CHANGE IN THE ELECTROCHEMICAL PROPERTIES OF SKIN AND THE LIPID PACKING IN STRATUM-CORNEUM BY ULTRASONIC IRRADIATION

Effect of ultrasound on the skin permeation of benzoate anion (BA), a model compound, through excised hairless rat skin was investigated usi ng electrochemical techniques. When the skin surface was sonicated at 150 kHz frequency and 111 mW/cm(2) intensity, skin impedance measured by alternative current with 10 Hz frequency was decreased and skin per meation rate (flux) of BA and deuterium oxide was correspondingly incr eased. A constant current iontophoresis with 0.1 mA/cm(2) after pretre atment of skin by the ultrasound for 60 min significantly increased th e BA flux through the skin compared to that without the pretreatment. In contrast, electric potential difference across the skin during iont ophoresis with the ultrasonic pretreatment was one-third lower than th at without the pretreatment. Analysis of these results using the Nerns t-Planck equation suggests that the ultrasound increased aqueous regio n in the stratum corneum (s.c.) as well as effective diffusivity of BA in the skin as a result of a structural disorder in the stratum corne um lipids, The ultrasound significantly increased leaching of sterols and ceramides, typical lipids, from the skin when 0.1% Tween 20 was us ed as a donor solution, and thus disordered the lipid packing of s.c, to lower the skin impedance and to increase the diffusivity in the s.c . We concluded that ultrasound acts on the s.c. lipids and increases t he diffusivity of polar molecules in the skin barrier.