Artificial skin is essential for bionic robotics, facilitating human skin-like functions such as sensation, communication, and protection. However, replicating a skin-matched all-in-one material with excellent mechanical properties, self-healing, adhesion, and multimodal sensing remains a challenge. Herein, we developed a multifunctional hydrogel by establishing a consolidated organic/metal bismuth ion architecture (COMBIA). Benefiting from hierarchical reversible noncovalent interactions, the COMBIA hydrogel exhibits an optimal combination of mechanical and functional properties, particularly its integrated mechanical properties, including unprecedented stretchability, fracture toughness, and resilience. Furthermore, these hydrogels demonstrate superior conductivity, optical transparency, freezing tolerance, adhesion capability, and spontaneous mechanical and electrical self-healing. These unified functions render our hydrogel exceptional properties such as shape adaptability, skin-like perception, and energy harvesting capabilities. To demonstrate its potential applications, an artificial skin using our COMBIA hydrogel was configured for stimulus signal recording, which, as a promising soft electronics platform, could be used for next-generation human-machine interfaces.

Janus hydrogels for human motion monitoring are thriving due to their conductivity, flexibility, anisotropy and self-adhesion, etc. However, most of them face challenges such as complex processes, interlayer detachment, and surface contamination, which degrade their sensing accuracy and sensitivity. Hence, this study proposes a facile strategy using the cellulose and lignin as building blocks to construct a Janus hydrogel for accurate and sensitive sensing. The proposed process involves sequential pouring and in-situ thermal-induced polymerization. Initially, the cellulose dissolved in ZnCl2 solution, along with acrylic acid (Cel/ZnCl2-AA), forms the antifouling and conductive precursors in the top layer, while lignosulfonate and acrylamide (LS-AM) function as the adhesive precursor in the bottom layer. The viscosity difference of the two precursors allows local diffusion and polymerization at the interlayer interface, thereby developing a bilayer structure with strong interface bonds. Consequently, the Janus hydrogel exhibits high conductivity (1.2 S/m), excellent asymmetric adhesion, good stretchability (520 %) and compressive strain (∼70 %). These properties enable the hydrogel to accurately monitor both large (elbow and wrist bending) and small (swallowing and frowning) human movements with accurate sensitivity (gauge factor up to 2). This work offers new insight and synthesis strategy to design the bilayer hydrogels for practical applications in electronic skin and flexible sensing.

Achieving the integration of multiple properties in a single hydrogel system faces significant challenges. This research presents a simple approach to developing a multifunctional conductive hydrogel with high stretchability (>740 %), electrical conductivity, frost resistance and self-adhesiveness. It serves as a wearable, flexible electronic material, it remains functional even in low-temperature environments. The hydrogel is synthesized by incorporating a uniformly mixed solution of carboxymethyl cellulose (CMC) and aminated carbon nanotubes (NH2-CNTs) into a polyacrylamide (PAM)/gelatin dual-network hydrogel. By adjusting the CMC mass fraction, the optimal composite hydrogel is obtained within a specified gradient. After cross-linking modification with a calcium chloride (CaCl2) solution, enhances its mechanical properties, resulting in a final hydrogel with excellent stretchability (strain = 749 %), strong adhesion, frost resistance, moisture retention, and conductivity. Additionally, this research explores the hydrogel's potential for anti-counterfeiting and salt ion monitoring by analyzing changes in mechanical properties and transparency. The hydrogel exhibits high sensitivity to external strains and effectively monitors human signals such as finger bending, head movement, and speech, even at low temperatures. This research provides new insights into flexible electronic skin, wearable sensors and human-computer interaction, expanding the potential applications of multifunctional 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.

Hydrogels, known for their outstanding water absorption, flexibility, and biocompatibility, have been widely utilized in various fields. Nevertheless, their application is still limited by their relatively low mechanical performance. This study has successfully developed a dual-network hydrogel with exceptional mechanical properties by embedding amino-functionalized polysiloxane (APSi) networks into a polyvinyl alcohol (PVA) matrix. This hydrogel effectively dissipates energy through dense sacrificial bonds between the networks, allowing for precise control over its tensile strength (ranging from 0.07 to 1.46 MPa) and toughness (from 0.06 to 2.17 MJ/m3) by adjusting the degree of crosslinking in the polysiloxane network. Additionally, the hydrogel exhibits excellent conductivity (10.97 S/cm) and strain sensitivity (GF = 1.43), indicating its potential for use in wearable strain sensors. Moreover, at the end of its life (EOL), the sensor waste can be repurposed as an adsorbent material for metal ions in water treatment, achieving the recycling of hydrogel materials and maximizing resource utilization.

Cellulose nanofiber (CNF) is often incorporated as reinforcements into various matrices to optimize the mechanical properties of composites. However, the role of CNF in structural design interface components has been mostly neglected. Inspired by the architectural structure of "straw and clay", CNF and polydopamine (PDA) were used as the "straw phase" and "clay phase", respectively, to construct PDA/CNF self-assembled coatings on the carbon fiber (CF) surface via covalent bonding and non-covalent self-assembly. The organic coatings endowed the CF with high specific surface area, roughness and polarity, as well as a broad and gentle interfacial layer of the CF/epoxy resin composites. After self-assembly, the monofilament tensile strength (TS) of the fiber and the interlaminar shear strength (ILSS) of the CF/epoxy resin composites were increased by 13.44 % and 31.88 %, respectively. This investigation furnishes ideas for improving the mechanical performances of composites from the viewpoint of surface structure design and interface modulation.

The biocompatibility and salient gelling feature of alginate via forming the interpenetrating network structure has received extensive interests for different applications. Traditional alginate hydrogels freeze at low temperature and evaporate easily at room temperature, leading to reduced performance. Consequently, it is crucial to develop methods to prevent alginate hydrogel from freezing at subzero temperature and dehydration at normal temperature to maintain the performance stability. Utilizing polyacrylic acid, sodium alginate, and acrylamide-hydroxyethyl methacrylate copolymers as flexible matrix materials, this study develops a wearable silica (SiO2)/carbon nanotubes (CNT)/sodium ions (SiO2/CNT/Na+) modified sodium alginate hydrogel strain sensor characterized by high sensitivity, flexibility, and anti-freezing and anti-drying properties. The hydrogel doped with NaCl (50 mg), CNT (10 mg) and M-SiO2 (200 mg) shows excellent mechanical and electrical properties, the tensile strength is 436 KPa, the break elongation is 426 %, the elastic modulus is 99 KPa, and the toughness is 897 kJ/m3. The modified sodium alginate hydrogel used as strain sensor shows fast response time (∼100 ms), high sensitivity factor and excellent stability. The strain sensor exhibits excellent flexibility, ductility, self-adhesion, anti-freezing and anti-drying properties, significantly enhancing its strain sensing application field.

The green synthesis strategy for cellulose-containing hydrogel electrolytes is significant for effectively managing resources, energy, and environmental concerns in the contemporary world. Herein, we propose an all-green strategy using AlCl3/ZnCl2/H2O solvent to create cellulose/polyacrylamide-based hydrogel (AZ-Cel/PAM) with expanded hierarchical topologies. The aqueous AlCl3/ZnCl2 facilitates the efficient dissolution of cellulose at room temperature, and the dispersed Al3+-Zn2+ ions autocatalytic system catalyzes in-situ polymerization of acrylamide (AM) monomer. This expands the AM network within the cellulose framework, forming multiple bonding interactions and stable ion channels. The resulting hybrid hydrogel exhibits improved mechanical properties (tensile strength of 56.54 kPa and compressive strength of 359.43 kPa) and enhanced ionic conductivity (1.99 S/m). Furthermore, it also demonstrates excellent adhesion, freeze resistance (-45 °C), and water retention capabilities. Quantum simulations further clarify the mechanical composition and ion transport mechanism of AZ-Cel/PAM hydrogels. The assembled supercapacitor with the hydrogel electrolyte, demonstrates an ideal area-specific capacitance of 203.80 mF/cm2. This all-green strategy presents a novel approach to developing sustainable energy storage devices.

Conductive hydrogels with high performance and frost resistance are essential for flexible electronics, electronic skin, and soft robots. Nonetheless, the preparation of hydrogel-based flexible strain sensors with rapid response, wide strain detection range, and high sensitivity remains a considerable challenge. Furthermore, the inevitable freezing and evaporation of water in sub-zero temperatures and dry environments lead to the loss of flexibility and conductivity in hydrogels, which seriously limits their practical application. In this work, ionic liquids (ILs) and MXene are introduced into gelatin/polyacrylamide (PAM) precursor solution, and a PAM/gelatin/ILs/MXene/glycerol (PGIMG) hydrogel-based flexible strain sensor with MXene co-ILs ion-electron composite conductive network is prepared by combining the electrohydrodynamic (EHD) printing method and in-situ photopolymerization. The introduction of ILs provides an ionic conductive channel for the hydrogel. The introduction of MXene nanosheets forms an interpenetrating network with gelatin and PAM, which not only provides a conductive channel, but also improves the mechanical and sensing properties of the hydrogel-based flexible strain sensor. The prepared PGIMG hydrogel with the MXene co-ILs ion-electron composite conductive network demonstrates a tensile strength of 0.21 MPa at 602.82 % strain, the conductivity of 1.636 × 10-3 S/cm, high sensitivity (Gauge Factor, GF = 4.17), a wide strain detection range (1-600 %), and the response/recovery times (73 ms and 74 ms). In addition, glycerol endows the hydrogel with excellent freezing (-60 °C) and water retention properties. The application of the hydrogel-based flexible strain sensor in the field of human motion detection and information transmission shows the great potential of wearable devices, electronic skin, and information encryption transmission.

Most tissues of the human body present hierarchical fibrillar extracellular matrices (ECMs) that have a strong influence over their physicochemical properties and biological behavior. Of great interest is the introduction of this fibrillar structure to hydrogels, particularly due to the water-rich composition, cytocompatibility, and tunable properties of this class of biomaterials. Here, the main bottom-up fabrication strategies for the design and production of hierarchical biomimetic fibrillar hydrogels and their most representative applications in the fields of tissue engineering and regenerative medicine are reviewed. For example, the controlled assembly/arrangement of peptides, polymeric micelles, cellulose nanoparticles (NPs), and magnetically responsive nanostructures, among others, into fibrillar hydrogels is discussed, as well as their potential use as fibrillar-like hydrogels (e.g., those from cellulose NPs) with key biofunctionalities such as electrical conductivity or remote stimulation. Finally, the major remaining barriers to the clinical translation of fibrillar hydrogels and potential future directions of research in this field are discussed.

The development of flexible wearable multifunctional electronics has gained great attention in the field of human motion monitoring. However, developing mechanically tough, highly stretchable, and recyclable composite conductive materials for application in multifunctional sensors remained great challenges. In this work, a mechanically tough, highly stretchable, and recyclable composite conductive elastomer with the dynamic physical-chemical dual-crosslinking network was fabricated by the combination of multiple hydrogen bonds and dynamic ester bonds. To prepare the proposed composite elastomers, the polyaniline-modified carboxylate cellulose nanocrystals (C-CNC@PANI) were used as both conductive filler to yield high conductivity of 15.08 mS/m, and mechanical reinforcement to construct the dynamic dual-crosslinking network with epoxidized natural rubber latex to realize the high mechanical strength (8.65 MPa) and toughness (29.57 MJ/m3). Meanwhile, the construction of dynamic dual-crosslinking network endowed the elastomer with satisfactory recyclability. Based on these features, the composite conductive elastomers were used as strain sensors, and electrode material for assembling flexible and recyclable self-powered sensors for monitoring human motions. Importantly, the composite conductive elastomers maintained reliable sensing and energy harvesting performance even after multiple recycling process. This study provides a new strategy for the preparation of recyclable, mechanically tough composite conductive materials for wearable sensors.

Hydrogels are extensively utilized in the fields of electronic skin, environmental monitoring, biological dressings due to their excellent flexibility and conductivity. However, traditional hydrogel materials possess drawbacks such as environmental toxicity, low strength, poor stability, and water loss deactivation, which limited its frequent applications. Here, a flexible conductive hydrogel called wood-based DES hydrogel (WDH) with high strength, high adhesion, high stability, and high sensitivity was successfully synthesized by using environmentally friendly lignocellulose as skeleton and deep eutectic solvent as matrix. The strength of WDH prepared from lignocellulose framework is approximately 50 times higher than poly deep eutectic solvent hydrogel, and about 4.5 times higher than that prepared from cellulose skeleton. The WDH exhibits stable adhesion to most common materials and demonstrates exceptional dimensional stability. Its conductivity remains unaffected by water, even after prolonged exposure to air, maintaining a value of 0.0245 S/m. The anisotropy inherent in the system results in three distinct linear sensing intervals for WDH, exhibiting a maximum sensitivity of 5.45. This paper verified the advantages of lignocellulose framework in improving the strength and stability of hydrogels, which provided a new strategy for the development of sensor materials.

Hydrogel-based functional materials had attracted great attention in the fields of artificial intelligence, soft robotics, and motion monitoring. However, the gelation of hydrogels induced by free radical polymerization typically required heating, light exposure, and other conditions, limiting their practical applications and development in real-life scenarios. In this study, a simple and direct method was proposed to achieve rapid gelation at room temperature by incorporating reductive MXene sheets in conjunction with metal ions into the chitosan network and inducing the formation of a polyacrylamide network in an extremely short time (10 s). This resulted in a dual-network MXene-crosslinked conductive hydrogel composite that exhibited exceptional stretchability (1350 %), remarkably low dissipated energy (0.40 kJ m-3 at 100 % strain), high sensitivity (GF = 2.86 at 300-500 % strain), and strong adhesion to various substrate surfaces. The study demonstrated potential applications in the reliable detection of various motions, including repetitive fine movements and large-scale human body motions. This work provided a feasible platform for developing integrated wearable health-monitoring electronic systems.

Conductive hydrogels have shown a great potential in the field of flexible electronic devices. However, conductive hydrogels prepare by traditional methods are difficult to combine high strength and toughness, which limits their application in various fields. In this study, a strategy for preparing conductive hydrogels with high strength and toughness by using the synergistic effect of biomineralization and salting-out was pioneered. In simple terms, by immersing the CaCl2 doped soy protein isolate/poly(vinyl alcohol)/dimethyl sulfoxide (SPI/PVA/DMSO) hydrogel in Na2CO3 and Na3Cit complex solution, the biomineralization aroused by Ca2+ and CO32-, and the salting-out effect of both NaCl and Na3Cit would enhance the mechanical properties of SPI/PVA/DMSO hydrogel. Meanwhile, the ionic conductivity of the hydrogel would also increase due the introduction of cation and anion. The mechanical and electrical properties of SPI/PVA/DMSO/CaCO3/Na3Cit hydrogels were significantly enhanced by the synergistic effect of biomineralization and salting-out. The optimum tensile strength, toughness, Young's modulus and ionic conductivity of the hydrogel were 1.4 ± 0.08 MPa, 0.51 ± 0.04 MPa and 1.46 ± 0.01 S/m, respectively. The SPI/PVA/DMSO/CaCO3/Na3Cit hydrogel was assembled into a strain sensor. The strain sensor had good sensitivity (GF = 3.18, strain in 20 %-500 %) and could be used to accurately detect various human movements.

Conductive hydrogels, exhibiting wide applications in electronic skins and soft wearable sensors, often require maturely regulating of the hydrogel mechanical properties to meet specific demands and work for a long-term or under extreme environment. However, in situ regulation of the mechanical properties of hydrogels is still a challenge, and regular conductive hydrogels will inevitably freeze at subzero temperature and easily dehydrate, which leads to a short service life. Herein, a novel adhesive hydrogel (PAA-Dopa-Zr4+) capable of strain sensing is proposed with antifreezing, nondrying, strong surface adhesion, and tunable mechanical properties. 3,4-Dihydroxyphenyl-l-alanine (l-Dopa)-grafted poly(acrylic acid) (PAA) and Zr4+ ion are introduced into the hydrogel, which broadly alters the mechanical properties via tuning the in situ aggregation state of polymer chains by ions based on the complexation effect. The catechol groups of l-Dopa and viscous glucose endow the hydrogel with high adhesiveness for skin and device interface (including humid and dry environments) and exhibit an outstanding temperature tolerance under extreme wide temperature spectrum (-35 to 65 °C) or long-lasting moisture retention (60 days). Furthermore, this PAA-Dopa-Zr4+ can be assembled as a flexible strain-sensitive sensor to detect human motions based on specific mechanical properties requirements. This work, enabling superior adhesive and temperature tolerance performance and broad mechanical tenability, presents a new paradigm for numerous applications to wearable sensing and personalized healthcare monitoring.

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.

Ion conductive hydrogels (ICHs) have attracted great interest in the application of ionic skin because of their superior characteristics. However, it remains a challenge for ICHs to achieve balanced properties of high strength, large fracture strain, self-healing and freezing tolerance. In this study, a strong, stretchable, self-healing and antifreezing ICH was demonstrated by rationally designing a multiphysically cross-linked network structure consisting of the hydrophobic association, metal-ion coordination and chain entanglement among poly(acrylic acid) (PAA) polymer chains. The deliberately designed Brij S 100 acrylate (Brij-100A) micelle cross-linker can effectively dissipate energy and endow hydrogels with desirable stretchability. The self-healing ability of hydrogels originates from the reversible hydrophobic association in micelles and Fe³⁺-COO^- coordination. After the addition of NaCl, the chain-entangled physical network caused by the salting-out effect can both enhance mechanical strength and promote electron transport. With the synergy of hydrophobic association, mental-ligand coordination and chain entanglement, the PAA/Brij-100A/Fe³⁺/NaCl (PAA/BA/Fe³⁺/NaCl) hydrogels exhibited a high tensile strain of 1140%, a tensile strength of 0.93 MPa and a toughness of 3.48 MJ m^-3. Besides, the PAA/BA/Fe³⁺/NaCl hydrogels exhibited a high conductivity of 0.43 S m^-1 and good freezing resistance. The ionic skin based on the PAA/BA/Fe³⁺/NaCl hydrogels showed high sensitivity (GF = 5.29), wide strain range (0-950%), fast response time (220 ms) and good stability. Also, the self-healing ability of the ionic skin can significantly prolong its service time, and the antifreezing property can broaden its applicable temperature. This study offers new insight into the design of multifunctional ionic skin for wearable electronics.

In recent years, hydrogels have attracted extensive attention in smart sensing owing to their biocompatibility and high elasticity. However, it is still a challenge to develop hydrogels with excellent multiple responsiveness for smart wearable sensors. In this paper, a facile synthesis of carbon quantum dots (CQDs)-doped cross-linked chitosan quaternary/carboxymethylcellulose hydrogels (CCCDs) is presented. Designing of dual network hydrogels decorated with CQDs provides abundant crosslinking and improves the mechanical properties of the hydrogels. The hydrogel-based strain sensor exhibits excellent sensitivity (gauge factor: 9.88), linearity (R² : 0.97), stretchable ability (stress: 0.67 MPa; strain: 404%), good cyclicity, and durability. The luminescent properties are endowed by the CQDs further broaden the application of hydrogels for realizing flexible electronics. More interestingly, the strain sensor based on CCCDs hydrogel demonstrates photo responsiveness (ΔR/R₀ ≈20%) and pH responsiveness (pH range ≈4-7) performance. CCCDs hydrogels can be used for gesture recognition and light sensing switch. As a proof-of-concept, a smart wearable sensor is designed for monitoring human activities and detecting pH variation in human sweat during exercise. This study reveals new possibilities for further applications in wearable health monitoring.