26 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 2.00 1.75 1.50 1.25 1.00 ß EPIDERMAL MEMBRANE (THICKNESS) o EPIDERMAL MEMBRANE (CW LENGTH) EACH POINT IS AN AVERAGE OF I0 REPLICAS HAIR DIAMETER FROM STAM et. al. 0 I0 20 30 40 50 60 70 80 90 I00 % RH Figure 6. Swelling of hair and epidermal membrane. length of human EM, and furthermore the differences in these two substrates are greatest at the highest relative humidities (11). Several authors have demonstrated the effects of RH or corneum moisture content on its elasticity (12, 13, 14). Wolfram (15) has compared the effects of RH on the extensibility of corneum and hair, and Middleton (16) has demonstrated the effects of temperature on corneum extensibility. In addition, skin dimensional changes com- pound this situation because the innermost layers of the corneum are exposed to approximately 86% RH (17) and the outermost layers to the external environment. Thus, when the skin is subjected to a change from high to low RH, the horny layer tends to shrink (See Table II), but this shrinkage is not uniform throughout its depth, therefore shearing stresses are produced. So, under conditions of temperature and humidity extremes, the horny layer develops a Table II Changes in Skin Dimensions by RH from 86% ARH % Area Change* % Thickness Change 86 to 20 --6.9 -- 16.4 86 to 30 --6.1 -- 15.8 86 to 40 -- 5.5 -- 15.6 86 to 50 --4.7 -- 14.8 86 to 60 --3.8 -- 12.6 86 to 70 --2.8 --9.5 *Obtained by Squaring % A CWLength

SWELLING OF EPIDERMAL MEMBRANE 27 dimensional or shrinkage-swelling gradient as well as an elasticity gradient as it is flexed, the resultant shearing stresses can produce cracking of the skin. LIQUID WATER EM swells more than an order of magnitude greater in volume than human hair, after 24 hours exposure to liquid water. And while hair length and diameter equilibrate rapidly in water (18), as does the CW length of EM (Figure 7), EM thickness does not 1.14 1.12 1.08 1.06 1.04 1.02 EACH POINT IS AN AVERAGE OF I0 REPLICAS 1.00 0 I0 20 30 40 50 60 70 TIME (HOURS} Figure 7, Rate of CW length swelling in H20. level at 24 hours (Figure 8), but continues to expand through several days. A similar result was obtained by Scheuplein and Ross (19) on stratum corneum using an infrared cell spacer technique to measure thickness. The orientation of the helical proteins in both hair and stratum corneum suggests that the diameter of hair is analogous to the thickness of EM, while the length of hair and the CW length of EM are analogous. The data of Table III confirm this conclusion and show further that EM is more responsive than hair to the swelling-action of water. To help explain the differences in swelling characteristics between these two keratins we turn to the extended matrix model of Menefee (1). Fraser (20) has shown that most of the volume swelling that takes place on hydration of keratins occurs in the matrix. In addition, matrix proteins have a higher disulfide bond content than helical proteins of the microfibrils (21) the epidermal membrane, and therefore its matrix, has a much lower disulfide content than human hair (Table IV). Also, larger amounts of high sulfur proteins exist in hair vs. stratum corneum (22). Thus, the disulfide bonds of hair must inhibit its swelling. Furthermore, the matrix of stratum corneum, unlike human hair, contains nonprotein structural matter, some of which is water soluble (23). Since extended soaking of EM in water (through 5 days) produces changes in its thickness