CONTROLLING DRUG RELEASE FROM SMART HYDROGELS

MOLINA M.A.; RIVAROLA C.R.; BARBERO C.A.

Congreso; 10th International Conference on Frontiers of Polymers and Advanced Materials; 2009

Hydrogels are one promising types of polymer materials which do not dissolve in water but swell considerably in aqueous medium. Smart hydrogels are stimuli responsive materials which suffer a phase transition, with volume change, in response to changing environmental conditions such as temperature, pH, solvent composition or electrical stimuli. One of the most thoroughly studied smart hydrogels is poly(acrylic acid) (pAA). The carboxylate group in the polymer backbone can be easily converted into carboxylic acids at low pH (pKa 4.3). A water soluble cationic metal complex (tris(2,2´-bipyridine)ruthenium(II)) can be loaded into hydrogels containing acrylamide and/or acrylic acid units. The cationic complex is retained in polymer containing acrylic acid units at high pH and released at low pH. This is likely due to electrostatic interactions of the cation with the carboxylate anions, present at high pH, which are converted into neutral carboxylic acid at low pH, releasing the metal complex. Since the gel contract at low pH, the water soluble cation is also released with the water expelled from the gel. However, a strong retention of the cation inside the gels is observed when acrylamide units are present. A possible explanation is a hydrophobic interaction of the large metal complex with the polyacrylamide network. Using the counteracting electrostatic and hydrophobic interactions of probe molecules with smart hydrogel matrixes it is possible to tune the pH of maximum release away from the pKa of the ionizable group.