Decoupled Attraction and Repulsion for Fast, Large, and Reversible Thermal Actuation in Hydrogels

In thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels, strategies that accelerate heating-induced shrinking often slow down subsequent re-swelling upon cooling, because the driving forces favorable for shrinking and those favorable for re-swelling are fundamentally opposed within a single homogeneous network. This kinetic asymmetry limits the reversibility and cyclability of thermoresponsive actuators. To resolve this trade-off, we propose decoupled attraction and repulsion for thermal actuation (DART), in which the shrinking and re-swelling driving forces are assigned to separate structural components of a hydrogel rather than sharing a single network. Water-swollen, negatively charged poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) nanogels, produced by planetary milling of bulk PAMPS hydrogels, were dispersed throughout a phase-separated PNIPAAm matrix to construct a composite hydrogel (PSG-Pn). The porous, phase-separated matrix enables rapid heating-induced water expulsion, while thermally modulated electrostatic repulsion among the PAMPS nanogels, governed by the temperature-dependent dielectric constant and Debye screening length, drives fast re-swelling upon cooling without impeding the shrinking process. PSG-Pn exhibited shrinking and re-swelling relaxation times of 2.24 and 219 s, respectively, approximately 1250-fold and 14-fold faster than those of a conventional non-phase-separated PNIPAAm hydrogel. PSG-Pn sustained ∼90% volume recovery over 20 heating-cooling cycles in water, and it also maintained repeated shrinking and re-swelling under open-air conditions without drying or structural failure. Building on this response, a near-infrared-responsive bilayer combining PSG-Pn with graphene oxide was steered to crawl in either direction across an unpatterned flat surface simply by switching the irradiation site, without any built-in ratchet or friction asymmetry, a behavior further supported by finite-element analysis of the localized contraction-recovery cycle. Together, these results indicate that partitioning attractive and repulsive driving forces into distinct structural elements offers a broadly applicable strategy for engineering thermoresponsive soft actuators with fast, large-amplitude, and reversible motion.