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.

Dynamic noncovalent interactions with reversible nature are critical for the integral synthesis of self-healing biological materials. In this work, we developed a simple one-pot strategy to prepare a fully physically cross-linked nanocomposite hydrogel through the formation of the hydrogen bonds and dual metal-carboxylate coordination bonds within supramolecular networks, in which iron ions (Fe) and TEMPO oxidized cellulose nanofibrils (CNFs) acted as cross-linkers and led to the improved mechanical strength, toughness, time-dependent self-recovery capability and self-healing property. The spectroscopic analysis and rheological measurements corroborated the existence of hydrogen bonds and dual coordination bonds. The mechanical tests and microscopic morphology were explored to elucidate the recovery properties and toughening mechanisms. The hydrogen bonds tend to preferentially break prior to the coordination bonds associated complexes that act as skeleton to maintain primary structure integrity, and the survived coordination bonds with dynamic feature also serve as sacrificial bonds to dissipate another amount of energy after the rupture of hydrogen bonds, which collectively maximize the contribution of sacrificial bonds to energy dissipation while affording elasticity. Additionally, the multiple noncovalent interactions in diverse types synergistically serve as dynamic but highly stable associations, leading to the effective self-healing efficiency over 90% after damage. We expect that this facile strategy of incorporating the biocompatible and biodegradable CNFs as building blocks may enrich the avenue in exploration of dynamic and tunable cellulosic hydrogels to expand their potential applications in the biomedical field.

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.

Self-healing hydrogels are particularly desirable for increased safety and functional lifetimes because of stress-induced deformation and propagation of cracks. In this paper, we report a tough, highly resilient, fast self-recoverable, and self-healing nanocomposite hydrogel, which builds an interpenetrated network encapsulating rod-like cellulose nanocrystals (CNCs) by flexible polymer chains of poly­(ethylene glycol) (PEG). A thermally reversible covalent Diels–Alder click reaction between furyl-modified CNCs and maleimide-end-functionalized PEG was confirmed by Fourier transform infrared spectroscopy. Uniaxial tensile tests and unconfined compression tests displayed outstanding mechanical properties of the hydrogels with a high fracture elongation up to 690% and a fracture strength up to 0.3 MPa at a strain of 90%. Cyclic loading–unloading tests showed excellent self-recovery and antifatigue properties of the nanocomposite hydrogels. The self-healing capability of nanocomposite hydrogels assessed by tension tests was found to be as high as 78%. The self-healing CNC-PEG nanocomposite hydrogels would shed insight into designing reusable and renewable polymeric hydrogels.

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.