A Novel Mechanism for Untethered Crawling Gel Robots

The development of untethered soft robots capable of locomotion in response to environmental stimuli is important for soft robotics, drug delivery, and autonomous smart devices. The absence of tethers allows them to be fabricated and operated en masse with high maneuverability and at small size scales. Of special relevance are devices that can operate in the range of human physiological and ambient temperature range. Reversible thermoresponsive hydrogels that swell and shrink in the temperature range of (30-60 °C) provide an attractive material class for the construction of these untethered robots. Previously, many thermoresponsive hydrogel crawlers have been demonstrated but they require substrate engineering such as the use of patterned or ratcheted surfaces and/or constrained channels or tubes to break the symmetry which limits their applicability.<br/>Here, we demonstrate a novel mechanism to break the symmetry of gel crawlers by harnessing the asymmetry in contact adhesion during asynchronous swelling and deswelling of an asymmetric segmented robot with suspended linkers. We tune the swelling and deswelling kinetics of 3-D printed bilayers consisting of active, thermoresponsive poly(N-isopropyl acrylamide) (pNIPAM) and passive polyacrylamide (pAAM) inks by varying their thickness ratios. By connecting two bilayer segments having different geometric properties and thickness ratios with a suspended linker we generate a curvature and temporal swelling difference between two bilayers. This results in dissimilar anchoring and release of the robot parts on the substrate during thermal cycling resulting in a forward translation during the heating cycle. Actuation studies show the consistent unidirectional movement of our hydrogel crawler across multiple thermal cycles on flat, unpatterned surfaces. We explain the mechanism using finite element simulations and varying experimental parameters like the number of legs and linker size and design. Experiments, image analysis, and models are compared and validated 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.