Effect of the Electric Field Vector Change on the Efficacy of Nanosecond Pulse Trains in 2D and 3D Cell Models

A phenomenon of bipolar cancellation stands for the suppression of bioeffects of nanosecond electric pulses (nsEP) when the electric field polarity is reversed. In contrast, the bioeffects of unipolar nsEP can be increased at sub-MHz frequency (summation effect). To explore cell responses at all other angles of electric field vector change, we employed an isosceles triangle and a pyramid electrode configurations for 2D and 3D cell models, respectively. In both cases, the electrode at the apex was always connected to the ground, while the other two electrodes were energized in alternation. This way, the electric field lines from either active electrode converged on the apex electrode, merging into a single long pulse. In other words, the electric field direction between the electrodes at the base of the triangle (pyramid) changed by 180°, but this angle gradually decreased to 0° closer to the apex electrode. Experiments with ablation of BPAE cell monolayers by 0.833 kHz 600-ns pulses showed maximum bipolar cancellation (13-fold vs a unipolar pulse) at 180° vector change. At angles less than 90°-120°, cancellation was replaced by summation, i.e., the effects became stronger (3-fold at 22°). The cancellation of nsEP effects near the base electrodes and the summation of them close to the apex electrode led to focal (one electrode) cell electroporation. Similarly, experiments with 3D cell model (potato tissues) and the pyramid electrode configuration qualitatively showed the ablated area near the apex electrode only. Additionally, we employed this technique to evoke electroporation and excitation (measured by Ca2+ mobilization) in smooth muscle cell monolayers. The cell response was observed near the apex electrode while avoiding it at two base electrodes, despite the stronger electric field there. Lastly, strobe imaging revealed a nanosecond kinetics of membrane potential change in single CHO-K1 cells upon sub-MHz nsEP train stimulation. The temporal summation of membrane polarization occurred near the apex electrode, in contrast to area between the base electrodes. These findings can be essential for multielectrode treatments including focal cancer ablation and brain stimulation