The chain entanglement and the molecular bonding that occur between the mucus and the mucoadhesive lead to changes in the rheological behavior of the two materials (Rossi et al., 2001). Since changes in the rheological properties reflect the degree of interaction between mucus and mucoadhesive, rheological methods constitute a common way to evaluate the strength of mucoadhesion. Mucoad-hesive systems with a high elastic component showed good mucoadhesiveness (Tamburic and Craig, 1997). Moreover, a high viscosity and viscoelasticity of the mucus-mucoadhesive system indicates improved cohesiveness and resistance to deformation (Madsen et al., 1998). A number of authors (Huang et al., 2000; Madsen et al., 1998; Rossi et al., 1994; Caramella et al., 1994) found experimentally that the viscosity of the mucus-mucoadhesive system can be larger as the sum of the separate viscosities. This phenomenon is called "rheological synergism." High rheological synergism indicates extensive chain entanglement (diffusion theory) and thus good mucoadhesiveness.
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