JOURNAL OF COSMETIC SCIENCE 244 deviation sometimes noted between the in vitro and in vivo results. Among the factors impacting on the in vitro SPF results, the most common are the device used to measure the transmission spectra, the amount of product applied, the application, the experience of the operator in the spreading process, and the substrate used to apply the product, or more precisely its roughness characteristics. One objective of Coty is to control more carefully these factors. Particular attention is paid to the substrate used for in vitro tests. In a previous study, we demonstrated how in vitro SPF reproducibility (5) can be improved by controlling the substrate’s surface micro- topography of the injected PMMA plates. As the roughness affects the SPF value, this factor must be fully controlled (6), but in spite of a constant PMMA roughness control, in vitro/vivo deviations are still observed. These experiments showed different product behavior during the product’s application on the surface of the HD6 PMMA, although the roughness remained constant. These phenomena clearly show that the physical properties of the PMMA molded plate are essential but are not the only substrate characteristics that play a key role in SPF results. In the course of our research on improving the in vivo/vitro correlation, the “physicochemical aspect” of the spreading process on the HD6 substrate was investi- gated. The cases of two different sunscreens offering a good and a bad adherence on the chosen substrate were compared. The objective was to study how the interfacial tension (I.T.) between product and HD6 molded plates can affect SPF results. The following study raises the possibility of modifying this I.T. by the use of a specifi c HD6 pretreatment with an amphoteric surfactant, cocamidopropyl betain. This paper describes how the amphoteric pretreatment led to a more universal in vitro SPF method suitable for every type of product tested, resulting in more reliable results. MATERIALS AND METHODS SUBSTRATE USED FOR IN VITRO SPF MEASUREMENT At the present time, PMMA is internationally recognized as a reliable substrate for in vitro sunscreen assay (1). The fi rst generation of PMMA plates was sandblasted, but in 2008 a new type of PMMA plate was introduced using injection molded manufacturing. Such PMMA plates offer a better batch-to-batch reproducibility. For this reason, the present study used the high-roughness molded PMMA plates supplied as Helioplate® HD6 (Helioscreen®, Creil, France). Each batch is validated by a control chart including ten roughness parameters (5). SUNSCREEN PRODUCTS SELECTED FOR IN VITRO SPF EVALUATION ON HD6 The in vitro SPF values of two different sunscreens, A and B, were investigated on the HD6 with exactly the same spreading procedure. Product A is an O/W emulsion, whereas product B is a polymer gel. Both products were selected for this study be- cause of their different in vitro behavior during spreading on HD6 as described in Table I. The second criterion of selection was the comparison of the in vivo to the in vitro SPF values.

IN VITRO SPF DETERMINATION ON HD6 PMMA 245 STUDY OF AN AMPHOTERIC HD6 PRETREATMENT Surfactants properties. The aim of the present study was to control more reliably the inter- action between products and the HD6 substrate during spreading. In this regard, the properties of the surfactant were used in order to modify the molded PMMA plates on a more strongly hydrophilic surface. Indeed, surfactants are usually amphiphilic compounds (containing both hydrophobic and hydrophilic groups). Very few amounts of surfactant can modify the interfacial tension, particularly those of water, by adsorbing at the inter- faces thanks to their amphoteric nature. Interfaces are characterized by an interfacial level of energy that depends on the properties of the two separated phases (chemical composi- tion and the nature of both phases) (7–10). As a result of their properties, surfactants are able to increase surface wetability, which corresponds to the ability of a drop to spread on a solid surface (Figure 1). Wettability properties have great application in the areas of painting and surfaces (11,12), and the interfacial tensions between the different phases are related by the following equation: T J J J SL SV LV cos where JSV J J SL LV =interfacial tension of the interface solid gas =interfacial tension of the interface solid liquid =interfacial tension of the interface liquid gas The deposition of a drop of sunscreen on the HD6 offers different degrees of wettability according to the product’s nature: the more the contact angle formed on the PMMA Table I Sample A and B Description Products In vivo SPF Base Adherence product/substrate In vitro SPF on HD6 vs in vivo Sample A 16 Steareth-21, Steareth-2 Bad Poor correlation Sample B 30 Acrylate polymer Good Good correlation Figure 1. A. High interfacial tension. B. Low interfacial tension.