Autonomous Soft Robotics Using Chemical Fuels

Hydrogel actuators generate mechanical motion through volume changes in response to stimuli as varied as pH, heat, light or magnetic fields. Methods of controlling pH have primarily focused on sequential addition of reagents to alternately raise and lower pH, use of photoacids, or multi-component enzymatic reaction networks. We demonstrate the coupling of a one-molecule dissipative reaction network to a pH-responsive hydrogel actuator, leading to transient motion. Exploiting the rapid proton release upon addition of fuel, followed by spontaneous decarboxylation and proton consumption by the resulting intermediate species, transient pH drops in the timescale of minutes to days can be achieved. Matching reaction cycle timescale to the rate of gel (de)swelling results in an autonomous system where fully reversible contraction of gel actuators and bending of bilayer devices is obtained. Our results illustrate the use of a tunable chemical reaction network to control centimeter-scale hydrogel actuators and exploit their motion to generate mechanical force. Due to its simplicity and cyclability derived from its resistance to waste accumulation, this method of control is likely to prove attractive for a wide variety of pH-responsive hydrogel systems, and can be readily integrated into larger reaction systems and more complex device architectures displaying chemomechanical feedback.