258 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS are from the same stratum corneum specimen. A comparison within the vari- ous columns may lead to erroneous .conclusions. A decrease in the elastic modulus below that of the control (recorded as +) is indicative of an in- crease in the elasticity of the stratum corneum. In the case of the stress relaxa- tion modulus, an increase in the value over that of the control (recorded as +) describes the skin's ability to relieve a strain by viscoelastic response. The wide variations in the values for the controls are due to the fact that dif- ferent specimens of epidermis were used, but this in no way reduces the value of the data. Each value recorded in Table I represents an average of 4 to 6 determinations on the same specimen of human epidermis. Treatment with either acid or base increases the elasticity of the stratum corneum substantial- ly but does not appreciably alter its ability to relieve stress. Generally, the elastic and the stress relaxation moduli results after treatment parallel each other. Particularly noticeable is the remarkable "plasticizing" effect of dilute solutions of sodium pyrrolidone carboxylate and of sodium lactate. It is be- lieved that the influence of cosmetic ingredients on the mechanical perform- ance of stratum corneum correlates directly with their beneficial effect in skin preparations. Moisture Absorption The equilibrium moisture absorption of several important cosmetic mois- turizers on glass filter cloth was studied after equilibration at several relative humidities. The data in Table II show that sodium pyrrolidone carboxylate and sodium lactate are capable of holding relatively large quantities of water even at intermediate relative humidities, i.e. between 50 and 70% RH. It proved impossible to conduct similar experimentation with propylene glycol because of its unexpectedly high volatility. Data for stratum corneum and some hydrophobic materials are included merely as a matter of orientation. In view of the large effect of glycerin on the mechanical behavior of stratum corneum as a function of temperature (Fig. 2), it was considered conceivable that there is a specific interaction between glycerin, stratum corneum, and water which may increase the water-holding properties of stratum corneum. Similarly, the high hygroscopicity of sodium pyrrolidone carboxylate makes it possible that it too might impart some special water-holding properties to stratum corneum. In order to determine whether any such interactions occur, the isothermal absorptions of water by combinations of known weights of unextracted stratum corneum and glycerol or sodium pyrrolidone carboxylate were determined. The results shown in Table III clearly show that water absorption by stratum corneum is merely additive to that by the humectant. In other words, the moisture absorption by stratum corneum treated with a humectant is the sum of the absorptions of the individual components, with no evidence for any synergistic effects. These results are in agreement with the data reported by Fox et al. (9).

EFFECTS OF COSMETIC INGBEDIENTS 9.59 Table II Equilibrium Moisture Absorption Material Water Absorbed at 31% RH 52% RH 76% RH 90% RH Stratum corneton Glycerol Sodium lactate Sodium pyrrolidone carboxylate Mineral oil Safflower oil 7 9 11 26 19 40 17 45 0.2 0.4 0.2 0.5 13 22 67 240 104 ... 210 ... 0.9 ... 1.0 ... Table III Weight Gain (Water Content) of Stratum Corneum and Stratmn Corneum Plus Hnmeetant Sample Amount of Water (rag) after Equilibra•ion at Initial Weight a (mg) 35% RH 75% RH Stratum corneum Glycerol ø Stratum corneum q- glycerol Experimental Calculated Stratum corneum Sodium pyrrolidone carboxylate c Stratum corneum q- sodium pyrrolidone carboxylate Exp•Hmental Calculated 3.816 3.058 6.874 . . . 3.784 3.468 7.252 0.226 0.530 0.339 2.058 0.b'•72 2.575 0.15615 2.588 0.183 0.515 0.720 3.488 0.892 3.892 0.903 4,003 •At equilibrium with air at 0% RH. •U.S.P. CApp.•ied from a 50% solution. Water Vapor Transmission The loss of moisture from the skin to the atmosphere is a continuous pro- cess, and excessive loss at low humidity and low temperatures is generally associated with skin dryness and chapping. It seemed particularly important to determine how cosmetic ingredients applied to stratum corneum might al- ter this tissue's ability to "transpire" water to an essentially dry atmosphere. Some typical in vitro water vapor transmission rates through stratum corneum are summarized in Table IV. This table also includes some common cosmetic humectants, some occlusive lipid materials, and Lotion #78. This last prep- aration, which is Formula 78 described by Barnett (10), was employed here because it wa• also studied in vivo by Berube et al. (11).