Chapter Seven Electroporation of Planar Lipid Bilayers and Membranes

Abstract

Abstract Strong external electric field can destabilize membranes and induce formation of pores thus increasing membrane permeability. The phenomenon is known as membrane electroporation, sometimes referred to also as dielectric breakdown or electropermeabilization. The structural changes involving rearrangement of the phospholipid bilayer presumably lead to the formation of aqueous pores, which increases the conductivity of the membrane and its permeability to water-soluble molecules which otherwise are deprived of membrane transport mechanisms. This was shown in variety of experimental conditions, on artificial membranes such as planar lipid bilayers and vesicles, as well as on biological cells in vitro and in vivo. While studies of electroporation on artificial lipid bilayers enabled characterization of the biophysical processes, electroporation of biological cells led to the development of numerous biomedical applications. Namely, cell electroporation increases membrane permeability to otherwise nonpermeant molecules, which allows different biological and medical applications including transfer of genes (electrogene transfer), transdermal drug delivery and electrochemotherapy of tumors. In general, the key parameter for electroporation is the induced transmembrane voltage generated by an external electric field due to the difference in the electric properties of the membrane and the external medium, known as Maxwell–Wagner polarization. It was also shown that pore formation and the effectiveness of cell electroporation depend on parameters of electric pulses like number, duration, repetition frequency and electric field strength, where the later is the crucial parameter since increased transmembrane transport due to electroporation is only observed above a certain threshold field. Two main theoretical approaches were developed to describe electroporation. The electromechanical approach considers membranes as elastic or viscoelastic bodies, and applying principles of electrostatics and elasticity predict membrane rupture above critical membrane voltage. A conceptually different approach describing formation and expansion of pores is based on energy consideration; it is assumed that external electric field reduces the free energy barrier for formation of hydrophilic pores due to lower polarization energy of water in the pores compared to the membrane. Combined with stochastic mechanism of pores expansion it can describe experimental data of bilayer membranes. Still, the molecular mechanisms of pore formation and stabilization during electroporation are not fully understood and rigorous experimental conformation of different theories is still lacking. The focus of this chapter is to review experimental and theoretical data in the field of electroporation and to connect biophysical aspects of the process with the phenomenological experimental observations obtained on planar lipid bilayers, vesicles and cells.