Spatially resolved two-color diffusion measurements in human skin applied to transdermal liposome penetration

A multiphoton excitation-based fluorescence fluctuation spectroscopy method, Raster image correlation spectroscopy (RICS), was used to measure the local diffusion coefficients of distinct model fluorescent substances in excised human skin. In combination with structural information obtained by multiphoton excitation fluorescence microscopy imaging, the acquired diffusion information was processed to construct spatially resolved diffusion maps at different depths of the stratum corneum (SC). Experiments using amphiphilic and hydrophilic fluorescently labeled molecules show that their diffusion in SC is very heterogeneous on a microscopic scale. This diffusion-based strategy was further exploited to investigate the integrity of liposomes during transdermal penetration. Specifically, the diffusion of dual-color fluorescently labeled liposomes--containing an amphiphilic fluorophore in the lipid bilayer and a hydrophilic fluorophore encapsulated in the liposome lumen--was measured using cross-correlation RICS. This type of experiment allows discrimination between separate (uncorrelated) and joint (correlated) diffusion of the two different fluorescent probes, giving information about liposome integrity. Independent of the liposome composition (phospholipids or transfersomes), our results show a clear lack of cross-correlation below the skin surface, indicating that the penetration of intact liposomes is highly compromised by the skin barrier.