Optimizing tribological, tensile & in-vitro biofunctional properties of UHMWPE based nanocomposites with simultaneous incorporation of graphene nanoplatelets (GNP) & hydroxyapatite (HAp) via a facile approach for biomedical applications

Abstract

The present study focuses on simultaneous influence of graphene nanoplatelets (GNP) and hydroxyapatite (HAp) nanopowder on microstructural, wear, tensile and biofunctional behavior of UHMWPE based nanocomposites used in biomedical applications, with the aim to utilize GNP's mechanical strength and wear resistance, while benefitting from HAp's biocompatibility at the same time. 0.1, 0.5 and 1 wt% GNP with 10 wt% optimized concentration of HAp were added to the UHMWPE matrix through an easy two-step approach consisting of solvent mixing and ultrasonication in ethanol as a liquid media. The dried nanocomposite samples of powder were then hot pressed at an optimized temperature and pressure to ensure complete melting and flowing of the polymeric material. Tensile testing results indicated a 114% & 24% increase in elastic modulus and yield strength in the sample containing 1 wt% GNP and 10 wt% HAp, respectively, as compared to UHMWPE. However, the sample containing 0.5 wt% GNP showed greatest tensile performance with an increase of 101% and 31% improvement in elastic modulus and yield strength, which proves that the strengthening mechanism is influenced by the content of the reinforcement, especially in case of 2D reinforcing phases such as GNP. Microstructural analysis revealed the nucleating effect of GNPs on the crystalline structure of UHMWPE, to which the escalated mechanical properties could be attributed. Furthermore, the assessments disclosed the dependency of nucleating, and in consequence strengthening effect of GNPs to their concentration and apparent clustering threshold. Moreover, pin-on-disk tribological testing results showed a somewhat similar result for the coefficient of friction, which decreased by 50% with 1 wt% GNP, while the similar parameter for the sample containing 0.5 wt% GNP underwent 54% reduction. Whereas a steady decreasing pattern was observed in the case of wear rate with an 82% decrease in the sample containing 1 wt% GNP, coming to a conclusion that GNP is much more effective in improving wear properties rather than in mechanical strengthening. Biological examinations also demonstrated that HAp promises biocompatibility, osteoconductivity and the elimination of adverse cellular response, while cell adhesion was still dependent on the concentration and was affected adversely with increasing GNP. This destructive biological effect of GNPs was seemingly attributed to the functional groups of the material, or to the inherent edge-shaped structure by means of FTIR, wettability and compaction analyses.