(318c) Designing a Self-Propelled Microswimmer Made of a Responsive Hydrogel

We use computer simulations to design a new self-propelling hydrogel microswimmer. The microswimmer features a simple design and represents an X-shaped gel layer. The gel has two distinct layers: one layer is responsive and expands in response to an external stimulus, whereas the other layer is passive. When an external stimulus is applied periodically, we find that the swimmer undergoes periodic shape changes that lead to its unidirectional propulsion in a highly viscous fluid. Specifically, when the stimulus is applied, the responsive layer swells, inducing internal stresses in the gel causing both lateral expansion and bending. When the external stimulus is removed, the responsive layer contracts, and elastic forces cause the microswimmer to straighten and to recover its initial shape. This combination of sequential expansion, bending, contraction, and straightening yields a time irreversible motion pattern, generating net propulsion at low Reynolds number. Moreover, we find that the swimming speed depends on the geometry and material properties. Specifically, we find that swimming velocity is maximized at an optimal ratio between the thickness of the passive and active layers when all other parameters are held constant.