All processes mentioned above require purification except for the emulsion polymerization process in some cases, when no surfactant and no organic solvents are used to produce nanoparticles. In general, the organic solvent is removed under reduced pressure and the resulting particles are purified by ultracentrifugation, ultrafiltration, gel chromatog-raphy, dialysis, or a combination of these methods. The choice of the appropriate purification method depends on the release properties of the nanoparticles. Inappropriate purification methods can lead to the loss of biologically active agent. Stabilization of the nanoparticles is usually achieved by lyophilization using a cryoprotector (such as mannitol) to prevent agglomerations .
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