Tri-state recyclable multifunctional hydrogel for flexible sensors

Recent developments in hydrogel functional materials have created opportunities for building flexible sensors with multiple kinetic properties. Despite this, the research of multifunctional hydrogels for sensor applications is still a challenge. Herein, we present a recyclable multifunctional hydrogel synthesized from polyvinyl alcohol (PVA), hydroxyethyl cellulose (HEC), glycerin, and borax with three different states (named PHG, PHG-F, and PHG-S, respectively). Among them, PHG hydrogels have unique transparency, plasticity, self-healing, injectability, electrical conductivity and thermal sensitivity, and can be used to detect temperature changes with high accuracy and fast response. PHG-F hydrogels are obtained by cryogenic freezing of PHG hydrogels, which have strong mechanical properties and toughness (up to 0.66 MPa) and can provide excellent performance for human motion monitoring applications. PHG-S hydrogels, obtained by swelling of dried PHG hydrogels, have stronger mechanical properties and toughness (up to 1.31 MPa), and high conductivity (up to 2.3 S m(-1)) obtained by immersion in ionic salt solutions, making them very suitable for strain sensing. The tri-state and recyclable multifunctional hydrogels we developed might even prove useful for practical/smart materials, wearable devices, soft machines, and biomedical uses.