WATER AND THE HORNY LAYER Sample b Table II Effect of Heat on SC Hydration Hydration (mg H20/mg SC) • Control After 10 min. at 60øC 67 Trypsin -- 10 0.57 0.62 -- 11 O.63 0.57 Cantharidin -- 10 0.68 0.71 -- 11 0.88 0.75 Sunburn exfoliate - 10 1.03 1.14 - 11 1.12 1.16 •'Hydration from dry weight to hydrated weight at 97 per cent BH and 30øC for 48 hours. "Samples harvested by the techniques indicated numbers 10 and 11 designate 2 different 8 mm punches from the same specimen. Table III Effect of Aqueous Exposure with Different Harvest Methods Dry Weight following Aqueous Exposure (hour) ' Harvest Method Control: 0 1 hour 3 hours 10 hours 24 hours Trypsin 0.522 0.518 0.537 0.507 0.497 Cantharidin 0.469 0.457 0.480 0.457 0.463 Sunburn exfoliate 0.362 0.349 0.312 0.259 0.337 Hydration following Aqueous Exposure (hour) b Trypsin 0.91 0.92 0.85 0.72 0.51 Cantharidin 0.90 0.86 0.89 0.76 0.85 Sunburn exfoliate 1.22 1.07 0.97 0.63 0.67 •Dry weight (in milligrams) of 1 of 5, 8 mm punches from the same specimen, following water exposure by immersion in distilled water for the time indicated. •'Hydration at 97 per cent RH, 30øC for 48 hours (rag H20/mg SC). Alternately, hydration of the SC has been treated as a pseudo-first order process, since H,20 is present in excess (8,9). The vater veight gain is ex- pressed dW/dt = K• u (Wm•.,- W) (2) Rearranging eq. 2 gives dW = r• • dt (3) (Wmax -- •V)

68 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.00 (Wmax-W) 0.10 (rag H20/mg S.C.) 0.01 i o ,oo o[o 6OO Figure 2. First-order hydration kinetics of SLC. Plot Of In (Wmax -- W) vs. t with slope in equals K• • for trypsin-harvested 8 mm punch of SC (lateral thigh of caucasian male, age 45 years). Wma• equals 0.90 mg H20/mg SC after 22'80 rain at 97 per cent RH and 30 ø C Integration from initial hydkation at t equals 0 to time t when Wmax water has been absorbed yields where Ktn equals the first-order rate constant for hydration. A typical plot of the hydration process using the variable in (Wmax- W) versus time is illustrated in Fig. 2. The first nonlinear portion of the curve has been ex- plained by Anderson et al (8) as the initial hydration of tightly bound water however, this nonlinearity leads to an alternate treatment of hydration kinetics. If the hydration occurs from a constant surface concentration determined by the constant RH, and if the rate of reaction is limited by both diffusion into the membrane and by reaction of available hydration sites, then the rate constant of the rate expression (2) will be a function of the diffusion constant of water into the membrane, and consequently, a function of the dif-