Materials Science

Untethered unidirectionally crawling gels driven by an asymmetry in contact forces

Authors

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.

Content

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Supplementary material

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Supplementary Information
Supplementary Methods, simulation details and supplementary figures

Supplementary weblinks

Supplementary Videos
Movie S1. Experimental video of the crawling of the two-segment robot with a stiff linker over three thermal cycles. Movie S2. Simulation of the two-segment robot crawling with a flexible linker over one thermal cycle. Movie S3. Simulation of the two-segment robot crawling with a flexible linker and interchanged bilayers over one thermal cycle. Movie S4. Experimental video of the crawling of the three-segment robot with an alternating linker over three thermal cycles. Movie S5. Experimental video of the crawling of the three-segment robot with a flexible linker over three thermal cycles. Movie S6. Experimental video of the crawling of the four-segment robot with an alternating linker over three thermal cycles.