A rheological examination of the mucoadhesive/mucus interaction: the effect of mucoadhesive type and concentration.

The ability of mucoadhesive polymers to produce a large increase in the resistance to deformation when incorporated into a mucus gel, relative to when the mucus gel and test materials are evaluated separately at the same concentrations, has been reported in several previous studies. It has been proposed that this phenomenon, termed rheological synergism, can be used as a measure of the strength of the mucoadhesive interaction. In this investigation rheological synergism was investigated for a large range of putative mucoadhesive gels by dynamic oscillatory rheology. Changes in the storage modulus (G'), loss modulus (G"), and loss tangent (tan-delta) were found and the relative rheological synergism calculated. Rheological synergism was evident for a range of materials with known mucoadhesive properties giving behaviour between that of strongly cross-linked polymers gels and a physically entangled system. This effect was most marked with materials known to be of high mucoadhesive strength even when fully hydrated. Hence, polymers like sodium carboxymethylcellulose and high molecular weight poly(ethylene oxide) showed limited rheological synergism. When the effect of mucoadhesive concentration was investigated, relative rheological synergism occurred within a limited concentration range only, the values of which varied between materials. It was concluded that macromolecules possessing numerous hydrogen bond-forming groups and an open expanded network in the test environment gave pronounced rheological synergism, and the relevance of this to mucoadhesion studies will be investigated in further work. This study also confirmed the advantages of dynamic oscillatory rheology over simple viscosity measurement in the study of these systems.