Nanocellulose-Mediated Electroconductive Self-Healing Hydrogels with High Strength, Plasticity, Viscoelasticity, Stretchability, and Biocompatibility toward Multifunctional Applications

Conducting polymer hydrogels (CPHs) have emerged as a fascinating class of smart soft matters important for various advanced applications. However, achieving the synergistic characteristics of conductivity, self-healing ability, biocompatibility, viscoelasticity, and high mechanical performance still remains a critical challenge. Here, we develop for the first time a type of multifunctional hybrid CPHs based on a viscoelastic polyvinyl alcohol (PVA)-borax (PB) gel matrix and nanostructured CNFs-PPy (cellulose nanofiberspolypyrrole) complexes that synergizes the biotemplate role of CNFs and the conductive nature of PPy. The CNF-PPy complexes are synthesized through in situ oxidative polymerization of pyrrole on the surface of CNF templates, which are further well-dispersed into the PB matrix to synthesize homogeneous CNF-PPy/ PB hybrid hydrogels. The CNF-PPy complexes not only tangle with PVA chains though hydrogen bonds, but also form reversibly cross-linked complexes with borate ions. The multi-complexation between each component leads to the formation of a hierarchical three-dimensional network. The CNF-PPy/PB-3 hydrogel prepared by 2.0 wt % of PVA, 0.4 wt % of borax, and CNF-PPy complexes with a mass ratio of 3.75/1 exhibits the highest viscoelasticity and mechanical strength. Because of a combined reinforcing and conductive network inside the hydrogel, its maximum storage modulus (similar to 0.1 MPa) and nominal compression stress (similar to 22 MPa) are 60 and 2240 times higher than those of pure CNF/PB hydrogel, respectively. The CNF-PPy/PB-3 electrode with a conductivity of 3.65 +/- 0.08 S m(-1) has a maximum specific capacitance of 236.9 F g(-1), and its specific capacitance degradation is less than 14% after 1500 cycles. The CNF-PPy/PB hybrid hydrogels also demonstrate attractive characteristics, including high water content (similar to 94%), low density (similar to 1.2 g cm(-3)), excellent biocompatibility, plasticity, pH sensitivity, and rapid self-healing ability without additional external stimuli. Taken together, the combination of such unique properties endows the newly developed CPHs with potential applications in flexible bioelectronics and provides a practical platform to design multifunctional smart soft materials.