In the past several years, lipid-encapsulated microbubbles have demonstrated utility in biomedical applications as ultrasound contrast agents and drug and gene delivery vehicles. Current production methods of these microbubbles result in distributions with a large size variance. This limits imaging sensitivity since only a small percentage of the contrast agents have diameters, which result in resonant frequencies optimized for the limited bandwidth of a clinical imaging system. The size and monodispersity are important in diagnostic and therapeutic applications due to the relationship between bubble diameter and resonant frequency.

Microfluidic chambers provide an ideal platform for the controlled production of microbubble contrast agents with a much smaller size distribution. Our 2-dimensional flow-focusing device, fabricated in poly(dimethylsiloxane) (PDMS) with standard soft lithography and rapid prototyping techniques, features expanding nozzle geometry to generate monodisperse microbubbles. The focusing of the bubble break-off location to one single point located at the orifice provides uniform control of bubble sizes. The geometry of the channel junctions in addition to the liquid and gas flow rates is used to control the bubble sizes. With microfluidics technology, we can produce microbubbles in a size range compatible with ultrasound imaging.

Several groups have recently developed microfluidic technologies for the generation of microbubbles, but to date no group has demonstrated microbubble production in the diameter range required for use as ultrasound contrast agents or the feasibility of producing lipid shell-based perfluorocarbon and nitrogen gas microbubbles. The microbubbles were produced by forcing a central stream of gas and two side sheath flows of an aqueous glycerol/propylene glycol mixture with the stabilizing lipids 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-distearoyl-sn-glycero-3-phoshoethanolamine-N-[Poly(ethylene glycol)2000] (DSPE-PEG2000) (This is a similar coating as the FDA approved contrast agent Definity).

The stabilization of microbubbles produced by microfluidic chamber is demonstrated using lipid encapsulation. The effect of the lipid, emulsifier and viscosity agent concentration was found to be critical to stabilize the microbubbles. High-speed camera images and particle sizing analysis were combined to study the coalescence and dissolution phenomena as well as the size distribution. The concentration of the PEG-emulsifier and the existence of the viscosity agents determined the microbubble coalescence rate. Our experiments show that increasing the concentration of glycerol and propylene glycol in the liquid phase mixture reduces coalescence. Monodisperse microbubbles coated with a lipid-shell in a viscous solution were found to be stable up to three months.