Swellable hydrogels and cross linking Agents - Their role in drug delivery system

Abstract

Hydrogels are cross linked polymers with the ability to swell in an aqueous medium. Cross linking in hydrogels occurs by chemical or physical means depending on the polymer properties and experimental conditions. PH-Responsive Polymers containing carboxyl groups or amino groups respond to the pH changes by changing their size in the swollen state. At low pH values, the carboxyl containing anionic polymers display minimum ionization and hence reduced hydration. Once the pH of the swelling medium rises above the pKa of the polymer, the carboxyl groups start to ionize and hydrate, which results in polymer expansion and hence higher swelling. On the contrary, cationic polymers containing amino groups (quaternary ammonium salts) display a stronger ionization and hence higher swelling at low pH. Higher cross-link density increases the surface area and pore volume whereas decreases the pore size of polymer beads and vice-versa for lower cross-link density polymer. Further, rigid cross-linker maximizes whereas flexible cross-linker lowers both thermo stability and glass transition temperature