Magnetically controlled multimodal motion for environmentally adaptive soft millirobots with transformable wheel-leg morphology

Small-scale soft robots with high morphological flexibility show significant potential for precise operation and sensing in confined environments. However, due to the coupled driving mechanism and the influence of environmental disturbances, the highly adaptable and stable navigation across diverse terrains through multimodal motion, which involves morphing shape and maintaining the reshaped configuration, still presents a major challenge for soft millirobots. Here, we develop a multi-stimuli-responsive millirobot with a multimodal locomotion adaptive control method, enhancing environmentally synergistic interactions and tasking capabilities. Constructed from materials responsive to temperature, humidity, and magnetic fields, the millirobot precisely navigates unstructured environments and independently controls deformation and locomotion. Theoretical models guide its polymorphic locomotion with optimal actuating parameters, such as bipedal walking in the two-leg mode and rolling in the wheel mode. A hierarchical dual-layer path-following controller manages path information and adjusts movement patterns. Experiments demonstrate the millirobot's environmental adaptability, morphological complementarity, and functional diversity. With various locomotion modes across different morphologies, the millirobot can traverse slopes, curved surfaces, stairs, slits, and gaps. It also performs tasks, such as cargo capture and transport, through morphological transformation. The proposed multimodal motion strategy based on polymorphism makes the soft millirobot a promising candidate for applications in micro-object manipulation and crevice inspection at confined, varied, and unstructured terrains.