Synthesis and Characterization of Transiently Stable Albumin-Coated Microbubbles via a Flow-Focusing Microfluidic Device

Abstract

We present a method of synthesizing albumin-shelled, large diameter (>10 μm), transiently-stable microbubbles using a flow-focusing microfluidic device (FFMD). Microfluidic device production enables microbubbles to be produced immediately prior to insonation, thus relaxing the requirements for stability. Both reconstituted fractionated bovine serum albumin (BSA) and fresh bovine blood plasma were investigated as shell stabilizers. Microbubble coalescence was inhibited by the addition of either dextrose or glycerol and propylene glycol. Microbubbles were observed to have an acoustic half-life of approximately 6 s. Microbubbles generated directly within a vessel phantom containing flowing blood produced a 6.5 dB increase in acoustic signal within the lumen. Microbubbles generated in real-time upstream of in vitro rat aortic smooth muscle cells under physiological flow conditions successfully permeabilized 58 % of the cells upon insonation at a peak negative pressure of 200 kPa. These results demonstrate that transiently-stable microbubbles produced via flow-focusing microfluidic devices are capable of image enhancement and drug delivery. In addition, successful microbubble production with blood plasma suggests the potential to utilize blood as a stabilizing shell.

In vivo imaging of microfluidic-produced microbubbles

“…As a result, few microbubbles survive the transit through the lung vasculature to provide contrast in the left ventricle. This behavior is not unexpected as these microbubbles were designed to be unstable by using highly soluble nitrogen gas and non-crosslinked albumin (Chen et al 2014). Furthermore, passage through the lungs is not a requirement as the intended application of these microbubbles would be local production and administration from a catheter tip, thus eliminating the need for systemic circulation.…”

“…These values may explain our unsuccessful attempts at measuring microbubble distribution using a Coulter counter as the measurement requires 30 seconds to acquire a measurement. Previous work using an acoustic metric of microbubble half-life also suggests lifetimes on the order of 10 seconds (Chen et al 2014). It should be noted that although microbubbles with no surfactant are predicted to have lifetimes upward of 8 seconds, microfluidic-produced microbubbles produced without a surfactant coalesce immediately upon formation within the microfluidic device.…”

“…cells are not in static contact with the microbubbles) (Dixon et al 2013). In addition, we demonstrated microfluidic-produced microbubbles stabilized with albumin that have a half-life under 10 s, yet still produce a strong acoustic signal when imaged with a clinical ultrasound scanner (Chen et al 2014). In this paper we report on the administration of microbubbles directly from the output of a microfluidic device, through a tailvein catheter, and into the mouse vasculature along with the simultaneous in vivo ultrasound imaging of the injected microbubbles.…”

Microbubble-mediated intravascular ultrasound imaging and drug delivery

“…However, microfluidics technology permits real-time, on-chip MB fabrication that, when combined with an IVUS catheter, can provide a fully integrated approach for contrastenhanced IVUS imaging and therapeutic delivery [125]- [127]. Microfluidically-produced MBs have been applied to contrast-enhanced imaging [126]- [129], molecular imaging [130], [131], and therapeutic delivery applications [132]- [134], and a 1.5-mm-diameter catheter-based microfluidic device was recently developed for use in sonothrombolysis applications [135]. On-chip MB fabrication directly within the catheter permits the use of unconventional gases and shell materials, thereby offering the option to modify MB size, concentration, and composition in real-time during an intervention [127], [133], [134].…”

“…Microfluidically-produced MBs have been applied to contrast-enhanced imaging [126]- [129], molecular imaging [130], [131], and therapeutic delivery applications [132]- [134], and a 1.5-mm-diameter catheter-based microfluidic device was recently developed for use in sonothrombolysis applications [135]. On-chip MB fabrication directly within the catheter permits the use of unconventional gases and shell materials, thereby offering the option to modify MB size, concentration, and composition in real-time during an intervention [127], [133], [134]. This MB production paradigm also permits the use of new MB formulations, such as transiently-stable MBs that are comprised of highsolubility gases [134], or MBs larger than 10 µm in diameter that provide enhanced acoustic backscatter, improved IVUS contrast enhancement [129], and enhanced therapeutic efficacy [133].…”