The effect of an addition of sodium hydrogenphosphate to poly(D,L)lactide--results of in vitro examinations.

Abstract

Aim of the study was to examine the influence of sodium hydrogenphosphate (NaP) on the pH value and the mechanical characteristics of degrading poly(D,L)lactide (PDLLA). Test rods of PDLLA with or without NaP amounting to 1, 10, 25 or 50 mol per 100 mol lactate, the degradation product of PDLLA, were produced by injection molding. Molecular weight and bending strength of the rods were measured before and after an accelerated in vitro-test (55 degrees C, Ringer's solution (RS)). For a long-time degradation test PDLLA-rods with or without 1 mol% NaP were placed in RS or Soerensen buffer solution (SB, pH 7.4) at 37 +/- 1 degrees C. Measurements of pH and determination of length, volume and weight of the samples were done in 2-4 week intervals up to the 52nd week after incubation. A pH-drop was measured in RS or SB containing pure PDLLA after 28 or 36 weeks respectively. Stabilization of the pH value due to admixed NaP delayed the degradation related pH drop for 8 weeks in RS or SB. A strong increase of length, volume and weight was measured in PDLLA + NaP-rods. In conclusion minimal stabilization of pH but also an increase of outer dimensions of the samples was found due to the admixture of NaP to PDLLA. Thus, an addition of substantially higher amounts than 1 mol% NaP in PDLLA can not be recommended, regardless of the positive effects on pH stabilization.