1.O.2. Electroporation-induced Pore (EiP) Family
Electroporation, the transient increase in the permeability of cell membranes when exposed to a high electric field, is an established in vitro technique and is used to introduce DNA or other molecules into cells. When the trans-membrane voltage induced by an external electric field exceeds a certain threshold (normally 0.2-1 V), a rearrangement of the molecular structure of the membrane occurs, leading to pore formation in the membrane and a considerable increase in the cell membrane permeability to ions, molecules and even macromolecules. This phenomenon is, potentially, the basis for many in vivo applications such as electrochemotherapy and gene therapy (Chen et al. 2006). Electrochemotherapy (ECT) can be used for the treatment for metastatic nodules of solid tumors on the skin or subcutaneous tissue. ECT is a combination of a physical effect, cell membrane poration, and cytotoxic drug administration (Giardino et al. 2006).
Single-cell electroporation (SCEP) has emerged for single-cell studies. When a large enough electric field is applied to a single cell, transient nano-pores form in the cell membrane allowing molecules to be transported into and out of the cell (Wang et al. 2010). Unlike bulk electroporation, in which a homogenous electric field is applied to a suspension of cells, in SCEP, an electric field is created locally near a single cell. Pore formation has been discussed from theoretical and experimental approaches. Current SCEP techniques using microelectrodes, micropipettes, electrolyte-filled capillaries, and microfabricated devices are all thoroughly discussed for adherent and suspended cells. SCEP has been applied in in-vivo and in-vitro studies for delivery of cell-impermeant molecules such as drugs, DNA, and siRNA, and for morphological observations (Wang et al. 2010). Lipid pores can be induced by external fields, stress, and peptides (Kirsch and Böckmann 2016). Electroporation induced by long monopolar and short bipolar pulses on pore formation in realistic 3D irregularly shaped cells has been studied (Chiapperino et al. 2020).
The applications of nanosecond, megavolt-per-meter electric field pulses to biological systems show cellular and subcellular electric field induced effects of electroporation. The absolute rate theory, with experimentally based parameter input, is consistent with membrane pore creation on a nanosecond time scale. Use of a Smoluchowski equation-based model for a planar cell membrane patch exposed to a 10 ns trapezoidal pulse with 1.5 ns rise and fall times, reversible supraelectroporation behavior in terms of transmembrane voltage, pore density, membrane conductance, fractional aqueous area, pore distribution, and average pore radius has been determined (Vasilkoski et al. 2006). Pore creation dominates the electrical response, and pore expansion proved to be negligible on this ns time scale. The electric field induces lateral stress on the bilayer, manifested by surface tensions of magnitudes in the order of 1 mN.m-1 (Tarek 2005).
Electroporation (EP) involves both nonlinear biophysical processes and complex geometries. When exposed to strong electric fields, the formation of pores within a cell membrane increases the membrane permeability. Discontinuous Galerkin (DG) finite element methods can directly enforce these flux jumps across the thin cell membrane interface. Sweeney and Davalos 2018 implemented a DG finite element method to model the electric field, pore formation, and transmembrane flux of charged solutes during EP. Kotnik et al. 2019 reviewed the field of electroporation. They first reviewed evidence for electrically induced membrane permeability, its correlation with transmembrane voltage, and continuum models of electropermeabilization that disregard the molecular-level structure and events. Second, they present insights from molecular-level modeling, particularly atomistic simulations that enhance an understanding of pore formation. Third, they discuss evidence of chemical modifications of membrane lipids and functional modulation of membrane proteins affecting membrane permeability. Finally, they consider remaining challenges for a full understanding of electroporation and electropermeabilization (Kotnik et al. 2019). Flow Micropillar-array Electroporation (FME) systems have been developed to wisely regulate an important transmembrane-determining factor, namely cell size variations among individual cells, to achieve more effective transfection (Zu et al. 2019). Extracellular calcium ions inhibit the electrotransfer of small charged molecules, an effect that is related to an increased rate of membrane resealing (Navickaite et al. 2020).