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Drug-delivery microspheres are tiny spheres or sphere-like bodies formed by dissolving or dispersing a drug in a polymeric material, with a particle size generally in the range of 1 to 250 μm. The drug delivery principle of microspheres is to embed or adsorb the drug on the surface or inside the polymer by physical means, and the stability of the polymer ensures the slow release of the drug. Compared with ordinary formulations, microspheres can enable long-term stable release of drugs in the body, which can be used for immunization, gene therapy, tumor chemotherapy, etc. Microsphere formulation can greatly enhance the convenience and compliance of patients' medication, and has been highlighted as a promising dosage form in clinical practice.
Polymeric microspheres, including poly(lactic acid-glycolic acid) copolymers (PLGA), poly(lactic acid) (PLA) and polycaprolactone (PCL) microspheres, are highly biocompatible and are widely used to develop particulate drug delivery systems. By introducing a variety of small molecules such as coordinating groups, hormones, proteins, peptides, nucleotides, enzymes, carriers, and antibodies, they can be encapsulated into microspheres or attached to the particle surface for specific drug delivery applications such as sustained release, targeted drug delivery, and protection of APIs from premature degradation. Following administration of drug-loaded biodegradable particles, biocompatible polymers degrade in vivo by hydrolyzing the ester backbone into a non-toxic product. The rate of drug release can be regulated by selecting the right type of polymer and adjusting the encapsulation process.
The solvent volatilization method is the most widely used method for the preparation of microspheres, which can be prepared in industrial production processes using reactors and static mixers.
The principle of phase separation method for the preparation of microspheres is to add a third component, usually an organic non-solvent, to the polymer-drug-solvent system under stirring. The solubility of the polymer decreases with the addition of the third component, so that the two phases are at a specific point where the solvent and the polymer undergo phase separation to form very soft drug-carrying droplets. The system is then transferred to another organic non-solvent, allowing the microspheres to cure to obtain the final microspheres.
The spray drying method for the preparation of microspheres is a method of spraying a liquid raw excipient into a hot drying medium to convert the raw excipient into a dry powder in a spray drier.
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