most common three methods used to prepare the nanoparticles are: (1) dispersion of preformed polymers; (2) direct polymerization of monomers using classical polymerization reactions; and (3) ionic gelation or coacervation of hydrophilic polymers. However, other methods like supercritical fluid technology [140] and particle replication in non-wetting templates (PRINT) [141] can be also used to prepare nanoparticles.
Polymeric nanoparticles (PNPs) can be prepared from preformed polymers through several methods such as solvent evaporation, salting-out, dialysis and supercritical fluid technology which involving the rapid expansion of a supercritical solution or rapid expansion of a supercritical solution into a liquid solvent. In contrast, PNPs can be directly synthesized through the polymerization of monomers using a variety of polymerization techniques like mini-emulsion, micro-emulsion, surfactant-free emulsion and interfacial polymerization.

The choice of preparation method is made on the basis of a number of factors such as the type of polymeric systems, the area of application, size requirement, and the drug to be loaded. Since, method of preparation mainly affect the properties of produced nanoparticles, it is highly advantageous to have preparation techniques at hand to get PNPs with the required properties for a particular application.

The term nanoprecipitation refers to a quite simple processing method for the fabrication of polymeric nanoparticles. The level of interest in nanoprecipitation waned for some decades, and the method regained recognition in the 50’s as a means of preparing colloids for stabilizing pigments [142], as well as industrially important components in paints, lacquers, and other coatings [142], While it had already been reported at least as early as the 1940’s as a way for isolation of purified analytical samples of synthetic polymers , nanoprecipitation regained a heightened level of patent interest in the 1950’s and 60’s this time as a cost effective method for purifying synthetic polyolefins [142].
In the late 80’s and early 90’s, Fessi et al.,1989 [143] patented the nanoprecipitation method as a procedure for the preparation of eligible colloidal systems of a polymeric substance in the form of nanoparticles [143].
Nanoprecipitation is also called solvent displacement method or interfacial precipitation method [143-148] .It depends on the precipitation of a preformed polymer from an organic solution and the diffusion of the organic solvent in the aqueous phase either in the presence or absence of a surfactant [143, 149-151].
The main principle of this technique is based on the interfacial deposition of a polymer after displacement of a semi polar solvent, miscible with water, from a lipophilic solution. Rapid diffusion of the solvent into non-solvent phase results in the decrease of interfacial tension between the two phases, which increases the surface area and causes the formation of small droplets of organic solvent [143, 152].
Nanoprecipitation system composed of three basic components: the polymer (synthetic, semi synthetic or natural), the polymer solvent and the non-solvent of the polymer. Organic solvent (i.e., Ethanol, acetone, hexane, methylene chloride or dioxane) which is miscible in water and can be easily removed by evaporation is chosen as polymer solvent. Because of this reason, acetone is considered to be the most commonly used polymer solvent in this method [143,153, 154].
Sometimes, it consists of binary solvent blends, acetone with small volume of water [155], blends of acetone with ethanol [156-158] and methanol [159].
The polymers commonly used are biodegradable polyesters