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Stratum corneum, the outermost layer of skin, consists of keratin filledrigid non-viable corneocyte cells surrounded by multilayers of lipids. Thelipid layer is responsible for the barrier properties of the skin. We calculatethe excess chemical potential and diffusivity of water as a function of depthin lipid bilayers with compositions representative of the stratum corneum usingatomistic molecular dynamics simulations. The maximum in the excess free energyof water inside the lipid bilayers is found to be twice that of water inphospholipid bilayers at the same temperature. Permeability, which decreasesexponentially with the free energy barrier, is reduced by several orders ofmagnitude as compared to with phospholipid bilayers. The average time it takesfor a water molecule to cross the bilayer is calculated by solving theSmoluchowski equation in presence of the free energy barrier. For a bilayercomposed of a 2:2:1 molar ratio of ceramide NS 24:0, cholesterol and free fattyacid 24:0 at 300K, we estimate the permeability P=3.7e-9 cm/s and the averagecrossing time \tau_{av}=0.69 ms. The permeability is about 30 times smallerthan existing experimental results on mammalian skin sections.

Water permeation through stratum corneum lipid bilayers from atomistic simulations