The remarkable progress in efforts to prepare conductive self-healing hydrogels mimicking human skin's functions has been witnessed in recent years. However, it remains a great challenge to develop an integrated conductive gel combining excellent self-healing and mechanical properties, which is derived from their inherent compromise between the dynamic cross-links for healing and steady cross-links for mechanical strength. In this work, we design a tough, self-healing, and self-adhesive ionic gel by constructing synergistic multiple coordination bonds among tannic acid-coated cellulose nanocrystals (TA@CNCs), poly(acrylic acid) chains, and metal ions in a covalent polymer network. The incorporated TA@CNC acts as a dynamic connected bridge in the hierarchically porous network mediated by multiple coordination bonds, endowing the ionic gels the superior mechanical performance. Reversible nature of dynamic coordination interactions leads to excellent recovery property as well as reliable mechanical and electrical self-healing property without any assistance of external stimuli. Intriguingly, the ionic gels display durable and repeatable adhesiveness ascribed to the presence of catechol groups from the incorporated tannic acid, which can be adhered directly on human skin without inflammatory response and residual. Additionally, the ionic gels with a great strain sensitivity can be employed as flexible strain sensors to monitor and distinguish both large motions (e.g., joints bending) and subtle motions (e.g., pulse and breath), which enable us to analyze the data on the user interface of smart phone via programmable wireless transmission. This work provides a new prospect for the design of the biocompatible cellulose-based hydrogels with stretchable, self-adhesive, self-healing, and strain-sensitive properties for potential applications in wearable electronic sensors and healthcare monitoring.

As a promising functional material, hydrogels have attracted extensive attention, especially in flexible wearable sensor fields, but it remains a great challenge to facilely integrate excellent mechanical properties, self-adhesion, and strain sensitivity into a single hydrogel. In this work, we present high in strength, stretchable, conformable, and self-adhesive chitosan/poly­(acrylic acid) double-network nanocomposite hydrogels for application in epidermal strain sensor via in situ polymerization of acrylic acid in chitosan acid aqueous solution with tannic acid-coated cellulose nanocrystal (TA@CNC) acting as nanofillers to reinforce tensile properties, followed by a soaking process in a saturated NaCl solution to cross-link chitosan chains. With addition of a small amount of TA@CNC, the double-network nanocomposite hydrogels became highly adhesive and mechanically compliant, which were critical factors for the development of conformable and resilient wearable epidermal sensors. The salt-soaking process was applied to cross-link chitosan chains by shielded electrostatic repulsions between positively charged amino groups, drastically enhancing the mechanical properties of the hydrogels. The obtained double-network nanocomposite hydrogels exhibited excellent tunable mechanical properties that could be conveniently tailored with fracture stress and fracture strain ranging from 0.39 to 1.2 MPa and 370 to 800%, respectively. Besides, the hydrogels could be tightly attached onto diverse substrates, including wood, glass, plastic, polytetrafluoroethylene, glass, metal, and skin, demonstrating high adhesion strength and compliant adhesion behavior. In addition, benefiting from the abundant free ions from strong electrolytes, the flexible hydrogel sensors demonstrated stable conductivity and strain sensitivity, which could monitor both large human motions and subtle motions. Furthermore, the antibacterial property originating from chitosan made the hydrogels suitable for wearable epidermal sensors. The facile soaking strategy proposed in this work would be promising in fabricating high-strength multifunctional conductive hydrogels used for wearable epidermal devices.

Natural biopolymer-based conductive hydrogels, which combine inherent renewable, nontoxic features, biocompatibility and biodegradability of biopolymers, and excellent flexibility and conductivity of conductive hydrogels, exhibit great potential in applications of wearable and stretchable sensing devices. Compared to traditional flexible substrates deriving from petro-materials-derived polymers, conductive hydrogels consisting of continuous cross-linked polymer networks and a large amount of water exhibit more fantastic combination of stretchability and conductivity because their polymer networks endow the hydrogels with mechanical flexibility and the water offers them a consecutive ionic transport property. Different from petro-materials-derived polymers, biopolymers that are extracted from bioresource with intrinsic biocompatibility and biodegradability are commonly considered as appropriate candidates for constructing wearable devices. For example, biopolymers such as cellulose, chitosan, and silk fibroin are usually chosen as promising candidates to construct conductive hydrogels, endowing the hydrogels with enhanced mechanical properties and remarkable biocompatibility. This review summarizes the recent progress of natural biopolymer-based conductive hydrogels that are utilized for electrical sensing devices with a series of typical biopolymers including cellulose, chitosan, silk fibroin, and gelatin. The chemical structures and physicochemical properties of the four typical biopolymers are demonstrated, and their applications in diverse conductive hydrogel sensors are discussed in detail. Finally, the remaining challenges and expectations are discussed.

Flexible zinc-ion hybrid capacitors (ZIHCs) based on hydrogel electrolytes are an up-and-coming and highly promising candidate for potential large-scale energy storage due to their combined complementary advantages of zinc batteries and capacitors. However, the freezing induces a sharp drop in conductivity and mechanical property with tremendous compromise of the interfacial adhesion, thereby severely impeding the low-temperature application of such flexible ZIHCs. To achieve the flexible ZIHCs with excellent low-temperature adaptability, an antifreezing and self-adhesive polyzwitterionic hydrogel electrolyte (PZHE) is engineered via a self-catalytic nano-reinforced strategy, affording unparalleled conductivity and robust interfacial adhesion, together with superhigh mechanical strength over a broad temperature ranging from 25 to −60 °C. Meanwhile, the water-in-salt-type PZHE filled with ZnCl2 can provide ion migration channels to enhance the reversibility of Zn metal electrodes, thus greatly reducing side reactions and extending the cycling life. With distinctive integrated merits of the water-in-salt type PZHE, the as-built ZIHCs deliver a high-level energy density of 80.5 Wh kg–1, a desired specific capacity of 81.5 mAh g–1, along with a long-duration cycling lifespan (100 000 cycles) with 84.6% capacity retention at −40 °C, even outperforming the state-of-the-art ZIHCs at room temperature. More encouragingly, the extraordinary temperature-adaptability for both electrochemical and mechanical performance under severe mechanical challenges is achieved for the flexible ZIHCs at extremely low temperature. Noticeably, the ZIHC is also capable of operating in an ice–water bath and vacuum. It is believed that this strategy makes contributions to inspire the design and application of high-performance PZHEs in fields of flexible and wearable electronics that can work in extremely cold environments.

Although self-healing gels with structural resemblance to biological tissues attract great attention in biomedical fields, it remains a dilemma for combination between fast self-healing properties and high mechanical toughness. On the basis of the design of dynamic reversible cross-links, we incorporate rigid tannic acid-coated cellulose nanocrystal (TA@CNC) motifs into the poly­(vinyl alcohol) (PVA)–borax dynamic networks for the fabrication of a high toughness and rapidly self-healing nanocomposite (NC) hydrogel, together with dynamically adhesive and strain-stiffening properties that are particularly indispensable for practical applications in soft tissue substitutes. The results demonstrate that the obtained NC gels present a highly interconnected network, where flexible PVA chains wrap onto the rigid TA@CNC motifs and form the dynamic TA@CNC–PVA clusters associated by hydrogen bonds, affording the critical mechanical toughness. The synergetic interactions between borate–diol bonds and hydrogen bonds impart a typical self-healing behavior into the NC gels, allowing the dynamic cross-linked networks to undergo fast rearrangement in the time scale of seconds. Moreover, the obtained NC hydrogels not only mimic the main feature of biological tissues with the unique strain-stiffening behavior but also display unique dynamic adhesiveness to nonporous and porous substrates. It is expected that this versatile approach opens up a new prospect for the rational design of multifunctional cellulosic hydrogels with remarkable performance to expand their applications.

The application of conductive hydrogels in intelligent biomimetic electronics is a hot topic in recent years, but it is still a great challenge to develop the conductive hydrogels through a rapid fabrication process at ambient temperature. In this work, a versatile poly(acrylamide) @cellulose nanocrystal/tannic acid−silver nanocomposite (NC) hydrogel integrated with excellent stretchability, repeatable self-adhesion, high strain sensitivity, and antibacterial property, was synthesized via radical polymerization within 30 s at ambient temperature. Notably, this rapid polymerization was realized through a tannic acid−silver (TA-Ag) mediated dynamic catalysis system that was capable of activating ammonium persulfate and then initiated the free-radical polymerization of the acrylamide monomer. Benefiting from the incorporation of TA-Ag metal ion nanocomplexes and cellulose nanocrystals, which acted as dynamic connecting bridges by hydrogen bonds to efficiently dissipate energy, the obtained NC hydrogels exhibited prominent tensile strain (up to 4000%), flexibility, self-recovery, and antifatigue properties. In addition, the hydrogels showed repeatable adhesiveness to different substrates (e.g., glass, wood, bone, metal, and skin) and significant antibacterial properties, which were merits for the hydrogels to be assembled into a flexible epidermal sensor for long-term human− machine interfacial contact without concerns about the use of external adhesive tapes and bacterial breeding. Moreover, the remarkable conductivity (σ ∼ 5.6 ms cm −1 ) and strain sensitivity (gauge factor = 1.02) allowed the flexible epidermal sensors to monitor various human motions in real time, including huge movement of deformations (e.g., wrist, elbow, neck, shoulder) and subtle motions. It is envisioned that this work would provide a promising strategy for the rapid preparation of conductive hydrogels in the application of flexible electronic skin, biomedical devices, and soft robotics.

Conductive hydrogels are receiving considerable attention because of their important applications, such as flexible wearable electronic, human-machine interfaces, and smart/soft robotics. However, the insufficient mechanical performance and inferior adhesive capability severely hinder the potential applications in such an emerging field. Herein, a highly elastic conductive hydrogel that integrated mechanical robustness, selfadhesiveness, UV-filtering, and stable electrical performance was achieved by the synergistic effect of sulfonated lignin-coated silica nanoparticles (LSNs), polyacrylamide (PAM) chains, and ferric ions (Fe 3+ ). In detail, the dynamic redox reaction was constructed between the catechol groups of LSNs and Fe 3+ , which could promote the rapid gelation of the acrylamide (AM) monomers in 60 s. The optimized conductive hydrogels containing 1.5 wt % LSNs as the dynamic junction points exhibited the excellent elasticity (<15% hysteresis ratio), high stretchability (∼1100% elongation), and improved mechanical robustness (tensile and compressive strength of ∼180 kPa and ∼480 kPa). Notably, the abundant catechol groups of LSNs endowed the conductive hydrogels with the long-lasting and robust self-adhesion, enabling seamless adhesion to the human skin. Meanwhile, the catechol groups also provided an exceptional UV-blocking capability (∼95.1%) for the conductive hydrogels. The combined advantages of the conductive hydrogels were manifested in flexible sensors for the high-fidelity detection of various mechanical deformations over a wide range of strain (10−200%) with good repeatability and stability. We believed that the designed conductive hydrogels may become a promising candidate material in future flexible wearable electronics for long-term and stable human movements monitoring.

Ionic hydrogels hold substantial promise as soft materials for achieving versatile wearable ionotronics due to the integrated merits of appropriate mechanical properties, excellent conductivity, and good conformability. However, overcoming freezing at subzero temperatures and hindering the evaporation of water are still huge challenges for ionic hydrogels. Herein, a dual-cross-linked ionohydrogel was designed using Al3+ to cross-link with the polymer network through dynamic metal coordination bonds in the water and ionic liquid (IL) binary solvent system, allowing for excellent mechanical properties (∼1 MPa, ∼600%), transparency (>90%), high ionic conductivity (∼12.40 mS cm–1), and robust adhesion, along with the advantages of superior antifreezing and long-term antidehydration properties. These exceptional characteristics inspired us to fabricate dual-responsive sensors, which could simultaneously detect human motion signals and a wide range change of temperatures (from −30 to 40 °C) with an impressive temperature coefficient of resistance (TCR) value (from −0.035 to −0.44 °C–1). More promisingly, benefiting from the superior interfacial adhesion between the poly­(dimethylsiloxane) (PDMS) and the ionohydrogels, a triboelectric nanogenerator was assembled with a single-electrode mode that was capable of providing sustainable energy for wearable ionotronic devices even at subzero temperatures. This work opens up an effective strategy to design a multifunctional ionohydrogel, enabling various applications integrated into the single device.