176 JOURNAL OF COSMETIC SCIENCE dermal delivery of an active ingredient Some of the emulsifiers studied in this investigation induced liquid crystal formation in the formulations and were found to interact in two different ways. On the one hand, they prevented the eva po ration of water from the formulation when applied on the skin and in doing so prolonged the delivery phase of water-soluble active ingredients as only solubilized molecules penetrate the skin at reasonable rates. On the other hand, when combined with lipophilic active ingredients, the location to which the active ingredient is delivered was changed. This can be explained by a po ssible interaction of these emulsifiers with the skin lipid bilayers. These are present in an orthorhombic, an hexagonal and a liquid packing, which co-exist simultaneously and are characterized by a relatively low, an intermediate and a high permeability, respectively. The results obtained in this study suggest that the use of these liquid crystal formation inducing surfactant systems favors the presence of the more permeable packing states. In a given time, an active ingredient will therefore penetrate deeper into skin with emulsifier systems that induce liquid crystal formation than with systems that do not. It is therefore suggested that such emulsifier systems be used when fast delivery of a lipophilic active ingredient or more delivery of a hydrophilic active ingredient is required, and non-liquid crystalline systems if slower or less delivery is required. The use of liquid crystalline emulsifier systems therefore allows cosmetic formulators to regulate the extent (hydrophilic active ingredients) or speed (lipophilic active ingredients) of active ingredient delivery and thus to formulate for fast efficacy.

J. Cosmet. Sci., 57, 177-204 (March/April 2006) Papers Presented at the 2005 Annual Scientific Meeting and Technology Showcase (Friday's Program) December 8-9, 2005 New York Hilton New York, NY 177