The staratum corneum accounts for most of the permeability barrier that is maintained by the embedding of keratinocytes into a lipid (oil) rich extracellular matrix. Chemical analysis has shown that the intercellular cement or lipids of the stratum corneum are mainly composed of ceramides, cholesterol, cholesterol etsers and fatty acids themselves which are added or synthesized by the keratinocytes.
In addition to the intercellular lipid lamellae (shhets or scales) other classes of lipids originating directly at the skin surface play important roles in the stratum corneum. The whole sheet formed is the hydrolipidic film. This film in istself is important in maintaining the integrity of the lipid configuration of much deeper layers beyond the startum corneum. There is a gradient of skin lipids in the skin. The hydrolipidic fiilm intermixes with lipid mastrix which improves the skin barrier. Making better skin texture, hydration and elasticity.
One application of a suitabaly balanced formula should prove active for many hours. The art or science is to mimic a skin like fatty acid profile. This makes finding the right oil blend important. To be effective the oil must permeate the skin and integrate with the existing lamellar lipid mix. The fatty acid match provides an entry pass to the deeper layers. A mismatch just allows a surface action as with massage oil. This affinity with the skin is crucial to performance.
As oils are expensive media spreadability is a consideration especially with face oils. The ideal is to mimic the spreadability of squalene or squalane. This spreadability is related to hydration by providing a barrier function.
The hydrolipidic film (Pale area) covers the epidermis and diffuses within the lipid lamellar matrix (yellow lines). Skin elasticity (bounce or tone), the skin barrier and smoothness is enhanced, and surface smoothness noticeable.