Liposome electroformation

Abstract

Liposome formation and lipid swelling on platinum electrodes in distilled water and water solutions in d.c. electrical fields have been investigated for different amounts of a negatively charged lipid (mixture from 71% PC, 21.5% PE and 7.5% PS), and a neutral lipid (DMPC). Negatively charged lipids do not form liposomes without field when the thickness of the dried lipid layer is of the order or less than that corresponding to 90 bilayers. The rate and extent of swelling of layers thicker than 90 bilayers is largest on the cathode, smaller without fields and smallest on the anode. The theory, based on the assumption that osmotic and electrostatic forces drive lipid swelling and liposome formation, is in semi-quantitative agreement with the experimental data; in particular, it gives the observed linear dependence of the rate of swelling on the inverse lipid layer thickness. To induce liposome formation for layers thinner than 90 bilayers it was necessary to apply a negative potential which is proportional to the logarithm of the inversed layer thickness. The characteristic critical potential is proportional to RTk/F; R being the gas constant, Tk the absolute temperature and F the Faraday constant. This indicates that redistribution of counterions may be the cause which increases the repulsive electrostatic intermembrane forces to overcome van der Waals attraction. For thicknesses below 10 bilayers, formation of very thin-walled liposomes of narrow size distribution and mean diameter of the order of 30 µm was observed. These liposomes grow in size before detachment and a formula for the kinetics of growth has been derived, which is in very good agreement with the experimental data. The effects of d.c. fields on DMPC swelling are smaller and lead to formation of liposome-like structures of different appearence. Bilayer separation and bending are prerequisites for liposome formation from hydrating lipids. Therefore, a possible molecular mechanism is that membranes should be destabilized to bend and fuse to form liposomes. This requires the right proportion between structured regions, in the form of bilayers, and defects and/or non-bilayer structures, and in many cases external constraints, in particular, electrical fields.