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 studing the electrical properties of the skin using a newly developed instrument that can be measure the resistance and capacitance to the high frequency current of 3.5 MHz, we have found that by employing dry electrodes we 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 corundum on the basis of the following finding: (i) contact of the skin with water even for a second caused a great increase in Vr: (ii) Vr progressively increased as deeper layers of the stratum corundum was 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 corundum was not influenced by the amount of tissue fluid contained in the underlying epidermis; and (iii) in vitro the Vr of pieces of fully hydrated stratum corundum rapidly decreased in a dry atmosphere with evaporation of the water contained in their superficial portion. Furthermore, we have been able to demonstrate the highly hygroscopic nature of the stratum corneum, (b) lesser hydration on the extremities that on the trunk, (c) the decreased water-holding capacity of scaly lesions despite the increased transepidermal water loss, and (d) the water supplying ability of skin emollients.