Untethered unidirectionally crawling gels driven by an asymmetry in contact forces

The development of untethered soft robots capable of locomotion in response to environmental stimuli is important for biomimetics, drug delivery, and non-minimally invasive surgery. Reversible thermoresponsive hydrogels, which swell and shrink in the temperature range of (30-60 °C), provide an attractive material class for operating such untethered soft robots in human physiological and ambient conditions. Crawling has been demonstrated previously with thermoresponsive hydrogels but needs a patterned or ratcheted surface to break symmetry for unidirectional motion. Here, we demonstrate a new locomotor mechanism for unidirectionally crawling gels driven by spontaneous asymmetries in contact forces during swelling and deswelling of segmented active thermoresponsive poly (N-isopropyl acrylamide) (pNIPAM) and passive polyacrylamide (pAAM) bilayers with suspended linkers. Experiments demonstrate consistent unidirectional movement of hydrogel crawlers across multiple thermal cycles on flat, unpatterned surfaces. We explain the mechanism using finite element simulations and varying experimental parameters such as the number of segments, linker size, and design. We compare and validate experiments, image analysis, and models to elucidate design and engineering principles. We anticipate that this mechanism could be widely applied and adapted to create a variety of shape-changing and smart locomotors.