FILM PROPERTIES OF POLYMERS USED IN ANHYDROUS SUNSCREEN FORMULATIONS 211 similar to the results already presented in this report, as well as additional unpublished data, and was independent of the composition of sunscreen phase. The effect of polymer concentration on the fi lm characteristics was also studied. We in- creased the concentration of polymer in Formulation D from 1%–3% (w/w). An SEM micrograph taken of this composition is displayed in Figure 5, which clearly demon- strates that the polymer formed a very defi ned fi lm over the corneocyte surface. The fi lm characteristics were quite similar, but because of the increased concentration of the fi lm former to 3% (w/w), there was better surface coverage. Figure 5 also displays a 3D micro- graph of the fi lm formed over the surface of the skin and sunscreen phase. In summary, this method confi rms earlier fi ndings that a polymeric fi lm typically forms over/above the sunscreen fi lm when an anhydrous sunscreen is sprayed on the skin (5). Because the polymeric fi lm is present as the uppermost layer on the skin, it will not only infl uence water resistance but will also affect the aesthetics of formulations. This makes selection of the correct polymer or polymer combination, as well as their levels in the formulation, of paramount importance. DISCUSSION The work presented in this article is the result of several years of investigation. The meth- odology presented appears quite simple and straightforward but the authors investigated many other methods and substrates that did not provide the same clarity and visuals. Among substrates investigated, we conducted studies with Vitro Skin™, silicone elasto- mers, and pig skin. None of these substrates had similar surface energy or topology like human corneocytes. In addition, standardization of the methodology was quite important to achieve reproducible results. All formulations (except the commercial control) were sprayed from the same size and type can/nozzle and contained the same propellant and were pressurized similarly. Spray rate and velocity were standardized as well. From an imaging stand point, multiple images were captured from each sample to ensure repro- ducibility of the methodology. The work presented in this article elucidated mechanistic information about polymer behavior in sunscreen formulation. One striking myth that this article uncovered is that Figure 5 . SEM micrographs of Formulation D with an adjusted concentration of 3% (w/w) VA/butyl maleate/isobornyl acrylate copolymer. (A) Conventional backscatter and (B) 3D images are shown at ×120 and ×250, respectively. The 3D image can be viewed with red and cyan 3D glasses.

JOURNAL OF COSMETIC SCIENCE 212 polymers do not form continuous fi lms on the surface of the skin. Polymers really form networks that contain mesh-like structures rather than continuous fi lms. This is of course intuitive since the level of polymers in the sunscreen formulation is only 1–2% (w/w). However, because the network is quite hydrophobic, it is diffi cult for water to penetrate due to the sunscreen system’s high surface tension. One other interesting fi nding is that immiscible polymers will not be miscible on the skin after the alcohol evaporates, and will not form continuous fi lms, as in the case with VA/butyl maleate/isobornyl acrylate copolymer and acrylates/dimethicone copolymer. On the other hand, miscible polymers will form continuous fi lms on the skin such as in the case of VA/butyl maleate/isobornyl acrylate copolymer and hydroxypropyl cellulose. A chemist can run simple miscibility tests before adding such polymers to the formulation to avoid any future incompatibilities. CONCLUSIONS In this article, we present an innovative technique that enables scientists to visualize sunscreen fi lms created on a stratum corneum substrate. Furthermore, it allows for the morphological investigation of polymeric fi lm formers in sun care formulations and helps to elucidate their interactions with fi lms formed by sunscreen actives. We used SEM to visualize the deposition of fi lms from sun care formulations on layers of stratum cor- neum, obtained from tape stripping studies. Overall, we found that sunscreen fi lms formed by sunscreen actives resulted in a continuous fi lm on the surface, as evidenced by SEM studies and evaporimetry. Introduction of polymeric additives to the formulation allowed for the formation of separate fi lms that contained a network architecture— dependent on the physicochemical properties of the polymer—resulting in unique inter- actions between fi lms comprising the sunscreen fi lters and the polymer. ACKNOWLEDGMENTS The authors of this article would like to thank Dr. Donald Prettypaul, Ms. Ritamarie Guerrero, Mr. Anthony Luschen, and Mr. David Streuli for their valuable contributions to this work. REFERENCES (1) S. Marguerie, M. Pis savini, A. Baud, and T. Carayol, A new chemical approach to optimize the in vitro SPF method on the HD6 PMMA plate, J. Cosmet. Sci., 63, 243–254 (2012). (2) X. Qu, X. Zhao, and Z. Chen, A new in vitro method to determine sun protection factor, J. Cosmet. Sci., 67, 101–108 (2016). ( 3) D. Moyal, V. Alard, C. Bertin, F. Boyer, M. W. Brown, L. Kolbe, P. Matts, and M. Pissavini, The revised COLIPA in vitro UVA method, Int. J. Cosmet. Sci., 35, 35–40 (2013). (4) O. Dueva-Koganov, B. S. Jaynes, C. Rocafort, S. Barker, and J. Mao, Correlating water contact angles and moisturization/sensory claims, Cosmet. Toil., 122, 20–27 (2007). (5) D. Prettypaul and H. Fares, Microscopic evaluation of polymeric fi lm properties of anhydrous sunscreen compositions and their relation to absorption and water resistance, J. Cosmet. Sci., 63, 213–221 (2012). ( 6) O. Dueva-Koganov, M. Russell, S. Misseri, and A. Duev, Comparative Evaluation of Film-Forming Polymers in Ethanol-Based Sunscreen Spray. (Sunscreen Symposium, Orlando, FL, 2011).