High-speed imaging of microbubble formation in a novel flow focusing microfluidics chip

Gas-filled lipid microbubbles are emerging as a next generation ‘theranostic’ tool in the medical arena. Initially developed as a diagnostic aid to improve echogenicity in ultrasound imaging, their therapeutic potential has now been realized also, through their somewhat unique ability to deliver molecular species such as drugs and genes by disrupting the cell membrane, again in response to ultrasound wave stimulus. The distinct aim for the present study was to produce monodisperse microbubbles, which it is hoped will have a more easily controlled, and thus clinically reliable, response to pulsed ultrasound. In pursuit of this, we designed a glass microfluidic chip with a distinctive flow focussing junction where the liquid and gas phases meet and are collected into an outlet channel [1] (see figure below). The shell material was composed of lipid mixed with poly ethylene glycol 2000 (PEG2000) and the gas core was either nitrogen or perfluorobutane. The chip and associated apparatus were set up under an inverted microscope (Axiovert 200, Zeiss, Germany) in a clean room, and generation of microbubbles was readily observed. The diameter of the microbubbles formed was 2 – 10 µm, depending upon the liquid and gas flow parameters. At the high flow rates required [at 152 kPa gas pressure and 1.5 – 2 µl min−1 liquid flow rate] for reasonable throughput, specific nuances of the bubble formation process such as pinch-off and location within the flow arc of the microfluidic channels could be readily discerned, parameters that will inform next generation designs for our chip. The high-speed camera used for all of this work was a HPV-1 (Shimadzu, Japan).