Microbubbles. Computer illustration of a group of microbubbles (large spheres), showing their shells (small spheres). 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.