Conductive polymer based hydrogels and their application in wearable sensors: a review

Hydrogels have been attracting increasing attention for application in wearable electronics, due to their intrinsic biomimetic features, highly tunable chemical-physical properties (mechanical, electrical, etc.), and excellent biocompatibility. Among many proposed varieties of hydrogels, conductive polymer-based hydrogels (CPHs) have emerged as a promising candidate for future wearable sensor designs, with capability of realizing desired features using different tuning strategies ranging from molecular design (with a low length scale of 10^-10 m) to a micro-structural configuration (up to a length scale of 10^-2 m). However, considerable challenges remain to be overcome, such as the limited strain sensing range due to the mechanical strength, the signal loss/instability caused by swelling/deswelling, the significant hysteresis of sensing signals, the de-hydration induced malfunctions, and the surface/interfacial failure during manufacturing/processing. This review aims to offer a targeted scan of recent advancements in CPH based wearable sensor technology, from the establishment of dedicated structure-property relationships in the lab to the advanced manufacturing routes for potential scale-up production. The application of CPHs in wearable sensors is also explored, with suggested new research avenues and prospects for CPHs in the future also included.