Biomimetic Photothermal Actuator Exhibits Robust Motion and Omnidirectional Phototropism

Abstract

Organisms in the natural world receive signals from the external environment and exhibit continuous propulsion. However, creating artificial systems that can rapidly track energy sources in real-time without human intervention and electronic circuit programming poses a significant challenge. Here, we have developed a soft actuator that mimics the phototropism of plants and possesses autonomous sensing and driving capabilities. The soft actuator is based on a thermo responsive PNIPAAM hydrogel, and its uniform incorporation of light-absorbing agents imparts photothermal response characteristics. Our preparation method involves a combination of ice templating and UV cryo-polymerization techniques, utilizing water as the primary solvent. This process results in the formation of substantial microscopic pores within the PNIPAAM hydrogel, greatly enhancing the rate at which water enters and exits the polymer network. The rapid response and recovery properties of the hydrogel soft actuator allow it to quickly track and align with a stimulus light source. Moreover, it can bend to 90° and undergo continuous oscillations, representing a substantial enhancement in performance compared to conventional composite PNIPAAM hydrogels. Furthermore, the soft actuator exhibits a low dependence on the type of incident light, as it can respond not only to pinpoint laser beams but also to broad-spectrum white light. This intelligent soft actuator holds promising applications in non-contact soft robotics, underwater propulsion systems, and various other fields.