A SURFACE-ENERGY ANALYSIS OF MUCOADHESION .2. PREDICTION OF MUCOADHESIVE PERFORMANCE BY SPREADING COEFFICIENTS

Mucoadhesion refers to the adhesion of polymeric materials to mucosal tissues. Some mucoadhesive polymers show appreciable binding to mucus even when hydrated and in the presence of an interstitial aqueous medium. The mucoadhesive performance (i.e. binding strength) is expected to depend on the interfacial energy thermodynamics in a three phase (solid-liquid-solid) system. According to known theories, such a system can be described in terms of spreading coefficients which are given by the free surface energy of each phase. Dispersion (alpha) and polar (beta) surface energy parameters were available from previously reported contact angle measurements of captive air/octane bubbles on polymeric hydrogels and pig intestinal mucosa immersed in aqueous test media. It was found that individual spreading coefficients, as calculated from these surface energy parameters by the geometric mean equation, can be combined to one single parameter, called combined spreading coefficient S(c). The latter is defined by the geometric mean of the polymer spreading coefficient and the Griffith fracture energy. This criterion provided a good correlation between predicted and measured mucoadhesive performance under various experimental circumstances. In its present form, this approach is based only on dispersion and polar surface energy components, without taking into account other forces due to ionic interactions, hydrogen bonding or acid-base interactions and eventual polymer chain interpenetration. Nevertheless, correct predictions were obtained. It is therefore concluded that the formation of a mucoadhesive bond is primarily governed by the aforementioned surface energy effects and spreading processes. The surface energy concept provides useful information for the search for better mucoadhesive materials and the identification of favored target sites in the human body for mucoadhesive drug delivery systems.