b. 30 Table 2.5 Embroidered textile electrodes for ECG monitoring Embroidery yarn/thread, substrate fabric Performance Properties Reference -Shieldex (Statex, Germany) Ag-nylon, 117/17 dtex, 2-ply thread -Machine sewn onto elastic chest belt -Active areas investigated: 1, 2, 4, 8, 16 cm2 -Linear resistance of yarn: 2 Ω/cm -Signal quality (visual, baseline drift -mean and SD, broadband noise at rest, exercise, recovery phases, influence of electrode area on signal morphology) [87] -Ag-nylon base yarn, 140 denier, 2-ply; electrolytic deposition of Ag/Cl on yarn; 100% polyester plain weave fabric -Terry structure (pile loops) machine embroidered on polyester fabric, 30 mm electrode diameter, 3 mm height -Electrode impedance spectroscopy (2 electrode configuration, 10 mV vs. open circuit potential, sweep 0.1 Hz-1000 Hz, 30-480 s) -Signal quality (visual) [84] -100% stainless steel yarn, 100 f/2; -Base fabric A: 100% cotton, plain weave (Density: 100x110 /cm; Weight: 154.9 g/m2; Thickness: 0.39 mm) -Base fabric B:50/50% stainless steel/cotton, single jersey knit (Density: 57x49 /cm; Weight: 155 g/m2; Thickness: 0.44 mm) -Electrode size: 15x30 mm -Linear resistance of SS yarn: 3.3 Ω/cm -Signal quality (R-peak detection rate from 60 s measurement, %) [30] -PET yarn, 50 µm diameter (150/48f dtex), sputter coated with metallic Ag (100 nm), followed by metallic Ti (4-7 nm); 99.99% purity -Polyester woven substrate fabric -Active area: 14 cm2 (20x70 mm) -Signal quality (visual at rest and in motion) [88] -silver.STAT® silver-coated nylon fibres (R.STAT, France) embroidered on non-stretch woven fabric substrate; Active area: 8 cm2 (40x20 mm) -Cu/Ag hybrid thread (Ag coated Cu microwires); Active area: 6 cm2 (30x20 mm) -Linear resistance: silver.STAT yarn: 0.459 kΩ/m; Cu/Ag hybrid thread: 7.6 Ω/m -Signal quality (visual) [62] -Three types of Ag-nylon threads used in embroidery patterns – 6-needle machine embroidery on 92% nylon-elastane tubular knit fabric; Upper thread 1: Less EMF Inc. (USA), Upper 2: TibTech (France); Lower: Imbut GmbH (Germany) -Electrode diameter: 16 mm -Linear resistance of threads: Upper 1: 400 Ω/cm; Upper 2: 2 Ω/cm; Lower: 0.7 Ω/cm -Surface resistance of embroidered electrode: 0.1 Ω/sq. -Signal quality (visual, SNR, baseline drift) -Wear resistance (abrasion-30,000 cycles, laundering-30 cycles; comparison of resistance and SNR before and after) [89] -Zig zag overlapping pattern, Ag-nylon, 2-ply thread -Electrode size: 30x30 mm -Resistance (multimeter measurement: 0.3 Ω -Signal quality (visual, HR, R-R intervals) -Resistance with stretch (0,12.5,25%) and bending (180⁰-control,135⁰,90⁰) -Resistance before and after washing cycles-up to 8 -Subjective/comfort rating (rating of descriptor words) [90] 31 Embroidery yarn/thread, substrate fabric Performance Properties Reference -Ag-nylon yarns, labelled as E1 (Statex, Germany 117f 17 dtex HC+B) and E2 (Madeira, HC-40 117x2 dtex) -Electrode diameter: 30 mm -Surface resistance- E1: 0.14 Ω/sq; E2: 0.23 Ω/sq. -Signal quality (visual), compared before and after wash -Power spectral density (0-140 Hz), compared before and after wash -Abrasion, 9 kPa load on samples up to 10,000 cycles, 0.8 s/cycle; compared resistance before and after -Washing cycles – up to 50 cycles; compared resistance before and after -Immersion in water and water + detergent solutions, compared resistance before and after [69] -Ag-nylon thread – 117 dtex, 2-ply (Madeira, Germany), 290 dtex±6 dtex (after plating) -Embroidered on top of non-woven stabilizer and foam material -Electrode sizes: Positive and negative electrodes- 30x60 mm, Ground electrode- 15x60 mm -Linear resistance of thread: <300 Ω /m -Visual examination of ECG data (raw, filtered, filtered zoomed) under non-active and active conditions [91] 2.6 Performance Properties of Textile ECG Electrodes The understanding of performance properties of textile electrodes is necessary to facilitate appropriate materials selection and development, and to gauge accurate and reliable biopotential signal recording and monitoring. In addition, material durability and signal quality over time with washing and use must also be considered for the development of e-textile sensors for long-term health monitoring [2]. The focus of this work is on the performance of ECG sensing electrode. At present there is no standardized evaluation method for the performance assessment of textile-based ECG electrodes. Most developments for textile electrodes have referred to performance limits reported by ANSI/AAMI EC12:2000/(R)2015[92] for disposable ECG electrodes, which cover electrical performance (Table 2.6), safety requirements, and adhesive performance. 32 Table 2.6 Some Performance Requirements for Disposable ECG Electrodes. (Reproduced from ANSI/AAMIEC12:2000/R2015, Table 2 [91]) Electrode characteristics Requirements DC resistance < 2 kΩ AC impedance < 3 kΩ Internal noise < 150 µV DC offset voltage < 100 mV Based on the understanding gathered for commercially used electrodes, and textile electrode development on the research scale, performance characterization for electrode materials includes two categories of metrics: (i) electrical and material properties, and (ii) signal quality analysis. Specific to textile electrodes, other factors relating to structural design, such as fabric structure, applied pressure, construction, and electrode geometry, and impacts on material properties and signal quality, must also be considered. The assessment of physiological comfort and subjective evaluation of e-textiles to quantify fabric handle must also be considered, as these properties impact long-term comfort and wearability of e-textiles integrated into apparel products. 2.6.1 Electrical Properties 2.6.1.1 Conductivity and Contact Impedance Conductivity-related properties of textile electrode materials are largely influenced by the fibrous structure. Surface resistance is associated with the contact resistance between neighbouring yarns through contact points as crossovers [93]. Castrillon et al. found that the impact of anisotropy in the fabric structure, resulting from the different number of warp and weft yarns per unit length, 33 caused a nonuniform spread of current on the electrode surface, thereby resulting in lower electroconductive performance [65]. Since biopotential signals exhibit a frequency range, contact impedance properties, which depend on the frequency of signal, must be characterized to provide a more complete understanding of electrical properties at the skin-electrode interface. Electrode contact impedance must be minimized to acquire high quality signal, with low noise levels [6], [94]. Li et al. outlined several factors influencing the electrode-skin impedance including electrode type (wet, semi-dry, dry), location on body/skin (differences in thickness, sweat glands etc.), individual skin types, applied pressure, and contact area. In addition, the moisture content of the stratum corneum is dependent on the humidity of the surrounding air/environment [18]. Skin is a solid-state electrolyte, in which the conductance can increase with increasing hydration (Dry human skin: 10-7 S/m; Wet human skin: 10-5 S/m) [25]. Skin impedance ranges from 1 kΩ (normal skin) to 1000 kΩ (dry skin), and depends on both the skin condition and location on the body [95]. Conventional methods of minimizing skin impedance include skin preparation procedures (depending on type of biological signal being collected), such as abrading the outer dead layer of skin, removing hair, or applying electrolytic gel on the skin surface, prior to applying electrodes [94]. Advances in electrode materials have sought to replace the need for conventional skin preparation, through novel structures such as hydrogel electrodes [96], micromachined spiked electrodes [97], [98], or micro- and nano-patterned surfaces [99] to increase the surface topography and effective surface area in contact with skin. Several researchers have explored the impact of increasing applied pressure of the electrode next to skin to minimize contact impedance, due to 34 increased coupling and contact area between the electrode and skin, and reduced interface capacitance [52], [58]. Textile electrodes generally show much higher impedance than conventional wet/gel electrodes when first applied to skin, owing to the strong capacitive behaviour of dry textile electrodes from the absence of an electrolyte. Over time, impedance may decrease due to skin moisture and perspiration accumulating under the electrode [100]. Research has been carried out to understand the influence of textile electrode structure on contact impedance, however, results have varied, due to different measurement methods and fabric properties. Impedance spectrum measurements to evaluate dry electrode performance have been performed in various ways: a. Measurements on the skin of human test subjects (generally performed on forearm) [101], [102] b. Simulated human skin such as an electrolyte cell containing 0.9% NaCl solution [51], [84] c. Skin dummies made from agar-agar (gelatinous substance derived from seaweed), distilled water, salt, and disinfectant agent [52], [103], [104] d. AC impedance measurements as specified in the ANSI/AAMI standard for commercial medical disposable electrodes, where electrodes are connected face-to-face [92] The reported AC impedance values for different textile electrodes in literature have ranged from less than 10 Ω up to 150 kΩ across frequency bands of 1 Hz – 1 MHz [18], [63]. Ask et al. determined that an electrode impedance of up to 50 kΩ at 50 Hz was acceptable for ECG signal acquisition [105]. Priniotakis et al. demonstrated that textile electrode structure may influence charge transfer, with many variables inherent to textile materials contributing to variability in contact 35 impedance, such as the stitch density, fibre properties, fibres density, and hairiness [106]. Castrillon et al. demonstrated that textile electrode structures with fibres tightly arranged resulted in lower contact impedance values, possibly due to a better contact area available [65]. Wu et al. found that plain knit fabric structures (1.1-1.27 MΩ) resulted in lower contact impedance than honeycomb stitch knitted structures (>1.4 MΩ) due to the higher presence of non-contact areas in the structure [66]. Ruffini et al. cited that the preferred value for contact impedance should be less than 20 kΩ [107]. Several researchers have reported the dry skin-electrode impedance of textile electrodes to be in the range of 10 kΩ to 5 MΩ in the 5-100 Hz frequency range, compared to less than 10 kΩ for commercial/gel electrodes [66], [85], [108], [109]. 2.6.1.2 Polarization Potential Electrode polarization describes the resistance or barrier that develops at the interface between an electrode and electrolyte, referred to as a galvanic cell. This results in resistance to current flow in one direction versus the other, causing variations in the half cell polarization potential, which deviates from standard value. It is desirable for electrodes/electrolyte materials to be electrochemically reversible and stable, with minimal fluctuation in the potential over time [110]. Electrode polarization is associated with electric instability, giving rise to noise and signal distortion, high resistance, offset potentials between electrodes, and changes in power spectral density curves of long-term signal recording over time [56], [111]. Cooper et al. examined the pick-up voltage from metal electrodes and found silver-chloride electrode pair to be the best performing, while silver, gold, and stainless steel demonstrated distorted waveforms, indicating high polarizability [112]. While these distortions in signal quality can be corrected with filters, care must be taken in appropriate filter selection that would not distort the signal. 36 Electrodes made from conductive metals i.e., pure Ag, Cu, Sn, Al. are not electrochemically reversible, as they eventually corrode. Metals in combination with metallic salts (usually chlorides) are ideal bioelectric interfaces. The metallic salt coating acts as an intermediate bridge between the electrode to electrolyte. Silver-silver chloride is deemed to be the electrode material of choice and is the gold standard material for electrodes used for biopotential monitoring in clinical settings, namely the adhesive gel-type form [111]. Tallgren et al. evaluated the polarization drift for metal EEG electrodes (silver, gold-plated silver, platinum, stainless steel), and found that appropriate stability could only be achieved when using Ag/AgCl electrodes, or if the metal electrodes were coupled with a chloride containing gel [113]. For newly developed electrodes for ECG monitoring (or other biopotential signals), it is necessary to characterize the polarization potential stability over time, in order to ensure that the fluctuations do not interfere with the signal [53]. Polarization potential measurement examined in literature have demonstrated that while textile electrodes (~15.3 mV) may show stable potentials, their performance is still lower and show significant differences from standard reference (Ag/AgCl gel) electrodes (~2.67 mV) [53], [65]. General observations in tests conducted demonstrate the need to study a means of reducing high amplitude transient polarization artifacts generated by movement of electrodes next to skin, and muscle activity [65]. 2.6.2 Textile Material Properties – Stability and Durability of Sensors Electrode stability and durability encompass stress tolerance (mechanical properties/deformation), water tolerance and impact of washing cycles, as well as chemical and electrochemical stability of the textile electrode during repeated wear and usage, which can result in changes in electrical properties and signal quality over time. Common failure modes for e-textiles subjected to mechanical stresses (i.e. stretching and bending from use, abrasion, and 37 washing) include, the formation of cracks and microcracks in the conductive coating of the fibre/yarn, or surface coating, or connection failure between textile interconnects [114], [115]. Future studies into the long-term effects of stretch and bending cycles/holding times on resistance change, combined with signal quality, material structure and recovery would provide a better understanding into the durability of textile electrodes for biological signal monitoring. 2.6.2.1 Mechanical Properties E-textile sensors worn on the body are subjected to stretching and bending in different directions. Important mechanical properties include tensile, and tearing strength, bending properties, and friction resistance of the conductive coating layer. Xu et al. recommended that electrode impedance before and after subjecting the material to mechanical stresses should be measured and compared with acceptable ranges specified by the electrode performance standard (ANSI/AAMI limit [92]) [18]. These properties are dependent on materials and processing properties from the molecular structure, fibre to fabric levels, and integration in the apparel system. The change in electrical properties such surface resistance or impedance with respect to stretching and bending cycles have been used to characterize stability and durability of e-textile materials. Much of this work has been carried out for strain sensing and e-textile interconnect materials, while few studies have directly characterized these properties for textile sensing electrodes for biological signal monitoring. Insights have included the impact of stretch/extension on subsequent recovery (material may not recover fully, 100%), and micro- or nano- crack formation (Figure 2.9) [116], [117], especially prevalent for e-textile materials with conductive surface coatings, which thereby reduce electrical conductance properties, and ultimately lower the signal quality from sensors. Table 2.7 summarizes studies that have examined the impact of stretch and or bending on electrode sensor performance properties, namely surface resistance, and impedance change. 38 Figure 2.9 (a) SEM of platinum thin film strain sensor with microcrack junctions at different strain. Reproduced under terms of the CC-BY license [118]. Copyright 2018, J. Heikenfeld, et al., Royal Society of Chemistry. (b) SEM of polypyrrole-coated elastane fibre Reproduced with permission [117]. Copyright 2011, Elsevier. Table 2.7 Studies investigating effects of mechanical deformation of textile electrodes on electrical performance properties Textile electrode form Strain-Stretch (%) Property change / Results Reference PEDOT:PSS (1.0-1.3 wt.% polymer in water) screen printed on weft knit (84% polyester, 16% elastane) fabric 3 cycles of 100% stretch in length (repeated, no significant variations to one cycle of stretch) -Impedance (243 ± 1 Ω to 670 ± 30 Ω, 2.8x increase) and noise (48 ± 2 μV to 139 ± 2 μV, 2.9x increase) -Deterioration of impedance was still within ANSI/AAMI limits, but slightly exceeded threshold for noise. [119] Ag-nylon thread stitched in zig zag pattern on jersey knit fabric (composition not specified) Electrode size: 30x30 mm Stretch: 0% (control), 12.5%, 25%, in four directions across electrode – vertical (along stitch direction), horizontal (perpendicular to stitch dir.), corner to corner diagonally (bias direction) with two sets of corners; repeated three times each Bend angles: 180° (control), 135°, 90° – chosen based on bending scenario of fabric on body; vertical and horizontal directions; repeated three times each -Resistance change with respect to stretch and bend tests – no significant changes were observed for all tests [90] a. b. 39 Textile electrode form Strain-Stretch (%) Property change / Results Reference Two types of nanocomposite inks: i) silver flake, 10 µm traces ii) silver powder 2-3.5 µm traces, printed onto nonwoven electrospun thermoplastic polyurethane substrate as rectangular strips Electrode size: 40x4 mm Stretch: Five stretch-release cycles up to 50% strain to observe recovery properties (tested on bare textile substrate and printed textiles); uniaxial strain to break tests -Stress-strain curve comparison for three materials, all materials showed full recovery at end of each cycle -Resistance change (R/R0) with respect to strain to break point (%); -Maximum strains at break: 115% for substrate, 240% for flake-based ink, 250% for powder-based ink [120] Cotton knit fabric, 160 g/m2; Polyester fabric, 140 g/m2 Silver ink (printed on fabric) with 75% solid content, particle size 1.5 µm; Electrode size: 30x20 mm Stretch: 5% increments from 0% to 30% of original length -Relative resistance change (Ri/R0) with respect to stretch. -Slight increase in resistance with stretching up to 30%. Cotton-substrate electrode increased by 1.06x, polyester-substrate electrode increased 1.23x [121] 2.6.2.2 Effects of Cleaning and Washing Washability of e-textile materials is an important consideration for the long-term performance, stability, and durability. In general, deterioration of properties from cleaning and washing may be a result of mechanical stresses (i.e. bending, stretch, friction, etc.) during the process, as well as changes in the chemical or electrochemical stability on the conductive surface coating due to thermal (temperature) stress, water stress, and chemical influence (detergent) [122], [123]. The evaluation of e-textile performance after multiple wash cycles has varied in terms of methods selected and modified, owing to the lack of e-textile specific standards, which have only recently been finalized and developed in the last few years. Table 7 lists some existing standards for textile washing that have been utilized or modified for use in many e-textile studies, as well as newly developed standards that have yet to be widely implemented into new material developments and studies. 40 Table 2.8 Washing-laundering standards used or modified for e-textiles Standard Method Title ISO 6330 International Organization for Standardization (ISO): Textiles – Domestic washing and drying procedures for textile testing ISO 105 Textiles – Tests for Colour Fastness - Parts C06, C08, C09, C10, C12 C06: Colour fastness to domestic and commercial laundering C08: Colour fastness to domestic and commercial laundering using a non-phosphate reference detergent incorporating a low-temperature bleach activator C09: Colour fastness to domestic and commercial laundering –oxidative bleach response using a non-phosphate reference detergent incorporating a low temperature bleach activator C10: Colour fastness to washing with soap or soap and soda C12: Colour fastness to industrial laundering AATCC-61 American Association of Textile Chemists and Colorists (AATCC) Colorfastness to laundering: accelerated AATCC-132 Colorfastness to drycleaning AATCC 135 Dimensional changes to fabrics after home laundering AS 2001.5.4 Australian Standard™ Methods of test for textiles – Method 5.4 Dimensional change –Domestic washing and drying procedures for textile testing (modified and reproduced from ISO 6330:2000) JSA-JIS L 0217 #103, #105 Japanese Standards Association – Care labelling of textile goods (includes washing machine conditions for woven and knit fabrics) JSA-JIS L 0844 Test methods for colour fastness to washing and laundering (performed in accordance with ISO 105 C08, C09, C12) New standards specific to e-textiles AATCC TM 210-2020 Test Method for Electrical Resistance Before and After Various Exposure Conditions IEC 63203-204-1-2021 Wearable electronic devices and technologies - Part 204-1: Electronic textile - Test method for assessing washing durability of leisurewear and sportswear e-textile systems Standard washing tests reported for e-textiles in literature have generally followed ISO 6330:2012, Textiles –domestic washing and drying procedures for textile testing [124]. The standard covers procedures for household washing and drying machines. Within the standard, testing procedures cover the following parameters: - Washing device - Load (generally 2 kg) - Wash temperature - Program and duration - Detergent - Drying 41 Studies that have utilized ISO 6330 for textile electrodes have generally applied their own modified methods, reporting results for up to 50 wash cycles at 40°C for 30 min, with variable rotation speeds (30 to 1200 rpm reported [63], [125]), and a 2 kg standard load. Before and after the 50 wash cycles, or increments of 5-10 cycles, material, and electrical properties such as weight, surface resistance and ratio before and after washing (Ri/R0, where i refers to the ith wash cycle, and R0 is resistance before washing), and impedance have been measured [125]. Signal quality has also been examined and characterized through visual observation of the signal in the time domain, and presence of noise [125], [126]. Varied performance properties have been reported for the impact of washing on textile electrodes, summarized in Table 2.9. Variations across wash programs/methods, as well as detergents and cleaning chemicals used, contribute to the differences in overall performance properties among reported studies. In general, all types of washing, from gentle to more regular/normal duty wash cycles were found to deteriorate electrical properties of e-textiles to varying extents. In the studies reviewed, it was found that a slight deterioration of properties, namely surface resistance, signal quality, or weight change was observed as early as 8-10 wash cycles, and an upwards of 30-50 cycles. A maximum of 150 wash cycles have been reported by Tsukada et al., with textile conductive properties maintained for up to 100 cycles under gentle machine-washing conditions. Gaubert et al. found that the combination of mechanical action from machine washing, and bleaching chemicals had the highest impact on yarn conductivity (decrease). Based on the studies reviewed, washing conditions that utilized gentle wash cycles/stirring, and mild detergent concentrations were better in retaining e-textile material properties. 42 Table 2.9 Studies investigating textile electrode property evaluation before and after launder/wash cycles Electrode Material Test Standard Followed/Adapted No. of Washes Property Comparison Reference Screen-printed PEDOT:PSS ISO-105-C10:2006 Textiles placed in aqueous solution (soap and water) containing 5% detergent, stirred at 40⁰C for 20 min; rinsed off soap with water, pressed fabrics with paper towels; oven dried at 70⁰C for 15 min. Up to 20 -Sheet resistance (Ω/sq.), skin contact impedance (Ω), noise (µV), properties all increased after each set of washing cycles [119] Cotton and polyamide, dip coated with PEDOT:PSS N/A Domestic washing machine, commercial detergent (X.TRA Total), 35 min. wash cycle, 40⁰C, 30 mL detergent, 2.5kg machine load; dry spinning speed 600 RPM Up to 50 -Sheet resistance (Ri/R0) increased up to 5x -Signal quality under stationary and movement conditions [64] Various metals (platinum, silver, stainless steel), conductive polymers, and Ag-knit textile ISO 6330 (2012) Electrode placed in washing bag, 40°C, 1200 RPM washing program; 2 kg standard load (machine filled with cotton towels); liquid detergent pH 8; specimens laid flat to dry Up to 50 -Surface resistance (Ω) – Ag-knit surface resistance increased approx. 3x (0.7-0.9 Ω to 2.1-2.7 Ω) -AC impedance (200% increase for Ag-knit) -Weight change before and after 50 washes (gsm) – Ag-knit decreased approx. 0.3% [63] Woven textile electrode from polyester nanofibre, coated with PEDOT:PSS JIS 103, 105 Gentle wash (JIS#105): 30°C, 5 min. wash cycle Slightly harsher wash (JIS#103): 40⁰C, 5 min. wash cycle, two rounds of machine rinse for 2 min., drying at <80⁰C 5 min. spin dry cycle performed between each machine wash cycle Compared washing effects with and without mesh laundry bag to hold specimens Up to 150 -Surface resistance measured after each set of 25 wash cycles. -Specimens subjected gentle machine washing (JIS #105) found to maintain conductive properties for up to 100 washes -Up to 50 washes, conductance properties were maintained for electrodes placed in mesh laundry bag -Specimens subjected to harsher conditions (JIS #103) without use of mesh bag resulted in sharp increase in resistance (from <200 Ω to <1 kΩ) [77] Ag-nylon thread stitched in zig zag pattern on jersey knit Washed in “Regular”-sized laundry load, with commercial liquid detergent in “normal” wash cycle type; hung to dry for 2 h Baseline scenario: garment worn 4x/week, washing 2x/week, for total of 8 washes per month Up to 8 -Surface resistance measured before first wash cycle, and after each complete wash and dry cycle (8 total) -Slight increase in resistance over 8 wash cycles for each electrodes; all found to be below the base measurement of 3 Ω [90] 43 Electrode Material Test Standard Followed/Adapted No. of Washes Property Comparison Reference Various commercial conductive fabrics (woven and knit), and embroidered electrodes ISO 6330 (used and compared “Express” and “Silk” wash programs from previous work) 15 and 38.rpm for silk and express respectively; 400 RPM tumbling speed; 35 min. wash time; 40°C water temperature; X.TRA Total detergent (20 g, based on 1.25 g/L standard); 2 kg load Up to 50 -Surface resistance change (Ri/R0); -ECG signal collection before and after 50 wash cycles (identification of PQRST waves, QRS complex) -Power spectral density – observations of <5 Hz frequency domain for any change from electrode degradation -Mild silk wash program gave acceptable ECG signals after 50 wash cycles; express wash program found to deteriorate conductive properties of two of the electrodes after 10 wash cycles, also translated into non-detectable ECG signal, and PSD degradation [122] Cotton and polyester knit fabrics, Silver ink printed on fabrics with 75% solid content, particle size 1.5 µm ISO 105-C06-A1S, 2010 4 g/L non-ionic detergent at 40°C for 30 min.; dried at room temperature Up to 10 -Surface resistance (Ω/sq.) measured before and after each wash cycle -Polyester substrate electrode surface resistance increased 9x original value; cotton substrate was more stable, 3.72x increase after 10 wash cycles [121] Double jersey knit fabrics made from silver-plated nylon yarns face side, cotton on back side -Custom method, eight different washing conditions evaluated (M1-M4, S1-S4) -Examined impact of powder and liquid detergents, tap water, tap water + sodium percarbonate -M1-M4: machine wash, delicate cycle 30°C, 1 h, max. 400 rpm -S1-S4: soak for 30 h (fabrics withdrawn every 5 h of soaking until 30 h) -Used AATCC135 as reference for wash cycle load weight, 1.8 kg. Up to 30 -All washing conditions decreased surface resistance of fabrics -X-ray photoelectron spectroscopy (XPS) to characterize surface composition of material – silver chloride formation observed -Thermogravimetric analysis (TGA) to quantify impact of laundering on silver content -decreased silver content -Combination of mechanical action from machine wash and bleaching chemicals found to have the highest impact on decreasing yarn conductivity after 30 washes -Colour evaluation (spectrophotometer) [123] 44 2.6.3 Signal Quality Analysis In biopotential measurements, signal quality refers to the preservation of morphological features associated with the given biosignal acquired [127]. In the development of new electrode materials for biosignal monitoring, the quality of the signal acquired is an important measure of electrode accuracy and is also largely influenced with material and electrical properties. ECG is one of the most studied and recorded biosignals, and its distinctive morphology can provide a vast amount of information about the condition and function of the heart, to a clinically trained observer or cardiologist. Given the large body of work in e-textiles for health monitoring focusing on electrode development for ECG, this section outlines techniques proposed and applied in literature for the assessment of ECG signal quality. Most approaches have relied on the visual (subjective) comparison of recorded ECG signals from investigated electrode developments with standard reference electrodes [128]–[130], which limits the reliability and robustness of the results [127]. There is a need to standardize the framework for ECG signal quality evaluation, along with recommended limits for analysis applied to e-textile or wearable applications. Signal quality assessment approaches examined from literature include morphological feature extraction and correlation (time domain), signal quality indices (SQIs), and spectral characteristics (power spectral density, cross-correlation, coherence), and are summarized in this section. ECG signal quality considerations are dependent on the intended application [131]: Basic quality: identifiable R-peaks, HR detection and extraction, comparison of peak amplitudes in waveform, identification of some arrhythmia types, basic spectral features and HRV information Diagnostic quality: P, QRS, and T segments identifiable, for use in clinical diagnosis of conditions (i.e., myocardial ischemia and coronary heart disease) 45 2.6.3.1 Point Scale Weighing Criteria and Weighting Coefficients [66], [85], [132] Pola et al. introduced a point-based (out of 2 points) weighing criterion for the identification of the ECG wave form components (QRS complex, P-, T- waves, R peaks), and extraction of amplitude and baseline levels in the signal, in order to compare between developed textile electrodes [85]. Based on the point scale evaluation, weighting coefficients (out of 100%) were applied to each parameter, from which a score was calculated for each material category (knit, woven, embroidered textile electrodes, and disposable gel electrodes), and condition (dry, moist, skin prepared). The final score calculated determined the embroidered electrode to be the best textile structure for measuring ECG (average: 1.54, std. dev.:0.39), while disposable gel electrodes still showed the highest performance, with an average of 1.65 (std. dev.: 0.08). However, since the weighting criteria in this method can be modified based on selected material, electrical, and comfort properties within individual investigations, results are not comparable across studies, and therefore, not suitable for benchmarking performance. Other researchers have also applied variations of this analysis technique in electrode material comparison. Kannaian et al. applied some parameters in Pola et al.’s point scale evaluation to compare signal quality of embroidered electrodes with different proportions of conductive threads against standard Ag/AgCl gel electrodes and evaluated signal performance before and after laundering (15 washes) [132]. Calculated results were 12 and 11 for the commercial and embroidered textile electrodes, respectively, thereby demonstrating similar performance. Wu et al. applied the point scale evaluation from Pola et al., to some parameters in the evaluation method to compare different textile electrodes of various knitted structures, with standard gel electrodes [66]. Similar scores (11-12) were obtained for the material groups. Other 46 signal quality parameters such as, material, electrical, and comfort properties were compared alongside the evaluation method. 2.6.3.2 ECG Signal Identification and Comparison – General Approaches Several researchers have proposed identification methods, quality evaluation algorithms, training, validation, and assessment, for the classification of ECG signals [131], [133]–[136]. Methods are summarized in Table 2.10. The assessment methods and features listed serve as a guideline for reference, as signal features, and comparison of signal quality is dependent on the data sampling rate, collection location on the body, and collection system used. Data sampling rate is dependent on application area, and desired level of accuracy of the ECG signal collected. Therefore, not all features listed can be extracted from signals collected. Where applicable, performance criteria for signal features were provided. Table 2.10 Assessment methods applied to ECG signals from textile electrodes and electrodes integrated in wearable devices/systems Type Domain Feature Criteria Signal characteristics Basic feasibility Time Amplitude (median, interquartile range, range, min./max.) Minimum cutoff: 0.2 mV Maximum cutoff: 15 mV Baseline wander/drift Time & Frequency Noise-Power measurements: Root mean square (RMS), signal-to-noise ratios (SNR) of R-peak amplitude, root-sum-of squares (or signal magnitude vector); -Ratio between in-band and out-of-band power spectra 5-40 Hz [133] Physiological feasibility Time (temporal HRV) Average heart rate (HR) 40-180 beats/min (0.67-3.0 Hz); up to 300 beats/min. (5 Hz) for exercise [131] Root mean square of differences between adjacent R-R (RMSSD); Number of pairs of adjacent RR differing by > 50 ms in entire recording (NN50); 47 Type Domain Feature Criteria Proportion of NN50 divided by total number of R-R interval (pNN50) Maximum R-R interval 3 s [131] Ratio of maximum and minimum R-R interval Should not exceed 1.1 s; change of over 10% in HR within 10 s improbable; accounting for single missed beat, ratio up to 2.2 s acceptable [131] Frequency (spectral HRV) Total Power in frequency bands [137], [138] (units: ms2) ULF: 0.0001-0.003 Hz or ≤0.003 Hz VLF: very low frequency, 0.003-0.04 Hz (long-term recordings, 24 h) or ≤0.04 Hz (short-term recordings, 5 min.) LF: low frequency, 0.04-0.15 Hz HF: high frequency, 0.15-0.4 Hz ToP: total power, 0.04-0.4 Hz LF/HF ratio[138], [139] ULF, VLF, LF, HF can be assessed for long-term recordings (24 h) [137] VLF, LF, HF components can be assessed at minimum ≥ 5 min. (short-term recordings) [137] Statistical/ trend-based characteristics Time R-R interval median, variance, standard deviation Pearson coefficients Cross-correlation – comparison of simultaneous signal recording (comparison of electrode types) Reported values: > 88% [65] Average 88.3%, max.97% [61] Template matching – extraction of QRS segment over 10 s segments, calculation of Pearson average correlation coefficient r ≥0.66 acceptable [140] Frequency Fourier coefficients Power distribution in bandwidths of ECG signal, and calculation of signal strength (RMS) from each bandwidth Low frequency (LF: 0.05-0.25 Hz) [141] Lower ECG bandwidth (ECG1: 0.25-10 Hz) [141] Higher ECG bandwidth (ECG2: 10-20 Hz) [141] Medium frequency (MG: 20-48 Hz) [141] Main powerline noise (50/60, 48-52 Hz), removed by applying digital notch filter [141] High frequency (HF: 52-100 Hz) [141] 48 Type Domain Feature Criteria Energy Wavelet features Power spectral density (PSD) and Magnitude squared coherence Reported value: Used for comparison of power spectra between ECG signal from textile and clinical gel electrodes 0.95 [56] Spectral distribution ratio (SDR) – ratio between PSD between 5-14 Hz frequency band, and 5-50 Hz frequency band 0.5-0.8, acceptable (outside of range is unacceptable) [131], [134] Signal quality indices (SQI) Time R peak detection matching degree (qSQI) Skew (sSQI) Larger absolute value Kurtosis (kSQI) Larger than 5 Percentage of beats detected by both wqrs and Pan-Tompkins methods (bSQI) Close to 1 Baseline drift in time domain (bsSQI) Larger Redundancy in ECG signal (using principal component analysis of time-aligned waveform – QRS complex (pcaSQI) Close to 1 Relative energy distribution of QRS complex (eSQI) Close to 1 Complexity of ECG signal, based on entropy of waveform (enSQI) Close to 0 Frequency Relative power in QRS complex (pSQI) In [0.5, 0.8] Relative power in baseline [0, 1] Hz frequency band (basSQI) Close to 1 ECG signal purity; measure of how well signal is delineated by single frequency (purSQI) Close to 1 Relative amplitude of high frequency noise (hfSQI) 49 2.7 Influence of E-Textile Design on Performance Properties Through literature review of the influence of e-textile design on performance properties, various qualitative and quantitative insights have been reported, and are deemed to be beneficial to guiding the design and development of wearable e-textile systems. 2.7.1 Electrode Size, Geometry, and Construction Several researchers have investigated the impact of textile electrode position, size, geometry, and construction on signal quality and electrical properties. Electrode distance and positioning on body have been observed to affect signal features and quality, due to differences in the distribution of body surface potentials during each cardiac cycle [26]. Yokus et al. studied the influence of electrode pair distance (2 to 12 cm, increasing increments of 2 cm) for Lead I placements between the left and right arm, and signal features including, R-wave peak amplitude, signal and noise power, and signal-to-noise ratio [26]. They found that an electrode pair distance of 8 cm yielded a higher R-peak amplitude and SNR value. Skin-electrode impedance is also influenced by electrode size and position, with better signal quality observed for larger electrodes; however, an optimum size must be specified given restrictions in available skin area. Table 2.11 summarizes some studies that have investigated the influence of textile electrode size on signal quality. From the studies examined, it was found that using a larger electrode area corresponded to better signal quality, and less noise – with a maximum area of 16 cm2 used. The minimum electrode size of 4 cm2 was specified by Marozas et al., with electrodes smaller than this area found to distort the lower frequency (0-0.67 Hz) components of the ECG signal spectrum [87]. In general, irrespective of the geometry of fabric electrodes, in circular or rectangular forms, the level of signal quality depends on the actual surface or contact area next-to-50 skin. This is thought to be correlated to the number of conductive points or yarn junctions per unit area in a textile electrode. Table 2.11 Studies investigating effect of electrode size on performance and signal quality Size(s) Material/Type Findings Reference Area (cm2): 0.4, 0.8, 1.9, 3.1, 7.1 Corresponding Diameters (cm): 0.7, 1,1.5, 2, 3 Embroidered silver coated polyester (tested under dry, wet-moistened with water, covered with hydrogel) Increasing electrode diameter reduced noise, lowered signal power, decreased impedance [130] Area (cm2): 1, 2, 4, 8, 16 Zig-zag stitch sewn in rectangular configuration on chest belt using silver-plated nylon yarn (117D/17f, 2 ply) Electrodes with an area smaller than 4 cm2 could distort very low (VLF) and low (LF) frequency ECG signal spectrum (0-0.67 Hz) [87] Area (cm2): 2.25 (wxl, 1.5x1.5), 4.5 (1.5x3), 8.0 (2x4) Weft knitted silver plated nylon (78%), elastane (22%) Largest electrode size (2 x 4 cm; 8 cm2) tested had equal performance characteristics (contact impedance, signal quality) to standard gel electrodes [35] Area (cm2): 0.8, 3.1, 7.1 Corresponding Diameters (cm): 1, 2, 3 Ag/AgCl conductive ink printed onto polypropylene-based nonwoven fabric, encapsulated by PU film Larger electrode area resulted in higher ECG amplitudes, signal power, and SNR [26] Comert et al. examined the effect of electrode construction on signal quality, identifying that the stabilization of skin around the electrode can distribute force and movement to an area beyond the electrical contact area, which can reduce motion artifacts [142]; a similar phenomenon to standard gel electrodes which comprise a surrounding adhesive structure. A support structure to stabilize the area around the electrode can restrict epidermis deformation and relocate or extend the movement applied to the skin beyond the electrode surface. For dry electrodes, soft foam padding behind the electrode is thought to reduce changes in pressure under the electrode caused from movement or muscular contraction. Several researchers have cited the benefits of integrating foam backings or conductive foams into textile electrode structures and observed improvements in signal quality and lower contact impedance [72],[73],[75],[127],[143],[144]. Electrode 51 integration into clothing systems can further reduce instances of motion artifacts by providing a support structure around the electrodes. 2.7.2 Applied Pressure According to Li et al., in apparel design, desirable pressure ranges to consider for comfortable and uncomfortable fitted clothes are 1.96-3.92 kPa and 5.88-9.80 kPa, respectively [55]. Various studies have examined the impact of applied external pressure to ECG textile electrodes for minimizing contact impedance and improving signal quality, with some elucidation into the impact on apparent contact area at the skin-electrode interface. According to Taji et al., applied pressure (from 0-4 kPa, or 0-12 mmHg) increases the skin-electrode contact area, and minimizes interfacial impedance, which impacts the ECG signal quality [58]. Ottenbacher et al. observed that baseline drift and noise could be reduced by applying a stabilizing contact pressure (such as foam beneath the electrode), which could thereby increase contact area [145]. Soroudi et al. found that at lower pressure, 3 mmHg, the electrode-skin contact impedance was not measurable when the textile electrode was first placed on the skin, whereas impedance for the reference Ag/AgCl electrode was measurable [146]. However, after a 5-minute resting period at the same applied pressure, the impedance could be detected and measured for the textile electrode that was coated with an additional elastic conductive coating layer. This was thought to be due to the better contact and humidity barrier properties of the electrode coating and skin. Several studies that have investigated the impact of applied pressure on contact impedance and signal quality are summarized in Table 2.12. In general, the average optimum pressure range for textile electrodes investigated has been identified to range from 1-3 kPa. 52 Table 2.12 Studies investigating the effect of applied pressure on electrode performance/comfort Electrode Size Applied Pressures Optimum Pressure Range Reference 50 x 50 mm 0.2 s, 1,2,4,6 kPa 1-4 kPa [57] 3 x 100 mm 5 x 300 mm 17 x 300 mm 2.1 to 23.5 N 1.96, 6.67, 11 kPa (based on electrode area, calculated from <10N) [52] 50 x 50 mm 3.9 kPa (40 gf/cm2) N/A [71] 25 mm diameter 0.67-3.33 kPa (5-25 mmHg, increased in 5 mmHg increments) 2, 2.67 kPa (15, 20 mmHg) [142] 11 x 11 mm 1 kPa N/A [132] 30 x 180 cm Alternated between 0 Pa or 4 kPa N/A; Skin-electrode impedance for conductive textile electrode found to decrease with increased applied pressure (at 4 kPa). [102] 45 x 45 mm 1 kPa N/A [66] 10 x 10 mm 0.2, 0.5, 1, 2, 4 kPa 1, 2 kPa [147] 30mm diameter, 1.154g Initial conditions: 8.996 Pa (0.068 mmHg), ~1.6 kPa (12 mmHg), >4 kPa (>30 mmHg) Pressure > 4 kPa caused discomfort; mild pressure ~2.7 kPa (20 mmHg) found to be ideal [26] 40 x 60 mm 0.4-3.7 kPa (3, 9, 18, 28 mmHg) 1.2-3.7 kPa (9-28 mmHg) [146] 30 x 20 mm 0.8-2 kPa (6-15 mmHg, increments of 3 mmHg) ~2 kPa (15 mmHg) [121] 18, 22, 26 cm diameter 500-1500 Pa D=22 cm ideal size 1297±102 Pa had best performance; similar SNR compared to wet electrode benchmark [148] 2.8 Fabric Handle and Physiological Comfort Evaluation of E-Textiles Fabric handle and clothing comfort are concepts introduced from the apparel and textile industry to measure the total sensation experience when a fabric is touched, and physiological responses of the human body to combinations of clothing and environment [149]. According to Kawabata and Niwa, the performance of textile and clothing should be evaluated in terms of three categories [150]: 1. Utility performance (i.e. strength); 2. Comfort performance (fitting to the human body, mechanical comfort, thermal comfort); 53 3. Mechanical properties and fabric performance for the engineering of clothing manufacture. By considering the three categories, the fabric performance, which relates to its mechanical properties and comfort can be applied towards a subjective method known as fabric handle judgement. The method was developed as the Kawabata Evaluation System (KES) and has been utilized in both industry and research to assess manufactured fabrics used in a variety of applications. The KES includes objective fabric property measures, that can be related to subjective ratings, including an evaluation by a human panel. While one of the main influences for textile electrode developments applied to long-term health monitoring is greater comfort and flexibility than conventional rigid electrodes, few studies have investigated the physiological comfort and subjective evaluation of e-textile materials. This is largely due to the subjective and qualitative nature of most comfort measurement indices, and difficulties in quantifying comfort which may vary among individuals. Reliable and reproducible comfort measures of textile electrodes developed for next-to-skin applications are beneficial to the overall assessment of whether the electrode can be used for long-term monitoring. Some work has sought to identify important and measurable factors that can be utilized to determine clothing comfort levels such as, thermal and moisture comfort, tactile comfort, and pressure comfort -with pressure dominating overall clothing comfort [151]. Most studies have applied physiological measures alongside tactile comfort assessments and fabric hand sensory properties (evaluation panel AATCC 5-2011) to evaluate comfort of e-textile and smart materials. Commonly applied physiological measures include air permeability,[66],[75],[152] water/moisture vapour management properties [152], and thermal resistance [153]. Ali et al. assessed the physiological comfort properties of silver coated cotton fabrics and observed no significant decrease in air permeability or water vapour permeability but noted the 54 partial coverage of fabric pores during the coating process could attribute to small losses in fabric porosity [152]. Xiao et al. compared electrical, signal and comfort properties of woven e-textiles for ECG monitoring. They observed that a honeycomb weave structure (highly air permeable, while exhibiting lower heat conduction and convection) provided acceptable signal quality, and higher comfort levels from subjective and objective evaluation, compared to plain weave structures (tighter construction, lower air permeability) [153]. Wu et al. assessed signal quality, utilizing the signal weighing criteria and weighting coefficients developed by Pola et al. [85], and comfort properties for various knitted textile electrodes [66]. The signal quality analysis for the different electrode materials showed similar performance, while the comfort assessment, which included air resistance, thermal conductivity, and tactile comfort (subjective, panel evaluation) enabled a better understanding of the influence of fabric structure on comfort properties. The conductive plain knitted structure was selected due to its combined comfort and electrical characteristics for signal monitoring. Tadesse et al. assessed the mechanical properties of conductive knitted fabrics using the Kawabata fabric evaluation system, calculating a reduction in tactile comfort by 69% due to the incorporation of a conductive copper yarn in the structure [154]. They identified that low stress properties such as surface friction and compression could provide better distinction of tactile properties of e-textile materials with different structures, than tensile, bending and shearing properties. In another study performed, they assessed the effectiveness of the subjective evaluation method (AATCC 5-2011) applied to various functional fabrics (e-textiles, inkjet printed, screen printed, coated) [155]. Visual subjective evaluation provided better results in terms of consistency, compared to blind evaluation. It was also found that for the bipolar fabric hand parameters 55 assessed, the most easily identifiable properties were noncompressible/compressible, non-stretchable/stretchable, stiff/flexible, and hard/soft attributes, while heavy/light and thick/thin were most difficult. Arquilla et al. utilized a comfort survey of their stitched ECG electrodes before and after each test was performed on human subjects [90]. The survey developed by Hollies et al. consists of 15 words that describe sensations that the person may feel, which are rated (4-partially, 3-mildly, 2-definitely, 1-totally, unmarked if no sensation is felt; + and – for each word based on desirable qualities) and ranked to a combined comfort score [156]. It was found that there was no significant difference in comfort between the standard gel electrode and sewn textile electrode. While physiological measures can provide a baseline understanding of fabric properties under static conditions, when correlated with objective measures, the performance and comfort properties on-body, under various microclimate, applied pressure, and dynamic conditions experienced by the person may be a vastly different experience. Therefore, it is necessary to utilize a combination of sensory perception evaluation, objective fabric properties, and physiological measures. Where possible, the development of on-body measurement, such as temperature/humidity comparison between body and environment, during biological signal collection could greatly complement these methods. 2.9 Relevant and Upcoming Standards for E-Textiles Testing The rapid growth of e-textiles over the past decade is expected to continue in the coming years, and this warrants the need for more standards to be developed and adopted from the combined textiles and electronics industries. The wide utilization and development of standards specific to e-textiles will enable product safety, better understanding and comparison of product quality reliability, durability, and long-term performance. Currently, there are several standards 56 organizations working on the development of new standards for the e-textiles industry, with new standards that have been recently published. The organizations include the American Association of Textile Chemists and Colorists (AATCC), ASTM International, the International Electrotechnical Commission (IEC), the IPC (Association Connecting Electronics Industries), and International Organization for Standardization (ISO). The associated organization committees and method developments are listed in Table 2.13. Table 2.13 Standards development technical committees/working groups for e-textiles, existing and upcoming standards for e-textiles Standards Organizations - Committees AATCC RA111 Committee on Electronically Integrated Textiles Test Methods ASTM D13.50 Committee on Smart Textiles IEC Technical Committee – TC124 Wearable Electronic Devices and Technologies IPC D-70 E-Textiles Committees D-71 E-Textiles Joining and Interconnection Techniques Subcommittee D-72 E-Textiles Materials Subcommittee D-73a E-Textiles Printed Electronics Design Standard Task Group D-74 E-Textile Test Methods Development and Validation Subcommittee D-75a-EU E-Textiles Wearables Standard Task Group in Europe ISO TC38/WG32 Smart Textile Committee ISO/TR 23383:2020(en) Textiles and textile products –Smart (Intelligent) textiles – Definitions, categorisation, applications and standardization needs Standard Methods Published AATCC EP013 EP13, Evaluation Procedure of Electrical Resistance of Electronically-Integrated Textiles AATCC TM76, Test Method for Electrical Surface Resistivity of Fabrics (applies to woven fabrics) AATCC TM84, Electrical Resistance of Yarns AATCC TM210, Test Method for Electrical Resistance Before and After Various Exposure Conditions (includes laundering, dry-cleaning, water, perspiration, acids, alkalis, ultraviolet radiation, and/or microbes; applicable to e-textile fabrics or end products with incorporated conductive traces) ASTM D8248-20 Standard Terminology for Smart Textiles CSN EN 16812 Textiles and textile products – Electrically conductive textiles – Determination of linear electrical resistance of conductive tracks IEC 63203-201-1: Electronic Textile – Measurement methods for basic properties of conductive yarns IEC 63203-201-3:2021 Wearable electronic devices and technologies –Part 201-3: Electronic textile – Determination of electrical resistance of conductive textiles under simulated microclimate IEC 63203-204-1:2021 Wearable electronic devices and technologies – Part 204-1: Electronic textile – Test method for assessing washing durability of leisurewear and sportswear e-textile systems IEC 63203-402-1 Devices and Systems – Accessory – Test methods of glove-type motion sensors for measuring finger movements IEC TR 63203-250-1:2021 Electronic Textile - Snap button connectors for e-textile wears and detachable electronic devices (Technical Report) IPC-8921, Requirements for Woven and Knitted Electronic Textiles (E-Textiles) Integrated with Conductive Fibers, Conductive Yarns, and/or Wires 57 Under development ASTM WK58009/WK61478 New Terminology for Smart Textiles ASTM WK61479 New Test Method for Durability of Smart Garment Textile Electrodes Exposed to Perspiration ASTM WK61480 New Test Method for Durability of Smart Garment Textile Electrodes after Laundering IEC 63203-201-2: Electronic Textile. Measurement methods for basic properties of conductive fabric and insulation materials IEC 63203-401-1: Devices and Systems – Functional elements – Evaluation method of stretchable resistive strain sensor IPC-8941 Guideline for Connectors for E-Textiles IPC-8952, Design Standard for Printed Electronics on Coated or Treated Textiles and E-Textiles IPC-8981, Quality and Reliability of E-Textiles Wearables ISO/DIS 24584 Textiles – Smart textiles – Test method for sheet resistance of conductive textiles using non-contact type 2.10 Conclusions The review presented organizes the understanding of e-textile structure-properties applied to sensing electrodes for electrocardiogram. The influence of specific textile properties at the different levels of the textile structural hierarchy (fibre to fabric), and design (electrode geometry, size, and construction), to performance properties (material and electrical properties, signal quality, durability, and comfort) must be considered prior to integration into smart apparel systems. Several methods to characterize these properties have been described and quantified in this work. In conjunction with newly developed test standards for e-textiles as well as forthcoming ones, this will allow for a greater understanding of considerations required for appropriate comparison and materials selection of e-textile materials in smart apparel. This review is aimed to serve as a basic reference standard which could help e-textile design and innovation be developed from concept inception to product. Looking to the future, several challenges that exist in the development of smart apparel sensing systems for physiological monitoring must be addressed to enable commercialization and widescale adoption: - Better understanding of structure-property parameters relating fibre/fabric morphology/composition to parameters such as signal quality and performance. 58 - Long term durability (i.e., sweat, moisture, temperature, mechanical deformation) and washability of wearable e-textile for a greater number of cycles. - Maintained stability and accuracy of wearable e-textiles under long-term monitoring. - Considerations into comfort, flexibility, biocompatibility of sensor materials selected. - Apparel system must have good fit on body, with electrodes having good contact with skin/body; therefore, appropriate sizing is required. - Power supply/source and storage capacity must be reliable for long-term measurement, and miniaturization of hardware should be considered to reduce bulkiness/rigidity of such components. - Control, privacy and security of data collected from smart apparel system must be addressed [157]. - Smart garments/apparel systems must reach higher Technology Readiness Level (TRL) [158]. 59 Chapter 3: Performance and Signal Quality Analysis of ECG Textile Electrodes for Smart Apparel Applications Note that the following chapter has been reproduced with minor edits from the publication: Le K, Servati A, Soltanian S, Servati P and Ko F (2021) Performance and Signal Quality Analysis of Electrocardiogram Textile Electrodes for Smart Apparel Applications. Front. Electron. 2:685264. doi: 10.3389/felec.2021.685264 Electronic textile systems applied to biological signal monitoring are of great interest to the healthcare industry, given the potential to provide continuous and long-term monitoring of healthy individuals and patients. Most developments in e-textiles have focused on novel materials and systems without systematic considerations into how the hierarchical structure of fibrous assemblies may influence performance and compatibility of the materials during use. This study examines mechanisms underlying the stability and quality of textile-based electrocardiogram (ECG) electrodes used in a smart bra. Signal quality of the biometric data obtained affects feedback and user experience and may be influenced by characteristics and properties of the material. Under stationary and dynamic conditions, analysis of the raw ECG signal and heart rate, with respect to textile-electrode material properties have been performed. Currently, there is no standardized procedure to compare the ECG signal between electrode materials. In this study, several methods have been applied to compare differences between silver-based textile electrodes and silver/silver-chloride gel electrodes. The comparison methods serve to complement visual observations of the ECG signal acquired, as possible quantitative means to differentiate electrode materials and their performance. From the results obtained, signal quality, and heart rate (HR) detection were found to improve with increased skin contact, and textile structures with lower stretch and surface 60 resistance, especially under dynamic/movement test conditions. It was found that the performance of the textile electrode materials that were compared exceeded ECG signal quality thresholds previously established for acceptable signal quality, specifically for the kurtosis (K>5), and Pearson correlation coefficients (r ≥ 0.66) taken from average ECG waveforms calculated. 3.1 Introduction Wearable products used in healthcare have focused on monitoring activity in the form of accessories. The potential for textile-based integrated wearables is significant due to their comfort, ease of use, ability to provide longer-term monitoring for patients, and to help guide treatment for practitioners. Additionally, there are many opportunities for integration of e-textiles into consumer grade products and applications, for improving lifestyle and health of the general population. Textile structures with flexible electronic components integrated into clothing are advantageous as they offer inherent flexibility, conforming to the shape of the human body, and can be designed as a network of sensors working in combination to measure various vital signs [70]. There are four main components of a smart textile system for sensing applications: 1) Sensor, which takes the form of an electronic/smart textile 2) Power source 3) Microcontroller and circuit 4) Data storage In this study sensing textiles for electrocardiography is explored. Electrocardiography is a technique in which the electrical activity of the heart is recorded in the form of an electrocardiogram (ECG – voltage vs. time graph) through electrodes that are placed on the skin [159]. The ECG signal provides information and assessment of cardiac rhythm, and information relevant for the prediction and treatment of coronary events [18], [160]. Electrical currents 61 associated with the contractions produced by the heart muscle can be detected on the skin surface (milivolt scale) by electrodes [18]. In the clinical setting, the standard ECG utilizes 12 viewpoints of the heart’s structure and function through recording associated electrical activity. Each view reflects a potential difference between a positive and negative electrode, referred to as a lead. The focus of this research is on the chest lead, which measures the potential difference between the left and right arm [18]. ECG electrodes are sensors used to transduce ionic current from the body to electron current through the electrode. Gel type silver/silver chloride (Ag/AgCl) electrodes, comprising electrolyte (gel) between the skin and electrode are most widely used in clinical settings. However, these materials are unsuitable for long-term use due to dehydration over time, thus affecting signal quality, as well as possible skin irritation and allergic reactions caused by their prolonged application [18], [129]. Consequently, alternatives to wet-gel electrodes for long-term monitroing have been explored, which have largely included flexible textile-based electrodes. Advantages of textile-based dry electrodes include greater comfort next to skin due to inherent flexibility to the structures, breathability, and unobtrusive integration into apparel. However, drawbacks of these electrode materials include higher sensitivity to motion and muscle artifacts, and lack of long-term durability. In this study, the performance and signal quality of commerically available silver-based textile electrodes incorporated into the underband region of a prototype bra have been investigated and compared with traditional Ag/AgCl gel electrodes. Most quality assessments for wearable health sensors apply basic quality assessments rather than diagnostic quality approaches. Basic quality assessments refer to the identification and extraction of heart rate (from R-peaks), identification of some arrhythmia types, basic spectral features, and heart rate variability (HRV) 62 information. Diagnostic quality assessments include the identification of other signal segments and features, and can is used for clinical diagnosis of cardiovascular conditions (i.e., myocardial ischemia, coronary heart disease). Diagnostic quality must be performed under static conditions for accurate detection, with minimal instances of motion artifacts. Several methods were applied in this study to compare differences between silver-based textile electrodes and silver/silver chloride gel electrodes, and quantify their performance, to ultimately guide materials selection. The comparison methods in the time domain included, heart rate variability, skewness, kurtosis, and average waveform template matching, and comparison of power spectral densities of the materials in the frequency domain. The performance of the textile electrode materials compared were found to match the gel electrodes and exceeded ECG signal quality thresholds previously established for acceptable signal quality. Dry textile electrodes can offer greater comfort and longevity for long-term ECG monitoring. 3.2 Materials and Methods 3.2.1 Electrodes The performance of silver-based textile electrodes was compared to commercially available 2 cm2 surface area disposable Uni-gel Ag/AgCl electrodes (Thought Technology Ltd., Canada). Silver plated nylon-based conductive fabrics of varying structures were used as dry electrodes, summarized in Table 3.1. Since the materials were sourced from various suppliers, and had different yarns and silver coating properties, a detailed comparison of the textile structural properties on performance was not included in this study. Textile electrodes, corresponding to the positive, negative, and ground ECG electrodes were cut to 7 cm2 rectangles (2 cm x 3.5 cm), bonded with sheet glue, and heat pressed onto the foam underband region of a women’s prototype bra. The positive and negative electrodes were spaced approximately 20 cm apart on the right and 63 left underband region respectively, and ground was placed 0.5 cm from the negative electrode on the right side of the underband. Table 3.1 Conductive fabric (commercially sourced) information Fabric ID Structure Stitch Density (W/C) Weight Thickness Sheet Resistance Composition (99% silver plating) Wales per in. / Courses per in. g/m2 ± 10% mm ± 10% Ω/sq. (fabric face, warp direction) A Warp knit 60/78 133 0.25 0.10 Nylon-Elastane (unknown %) B Warp knit 72/92 136 0.28 0.68 Nylon-Elastane (unknown %) C Warp knit 66/66 130 0.22 0.68 Nylon-Elastane (unknown %) D Warp knit 55/33 100 0.35 1.70 79% Nylon, 21% Elastane E Warp knit; 1-dir. stretch 52/62 175 0.55 0.32 94% Nylon, 6% Elastane F Warp knit; 1-dir. stretch 66/81 244 0.55 1.70 94% Nylon, 21% Elastane G Plain weave 138 x 107 (picks x ends/in.) 44 0.09 0.05 100% Nylon 3.2.2 Geometric Roughness Measurement Geometric roughness of the fabrics was measured using the KES-SE (Kato Tech, Japan). The method uses a 5 mm sq. steel piano wire (0.5 mm diameter) sensor that applies a contact force of 0.1 N and enables quantification similar to the human fingertip. The properties were measured in the warp/wale and weft/course directions for all fabrics. The average roughness (SMD) was calculated from the results from both directions. The results are summarized in Table 3.2, with additional results included in Appendix A.3. Table 3.2 Average surface roughness results Fabric ID (W/C) E (52/62) A (60/78) C (66/66) D (55/33) F (66/81) G (woven, 138x107) SMD (µm) 4.96 4.03 3.11 1.76 1.43 1.09 ±1.93 ±2.23 ±0.54 ±0.68 ±0.16 ±0.24 *Note measurement was not performed on fabric B due to insufficient amount of fabric available. 64 3.2.3 Sheet Resistance Measurement Sheet resistance was measured using a four-point probe. Sheet resistance measurements were made for all fabrics tested (face and back of plain fabric, and face of fabric bonded to foam backing). The four-point probe setup consisted of four equally spaced (20mm) gold metal tips supported by springs, to minimize damage to the sample, and attached to an adjustable stage. The stage height was modified to accommodate different sample thicknesses. A current source (Keithley 2400 Sourcemeter) supplied current through the two outer probes, and a multimeter (Tektronix DMM2040) was used to measure the voltage change across the inner two probes, from which the resultant surface resistance was calculated. For a square sample, the sheet resistance is: 𝑹𝒔 = 𝑹(𝒐𝒇 𝒔𝒒𝒖𝒂𝒓𝒆 𝒇𝒊𝒍𝒎) = 𝑽/𝑰 , (1) where V is the voltage measured by the voltmeter (V), and I is the current that flows through the sample (A). For samples of arbitrary shape, the sheet resistance is defined as: 𝑹𝒔 = (𝒌𝑽)/𝑰, (2) where k is the geometric correction factor commonly used for non-ideal samples. k = 4.532 is commonly used when assuming a semi-infinite thin sheet [161]. From the sheet resistance measurements performed (Figure 3.1), differences were observed when the probe was positioned on the warp and weft directions of the knitted fabrics. In general, lower sheet resistance values were observed in the warp than weft direction, along with lower values for fabrics with higher stitch densities. A possible explanation for this observation may be related to the higher contact points from yarn loops aligned in the warp direction, which is inversely proportional to contact resistance [162], [163]. 65 Figure 3.1 Sheet resistance measurement results for conductive fabric. Input source current: 1 mA. (Note: Insufficient material quantity to test Fabric B and F; Fabric G – plain weave, did not display directionality in sheet resistance measurements). Sheet resistance values were measured to be lower when fabrics were bonded to foam sheets (same foam material used in bra prototype) than plain fabrics (no backing). A possible explanation for this may be due to the material being fixed and slightly contracted when bonded, which could potentially improve and increase the number of contact points from electrode to skin. Sheet resistance ranking (Table 3.3), from lowest to highest resistance, specifically taking the warp direction on the face side of the fabrics are listed. Table 3.3 Sheet resistance measurement ranking – lowest to highest Lowest Highest Fabric ID G A F E C B D Sheet Resistance, Ω/sq. (fabric face, warp direction) 0.05 0.10 0.21 0.32 0.68 0.68 1.70 66 3.2.4 Electrode Pair Impedance Following ANSI/AAMI EC12:2000/(R)2015, Section 4.2.2.1 [92], the average impedance of clamped electrode pairs (7 cm2 rectangles, bonded to 3 mm foam fabric) connected face-to-face was measured using an LCR-bridge meter (HM8118, Hameg Instruments, Germany), summarized in Figure 3.2. Impedance was scanned from 20 Hz to 10 kHz frequencies. This initial measurement excludes effects from skin-electrode interface and tissue impedance, with ECG lead clamps included in the results. The observed results meet the ANSI/AAMI standard requirement, less than 2 kΩ. Figure 3.2 Average face-to-face impedance from three measurement trials of electrode pairs. 67 3.2.5 Electrocardiogram Measurement ECG was measured using a clinical 1-lead ECG system (2048 samples/s) from Thought Technology Ltd. comprising a data acquisition, processing, and analysis software (BioGraph Infiniti). Figure 3.3 depicts the experimental set up of the clinical system and electrodes. Tests under stationary (sitting) and movement conditions (running on the spot) were performed, with at least three 6-minute trials. Electrode-skin contact pressure was also monitored during one test involving stationary followed by movement using a 0.204 cm2 force resistive sensor (Pololou Electronics, USA). The contact pressures ranged from 0.04 to 1.8 kPa (results in Appendix A.2). Figure 3.3 Schematic of clinical ECG system setup. The placement for both gel and fabric electrodes followed the configuration on the underband positions specified for the women’s bra is shown in Figure 3.4a. The positive and negative electrodes (L x W: 2.5 cm x 2.0 cm) were spaced approximately 20 cm apart on the right and left underband region respectively, and ground was placed 0.5 cm from the negative electrode on the right side of the underband. With skin electrical properties known to vary along different parts of the body, this results in differing contact impedance values for different electrode placements, thereby affecting the signal morphology. As a result, tests for the gel and textile electrodes could not be performed simultaneously, to ensure the same recording locations on the 68 body were used. A set of tests performed with gel electrodes were performed prior to each test with textile electrodes. Figure 3.4 a. Underband electrode placement locations (not to scale). b. ECG waveform. c. Overlaid signals for visual comparison -recording of conductive textile and gel electrodes (not performed simultaneously). 3.2.6 Electrocardiogram Signal Performance Comparison The characteristic ECG waveform and its respective components are displayed in Figure 3.4b. The P wave results from atrial depolarization. The QRS complex corresponds to the ventricular depolarization, and typically is the largest wave, as the ventricles have larger muscle mass. The T wave corresponds to the ventricular repolarization. a. b. c. 69 Most approaches for signal and textile ECG electrode comparison have relied on qualitative visual comparison of the signal (Figure 3.4c) which limit the ability to compare between candidate materials from standard gel electrodes. Several methods were investigated to compare ECG signal quality from the different electrode materials. 3.2.6.1 Time Domain 3.2.6.1.1 Variability (coefficient of variation, % CV) in R-R intervals (or inter-beat interval, IBI) The inter-beat interval (IBI) can be obtained from the ECG signal by measuring the time difference between successive R-R intervals across a test, measured in milliseconds (ms). The number of R wave occurrences in one minute is used to calculate the heart rate (HR) in beats per minute (bpm). For example: If one IBI or one beat spans 1000 ms, then over a period of one minute, the number of beats is 60. The IBI data across each trial performed was obtained from the BioGraph Infiniti software. The average heart rates were calculated and compared for the materials. From the raw data (IBI values), standard deviations (SD) were calculated, and the average coefficient of variation (the ratio of the SD to Mean, expressed in %) for each material was obtained. For each set of textile electrodes tested (stationary and movement conditions) a set of gel electrode tests (stationary and movement, standard chest placement) were performed on the same day/session serving as the baseline comparison tests results were compared in the same manner. Given that the gel and textiles could not be compared simultaneously, owing to variability in signal morphology at different locations on the body, the average HR could not be directly compared. However, it was deemed practical to compare the % CV or spread of the data sets. 70 3.2.6.1.2 Skewness and Kurtosis Skewness and Kurtosis are the third and fourth standardized moments of a probability distribution, respectively. Skewness (Equation 3) is the measure of symmetry in the distribution, taking positive or negative values, which correspond to the right or left tail of the distribution, respectively [131]. Kurtosis (Equation 4) is the measure of sharpness in the peak of the distribution [131]. Distributions with many outliers are expected to have lower values of kurtosis, since the probability distribution will be flatter and less symmetrical, while distributions with little to no outliers are expected to be symmetric, with sharper peaks (higher skewness and kurtosis values) [131]. Skewness and kurtosis were proposed as quality indices for outliers and noise present in ECG segments, where the kurtosis of 10-second ECG segments was measured and used as signal quality index [133], [134]. Clifford et al. demonstrated muscle artifacts to have a kurtosis (K) of approximately 5, and baseline wander and power-line interference lower than 5. Orphanidou et al. found the kurtosis of a clean ECG in sinus rhythm to have kurtosis values larger than 5 [131]. Expected values for skewness were not defined in ECG quality index methods reported. Skewness and kurtosis functions were calculated in MATLAB® (R2019a, The Mathworks Inc., RRID:SCR_001622). S = 𝟏𝑵∑ (𝒙𝒊−?̂?)𝟑𝑵𝒊=𝟏?̂?𝟑, (3) 𝑲 = 𝟏𝑵∑ (𝒙𝒊−?̂?)𝟒𝑵𝒊=𝟏?̂?𝟒, (4) Figure 3.5 a-c display three examples of 10-second ECG segments and corresponding distributions with skewness and kurtosis values for different signal quality features. Figure 3.5a shows a clean ECG signal with a large kurtosis value (K=27.53, S=4.06). Figure 3.5b shows an ECG segment where the signal contains a large amount of noise and motion artefact, and a 71 corresponding low kurtosis value (K=4.27), and relatively symmetric distribution, with a low skewness value (S=0.29). Figure 3.5c shows an ECG segment with baseline wander, with K=3.25, S=0.54. Signal features from Figure 3.5b and c follow observations from Clifford et al. with muscle or motion artefacts shown to have a kurtosis value of around 5, and baseline wander to have a kurtosis value lower than 5 [133]. Work by Qin et al. investigated ECG signal quality of nanowire electrodes with gel electrode materials and applied several statistical signal quality indices to their analysis, including the kurtosis value of greater than or equal to 5 as being “Good Quality” signal [138]. 72 Figure 3.5 Examples of 10 s ECG segments containing notable signal quality features and corresponding distributions, with calculated kurtosis and skewness values. (a) A clean ECG segment with a large kurtosis value (K = 27.53, S = 4.06). (b) The segment contains a large amount of noise and motion artifacts, corresponding to a low kurtosis value (K = 4.27), and relatively symmetric distribution (S = 0.29). (c) Baseline wander is observed in this segment (K = 3.25, S = 0.54). 73 3.2.6.1.3 Average waveform – Template matching Figure 3.6 Data windows for reach waveform in the ECG signal collected was obtained by centering the R-peak around the medial R-R interval duration, from which the average waveform was obtained. The average ECG waveform for each electrode material was obtained across the signals collected (example shown in Figure 3.6). This trend-based method compares the similarities of the morphology of the ECG waveform. The method was carried out in MATLAB® (R2019a, The Mathworks Inc., RRID:SCR_001622), and modified from Orphanidou’s work [131] to use the entire PQRST waveform, as opposed to only the QRS complex. The following steps were carried out: 1. Determine the location of R peaks in 6-minute signal trial (2048 samples/second, 737,280 data points). 2. Determine duration of R-R intervals (number of points) across entire trial, obtain the median value. 3. Apply median as the total time for each data window. 4. Create data windows, centering each R peak around the median (Figure 3.6). 74 5. Average all data windows created to obtain the final average wave form across the 6-minute trial recording. 6. Calculate the Pearson correlation coefficient between corresponding sets of gel and textile electrodes. From the final average waveform, the Pearson’s correlation coefficient, PCC (Equation 5) was calculated between each textile electrode material and corresponding gel electrode trial. Results were compared to the assigned metrics for template matching quality indices determined empirically by Orphanidou [131]: PCC >/= 0.66 acceptable, and <0.66 unacceptable. r =∑ (𝒙𝒊−?̂?)(𝒚𝒊−?̂?)𝑵𝒊=𝟏√∑ (𝒙𝒊−?̂?)𝟐𝑵𝒊=𝟏 √∑ (𝒚𝒊−?̂?)𝟐𝑵𝒊=𝟏, (5) 3.2.6.2 Frequency Domain: Power spectral density (PSD) comparison between ECG signals of textile and gel electrodes was carried out, with PSD curves obtained from the averaged time domain waveforms overlapped for a visual examination of the frequency components. 3.3 Results and Discussion 3.3.1 Coefficient of Variation (C.V.) in R-R Intervals The coefficient of variation across R-R intervals of the 6-minute trials were measured under stationary and movement conditions and ordered from lowest to highest % CV (for movement conditions), in Table 3.4. During stationary tests, it was observed that across the two material groups (gel and textile), the average % CV were within the same range, with an average of 6.88% and 6.47% respectively. As expected, the recorded R-R intervals from the gel electrodes were found to have lower variation under movement conditions than textile electrodes, likely owing to increased contact with the skin surface by adhesion and wet gel electrolyte material. From the 75 textile electrodes compared, Fabrics G and F showed lowest variation under movement conditions, likely because the materials have limited stretch (Fabric G is a non-stretch woven, Fabric F has stretch in 1-direction), this is thought to result in greater skin contact area and minimal movement next to skin compared to the other fabric materials tested. Furthermore, the two fabrics were also found to have the lowest average geometric surface roughness (1.09 ±0.27 µm and 1.43 ±0.14 µm, for G and F, respectively), and could thereby provide a higher apparent contact surface between the electrode and skin. Table 3.4 Average coefficient of variation (%) from three trials, stationary and movement conditions. Sample ID GEL F G A D E C B Stationary 6.47 6.92 6.86 5.67 6.61 6.67 7.21 8.26 Movement 11.97 17.69 20.42 22.54 37.40 47.92 51.56 N/A 3.3.2 Kurtosis and Skewness Table 3.5 Average kurtosis values, and standard deviation across 10s ECG segments from 6-minute trials (stationary condition) ranked in descending order of results. Sample ID GEL A G E D C F B Ave. Kurtosis 23.91 19.30 15.28 14.93 14.48 13.67 13.38 11.95 Std. Dev. 0.88 0.71 0.73 0.54 0.55 1.18 1.15 1.16 Table 3.6 Average skewness values from three 6-minute ECG trials ranked in descending order of results. Sample ID A GEL G D E C F B Ave. Skewness 3.32 2.98 2.67 2.55 2.38 2.19 2.14 1.35 76 Results obtained from 6-minute trials (approx. 737,280 data points) from gel and textile electrodes under stationary conditions are summarized for kurtosis and skewness in Table 3.5 and 3.6, respectively, with textile electrodes ordered from highest to lowest values. The average standard deviations of the measured kurtosis values across the 10-second segments from each 6-minute trial are also reported. There is a general trend of increasing standard deviation with decreasing kurtosis values, which may be indicative of more signal variation or outliers. Figure 3.7 shows an example plot of calculated kurtosis values from 10-second segments of a selected test trial for Fabric A, compared to the gel electrode measurement (performed in the same-day session). Figure 3.7 Example plot of calculated kurtosis values from 10-second segments of a selected test trial for Fabric A, compared to the gel electrode measurement (performed in the same-day session). The reported results all have kurtosis values larger than 5, which corresponds to a clean ECG signal as defined and observed by Orphanidou [131]. The gel electrodes have the highest kurtosis and skewness values, compared to the textile electrodes. The top three fabrics with higher 77 kurtosis values (A, G, E) were also observed to match the lowest sheet resistance values. This indicates that fabrics with lower sheet resistance (or higher surface conductance) correspond to good signal quality. 3.3.3 Template Matching – Average Waveform The average ECG waveforms were obtained for stationary ECG tests for each textile electrode material, compared with the standard gel electrode (example waveform for each material shown in Figure 3.8 (a-g)). It can be observed that the amplitude of the average wave form for the gel electrodes were higher than textile electrodes; however, this difference does not affect the heart rate determination from the signal collected. The Pearson’s correlation coefficient (PCC) values were calculated for each textile electrode compared against the standard gel electrode and summarized in Table 3.7. PCC values exceeded the minimum acceptable value (>/= 0.66) defined by Orphanidou [131], further demonstrating similarities between the two material groups. Table 3.7 Summary of PCC calculated from textile electrodes compared with standard gel electrode. Fabric ID A G D B E C F Pearson, r 0.98 0.97 0.97 0.96 0.96 0.93 0.91 78 Figure 3.8 a-g Example average waveform for each electrode material compared with gel electrode (Fabrics A-G) 79 3.3.4 Power Spectral Density Comparison The comparison between PSDs for the different electrode materials are displayed in Figure 3.9. The analysis was carried out on the ECG signals over the 6-minute trials carried out for each material. In general, from a visual observation of the spectral plots, the textile electrodes follow the same trend as the gel electrode, with the corresponding main frequency components, but higher magnitudes, which is thought to be due to higher instances of noise in the measured signal. Figure 3.9 Spectral plots for the different electrode materials compared demonstrate that the textiles follow the same trend as the gel electrode, with higher overall magnitudes observed. 3.4 Conclusions The preliminary work presented aims to establish the foundation for the objective evaluation and comparison of signal quality for ECG electrode materials. Under stationary and movement conditions, analysis of ECG signal with respect to textile material properties has been performed. It was found that the fabric properties (stitch density, weight, thickness) did not have a significant effect on the ECG signal quality. Signal quality and HR detection are improved with increased skin contact, and textile electrodes having little to no stretchability, lower surface 80 resistance and roughness. Several methods have been identified to quantify and compare differences between the textile and gel electrodes in time and frequency domains. The methods applied serve to complement visual observations of the ECG signal acquired, as possible quantitative means to differentiate electrode materials and their performance. From the signal quality comparison methods selected, the performance of textile electrodes exceeded existing quality thresholds established, specifically for the kurtosis results (K greater than 5), and acceptable Pearson correlation coefficients (r greater than 0.66) taken from the average ECG wave form morphologies. Textile electrodes are promising options for long-term ECG monitoring, and the research has identified the need to establish a standardized approach to compare differences between electrode materials. This can subsequently be used to guide the development and materials selection of the components that form part of e-textile systems for health monitoring. 81 Chapter 4: Influence of Yarn Geometry and Twist on the Electrical Properties of Conductive Silver-Nylon Yarns for E-Textiles The following chapter is reproduced with minor revisions from a manuscript in preparation. Le, K., Narayana, H., Servati, A., Soltanian, S., Servati, P., Ko, F. “Influence of Yarn Geometry on Electrical Properties of Conductive Silver-coated Nylon Filaments for E-Textiles: A Fundamental Study.” A conductive fibre strand that forms part of a yarn is the basic element in an e-textile assembly. This study investigates the impact of yarn twist and geometric parameters on linear resistance properties of silver-coated nylon multifilament yarns. Single plied and up to 4-ply yarns were investigated from twists levels of 30 twists per meter (TPM) up to 600 TPM (0.5 to 15 twists per inch, TPI). It was found that with increasing twist levels, yarn linear resistance decreases and reaches a plateau at 300 TPM. Similarly, it was observed that the yarn specific volume decreases with increasing twist factor, occurring more rapidly at lower twist levels, approaching a limiting range of 1.6-1.9 g/cm3. Yarn volume fraction was found to approach a value of 0.5, which was also the point at which the yarn reached it tightest structure, with yarn jamming occurring between the angle of 12 to 18⁰. The increased conductivity of yarns observed at higher twist levels can be elucidated by greater contact between the fibrous assembly in the yarn, which bridges electrically conductive pathways in the given structure. Based on the findings, 2-ply twisted yarns were selected to fabricate embroidered electrode structures for skin contact impedance comparison. It was found that the contact impedance increases from electrodes embroidered from the 30 TPM yarns, followed by a general plateau in the range of impedance values after 210 to 300 TPM. This observation can be attributed to the decrease in the yarn specific volume, and subsequently the 82 longitudinal diameter with increasing twist level, which decreases the overall contact area between the skin and electrode interface. This experimental and theoretical work forms the basis in defining relationships between established yarn twist mechanics and geometrical properties, with electrical properties. This can be used to guide the materials selection of suitable conductive yarn parameters for e-textile applications such as electrodes for health monitoring and wearable electronics. 4.1 Introduction Yarn assemblies comprise continuous filaments that are spun into yarns, and are used in textile structures or as individual threads, which can be arranged or formed through twisting (spiral arrangement of fibres around yarn axis) in static lengths or feeding [164], [165]. Established theories and studies relating yarn parameters including specific volumes, packing densities, geometry, and yarn twist from different fibre types have been developed in the last century. This has contributed to the understanding of and ability to predict yarn mechanical properties, packing geometry, and subsequent behavior in textile structures. Hearle et al. found that with increase in twist factor (Eq. 1a), yarn diameter decreases, yarn specific volume (Eq. 1b, and Figure 7.1a) decreases, and approaches a limiting value. Theoretical limits for fibre packing in yarn bundles defined by Du and Ko [80], defined limits for fibre packing in linear fibrous assemblies applied to textile preforms based on fibre volume fraction, fibre orientation (surface helix angle), and the twist (Eq. 2a-b, Figure 7.1b). From Equations 2a-b, parameters related to yarn jamming, which is the tightest structure the yarn can take is defined from the maximum fibre orientation angle and corresponding fibre volume fraction. - Twist factor (tex1/2 turns/cm) is calculated as: τ = C1/2T , (1a) where C is the yarn count in tex, T is the twist level (or turns) per cm. 83 - Yarn specific volume (cm3/g) is calculated as: vy = 𝜋𝑑2𝑙0.4ω, (1b) where d is the yarn diameter, mm; l is the specimen length as cut (mm); ω is the mass of the yarn specimen, mg as cut. - The orientation (helix) angle of the fibres in the outer layer of a yarn bundle is calculated as: θ = tan-1[π (D - d) T], (2a), where D is the diameter of the yarn bundle, d is the fibre diameter, T is the twist level. - Fibre volume fraction is calculated as: Vf = n 1 + tanθπ d T -2, (2b), where n is the number of fibres in the yarn bundle, θ is the helix angle, d is the fibre diameter, T is the twist level. Figure 4.1 a. Nylon yarn specific volume with twist factor [165]. b. Relationship of fibre volume fraction to fibre orientation at various twist levels [79]. 84 With more recent developments in e-textile materials, such as hybrid conductive fibres and yarns fabricated from twisting metal wires, metal coatings of synthetic fibres/filaments, conductive fillers, and polymers, it is highly beneficial to combine their additional electrical functional properties with the basic textile structural processing parameters, for incorporation into hierarchical structures. The conductivity of e-textile yarns that have been fabricated range from the order of Ω/m to several kΩ/m [70]. The mechanism of conduction of e-textile yarns is determined by the contact points between adjacent conductive fibres in the yarn assembly, which enable electric current flow. The electrical properties of yarns containing a mixture of conductive and non-conductive fibrous components are determined by the probability of conductive fibres present and adjacent or connected to each other within the structure [166]. Few studies have examined the relationship between yarn twist on electrical properties of conductive yarns with respect to mechanical properties, and related changes to yarn geometry. Findings of studies reviewed have all found that in general, higher yarn twist results in yarn electrical conductivity increase, due to greater contact between fibres in the yarn, which promote the formation of electrically conductive pathways, and in part, due to slight contraction in yarn length [167], [168]. However, at higher twist levels, and increased axial tension in fibres, breakage may occur, resulting in reduced conductance due to fragmented and discontinuous fibres and yarns [167]–[169]. A novel attempt has been made in this study to investigate the relationship between the yarn twist level and corresponding linear resistance of conductive silver-coated nylon filament yarns, with considerations into established relationships to yarn geometrical parameters. Based on the findings from the yarn twist relationship to linear resistance, twisted yarns were selected for 85 fabrication into embroidered electrodes, from which contact impedance measurements were performed to identify and compare performance properties at the fabric level. 4.2 Experimental Materials and Procedures 4.2.1 Twisted Yarn Preparation An untwisted 1-ply silver (99.9%) coated nylon yarn (Circuitex™, Noble Biomaterials, USA) having a 100-denier linear density and 34 filaments, was used as the starting yarn. Initial twist levels were selected based on twist multipliers (measure of spiral orientation of fibres in a yarn) from 1 to 5 (30 to 150 TPM), and then expanded up to 600 TPM. The number of plied yarns was investigated from 1- to 4-plies. Table 4.1 summarizes the parameters selected. Yarns of 20 cm in length were twisted using a controlled electric motor the Z direction. Five specimens for each twist level and ply were prepared (Figure 4.2 a-n). Table 4.1 Yarn twist parameters selected for 1- to 4-plied yarns investigated. Twist/metre (TPM) 30 90 150 210 300 450 600 Twist/inch (TPI) 0.8 2.3 3.8 5.3 7.6 11.4 15.2 86 Figure 4.2 Selected optical images of twisted yarns. 1-ply a. 30 TPM b. 300 TPM c. 600 TPM; 2-ply d. 30 TPM e. 300 TPM f. 600 TPM; 3-ply g. 30 TPM h. 300 TPM i. 600 TPM; 4-ply j. 30 TPM k. 300 TPM l. 600 TPM 87 4.2.2 Yarn Linear Resistance Measurement The linear resistance across the longitudinal section of each twisted yarn group was measured using a two-probe multimeter (Tektronix DM2040, USA), with a copper clip attachment used for each point measurement. The resistance at 1 cm increments to a total of 10 cm was measured from each 20 cm twisted yarn length, accounting for differences in twist level at the 5 cm ends of the yarns. 4.2.3 Imaging and Cross-Sectional and Longitudinal Section Measurements The longitudinal sections of yarns were imaged by optical microscopy (Leica, Germany), and yarn cross-sectional images were taken using both optical (Nikon, Japan) and scanning electron (Zeiss, Germany; secondary electron signal, 10 kV accelerating voltage) microscopes to determine fibre diameters. Measurements were made using ImageJ software (National Institutes of Health and Laboratory for Optical and Computational Instrumentation, USA). For cross-sectional examination, yarns were wrapped around two metal wires, cast in epoxy (Epofix resin and hardner, Struers, Denmark), then cured for 48 hours (Figure 4.3). The cast samples were cut with a saw to expose the suspended yarn cross-section, and polished to obtain a flat surface. A challenge in obtaining the true circular cross-section of a yarn is the fact that it may be subject to compression in the cast medium (Fig 4.4a). However, considering that during linear resistance measurements of yarns, the probe clip compresses the yarn, the longitudinal section was therefore considered as apparent diameters in calculations. Figures 4.4b-c shows the fiber and yarn cross-section images of the 1-ply untwisted silver-nylon yarn, used for diameter estimations. Figure 4.3 Yarns wrapped between metal wires, cast in epoxy for subsequent cutting and surface preparation. 88 Figure 4.4 Cross-sectional images of compressed yarns. a. SEM micrograph of compressed 2-ply yarn b. Optical image of 1-ply cross-section (approx. 218 ±5 µm) c. SEM micrograph of fibre cross-section (approx. 19.5 ±0.8 µm) 4.2.4 E-Textile Electrode Fabrication Two-ply yarns from 30 TPM to 600 TPM, and all plied yarns at 300 TPM were embroidered into square electrodes having dimensions of 1x1 cm for electrical impedance spectroscopy (EIS), and electrocardiogram testing. The two-ply yarn group was selected based on the observed twist balance of the yarn bundles after the twisting process. A 2-metre length of yarn was prepared for the twisted yarn groups. The yarn was stitched onto two layers of woven fabric (244 g/m2 cotton, and 44 g/m2 silver-nylon). A zig-zag lock-stitch stitch of 5 mm width, and 0.8 mm spacing was patterned into three columns (two-layers were stitched), using one continuous 89 conductive yarn for each electrode (Figure 4.5 a-b). The embroidered woven fabrics were stitched to a 3 mm spacer foam fabric. 4.2.5 Electrical Impedance Spectroscopy Measurement Electrical impedance spectroscopy (EIS) using a three-electrode setup was performed on the left palmar forearm of a 30-year-old female subject. The embroidered electrode was the working electrode (WE), while standard Ag/AgCl gel electrodes (UniGel, USA) served as counter and reference electrodes (CE, RE, respectively). Electrode positions followed the setup used by Euler et al. [170], with a 10 cm distance between the WE and CE, and 1 cm distance between the WE and RE (Figure 4.5c). Light pressure (approx. 0.5 kPa) was applied to the electrodes with an arm wrap. The EIS scans were made using an electrochemical workstation (BioLogic VMP300, France), in potentiostatic mode, in which a sinusoidal voltage of 1 V was applied, using a frequency scan from 10 kHz to 0.1 Hz, 10 points per decade, and three measurements per reading. The data was analyzed by comparing impedance measurements at frequency points of interest, in the range of 0.5-1500 Hz, due to most biopotential signals being in this range. Figure 4.5 a. Embroidered electrode from 2-ply twisted yarn b. Schematic of electrode layers c. Schematic of EIS experimental set-up on planar forearm. 90 4.3 Results and Discussion 4.3.1 Yarn Twist Properties The linear resistance of the yarns (1, 2, 3, & 4-ply) was measured at variable twist levels (30 to 600 TPM), and the results are shown in Figure 4.6 a-b. It was observed that the linear resistance decreases and reaches a plateau at approximately 300 TPM for all the plies that were investigated. With a higher twist level, there is greater inter-filament/fibre coherence and contact between the conductive strands in the yarn, and a slight contraction in the yarn length, which are thought to be the main contributors to the decrease in linear resistance. In addition, the apparent yarn diameters measured across the longitudinal sections follow this relationship and observed trend (Figure 4.6c). 91 Figure 4.6 Effect of twist level on plied yarn linear resistance and yarn diameter. a. & b. Linear resistance as a function of TPM; c Yarn diameter (longitudinal section) as a function of TPM. Theoretical yarn diameter (Eq. 3) can be calculated from the yarn linear density (denier: weight in grams of 9000 metres length) and mass density of the fibres or measured experimentally either by cross-sectional or longitudinal section observation under a microscope. Since the theoretical yarn diameter calculation does not account for the effect of twist, in this work, the longitudinal section diameter measurements were used for the specific volume calculations. This is to account for the change in diameter with twist, and to reflect the specimen morphology under compression during resistance measurements. Table 4.2 provides a comparison of the theoretical yarn diameter calculation and the longitudinal section measurements, with the measured difference 92 being nearly two times larger than the estimation. The longitudinal section diameter was taken as an average from the seven twist levels. It can be noted that the longitudinal section diameter measurement of the 1-ply yarn corresponds well to the cross-sectional measurement (Fig 4.4b) of the epoxy mounted yarns, which was roughly measured to be approximately 0.218 ±0.005 mm. Theoretical yarn diameter is calculated as: Dy = √𝑑𝑒𝑛2250×𝜋×𝜌𝑓 (3), where den is the yarn denier (g/9000 m), ρf is fibre density (taken as 1.14 g/cm3 for nylon). Table 4.2 Calculated theoretical yarn diameter, and measured longitudinal cross-section diameter Theoretical (mm) Longitudinal Section (mm) 1-ply (100 denier) 0.111 0.264 ±0.083 2-ply (200D) 0.158 0.335 ±0.099 3-ply (300D) 0.193 0.383 ±0.081 4-ply (400D) 0.223 0.430 ±0.069 *Longitudinal section diameter taken as an average from the seven twist levels (measured results for each twist level reported in Appendix A.4) Based on the initial observations of the electrical conductance on e-textile yarn twist, further investigation was carried out to examine contributing geometric properties of the twisted yarns, from relationships established by Hearle et al. [171], and theoretical limits for fibre packing in yarn bundles defined by Du and Ko [80]. Figure 4.7a shows the calculated yarn specific volume for the 1- to 4-ply yarns plotted as a function of twist factor. As with the relationship demonstrated from work by Hearle et al.(Figure 4.1a), the specific volume of the twisted silver-coated nylon yarns decreases with increasing twist factor, with a rapid decrease at lower twist levels, eventually approaching a limiting value between 1.6-2.0 cm3/g [171]. Figure 4.7b shows the influence of fibre orientation angle in the outer layer 93 of the yarn bundle on fibre volume fraction in the yarn. The fibre volume fraction approaches a limiting value, occurring between 6°-8° for the 1-ply yarn, and between 12°-18° for the 2-,3-, and 4-ply yarns. Yarn packing fraction with respect to yarn twist is also observed to reach a plateau (Figure 4.7c). These values correspond to the yarn jamming condition, which represents the tightest structure the twisted yarns reach when the filaments in the yarn are in contact with their adjacent filaments. The calculation results shown in both Figures 4.7a-c follow the observed twist level of up to 300 TPM, where the yarn linear resistance is found to reach a plateau or limiting minimum value. These results show the significance of the study and can be referred to these values to be considered for deciding the optimum twist level for a given conductive yarn. It is important to not to go beyond the optimum twist level to avoid conductivity loss caused by over tension, torsion and abrasion between the strands. 94 Figure 4.7 Effect of twist and fibre orientation angle on geometrical properties. a. Calculated yarn specific volume with twist factor b. Calculated fibre volume fraction with orientation angle c. Yarn packing fraction with respect to twist (TPM). These observed electrical properties of conductive coated yarns at varying twist levels correspond to the geometry of fibre packing in a yarn structure on the mechanical property level, and parallel models proposed for the conductivity behavior of carbon nanotube (CNT), and carbon/polymer filler-based composites [172]. The models for conductive composites have attributed the influence of filler material volume on electrical conductivity, based on the formation 95 of percolation paths [172]. At the critical volume, known as the percolation threshold, the conductivity of the composite increases by several orders of magnitude due to the continuous network of fillers formed within the polymer matrix. After this point, further addition of fillers has limited effect on the conductivity, which approaches a maximum value. Based on the observed features of changes in the geometric parameters of the twisted silver-nylon yarn structures, it is deduced that the electrical conductance of the investigated yarn is dependent on the fibre packing and orientation in the yarn bundles (Figure 4.8). This subsequently influences the contact resistance and conductive pathways formed in the structure. Figure 4.8 Relationship between yarn twist level and electrical conductance properties. Observable changes in fibre packing and orientation in yarn bundles visible from optical images shown. 4.3.2 Twisted Yarns Employed in Embroidered Fabric Structures The skin-electrode contact impedances of the embroidered fabric electrodes made from the twisted two-ply yarns were measured and compared. The Bode plots of the yarns are shown in Figure 4.9a-b. Since biopotential signals generally fall in the frequency range of 0.5-1500 Hz, impedance values at selected frequency points in the scanned range are presented in Table 4.3 and 96 plotted in Figure 4.9c-d. In general, a similar trend to the twisted yarn linear resistance results is observed, with the measured impedance increasing from the initial 30 TPM, followed by a general plateau in impedance value after 210-300 TPM. The lower impedance at lower twist levels can be explained by the influence of a larger longitudinal diameter, and higher yarn specific volume, which thereby increases the apparent contact area of the electrode and skin at the fabric level. This can also be explained by the fact that the fibres in the yarn cover a higher surface area at lower twist levels, which in turn, make more contact with the skin. With increasing twist levels, the yarn specific volume decreases, and as the structure compacts, the longitudinal diameter also decreases, and reaches a plateau (Figure 4.6c). The fabric structure embroidered from the given yarn length (2 m) at higher twist levels are expected to have a lower exposed surface area, reduced apparent contact area between electrode and skin, and a higher contact impedance (Table 4.3). Table 4.3 Measured impedance values at selected frequencies. Twist Z @ 1 Hz Z @ 10 Hz Z @ 145 Hz Z @ 295 Hz Z @471 Hz Z @ 1206 Hz 30 TPM 12.16 3.82 0.73 0.44 0.31 0.16 90 TPM 13.35 4.30 0.80 0.49 0.35 0.18 150 TPM 15.49 5.35 1.01 0.61 0.44 0.22 210 TPM 16.08 5.38 1.00 0.64 0.45 0.23 300 TPM 16.30 5.44 1.00 0.60 0.43 0.22 450 TPM 16.39 5.18 0.96 0.58 0.41 0.21 600 TPM 16.56 5.22 0.93 0.56 0.40 0.20 97 Figure 4.9 a-b Contact impedance results from twisted 2-ply embroidered electrodes. c-d Contact impedance results at selected frequencies. 98 4.4 Conclusions This work has examined the impact fibre geometric parameters on electrical properties of silver-coated nylon yarns, as well as preliminary work into fabric level properties. The relationship between fibre packing and orientation angle from increasing yarn twist level is found to influence the yarn electrical resistance. It is found that with increasing twist level of up to 300 TPM, linear resistance decreases and reaches a limiting value for the four yarn plies investigated (1 to 4). Yarn specific volume, and orientation angle also approach limiting values of 1.6-1.9 g/cm3, and 12-18° respectively, from the observed twist level. Embroidered electrodes fabricated from 2-ply yarns at twist levels from 30 TPM to 600 TPM are observed to show increasing contact impedance from the initial 30 TPM, followed by a general plateau in the range of impedance values after 210-300 TPM. Similar to observations at the yarn level, this trend can be attributed to the decrease in the yarn specific volume at increasing twist levels due to the compacting of the structure, fibre/filament coherence, which thereby decreases the longitudinal diameter, and overall apparent contact area of the electrode. Overall, these findings follow established relations of yarn twist mechanics, and processing parameters defined for linear fibrous assemblies. The limiting twist value for any given conductive yarn can be determined by the experimental methods reported in this work, and objectively confirmed by measuring geometrical parameters such as fibre orientation angle, and fibre/yarn packing fraction up to the yarn jamming point. Detailed understanding of fibre and yarn to fabric geometric properties for e-textiles, allows for the prediction of material and electrical properties, which can guide raw materials selection and design. Future work should explore whether these relations are observable for other e-textile yarn materials/assemblies, and the translation of geometric and electrical properties to additional structures at the fabric level. 99 Chapter 5: Structure-Property Evaluation and ECG Signal Quality Investigation of Knit Textile Electrodes 5.1 Introduction To better understand the impact of e-textile structure-properties on performance in biological signal collection, skin surface morphology and roughness were investigated for dry and wet skin conditions. This was followed by a systematic study on of the effect of yarn and knit structure selection on electrocardiogram signal quality. While there have been many studies reporting important structure-property insights on e-textile performance for biological signal collection [26], [51], [56], [63], [142], [173], few studies have approached the understanding of properties through addressing specifications from fibre to fabric levels, with a combined investigation on skin-electrode interaction. This may be due to variation in skin conditions that impact electrical measurements (contact impedance), and challenges in direct measurements of textile and skin interaction. In this work, effects of skin-electrode interfacial properties, and electrode design parameters (yarn plies, type of knit structure, and loop length), have been explored. Preliminary attempts to quantify skin-electrode interaction have been made through measuring skin and textile surface roughness. The measured parameters have been analyzed to examine the relationship to e-textile material and electrical properties, and ECG signal quality. 100 5.2 Materials and Methods 5.2.1 Selection and Fabrication of Knitted Silver-Nylon E-Textiles Silver (Ag) coated nylon-yarns of varying plies (1, 2, and 3-ply) were sourced from Noble Biomaterials Ltd. (Scranton, USA). Yarn details and properties are listed in Table 5.1, and images are shown in Figure 5.1. Rib and jersey knit structures were investigated, with three different machine loop lengths (no unit) selected for each structure. The fabrics were knitted using a 14-gauge flatbed knitting machine (Manufacturer: Universal) at the Groupe CTT facility (Saint-Hyacinthe, Canada). The 18 fabric structures corresponding to each yarn ply group and loop length (“LL” corresponds to unitless machine loop length) are listed in Figure 5.2. It was noted by the prototyping technician after fabrication, that the machine did not specify units for the loop lengths provided. Measurements were made after the fabrics were received. In addition, after the structures were fabricated, we learned that it was not possible to specify the same measurable loop lengths between the knitted structures (Table 5.2). Therefore, a direct comparison between the two knitted structures was not possible. Table 5.1 Ag-coated nylon yarn properties Plies Twists per inch, Direction (Z/S) Linear density Reported Linear Resistance (Ω/cm) 1 4, Z 100 denier/34 filaments 1.6 2 14, Z (single plies, untwisted) 100D/34f 2.0 3 16, S (single plies); Z (3-ply) 100D/34f 0.8 101 Figure 5.1 Microscope images (optical and SEM) of Ag-coated nylon yarns for prototype fabrication. a. Optical images of three yarns selected for prototype fabrication, b. 1-ply, c. 2-ply, d. 3-ply. Figure 5.2 Summary of knit fabric structures investigated, organized into ply groups and machine loop lengths (higher loop length number corresponds to larger length) 102 The measured loop lengths between the rib knit structures made from different plied yarns were found to be different across sample groups, therefore, analysis for this structure was excluded (selected results for rib fabric group included in Appendix A.5). Detailed interpretation for results obtained from the nine jersey knit fabrics are presented in this chapter. The electrodes were made by bonding the knitted fabric to 3mm thick spacer foam to sheet adhesive, using a heat press at 80°C, and cut into 2 cm x 2 cm squares (Figure 5.3). Figure 5.3 Schematic of electrode structure, and images of top layer knit fabrics. Table 5.2 Summary of loop length measurements for each yarn ply group (jersey and rib knit structures) JERSEY KNIT Actual LL, cm RIB KNIT Actual LL, cm Machine LL (no units) 1-ply 2-ply 3-ply Machine LL (no units) 1-ply 2-ply 3-ply 20 0.36 ±0.014 0.37 ±0.014 0.37 ±0.008 15 0.42 ±0.006 0.47 ±0.007 0.53 ±0.011 25 0.40 ±0.006 0.39 ±0.002 0.40 ±0.008 20 0.45 ±0.005 0.52 ±0.01 0.66 ±0.009 30 0.47 ±0.022 0.46 ±0.003 0.46 ±0.013 25 0.52 ±0.011 0.57 ±0.006 0.68 ±0.004 103 5.2.2 Characterization of Knitted E-Textile Electrodes 5.2.2.1 Material and Electrical Properties Figure 5.4 Schematic of knit structure elements The following knitted textile material (Figure 5.4 schematic) and electrical properties were measured: • Loop Length (cm): From a 2x2 cm fabric swatch, the number of loops in the wales (horizontal row) was counted, then the yarn from each row was removed by pulling the loop, and yarn length was measured by pulling the yarn straight. The loop length was calculated by dividing the number of loops counted by the length of the stretched yarn. • Stitch Length (warp/cm, weft/cm) This is measured by counting the number of warp or weft loops per centimeter of fabric. • Stitch Density: Calculated by multiplying warp/cm and weft/cm for each fabric. 104 • Thickness (mm): Measured using a thickness gauge. • Weight (g/m2 or gsm): Mass of a given area of fabric. • Surface (sheet) Resistance: Measured using a four-point probe setup of four equally spaced spring-loaded gold tips (20 mm). A Sourcemeter (Keithley 2400, USA) was used to supply a constant current to the outer tips, and a multimeter (Tektronix DM2040) measured the voltage across the two inner probes. From the configuration, sheet resistance is obtained by Rs = (kV)/I, where k is the geometric correction fabric (k=4.532) for a semi-infinite sheet. The sheet resistances of the fabrics were measured in the warp and weft orientations. • Contact Impedance: Electrical impedance spectroscopy (EIS) was performed using a three-electrode measurement setup on the palmar forearm. The knitted e-textile electrode was used as the working electrode (WE), and standard Ag/AgCl gel electrodes (UniGel, USA) served as counter (CE) and reference (RE) electrodes. The working and reference electrodes were positioned 1 cm apart, and the working and counter electrodes were positioned at a 10 cm distance (Figure 5.5a). Figure 5.5 a. Schematic of EIS experimental set-up on planar forearm b. Skin-electrode circuit model 105 The EIS tests were performed in potentiostatic mode, with a 1 V sinusoidal voltage applied, frequency scanned from 10 kHz to 0.1 Hz, with 10 points per decade and three measurements per reading. Analysis was performed by comparing the recorded EIS curves with equivalent circuit models to calculate the circuit components (refer to Appendix A.9 for method details). The Simplified Randle’s circuit was selected to represent the skin-electrode interface and used for the impedance estimation. The model comprises a resistor Rs in series with a parallel resistor (Rct) and capacitor (Cd) (Figure 5.5b). The capacitor represents the electrical charge between the electrode and skin, Rct represents the resistance between skin and electrode during the charge transfer process, and Rs represents the total resistance of the body/electrolyte, wires, and electrodes [59],[173]. In addition, impedance values at specific frequency points were compared. 5.2.3 Skin Replicas and Surface Roughness Measurements It is hypothesized that contact area between the skin-electrode interface impacts signal quality. There are various influencing factors that may impact the contact area between the two surfaces as demonstrated in the literature, which include applied pressure [52],[59],[121],[174], skin moisture and hydration (or sweat) [23],[67], [176], [177], and electrode structure. Skin surface replicas representing dry and wet conditions from the forearm were fabricated from PDMS, through a process adapted from Lee et al. [178] (details outlined in Appendix A.6). The aim of recreating these surfaces was to demonstrate and quantify the differences in the skin surfaces, to subsequently relate this to a better understanding of skin-electrode interaction, and electrode performance. The surface roughness measurements were performed using the Olympus LEXT OLS3100 laser confocal microscope (408 nm laser, 120 nm lateral, 10nm axial resolution). For the PDMS skin surface replicas, surface roughness was analyzed from area roughness scans, in which the root 106 mean square height averages were measured from the specimens. The knitted e-textile electrodes were analyzed for mean peak height of roughness, in the areal surface roughness analysis mode. Specimens were measured at 5x zoom magnification, which corresponded to a 2x2 mm scanning area. 5.2.4 Effective Skin Contact Length and Area based on Plain Knitted Fabric Model An et al. utilized the understanding of loop length, from Munden’s loop model to estimate the effective skin contact length of a single loop in a knitted textile electrode structure, and extended it into the estimation of the effective contact area per square centimeter[35]. Work by Munden in modelling the geometry of a plain knitted fabric consists of a loop formed by a series of circular portions joined by straight lines [179] (Figure 5.6). Figure 5.6 Single knitted loop based on Munden loop model [179]. Reproduced under terms of the CC-BY 4.0. In this model, assumptions include negligible bending resistance of the yarns, and a yarn of circular cross-section [180]. Accordingly, the length of a single loop can be calculated by Equation 1a [181], in combining Peirce’s plain weave model (Equation 1b) [181], and Munden’s calculation (Equation 1c) [179]. 107 𝑙 = 2𝑙𝐴𝐵 + 2𝑙𝐵𝐶, (1a) with 𝑙𝐴𝐵 = 𝑐 [1 + 916(𝑑𝑐)2], (1b) and 𝑙𝐵𝐶 = 0.544𝑐2𝑑, (1c) where lAB represents the straight length between A and B in the loop (Figure 5.6), lBC represents the semicircle length from B to C, c is the course spacing, and d is the yarn diameter [35]. In considering the three-dimensional structure of a loop in a knitted fabric, the area between B and C is partly covered by a loop in the previous row, while segment AB and its mirror would be exposed. Therefore, in the context of a knitted textile electrode in contact with skin, the effective skin contact length in a single loop is estimated by Equation 2 [35]: 𝑙1 = 2𝑐 [1 +916(𝑑𝑐)2], (2) and subsequently, the effective contact area per square centimetre is estimated by Equation 3 [35]: 𝑆 = 2𝐶𝑊𝑐 [1 +916(𝑑𝑐)2] 𝐷, (3) where W is the number of wales per centimetre, C is the number of courses per cm, c is the course spacing (1/C), D is the effective contact width between the yarn and skin, which includes the contribution from fibre and yarn diameters, and deformation rate of the yarn and skin [35]. From Equation 3, the stitch density (C x W) is positively correlated with the skin contact area, which signifies that as stitch density increases, this is expected to increase the skin-electrode contact area. Yarn diameters specific to this investigation were calculated from theoretical equation (Equation 4) from Hearle et al. [171]. 𝑑 (𝑐𝑚) = √4𝜋(𝑑𝑒𝑛𝑖𝑒𝑟9×105×𝜌) , (4) where d is the fibre density, taken as 1.14 g/cm2 for nylon. 108 Combining this understanding with skin-electrode impedance properties, which include resistive and capacitive components, it is expected that the increase in effective skin-electrode contact area will result in decreased resistance, increased capacitance at the interface, and an overall decrease in impedance. 𝑅 = 𝜌𝑙𝐴, (5) 𝐶 = 𝜀𝐴𝑑, (6) R and C are resistance, ρ is the material electrical resistivity, l represents material/fabric thickness, A is the skin-electrode contact area, ε is the dielectric layer permittivity, and d is the electrode and skin distance. Estimations of effective skin-electrode contact length of a single loop and contact area per square centimeter were made for the jersey knit textile electrodes. These estimations were then compared with the electrode surface resistance, skin-electrode contact impedance, and surface roughness results to analyze the structure-property relationships. It is important to note the high variability within the structure (loop shapes, stitches) of a finished knitted fabric, which may impact comparisons made with theoretical properties. 5.2.5 Electrocardiogram Testing and Signal Performance Evaluation ECG was measured using a clinical 1-lead ECG system (Thought Technology Ltd., Canada), with a sampling rate of 2048 samples/s, and comprising a data acquisition and processing software (Biograph Infiniti). Signals were collected from two channels simultaneously to compare the performance of the e-textile electrodes with the standard Ag/AgCl gel electrodes. The sets of electrodes were attached side-by-side in a chest placement configuration, following the manufacturer’s guideline (Figure 5.7). It is important to note that the simultaneous recordings do 109 not produce identical ECG signals, since they will pick up slightly different phase and amplitude differences due to relative positioning. The e-textile electrodes were attached to the skin using a snap clip connector, compatible with ECG snap leads, and secured with medical tape (3M, USA). Six-minute test sessions with three trials for each electrode set (Ag and Ag/AgCl e-textiles) were performed in a stationary seated position. 110 Figure 5.7 Schematic of ECG collection setup 111 Methods in the time and frequency domain were used to compare ECG signal quality of the different electrode materials. For time domain comparison, variability (coefficient of variation, % C.V.) in the R-R (or inter-beat-interval IBI) intervals were compared, as well as a visual comparison of the waveforms. The template matching method was also employed, which compares similarities in the ECG waveform morphology (summarized in Chapter 3). From the resulting average wave forms, the Pearson’s correlation coefficient (PCC) was calculated between each e-textile electrode material set and the corresponding gel electrode. The PCC results were compared to the metrics for template matching quality indices specified by Orphanidou, with a PCC of greater than or equal to 0.66 deemed as acceptable [131]. From the results reported in Chapter 3 [182], commercial Ag-nylon e-textiles of acceptable signal quality had PCC values of approximately 0.94 (range: 0.91 to 0.98). In the frequency domain, spectral features including maximum power, and occupied bandwidth (width of frequency containing 99% of power in signal) were compared, with key results summarized in for the different jersey knit structures. 5.3 Results and Discussion 5.3.1 Textile Electrode Properties and Skin-Electrode Interface Table 5.3 displays images of surface morphology, roughness profiles, and calculated areal mean roughness values for the scanned PDMS skin replicas, that can be compared to the electrolyte gel surface used in standard Ag/AgCl gel electrodes. There is a visible contrast between the dry and pre-wetted skin surfaces, and it can be observed that the wet skin sample has a similar average roughness (1.55 µm) to the electrolyte gel surface (1.38 µm). According to Vilhena et al., increased water in the stratum corneum (top layer of the skin), lowers the elastic modulus, and has a plasticizing effect due to the smoothening of roughness asperities, which thereby increases the real contact area [183]. This measurement quantitatively shows that with the similar roughness values 112 for the wet skin and gel electrolyte, there is a strong likelihood for surface compatibility and higher apparent contact area between the two surfaces. This corresponds to the general practice of skin preparation (hair removal, application of gel to wet skin, etc.) prior to electrode attachment for biopotential signal monitoring. Furthermore, when biopotential measurements are performed, sweat tends to accumulate between the skin-electrode interface over time, which contributes to further improving the contact area between the two surfaces [23]. These observations follow the findings from work by Derler et al., who found that during friction measurements performed between textile materials and skin, surface structures that were most like that of skin, demonstrated better correspondence under dry conditions, with further increases to friction coefficients with higher moisture content in the textile [184]. Table 5.4 presents the measured textile material properties of the knitted fabrics, which show expected observable trends: decreased stitch density (cm2) with increased loop length (cm), increased thickness (mm) and weight (grams per square metre, g/m2 or gsm) with increased yarn ply for a given loop length and decreased thickness and weight for with increased loop length for each yarn ply group. The minor differences observed in the actual loop lengths measured across the yarn ply groups, and calculated stitch density (from measured wales/cm and courses/cm) may be due to slight variations in the knitted fabric structure. The sheet resistance results presented in Table 5.5 show the general trend of increased resistance with increased loop length for a given ply group (1-,2-,3-ply), which is expected due to material resistance being proportional to length. Decreased resistance with increasing number of yarn plies for a given loop length group (0.37 cm, 0.40 cm, 0.46 cm) is observed, due to the increased volume of material from the additional yarn plies. In addition, on the textile level, the higher twist across yarn ply groups (Table 5.1 properties of commercially sourced yarns), 113 contribute to lower yarn linear resistance (Chapter 4), due to higher contact points within the yarn structure, which translate into lower surface resistance on the fabric structure level114 . Table 5.3 Surface roughness profiles of dry skin, wet skin, and the gel electrode 115 Table 5.4 Jersey knit e-textile material properties Actual LL (cm) Wales/cm Courses/cm Stitch Density (cm2) Thickness (mm) Weight (g/m2) Machine LL (no units) 1-ply 2-ply 3-ply 1-ply 2-ply 3-ply 1-ply 2-ply 3-ply 1-ply 2-ply 3-ply 1-ply 2-ply 3-ply 1-ply 2-ply 3-ply 20 0.36 0.37 0.37 15 14 14 11 10 9 165 140 126 0.21 ±0.005 0.44 ±0.014 0.51 ±0.011 55 ±2.18 154 ±4.37 235 ±2.36 25 0.40 0.39 0.40 13 12 12 10 9 8 130 108 96 0.17 ±0.007 0.34 ±0.017 0.46 ±0.009 54 ±3.79 155 ±3.27 184 ±3.66 30 0.47 0.46 0.46 11 11 11 9 8 8 99 88 88 0.16 ±0.005 0.32 ±0.014 0.44 ±0.008 51 ±2.68 101 ±1.48 170 ±3.14 Table 5.5 Jersey knit e-textile surface resistance properties Actual LL (cm) Surface Resistance, Ω/sq. Machine LL (no units) 1-ply 2-ply 3-ply 1-ply Warp | Weft 2-ply Warp | Weft 3-ply Warp | Weft 20 0.36 0.37 0.37 0.48 ±0.07 0.50 ±0.06 0.34 ±0.02 0.34 ±0.02 0.30 ±0.05 0.28 0.05 25 0.40 0.39 0.40 0.53 ±0.09 0.57 ±0.08 0.47 ±0.08 0.44 ±0.08 0.33 ±0.03 0.29 ±0.02 30 0.47 0.46 0.46 0.59 ±0.08 0.66 ±0.09 0.53 ±0.05 0.47 ±0.09 0.34 ±0.05 0.31 ±0.03 116 Table 5.6 and 5.7 present effective contact length for a single knitted loop, and effective contact length calculation results (Equations 2-3 from Section 5.2.4) for the jersey knitted fabrics investigated. These calculations were based on work by X. An et al. [35], who established estimations for e-textile and skin contact behavior from geometrical models of plain knitted fabrics from Munden and Peirce [179], [181]. For a single knitted loop, the calculated results show the expected increase in contact length with increasing loop length. The effective contact areas per unit centimeter were calculated accounting for the theoretical yarn diameters (Equation 4) for each of the yarn ply groups. The theoretical yarn diameters taken from the 1-ply to 3-ply yarns (each ply with linear density of 100D/34f), were 0.111 mm, 0.158 mm, and 0.195 mm, respectively. The results presented show an increase in effective contact area (cm2) across ply groups for a given loop length, due to the increase in theoretical yarn diameter from 1-ply to 3-ply. Within the ply groups, the effective contact area is decreased with increasing loop length, due to the decreased stitch density per unit area. Table 5.6 Calculated effective contact length (cm) for single loop Ply group, MLL (cm) 1p-MLL20 0.18 2p-MLL20 0.20 3p-MLL20 0.23 1p-MLL25 0.20 2p-MLL25 0.22 3p-MLL25 0.24 1p-MLL30 0.23 2p-MLL30 0.25 3p-MLL30 0.27 Table 5.7 Calculated effective contact area per unit cm (considering circular yarn cross-section) Ply group, MLL, (cm2) 1p-MLL20 0.53 2p-MLL20 0.71 3p-MLL20 0.85 1p-MLL25 0.47 2p-MLL25 0.59 3p-MLL25 0.74 1p-MLL30 0.37 2p-MLL30 0.54 3p-MLL30 0.64 *The total length per unit area from Eq. 3 was multiplied by the half circumference of the yarn (assuming circular cross-section), based on theoretical yarn diameters for 1-, 2-, 3-ply yarns, to represent a proportion of D, the effective contact width of yarn and skin, which is related to yarn diameter, and deformation rate of yarn and skin [35]. 117 Table 5.8 summarizes the contact impedance and circuit element estimations for the knitted electrode structures. The measured values fall within the appropriate performance range specified for ECG textile electrodes from 1 kΩ-5 MΩ [66],[85],[108],[109]. The overall trends observed are decreased impedance with increased loop length within the yarn ply groups, and increased impedance with higher yarn ply. These results do not follow the expected theoretical properties, in which the effective increase in skin-electrode contact area (higher stitch density for smaller loop lengths) would be expected to lead to a reduction in skin-electrode impedance (with decreased resistance and increased capacitance), according to Equations 5-6. Nevertheless, it is important to note that the measured impedance and estimated circuit element results do not show large enough difference in range of values that are significant enough to draw conclusive findings. Inconsistencies in the collected data could be due to numerous factors in the experimental conditions, potentially leading to systematic errors. This is thought to include the test subject’s skin hydration, effective contact pressure of the electrode on skin over the course of the measurement, order of electrode testing, and electrode construction. Table 5.8 Estimated circuit element parameters, and selected contact impedance results at 1 Hz, 10 Hz, 145 Hz SAMPLE ID Rs (kΩ) Rct (MΩ) Cd (F x10-10) Z @ 1 Hz (MΩ) Z @ 10 Hz (MΩ) Z @ 145 Hz (MΩ) 1p-MLL20 11.77 6.39 2.87 3.25 0.86 0.19 1p-MLL25 11.11 5.37 3.37 3.21 0.86 0.19 1p-MLL30 9.86 5.19 3.67 2.75 0.70 0.15 2p-MLL20 12.68 8.63 3.41 4.95 1.70 0.35 2p-MLL25 14.44 9.89 2.60 5.72 1.76 0.36 2p-MLL30 13.06 7.72 2.86 5.63 1.69 0.34 3p-MLL20 19.93 12.96 0.928 7.36 1.99 0.44 3p-MLL25 20.31 12.18 1.30 7.33 1.90 0.42 3p-MLL30 17.66 9.64 1.80 5.08 1.41 0.30 118 From additional electrode surface roughness measurements that were performed, and qualitative observations from the visual examination of the electrode structure, an attempt was made to explain the observed results. The influence of surface roughness, and relationship to apparent electrode-skin contact area were considered. From initial visual observation of the knitted electrode structures (Figures 5.8), it can be hypothesized that the structures with higher loop length and lower ply may have a higher exposed surface and roughness, partly due to the contribution of the sheet glue used in the structure, which has acted to fix the knitted fabric in place. Furthermore, from observations in Chapter 4, with the lower twist level in the 1-ply yarn, this could be due to a higher longitudinal cross-sectional area exposed on the surface, which may contribute to a higher contact surface in the structure. This could contribute to increased contact between the electrode and skin. Figure 5.8 Visual comparison of jersey knit e-textile electrode structures. 119 From the surface roughness profiling carried out by confocal microscopy, the mean peak heights of roughness (SRc) were analyzed and presented in Table 5.9. The results show that the mean peak heights of roughness are largest for the lower ply yarns (decreasing roughness peak heights from 1-ply to 3-ply). It can be observed that with increasing loop length, the mean peak heights increase (note standard deviation values between the measurements show that the measured values within ply groups are relatively similar in value). This can be explained by the larger number of gaps in the structure (Figure 5.8), observed for the electrodes with larger loop lengths and made from a lower yarn ply, that result in the higher measurable contrasts in asperity height across the surface of the fabric. Figures 5.9 a-c show sample profiles of the fabric surfaces scanned. Table 5.9 Mean surface peak height measurement results, with comparison to skin-electrode resistance estimations SAMPLE ID Loop Length (mm) Rct (MΩ) Mean Surface Peak Height, SRc (µm) 1p-MLL20 0.36 ±0.014 6.39 ±1.09 181 ±2.71 1p-MLL25 0.40 ±0.006 5.37 ±1.16 188 ±3.44 1p-MLL30 0.47 ±0.022 5.19 ±1.22 189 ±4.05 2p-MLL20 0.37 ±0.014 8.63 ±2.89 164 ±7.36 2p-MLL25 0.39 ±0.002 9.89 ±3.29 167 ±8.03 2p-MLL30 0.46 ±0.003 7.72 ±3.40 174 ±8.69 3p-MLL20 0.37 ±0.008 12.96 ±3.72 141 ±4.05 3p-MLL25 0.40 ±0.008 12.18 ±3.33 140 ±4.06 3p-MLL30 0.46 ±0.013 9.64 ±2.64 149 ±7.72 120 Figure 5.9 Surface roughness 3D profiles of knitted electrode samples a) 1-ply, LL: 0.46 mm b) 2-ply, LL: 0.46 mm, c) 3-ply, LL: 0.37 mm121 Based on the Whitehouse-Archard model, which describes general theory for contact of random surfaces, as surfaces are pressed together, contact between the two surfaces will occur at higher asperities in each structure [185]. The highest asperities produce higher stresses, and contribute to a larger proportion of the contact load [185]. It was also found that as the level of roughness increases, contact area varies with the applied load; however, the ratio of load to real contact area, particularly at increased loads, is more influenced by the material properties, including the Young’s modulus, and resulting elastic or plastic deformation [185]. From observations in the literature on the effect of pressure and padding on motion artifacts in textile electrodes, higher applied pressures, will have a greater overall influence on reducing biopotential signal motion artifacts, and interfacial impedance than the electrode surface structure. The insights from these results can be used as a potential explanation to the discrepancies in the structure-property results observed from the electrode structure. However, future work in examining skin-textile electrode interfacial properties, through investigating surface interaction and structure is necessary to explain the findings presented in this study. 5.3.2 Analysis of ECG Signal Quality based on Electrode Structure-Properties Signal quality measures in the time domain for ECG collected from the knitted e-textile electrodes are presented in Figures 5.10 and 5.11a-c. The inter-beat (R-R) intervals (reported in milliseconds) obtained from the ECG signal, are used to calculate heart rate (60 ms/IBI). It was observed that all textile electrode groups show less than 0.1% difference in coefficient of variation between the corresponding gel electrode pairs they were measured with (Table 5.10). This demonstrates that the electrode materials are suitable for ECG signal recording, and heart rate determination. 122 Table 5.10 Average IBI coefficient of variation (%) over three ECG trials Textile Electrode Gel Electrode SAMPLE ID CV, % CV, % 1p-MLL20 5.07 5.07 1p-MLL25 5.02 5.02 1p-MLL30 5.00 5.00 2p-MLL20 6.59 6.59 2p-MLL25 6.21 6.21 2p-MLL30 6.06 6.06 3p-MLL20 12.15 12.13 3p-MLL25 11.24 11.33 3p-MLL30 6.27 5.84 Figure 5.10 Plot of average inter-beat intervals (ms) from simultaneous e-textile and gel electrode signal recordings.123 -0.50.00.51.01.52.0-0.50.00.51.01.52.0-0.50.00.51.01.52.0 Gel Ag/AgCl Electrode E-Textile - 3p-MLL20 Gel Ag/AgCl Electrode E-Textile - 1p-MLL25c.b.ECG Amplitude, mV Gel Ag/AgCl Electrode E-Textile - 2p-MLL30a. Figure 5.11 Example average waveform plots for knitted e-textiles compared with standard gel electrodes. a. 3-ply MLL 20, b. 1-ply MLL 25, c. 2-ply MLL 30 Table 5.11 Summary of PCC results calculated from e-textiles compared with standard gel electrodes SAMPLE ID 1p-MLL20 0.983 2p-MLL20 0.988 3p-MLL20 0.991 1p-MLL25 0.993 2p-MLL25 0.992 3p-MLL25 0.976 1p-MLL30 0.993 2p-MLL30 0.988 3p-MLL30 0.987 Average 0.989 0.990 0.985 124 The average signal waveform, and Pearson’s correlation coefficient (PCC) comparing the simultaneous Ag/AgCl gel and textile electrode signal measurements were calculated for each trial performed and summarized in Table 5.12. Results for both sets of e-textiles have an average PCC of 0.98. This exceeds the metric defined by Orphanidou, with PCC >/= 0.66 [131] deemed as acceptable, and fall in the higher end of the acceptable PCC range from previous work (Chapter 2) comparing commercial Ag-nylon textiles, with values of approximately 0.94 (range: 0.91 to 0.98). A notable observation from the average signal waveform is the higher R-peak amplitude of the textile electrodes compared to gel electrodes, which is thought to be due to the relative placement of the textile electrodes, closer to the heart centre. Overall, the textile electrodes knitted from the 2-ply yarns show the highest correlation values, closely followed by the 1-ply group. Signal quality features in the frequency domain are summarized in Table 5.12 (organized by loop length groups). The textile electrodes had a higher average band power than the gel electrodes they were compared with. This is indicative of higher instances of noise in the signal, as the bandwidth results show that the e-textiles occupy lower frequency bounds than the gel reference electrodes (Table 5.12 and Figure 5.12 periodogram overlay). Instability at lower frequencies from impedance scans was also observed for some of the e-textile materials (Figure 5.13a-b). The 2-ply e-textile group showed a lower band power overall compared to the other e-textile groups. It is also of interest to note that the lower bounds for higher loop length groups of all three yarns investigated trend closer to the lower bounds of the gel electrodes, which may also be demonstrative of less noise pickup in the lower frequency range. However, the occupied band power trends in the higher range of values than that of the lower loop length groups. The findings from the signal quality analyses appear to follow similar trends observed for the textile electrode 125 contact impedance property results. The 1- and 2-ply electrode groups, and higher loop length structures show greater stability and signal quality overall. Table 5.12 Summary of frequency domain signal quality features, organized with respect to loop length groups LL: 0.37 cm (MLL 20) 1 -PLY 2-PLY 3-PLY Gel Ref. E-Textile Gel Ref. E-Textile Gel Ref. E-Textile Avg. Band Power 0.031 0.059 0.030 0.060 0.036 0.069 Avg. Occupied Bandwidth 42.9 37.3 42.3 37.6 43.0 38.4 Lower Bound 0.99 0.54 0.92 0.71 0.93 0.46 Upper Bound 43.9 37.8 43.2 38.3 43.9 38.8 LL: 0.40 cm (MLL 25) 1 -PLY 2-PLY 3-PLY Gel Ref. E-Textile Gel Ref. E-Textile Gel Ref. E-Textile Avg. Band Power 0.032 0.058 0.027 0.049 0.031 0.076 Avg. Occupied Bandwidth 42.4 39.2 42.8 39.8 43.9 39.1 Lower Bound 0.97 0.52 0.83 0.53 0.98 0.61 Upper Bound 43.4 39.7 43.6 40.3 44.8 39.7 LL: 0.46 cm (MLL 30) 1 -PLY 2-PLY 3-PLY Gel Ref. E-Textile Gel Ref. E-Textile Gel Ref. E-Textile Avg. Band Power 0.033 0.077 0.028 0.054 0.033 0.070 Avg. Occupied Bandwidth 42.4 36.9 43.1 39.1 43.1 38.0 Lower Bound 0.96 0.73 0.79 0.76 0.96 0.68 Upper Bound 43.4 37.6 43.9 39.9 44.0 38.7 126 Figure 5.12 Example periodograms of e-textile and gel electrodes overlaid to demonstrate differences frequency spread and power. 127 Figure 5.13 Impedance plots from EIS measurements displaying low frequency instabilities, a) Nyquist plots, b) Bode plots 128 5.4 Conclusions This study has demonstrated the importance of combining the understanding of material and electrical properties with signal quality analysis, to evaluate the impact of structure-property, and performance of newly developed textile electrodes. The design of textile electrode structures may impact electrical and signal properties and their stability in various ways based on the interaction at the skin-electrode interface. In this work, it has been shown that surface compatibility at the skin-electrode interface is impacted by factors such as skin hydration, roughness, and applied pressure. This may influence contact impedance and signal quality properties. Electrodes fabricated from lower ply yarns with higher knitted loop lengths were measured to have larger surface asperities (surface roughness peaks). Furthermore, they were found to have low contact impedance, despite their estimated lower contact surface per unit area, compared to structures fabricated from higher plies with lower loop lengths. This is thought to be due to the high surface roughness asperities contributing to the contact load at the skin-electrode interface. However, this result is still inconclusive, and future work is necessary to further explore this relationship. The power spectral density results for textile electrodes were observed to occupy more power in the lower frequency bounds (observed from 0.3 Hz), with higher total occupied band power, compared to standard Ag/AgCl gel electrodes. This is thought to correspond to lower signal quality and instabilities observed during EIS measurements in the lower frequency range, and the higher raw signal amplitude in the time domain. Overall, the textile electrodes showed less than 0.1% difference in coefficient of variation between inter-beat intervals, compared to the standard Ag/AgCl gel electrode pairs they were measured against. This indicates that the e-textile electrodes can be used to provide accurate heart rate measurements.129 Chapter 6: Roll-to-Roll Electrochemical Fabrication of Silver/Silver Chloride Coated Yarns for E-Textile Electrodes The following chapter is reproduced with minor revisions from a manuscript in preparation. Le, K., Narayana, H., Soltanian, S., Servati, A., Ko, F., Servati, P., “Roll-to-Roll Fabrication of Silver/Silver Chloride Coated Yarns for Developing E-Textile Dry Electrodes, and Applications in Biopotential Signal Monitoring.” This work presents a roll-to-roll electrochemical coating system for producing silver/silver chloride (Ag/AgCl)-coated yarns and its application in developing e-textile electrodes for biological signal monitoring. Ag/AgCl is one of the most preferred electrode materials as an interface between the conductive backbone of an electrode and skin, for biopotential signal monitoring. E-textile Ag/AgCl-coated nylon yarns offer an accurate, stable, flexible, and breathable alternative to currently available rigid or flexible film-based Ag/AgCl electrode. The developed system allows for highly controlled process parameters to achieve stable and uniform AgCl film deposition on commercially available Ag-coated nylon yarns. The electrical, electrochemical, micro- and nanostructure, and crystal structures of the yarns were characterized to obtain and optimize process parameters. The produced yarns were used to develop flexible dry electrodes, which were used as electrocardiogram (ECG) signal detection. The fabricated electrodes using the Ag/AgCl coated yarn demonstrated high stability, with low average polarization potential (1.22 mV/min) compared to electrodes made from Ag-nylon yarns (3.79 mV/min), and low impedance (below 2 MΩ between 0.1 to 150 Hz). Furthermore, the e-textile electrodes were able to measure ECG signals, with comparable performance to standard gel electrodes. The results demonstrated that the developed Ag/AgCl e-textile based electrodes are 130 more stable compared to other metal alternatives and are excellent candidates for heart rate detection and monitoring. 6.1 Introduction Biopotential signals can be detected and monitored from various locations of the body, by employing electrode sensors. Electrodes used for biological signal monitoring function by transducing ionic current (i.e. sodium, potassium, calcium, chloride ions) from the skin surface, into electron current, through metallic wires of a given recording system/instrument [186],[110]. Silver/Silver chloride (Ag/AgCl) electrode material is the gold standard for biological signal acquisition (electrocardiogram, electroencephalography, endodermal activity etc.), owing to its low and stable half-cell potential [110]. The electrode is composed of a pellet of sintered silver and silver chloride, with a thin layer of conductive gel electrolyte on its top surface. This configuration provides good electric contact with the skin, and the metallic salt coating acts as an intermediate bridge between electrode and electrolyte, thereby reducing skin-electrode impedance. In turn, this facilitates the collection and measurement of high-quality signal. It is desirable for electrodes-electrolyte materials to be electrochemically reversible and stable, with minimal fluctuations in potential over time [2],[9]. While standard Ag/AgCl gel electrodes can provide accurate ECG signal quality, their rigid form factor may be uncomfortable for patients to wear for long periods of time. Furthermore, they are not suitable for prolonged use due to the electrolyte gel drying out over time, and potential to cause skin irritation for some patients. Electrodes made from conductive metals or polymer films are commonly used, but tend to have higher sensitivity to motion artifacts, produce fluctuations in potential, and cause signal distortion over time, owing to the absence of an electrolyte layer [56]. However, sweat and moisture that develop on the skin surface over time is thought to act as an electrolyte layer, thereby 131 reducing skin impedance [26]. While these distortions in features can be corrected by signal processing methods, it is more ideal to develop materials with polarization potential stability, to reduce interference with the collected signal [112],[25]. Other considerations to be made when developing electrode materials for biopotential signal monitoring include low electrical resistance of the electrode compared to that for skin surface [188], skin contact, conformability, durability, and biocompatibility [52]. Extensive efforts have been devoted to develop dry electrodes for biological signal monitoring, such as printed Ag/AgCl-based inks on flexible substrates or textiles [25],[188]. Work by Xu et al. and Haddad et al. have demonstrated the fabrication of Ag/AgCl based e-textiles comparable with Ag/AgCl solid gel electrodes for biological signal monitoring [18],[189]. Ag/AgCl coated yarn is the key element in developing e-textile based biopotential electrodes. A wide range of Ag coated yarns made from a variety of core materials, fibre diameter, Ag film thickness, and quality are available on the market and can be used as starting materials for making Ag/AgCl coated yarns. However, formation of uniform, thin, robust, and stable Ag/AgCl coatings on the yarns is challenging due to the sensitivity of the process on multiple factors such as, the coating medium, applied current, thickness of the starting Ag coating on the surface, and susceptibility of the coating itself to mechanical abrasion during long-term use. In furthering the development of the processes that have been demonstrated, investigations on the formation of AgCl on the surface of Ag-coated yarns is necessary. The current understanding of the formation process has been taken from studies on the formation and electrochemical behaviour of Ag/AgCl reference electrodes [191]–[193]. This work reports a roll-to-roll electrochemical system for the lab scale fabrication of Ag/AgCl-coated multi-filament nylon yarn that has been developed by the authors [190], electrochemical characterization of the coating process of AgCl on Ag-nylon yarns, 132 design and performance of embroidered electrodes for ECG measurement, which provide insights into the stability and performance as an electrode material. 6.2 Materials and Methods 6.2.1 Fabrication of Ag/AgCl E-Textile Yarns - Roll-to-roll coating system The method used to fabricate Ag/AgCl yarns is a modified setup from previous reported work [190]. The setup allows for a higher throughput of Ag/AgCl yarn, as well as modifiable process parameters, which allow for accurate control of coating thicknesses and yarn conductance to meet material performance requirements. Commercial silver (99.9%) coated nylon two-ply yarn (Circuitex™, Noble Biomaterials, USA) with each ply having a 100-denier linear density and 34 filaments, was used as the starting base yarn to be coated. A schematic of the setup is shown in Figure 6.1 (photograph of setup Appendix A.7, Figure A.7.1). The system comprises several components. First, a manual spring tensioner controls the tension of the yarn through the process. A rectangular glass (Pyrex) container mounted on top of a magnetic stirrer hot plate is used as the electrochemical reactor. A plexiglass holder consisting of nylon pulleys guides the yarn (working electrode and anode) into the glass reactor to pass through the electrolyte. It is designed to allow a certain length of yarn (L) to move horizontally in the electrolyte. Two platinum wires (counter electrode, cathode) with the same length (L) are horizontally mounted on the holder, and are fixed on both sides, running parallel and equidistant to the yarn. Upon exit from the reaction bath, the yarn may be directed to a second bath that passes through DI water to remove any loose particles. The yarn is then wound onto a rotating spool powered by a direct current (DC) geared motor, equipped with a speed controller, from which it is then dried with a small fan. The electrochemical process of Ag/AgCl nylon yarn was carried out in a 0.9 wt.% NaCl (VWR International, USA). A Keithley 2400 Sourcemeter (Keithley, USA) was used as a current 133 source for the reaction to occur (Figure 6.1-d.). As the anode, and working electrode, the as-obtained Ag-coated yarn passes through and (Figure 6.1-a.) runs through a fixture comprising metal screws/washers, and a tension controller (Figure 6.1-b.). In the setup, this fixture is connected to (Figure 6.1-c.) the positive terminal of the Sourcemeter (Figure 6.1-d.). The negative terminal of the Sourcemeter is connected to the platinum wires (counter electrodes, Figure 6.1-e.) that are mounted in parallel and run horizontally along both sides of the yarn at the bottom of the reaction container (Figure 6.1-g.). The yarn travels through the solution bath and through rollers, housed by an acrylic frame (Figure 6.1-f.) that carries it to the final collecting spool (Figure 6.1-j.) at the end of the system. The collecting spool is connected to a 20 V DC geared motor (Figure 6.1-k.) attached to a power supply and is used to control the reaction time and rate of yarn movement through the chamber. At the end of the process, the yarn is removed from the spool and dried for at least 10 hours at 60°C prior to characterization and use. Relevant process parameters based on the yarn winding motor speed, and source meter applied current were identified, and informed by electrochemical characterization tests carried out, and resulting Ag/AgCl yarn resistance. 134 Figure 6.1 Roll-to-roll Ag/AgCl electrochemical coating system. 6.2.2 Characterization of E-Textile Yarns and Electrodes 6.2.2.1 Electrochemical characterization Cyclic voltammetry (CV), and potentiodynamic polarization (PDP) tests were carried out using the Biologic VMP-300 electrochemical workstation (BioLogic, France) to characterize the formation of AgCl on the Ag-nylon yarn, and to identify appropriate operating range for a stable reaction process. The tests were carried out in the 0.9 wt.% NaCl solution, using a three-electrode cell configuration, with the Ag-coated nylon yarn as the working electrode (WE), a platinum wire as the counter electrode (CE), and a saturated calomel electrode (SCE) as the reference electrode (RE). Parameters were selected based on existing work on Ag/AgCl reference electrode characterization and preparation [191],[192]. The CV scans were performed under both steady-state and dynamic conditions, from -400 mV to +150 mV (vs. ref.) at a scan rate of 100 mV/s for 10 cycles. The reference value for the standard electrode potential of Ag/AgCl vs. SCE is -0.022 V. The CV scans display the redox 135 reactions in the NaCl solution and were used to qualitatively identify points of nucleation and growth of AgCl. The PDP tests were performed from -200 mV to +1600 mV (vs. ref.) at a scan rate of 0.5 mV/s. The results of the test are used to identify a suitable and stable operating window for the anodizing process of Ag to AgCl. 6.2.2.2 Material characterization The linear resistance of yarn samples was measured using a two-probe multimeter (Tektronix DM2040, USA). The reproducibility of the process and uniformity of the coating was evaluated by measuring the resistances of three separate sets of yarns (each 10 cm long) coated under the same processing parameters. Sheet resistance of the embroidered e-textile electrodes was measured using a four-point probe setup, consisting of four equally spaced spring-loaded gold tips (20 mm), attached to an adjustable stage to minimize sample damage, and accommodate various sample thicknesses. A source meter (Keithley 2400 Sourcemeter, USA) was used to supply constant current to the two outer tips, and a multimeter (Tektronix DM2040) was used to measure the voltage across the two inner probes. From this configuration, the sheet resistance can be obtained by (Eq.1): Rs = (kV)/I, (1) where k is the geometric correction factor for non-ideal samples, with k = 4.532 for a semi-infinite sheet [161]. The surface resistance of the embroidered electrodes was measured in both warp and weft (vertical and horizontal) orientations. X-ray diffraction (XRD) was carried out using a diffractometer (Rigaku Ultima IV, Japan), with CuKα radiation (1.54 Å), 2θ ranging from 5° to 90°, scan step size of 0.02°, scan speed of 3°/min, to verify the crystal structure of the deposited Ag/AgCl. 136 Scanning electron microscopy (SEM) (Carl Zeiss NTS Ltd., Germany) was used characterize the surface morphology and thickness of the Ag and Ag/AgCl layers (secondary electron signal, 10 kV accelerating voltage). Energy-dispersive x-ray spectroscopy (EDX) was used to qualitatively verify the presence of Ag and AgCl on the coated yarns. To examine the cross-section of the yarns, specimens were cast in epoxy resin (Epofix Struers, Denmark), cured for 48 hours, and polished to obtain a flat surface with roughness less than 2 µm. 6.2.3 Electrode Fabrication by Embroidery The e-textile yarns were used to develop new electrodes for biopotential signal detection. The Ag/AgCl coated yarns were embroidered onto two layers of woven fabric, a 244-gsm cotton woven fabric, and 44-gsm silver-nylon woven fabric (Statex, Germany), using a sewing machine (Baby Lock Aventura Sewing Machine, Suzuki Machinery Co., Ltd., Japan). The silver-coated nylon woven fabric (Statex, Germany) was placed as a backing material for the electrode, to serve as an electrical connection for the electrodes during testing. A zig-zag lockstitch of 5mm width was patterned into six columns, both the horizontal and vertical directions using one continuous conductive yarn for each electrode (Figure 6.2). Two sets of 2.5x2.5 cm electrodes were made from the original Ag and coated Ag/AgCl nylon yarns. The embroidered woven fabrics were stitched to a 3 mm spacer foam. Figure 6.2 Embroidered e-textile electrodes: a. Schematic of structure, b. Ag-nylon (left), Ag/AgCl-nylon (right), 137 6.2.4 Electrochemical Characterization of E-Textile Electrodes The electrochemical characterization was conducted using an electrochemical workstation (BioLogic VMP 300, France). Electrical impedance spectroscopy (EIS) was performed on the left palmar forearm of a 30-year-old female, using a three-electrode measurement setup. The embroidered e-textile electrode was used as the working electrode (WE), while standard Ag/AgCl gel electrodes (UniGel, USA) served as counter and reference electrodes (CE, RE, respectively). The electrodes were positioned following the setup used by Euler et al., with 10 cm distance between the WE and CE, and 1 cm distance between the WE and RE (Figure 6.3a). Using an arm wrap, light pressure (~ 0.5 kPa) was applied to the electrodes to ensure direct contact to skin. The EIS tests performed in potentiostatic mode, in which a sinusoidal voltage of 1 V was applied, using a frequency scan from 10 kHz to 0.1 Hz, with 10 points per decade, sinus amplitude of 10 Vrms and three measurements per reading. Analysis was performed by comparing the recorded EIS curves with equivalent circuit models to calculate circuit components. The Simplified Randle’s circuit was selected to represent the skin-electrode interface and used for impedance circuit modeling. The model comprises a resistor Rs in series with a parallel resistor (Rct) and capacitor (Cd) (Figure 6.3b). The capacitor represents the electrical charge between the electrode and skin, Rct represents the resistance between skin and electrode during the charge transfer process, and Rs represents the total resistance of the body/electrolyte, wires and electrodes [21], [25], [174]. 138 Figure 6.3 a. Electrode configuration for EIS measurement on planar forearm, b. Simplified Randle’s Circuit representing skin-electrode impedance [174], c. Electrode configuration for polarization potential measurement with two sets of electrodes (Ag or Ag/AgCl e-textile and standard Ag/AgCl gel) Electrode polarization occurs due to changes in the charge distribution at the skin-electrode interface, and may cause baseline drift, or DC offset in ECG signals [65]. Therefore, electrodes suitable for ECG or biopotential measurements must be stable enough so that fluctuations do not interfere with the measured signals. This is generally controlled by appropriate material selection (use of Ag/AgCl), or through applying a high pass filter during signal processing [53]. Polarization potential was measured by utilizing the open circuit potential mode. The potential difference was recorded every 0.1 seconds for 10 minutes for each e-textile electrode set, placed 5 cm apart on the left planar forearm, and compared with standard gel Ag/AgCl electrodes (Figure 6.3c). Mean 139 potential drift for each measurement series (per minute) was calculated according to Equation 2 [53]. ∑ |𝑥𝑖−𝑥𝑖−1|𝑖𝑖+1, where i = 1, … n (2) 6.2.5 Electrocardiogram Measurements and Analysis Methods Figure 6.4 ECG electrode configuration and measurement system schematic ECG was measured using a clinical 1-lead ECG system (Thought Technology Ltd., Canada), equipped with a data acquisition board and processing software (Biograph Infiniti), at a sampling rate of 2048 samples/s. Signals were simultaneously collected from two channels to compare the performance of the e-textile electrodes with the standard Ag/AgCl gel electrodes. The sets of electrodes were attached side-by-side in a chest placement configuration, following the manufacturer’s guideline (Figure 6.4). It should be noted that the simultaneous recordings do not 140 produce identical ECG signals, since they will pick up slightly different phase and amplitude differences due to relative positioning of the electrodes. The e-textile electrodes were attached to the skin using a snap clip connector compatible with ECG snap leads, and secured with medical tape (3M, USA). Three trials, six-minutes long were conducted in a stationary seated position for each electrode set (Ag and Ag/AgCl e-textiles). All measurements were performed on one subject (30-year-old, female). Methods in the time and frequency domain were used to compare ECG signal quality of the different electrode materials. For time domain comparison, variability (coefficient of variation, % C.V.) in the R-R (or inter-beat-interval IBI) intervals were compared, as well as a visual comparison of the waveforms. In addition, average waveforms were calculated by template matching, from which the Pearson’s correlation coefficient (PCC) was calculated between each e-textile electrode material set and the corresponding standard reference electrode. The template matching method compares similarities in the ECG waveform morphology. A modified model from Orphanidou [131] was employed, using the entire PQRST waveform [182], and was carried out in MATLAB (R2021b, The Mathworks Inc., RRID:SCR_001622). PCC results were compared to the metrics for template matching quality indices specified by Orphanidou, with a PCC of greater than or equal to 0.66 deemed as acceptable. Previous work employing this method for commercial Ag-nylon e-textiles (Chapter 3) with acceptable signal quality, reported PCC values of approximately 0.94 (range: 0.91 to 0.98) [182]. For frequency domain comparison, power spectral density plots were calculated and compared in terms of occupied bandwidth (total, lower, and upper bounds, in Hz), which represents the data region containing 99% of the power spectral density estimate [194]. 141 6.3 Results and Discussion 6.3.1 Roll-to-Roll System Process Parameters and Yarn Electrical Properties The winding speed for the spool collecting the coated yarn was measured from 0.75 V to 10 V in 0.5 V increments. The corresponding yarn speed as well as the corresponding reaction time were measured, and the results are displayed in Figure 6.5a and Table 6.1. The reaction time was considered as the total time that a certain point of the yarn is immersed in electrolyte between the Pt electrodes in the reaction container. Figure 6.5b displays a typical plot of resistance of the as-obtained Ag-yarn across a 25 cm segment, measured at every 1 cm. The linear relation of the resistance vs. length of the yarn shows uniform Ag coating with an average resistance of 1.8 Ω/cm. Following previous work [190], the initial yarn coating was performed at various speeds from 0.75 V to 8 V, and at different applied deposition currents, varying from 0.5 mA to 2.0 mA. This was performed to obtain a general understanding of operating parameters for the modified roll-to-roll system. Like the Ag-coated yarn, the average linear resistance was measured for the Ag/AgCl-coated yarns, and results are plotted in Figure 6.5c. Based on the results, operating parameters which result in yarn resistances between 10-30 Ω/cm were explored, due to the sharp increase in yarn resistance observed thereafter (for samples measuring >30 Ω/cm). This is likely due to the full dissolution of the thin Ag-coating on the yarn, measured to be approximately 265 nm (Figure 6.5d). 142 Figure 6.5 a. Yarn winding speed (cm/s) calculated from motor speed (V) b. Linear resistance of the original Ag coated nylon yarn measured across a 25cm segment. Average resistance 1.8 ±0.2 Ω/cm. c. Linear resistance was measured for the Ag/AgCl coated nylon yarns at different applied currents between 0.5 mA to 2.0 mA, and winding speeds (0.022 cm/s to 0.272 cm/s) d. Fibre cross-section, showing Ag coating thickness (7.5K x)143 Table 6.1 Reaction time of 10 cm yarn segment in coating bath with respect to yarn winding speed Motor Speed, V Winding Speed, cm/s Reaction Time, s (10 cm yarn) 0.75 0.022 447 1.5 0.046 218 2.0 0.063 159 2.5 0.080 126 3.0 0.095 106 3.5 0.112 89 4.0 0.131 76 4.5 0.147 68 5.0 0.165 61 5.5 0.181 55 6.0 0.198 51 6.5 0.216 46 7.0 0.235 42 7.5 0.250 40 8.0 0.272 37 8.5 0.287 35 9.0 0.317 32 9.5 0.321 31 10.0 0.349 29 6.3.2 Electrochemical characterization There have been numerous investigations that have studied the formation of AgCl on Ag metal substrates in chloride solutions through both CV and PDP [191]–[193], [195]. The electrochemical tests allow for the interpretation of the formation and growth processes of AgCl nuclei to films on Ag surfaces, as well as from corresponding observations of the layer morphology and microstructure from different operating conditions [192], [195]. The formation of AgCl begins with initial AgCl nuclei formation on preferential sites of the Ag surface, or adsorption of chloride 144 ions through a partial charge transfer with the substrate. This is then followed by the nucleation and growth of the three-dimensional AgCl film in patches across the substrate, until a continuous film is formed [191], [192], [196]. This part of the process is thought to be controlled by either interfacial, and diffusion-controlled growth kinetics [192]. The CV tests were used as a tool to understand the redox reaction window of the electrochemical coating process of Ag-nylon yarn in 0.9wt.% NaCl solution. Figure 6.6a displays recorded results of 10 CV-cycles (0.5 V to 0.2 V) carried out for the process under steady-state conditions, in which the yarn was kept stationary (no yarn movement) in the electrolyte solution. As can be seen, an anodic peak was observed at scan potential of ~148 mV. The intensity of the anodic peak decreases upon subsequent scans, owing to the stripping of the Ag layer from the yarn surface (Equation 3a). Furthermore, a broad shift is observed from the anodic to cathodic peak in the reverse scan and demonstrates a multi-step process occurring in the anodic region, described by Equation 3b, representing the formation of silver chloride nuclei. In the cathodic scan, the peak potential shows a positive shift, increasing from -21 mV to -15 mV. This positive shift may indicate a more efficient electron transfer process due to the formation of conductive Ag and Ag adatoms on the surface of the yarn. However, over the increasing scans, the decreased peak area demonstrates a lower reaction rate with increasing scans. As this test was carried out under stationary conditions (yarn in solution), this is indicative of the conversion of Ag to Ag/AgCl being limited by the available Ag coating on the nylon yarn surface. The reactions occurring in the cathodic scan can be explained by work from Birss et al., who studied the formation of AgCl through surface enhanced Raman scattering effect (SERS) [191], to involve the deposition of silver adatoms when AgCl nuclei reduce (Equations 4a-b). The adatoms 145 are active forms of silver that may enable the formation of the AgCl film in subsequent anodic cycles. 1. Anodic peaks (multi-step process) Ag(s) ⇌ Ag+ + e- (3a) Ag(s) + Cl- ⇌ AgCl(s) + e- (3b) 2. Large cathodic peaks (Ag adatoms) AgCl(s) + e- ⇌ Ag(s) + Cl- (4a) Ag+ + e- ⇌ Ag(s) (4b) The CV tests were also performed under dynamic condition when the Ag-coated yarn was moving through the solution bath at a speed of 0.13 cm/s (4 V motor speed). The recorded results display similar behaviour to the steady-state condition, indicating no significant difference between the reactions occurring under the two conditions (Figure 6.6b). It should be noted that smaller shifts were observed in the anodic and cathodic peaks with increasing scans, owing to continuous movement of the yarn, working electrode, and continuous exposure of fresh yarn to the electrolyte in the reaction bath. 146 Figure 6.6 a. CV scans (10 cycles) under static condition in 0.9wt.% NaCl solution. b. CV scans (10 cycles) under dynamic condition, with yarn winding speed of 0.13 cm/s in 0.9 wt.% NaCl solution. 147 Potentiodynamic polarization (PDP) was performed to determine the suitable operating range of applied currents for AgCl formation. The test follows the process parameters specified by Pargar et al. [193]. Shown in Figure 6.7a, the initial potential increases with current (active corrosion), until it reaches the primary passive potential, Epp, at approximately 0.27 V, and up to 2.4 mA. Thereafter, current and corrosion rate decrease (passive region), in which an external applied potential is needed to activate the process. This can be interpreted as Ag beginning to passivate after 0.27 V, forming AgO. Therefore, as only Ag dissolution is desired to subsequently form AgCl, it is best to operate the process at potentials lower than 0.27 V, or current lower than 2.4 mA. Based on the above observation, the applied current of 2 mA (or 2.3 mA/cm2) was selected for Ag/AgCl yarn fabrication. The CV tests were also performed under dynamic condition when the Ag-coated yarn was moving through the solution bath at a speed of 0.13 cm/s (4 V motor speed). The recorded results display similar behaviour to the steady-state condition, indicating no significant difference between the reactions occurring under the two conditions (Figure 6.6b). It should be noted that smaller shifts were observed in the anodic and cathodic peaks with increasing scans, owing to continuous movement of the yarn, working electrode, and continuous exposure of fresh yarn to the electrolyte in the reaction bath. 148 Figure 6.7 a. PDP measurement for AgCl formation on Ag nylon yarn in 0.9 wt.% NaCl b. Open circuit potential measurement to assess stability of AgCl coating process over a 3-minute period. 149 6.3.3 Yarn electrical and material characterization The measured resistance as a function of yarn length for the original Ag-coated yarn, and Ag/AgCl-coated yarn, processed at 2 mA (0.13 cm/s, 4V motor speed) are shown in Figure 6.8. The average linear resistance of the original Ag yarn is 1.8 Ω/cm ± 0.2 Ω/cm (n=25), while the coated Ag/AgCl yarn is 14.7 Ω/cm ± 4.7 Ω/cm (n=50). 0 5 10 15 20 25050100150200250300350400 Ag coated nylon Ag/AgCl coated nylonResistance ()Length (cm) Figure 6.8 Resistance of Ag-coated nylon (original), and Ag/AgCl coated nylon across 25 cm yarn length. Further qualitative material characterization was performed on the Ag-coated and Ag/AgCl-coated nylon yarns by SEM-EDX (Figure 6.9) and XRD (Figure 6.10) analysis. Figures 6.9a-b show the EDX maps of the surfaces of the Ag and AgCl coated yarns (spectrum analysis in Appendix A.7, Figure A.7.2a-b). Both Ag and Cl were observed all over the yarns. This confirms the uniform distribution of Ag on the original yarns, and uniform reaction process of Ag/AgCl coating on the surface of the processed yarns. The SEM micrographs of the surfaces (Figure 6.11) show differences in the morphology of the Ag and Ag/AgCl yarns. The Ag yarns appear to have a smoother and continuous surface, 150 whereas the Ag/AgCl coated yarns show a granular surface with varied grain size (average ~220 nm ±30nm) and stacked layers. In addition, at higher magnifications, pores are visible on the outer layers, characterized as microchannels, that facilitate the ionic transport through the film [192], [193], [195]. According to characterization work by Ha et al., initial AgCl layer growth begins in a non-continuous manner, in which Ag dissolution kinetics can be described as activation control [192]. With a thin layer formed, and patches of AgCl expanding laterally across the substrate, dissolution kinetics become mixed control, with ohmic overpotential through the AgCl layer, and activation overpotential in Ag substrate regions. Ionic transport occurs through the newly formed film through spaces in between the AgCl grains. As a thicker film forms, ionic transport occurs through micro-channels through the grain, upon which any further ionic dissolution occurs as ohmic overpotential through the AgCl layer. As observed from the XRD analysis spectrum, AgCl forms in polycrystalline structure, with clear peaks associated with (220), and (111) crystal planes, which reflect the angular and spherical grains as described by Zhang et al. [195]. Those grains are clearly visible on the microstructure surface of the coated yarns, shown in Figure 6.10b. The peaks observed below 25° could not be identified, they likely reflect the exposed nylon 6,6 core of the yarn specimen that was cut and attached to a thin polymer sheet placed on the sample holder. 151 Figure 6.9 a. Ag-coated nylon yarn, with EDX map of Ag. b. Ag/AgCl-coated nylon yarn, with EDX map of Ag and Cl. 152 Figure 6.10 XRD patterns for yarn surfaces of Ag-coated nylon yarn, and Ag/AgCl-coated nylon yarns, with annotated peaks corresponding to silver and silver chloride. 153 Figure 6.11 SEM micrographs of Ag-coated nylon yarn surfaces at a) 3.5k and b) 8.5k magnifications; SEM micrographs of Ag/AgCl-coated nylon yarn surfaces (2.0 mA applied current) at c) 4k and d) 12k magnifications. 6.3.4 Electrode Characterization 6.3.4.1 Surface resistance The surface resistances of a set of four of embroidered electrodes were measured and summarized (Figure 6.12a-b). It should be noted that the error for the measurements are high due to the uneven contact points between the probe and the embroidered surface. The average surface resistance measured in the vertical and horizontal directions for the four Ag electrodes are measured as 0.15 ±0.04 Ω/sq. and 0.14 ±0.03 Ω/sq., respectively. There was no significant difference between the surface resistance in the two measurement directions for the Ag e-textile electrodes (p-value = 0.33). 154 The average surface resistance for the four embroidered Ag/AgCl electrodes in the vertical and horizontal directions were measured to be 0.25 ±0.12 Ω/sq. and 0.25 ±0.10 Ω/sq., respectively. Again, no significant differences between measured surface resistance for the two directions were observed (two measurement directions for Ag/AgCl e-textile electrodes: p-value = 0.49). Figure 6.12 Measured surface resistance for a) Ag-nylon embroidered electrodes, and b) Ag/AgCl-nylon embroidered electrodes. 6.3.4.2 Polarization potential The polarization potential was measured for a period of 10-minutes for the electrodes made of the Ag-coated and Ag/AgCl coated yarns, as well as the commercially available standard sintered electrodes (with gel electrolyte). The drift for the electrodes were then calculated (measurement taken every 0.1 seconds, averaged per minute) based on Equation 2, and summarized in Table 6.2 and Figure 6.13. Polarization potential drift is a measure of electrode stability over time. It is important that electrodes have minimal polarization potential fluctuations to minimize interference with biopotential signals being collected. As observed from the summary table, and Figure 6.13a, the Ag e-textile electrodes show higher variation in mean polarization potential, notably after 5 minutes of measurement (average drift 3.79 mV/min, between 0.71 to 155 6.44 mV/min), and a higher overall average compared with the Ag/AgCl e-textiles (average drift 1.22 mV/min, between 0.41 to 2.33 mV/min), and reference gel electrodes (average 0.18 mV/min, between 0.06 to 0.32 mV/min). It should be noted that dry textile electrodes have been compared with commercially available standard electrodes that contain gel electrolyte. The gel fills the gaps between the electrode and skin, which minimizes drift. The results thereby demonstrate the stability of Ag/AgCl electrode materials observed from the low half-cell potentials measured. To estimate the potential effect of drift on the measured signal, a calculation method utilized by Rattfalt et al. is followed [197]. The QRS wave of ECG can be considered: assuming an amplitude of 1 mV and 80 ms duration [198], this yields a rate of 12.5 mV/s. Scaling the results for the average drift of e-textiles yields 0.06 mV/s, and 0.02mV/s for the Ag and Ag/AgCl e-textile electrodes, respectively. This represents a factor of 103 difference, which indicates that the e-textiles would be unlikely to cause significant drift on the ECG signal. In comparison, work by Rattfalt et al. reported mean polarization potential drift for textile electrodes (stainless steel and polyester blended) between 2 mV/min up to 16 mV/min [53]. Overall, the low potentials measured demonstrate the adequate stability of Ag/AgCl e-textile electrode materials for ECG and heart rate measurement. To further reduce the potential drift for future studies, a denser embroidered structure of Ag/AgCl yarn can be considered. Table 6.2 Mean potential drift results calculated from 10-minute measurement period Electrode Material Potential Drift (mV/min) Ag 3.79 Ag/AgCl 1.22 Std. Gel Ref. 0.18 156 Figure 6.13 a. Mean polarization potential across 10-minute measurement period. b. Boxplot displaying range/spread of potentials measured across 10-minute period. 157 6.3.4.3 Electrode impedance The skin-electrode impedances of the e-textile electrodes were measured and compared with the standard Ag/AgCl gel electrodes. Estimates for the components of the skin-electrode impedance model applied the Simplified Randle’s circuit (Figure 6.3b), and Rs, Rct, and Cd were calculated in MATLAB (R2022a, Mathworks) using the least squares nonlinear curve fitting method. Example results are plotted in Figure 6.14a and summarized in Table 6.3. Model parameters were estimated from the following equations: 𝑍 = 𝑅𝑠 + 𝑅𝑑(1+𝑋𝑑2) − 𝑗𝑅𝑑𝑋𝑑(1+𝑋𝑑2) (5a) where Xd = Rd.Cd.ω, ω = 2πf (5b) |Z|2 = A2 + B2 , (6a) where the real and imaginary components of |Z| as A and B as shown: 𝐴 = 𝑅𝑠 + 𝑅𝑑(1+𝑋𝑑2) (6b) 𝐵 = − 𝑅𝑑𝑋𝑑(1+𝑋𝑑2) (6c) Overall, both Ag and Ag/AgCl e-textile electrode showed nearly identical impedance across the range of 0.1 to 10 kHz (Figure 6.14b), but higher than that for the standard gel reference electrodes. In general, impedance between 0.01-5 MΩ in the 5-100Hz range is deemed as acceptable for ECG e-textiles [66], [73], [85], [109]. The impedance of the e-textile electrodes is within this range, lower than 1.9 MΩ, as summarized in Table 6.3. The results for the circuit elements for the sets of electrode materials are all within the same order of magnitude and range (Table 6.3). From the Nyquist plots (Figure 6.14a), it is observed that the ohmic resistance contribution is negligible, as the intercept at the highest frequency lies close to the origin. This represents Rs, total resistance of the body/electrolyte, wires, and electrode. Furthermore, the plots 158 (Figure 6.14a) show visible differences in the charge transfer resistance between the electrode materials (distance between the two axis intercepts). The standard gel electrodes have the lowest, followed by the Ag/AgCl e-textiles, and Ag-e-textiles. This is expected, as the gel electrolyte and AgCl layer on the standard electrodes, and the coated yarns serve to reduce the resistance of ion to electron charge transfer. Both sets of e-textile electrodes have similar estimated values for the components of the circuit model, with the Ag/AgCl e-textiles having a slightly lower resistance and capacitance for the skin-electrode interface. The standard gel reference electrodes have slightly higher Cd values compared to the e-textile electrodes likely due to the gel electrolyte layer present on the surface of the electrode, which facilitates the ionic to electron charge transfer between the skin to electrode. As expected, the resistance between skin-electrode, Rct for the gel electrode is lower than the e-textile electrodes due to high contact surface with the skin, and high ionic conductivity of the gel, which facilitates ion to electron charge transfer. 159 Figure 6.14 a. Example Nyquist plot with measured and fitted data, b. Bode plot displaying impedance vs. frequency for e-textile and standard gel Ag/AgCl electrodes Table 6.3 Summary of average impedance results at 1 Hz, 10 Hz, 145 Hz, and estimates of circuit model parameters for electrode materials investigated Z @ 1Hz (MΩ) Z @ 10Hz (MΩ) Z @ 145Hz (MΩ) Rs (MΩ) Rct (MΩ) Cd (F) Ag 1.890 0.300 0.034 6.33x10-4 5.62 2.41x10-8 Ag/AgCl 1.892 0.306 0.035 6.50x10-4 5.35 2.36x10-8 Ref. Gel 1.288 0.241 0.029 6.37x10-4 3.53 2.52x10-8 6.3.5 Electrocardiogram of E-Textile Electrodes – Signal Quality Evaluation Figure 6.15a shows a characteristic ECG waveform, which represents atrial (P wave), and ventricular (QRS complex) depolarization, followed by the ventricular repolarization (T wave) of the heart. Methods in the time and frequency domain have been applied to compare signal quality 160 between standard Ag/AgCl gel electrodes and the embroidered Ag and Ag/AgCl coated e-textile electrodes. Figure 6.15 a. Characteristic ECG waveform; Sample visual comparison of ECG recording collected for b. Ag- and c. Ag/AgCl e-textiles simultaneously with standard gel Ag/AgCl electrodes. 161 Initial visual comparison of ECG collected for each e-textile electrode group and corresponding the standard gel electrodes (simultaneous measurement side-by-side) show similar signals, with the embroidered electrodes showing higher R-peak amplitudes, and lower S-waves (Figures 6.15b-c). However, the amplitude differences do not impact the heart rates (HR) calculated from the raw signal. The differences are thought to be due to the closer proximity and placement of the textile electrodes to the heart centre compared to the gel electrodes in the simultaneous recording configuration. Another possible reason is due to the higher surface area of the textile electrode compared to the standard gel electrode, which could contribute to better contact between the skin and electrode. Measurements to compare differences in electrode placement on signal R-peak amplitude are summarized in Figures A7.3-A.7.4 (Appendix A.7). Corresponding average heart rates (HR) calculated from the inter-beat intervals (time difference between R-R peaks across a measurement period), standard deviations (SD) and coefficient of variation (CV, %) for all sample groups of the five trials performed demonstrate nearly identical HR results (Table 6.4). This indicates that the embroidered electrodes are excellent alternatives for collecting ECG signals. 162 Table 6.4 HR (beats/min) results of ECG trials for Ag and Ag/AgCl e-textile electrodes, measured simultaneously with standard gel electrodes, calculated standard deviation, and coefficient of variation (%) results. Trial 1 T2 T3 T4 T5 Gel Ref. Ag e-textile Gel Ref. Ag e-textile Gel Ref. Ag e-textile Gel Ref. Ag e-textile Gel Ref. Ag e-textile Average 56.96 56.96 57.82 57.82 55.99 55.99 58.17 58.17 58.86 58.86 Std. Dev. 3.09 3.09 3.02 3.02 3.16 3.16 3.84 3.84 3.35 3.35 Var. (%) 5.42 5.42 5.23 5.23 5.64 5.64 6.61 6.61 5.70 5.70 Trial 1 T2 T3 T4 T5 Gel Ref. Ag/AgCl e-textile Gel Ref. Ag/AgCl e-textile Gel Ref. Ag/AgCl e-textile Gel Ref. Ag/AgCl e-textile Gel Ref. Ag/AgCl e-textile Average 57.33 57.33 57.05 57.05 57.26 57.26 56.02 56.02 55.61 55.61 Std. Dev. 3.22 3.22 3.50 3.54 3.97 3.97 3.24 3.24 2.68 2.68 Var. (%) 5.62 5.62 6.14 6.20 6.93 6.93 5.78 5.78 4.82 4.82 163 The average ECG waveform for the e-textile electrodes were obtained and compared with the respective standard reference (gel) electrode using MATLAB (R2022a, Mathworks, Inc., RRID:SCR_001622). Typical results for the Ag and Ag/AgCl textile electrodes are shown in Figure 6.16. Again, differences between the signals are visible, specifically the higher R-peak amplitudes, and lower S-T segments for e-textile electrodes. This is thought to be due to the presence of more motion artifacts, based on visual observation of the raw ECG signal. The Pearson’s correlation coefficient (PCC) comparing the simultaneous Ag/AgCl standard electrode and e-textile electrode signals were calculated for each trial and summarized in Table 6.5. Results for both sets of e-textile electrodes have an average PCC value of 0.98, which exceed the acceptable range defined by Orphanidou, with PCC >/= 0.66 [131]. It falls in the higher end of the acceptable PCC range from previous work comparing Ag-nylon e-textile electrodes, with calculated values of approximately 0.94 (range: 0.91 to 0.98) [182]. Figure 6.16.Example average ECG waveform (PQRST) for Ag and Ag/AgCl coated nylon e-textiles. 164 Table 6.5 PCC results for e-textile trials compared with respect to standard gel electrodes E-textile group Trial 1 Trial 2 Trial 3 Trial 4 Average Ag 0.98 0.98 0.98 0.98 0.98 Ag/AgCl 0.98 0.97 0.97 0.98 0.97 Analysis of the ECG signals in the frequency domain included the calculation of power spectral density, and occupied bandwidth, representing the data region containing 99% of the total power, with example results displayed in Table 6.6. The corresponding power spectral density plots for the embroidered e-textile electrodes compared to standard gel electrodes (Figure 6.17a-b) show the higher power magnitude in the low frequency range (1-10 Hz) for the e-textile electrodes, which reflect the higher signal peak amplitude and noise observed in the raw signal (time domain). This observation also corresponds to the occupied bandwidth results that show 99% of the total power to be in a slightly smaller range for the e-textile electrodes (34.56 and 33.86 for Ag and Ag/AgCl e-textiles, respectively), compared with the Ag/AgCl gel reference electrodes (approximately 38.8 for both sets tested simultaneously with e-textile electrodes). Despite these differences, the results for the textile electrodes are within close range to the standard gel electrodes and demonstrate acceptable ECG signal quality based on the signal quality comparison methods employed in this study. Table 6.6 Example results for occupied bandwidth calculations for e-textile and gel electrode ECG signals Occupied Bandwidth Lower Bound (Hz) Upper Bound (Hz) Gel Ref. 36.80 0.87 37.67 Ag e-textile 34.56 0.88 34.49 Occupied Bandwidth Lower Bound (Hz) Upper Bound (Hz) Gel Ref. 36.52 0.82 37.34 Ag/AgCl e-textile 33.86 0.81 34.67 165 Figure 6.17 Example spectral plots for a) Ag coated nylon and b) Ag/AgCl coated nylon embroidered e-textile electrodes compared to standard Ag/AgCl gel electrodes. 6.4 Conclusions This work has demonstrated an electrochemical roll-to-roll system for fabrication of Ag/AgCl coated nylon yarns, which offers high material throughput, and tunable process parameters to obtain high performance materials for fabrication of textile-based electrodes for biopotential signal monitoring. The AgCl film growth and morphology were investigated through electrochemical methods and observation by scanning electron microscopy. The appropriate operating current range specific to the yarn studied in this work has been identified as 0.1 to 2.4 mA. The selected operating parameters of 2 mA applied current, and 0.13 cm/s winding speed was 166 observed to be stable and reproducible over multiple coating periods, producing Ag/AgCl-nylon yarns with an average resistance of 14.7 Ω/cm ± 4.7 Ω/cm. The electrodes fabricated by embroidery using the coated yarns demonstrated high stability, with low average polarization potential (1.22 mV/min) compared to electrodes embroidered from Ag-nylon yarns (3.79 mV/min), and low impedance (below 2 MΩ between 0.1 to 150 Hz). This demonstrates the improved effect from the metallic salt layer at the skin-electrode interface. The e-textile electrodes were used for ECG signal collection and exceeded the comparative measures for acceptable and good signal quality. Ag/AgCl based e-textiles are flexible, comfortable, and breathable alternatives to standard rigid gel electrodes and have the potential to be integrated unobtrusively into wearable systems for health monitoring applications. Further research for developing Ag/AgCl e-textiles is necessary to address improvements to durability, washability, and long-term performance. 167 Chapter 7: Stretchable, Self-Healing, Biocompatible and Durable Ionogel for Continuous Wearable Strain and Physiological Monitoring The following chapter is reproduced with minor revisions from a manuscript in preparation. Le, K., Sun, X., Chen, J., John, J., Servati, A., Heidari, H., Khademhosseini, A., Ko, F., Jiang, F., Servati, P. “Stretchable, self-healing, biocompatible, and durable ionogel for continuous wearable strain and physiological monitoring.” Electronic textiles (e-textiles) have great potential for use in wearable systems worn next to skin for continuous physiological monitoring, owing to their flexibility and conformability to the body. However, challenges exist in the long-term functionality and durability of these materials. This work presents a highly stretchable, self-healing, and biocompatible chloride-based ionogel fabricated from polymerization of thioctic acid, and 1-n-Butyl-3-methylimidazolium chloride ([BMIM]Cl) ionic liquid in ethanol. With its adaptable mechanical properties and tunable viscosity by simple dilution, the ionogel precursor can be either cast into an ionogel film used for strain sensing applications up to 500% strain, or directly sprayed as a coating for e-textile electrodes for biopotential signal monitoring. The ionogel can reform at room temperature, demonstrating 73.5% stress recovery, and 84.5% modulus recovery after a 30-minute healing period. When used as a spray barrier coating on e-textiles, the ionogel displayed high resistance to abrasion, and reformed after adding ethanol to the surface. The electrical properties of the ionogel coated e-textiles satisfies the performance range desired for textile sensing electrodes. Further development and application of the ionogel material can address long-term durability concerns by extending the usable lifetime of the sensors for wearable applications. 168 7.1 Introduction E-textiles functionalized with electrically conductive or active materials have many applications in biological signal monitoring, environmental sensing, energy generation, and electromagnetic shielding. These materials must retain their functional properties and performance during use, and in the long term, provide reliable measurements within the systems they are incorporated in. Exposure to temperature, air flow, humidity, and moisture changes, or use conditions that involve stretching, bending, abrasion, and stability after cleaning can impact material properties and performance. Specific to sensing electrodes used next-to-skin, consideration of durability is an important factor for long-term reliability and use, particularly for sensors intended for continuous monitoring. Sensing electrodes used for biopotential signal monitoring are often fabricated from metals paired with metallic salts (i.e. chlorides), which function by transducing ionic current (i.e. sodium, potassium, calcium, chloride ions) from the skin surface, into electron current, through metallic wires of a given recording system/instrument. Disposable Silver/Silver chloride (Ag/AgCl) electrode materials are the gold standard material system for biopotential signal monitoring (electrocardiogram, electroencephalography, endodermal activity etc.) in clinical settings, owing to its low and stable half-cell potential. It is preferable for sensing electrodes to be electrochemically reversible and stable, as this in turn reduces interference or noise in the signal recorded. Ag/AgCl gel electrodes take the form of pellet sintered high purity silver and silver chloride, with a conductive gel electrolyte coating on the top surface. This configuration provides good contact with the skin, as both the gel electrolyte paired with the metallic salt provide an intermediate bridge between the skin and electrode, thereby reducing skin-electrode impedance, translating into good signal quality. However, these electrodes are not suitable for repeated and 169 long-term use, as the gel electrolyte material dries out over time. In the development of dry electrodes for long-term biological signal monitoring, the use of flexible Ag/AgCl textile-based materials is uncommon, due to the instability of the AgCl layer with continuous use. Electrodes made from conductive metals or polymer films are commonly used, but tend to have higher sensitivity to motion artifacts, produce fluctuations in potential, and cause signal distortion over time, owing to the absence of an electrolyte layer [18]. However, sweat and moisture that develop on the skin surface over time are thought to act as an electrolyte layer, thereby reducing skin impedance [23]. While these distortions in features may be corrected by signal processing methods, it is more ideal to develop materials with polarization potential stability over time, to ensure minimal interference with the collected signal. Encapsulation and protective barrier coatings of e-textiles can offer protection to the functionalized materials, with several studies that have examined the change in resistance and sensing properties after washing cycles [125],[198],[199]. Protective encapsulation methods that have been employed include: the use of polymer films or tapes, dip-coating or immersion in polymeric materials that may be cross-linked, hydrophobic treatment sprays, or core-spun and core-sheath yarns in which metallized filaments are used in the core and covered or wrapped in insulating fibres [201]–[204]. Commonly used materials have included thermoplastic polyurethane (TPU), polyethylene, polyimide, silicone, epoxy resin, or PDMS for the protection of conductive embroidered tracks, printed conductive structures, electrodes, or circuit boards [199],[204]–[208]. While there has been reported success in these methods, the main challenges in the context of next-to-skin sensing electrode applications include the balance in maintaining the electrical conductance and performance of the material with the added protective barrier, and moisture permeability of the textile, which can impact the wear and comfort properties. 170 Ionic liquids (IL) are molten salt compounds comprising organic/inorganic cations and anions with low melting temperatures, good ionic conductivity, high thermal and chemical stability, unique, tunable physical and chemical properties that have been applied in various areas [210]–[213]. They have been utilized in numerous applications for sensors, actuators, solvents in chemical processes, energy devices including batteries and fuel cells, biomaterials, biological processes in pharmaceutics and biomedicine. IL materials have been polymerized into ionogel materials, used as quasi-solid state electrolytes, strain sensors for movement detection [214]–[218], and reported in cutaneous electrode applications for biological sensing, specifically for improving skin contact, and maintaining low contact impedance with skin [33], [211], [216], [219]–[221]. In this work, a stretchable, self-healing, biocompatible, and recyclable chloride-based ionogel material is presented. The material is prepared through the polymerization of thioctic acid (TA) and 1-butyl-3-methyilimidazolium chloride ([BMIM]Cl), dissolved in ethanol. [BMIM]Cl ionic liquids have been used a biocompatible stabilizer of proteins and enzymes in biopharmaceuticals and drug delivery systems [221],[222], ionic electroactive films for muscle tissue engineering [224], as well as additives in cosmetic formulations used for skin and hair cleansing [225]. The ionogel is demonstrated as a standalone ionically conductive material for strain sensing applications, and a functional and self-healing protective electrolyte coating layer for e-textile sensors used for biopotential signal monitoring. The functional and adaptive properties of the material enable good skin-electrode contact, thereby lowering contact impedance, while the healable properties have the potential to extend the durability of coated textile sensors for long-term use. 171 7.2 Materials and Methods 7.2.1 Materials Thioctic acid (TA, or α-lipoic acid), 98.0% was obtained from Tokyo Chemical Industry (TCI America). 1-n-Butyl-3-methylimidazolium chloride, ≥98.0% ([BMIM][Cl]) and ethanol were obtained from Sigma-Aldrich. Polyurethane pellets (Texin® 285A) were obtained from Covestro AG. N,N-Dimethylformamide (DMF) was purchased from Sigma-Aldrich. Silver coated (99.9%) plain woven nylon fabric (Bremen, Statex, Germany) was obtained from VTTech Textiles (USA). 7.2.2 Preparation of pTA Ionogel The poly-thioctic acid (pTA) ionogel was prepared by an evaporation induced polymerization process. 3 g of TA monomer was dissolved in 30 mL ethanol, and 0.507 g [BMIM][Cl] (molar ratio, IL:TA - 1:5) was added (1:3, 1:4, 1:10 ratios prepared, results summarized in Appendix A.8). After stirring, a clear and transparent yellow TA solution was obtained (Figure 7.1a). The solution was cast into a rectangular mould and vacuum degassed to remove entrapped air bubbles prior to curing. The mould with TA-IL solution was placed at room temperature for 3 days to allow for ethanol evaporation, after which transparent pTA ionogels were formed (Figure 7.1b). Separate pTA-IL solutions were prepared for spray coating on conductive silver plain weave fabric (Figure 7.1c). An airbrush system at 100 psi (690 kPa) was used to spray the coatings onto the fabric. A solution containing 1% PU-DMF was prepared and coated onto the fabric to provide a thin and protective layer of the metallic coating, and to lower the surface energy for better adhesion for the subsequent spraying of the pTA-IL layer. 172 Figure 7.1 a. IL:pTA solution b. IL:pTA cast film c. IL:pTA spray coated film (left) on conductive fabric 7.2.3 Preparation of Skin Phantom A skin phantom was prepared, containing 30% gelatin (14.65 g), 1% NaCl (0.489 g) heated and mixed at 85°C. After the gelatin was fully dissolved, 0.4% formaldehyde (37% solution, 0.17 mL) was added for room temperature stability (Figure 7.2a). The mixture was poured into a mould and set for 24 hours (Figure 7.2b). The contact resistance of the phantom was approximately 90-100 kΩ/cm, which is within close range to the resistance of human skin. Figure 7.2 a. Gelatin dissolution b. Cast skin phantom c. b. a. a. b. 173 7.2.4 Mechanical Properties The tensile mechanical properties of pTA ionogels were measured on an Instron machine (Instron 5969) with a load cell of 500 N, and a crosshead speed of 50 mm/min. The samples were cut into rectangular shapes (fixture length: 10mm, width: 5 mm, thickness: ~0.4 mm). For cyclic tests, the samples were stretched to fixed strain levels (50%, 100%, 200% and 300%) for different cycles. For energy dissipation tests, samples were stretched to different strain levels (50%, 100%, 200% and 300%). The toughness (W, kJ/m3) was calculated by integrating the area under the stress-strain curves, while the dissipated energy was calculated by integrating the area between the loading and unloading curves. 7.2.5 Conductivity and Strain Sensitivity Tests The conductivity was measured on an electrochemical workstation (Gamry Interface E, Gamry Instruments Inc). An ionogel specimen (S = 1.76 mm2, thickness ≈ 0.4 mm) was sandwiched between a pair of stainless-steel electrodes (S = 1.76 mm2, thickness = 1 mm) and the resistance was obtained from the intercept of the x-axis of electrochemical impedance spectroscopy (EIS) Bode plot from 106 to 10 Hz. The conductivity (σ, mS/m) was calculated by σ=d/R*A, where d was the distance between two electrodes, R was the resistance and A was the contact area between electrodes and ionogel. The strain sensitivity was tested by combining the Instron machine and a digital source meter (Keithley 2450, USA). The ionogel sample was fixed on the Instron machine connected with the source meter copper wires, and when the sample was stretched, the real-time resistance change (ΔR/R0) was recorded by the source meter. The ΔR/R0 of different strains and the cyclic performances were examined. 174 7.2.6 Thickness The thickness of the coated textile specimens was measured using a Mitutoyo Digimatic Micrometer (MDC-1” SX, Japan), with results presented in Table 7.1. Table 7.1 Summary of coating layer thicknesses, ionogel spray coated e-textile Total Thickness of Fabric (mm) Coated Layer Thickness (mm) Woven Ag fabric (bare) 0.052 ±0.0018 + PU layer 0.062 ±0.015 0.010 +PU+IL-pTA 0.073 ±0.013 0.011 (IL-pTA) 7.2.7 Bending Characterization Bending properties of the ionogel coated and bare Ag-nylon woven fabrics were compared through characterization of bending rigidity and hysteresis (recoverability of textile). The KES-FB20S bending tester (Kato Tech, Japan) and software (KES-FB Measurement Program, Version 8.07, Japan) were used to measure these characteristics. For each test, the bending moment-curvature curves were plotted, and bending rigidity and hysteresis were calculated. The measurement sensitivity was 0.49 N*cm (50 gram force*cm), bending displacement velocity was 0.5 cm-1/s, and bending curvature was 2.5 cm-1. Bending rigidity describes the fabric resistance to bending and is defined as the first derivative of the moment-curvature plot [226]. Bending hysteresis is the energy loss within a bending cycle when a fabric deforms and recovers and is calculated as the difference in bending moment from the loading and unloading plot at a fixed bending curvature [226]. Lower values for bending rigidity and hysteresis indicate that the fabric has low rigidity and good recoverability. The bending tests for the uncoated and coated Ag-nylon 175 plain weave fabrics (L: 10 cm, W: 4 cm; 3 measurements per fabric, n=3) were measured, with fabrics rotated through to allow the material to recover after each trial. 7.2.8 Sheet Resistance Sheet resistance of the ionogel films and ionogel-coated conductive fabric was measured using a four-point probe setup, comprising equally spaced spring-loaded gold tips (20mm) mounted to an adjustable stage, to accommodate various specimen thicknesses. A source meter (Keithley 2400, USA) was used to supply constant current to the two outer probes, and a multimeter (Tektronix DM2040) was used to measure the voltage across the two inner probes. Sheet resistance is calculated by: Rs = (kV)/I, where k is the geometric correction factor for non-ideal samples, with k=4.532 for a semi-infinite sheet [161]. 7.2.9 Self-Healing Properties The self-healing properties were explored by tensile tests. An ionogel specimen (length: 20 mm, width: 5 mm, thickness: ~0.4 mm) was cut into two pieces, the two cut pieces were combined immediately. Different healing time periods were investigated (0.5 h, 2 h and 24 h, respectively), after which the ionogel was subjected to tensile tests, using the same procedure as the mechanical tests. 7.2.10 Electrochemical Impedance Spectroscopy Electrical impedance spectroscopy (EIS) was performed on a skin phantom, using a three-electrode measurement setup. The Ag-nylon woven fabric (2 cm2) spray coated with the ionogel was the working electrode (WE), and sintered Ag/AgCl electrodes served as counter (CE) and reference (RE) electrodes. The WE and CE were placed 5cm apart, and the WE and RE were 1 cm 176 apart. The results for the ionogel coated fabrics were compared with an uncoated Ag-nylon woven fabric, and sintered Ag/AgCl tested as WEs. The EIS scans were performed using an electrochemical workstation (BioLogic VMP300, France), in potentiostatic mode, in which a sinusoidal voltage of 1 V was applied, using a frequency scan from 10 kHz to 0.1 Hz, with 10 points per decade, and three measurements per reading. Analysis of the measurements was performed by comparing impedance values at frequency points of interest, and equivalent circuit modeling to calculate circuit component parameters. The Simplified Randle's circuit was selected to represent the skin-electrode interface and used for the impedance estimation. The model comprises a resistor Rs in series with a parallel resistor (Rd) and capacitor (Cd) [173]. Rs represents the total resistance of the skin, wires and electrodes, the capacitor Cd represents the electrical charge between the electrode and skin, Rd represents the resistance between skin and electrode during the charge transfer process [58]. 7.2.11 X-Ray Diffraction (XRD) XRD (Rigaku Ultima IV, Japan) was carried out for phase identification. The measurement was recorded using CuKα radiation (1.54 Å) with 2θ ranging from 5° to 90°, scan step size 0.02°, and scan speed 3°/min. 7.2.12 Field Emission Scanning Electron Microscopy (FE-SEM) FE-SEM (Zeiss Sigma, Germany) was used to observe ionogel film coated in the conductive textile (secondary electron signal, 10 kV accelerating voltage, 8 mm working distance). Micrographs of the coated textile surfaces were taken after the coating process, after mechanical abrasion, and after ethanol was applied to the abraded textile surface to demonstrate the self-healing capability. 177 7.2.13 Biopotential Signal Measurement Pre-recorded ECG data was loaded into a programmable function generator (GW Instek AFG-3051, Taiwan) to generate a simulated human ECG signal. The ionogel film was placed onto the surface of two standard rigid Ag/AgCl electrodes and connected to a 1-lead clinical ECG system for signal measurement. The average heart rate (beats/min) was calculated from the inter-beat intervals and compared with the signal recording from a pair of standard Ag/AgCl gel electrodes. Simultaneous ECG recordings were performed following procedures specified in Sections 5.2.5 and 6.2.5, for standard Ag/AgCl gel electrodes, and ionogel spray coated onto plain weave silver-coated nylon fabrics made into electrodes. A schematic of the electrode construction is shown in Figure 7.3a. The signals were compared visually, and average inter-beat intervals (IBI) were compared. EMG recordings were collected using a clinical system (Thought Technology Ltd., Canada) equipped with a data acquisition board, and processing software (Biograph Infiniti), at a sampling rate of 256 samples/s. Signals were simultaneously collected from two channels to compare the performance of the ionogel coated textile electrode, with the standard Ag/AgCl gel electrodes. The sets of electrodes were attached on the middle bicep (Figure 7.3b). It should be noted that the simultaneous recordings do not produce identical EMG signals, since they will pick up slightly different signals due to their relative positioning on the muscle. The e-textile electrodes were attached to the skin using a snap clip connector compatible with the EMG snap leads, and secured with medical tape (3M, USA). The EMG signals were compared by visual observation, based on the flexion and extension movements during the recording. 178 Figure 7.3 a. Schematic of ionogel coated Ag-woven fabric electrode. b. Schematic of EMG setup for simultaneous recording using standard gel electrodes and ionogel coated textile electrode 7.2.14 Biocompatibility Testing Cell viability assessment Human dermal fibroblast (HDF) cells (ATCC PCS-201-012) were grown in Dulbecco's modified Eagle's medium (Gibco, 1165092), and supplemented with 10% heat-inactivated fetal bovine serum (GibcoTM), 1% Pen-strip (antibiotics) at 37 °C, and 5% CO2. A 24-well plate was seeded with 5 X 104 cells per well. The 4 mm of control, ionogel film, bare Ag-nylon fabric, and the ionogel coated Ag-nylon fabric were inserted into trans well inserts (Costar, 3396) and sterilized via UV sterilization for 30 min. The eluates from the samples in the trans well inserts were contacted to the 24-well plate with HDF cells, and fresh complete growth media was added (1 mL) to the T-STH samples in the transwell inserts, and changed after 12h periods throughout 179 the experiment. Cytotoxicity was assessed at 12h and 24h using PrestoBlue™ cell viability reagent (A13261, ThermoFisher) following the manufacturer's protocol. Trans well inserts without any T-STH were used as controls. Fluorescence was read at 560nm (excitation), 590 nm (emission) using a plate reader. All data were normalized to the controls and reported as mean, and standard error means of all replicates. Live/Dead Assay A standard protocol was followed for identifying live/dead assay (Invitrogen). At each time point (12h and 24h), each sample-treated HDF cell-well was washed with PBS and stained with calcein-AM (4 mM) and ethidium homodimer-1 (2 mM) in PBS, and incubated for 30 minutes in the dark at 37oC in a 5% CO2 humidified atmosphere. All samples were photographed using a fluorescent microscope (Keyence Fluorescence microscope-BZ-X710), which showed live cells as green, and dead cells as red (results in Appendix A.8) 180 7.3 Results and Discussion 7.3.1 Materials Characterization Figure 7.4 a. Schematic illustration of TA and [BMIM]Cl polymerization b. Structure of ionogel c. XRD results for ionogel film (1:5, IL:TA) compared with TA powder, d. SEM of ionogel film spray coated on Ag-nylon plain weave fabric 55x, e. 300x The ionogel preparation involves a concentration induced polymerization process depicted in Figure 7.4a. The process involves the dispersion of thioctic acid (TA) and ion liquid ([BMIM][Cl]) in ethanol. Upon evaporation of ethanol, the disulfide bonds on the TA rings undergo ring-opening polymerization, forming pTA ionogel, as shown in Figure 7.4b. The resulting material can be diluted to any concentration and is adjustable over a wide range. As a result, these properties were leveraged with the material prepared as a film, and spray coated on conductive fabric for use as strain sensors and e-textile electrodes. The XRD result (Figure 7.4c) 181 for the ionogel film shows that the material is amorphous. XRD for TA powder is shown for comparison. The SEM micrographs for the ionogel spray coated onto Ag-nylon woven fabric is displayed in Figure 7.4d-e. The images and thickness measurements (0.073 ±0.013 mm, Table 7.1) show that the ionogel material is uniformly incorporated between the fibres and across the surface of the fabric. 7.3.2 Mechanical Properties and Self-Healing The pTA ionogel exhibited good mechanical properties as well as excellent self-healing properties due to the reconstruction of dynamic S-S bonds. The mechanical properties of pTA ionogel were measured by tensile tests, with an example stress-strain curve was shown in Figure 7.5a. The ionogel showed a high strain of 721.3 ±80.4% and a tensile strength of 0.090±0.003 MPa, due to the dynamically crosslinked network. The ionogel also displayed a moderate modulus of 0.158 ±0.002 MPa, which is beneficial for human-machine interface and applications as cutaneous electrodes. Figure 7.5b shows the ionogel subjected to high strain in tensile testing. Energy dissipation behavior was investigated and presented in Figure 7.5c-d. It was obvious that with the increase of external strain, the dissipated energy also increases, which indicates that more S-S bonds and physical interactions were broken during the stretching. For example, when the strain was 50%, the dissipated energy was 1.84 kJ/m3, while at 300% strain, the dissipated energy increased to 57.8 kJ/m3. Under low strain, the S-S bond as well as the abundant hydrogen bonds could quickly reform after deconstruction, while under high strain, these dynamic covalent bonds or physical interactions could not completely recover, thereby inducing a higher dissipated energy. Moreover, the reversible S-S bonds and the hydrogen bonds also enables high durability and self-healing properties. Figure 7.5e displays the 85 cycles of cyclic stress-strain curves under 50% strain. Apart from the first cycle, the curves nearly overlap, with a slight 182 decrease, which suggests that the ionogel has high elastic recovery. The maximum tensile stress during each cycle was recorded and shown in Figure 7.5f. During the process, the maximum tensile strength only exhibited a minor decrease from 0.044 MPa to 0.032 MPa (remaining 73% of the first cycle), and in the last few cycles, the maximum tensile strength remained nearly constant. To further demonstrate the durability of the pTA ionogel, additional cyclic tests were carried out under different strains (100%, 200% and 300%, respectively) reported in Appendix A.8 Similarly, the ionogel showed overlapping curves under different strains, further indicating excellent durability. The self-healing properties were also measured by the tensile tests, and the stress-strain curves under different healing times are presented in Figure 7.5g. With the prolonged healing time, the tensile strength, tensile strain, modulus, and toughness (Appendix A.8) were gradually recovered, due to the reconstruction of S-S bonds and hydrogen bonds at the healing interface. After 30 minutes of healing, the stress could recover 73.5%, while the modulus could recover 84.8%, revealing a fast and highly efficient healing process. Although the strain and toughness showed relatively low healing efficiency due to the stress concentration at the interface, this stretchable and durable ionogel demonstrates high potential for healable coatings in cutaneous electrode applications. Further improvements to the self-healing properties can be made by heating the material above the transition temperature of 52°C to thermally activate the dimeric H-bonds in the pTA structure [215]. Wang et al. demonstrated that after heating a fractured pTA-[EMIM][ES] ionogel specimen at 55°C for 6 hours, the material almost recovered its original tensile strength and elongation values [215]. 183 Figure 7.5 Mechanical properties and self-healing properties. (a) Stress-strain curve of pTA ionogel. (b) Photo of pTA ionogel under stretching. (c) Cyclic stretching-releasing stress-strain curves under different strains and (d) the corresponding dissipated energy. (e) Stress-strain curves of 85 cycles under 50% strain and (f) the corresponding maximum tensile stress. (g) Stress-strain curves of pTA ionogel after different healing periods.184 7.3.3 Bending Properties of E-Textiles The bending properties were characterized to observe and quantify the change in flexibility of the ionogel coated e-textile (Figure 7.6a). The bending moment versus curvature for one cycle is shown for both uncoated and coated Ag-nylon plain weave textiles tested (Figure 7.6b). Results are summarized in Table 7.2. Lower values for bending rigidity and hysteresis indicate that the material is more flexible and has good recoverability. While the ionogel coating increases the overall bending rigidity and hysteresis substantially, the overall measured values for both e-textiles are very low as the fabrics are very thin. In comparison to results reported for a nylon plain weave textile substrate (thickness: 0.097 mm) for e-textile embroidery, the results are lower in comparison [227]. Table 7.2 Summary of average bending rigidity and bending hysteresis results and comparative nylon textile substrate [227]. Bending Rigidity (10-4 Nm/m) Bending Hysteresis (10-2 Nm/m) Uncoated e-textile (0.052 mm thickness) 2.73x10-2 ±0.002 7.69x10-3 ±0.0002 Iongel coated e-textile (0.073 mm thickness) 4.62x10-2 ±0.001 1.43x10-2 ±0.001 Nylon textile (0.097 mm thickness) 8.00x10-2 ±0.001 5.37x10-2 ±0.001 185 Figure 7.6 Bending characterization (a) Fabric being tested in bending tester, KES-FB2-S (b) Example bending moment vs curvature plot for uncoated and coated e-textiles 7.3.4 Electrical Characterization 7.3.4.1 Sheet Resistance and Ionic Conductivity Results for the sheet resistance of the coated fabric layers and ionogel film are summarized in Table 7.3. The sheet resistance values fall within reasonable range for e-textile applications. Table 7.3 Sheet resistance results for coated e-textile layers, and ionogel film Sheet Resistance (Ω/sq.) S.D. Woven Ag fabric (bare) 0.176 ±0.007 + PU layer 0.208 ±0.007 +PU+IL-pTA 0.248 ±0.010 1:5 (TA:IL) ionogel film 0.120 ±0.012 b -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0-0.15-0.10-0.050.000.050.100.15Bending Moment (N/m x10-2) Uncoated E-Textile Coated E-TextileBending Rate (cm-1)a. b. 186 With the addition of the ionic liquid, the ionogel exhibits considerable ionic conductivity, which is advantageous in sensing electrode applications in serving as an electrolyte coating. As shown in Figure 7.7, the Nyquist plot consists of two regions, the low frequency region which indicates the capacitive characteristics, and high frequency region related to the equivalent series resistance (ESR) of the electrolyte. The form of the curve shows low ESR due to the transportation of charge carriers at electrode-electrolyte interface, indicating that the Cl- ions of the ionic liquid can facilitate high transport in the ionogel electrolyte, and diffuse to the surface of electrodes to facilitate electron transfer. The ionic conductivity was calculated as 2.27 ±0.23 mS/m, which was comparable to some reported ionogels [213],[228]. Figure 7.7 Nyquist plot and calculated ionic conductivity. 187 7.3.4.2 Electrical Impedance Spectroscopy (EIS) EIS results for the ionogel coated Ag-nylon textile were measured and compared with bare Ag-nylon textile, and standard gel electrodes using the prepared skin phantom. Estimates for the components of the skin-electrode impedance model applied the simplified Randle’s circuit, with parameters, Rs, Rd, and Cd calculated in MATLAB (R2022a, Mathworks) using a parameter modelling approach (Appendix A.9) and least squares nonlinear curve fitting method. Example results showing the Nyquist and Bode plots from the measurements are presented in Figure 7.8a and summarized in Table 7.4. Model parameters were estimated from the following equations: 𝑍 = 𝑅𝑠 + 𝑅𝑑(1+𝑋𝑑2) − 𝑗𝑅𝑑𝑋𝑑(1+𝑋𝑑2) (5a) where Xd = Rd.Cd.ω, ω = 2πf (5b) |Z|2 = A2 + B2 , (6a) Where the real and imaginary components of |Z| as A and B as shown: 𝐴 = 𝑅𝑠 + 𝑅𝑑(1+𝑋𝑑2) (6b) 𝐵 = − 𝑅𝑑𝑋𝑑(1+𝑋𝑑2) (6c) Overall, the results for all electrode materials tested fall within the same order of magnitude and range. The reference Ag/AgCl sintered electrodes have the lowest impedance, estimated resistance and capacitance values overall, closely followed by the Ag-nylon textile coated with the ionogel film, while the uncoated Ag-nylon textile has the highest overall measured and estimated values (Figure 7.8a-b). The results show that the addition of the ionogel coating to the Ag-nylon fabric lowers the impedance and resistance at the skin-electrode interface through providing a medium for ionic to electron current transfer. This is also demonstrated from the observable 188 reduction in the charge transfer resistance (width of the Nyquist plot, Figure 7.8b) between the uncoated and coated Ag-nylon textiles. Furthermore, an additional metallic salt coating of AgCl onto the Ag-textile is thought to further lower impedance and resistance as shown from the results of the sintered Ag/AgCl electrodes. The electrode impedance of the ionogel coated Ag-nylon textile meet the specified performance criteria for use in electrocardiogram signal measurement, with values lower than 1 MΩ (0.01-5 MΩ in 5-100 Hz range) [66],[75],[85], [109],[227]. In addition, the impedance results of the ionogel coated e-textile from this work have been compared to other studies that have utilized ionic liquid coating for e-textile electrode applications and are found to be within close range or lower than reported results (Table 7.6). This demonstrates their suitability for biopotential signal monitoring. 189 Figure 7.8 a) Example Bode plots displaying impedance vs. frequency for uncoated, coated ionogel Ag-nylon fabrics, and sintered Ag/AgCl electrodes b) Example Nyquist plots with measured and fitted data for uncoated, coated ionogel Ag-nylon fabrics, and sintered Ag/AgCl (reference electrode)190 Table 7.4 Summary of average impedance results at 1 Hz, 10 Hz, 145 Hz, and estimates of circuit model parameters for electrode materials investigated. Z @ 1Hz (kΩ) Z @ 10Hz (kΩ) Z @ 145Hz (kΩ) Rs (kΩ) Rd (kΩ) Cd (F) Ionogel + Ag-textile 1.70 1.58 1.27 0.38 1.25 7.93x10-8 Ag-nylon textile (bare) 2.86 2.84 2.22 0.59 2.34 1.64x10-8 Ag/AgCl sintered electrode 1.35 1.10 0.87 0.21 0.89 9.85x10-8 Table 7.5 Comparison of electrode impedance values for studies investigating IL coatings Material Z (Ω), Frequency (Hz) Range Ref. Au + IL gel (EMISE + PEGDA) & PEDOT:PSS + IL gel (EMISE + PEGDA) 105-106 Ω, 0.1-10 Hz [211] PEDOT:PSS + IL gel (2-cholinium lactate methacrylate, cholonium lactate, PEGDA), 30-50wt.% IL 2-4 x104 Ω, 1-10 Hz [220] PEDOT:PSS + IL gel (EMISE, PEGDA, 2-benzoyl-2-propanol) 1-2 x105Ω, 1-10 Hz [33] 1. Carbon coating on textile + 2.5%, 5% IL (EMIM-TFSI) 2. PEDOT:PSS + 6.25% PDMS + 2.5%, 5% IL (EMIM-TFSI) 1. 1-3 x 103Ω, 5 Hz 2. 20-30 Ω, 5 Hz [219] Ag-nylon plain weave fabric + PU-pTA-[BMIM]Cl IL 1-2 x 103 Ω, 1-10 Hz This work 191 7.3.5 Mechanical Abrasion, Surface Reformation, and Recyclability Ionogel-coated fabrics (unabraded specimen Figure 7.9a.) were abraded with a rotating fabric covered motorhead for approximately 30 seconds. The surface after abrasion is shown in Figure 7.9b, which shows that the film remains intact, with some displacement of the coating layer. To examine the effect of surface reformation and potential sanitizing method, approximately 1 mL of ethanol was added (dropwise) to the abraded textile surface. It is expected that the ethanol can allow for the redispersion of the displaced regions of the film. Figure 7.9c-d shows the reformed surface after ethanol addition, in which the darker regions on the fabric structure are indicative of the abraded regions of the fabric. The visual observations indicate that the ionogel coating on the fabric can be redispersed and reformed after mechanical abrasion to the surface. Furthermore, both the surface coating and standalone film material can be recycled by dissolution in ethanol, heating to approximately 55°C, above the transition temperature of 52°C, and be reformed into new ionic conductors, sensors, or adhesives [215], [217]. Figure 7.9 SEM micrographs of ionogel-coated fabric a. Coated fabric b. Abraded fabric c. Abraded fabric after ethanol addition (55x) d. Regenerated film on abraded region after ethanol addition (300x). 192 7.3.6 Biocompatibility The cell proliferation and viability test results demonstrate that the ionogel coated fabric is biocompatible after 12 and 24 hours (Figure 7.10a-b). Cell proliferation results show that that the ionogel coated fabric is comparable to the control sample, while also performing better than the standard reference gel electrolyte material. The cell viability (%) results show statistically similar results between the fabrics and gel electrolyte material after 12 hours, with cell viability maintained for the fabrics after 24 hours, while the viability for the gel electrolyte is reduced. Figure 7.10 Biocompatibility results a. Cell proliferation (relative fluorescence units, optical density, ex 560 / em 590) b. Cell viability (%) 193 7.3.7 Application as Strain Sensors and Biopotential Electrodes 7.3.7.1 Strain sensitivity Incorporating the [BMIM]Cl ionic liquid softened and stabilized the pTA network and induced considerable ionic conductivity to the ionogel material. For amorphous polymer electrolytes or ionic conductors under external strains, change in geometry via stretching can induce the ion conduction in the material through the opening or movement of the polymer chains, which enhances and increases ion conduction by creating more coordination sites [230]–[232]. This subsequently results in a change in the material’s resistance. The strain sensitivity of the pTA ionogel was tested under various strain scenarios and cycling. As shown in Figure 7.11a, the relative resistance change (ΔR/R0) increased with the increase in applied strain. When the strain was 500%, the ΔR/R0 increased to 1507%, showing a very high sensitivity. Gauge factor (GF) was employed to determine the strain sensitivity, which was calculated as (ΔR/R0)/strain, and the result is presented in Figure 7.11b. The GF also increased with the applied strain, and when the strain was 500%, the GF was 3.01, which is within similar range to reported pTA-based ionogel materials [217]. Considering the durability of the ionogels revealed by the cyclic tensile tests, it is expected that the ionogel would exhibit high stability after cyclic tensile loading. Figure 7.11c shows the ΔR/R0 in the 300 cycles at 100% strain; the resistance signal remains stable with a baseline shift, indicating high stability and reliability. Furthermore, the ΔR/R0 responses under different strains were also investigated, and as shown in Figure 7.11d-f, when the ionogel was subjected to small (1%, 2% and 5%) and large strains (100%, 200% and 500%), the output signals were all stable and highly repeatable. This demonstrates that the ionogel can be used to sense different external strains with high sensitivity (detection limit, 1%). Based on the results presented, the pTA ionogel shows high stability and sensitivity for strain sensor applications. 194 Figure 7.11 Strain sensitivity of pTA ionogel. (a) ΔR/R0 versus strain (0-500%) and (b) the corresponding GF. (c) ΔR/R0 under 100% strain for 300 cycles. ΔR/R0 under different strains: (d) 1%, 2% and 5%; (e) 10%, 20% and 50%; and (f) 100%, 200% and 500%. 195 7.3.7.2 Biopotential signal monitoring Pre-recorded ECG signal Ag/AgCl electrodes coated with the ionogel film, and a set of reference Ag/AgCl gel electrodes were used to measure a pre-recorded ECG signal from a programmable function generator (Figure 7.12a). The signal collected from the electrodes were compared visually, and the average inter-beat interval (milliseconds) was compared between the two readings (Figure 7.12b). The signals presented in Figure 7.12b show that both electrodes were able to read the ECG signal, with waveform morphology and inter-beat intervals (IBI) of similar quality. In both signals, instances of baseline drift in the signal are present between 1.5 to 5.0 seconds, as well as 6.5-7.0 seconds. The preliminary results show that the ionogel coating has good potential for ECG signal collection of acceptable signal quality. Figure 7.12 a. Schematic of experimental design to measure pre-recorded ECG signal from electrodes. b. ECG signal obtained from Ag/AgCl-gel and Ag/AgCl-ionogel electrodes. 196 Figure 7.13 a. Example simultaneous ECG signal recordings from standard reference electrodes and ionogel-coated Ag-nylon e-textile electrodes b. Example simultaneous EMG recordings from standard reference electrodes and ionogel-coated Ag-nylon e-textile electrodes 197 ECG and EMG Recordings Results for ECG and EMG recordings are displayed in Figure 7.13. The signals from a set of disposable commercial electrodes, and the ionogel coated fabrics were recorded simultaneously. The ECG recordings show nearly identical signal quality between the two sets of electrodes from visual observation. The same average inter-beat interval of 851 ±49 ms was calculated, corresponding to approximately 70 beats/min. The amplitude for the ECG signal recorded by the ionogel-coated textile electrode is slightly higher than the standard gel electrode, likely due to differences in the electrode placement with respect to the heart centre. However, this does not impact the basic signal quality features (heartbeat and heart rate variability). Overall, the EMG recording from the two sets of electrodes show the same signal responses for bicep flexion and extension, with some observable differences in signal amplitude and noise between the electrodes. The results show that the ionogel coated Ag-textile electrodes can record biopotential signals, with good quality recordings demonstrated for both ECG and EMG. Basic features observed for both signals recorded showed comparable performance to standard Ag/AgCl gel electrodes. 7.4 Conclusions In this study, a stretchable, self-healing, biocompatible, and highly durable ionogel material prepared from [BMIM]Cl and thioctic acid has been demonstrated. This material has high adaptability in form, from cast and dilutable films, to sprayable coatings for strain sensing and biopotential monitoring applications. The material demonstrated excellent self-healing capacity from being reformed at room temperature, with 73.5% stress recovery, and 84.5% in modulus recovery after a 0.5 h healing time. As a spray coated barrier on e-textiles, the ionogel demonstrated high resistance to abrasion, and was able to be reformed by the addition of ethanol to the surface. The material can be produced in various concentrations under room temperature 198 condition, through the mixing of TA, IL, and ethanol, with highly adjustable mechanical properties. The ionogel (1:5 IL-TA concentration) demonstrated up to 721.3 ±8.4% strain, 0.090 ±0.003 MPa stress, and a modulus of 0.158 ±0.002 MPa, indicating its suitability for cutaneous applications. The electrical properties of the ionogel coated e-textiles were measured, including sheet resistance (0.248 ±0.01 Ω/sq.) and skin-electrode impedance (below 2 kΩ from 0.1-1 kHz), demonstrating excellent performance, meeting specified performance properties, and within reported range of values for e-textile sensing electrodes. Strain sensitivity of the ionogel showed high stability under cycling loading at various strain levels, and a gauge factor of 3.0 at 500% strain. Biocompatibility test results demonstrated that the ionogel coated fabrics are safe to use in next-to-skin applications for up to 24 hours, with performance (cell proliferation and viability) exceeding that of the standard reference gel electrolyte material. Results for biopotential signal recordings demonstrate that the ionogel-coated Ag-textile electrodes can collect ECG and EMG signals, showing comparable quality to standard Ag/AgCl gel electrodes. The demonstrated functional and self-healing properties of the ionogel show great potential to extend the lifetime and enable continuous use of e-textile electrode materials. Future work should include long-term studies on the ionogel durability (varying thickness) as a strain sensor, and as a functional, protective coating for e-textile electrodes and sensors. 199 Chapter 8: Conclusions 8.1 Summary The study of structural hierarchy in e-textile materials, and relationship to functional and performance properties of textile sensors used for biopotential signal monitoring, specifically applied to ECG has been investigated in this work. In reviewing the literature on e-textile materials from fibre to fabric levels, and demonstrated applications in ECG monitoring, existing design and performance criteria for material properties and signal quality have been organized as a guide for textile electrode development (Chapter 2). Key design aspects have considered the influence of electrode size (minimum 4 cm2 [87]), construction of electrodes with stabilizing support structure around the electrode, or soft padding for better conformation with skin, and applied pressure (optimal range 1-3 kPa). Performance criteria include electrical properties (contact resistance, contact impedance, polarization potential), stability and durability of material properties (mechanical bending and stretching, effects of laundering and cleaning), and signal comparison (signal trend characteristics, quality indices). Signal quality indices were applied to the assessment of commercially sourced conductive silver-nylon fabrics of varying knitted and woven structures for use as ECG electrodes to be integrated into a women’s smart bra (Chapter 3). Methods included comparison of coefficient of variation between HR detected from stationary to dynamic movement scenarios, skewness, kurtosis, and Pearson correlation coefficient for calculated average ECG wave form morphologies between Ag-textile electrodes and standard Ag/AgCl gel electrodes. ECG signals were compared under stationary and movement conditions. The performance of textile electrodes exceeded existing quality thresholds established, for kurtosis results (K greater than 5), and Pearson correlation coefficients (r), exceeding acceptable quality measures (r greater than 0.66 deemed as 200 acceptable [131]), with r values ranging from 0.91-0.98 for the fabric structures compared. Highest signal quality levels, and stability under dynamic (movement) conditions were observed for textile electrodes with lower surface resistance and stretch, and lower geometrical surface roughness, which was attributed to increased skin-electrode contact surface area. For e-textile yarns, the influence of geometry parameters including fibre packing and orientation angle from increasing yarn twist, were found to influence yarn electrical resistance properties for silver-coated nylon yarns (Chapter 4). Specifically, it was found that linear resistance approaches a limit, with increasing yarn twist levels up to 300 TPM for the 1- to 4-ply yarn structures investigated. Following this, yarn specific volume, fibre volume fraction, and orientation angles also approached limiting values of 1.6-1.9 cm3/g, 0.5, and 12-18°, respectively. These observed geometric property changes, are thought to contribute to the increase in the direct contact between fibres in the yarn structure, which reduces the total electrical resistance of the yarns overall. Taking these measured observations to the fabric level, embroidered electrodes fabricated from 2-ply yarns at twist levels from 30 TPM to 600 TPM, showed an increasing to plateauing trend for measured contact impedance (from 210-300 TPM). These findings followed the observed plateauing trend at the yarn level, described by influence from changes to the yarn geometry and structure, which was thought to subsequently impact contact area along with impedance properties on the fabric level. Knitted e-textiles were fabricated from three different yarn ply levels (1-, 2-, 3-ply) and three loop lengths, to investigate the impact of fabric structure on electrode material properties and ECG signal quality (Chapter 5). Influence of surface roughness between electrode and skin was also examined to elucidate the possible relationship of measured properties on interfacial interaction. It was shown that electrodes knitted from lower plied yarns and larger loop lengths 201 had higher measured surface asperity roughness, and lower contact impedance overall, compared to structures knitted with smaller loop lengths, and higher plied yarns. Additional and repeated impedance measurements over long-term periods on skin are required to confirm these findings. The ECG signal amplitude for the textile electrodes was found to be higher than gel reference electrodes, with analysis of power spectral density in the frequency domain showing higher occupied band power in the low frequency range (0.40-0.76 Hz). This is thought to be due to textile electrodes capturing lower frequency motion artifacts during measurement. All textile electrode groups showed less than 0.1% difference in coefficient of variation for the inter-beat intervals (corresponds to heart rate) compared with the standard Ag/AgCl gel electrode pairs they were measured against. The results demonstrated that the knitted electrodes can be used to measure good quality ECG and provide accurate heart rate information. The process for electrochemical coating of Ag/AgCl yarns from coating Ag-nylon yarns as a base material was characterized through cyclic voltammetry and potentiodynamic polarization, with key process parameters identified for fabrication (Chapter 6). The operating current range specific to the Ag-nylon yarn studied in this work was identified as 0.1 to 2.4 mA. The selected operating parameters of 2 mA applied current, and 0.13 cm/s were found to be stable based on the potential measured between the working and counter electrode during the coating process. The e-textile yarn was used to fabricate embroidered electrodes for ECG monitoring, from which electrode and signal performance properties were evaluated, by comparing the dry Ag/AgCl e-textile electrodes, with Ag e-textile electrodes and standard sintered Ag/AgCl electrodes. The embroidered Ag/AgCl e-textile electrodes demonstrated high stability, with low average polarization potential (1.22 mV/min) over a 10-minute measurement period compared to 202 electrodes embroidered from Ag-nylon yarns (3.79 mV/min), and low impedance (below 2 MΩ between 0.1 to 150 Hz). Considerations into e-textile electrode durability were explored through the development of a stretchable, biocompatible, self-healable chloride-based ionogel coating fabricated from the polymerization of thioctic acid, [BMIM]Cl ionic liquid in ethanol (Chapter 7). With its adaptable mechanical properties and ability to be diluted, the material was demonstrated as a cast film and sprayable coating applied in strain sensing and biopotential monitoring, respectively. The ionogel can reform at room temperature, demonstrating 73.5% stress recovery, and 84.5% modulus recovery after a 30-minute healing time. Higher recovery properties are expected by heating the material up to 55°C to thermally activate the dimeric H-bonds in the polyTA structure. As a spray coated barrier on e-textiles, the ionogel displayed higher resistance to abrasion, and reformed after the addition of ethanol to the surface. Electrical properties of the ionogel coated e-textiles met the specified performance range reported for textile sensing electrodes, with an average sheet resistance of 0.25(±0.01) Ω/sq., and skin-electrode impedance below 2 kΩ from 0.1-1kHz. Strain sensitivity of the ionogel in the form of a film showed high stability under cyclic loading at various strain levels, and a gauge factor of 3.0 at 500% strain. Biocompatibility test results showed that the ionogel coated fabrics are safe to use in next-to-skin applications for up to 24 hours, with performance (cell proliferation and viability) exceeding standard gel electrolyte material. Results for biopotential signal pickup demonstrated that the Ag/AgCl ionogel-coated electrodes could read the pre-recorded ECG signals, showing comparable quality to standard Ag/AgCl gel electrodes (0.3% difference in average inter-beat interval). Results for biopotential signal recordings demonstrated that the ionogel-coated Ag-textile electrodes can read and collect ECG and EMG signals, showing comparable quality to standard Ag/AgCl gel electrodes. 203 8.2 Contributions to Knowledge This work has approached the understanding of e-textiles applied to biopotential electrodes through the lens of materials engineering and design. Specifically, this has encompassed the understanding of interrelationships between material structures, properties, to processing and performance. Contributions are made to the understanding of Ag- and Ag/AgCl-based e-textile structural hierarchy, processing, and relationship to functional and performance properties as ECG electrodes. 8.2.1 General framework for characterization and development of textile electrodes for ECG monitoring Despite the multitude of material developments in the field of e-textiles and wearables for long-term and continuous health monitoring, it was identified that there is an absence of a systematic approach to characterizing and comparing the properties and performance of new electrode developments. This work has organized existing literature related to e-textile electrodes for electrocardiogram monitoring, through the approach of presenting material properties at different levels of textile structural hierarchy (fibre to fabric), interrelationships to design (electrode geometry, size, construction) and performance properties (materials and electrical properties, signal comparison and quality, durability, comfort). Additionally, existing, and upcoming standards from international organizations were outlined. Challenges to be addressed and recommendations for the advancement and adoption of smart apparel systems for biological signal monitoring were provided. 204 8.2.2 Structure-property effects from fibre to fabric levels for selection and design Ag e-textiles Several studies on e-textile electrode development have identified design parameters that influence performance. However, there has not been any work that has approached the entire design and materials selection from fibre and yarn to fabric levels. This work has studied the structural hierarchy effects of the Ag-nylon textile system, a commonly used conductive material in e-textile applications. Twisting is a means of producing or forming yarn systems that comprise filaments or staple fibres and can provide coherence and strength to the yarn. This can have influencing effects when the yarn is integrated to a fabric structure. In the study of yarn mechanics, the resulting yarn geometry (twist level, denier, etc.) is based on fibre material properties in the yarn bundle, which in turn determines the performance properties of the yarn, and dictates suitable processing methods and applications [233]. In extending this understanding to the materials selection and design of e-textiles, yarn twist and fibre packing properties were investigated to examine the impact on yarn electrical properties. The results showed that increasing twist to certain level (around 300 TPM) increased electrical conductance to a point where it plateaued thereafter. These findings were explained through both electrical contact theory as well as yarn geometry properties. The effects on the fabric were investigated by incorporating yarns of varying twist into the design of textile electrodes, with observed plateauing of skin-electrode contact impedance properties. The influencing factors were described through yarn properties, including apparent longitudinal diameter, and skin-electrode contact surface. These results on the yarn and fabric levels have highlighted the importance of structure-property to allow for the prediction of material and electrical properties. This understanding has implications on material cost and can be used to guide raw materials sourcing for e-textile applications. 205 8.2.3 Electrochemical fabrication, characterization, and integration of Ag/AgCl e-textiles Metals paired with metallic salts, notably Ag/AgCl electrodes are preferred material systems for biological signal monitoring, since they are electrochemically reversible and stable, with minimal fluctuations in half-cell potential over time. However, few studies on Ag/AgCl-based dry textile electrodes have been reported. Ag/AgCl based e-textiles are flexible, comfortable, and breathable alternatives to standard rigid gel electrodes and have the potential to be integrated unobtrusively into wearable systems for health monitoring applications. The lack of development of this material is thought to be due to the instability of AgCl layer during long-term use because of contamination by sweat and moisture from it use next-to-skin. Reported fabrication methods have included screen printing of conductive inks or paints [26], [197], and an electrochemical batch process for Ag/AgCl nylon yarn coating [18], from which work by our research group was referenced to create a continuous roll-to-roll system for Ag/AgCl coated nylon yarns [190]. The developed electrochemical coating system addressed improvements to yarn throughput and control of winding speed. The process was characterized electrochemically to gain a better understanding of the mechanism of AgCl formation on Ag yarn, reaction windows, stability of the coating process, and suitable process parameters. As process characterization has not been carried out for Ag/AgCl yarn fabrication, this research contributes to this understanding, from which further advancements to optimize both the process and material durability for long-term use in e-textile sensors can be made. The Ag/AgCl-nylon yarns were integrated as embroidered textile electrodes for ECG monitoring. Electrode performance (polarization potential and contact impedance) and ECG signal quality were compared to Ag-nylon embroidered electrodes. Both sets of e-textile electrodes collected comparable ECG signals to standard Ag/AgCl gel electrodes, with low contact 206 impedance. The Ag/AgCl e-textile electrodes showed greater stability than Ag e-textile electrodes from polarization potential measurements over a 10-minute period (Ag/AgCl e-textile: 1.22 mV/min, Ag e-textiles: 3.79 mV/min), which demonstrates the improved effect from the metallic salt layer at the skin-electrode interface. 8.2.4 Factors influencing skin-electrode interaction, electrode performance and biopotential signal quality Factors influencing electrode-skin interaction, and signal quality during biopotential signal monitoring were explored through attempts to characterize and quantify skin and electrode surface features. In this work, it was shown that surface compatibility at the skin-electrode interface is impacted by factors such as skin hydration, surface roughness, and applied pressure. This was investigated and quantified by surface roughness, and average surface asperity roughness measurements. Similar surface roughness measurements were obtained between the pre-wetted skin condition (representing scenarios of skin preparation or sweat accumulation between skin-electrode interface), gel electrolyte layer, and the best performing plain weave Ag-nylon conductive fabric (1.55, 1.38, 1.09 µm, respectively). In the structure-property investigation for knitted Ag-nylon electrodes, it was found that the larger surface roughness asperities in a structure may contribute to a larger proportion of the contact load, for a given applied pressure. Subsequently, this may have a greater influence on the apparent skin-electrode contact area, and result in lower contact impedance. 207 8.2.5 Development of a functional, biocompatible, self-healable, and adaptive chloride-based ionogel coating for e-textiles A stretchable, biocompatible, self-healing, and recyclable chloride-based ionically conductive ionogel material was developed to address the long-term functionality, reliability, and durability aspects of e-textile electrodes used in next-to-skin applications. The material was prepared using thioctic acid (TA) and 1-butyl-3-methyilimidazolium chloride (BMIM[Cl]), polymerized in ethanol under ambient conditions. The material can be recycled through redissolution and heating in ethanol (55°C). The ionogel demonstrated recovery of stress and modulus (>70%), reforming after a 30-minute healing period at room temperature. As both a film and spray coated layer on e-textiles, the material exhibited good electrical properties (sheet resistance and contact impedance), and stable strain sensitivity under cyclic loading. The material demonstrated excellent self-healing capacity from being reformed at room temperature, with 73.5% stress recovery, and 84.5% in modulus recovery after a 0.5 h healing time. As a spray coated barrier on e-textiles, the ionogel demonstrated high resistance and wear to abrasion, reforming by the addition a small volume of ethanol to the surface. The ionogel coated fabrics were found to be biocompatible, and safe for use in next-to-skin applications for up to 24 hours, with performance (cell proliferation and viability) exceeding that of the standard reference gel electrolyte material. Demonstrated applications of the ionogel material included its use as a functional electrolyte layer for continuous cutaneous biopotential signal monitoring (ECG and EMG), and a strain sensor that can be employed for movement detection applications. Further development and application of this ionogel coating material for e-textile applications can address long-term durability concerns by extending their usable lifetime. 208 8.2.6 Application of performance evaluation methods to newly developed e-textile and wearable systems Electrode and signal quality evaluation methods developed in this work have been used in various e-textile and wearable applications: - Evaluation of commercially sourced silver-nylon fabrics for ECG textile electrodes in a women’s smart bra (industry project – Chapter 3, additional details Appendix A.1) - Incorporation of e-textile electrodes and sensors into textile wristband for multimodal monitoring of physiological parameters. Sensors included Ag/AgCl plain woven e-textile ECG electrodes, and a newly developed carbon black-based yarn-based pulse sensor, and performance was compared based on signals obtained from standard Ag/AgCl gel electrodes, and a photoplethysmography (PPG) sensor, respectively. Interbeat intervals between signal peaks, and corresponding heart rate information were extracted and compared from the signals. Preliminary results demonstrated that the proposed wearable textile wristband could provide viable measurements of physiological signals. Results were presented in a conference presentation: K. Le, A. Servati, H. Narayana, Y. Shao, S.T. Htun, A. Imaizumi, S. Srinivasan, Z. Jiang, S. Soltanian, F. Ko, P. Servati, “Sensor fusion wearable textile wristband for multimodal monitoring of physiological parameters,” at SPIE BiOS Photonics West, San Francisco, USA, Feb. 2020. - Ag/AgCl coated yarns fabricated from the method developed in this work were embroidered into electrodes for electroencephalogram (EEG) monitoring application, meeting the impedance requirement of <1 kΩ at 10 Hz. Furthermore, results from preliminary EEG tests comparing the signal from embroidered electrodes with standard 209 reference electrodes showed high correlation (Pearson’s r: 0.98; details summarized in Appendix A.10) - Application of ECG average waveform calculation, as an evaluation method for comparing the performance of a liquid-free ionic conducting gel made from deep eutectic solvent hydrolyzed cellulose fibres (PDES/CMF), as a sustainable replacement to the gel electrolyte commonly used for standard Ag/AgCl electrodes. The PDES/CMF-containing electrodes showed nearly identical ECG signal as the standard gel electrodes, with calculated PCC of 0.999, indicating the suitability of the PDES/CMF material as a potential replacement for the gel electrolyte layer used in commercial electrodes. Results were reported in the following article: X. Sun, Y. Zhu, J. Zhu, K. Le, P. Servati and F. Jiang (2022) Tough and ultrastretchable liquid-free ion conductor strengthened by deep eutectic solvent hydrolyzed cellulose microfibres. Adv. Func. Mater. doi: 10.1002/adfm.202202533 210 8.3 Recommendations and Future Work The findings presented from this dissertation can be used to inform research on e-textile electrode materials and provide insights into materials selection and sourcing for the design and development of smart wearable systems for biological signal monitoring. Recommendations and future research directions are detailed below. - Investigate properties influencing skin-electrode interaction, including thermal resistance, breathability, sweat levels, bending, and more detailed studies on surface roughness. Coupling this with subjective evaluation (fabric handle and comfort properties) can further contribute to a holistic performance evaluation methodology. - Investigate e-textile electrode performance in dynamic conditions and identification of ways to improve signal quality – either through electrode design, or signal processing. - Expand the capabilities of wearable e-textile systems to be used for diagnostic signal quality measurements. - Further characterize and examine the long-term performance and durability properties of the chloride-ionogel material as a functional electrolyte layer next-to-skin. Explore other biocompatible ionic liquids or ionically conducting polymers for electrolyte layer or coatings. Novel durable materials, or solutions that extend the lifetime and use of e-textile electrodes for continuous and long-term biopotential monitoring, with the ability to withstand washing or sanitizing, would greatly advance the progress and adoption of smart apparel systems. - Further investigate relationship between effects of yarn twist on linear resistance, related to mechanical properties (modulus, tenacity, etc.), packing fraction, and yarn specific volume. 211 - Assess fibre-yarn geometry and twist effects on yarn electrical properties for other fibre/yarn types and structures (stainless steel, copper, other metal coated yarns). - Examine the impact of yarn selection on other textile structures (jacquard weaving or knitting, warp knit, tufting stitch embroidery pattern etc.) on electrode properties and performance. - Examine spacer knit fabric construction for e-textile electrodes. This fabric structure could comprise of a conductive top layer, separated by a mix of conductive, and non-conductive inlay and spacer yarns, with an insulating bottom layer fabric. This configuration would offer excellent conformation with the skin. Furthermore, the three-dimensional structure can allow for good thermal or moisture transfer/permeability, to address fabric handle and comfort properties. - Explore the effect of changing Ag coating thickness (a thicker Ag layer) on AgCl coating durability, process characterization, and scale-up of process. Investigate performance of other e-textile electrode structures fabricated from Ag/AgCl coated yarns. - Expand the use of Ag/AgCl yarn-based materials to other biopotential signal monitoring applications (electrodermal activity, EDA; electromyography, EMG; electroencephalogram, EEG, etc.), and combine sensors into a multimodal monitoring wearable system. - Explore the integration of e-textile electrodes in a smart wearable system, with consideration into requirements for interconnects, circuitry and microcontrollers, data collection/storage, wireless communication, and power source. 212 8.4 Publications 8.4.1 Journal Articles • K. Le, A. Servati, S. Soltanian, P. Servati and F. Ko (2021) Performance and Signal Quality Analysis of Electrocardiogram Textile Electrodes for Smart Apparel Applications. Front. Electron. 2:685264. doi: 10.3389/felec.2021.685264 • K. Le, H. Narayana, A. Servati, S. Soltanian, A. Bahi, P. Servati and F. Ko (2022) Electronic-textiles (e-textiles) for electrocardiogram (ECG) monitoring: A review on structure-property and performance evaluation from fibre to fabric. Textile Research Journal. doi:10.1177/00405175221108208 • X. Sun, Y. Zhu, J. Zhu, K. Le, P. Servati and F. Jiang (2022) Tough and ultrastretchable liquid-free ion conductor strengthened by deep eutectic solvent hydrolyzed cellulose microfibres. Adv. Func. Mater. doi: 10.1002/adfm.202202533 Manuscripts in preparation for submission: • K. Le, H. Narayana, S. Soltanian, A. Servati, F. Ko, P. Servati “Roll-to-Roll Fabrication of Silver/Silver Chloride Coated Yarns for Dry Electrodes and Applications in Biopotential Signal Monitoring.” • K. Le, H. Narayana, A. Servati, S. Soltanian, P. Servati, F. Ko “Influence of Yarn Geometry on Electrical Properties of Conductive Silver-coated Nylon filaments for E-Textiles: A Fundamental Study.” • K. Le, X. Sun, J. Chen, J. V. John, A. Srvati, H. Heidari, A. Khademhosseini, F. Ko, F. Jiang, P. Servati “Stretchable, self-healing, biocompatible, and durable ionogel for continuous wearable strain and physiological monitoring.” 213 8.4.2 Conference Papers & Presentations • K. Le, A. Servati, F. Ko, P. Servati, “Signal Quality Analysis of Electrocardiogram Textile Electrodes for Smart Apparel Applications,” at IFETC 2019, Vancouver, Canada, Aug. 2019. • K. Le, A. Servati, F. Ko, P. Servati “Mechanisms Underlying Stability and Accuracy of Electrocardiogram Textile Electrodes for Smart Textile Systems,” at SAMPE-CAMX North America, Anaheim, USA, Sept. 2019. • K. Le, A. Servati, H. Narayana, Y. Shao, S.T. Htun, A. Imaizumi, S. Srinivasan, Z. Jiang, S. Soltanian, F. Ko, P. Servati, “Sensor fusion wearable textile wristband for multimodal monitoring of physiological parameters,” at SPIE BiOS Photonics West, San Francisco, USA, Feb. 2020. • H. Narayana, A. Servati, Y. Shao, Z. Jiang, S. Srinivasan, K. Le, S.T. Htun, A. Imaizumi, S. Soltanian, F. Ko, B. Shadgan, P. 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Rep., vol. 4, no. 1, p. 6074, May 2015, doi: 10.1038/srep06074. 231 Appendices Appendix A A.1 General framework for materials testing and signal quality analysis (MITACS Accelerate Project, VITALI Wear) 232 A.2 Contact pressure measurement from e-textile electrodes placed at underband Contact pressure across 6-minute ECG test using force resistive sensor from electrodes placed at underband region. -Active area of FSR – 0.204 cm2 (Pololou Electronics, D: 0.2”/0.51cm) -Protocol: Baseline (sitting, first 3 min), Movement (jogging on spot, last 3 min) -Range of values: 0.04 to 1.8 kPa (0 kPa corresponds to no pressure detected) 233 A.3 Surface roughness KES-SMD geometrical roughness data Kawabata Evaluation System – KES-SE Tester Measurement: Steel piano wire, diameter 0.5mm, length 5mm, contact force 0.1N SMD (μm) Forward Forward Average Average total (warp and weft) A Wale↕︎ 1 1.795 (60/78) 2 1.809 3 1.830 1.811 4.030 Course↔︎ 1 5.897 (SD: 2.228) 2 6.613 3 6.234 6.248 G Warp 1 1.300 (woven 138x107) 2 1.351 3 1.334 1.328 1.087 Weft 1 0.866 (SD: 0.242) 2 0.856 3 0.817 0.847 C Wale↕︎ 1 2.665 (66/66) 2 2.612 3 2.525 2.600 3.112 Course↔︎ 1 3.853 (SD: 0.539) 2 3.308 3 3.710 3.624 D Wale↕︎ 1 2.731 (55/33) 2 2.293 5 2.2105 2.411 1.759 Course↔︎ 1 1.230 (SD:0.677) 2 0.974 3 1.113 1.106 E Wale↕︎ 1 6.703 (52/62) 2 6.748 3 7.165 6.872 4.956 Course↔︎ 1 3.195 (SD:1.932 2 2.645 3 3.282 3.041 F Wale↕︎ 1.161 (66/81) 2 1.099 3 1.210 1.132 1.434 4 0.997 (SD:0.165) 5 1.220 Course↔︎ 1 1.531 2 1.179 3 2.502 1.737 234 A.4 Twisted Yarn Diameters Yarn D app (mm) TPM 1-ply 2-ply 3-ply 4-ply 0 0.492 0.501 0.546 0.991 30 0.425 0.499 0.576 0.583 90 0.252 0.346 0.390 0.475 150 0.211 0.295 0.354 0.397 210 0.189 0.268 0.349 0.395 300 0.186 0.268 0.345 0.393 450 0.177 0.257 0.333 0.385 600 0.177 0.251 0.333 0.382 235 A.5 Rib knit textile electrode properties & signal quality results Jersey knit e-textile material properties RIB KNIT FABRIC Wales/cm Courses/cm Stitch Density (cm2) Thickness (mm) Weight (gsm) Machine LL (no units) 1-ply 2-ply 3-ply 1-ply 2-ply 3-ply 1-ply 2-ply 3-ply 1-ply 2-ply 3-ply 1-ply 2-ply 3-ply 15 11 11 11 6 6 6 60 64 68 0.38 ±0.02 0.63 ±0.02 0.93 ±0.02 114 ±5.74 220 ±9.20 311 ±9.91 20 7 9 8 4 6 5 26 52 44 0.27 ±0.01 0.53 ±0.02 0.72 ±0.04 119 ±1.93 190 ±8.76 256 ±8.66 25 6 9 7 4 5 5 22 43 36 0.19 ±0.02 0.45 ±0.02 0.69 ±0.03 115 ±1.88 161 ±2.01 218 ±9.36 Jersey knit e-textile surface resistance properties Actual LL, cm Surface Resistance, Ω/sq. Machine LL (no units) 1-ply 2-ply 3-ply 1-ply Warp | Weft 2-ply Warp | Weft 3-ply Warp | Weft 15 0.42 ±0.006 0.47 ±0.007 0.53 ±0.011 0.33 ±0.05 0.40 ±0.09 0.25 ±0.08 0.34 ±0.07 0.31 ±0.06 0.37 0.06 20 0.45 ±0.005 0.52 ±0.01 0.66 ±0.009 0.45 ±0.09 0.44 ±0.09 0.32 ±0.09 0.41 ±0.10 0.34 ±0.09 0.39 ±0.10 25 0.52 ±0.011 0.57 ±0.006 0.68 ±0.004 0.49 ±0.08 0.64 ±0.09 0.42 ±0.09 0.45 ±0.10 0.43 ±0.10 0.43 ±0.12 236 Summary of PCC results calculated from e-textiles compared with standard gel electrodes SAMPLE ID 1p-MLL15 0.956 2p-MLL15 0.980 3p-MLL15 0.960 1p-MLL20 0.948 2p-MLL20 0.960 3p-MLL20 0.976 1p-MLL25 0.952 2p-MLL25 0.977 3p-MLL25 0.968 Average 0.952 0.972 0.968 Summary of frequency domain signal quality features, organized with respect to loop length groups MLL 15 1 -PLY 2-PLY 3-PLY Gel Ref. E-Textile Gel Ref. E-Textile Gel Ref. E-Textile Avg. Band Power 0.032 0.077 0.023 0.048 0.032 0.089 Avg. Occupied Bandwidth 39.6 35.4 44.7 37.6 41.4 36.7 Lower Bound 0.90 0.49 0.74 0.35 1.03 0.41 Upper Bound 40.4 35.4 45.4 37.6 42.4 36.7 MLL 20 1 -PLY 2-PLY 3-PLY Gel Ref. E-Textile Gel Ref. E-Textile Gel Ref. E-Textile Avg. Band Power 0.029 0.067 0.029 0.064 0.033 0.076 Avg. Occupied Bandwidth 37.9 35.0 43.3 38.0 41.2 36.9 Lower Bound 0.84 0.85 1.05 0.37 0.95 0.38 Upper Bound 38.7 35.9 44.3 38.4 42.1 37.3 237 MLL 25 1 -PLY 2-PLY 3-PLY Gel Ref. E-Textile Gel Ref. E-Textile Gel Ref. E-Textile Avg. Band Power 0.030 0.068 0.031 0.066 0.032 0.081 Avg. Occupied Bandwidth 38.3 35.1 43.2 38.5 39.6 36.7 Lower Bound 0.85 0.51 0.82 0.41 0.91 0.44 Upper Bound 39.1 35.6 44.0 38.9 40.5 37.1 238 A.6 Skin surface replicas for surface roughness observation The process for skin surface preparation was adapted from work by: Lee et al., “Self-adhesive epidermal carbon nanotube electronics for tether-free long-term continuous recording of biosignals.” 2014 [234]. Square frames (3x3cm) were 3-D printed and glued to the skin surface. Surface moulds (3x3cm) of the skin surface from the forearm were prepared with silicone (Ecoflex®00-35), and cast with PDMS (SYLGARD 184). The PDMS specimens was degassed for 1 hour, cured at room temperature for 48 hours. Schematic of skin surface replica preparation (adapted from Lee et al.)239 A.7 Roll-to-roll fabrication of Ag/AgCl coated yarns – Additional figures Figure A.7.1 Photograph of roll-to-roll fabrication system. 240 Figure A.7.2 EDX spectrum of a. Ag and b. Ag/AgCl yarns 241 Figure A.7.3 (a) Electrode placement for ECG recording (separate measurements made for standard electrode, and and e-textile set) (b) Average ECG waveform from standard Ag/AgCl gel electrode (R-peak amplitude: 1.27 mV), and Ag/AgCl e-textile electrode (R-peak amplitude: 1.42 mV) Figure A.7.4 (a) Electrode placement for simultaneous ECG recording with standard electrode, and e-textile electrode sets (b) Average ECG waveform from standard Ag/AgCl gel electrode (R-peak amplitude: 1.27 mV), and Ag/AgCl e-textile electrode (R-peak amplitude: 2.36 mV) The results show that for ECG recordings made at the same chest location, the R-peak amplitude between the two material sets (gel and textile) are relatively similar. With the simultaneous recording configuration, higher R-peak amplitude is measured for the e-textile electrode set that is placed closer to the heart. 242 A.8 Chloride-based ionogel – Additional results XRD results for ionogel films (IL:TA) compared with TA powder Sheet resistance of [BMIM]Cl-pTA film and coated fabric Ratio Sheet resistance (Ω/sq.) Film Coated Fabric 1:3 0.003 ± 0.001 0.291 ± 0.040 1:4 0.035 ± 0.007 0.230 ± 0.017 1:10 0.404 ± 0.081 0.261 ± 0.029 243 Contact impedance results of ionogel coated fabrics at selected frequencies Z @ 1 Hz Z @ 10 Hz Z @ 145 Hz 1:3 1.37 1.20 0.95 1:4 1.41 1.31 0.95 1:10 2.13 2.10 1.30 Ag-nylon bare fabric 2.86 2.84 2.22 Ag/AgCl ref. electrode 1.35 1.10 0.87 244 Tensile tests after healing periods compared (left), and tensile stress and modulus values for each period (right) Tensile strain and toughness value for different healing periods 245 Live/dead assay fluorescent microscope images – live cells shown as green, and dead cells as red.246 A.9 Modelling Approaches in Experimental Curve Fitting and Equivalent Circuit Parameter Identification Using the Simplified Randle’s Circuit as a Study Interfacial impedance between the electrode and skin is studied using electrochemical impedance spectroscopy. From these measurements, the data is fitted with equivalent circuits that are used to explain interfacial activity. The non-linear least squares regression is often performed to fit the circuit model to measured impedance data, by minimizing the sum of squares of the errors between the model and data. In this work, two modelling approaches have been derived to solve for equivalent electrical circuit (EEC) parameters for the Simplified Randle’s Circuit, with results compared to the conventional fitting approach. The new approaches are proposed for the following reasons: - To avoid the need to provide initial values for the conventional non-linear least squares fitting technique - To provide alternate approaches for circuit model fitting to compare results with conventional estimations. This is to ensure that the best fit results from experimental data are selected, given that they are often associated with some degree of uncertainty due to the instability between electrode-skin interface during measurements. The simplified Randle’s circuit for skin-electrode impedance is shown in Figure A7.1. It includes a resistor (Rs) in series with a parallel resistor (Rd) and capacitor (Cd). In the context of skin-electrode impedance, Rs represents the resistance of skin tissue, electrolyte or sweat, Rd is the electrical charge transfer resistance between skin and electrode, and Cd is the double-layer capacitance, or charge between skin and electrode. 247 Figure A9.1 – Simplified Randle’s Circuit Total impedance expressed as a function of frequency is represented in terms of real and imaginary components, from which magnitude of impedance can be calculated. 𝑍 = 𝑅𝑠 + 𝑅𝑑(1+𝑋𝑑2) − 𝑗𝑅𝑑𝑋𝑑(1+𝑋𝑑2) (where, Xd = Rd.Cd.ω, and ω = 2πf ) (1.1) 1. General approach to modelling results from Nyquist plot – Imaginary (B) Form Taking the real and imaginary components of |Z| as A and B, Equation 1 can be expressed as follows: 𝐴 = 𝑅𝑠 + 𝑅𝑑(1+𝑋𝑑2) (1.2) 𝐵 = − 𝑅𝑑𝑋𝑑(1+𝑋𝑑2) (1.3) Combining Eqs. 1.2 and 1.3, and expressing the data in terms of the imaginary component (-B) the expression in the following form is obtained: 𝑩 = 𝑨. 𝑿𝒅 − 𝑹𝒔. 𝑿𝒅 (2) 𝒘𝒉𝒆𝒓𝒆 𝒄𝒐𝒎𝒑𝒐𝒏𝒆𝒏𝒕𝒔 𝒄𝒂𝒏 𝒃𝒆 𝒔𝒐𝒍𝒗𝒆𝒅 𝒃𝒚: 𝑭𝟏. 𝑿𝟏 = [−𝝎𝟏 𝑨𝟏𝝎𝟏⋮ ⋮−𝝎𝒏 𝑨𝒏𝝎𝒏] . [𝑹𝒔𝑹𝑪] The value of Rd can be determined from the largest value of the real component of impedance Z, MxA as shown: Rd = MxA – Rs Eq. 2 can be used to solve for the circuit elements, Rs, Rd, Cd using MATLAB (R2021B, MathWorks, USA). Furthermore, the results from the direct method using Eq. 2, can be compared 248 to MATLAB optimization solver, using the “lsqcurvefit” function. Fitted results for both methods are shown in Figure A9.2 using an example data set and show similar results. The fitted curves do not match the experimental data very well. Therefore, an alternate approach is introduced in the next section. Figure A9.2 – Plotted results comparing experimental data with curve fitting methods from Eq. 2 model, and non-linear least squares regression function. 2. New approach to modelling results from Nyquist plot – integration of characteristic values A new approach is derived to model the Nyquist plot form of experimental results through the following form of the magnitude of impedance and frequency response: 𝑨𝟐 − 𝟐𝑹𝒔. 𝑨 + 𝑹𝒔𝟐 + 𝑩𝟐 = 𝑹𝒅.𝑩𝑿𝒅 (3) Derivation of Eq. 3 is outlined: From expansion of Eq. 1.2, 0 1x1062x1063x1064x1065x1066x1060.05.0x1051.0x1061.5x1062.0x1062.5x1063.0x106 B (experimental) B (calc. Eq. 2) B (NLSq.)-Z", Z', 249 𝐴 (1 + 𝑋𝑑2) = 𝑅𝑠. (1 + 𝑋𝑑2) + 𝑅𝑑 Rearranging, 𝐴 = ( 𝑅𝑠 + 𝑅𝑑 ) + 𝑅𝑠. 𝑋𝑑2 − 𝐴. 𝑋𝑑2 From Eq. (2) and Eq.(3), the expressions can be written as: 𝐵𝑋𝑑= − 𝑅𝑑(1+𝑋𝑑2) => 𝐴 − 𝑅𝑠 = −𝐵𝑋𝑑 Re-arranging Eq. 1.1: Z = |Z| = 𝑅𝑠 + 𝑅𝑑(1+𝑋𝑑2) (1 − 𝑗 𝑋𝑑) Z = 𝑅𝑠 + 𝑅𝑑(1+𝑋𝑑2) (1 − 𝑗 𝑋𝑑) , ( Z = A – jB, a complex vector ) 𝐴 − 𝑅𝑠 = 𝑅𝑑(1+𝑋𝑑2) (1 − 𝑗 𝑋𝑑) (𝐴 − 𝑅𝑠) − 𝑗𝐵 = 𝑅𝑑(1+𝑋𝑑2) (1 − 𝑗 𝑋𝑑) Taking the | . |2 magnitude square of both sides of the equation: (𝑍 − 𝑅𝑠)2 = [𝑅𝑑(1+𝑋𝑑2)]2∗ (1 + 𝑋𝑑2) =𝑅𝑑2(1+𝑋𝑑2)2 (1 + 𝑋𝑑2) = 𝑅𝑑(1+𝑋𝑑2) (𝐴 − 𝑅𝑠)2 + 𝐵2 = [𝑅𝑑(1+𝑋𝑑2)]2∗ (1 + 𝑋𝑑2) =𝑅𝑑2(1+𝑋𝑑2)2 (1 + 𝑋𝑑2) = 𝑅𝑑(1+𝑋𝑑2) (𝑍 − 𝑅𝑠)2 = 𝑅𝑑2(1+𝑋𝑑2) Hence, 𝑨𝟐 − 𝟐𝑹𝒔. 𝑨 + 𝑹𝒔𝟐 + 𝑩𝟐 = 𝑹𝒅𝟐(𝟏+𝑿𝒅𝟐) From the example Nyquist plot of the Simplified Randle’s circuit (Figure A9.3), the high-frequency intercept yields Rs (frequency-independent contributions, ohmic resistance of electrolyte), and the low-frequency intercept is expressed as Rs + Rd, which yields Rd, after subtracting Rs. In determining Rd, the characteristic capacitance and corresponding frequency from 250 the maximum point of the plot can be expressed as C = 1/Rdω, or as, Xd = Rd.Cd.ω = 1. The maximum value on the Nyquist plot is located ½ Rd on the x-axis as indicated by Eq. 1.3: 𝐵 = 𝑅𝑑𝑋𝑑(1+𝑋𝑑2) Figure A9.3 Example Nyquist plot from Simplified Randle’s Cell showing characteristic features. (https://www.cei.washington.edu/wp-content/uploads/2018/05/EIS-and-NLEIS-Wiki.pdf) Thus, by using the concept of Characteristic Frequency and Capacitance from the Nyquist plot, this transforms the equation Eq. 3 to be independent of frequency. This also simplifies the formulation of B, with fewer unknown parameters to be determined, specifically with the three distinctive components Xd = Rd.Cd.ω set to 1. To prove that the characteristic value of Xd is equal to 1, the derivative of B is taken. Based on the product rule: y = u/v => y’ = (u’.v – u.v’) / v2 Let 𝑋𝑑 = 𝑅𝐶𝜔, then B can be written as: 𝐵 = 𝑅𝑑𝑅𝐶𝜔(1+𝑅𝐶2𝜔2) Differentiating B with respect to ω: 251 𝑑𝐵𝑑𝜔 = 𝑅𝑑.𝑅𝐶(1+𝑅𝐶2 𝜔2 )−(𝑅𝑑.𝑅𝐶.𝜔).(2𝑅𝐶2 𝜔) (1+𝑅𝐶2ω2)𝟐 = 𝑅𝑑.𝑅𝐶+ 𝑅𝑑.𝑅𝐶3𝜔2 −2.𝑅𝑑.𝑅𝐶3𝜔2 (1+𝑅𝐶2ω2)𝟐 𝑑𝐵𝑑𝜔 = 𝑅𝑑.𝑅𝐶− 𝑅𝑑𝑅𝐶3𝜔2 (1+𝑅𝐶2ω2)𝟐 Then set the derivative to zero to determine its extremum: 𝑑𝐵𝑑𝜔 = 0 => 𝑅𝑑 . 𝑅𝐶 − 𝑅𝑑 . 𝑅𝐶3𝜔2 = 0 Hence, 𝑅𝑑 . 𝑅𝐶(1 − 𝑅𝐶2𝜔2 ) = 0 or (1 − 𝑅𝐶2𝜔2 ) = 0 => 𝑅𝐶2𝜔2 = 1 Thus, 𝑅𝐶𝜔 = 𝑋𝑑 = 1 (4.1) The extremum happens to be a maximum from the second derivative, and it is shown that it is negative. 𝑑2𝐵𝑑𝜔2 = −2(𝑅𝑑.𝑅𝐶3𝜔)(1+𝑅𝐶2ω2)− 2(𝑅𝐶2𝜔)(𝑅𝑑.𝑅𝐶− 𝑅𝑑𝑅𝐶3𝜔2 ) (1+𝑅𝐶2ω2)𝟑 𝑑2𝐵𝑑𝜔2 = −4(𝑅𝑑.𝑅𝐶3𝜔) (1+𝑅𝐶2ω2)𝟑 < 0 (4.2) The model expressed in terms of B, from Eq. 3 with Xd = 1 now becomes: 𝑅𝑑 . 𝐵 = 𝐴2 − 2𝑅𝑠. 𝐴 + 𝑅𝑠2 + 𝐵2 (4.3) 𝐵 = (𝐴2 − 2𝑅𝑠. 𝐴 + 𝑅𝑠2 + 𝐵2 )/𝑅𝑑 Defining x(1-4), x = [ 𝑹𝒔𝟐𝑹𝒅 𝑹𝒔𝑹𝒅 𝟏𝑹𝒅 𝟏𝑹𝒅 ] (4.4) 252 In matrix format, it can be expressed as shown: [ 𝑰 − 𝟐𝑨 𝑨𝟐 𝑩𝟐 ]. 𝒙 = 𝑩 (4.5) Eq. 4.5 represents the model for B vs. A in the Nyquist plot featuring the characteristic circuit parameter values. The solution of this model can be used to verify if 𝑋𝑑 = 𝑅𝐶2𝜔2 = 1 if the result of Rd obtained from solution, 𝑅𝑑 = 1/𝑥(4) is known. Moving Rd to the left-hand side, the following equation is obtained: 𝑩 = [ 𝑨𝟐/𝑹𝒅 − 𝟐𝑹𝒔. 𝑨/𝑹𝒅 + 𝑹𝒔𝟐/𝑹𝒅 + 𝑩𝟐/𝑹𝒅 ] . 𝒚 (4.6) where y = [ 𝑹𝒔𝟐 𝑹𝒔 𝒌𝑨 𝒌𝑩 ] (4.7) Upon solving Eq. 4.6 for y, kB = y(4) = 1. This is to confirm that 𝑋𝑑 = 𝑅𝐶2𝜔2 = 1. On the other hand, if Rd is extracted from result x(3) of Eq. 4.6 model presented, i.e., 𝑅𝑑 = 1/𝑥(3), it is shown that kA is equal to 1. Furthermore, since the model is derived from the same EEC impedance formulation with some modifications based on the solution obtained from Eq. 4.5, it is expected that the models should yield identical EEC parameters. The calculated solutions from the models presented yield a very good fit with the experimental data (Figure A9.4). 253 Figure A9.4 – Plotted results comparing experimental data with models presented in Eq. 3/4.5 and 4.6, and non-linear least square curve fitting results for comparison (Z’ x-axis is experimental data) EEC Parameters from Solution x = [ 𝑅𝑠2𝑅𝑑 𝑅𝑠𝑅𝑑 1𝑅𝑑 1𝑅𝑑 ] from Eq.3/4.5 From the description of x, there will be multiple values of Rs. Calculation methods are summarized as follows: a) Determining the value of Rd uniquely: 2/Rd = x(3) + x(4) = Kx => Rd = 2/Kx , Cx = x(3) + x(4) (5.1) b) Determining value of Rs uniquely if the 2nd order equation solution exists: After x has been computed, to extract the value of Rs, using Rd above (Eq. 5.1), the following approach is taken: x(4) = 1/Rd2 => 𝑅𝑑2 =1𝑥(4) 𝑅𝑠2 = 𝑥(1) ∗ 𝑅𝑑 , 𝑅𝑠 = 𝑥(2) ∗ 𝑅𝑑 0 1x1062x1063x1064x1065x1066x1060.05.0x1051.0x1061.5x1062.0x1062.5x1063.0x106-Z", Z',  B (experimental) B (NLSq.) B (charac. param, Eq. 3/4.5) B (charac. param, verific. Eq. 4.6)254 𝑅𝑠2 + 𝑅𝑠 = 𝑅𝑑 (x(1) + x(2)) = 𝐾𝑠 𝑹𝒔𝟐 + 𝑹𝒔 − 𝑲𝒔 = 0 𝑲𝒔 = 𝑹𝒅 (𝐱(𝟏) + 𝐱(𝟐)) (5.2) Solve for Rs if all the conditions are satisfied for Rs as a real and positive number. c) Other ways to extract the solution for Rd & Rs, where values are not unique: Rd1 = 1/kA => Rs1 = MxA – Rd1 Rd2 = 1/kB => Rs2 = MxA – Rd2 Rs3 = x(1)/x(2) => Rd3 = MxA – Rs3 Rs4 = x(2)/x(3) => Rd4 = MxA – Rs4 Rs5 = x(2)/x(4) => Rd5 = MxA – Rs5 EEC Parameters from Solution y = [ 𝑅𝑠2 𝑅𝑠 𝑘𝐴 𝑘𝐵 ] from Eq. 4.6 Depending on how Rd is extracted from the solution for x (Eq. 5.2), the parameters kA and kB are used to check the validity of the characteristic value of 𝑋𝑑 = 𝑅𝐶2𝜔2 = 1 It is obvious from the description of y, there will be multiple values of Rs. Calculation methods are summarized as follows: d) Determining the value of Rs uniquely if the 2nd order equation solution exists: 𝑅𝑠2 + 𝑅𝑠 = x(1) + x(2) = 𝐾𝑠 𝑹𝒔𝟐 + 𝑹𝒔 − 𝑲𝒔 = 0 The second order equation can be solved if all the conditions are satisfied, with Rs as a real and positive number. e) Other ways to extract the solution for Rd & Rs, where values are not unique: 255 Rs1 = y(1)/y(2) => Rd1 = MxA – Rs1 Rs2 = y(2)/y(3) => Rd2 = MxA – Rs2 Rs3 = y(2)/y(4) => Rd2 = MxA – Rs3 Calculated results for all models used in the example data presented are presented in Table A9.1. Table A9.1 – Summary of estimated EEC parameter results Model from Eq. 2 Non-linear least squares curve fit (Eq. 2) Model from Eq. 3/4.5 Model from Eq. 4.6 Rs 6.15E+02 6.15E+02 3.49E+03 3.49E+03 Rd 5.59E+06 5.59E+06 5.58E+06 5.58E+06 MxA 5.59E+06 5.59E+06 5.59E+06 5.59E+06 RC 1.36E-01 1.29E-01 5.32E-02 5.32E-02 Cd 2.44E-08 2.31E-08 2.44E-08 2.44E-08 Rs1 6.15E+02 3.40E+06 3.40E+06 Rs2 3.59E+06 3.59E+06 Rs3 3.84E+06 3.84E+06 Rd1 5.59E+06 2.18E+06 2.18E+06 Rd2 2.00E+06 2.00E+06 Rd3 1.75E+06 1.75E+06 256 A.10 Embroidered Ag and Ag/AgCl e-textiles for EEG monitoring The following section details the fabrication, and preliminary signal collection performance comparison of Ag-nylon and Ag/AgCl-nylon embroidered textile electrodes for EEG signal measurements during sleep (UBC Pacific Parkinson’s Research Centre). • Three sets of e-textile electrodes (13mm diameter) were fabricated by stitching e-textile yarns on a silver-nylon coated plain weave fabric Sewing machine settings: 7.0 mm width, 2.5 mm length, no tension; stitch design shown below 1) Ag-nylon yarn based (AG) – 4 passes of stitch 2) Low density AgCl (LDAGCL) – 2 passes of stitch 3) High density AgCl (HDAGCL) – 4 passes of stitch Left to right: Stitch design, sample embroidered e-textile electrodes, integration of electrode in EEG cap • Initial impedance checks for both electrode materials met requirement <1 kΩ at 10 Hz. • Results from EEG test compared e-textile electrodes with standard Ag/AgCl rigid electrodes (and layer of electrolyte paste) (Pearson r, EEG with gel vs. textile electrodes. o Pearson r was calculated from comparison of filtered EEG signal of textile electrodes vs. standard electrodes; excellent correlation between the three sets of materials- AG: 0.9625; LDAGCL: 0.9368; HDAGCL: 0.9839
Thesis/Dissertation
eng
Materials Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Ko, Frank K.
Servati, Peyman
Design and evaluation of textile electrodes for biological signal monitoring
Text