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

Abstract

Conducting polymer hydrogels (CPHs) emerge 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-borax (PB) gel matrix and nanostructured CNF-PPy (cellulose nanofibers-polypyrrole) 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 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 crosslinked complexes with borate ions. The multi-complexation between each component leads to the formation of hierarchical 3D 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. Due to a combined reinforcing and conductive network inside the hydrogel, its maximum storage modulus (~0.1 MPa) and nominal compression stress (~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 (~94%), low density (~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.