j. Cosmet. sci., 52, 13-22 (January/February 2001) The ability of electrical measurements to predict skin moisturization. I. Effects of NaCl and glycerin on short-term measurements FANG LI, EILEEN CONROY, MARTY VISSCHER, and R. RANDALL WICKETT, College of Pharmacy, University of Cincinnati (F.L., E.C., R.R.W.), and The Skin Sciences Institute, Children's Hospital Medical Center (M. V.), Cincinnati, OH 45267. Accepted for publication January 15, 2001. Presented as a poster at the Annual Scientific Seminar of the Society of Cosmetic Chemists, Chicago, May 6-7, 1999. Synopsis Non-invasive methods to evaluate skin hydration by measuring electrical properties are widely used in the cosmetic industry. However, there is still some controversy about factors that affect measurement. For example, concerns have often been expressed about the possible confounding effect of salts, either in the formulation or on the skin. Ionized salts on the skin may increase electrical conductivity and may lead to changes in electrical properties that are not related to increased water content. We have performed a systematic study of the effects of salt, i.e., sodium chloride, and glycerin on the electrical properties of skin as measured by the three most commonly used instruments, the Nova © DPM 9003, the Corneometer © CM 825, and the Skicon © 200. Formulations containing salt from 0-3% and glycerin from 0-] 0% were tested for their effects at one and two hours after a single application. Salt lowered the readings in the absence of glycerin and increased the reading in the presence of glycerin. For all three instruments, there was a linear correlation between the measurement and the glycerin level in the presence or absence of salt. INTRODUCTION The term dry skin refers to skin features that give an impression of dryness, roughness, scaling, fissures, and cracks (1). Dry skin develops because of decreased water content in the horny layer (2,3). More recently, Warner and Lily have shown conclusively that dry skin lacks water (4). Moisturizing lotions are widely used to hydrate the skin surface and relieve the symptoms of dry skin. Evaluating the efficacy of these products constitutes an important task, especially in the course of substantiating the moisturizing claims of cosmetic products. In recent years, non-invasive methods to evaluate, quantify, and compare skin hydration by measuring electrical properties have become increasingly popular (5). Commercially available instruments, based on principles of measuring skin capacitance, conductance, or impedance (3,5-8), are becoming mature and have been widely used for this purpose. There is ample evidence that these measurements do 13

14 JOURNAL OF COSMETIC SCIENCE correlate to the extent of skin hydration (5-10) however, there is still controversy about what they actually measure. For example, there is concern about the possible confound- ing effect of salts (9), either in the formulation or on the skin. Salt may increase the conductance of electricity by the skin and may lead to changes in electrical properties that are not related to increased water content. This may be an especially important consideration with alpha hydroxy acid products since they will necessarily contain significant quantities of salts. While there have been several studies comparing the ability of the various commercial instruments to measure the moisturizing effect of lotions (10,11), no systematic study on the possible interference of salts, such as NaC1, has been published. In this study, simple lotions containing NaC1 and glycerin in combination were formulated to investigate whether there is interference by salt. The NaC1 levels of 0-3 % were selected based upon the anticipated range of ionic species that might be present in commonly used topical formulations, e.g., alpha hydroxy acid moisturizers with partially ionized species. MATERIALS AND METHODS APPARATUS Corneometer CM 825. The Corneometer © CM 825 (Courage-Khazaka Electronic, Co- logne, Germany) measures skin hydration by determination of skin capacitance. The measuring probe (surface 0.65 cm 2) consists of an interdigitated grid of gold-covered electrodes. There is no direct galvanic contact between the electrode and the skin surface. The whole system (interdigitated electrode and upper part of the epidermis) works as a variable capacitor. The probe of the CM 825 is electrically isolated from the measuring electronics to limit the influence of ground capacitance and salty skin surface (Courage, 1994). The CM 825 operates at a mean frequency of 1 MHz (1.15 MHz, very dry medium 0.95 MHz, very hydrated medium). The measurements are given in arbitrary units (AU) ranging from 0 to 120 AU (10-12). Skicon 200. The Skicon © 200 (IBS Company, Hamamatsu, Japan) measures conductance at a fixed current of 3.5 MHz. The measuring probe (surface 0.28 cm 2) consists of two concentric gold-covered electrodes (with respective external diameters of 2 mm and 5 mm). The distance between the inner and the outer electrode is 1 mm. A high frequency current of a few pA flows when the probe is placed on the measurement area. The measurement values are given in micro-siemens units (ps), ranging from 0 to 1999 ps (10,11,13). Nova DPM 9003. The Nova © Dermal Phase Meter 9003 (NOVA Technology Corpo- ration, Portsmouth, NH) is a capacitance instrument. Measurement values are obtained by integrating measurements at different frequencies of the applied alternating current at preselected variable frequencies up to 1 MHZ. Capacitance values are calculated from the phase delay of the signal. The standard probe has two concentric brass ring electrodes separated by an isolator (with respective inner and outer diameters of 4.34 mm and 8.76 mm). The distance between the inner and outer electrodes is 1 mm. There is direct galvanic contact between the electrodes and the skin. Measurement modes can be selected in this instrument: CON : continuous readings d15 : reading after a 5-s measurement interval d10 = instantaneous reading. The final readout is given in arbi- trary DPM units (AU), ranging from 90-999 (10,11,14).