A brief introduction of microsphere

What are microspheres?
Microspheres 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. However, microsphere is different from microcapsules for the latter one has a distinct outer layer which lead to a different release dynamic process.
Generally in pharmaceutics industry, microspheres can be applied to encapsulate drugs, in order to attain sustained-release, protect sensible drugs or special delivery methods, etc. Drugs are mainly dispersed in the entire spheres and minority embedded onto the surfaces. Figure 1 shows some typical microspheres by microscope.
Fig.1 typical microspheres
What are the materials of microspheres?
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.
Biodegradable polymer microspheres can work as miniature time release capsules for parenteral drugs, thus have special values within pharmaceutical and biotechnology industries. Among the polymers - PLGA and PLA – their safety has been evaluated completely and they are approved by FDA. Many seasoned products emerged are based on these two polymers.
What is crucial to manufacturing microspheres?
Production process sometimes for injectable drugs requires an aseptic manufacturing process and an accurate selection of the microsphere sizes, typically several hundreds nanometers up to 250μm. Formulation of polymer microspheres is a complex process where classification of the correct size microspheres remains the most difficult task. Typically with microsphere processes, large liquid volumes with a small ratio of suspended solids have to be handled. Thus some processes require a scalping pre-filtration step in order to reduce the liquid volume and eliminate the over-sized microspheres. Suspended microspheres obtained from various micro encapsulation processes then require unique handling that differ from a typical filtration and drying operation. Following the synthesis stage, microspheres require being washed, classified by size, filtered then dried under appropriate conditions to gain the final free flowing injectable or inhalation microsphere product. Microspheres are random in size and need to be filtered and classified into the micron size range desired before drying.
n the drying process usually freeze-drying is adopted for it can minimize the side effects of drying on the drugs and microspheres. Merits such as protecting heat-sensible drugs, can be re-dispersed for injection readily and fine appearance of products render freeze-drying rife in pharmaceutics.