PLASTIC YIELDING IN HAIR CUTICLES 77 fatigue required to fracture hair longitudinally increases with moisture. Thus, cosmetic products that help to moisturize or plasticize the cuticles should have a preventive effect against "split end" formation. ACKNOWLEDGMENTS The author wishes to thank Herb Eldestein for his helpful discussions. REFERENCES (1) V. N. E. Robinson, A study of damaged hair, J. Soc. Cosmet. Chem., 27, 155-161 (1976). (2) R. Robbins, Chemical aria'Physical Behavior of Human Hair, 3rd ed. (Springer-Verlag, New York, 1994), pp. 211-206. (3) J. A. Swift, "The Hair Surface," in Hair Research, Orfanos, Montagna, Stuttgen, Eds. (Springer-Verlag, New York, 1994), pp. 211-226. (4) M. L. Garcia, J. A. Epps, R. S. Yane, and L. D. Hunter, Normal cuticle wear patterns in human hair, J. Soc. Cosmet. Chem., 29, 155-178 (1978). (5) J. A. Swift, Mechanism of split-end formation in human hair, J. Soc. Cosmet. Chem., 48, 123-126, (1997). (6) S. B. Reutsch and H. D. Weigrnann, Mechanism of tensile stress release in the keratin fiber cuticle. I, J. Soc. Cosmet. Chem., 47, 13-26, (1996). (7) M. Gamez-Garcia, Cuticle decementation and cuticle buckling produced by Poisson contraction on the cuticular envelope of human hair, J. Soc. Cosmet. Chem., 49, 213-222, (1998). (8) L.J. Wolfram and L. Albrecht, Torsional behavior of human hair,J. Soc. Cosmet. Chem., 36, 87 (1987). (9) L. H. Sperling, "Mechanical Behavior of Polymers," in Introa'uct]on to Physical Polymer Science (John Wiley & Sons, New York, (1992), pp. 503-547. (10) S. Rabinowitz and P. Beardmore, CRC critical review, •Iarcomo/ecu/ar Sci., I, 1 (1972). (11) M. Gamez-Garcia, fiqor,•ho/ogica/ Changes ]n Human Hair Cuticles Upon the Simultaneous Action of Cyclical fiqechan/ca/ aria' Therma/Stresses: Their Relevance to Grooming Practices, presented at the Annual Conference of the Society of Cosmetic Chemists, New York, December 1997. (12) M. Gamez-Garcia, "The crackling of human hair cuticles by cyclical thermal stresses, fl. Soc. Cosmet. Chem., 49, 141-153 (1998). (13) W. A. Nash, Strength offiqateria/s, Schaum's Outline Series (McGraw-Hill, New York, 1972), Chap. 5, pp. 61-79. (14) L. Mascia, The Role of Aa'a'itives in Plastics (Edward Arnold, London, 1974). (15) R. P. Karobout and R. E. Robertson, Chapter 11 in Polymer Sciences. A. D. Jenkins, Ed. (Elsevier-North Holland, Amsterdam, 1972). (16) G. Danilatos and M. Feughelman, Dynamic mechanical properties ofa-keratin fibers during extension, J. fiqacromoL So/. Phys. B, 16(4), 581-502 (1979).

j. Cosmet. Sci., 50, 79-90 (March/April 1999) In vitro method for modeling water resistance of sunscreen formulations VINOD K. GUPTA and JOEL L. ZATZ, Organon Inc., 3 75 Mt. Pleasant Ave., West Orange, NJ 07052 (V.K.G.), and Department of Pharmaceutics, Rutgers University College of Pharmacy, 160 Frelinghuysen Road, Piscataway, NJ 08854 (J.L.Z.). Accepted for publication March 31, 1999. Synopsis Two of the most frequently used sunscreens, benzophenone-3 and octyl methoxycinnamate (OMC), were formulated into hydroalcoholic, diisopropyl adipate oil prototype formulations and o/w and w/o emulsions. Various polymers at a concentration of 5% w/w were included in these formulations. We developed an in vitro procedure for measuring the water resistance of sunscreens and assessing their ability to resist washoff. Results of our in vitro procedure were found comparable to in vivo findings reported in the literature. With tricontanyl PVP copolymer present in the o/w emulsion, a statistically significant increase in retention of benzophenone-3 by stratum corneum (SC) was observed. INTRODUCTION In order for sunscreens to provide prolonged photoprotection they should resist removal by sweat, immersion in water, or mechanical wipe off. The use of complex polymers and the selection of solvents that increase sunscreen substantivity are some of the means used to enhance water resistance of sunscreen products. Several tests have been recommended for evaluating the degree of sunscreen substan- tivity. These include both in vitro and in vivo measurements. In vitro methods usually involve measurements of the amounts of sunscreens that can be "leached out" following a water immersion procedure or some other standardized exposure to water (1). A more accurate method involves the use of excised animal skin such as hairless mouse epidermis that is treated with sunscreens and then exposed to water in a bath at controlled temperatures for a certain time interval. Residual photoprotection can then be measured by forward-scattering spectrophotometry (2). Although these in vitro procedures have not gained widespread use, they are often employed in pilot studies to provide rough estimates of substantivity prior to human testing. Address all correspondence to Joel L. Zatz. 79