VISUALIZATION STUDIES OF THE MUCOADHESIVE INTERFACE

Aim of this study was to prove the occurrence of intermixing ("interpenetration") at the interfacial zone between the mucoadhesive polymer Polycarbophil and intestinal mucus by (electron)-microscopical examination. As both phases are essentially hydrogels, various cryotechniques were tried to achieve dehydration of the samples without destroying the polymeric network structure of the native gels. Freeze fracture replication or sputter-coating of freeze-dried samples for transmission or scanning electron microscopy did not allow to identify the two hydrogel phases by characteristic network structures. Best results were obtained with plastic sections of freeze substituted samples. With light microscopy, mucus glycoproteins could be identified unambiguously by specific histochemical reactions. The mucoadhesive interface appeared as an irregular borderline with many coves and invaginations, but was sharp rather than hazy. There was no evidence for intermixing between mucus and mucoadhesive hydrogel to occur in the mu-m-range. Interpenetration of free polymer chain ends, however, mat still be possible in the nm-range.