Diffusion in a realistic simulated model for the stratum corneum

The outermost layer of mammalian skin, called the stratum corneum (SC), constitutes a self-healing barrier against moisture loss and ingress of foreign substances. The SC comprises flat ``bricks'' (50-100 micron wide and 1 micron thick corneocytes largely filled with keratins) held together by a ``mortar'' of 6-10 layers of lipids (100 nm thick). The corneocytes are hydrophilic, while the lipid matrix is hydrophobic. The ability and way of a chemical to pass the SC is a key point for risk assessment and development of cosmetics. 

Here we propose a realistic simulation model with which to study transport through the SC. We explicitly calculate the layered structure between corneocytes and endow it with the local anisotropy in mobility such that diffusion across layers differs from diffusion within layers. By investigating steady-state transport in the SC our results suggest that the crossover between transcellular and extracellular localization during transport is set by an interplay between

(1) the relative hydrophobicity (or lipophilicity) of a species, 

(2) the anisotropy of diffusion within in the lipid matrix, 

(3) the difference in mobilities within the lipid matrix and the corneocytes

(4) the aspect ratio of the corneocytes and the thickness of the lipid “mortar".