Liposome Structure Imaging by Atomic Force Microscopy: Verification of Improved Liposome Stability during Adsorption of Multiple Aggregated Vesicles

Abstract

Atomic force microscopy has enabled direct visualization of the liposome structure supported on mica surfaces in air and silanized mica surfaces in aqueous media. The images display distinct patterns of adhered liposomes: multiple and single vesicle liposomes and flat supported bilayers. The multiple vesicle liposome structure is not visible by optical microscopy since the vesicles forming the liposome have diameters as small as 20 nm. Molecularly resolved force versus distance curves displaying the organization of hydrocarbon chains (mono- or bilayers) corroborate the presence of distinct adsorbed structures observed by scanning the surface. The high resolution of the observed liposome images allows the visualization of the aggregation of the multiple vesicles forming liposomes which were shown to have their origin in the liposome formation process and not during adsorption. Since most of the observed liposomes are aggregated vesicles, this aggregated structure has a substantially larger stability during adsorption than the single vesicle structure and consequently a larger resistance in maintaining its shape and function as a carrier of cosmetics, food additives, and drugs. This observation also has some important consequences in the liposomes' selective permeability when they are used as carriers.