2 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Still, dermatologist expert visual grading currently is the most widely used evaluative technique for assessing in vivo product efficacy, perhaps due to lack of validated and widely accepted alternative measures. Many other instrumental skin measurement techniques are under evaluation, both in our laboratories and at other academic/ industrial institutions. Among these are: examination of resonance frequency (1) determination of the complex electrical impedance of in vivo skin (2-9) monitoring the fluorescence of an infused marker to determine the stratum corneum turnover time (10) measurement of transepidermal water loss across the skin barrier (11-12) quantification of the surface texture of the skin using replicate molding techniques coupled with microscopy and profilometry (13,14) infrared absorption/reflection (15) and ellipsometric (16) spectrophotometric characterization as well as other techniques. Several of these methods are now employed in the clinical environment. This paper establishes the utility of sonic velocity measurements as a novel skin measurement tool. A recent report mentions the routine use of resonance frequency (1) (closely related to the velocity measurement discussed herein) by the Juvena Company in Switzerland (17) to assess skin condition. To our knowledge, however, the Juvena method has not been published. Also, measurement of elasticity changes in the skin, to which we relate the sonic velocity alteration reported herein (vide infra.), have been investigated by others both in vivo (18) and in vitro (19). Having established the background and objective of this work, the remainder of this article will consider: 1) the conditions for modifying commercially available instrumen- tation in order to obtain sonic velocity measurements and the human factors/operating conditions necessary to secure reproducible data 2) an experimental study to discover the physical/biological origin of the changes in sonic velocity observed and 3) representative clinical data demonstrating the utility of the technique in establishing treatment differences. EXPERIMENTAL CONDITIONS A commercially available instrument, the Dynamic Modulus Tester © (Model PPM-5), produced by the H.M. Morgan Company (20) was modified to measure the velocity of sonic propagation through skin in vivo. The instrument was designed to measure the elasticity of sheet and cord materials. Its operational principle is simple and is set out in the instrument block diagram of Figure 1. The main control unit generates electrical pulses of acoustically white noise at a repetition rate of 60 Hz. As each pulse is generated, a timing circuit is triggered. These pulses are conducted to a piezoelectric crystal which produces acoustic (mechanical) pulses that are directed through a known distance of skin and detected by a second piezoelectric crystal. Both crystals are mounted so that the mechanical vibrations transmitted or received are parallel to the line between the two transducers no transverse motion is generated or detected. The electrical signal from the receiving transducer crystal is monitored, and when the level crosses a preset threshold value the timing circuit is halted. A signal is produced proportional to the time delay between transmission and reception and averaged over the sixty pulses per second. Relating this output voltage to the delay time, and with knowledge of the transmission distance through the skin, the sonic propagation velocity is calculated directly. The instrument is calibrated as per the specifications in Product Bulletin No. PM3-8, H.M. Morgan Company.

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