Influence of processing conditions on the release of sodium fluoride from oligomeric L-lactic acid matrices.

Matrices were prepared in the shape of rods with a diameter of 4 mm and a length of 10 mm. They were composed of oligomers of L-lactic acid (Mw 2550-11730 Daltons) and micronized sodium fluoride particles. Sodium fluoride concentration was kept in the range 5-60%. Matrices were prepared by melting the mixture of L-lactic acid oligomer and sodium fluoride. Two types of products according to the applied cooling rate were prepared and evaluated for release characteristics in vitro. Sink conditions of isotonic phosphate buffer solution pH = 7.4 at 37 degrees C were maintained throughout the experiment. The concentration of dissolved sodium fluoride was determined by ion-selective electrode. It was documented that the release rate of sodium fluoride is strongly dependent on the molecular weight of oligomer. The effects of sodium fluoride concentration and cooling rate of melted mixture were evaluated by ANOVA, separately for each oligomer. The effect of sodium fluoride concentration was significant at each oligomer studied, while the influence of cooling rate was significant with higher molecular weight oligomers. Moreover, this effect is ambiguous and dependent on the capability of oligomers to form crystalline phases and on the form and geometric arrangement of this phase.