Poly(acrylic acid) microspheres containing beta-cyclodextrin: loading and in vitro release of two dyes

Microspheres containing poly(acrylic acid) and beta-cyclodextrin or maltose were prepared by a w/o solvent evaporation technique. The dispersed aqueous phase contained poly(acrylic acid) (800 mg) and beta-cyclodextrin or maltose (0, 200 or 800 mg). Food-grade olive oil was the continuous phase. Microsphere particle size was consistently between 15 and 25 microm, and carbohydrate content was in good agreement with that added to the dispersed phase in all cases. Two dyes, phenolphthalein and rhodamine B, having different solubility characteristics and strengths of association with beta-cyclodextrin, were selected for loading and in vitro release studies. Microspheres were loaded by soaking in a saturated propan-2-ol solution of the appropriate dye (6 h). Microsphere dye content ranged between 2.8 and 4.8 mg/g microspheres for phenolphthalein and between 2.2 and 3.7 mg/g for rhodamine B. Release studies were performed in phosphate buffer (pH 7.4; 37 degrees C). No difference in the release profile of either dye was observed between microspheres. The failure of microspheres containing beta-cyclodextrin in particular, to alter the in vitro release kinetics of either dye may be due to a number of factors and include: (i) limited cross-linking giving rise to a the rapid hydration of the polymer matrix; (ii) perturbation of the dye-beta-cyclodextrin complex by oil and/or organic solvent residues; and (iii) conformational changes/steric hindrance of the beta-cyclodextrin cavity (due to its covalent binding with PAA) resulting in a reduction in its ability to form inclusion complexes.