Highly stretchable polyacrylamide/silk fibroin double network self-adhesive hydrogel for flexible wearable sensors

Silk fibroin (SF) is a natural biopolymer that is highly suitable for preparing biocompatible composite hydrogel strain sensors due to its excellent biocompatibility, amphiphilicity, and biodegradability. However, the excessive crystallization of β-sheet structures in SF limits the tensile and adhesive properties of pure SF hydrogels, severely restricting their application in hydrogel sensors. This study addresses this problem by introducing polyacrylamide (PAM) into the SF hydrogel system to construct a double-network structure. The flexible PAM chains regulate the β-sheet crystallization in SF through intermolecular forces, simultaneously altering the network morphology of the PAM/SF hydrogel, thereby improving its tensile and adhesive properties. Furthermore, this study introduces tannic acid-modified reduced graphene oxide (TA-rGO) into the PAM/SF double-network hydrogel system. This not only imparts conductivity to the hydrogel but also further enhances its adhesion. After in-situ reduction, some oxygen-containing functional groups in graphene oxide (GO) are retained, thus maintaining the integrity of the hydrogel structure. By optimizing the ratio of SF and GO, the PTSG-4 hydrogel with excellent mechanical and adhesive properties was obtained. This hydrogel exhibits a maximum tensile stress of 221.65 kPa, a fracture elongation of 1467.36%, and an adhesion strength of 32.48 kPa on pigskin. The strain sensor assembled based on this hydrogel demonstrates excellent stability (500 cycles at 60% strain), high sensitivity (GF = 9.43), and a wide measurement range (1350%). It can also self-adhere to various parts of the human body, enabling real-time detection of human movement, providing an innovative method for the development of next-generation flexible electronic devices.