Drug delivery microbubbles. Computer illustration of microbubbles (large spheres) used to deliver drugs, showing their shells (small spheres) surrounding the bubble containing the drug inside. 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.