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.