Electroporation

The formation of pores in the plasma membrane is induced by the application of electric fields, normally in the form of a direct current (DC) applied across an electrode chamber in which the living cells are suspended. Due to electrostatic conduction, ion migration occurs within cells located between, but not in direct contact with, two oppositely energized electrical conductors (electrodes). These charge within the cells move along the electrical lines of force to take up energetically favorable position within the cells; that is, they separate and form charged poles within the cells along the electrical field lines, corresponding to oppositely charged electrode plates.

Three electrical parameters are used to control electroporation; namely, the electrical field strength, the pulse duration, and the number of pulses applied. Cell lysis may ensue if one of these parameters is too high or if the osmotic potential of the electroporation solution is incorrect. The electrical field strength (E) is the force with which the field acts on a unit charge situated at a particular point in space and is determined by the applied electrical potential (voltage, V) and the distance (d) between the electrodes; that is E=V/d. if the distance is measured in centimeters (cm), then the electrical field strength is given as V / cm.

The number and duration, ranging from tens of microseconds to a few milliseconds, of the applied electrical pulses can affect the number and size of the resultant pores in biological membranes. A certain critical field strength, however, must be reached before pore formation will occur. Theoretically, the field strength required to produce a localized breakdown of the membrane at the field-induced poles can be calculated from the equation Ecr= Vcr / 1,5r, where Vcr is the transmembrane potential difference required to induce localized breakdown of the membrane and r is the radius of the cell. Vcr values of 0,5 to 1,5 V have been found, according to the cell type and the experimental conditions. Depending on the state of the membrane and the external environment such as the temperature, electroinduced membrane pores may remain open for seconds or hours and exogenously located macromolecules such as DNA may cross the cell membrane (electrotransfection). The molecules to be transferred are usually included in the electroporation solution before application of the electric field.

The term electroporation, although commonly used when referring to electrotransfection, is also a fundamental event in the electrofusion process. When electroporation occurs in two or more closely aligned cells, the membranes of the adjacent cells may collapse at their points of contact, followed by cytoplasmic mixing to instigate cell fusion (Zimmermann and Vienken, 1982).