Alternative In-Vitro SPF Sunscreen Test Method

TAGS:  Sun Care   

In the past years, sun protection has been deeply improved in terms of:

• Balance (UVB, UVA, Blue Light, Infrared, etc.)
• Claims (Sun Protection Factor – SPF, UVA Protection Factor – UVAPF, Critical Wavelength – CW, Water Resistance – WR, etc.), Formulation
• Understanding sunlight and testing methods
• Regulations, awareness from consumers, etc.

There are different methods available for the sunscreen testing procedure. These methods are relatively complex and require significant experience, processes and equipment. Fortunately, standards are available in order to harmonize these methods worldwide, mainly, according to ISO (International Organization for Standardization).

Nowadays, from the ISO projects, several methods are already available or under development, such as:

• In-Vivo SPF according to ISO 24444 (published)
• In-Vivo UVAPF according to ISO 24442 (published)
• In-Vitro UVAPF – CW according to ISO 24443 (published)
• In-Vivo WR procedure according to ISO 16217 (development)
• In-Vivo WR percentage calculation according to ISO 18861 (development)
• In-Vitro SPF according to ISO 23675 (development)
• In-Vivo/In-Vitro Hybrid Diffuse Reflectance Spectroscopy SPF – UVAPF according to ISO 23698 (development)

(Other methods may be added in the future for standardization process such as the Blue Light assessment, In-Vitro Water Resistance, etc.).

Among all these methods, the In-Vitro SPF is strongly required by the industry and governmental organizations delivering results equivalent to the ISO 24444 method. As evidence, the degree of protection should be measured using standardized, reproducible testing methods and take photo-degradation into account as recommended by the European Commission^[1].

Let’s study the alternative In-Vitro sunscreen test method under the project ISO 23675.

In-Vitro SPF Method Principle

The In-Vitro SPF method in progress at the ISO level (projects ISO 23675) includes new requirements and appliances to ensure the reliability of the results for the In-Vitro determination of UVA protection. The method is validated by Cosmetics Europe (CE) in a recent publication^[2] and incorporates many of the recommendations issued by the ISO 24443. Given below are the different simplified steps^[3-4]:

1. As for the ISO 24443, topographic parameters of the substrate shall be controlled and respected as a control chart for both molded and sandblasted PMMA plates. Both substrates are used with different quantity of products (1.3 mg/cm² for molded and 1.2 mg/cm² for sandblasted).
   
   
   
   Molded (L) and Sandblasted (R) PMMA Plates
   Source: HelioScreen


2. The temperature of the interface substrate/sample shall be controlled during the whole process.
   
   
   
   HD-THERMASTER – An Instrument that Controls the Temperature of Substrate/Sample
   Source: HelioScreen


3. As the method consists of assessing the residual transmission of a product spread in a thin layer on substrates, to ensure reproducibility, it has been demonstrated that automated spreading is the only way by using a robotic arm with specific characteristics.
   
   
   
   HD-SPREADMASTER – An Automated Sunscreen Product Spreading Machine Using a Robotic Arm
   Source: HelioScreen


4. After the drying step and prior to any UV irradiation, the acquisition of the initial UV absorbance spectrum, the characteristics and the performance of the spectrophotometers and their regular checking with specific calibration are necessary.
5. After a mathematical adjustment of the initial UV absorbance spectrum using correction factors, a single UV exposure dose D (MED/h) is applied with a solar spectrum similar to the In-Vivo SPF ISO 24444. It will be mandatory to control the emission of the source in order to calculate and justify the dose.
6. Finally, a second In-Vitro absorbance measurement of the sunscreen product after UV exposure is required in order to calculate the final In-Vitro SPF after mathematical adjustment.
   
   
   
   In-Vitro Absorbance Measurement of the Sunscreen

Acceptance of the Alternative SPF Method

Even if it was never raised so far for other sun protection testing methods, one of the problems created for the In-Vitro SPF method validation was that no acceptance criteria existed among the ISO TC217 (Technical Committee for Cosmetics) WG7 (Working Group for Sun Protection Test Methods) consensus.

To respond to this issue, after a huge work from different parties (ISO TC217 WG7 Ad Hoc group, CE, statisticians, experts, etc.), an international consensus was proposed and accepted years ago by checking:

• The ascertain minimal method bias (matrix effect, overall bias, etc.).
• At least 95% of the paired SPF values for 24 products, derived from the 3 in-vivo test institutes (at least 5 test subjects per laboratory) and the 3 in-vitro testing labs (both in a blinded fashion), fit within the upper and lower limits of a funnel across the full range of labeled SPF categories (SPF 6, 10, 15, 20, 25, 30, 50 and 50+).

During this time, even if other proposals were proposed regarding validation criteria^[5] and even if some of them may be interesting, no new consensus from the different experts in the sun protection testing fields changed at the international level.

In other terms, the minimum criteria proposed and accepted by the ISO/TC217/WG7 years ago remain valid and already include a balance between the statistical requirements, the cost efficiency and the realistic feasibility.

Conclusion

Today, all barriers are solved for the alternative In-Vitro SPF method according to the ISO 23675 projects, including:

• The technical limits (the method is reproducible and correlated to the In-Vivo SPF values).
• The fulfillment of the ISO acceptance criteria (as explained in a publication about the CE method^[2]).
• The established international majority consensus.

Upgrade your SPF Test Strategy for Regulatory Compliance

Talk to Julian Hewitt where he will help you avoid running expensive tests and show you how to support formulation work & UVA/SPF claims for your targeted markets (EU, US, Japan). He will also help you learn about conducting right test at the right time (in-vitro or in-vivo) & how to interpret test results better.