SUN CARE

BUY THI XUAN HOANG, IULIANA POPA* *Corresponding author University Paris Sud XI, Faculty of Pharmacy, UMR 8612, Chatenay Malabry, France

Iuliana Popa

Innovation in inorganic UV filters in

KEYWORDS: inorganic filter, metallic inorganic UV filters, sunscreen

WHY SUN PROTECTION IS IMPORTANT? health as well. The news inorganic based on TiO2 deliver an optimal balance of UVA, UVB and the Near-UV (HEV) light There are many benefi ts that are brought by sun, such attenuation. as positive feeling and mood and trigger the production This article is a review of several papers and patents in of vitamin D in skin. But giving to the skin an overdose of recent years concerning the innovations in inorganic fi lters rays (UV) may be harmful to our bodies and result in for sunscreen, including surface changes to conventional premature skin ageing, skin cancers, eye damage and photo- fi lters and new promising fi lters to completely replace immunosuppression. conventional fi lters. UV radiation is made up of three types of rays with different wavelength (λ): ultraviolet A (UVA) (λ = 315–380 nm), ultraviolet B (UVB) (λ = 280–315 nm), and ultraviolet C (UVC) (λ TYPES OF INORGANIC : ZnO AND TiO2 AND OTHER = 100–280 nm). The UVA is weaker than UVB but passes further METALLIC INORGANIC SUNSCREENS AND DERIVATIVES into the skin. Lately, it was defi ned the Near-UV (HEV) light is just beyond the cut-off from the UVA to the visible region The more commonly used inorganic UV fi lter are of the electromagnetic spectrum (380 nm-500 nm), being (TiO2) and (ZnO). Due to their large particle size, they related directly to the degradation of collagen and elastin leave on the skin a white fi lm. They also have a low dispersity, resulting in formation of wrinkles and premature aging. leaving users with a gritty feeling. Since 90-thies ZnO and TiO2 nanoparticles gradually replaced the large TiO2 and ZnO particles in order to overcome these drawbacks. However, the IMPORTANCE OF THE INORGANIC SUNSCREENS decrease in the size results in photo-catalysis and generation of .

Sunscreens are a form of photo-protection most widely used In this respect, some surface modifi cations were made for TiO2 by the public to reduce UV damage. They contain UV fi lters, and ZnO in order to reduce the photo-catalytic activity at the organic and inorganic, aiming specifi cally to fi lter the UV surface of nanoparticles, to alter their hydrophobic/hydrophilic light to a certain level to protect the human skin from the character to improve their dispersibility in various media, or to harmful affections of sunlight. The sunscreen consisting of enhance their compatibility with organics matrix by introducing inorganic UV fi lter such as titanium dioxide (TiO2) and zinc new functional groups that can react with organic molecules. oxide (ZnO) has long been considered safe and effective. These modifi cations require coating the particles with certain They are particularly preferred by individuals having high substances. There are direct coating and indirect coating. propensity for skin irritation from sunscreens containing Direct coating is a passive protection. A protective fi lm of organic UV fi lters. Despite these advantages, the use of this organic or inorganic material envelops the nanoparticle to type of fi lter has been limited because of their poor aesthetics prevent direct contact with the viable cells and generate due to their large particle size. The decrease in size results in dangerous free radicals. Although the inner part of the photo-catalysis and generation of reactive oxygen species, nanoparticles is quite active and might generate electrons and this stimulates scientists to invent new inorganic fi lters, under UV irradiation, these generated electrons cannot migrate which would not only provide full protection against UV rays, to the particle surface and therefore avoid any contact with the but should not cause damage to the skin and to the users’ aqueous solution or oxygen to create free radicals.

H&PC Today - Household and Personal Care Today, Vol. 9 nr. 3 May/June 2014 35 Some inert materials such as Al2O3 (1) and SiO2 are used and with further organic molecules in the composition, or to coat the particles to suppress their photo-catalytic greying of the formulations upon UV irradiation. Organic activity. Metal-doped nanoparticles showed a decrease in phosphorus compounds selected from the group of the the degradation of ingredients in the fi nal products under hydroxyalkyldiphosphonic , alkylphosphonic acids, the effect of sunlight. Other suitable dopants for the oxide aminoalkylenephosphonic acids or organic phosphoric particles include manganese (especially in form of Mn3+), esters help reduce or prevent the degradation of vanadium, chromium, cerium, selenium, iron, nickel, copper, the dihydroxyacetone in the cosmetic formulations. The tin, aluminium, lead, silver, zirconium, zinc and cobalt. titanium dioxide particles post treated by this method have It has been found that an UV protection obtained by also the property of preventing colour reactions (9). hydrothermal treatment of a particulate metal oxide and The surface of nanoparticles is also modifi ed by grafting subsequent application of a manganese-containing coating or anchoring polymers such as polymethacrylic acid is possible to suppress the pro-oxidative properties. The (10) polymethylmethacrylate (11), polystyrene (12) and hydrothermal treatment leads to the formation of stable polyvinylpyrrolidone (13). These polymers can penetrate the nanocrystallites of uniform size and shape, resulting in a aggregated nanoparticles and react with the activated reduction in reactivity and photo-activity. It is preferred for sites on the nanoparticle surface. The interstitial volume the metal oxide parent substance to carry a fi rst coating inside the nanoparticle aggregates becomes partially fi lled essentially consisting of manganese compounds and a with grafted macromolecular chains, and the aggregated second coating essentially consisting of aluminium and/ nanoparticles become further separated. In addition, the or silicon compounds or vice versa. It may be furthermore surfaces of the nanoparticles become hydrophobic which preferred for the particulate UV protection agent to be is important for the miscibility of the fi ller and matrix (14). hydrophobicized by application of a further organic coating Contrary to the direct coating, the method of indirect or layer. The organic outer coating or layer can consist, for coating is based on the addition of semi-stable radical example, of silicone oils, alkylsilanes, olefi nic acids, polyols or scavengers such as MeO – MnO, MgO, nitroxides NO, organophosphonic acids, or mixtures thereof (2). antioxidants-phenols to the nano-fi lter composition. They Inorganic layer with metal oxides or silicon oxides produces provide an active intervention in the radical generating stable coating. But the density of the fi lm seems to be a process at the surface of the crystal fi lter. However, it is problem that decreases UV-shielding ability of original recommended to combine direct and indirect coating in particles powder (3). Thus, the effi cient inhibition of photo- which the indirect coating can act as a second defense catalytic activity with less coating amounts will be essential. line against a generation of free radicals caused by an The organic substances are preferred for coating the particles imperfect direct coating (15). because they form a thinner layer covering the surface of A good example is SOLAVEIL SPEXTRA (Croda, particles. There are many agents that are proved to be able INCOSMETCS,I nnovation 2013), providing protection across to decrease the photo-catalytic activity of particles without the UVA and near–UV (HEV) light. The innovative dispersion much affecting its capacity to protect against the UV. It helps of TIO2 in a mixture of Polyglyceryl-2 caprate / Sucrose also improving the compatibility between the particles and Stearate /Simmondsia chinensis (Jojoba) Seed oil / Stearic the surface of the polymers and increasing the dispersion Acid /Alumina /Glyceryl caprylate / Squalane is giving a stability of particles in various organic media. talc-like feel on skin and protection against fee radicals The most popular are silane coupling agents used damage in the near-UV part of the spectrum. for particle modifying. 3-aminopropyltriethoxysilane, Another example is the UV-TITAN M040 (Merck) in which n-propyltriethoxysilane (4) 3-methacryloxypropyl the TIO2 is coated with silica and glycerin for a perfect trimethoxy-silane, γ-methacrylic acyloxy dispensability in water phase and a good stability versus propyltrimethoxysilane (5), 3-aminopropyltrimethoxysilane, antioxidants and . Kobo company developed the

3-isocyanatopropyltrimethoxysilane (6), TIO2 (KSL-026I) and ZnO (KSL-016I) with particles sized over 3-glycidyloxypropyltrimethoxysilane (7), 3-methacryloxypropyl 100 nm after surface treatment with jojoba esters. trimethoxysilane (8) and some other silane coupling agents Nevertheless we should mention the new ecological have been developed to modify the surface of nanoparticles combination of TIO2/ZnO with coconut oil in the mixture used in sunscreen (Z-Cote and Z-Cote HP1 from BASF) (UV- VEGELIGHT TIO2/ZnO (30 dispersion) by the Grant Industries, TITAN M160, M212, M262, M195 from Merck, (SOFTITAN 85 from that is an innovation product with a biodegradability SUNJIN). (ECOCERT) and water resistance presented at Besides an enhanced aesthetic light formulation of Solaveil INCOSMETICS in 2013. The particle size is about 150 nm ST-100 that offers transparency to the skin due to the in the coconut oil, presenting low whitening and a silky silane coating (Croda, INCOSMETCS, Innovation 2013), this touch along with a high SPF value with a guaranteed UVA technology provides an excellent UVB protection due to protection. SUNJIN Company obtained the non-nano TIO2 the tightly controlled particle size distribution of the new (SOSTITAN 60) with silica, alky sylane and lauroyl-lysine for

Titanium Dioxide (TiO2) dispersion. an ECOCERT grade. Coating of a specifi c organic phosphorus compound was also developed for UV protection particles that are used in self-tanning products containing dihydroxyacetone NEW INORGANIC SUNSCREENS AND DERIVATIVES: (example: RonaCare® Bronzyl™, MERCK). This innovation HYDROXYAPATITE, TRICALCIUM PHOSPHATE, CERIUM, SILICON came from a trend of combination of self-tanning MICROSPHERES, GLASS MICROSPHERES AND HAIRY ROOTLES products with UV inorganic fi lters. However, the presence OF ENGLISH IVY of metal oxides like TiO2 causes many unwanted effects such as degradation of dihydroxyacetone, colour Besides the development of conventional fi lter TiO2 and ZnO, reactions of the TiO2 particles with dihydroxyacetone many new fi lters are being also developed to meet the high

36 H&PC Today - Household and Personal Care Today, Vol. 9 nr. 3 May/June 2014 demands of consumers. These advanced filters are proven to is Ca2+. With Ca-doped particles, the photo-catalytic be more efficient, safer not only for the health of users but also activity of CeO2 can be nearly removed without loss of UV for the environment, and can be replacements for TiO2 or shielding ability and transparency in the visible light region ZnO to be used in future sunscreens. (25). Free radicals are not generated in totally stripped

Hydroxyapatite is one of the potential candidates. It is dermis if CeO2-formulations are applied (3). A blue shift of a compound already present in the human body, being the absorption is observed for the doped CeO2. This blue the main mineral constituent of bone tissue. Its nature shift allows for better screening of short UVA, the most leads to a complete dermal and systemic tolerance. harmful UVA wavelengths that are involved in skin cancers

More important, hydroxyapatite is found to be highly (22). In sunscreen preparation, Ca-doped CeO2 can be effective as a physical sunscreen agent to protect the used in association with TiO2 as an alternative to ZnO. The skin from UV rays. This effectiveness is even greater than combinations TiO2/Ca-doped CeO2 gave better photo- that shown by zinc oxide and titanium dioxide. Besides protection than the classical combination TiO2/ZnO (22). being an excellent physical UV filter, it also acts as an In 2004, nanoparticles of calcium-doped ceria were SPF booster that advantageously helps to reduce the coated with amorphous silica that was proved to be concentration of chemical agents typically used in known safe when used to provide high UV-shielding ability while sunscreen products. In addition, hydroxyapatite shows an maintaining a natural appearance when applied to the extremely low whitening effect, thus overcoming aesthetic human skin (26). drawbacks and problems of proper skin transpiration (16). Recently, it was shown (27) that cerium phosphate

The optical absorption of hydroxyapatite can be further (CePO4) is a new UV filter with excellent morphological improved by doping with Zn2+ and Mn2+ ions (17). characteristics, high UV absorption features, low toxicity,

Tricalcium phosphate Ca3(PO4)2 is another phosphate low photocatalytic activity and ideal particle size particle which has the potential to be an active ingredient for application in sunscreen formulations which can in sunscreens as it is non-toxic, non-mutagenic, and non- substitute the currently employed inorganic UV filters ZnO irritating (18).The particles of Ca3(PO4)2 alone are not and TiO2 (27). able to absorb UV rays. However, when they are doped Silicon microspheres with the size from 0.3 to 5.0 with metal ions, they can play a role of UV filter. Fe3+ is a micrometers proved to be good candidates for metal that may generate optical absorption bands in the developing effective broad-spectrum radiation filters UV range when incorporated in a crystalline matrix. The from UV to visible and IR. These highly refractive materials biocompatibility of Ca3(PO4)2 with the optical activity of represent a new generation of solar filters since they are the Fe3+ makes it very interesting as an active ingredient chemically inert, able to absorb UV and visible rays, and for inorganic sunscreens (19). they strongly scatter IR radiation. Moreover, the spherical

Nano-sized particles of TiO2-doped SiO2 gels have been shape and smoothness of the silicon microspheres can used as composites to enhance the UV absorption impart desirable properties in cosmetic preparations, efficiency (20). SiO2 gels having mesoporous morphology providing a soft feel when applied onto the skin. Finally, have been synthesized via a sol-gel processing route from their high IR blocking power enables protection from rice husk. Some studies have shown that amorphous SiO2 thermal effects, suggesting new benefits for sun care particle have not only a size in the nano-scale range, but products (28). also the typical characteristics of the particles (21). Glass microspheres are also used in the sun care product.

Nano-structured CeO2 appears to be a promising Although they are not UV filters, the final formulations inorganic material for use as a UV filter in sunscreen containing them are aesthetically elegant and have a cosmetic products (22). Lima has suggested that soft, non-greasy and non-sticky sensation when applied to

ZnO:CeO2 systems are promising candidates to be used the skin, the lips, the mucous membranes and/or the hair, as optical materials in UV-filters that present higher UV and compared with identical formulations not containing absorption and better transparency in the visible region glass microspheres. Their presence in the sun care product and less photo-catalytic activity than the conventional also increased in vitro FPS. system TiO2:ZnO (23). Particles extracted from hairy rootlets of English Ivy Although cerium manifests a lower photocatalytic (scientific name Hedera helix), a popular evergreen activity than that of zinc and titanium oxides, it still has plant in gardens, are an exception for inorganic UV an important catalytic behaviour for the oxidation of filters. They are different from the filters mentioned above (24). It can oxidize under light and because they are organic particles. These particles degrade the other compounds present in the product. block ultraviolet light from the sun much better than This characteristic feature makes the pure material conventional sunscreens containing nanoparticles. Strong incompatible with a use in commercial cosmetic products. ultraviolet extinction and excellent visible transparency

Additionally, it also slightly absorbs in the visible range are observed, compared to the inorganic TiO2 and producing a yellowish colouring in the products, mostly ZnO nanoparticles at similar concentrations (29). Ivy undesired in specific applications. nanoparticles exhibited much less toxicity than widely Doping with metal ions is a solution for the drawbacks used TiO2 nanoparticles. Ivy nanoparticles with a diameter of ceria. Yabe et al. have shown that the substitution of 65.3 nm do not reach the bottom of the stratum of larger and/or less positively charged cations (Ca2+, corneum layer in normal conditions for short periods of Sr2+, Ba2+, Mg2+, Zn2+) for Ce4+ can reduce the oxidation time after application. The biodegradability of these catalytic activity. Furthermore, the presence of the ivy nanoparticles also reduces all concerns regarding dopant ions caused significant absorption shifts into the an environmental contamination and problems arising visible region that significantly improves their optical from the entry into the body (30). They are degradable activity (25). The most preferred metal ion for doping by common proteinase. In addition, the ivy particles are

38 H&PC Today - Household and Personal Care Today, Vol. 9 nr. 3 May/June 2014 sticky, and should adhere to skin effectively. For these 15. Herrling T, Seifert M and K. Jung, Characterization of the advantages, the ivy nanoparticles have the potential to Coating Efficacy of Inorganic UV Filters by Using Electron Spin act as a very effective sunscreen that is also safer than Resonance Spectroscopy, SOFW-Journal,137:10-17(2011) present inorganic filters like TiO and ZnO. 16. EP 2410974 A2, Gazzaniga G, Gianbattista R and L Rigano, 2 Sunscreen composition comprising hydroxyapatite as physical In conclusion, concerning the classical ZnO and TiO filters, 2 solar filter., Kalichem Italia S.r.l (2010). many achievements are reported in the development of 17. De Araujo T S, De Souza S O, De Sousa EM et al., Phosphates their formulation and association with the new inorganic nanoparticles doped with zinc and manganese for sunscreens, filters such as polylmer grafting, TiO2-doped SiO2 or Mat. Chem. and Phys. 124:1071–6(2010) TiO2/Ca-doped CeO2 to ensure a perfect coating and 18. WO 2011060099 A2, Mansouri Z, Canelide R, Thomas D et al., efficiency for UV rays by overcoming the aesthetic Sunscreen compositions comprising uniform, rigid, spherical, drawbacks and skin transpiration, and no photo-catalytic nanoporous calcium phosphate particles, and methods of reaction. Beside these improvements, there are new making and using the same, Laboratory Skin Care LTD. (2011) ways of research and application for new filters such as 19. De Araujo TS, De Souza SO, De Sousa EMB and et al., Analysis of a new sunscreen active ingredient based on β-FeTCP hydroxyapatite, cerium or roots Ivy nanoparticles. nanoparticles.11th Int. Conf. on Adv. Mat., ICAM (2009) 20. Jaroenworaluck A, Pijarn N, Kosachan N et al., Nanocomposite

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