Multi‐Responsive Bilayer Hydrogel Actuators with Programmable and Precisely Tunable Motions

Abstract

Multi‐responsive hydrogel actuators show promising applications for soft robotics, biomedical engineering, and artificial muscles, but the uncontrollable nature of their motions poses a barrier to practical applications. Herein, a novel type of bilayer hydrogel actuators (BHAs) is presented comprising of a poly(N‐isopropylacrylamide) (PNIPAm) and a poly (N‐hydroxyethyl acrylamide) (PHEAm) hydrogel layer with various compositions, which demonstrate thermal‐responsive and novel solvent‐responsive actuation under water or within solvents. These BHAs exhibit a wide range of regulable bidirectional motions due to the simultaneous co‐nonsolvency property of PNIPAm and the shrinking behavior of PHEAm in ethanol/water mixtures with various ethanol contents. By adjusting the compositions of ethanol/water mixtures, the bending directions and amplitudes of BHAs are precisely regulable and the curvatures of actuators are tunable between −0.34 and 0.3. Because of the temperature‐responsive character of PNIPAm, BHAs fulfilled thermal‐driven motions to lift items. By reinforcing PHEAm layers with cellulose nanocrystals (CNCs) or CNCs bearing methacylamide moieties on the surface, the weight‐lifting capability of BHAs is highly improved to 18 times the weight of their own polymer weights. This design concept with bilayer structures provides a new strategy for the construction of precisely regulable hydrogel actuators, which allows using solvents to exactly control their motions.