j. Soc. Cosmet. Chem., 35, 1-19 (January/February 1984) Measurement of skin condition by sonic velocity R. MARC DAHLGREN and W. H. ELSNAU, The Procter & Gamble Company, Beauty Care Product Development Division, Cincinnati, OH 45241. Received June 22, 1983. Presented at the Annual Scientific Seminar of the Society of Cosmetic Chemists, CincinnatL OH, May 5, 1983. Synopsis Sonic velocity measurements through skin were made as a means of evaluating skin condition. These velocities were shown to be a measure of skin elasticity and can be used to assess product-induced changes in skin condition. A commercially available instrument was modified for these studies to measure transmission velocity through forearm and dorsal hand skin. Tape stripping experiments indicated that observed velocity changes originate in the upper layers of the stratum comeurn and were directly related to changes in elasticity induced by product treatments. Sonic velocity measurements allow quantification of the consumer-perceived effectiveness of skin care products both accurately and reproducibly as a function of chronic product use. Additionally, the dynamic effects of acute product exposure could be shown. INTRODUCTION The goal of this research was to develop a method which would provide a material, instrumental measure of skin condition and discern the effects of product treatments on a physically meaningful variable of the skin. Today, most clinical studies of product efficacy still depend on expert dermatological visual and/or tactile grading of the treated region over time as the principal means of evaluation. Such measures are necessarily qualitative and, consequently, the least significant difference between treatments which may be detected within a given population is highly dependent on the dermatologist's experience and the reproducibility of his grades. Further, the treatment period needed to differentiate effects is also a function of the sensitivity of the dermatologist and the induction period before these effects are visually apparent a small signal-to-noise ratio within the dermatologist's grades requires larger populations and longer treatment regimens to detect product-related effects. Finally, because different dermatologists rarely use the grading scales identically, intercomparisons and pooling of data between tests graded by different individuals is limited if high confidence levels are required. In large, costly studies, this inability to intercompare dermatologist grades often leads to the added expense of maintaining a second, backup expert grader in case the principal investigator is unable to complete the study.

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.