P2A-8 Lipid-Stabilized Monodisperse Microbubbles Produced by Flow Focusing for Use as Ultrasound Contrast Agents

Lipid-encapsulated microbubbles have demonstrated utility in biomedical applications as ultrasound contrast agents and drug delivery vehicles. Current production methods of these microbubbles result in distributions with a large size variance. The size and monodispersity of ultrasound contrast agents are fundamentally important due to the relationship between bubble diameter and resonant frequency, destruction threshold, and susceptibility to radiation force. Several groups have recently developed microfluidic technologies for generation of microbubbles, but to date no group has demonstrated the production of shell-stabilized monodisperse contrast agents using these techniques. In this work, we use microfluidic-based flow focusing methods to produce monodisperse microbubbles in the diameter range required for in-vivo imaging. The diameter of the bubbles produced using this technique can be precisely tailored by adjusting the gas and liquid flow rate parameters. In order to stabilize the microbubbles for use as ultrasound contrast agents, we examine aqueous mixtures of glycerol/propylene glycol as well as stabilizing lipids. 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 an optimum 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