The mechanics of hydrogel crawlers in confined environment.

We present theoretical and experimental results regarding the development of temperature-sensitive hydrogel particles that can display self-motility in confined channels. Inspired by the motility of living organisms such as larva, the motion of the particle relies on the combination of two key mechanisms. The first, referred to as actuation, is enabled by the cyclic extension and retraction of the particle owing to oscillations of its temperature around the so-called lower critical solution temperature. The second, referred to as symmetry breaking, transforms the isotropic particle actuation into a directed motion owing to the asymmetric friction properties of the channel's surface. The role of particle confinement in these processes is, however, less intuitive and displays an optimal value at which the particle's step size is maximum. These observations are supported by a model that identifies the underlying locomotion mechanisms and predicts the dependency of the particle motion efficiency on the confinement condition, as well as frictional properties of the substrate. Our analysis suggests that the existence of a lubrication layer around the particle hinders its motion at low confinement, while an excessive degree of confinement is detrimental to the particle's overall deformation and, thus, to its locomotion efficiency.