Soft Crawling Robots Based on Thin-Film and Gel-Based Actuators

Soft crawling robots offer high compliance, large deformation, and adaptability to complex, unstructured environments. These properties make them promising for miniature inspection systems, biomedical applications, intelligent manufacturing, and operation in extreme environments. Conventional rigid robots rely on complex mechanical structures and articulated joints with multiple degrees of freedom, whereas soft robots achieve actuation and locomotion primarily through the controlled deformation of smart materials. Thin films and gel-based materials (such as hydrogels and ionic gels) are particularly attractive because of their low mass, high flexibility, large strain capacity, and ease of integration. Functionalization, including the incorporation of specific functional groups, enables responses to electrical, thermal, optical, magnetic, and humidity stimuli. These materials are therefore becoming key functional components of next-generation soft crawling robots. This review examines recent advances in soft crawling robots actuated by thin-film and gel materials. The principal smart materials are classified by actuation mechanism into dielectric elastomer actuators (DEAs), liquid crystal elastomers (LCEs), magnetoactive elastomers (MAEs), ionic polymer–metal composites (IPMCs), hydrogels, and shape-memory polymers (SMPs). Their actuation mechanisms, mechanical properties, and application scenarios are analyzed. The material systems are compared in terms of deformation capability, force output, response time, energy requirements, durability, fabrication complexity, robot integration and miniaturization, motion control, environmental adaptability, and functional versatility. The review also identifies major challenges related to material stability, cyclic durability, electrical and thermal requirements, dehydration, external-field dependence, interfacial reliability, fabrication reproducibility, and scalable manufacturing. Advances in multimaterial integration, hybrid actuation, intelligent manufacturing, self-sensing, and closed-loop control are expected to improve system integration, adaptability, autonomy, and multifunctionality.