Serving again as a judge for the local science fair, this author reflected on the year’s prior event and a project adjacent to the garlic-for-acne study discussed in the last edition of this column. The project was an attempt to measure the in vitro SPF of commercial products using photosensitive paper from a child’s hobby kit. Expecting to find a solar focus in this year’s entries, it was surprising to see submissions take a different direction—toward capturing the electricity generated by UV light. Both titanium dioxide and plant pigments were the absorptive molecules of interest, and demonstrations showed how their energy transfer abilities could be capture UV energy rather than just deflect it.
In contrast, the industry continues to seek protection from the sun without chemical reactions on the skin, in the body or in the environment. A keynote lecture from the most recent International Federation of the Societies of Cosmetic Chemists (IFSCC) Conference in Thailand identified four challenges to address for eco-friendly photoprotection, including: improving consumer understanding of adequate sun protection, increasing compliance by formulating more esthetic products, increasing unit effectiveness of actives in products to minimize environmental impact, and improving assessment methods for predicting ecotoxicity to guide formulation.1 These key points comprise a good framework from which to build new products that balance individual consumer needs with effects on both the user and the environment. Natural formulations offer one avenue of approach to meet these challenges.
Inorganic Mineral Sunscreen Actives
Although the U.S. Food and Drug Administration (FDA) has questioned the use of “natural” marketing claims in its 1999 Final Rule, the latest proposed rules in 2011 still do not specifically address them.2 The agency side-stepped definitive guidelines by simply saying in the 1999 Federal Register that “natural” claims would likely require context-specific analysis but could not be intermingled in required FDA labeling; it was stronger about citing “chemical-free” statements as likely to be considered unacceptable.
When formulators talk about natural sunscreens, they may automatically think of inorganic particulate actives, i.e., zinc oxide and titanium dioxide, and in most cases, suspend judgment on the inactive ingredients in the formula. The industry hears both zinc oxide and titanium dioxide touted as natural mineral compounds but neither is used in its native raw form. Sometimes still branded as “chemical-free,” these purified inorganic actives undergo processes that are themselves intensive chemical transformations to convert the source minerals into pure compounds and thus comply with US Pharmacopeia (USP) specifications for suitable OTC drug actives. Analogously, inorganic pigments such as iron oxides, i.e., “rust” by any other name, are chemically purified to meet federal regulation standards, i.e., 21 CFR, for color additives and not simply pulled out of the earth. Most commercial versions of these inorganic oxides are then coated to attenuate surface chemical reactions or catalysis, and to convert the surface to an easier-to-disperse particle designed for its vehicle.
Conversely, the industry might view many of the organic sunscreen actives, as originally proposed in 1978, as derivatives of naturally occurring aromatic chemicals including salicylates, aminobenzoates and hydroxycinnamates. Inferences may be made for their natural derivation, akin to the citation of natural occurrences for certain food preservative chemistries, for acceptance by standards organizations such as Natrue.3 In fact, many of the original manufacturers of organic sunscreens were primarily fragrance chemical suppliers due to the similar chemistries and starting materials involved. Issues of skin penetration and potential endocrine disruptive effects are still controversial but continue to drive the restriction of organic actives by natural product certifying organizations (see Table 1).4
Inorganic sunscreens work by a dual mechanism: light scattering and photon absorption over a wide band width.5 Radiant UV energy can be absorbed up to a maximum wavelength of approximately 380 nm based on the excitation of electrons in these semiconductor compounds from a valence band to a conduction band of higher energy. Any wavelength photon shorter than the minimum energy wavelength, i.e., with greater energy—recall that light energy is inversely proportional to wavelength—can elevate that electron’s energy level and be absorbed. The absorption spectra of these compounds appear as a plateau that drops off abruptly beyond that minimum energy/maximum wavelength; i.e., the highest wavelength detected as significant absorption. Additionally, light of wavelengths higher than that, approximately double the effective particle size, will be scattered according to Mie light scattering theory.5 This dual mechanism gives broad-spectrum UVR attenuation. Mie theory itself is used to determine particle size in dilute solutions but here, formulators are working with complex concentrated systems with multiple effects governing the final biological end point—the product SPF.
From a regulatory perspective, titanium dioxide appeared on the list of Category 1, “safe and effective” actives in the original Proposed Monograph4 at 2–25%; it is now simply 25% maximum and the same virtually worldwide. Zinc oxide, although cited as effective in the original 1978 proposed rule, surprisingly did not appear as a Category 1 sunscreen active until 20 years later in 1998, after a number of laboratories demonstrated an efficacy efficiency of about one SPF unit per percentage use level of zinc oxide (range 0.66–1.5), and increased activity when mixed with titanium dioxide. The results demonstrated that zinc oxide has a modest UV radiation attenuation over a wider wavelength range, and titanium dioxide a stronger UV radiation attenuation primarily in the UVB range where SPF endpoints are actually determined. Zinc oxide had been judged as safe and effective in the catch-all category of “skin protectant” for both its absorbent and lubricant properties at levels of 1–25% and remains in the same place in the Final Monograph from 2003; it was judged as an effective sunscreen years later based on review of the described lab results. There are slightly reduced maximums of 20% for zinc oxide in a few markets, notably Australia/ New Zealand and Canada.4
Natural Sun Care Market
The market for natural sun care products is growing, with notable recent additions in the past year or more. Among these was the introduction in 2011 of the Sunology range,6 which makes use of a ferulic acid and soybean oil compound, the FSG Complex. This complex was invented at a United States Department of Agriculture (USDA) research center as an adjunct UV-absorbing compound to supplement the two inorganic mineral-based actives zinc oxide and titanium dioxide. With the INCI name Feruloyl Soy Glycerides, the material is an enzymatically synthesized glyceride that substitutes one fatty acid chain on the soy triglyceride with ferulic acid, a phenolic phytochemical with UVB-absorbing and antioxidant capacity. Both the absorbing glyceride and formulations may be covered by these US patents.7 The initial formulation uses 10% zinc oxide and 7.5% titanium dioxide in an SPF 50 anhydrous gel suspension labeled “Sunscreen for Skin That Prefers No Chemicals.”
Overshadowing most products in the commercial naturals category this year—and there are dozens—is a line from Banana Boat branded as “natural reflect” that employs much lower levels, i.e., less than half, of inorganic actives—titanium dioxide at 3.6% and zinc oxide at 4%, in a very water-resistant water-in-oil emulsion. At an SPF 50+, it makes efficient use of both particulate sunscreen actives yet contains two aromatic compounds, butyloctyl salicylate and ethylhexyl methoxycrylene, that help to attenuate the ultraviolet radiation (UVR) by one mechanism or another. This unctuous emulsion goes on with much effort and rubs out to a heavy, although virtually transparent, film. Based on a look at the additional ingredients, the titanium dioxide is likely coated with both alumina and dimethicone to retard potential redox catalysis at the surface, with or without UV exposure, and to convert the naturally hydrophilic surface to a hydrophobic one, facilitating dispersion in the outer phase of this inverse emulsion. Likewise, the zinc oxide probably has a light dimethicone coating to improve dispersibility, although silicone-based coatings may have difficulty being accepted under most current natural standards.
The rest of the commercial field, as evidenced online and on store shelves, is not quite as ambitious. A good many of these natural formulations stick to vegetal ingredients like jojoba oil, beeswax and shea butter with aloe juice, yet one marvels that many reveal no emulsifiers or preservatives in formulas that look to be in need of same. Others use the full range of synthetic excipients to formulate optimal vehicles including w/o silicone-based emulsions. A brief survey of two dozen formulations listed online and classified as “natural” revealed that at least half make use of a mixture of both inorganics, followed by those using only zinc oxide and a few relying on titanium dioxide as their sole active. A few entries that call themselves “natural” use the full complement of organic chemical actives not allowed under any of the natural standards with the inclusion of aloe, a few botanical extracts and safflower oil as justification for being in the natural category.
A surprising synergy seems to manifest itself in the combined mineral-based products. If one calculates the efficiency of the sunscreen actives by a simple ratio of SPF to percent use level, although the dose/effect curve is not usually linear, the best performance comes from the mixed inorganic active products with titanium dioxide/zinc oxide combinations giving SPF efficiencies of near 3—even before averaging in the especially effective “natural reflect” products, which brings that ratio up to about 3.8. In comparison, all zinc oxide products as well as primarily organic actives products come in closer to 1.5. Titanium dioxide itself has good UVB attenuation with overlap into the UVA, as was shown years ago in a petition to the FDA for identifying it as a broad-spectrum active. In the few cases where it was the sole active, it averaged about 2.5 efficiency, although a high outlier in this efficiency race might be one of the latest Anthelios range from L’Oréal: the La Roche-Posay Anthelios Mineral Ultra Light Sunscreen Fluid, boasting an SPF 50 for 11% titanium dioxide in a decidedly synthetic-based formulation.
It appears that the combination of titanium dioxide and zinc oxide can yield high efficiency and broad-spectrum products. In these examples, simply multiply by 100 to arrive at the Formulation Efficiency Factor (FEF); this was devised by O’Lenick8 in 2011 as a numerical way to compare formulations and the effects of additives. Actives have varying maximum absorbance wavelengths, i.e., lambda MAX, and molar absorbtivities also stray from Beer-Lambert linearity in their biological effect, so formula comparisons are complicated but within this random sample of commercial products there seems to be consistency in pointing to a mixed active system for the best effect. Although broad-spectrum protection can be achieved with only titanium dioxide, the inclusion of zinc oxide makes it virtually assured.
To see how inorganic sunscreens are formulated, a look at ingredient supplier prototypes is instructive. Pre-formulated dispersions are a popular method for incorporating these fine powders to eliminate the inconvenience, variability and respiratory issues of working with the dry powders. Many of the dispersions available are prepared in lipophilic vehicles with hydrophobically coated particles. This favors better continuous film formation on skin dry down and optimal efficacy, although some formulas using hydrophilic titanium dioxide can achieve the same efficiency.
Suppliers have begun to consider certification by certain private organizations in the natural and organic market as an avenue to that market. Foremost among these worldwide is Ecocert, based in France, now with more than 2,500 commercial personal care ingredients in its database. Among these are more than 100 titanium dioxide-containing products, of which many are pearlescent pigments, but also many UVR-attenuating active sunscreen dispersions and powders, and nearly 30 zinc oxide dispersions and active powders. The other major certifiers including NSF/ANSI 305 standard, through Quality Assurance International, NPA and Natrue focus more on finished products to appeal to the final consumer and attract cosmetic marketers looking for third party organizations for credibility when they claim the natural mantle.
Validation of “natural” has not yet been standardized but a look at the treatment of actives can be a guide; see Table 1. It is recommended to use inorganics, preferably larger than nanoparticles, i.e. > 100 nm, and formulation vehicles with ingredients acceptable to all of these certifying groups. The standards of these natural certifying bodies are becoming more and more alike, and some organizations are cross-certifying, such as NSF/ANSI 305 and Natrue. For a look at acceptable raw materials, an instructive Web course is available at learn.cosmeticsandtoiletries.com.
For efficacy and long lasting water and wear resistance, thick hydrophobic films work best but usually are challenged by unappealing esthetics in feel and appearance. Beach products can withstand this drawback since consumers are looking for efficacy and especially equate natural formulations with mildness. W/O formulations can be made with low-HLB polyglyceryl esters, which are acceptable by most natural standards. For lighter facial products, water-based o/w emulsions using alkyl glycosides in conventional lotions with fatty alcohols and light esters such as caprylic/capric triglycerides are preferable.
Investigating further into the improved efficacy of some recent products reveals the use of adjunct technologies to boost the SPF and overall effectiveness of sun care products. Much of this technology cannot find its way into the naturals market due to its synthetic origin but there is potential for cross-over. The three major categories of boosters that work are: path length extenders, antioxidant/anti-inflammatories and stealth sunscreens.
The optimization of an active’s solubility and dispersion in the product film magnifies the final product effectiveness up to its potential maximum but arguably can only reach 100% and not more; boosters actually move the potential maximum by adding to the base. Path length extenders can be hollow polymer microspheres, ~100 microns, which diffract light due to refractive index differences and bounce UV light through thicker film—allowing less of the UVR to reach the living skin. Inorganic particles such as mica and hydroxyapatite are also being used for this application but a more sophisticated inorganic version of these spheres might be acceptable to the natural market, such as hollow glass microspheres.9
The second technology, promoted by many botanical and natural product companies, is the use of natural antioxidants and anti-inflammatories to enhance the protective properties of sunscreens in the sunscreen product itself. The list includes the usual candidates: vitamin E, ascorbates, ubiquinone, glutathione and carotenoids, as well as naturally derived and nature-identical aromatics like alpha-bisabolol and various glycyrrhizinates. Green tea extracts and any number of botanicals have multiple functions to quench free radicals and interfere with the biochemical inflammatory cascades that UVR sets in motion.10
Some of these botanical actives have the added advantage of UV absorbance, which itself adds to the overall effectiveness of a product’s physical absorption (see Table 2). The industry made use of these multifunctional properties of phytochemicals, here troxerutin, as a strategy to improve sunscreens for skin care in the 1980s after the issue of the proposed sunscreen monograph in 1978 directed many formulators to focus skin care efforts on sun protection.11
Mention here also should be made of melanin, the natural protective agent in skin throughout the animal kingdom. Under investigation for UVR absorption and free radical quenching, melanin could find more extensive application were it not for its inherent deep color. Synthetic melanins represented both by a glycine-tyrosine and a dihydroxyphenylalanine-based polymer were shown to significantly reduced pro-inflammatory cytokines including tissue necrosis factor (TNF) and Interleukins (IL)-1β, IL-6 and IL-10.12 Synthetic melanins have also been the subject of patents for cosmetic use, from skin protection to hair coloration. The main drawback of all available melanin type polymers is their intense coloration, lending a cocoa brown to a dark gray-black to delivery vehicles.
The third approach, referred to here as stealth sunscreens, are ingredients added by the formulator for one functionality with the added benefit of UV absorption. Many botanical actives with phenolic functionality have UVB absorption for single ring species and UVA and visible light absorption for multiple phenolic rings. These compounds encompass a large group of the many plant pigments in flowers and fruits, as well as numerous colorless phytochemica. If the technologies used to hybridize soybean oil with ferulic acid in FSG cited above could be summoned to graft the many phytochemicals available to natural lipids for use in creams and lotions, the industry might have a larger array of active excipients at its command.
All of the natural product standards consider esterification an acceptable chemical process and these natural UVR absorptive chemical moieties have been cited as justification for acceptance of alternative actives; for example, isoamyl hydroxycinnamte as a constituent of galangal extract. The ethyl ester of p-methoxycinnamic acid is a major constituent of the essential oil of rhizomes of Kaempferia galangal (botanical family: Zingiberaceae) and the amyl ester is present in small quantities.13
Take note that some natural waxes also contain aromatic esters that can have UVR-attenuating properties. Carnauba wax for instance can have as much as 25% esterified hydroxycinnamic (coumaric) acid. Formulators use this stealth sunscreen strategy knowingly or not when they employ some of the photostabilizer, antioxidant and solubilizing ingredients recently launched on the market. These chemicals, whether salicylates, substituted benzoates or multi-ring aromatic compounds with delocalized electron clouds, also absorb in the UV range like their OTC sunscreen counterparts and have varying UVR attenuating activity by photochemical mechanisms.
Sunscreen formulations for the natural market may appear to be very limited when it is first discovered that formulators are basically limited to the two FDA Category 1 inorganic particulate sunscreens, i.e., titanium dioxide and zinc oxide. The challenge is to formulate them into esthetic, effective products with the use of adjunct technologies, yet keep within the overall natural certification framework by using formulation ingredients that are acceptable to the target consumers for natural and organic products.
- D Moyal, Towards an Eco-Friendly Photoprotection, IFSCC Magazine 15(2) (Apr/Jun 2012) pp 67–70
- N Shaath, What’s “Not Final” in the Final Rule, HAPPI Webinar (Aug 11, 2011)
- www.natrue.org/fileadmin/natrue/downloads/Annexes_V2_2.xls (Jul 5, 2012)
- N Shaath, Ultraviolet filters and other issues, ch 2, The Encyclopedia of Ultraviolet Filters, Allured Business Media, Carol Stream, IL USA (2007) p 25
- MW Anderson et al, Broad spectrum physical sunscreens: Titanium dioxide and zinc oxide, in Sunscreens—Development, Evaluation and Regulatory Aspects, 2nd edn, N Lowe et al, eds, Marcel Decker, New York (1997)
- www.sunology.com (Accessed Jul 3, 2012)
- US Pat 7,744,856, Formulations with feruloyl glycerides and methods of preparation, DeFilippi et al, assigned to Biotech Research and Development Corp. (Jun 29, 2010)
- www.cosmeticsandtoiletries.com/formulating/category/suncare/128710373.html (Aug 30, 2011)
- http://cenotechnologies.com/microspheres.php (Accessed Jul 24, 2012)
- S Saraf, Phytoconstituents as photoprotective novel cosmetic formulations, Pharmacogn Rev 4(7) 1–11 (Jan–Jun 2010)
- US Pat 4,603,046, Improved sunscreen or sunblock composition, A Georgalas et al, assigned to Charles of the Ritz Group Ltd (Jul 29, 1986)
- N Mohagheghpour et al, Synthetic melanin suppresses production of pro-inflammatory cytokines, Cellular Immunology 199(1) 25–36 (Jan 10, 2000)
- IL Gatfield et al, Enzymatic synthesis of isoamyl p-methoxycinnamate from the natural ethyl ester, Cosm Toiletries 122(6) 63 (Jun 2007)