Formulating for protection

Sun care products play a crucial role so not surprisingly a lot is expected from them in terms of performance and aesthetics. John Woodruff suggests formulation routes to achieving sun care product optimisation

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Sun care products play a crucial role so not surprisingly a lot is expected from them in terms of performance and aesthetics. John Woodruff suggests formulation routes to achieving sun care product optimisation

Sun care for people means protection from the harmful effects of solar radiation, in particular ultra violet radiation of wavelengths 290-320nm (UVB) and 320-400nm (UVA). This is achieved by preventing UV radiation reaching the skin, either by staying in the shade, wearing suitable clothing or by applying sun screening products. Sun care also includes the application of products that mitigate the damage caused by UV radiation, usually by boosting the body’s own protection mechanisms using antioxidants and materials said to enhance DNA repair.

Launches of new organic sunscreens permitted for use in Europe are rare. All UV filters in sun protection products have to undergo a stringent safety assessment in accordance with the EU Cosmetics Directive before they can be used in cosmetic products on sale in Europe. The list of permitted UV filters, the level allowed and conditions of use are shown in Annex VII of the Directive. Any new UV filters must be approved by the European Commission’s Scientific Committee for Consumer Products (SCCP) before they can be added to the list, which requires submission of a comprehensive safety dossier. Similar restrictions apply throughout the world but there is not worldwide harmony about which sunscreens may or may not be used.

There is even a lack of agreement about the use of micronised zinc oxide as a sunscreen. It is a permitted material in the US at up to 25% but in Europe it is not allowed because of public concerns about nanoparticles. For either titanium dioxide or zinc oxide to provide protection against UV radiation without painting the skin white the particles must be smaller than 100nm, which brings them into the definition of nanoparticles. Titanium dioxide is allowed but not zinc oxide, although reference to ingredient listings on sun protection products will show that it is often included.

UVA light makes up 90% of solar radiation and penetrates deep into the layers of the dermis where it is responsible for a number of detrimental long-term effects. It increases degradation of the skin matrix proteins, elastin and collagen. It causes loss of skin elasticity and moisture and an increase in pigment disorders, all of which contribute to premature skin ageing. UVA radiation also causes damage indirectly via the generation of aggressive free radicals and oxygen species. These highly reactive species initiate a number of adverse reactions and are the most important source of UV-induced skin damage, including skin cancers.

According to Cancer Research UK more than 72,000 cases of non-melanoma skin cancer are registered per annum but it is estimated that the actual number is at least 100,000 in the UK each year. The most recent statistics show that more than 9,500 cases of malignant melanoma are diagnosed and more than 2,300 people die of skin cancer every year in Britain.[1] It is the author’s opinion that to deny the public the use of micronised zinc oxide, an efficient UVA absorber, because its safety has not been proven beyond all possible doubt shows a lack of understanding of risk assessment.

Despite such hindrances to innovation, ingredient suppliers continue to offer materials that will enhance the efficacy of existing materials, improve transparency and cosmetic acceptability of products containing micronised oxides and mitigate the effects of solar radiation when it does reach the skin. It is the purpose of this feature to introduce such materials.

RISK REDUCTION

A sunscreen that not only protects against UVA but also reduces the risk of reactive oxygen species being formed is the claim of Uvinul A Plus [INCI: Diethylamino hydroxybenzoyl hexyl benzoate (DHHB)] from BASF. It is permitted in sun protection products at up to 10% in the EU, China, Japan and South Africa but is not approved for use in Australia, the US or Canada. DHHB is an oil-soluble solid showing maximum absorption at 354nm and is available either as Uvinul A Plus in the form of yellow crystals or Uvinul A Plus Granular as off-white granules. It is also available as Uvinul A Plus B, a yellow liquid with 36% DHHB dissolved in ethylhexyl methoxycinnamate (EHMC).

The skin has an intrinsic antioxidative defence system comprising enzymatic and non-enzymatic antioxidants like superoxide dismutase or catalase, as well as ascorbic acid and tocopherol to deal with free radicals and reactive oxygen species. Topically applied sunscreens are able to protect this defence system and antioxidants can enhance it. As well as its UVA absorption properties Uvinul A Plus shows good protection capabilities against free radical formation and tests by BASF show that 3% of this with just 1% ethylhexyl triazone has SPF 6 and provides almost 100% protection to the skin’s own defence mechanisms. It is also claimed to improve photostability of other sunscreens and to act in synergy to boost SPF values of UVB absorbers.

PERFORMANCE ENHANCERS


Improving the performance of existing filters is the subject of much research and many different systems now claim to achieve this ability. It can be accomplished by finding a number of absorbers that appear to work in synergy, by ensuring the photostability of the filters used, by careful selection of solvents that boost expected SPF levels and by modifying the formula to provide a continuous film on the skin of sufficient depth to meet the Colipa recommended level of application in its test method. It is also important that whatever system is used UVA protection is proportional to the protection provided against UVB and it should be a minimum of one third the labelled SPF factor at the critical wavelength of 370nm.

DSM claims to achieve high SPF protection by combining optimum levels of its different absorbers.

Of the filters featured is PBSA, phenylbenzimidazole sulfonic acid, one of the few water-soluble sunscreen actives and it can be used to produce clear gel sunscreens. BMDBM is butyl methoxydibenzoylmethane, a UVA absorber with worldwide approval, and Parsol SLX is Polysilicone-15, a UVB absorber that shows good compatibility with other sunscreens. Parsol SLX is liquid at room temperature and its silicone backbone provides a pleasant skin feel and adds water-resistance to the formulation. It is particularly recommended for protection of hair and hair colour and is substantive to hair keratin.

ISP Europe provides a synergistic mixture of sunscreen actives already dissolved in a solvent that itself enhances SPF values. Trade named Sun Complex 100 it is a combination of ethylhexyl salicylate, BMDBM, EHMC and benzophenone-3 dissolved in phenethyl benzoate. The combination of actives is such that the desired SPF with proportional UVA protection can be attained by adding the calculated quantity of Sun Complex 100 to products suitable for topical application. It is liquid and can be cold-processed and ISP claims that its ester content reduces the greasy feel noticeable with many sunscreen products.

INSTABILITY & STABILITY

Molecules that absorb the energy contained in ultraviolet radiation (UVR) and visible light are called chromophores. The organic UV filters in sunscreens are chromophores and they convert the energy in UVR to electronic excitation energy. It was long thought that they then dissipated this extra energy as heat and returned to their ground state unharmed. However, it is now known that chromophores can be destroyed by UVR and therefore some sunscreens are prone to photo-instability and this instability is affected by the other components of the formulation.

In 2007 the US Food and Drug Administration (FDA) published its proposal to amend the regulations governing sunscreens sold to consumers in the US. In it the FDA proposed new standards for formulating, testing and labelling sunscreen products with UVA and UVB protection. The FDA also addressed the issue of sunscreen photostability and its importance to sunscreen performance: “The FDA agrees that it is important to address the photostability for sunscreen drug product formulations. Unstable product formulations present the problem of degradation of product effectiveness during actual use... The FDA is proposing to address photostability by adding a pre-irradiation step to the in-vitro test method for measuring UVA protection.”

Craig Bonda of The Hallstar Company has long worked on this problem and has published many papers. Of these, Research pathways to photostable sunscreens[2] is a complete and understandable article on the subject and is recommended to all formulators that use UV absorbers. In it Bonda describes the theory behind photostability of sunscreens and then ways of improving it. A simple test has been developed in the Hallstar laboratory to determine which combinations of sunscreens are more stable. In summary, benzophenone-3 stabilises BMDBM weakly, octocrylene stabilises EHMC strongly but does not stabilise BMDBM and ethylhexyl methoxycrylene stabilises BMDBM strongly.

The UVA filter BMDBM is not photostable and may lose 60% or more of its activity at 360nm when exposed to solar radiation. Previous work by Bonda showed that improved photostability can be achieved by careful formulation of the oil phase. He found a relationship between the rate of photo decay of BMDBM and the dielectric constant of the oil phase. As the dielectric constant was increased, BMDBM became progressively more stable until it reached maximum stability at about 8.8, after which its rate of decay increased again.[3]

Bonda has patented many solvent systems for enhancing sun product efficacy, including one that claims diethylhexyl naphthalate (DEHN) significantly improves the photostability of BMDBM. DEHN is a semi-viscous oily liquid with a refractive index of 1.53 and a specific gravity of 1.02. It is insoluble in water, propylene glycol and glycerin but freely soluble in most cosmetic oils and esters. It is a good solvent for benzophenone-3, BMDBM, MBDC and ethylhexyl triazone.

SOLUBILITY ISSUES

Because of the comparatively high levels of organic sunscreen actives required by today’s demand for products with high SPF, dissolving the active can represent a problem. LexFilm Sun from Inolex is a waterproof polymer and solubiliser for sun care. It is a synergistic combination of the high molecular weight, water-resistant polymer Polyester-7 with the light, dry emollient ester, neopentyl glycol diheptanoate. This combination yields a fluid claimed to have superior solubility for organic UV filters while enhancing water resistance. From the same supplier, LexFilm Spray is a mixture of Polyester-10 and propylene glycol dibenzoate is specially recommended for sun product spray applications. The high dielectric constant of the propylene glycol dibenzoate promotes photostability and the polyester contributes towards the total SPF of the product.

HallBrite BHB from Hallstar is butyloctyl salicylate, a solvent suitable for many oil-soluble UV filters. It is said to provide excellent spreadability, added substantivity in the presence of water and to impart a pleasant, moisturising skin feel. It is compatible with most lipophilic cosmetic additives and helps improve stability of BMDBM. Elefac I-205 from Alzo International is octyldodecyl neopentanoate, which is also a good solubiliser for organic sunscreens and an excellent dispersant for inorganic oxides. Lubrizol suggests light esters to impart a softer skin feel and highly branched esters such as Schercemol NGDO ester [INCI: Neopentyl glycol diethylhexanoate] to mitigate the oiliness and greasiness caused by oil-soluble organic sunscreens.

Hydramol TGL, INCI: Polyglyceryl-3 laurate from Lubrizol exhibits very good pigment dispersant properties which suit it to applications containing inorganic sunscreens. It is a water-soluble emollient ester that delivers moisturisation from the water phase. E-xtend-226 from ISP is

2-phenylethyl benzoate, a polar ester with good solvent properties for BMDBM and benzophenone-3 and this is said to boost the critical wavelength and UVA/UVB ratio of BMDBM.

Two relatively new sunscreens that may cause solubility problems are Tinosorb S and Tinosorb M from Ciba Speciality Chemicals. Tinosorb M is an aqueous dispersion of methylene bis-benzotriazolyl tetramethylbutylphenol, approved for use as a broad-spectrum UV absorber in Europe, South America, China and Australia. It is not soluble in commonly used emollients or solvents and Ciba suggests adding the material to a preformed emulsion. Pheonix Chemical reports that it has successfully incorporated one part Tinosorb M in four parts of a pre-emulsion comprising Pelemol BIP-PC, a mixture of butylphthalimide and isopropylphthalimide, with PhoenoMulse 100, a mixture of polyhydroxystearic acid, isononyl isononanoate, ethylhexyl isononanoate and sodium cocamidopropyl PG dimonium chloride phosphate.

Much can be done to improve sun product compositions. The high levels of organic actives or powder dispersion necessary to create high SPF products can cause difficulties in emulsion formulation. Cognis is launching Eumulgin Prisma at in-cosmetics 2009. Described as a high performance anionic emulsifier its INCI name is sodium cetearyl sulfosuccinate. It has excellent compatibility with different actives and can be used with both physical and chemical sunscreens. It is said to work well with high levels of physical sunscreens either in the powder or dispersion forms. Cognis also markets Cosmedia DC [INCI: Hydrogenated dimer dilinoleyl/dimethylcarbonate copolymer] as an additive to improve sunscreen water-resistance. This and many other ideas for improving sunscreen formulation are described in its presentation entitled Sensational suncare, which although produced in 2004 still provides good background information.

ENCAPSULATED EFFICIENCY

Improving efficacy and stability of the actives are the twin claims of other additives such as that being launched by Merck at in-cosmetics this year. Trade named Eusolex UV Pearls B-O, this product is a new combination of BMDBM and octocrylene in a stable encapsulation system to give UVA protection that can be added to the water phase without solubilisation. Eusolex pearls consist of sunscreen actives encapsulated in micro-sized glass spheres. They are very robust and as well as separating the active against possible incompatibilities with other ingredients they prevent penetration of the active through the skin.

The encapsulation process creates aqueous dispersions irrespective of the solubility of the UV filter and the hydrophilic nature of the capsule wall can be adjusted irrespective of the solubility of the active encapsulated within. This makes it possible to incorporate hydrophobic UV filters into the aqueous phase and even into purely aqueous cosmetic formulations. Particularly useful is the physical separation of incompatible actives such as EHMC and BMDBM. When present together in a conventional sun product BMDBM is rapidly lost through photo-degradation but if it is encapsulated in Eusolex pearls the process is slowed considerably. It is not necessary to encapsulate both actives – either is available in this form as are other actives.

A US Patent granted 26 February 2009 to the Sunjin Chemical Co describes the encapsulation of various sunscreen actives in methyl methacrylate crosspolymer or PMMA.[4] By encapsulating them in an inert polymer the stability of the actives is improved, direct contact with skin is avoided and processing of the materials is simplified. As well as encapsulated organic sunscreens the same technology has been applied to TiO2 and ZnO. Although the oxides consist of nano-sized particles, the bead that encapsulates them is larger and this has gained Ecocert approval.

Nanospheres from Exsymol are microspheres of polystyrene/acrylic acid copolymer loaded with active ingredients for a sustained, time release. The porous nature of the polymer allows for excellent absorption and dispersion and, unlike liposomes, they release their active over a time period of variable duration giving a constant availability profile. Nanospheres 100 OMC are loaded with EHMC to provide day-long UVB protection and are particularly suitable for skin care products.

PROTECTION BOOSTERS

The use of non-active polymers to enhance SPF is widely practised. Syntran PC 5227 from Interpolymer Corporation is a mixture of polyacrylate-15 and polyacrylate-17 designed specifically to increase the efficacy of UV absorbers. The addition of 3% to an existing formulation may boost SPF by more than 50%. SolTerra Boost is an ultraviolet protection-boosting polymer from Amerchol that is compatible with both inorganic and inorganic/organic blends. Based on methyl cellulose, 3-5% is said to dramatically increase the SPF and UVA protection when incorporated into skin care products containing inorganic UV filters, such as zinc oxide and titanium dioxide.

Orgasol is the family name of materials from Arkema that contain either Nylon-6 or Nylon-12 with EHMC. They provide SPF protection boosted by the presence of the nylon polymers. Unimers come from Induchem and rely on PVP polymers to improve SPF results and the tactile properties of the product. Also utilising PVP polymer chemistry, SolPerForm 100 [INCI: Aqua, hydrolyzed wheat protein/PVP crosspolymer] is a novel film-forming polymer from Croda Europe (see p67-68 for further information). It optimises the film forming properties of sunscreen emulsions and hence enhances the efficacy of the active ingredients and it is claimed that just 2% is sufficient to raise the SPF by between 30% and 70%. SolPerForm 100 works with both organic and inorganic UV filters and enhances UVA efficacy as well as improving SPF results.

SunSpheres SPF Booster is a styrene/acrylates copolymer manufactured via emulsion polymerization and marketed by Rohm and Haas. Sunspheres consist of hollow spheres with an external size of approximately 325nm. As light passes through the sphere it is scattered by refraction. The air in the voided sphere has a refractive index of 1.0, the refractive index of the polymer shell is approximately 1.6 and in a sunscreen formulation, the refractive index of the dried film is typically 1.4 to 1.5. When SunSpheres SPF Booster is added to a sunscreen they raise the UV protection over the whole UVA/UVB spectrum and work equally well with organic and inorganic sunscreen actives. SunSpheres are also compatible with a broad range of cosmetic ingredients, including emollients and skin conditioners.

MICRONISED MATERIALS

The use of polymers to increase UV protection is an interesting concept that would appear to circumnavigate the restrictions placed on the use of conventional sunscreen actives. The same was expected when micronised inorganic oxides were first proposed as sunscreen additives, but although the two principal materials – titanium dioxide and zinc oxide – had been used as cosmetic ingredients with an excellent safety record for many decades it took time for them to be approved in micronised form and, as already mentioned, zinc oxide particles of less than 100nm are not approved for use within Europe, with the exception of Germany.

Antaria of Australia is offering transparent zinc oxide particles one micron in diameter, which takes them out of the nanoparticle arena. Generally transparency is lost as particle size exceeds 100nm but Antaria claims to have overcome this difficulty by adjusting the refractive index of the particles to provide a UVA sunscreen permitted for use throughout Europe that is transparent on the skin. Kobo Products also produces ZnO particles guaranteed to be greater than 100nm but small enough to absorb UVA with acceptable transparency. Kobo also recommends the use of mixtures of inorganic oxides of various particle sizes to provide balanced UVB and UVA protection and supplies such mixtures as dispersions.

There are many sources for zinc oxide of sunscreen quality and it is supplied in many different formats, including mixtures with titanium dioxide to provide broad spectrum protection. Umicore produces microfine zinc oxide for sunscreen applications under the trade names Zano 10 and Zano 10 Plus. The particle size is in the range of 35-40nm, giving good absorption in both the UVA and UVA region with minimal whitening. Zano 10 Plus is surface treated with triethoxycaprylyl silane, rendering it hydrophobic and assisting its dispersion in the oil phase of emulsions.

Grant Industries supplies UV Cut TiO2 as a 40% dispersion of surface-treated titanium dioxide having a primary particle size of 16nm in cyclopentasiloxane. This material is compatible with silicone elastomer gels so products with a very light, smooth feel are possible. Croda now markets the Solaveil Clarus range of micronised oxide dispersions known for their controlled particle size combined with excellent transparency that were first produced by Uniqema. These are available in a number of different carriers, giving a wide range of possible formulations.

As part of sun protection technology there are many ingredients available to mitigate the effects of UVR that does reach the skin. A future article will look at these materials with emphasis on natural ingredients and peptide technology.

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