RESUMO
OBJECTIVES: The evaluation of the SPF by the measurement of light transmission through a thin film on PMMA plates is an interesting alternative to the conventional in vivo method. However, during the last years an internationally acknowledged in vitro SPF measurement could not be established mainly due to bad reproducibility. With this work, we would like to give new suggestions for an improved reproducibility and to open new space for discussion. METHODS: A total of 22 o/w emulsions with well-established in vivo SPF were measured in vitro close to the international Standard ISO 24443 on two different substrates (PMMA plates: WW5 and HD6). For each product, the ratio 'in vitro SPFraw /in vivo SPF' was calculated. The composition of the products was analysed regarding a parameter that correlates with the aforementioned ratio. Additionally seven suitable calibration products were determined to transfer the in vitro SPFraw to the calibrated in vitro SPFcal . RESULTS: We could show that the results for the SPFcal matched very well for 19 measured o/w emulsions on WW5 plates. However, we found two products where the in vitro SPF was much lower than the in vivo SPF. The Dispersal Rate allows us to identify a product characteristic parameter to predict too low measured in vitro SPFs on WW5 plates. This parameter mainly refers to the ratio of water to lipids in an emulsion. We found that products with few emollients and few emulsifiers are measured too low in vitro. CONCLUSION: We suggest implementing an individual calibration of the in vitro SPF to improve the reproducibility of in vitro SPF measurements between different laboratories. Considering the Dispersal Rate helps to estimate the reliability of the in vitro SPF measured on WW5 plates. In order to evaluate whether those products with a high Dispersal Rate can also be calibrated with special standards, further measurements need to be done. We demonstrate that, besides the known parameter, also the composition of the products should be considered for the interpretation of the in vitro SPF. Our findings could explain some multiple reported problems in correlation between in vitro and in vivo SPF, especially for higher SPFs (Personal Care, November, 2014, 27).
