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.

中文翻译：

---

接触力不对称驱动的无束缚单向爬行凝胶

可逆热响应水凝胶在 30° 至 60°C 的温度范围内膨胀和收缩（消溶胀），为在人体生理和环境条件下操作不受束缚的软体机器人提供了一种有吸引力的材料类别。之前已经用温敏水凝胶证明了爬行，但需要图案化或约束凝胶或基底来打破单向运动的对称性。在这里，我们展示了一种单向爬行凝胶的运动机制，该凝胶在分段活性热响应聚（否-异丙基丙烯酰胺) (pNIPAM) 和带有悬浮接头的被动聚丙烯酰胺 (pAAM) 双层。驱动研究表明，这些凝胶履带在平坦、无图案的基板上跨多个热循环进行一致的单向运动。我们使用有限元模拟并通过改变实验参数（例如链接器刚度和双层片段的数量）来解释该机制。我们使用图像分析和有限元模型阐明设计标准并验证实验。我们预计这种机制可能会应用于其他变形运动。