Bioinspired cellulose nanofibrils and NaCl composited polyacrylamide hydrogels with improved toughness, resilience, and strain-sensitive conductivity

Hydrogels are regarded as excellent substrates for flexible materials owing to their porous structure, softness, and flexibility. However, the requirement of enhancement and functionalization for a hydrogel is still an important issue. Cellulose nanomaterials present promising reinforcing potentials for hydrogels thanks to their high strength, good compatibility, and environmental friendliness. Inspired by the composition of the dermis, we use carboxymethyl cellulose nanofibrils (CMCNFs) and NaCl as additives to participate in the crosslinking polymerization of acrylamide (AM), fabricating PAM/CMCNFs/NaCl composite hydrogels with excellent mechanical properties, good resilience, and strain-sensitive conductivity. The interactions between CMCNFs and PAM chains contribute to mechanical improvement of composite hydrogels; 0.5 M NaCl in precursor solutions imparts further synergistic reinforcement and ionic conductivity. The compressive stress, tensile strength and elongation at break values of PAM-based hydrogels are increased with the incorporation of CMCNFs, and the optimal CMCNFs dosage is found at 4 wt% of AM. The resilience of composite hydrogels can be maintained at approximately or over 90% after five compression or stretching cycles. Moreover, the strain-sensitive conductivity of a composite hydrogel demonstrates their potential application in flexible sensors. In brief, this work provides a facile one-step approach to fabricate composite hydrogels suitable for application as flexible sensors.