Robust Fabrication of Anisotropic Bilayer Hydrogel Embedded With a Gradient Structure in One Layer: Enhanced Temperature-Responsive Bending, Shape Programmability, and Actuator and Sensor Applications

Anisotropic hydrogel actuators with gradient structures offer tunable mechanical properties, like directional stiffness or bending. However, creating these gradient-structured bilayer hydrogels is challenging, as current methods rely on complex, single-force programming. Developing a double-actuating bilayer hydrogel with temperature-responsive and auxiliary layers could address these limitations and enhance their applicability. In this study, an anisotropic hydrogel actuator was developed using a simple, cost-effective method to create a unique multi-asymmetric structure with an embedded gradient in one layer. The resulting hydrogels exhibited excellent temperature-responsive bending (360° within 9 s) and adaptive, complex shape-programmable deformation (2D letters, flowers, butterflies, leaves). In addition, the hydrogels demonstrated good shape memory, mechanical strength, and conductivity. Prototypes of a hydrogel gripper and humidity alarm were successfully fabricated, showcasing the potential of this strategy for designing smart hydrogels for applications in sensors, smart humidity alarms, grippers, and actuators.