Polymeric microspheres and nanoparticles are small particles formed from natural or synthetic polymers. Novel drug delivery systems based on polymeric nanoparticles and microspheres have shown great potential in enhancing drug therapeutic effects. Active pharmaceutical ingredients including small molecules and biological compounds can be encapsulated to microspheres and nanoparticles, or adhere to the particle surface for a specific drug delivery application.
Microspheres offer a myriad of opportunities to control administration of pharmaceutical compounds, facilitating the precise release of the desired amount of components at target sites in the body to quickly achieve an effective concentration and then maintain it for a given time. In addition, microspheres can guarantee protection of the compound before and after administration and manipulate the targeting of compounds by coupling recognition molecules. Therapeutic effects can be improved through exploiting these changes in pharmacokinetic behavior. One of the most important factors affecting properties of a microsphere is the choice of the type of polymer, which must be based on the physicochemical properties of the pharmaceutical compounds, the loading requirements, and the speed and time of release. The performance of microspheres is determined by the properties of polymer materials, such as the chemical composition, physical and mechanical properties, mechanism and rate of biodegradation, and toxicological profile. The most commonly used synthetic polymers include polymethyl methacrylate, glicycidyl-epoxy-methacrylate, lactic acid and glycolic acid polymers, and poly-(alkyl cyanoacrylate). Commonly used natural polymers include albumin, gelatin, collagen, and carbohydrates.
Common forms of polymeric nanoparticles are nanocapsules and nanospheres, which are further divided into polymersomes, micelles, and dendrimers. Polymeric nanoparticles can be formulated to allow precise control of multiple characteristics and are generally good delivery vectors because of their biocompatibility and simple formulation parameters. Therapeutics can be encapsulated within the nanoparticle core, entrapped in the polymer matrix, chemically conjugated to the polymer or bound to the nanoparticle surface. This enables polymeric nanoparticles to deliver a variety of payloads, including hydrophobic and hydrophilic small molecules, biomacromolecules, and more. Polymeric nanoparticles can be synthesized from synthetic or natural, and non-biodegradable or biodegradable polymers using various techniques, thus allowing for a variety of possible structures and characteristics. Synthetic nanoparticles include polylactic acid-glycolic acid copolymer (PLGA), polyvinyl imine (PEI), polycaprolactone (PCL), polyvinyl alcohol (PVA), etc. Synthetic polymers are favored for their low cost, high reproducibility, and controlled release effects. The common materials of natural polymeric nanoparticles include polysaccharides, peptides, cholesterol and cyclodextrin inclusion complexes, etc. Natural polymers are widely used because of their good biocompatibility, non-toxicity and degradability. Biodegradable polymers such as poly(D, L-lactic acid) and poly(D, L-lactic-co-glycolic acid) are commonly used to encapsulate high-load, low-soluble materials.
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