Dr Robert M Sayer, Dr Paul Staniland – Croda, UK
SUN CARE
Broad band, photostable prevention of UV damage Exposure to sunlight is of the utmost importance in vitamin D3 formation and has the additional cosmetically desirable effect of imparting a bronzed tanned glow to our skin in the summer months. Indeed, when people sunbathe their bodies produce endorphins, which contribute to a ‘feel-good factor’ and enhance a person’s mood. However, it is now completely indisputable that the sun’s rays can damage the skin. The short-term damaging effects of overexposure to UV radiation to the skin are well recognised by consumers, including skin reddening, blistering and burning. Furthermore, consumers are currently becoming more aware of the associated longer-term adverse effects, and it has been widely reported and well documented that excessive exposure to UV radiation can cause sun burn, photoageing, photoimmunosuppression and photocarcinogenesis.1–4 Table 1 summarises how light can be categorised according to its wavelength range and the potentially dangerous effects on the skin. While many people have protected themselves with sunscreens against UVB radiation, and therefore burning for many years, this only comprises 5% of the total UV radiation that reaches the Earth’s surface. 95% of the total UV radiation reaching the Earth is made up from UVA radiation, therefore it is important when formulating anti-ageing skin and sun care
ABSTRACT
The effects of UVB radiation have been known for many years with most people protecting themselves from induced burning. More recently the damaging effects of UVA radiation have been highlighted and sunscreens are now formulated to include protection against induced premature ageing. New skin research is showing the potential of near-UV or HEV light to induce premature ageing, the formation of wrinkles and DNA damage. A novel Electron Spin Resonance (ESR) technique has been developed to investigate the effect of inorganic and organic sunscreens in preventing radical generation in skin substitutes under the action of UV and near-UV radiation. A spin trap (5,5-Dimethyl-1-Pyrroline-N-
Oxide, DMPO) has been used to form spin adduct, thus prolonging the lifetime of radicals formed in the skin substitutes. Water in oil sunscreen formulations containing titanium dioxide (TiO2) with different levels of UVA/UVB attenuation have been used to protect the skin, and the number of radical adducts formed has been investigated. The broad spectrum, photostable TiO2 used in the investigation has been shown to reduce the number of radical adducts formed by 70%, when compared to unprotected skin. Thus highlighting the necessity for broad spectrum protection in sunscreen products when looking to reduce the numbers of radicals generated from UV and near-UV radiation.
products that full UV spectral coverage is attained. The longer term effects of exposure to UVA radiation have been linked to skin ageing, wrinkling, vascular and lymphatic damage5,6 and more recently DNA damage.7 Although of lower energy, UVA radiation penetrates deeper into the skin than higher energy UVB radiation, and has the ability to cause damage in the deeper layers of the dermis and epidermis. Having penetrated these layers of the skin, UVA radiation produces free radicals or reactive
oxygen species (ROS) such as hydroxide radicals (·OH), singlet oxygen (1O2) and superoxide anion radicals (·O2–) (Fig. 1). These species cause degradation of proteins and dermal tissue, leading to visible signs of skin damage. It is therefore reasonable to assume that preventing UVA radiation in particular from reaching the skin is extremely important in reducing premature skin ageing and photocarcinogenesis. Although wearing clothing and seeking shade is the most effective method of reducing UV damage, sunscreen is the most widely used form of photoprotection by the public.
Table 1: Summary of how UV light can be categorised according to wavelength and the corresponding potential damaging effects to the skin. Region
UVB
Wavelength range (nm) 280-320
UVA
HEV (Near-UV)
320-400
380-500
Decreasing energy Increasing penetration Penetration
Epidermis to basal cell Dermis
Damage caused to skin •Reddening •Blistering •Skin cancer
•Free radical formation •Skin ageing •Vascular and lymphatic damage •DNA damage
Dermis to blood cells •Degrades elastin and collagen •Formation of glycation wrinkles •Premature ageing
Inorganic sunscreens Inorganic based sunscreen composed of mineral UV filters, such as titanium dioxide (TiO2) and zinc oxide (ZnO) have long been regarded as safe and effective.8 They are especially preferred by individuals with a high propensity for skin irritation over sunscreen containing organic UV filters. Organic (or ‘chemical’) sunscreens, such as butyl methoxydibenzoylmethane and benzophenone-3, absorb high intensity UV radiation whereas inorganic (or ‘physical’) September 2014 P E R S O N A L C A R E
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sunscreens, by contrast, attenuate UV by three mechanisms: absorption, reflection and scattering, as shown in Figure 2. As a result of these different mechanisms, physical sunscreens attenuate UV over a broader wavelength range and this is one of the many key advantages of these materials. Despite these benefits, the use of older generations of sunscreen based on inorganic UV filters was limited because of poor cosmetic elegance. The large particle sizes of TiO2 and ZnO left a white film on the skin. In addition, the inorganic filters had poor dispersive qualities, leaving users with a grainy after-feel. Over the last number of years, patented sun care technologies have been developed that deliver true innovation in the sun protection arena and as a result, sun protection products have been dramatically refashioned and improved. Croda have a wealth of expertise surrounding the progression towards more efficient and aesthetically pleasing sun protection products and were the first to develop and market TiO2 dispersion technology, thus improving product performance, enhancing product stability and promoting ease of use. Solaveil Clarus was a breakthrough in terms of product aesthetics. It was the first inorganic sunscreen dispersion available on the market offering true transparency on skin. For the first time ever TiO2 could be incorporated into a formulation while providing transparency comparable to that of an organic sunscreen. In terms of their performance, the UV absorption profiles of TiO2 and ZnO are largely dependent on the aggregate size of the metal oxides. Sun protection factor (SPF) varies significantly if sunscreen formulations are made with the same concentration of inorganic particles, and if the sizes of the particles in the formulation vary. In contrast, the UV absorption profiles of organic UV filters are more closely related to the concentration of the filters. Figure 3 shows the UVvisible properties of TiO2 for three different aggregate sizes. TiO2 particles with an average aggregate size of approximately 100 nm (red curve) offer effective UVA and UVB protection. Significant scattering is noted in the visible region of the spectrum, and is due to the size of the aggregates and a significantly larger portion of the total attenuation is made up of scattering from larger particles in comparison to smaller particles. The cosmetic regulatory requirements dictate what is classed as effective and sufficient protection against both UVA and 2
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UVA
UVA
Organic materials
ROS
(eg pollutants)
ROS Dermis
Photoageing and photocarcinogenesis Figure 1: Schematic representation of processes in the skin caused by UVA radiation.
UVB radiation. Focus on the attenuation of UVA radiation has highlighted the requirement for products which offer broad spectrum protection. However, the creation of globally approved UV protection products can be a frustrating challenge, since UVA protection and labelling requirements vary between regions. In Europe, Colipa recommends that the UVA Protection Factor (UVAPF) of a sunscreen should be at least one third of the labelled SPF.9 In addition, the critical wavelength of the product should be at least 370 nm.9 The FDA also requires that sun protection products in America meet this critical wavelength.10 Asia meanwhile uses an in vivo Persistent Pigment Darkening (PPD) method to assign a PA label to a product, based upon its UVAPF. There are three PA rankings available, with PA+ corresponding to a product with a
UVAPF of between 2 and 4, PA++ a UVAPF of between 4 and 8 and PA+++ products offering the highest protection, with a UVAPF of greater than 8. As well as meeting the UVA and UVB requirements, sunscreen formulators face the same challenges with regards to which approved UV filters can be used. There are a number of common approaches in designing broad spectrum (UVA and UVB) UV protection formulations. The first of these is to use organic, or chemical, UVA and UVB filters to provide high SPF broad spectrum protection. However this type of system cannot be labelled ‘natural’. Organic sunscreens, sometimes known as ‘chemical’ UV filters, can lead to skin irritation. This makes it difficult to formulate a globally approved, organic-only sun protection product with additional sensorial benefits. Inorganic sunscreen
Organic sunscreen UV
UV
TiO2 or ZnO particles
Sunscreen
Skin UV light is absorbed as it passes through the film
UV light is scattered and absorbed by TiO2 or ZnO particles
Figure 2: Mode of action of organic and inorganic sunscreens.
SUN CARE
delivers high SPF, broad spectrum photostable protection, that is globally approved and mild on skin, from a single active ingredient. Proven to reduce free radical damage, it is ideal for anti-ageing skin and sun care.
70
UVB
UVA
Visible
60
■ 20 nm ■ 50 nm ■ 100 nm
Attenuation
50
Infrared and high-energy visible radiation
40
30
20
10
0 290
310
330
350
370
390
410
430
450
470
490
510
530
550
Wavelength (nm) Figure 3: UV-visible attenuation versus wavelength for spherical titanium dioxide of varying particle size. These are theoretical UV-visible profiles based on Mie theory.
Another solution commonly used by global formulators uses a combination of organic and inorganic, or physical, UV filters such as TiO2 and ZnO. However incompatibilities between ingredients and the global IP status of some UV filters can complicate the formulation process. The final option is to use inorganic, or physical, UV active ingredients, only.
Formulations based on these actives ensure a ‘natural’, mild and safe product can be formulated although care has to be taken to ensure the aesthetics meet the consumer demands. A single UV filter from Croda has been developed and is able to meet all of the necessary criteria that have been previously outlined. Solaveil SpeXtra
Over the last few years, an increased demand for cosmetic anti-ageing products has driven intense research into the processes of skin-ageing caused by solar radiation. In addition to the skin damage caused by UV radiation, there is now increased awareness around other segments of the electromagnetic spectrum and their effects on the skin. The importance of infrared (IR) radiation is only recently becoming realised. It is now known that this short-wave, non-thermal producing infrared light, in particular infrared-A (760 nm-1400 nm), causes damage to the skin and plays a part in the formation of wrinkles. More recently, scientists have also studied the potential of near-UV light, also known as high energy visible (HEV) light, which is just beyond the cut-off from the UVA to the visible region of the electromagnetic spectrum (380 nm-500 nm), to cause damage to skin.11,12 HEV light
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is said to penetrate deep into the skin and has been linked to age-related macular degeneration by activating metalloproteinases, which promote degradation of collagen and elastin, resulting in the formation of glycation wrinkles and premature ageing.11,12 The mechanisms of the damage trace back to the formation of reactive oxygen species. The increased production of singlet oxygen, superoxide and hydroxyl radical species (as shown in Figure 1) is the possible and plausible cause of damage to cell DNA. In response to the recent concerns over free radical induced skin damage, the question that emerges is ‘can sunscreens that absorb in the UVA-region sufficiently counteract the formation of radicals caused by near-UV (HEV) light?’ In response to this question, Croda have developed a new methodology based on electron spin resonance (ESR) spectroscopy, which has been used to investigate the formation of radicals in skin substitutes following irradiation by UVvisible and high energy visible wavelengths of light. The technique measures the numbers of radical adducts generated in a synthetic skin replica when exposed to UV radiation from a solar simulator. The extent to which sunscreen formulations protect skin from free radical adducts can be measured by comparing the results for unprotected synthetic skin, against those for skin protected by the test formulations. The experimental set-up is shown schematically in Figure 4. An EpiDerm Full Thickness skin substitute (MaTek Corporation) was used as the skin sample. A 5,5-Dimethyl-1-Pyrroline-N-Oxide (DMPO) spin trap (Fig. 5) was used to create a spin adduct, which prolonged the lifetime of the radical species and created more persistent adducts that could be easily detected and investigated. In a comparative study, two different water-in-oil sunscreen formulations of SPF 15 were investigated. Repeated ESR trials were run using a UV-visible light source upon unprotected skin, and with skin protected by two sunscreen formulations, containing: w Solaveil CT-100, a transparent TiO2 dispersion with strong UVB attenuation w Solaveil XT-100 (from the Solaveil SpeXtra range), a broad spectrum TiO2 dispersion with extra UVA attenuation. During the first test series, the skin models were irradiated with UV-visible light over a time period of 60 minutes. The overlaid ESR spectrum of the skin replica irradiated by UV-visible light (Fig. 6) shows broad anisotropic peaks which are common to protein-radical adducts with restricted mobility. 4
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UV radiation
Final formulation
ESR cell within ESR chamber
EFT skin substitute Figure 4: Schematic of the ESR equipment.
DMPO–5,5-dimethyl-1-pyrroline-N-oxide
R H N+
+R·
H N
O–
O· Nitroxide radical (spin-adduct)
Figure 5: Experiments utilised a spin trap to prolong the lifetime of the radical species for study.
Integrating the spectra allows the number of radical adducts formed to be determined over the time period of the experiment. The results of the experiments are shown in Figure 7 and show that both formulations reduce the number of radical adducts formed compared to unprotected skin. Further, the formulation containing XT-100 provides significantly better protection (fewer radicals generated) compared to the CT-100 formulation, despite having similar SPF values. The UVB absorbing TiO2, CT-100 reduces the number of free radical adducts formed
by 21%, whereas the additional UVA absorbing properties of XT-100 mean that the number of radical adducts formed is reduced by 70%. A second test series was carried out wherein irradiation took place with HEV light in the wavelength range 415 nm-485 nm. The ESR signals were much less intense than those generated by UV-visible light, however the broad peaks assigned to the radical-adducts still dominated the spectra. The results are shown in Figure 8. The broad spectrum filter Solaveil XT-100 again proved to be incredibly efficient here and radical-adduct formation
Protected Solaveil XT-100
Protected Solaveil CT-100
Unprotected
328
329
330
331
332
333
334
335
336
Magnetic field/mT
Figure 6: An overlaid ESR spectrum. By integrating the area within the profile, the number of radicals formed can be calculated.
337
338
70
70
60
60
Number of radical adducts formed
Number of radical adducts formed
SUN CARE
50
40
30
20
10
0
50
40
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20
10
0
Unprotected skin
Skin protected Solaveil CT-100
Skin protected Solaveil XT-100
Figure 7: Results of the ESR experiment when skin models were irradiated with UV light.
was reduced by 75%. CT-100 decreased the number formed by only 20% compared to the untreated sample. The noticed effect can be explained by means of the absorption spectrum of the filter. XT-100 provides significant attenuation across the HEV wavelengths, and therefore offers considerable protection against the production of radical species. The application of XT-100 does not only enable the fulfilment of required legal guidelines with regards to UVB and UVA protection as a single active ingredient, but also offers further protection against the HEV portion of the electromagnetic spectrum, which also contributes to skin ageing.
Conclusion In recent years, heightened consumer awareness of the damaging and ageing effects of UV radiation on the skin has created more challenges for formulators of sun and skin care products. Consumers have long been aware of the fact that ultraviolet (UV) radiation emitted by the sun can cause damage to the skin and many people have protected themselves against UVB radiation (290 nm-320 nm) and sunburn. More recently improved knowledge about the risks of UVA and premature ageing has led to the demand for UVA protection not just in sun care products but also in every day skin care products and cosmetics. As a result formulators are now challenged to create high SPF, broad spectrum products with aesthetics suitable for everyday use. Furthermore, progressions made in instrumental analysis have enabled more and more thorough and precise investigations into the processes that occur within the skin and also their mechanisms of action. From this has stemmed increased understanding and important new insights into the effects of sunlight
Unprotected skin
Skin protected Solaveil CT-100
Skin protected Solaveil XT-100
Figure 8: Results of the ESR experiment when skin models were irradiated with HEV light.
on our skin. This allows more efficient sunscreen products to be developed, which encapsulate all of the protective requirements of skin. The importance of sunscreens in skin cancer prevention is indisputable. Moreover their active role in preventing the ageing of skin by solar radiation is fast becoming recognised. To this end, Electron Spin Resonance Spectroscopy has been proven to be a highly effective technique to measure the generation of radical adducts in skin by UV-visible and HEV light. Protecting the skin substitutes with organic and inorganic UV sunscreens reduces the number of radicals formed in the samples. Sunscreens containing Solaveil SpeXtra, a TiO2 product with enhanced UVA protective properties, reduced the number of radical adducts formed in the skin by a much greater extent than a UVB titanium dioxide based sunscreen, indicating that free radicals in the skin are produced by both UVA and UVB light. Irradiating the skin with HEV light produces radical adducts in the skin which are similar to those generated by UV-visible light on the basis of the peaks in the ESR spectra. The enhanced UVA product, Solaveil SpeXtra, TiO2 gives a remarkable reduction in the number of free radicals generated in the skin as a result of its attenuation profile in the HEV region of the spectrum. The sunscreen containing Solaveil Clarus, which offers enhanced UVB protection, provides relatively less protection from HEV light. This highlights the importance of a broad spectrum physical shield in preventing both UV and near-UV (HEV) PC induced radical damage.
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2 Hersey P, Bradley M, Hasic E, Haran G, Edwards A, McCarthy WH. Immunological effects of solarium exposure. Lancet 1983; 1 (8324): 545-8. 3 Gilchrest BA, Yaar M. Ageing and photoageing of the skin. Observations at the cellular and molecular level. Br J Dermatol 1992; 127 (Suppl 41): 25-30. 4 Brash DE, Rudolph JA, Simon JA et al. A role for sunlight in skin cancer: UV-induced p53 mutations in squamous cell carcinoma. Proc Natl Acad Sci USA 1991; 88 (22): 10124-8. 5 World Health Organization. Solar ultraviolet radiation: Global burden of disease from solar ultraviolet radiation. Environmental Burden of Disease Series, No. 13. 2006. 6 Bickers DR, Lim HW, Margolis D et al. The burden of skin diseases: 2004 a joint project of the American Academy of Dermatology Association and the Society for Investigative Dermatology. J Am Acad Dermatol 2006; 55 (3): 490-500. 7 Cadet J, Douki T. Oxidatively generated damage to DNA by UVA radiation in cells and human skin. J Invest Dermatol 2011; 131 (5): 1005-7. 8 Nohynek GJ, Dufour EK. Nano-sized cosmetic formulations or solid nanoparticles in sunscreens: a risk to human health? Arch Toxicol 2012; 86 (7): 1063-75. 9 European Commission Recommendation of 22 September 2006 on the efficacy of sunscreen products and the claims made relating thereto. Official Journal of the European Union 26.9.2006; L265/39-L265/43. 10 Food and Drug Administration. Final rule: Labelling and effectiveness testing; sunscreen drug products for over-the-counter human use. Federal Register June 2011; 76 (117). 11 Boulton M, Rózanowska M, Rózanowski B. Retinal photodamage. J Photochem Photobiol B 2001; 64 (2-3): 144-61. 12 Godley BF, Shamsi FA, Liang FQ, Jarrett SG, Davies S, Boulton M. Blue light induces mitochondrial DNA damage and free radical production in epithelial cells. J Biol Chem 2005; 280 (22): 21061-6.
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