Skin dryness is related to the water content of the stratum corneum. The stratum corneum hydration interacts with the barrier function, permeability, and mechanical properties of the skin and is related to the water-binding capacity of the stratum corneum lipids (29). Objective quantification of skin hydration by bioengineering tools has gained wide popularity, as it provides fundamental information of the skin function and is comparatively easy to perform. Three electrical methods for the skin hydration measurements are used currently, based on capacitance, impedance, and conductance measurements (30). The Corneometer® (Courage & Khazaka, Cologne, Germany) is based on a capacitance measurement. Two metal plates with an electric field in between are integrated in the electrode surface. Capacitance is the capability to store the electrical charge that is built up by the electron excess at one plate and an electron deficit at the other plate. It is influenced by the dielectric constant of the material between the plates, which changes with water content. The device estimates the water content in the epidermis up to an approximate depth between 60 and 100 ^m (29). The Corneometer CM 825® is the most recent version. The Skicon® principle is based on the conductance measurement of a fixed high-frequency current of 3.5 MHz with a probe consisting of two concentric electrodes (I.B.S. Hamamatsu-shi, Japan). It measures more superficial depths as compared to the Corneometer. The Nova Dermal Phase Meter® (Nova Technology Corporation, Gloucester, Massachusetts, U.S.A.) is an impedance-based capacitance instrument (30). Fluhr et al. (31,32) have undertaken comparative and systematic studies of five instruments.
In order to gain accurate and reliable results with any of the devices, a considerable number of individual and environmental factors must be recognized. Dependence of the position and pressure exerted on the probe must be considered. Because of occlusion, values can increase in repeated measurements. Thus, waiting periods of at least five seconds are recommended. In addition, environmental conditions must be considered; constant room temperature and relative humidity should be ascertained (30).
Capacitance measurements with the use of the Corneometer have had widespread application, as does the measurement of the TEWL, in studies of vulvar skin physiology and experimental contact dermatitis (2,5,7,8,23,24,27). Baseline capacitance values of unaffected, healthy skin were found to be significantly higher at the vulva as compared to the forearm. However, reactivity of female genital skin after exposure to typical detergent irritants such as SLS was not higher as compared to the forearm (28). Age-related differences were detected in pre- and post-menopausal vulvar skin. The dehydrating capacity of SLS was less pronounced in postmenopausal as compared to premenopausal women (27).
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