8 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS altered (To changed) and the modulus, E, relating AT and AV will not change as the slope of the curve remains constant. The observation that further extension of the skin does not alter the recorded velocity is consistent with this model as the modulus does not change with a slight increase in stress. Viscous damping forces (i.e., forces in the transmission medium resisting the force perturbation, AT, acting over the distance of transmission), both within the outer elastic skin structure and arising from the viscous tissue upon which this structure resides, will reduce the magnitude of AT at the receiving transducer. Consequently, the pressure wave amplitude will be attenuated with distance. This behavior corresponds to the observations of this study. Separation of the probes over longer distances results in increased attenuation of the signal but does not affect the velocity, as the elastic modulus is not a function of separation. Further, the viscous damping forces in the underlying skin tissues are not isotropic but are directed perpendicular to Langer's lines which align along the direction of initial extension of the skin. When sound is transmitted parallel to these lines, the attenuation is low. Experiments in the same volume of skin oriented perpendicular to these lines result in high attenuation. The velocity, however, remains constant in both cases. All of these observations support the conclusions that the elastic outer layer of the skin influences the velocity alterations observed in this work and that this parameter is separable from attenuation and is dependent on the elastic modulus of the outer layers of the stratum corneum. It has long been known that hydration softens the stratum corneum (32). It has also been observed that some skin care product components penetrate into the near-surface layers of the stratum corneum (33) and one researcher (21) reports a lubricating/ softening effect from this product penetration. The effect of this softening would be a lowered elastic modulus in this skin region as occurs with plastics to which skin has been likened (21). The elastic moduli for soft materials are low, while those for hard materials are high it takes less force to deform soft rubber a unit length than to deform steel the same length. It must be considered that, in fact, changes in the density, rather than elasticity, of the outermost skin layers are measured in this experiment. The density of the skin involved in the transmission process was assumed to remain constant (1.30 g/ml from ref. 34) when using the equation above relating sonic velocity to Young's modulus. It is not possible to deconvolute changes in elasticity and skin density in the upper cornified cell layers with sonic velocity data alone. The results above are consistent with either interpreta- tion, or with both effects (density and elasticity alterations) occurring simultaneously. However, the effect of hydration of the outer layers of the stratum corneum would be expected to result in a decrease in the density of this material just as the hydration of hair or wool (made up of materials similar to the cornified layer of skin) results in a decrease in their density. At 0% relative humidity the density of wool fiber is 1.304 g/ml while at 100% humidity it decreases to 1.268 g/ml, a 2.6% drop in density (35). Inspection of the equation above shows that squared velocity is directly proportional to the elastic modulus and inversely proportional to the density. Consequently, after product treatments which are hypothesized to hydrate and plasticize the outer skin layers, a hydration-induced decrease in density should increase the measured sonic velocity. Therefore, although a density change resulting from product treatment probably occurs, it most likely is in the wrong direction to explain the product-induced decreases in sonic velocity and one may conclude that by ignoring density alterations, the

SKIN CONDITION MEASURED BY SONIC VELOCITY 9 calculated elastic modulus changes are a lower limit of the actual decrease in outer layer skin elasticity the actual change in the modulus is greater than that calculated. Finally, lower limit elastic toodull obtained in this study may be compared to those determined by other workers using different techniques (18,19) as set out in Figure 5. In 12 10 8 Elasticity (Newton/m 2 X 10 ?) 6 4 2 0 _ ,._ .._ .._ .._ ,•,,• "' "' "' ' •.,.•,--, - 18-35 36-50 50 Age (Yrs.) Figure 5. Young's modulus of elasticity as a function of age and body site. A. Torso skin from Rollhauser (1950) B. Forearm skin from Grahame (1%9) C. Hand skin (this study). this figure, the effects of both age and body site are examined. The moduli calculated from this work for dorsal hand skin are consistent with the other investigators' results in that an age-related increase in elastic modulus (vide infra.) was observed as well as a body site effect. Comparing the three sets of observations, a lower elastic modulus was found in the more flexible skin areas (i.e., torso skin modulus forearm skin modu- lus hand skin modulus). Direct intercomparison of skin elasticity measured by different techniques must be viewed with caution, however. SONIC VELOCITY MEASUREMENTS IN THE CLINICAL ENVIRONMENT Having established the technique for making sonic velocity measurements and the probable physical basis for the changes in velocity, two examples of the use of the method in skin clinical studies will be detailed. These studies are examined, not to document the performance of competitive products (which will not be named), but rather to demonstrate the ability of sonic velocity measures to discriminate between treatments establish the sensitivity of the technique examine the effects of the clinical regimen and base size on sonic velocity results and explore the relationship between observed sonic velocity trends, the dermatologist's evaluations, and consumer percep- tion of treatments/products. A small base, acute product treatment forearm study and a large base, ad. lib. usage hand study will be considered. The data from these investigations will establish that sonic velocity maps differences between product treatments at a statistically significant level before visual differences among the treatments occur. It will also be shown that the early detection of elasticity changes calculated utilizing this technique are consistent with the final, longer term visual effects of products and the consumer