56-158] and methanol [159].
The polymers commonly used are biodegradable polyesters, especially poly (Ɛ-caprolactone) (PCL) [160-164], polylactide (PLA) [165, 166] and poly (lactide-co-glycolide) (PLGA) [167, 168]. Eudragit [156] can also be used as many other polymers such as polyalkylcyanoacrylate (PACA) [169-171].
Natural polymers such as allylic starch [172], dextran ester [173], were also used ,though synthetic polymers have higher purity and better reproducibility than natural polymers [174]. On the other hand, some polymers are PEG copolymerized in order to decrease nanoparticle recognition by the reticular endothelial system [159].
PNP characteristics are influenced by the nature and concentration of their components [162, 164]. The key variables determining the success of the method and affecting the physicochemical properties of the PNP are those associated with the conditions of adding the organic phase to the aqueous phase, such as organic phase injection rate, aqueous phase agitation rate, the method of organic phase addition and the organic phase to aqueous phase ratio.
Lince et al.,2008 [175] indicated that the process of particle formation in the nanoprecipitation method includes three stages: nucleation, growth and aggregation. The separation between the nucleation and the growth stages is the key factor for formation of uniform particles. Ideally, operating conditions should allow a high nucleation rate strongly dependent on super saturation and low growth rate.

Nanoprecipitation method has some advantages over other method used for preparation of nanoparticles, which include that: (1) Major advantage of this method is the use of the solvents (Acetone/Ethanol) which are considered to be less toxic than water- immiscible solvents like dichloromethane and chloroform, (2) in this method, nanoparticles are formed spontaneously with high shear , (3) Further purification is not required because of the surfactant and solvent and (4) it is a simple, fast and reproducible method which is commonly used for the preparation of both nanocapsules and nanospheres.
The solvent displacement technique can be used to formulate nanocapsules by incorporating a small amount of nontoxic oil in the organic phase. Considering the oil-based central cavities of the nanocapsules, high loading efficiencies are usually obtained for lipophilic drugs when nanocapsules are prepared.
The major drawbacks of this preparation method include that: (1) the use of this simple method [176] is restricted to water-miscible solvents, in which the diffusion rate is sufficient to produce spontaneous emulsification. In some cases, spontaneous emulsification is not observed when the coalescence rate of the formed droplets is sufficiently high and this usually occurs with some water-miscible solvents that produce a certain instability when mixed with water [177] and (2) the nanoprecipitation technique possesses poor encapsulation efficacy in the case of hydrophilic drugs, because the drug can diffuse to the aqueous outer phase during polymer precipitation [151]. The encapsulation efficiency has been increased through modifying the drug solubility by changes in pH [178-180].So that, this technique is mostly appropriate for lipophilic drugs due to the miscibility of the organic solvent with the aqueous phase.
Although, a surfactant is not necessary to ensure the formation of PNP by nanoprecipitation method, the particle size is affected by the surfactant nature and concentration [160, 164]. Furthermore, the addition of surfactants helps to maintain the stability of nanoparticle suspensions and prevents agglom