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Based on the evidence from our previous studies that we can evaluate the hydration state of the skin surface quickly and quantitatively in terms of conductance to the high frequency electric current of 3.5 MHz, we have established a simple in vivo function test that furnishes information on the hygroscopic property and water-holding capacity of the stratum corneum in a few minutes. The test procedure consists of electromeasurements before and after application of a droplet of water on the skin for 10 seconds to obtain data on the hygroscopic property of the skin surface and later serial measurements at an interval of 30 seconds for 2 min to evaluate the water-holding capacity. Under usual ambient conditions normal skin surface showed a high rise in conductance just after application of water, which was followed by a rapid fall-off within 30 seconds, thereafter by gradual return to the prehydration levels by 2 min. By this method we have demonstrated that (i) the superficial horny layer of normal skin is much less hygroscopic and less capable of holding water than the corresponding deeper portions and that (ii) scaly skin shows functional defects in both hygroscopicity and water-holding capacity, between which the former normalizes much faster than the latter.

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.