Efficacy and Elegance in 21st Century Sun Care

Mar 20, 2014 | Contact Author | By: Julian P. Hewitt, JPH SunCare Technologies Ltd., Durham, UK; Jennie Teague and Paul Sutcliffe; Vivimed Labs Europe Ltd., Huddersfield, UK
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Title: Efficacy and Elegance in 21st Century Sun Care
sunscreen synergyx SPFx UV filtersx efficacyx distributionx solubilityx organicx inorganicx
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Keywords: sunscreen synergy | SPF | UV filters | efficacy | distribution | solubility | organic | inorganic

Abstract: This article discusses formulating strategies to achieve high-SPF sunscreen formulations that are cosmetically elegant and also cost-effective. By carefully choosing actives that complement one another in terms of spectral coverage, and physical and chemical properties, synergistic combinations of relatively inexpensive UV filters can be identified to deliver effective and elegant formulations.

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J Hewitt, J Teague and P Sutcliffe, Efficacy and Elegance in 21st Century Sun Care, Cosm & Toil 129(3) 58 (Apr 2014)

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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.

Sunscreen Fundamentals

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.

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Table 1. SPF Levels Achieved Using Example UV Filters

Table 1. SPF Levels Achieved Using Example UV Filters

The results shown here 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 analyzer with tape as the substrate.

Table 2. SPF Data for Formulas 1 and 2

Table 2. SPF Data for Formulas 1 and 2

The in vitro SPF data for Formulas 1 and 2; this data was obtained with a sample analyzer using PMMA plates as the substrate.

Table 3. In vitro SPF Data for Formulas 3a-b and 4a-b

Table 3. In vitro SPF Data for Formulas 3a-b and 4a-b

The formulations developed are shown in Formula 3a-b (o/w system) and Formula 4a-b (w/o system); shown here is the in vitro SPF data for the four formulations.

Figure 1. Effect of the solubility of organic UV filters on SPF

Figure 1. Effect of the solubility of organic UV filters on SPF

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.

Figure 2. Solubility of avobenzone in various emollients

Figure 2. Solubility of avobenzone in various emollients

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 here.

Footnotes [Hewitt 129(3)]

a SPF-290 analyzer, Optometrics

b Transpore tape, 3M

c UV-2000S, Labsphere

Formula 1. Synergistic organic and inorganic sunscreen

Formula 1. Synergistic organic and inorganic sunscreen

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.

Formula 2. Additional synergistic organic and inorganic sunscreen example

Formula 2. Additional synergistic organic and inorganic sunscreen example

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.

Formula 3. Example o/w systems

Formula 3. Example o/w systems

Implementing the strategies discussed in the present article, o/w and w/o sunscreen formulation systems were developed using a “building-block” approach.

Formula 4. Example w/o systems

Formula 4. Example w/o systems

Implementing the strategies discussed in the present article, o/w and w/o sunscreen formulation systems were developed using a “building-block” approach.

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