Abstract/Summary

In this PhD thesis we investigate novel aspects of the fundamental chemical-mechanical coupling in Belousov-Zhabotinsky hydrogels, in order to draw new and strong parallels between the self-oscillation of this non-living biomimetic material and the chemical-mechanical aspects of living systems, such as the beating of heart cells. Chemical oscillation as an underlying non-equilibrium, physical-chemical phenomenon has been shown to be a pivotal feature of both cardiac cells and BZ hydrogels, and in both systems a mechanical oscillation is coupled to this chemical oscillation, namely the contractile beating of heart cells and the swelling-deswelling volume changes of BZ gels. Due to these close parallels, BZ hydrogels have long been considered as simplified non-living models of cardiac cells, and questions have been raised whether and how the investigation of the chemomechanical coupling in BZ gels might reveal new information about the mechanism of the heartbeat as well. In some significant biophysical studies of recent years, cardiac cells have been shown to be able to change their inherent beating rhythm when they were stimulated by a regular mechanical pacemaker: not only were they able to directly adopt the frequency of the pacemaker and become synchronised to it in a wide parameter range, in certain cases they also managed to maintain this adopted frequency for a long while even after stimulations were stopped, in other words cardiac cells could be successfully entrained by a mechanical pacemaker. Motivated by these results, we aimed to investigate if such robust and sustained synchronisation and entrainment effects could also be achieved in BZ hydrogels, in response to mechanical stimulation. We designed a forced oscillator system where we could subject samples of various shapes and sizes to rhythmically applied mechanical perturbations, in the form of compressions, and observe how the inherent chemomechanical oscillation of the hydrogel changed as a result. We found that BZ gels could indeed become synchronised to the external stimulations in an incredibly wide range of parameters, leading to a rich variety of potential interactions. Most significantly, depending on the frequency of compressions and the magnitude of the applied force, it was possible for the hydrogel to slow down its oscillation and enter into a distinct, resonant state with the forcing oscillator; this could occur either in a fundamental, one-to-one frequency ratio, or with a multiple/fraction of the stimulation frequency, resulting in harmonic resonance. On top of this, observations were not limited to the application time of compressions only, but also continued post-stimulation, to assess whether any changes induced in the dynamic behaviour of BZ gels were sustained, as it had been for cardiac cells. Our data clearly revealed that resonant BZ hydrogels became entrained to the rhythmic stimulation, since they managed to keep a form of memory of the compressions, at least for a short time post-stimulation, and then they followed a long and gradual relaxation process back to their natural state. To build a comprehensive picture, our mechanical stimulation experiments were performed in both larger hydrogel samples that displayed chemomechanical wave propagation and smaller samples giving isotropic swelling-deswelling, to draw parallels between multiple cardiac behaviours like large-scale tissue-level signal propagation or the microscopic beating of individual cells, respectively.