Rheological properties and drug release characteristics of pH-responsive hydrogels

Rheological properties, blend compatibility, and gel-forming capacity of carbopol 940 (CP-940), sodium alginate (NaAlg), and guar gum (GG) have been studied. These matrices have been used in delivery of timolol maleate for ophthalmic applications. Aqueous solutions of CP940, NaAlg, and GG in concentrations between 0.1 and 1% (wt/vol) acid their blends have been prepared. In situ gel forming polymeric solutions have shown an increase in viscosity upon exposure to specific pH, ions, and temperature of the eyeball. Blend miscibility was studied by calculating polymer-polymer interaction parameters using viscosity data. Rheological properties viz., torque, viscosity, shear stress, and shear rate were obtained using a Brookfield rheometer. Viscosities of polymer solutions were obtained by a Schott Gerate viscometer. Rheological data were analyzed using Bingham, Casson Standard, and Casson Chocolate equations. The hydrogels were subjected to ex vivo release studies on timolol maleate through the excised bovine cornea using a modified Franz diffusion cell. Results were compared with the conventional drug solution. The release could be extended when the drug is incorporated into hydrogel-forming solution. (C) 2004 Wiley Periodicals.