g49. JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS response to the external stress consists primarily of local adjustments since the chain backbone configuration is essentially immobilized. Technique A model TM Instron Tensile Tester©* was used for measuring both elastic toodull and relaxation spectra. The rate of travel of the cross head was 2.54 mm/min, which was equivalent to an extension of about 10-15%/min. (The sample length varied between 17 and 25 mm. ) Strips of stratum corneum, 0.5 cm wide by approximately 3.5 cm long, were attached to Bakelite©t tabs with a commercial epoxy cement or Duco©* cement. The length of each strip (between tabs) xvas measured to the nearest 0.01 mm with a cathetometer.õ The samples were stored at the desired rela- tive humidity (RH) for at least 24 hours and then suspended in a cvlinder (diameter, 7 cm length, 15 cm) mounted on the Instron. Air at the same RH was passed through the chamber for 20 rain at which time the samples were extended to a load of 4.0 g. Some obvious minor modifications of the above procedure were required for those stratum corneum strips which were extended under deionized water or test solutions. Water Absorption A Cahn RG Electrobalance©// was used to measure moisture absorption by stratum corneum or of cosmetic materials applied to samples of stratum corneum or to glass filter paper. The Electrobalance was fitted with an X-Y recordertl which has an input range of 0.1 mg/2.54 cm, while the graph can be read to 0.002 mg. Equilibrium weights can be estimated to 0.001 mg using a null system. The balance was also fitted with a cube-shaped plastic chamber (7.5 cm on each side) into which air of controlled humidity was blown at a rate of 1200 ml/min. The determination of equilibrium water content was made immediately after stopping the air flow so that air currents would not affect the reading. Equilibration occurs quite rapidly due to the moving air stream (absence of an unstirred layer and the small size of the sample). At- tainment of equilibrium •vas evidenced by constancy of •veight for 4 to 5 hours. *Instron Corp., Canton, Mass. pUnion Carbide Corp., New York, N.Y. E. I. du Pont de Nemours & Co., Wilmington, Del. õEa]ing Corp., Cambridge, Mass. //Cahn Instrument Co., Paramount, Calif. •TModel 7035 B xvith internal time base generator, Hexvlett-Packard Co., Palo Alto, Calif.

WATER LOSS OF STRATUM CORNEUM 243 Figure 2. Construction of water vapor transmission cell. Top row, left to right: body of cell, lower silicone gasket, stratum corneum. Bottom row, left to right: upper silicone gasket, top plate, closure nuts) Water Vapor Transmission Cells for measuring the rate of diffusion of water vapor through stratum corneum were constructed of cylindrical aluminum stock (3.7 cm in diameter x 2.2 cm high) into which was drilled a 1.5-cm deep hole having a diameter of 1.3 cm (Fig. 2). This depression has a capacity of approximately 2 ml. An aluminum top provided with a 1.3-cm hole could be attached to the lower chamber with four bolts. The stratum corneum was placed between two sili- cone rubber gaskets which were then placed between the chamber and the top. The bolts were attached "finger tight" in order not to deform the silicone gasket. After mounting the stratum corneum on a cell containing 0.3 ml of wa- ter, the cell was placed into a chamber containing a constant RH solution or exposed to a stream of humidified air. The cell was weighed every 24 hours until the rate of water loss became constant. Initial rates of transep'idermal water loss are fairly high, and up to 4 days may be required to reach a steady rate. Only those rates were utilized in this study which remained constant for 2 to 3 days after the initial equilibration. Whenever long periods of time were required for testing at several humidi- ties, the sample was examined for signs of mold growth or stretching of the epidermis due to the "vacuum" formed inside the cell as water leaves the cell. The loss of water was computed as mg/cm2/hr.