Spatiotemporal control of hydrogel actuators by autocatalytic reaction networks

Regulating hydrogel actuators using chemical reaction networks is essential for the creation of bio-inspired smart materials. In this study, we have engineered hydrogel actuators controlled by thiol-based autocatalytic reaction networks. We developed two types of actuators. The first type of actuator uses the principle of a loaded spring. It comprises two distinct layers. The first layer consists of regular polyacrylamide hydrogel in a strained conformation. The second layer is composed of polyacrylamide hydrogel with disulfide crosslinks, which helps maintain the strain in the first layer. When thiols released by the autocatalytic front reduce the disulfide crosslinks, the hydrogel actuates by releasing the mechanical strain from the first layer (Figure 1). The second type of actuator uses a swelling-deswelling mechanism to achieve actuation. It also consists of two layers: one with disulfide crosslinks and another one without. However, the disulfide crosslinker is more responsive to thiols, and no initial strain is introduced in the system. Because of the high responsiveness of the crosslinker, its reduction by thiols causes a sufficient change in the degree of swelling to induce movements in the bilayer system.
Regulation of actuators relies on a complex reaction network that utilizes thiouronium salts, disulfides of β-aminothiols, and maleimide as starting components. The gradual actuation by the autocatalytic front enables a variety of movements, including gradual unrolling, screwing, and sequential closing of "fingers." Moreover, this actuation allows for the transmission of chemical signals in a relay fashion and the conversion of a chemical signal into an electrical signal. Additionally, the locations and times of spontaneous initiation of autocatalytic fronts can be preprogrammed by strategically distributing the reactants within the hydrogel. To approach the functionality found in living organisms, these actuators triggered by autocatalytic fronts should be integrated into smart materials governed by chemical circuits.