Creative Developments (Cosmetics) Limited

Sun Products: SPC 2003

John Woodruff

Sun protection products remain an active area of development for the formulating chemist, and one where science is more important than folklore. Its importance can be assessed by the number of papers and posters devoted to the subject at the IFSCC Congress, Edinburgh, 2002. Professor Antony Young [Ref 1] asked “Do Sunscreens Offer Long-term Protection”? Young said that skin cancer is more common than any other type of cancer and solar UV radiation has been implicated as a major cause. Sunscreens are designed to prevent erythema and to do this they must attenuate UVB, but increasingly sunscreens also attenuate UVA. There is also an expectation that sunscreens should prevent the long-term effects of solar exposure such as photoageing and skin cancer. Mouse studies have shown that sunscreens do in fact prevent squamous cell carcinoma but it is clearly more difficult to study the effectiveness of sunscreen use in the prevention of human skin cancer. In April 2000, the International Agency for Research on Cancer evaluated the available evidence for the cancer-preventative activity of sunscreens in humans and concluded that it was inadequate to support cancer-preventative claims against cutaneous malignant melanoma and basal-cell carcinoma but there was limited evidence to support the topical use of sunscreens against squamous-cell carcinoma. There is also concern that the use of sunscreens extends the duration of intentional sun exposure, which may lead to an increased risk for cutaneous melanoma.

Various methods of measuring the deleterious effects of solar radiation on human skin and of the effectiveness of different sunscreen agents were described at the IFSCC Congress. Also described was a real-life exercise [Ref 2] whereby employees of Sara Lee, Germany, wore radiation monitors on their heads as they commuted to work and throughout the working day. As well as providing amusement to the local population and to Congress delegates, the exercise also provided some useful data. Results indicated that erythemal UVB in mid-Europe is of minor importance for daily skin care products while UVA is present all year round in significant doses that are received not only out-doors but in-doors as well and it was concluded that daily use skin care products for mid-Europe require a minimal SPF but should provide adequate UVA protection.

The introduction of new approved sunscreen agents is a rare occurrence. Bis-ethylhexyloxyphenol methoxyphenyl triazine (BEMT) was approved in year 2000 as the first oil-soluble sunscreen effective in the UVA range for general distribution. Until that time the only other oil-soluble UVA screen approved for use in Europe was drometrizole trisiloxane, and this is exclusive to L’Oreal. A paper describing the development and application of BEMT was presented at the IFSCC Congress [Ref 3]. Due to the presence of two electronic transitions with strong dipole moments perpendicular to each other hydroxyphenyltriazine exhibits a UV spectrum with two distinctive absorptions. By building on the basic hydroxyphenyltriazine molecule it is possible to produce an oil-soluble material that combines good photostability with long lasting, full spectrum protection. For rating UV-A performance the Australian standard is frequently used. UV-A protection is recognised when a sunscreen preparation transmits less than 10% of the incoming light between 320 nm and 360 nm at a path length of 8 µm. Only 2% BEMT is required to meet this standard. Also it is said to stabilise and boost the performance of ethylhexyl methoxycinnamate and of butyl methoxydibenzoylmethane (Avobenzone).

Sunglasses for the skin was the intriguing title of a paper presented by R. Chaudhuri at the congress [Ref 4] that described the production and use of UV absorbers entrapped in glass microcapsules. The microcapsules have an average particle size of about 1 micron and consist of a core of UV absorber entrapped within a silica shell. The UV absorber, which is usually an oil or oil soluble compound, constitutes about 80% of the capsule weight. The products are manufactured as an aqueous dispersion, containing 35% by weight of UV absorber(s). The refraction index of the particles is small enough that they appear transparent when applied topically; they do not agglomerate and impart a pleasant feel when applied to the skin.

The glass microcapsules were shown to effectively retain the encapsulated UV absorbers in a series of stress experiments. These included elevated temperatures, drying, pressure spraying, and incubation with emollients, tensides and other raw materials, which could have the potential to extract the capsules content. All experiments clearly demonstrated the stability of the capsules. Skin penetration experiments showed the maintenance of sunscreen actives on or near the surface of the skin and electron microscopy was used to determine that the capsules remained intact. Encapsulation of incompatible UV absorbers that are known to photo-degrade others showed improved photo-stability and contact between the active ingredients and skin is eliminated.

Despite the introduction of several new organic actives in recent years the use of inorganic oxides continues to grow and the number of variations available to the formulator are also forever increasing. One claim that is often made for the use of titanium dioxide is that although it is primarily used to provide high SPF values, it has broad-spectrum activity. A paper presented by Art Georgalas [Ref 5] described the detailed work undertaken to substantiate claims of broad spectrum and UVA radiation attenuation for titanium dioxide. A series of in-vitro and in-vivo evaluations were conducted on two sunscreen formulations; a water-in-oil (w/o) silicone base cream and a conventional oil-in-water (o/w) type cream; each containing varying concentrations of a range of titanium dioxide particle powders and dispersions. The authors included hydrophobically and hydrophilically coated particles of 15 nm and 35 nm microfine material and a pigment grade of 180 nm. Pre-made dispersions of the microfine pigments make formulation easier and also improve efficacy and aesthetics in formulations and were also evaluated.

Findings indicated effective sunscreen activity from all grades of titanium dioxide with attenuation of radiation in both the UVB and UVA solar spectrum. Preliminary results with pre-made dispersions indicated more UVB versus UVA attenuation as less and less agglomerates form, which drives the critical wavelength lower, but these formulations still showed significant UVA effect. In addition to the effects of concentration and formulation on efficacy, inorganic sunscreen activity is governed by the effective particle size present in the product film as deposited on the skin which itself is dependant on the overall state of particle dispersion, stated Georgalas. The paper was accompanied by extensive tabulated resultsand there was good correlation between the in-vitro and in-vivo results.

The validity of different test methods for determining UVA protection values has been the subject of much discussion over recent years. There are three in-vitro methods in common use: the determination of critical wavelength and the determination of the UVA/UVB ratio, each of which gives results relative to the sun protection factor of the product; and the technique described in the Australian standard 2604 that gives an absolute criterion based on the transmission in the spectral range between 320 and 360 nm. The most widely used in-vivo method is the Persistent Pigment Darkening (PPD) method, the results of which are expressed in terms of UVA-protection factors (UVA-PFs). A paper by Herzog et al described the in-vivo and in-vitro assessment of UVA protection by sunscreen formulations containing a variety of UVA absorbers [Ref 6].

The paper is too extensive to summarise here but the results are interesting. In the absence of UVB filters the UVA/UVB-ratio and the critical wavelength (λc) would be expected to be constant, irrespective of increased levels of UVA active. In the presence of a constant amount of UVB filter increasing the concentration of the UVA active should increase the UVA/UVB ratio and λc. This is so with the UVA/UVB measurements but measurement of λc showed it to be less sensitive to change. Changes in UVA-protection after irradiation due to photo-instabilities can be determined by measuring λc or the UVA/UVB ratio. Measurement of PPD gives results dependent on the concentration of UVA filter and are reported as UVA-protection factors (UVA-PFs). The Australian method is shown as a pass/fail figure and the authors concluded that formulations that meet the Australian standard are likely to show UVA-PFs results greater than 3.9+/-0.5.

Poster presentations at IFSCC Congresses are usually of a high standard, and generate much interest. There were many posters relevant to sun protection products including a poster by J. Hewitt on the influence of emulsion structure on SPF in physical sunscreen formulations [Ref 7] and one by W. Johncock describing the use of remission spectroscopy to measure the degree of broadband protection afforded by different products when topically applied [Ref 8]. Hewitt looked at the effect additions of waxes had on the rheological and SPF properties of w/o emulsions and found that additions of wax that increased high-shear viscosity decreased SPF but if the additions shortened viscosity recovery times the SPF was increased. A balance has to be found between these opposing effects; which was approximately 1% for the systems examined. Fatty alcohols are often used as viscosity modifiers in o/w systems; Hewitt found that when using hydrophobic microfine dispersions of TiO2 SPF was increased with increasing carbon chain length of the fatty alcohol.

Remission spectroscopy on human skin was used by Johncock & Langner to measure the performance of disodium phenyl dibenzimidazole tetrasulfonic acid (DPDT) and butyl methoxydibenzyolmethane (BMDBM) and they compared the results with those obtained by measuring PPD and to those observed in dilute solution. They also determined how changes in the neutralising base and the emulsion composition affected the absorbance characteristics of UVA filters in thin films as measured on human skin and by in vitro absorbance measurements on thin films of emulsions spread on quartz glass plates. It was concluded that the data gives some explanation as to why higher than expected PPD protection factors are obtained with DPDT, which are similar to those observed with photo protected BMDBM, and that the UVA protection obtained with DPDT may be further improved by changing the inert ingredients of the emulsion. It was also demonstrated that the absorbance of UV filters observed in dilute solution have little relevance to their behaviour as thin films of emulsion applied to human skin.

The academic nature of papers and posters at scientific gatherings is very useful for giving an insight into the background of the ingredients available to the formulating chemist but it is also worthwhile examining the information offered by the material supplier. Thus Merck KgaA offers the sunglasses for skin referred to at the IFSCC Congress as Eusolex UV-Peals. The first commercially available product contains ethylhexyl methoxycinnamate, still commonly referred to as OMC in memory of its previous INCI designation of octyl methoxycinnamate. The supplier’s literature emphasises the non-irritant nature of active ingredients isolated from direct contact with the skin and also the improved photostability of systems containing BMDBM when used in combination with encapsulated OMC.

Two suppliers each provide excellent background literature on sun care. That supplied by Cognis is entitled Sun Protection – a Global Market? It compares EU vs. USA regulations and describes marketing trends in the two areas. Of the sixty sunscreens in use worldwide only eleven are common to the USA and Europe, and only three of these can be used at the same concentration in the finished product. Only nine are common to the USA, Europe and Japan. Microfine oxides are allowed in all three markets and because of this and their safety record they are the most common choice for children’s and high SPF products. The literature supplied by Cognis gives useful tips on formulating with microfine oxides and other actives and also on the use of PIT technology for formulating sprayable sunscreen products.

Sun Products is the simple title of the literature supplied by Croda Oleochemicals but it contains a wealth of information on all aspects of sun care, from a basic description of solar radiation and its effects on human skin through market trends and onto help with formulation. Because Croda supplies such an extensive range of cosmetic ingredients the literature covers all aspects of formulation including the selection of emulsifier, the effects of different emollients on SPF, on imparting water-resistance and it gives formulation guidelines for every physical type of product. The literature is profusely illustrated with suggested formulations and it concludes with descriptions of after-sun and artificial tanning products.

Sun care is not just about providing high SPF and UVA protection figures but also about combating photo-induced radiation damage. Solar radiation that penetrates the skin causes the formation of reactive oxygen species within skin cells. These cause the peroxidation of fatty acids, generating inflammatory reactions. They also polymerise proteins in the dermis leading to complex brittle structures, which lead to slack, wrinkled skin. Although most sun damage is caused by UV light blue light of 445 nm is also a cause of free-radical formation in skin cells. Various anti-free radical materials are available for inclusion in sun care products. Recently introduced is Helioxine from Silab, which is extracted from sunflowers (Helianthus annuus). Described as a biological photo protective shield it is said to be rich in photo protectors and polyphenols that have a free-radical quenching effect. It can be used in daily skin care products, be added to sun protection products or used to prevent hair damage by UV radiation, a method that might be more effective than trying to coat the hair shaft with a UV filter. Also from Silab is Bioprotectyl, a material extracted from apple skins that is rich in flavonoids and chalcones and is a natural anti-oxidant.

Lutein is a mixture of carotenoids found in the chlorophyll complex of many green-leafed plants, and also in human skin. It absorbs radiation at 445nm and protects the epidermis from peroxide formation. It also reduces UVA-induced oxidative stress and UVB-induced inflammation. Kemin Personal Care supplies Lutein as an oil-soluble serum extracted from lettuce, broccoli, spinach and kale. Floraglo Lutein is a crystalline form extracted from marigold that is suspended in the water phase of products to provide similar protection to Lutein. Other materials of natural origin suggested for sun protection include Pronalen Sunlife, Provital SA, an antioxidant derived from malt, Ginkgo Biloba extract, Vitus Vinefera (Grape) seed extract and Soy Isoflavone Extract, all available from Carrubba Inc.

Finally, although pale-skinned Europeans are fully aware of the dangers of solar radiation, many still cherish a suntan. Tyrostan is a tan activator from Chemlink UK that has been synthesised as the condensation product of tyrosine and decanoic acid; it is water-soluble and has the INCI designation Potassium Capryl Tyrosine. Adding from 2% to 5% of this to a suitable formulation significantly shortens the exposure time necessary to achieve a natural suntan.

All references are to papers and posters presented at the IFSCC Congress, Edinburgh, 2002.

1. Antony R Young, King’s College London, St Thomas’ Hospital, London

2. Using real life UV monitoring to design a daily skin care product; Rudolph T., Träger Ch. Sara Lee

3. Hydroxyphenyltriazines: A new generation of cosmetic UV filters with superior photoprotection; Hueglin D., Herzog B., Mongiat S. Ciba Specialty Chemicals Inc.

4. Sunglasses for the skin: UV absorbers entrapped in glass microcapsules; Chaudhuri R, Lapidot N et al

5. Inorganic Sunscreens: Substantiation of Broad Spectrum and UVA Radiation Attenuation Claims; Art Georgalas & Elizabeth Kasprzyk, TRI-K Industries, Inc.

6. In vivo and in vitro assessment of UVA protection by sunscreen formulations containing either ZnO, Butyl Methoxy Dibenzoyl Methane, Methylene Bis-Benzotriazolyl Tetramethylbutylphenol, or Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine; Herzog B., Mongiat S., Deshayes C., Neuhaus M., Quass K., Mantler A., Comte C., Ciba Specialty Chemicals Inc.

7. The influence of emulsion structure on SPF in physical sunscreen formulations; JP Hewitt, S Housley, AC Pool. Uniqema, Wilton, UK

8. Broadband UVA Protection Demonstrated by Remission Spectroscopy on Human Skin; Dr. William Johncock, Roland Langner. Haarmann & Reimer GmbH.