184 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The percentage of reduction in TFML is then given by equation (2). Percentage of reduction in TFML = [(TFML-B) -- (TFML-S)] x 100 (TFML-B) Where (TFML-B) the trans film moisture loss of the polymeric film (TFML-S) the trans film moisture loss of the product and polymeric film (2) 2.5 2.0 1.5 1.0 27 r=0.79 t • i | 28 29 30 31 TEMPERATURE øC A, 0% RH 2.5 ß ß 2.0 * 1.5 1.0 27 r=0.75 I I I I 28 29 30 31 TEMPERATURE øC B, 20% RH Figure 3. The effect of temperature on the Trans Film Moisture Loss rate A. at 0% relative humidity and B. at 20% relative humidity. r = correlation coefficient.

TRANSEPIDERMAL MOISTURE LOSS 185 The percentage of reduction in TFML can be determined at a variety of temperatures and relative humidities. RESULTS TFML CHARACTERISTICS OF THE SYNTHETIC SUBSTRATE Figure 2 shows the TFML rates for three samples of our polymeric substrate taken from beginning, middle and end of the roll. Two important factors should be noted here: 1) The shape of the humidity response curve of TFML is similar to that shown for human skin by Grice, et al. (7). This shape has also been observed by us in experiments with our in vivo apparatus. It is our belief that the shape of the curve is generated by water plasticization of the permeability constant (rising portion of the curve) and the effect of decreasing gradient with increased relative humidity (descending portion of the curve). The relatively large differences in the rates of the three samples from different portions of the film are most likely due to minor variations in film thickness and density 2) We have chosen a 2.5 ß ß 2.0 ß ß • '1.5 '• 1.0 • 0.5 0 0 10 20 30 40 50 % RELATIVE HUMIDITY Figure 4. Trans Film Moisture Loss rates (TFML) in mg/cm2/hr for a blank polymeric substrate (o), a Cream A coated polymeric substrate (,), and a USP Petrolatum coated polymeric substrate •!). Results are shown as a function of relative humidity.