Highly Adhesive,\nStretchable, and Antifreezing Hydrogel\nwith Excellent Mechanical Properties for Sensitive Motion Sensors\nand Temperature-/Humidity-Driven Actuators

Abstract

Conductive hydrogels as flexible wearable devices have\nattracted\nconsiderable attention due to their mechanical flexibility and intelligent\nsensing. How to endow more and better performance, such as high self-adhesion,\nstretchability, and wide application temperature range for traditional\nhydrogels and flexible sensors is a challenge. Herein, a stretchable,\nself-adhesive, and antifreezing conductive hydrogel with multiple\nnetworks and excellent mechanical properties was prepared by a two-step\nmethod for its application in sensitive motion sensors and temperature-/humidity-driven\nactuators. First, quaternary chitosan (QCS) was introduced into the\nnetwork of an acrylamide (AM) and 1-vinyl imidazole (VI) copolymer\ninitiated by UV-photoinitiated radical polymerization. Then, the double-network\nhydrogel was immersed in a FeCl₃ solution to fabricate\nthe P­(AAm-<i>co</i>-VI)/QCS-Fe³⁺ ionic hydrogel\nwith multiple physical networks. The properties of the hydrogel were\ncontrollable and adjustable. The toughness of the ionic hydrogel could\nreach up to 654.4 kJ/m³, the fracture strength could reach\n253.1 kPa, and the compressive strength reached 8.4 MPa at an 80%\ncompression strain. The multiple physical networks improved the mechanical\nproperties and the quick resilience of the hydrogel. A large amount\nof FeCl₃ in the network greatly enhanced the ionic conductivity.\nMeanwhile, hydrogen bonds with water molecules inhibit the formation\nof ice crystals between zero water molecules and enhance the freezing\nresistance of P­(Aam-<i>co</i>-VI)/QCS hydrogels. The active\ngroup on the QCS chain provided adhesiveness to various substrates\nfor hydrogels. The P­(AAm-<i>co</i>-VI)/QCS-Fe³⁺ hydrogel-based sensor showed high sensitivity, which can detect\nhuman movement and pulse, with a gauge factor of 2.37. Finally, due\nto the different dehydration rates of the P­(AAm-<i>co</i>-VI)/QCS-Fe³⁺ and P­(AAm-<i>co</i>-VI)/QCS hydrogel,\na double-layer temperature/humidity-driven actuator was fabricated,\nexpanding the application of conductive hydrogels.