Green synthesis of graphene-hydroxyapatite nanocomposites with improved mechanical properties for bone implant materials

Herein, an in-situ synthesis of reduced graphene oxide-hydroxyapatite (RGO-HA) nanocomposites via a green reduction protocol is presented. 0.5, 1.0, and 1.5 wt% of RGO in HA are obtained by reducing the precursor solution with custard apple leaf extract. The 10x5 mm-sized discs of the RGO-HA nanocomposites are prepared and investigated for their mechanical properties. The presence of graphene in the composites is confirmed by UV–Visible, Raman spectroscopic techniques while XRD spectral analysis confirmed the α-tricalcium phosphate (TCP, JCPDS PDF No 00-009-0432) structure of HA. Nanoindentation and Vicker indentation studies revealed the enhancement in the mechanical properties of RGO-HA nanocomposites: elastic modulus, microhardness, and fracture toughness values are enhanced by 100%, 33.3%, and 64.5 %respectively for 1.5 wt% of graphene in HA matrix. The FE SEM analysis of the indentation cracks revealed that the wrapped and tucked graphene sheets in the HA matrix have hindered the propagation of the crack through pull-out, ridge growth, bridging, and deflection mechanisms. Nano scratch analysis revealed the enhanced surface roughness (120%) and coefficient of friction (102%) for 1.5 wt% composites than pure HA. The same composite showed 100% viability of HEK 293 cells at 12.5 μg/mL concentration and in vitro apatite mineralization when immersed for 7 days under SBF solution.