(438b) Measuring the Strength of Ligand-Receptor Mediated Adhesion of Ultrasound Contrast Agent Microbubbles

Ultrasound-activated microbubbles are employed in several current and emerging biomedical applications. They are routinely used as contrast agents for ultrasonography, and can be functionalized with targeting ligands for selective imaging of cells presenting markers of disease. Furthermore, hydrodynamic stresses exerted by ultrasound-activated microbubbles may be harnessed to controllably permeabilize cell membranes. A deeper understanding of the hydrodynamic and acoustic forces acting on, and exerted by, microbubbles in ultrasound, along with knowledge of the mechanics of strongly deforming phospholipid monolayers and membranes, are therefore key to improved medical imaging and drug delivery protocols.

The effects of hydrodynamic and acoustic forces on the stability of targeted microbubbles adherent to cell membranes through ligand-receptor interactions are studied here. In particular, we focus on the effects of secondary acoustic radiation force, which causes bubbles to attract each other during activation with ultrasound. Previously, we developed a model to describe the dynamics of bubbles propelled by the secondary acoustic radiation force; the unsteady contribution to the viscous dissipation was found to be crucial to correctly predict bubble displacement.

We performed experiments on phospholipid-coated microbubbles (2-3 microns) functionalized with anti-fluorescein antibody and allowed to adhere to a fluorescein-functionalized polystyrene substrate. The bubble dynamics in ultrasound (1.7 to 2.5 MHz) was recorded at 15 million frames per second using a custom ultra-high speed camera. By increasing the ultrasound pressure, and therefore the magnitude of the secondary acoustic radiation force, a threshold was found above which the adhesion of targeted microbubbles was disrupted. This observation points to the fact that the secondary acoustic radiation force may alter the spatial distribution of targeted contrast agents bound to tissues during activation with ultrasound. The net force pulling on the bubbles at the time of unbinding was extracted from the force balance, and was found to be up to 100 nN. While the mechanism of unbinding (rupture of intermolecular bonds, disruption of the phospholipid layer) remains elusive, it is shown that secondary acoustic radiation force can be used to quantify the binding force of targeted microbubbles.