Self-Regulated Hydrogel Actuators - Tuning Actuation with a Pinch of Salt

Self-regulation is an essential property for maintaining the vitality of living organisms. Helping the organisms respond to internal and external changes, self-regulation is also the basis of the autonomy of all living systems. The complex biochemical feedback for self-regulation is hard to replicate directly in artificial systems. However, it was previously shown that artificial self-regulation can also be achieved in material systems by designing proper combinations of the systems chemical and physical features. Previously, we constructed electronics-free artificial systems mimicking heliotropism (sun tracking) and nyctinasty (leaf opening) in plants. In these systems, dehydration/rehydration cycles were employed to cause the actuation and feedback. In this work, we use some common salts doped in the hydrogels to tune the bending behavior of hydrogel actuators. The bending rate of the ‘salty’ actuators parallels the physical-chemical differences of the ions in the salt. With the actuator parts made up of essentially the same hydrogel, including different salts, responding to the same input at different rates, the programmability of an overall hydrogel material for more complex motion can be achieved. The straightforwardly achievable, programmable, and tunable ‘embodied intelligence’ in autonomous hydrogel materials can be used in soft robotics, soft sensors, and environmental monitoring.