SURFACTANT PENETRATION INTO HUMAN SKIN AND RESULTING SKIN DRYNESS 43 Figur e 3. CAPB extracted from skin using fi ve pooled tape strips or cup scrubs in vivo versus (A) corneometer CFB reading determined in vivo (B) visual dryness CFB scores determined in vivo and (C) 14 C-SDS skin pen- etration determined ex vivo. Data points represent mean ± SEM (some error bars are too small to see). (▲) Tape strip extraction (fi ve pooled extracts) after multiple applications of an anion-based formulation ( ) tape strip extraction (fi ve pooled extracts) after multiple applications of a nonionic-based formulation ( ) cup scrub extraction after multiple applications of an anion-based formulation ( ) cup scrub extraction after multiple applications of a nonionic-based formulation.

JOURNAL OF COSMETIC SCIENCE 44 more nonionic-based formulations would need to be examined to investigate this theory. The fact that the correlations between CAPB extractions and the skin hydration measures only approached statistical signifi cance is likely because of the small number of formula- tions tested here. A follow-up study to further examine the relationship between CAPB penetration into the skin, SLE1S penetration into the skin, and skin hydration has been completed and will be reported separately. The observation that SLE1S and CAPB have opposite relationships with the clinical mea- sures may be because of differences in these surfactants’ physical properties, and in particu- lar in their structures and/or charges. The structure–function relationships of these surfactants and how they impact the mechanisms of surfactant penetration into human skin are currently unknown. We also noted that the individual surfactants did not penetrate the skin in quantities that are consistent with their bulk solution compositions. For example, the ratio of SLE1S to CAPB in formulation A is 6:1, but CAPB made up 33–39% of the total surfactant mass found in the skin after treatment with formulation A. It is well under- stood that mixed micelle composition often differs from bulk solution composition because of interactions between the surfactants (22). Our results indicate that mixed micelle com- position may play a major role in the mechanism of surfactant-induced skin dryness. More work will need to be carried out in this area to fully understand this phenomenon. CONCLUSIONS Surfactant-induced skin dryness and individual surfactant penetration into human skin were examined using a clinical FCAT study and an ex vivo study using a 2-min exposure protocol. Our results indicate that cup scrub extraction is a suitable substitute for tape stripping in surfactant skin penetration analysis, and may be the preferred method as cup scrubs extract more material and the method is less time-consuming and less expensive to perform. SLE1S and SDS were found to be good predictors of clinical hydration for the anion-based surfactant systems examined here. Furthermore, we found that 14 C-SDS skin penetration from surfactant systems trends with clinical skin hydration induced by the surfactant systems as assessed by corneometry and visual dryness grading, indicating that this ex vivo method may be a useful preclinical test for sulfate-based rinse-off products. More work will need to be completed to understand the structure–function relationships between SLE1S and CAPB and skin penetration, which resulted in oppositely trending relationships for the two surfactants with skin hydration measures. ACKNOWLEDGMENTS Financial support for this research was provided by the Procter & Gamble Company. We thank Chandra Ade-Browne and Tiffany Brooks for their help in conducting the ex vivo skin penetration experiments. REFERENCES (1) P. N. Moore, S. Puvvada, and D. Blankschtein, Challenging the surfactant monomer skin penetration model: penetration of sodium dodecyl sulfate micelles into the epidermis. J. Cosmet. Sci., 54, 29–46 (2003).