Mechanisms of ultrasonically-mediated drug delivery: high-speed camera observations of microbubbles with attached microbeads

Ultrasonically-triggered drug delivery may be ac- complished through the attachment of drug-enriched liposomes to microbubble contrast agent. Understanding of the dynamics and mechanisms of drug release is needed to ensure robust and reproducible delivery of pharmaceutical payloads. Fluorescent microbeads were attached to stabilized perfluorobutane mi- crobubbles with avidin-biotin bonding. The resulting bubble-bead complex is a model for a drug-loaded ultrasound contrast agent. In a preliminary experiment using fluorescent microscopy, the destruction of microbubbles and release of beads were observed during exposure to diagnostic ultrasound. Detailed observations were made using the Brandaris-128 camera operated at 13 million frames per second (Mfps) speed. The bead-bubble complexes were exposed to 2.0 MHz, 10-cycle ultrasonic pulses at up to 0.65 MPa peak negative amplitude. Release and displacement of fluorescent microbeads from their original position was observed during insonation. Detached beads track the fluid flow induced by the bubble compression and expansion, at velocities greater than 10 m/sec. The translational and oscillatory movement of detached beads diminishes as the distance between the bubble and the bead increases. The results did not establish the capability of ultrasound to propel microbeads to a distance greater than a few cell layers. In short, this study exposed the process of the ultrasound-mediated release and subsequent movement of shell- associated microbeads with high temporal resolution.