Light-Responsive Hydrogels for Soft Robotics: From Multiscale Actuation Mechanisms to Programmable Locomotion and Functional Applications

Light-responsive hydrogels combine large reversible deformation with remote, non-contact, and spatially selective control, providing a versatile material basis for untethered soft robots. This review critically examines recent progress from photothermal conversion, molecular photoswitching, network photochemistry, and interfacial regulation to fabrication, structural design, locomotion, sensing, and biomedical operation. Comparative analysis indicates that robotic performance is governed not by hydrogel responsivity alone, but by the coupling of network strain, heat and solvent transport, programmed anisotropy, body geometry, and environmental interactions. Photothermal systems provide efficient remote activation and broad material compatibility, yet their speed and operating stability remain constrained by thermal dissipation and water migration. Photochemical systems offer finer spatial and wavelength selectivity, but optical penetration, chromophore fatigue, chemical reversibility, and auxiliary reagents remain important limitations. Bilayers and continuous gradients reliably transform differential swelling into bending, whereas anisotropic networks, additive manufacturing, and femtosecond-laser processing enable rapid actuation, addressable joints, and programmable three-dimensional architectures. Across crawling, rolling, jumping, swimming, and multimodal operation, net motion requires spatiotemporal symmetry breaking through friction, self-shadowing, topology, or fluid coupling. The review therefore distinguishes directly deformation-driven robots from hybrid platforms in which light regulates adhesion, charge, reconfiguration, or fluid flow while magnetic or electric fields provide propulsion. Emerging applications include intrinsic strain sensing, cargo manipulation, localized cellular stimulation, and three-dimensional drug screening. Future progress requires standardized performance metrics, improved optical delivery, faster mass transport, reduced thermal exposure, long-term cyclic stability, and biodegradable material systems.