Development and testing of a biphasic electrical stimulator for cardiac tissue engineering applications

The development of functional cardiac in vitro models is essential to improve drug screening and to understand fundamental cardiac regeneration mechanisms. For the development of Engineered Cardiac Tissues are essential the presence of cells housed in scaffolds, which provide the architectural and biochemical support, but a paramount role is played by the physical stimuli applied. Electrical stimulation is one of the key physical stimuli capable to greatly support cardiac maturation and functionality. In the heart, the electrical stimuli are useful for generating impulses that allow the coordinated contraction of cells. In vitro, several studies demonstrated that electrical stimulation influences the rate and duration of action potentials of cardiomyocytes, increasing the percentage of beating cells, directing the organization of cardiac cells and promoting cell-cell coupling and calcium handling. This study focused on rigorously compare for the first time the effects of monophasic and asymmetrical biphasic pulses on cardiac maturation and functionality delivering the same amount of charge. The software of ELETTRA, a previously developed electric stimulator at the Industrial Bioengineering Lab (Politecnico di Torino, Italy), was updated so that it could also provide an asymmetrical biphasic wave. Its core consisted of an Arduino Due board running a purpose-built software, interfaced to analog and digital peripherals. Its performances were first tested in a study of different culture conditions to evaluate their functional parameters using the monophasic pulse. Finally, the quality of its monophasic signal was compared with another custom-made electrical stimulator in order to verify the reliability in delivering electrical impulses. It was also modified the interface of the system to the biological targets and created a PDMS culture chamber to electrically stimulate the cells on: 1) two-dimension (2D) monolayer of neonatal rat cardiomyocytes; 2) three-dimension (3D) fibrin gel constructs, encapsulating neonatal rat cardiomyocytes, cultured on a smooth PDMS membrane. Computational simulation has been evaluated to assess the behavior of the chamber when subjected to electrical stimulation using Comsol Multiphysics 5.5. The findings showed that ELETTRA was successfully modified to provide the asymmetrical biphasic stimuli, the biological tests and the comparison with another custom-made electrical stimulator showed that the developed bioreactor was compatible for CTE culture applications. Computational simulation showed the presence of a central area on the bottom of the chamber in which there was a relatively uniform electric field over a wide region of 6 mm between the electrodes, with an average value similar to that set by the stimulator. Since the area under the current curve was similar in the two different electrical waveforms chosen, the cells were stimulated with the same amount of charge making it possible to compare fairly the effects of the two electrical stimulations. Benefit of the asymmetrical biphasic stimulation were observed on the contractility of neonatal cardiac cells cultured both in monolayer and in 3D fibrin-based constructs, in terms of lower Excitation Threshold (ET) and higher Maximum Capture Rate (MCR) values. Clear benefits on cardiac maturation were also observed in 2D culture with the Connexin-43 mainly localized at the cell membrane in proximity of neighbouring cardiomyocytes.