In Situ Microbubble Production via Flow-Focusing Microfluidic Devices as a Means to Achieve Large Transiently-Stable Microbubbles to Improve Image Contrast and Drug Delivery

Ultrasound remains a mainstay of diagnostic imaging due to its portability, cost, and safety profile. Similarly, gas filled microbubbles remain the agent of choice for enhancing ultrasound contrast and therapy. Currently, microbubbles are designed to be stable in order to facilitate storage and provide an acceptable duration of contrast in vivo. This requirement to be stable reduces the acceptable range of microbubble diameters since microbubbles that are both large (>10 µm diameter) and stable pose an increased risk of gas emboli formation. Unfortunately, this limitation in diameter also limits acoustic contrast as the reflected acoustic power from a microbubble scales with the square of the microbubble diameter. With access to transiently-stable microbubbles, however, large diameter microbubbles could potentially be investigated and utilized as rapid dissolution could be relied upon to prevent gas emboli formation,

Microbubble production via microfluidic devices presents an opportunity to rethink microbubble design and potentially gain access to larger diameter microbubbles and the concomitant improvements in contrast and therapy. Microfluidics enables microbubble production with devices capable of being miniaturized to vascular compatible dimensions. By integrating a microfluidic device within a catheter, microbubbles could be produced in situ, directly within the vasculature. Direct microbubble production within the vasculature could eliminate the need for highly stable microbubbles since there would be no requirement for long-term storage or multiple passes through the circulation to reach the target of interest.

By removing the stability requirement, the in situ microbubble production paradigm enables the development of transiently-stable microbubbles. In this dissertation, I present work towards the development of microfluidics as a means to produce transiently-stable microbubbles in order to gain access to large diameter microbubbles that would present minimal risk of gas emboli formation.

First, I present a method to reduce the footprint of microfluidic devices by simplifying tubing interconnects. In this method, a microfluidic device is directly connected to a chamber containing pressurized liquid, thus eliminating bulky interconnects. Next, I describe a model to predict microbubble production rate as a function of gas and liquid input parameters. Access to models such as this helps to guide device development and operation. Third, I present experiments towards the optimization of albumin and nitrogen for the production of transiently stable microbubbles. To investigate drug delivery, I present results characterizing the drug delivery enhancement achieved in vitro using in situ produced microbubbles. Finally, these studies culminate in the in vivo characterization of transiently-stable microfluidic-produced microbubbles directly administered from the microfluidic device into the mouse vasculature.

Overall, I observed that transiently-stable microbubbles can be fabricated with albumin and nitrogen and have a half-life of under 10 s. Despite the short half-life, these microbubbles are capable of providing sufficient acoustic contrast and can enhance drug delivery. Furthermore, stabilization with albumin suggests the possibility of directly using plasma for microbubble stabilization. Most significantly, however, when administered in vivo transiently-stable microbubbles provide acoustic contrast without causing harm.