carriers show rapid drug release and then high drug concentration gradient as they undergo quick dissolving at definite pH and definite sites. The phenomenon is beneficial for the drug absorption. (c) They improve drug absorption comparing to the other conventional nanoparticles as they turn from the solid state to the hydrogel state at certain dissolution pH and so, the bioadhesion of the carrier to the mucosa becomes greater at specific fragment. (d) The drug stability can be enhanced more effectively using pH sensitive nanoparticles.
Different materials can be used for preparation of pH-sensitive nanoparticles:
a- pH-sensitive nanoparticles prepared from polyanions:
Such as Eudragits and HPMC phthalate.
b- pH-sensitive nanoparticles prepared from publications:
Chitosan is the main cationic polymer used to prepare pH-sensitive nanoparticles. It is the second most plentiful polymer in nature after cellulose.
c- pH-sensitive nanoparticles prepared from the mixture of polyanions and polycations:
Some techniques have been improved using the advantages of both polyanions and polycations [97,108, 113-116]. Most of the nanoparticle systems related consist of the positive-charged chitosan and a negative-charged polymer, such as Eudragit [97, 115, 117], poly (g-glutamic acid) [113, 114, 116], alginate [118], methacrylic acid [119] and polyaspartic acid [120].
d- Cross-linked polymers pH-sensitive nanoparticles (nanogels)
e- pH-sensitive nanomatrix prepared from Eudragit and nano porous silica:
A novel nanomatrix system for oral administration was developed in order to overcome the main problems of the nanoparticle colloid system which are its stability and scaling up. The system was composed of the pH-sensitive Eudragit and nano-porous silica previously used in pharmaceutical processes.

Historically, gelatin and cross-linked albumin were used to prepare the first nanoparticles proposed as carriers for therapeutic applications [121, 122]. Synthetic polymers were used to prepare the nanoparticles to avoid the usage of the proteins as they may stimulate the immune system and also, to minimize the toxicity of the cross-linking agents. At first, the nanoparticles were prepared using emulsion polymerization of acryl amide and by dispersion polymerization of methylmethacrylate [123, 124]. These nanoparticles were proposed as adjuvants for vaccines. Polymethacrylate (PMA) and polymethyl methacrylate (PMMA) have been broadly used in a variety of pharmaceutical and medical applications. Specifically, PMMA Eudragit® nanoparticles can be prepared by nanoprecipitation method [125]. PMMA c