Bubble cycling and standing waves in ultrasonic cell lysis.

Two quite different but potentially complementary hypothetical mechanisms have been proposed to explain the dependence of ultrasonic cell lysis in vitro on exposure vessel rotation. One mechanism proposes that resonant-size bubbles are trapped in planar arrays by standing waves in the sample and that exposure vessel rotation sweeps cells through the bubble arrays. The other mechanism, for which there is now considerable support, proposes that exposure vessel rotation simply allows bubbles to recycle after having been driven through the medium by the ultrasound. We report here the results of efforts to further discriminate between, and quantify, the relative contributions of these two mechanisms. Murine P388 cells were exposed to 1.07 MHz continuous-wave (CW) ultrasound for 5 min at 5 W/cm2 (ISP) using various exposure vessel compositions and exposure configurations. Experimental treatments were designed either to minimize or maximize standing waves in the sample tube, or to minimize the potential for bubble recycling while maximizing standing wave formation. The results of these experiments indicate that bubble cycling is responsible for the majority of cell lysis occurring in the rotating exposure vessels, but that a significant contribution to total lysis is provided by trapped bubbles under some conditions.