Chitosan gel beads were prepared using an in-liquid curing method by ionotropic crosslinking or interpolymer Linkage with tripolyphosphate (TPP) or polyphosphate (PP). The ionic interaction of chitosan with TPP or PP is pH-dependent due to the transition of "ladder-loop" complex structures. Chitosan gel beads cured in a pH value lower than 6 of a TPP solution was a controlled homogeneous ionic-crosslinking reaction, whereas chitosan gel beads cured in a lower pH PP solution was a nonhomogeneous interpolymer complex reaction due to the mass-transfer resistance for the diffusion of macromolecular PP. According to the results of FTIR and EDS studies, it was suggested that significantly increasing the ionic-crosslinking density or interpolymer linkage of a chitosan-TPP or chitosan-PP complex could be achieved by transferring the pH value of curing agent, TPP or PP, from basic to acidic. The swelling behavior of various chitosan heads in acid medium appeared to depend on the ionic-crosslinking density or inter-polymer linkage of the chitosan-TPP or chitosan-PP complex, which were deeply affected by the in-liquid curing mechanism of the chitosan gel beads. By the transition of the in-liquid curing mechanism, the swelling degree of chitosan-TPP or chitosan-PP beads was depressed and the disintegration of chitosan-TPP or chitosan-PP beads did not occur in strong acid. The drug-release patterns of the modified chitosan gel beads in simulated intestinal and gastric juices were sustained for 20 h. These results indicate that the sustained release of anticancer drugs could be achieved due to the variation of the reaction mechanism of a chitosan-polyelectrolyte pH-dependent ionic interaction. (C) 1999 John Wiley & Sons, Inc.
