The outcome is that PMMA without glycerine,Teflon and quartz plates resulted photo-stable both to full spectrum exposure and to vis + IR spectral range irradiance. This fact suggests that these substrates are suitable for sunscreen photo-stability testing. In contrast, Transpore and hydrated Vitro-Skin are not suitable for such test.
Transpore resulted stable only to vis + IR exposure but not to full spectrum exposure (Fig. 5). Therefore, we can assume that UV radiation alters Transpore properties. The transmittance variation was mainly concentrated in the UVB part of the spectral range which is an UV region that strongly affects the SPF computation; the maximum differenceof transmittanceabsolute value was 8% at 310 nm for 10 min of exposure. This result can partially explain some results reported in the literature using this substrate for UV
in vitro testing. To this regard, Gers-Barlag et al. [6] demonstrated significant differences in their protection factor results using quartz plates or Transpore, especially when the applied product was irradiated before or during the tests. They also demonstrated that Transpore is inappropriate for such tests because of an unspecified interaction between sunscreen filters and the sub- strate. Vitro-Skin has been seldom used for photo-stability sunscreen tests [23].Whenexposedtoeithervis/IRorfullspectrum irradiation, the hydrated samples of Vitro-Skin underwent a tremendous damaging. In particular, its hydrating component evaporated rapidly wrinkling the sample. Moreover, the transmit- tance curve of the sample changed rapidly during the exposure as illustrated in Fig. 6a. The not-hydrated sample did not undergo a visible damaging when exposed to vis/IR irradiation, but when exposed to full spectrum radiation it resulted not photo-stable (Fig.6b),withamaximumdifferenceoftransmittanceabsolute value of 10% at 300 nm for 10 min of exposure. Therefore, also Vitro-Skin should not be used for photo-stability measurements. In Fig. 7 the curves of absorption for each substrate considered, calculated as A =1 R T where R and T are the total reflectance and transmittance as in Fig. 3, are reported. It is interesting to notice that both Vitro-Skin and Transpore absorb UV radiation mainly in the spectral region in which they are photo-unstable.
3.3. Total absorbance/transmittance measurements of sunscreen
Each substrate has been tested with the application of seven different sunscreen products. The total absorbances A(l) are reported in Fig. 8. The curves are calculated as the mean of several data acquired; the reproducibility of the values has been found within 10%. The absorbance values give the respective transmit-
Fig. 5. Full spectrum photo-stability of Transpore (3M).
Fig. 6. (a) Full spectrum photo-stability of hydrated Vitro-Skin and (b) full spectrum photo-stability of not hydrated Vitro-Skin.
D.