Anisotropic actuation in salty agarose gel actuators

Abstract

When hydrogels that can reversibly dehydrate/rehydrate are physically combined with a constant water supply, the cycles can be controlled by on/off states and the positions of an external light source. The shrinking upon dehydration upon illumination causes bending towards the light source, and rehydration in the light-off state restores the initial shape. This simple material feedback mechanism mimics the self-regulating heliotropism (sun tracking) and nyctinasty (leaf opening) movements of plants. In this work, we show the effect of some common salts on the bending behavior of actuators entirely made of hydrogel. The ‘salty’ actuators exhibit different motion kinetics regarding the unique chemical characteristics of each ion. We display that this chemistry of ions also enables us to program the kinetics in a single actuator using the differences in evaporation/diffusion rate of water in the salty gels. This programmability of the motion in a hydrogel actuator with the inclusion of salts can be used to achieve complex behavior observed in living organisms straightforwardly.