Introduction

The application of short, intense, electric fields across the bilayer–lipid membranes of natural or artificial cells can create transient, self-resealing holes in the bilayer lamellae. This hole-forming or electroporative effect was first identified from the collective results of early studies which applied transverse fields to lipid membranes in an effort to probe their ultrastructure. The phenomenology of electroporation was established and systematized in later studies by Zimmermann et al., Tsong et al., and Abidor, Chizmadzhev and co-workers. (Cf. also the review articles by Zimmermann, and the monograph by Neumann, Sowers and Jordan). As identified in these experimental studies, the usual methodology for electroporating cells is to apply a direct current field of a few kV/cm to a cell or vesicle suspension in the form of a pulse (or pulses) of the order of 10 μs in duration. The pulse waveform is usually rectangular or of exponential decay (cf. Potter for a summary of methods), although recent studies have demonstrated that a pulse of a dc shifted radio-frequency field can efficiently electroporate cells. Resealment of the holes takes place approximately 10–100 s after application of the pulse. The holes which are created have been estimated to be of the order of 5–10 nm in diameter and cover the cell surface with a density of 10 pores/cm. A critical field strength appears necessary to cause the electroporation; however, if the field strength is too high, or the duration or number of pulses too great, this prompt, reversible response is replaced by an irreversible breakdown of the membrane.