title: A study of chemical-mechanical coupling in Belousov-Zhabotinsky self-oscillating hydrogels, towards the physical-chemical model of the heart
Creators: name: Geher-Herczegh, Tunde
Publisher: name: University of Reading
Year: 2023

description: 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.