Synchronicity in Composite Hydrogels: Belousov–Zhabotinsky (BZ) Active Nodes in Gelatin

Synchronization of motion, task, or communication is responsible for the successful function of many living systems. Composite Belousov-Zhabotinsky (BZ) self-oscillating hydrogels exhibit a sufficiently complex chemical-mechanical feedback to develop synchrony and other dynamical behaviors. In the context of BZ gels, synchrony is the sustained, oscillating oxidation with constant phase of two or more catalyst-immobilized gel segments. However, design criteria to control chemical-mechanical synchronization through patterning of the reaction catalyst are lacking. To characterize the fundamental units of composite device design, the periodic oxidation behavior of isolated nodes, node pairs, and multinode systems were investigated. Isolated nodes of Ru-immobilized gelatin exhibited three distinct, volume-dependent, regimes of oscillation: (i) long period (10-40 min), (ii) biperiod (mix of long and short), and (iii) short period (2.5 min). Node pairs and multinode grids of Ru gelatin were embedded in plain gelatin through a film stacking or 3D printing technique. The fraction of synchronized node pairs decreased with increasing interspace distance. Embedment increased the probability of synchronization, with 100% synchronization for interspace distances of less than 10 times the characteristic length of the reaction-diffusion process. The phase difference between synchronized node pairs transitioned from in-phase at small interspace distances to antiphase at large distances, providing the first experimental verification of antiphase synchrony in composite BZ gels. From these design criteria and fabrication techniques, the chemical-mechanical feedback of BZ composites can be programmed through strategic patterning of the catalyst to build BZ devices for sensor, trigger, or chemical computing applications.