[3] The composition of the bubble shell and filling material determine important design features such as buoyancy, crush strength, thermal conductivity, and acoustic properties.
[4] The gas-filled microbubbles, typically air or perfluorocarbon, oscillate, and vibrate if a sonic energy field is applied and may reflect ultrasound waves.
Microbubbles with one or more incompressible liquid or solid cores surrounded by gas are referred to as microscopic or endoskeletal antibubbles.
For increased stability and persistence in the bloodstream, gases with high molecular weight as well as low solubility in the blood are attractive candidates for microbubble gas cores.
Hence, microbubbles that have a core with a density orders of magnitude lower than and compress more readily than the surrounding tissues and blood, afford high contrast in imaging.
The resulting force from destruction can dislodge the therapeutic payload present on the microbubble and simultaneously sensitize the surrounding cells for drug uptake.
Furthermore, through attachment of these particles to microbubbles as opposed to co-injection, the drug is confined to the blood stream instead of accumulating in healthy tissues, and the treatment is relegated to the location of ultrasound therapy.
[29] An analysis of nanoparticle infiltration due to microbubble destruction indicates that higher pressures are necessary for vascular permeability and likely improves treatment by promoting local fluid movement and enhancing endocytosis.
The transient pores formed by microbubble collapse allow the genetic material to pass into the target cells in a safer and more specific manner than current treatment methods.
In addition to permeating the blood brain barrier, ultrasound and microbubble therapy can alter the tumor environment and serve as an immunotherapeutic treatment.
Low-intensity focused ultrasound (LIFU) in combination with microbubbles has also shown to stimulate immunostimulatory effects, inhibiting tumor growth and increasing endogenous leukocyte infiltration.
[54] With their immune stimulating mechanism, ultrasound and microbubbles offer a unique ability to prime or enhance immunotherapies for more effective cancer treatment.