Microsphere

Not to be confused with microparticle.

Microsphere are small spherical particles, with diameters in the micrometer range (typically 1 μm to 1000 μm (1 mm)). Microspheres are sometimes referred to as microparticles.

Microspheres can be manufactured from various natural and synthetic materials. Glass microspheres, polymer microspheres and ceramic microspheres are commercially available. Solid and hollow microspheres vary widely in density and, therefore, are used for different applications. Hollow microspheres are typically used as additives to lower the density of a material. Solid microspheres have numerous applications depending on what material they are constructed of and what size they are.

Polyethylene and polystyrene microspheres are two most common types of polymer microspheres.

Polystyrene microspheres are typically used in biomedical applications due to their ability to facilitate procedures such as cell sorting and immunio precipitation. Proteins and ligands adsorb onto polystyrene readily and permanently, which makes polystyrene microspheres suitable for medical research and biological laboratory experiments.

Polyethylene microspheres are commonly used as a permanent or temporary filler. Lower melting temperature enables polyethylene microspheres to create porous structures in ceramics and other materials. High sphericity of polyethylene microspheres, as well as availability of colored and fluorescent microspheres, makes them highly desirable for flow visualization and fluid flow analysis, microscopy techniques, health sciences, process troubleshooting and numerous research applications. Charged polyethylene microspheres are also used in electronic paper digital displays.^[1][2]

Glass microspheres are primarily used as a filler and volumizer for weight reduction, retro-reflector for highway safety, additive for cosmetics and adhesives, with limited applications in medical technology.

Ceramic microspheres are used primarily as grinding media.

Microspheres vary widely in quality, sphericity, uniformity, particle size and particle size distribution. The appropriate microsphere needs to be chosen for each unique application.

Applications

New applications for microspheres are discovered every day, below are just a few:

• Assay - Coated microspheres provide measuring tool in biology and drug research
• Buoyancy - Hollow microspheres are used to decrease material density in plastics (glass and polymer)
• Ceramics - Used to create porous ceramics used for filters (microspheres melt out during firing, Polyethylene Microspheres)
• Cosmetics - Opaque microspheres used to hide wrinkles and give color, Clear microspheres provide "smooth ball bearing" texture during application (Polyethylene Microspheres)
• Drug delivery - As miniature time release drug capsule made of, for example, polymers. A similar use is as outer shells of microbubble contrast agents used in contrast-enhanced ultrasound.
• Electronic paper - Dual Functional microspheres used in Gyricon electronic paper
• Personal Care - Added to Scrubs as an exfoliating agent (Polyethylene Microspheres)
• Spacers - Used in LCD screens to provide a precision spacing between glass panels (glass)
• Standards - monodispere microspheres are used to calibrate particle sieves, and particle counting apparatus.
• Retroreflective - added on top of paint used on roads and signs to increase night visibility of road stripes and signs (glass)
• Thickening Agent - Added to paints and epoxies to modify viscosity and buoyancy

Biological protocells

Some refer to microspheres or protein protocells as small spherical units postulated by some scientists as a key stage in the origin of life.

In 1953, Stanley Miller and Harold Urey demonstrated that many simple biomolecules could be formed spontaneously from inorganic precursor compounds under laboratory conditions designed to mimic those found on Earth before the evolution of life. Of particular interest was the substantial yield of amino acids obtained, since amino acids are the building blocks for proteins.

In 1957, Sidney Fox demonstrated that dry mixtures of amino acids could be encouraged to polymerize upon exposure to moderate heat. When the resulting polypeptides, or proteinoids, were dissolved in hot water and the solution allowed to cool, they formed small spherical shells about 2 μm in diameter—microspheres. Under appropriate conditions, microspheres will bud new spheres at their surfaces.

Although roughly cellular in appearance, microspheres in and of themselves are not alive. Although they do reproduce asexually by budding, they do not pass on any type of genetic material. However they may have been important in the development of life, providing a membrane-enclosed volume which is similar to that of a cell. Microspheres, like cells, can grow and contain a double membrane which undergoes diffusion of materials and osmosis. Sidney Fox postulated that as these microspheres became more complex, they would carry on more lifelike functions. They would become heterotrophs, organisms with the ability to absorb nutrients from the environment for energy and growth. As the amount of nutrients in the environment decreased, competition for those precious resources increased. Heterotrophs with more complex biochemical reactions would have an advantage in this competition. Over time, organisms would evolve that used photosynthesis to produce energy.

Cancer research

One useful discovery made from the research of microspheres is a way to fight cancer on a molecular level. According to Wake Oncologists, "SIR-Spheres microspheres are radioactive polymer spheres that emit beta radiation. Physicians insert a catheter through the groin into the hepatic artery and deliver millions of micropheres directly to the tumor site. The SIR-Spheres microspheres target the liver tumors and spare healthy liver tissue.Cancer microsphere technology is the latest trend in cancer therapy. It helps the pharmacist to formulate the product with maximum therapeutic value and minimum or negligible range side effects. A major disadvantage of anticancer drugs is their lack of selectivity for tumor tissue alone, which causes severe side effects and results in low cure rates. Thus, it is very difficult to target abnormal cells by the conventional method of the drug delivery system. Microsphere technology is probably the only method that can be used for site-specific action, without causing significant side effects on normal cells.^[3]

See also

• Coacervate
• Luminescent
• Proteinoid
• Microparticles
• Microbeads

References

1. ^ Paint and Coatings Industry Magazine, January 1st, 2010 : Opaque Polyethylene Microspheres for the coatings applications
2. ^ Cosmetics and Toiletries, April 2010 Issue: Solid Polyethylene Microspheres for effects in color cosmetics
3. ^ Mithun Singh Rajput, Purti Agrawal. Microspheres in Cancer Therapy. Indian Journal of Cancer. 2010;47(4):458-468. http://www.indianjcancer.com/text.asp?2010/47/4/458/73547

External links

• Images of microspheres for various applications
• Microsphere Comparison: Glass vs. Polyethylene