WATER AND THE HORNY LAYER 69 [erenee between the water content at the surface and water within the mem- brane. Allowing for a suitable lag time for equilibrium to be established, the surface layer of the SC is in equilibrium with the water vapor in the atmos- phere. Therefore, k will be large initially and decrease as hydration ap- proachs an equilibrium SS. The surface layer, at equilibrium with water vapor in the atmosphere, will have a water content of Wmax (rag H20/mg surface layer). The interior water content, taken as an average, is represented by W. If K• n is not actually a constant but rather a function of the water con- tent, one might express the rate function K(W) in terms of the degree of hydration as K(W) = K' (Wm•x/W) (5) which is large initially, decreasing as the extent of hydration increases. Substituting eq. 1 in 5 gives K(W)---- K'• wl2 '• L l•J (6) Applying the resulting expression K(W) for K• • in eq. 3 dW [ •rF 71/2 (Wm•x W) =K' L l•j t-'/• dt - (7) Integrahng over the limits of 0 to Wm•x and 0 to t (s) w•ch is cons•ained to descdphon of initial so•tion data as is 1. A •pical plot of eq. 8 is line• over the initial 300 min. of hydration. It should be noted that the linear porhon o• the cu•e beans after t equals 1 rain, as a result of a lag •e •or formahon of equilibrium between water vapor and the suEace ol the sample. Discussion Comparison of hydration parameters in Table I indicates that average dry weights and water content at 97 per cent RH are similar for SC samples harvested by cantharidin blister or trypsin techniques. Sunburn exfoliated samples have lower dry weight, indicating a lower density than samples har- vested by the other methods. In addition, sunburn samples retain more water than either cantharidin or trypsin samples. Some harvest methods involve immersion in water at 60øC for separation of epidermis from dennis (7). In the current work, exposure to 60øC had no detectable effect on hydration of any samples at 97 per cent RH (Table II). However, SC samples were pro-

70 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tected from water exposure by plastic therefore, no statement can be made concerning the wash effect of warm water on water soluble fraction in the membrane. Excessive aqueous exposure of SC causes a reduction in dry weight and ability of the SC to retain water at 97 per cent RH (Table II! ). With 3 hours of exposure, the effect is small in both cantharidin and trypsin harvested samples however, sunburn exfoliated tissue loses weight and ability to absorb water with increasing water exposure. The trend in weight loss indicates that sunburn exfoliated membranes lose more water soluble components and that the ability of sunburn membrane to retain water is reduced by the loss of these components. Since sunburn exfoliated skin has been reported to have incompletely formed keratin, caution should be taken in comparing data from sunburn exfoliated SC with normal SC harvested by other methods. The hydration of SC has been described in terms of three types of water (8, 10, 11). The first type, primary bound water, is tightly bound water asso- ciated with strong protein hydrogen-bonded interactions. Primary bound wat- er constitutes the first 0.10 to 0.15 mg H20/mg SC absorbed by dry SC at 60 per cent RH (Fig. 1 ) and is necessary for normal pliability of SC (1). As the activity of water (RH) increases above 60 per cent, water sorption ap- pears to increase exponentially in relation to RH (Fig. 1). The water absorbed in this region is secondary water, bound to the primary water layer, constitut- ing 0.40 to 0.50 mg H20/mg SC (Fig. 1). At RH above 94 to 96 per cent, water content increases rapidly as free water, not directly bound to the mem- brane, is absorbed. In this region, SC will continue to absorb water xmtil the tissue begins to break down mechanically (7,8). Spectroscopic observations by Hansen and Yellin (12) indicate that continued water uptake in this re- gion results in an increase in primary water binding also hence, excessive free water opens up the molecular lattice exposing more primary hydration sites. Irreversible alteration of these primary binding sites or hydroscopic components affecting these sites may play a role in the effect of water expo- sure on the loss of mechanical integrity when the SC is hydrated (13,14). Since hydration can be described in terms of a single function (Fig.3), which holds over the region in which bound water is being absorbed, both primary and secondary water are defined by a single parameter, implying that they are interchangeable. Nuclear magnetic resonance (NMR) results dis- covered by Hansen and Yellin (12) also indicate that the interchange be- tween primary and secondary bound water is very rapid. Hence, if a signifi- cant amount of secondary water is present, one can assume that all the pri- mary bound sites are hydrated due to the rapid equilibrium As observed in a previous report (3), low temperatures on the order of 10 to 20øC reduce the ability of the SC to retain water at relative humidi- ties below 60 per cent. For example, a SC sample at 60 per cent RH retains