Reversible thermoresponsive hydrogels, which swell and shrink (deswell) in the temperature range of 30° to 60°C, provide an attractive material class for operating untethered soft robots in human physiological and ambient conditions. Crawling has been demonstrated previously with thermoresponsive hydrogels but required patterned or constrained gels or substrates to break symmetry for unidirectional motion. Here, we demonstrate a locomotion mechanism for unidirectionally crawling gels driven by spontaneous asymmetries in contact forces during swelling and deswelling of segmented active thermoresponsive poly( N -isopropylacrylamide) (pNIPAM) and passive polyacrylamide (pAAM) bilayers with suspended linkers. Actuation studies demonstrate the consistent unidirectional movement of these gel crawlers across multiple thermal cycles on flat, unpatterned substrates. We explain the mechanism using finite element simulations and by varying experimental parameters such as the linker stiffness and the number of bilayer segments. We elucidate design criteria and validate experiments using image analysis and finite element models. We anticipate that this mechanism could potentially be applied to other shape-changing locomotors.

No abstract

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.

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