Thermoresponsive Deformable Actuators Prepared by Local Electrochemical Reduction of Poly(N-isopropylacrylamide)/Graphene Oxide Hydrogels

Inspired by the shape deformations of plants with inhomogeneous structures, more and more attention has been paid to prepare deformable actuators with inhomogeneous structures and precisely program their shape deformations. Here, we report a simple and novel method to prepare graphene oxide/reduced graphene oxide (GO/RGO) nanocomposite hydrogel-based deformable actuators by local electrochemical reduction of homogeneous poly(N-isopropylacrylamide)/GO (PNIPAM/GO) hydrogels. GO nanosheets in PNIPAM/GO hydrogels can be reduced in an electrolytic cell made of two indium-tin oxide (ITO) glass plates, and local reduction can be realized by pasting patterned insulating stickers onto the cathodal ITO glass plate. The reduction of GO nanosheets leads to the breakage of hydrogen bonding between GO nanosheets and PNIPAM chains and hence lower cross-linking densities of the reduced regions. Therefore, the difference in the swelling/deswelling behaviors between the reduced and unreduced sides (or regions) enables the shape deformations of the PNIPAM/GO hydrogels, and their deformation degrees can be adjusted by changing their composition and thickness as well as the parameters of electrochemical reduction (e.g., applied voltage and reduction time). Very impressively, shape deformations from one-dimensional (1D) to two-dimensional (2D) and 2D to three-dimensional (3D) can be programmed by appropriate designing reduced regions on 1D hydrogel strips or 2D hydrogel sheets. As a representative example, a 3D "scorpion" with uplifted head/tail and an arched belly could deform from a 2D "scorpion" with the reduced head/tail on the same side and the reduced belly on the opposite side.