**3.2 Polymeric nanoparticle preparation**

The preparation of nanospheres and nanocapsules follows an uncomplicated basic principle, the emulsification solvent evaporation method. The guiding principles are that the drug and polymer dissolve in each other during nanosphere preparation. As a result, the drug molecules are uniformly distributed in the polymeric matrix. Examples of hydrophobic drugs with hydrophobic polymers were previously discussed in the section on nanosphere molecular orientation. However, nanocapsules are prepared from drugs and polymers that are essentially insoluble in each other. This property promotes the formation of core-shell structures to load drugs.

The preparation of both types of nanoparticles depends upon drug solubility and polymer solubility. Therefore, drug solubility is a strict limitation in the use of aliphatic polyesters, as drug solubility affects the type of nanoparticles that can be prepared because polyesters are hydrophobic polymers. This indicates that the use of water-soluble drugs could produce nanocapsules, but insoluble drugs yield nanospheres. In addition to drug solubility, nanoparticles of aliphatic polyesters can be easily prepared. This section describes the preparation of nanoparticles via emulsification methods for these polyester-controlled drug delivery systems.
