Triple-cross-linked composite hydrogels based on EGaIn liquid metal with controllable degradability for flexible strain sensors

Conductive hydrogels with intrinsic flexibility, high sensitivity, antibacterial activity, and repeatability are used in wearable devices. The development of conductive hydrogels with simultaneously high conductivity, reliability, and controllable degradability remains a challenge. Therefore, conductive composite hydrogel polyvinyl alcohol (PVA)-stabilized eutectic gallium and indium (EGaIn)/polyacrylamide (PAAm)/polyacrylic acid (PAA)@FeCl3@polyprrole (PPy) (PVA-EGaIn-x@PAAm/PAA@FeCl3@PPy, where x represents EGaIn content) was employed to fabricate a multi-cross-linked network and interpenetrating hydrogel with high conductivity, mechanical properties, and controllable degradability. Ga3+/Fe3+–carboxyl group coordination bonds can act as a dynamic cross-linking network to promote controllable degradability. Meanwhile, EGaIn liquid metal and PPy, as electronic conductors, conferred the composite hydrogel with good conductivity. The effects of EGaIn liquid metal content on mechanical properties and conductivity were investigated in detail. A simple strain sensor fabricated by using PVA-EGaIn-5@PAAm/PAA@FeCl3@PPy achieved the detection of the current signal of human motions. The conductive composite hydrogel had good conductivity (8.1 S m−1), acceptable sensitivity (gauge factor of 0.28), and reliability (500 cycles at 200% strain). Furthermore, the prepared composite can be degraded in acid and base solutions for environmental preservation. This work provides a feasible method for the construction of multifunctional robust hydrogel strain sensors to facilitate the practical development of sensors for monitoring human motions.