Sustaining Microbubbles Derived from Phase Change Nanodroplet by Low-Amplitude Ultrasound Exposure

To improve the short lifetime of microbubbles induced upon application of triggering ultrasound pulse to a phase change nanodroplet (PCND), the effect of low-pressure continuous ultrasound for sustaining microbubbles was studied in a gel phantom. A pulse with 100 cycles of 1.1MHz ultrasound with a peak negative pressure of 2.4 MPa was used for the generation of microbubbles while superimposing a bubble-sustaining ultrasound at a frequency of 1.1 MHz with a relatively low-pressure amplitude. It was found that a peak negative pressure in the range from 0.01 to 0.1MPa was suitable for sustaining the microbubbles without inducing cavitation. The presence of sustained bubbles could be echographycally observed as a beam-shaped brightness change. Moreover, the sustained microbubbles induced cavitation upon additional application of ultrasound pulse at a peak negative pressure of 0.2 MPa. The results obtained suggested that not only the lifetime but also the activity of the microbubbles can be controlled. (C) 2010 The Japan Society of Applied Physics