The presence of suitable molecular groups in the mucoadhesives leads to the formation of covalent bonds (e.g., disulfide bonds), as well as noncovalent bonds (e.g., ionic, hydrogen, and van der Waals bonds) with the mucus layer. These molecular bonds contribute considerably to good adhesion, according to the electronic and the adsorption theory. The advantage of covalent bonds may be that they are stronger than the noncovalent bonds, which may result in higher mucoadhesive forces (Bernkop-Schniirch and Steiniger, 2000). However, covalent bonds require time to be created, whereas noncovalent bonds are formed immediately as soon as the mucus and the mucoadhesive polymer come into contact. The delay time that is required for covalent bonding does not play an impeding role for such drug delivery systems, in which maintaining the delivery system at a particular location for an extended period of time (about 3 h in gastrointestinal delivery) is advantageous (Lee et al., 2000; Junginger et al., 2002), whereas a desired longer residence time at the intestinal gut surface is most likely not possible as the turnover time of mucus is estimated to be in the order of 47-270 min in the rat (Lehr et al., 1991).
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