A multi-model, large range and anti-freezing sensor based on a multi-crosslinked poly(vinyl alcohol) hydrogel for human-motion monitoring

Mechanical Field Guiding Structure Design Strategy for Meta-Fiber Reinforced Hydrogel Composites by Deep Learning

Fiber-reinforced hydrogel composites are widely employed in many engineering applications, such as drug release, and flexible electronics, with more flexible mechanical properties than pure hydrogel materials. Comparing to the hydrogel strengthened by continuous fiber, the meta-fiber reinforced hydrogel provides stronger individualized design ability of deformation patterns and tunable stiffness, especially for the elaborate applications in joint, cartilage, and organ. In this paper, a novel structure design strategy based on deep learning algorithm is proposed for hydrogel reinforced by meta-fiber to achieve targeted mechanical properties, such as stress and displacement fields. A solid mechanic model for meta-fiber reinforced hydrogel is first developed to construct the dataset of fiber distribution and the corresponding mechanical properties of the composite. Generative adversarial network (GAN) is then trained to characterize the relationship between stress or displacement field, and meta-fiber distribution. The well-trained GAN is implemented to design meta-fiber reinforced hydrogel composite structure under specific operation conditions. The results show that the deep learning method may efficiently predict the structure of the hydrogel composite with satisfied confidence, and has great potential for applications in drug delivery and flexible electronics.

Hydrogen Bonds Reinforced Ionogels with High Sensitivity and Stable Autonomous Adhesion as Versatile Ionic Skins

Flexible wearable sensors have demonstrated enormous potential in various fields such as human health monitoring, soft robotics, and motion detection. Among them, sensors based on ionogels have garnered significant attention due to their wide range of applications. However, the fabrication of ionogels with high sensitivity and stable autonomous adhesion remains a challenge, thereby limiting their potential applications. Herein, we present an advanced ionogel (PACG-MBAA) with exceptional performances based on multiple hydrogen bonds, which is fabricated through one-step radical polymerization of N-acryloylglycine (ACG) in 1-ethyl-3-methylimidazolium ethyl sulfate (EMIES) in the presence of N,N'-methylenebis(acrylamide) (MBAA). Compared with the ionogel (PAA-MBAA) formed by polymerization of acrylic acid (AA) in EMIES, the resulting ionogel exhibits tunable mechanical strength (35-130 kPa) and Young's modulus comparable to human skin (60-70 kPa) owing to the multiple hydrogen bonds formation. Importantly, they demonstrate stable autonomous adhesion to various substrates and good self-healing capabilities. Furthermore, the ionogel-based sensor shows high sensitivity (with a gauge factor up to 6.16 in the tensile range of 300-700%), enabling the detection of both gross and subtle movements in daily human activities. By integration of the International Morse code, the ionogel-based sensor enables the encryption, decryption, and transmission of information, thus expanding its application prospects.

Copolymer-grafted cellulose nanocrystal induced nanocomposite hydrogels with enhanced strength, high elasticity and adhesiveness for flexible strain and pressure sensors

Recently, the application of cellulose nanocrystals (CNCs) in the field of hydrogel sensors has attracted much attention. However, it remains challenging to construct CNC-reinforced conductive hydrogels with a combination of enhanced strength, low hysteresis, high elasticity and remarkable adhesiveness. Herein, we present a facile method to prepare conductive nanocomposite hydrogels with the above-mentioned properties by reinforcing chemically crosslinked poly(acrylic acid) (PAA) hydrogel with rational-designed copolymer-grafted CNCs. The copolymer-grafted CNCs interact with PAA matrix to form carboxyl-amide conventional hydrogen bonds and carboxyl-amino ionic hydrogen bonds, among which the ionic hydrogen bonds with rapid recovery capability are critical to the low hysteresis and high elasticity of hydrogel. The introduction of copolymer-grafted CNCs endowed the hydrogels with enhanced tensile/compressive strength, high resilience (>95 %) during tensile cyclic loading, rapid self-recovery during compressive cyclic loading and improved adhesiveness. Thanks to the high elasticity and durability of hydrogel, the assembled hydrogel sensors exhibited good cycling repeatability and durability in detecting various strains, pressures and human motions. The hydrogel sensors also showed satisfying sensitivity. Hence, the proposed preparation method and the obtained CNC-reinforced conductive hydrogels would open new avenues in flexible strain and pressure sensors for human motion detection and beyond.

A Strong and Double-sided Self-Adhesive Hydrogel Sensor

Flexible self-adhesive hydrogel sensors are attracting considerable concerns in recent years. However, creating a self-adhesive hydrogel sensor with excellent mechanical properties remains to be challenging. Herein, a double-sided self-adhesive hydrogel capable of strain sensor with high strength is demonstrated by penetration strategy. The middle poly(acrylic acid)-polyacrylamide/Fe3+ (PAA-PAM/Fe3+ ) tough layer endows the double-sided self-adhesive hydrogel with high mechanical properties, while the bilateral poly[2-(methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide-polyacrylamide (PSBMA-PAM) adhesive layers are used to ensure excellent adhesiveness on diverse substrates. The tough layer of the double-sided self-adhesive hydrogel sensor shows a strong interface bonding force against the adhesive layer. The double-sided self-adhesive hydrogel sensor enables excellent adhesiveness on diverse substrates. More importantly, it can accurately detect different strains and human motions as a self-adhesive hydrogel strain sensor. This work manifests a new route of structural design to develop a self-adhesive hydrogel sensor with excellent mechanical properties that is suitable for a wide range of applications.

Self-healing liquid metal hydrogel for human-computer interaction and infrared camouflage

Due to their mechanical flexibility, conductive hydrogels have been widely investigated in the fields of flexible electronics and soft robots, but their non-negligible disadvantages, such as poor toughness and limited self-healing, severally restrict their practical application. Herein, gallium indium alloy (EGaIn) is utilized to initiate the polymerization and simultaneously serve as flexible fillers to construct a super-stretchable and self-healing liquid metal/polyvinyl alcohol/p(acrylamide-co-octadecyl methacrylate) (liquid metal/PVA/P(AAm-co-SMA)) double network hydrogel (LM hydrogel). The synergistic effect of the rigid PVA microcrystal network and the ductile P(AAm-co-SMA) hydrophobic network, together with the ionic coordination and hydrogen bonds between polymer networks (multiple physical cross-links), endow the LM hydrogel with excellent super-stretchability (2000%), toughness (3.00 MJ m-3), notch resistance, and self-healing property (healing efficiency > 99% at 25 °C after 24 h). The LM hydrogel exhibits sensitive strain sensing behavior, allowing human-computer interaction to achieve motion recognition and health monitoring. Significantly, owing to the excellent photothermal effect and low infrared emissivity of EGaIn, the LM hydrogel reveals great potential in infrared camouflage. The work of self-healing conductive liquid metal hydrogels will promote the research and practical application of hydrogels and liquid metal in intelligent devices and military fields.

Environmentally adaptive polysaccharide-based hydrogels and their applications in extreme conditions: A review

Polysaccharide hydrogels are one of the most promising hydrogel materials due to their inherent characteristics, including biocompatibility, biodegradability, renewability, and easy modification, and their structure and functional designs have been widely researched to adapt to different application scenarios as well as to broaden their application fields. As typical wet-soft materials, the high water content and water-absorbing ability of polysaccharide-based hydrogels (PHs) are conducive to their wide biomedical applications, such as wound healing, tissue repair, and drug delivery. In addition, along with technological progress, PHs have shown potential application prospects in some high-tech fields, including human-computer interaction, intelligent driving, smart dressing, flexible sensors, etc. However, in practical applications, due to the poor ability of PHs to resist freezing below zero, dehydration at high temperature, and acid-base/swelling-induced deformation in a solution environment, they are prone to lose their wet-soft peculiarities, including structural integrity, injectability, flexibility, transparency, conductivity and other inherent characteristics, which greatly limit their high-tech applications. Hence, reducing their freezing point, enhancing their high-temperature dehydration resistance, and improving their extreme solution tolerance are powerful approaches to endow PHs with multienvironmental adaptability, broadening their application areas. This report systematically reviews the study advances of environmentally adaptive polysaccharide-based hydrogels (EAPHs), comprising anti-icing hydrogels, high temperature/dehydration resistant hydrogels, and acid/base/swelling deformation resistant hydrogels in recent years. First, the construction methods of EAPHs are presented, and the mechanisms and properties of freeze-resistant, high temperature/dehydration-resistant, and acid/base/swelling deformation-resistant adaptations are simply demonstrated. Meanwhile, the features of different strategies to prepare EAPHs as well as the strategies of simultaneously attaining multienvironmental adaptability are reviewed. Then, the applications of extreme EAPHs are summarized, and some meaningful works are well introduced. Finally, the issues and future outlooks of PH environment adaptation research are elucidated.

Room Temperature Ca2+-Initiated Free Radical Polymerization for the Preparation of Conductive, Adhesive, Anti-freezing and UV-Blocking Hydrogels for Monitoring Human Movement

In recent years, conductive hydrogels have received increasing attention as wearable electronics due to the electrochemical properties of conductive polymers combined with the softness of hydrogels. However, conventional hydrogels are complicated to prepare, require high temperature or UV radiation to trigger monomer polymerization, and are frozen at low temperatures, which seriously hinder the application of flexible wearable devices. In this paper, a conductive sensor integrating mechanical properties, adhesion, UV shielding, anti-dehydration, and anti-freeze was prepared based on Ca²⁺-initiated radical polymerization at room temperature using the synergy of sodium lignosulfonate, acrylamide (AM), and calcium chloride (CaCl₂). Metal ions can activate ammonium persulfate to generate free radicals that allow rapid gelation of AM monomers at room temperature without external stimuli. Due to ionic cross-linking and non-covalent interaction, the hydrogels have good tensile properties (1153% elongation and 168 kPa tensile strength), high toughness (758 KJ·m^-3), excellent adhesive properties (48.5 kPa), high ionic conductivity (7.2 mS·cm^-1), and UV resistance (94.4%). CaCl₂ can inhibit ice nucleation, so that the hydrogels have anti-dehydration and frost resistance properties and even at -80 °C can maintain flexibility, high conductivity, and adhesion. Assembled into a flexible sensor, it can sense various large and small movements such as compression, bending, and talking, which is a flexible sensing material with wide application prospects.

Assembling a supramolecular 3D network with tuneable mechanical properties using adamantylated cross-linking agents and β-cyclodextrin-modified hyaluronan

Hydrogels based on the supramolecular host-guest concept can be prepared if at least one constituent is a polymer chain modified with supramolecular host or guest (or both) units. Low-molecular-weight multitopic counterparts can also be used, however, guest molecules in the role of cross-linking agents are seldom reported, although such an approach offers wide-ranging possibilities for tuning the system properties via easily achievable structural modifications. In this paper, a series of adamantane-based star-like guest molecules was used for cross-linking of two types of β-cyclodextrin-modified hyaluronan (CD-HA). The prepared 3D supramolecular networks were characterised using nuclear magnetic resonance, titration calorimetry and rheological measurements to confirm the formation of the host-guest complexes between adamantane moieties and β-cyclodextrin units, including their typical properties such as self-healing and dynamic nature. The results indicate that the nature of the cross-linker (amides versus esters) has a greater impact on mechanical properties than the length of the guest's arms. In addition, the results show that the length of the HA polymer chain is more important than the degree of modification with supramolecular units. In conclusion, it was proven that the modular concept employing low-molecular-weight cross-linking guests is valuable for the formulation of supramolecular networks, including hydrogels.

A low-power and flexible bioinspired artificial sensory neuron capable of tactile perceptual and associative learning

Biomimetic haptic neuron systems have received a lot of attention from the booming artificial intelligence industry for their wide applications in personal health monitoring, electronic skin, and human-machine interfaces. In this work, inspired by the human tactile afferent nerve, we developed a flexible and low energy consumption artificial tactile neuron, which was constructed by combining a dual network (DN) hydrogel-based sensor and a low power memristor. The tactile sensor (ITO/PAM:CS-Fe³⁺ hydrogel/ITO) serves as E-skin, with mechanical properties including pressure and stretching. The memristor (Ti:ITO/BiFeO₃/ITO) serving as an artificial synapse has low power (∼3.96 × 10^-7 W), remarkable uniformity, a large memory window of 500 and excellent plasticity. Remarkably, the pattern recognition simulation based on a neuromorphic network is conducted with a high recognition accuracy of ∼89.81%. In the constructed system, the artificial synapse could be activated by the electrical information from the E-skin induced by an external pressure, to generate excitatory postsynaptic currents. The system shows functions of perception and memory functions, and it also enables tactile associative learning. The present work is important for the development of empowering robots and prostheses with the capability of perceptual learning, and it provides a paradigm for next-generation artificial sensory systems with low-power, wearable and low-cost features.

Temperature-Modulated Changes in Thin Gel Layer Thickness Triggered by Electrochemical Stimuli

A series of thermoresponsive hydrogels containing positively charged groups in the polymeric network were synthesized and modified with the electroactive compound 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS). ABTS, which forms a dianion in aqueous solutions, acts as an additional physical cross-linker and strongly affects the swelling ratio of the gels. The influence of the amount of positively charged groups and ABTS oxidation state on the volume phase transition temperature was investigated. A hydrogel that possesses a relatively wide and well-defined temperature window (the temperature range where changes in the ABTS oxidation state affects the swelling ratio significantly) was found. The influence of the presence and oxidation state of ABTS on mechanical properties was investigated using a tensile machine and a rheometer. Then, a very thin layer of the gel was deposited on an Au electrochemical quartz crystal microbalance with dissipation (EQCM-D) electrode using the electrochemically induced free radical polymerization method. Next, chronoamperometry combined with quartz crystal microbalance measurements, obtained with an Au EQCM-D electrode modified by the gel, showed that the size of the thin layer could be controlled by an electrochemical trigger. Furthermore, it was found that the electrosensitivity could be modulated by the temperature. Such properties are desired from the point of view construction of electrochemical actuators.

Rapid Preparation of Antifreezing Conductive Hydrogels for Flexible Strain Sensors and Supercapacitors

Conductive hydrogels have shown great promise in flexible electronics, but their practical applications may be impeded by the time-consuming and energy-consuming polymerization process. We proposed a sodium lignosulfonate-Fe (SLS-Fe) strategy to address this challenge and took advantage of carboxymethyl cellulose (CMC) and poly(acrylic acid) to prepare the CMC/PAA/Fe3+/LiCl interpenetrating conductive hydrogels with good self-healing properties, antifreezing properties, and a 6-fold increase in conductivity in this study. The hydrogel-based flexible strain sensors demonstrated a broad detection range (400%), high sensitivity (GF = 6.19 at 200-400%), and human motion detection capability. The hydrogel-based supercapacitor exhibited a single-electrode specific capacitance of 122.36 F g-1 which successfully powered LEDs. Furthermore, the supercapacitor showed a single-electrode specific capacitance of 83.16 F g-1 at -23 °C (68% of the one exhibited at 25 °C). Therefore, the multifunctional performance of the CMC/PAA/Fe3+/LiCl hydrogel is anticipated to play an exemplary role in a new generation of flexible electronics.

Multiply cross-linked poly(vinyl alcohol)/cellulose nanofiber composite ionic conductive hydrogels for strain sensors

Building multiple chemical crosslinks is an effective strategy to improve mechanical properties and to diversify final application of polysaccharide nanoparticles reinforced poly(vinyl alcohol) (PVA) physical hydrogels. In this work, PVA/cellulose nanofibers (CNFs) were used as composite substrate to fabricate ionic conductive hydrogels for strain sensor. Three types of characteristic crosslinks, including chemical crosslinking via boronic ester covalent bonds only, and with additional metal coordination bonding, as well as coexistence of physical crosslinks via PVA crystallites and aforementioned two kinds of chemical crosslinks, were constructed. The sample with triple crosslinks has superior mechanical strength and resistance to fatigue, and the polydopamine/Fe³⁺ ratio act as key to tune final performance because double-network structure prefers to form as Fe³⁺ is superfluous, while dual-crosslink one forms in the case of insufficient Fe³⁺. As-optimized ionic conductive hydrogel is suitable as strain sensor for probing human motions. This work provides an interesting insight into the network structure and property regulation for PVA/CNF composite hydrogels with multiple crosslinks.

Bioinspired Freeze-Tolerant Soft Materials: Design, Properties, and Applications

In nature, many biological organisms have developed the exceptional antifreezing ability to survive in extremely cold environments. Inspired by the freeze resistance of these organisms, researchers have devoted extensive efforts to develop advanced freeze-tolerant soft materials and explore their potential applications in diverse areas such as electronic skin, soft robotics, flexible energy, and biological science. Herein, a comprehensive overview on the recent advancement of freeze-tolerant soft materials and their emerging applications from the perspective of bioinspiration and advanced material engineering is provided. First, the mechanisms underlying the freeze tolerance of cold-enduring biological organisms are introduced. Then, engineering strategies for developing antifreezing soft materials are summarized. Thereafter, recent advances in freeze-tolerant soft materials for different technological applications such as smart sensors and actuators, energy harvesting and storage, and cryogenic medical applications are presented. Finally, future challenges and opportunities for the rapid development of bioinspired freeze-tolerant soft materials are discussed.

Antifreezing, Ionically Conductive, Transparent, and Antidrying Carboxymethyl Chitosan Self-Healing Hydrogels as Multifunctional Sensors

Through a simple strategy of immersion in a mixed solution of water/ethylene glycol (EG)/lithium chloride (LiCl), self-healing carboxymethyl chitosan (CA) hydrogels, that is, CA/N-vinylpyrrolidone-EG-Li⁺ hydrogels (CEH) with an ultra-low-temperature freezing resistance below -70 °C were fabricated. The introduction of electrolyte ions and small-molecule polyol also made these hydrogels highly conductive (0.8 S m^-1) and imparted antidrying property to them, showing stable and reversible sensitivity to finger-wrist bending as well as 150 cycles of stretching. Such hydrogels also presented highly efficient self-healing ability, with a stress-strain healing efficiency of over 90%. Furthermore, the CEH-based sensors maintained a stable sensing performance over a wide range of temperatures below the freezing point (from -10 to -70 °C) and exhibited stable sensitivity to temperatures with fast response and no significant hysteresis. The present work is expected to provide a simple and sustainable route for the preparation of multifunctional antifreezing conductive hydrogels based on CA, leading to a wide range of potential applications in soft sensor devices.

A Highly Mechanical, Conductive, and Cryophylactic Double Network Hydrogel for Flexible and Low-Temperature Tolerant Strain Sensors

Due to their stretchability, conductivity, and good biocompatibility, hydrogels have been recognized as potential materials for flexible sensors. However, it is still challenging for hydrogels to meet the conductivity, mechanical strength, and freeze-resistant requirements in practice. In this study, a chitosan-poly (acrylic acid-co-acrylamide) double network (DN) hydrogel was prepared by immersing the chitosan-poly (acrylic acid-co-acrylamide) composite hydrogel into Fe₂(SO₄)₃ solution. Due to the formation of an energy-dissipative chitosan physical network, the DN hydrogel possessed excellent tensile and compression properties. Moreover, the incorporation of the inorganic salt endowed the DN hydrogel with excellent conductivity and freeze-resistance. The strain sensor prepared using this DN hydrogel displayed remarkable sensitivity and reliability in detecting stretching and bending deformations. In addition, this DN hydrogel sensor also worked well at a lower temperature (−20 °C). The highly mechanical, conductive, and freeze-resistant DN hydrogel revealed a promising application in the field of wearable devices.

Dual-Sensing, Stretchable, Fatigue-Resistant, Adhesive, and Conductive Hydrogels Used as Flexible Sensors for Human Motion Monitoring

Hydrogel-based sensors serve as an ideal platform for developing personalized wearable electronics due to their high flexibility and conformability. However, the weak stretchability and inferior conductivity of hydrogels have severely restricted their large-scale application. Herein, a natural polymer-based conductive hydrogel integrated with favorable mechanical properties, good adhesive performance, and excellent fatigue resistance was fabricated via interpenetrating tannic acid (TA) into a chitosan (CS) cross-linked network in an acidic aqueous solution. The hydrogel was composed of a regular hierarchical porous structure, which was built by the hydrogen bonding between TA and CS. In addition, the hydrogels exhibited adjustable mechanical properties (maximum yield stress of 7000 Pa) and good stretchability (strain up to 320%). Benefiting from the abundant catechol groups of TA, the proposed hydrogels could repeatedly adhere to various material surfaces and could be easily peeled off without residue. Moreover, the hydrogel exhibited stable conductivity, high stretching sensitivity (gauge factor of 2.956), rapid response time (930 ms), and excellent durability (>300 cycles), which can be assembled as a strain sensor to attach to the human body for precise monitoring of human exercise behavior, distinguishing physiological signals, and recognizing speech. Furthermore, the prepared hydrogels also exhibited stable sensing performance to temperature. As a result, the hydrogels exhibited dual sensory performance for both temperature and strain deformation. It is anticipated that the incorporation of strain sensors and thermal sensors will provide theoretical guidance for developing multifunctional conductive hydrogels and pave a way for the versatile application of hydrogel-based flexible sensors in wearable devices and soft actuators.

Chitosan-based double cross-linked ionic hydrogels as a strain and pressure sensor with broad strain-range and high sensitivity

A conductive hydrogel P(AAm-co-AA)/CS-Fe³⁺ with double cross-linked networks was fabricated using a one-step polymerization by UV irradiation and a soaking process in Fe(NO₃)₃ solution. In this hydrogel, the rigid chain of chitosan (CS) and the soft chain of copolymer P(AAm-co-AA) with acrylic acid (AA) and acrylamide (AAm) act as the backbone, among which large amounts of hydrogen bonds are formed by the amino, hydroxyl, and carboxyl groups on the two polymers. Ferric irons (Fe³⁺) are introduced and form coordination interactions with carboxyl and amino groups of the polymers. The double cross-linked interactions in the system can enhance the tensile strength and toughness of the hydrogel. Thus, the prepared P(AAm-co-AA)/CS-Fe³⁺ hybrid network hydrogel shows excellent mechanical properties in many aspects: a strength of up to 550 kPa, a broad strain-range up to 800%, fast self-recovery ability (30 min), and low hysteresis strain (<100%). The conductive hydrogel demonstrates high strain sensitivity (gauge factor (GF) = 6.6 at a strain of 700%) as a flexible sensor. Human movements (for example, finger bending, vocal cord vibration, and other human activities) can be sensitively detected using the P(AAm-co-AA)/CS-Fe³⁺ hydrogel sensor.

Progress in hydrogels for skin wound repair

As the first defensive line between the human body and the outside world, the skin is vulnerable to damage from the external environment. Skin wounds can be divided into acute wounds (mechanical injuries, chemical injuries and surgical wounds, etc.) and chronic wounds (burns, infections, diabetes, etc.). In order to manage skin wound, a variety of wound dressings have been developed, including gauze, films, foams, nanofibers, hydrocolloids and hydrogels. Recently, hydrogels have received much attention because of their natural extracellular matrix (ECM)-mimik structure, tunable mechanical properties, and facile bioactive substance delivery capability. They show great potential application in skin wound repair. This paper first introduces the anatomy and function of the skin, the process of wound healing and conventional wound dressings, and then introduces the composition and construction methods of hydrogels. Next, this paper introduces the necessary properties of hydrogels in skin wound repair and the latest research progress of hydrogel dressings for skin wound repair. Finally, the future development goals of hydrogel materials in the field of wound healing are proposed. This article is protected by copyright. All rights reserved.

Hydrogels as Soft Ionic Conductors in Flexible and Wearable Triboelectric Nanogenerators

Flexible triboelectric nanogenerators (TENGs) have attracted increasing interest since their advent in 2012. In comparison with other flexible electrodes, hydrogels possess transparency, stretchability, biocompatibility, and tunable ionic conductivity, which together provide great potential as current collectors in TENGs for wearable applications. The development of hydrogel-based TENGs (H-TENGs) is currently a burgeoning field but research efforts have lagged behind those of other common flexible TENGs. In order to spur research and development of this important area, a comprehensive review that summarizes recent advances and challenges of H-TENGs will be very useful to researchers and engineers in this emerging field. Herein, the advantages and types of hydrogels as soft ionic conductors in TENGs are presented, followed by detailed descriptions of the advanced functions, enhanced output performance, as well as flexible and wearable applications of H-TENGs. Finally, the challenges and prospects of H-TENGs are discussed.

Stretchable, self-healing and adhesive sodium alginate-based composite hydrogels as wearable strain sensors for expansion-contraction motion monitoring

Developing multifunctional hydrogels with stretchability, self-healing ability, adhesiveness, and conductivity into flexible strain sensors for human motion and health monitoring has attracted great attention and is highly desired. However, the present motion detectors mainly focus on stretching, bending, and twisting of different body parts while the expansion-contraction motion has been rarely investigated. In this study, along with carbon nanotubes (CNTs) as conductive components, sodium alginate (Alg) modified with 3-aminophenylboronic acid (PBA) and dopamine (DA) were synthesized and employed as precursors to prepare a multifunctional Alg-CNT hydrogel. The formed dynamic covalent bonds between PBA and DA endowed the hydrogel with a rapid self-healing property (30 s) while the introduction of CNTs remarkably enhanced the mechanical strength and electrical conductivity of the hydrogel. Moreover, the as-prepared hydrogel displayed a satisfactory stretchability (500%) and self-adhesiveness to various substrates. When used as a strain sensor, the Alg-CNT hydrogel that exhibited a fast response (150 ms) and ultra-durability (over 30 000 cycles) was demonstrated to be capable of monitoring subtle expansion-contraction motions (e.g., human breathing and mouse heart beating) via periodic and repeatable electrical signals. Therefore, this multifunctional hydrogel is highly suitable for monitoring expansion-contraction motions, indicating its potential applications in personal health monitoring.

Gelatin-Reinforced Zwitterionic Organohydrogel with Tough, Self-Adhesive, Long-Term Moisturizing and Antifreezing Properties for Wearable Electronics

Strong mechanical performance, appropriate adhesion capacity, and excellent biocompatibility of conductive hydrogel-based sensors are of great significance for their application. However, conventional conductive hydrogels usually exhibit insufficient mechanical strength and adhesion. In addition, they will lose flexibility and conductivity under subzero temperature and a dry environment owing to inevitable freezing and evaporation of water. In this study, a tough, flexible, self-adhesive, long-term moisturizing, and antifreezing organohydrogel was prepared, which was composed of gelatin, zwitterionic poly [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) (PSBMA), MXene nanosheets, and glycerol. Natural gelatin was incorporated to enhance mechanical performance via the entanglement of a physical cross-linked network and a PSBMA network, which was also used as a stabilizer to disperse MXene into the organohydrogel. Zwitterionic PSBMA endowed the organohydrogel with good adhesion and self-healing properties. Long-term moisturizing properties and antifreeze tolerance could be achieved owing to the synergistic water retention capacity of PSBMA and glycerol. The resulting PSBMA-gelatin-MXene-glycerol (PGMG) organohydrogel exhibited high mechanical fracture strength (0.65 MPa) and stretchability (over 1000%), excellent toughness (3.87 MJ/m³), strong and repeated adhesion to diverse substrates (e.g., paper, glass, silicon rubber, iron, and pig skin), good fatigue resistance (under the cyclic stretching-releasing process), and rapid recovery capacity. Moreover, the PGMG organohydrogel showed good stability under -40 °C. The sensor based on PGMG organohydrogel could tightly attach to the human skin and real-time-monitor the motions of joints (e.g., bending of the finger, wrist, elbow, and knee) and the change in mood such as smiling and frowning. Therefore, PGMG organohydrogels have a huge potential for wearable sensors under room temperature or extreme environments.

Multifunctional bacterial cellulose-based organohydrogels with long-term environmental stability

Sensitive strain sensors have attracted more attention due to their applications in health monitoring and human-computer interaction. However, the problems existing in conventional hydrogels, such as inherent brittleness, freezing at low temperature, low toughness, and water evaporation, have greatly hindered the practical applications. In order to solve the above problems, herein, we designed dual network multifunctionality organohydrogels using polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA) covalent cross-linking polymer as the first network, the bacterial celluloses (BCs) and calcium chloride by ligand binding as the second network. The prepared organohydrogels showed good conductivity and sensitivity over a wide temperature range (-20 ∼ 40 ℃), and maintained long-term stability (>15 days) in the air. In addition, the dynamic combination of BCs-Ca^{2 +} and hydrogen bonds in the binary system further endows the organohydrogels with excellent tensile strength (≈1.0 MPa), tensile strain (≈1300%), toughness (≈6.2 MJ m^-3), conductivity (3.4 S m^-1), gauge factor (≈1.24), adhesion (≈0.3 MPa), and self-healing properties (self-healing tensile strain to 632%). Therefore, this organohydrogel has potential candidates for flexible electronic skin, motion monitoring, and soft robotics.

Robust, anti-freezing and conductive bonding of chitosan-based double-network hydrogels for stable-performance flexible electronic

Unstable hydrogel-substrate interfaces and defunctionalization at low temperature severely restrict versatile applications of hydrogel-based systems. Herein, various chitosan-polyacrylamide double-network (CS-PAM DN) ionic hydrogels were chemically linked with diverse substrates to construct robust and anti-freezing hydrogel-substrate combination, wherein the destructible CS physical network rendered effective energy dissipation mechanism to significantly enhanced the cohesion of hydrogels and the covalent linkage between PAM network with substrate surface strongly improved the interfacial adhesion. The synergistic effects enabled the CS-PAM DN hydrogels to be tightly bonded on diverse metals and inorganics. Impressively, the hydrogel-substrate combinations were freezing tolerant to well-maintain high interfacial toughness at low temperature. Notably, due to the high toughness and conductivity of hydrogel-metal interface, the hydrogel-metal combination can be utilized as a multi-model flexible sensor to detect strain and pressure within broad temperature range. This work may provide a platform for construction and emerging application of robust, anti-freezing and stable-performance hydrogel-based systems.

Cellulose nanocrystal reinforced conductive hydrogels with anti-freezing properties for strain sensors

High strength hydrogels with frost resistance can be used as human body sensors in low temperature environment.

One-pot freezing-thawing preparation of cellulose nanofibrils reinforced polyvinyl alcohol based ionic hydrogel strain sensor for human motion monitoring

Ionic conductive hydrogels have been widely applied in sensors, energy storage and soft electronics recently. However, most of the polyvinyl alcohol (PVA) based ionic hydrogels are mainly fabricated by soaking the hydrogels in high concentration electrolyte solution which can induce the waste of electrolyte and solvent. Herein, we have designed cellulose nanofibrils (CNF) and ZnSO₄ reinforced PVA based hydrogels through a one-pot simple freezing-thawing method at low ZnSO₄ concentration without any soaking process. Furthermore, the hydrogel with 0.4% CNF exhibited stress up to 0.79 MPa (242% strain) and high ionic conductivity of 0.32 S m^-1 (0.07 M ZnSO₄). Moreover, hydrogel sensor displayed high linear gauge factor 1.70 (0-200% strain), excellent stability, durability and reliability. The integrated hydrogel sensor also showed excellent sensor performance for human motion monitoring. This work provides a new prospect for the design of cellulose reinforced conductive hydrogels via a facile method.

Preparation of stretchable and self-healable dual ionically cross-linked hydrogel based on chitosan/polyacrylic acid with anti-freezing property for multi-model flexible sensing and detection

As a kind of promising material for flexible wearable electronics, conductive hydrogels have attracted extensive interests of researchers for their inherent merits such as superior mechanical properties, biocompatibility, and permeability. Herein, we constructed a new type of highly stretchable, anti-freezing, self-healable, and conductive hydrogel based on chitosan/polyacrylic acid. The large amount of ions inside the network had five functions for the proposed hydrogel, including excellent mechanical behaviors, high conductivity, self-recovery, self-healing and anti-freezing capability. Consequently, the proposed hydrogel possessed tunable stretchability (1190-1550%), tensile strength (0.96-2.56 MPa), toughness (5.7-14.7 MJ/m³), superior self-healing property (self-healing efficiency up to 83.7%), high conductivity (4.58-5.76 S/m), and excellent anti-freezing capability. To our knowledge, the self-healable hydrogel with balanced tensile strength, toughness, conductivity, and low-temperature tolerance can hardly be achieved till now. Furthermore, the conductive hydrogels exhibited high sensitivity (gauge factor up to 10.8) in a broad strain window (0-1000%) and could detect the conventional motion signals of human body such as bending of a knuckle, swallowing, and pressure signal at both room temperature and -20 °C. Moreover, the hydrogels could also be fabricated as flexible detectors to identify different temperatures, different kinds of solutions, and different concentrations of the solution.