Effect of biodegradation on thermo-mechanical properties and biocompatibility of poly(lactic acid)/graphene nanoplatelets composites

Two types of graphene nanoplatelets (GNP-M and GNP-C) were incorporated in PLA by melt-blending at 0.25 wt.% loading, and the resulting composites subject to hydrolytic degradation for 6 months in phosphate-buffered saline (PBS) at 37 °C. The materials were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), size exclusion chromatography (GPC-SEC), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile testing, creep-recovery testing, and biocompatibility assays. After two months degradation, all materials presented a low decrease in molecular weight (about 10%), while after six months the decrease was higher than 85%. For this degradation time, temperatures of onset of intense thermal degradation decreased by about 10 °C for all samples. Both fillers were able to improve the mechanical properties of PLA, and to reduce the decay of its mechanical performance after 6 months biodegradation. Unfilled PLA showed a 10-fold decrease in toughness (AUC) after 6 months degradation, while toughness was only reduced by 3.3 and 1.7-fold, respectively, for the GNP-M and GNP-C composites. In addition, the composites had stable behaviour under cyclic creep-relaxation testing, while PLA exhibited significant cumulative permanent stain and ruptured after only 4 cycles. Comparing with PLA, the GNP-based composites presented similar human foreskin fibroblasts (HFF-1) adhesion and growth at the surface until 72 h, and did not release toxic products after the degradation period.