12:3/ Destructive Effects of High Intensity Ultrasound 225 



cavitating ultrasonic field. The heat-destroyed cells break into small 

 spherical pieces, each containing some methemoglobin. 1 However, the 

 red blood cells which are "ultrasonerated" form hemoglobin free ghosts 

 and cellular debris; the hemoglobin itself is slowly converted to the met 

 form. 



Another action of cavitation is to break molecular bonds. For 

 instance, H 2 molecules are fractured into H, OH, and H0 2 radicals 

 and H 2 2 is produced. Similarly, when a solution of NaCl is exposed 

 to a cavitating ultrasonic field, CI is produced. The CI is apparently 

 formed directly, for much more is present than could be formed by the 

 H 2 2 produced by the cavitation. Either free CI atoms or H 2 2 

 molecules could destroy biological cells. Quantitative studies, however, 

 indicate that the concentrations of H 2 2 and CI are several orders of 

 magnitude too small to account for the cellular disruption. 



Electron-microscope studies of cells' exposed to ultrasonic fields show 

 that they are torn mechanically. Some typical electron micrographs 

 are shown in Figure 2. This tearing could occur in any of several closely 

 related fashions. In the presence of an expanding and collapsing 

 bubble (that is, cavity), there will be very violent motions close to the 

 bubble and relatively weak ones several diameters away. Thus, a part 

 of the cell wall near the bubble will execute large motions relative to the 

 rest of the cell wall. The resulting shearing strains could easily rip the 

 cell wall. A model of this action is shown in Figure 3. Near the 

 collapsing cavities, there is also an extremely vigorous stirring type of 

 turbulence. The cell walls might be broken by the shearing stresses 

 set up by this turbulence. 



Similar mechanical rupture of cell walls of many types such as those 

 of amoebae, yeast, and white blood cells can be produced by rapid local 

 shearing stresses. For instance, a micromanipulator needle can be 

 slowly inserted into and then removed from these cells without damaging 

 them. However, a rapid jab will permanently destroy the cell wall. 

 By analogy, one might suspect that not only the shearing stresses pro- 

 duced by cavitation but also the rapidity of their application are 

 important. 



Thus, cavitation may tear cellular structures by a combined effect of 

 local shearing stress, local turbulence, and rapidly applied shearing 

 action. When, and only when, cavitation occurs, are these effects 

 observed. 



1 Methemoglobin is an altered form of hemoglobin with an optical absorption 

 spectrum differing from that of the hemoglobin occurring normally in red blood 

 cells. 



