Highly Bidirectional Actuation of LCST- and UCST-Type Bilayer Hydrogel Actuators with Enhanced Mechanical Properties and Interfacial Toughness

Bilayer hydrogel actuators, with their anisotropic structure akin to biological tissues, are poised to revolutionize the fields of soft robotics and tissue engineering. Despite their promise, achieving a harmonious balance between high bulk toughness, robust interface toughness, and actuating property has been a formidable challenge, significantly hindering their broader application. By employing a dual-strategy approach, we successfully developed a poly(N-isopropylacrylamide-co-acrylamide)-agar/poly(acrylamide-co-acrylic acid) [p(NIPAM-co-AAM)-agar/p(AAM-co-AAC)] (NM–AMC) LCST- and UCST-type double-layer hydrogel actuator. First, the double-network strategy enhanced the mechanical properties of the hydrogel. Second, the synchronous UV polymerization method effectively improved the toughness of the interface between the two layers of hydrogels. The NM–AMC double-layer hydrogel actuator integrates the low-temperature response characteristics of NM (lower critical solution temperature-type) and the high-temperature response characteristics of AMC (upper critical solution temperature-type). This design confers a unique dual-thermal response behavior on the actuator, enabling it to achieve reversible bending angle changes from +361.33 to −417.33° over a temperature range of 5–85 °C. More importantly, after 5 cycles, the NM–AMC bilayer hydrogel actuator did not have obvious delamination, indicating that the NM–AMC bilayer hydrogel shows an excellent interface toughness between the two layers of hydrogels. Inspired by the adaptive mechanisms of plants in nature, a series of NM–AMC biomimetic hydrogel actuators were designed to emulate the morphology of flowers and leaves. This innovation not only substantially expands the actuating range of hydrogel actuators but also offers new insights into the intelligent design and manufacture of responsive hydrogel systems, thereby advancing the development of biomimetic hydrogel technology.