Light-driven Self-sustained Rolling of Cylinder Hydrogels with Fast and Anisotropic Responses

Achieving continuous motions typically requires dynamic external stimuli for cyclic deformation, or crafted geometries with intricate modules to form a self-regulated feedback loop upon static stimulation. It is still a grand challenge to realize self-sustained motion in soft robots subject to unchanging environment, without complex geometry or a control module. In this work, we report soft robots based on an anisotropic cylindrical hydrogel showing self-regulated, continuous rolling motions under constant light irradiation. The robots are animated by mirror-symmetry-breaking induced by photothermal strain gradient. The self-sustained motion is attributed to the fast and reversible deformation of the gel and the autonomous refresh of the irradiated region during the rolling motion. The hydrogel robots can reach a rolling speed of 1.27 mm·s⁻¹ on a horizonal surface and even climb a ramp of 18° at a speed of 0.57 mm·s⁻¹ in an aqueous environment. Furthermore, the hydrogel robots can overcome an obstacle, with rolling direction controllable through irradiation angle of the light and local irradiation on selective regions. This work suggests a facile strategy to develop hydrogel robots and may provide unforeseen inspirations for the design of self-regulated soft robots by using other intelligent materials.