Ultrafast imaging of microbubble cavitation using integrated optical trapping for spatial control: Progress and prospects

Cavitation, which involves the formation and dynamic evolution of bubbles, is a ubiquitous phenomenon in fluids. Recently, the area has received heightened interest in medical contexts due to the utility of [shelled] bubbles for exploitation both as contrast agents for clinical diagnostic ultrasound imaging, and increasingly, for their emerging potential as microscopic drug delivery systems. Micrometer-sized bubbles [microbubbles (µBs)] are most clinically relevant as their small size allows them to flow easily within even the smallest vessels of the vascular system. This size constraint results in bubble resonant frequencies lying in the MHz range, and in turn, requires [ultrasonic] driving frequencies in a comparable range. Accurate monitoring of µB response at sampling rates greater than the Nyquist limit thus requires the use of ultra-high speed microphotography. Moreover, in order to develop a fundamental understanding of bubble behavior over a range of clinically relevant scenarios, it is imperative that spatial control be exercised over their whereabouts relative to any proximal surfaces, and indeed, to other bubbles, so that the effects of any boundary constraints and acoustic cross-talk can be discriminated. Only one technique can be applied readily to this situation, that neither interferes with the incident ultrasound wave, nor perturbs any resultant hydrodynamical response in the surrounding fluid: that technique is optical trapping.