Calcium-induced gelation of alginic acid and pH-sensitive reswelling of dried gels.

Calcium-induced alginate gelation was examined in terms of water content changes. The gelation was accompanied with considerable water loss, which reached about 50-60% (w/w) reduction in the fully-cured state. The calcium association with the polymer was strong enough to maintain the shape of fully-cured beads in distilled water and the amount of the ions associated with the alginate used in the present study was 1.6 × 10 -3mol/g of polymer. The diffusion coefficients of several model compounds having molecular weights ranging from 122 to 1050 were estimated as a function of the polymer concentration in the fully-cured beads.The swelling property of dried gel particles prepared from fully-cured hydrogels was of interest : the particles remained unchanged in distilled water or acidic medium (pH 1.5 KCl-HCl) but swelled rather rapidly in pH 7.0 phosphate buffer to a size greater than their original size before being dried. Such a pH-sensitive swelling property could be advantageous for orally-administered drug vehicles, especially when an acid-sensitive drug is incorporated in the gel.