Sun care technology – bright ideas

While regulatory hurdles may slow the development of new UV filters, there is no shortage of innovation in sun care formulation. Julian Hewitt examines the background and identifies the ingredients and technologies providing the protection of the future

For the sun care formulator, life isn’t getting any easier. Globally sun care is one of the fastest growing sectors in personal care, leading to increased competition as more manufacturers look for a slice of the pie. This situation, combined with increasing regulatory pressures and a growing list of demands from ever more discerning consumers, means that it is no longer enough to create a product that is safe and meets the required SPF. Beyond these fundamental requirements, sun protection products must also meet UVA protection standards, and consumers demand products that are easy to use, aesthetically pleasing and of course as inexpensive as possible. Depending on the target market, other requirements might include ‘natural’ ingredients, avoidance of certain ingredients or inclusion of other actives to create additional marketing or performance claims.

Test methods

The efficacy of a sun protection product is defined by two parameters: the Sun Protection Factor and a UVA protection parameter. The definitive measure of SPF is provided by in-vivo testing on a panel of human volunteers; there are different protocols for doing this in different parts of the world, but recent years have seen significant progress in global harmonisation of test methods.

Most recently the International Standards Organisation (ISO) published its harmonised in-vivo SPF test method and it is expected that many countries, including Australia, New Zealand, Japan, the EU and South Africa will adopt this in due course. It remains to be seen whether the US FDA will adopt the ISO method or maintain its own protocol. Differences in the detail of SPF test methods can make a significant difference to results, but the fundamental principle behind the tests (measuring the UV dose required to cause the skin to begin to turn red, with and without protection) is the same in all cases.

The same cannot be said for UVA performance parameters. Methods for measuring UVA protection have been the subject of heated debate for as long as this author has been involved in this industry (20 years and counting!).

Although ISO is working towards harmonisation of UVA test methods – and has already published an in-vivo UVA method – a number of different test methods, performance criteria and labelling systems are currently in use in different parts of the world. In Europe performance standards were laid down in the European Commission Recommendation of 22 September 2006.^[1] According to these guidelines, the UVA Protection Factor of a product should be at least one-third of the labelled SPF. The UVAPF can be measured in-vivo or in-vitro, and European cosmetics association Colipa developed a method for the in-vitro measurement.^[2] A further requirement is that the critical wavelength should be at least 370nm. It is important to note that these are guidelines rather than actual regulations, but the industry has effectively self-regulated on this issue and meeting the one-third criterion is now seen as a prerequisite for any sun care formulation.

In the UK, the Boots Star Rating System, based on an in-vitro measurement of the mean UVA/UVB absorbance ratio, is still used. The system was revised in 2008 to take account of the new European requirements. The one- and two-star ratings were abolished, as products in these categories would not meet the EU one-third criterion. Also, a pre-irradiation step was introduced to take account of any photo-instability in the formulation; the Colipa in-vitro UVA test uses a different pre-irradiation protocol.

Meanwhile in the US, the sun care industry still awaits publication of the finalised version of the FDA Sunscreen Monograph, the development of which dates back to 1978. A ‘final’ monograph was issued in 1999,^[3] but made no mention of UVA test methods or labelling. The FDA sought to remedy this, and some other issues, with proposed amendments to the monograph, published in 2007.^[4] The UVA test methods in particular have been debated at length, and at the time of writing the complete, final monograph has yet to be published.

Table 1: Recent developments in organic UVA filters
INCI name	Acronym	Trade name	Supplier
Bis-ethylhexyloxyphenol methoxyphenyl triazine	BEMT	Tinosorb S/Escalol S	BASF/ISP
Methylene bis-benzotriazolyl tetramethylbutylphenol	MBBT	Tinosorb M	BASF
Diethylamino hydroxybenzoyl hexyl benzoate	DHHB	Uvinul A Plus	BASF
Disodium phenyl dibenzimidazole tetrasulfonate	DPDT	Neo Heliopan AP	Symrise

UV filters

The development of new UV filter molecules is hindered by the substantial regulatory barriers to the introduction of such materials. In Europe, the new Cosmetics Regulation was published in December 2009 and came into force on 11 January 2010, but will not be fully implemented until July 2013. In the meantime the Cosmetics Directive (76/768/EEC) remains the current applicable legislation. Some of the Annexes have been renumbered between the Directive and the Regulation; for example the list of permitted UV filters is Annex VII in the Directive but is Annex VI in the Regulation. This Annex currently lists 27 permitted UV filters. The most recent additions to this list have mainly been UVA or broad-spectrum filters, such as those shown in table 1.

When it comes to choosing which filters to use, the author has found BASF’s sunscreen simulator (www.basf.com/sunscreen-simulator) to be very useful. This online tool, originally developed by Ciba and now revamped and relaunched by BASF, allows registered users to input their proposed combination of UV filters and calculates predicted SPF and UVA protection parameters. Of course the formulator still has to actually make the formulation and test it, but the simulator often provides a useful starting guideline.

With the difficulty and expense involved in developing new UV filters and gaining regulatory approval for them, further recent developments in this area have been confined to new physical forms or delivery systems for existing filters. BASF’s Tinosorb S Aqua is an aqueous dispersion of BEMT in a PMMA polymer matrix. The dispersion contains 20% BEMT. This allows BEMT to be incorporated in the water phase, which increases formulation flexibility by reducing the amount of UV filter that needs to be in the oil phase, and also improves the overall distribution of the actives throughout the emulsion.

Inorganic filters provide considerable scope for new variations, without changing the fundamental identity of the active. For example, Croda’s Solaveil SpeXtra dispersions, launched last year, contain a new grade of TiO₂ that provides substantially more UVA protection than conventional grades. This allows high SPF and the EU UVA criteria to be achieved with a single active ingredient. One of the dispersions is also Ecocert approved.

Another interesting new variant is Miyoshi Kasei’s TTC-30 (available from Azelis), which consists of ultrafine TiO₂ deposited on talc; this is especially useful for powder make-up products where an SPF is required. Sensient’s Matlake TMO (available from Adina) consists of ‘nanofine TiO₂ co-processed with alumina’, resulting in a material that is claimed to be ‘nanofine-free’. This is just one example of a current trend in inorganic sunscreens. Ill-founded concerns over the safety of nano-sized inorganic sunscreens, perpetuated by misinformation in the popular press, have caused some finished product manufacturers to avoid the use of nanoparticles. In turn, a number of suppliers now offer ‘non-nano’ products.

This trend is further fuelled by the fact that when the Cosmetics Regulation comes into force in 2013, products containing nanoparticles will be required to state this in the ingredient listing, for example ‘titanium dioxide (nano)’. At present however there is still no clear regulatory definition of nanoparticles. Once this definition, and the recognised particle sizing method(s), has been decided, we will have a clearer view of which inorganic sunscreen grades will be defined as nano and which will not.

Most other current innovations in the sun care field are related to maximising the efficacy of the existing actives or incorporating additional benefits or claims into sun protection products. Some of these developments include the following areas.

SPF boosters

The last couple of years have seen the launch of several new ingredients promoted as SPF boosters, helping formulators to develop high SPF systems while minimising the content of UV filters. Some of these have been reported in previous SPC sun care articles. For example: Croda’s SolPerForm 100 [INCI: Aqua (and) hydrolyzed wheat protein/PVP crosspolymer]; Interpolymer’s Syntran PC 5227 (available from Azelis); and Dow Chemical’s SolTerra Boost [INCI: Methylcellulose], available via Univar Europe, and SunSpheres SPF Boosters [INCI: Styrene/acrylate copolymer]. SolTerra Boost is designed to boost the SPF of formulations containing inorganic filters, while SunSpheres SPF Boosters can be used with organic and inorganic filters.

Interpolymer has launched a new paraben-free version of Syntran PC 5227, called Syntran PC 5227-CG [INCI: Aqua, polyacrylate-15, polyacrylate-17, C11-17 pareth-7, C11-15 pareth-40, sodium laureth-12 sulfate, disodium lauryl sulfosuccinate, phenoxyethanol, caprylyl glycol].

Another film-forming polymer claimed to show SPF boosting properties is Baycusan® C1000 [INCI: Polyurethane-34] from Bayer Material Science. New test results show this material to have an SPF boosting effect in formulations based on broad-spectrum UV absorbers such BEMT as well as systems based on only UVB filters (octocrylene and ethylhexyl triazone). It is further claimed that formulations have a pleasant skin feel, meaning they aren’t sticky, waxy or greasy. Other benefits of Baycusan® C1000 include stabilisation of o/w emulsion systems and enhancement of water resistance.

The natural megatrend has made its way into sun care too, and a naturally derived ingredient that is claimed to show SPF boosting effects is Phytoterra Organic Maté [INCI: Ilex paraguariensis leaf extract & maltodextrin] from Arch (distributed by Adina). This natural antioxidant, certified organic by the USDA, contains a combination of phytochemicals and free radical scavengers, including 18-20% polyphenols. In-vitro SPF tests, on a formulation containing butyl methoxydibenzoylmethane (BMDM), homosalate and ethylhexyl salicylate, showed that SPF was increased from 14.9 to 23.4 when 1.5% of Phytoterra Organic Maté was included. Supplier literature indicates that this boost is due to the presence in the ingredient of around 5% chlorogenic acid, which absorbs UVA in the wavelength range 320-329nm. However, the literature is also careful to point out that Phytoterra Organic Maté by itself yields no SPF and so is not a UV filter in its own right.

Continuing with the natural trend, Lonza’s Laracare A200 [INCI: Galactoarabinan], available from Cornelius, is a naturally derived and Ecocert approved polysaccharide that aids film formation and is claimed to boost SPF by up to 50%.

Water resistance

Several of the polymers promoted as SPF boosters are also claimed to enhance water resistance, which remains a key requirement for most beach sun care products. Other ingredients that can help to improve water resistance include AkzoNobel’s Dermacryl AQ-F [INCI: Acrylates copolymer], available from Azelis. This polymer is supplied as a liquid aqueous emulsion for ease of incorporation into o/w formulations and is claimed to be compatible with most commonly used UV filters, including zinc oxide. The polymer is reported to be sensitive to some coatings used in TiO₂ dispersions, but AkzoNobel’s literature includes a list of TiO₂ dispersions that are compatible. Dermacryl AQ-F requires no heat or neutralisation and shows good salt tolerance. It is also claimed to be particularly well suited for sprayable o/w sunscreen systems, an important advantage bearing in mind the current popularity of this type of formulation.

For some time now, UV filters have been finding their way into other skin care and cosmetic products as well as the traditional beach sun care formulations, and at the same time, ingredients originally developed for other applications are crossing over into sun care. An example of this is Dow Corning MQ-1640 Flake Resin [INCI: Trimethylsiloxysilicate (and) polypropylsilsesquioxane]. This material (available from Azelis in the UK and from Univar Europe in several other countries in the EMEA region) is a combination of MQ and T-propyl type silicone resins. It was originally developed for use in decorative cosmetics where it offers advantages such as wash-off resistance, non-transfer properties, and improved flexibility for more comfortable wear in foundations and lipsticks. The wash-off resistance and film flexibility make this an obvious candidate for improving water resistance in sunscreen products. Experiments with a dispersion of zinc oxide showed that incorporation of MQ-1640 resin improves resistance to both rub-off and wash-off.^[5]

Another such crossover ingredient is ISP’s Advantage Plus [INCI: VA/Butyl maleate/isobornyl acrylate copolymer], which was developed as a hair styling polymer but has found application in anhydrous, continuous spray sunscreen formulations, where ISP has found that it boosts both SPF and water resistance.

A challenge that sun care formulators can face is how to achieve water resistance in a formulation while ensuring the sensorial profile of the final products remains good and more than acceptable for the consumer. The use of water-soluble UV filters is often desirable for reasons of improved skin feel, but also because a combination of oil-soluble and water-soluble UV filters can give higher SPF than oil-soluble actives alone. But using water-soluble UV filters has a negative influence on water resistance because they are easily removed after bathing.

A recent study by DSM sought to address this problem with an optimised combination of filters to achieve high protection and water resistance while giving the formulation a very good skin feel. A product combining Parsol SLX [INCI: polysilicone-1], Parsol HS [INCI: phenylbenzimidazole sulfonic acid], Parsol 1789 [INCI: BMDM] and Parsol 340 [INCI: octocrylene] was tested against two market benchmarks in a sensory study, which showed the superiority of the Parsol formulation when different sensorial parameters were evaluated by trained panellists.

Solubility

Another area of development in sun care is new emollients to improve the solubility of UV filters and hence optimise their efficacy. An ingredient that has been previously reported here is LexFilm Sun from Inolex [INCI: Polyester-7 (and) neopentyl glycol diheptanoate], available from Azelis, which combines a waterproofing polymer with a light, dry emollient that is also an effective solvent for organic UV filters.

Croda has launched Crodamol SFX [INCI: PPG-3 benzyl ether ethylhexanoate], a patented emollient ester that provides many of the sensory benefits of cyclomethicone D5 while also being an effective solvent for organic UV filters. New solubility data show that Crodamol SFX outperforms many other emollients, including C12-15 Alkyl benzoate, as a solvent for both benzophenone-3 and BMDM.

A presentation by Evonik at the last Florida Sunscreen Symposium^[6] discussed the design and development of new emollients to give enhanced sunscreen solubility. Using solubility measurements in existing and developmental emollients, the structural elements that give rise to good solubility were identified. The product of this work is a new emollient, Tegosoft XC [INCI: Phenoxyethyl caprylate], which is launching at in-cosmetics in Milan. Data show that this is a more effective solvent than C12-15 alkyl benzoate for benzophenone-3, BEMT, BMDM and ethylhexyl triazone. Tegosoft XC is also reported to give improved skin feel (less oily than C12-15 alkyl benzoate), improved wetting for hydrophilic pigments, and improved stability in low viscosity o/w emulsions.

Photostability

Another prerequisite for the modern sun care formulation is photostability. This is especially necessary to pass the various UVA tests (such as the Colipa test), all of which include pre-irradiation steps. The most notoriously photolabile UV filter is BMDM, and much research effort has been expended in recent years to identify ways to photostabilise this.

Lancaster

One of the best known approaches is to combine BMDM with octocrylene, and this strategy is embodied in the latest addition to Merck’s Eusolex UV-Pearls line of encapsulated organic sunscreens. Eusolex UV-Pearls OB-S is an aqueous dispersion of silica beads containing a combination of octocrylene and BMDM. As previously reported, encapsulating the actives in this way carries a number of advantages. Photostability is enhanced not only by the combination with octocrylene, but also by separating the BMDM from other ingredients (for example ethylhexyl methoxycinnamate) that might otherwise increase photo-degradation. Formulation and handling is made easier by allowing hydrophobic UV filters to be incorporated in the water phase of the emulsion, which also improves the sensory properties of the formulation. Encapsulation also prevents the actives from directly contacting the skin, reducing dermal uptake and hence the potential for any irritant or allergic reactions.

Several ingredients that have the ability to photostabilise BMDM and other photolabile sunscreens have been developed in recent years. To understand how these work it is useful to have an understanding of the photochemistry of sunscreens and how they photo-decay. A useful basic introduction to this subject was published in 2009 by Craig Bonda of the HallStar Company.^[7] This article shows a graphical representation of the processes by which the excited state of a sunscreen molecule releases its energy and returns to the ground state. Briefly, when a sunscreen molecule absorbs UV, it is elevated to the singlet excited state. From here it can either return to the ground state via various processes or undergo intersystem crossing to the triplet excited state. There are further processes by which the molecule can return to the ground state from the triplet state but, crucially, the molecule can also undergo photochemical reactions from the triplet state, and it is these reactions that lead to loss of UV protective efficacy as the molecule is converted to a form that does not absorb UV (or absorbs at a different wavelength). Whether a particular UV filter is photostable or not depends on which processes predominate.

Photostabilisers work by quenching the excited states, returning the molecule to the ground state before it can undergo photochemical reactions. Most photostabilisers are triplet quenchers, meaning that they quench the triplet excited state. These are effective in many cases but not fully effective in the more challenging photolabile systems, for example when BMDM is used in combination with ethylhexyl methoxycinnamate (EHMC).

However, HallStar’s most recent photostabiliser, SolaStay S1 [INCI: Ethylhexyl methoxycrylene], works by quenching the singlet state. At one time this was thought to be impossible because the singlet states of sunscreen molecules are very short lived. However SolaStay S1 works fast enough to quench it. Data from HallStar show that this ingredient can even stabilise BMDM/EHMC combinations. Most recently, Bonda has also shown that SolaStay S1 photostabilises retinol and retinyl palmitate.^[8] Another study^[9] shows that ethylhexyl methoxycrylene can almost completely photostabilise BMDM in the presence of TiO₂.

Beyond SPF

The EU sun care guidelines limit SPF label claims to a specified set of numbers (6, 10, 15, 20, 25, 30, 50, 50+), while the UVA protocols involve simple pass/fail criteria. It is therefore becoming increasingly difficult for manufacturers to differentiate their products based only on SPF and UVA claims. This has led to a trend for additional claims, for example DNA protection, anti-free radical protection, skin repair etc.

With this in mind, Croda has released new data for its Solaveil SpeXtra TiO₂ dispersions. An electron spin resonance (ESR) technique was used to measure the concentration of free radicals in a synthetic skin substitute, irradiated with UV from a solar simulator. Using this technique, measurements were made on unprotected skin, and skin protected with three different SPF15 formulations, based on the following actives:
• EHMC and BMDM (not photostabilised)
• Solaveil CT-100 (transparent TiO2 dispersion, providing high UVB protection but limited UVA protection)
• Solaveil XT-100 (Solaveil SpeXtra broad-spectrum TiO2 dispersion, offering high UVB and UVA protection)

All three formulations gave some reduction in the level of free radicals generated by UV irradiation, but the Solaveil XT-100 formulation gave by far the largest reduction. This shows the importance of photostable broad-spectrum protection in preventing the photo-induced formation of free radicals in the skin. This data will be presented by Croda at an Innovation Seminar at in-cosmetics in Milan.

A new ingredient from Symrise is targeted at preventing a newly identified mechanism of UVB damage to skin, which was the subject of a paper presented at the 2009 Florida Sunscreen Symposium.^[10] UVB rays cause a toxic response in the skin via activation of the aryl hydrocarbon receptor (AhR), a protein complex found in the cytoplasm of every cell in all vertebrates. When this binds to toxins it releases some of its chaperone proteins and then translocates into the nucleus where it interacts with DNA, causing the release of enzymes such as cytochrome P1 A1, COX-2 and MMP-1. These are responsible for long-term skin damage, for example skin irritation, atopic dermatitis, skin ageing and cancer. Symhelios 1031 [INCI: Benzylidene dimethoxy-dimethylindanone] is an AhR antagonist; in other words it inhibits the UVB-induced activation of AhR in the skin. Since AhR activation can also be caused by environmental toxins such as those found in tobacco smoke and car exhausts, Symhelios 1031 also protects the skin against the effects of these pollutants.

Another ingredient claimed to offer protection against DNA damage is Pepha-Protect from Pentapharm/DSM (available from Azelis). This material [INCI: Aqua, glycerine, Citrullus vulgaris (watermelon) fruit extract, sodium benzoate, potassium sorbate, phenoxyethanol, citrullin)] is a highly purified watermelon extract that contains small, water-soluble molecules called compatible solutes. These are produced by organisms under extreme conditions and protect cell structures such as DNA from damage. Supplier data (comet assay) for Pepha-Protect shows a 68% reduction in DNA damage.

Novel delivery systems

Several new technologies now allow formulators to develop novel formulations to deliver sunscreens. One particular area of current research is that of gelling technologies. A poster presented by Dow Corning at last year’s IFSCC Congress[11] showed how clear sunscreen gels can be formulated with a new silicone elastomer, dimethicone bis-isobutyl PPG-20 crosspolymer.

Another silicone polymer technology is the NuLastic Silk range from Alzo (available from Lehvoss). This is a range of silicone crosspolymers produced in situ in various solvents to create a continuous polymer network, not a colloidal suspension. It is claimed that this material can contain organic sunscreens within the lipophilically activated structure at high levels, whilst maintaining viscosity and providing excellent film forming on the skin.

On the subject of gellants, Sucragel from Alfa is a range of ingredients based on sucrose laurate, which function as emulsifiers and oil gellants. Alfa further claims that these materials can gel a range of sun filters to form a stable microemulsion gel. These gels can then be diluted with water to form thin sprayable sun care milks and lotions, stabilised with a small amount of gellan gum.

References
1. European Commission Recommendation of 22 Sep 2006, on the efficacy of sunscreen products and the claims made relating thereto; Official Journal of the European Union, p L 265/39 - L 265/43, 26 (Sep 2006)
2. Colipa, Method for in-vitro determination of UVA protection (2009) www.colipa.eu/publications-colipa-the-european-cosmetic-cosmetics-association/guidelines.html?view=item&id=33
3. Food & Drug Administration, Sunscreen Drug Products for Over-The-Counter Human Use: Final Monograph, Fed Reg 64:27666-27693 (21 May 1999)
4. Food & Drug Administration, Sunscreen Drug Products for Over-The-Counter Human Use: Proposed Amendment of Final Monograph, Fed Reg 72:49119-49120 (27 Aug 2007)
5. Univar Europe, private communication
6. Springer O & Jenni K, The Route to New Emollients for Sunscreen Formulations, Florida Sunscreen Symposium, Orlando, Sep 2009
7. Bonda C, Sunscreen Photostability 101, Happi (Oct 2009)
8. Bonda C & Zhang J, Photostabilization of Retinol and Retinyl Palmitate by Ethylhexyl Methoxycrylene, Cosmetics & Toiletries, 126(1), p40-48 (Jan 2011)
9. Bonda C, Neudahl G & Zhang J, Formulating Photostable Sunscreens with Titanium Dioxide and BMDM, Personal Care Europe (due for publication March 2011)
10. Johncock W, New findings in the causes and prevention of photodamage of human skin, Florida Sunscreen Symposium, Orlando (Sep 2009)
11. Starch M & Pretzer M, Formulating Transparent Sunscreen Gels with Silicone Elastomers, poster presentation, IFSCC Congress, Buenos Aires (Oct 2010)

Author
Julian Hewitt
email jp.hewitt@btinternet.com