Effect of pulsed, high-power radiofrequency radiation on electroporation of mammalian cells

Experiments have been performed to investigate the nonthermal response of biological cells to pulsed, high-power (∼GW/m³), radiofrequency (RF) radiation. Effects of 25-kHz RF-modulated-unipolar pulses, 20-kHz bipolar RF pulses, and 13.56-MHz bipolar RF pulses have been evaluated on nonexcitable COS-1 (African green monkey kidney) and electrically excitable N1E-115 (mouse brain neuroblastoma) cell electroporation. Trypan blue exclusion has been used to determine electroporation induced cell death; 4´,6-Diamidino-2-phenylindole hydrochloride (DAPI) staining of DNA has been used to measure cell membrane permeabilization during electroporation. Experiments using 25-kHz modulation of a unipolar pulse indicate that the greatest effect of RF modulation on survivability and electroporation efficiency occurs at electric fields where a majority of the cells are killed in the absence of RF modulation: 1.6-2 kV/cm. RF modulation at these voltages protects the cells from electroporation induced death but it also reduces the membrane permeabilization. Bipolar RF pulses with no dc offset did not cause electroporation at 20 kHz or 13.56 MHz. RF power, pulselengths, and sample volumes used in experiments allowed only negligible amounts of sample heating. A simplified circuit analysis of an electrical model of the cell predicts that the transition frequency for transmembrane voltage in a bipolar, sinusoidal external electric field is 1 MHz.