116 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ß Untreated o Mineral Oil A Sesame Oil [] Glycerin 500 400 $oo 200 - 100 - 0 i 2 5 4 5 6 7 24 Time (hours) Figure 5. TEWL rates vs time values for untreated skin and skin treated with mineral oil, sesame oil, and 25% glycerin in water. Bars refer to standard error of measurement. A two-week conditioning study was carried out with mineral oil, the most effective of the occlusive agents tested. This test provided us with some insight into the mechanism of mineral oil as a moisturizer. Results of this study are reported in Table II. No significant differences were found between treated and untreated ear skin samples. It was concluded that for mineral oil to act to decrease TEWL rate in this procedure, it must be able to bind to the skin. The protective effects of mineral oil were experimen- tally removed during sample preparation. Thus we conclude that mineral oil acts to decrease TEWL by imparting a physical barrier to the transport of water from the membrane. Conditioning the skin with mineral oil appeared to be an ineffective treat- ment modality. CONCLUSIONS Previously developed in vitro techniques for measuring TEWL are tedious and time- consuming. Most of these in vitro methods employ a gravimetric technique for measure- ment. Steady state TEWL rates cannot be achieved in less than 24 hours. Blank's in vitro method (30) of using tritium-labeled water to measure water loss was combined

METHOD FOR TRANSEPIDERMAL WATER LOSS 117 ß Untreated •- t v Castor Oil E-= 500 ..... .a 4oo- •._. 300 •- o 200 o._j 100 - o 1 2 3 4 5 6 7 24 Time (hours) Figure 6. TEWL rates vs time values for untreated skin and skin treated with castor oil. Bars refer to standard error of measurement. with adsorption of water onto a desiccant. The new method proved to be very efficient. TEWL equilibrium could be achieved within three hours and a full study can be com- pleted within eight hours. Cartilage-stripped hamster ear skin was successfully used as an in vitro membrane for TEWL measurement. The barrier properties of the hamster ear skin compared favorably to those of human skin. TEWL rate values agreed with values previously reported for human skin obtained under similar environmental conditions (1,2). The membrane exhibited an exponential increase in TEWL with increased temperature. A calculated energy of activation for water permeation of 13 Kcal/mole was in general agreement with the value reported previously by Scheuplein for human skin (9). Table II Transepidermal Water Loss Values for Mineral Oil Conditioning Ear After Various Time Periods Time Periods: 1 hr 3 hr 6 hr 24 hr Treated Ear Mean TEWL I-tg/cm2/hr Mean TEWL }zg/cm2/hr 391 269 268 2O6 Untreated Ear 437 315 226 2O2 p 0.3 p 0.3 p 0.3 p 0.3