(489ab) Stability of Size-Selected Microbubbles

Mechanical agitation is the only method currently available to produce lipid-coated microbubbles in sufficient yields for ultrasound imaging and targeted drug delivery. However, the polydispersed nature of microbubbles formed by mechanical agitation is problematic for biomedical applications, since microbubble behavior is highly dependent on size. As a result, a means of separating microbubble size classes has been adapted that utilizes Stokes equation for the rise velocity of a buoyant particle relative to the bulk fluid under creeping flow conditions and based on its migration in a centrifugal field. Polydispersity of the freshly sonicated suspension was characterized by particle sizing and counting through light obscuration/scattering and electrical impedance sensing, fluorescence and bright-field microscopy and flow cytometry. The differential centrifugation technique was used to successfully isolate the 1?2, 4?5 and 6-8 μm diameter fractions with low polydispersities. The polydispersity index was defined as the ratio of volume average to number average mean diameter. Isolated microbubbles were stable up to two weeks. After two weeks, however, more dilute suspensions (<1 vol%) were susceptible to Ostwald ripening as apparent by the light obscuration sizing technique. For example, 4?5 μm microbubbles disintegrated into 1?2 μm microbubbles. This latter observation indicated the existence of an optimally stable diameter in the 1?2 μm range for these lipid-coated microbubbles. Differential centrifugation provided a rapid and robust means for size selection and reduced polydispersity of lipid-coated microbubbles. Currently, these size-isolated microbubbles are being tested as improved ultrasound contrast agents and for imaging and drug delivery applications.