A transparent, self-adhesive and fully recyclable conductive PVA based hydrogel for wearable strain sensor

Hydrogel-based wearable sensors have attracted tremendous attention in the sensing electronics field due to their unique properties. However, integrating hydrogel sensors with transparency, biosafety, self-adhesiveness, recyclability and conductivity remains a great challenge. Here, we report an ionic conductive hydrogel prepared using poly (vinyl alcohol) (PVA) grafted with the 2,3-epoxypropyltrimethylammonium chloride (TMAC) through a facile freezing-thawing (F-T) process. The obtained PVA grafted with TMAC hydrogel (PVATH) exhibited relatively high transparency (similar to 87% transmittance) and strong self-adhesion to different substrates with high stretchability (544%) and good conductivity (0.038 S m(-1)). Most notably, the hydrogel can be fully recycled by a simple and green process on account of a fully physical cross-linking network. Even after three recycling cycles, the hydrogel sensor exhibited almost identical electromechanical performance with respect to the original sensor. The hydrogel was further assembled as a strain sensor for monitoring real-time human motions, including finger, wrist, elbow, knee joint and neck movement. This work provides an innovative design strategy to fabricate ionic conductive hydrogels and reduce electronic waste, which will broaden the applications of hydrogels in wearable devices.