All polymeric conductive strain sensors with excellent skin adhesion, recovery, and long-term stability prepared from an anion-zwitterion based hydrogel

Developing a high-performing hydrogel with long-lasting skin adhesion, high ionic conductivity, mechanical stability, and fatigue resistance is a crucial issue in the field of wearable electronic devices. Because of their weak mechanical properties, zwitterion-based hydrogels are not suitable for application in wearable strain sensors despite their excellent adhesion to the skin. In this study, a hydrogel of polymer without additive was prepared by using polymerizable monomers consisting of zwitterionic 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS), anionic 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (AMPSs), and acrylamide (AAm); the hydrogel is abbreviated as P(AMPSs/VIPS-co-AAm). The P(AMPSs/VIPS-co-AAm) hydrogel shows exceptional adhesive strength, reaching up to 26.29 kPa (lap shear to porcine skin) and high stretchability (with a fracture strain of 1282% and stress of 40 kPa). The high polarity of the AMPSs/VIPS pair improves the interfacial adhesion to the skin, the internal cohesion and recovery tendency. Unique structural characteristics of the hydrogel impart excellent fatigue resistance, network toughening, and electrical stability after multiple deformations. Thus, the prepared hydrogel has an ionic conductivity (0.51 S m^-1), strain sensitivity, and long-term skin adhesion, and it demonstrates potential to be applied for wearable strain sensors.