Gradient Assembly of Polymer Nanospheres and Graphene Oxide Sheets for Dual-Responsive Soft Actuators

Bimorph actuators hold great promise for developing soft robots. However, poor interlayer adhesion between different materials always threatens their stability for long-term usage. In this paper, instead of using a bilayer structure, we reported the gradient assembly of graphene oxide (GO) sheets and polymer nanospheres for developing robust moisture and light dual-responsive actuators. The distribution gradient of poly(methyl methacrylate) (PMMA) nanospheres along the normal direction of a GO paper leads to an asymmetric structure. The front side that mainly consists of GO is quite sensitive to water molecules, which swells upon exposure to moisture, whereas the back side that is rich in PMMA nanospheres expands obviously due to the photothermal effect. The distinct properties of the two sides endow the composite paper with moisture and light dual-responsiveness. Moreover, since GO has been used as a host material, the composite paper shows a moisture-triggered self-healing property, which permits front-to-front and front-to-back healing. The self-healed paper can maintain similar responsive property and reasonable mechanical strength to the pristine one. As a proof of concept, a dual-responsive gripper actuator and a scorpion robot have been fabricated for light and moisture cooperative manipulation. The gradient assembly strategy may open up a new way for developing robust multiresponsive actuators beyond bilayer structures.