JOURNAL OF COSMETIC SCIENCE 252 correlation for 21 of the 30 products tested is acceptable. Only nine products appeared to be outliers. The signifi cant improvement in the in vitro/vivo correlation by the use of the pretreatment is shown in Figure 6, which demonstrates that the C.B. limits the aberrant values some- times obtained with the classic application procedure of in vitro SPF evaluation. The pretreatment investigation shows the benefi ts in the quality of spreading in terms of in- terfacial tension between product and substrate, and in wettability during spreading on the PMMA plate. Hence there is a better in vitro/in vivo correlation. CONCLUSIONS In vitro spectroscopic methods are very useful tools in the development of sunscreen prod- ucts. The goal of industry is to fi nd a universal SPF in vitro method. Nowadays it remains a challenge because of the multiple parameters that impact upon the results. Among these factors the substrate used is one of the most important, which is why a big effort was made to understand more clearly its role. Figure 6. Correlation of in vivo/in vitro SPF: untreated HD6 and pretreated HD6. Table VIII Values of the Contact Angle (θ) between Sunscreen B and the Substrate before and after Pretreatment Sample B Untreated PMMA plate PMMA plate + TegoBetain F50 PMMA plate n° 1 56.7° 44.8° PMMA plate n° 2 53° 40.5° PMMA plate n° 3 58.2° 42.6° Mean of the contact angle (θ) 55.9° 42.6°

IN VITRO SPF DETERMINATION ON HD6 PMMA 253 A previous study showed the importance of the physical characteristics of the surface substrate used. The reproducibility of the in vitro SPF results can readily be improved by controlling roughness by using the HD6 molded plates. Although the method proposed on the HD6 plates showed good correlation with the in vivo results for the majority of products, we still observed deviations for certain types of products. In order to go further into the reliability of the method, we investigated the behavior of the formulae on the HD6 during spreading. Upon consideration of the hydrophilicity level of the molded plate, the present study was inspired by the interfacial tension phe- nomena that are present between product and substrate. With the aim of modifying this interfacial tension, an amphoteric pretreatment with C.B. was investigated. The results obtained showed the benefi ts in the spreading quality and, as a consequence, in the in vitro/in vivo correlation. In the case of high interfacial tension between product and HD6 PMMA plates, it will decrease to enable better adherence so as to obtain suitable spreading. In the case of low interfacial tension, it isn’t affected, but it slightly improves the repartition of the vehicle, resulting in better homogeneity of the results on the whole surface without any infl uence on the SPF result. With this pretreatment, the chemical characteristics of the spread- ing were improved in terms of wettability and in adherence of the product onto the HD6 PMMA. Of the 30 products tested, the C.B. pretreatment improved the results for the nine non-correlated SPF values obtained without pretreatment. Interestingly, the C.B. application had no effect on the SPF values of the 21 well-correlated products. The C.B. pretreatment is, however, suitable for both correlated and non-correlated products. The control of the chemical aspect of spreading enabled us to access a more universal method, adapted for each type of vehicle (O/W, W/O, gel, oil). The future goal is to evaluate the pretreatment technique at different laboratories in order to extend the results and validate inter-laboratory variability. ACKNOWLEDGMENT The authors thank Dr Alice Mija (Laboratory of Chemistry of Organic Materials and Metal, University of Nice-Sophia Antipolis) for her excellent support in the wettability measurements. REFERENCES (1) The Colipa In Vitro Photoprotection Methods Task Force, Method for in vitro determination of UVA protec- tion. Colipa web site is available at www.colipa.eu/publications-colipa-the-european-cosmetic-cosmetics- association/guidelines.html?view=item&id=33 (2011) (accessed May 2011). (2) M. Pissavini and L. Ferrero, In vitro determination of sun protection factor, Business Briefi ng, Global Cosmetics Manufacturing (2004). (3) M. Pissavini, L. Ferrero, V. Alard, U. Heinrich, H. Tronnier, D. Kockott, D. Lutz, V. Tournier, M. Zambonin, and M. Meloni. Determination of the in vitro SPF, Cosmet. Toiletr., 118, 63–72 (2003). (4) M. Rohr and E. Klette, In vitro sun protection factor: Still a challenge with no fi nal answer, Skin Pharmacol. Physiol., 23, 201–212 (2010). (5) M. Pissavini, S. Marguerie, A. Dehais, L. Ferrero, and L. Zastrow, Characterizing roughness: A new substrate to measure SPF, Cosmet. Toiletr., 124(9), 56–64 (2009). (6) L. Ferrero, M. Pissavini, S. Marguerie, and L. Zastrow. Importance of substrate roughness for in vitro UV protection assessment, IFSCC, 9(2), 2–13 (2006).