External Force-free Unpinning and Wave-Break of Pinned Spirals in a Chemical Reaction-Diffusion System

Excitable media demonstrate patterns; the spiral is one of them. These spiral patterns are also present in the human cardiac system, where they can get pinned in the presence of spatial heterogeneities. Here, we investigate the response of pinned spiral waves in a Belousov-Zhabotinsky (BZ) reaction to an internally generated and temporally evolving chemical heterogeneity, in the form of a gradient produced by Amberlite IR-120 ion-exchange resin beads. The resin establishes a mild concentration gradient and modifies the medium’s local chemical environment, producing spatial changes in ion concentrations and altering local excitability. Under these conditions, pinned spiral tips slowly detach from the glass-bead obstacles without the application of an external electric or thermal field. We observed unpinning and wave fragmentation near the resin line. The unpinning occurred at the tip closer to the resin in all our experiments with 10 and 12 resin beads, whereas nearly 36.4% of the experiments with 8 beads showed the first unpinning away from the resin line. These results indicate that the evolution of chemical heterogeneity and local excitability can jointly regulate the stability of pinned spiral waves. The work provides a physicochemical framework for studying unpinning of spiral waves under a mild gradient and with evolving heterogeneity in excitable media; it will be relevant to biological excitable systems, including cardiac tissue.