Directional Natural Rubber Latex–Polyacrylamide/Graphene–PNIPAM Bi-Hydrogel for Programmable Complex Stimuli-Responsive Actuations

Intelligent hydrogels can provide various biomimetic actuations in response to external stimuli, but it is still difficult to program their actuating complexity, which severely limits their further applications. Herein, a stimuli-responsive actuating bi-hydrogel with a directional negative layer and a homogeneous positive layer has been explored. The stretched natural rubber latex–polyacrylamide (NRL-PAAm) hydrogel layer can partly maintain this directional structure, which can further integrate with the photothermal-responsive graphene-poly(N-isopropylacrylamide) (G-PNIPAM) hydrogel layer to obtain the anisotropic bilayer hydrogel. First of all, different from bilayer PNIPAM-based hydrogels, which commonly only achieve two-dimensional (2D) complex actuations, this bi-hydrogel can achieve various not only 2D but also three-dimensional (3D) complex programmable actuations, especially because of its double anisotropy of both bilayer and directional structures. Furthermore, this bi-hydrogel can own a rapid actuating speed reaching 27.7°/s of bending and 18.0°/s of folding in response to non-contact near-infrared (NIR) irradiation, mainly owing to the high photothermal conversion efficiency of composited graphene. Last but not least, thanks to the highly enhanced mechanical performance of the directional NRL-PAAm hydrogel layer, this bi-hydrogel is robust, reaching 4.0 times the tensile strength of pure G-PNIPAM, which owns a relatively powerful force to lift more than 50 times its self-weight. This work provides a promising intelligent hydrogel integrating programmable 2D/3D complex actuation, rapid actuating speed, and powerful force, which will also inspire exploration of other soft intelligent materials.