Delivery of molecules to cells via electropermeabilization (electroporation) is a common procedure in laboratories and clinics. However, despite a long history of theoretical effort, electroporation protocols are still based on trial and error because the biomolecular structures and mechanisms underlying this phenomenon have not been established. Electroporation models, developed to explain observations of electrical breakdown of lipid membranes, describe the electric field-driven formation of pores in lipid bilayers. These transient pore models are consistent with molecular dynamics simulations, where field-stabilized lipid pores form within a few nanoseconds and collapse within tens of nanoseconds after the field is removed. Here we experimentally validate this nanoscale restructuring of bio-membranes by measuring the kinetics of transport of the impermeant fluorescent dye calcein into lipid vesicles exposed to ultrashort electric fields (6 ns and 2 ns), and by comparing these results to molecular simulations. Molecular transport after vesicle permeabilization induced by multiple pulses is additive for interpulse intervals as short as 50 ns, while the additive property of transport is no longer observed when the interval is reduced to 0 ns, consistent with the lifetimes of lipid electropores in molecular simulations. These results show that lipid vesicle responses to pulsed electric fields are significantly different from those of living cells where, for similar pulse properties, the uptake of fluorescent dye continues for several minutes.

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