A composition that is applied to healthy, intact skin will partially absorb into the stratum corneum (SC) and its components will partition into the bricks-and-mortar structure of the SC depending on their chemical and physical properties. In addition, the vehicle or formulation can affect the SC either by lowering its resistance and thus accelerate penetration, or by providing additional strength and protection to retard penetration.
Although not designated as such, many materials are included in cosmetic formulations as permeation enhancers and these hold the potential to affect skin barrier integrity. For example, ethanol, oleic acid and some surfactants are known to affect the thermodynamics of intercellular lipids in the SC, changing their organization from rigid gel crystalline to liquid.1 This process accelerates penetration of the enhancers and other components in the formula. Delivery systems such as vesicles, nanoparticles and other solubilizers may affect profiles of partitioning as well. This means that two similar formulations containing the same percentage of active compound, but differing in vehicle, can exhibit different penetration profiles into and through the skin. Therefore, the study of skin penetration is a valuable tool for both the safety and efficacy assessment of topically applied formulations.
Excerpt Only This is a shortened version or summary of the article that appeared in the Mar. 1, 2009 issue of Cosmetics & Toiletries magazine. The full content is not currently available online.