A new class of single-material, non-reciprocal microactuators

A crucial component in designing soft actuating structures with controllable shape changes is programming internal, mismatching stresses. In this work, we demonstrate a new paradigm for achieving anisotropic dynamics between isotropic end-states --- yielding a non-reciprocal shrinking/swelling response over a full actuation cycle --- in a microscale actuator made of a single material, purely through microscale design. Anisotropic dynamics is achieved by incorporating micro-sized pores into certain segments of the structures; by arranging porous and non-porous segments (specifically, struts) into a two-dimensional hexagonally-shaped microscopic poly(N-isopropyl acrylamide) hydrogel particle, the rate of isotropic shrinking/swelling in the structure is locally modulated, generating global anisotropic, non-reciprocal, dynamics. A simple mathematical model is introduced that reveals the physics that underlies these dynamics. Our design has the potential to be used as a foundational tool for inducing non-reciprocal actuation cycles with a single material structure, and enables new possibilities in producing customized soft actuators and modular anisotropic metamaterials for a range of real-world applications, such as artificial cilia.