Model-based interpretation of creep profiles for the assessment of polymer-mucin interaction

It is recognized that the interaction between polymer and mucin in the bioadhesion phenomenon results in rheological changes that can be described by the parameters obtained with oscillatory (dynamic) tests. This paper presents a new rheological approach, based on a stationary viscoelastic test (creep test), to describe the interaction between a mucoadhesive polymer and mucin. The interpretation of the experimental data is made possible by an automated model builder tool which is able to identify accurate compliance models. Both the number of the parameters (model order) and their values are estimated to give a measure of the complexity of the material. This approach has been here applied to study the interaction of gastric porcine mucin with three viscosity grades of a known mucoadhesive polymer, sodium carboxymethylcellulose. To this end, solutions of polymer and polymer added with mucin were tested at different concentrations and concentration ratios. By comparing the compliance models of the polymers and the polymers added with mucin, we observed an increase in the order of the model of the mixture at the lowest polymer concentration for all the three polymer grades; this effect is more pronounced for the low viscosity grade. On the other hand, an increase in mucin concentration does not result in a further increase in model order, but rather in a decrease in retarded compliance and an increase in newtonian viscosity. In our interpretation, the model order represents the number of different interactions between polymer and mucin, while the parameter values are related to their strength. The results are in accordance with those obtained in the usual dynamic tests, and indicate the suitability of the approach, that, on the other hand, allows for a deeper characterization of the interactions involved in mucoadhesion.