Comparative rheological investigation of crude gastric mucin and natural gastric mucus.

Mucus is a viscoelastic gel covering the entire alimentary tract. It represents a pre-epithelial diffusion barrier influencing the drug absorption process. In this way the rheological properties of mucus play an important role. The purpose of our investigation was to evaluate and compare the rheological properties of rehydrated dried crude porcine gastric mucin (Sigma mucin) and isolated natural porcine gastric mucus. Two different rheological approaches were used: constant shear and oscillatory shear conditions. The results obtained under steady shear conditions show that the reduction of apparent viscosity is caused by time, shear rate and temperature. With increasing Sigma mucin concentration the rheological behaviour changes from time-independent systems fitting in with the Casson model to time-dependent systems fitting in with the Herschel-Bulkley rheological model. Measurements performed under oscillatory shear conditions clearly show how the mechanical response of different concentrated systems has changed from dispersions with prevailing plastic to prevailing elastic properties. Mechanical deformation and frequency response of natural gastric porcine mucus have shown a strong gel structure with elastic properties. Comparing both systems it can be concluded that after rehydration of dried crude gastric porcine mucin, a model mucus system with rheological properties equivalent to natural mucus cannot be obtained.