Interactions between bioadhesive poly(acrylic acid) and calcium ions

Polycarbophil (PC) is a weakly crosslinked poly(acrylic acid) used for bioadhesive delivery systems. Complexation of calcium ions by this polymer depends on the accessability of carboxylate functions in the polymer. Therefore, the calcium binding capacity increased linearly with the degree of neutralisation. Crosslinking rendered 20% of the carboxyl inaccessable to calcium whereas in water-soluble non-crosslinked poly(acrylic acid) (PAA), all carboxylate groups bound calcium. Addition of ionic substances reduced the fraction of calcium ions bound due to a dehydration of the polymer and a competition between the ions for binding. In a physiological buffer, PC chelated maximally 80% of the total calcium concentration, and water-soluble PAA bound 95%. In comparison to calcium chloride solution, the binding constant decreased from 51400 l/mol to 1800 l/mol in Tyrode's solution, moreover, the number of binding sites in the polymer was reduced. A dispersion of PC in water yielded swollen particles with a size of ca. 2.5 mu m, depending on the degree of neutralisation and on the concentration of electrolytes, especially calcium ions. Addition of electrolytes caused a decrease of the particle size due to dehydration of the polymer. The reduction was more pronounced with calcium. Chelation of calcium by PC could be an explanation for the increase in bioavailability of drugs observed with bioadhesive PC delivery systems. The function and structure of epithelial tissue could be affected by chelation of extracellular calcium.