j. Cosmet. Sci., 50, 147-157 (May/June 1999) Skin penetration properties of cosmetic formulations using a perfused bovine udder model TH. F(SRSTER, W. PITTERMANN, M. SCHMITT, and M. KIETZMANN, Henkel KGaA, D-40191 Diisseldorf and Institute of Pharmacology, Toxicology and Pharmac•y, School of Veterinary Medicine, D-30559 Hannover, Germany Accepted for publication April 30, 1999. Presented at the XXth Congress of the International Federation of the Societies of Cosmetic Chemists, Cannes, France, September 14-18, 1998. Presented in part at Penetration, Leiden, The Netherlands, September 22-26, 1998. Synopsis Bioactives are a major focus of the recent developments in cosmetics. Most of these ingredients will work only if they penetrate the stratum corneum and reach the living epidermis. The extent and rate of pen- etration of bioactive ingredients from cosmetic products is of paramount interest. A systematic in vitro study of penetration into stratum corneum and viable skin was performed using the isolated perfused bovine udder skin (BUS) model. As bioactive ingredients, oil-soluble vitamin E, vitamin E acetate, and water-soluble D-panthenol were incorporated into different cream formulations and a shower gel. Under leave-on conditions (exposure periods of one hour and five hours), the changes in phase behavior and viscosity have a strong impact on the penetration pattern. Generally, the water-soluble D-panthenol penetrates faster and in a higher amount into the stratum corneum than the oil-soluble vitamin E. Regarding the choice of the galenic vehicles, the penetration of both the oil-soluble and water-soluble vitamin is faster and much higher from w/o-cream than from lameIlar o/w-cream due to changes in phase behavior. In contrast to the lamellar o/w-emulsion, for the w/o-emulsion there is no difference in the penetration of vitamin E between one hour and five hours (e.g., a steady state is already reached after an exposure period of 1 h). The top ten skin layers (200 pm depth) again reflect the influence of the emulsion formulation on the vitamin E absorption. From the w/o-cream the penetration is faster and more efficient than from the lameliar o/w-cream. Under rinse-off conditions (3*2 minutes), the water-soluble D-panthenol penetrates in a higher amount than the oil-soluble vitamin. However, the penetration effect is detectable even after the extremely short period of exposure. Due to the fact that the shower gel will not change the phase behavior, different routes of penetration may be considered for water- and oil-soluble vitamins. INTRODUCTION Today's cosmetic emulsions are not simply mixtures of oil, water, and emulsifier, but they also contain a lot of active ingredients whose function is to ameliorate the condition 147

148 JOURNAL OF COSMETIC SCIENCE of the skin, e.g., use in skin hydration or on the barrier properties of the stratum corneum. Most of these ingredients will work only if they penetrate into the stratum corneum or the deeper layers of the epidermis. The extent and rate of penetration of an active ingredient from a topical formulation depends on its passive diffusion into the skin, which in principle obeys Fick's law of diffusion. This takes account of the con- centration of the active ingredient in the galenic vehicle, the mobility (diffusion con- stant) of the active ingredient molecule, and--in the form of the distribution coeffi- cient-the interaction of the active ingredient with the vehicle and the skin (overview in references 1 and 2). In practice, the application of this simple law is frequently difficult, because the composition of a topical formulation only rarely remains constant during the course of application volatile components vaporize, water evaporates (3-5), and the mobility and the distribution coefficient of the active ingredient change as a function of its concentration and of the viscosity and phase behavior of the galenic vehicle. Moreover, the penetration properties of a formulation are also dependent on the inter- action of the vehicle with the skin (2,6). The application of the emulsion can cause an occlusive effect, under which the hydration of the horny layer increases, resulting in increased penetration. Certain components of the formulation may accumulate in the outermost skin layer, the actual diffusion barrier, thus accelerating penetration (2,7). Simple penetration models cannot take these different, mutually interacting effects into account, so that an optimal galenical can only be developed on the basis of experimental penetration studies. On ethical grounds, in vivo studies on humans and animals are only carried out on a limited scale, mainly in connection with dermatics (8). The isolated perfused bovine udder skin (BUS) model is a good substitute for in vivo tests on humans (9). As a living skin model it takes also into account metabolic processes in the skin, so that it can be used to study both skin penetration and skin irritation by cosmetic formulations (10,11). Due to the different rheology of the emulsions under living conditions (physiological surface temperature, physiological TEWL) and the expected identical substantivity (same components as far as possible), it was intended to compare the emulsions under the conditions of infinite dose, i.e., more than 10 or 20 mg/cm 2. The high dosage prevents any depletion of vitamin concentration in the vehicle during the penetration experiment. Furthermore, the study design allows a comparison with the results of the testing of certain types of ointments and dermatics using this in vitro model (9). Based on an extensive investigation of the influence of emulsion type and structure on vitamin penetration into the skin (10), the objective of this work was to study the kinetics of the penetration of oil- and water-soluble vitamins such as vitamin E, vitamin E acetate, and D-panthenol. An emulsion of the water-in-oil (w/o) type and a lamellar oil-in-water (o/w) emulsion were chosen as leave-on products since they showed big differences in phase behavior and penetration properties (10). At an extreme, the short- term penetration from a surfactant-based shower gel is investigated under rinse-off conditions. MATERIALS AND METHODS CHEMICALS The galenic formulations selected for the penetration studies (Table I) exhibit basic