Glass-ionomer cements based on poly(acrylic acid-co-vinyl alcohol) in drug release model formulations

The release of methylene blue from glass-ionomer cements based on a commercial glass powder and partly hydrolysed poly(acrylic acid-co-vinyl alcohol) has been studied. A series of random copolymers were synthesized from acrylic acid and vinyl acetate monomers, using feed ratios of 10-90 mol% acrylic acid. The acetate group was hydrolysed using sodium hydroxide. According to proton nuclear magnetic resonance analysis, the degree of hydrolysis was about 40%. The residual Na+ content in the samples after hydrolysis was in the vicinity of 3 mmol g-1 polymeric sample. The viscosity average molecular weight of the copolymers was the region of 60,000 g mol-1 for samples with low acrylic acid content and in the region of 300,000 g mol-1 for samples with a larger fraction of acrylic acid. Cement sample discs were prepared and the porosity of the cement was determined using N2 adsorption-desorption isotherms. The specific surface area was 10 m2 g-1, and the volume of the pores (< 20 nm) 0.035 cm3 g-1. The swelling indices and the release rates in two different buffers, pH 2.0 and pH 7.4, were determined for fresh and dried samples. An acidic environment did not bring about significant swelling in the samples and release of the model agent at pH 2.0 was limited. At a physiological pH value the cements swelled in a hydrogel manner and the fractional release of model agent increased.