Powder SPF in-vitro method

Consumers are looking for far more than a standard Sun Protection Factor (SPF) to add to their daily beauty routines – cue the host of hybrid skin and sun care products dominating the category. Skincare products include formulations designed for the care and protection of the skin, including sunscreen. In 2024, the market value of the global sunscreen industry is expected to grow. Brightening specialities are a leading sector in sun care, followed by acne treatment, anti-aging products, and skincare sets.

Sunscreens trends have been evolving to meet consumers’ changing needs and preferences. Some notable trends in the sunscreen industry include innovative formats. Today, sunscreens are available in various formats, such as creams, lotions, sprays, gels, sticks, and powders. Various formats allows consumers to choose the one that best suits their preferences and lifestyle.
A renewed consumer interest in healthy lifestyle choices also drives innovation in this space as UV protection products feature claims such as broad spectrum, blue light, anti-pollution, and infrared protection. As consumers try to find better ways to protect their skin from the sun’s damaging rays, the demand for new sun care products is growing. Sunscreens require substantiation of their sun protection claims. Testing can be expensive, yield highly variable results, and potentially harm human test participants. Therefore, establishing an in vitro SPF testing method for different sunscreen textures is critical to improving the test’s efficiency before testing in vivo.

The powder SPF market has grown in recent years due to the increasing demand for convenient and versatile sun protection options. These products offer a unique combination of sun protection and ease of use, appealing to consumers who want to reapply sunscreen throughout the day without disrupting their makeup or skincare routine. The powder SPF market will likely continue evolving with advancements in formulations and increased awareness about the need for sun protection³.
Most powders contain mineral sunscreen ingredients like zinc oxide and titanium dioxide, providing a barrier against UV rays. Several types of SPF powder products are available on the market: some common types are brush-on powders, powder sunscreen with applicator pads, coloured or tinted powders, translucent powders, hair and scalp powders, and powder sunscreen with blotting paper. 

Successful testing is one of the most critical steps in taking a product to market. Many in vitro protocol tests exist. All generally involve applying a layer of sunscreen to an artificial substrate, exposing it to UV light from a solar simulator, and measuring the UV transmittance through the product and film by spectrophotometry.
However, studies on powder products are limited. This is attributed to the characteristics of powder products, which show weak adhesive force (low oil content) and powder scattering due to static electricity.
In vitro testing offers several benefits. It allows for controlled and standardised conditions, reducing the variability in human-based testing. In vivo testing is expensive, yields highly variable results, and can harm human test participants. Therefore, in vitro testing allows manufacturers to assess the initial effectiveness of a product before proceeding to human trials.
In vitro testing also helps manufacturers determine the appropriate formulation and concentration of sunscreen ingredients to achieve the desired SPF level. It helps to narrow down the list of potential sunscreen candidates for further evaluation. Products that demonstrate low efficacy in vitro may not proceed to costly and time-consuming in vivo testing. Besides, in vitro testing supports new screenformulation and innovative delivery methods such as powders or sprays.
It is important to note that while in vitro testing provides valuable data on a product’s UV-blocking capabilities, it does not fully replicate the complexity of interactions that occur when sunscreen is applied to human skin. Factors like skin’s natural oils, sweat, and other environmental conditions can affect real-world effectiveness. Thus, in vitro results are often complemented by in vivo (human-based) testing to provide a more comprehensive understanding of a product’s performance.

Univar Solutions has adapted our SPF in vitro testing methodology to suit powder products such as foundations or oil-absorbing mattifying powders. In vitro SPF is measured using the Optometrics SPF Analyzer, Model SPF-290S, on a polymethyl methacrylate (PMMA) substrate.
A solvent was applied on the PMMA substrate before applying the powder product to adhere the product to the plate. Alternate studies have used mineral oil or petrolatum to conduct powder SPF studies as UV light does not absorb them (Watanabe A. et al.). While these solvents can be used for testing, they are irrelevant for real-life applications.
C12-C15 alkyl benzoate was applied onto the PMMA plate to adhere the powder product for this method development. This solvent has no absorption to UV light and is commonly used in sunscreen formulations. The powder was added using a sieve to avoid accumulation on the plate and the formation of clumps. Then, the powder-solvent mixture was spread homogeneously on the PMMA plate. The SPF value was measured using spectrophotometer equipment.
SPF results demonstrated that by sprinkling the powder after applying the solvent, SPF values were more accurate, and the standard deviation was minimised. 

A commercial benchmark was selected for use in the method development trials for in vitro SPF. The formula claims SPF 50. The sample was a pressed mineral powder compact made with non-nano Zinc Oxide (ZnO). The pressed compact was broken into a fine powder and mixed thoroughly before use to ensure the sample was homogenous. Particle size analysis was measured, and found that the lower particle size distribution of the benchmark improved dispersion and had higher in vitro SPF values. In vitro SPF data for powder sunscreen is summarised in Table I. The solvent used was verified to have no significant effect on in vitro SPF results. When the powder was applied to the PMMA plate alone, the result was poor adhesion to the plate and poor in vitro SPF results (Figure 1). During trial applications, it was found that applying and evenly spreading the C12-15 alkyl benzoate to the PMMA plate before sprinkling the powder onto the plate improved reproducibility. Various concentrations of solvent to powder ratios were applied to the PMMA plates and tested in vitro. Previous studies have used different coating weights (CN patent 201410179313.8A). For this case, 1 mg/cm² of solvent with 2 mg/cm2 of powder yielded in vitro SPF results similar to the label claims. High standard deviations are expected at high SPF values. Studies of solvent-to-powder ratios will likely be required for each new type of powder SPF formula. In this instance, this ratio allowed the instrument to measure similar results to the label and have a powder-like appearance (Figure 2). Adding increased solvent ratios created a lotion-like texture and higher than expected in vitro SPF values (Figure 3).
The commercial benchmark product dispersed evenly in the solvent, creating an even application.

C13-C15 alkyl benzoate (Making Cosmetics Inc.) was used as a solvent since it doesn’t present ultraviolet-protection properties and is commonly used in sunscreens. SPF analysis was simulated in vitro following a modified method for powders of the lab’s internal SOP procedure, which used the following parameters:
• Substrate: HelioPlate HD6 PMMA plates
• Sunscreen application: 2 mg/cm² powder and 1 mg/cm² of solvent using an analytical balance
• 1 mg/cm² of C12-15 Alkyl Benzoate solvent was applied to the HelioPlate and spread evenly
• Sunscreen was premixed immediately before sampling for application
• Sunscreen was dusted over a prepared plate and rubbed in to create an even mixture
• Three test substrates per sunscreen
• Instrument: Optometrics (now Solar Light) SPF-290AS automated sunscreen analyzer
• 9 SPF readings per substrate (reported as an average of nine readings and the standard deviation)
• SPF standard: FDA SPF Standard/P2 High SPF Standard, Lot# 1902AB (Solar Light Company)
• Static SPF measured 15 minutes post-application

Powder sunscreens are becoming popular on the market, and substantiation of their performances in vitro is needed. Using in vitro testing helps reduce the need for in vivo testing and minimises the ethical concerns associated with human testing.
Formulating sunscreens with a high enough SPF factor and conducting appropriate SPF testing to ensure that the formulation will perform as desired and required is necessary for any brand. Formulating a sunscreen that meets these requirements while maintaining good skin feel, performance, and stability can be challenging. Therefore, it’s essential to consider the types of ingredients chosen for a sunscreen formula and understand how ingredient quality and proportions can impact the desired SPF value, performance, and stability.
In this study, we adapted our SPF in vitro testing methodology to suit powder products like foundations or oil-absorbing mattifying powders. The objective was to solve the difficulty of testing powder products due to their weak adhesion to substrates and powder scattering due to static electricity.
C12-C15 alkyl benzoate was applied onto the PMMA plate to adhere to the powder product. This solvent has no absorption of UV light and is commonly used in sunscreen formulations. In vitro SPF was measured using the Optometrics SPF Analyzer, Model SPF-290S, on a polymethyl methacrylate (PMMA) substrate.
Depending on powder particle size, studies of solvent-to-powder ratios will be required for each new type of powder SPF formula. In this illustration, the ratio of 1 mg/cm2 of solvent with 2 mg/cm2 of powder allowed the instrument to measure results similar to the label while also having a powder-like appearance. Adding increased solvent ratios created a lotion-like texture and higher than expected in vitro SPF values, reducing SPF standard deviation.
Final in vitro SPF results demonstrated that, by sprinkling the powder after applying the solvent, SPF values were more accurate, and the standard deviation was minimised.

References:  

¹ Chinese patent CN201410179313.8A. Method for in-vitro determination of sun protection factor of powder.

² Watanabe A. et al., J. Soc. Cosmet. Chem. Jpn. 954 (4):340-350. Method for in-vitro determination of sun protection factor of powder

³ Mintel. Sunscreen market trends.