Rhythmic cardiac activity is regulated by the synchronized electrical activity of cardiac pacemaker cells, which is vital for maintaining proper heart function. Disturbances in rhythmicity, often arising from factors such as heart damage, stress, or ischemia, can result in arrhythmic contractions, posing severe health risks [1]. The heart employs protective mechanisms to stabilize its rhythm, one of which is called overdrive suppression. Overdrive suppression occurs when cardiac tissue is depolarized at frequencies higher than its intrinsic beating rate, causing a temporary pause in electrical activity [2]. The fast stimulation overwhelms the cellular signaling, hindering the restoration of intra- and extracellular ionic concentrations between stimuli [2]. This effect has been utilized in the treatment of cardiac arrhythmias and to override malignant tachycardias [3]. Our research focuses on the use of organic photovoltaic devices (OPDs) to modulate the activity of excitable cells [4,5], which could be a potential candidate as an extracellular stimulation device for cardiac tissue.

While previous studies explored the impact of various stimulation protocols on the duration of this pause in activity in cardiac cell aggregates [2], in this study, we are interested in the effects of localized stimulation on the synchronized activity in different parts of cardiac tissue. Therefore, we used multielectrode arrays (MEAs) to investigate the signal propagation originating from cardiac pacemakers [6]. Embryonic chicken cardiomyocyte monolayers were cultured on the surface of MEAs and their extracellular electrical activity observed by a grid of electrodes positioned at a distance of 200 µm from each other. The same electrodes were used to electrically stimulate the cell culture and induce overdrive suppression, whereby the stimulation parameters, such as amplitude, pulse length, frequency, and number of pulses, could be adjusted. The changes in electrical activity were acquired during stimulation, while movements of the cardiac tissue were recorded using a microscope camera. The video recordings were then analyzed using object recognition on selected regions of interest to quantify the contractions and correlate them with the electrical activity observed in the MEA recordings [7].

These findings were then used to investigate the effects of OPD stimulation. Cardiac monolayers were cultured on the surface of OPDs, which were stimulated by light pulses with similar parameters that were used to induce overdrive suppression with MEAs. Cellular contractions were again recorded with a microscope camera, analyzed, and compared to the activity observed in MEA experiments. The results confirm that the OPDs can modulate the intrinsic beating frequency of the cardiac cell cultures resulting in overdrive suppression, which may lead to new insights on the mechanics of OPD stimulation and potential applications as an external cardiac stimulator.