SKIN CONDITION MEASURED BY SONIC VELOCITY 7 elastic outermost skin layers. Resting the transducer probes directly on the exposed under layers after stripping results in a decreased observed velocity, whereas shorter pulse transit times are recorded if the probes are on the outer layers. It is highly unlikely that removal of the stratum corneum would immediately lead to a decrease in the propagation velocity of sound through the lower skin layers (i.e., the transmission velocity through these layers would remain the same whether the stratum corneum was present or not). Therefore, the fastest propagating wavefronts, those which reach the receiving transducer first after an input pulse and trigger the timing circuit, must travel longitudinally (the geometry of the transducers on the skin insures that only longitudinal waves are generated and detected) for at least part of their path through the skin structure in which propagation is the fastest, the very outer layer of the stratum corneum. Because the stripping experiments indicate that this layer is extremely thin (being removed in five tape strippings), a one-dimensional, longitudinalphysical model for the sonic propagation process in the skin will be assumed. Applying this simple model, Young's modulus of elasticity, E (slope of the stress-strain curve, definitionally the stress required to produce unit strain), is directly related to the squared sonic transmission velocity, c2: E = d c 2 ß 10 3 where E is in newtons/meter 2, d is the density of skin in g/ml, and c is in m/sec (31). An experimentally determined stress-strain curve for skin from the data of Grahame (18) is set out in Figure 4. Referencing this figure provides a qualitative interpretation of 5 To • Stress (Newton/m X 10 2) 2 1 0 2 4 6 8 Strain (m X 10 -•) Figure 4. Experimental stress--strain curve for in vivo skin from the data of Grahame (ref. 18). sonic transmission. Before sonic velocity measurements, the skin is extended by application of a force, To, into the linear portion of the stress-strain curve. A time-dependent compression and rarification of the tissues involved in longitudinal wave propagation may be represented by the alteration AV, or volume change along the abscissa as pressure alterations AT are applied by the transmitting crystal. As long as one remains in the linear elastic region of the curve, the tension of the skin may be

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