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By John G. Webster (Editor)

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Typical ratios of ultrasound-enhanced flux of permeants to the passive flux of permeants are less than or equal to a factor of 10. Even more successful (by a factor of 1000 or so) has been work (102,103) at lower frequencies (around 20 kHz) using pulsed ultrasound (with pulse lengths on the order of 100 ms applied every second for as long as one hour) with smaller average intensities, typically a few hundred milliwatts per square centimeter. ] We take the time to discuss sonophoresis in some detail because its study sheds light on a number of bioacoustic phenomena and illustrates the complexity of identifying acoustic mechanisms behind bioeffects.

Away from hair follicles, the stratum corneum has a thickness of 15 Ȑm. It is made of a combination of keratinocytes (which are 1 Ȑm thick and about 23 Ȑm long) stacked like mortared bricks with lamellar lipid bilayers that have a net thickness of 50 nm acting as the mortar. Near the base of 378 BIOLOGICAL EFFECTS OF ULTRASOUND the hair follicles (whose horizontal cross section measures about 50 nm), the stratum corneum thins considerably. The best in vitro study of possible mechanisms behind sonophoresis at 1 MHz (106) involves a series of in vitro experiments using a special two-compartment tank, where a layer of stratum corneum taken from heat-stripped, then hydrated human cadaver skin separates the compartments.

For example, shock waves in the form of individual high amplitude (5 MPa to 10 MPa), short pulses (5 ms) of sound generated by a lithotripter have caused premature ventricular contractions in vivo, as demonstrated in frogs (82). Longer pulses of sound with high amplitude also alter the contraction of heart muscle in frogs (83). As yet no mechanism to create these bioeffects has been identified, although cavitation is likely, given the acoustic regime brought to bear in their studies. In principle, these results could be of concern to those who wish to use ultrasound only for diagnostic purposes.

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61.Ultrasonics, Ferroelectrics, and Frequency Control by John G. Webster (Editor)

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