In this study, a novel chitosan-based polymeric network was synthesized by crosslinking with a naturally occurring crosslinking agent-genipin. The results showed that the crosslinking reactions were pH-dependent. Under basic conditions, genipin underwent a ring-opening polymerization prior to crosslinking with chitosan. The crosslink bridges consisted of polymerized genipin macromers or oligomers (7 ϳ 88 monomer units). This ring-opening polymerization of genipin was initiated by extracting proton from the hydroxyl groups at C-1 of deoxyloganin aglycone, followed by opening the dihydropyran ring to conduct an aldol condensation. At neutral and acidic conditions, genipin reacted with primary amino groups on chitosan to form heterocyclic amines. The heterocyclic amines were further associated to form crosslinked networks with short chains of dimmer, trimer, and tetramer bridges. An accompanied reaction of nucleophilic substitution of the ester group on genipin by the primary amine group on chitosan would occur in the presence of an acid catalysis. The extent in which chitosan gels crosslinked with genipin was significantly dependent on the crosslinking pH values: 39.9 Ϯ 3.8% at pH 5.0, 96.0 Ϯ 1.9% at pH 7.4, 45.4 Ϯ 1.8% at pH 9.0, and 1.4 Ϯ 1.0% at pH 13.6 (n ϭ 5, p Ͻ 0.05). Owing to the different crosslinking extents and different chain lengths of crosslink bridges, the genipincrosslinked chitosan gels showed significant difference in their swelling capability and their resistance against enzymatic hydrolysis, depending on the pH conditions for crosslinking. These results indicated a direct relationship between the mode of crosslinking reaction, and the swelling and enzymatic hydrolysis properties of the genipin-crosslinked chitosan gels. The ring-opening polymerization of genipin and the pH-dependent crosslinking reactions may provide a novel way for the preparation and exploitation of chitosanbased gels for biomedical applications.
ABSTRACT:Hydrogel membranes prepared from polyelectrolyte complex (PEC) have been used for repair of wounds and controlled antibacterial release. A simple method, based on homogenizing interpolyelectrolyte complex, has been developed to prepare a chitosan-alginate sponge with high stability. The spongelike chitosan-alginate hydrogel can be used as a wound dressing for the sustained release of silver sulfadiazine (AgSD) in a controlled way. In this study, we evaluated the effect of electrolyteic properties of chitosan and alginate on the characteristics of the prepared chitosan-alginate PEC. All types of the spongelike chitosan-alginate hydrogels exhibited superabsorbent properties. However, only the chitosan-alginate hydrogel prepared by the interpolyelectrolyte complex of alginate with low pH of chitosan, and that prepared by the interpolyelectrolyte complex of chitosan with high pH of alginate, can keep their stability after swelling in PBS solution. FTIR analysis suggests that the protonated amino groups on chitosan and the ionized carboxylic groups on alginate should be responsible for the formation of a stable ladder-type of chitosan-alginate PEC. Ionic crosslinking is helpful to increase the stability of the loop-type of chitosan-alginate PEC. The release of AgSD from chitosan-alginate PEC sponges could be controlled by the variation of ladder-loop structural transition of chitosan-alginate PEC and the ionic crosslinking of the chitosan-alginate complex. The antibacterial ability of AgSD-incorporated PEC sponges was examined in agar plate against Pseudomonas aeruginosa and Staphylococcus aureus. The result suggests that the PEC sponges containing antimicrobial agents should effectively suppress bacterial proliferation to protect the wound from bacterial invasion.
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