for each type of product formulation. This variability is well
recognized when testing various types of products with different
formulation bases which yield different correlation slopes. Here,
we have demonstrated that differences in correlation slopes can
also derive from the use of different substrate materials.
4. Conclusions
In this work different substrate materials for sunscreens testing
have been investigated. Transpore and laboratory prepared PTFE
are cheap and easily available on the market, while Vitro-Skin,
roughened quartz plate and roughened PMMA plate are more
expensive. Roughened quartz and PMMA plate can be re-used, but
the cleaning process is difficult and very time consuming. VitroSkin must be hydrated and this must be performed 24 h before the
use. Application of sunscreen products at a rate of 1 mg/cm2 on
Transpore, Quartz, PMMA, PTFE and Vitro-Skin have been
performed quite easily, although with hydrated Vitro-Skin the
application must be performed quickly in order to weight correctly
the amount applied. Controlling the amount of product applied on
roughened quartz plate is difficult to achieve because of the low
adherence of the sunscreen. Self-standing Transpore tends to break
down and, for this reason, it is used coupled to a smooth quartz
plate; this results in lower transmittance values. Not all substrates
were photo-stable and only PMMA plate and roughened quartz
plate can be recommended for photo-stability tests. PTFE also
showed to be suitable for photo-stability tests and can be regarded
as an alternative option.
From our results, self-standing Transpore, quartz plate and
Vitro-Skin are preferable for SPF tests due to a better correlation
between in vitro and in vivo measurements. To recover the linear
regression slope, an initial calibration must be performed for
different sunscreen products labelled with different SPF values.
Calibration should be performed for each instrumentation used,
each method adopted (substrates, amount of sunscreen applied
etc.), each type of sunscreen formula (lait, gel, etc.). Finally, the
results of our measurements demonstrate that, while a correlation
between in vitro and in vivo SPF results can be established, this
correlation is never exactly 1:1; the linear regression slope must be
always used to correct the data in order to use them for labelling.
Therefore in vitro SPF testing, regardless of substrate, is not a viable
substitute for in vivo SPF testing, and should not be used as such by
any regulatory bodies for ‘‘quality control’’ of sunscreens. Further
studies are needed in order to achieve a 1:1 correlation.