Method to Reproduce In vitro Cosmetic Product Photostability Findings

Mar 1, 2012 | Contact Author | By: Marc Pissavini, PhD, Adeline Baud, Stéphanie Marguerie, Karine Desseille and Olivier Doucet, PhD, Coty-Lancaster
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Title: Method to Reproduce In vitro Cosmetic Product Photostability Findings
photostability percentagex PMMA substratex reproducibilityx in vitro spectroscopyx
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Keywords: photostability percentage | PMMA substrate | reproducibility | in vitro spectroscopy

Abstract: The present article describes a reproducible method for determining the photostability of sunscreen products. This method is based in part on the in vitro determination of the UVA protection factor as proposed by Colipa for the irradiation aspect, and on the spectroscopy of a sunscreen in dilute solution for the absorbance measurement aspect.

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M Pissavini, A Baud, S Marguerie, K Desseille and O Doucet, Method to reproduce in vitro cosmetic product photostability findings, Cosm & Toil 127(3) 208-218 (Mar 2012)

Market Data

  • Awareness among consumers about the harmful effects of UV boosted sun care sales by 6.5% in 2012 in the United States.
  • Sun care marketers are diversifying their product offerings; a common trend emerging is to include tint.
  • Although spray-on sun care products are popular, there are rising concerns associated with the inhalation of nanoparticles.
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Recently, an increased number of methods for determining the ultraviolet (UV) protection factor have been proposed. In fact, three major methods have been validated by the ISO to evaluate sun protection, including in vivo SPF (ISO 24444), in vivo persistent pigment darkening (PPD, ISO 24442) and in vitro PPD (ISO 24443, to be published in 2012). Nevertheless, despite great effort, some methods based on photostability percentage are still not validated, and while several methods are published, none is generally used. This is mainly due to their lack of inter-laboratory reproducibility or their use of analytical chemistry techniques such as HPLC, which are excessively burdensome to implement. Clearly there is compelling interest in validating a method to determine the percentage of photo-protection that remains in a product after UV exposure, i.e., the photostability percentage.

The present article describes a reproducible method for determining the photostability of sunscreen products. This method is based in part on the in vitro determination of the UVA protection factor as proposed by Colipa for the irradiation aspect, and on the spectroscopy of a sunscreen in dilute solution for the absorbance measurement aspect.

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This is an excerpt of an article from GCI Magazine. The full version can be found here.

 

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Table 1. Active filters in the 11 test products

Table 1. Active filters in the 11 test products

Regarding the test procedure, 28.7 mg of each product was dispensed and spread carefully over the PMMA plate. The samples were allowed to settle for 15 min in the dark at room temperature to ensure leveling of the products. A total of nine UV transmission spectra—i.e., from 290 to 400 nm, in 1-nm increments—were recorded at different locations on each plate. A transmittance analyzer was then used to determine the diffuse transmission spectrum of UV radiation through the substrate before and after irradiation, followed by the photostability percentage.

Table 2. Photostability percentage for the 11 test products using protocol 1

Table 2. Photostability percentage for the 11 test products using protocol 1

Products C4, C2 and H1 showed a photostability percentage above 100%, as the SPF (or PPD) after irradiation was higher than the SPF (or PPD) before irradiation. In addition, Products C1 and C3 had the same filter combinations but different photostability percentage.

Table 3. Extraction percentage calculated for the 11 test products

Table 3. Extraction percentage calculated for the 11 test products

The extraction percentages were first calculated to verify that they were all sufficiently high and relatively homogeneous according to the formulations; all, except for F2, were well above 75%.

Table 4. Photostability percentage for the 11 test products using protocol 2

Table 4. Photostability percentage for the 11 test products using protocol 2

Shown here is the improvement brought about by this new protocol on the calculations of photostability percentage in both the UVA and UVB range. Except for Product C5 (101% on test 1), no product exceeded 100%.

Table 5. Comparison of the photostability and extraction percentages for products C6 and F3

Table 5. Comparison of the photostability and extraction percentages for products C6 and F3

Photostability percentage does not seem to be directly related to the extraction percentage; while Products C6 and F3 had the greatest difference in extraction percentage, they had a similar photostability percentage, as shown here.

Table 6. Photostability percentages in the UVB range by protocols 1 and 2 and HPLC

Table 6. Photostability percentages in the UVB range by protocols 1 and 2 and HPLC

With some products (C4, C2, C3, H1, C7 and C6), HPLC method and protocol 1 gave very different results (> 10% difference). Conversely, the results obtained by HPLC and protocol 2 were similar for 10 products, whether in the UVB or UVA range.

Table 7. Photostability percentages in the UVA range by protocols 1 and 2 and HPLC

Table 7. Photostability percentages in the UVA range by protocols 1 and 2 and HPLC

With some products (C4, C2, C3, H1, C7 and C6), HPLC method and protocol 1 gave very different results (> 10% difference). Conversely, the results obtained by HPLC and protocol 2 were similar for 10 products, whether in the UVB or UVA range.

Figure 1. Calculation for percentage of photostability (PS) based on SPF

Figure 1. Calculation for percentage of photostability (PS) based on SPF

Two percentages of photostability were therefore calculated using both protocols for the 11 test products, one in the UVB range (SPF) and the other in the UVA range (PPD), to get an overall idea of the quality of solar products studied.

Figure 2. Calculation of SPF

Figure 2. Calculation of SPF

Figure 2. Calculation of SPF; dl = wavelength step (1 nm); = erythema action spectrum (CIE, 1987); Sλ = spectral irradiance received from the UV source (SSR for SPF testing); and Tλ = mean transmittance of the test product layer (9 spots per plate)

Figure 3. Photostability percentage based on PPD

Figure 3. Photostability percentage based on PPD

Two percentages of photostability were therefore calculated using both protocols for the 11 test products, one in the UVB range (SPF) and the other in the UVA range (PPD), to get an overall idea of the quality of solar products studied.

Figure 4. Calculation of PPD

Figure 4. Calculation of PPD

Calculation of PPD;1 P(λ) = PPD action spectrum; I(λ) = spectral irradiance received from the UV source (UVA 320-400nm for PPD testing) ; and Ac(λ) = mean monochromatic absorbance of the test product layer (9 spots per plate)

Figure 5. Beer-Lambert Law

Figure 5. Beer-Lambert Law

Beer-Lambert Law; Aext(λ) = absorbance of the extraction solution at wavelength λ and Adil (λ) = absorbance of the dilution at wavelength λ

Figure 6. Absorption calculation

Figure 6. Absorption calculation

For protocol 2, the percentage of extraction must first be verified. This calculation is based on the additivity absorbances for a product in solution per the Beer-Lambert Law (see Figure 5). The absorbance taken into account in this calculation is the absorbance corresponding to a mass of 28.7 mg and is derived by the equation shown here.

Figure 7. Influence of a black sheet under the PMMA plate

Figure 7. Influence of a black sheet under the PMMA plate

Influence of a black sheet under the PMMA plate with protocols 1 and 2 on products H1 and F3

Figure 8. Photostability percentages in the UVB range by protocol 1 (PMMA), protocol 2 (cuvette) and HPLC

Figure 8. Photostability percentages in the UVB range by protocol 1 (PMMA), protocol 2 (cuvette) and HPLC

With some products (C4, C2, C3, H1, C7 and C6), HPLC method and protocol 1 gave very different results (> 10% difference). Conversely, the results obtained by HPLC and protocol 2 were similar for 10 products, whether in the UVB or UVA range.

Figure 9. Photostability percentages obtained in the UVA range by protocol 1 (PMMA), protocol 2 (cuvette) and HPLC

Figure 9. Photostability percentages obtained in the UVA range by protocol 1 (PMMA), protocol 2 (cuvette) and HPLC

With some products (C4, C2, C3, H1, C7 and C6), HPLC method and protocol 1 gave very different results (> 10% difference). Conversely, the results obtained by HPLC and protocol 2 were similar for 10 products, whether in the UVB or UVA range. Only Product F3 gave results significantly different from the HPLC results.

Rearrangement

In some cases (C4, C2 and H1, here), the rearrangement of the product can form a film distributed more evenly than before irradiation, leading to an artificial increase of absorbance values. With protocol 1, this results in a photostability percentage that is higher than that found by HPLC, and which can be more than 100%.

In other cases (C3, C7 and C6, here), rearrangement can form a film distributed less evenly than before irradiation, resulting in lower artificial absorbance values than with protocol 1, and a photostability percentage lower than that found by HPLC.

It is possible the formula self-levels due to certain conditions such as UV and/or thermal exposure or other raw materials in the formula. The film product can slip to the deeper valleys of the PMMA plate, leaving the upper peak unprotected. Conversely, a formula may tighten or tense itself and cover the valleys or peaks of the PMMA plate more equally.

Footnotes [Pissavini 127(3)]

a The Suntest solar simulator, type CPS+, is manufactured by Atlas Material Testing Technology LLC.

b Helioplate HD6 PMMA plates are manufactured by Helioscreen.

c The UV-2000S transmittance analyzer is manufactured by Labsphere.

d The type N° 110-QS 1-mm quartz cells used for this study are manufactured by Hellma Analytics.

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