Micron sized, lipid stabilized bubbles of gas or 'microbubbles' are in medical use as contrast agents for ultra-sound imaging. Due to their compressibility they reflect and echo sound waves very efficiently thus enhancing diagnostic ultra-sound imaging.
Microbubbes are also being developed as vehicles for targeted drug delivery by functionalising the shell with a drug payload and targeting ligands. By combining the imaging and delivery properties of the microbubbles they can be traced in real time as they accumulate around a tumour. High powered ultra-sound can then be used to destroy the bubbles and release the drug directly to the tumour.
The size and concentration of bubbles needs to be accurately measured to establish the drug payload, but due to their small size they cannot be counted with traditional optical techniques.
TRPS has been successfully used to characterise these bubbles in a number of research groups.