Many living organisms track light sources and adaptively stop their movement when the tracking is achieved. This phenomenon, known as phototropism , occurs as plants self-orient to face the sun perpendicularly throughout the day. While many smart materials exhibit nastic behaviors in response to external stimuli, no synthetic material can intrinsically detect and accurately track the direction of stimuli, i.e., exhibit tropistic behaviors. Other than the phototropism in dynamic equilibrium state, oscillations are also widely found in living organisms to generate propulsion-based locomotion often driven by constant ambient conditions, such as phototactic movements   . However, most man-made oscillations require non-constant energy input and cannot perform environment-dictated movement. Here we report an artificial phototropic and an artificial phototactic system by using the same light-sensitive hydrogel system, respectively, termed sunflower-like biomimetic omnidirectional tracker (SunBOT) and all-soft swimming robot (OsciBOT). The SunBOT can instantaneously aim to incident light in three-dimensional space over broad ambient temperatures. Phototropism successfully enabled plant-like maximization of energy harvesting through SunBOT-based omnidirectional solar vapor generation, achieving up to 400% energy harvesting enhancement over non-tropistic materials by maintaining normal to oblique illuminations. The artificial phototropic system would open new avenues for intelligent energy generations, solar sails, smart windows for energy-efficient architecture, and self-reconfigurable optics, as well as energetic emission detection and tracking with telescope, radar, and hydrophones. OsciBOT showcases agile life-like omnidirectional oscillatory motion in entire 3D space with near-infinite degrees of freedom. The large force generated by the powerful and long-lasting oscillation of OsciBOT can effectively self-propel away from light source, showing high-speed and controllable phototactic locomotion. This autonomous robot features battery-free, deployable, scalable, and integratable. The artificial phototaxis opens broad opportunities in maneuverable marine automated systems, miniaturized transportation, and solar sails.

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Artificial Phototropism and Artificial Phototaxis based on Light-sensitive Hydrogels

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