J. Cosmet. Sci., 68, 219–231 (May/June 2017) 219 Development of a preclinical surfactant skin penetration assay to refl ect exposure times typical of consumer use NICOLE MCCARDY, RYAN THOMPSON, MATTHEW MILLER, PETER STYCZYNSKI, STEPHANIE VENTURA, ROBERT GLENN, and GERALD B. KASTING James L. Winkle College of Pharmacy, University of Cincinnati, Cincinnati, OH (N.M., M.M., S.V., G.B.K.), and Research and Development Department, The Procter & Gamble Company, Cincinnati, OH (R.T., P.S., R.G.). Accepted for publication May 30, 2016. Synopsis Mixed surfactant and surfactant–polymer compositions have been reported to decrease surfactant deposition onto and penetration into the skin relative to single surfactant compositions, potentially improving the mildness of the product. Previous workers in this area [see Moore et al., J. Cosmet. Sci. 54:29–46 (2003), and subsequent publications] employed a procedure in which excised porcine skin was exposed to a surfactant solution containing radiolabeled sodium dodecyl sulfate (14C-SDS) for 5 h. We have developed an improved SDS penetration assay using excised human skin that refl ects typical consumer exposure times for rinse-off products. Using the new protocol, we were able to see a signifi cant decrease in 14 C-SDS penetration from a sodium lauryl sulfate (SLS)/polyethylene oxide composition applied to excised skin for either 2 or 10 min, as compared to SLS only. Furthermore, differences between the SDS penetration patterns on porcine skin and human skin were seen with a second SLS/polymer system consequently, we do not recommend porcine skin for routine mildness screening by 14 C-SDS penetration. INTRODUCTION Surfactants are amphiphilic molecules that adsorb at air–liquid, liquid–liquid, and liquid– solid interfaces, reducing surface and interfacial tensions, respectively. Surfactants have multiple industrial uses as emulsifying and cleansing agents, especially in the cosmetic and personal care industry. Human skin can be exposed to surfactants from short periods of time (for rinse-off cleansers) to long periods of time (for leave-on emulsions). Depend- ing on how long the skin is exposed, a surfactant can penetrate to a greater or lesser extent into the layers of the skin. This penetration has been linked to skin irritation (1–3). Sodium dodecyl sulfate (SDS) is a common anionic surfactant often found in cleansing products. It has been shown that with increasing concentrations of SDS above the critical Address all correspondence to Gerald B. Kasting at Gerald.Kasting@uc.edu. Present address for Nicole McCardy, Dermazone Solutions, St. Petersburg, FL 33713.

JOURNAL OF COSMETIC SCIENCE 220 micelle concentration (CMC), there is a dose-dependent increase in the amount of SDS measured in the skin (1). When certain polymers are added to SDS and other surfactant systems, SDS penetration is reduced (1,3,4). Polymers can interact with surfactant micelles and modify the adsorption behavior of the surfactant (1,3,5,6). For such interaction, a “pearls on a string model” has been proposed, where surfactants self-assemble around the hydrophobic portions of the polymers to form hemimicelles (5,6). In general, for water- soluble polymers, the more hydrophobic the polymer, the stronger the interaction (6). In the case of SDS, the ionic repulsion between the “micellar pearls” leads to an expansion of the polymer chain, which causes an increase in blob size (i.e., the size/length of the poly- mer molecule). This model has been validated through viscosity measurements and neu- tron scattering. When the surfactant concentration is above the CMC, micelles can begin to form. A widely accepted view of surfactant penetration through the skin, as reviewed by Moore et al. (1), is that “at surfactant concentrations that exceed the CMC, where surfactant micelles fi rst form, only surfactant monomers can penetrate into the skin, because the surfactant micelles are not surface active, or they are too large to penetrate into the stratum corneum (SC).” This theory is known as the monomer penetration model (7) it is largely based on clinical observations using surfactant mixtures. This view was challenged in 2003 by the Blankschtein group at the Massachusetts Insti- tute of Technology (1), who showed that addition of polyethylene oxide (PEO, MW ~8000) to SDS solutions reduced the penetration of 14 C-radiolabeled SDS into porcine SC at levels well above the CMC (1). The Moore et al. study (1) and subsequent publications from this group (3) employed a 5-h exposure time of the skin to the surfactant solution. The objectives of the present study were to confi rm the effects of polymer addition on SDS penetration into human skin and to determine whether the exposure time could be further reduced to refl ect conditions closer to consumer usage of rinse-off products. We furthermore sought to simplify the assay and maximize its sensitivity. Because the envi- sioned use of the assay was to screen prototype rinse-off product formulations, we used commercial-grade surfactants and polymers rather than highly purifi ed materials. A lim- ited study of the solution properties of these materials was conducted to provide partial characterization. Notably, the bulk surfactant was sodium lauryl sulfate (SLS), which contains a natural mixture of alkyl chain lengths as well as residual impurities, whereas the radiolabeled marker was the purifi ed C12 homolog, SDS. We will maintain this dis- tinction throughout the article. Experimental SDS penetration trials on human skin were then conducted using exposure times of 10 and 2 min. A simplifi ed protocol, in which the tape-stripping step was eliminated, was employed for the 2-min exposure protocol fur- thermore, a random controlled block design, followed by a two-way analysis of variance of log10-transformed data, was employed to increase sensitivity (8). The report presents the details of these studies and provides a recommendation for further use of this assay. MATERIALS AND METHODS Aqueous solutions of SLS (50 mM), SLS with 2% polyethylene glycol (PEG 8000, here- after referred to as PEO) and SLS with 2% polyvinyl alcohol (PVA) were provided by the Procter & Gamble (P&G) Company (Cincinnati, OH). The SLS sample was a commercial- grade material showing evidence of surface-active impurities. The PVA raw material had