Theories of mucoadhesion

Mucoadhesion is said to occur when two surfaces, one of which is a mucous membrane, adhere to each other. Mucoadhesive materials are typically hydrophilic macromolecules containing numerous hydrogen bond forming groups, the carbomers, cellulose derivatives and chitosans being well-known examples. Mucoadhesion is said to occur in two steps, the initial contact (wetting) stage followed by the consolidation stage (the establishment of the adhesive interactions). The relative importance of each stage will depend on the individual application. For example, contact / adsorption is a key stage if the dosage form cannot be applied directly to the target mucosa, while consolidation is important if the formulation is exposed to significant dislodging stresses, such as those experienced in the oral cavity. The mucoadhesive will stay in place until dislodged, which can arise from either overhydration of a dosage form, epithelial cell or mucus turnover, the application of force that overcomes the adhesive properties of the formulation or a combination of these. Mucoadhesion can also include the retention of semisolids and liquids on a mucous membrane, and the adsorption of materials from solution, the mechanism of which differs from that of solids. Modified and new mucoadhesive materials with optimal adhesive properties have been developed that can further enhance the potential of this technology.