Establishing an <i>In Vitro</i> Sun Protection Factor Test for Powder Products

Abstract

It is necessary to establish an in vitro sun protection factor (SPF) test that considers costs, time requirements, and ethical problems. However, few studies on in vitro SPF tests for powder products, which do not easily create uniform layers on polymethyl methacrylate (PMMA) plates, have been reported so far. To produce uniform layers, solvents were applied on PMMA plates before applying powder products. This study aims to clarify the impact of various factors on in vitro SPF values of the powder products and establish an in vitro SPF test method that correlates with in vivo SPF values. Temperature variations affect the solvent viscosity, leading to significant differences in the in vitro SPF values. Therefore, we selected mineral oil because its viscosity is not strongly affected by the temperature. Based on the combinations of the amounts of solvents and samples, the confidence interval (CI[%]) varied. Therefore, we selected 0.3 mg/cm2 of sample against 0.1 mg/cm2 of solvent for the combination with CI [%]≤17. We identified significant differences in the in vitro SPF values depending on the number of spreads. Therefore, we defined the number of spreads as 10 reciprocations × 4, which had the smallest 95% confidence intervals (95% CI). We could not identify significant differences upon varying the spreading pressure. However, when the spreading pressure was (1.18±0.20)N, the CI [%] was higher than 17. Therefore, we chose (0.69±0.20)N, which had the smallest 95% CI. A single-regression analysis between the in vitro SPF values obtained by applying the conditions established in this study and the in vivo SPF values according to ISO 24444:2010 showed that the coefficient of determination was 0.9486 (r=0.9739). Thus, we successfully established an in vitro SPF testing approach for powder products.