SKIN CONDITION MEASURED BY SONIC VELOCITY 5 1. EMPIRICAL OBSERVATIONS During the instrumental design and validation work, the following empirical observa- tions were noted. The propagation velocity was unchanged by: changing the direction of transmission through the skin changing the normal pressure of the transducers on the skin from 20 to 50 g increasing the transducer spacings (accounting for the lengthened transmission distance in the velocity calculation) or, further extension of the skin. The velocity did change if the initial slack in the skin was not removed or with alteration of the transducer contacts such that they could slip over the skin surface. Large changes in the attenuation of the sonic pulses were monitored by observing the minimum detection threshold level at which stable delay time readings could be obtained. These observations indicated that increased sonic attenuation resulted if the separation of the probes was lengthened or if the sonic pathway was perpendicular to the direction of initial skin extension. 2. SKIN STRIPPING EXPERIMENTS Figure 2 graphically displays the changes in recorded sonic velocity in the dorsal region of the hand on a single subject as a function of repeated tape stripping. After only 5 strips the measured velocity had decreased by approximately 15% to a level which remained relatively constant through the next 30 strippings. Even after 35 strippings, however, the glistening layer had not been reached, providing evidence that the entire stratum corneum had not been removed from that site. Removal of only the outermost surface layers led to the observed velocity reduction. When the same site was remeasured 24 hours after the stripping, a new, brittle outer layer of cellular material was evident upon visual inspection, and the measured sonic velocity returned to its initial value. The effect of removal of the very outer layers of the stratum corneum mimics product treatment as set out in Figure 3. In this study, two sites on the dorsal region of the hands of eight individuals were studied. One site (x) was treated with a fully formulated Hand and Body Lotion product containing glycerin and petrolatum after Sonic Velocity (m/sec.) 120- 100- 80 Stripped Regimen Figure 3. Sonic velocities as a function of skin treatment and skin stripping. O = control site (stripped but not treated). x = treated site. Data are the average of eight subjects and error bars represent o• .05 confidence limits. For regimen details see text.

6 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS an initial reading (pt. A on the abscissa). The control site (o) was left untreated. After two hours the velocity through the treated site had reduced significantly relative to the control. Immediately after this reading (B) both sites were tape stripped to remove the outer stratum corneum layers and the sonic velocity was measured again (C). The untreated control site velocity decreased to a level equivalent with the treated site, while on the treated site the velocity remained constant after stripping (the criterion of c• .05 was used to determine if statistically significant differences existed). In another experiment, application of glycerin/petrolatum products or pure water to freshly stripped skin was found not to produce a statistically significant alteration in the sonic propagation velocity compared to untreated stripped skin. These stripping studies are interpreted as indicating that the observed differences in sonic velocity which occur ajQer acute product treatment are the result of changes in the near surface layers of the stratum corneum induced by the treatment. Both product treatment and removal of the uppermost cell layers of the skin produce a sonic velocity decrease of the same magnitude (15%). Once these outer layers are removed, a constant lower velocity is observed which is not further decreased by continued cell removal (stripping) or by application of either products or water to the stripped site. 3. SONIC VELOCITY AND THE VISCOELASTIC PROPERTIES OF SKIN The physical properties of skin have been widely studied (18,19,21-29). While all of these workers agree that human skin is a viscoelastic material, an examination of this literature reveals different descriptions of its viscoelastic behavior. Some researchers (19,21,24) report that with continued stretching (increasing strain), skin goes through four distinct physical behavior patterns as follows: Phase 1: Removal of "slack" Phase 2: Purely elastic upon release, the skin immediately returns to its original form. Phase 3: Viscoelastic upon release, the skin slowly returns to its original form (hysteresis). Phase 4: Purely viscous the skin is permanently damaged and does not return to its original form (as manifested by the striae gravidarum resulting from pregnancy or obesity--ref. 30). The sonic velocity measurements in this work were taken while the skin was in the elastic region (phase 2 of the model), i.e., just after the slack was removed. In phase 2, the in vivo stress-strain characteristics of skin are linear (18). Indeed linearity was evidenced by the fact that slight further manual stretching of the skin did not change the observed sonic velocity which is related to the elastic modulus (vide infra.). Others (23,25) describe the skin's viscoelastic behavior as an interaction between predominantly elastic components such as collagen, elastin, and keratin in the outer layers, and the purely viscous interstitial fluids. It is this keratinous outer layer that skin care products are thought to affect and the skin stripping studies described above confirm that it is this very layer which affects the observed changes in sonic velocity. These models, together with the tape stripping results, suggest that the physical property measured in sonic transmission studies may be made up of two components: transmission through the viscous, inner layers of the skin (lower stratum corneum, epidermis, and perhaps the dermis) and transmission across or through the dehydrated,