Creative Developments (Cosmetics) Limited
Sun
Care 2000
Advances
in the formulation of sun protection products have been fuelled by advances in
our understanding of the deleterious effects of solar radiation. Formulations
are no longer restricted to sun tan oils that promise a glorious tan but add UV-A
to UV-B protection, offer free radical scavenging and claim to boost the skins
natural immune system. Broad-spectrum is a term to be found on a significant
proportion of products and nor is such protection restricted to items for the
beach; it also appears on moisturisers and other skin care products and
compositions for the hair and nails. With such dynamic interest in the subject
it is perhaps not surprising that there is a corresponding growth in skin
lightening and artificial tanning products.
Perhaps
the single most obvious change in the sun care market is the renaming of octyl
methoxycinnamate as ethylhexyl methoxycinnamate under the INCI system. However
there have been other changes and three major conferences on cosmetic science
in the last twelve months featured papers on the dire effects of sunlight and
cosmetic means of sun protection. South Africa was an appropriate venue for the
1999 IFSCC Congress that included papers on the inhibition of photo-ageing; the
use of glutathione to protect against UV-induced immune suppression; two
methods of controlling the release rate of organic actives and one paper on
factors influencing the efficacy of oil-dispersed physical sunscreens in which
Hewitt [Ref 1] listed the three fundamental requirements for successful
formulations containing physical sunscreens as follows: -
1.
Select the correct primary particle size
2.
Maintain this particle size in the final formulation
3.
Ensure that the particles are evenly distributed
on the skin
Hewitt
said that selecting material with the correct particle size is not enough; if
the particles are allowed to agglomerate, either in the formulation or when
applied to the skin, they will behave optically as larger particles. This
reduces UV extinction, thus reducing SPF and increasing attenuation of visible
light causing whitening on the skin. An even coverage of the skin by the active
is essential in order to achieve high SPF and this is influenced by the
rheology of the emulsion, which determines how the product itself spreads, and
also by the location of the physical sunscreen particles after application. For
optimal efficacy of physical sunscreens, the maximum amount of active agent
must be incorporated into the product film that remains on the skin after
dry-down.
Hewitt
listed the principal factors that can affect the efficacy of the system as
follows: -
1.
Particle size distribution influences not only efficacy but
also transparency.
2.
Coating of the particles can improve
compatibility with oil phases, thereby rendering the TiO2 more
versatile in terms of the formula systems into which it can be effectively
incorporated.
3.
Pre-dispersion of the particles with optimised
dispersion processes and a carefully selected dispersant, maintains the
“positive” interactions in a “suspo-emulsion” system while eliminating the
“negative” interactions.
Added
emollients can influence SPF via different mechanisms. Examination of the UV
attenuation spectra allows us to deduce which mechanism is operating in any
given case. If a particular emollient has the effect of improving the degree of
dispersion, this has benefits in terms of both SPF and transparency. Rheology
modifiers, such as waxes in w/o emulsions, can improve SPF by helping to ensure
an even film over the skin; but excessive use of waxes can be detrimental to
SPF, if spreading is adversely affected.
Paris
in November was the less appropriate venue for the 2nd European
Sunfilters Conference, which included several papers on using test methods to
improve product performance and formulating better products. Whereas proving
the efficacy of the majority of skin care compositions is highly subjective and
the effects are of short duration, the main criteria of sun protection products
may be reliably measured and quantified. Also these parameters may be used to
improve the performance of the protective ingredients. SPF may be measured in-vivo
and there is a standardised method developed by COLIPA that has gained
worldwide recognition.
In-vitro
testing is both quicker and less costly than in-vivo but the results for
SPF are not as reliable as the in-vivo method. However in-vitro
testing quantifies the ratio of UV-A to UV-B protection and is the basis for
the Boots Star Rating system that is featured on the majority of sun protection
products retailed in Great Britain.
Much work has been done to measure the correlation between the two
methods. A paper by Dominique Lutz [Ref 2] described both methods and the
discrepancies between results. Lutz found the main differences between
successive in-vitro measurements were due to the substrate, the
spreading technique and the nature of the product. If these discrepancies can
be reduced correlation between in-vivo and in-vitro is improved.
Spectroscopy
is the basis of the in-vitro tests and has long been used for the
quantitative assay of organic filters. L. Ferrero [Ref 3] described how
spectroscopy could be used to characterise microfine powders and then related
this to in-vitro SPF determinations. The technique was used to
illustrate the fundamental requirement of good dispersion without agglomeration
that is required for maximum efficacy when using microfine oxides. Ferrero also
described the application of step film geometry to SPF determinations obtained
when using Transpore tape for in-vitro measurements and how this
explained some apparent anomalies in results. Janet Blakely [Ref 4] described
the application of current and new test methods to investigate product
improvements by formulation changes. Blakely undertook in-vivo and in-vitro
SPF determinations, measured wash-off resistance and used microscopy, rheology
and three different techniques of sensory evaluation to evaluate the effects of
alkylmethylsiloxanes on various formulations. It was shown that the physical
methods were very useful for screening candidate formulations and that
alkylmethylsiloxanes could impart positive product benefits by enhancing SPF
values, adding water-resistance and improving sensorial attributes on
application. Sensory evaluation was also described by Tracey Sanderson [Ref 5]
who applied descriptive sensory profiling to a number of branded sunscreens and
was able to objectively measure and compare the sensory and performance
characteristics of the various products.
The
safety of sunscreens is a well-aired subject. The organic materials are by necessity
active molecules and cases of irritancy and allergic reactions to either the
material or the product of its photodegradation have been reported. A novel way
of encapsulating the active in microcapsules of glass was presented by Noa
Lapidot [Ref 6]. After describing the manufacturing process Lapidot showed how
leaching tests were used to demonstrate the effectiveness of the encapsulation
process. The safety of microencapsulated organic actives was proven by the
absence of plasmid DNA nicking in photobiological tests and of phototoxicity on
yeast cells. There is very little loss of efficacy of the active and the
encapsulated materials are cosmetically acceptable, being transparent and of
pleasant feel on the skin.
The
third event was the In-Cosmetics 2000 Conference in Barcelona in April 2000.
Day three was entitled Skin Photoprotection, an Update, and included papers on
sunscreen rheology, optimising the efficacy of physical sunscreens, the use of
carotenoids for UV protection and on the formulation of non-aqueous and
sprayable sunscreen emulsions. The morning
of the first day concentrated on skin ageing, which is a related subject. Many
papers described the use of vitamin C and retinoids and cosmetic skin
lightening was strongly featured.
The
theme throughout the three conferences was how to improve the product by
maximising the effect of the sunscreen active. With legislation in all the
major markets of the world restricting the materials that may be used and
limiting the concentration of those that are allowed, achieving the optimum
effect is of paramount importance. The first requirement with the traditional
oil-soluble actives is to ensure complete solution in the oil phase. Butyl
methoxydibenzoylmethane and 4-methylbenzilidene camphor are two crystalline sun
filters that can cause problems and both are soluble in dibutyl adipate and
cocoglycerides. These two emollients have also been found to improve the SPF of
both inorganic and organic actives. Being polar materials they improve the
particle distribution of inorganic oxides and increase the viscosity of
dispersions. Increased polarity also enhances the SPF obtained with organic
filters and work by Leonard et al [Ref 7] shows a 15% increase in
absorption obtained with ethylhexyl methoxycinnamate when dissolved in dibutyl
adipate compared with those obtained with paraffinum liquidum and results
with cocoglycerides were even higher. Leonard also describes the benefit of a
mixture of quick, medium and slow spreading emollients in the product to extend
application time and impart a feeling of smoothness. The same paper describes
the use of a new emulsifier for cold-process o/w emulsions and the application
of PIT technology to create a sprayable o/w sun care lotion.
The
question of sunscreen solubility was also discussed in a paper by M. Luder [Ref
8] who observed the shift in maximum absorption wavelengths when actives were
dissolved in various solvents. Working with ethylhexyl triazone and
combinations of isostearic acid and isostearyl alcohol, Luder found absorption
peaks between 306.0 and 310.8 nm. He also found a difference in SPF of 7.2 to 9.3
depending on the ratio of the two solvent oils.
Silicone
compounds are used to advantage in most types of cosmetic and sun care is no
exception. The paper by Blakely [Ref 4] showed the advantages bestowed by
alkylmethylsiloxanes when used in emulsions. Dimethicone copolyol can be used
to improve the dispersion of hydrophobic TiO2 in water-based systems
and can be used to particular advantage to disperse TiO2 in
cyclomethicone. Work in the author’s laboratory discovered significant
advantages if a silicone ester was used to improve the solubility of an organic
active or the dispersion of a microfine oxide and SPF could be increased by 25%
by utilising diisostearoyl trimethylolpropane siloxy silicate [Ref 9] in a
formulation. This improvement is also due to the film forming and spreading
characteristics of the ester; it is essential that product covers the skin and
does not sink into the cracks and crevices leaving the ridges and peaks
exposed. Rheological properties over a wide temperature range are as important
as other aspects of the formulation in maximising the SPF. It is also important
that organic actives are not absorbed too deeply into the stratum corneum as
this too, impairs efficacy. A novel approach suggested by Diana Smith [Ref 10]
in a paper given at the European Sunfilters Conference was the use of polymeric
esters. These materials are based on polymers of trimethylpentanediol or
neopentylglycol and Smith showed how, by using a cross-linked polymer and
selecting the optimum molecular weight, retention of the sunscreen by the upper
layers of the stratum corneum could be increased. The polymers could be
terminated with an –OH group or capped with an alkyl grouping, which improves
the water-resistance properties.
Patent
activity has concentrated on improving performance of existing ingredients. A
patent granted to Roche in April 2000 (USP 6,048,516) described the requirement
to photo stabilise dibenzolymethane compounds if they are to be used in sun
care products. It is common to find the UV-A screening agent, butyl
methoxydibenzoylmethane, stabilised with octocrylene. The Roche patent cites
the use of dicyanodiphenylethylene derivatives that may be used instead of
octocrylene with comparative effect and it leaves the formulator the freedom to
select alternative UV-B filters.
In
general consumers prefer the sensorial characteristics of o/w emulsions to
those of w/o but the latter generally provide enhanced SPF and improved
water-resistance. A patent (USP 5,866,148) granted to L’Oreal in February 1999 combined
the two characteristics by incorporating two mutually incompatible oil-phases
into one o/w emulsion. The composition contains at least one hydrophilic and
one lipophilic sunscreen in an oil-phase that is added to a preformed o/w
emulsion and the final product may also contain microfine oxides. Another
recent patent (USP 6,030,629, L’Oreal) claims remarkable synergy between
phenylbenzimidazole sulfonic acid and benzotriazole silicone whereby 5% of a
1:1 mix of the two materials gave an in-vitro SPF of 17.8. Used alone 2.5%
phenylbenzimidazole sulfonic acid yields an SPF of 4.2 and 2.5% of
benzotriazole silicone gave 5.2.
Finally,
there has recently been a case of misrepresented SPF on a sun care product that
resulted in substantial fines for the manufacturing and the marketing company
and much adverse publicity in the media. There is no substitute for expert
in-vivo SPF testing according to recognised protocols.
Ref
1. J.Hewitt, J.Woodruff,
Factors influencing efficacy of oil-dispersed physical sunscreens, IFSCC
Congress, South Africa 1999
Ref
2. D.Lutz, Some keys for the
reliability of sunscreen spectrophotometric evaluation; 2nd European
Sunfilters Conference, Paris 1999.
Ref
3. L.Ferrero, Spectroscopy
of sunscreen products; ibid
Ref
4. J.Blakely et al,
Use of current and new test methods to demonstrate the benefits of
alkylmethylsiloxanes in suncare products; ibid
Ref
5. T.Sanderson, Brand
Fingerprinting – using sensory evaluation to develop quality products; ibid
Ref
6. N.Lapidot, Superior
properties of sunscreen UV absorbers entrapped in glass microcapsules; ibid
Ref
7. Leonard M, Ansmann A, Jackwerth B,
The synergistic sun systems concept; publication by Care
Chemicals,
Dusseldorf.
Ref
8. M.Luder, Effective sun
care formulations using less sunscreen; In-Cosmetics Conference, Paris 1999.
Ref
9. The use of silicone esters to
enhance SPF values; publication by A&E Connock Ltd.
Ref
10 D.Smith, Polymeric ester technology
for photoprotection enhancement; 2nd European Sunfilters Conference,
Paris 1999.