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Microbubbles are the ultimate mechanically responsive colloidal particles. The compressible gas core and resonant response of the microbubble at MHz frequencies makes it the ideal ultrasound contrast agent and drug delivery vehicle. No other particle comes close to the acoustic echo – a single microbubble can be detected in vivo with current clinical scanners. Each microbubble is only 1-10 μm diameter and stabilized by a lipid shell, so they are safe for intravenous injection. In fact, they have been FDA approved for echocardiography for more than a decade. Ultrasound can also be used to push microbubbles (radiation force) and destroy them in the beam focus for targeted delivery. Our goal is to engineer innovative microbubble suspensions for imaging, therapy and theranostics (therapy + diagnostics). Our approach is to study the relevant intermolecular and surface forces that control the structure, properties and performance of microbubble suspensions, and to exploit these for rational design.
Ultrasound-Stimulated Coalescence: two DSPC-coated perfluorobutane microbubbles exposed to brief ultrasound pulses every two seconds attract by secondary radiation force and then coalesce.
Nanodrops are condensed microbubbles – they offer the same acoustic effects as microbubbles, but in a much more compact form. In a method pioneered by Dayton’s group at UNC, a nanodrop is formed by condensing the microbubble’s fluorocarbon vapor core into liquid. Once condensed, the nanodrops are remarkably metastable and remain in liquid form until activated by acoustic or optical energy. User-controlled activation stimulates conversion of the liquid core back into a vapor, transforming the nanodrop back into a microbubble. This is the hot new area in ultrasound contrast agent design! Again, our goal is to engineer nanodrops for innovative biomedical applications, and our approach is to focus on the relevant intermolecular and surface forces.
Novel Soft Colloids
Our group is constantly synthesizing and characterizing novel colloidal constructs with multiple therapeutic and diagnostic functions. Examples include Gd-labeled microbubbles for MRI-guided focused ultrasound surgery, lung surfactant microbubbles for enhanced lipophilic drug payload, and polyplex-loaded microbubbles for ultrasound-targeted gene therapy. The example below shows recent work on gold nanoparticle-coated nanodrops for photoacoustic imaging and photothermal therapy.