The importance of autocatalysis spans from practical applications such as in chemically amplified photoresists, to autocatalysis playing a fundamental role in evolution as well as a plausible key role in the origin of life. The phenomenon of autocatalysis is characterized by its kinetic signature rather than by its mechanistic aspects. The molecules that form autocatalytic systems and the mechanisms underlying autocatalytic reactions are very diverse. This chemical diversity, combined with the strong involvement of chemical kinetics, creates a formidable barrier for entrance to the field. Understanding these challenges, we wrote this Review with three main goals in mind: (i) To provide a basic introduction to the kinetics of autocatalytic systems and its relation to the role of autocatalysis in evolution, (ii) To provide a comprehensive overview, including tables, of synthetic chemical autocatalytic systems, and (iii) To provide an in‐depth analysis of the concept of autocatalytic reaction networks, their design, and perspectives for their development.

stimuli such as temperature, [5] pH, [6] metal ions, [7] or ion strain. [8] Nevertheless, the complexity of motion executed by these actuators is limited by two factors: i) the whole volume of the material responses simultaneously to the stimuli; ii) the response lacks temporal regulation (that is, the response coincides with the stimulus, when the stimulus is ON, the response is ON, and no delays, periodicities, or other complex temporal patterns take place). To overcome these limitations, actuation should be spatially and temporarily regulated.Inspired by biological organisms where responses are controlled by biochemical signaling networks, [9] researchers have been developing hydrogel materials and actuators that are autonomously regulated by synthetic chemical reaction networks. [10] Initially, the research focused on using the Belousov-Zhabotinsky (BZ) reaction-the best-known chemical oscillator-or its close analogs to create periodical swelling/shrinking hydrogels. [11] Lately, however, the focus has moved towards using de novo enzymatic, [12] DNA, [13] and organic [14] reaction networks to regulate hydrogel materials. These reaction networks provide control over individual interactions between the system's components and therefore, they are more desirable than the BZ reaction from a design standpoint. Schulman, Gracias, and co-workers demonstrated the selective actuation of various parts of the hydrogel by specific DNA strands. [13b] Walther and coworkers [14a-c,15] and Huck and co-workers [12a] pioneered temporal control of sol-gel transitions by organic or enzymatic reaction networks. Pojman, Taylor, and co-workers demonstrated spatiotemporal regulation of gelation by an autocatalytic front. [12b,16] Nevertheless, autonomous spatiotemporal regulation of hydrogel actuators by de novo reaction networks remains elusive.In this work, we regulated the actuation of disulfide-containing hydrogel materials by autocatalytic fronts that release thiols. These actuators can perform complex motions (that is, gradual unrolling, wave movement, or sequential actuation) otherwise inaccessible to hydrogel actuators without an external control. Results and DiscussionDesigning hydrogel actuators that can be regulated by the autocatalytic front requires developing two compatible Regulating hydrogel actuators with chemical reaction networks is instrumental for constructing life-inspired smart materials. Herein, hydrogel actuators are engineered that are regulated by the autocatalytic front of thiols. The actuators consist of two layers. The first layer, which is regular polyacrylamide hydrogel, is in a strained conformation. The second layer, which is polyacrylamide hydrogel with disulfide crosslinks, maintains strain in the first layer. When thiols released by the autocatalytic front reduce disulfide crosslinks, the hydrogel actuates by releasing the mechanical strain in the first layer. The autocatalytic front is sustained by the reaction network, which uses thiouronium salts, disulfides of β-aminothiols, and maleimide...

Hydrogel Actuators Biochemical circuits regulate the movement of living organisms. In article number 2106816, Sergey N. Semenov and co‐workers use a synthetic chemical reaction network based on thiols to regulate hydrogel actuators. The signal inducing the actuation is propagated by the autocatalytic front of the thiols. It can jump from one actuator to another or be initiated by the chemical circuit with preprogrammed delay.