Microbubbles are small spherical gas filled bubbles in the size range of 1-10 microns. Their external coat is
made of polymers or phospholipids. In combination with ultrasound, they have been explored as contrast agents for
ultrasound and also carriers for drug and gene delivery. In response to ultrasound of lower mechanical index,
microbubbles oscillate and vibrate to give a distinct signal in ultrasound imaging. At a higher mechanical index, these
microbubbles rupture and break to deliver the drugs or genes enclosed in them or attached to their surface. The behaviour
of microbubbles in response to ultrasound is a characteristic of the properties of microbubbles like its shell composition,
shell thickness and the density and compressibility of the enclosed gas. Microbubbles have various applications in
diagnostic imaging like echocardiography, and imaging of cancer cells, inflammed cells etc. They are also used as a
medium for drug and gene delivery. Microbubbles can be further modified by binding specific ligands to their surface
which specifically attach to certain cells so that selective action of the microbubbles can be seen at that location of cells.
Keywords: Acoustic impedance, Contrast agent, Drug delivery, Ligands, Microbubbles, Targeting, Ultrasound.
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