Mechanical And Biocompatibility Properties Of A Graphene Nanosheet Toughened Hydroxyapatite Ceramic Composite

Hydroxyapatite (HA) is a biocompatibility ceramic with excellent bioactivity andosteoconductivity owing to its chemical composition and crystal structure similar to theapatite in human skeletal system, leading to capability of good osteoblast adhesion andproliferation, new bone growth and integration. However, the intrinsic brittleness of HA,i.e., low fracture toughness and low toughness-induced poor wear resistance, still restrictsits clinical applications. Therefore, toughening of HA with a second phase has beenextensively explored to overcome the deficiencies of pure HA.Graphene nanosheet (GNS) exhibits excellent performance in mechanical andbiocompatibility properties, and therefore GNS was proposed to toughen HA in thisresearch. Ultrasonic dispersion with the addition of surfactant was applied to disperseGNS/HA mixture powders, the compositions chosen here were pure HA,0.5wt.%GNS/HA and1.0wt.%GNS/HA. The resulting powders were consolidated by sparkplasma sintering (SPS), followed by careful analysis of phase constituents andmicrostructure, and assessment of mechanical properties and biocompatibilities of thesintered composites.X-ray diffraction and Raman spectra were used for phase analysis of the sinteredcomposites. Results showed that optimization of sintering processing parameters caneffectively avoid HA decomposition, and retain original sheet-like structure of GNSs evenafter high temperature sintering. Careful observation of fracture surface of the sinteredcomposites through scanning electron microscopy depicted GNSs are uniformly dispersedin the composite.Micro-hardness, fracture toughness and micro/nano indentation were carried out usingmicrohardness and nanoindentation testers. Results showed that the elastic modulus,hardness and fracture toughness of GNS/HA composite were improved as the increasing ofGNS addition at a maximum1.0wt.%addition in this research. The improved toughness ofthe sintered composites ascribes to the toughening mechanisms including GNS pullout, GNS bridging between the grains, crack bridging and crack deflection. An analyticalmethod of interfacial stresses was employed to investigate the interfacial behaviors of theGNS/HA composite. The interfacial behaviors such as debonding of a GNS/HA interfaceand rupturing of a damaged GNS were identified. Moreover, a modified model was alsoapplied to evaluate the effect of the number of GNS layers on the energy dissipation byGNS pullout. The simulation results sufficiently manifest that GNS pullout contributesgreatly to the improved fracture toughness of GNS/HAcomposite.Osteoblast adhesion, proliferation and mineralization were evaluated for GNS/HAin-vitro biocompatibility. Results showed that the amount of the adherent osteoblasts andosteoblast proliferation increase with the additive GNS content at a maximum1.0wt.%addition. The added GNSs could be detected by osteoblasts as extra suitable locations toadhere, and subsequently leading to the improved osteoblast adhesion on the GNS/HAcomposites. Osteoblast mineralization experiments illustrated that the surface of GNS/HAcomposite is more suitable for mineralization. It is hypothesized GNSs not only providemore preferable locations for osteoblast adhesion, but also create more nucleation sitesfacilitating apatite mineralization.Abovementioned results sufficiently illustrate that GNS is an excellent tougheningphase for HA matrix ceramic composite and the GNS/HA composite is expected to be apromising material for load-bearing orthopedic implant.