By dipping chitosan films into saturated NO 2 -glacial acetic acid solution, the hydromethyl groups on the film surface could be oxidized to carboxyl groups and the blood compatibility of the films improved. Fourier transform infrared (FTIR) spectra indicated the presence of many À ÀCOOH and À ÀCOO À groups on the modified membrane surface. Scanning electron microscopy (SEM) showed that the surface of the modified membrane was rough, as compared with the chitosan film, which possessed a smooth surface. In the oxidation process, with increased dipping time of the films in saturated NO 2 -glacial acetic acid solution, the tensile strength of the films decreased slowly initially, and rapidly 10 h later. The swelling ratio of the modified chitosan film increased obvious noticeably as the degree of oxidation of the film increased. All antithrombosis and hemolysis tests and blood cell morphology observation with SEM revealed that the blood compatibility of modified chitosan membranes is superior to that of chitosan films.

ABSTRACT:The hydroxymethyl groups of 2-amino-2-deoxy-d-glucose units in chitosan were selectively oxidized to carboxyl groups with NO 2 gas to form 6-carboxy-chitosan in aqueous 0.5 mol/L acetic acid. In optimal conditions, the 52.5% hydroxymethyl groups in chitosan could be converted into carboxyl groups. When the pH value of the solution is lower than 4.0 or more than 5.4, 6-carboxy-chitosan has common characteristics of a polycation or polyanion electrolyte, respectively, and when the pH value of the solution is 4.0 -5.4, 6-carboxy-chitosan has common characteristics of an amphoteric polyelectrolyte. 6-Carboxy-chitosan has same macromolecular backbones as chitosan, and its isoelectric point (pI) value is 4.9.