Formulating High SPF Sun Care Products With A - External Factor ...

SUN CARE
LIQUID-MINERAL UV FILTER
This purely mathematical model shows
that the transmission properties of a homogenous
film decreases significantly
with increasing film irregularities represented
by the step film parameters f and
g and can be calculated as the sum of the
transmissions through the two fractions
of the film (Fig. 5).
Herzog et al. (10,11) refined this simple
model by correlation of calculated data
with measured in vivo SPF data and
hereby defining the parameters f and g
to be g = 0.269 and f = 0.935. These values
reflect a considerable roughness of a
sunscreen film as visualized in Fig. 6.
Using these parameters they have a
calibrated tool at their hands, called ‘the
sunscreen simulator’ (16). This tool allows
a good prediction of in vivo SPF’s
for a given roughness of the film and
in dependency of the UV-filter concentration
(Table 1, B1-B3). However, this
method fails in the prediction of the in vivo
performance for formulations containing
Polysilicone-15 (Table 1, A1-A3).
This result is not surprising as Polysilicone-15
has a comparable low UV specific
extinction. Nevertheless, there are
other effects compensating the low UV
specific extinction which can be explained
using the same mathematical
model based on the step film theory. Traditional
organic UV-filters as well as inorganic
or organic pigments are concentrated
on the skin surface after application
and evaporation of the water phase.
During this concentration process, traditional
organic UV-filters tend to aggregate
(13) as well as pigments tend to agglomerate.
Due to these reasons, it is difficult
to distribute the UV-filters evenly
over the inherently rough skin surface.
The model proposed by Herzog (16) using
formulation containing traditional
UV filters takes these effects into account
through the choice of the parameters
f and g. However, this model does
not consider effects which lead to a more
even distribution of the UV-light absorbing
units on the skin which would
consecutively be reflected in higher values
for g and lower values for f.
Through the unique structure of Polysilicone-15
the aggregation of the chromophoric
systems is significantly reduced.
This is the case as the chromophore units
are hold apart by the dimethylsiloxy-
Fig. 5 The step film model as introduced by O´Neill (12)
Fig. 6 The calibrated step film model by Herzog et al (10,11,16) reflecting
in vivo conditions
INCI A1 B1 A2 B2 A3 B3
Polysilicone-15
Ethylhexyl
PARSOL® SLX 5 3 3
Methoxycinnamate
Butyl Methoxy-
PARSOL® MCX 5 3 3
dibenzoymethane
Phenylbenzimidazol
PARSOL® 1789 2 2 2.5 2.5
Sulfonic Acid
4-Methylbenzylidene
PARSOL® HS 2 2 2 2
Campher PARSOL® 5000 3 3 4 4
Titanium dioxide Uvinul® TiO2 4 4
Calculated in vivo SPF according to (16) 4 7 14 20 33 44
Measured in vivo SPF* 10 11 20 21 44 41
*International Sun Protection Factor (SPF) Test Method, COLIPA, February 2003
Table 1 The ‘sunscreen simulator’
4 SÖFW-Journal | 131 | 7-2005