The Effect of Intense Subnanosecond Electrical Pulses on Biological Cells

Nanosecond electrical pulses have been successfully used to treat melanoma tumors by using needle arrays as pulse delivery systems. Reducing the pulse duration of intense electric field pulses from nanoseconds into the subnanosecond range will allow us to use wideband antennas to deliver the electromagnetic fields into tissue with a spatial resolution in the centimeter range. To explore the biological effect of intense subnanosecond pulses, we have developed a generator that provides voltage pulses of 160 kV amplitude, 200 ps rise time, and 800 ps pulse width. The pulses are delivered to a cylindrical Teflon chamber with polished flat electrodes at either end. The distance between the electrodes is variable and allows us to generate electric fields of up to 1 MV/cm in cell suspensions. The pulses have been applied to B16 (murine melanoma) cells, and the plasma membrane integrity was studied by means of trypan blue exclusion. For pulse amplitudes of 550 kV/cm, approximately 50% of the cells took up trypan blue right after pulsing, whereas only 20% were taking it up after 1 h. This indicates that the plasma membrane in a majority of the cells affected by the pulses recovers with a time constant of about 1 h. The cells that show trypan blue uptake after this time suffer cell death through apoptosis. Evaluation of the experimental results and molecular dynamics modeling results indicate that with a pulse duration of 800 ps, membrane charging and nanopore formation are the dominant bioelectric effects on B16 cells. This information has been used in a continuum model to estimate the increase in membrane permeability and, consequently, the increase in pore size caused by repetitive pulsing.