Medical imaging microbubble. Computer illustration of a microbubble (blue) being stimulated by ultrasound waves inside a blood vessel, during an ultrasound scan. Microbubbles are miniature bubbles smaller than one hundredth of a millimetre in diameter, but larger than one micrometre. They have numerous applications in science and medicine. They may be used for drug delivery, biofilm removal, membrane cleaning/biofilm control and water/waste water treatment purposes. They are also used in medical diagnostics as a contrast agent for ultrasound imaging. Because gas bubbles are unstable in liquid and would quickly dissolve, microbubbles must be coated with a solid shell. This shell is usually made from either a lipid or a protein. Here, the microbubble releases a contrast agent when stimulated by the ultrasound waves to aid in medical imaging.