Highly Adhesive, Stretchable, and Antifreezing Hydrogel with Excellent Mechanical Properties for Sensitive Motion Sensors and Temperature-/Humidity-Driven Actuators

Abstract

Conductive hydrogels as flexible wearable devices have attracted considerable attention due to their mechanical flexibility and intelligent sensing. How to endow more and better performance, such as high self-adhesion, stretchability, and wide application temperature range for traditional hydrogels and flexible sensors is a challenge. Herein, a stretchable, self-adhesive, and antifreezing conductive hydrogel with multiple networks and excellent mechanical properties was prepared by a two-step method for its application in sensitive motion sensors and temperature-/humidity-driven actuators. First, quaternary chitosan (QCS) was introduced into the network of an acrylamide (AM) and 1-vinyl imidazole (VI) copolymer initiated by UV-photoinitiated radical polymerization. Then, the double-network hydrogel was immersed in a FeCl₃ solution to fabricate the P(AAm-co-VI)/QCS-Fe³⁺ ionic hydrogel with multiple physical networks. The properties of the hydrogel were controllable and adjustable. The toughness of the ionic hydrogel could reach up to 654.4 kJ/m³, the fracture strength could reach 253.1 kPa, and the compressive strength reached 8.4 MPa at an 80% compression strain. The multiple physical networks improved the mechanical properties and the quick resilience of the hydrogel. A large amount of FeCl₃ in the network greatly enhanced the ionic conductivity. Meanwhile, hydrogen bonds with water molecules inhibit the formation of ice crystals between zero water molecules and enhance the freezing resistance of P(Aam-co-VI)/QCS hydrogels. The active group on the QCS chain provided adhesiveness to various substrates for hydrogels. The P(AAm-co-VI)/QCS-Fe³⁺ hydrogel-based sensor showed high sensitivity, which can detect human movement and pulse, with a gauge factor of 2.37. Finally, due to the different dehydration rates of the P(AAm-co-VI)/QCS-Fe³⁺ and P(AAm-co-VI)/QCS hydrogel, a double-layer temperature/humidity-driven actuator was fabricated, expanding the application of conductive hydrogels.