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

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, wer e produced by injection molding. Molecular weight and bending strength of t he rods were measured before and after an accelerated in vitro-test (55 deg reesC, 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 degreesC. Measurements of pH and determination of l ength, volume and weight of the samples were done in 2-4 week intervals up to the 52(nd) week after incubation. A pH-drop was measured in RS or SB con taining 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 w eeks in RS or SB. A strong increase of length, volume and weight was measur ed 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 I mol% NaP in PDLLA can not be recommended, regardless of the positive effect s on pH stabilization.