Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Amini, Majed
Doctor of Philosophy - PhD
University of British Columbia
This thesis presents the development of three-dimensional (3D)-printed conductive cellulose cryogels designed for electromagnetic interference (EMI) shielding. The motivation for this work arises from the need to find environmentally friendly alternatives to petroleum-based conductive polymers, turning attention to biopolymers derived from natural carbohydrates like cellulose. The research began with optimizing cellulose nanocrystal (CNC) inks to achieve high-resolution 3D structures. This involved creating a "3D-printing" map to demonstrate the relationship between CNC/salt ink rheology and printing fidelity. This map revealed that high fidelity could be achieved within specific concentration ranges, significantly influenced by the type of salt used. Among the ten salts evaluated, FeClâ, CaSOâ, and NaHCOâ were particularly effective, enhancing the ink's rheological properties and enabling precise 3D printing. To impart conductivity to the cellulose inks, poly(3,4-ethylenedioxythiophene) (PEDOT) was used. This step was crucial for the application of cryogels in EMI shielding. At the macro scale, manipulating the grid dimensions of the 3D-printed cryogels revealed a clear relationship between grid size and shielding effectiveness (SE). Larger grid sizes, ranging from 1 mm to 3 mm, resulted in a 30% reduction in SE and shifted the shielding mechanism from reflection to absorption. At the micro-scale, altering pore sizes from approximately 50 microns to 170 microns demonstrated a similar shift from reflection to absorption, highlighting the impact of pore structure on performance. On the nano-scale, modifications using TEMPO-oxidized cellulose nanofibers (CNFs) coated with PEDOT achieved a high conductivity of up to 546 S/m. This enhancement in conductivity greatly improved the EMI shielding effectiveness, outperforming traditional fabrication methods. Overall, this thesis not only advances the understanding of 3D-printed cellulose cryogels but also establishes their practical applicability in EMI shielding. The innovative methodologies and findings presented offer new avenues for developing sustainable, biocompatible, and lightweight materials, aligning with global demands for eco-friendly technological advancements.
3D-Printed Conductive Cellulose Cryogels for Electromagnetic Interference Shielding: Exploring Structural Design, Printability, and Shape Fidelity by Majed Amini B.Sc., Sahand University of Technology, 2015 M.Sc., Sharif University of Technology, 2018 A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE COLLEGE OF GRADUATE STUDIES (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) October 2024 © Majed Amini, 2024ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a dissertation entitled: 3D-Printed Conductive Cellulose Cryogels for Electromagnetic Interference Shielding: Exploring Structural Design, Printability, and Shape Fidelity submitted by Majed Amini in partial fulfillment of the requirements of the degree of Doctor of Philosophy Dr. Mohammad Arjmand, School of Engineering Supervisor Dr. Abbas Milani, School of Engineering Supervisory Committee Member Dr. Orlando Rojas, University of British Columbia (Vancouver) Supervisory Committee Member Dr. Shahria Alam, School of Engineering University Examiner Dr. Nicole R. Demarquette, Ãcole de technologie supérieure External Examiner iii Abstract This thesis presents the development of three-dimensional (3D)-printed conductive cellulose cryogels designed for electromagnetic interference (EMI) shielding. The motivation for this work arises from the need to find environmentally friendly alternatives to petroleum-based conductive polymers, turning attention to biopolymers derived from natural carbohydrates like cellulose. The research began with optimizing cellulose nanocrystal (CNC) inks to achieve high-resolution 3D structures. This involved creating a "3D-printing" map to demonstrate the relationship between CNC/salt ink rheology and printing fidelity. This map revealed that high fidelity could be achieved within specific concentration ranges, significantly influenced by the type of salt used. Among the ten salts evaluated, FeCl3, CaSO4, and NaHCO3 were particularly effective, enhancing the ink's rheological properties and enabling precise 3D printing. To impart conductivity to the cellulose inks, poly(3,4-ethylenedioxythiophene) (PEDOT) was used. This step was crucial for the application of cryogels in EMI shielding. At the macro scale, manipulating the grid dimensions of the 3D-printed cryogels revealed a clear relationship between grid size and shielding effectiveness (SE). Larger grid sizes, ranging from 1 mm to 3 mm, resulted in a 30% reduction in SE and shifted the shielding mechanism from reflection to absorption. At the micro-scale, altering pore sizes from approximately 50 microns to 170 microns demonstrated a similar shift from reflection to absorption, highlighting the impact of pore structure on performance. On the nano-scale, modifications using TEMPO-oxidized cellulose nanofibers (CNFs) coated with PEDOT achieved a high conductivity of up to 546 S/m. This enhancement in conductivity greatly improved the EMI shielding effectiveness, outperforming traditional fabrication methods. Overall, this thesis not only advances the understanding of 3D-printed cellulose cryogels but also establishes their practical applicability in EMI shielding. The innovative methodologies and findings presented offer new avenues for developing sustainable, biocompatible, and lightweight materials, aligning with global demands for eco-friendly technological advancements. iv Lay Summary In recent years, there's been a growing need for environmentally friendly materials in various industries. My PhD research focuses on creating a new type of material that's sustainable and useful in protecting against electromagnetic interference (EMI), a kind of environmental pollution caused by the electronic devices surrounding us. To do this, I worked with a plant-based substance called cellulose, which is abundant in nature. Think of it as the structural building block of plants. I developed a special ink made from tiny cellulose particles, which can be used in 3D printers to create ultra-light, sponge-like materials known as aerogels. The most exciting part? The size and structure of these cryogels can be tweaked to change how they block EMI, either by reflecting it away or absorbing it. This means we can customize them for different uses, from protecting sensitive medical equipment to improving the safety of electronic gadgets at home. v Preface Majed Amini authored this doctoral dissertation under the guidance of Dr. Mohammad Arjmand at the School of Engineering at the University of British Columbia. Various sections of this dissertation have resulted in journal articles. The following sections summarize the publications derived from this dissertation and highlight the author's contributions. In addition, in writing this thesis, I used AI tools to help edit and polish some of my sentences to make sure they were grammatically correct and clear. I only used the AI to improve the language and not to generate any scientific content or ideas. All the research, findings, and conclusions in this thesis are my own work. Journal Publications: â¢ A version of Chapter 2 has been published in the following article. Amini, M., Kamkar, M., Rahmani, F., Ghaffarkhah, A., Ahmadijokani, F. and Arjmand, M., 2021. Multilayer structures of a Zn0.5Ni0.5Fe2O4-reduced graphene oxide/PVDF nanocomposite for tunable and highly efficient microwave absorbers. ACS Applied Electronic Materials, 3(12), pp.5514-5527. Contribution: I identified and designed the research, contributing to all aspects of the study, including literature reviews, experiments, data collection, and interpretation of results. I analyzed the research data and prepared manuscripts based on the research findings. â¢ A version of Chapter 4 has been published in the following article. Amini, M., Kamkar, M., Ahmadijokani, F., Ghaderi, S., Rojas, O.J., Hosseini, H. and Arjmand, M., 2022. Mapping 3D printability of ionically cross-linked cellulose nanocrystal inks: architecting from Nano-to macroscale structures. Biomacromolecules, 24(2), pp.775-788. Contribution: I identified and designed the research, contributing to all aspects of the study, including literature reviews, experiments, data collection, and interpretation of results. I analyzed the research data and prepared manuscripts based on the research findings. â¢ A version of Chapter 5 has been published in the following article. Amin Ojagh, S.M., Amini, M., Cranmer-Smith, S., Vahabzadeh, F., Arjmand, M., Tam, K.C., Rojas, O.J., Kamkar, M. and van de Ven, T.G., 2023. Crystalline and hairy nanocelluloses for 3D printed hydrogels and strongly structured cryogels. ACS Sustainable Chemistry & Engineering, 11(14), pp.5674-5684. Contribution: I identified and designed the research, contributing to all aspects of the study, including literature reviews, experiments, data collection, and interpretation of results. I vi was responsible for writing half of the manuscript and collecting data, specifically focusing on rheology, 3D printing, morphological and mechanical properties. â¢ A version of Chapter 6 has been published in the following article. Amini, M., Hosseini, H., Dutta, S., Wuttke, S., Kamkar, M. and Arjmand, M., 2023. Surfactant-mediated highly conductive cellulosic inks for high-resolution 3D printing of robust and structured electromagnetic interference shielding aerogels. ACS Applied Materials & Interfaces, 15(47), pp.54753-54765. Contribution: I identified and designed the research, contributing to all aspects of the study, including literature reviews, experiments, data collection, and interpretation of results. I analyzed the research data and prepared manuscripts based on the research findings. â¢ A version of Chapter 7 is under review in the following article. Amini, M., Hashemi, SA., Yu, Z., Kamkar, M., Jiang, F., & Arjmand, M., 2024. Flexible 3D-Printed Cellulosic Constructs for EMI Shielding and Piezoresistive Sensing. Advanced Functional Materials. Contribution: I identified and designed the research, contributing to all aspects of the study, including literature reviews, experiments, data collection, and interpretation of results. I analyzed the research data and prepared manuscripts based on the research findings. Conference Publications and Presentations â¢ Amini, M., Hosseini, H., Arjmand, M. Robust Three-Dimensional (3D) Printed Biocomposite Aerogel for Electromagnetic Interference Shielding. The 49th World Polymer Congress (Macro, 2022). July 17-21, 2022, Winnipeg, Canada. (Best Poster Award) â¢ Amini, M., Kamkar, M., Arjmand, M. 3D printing of physically crosslinked cellulose nanocrystal inks. The 49th World Polymer Congress (Macro, 2022). July 17-21, 2022, Winnipeg, Canada. â¢ Amini, M., Hosseini, H., Kamkar, M., Arjmand, M. High-Resolution 3D-printed and Mechanically Robust Bio-nanocomposite Aerogel toward Electromagnetic Interference Shielding. The 38th International Conference of the Polymer Processing Society (PPS-38, 2023). May 22-26, 2023, St. Gallen, Switzerland. (Graduate Travel Award from Polymer Processing Society - only two researchers worldwide receive this award annually) â¢ Amini, M., Hashemi, SA., Yu, Z., Jiang, F., & Arjmand, M. (2024) Multifunctional 3D-Printed Cellulosic Constructs via a Sequential Cold Chemical Vapor Polymerization Technique. The 39th International Conference of the Polymer Processing Society (PPS-39, 2024). May 21-24, 2024, Colombia. (Keynote Speaker, Graduate Travel Award from Polymer Processing Society - only two researchers worldwide receive this award annually) vii viii Table of Contents Abstract .......................................................................................................................................... iii Lay Summary ................................................................................................................................. iv Preface..............................................................................................................................................v Table of Contents ......................................................................................................................... viii List of Tables ................................................................................................................................ xii List of Figures .............................................................................................................................. xiii List of Symbols ........................................................................................................................... xxii List of Abbreviations ................................................................................................................. xxiv Acknowledgments........................................................................................................................xxv Dedication .................................................................................................................................. xxvi Chapter 1: Background and Thesis Organization ............................................................................1 1.1 Motivation ....................................................................................................................... 1 1.2 Research objectives and hypothesis ................................................................................ 1 1.3 Thesis organization and novelty ..................................................................................... 5 Chapter 2: Background and Literature Review ...............................................................................8 2.1 Beyond cellulose-based inks: Connecting molecular details to macroscopic realities ... 8 2.1.1 Cellulose: background, chemistry, and structure .................................................... 8 2.2 General classification of cellulose ................................................................................ 12 2.2.1 Cellulose nanofibers (CNF) .................................................................................. 13 2.2.2 Cellulose nanocrystals (CNCs) ............................................................................. 14 2.2.2.1 Structures in CNC suspensions ......................................................................... 15 2.2.3 Bacterial nanocellulose (BNC) ............................................................................. 19 2.3 Cellulose interactions: crafting mono, co-polymer gels ............................................... 19 2.3.1 Physical interaction forces .................................................................................... 20 2.3.2 Chemical interaction forces .................................................................................. 23 2.4 Printability and shape fidelity ....................................................................................... 25 2.4.1 Three-dimensional-printing techniques ................................................................ 25 2.4.2 Shape fidelity ........................................................................................................ 27 2.4.2.1 Initial screening - inverted vial test ................................................................... 29 2.4.2.2 Rheological assessments ................................................................................... 29 ix 2.4.3 Before deposition - filament formation................................................................. 41 2.4.4 After deposition â assessing printability through imaging techniques ................. 42 2.4.5 Cryogels/Aerogels................................................................................................. 46 2.4.5.1 Optimizing the dispersion and gelation processes of nanocellulose for enhanced cryogel performance ......................................................................................................... 47 2.4.5.2 Drying processes ............................................................................................... 48 2.5 Shielding setup and background concept ...................................................................... 50 2.5.1 Transmission line theory and conceptual mechanism .......................................... 50 2.5.2 EMI shielding parameters and equations .............................................................. 52 2.5.3 Shielding mechanism evaluation criteria and common mistakes ......................... 56 2.5.4 3D-printed cellulose for EMI shielding application ............................................. 56 Chapter 3: Synthesis and Characterization of Materials ................................................................59 3.1 3D printing .................................................................................................................... 59 3.2 Cryogel preparation ...................................................................................................... 59 3.3 Characterizations........................................................................................................... 59 Chapter 4: Mapping 3D Printability of Ionically Cross-linked Cellulose Nanocrystal Inks: Architecting from Nano to Macro-scale Structure.........................................................................64 4.1 Introduction ................................................................................................................... 64 4.2 Experimental section ..................................................................................................... 66 4.2.1 Materials ............................................................................................................... 66 4.2.2 CNC inks preparation ........................................................................................... 67 4.2.3 3D printing ............................................................................................................ 67 4.2.4 Cryogels preparation ............................................................................................. 68 4.2.5 Characterization .................................................................................................... 68 4.3 Theory ........................................................................................................................... 68 4.3.1 Electrical double layer (EDL) ............................................................................... 69 4.3.2 Derjaguin-Landau-Verwey-Overbeek (DLVO) theory ........................................ 69 4.3.3 Cation charge number ........................................................................................... 69 4.3.4 Cation and anion size ............................................................................................ 70 4.3.5 Hydrogen bond (H-bond) ...................................................................................... 70 4.3.6 Dipole-dipole interactions ..................................................................................... 70 x 4.3.7 Ion-dipole interaction ............................................................................................ 70 4.4 Results and discussion .................................................................................................. 71 4.4.1 Rheological properties .......................................................................................... 71 4.4.1.1 Effect of cation charge number (valence) ......................................................... 76 4.4.1.2 Inter- and intramolecular interactions ............................................................... 76 4.4.1.3 Effect of ionic radius ......................................................................................... 77 4.4.2 Materials shaping .................................................................................................. 79 4.4.3 Tailoring mechanical performances of hydrogels and structural properties of the cryogels 83 4.5 Conclusion .................................................................................................................... 86 Chapter 5: Crystalline and Hairy Nanocelluloses for 3D-printed Hydrogels and Strongly Structured Cryogels .......................................................................................................................88 5.1 Introduction ................................................................................................................... 88 5.2 Experimental section ..................................................................................................... 90 5.2.1 Materials ............................................................................................................... 90 5.2.2 Synthesis of ENCC ............................................................................................... 90 5.2.3 Preparation of nanocrystalline and hairy nanocrystalline cellulose hybrid inks ... 91 5.2.4 3D printing ............................................................................................................ 91 5.2.5 Cryogels ................................................................................................................ 91 5.2.6 Characterization .................................................................................................... 92 5.3 Results and discussion .................................................................................................. 92 5.3.1 Carboxylated hairy nanocellulose synthesis ......................................................... 92 5.3.2 Surface properties ................................................................................................. 92 5.3.3 Morphology........................................................................................................... 93 5.3.4 Size distribution and surface charge ..................................................................... 95 5.3.5 Rheology and 3D printing ..................................................................................... 96 5.3.6 Material shaping.................................................................................................. 100 5.3.7 Microstructure and mechanical performances of 3D-printed CNC/ENCC cryogels 102 5.4 Conclusion .................................................................................................................. 104 xi Chapter 6: Surfactant-mediated Highly Conductive Cellulosic Inks for High-resolution 3D Printing of Robust and Structured Electromagnetic Interference Shielding Cryogels ................106 6.1 Introduction ................................................................................................................. 106 6.2 Experimental section ................................................................................................... 109 6.2.1 Materials ............................................................................................................. 109 6.2.2 Sample preparation ............................................................................................. 109 6.2.3 3D printing and cryogel preparation ................................................................... 110 6.2.4 Characterization .................................................................................................. 110 6.3 Results and discussion ................................................................................................ 110 6.3.1 Interfacial interaction mechanisms ..................................................................... 110 6.3.2 Rheology and 3D printing ................................................................................... 112 6.3.3 Morphology and microstructure analysis ............................................................ 117 6.3.4 Mechanical and thermal insulation performances .............................................. 118 6.3.5 Electromagnetic interference (EMI) shielding performance .............................. 121 6.4 Conclusion .................................................................................................................. 124 Chapter 7: Multifunctional 3D-Printed Cellulosic Constructs via a Sequential Cold Chemical Vapor Polymerization Technique ................................................................................................125 7.1 Introduction ................................................................................................................. 125 7.2 Results and discussion ................................................................................................ 127 7.2.1 Design principles: from 3D printing to cold chemical vapor polymerization .... 127 7.2.2 Evaluating 3D printability, volume retention, density, and water uptake .......... 129 7.2.3 Characterization: after printing ........................................................................... 131 7.2.4 Morphological assessments ................................................................................ 133 7.2.5 Electromagnetic interference (EMI) shielding. ................................................... 136 7.3 Conclusion .................................................................................................................. 139 Chapter 8: Conclusion and Future Work .....................................................................................140 8.1 Summary of main findings.......................................................................................... 140 8.2 Future work recommendations ................................................................................... 142 Bibliography ................................................................................................................................144 xii List of Tables Table 4-1: Materials' information ................................................................................................ 67 Table 4-2: Yield stress and GÊ¹/GÊº ................................................................................................ 74 Table 4-3: The acquired parameters of the power-law model via fitting procedures for CNC inks containing various salts. ........................................................................................................ 75 Table 4-4: Properties of the cations 192,213 ................................................................................... 79 Table 4-5: Yield stress, storage modulus over loss modulus, line width (hydrogel and cryogel state), and area of 3D-printed samples over designed shape (%). ................................................ 80 Table 4-6: Sample shrinkage characteristics ............................................................................... 85 Table 5-1: Specification of the prepared CNC/ENCC hybrid inks. ............................................. 91 Table 5-2: DLS/ELS measurements for the prepared samples (without salt).............................. 96 Table 5-3: Yield stress and storage modulus over loss modulus for different samples. ............ 100 Table 6-1: Naming and specifications of samples .................................................................... 112 xiii List of Figures Figure 1-1: Roadmap of this thesis. ............................................................................................... 7 Figure 2-1: (a) The plant cell wall structure consists of lignin, hemicellulose, and cellulose. Reproduced with permission 16. Copyright 2022, Elsevier. (b) Cellulose, cellobiose, and anhydroglucopyranose structures. (c) α and β form the linkages. (d) gt, gg, and tg conformers. Reproduced with permission 3. Copyright 2010, American Chemical Society. (e) Crystal structures of cellulose Allomorphs Iα, Iβ, II, and III. Carbon atoms are denoted by gray spheres, oxygen atoms by red, and hydrogen atoms by white. The hydrogen bonds interconnecting these atoms are illustrated with dashed lines. Reproduced with permission 17. Copyright 2020, Elsevier. ........... 12 Figure 2-2: Self-assembly of CNCs in a suspension at rest with increasing concentration (upper row) and corresponding assembly in shear (bottom row). Reproduced with permission 46. Copyright 2021, American Chemical Society. (b) Polarized optical microscopy of the sonicated and nonsonicated CNC suspensions and Figure representation of the CNC particle arrangement in the isotropic, chiral nematic, and gel phases. Reproduced with permission 49. Copyright 2022, American Chemical Society. (c) Figure showing how the domain alignment. Reproduced with permission 45. Copyright 2022, Wiley-VCH. (d) Figure of the DIW 3D printing and a 3D-printed âgecko.â Reproduced with permission 45. Copyright 2022, Wiley-VCH and (e) a Figure of the printing pattern and pictures of the evolution of the anisotropic actuation of the printed structures. Reproduced with permission 52. Copyright 2021, Elsevier........................................................... 18 Figure 2-3: Possible physical interaction forces in cellulose inks for DIW printing. .................. 22 Figure 2-4: Different printing techniques: (a) Nozzle-based, (I) meniscus-guided printing (MGP), (II) inkjet printing (IGP), and direct ink writing (III). (b) light-based printing, (IV) SLA, (V) TLP, and (VI) DLP. ............................................................................................................................... 27 Figure 2-5: Detail of different printing techniques: (a and b) are light-based printing, (a) TLP or two-photon laser printing, (b) computed axial lithography (CAL), (c-e) nozzle-based printing, (c) rotational printing, (d) triple-nozzle printing, (e) co-axial printing. ............................................. 27 Figure 2-6: (a) Inverted vial test as an initial screening test to evaluate the quality of the ink before printing, (a1-a10) rheological assessments before 3D printing; (a1 and a2) the viscosity versus shear rate for different types of materials, (a3) behavior of solid, gel and liquid-like materials, (a4 and a5) flow test, (a6 and a7) different methods to determine yield stress from a strain sweep test include: in method a6, the possible yield point is identified at the intersection of the G' and the G''. xiv According to another method (a7), the yield point is defined as the point where there is a 5-10% reduction in the G', (a8 and a9) elastic recovery, (a10) frequency sweep, (b) 3D printing fidelity assessments by watching a single filament, (b1) droplet formation due to lack of proper rheological properties, (b2) uniform filament formation, (b3) non-uniform filament formation due to high viscosity or yield stress, (c) 3D printing fidelity assessments by using optical imaging, (c1) comparing the diameter of a single filament at different points, (c2 and c3) top view of uniformity assessments of a single filament, (c4) transverse pore geometry with an optimal rectangular shape for ideal filament stacking (A=1), along with (c5 and c6) top and 3D views of multilayer 3D-printed sample showing the circularity of filaments. ................................................................................ 35 Figure 2-7: A summary/roadmap of the required rheological characterizations before printing. 35 Figure 2-8: (a) Top: Steady-state viscosity as a function of shear rate, (a) bottom: Rheological storage and loss modulus as a function of oscillation stress, (b) three-interval thixotropy test of the printed cellulose ink (7 wt%), (c) printability diagram as a function of cellulose concentration and nozzle diameter, including the relationship between nozzle diameter, pressure, and speed, along with 3D products printed using cellulose inks of different concentrations. The inset shows an image of the printed cellulose filament just after extrusion and (d) images of DIW-printed cellulose objects with fully 3D configurations. Reproduced with permission 134. Copyright 2022, Elsevier........................................................................................................................................................ 37 Figure 2-9: (a) Diagram illustrating the rheological properties of ink necessary for DIW 3D printing, (b) graph showing viscosity as a function of shear rate, with an inset displaying viscosity at a shear rate of 10â2 s-1, (c) plot of viscosity changes over time when alternating between low (10â2 s-1) and high shear rates (102 s-1), (d) graph of Gâ² and Gâ³ versus oscillatory stress, (e) filament width at various printing speeds under 45 KPa air pressure, with insets showing digital images of the extruded filaments, (f) various 3D-printed structures using MXene/CNF10 ink. Reproduced with permission 136. Copyright 2023, Wiley-VCH. (g) Viscosity versus shear rate with insets showing digital images of pure MXene, MXene/CNF5, and MXene/CNF10 gel inks, (h) shear stress versus shear rate fitted by the power law, and (i) viscosity changes over time for alternating low (10â2 s-1) and high shear rates (102 s-1). Reproduced with permission 135. Copyright 2022, Wiley-VCH. .................................................................................................................................. 39 Figure 2-10: (a) Digital images showing 3D-printed structures, each about 10 mm in height, using 4:1 WPUCNF/CNF inks with 20%, 30%, and 40% dewatering levels, (b) viscosity versus shear xv rate for 4:1 WPUCNF/CNF inks at 20%, 30%, and 40% dewatering, and (c) loss and storage moduli versus oscillatory stress for 4:1 WPUCNF/CNF inks at 20%, 30%, and 40% dewatering. Reproduced with permission 137. Copyright 2021, Elsevier. (d) Storage modulus and loss modulus as a function of the strain (γ) and angular frequency, (e) optical images of different inks forming droplets to the filament by changing rheological properties, and (f) 3D-printed samples with (1) 10 layers, (2) 50 layers and (3) 100 layers. Reproduced with permission 138. Copyright 2019, Springer Nature. ............................................................................................................................ 41 Figure 2-11: (a) Evaluation of printing quality by optical images and profilometers. Reproduced with permission 58. Copyright 2023, American Chemical Society. (b) Fluorescent microscopic images displaying the cross-sectional shapes of printed lines under various CNF matrix conditions: 0.01 mmol/g with 0.75%, 1.14 mmol/g with 0.75%, and 1.14 mmol/g with 1.5%, along with surface profiles of the printed features. (c-h) Analysis of printing accuracy by assessing the sharpness of an angled line, (c) schematic diagram illustrating the measurement of the difference (θd) between the ideal angle (θi) and the actual measured angle (θm), (d) depiction of the overlapping area formed when printing the angled line, (e) variation of θd based on carboxylic content and concentration, (f) yield stress of the CNF matrix as influenced by carboxylic content and concentration, (g) fluorescent microscopic images of printed features at different angles under three CNF matrix conditions: 0.66 mmol/g with 0.75%, 1.14 mmol/g with 0.75%, and 1.14 mmol/g with 1.5%, including a schematic showing the relationship between ink yield stress and matrix yield stress on sharpness development, and (h) variation of θd with different extrusion pressures and rates. Reproduced with permission 104. Copyright 2021, Elsevier. ........................................ 44 Figure 2-12: (a) Evaluation of lateral pores using both qualitative and quantitative methods. Reproduced with permission 142. Copyright 2018, MDPI, (b, c) ImageJ macro process: the CAD model demonstrates mechanical (cylinder) and biological (cube) performance with representative printed samples. A magnified region of interest (ROI) in the CAD model highlights pore size (P), strut size (S), and the distance between strut centers (DS). Captured images show the ROI with pores of interest, which are then converted using a threshold to measure orthogonal distances between adjacent pores (S). Reproduced with permission 103. Copyright 2012, Elsevier, (d) as-printed 40-layer scaffolds with different wt% of CNC using a 200 μm nozzle, along with corresponding top-view optical images. Scale bars are 1 mm. Reproduced with permission 143. Copyright 2024, Elsevier, and (e) Overview and macroscopic characteristics of the bioinks and xvi their corresponding as-printed thin grid scaffolds. Reproduced with permission 144. Copyright 2022, Springer. .............................................................................................................................. 46 Figure 2-13: (a) Figure illustration of the preparation of bidirectional aerogel and SEM images for the cross-section of the bidirectional aerogel, (b) figure illustration of the preparation of unidirectional aerogel and its SEM images for the cross-section, (c) figure illustration of the preparation of common aerogel and its SEM. Reproduced with permission 154. Copyright 2021, Elsevier, and (d) Pressure-temperature diagram significant for hydrogel drying by freeze-drying and supercritical drying to obtain aerogels. .................................................................................. 49 Figure 2-14: (a) Schematic illustration of the preparation steps of MWCNT/CNF inks and their 3D printing toward EM wave shielding; (b) controlling the micro-scale porosity or density of the 3D-printed MWCNT/CNF inks through variating the printing parameters; (c) the average SSE, and (d) EM shielding parameters of the developed shielding systems with different levels of porosities. (e) EM shielding mechanism of the developed 3D-printed constructs with well-defined hierarchical pores. Reprinted with permission from Ref. 160, Copyright 2023, Elsevier. (f) The overall process of the shielding design starts from the MXene synthesis, MXene/CNF ink preparation, and 3D printing of MXene/CNF scaffold toward potent EM shielding constructs with the ability to tune the 3D spatial arrangement and porosity. (g) EM shielding mechanism of the developed hierarchical porous 3D-printed cellulose-based structure. Reprinted with permission from Ref. 161, Copyright 2023, Elsevier. ....................................................................................... 58 Figure 4-1: Figure illustration of 3D printing of inks composed of CNC and given salts. .......... 68 Figure 4-2: Effective parameters and possible interactions between CNCs, water, and salt: (a) Electrical double layer, (b) effect of charge number increase (for chloride salts) on the attractive forces between CNCs, (c) effect of cation radius size increase with the same charge number on the attractive forces between CNCs, (d) possible interaction between CNCs/CaSO4, and CNCs/NaHCO3, (E) inter- and intramolecular forces and H-bond, and (f) dipole-dipole interaction........................................................................................................................................................ 71 Figure 4-3: (a-c) Photographs of the inks stored in inverted vials to show fluidity: (a) Pristine CNC at various concentrations (3 wt.%, 5 wt.%, and 7 wt.%), (b) CNC/salt (3 wt.%/100 mM), (c) CNC/salt (5 wt.%/100 mM), (d, and e) Strain-sweep test at a fixed frequency of 10 radâ s- 1. ..... 72 Figure 4-4: (a-d) Rheological properties of CNC/salt (5 wt.%/100 mM) inks: (a) shear stress-sweep at 25 â. (b and c) frequency-sweep at a fixed shear strain of 0.10%. In figures a, b, and c, xvii the storage and loss modulus are indicated by Gâ² and Gâ²â², respectively, and (d) viscosity curves versus shear rate (Î³Ë). .................................................................................................................... 78 Figure 4-5: Yield stress for all samples (different colors show different CNC concentrations and maximum shear stress is indicated by pink diamond). ................................................................. 79 Figure 4-6: (a) Drop and filaments of the inks depending on the concentration of the CNC in the inks: 3 wt.% CNC inks formed droplets, 5 wt.% CNC inks formed uniform filaments and 7 wt.% CNC inks formed non-uniform filaments, leading to nozzle clogging. (b-k) Digital images of the 3D-printed structures with CNC (5 wt.%) inks containing different types of salt at a fixed loading of 100 mM (for all samples except CNC/NaHCO3, which includes 3 wt.% of CNC and 75 mM salt) (scale bars are 2 mm). ........................................................................................................... 81 Figure 4-7: SEM images of different 3D-printed cryogels with CNC (5 wt.%) inks containing different types of salt at a fixed loading of 100 mM (for all samples except CNC/NaHCO3, which includes 3 wt.% of CNC and 75 mM salt) at different magnifications. ....................................... 82 Figure 4-8: Concentration mapping of the salt-incorporated CNC inks. CNC concentrations are 3, 5, and 7 wt.%, and salt concentrations are 25, 50, 75, and 100 mM. The definition of each printing quality can be found in Section 3.2 of this chapter. ...................................................................... 83 Figure 4-9: SEM micrographs of cryogels (a) CNC (5 wt.%) without salt, (b) CNC/LiCl (5 wt.%/100 mM), (c) CNC/NaCl (5 wt.%/100 mM), (d) CNC/KCl (5 wt.%/100 mM), (e) CNC/NH4Cl (5 wt.%/100 mM), (f) CNC/CaCl2 (5 wt.%/100 mM), (g) CNC/AlCl3 (5 wt.%/100 mM), (g) CNC/MgCl2 (5 wt.%/100 mM), (i) CNC/FeCl3 (5 wt.%/100 mM), (j) CNC/CaSO4 (5 wt.%/75 mM), and (k) CNC/NaHCO3 (3 wt.%/75 mM). ............................................................. 84 Figure 4-10: (a) Images of the 3D-printed cryogels; all scale bars are 5 mm, (b) the size of the 3D-printed cryogels made with CNC/NaHCO3 (3 wt.%/75 mM) at different dimensions (all sizes were measured by ImageJ, and this part (using caliper) is just for further justification), (c) compression stress-strain curves for 3D-printed hydrogels, (d) compressive modulus for different 3D-printed hydrogels, and (e) compressive strength at 50% strain for different 3D-printed hydrogels. ........ 86 Figure 5-1: Illustrative presentation of synthesis of ENCC ......................................................... 91 Figure 5-2: The FTIR spectra of (a) CNC, (b) CNC/ENCC (2:1), (c) CNC/ENCC (1:1),(d) CNC/ENCC (1:2), and (e) ENCC (samples contained no salt). ................................................... 93 Figure 5-3: AFM images of (a) CNC and (b) ENCC particles. ................................................... 94 xviii Figure 5-4: TEM images of (a) CNC, (b) ENCC (c) CNC/ENCC (2:1), (d) CNC/ENCC (1:1), (e) CNC/ENCC (1:2) (samples contained no salt). ............................................................................ 95 Figure 5-5: (a-d) Rheological properties of CNC/ENCC hybrid inks containing salt (NaHCO3): (a) Steady-shear viscosity versus shear rate, (b) storage and loss modulus versus shear stress (stress sweep test), (c) strain sweep test, and (d) frequency-sweep at a fixed shear strain of 0.10%. In figures a-d, the storage and loss modulus are indicated by Gâ² and Gâ²â², respectively. ................... 99 Figure 5-6: (a) Photographs of the hybrid inks stored in inverted vials to show fluidity. (b) Digital images of the 3D-printed structures with ENCC1/CNC1 hybrid inks containing 50 mM NaHCO3. (b2) and (b3) Zoom-in view of as printed ENCC1/CNC1, and (b4) as printed sample after drying. (c) Digital images of the 3D-printed structures with ENCC1/CNC2 hybrid inks containing 50 mM NaHCO3. (c2) and (c3) Zoom-in view of as printed ENCC1/CNC2, and (c4) as printed sample after drying. (d) Digital images of the 3D-printed structures with CNC ink containing 50 mM NaHCO3. (d2) and (d3) Zoom-in view of as printed CNC1, and (d4) as printed sample after drying (all scale bars are 5 mm). ............................................................................................................ 101 Figure 5-7: SEM micrographs of cryogels (a) CNC (10 wt.%) with 50 mM NaHCO3, (b) CNC/ENCC (2:1), and (c) CNC/ENCC (1). ............................................................................... 103 Figure 5-8: Mechanical properties of the 3D-printed cryogels, (a) compressive stress versus strain, and (b) porosity and (c) density of different 3D-printed cryogels. ............................................. 104 Figure 6-1: From biopolymers to 3D-printed conductive cryogels for EMI shielding applications...................................................................................................................................................... 109 Figure 6-2: Different grid sizes (G1: 1 mm distances between grids, G2: 2 mm distances between grids, and G3: 3 mm distances between grids). .......................................................................... 110 Figure 6-3: (a) Proposed mechanism for the polymerization of EDOT. (b) Proposed interactions: (I) PEDOT doped with DBSA, (II) CNC-PEDOT, and (III) CNC-DBSA. (c) Figures of hydrophobic attraction of DBSA onto the CNC surface, and (d) Fe3+ cation or PEDOT positive bridging between DBSA and CNC. ............................................................................................ 112 Figure 6-4: (a-d) Ink rheological characterization: (a) the viscosity of the prepared samples versus shear rate (s-1). (b) Storage and loss modulus of the inks versus shear stress (Pa). (c) Yield stress for all samples. (d) Storage and loss modulus of the inks as a function of frequency (rad/s), and (e) gel-like behavior of the inks. (e1-e6) CNC 3 wt%, CNC 3 wt%/DBSA, CNC 3 wt%/DBSA/PEDOT (1 wt% EDOT), CNC 3 wt%/PEDOT (1 wt% EDOT) without DBSA, CNC xix 3 wt%/PEDOT (0.75 wt% EDOT) with DBSA, and CNC 3 wt%/PEDOT (0.75 wt% EDOT) with DBSA, respectively. (f) Photograph of the fabricated microstructures on various substrates. From left to right: PEDOT structure on polycarbonate, PEDOT structure on glass, and UBC logo on polycarbonate. (g) The progression of the structures formed by the deposition of 55 layers (all scale bars are 25 mm). ................................................................................................................ 115 Figure 6-5: (a) Figure illustration of 3D printing. DBSA acts as a surfactant and causes PEDOT to disperse better into CNC crystals. (b) Pictures of the extrusion printed line. (c-f) The width variation and average width of the extrusion printed lines of (c) C3P0.1, (d) C3P0.5, (e) C3P0.75, and (f) C3P1. (g) The result of the profilometer for C3P0.75, the x-axis, and the y-axis are the width and height of each line, respectively. (h) Inks printing quality (y-axis is filament diameter over needle diameter) compared with other previously published papers, including MAHM: Methacrylated hyaluronic acid, dECM= Decellularized extracellular matrix-based bio-ink, CNF: Cellulose nanofiber, PVA/GO-HA: Polyvinyl alcohol/Graphene oxide- Hyaluronic acid, PEDOT:PSS: Poly(3,4-ethylenedioxythiophene):Polystyrene sulfonate 226,254â262. ................... 116 Figure 6-6: (a) Different views of cubic grids printed with C3P0.75. (b) FESEM micrographs of C3P0.75 at different scales. (c, d) FESEM micrographs of (c) pure CNC and (d) C3P0.75 cryogels, and (e, f) EDX mapping result for S, C, Cl, Fe, and O elements. ............................................... 118 Figure 6-7: (a) Compressive stress-strain curves. (b) Compressive strength values of samples at 40% and 80% strain and compressive modulus of cryogels. (c) Images of a piece of lightweight 3D-printed C3P1 supported by a flower and a piece of robust, lightweight, and conductive C3P1 cryogel (0.64 g) supporting a 2.5 kg autoclave reactor. (d) Porosity and density of different samples, and (e) comparison of Young's modulus vs. density of the present work with the literature, PU: Polyurethane, MFC: Microfibril cellulose, CNF: Cellulose nanofiber, MCM: Microcrystalline cellulose, T-CNF: Tempo oxidize cellulose nanofiber, CNF-F: Cellulose nanofiber foam, RGO: Reduced graphene oxide, Ca: Calcium, CNC: Cellulose nanocrystal 167,169,170,194,269â279. .......... 119 Figure 6-8: (a) Thermal insulation performance of molded conductive cryogels with (C3P1) and without the PEDOT (C3). (b) 2D temperature gradient distribution of the C3 and C3P1 cryogels at the initial stage and after 60 min. (c, d) A flower placed on the (c) pure CNC cryogel heated by butane torch for 1 minute and (d) C3P1 cryogel composite heated by butane torch for 5 minutes...................................................................................................................................................... 121 xx Figure 6-9: (a) Electrical conductivity and EMI shielding effectiveness (EMI SE) (average SET, SEA, and SER) of the C3P1-G2 composites with different PEDOT contents. (b) EMI SE of the C3P1-G2 composites with different PEDOT contents over the X-band frequency range (8.2-12.4 GHz). (c) R and A coefficients of the C3P1-G2 composites with different PEDOT contents. (d) EMI SE (average SET, SEA, and SER) of the C3P1 composites with different grids. (e) The values of EMI SE for the C3P1 composites with a different grid over the X-band frequency range (8.2-12.4 GHz), (f) R and A coefficients of the C3P1 composites with different grids. (g) EMI shielding mechanism (g1: reflection mechanism by electron migration and hopping and multiple internal reflection inside the micropores of the shields, g2: the effect of different grid sizes on the EMI shielding mechanism), and (h) Specific EMI shielding effectiveness versus A coefficient for different composites; CNF: Cellulose nanofiber, AgNW: Silver nanowires, CNT: Carbon nanotube, CNC: Cellulose nanocrystal 218,288â297. MXene/rGO, Ti3C2Tx-PEDOT:PSS, CNT/Chitosan, TiO2-Ti3C2Tx/rGO/PDMS, MXene/CNT/polyimide 298â305. (Grid G1: 1 mm distance between grids, grid G2: 2 mm distance between grids, and grid G3: 3 mm distance between grids). ............................................................................................................................ 123 Figure 7-1: (a) Figure illustration of the 3D printing process of CNF, enabling controlling macro-scale porosity of the 3D-printed constructs. (b) The progression of the structures formed by the deposition of 35 layers. (c: I-IV) Photograph of the fabricated 35-layer microstructures, (c-I) pure CNF, (II) CNF incorporated with FeCl3, (III) freeze-dried CNF incorporated with FeCl3, (IV) conductive CNF/PEDOT; scale bar: 1 cm. (d) Figure illustration of the CCVP method enabling the polymerization of the EDOT monomer on the oxidative FeCl3 (I) at the initial step, and (II) after eight days, generating a lightweight conductive 3D-printed CNF/PEDOT construct. Additionally, the middle portion of (d) illustrates a Figure representation of the polymerization of EDOT (depicted in black) on the surface of CNF through the use of Fe3+ bridges (represented as golden spheres). .......................................................................................................................... 129 Figure 7-2: (a) 3D-structured CNF hydrogel after impregnation with FeCl3. (b) Free-standing 3D-printed CNF decorated with FeCl3, (c) after freezing, lyophilization, and drying steps. (d) Zoom-in images of the high-quality 3D-printed sample after lyophilization. (e) Ultra-lightweight 3D-printed conductive CNF/PEDOT structure on the very light hair of a cactus. (f) A view of the conductive 3D-printed CNF/PEDOT structure capable of lighting up an LED lamp. Scale bars in parts (a, c, and e) are 2 cm, and in part d, the scale bar is 2 mm. General characterization of the xxi CNF and cold chemical vapor polymerized CNF/PEDOT monoliths at 43% RH, including (g) volume retention value, (h) density, (i) water uptake, and (j) cyclic water uptake of CNF 1/EDOT 1 monoliths at 43% RH for up to 10 cycles (during 10 days). The monoliths were firstly saturated at 43% RH for 1 day, followed by drying at 70 â for 8 h to complete one absorption-drying cycle. (k) FTIR spectroscopy, and (h) micro-Raman spectroscopy of CNF, CNF/Fe, and CNF/PEDOT. In all Figures labeled g through i, the color coding is as follows: green represents TEMPO-oxidized CNF, orange denotes TEMPO-oxidized CNF treated with FeCl3, and blue signifies the conductive TEMPO-oxidized CNF that has been integrated with PEDOT. ................................................. 133 Figure 7-3: (a, b) SEM micrograph of pure CNF and CNF/Fe at different magnifications, showcasing their abundant micro-scale porosities, respectively, and (c) SEM micrographs of 3D-printed conductive CNF/PEDOT at various magnifications. ..................................................... 135 Figure 7-4: (a-c) SEM micrographs of CNF/PEDOT monolith reveal the formation of the PEDOT throughout the fibrillated cellulose-based construct, (d) the EDX mapping of the CNF/PEDOT sample reveals a homogeneous distribution of elements across the structure, and (e) SEM micrographs of PEDOT on the CNF structure. ........................................................................... 136 Figure 7-5: (a) Controllable macro-scale and micro-scale porosity levels of the 3D-printed CNF/PEDOT constructs; different grid sizes for 3D-printed samples (G1: 1 mm distances between grids, G2: 2 mm distances between grids, and G3: 3 mm distances between grids). The number of printed layers is 10 for EMI shielding measurement. (b) Electrical conductivity and EMI shielding effectiveness (EMI SE) (average SET, SEA, and SER) of the conductive CNF/PEDOT-G2 constructs with different PEDOT contents. (c) R and A coefficients of the CNF/PEDOT-G2 construct with different PEDOT contents. (d) EMI SE of the conductive CNF/PEDOT-G2 structures with different PEDOT contents over the X-band frequency range (8.2-12.4 GHz). (e) Shielding characteristics (average SET, SEA, and SER) of the 3D-printed CNF1-PEDOT constructs with different grid sizes. (f) R and A coefficients of the CNF1-PEDOT constructs with different grid sizes. (g) The total EMI SE of the 3D-printed CNF1/PEDOT structures with different grid sizes over the X-band frequency range (8.2-12.4 GHz). (h) EMI shielding mechanism of the developed 3D-printed samples. (i) The absorbance of the shields compared with previous practices in the literature. ........................................................................................................................... 138 xxii List of Symbols A Absorbance d Thickness of sample dB Decibel (unit of shielding effectiveness) e Charge of an electron E Electric field EI Incident electric field ET Transmitted electric field f Frequency of electromagnetic wave Gâ² Storage modulus Gâ²â² Loss modulus H Magnetic field I Electric current J Current density PI Incident wave power Pr Pore geometry PR Reflectance wave power PT Transmitted wave power R Reflectance S Siemens (unit of electrical conductivity) SEA Shielding by absorption SEM Shielding by multiple reflections SER Shielding by reflection SET Total shielding effectiveness SSE/t Specific shielding effectiveness S11 Ratio of reflected power to incident power in port 1 S12 Ratio of transmitted power from port 1 to port 2 to incident power in port 1 S21 Ratio of transmitted power from port 2 to port 1 to incident power in port 1 S22 Ratio of reflected power to incident power in port 2 T Transmittance Zâ² Real impedance xxiii Zâ²â² Imaginary impedance ð Angular frequency ð¼ Attenuation constant Ï Conductivity Æ* Complex viscosity Ï Density Ïa Density of air-dried samples Ïf Density of freeze-dried samples ð Dielectric permittivity É³ Impedances É³ð Impedances of air μ Permeability ?Ì? Shear rate γ0 Shear strain Ïr Shear stress δ Skin depth λ Time constant η Viscosity Ïy & ðð Yield stress Æ0 Zero shear viscosity xxiv List of Abbreviations 2D Two dimensional 3D Three dimensional AFM Atomic force microscopy CNC Cellulose nanocrystal DIW Direct ink writing EDS Energy dispersive spectrometer ENCC Hairy cellulose nanocrystal EMI Electromagnetic interference EMI SE Electromagnetic interference shielding effectiveness FESEM Field emission scanning electron microscope FTIR Fourier-transform infrared spectroscopy HRTEM High-resolution transmission electron microscope PEDOT Poly(3,4-ethylenedioxythiophene) PET Polyethylene terephthalate SE Shielding effectiveness SEM Scanning electron microscopy SSE Specific shielding effectiveness TEMPO-CNF 2,2,6,6-tetramethylpiperidine-1-oxyl-cellulose nanofiber TGA Thermogravimetric analyzer VAF Vacuum-assisted filtration VNA Vector network analyzer XRD X-ray diffraction xxv Acknowledgments I take this moment to express my profound gratitude for the support and guidance I have received throughout the past four years. My deepest thanks are extended to my esteemed supervisor, Dr. Mohammad Arjmand. His exemplary mentorship and continual source of inspiration have greatly enriched my academic journey. Dr. Arjmand's relentless dedication and extraordinary work ethic have made my experience as a PhD student exceptionally rewarding. I am also profoundly indebted to Dr. Milad Kamkar, a former postdoctoral researcher in our group and now an assistant professor at the University of Waterloo. His early guidance and astute advice laid a robust foundation for my PhD journey, ensuring a seamless and fruitful academic pursuit. The successful completion of this thesis owes significantly to collaborative efforts with various international universities. I am particularly grateful for the mentorship and support provided by luminaries such as Professor Orlando J. Rojas from the Bioproducts Institute at the University of British Columbia and Professor Abbas Milani from the University of British Columbia, Okanagan campus. These collaborations have not only broadened my academic horizons but also ingrained in me the value of teamwork, an ethos I will carry throughout my academic career. xxvi Dedication To my beloved wife, Zhaleh, for her unwavering support, constant motivation, and ceaseless inspiration. To my parents, Parvin and Hasan, for their endless support and patience. I am profoundly grateful for all your sacrifices and efforts; without them, I would not be where I am today. To my sisters, Mahnaz and Mehri, who are not just siblings but also my closest friends. 1 Chapter 1: Background and Thesis Organization Creating bio-inspired structures with low density, high porosity, and mechanical strength for practical applications remains a challenge. Traditional techniques, including vacuum-assisted filtration and ink-jet printing, face limitations like low aspect ratio patterns and viscosity requirements. Additive manufacturing (AM), particularly 3D extrusion printing, offers a solution that is able to produce complex microstructures efficiently. The primary aim of this thesis is to investigate the utilization of bio-based polymers, particularly cellulose, in crafting conductive structures. This inquiry spans three distinct scales: nano, micro, and macro. Techniques vary across these scales, involving diverse synthesis methods for nano-scale structures, modifying drying conditions to yield cryogels with varied pore sizes at the micro-scale, and employing extrusion 3D printing for the macroscale architecture. The study critically evaluates the effect of these scales on the EMI shielding effectiveness (EMI SE) of the developed cryogels. The subsequent sections will thoroughly discuss the motivations behind this research and chronologically detail the investigative process as presented in this thesis. 1.1 Motivation In the current research environment, the focus is shifting from traditional conductive metals to synthetic conductive structures, mainly due to the limitations of metals in modern electronic applications such as EMI shielding. The weight, bulkiness, and corrosion susceptibility of metals, along with their tendency to create secondary pollution through reflection-based shielding, have led to a preference for absorption-based, lightweight alternatives. This thesis explores the potential of absorption-based 3D-printed conductive cryogels for EMI shielding, beginning with the optimization of 3D printing processes for cellulose and its ink rheology. The aim is to establish a connection between rheological properties and the structural integrity of the final product, underscoring the critical role of structural design in the development of multifunctional materials. Our work then progresses to advanced conductive 3D printing cryogels, highlighting the essential role that structural design plays in the development of multifunctional materials. 1.2 Research objectives and hypothesis To accomplish the above-mentioned goals, the research is structured into three main streams (nano-, micro-, and macro-scale design), each with specific sub-objectives, hypotheses, and tasks as follows: 1- Macro-scale design 2 This stream has two sub-objectives: 1.a) Investigating rheological properties for high-quality cellulose ink printing Hypothesis: High printing fidelity and shape integrity are achieved within a specific range of cellulose and salt concentrations, which varies depending on the type of salt used. The incorporation of trivalent salts, such as FeCl3, enhances the printing quality of cellulose-based inks by increasing intramolecular and intermolecular interactions within the polymer matrix. Trivalent salts introduce stronger ionic crosslinking, leading to better structural cohesion during the printing process, which results in improved shape retention and precision. This enhanced network formation stabilizes the ink during extrusion, allowing for more consistent layer deposition and better control over the final printed geometry. By optimizing the salt type and concentration, the mechanical properties and printability of cellulose inks can be significantly improved. Tasks: To achieve this aim, 10 different salts (monovalent, divalent, and trivalent), including LiCl, KCl, NaCl, MgCl2, CaCl2, CaSO4, AlCl3, and FeCl3, as well as salts capable of forming hydrogen bonds (e.g., NH4Cl and NaHCO3), will be chosen to treat CNC inks physically. The microstructural features of the developed suspensions/hydrogels will be examined via rheological tests. These results will be used to optimize the composition of the CNC/salt inks to achieve high-quality 3D-printed structures, providing a printing map for each salt based on CNC loading and salt content. Finally, using freeze-drying, cryogels derived from the 3D-printed CNC/salt structures will be made to unravel the effect of salt type on the structural stability of the resultant cryogels. Outcome: This sub-objective has been successfully concluded, and its results are presented in Chapters 4 and 5. 1.b) Creating conductive, absorption-based EMI shields via 3D printing with adjustable macro-scale porosity Hypothesis: Varying the grid size in 3D-printed structures from 1 mm to 3 mm will influence the EMI shielding properties by changing the interaction between electromagnetic waves and the internal structure of the material. Larger grid sizes 3 create more open and interconnected pore networks, which lead to increased internal scattering of the waves. This scattering causes electromagnetic energy to be trapped and dissipated within the material, thereby enhancing absorption. As the grid size increases, the path length for wave interaction inside the structure also increases, allowing for multiple internal reflections and enhanced energy dissipation. This transition from a reflection-dominant mechanism, where waves are primarily reflected off the surface, to an absorption-dominant mechanism enhances the material's ability to attenuate electromagnetic waves through absorption without requiring any changes to its composition.This design strategy leverages geometric factors to enhance performance, aligning with established principles in EMI shielding materials. Tasks: To achieve this objective, we will first synthesize CNC/PEDOT ink using the in-situ polymerization method, incorporating DBSA as a surfactant to enhance conductivity, improve dispersion, and optimize the mechanical properties and printability of the ink for high-precision macro architecture. Next, we will 3D print three different grid sizes (1 mm, 2 mm, and 3 mm) using this ink. The printed structures will be converted into cryogels using a freeze dryer. These cryogels will then be tested using a Vector Network Analyzer (VNA) to evaluate their EMI shielding properties, with a particular focus on the mechanism of shielding. Outcome: This sub-objective has been successfully concluded, and its results are presented in Chapters 6 and 7. 2- Micro-scale design To examine the influence of pre-drying conditions on the micro-scale porosity of cryogels and their subsequent impact on EMI shielding performance, this study aims to adjust the EMI shielding mechanism by precisely controlling micropore size within the cryogel structure. Modifying the micropore sizes is expected to enhance the effectiveness of shielding by optimizing the reflection and absorption processes that are critical for EMI attenuation. 4 Hypothesis: We hypothesize that modifying pre-drying conditions, such as utilizing liquid nitrogen, varying freezing methods, or adjusting the direction of liquid nitrogen application, will influence the micro-scale porosity of cryogels, thereby impacting their EMI shielding performance. Specifically, by increasing pore sizes through controlled modifications, we anticipate enhanced internal scattering and electromagnetic wave trapping, driving a shift in the shielding mechanism towards absorption.This is supported by the fact that larger pore structures increase the surface area for wave interaction, leading to multiple internal reflections and higher energy dissipation. This strategic manipulation of porosity will contribute to the development of more effective absorption-based EMI shielding materials. Tasks: To systematically explore the relationship between the microscale porosity of cellulose cryogels and their EMI shielding effectiveness, we will conduct the following tasks: First, we will synthesize conductive cellulose nanofiber constructs. Next, we will adjust the pre-drying conditions to create variations in pore sizes within the constructs. Following this, we will use a freeze dryer to convert the constructs into cryogels with different pore sizes and structures. Once the cryogels are prepared, we will utilize SEM to examine and measure the pore sizes. Finally, we will evaluate the EMI shielding effectiveness of these cryogels using a VNA within the X-band frequency range (8.2 â 12.4 GHz). Outcome: This sub-objective has been successfully concluded, and its results are presented in Chapter 7. 3- Nano-scale design Employing conductive polymers, like PEDOT, will optimize electrical conductivity and maintain low density, reducing the need for excessive filler material and preventing nanoparticle agglomeration. Hypothesis: Our hypothesis suggests that different synthesis methods result in varied morphologies, and by controlling synthesis conditions and polymerization rates, we can improve the conductivity of the material. FeCl3, as identified in 5 Chapter 4, is highly effective in improving print quality due to its multiple roles. It crosslinks the cellulose-based ink, strengthening the printed structure and enhancing its resolution. Additionally, FeCl3 is a hygroscopic salt, meaning it absorbs moisture from the environment. This absorbed moisture interacts with the cellulose matrix, reducing intermolecular hydrogen bonding and acting as a plasticizer. Increasing the mobility of cellulose chains introduces flexibility to the structure, allowing it to deform more easily under stress. It also catalyzes the oxidative polymerization of PEDOT, promoting the formation of conductive pathways. This hypothesis explores the potential to leverage these combined effects to enhance both the mechanical and conductive properties of the material. Tasks: We begin with the 3D printing of cellulose nanofiber (CNF), and then these structures are seeded with Fe3+ ions (using FeCl3 salt). Fe3+ ions serve as the initiator for the polymerization of EDOT monomers via cold chemical vapor polymerization (CCVP), resulting in a conductive network of poly(3,4-ethylenedioxythiophene) (PEDOT) throughout the structure. This technique yields 3D-printed CNF structures that are electrically conductive, mechanically robust, and remarkably flexible. Outcome: This sub-objective has been successfully concluded, and its results are presented in Chapter 7. 1.3 Thesis organization and novelty Our journey in this thesis begins with a comprehensive literature review in Chapter 2. Here, we investigate the fundamentals of cellulose, provide background on EMI shielding, and offer an overview of 3D printing and rheology. In Chapter 3, we will discuss all materials, methods, and characterizations used in this thesis. Chapter 4 takes us through the correlation of rheology and 3D printing. Overall, this chapter (1) provides a detailed view of the 3D structuring of CNC/salt inks, (2) focuses on the interrelationship of the rheological properties and shape fidelity of 3D-printed CNC-based inks, and (3) broadens the horizon of the additive manufacturing of CNC-based inks and the design of new functional materials. These 3D-printed scaffolds can be used as a versatile substrate for a multitude of applications, including gas adsorption, wastewater treatment, and electromagnetic interference shielding. In Chapter 5, we take advantage of carboxylate cellulose 6 nanocrystals to boost the mechanical properties of 3D-printed cellulose cryogels. By fine-tuning the rheological properties, hybrid inks (CNC/ENCC) with high printing fidelity are achieved. We also investigate the morphological and mechanical features of the cryogels derived from the cellulose nanocrystal inks. Thus, this chapter proposes new cellulosic inks and cryogels that have both great printing quality and notable mechanical qualities. In chapter 6 we introduce one of the pioneering instances of 3D-printed structures featuring cellulose/PEDOT. These structures find applications in the areas of EMI shielding. We grapple with three key challenges inherent to extrusion printing: poor electrical conductivity, inferior mechanical strength, and low printing resolution. The hydrogel inks, namely CNC/PEDOT-DBSA, feature a unique formulation containing well-dispersed cellulose nanocrystals decorated by PEDOT coupled with dodecylbenzenesulfonic acid (DBSA). The rheological properties are precisely engineered by manipulating the solid content and the intermolecular interactions among the constituents, resulting in 3D-printed structures with excellent resolution. Importantly, it is shown that the shielding mechanism of the 3D cryogels could be manipulated by their geometrical features, unraveling the undeniable role of additive manufacturing in materials design (macro-scale design). Chapter 7 introduces a fresh perspective on EMI shielding structures, focusing on cryogels. In our pursuit of cryogel fabrication, we tried to tune micro porosities by changing pre-drying conditions (micro-scale design). To do so, we change the freezing temperature before freeze-drying. By this, the size and shape of the pore sizes change, and consequently, the shielding mechanism changes from reflection to absorption. Aside from that, in this chapter, we follow our third hypothesis, which is taking advantage of the triple functions of FeCl3. In this chapter, we introduce a versatile multistep approach titled "cold chemical vapor polymerization" (CCVP) for making conductive 3D-printed constructs. This method starts with direct ink writing of an engineered TEMPO-oxidized CNF hydrogel, followed by impregnation with Fe3+ oxidation sites, freezing and lyophilization, and subsequent CCVP to integrate interconnected conductive pathways of PEDOT throughout the 3D structure. This process enables the precise customizability of electrical conductivity, allowing the generation of highly conductive (560 S/m) and mechanically flexible (70% compressible) patterned constructs. This shape, mechanical, and electrical tunability broadens the application horizon of 3D-printed bio-based structures. Herein, we capitalize on the applicability of these advanced multifunctional 3D constructs in absorption-dominant shields (absorbance (A) of 0.71). The introduced method paves the way for generating functional ultra-7 lightweight systems through green and sustainable approaches. What sets these cryogels apart is their significant advantages: they require no chemical or thermal reduction or extensive pre-processing for high conductivity. In terms of EMI shielding attributes, the presence of micro- and macro-scale porosities substantially enhances the absorption characteristics of these cryogels. This pivotal observation underscores the pivotal role of structural design in shaping the EMI shielding capabilities of conductive materials, which serves as the core theme of this thesis. Finally, Chapter 8 contains the key conclusions as well as recommendations and suggestions for future studies. Figure 1-1 shows the overall roadmap and structure of this thesis. Figure 1-1: Roadmap of this thesis. 8 Chapter 2: Background and Literature Review 2.1 Beyond cellulose-based inks: Connecting molecular details to macroscopic realities 2.1.1 Cellulose: background, chemistry, and structure Cellulose, the most prevalent biopolymer on Earth, constitutes roughly 33% of all plant matter 1. The core of this biopolymer's abundance lies in the plant cell wall, which primarily consists of cellulose, hemicellulose, and lignin (Figure 2-1a). Serving as the backbone of the cell wall, these components ensure both strength and protection 2. The role of cellulose in plants cannot be disputed, considering its engagement in structures. However, discoveries and vast applications in the human world are also of immense interest. Its life can be traced through various ordinary products ranging from paper, textiles, and food 3. Its journey of scientific understanding began in 1838 when Anselme Payen first isolated cellulose while exploring the cell walls of plants 4. He discovered a robust, fibrous material, a remnant that emerged after subjecting diverse plant tissues to a series of treatments with acids and ammonia, followed by extraction using water, alcohol, and ether. This substance, which Payen identified through elemental analysis to have the molecular formula C6H10O5, showed remarkable isomerism with starch 4. The following year, in 1839, the term âcelluloseâ was introduced in a report by the French Academy, celebrating Payen's work 5. Long before its scientific discovery, it was present in everyday life as wood, cotton, and other plant fibers. These natural resources were central to human survival and progress, offering energy, building materials, and clothing. The use of cellulose can be traced back to the days of the Egyptian papyri, significantly shaping a considerable portion of human culture 5. Cellulose has been used as a chemical raw material for about 0.5 million years. A significant milestone in this field was the formation of cellulose nitrate through its reaction with nitric acid. This reaction was pivotal in the synthesis of the first thermoplastic polymer material, celluloid, developed by the Hyatt Manufacturing Company in 1870 6. Celluloid, with camphor as a plasticizer, was a testament to the industrial-scale production of new materials through the chemical modification of cellulose 6. The fascination with cellulose stems from its unique structure, which combines carbohydrate and polymer chemistry. Its macromolecular composition is made up of repeating glucose units, gives it specificity and an array of diverse architectures, reactivities, and functions. Unlike low molar mass carbohydrates, the reactions and properties of cellulose are influenced by its intermolecular interactions, cross-linking reactions, chain lengths, and the distribution of 9 functional groups along its chains. The polymeric structure of cellulose was discovered in the 1920s by Hermann Staudinger 3. Through his experiments, he disproved the idea that cellulose was just a collection of glucose units. Instead, he showed that these units are chemically bonded to form long molecular chains (each glucose unit in a cellulose chain is connected to the next by a covalent C-O-C bond, with two hydrogen bonds surrounding it). He found that cellulose is a linear polymer consisting of β-1,4-linked D-glucose units (Figure 2-1b) 3. In other terms, the glucose units within the cellulose chain connect via β-1,4 glycosidic bonds, with the β crystalline form presenting a parallel chain structure, leading to the formation of equatorial linkages. On the other hand, the α crystalline form results in axial linkages (Figure 2-1c) 3. X-ray diffraction studies subsequently validated this significant proposition by Staudinger. The α crystalline form is dominant in cellulose derived from bacteria (bacterial cellulose). Additionally, in terms of the degree of crystallinity, bacterial cellulose holds a higher value (>80%) than that of plant-based cellulose. As mentioned, the β-1,4 glycosidic bond is a type of chemical bond that forms between two glucose units. The bond is formed when the hydroxyl group on the first glucose unit reacts with the aldehyde group on the second glucose unit 7,8. The cellulose molecule demonstrates chemical asymmetry: one end showcases a chemically reducing functionality (has a free aldehyde group, and it can participate in redox reactions, typically acting as a reducing agent), while the opposite terminus carries a pendant hydroxyl group, known as the nonreducing end. This end of the molecule is typically involved in forming glycosidic bonds with other molecules (Figure 2-1b). According to Hermann Staudinger's hypothesis, cellulose comprises a large amount of anhydroglucose units (AGU) that are covalently bound to one another to form a high molecular weight linear polymer. Each AGU of the cellulose structure contains three active hydroxyl groups: a primary one at C6 and two secondary ones at C2 and C3, which align with the ring's plane (Figure 2-1b) 3. The length of a cellulose chain, shown by the number of its constituent AGUs, reflects its degree of polymerization (DP) 9. This characteristic varies significantly depending on the source of the cellulose and the methods used to process the raw material. For instance, wood pulp cellulose typically has between 300 to 1700 glucose units, while cotton and other plant fibers can range from 800 to 10,000 units. Through specific treatments like acid hydrolysis, cellulose can be broken down into D-glucose or transformed into microcrystalline types with shorter chains 5. 10 Before talking about the crystalline structure of cellulose, it is critical to know the possible orientations of the hydroxymethyl group (which are defined according to the placement of the O6âC6 bond with respect to O5âC5 and C4âC5 bonds) affect the formation of hydrogen bonds within and between cellulose molecules 10. These hydrogen bonds are key to the formation of cellulose's crystalline structure. Different orientations can lead to variations in the crystallinity of cellulose, which in turn influences its mechanical strength, chemical reactivity, and interaction with water and other substances. The most common orientations are the tg (trans-gauche, 300º) and gt (gauche-trans, 180º) (Figure 2-1d) 3. The tg orientation causes the hydroxymethyl group to extend outward from the glucose ring plane, influencing the formation of hydrogen bonds crucial for the material's crystalline structure. Apart from these, the gg (gauche-gauche, 60º) orientation, although less common, contributes to the diversity of cellulose's structural forms 10. The occurrence of the gg orientation, primarily in modified or less common forms of cellulose, results from specific chemical treatments or conditions during synthesis. It modifies the hydrogen bonding pattern and the cellulose's three-dimensional structure, thereby affecting crystallinity, solubility, and reactivity 3. In nature's grand design, cellulose molecules rarely stand isolated. During their biosynthesis, singular cellulose molecules undergo a systematic process, forming fibers through progressive stages. Initially, these molecules come together to form elementary fibrils. These elementary units then merge, forming more substantial entities known as microfibrils, which subsequently amalgamate to birth recognizable cellulose fibers 5,6. The exact configuration of these fibers can vary and be influenced by the conditions during their biosynthesis. This entire assembly process is orchestrated within a distinct rosette-shaped structure, facilitated by specialized enzyme-based terminal complexes (TCs) 11. For more comprehensive reviews of biosynthesis mechanisms and TCs, we guide the readers to relevant publications in the field 3,7,12. From these organized assemblies emerges a deeper complexity within cellulose's structure. Its fibers encapsulate a variety of crystalline and amorphous domains, forming an intricate hierarchy that leads to the identification of six polymorphs (I, II, IIII, IIIII, IVI, and IVII). The shape and the arrangement of the TCs predominantly influence the structure of these six cellulose crystals. This correlation was suggested by Tsekos 13. The primary class of cellulose is considered to possess the cellulose I structure (which contains two cellulose chains in a parallel orientation with a twofold screw axis), further refined into two distinct sub allomorphs, Iα (triclinic) and Iβ 11 (monoclinic), each with its own hydrogen-bonding pattern and chain arrangement that contribute to the overarching complexity of cellulose's architecture (see Figure 2-1e) 3,7,11. Comprehension of cellulose's polymorphic nature extends beyond theoretical significance and serves practical applications by guiding the transformation of cellulose across its various forms. For example, the transition from cellulose I to cellulose II (Figure 2-1e), which is affected by processes such as mercerization and chemical regeneration, is driven by the need for enhanced material properties in the textile and paper industries 3,12. Moreover, the transformation of cellulose types I and II into cellulose III can be achieved through ammonia treatment. Subsequently, cellulose IV is obtained by heating cellulose III in glycerol at a temperature of 260 °C 14. These transformations may arise from two primary factors: as mentioned, the synthesis of varied cellulose forms by distinct terminal complexes and the potential impact of post-synthetic processes, including mechanical deformation like bending, on the cellulose structure 15. This transition exemplifies the application of fundamental knowledge of cellulose biosynthesis in the engineering of materials with tailored characteristics. The mechanistic insight gained from cellulose synthesis in nature, particularly the role of terminal complexes, underpins the development of techniques for manipulating cellulose structure for advanced material applications 12. 12 Figure 2-1: (a) The plant cell wall structure consists of lignin, hemicellulose, and cellulose. Reproduced with permission 16. Copyright 2022, Elsevier. (b) Cellulose, cellobiose, and anhydroglucopyranose structures. (c) α and β form the linkages. (d) gt, gg, and tg conformers. Reproduced with permission 3. Copyright 2010, American Chemical Society. (e) Crystal structures of cellulose Allomorphs Iα, Iβ, II, and III. Carbon atoms are denoted by gray spheres, oxygen atoms by red, and hydrogen atoms by white. The hydrogen bonds interconnecting these atoms are illustrated with dashed lines. Reproduced with permission 17. Copyright 2020, Elsevier. Exploring the chemistry and structure of cellulose reveals that this abundant biopolymer is unique in its composition and applications. The chemical complexity of cellulose leads to its classification into different types. Understanding these categories helps us appreciate its diverse uses and characteristics in nature and industry. 2.2 General classification of cellulose Given the numerous types of cellulose available, it's pivotal to narrow our focus to understand its intricacies and applications better. Therefore, in this chapter, we will be concentrating specifically on the nanoscale variants of cellulose, delving into their unique properties, behaviors, and potential applications. Nanocellulose can be broadly categorized into three primary classes: cellulose nanocrystals (CNCs), cellulose nanofibers (CNFs), and bacterial nanocellulose (BNC). These classifications are rooted in both their distinctive morphologies and their sources. The derivation of CNCs and CNFs employs a top-down methodology in which plant matter is methodically broken down, either chemically or mechanically 18,19. Within plants, 13 cellulose undergoes biosynthesis, leading to the formation of semi-crystalline fibers 3. The process of either mechanical shearing or acid hydrolysis targets the more fragile, non-crystalline regions of these fibers, culminating in the formation of nanocellulose. Intriguingly, the amorphous sections display distinct physical properties compared to their crystalline counterparts 20. The segmentation of cellulose fibers at these amorphous junctures creates the respective nanoparticles. When acid hydrolysis is employed, the outcome is highly crystalline, nanometric rod-like structures, commonly referred to as CNCs 21â24. In contrast, mechanical shearing fractures cellulose fibers down to their foundational nanoscale elements, giving rise to CNFs. These CNFs tend to be more elongated, often reaching micrometric dimensions 22â24. On the other hand, BNC emerges from a bottom-up strategy, relying on bacterial cultures for its synthesis 25. In the following sections, we will explore in greater detail the distinct characteristics of each class of nanocellulose. Additionally, we will examine the various modifications necessary to render these nanocelluloses suitable for printing applications. 2.2.1 Cellulose nanofibers (CNF) CNF is derived from the hierarchical disintegration of cellulose fibers into their most rudimentary nanoscale structures 26. The production of CNF typically involves a top-down approach, where bulk cellulose sources undergo rigorous mechanical or chemical treatments to release the nanofibrils 27,28. The initial mechanical refining facilitates the separation of cellulose bundles into micro-sized fibrils. However, to achieve true nanoscale dimensions, further treatments, often a combination of enzymatic hydrolysis and high-pressure homogenization, are employed 29,30. These methods ensure that the cellulose fibers are disintegrated into their nano-scaled fibrils while preserving the structural integrity of the fibrils. One of the distinguishing features of CNF is its rich surface chemistry. The surface of CNF has many hydroxyl groups, making it highly reactive and suitable for various chemical modifications 31. Through targeted chemical reactions, the surface properties of CNF can be tailored for specific applications. For instance, esterification 9,32,33 or etherification 34 can render CNF more hydrophobic, while grafting reactions can introduce functional groups that facilitate its compatibility with polymers or improve its mechanical properties 35. This tunable surface chemistry has paved the way for CNF's integration into a diverse array of products and materials. Traditionally, wood pulp has been the primary source of CNF, given its abundance and well-established extraction methods. Softwood, 14 like pine and spruce, and hardwoods, such as birch and eucalyptus, are commonly used for CNF extraction 36. However, with increasing environmental concerns and the pursuit of sustainable alternatives, non-wood sources, including agricultural residues like sugarcane bagasse 37, wheat straw 38, and even certain algae and bacteria 39, are being explored. These alternative sources not only offer a sustainable edge but also provide unique properties to the extracted CNF due to their inherent structural differences. 2.2.2 Cellulose nanocrystals (CNCs) CNCs possess a distinct rod-like or whisker-like morphology with a high aspect ratio, which is a characteristic length-to-diameter proportion 40. These nano-sized entities generally exhibit lengths ranging from a few to several hundred nanometers, while their diameters are typically between 3 to 20 nm, contingent upon the cellulose source and the extraction method employed 3. Their remarkable properties are attributed to the organized and tight hydrogen bonding networks within the cellulose chains in the crystal lattice. Their crystallinity index is often above 70%, attributed to their superior mechanical properties 3,41. In fact, Young's modulus and tensile strength can approach values as high as 150 GPa and 9 GPa, respectively, which makes them stiff, rivaling even that of Kevlar fibers 3. Furthermore, CNCs exhibit a high surface area, which provides an ample platform for functionalization and broadens their utility in composite materials where surface interactions are critical. Their thermal stability is also commendable, usually remaining stable up to 350 °C, though this can vary based on the cellulose source and any post-synthesis modifications 42. Additionally, CNCs possess a notable refractive index, making them suitable candidates for optics and photonics applications 43. CNCs are typically produced through acid hydrolysis, where plant fibers are treated with strong acids like sulfuric or hydrochloric acid 3. The acid breaks down the amorphous regions of the cellulose, leaving behind the crystalline segments, which are inherently resistant to acidic degradation 44. The end product is a suspension of rod-like or needle-shaped crystals, which, when centrifuged and washed, yield pure CNCs. CNCs inherently possess numerous hydroxyl groups that make them highly hydrophilic; however, their main backbone exhibits hydrophobic characteristics 3. Depending on the hydrolysis conditions, especially with sulfuric acid, CNCs can also have sulfate groups on their surface, imparting them with a negative charge. This surface chemistry allows for various chemical modifications, facilitating compatibility with different polymers or introducing new functionalities to the CNCs. 15 In addition to their notable properties, the inherent characteristics of CNCs, such as a high aspect ratio, rigid rod-like structure, and the amalgamation of hydrophilic and hydrophobic domains, endow them with the fascinating ability to form liquid crystal phases 43. When suspended in a suitable medium, CNCs can exhibit a range of liquid crystalline behaviors, transitioning through various phases such as nematic, cholesteric, or even smectic, depending on factors like concentration, temperature, ionic conditions, and shear stress 45. Given that CNC comprises a collection of anisotropic particles whose alignment during ink deposition significantly influences the microstructure and properties of 3D-printed materials, understanding this property is crucial 45. This alignment is not just indicative of CNC's complex physicochemical nature but also opens pathways to its diverse applications. While the alignment of particles in diluted suspensions is extensively described in existing literature, the behavior and dynamics of CNC particle orientation within the highly concentrated inks typically employed in DIW are not as thoroughly understood. Understanding these liquid crystal phases is crucial, as they influence the processing characteristics and the final properties of CNC-based materials, particularly in extrusion 3D printing, where control over material behavior and structure is paramount. In the subsequent subsection, we will investigate the various liquid crystal phases found in CNC, along with their application in DIW 3D printing. 2.2.2.1 Structures in CNC suspensions Under different conditions, various structural formations are observed in CNC suspensions. As mentioned, the physicochemical properties of CNCs and their potential applications in 3D printing are heavily reliant on these structures, which range from simple isotropic to complex coacervates 46. Generally, CNCs form an isotropic phase in low-concentration suspensions. In this case, CNC particles are randomly oriented and evenly distributed (Figure 2-2a) 46. As the nanocrystals lack orientational order, this phase is transparent and lacks birefringence. The isotropic phase, notable for its uniform dispersion of CNCs without distinct directional properties, is vital for applications that demand such uniformity. However, this phase will not be the focus of our work, as it does not align with the specific requirements of DIW 3D printing and thus falls outside the scope of our discussion. As CNC concentrations reach a critical threshold of ~ 4 wt% 43, they begin forming the initial stages of liquid crystalline organization. This development causes a condition where isotropic and liquid crystalline domains coexist in a state of equilibrium (biphasic-Figure 2-2a). With increasing 16 CNC concentration, a full liquid crystalline order is established. The progression from this state to a gel phase is governed by a combination of concentration levels and the forces of attraction between particles 47. The presence of electrolytes such as NaCl can compress the electrical double layer surrounding the CNC particles, diminishing repulsion among these charged entities 46. This compression is known to catalyze the transition from liquid to solid states. Furthermore, the nature of this transitional structure is influenced by the balance between anisotropic and isotropic ordering within the CNC suspension 48. Depending on the CNC concentration and certain physical modifications, such as ultrasonic treatment 49 or adding salt to tune electrostatic repulsion among CNC particles (which will be discussed in detail in sections 2-4), chiral nematic (cholesteric, characterized by a helical arrangement of nanocrystals), and nematic (gel-like, where CNC particles align parallel to each other, displaying long-range orientational order without positional order) structures can form in the CNC suspensions 48,50. Esmaeili et al. 49 investigated the influence of sonication treatment on the formation of chiral structure for CNC suspensions with various concentrations at 3, 5, 7, and 9 wt% of CNC. Based on the polarized optical microscopy images, they figured out that there were no distinct structural differences for a sample containing 3 wt% of CNC in both sonicated and nonsonicated conditions. In the chiral nematic and nematic phase, the shear flow in the 3D printing has a distinct effect on the alignment of the CNC particles (Figure 2-2b, c), and these particle alignments have a promising effect on final properties 51. For example, Chan et al. 45 showed how these randomly oriented domains can be aligned after printing with tailored final properties. They selected a mesophase cellulose ink (hydroxypropyl cellulose functionalized with methacrylic anhydride) with a 64 wt% concentration, which exhibited characteristics of a cholesteric phase under these conditions. Their research demonstrated that this cholesteric phase could fluidly pass through the printing nozzle and then be rapidly solidified by photo-crosslinking, maintaining the intended shape post-extrusion. The visual attributes of the printed cholesteric phase items were notably influenced by the spatial arrangement and orientation of the phase domains. One of their notable creations, a 3D-printed âgeckoâ model (Figure 2-2d), showcased structural coloration derived from the 64 wt% cellulose concentration 45. This model vividly illustrates the potential of using cellulose-based inks to produce objects with intricate color patterns and complex structural details. Fourmann et al. also reported the alignment of high-aspect-ratio CNC particles along the ink flow direction 52. They claimed that the alignment imparts anisotropic mechanical properties and 17 distinct swelling behaviors to the printed structures, leading to a notable enhancement in their mechanical attributes. Based on their research, the high degree of CNC alignment during printing allowed for the fabrication of hydrogel structures with intricate architectures, including varying angles and textures, and enabled programmable self-shaping actions. An example of this can be seen in Figure 2-2e, which illustrates the combination of two bilayer strips created through 3D printing of CNC ink (20 wt%). The figure showcases synthetic structures that twist based on printing patterns. This technique represents a sophisticated approach to artificially engineering structures capable of dynamic, responsive behavior. While the alignment of CNC particles and the formation of various nematic phases have widespread applications, scaling up the production of aligned CNC-based materials poses challenges, including prolonged self-assembly times, assembly defects, and inherent brittleness. To overcome these obstacles, Cheng and colleagues 53 developed flexible hydrogels with chromatic patterns using CNC-based inks. They innovatively manipulated the ink's viscosity by adding polyacrylamide, demonstrating that forming a nematic phase is not limited to high CNC concentrations. Employing CNC concentrations as low as 0.1 - 2 wt%, their structures still exhibited large, monodomain color patterns when viewed under crossed polarizers. Furthermore, by adjusting the 3D printing conditions, they opened a new pathway for creating colored CNC structures. Their approach proved that altering the number of printed layers and the direction of printing could precisely control the CNCs' photonic properties 53. 18 Figure 2-2: Self-assembly of CNCs in a suspension at rest with increasing concentration (upper row) and corresponding assembly in shear (bottom row). Reproduced with permission 46. Copyright 2021, American Chemical Society. (b) Polarized optical microscopy of the sonicated and nonsonicated CNC suspensions and Figure representation of the CNC particle arrangement in the isotropic, chiral nematic, and gel phases. Reproduced with permission 49. Copyright 2022, American Chemical Society. (c) Figure showing how the domain alignment. Reproduced with permission 45. Copyright 2022, Wiley-VCH. (d) Figure of the DIW 3D printing and a 3D-printed âgecko.â Reproduced with permission 45. Copyright 2022, Wiley-VCH and (e) a Figure of the printing pattern and pictures of the evolution of the anisotropic actuation of the printed structures. Reproduced with permission 52. Copyright 2021, Elsevier. In summary, aligned CNC particles often exhibit enhanced mechanical properties, but the relationship between alignment and properties like tensile strength, modulus, and thermal stability is complex. Factors like the degree of polymerization, crystallinity, and the interaction between CNC and the other components significantly influence the final properties. In addition, researchers should evaluate the suitability of liquid crystalline CNC on a case-by-case basis, considering factors such as printability, post-processing, and end-user requirements. The use of liquid crystalline CNC in 3D printing is highly dependent on the specific application and desired properties. For instance, while liquid crystalline phases can facilitate alignment and thus enhance mechanical properties, they might not be suitable for all printing applications, particularly where flexibility or specific interactions with other materials are required. 19 2.2.3 Bacterial nanocellulose (BNC) BNC is an intriguing biopolymer synthesized by select bacteria strains, primarily from the genera Gluconacetobacter, Agrobacterium, and Rhizobium 39,54. Unlike plant-derived cellulose, BNC is produced in a pure form, free from lignin, hemicellulose, and pectins, which are typical components of the plant cell wall. This purity affords BNC its distinctive and often superior properties when compared to plant-based cellulose 55. Morphologically, BNC is characterized by a unique three-dimensional network of interconnected fine fibrils. This nano-structured fibrous web endows BNC with a high water-holding capacity, making it incredibly hydrophilic 56. This property is especially beneficial in specific applications, like wound care, where moisture retention is crucial 25. Furthermore, the nanofibers of BNC are highly crystalline, which translates to commendable mechanical strength (elastic modulus of 78 GPa) and flexibility 29. In many instances, BNC surpasses the mechanical properties of plant-based nanocellulose due to its uniform fiber distribution, high crystallinity, and high degree of polymerization. From a functional perspective, BNC offers a suite of advantageous properties. Its high purity means that it exhibits biocompatibility, making it an excellent candidate for biomedical applications, from wound dressings to scaffolds in tissue engineering 57. Moreover, due to its high crystallinity, BNC demonstrates impressive thermal stability, often surpassing that of its plant-derived counterparts. The production of BNC, primarily through static and agitated fermentation processes, allows for the tuning of its properties. By adjusting parameters like fermentation time and bacterial strain or adding modifying agents, the properties of BNC, such as its mechanical strength or porosity and crystallinity, can be tailored to specific applications. 2.3 Cellulose interactions: crafting mono, co-polymer gels Building on our understanding of the diverse microstructures of cellulose, this section scrutinizes the complex force interactions within pure cellulose ink, which are critical in shaping its behavior and functionality, especially in DIW 3D printing. DIW is a sophisticated fabrication process where the optimal performance of the printed structures largely depends on the ink formulation, which includes a fine balance of viscosity, yield stress, and thixotropy 58,59. These properties govern how the ink behaves under pressure and its ability to retain shape after being deposited. The complexity and functionality of the 3D objects crafted through DIW demand a meticulous understanding of the interactions among cellulose molecules and any additives, aiming 20 for a formulation that ensures smooth printing and post-deposition structural integrity. Gelation or hydrogel formation (increase in viscosity) is the first step in DIW 58. Gelation is achieved by creating cross-linked networks formed by a spectrum of interactions ranging from weak, noncovalent bonds like hydrogen bonds, van der Waals force, hydrophobic and ionic interactions to more complex host-guest chemistry, metal coordination, Ï-Ï stacking, and covalent bonds 60â62. These dynamic bonds afford the printed material the requisite properties for successful DIW. Specifically, hydrogen bonding, ionic linkages, and covalent bonds play vital roles in forming these networks. Herein, we provide a comprehensive discussion of these interactions, followed by an analysis of rheological properties and their correlation with shape fidelity in the following two sections. 2.3.1 Physical interaction forces Formulating cellulose ink begins with the strategic tuning of physical interactions, an approach that is both feasible and cost-effective. By carefully engineering key factors such as hydrogen bonding and polymer chain entanglement and incorporating salts to enable dipole-dipole interaction or ionic bonding (to reduce electrostatic repulsion), we can improve the ink's functional properties. This section explores how such molecular-level manipulations contribute to enhanced rheological characteristics, thereby elevating the efficacy of cellulose inks in DIW. We start with hydrogen bonds (H-bonds); H-bonds are critical noncovalent interactions that occur between a hydrogen atom and another atom with a strong electron affinity, typically fluorine (F), nitrogen (N), or oxygen (O) 59. Within cellulose structures, these bonds manifest both between individual particles/chains (intramolecular interactions) and within their stacked layers (intermolecular forces). The surface hydroxyl groups of cellulose are active in forming these H-bonds. Water molecules can infiltrate these clusters, interacting with the three hydroxyl groups present on each repeating unit of the cellulose, which contributes significantly to the overall bonding network 9. Additionally, the inclusion of salts that contain F, N, or O atoms can boost these H-bond interactions, enhancing the rheological properties of cellulose-based ink 6. This reinforcement of H-bonds by strategic salt addition can substantially elevate the print quality of cellulose-based inks, optimizing them for sophisticated 3D printing applications. Hydrogen bonds, although weaker than covalent or ionic bonds, can be multiplied within a polymer matrix to enhance overall structural integrity. Similarly, ionic interactions and dynamic covalent bonds contribute to the stability and functionality of the resulting 3D-printed constructs. 21 Ionic linkages are the next important linkage to form a 3D printable cellulose ink. Once cellulose is ionically modified, it can establish ionic linkages with various counterparts. For example, it was reported that in anionic cellulose, some ionic linkages can be formed with carboxylate groups. In this case, the cellulose can interact with multivalent cations like Ca²âº, Mg²âº, or Al³âº. These cations can form cross-links between individual cellulose chains/particles, creating a network structure that increases the viscosity and stability of the ink. In addition, when functionalized with cationic groups, cellulose can form ionic bonds with multivalent anions or with negatively charged substances like clays or polyanions. This interaction can also lead to cross-linking and the formation of a robust 3D network. Generally, the ionic cross-linking mechanism is driven by the electrostatic attraction between opposite charges. When an ionically modified cellulose is mixed with a solution containing a multivalent counter-ion, intermolecular cross-linking can occur. The multivalent ions can bridge two or more cellulose chains by binding to their ionized groups. Amini and colleagues 59 conducted a thorough investigation into how various ions impact the viscoelastic properties of cellulose-based inks. Their research revealed that viscosity increases with the cation charge number (for chloride salts) following the sequence (Li+, Na+, K+) < (Ca2+) < (Al3+, Fe3+). This pattern aligns with prior studies on gels formed by CNCs, underscoring the notion that a higher cation charge number strengthens the CNC network structure 63â68. This strengthening occurs due to a decrease in the Debye length, enhanced screening effects, and compression of the Electric Double Layer (EDL). Researchers showed that in environments with higher ionic strengths, the negative surface charge of CNCs is largely neutralized by more counterions, leading to segmental aggregation between CNCs and the formation of an interconnected network. Consequently, an increase in the cation charge number results in higher viscosity 66,69,70. However, this correlation between viscosity and cation charge number does not always adhere strictly to this trend. For example, some works claimed that the viscosity of CNC/CaSO4 suspensions was significantly higher than that of suspensions with trivalent chloride salts like AlCl3 and FeCl3. This suggests that other factors, such as the type of bonding (inter- and intramolecular interactions) and ionic radius, also influence viscosity 58,59,71. Interestingly, different trends for gelation mechanisms are observed with divalent chloride metal salts. For instance, although MgCl2 has a smaller ionic radius compared to CaCl2, suspensions of CNC with MgCl2 exhibit a higher viscoelastic profile (viscosity and storage 22 modulus) than those with CaCl2. This phenomenon can be attributed to the different solubilities of these metal chlorides in water: MgCl2 and CaCl2 have solubilities of 54.3 and 75.5 g/100 g in water, respectively. Therefore, divalent cations with lower solubility in water may better bridge functional groups of adjacent CNCs, leading to the formation of stronger gels 71. Aside from metal ions, the interplay of these forces can be influenced by various factors, such as pH, surrounding medium (adding polymers-chain entanglement), and the presence of other chemical entities (conductive filler and polymers). For instance, altering the pH can change the charge density on CNC surfaces 67, thereby affecting the electrostatic interactions. The next method for inducing gelation in cellulose involves increasing the concentration of cellulose, leading to chain entanglement in CNF and cluster formation in CNC. While this method is straightforward and practical, its drawback is the increased material usage, making it less appealing from an efficiency and cost perspective. The final interaction mechanism in the context of cellulose-based gels is Ï-Ï interactions. These are non-covalent interactions occurring between aromatic rings, which are sometimes present in modified cellulose derivatives. Ï-Ï interactions play a significant role in forming structured networks within gels. Although this concept is a well-known principle in polymer science, its specific application to cellulose-based gels may not be extensively documented or explored in existing literature 72â75. All of these interactions are shown in Figure 2-3. Figure 2-3: Possible physical interaction forces in cellulose inks for DIW printing. 23 2.3.2 Chemical interaction forces The evolution of 3D bioprinting is witnessing a remarkable transformation with the advent of dynamic covalent hydrogels. Traditional hydrogels have limitations: simple chemically cross-linked ones are too rigid, and physical ones lack durability. This has led researchers to develop innovative materials that combine strength with flexibility 76,77. The journey of chemical crosslinking in cellulose gel synthesis begins with the modification of hydroxyl and carboxyl groups. These functional groups are important in the advanced fabrication of cellulose-based materials, serving as the primary active sites for crosslinking. Their strategic manipulation and alteration are essential for the effective synthesis of cellulose gels, underpinning the structural integrity and functionality of the final product. These include dynamic covalent bond, esterification using condensing agents such as N, Nâ²-dicyclohexylcarbodiimide (DCC) or 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), Michael addition where hydroxyl groups react with carbon-carbon double bonds under alkaline conditions, interactions between epoxides and hydroxyl groups, the coupling of alkyl halides with hydroxyl groups, and the chain transfer reaction involving hydroxyl groups in free radical polymerization. In-depth discussions of these chemical processes are presented in 78. For instance, the concept of dynamic covalent bonds is explored as a prime example. Dynamic covalent inks advance with a variety of bonds, including imines, hydrazones, oximes, disulfides, and boronic esters 77,79. The term "dynamic" signifies their rapid responsiveness to suitable external triggers, like heat or catalysts. Upon exposure to these stimuli, these bonds exhibit reversible behavior and tend toward an equilibrium state. This reversible nature under specific conditions underscores their dynamic quality, allowing for adaptability and modification in various chemical processes 78. Such versatile bonding not only imparts the 3D-printed inks with the ability to self-heal but also endows them with a unique resilience to reconfigure under stress. Consider the 3D structure formed through imine linkages, which exhibit self-healing and are malleable under gentle conditions. A 3D printable dynamic nano-polysaccharide (carboxyl methyl chitosan, oxidized cellulose nanofibers, and chitin nanofibers) self-healing hydrogel, suitable for flexible strain sensing applications by incorporating three distinct dynamic bonds: imine, hydrogen, and catecholate-metal coordination bonds, was fabricated by Nasri-Nasrabadi, and Kouzani et al. 80â82. This 3D-printed structure has become particularly valuable in precision medicine, including targeted drug delivery in cancer therapies. In another work, Yang et al. 76 24 successfully developed a hydrogel composed solely of CNCs, with a CNC content below 2 wt.%. This was achieved by functionalizing the CNC surfaces with aldehyde and amino groups, resulting in the formation of NHNH2-CNC and CHO-CNC. When these two types of modified CNCs were combined, they formed robust hydrazone crosslinking bonds. The CNC hydrogel crafted through this method displayed enhanced mechanical properties compared to hydrogels that were physically crosslinked. This improvement is largely attributed to the higher energy associated with the chemical bonds in the crosslinking process. An innovative extrudable hydrogel was synthesized by combining carboxymethyl cellulose modified with adipic acid dihydrazide (CMC-NHNH2) and a suspension containing aldehyde-functionalized cellulose nanocrystals (CHO-CNC). This formulation quickly gels, indicating the formation of Schiff base linkages, which effectively trap CNC within the hydrogel matrix 83. The real genius of these printable ink lies in their quick rebound from physical disruptions and their adaptability in rheology, essential traits for printing intricacies. They shine particularly in healing applications, such as treating skin burns, where their adherence to tissue and absorption capabilities come to the fore, marking a significant milestone in regenerative medicine and the engineering of complex tissues through 3D bioprinting. Further advancements in this field led to the development of a specialized hydrogel for tissue engineering, utilizing a similar crosslinking technique with adipic acid dehydrazide-modified hyaluronic acid and CHO-CNC. This new hydrogel variant demonstrated lower swelling and increased resistance to deformation due to a denser crosslink network 84. Following this trend, a unique bio-based hydrogel was created using aldehyde-modified cellulose and sodium alginate, polymerized with vinyl monomers containing amine groups. This process resulted in a hydrogel with enhanced mechanical properties, where the modified cellulose and alginate acted as effective crosslinkers 85. Further exploiting the benefits of this linkage, a novel approach involved introducing dialdehyde groups to CNC surfaces. These modified CNCs were mixed with water-soluble carboxymethyl chitosan, yielding a self-healing hydrogel aimed at enhancing the healing process of deep partial-thickness burns 86. The pioneering use of dynamic covalent hydrogels signals a potential paradigm shift in tissue engineering, delivering the requisite structural integrity, versatility, and biocompatibility needed for crafting stable, functional tissue constructs 83,85,86. These materials are now at the forefront, spearheading the journey towards more refined and personalized therapeutic strategies. Within this 25 exciting field, disulfide bond-based inks have shown exceptional promise. Through the work of researchers like Heidarian 80 and others, these inks can be fine-tuned, offering self-healing properties and responsiveness to varying conditions. Creative solutions have also been developed to address challenges, such as oxidative reactions that affect the stability of thiol-disulfide exchanges. Turning to boronic ester bonds, researchers have overcome the challenge of stabilizing these bonds at physiological pH by pioneering new ways to create self-healing, responsive cellulose hydrogels without auxiliary compounds 80,81. The introduction of additional dynamic bonding patterns has further enriched the mechanical and rheological features of these hydrogels. Generally, by employing a mix of reversible and irreversible interactions within the hydrogel, these CNC-based composites maintain their mechanical integrity through chemical bonds while preserving flexibility due to the reversible interactions. Such a combination of properties makes these hydrogels particularly suitable for 3D printing applications, where specific rheological characteristics, like shear-thinning and strong viscoelasticity after printing, are essential. 2.4 Printability and shape fidelity 2.4.1 Three-dimensional-printing techniques In the field of additive manufacturing, 3D printing stands as a pivotal innovation, enabling the precise assembly of cellulose-based inks into structures tailored to a wide spectrum of applications 87. This technology spans an impressive dimensional range, from the sub-micrometer to the sub-meter, addressing diverse application needs. Additive manufacturing bifurcates into two principal categories: light-based and nozzle-based printing (Figure 2-4) 87. Light-based printing, distinguished by its superior resolution and rapid fabrication, employs light irradiation to pattern and crosslink photosensitive polymers, thus ensuring high fidelity in the final structure 88. This category encompasses techniques such as stereolithography (SLA) utilizing laser systems 89, Two-photon laser printing (TLP) 90 based on biphotonic interactions, and digital light processing (DLP) 88, and computed axial lithography (CAL) employing light projection (Figures 2-4 IV, V, and VI). Conversely, nozzle-based printing (Figures 2-4 I, II, III) intricately deposits precursor inks using printheads, transforming these fluids into gel/solid states through subsequent processing. The process efficacy is intricately linked to the rheological properties of the inks, which demonstrate diverse shear-thinning behaviors 91,92. Within this category, notable techniques 26 include meniscus-guided printing (MGP), inkjet printing (IGP), characterized by droplet formation, and direct ink writing (single nozzle, co-axial, and rotational printing) known for its filament production (Figure 2-5). The focus of this review is on DIW, a technique renowned for its cost-efficiency and versatility. DIW involves the extrusion of inks into continuous fibers, their dynamic alteration during printing, and eventual deposition onto substrates. This method necessitates inks with specific viscoelastic properties, such as high viscosity and thixotropy, to ensure compatibility with a broad range of formulations. DIW facilitates the fabrication of complex designs like helical structures 93, fractal lattices, in-tubule stents, and vascular mimetic conduits 94. Its compatibility with various ink formulations allows for the continuous extrusion of multi-material inks, which is crucial for the development of integrated tissue engineering, robotic systems, and other applications such as sensor and EMI shielding. The resulting polymers demonstrate sufficient mechanical strength to form intricate 3D architectures, including hollowed structures and overhanging frameworks 95,96. A critical challenge in DIW is die swelling, where extruded inks expand beyond the die's cross-section, impacting resolution and shape fidelity 91. To mitigate this, the rheological or viscoelastic properties of inks and printing parameters can be modulated. Strategic approaches involve optimizing nozzle speed to manipulate the extruded ink pre-deposition and exploiting in-flight ink deformation, thus enabling the fabrication of ultrafine fibers. For the long-term stability of cellulose-based inks in 3D structures, the formation of a robust polymer network is essential 97,98. Various techniques, including the addition of salts 59, ionic liquids 99, coagulation baths 100, and freeze-thaw cycles 101, are employed to enhance the structural integrity and shape fidelity of the printed objects. These methodologies, particularly those based on reversible physical bonding, provide streamlined processing and reinforce the nascent polymer network for subsequent treatments. 27 Figure 2-4: Different printing techniques: (a) Nozzle-based, (I) meniscus-guided printing (MGP), (II) inkjet printing (IGP), and direct ink writing (III). (b) light-based printing, (IV) SLA, (V) TLP, and (VI) DLP. Figure 2-5: Detail of different printing techniques: (a and b) are light-based printing, (a) TLP or two-photon laser printing, (b) computed axial lithography (CAL), (c-e) nozzle-based printing, (c) rotational printing, (d) triple-nozzle printing, (e) co-axial printing. 2.4.2 Shape fidelity In this context, it is crucial to clarify the concepts of 'printability' and 'fidelity'. Although 'printability' is frequently mentioned in bio fabrication literature, its definition varies. First of all, it was defined as the ability of a material to achieve favorable outcomes under specific printing 28 conditions for a particular application 92. This concept encompasses diverse parameters of the printing process, affecting the comparability of different inks in individual studies. In some instances, within the literature, there is a tendency to mistakenly use 'printability' interchangeably with 'fidelity' 87. However, this is not accurate. While it is understandable to see a close relationship between printability and shape fidelity concepts, particularly in DIW, equating the two terms is not entirely accurate. Printability, as defined, primarily concerns the physical and rheological properties of the material (ink) that allow it to be extruded through a printer nozzle and deposited layer by layer to form a structure, as entirely described in Chapter 4 59. It includes considerations like the ease of flow through the nozzle and the formation of a continuous filament. Essentially, printability is about the material's suitability and behavior during the printing process. On the other hand, shape fidelity is about the outcome of the printing process 102,103. It refers to how accurately and precisely the printed structure replicates the intended design or model. This includes aspects like geometric accuracy, dimensional precision, and surface finish. Shape fidelity is a measure of the final result, reflecting how closely the final product matches the original design. Although there is overlap, for instance, a material's printability directly influences the shape fidelity. Printability is a prerequisite for high fidelity; good printability means the material is capable of being printed with high fidelity, but high fidelity also depends on other factors like the precision of the printer, the quality of the design, and the post-processing steps 104,105. Having established the distinct differences between printability and fidelity, it is now pertinent to explore the various methodologies employed in assessing both printability and shape fidelity in DIW 3D printing. Evaluating cellulose-based inks for DIW involves diverse approaches, each corresponding to different stages of the printing process. When developing a cellulose-based ink, key considerations include the gelation mechanism, uniformity, and behavior upon swelling and deposition. Printability assessment may begin with analyzing the ink's key rheological properties, or even earlier just by initial observations as a screening step, serving as predictors of shape fidelity. The formation, orientation, and stacking of hydrogel filaments during printing are also critical to evaluate. Advanced imaging technologies now enable real-time monitoring of the printing process, providing direct insights into the fidelity of both individual layers and the assembled 3D constructs. The methods for evaluating these metrics are detailed below. 29 2.4.2.1 Initial screening - inverted vial test The inverted vial test is popular due to its simplicity and the minimal equipment required. It provides a quick and qualitative assessment of an ink's suitability for DIW, particularly in terms of its ability to be extruded and maintain structure post-deposition. However, for a comprehensive understanding of an ink's printability, this test is often used in conjunction with more detailed rheological assessments. In this method, the vial containing the ink is turned upside down (Figure 2-6a). The key observation is whether the ink stays in place or begins to flow down the sides of the vial. An ink that remains in place without flowing indicates a high yield stress and sufficient viscosity, qualities that are desirable for printability in DIW 59. This behavior suggests that the ink can maintain its shape and resist deformation under its own weight, which is crucial for layer-by-layer deposition in 3D printing. If the ink flows or slumps significantly, it may be too low in viscosity or yield stress for effective 3D printing, indicating poor printability. Such ink might not hold its shape when extruded, leading to poor definition and resolution in the printed object. For instance, as depicted in 59, suspensions devoid of salt exhibited liquid-like characteristics, flowing readily upon the inversion of their vials. Moreover, within 30 minutes, samples with a composition of 3 wt.% CNC and 100 mM salt commenced flowing, whereas the gel-like suspensions containing 5 wt.% CNC and 100 mM salts maintained their integrity, showing no flow upon vial inversion. Consequently, samples demonstrating liquid-like behavior were not considered for further analysis, while those exhibiting gel-like properties were earmarked for more detailed investigation. This straightforward method has been widely employed in numerous research studies 66,68,106,107. 2.4.2.2 Rheological assessments Rheological features of hydrogel inks are known as their most critical physiochemical fingerprints for assessing the quality of printing and shape fidelity. For an accomplished extrusion-based 3D printing, specifically DIW, prepared hydrogel inks should meet several requirements from a rheological viewpoint. A hydrogel should be able to extrude through a narrow nozzle and form a stable structure after deposition. In the DIW process, a hydrogel ink that is initially resting experiences a high shear deformation by transmission within the nozzle 108. As a result, it obtains a filamentous shape after extrusion under a new resting condition. For describing these transitions, several vital rheological factors such as viscosity (Æ), viscoelastic shear moduli, elastic recovery, 30 storage (Gâ²) and loss moduli (Gâ²â²), shear stress (Ï), damping factor or tan (δ) and yield stress (Ïy or Ïy) should be considered 87,95. These assessments are conducted through a series of systematic steps, each designed to elucidate specific aspects of the ink's behavior under conditions mimicking the DIW process. Therefore, first, we plan to describe and define these parameters to gain more insights into the correlation between rheological features and the extrusion-based 3D printing process. Afterward, a rheological fingerprint window will guide scholars in this area to design next-generation 3D-printed structures. Finally, a literature survey will focus on the rheological properties of prepared cellulose-based inks. The first critical parameter is viscosity. The viscosity property measures a fluid's internal resistance to deformation or flow in response to an externally applied force or stress 95. It has a dramatic impact on printability, filament formation, and shape fidelity. Overall, higher printing fidelity arises from greater viscosity. Nevertheless, as viscosity increases, shear stress increases (Figures 2-6a1 and a2), which can negatively impact filament formation 109. A fluid following Newtonian behavior principles relates the applied shear stress linearly to the shear rate or velocity gradient 57. Any deviation from this linear relation can be ascribed to non-Newtonian fluids. Non-Newtonian fluids such as shear-thinning and shear-thickening are considered time-independent, while thixotropic or rheopectic are time-dependent fluids (Figures 2-6a1 and a2) 57. Shear-thinning behavior is one of the most striking features of complex fluids, such as polymer melts and solutions, for extrusion printing. All mentioned flows and rheological behaviors are shown in Figures 2-6a4 and a5. In extrusion printing, shear-thinning behavior has a significant influence on both facilitating the printing process and preservation of the ink's initial shape. During the extrusion printing process, the viscosity experiences a steady decrease, whereas the shear force on the ink rises permanently 110,111. When the extrusion printing process is completed, the shear rate falls, and concurrently, viscosity increases, preserving the printed shape. The extent of shear-thinning can vary based on the ink type. In the case of inks based on colloidal dispersion and solid suspensions like cellulose-based ink, the interaction among the solid particles would disrupt upon inducing the shear, leading to the incidence of shear-thinning behavior 112. Under the influence of shear deformation, rod-shaped or fibril-shaped particles respond due to the alignment of their one-dimensional structure parallel to the flow direction 113,114. Another non-Newtonian material category is the thixotropic materials, which possess time dependency rather than dependence on the shear 115. In this case, the viscosity falls with time, 31 followed by returning to its initial value, overtaking a rest. Generally, the time-dependent viscosity profile can complicate extrusion printing and is not usually considered during ink design. Because a few printing parameters, such as extrusion pressure, require continuous adjustment to maintain a constant flow 87. Nevertheless, in some cases, as discussed in the next section, this parameter was taken into consideration by some researchers in DIW of cellulose-based inks. As expressed, essential requirements for successful 3D printing processes are proper flowability and elastic shape retention properties. The ink should hold a minimum internal resistance while transferring through a nozzle. Once the process is finished and the ink is dispensed on the substrate, an opposite behavior, namely elastic shape retention, would be desired. The coexistence of the viscous flow (flowability) and elastic shape retention is recognized as viscoelasticity 59. Two critical parameters explain viscoelasticity: the loss (viscous) modulus, Gâ²â², and the storage (elastic) modulus, Gâ². The storage modulus represents the stored energy elastically, whereas the loss modulus represents the dissipated energy. Viscoelastic properties of inks could be measured through small or large amplitude oscillatory tests such as strain amplitude or frequency sweep. In the frequency sweep test, different types of viscoelastic behavior, including solid-like, liquid-like, and gel-like, can be inferred from the graphs of Gâ² and Gâ²â² versus frequency. In solid-like behavior, Gâ² is frequency invariant in the whole probed frequency window, whereas Gâ²â² experiences a linear proportion to the frequency 87. For gel-like materials, both Gâ² and Gâ²â² stay roughly autonomous of the frequency. In the case of liquid-like viscoelastic behavior, Gâ² and Gâ²â² display frequency-independent trends (Figures 2-6a3 and a10). Transitioning to another crucial rheological characteristic, the damping factor, denoted as tan (δ), is of significant interest. This parameter represents the ratio of the loss modulus to the elastic modulus, thereby conveying vital insights into the balance between the viscous and elastic deformation properties of a material. The measurement of tan (δ) is both rapid, typically taking just a few seconds, and remarkably resistant to experimental bias, making it a reliable and efficient quantitative tool. This quick and accurate assessment makes tan (δ) particularly valuable for the prompt evaluation of cellulose-based inks, providing essential data on their rheological behavior 87. Besides Gâ², the yield stress is another outstanding parameter that significantly impacts the shape retention of printed inks. The yield stress or yield point is when the ink starts to deform without reversible deformation, followed by flowing 116,117. The yield (apparent yield) stress 32 (Figures 2-6a6 and a7) is considered an essential rheological parameter for designing a proper process for use, notably in the pulp and paper industry 116,117. Yield stress can be determined via various approaches based on curve-fitting onto experimental data. For instance, fitting complex viscosity versus frequency via the four-parameter Cross model (equation 2-1) determines apparent yield stress 118,119. ð =ðð¦ð+ ð(1 + ððð)â1 2-1) Herein, the model parameters are denoted by d (dimensionless), c (s), b (Pa·s), η (viscosity-Pa.s), and Ï (frequency- Hz). The modified CarreauâYasuda model can also be employed as an alternative for the quantification of yield stress in the frequency sweep test by the fitting of complex viscosity versus frequency (equation 2) Î·â =ÏyÏ+ η0(1 + (Î»Ï)a)(nâ1/a) 2-2) Where Ïy, Ï, Æ0, Æ*, λ, a, and n denote apparent yield stress, angular frequency, zero shear viscosity, complex viscosity, time constant, Yasuda parameter, and power law exponent, respectively 57. Apparent yield stress can also be extracted via the curve fitting of the shear stress versus shear rates (steady shear test) using the Herschel-Bulkley model (equation 2-3). Ï = Ï0 + k(Î³Ì)n 2-3) Herein, Ï, Ï0, k, ?Ì? and n are shear stress, apparent yield stress, consistency factor, shear rate, and fluid behavior index, respectively 120. It is worth mentioning that the above-mentioned equations are only validated for extrusion-based 3D printing. In other words, other 3D printing techniques, such as lithography-based methods, encounter totally different rheological requirements. For instance, in this type of 3D printing, it has been reported that the prepared ink should hold a low viscosity ranging from 0.25 to 10 Pa.s 121,122. A discrepancy might occur between apparent yield stress obtained by various methods, which can be explained by the deviation from the Cox-Merz rule, suggesting the 3D network of cellulose within produced gels 118,123,124. Nevertheless, obtaining yield stress via the above-mentioned equations depends on the fitting parameters and their precision. As a result, the obtained values might not be accurate, and we should find another alternative approach without relying on trial-and-error fitting-based methods. 33 Another conventional way to determine the yield stress is using Gâ² and Gâ²â² curves plotted against shear stress in the oscillatory test, where Gâ²â²= (Figures 2-6a6 and a7) 125. In other words, the crossover points between Gâ² and Gâ²â² in the stress sweep test in oscillatory mode at a constant low frequency represent the yield stress. This method has been frequently used in determining the yield stress for 3D printing applications due to its precision and straightforwardness compared to fitting-based methods 126â128. In fact, this method has no trial-and-error procedures; consequently, it has no sensitivity to fitting parameters and their precisions. During the printing process, once the highest shear stress produced within the nozzle is insufficient to prevail over the yield stress of the ink, the ink will be encountered with a plug flow regime 109. This hinders the uniform extrusion of the ink and can disrupt the direct ink-writing process. As a result, an estimation regarding the shear stress in the radial direction of the deposition nozzle could be performed via the following equation: Ï =ÎP2Lr 2-4) Herein, ÎP, r, and L display the highest employed pressure at the nozzle, radius, and nozzle length, respectively. The maximum shear stress can be calculated by substituting these parameters from the 3D printer device (Ï). Subsequently, this data could be compared with the dynamic yield stress of the inks, obtained through the plots of Gâ² and Gâ²â² versus shear stress. In the 3D extrusion printing of cellulose ink, plug flow assumptions include considering the ink as a non-Newtonian fluid with a distinct yield stress, where it behaves like a solid below this stress and flows above it. The flow inside the nozzle is assumed to be laminar, with a uniform cross-sectional area, and there is no slip at the nozzle walls 129. Additionally, the ink's rheological properties are assumed to remain constant, and the material is considered incompressible throughout the printing process 129,130. These simplifications allow for the calculation of yield stress based on the plug flow behavior. Another important rheological parameter is the elastic recovery that signifies the evolution from fluid-like flow to elastic shape preservation. This parameter can be measured via the recovery of the storage and loss moduli 131,132 or the viscosity 133 Over time, under low deformation, immediately after cessation of the application of high deformation, It is worth mentioning that the transition between these two high and low deformation regimes occurs instantaneously over several consecutive cycles. A Figure graph regarding this test is shown in (Figures 2-6a8 and a9). 34 In conclusion, the shear-thinning behavior, elastic recovery, and prompt reformable solâgel transfer can be considered the most critical features of inks for printability and shape fidelity in the extrusion printing process. Given the above explanations, we can propose a rheological protocol to examine the rheological parameters required for extrusion-based 3D printing (i.e., DIW). First, the linear viscoelastic region (LVR) should be determined via the strain amplitude sweep test (Gâ² and Gâ²â² versus strain amplitude). After that, the apparent yield stress could be obtained via Gâ² and Gâ²â² curves versus shear stress. The obtained yield stresses can be compared with the highest shear stress procreated within the nozzle, Equation 4, to realize whether the latter can overcome the shear yield stresses of inks. Subsequently, small amplitude oscillatory shear (SAOS) experiments can be performed by conducting frequency sweep tests (for example, 0.05â100 Hz) at room temperature. This test can provide us with critical viscoelastic factors, such as complex viscosity, Gâ², and Gâ²â², as well as loss factor. These parameters allow for deeper insight into the viscoelastic behavior of the prepared inks, particularly solid-like or liquid-like behaviors. In the next step, the steady state shear test, known as the flow curve test, should be conducted to evaluate the shear-thinning characteristic of the provided inks. As mentioned before, the shear-thinning behavior enables a smooth flow of inks across a narrow nozzle. Finally, the elastic recovery test can be performed by measuring the Gâ² and Gâ²â² or viscosity versus time at low and high deformations. As a summary, the roadmap of these rheological characterizations is illustrated in Figure 2-7. 35 Figure 2-6: (a) Inverted vial test as an initial screening test to evaluate the quality of the ink before printing, (a1-a10) rheological assessments before 3D printing; (a1 and a2) the viscosity versus shear rate for different types of materials, (a3) behavior of solid, gel and liquid-like materials, (a4 and a5) flow test, (a6 and a7) different methods to determine yield stress from a strain sweep test include: in method a6, the possible yield point is identified at the intersection of the G' and the G''. According to another method (a7), the yield point is defined as the point where there is a 5-10% reduction in the G', (a8 and a9) elastic recovery, (a10) frequency sweep, (b) 3D printing fidelity assessments by watching a single filament, (b1) droplet formation due to lack of proper rheological properties, (b2) uniform filament formation, (b3) non-uniform filament formation due to high viscosity or yield stress, (c) 3D printing fidelity assessments by using optical imaging, (c1) comparing the diameter of a single filament at different points, (c2 and c3) top view of uniformity assessments of a single filament, (c4) transverse pore geometry with an optimal rectangular shape for ideal filament stacking (A=1), along with (c5 and c6) top and 3D views of multilayer 3D-printed sample showing the circularity of filaments. Figure 2-7: A summary/roadmap of the required rheological characterizations before printing. Next, we plan to conduct a literature survey on the 3D printing of cellulose-based materials from a rheological point of view. In the innovative field of cellulose 3D printing, achieving 36 dimensional accuracy and true 3D configurations remains a formidable challenge, necessitating meticulous evaluation of ink rheology. Rui Yuan et al. 134 made significant advancements by successfully printing several complex 3D shapes with high precision, linking these achievements directly to the ink's rheological properties. Figure 2-8a highlights that both the storage modulus under minimal shear stress (0.1 Pa) and the viscosity at a low shear rate (0.1 s-1) escalate with an increase in cellulose concentration. This trend suggests the formation of an interactive network crucial for maintaining the shape fidelity of the extruded ink. Moreover, inks possessing higher viscosity and shear-thinning properties enable continuous printing at lower pressures. However, the study also revealed that when the cellulose concentration surpasses 8 wt%, microfiber agglomeration within the ink occurs, often exceeding the nozzle's diameter and potentially leading to blockages. Additionally, the crucial role of thixotropic behavior and self-supporting ability in determining the ink's formability was discussed (Figure 2-8b) 134. This behavior becomes particularly evident when the ink undergoes transitions from low to high shear stress and back, mimicking the actual printing process. Additionally, the study presented an informative map (Figure 2-8c) outlining the various printing qualities based on cellulose concentration and nozzle diameter, serving as a valuable guide for researchers in this field. The research pinpointed a cellulose concentration of 7 wt% as the most suitable for 3D printing, achieving an ideal balance between printability and rheological properties. Examples of 3D structures printed under these optimal conditions and cellulose concentration are illustrated in Figure 2-8d. 37 Figure 2-8: (a) Top: Steady-state viscosity as a function of shear rate, (a) bottom: Rheological storage and loss modulus as a function of oscillation stress, (b) three-interval thixotropy test of the printed cellulose ink (7 wt%), (c) printability diagram as a function of cellulose concentration and nozzle diameter, including the relationship between nozzle diameter, pressure, and speed, along with 3D products printed using cellulose inks of different concentrations. The inset shows an image of the printed cellulose filament just after extrusion and (d) images of DIW-printed cellulose objects with fully 3D configurations. Reproduced with permission 134. Copyright 2022, Elsevier. One of the primary challenges in developing 3D printable inks for DIW lies in the necessity to increase ink concentration, particularly for conductive materials like MXene or MWCNT pastes. This often results in self-restacking structures after drying. Addressing this, Zhou et al. 135,136 employed a series of CNFs with varied morphologies and surface chemistries to enhance the rheology of MXene- and CNT-based inks. This innovation facilitated 3D printability even at lower solid contents. The team crafted a CNF/MWCNT and CNF/MXene hybrid suspension with varying mass ratios, keeping the total solid content between 0.5 to 6 wt%. Comprehensive rheological tests were conducted, with each test's applicability during the 3D printing process demonstrated in Figure 2-9a. A notable observation was the shear-thinning behavior of the ink (Figure 2-9b), which significantly eased the printing process and contributed to the superior shape fidelity of the printed structures. However, the inks with lower concentrations (1 wt%) faced challenges. The viscosity recovery post-printing was slow, leading to potential structural collapse and deformation, as evidenced by the evolution of viscosity over time (Figure 2-9c) 136. The study also examined the inks' viscoelastic properties through Gâ² and Gâ²â² curves versus shear stress (Figure 2-9d), revealing a predominantly solid-like (elastic) behavior. However, as explained in the previous section, the solid-like behavior can be inferred from the graphs of Gâ² and Gâ²â² versus frequency, featuring Gâ² independent of frequency and linear relation of Gâ²â² with 38 frequency. Having said that, the dominance of the elastic nature of prepared inks is due to higher values of Gâ² than Gâ²â² (Gâ²>Gâ²â²). The frequency sweep tests provided further insights, where a linear dependence of Gâ² and Gâ²â² with frequency indicated liquid-like behavior, whereas solid-like behavior was characterized by a stable Gâ² across frequencies and a linear relationship of Gâ²â² with frequency. An intriguing aspect of their research involved exploring the effect of printing speed on the quality of 3D printing. As demonstrated in Figure 2-9e, an increase in printing speed resulted in a decrease in filament width, ostensibly improving printing quality 136. However, this approach had limitations, particularly in creating inconsistencies in structure, especially at the edges. They also showcased different 3D-printed gel architectures using the 6 wt.% ink, which exhibited the highest rheological properties. This study not only advances the field of 3D printable conductive inks but also offers critical insights into optimizing printing parameters for achieving high-quality, structurally sound 3D-printed objects. In their comprehensive evaluation of printability, Zhou et al. employed a multifaceted testing approach to assess the feasibility of their ink formulations 135. A straightforward yet telling test involved the inverted vial method, as depicted in the inset of Figure 2-9g. This test revealed that inks devoid of CNF were not suitable for printing, evidenced by their inability to maintain consistency when the vial was tilted, resulting in the ink flowing freely. To further study the rheological characteristics of the inks, they conducted extensive studies, including analyzing the relationship between shear stress and shear rate (Figure 2-9h). These curves were adeptly fitted using the power law, demonstrating a clear correlation between CNF concentration and rheological properties. Inks with higher CNF concentrations exhibited greater values of the flow behavior index (n) and consistency coefficient (k) 135. A critical observation was made regarding the viscoelastic properties of the inks, particularly the magnitude of the Gâ² in the plateau region and the yield stress. As the CNF loading increased, both Gâ² and yield stress showed a marked increase 135. This trend was most pronounced in the MXene/CNF10 (10 wt% CNF) ink, where Gâ² in the plateau region and yield stress reached their peak values at approximately 5100 and 191.56 Pa, respectively. These findings underscored that the MXene/CNF10 ink not only exhibited superior rheological properties but also achieved the highest quality in terms of 3D printability 135. 39 Figure 2-9: (a) Diagram illustrating the rheological properties of ink necessary for DIW 3D printing, (b) graph showing viscosity as a function of shear rate, with an inset displaying viscosity at a shear rate of 10â2 s-1, (c) plot of viscosity changes over time when alternating between low (10â2 s-1) and high shear rates (102 s-1), (d) graph of Gâ² and Gâ³ versus oscillatory stress, (e) filament width at various printing speeds under 45 KPa air pressure, with insets showing digital images of the extruded filaments, (f) various 3D-printed structures using MXene/CNF10 ink. Reproduced with permission 136. Copyright 2023, Wiley-VCH. (g) Viscosity versus shear rate with insets showing digital images of pure MXene, MXene/CNF5, and MXene/CNF10 gel inks, (h) shear stress versus shear rate fitted by the power law, and (i) viscosity changes over time for alternating low (10â2 s-1) and high shear rates (102 s-1). Reproduced with permission 135. Copyright 2022, Wiley-VCH. In their research, Jiang et al. 137 demonstrated that dewatering significantly enhances the rheological properties and printing fidelity of waterborne polyurethane (WPU) and CNF-based inks. By meticulously dewatering the WPUCNF/CNF suspension, they achieved composite inks with remarkable printability. They specifically focused on a WPUCNF/CNF ink with a 4:1 ratio, exploring various dewatering degrees of 20%, 30%, and 40%, corresponding to solid concentrations of 5.51%, 6.30%, and 7.36%, and CNF concentrations of 1.10%, 1.26%, and 1.47%, respectively 137. Their findings revealed that with increasing solid and CNF concentrations, printability and shape fidelity notably improved, particularly for the 4:1â40% ink. This ink exhibited edge stability and the highest shape retention angle of 81 degrees (Figure 2-10a). All WPUCNF/CNF inks demonstrated shear-thinning properties, with low-shear viscosities increasing with the degree of dewatering, indicating enhanced shape fidelity under low shear forces (Figure 2-10b). 40 Additionally, the Gâ² increased alongside the dewatering content, further contributing to shape fidelity post-printing. The yield stress also escalated with greater dewatering, displaying values of 10, 20, and 40 Pa for 20%, 30%, and 40% dewatering levels, respectively (Figure 2-10c). Crucially, the inks that exhibited the best shape fidelity, specifically those with 30% and 40% dewatering in 1:1 and 4:1 ratios, contained higher CNF concentrations (1.38â1.47%). Similarly, the moduli against shear stress and frequency were evaluated by Davila et al. 138 in the case of double network (DN) hydrogel. They stated that the appearance of shear thinning behavior could assist the flow of inks across the fine nozzle. Moreover, paste-like behavior at low shear rates (high viscosity) can maintain the shape of the printed texture immediately following ink deposition. They found that the increase in concentration enhanced the viscosity and shear moduli of the inks. Furthermore, the crossover between storage and loss moduli plots versus shear stress (Figure 2-10d) was used to determine the gel point of the inks. This gel point may refer to the yield point. The liquid-like behavior of the inks has been attributed to the higher values of Gâ²â² than Gâ² at high shear stress values, while the solid-like behavior was located prior to the gel point in which Gâ² was greater than Gâ²â². Also, Figure 2-10e presents optical microscopy images of the extruded filament, vividly illustrating how its shape transitions from droplets to uniform filaments in response to variations in viscoelastic properties. As the moduli increases, a more uniform filament is formed. This is crucial for the successful development of complex structures, as evidenced in Figure 2-10f, which shows a 100-layer scaffold printed without any deformation. This progression highlights the critical role of viscoelasticity in achieving consistent and structurally sound 3D-printed objects 138. 41 Figure 2-10: (a) Digital images showing 3D-printed structures, each about 10 mm in height, using 4:1 WPUCNF/CNF inks with 20%, 30%, and 40% dewatering levels, (b) viscosity versus shear rate for 4:1 WPUCNF/CNF inks at 20%, 30%, and 40% dewatering, and (c) loss and storage moduli versus oscillatory stress for 4:1 WPUCNF/CNF inks at 20%, 30%, and 40% dewatering. Reproduced with permission 137. Copyright 2021, Elsevier. (d) Storage modulus and loss modulus as a function of the strain (γ) and angular frequency, (e) optical images of different inks forming droplets to the filament by changing rheological properties, and (f) 3D-printed samples with (1) 10 layers, (2) 50 layers and (3) 100 layers. Reproduced with permission 138. Copyright 2019, Springer Nature. While numerous studies have explored the relationship between rheology and printing fidelity in DIW, a gap remains in the literature regarding the precise viscoelastic properties conducive to optimal printing. Many of these studies detail various rheological properties and their corresponding print quality. Yet, they often fall short of specifying exact values or ranges for viscosity, Gâ², Gâ²â², or yield stress that are ideal for DIW. This specificity is crucial for quantifying print quality based on rheological characteristics. According to the above-mentioned explanations, it can be concluded that prepared inks should meet several requirements from rheological viewpoints to fabricate stable and well-structured 3D-printed samples. Nevertheless, the rheological parameters have not been studied deeply and have been ignored in some previous research compared to the rheological windows defined in Figure 2-7. 2.4.3 Before deposition - filament formation In this step, the ink undergoes practical extrusion tests. These involve extruding the ink through a DIW nozzle under controlled conditions to observe its real-time behavior. This step verifies the rheological data, providing a direct correlation between the measured properties and the ink's performance during the printing process. Observations such as stringing, droplet, or inconsistent extrusion provide further fine-tuning parameters for rheological properties, ensuring 42 the ink formulation is optimized for maximum shape fidelity in the final printed structure (see Figures 2-6b1, b1, and b3). 2.4.4 After deposition â assessing printability through imaging techniques Once 3D structures are successfully printed, a critical phase is the evaluation of their shape fidelity. This step is crucial to confirm the shape fidelity and accuracy of the deposition process. Various imaging techniques are indispensable in this assessment, offering detailed insights into the structural integrity, resolution, and overall quality of the prints. Beyond merely assessing filament formation, it is essential to examine filament homogeneity and uniformity, key factors in creating high-quality 3D structures 87. To investigate shape fidelity comprehensively, several parameters must be considered. The primary step involves measuring the filament width and comparing it to the nozzle diameter, intending to determine the filament-to-nozzle diameter ratio. A ratio close to 1 indicates optimal filament formation (Figures 2-6c1-c4) 87. The next phase involves assessing the printed angles. Typically, a designed angle of 90° is the benchmark. By measuring the printed angle and comparing it to this standard, a closer alignment to 90° signifies improved fidelity. Pore geometry is another critical aspect, especially when assessing the overall structure. This involves comparing the area of the printed cell to that of the designed cell, with a ratio closer to 1, indicating enhanced shape fidelity 59. For 3D-printed structures, measuring the height of the printed sample and comparing it to the design height is an essential final step. This measurement confirms whether the 3D structure adheres to the intended dimensions (Figures 2-6c1-c4). Various techniques can be employed for these assessments, including various microscopic techniques such as optical microscopy, SEM, computed tomography (CT) scanning, atomic force microscopy (AFM), and profilometry. Each of these techniques offers unique insights, making them collectively invaluable in a comprehensive evaluation of print quality (Figures 2-6c5-c6). For example, in evaluating the dimensions of a single printed line, such as its width and height, a profilometer is often employed. Amini et al. 58 conducted a detailed study on the print quality of conductive CNC, paying special attention to a sample with the highest solid content (viscosity and yield stress). This sample was selected for its outstanding rheological properties, making it a prime candidate for an in-depth analysis of its printing characteristics. As illustrated in Figure 2-11a, a line printed using a nozzle with a 250-micron diameter exhibited a width of approximately 310 microns and a height of 150 microns. This result is 43 noteworthy, particularly considering that the filament spreading was limited to only 22%. Such precision in printing a single line is instrumental in building 3D structures with high fidelity 58. Another key factor in evaluating print quality is the straightness of the printed features. The fidelity of printing straight lines is reflected in the surface smoothness of these lines 104. In instances where 0.75% CNF with a lower level of nanofibrillation was used, the printed lines exhibited roughness. However, by increasing the degree of carboxymethylation at the same CNF concentration, the lines became smoother (Figure 2-11b). This improvement was evident in the surface line profile, which displayed a significant reduction in height deviation and became thinner with higher CNF concentration (Figure 2-11b) 104. These observations highlight the importance of fine-tuning material composition and printer settings to achieve optimal print quality. The studies demonstrated that careful consideration of factors like filament width, height, and surface texture is essential for creating precise and high-quality 3D-printed structures. Evaluating printing fidelity can also involve analyzing the sharpness of printed angles 104. In this process, the printing path is configured to create a 90° angle, allowing for a comparison between the intended design angle (θi=90°) and the actual angle achieved in the printed ink feature (θm), as depicted in Figure 2-11c. Occasionally, an overlap can occur at the angled edges where the ink is printed twice, leading to deformation in these areas (Figure 2-11b). Enhancing the ink's rheological properties is key to achieving precise printing. For instance, a sample with 1.5% CNF concentration demonstrated superior performance due to its higher viscosity and yield stress. Figures 2-11e, and f illustrated that samples with higher yield stress exhibited a smaller deviation between the printed and designed angles. In situations where the ink has low yield stress, the printed features tend to deform easily 104. Conversely, when the yield stress reaches a threshold that counteracts the ink's expanding force, the printing of accurate angles becomes feasible. Ink with yield stress below this critical value fails to resist the outward extrusion of the ink, resulting in the printing of acute angles at the edges. On the other hand, when the yield stress surpasses this critical point, obtuse angles are more likely to be printed, as shown in Figure 2-11g. To maintain consistency in the extrusion rate of ink at angled regions, the printing speed is adjusted relative to the applied pressure, ensuring it matches the rate at straight lines 104. As the extruding pressure increases from 20 psi to 50 psi, the printed features tend to deviate more acutely from the designed angle due to the increased volume of ink in that region. However, by carefully 44 controlling the applied pressure and printing speed, the angle difference can be minimized, as evidenced in Figure 2-11h, resulting in a more faithful replication of the intended design 104. Figure 2-11: (a) Evaluation of printing quality by optical images and profilometers. Reproduced with permission 58. Copyright 2023, American Chemical Society. (b) Fluorescent microscopic images displaying the cross-sectional shapes of printed lines under various CNF matrix conditions: 0.01 mmol/g with 0.75%, 1.14 mmol/g with 0.75%, and 1.14 mmol/g with 1.5%, along with surface profiles of the printed features. (c-h) Analysis of printing accuracy by assessing the sharpness of an angled line, (c) schematic diagram illustrating the measurement of the difference (θd) between the ideal angle (θi) and the actual measured angle (θm), (d) depiction of the overlapping area formed when printing the angled line, (e) variation of θd based on carboxylic content and concentration, (f) yield stress of the CNF matrix as influenced by carboxylic content and concentration, (g) fluorescent microscopic images of printed features at different angles under three CNF matrix conditions: 0.66 mmol/g with 0.75%, 1.14 mmol/g with 0.75%, and 1.14 mmol/g with 1.5%, including a schematic showing the relationship between ink yield stress and matrix yield stress on sharpness development, and (h) variation of θd with different extrusion pressures and rates. Reproduced with permission 104. Copyright 2021, Elsevier. During 3D printing, two distinct qualitative tests are crucial to evaluate their structural integrity. The first test involves suspending each composition in open air under an 8-psi air pressure at a distance of 10 mm from the nozzle. The objective is to observe whether the ink forms a continuous filament or breaks into droplets (as discussed in section 3-2-3). Continuous filament formation, as depicted in Figure 2-12a, indicates proper gelation and suggests that the composition is likely to maintain shape fidelity in the final scaffold 139â141. Conversely, compositions that form droplets, a phenomenon characterized as under-gelation, tend to lose shape fidelity during scaffold fabrication. The second test focuses on assessing the lateral porosity of the scaffold for each composition. This is done by visually/microscope examining the structure to identify any potential collapse in the overhang regions between two adjacent filaments of the lower layer 142. Compositions with lower viscosity are more prone to collapsing, leading to a reduction or disappearance of lateral pores. Quantitative assessment of shape fidelity can be done in terms of 45 lateral porosity, as illustrated in Figure 2-12a 142. The lateral collapse area factor is calculated using the following formula: Cfl =AalFD Ã ð¿ð â Ïrf2 Ã 100% 2-5) Here, Aal represents the actual area of the lateral pore, while LT, rf, and FD are the theoretical layer thickness, the radius of the filament, and filament-to-filament distance, respectively. A higher value of Cfl close to 1 signifies better preservation of lateral pores and, consequently, higher scaffold shape fidelity 142. Besides the lateral collapse area factor, assessing shape fidelity also involves measuring key structural elements like pore size (P), strut size (S), and the distance between the centers of struts (DS), as illustrated in Figure 2-12b. For example, the orthogonal distances between two adjacent pores and strut size were reported as a key factor to show the shape fidelity (Figure 2-12c, yellow lines) 103. These measurements are crucial for a comprehensive evaluation of the 3D-printed structural integrity. An additional intriguing parameter for assessing shape fidelity is the pore area ratio index. This index is calculated based on the average pore area of the first printed layer (A1) and the final printed layer (AF), as depicted in Figure 2-12d. An ideal self-supporting structure would have a pore area ratio index value of 1, characterized by the absence of shadow in the top view. Referring to Figure 2-12d, particularly insets (ei-hi), it is observed that samples with higher concentrations of CNC and enhanced viscoelastic properties exhibit less shadowing on the top layer. Therefore, this reduced shadowing effect is also indicative of improved shape fidelity 143. Ultimately, the measurement of the printability index (Pr) is a critical parameter for assessing shape fidelity. Any deviation from a Pr value of 1 indicates compromised shape fidelity, with a value of 1 representing the pinnacle of shape fidelity 144. We believe that this parameter should be more aptly termed the 'shape fidelity factor' rather than the 'printability index'. This renaming aligns better with its purpose, as it encompasses the final shape of the printed structure rather than focusing solely on the formation of a single filament. While some research has made a direct correlation between printability and overall shape fidelity (as shown in Figure 2-12e), it is essential to consider additional factors that influence the final structure. These include gravity, surface tension, and the time-dependent flow behavior of the ink, which are particularly impactful in the context of the entire printed structure and may not be evident in the examination of a single filament. Therefore, the 'shape fidelity index' would be a 46 more fitting term, capturing the broader scope of factors that define the final quality of the printed object. Figure 2-12: (a) Evaluation of lateral pores using both qualitative and quantitative methods. Reproduced with permission 142. Copyright 2018, MDPI, (b, c) ImageJ macro process: the CAD model demonstrates mechanical (cylinder) and biological (cube) performance with representative printed samples. A magnified region of interest (ROI) in the CAD model highlights pore size (P), strut size (S), and the distance between strut centers (DS). Captured images show the ROI with pores of interest, which are then converted using a threshold to measure orthogonal distances between adjacent pores (S). Reproduced with permission 103. Copyright 2012, Elsevier, (d) as-printed 40-layer scaffolds with different wt% of CNC using a 200 μm nozzle, along with corresponding top-view optical images. Scale bars are 1 mm. Reproduced with permission 143. Copyright 2024, Elsevier, and (e) Overview and macroscopic characteristics of the bioinks and their corresponding as-printed thin grid scaffolds. Reproduced with permission 144. Copyright 2022, Springer. In sum, to accurately assess shape fidelity, it is essential to consider all factors and indexes that give valuable information regarding printed inks. Although the printability index has been suggested as a straightforward indicator of shape fidelity, it is crucial not to overlook a comprehensive imaging analysis and other significant parameters, such as the pore area ratio index. The integration of advanced imaging technologies and predictive methods for shape fidelity could be instrumental in achieving flawless 3D-printed structures. Recognizing the significance of these aspects, these technologies and methodologies have also been extensively explored and documented in other valuable scholarly reviews 87,145â147. 2.4.5 Cryogels/Aerogels Cryogels/aerogels are a unique category of synthetic, ultra-lightweight materials with a porous structure originating from a gel 148. These materials are produced by replacing the liquid in 47 the gel with air, a process that preserves the intricate network of the original gel structure without significant collapse. Characterized by their porosity, often exceeding 80%, and a vast specific surface area ranging from 100 to 1000 m²/g, coupled with an ultra-low density (0.05-0.004 g/cm3), aerogels have found extensive applications 58. They are particularly beneficial in fields like aerospace engineering, energy harvesting and storage, tissue engineering, and environmental remediation. The advent of nanotechnology has further expanded the scope of aerogels. By integrating nanomaterials with various matrix materials, the fabrication of multifunctional composite aerogels has surged. These advanced aerogels serve a wide array of purposes, from thermal insulation to pollutant detection 149. In the field of EMI shielding, porous structures have demonstrated significant efficacy in attenuating incident electromagnetic waves while effectively minimizing secondary reflections. This effectiveness is contingent on the precise engineering of their electrical properties. These structures offer the potential for outstanding shielding performance, primarily due to their essential physicochemical properties, namely low density and high porosity 58,150. The upcoming section sheds light on the recent developments in cellulose-based functional aerogels, focusing on their fabrication processes. The insights and guidelines discussed here will be instrumental in chapters 6 and 7, specifically for evaluating the structure of fabricated pores. 2.4.5.1 Optimizing the dispersion and gelation processes of nanocellulose for enhanced cryogel performance Due to the abundance of active hydroxyl groups on its surface, nanocellulose is prone to forming both intermolecular and intramolecular hydrogen bonds, leading to self-aggregation and the intertwining of nanocrystals or nanofibrils 151. To achieve a stable and even distribution in water, various negatively charged groups like carboxyl, carboxymethyl, or sulfonic can be attached to the nanocellulose surface, facilitating electrostatic repulsion between the charged particles. This is evident in TEMPO-oxidized CNFs and sulfonated CNCs. In aqueous solutions, CNFs can form a three-dimensional network through hydrogen bonding and the entanglement of lengthy fibrils, enhancing the strength and elasticity of the resulting gel. This network structure prevents significant shrinkage during drying, allowing the resulting aerogel to maintain its integrity even with a low solid nanocellulose content 151. The production of nanocellulose aerogels typically involves a gelation process to strengthen the internal 3D network. This gelation can be categorized into chemical and physical crosslinking. 48 Chemical crosslinking involves adding crosslinking agents like citric acid or glutaraldehyde, which react with nanocellulose to form stable covalent bonds between cellulose chains. An example is the use of 1,2,3,4-butanetetracarboxylic acid (BTCA) with TEMPO-oxidized CNFs to form a network structure 152. Physical gelation, on the other hand, relies on weaker interactions like van der Waals forces and hydrogen bonds. Additionally, inorganic salts like FeCl3 and CaCl2 can be added to create reversible crosslinks. Carboxylic CNFs, for instance, can form hydrogels through ionic gelation with multivalent metal ions, which also facilitate the formation of metal-organic frameworks within the CNF network. Generally, chemical methods offer better control over the gel's porosity and specific surface area but are more costly and time-consuming than physical methods 151,152. Following the gelation process, adjusting the pre-drying conditions, particularly the freezing parameters, enables the formation of tailored structures. Adjustments can be made to the freezing temperature, the application of freeze casting techniques, or the altering of the freezing direction (refer to Figure 2-13 a-c). These modifications primarily influence ice crystal formation and subsequently affect the final characteristics of the material, especially morphological properties (see Figure 2-13 a-c). This strategy employs ice crystals as molds during the solidification of water, promoting efficient phase separation. The removal of the frozen solvent by sublimation then leads to the formation of a three-dimensional hierarchical network characterized by aligned pores. This method is essential in defining the structural and functional properties of the final product. 2.4.5.2 Drying processes The key objective in creating cellulosic cryogels/aerogels is to prevent network damage, such as the collapse of pore structure, which occurs due to capillary pressure induced by the bending of the air-liquid interface during solvent removal or drying of wet gels. Two main drying techniques are proposed: freeze-drying and supercritical drying. Although freeze-drying typically results in open-porous foams, known as cryogels rather than aerogels, due to their lack of mesoporous porosity, it is discussed here for a comprehensive overview. Freeze-drying: Excels in removing solvent from wet gels or hydrogels, leading to cellulosic cryogels and sponges with high porosity, extensive surface area, and low density. This method avoids the formation of a liquid/vapor interface through ice sublimation, known variably as lyophilization, cryodesiccation, or ice templating 153. As shown in Figure 2-13 d, it involves reducing the temperature below the liquid's triple point, followed by vacuum application and 49 controlled sublimation (isobaric condition). Ice crystal growth during freezing forms a dendritic network, impacting the morphology established in the gelation process. Therefore, controlling ice crystal growth is crucial to maintaining structural integrity and limiting shrinkage. The rate of sublimation, influenced by temperature, gel size and shape, and cellulose concentration, significantly affects the porous framework 151,152. Rapid cooling in liquid nitrogen results in smaller ice crystals and better preservation of the original dispersion structure. However, freeze-drying has downsides like high energy consumption, lengthy processing times, and the formation of microcrystals as the primary structural element. Supercritical drying: In supercritical drying, the liquid in the pores is replaced with a supercritical fluid, preserving the 3D porous structure of cellulosic aerogel. The process begins with immersing wet gels in a transitional solvent, such as ethanol or acetone, which is miscible with CO2. This method reduces the chances of meniscus formation and capillary pressure during solvent removal. Supercritical CO2 drying maintains the wet gel's porous structure with smaller pores after drying. Pressure and decompression rates are adjusted for optimal aerogel production, with CO2 being a popular choice due to its lower critical point, non-toxicity, cost-effectiveness, and environmental friendliness. Despite its advantages, supercritical drying is not widely used due to its complexity and the high cost of equipment 152,153. Figure 2-13: (a) Figure illustration of the preparation of bidirectional aerogel and SEM images for the cross-section of the bidirectional aerogel, (b) figure illustration of the preparation of unidirectional aerogel and its SEM images for the cross-section, (c) figure illustration of the preparation of common aerogel and its SEM. Reproduced with permission 154. Copyright 2021, Elsevier, and (d) Pressure-temperature diagram significant for hydrogel drying by freeze-drying and supercritical drying to obtain aerogels. 50 2.5 Shielding setup and background concept This section explores the fundamental aspects of shielding and the various methods used to analyze different samples. The focus of the EMI shielding assessments in this thesis is on the X-band frequency range (8.2-12.4 GHz). The choice of this frequency range is driven by its extensive use in areas such as radar systems, satellite communications, medical equipment, and aerospace 155,156. These fields require effective shielding, making the X-band significant. Additionally, the X-band is commonly used for testing purposes due to the widespread availability and standardization of testing equipment designed for this frequency range. The evaluation of EMI shielding capabilities of materials in the X-band is conducted using a Vector Network Analyzer (VNA), specifically the Keysight model P9374A and a WR-90 rectangular waveguide. The samples are attached to an offset and placed between WR-90 waveguide adapters, which are then connected to the VNA via microwave cables. Detailed explanations of the underlying principles of EMI shielding and the specific approaches used will be provided in later sections. 2.5.1 Transmission line theory and conceptual mechanism Transmission line theory is a theoretical framework used to analyze and design electrical transmission systems, such as waveguides and other interconnects. It is based on the principles of electromagnetic wave propagation, where electrical signals are treated as waves that travel through a medium. In transmission line theory, the electrical properties of the transmission line, such as its impedance, attenuation, and reflection coefficient, are modeled mathematically using a set of differential equations. These equations describe how the voltage and current signals change along the length of the transmission line and how they interact with the surrounding electromagnetic environment. In EMI shielding, transmission line theory is used to model and analyze the propagation of electromagnetic waves in a shielded environment. In this scenario, the EMI shielding material plays the role of the receiver, while the incident EM wave acts as the source. The air, which acts as the substrate of the EMI shielding material, should also be considered as an impedance section within the circuit. According to this theory, the shielding mechanism can be divided into three parts: reflection, absorption, and multiple reflections. An additional shielding phenomenon known as multiple internal reflections or internal scattering may occur due to certain structural features and material choices in EM shields. Reflection plays a fundamental role in the context of EMI shielding, wherein the incident electromagnetic wave is redirected upon encountering the surface of the shielding material, thereby preventing its penetration into the 51 interior of the shielded space 157. This redirection occurs due to an impedance mismatch between air (377 â¦) and the conductive surface (â0.8 â¦). Upon reaching the material's surface, the electrons present absorb a portion of the wave's energy, resulting in their oscillation. Consequently, a new electromagnetic wave is generated, propagating in the opposite direction to that of the incident wave and emanating from the surface. The magnitude of the reflected wave depends on the properties of the material, particularly its electrical conductivity. In addition to the reflection mechanism, another phenomenon arises when electromagnetic waves interact with EMI shielding materials or structures. A fraction of these waves undergo a process known as "absorption," wherein they are converted into heat. This absorption phenomenon manifests within materials or structures possessing finite electrical conductivity and electrical or magnetic dipoles. The attenuation of electromagnetic waves encompasses several types of losses, including resistance loss, dielectric loss, magnetic loss, and other losses resulting from factors such as destructive interference between incident and reflected waves. To achieve optimal EMI shielding performance, materials must possess either mobile charge carriers or electric and magnetic dipoles capable of interacting with incident waves, thereby reflecting and absorbing them. Furthermore, it is important to acknowledge that a certain portion of the incident electromagnetic waves also traverses through the shielding structures, referred to as "transmission." Multiple reflections and internal scattering are two more mechanisms that can greatly impact the overall shielding properties. Unfortunately, previous studies have mistakenly confused these concepts with each other. The multiple reflections mechanism occurs within thin EMI shielding films. When incident electromagnetic waves pass through the front interface of a thin EMI shield, they may reflect back to the front interface upon interacting with the back interface. Some of the reflected waves exit the shield, while the remaining waves continue to reflect within the shielding structure, contributing to subsequent transmissions. This process continues until the incident waves completely lose their energy and dissipate entirely. The multiple reflection process mainly depends on the shield's thickness and becomes insignificant when the thickness approaches or exceeds the skin depth. The skin depth is the distance below the shield's front surface where the electric field intensity decreases to approximately 37% of the incident wave's intensity. Conversely, if the thickness of the thin film EM shield is smaller than the skin depth, multiple 52 reflections between the front and back interfaces of the shield decrease the shielding effectiveness of the final structure 157. On the other hand, internal scattering or multiple internal reflections occur within materials that have numerous internal interfaces or designed porosities, such as aerogels, foams, segregated structures, and multi-layered shields. Unlike multiple reflections, internal scattering can enhance the shielding performance of a shield and improve its effectiveness by absorbing infiltrated electromagnetic waves and dissipating their energy within the structure with multiple interfaces or porosities. In this case, the numerous interfaces within a porous or smartly designed configuration with mismatching impedances contribute to multiple back-and-forth reflections within the shield. This prolongs the propagation pathway of electromagnetic waves before transmission, leading to an improved interaction between the shield and the waves, resulting in energy dissipation. The multiple reflections between the interfaces decrease the energy of the electromagnetic wave in the form of heat, thereby increasing the absorption rate of the shield and contributing to the absorption mechanism. This thesis focuses on strategies to enhance the internal porosities of 3D-printed cryogel. This approach increases the internal scattering of the shield by promoting back-and-forth reflections between the surfaces of the conductive path, resulting in significant dissipation of infiltrated or trapped electromagnetic waves within the structure. This design leads to a decent shielding performance through a highly efficient absorption-based mechanism. The following section will discuss the formulations and EMI shielding parameters in detail. 2.5.2 EMI shielding parameters and equations The efficacy of a shield can be quantified by assessing the degree of attenuation it provides to incident EM waves. The underlying mechanisms responsible for shielding can be elucidated through understanding the interactions between EM waves and the shielding material. The EMI Shielding Effectiveness (SE) of shielding materials is determined by calculating the (scattering) S-parameters, which are measured using a vector network analyzer (VNA). Various methods can be used to measure the EMI SE, including the co-axial transmission line, open field, shielded box, and shielded room methods. Among these methods, the co-axial transmission line method is commonly used to record the S-parameters over a wide range of frequencies. The total EMI SE, SE absorption, and SE reflection can be obtained by employing the waveguide method. Additionally, the corresponding power coefficients of reflectivity (R), absorptivity (A), and 53 transmissivity (T) can be calculated to find the EMI shielding mechanism. Generally, this approach involves sending a signal with a specific frequency from port one (S1) to the sample. The receiver then detects the transmitted signal (S21) and the reflected signal (S11), measuring the magnitude and phase of each received signal. The same procedure is repeated using port two (S2), where the system sends the same signal at the same frequency to the sample. The receiver on the opposite side measures the transmitted signal (S12) and the reflected signal (S22). This process is repeated for each frequency of interest. The reflectivity is defined by equation (2-6) and is a function of output impedance (Zout) and input impedance (Zin). Notably, Zout represents the impedance of free space (377Ω) 157. R = (Zin â ZoutZin + Zout)2 2-6) Also, Zin can be defined as a function of electric (E) and magnetic (H) field strength; Zin = |EH| 2-7) By considering the intrinsic impedance (ηi), propagation constant (γi), thickness (d), and the input impedance of downstream sections (Zi-1), equation (2-7) can be written like the following equations; Zin = Î·iZiâ1 cosh( γid) + ηi sinh( γid)ηicosh( γid) + Ziâ1 sinh( γid)= Î·iZiâ1 + ηi tanh( γid)ηi+ Ziâ1 sinh( γid) 2-8) ðð = âðððððð + ðððð 2-9) γi= âjÏÎ¼i(Ïi + jÏÎµi) 2-10) Where ð represents the angular frequency, and ðð, εi, and ðð, denote the permeability, permittivity, and conductivity of the ith layer of the shielding material, respectively. The permeability ðð can be expressed as ðð = ð0ðð, where ð0is the permeability of free space (1.26Ã10-6 H/m), and ðð represents the relative complex permeability of the material. Similarly, the permittivity ε can be expressed as ðð = ð0ðð, where ð0 is the permittivity of free space (8.85Ã10-12 F/m), and ðð represents the relative complex permittivity of the material. Additionally, η0 denotes the impedance of air, 377 Ω, and c represents the speed of light, 3Ã108 m/s 157. As mentioned, the obtained scattering parameters from the measurements are utilized to determine the EMI shielding coefficients, R, T, and A. The R coefficient represents the power of the reflected wave (PR) divided by the power of the incident wave (PI). It can be calculated using the complex S11 parameter as follows: 54 R = |S11|2 2-11) Similarly, the T coefficient corresponds to the power of the transmitted wave (PT) divided by the power of the incident wave. It can be calculated using the complex S21 parameter as follows: T = |S21|2 2-12) The A coefficient is of particular interest as it represents the ability of the shield to absorb electromagnetic energy. It can be calculated using the following Equation: A = 1 - R - T 2-13) The absorbance coefficient quantifies the amount of energy that is absorbed by the shield, and a higher value indicates a more effective shielding material. Another crucial parameter for assessing the overall performance of a shield is shielding effectiveness (SE). SE is the ratio of the total power of transmitted EM waves (PT) to the total power of incident EM waves (PI), expressed on a logarithmic scale. It can be calculated using the following equation; SET(dB) = 10 logPTPI= 20 logETEI 2-14) Where P and E represent the power intensity and electric field intensity, respectively. According to Schelkunoff's theory, the EMI shielding effectiveness (SE) can be decomposed into three components: reflection loss (SER), absorption loss (SEA), and multiple reflections loss (SEM). These components can be expressed as follows 157. SET = SER + SEA + SEM 2-15) The SER can be accurately measured using a simplified version of Fresnel's Equation for a monolithic conductive shield. It is given by: SER(dB) = 20 log(η + η0)4ηη0= 39.5 + 10 logÏ2ÏfÎ¼= 10 log(11 â R) 2-16) Here, ð and ð0 represent the impedance of the shield and air, respectively. Parameters ð, ð, and ðcorrespond to the magnetic permeability, electrical conductivity, and incident EM wave frequency, respectively. The electrical conductivity, magnetic permeability, and frequency of the incident EM waves directly influence the reflection loss. When an EM wave propagates through a lossy medium with finite electrical conductivity and electrical/magnetic dipoles, the wave is attenuated due to energy dissipation. The dissipated energy is converted into heat, and the attenuation constant (α) can be calculated using the following equation: 55 α = âÎ¼Îµ2[â1 + (ÏÏÎµ)2 â 1] 2-17) Where ε and Ï represent the dielectric permittivity and angular frequency, respectively. The total absorption loss (SEA) of the shield can be calculated using the following equation 157: SEA(dB) = 20 log eαd = 20(dδ) log e = 8.68dâÏfÎ¼Ï = 10 log(1 â RT) 2-18) Here, δ represents the skin depth or the penetration depth of EM waves into the shield, expressed in micrometers (µm). The absorption loss is influenced by the electrical conductivity, thickness of the shield, permittivity, and permeability. In shields with small thicknesses, the reflection from the bottom interface affects the transmission due to multiple reflections. The reflected wave from the bottom interface interacts with the top interface, causing subsequent reflections and resulting in a lower value of SET for the shield. The multiple reflection loss (SEM) can be calculated using the following equation: ðEM(dB) = 20 log( 1 â eâÎ±d) = 20 log( 1 â eâ2dÎ´ ) (2-19) In addition to the above parameters, another important parameter for lightweight shields, especially cryogels, is the specific shielding effectiveness (SSE) (dB.cm2.g-1), which considers both the density and thickness of the shield. SSE can be calculated using the equation: SSE =SETÏ Ã t 2-20) Where ð is the density (g cm-3), t is the thickness (cm), and SET is the EMI shielding effectiveness (dB). This parameter provides a measure of the shield's effectiveness in applications where weight is a critical factor, such as lightweight shields based on cryogel systems. In lots of studies involving semiconductive materials, it is assumed that Ï << ÏÎµ, and thus, the Ï term is neglected in the impedance equations for simplification purposes. However, this may introduce a non-negligible error when applying the Equation to conductive polymer composites with a conductivity of more than 0.01 S/cm. In this work, porous 3D-printed cellulose is considered as the EMI shielding material. Therefore, the circuit consists of two impedance components: the 3D-printed conductive structure and the air substrate. In section 3, we provide a more detailed explanation of the effect of macro-scale structures, specifically 3D-printed samples with rectangular geometries, on the EMI shielding performance. We explore how the arrangement and design of these structures influence shielding effectiveness and other relevant parameters. By 56 analyzing the behavior of the rectangular samples, we aim to gain insights into the relationship between the geometry of the printed structures and their ability to attenuate electromagnetic waves. 2.5.3 Shielding mechanism evaluation criteria and common mistakes To accurately assess the predominant shielding mechanism of an EMI shielding structure, it is crucial to analyze the shielding coefficients, namely the R and A values. By comparing the power of incident electromagnetic waves to the powers of the reflected and transmitted waves, one can determine whether the shielding operates primarily through absorption or reflection. If the R coefficient is greater than the A coefficient, it indicates a reflection-based mechanism. This is common in highly conductive shields due to the impedance mismatch between the shield and free space, resulting in secondary reflections in response to incident electromagnetic waves. Conversely, an A value exceeding R signifies that absorption is the main mechanism, where A > 0.5 indicates absorption dominance, and A < 0.5 or R > 0.5 suggests reflection dominance. This reflects a higher rate of electromagnetic wave dissipation and conversion to heat. However, some studies have incorrectly used the logarithmic values of SEA and SER to determine the shielding mechanism. They argue that if SEA > SER, absorption is the dominant mechanism. This approach is flawed because the shielding mechanism should be evaluated based on the total power of electromagnetic waves as indicated by the shielding coefficients, not the SE parameters that represent logarithmic power ratios. 2.5.4 3D-printed cellulose for EMI shielding application Cellulose-based 3D-printed shielding systems offer flexibility in design by computer-guided control of the 3D spatial arrangement, leading to hierarchal porous shields with controllable electrical conductivity. This minimizes the surface reflection and allows EM waves to penetrate and be trapped in the shield, highly dissipating their energy upon boosting the internal scattering between interfaces. The joint action of internal scattering with dielectric and/or magnetic losses makes 3D cellulose constructs formidable shields for attenuating EM waves 158,159. Several experimental attempts recognized the potential of 3D-printed cellulose constructs for shielding. Accordingly, Jiang et al. 160 developed a functional hybrid cellulose ink upon solution-blending of CNFs as a viscoelastic regulator with multi-walled carbon nanotubes (MWCNTs) as a functional additive to promote the electrical conductivity toward EM shielding. MWCNTs were selected because of their high aspect ratio, which eases the formation of a conductive pathway in the CNF framework. This enables a stable dispersion with high solid content, leading to the desired 57 viscoelasticity for DIW plus the required electrical conductivity to facilitate the EM shielding. The process starts with the ink preparation and rheology tunning toward pre-programmed 3D printing of scaffolds with engineerable hierarchal porosities (Figure 2-14a, b). Furthermore, benefiting from the controllable nature of 3D printing, the porosity of 3D-printed MWCNT/CNF scaffolds was tuned to alter their micro-scale pores and density on demand. In this regard, increasing the size of micro-scale porosities promoted the SSE and shifted the porous MWCNT/CNF framework toward a more absorption-dominant shielding construct (Figure 2-14c, d). The optimized 3D-printed MWCNT/CNF framework showcased an ultralightweight shield with density and SET of 0.11 g/cm3 and 43.5 dB, respectively, in the X-band frequency range (8.2-12.4 GHz). This shielding capability arises from the multi-scale porous framework of the 3D-printed MWCNT/CNF structure that facilitates the internal scattering among macroscopic and microscopic pores and dissipates the energy of EM waves in the form of heat (Figure 2-14e). More interestingly, the applicability of the shield was also proved by fluctuating the intensity of EM waves between the range 0 and 1500 μT/cm2 through unloading and loading the 3D-printed scaffold on an EM source. In another attempt, Chen et al. 161 developed a potent shielding system upon physical grinding of multilayer MXene sheets (m-Ti3C2Tx) and CNFs toward DIW of lightweight and highly conductive porous scaffolds (Figure 2-14f). The formed ultralightweight (density of 139.3 mg cm-3) 3D structured conductive scaffold benefited from abundant macro and micro-scale porosities, leading to the superior SET of 110 dB in the X-band frequency range. More interestingly, adjusting the number of layers and filament spacing during the 3D printing process allowed for generating hierarchical architecture with adjustable EM shielding between the range of 6.8-110 dB. It could also reach the proper shielding of 45 dB at gigahertz broadband (8.2-40GHz), enabling blocking a wide range of frequency range. However, owing to the high electrical conductivity of the shields arising from m-Ti3C2Tx, the 3D-printed construct showed a reflection-dominant shielding mechanism (reflection loss (SER)Ë3dB or reflectanceË0.5). The giant EM shielding capability of the developed scaffold roots in high electrical conductivity derived from m-Ti3C2Tx combined with the multiscale porosities that facilitate the internal scattering (Figure 2-14g). Moreover, the charge carriers in the conductive MXene/CNF network migrate and jump in response to an alternating EM field, forming microcurrents that promote conduction loss. Likewise, upon subjecting the 3D-printed scaffold to an alternating EM field, the generated local 58 dipoles between surface functionalities, i.e., =O, -F, or -OH, and Ti of m-Ti3C2Tx, generate dipole polarization 160,161. This, along with the polarization loss caused by CNFs and m-Ti3C2Tx plus the interfacial polarization generated via heterogeneous interfaces, highly dissipates the energy of EM waves, leading to a potent 3D-printed hierarchical cellulose-based construct. Figure 2-14: (a) Schematic illustration of the preparation steps of MWCNT/CNF inks and their 3D printing toward EM wave shielding; (b) controlling the micro-scale porosity or density of the 3D-printed MWCNT/CNF inks through variating the printing parameters; (c) the average SSE, and (d) EM shielding parameters of the developed shielding systems with different levels of porosities. (e) EM shielding mechanism of the developed 3D-printed constructs with well-defined hierarchical pores. Reprinted with permission from Ref. 160, Copyright 2023, Elsevier. (f) The overall process of the shielding design starts from the MXene synthesis, MXene/CNF ink preparation, and 3D printing of MXene/CNF scaffold toward potent EM shielding constructs with the ability to tune the 3D spatial arrangement and porosity. (g) EM shielding mechanism of the developed hierarchical porous 3D-printed cellulose-based structure. Reprinted with permission from Ref. 161, Copyright 2023, Elsevier. 59 Chapter 3: Synthesis and Characterization of Materials This chapter details the synthesis and characterization techniques used to develop and analyze the materials studied in this research. The processes involved in the preparation of these materials are described step-by-step, highlighting the methodologies employed to achieve desired properties. Additionally, this chapter presents the various characterization methods used to examine the structural, chemical, and physical properties of the synthesized materials. 3.1 3D printing A 3D bioprinter (Allevi 2, USA) equipped with 10 mL syringes was used to deposit the printed cellulose inks for this thesis. To command the x-y-z motion of the printer head, CAD drawing paths (SolidWorks 2016, Dassault Systems) were converted into G-code by commercial software. To optimize the resolution, the applied pressure and the print head speed were in a range of 15 (the minimum pressure that the inks flowed) - 65 (the maximum pressure to have a filament) psi and 6 (the minimum printer speed) - 15 (the maximum speed at which the ink could be printed entirely) mm/s, respectively. Considering the different hydrogel formulations and their viscosity, the print head speed and pressure were adjusted for each sample. Finally, the prepared inks were deposited onto different substrates. 3.2 Cryogel preparation The 3D-printed cellulose inks were frozen at -81 â using an ultra-low-temperature freezer for 24 h. Thereafter, these frozen samples were freeze-dried at -108 â and 0.005 mbar using a freeze-dryer (LabConco, 4liter) for two days, and finally, cellulose-based cryogels were obtained. 3.3 Characterizations Rheology: The rheological properties of the cellulose-based inks were measured at room temperature with a rheometer (Anton-Paar MCR 102) equipped with a 25 mm parallel-plate geometry at a gap size of 0.8 mm. The flow behavior of the prepared inks was evaluated by steady-state shear flow test at shear rates ranging from 0.01 to 1000 1/s. First and foremost, small amplitude oscillatory shear (SAOS) rheology tests were performed at 0.10-1000% strain at 1 Hz frequency to determine the linear viscoelastic region (LVR); a strain amplitude of ð¸ð = 0.10% was deemed safe to ensure that the frequency sweep tests are conducted within the LVR. Then, the SAOS test was performed by frequency ranging from 0.10 to 100 rad/s at 0.10% strain (frequency 60 sweep test). All the rheological tests were performed at ambient temperature. Due to its strong relationship with microstructural features, rheology is a robust tool for assessing ink printability. A proper ink for 3D printing needs to flow smoothly through the printer nozzle and sustain good shape fidelity when deposited on the collecting bed. Therefore, two important sets of rheological tests must be performed: 1) oscillatory stress/strain amplitude sweep test to explore the viscoelastic characteristics (storage (GÂ´) and loss (GË) moduli) and to find the LVR and yield stress/strain, 2) steady-state shear flow test to investigate the shear rate dependence of the viscosity, and to determine the minimum shear stress/rate needed for the ink to flow through the printing nozzle. Yield stress is the foremost rheological property to be assessed for high-quality 3D printing. Finally, the elastic recovery test was performed by dynamic oscillatory strain cycles at 1 Hz at ten consecutive cycles of low-high strains, wherein each cycle, the very low shear strain of 1% was applied for 5 min, followed by applying the high strain of 300% over the next 5 min. All tests were carried out at room temperature. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS): SEM was employed to analyze the microstructural and morphological characteristics of the 3D-printed cellulose-based constructs. Samples were prepared by carefully mounting small sections of the cryogels onto aluminum stubs using conductive carbon tape to ensure stability and electrical conductivity during imaging. The mounted samples were then sputter-coated with a thin layer of gold to prevent charging and enhance image resolution. SEM imaging was conducted using a high-resolution microscope (model: Tescan Mira 3 XMU equipped with an EDS detector (Oxford Instruments)), operating at an accelerating voltage of 10-20 kV. The electron beam interacted with the sample surface, producing secondary electrons that were detected to form detailed images of the sample's topography. Various magnifications were utilized to capture both the overall structure and fine details of the porous networks. The images provided insights into the uniformity, porosity, and interconnectivity of the PEDOT-decorated cellulose nanofibers, as well as the distribution and size of pores within the cryogels. Optical microscopy, ImageJ software, shrinkage, porosity, and density measurements: Morphological analyses are critical for understanding the relationship between the processing parameters, structural integrity, and functional properties of the fabricated materials. A digital microscope (Dcorn 7) was also used to capture images of the 3D-printed samples. The dimensions of the samples were measured using ImageJ software (spreading was calculated based on the inner 61 diameter of the nozzle). The exterior volumes of the printed samples before (Vw) and after (Vd) drying were measured using ImageJ and also a digital caliper, and the following equation was used to calculate the shrinkage. Shrinkage (%) =VwâVdVwà 100 (3-1) The density of freeze-dried samples was measured according to the following equation: Ï= m/V (3-2) Where "m" is the mass of the solid and "V" is the volume of 3D-printed samples, and Ï is the density of freeze-dried samples. Also, we measured the porosity based on the density of the freeze-dried and air-dried samples. Of note, following air drying, there is no void or empty space due to collapsing. So, porosity can be calculated using the following equation: Porosity = 1- (Ïf /Ïa) (3-3) Where Ïf is the density of freeze-dried samples and Ïa density of air-dried samples Mechanical properties: Mechanical properties of cryogels were examined via a Z010 Zwick/Roell universal tensile machine equipped with a 200 N load cell. To examine the ultimate tensile strength of samples, the cryogels were compressed up to 70% with a speed of 1 mm/min, and their maximum applied stress and Young's modulus were measured. Young's modulus was measured from the slope of the elastic region of the stress-strain curves. The cyclic compression (100 cycles) tests were performed at 20, 40, 60, and 70% compressive strains through loading and unloading at the speed of 1 mm/min. Tests were performed in triplicate for each formulation; the average results are presented in this thesis. Fourier transform infrared (FT-IR) spectroscopy: To have a better understanding regarding functional groups and their variations upon polymerization and interaction between cellulose, DBSA, and PEDOT, FT-IR spectra at ATR mode were evaluated on a Nicolet iS20 (Thermo Fisher Scientific) in the wavenumber ranging from 4000 to 500 cm-1. All the spectra of the samples were averaged from 32 scans with a resolution of 4 cm. X-ray diffraction (XRD): XRD patterns of the prepared cryogels were obtained via an XRD diffractometer (D8- Advance, Bruker, Germany). The radiation of Cu Kα (λ =0.15418 nm) at 40 kV and 30 mA was used to record the diffraction data in the 2θ region from 10 to 60Ë at a scanning speed of 5Ë/ min. Raman spectroscopy was analyzed using a Raman imaging microscope (Thermo Scientific DX xi, USA). 62 Thermogravimetric analysis (TGA): Thermogravimetric (TG) analysis was carried out on a TGA Q500 instrument (TA Instruments, USA) at a heating rate of 10 ËC/min from room temperature to 800 ËC under an N2 atmosphere to provide us with valuable information regarding the thermal degradation behavior of prepared cryogels. Based on the proposed method,1 the carboxylic content of CNC, ENCC, and their mixtures (without salt) was determined by conductometric titration on an 836 Titrando titrator (Metrohm, Switzerland, Department of Chemistry, McGill University). 30 mg of sample was added to 2 mL of 0.02 M NaCl solution and 100 mL Milli-Q water. By adding 0.05 M HCl dropwise while stirring, the pH was adjusted to 3.5. Then, 0.01 M NaOH was added at a rate of 0.1 mL/min until the pH reached around 11. The carboxyl content of the sample was determined from the middle part of the titration curve (which represents the volume of the weak acid). Atomic force microscopy (AFM): In order to investigate the morphological characterization of CNC, ENCC, and their mixtures (without salt), AFM images were obtained with a Cypher-S scanning probe atomic force microscope (Asylum Research, UK, Department of Chemistry, McGill University,). The samples were prepared according to the method developed by Yang et al.2 A freshly cleaved mica was attached to the magnetic stainless-steel disc and coated with 0.1 wt.% poly-L-lysine (PLL). The excess PLL was rinsed with Milli-Q water. A drop of the diluted sample (0.1 mg/mL) was directly placed on the surface of the mica, and the excess was washed with Milli-Q water. The prepared sample was air-dried overnight. The imaging procedure was performed in tapping mode using silicon cantilevers with a tip radius of 6 nm, a force constant of 37 N m-1, and a frequency of 300 kHz. Transmittance electron microscopy (TEM): To better understand the shape and size of CNC, ENCC, and their mixtures (without salt), TEM images were obtained using a low-voltage benchtop TEM (LVEM5 Benchtop Electron Microscope, Delong America, Department of Chemistry, McGill University,) with a maximum accelerating voltage of 5 kV. A 5 ï­L drop of a diluted sample (0.5 mg/mL) was placed for 5 min on a copper grid coated with a thin carbon film. Dynamic light scattering (DLS): By using DLS by Brookhaven light scattering instrument BI9000 AT digital correlator, the effective diameter of CNC, ENCC, and their mixtures (without salt) was determined. All the experiments were carried out by monitoring the scattered light at 90ï° scattering angle at room temperature (25 ï°C), and the concentration of the samples was kept at 0.1 wt.%. The surface charge of the prepared samples with a concentration of 0.1 wt.% was determined 63 by electrophoretic light scattering (ELS) using a Zeta-sizer Nano ZS (Malvern Instruments Ltd., UK, Department of Chemistry, McGill University,). Conductivity and electromagnetic interference shielding assessment: To determine the samples' conductivity, a Loresta GX resistivity meter (MCP-T700 model, Mitsubishi Chemical Co., Japan) was used. EMI shielding measurements were conducted in a WR-90 rectangular waveguide via a Keysight vector network analyzer (VNA) in the X-band (8.2â12.4 GHz) frequency range. All equations and measuring methods for this part were discussed in Chapter 2, Section 2.5.2. 64 Chapter 4: Mapping 3D Printability of Ionically Cross-linked Cellulose Nanocrystal Inks: Architecting from Nano to Macro-scale Structure Engineering the rheological properties of colloidal inks is one of the main challenges in achieving high-fidelity 3D printing. Herein, we provide a comprehensive study on the rheological behavior of inks based on CNC in the presence of given salts to enable high-quality 3D printing. The rheology of CNC suspensions was tailored by considering the nature of the electrolyte (10 different types of salts were tested) and their ion size, charge number, inter- and intramolecular interactions, as well as salt concentration (25â100 mM). A high printing fidelity was achieved in a narrow CNC and salt concentration range, significantly depending on the salt type. The structure-property relationship was explored in a "3D-printing" space (map), introducing a guideline for researchers active in this field. To further unravel the effect of salt type on morphological properties, CNC cryogels were developed by freeze-drying the printed structures. The results illustrated that enhancing viscoelastic properties rendered a denser structure with smaller pores. 4.1 Introduction 3D bioprinting, particularly DIW of biomaterials, has become a present-day technology to fabricate complex structures in the field of tissue engineering 162,163,164. Building up layer-by-layer biomaterials to create complex structures offers researchers abundant opportunities to mimic active organs and tissues 164. However, the quality of the printed structure significantly relies on the rheological features of the ink. A desired printable ink features shear thinning behavior under high shear rates and rapid elastic recovery after relaxation 165. The shear-thinning behavior guarantees a smooth and uniform flow during extrusion, and quick elastic recovery prevents the deposited filaments from lateral spreading, endowing printing fidelity 166,167. Aqueous suspensions of cellulose-based biomaterials in nanoscale forms (nanocelluloses), e.g., CNC or CNF, have a balanced rheological profile, rendering them suitable for constructing 3D structures 168,169. This is due to nanocellulose's outstanding properties, such as tunable surface chemistry, good mechanical strength, and adjustable rheology (shear thinning and quick elastic recovery) 170. Recent advancements in additive manufacturing of nanocellulose-based suspensions/gels have significantly expanded the library of available materials for 3D printing. However, the corresponding inks must meet some requirements to offer proper printability. For instance, CNC-based inks are not printable at low concentrations due to insufficient mechanical 65 integrity and poor rheological properties 171,172. Therefore, varying CNC concentration and tuning the inter- and intramolecular forces by adding secondary components to the ink, such as surfactants, polymers, salts, and surface modifications, have been practiced improving the printability of the inks 173. For instance, rheological studies on CNC inks in the 0.50â25 wt.% concentration range showed that the hydrogel with 20 wt.% CNC has optimal print resolution and fidelity 174. Knowing the microstructure of aqueous inks is required to gain a deeper insight into the effect of the above-mentioned approaches to achieve high-fidelity ink printability. Accordingly, CNC-based aqueous suspension can be categorized into the jammed or gelled state by manipulating governing forces among particles, i.e., van der Waals and electrostatic forces, as well as hydrogen bonding 170,175,176. The colloidal transition of CNC suspensions, including liquid-solid and isotropic-anisotropic transitions, dictates their final physical properties and structural features. These transitions depend on many factors, among which CNC concentration and stability in the aqueous phase are the most important ones. In this regard, at large, increasing the colloidal loading or decreasing CNC stability in water leads to the formation of colloidal glass or gel, respectively, making them suitable for 3D printing 177â179. Various chemical and physical approaches have been followed to manipulate the colloidal stability of CNC suspensions 180. Approaches such as chemical cross-linking using aldehyde or hydrazide surpass the physical methods (for example, dehydrothermal treatment) and regulate the CNC gel properties such as porosity. For example, Yang et al 76 reported an ultralightweight (5.6 mg/cm3) and highly porous (99.6%) CNC cryogel made by the chemical cross-linking approach. However, chemical methods suffer from being costly and time-consuming. Hence, as an alternative, enhancing the ionic strength of CNC suspensions by salt incorporation is among the most practical, cost-effective, and reliable physical methods. Adding salts can shorten the Debye length of CNC and reduce their electrostatic repulsive forces 181. So far, numerous research studies have explored the effect of mono and multivalent salts on the rheological properties of CNC-based suspensions and gels. It has been frequently observed that rheological properties (e.g., viscosity and storage modulus) are enhanced with increasing cation charge number and ionic radius 182â26187. 66 Although many efforts have been made to adjust the rheological features of CNC-based inks via salt addition, the effect of salt type, as a determining factor in the printability of CNC hydrogels, has remained untouched. So, to shed light on extending the advanced applications of 3D-printed hydrogels, such as the fabrication of ultralight cryogels with tunable micro/macroscale structures, a comprehensive study on the impact of salt type on the mechanical properties of 3D-printed hydrogel has yet to be carried out. Herein, we explore the effect of the size and strength of ions on the printing fidelity of CNC hydrogels and the mechanical integrity of their corresponding cryogels. To this aim, ten different salts (monovalent, divalent, and trivalent), e.g., LiCl, KCl, NaCl, MgCl2, CaCl2, CaSO4, AlCl3, and FeCl3, as well as salts with the capability of forming hydrogen bonds, e.g., NH4Cl and NaHCO3, were chosen to treat CNC inks physically. Accordingly, the microstructural features of the developed suspensions were examined via an oscillatory rheometer. These results were employed to optimize the composition of the CNC/salt inks to achieve high-quality 3D-printed structures, providing a printing map for each salt. Finally, cryogels derived from the 3D-printed CNC/salt structures were made by freeze-drying. Overall, our systematic study (1) provides a detailed view of the 3D structuring of CNC/salt inks, (2) focuses on the interrelationship of the rheological properties and shape fidelity of 3D-printed CNC-based inks, and (3) broadens the horizon of the additive manufacturing of CNC-based inks and the design of new functional materials. These 3D-printed scaffolds can be used as a versatile substrate for a multitude of applications, including gas adsorption, wastewater treatment, and electromagnetic interference shielding. 4.2 Experimental section 4.2.1 Materials Spray-dried cellulose nanocrystals were provided by CelluForce, Inc. (Montréal, Canada). NaCl, AlCl3 (anhydrous), KCl, LiCl, and FeCl3 were procured from Sigma-Aldrich. MgCl2 (anhydrous), CaCl2 (anhydrous), NH4Cl, NaHCO3, and CaSO4 were purchased from Fisher Scientific (see Table 4-1 for all materials' information). 67 Table 4-1: Materials' information Material name Lot-number Production date Cellulose nanocrystal (CelluForce NCV100-NAL90) C1A21001 2021-01-15 NaCl SLCC4856 - KCl SLCB4652 - LiCl MKCJ5259 - NaHCO3 SLBJ8214V 2018-05-01 CaSO4 W25D007 2021-06-15 MgCl2 Y19G027 - CaCl2 SLCB2168 - FeCl3 STBK1163 - AlCl3 SDZZC-WE - NH4Cl SLBQ8047V 2018-06-05 4.2.2 CNC inks preparation CNC aqueous suspensions were prepared at concentrations of 3, 5, and 7 wt.% at room temperature, followed by homogenization under magnetic stirring (2000 rpm for 5 min). CNC inks were prepared by adding salts to the CNC aqueous suspensions to reach salt molarities in the range of 25â100 mM. Immediately after salt addition, the ink was homogenized by mechanical stirring (2000 rpm, 2 min) to achieve a uniform gel. 4.2.3 3D printing The methodology for 3D printing is derived from the guidelines and methods described in Chapter 3 of this thesis. Figure 4-1 illustrates the 3D printing process of CNC/salt inks and the possible interaction between salt and CNCs. 68 Figure 4-1: Figure illustration of 3D printing of inks composed of CNC and given salts. 4.2.4 Cryogels preparation The methodology for cryogel preparation is derived from the guidelines and methods described in Chapter 3 of this thesis. 4.2.5 Characterization The methods of characterization and their detailed explanations are elaborated in Chapter 3 of this thesis. 4.3 Theory The rationale behind the choice of salts alongside possible interactions between constituents (e.g., CNC, water, and salts) is discussed in this section. This section aims to enlighten possible interactions between CNCs and salts by delineating the effect of various parameters such as the ion size, charge number, and inter- and intramolecular interactions (hydrogen bond (H-bond) and electrostatic interactions). This allows us to infer reasonably the influence of each salt on the properties of hydrogels (mechanical, rheological, and shape fidelity) and establish a structure-property relationship. In the present study, seven chloride salts (mono, di, and trivalent chloride salts) were chosen to explore the effect of cation charge number on the final properties of the CNC-based hydrogels. For instance, salts with the same charge number but different cation sizes, such as LiCl, NaCl, and KCl, were considered to study the sole effect of cation size. In addition, CaSO4 and NaHCO3 were selected to investigate the effect of anion size by comparing their results with those of chloride salts such as CaCl2 and NaCl. Finally, the impact of the H-bond, formed by the presence of N and O atoms, was investigated by introducing NH4Cl and NaHCO3 into the CNC 69 suspensions. As a result, the following parameters are defined to enable the establishment of structure-property relationships (the influence of each salt's chemistry on the rheological, mechanical features, and 3D printing fidelity of the prepared inks). Also, all possible interactions are shown in the Figure 3-2. 4.3.1 Electrical double layer (EDL) The particle's surface charge and its counterions form an ionic cloud around the particle 188. When a particle is negatively charged, it attracts the counterions, i.e., positive charges, surrounding it. As a result, a dispersed particle has an electrical double layer around it, including ions adsorbed on its surface and a film of countercharged dispersion medium), as shown in Figure 4-2a. In the case of CNC/salt suspensions, EDL of the CNC can be compressed by changing ionic strength. Therefore, a stronger attractive force between CNC particles compresses EDL. Debye length: Effective distance of the electrostatic interaction between particles (in our system, CNC particles) is the Debye length. Generally, Debye length decreases with increasing CNC concentration and ionic strength, resulting in more clusters and stronger network formation between CNCs 181,186. 4.3.2 Derjaguin-Landau-Verwey-Overbeek (DLVO) theory DLVO theory includes attractive van der Waals interactions and electrostatic double-layer interactions, which may be either attractive or repulsive. DLVO theory describes the balance between these two forces 178,189 4.3.3 Cation charge number CNCs have an intrinsic negative charge due to hydroxyl and sulfate half esters groups, so they are electrostatically stabilized in aqueous media 190,191. Salt can compress the EDL of CNC by offering enough counterions to balance surface charges 185. Also, adding ions to CNC suspensions reduces the Debye length by balancing repulsive and attractive forces. EDL is further compressed as the charge number increases because more positive charges are able to interact with negatively charged surface groups of CNCs 192â194. Therefore, as can be seen from Figure 4-2b, CNC layers are packed closer together as a direct result of reducing repulsive forces between CNCs. This is due to screening negative charges upon incorporating salts with a greater cation charge number, resulting in enhanced rheological properties 189. 70 4.3.4 Cation and anion size When cationic concentration and charge number are equal, larger cations interact more strongly with CNCs and compress the EDL more efficiently than smaller ones. It means that larger cations can provide positive charges in a larger spherical area, thereby increasing the screening efficacy of negative charges 192â194. The same trend (larger anions can increase the screening efficacy) can also be observed for anions (see Figure 4-2c and d). 4.3.5 Hydrogen bond (H-bond) An interaction between a hydrogen atom and another atom with a high affinity for electrons (electronegativity), such as fluorine (F), nitrogen (N), or oxygen (O), can form H-bond 186. CNC clusters are arranged in flat sheets (see Figure 4-2e) with hydrogen bonds between neighboring particles (intermolecular interactions) and adjacent layers (intramolecular forces) 186. Surface hydroxyl groups of the CNCs play a key role in such interactions. Water can penetrate into the clusters and interact with the CNC particles (three hydroxyl groups exist on the surface of CNC). Also, as mentioned, F, N, and O can form H-bond; thus, using salts containing these atoms could improve the rheological properties of CNC suspensions by forming stronger H-bonds, resulting in improved printing quality. 4.3.6 Dipole-dipole interactions An attractive force between two polar molecules (one partially positive and one partially negative) is called a dipole-dipole interaction 195. For example, the more electronegative fluoride atoms pull electron density towards themselves in hydrogen fluoride. So, fluoride will be partially (δ-) negative, and hydrogen will be partially positive (δ+) 185,186,193. Therefore, molecules with permanent dipoles attract each other electrostatically. Although such interactions are weak, they can play a role (Figure 4-2f). 4.3.7 Ion-dipole interaction This is a type of attractive force between a charged ion and a polar molecule. It can be formed when ionic compounds are dissolved in polar solvents like water. Also, this interaction turns stronger as the ion charge and size increase 196. 71 Figure 4-2: Effective parameters and possible interactions between CNCs, water, and salt: (a) Electrical double layer, (b) effect of charge number increase (for chloride salts) on the attractive forces between CNCs, (c) effect of cation radius size increase with the same charge number on the attractive forces between CNCs, (d) possible interaction between CNCs/CaSO4, and CNCs/NaHCO3, (E) inter- and intramolecular forces and H-bond, and (f) dipole-dipole interaction. 4.4 Results and discussion 4.4.1 Rheological properties The range of the linear viscoelastic response of the CNC gels was determined by performing a strain sweep test (Figure 4-3). Almost all samples showed a plateau-like behavior at shear strain amplitudes at ð¾0 = 0.1%, indicating the LVR limit. Inverting the vial containing the inks provides a visual observation of the viscosity and yield stress. Inks flowing upon inverting a vial, corresponding to low shear rates, lack yield stress required for 3D printing 197,198,199. As shown in Figure 4-3a, salt-free suspensions feature a liquid-like behavior (i.e., they flow upon inverting the respective vial). In addition, after 30 min, samples containing 3 wt.% CNC and 100 mM salt began to flow, while the gel-like suspensions containing 5 wt.% CNC and 100 mM salts did not flow when the vials were inverted (Figure 4-3b and c). As a result, samples with liquid-like behavior were excluded from further investigation, while those featuring gel-like behavior were considered for further investigation. To systematically evaluate 72 the relationship between the molecular structure and viscoelastic behavior, rheological measurements should be conducted in the region where the viscoelastic properties are independent of the applied strain (i.e., linear viscoelastic region (LVR)) 200,170. Accordingly, the range of the linear viscoelastic response of the CNC gels was determined by performing a strain sweep test (Figure 4-3 d and e). Figure 4-3: (a-c) Photographs of the inks stored in inverted vials to show fluidity: (a) Pristine CNC at various concentrations (3 wt.%, 5 wt.%, and 7 wt.%), (b) CNC/salt (3 wt.%/100 mM), (c) CNC/salt (5 wt.%/100 mM), (d, and e) Strain-sweep test at a fixed frequency of 10 radâ s- 1. Along with shear-thinning behavior, the inks should flow through the nozzle under the applied pressure. To learn about the required applied pressure for extruding an ink, the nozzle's shear stress (Ïr, see equation 4-1) should be more than the ink's yield stress (Ïy) 163,184,186,201. Shear stress applied on a fluid in a cylindrical geometry can be expressed by the equation 2-4 (plug flow). During 3D printing, the maximum applied pressure was 300,000 Pa, and the nozzle's diameter and length were 0.33 mm and 38.1 mm, respectively. As a result, the maximum shear stress, i.e., on the nozzle wall, was approximately 650 Pa, which was more than the yield stress of the samples measured via the stress sweep test (Figure 4-4a), indicating that these inks would flow inside the nozzle. According to Figure 4-4a, the obtained Ïy values for CNC containing mono, di, and trivalent chloride salt is less than 300 Pa. It shows the possible flow inside the nozzle. However, inks with higher yield stress generally produce a stiffer construct and better filament formation 87. However, high yield stress can also cause nozzle clogging. Take CNC/CaSO4 (5 73 wt.%/100 mM) as an example. Its yield stress is higher than the maximum shear stress in the nozzle; therefore, it causes nozzle clogging. As a result, the best condition for 3D printing is having the highest yield stress but not exceeding the nozzle's shear stress. These results can offer a better comprehension of the salt type, leading to improved printing quality. Figures 4-4b and c indicate the frequency dependence of the storage modulus (Gâ²) and loss modulus (Gâ²â²) of the CNC/salt suspensions. As seen, all suspensions showed a plateau-like regime for GÊ¹ and GÊº in the low-frequency region. In other words, both parameters are roughly frequency invariant at low frequencies, suggesting the formation of a 3D interconnected network throughout the aqueous media. Additionally, the linear viscoelastic response of the samples featured GÊ¹ > GÊº condition, confirming their gel-like behavior (i.e., elastic-dominant nature). The GÊ¹ of the prepared gels is more than one order of magnitude greater than GÊº, which is comparable with those of strong gels reported previously 187,202,182. Nevertheless, the strength of each gel depends on the salt type and can be compared by the values of their GÊ¹ and GÊº, which will be discussed later. Flow curve tests were also carried out on suspensions containing 5 wt.% CNC and 100 mM salts to investigate the shear rate dependence of the viscosity. From Figure 4-4 d, it can be seen that all samples exhibited a shear-thinning behavior, which is favorable for extrusion printing 163. For these samples, viscosity significantly decreased (for instance, for CNC/NaHCO3 (3 wt.%/75 mM) from 173.29 to 0.0048 Pa.s) as the shear rate increased from 0.01 to 1000 1/s, allowing a smooth extrusion of the inks through a narrow nozzle. Herein, as follows, we will explore other features of the inks in order to gain an in-depth understanding of the rheological results (rheological results for all samples are presented in Table 4-2 and Figure 4-5). To shed light on the shear thinning behavior of the prepared CNC-based gels (just the printed samples), the power-law model was fitted to the experimental data, shown in Figure 4-4d. ð = ð?Ì?ðâ1 (4-1) herein, k and n denote the consistency and flow indexes, respectively. The results of curve fitting based on Equation 4-1 are summarized in Table 4-3. As seen, the CNC suspensions interacting with NaHCO3 possess the highest consistency value (at 13,889) and a relatively low flow index at 0.0067 compared to the other specimens, revealing its superior shear thinning behavior. 74 Table 4-2: Yield stress and GÊ¹/GÊº Sample Yield Stress (Pa) GÊ¹/GÊº CNC (3 wt.%) 1.15 2.66 CNC/LiCl (3 wt.%/25 mM) 17.07 3.01 CNC/NaCl (3 wt.%/25 mM) 33.72 3.22 CNC/KCl (3 wt.%/25 mM) 42.12 2.41 CNC/NH4Cl (3 wt.%/25 mM) 117.65 2.11 CNC/CaCl2 (3 wt.%/25 mM) 51.06 2.86 CNC/MgCl2 (3 wt.%/25 mM) 106.3 2.79 CNC/AlCl3 (3 wt.%/25 mM) 120.88 3.01 CNC/FeCl3 (3 wt.%/25 mM) 130.41 2.59 CNC/CaSO4 (3 wt.%/25 mM) 170.94 3.76 CNC/NaHCO3 (3 wt.%/25 mM) 230.84 6.11 CNC/LiCl (3 wt.%/50 mM) 38.16 2.87 CNC/NaCl (3 wt.%/50 mM) 48.12 3.01 CNC/KCl (3 wt.%/50 mM) 52.68 2.11 CNC/NH4Cl (3 wt.%/50 mM) 54.48 2.53 CNC/CaCl2 (3 wt.%/50 mM) 118.54 2.56 CNC/MgCl2 (3 wt.%/50 mM) 170.16 3.76 CNC/AlCl3 (3 wt.%/50 mM) 127.36 3.28 CNC/FeCl3 (3 wt.%/50 mM) 199.11 2.87 CNC/CaSO4 (3 wt.%/50 mM) 291.64 5.32 CNC/NaHCO3 (3 wt.%/50 mM) 298.48 8.16 CNC/LiCl (3 wt.%/75 mM) 99.03 2.01 CNC/NaCl (3 wt.%/75 mM) 106.84 2.53 CNC/KCl (3 wt.%/75 mM) 116.03 2.12 CNC/NH4Cl (3 wt.%/75 mM) 129.34 2.65 CNC/CaCl2 (3 wt.%/75 mM) 130.77 2.54 CNC/MgCl2 (3 wt.%/75 mM) 133.07 2.76 CNC/AlCl3 (3 wt.%/75 mM) 213.75 2.11 CNC/FeCl3 (3 wt.%/75 mM) 240.52 3.42 CNC/CaSO4 (3 wt.%/75 mM) 291.28 4.12 CNC/NaHCO3 (3 wt.%/75 mM) 327.15 9.03 CNC/LiCl (3 wt.%/100 mM) 40.11 2.28 CNC/NaCl (3 wt.%/100 mM) 51.24 2.44 CNC/KCl (3 wt.%/100 mM) 58.34 2.48 CNC/NH4Cl (3 wt.%/100 mM) 118.67 2.79 CNC/CaCl2 (3 wt.%/100 mM) 120.88 2.64 CNC/MgCl2 (3 wt.%/100 mM) 160.99 5.38 CNC/AlCl3 (3 wt.%/100 mM) 226.41 2.71 CNC/FeCl3 (3 wt.%/100 mM) 480.67 2.47 CNC/CaSO4 (3 wt.%/100 mM) 747.18 5.45 CNC/NaHCO3 (3 wt.%/100 mM) 752.64 9.63 Plug flow 649.61 CNC (5 wt.%) 76.12 2.66 CNC/LiCl (5 wt.%/25 mM) 111.06 3.13 CNC/NaCl (5 wt.%/25 mM) 122.67 3.18 CNC/KCl (5 wt.%/25 mM) 139.13 3.98 CNC/NH4Cl (5 wt.%/25 mM) 130.22 3.26 CNC/CaCl2 (5 wt.%/25 mM) 207.79 2.73 CNC/MgCl2 (5 wt.%/25 mM) 209.42 4.79 CNC/AlCl3 (5 wt.%/25 mM) 182.77 3.36 CNC/FeCl3 (5 wt.%/25 mM) 232.19 4.04 CNC/CaSO4 (5 wt.%/25 mM) 291.45 5.59 CNC/NaHCO3 (3 wt.%/25 mM) 778.09 11.28 CNC/LiCl (5 wt.%/50 mM) 81.22 3.13 CNC/NaCl (5 wt.%/50 mM) 112.54 3.74 75 Sample Yield Stress (Pa) GÊ¹/GÊº CNC/KCl (5 wt.%/50 mM) 142.98 3.11 CNC/NH4Cl (5 wt.%/50 mM) 138.16 3.71 CNC/CaCl2 (5 wt.%/50 mM) 188.43 4.08 CNC/MgCl2 (5 wt.%/50 mM) 291.09 4.91 CNC/AlCl3 (5 wt.%/50 mM) 215.03 4.05 CNC/FeCl3 (5 wt.%/50 mM) 275.81 4.46 CNC/CaSO4 (5 wt.%/50 mM) 342.1 6.96 CNC/NaHCO3 (5 wt.%/50 mM) 711.57 9.31 CNC/LiCl (5 wt.%/75 mM) 101.51 3.19 CNC/NaCl (5 wt.%/75 mM) 121.54 4.98 CNC/KCl (5 wt.%/75 mM) 125.19 4.15 CNC/NH4Cl (5 wt.%/75 mM) 181.64 4.35 CNC/CaCl2 (5 wt.%/75 mM) 201.41 5.42 CNC/MgCl2 (5 wt.%/75 mM) 241.56 5.79 CNC/AlCl3 (5 wt.%/75 mM) 215.75 5.26 CNC/FeCl3 (5 wt.%/75 mM) 240.61 6.9 CNC/CaSO4 (5 wt.%/75 mM) 308.76 10.15 CNC/NaHCO3 (3 wt.%/75 mM) 918.06 11.22 CNC/LiCl (5 wt.%/100 mM) 129.06 6.15 CNC/NaCl (5 wt.%/100 mM) 132.57 6.06 CNC/KCl (5 wt.%/100 mM) 162.83 7.64 CNC/NH4Cl (5 wt.%/100 mM) 207.09 7.23 CNC/CaCl2 (5 wt.%/100 mM) 216.81 9.76 CNC/MgCl2 (5 wt.%/100 mM) 340.24 8.91 CNC/AlCl3 (5 wt.%/100 mM) 322.29 8.51 CNC/FeCl3 (5 wt.%/100 mM) 387.67 8.79 CNC/CaSO4 (5 wt.%/100 mM) 550.65 13.91 CNC/NaHCO3 (5 wt.%/100 mM) 1069.54 11.95 CNC (7 wt.%) 138.69 2.58 CNC/LiCl (7 wt.%/25 mM) 349.73 9.78 CNC/NaCl (7 wt.%/25 mM) 388.04 9.92 CNC/KCl (7 wt.%/25 M) 376.86 11.82 CNC/NH4Cl (7 wt.%/25 mM) 732.4 8.28 CNC/CaCl2 (7 wt.%/25 mM) 734.33 11.07 CNC/MgCl2 (7 wt.%/25 mM) 1068.75 8.24 CNC/AlCl3 (7 wt.%/25 mM) 1222.84 11.24 CNC/FeCl3 (7 wt.%/25 mM) 1806.77 11.32 CNC/CaSO4 (7 wt.%/25 mM) 998.11 10.29 CNC/NaHCO3 (7 wt.%/25 mM) 2295.54 11.04 Table 4-3: The acquired parameters of the power-law model via fitting procedures for CNC inks containing various salts. Salt k n R2 NaHCO3 13889 0.0067 0.9994 CaSO4 1430.92 0.0480 0.9995 FeCl3 411.53 0.0248 0.9993 AlCl3 9.15 0.1112 0.9974 MgCl2 160.15 0.0071 0.9992 CaCl2 4.42 0.0241 0.9947 NH4Cl 1.58 0.1383 0.9966 NaCl 0.66 0.1628 0.9918 KCl 0.74 0.0555 0.9906 LiCl 0.3238 0.0854 0.9992 Nevertheless, in order to comprehend the results better, we will examine the effect of other parameters, as follows: 76 4.4.1.1 Effect of cation charge number (valence) The results presented in Figure 4-4d indicate that the viscosity increased with an increase in cation charge number (for chloride salts) in the order of (Li+, Na+, K+) < (Ca2+) < (Al3+, Fe3+). This trend is in agreement with the previous studies of the gels formed by cellulose nanocrystals 167,189,192,203â205. It confirms that increasing the cation charge number strengthens the CNC network structure due to reduced Debye length, improved screening effects, and compressed EDL 206. In addition, ionic strength increases with increasing cation charge number, as shown in Table 4-4. In the presence of higher ionic strengths, the negative surface charge of CNC is largely screened out by more counterions, causing segmental aggregation between CNCs to form networks. Thus, as the cation charge number increased, the viscosity increased. However, the correlation between viscosity and cation charge number does not always follow the above trend (for example, the viscosity of CNC/CaSO4 suspension was much higher than that of trivalent chloride suspension). Hence, it can be deduced that other factors, such as bonding type (inter- and intramolecular interactions) and ionic radius, may also affect viscosity, which will be discussed in the following sections. 4.4.1.2 Inter- and intramolecular interactions It has been reported that van der Waals and hydrogen bonding interactions are dominant forces favoring CNC cluster formation, in line with the DLVO theory 207,208. As evident in Figure 4-4d, a higher viscosity is observed in the case of CNC suspensions containing CaSO4 and NaHCO3 salts compared to those containing mono, di, and trivalent chloride metal salts. A possible reason could be ascribed to the formation of different bonds in each system. For example, when CaSO4 and NaHCO3 dissociate in water, the chance of hydrogen bond formation in the system will increase significantly (see equations 4-2 and 4-3) 195. In the case of NaHCO3, when it dissolves in water, several types of interactions occur: the bicarbonate ion (HCO3-) forms hydrogen bonds with water molecules, both as an acceptor (through its oxygen atoms) and as a donor (through its hydroxyl group). Then sodium ions (Na+) form ion-dipole interactions with water molecules. And finally, bicarbonate ions (HCO3-) form ion-dipole interactions with water molecules. NaHCO3 â Na+ + HCO3- (4-2) The bicarbonate ion can then participate in an equilibrium reaction with water: HCO3- + H2O â H2CO3 + OH- (4-3) 77 On the other hand, oxygen atoms in CaSO4 could provide potential spots to make an H-bond with the possible OH groups in the system. Therefore, enhancing network formation in the suspension enhances the gel's viscosity by impeding the CNC's cluster mobility. Another example is NH4Cl, which is monovalent, but its viscosity is higher than the other monovalent chloride salts. Again, this result could be due to the hydrogen bonding between N in the NH4+ or H in NH4+ with the OH group of CNCs. As a result, the chance of H-bond formation in the CNC inks containing NaHCO3, CaSO4, and NH4Cl is higher than that of the corresponding chloride salts. With CNC/FeCl3, exciting interactions can be observed. FeCl3 can form hydrated complexes in an aqueous solution in which Fe3+ cations can be coordinated with the glycosidic oxygen of cellulose, forming an intermediate oxygen-metal coordination complex 207. Additionally, Fe3+ possesses a powerful ability to attract electron pairs and, thus, could potentially interact with the β-1,4 glycosidic bonds of the cellulose chain, weakening them. Further, Fe3+ might attack the oxygen atoms in the glucose ring's C2, C3, and C6 positions in the β-D-glucopyranose ring, breaking the hydrogen bonds 207,209,210. Generally, there is a competition between intramolecular electrostatic interaction formed in CNC/FeCl3 and intramolecular H-bonds within the CNC. The former can be considered as the screening effect that can weaken or break up the latter. As a result, broken H-bonds and their affiliated O and H atoms (in CNC) are able to take part in the formation of new interfacial interactions with the salt molecules. Furthermore, chloride salts can form complex cation formation with water molecules through a hydrolysis reaction. Take both FeCl3 and AlCl3 as examples; they possess similar reaction paths in which six coordination bonds would be created between cations and water molecules 211. Nevertheless, this character (Lewis acid character) would disappear faster in the case of FeCl3 than that of AlCl3 212. This leads to a higher viscosity of FeCl3 than AlCl3. In other words, FeCl3 contributed to the hydrolysis reaction rate, cleaving more of the glycosidic linkage and yielding a lower aspect ratio for the CNC rod-like particles. As a result, a thicker gel with higher viscosity would be observed in the case of CNC/FeCl3 hydrogel. It can be postulated that several factors influence the viscosity of this sample. In the rivalry for dominance, the screening effect or the ability of Fe3+ to form coordination complexes may prevail over other factors 209,210. 4.4.1.3 Effect of ionic radius Table 4-4 provides the ionic radius of the studied salts. For monovalent and trivalent chloride metal salts, the increase in viscoelastic properties (yield stress, viscosity, and G', Gâ²â²) with 78 increasing ionic radius is correlated with two parameters: first, a reduction in the Debye length (by increasing ionic radius) causes attractive electrostatic force between the CNCs. Second, a cation with a larger size can provide a positive charge across a larger area, enhancing the efficacy of screening 190,213. Both effects give rise to the formation of stronger gels of CNCs. Furthermore, a larger component (cation or anion) can hinder the free movement of CNC particles, resulting in increased viscosity. However, different trends are observed for divalent chloride metal salts. MgCl2 has a smaller ionic radius than CaCl2, but CNC/MgCl2 has a higher viscoelastic fingerprint than CNC/CaCl2. This can be explained by the decrease in solubility of the corresponding metal chloride in water: MgCl2 and CaCl2 have solubilities of 54.3 g/100 g and 75.5 g/100 g, respectively 213. Based on this trend, divalent cations with lower water solubility could bridge adjacent functional groups of CNCs, resulting in stronger gels 213. A similar trend has been reported for the adsorption of metal ions with varying valences via various polysaccharides, including chitosan and cellulose 187. Figure 4-4: (a-d) Rheological properties of CNC/salt (5 wt.%/100 mM) inks: (a) shear stress-sweep at 25 â. (b and c) frequency-sweep at a fixed shear strain of 0.10%. In figures a, b, and c, the storage and loss modulus are indicated by Gâ² and Gâ²â², respectively, and (d) viscosity curves versus shear rate (Î³Ë). 79 Figure 4-5: Yield stress for all samples (different colors show different CNC concentrations and maximum shear stress is indicated by pink diamond). Table 4-4: Properties of the cations 192,213 Sample Cation radius (Ã ) Cation charge number Ionic strength at 100 mM salt LiCl 0.90 1 0.1 NaCl 1.12 1 0.1 KCl 1.52 1 0.1 NH4Cl - 1 0.1 CaCl2 1.12 2 0.3 MgCl2 0.86 2 0.3 AlCl3 0.54 3 0.6 FeCl3 0.61 3 0.6 CaSO4 1.12 2 0.4 NaHCO3 1.02 1 0.15 So far, a thorough picture of the rheological response of the CNC-based suspensions in the presence of different salts has been furnished. It was shown that the rheological parameters of the CNC suspensions could be controlled by the type of salt, making these systems a versatile candidate for 3D printing, bringing us to the next section. 4.4.2 Materials shaping The low or high rheological fingerprints (viscosity, storage modulus, loss modulus, and yield stress) of the CNC/salt inks, depending on the CNC concentration, salt concentration, and salt type, cause droplet formation or nozzle clogging during printing, respectively (Figure 4-6a). Hence, we found out that an intermediate amount of the CNC concentration (5 wt.%) features the optimal rheological properties for most salts for 3D printing. Furthermore, it has been shown that both elastic modulus and yield point are correlated with the number of cross-links or clusters within the 80 ink 87. As a result of these interactions, the material behaves like an elastic solid for small perturbations, recovering its original shape. Table 4-5 shows that CNC/NaHCO3 (3 wt.% 75 mM) exhibits the highest yield stress and remains below shear stress (650 Pa), thus providing the most reliable print quality. GÊ¹/GÊº should also be considered in addition to yield stress. A higher GÊ¹/GÊº resulted in better printing quality, and high-quality printing can only be seen for samples with a GÊ¹/GÊº higher than 5 (see Table 4-5 and Figure 4-6b-k). Table 4-5: Yield stress, storage modulus over loss modulus, line width (hydrogel and cryogel state), and area of 3D-printed samples over designed shape (%). Sample Yield Stress (Pa) GÊ¹/GÊº Hydrogel Average Line Width (µm) Cryogel Average Line Width (µm) Printed Cell Area/Designed Cell Area (%) CNC/LiCl (5 wt.%/100 mM) 129 ± 6 6.15 ± 0.13 1748 ± 49 1158 ± 81 16.23 ± 3.5 CNC/NaCl (5 wt.%/100 mM) 132 ± 5 6.06 ± 0.09 1040 ± 61 802 ± 59 25.91 ± 4.3 CNC/KCl (5 wt.%/100 mM) 162 ± 4 7.64 ± 0.21 1354 ± 52 593 ± 74 54.60 ± 5.2 CNC/NH4Cl (5 wt.%/100 mM) 207 ± 8 7.23 ± 0.15 1287 ± 38 515 ± 57 56.09 ± 4.8 CNC/CaCl2 (5 wt.%/100 mM) 217 ± 7 9.76 ± 0.14 758 ± 41 497 ± 51 62.03 ± 6.1 CNC/MgCl2 (5 wt.%/100 mM) 330 ± 8 8.91 ± 0.09 514 ± 24 431 ± 38 73.59 ± 4.5 CNC/AlCl3 (5 wt.%/100 mM) 322 ± 12 8.51 ± 0.43 520 ± 16 389 ± 34 71.14 ± 6.2 CNC/FeCl3(5 wt.%/100 mM) 328 ± 9 8.79 ± 0.16 563 ± 11 371 ± 22 75.96 ± 5.4 CNC/CaSO4 (5 wt.%/75 mM) 339 ± 6 10.15 ± 0.10 399 ± 19 359 ± 19 84.59 ± 3.1 CNC/NaHCO3 (3 wt.%/75 mM) 367 ± 8 9.03 ± 0.05 404 ± 17 338 ± 16 85.17 ± 4.0 Figure 4-6b-k demonstrates a 30-layer structure printed with CNC/salt inks. As can be seen, monovalent metal chloride salts did not bestow the desired printing fidelity on the CNC suspensions, and printed structures with these inks lack high printing quality. Samples containing divalent and trivalent chloride metal salts showed no deformation, needle clogging, nor filament spreading. A slightly distorted edge can be observed for the CNC/MgCl2, CNC/CaCl2, and CNC/AlCl3 inks, which could be caused by the high ink viscosity, leading to difficulty in printing. In terms of printing quality, the best sample at 3 wt.% CNCs and 75 mM salt was CNC/NaHCO3, exhibiting no structural defects. In addition, the printed line width for this sample was 0.412 mm (see Table 4-5 and Figure 4-7), showing one of the best printing resolutions among the printed inks; for instance, this value was 0.758 mm for CNC/CaCl2 (5 wt.%/100 mM) ink. In addition to the digital microscope, SEM was used to investigate the printing quality in detail. Figure 4-7 shows the structure and dimension of the freeze-dried samples, and the results of this figure are presented in Table 4-5 (cryogel line width (µm) and the percentage of printed cell area over 81 designed cell area). The results for cryogels are similar to those for hydrogels. Likewise, both CNC/NaHCO3 and CNC/CaSO4 maintain their structure very well, and the final shapes of these two samples are 85.17 and 84.59% of the original designs, respectively. Figure 4-6: (a) Drop and filaments of the inks depending on the concentration of the CNC in the inks: 3 wt.% CNC inks formed droplets, 5 wt.% CNC inks formed uniform filaments and 7 wt.% CNC inks formed non-uniform filaments, leading to nozzle clogging. (b-k) Digital images of the 3D-printed structures with CNC (5 wt.%) inks containing different types of salt at a fixed loading of 100 mM (for all samples except CNC/NaHCO3, which includes 3 wt.% of CNC and 75 mM salt) (scale bars are 2 mm). 82 Figure 4-7: SEM images of different 3D-printed cryogels with CNC (5 wt.%) inks containing different types of salt at a fixed loading of 100 mM (for all samples except CNC/NaHCO3, which includes 3 wt.% of CNC and 75 mM salt) at different magnifications. The printability of the inks was classified into five different categories to represent the whole spectrum: non-printable, low-quality printable, fairly printable, high-quality printable, and clogged. Clusters or droplets formed immediately upon ink extrusion from the nozzle were considered non-printable. Inks with extremely high rheological properties formed non-uniform filaments, leading to nozzle clogging. Finally, printability was defined as the successful formation of the filaments on the collecting bed with different qualities (low, medium, and high). Figure 4-8 (bottom row) shows the digital images for the above-mentioned printing conditions. As can be seen in Figure 4-8, in contrast to the other samples, a high printing quality at the lowest CNC concentration (3 wt.%) was only achieved for the inks containing 50 and 75 mM NaHCO3 salt. The CNC/CaSO4 inks showed a similar behavior at 5 wt.% CNC concentration. However, the other salts could not bestow printability on 5 wt.% CNC suspensions at 50 and 75 mM. FeCl3 and AlCl3 made the 5 wt.% CNC suspension printable only at the high concentration of 100 mM. These results are consistent with the rheological characterizations, where NaHCO3 and CaSO4 showed the best performance in enhancing the viscoelastic properties of the CNC-83 based inks. Indeed, as the viscoelastic properties increased, printability improved to a critical point, above which dramatically high viscoelasticity caused nozzle clogging and non-uniform filament formation 214,215. For instance, we observed nozzle clogging in samples containing 5% CNC upon incorporating the strongest salt, i.e., NaHCO3, even at the low salt concentration of 25 mM. Hence, the rheological characteristics and, thus, the printability of the inks notably depend on the salt type in conjunction with the CNC and salt loadings. This triggered us to design a 2D map as a benchmark for 3D printing of the salt-incorporated CNC inks. To recapitulate, it is feasible to achieve high printing fidelity at low concentrations of CNC ranging from 3 to 7 wt.% by tuning the intermolecular interactions manipulated by salt. In other words, the wide variety of the salts employed in this study renders the CNC inks 3D printable at a specific CNC and salt concentration range. From another perspective, if one is restricted in using limited types of salt for a specific application, 3D printability can be achieved by tuning the concentration of the CNC. In conclusion, the salt and CNC concentrations should be optimized to yield high-quality printing structures. This can be simply achieved by the map provided in Figure 4-8, which can be used by researchers in the field for high-quality 3D printing of CNC-based inks. Figure 4-8: Concentration mapping of the salt-incorporated CNC inks. CNC concentrations are 3, 5, and 7 wt.%, and salt concentrations are 25, 50, 75, and 100 mM. The definition of each printing quality can be found in Section 3.2 of this chapter. 4.4.3 Tailoring mechanical performances of hydrogels and structural properties of the cryogels The cryogels were made from the printed structures by the one-step freeze-drying of the 3D-printed shapes to determine the effect of salt type on the mechanical and morphological properties 84 of the CNC-based ultra-light solids. The morphological characteristics of the cryogels were assessed using SEM imaging (Figure 4-9). 3D porous structures are evident in all cryogels. The inks with low viscoelastic properties (CNC without salt or CNC with monovalent metal chloride salts) render rough surfaces and highly porous structures with pore sizes ranging from a few tens to a few hundred micrometers. Large porous structures (for CNC and CNC/KCl samples) implied loosely integrated CNCs due to the lack of sufficient cross-linking sites. As the viscoelastic fingerprints of the hydrogels increased, the pores size of the corresponding cryogels decreased, leading to a more compact microstructure (Figure 4-9e-k). Hence, uniform and denser structures could be observed with divalent and trivalent metal chloride salts, CaSO4, and NaHCO3. This could be due to either the formation of strong interparticle bonding and H-bonding among CNCs or the fact that divalent and trivalent cations decrease the bond distance between CNCs 216. Figure 4-9k shows the SEM image of CNC/NaHCO3 cryogel, demonstrating a compact and homogeneous network. As a result of incorporating Na+, CNCs could form clusters via screening surface charges, leading to a more condensed network 180,217. According to Ping-Li et al. 190, by using sodium bicarbonate, the pH of the solution will rise significantly, resulting in an alkali solution (when compared with sodium chloride, the pH increases by about 1.20 units). Because sodium bicarbonate is a strong-alkali salt with a powerful buffer, more OHâ are formed. Therefore, they are able to form stronger interactions with cellulose crystals. Figure 4-9: SEM micrographs of cryogels (a) CNC (5 wt.%) without salt, (b) CNC/LiCl (5 wt.%/100 mM), (c) CNC/NaCl (5 wt.%/100 mM), (d) CNC/KCl (5 wt.%/100 mM), (e) CNC/NH4Cl (5 wt.%/100 mM), (f) CNC/CaCl2 (5 wt.%/100 mM), (g) CNC/AlCl3 (5 wt.%/100 mM), (g) CNC/MgCl2 (5 wt.%/100 mM), (i) CNC/FeCl3 (5 wt.%/100 mM), (j) CNC/CaSO4 (5 wt.%/75 mM), and (k) CNC/NaHCO3 (3 wt.%/75 mM). 85 Shape fidelity and integrity can significantly impact the overall performance of 3D-printed cryogel structures. Accordingly, the shrinkage behavior of the 3D-printed samples was evaluated by monitoring their volume change after freeze-drying. After freeze-drying, the CNC/NaHCO3 sample (3 wt.%/75 mM) retained its shape to a reasonable extent with a shrinkage percentage of less than 20% (see Table 4-6, and Figures 4-10a and b). In addition, 3D structures containing monovalent metal chloride salts were not even able to retain their complete shape, and the 3D-printed structure collapsed during freezing-drying (see Figure 5a for LiCl). Generally, the higher the rheological fingerprints of the hydrogels, the higher the volume retention of the corresponding cryogels. This is attributable to a more robust network structure formed by CNCs and salts, preserving the shape from over-shrinkage upon drying. Hence, the intermolecular interactions tuned by salt addition dictate the printing quality in the gel state and control the shape retention during drying. Table 4-6: Sample shrinkage characteristics Sample Vw (mm3) Vd (mm3) Shrinkage (%) CNC/LiCl (5 wt.%/100 mM) 1091.2 726.03 34 ± 4 CNC/NaCl (5 wt.%/100 mM) 1091.2 729.11 33 ± 4 CNC/KCl (5 wt.%/100 mM) 1091.2 743.4 32 ± 3 CNC/NH4Cl (5 wt.%/100 mM) 1091.2 756.43 30 ± 3 CNC/CaCl2 (5 wt.%/100 mM) 1091.2 761.02 30 ± 2 CNC/MgCl2 (5 wt.%/100 mM) 1091.2 794.78 27 ± 3 CNC/NaHCO3 (3 wt.%/50 mM) 1091.2 880.04 19 ± 2 CNC/CaSO4 (5 wt.%/50 mM) 1091.2 846.32 22 ± 3 CNC/AlCl3 (5 wt.%/100 mM) 1091.2 773.32 29 ± 3 CNC/FeCl3 (5 wt.%/100 mM) 1091.2 825.87 24 ± 3 As discussed, changing the salt type significantly alters morphology, drastically affecting the mechanical properties. Compressive stress-strain curves for the 3D-printed hydrogel samples at a maximum deformation of 50% are presented in Figure 4-10c-e. It is worth mentioning that the pure CNC ink suffered a lack of printability; hence, the compressive stress-strain curve of the 3D-printed CNC hydrogel is not presented here. Furthermore, pure CNC lacked a stable cylindrical shape in the hydrogel state, which was not suitable for a compressive test. As depicted in Figure 6c-e, the mechanical properties of the hydrogels are in line with rheological data. Figure 4-10c-e shows that samples with higher rheological fingerprints (viscosity and modulus) are mechanically 86 stronger (for instance, compression modulus increased dramatically upon increasing the rheological fingerprints, from 0.67 kPa in CNC/LiCl (5 wt.%/100 mM) to 24.67 kPa in CNC/NaHCO3 (3 wt.%/75 mM)), which is consistent with previous reports 167,169,179,218â220. The interactions between cellulose and salts could explain this. As discussed, the screening effect between CNC and salts causes the attenuation of Debye length, which could account for the integrity and stress transfer throughout the nanocrystals. Since incorporating NaHCO3 and CaSO4 into CNC showed the strongest screening effect (by forming strong electrostatic and H-bonds with CNC functional groups), the strengthening of CNC's H-bonds is more intense in these samples. Upon incorporating metal chloride salts into the CNC suspension, followed by gentle mixing via a mechanical mixer, the facile diffusion of metal cations and Cl anion would enhance the possibility of network formation, leading to the improvement of the mechanical strength 221. Figure 4-10: (a) Images of the 3D-printed cryogels; all scale bars are 5 mm, (b) the size of the 3D-printed cryogels made with CNC/NaHCO3 (3 wt.%/75 mM) at different dimensions (all sizes were measured by ImageJ, and this part (using caliper) is just for further justification), (c) compression stress-strain curves for 3D-printed hydrogels, (d) compressive modulus for different 3D-printed hydrogels, and (e) compressive strength at 50% strain for different 3D-printed hydrogels. 4.5 Conclusion In summary, ten different salts (monovalent, divalent, and trivalent) were selected in order 87 to cover the entire periodic table. As a first step, we were able to fabricate CNC/salt inks that were printed with high precision and fidelity without running into clogging or gel spreading issues. For example, the width of the printed tracks of the CNC/NaHCO3 ink was almost similar to the inner diameter of the extrusion nozzle, with only 26% of filament spreading. Additionally, compared to CNC/monovalent chloride salt, CNC/divalent chloride salt and CNC/trivalent chloride salt displayed higher rheological fingerprints in the following order: CNC/LiCl > CNC-NaCl > CNC-KCl > CNC/NH4Cl > CNC/CaCl2 > CNC/AlCl3 > CNC/MgCl2> CNC/FeCl3 > CNC/CaSO4 > CNC/NaHCO3. The same order was observed for printing quality. In the last step, the 3D-printed CNC/salt structures cryogels were achieved via a facile one-step freeze-drying method. The result showed that the type of salt greatly impacts the morphology of the resultant cryogels from the 3D-printed CNC/salt hydrogels. Also, the mechanical properties of the 3D-printed samples were examined. Generally, it was observed that the 3D-printed hydrogels prepared from CNC inks of higher rheological fingerprints were mechanically more robust. For instance, compression modulus increased dramatically upon increasing the rheological fingerprints, from 0.67 kPa in CNC/LiCl (5 wt.%/100 mM) to 24.67 kPa in CNC/NaHCO3. The overall aim of this systematic study was to significantly broaden the horizons of additive manufacturing of biomaterials by providing a detailed analysis of the 3D structure of CNC/salt inks, focusing on their rheological properties and shape fidelity. 88 Chapter 5: Crystalline and Hairy Nanocelluloses for 3D-printed Hydrogels and Strongly Structured Cryogels This work addresses two main challenges in the additive manufacturing of cellulosic inks and cryogels. Those are (1) the inability of electrostatically stabilized nanocrystalline cellulose (ENCC, a member of the family of hairy nano celluloses) to form three-dimensional (3D) printable hydrogels with sufficient rheological properties and (2) cellulose nanocrystal (CNC) cryogels' poor mechanical properties. The above limitations are effectively addressed by the hybridization of CNC/ENCC at an optimized weight ratio. Controlling the CNC/ENCC weight ratio leads to high customizability of the rheological properties of the hybrid inks, resulting in high-fidelity direct ink writing (DIW) into 3D objects. This stems from the solidification of hybrid inks via the hydrogelation of CNCs. Similarly, high tunability in the mechanical performance of the cryogels, derived upon freeze drying the printed hybrid inks, is achieved by manipulating the CNC/ENCC ratio. Compared to the neat CNC cryogels, the compressive strength of the CNC/ENCC (1:1) cryogels is 78% higher. Combined, CNC bestows high printability on the hybrid inks, while ENCC presence leads to the strengthening of the corresponding super porous solid materials. Our findings open new opportunities for sustainable, biocompatible, and lightweight cellulosic structured scaffolds that can be tailored for a broad spectrum of applications. 5.1 Introduction In recent years, there has been an increasing interest in 3D printing as a highly effective technology that can produce complex shapes and structures for various applications, particularly in biomedical engineering. Extrusion-based 3D printing, such as DIW, is the most common method. Through DIW, several colloidal gels, including those formed by nanocellulose, two-dimensional particles such as graphene, metal carbides and nitrides (MXenes), and polymer nanocomposites, have been explored. Although these systems have unique physicochemical properties, cellulosic gels are among the most versatile candidates for 3D printing, given their non-toxic, sustainable, biocompatible, and environmentally sound features. Nanoscale cellulose particles are available as stiff crystals or flexible fibrils, i.e., CNC and CNF. CNC has a relatively high degree of crystallinity, aspect ratio, and mechanical strength. Controlled oxidation of cellulose chains while maintaining the crystalline structure by cleaving the amorphous chains leads to nanoscale particles with a needle-like crystalline part and amorphous chains ("hairs") protruding out of the particle ends, called hairy nanocellulose (HNC). The 89 protruding soft biopolymeric chains bestow significant colloidal properties on the CNC, including tunable rheology, high stability, and charge density. HNC modification with carboxyl groups is referred to as electrostatically stabilized nanocrystalline cellulose (ENCC). Compared with CNC, ENCC has a higher charge density, rendering it more colloidally stable, and can be easily functionalized via chemical modification. ENCC has been used to prepare transparent and super hydrophobic films, to reinforce polymer composites, for directed metal nanoparticle synthesis, as a flocculant, a scale inhibitor, and a rheological modifier. The advantages of CNC and CNF in 3D printing include their surface modification characteristics and shear-thinning behavior in aqueous media. However, dilute suspensions of CNC and CNF are not fully suitable for 3D printing due to their low viscosity and drying shrinkage. Increasing the nanocellulose solid content (e.g., 30-40 wt.% CNC) surmounts these issues but causes other problems, such as nozzle clogging. Thus, optimizing the solid content in inks is a major challenge. A viable approach to tuning ink rheology for high-fidelity 3D printing is to tune the inter- and intra-molecular interactions, which can be achieved by the addition of secondary materials such as polymers, nanoparticles, and salts. ENCC suspensions offer lower viscosity compared to CNC at the same volume loading. This can be attributed to the extremely high charge density of ENCC, leading to higher colloidal stability, which prevents network formation. Therefore, formulating ENCC-based 3D printable hybrid inks is an arduous challenge. It has been widely shown that such CNC networks can be strengthened in the presence of mono-, di-, and trivalent salts. In order to design ENCC-based printable hybrid inks, we combined ENCC with CNC to trigger gelation in the presence of a strong salt. In our hybrid ink design, the total solid content (ENCC+CNC) and the amounts of salt were fixed at 10 wt.% and 50 mM, respectively, and the printability of the hybrid inks was manipulated by varying the ENCC/ENC ratio. By fine-tuning the rheological properties, hybrid inks with high printing fidelity were achieved. As there is no report in the literature on the 3D printing of ENCC, this study offers a comprehensive account of ENCC 3D printability. We also investigated the morphological and mechanical features of the cryogels derived from the CNC/ENCC hybrid inks. Thus, this study proposed new cellulosic inks and cryogels that have both great printing quality and notable mechanical qualities. 90 5.2 Experimental section 5.2.1 Materials ENCC was synthesized using bleached softwood Kraft pulp and cellulose nanocrystals were obtained via spray drying from FPInnovations (Montreal, QC, Canada). Sodium (meta)-periodate (NaIO4), sodium chloride (NaCl), ethylene glycol (C2H6O2), sodium chlorite (NaClO2), poly-L-lysine (PLL), and sodium bicarbonate (NaHCO3) were purchased from Sigma-Aldrich. Hydrogen peroxide solution (H2O2; 30%) and anhydrous ethanol (C2H5OH; 95.27%) were obtained from Fisher Scientific. 5.2.2 Synthesis of ENCC ENCC was synthesized according to the previous report. Briefly, 20 g of softwood pulp was added to a solution consisting of 77.4 g NaCl, 26.4 NaIO4, and 1.25 L DI water. The mixture was stirred at ambient temperature for 42 h, and the reaction beaker was sealed with aluminum foil to avoid the deactivation of periodate. After adding 20 mL of ethylene glycol, additional stirring was performed for 15 min, leading to the consumption of the residual periodate. Dialdehyde-modified cellulose (DAMC) was vacuum filtered with DI water through a 4-ï­m-pore-size nylon cloth five times. The dialdehyde groups were converted to carboxylic groups by adding DAMC to a solution containing 13.8 g NaClO2, 12.8 g H2O2, and 56.5 g NaCl in 965 mL DI water and then stirring at ambient temperature for 24 h. During the oxidation time, the pH was adjusted to ~5 with the intermittent addition of 0.5 M NaOH to monitor the color changes. In general, dark yellow indicates that more base is needed, light yellow suggests that the reaction condition is favorable, and white signifies the end of the reaction. Centrifugation was performed to remove any micro-fibrous pulp from the suspension (5000 rpm, 20 min). The addition of ethanol to the supernatant leads to the precipitation of ENCC. This was followed by centrifugation at 5000 rpm for 30 min and resuspension in DI water. Finally, purification of ENCC was achieved by dispersing and dialyzing (MW cut-off = 8â10 kDa; Fisher Scientific) against DI water for 24 h. The synthesis procedure is graphically depicted in Figure 5-1. 91 Figure 5-1: Illustrative presentation of synthesis of ENCC 5.2.3 Preparation of nanocrystalline and hairy nanocrystalline cellulose hybrid inks In order to investigate the effect of functional groups on the physicochemical properties of CNC and ENCC, and their 3D printing capability, five samples were prepared with a total of 10 wt.% solid content and different weight ratios. CNC/ENCC hybrid inks were prepared by adding salt (NaHCO3) to the aqueous suspensions to reach salt molarities of 50 mM (Table 5-1). Immediately after the salt addition, the hybrid ink was mixed by mechanical stirring (2000 rpm, 2 min) to achieve a uniform gel. Finally, the CNC/ENCC hybrid inks were placed in a 10 mL syringe and centrifuged at 1000 rpm for 2 min to remove air bubbles. Table 5-1: Specification of the prepared CNC/ENCC hybrid inks. Sample Total solid (wt.%) CNC weight ratio ENCC weight ratio NaHCO3 (mM) CNC 10 - - 50 CNC/ENCC (2:1) 10 2 1 50 CNC/ENCC (1:1) 10 1 1 50 CNC/ENCC (1:2) 10 1 2 50 ENCC 10 - - 50 5.2.4 3D printing The 3D printing methodology is described in Chapter 3 of this thesis. 5.2.5 Cryogels The methodology for cryogel preparation is derived from the guidelines and methods described in Chapter 3 of this thesis. 92 5.2.6 Characterization The methods of characterization and their detailed explanations are elaborated in Chapter 3 of this thesis. 5.3 Results and discussion 5.3.1 Carboxylated hairy nanocellulose synthesis The ENCC synthesis included periodate and chlorite oxidation, where the first resulted in breaking the C2 and C3 bonds of glucose units and the introduction of two aldehyde groups while cleaving the chains of the amorphous regions via a side reaction. Two of three hydroxyl groups of cellulose will be converted to aldehyde groups during full periodate oxidation, leading to ~ 12 mmol aldehyde per gram of cellulose. The hydroxylamine hydrochloride titration method was used to determine the aldehyde content of the synthesized dialdehyde-modified cellulose, which was found to be 7.2 mmol per gram of cellulose, corresponding to a conversion of 60%. Subsequently, the DAMC was converted to dicarboxylated cellulose (DCC) via chlorite oxidation. Depending on the number of carboxyl groups, the fibers remain intact or fall apart in a mixture of ENCC and dissolved DCC. In order to disintegrate the cellulosic fibers, the charge content should exceed 3 mmol per gram of cellulose; if it is lower, the fibers remain intact. The larger the charge density, the lower the ENCC content and the larger the dissolved DCC content. All cellulose above about 10 mmol/g goes into the solution. The addition of ethanol (or isopropanol) as a poor co-solvent can separate the mixture of ENCC and DCC at an intermediate charge content. As determined by conductometric titration, the charge densities (carboxyl content) of ENCC and CNC were found to be 4.37 and 0.09 mmol per gram of cellulose, respectively. This indicates that CNC has a very low charge density, while ENCC features a charge density higher than the maximum that crystalline regions can carry (~1 mmol per gram of cellulose), attributed to the existence of the amorphous regions in the structure. In the case of mixtures, the charge densities of CNC/ENCC (2:1), CNC/ENCC (1:1), and CNC/ENCC (1:2) were 0.61, 1.40, and 2.50 mmol per gram of cellulose, respectively. As expected, the number of carboxyl groups in the mixtures gets more abundant as the loading of ENCC increases. 5.3.2 Surface properties FTIR spectroscopy was used to determine the presence of functional groups in the prepared samples, as shown in Figure 5-2. The peak at 3340 cm-1 corresponds to the hydroxyl groups (-OH) stretching, while the peaks at 2894, 1416, 1315, and 1061 cm-1 correspond to C-H stretching, 93 -CH2 scissoring, -OH bending, and CH2-O-CH2 stretching vibrations, respectively. In the case of ENCC, the hydroxyl groups (-OH) stretching is evident from the wide peak at 3342 cm-1. Carboxyl groups account for the apparent peak at 1606 cm-1, indicating that ENCC was successfully synthesized. The other peaks at 2900, 1414, 1310, and 1054 cm-1 are assigned to C-H, -CH2 scissoring, -OH bending, and CH2-O-CH2 stretching vibrations, respectively. In comparison with ENCC (depending on the weight ratio), CNC/ENCC mixtures did not show any significant differences in their spectra. Figure 5-2: The FTIR spectra of (a) CNC, (b) CNC/ENCC (2:1), (c) CNC/ENCC (1:1),(d) CNC/ENCC (1:2), and (e) ENCC (samples contained no salt). 5.3.3 Morphology CNC (Figures 5-3a and 5-4a) features rod- or needle-like morphology, which is in good agreement with the literature. The nanoparticles are visualized in the TEM and AFM images. In the case of CNC, the nanoparticle's average length and diameter were 207 ± 40 and 22 ± 4 nm, respectively. For the pure ENCC (Figure 5-4b), the nanoparticles are well-dispersed in the system, and a reduction of size was observed in which the length and diameter of the nanoparticles were 115 ± 15 nm and 5 ± 2 nm, respectively. These results are in accord with previous results. 94 Figure 5-3: AFM images of (a) CNC and (b) ENCC particles. In the case of mixtures, a higher ratio of CNC leads to aggregation in the system, and the average size of the nanoparticles in CNC/ENCC (2:1) was reduced (in comparison with CNC), in which their length and diameter were found to be 160 ± 35 nm and 18 ± 5 nm, respectively (Figure 3c) which confirms the smaller size of the ENCCs. At a ratio of nanoparticles of unity, CNC/ENCC (1:1), the average length and diameter were found to be 145 ± 30 and 15 ± 5 nm, respectively (Figure 5-4d). Based on Figure 5-4e, a further increase in the ratio of ENCC in the mixture led to a reduction in the average length (122 ± 25) and diameter (9 ± 4 nm). 95 (a) (b) (c) (d) (e) Figure 5-4: TEM images of (a) CNC, (b) ENCC (c) CNC/ENCC (2:1), (d) CNC/ENCC (1:1), (e) CNC/ENCC (1:2) (samples contained no salt). 5.3.4 Size distribution and surface charge DLS was used to determine the dispersed particle sizes in the suspensions (0.1 wt.%), with the results compiled in Table 5-2. For ENCC, the effective diameter was 107 nm, which is close to the length of the individual ENCC obtained based on TEM and AFM images. This indicates the appropriate dispersion of nanoparticles in the water phase. Increasing the CNC ratio in the mixture resulted in an increase in the effective diameter of CNC-rich domains, which is in line with previous results.2, 3 The polydispersity index (PDI) was used as an indicator to further evaluate the dispersion characteristics of the nanoparticle in the aqueous phase. The incorporation of CNC in the ENCC suspensions increased the PDI, with the highest value corresponding to the neat CNC suspension. More importantly, the negative charge of the suspensions significantly decreased from -21 mV for CNC suspension (mainly due to the presence of sulfate half-ester groups) upon addition and increase in ENCC concentration and reached -69 mV for pure ENCC suspension. This is a 200 nm 200 nm 200 nm 200 nm 200 nm 96 direct result of attaching carboxyl groups to ENCC amorphous regions (hairs), leading to a more stable colloidal suspension. Table 5-2: DLS/ELS measurements for the prepared samples (without salt). Sample Effective diameter (nm) Polydispersity index (PDI) Zeta potential (mV) CNC 190 ï± 3 0.30 ï± 0.02 -21 ï± 2 CNC/ENCC (2:1) 180 ï± 4 0.28 ï± 0.03 -39 ï± 3 CNC/ENCC (1:1) 172 ï± 3 0.29 ï± 0.04 -58 ï± 3 CNC/ENCC (1:2) 159 ï± 5 0.30 ï± 0.03 -66 ï± 1 ENCC 107 ï± 1 0.16 ï± 0.03 -69 ï± 2 5.3.5 Rheology and 3D printing In this section, the rheological properties of the hybrid inks, including flow behavior and viscoelastic parameters, are presented.4 Hybrid inks suitable for DIW should feature shear-thinning behavior to ensure a smooth flow through the narrow printing nozzles and must spontaneously recover their gel-like behavior after deposition on the substrate. The gel-like nature stems from the microstructural features of the 3D printable hybrid inks, leading to a sufficiently high storage modulus and yield stress (Ïy). Due to these features, the extruded hybrid ink tends to retain its filamentous shape after deposition. Figures 5-5a-d show the rheological response of the hydrogels containing 10 wt.% of CNC/ENC in the presence of 50 mM salt (NaHCO3). We provided a comprehensive study on optimizing CNC concentration in the presence of 10 different salts for high-fidelity 3D printing. According to these results, CNC suspensions containing NaHCO3 can achieve the highest Gâ² and Gâ²â² values compared to other salts. Lenfant et al. 63 investigated the rheological properties of CNC and ENCC suspensions. Based on their report, CNC tends to form a gel above 1.0 wt.% via salt addition. This is a result of the network formation of CNC upon screening the negative surface charges. However, in the case of ENCC suspensions (<10 wt.%), the addition of NaCl has a weak effect, and CaCl2 leads to the formation of weak agglomerates. This can be attributed to the bridging between calcium ions and the carboxylic acid groups, causing ENCC networking. With regards to concentration, weak liquid-to-solid-like viscoelastic transition takes place at ENCC concentrations higher than 10 wt.%. At a concentration of 10 wt.%, the storage modulus of CNC 97 suspensions is almost four orders of magnitude greater than that of the ENCC. Thus, ENCC cannot form a substantial gel-like structure necessary for 3D printing. As can be seen in Figure 5-5, salt-incorporated suspensions behave differently depending on the ratio of CNC and ENCC. In the case of neat ENCC (10 wt.%) containing NaHCO3, the viscosity was roughly constant at low shear rates. This behavior confirmed that there is no substantial network formation in this system. However, a slight shear-thinning response at sufficiently high shear rates took place, which can be attributed to the preferred orientation of ENCC nanoparticles parallel to the flow direction. A strong shear-thinning behavior was observed when CNC was added to the ENCC suspensions. The shear viscosity decreased from ~104 Pa.s at a shear rate of ~ 0.1 sâ1 to ~ 0.1 Pa.s at a shear rate of ~1000 sâ1. This can be attributed to the disruption of the CNC/ENCC network structure under sufficiently high shear rates. Additionally, CNC caused the hydrogel viscosity to increase dramatically, which is a sign of a strong network formation. For instance, at a shear rate of 0.1 s-1, CNC/ENCC (1:2) has a viscosity of 386 Pa.s, and it jumped to 5260 Pa.s for CNC/ENCC (2:1). The viscosity increase can be attributed to the fact that ENCC is a highly anionic nanomaterial. Therefore, there was a strong electrostatic repulsion between ENCC nanoparticles, which was disrupted in the presence of CNC, bearing lower negative charges and longer length, resulting in a weaker electrostatic repulsion and a stronger network by bridging between nanoparticle-rich domains for high ENCC concentrations. Hybrid inks should flow smoothly through the printing nozzles with minimal internal resistance. To cause deformation or flow, the yield stress (Ïy) has to be less than the shear stress generated in the nozzle (Ïr). When a material reaches a certain stress threshold, known as the yield point or flow point, it becomes permanently deformed. There is a correlation between yield stress and the network characteristics within a hybrid ink, defined by cross-linking and entanglement density. As a result of these physical interactions, after deposition, a structured fluid maintains its original shape with small perturbations. During 3D printing, the maximum applied pressure was around 485 kPa, and the nozzle's diameter and length were 0.33 mm and 25 mm, respectively. Therefore, the nozzle applied stress on the wall was approximately 1600 Pa based on the plug flow equation in the cylindrical geometry. By plotting storage and loss moduli versus shear stress (stress amplitude test), the yield 98 stress (Ïy), where both Gâ²â² and Gâ² are equal, could be estimated for all samples. The yield stress determined in this way is equivalent to the yield stress derived from a stress-strain curve. According to Figure 5-5b, the obtained Ïy values for CNC/ENCC (1:2), CNC/ENCC (1:1), CNC/ENCC (2:1), and CNC (10 wt.%) were 16, 133, 145, and 1016 Pa, respectively (derived from the stress where the modules suddenly approach zero). A hybrid ink with high-yield stress generally produces a stiffer construct. In particular, the yield stress is suggested as a predictor of potential filament formation, and increasing values of this parameter are correlated to better filament formation. However, overly high yield stress causes nozzle clogging. The low yield stress of the ENCC makes the hybrid ink non-printable, as droplets are formed upon extrusion. Indeed, the ENCC and CNC/ENCC (1:2) featured a liquid-like behavior (see the inverted vial test). However, suspensions with a higher CNC loading, featuring sufficient yield stress, did not flow in inverted vials. Linear and nonlinear viscoelastic properties of the CNC/ENCC suspensions (containing salt) were analyzed to further understand the particles' ability to form a network structure and their impact on shape fidelity. As shown in Figure 5-5c, the hydrogels' linear and nonlinear viscoelastic regions were distinguished by a strain amplitude test. Figure 5-5c indicates that 0.1% strain amplitude is a suitable value for conducting the frequency sweep test in the linear viscoelastic region (LVR). The storage and loss moduli of the hybrid inks were thus monitored as a function of frequency in the LVR (Figure 5-5d). Generally, as mentioned in the literature,5, and according to Figures 5-5c, d, the ENCC suspension showed low viscoelastic properties. With increasing frequency, both loss and storage moduli of the ENCC suspensions increased slightly. Still, the loss modulus was higher than the storage modulus in the entire probed frequency window, confirming the ENCC suspensions' liquid-like behavior. Having amorphous, highly charged domains, ENCCs exhibit higher electrostatic repulsions, preventing the nanoparticles from physical associations. When CNC was introduced into the system, the hybrid inks exhibited a gel-like behavior (Gâ² > Gâ²â²) over the entire frequency range. Moreover, upon the increase in the loading of CNC, the viscoelastic properties were improved, and Gâ² reached its highest value for pure CNC ink. Hence, the ENCC presence in the hybrid inks deteriorated their rheological properties mainly due to two factors: 1) its high negative charge, preventing the nanoparticles from attractive physical interactions, and 2) its shorter length compared to CNC, corroborated by imaging 99 techniques and DLS. Indeed, the rheological percolation threshold depends on the aspect ratio of the nanoparticles, and the longer the nanoparticle is, the lower the percolation threshold will be. This fact can be justified by an increase in the number density of contact points and load-bearing junctions upon increasing the aspect ratio of the nanoparticles. Hence, physical characteristics could significantly alter the rheological response of the mixtures. Figure 5-5: (a-d) Rheological properties of CNC/ENCC hybrid inks containing salt (NaHCO3): (a) Steady-shear viscosity versus shear rate, (b) storage and loss modulus versus shear stress (stress sweep test), (c) strain sweep test, and (d) frequency-sweep at a fixed shear strain of 0.10%. In figures a-d, the storage and loss modulus are indicated by Gâ² and Gâ²â², respectively. In addition, salt addition (NaHCO3) is expected to screen the negative charges. Therefore, NaHCO3 acts as a physical cross-linker by screening the negative charges of the nanoparticles, specifically CNC, signifying the viscoelastic response of the formed network within the hybrid inks. Hence, Gâ² of the CNC/ENCC (1:2) experienced a dramatic increase, from ~1 kPa to 21 kPa, upon increasing the CNC ratio (CNC/ENCC (2:1)). Considering the rheological properties, CNC/ENCC (2:1) hybrid hydrogel can be considered a promising hybrid ink for DIW. These unique properties include shear-thinning behavior, sufficient viscoelastic response, and yield stress. So far, a thorough picture of the rheological response of the CNC/ENCC-based suspensions has been established. Moreover, it was shown that the rheological response of the CNC/ENCC 100 suspensions can be readily tuned by varying the ratio of CNC/ENCC, rendering these systems a versatile candidate for 3D printing, a subject that is discussed in the next section. 5.3.6 Material shaping Printability refers to both the quality of the 3D-printed structure compared to the original design and the shape fidelity of the extruded filaments. Different approaches can be practiced to evaluate hybrid ink extrusion printability. Images were visually taken with a digital camera (Figure 5-6) to assess and visualize the rheological features, shape fidelity, and filament formation of the devised hybrid inks. These results are in accord with the quantitative rheological properties discussed in the previous section. ENCC and CNC/ENCC (1:2) lack sufficient yield stress (see Figure 5-6a and Table 5-3), and due to their liquid-like behavior, they were excluded from further investigation. The yield stress of the samples was visualized by the inverted vial test (Figure 5-6a). Also, Table 5-3 is a summary of the yield stress and storage modulus over loss modulus for all samples, which can provide a correlation between rheological results and printing fidelity. As shown in Table 3, CNC/ENCC (2:1) exhibited the highest yield stress among CNC/ENCC hybrid inks and remained below the generated shear stress (1600 Pa) in the nozzle, thus providing a reliable print quality. GÊ¹/ Gâ²â² should also be considered in addition to yield stress. A higher GÊ¹/ Gâ²â² resulted in better printing quality, and based on Figure 5-4b, high-quality printing can only be seen for samples with a GÊ¹/GÊº higher than 5 (see Table 5-3 and Figure 5-5). The value of GÊ¹/GÊº depends very little on frequency, except for ENCC (for this sample, the average of GÊ¹/GÊº in the frequency range of 0.1-1000 rad/s is reported in Table 5-3). Table 5-3: Yield stress and storage modulus over loss modulus for different samples. Sample Yield Stress (Pa) GÊ¹/GÊº ENCC (10 wt.%) 0 0.14 ± 0.03 CNC/ENCC (1:2) 16 ± 2 2.35 ± 0.19 CNC/ENCC (1:1) 133 ± 4 6.40 ± 0.23 CNC/ENCC (2:1) 145 ± 3 8.59 ± 0.13 101 Figure 5-6: (a) Photographs of the hybrid inks stored in inverted vials to show fluidity. (b) Digital images of the 3D-printed structures with ENCC1/CNC1 hybrid inks containing 50 mM NaHCO3. (b2) and (b3) Zoom-in view of as printed ENCC1/CNC1, and (b4) as printed sample after drying. (c) Digital images of the 3D-printed structures with ENCC1/CNC2 hybrid inks containing 50 mM NaHCO3. (c2) and (c3) Zoom-in view of as printed ENCC1/CNC2, and (c4) as printed sample after drying. (d) Digital images of the 3D-printed structures with CNC ink containing 50 mM NaHCO3. (d2) and (d3) Zoom-in view of as printed CNC1, and (d4) as printed sample after drying (all scale bars are 5 mm). CNC and CNC/ENCC hybrid inks are used to print a 40-layer 3D structure before and after drying (see Figures 5b-d). The samples containing higher CNC concentrations showed no deformation or filament spreading, confirming their good printing quality. Generally, by increasing the CNC concentration, the printing quality was enhanced, which correlates with the enhancement of the rheological properties, as shown in Figure 5-5. A slightly distorted edge can be observed for the CNC/ENCC (1:1) hybrid ink (see blue lines in Figures 5b2 and 5b3), which could be caused by the low hybrid ink viscosity and viscoelastic properties, leading to filament spreading. According to Figure 5b-d, the best printing quality was achieved for pure CNC ink containing 50 mM salt, which had no structural defects, followed by CNC/ENCC (2:1) hybrid ink. All in all, CNC/ENCC (2:1) hybrid inks provide excellent print quality, although it is not as good as CNC by itself. These results show that the 3D structure of CNC/ENCC was kept intact by intermolecular bonds alone, without the need for any chemical cross-linkers or adhesives. The 3D-102 printed samples showed impressive shape fidelity even after being freeze-dried, as illustrated in Figures 5-6b4, c4, and d4. 5.3.7 Microstructure and mechanical performances of 3D-printed CNC/ENCC cryogels Cryogels' morphologies were assessed using SEM imaging (Figure 5-7) upon freeze-drying the hybrid inks. In all cryogels, 3D porous structures are evident. Hybrid inks with the highest viscoelasticity (CNC 10 wt.% containing salt) produced highly porous structures with pore sizes between 10 to 200 ï­m (Figure 5-7a). A surprising feature of Figure 5-7a is the elongated or platelet-like pore morphology of the CNC cryogel. This observation is because the number of contact points between CNC particles increased in the presence of the salt. Increasing the ratio of ENCC significantly changed the morphological features. That is, the elongated morphology has been altered to disorder pores morphology. Moreover, as the ENCC concentration increased, the pore size in the hybrid cryogel decreased. This can be attributed to the interaction between CNC and ENCC, which brought the particles closer to form a close-packed structure during freezing. In other words, by increasing ENCC concentration, a higher number of contact points per gel volume (more interaction points) are formed during freezing and ice crystal formation. Thus, these contacts lead to the formation of smaller ice crystals, which results in smaller macropores as the ice crystals are removed upon freeze-drying. It is hypothesized that upon freezing, as the ice crystals are formed, the amorphous parts of the ENCC are brought together due to volume exclusion, leading to the formation of physical interlockings in these amorphous regions and, thus, stronger cell walls, preventing ice crystals growth. To verify this fact, we conducted mechanical tests on the cryogels. In the next section, we also provide a comprehensive discussion on the rationale behind the superior mechanical properties of the amorphous region upon drying. 103 Figure 5-7: SEM micrographs of cryogels (a) CNC (10 wt.%) with 50 mM NaHCO3, (b) CNC/ENCC (2:1), and (c) CNC/ENCC (1). All in all, the morphology is significantly altered by increasing ENCC concentration, drastically affecting the mechanical properties. Here, we evaluated the mechanical features of the samples under compressive deformation (Figure 5-8). The compression curves for porous materials usually show two distinct regions: a small-strain region with collapsing walls and a large-strain region with the pores' cell walls in contact with opposite-facing walls. As shown in Figure 5-8a, in contrast to rheological properties (mechanical properties in the wet state), cryogels derived from pure CNC ink are mechanically softer in the dry state. As evident in Figure 5-8a, CNC (10 wt.%) cryogel's maximum stress (compressive strength) reached 104.5 kPa when compressed by 70%. Additionally, the CNC (10 wt.%) cryogel had the lowest compressive modulus (0.25 kPa) compared to other cryogels. The poor mechanical properties of the pure CNC cryogel can be attributed to weak physical bonding among CNC particles (i.e., they are only held together with weak cellulose-cellulose bonds, e.g., hydrogen bonding and van der Waals forces). Additionally, acid hydrolysis in the synthesis of CNC leads to the elimination of amorphous regions (e.g., cellulose chains "hairs") and hemicelluloses (acting as a binder which provides a more dense and compact structure). This results in poor mechanical properties for the CNC cryogel. The observed inferior mechanical response of CNC (10 wt.%) cryogel is in accordance with the previously reported data in this field. Compressive stress increases in the entire probed strain window upon increasing the ENCC concentration in the suspensions. CNC/ENCC (1:1) had the highest compressive strength and Young's modulus, 489 kPa and 1.8 MPa, respectively. Compared to CNC (10 wt.%) cryogel, the 104 compressive strength increased by 78% in the case of CNC/ENCC (1:1) cryogel. The improvement of mechanical properties in CNC/ENCC (1:1) can be attributed to the following reasons: (1) the presence of dicarboxylated cellulose (DCC) chains (hairs) in ENCC structure leads to the formation of ester bonds between carboxyl and hydroxyl groups and as a result, provides a more compact structure. The removal of water during freeze drying favors the formation of ester bonds. (2) The smaller size of ENCC particles (in comparison with CNC particles according to the AFM image and DLS results) facilitates more bonding and inter-chain linkage between functional groups. (3) The high carboxyl content of ENCC results in more swelling in which, upon drying, the DCC chains (hairs) protruding from the nanoparticles enable interfacial adhesion and act as a binder, thus playing a key role in reinforcing the cryogels' structure. Generally, the poor rheological properties of hairy nanocellulose (ENCC) make it impossible to print high-quality 3D structures, and cellulose nanocrystal (CNC) cryogels' poor mechanical properties make them impractical for real-world applications. Based on the appreciable mechanical properties and excellent printability of the CNC/ENCC hydrogels, this research has bridged the gaps in this field. Figure 5-8: Mechanical properties of the 3D-printed cryogels, (a) compressive stress versus strain, and (b) porosity and (c) density of different 3D-printed cryogels. 5.4 Conclusion In summary, without requiring any external agents, ENCC suspensions were made printable while significantly improving the mechanical strength of the CNC cryogels. To this aim, CNC/ENCC systems were developed using a simple mixing technique. Printing quality and mechanical properties, two major challenges in the area, were examined. Different samples (10 wt. % total solids), including CNC, CNC/ENCC (2:1), CNC/ENCC (1:1), CNC/ENCC (1:2), and 105 ENCC, were prepared. Based on the rheological characterization and material 3D shaping via direct ink writing, we concluded that CNC modifies the rheological features of ENCC suspensions, and the order of the printing quality is as follows: CNC> CNC/ENCC (2:1)> CNC/ENCC (1:1). Interestingly, an opposite trend was observed in the mechanical integrity of the samples in the solid state (cryogels) upon freeze dying the 3D-printed hydrogels. Regarding microstructural features of the cryogels, a more compact structure was obtained by increasing the loading of ENCC, which provided a better mechanical strength: CNC/ENCC (1:1)> CNC/ENCC (2:1) > CNC. Combined, by taking advantage of CNCs to improve the printing quality of the cellulosic hydrogels and ENCCs to enhance the mechanical properties of the cellulosic cryogels, CNC/ENCC hybrid inks are shown to be excellent candidates to develop rationally designed 3D lightweight scaffolds. 106 Chapter 6: Surfactant-mediated Highly Conductive Cellulosic Inks for High-resolution 3D Printing of Robust and Structured Electromagnetic Interference Shielding Cryogels Technological fusion of emerging three-dimensional (3D)-printing of cryogels with gel processing enables the fabrication of lightweight and functional materials for diverse applications. However, 3D-printed constructs via direct ink writing (DIW) for fabricating electrically conductive structured bio-based cryogels suffer several limitations, including poor electrical conductivity, inferior mechanical strength, and low printing resolution. This work addresses these limitations via molecular engineering of conductive hydrogels. The hydrogel inks, namely CNC/PEDOT-DBSA, featured a unique formulation containing well-dispersed cellulose nanocrystal (CNC) decorated by poly(3,4-ethylene dioxythiophene) (PEDOT) coupled with dodecylbenzenesulfonic acid (DBSA). The rheological properties were precisely engineered by manipulating the solid content and the intermolecular interactions among the constituents, resulting in 3D-printed structures with excellent resolution. More importantly, the resultant cryogels following freeze-drying exhibited a high electrical conductivity (110±12 S.m-1), outstanding mechanical properties (Young's modulus of 6.98 MPa), and fire-resistance ability. These robust cryogels were employed to address pressing global concerns about electromagnetic pollution, with specific shielding effectiveness of 4983.4 dB.cm2.g-1. Importantly, it was shown that the shielding mechanism of the 3D cryogels could be manipulated by their geometrical features, unraveling the undeniable role of additive manufacturing in materials design. 6.1 Introduction Shaping bio-inspired patterned structures featuring low density (less than 0.5 g.cm-3), high porosity (> 80%), and mechanical strength (greater than 1 MPa) is a demanding endeavor to further broaden their practical application range. So far, several conventional techniques, such as vacuum-assisted filtration (VAF) 222, dip-coating 223, solution blending 199, ink-jet printing 224 or screen printing 225 have been utilized to develop patterned structures and highly integrated electronics based on biopolymers. However, all of these techniques have limitations, such as two-dimensional (e.g., low aspect ratio) patterns, and for example, in the case of ink-jet printing, there is also a requirement for low-viscosity inks, which rules out several effective inks from being used with 107 this method. These limitations restrict the innovations and applications of patterned conducting polymers using these methods 226. Contrary to conventional methods, additive manufacturing (AM) technologies enable the production of customized samples with intricate microstructures in a cost-effective and time-reasonable manner, which can be utilized in a wide variety of applications, including tissue engineering, electronics, and so on 227â229. AM can also efficiently produce rapid and reproducible micro-scale structures, even with very complex shapes. Among various AM techniques, three-dimensional (3D) extrusion printing offers excellent design flexibility, programmability, and straightforwardness when fabricating micro-scale structures 181,226. This emerging technology is broadly applied in both scientific and industrial applications, such as the manufacturing of various parts of vehicles, aerospace, and medical applications. From the materials point of view, bio-based structures obtained from natural biopolymers such as cellulose, chitin, chitosan, starch, and alginate are in high demand owing to their environmentally friendly features such as biodegradability, non-toxicity, and low carbon footprint 230,231. Biopolymers offer several advantages, such as adjustable biophysical and biochemical features, similarity to the extracellular matrix, and capability to encapsulate living cells, making them paramount in soft tissue engineering fields 230,231. 3D extrusion printing of biopolymers is believed to be the way toward green and sustainable development of such shapeable materials. Among different biopolymers, cellulose, the most abundant natural biopolymer on earth, has been widely used in the production of multifunctional bio-composites. The nanostructured cellulose family includes cellulose nanofibers (CNF), lignocellulose, and crystalline nanocellulose (CNC), among which CNC with facile processability in water and tunable surface chemistry has harnessed great attention. Nevertheless, the 3D printing of CNC-based inks is hindered due to the lack of desired viscoelastic properties, resulting in structure collapse over the printing process. In order to meet the requirement of the 3D bioprinting process, the prepared ink must hold sufficient viscosity to retain its shape during the process. Furthermore, the ink must have a solid-like behavior to preserve the 3D structure after the 3D printing. A state-of-the-art approach of in situ polymerization of an intrinsically conductive polymer, i.e., PEDOT (Poly(3,4-ethylenedioxythiophene)) in the presence of a surfactant and in combination with a salt can be devised to make cellulose conductive 177. PEDOT polymer is highly favorable for electronic applications due to its high electrical conductivity (up to 6259 S.cmâ1 for 108 thin films and 8797 S.cmâ1 for single crystals) 232. However, its insolubility (both its monomer and polymer) in some solvents hinders its potential for extrusion 3D printing of aqueous-based PEDOT inks 232. Thus, incorporating PEDOT into the CNC framework via the in situ polymerization method and establishing grafting between PEDOT and CNC's functional groups could pave the way to address the limitations of both material classes. Electromagnetic (EM) waves are becoming more prevalent due to the rapid development of electronics, which pose a severe risk to human health, electrical devices, and the environment 233,234. Biological damage may result from excessive electromagnetic waves, especially for pregnant women and people with weak immune systems 226. In order to overcome this crucial problem, the development of efficient and low-cost EM wave-protecting materials is in high demand. In recent years, numerous studies have been conducted on developing materials to shield unwanted EM radiation, ranging from metals to petroleum-based conductive polymers 235â239. However, because of their high conductivity and, thus, impedance mismatches, such materials reflect a major portion of incident waves, resulting in secondary environmental electromagnetic pollution 240â243. To this end, using 3D cellulose-derived patterned structures for EMI shielding can offer a viable solution to the issues of non-renewable resources and provide enhanced shielding performance with minimized reflection due to the porous structures of cryogels 244. Indeed, the fabrication of 3D porous conductive networks with engineering structures from nano- to macro-scale can be an efficient strategy to solve the impedance mismatching issue, thereby resulting in high-performance shields with low reflection 243,245. The distinctive micro/macro porous structures of electrically conductive cryogels promote the induction of local capacitive-like structures through a large number of dipoles, leading to an enhanced dielectric loss of electromagnetic waves 246,247. Additionally, the porous structures lengthen the transmission path of electromagnetic waves by means of multiple internal reflections or scattering, facilitating the absorption of the waves and hence resulting in higher EMI shielding effectiveness values 246,247. Considering all these existing challenges and the windows of opportunities, as seen in Figure 6-1, we have strategically designed, synthesized, and 3D-printed a cellulose-based aqueous ink of CNC/PEDOT-DBSA and then developed its cryogel. Upon fine-tuning the rheological properties via optimizing the constituent concentrations, we achieved a minimal ratio of filament diameter over nozzle diameter (1.26), denoting high-fidelity printing ability. Furthermore, the printed 109 structures of CNC/PEDOT-DBSA composite cryogels (after drying) showed concurrent high electrical conductivity, EMI shielding, and mechanical properties along with fire-resistance ability. These all-in-one features highlight the superior efficacy of our state-of-the-art approach in designing multifunctional materials and open a new avenue in the field of cellulose-based 3D-printed cryogels. Last but not least, for the first time, switching the EMI shielding mechanism from reflection to absorption has been achieved by simply altering the shape of 3D-printed shields without modifying their chemical composition. Figure 6-1: From biopolymers to 3D-printed conductive cryogels for EMI shielding applications. 6.2 Experimental section 6.2.1 Materials Spray-dried cellulose nanocrystals were acquired from CelluForce, Inc. (Montréal, Canada). FeCl3.6H2O, the 4-dodecylbenzene sulfonic acid mixture of isomers (â¥ 95%), and 3,4-ethylenedioxythiophene were all purchased from Sigma-Aldrich. 6.2.2 Sample preparation Firstly, 3 g CNC was dispersed in 70 mL water under intensive stirring at 2000 rpm for 15 min. Subsequently, the EDOT monomer in various weights (0.1 g to 1 g) was incorporated into the above suspension and stirred for 1 hour. Then, according to 248, DBSA was added in the fixed molar ratio with EDOT and stirred for 30 min at room temperature (molar ratio of DBSA: EDOT was 2:1). Next, 30 mL of aqueous FeCl3 (2.5 mmol) as an oxidizing agent was added dropwise to 110 the mixture of CNC/EDOT-DBSA. The mixtures were allowed to stay for 48 hours at room temperature until the polymerization process was completed. At the end of the polymerization process, the resultant material was gathered and underwent centrifuge (3000 rpm for 2 min), followed by washing with a copious amount of water. The latter was performed to ensure all mixtures became neutral. 6.2.3 3D printing and cryogel preparation A 3D bioprinter (Allevi 2, USA) equipped with 10 mL syringes was used to deposit the CNC-based inks. Ink flow could be controlled by applying proper pressure and printer speed to form filaments, which were around 80 psi and 10 mm/s, respectively. The speed and pressure of printing were adjusted according to the different ink formulations and their viscosities. The prepared inks were deposited onto glass and polycarbonate substrates with different grid sizes from 1 to 3 mm (see Figure 6-2). Finally, 3D-printed samples were frozen at -86 â using an ultra-low-temperature freezer for 24 h. Thereafter, these frozen samples were freeze-dried for 2 days to CNC-based cryogels were obtained. Figure 6-2: Different grid sizes (G1: 1 mm distances between grids, G2: 2 mm distances between grids, and G3: 3 mm distances between grids). 6.2.4 Characterization The methods of characterization and their detailed explanations are elaborated in Chapter 3 of this thesis. 6.3 Results and discussion 6.3.1 Interfacial interaction mechanisms The polymerization mechanism and possible interactions between inks' constituents (i.e., CNC, PEDOT, DBSA, and salt) are discussed in this section. This allows us to rationally infer 111 each component's influence on the final properties (mechanical, rheological, and shape fidelity) of the hydrogels and the resultant cryogels to establish a structure-property relationship. The engineered CNC/PEDOT-DBSA hybrid ink is formed in an aqueous medium. DBSA acts as a surfactant, emulsifying EDOT monomers to form micro micelles 248â250. Micelles in aqueous solutions usually form hydrophilic head-rich regions in contact with water and hydrophobic tail-rich regions in the center. Upon adding FeCl3, the polymerization is initiated, and cationic free radicals of EDOT are formed 248. Figure 6-3a shows that Fe3+ serves as an oxidizing agent, causing EDOT oxidation; then, two oxidized EDOT dimers combine to form a 2-EDOT dimer that is deprotonated by the surrounding water molecules. A polymer chain is then formed by repeating the previous steps. Next, DBSA is inserted as counter-anions to stabilize the PEDOT. By using DBSA during polymerization, PEDOT dispersibility, conductivity, and interaction with CNCs can be improved.248 DBSA is an electron-deficient entity that can increase the conductivity by creating positive holes, leading to better charge transport pathways and connections between PEDOT-rich grains.251 Figure 6-3b shows the possible mechanisms among constituents in which a strong electrostatic interaction might be formed between the SO3- of DBSA and positively charged PEDOT (section I). Additionally, supramolecular interaction might be established between the electron-rich -OH group of CNCs and the S atom of PEDOT (section II). Concurrently, in section III, there would be a hydrogen bond between OH functional groups of CNC and those of DBSA. DBSA surfactant molecules and CNCs can attract each other in two ways. First, the carbon tails of surfactant molecules are hydrophobic and can be attracted to the hydrophobic part of the CNCs, i.e., the backbone of the cellulosic chains (see Figure 6-3c). In addition, the surfactant molecules can be attracted to the negative charges on the CNC surface through Fe3+ cation or PEDOT (positive charge) bridging (Figure 6-3d). The interplay between these two interactions affects the rheological features of these suspensions and, thus, the quality of the 3D-printed structures. 112 Figure 6-3: (a) Proposed mechanism for the polymerization of EDOT. (b) Proposed interactions: (I) PEDOT doped with DBSA, (II) CNC-PEDOT, and (III) CNC-DBSA. (c) Figures of hydrophobic attraction of DBSA onto the CNC surface, and (d) Fe3+ cation or PEDOT positive bridging between DBSA and CNC. 6.3.2 Rheology and 3D printing The prepared samples were labeled as C3-PY, in which C3, P, and Y denote CNC (3wt%), PEDOT, and the weight content of EDOT in the prepared samples, respectively. For example, C3-P0.1 denotes 3 wt% of CNC and 0.1 wt% of EDOT. The details of the samples' names and combinations are given in Table 6-1. Table 6-1: Naming and specifications of samples Sample Name CNC concentration (wt %) EDOT concentration (wt %) DBSA:EDOT (molar ratio) C3 3 0 2:1 C3P0.1 0.1 C3P0.5 0.5 C3P0.75 0.75 C3P1 1 Rheological properties play a vital role in the 3D printability of soft materials 87. Therefore, critical rheological properties, such as viscosity, flow behavior, viscoelastic parameters, and yield stress, must be investigated before printing. For instance, in order to ensure a smooth flow through the printing nozzles, inks must feature a shear-thinning behavior and quickly recover their solid-113 like nature after being deposited on the bed. The gel-like nature of the inks, stemming from sufficiently high viscoelasticity, yield stress (Ïy), and storage modulus, guarantees the filamentous shape of the extruded inks after deposition . The rheological findings of the prepared CNC/PEDOT-DBSA inks are presented in Figures 6-4a-d. As illustrated in Figure 6-4a (flow curves of the prepared inks), all the prepared inks exhibited a shear-thinning behavior. The shear viscosity decreased by almost four orders of magnitude for all the samples by increasing the shear rate from 0.01 to 1000 s-1. Shear-thinning behavior could emerge as a result of anisotropic CNCs orientation and/or their network rupture caused by sufficiently high shear stress. Furthermore, the interaction of DBSA-PEDOT with CNC increased the viscosity of the CNC-based inks, contributing to the formation of a more robust network. For instance, at the shear rate of 0.1 s-1, C3P1, containing the highest amount of PEDOT, showed the highest viscosity, i.e., 105 Pa.s. The viscosity increase can be attributed to two reasons: firstly, CNC is a negatively charged nanomaterial; thus, there is a strong electrostatic repulsion among CNCs, which was disrupted in the presence of PEDOT with positive charges and DBSA (bearing lower negative charges compared to CNCs) 251. Secondly, adding FeCl3 (initiator for in-situ polymerization of EDOT) also screened the negative charges (refer to the previous section regarding the possible mechanism of interactions between CNCs, EDOT, and DBSA in CNCs/PEDOTâDBSA inks). The next crucial rheological property is yield stress. According to Figure 6-4b, C3P1 has the highest yield stress, making it the best ink for 3D printing (inks with a higher yield stress generally produce a better filament formation). However, because its yield stress is higher than the shear stress generated in the nozzle, it cannot flow smoothly, leading to nozzle clogging. As a result, the best condition for 3D printing is having sufficiently high yield stress but not exceeding the nozzle's shear stress. These results offer a better comprehension of the impact of PEDOT concentration on improved printing quality. It is worth mentioning that the apparent yield stress of the prepared inks was also determined via other methods, such as fitting Herschel-Bulkley and the modified four-parameter cross models onto the rheological data, including shear stress versus shear rate and complex viscosity versus frequency, respectively. The sufficient viscoelastic properties of the inks are a positive sign for printing fidelity and shape maintenance after printing 165,196,252,253. Figure 6-4d shows the G' and G" of CNC-based inks as a function of angular frequency. It is obvious that G' is dramatically higher than G" in the entire 114 frequency window. Both G' and G" show almost a frequency-independent trend, suggesting the dominance of a gel-like behavior of the produced inks 57. Another noticeable feature of the dynamic moduli of CNC-based inks is the increased dynamic moduli upon adding DBSA as a surfactant agent. The same trend can be noticed upon raising the amount of the synthesized PEDOT onto CNC aggregates by increasing the concentration of the EDOT in the precursor. This can be justified by increasing the density of contact points and load-bearing junctions upon increasing the solid content in the final inks. The gel-like nature of the CNC-based inks after the in-situ polymerization process and incorporation of DBSA can also be observed in the inverted vial test (Figures 6-4e1 and e3). In this regard, although the C3 sample flowed (Figure 6-4e1) upon flipping the vial, the sample containing DBSA and PEDOT showed a behavior similar to yield-stress gel and did not flow. Also, a comparison of the ink structure with and without DBSA is shown in Figures 6-4e4-e6. To recapitulate, considering its ideal rheological properties, C3P0.75 is deemed a promising ink for extrusion printing. These unique properties are shear thinning behavior, sufficient viscoelastic response, and yield stress lower than the nozzle's stress. Figure 6-4f shows the excellent printability of this ink on different substrates. Micrometer-thick structures were printed on glass and polycarbonate (Figure 6-4f). In addition to these micrometer-thick layers, a 55-layer C3P0.75 structure is printed to showcase its outstanding 3D printability, with several millimeters in height (Figure 6-4g). Indeed, the C3P0.75 offers the highest printing quality (which will be discussed in detail) among the devised inks in this work when many layers are deposited layer by layer. 115 Figure 6-4: (a-d) Ink rheological characterization: (a) the viscosity of the prepared samples versus shear rate (s-1). (b) Storage and loss modulus of the inks versus shear stress (Pa). (c) Yield stress for all samples. (d) Storage and loss modulus of the inks as a function of frequency (rad/s), and (e) gel-like behavior of the inks. (e1-e6) CNC 3 wt%, CNC 3 wt%/DBSA, CNC 3 wt%/DBSA/PEDOT (1 wt% EDOT), CNC 3 wt%/PEDOT (1 wt% EDOT) without DBSA, CNC 3 wt%/PEDOT (0.75 wt% EDOT) with DBSA, and CNC 3 wt%/PEDOT (0.75 wt% EDOT) with DBSA, respectively. (f) Photograph of the fabricated microstructures on various substrates. From left to right: PEDOT structure on polycarbonate, PEDOT structure on glass, and UBC logo on polycarbonate. (g) The progression of the structures formed by the deposition of 55 layers (all scale bars are 25 mm). High-fidelity printing of bio-based inks allows for a variety of advanced 3D printing capabilities, including high resolution, high aspect ratio, and overhanging structures. Figure 2a shows the 3D printing process of the devised CNC-based inks. A homogeneous line formation is essential for reproducible electrical characteristics and mechanical performances in 3D-printed structures.226 In this case, line width variations and the difference between the needle's inner diameter and the diameter of the extruded filaments (demonstrating die-swell, Figures shown in Figure 6-5a) were examined to determine the printing fidelity (Figures 6-5b-g). Inks containing 0.75 wt% and 1 wt% EDOT (C3P0.75, and C3P1, respectively) showed an acceptable variation in line width. The average width of the printed lines for these two samples is 315 µm, which is only 116 26% larger than the inner diameter (250 µm) of the nozzle, making the production of highly accurate digitally designed patterns possible. This value for the inks holding 0.1 and 0.5 wt% EDOT is higher (71% and 49%, respectively). These differences can be attributed to their rheological properties. C3P0.75 exhibited one of the highest yield stresses (560 Pa), which is below the shear stress inside the nozzle (2800 Pa), thus providing the most reliable print quality. GÊ¹ should also be considered in addition to yield stress. A higher GÊ¹ resulted in better printing quality, and high-quality printing in this work can only be seen for samples with a GÊ¹ higher than 4Ã104 Pa. Consequently, the filaments of the prepared inks (C3P0.75) maintained their original shape with minimal spreading (1.26), which allowed high ratio printing, i.e., 55 layers to be stacked without deformation or sagging. To the best of our knowledge, these numbers are the best reported in the literature (see Figure 6-5h). Thus, our CNC/PEDOT-DBSA inks offer unique advantages for high-fidelity conductive structure printing. Figure 6-5: (a) Figure illustration of 3D printing. DBSA acts as a surfactant and causes PEDOT to disperse better into CNC crystals. (b) Pictures of the extrusion printed line. (c-f) The width variation and average width of the extrusion printed lines of (c) C3P0.1, (d) C3P0.5, (e) C3P0.75, and (f) C3P1. (g) The result of the profilometer for C3P0.75, the x-axis, and the y-axis are the width and height of each line, respectively. (h) Inks printing quality (y-axis is filament diameter over needle diameter) compared with other previously published papers, including MAHM: Methacrylated hyaluronic acid, dECM= Decellularized extracellular matrix-based bio-ink, CNF: Cellulose nanofiber, PVA/GO-HA: Polyvinyl alcohol/Graphene oxide- Hyaluronic acid, PEDOT:PSS: Poly(3,4-ethylenedioxythiophene):Polystyrene sulfonate 226,254â262. 117 6.3.3 Morphology and microstructure analysis Considering their low density, large specific surface area, and high porosity, cryogels are appealing, lightweight materials for advanced functions and applications. This section thoroughly characterizes cryogels derived from one-step freeze drying of the 3D-printed inks. The interfacial interactions between the functional groups of CNC/PEDOT-DBSA, crystalline behavior of the CNCs before and after in-situ polymerization, and thermal properties of the cryogels were studied using a broad spectrum of analyses such as FTIR, Raman, XRD, and TGA. The shape fidelity and structural integrity of 3D-printed structures play an essential role in advanced applications, such as EMI shielding and biological applications. As shown in Figure 6-6a, 3D-printed samples are stable and structurally accurate. The skeleton morphology of the prepared pure CNC cryogel (C3) and C3P0.75 samples after the freeze-drying process was evaluated by FESEM analysis, and the results are illustrated in Figures 6-6b-d. Pure CNC cryogel (Figure 6-6c) shows a highly porous and hierarchical 3D structure, having an average pore size of 62.5±4.1 µm (using Image J). In contrast, after incorporating DBSA and PEDOT, a denser structure and a smaller pore size (15.6±2.5 µm) are observed in the case of C3P0.75 cryogel. The formation of such a unique structure could be favorable to enhance the mechanical properties of the final cryogels. In addition, the in-situ polymerization of PEDOT can establish noncovalent interactions with the CNCs and DBSA, further enhancing the 3D network connection and decreasing the pore size. It can be concluded that CNC nanoparticles with rod shape geometry and large surface area serve as a host for PEDOT-rich domains.263 Indeed, nucleation and growth of PEDOT macromolecules are guided on the entire surface of the rods, leading to the lack of any agglomeration of PEDOT during in-situ polymerization.264 Thus, an attractive interaction occurs between CNC and PEDOT 265. Another reason for the denser morphology of C3P0.75 can be ascribed to the higher nucleation density of ice crystals in the hybrid sample, creating more pore walls 266. The existence of numerous pores in such a large surface area of 3D structured cryogels can facilitate electron transport and fast ion penetration; hence, it is favorable for soft electronic applications. Also, the EDX mapping image illustrates the presence of PEDOT (S, O, Cl elements) within the cryogel cellulosic structures (Figures 6-6e and f) and confirms the deposition of PEDOT on the CNCs' surface. Also, to show that the cryogels are conductive, the printed C3P0.75 patterns on polycarbonate were used as conductive tracks to connect an LED light, displaying bright illumination when powered by a 5 V power supply with different patterns. 118 Figure 6-6: (a) Different views of cubic grids printed with C3P0.75. (b) FESEM micrographs of C3P0.75 at different scales. (c, d) FESEM micrographs of (c) pure CNC and (d) C3P0.75 cryogels, and (e, f) EDX mapping result for S, C, Cl, Fe, and O elements. 6.3.4 Mechanical and thermal insulation performances Lightweight porous materials frequently suffer from poor mechanical characteristics, limiting their utilization in advanced applications 267. The excellent mechanical properties of the devised hybrid cryogels in this work render them feasible in real-life applications. The mechanical properties of the prepared cryogels in the compression mode were investigated, and the results are shown in Figure 6-4. As expected, the pure CNC cryogel experienced an inferior mechanical property due to the weak physical bonding among the CNC nanoparticles (see Figures 6-7a-c). The inferior mechanical character of pure CNC cryogels has been known in this field 244,268. It is evident that the maximum compressive stress dramatically increased upon raising the PEDOT concentration. It can be observed that C3P1 had the highest compressive strength and Young's modulus of 1.85 MPa and 6.98 MPa, respectively. Compared to pure CNC cryogel, the compressive strength increased by 85% in the case of C3P1 cryogel. The formation of strong interfacial interaction between CNC functional groups and those of DBSA and PEDOT, as well as the denser porosity of C3P1, could account for observing such behavior 268. Compared with 119 previously reported cellulose cryogels, the C3P1 cryogel presented in this study illustrated superior mechanical performance, low density, and proper porosity (see Figures 6-7c-e). Figure 6-7: (a) Compressive stress-strain curves. (b) Compressive strength values of samples at 40% and 80% strain and compressive modulus of cryogels. (c) Images of a piece of lightweight 3D-printed C3P1 supported by a flower and a piece of robust, lightweight, and conductive C3P1 cryogel (0.64 g) supporting a 2.5 kg autoclave reactor. (d) Porosity and density of different samples, and (e) comparison of Young's modulus vs. density of the present work with the literature, PU: Polyurethane, MFC: Microfibril cellulose, CNF: Cellulose nanofiber, MCM: Microcrystalline cellulose, T-CNF: Tempo oxidize cellulose nanofiber, CNF-F: Cellulose nanofiber foam, RGO: Reduced graphene oxide, Ca: Calcium, CNC: Cellulose nanocrystal 167,169,170,194,269â279. The versatile thermal insulation capability of the constructed cryogels can be attributed to their abundant micro-scale porosities. In Figure 6-8a, we showcased the impressive thermal insulation performance of the cryogels (molded cryogels). To this aim, the cryogels were placed on a hot plate at a temperature of 140 °C, and their dynamic temperature variation was monitored via a thermal infrared camera (as depicted in Figure 6-8b). Our findings indicate that the pure CNC cryogel (C3), and conductive CNC-based cryogel (C3P1) exhibited a temperature of 30.4 °C and 27.1 °C after 5 seconds of being placed on the hot plate. Subsequently, the temperature leveled off at 51.2 °C and 39.2 °C after 60 minutes, respectively. These results clearly demonstrate the potential of our engineered conductive cellulosic cryogels in insulating and blocking heat. The thermal insulation property of the cryogels is rooted in the abundant free spaces within the cryogel framework along with interfacial thermal resistances, which reduce thermal conductivity through the solid phase due to phonon scattering at the interfaces 280. The conductive PEDOT structure integrated into the cryogel network creates pathways for electrons to flow through the material, allowing for heat to be dissipated more effectively. This mechanism is known as the "phonon scattering effect", where the conductive component scatters the phonons (vibrational waves that 120 carry thermal energy) and impedes their transfer, reducing the thermal conductivity of the material 281. The phonon scattering that occurs at solid-solid and solid-gas interfaces decreases the cryogels' solid conduction and promotes their thermal insulation property. All combined, these results highlight the promising potential of the devised conductive cellulose-based cryogels as effective thermal insulation materials. The insights gained from our research provide a valuable foundation for developing future cryogel-based insulation systems. In addition, to visually demonstrate the fire-resistance ability along with the thermal insulation performance of these cryogels, we placed a flower on top of pure CNC and C3P1 cryogels exposed to the flame of a butane torch. In the case of pure CNC cryogel, after 60 seconds, the flower wilted, and after 105 seconds, the flower burned. We observed that C3P1 exhibited excellent thermal insulation and fire resistance properties when placed between the flower and the flame. Even after 300 seconds, the flower remained intact (Figure 6-8d). This exemplary behavior can be interpreted by TGA findings, in which C3P1 displayed low weight loss and high residual in TGA curves, unfolding the excellent thermal stability of CNC/PEDOT-DBSA cryogel. 121 Figure 6-8: (a) Thermal insulation performance of molded conductive cryogels with (C3P1) and without the PEDOT (C3). (b) 2D temperature gradient distribution of the C3 and C3P1 cryogels at the initial stage and after 60 min. (c, d) A flower placed on the (c) pure CNC cryogel heated by butane torch for 1 minute and (d) C3P1 cryogel composite heated by butane torch for 5 minutes. 6.3.5 Electromagnetic interference (EMI) shielding performance The versatility and advantages of the 3D-printed highly conductive cryogels are capitalized for the fabrication of complex EMI shields with patterned geometries and a customizable shielding mechanism. Figure 6-2 shows the geometry and Figure of the 3D-printed samples (grid G1: 1 mm distance between grids, grid G2: 2 mm distance between grids, and grid G3: 3 mm distance between grids). The excellent rheological properties of the CNC/PEDOT-DBSA inks allow the development of 3D EMI shields with less material than shields prepared by conventional processing techniques. A decrease in the density and increase in porosity could be observed by gradually decreasing the PEDOT content and increasing the grid distance (discussed in detail in the previous section). Generally, the EMI shielding performance of the porous shields depends on their inherent electrical conductivity, micro-scale structural porosity, and macro-scale geometry. So, for the first time, here, we explore how 3D-printed macro-scale patterns (i.e., grid size) can affect the EMI shielding mechanisms. To the best of our knowledge, EMI absorbers (low reflectance), previously described in the literature 243,282â285, are primarily composed of magnetic nanomaterials or multilayer structures with a gradual change in impedance. However, the results presented in this section introduce the 3D printing approach as a powerful tool for designing absorption-dominant shields based on conductive materials routinely known to have a high reflection. First of all, we explore the effect of PEDOT concentration on the shielding performance and mechanism by fixing the macro-scale design (grid size). Then, the effect of macro-scale (different grid sizes at a fixed PEDOT concentration) will be discussed. Figures 6a-c highlight the interplay between the electrical conductivity of the printed cryogels and the EMI shielding behavior of the grid G2 samples. Upon increasing the PEDOT loading in the structures, the electrical conductivity and, thus, the amount of mobile charges in the printed paths significantly increased (i.e., the conductivity increased from 19 to 148 S/m when the EDOT content was enhanced from 0.1 to 1 wt% (Figure 6-9a)), resulting in higher total shielding effectiveness (SET). The C3P1-G2 cryogel showed the highest SET, which is 31.82 dB (Figure 6-9b). This means that this grid can shield more than 99.91% of the incident wave. Therefore, these grids have the potential to be used for commercial applications, as the minimum required SET for most industries is 20 dB. In addition, 122 the reflectance and absorbance coefficients of the printed G2 grids containing different amounts of PEDOT are shown in Figure 6-9c. As expected, the reflectance coefficient shifted toward higher values by increasing the PEDOT loading in the printed structures. Interestingly, despite the high electrical conductivity of the cryogels, the absorbance coefficient (A) is higher than the reflectance coefficient (R) for all the G2 grids shown in Figure 6-9c. Therefore, the primary shielding mechanism is absorption, denoting an absorption-dominant shield 226,243. The absorption-dominant mechanism is related to the nano-, micro-, and macro-scale design of the structures, as shown in Figure 6-9g. In this regard, due to mobile charges (electron hopping, jumping, tunneling, and migrating), the nanoscale-synthesized highly conductive PEDOT domains reflect the waves (Figure 6-9g1). Then, the reflected waves travel through the micro-scale porous structure of the 3D-printed cryogels. The waves are reflected multiple times throughout the microstructure as they are entrapped in the pores (Figure 6-9g1). This zigzag reflection prolongs the wave's path, leading to wave dissipation (Figure 6-9g2). Concerning the macro-scale design of the shields, complex structures and patterns provide more interfaces for scattering the incident waves and, thus, influence the impedance of the structures. In the next step, we investigated the effect of macro-scale on the EMI shielding performance. With C3P1 ink, various printed shields with different grid spacings and a completely filled shield were prepared. The macro-scale design of the shields affected the overall shielding effectiveness as well as the shielding mechanisms, see Figures 6-9d-f. SET for the sample with 3 mm grid spacing was 22.59 dB, while this value was 35.82 dB for the 100% filled structure. Large grid spacing reduces the possibility of the waves' interaction with the printed conductive paths, resulting in a lower SET. More importantly, the printed shields' macro-scale design significantly affects the EMI shielding mechanism. In this regard, for a 100% filled structure, the reflectance and absorbance coefficients were 0.75 and 0.24, respectively, showing a reflection-dominant mechanism. In contrast, an absorption-dominant shielding mechanism was observed with a 0.65 absorption coefficient for a printed structure with a grid spacing of 3 mm. Increasing the interlayer spacing reduces reflectance and shielding effectiveness, as shown in Figure 6-9d-f. These results confirm the importance of macro-scale design on the shielding mechanism of highly conductive printed structures. Thus, the 3D extrusion printing method provides a platform for fabricating patterned 123 lightweight EMI shields that require less conductive material and a way to control the shielding mechanism. To highlight the performance of the printed structures, the specific EMI shielding effectiveness (SSE/t= (SET/density)/thickness)286,287 of the samples is compared with the lightweight porous cellulosic shields previously reported in the literature (Figure 6-9h). These 3D-printed grids with various inter-filament spacing demonstrate lower density and greater SSE/t compared to common, porous cellulosic EMI shields. Therefore, the cost-effective, robust, lightweight, and absorption-dominant structures reported in this study have great potential to be utilized effectively as green and sustainable materials in different sectors for addressing the current demands for EMI shields. Figure 6-9: (a) Electrical conductivity and EMI shielding effectiveness (EMI SE) (average SET, SEA, and SER) of the C3P1-G2 composites with different PEDOT contents. (b) EMI SE of the C3P1-G2 composites with different PEDOT contents over the X-band frequency range (8.2-12.4 GHz). (c) R and A coefficients of the C3P1-G2 composites with different PEDOT contents. (d) EMI SE (average SET, SEA, and SER) of the C3P1 composites with different grids. (e) The values of EMI SE for the C3P1 composites with a different grid over the X-band frequency range (8.2-12.4 GHz), (f) R and A coefficients of the C3P1 composites with different grids. (g) EMI shielding mechanism (g1: reflection mechanism by electron migration and hopping and multiple internal reflection inside the micropores of the shields, g2: the effect of different grid sizes on the EMI shielding mechanism), and (h) Specific EMI shielding effectiveness versus A coefficient for different composites; CNF: Cellulose nanofiber, AgNW: Silver nanowires, CNT: Carbon nanotube, CNC: Cellulose nanocrystal 218,288â297. MXene/rGO, Ti3C2Tx-PEDOT:PSS, CNT/Chitosan, TiO2-124 Ti3C2Tx/rGO/PDMS, MXene/CNT/polyimide 298â305. (Grid G1: 1 mm distance between grids, grid G2: 2 mm distance between grids, and grid G3: 3 mm distance between grids). 6.4 Conclusion In conclusion, our strategic design and fabrication of cellulose bio-ink-based conductive cryogels address two significant challenges associated with the 3D printing of conductive biopolymers and cryogels: 1) the low printing quality of the inks and 2) the poor mechanical integrity of their cryogel counterparts. The judicious choice of the constituents and precise control over intermolecular engineering yielded in the formulation of the novel conductive bio-ink, CNC/PEDOT-DBSA, which not only featured the highest 3D printing resolution among the CNC-based inks but also exhibited the strongest mechanical performance among the CNC-based cryogels. The design's simplicity, straightforwardness, and inexpensive materials utilization provide significant customizability to engineer the micro- and macro-structure of the fabricated 3D-printed conductive cryogels. The precise 3D structuring of these multifunctional inks allowed us to exploit space efficiently with the lowest possible material content for targeted applications. The engineered multifunctional inks were employed to fabricate ultra-lightweight robust 3D electromagnetic shields, which featured a unique combination of properties such as high electrical conductivity and thermal insulation. In addition, for the first time, the EMI shielding mechanism was switched from reflection to absorption by simply altering the shape of the 3D-printed shields. We expect that the precise additive manufacturing of biopolymers and bio-composites will enable next-generation solutions to high-demand material applications by controlling their chemical structure and physical form along with their structure-property performance. 125 Chapter 7: Multifunctional 3D-Printed Cellulosic Constructs via a Sequential Cold Chemical Vapor Polymerization Technique The rapidly expanding domain of materials science, coupled with the pursuit of sustainability, has experienced a remarkable breakthrough in leveraging cellulose nanofibers (CNFs), a widely available nanomaterial sourced from nature, for fabricating intricate 3D shapes via diverse 3D printing techniques. This innovative approach, especially promising for electronics applications, confronts significant obstacles, notably the intrinsic low electrical conductivity and limited mechanical flexibility of conventional CNF-based materials. Our research addresses these challenges by integrating ion-seeded CNF-based 3D-printed frameworks with a conductive polymer via a feasible process known as "cold chemical vapor polymerization" (CCVP). The procedure initiates with the precision direct ink writing (DIW) of a specially formulated 2,2,6,6-tetramethylpiperidine-1-oxy-oxidized (TEMPO-oxidized) CNF hydrogel, which then undergoes saturation with Fe3+ ions and freeze-drying to produce ion-embedded CNF frameworks. Subsequently, interconnected conductive pathways of poly(3,4-ethylenedioxythiophene) (PEDOT) are generated within these structures using CCVP. This methodology allows for precise customization of electrical conductivity, resulting in the production of highly conductive (546 S/m) and mechanically flexible (70% compressible) patterned constructs. This advancement is highlighted by the development of grid-based structures designed for absorption-dominant electromagnetic interference (EMI) shields. These innovative shields demonstrate an absorbance (A) of 0.71 and a specific EMI shielding effectiveness (SSE/t) of 3406.45 dB cmâ2 gâ1. This achievement directly tackles a significant issue inherent in synthetic EMI shields: their predominantly reflective nature. Furthermore, these constructs serve as sensitive piezoresistive sensors, showcasing the versatility of this sustainable method for the development of advanced wearable electronics. 7.1 Introduction The burgeoning field of 3D printing technology has paved the way for innovative applications in creating flexible and conductive structures that are essential in various domains, including electronics 306, energy storage 307, wearable technologies 308, and electromagnetic interference (EMI) shielding 58. Central to this advancement are conductive materials such as MXene 309, PEDOT:PSS 310, and reduced graphene oxide (rGO) 311. These materials, when utilized in 3D printing processes, enable the fabrication of structures that are not only conductive but also 126 boost a degree of flexibility and three-dimensional complexity. This capability has opened new horizons in the design and development of functional materials and devices. However, alongside these technological strides, a growing need for sustainable and environmentally friendly alternatives in the field of conductive materials emerges. This necessity is where cellulose nanofibers (CNF) enter the scene, heralding a new era of sustainable materials science 312. The integration of sustainability with functionality through incorporating CNF in 3D-printed structures poses a challenging yet vital goal, marking a pivotal step forward in advancing 3D printing technology. CNFs, owing to their natural abundance and biodegradability, have emerged as promising candidates for constructing 3D-printed structures 137,312,313. By amalgamating CNF with conductive materials such as carbon nanotubes (CNC) 314, MXene 315, and rGO 316, researchers have been able to fabricate structures that exhibit desirable conductive properties. However, a major limitation of these CNF-based structures is their inelastic nature. Although there have been attempts to create CNF-based conductive 3D structures in porous forms, the elasticity predominantly originates from the conductive components rather than the CNF matrix 137,312,317,318. Also, such elasticity is only achieved at a low material density, and this reveals a gap in having both conductivity and flexibility at the same time. Most 3D-printed CNF frameworks exhibit a transition to elastic states upon immersion in water 312. This transformation is primarily due to the plasticizing effect of water, where the interaction between water molecules and nanocellulose through competitive hydrogen bonding plays a pivotal role 319,320. However, the conventional method of immersing these structures in water to induce elasticity lacks precision in controlling the water content. While in its saturated state, the resultant hydrogel often demonstrates suboptimal mechanical strength and tends to release excess water under compression. Recent advancements have identified the potential of hygroscopic salts, such as calcium chloride (CaCl2) 313, to meticulously regulate water absorption from the surrounding air, even under low-humidity conditions. Based on this insight, we hypothesize that integrating a hygroscopic salt might achieve a superelastic conductive 3D-printed CNF structure. This method aims to leverage ambient moisture to induce and maintain the desired elastic properties of the conductive CNF. Moreover, the quest for multifunctionality in 3D-printed CNFs necessitates the incorporation of materials that not only enhance structural integrity and resolution but also 127 introduce additional functionalities. Within this framework, the integration of a particular hygroscopic trivalent salt is posited to serve a threefold purpose. Firstly, it is anticipated to function as a crosslinking agent, enhancing the CNF hydrogel matrix's integrity and consequently refining its structural definition. Secondly, the introduced hygroscopic salt absorbs moisture from the ambient environment, enhancing the elasticity of the 3D-printed CNF structure. Thirdly, the incorporation of this select salt is expected to endow the CNF infrastructure with conductive capabilities, thus opening avenues for its innovative domain of flexible and conductive materials. Herein, we begin with the 3D printing of TEMPO-oxidized CNF (CNF), and then these structures are seeded with Fe3+ ions (using FeCl3 salt). Fe3+ ions serve as the initiator for the polymerization of EDOT monomers via cold chemical vapor polymerization (CCVP), resulting in a conductive network of poly(3,4-ethylenedioxythiophene) (PEDOT) throughout the structure. This technique yields 3D-printed CNF structures that are electrically conductive (achieving conductivity of up to 546 S/m), mechanically robust, and remarkably flexible. These 3D-printed entities mark a significant leap forward in the development of absorption-dominant electromagnetic interference (EMI) shielding, demonstrating an absorbance (A) of 0.71. Moreover, their application as sensitive piezoresistive sensors further highlights the versatility and sustainability of this approach, propelling advancements in wearable electronics. 7.2 Results and discussion 7.2.1 Design principles: from 3D printing to cold chemical vapor polymerization Herein, 3D-printed hierarchical constructs made of CNFs were fabricated using a commercial 3D bioprinter. Accordingly, 3D-structured CNF-based grids were printed with controllable macro-scale patterns (size of grids: 1, 2, and 3 mm, designated as G1, G2, and G3, respectively) and well-arranged micro-scale porosity (Figure 7-1a). The methodology allows controlling the number of layers and thickness of the 3D constructs (Figure 7-1b) and engineering morphology from micro- to macro-scale. This is beneficial for advanced EMI shields and sensors, owing to the crucial effect of the hierarchical porosity of the assembly on final related properties. This methodology marks a significant advancement in the field of 3D printing, particularly in fabricating constructs that excel in flexibility and conductivity. At the heart of this approach is the strategic formation of active localized FeCl3 sites on the surface of CNF. FeCl3, known for its hygroscopic nature, plays a multiple role in this process. Firstly, the trivalent Fe3+ can crosslink the CNF by chelating with the carboxylate groups on the CNFs 321; Secondly, FeCl3 imbues the 128 3D-printed CNF with enhanced flexibility by absorbing environmental moisture, and thirdly, it acts as a catalyst for the polymerization of the EDOT monomer, facilitating this reaction through a unique CCVP technique. Consequently, the polymerization of the EDOT monomer at these moisture-enriched active FeCl3 sites leads to a final construct with unparalleled flexibility and conductivity. This process leads to the formation of highly conductive interconnected PEDOT domains throughout the construct. For this aim, the free-standing, freshly 3D-printed CNF construct (Figure 7-c) was impregnated with a 0.5 M FeCl3 solution for 30 min at room temperature (Figure 7-c-II, III). Freezing (at -85 °C for 48 h) and subsequent lyophilization (0.005 mbar for 48 h) of the 3D-printed CNF construct enriched with FeCl3 led to the formation of robust 3D porous structures for polymerization of EDOT monomer via CCVP (Figure 7-1c-IV). Gradual CCVP of the EDOT monomer at the FeCl3 oxidating sites was conducted by placing the FeCl3-enriched 3D-printed CNF construct within a sealed container containing 0.1-1 mg EDOT monomer at 0 °C for eight days (Fig. S3). This allows the formation of a conductive CNF construct decorated with interconnected conductive PEDOT domains (Figure 7-1d). In other words, the metal-coordination bond between the oxygen electron pairs and Fe3+ enables uniform nucleation for PEDOT growth through a slight polymerization process (Figure 7-1d) 322,323. 129 Figure 7-1: (a) Figure illustration of the 3D printing process of CNF, enabling controlling macro-scale porosity of the 3D-printed constructs. (b) The progression of the structures formed by the deposition of 35 layers. (c: I-IV) Photograph of the fabricated 35-layer microstructures, (c-I) pure CNF, (II) CNF incorporated with FeCl3, (III) freeze-dried CNF incorporated with FeCl3, (IV) conductive CNF/PEDOT; scale bar: 1 cm. (d) Figure illustration of the CCVP method enabling the polymerization of the EDOT monomer on the oxidative FeCl3 (I) at the initial step, and (II) after eight days, generating a lightweight conductive 3D-printed CNF/PEDOT construct. Additionally, the middle portion of (d) illustrates a Figure representation of the polymerization of EDOT (depicted in black) on the surface of CNF through the use of Fe3+ bridges (represented as golden spheres). 7.2.2 Evaluating 3D printability, volume retention, density, and water uptake Rheological features of hydrogel inks are known as their most critical physiochemical fingerprints for assessing the quality of printing and shape fidelity. The hydrogel inks used in extrusion 3D printing, especially in DIW, must meet several rheological criteria. They must be able to be extruded through a fine nozzle and retain a stable shape once applied 87,95. To accurately describe this process, it is crucial to consider various rheological factors such as viscosity (Æ), storage (G') and loss moduli (G"), elastic recovery, shear stress (Ï), and yield stress (Ïy) 87,95. These are assessed through methodical testing to understand the ink's behavior under DIW-like conditions. 130 In our prior research 313, we established that 2.0% CNF ink exhibits optimal print quality; hence, we opted for this concentration in the current study. Submerging 3D-printed constructs in the FeCl3 solution initiates crosslinking, resulting in robust, free-standing 3D structures that maintain their integrity post-freezing and lyophilization (Figures 7-2a-f). After being freeze-dried, the FeCl3 crosslinked CNF structures turned into yellowish monoliths that preserved their original form without noticeable distortion or collapse (Figures 7-2c and d). The superb rheological attributes of the CNF inks allowed for the retention of over 91% of the intended volume in the G2 sample (with a grid size of 2 mm (Figure 7-2g). However, this volume retention dropped slightly to 84% after FeCl3 crosslinking, likely due to the densification of the CNF network via ionic bonds with iron ions, resulting in a compact structure that occupies less space. Moreover, the volume retention for the G2 sample further decreased to 75% after freeze-drying, a consequence of the hydrogen bonding-driven association among CNFs. An increase in infill density (from G3 to the closely packed printed samples) led to enhanced volume retention, which is attributed to the higher solid content limiting structural shrinkage.137,313 Remarkably, the incorporation of PEDOT into our structures did not alter volume retention significantly post-freeze drying, demonstrating the gentle nature of the polymerization environment, which avoids direct contact and mixing with the sample. Additionally, we calculated the densities of all monoliths to be between 0.07â0.13 g cm-3 under 43 ± 3% relative humidity (RH), indicating a lightweight structure (Figure 7-2h). Water uptake assessments revealed lower absorption at higher infill densities, indicating decreased porosity in denser structures. All assessments were conducted on samples maintained at a 43% RH. At 43% RH, for the G2 sample, the non-crosslinked CNF monolith showed minimal water absorption of 11.38%, which dramatically increased to 53% upon adding salt. The introduction of FeCl3 to the CNF monoliths enhanced their water uptake, primarily due to the hygroscopic characteristics of the salt 313. Despite anticipating a significant increase in water absorption with FeCl3, the actual rise was modest (up to 58% for the G3 sample). We postulate that incorporating FeCl3 significantly changes the water absorption capacity of CNF monoliths via interactions with functional groups. This can be through interactions with functional groups, as its hygroscopic nature predominantly dictates the moisture levels, overshadowing the impact of the denser structure and reduced pore volume. Finally, adding PEDOT to the mix further reduced water uptake, which was attributed to the hydrophobic properties of PEDOT (Figure 7-2i). 131 This outcome highlights the performance and simplicity of fabricating these 3D-printed CNF structures for prospective uses such as water harvesting 324. Additionally, the water absorbed by these structures could be easily expelled by heating them at 70 °C, allowing the monoliths to recover more than 72% of their original water absorption capacity after 10 cycles (during 10 days), as shown in Figure 7-2j. This innovative method paves the way for creating a water-absorbing, conductive, and interconnected 3D-structured CNF, resulting in a lightweight framework. This framework is so delicate that it can be supported by an old-man cactus's hair (Figure 7-2e), and its electrical conductivity can be visualized by lightening an LED lamp (Figure 7-2f). 7.2.3 Characterization: after printing The formation of a monolithic CNF structure and the polymerization on its surface with PEDOT are validated through different techniques. As depicted in Figure 7-2k, the FTIR spectrum of CNF exhibits characteristic peaks associated with cellulose nanofibrils. The broad absorption band around 3300 cm-1 can be attributed to the O-H stretching vibrations, indicative of the hydroxyl groups in the cellulose. The peak around 1621 cm-1 is representative of -COO-Na+, and there is a small peak at 1735 cm-1, which is typically assigned to the COOH. Additional small peaks, including those near 1420 cm-1 and 1370 cm-1, are assigned to the symmetric bending of CH2 and C-H deformation in CNF, respectively. The significant peak at approximately 1090 cm-1 corresponds to the antisymmetric bridge C-O-C stretching vibration, a signature of the glycosidic linkages between glucose units in the cellulose chains. The introduction of FeCl3 to form CNF/Fe results in some changes in the FTIR spectrum. The carboxylate stretching vibration (COO-) band appears to shift slightly to 1611 cm-1, suggesting an interaction between the carboxylate groups of CNFs and the iron ions. This is due to the formation of coordination bonds between Fe3+ and the carboxylate groups. The presence of the PEDOT on the CNF is validated by the presence of the peaks at around 1430 cm-1 and 1260 cm-1, attributed to the C-H bending vibration and C-O-C stretching vibration of the ethylenedioxy group in PEDOT, respectively 232. These peaks, along with the stretching vibrations of the C-S-C bond in the thiophene ring at 754, 812, 889, and 951 cm-1, confirm the successful synthesis of the PEDOT by CCVP. Besides, the disappearance or shift of the main carboxyl group (C=O) fingerprint peak of the backbone CNF to lower frequencies clearly proves the presence of the PEDOT on the CNF structure 177,232. 132 Following previous analyses, the micro-Raman spectroscopy also endorses the successful assembly of CNF with PEDOT (Figure 7-2i). As such, the micro-Raman spectrum of CNF showcases the characteristic peaks of CNF at 1098, 1122, and 1365 cm-1, attributed to the C-O-C stretching vibration, C-O stretching vibrations of the C-OH side groups in CNF, and C-C stretching vibration, respectively 57. For the CNF with FeCl3, shifts in the existing peaks are observed. These may indicate the interaction between the iron ions and the TEMPO-oxidized cellulose. Also, the fingerprint region (below 1500 cm-1), which contains many complex overlapping bands due to various bending and deformation modes, shows shifts and intensity changes as the entire molecular structure of the CNFs is affected by the binding of Fe3+ ions. Upon decoration of the CNF with PEDOT, a set of functional groups appeared in the micro-Raman spectrum, validating the successful formation of the monolithic structure of CNF/PEDOT. Accordingly, the appearance of characteristic peaks at around 559, 945, 949, and 1178 cmâ1 correspond to the C-O-C deformation or symmetric C-S-C deformation, oxyethylene ring deformation Cα-Cα inter-ring stretching, and C-H bending mode, respectively 232. 133 Figure 7-2: (a) 3D-structured CNF hydrogel after impregnation with FeCl3. (b) Free-standing 3D-printed CNF decorated with FeCl3, (c) after freezing, lyophilization, and drying steps. (d) Zoom-in images of the high-quality 3D-printed sample after lyophilization. (e) Ultra-lightweight 3D-printed conductive CNF/PEDOT structure on the very light hair of a cactus. (f) A view of the conductive 3D-printed CNF/PEDOT structure capable of lighting up an LED lamp. Scale bars in parts (a, c, and e) are 2 cm, and in part d, the scale bar is 2 mm. General characterization of the CNF and cold chemical vapor polymerized CNF/PEDOT monoliths at 43% RH, including (g) volume retention value, (h) density, (i) water uptake, and (j) cyclic water uptake of CNF 1/EDOT 1 monoliths at 43% RH for up to 10 cycles (during 10 days). The monoliths were firstly saturated at 43% RH for 1 day, followed by drying at 70 â for 8 h to complete one absorption-drying cycle. (k) FTIR spectroscopy, and (h) micro-Raman spectroscopy of CNF, CNF/Fe, and CNF/PEDOT. In all Figures labeled g through i, the color coding is as follows: green represents TEMPO-oxidized CNF, orange denotes TEMPO-oxidized CNF treated with FeCl3, and blue signifies the conductive TEMPO-oxidized CNF that has been integrated with PEDOT. 7.2.4 Morphological assessments The chemical properties of CNF include a high density of carboxylate groups, which are negatively charged at neutral to basic pH levels. FeCl3, being a Lewis acid, has a high affinity for electrons and can interact with electron-rich species. When CNF is immersed in FeCl3 solution, one of the primary expected interactions is ionic bonding. The carboxylate groups (âCOOâ») on CNF can attract and potentially form ionic bonds with the Fe3+ ions. This interaction can lead to the formation of a coordination complex where the iron ions are coordinated with the oxygen atoms of the carboxylate groups 63â65. In addition, the multiple carboxylate groups on a single CNF could potentially chelate the Fe3+ ions. This chelation would involve the formation of a more stable structure with the metal ion at its center, altering the physical and chemical properties of the CNF. 134 Aside from crosslinking of CNFs, FeCl3 can act as an initiator for the polymerization reaction. During the polymerization process, CNFs act as a host (substrate) for the growth of the PEDOT domains, forming a CNF/PEDOT framework with enhanced electrical properties. Firstly, two EDOT monomers combine to form a 2-EDOT dimer, deprotonated by the surrounding water molecules. Subsequently, a polymer chain is formed by repeating the previous steps, creating a robust, interconnected conductive pathway of PEDOT throughout the 3D-printed CNF construct. The proposed method offers the capability to manipulate the distribution of macro-scale porosity as well as the thickness and 3D morphology of the final structure, rendering it particularly suitable for uses requiring large surface areas, like EMI shielding and piezoresistive sensing. The pure CNF monolith exhibited a micro-scale porosity with larger pore sizes compared to the other samples, as illustrated in Figure 7-3a. Remarkably, upon CCVP of the EDOT monomer at the FeCl3 oxidation sites on the CNF, the resultant structure retained its outstanding stability and precision in printing, a testament to the structural integrity inherent in the pristine CNF construct, as shown in Figures. 2b and c. The CCVP of EDOT across the oxidation sites throughout the 3D-printed CNF structure resulted in a more compact structure with reduced pore size. This process induces a series of noncovalent interactions between the polymerized EDOT monomer and CNFs, culminating in an interconnected 3D hierarchical network with diminished pore sizes. In this scenario, CNF serves as a scaffold for PEDOT-rich areas, promoting the nucleation and growth of PEDOT macromolecules across the CNF surface. Such characteristics afford precise control over micro-scale porosities and their configuration, which is essential for fabricating high-performance EMI shields through the enhancement of multiple internal reflections. 135 Figure 7-3: (a, b) SEM micrograph of pure CNF and CNF/Fe at different magnifications, showcasing their abundant micro-scale porosities, respectively, and (c) SEM micrographs of 3D-printed conductive CNF/PEDOT at various magnifications. At higher magnification (on the micrometer scale), the SEM image uncovers more intricate details of the CNF/PEDOT structure, revealing the sheet-like morphology of the nanofibrils and a smoother surface. This appearance hints at a potential coating or interaction with PEDOT, as indicated in Figure 7-4a. Further examination of the surface presents a granular texture, which could be indicative of crosslinking by FeCl3 or perhaps related to the PEDOT matrix within which the CNFs are integrated, as shown in Figures 7-4b and c. Additionally, the energy-dispersive X-ray (EDX) mapping images are depicted in Figure 7-4d, illustrate the spatial distribution of elements such as carbon (C), sulfur (S), chlorine (Cl), oxygen (O), and iron (Fe) throughout the sample. The maps of carbon and oxygen are likely the representative of the CNF/PEDOT composition. In contrast, the distribution of sulfur underscores the presence of PEDOT, which incorporates sulfur within its thiophene rings, providing valuable insights into the chemical and structural composition of the sample. The uniformity of the elements in the EDX mapping suggests a thorough integration of the PEDOT into the CNF matrix, which can be crucial for the conductive properties of the nanocomposite material. The even distribution of all elements, particularly sulfur, indicates the successful synthesis process of the nanocomposite material, potentially leading to consistent electrical and mechanical properties throughout the material. Finally, a detailed view of the CNF/PEDOT nanocomposite at a higher magnification is shown in Figure 7-4e, where we can observe a dense and uniform coverage of the CNFs by PEDOT, which appears as a textured layer on the CNFs, indicating a successful nanocomposite formation. In summary, the SEM images combined with the EDX mapping indicate a well-integrated nanocomposite of TEMPO-oxidized CNFs with PEDOT, showcasing a uniform elemental distribution and a porous yet textured surface morphology that could be advantageous for various applications such as filtration, catalysis, or as electrodes in electronic devices. 136 Figure 7-4: (a-c) SEM micrographs of CNF/PEDOT monolith reveal the formation of the PEDOT throughout the fibrillated cellulose-based construct, (d) the EDX mapping of the CNF/PEDOT sample reveals a homogeneous distribution of elements across the structure, and (e) SEM micrographs of PEDOT on the CNF structure. 7.2.5 Electromagnetic interference (EMI) shielding. Comprehensive tests were carried out to illustrate the potential of 3D printing for fabricating structures with large-scale porosity aimed at absorption-dominant, conductive EMI shields. These experiments assessed the EMI shielding effectiveness (EMI SE) of these structures throughout the X-band frequency spectrum. Utilizing structural design alone for optimization, these CNF/PEDOT structures with a grid-like pattern allow for precise electromagnetic absorption control through adjustments in geometry, particularly filament spacing. The study revealed that the EMI shielding capabilities can be finely tuned by modifying the electrical conductivity and geometric parameters of the printed grids, which directly influence impedance and reflection levels. The geometry of the 3D-printed CNF/PEDOT shields is showcased in Figure 7-5a. Three different interfilament spacings of 1 mm (G1), 2 mm (G2), and 3 mm (G3) are considered for these structures to evaluate the effect of geometrical characteristics of the printed grids on the overall EMI shielding characteristics. Initially, we explore the outcomes for the G2 samples chosen for their absorption-dominant shielding mechanism. The EMI shielding characteristics of grid G2 with different loadings of PEDOT are evaluated to unravel the importance of electrical conductivity on EMI shielding effectiveness (SE). In this regard, changing the weight of the EDOT monomer from 0.1 to 1 g enhanced the electrical conductivity from 13.5 S/m to 546 S/m, leading to a notable rise 137 in the total shielding effectiveness (SET) from 15.81 dB to 33.75 dB (Figure 7-5b and Figures. S6-S9). Moreover, it was also unveiled that the increase in the weight of the EDOT monomer could increase the reflectance (R) from 0.29 to 0.44 (Figure 7-5c), which is due to the enhanced electrical conductivity, leading to an impedance mismatch between the shield and the free space. The shields (CNF 1/EDOT 1 samples) exhibited a frequency-independent EMI shielding behavior over the X-band frequency range (8.2-12.4 GHz) (Figure 7-5d). These outcomes vividly validate the capability of the CCVP method in controlling the electrical conductivity of the hierarchically structured CNF monolith, which is an essential factor in governing the shielding mechanisms. Further investigations unveiled the direct relationship of the grid size with the shielding mechanism and SET of the 3D-printed CNF/PEDOT constructs. In the case of closely packed printed samples with no interfilament spacing, we observed a SET of 38.31 dB. However, for the grid-like structures, the SET value was lower, with the minimum and maximum values of 20.02 and 35.06 dB for grids G3 and G1, respectively (Figures 7-5e-g). This reduction in SET can be attributed to a lower amount of conductive paths and mobile charges in the case of larger interfilament spacing. The underlying mechanism for tuning EMI shielding performance via manipulating the interfilament spacing of the printed structure has been further investigated. In the case of a closely packed sample, the recorded absorbance (A) and reflectance (R) coefficients were 0.21 and 0.79, respectively, demonstrating a reflection-dominant EMI shielding mechanism. However, in the case of the grids, the absorbance value was relatively higher. For example, we observed the A coefficient of 0.71 and 0.56 for G3 and G2 grids, respectively, which showcase a unique absorption-dominant mechanism. The equivalent impedance (ðð ) could explain such an increase in the absorption capacity of the printed structure of the printed grids. By increasing the interfilament spacing of these printed structures, the value of impedance increases, which lowers the corresponding reflectance (R). In this scenario, a larger portion of the EM waves penetrates the printed structure, which can be further dissipated due to numerous interfaces inside the structure, dipole polarization loss (caused by the presence of water molecules in the 3D-printed samples and CNF polar structure), and Ohmic loss (due to the presence of freely movable electrons inside the printed paths) (Figure 7-5h). As a result, grid-like structures of CNF/PEDOT demonstrate higher absorbance than closely packed printed counterparts. We also compared the A coefficients of all printed structures with the recent literature (Figure 7-5i). This comparison disclosed the 138 unparalleled absorbance of the 3D-printed shields, which the processing parameters can tailor. Thus, the lightweight and flexible printed structures with an absorption-dominant mechanism have great potential for use as green and sustainable materials in various sectors to address the current demands for EMI shielding. The 3D extrusion printing method provides a platform for fabricating patterned lightweight EMI shields that require less conductive material and offer a way to control the shielding mechanism. Figure 7-5: (a) Controllable macro-scale and micro-scale porosity levels of the 3D-printed CNF/PEDOT constructs; different grid sizes for 3D-printed samples (G1: 1 mm distances between grids, G2: 2 mm distances between grids, and G3: 3 mm distances between grids). The number of printed layers is 10 for EMI shielding measurement. (b) Electrical conductivity and EMI shielding effectiveness (EMI SE) (average SET, SEA, and SER) 139 of the conductive CNF/PEDOT-G2 constructs with different PEDOT contents. (c) R and A coefficients of the CNF/PEDOT-G2 construct with different PEDOT contents. (d) EMI SE of the conductive CNF/PEDOT-G2 structures with different PEDOT contents over the X-band frequency range (8.2-12.4 GHz). (e) Shielding characteristics (average SET, SEA, and SER) of the 3D-printed CNF1-PEDOT constructs with different grid sizes. (f) R and A coefficients of the CNF1-PEDOT constructs with different grid sizes. (g) The total EMI SE of the 3D-printed CNF1/PEDOT structures with different grid sizes over the X-band frequency range (8.2-12.4 GHz). (h) EMI shielding mechanism of the developed 3D-printed samples. (i) The absorbance of the shields compared with previous practices in the literature. 7.3 Conclusion In summary, our study introduces a customizable method for creating lightweight, CNF-based, 3D-printed structures with outstanding conductivity and compressibility. This method involves ion-seeded 3D printing of CNF inks treated by the CCVP technique to produce conductive PEDOT/CNF 3D frameworks. Our approach spans macro-scale production, micro-scale assembly, and molecular-scale polymerization, effectively overcoming prevalent issues in CNF-based architectures. We achieved controlled spatial organization and compressible hierarchical designs (up to 70%) and set a new benchmark for electrical conductivity in 3D-printed CNF structures at 546 S/m. This versatility enables the fabrication of grid-shaped 3D monoliths with adjustable electrical conductivity (15.6 to 546 S/m) and customizable interfilament distances. As a result, we fine-tuned the EMI shielding capabilities of these monoliths, generating absorption-dominant shields that overcome the typical high reflectance issue of common EMI shields. Furthermore, the integration of superior elastic resilience, multi-scale porosity, and uniform conductive networks within the 3D-printed structures facilitated the development of sensitive piezoresistive sensors for tracking human movement. The innovative technique presented in this paper not only tackles existing challenges but also lays the groundwork for future advancements in 3D-printed CNF monoliths, employing a sustainable and eco-friendly strategy. 140 Chapter 8: Conclusion and Future Work This thesis presents a significant advancement in the field of synthetic conductive structures, with a focus on developing bio-inspired structures featuring low density, high porosity, and superior mechanical strength for practical applications like EMI shielding. The traditional challenges posed by conventional manufacturing methods, like vacuum-assisted filtration and ink-jet printing, are addressed through the innovative use of AM, particularly 3D extrusion printing. This method proves advantageous in fabricating complex microstructures efficiently and effectively. The core of this research lies in exploring the potential of bio-based polymers, especially cellulose, in developing conductive structures. The study spans nano, micro, and macro scales, employing diverse synthesis methods, modifying drying conditions, and utilizing extrusion 3D printing techniques. One of the most critical outcomes of this research is the understanding of how these different scales influence EMI SE. The thesis effectively highlights a shift from traditional conductive metals to synthetic structures. This change is driven by the drawbacks of metals, including their weight, bulkiness, corrosion, and secondary EM pollution. The research emphasizes the development of absorption-based, lightweight alternatives using 3D-printed conductive cryogels, offering a significant advancement in the field. The research objectives were connected with addressing the limitations in current synthetic conductive structures for EMI shielding. The thesis tackled key challenges like reflection-based EMI shielding mechanisms and the environmental impacts of petroleum-based polymer shields. The innovative approach combined material selection, processing methods, and structural design to achieve the desired outcomes. 8.1 Summary of main findings â Chapter 4: This chapter presented a comprehensive study in the field of additive manufacturing, particularly focusing on the optimization of rheological properties for enhanced printability of CNC-based hydrogels. We systematically investigated ten different salts, including mono-, di-, and trivalent types, to fine-tune the CNC-based hydrogels' rheological characteristics. This meticulous approach led to the development of CNC/salt inks that could be printed with high precision and fidelity, effectively overcoming common challenges such as clogging and filament spreading. â We achieved near-perfect width control in the printed structures, exemplified by the CNC/NaHCO3 ink, which showed only a 26% filament spreading compared to the nozzle 141 diameter. We also showed a clear correlation between the type of salt used and the rheological and mechanical properties of the hydrogels. It was observed that hydrogels with higher rheological fingerprints, particularly those with divalent and trivalent chloride salts, resulted in superior printing quality and mechanical robustness. â Finally, we demonstrated that achieving perfect printing fidelity requires not only a decent yield stress but also a storage modulus that is almost four times higher than the loss modulus. This finding helps researchers in the field of cellulose 3D printing by emphasizing the importance of considering quantitative rheological data before proceeding with 3D printing. â Chapter 5: This chapter's main finding revolved around the successful development of printable CNC and ENCC suspensions, which notably enhance the mechanical strength of CNC cryogels without requiring external agents. By employing a straightforward mixing technique, we created various CNC/ENCC systems (in ratios of 2:1, 1:1, and 1:2, alongside pure CNC and ENCC samples) and evaluated their printing quality and mechanical properties - two significant challenges in the field. â Rheological characterization and 3D shaping through direct ink writing revealed that CNC effectively modified the rheological properties of ENCC suspensions, with the order of printing quality observed as CNC > CNC/ENCC (2:1) > CNC/ENCC (1:1). Intriguingly, an opposite trend was found in the mechanical integrity of the solid-state samples (cryogels), where increasing the ENCC proportion led to a more compact structure and enhanced mechanical strength, following the order CNC/ENCC (1:1) > CNC/ENCC (2:1) > CNC. â Chapter 5 concludes that CNC/ENCC mixtures are promising for developing rationally designed, lightweight, 3D-printed scaffolds, leveraging the advantages of CNCs for improved printability and ENCCs for superior mechanical properties. â Chapter 6: This chapter successfully formulated a novel conductive bio-ink (CNC/PEDOT-DBSA) for high-resolution 3D printing. This bio-ink demonstrated excellent printing fidelity with minimal filament spreading and a high aspect ratio, enabling the fabrication of intricate microstructures with superior electrical conductivity and mechanical properties. 142 â The resultant cryogels, derived from freeze-drying the 3D-printed structures, exhibited high electrical conductivity (110±12 S/m), outstanding mechanical strength (Young's modulus of 6.98 MPa), and remarkable fire resistance. These features make the cryogels suitable for practical applications, including EMI shielding and thermal insulation. â The main achievement of this chapter was the ability to switch the EMI shielding mechanism from reflection to absorption by altering the geometrical features of the 3D-printed shields (macro-scale design). The innovative 3D-printed cryogels showed high specific shielding effectiveness (4983.4 dBÂ·cm²·gâ»Â¹) with a primary focus on absorption, addressing the limitations of traditional reflective EMI shields. â The strategic design and fabrication process utilized environmentally friendly materials, emphasizing sustainability. The multifunctional 3D-printed cryogels combine high electrical conductivity, mechanical robustness, and thermal insulation, showcasing the potential for next-generation applications in electronics, thermal management, and EMI shielding. â Chapter 7: This chapter introduced a method for producing lightweight, CNF-based 3D-printed structures with high precision and customization. This method involved ion-seeded 3D printing combined with cold chemical vapor polymerization (CCVP), achieving a decent electrical conductivity (up to 546 S/m) and compressibility (up to 70%). â This chapter focused on nano-, micro, and macro-scale design to tune the EMI shielding mechanism. This chapter successfully created grid-shaped 3D monoliths with adjustable electrical conductivity and customizable interfilament distances. These structures demonstrated superior EMI shielding capabilities, focusing on absorption rather than reflection, which addresses a common limitation of traditional EMI shields. 8.2 Future work recommendations Standardization of Geometric Dimensioning and Tolerancing (GD&T): Introduce Geometric Dimensioning and Tolerancing standards similar to those used for metals. This standardization is necessary to evaluate printing fidelity and correlate it with rheological properties, ensuring consistent and precise 3D-printed structures. Machine learning (ML) techniques could be applied to predict and optimize the impact of various parameters on the 3D printing of cellulose ink. By training ML models on experimental data, the 143 relationships between rheological properties, printing parameters, and print quality can be identified more efficiently. This approach would enable real-time adjustments during printing, improving precision and reducing trial-and-error experiments. The characterization of cellulose with different charge densities could involve studying how various ions and salts influence the material's properties. By adjusting the type and concentration of salts such as FeCl3, NaHCO3, or CaSO4, which carry different ionic charges, the surface charge and electrostatic interactions of cellulose can be modified. This would impact factors like dispersion, printability, and mechanical strength. Investigating the charge effect could reveal how cellulose structure and functionality change with ionic interactions, providing insight into tailoring cellulose materials for specific applications such as conductivity and EMI shielding. Synthesis Methods for PEDOT-Cellulose: Investigate various synthesis methods for PEDOT-cellulose composites, aiming to create chain-like and uniform conductive structures. One approach involves growing iron-based metal-organic frameworks (MOFs) on the surface of individual cellulose nanocrystals. These MOFs can serve as initiator sites for EDOT polymerization, ensuring uniformity and enhancing both electrical conductivity and mechanical performance. This research can significantly improve the properties and applications of PEDOT-cellulose composites. Future Development of Magnetic-Conductive Cellulose Structures: Explore the production of core-shell or Janus conductive-magnetic structures by integrating functional materials with cellulose. Combining 3D-printed CNF structures with materials such as magnetic nanoparticles or responsive polymers can create multifunctional composites. This approach can significantly enhance the absorption component of the EMI shielding mechanism, improving the effectiveness of EMI shields. 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Advanced Materials 36, 2306653 (2024).
Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Arjmand, Mohammad
3D-Printed conductive cellulose cryogels for electromagnetic interference shielding : exploring structural design, printability, and shape fidelity
Text