Hydrogel-polymer hybrid actuator with soft lattice skeleton for excellent connectivity

Abstract

Soft actuators, which integrate stimuli-responsive hydrogels with flexible polymers, have been extensively studied to develop in vivo soft robots for drug delivery systems and non-invasive treatments. However, conventional bilayer actuators, consisting of hydrogel and polymer layers, pose a challenge: the necessity for chemical modification to connect the two layers imposes limitations on the selection of usable materials. This study presents a hydrogel-polymer hybrid actuator comprising a stimuli-responsive hydrogel layer and a hydrogel-polymer hybrid layer with a three-dimensional (3D) porous polymer “soft lattice skeleton.” The soft lattice skeleton improves connectivity between layers through a mechanical interlock, eliminating the need for chemical modification. A temperature-responsive poly(N-isopropylacrylamide) hydrogel was used as the driving source of the actuator. The soft lattice skeleton was fabricated using multi-directional photolithography. Connectivity improvement was demonstrated by comparing the hybrid actuator to a conventional bilayer actuator after thermal deformation cycles. Specifically, the hybrid actuator maintained connectivity even after multiple cycles, while the bilayer actuator showed complete exfoliation. Deformation behavior was controlled by adjusting the design of the soft lattice skeleton, specifically the width of its constituent micropillars. Curved actuators with various pillar widths exhibited distinct deformation patterns, including bending in the same or opposite direction as the preset curve or twisting. This hybrid actuator design offers improved durability and programmable deformation without chemical modification. It will enable the selection of various hydrogel materials for implementing diverse functions, including pH and glucose sensing. Future research should explore additional structural parameters of the soft lattice skeleton to further control deformation behavior.