SKIN CONDITION MEASURED BY SONIC VELOCITY 17 treatment and the final visual grade are not significantly different in their ability to predict consumer reaction. Together they predict 20% of the variance in the data concerning the panelists' product impression. Finally, further investigations should be undertaken to relate sonic and visual measures with consumer data, as it is hypothesized that the large unexplained variance in the panelists' responses could be reduced if random noise was suppressed by designing better questions and giving the panelists standards against which to mark the scales. CONCLUSIONS Changes in skin condition resulting from treatment with skin care products are measurable by sonic velocity changes and also by visual grading methods. The changes induced by products result in a decrease of approximately 15% of the total measured sonic velocity. These changes can be explained as resulting from product-induced changes in the elastic modulus of the outermost layers of the stratum corneum. The Dynamic Modulus Tester method provides reproducible and sensitive measurements, often showing statistical significance with relatively small differences. It has demon- strated an ability to rank products earlier than visual grading methods. The sonic velocity/elasticity method may easily be applied to skin condition evaluation in a clinical setting and is implemented simply from inexpensive, readily available instrumentation. In clinical tests, both chronic and acute product exposures result in measured velocity changes. Also, the dynamic development of skin elasticity after acute exposure may be monitored. Further, objective measures of elasticity are obtained which are intercomparable between studies conducted with different princi- pal investigators. It must be recognized in this regard, however, that age and body site both influence the absolute measured values, although calculated elasticity changes are independent of age. The value of sonic velocity measures in clinical testing rests on the observations that effects among products are noted earlier than with visual grading, and the correlation between visual grades and sonic velocity-derived elasticities are low and thus the measures describe different skin condition attributes. Moreover, the origin of the sonic velocity changes may be attributed to a physically meaningful property of the upper layers of the stratum corneum. Elasticity changes induced by products cannot be observed visually, although that is not to say the consumer does not experience this change through other sensory mechanisms. However, these changes in elasticity, for the products tested, foreshadow improvements in skin appearance (manifested as a decrease in visual dry skin pathology) over a much longer time scale. In fact, they may signal changes in the biological processes within the tissues which, over time, lead to elimination of the perception of dry skin. The power of the sonic velocity technique is that it quickly and simply measures a new dimension of skin not amenable to visual investigation, yet appears to be predictive of the eventual visual condition and consumer perception. REFERENCES (1) H. Tronnier, Der hydratationszustand der haut,J. Soc. Cosmet., Chem., 32, 175-192 (1981). (2) D.C. Salter, The form of the alternating current electrical properties of human skin measured in vivo, preprint.

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