title: Contact angle measurement - a reliable supportive method for screening water-resistance of ultraviolet-protecting products in vivo.
abstract: Substantivity of sunscreen formulations is affected by the wash-out rate of ultraviolet-absorber and -reflector compounds in water. Water-resistance of sunscreen formulations is currently determined according to a standardized European Cosmetic Toiletry and Perfumery Association (COLIPA) protocol, encompassing the determination of a minimal erythemal dose before and after a defined immersion step in water. It can be supposed that the higher the wettability of a treated skin area, the higher is the wash-out rate of sunscreen compounds. This present report addresses the validity of determining the wettability of treated skin alone as a measure for the water-resistance of sunscreen products. The report addresses the robustness, accuracy and congruence of a recently developed wettability test, based on the measurement of the contact angle (CA) of a sessile water drop on treated skin areas. Contact angle data of 66 sunscreen formulations are compared with the corresponding results of 81 water-resistance tests, using the sun protection factor (SPF)/immersion/SPF method. Sunscreen products tested by the CA method were applied to the skin of the volar forearm of test subjects at a defined dose and drying-time, using a standardized application and recording device. Contact angles between a sessile water drop and skin were recorded by a Charge-Coupled Device (CCD) camera and subjected to automatic contour analysis. Taking the SPF/immersion/SPF method as gold standard, accuracy parameters of the CA method were determined. By using an appropriate cut-off level of CAs, the CA method has a specificity and positive-predictive value of 100%, and turns out to be a reliable screening method to identify water-resistant formulations. Based on our findings, those formulations that give CAs above 30 degrees may be categorized water-proof without further testing by the COLIPA water-resistance method.
authors.affiliation: Beiersdorf AG, Research Bioengineering, Unnastrasse 48, D-20245 Hamburg, Germany. ralf.hagens@beiersdorf.com
authors.forename: R
authors.initials: R
authors.lastname: Hagens
authors.forename: T
authors.initials: T
authors.lastname: Mann
authors.forename: V
authors.initials: V
authors.lastname: Schreiner
authors.forename: H G
authors.initials: HG
authors.lastname: Barlag
authors.forename: H
authors.initials: H
authors.lastname: Wenck
authors.forename: K-P
authors.initials: KP
authors.lastname: Wittern
authors.forename: W
authors.initials: W
authors.lastname: Mei