610 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ether and then water causes about a 30 per cent dry weight loss and nearly a two-fold decrease in stiffness as measured by the work index. A contacting probe balance, developed by E. M. Buras at our laboratory,* has been employed for measuring the cross-sectional swelling of specimens of stratum corneum (8). This instrument permits the rapid and accurate record- ing of the displacement of a probe placed in contact with the dry surface of dry cornified tissues. After measuring the dry state thickness, each sample is submerged in 0.1 per cent aqueous Triton X-100,* and the displacement due to swelling is continuously monitored. The final thickness is determin,ed after equilibrium is reached, usually within 10 min. The percentage swelling is calculated by comparing the initial displacement with that following imbibi- tion. Changes in the remaining two dixnensions (termed in-plane swelling) are measured directly for square samples (20 x 20 mm) before and after immer- sion in 0.1-per cent aqueous Triton X-100. After 16 h, all squares are removed, and their perimeters remeasured to the nearest 0.1 mm to determine the per- centage change. To test the effects of delipidization, squares have been pre- extracted with diethyl ether for 1.5 h, air dried, incubated as above for 16 h, and then remeasured. The pronounced weakening of the ether-pretreated specimens as reported in the Instron study correlates well with distinct increases in both cross-sec- tional and in-plane swelling. Ether-water extraction and concomitant loss of NMF causes about a 3-fold increase in thickness when subsequently re- swollen, but only a 5 per cent enhancement in area (8), as compared with xvater-soaked stratum corneum which is not preextracted. Ether pretreatment, therefore, alters not just the lipid content and moisture retaining capacity through loss of NMF, but the physical dimensions, strength, elasticity, and •nembrane permeability as well. The high swelling and reduction in the rate of strain recovery (i.e., decrease in the viscous component of elasticity) of the ether-treated samples may be explained by marked alteration in the confor- mation of keratin •nolecules, which is brought about by the breakdown of hydrogen bonds and accompanying aqueous exposure of previously buried *T,he probe balance consists of a freely moving aluminum arm suspended on aluminum- coated Mylar flexures. T'he balance has a 2 mm 2 probe at one end, which contacts the corneum specimen placed on a fiat surface. At the other end of the balance armø there is a position transducer which consists of a vane and a proximity probe. Displacement is measured by change in capacitance which varies with the length of a cylindrical probe inserted into the vane. The proximity probe of this dynamic balance is wired into a commercial driver unit, and then into the Y-axis of the recorder. An aluminum cylin- der coil fastened to the balance arm above a magnet constitutes a damping system. The instrument has also been used to measure diametrical swelling of hairs. ?Rohm and Haas Co., Philadelphia, PA 19105.

ASSESSMENT OF SKIN MOISTURIZATION 611 hydrophilic and hydrophobic groups within the keratin of the delipidized corneum cells. Instead of the work index, Rieger and Deem (13, 14) have analyzed the elastic modulus (i.e., the ratio of stress imposed on stratum corneum to the strain applied at a constant strain rate) and the relaxation function (i.e., the decay in the strain rate of the tissue while a constant stress is imposed). Both the elastic modulus and the ti•ne constant of relaxation of unmodified stratum corneum decreases with increasing RH, providing an objective characteriza- tion of pliability (13). Upon application of known humectants such as 4 per cent sodium pyrro]idone carboxylate and 50 per cent glycerol in water, there is a decided increase in elasticity and a faster relaxation time in comparison with dried tissue (14). Light mineral and safflower oils have the opposite ef- fect, suggesting an increase in the stiffness and a decrease in the pliability of the treated samples. In Vivo METI-IODOLOGIES A. Transpirometry Some of the current techniques, which we utilize in vivo, have aided us greatly in directly evaluating cutaneous moisturizers. Several types of instru- mentation a. re described in the literature for application in the direct determi- nation of transpiration rates (15-21). We employ an apparatus designed by Slegers and Dobson for measuring the rate of moisture release from the skin into a stream of dry nitrogen (Fig. 1 ) .* This stream, passing in a flow-through chamber on the skin, and a stream of identical pressure flowing independently of the skin are compared for their thermal conductivity in a gas chromato- graph. Two of these systems, each equipped with integrators, allow for simul- taneous measurement of the rate of moisture loss at two separate sites (i.e., a control and a test). We have observed all three sources of water (i.e., surface moisture, trans- pired water, and eccrine sweating) which Berube et al. have mentioned (15). After an equilibration and "calm-down" period of 30 min, surface moisture is eliminated and most panelists become sufficiently conditioned to a room tem- *The transpirometer consists of two thermal conductivity gas chromatographs. For each unit, streams of dry nitrogen at 2,00 ml/min/cm"are split into two equal components, one passing directly into the chromatography unit, while the other streams into a flow- through probe on the skin before entering this thermal conductivity analyzer. The dif- ference in the conduc{ance between the split streams is measured, and a signal from each chromatograph is sent to a dual pen recorder. The latter is equipped with two re- peating potentiometers allowing for integration of each signal. Standard curves are ob- tained for each system before use each day by application of known quantities of wa- ter (0.1 to 1.0 t•l, for the 0.05 mV sensitivity range) to filter paper sealed within each chamber.