GENERATION AND GROWTH OF BILAYER DEFECTS INDUCED BY ULTRASOUND

Transdermal drug delivery is handicapped by the low skin permeability caused by highly ordered structure of lipid bilayers in the outer huma n skin layer. It has been reported that ultrasound can increase the pe rmeability of human skin. The enhancement was attributed to acoustic c avitation but the underlying physical mechanism is not fully understoo d. As a model, dipalmitoylphosphatidylcholine (DPPC) lipid bilayers ar e insonicated by ultrasound of two submegahertz frequencies (168 kHz a nd 707 kHz). The free-field spatial peak pressure amplitudes of both a re measured to be 6 x 10(5) Pa. Bilayer defects, which have average di ameters of tens to hundreds of nanometers and can be detected by an at omic force microscope, are generated within less than 0.5 min. The num ber of the defects grows with time. The defect growth rate at the 168- kHz frequency is about 3.5 times that at the 707-kHz frequency. (C) 19 98 Acoustical Society of America.