A new in vitro screening method to assess water resistancy
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Presented by: Marc Pissavini
Objective:
Recently, an increased number of methods for determining the ultraviolet (UV) protection factor have been proposed. But water resistance in vitro is still not validated, although several methods are published.
In previous studies, the authors demonstrated that the absorbance value of sunscreen depends on its uniform spreading on the test substrate. This uniformity of spreading can be modified by different factors such as heat or ventilation as has already been demonstrated elsewhere, but also by the action of water which can modify the film homogeneity of the sunscreen. This is one reason that spectroscopic methods based only on UV transmission to measure the water-resistance in vitro can lead to issues of reproducibility.
Here, the authors describe a new method for determining the water-resistance in vitro of sunscreen products based on the SPF in vitro determination as recommended by Cosmetics Europe for the sunscreen application and spreading and on a previous publication for the immersion aspect and the absorption of sunscreens in dilute solution.
Materials and Methods:
This protocol in 4 steps consists in studying the absorbance of sunscreens extracted from a product before and after rinsing to determine a water resistance index correlated to the in vivo ISO 18861 & ISO 16217.
Step 1: Sunscreen application following a protocol based on the Cosmetics Europe recommendation
Step 2: Plates rinsing with demineralized water.
Step 3: Sunscreen filters extraction thanks to isopropanol following a method published elsewhere.
Step 4: Absorbance measurements of the extracted solutions with and without rinsing thanks to an ultraviolet spectrophotometer in 1cm cuvet.
Results:
Extracted solution absorbance is used to calculate the percentage of water resistance. The statistical analysis of the obtained results demonstrated that the method is repeatable and reproducible. The correlation with the results of the in vivo water-resistance determination was satisfactory whatever the SPF level or the % of water-resistance of the product.
Conclusion:
A method for determining in vitro the water resistance percentage based solely on UV transmission is often inaccurate since it involves taking the absorbance measurement twice in the calculations—once before and once after immersion. The resulting ratio will therefore show significant variations.
The protocol was thus changed to involve a spectroscopic technique in dilute solution in order to avoid absorbance measurements of thin films.
The in vitro water resistance percentages obtained for the 10 products tested were then found to be reproducible and in line with the results obtained in vivo. This new Water Resistant method seems to provide a more accurate water resistance percentage from the product tested, since they are not/less influenced by the redistribution of the thin film. We concluded that this new technical approach seems promising and might become a good candidate for validation in a ring test.
Recently, an increased number of methods for determining the ultraviolet (UV) protection factor have been proposed. But water resistance in vitro is still not validated, although several methods are published.
In previous studies, the authors demonstrated that the absorbance value of sunscreen depends on its uniform spreading on the test substrate. This uniformity of spreading can be modified by different factors such as heat or ventilation as has already been demonstrated elsewhere, but also by the action of water which can modify the film homogeneity of the sunscreen. This is one reason that spectroscopic methods based only on UV transmission to measure the water-resistance in vitro can lead to issues of reproducibility.
Here, the authors describe a new method for determining the water-resistance in vitro of sunscreen products based on the SPF in vitro determination as recommended by Cosmetics Europe for the sunscreen application and spreading and on a previous publication for the immersion aspect and the absorption of sunscreens in dilute solution.
Materials and Methods:
This protocol in 4 steps consists in studying the absorbance of sunscreens extracted from a product before and after rinsing to determine a water resistance index correlated to the in vivo ISO 18861 & ISO 16217.
Step 1: Sunscreen application following a protocol based on the Cosmetics Europe recommendation
Step 2: Plates rinsing with demineralized water.
Step 3: Sunscreen filters extraction thanks to isopropanol following a method published elsewhere.
Step 4: Absorbance measurements of the extracted solutions with and without rinsing thanks to an ultraviolet spectrophotometer in 1cm cuvet.
Results:
Extracted solution absorbance is used to calculate the percentage of water resistance. The statistical analysis of the obtained results demonstrated that the method is repeatable and reproducible. The correlation with the results of the in vivo water-resistance determination was satisfactory whatever the SPF level or the % of water-resistance of the product.
Conclusion:
A method for determining in vitro the water resistance percentage based solely on UV transmission is often inaccurate since it involves taking the absorbance measurement twice in the calculations—once before and once after immersion. The resulting ratio will therefore show significant variations.
The protocol was thus changed to involve a spectroscopic technique in dilute solution in order to avoid absorbance measurements of thin films.
The in vitro water resistance percentages obtained for the 10 products tested were then found to be reproducible and in line with the results obtained in vivo. This new Water Resistant method seems to provide a more accurate water resistance percentage from the product tested, since they are not/less influenced by the redistribution of the thin film. We concluded that this new technical approach seems promising and might become a good candidate for validation in a ring test.