IN VITRO SPF TESTING ACCORDING TO ISO 23675: PRINCIPLES, REQUIREMENTS, AND PRACTICAL GUIDELINES.

ISO 23675:2024 is the latest standard defining the principles for determining the sun protection factor (SPF) fully in vitro. It represents the culmination of many years of research and intensive efforts by the entire sunscreen industry to develop alternative methods for testing sun protection efficacy. This standard not only supports an ethical approach to testing (the 3R principle), but above all ensures a high level of correlation with results obtained using in vivo methods, provided that specific technical and procedural requirements are met.

The method described in ISO 23675:2024 assesses the level of sun protection by measuring the UV transmittance of a product layer applied to appropriate substrates, which are specially prepared PMMA plates. The SPF value is calculated based on the resulting transmittance spectrum.

Key elements of the method include:

• the use of two types of plates (molded and sandblasted) with different surface characteristics, forming pairs for which the results are integrated,
• application of the sunscreen product to each type of substrate in a strictly defined amount,
• uniform spreading of the product using a dedicated robot in accordance with the procedure specified in the standard,
• operation on equipment that is regularly checked and compliant with the requirements of the standard,
• strict adherence to the defined time regime during the test procedure,
• maintenance of stable thermal conditions throughout the entire process.

The plates used in in vitro SPF testing according to ISO 23675:2024 must exhibit strictly defined and reproducible optical and surface properties in order to ensure reliable and repeatable results. The requirement to use two types of plates with different surface characteristics significantly improves correlation with in vivo SPF results. The condition of the plates should be checked before each use to exclude contamination or damage that could affect transmittance.

The standard also defines the exact amount of product per unit area that should be applied to each type of plate. For molded plates, 1.2 µg/cm² (±1.6%) should be applied, while for sandblasted plates the required amount is 1.3 µg/cm² (±1.5%). These quantities are measured with high precision and then evenly spread over the entire active surface of the plate. Any deviation from the specified amounts leads to significant changes in film thickness and thus directly affects the determined SPF value.

The use of a robot for product application in testing according to ISO 23675:2024 is crucial for the quality and repeatability of the method. The robot ensures constant and controlled force, speed, and spreading pattern, eliminating variability resulting from operator-dependent application.

Devices used for the measurement and irradiation of sunscreen product samples should  be calibrated and checked regularly, and thermal conditions should be stabilized. Maintaining the temperature within a narrow, defined range enables standardization of the film formation process and limits the negative impact on the optical properties of the samples.

The standard also specifies minimum and maximum time limits for key stages of the procedure. Compliance with these limits is essential for the comparability and repeatability of results.

The in vitro method is based on a physico-optical assessment of a cosmetic product’s ability to absorb and scatter UV radiation, rather than on its interaction with a biological system such as human skin. Therefore, achieving the declared SPF value requires consideration of certain technological aspects already at the product formulation stage.

The first and key factor is the stability of the cosmetic formulation. Only stable formulations ensure reliable, repeatable, and reproducible results. Product stability should take into account the real conditions to which the product will be exposed during use.

The selection of UV filters should consider both their absorption range and photochemical stability. Degradation of filters upon exposure to radiation significantly reduces the resulting SPF value. Equally important are the interactions between the filters themselves, as well as between the filters and other formulation components such as active ingredients, preservatives, fragrance, emulsifiers, and thickeners, all of which can significantly affect the final photoprotective performance. Potential interactions between UV filters and packaging components is also important.

The viscosity of the formulation affects the homogeneity, continuity, and stability of the product film on the substrate. Products with inappropriate rheological properties may form non-uniform layers, resulting in increased variability of test results.

Proper execution of the test in accordance with ISO 23675:2024 requires meeting specific instrumentation and procedural requirements; however, understanding and accounting for the specific characteristics of the standard, along with conscious formulation design, significantly increases the likelihood of obtaining satisfactory and reliable SPF results.

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