Characterization of poly-d-mannuronate and poly-l-guluronate block fractions from sodium alginate and preparation of hydrogels with poly(vinylalcohol).

Sodium salts of homopoly-d-mannuronic acid (MM) and of homopoly-l-guluronic acid (GG) from sodium alginates were characterized by NMR relaxometry. Determination of NMR spin-lattice and spin-spin relaxation times of water proton in homopolymeric block solutions and hydrogels indicated differences in homopolymeric blocks tertiary structure. Hydrogels of MM or GG blocks and poly(vinyl alcohol) (PVA) were prepared by freeze-thawing cycles method; their swelling properties and sensitivity to pH stimuli were assayed in control delivery of a model drug. MM/PVA hydrogels show better metformin release characteristics than GG/PVA hydrogels. It was found that release of the drug at pH 1.2 from hydrogels was minor to 5%. At the release equilibrium, 60 and 55% of the drug encapsulated were release from MM/PVA and GG/PVA hydrogels, respectively. Also, the release of metformin from hydrogels was studied by H NMR spectroscopy showing that 40 and 36% of drug were released after 4 h from MM/PVA and GG/PVA hydrogels, respectively.