Bidirectional Hydrogel Actuators with Tunable Bending under a Single Thermal Field

Abstract

Hydrogel actuators are intelligent soft materials capable of undergoing deformation in response to external stimuli, offering significant potential for applications in soft robotics and biomedicine. However, current systems are constrained by inherent limitations: single-stimulus-responsive actuators lack programmability and functional complexity, whereas multiresponsive designs are often hindered by material incompatibility and intricate fabrication processes. To address these challenges, we propose a bilayer hydrogel actuator comprising dual-active layers, a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM)/polysulfated betaine methacrylate (PSBMA) layer for contractile deformation, and a photothermal layer incorporating polydopamine nanoparticles for localized heating, thereby enabling bidirectional bending under a single thermal stimulus. Synergistic electrostatic interaction within the thermoresponsive layer notably enhances the contraction efficiency, with the T layer shrinking to 58.5% of its original length at 50 °C and the compressive modulus increasing by over 600%. Meanwhile, near-infrared-induced photothermal actuation drives the P layer to contract to ∼72.3% of its initial length, achieving reverse bending. By optimization of the thickness ratio between layers, the T2–P1 actuator demonstrates rapid response, large-amplitude bending (>180°), and controllable bidirectional deformation. This dual-layer system can be constructed solely through sequential radical polymerization of two pregel solutions, eliminating the need for additional chemical modification steps while retaining multifunctional driving properties. This work presents a simplified yet versatile strategy for realizing complex actuation behaviors, advancing the development of smart valves, soft grippers, and other multifunctional soft robotic systems without reliance on multistimulus architectures.