Acoustically driven microbubble dynamics near a tissue-mimicking soft substrate*

Micrometric coated bubbles are established as contrast agents in medical ultrasound imaging and are currently being investigated as ultrasound-responsive agents for therapies such as targeted drug delivery. In blood vessels, acoustically driven microbubbles undergo volumetric oscillations that can mechanically influence the surrounding tissue and release the therapeutic payloads, although their exact mechanisms of action remain unclear. In this study, we employ a horizontal ultrafast videomicroscope to visualize the acoustic response of single bubbles in contact with a flat substrate from a side-view perspective. We use clinically relevant bubble sizes and a particularly soft substrate (elastic modulus < 1 kPa) to provide a realistic model of biological tissue softness. We demonstrate that microjetting enabled by interfacial instabilities generating shape modes can occur and be directed toward the substrate without being caused by the boundary. We also investigate the relationship between the bubble dynamics and payload release using high-speed fluorescence imaging. Our findings elucidate the potential physical mechanisms responsible for bubble-mediated drug release and uptake.