Evaluation of the skin surface hydration in vivo by electrical measurement

Nobody denies the fact that cracking and scaling of the skin surface develop because of decreased water content in the horny layer. However, functional research of such stratum corneum has been greatly hampered by the lack of an adequate, noninvasive in vivo method to assess the hydration state of the skin surface. While studying the electrical properties of the skin using a newly developed instrument that can measure the resistance and capacitance to the high frequency current of 3.5 MHz, the authors have found that by employing dry electrodes they can evaluate the hydration state of the skin surface quickly and quantitatively in terms of output voltage of the loss resistance detector (Vr), which is in a reciprocal relation to resistance. The sensitivity is very high since it can detect a high Vr value at the site where a water droplet is applied and blotted out instantaneously and where no special change can be found with the naked eye. The principal hydration detected by this method seems to be that in the outermost portion of the stratum corneum on the basis of the following findings: (1) contact of the skin with water even for a second caused a great increase in Vr; (2) Vr progressively increased as deeper layers of the stratum corneum were serially exposed by cellophane tape-stripping until reaching a certain level of the hydrated, viable epidermis, while the Vr on the surface of the intact stratum corneum was not influenced by the amount of tissue fluid contained in the underlying epidermis; and (3) in vitro the Vr of pieces of fully hydrated stratum corneum rapidly decreased in a dry atmosphere with evaporation of the water contained in their superficial portion. Furthermore, the authors have been able to demonstrate (a) the highly hydroscopic nature of the stratum corneum, (b) lesser hydration on the extremities than on the trunk, (c) the decreased water-holding capacity of scaly lesion despite the increased transepidermal water loss, and (d) the water supplying ability of skin emollients.