Today’s sun care formulator must achieve high SPF and challenging UVA protection standards while also making products elegant to encourage consumer compliance—and cost-effective enough to be affordable in difficult economic times. Efficacy and elegance are dependent on one another; maximizing the efficacy of UV filters enables the creation of high-SPF products using minimal amounts of said filters, which allows the formulator greater freedom to optimize skin feel. Conversely, good product aesthetics encourage consumers to apply more product, therefore moving closer to the labeled SPF.
In recent years, several highly effective and broad spectrum UV filters have been developed, enhancing the formulator’s toolbox for the development of effective sunscreen formulations. However, most new UV filters are either proprietary to one user—e.g., drometrizole trisiloxane and terephthalylidene dicamphor sulfonic acid, or are still under patent and only available from one supplier, as in the case of diethylamino hydroxybenzoyl hexyl benzoate and methylene bis-benzotriazolyl tetramethylbutylphenol. These latter materials can be relatively expensive, sometimes prohibitively so for manufacturers catering to the mass market. Thus, the work described in this paper is aimed at identifying efficient UV filter combinations without incorporating proprietary or more costly materials.
There are four key requirements to designing an efficacious sunscreen formulation. These are: the right UV filters, homogenous distribution in the formulation, homogeneous distribution on the skin, and efficacy after application. These steps will be discussed here.
Choosing UV filters: SPF depends primarily on UVB protection, although UVA protection also provides a significant contribution to SPF. Therefore, the most efficient sunscreen combinations are those that address all parts of the UVA/UVB spectrum by combining UVB filters with UVA filters. For example, the results shown in Table 1 were achieved in an o/w sunscreen formulation. This data was generated using the in vitro SPF method of Diffey and Robson,1 as assessed by an SPF analyzera with tapeb as the substrate. While octocrylene gives some UVA absorbance here, it is primarily a UVB filter and is limited in terms of its individual SPF. However, including a UVA filter in the formulation allows for a considerably higher SPF without increasing the concentration of the octocrylene.
Also, combinations of organic UV filters with inorganic filters often show “synergistic” effects,2-7 delivering greater SPF efficacy than might be expected from the additive effects of the individual actives. A striking example is shown in Formula 1 and Formula 2, the in vitro SPF data for which is shown in Table 2. In this case, the data was obtained with a sample analyzerc using PMMA plates as the substrate.
Homogeneous distribution: With organic UV filters, a key factor in optimizing efficacy is to ensure good solubility in the formulation. As an example, Figure 1 shows in vitro SPF values for a series of simple formulations in which butyl methoxydibenzoylmethane (BMDM), benzophenone-3 (Bz-3) and ethylhexyl triazone (EHT) were incorporated with various emollients.8 Here, it is apparent that SPF tended to increase with the increasing solubility of the active in the emollient. However, to achieve good skin feel, the emollient must also have pleasant aesthetic properties. Fortunately, a number of new emollients have been launched that show excellent skin feel combined with good solvency properties for organic UV filters, as illustrated by the solubility data for BMDM shown in Figure 2.
With inorganic sunscreens, it is critical for particles to be dispersed evenly throughout the formulation. In emulsion systems, the inorganic can be incorporated into the external phase, internal phase or both. The best results tend to be achieved when it is in the external phase, but good efficacy can also be achieved with oil-dispersed inorganics in o/w emulsions. Also, pre-dispersions usually give better SPF efficacy9 and stability than dry powder forms.
To achieve water-resistance, many sunscreen formulations contain only oil-soluble or oil-dispersed actives. However, in emulsion systems, a better overall distribution can be achieved by including water-based UV filters alongside oil-based ones since even relatively low levels of water-soluble filters, or aqueous dispersions of inorganics, can have a dramatic effect on overall SPF efficacy.
Homogeneity on skin: The ideal scenario for optimum sun protection is the sun product forming a film of even thickness across the whole skin surface, with UV filters evenly distributed throughout this film. In the real world, the topography of skin makes the product film vary in thickness (see Lutz/Miksa article for more on skin topography), and this is a critical limiting factor on the achievable SPF. Creating a more even film can therefore enhance SPF substantially. This can be achieved in part by optimizing the rheology of the formulation, particularly in the case of w/o emulsions.10 In o/w systems, the formation of an even product film during application is dependent on the coalescence of the oil droplets as the water evaporates. This can be enhanced by the inclusion of certain film-forming polymers, for example PVP copolymers and crosspolymers, cellulose derivatives, polyurethanes and polyesters. The use of such SPF boosters has been shown to significantly improve SPF.11
Efficacy after application: Finally, it is important for UV filters to maintain their efficacy when exposed to UV radiation. Since in vivo SPF measurements are time-resolved experiments, any decay in the activity of filters is reflected in the measured SPF. The same applies to the in vivo UVA protection factor (UVA-PF) measurement—and modern in vitro UVA techniques include a pre-irradiation step to account for this potential for photo-decay. Inorganic sunscreens and certain organic filters, e.g., Bz-3, are inherently photostable, but a number of organic filters such as BMDM and ethylhexyl methoxycinnamate (EHMC) are photo-labile. Therefore, when such filters are used, it is important to also incorporate a photostabilizing ingredient that can prevent or “quench” the photo-reactions that lead to the decay of these materials.12
Implementing the strategies discussed above, o/w and w/o sunscreen formulation systems were developed using a “building-block” approach, as described in the following steps.
Step 1: Select one or more broad-spectrum filters that provide good efficacy across both UVB and UVA. In this example, a broad-spectrum grade titanium dioxide could be used. For the w/o emulsion, titanium dioxide could be used as an oil-based dispersion. In the o/w system, a water-based dispersion was used, supplemented by the broad-spectrum water-soluble filter benzophenone-4 (Bz-4).
Step 2: Build on Step 1 with high-efficacy UVB and UVA filters. In this example, BMDM was used as the UVA filter, and iso-amyl methoxycinnamate (IMC) was selected as the primary UVB filter. In each formula system, two versions were developed. The first contained only IMC as the UVB filter, the second also contained the water-soluble filter phenylbenzimidazole sulfonic acid (PBSA) and the oil-soluble filter EHT.
Step 3: Select emollients appropriate for the filters used. As discussed, it is crucial to include emollients that are effective solvents for the UV filters, while also giving good skin feel.
Step 4: Include a suitable film-former and/or rheology modifier. For example, in o/w emulsions, film-formers of the types mentioned earlier (see Homogeneity on skin, above) are useful. In w/o systems, inclusion of waxes or other rheology modifiers is often beneficial.
Step 5: Include a photostabilizer to prevent or at least inhibit photo-decay.
Results and Constraints
Using the above approach, SPF values of 50 were achieved—as measured by a sample analyzerc and using PMMA plates—with relatively low levels of active, i.e., 13.3% in the o/w emulsion and 15.3% in the w/o system. Especially striking is the effect of two filters, EHT and PBSA, in boosting the overall efficacy of the systems.
It is important to note that the described formulations were developed for the European market. Unfortunately, they are not currently suitable for the U.S. market, as IMC and EHT are still awaiting approval by the U.S. Food and Drug Administration (FDA) via the Time and Extent Application (TEA) system along with a number of other sunscreen actives. While IMC could probably be replaced by EHMC since it has a similar absorption spectrum, EHT has a uniquely high UVB absorbance that is not replicated by any FDA-approved filter. Also, the combination of BMDM with either PBSA or TiO2 is still not permitted under the FDA Sunscreen Monograph. Finally, while most countries allow up to 5% BMDM in sunscreen formulations, the limit in the United States remains at 3%.
These restrictions are preventing U.S. sunscreen formulators from being able to create more efficient sunscreen products, such as those widely seen in Europe and elsewhere. In turn, U.S. consumers are denied access to such products. While the FDA has been much criticized for the long delays in approving new sunscreen ingredients and combinations, as was pointed out in a recent article,13 the solution to this issue does not lie with the FDA alone. Congress, the sunscreen industry and the scientific community all have roles to play in enabling the agency to resolve the various TEA applications. It is to be hoped that the FDA will, in the not too distant future, be able to approve these filters and filter combinations that have been widely—and safely—used for more than a decade in other parts of the world, thus providing U.S. consumers with a new generation of safe, elegant and cost-effective sunscreen formulations.
Formulating the type of effective, high-SPF, broad-spectrum and elegant sunscreens in demand by today’s market requires careful choice of both the UV filters used and the formulation excipients to deliver the filters onto the skin. By selecting filters that complement each other in terms of UV spectrum, physical/chemical properties and placement in the emulsion, and then selecting other ingredients to maximize the efficacy of the filters used, a high SPF can be achieved with relatively low levels of actives, and without the need to resort to the more expensive actives. The recent development of new emollients, film-formers and photostabilizers has facilitated improvements in overall formula efficacy. However, further regulatory change is needed to make these strategies available to all markets.
- BL Diffey and J Robson, A new substrate to measure sunscreen protection factors throughout the ultraviolet spectrum, J Soc Cosmet Chem 40 127-133 (1989)
- GB Pat 2279007, Compositions containing sunscreens, assigned to Tioxide Specialties Ltd (1994)
- GB Pat 2278055, Compositions containing sunscreens, assigned to Tioxide Specialties Ltd. (1994)
- European Pat 0456458A2, Cosmetic composition, assigned to Unilever (1991)
- US Pat 5 417 961, Sunscreen compositions, assigned to Colgate-Palmolive Company (1995)
- International Pat WO 94/04131
- SR Spruce, 5th Florida SCC Sunscreen Symposium, Orlando (1995)
- C Wright, Effects of Emollients on Efficacy of UV Filters, MChem Report, York University (2002)
- J Woodruff, Formulating sun care products with micronized oxides, Cosm & Toil Mfr Worldwide 179-185 (1994)
- JP Hewitt and GH Dahms, The influence of rheology on efficacy of physical sunscreens, IFSCC Conference Proceedings, Montreux, Switzerland 313-323 (1995)
- JW Hart and JP Hewitt, Aiming higher, Soap, Perfumery & Cosmetics 82(4) 67-68 (2009)
- C Bonda, Sunscreen photostability 101, Happi (Oct 2009)
- NA Shaath, The archaic TEA process revisited, Happi (May 2013)