METHOD FOR TRANSEPIDERMAL WATER LOSS 113 adapted from Solomon (36). Since the apparatus operates in the presence of a dry atmo- sphere, this transit time period results in an initial evaporation of water from the mem- brane. Initially, the membrane is fully hydrated, swollen, and is more permeable. This provides an "increased push" effect to pass water through the membrane towards the anhydrous desiccant. This transit time period is designated the "burst" effect. At steady state, the amount of water adsorbed onto the desiccant is in equilibrium with the amount above the membrane. Also, the membrane is approaching a drier state and is less permeable. Equilibrium TEWL rate after six hours was approximately 300 Ixg/cm2/hr at 32øC. Table I summarizes previously reported TEWL values based upon the various in vitro techniques currently employed for measurement. Blank (1,2) utilized full-thickness skin membrane from the human abdomen in his studies and reported TEWL values of between 130 and 270 Ixg/cm2/hr (23øC, 23% RH). Our value of 152 Ixg/cm2/hr (22øC, 20-25% RH) for the hamster skin is in excellent agreement with Blank's value for human skin. TEMPERATURE STUDY A plot of TEWL rate-vs-temperature is shown in Figure 3. An exponential increase in TEWL was observed, which implies that water vapor permeates by a diffusional pro- cess. The TEWL rate between 5 and 12øC, however, showed little change. Table I Summary of Transepidermal Water Loss (TEWL) Rate Data for Various In Vitro Skin Models Investi- gator TEWL Temper- Refer- Membrane rate ature Year ence used Method (•g/cm2/hr) øC Relative humidity in % Blank Mali Mali Reiger, Deem Wu Blank et al. This work 1952 (1,2) Human, full- Gravimetric 100-200 23 thickness skin 130- 270 23 from abdomen 400-800 35 1956 (8) Human, epider- Gravimetric, 500-600 21 mis from trunk desiccator 1956 (8) Human, epider- Gravimetric, 3000 21 mis from sole desiccator 1974 (30) Human, stratum Gravimetric 250- 300 21 corneum from abdomen 1983 (19) Fetal hog Saturated 251 21 periderm salt solutions 130 21 85 21 1984 (33) Human, stratum Saturated salt 600-1200 30 comeurn solutions and tritiated water 1987 Hamster ear, Tritiated water 250-300 32 full-thickness 152 22 skin 40-50 20-30 57 0 0 0-20 5O 4O 3O 0-80 20-25 20-25

114 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 500 400 lOO I 0 5 10 15 20 25 :50 :55 40 Temperature øC. Figure 3. TEWL rate vs temperature at steady-state conditions (after 3 hours). Bars refer to standard error of measurement. If the reciprocal temperatures in degrees Kelvin are plotted vs the natural logarithm of the TEWL rate in the temperature ranges between 12 and 37øC, linearity is observed (Figure 4). It is difficult to explain the lack of linearity at 5øC. The water permeation may have been disrupted at this near-freezing temperature. The energy of activation was calculated by slope analysis. Our calculated value of 13.1 Kcal/mole compares favorably with an activation energy value of 15 Kcal/mole previously reported by Scheuplein for human skin (9). MOISTURIZER STUDY Four experimental cells and two blank cells were prepared for each moisturizing agent tested. Ten •L of each agent was applied to the top of the membrane surface, prior to attachment of the receptor cell filled with desiccant and placement of the unit into the 32øC heating block. Plots of TEWL-vs-time values for membranes treated with various potential moisturizers are shown in Figures 5 and 6. The figures show individual TEWL values vs time over a 24-hour time period for the untreated sample and skin treated with agents. The untreated sample shows a result similar to that reported in Figure 2. Transepidermal water loss reaches steady-state conditions after three hours, except for that sample treated with 25% glycerin. When compared to the untreated sample, castor oil is the only agent that exhibited a "burst" effect. Skin treated with sesame oil, mineral oil, and 25% glycerin in water all appear to inhibit this effect. Sato and Nagai (37) compared the effects of different