A stretchable, self-healing conductive hydrogels based on nanocellulose supported graphene towards wearable monitoring of human motion

Lignosulfonate-enhanced dispersion and compatibility of liquid metal nanodroplets in PVA hydrogel for improved self-recovery and fatigue resistance in wearable sensors

Stretchable and resilient conductive hydrogels, incorporating flowable liquid metals (LM) into polyvinyl alcohol (PVA), have emerged as promising materials for wearable sensors due to their exceptional mechanical properties and sustainability. However, the fluidity and compatibility of LM with the hydrogel matrix limit the construction and performance of LM/PVA conductive hydrogels. This study aimed to develop a flexible, high-performance hydrogel for advanced wearable sensors by introducing LM nanoparticles encapsulated in sodium lignosulfonate (LS-LM) into the PVA matrix. The renewable natural macromolecule LS, rich in functional groups, enhanced the compatibility between LM and the PVA matrix. Moreover, LS formed a stable shell around the LM droplets, preventing rupture and leakage of LM, ensuring uniform dispersion within the hydrogel and significantly improving its durability by preventing phase separation. The optimized conductive lignosulfonate-liquid metal/polyvinyl alcohol hydrogel (LS-LM/PVA) exhibited a tensile stress of 1.60 MPa, a compressive strength of 0.53 MPa under 70 % strain, and electrical conductivity (4.87 S m-1). The hydrogel-based sensor demonstrated excellent sensitivity (GF = 2.40) and outstanding fatigue resistance (over 500 cycles). A Life Cycle Assessment (LCA) was conducted to evaluate the environmental impacts of LS-LM/PVA hydrogel production. The composite hydrogel-based sensor shows significant promise for advancing human motion tracking and information recognition.

Self-Healing Hydrogels: Mechanisms and Biomedical Applications

Hydrogels have emerged as dependable candidates for tissue repair because of their exceptional biocompatibility and tunable mechanical properties. However, conventional hydrogels are vulnerable to damage owing to mechanical stress and environmental factors that compromise their structural integrity and reduce their lifespan. In contrast, self-healing hydrogels with their inherent ability to restore structure and function autonomously offer prolonged efficacy and enhanced appeal. These hydrogels can be engineered into innovative forms including stimulus-responsive, self-degradable, injectable, and drug-loaded variants, thereby enhancing their applicability in wound healing, drug delivery, and tissue engineering. This review summarizes the categories and mechanisms of self-healing hydrogels, along with their biomedical applications, including tissue repair, drug delivery, and biosensing. Tissue repair includes wound healing, bone-related repair, nerve repair, and cardiac repair. Additionally, we explored the challenges that self-healing hydrogels continue to face in tissue repair and presented a forward-looking perspective on their development. Consequently, it is anticipated that self-healing hydrogels will be progressively designed and developed for applications that extend beyond tissue repair to a broader range of biomedical applications.

Highly sensitive and multifunctional Fe3+ enhanced PVA/gelatin multi-network hydrogels with wide temperature range environmental stability for wearable sensors

Hydrogels are one of the ideal materials for preparing new flexible sensor devices. In this work, based on the freeze-thaw cycle and Hoffmeister effect to improve the mechanical properties, a low-temperature resistant multifunctional conductive hydrogel with excellent mechanical properties, high transparency, high adhesion, high conductivity, high sensitivity, excellent sensing ability and self-healing ability was prepared by diffusing Fe3+ and ethylene glycol (EG) in the hydrogel network via immersion method. Moreover, wearable sensors assembled from it could accurately monitor the movement of various parts of the human body, and it could accurately identify tiny strain and pressure sensing. In addition, the hydrogel had long-term stability (after 7 days at room temperature, it still maintained 88.03 % of its original weight), strong adhesion (after repeated adhesion for 3 times, the adhesion to 6 materials was more than 72 % of the original value), excellent frost resistance (not freeze under -50 °C), and good self-repairing ability (the cut hydrogel would be self-repaired after 5 min and could be stretched to more than 2 times the original length). The hydrogel's multifunctionality and high sensitivity to small strains will make it show a high application prospect in the field of precision sensing.

Recent Progress in Cellulose-Based Conductive Hydrogels

Cellulose, a widely abundant natural polymer, is well recognized for its remarkable properties, such as biocompatibility, degradability, and mechanical strength. Conductive hydrogels, with their unique ability to conduct electricity, have attracted significant attention in various fields. The combination of cellulose and conductive hydrogels has led to the emergence of cellulose-based conductive hydrogels, which show great potential in flexible electronics, biomedicine, and energy storage. This review article comprehensively presents the latest progress in cellulose-based conductive hydrogels. Firstly, it provides an in-depth overview of cellulose, covering aspects like its structure, diverse sources, and classification. This emphasizes cellulose's role as a renewable and versatile material. The development and applications of different forms of cellulose, including delignified wood, bacterial cellulose, nanocellulose, and modified cellulose, are elaborated. Subsequently, cellulose-based hydrogels are introduced, with a focus on their network structures, such as single-network, interpenetrating network, and semi-interpenetrating network. The construction of cellulose-based conductive hydrogels is then discussed in detail. This includes their conductive forms, which are classified into electronic and ionic conductive hydrogels, and key performance requirements, such as cost-effectiveness, mechanical property regulation, sensitive response to environmental stimuli, self-healing ability, stable conductivity, and multifunctionality. The applications of cellulose-based conductive hydrogels in multiple areas are also presented. In wearable sensors, they can effectively monitor human physiological signals in real time. In intelligent biomedicine, they contribute to wound healing, tissue engineering, and nerve regeneration. In flexible supercapacitors, they offer potential for green and sustainable energy storage. In gel electrolytes for conventional batteries, they help address critical issues like lithium dendrite growth. Despite the significant progress, there are still challenges to overcome. These include enhancing the multifunctionality and intelligence of cellulose-based conductive hydrogels, strengthening their connection with artificial intelligence, and achieving simple, green, and intelligent large-scale industrial production. Future research directions should center around exploring new synthesis methods, optimizing material properties, and expanding applications in emerging fields, aiming to promote the widespread commercialization of these materials.

Thermoresponsive and Strain-Sensitive Hydrogels with Inscribable Transparency-Based Dynamic Memory Behaviors

Personal health management drives the development of intelligent hydrogel dressings, which pursue optical transparency, stretchability, and conductivity and are required to perceive specific environmental stimuli by dynamic structure, shape, or color memory. However, the incorporation of weak perceptive elements or black conductive polymers limits the fabrication of these hydrogels. Herein, we propose smart hydrogels with inscribable dynamic memorizing-forgetting transparency behavior by in situ degrading and immobilizing conductive polydopamine-doped polypyrrole (PDA-PPy) nanodots into an interpenetrating poly(NIPAm-co-acrylic acid) copolymer/polyacrylamide (PNAc/PAM) network. These hydrogels are not only optically transparent (∼64.99%), stretchable (∼1052%), self-adhesive (21-105 kPa), and highly conductive (∼0.8 S/m), but also can perceive temperature changes via structure shifts, which enables temperature-induced reversible transparency control. Especially, the temperature-dependent transparent-opaque transition kinetics of the hydrogels are tuned by the protonation of -COOH groups at pH < pKa, utilizing which the hydrogels achieve inscribed programmed dynamic memory for information memorizing-forgetting-recalling based on a pH-engraved dynamic transparency evolution of the hydrogel in response to temperature changes. These intelligent hydrogels can not only be used as efficient near-infrared (NIR) light-controlled drug release carriers to realize on-demand drug release, but also serve as a soft sensor to recognize different body postures and movement behaviors with high strain sensitivity (gauge factor, GF = 5.98), broad working strain (5-500%), rapid response (139 ms), and excellent sensing reliability (≈1000 cycles at 50% strain).

Advances in conducting nanocomposite hydrogels for wearable biomonitoring

Recent advancements in wearable biosensors and bioelectronics have led to innovative designs for personalized health management devices, with biocompatible conducting nanocomposite hydrogels emerging as a promising building block for soft electronics engineering. In this review, we provide a comprehensive framework for advancing biosensors using these engineered nanocomposite hydrogels, highlighting their unique properties such as high electrical conductivity, flexibility, self-healing, biocompatibility, biodegradability, and tunable architecture, broadening their biomedical applications. We summarize key properties of nanocomposite hydrogels for thermal, biomechanical, electrophysiological, and biochemical sensing applications on the human body, recent progress in nanocomposite hydrogel design and synthesis, and the latest technologies in developing flexible and wearable devices. This review covers various sensor types, including strain, physiological, and electrochemical sensors, and explores their potential applications in personalized healthcare, from daily activity monitoring to versatile electronic skin applications. Furthermore, we highlight the blueprints of design, working procedures, performance, detection limits, and sensitivity of these soft devices. Finally, we address challenges, prospects, and future outlook for advanced nanocomposite hydrogels in wearable sensors, aiming to provide a comprehensive overview of their current state and future potential in healthcare applications.

Fracture Behavior and Biocompatibility of Cellulose Nanofiber-Reinforced Poly(vinyl alcohol) Composite Hydrogels Cross-Linked with Borax

We investigated the fracture behavior of cellulose nanofiber (CNF)-reinforced poly(vinyl alcohol) (PVA) hydrogels cross-linked with borax and the effect of freeze-thaw (FT) cycles on it. The CNF/PVA/Borax hydrogel not subjected to FT achieved a fracture energy of 5.8 kJ m-2 and a dissipative length of 2.3 mm, comparable to those of tough hydrogels. Lacking either CNF or borax remarkably decreased the fracture energy and the dissipative length; CNF contributed to a physical blocking of the crack growth, whereas the complexations between CNF and borate yielded nonlocalization of energy dissipation. Repeated FT cycles markedly improved the mechanical performance of unnotched samples, but they decreased the fracture energy due to the lowering of the dissipative length. Besides, CNF/PVA/Borax hydrogels were suitable for cell scaffold materials. The culture of umbilical cord mesenchymal stem cells (UC-MSCs) revealed a positive correlation between culture duration and the number of UC- MSCs adherent to the material.

Enhancing anticorrosion performance of metals by incorporating cellulose nanofibrils/α-ZrP composite as nanofiller into water-based coating

The anticorrosion of metals has gain considerable interest in view of economic and environmental issues. Coating protection is one of the most effective and cost-effective methods for anticorrosion of metals. However, the traditional coatings often suffer from many issues such as poor performance or high cost. For the first time, a strategy was proposed by constructing cellulose nanofibrils (CNF)/alpha‑zirconium phosphate (α-ZrP) composite as nanofiller and incorporating into water-based coatings for anticorrosion of metals. The successful coordination of α-ZrP nanosheet and CNF were characterized. The effects of the resultant composite on anticorrosion performance were investigated. The results showed that, the as-prepared coating exhibited superior anticorrosion performance to commercial coatings. The impedance of the test sample coated with the as-prepared coating reached up to 4.38 × 105 Ω when it was immersed in 3.5 % NaCl solution with few corrosions fragmentation on metal surface, exhibiting a favorable long-term anticorrosion performance. Meanwhile, the anticorrosion mechanism was proposed. It is expected that this strategy would provide novel solutions for developing highly efficient water-based anticorrosive coatings of metals.

Ultra-Stretchable Composite Organohydrogels Polymerized Based on MXene@Tannic Acid-Ag Autocatalytic System for Highly Sensitive Wearable Sensors

Conductive hydrogels have attracted widespread attention in the fields of biomedicine and health monitoring. However, their practical application is severely hindered by the lengthy and energy-intensive polymerization process and weak mechanical properties. Here, a rapid polymerization method of polyacrylic acid/gelatin double-network organohydrogel is designed by integrating tannic acid (TA) and Ag nanoparticles on conductive MXene nanosheets as catalyst in a binary solvent of water and glycerol, requiring no external energy input. The synergistic effect of TA and Ag NPs maintains the dynamic redox activity of phenol and quinone within the system, enhancing the efficiency of ammonium persulfate to generate radicals, leading to polymerization within 10 min. Also, ternary composite MXene@TA-Ag can act as conductive agents, enhanced fillers, adhesion promoters, and antibacterial agents of organohydrogels, granting them excellent multi-functionality. The organohydrogels exhibit excellent stretchability (1740%) and high tensile strength (184 kPa). The strain sensors based on the organohydrogels exhibit ultrahigh sensitivity (GF = 3.86), low detection limit (0.1%), and excellent stability (>1000 cycles, >7 days). These sensors can monitor the human limb movements, respiratory and vocal cord vibration, as well as various levels of arteries. Therefore, this organohydrogel holds potential for applications in fields such as human health monitoring and speech recognition.

Extracellular Matrix-Surrogate Advanced Functional Composite Biomaterials for Tissue Repair and Regeneration

Native tissues, comprising multiple cell types and extracellular matrix components, are inherently composites. Mimicking the intricate structure, functionality, and dynamic properties of native composite tissues represents a significant frontier in biomaterials science and tissue engineering research. Biomimetic composite biomaterials combine the benefits of different components, such as polymers, ceramics, metals, and biomolecules, to create tissue-template materials that closely simulate the structure and functionality of native tissues. While the design of composite biomaterials and their in vitro testing are frequently reviewed, there is a considerable gap in whole animal studies that provides insight into the progress toward clinical translation. Herein, we provide an insightful critical review of advanced composite biomaterials applicable in several tissues. The incorporation of bioactive cues and signaling molecules into composite biomaterials to mimic the native microenvironment is discussed. Strategies for the spatiotemporal release of growth factors, cytokines, and extracellular matrix proteins are elucidated, highlighting their role in guiding cellular behavior, promoting tissue regeneration, and modulating immune responses. Advanced composite biomaterials design challenges, such as achieving optimal mechanical properties, improving long-term stability, and integrating multifunctionality into composite biomaterials and future directions, are discussed. We believe that this manuscript provides the reader with a timely perspective on composite biomaterials.

Rapid prototyping and facile customization of conductive hydrogel bioelectronics based on all laser process

Proper customization in size and shape is essential in implantable bioelectronics for stable bio-signal recording. Over the past decades, many researchers have heavily relied on conventional photolithography processes to fabricate implantable bioelectronics. Therefore, they could not avoid the critical limitation of high cost and complex processing steps to optimize bioelectronic devices for target organs with various sizes and shapes. Here, we propose rapid prototyping using all laser processes to fabricate customized bioelectronics. PEDOT:PSS is selectively irradiated by an ultraviolet (UV) pulse laser to form wet-stable conductive hydrogels that can softly interact with biological tissues (50 μm line width). The encapsulation layer is selectively patterned using the same laser source by UV-curing polymer networks (110 μm line width). For high stretchability (over 100%), mesh structures are made by the selective laser cutting process. Our rapid prototyping strategy minimizes the use of high-cost equipment, using only a single UV laser source to process the electrodes, encapsulation, and substrates that constitute bioelectronics without a photomask, enabling the prototyping stretchable microelectrode array with an area of 1 cm2 less than 10 min. We fabricated an optimized stretchable microelectrode array with low impedances (∼1.1 kΩ at 1 kHz) that can effectively record rat's cardiac signals with various health states.

Strong, tough, conductive and transparent nanocellulose hydrogel based on Ca2+-induced cross-linked double-networks and its adsorption of methylene blue dye

A novel multi-performance SHNC/SA/CaCl2 hydrogel with multi-performance was prepared via ultra-low-temperature freeze-thaw cycling and Ca2+ cross-linking for the removal of methylene blue (MB) from industrial wastewater. Various methods were used to characterize the structure and properties of hydrogel, and the internal structure of hydrogel showed a three-dimensional network with hydrogen and ester bonds. The SHNC/SA/CaCl2-15 hydrogel exhibited the highest tensile properties (elongation = 800 %), viscoelasticity (90 kPa), compressive strength (0.45 MPa), tensile strength (0.47 MPa) and ionic conductivity (4.34 S/cm). The maximum adsorption capacity of 2 g SHNC/SA/CaCl2-15 hydrogel was 608.49 mg/g at 40 °C, pH = 8 and adsorption 24 h. The adsorption process of hydrogel toward MB was more consistent with the second-order kinetic model and Langmuir isothermal adsorption model. According to the Langmuir isotherm model, the maximum monolayer adsorption capacity of SHNC/SA/CaCl2-15 hydrogel toward MB can reach 613.88 mg/g. Finally, it was found that the removal rate of SHNC/SA/CaCl2-15 hydrogel for MB was still as high as 90 % after five cycles of the adsorption-desorption test, and it could be reused. The hydrogel can be used as cheap and reusable adsorption material for cationic dyes. Our study provides a new perspective for the development of multifunctional cellulose hydrogel adsorbent materials.

Recent Advances in the 3D Printing of Conductive Hydrogels for Sensor Applications: A Review

Conductive hydrogels, known for their flexibility, biocompatibility, and conductivity, have found extensive applications in fields such as healthcare, environmental monitoring, and soft robotics. Recent advancements in 3D printing technologies have transformed the fabrication of conductive hydrogels, creating new opportunities for sensing applications. This review provides a comprehensive overview of the advancements in the fabrication and application of 3D-printed conductive hydrogel sensors. First, the basic principles and fabrication techniques of conductive hydrogels are briefly reviewed. We then explore various 3D printing methods for conductive hydrogels, discussing their respective strengths and limitations. The review also summarizes the applications of 3D-printed conductive hydrogel-based sensors. In addition, perspectives on 3D-printed conductive hydrogel sensors are highlighted. This review aims to equip researchers and engineers with insights into the current landscape of 3D-printed conductive hydrogel sensors and to inspire future innovations in this promising field.

Temperature-Tolerant Versatile Conductive Zwitterionic Nanocomposite Organohydrogel toward Multisensory Applications

Conductive hydrogels (CHs) are emerging materials for next generation sensing systems in flexible electronics. However, the fabrication of competent CHs with excellent stretchability, adhesion, self-healing, photothermal conversion, multisensing, and environmental stability remains a huge challenge. Herein, a nanocomposite organohydrogel with the above features is constructed by in situ copolymerization of zwitterionic monomer and acrylamide in the existence of carboxylic cellulose nanofiber-carrying reduced graphene oxide (rGO) plus a solvent displacement strategy. The synergy of abundant dipole-dipole interactions and intermolecular hydrogen bonds enables the organohydrogel to exhibit high stretchability, strong adhesion, and good self-healing. The presence of glycerol weakens the formation of hydrogen bonds between water molecules, endowing the organohydrogel with excellent environmental stability (-40 to 60 °C) to adapt to different application scenarios. Importantly, the multimodal organohydrogel presents excellent sensing behavior, including a high gauge factor of 16.3 at strains of 400-1440% and a reliable thermal coefficient of resistance (-4.2 °C-1) over a wide temperature widow (-40 to 60 °C). Moreover, the organohydrogel displays a highly efficient and reliable photothermal conversion ability due to the favorable optical absorbing behavior of rGO. Notably, the organohydrogel can detect accurate human activities at ambient temperature, demonstrating potential applications in flexible intelligent electronics.

Single/Multi-Network Conductive Hydrogels-A Review

Hydrogels made from conductive organic materials have gained significant interest in recent years due to their wide range of uses, such as electrical conductors, freezing resistors, biosensors, actuators, biomedical engineering materials, drug carrier, artificial organs, flexible electronics, battery solar cells, soft robotics, and self-healers. Nevertheless, the insufficient level of effectiveness in electroconductive hydrogels serves as a driving force for researchers to intensify their endeavors in this domain. This article provides a concise overview of the recent advancements in creating self-healing single- or multi-network (double or triple) conductive hydrogels (CHs) using a range of natural and synthetic polymers and monomers. We deliberated on the efficacy, benefits, and drawbacks of several conductive hydrogels. This paper emphasizes the use of natural polymers and innovative 3D printing CHs-based technology to create self-healing conductive gels for flexible electronics. In conclusion, advantages and disadvantages have been noted, and some potential opportunities for self-healing single- or multi-network hydrogels have been proposed.

Nanocellulose-mediated bilayer hydrogel actuators with thermo-responsive, shape memory and self-sensing performances

Inspired by creatures, abundant stimulus-responsive hydrogel actuators with diverse functionalities have been manufactured for applications in soft robotics. However, constructing a shape memory and self-sensing bilayer hydrogel actuator with high mechanical strength and strong interfacial bonding still remains a challenge. Herein, a novel bilayer hydrogel with a stimulus-responsive TEMPO-oxidized cellulose nanofibers/poly(N-isopropylacrylamide) (TOCN/PNIPAM) layer and a non-responsive TOCN/polyacrylamide (TOCN/PAM) layer is proposed as a thermosensitive actuator. TOCNs as a nano-reinforced phase provide a high mechanical strength and endow the hydrogel actuator with a strong interfacial bonding. Due to the incorporation of TOCNs, the TOCN/PNIPAM hydrogel exhibits a high compressive strength (~89.2 kPa), elongation at break (~170.7 %) and tensile strength (~24.0 kPa). The prepared PNIPAM/TOCN/PAM hydrogel actuator performs the roles of an encapsulation, jack, temperature-controlled fluid valve and temperature-control manipulator. The incorporation of Fe3+ further endows the bilayer hydrogel actuator with a synergistic performance of shape memory and temperature-driven, which can be used as a temperature-responsive switch to detect ambient temperature. The PNIPAM/TOCN/PAM-Fe3+ conductive hydrogel can be assembled into a flexible sensor and generate sensing signals when driven by temperature changes to achieve real-time feedback. This research may lead to new insights into the design and manufacturing of intelligent flexible soft robots.

Flexible vanillin-polyacrylate/chitosan/mesoporous nanosilica-MXene composite film with self-healing ability towards dual-mode sensors

Manufacturing flexible sensors with prominent mechanical properties, multifunctional sensing abilities, and remarkable self-healing capabilities remains a difficult task. In this study, a novel vanillin-modified polyacrylate (VPA), which is capable of forming green dynamic covalent crosslinking with chitosan (CS), was synthesized. The synthesized VPA was combined with mesoporous silica-modified MXene (AMS-MXene) and covalently cross-linked simultaneously with CS, resulting in the formation of a flexible composite conductive film designed for dual-mode sensors. Due to the multidimensional structure formed by the mesoporous silica and MXene layers, the resulting composite film is not only suitable for strain sensing but also excels in gas response sensing. Most importantly, the composite films demonstrate a remarkable self-healing capability through reversible dynamic covalent bonds, specifically Schiff base bonds, coupled with multiple hydrogen bonding interactions with AMS-MXene. This robust self-repair functionality remains effective even at a low temperature of 30 °C. Additionally, the synergistic antibacterial effect exerted by vanillin and CS in the film can endow the composite sensor with excellent antimicrobial properties. This multifunctional composite film holds tremendous potential for applications in green flexible wearable sensors. Furthermore, it can show diverse applications in a wide variety of fields, driving advances in wearable technology and human health monitoring.

Flexible Strain Sensor Based on Nickel Microparticles/Carbon Black Microspheres/Polydimethylsiloxane Conductive Composites for Human Motion Detection

Herein, we report a dual-functional flexible sensor (DFFS) using a magnetic conductive polymer composed of nickel (Ni), carbon black (CB), and polydimethylsiloxane (PDMS). The material selection for the DFFS utilizes the excellent elasticity of the PDMS matrix and the synergistic interaction between Ni and CB. The DFFS has a wide strain range of 0-170%, a high sensitivity of 74.13 (140-170%), and a low detection limit of 0.3% strain. The DFFS based on superior performance can accurately detect microstrain/microvibration, oncoming/contacting objects, and bicycle riding speed. Additionally, the DFFS can be used for comprehensive monitoring of human movements. Therefore, the DFFS of this work shows significant value for implementation in intelligent wearable devices and noncontact intelligent control.

Self-healing, antioxidant, and antibacterial Bletilla striata polysaccharide-tannic acid dual dynamic crosslinked hydrogels for tissue adhesion and rapid hemostasis

Biomaterials capable of achieving effective sealing and hemostasis at moist wounds are in high demand in the clinical management of acute hemorrhage. Bletilla striata polysaccharide (BSP), a natural polysaccharide renowned for its hemostatic properties, holds promising applications in biomedical fields. In this study, a dual-dynamic-bonds crosslinked hydrogel was synthesized via a facile one-pot method utilizing poly(vinyl alcohol) (PVA)-borax as a matrix system, followed by the incorporation of BSP and tannic acid (TA). Chemical borate ester bonds formed around borax, coupled with multiple physical hydrogen bonds between BSP and other components, enhanced the mechanical properties and rapid self-healing capabilities. The catechol moieties in TA endowed the hydrogel with excellent adhesive strength of 30.2 kPa on the surface of wet tissues and facilitated easy removal without residue. Benefiting from the synergistic effect of TA and the preservation of the intrinsic properties of BSP, the hydrogel exhibited outstanding biocompatibility, antibacterial, and antioxidant properties. Moreover, it effectively halted acute bleeding within 31.3 s, resulting in blood loss of 15.6 % of that of the untreated group. As a superior hemostatic adhesive, the hydrogel in this study is poised to offer a novel solution for addressing future acute hemorrhage, wound healing, and other biomedical applications.

Development of nanocellulose hydrogels for application in the food and biomedical industries: A review

As the most abundant and renewable natural resource, cellulose has attracted significant attention and research interest for the production of hydrogels (HGs). To address environmental issues and emerging demands, the benefits of naturally produced HGs include excellent mechanical properties and superior biocompatibility. HGs are three-dimensional networks created by chemical or physical cross-linking of linear or branched hydrophilic polymers and have high capacity for absorption of water and biological fluids. Although widely used in the food and biomedical fields, most HGs are not biodegradable. Nanocellulose hydrogels (NC-HGs) have been extensively applied in the food industry for detection of freshness, chemical additives, and substitutes, as well as the biomedical field for use as bioengineering scaffolds and drug delivery systems owing to structural interchangeability and stimuli-responsive properties. In this review article, the sources, structures, and preparation methods of NC-HGs are described, applications in the food and biomedical industries are summarized, and current limitations and future trends are discussed.

One-step of ionic liquid-assisted stabilization and dispersion: Exfoliated graphene and its applications in stimuli-responsive conductive hydrogels based on chitosan

Conductive hydrogels, as novel flexible biosensors, have demonstrated significant potential in areas such as soft robotics, electronic devices, and wearable technology. Graphene is a promising conductive material, but its dispersibility in aqueous solutions exists difficulties. Here, we discover that untreated graphene, after exfoliation by different ionic liquids, can disperse well in aqueous solutions. We investigate the impact of four ionic liquids with varying alkyl chain lengths ([Bmim]Cl, [Omim]Cl, [Dmim]Cl, [Hmim]Cl) on the dispersibility of grapheme, and a dual physically cross-linked network hydrogel structure is designed using acrylamide (AM), acrylic acid (AA), methyl methacrylate octadecyl ester (SMA), ionic liquid@graphene (ILs@GN), and chitosan (CS). Notably, SMA, CS, AA and AM act as dynamic cross-linking points through hydrophobic interactions and hydrogen bonding, playing a crucial role in energy dissipation. The resulting hydrogel exhibits outstanding stretchability (2250 %), remarkable toughness (1.53 MJ/m3) in tensile deformation performance, high compressive strength (1.13 MPa), rapid electrical responsiveness (response time ∼ 50 ms), high electrical conductivity (12.11 mS/cm), and excellent strain sensing capability (GF = 12.31, strain = 1000 %). These advantages make our composite hydrogel demonstrate high stability in extensive deformations, offering repeatability in pressure and strain and making it a promising candidate for multifunctional sensors and flexible electrodes.

Conductive Gels for Energy Storage, Conversion, and Generation: Materials Design Strategies, Properties, and Applications

Gel-based materials have garnered significant interest in recent years, primarily due to their remarkable structural flexibility, ease of modulation, and cost-effective synthesis methodologies. Specifically, polymer-based conductive gels, characterized by their unique conjugated structures incorporating both localized sigma and pi bonds, have emerged as materials of choice for a wide range of applications. These gels demonstrate an exceptional integration of solid and liquid phases within a three-dimensional matrix, further enhanced by the incorporation of conductive nanofillers. This unique composition endows them with a versatility that finds application across a diverse array of fields, including wearable energy devices, health monitoring systems, robotics, and devices designed for interactive human-body integration. The multifunctional nature of gel materials is evidenced by their inherent stretchability, self-healing capabilities, and conductivity (both ionic and electrical), alongside their multidimensional properties. However, the integration of these multidimensional properties into a single gel material, tailored to meet specific mechanical and chemical requirements across various applications, presents a significant challenge. This review aims to shed light on the current advancements in gel materials, with a particular focus on their application in various devices. Additionally, it critically assesses the limitations inherent in current material design strategies and proposes potential avenues for future research, particularly in the realm of conductive gels for energy applications.

Self-Healing Hydrogel Bioelectronics

Hydrogels have emerged as powerful building blocks to develop various soft bioelectronics because of their tissue-like mechanical properties, superior bio-compatibility, the ability to conduct both electrons and ions, and multiple stimuli-responsiveness. However, hydrogels are vulnerable to mechanical damage, which limits their usage in developing durable hydrogel-based bioelectronics. Self-healing hydrogels aim to endow bioelectronics with the property of repairing specific functions after mechanical failure, thus improving their durability, reliability, and longevity. This review discusses recent advances in self-healing hydrogels, from the self-healing mechanisms, material chemistry, and strategies for multiple properties improvement of hydrogel materials, to the design, fabrication, and applications of various hydrogel-based bioelectronics, including wearable physical and biochemical sensors, supercapacitors, flexible display devices, triboelectric nanogenerators (TENGs), implantable bioelectronics, etc. Furthermore, the persisting challenges hampering the development of self-healing hydrogel bioelectronics and their prospects are proposed. This review is expected to expedite the research and applications of self-healing hydrogels for various self-healing bioelectronics.

Nanocellulose-stabilized nanocomposites for effective Hg(II) removal and detection: a comprehensive review

Mercury pollution, with India ranked as the world's second-largest emitter, poses a critical environmental and public health challenge and underscores the need for rigorous research and effective mitigation strategies. Nanocellulose is derived from cellulose, the most abundant natural polymer on earth, and stands out as an excellent choice for mercury ion remediation due to its remarkable adsorption capacity, which is attributed to its high specific surface area and abundant functional groups, enabling efficient Hg(II) ion removal from contaminated water sources. This review paper investigates the compelling potential of nanocellulose as a scavenging tool for Hg(II) ion contamination. The comprehensive examination encompasses the fundamental attributes of nanocellulose, its diverse fabrication techniques, and the innovative development methods of nanocellulose-based nanocomposites. The paper further delves into the mechanisms that underlie Hg removal using nanocellulose, as well as the integration of nanocellulose in Hg detection methodologies, and also acknowledges the substantial challenges that lie ahead. This review aims to pave the way for sustainable solutions in mitigating Hg contamination using nanocellulose-based nanocomposites to address the global context of this environmental concern.

A Novel Cryogenic Approach to 3D Printing Cytocompatible, Conductive, Hydrogel-Based Inks

In the field of tissue engineering and regenerative medicine, developing cytocompatible 3D conductive scaffolds that mimic the native extracellular matrix is crucial for the engineering of excitable cells and tissues. In this study, a custom cryogenic extrusion 3D printer was developed, which afforded control over both the ink and printing surface temperatures. Using this approach, aqueous inks were printed into well-defined layers with high precision. A conductive hydrogel ink was developed from chitosan (CS) and edge-functionalised expanded graphene (EFXG). Different EFXG:CS ratios (between 60:40 and 80:20) were evaluated to determine both conductivity and printability. Using the novel customized cryogenic 3D printer, conductive structures of between 2 and 20 layers were produced, with feature sizes as small as 200 μm. The printed structures are mechanically robust and are electrically conducting. The highest Young's modulus and conductivity in a hydrated state were 2.6 MPa and ∼45 S/m, respectively. Cytocompatibility experiments reveal that the developed material supports NSC-34 mouse motor neuron-like cells in terms of viability, attachment, and proliferation. The distinctive mechanical and electrical properties of the 3D-printed structures would make them good candidates for the engineering of 3D-structured excitable cells. Moreover, this novel printing setup can be used to print other hydrogel-based inks with high precision and resolution.

Ultrastretchable E-Skin Based on Conductive Hydrogel Microfibers for Wearable Sensors

Conductive microfibers play a significant role in the flexibility, stretchability, and conductivity of electronic skin (e-skin). Currently, the fabrication of conductive microfibers suffers from either time-consuming and complex operations or is limited in complex fabrication environments. Thus, it presents a one-step method to prepare conductive hydrogel microfibers based on microfluidics for the construction of ultrastretchable e-skin. The microfibers are achieved with conductive MXene cores and hydrogel shells, which are solidified with the covalent cross-linking between sodium alginate and calcium chloride, and mechanically enhanced by the complexation reaction of poly(vinyl alcohol) and sodium hydroxide. The microfiber conductivities are tailorable by adjusting the flow rate and concentration of core and shell fluids, which is essential to more practical applications in complex scenarios. More importantly, patterned e-skin based on conductive hydrogel microfibers can be constructed by combining microfluidics with 3D printing technology. Because of the great advantages in mechanical and electrical performance of the microfibers, the achieved e-skin shows impressive stretching and sensitivity, which also demonstrate attractive application values in motion monitoring and gesture recognition. These characteristics indicate that the ultrastretchable e-skin based on conductive hydrogel microfibers has great potential for applications in health monitoring, wearable devices, and smart medicine.

Highly stretchable polyvinyl alcohol composite conductive hydrogel sensors reinforced by cellulose nanofibrils and liquid metal for information transmission

Conductive hydrogels have received widespread attention in the field of flexible sensors. However, a single network structure inside the hydrogel sensor usually makes it difficult to bear larger mechanical loadings, greatly limiting practical applications. Developing a recoverable conductive hydrogel sensor with high toughness and adaptability is still challenging. Herein, a high-performance polyvinyl alcohol (PVA)-based conductive composite hydrogel was constructed, assisted by green cellulose nanofibrils (CNFs), magnesium chloride (MgCl2), ethylene glycol (EG), and liquid metal (LM). The synergistic effects between CNFs and LM enhanced the network structure inside the recoverable hydrogel. This resulted in an excellent tensile strength of 3.86 MPa with an elongation at break of as high as 918.4 % and compressive strength of 4.04 MPa at 80 % strain. In addition, the conductive network composed of MgCl2 and LM endowed the hydrogel good electrical conductivity. Moreover, it could be used as a flexible strain sensor for various application scenarios, e.g., micro-stress monitoring (water droplet falling) and information encryption transmission of Morse code. Such uniqueness will provide a design strategy for developing a new generation of hydrogel sensors.

A highly sensitive and anti-freezing conductive strain sensor based on polypyrrole/cellulose nanofiber crosslinked polyvinyl alcohol hydrogel for human motion detection

Electro-conductive hydrogels emerge as a stretchable conductive materials with diverse applications in the synthesis of flexible strain sensors. However, the high-water content and low cross-links density cause them to be mechanically destroyed and freeze at subzero temperatures, limiting their practical applications. Herein, we report a one-pot strategy by co-incorporating cellulose nanofiber (CNF), Poly pyrrole (PPy) and glycerol with polyvinyl alcohol (PVA) to prepare hydrogel. The addition of PPy endowed the hydrogel with good conductivity (∼0.034 S/m) compared to the no PPy@CNF group (∼0.0095 S/m), the conductivity was increased by 257.9 %. The hydrogel exhibits comparable ionic conductivity at -18 °C as it does at room temperature. It's attributed to the glycerol as a cryoprotectant and the formation of hydrated [Zn(H2O)n]2+ ions via strong interaction between Zn2+ and water molecules. Moreover, the cellulose nanofiber intrinsically assembled into unique hierarchical structures allow for strong hydrogen bonds between adjacent cellulose and PPy polymer chains, greatly improve the mechanical strength (stress∼0.65 MPa, strain∼301 %) and excellent viscoelasticity (G'max ∼ 82.7 KPa). This novel PPy@CNF-PVA hydrogel exhibits extremely high Gauge factor (GF) of 2.84 and shows excellent sensitivity, repeatability and stability. Therefore, the hydrogel can serve as reliable and stable strain sensor which shows excellent responsiveness in human activities monitoration.

Secondary Chemical Cross-Linking to Improve Mechanical Properties in a Multifaceted Biocompatible Strain Sensor

A new conductive and transparent organohydrogel is developed with high stretchability, excellent mechanical, self-healing, antifreezing, and adhesive properties. A simple one-pot polymerization method is used to create polyacrylamide cross-linked through N,N'-methylenebis(acrylamide) (MBAA) and divinylbenzene (DVB). The dual chemical cross-linked gel network is complemented by several physical cross-links via hydrogen bonding and π-π interaction. Multiple chemical and physical cross-links are used to construct the gel network that allows toughness (171 kPa), low modulus (≈45 kPa), excellent stretchability (>1100%), and self-healing ability. The use of appropriate proportions of the water/glycerol binary solvent system ensures efficient environment tolerance (-20 to 40 °C). Phytic acid is used as a conductive filler that provides excellent conductivity and contributes to the physical cross-linking. Dopamine is incorporated in the gel matrix, which endows excellent adhesive property of the gel. The organohydrogel-based strain sensors are developed with state-independent properties, highly linear dependence, and excellent antifatigue performance (>100 cycles). Moreover, during the practical wearable sensing tests, human motions can be detected, including speaking, smiling, and joint movement. Additionally, the sensor is biocompatible, indicating the potential applications for the next generation of epidermal sensors.

Comprehensive Insights and Advancements in Gel Catalysts for Electrochemical Energy Conversion

Continuous worldwide demands for more clean energy urge researchers and engineers to seek various energy applications, including electrocatalytic processes. Traditional energy-active materials, when combined with conducting materials and non-active polymeric materials, inadvertently leading to reduced interaction between their active and conducting components. This results in a drop in active catalytic sites, sluggish kinetics, and compromised mass and electronic transport properties. Furthermore, interaction between these materials could increase degradation products, impeding the efficiency of the catalytic process. Gels appears to be promising candidates to solve these challenges due to their larger specific surface area, three-dimensional hierarchical accommodative porous frameworks for active particles, self-catalytic properties, tunable electronic and electrochemical properties, as well as their inherent stability and cost-effectiveness. This review delves into the strategic design of catalytic gel materials, focusing on their potential in advanced energy conversion and storage technologies. Specific attention is given to catalytic gel material design strategies, exploring fundamental catalytic approaches for energy conversion processes such as the CO2 reduction reaction (CO2RR), oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and more. This comprehensive review not only addresses current developments but also outlines future research strategies and challenges in the field. Moreover, it provides guidance on overcoming these challenges, ensuring a holistic understanding of catalytic gel materials and their role in advancing energy conversion and storage technologies.

3D printable, stretchable, anti-freezing and rapid self-healing organogel-based sensors for human motion detection

Self-healing hydrogels have promising applications in sensors and wearable devices. However, self-healing hydrogels prepared with water as the dispersion medium inevitably freeze at sub-zero temperature, resulting in a loss of the self-healing and sensing ability. The black phosphorene / ethylene glycol / polyvinyl alcohol / sodium tetraborate / sodium alginate (BP/EG-SPB) organogels were prepared by 3D printing technology and solvent displacement method. The organogel exhibits high stretchability (1900 % strain), excellent self-healing property (25 s) and outstanding anti-freezing property (lower than -120 °C freezing point). Furthermore, the organogel can rapidly self-healed (150 s) at a low temperature (-80 °C) without any external stimulation. Additionally, this organogel-based flexible sensor possesses excellent sensitivity (gauge factor: 28.66 at 1900 % strain) and fast response capability, allowing for effective detection of human motion. This work provides a novel method for preparing multifunctional organogel-based sensors for use in harsh climates.

Low-temperature strain-sensitive sensor based on cellulose-based ionic conductive hydrogels with moldable and self-healing properties

Bioelectronics based on high-performance conductive ionic hydrogels, which can create novel technological interfaces with the human body, have attracted significant interest from both academia and industry. However, it is still a challenge to fabricate hydrogel sensor with integration of good mechanical properties, fast self-healing ability and flexible strain sensitivity below 0 °C. In this paper, we present a moldable, self-healing and adhesive cellulose-based ionic conductive hydrogel with strain-sensitivity, which was prepared by forming dual-crosslinked networks using poly(vinyl alcohol) (PVA) with borax, calcium chloride (CaCl2), zinc chloride (ZnCl2) and 2,2,6,6-tetramethylpiperidine-1-oxyl oxidized cellulose nanofibril (TCNF). The hydrogel exhibited fast self-healing within 10 s, moderate modulus of 5.13 kPa, high elongation rate of 1500 % and excellent adhesion behavior on various substrates. Due to multiple hydrogen bonding and the presence of CaCl2 and ZnCl2, the hydrogel presented a reduced freezing point as low as -41.1 °C, which enabled its application as a low-temperature strain sensor. The proposed hydrogel provides a simple and facile method for fabricating multi-functional hydrogels that can be used as suitable strain sensors for applications such as wearable electronic sensor, soft robotics and electronic skins in a wide temperature range.

Rheological insights on Carboxymethyl cellulose hydrogels

Hydrogels are copiously studied for tissue engineering, drug delivery, and bone regeneration owing to their water content, mechanical strength, and elastic behaviour. The preparation of stable and mechanically strengthened hydrogels without using toxic crosslinkers and expensive approaches is immensely challenging. In this study, we prepared Carboxymethyl cellulose based hydrogels with different polymer concentration via a less expensive physical crosslinking approach without using any toxic crosslinkers and evaluated their mechanical strength. In this hydrogel system, the carbopol concentration was fixed at 1 wt/v% and the Carboxymethyl cellulose concentration was varied between 1 and 5 wt/v%. In this hydrogel system, Carbopol serves as the crosslinker to bridge Carboxymethyl cellulose polymer through hydrogen bonds. Rheological analysis was employed in assessing the mechanical properties of the prepared hydrogel, in particular, the viscoelastic behaviour of the hydrogels. The viscoelastic nature and mechanical strength of the hydrogels increased with an increase in the Carboxymethyl cellulose polymer concentration. Further, our results suggested that gels with Carboxymethyl cellulose concentration between 3 wt/v % and 4 wt/v % with yield stresses of 58.83 Pa and 81.47 Pa, respectively, are potential candidates for use in transdermal drug delivery. The prepared hydrogels possessed high thermal stability and retained their gel network structure even at 50 °C. These findings are beneficial for biomedical applications in transdermal drug delivery and tissue engineering owing to the biocompatibility, stability, and mechanical strength of the prepared hydrogels.

Nanocellulose-graphene composites: Preparation and applications in flexible electronics

In recent years, the pursuit of high-performance nano-flexible electronic composites has led researchers to focus on nanocellulose-graphene composites. Nanocellulose has garnered widespread interest due to its exceptional properties and unique structure, such as renewability, biodegradability, and biocompatibility. However, nanocellulose materials are deficient in electrical conductivity, which limits their applications in flexible electronics. On the other hand, graphene boasts remarkable properties, including a high specific surface area, robust mechanical strength, and high electrical conductivity, making it a promising carbon-based nanomaterial. Consequently, research efforts have intensified in exploring the preparation of graphene-nanocellulose flexible electronic composites. Although there have been studies on the application of nanocellulose and graphene, there is still a lack of comprehensive information on the application of nanocellulose/graphene in flexible electronic composites. This review examines the recent developments in nanocellulose/graphene flexible electronic composites and their applications. In this review, the preparation of nanocellulose/graphene flexible electronic composites from three aspects: composite films, aerogels, and hydrogels are first introduced. Next, the recent applications of nanocellulose/graphene flexible electronic composites were summarized including sensors, supercapacitors, and electromagnetic shielding. Finally, the challenges and future directions in this emerging field was discussed.

Constructing triple-network cellulose nanofiber hydrogels with excellent strength, toughness and conductivity for real-time monitoring of human movements

In recent years, there has been a lot of interest in developing composite hydrogels with superior mechanical and conductive properties. In this study, triple-network (TN) cellulose nanofiber hydrogels were prepared by using cellulose nanofiber as the first network, isotropic poly(acrylamide-co-acrylic acid) as the second network, and polyvinyl alcohol as the third network via a cyclic freezing-thawing process. The strong (9.43 ± 0.14 MPa tensile strength, (445.5 ± 7.0)% elongation-at-break), tough (15.12 ± 0.14 MJ/m3 toughness), and conductive (0.0297 ± 0.00021 S/cm ionic conductivity) TN cellulose nanofiber hydrogels were effectively created after being pre-stretched in an external force field, cross-linked by Fe3+ and added Li+. The produced composite TN cellulose nanofiber hydrogels were successfully used as a flexible sensor for real-time monitoring and detecting human movements, highlighting their potential for wearable electronics, medical technology, and human-machine interaction. CHEMICAL COMPOUNDS STUDIED IN THIS ARTICLE: Acrylamide (PubChem CID: 6579); Acrylic acid (PubChem CID: 6581); Ammonium persulfate (PubChem CID: 6579); N, N'-methylene bisacrylamide (PubChem CID: 17956053); Sodium bromide (PubChem CID: 253881); Sodium hydroxide (PubChem CID: 14798); Sodium hypochlorite (PubChem CID: 23665760); Sodium chlorite (PubChem CID: 23668197); 2,2,6,6-tetramethylpiperidinyl-1-oxide (PubChem CID: 2724126); Polyvinyl alcohol (PubChem CID: 11199); Lithium chloride (PubChem CID: 433294); Iron nitrate nonahydrate (PubChem CID: 129774236).

Super-stretching and high-performance ionic thermoelectric hydrogels based on carboxylated bacterial cellulose coordination for self-powered sensors

Self-powered sensors that do not require external power sources are crucial for next-generation wearable electronics. As environment-friendly ionic thermoelectric hydrogels can continuously convert the low-grade heat of human skin into electricity, they can be used in intelligent human-computer interaction applications. However, their low thermoelectric output power, cycling stability, and sensitivity limit their practical applications. This paper reports a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized carboxylated bacterial cellulose (TOBC) coordination double-network ionic thermoelectric hydrogel with lithium bis(trifluoromethane) sulfonimide (LiTFSI) as an ion provider for thermodiffusion, as LiTFSI exhibits excellent thermoelectric properties with a maximum power output of up to 538 nW at a temperature difference of 20 K. The interactions between the ions and the hydrogel matrix promote the selective transport of conducting ionic ions, producing a high Seebeck coefficient of 11.53 mV K-1. Hydrogen bonding within the polyacrylamide (PAAm) network and interactions within the borate ester bond within the TOBC confer excellent mechanical properties to the hydrogel such that the stress value at a tensile deformation of 3100 % is reaches 0.85 MPa. The combination of the high ionic thermovoltage and excellent mechanical properties ionic thermoelectric hydrogels provides an effective solution for the design and application of self-powered sensors based on hydrogels.

Enhanced Mechanical Strength of Metal Ion-Doped MXene-Based Double-Network Hydrogels for Highly Sensitive and Durable Flexible Sensors

Development of conductive hydrogels with high sensitivity and excellent mechanical properties remains a challenge for constructing flexible sensor devices. Herein, a universal strategy is presented for enhancing the mechanical strength of Mxene-based double-network hydrogels through metal ion coordination effects. Polyacrylamide (PAM)/sodium alginate (SA)/Mxene double-network (PSM-DN) hydrogels were prepared by metal ion impregnation of PAM/SA/Mxene (PSM) hydrogels. High electrical conductivity is achieved due to MXene nanosheets, while the strong coordination bond between metal ions and SA constructs a second network that increases the mechanical strength of the hydrogel by an order of magnitude. Mechanical tests demonstrated that the elastic modulus of hydrogels matches that of human tissues. Hence, they can be used as a highly sensitive electronic skin sensor to recognize the movement of different joints in humans and also as a pressure sensing interface to recognize characters for anticounterfeiting and information transfer. This work can promote the practical application of conductive hydrogels in high-tech fields, such as flexible electronic skin and interface interaction.

Advances in the Preparation of Tough Conductive Hydrogels for Flexible Sensors

The rapid development of tough conductive hydrogels has led to considerable progress in the fields of tissue engineering, soft robots, flexible electronics, etc. Compared to other kinds of traditional sensing materials, tough conductive hydrogels have advantages in flexibility, stretchability and biocompatibility due to their biological structures. Numerous hydrogel flexible sensors have been developed based on specific demands for practical applications. This review focuses on tough conductive hydrogels for flexible sensors. Representative tactics to construct tough hydrogels and strategies to fulfill conductivity, which are of significance to fabricating tough conductive hydrogels, are briefly reviewed. Then, diverse tough conductive hydrogels are presented and discussed. Additionally, recent advancements in flexible sensors assembled with different tough conductive hydrogels as well as various designed structures and their sensing performances are demonstrated in detail. Applications, including the wearable skins, bionic muscles and robotic systems of these hydrogel-based flexible sensors with resistive and capacitive modes are discussed. Some perspectives on tough conductive hydrogels for flexible sensors are also stated at the end. This review will provide a comprehensive understanding of tough conductive hydrogels and will offer clues to researchers who have interests in pursuing flexible sensors.

Preparation and properties of lignin-based dual network hydrogel and its application in sensing

Ionic conductive hydrogels prepared from various biological macromolecules are ideal materials for the manufacture of human motion sensors from the perspective of resource regeneration and environmental sustainability. However, it is now difficult to develop conductive hydrogels including excellent self-healing and mechanical properties, mainly due to their inherent trade-off between dynamic cross-linked healing and stable cross-linked mechanical strength. In this work, alkali lignin-Polyvinyl alcohol-polyacrylic acid double network conductive hydrogels with high mechanical strength and good self-healing properties were prepared. We formed the primary network structure by hydrogen bonding interaction between polyvinyl alcohol, alkali lignin and polyacrylic acid, and the secondary network structure by coordination interaction with polyacrylic acid through the addition of Fe3+. The added lignin acts as a dynamic linkage bridge in a porous network mediated by multiple ligand bonds, imparting superior mechanical properties to the hydrogels. The relationships between the alkali lignin and iron ion dosage and the comprehensive properties of hydrogels (adhesion, antibacterial, self-healing, electrical conductivity and mechanical properties) were studied in detail. On this basis, the hydrogels explored the role of lignin in the regulation of hydrogels properties and revealed the self-healing and conductive mechanism.

Gesture Recognition System Using Reduced Graphene Oxide-Enhanced Hydrogel Strain Sensors for Rehabilitation Training

Gesture recognition systems epitomize a modern and intelligent approach to rehabilitative training, finding utility in assisted driving, sign language comprehension, and machine control. However, wearable devices that can monitor and motivate physically rehabilitated people in real time remain little studied. Here, we present an innovative gesture recognition system that integrates hydrogel strain sensors with machine learning to facilitate finger rehabilitation training. PSTG (PAM/SA/TG) hydrogels are constructed by thermal polymerization of acrylamide (AM), sodium alginate (SA), and tannic acid-reduced graphene oxide (TA-rGO, TG), with AM polymerizing into polyacrylamide (PAM). The surface of TG has abundant functional groups that can establish multiple hydrogen bonds with PAM and SA chains to endow the hydrogel with high stretchability and mechanical stability. Our strain sensor boasts impressive sensitivity (Gauge factor = 6.13), a fast response time (40.5 ms), and high linearity (R2 = 0.999), making it an effective tool for monitoring human joint movements and pronunciation. Leveraging machine learning techniques, our gesture recognition system accurately discerns nine distinct types of gestures with a recognition accuracy of 100%. Our research drives wearable advancements, elevating the landscape of patient rehabilitation and augmenting gesture recognition systems' healthcare applications.

Efficient removal of chloroform from groundwater using activated percarbonate by cellulose nanofiber-supported Fe/Cu nanocomposites

Chloroform (CF) is a recalcitrant halogenated methane (HM) that has received widespread attention due to its frequent detection in groundwater and its potential carcinogenic risk. In this study, TEMPO-oxidized cellulose nanofiber-supported iron/copper bimetallic nanoparticles (TOCNF-Fe/Cu), a novel composite catalyst, was synthesized to activate sodium percarbonate (SPC) for the removal of CF from groundwater. The results showed that over 96.3% of CF could be removed in a neutral reaction medium (pH 6.5-9) within 180 min using 0.66 g L-1 of TOCNF (0.32)-Fe/Cu (1) and 1 mM of SPC, which outperforms typical advanced oxidation processes. The reaction mechanism of the TOCNF-Fe/Cu-SPC system for the CF removal was elucidated. As demonstrated through electron paramagnetic resonance and quenching experiments, the TOCNF-Fe/Cu-SPC system was found to include •OH and O2•-, where the latter played a dominant role in the CF removal. DFT calculations indicated that TOCNF improved the electron transport capability of Fe/Cu and reduced the transition state energy. The Fe species on the surface of TOCNF-Fe/Cu were identified as the primary active sites for SPC activation, whereas the Cu species were beneficial to the regeneration of the Fe species. Additionally, TOCNF-Fe/Cu was found to have good recyclability and stability. The feasibility of the TOCNF-Fe/Cu-SPC system was further confirmed by applying it for the efficient removal of composite HMs from actually contaminated groundwater. Overall, the TOCNF-Fe/Cu-SPC system is an attractive candidate for the treatment of HM-contaminated groundwater.

Advanced Metallized Nanofibers for Biomedical Applications

Nanofibers are long, wire-like materials with nanoscale diameters and specific length diameter ratios. Nanofibers have porous reticular networks with remarkably high specific surface areas and significant interconnectivity between pores, allowing for the chemical modification and loading of drugs. Metallized nanofibers are novel materials that enhance the performance of attributes of conventional nanofibers by combining metals with nanofibers through electrostatic spinning doping, chemical modification, and loading approaches. Due to their unique physical and chemical properties, metallized nanofibers are diverse, rapidly developed materials in the fields of physical chemistry, materials science, and battery preparation. To date, with improvement in advanced preparation techniques and biocompatibility levels for materials, metallized nanofiber applications are gradually expanding into the biomedical field due to their excellent thermal and electrical conductivities and unique metal properties. In this review, the applications of metallized nanofibers in biomedicine are summarized. It is suggested to prepare metallized multifunctional nanofibers for tissue engineering, drug delivery, tumor treatment, wound healing, and biosensing applications by taking safety and stability as the main material selection guidelines. Finally, the development of nanofibers for biomedical applications is summarized and discussed.

Highly stretchable, self-healable and adhesive, thermal responsive conductive hydrogel loading nanocellulose complex for a flexible sensor

Currently, with the widespread concerns of smart soft sensors in wearable electronics, human health detection and electronic skin, flexible conductive hydrogels have been extensively studied. However, it remains a great challenge to develop hydrogels that have both satisfactory mechanical performance with stretchable and compressible and high conductive. Herein, based on synergistic dynamic hydrogen and metal coordination bonds, polyvinyl alcohol (PVA)/poly (2-hydroxyethyl methacrylate) (PHEMA) hydrogels doped with polypyrrole decorated cellulose nanofibers (CNFs@PPy) are developed via free radical polymerization. The loading versatile CNFs@PPy highlighted the complex hydrogels super-stretchability (approximately 2600 % elongation) and excellent toughness (2.74 MJ/m3) properties to tensile deformation, strong compressive strength (1.96 MPa), fast temperature responsiveness and outstanding strain sensing capability (GF = 3.13). Moreover, the PHEMA/PVA/CNFs@PPy hydrogels possessed rapid self-healing and powerful adhesive abilities to various interfaces without extra assistance, as well as distinguished fatigue resistance performance. Such advantages make the nanocomposite hydrogel displayed high stability and repeatable to both pressure and strain in a wide range of deformations, enabling a promising candidate in the fields of motion monitoring and healthcare management.

Highly stretchable, fast self-healing nanocellulose hydrogel combining borate ester bonds and acylhydrazone bonds

Self-healing hydrogels have received considerable attention as a promising material for flexible electronic devices given their mechanical durability and structurally tunable properties. In this study, a highly stretchable self-healing hydrogel with dual cross-linking network was developed via borate ester bonds generated by polyvinyl alcohol and borax, and acylhydrazone bonds formed by aldehyde nanocellulose with adipic acid dihydrazide-modified alginate. Compared with the single network hydrogel composed of polyvinyl alcohol and borax, the introduction of dynamic acylhydrazone bonds greatly increases the flexibility of the hydrogel. The elongation rate increased from 480 % to approximately 1440 %, and the self-healing efficiency increased from 84.6 % to 92.7 % after healing for 60 min at ambient temperature without any stimulus. Moreover, the longer the self-healing time, the more evident the self-healing effect of the acylhydrazone bonds. In addition, electrical measurements confirmed a wide working strain range (ca.1000 %), durability, and reliability. Once assembled as a strain sensor, the hydrogel is able to monitor both large and subtle human motions. Besides, this hydrogel exhibited desirable biocompatibility, as demonstrated by in vitro cytotoxicity towards NIH 3T3 cells. These integrated properties make this nanocomposite hydrogel a promising candidate for future applications as green, flexible, and smart sensors.

High-Performing Conductive Hydrogels for Wearable Applications

Conductive hydrogels have gained significant attention for their extensive applications in healthcare monitoring, wearable sensors, electronic devices, soft robotics, energy storage, and human-machine interfaces. To address the limitations of conductive hydrogels, researchers are focused on enhancing properties such as sensitivity, mechanical strength, electrical performance at low temperatures, stability, antibacterial properties, and conductivity. Composite materials, including nanoparticles, nanowires, polymers, and ionic liquids, are incorporated to improve the conductivity and mechanical strength. Biocompatibility and biosafety are emphasized for safe integration with biological tissues. Conductive hydrogels exhibit unique properties such as stretchability, self-healing, wet adhesion, anti-freezing, transparency, UV-shielding, and adjustable mechanical properties, making them suitable for specific applications. Researchers aim to develop multifunctional hydrogels with antibacterial characteristics, self-healing capabilities, transparency, UV-shielding, gas-sensing, and strain-sensitivity.

Highly stretchable, tough and conductive chitin nanofiber composite hydrogel as a wearable sensor

To meet the requirements of eco-friendly and sustainability in the 21st century, hydrogels based on biopolymer with conductivity and stretchable property have attained increasing attention for strain sensor. However, the as-prepared of hydrogel sensor with excellent mechanical property and high strain sensitivity is still a challenge. In this study, chitin nanofiber (ChNF) reinforced composite hydrogels of PACF are fabricated via a facile one-pot method. The obtained PACF composite hydrogel exhibits good transparency (80.6 % at 800 nm)and excellent mechanical properties (tensile strength, 261.2 kPa; tensile strain as high as 550.3 %). Moreover, the composite hydrogels also demonstrate excellent anti-compression performance. The composite hydrogels own good conductivity (1.20 S/m) and strain sensitivity. Most importantly, the hydrogel can be assembled as a strain/pressure sensor for detecting large-scale and small-scale human motion. Therefore, flexible conductive hydrogel strain sensors will have broad application prospects in artificial intelligence, electronic skin, and personal health.

Co-Self-Assembly of Amphiphiles into Nanocomposite Hydrogels with Tailored Morphological and Mechanical Properties

As one of the most amazing aspects of life, all living organisms are formed by self-assembly, a fundamental biological design process in which ordered nanostructures are assembled from small parts. For example, most of the biological tissues contain structurally soft and hard parts that are usually hierarchically organized at nano or micro levels to achieve specific functions. Hydrogels are one of the most promising soft materials owing to their potential applications in building of biological tissues and stretchable sensors. In this work, a series of hydrogels are synthesized through the co-self-assembly of two types of amphiphiles in their aqueous solution prior to polymerization. Soft and hard parts with nanostructures of different order parameters are incorporated into the hydrogels. The hydrophilic segment (as soft phases) of the polymer network provides water absorption, fluid flow, and softness, whereas the hydrophobic segment (as hard phases) provides strength and tearing and fracture resistance. Appropriate soft/hard nanostructures and their interfaces allow for the tailoring of the desired morphological and mechanical properties, including a different wetting ability, toughness, energy dissipation, self-recovery, and fracture resistance arising from their nanostructures. This work provides insights into the design of nanostructured anisotropic hydrogels with controlled morphological and mechanical properties.

Self-Healing MXene- and Graphene-Based Composites: Properties and Applications

Today, self-healing graphene- and MXene-based composites have attracted researchers due to the increase in durability as well as the cost reduction in long-time applications. Different studies have focused on designing novel self-healing graphene- and MXene-based composites with enhanced sensitivity, stretchability, and flexibility as well as improved electrical conductivity, healing efficacy, mechanical properties, and energy conversion efficacy. These composites with self-healing properties can be employed in the field of wearable sensors, supercapacitors, anticorrosive coatings, electromagnetic interference shielding, electronic-skin, soft robotics, etc. However, it appears that more explorations are still needed to achieve composites with excellent arbitrary shape adaptability, suitable adhesiveness, ideal durability, high stretchability, immediate self-healing responsibility, and outstanding electromagnetic features. Besides, optimizing reaction/synthesis conditions and finding suitable strategies for functionalization/modification are crucial aspects that should be comprehensively investigated. MXenes and graphene exhibited superior electrochemical properties with abundant surface terminations and great surface area, which are important to evolve biomedical and sensing applications. However, flexibility and stretchability are important criteria that need to be improved for their future applications. Herein, the most recent advancements pertaining to the applications and properties of self-healing graphene- and MXene-based composites are deliberated, focusing on crucial challenges and future perspectives.