The not-hydrated sample was suitable as a substrate,
nonetheless it must be underlined that hydrating the substrate is necessary in order to break down sunscreen emulsion in a similar way as in human skin [14,22].Theshapeoftransmittanceand reflectance curves was not affected by hydrating the substrate. However, an increase in transmittance, with a consequent decreasing in reflectance values, was observed in the hydrated sample. With regard to PMMA covered with glycerine, a small difference in the shape of the transmission curve was found when compared with substrates without glycerine (Fig. 3). Nevertheless, it must be underlined that PMMA treated with glycerine is only used to recover the transmission of the plate without applied sunscreen; therefore the impact of this difference is only in the SPF evaluation. Many authors do not use self-standing Transpore during their measurement, since itbreaks downeasily, butthey prefer toattach it on a smooth quartz plate in order to support it. We have also characterized this composed system as reported in Fig. 4.
3.2. Photo-stability tests on substrates
In our study, we investigated for the first time the photo- stability of substrate materials alone. The photo-stability of the substrates is fundamental to correctly understand the outcome of experiments, and some of the controversial outcomes can be explained by the instability of the substrates [7]. All substrates investigated have been exposed under vis + NIR (visible–near infrared) and full UV + vis + NIR (ultraviolet–visible– near infrared) radiation. In the case of full spectrum exposure the tests have been repeated by using three different low wavelength band-pass filters. In three cases the same dose of exposure has been used. Here we report the data relative only to the exposure with the WG 295 filter. The results obtained with the other two filters were perfectly compatible. The outcomeis thatPMMAwithout glycerine,Teflonandquartz 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.
The not-hydrated sample was suitable as a substrate,
nonetheless it must be underlined that hydrating the substrate is necessary in order to break down sunscreen emulsion in a similar way as in human skin [14,22].Theshapeoftransmittanceand reflectance curves was not affected by hydrating the substrate. However, an increase in transmittance, with a consequent decreasing in reflectance values, was observed in the hydrated sample. With regard to PMMA covered with glycerine, a small difference in the shape of the transmission curve was found when compared with substrates without glycerine (Fig. 3). Nevertheless, it must be underlined that PMMA treated with glycerine is only used to recover the transmission of the plate without applied sunscreen; therefore the impact of this difference is only in the SPF evaluation. Many authors do not use self-standing Transpore during their measurement, since itbreaks downeasily, butthey prefer toattach it on a smooth quartz plate in order to support it. We have also characterized this composed system as reported in Fig. 4.
3.2. Photo-stability tests on substrates
In our study, we investigated for the first time the photo- stability of substrate materials alone. The photo-stability of the substrates is fundamental to correctly understand the outcome of experiments, and some of the controversial outcomes can be explained by the instability of the substrates [7]. All substrates investigated have been exposed under vis + NIR (visible–near infrared) and full UV + vis + NIR (ultraviolet–visible– near infrared) radiation. In the case of full spectrum exposure the tests have been repeated by using three different low wavelength band-pass filters. In three cases the same dose of exposure has been used. Here we report the data relative only to the exposure with the WG 295 filter. The results obtained with the other two filters were perfectly compatible. The outcomeis thatPMMAwithout glycerine,Teflonandquartz 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.
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