Enhancing photo-stability of sunscreens

Testing procedures
Vitro-skin (IMS Inc., US) was hydrated for 16-24 hours in a hydration chamber containing glycerine (15% w/v) in water, at room temperature. Formulation samples were applied to the hydrated Vitro-skin at a dose of 2 mg/cm2 and rubbed in with a gloved finger. The film was then mounted in a 6 x 6 cm glassless slide mount and left to dry for 15 minutes. In vitro SPF was measured with a Labsphere UV-1000S UV transmittance analyser.

Samples were then irradiated in a Honle Sol-2 solar simulator, with an output energy of 8 mW/cm2. Samples were irradiated for four hours, and then the measurement of in vitro SPF was repeated.

From the absorbance data measured in the in vitro SPF tests, the following UVA performance parameters were calculated:

1 Predicted UVA protection factor (UVA-PF): This is an in vitro prediction of the UVA protection factor as measured by persistent pigment darkening. It is calculated by using the following formula: Predicted UVA-PF (PPD)= 400 òE(l).S(l)PPD.dl 320 400 òE(l).S(l)PPD.T(l).dl 320 where: E(l) = spectral irradiance at wavelength l; S(l)PPD = PPD action spectrum at wavelength l; T(l) = spectral transmittance at wavelength l. The irradiance spectrum used corresponds to that of the light source defined for in vivo PPD tests, as defined in German standard DIN67502.

2 UVA-PF/SPF ratio: This ratio forms part of the new EU guidelines on efficacy and labelling of sun protection products. The guidelines state that the UVA-PF should be at least one-third of the SPF. An in vitro method for determining UVA-PF (and hence the UVA-PF/SPF ratio) has been developed by COLIPA.8 This method requires a pre-irradiation of the sample to take account of photodegradation of the actives, so good photo-stability is vital to achieving the specified criterion.

3 UVA/UVB ratio: This is used as the basis for the “Boots Star Rating System” in the UK, and is calculated from the area under the absorbance curve, as follows: UVA/UVB ratio = UVA area per unit wavelength UVB area per unit wavelength

4 UVA-I/UV ratio: This is a modification of the Boots system, and has been proposed by the FDA as an in vitro method to be used in the US.9 It is calculated as follows: UVA-I/UV ratio = UVA-I area per unit wavelength Total UV area per unit wavelength (Note: UVA-I is the wavelength range from 340 to 400 nm).

Results

Figure 1 shows the absorbance curves for the three formulations as measured before irradiation (t=0) and after 4 hours in the solar simulator (t=240). With the conventional TiO2, there is a significant loss of protection, especially in the UVA, after four hours irradiation. In this study, there was no formulation based only on organic filters. However, from the previous work,7 it is to be expected that such a formulation would show an even greater loss of protection.

The manganese-doped TiO2 shows significantly improved photo-stability compared to the conventional TiO2. However, when the manganese-doped material is incorporated as a predispersion, there is an even greater improvement, both in the initial absorbance and also in the photo-stability. Figures 2, 3, and 4 show the UVA parameters for the three formulations, pre- and post-irradiation. Once again, it is clear that the major improvement in photo-stability comes from the presence of manganese. The formulations containing manganese-doped TiO2 maintain a much better balance between UVA and UVB protection after irradiation, compared to the formulation containing conventional TiO2. Also, the formulation containing the pre-dispersion of Mn-doped material gives improved photo-stability compared to the powder.

Discussion

From their studies, Wakefield and Stott7 concluded that conventional TiO2 photostabilises organic UV filters primarily as a result of its UV screening properties. The additional photo-stabilising effect of Mn-doped TiO2 is believed to arise from scavenging of reactive oxygen species (ROS) by manganese ions at the surface of the particles, according to the following reaction scheme: Mn2+ ÜÞ Mn3+ + e– Mn2+ + OH• Þ Mn3+ + OH– Mn3+ + •O2 Þ Mn2+ + O2 Thus we have two possible explanations for the improved photo-stabilisation effect with a pre-dispersion of Mn-doped TiO2. It is known that, as UV filters in their own right, dispersions of physical sunscreens are more effective than powders.10 Therefore, the improvement could simply arise from the fact that the pre-dispersed material absorbs more of the incident UV light, and protects the organic filters by doing so. Formulation trials have been conducted with the Mn-doped TiO2 by itself, and the pre-dispersion does indeed show increased absorbance compared to the powder.

The second possible reason is that predispersion of the Mn-doped TiO2 means that the particles agglomerate to a lesser extent than is the case with the powder form. As a result, more particle surface is exposed. As explained above, the ROSscavenging occurs at the particle surfaces, so with more surface available there is likely to be more scavenging of ROS and hence less degradation of the organic filters.

Unfortunately, the complexity of UV absorption in a multi-absorber system means that simple absorbance data is not sufficient to quantify how much of the organic absorbers remain after irradiation. Therefore we cannot determine, based on the available data, whether the improved UV screening by the pre-dispersion is sufficient to account for the improved photo-stabilisation effect observed, or whether the second mechanism (more available surface) is more important. This would require a quantitative analysis of the amounts of organic filters in the formulations, before and after irradiation.

Conclusion

Previous work has shown that titanium dioxide doped with manganese is more effective than titanium dioxide alone in reducing photo-degradation of organic UV filters. From the data presented in this article, we can conclude that, by incorporating the Mn-doped TiO2 as a predispersion, an even greater improvement in photo-stability is achieved.

References

1 Bonda C.A. The Photostability of Organic Sunscreen Actives: A Review, in Sunscreens: Regulations and Commercial Development, 3rd edition, Shaath N.A., ed., New York: Taylor and Francis, 2005, pp. 321-349.

2 Deflandre A., Lang G. Photostability Assessment of Sunscreens. Benzylidene Camphor and Dibenzoylmethane Derivatives, Int. J. Cos. Sci, 10, 53-62 (1988).

3 Shaath N.A., Fares H.M., Klein K. Photodegradation of Sunscreen Chemicals: Solvent Considerations, Cos.Toil., 105 (12), 41-44 (Dec. 1990).

4 Schwack W., Rudolph T. Photochemistry of Dibenzoyl Methane UVA Filters Part 1, J. Photochem. Photobiol. B, 28, 229-234 (1995).

5 Marginean-Lazar G. et al, Sunscreens’ photochemical behaviour: in vivo evaluation by the stripping method, Int. J. Cos. Sci., 19, 87-101 (1997).

6 Bonda C.A., Steinberg D.C. A new photostabilizer for full spectrum sunscreens, Cos. Toil., 115 (6), 37-45 (June 2000).

7 Wakefield G., Stott J. Photostabilization of organic UV-absorbing and anti-oxidant cosmetic components in formulations containing micronized manganese-doped titanium oxide, J. Cosmet. Sci., 57, 385-395 (2006).

8 COLIPA Guideline: Method for the in vitro determination of UVA protection provided by sunscreen products, 2007a (http://www.colipa.com/site/index.cfm?SID= 15588&OBJ=28546&back=1)

9 Food & Drug Administration, “Sunscreen Drug Products for Over-The-Counter Human Use: Proposed Amendment of Final Monograph” 21 CFR Part 352, §352.71 paragraph (h) (72 FR 49119 – 49120), Aug. 27, 2007.

10 Woodruff J. Formulating Sun Care Products with Micronised Oxides, Cosmetics & Toiletries Manufacture Worldwide, Aston Publishing Group, 179-185 (1994).

ABSTRACT
The aim of the work described in this article was to investigate the effects of pre-dispersion on the ability of inorganic sunscreens to photo-stabilise organic UV filters.

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 photo-stabilising effect on photo-labile 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 photo-stabilise 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 manganesedoped materials stabilised the organic filters to a much greater extent. Also, the pre-dispersion further enhanced photo-stability (and SPF) compared to the powder. Possible explanations for this effect are discussed.