Coexistence effect of UVA absorbers to increase their solubility and stability of supersaturation.

OBJECTIVE: Sunscreens containing UVA absorbers in high concentrations are expected to be developed, since recent studies have suggested the possibility of involvement of UVA ray in skin cancer and early skin aging. Solubility and stability of supersaturation of UVA absorbers in UVB absorber were determined in the absence and the presence of cosmetic oil. Coexistence effect of UVA absorbers was analyzed to dissolve them in high concentrations. METHODS: Two UVA absorbers, diethylamino hydroxybenzoyl hexyl benzoate (DHHB) and butyl methoxydibenzoylmethane (BMDM), a UVB absorber, 2-ethylhexyl methoxycinnamate (EHMC), and a cosmetic oil, 2-ethylhexyl ester of oligomer of hydroxystearic acid (EH-O-HSA), were used. Their solutions were prepared at 80°C and cooled to 5°C. The solid DHHB and/or BMDM were added to it, and the time evolution of concentrations of the UVA absorbers in the solution phase was monitored. RESULTS: At the saturation in the absence of EH-O-HSA at 5°C, weight ratio of DHHB and BMDM to EHMC was 0.39/1.00 and 0.22/1.00, respectively. Addition of EH-O-HSA slightly changed the solubility of DHHB and BMDM. When the weight ratio of EH-O-HSA to EHMC was 0.20/1.00, weight ratio of DHHB and BMDM to EHMC was 0.35/1.00 and 0.25/1.00, respectively at the saturation at 5°C. In the presence of EH-O-HSA, a strong coexistence effect of DHHB and BMDM was found on their solubility. A thermodynamically stable saturated solution at 5°C having the composition that DHHB: BMDM: EHMC: EH-O-HSA = 0.47: 0.46: 1.00: 0.20 was obtained by the simultaneous addition of solid DHHB and BMDM into the initial solution. CONCLUSION: The solution type composite having the highest concentrations of DHHB and BMDM prepared in this study exhibited critical wavelength at 368 nm that was just below the border for sunscreens being qualified as 'Broad Spectrum' protection under the new rule launched by US FDA.

Problems of in vitro SPF measurements brought about by viscous fingering generated during sunscreen applications.

Up to date, no worldwide standard in vitro method has been established for the determination of the sun protection factor (SPF), since there are many problems in terms of its repeatability and reliability. Here, we have studied the problems on the in vitro SPF measurements brought about by the phenomenon called viscous fingering. A spatially periodic stripe pattern is usually formed spontaneously when a viscous fluid is applied onto a solid substrate. For the in vitro SPF measurements, the recommended amount of sunscreen is applied onto a substrate, and the intensity of the transmitted UV light through the sunscreen layer is evaluated. Our theoretical analysis indicated that the nonuniformity of the thickness of the sunscreen layer varied the net UV absorbance. Pseudo-sunscreen composites having no phase separation structures were prepared and applied on a quartz plate for the measurements of the UV absorbance. Two types of applicators, a block applicator and a 4-sided applicator were used. The flat surface was always obtained when the 4-sided applicator was used, while the spatially periodic stripe pattern was always generated spontaneously when the block applicator was used. The net UV absorbance of the layer on which the stripe pattern was formed was found to be lower than that of the flat layer having the same average thickness. Theoretical simulations quantitatively reproduced the variation of the net UV absorbance led by the change of the geometry of the layer. The results of this study propose the definite necessity of strict regulations on the coating method of sunscreens for the establishment of the in vitro SPF test method.