Abstract
Microbubbles form aggregates by secondary Bjerknes force, which behaves as attractive or repulsive force between the bubbles, in an ultrasound field. However, not only the mechanism to form aggregates but also the size of aggregates, or the length of time for the aggregate formation were not entirely clarified yet because of the complexity of the parameters involved. In this study, we have observed aggregate size development with an elapse of time and measured their average size under various conditions of ultrasound field by using a high-speed camera. We also evaluated the influence of the presence of red blood cells in the suspension with microbubbles. We prepared microbubbles (microcapsules) with a hard shell of poly(vinyl chloride) to compare with Sonazoid, which is commercially available for clinical use. First we have measured their resonance frequency and determined the range of frequency varying from 3 to 10 MHz. We found that the saturation size of aggregates at 3 MHz of central frequency was two times greater than that at 10 MHz in both types of microbubbles and the shortest formation time of Sonazoid aggregates that confirmed at the resonance frequency. Then, we showed that the saturation size of aggregates was inversely proportional to the ultrasound frequency and the formation time became short at the resonance of bubbles. In addition, we confirmed that the aggregate formation of bubbles is unaffected by the presence of red blood cell with the concentration less than 6.50×106 cells/ml.
