UREA ANALYSIS OF STRATUM CORNEUM 161 11o - lOO c 90 ,,.. 80 c 70 P 60 c 50 c 40 o 30 • 20 lO [] volunteer A [] volunteer B [] volunteer C [] volunteer D [] volunteer E SDS-solution 4% shower gel shower gel with 10 % urea cleansing product Figure 4. Comparison of urea content (in percent of control) of stratum corneum after treatment with different cleansing products. method described by Schwarz (10-12) and Kloss and Schwarz (13). Schwarz examined the urea content of stratum corneum in samples scraped from the skin surface. A similar extraction method was introduced by Koyama et aL (14). However, in our protocol, water is the only extraction solvent instead of a sequence of an organic and an inorganic solvent. The rapidity and the safety of our method offers the possibility of using it routinely on human volunteers when testing skin care formulations and cleansing prod- ucts. We observed a good reproducibility of the urea concentration in different skin extracts taken from the volar forearms of an individual volunteer even when taken on separate days. Therefore, it appears possible to compare different volar forearm skin sites when the testing of a number of products is required. Repeated extractions from an identical skin area resulted in a tenfold lower urea concentration in extracts when compared to the initial extraction. The extraction procedure causes a swelling of stratum corneum that might cause a slightly enhanced mobilization of urea in repeated extracts. The extension of extraction time will therefore produce only a slight increase of urea extracted from stratum corneum. From these results we conclude that the amount of extracted urea by the given method is representative of the urea content of stratum corneum. Treatment of skin with water or cleansing products dramatically decreases the urea content and, therefore, one important component of the NMF of stratum corneum. This might be a possible factor in the development of a dry and scaly skin after prolonged skin cleansing with water exclusively (15). In our experiments neither the presence nor the actual concentration of detergents in cleansing products had any influence on the de- crease of urea content in stratum corneum. However, a supplementation of cleansing product with 10% urea has a measurable positive effect on the urea amount that can be extracted from the skin surface. This positive effect on extractable urea might be related

162 JOURNAL OF COSMETIC SCIENCE 135 130 '• 125 i • 120 - __. a= T ..... 105 100 95 90 [] Laceran Spezial Creme ß Laceran Spezial Creme 5% Urea ..: E F G H I volunteer 4OO 300 200 A B C D E F G H b volunteer J K L [] Laceran Spezial Creme ß Laceran Spezial Creme 5% Urea J K L Figure 5. Comparison of Laceran © Spezial Creme and Laceran © Spezial Creme 5 % Urea in relation to their hydration (Figure 5a) and the increasing urea content of stratum corneum (Figure 5b). Means and standard deviations of the estimation of skin hydration were calculated from six measurements at each test site. The corneometer units of the test site after the treatment were related to the value before treatment (tO) and the corneometer units of the control site. to a reduced concentration gradient of urea between stratum corneum and wash solution. Only a limited compensation of urea loss during the washing procedure can be achieved with urea supplementation of cleansing products. On the other hand, a considerable supply of urea to stratum corneum can be achieved with urea-containing skin care formulations. For example, a prolonged treatment of skin with Laceran © Spezial Creme