262 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Recovery of •4C-Labeled 0.14% w/v Lactic Acid at pH 7 With a Specific Activity of 12 •Ci/ml From Four Experimental Sites of a Nude Mouse at Zero Time % of Applied Radioactivity Recovered in 20 Strips Left Right Left Right Front Front Back Back Swabs 99.28 98.71 98.65 98.92 Tape Strippings 0.56 0.53 0.74 0.69 Total Recovery 99.84 99.24 99.39 99.61 Each result is the mean of the readings obtained using two mice. Applied amount is 5 microliters. Table II Stratum Corneum Uptake of Some of the Main Ingredients of NMF % of Applied Radioactivity Recovered in 20 Strips Specific Concentration Human Mouse Skin Mouse Skin Mouse Skin Ingredient Activity % w/v pH Transplant Right Front Left Back Right Back Lactic Acid 12 •Ci/ml 0.14 7.0 6.78 4.23 5.43 7.00 Alanine 7 •Ci/ml 0.07 6.0 5.93 4.05 4.53 4.25 Urea 7 •Ci/ml 0.1 7.2 17.13 14.76 16.54 15.41 Recovery time is four hours. Each result is the mean of the data obtained using two mice. 5 microliters of test material was applied to 1 cm 2 test area. Table III Stratum Corneum Uptake of 10% w/w Radiolabeled NMF Condensate and Aqualizer From Different Cosmetic Bases % of Applied Radioactivity Recovered in 20 Strips Type Human Skin Mouse Skin Mouse Skin Mouse Skin Applied Substance of Base pH Transplant Right Front Left Back Right Back NMF Condensate Aqueous 5.42 1.36 1.29 2.11 1.45 in Water Solution Aqualizer in water Aqueous 6.28 3.45 3.51 2.48 Solution Aqualizer in A Enriched 5.88 1.11 0.69 1.52 0.68 Cream, w/o Aqualizer in B Moisture 5.87 1.14 1.05 1.28 0.28 Cream, w/o Aqualizer in C Moisture 4.28 2.03 1.19 2.11 1.48 Lotion, o/w Aqualizer in D Moisture 5.86 1.45 2.26 2.07 2.14 Lotion for Oily Skin o/w Recovery time = 4 hrs. Each value is the mean of three replicates.

HUMAN SKIN GRAFTED NUDE MOUSE 263 type bases (C and D) appeared to promote the diffusion of the NMF to the lower layers of stratum corneum. This could be the effect of the viscosity or composition variations in those bases. Figure 6 demonstrates the kinetics of the stratum corneum uptake of PABA (5%) in a hydro-alcoholic vehicle (22). For this substance, the corneocytes of the human skin transplant consistently retained a higher concentration of radioactivity for all experi- mental intervals. A decreasing rate of uptake with time was demonstrated. All the preliminary results show that human skin transplanted nude mouse can be a useful model to study the stratum corneum uptake, diffusion kinetics, and even desorp- tion properties of several cosmetic ingredients and formulations working with minute amounts of test materials. This can be particularly convenient in cases where the em- ployment of radiolabeled substances is required. CONCLUSIONS The experimental results obtained clearly indicate that the human-skin-grafted athymic I[ [ I I • I i I 0 I 2 3 5 6 7 8 T I M E hrs Figure 6. O, Percent •4C-labeled PABA recovered in 20 strippings of grafted human skin treated with 5 •tl/cm 2 of a 5% solution of PABA in a vehicle of 50% ethanol, 20% glycerin and 30% water. n = 4 for each data point. &, Percent •4C-labeled PABA recovered in 20 strippings of nude mouse skin by the same methods. n = 4 for each data point.