Nanoparticles are promising carriers due to their ability to deliver molecules, and this capability is based on their swelling behavior. One of the critical elements of nanoparticle design that affects pharmacokinetics and cellular uptake is nanoparticle morphology. The aim of this study is to compare the swelling rate of chitosanpolyacrylic acid nanoparticle at different pH and show the morphology of this nanoparticle. Chitosan-polyacrylic acid nanoparticles were synthesized. A certain amount of the nanoparticle was placed in three KCl-HCl buffers (pH=1.2), Acetate buffer (pH=5) and Tris-HCl buffer (pH=7.4), and their swelling was checked in certain time period. Also, the morphology of the nanoparticle was analyzed by SEM. Chitosan-polyacrylic acid nanoparticle was synthesized by polymerization of acrylic acid in chitosan. Among the advantages of this system, it can be mentioned that the constituent polymers are hydrophilic, non-toxic and biodegradable. Also, the investigation of swelling showed a positive pH-sensitive behavior of chitosanpolyacrylic acid nanoparticle. The results indicate a remarkable difference in the swelling rate of chitosan-poly acrylic acid nanoparticle at pH=1.2 compared to pH=5 and pH=7.4; As a result, this nanoparticle can be an appropriate carrier for the delivery of drugs and other substances to the stomach cavity
