In the in vitro evaluation of Sun Protection Factor (SPF), the photostability of the ultraviolet (UV) filters can have a major impact, especially for high-SPF formulations, but is generally not taken into consideration. In this study, we present a UV transmission spectrum measurement system utilizing a high-sensitivity UV photomultiplier tube with concomitant evaluation of photostability. We have developed an algorithm to estimate SPF in vitro by converting the amount of UV light transmission through the sunscreen layer into cumulative relative erythema effectiveness to obtain one minimal erythema dose. Thus, the algorithm uses the same endpoint as in vivo SPF methods, but with a photomultiplier tube as the detector instead of skin. The values obtained showed an excellent correlation with in vivo SPF values, even for high-SPF sunscreens exceeding SPF 50. This method should be suitable as an in vitro SPF testing method for regulatory purposes.
IFSCC Magazines, 12 (2009) (1) 3–7
It is generally well known that the surface of an inorganic UV‐protective powder used in a sunscreen formula is treated with a fatty acid or other hydrophobic chemicals to obtain a high water resistance. Due to the resulting strongly hydrophobic surface, the treatment has a clear drawback in that it allows the sunscreen to leave its base components on the skin after washing with regular soap. Therefore, in this study to develop an intelligent sunscreen formula that has a high water resistance and can be washed away easily with regular soap, we focused on a pH‐responsive polymer as the surface‐treatment agent for a UV‐protective powder. Numerous experiments led to the synthesis of the 2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid/11‐methacrylamidoundecanoic acid (AMPS/MAU) copolymer, which is hydrophobic in acidic to neutral solution and hydrophilic in basic solution. Titanium dioxide subjected to surface treatment was treated with the AMPS/MAU copolymer showed a high pH‐responsiveness that was similar to that of the polymer. A W/O‐type sunscreen containing the pH‐responsive titanium dioxide showed a high water resistance as well as a high washability with regular soap.
Keywords: Detergent‐washable, pH‐responsive polymer, pH‐responsive powder, sun care product, water‐resistant
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