Forestry, Faculty of
DSpace
UBCV
Zhang, Yifan
Master of Science - MSc
University of British Columbia
To address the increasing environmental concerns related to solid waste disposal, specifically non-biodegradable electronic sensors laden with heavy metals, this thesis investigates an environmentally friendly substitute. We delve into the potential of cellulose, the most abundant natural polymer, in developing biodegradable and flexible 3D sensing materials. In the first part, we create a super-stretchable and self-healing all-cellulose hydrogel. Our technique involves molecular engineering of cellulose chains and their hydrogen bonding network through periodate oxidation ring-opening reactions, followed by borohydride reduction. This innovative approach alleviates chain rigidity, enhances chain mobility, and leads to the production of hydrogels that showcase record-breaking stretchability, quick self-healing properties, and non-Newtonian behavior. The hydrogels are further tested as human motion sensors and ECG electrodes, marking significant strides in wearable technology. The second part of this work presents a super-elastic Carbon Black (CB) coated cellulose sub-micron fiber aerogel sensor, which is developed through ice-templating and electrostatic assembly. By controlling temperature, the 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNF) assembles into a 3D sub-micron fiber (SMF) network, achieving elasticity in the aerogel. The CB forms a conductive network structure on the TOCNF SMF surface through electrostatic adsorption. This unique strategy results in a SMF/CB aerogel with super-elasticity, high sensitivity, fast response and recovery times, which opens new avenues for wearable sensor applications. In summary, this thesis advocates for the potential of cellulose in developing eco-friendly, high-performance sensors, offering key insights into cellulose-based hydrogel and aerogel that manifest extraordinary properties, with potential applications in wearable technology and human health monitoring.
Preparation and Application of Cellulose Based Sensing Materials by Yifan Zhang A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Forestry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) August 2023 ©Yifan Zhang, 2023 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: Preparation and Application of Cellulose Based Sensing Materials submitted by Yifan Zhang in partial fulfilment of the requirements for the degree of Master of Science in Forestry Examining Committee: Feng Jiang, Associate Professor, Wood Science, UBC Supervisor Jack Saddler, Professor, Wood Science, UBC Supervisory Committee Member Scott Renneckar, Professor, Wood Science, UBC Supervisory Committee Member Tianxi Yang, Assistant Professor, Food Science, UBC Additional Examiner iii Abstract To address the increasing environmental concerns related to solid waste disposal, specifically non-biodegradable electronic sensors laden with heavy metals, this thesis investigates an environmentally friendly substitute. We delve into the potential of cellulose, the most abundant natural polymer, in developing biodegradable and flexible 3D sensing materials. In the first part, we create a super-stretchable and self-healing all-cellulose hydrogel. Our technique involves molecular engineering of cellulose chains and their hydrogen bonding network through periodate oxidation ring-opening reactions, followed by borohydride reduction. This innovative approach alleviates chain rigidity, enhances chain mobility, and leads to the production of hydrogels that showcase record-breaking stretchability, quick self-healing properties, and non-Newtonian behavior. The hydrogels are further tested as human motion sensors and ECG electrodes, marking significant strides in wearable technology. The second part of this work presents a super-elastic Carbon Black (CB) coated cellulose sub-micron fiber aerogel sensor, which is developed through ice-templating and electrostatic assembly. By controlling temperature, the 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNF) assembles into a 3D sub-micron fiber (SMF) network, achieving elasticity in the aerogel. The CB forms a conductive network structure on the TOCNF SMF surface through electrostatic adsorption. This unique strategy results in a SMF/CB aerogel with super-elasticity, high sensitivity, fast response and recovery times, which opens new avenues for wearable sensor applications. In summary, this thesis advocates for the potential of cellulose in developing eco-friendly, high-performance sensors, offering key insights into cellulose-based hydrogel and aerogel that manifest extraordinary properties, with potential applications in wearable technology and human health monitoring. iv Lay Summary As technology progresses, more sensors integrated into electronic products. However, as these devices become obsolete, they are often not properly disposed of, leading to environmental damage. The heavy metals contained within sensors, such as germanium, gallium arsenide, indium antimonide, becoming one of the main environmental hazards. Cellulose, the most abundant polymer in nature, has high aspect ratio, biodegradability, and abundant surface groups, making it a promising candidate for environmentally friendly and high-performance sensors. However, the inherent rigidity of cellulose impedes high-performance sensor development from it. Herein, we report the synthesis of two flexible 3D network cellulose sensing materials: an ultra-stretchable, all-cellulose hydrogel as strain sensor, and a superelastic cellulose aerogel as pressure sensor. Remarkably, the former can be stretched beyond 40000%, while the latter exhibits 94.6% height retention rate post 10,000 compressions, with an impressive response time of 7.7ms. v Preface In this thesis, under the guidance of my supervisor, Prof. Feng Jiang, I have made the most contributions, including the identification and design of cellulose-based sensing materials, as well as the performance of major parts of the experiments and data analysis. In Chapter 3, TEM and POM image was performed by Xia Sun. Dr. Hale Oguzlu assisted in conducting rheological tests of hydrogel samples. The FTIR test was carried out with the help of Dr. Yeling. Prof. Feng Jiang and Dr. Yuhang Ye provided advice on the experiments and contributed to the manuscript revisions. This chapter has been primed for publication (Y Zhang, Dr. Y Ye, X Sun, Dr. H Oguzlu, Dr. Y Zhu, Prof. F Jiang). In Chapter 4, SEM images was conducted with the help of Dr. Penghui Zhu, and Xia Sun contributing to the TEM image. Hao Sun helped with the mechanical tests. Prof. Feng Jiang and Dr. Penghui Zhu both offered advice regarding the experiments and contributed significantly to manuscript revisions. This chapter has been prepared for publication (Y Zhang, Dr. P Zhu, H Sun, X Sun, Prof. F Jiang). vi Table of Contents Abstract ............................................................................................................................. iii Lay summary .................................................................................................................... iv Preface ................................................................................................................................. v Table of contents .............................................................................................................. vi List of figures .................................................................................................................... ix List of abbreviations ...................................................................................................... xiv Acknowledgements ........................................................................................................ xvi Dedication ...................................................................................................................... xvii Chapter 1: Literature review ............................................................................................ 1 1.1 Introduction ........................................................................................................ 1 1.2 Cellulose and nanocellulose ............................................................................... 2 1.2.1 Cellulose ...................................................................................................... 2 1.2.2 Nanocellulose .............................................................................................. 4 1.3 Chemical pretreatment ....................................................................................... 7 1.3.1 TEMPO oxidation ....................................................................................... 7 1.3.2 Periodate oxidation ...................................................................................... 9 1.3.2.1 Reaction mechanism .............................................................................. 11 1.3.2.2 Hairy cellulose nanocrystals .................................................................. 13 1.4 Cellulose-based functional materials ................................................................ 15 1.4.1 Cellulose-based hydrogel .......................................................................... 15 1.4.2 Cellulose-based aerogel ............................................................................. 17 1.4.3 Cellulose-based sensing materials ............................................................. 20 1.5 Objectives and hypothesis ................................................................................ 22 Chapter 2: Materials and methods ................................................................................. 24 vii 2.1 Materials ........................................................................................................... 24 2.2 Synthesis of dialdehyde cellulose ..................................................................... 24 2.3 Synthesis of dialcohol cellulose nanorods (DCNRs) and hydrogel ................. 24 2.4 Preparation of CNF, SMF and SMF/CB aerogels ............................................ 25 2.5 Characterization of DCNRs .............................................................................. 25 2.5.1 UV-vis spectroscopy .................................................................................. 25 2.5.2 Transmission electron microscopy (TEM) ................................................ 26 2.5.3 Atomic force microscopy (AFM) .............................................................. 26 2.5.4 Dynamic light scattering (DLS) ................................................................ 26 2.5.5 Fourier transform infrared (FTIR) ............................................................. 26 2.5.6 X-ray diffraction (XRD) ............................................................................ 27 2.6 Characterization of super stretchable hydrogel and drived fiber...................... 27 2.6.1 Rheological properties of hydrogel and mechanical properties of derived fibers ........................................................................................................... 27 2.6.2 Hydrogel sensor fabrication ...................................................................... 28 2.6.3 Electrocardiogram (ECG) .......................................................................... 28 2.7 Characterization of flexible SMF/CB Aerogel ................................................. 28 2.7.1 Scanning electron microscope (SEM) ....................................................... 28 2.7.2 Mechanical test .......................................................................................... 29 2.7.3 Sensing performance ................................................................................. 29 Chapter 3: Super-stretchable and fast self-healing all-cellulose hydrogel ................. 30 3.1 Introduction ...................................................................................................... 30 3.2 Formation of super-stretchable all-cellulose hydrogel ..................................... 32 3.3 Characterization of DCNRs .............................................................................. 33 3.4 Characteristics of the DCNR hydrogel ............................................................. 37 3.5 High stretchability of the DCNR hydrogel ....................................................... 41 3.6 Applications as sensors and electrodes............................................................. 43 viii 3.1 Conclusion ........................................................................................................ 45 Chapter 4: Electrostatic self-assembly enabled superelastic cellulose aerogel for pressure sensing ............................................................................................................... 47 4.1 Introduction ...................................................................................................... 47 4.2 Fabrication of the superelastic SMF/CB Aerogel ............................................. 49 4.3 Characterization of SMF/CB suspension and SMF/CB aerogel ...................... 50 4.4 Compressibility and elasticity of SMF/CB aerogel .......................................... 54 4.5 Sensing performance of SMF/CB aerogel ........................................................ 57 4.6 Potential application of SMF/CB Aerogel ........................................................ 61 4.7 Conclusion ........................................................................................................ 63 Chapter 5: Conclusion and future works ................................................................... 64 Bibliography ..................................................................................................................... 66 Appendices ........................................................................................................................ 85 Appendix A Photographic of self-healing all-cellulose hydrogel in Chapter 3 ................. 85 Appendix B Photographic of all-cellulose hydrogel in Chapter 3 during stretching ......... 86 Appendix C Keithley 2450 setup for measuring all-cellulose hydrogel sensing performance. ...................................................................................................................... 87 Appendix D Keithley 2450 setup for measuring superelastic aerogel sensing performance. ...................................................................................................................... 88 Appendix E Resistance changes of SMF0.2/CB0.06 aerogel under pressure ................... 89 Appendix F The resistance change of SMF0.2/CB0.1 aerogel response to the pressure of water drops. ..................................................................................................... 90 Appendix G The gauge factor of SMF0.2/CB0.1 aerogel within 0-50% strain ................ 91 ix List of Figures Figure 1. The hierarchical structure of wood from tree to cellulose molecular structures. Reprinted with the permission from ref19, Copyright 2013, Elsevier. ................................. 3 Figure 2. morphology image of cellulose nanocrystals (CNCs TEM), cellulose nanofibrils (CNFs TEM), and bacterial cellulose (BC SEM). Reprinted with the permission from ref27, Copyright 2021, Royal Society of Chemistry. ................................. 4 Figure 3. Selective oxidation of C6 primary hydroxyl groups to carboxylate groups by TEMPO/NaBr/NaClO oxidation in water at pH 10–11. Reprinted with the permission from ref65, Copyright 2011, Royal Society of Chemistry. ................................................... 8 Figure 4. Selective oxidation of C6 primary hydroxyl groups to carboxylate groups by TEMPO/NaClO/NaClO2 oxidation in water at pH 4.8–6.8. Reprinted with the permission from ref65, Copyright 2011, Royal Society of Chemistry. ................................. 9 Figure 5. Schematic of periodate oxidation of cellulose. Reprinted with the permission from ref85, Copyright 2012, Springer. ................................................................................ 11 Figure 6. Periodate oxidation model of cellulose. Reprinted with the permission from ref95, Copyright 2021, Elsevier. ......................................................................................... 12 Figure 7. Trimmed Hairy CNC using 3 N HCl and carboxyl contents after trimming. Reprinted with the permission from ref100, Copyright 2020, Elsevier. .............................. 14 Figure 8. physical and chemical crosslinking mechanisms during hydrogel formation. Reprinted with the permission from ref112, Copyright 2023, Royal Society of Chemistry. ............................................................................................................................................ 15 Figure 9. Applications of cellulose based flexible sensors. Reprinted with the permission from ref180, Copyright 2021, American Chemical Society. ............................. 20 Figure 10. Schematic illustration showing that a) cleaving the rigid anhydroglucose unit (AGU) rings will increase chain flexibility and promote the re-formation of hydrogen bonds during stretching. ..................................................................................... 32 Figure 11. Schematic illustration of the fabrication process of DCNRs hydrogel............. 33 x Figure 12. Photos of a) Aqueous NBSK pulp, DACs and DCNRs suspension. b) Polarized microscope image of NBSK pulp ...................................................................... 34 Figure 13. a) Transmittance of 0.5wt% DACs and DCNRs suspension. b) FTIR spectra of NBSK pulp, DACs and DCNRs. ................................................................................... 35 Figure 14. a) TEM micrographs of DCNRs suspension. b) DCNRs width and length distributions. ....................................................................................................................... 35 Figure 15. AFM image of DCNRs ..................................................................................... 36 Figure 16. XRD diffractogram of NBSK pulp and DCNRs .............................................. 37 Figure 17. Z-average hydrodynamic diameter of a) 0.025wt% dialyzed DCNRs suspension, redispersed DCNRs hydrogel suspension, dialyzed redispersed DCNRs hydrogel suspension. .......................................................................................................... 38 Figure 18. Characteristics of DCNRs hydrogel. a) Photos showing the fluid behavior of DCNRs hydrogel under static condition. b) Snapshots showing the DCNRs hydrogel ball bouncing back after being dropped from a height of 0.5 m above the ground. .......... 39 Figure 19. Frequency sweep of DCNRs hydrogel. ............................................................ 39 Figure 20. Photos of self-healing process of the DCNRs hydrogel (with one piece being stained with methylene blue) ............................................................................................. 40 Figure 21. a) The storage modulus G′ of the DCNRs hydrogel during the strain amplitude sweep (from 0.1% to 1000% and back to 0.1%). b) Time sweep analysis of DCNRs hydrogel with alternative strain from 0% to 150% .............................................. 41 Figure 22. . Photographic of a) DCNRs hydrogels as extrusion printing inks. b) Printed “cellulose” using DCNRs hydrogels ink. ........................................................................... 41 Figure 23. Photographs of DCNRs hydrogels before and after stretching. ....................... 42 Figure 24. SEM images of dried DCNR fiber with knots. ................................................. 42 Figure 25. a) Polarized optical microscopy (POM) image of the dried DCNRs fiber. b) Average stress-strain curve of dried DCNRs fiber. c) Photos of DCNRs fiber holding a 100 g weight ....................................................................................................................... 43 xi Figure 26. a) The relative resistance changes of sensor during stretching and releasing. b) Time-dependent relative resistance changes of the sensor with different tensile strains. c) Photographs of sensor attached to elbow for monitoring elbow bending in real-time ............................................................................................................................. 44 Figure 27. Spectrum of ECG signal obtained from commercial electrodes and DCNRs electrodes. .......................................................................................................................... 45 Figure 28. Schematic illustrating the design strategy of SMF/CB aerogel. (a) Self-assembly of CNF into SMF through ice-templating. (b) Adsorption of CB on the surface of SMF through electrostatic assembly. (c) Structure of the prepared SMF/CB aerogel. ............................................................................................................................... 50 Figure 29. Morphology characterization of CNF, SMF, and SMF/CB suspensions and aerogels. (a) Photographs showing the appearance of CNF, SMF, and SMF/CB suspensions. TEM images of (b) CNF, (c) SMF, and (d) SMF/CB. Photographs demonstrating € CNF, (f) SMF, and (g) SMF/CB aerogels before and after compression by a 100 g weight. SEM images showing the morphologies of (h-j) CNF0.2, (k-m) SMF0.2, and (n-p) SMF0.2/CB0.1 aerogels. ..................................................................... 52 Figure 30. Laser experiment demonstrating the transparency of (a) CNF, (b) SMF, and (c) SMF/CB suspensions. ................................................................................................... 52 Figure 31. Photograph of CB suspensions (a) with and (b) without SMF after standing one week and 12 h, respectively. ....................................................................................... 54 Figure 32. Photographs demonstrating the shape recovery property of (a) CNF, (b) SMF, and (c) SMF/CB aerogels after compression with a 10 g weight. ..................................... 54 Figure 33. Mechanical properties of SMF and SMF/CB aerogels. (a) Compressive stress-strain curves of SMF0.4 aerogel at strains from 10% to 50%. Maximum stress remaining of (b) SMF aerogels with various densities and (c) SMF/CB aerogels with various CB contents at 40% strain during 50 cyclic tests. ................................................. 55 Figure 34. Density of SMF0.2, SMF0.3, SMF0.4, SMF0.2/CB0.06, SMF0.2/CB0.1, xii SMF0.2/CB0.2 aerogels. .................................................................................................... 56 Figure 35. Mechanical properties of SMF0.2/CB0.1aerogels. (a) Compressive stress-strain curves of SMF0.2/CB0.1aerogel at strains from 10% to 50%. (b) Cyclic compressive stress-strain curves of SMF0.2/CB0.1 aerogel at 40% strain. (c) Height retention of SMF0.2/CB0.1 aerogel during 10000 cyclic tests. ......................................... 57 Figure 36. a) The energy loss coefficient curves of SMF0.2/CB0.1 aerogel. b) Height retention of SMF0.2/CB0.1 aerogel during 10,000 cyclic tests. ........................................ 57 Figure 37. Photograph showing the assembled SMF/CB aerogel sensor. ......................... 58 Figure 38. Sensing performance of SMF/CB aerogel. (a) Current-voltage (I-V) curves of SMF/CB aerogel under various applied pressures. (b) Relative resistance changes of SMF0.2/ CB0.1 aerogel under various applied pressures. ............................................ 58 Figure 39. Sensitivity of SMF0.2/CB0.1 2 aerogel in the range of 0-6.5 kPa ................... 59 Figure 40. (a) Relative resistance changes of SMF0.2/CB0.1 aerogel under cyclic compressing/releasing tests for 1000 cycles with 1.5 kPa applied pressure. (b) Response and recovery time of SMF0.2/CB0.1 aerogel during compressing/releasing process with 0.5 kPa applied pressure. (c) Response and recovery time of SMF0.2/CB0.1 aerogel during compressing/releasing process with 0.5 kPa applied pressure. (f) Comparison of response and recovery time between this work and MXene/rGO,248 graphene,270 carbonized cellulose,292 ODA-rGO@PANFs/CNT,284 aPANFs/GA,285 PPy@rGO,286 brophene-graphene,287 PINF/MXene,288 PPy/ PAAm,289 CA/PPy,290 cotton/PPy,291 PI/CNT,293 CNTs/graphene/WPU/CNC,294 AgNWs/CNFs266, aPANFs/MX–rGA283 based pressure sensors. ................................................................... 60 Figure 41. Demonstrations of SMF/CB aerogel for the applications in detecting various human body motions. Relative resistance changes of SMF0.2/ CB0.1 aerogel in detecting (a) coughing, (b) swallowing ............................................................................. 61 Figure 42. Demonstrations of SMF/CB aerogel for the applications in detecting small human body motions. Relative resistance changes of SMF0.2/ CB0.1 aerogel in xiii detecting (a,b) speaking, (c) artery pulse. .......................................................................... 62 Figure 43. Demonstrations of SMF/CB aerogel for the applications in detecting large human body motions. Relative resistance changes of SMF0.2/ CB0.1 aerogel in detecting (a) elbow bending, and (b) knee bending. .......................................................... 63 xiv List of Abbreviations AgNWs: Sliver nanowires AGU: Anhydroglucose unit BNC: Bacterial nanocellulose CNFs: Cellulose nanofibrils CA: Citric acid CB: Carbon Black CMC: Carboxymethyl cellulose CNCs: Cellulose nanocrystals CUAs: Celluluronic Acids DAC: Dialdehyde cellulose DCNRs: Dialcohol cellulose nanorods DLS: Dynamic light scattering DO: Degree of oxidation ECG: Electrocardiography ECNC: Electrosterically stabilized cellulose nanocrystal HCNC: Hairy cellulose nanocrystals MFC: Micro fibrillated cellulose MTMS: Methyltrimethoxysilane NBSK: Northern bleached softwood Kraft PANI: Polyaniline POM: Polarizing microscopes PVA: Polyvinyl alcohol SMF: Sub-micron fiber SNCC: Sterically stabilized nanocrystalline cellulose TEMPO: 2,2,6,6-tetramethylpiperidine-1-oxyl TMCS: Trimethylchlorosilane xv TOCNFs: TEMPO-oxidized cellulose nanofibrils xvi Acknowledgements I would like to express my sincere gratitude to my supervisor, Prof. Feng Jiang, for his unceasing guidance, patience, and support throughout my three-year journey in pursuing this thesis. His expertise, understanding, and encouragement have been instrumental to my academic and personal growth. I also offered my enduring thanks to my committee members, Prof. Jack Saddler and Prof. Scott Renneckar. Their valuable insights and suggestions have greatly enriched this work and expanded my understanding of the field. My profound thanks go to Dr. Yuhang Ye and Dr. Penghui Zhu for their contributions to my experimental work. Their depth of knowledge was invaluable in the progress and successful completion of this project. Thanks also to my colleagues at the Sustainable Functional Biomaterials Laboratory. To Dingyuan Zheng, Hao Sun, Jiaying Zhu, Pu Yang, Xia Sun, and Dr. Yeling Zhu for your support and camaraderie. I am grateful to Dr. Jie Wu and Xuan Jun for their help in both academic and life. Their friendship and support were greatly appreciated during challenging times. Lastly, I want to convey my deepest gratitude to my parents, Li Jia and Lifeng Zhang, for their unconditional love and support. Their belief in me and their encouragement have been my source of strength and resilience throughout this journey. xvii Dedication This work is dedicated to my family, supervisor, friends. whose unwavering support and encouragement have been the pillar of my journey 1 Chapter 1: Literature Review 1.1 Introduction As the economic development and the improvement of living standards, solid wastes pollution has become one of the top pollution sources on earth.1-3 Every year, the world generates 2.01 billion tons of solid waste, and 33% of it are non-degradable substances, such as plastic, rubber, metal, and glass.4 Dealing with solid waste is getting to be a real headache for most countries. So far, most solid waste was disposed in open landfills by local public entities with poor operating procedures, and the inappropriate disposal has caused serious air pollution, groundwater pollution, health issues, as well as energy consumption and global warming.3, 5, 6 The non-degradable solid waste like rubber and plastic are often burnt, causing fumes and releases of noxious gases into atmosphere. Meanwhile, the growing quantity of electronic waste in solid waste streams is becoming an increasing concern.7 Electronic products today use various sensors for data collection and human-machine interaction, including optical fiber sensors, proximity sensors, gyroscopes, accelerometers, barometric sensors, pressure sensors, and sound sensors, among others. To prolong their operational lifespan, many of these sensors employ materials like germanium, gallium arsenide, indium antimonide, lead telluride, and cadmium sulfide. The rapid pace of technological progress has led to an escalating number of electronic devices being discarded and subjected to improper disposal methods, posing a potential risk to local communities due to the heavy metals present in these waste sensors, which can have toxic effects and result in groundwater pollution, making entire regions uninhabitable.7-10 Hence, the development of environmentally friendly and biodegradable sensing materials is a pressing requirement. Cellulose, as the most abundant nature polymer on earth, has attracted a lot of attention due to its sustainability, biodegradability, low cost, and renewability.11 It has been used in our 2 society for thousands of years as paper and building blocks. As the primary constituent of plant cell walls, cellulose can be obtained from a diverse range of natural sources, such as wood, fruits, leaves, grasses, and seeds, etc. Among them, wood is the main source for cellulose industry as kraft pulp and dissolving pulp can be easily prepared by removing most lignin and hemicellulose.12 Meanwhile, high aspect ratio, low density and abundant surface groups make cellulose become an ideal material on modern biopolymer composites industry. Potential utilizations extend to various domains such wound treatment, tissue engineering, cosmetic, flexible electronics, medical implants, fibers and textiles, water treatment, separation membranes, batteries, and supercapacitors.13-18 1.2 Cellulose and Nanocellulose 1.2.1 Cellulose Cellulose is the main reinforcement structure of plants, and it has the repeating unit of two glucose rings connected by β 1–4 glycosidic bond. As shown in Figure 1, the size of cellulose ranges from several nanometers to micrometers with the structure changing from nanocellulose to macrofibers.19 The hydrogen bond as well as Van Der Waals force between hydroxyl groups stabilize the linear structure and promote the association of cellulose chain to form elementary fibrils. In these, highly ordered arrangements constitute the crystalline region, while those with no discernable alignment or pattern are termed as the amorphous region. The crystalline region of cellulose boasts an impressive theoretical modulus reaching 100–200 GPa, and a tensile strength of about 4.9–7.5 GPa, outperforming most metals, alloys, synthetic polymers, and even many ceramics.11 Furthermore, the abundant surface groups of cellulose could act as anchoring points to introduce new functionalities. For instance, polyelectrolytes can be physically adsorbed to achieve hydrophobicity or antibacterial properties, or polymer grafting can enhance mechanical properties or create 3 luminescent CNFs. Thus, cellulose holds great promise for high-value applications.20-23 Figure 1. The hierarchical structure of wood from tree to cellulose molecular structures. Reprinted with the permission from ref19, Copyright 2013, Elsevier. There are four common polymorph types of cellulose (I, II, III, IV) depending on different cellulose chain arrangement. Cellulose I, also known as “natural” cellulose, is extracted from natural materials like wood, plants, and bacteria. It also has two allomorphs: triclinic structure (Iα) and monoclinic structure (Iβ). The triclinic structure (Iβ) is more common in plant cell walls, while Iα is typically to find in most bacteria and algae.24 Cellulose II is the crystalline modifications of Cellulose I, and it is the most thermodynamically stable polymorph of cellulose. Except directly obtained from Halicystis, it can be prepared by recrystallization or mercerization with aqueous sodium hydroxide. Cellulose III can be derived via the ammonia treatment of cellulose I or II, while cellulose IV emerges as a product of subjecting cellulose III to elevated temperature in the glycerol.12, 25, 26 4 1.2.2 Nanocellulose Compared to cellulose, nanocellulose has higher strength, modulus, specific surface area and unique crystalline properties, making it an ideal material for many different applications. It can be classified into three major categories: cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and bacterial nanocellulose (BNC) (Figure 2). Figure 2. morphology image of cellulose nanocrystals (CNCs TEM), cellulose nanofibrils (CNFs TEM), and bacterial cellulose (BC SEM). Reprinted with the permission from ref27, Copyright 2021, Royal Society of Chemistry. In 1950, Randy utilized sulfuric acid-catalyzed degradation of cellulose for the first time to produce a suspension of cellulose nanocrystals (CNCs).28 He broke down the amorphous region of cellulose in an acidic solution, resulting in rod-like particles that were observed through transmission electron microscopy (TEM) and electron diffraction, and identified as CNCs. The primary methods of preparing CNCs include mechanical processing, acid hydrolysis, and enzyme hydrolysis. Mechanical treatments currently include high-pressure homogenization,29, 30 high-intensity ultrasonic processing,31 and bead milling,32, 33 among others. During mechanical processing, mechanical shearing breaks down the amorphous regions to produce the desired CNCs. However, the presence of an ordered hydrogen bond network in cellulose requires the application of a significant amount of energy during mechanical processing. Furthermore, the crystallinity of CNCs obtained through mechanical processing varies considerably depending on the method employed. CNCs produced through high-pressure homogenization can achieve a crystallinity of 76%,29 5 while those obtained through high-intensity ultrasonic processing can reach 81%,31 which is still lower compared to CNCs generated via acid hydrolysis (85-90%).34 However, CNCs prepared by bead milling can achieve a crystallinity level of 85-95%,32 surpassing even the levels attained through acid hydrolysis. Strong acid hydrolysis is the most widely used method, typically using sulfuric acid and hydrochloric acid.35-37 Some weak acids, such as phosphoric acid, citric acid, and formic acid, are also utilized.38-41 During acid hydrolysis, the oxygen (cyclic or glycosidic) of cellulose is rapidly protonated under the influence of the H+. Sulfuric acid reacts with the hydroxyl groups on the surface of cellulose, resulting in sulfated CNCs with excellent dispersibility due to the charged surface sulfate groups.42 Hydrochloric acid hydrolysis results in non-charged CNCs, which tend to flocculate in water.43-45 Biological enzymes can catalyze the hydrolysis of cellulose, commonly utilizing a cellulase system composed of one or more endoglucanases, xylanases, or cellobiohydrolases (CBH).46-49 For example, endoglucanase can act on the β-1,4 glycosidic bond, disrupting the amorphous regions of cellulose and yielding CNCs with a width of 6-10 nm.49 CNCs prepared through enzyme hydrolysis typically exhibit poor dispersion in aqueous suspensions but have a higher aspect ratio. However, the yield and efficiency of enzyme hydrolysis are relatively low, so it is commonly combined with mechanical processing as an environmentally friendly method for preparing CNCs.50-52 In 1983, Turbak firstly utilized high-pressure homogenization to produce microfibrillated cellulose, which were observed to have a diameter of 25-100 nm through scanning electron microscopy (SEM) and primarily demonstrated their potential use as a food additive.53 This marked the beginning of extensive research on the properties and characterization of nano- or micro-fibrillated cellulose, or referred as cellulose nanofibrils (CNFs) when the lateral dimensions are in the nanoscale range. CNFs offer several advantages over CNCs due to their composition of long cellulose chains that include both crystalline and amorphous regions, providing greater flexibility. Nowadays, it are widely used in various industries 6 such as 3D printing, wound dressings, construction materials, paper, and packaging.54-57 CNFs can be produced through a variety of mechanical, chemical, and enzymatic methods, including high-pressure homogenization, microfluidization, ball milling, grinding, cryocrushing, and steam explosion. Conventional mechanical processes, such as homogenization, grinding, and refining, have been scaled up for industrial production.58 However, the uneven size distribution of CNFs produced through mechanical methods makes characterization challenging.59 To develop more uniform CNFs, the commonly used method is a combination of chemical pretreatment and mechanical processing, including carboxymethylation, phosphorylation, acetylation, and periodate oxidation, as well as 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO).60-64 TEMPO-oxidized cellulose nanofibrils (TOCNFs) are the most commonly investigated, which present as a stable aqueous suspension due to the electrostatic repulsion or osmotic effects between anionically-charged cellulose microfibrils (Refer to Chapter 1.3 for further details).65 In 1886, Brown demonstrated the ability of bacteria to produce cellulose.66 Unlike CNCs and CNFs, which are produced through a top-down approach, bacterial nanocellulose (BNC) is obtained through a bottom-up method. The synthesis process of bacterial cellulose involves four steps: activation of monosaccharides, polymerization to form repeating sugar units, addition of acyl groups, and excretion of BNC from the cell into the environment. A variety of bacteria, including Gluconacetobacter xylinus, Alcaligenes, Pseudomonas, Sarcina, Rhizobium, and Enterobacter, are known to produce cellulose.67-70 Among these, Gluconacetobacter xylinum is the most commonly used due to its ability to produce high-purity, high-crystallinity gel-like BNC films without the need for pre-treatment to remove lignin and hemicelluloses.71, 72 BNC is highly biocompatible and safe, making it popular for use in the biomedical field for applications such as wound dressings, tissue regeneration, drug delivery, and cell culture and tissue engineering.73-76 However, its high cost and slow production process are limitations in its commercialization. 7 1.3 Chemical pretreatment 1.3.1 TEMPO Oxidation 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), is a water-soluble, stable nitroxyl radical capable of oxidizing primary hydroxyl groups to aldehyde or carboxyl groups under mild conditions. In 1984, Semmelhack et al. demonstrated the ability of TEMPO-mediated selective oxidation of primary alcohols in their published work.77 Subsequently, in 1994, de Nooy et al. conducted a study where they utilized TEMPO-mediated oxidation on water-soluble polysaccharides, including potato starch, pullulan, and amylodextrin. This oxidation process successfully converted the hydroxyl group on the C6 of these polysaccharides into a carboxyl group.78 Figure 3 depicts the mechanism of TEMPO-mediated oxidation of cellulose. In this system, TEMPO and NaBr are dissolved in a cellulose suspension with a pH value of 10-11, and the oxidation process is initiated by adding sodium hypochlorite (NaClO) solution. Through an aldehyde intermediate, the primary hydroxyl groups at C6 position on cellulose chains undergo oxidation to form carboxyl groups. Throughout the oxidation, TEMPO functions as catalyst, while NaClO is the primary oxidant that will be consumed. To maintain the optimal pH during this selective TEMPO/NaBr/NaClO-mediated oxidation, a small quantity of NaOH must be added, accounting for the generation of deprotonated carboxyl groups.65 After being treated with a homogenizer or ultrasonic homogenizer, TEMPO-oxidized cellulose yields a transparent CNF dispersion. CNFs with low oxidation levels tend to aggregate, whereas those with a higher carboxylic acid content exhibit more individual nanofibrils with widths around 3-5 nm due to the presence of anionic surface charges groups.65 As a result, the suspension displays improved transparency as compared 8 to those less oxidized. Figure 3. Selective oxidation of C6 primary hydroxyl groups to carboxylate groups by TEMPO/NaBr/NaClO oxidation in water at pH 10–11. Reprinted with the permission from ref65, Copyright 2011, Royal Society of Chemistry. Numerous studies have demonstrated that Guluronic acids undergo unavoidable depolymerization via a β-elimination mechanism in the pH range of 9 to 12.79 To address this issue, Saito et al. proposed the TEMPO/NaClO/NaClO2 oxidation system, which is illustrated in Figure 4.80, 81 This oxidation system employs NaClO2 as the primary oxidizing agent under conditions of 60 °C and pH 6.8. The resulting product is individual CNFs with a degree of polymerization exceeding 900, a diameter of 5 nm, and a length of 2 μm. 9 Figure 4. Selective oxidation of C6 primary hydroxyl groups to carboxylate groups by TEMPO/NaClO/NaClO2 oxidation in water at pH 4.8–6.8. Reprinted with the permission from ref65, Copyright 2011, Royal Society of Chemistry. 1.3.2 Periodate Oxidation The periodate oxidation was firstly observed by Malaprade in 1928 where he observed the degradation of mannitol through the reaction. Consequently, the process became known as 10 the Malaprade reaction.82 In 1952, periodate oxidation was utilized for molecular analysis of polysaccharides and oligosaccharides.83 It selectively cleaves the furanose or pyranoses rings and can provide information on the composition, sequence, and location of glycosidic bonds in polysaccharides by detecting mono-/oligomers. In 2002, Varma et al. conducted a detailed study on the oxidation of cellulose with sodium periodate to produce 2,3-dialdehyde cellulose to determine the effect of periodate concentration, reaction temperature, medium pH, and cellulose morphology on the process.84 Figure 5 illustrates the mechanism of periodate oxidation of cellulose.85 The first step involves two consecutive attacks of iodine by lone pairs on the hydroxyl groups (nucleophilic addition), followed by a proton transfer. The second step results in the formation of a cyclic periodate ester, which then decomposes to form dialdehyde. In other words, the periodate salt can oxidize the hydroxyl groups at C2 and C3 of cellulose to form aldehydes and cleave the C2-C3 bond, ultimately yielding 2,3-dialdehyde cellulose (DAC). The aldehydes can serve as chemical anchors for reactions with various functional groups, leading to the preparation of DAC derivatives. For instance, Liimatainen et al. used NaClO2 to oxidize the aldehydes to carboxyls, promoting the nanofibrillation of cellulose.86 They homogenized samples with carboxyl content ranging from 0.38 to 1.75 mmol/g into a transparent gel. Zhang et al. synthesized a cellulose-based Schiff base through the condensation of DAC and L-lysine, which exhibited good antibacterial properties, nontoxicity, and biocompatibility.87 Kasai et al. reduced dialdehyde cellulose to dialcohol cellulose using NaBH4, yielding a unique transparent film with remarkable tensile properties.88 Rajalaxmi et al. sulfonated DAC using sodium sulfite to prepare a novel water-soluble cellulose derivative.89 Abboud et al. developed a novel eco-friendly flame-retardant nanocomposite material by combining DAC with amino-functionalized mesoporous silica MCM-41 (N-M41).90 Muchová et al. found that DAC can serve as an efficient crosslinking agent for polyvinyl alcohol (PVA) hydrogel for drug delivery and 11 wound dressings.91 Figure 5. Schematic of periodate oxidation of cellulose. Reprinted with the permission from ref85, Copyright 2012, Springer. 1.3.2.1 Reaction mechanism The characteristics of DAC are significantly dependent on the degree of oxidation (DO) and crystallinity, and the mechanism of the oxidation reaction greatly influences the final morphology and properties of the product. Periodate oxidation is a self-accelerating process, as the surface glucopyranose ring is disrupted, neighboring groups become more easily oxidized, and this oxidation leads to a decrease in cellulose crystallinity. As the DO increases, the crystallinity of cellulose gradually weakens and eventually disappears when the DO reaches 87%.92 Periodate oxidation produces three products: fibrous cellulose, sterically stabilized nanocrystalline cellulose (SNCC), and dialdehyde-modified cellulose.93 Some research proposed a model for periodate oxidation of cellulose, where periodate reacts preferentially 12 with the amorphous regions of cellulose, breaking the cellulose glycosidic bond while opening the C2 and C3 bonds.94 Then, periodate has a preferential oxidation reaction at the interface between the amorphous and crystalline regions, pushing into the crystalline region, leading to a reduction in the length of the crystalline region (Figure 6).94, 95 It is noteworthy that the diameter of cellulose before and after oxidation is very similar, indicating that periodate oxidation is highly heterogeneous, mainly causing the breakage of cellulose chains and the destruction of the glucopyranose ring. Figure 6. Periodate oxidation model of cellulose. Reprinted with the permission from ref95, Copyright 2021, Elsevier. Based on this periodate oxidation model, we can find that the crystallinity of the raw material also affects the reaction rate. The rate of oxidation is faster for cellulose I than for cellulose II, as well as for cotton and microcrystalline cellulose, which have higher 13 crystallinity.96 Furthermore, the morphology of cellulose nanofibrils can be controlled by adjusting the periodate concentration and reaction time.97 At an aldehyde content of 0.45 mmol/g, nanofibrillar bundles with widths of 10-50 nm and lengths of several micrometers can be produced. By increasing the aldehyde content to approximately 0.8 mmol/g, it is possible to obtain nanofibrillar bundles with widths of 5-10 nm and cellulose nanocrystals (CNCs). When the aldehyde content is further increased to 1.5 mmol/g, only CNCs with a width of approximately 5 nm can be observed in the suspension.98 1.3.2.2 Hairy cellulose nanocrystals As discussed, as the periodate oxidation progresses towards the crystalline region, aldehyde cellulose chains remain attached to the ends of the crystalline region.94, 95, 98-100 This morphology of CNC is therefore referred to as hairy cellulose nanocrystals (HCNC). When the temperature is increased above 80 C, the dialdehyde modified cellulose chains at the ends of the hairy cellulose nanocrystals may dissolve. The dissolved dialdehyde modified cellulose remains attached to the ends of the CNCs, improving the spatial stability of the CNCs. This cellulose nanocrystal produced after heating at 80 C is referred to as sterically stabilized cellulose nanocrystals (SNCC). The aldehyde groups can then undergo oxidation with the chloric acid to form a carboxyl group. This CNC with negatively charged dicarboxyl cellulose chains at both ends provides electrostatic and steric (electrosteric) stability to the CNCs, and is therefore referred to as electrosterically stabilized CNC (ECNC).99-101 The cellulose chains at the ends of HCNC cannot be directly observed by AFM or TEM, but their existence can be confirmed by dynamic light scattering (DLS) or electroacoustic. In the study by van de Ven and Sheikhi (2016), the addition of salt shielded the electrostatic repulsion between the ECNC charge groups. The size of the ECNC measured by DLS was reduced from 220 nm (in distilled water) to 100 nm (at high potassium chloride 14 concentration), demonstrating the existence of ECNC cellulose chains.99 Additionally, the crosslinking of the hydrolysed ECNC through diamine or azide–alkyne cycloaddition can form rod-like particles, indicating that the charged groups were mainly located at the ends of the CNCs, successfully demonstrating the position of the cellulose chains. Also, the hydrodynamic diameter of CNC oxidized with sodium periodate increased from 95 nm to 120 nm, also demonstrating the existence of the chains in periodate oxidized cellulose.98 It is worth noting that both the crystalline region and the hairy part of HCNC are controllable (Figure 7), as the cellulose chains at both ends can be trimmed through a hydrolysis reaction catalyzed by hydrochloric acid, affecting the rheological properties of the HCNC.100 Figure 7. Trimmed Hairy CNC using 3 N HCl and carboxyl contents after trimming. Reprinted with the permission from ref100, Copyright 2020, Elsevier. Functionalized HCNC can also be prepared through simple physical assembly or chemical assembly to produce hydrogels with different rheological properties, transparent films, or aerogels.101-105 At the same time, due to the existence of cellulose chains at both ends, HCNC has many special properties. The film prepared from reduced HCNC has a 200% stretching performance, far exceeding ordinary cellulose.88 The surface charge of oxidized HCNC is greater than that of CNC oxidized by TEMPO, so it can be used to remove heavy metals in wastewater, achieving a removal capacity of 185 mg/g for copper ions.106 HCNC 15 functionalized with a high density of primary amine groups can be used to fabricate films with photocatalytic bactericidal properties and has potential for use in food packaging.107 1.4 Cellulose-based functional materials 1.4.1 Cellulose-based hydrogel Hydrogels are soft materials formed by crosslinking hydrophilic polymers into a network structure, enabling them to retain a significant amount of water by surface tension and capillary forces. It has found wide-ranging applications in diverse fields, including wound management, drug delivery, wearable sensors, and agriculture, etc.108-111 According to the crosslinking methods, hydrogels can be divided into physically crosslinked hydrogels and chemically crosslinked hydrogels (Figure 8). Figure 8. physical and chemical crosslinking mechanisms during hydrogel formation. Reprinted with the permission from ref112, Copyright 2023, Royal Society of Chemistry. Physically crosslinked cellulose hydrogels are formed by molecular self-assembly through ionic, hydrogen bonds, or macromolecular entanglement. This type of gel has a strong self-16 healing ability and can easily recover after damage due to the molecular network being tightly connected by hydrogen bonds.113-115 Freezing-thawing technique is a common method for producing physical hydrogels, which works by causing the solvent to crystallize during freezing, forcing the polymer to form a 3D network connected by hydrogen bonds.116 The properties of the hydrogel are primarily determined by the freezing time, freezing temperature, and the number of freezing-thawing cycles.117 Currently, the freezing-thawing technique is used to produce hydrogels from CNF, CNC/CNF, CNC/PVA, CNC/gelatin, and other materials.118-120 Since this technique does not affect the biocompatibility and biodegradability of cellulose, the resulting hydrogels are widely used in the fields of biology and medicine. Other methods for preparing physical hydrogels include the interaction of polyelectrolytes with opposite charges, such as the formation of hydrogels from negatively charged carboxymethyl cellulose (CMC) and protonated chitosan.121 CMC and CNF containing quaternary ammonium groups (CDAC) with opposite charges can also be used to produce cellulose polyion complex hydrogels.122 Additionally, metal ions as ionic crosslinkers are widely used to produce hydrogels, such as the use of Ca2+ or Fe3+ to crosslink TEMPO oxidized CNFs or the preparation of polydopamine (PDA)-CNF hydrogel with Ca2+.123-125 Other common strategies for producing physical hydrogels include host-guest interactions and hydrophobic interactions. These methods rely on non-covalent binding between molecules, often referred to as molecular recognition, which occurs due to their distinct structures.112, 126-128 Chemically crosslinked cellulose hydrogels rely on crosslinking agents to form stable covalent bonds between cellulose chains. Since this covalent bond is irreversible, chemically crosslinked hydrogels exhibit higher stability and greater mechanical strength compared to physically crosslinked hydrogels.129-131 It is worth noting that the functional groups on the surface of the crosslinking agent determine the characteristics of the hydrogels. For instance, epichlorohydrin (ECH) crosslinked with CNF has stronger 17 swelling and water absorption capabilities.132 Ethylene glycol diglycidyl ether (EGDE) can crosslink carboxymethyl cellulose (CMC), and this hydrogel shows protein adsorption and release abilities, demonstrating its potential in drug delivery.133 A mixture of poly(N-isopropylacrylamide) (PNIPAAm) and CNC forms a thermoresponsive hydrogel that can be used as a wound dressing.134 Citric acid (CA) is a natural organic substance that can enhance the tensile strength and thermal stability of cellulose hydrogels. It is also one of the most commonly used safe crosslinking agents for humans, and hydrogels crosslinked with it are widely used in the food industry.135-137 Other commonly used crosslinking agents include 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), genipin, glutaraldehyde, and divinylsulfone (DVS).138-141 In addition, surface-modified nanocellulose can act as a crosslinking agent as well. For example, furyl-modified CNC can crosslink with poly(ethylene glycol) to form tough, highly resilient, and fast self-recoverable hydrogels.142 Furfurylamine-modified TOCNF can react with water-soluble bismaleimide through Diels-Alder coupling to form thermally reversible hydrogels.143 1.4.2 Cellulose-based aerogel Cellulose aerogels are unique materials formed by replacing the liquid phase in a gel with a gas while preserving the 3D network structure of the gel. The first aerogel was created in 1932 by Kistler via supercritical CO2 drying of silica gel.144 Although various inorganic aerogels have since been developed, cellulose has attracted significant attention due to its sustainability and abundant surface groups, making it an ideal material for aerogel production. The synthesis of cellulose aerogels typically involves two steps: dissolving/dispersing cellulose and forming a cellulose gel through gelation, followed by drying the cellulose gel while preserving its 3D porous structure. The pore size, density, surface area of the aerogel 18 depends on the raw material, gelation method, and drying technique.145, 146 MFC, CNFs, and BCs are commonly used as starting materials due to their flexibility and ability to form a porous fibrous network via hydrogen bonding. Compared to other cellulose aerogels, CNC aerogels are more rigid and brittle due to their crystalline nature, necessitating physical or chemical crosslinking to strengthen the 3D network of CNC and prevent uncontrolled collapse during solvent removal.147, 148 Common chemical crosslinkers for cellulose aerogels include citric acid, glutaraldehyde, epichlorohydrin, and others, while physical crosslinking can be achieved through the use of multivalent metal ions or inorganic salts to enhance structural stability.149-152 Drying is the final and most crucial step in aerogel production. Capillary forces exerted by surface tension of solvent within the cellulose hydrogel on surrounding pore walls may cause uncontrollable collapse of the 3D network.153-155 The formula for capillary forces is shown below, wherein γ represents surface tension, r represents the radius of pores, and θ denotes the contact angle between the solvent and the pore walls.154 To mitigate the uncontrollable collapse of the three-dimensional network, one may reduce surface tension, increase pore diameter, or fortify the mechanical properties of the pore walls. 𝑃 =−2𝛾 cos 𝜃𝑟 Supercritical drying, freeze drying, and ambient drying are the most commonly employed drying methods. Supercritical drying uses supercritical fluid (usually CO2) to replace the solvent in the gel, eliminating capillary force.156-158 Freeze drying is the most widely used method for producing cellulose aerogels. This method typically involves two steps: freezing and sublimation of the ice under high vacuum conditions. During the freezing process, the growth of ice crystals results in increased cellulose concentration among the ice crystals, and these concentrated celluloses self-assemble with each other to form pore walls. During the drying process, water molecules acquire sufficient energy to transform directly from the solid state to the gas phase by dissociating from surrounding molecules, 19 leaving pores that were previously occupied by the ice crystals159 Directional freeze drying, including unidirectional and bidirectional freeze drying, yields anisotropic aerogels with improved insulation and mechanical properties.160-162 It should be noted that aerogel produced using freeze drying methods sometimes is referred as cryogel. Ambient drying employs low surface tension solvents or strengthened pore walls to minimize capillary force and wall destruction. While ambient drying is low-cost, aerogels produced using this method tend to have relatively high density and low specific surface area, and it commonly referred as xerogel163-165 Cellulose aerogels have numerous advantages, including a large specific surface area, a porous structure, low density, and controllable mechanical strength. Because of their uniform porous structure, cellulose aerogels can be utilized as thermally insulating materials, exhibiting thermal conductivity lower than 25 mW/(m·K), and their thermal stability can be further improved by incorporating inorganic additives, carbon materials, or thermally stable polymers.166-169 Moreover, cellulose aerogels have an immense specific surface area, making them applicable in adsorption and environmental protection. Silanized cellulose aerogels, hydrophobically modified with MTMS or TMCS, exhibit an impressive adsorption capacity of 489 g/g for chloroform, as well as excellent performance for adsorbing other organic solvents and oils.170 Furthermore, modified cellulose aerogels with functional groups on their surfaces can adsorb corresponding metal ions, making them ideal for removing pollutants.171 Cellulose aerogels are also frequently used as three-dimensional scaffolds that can be functionalized by incorporating other substances. For example, they can bind with conductive materials through covalent bonds or hydrogen bonds to function as high-performance sensors or electrodes, or can be combined with AgNWs to produce electromagnetic interference (EMI) shielding materials.172 20 1.4.3 Cellulose-based sensing materials Sensor materials are often classified into four categories: semiconductors, ceramics, metals or alloys, and organics. Of these, semiconductors and ceramics are the most frequently encountered sensor materials in daily life, including silicon (Si), germanium (Ge), silicon nitride (Si3N4), aluminum nitride (AlN), aluminum oxide (Al2O3), zirconium oxide (ZrO2), and strontium aluminate (SrAl2O4).173-179 However, these materials are generally plagued by challenges such as problematic waste disposal, susceptibility to oxidation, and poor biocompatibility. Figure 9. Applications of cellulose based flexible sensors. Reprinted with the permission from ref180, Copyright 2021, American Chemical Society. In contrast, cellulose, as the most abundant polymer in nature, boasts not only nontoxicity and biocompatibility, but also remarkable durability, strength, and flexibility, making it an ideal material for the fabrication of next-generation flexible sensors(Figure 9).180-182 The structures of cellulose-based functional sensing materials can be divided into four groups: hydrogels, aerogels, films/membranes, and fibers. The primary methods for preparing 21 cellulose film sensors include vacuum filtration, solution regeneration, electrospinning, solution casting, phase inversion, and other preparation methods.183-187 These materials often comprise cellulose nanofibrils (CNFs), cellulose nanocrystals (CNCs), cellulose acetate, and are frequently blended with other metal nanomaterials to achieve superb sensing and mechanical performance. The preparation methods for cellulose fiber sensors include dip coating, mold processing, dry-wet spinning, and 3D printing.188-190 Compared with traditional 1D and 2D sensors (films/membranes, fibers), 3D materials with high porosity and web-like entangled structures have led to a growing preference for three-dimensional network sensors (hydrogels and aerogels).191-193 However, even these 3D cellulose materials, such as hydrogels and aerogels, have their own flexibility constraints. For instance, pure cellulose nanofibril (CNF) aerogels, characterized by their interconnected 3D network and robust structural integrity, exhibit excellent compressibility. However, their inability to recover to their original shape after intense mechanical loading limits their potential applications in flexible pressure sensors.170 In the pursuit of enhancing the flexibility of 3D cellulose materials, researchers have employed strategies such as unidirectional freeze drying and incorporation of reinforcing polymers into the cellulose-based structures.194-196 Nevertheless, both approaches present challenges. Anisotropic elasticity was observed in the elastic CNF aerogel prepared via unidirectional freezing, showing poor or lack of elasticity along the longitudinal direction.194 On the other hand, the integration of various polymer matrices to create flexible composite cellulose materials can lead to complex synthesis procedures, limiting large-scale production. In terms of the limitation of cellulose-based hydrogel, all cellulose hydrogels are inherently stiff due to the presence of rigid anhydroglucose units and highly ordered structures resulting from extensive intra- and inter-chain hydrogen bonding. Even though stretchable cellulose-based hydrogels have been fabricated, cellulose only contributes a minor fraction in these systems, primarily serving as a reinforcement filler.113, 197-199 Therefore, finding an all cellulose based flexible 3D material with all desired properties is impactful. 22 1.5 Objectives and hypothesis The central objective of this thesis is to investigate the potential of cellulose, an abundantly available, biodegradable natural polymer, to be transformed into an environmentally friendly and flexible 3D functional material for application as sensors. The targeted preparation should be achieved without incorporating any polymer reinforcement, aiming to enhance the stretchability or compressibility of the resulting materials, and thus increase its performance in various applications. In Chapter 3, a super-stretchable and self-healing all-cellulose hydrogel was developed through the molecular engineering of cellulose chains and their hydrogen bonding network. We hypothesize that, through periodate oxidation ring-opening reactions and the following borohydride reduction, the rigidity of cellulose chains could be alleviated thereby forming the polymer strands with enhanced chain mobility. The applicability of these hydrogels will be demonstrated in the realm of sensing technology, specifically as strain sensors and electrodes for the detection of human physiological signals, such as electrocardiography (ECG). In Chapter 4, a super-elastic, multifunctional carbon black (CB) coated cellulose sub-micron fiber aerogel sensor was developed through ice-templating and electrostatic assembly. We hypothesize that by controlling freezing temperature, the TEMPO-oxidized cellulose can be assembled into a 3D sub-micron fiber network, thereby achieving elasticity in the cellulose aerogel matrix. Furthermore, we propose that a conductive network structure can be formed by electrostatically adsorbing CB onto the carboxyl groups on the TOCNF surface. With special hierarchical structure, we anticipate that this strategy will result in a SMF/CB aerogel with improved sensing properties, including super-elasticity 23 and high sensitivity, which could have significant application potential in the field of flexible sensors. 24 Chapter 2: Materials and methods 2.1 Materials Sodium meta-periodate (NaIO4, >99.8%), sodium chloride (NaCl, >99%), sodium borohydride (NaBH4, 99%) was obtained from Thermo scientific. 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 99.9%), sodium hypochlorite (NaClO, 11.9%), and sodium bromide (NaBr, > 99%) were obtained from Sigma-Aldrich. Northern bleached softwood kraft (NBSK) pulp was provided by Pulp and Paper Centre at UBC (Vancouver, Canada). Carbon black (CB) was purchased from MTI Corporation, Canada. All the reagents were used as received without further purification. 2.2 Synthesis of dialdehyde cellulose The DAC was prepared by adding 1 g of NBSK in 100 mL deionized water and then stirred for at least 12 h at room temperature, followed by adding NaIO4 (1.32 g) and NaCl (3.87 g) to the cellulose suspension and stirring with a magnetic stirrer for 72h. To protect the periodate from decomposition, the mixture was covered with aluminum foil during the oxidation process. The mixture was then dialyzed against deionized water for one week to remove residual chemicals. 2.3 Synthesis of dialcohol cellulose nanorods (DCNRs) and hydrogel The DCNRs were synthesized by reducing the DAC using sodium borohydride. Before borohydride reduction, the DAC suspension was blended by high-speed mechanical blending (Vitamix 5200) for 15 min to ensure complete dispersion. Then, 0.5 g NaBH4 (per gram of cellulose) and 0.025 mol/L of NaH2PO4 was added in the DAC suspension and the reaction was allowed to proceed for 12 h at room temperature under magnetic stirring, yielding DCNRs of approximately 0.6 wt% concentration. The residual salts from the 25 buffer and reducing agent were kept in the suspension, and the suspension was vacuum filtrated through a 0.45 μm nylon filter paper for 70 h at room temperature, leading the formation of ultrastretchable all-cellulose hydrogel. 2.4 Preparation of CNF, SMF and SMF/CB aerogels CNF was prepared from NBSK pulp by TEMPO oxidation (5 mmol NaClO per gram of NBSK) combined with high-speed blending (25 min) according to the previous literature.170 After that, the obtained CNF suspension was diluted to 0.05 wt% and then frozen at -196 °C using liquid nitrogen, followed by freeze-drying to obtain SMF. The dried SMF was re-dispersed in distilled water at various concentrations of 0.2 wt%, 0.3 wt%, and 0.4 wt%, and some 0.2 wt% SMF suspension were mixed with CB at 1:1, 2:1, and 3:1 mass ratio with respect to SMF. Subsequently, all mixtures were sonicated in a water bath (Branson 5200) for one hour to obtain a well-mixed suspension. All these suspensions were then frozen at -18 ℃ overnight and freeze-dried to obtain SMF and SMF/CB aerogels. The SMF aerogels are designated as SMFX, where X represents the percentage concentration of SMF in the suspension. The SMF/CB aerogel is designated as SMFX/CBY, where X and Y represent the percentage concentration of each component in the suspension, respectively. A CNF aerogel were prepared through freezing 0.2 wt% CNF suspension at -18 ℃ and freeze-drying and was used as a control sample. 2.5 Characterization of DCNRs 2.5.1 UV-vis spectroscopy Cary 60 UV−vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) was utilized to measure the transmittance and absorbance of 0.5 wt% DCNRs suspension at a wavelength of 220 to 800 nm with a scanning rate of 100 nm min-1. 26 2.5.2 Transmission Electron Microscopy (TEM) TEM images were acquired using a Hitachi H7600 transmission electron microscope operating at an acceleration voltage of 80 kV. For sample preparation, drops of a 0.01 wt% DCNRs suspension were carefully deposited onto glow-discharged TEM grid coated with a carbon layer. Excess liquid was removed by gently blotting with filter paper. Subsequently, the samples were dried at room temperature. To enhance contrast, a drop of a 2% uranyl acetate solution was applied to negatively stain the DCNRs. Widths and lengths of DCNRs were measured from TEM images using Nano Measurer 1.2 software of more than 200 individual nanorods. 2.5.3 Atomic force microscopy (AFM) The morphology of the DCNRs was characterized using an atomic force microscope (MultiMode8, Bruker USA Inc.). A drop of 0.001 wt% DCNR suspension was deposited onto a freshly peeled mica surface and allowed to dry at ambient condition before measurement. The experiments were conducted in peak force tapping mode using aluminum-coated antimony-doped silicon cantilevers with a spring constant of 40 N/m and a resonant frequency of 300 kHz. The 2D height images were processed using Nanoscope Analysis software. 2.5.4 Dynamic light scattering (DLS) The size distribution of each sample was analyzed using DLS. The Malvern Zetasizer Nano ZS was utilized for all measurements, with data collection based on monitoring the intensity of scattered light at a 90° scattering detector angle and a temperature of 21 °C. 2.5.5 Fourier transform infrared (FTIR) FTIR spectra of the samples were collected using an INVENIO-S attenuated total 27 reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Bruker Optics Pty. Ltd., Billerica, MA, USA) with single bounce diamond crystal. All the spectra of measured samples were collected from 400 to 4000 cm-1, comprising a total of 16 scans, with a resolution of 4 cm-1. 2.5.6 X-ray diffraction (XRD) X-ray diffraction (XRD) experiments were conducted using a D8 Advance Bragg-Brentano Diffractometer (Bruker, Billerica, MA, USA) equipped with a long fine-focus Co X-ray tube operating at 35 kV and 40 mA. The XRD diffractogram of NBSK pulp and freeze-dried DCNRs were recorded over a 2θ range of 5° to 65°, with a scan rate of 1° min−1. Subsequently, the obtained 2θ values were shifted to correspond to a Cu target using Jade software. The crystallinity index (CrI) was determined by applying the Segal empirical equation. 2.6 Characterization of super stretchable hydrogel and dried fiber 2.6.1 Rheological properties of hydrogel and mechanical properties of derived fibers The viscoelastic rheological properties of the hydrogel were assessed using a TA Instruments AR 200 rotational rheometer at room temperature (25°C). A parallel plate with a 25 mm diameter and a gap distance of 1 mm was utilized to generate a uniform shear rate. The sample was subjected to torsional flow in both CR and CS modes to determine its viscous behavior and yield stress (τy). In addition, oscillatory tests were carried out to evaluate the viscoelastic behavior of the sample. The sample was allowed to reach equilibrium temperature with the measuring system for at least 10 minutes. The mechanical properties of derived DCNRs fibers was tested by dynamic mechanical analysis (Dma Q800, TA Instruments, USA) from 0.01 N to 1.8 N at room temperature. 28 2.6.2 Hydrogel sensor fabrication The strain sensor was constructed by placing the hydrogel between two 3M VHB adhesive tapes. The VHB tapes were used to adhere the sensor to the skin and prevent solvent evaporation. The resistance of the superelastic hydrogel was measured in real-time using a digital source meter (Keithley 2450, USA). The relative change in resistance was calculated using the formula (R - R0)/R x 100%, where R represents the resistance under different strains, and R0 represents the resistance at the initial state without any strain. 2.6.3 Electrocardiogram (ECG) The performance of the fabricated electrodes was evaluated by monitoring ECG signals using the DCNRS hydrogel to replace the commercial gel of T3425 UniGel electrodes and comparing it to the ECG signals obtained from commercial T3425 UniGel electrodes using the T9306M EKG sensor package (Thought Technology, Canada). BioGraph Infiniti (Thought Technology, Canada) was utilized to visualize and display the ECG signals obtained. All signal measurements were performed at room temperature. Six electrodes were placed on the left and right forearms of the test subject and simultaneously connected to the T9306M EKG sensor to acquire accurate data. 2.7 Characterization of flexible CB/CNF Aerogel 2.7.1 Scanning electron microscope (SEM) The morphologies of CNF, SMF, and SMF/CB aerogels were characterized using a scanning electron microscope (Helios NanoLab 650, FEI, USA) at an accelerating voltage of 5 kV. 29 2.7.2 Mechanical Test The mechanical properties of aerogels were analyzed using Instron 3345 Materials Testing System with 2 kN load cell. Cylindrical aerogels (diameter: 20 mm, height: 10 mm) were compressed to 50% strain at 1mm/min plate displacement rate under temperature of 21oC and a relative humidity of 30%. The maximum stress retention (MSR) and Young's modulus of the aerogels were calculated from the stress-strain curves, where MSR = σ/σ0 × 100%, σ0 is the maximum stress of the aerogels before compression; σ is the stress of the aerogels after compression. 2.7.3 Sensing performance Conductive silver paste and aluminum foils were sequentially coated on both ends of the obtained cylindrical SMF/CB aerogels (with an approximate diameter of 22 mm and height of 21 mm) as contact electrodes to obtain the sensors. Then, copper wires were used to connect the aluminum foils at both ends of the aerogel to the analyzer for sensing measurement.The responsive properties of the aerogels were characterized using a digital source meter (2450, Keithley, USA). When the aerogel was subjected to different stress, the resistance of the aerogel was recorded in real time under an applied voltage of 1 V. The responsive properties of the sensors were reflected by sensitivity, which is calculated as (ΔR/R0)/ΔP, ΔR is the resistance change of the aerogels after compression and R0 is the initial resistance before compression; ΔP represents the change of applied pressure. . The GF was calculated as ΔR/ε, where ε represents the strain. 30 Chapter 3: Super-stretchable and fast self-healing all-cellulose hydrogel 3.1 Introduction Hydrogel, an emerging material composed of a polymeric network that encapsulates a significant amount of water, has gained significant attentions in diverse fields, including biomedical engineering,200, 201 flexible electronics,202, 203 and even energy storage and conversion.204-206 To ensure optimal performance in real-world application scenarios, the choice of an excellent polymeric backbone is crucial, as it plays a governing role in determining the overall performance. Synthetic polymers such as polyacrylic acid (PAA), polyacrylamide (PAAm), and poly vinyl alcohol (PVA) currently dominate both the research and commercial sectors in hydrogel fabrication, owing to their exceptional mechanical properties, significant water absorption capacity, and easy accessibility.207-210 However, these synthetic polymers, derived from petrochemicals, are not biodegradable, posing serious environmental concerns.211-213 Consequently, discovering an environmentally friendly alternative for high-performance hydrogel preparation holds significant importance. In nature, hydrogel-like structures are abundant in multicellular organisms.214-216 For instance, vascular and nonvascular (e.g., algae) plants can be considered as hydrogels during specific stages of their growth.215 In these cases, cellulose acts as the fundamental building block, forming a 3D interconnected network that swells in water while maintaining structural integrity, thanks to its strong hygroscopicity and extraordinary mechanical strength derived from inter- and intra-molecular hydrogen bonds.215 Due to its biodegradability, abundant availability, and a host of other beneficial properties, cellulose has gained widespread recognition as the most promising environmentally friendly candidate for the fabrication of hydrogels.217-221 Nevertheless, native cellulose hydrogels exhibit inherent stiffness, primarily attributed to the presence of rigid anhydroglucose unit (AGU) and the highly ordered, crystalline structures formed by extensive intra- and inter-31 chain hydrogen bonding. Although stretchable cellulose-based hydrogels have been reported, cellulose typically constitutes a minimal fraction within the hydrogel system, primarily functioning as reinforcement filler by harnessing its rigid and high modulus properties. The stretchability of these hydrogels primarily arises from the synthetic polymer network.220, 222, 223 Breaking and reconstructing intermolecular hydrogen bonds by dissolving cellulose into molecular chains and regenerating them in an anti-solvent bath has been proposed as a solution to this limitation.224-227 The transformation from Cellulose I to Cellulose II allomorphs has shown potential in enhancing the elasticity and stretchability of the resulting hydrogel.228 Furthermore, the presence of water molecules as a plasticizer contributes to the further increase in stretchability by disrupting the formation of hydrogen bonds within cellulose chains.229 Despite these advances, the stretchability of all-cellulose hydrogels remains limited, with a maximum value typically below 250%.225 Again, such limitation arises from the presence of rigid AGU rings and abundant uncontrolled hydrogen bonding networks, which restrict their applicability in areas where high stretchability is needed. In this study, we introduce a straightforward strategy to produce a highly stretchable all-cellulose-based hydrogel by precisely controlling the molecular architecture of cellulose chains and their associated hydrogen bonding network. The substantial reduction in rigidity of cellulose chains can be achieved through periodate oxidation ring-opening reactions, which result in the formation of polymer strands with enhanced chain mobility. The dialdehyde groups formed on the cellulose chains are subsequently converted into hydroxyl groups, providing a chemical foundation for the formation of inter-chain hydrogen bonds. The dialcohol nanocellulose tends to aggregate at a high concentration, allowing for its assembly into a hydrogel through the removal of excess water using a straightforward vacuum filtration process. The resulting hydrogel exhibits superb stretchability (>40000%) due to the combination of a relatively linear structure and a well-regulated hydrogen 32 bonding network. Furthermore, this fully physically crosslinked structure imparts the hydrogel with instant and efficient self-healing properties (with seconds). Based on these properties, we showcase the application of this all-cellulose hydrogel as strain sensors and electrodes for detecting human physiological signals, such as electrocardiography (ECG) and human motions. We believe that this molecular engineering strategy not only paves the way for comprehensive exploration of cellulosic materials but also holds tremendous potential for the development of high-performance hydrogel and other functional materials. 3.1 Formation of super-stretchable all-cellulose hydrogel All-cellulose hydrogels typically exhibit high stiffness with limited stretchability due to the presence of rigid anhydroglucose unit (AGU) ring structures in the backbone, along with the robust interchain hydrogen bonding interactions, as depicted schematically in Figure 10. Therefore, we hypothesize that by modifying the cellulose structure and regulating their interchain interactions, it is possible to create a highly stretchable all-cellulose hydrogel. Figure 10. Schematic illustration showing that a) cleaving the rigid anhydroglucose unit (AGU) rings will increase chain flexibility and promote the re-formation of hydrogen bonds during stretching. 33 Aming enhance the chain flexibility, periodate oxidation was employed to open the AGU ring structure by selectively cleaving C2-C3 linkage, converting secondary hydroxyls into aldehyde groups. Additionally, the two aldehyde groups were subsequently reduced to two primary hydroxyl groups, which are expected to facilitate the formation of hydrogen bonds among the dialcohol cellulose nanorods (DCNRs). To concentrate the DCNRs and promote their proximity for the formation of a stretchable hydrogel, vacuum filtration was employed to remove excess water. By implementing this molecular engineering strategy, we successfully developed an ultra-stretchable all-cellulose hydrogel that exhibits an exceptional strain exceeding 40000%. Furthermore, this hydrogel also demonstrates rapid self-healing capabilities (Figure 11). Figure 11. Schematic illustration of the fabrication process of DCNRs hydrogel 3.2 Characterization of DCNRs Throughout the chemical modification of the northern bleached softwood kraft (NBSK) pulp, the products resulting from each individual step were characterized to monitor the progression of the reaction process. As shown in Figure 12a, the original pulp suspension appeared opaque and exhibited severe aggregation, which can be attributed to the large dimensions of the pulp fibers (Figure 12b). Following periodate oxidization, the suspension remained opaque; however, it exhibited improved stability, which can be attributed to the reduction in fiber length caused by the oxidation. The high opaqueness of 34 the suspension suggested the presence of severe aggregation, which can be ascribed to the formation hemiacetal or acetal linkages between the aldehyde groups.230 Nevertheless, a transparent and stable suspension can be achieved following mechanical blending and borohydride reduction. Figure 12. Photos of a) Aqueous NBSK pulp, DACs and DCNRs suspension. b) Polarized microscope image of NBSK pulp UV-Vis spectra (Figure 13a) was utilized to show the transmittance of dialdehyde cellulose (DACs) and DCNRs suspension. The results clearly indicate a significant increase in the transmittance at 650 nm, rising from 25.8% form DACs to approximately 95.6% for DCNRs. Additionally, it is evident that the absorbance peak associated with carbonyl groups at around 250 nm disappears in the DCNRs suspension, further confirming the successful reduction of aldehyde groups. In order to further verify the transformation from cellulose to DCNRs, ATR-FTIR was employed. As shown in Figure 13b, the ATR-FTIR spectra of NBSK pulp, DACs and DCNRs all exhibit characteristic cellulose absorption peaks, including O-H stretching vibration at approximately 3330 cm−1, C-H stretching vibration at around 2900 cm−1, −CH2 symmetric bending vibration at 1456 cm−1, C-H bending vibration at 1371 cm−1, C-O-C glycosidic linkage stretching vibration at 1140 cm−1, and C-O stretching vibration at 1053 cm−1.231 After periodate oxidation, the DACs spectrum exhibited a small peak at 1730 cm-1 and an intensified peak at 880 cm-1, which can be assigned to the carbonyl groups and the newly formed hemiacetal linkages between 35 the aldehyde groups and hydroxyl groups, respectively, thus confirming the successful oxidization process. After borohydride reduction, the peaks observed at both 1730 cm-1 and 880 cm-1 vanished, while the peak at 3330 cm-1 becomes more pronounced, confirming the successful reduction of aldehyde groups to hydroxyl groups. Figure 13. a) Transmittance of 0.5wt% DACs and DCNRs suspension. b) FTIR spectra of NBSK pulp, DACs and DCNRs. The morphology of DCNRs was observed by TEM and AFM. As shown in Figure 14a, the DCNRs exhibited typical rod-like structure with an average length of 94.9 ± 1.6 nm and an average width of 8.0 ± 0.1 nm (Figure 14b). The AFM image (Figure 15) also displayed similar rod-like structure consistent with the TEM image. Figure 14. a) TEM micrographs of DCNRs suspension. b) DCNRs width and length distributions. 36 Figure 15. AFM image of DCNRs In addition, Figure 16 shows the X-ray diffraction patterns of NBSK pulp and DCNRs. The NBSK pulp exhibits typical diffraction peaks of cellulose type I at 2θ= 14.7° (11__0), 16.5° (110) and 22.6° (200).232 Interestingly, the XRD diffractogram of DCNRs revealed a noticeable reduction in peak intensity, particularly for the (200) peak at 22.6°. This suggests that the chemical reactions have disrupted the crystalline structure. The considerable decrease in peak intensity observed for the (200) crystal plane suggests that the crystal damage primarily occurs at the axial position.233 In cellulose crystals, the hydroxyl groups as situated at the equatorial position of the AGU ring, contributing to the intra-sheet hydrogen bonding interactions. Meanwhile, inter-sheet stacking is primarily governed by Van der Waals interactions. After the conversion of the secondary hydroxyl groups into primary hydroxyls through ring-opening interactions, the resulting hydroxyl groups exhibit a higher degree of freedom, enabling them to rotate away from the equatorial position. Such rotation ultimately leads to the disruption of the crystal plane, particularly affecting the (200) plane held together by Van der Waals interactions. Based on Segal empirical equation, the crystallinity index dropped from 70.9% to 26.7%. 37 Figure 16. XRD diffractogram of NBSK pulp and DCNRs 3.3 Characteristics of the DCNR hydrogel The all-cellulose hydrogel was prepared through a simple vacuum filtration of 0.6 wt% DCNR suspension. During the filtration process, the DCNRs tend to accumulate on the 0.45 mm nylon filter, leading to the formation of gel-like material. This is to our surprise since the measured particle sizes of the DCNRs from both AFM and TEM are close to 100 nm in length and a few nm in width. Therefore it is not expected that the DCNRs can be retrained by the 0.45 mm filtration membrane. It should be noted that no gel-like material can be obtained when filtering diluted 0.05 wt% DCNRs suspensions, suggesting that the DCNRs tends to aggregate at high concentration, possibly due to the strong hydrogen bonding and chain entanglement. To further verify the impact of concentration on the particle size of the DCNRs, dynamic light scattering (DLS) was employed to evaluate the hydrodynamic diameter of the DCNRs suspension at varied concentrations. As shown in Figure 17, the 0.05 wt% DCNRs showed two peaks centered at 8.7 nm and 78.8 nm. Notably, both peaks significantly shifted to larger values of 43.8 nm and 295.3 nm at 0.6 wt% concentration, and then to 122.4 nm and 712.4 nm at a concentration of 3 wt%, clearly indicating particles aggregation at increased concentration. Therefore, when filtering the DCNRs suspension at 0.6 wt%, more DCNRs aggregates can be retained on the filtration membrane as the concentration increases, resulting in a gel-like material. 38 Figure 17. Z-average hydrodynamic diameter of a) 0.025wt% dialyzed DCNRs suspension, redispersed DCNRs hydrogel suspension, dialyzed redispersed DCNRs hydrogel suspension. Further characterization of the prepared all-cellulose hydrogel revealed its characteristic non-Newtonian behavior, demonstrating a liquid-like behavior under low shearing rate and solid-like behavior under high shearing rate. This phenomenon is demonstrated in Figure 18a&b. Under static conditions, the all-cellulose hydrogel flow slowly over time due to gravity, displaying a liquid-like behavior (Figure 18a). However, when a hydrogel ball is dropped from a height, it demonstrates elastic and solid material characteristics by bouncing back (Figure 18b). Rheology tests were employed to further characterize the non-Newtonian behavior of the all-cellulose hydrogel. As shown in Figure 19, at low frequency, the loss modulus G’’ surpassed the storage modulus G’, indicating a liquid-like structure. However, with increasing frequency, the G’ and G’’ intersected, and in the high frequency range, the G’ exceeded G’’, signifying a solid-like behavior. This observation confirms the non-Newtonian nature of the hydrogel and is consistent with the findings presented in Figure 18a&b. 39 Figure 18. Characteristics of DCNRs hydrogel. a) Photos showing the fluid behavior of DCNRs hydrogel under static condition. b) Snapshots showing the DCNRs hydrogel ball bouncing back after being dropped from a height of 0.5 m above the ground. Figure 19. Frequency sweep of DCNRs hydrogel. In addition, the all-cellulose hydrogel also showcased remarkable self-healing capabilities attributed to its unique crosslinked structure. As shown in Figure 20, a piece of hydrogel was split into two parts, with one piece dyed blue for enhanced visibility. After being recombined for 2 mins, the two pieces seamlessly healed together, resulting a fully restored hydrogel. Remarkably, the healed hydrogel showed exceptional stretchability, proving its 40 high healing efficiency. The rheological test further confirmed the impressive recovery ability, as depicted in Figure 21a. Initially, as the strain increased, the storage modulus G’ remained stable, but under high strain, it started to decrease due to the disruption of the hydrogel network. However, as the strain gradually returned to a lower range, the G’ also recovered correspondingly, indicating the remarkable self-recovery ability. Additionally, Figure 21b also showed that the hydrogel was capable of recovering from various strains. Furthermore, the all-cellulose hydrogel exhibited outstanding printability, allowing it to be easily injected through a syringe (Figure 22a) and utilized as ink for writing letters (Figure 22b). This quality makes it promising candidate for advanced applications as an injectable ink. Figure 20. Photos of self-healing process of the DCNRs hydrogel (with one piece being stained with methylene blue) 41 Figure 21. a) The storage modulus G′ of the DCNRs hydrogel during the strain amplitude sweep (from 0.1% to 1000% and back to 0.1%). b) Time sweep analysis of DCNRs hydrogel with alternative strain from 0% to 150% Figure 22. . Photographic of a) DCNRs hydrogels as extrusion printing inks. b) Printed “cellulose” using DCNRs hydrogels ink. 3.1 High stretchability of the DCNR hydrogel In addition to the self-healing and printability, the all-cellulose hydrogel exhibited record-high and unprecedent stretchability exceeding 40000% strain. As shown in Figure 23, a 42 piece of hydrogel (1 cm) can be stretched to 4.42 m, corresponding to a strain of 44200%. After drying at 80°C for an hour, a flexible and smooth fiber with a width of 46.7 μm can be obtained. SEM was utilized to observe the morphology of the hydrogel derived fibers. As displayed in Figure 24a-c, the hydrogel derived fiber exhibited exceptional flexibility, allowing it to withstand twisting and be easily tied into various knotted structure. This manifests that the ultrastretchable all-cellulose hydrogel has the potential to fabricate flexible cellulose fibers through a straightforward process of stretching and drying. Figure 23. Photographs of DCNRs hydrogels before and after stretching. Figure 24. SEM images of dried DCNR fiber with knots. The remarkable ultrastretchability of the all-cellulose DCNR hydrogel can be attributed to its unique crosslinked network structure, as well as its non-Newtonian behavior. During the gradual stretching process, the hydrogel exhibited a characteristic viscous behavior, as depicted in Figure 19. The viscous behavior enables the DCNRs to be drawn out. The abundant primary hydroxyl groups facilitate for the formation of dynamic hydrogen bonds among the DCNRs during the stretching process, resulting in an exceptional level of stretchability that surpasses previous records. The fiber showed vivid birefringence under polarized optical microscopy (POM) (Figure 25a), indicating its crystalline structure. The DCNR fiber also demonstrated high mechanical strength, showing a high tensile stress 43 reaching up to 226 ± 34 MPa and a strain of 3.5%, with high Young's modulus of 9.4 ± 1.1 GPa (Figure 25b). As shown in Figure 25c, a dried DCNR fiber with the diameter of 87 μm can lift a weight of 100 g, showing its high mechanical performance. Figure 25. a) Polarized optical microscopy (POM) image of the dried DCNRs fiber. b) Average stress-strain curve of dried DCNRs fiber. c) Photos of DCNRs fiber holding a 100 g weight 3.2 Applications as sensors and electrodes For the preparation of the all-cellulose hydrogel, the DCNR suspension after borohydride reduction was employed as-is, without eliminating the residual salts from sodium borohydride (NaBH4) and monobasic sodium phosphate (NaH2PO4). After reduction, it is expected that the suspension encompassed the subsequent ions: Na+, H2PO42-, and B(OH)4-, which can offer ionic conductivity to the hydrogel. Given the ultrastretchability, flexibility as well as the existence of free ions, we speculated that the all-cellulose hydrogel could 44 serve as an ion-conductive motion sensor and apply as ECG electrode for monitoring physiological signals. Firstly, the sensing performance under different strain ratios was presented in Figure 26a. With the increased external strain from 0% to 300%, the relative resistance change (ΔR/R0) of the hydrogel gradually increased correspondingly from 0 to 136%. Surprisingly, after stepwise releasing the external strain, the ΔR/R0 could recover to the corresponding value, suggesting a low hysteresis and high reliability of transducing strain signals. Besides, cyclic stability under different strains was also evaluated from 100%-400% strain as shown in Figure 26b. Under the same strain, the output resistance signals were highly stable and repeatable, indicating a high stability of the hydrogel sensor. herefore, the all-cellulose hydrogel was employed as a wearable sensor to monitor human movements in real time. As shown in Figure 26c, when the sensor was mounted on the volunteer’s elbow, the relative resistance change was increased when the volunteer performed bending behavior and the bending angle could be clearly reflected by the peak intensity, signifying the potential for the all-cellulose hydrogel in wearable devices. Figure 26. a) The relative resistance changes of sensor during stretching and releasing. b) Time-dependent relative resistance changes of the sensor with different tensile strains. c) Photographs of sensor attached to elbow for monitoring elbow bending in real-time In addition to the strain sensitivity, the free ions in the hydrogel also enabled that the all-cellulose hydrogel to transduce signals as an ECG electrode. As shown in Figure 27a, all-cellulose electrodes and commercial gel electrodes were fixed on hands simultaneously to acquire the ECG signals in real time. Figure 27b showed the ECG signals obtained through commercial electrodes and all-cellulose hydrogel electrodes, respectively. It was obvious 45 that all PQST typical wave were clearly observed for both commercial and our hydrogel electrodes. Besides, by comparing the ECG signals from commercial gel electrodes and all-cellulose hydrogel electrodes, the similarity could reach 99.9%, indicating the potential of all-cellulose hydrogel for replacing the commercial counterparts. All these results suggest that our all-cellulose hydrogel is highly potential for the applications as wearable sensors and for physiological signal monitoring. These results highlighted that the ultrastretchable all-cellulose hydrogel was highly potential for the wearable devices. Figure 27. Spectrum of ECG signal obtained from commercial electrodes and DCNRs electrodes. 3.3 Conclusion To sum up, an ultrastretchable all-cellulose hydrogel was reported for the first time. Sequential periodate oxidation and borohydride reduction can lead to the breakage of rigid AGU ring structure, as well as the conversion of two secondary hydroxyl groups on each AGU ring to primary hydroxyl groups. An ultrastretchable all-cellulose hydrogel can be facilely formed by vacuum filtration of the dialcohol cellulose nanorods at high concentration, showing fast self-healing property and unique non-Newtonian behavior. Benefiting from the dynamic hydrogen bonding induced by the primary hydroxyl groups and opened ring structure, the hydrogel can rapidly self-heal after breaking and can be stretched to over 40000% strain without breaking. In addition, the DCNRs hydrogel can be stretched into a dried cellulose fiber with excellent flexibility and high mechanical strength. Furthermore, the hydrogel can serve as an ionic-conductive motion sensor and ECG 46 electrode due to its inherent free ions, showing excellent reproducibility as compared to commercial ECG gel electrode. This innovative approach offers a streamlined method for crafting ultrastretchable all-cellulose hydrogels, holding significant promise for advanced sensor and health monitoring applications. 47 Chapter 4: Electrostatic Self-assembly Enabled Superelastic Cellulose Aerogel for Pressure sensing 4.1 Introduction Pressure sensors have attracted extensive attention due to their wide range of applications in fields such as speech recognition,234-236 health monitoring,237-239 motion detection,240-242 and electronic skin243-245, etc. According to their different transduction mechanisms, pressure sensors can be classified into four categories, including piezoelectric,246-248 piezoresistive,249-252 capacitive,253-255 and triboelectric sensors.256-258 Among them, piezoresistive sensor stands out due to its feasible fabrication process, ease of signal collection, low cost, as well as fast response speed. Piezoresistive materials used in pressure sensing mainly include copper,259 silver,260 polyaniline (PANI),261 and polypyrrole (PPy),262 etc. Conventionally, metal-based sensors are incompatible with biological system due to their rigid structure and low stretchability. Even though conductive polymers-based sensors present excellent flexibility, the crosstalk caused by temperature or humidity limits their applications in a wide range.263, 264 Recently, due to their low density and high surface aera, porous cellulose aerogels and polymer sponges have been explored to develop piezoresistive pressure sensors.237, 265-268 However, the sensing performance is highly dependent on the density of the porous materials. Typically, for low-density materials with large interlayer distance, they cannot form effective conductive pathways when subjected to low pressure, resulting in a high detection limit and a long response time. While high-density materials are advantageous for constructing effective conductive networks, they often come at the cost of sacrificing the elasticity and sensitivity. Therefore, achieving porous material-based pressure sensor with high sensing performance while simultaneously retaining structural elasticity remains a challenge. Since the working principle of piezoresistive pressure sensors is based on the 48 mechanical deformation-induced electrical resistance changes, one of the effective approaches for improving the sensing performance is to optimize their structure, for example, incorporating micro/nano structures into the porous materials.269 Following this strategy, several novel structures, such as bioinspired interlocked hemisphere270 and fluffy structure,271 have been developed and exhibited great promise in reducing response time and detection limit of the porous materials. Specifically, inspired by human skin, a biomimetic interlocked structure was constructed by assembling natural hierarchical microcapsules in 2D Ti3C2 MXene nanosheets, which can significantly improve the deformability of the materials, thus resulting in a low detection limit of 8 Pa and a short response time of 14 ms and recovery time of 16 ms.270 Additionally, inspired by the fluffy leg of spiders, a graphene aerogel film with fluffy structure was developed via bidirectional freezing technique.271 This structure, featuring lamellar layers with interlayer dendrites, is capable of delivering a significant variation in electrical signal even at low pressure. As a result, the prepared sensor exhibits a low detection limit of 5 Pa, a response time of 31 ms and a recovery time of 38 ms. However, these processes involve intricate manufacturing techniques, high production costs, and the use of non-renewable and non-biodegradable elastic matrices. These factors have emerged as significant hurdles that impede the progress of these structures. To develop an aerogel-based sensor with both high sensitivity and a fast response speed, it is essential to employ a lightweight superelastic aerogel that exhibits fast shape recovery following compression. Despite the widespread use of cellulose in constructing lightweight aerogels, the resulting aerogels often encounter a limitation in terms of shape recovery.272-275 Recently, there have been significant progress in fabrication of elastic cellulose aerogels, showing rapid elastic shape recovery from compression.170, 276-279 Specifically, we have previously developed a dual ice-templating assembly (DITA) method, which enables the fabrication of a superelastic aerogel using cellulose nanofibrils (CNF).170 Beyond this, due to the inherently insulating of cellulose, CNF is commonly integrated 49 with carbon nanomaterials to achieve highly conductive CNF-based structure.280-283 For example, Kuang et al. have orchestrated a thick electrode, in which neutral CB nanoparticles were firmly assembled on the surface of TEMPO-oxidized CNF through electrostatic attraction.283 With this conformal structure, the electrode shows high mass load up to 60 mg cm−2. Inspired by this, we hypothesize that the superelastic aerogel can be utilized as a substrate for constructing a piezoresistive sensor through the incorporation of electrically conductive fillers. In this study, we employed the DITA technique to construct a conductive and superelastic aerogel, using assembled sub-micron fibers (SMF) from CNF as the elastic aerogel scaffold and carbon black (CB) nanoparticles as the conductive fillers. Based on the robust superelastic and conductive performance, the SMF/CB aerogel exhibits a low detection limit (20 Pa), high sensitivity (0.234 kPa–1), rapid response (7.7 ms) and recovery times (12.8 ms), as well as excellent durability (94.6% height retention after 10,000 compression/recovery cycles). In addition, the SMF/CB aerogel is capable of monitoring various human motions including pulse, swallowing, coughing, speech recognition, and limb movements, showing a great potential in human health monitoring. 4.2 Fabrication of the superelastic SMF/CB Aerogel To design pressure sensors with high sensitivity and rapid response time, it is essential to fabricate a lightweight and superelastic aerogel. In our previous work, we successfully fabricated a superelastic CNF aerogel using a dual ice-templating assembly (DITA) strategy.170 This innovative approach effectively induces the assembly of CNF into SMF, allowing the aerogel to maintain its fibrillar structure and exhibit remarkable elasticity. In this study, we employed the aforementioned strategy to develop a highly conductive and superelastic aerogel composed of SMF and CB. Firstly, CNF can be assembled into continuous SMF by subjecting the CNF suspension (0.05 wt%) to –196 °C through the first ice templating-induced self-assembly (Scheme 1a). Secondly, fluctuations of counterions 50 on the surface of TEMPO-oxidized cellulose fibers induce polarization of the sp2 electrons in CB, thereby generating an attractive interaction between SMF and CB.281, 284 Then, CB nanoparticles will be assembled onto the charged surface of SMF to make the fibers conductive through electrostatic assembly (Scheme 1b). Thirdly, the assembled SMF/CB complex fibers will undergo further assembly to form an aerogel by freezing (–18 °C) and freeze-drying, representing the second ice-templating assembly (Scheme 1c). Benefiting from the rational design, the SMF/CB conductive aerogel shows a supershort response time (7.7 ms) and recovery time (12.8 ms), a low detection limit (20 Pa), and an excellent dimensional stability (94.6% shape recovery) after compression at 40% strain for 10,000 cycles. Figure 28. Schematic illustrating the design strategy of SMF/CB aerogel. (a) Self-assembly of CNF into SMF through ice-templating. (b) Adsorption of CB on the surface of SMF through electrostatic assembly. (c) Structure of the prepared SMF/CB aerogel. 4.3 Characterization of SMF/CB suspension and SMF/CB aerogel Light transmittance was employed to visualize the assembly process from CNF to SMF, 51 and further to SMF/CB suspension. The nanoscale dimension of CNF leads to high light transmittance (Figure 29a) with minimal laser scattering (Figure 30a). When assembling into SMF, the suspension becomes white and turbid (Figure 29a) with severe scattering of incident light (Figure 30b), indicating enlarged fiber dimensions that will interact with light wave. After further assembling CB onto the surface of SMF, the SMF/CB suspension becomes completely black, and the laser cannot pass through (Figure 30c). The assembled SMF/CB suspension can maintain well dispersed even after standing for over a week (Figure 31a). In contrast, due to the hydrophobic nature of CB, CB shows poor dispersibility in water without SMF (Figure 31b). This phenomenon clearly indicates the ability of SMF to interact with CB nanoparticles and stabilize them within aqueous suspension. Clear morphological evolvement can be observed from the transmission electron microscopy imaging, showing nanoscaled CNFs (3-5 nm, Figure 29b) firstly assemble into continuous SMF (100-300 nm, Figure 29c), and then into SMF/CB complex fibers with a string and bead like structure (~30 nm for CB nanoparticles, Figure 29d). Such string and bead like structure with densely connected CB nanoparticles can ensure consistent electrical conductivity of the SMF/CB complex fibers, which is essential for serving as pressure sensor. 52 Figure 29. Morphology characterization of CNF, SMF, and SMF/CB suspensions and aerogels. (a) Photographs showing the appearance of CNF, SMF, and SMF/CB suspensions. TEM images of (b) CNF, (c) SMF, and (d) SMF/CB. Photographs demonstrating € CNF, (f) SMF, and (g) SMF/CB aerogels before and after compression by a 100 g weight. SEM images showing the morphologies of (h-j) CNF0.2, (k-m) SMF0.2, and (n-p) SMF0.2/CB0.1 aerogels. Figure 30. Laser experiment demonstrating the transparency of (a) CNF, (b) SMF, and (c) SMF/CB suspensions. The shape-recovery property of the aerogels was demonstrated by compression test using 53 a weight (100 g), and direct comparison was made among the CNF, SMF, and SMF/CB aerogels to demonstrate the effectiveness of the proposed strategy in introducing elasticity. CNF aerogel shows severe structural collapse and cannot recover to its original shape after removing the weight (Figure 29e; Figure 32a), while both SMF and SMF/CB aerogels can fully recover to their original shape in both two longitudinal and radial directions, showing excellent elasticity (Figure 29f, g; Figure 32b, c). To understand the mechanism of the elasticity of the aerogels, their microstructures were investigated using SEM. For CNF aerogel, it exhibits a honeycomb structure (Figure 29h), with a dense pore wall (Figure 29i, j). This compact pore wall structure is attributed to the formation of large ice crystals during freezing at -20 °C, which push CNF into the space between the ice crystal boundaries and further promote the hydrogen bonding-induced self-assembly of CNF. Consequently, such compact pore wall structure confers CNF aerogel with high stiffness but low elasticity. SMF aerogel presents a significant different morphology as compared to that of CNF aerogel (Figure 29k-m). The pore walls of SMF aerogel consist of entangled and interconnected SMF fibers, forming a cobweb-like structure, which favors more elastic deformation under compression, and thus rendering SMF aerogel shape recovery property. In addition, SMF/CB aerogel also shows a honeycomb structure (Figure 29n). However, it is noted that the pore walls of SMF/CB aerogel are comprised of both dense film and interconnected fibers, indicating that a portion of the porous fibrous network is occupied by CB nanoparticles. The presence of CB particles on the fiber surface can inhibit the formation of hydrogen bonding between the fibers. Therefore, the SMF/CB aerogel still maintains high elasticity. The CB nanoparticles are observed to be densely packed along the SMF matrix, which can lead to continuous electrical conductivity that favors piezoresistive sensing application. 54 Figure 31. Photograph of CB suspensions (a) with and (b) without SMF after standing one week and 12 h, respectively. Figure 32. Photographs demonstrating the shape recovery property of (a) CNF, (b) SMF, and (c) SMF/CB aerogels after compression with a 10 g weight in transverse direction. 4.4 Compressibility and elasticity of SMF/CB aerogel The mechanical properties of the aerogels are crucial in sensor applications. Herein, the compressibility and cyclic stability of SMF/CB aerogel were evaluated by compression tests. The initial evaluation of the stress-strain curve for the pure SMF aerogel revealed that the SMF0.4 aerogel demonstrated notable compressibility, with minor height loss under less than 40% strain (Figure 33a). As irreversible deformation was discernible at higher strain, 40% strain was selected for subsequent cyclic compression tests. Furthermore, the maximum stress retention from 40% strain compression was used to characterize the shape recovery performance of the aerogels. Apparently, the maximum stress retention of the 55 aerogels with various densities decreases with the increase of the compression cycles (Figure 33b). After 50 cycles of compression, the SMF0.3 and SMF0.4 aerogels retain 71.3% and 65.2% of the maximum stress, respectively, while SMF0.2 still retains 88.8% of its maximum stress. This high maximum stress retention value of the SMF0.2 aerogel suggests its excellent shape recovery performance, and will be used further to incorporate with CB nanoparticles. With the addition of CB nanoparticles at high loading (SMF0.2/CB0.2), the shape recovery performance was sacrificed, showing only 75% maximum stress retention after 50 cycles of compression (Figure 33c). In contrast, SMF0.2/CB0.1 and SMF0.2/CB0.06 wither lower CB nanoparticles loading demonstrate comparative stress retention values to SMF0.2 post 50 cycles, at 87.8% and 88.7% respectively. The reduced shape recovery performance of SMF0.2/CB0.2 aerogel could be due to its increased SMF0.2 density and relatively reduced elastic SMF network structures (Figure 34). When a small amount of CB nanoparticles was introduced, the negative impact on elasticity due to the increase density could be effectively compensated by the hydrophobic nature of CB nanoparticles, resulting in uncompromised shape recovery performance. Considering that SMF0.2/CB0.1 exhibits comparable elasticity with SMF0.2/CB0.06, but with higher conductive fillers, the SMF0.2/CB0.1 aerogel was further investigated for its sensing performance. Figure 33. Mechanical properties of SMF and SMF/CB aerogels. (a) Compressive stress-strain curves of SMF0.4 aerogel at strains from 10% to 50%. Maximum stress remaining of (b) SMF aerogels with various densities and (c) SMF/CB aerogels with various CB contents at 40% strain during 50 cyclic tests. 56 Figure 34. Density of SMF0.2, SMF0.3, SMF0.4, SMF0.2/CB0.06, SMF0.2/CB0.1, SMF0.2/CB0.2 aerogels. As depicted by the compressive stress-strain curve of SMF0.2/CB0.1 aerogel (Figure 35a), it almost shows a linear elastic region at strain lower than 40%, beyond which a steep increase in the stress can be observed, indicating the severe densification stage. Besides, SMF0.2/CB0.1 aerogel also exhibits excellent fatigue resistance, maintaining a similar stress-strain curve after 50 compression cycles with no noticeable deformation (Figure 35b). In the first cycle, owing to the destruction of unstable CB and SMF, a high energy loss coefficient of 60.4% can be observed. The energy loss coefficient eventually stabilizes around 39% after 25 cycles, suggesting the structural integrity of the SMF0.2/CB0.1 aerogel (Figure 36a). In addition, even after 10,000 compression cycles, the height retention of the aerogel only slightly decreases to 94.6% (Figure 36b), further indicating the excellent resilience of the SMF0.2/CB0.1 aerogel. 57 Figure 35. Mechanical properties of SMF0.2/CB0.1aerogels. (a) Compressive stress-strain curves of SMF0.2/CB0.1aerogel at strains from 10% to 50%. (b) Cyclic compressive stress-strain curves of SMF0.2/CB0.1 aerogel at 40% strain. (c) Height retention of SMF0.2/CB0.1 aerogel during 10000 cyclic tests. Figure 36. a) The energy loss coefficient curves of SMF0.2/CB0.1 aerogel. b) Height retention of SMF0.2/CB0.1 aerogel during 10,000 cyclic tests. 4.5 Sensing performance of SMF/CB aerogel To characterize the applications of SMF/CB aerogel as a pressure sensor, the aerogel was assembled into a sensor (Figure 37) and the sensing performance of the aerogel subjected to different pressure was investigated. 58 Figure 37. Photograph showing the assembled SMF/CB aerogel sensor. As shown in Figure 38a, the electrical characteristics of SMF0.2/CB0.1 aerogel were firstly evaluated by current-voltage (I-V) curves. The linear behavior between current and voltage over the testing range from -1 to 1 V under different pressures indicates an ideal ohmic contact between the aerogel and the electrodes. Furthermore, the real-time response of the aerogel was explored under various pressure. Apparently, the aerogel is highly sensitive to the applied pressure, showing a stepwise response to the intermittent pressure changes (Figure 38b). It is noting that a pressure as low as 20 Pa can be identified, indicating the low detection limit of the aerogel. Also, when the pressure increases to 3000 Pa, ΔR/R0 achieve a maximum value of 54.5%. These results suggest the aerogel show a high sensitivity and wide range of working pressure. Figure 38. Sensing performance of SMF/CB aerogel. (a) Current-voltage (I-V) curves of SMF/CB aerogel under various applied pressures. (b) Relative resistance changes of SMF0.2/ CB0.1 aerogel under various applied pressures. 59 To quantitatively evaluate the sensitivity, ΔR/R0 of the aerogel versus applied pressure was plotted and the sensitivity values were calculated from the slope of the linear fitting the curve (Figure 39). As can be observed, the relative resistance change vs. pressure curve can be divided into three stages. In first stage from 0 to 2 kPa, the SMF0.2/CB0.1 aerogel sensor presents a high sensitivity of 0.234 kPa-1, but this value decreases to 0.101 kPa-1 in the second stage of 2-3.8 kPa, and then further dropped to 0.047 kPa-1 in the third stage of 3.8-6.5 kPa. Such change in sensitivity can be explained as follows. Initially, the compression of macropores of the aerogel dominates the formation of new contact points, which can significantly reduce the resistance with small amount of pressure, resulting in a high sensitivity. The continuous increase in pressure in the second and third stages leads to a transition from point-to-point contact among the CB nanoparticles to a higher degree of surface-to-surface contact. However, this transition does not facilitate the formation of new conductive pathways during compression, ultimately leading to decreased sensitivity. Figure 39. Sensitivity of SMF0.2/CB0.1 2 aerogel in the range of 0-6.5 kPa Furthermore, the durability of SMF0.2/CB0.1 aerogel was evaluated by recording the resistance change during 1000 compressing/releasing cycles with 1.5 kPa applied pressure (Figure 40a). It can be observed that ΔR/R0 of the aerogel maintains a relatively stable 60 amplitude during the cyclic test. The enlarged inset in Figure 40a shows the variation curves at the initial and final stages of the test, which both exhibit almost identical forms, demonstrating the excellent durability of SMF0.2/CB0.1 aerogel. In addition to sensitivity and durability, response speed of the sensor is also crucial to its practical applications. As illustrated in Figure 40b, the SMF0.2/CB0.1 aerogel demonstrates a remarkably short response time of 7.7 ms and a recovery time of 12.8 ms, which can be attributed to the hierarchically porous nanostructure of SMF/CB within the aerogel. This unique architecture enables the material to undergo deformation even under low mechanical stress, thereby facilitating the formation of numerous conductive pathways. After releasing the stress, the conductive pathways can quickly disconnect due to the inherent elasticity of SMF. Compared to MXene249, 285, graphene271, 286-290, polypyrrole (PPy)291-293, carbonized cellulose294, carbon nanotubes (CNTs)295, 296, silver nanowires (AgNWs)267 based pressure sensors, the SMF0.2/CB0.1 aerogel shows the shortest response and recovery times (Figure 40c), making it more favorable for detecting real-time human movement. Figure 40. (a) Relative resistance changes of SMF0.2/CB0.1 aerogel under cyclic compressing/releasing tests for 1000 cycles with 1.5 kPa applied pressure. (b) Response and recovery time of SMF0.2/CB0.1 aerogel during compressing/releasing process with 0.5 kPa applied pressure. (c) Response and recovery time of SMF0.2/CB0.1 aerogel during compressing/releasing process with 0.5 kPa applied pressure. (f) Comparison of response and recovery time between this work and MXene/rGO,249 graphene,271 carbonized cellulose,294 ODA-rGO@PANFs/CNT,286 aPANFs/GA,287 PPy@rGO,288 brophene-graphene,289 PINF/MXene,290 PPy/ PAAm,291 CA/PPy,292 cotton/PPy,293 PI/CNT,295 CNTs/graphene/WPU/CNC,296 AgNWs/CNFs267, aPANFs/MX–rGA285 based pressure sensors. 61 4.6 Potential application of SMF/CB Aerogel The SMF/CB aerogel exhibits high elasticity and excellent sensing performance. To evaluate the practical applications of SMF/CB aerogel, it was demonstrated to detect various human motions. Figure 41. Demonstrations of SMF/CB aerogel for the applications in detecting various human body motions. Relative resistance changes of SMF0.2/ CB0.1 aerogel in detecting (a) coughing, (b) swallowing We firstly fixed the aerogel on a volunteer’s throat and recorded its resistance change during coughing and swallowing (Figure 41a, b). Due to the rhythmic movement of throat, continuous and regular signals change can be obtained throughout repeated trials, indicating the feasibility of human throat monitoring. Notably, there is a large difference in the response curves for coughing and swallowing, suggesting that the aerogel can distinguish subtle movements of the throat. Moreover, the aerogel was also demonstrated for speech recognition. The volunteer's articulation of the words “Nice”, “To”, “See”, and "You" shows distinctive signal waveforms, which are consistent across repeated attempts (Figure 42a). The signals for the complete sentence “Nice to See You” exhibits four unique peaks (Figure 42b), indicating the vibrations of different vocal cords. Not only that, the aerogel can also be applied to detect the human pulse in real time (Figure 42c). The beating times can be easily obtained from the number of peaks in the curves, with a fluctuation rate 62 of 74 beats per minute. Figure 42. Demonstrations of SMF/CB aerogel for the applications in detecting small human body motions. Relative resistance changes of SMF0.2/ CB0.1 aerogel in detecting (a,b) speaking, (c) artery pulse. In addition to small scale monitoring, SMF/CB aerogel can also be used for large-scale motion monitoring including elbow bending and leg movement. When attached to the lateral aspect of the elbow, the sensor generated a steady and distinguishable signal upon repeated flexing (Figure 43a). The bending angle can be easily identified from the amplitude of signal changes. Furthermore, securing the aerogel to the knee enabled the calculation of step frequency during stationary walking and running exercises. As shown in Figure 43b, the step frequencies for walking and running were determined as 92/min and 212/min, respectively, which is consistent with the real data. Overall, SMF/CB aerogel not only excels in small pressure detection but also has good applicability in large-scale movements, making it suitable for various applications including medical monitoring, sports data acquisition, and speech recognition. 63 Figure 43. Demonstrations of SMF/CB aerogel for the applications in detecting large human body motions. Relative resistance changes of SMF0.2/ CB0.1 aerogel in detecting (a) elbow bending, and (b) knee bending. 4.7 Conclusion In conclusion, a conductive and superelastic aerogel was successfully developed using a bottom-up approach that integrated CB nanoparticles with SMF through ice templating and electrostatic assembly processes. The SMF/CB aerogel exhibits excellent elasticity and durability, maintaining 88.7% maximum stress and 94.6% height after compression at 40% strain for 50 and 10,000 cycles, respectively. Besides, SMF/CB aerogel also shows good sensing performance, with as low as 20 Pa detection limit, a wide working range of 0-6.5 kPa, a high sensitivity of 0.234 kPa-1, and a rapid response time of 7.7 ms and a recovery time of 12.8 ms. Finally, SMF/CB aerogel demonstrates an excellent responsiveness towards various human motions including pulse, swallowing, coughing, speech recognition, and limb movements, indicating a wide range of applications in human health monitoring fields. 64 Chapter 5: Conclusion and future works The increasing reliance on non-degradable materials and heavy metals in sensors for electronic products has emerged as a significant environmental concern. As these products reach the end of their life cycle, improper disposal methods contribute to escalating pollution. This thesis offers an innovative solution to this issue by focusing on the development of flexible sensors using most abundant and environmentally friendly biopolymers – cellulose. In Chapter 3, a pioneering all-cellulose hydrogel is introduced, which is fabricated through innovative molecular engineering techniques. The impressive super-stretchability and self-healing properties of this hydrogel show substantial promise in wearable sensor technology. It presents an opportunity to use environmentally friendly materials without sacrificing functionality, thus alleviating the environmental pressure. Chapter 4 showcases a super-elastic, multifunctional carbon black (CB) coated cellulose sub-micron fiber aerogel sensor. Through a simple process of dual ice-templating and electrostatic assembly, this aerogel demonstrates superior elasticity and high sensitivity, capable of capturing both minuscule and substantial human movements. This technology expands the potential applications of environmentally friendly materials in wearable sensor technology and human health monitoring. Although this thesis contributes significantly to the realm of environmentally conscious in all cellulose based sensor technology, numerous aspects of DAC hydrogel and SMF aerogel warrant further investigation and enhancement. Firstly, the influential role of the water content within hydrogel in dictating its conductivity and mechanical durability necessitates further exploration. Future research may 65 concentrate on achieving a balance between stretchability and mechanical strength by modulating the water content, thereby facilitating the development of hydrogel sensors with enhanced elastic properties and unparalleled stretchability over 40000%. In the context of aerogels, the current fabrication procedure employs two freeze-drying stages to yield SMF. Yet, the obtaining an isotropic elastic aerogel of pure cellulose through mere mechanical blending of hemp fiber sets a precedent for process simplification.276 Future endeavors could focus on streamlining this process by adjusting the TEMPO oxidation level and mechanical processing time to acquire SMF. Investigating the consequential impact of the morphology of the obtained SMF on sensing performance would be a significant area of interest. Furthermore, the unique combination of elasticity, excellent conductivity, and large surface as well as 3D nano structure of the SMF/CB aerogel indicates its potential as energy storage and generation applications, which could be further explored to expand its applications. 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ACS Sustainable Chemistry & Engineering 2021, 9 (42), 14029-14039. 85 Appendices Appendix A Photographic of Self-healing all-cellulose hydrogel in Chapter 3 86 Appendix B Photographic of all-cellulose hydrogel in Chapter 3 during stretching 87 Appendix C Keithley 2450 setup for measuring all-cellulose hydrogel sensing performance. 88 Appendix D Keithley 2450 setup for measuring superelastic aerogel sensing performance. 89 Appendix E Resistance changes of SMF0.2/CB0.06 aerogel under different pressure. 90 Appendix F The resistance change of SMF0.2/CB0.1 aerogel response to the pressure of water drops. 91 Appendix G The gauge factor of SMF0.2/CB0.1 aerogel within 0-50% strain
Thesis/Dissertation
eng
Forestry
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Jiang, Feng
Preparation and application of cellulose based sensing materials
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