A photo-mechanically inactive tough gel exhibits multimodal, light-guided underwater navigation.

Soft robots exhibit flexural and active features that enabled their potential deployment in applications where rigid systems could not perform well. However, the locomotion of the soft robots still largely relies on the paddling that requires variety of active materials as artificial muscles. Hence, extreme conditions, i.e., under compressive or tensile loads, can cause irreversible damages that disable the actuation. Here, we report a tough underwater robot that exhibits an accurate, three-dimensional, and multimodal motion. Counterintuitively, the soft robot is composed of polymers that are not photo-mechanically responsive but are powered by the ambient fluid. The unique feature notably widens the pool of material selection and allows further treatment to strengthen the polymer matrix without concerns of trade-off between the mechanical properties with the actuation capability. The tough, photo-inactive gel maneuvers precisely, traveling effortlessly in and out of the tunnels, and enables versatile phototactic locomotion.