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Photostability Test for Additional Sunscreen Claims, Part II: Calculations and Results

October 27, 2015 | Contact Author | By: S. Miksa, D. Lutz and C. Guy, HelioScreen, Creil, France
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Keywords: photostability | in vitro | sunscreen | UV residual efficiency | solar simulator

Abstract: Proposed here is an in vitro method, based on UV transmission measurements at two irradiation doses, to test and rank sunscreens based on their photostabilities. This approach was used to assess some 107 sunscreens and shows how, by strictly controlling key parameters, comparisons between the photostabilities of products can be made, with potential for new label claims.

One paradox of organic sunscreens is that in order for them to protect against UV radiation, they must be exposed to it, which induces their photochemical degradation and, in turn, a progressive loss in efficacy. This intrinsic characteristic is due to the susceptibility of the filters, absorbing and restoring energy in different chemical ways.

While each UV filter has a specific photo behavior, that behavior is modified based on the stereochemical state of the formula. As a result, different formulas may have different photo behaviors even with the same sets of filters. This means photo-instability can be reduced and even avoided by the right balance in a formula; and there are plenty of patents on this topic. However, it also means photo-instability can be enhanced due to bad stereochemical conditions. So while photo-instability is not a contradictory property for UV filters, it can be for formulations. Therefore, this parameter must be studied for each formula; unfortunately, this is not always the case.

Recently, issues have been raised over unstable sunscreens and lost photoprotection. In relation, most experts would agree that indicating the photostability of a product would be of compelling interest for consumer safety. Different methods have been proposed to determine photostability, including: UV transmission measurements, analytical chemistry techniques based on HPLC dosage, and in vivo methods by means of Diffuse Reflectance Spectroscopy (DRS). Despite great efforts, methods based on photostability percentages are still not validated, and most are not used mainly due to their lack of inter-laboratory reproducibility.

The important task of developing a reliable and reproducible photostability test method was registered by the International Organization for Standardization (ISO) in 2012 as Preliminary Work Item (PWI) 18607. Unfortunately, it was cancelled due to controversy over the predictive value of in vitro observations in relation to in vivo realities during sun exposure. Other reasons included the complexity of the available methods, and the fact that photostability behavior, as defined by some countries, already appeared to account for official in vivo or in vitro methods.

Regarding this last point, it is important to clarify some common confusion between pre-irradiation and photostability behavior. Pre-irradiation is a necessary step in the process to obtain a Sun Protection Factor (SPF) or UVA Protection Factor (UVA-PF). It occurs without any knowledge of the photo-chemical behavior of the product itself. In the case of photostability, however, tests should be designed to understand the physical-chemical behavior of the product under UV exposure—and without any knowledge of the end SPF or UVA-PF results. Further, while various methods use SPF or UVA-PF values to calculate photostability percentages, few represent photodegradation in terms of the residual efficiencies of the UV A+B components of a product.

It is important to remember the photo-behavior of organic UV filters is a chemical process and not influenced by biological efficacy. In addition, most irradiation appliances deliver a great quantity of heat, so degradation may be due to effects of temperature. While improvements to cool the exposed surface with air have been made, they can lead to false results.18 Thus, it is not easy to determine, in a practical way, the real photostability of a formula.

The aim of the present work therefore was to develop a reproducible, dynamic method to measure photostability based on the residual efficacies of UV A+B parts in a formula. By using different doses of UV exposure, samples also could be ranked in order of stability—potentially providing new label claims. To accomplish these goals, 107 marketed sunscreens in different categories were evaluated using a strict procedure, described herein.

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Editor's note: Proposed here is an in vitro method to test sunscreens for photostability. Part I sets up the protocol, Part II details the test method, calculations and results, while Part III outlines a new photostability label claim concept based on the results.

Detailed Test Method

Using these described steps, 107 products were assessed as follows.

Product application: Each product was spread using an automated syringe across a substrate/plate surface at a rate of 1.3 mg/cm². Per the ISO 24443:2012 Standard, PMMA platesa having one reproducibly rough side to replicate topography parameters were used to ensure fairly consistent film-forming.21 The substrates were positioned in a horizontal plane so as to avoid uneven flow of the sunscreen, which could give varying results.

Immediately after weighing, the sunscreen was spread over the whole substrate surface using an automated robotb to ensure total reproducibility; previous work19 has shown manual spreading to cause variations in test films, challenging the reliability of results. Note that alternatives to robotic spreading may be used but should produce results similar to, comparable with and correlated to those achieved using robotic spreading. A similar number of products also must be used, in this case 107, to provide accurate and relevant results.

Temperature control: During spreading, the temperature of the substrate/product interface was controlled by a devicec designed to maintain 25°C ± 0.5°C.20 After the product was spread, the sample was allowed to dry and settle for 30 min in the dark for self-leveling, all while maintaining 25°C. As noted, strict control of the product/substrate interface temperature is compulsory for thermodynamic reasons, as fluctuations could challenge the relevance of this method. Control over the room temperature may also be useful but is not crucial.

UV transmittance: Before transmission tests, the transmittance analyzerd was calibrated both by internal specified standards and the linearity/additivity by calibrated reference standard PMMA platese, to which UV filters were incorporated.22 After samples were dry, their UV transmittance was measured. Blank transmittance also was obtained by covering a PMMA plate with a film of white petrolatum.

Three different plates were used per product, and nine UV transmission spectra were recorded at every nanometer from 290-400 nm for each plate at different application locations. For the present method, a minimum of three replicates at different placements on the plate were measured, representing a total 20 cm² of surface area.

Irradiation intensity: Prior to irradiationf, the intensity of the UV exposure source must be checked by means of a spectroradiometer or radiometerg sensitive to UVB erythemal effectiveness (see Erythemal Effectiveness sidebar). Of the total 290-400 nm UV range, the radiometric proportion of UVA II (320-400 nm) should be higher than 20%, and the proportion of UVA I (340-400 nm) should be higher than 60%, with respect to limits of the percentage of relative cumulative erythemal effectiveness (RCEE) (see Table 1).

UV irradiation: Samples were then exposed to the full spectrum of UV radiation according to doses of 0, 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 MEDs to determine product photodegradation categories; note that only 0, 4 and 8 MEDs are used in the final method, as explained in the next section of this paper.

While the erythemal power of UV radiation can be measured in terms of effective irradiance, as defined by UV A+B content, it is also related to the sensitivity of human skin to sunburn, expressed as Minimal Erythemal Doses per hour (MEDs/hr). Here, the well-accepted definition for an MED unit is used; i.e., the dose of UV exposure capable of causing minimum skin redness (erythema) in average type II skin: 200 J/m²-eff (erythema-effective Joules (J)/m²).

During UV irradiationf, the test procedure must take only the eventual photodegradation of the sunscreen into consideration. External parameters may not be imposed, according to specification published;18 for example, the sample must not be cooled by air flow, and the temperature during exposure must remain at 25°C ± 2°C.

Calculations

Before and following each UV irradiation step, transmittance measurements were taken using the same applianced at the same location for each plate. To determine the ability of each product to absorb or scatter UV light, monochromatic attenuation factors were evaluated at each wavelength between 290 nm and 400 nm; the monochromatic attenuation factor is calculated as the inverse of the value of transmission: 1/T(λ).

To express the percentage of residual efficiency for UV A+B parts, referred to as UVA+B %RE, Equation 1 was used, where n represents the number of plates.

Here, T(λ)0 is the transmission before irradiation, and T(λ)t is the transmission after irradiation. The same equation is used for any UV irradiation dose.

From a statistical point of view, the UVA+B %RE of the product is the arithmetic mean of individual plate UVA+B %REn values, obtained from the total number of plates used, n, and expressed to one decimal point according to Equation 2.

The standard deviation (SD), s, is obtained by Equation 3.

The final UVA+B %REf retained for photostability labeling is determined as follows:

1. Calculate the mean UVA+B %RE and the standard deviation, s, from the UVA+B %REn values.

2. Calculate the standard error (SE), which equals s/√n; where n equals the number of plates that provided valid test results.

3. Calculate the t value from Student’s t distribution table corresponding to the upper 5% point, with n-1 degrees of freedom.

4. Obtain the final UVA+B %REf, which equals the largest whole number less than UVA+B %RE – (t*SE).

Photodegradation Results

In the first step of this process, the photostability behaviors of the products were assessed and three common evolutions of residual efficacy were observed, depending on the photo-behavior of the product (see Figure 1): a) no photodegradation, b) linear photodegradation, or c) polynomial photodegradation.

Furthermore, different photodegradation behaviors were observed when viewing only UVB (290 nm to 320 nm) (see Figure 2); only UVA (320 nm to 400 nm) (see Figure 3); or both UV A+B (290 nm to 400 nm) (see Figure 4). These results confirm the need for product developers to consider the whole UV A+B spectrum absorbance for photostability calculations and consumer safety. Figure 5 attempts to summarize the photostability results for all 107 products.

(Continue to Part III)

References

18. S Miksa, D Lutz and C Guy, Improving the UV exposure of sunscreen during in vitro testing, Cosm & Toil, 129(7) 34-40 (Sept 2014)

19. S Miksa, D Lutz and C Guy, In vitro UV testing-robot vs. human spreading for repeatable, Reproducible Results, Cosm & Toil, 128(10) 742-752 (Oct 2013)

20. S Miksa, D Lutz and C Guy, UV transmission assessment: Influence of temperature on substrate surface, Cosm & Toil, 128(7) 484-494 (Jul 2013)

21. M Pissavini, S Marguerie, A Dehais, L Ferrero and L Zastrow, Characterizing roughness: A new substrate to measure SPF, Cosm & Toil 124(9) 56-64 (Sep 2009)

22. N Cariou and D Lutz, Sunscren in-vitro SPF determination inter and intra comparison tests between several measurement instruments, H&PC Monographic Supplement Series: Compendium on Sun Care 7(3) (Jul/Sep 2012)

 

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Table 1. %RCEE Limits for Solar Simulator

Table 1. %RCEE Limits for Solar Simulator

Of the total 290-400 nm UV range, the radiometric proportion of UVA II (320-400 nm) should be higher than 20%, and the proportion of UVA I (340-400 nm) should be higher than 60%, with respect to limits of the percentage of relative cumulative erythemal effectiveness.

Equation 1. UVA+B %RE

Equation 1. UV<sub>A+B</sub> %RE

This equation was used to express the percentage of residual efficiency for UV A+B parts.

Equation 2. Arithmetic mean of individual plate UVA+B %REn values

Equation 2. Arithmetic mean of individual plate UV<sub>A+B</sub> %RE<sub>n</sub> values

From a statistical point of view, the UVA+B %RE of the product is the arithmetic mean of individual plate UVA+B %REn values, obtained from the total number of plates used, n, and expressed to one decimal point

Equation 3. Standard deviation

Equation 3. Standard deviation

The standard deviation (SD), s, is obtained by this equation.

Figure 1. Different behavior of products under UV exposure

Figure 1. Different behavior of products under UV exposure

no photodegradation (a), linear photodegradation (b) and polynomial photodegradation (c)

Figure 2. Photodegradation mainly on UVB

Figure 2. Photodegradation mainly on UVB

Different photodegradation behaviors were observed when viewing only UVB (290 nm to 320 nm).

Figure 3. Photodegradation mainly on UVA

Figure 3. Photodegradation mainly on UVA

Different photodegradation behaviors were observed when viewing only UVA (320 nm to 400 nm).

Figure 4. Photodegradation of UV A+B

Figure 4. Photodegradation of UV A+B

Different photodegradation behaviors were observed when viewing both UV A+B (290 nm to 400 nm), compared with UVA and UVB individually.

Figure 5. Photostability behavior of the 107 marketed sunscreen products tested

Figure 5. Photostability behavior of the 107 marketed sunscreen products tested

This figure attempts to summarize the photostability results for all 107 commercial products.

Erythemal Effectiveness

The erythemal effectiveness of each waveband is expressed as the spectral irradiance of the source multiplied by the 1998 Commission International de L’Eclairage (CIE) standard skin erythemal action spectrum, to obtain the erythemal effectiveness of the source. The spectral erythemal effectiveness values of the source spectrum are then integrated from 290 nm to the various successive reference wavelength values in order to produce the cumulative erythemal effectiveness for each spectral waveband, and the total erythemal effectiveness is calculated up to 400 nm.

Footnotes [Miksa 130 (8)]

a Helioplates HD6, b HD-Spreadmaster and c HD-Thermaster, HelioScreen

d UV-2000S, Labsphere Inc.

e Helioplates HD0, HelioScreen

f Pre-Irradiation Solar Simulator Model 16S-300-009 and g UVB sensor PMA 2101 biologically-weighted erythema, Solar Light Company Inc.

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