Superhydrophobic Photothermal Paper Based on Ultralong Hydroxyapatite Nanowires for Controllable Light-Driven Self-Propelled Motion

Stimulus-responsive actuators that can respond to the external stimuli and convert external energy into dynamic movement behaviors are highly desired for many applications. Among various driving schemes, the Marangoni propulsion produced by the photothermal effect is promising for yielding stimulus-responsive motion in a contactless and switchable manner. However, challenges still remain, especially for the fluid drag and poor processability. Herein, a flexible, superhydrophobic, and photothermal paper based on ultralong hydroxyapatite nanowires has been developed for controllable light-driven self-propelled motion. The highly stable superhydrophobicity introduces a thin air layer between the floating superhydrophobic photothermal paper and water surface to reduce the fluid drag. In addition, the photothermal effect produces the Marangoni flow that results in the controllable light-driven motion. The motional direction and speed can be controlled by adjusting the position of the irradiated light and light power density. More importantly, because of the high flexibility and excellent processability of the as-prepared superhydrophobic photothermal paper, various light-driven self-propelled actuators with designed morphologies can be prepared through simple curling and cutting to achieve specific movement behaviors. The present work will inspire the development of high-performance paper-based self-propelled actuators for applications in various fields such as microrobots, environment protection, and biomedicine.