Sonofluidic fabrication of microbubbles for imaging and therapy

Abstract

Gas-filled microbubbles are routinely used in diagnostic ultrasound imaging as contrast agents, and are under investigation for therapeutic application including drug delivery and ablation enhancement. They typically consist of a high molecular weight gas surrounded by a coating of surfactant, denatured albumin and/or a polymer. The acoustic response of microbubbles is profoundly influenced by their physical characteristics, in particular their size, size distribution, and the rheological properties of the coating. These in turn depend on the chemical formulation of the microbubble shell and the production technique. Sonication is the most commonly employed method and can generate high concentrations of microbubbles rapidly but with a broad size distribution and wide variability in acoustic response. Microfluidic devices provide excellent control over size, but the small-scale architectures required are often challenging to manufacture, offer low production rates, and are prone to clogging. Microfluidic generated microbubbles may also have inferior surface characteristics and stability compared to those produced by sonication. In order to address the limitations of the existing methods, we have developed a hybrid “sonofluidic” device. Monodisperse bubbles of a few 100 µm in diameter are first produced using a simple T-junction with relatively large channel dimensions (254µm x 50µm). Consequently high flow rates can be used with minimal risk of clogging or leakage. After formation, the large bubbles are exposed to ultrasound from a transducer embedded within the device for 1s over a frequency range of 71-73kHz. This promotes controlled fragmentation of the large bubbles to generate bubbles of the size required for clinical applications. Microbubbles were prepared using the sonofluidic device, a conventional microfluidic system or a standard sonication protocol. They were compared in terms of their size, size distribution, concentration, stability, acoustic response, and surface molecular concentration using quantitative fluorescence microscopy. The characteristics of the microbubbles produced by the sonofluidic device were found to be equivalent in terms of production rate and stability to those formed by sonication; but to have a narrower size distribution, closer to that obtained with microfluidics. These differences were reflected in the measured acoustic response and surface properties.