Release of sodium fluoride from poly (L-lactic acid) implants characterized by thermal history

The method of injection molding of the melt of poly(L-lactic (PLLA), Mw 11 730, containing 7.5% of particles of sodium fluoride smaller than 1 microm was employed to prepare cylinders of a diameter of 2 and 10 mm in length. These matrices with an insoluble active ingredient with an amorphous structure of the carrier obtained by rapid cooling of the melt were annealed at temperatures from 70 to 140 degrees C in the medium of a hot-air drying plant and liquid paraffin. It has been found that crystallization of PLLA manifested by the rapidity of release of sodium fluoride does not take place at a temperature of 70 degrees C; at a temperature of 75 degrees C the effect of annealing of polyester matrices on the decrease in the rapidity of sodium fluoride release was already perceptible. Also in annealing of matrices at temperatures of 120 and 140 degrees C an identical deceleration of sodium fluoride release as that at 75 degrees C took place; the rapidity of the development of an effective crystalline phase was higher at higher temperatures. The method of interval cooling with the first stage of slow cooling and the second stage of rapid cooling of the melt of PLLA with 15% sodium fluoride fixed the structure achieved till the moment of the change in the rapidity of cooling. This revealed the temperature range of the development of the effective crystalline phase from the viewpoint of sodium fluoride release lying between 92 and 122 degrees C.