A new in vitro method to evaluate the Long-Lasting Performances of Sunscreen products

Abstract

Despite international guidelines recommending reapplying sunscreen every two hours, most consumers rarely apply sunscreen daily or fail to reapply later. Therefore, secondary sunscreen products such as moisturizer or makeup providing a sun protection appear as a more sustainable option for an everyday use, especially if that protection can last several hours. As a way to back up such claims with scientific data, the authors developed an in vitro method to measure the durability of the sun protection performance in time while considering its photostability.
The sunscreen is spread on PMMA plates by means of a robotic appliance before drying at 25°C ±2.0°C for 15 minutes. Several measurements are taken at T0, T2h, T4h and T8h in order to obtain the corresponding SPF. One set of plates is also exposed for 2,4 and 8 hours, with their absorbance measured to evaluate the photostability. The percentage of Long Lasting is calculated by a ratio of SPF before and after drying/exposure. The product can be considered as long lasting if its protection is maintained at 95% for both the drying and exposure percentages.
Initial results show that 25% of the products present a protection lasting for 8 hours. To conclude, this method demonstrates a good selectivity and reaffirms the need to reapply sunscreen every 2 hours.

Introduction

All worldwide experts, health agencies, cosmetic manufacturers, consumer’s associations, etc. involved in the sun protection field recommend reapplying sunscreen products frequently (at least every two hours and especially after swimming or sweating) and generously. Moreover, some regulations ban mentions which do not incite for reapplication.¹ This comes from a global effort to raise awareness of the damages caused by UV exposure, which includes sunburns, sun-induced skin aging and skin cancers. Nowadays, according to the French National Institute of Cancer, 80 000 skin cancers are diagnosed every year in France, including more than 15 000 melanomas in 2018. The International Agency for Research on Cancer (IARC) estimates that more than 80% of those cancers are caused by excessive sun exposure.²
The need of a protection against the sun is well understood, with the use of sunscreen products at the beach. Those are also called primary sunscreen products, which aim primarily at protecting the skin from UV radiation. However, the reapplication guidelines are often not followed, or not properly, as customers can find the process time consuming and annoying.
Secondary sunscreen products have a primary function that differs from sun protection whilst providing some protection of the skin from UV radiation. They include Make-up, foundations, anti-ageing, moisturizers, etc.^3–4 These products are used more often on a daily basis by customers, but they usually don’t consider that sunscreen products’ guidelines apply in their case. Yet, if they don’t plan to stay in the sun, most of the time people will put on primary and/or secondary products in the morning as part of their routine but will not reapply later. A survey conducted in 2020 in the USA indicates that only 12.3% of men and 29% of women over 18 years old always used sunscreen when going outside for more than one hour on a sunny day.⁵
Therefore, some consumers are looking for secondary products providing a sun protection during several hours, even if studies emphasized the decrease of the sun protection in time through UVA camera or SPF value analysis.⁶ Obviously, all claims such as the mention “Long Lasting”, “Once-a-day”, “8hr protect”, and recently “Durable sunscreen”, etc. should be based on a scientific proof. These proofs are generally obtained by an in vivo SPF assessment method considering a drying step longer (several hours compared to few minutes according to official methods) before the UV exposure and erythema reading. Nevertheless, this way of testing is biased for sunscreen products because, all day long, external factors challenge the sun protection such as clothes, sand, sweat, sun light (considering photostability), etc. Moreover, adapted methods are expensive and time consuming so brands may be interested to use in vitro methods to support these claims.
Thus, in this article, the authors tried to come up with an in vitro method to measure the durability of the sun protection performance in time while taking into account its photostability.

Materials and Methods

This study has been conducted on a panel of 17 products covering different galenic (emulsions, sticks, foundations, oils, powders, and alcoholic sprays) and ranging from SPF 6 to 50+.

1. Substrate
   The roughness of test substrates has been shown to affect the reproducibility of UV in vitro tests.^7–9 Thus, molded polymethyl methacrylate (PMMA) plates HD6 (WENEOS, ex HelioScreen) were used for the present study. The application area is 47 mm x 47 mm, i.e., more than 22 cm², and only one face of the plates is rough. All surface topography parameters were controlled with an ad hoc profilometer and were in total compliance with the specifications described in the ISO 24443:2021 standard for in vitro UVA-PF determination.

2. Product application
   Sunscreens are shaken to ensure good homogenization, then applied to the PMMA substrates in sixteen dots using a multipipette at a rate of 1.3 mg/cm². Immediately following, the product is spread using an automated device (HD-SPREADMASTER,
   WENEOS, ex HelioScreen) which is composed of two parts. The first one is a robotic arm, which performs precise and repeatable movements, both circular and linear strokes with controlled pressure. The second part is a silicone finger with a hard surface that simulates the human finger. This pairing perfectly imitates human spreading with better reproducibility, as it was previously demonstrated.^10–11 After spreading, samples are allowed to dry for 15 min (HD-THERMASTER, WENEOS ex HelioScreen) before the first SPF measurement, during which the temperature of surface of the substrates is maintained at 25°C ± 2.0°C.¹²

3. Solar simulator
   As explained previously, it is essential to consider any photostability of the tested products, which means that it is necessary to perform an irradiation step. It is realized in a calibrated testing device (SUNTEST CPS+, Ametek) that simulates solar irradiance at ground level under a standard zenith sun and within the acceptance limits specified by ISO 24443:2021 standard. The tests are conducted at a controlled temperature of 25°C ± 2.0°C.^13–14

4. Protocol
   After depositing 1.3 mg/cm² of product on the plates with a multipipette, it is then spread by means of a robotic appliance. The plates are left to dry at 25°C ± 2.0°C for at least 15 minutes in the dark. An initial measurement of the absorbance is performed between 290 and 400 nm to obtain the initial SPF. A first set of plates is left to dry for 8 hours at 25°C ±2.0°C in the dark, with several other absorbance measurements after 2, 4 and 8 hours, while the other set will be exposed to several exposure doses to challenge its photostability. Those doses represent 2, 4 and 8 hours of UV exposure as used in outdoor situation during a standard day light irradiance. Absorbance measurements are also included after each dose.

5. Evaluation of the Long-Lasting Potential
   SPF is determined using a spectrophotometric method. We use the Labsphere UV-2000S (Labsphere Inc.) to measure the intensity of the UV radiation going through the sunscreen product depending on the wavelength of the UV spectrum – here between 290 and 400 nm. Furthermore, blank transmissions were created using PMMA plates covered with glycerine. Each plate is measured in 9 different points across it and the SPF can be calculated with the following equation:

The ratio of the SPF before and after drying/exposure was used in order to assess the percentage of Long Lasting (%LongLasting):

The proposal is to consider a product as “Long Lasting” if the protection is maintained at 95% compared to the initial value for both the drying and the photostability percentages at 2, 4 and 8 hours.

Results and Discussion

This study has been conducted on a panel of 17 products covering different galenic (emulsions, sticks, foundations, oils, powders, and alcoholic sprays) and ranging from SPF 6 to 50+.
Table 1 presents the different results obtained for the tested sunscreen products at different drying and photostability times (2h, 4h and 8h).
For a product to be considered as “Long Lasting”, its resistance percentages of both drying time and photostability must be greater than 95% when compared to the initial SPF.
First of all, it is interesting to observe a rational decrease of the number of products with a “Long Lasting” performance when the time increases. After two hours, about ½ of the products can be considered as Long Lasting. This amount drops to 1/3 of the products after 4h and ultimately, only ¼ of them present a Long-Lasting performance after 8h. Moreover, the decrease in time is mainly noticeable for the photostability percentages. This illustrates the necessity of challenging sunscreen products with a UV exposure when assessing their protection performances, whether it is normal or Long-Lasting.
Secondly, it is possible to observe some continuity in the Long-Lasting performance from 2h to 8h. In other terms, if a product is resistant at 8h, it is reasonable to interpolate the Long-Lasting performance to 4h and 2h.
Finally, around 50% of the products appear to be Long-Lasting after 2h. Thus, the recommendation to frequently reapply sunscreen every 2h is extremely important for the majority of the products in order to conserve a high level of photoprotection.

Conclusion

The claim “Long Lasting” is most of the time not appropriate for a primary sunscreen product, as it may be leading to an overexposure to the sun with a product losing photoprotection in time. However, different brands already offer this claim to consumers. To be able to detect this protection level, this in vitro method was proposed considering the sun protection factor (SPF) at different times and including a photostability factor.
From this method and applying a high level of selectivity in terms of resistance percentage (time and photostability) for consumer’s health, we can conclude that about 25% of products maintain their protection throughout the day (about 8 hours).
Finally, as evidence, this proposal is a starting point, and these results should be completed by in vivo or in vitro analysis to ensure a higher security and include several other external factors from real life (sweat, sand, rub, etc.).

References

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