Development of chitosan–tripolyphosphate fibers through pH dependent ionotropic gelation

Incorporation of phosphate groups into a material may be of particular interest as they act as templates for hydroxyapatite growth through complexation with Ca2+ and thus improve the osteoconduction property. The phosphate groups can be incorporated into chitosan through ionotropic gelation with tripolyphosphate (TPP). Interestingly, the ion pairs formed through negatively charged phosphate groups with protonated amine functionality of chitosan in ionotropic gelation are expected to provide chitosan with an amphoteric character, which may facilitate protein adhesion following enhanced attachment of anchorage dependant cells than chitosan, which shows poor cell adhesion properties. In this study, chitosan–tripolyphosphate (TPP) fibers with varying phosphate contents were prepared through wet spinning in STPP baths of different pH. Gelation kinetics and gel strength of chitosan with STPP solutions of three different pH were evaluated and compared with that of NaOH solution for evaluation of their influence on nature of gelation. The solution pH of STPP baths was found to have significant control on the extent of ionic cross-linking and physico-chemical properties of the fibers. Moreover, this kinetically driven ionotropic gelation of chitosan by TPP results in low degree of crystallinity of chitosan–TPP fibers and consequently their lower thermal stability than chitosan fibers.

At STPP bath pH 8.6, the differential competitions between ionotropic cross-linking by TPP ions and neutralization through deprotonation of protonated chitosan occur, whereas, at pH 3, ionotropic cross-linking is the only way of neutralization of protonated chitosan by TPP ions.Figure optionsDownload as PowerPoint slideHighlights
► Chitosan fibers with varying phosphate contents were prepared by varying STPP bath pH.
► Degree of ionization of STPP is pH dependant owing to different pKa values.
► STPP bath pH has active control on nature of gelation of chitosan.
► At pH 3, ionotropic cross-linking is the way of neutralization of protonated chitosan.
► At pH 8.6, both deprotonation and ionic cross-linking occur simultaneously.