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

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.