Ionically Conductive Hydrogel with Fast Self-Recovery and Low Residual Strain as Strain and Pressure Sensors

High-strength, anti-fatigue, cellulose nanofiber reinforced polyvinyl alcohol based ionic conductive hydrogels for flexible strain/pressure sensors and triboelectric nanogenerators

Ionic conductive hydrogels (ICHs) have attracted great attention because of their excellent biocompatibility and structural similarity with biological tissues. However, it is still a huge challenge to prepare a high strength, conductivity and durability hydrogel-based flexible sensor with dual network structure through a simple and environmentally friendly method. In this work, a simple one-pot cycle freezing thawing method was proposed to prepare ICHs by dissolving polyvinyl alcohol (PVA) and ferric chloride (FeCl3) in cellulose nanofiber (CNF) aqueous dispersion. A dual cross-linked network was established in hydrogel through the hydrogen bonds and coordination bonds among PVA, CNF, and FeCl3. This structure endows the as-prepared hydrogel with high sensitivity (pressure sensitivity coefficient (S) = 5.326 in the pressure range of 0-5 kPa), wide response range (4511 kPa), excellent durability (over 3000 cycles), short response time (83 ms) and recovery time (117 ms), which can accurately detect various human activities in real time. Furthermore, the triboelectric nano-generator (TENG) made from PVA@CNF-FeCl3 hydrogel can not only supply power for commercial capacitors and LED lamps, but also be used as a self-powered sensor to detect human motion. This work provides a new approach for the development of the next generation of flexible wearable electronic devices.

Ureido-Ionic Liquid Mediated Conductive Hydrogel: Superior Integrated Properties for Advanced Biosensing Applications

Ionic conductive hydrogels (ICHs) have recently gained prominence in biosensing, indicating their potential to redefine future biomedical applications. However, the integration of these hydrogels into sensor technologies and their long-term efficacy in practical applications pose substantial challenges, including a synergy of features, such as mechanical adaptability, conductive sensitivity, self-adhesion, self-regeneration, and microbial resistance. To address these challenges, this study introduces a novel hydrogel system using an imidazolium salt with a ureido backbone (UL) as the primary monomer. Fabricated via a straightforward one-pot copolymerization process that includes betaine sulfonate methacrylate (SBMA) and acrylamide (AM), the hydrogel demonstrates multifunctional properties. The innovation of this hydrogel is attributed to its robust mechanical attributes, outstanding strain responsiveness, effective water retention, and advanced self-regenerative and healing capabilities, which collectively lead to its superior performance in various applications. Moreover, this hydrogel  exhibited broad-spectrum antibacterial activity. Its potential for biomechanical monitoring, especially in tandem with contact and noncontact electrocardiogram (ECG) devices, represents a noteworthy advancement in precise real-time cardiac monitoring in clinical environments. In addition, the conductive properties of the hydrogel make it an ideal substrate for electrophoretic patches aimed at treating infected wounds and consequently enhancing the healing process.

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.

Highly stretchable dynamic hydrogels for soft multilayer electronics

Recent progress in the development of synthetic polymer networks has enabled the next generation of hydrogel-based machines and devices. The ability to mimic the mechanical and electrical properties of human tissue gives great potential toward the fields of bioelectronics and soft robotics. However, fabricating hydrogel devices that display high ionic conductivity while maintaining high stretchability and softness remains unmet. Here, we synthesize supramolecular poly(ionic) networks, which display high stretchability (>1500%), compressibility (>90%), and rapid self-recovery (<30 s), while achieving ionic conductivities of up to 0.1 S cm -1. Dynamic cross-links give rise to inter-layer adhesion and a stable interface is formed on account of ultrahigh binding affinities (>1013 M-2). Superior adherence between layers enabled the fabrication of an intrinsically stretchable hydrogel power source, paving the way for the next generation of multi-layer tissue mimetic devices.

Tough, conductive hydrogels based on gelatin and oxidized sodium carboxymethyl cellulose as flexible sensors

Natural polymer-based hydrogels have been wildly used in electronic skin, health monitoring and human motion sensing. However, the construction of hydrogel with excellent mechanical strength and electrical conductivity totally using natural polymers still faces many challenges. In this paper, gelatin and oxidized sodium carboxymethylcellulose were used to synthesize a double-network hydrogel through the dynamic Schiff base bonds. Then, the mechanical strength of the hydrogel was further enhanced by immersing it in an ammonium sulfate solution based on the Hofmeister effect between gelatin and salt. Finally, the gelatin/oxidized sodium carboxymethylcellulose hydrogel exhibited high tensile properties (614 %), tensile fracture strength (2.6 MPa), excellent compressive fracture strength (64 MPa), and compressive toughness (4.28 MJ/m3). Also, the electrical conductivity reached 3.94 S/m. The hydrogel after salt soaked was fabricated as strain sensors, which could accurately monitor the movement of many joints in the human body, such as fingers, wrists, elbows, neck, and throat. Therefore, the designed hydrogel fully originated from natural polymers and has great application potential in motion detection and information recording.

Rapidly damping hydrogels engineered through molecular friction

Hydrogels capable of swift mechanical energy dissipation hold promise for a range of applications including impact protection, shock absorption, and enhanced damage resistance. Traditional energy absorption in such materials typically relies on viscoelastic mechanisms, involving sacrificial bond breakage, yet often suffers from prolonged recovery times. Here, we introduce a hydrogel designed for friction-based damping. This hydrogel features an internal structure that facilitates the motion of a chain walker within its network, effectively dissipating mechanical stress. The hydrogel network architecture allows for rapid restoration of its damping capacity, often within seconds, ensuring swift material recovery post-deformation. We further demonstrate that this hydrogel can significantly shield encapsulated cells from mechanical trauma under repetitive compression, owing to its proficient energy damping and rapid rebound characteristics. Therefore, this hydrogel has potential for dynamic load applications like artificial muscles and synthetic cartilage, expanding the use of hydrogel dampers in biomechanics and related areas.

Rational Design of Dynamically Super-Tough and Super-Stretchable Hydrogels for Deformable Energy Storage Devices

Hydrogels possess unique polymer networks that offer flexibility/stretchability, high ionic conductivity, and resistance to electrolyte leakage, making them suitable for deformable energy storage devices. Endowing the mechanical functionality of the hydrogel electrolytes focus on either enhancing the stretchability or the toughness. However, the stretchability and the toughness are generally a trade-off that the stretchable gels are intrinsically prone to damage and sensitive to notches and cracks. Here, the regulating strategies on the hydrogel's mechanical properties are provided to develop the designated hydrogel electrolyte, where different polymeric network structures are constructed, including single network structures, semi-interpenetrating network structures, and interpenetrating dual-network structures. A comprehensive comparison of these polymer network structures is conducted to evaluate their mechanical stretchability and toughness. Designing super-tough and super-stretchable hydrogels based on specific application requirements can be realized by striking a balance by regulating the hydrogel structure. In specific, incorporating semi-interpenetrating networks significantly can enhance stretchability to achieve a break elongation up to 1300%, while the interpenetrating dual-networks can largely improve the toughness to realize the extraordinary fracture toughness of 6.843 kJ m-2. These findings offer valuable designing guidance for designated hydrogel electrolytes and the deformable zinc-silver battery is demonstrated with high mechanical stability and electrochemical performance.

Mechanically Tough and Conductive Hydrogels Based on Gelatin and Z-Gln-Gly Generated by Microbial Transglutaminase

Gelatin-based hydrogels with excellent mechanical properties and conductivities are desirable, but their fabrication is challenging. In this work, an innovative approach for the preparation of gelatin-based conductive hydrogels is presented that improves the mechanical and conductive properties of hydrogels by integrating Z-Gln-Gly into gelatin polymers via enzymatic crosslinking. In these hydrogels (Gel-TG-ZQG), dynamic π-π stacking interactions are created by the introduction of carbobenzoxy groups, which can increase the elasticity and toughness of the hydrogel and improve the conductivity sensitivity by forming effective electronic pathways. Moreover, the mechanical properties and conductivity of the obtained hydrogel can be controlled by tuning the molar ratio of Z-Gln-Gly to the primary amino groups in gelatin. The hydrogel with the optimal mechanical properties (Gel-TG-ZQG (0.25)) exhibits a high storage modulus, compressive strength, tensile strength, and elongation at break of 7.8 MPa at 10 °C, 0.15 MPa at 80% strain, 0.343 MPa, and 218.30%, respectively. The obtained Gel-TG-ZQG (0.25) strain sensor exhibits a short response/recovery time (260.37 ms/130.02 ms) and high sensitivity (0.138 kPa-1) in small pressure ranges (0-2.3 kPa). The Gel-TG-ZQG (0.25) hydrogel-based sensors can detect full-range human activities, such as swallowing, fist clenching, knee bending and finger pressing, with high sensitivity and stability, yielding highly reproducible and repeatable sensor responses. Additionally, the Gel-TG-ZQG hydrogels are noncytotoxic. All the results demonstrate that the Gel-TG-ZQG hydrogel has potential as a biosensor for wearable devices and health-monitoring systems.

High-Strength, High-Swelling-Resistant, High-Sensitivity Hydrogel Sensor Prepared with Wood That Retains Lignin

Wood-derived hydrogels possess satisfactory longitudinal strength but lack excellent swelling resistance and dry shrinkage resistance when achieving high anisotropy. In this study, we displayed the preparation of highly dimensional stable wood/polyacrylamide hydrogels (wood/PAM-Al3+). The alkali-treated wood retains lignin as the skeleton of the hydrogel. Second, Al ions were added to the metal coordination with lignin. Finally, by employing free radical polymerization, we construct a conductive electronic network using polyaniline within the wood/PAM-Al3+ matrix to create the flexible sensor. This approach leverages lignin's integrated structure within the middle lamella to provide enhanced swelling resistance and stronger binding strength in the transverse direction. Furthermore, coordination between lignin and Al ions improves the mechanical strength of the wood hydrogel. Polyaniline provides stable linear pressure and temperature responses. The wood/PAM-Al3+ exhibits a transverse swelling ratio of 3.90% while achieving a longitudinal tensile strength of 20.5 MPa. This high-strength and high-stability sensor is capable of monitoring macroscale human behavior. Therefore, this study presents a simple yet innovative strategy for constructing tough hydrogels while also establishing an alternative pathway for exploring lignin networks in new functional materials development.

Developing conductive hydrogels for biomedical applications

Conductive hydrogels have attracted copious attention owing to their grateful performances, such as similarity to biological tissues, compliance, conductivity and biocompatibility. A diversity of conductive hydrogels have been developed and showed versatile potentials in biomedical applications. In this review, we highlight the recent advances in conductive hydrogels, involving the various types and functionalities of conductive hydrogels as well as their applications in biomedical fields. Furthermore, the current challenges and the reasonable outlook of conductive hydrogels are also given. It is expected that this review will provide potential guidance for the advancement of next-generation conductive hydrogels.

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

As noninvasive wearable electronic devices, epidermal sensors enable continuous, real-time, and remote monitoring of various human physiological parameters. Conductive biomaterials-based hydrogels as sensor matrix materials have good biocompatibility, biodegradability, and efficient stimulus response capabilities and are widely applied in motion monitoring, healthcare, and human-machine interaction. However, biomass hydrogel-based epidermal sensing devices still need excellent mechanical properties, prolonged stability, multifunctionality, and extensive practicality. Therefore, this paper reviews the common biomass hydrogel materials for epidermal sensing (proteins, polysaccharides, polyphenols, etc.) and the various types of noninvasive sensing devices (strain/pressure sensors, temperature sensors, glucose sensors, electrocardiograms, etc.). Moreover, this review focuses on the strategies of scholars to enhance sensor properties, such as strength, conductivity, stability, adhesion, and self-healing ability. This work will guide the preparation and optimization of high-performance biomaterials-based hydrogel epidermal sensors.

Dielectric Gels with Microphase Separation for Wide-Range and Self-Damping Pressure Sensing

Omnipresent vibrations pose a significant challenge to flexible pressure sensors by inducing unstable output signals and curtailing their operational lifespan. Conventional soft sensing materials possess adequate elasticity but prove inadequate in countering vibrations. Moreover, the utilization of conventional highly-damping materials for sensing is challenging due to their substantial hysteresis. To tackle this dilemma, dielectric gels with controlled in situ microphase separation have been developed, leveraging the miscibility disparity between copolymers and solvents. The resulting gels exhibit exceptional compression stress, remarkable dielectric constant, and exceptional damping capabilities. Furthermore, flexible pressure sensors based on these microphase-separated gels show a wide detection range and low detection limit, more importantly, excellent sensing performance on vibrating surfaces. This work offers high potentials for applying flexible pressure sensors in complex practical scenarios and opens up new avenues for applications in soft electronics, biomimetic robots, and intelligent sensing.

Janus-type ionic conductive gels based on poly(N,N-dimethyl)acrylamide for strain/pressure sensors

Strain/pressure sensors with high sensitivity and a wide operation range have broad application prospects in wearable medical equipment, human-computer interactions, electronic skin, and so on. In this work, based on the different solubilities of Zr4+ in the aqueous phase and the hydrophobic ionic liquid [BMIM][Tf2N], we used N,N-dimethylacrylamide (DMA) as a vinyl monomer to prepare a Janus-type ionic conductive gel with one-sided adhesion through "one-step" UV irradiation polymerization. The Janus-type gel has satisfactory mechanical properties (tensile strength: 217.06 kPa, elongation at break: 1121.01%), electrical conductivity (conductivity: 0.10 S m-1), one-sided adhesion (adhesion strength to glass: 72.35 kPa) and antibacterial properties. The sensor based on the Janus gel can be used not only for real-time monitoring of strain changes caused by various movements of the human body (such as finger bending, muscle contraction, smiling, and swallowing) but also for real-time monitoring of pressure changes (such as pressing, water droplets, and writing movements). Therefore, based on the simplicity of this method for constructing Janus-type ionic conductive gels and the excellent electromechanical properties of the prepared gel, we believe that the method provided in this study has broad application prospects in the field of multifunctional wearable sensors.

Strong and Ultra-tough Ionic Hydrogel Based on Hyperbranched Macro-Cross-linker: Influence of Topological Structure on Properties

The application of hydrogels often suffers from their inherent limitation of poor mechanical properties. Here, a carboxyl-functionalized and acryloyl-terminated hyperbranched polycaprolactone (PCL) was synthesized and used as a macro-cross-linker to fabricate a super strong and ultra-tough ionic hydrogel. The terminal acryloyl groups of hyperbranched PCL are chemically incorporated into the network to form covalent cross-links, which contribute to robust networks. Meanwhile, the hydrophobic domains formed by the spontaneous aggregation of PCL chains and coordination bonds between Fe3+ and COO- groups serve as dynamic non-covalent cross-links, which enhance the energy dissipation ability. Especially, the influence of the hyperbranched topological structure of PCL on hydrogel properties has been well investigated, exhibiting superior strengthening and toughening effects compared to the linear one. Moreover, the hyperbranched PCL cross-linker also endowed the ionic hydrogel with higher sensitivity than the linear one when used as a strain sensor. As a result, this well-designed ionic hydrogel possesses high mechanical strength, superior toughness, and well ionic conductivity, exhibiting potential applications in the field of flexible strain sensors.

Transparent, High Stretchable, Environmental Tolerance, and Excellent Sensitivity Hydrogel for Flexible Sensors and Capacitive Pens

The prospect of ionic conductive hydrogels in multifunctional sensors has generated widespread scientific interest. The new generation of flexible materials should be combined with superior mechanical properties, high conductivity, transparency, sensitivity, good self-restoring fatigue properties, and other multifunctional characteristics, while the current materials are difficult to meet these requirements. Herein, we prepared poly(acrylamide-acrylic acid) (P(AM-AA))/gelatin/glycerol-Al3+ (PG1G2A) ionic conducting hydrogel by one-pot polymerization under UV light. The prepared PG1G2A ionic conductive hydrogel had high tensile strength (539.18 kPa), excellent tensile property (1412.96%), good fast self-recovery and fatigue resistance, high transparency (>80%), excellent moisturizing, and antifreezing/drying properties. In addition, the ionic conductive hydrogel-based strain sensor can respond to mechanical stimulation and generate accurate, stable, and recyclable electrical signals, with excellent sensitivity (GF 5.81). In addition, the PG1G2A hydrogel could be used as flexible wearable devices for monitoring multiple strain and subtle movements of different body parts at different temperatures. Interestingly, the PG1G2A hydrogel capacitive pen embedded in the mold can be used to write and draw on the screen of a phone or tablet. This new multifunctional ionic conducting hydrogel shows broad application prospects in E-skin, motion monitoring, and human-computer interaction in extreme environments.

Cellulose-Based Ionic Conductor: An Emerging Material toward Sustainable Devices

Ionic conductors (ICs) find widespread applications across different fields, such as smart electronic, ionotronic, sensor, biomedical, and energy harvesting/storage devices, and largely determine the function and performance of these devices. In the pursuit of developing ICs required for better performing and sustainable devices, cellulose appears as an attractive and promising building block due to its high abundance, renewability, striking mechanical strength, and other functional features. In this review, we provide a comprehensive summary regarding ICs fabricated from cellulose and cellulose-derived materials in terms of fundamental structural features of cellulose, the materials design and fabrication techniques for engineering, main properties and characterization, and diverse applications. Next, the potential of cellulose-based ICs to relieve the increasing concern about electronic waste within the frame of circularity and environmental sustainability and the future directions to be explored for advancing this field are discussed. Overall, we hope this review can provide a comprehensive summary and unique perspectives on the design and application of advanced cellulose-based ICs and thereby encourage the utilization of cellulosic materials toward sustainable devices.

Tough Gelatin Hydrogel for Tissue Engineering

Tough hydrogel has attracted considerable interest in various fields, however, due to poor biocompatibility, nondegradation, and pronounced compositional differences from natural tissues, it is difficult to be used for tissue regeneration. Here, a gelatin-based tough hydrogel (GBTH) is proposed to fill this gap. Inspired by human exercise to improve muscle strength, the synergistic effect is utilized to generate highly functional crystalline domains for resisting crack propagation. The GBTH exhibits excellent tensile strength of 6.67 MPa (145-fold that after untreated gelation). Furthermore, it is directly sutured to a ruptured tendon of adult rabbits due to its pronounced toughness and biocompatibility, self-degradability in vivo, and similarity to natural tissue components. Ruptured tendons can compensate for mechanotransduction by GBTH and stimulate tendon differentiation to quickly return to the initial state, that is, within eight weeks. This strategy provides a new avenue for preparation of highly biocompatible tough hydrogel for tissue regeneration.

Chitosan-based transparent and conductive hydrogel with highly stretchable, adhesive and self-healing as skin-like sensor

Hydrogel sensors attained increasing attention due to their excellent mechanical and sensing properties. However, it is still a big challenge to fabricate hydrogel sensors with multifunctional properties of transparent, high stretchability, self-adhesive and self-healing ability. In this study, chitosan as a natural polymer has been employed to construct a polyacrylamide-chitosan-Al³⁺ (PAM-CS-Al³⁺) double network (DN) hydrogel with high transparency (>90 % at 800 nm), good electrical conductivity (up to 5.01 S/m) and excellent mechanical properties (strain and toughness as high as 1040 % and 730 kJ/m³). Moreover, the dynamic ionic and hydrogen bond interaction between PAM and CS endowed the PAM-CS-Al³⁺ hydrogel good self-healing ability. In addition, the hydrogel possesses good self-adhesive ability on different substrates, including glass, wood, metal, plastic, paper, polytetrafluoroethylene (PTFE) and rubber. Most importantly, the prepared hydrogel could be assembled into transparent, flexible, self-adhesive, self-healing and high sensitive strain/pressure sensor for monitoring human body movement. This work may pave the way for fabricating the multifunctional chitosan-based hydrogels which has potential application in the fields of wearable sensor and soft electronic devices.

Cellulose nanocrystals boosted hydrophobic association in dual network polymer hydrogels as advanced flexible strain sensor for human motion detection

Conductive hydrogels attract the attention of researchers worldwide, especially in the field of flexible sensors like strain and pressure. These flexible materials have potential applications in the field of electronic skin, soft robotics, energy storage, and human motion detection. However, its practical application is limited due to low stretchability, high hysteresis energy, low conductivity, long-range strain sensitivity, and high response time. It's still a challenging job to endow all these properties in a single hydrogel network. In the present work, cellulose nano crystals (CNCs) reinforced hydrophobically associated gels were developed using APS as a source of radical polymerization, acrylamide and lauryl methacrylate were used as a monomer. CNCs reinforced the hydrophobically associated hydrogels through hydrogen bonding to retain the hydrogel's network structure. Hydrogels consist of dual crosslinking, which demonstrate exceptional mechanical performance (fracture stress and strain, toughness, and Young's modulus). The low hysteresis energy (10.9 kJm^-3) and high conductivity (22.97 mS/cm) make the hydrogels a strong candidate for strain sensors with high sensitivity (GF = 19.25 at 700% strain) and a fast response time of 200 ms. Cyclic performance was also investigated up to 300 continuous cycles. After 300 cycles, the hydrogels were still stable and no considerable change was observed. These hydrogels are capable of sensing different human motions like wrist, finger bending, and neck (up-down and straight and right/left motion of neck). The hydrogels also demonstrate changes in current in response to swallowing, different speaking words, and writing different alphabets. These results suggest that our prepared materials can sense different small and large human motions, and also could be used in any electronic device where strain sensing is required.

Highly tough and ionic conductive starch/poly(vinyl alcohol) hydrogels based on a universal soaking strategy

High-performance hydrogels with favorable mechanical strength, high modulus, sufficient ionic conductivity and freezing resistance have far-ranging applications in flexible electronic equipment. Nevertheless, it is challenging to combine admirable mechanical properties and high ionic conductivity into one hydrogel. Herein, a facile strategy was developed for the preparation of the hydrogel with excellent strength (1.45 MPa), super Young's modulus (8.85 MPa) and high conductivity (1.47 S/m) using starch and poly(vinyl alcohol) (PVA) as raw materials. The starch/PVA/Gly/Na₃Cit (SPGN) gel was firstly cross-linked by crystalline regions of PVA upon freezing-thawing cycles. It was further immersed in the saturated Na₃Cit solution to enhance the interaction between the substrates through the salting-out effect. The effect of soaking time on the crystallinity, intermolecular interactions, mechanical and electrical properties of SPGN gel was demonstrated by X-ray diffraction, Fourier transform infrared spectroscopy, tensile and impedance testing measurements. The introduction of glycerol and Na₃Cit also endowed SPGN gels with favorable anti-freezing properties. The SPGN gel could maintain high mechanical flexibility and ionic conductivity at -15 °C.

A high-conductive, anti-freezing, antibacterial and anti-swelling starch-based physical hydrogel for multifunctional flexible wearable sensors

Flexible wearable sensors based on conductive hydrogels are attracting increasing interest. To meet the urgent demands of sustainability and eco-friendliness, biopolymer-based physically crosslinked hydrogels have drawn great attention. Starch has a great potential due to its renewability, biocompatibility, nontoxicity and low cost. However, poor mechanical property, low conductivity and lack of versatility are seriously limiting the applications of starch-based hydrogels in wearable sensors. Moreover, the development of starch hydrogel-based wearable sensors in harsh conditions remains a challenge. Herein, multifunctional and physical crosslinking hydrogels were developed by introducing ionic liquid (1-ethyl-3-methyl imidazolium acetate) and metal salt (AlCl₃) into starch/polyvinyl alcohol double-network structure. The hydrogel exhibited excellent stretchability (567%), tensile strength (0.53 MPa), high conductivity (2.75 S·m^-1), good anti-freezing, antibacterial and anti-swelling properties. A wearable sensor assembled from the starch-based hydrogel exhibited a wide working range, high sensitivity (gauge factor: 5.93) and excellent reversibility. Due to the versatility, the sensor effectively detected human motion in normal and underwater environment, and possessed a sensitive pressure and thermal response. Overall, the present work provided a promising route to develop multifunctional and "green" biopolymer-based hydrogels for wearable sensors in human health and sporting applications.

Hofmeister effect in gelatin-based hydrogels with shape memory properties

The soaking strategy with the Hofmeister effect has been proposed to fabricate gelatin- based hydrogels with excellent properties. However, the modulation mechanism of hydrogels lacks in-depth study. In this work, we studied in detail the effects of Hofmeister ions on the structural, thermal, viscoelastic and mechanical properties of gelatin hydrogels. The results showed that kosmotropic anions (Cit^3-, SO₄^2-, H₂PO₄^- and S₂O₃^2-) enhanced hydrogen bonds and hydrophobic interactions between gelatin molecules, resulting in increases in the length and content of triple helices and thus improving the properties of gelatin hydrogels. In contrast, chaotropic anions (I^- and SCN^-) weakened the interactions between gelatin molecules, and thus attenuated the properties. Based on the Hofmeister effect, we successfully fabricated gelatin poly N-methylolacrylamide (PNMA) double network hydrogels with shape memory properties. The Hofmeister effect provides an excellent route for the rational design and fabrication of functional gelatin-based hydrogels.

Preparation of PAA/PAM/MXene/TA hydrogel with antioxidant, healable ability as strain sensor

Conductive hydrogels based on MXene have gained more attention due to the excellent conductive property and biocompatibility. At present, they have great potential in electronic skins, personally healthcare monitoring and human motion sensing. However, MXene are prone to be oxidized due to the abundant hydroxyls, which results in the unstable conductive property of hydrogel. To improve the shortcoming, conductive PAA/PAM/MXene/TA hydrogel was prepared, in which the introduction of TA can prevent MXene from oxidation owing to the great deal of pyrogallol groups. Mechanical tests showed that the tensile strength, toughness and elongation at break of PAA/PAM/MXene/TA hydrogel are 0.251 ± 0.05 MPa, 0.895 ± 0.16 MJ/m³ and 560.82 ± 19.56%, respectively, indicating the hydrogel possess good stretchability. In addition, the MXene and TA were introduced into hydrogel through hydrogen bonds, which endow the hydrogel with good restorability and self-healing property. Resistance variation-strain curves demonstrated that the introduction of MXene endue the hydrogel with appreciable sensing performances. Moreover, in vitro cytotoxicity assay indicated that the hydrogel has good biocompatibility. In conclusion, PAA/PAM/MXene/TA hydrogel has great potential in flexible wearable sensor field.

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.

Development of Conductive Hydrogels for Fabricating Flexible Strain Sensors

Conductive hydrogels can be prepared by incorporating various conductive materials into polymeric network hydrogels. In recent years, conductive hydrogels have been developed and applied in the field of strain sensors owing to their unique properties, such as electrical conductivity, mechanical properties, self-healing, and anti-freezing properties. These remarkable properties allow conductive hydrogel-based strain sensors to show excellent performance for identifying external stimuli and detecting human body movement, even at subzero temperatures. This review summarizes the properties of conductive hydrogels and their application in the fabrication of strain sensors working in different modes. Finally, a brief prospectus for the development of conductive hydrogels in the future is provided.

Strong, tough, anti-freezing, non-drying and sensitive ionic sensor based on fully physical cross-linked double network hydrogel

Ionic conductive double network (DN) sensors have attracted increasing attention in wearable electronic devices. However, their low mechanical and sensing properties as well as poor moisture retention and freezing resistance restrict severely their applications. Herein, we synthesized a fully physical cross-linked poly (N-hydroxymethyl acrylamide)/agar/ethylene glycol (PHA/Agar/EG) ionic conductive DN hydrogel exhibiting high strength and toughness, fast self-recovery, good fatigue resistance and good self-healing. Agar could form a physical network via reversible sol-gel transition, and interact with physical cross-linked poly (N-hydroxymethyl acrylamide) and sodium chloride (NaCl) via hydrogen bonds and salting-out effect, respectively. Meanwhile, ethylene glycol and NaCl improved the mechanical properties, long-lasting moisture retention and anti-freezing ability. The PHA/Agar/EG gel-based flexible sensor possessed excellent long-lasting and fatigue resistant sensing properties, and could monitor various human activities stably and sensitively. Therefore, this work would provide a simple and promising strategy to fabricate flexible sensors with integrated high performances for smart wearable devices.

Self-Recovery, Fatigue-Resistant, and Multifunctional Sensor Assembled by a Nanocellulose/Carbon Nanotube Nanocomplex-Mediated Hydrogel

Flexible sensors have attracted great research interest due to their applications in artificial intelligence, wearable electronics, and personal health management. However, due to the inherent brittleness of common hydrogels, preparing a hydrogel-based sensor integrated with excellent flexibility, self-recovery, and antifatigue properties still remains a challenge to date. In this study, a type of physically and chemically dual-cross-linked conductive hydrogels based on 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofiber (TOCN)-carrying carbon nanotubes (CNTs) and polyacrylamide (PAAM) matrix via a facial one-pot free-radical polymerization is developed for multifunctional wearable sensing application. Inside the hierarchical gel network, TOCNs not only serve as the nanoreinforcement with a toughening effect but also efficiently assist the homogeneous distribution of CNTs in the hydrogel matrix. The optimized TOCN-CNT/PAAM hydrogel integrates high compressive (∼2.55 MPa at 60% strain) and tensile (∼0.15 MPa) strength, excellent intrinsic self-recovery property (recovery efficiency >92%), and antifatigue capacity under both cyclic stretching and pressing. The multifunctional sensors assembled by the hydrogel exhibit both high strain sensitivity (gauge factor ≈11.8 at 100-200% strain) and good pressure sensing ability over a large pressure range (0-140 kPa), which can effectively detect the subtle and large-scale human motions through repeatable and stable electrical signals even after 100 loading-unloading cycles. The comprehensive performance of the TOCN-CNT/PAAM hydrogel-based sensor is superior to those of most gel-based sensors previously reported, indicating its potential applications in multifunctional sensing devices for healthcare systems and human motion monitoring.

Highly Electrically Conductive Flexible Ionogels by Drop-Casting Ionic Liquid/PEDOT:PSS Composite Liquids onto Hydrogel Networks

Ionogels have been extensively studied as ideal flexible and stretchable materials by virtue of the unique properties of ionic liquids, such as non-volatility, non-flammability, and negligible vapor pressure. However, the generally low ionic conductivity of the current ionogels limits their applications in the market of highly conductive, flexible, and stretchable electrical devices. Here, the fabrication of highly electrically conductive ionogels is reported by combining composite liquids consisting of 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCA]) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with flexible negative-charged poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) hydrogel. The generated composite film exhibits high electrical conductivity up to about 38 S cm^-1 with the maximum tensile strain of 45% and fracture stress of 27 kPa. In addition, it is demonstrated that the composite film can maintain conductivity in a high level under different mechanical deformations, and can also be used as flexible sensors in a wide temperature range from -58 to 120 ℃. It is believed that the designed composite film would expand the applications of flexible conductive materials where both high conductivity and robust mechanical flexibility are required.

An extremely tough and ionic conductive natural-polymer-based double network hydrogel

Hydrogels are widely used in fields such as drug delivery, tissue regeneration, soft robotics and flexible smart electronic devices, yet their application is often limited by unsatisfactory mechanical behaviors. Among the various improvement strategies, double network (DN) hydrogels from synthetic polymers demonstrated impressive mechanical properties, while those from natural polymers were usually inferior. Here, a novel DN hydrogel composed fully of natural polymers exhibiting remarkable mechanical properties and conductivity is prepared by simply soaking a virgin gellan gum/gelatin composite hydrogel in a mixed solution of Na₂SO₄ and (NH₄)₂SO₄. This hydrogel exhibits a tunable Young's modulus (0.08 to 42.6 MPa), good fracture stress (0.05 to 7.5 MPa), good fracture stretch (1.4 to 7.1), high fracture toughness (up to 27.7 kJ m^-2), and high ionic conductivity (up to 11.4 S m^-1 at f = 1 kHz). The improvement in the mechanical properties of the DN gel is attributed to the chain-entanglement crosslinking points introduced by SO₄^2- in the gelatin network and the electrostatic interaction crosslinking points introduced by Na⁺ in the gellan gum network. The high ionic conductivity of the DN gel is attributed to the infiltration of the DN gel in a salt solution of high concentration. The developed gellan gum/gelatin DN hydrogel has shown a new pathway towards strengthening natural-polymer-based DN hydrogels and towards potential applications in biomedical engineering and flexible electronic devices.

Robust Conductive Hydrogels with Ultrafast Self-Recovery and Nearly Zero Response Hysteresis for Epidermal Sensors

Robust conductive hydrogels are in great demand for the practical applications of smart soft robots, epidermal electronics, and human–machine interactions. We successfully prepared nanoparticles enhanced polyacrylamide/hydroxypropyl guar gum/acryloyl-grafted chitosan quaternary ammonium salt/calcium ions/SiO₂ nanoparticles (PHC/Ca²⁺/SiO₂ NPs) conductive hydrogels. Owing to the stable chemical and physical hybrid crosslinking networks and reversible non-covalent interactions, the PHC/Ca²⁺/SiO₂ NPs conductive hydrogel showed good conductivity (~3.39 S/m), excellent toughness (6.71 MJ/m³), high stretchability (2256%), fast self-recovery (80% within 10 s, and 100% within 30 s), and good fatigue resistance. The maximum gauge factor as high as 66.99 was obtained, with a wide detectable strain range (from 0.25% to 500% strain), the fast response (25.00 ms) and recovery time (86.12 ms), excellent negligible response hysteresis, and good response stability. The applications of monitoring the human’s body movements were demonstrated, such as wrist bending and pulse tracking.

Transparent, Conductive Hydrogels with High Mechanical Strength and Toughness

Transparent, conductive hydrogels with good mechanical strength and toughness are in great demand of the fields of biomedical and future wearable smart electronics. We reported a carboxymethyl chitosan (CMCS)-calcium chloride (CaCl2)/polyacrylamide (PAAm)/poly(N-methylol acrylamide (PNMA) transparent, tough and conductive hydrogel containing a bi-physical crosslinking network through in situ free radical polymerization. It showed excellent light transmittance (>90%), excellent toughness (10.72 MJ/m3), good tensile strength (at break, 2.65 MPa), breaking strain (707%), and high elastic modulus (0.30 MPa). The strain sensing performance is found with high sensitivity (maximum gauge factor 9.18, 0.5% detection limit), wide strain response range, fast response and recovery time, nearly zero hysteresis and good repeatability. This study extends the transparent, tough, conductive hydrogels to provide body-surface wearable devices that can accurately and repeatedly monitor the movement of body joints, including the movements of wrists, elbows and knee joints. This study provided a broad development potential for tough, transparent and conductive hydrogels as body-surface intelligent health monitoring systems and implantable soft electronics.

Strengthening gelatin hydrogels using the Hofmeister effect

A simple yet effective soaking treatment has been proposed to fabricate hydrogels with desirable mechanical properties, but the strengthening mechanism of hydrogels lacks an in-depth study. Here, we investigated the influence of kosmotropic citrate anion on the structure and properties of immersed gelatin hydrogels. The obtained hydrogels possessed the properties of high strength, modulus and toughness simultaneously. The dehydration of hydrogels facilitated the interactions among gelatin molecules, resulting in the formation of helix structures. Both the content and length of the triple helices increase with an increase in citrate concentration, which in turn contributes to the strengthening of hydrogels. The excellent mechanical performances of these hydrogels may open up new applications for protein materials.

Three-Dimensional Printed Hydrogels with High Elasticity, High Toughness, and Ionic Conductivity for Multifunctional Applications

Hydrogels have drawn extensive attention due to their unique physical and biological properties. However, the relatively low mechanical strength and poor processability of hydrogels limit their applications. Especially, the emerging 3D printing technology for nontoxic hydrogels requires proper formability and controllable mechanical behaviors. In this study, a new strategy to construct a novel double-network biocompatible hydrogel from poly(ethylene glycol) diacrylate (PEGDA) and short-chain chitosan (CS) via ionic-covalent cross-linking is by a two-step method involving UV curing followed by immersion in an anionic solution. The CS-based ionic network and PEGDA-based covalent network as well as the hydrogen bonds between them jointly induce excellent mechanical properties, which can be regulated by changing the PEGDA/CS content and ionic cross-linking time. Compared with conventional hydrogels, this mechanically optimized hydrogel exhibits a superior elastic modulus (3.84 ± 0.4 MPa), higher tensile strength (7.23 ± 0.2 MPa), and higher tensile strain (162 ± 7%). Notably, its excellent printing capability through the citrate anionic solution adjustment enables 3D printing with precision, flexibility, and a complex inner structure by extrusion in air at room temperature. In addition, a number of citrate ions existed in the ionic network, giving the hydrogels good electrical conductivity. Therefore, this printable, conductive, and tough hydrogel exhibits potential for vascular engineering, cartilage tissue engineering, and wearable device applications.

Basic Approaches to the Design of Intrinsic Self-Healing Polymers for Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) as a revolutionary system for harvesting mechanical energy have demonstrated high vitality and great advantage, which open up great prospects for their application in various areas of the society of the future. The past few years have seen exponential growth in many new classes of self-healing polymers (SHPs) for TENGs. This review presents and evaluates the SHP range for TENGs, and also attempts to assess the impact of modern polymer chemistry on the development of advanced materials for TENGs. Among the most widely used SHPs for TENGs, the analysis of non-covalent (hydrogen bond, metal-ligand bond), covalent (imine bond, disulfide bond, borate bond) and multiple bond-based SHPs in TENGs has been performed. Particular attention is paid to the use of SHPs with shape memory as components of TENGs. Finally, the problems and prospects for the development of SHPs for TENGs are outlined.