METHOD FOR TRANSEPIDERMAL WATER LOSS 111 A. Receptor compartment B. Donor compartment C. Parafilm seal D. Receptor compartment filled with calcium chloride. E. "Screw-in" attachment functions to secure skin and hold receptor cell. F. Skin membrane placed upon small well of water which functions as donor compartment. Figure I. In vitro TEWL apparatus, representing a modification of Bronaugh's flow-through diffusion cell.

112 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS RESULTS AND DISCUSSION METHODOLOGY Calcium chloride was chosen as it is the most convenient desiccant available for study. Five individual measurements were made of the amount of anhydrous calcium chloride placed in the receptor compartment of the cell. The average weight recorded was 0.24 + 0.016 g. Cartilage-stripped hamster ear membrane is a new source for obtaining a fresh sample of full-thickness skin membrane. Percutaneous absorption theory states that the flux is inversely proportional to skin thickness (10). In actuality, we are referring to the thick- ness of the stratum corneum, as this is the rate-determining skin layer. Measured skin thickness was 0. 181 --- 0.003 mm. Since little or no variation in skin thickness was observed, it was assumed to be a constant when measuring TEWL. TIME STUDY Two blank and six experimental cells were prepared and placed into the heated alu- minum holding block at various temperatures. The receptor compartment containing the desiccant was exchanged for a fresh sample at the following time intervals: 0,0.5,1,2,3,4,5,6,7,8, and 20 hours. The TEWL flux was determined by calculating the cumulative counts at the particular time interval. A plot of the rate of TEWL-vs-time is shown in Figure 2. It was observed that at 0.5 hour and one hour, the rate increased rapidly from 340 Ixg/cm2/hr to 460 Ixg/cm2/hr, and then the TEWL decreased after two hours. Equilibrium was reached between the third and sixth hour. The apparatus operated in the presence of a dry atmosphere (less than 25 percent relative humidity), determined by a cobalt chloride paper technique 500 400 500 0 1 2 3 4 5 6 7 8 20 Time (hours) Figure 2. TEWL rate vs time. Bars refer to standard error of measurement.