REDUCING FACIAL WRINKLE SIZE USING POLYMERS 141 never unacceptable to most study participants. After application, the skin felt and ap- peared tightened for the polymer formulations, but not smoother than with the formula- tions without polymer. The tighter feel and appearance was confi rmed by the measurements of the mechanical properties of the skin surface using two independent instrumental methods. Moreover, the surface roughness and visibility of fi ne lines and wrinkles were reduced with the 4% AMC formulation. Total wrinkle volume decreased for 2% AMC, but total wrinkle area was unchanged, indicating that wrinkle depth decreased. These results are consistent with the effects on skin produced by a drying and contracting polymer fi lm. The mechanical measurements, optical observations and consumer feedback prove that the reduced wrinkle visibility is not due to an optical blurring effect. The poly- mer fi lms have an evident mechanical impact on the skin surface and skin microstructure. During drying of the aqueous polymer solution, a continuous thin polymer fi lm is formed covering all or most of the skin surface. This fi lm contracts and smoothens out the skin, reducing wrinkle depth and producing a perceivable tightening sensation to test subjects. MECHANISM OF SHRINKING STRESS GENERATION The fi lm contraction is analogous to fi lm shrinkage as observed in polymer latexes. The mechanism may be very similar as well, although the particle coalescence step would not occur. For our study we selected skin care polymers, AMC and PVP, with a relatively high Tg at high moisture levels. As the polymer fi lm dries, the moisture level in the fi lm decreases until the glass transition is reached and drying stresses are generated (Figure 9). How- ever, the fi lm at that level needs to be strong enough to sustain these stresses without cracking or breaking. If the water content of the fi lm is too low when Tg is reached, the fi lm could be too thin to sustain mechanical stresses without cracking. Hence, for a shrinking polymer fi lm, the water content of the fi lm should be above a certain percent- age when the Tg approximates the skin temperature. The mechanical stresses observed in the AMC and PVP fi lms confi rm this model. In future work, studying the drying phenomena in fi lms of polymers with lower Tg would be interesting as the drying stresses would potentially occur when the fi lm is relatively Figure 9. Glass tran sition temperature of polymer material exposed to different levels of relative humidity versus water content (same data as in Figure 1). As the water content decreases during drying, Tg reaches skin temperature (33°C) and drying stresses start.

JOURNAL OF COSMETIC SCIENCE 142 thin, leading to cracks that could limit the area of mechanical impact. The dependence of Tg on the polymer weight fraction in the emulsion system also remains to be elucidated, as does the infl uence of other emulsion components such as the oil phase. DIFFERENCES BETWEEN POLYMER FORMULATIONS Among the three polymer formulations studies, the formulation with 4% polyacrylate showed the highest effi cacy in skin tightening. It was signifi cantly better than the pla- cebo in the change in the R0 and R3 parameters and signifi cantly better than all other formulations in the change in CoR. Against baseline, it outperformed signifi cantly in the change in skin roughness in terms of parameters Sq and Sa, in the change in curvature density and area, and outperformed directionally in the change in total wrinkle volume. In consumer perception, it was signifi cantly better than the placebo formulation and the benchmark formulation with PVP. It was perceived as signifi cantly tackier on application than the PVP and placebo formulations, but this tackiness was acceptable. The formulation with 2% polyacrylate showed skin tightening as well, but not as strong and in most experiments not strong enough for statistical signifi cance. Also, the formula- tion with 1.5% PVP showed signifi cant tightening against baseline in the Cutometer measurements only, but not in any of the other experimental studies. Moreover, the test subjects were unable to perceive any tightening effect with this formulation. In summary, the polyacrylate polymer was superior to PVP in skin tightening effi cacy, particularly at the higher concentration of 4% in our model emulsion formulation. CONCLUSION We have shown that the skin care polymers studied reduce facial wrinkle size and make skin fi rmer. Skin fi rming measures indicated that both polymers instantly tightened facial skin, whereas the placebo product offered no signifi cant tightening benefi t. However, in clinical evaluation only the polyacrylate polymer produced clinically signifi cant improve- ments in wrinkle size and skin fi rmness on the face without signifi cant consumer use negatives such as tackiness. Skin care products with materials such as this have the poten- tial to offer immediate and visible benefi ts to consumers with aged skin. The tightened skin feel that users perceived is a direct and welcome signal of product effi cacy. ACKNOWLEDGEMENTS We would like to thank our clinical panelists for their participation in this study. REFERENCES (1) T. Okuzono and M. Doi, Effects of elasticity on drying processes of polymer solutions, Phys. Rev. E Stat. Nonlin. Soft Matter Phys., 77, 030501 (2008). (2) E. Bormashenko, S. Balter, R. Pogreb, Y. Bormashenko, O. Gendelman, and D. Aurbach, On the mechanism of patterning in rapidly evaporated polymer solutions: Is temperature-gradient-driven Marangoni instability responsible for the large-scale patterning? J. Colloid. Interface. Sci., 343, 602–607 (2010).