SKIN CONDITION MEASURED BY SONIC VELOCITY 3 Dynamic Modulus Tester PPM-5 Transmitter Receiver [ X I Skin =l I I Velocity t Figure 1. Instrument block diagram. The dynamic modulus tester (H. M. Morgan Company, Norwood, Mass.) as modified to evaluate skin condition. In order to measure transmission through skin, the transducers were modified as follows. Intimate transducer/skin contact was established with a minimum amount of contact pressure. The contact points of the transducers received with the Dynamic Modulus Tester © were modified to simulate phonograph styli, thus eliminating any tendency to skip or skid on the skin surface. To this end, straight pins were epoxyed to the transducer crystals and cut to about 3 mm length. These pins, having a diameter of 0.6 mm, were then polished smooth and round with fine emery cloth. With the transducers thus modified, changes in application of normal transducer force to the skin surface from 20 to 50 g had no effect on the measured sonic velocity and the instrument functioned well at a very low detection sensitivity (i.e., a high detection threshold value). An oscilloscopic examination of the signal at the receiving transducer revealed a significant amount of spurious signal transmitted through the standard factory- supplied mounting. In order to reduce or eliminate this unwanted signal, the individual transducer mounting rods were removed from their common rigid metal mounting bar and isolated in polyurethane foam padding. Within this foam assembly, the separation distance of the contact points of the transmitting and receiving transducers could be fixed at a known distance. For the majority of the work reported herein, a separation distance of 7 mm was employed, although velocity as a function of transmission distance was examined (vide infra.). This separation distance was found optimum for work on forearms and hands. It was short enough that significant sonic attenuation did not occur necessitating a low detection threshold sensitivity with a concomitant noise gain, yet long enough so that the delay time was well above the 50-microsecond minimum resolution of the instrument timing circuit. This contact assembly was then counterbalanced in a pivot apparatus with adjustable weights which maintained a normal force on the skin surface of 30 g (again, an empirically determined optimum), functioning in the same manner as a phonograph tone-arm. The biological test area (here the inner aspects of forearms and the dorsal regions of hands) had to be restrained in order to record reproducible velocities. Prior to measurement, the test area was immobilized and all slack was removed from the skin. Tregear (19) reported that elasticity in skin can be measured only after the slack has

4 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS been removed from the skin. Further, he reported that once the slack has been removed, the elasticity of skin remains constant through additional stretching until the breaking point is reached. For forearm measurements, this slack removal was accomplished by placing the forearm, outstretched at shoulder height, on a 10- cm-thick polyurethane foam block and letting the wrist bend down comfortably over the end of the pad. For measurements on the dorsal surface of the hand, the palm was placed on an inverted 250-ml beaker with the thumb and fingers hanging down over the sides of the beaker. The sonic propagation axis was kept parallel to the direction in which the skin was stretched. When skin is stretched, the collagen fibres align mostly parallel to the direction of extension (19). This alignment creates a more satisfactory pathway for the transmission of sonic energy. In use of the Dynamic Modulus Tester © , attenuation resulting from variation of the axis of stretching often led to signal levels at the receiving transducer below the detection threshold of the instrument. With the above modifications and conditions, sonic velocities in the range from 80 to 110 m/s were measured reproducibly through forearm and dorsal hand skin. Over repeated measurements, the accuracy of the recorded sonic velocities using this technique (the standard error) was found to be +0.8 m/s. THE PHYSICAL/BIOLOGICAL ORIGIN OF SONIC VELOCITY CHANGES The observation of product treatment-induced differences in measured sonic velocity values (vide infra.) identifies directly two critical issues: 1) What region of the skin most influences these velocity changes? and 2) What physical property of the skin in this region is being measured? In this section, evidence from empirical observation during the optimization of measurement conditions and tape stripping studies will be presented which establish that changes in the elasticity of the upper layers of the stratum comeurn adequately explain the observed treatment-induced velocity altera- tions. 120- 110- Sonic - Velocity (m/sec,) 100 90 , , , .... II , 0 5 10 15 20 25 30 35 24 Hrs. Number Of Strippings After Stripping Figure 2. The effect of skin stripping on sonic velocity. Data are from a single subject. Error bars represent • .05 confidence limits.