Behavior of the oscillating microbubble clusters trapped in focused ultrasound field

Microbubbles (MBs) are widely used in clinical site as contrast agents in ultrasound imaging due to its eminent responsiveness to several MHz band ultrasound. In recent years, ultrasound drug delivery system (DDS) using MBs are widely researched to achieve selective medication to the targeted diseased part. In the condition of high number density of bubbles to enhance the performance of DDS, it is essentially important to interaction between bubbles in the ultrasound field and establish a control method of bubble clusters. In previous works, we can manipulate MBs using focused ultrasound, forming a bubbles cluster at the focus of the ultrasound field. The purpose of this study is to clarify scaling behavior of bubble clusters with various physical properties, since such scaling parameters is highly relating various bubble parameters which have not been measured in other methods. We manipulated two types of MBs. One was Sonazoid with average diameter of 2.6 μm and resonant frequency of about 5.2 MHz. The other was developed by Teikyo University, Japan, with average diameter of 1.5 μm. Its resonant frequency was unknown. Each cluster of MBs was trapped at the antinode focal region of the ultrasound and had their own critical cluster size for each ultrasound frequency. Sonazoid formed larger clusters than MBs developed by Teikyo Univ., and Sonazoid cluster size was inversely proportional to the ultrasound frequency while the other MBs show the different scaling behavior compared to the free interface bubble cluster theory. These difference may be from diameter distribution of MBs and shell properties.