Characterization of chitosan hydrochloride--mucin rheological interaction: influence of polymer concentration and polymer:mucin weight ratio.

The aim of the present work was to investigate the influence of polymer concentration and polymer:mucin weight ratio on chitosan--mucin interaction, assessed by means of viscosimetric measurements. Two hydration media, distilled water and 0.1 M HCl, were used. Chitosan solutions were prepared at concentrations greater than the characteristic entanglement concentration and mixed with increasing amounts of porcine gastric mucin. Viscosity measurements were performed on the polymer--mucin mixtures and on polymer and mucin solutions having the same concentrations as in the mixtures. The flow curves were fitted according to a modified form of Cheng--Evans equation in order to obtain the relevant model parameters: low shear viscosity eta(0) and high shear viscosity eta(infinity), indexes of the sample structure at rest and upon high shear, respectively. The formation of chitosan--mucin interaction products was determined on the basis of the changes in eta(0) and eta(infinity) of the mixtures as a function of polymer:mucin weight ratio. Rheological synergism parameter was also calculated. The results obtained suggest that two different types of rheological interaction occur between chitosan and mucin in both media, depending on polymer concentration and polymer:mucin weight ratio: one is characterized by a minimum in viscosity and occurs at higher polymer:mucin weight ratio, the other one produces a positive rheological synergism and is observed in presence of an excess of mucin. Only the last one causes a 'strengthening' of the mucoadhesive interface and it is responsible for the mucoadhesive joint. This hypothesis is confirmed by tensile stress measurements performed on HCS solutions in presence of mucin dispersions at different concentrations.