344 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table II Variation in E' with Vertical Position Along the Lower Leg, Outer Aspect of the Calf (Mean Values for Five Panelists Selected for Dry Skin) Distance below Glycerol-treated Sites Water-treated Sites knee joint Initial Final Initial Final 10 cm 207 -- 71 142 + 49 149 --- 55 178 + 72 15 cm 353 ----- 119 252 ----- 55 304 _+ 99 319 --- 93 20cm 451 --- 46 313 -- 94 415 -- 166 448 + 184 amplitude and driving frequency in a complex manner, and the analysis procedure derived above is no longer valid. A theory describing the nonlinear behavior of stratum corneum under uniaxial tension in vitro has been presented recently (26). This approach would not be tractable for dynamic viscoelastic measurements such as ours. Rather, it is appropriate to apply harmonic analysis to the displacement and to apply equations 700 600 -- 500 3OO 2OO IO0 I I I I I • I 1 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 SKIN GRADE -" Better Skin Figure 7. Elastic modulus of skin (lower leg) vs. skin grade.

MECHANICAL PROPERTIES OF SKIN 345 Grams 10 Grade 3.75 Grade 1 : MM 2 -10 Figure 8. Stress-strain loops for soft and stiff skin (lower leg). (1-9) above to the fundamental component (23). The presence of higher odd harmonics, when not accounted for in this way, will result in large errors in measurement of the loss modulus and probably much smaller errors in the storage modulus. It was not necessary to use harmonic analysis for the range of displacements encountered in the studies presented here. We believe that nonlinear viscoelastic response may indicate a stronger dermal contribution to the measurement than is present during linear visco- elastic response (4, 5). The elastic modulus is linear with respect to the diameter of the tape disk as is shown in Figure 6 for two different subjects. This seems odd at first, since the elastic modulus is defined in such a way as to be independent of sample dimensions. This functional dependence of E' on disk diameter is a consequence of the fact that we have no control over the dimensions of the skin that are involved in the movement of the probe. In fact, the E' we measure is not equivalent with but only analogous to an elastic modulus, since the area cannot be defined in vivo for a formal definition of the applied stress. Therefore we report E' in units of force (gm) per unit displacement (cm). The linear dependence seems intuitively to be due to the fact that only the stratum corneum around the circumference of the disk is being stretched along the projection of motion. The stratum corneum attached to the tape is not being stretched and does not contribute to the measurement. The non-zero intercept reflects to some degree the contribution from the surrounding skin or underlying tissue. The mechanical properties are not very sensitive to the angle between the probe and the leg, but abrupt changes in the stress-strain loop can be observed and easily identified