24 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS sulfate (SLS) (See Table I). Note that treatment effects in this experiment are highly significant, and there are no direction-treatment interactions. Thus, the membranes do provide the same response in both dimensions, consistent with the fact that the fibrous proteins of stratum corneum cells are randomly arranged in planes parallel with the plane of the membrane (4, 5). We chose to measure swelling only in the CW length direction for subsequent experiments however, area estimates can be made by squaring the CW length measurement. Weights up to 0.5 grams were added to membranes with no change in the CW length response. Under extensive swelling, e.g., with sodium dodecyl sulfate or formic acid, membranes twisted during treatment, and the amount of twist seemed to increase with increasing CW length swelling. Therefore, the true swelling lengths are slightly larger than the indicated values nevertheless, this is still a highly reproducible measurement. WATER VAPOR The CW length of human EM was measured at different relative humidities (RH) (Figure 3). From a drierite atmosphere (assumed to be zero percent RH) to a water 5.6 5.5 5.2 5,1 5,0 - ß ABSORPTION - AcHEpSo RTP•iO•N AVERAGE OF I0 REPLICAS 4.9 I I I I I I I I I I 0 I0 20 30 40 50 60 70 80 90 I00 % RH Figure 3. CW length of epidermal membrane vs. RH. atmosphere, the membrane length increased by about 11%. Squaring this value approximates the maximum area change in isolated EM by humidity changes. This corresponds to a 23% increase and is larger than anticipated (9). Three dimensional swelling of human EM and human hair are compared in Figures 4, 5, and 6. Data for human hair by Stam et al. (10) are used in these plots. These data show that human hair, in any dimension, is not as responsive to changes in RH as the CW

SWELLING OF EPIDERMAL MEMBRANE 25 z 14_ 1.12 1.10 1.08 1.06 1.04 1.02 1.00 ß EPIDERMAL MEMBRANE (CW LENGTH) T - EACH POINT IS AN AVERAGE OF I0 REPLICAS ] _ o HAIR LENGTH / I I I I I I I I I I I0 20 30 40 50 60 70 80 gO I00 % RH Figure 4. Hair length and EM-CW length vs. RH. 2.00 1.75 1.50 1.25 1.00 ß EPIDERMAL MEMBRANE (THICKNESS) - EACH POINT IS AN AVERAGE OF I0 REPLICAS / • HAIR DIAMETER 0 I0 20 30 40 50 60 70 80 90 I00 %R.H. Figure 5. EM thickness and hair diameter.