Programmable Folding Kinetics in Single-Composition Hydrogel Actuators via UV-Induced Microphase Separation

Abstract

Hydrogel-based self-folding actuator systems offer significant potential for soft robotics due to their softness, high strain rates, and multi-stimuli responsiveness. However, the lack of precise control over the swelling kinetics of hydrogels remains a challenge for constructing sophisticated actuation systems. Herein, we demonstrate reversible actuators with programmable folding kinetics by modulating the degree of microphase separation in hydrogels that possess an identical chemical composition—comprising thermo-responsive poly(N-isopropylacrylamide) (PNIPAM), N,N′-methylenebisacrylamide crosslinker, and nanoclay (NC) rheological modifier. We utilized extrusion-based 3D printing to align the NC, while microphase-separated structures were formed by inducing nucleation and growth of NC through the regulation of the PNIPAM polymerization rate. NC serves as a physical crosslinker within the hydrogel matrix, its association and dispersity directly influence water diffusivity and polymer chain mobility. Therefore, a hydrogel with fast swelling and deswelling kinetics can be fabricated by inducing a microphase-separated structure. The programmed folding kinetics are achieved by integrating hydrogels with distinct microphase separation degrees into hinge-based soft actuators with a bilayer structure. Specifically, soft actuators incorporating highly phase-separated hydrogels exhibited deswelling and swelling kinetics that are 1.6 and 4.4 times faster, respectively, than those of non-phase-separated ones. Leveraging such different kinetics profiles, a sequentially foldable multi-hinge strip and a gripper from a single-composition hydrogel system are successfully realized.