Optimising sunscreen photostability

 It has been previously shown that doping of titanium dioxide with a low level of manganese ions produces a material which can act as a free radical scavenger, and as a result, this material exerts a photostabilising effect on photolabile organic UV filters such as ethylhexyl methoxycinnamate and butyl methoxydibenzoylmethane. The purpose of this work was to investigate whether pre-dispersion of this novel inorganic filter has any effect on its ability to Figure 1: Photostability data. photostabilise the organic filters. Formulations were prepared using a conventional coated TiO2, the manganese-doped TiO2 powder, and a pre-dispersion of this material based on a customised blend of carrier fluids and dispersing agents. Each formulation also contained photo-labile organic filters. All three types of TiO2 exerted a photostabilising effect, but the manganese-doped materials stabilised the organic filters to a much greater extent. Also, the pre-dispersion further enhanced photostability (and SPF) compared to the powder. Possible explanations for this effect are discussed.  

The ultraviolet (UV) part of the spectrum is divided into UVA (320 nm – 400 nm), UVB (290 nm – 320 nm) and UVC (<290 nm). Of those three sub-divisions of the UV spectrum only UVA and UVB are realised at sea level and exposure to both has important biological consequences for the skin. The shorter wavelength UVB is absorbed by DNA, causing sunburn and chromosomal damage, which may or may not lead to skin cancer. The longer wavelength UVA is absorbed by chromaphores in the skin resulting in the formation of reactive oxygen species (ROS), including free radicals, oxygen ions and peroxides, all of which contribute to skin ageing and carcinogenesis. Both UVA and UVB, however, will contribute to tanning or darkening of the skin which for a whitening product must be avoided. In order to prevent tanning, darkening of the skin and sun damage, modern sunscreen formulations must be highly effective in protecting against both UVB and UVA radiation. In order to achieve these high levels of protection, sunscreen actives must not only offer high UV absorbance, but also must be photostable, so that protection is maintained throughout the period of exposure to the sun. It is known that certain organic UV filters are subject to photodegradation,1-5 and therefore the development of technologies to improve the photostability of these filters has been the subject of much research in recent years.1,6 Wakefield and Stott7 studied the photostability of butyl methoxydibenzoylmethane (BMDM) and octyl methoxycinnamate (OMC) in O/W formulations containing fine particle titanium dioxide and manganese-doped titanium dioxide. They found that, while conventional coated TiO2 gave a modest improvement in the photostability of the organic filters, the manganese-doped TiO2 gave a much greater improvement. In the above study, both the conventional TiO2 and the manganesedoped material were incorporated into the formulations as dry powders. The aim of the work reported in this article was to determine whether pre-dispersion of the inorganic sunscreen has any effect on photostabilisation of the organic filters.

Experimentation