Adhesive hydrogels tailored with cellulose nanofibers and ferric ions for highly sensitive strain sensors

Conductive hydrogels are regarded as one of the promising synthetic materials for emerging applications such as soft robotics, bioelectronics, and wearable devices. Herein, poly (N- [tris (hydroxymethyl) methyl] acrylamide-co-acrylamide) (P (THAM/AM)) adhesive hydrogels were tailored by physically cross-linking with cellulose nanofiber (CNF). Compared with chemical crosslinker (N,N'-methylene bisacrylamide), CNF complexing with multiple hydroxyl groups on P (THAM/AM) chains led to the formation of numerous hydrogen bonds, endowing the hydrogel with additional energy dissipation and resulting in a better strength reinforcement for adhesive hydrogels. Furthermore, we introduced ferric ions (Fe3+) into the hydrogel to make the hydrogel conductive for producing a strain sensor with desired adhesion, high sensitivity, fast response time, and superior durability. The fabricated hydrogel strain sensor attached well to human skin and was able to translate both large and subtle human motions into relative resistance changes, showing its promising applications in wearable electronics for human motion detection.