Preparation And Properties Of Novel PH-Sensitive Carboxymethyl Chitosan-Based Hydrogels

Preparation of dialdehyde starch (DAS) was carried out at 0℃in"qausi-homogeneous phase"system by the oxidation of soluble starch with sodium periodate. The products were purified using dialysis followed by filtration, and then characterized by potentiometry and IR spectroscopy for the degree of oxidation and the structure of DAS.Different dialdehyde content of DAS can be prepared by changing the molar ratio of oxidant to starch or the pH of the reaction system. The results showed that the molar ratio of sodium periodate to starch and the pH of reaction system were important factors that considerably affected the dialdehyde content of DAS. The dialysis was used for purification and separation of DAS and the potentiometry for determination of the degree of oxidation, which were proved to be simple methods for preparation and characterization of DAS.Novel pH-sensitive carboxymethyl chitosan-based hydrogels were successfully prepared under mild conditions using dialdehyde starch (DAS) as a crosslinker. The cross-linked hydrogels with well-defined cylindrical shape were obtained by using a self-made cylindrical wax mold. The effects of the degree of oxidation and the concentrations of the cross-linker, and pH and ionic strength of release medium on the swelling ratios were investigated.The wet cross-linked hydrogels with well-defined cylindrical shape contained great amount of water, and further absorbed water to swell, hydrogels, however, considerably eroded during swelling. Thus, the wet hydrogels can not be used as such. Nevertheless, the strength of the hydrogels could be greatly improved by dryness in comparison with the wet counterparts.The results showed that the mechanical strength of the hydrogels were too weak to keep the cylindrical shape when the concentration of the DAS was lower than 2g/L. When the ionic strength of the swelling medium was greater than 0.05M or the concentration of the crosslinker greater than 15g/L, the swelling rates and the reduction in the swelling ratios of hydrogels decreased with increasing ionic strength of swelling medium and increasing concentration of the crosslinker. Hydrogels showed the highest swelling ratio in simulated gastric conditions, the lowest in the swelling medium with pH between 2 and 3, which was near the isoelectric point of the amphiphilic carboxymethyl chitosan. The pH-sensitive swelling behavior of the hydrogels could be modulated by changing the degree of oxidation of dialdehyde starch and its amount used to formulate the crosslinked carboxymethyl chitosan-based hydrogels.The release of the model drug theophylline could be sustained from the cross-linked hyrogels. The pH-sensitive swelling, erosion of the matrix and the corresponding release kinetics showed that the carboxymethyl chitosan hydrogels have a potential application for the oral stomach-retained drug delivery system or intestine specific drug delivery system with the addition of a coating on the hydrogel surface.