Mechanisms for hemolysis by ultrasonic cavitation in the rotating exposure system

The rotating chamber provides a useful system for enhancing cavitation bioeffects so that they may be more easily studied. A tube with acoustically transparent windows was rotated at 72 rpm perpendicular to its axis. The 1.61 MHz ultrasound beam was switched on and off in 42 ms bursts to expose the chamber while it was aligned with the beam. The hemolysis of 0.5% suspensions of canine erythrocytes was used as a measure of the efficacy of this rotating exposure system. Use of dialysis membrane, agar plugs, plastic sealing film or wax film for the windows produced no differences in the results for exposure intensities above an apparent threshold of 2-2.8 W/cm2, up to 16 W/cm2, at which essentially 100% lysis was observed. Changing the tube length from 1.4 cm to 4 cm, or partitioning it into three 0.5 cm segments had little influence on the results, but a short 0.5 cm chamber had reduced efficacy. Pressurizing the suspension filled chamber at 10 MPa to reduce the population of cavitation nuclei reduced the hemolysis. Separately pressurizing the suspension or the chamber produced indistinguishable, smaller reductions in hemolysis. The results lead to the hypothesis that the hemolysis occurs primarily in the bulk of the medium (rather than on surfaces). Bubbles recycled by rotation into the medium move toward the back of the chamber at estimated speeds of 10 m/s, generating hydrodynamic stresses sufficient to cause the observed hemolysis.