Synchronous Ultraviolet Polymerization Strategy to Improve the Interfacial Toughness of Bilayer Hydrogel Actuators

Bilayer hydrogel actuators are of great interest in mechanical valves, soft robots, and bionic devices benefiting from their flexibility and adaptability to actuate in different environments. They respond rapidly to external stimuli through differential deformation of the internal structure to achieve the actuation effect. However, the bilayer hydrogel is prone to delamination due to the low interfacial toughness of the two gel layers, thus they exhibit poor actuating performances. In this work, a synchronous ultraviolet (UV) polymerization strategy was proposed to enhance the interfacial toughness of bilayer hydrogel actuators. Based on the synchronous UV polymerization strategy, a gelatin/poly(N-hydroxyethyl acrylamide)–poly(N-isopropyl acrylamide-co-N-hydroxyethyl acrylamide) [gelatin/PHEAA–P(NIPAM-co-HEAA)] bilayer hydrogel actuator with gelatin/PHEAA functional layer and P(NIPAM-co-HEAA) actuating layer was prepared. The obtained bilayer hydrogel showed a maximum interfacial toughness of 508.11 ± 45.62 J/m2, which was attributed to the covalent bonding and topological entanglement of polymer chains at the gel–gel interface induced by the permeation–polymerization step. In addition, the copolymerization of NIPAM with the hydrophilic monomer N-hydroxyethyl acrylamide (HEAA) increased the lower critical solution temperature of the bilayer hydrogel actuator, which allowed the actuator to exhibit stable actuating ability at 90 °C and to be used as a bionic gripper for high-temperature pickup. Overall, a synchronous UV polymerization strategy was presented. It simplified the fabrication of bilayer hydrogel actuators and enhanced the interaction between bilayer hydrogels by forming strong covalent bonding and local topological entanglement structure at the hydrogel interface, which provided a new idea for preparing bilayer hydrogel actuators with high interfacial toughness.