The optical properties of hydrated and not-hydrated samples of
Vitro-Skin have been investigated in order to verify presence of
differences. 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]. The shape of transmittance and
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 it breaks down easily, but they prefer to attach
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 substratesIn our study, we investigated for the first time the photostability 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 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 difference of transmittance absolute 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