Synthesis and characterization of biodegradable magnesium zinc alloy reinforced with carbon nanofiber for potential medical implant applications

Abstract

Magnesium (Mg) is becoming a potential material to replace conventional stainless-steel and titanium alloy medical implants. However, non-adequate mechanical stability of Mg could lead to premature failure and corrosion. Carbon nano fiber (CNF) has revolutionised the composite industries and continue to show a great promise in improving the mechanical properties and corrosion resistance of Mg. Thus, the primary purpose of this study is to develop Mg composites reinforced with CNF using a powder metallurgy method. The significant factors that influenced the process design were screened using two-level factorial design. Four factors; the percentage of CNF (0.1 - 2.0%), compaction pressure (100 - 400 MPa), sintering temperature (300 - 500°C), and sintering time (1 - 4 hrs), were analysed for three responses, namely elastic modulus, hardness, and weight loss. The significant factors were further subjected to the Box-Behnken design (BBD) of response surface methodology to obtain the optimum parameters. Selected specimens were subjected to X-ray diffraction (XRD), attenuated total reflection-Fourier transform infrared (ATR-FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM), hydrophobicity, thermogravimetric (TGA), X-ray photoelectron spectroscopy (XPS) and biocompatibility analyses. The results show that the mechanical properties and corrosion resistance of the composites were optimum at 2% CNF, 400 MPa of compaction pressure, and 500°C of sintering temperature with a significant effect at P <0.05 for all variables except the sintering time (P >0.05). The elastic modulus and hardness of the composites peaked at 4685 MPa and 60 Hv, respectively. The nanomechanical analysis also revealed that the highest elastic modulus (766 MPa), hardness (539 MPa), and stiffness (575 N/m) were achieved at the same condition. After three days of immersion in phosphate buffered saline, the minimum and maximum weight loss were recorded at 54% and 100%, respectively. The CNF significantly improved the surface morphology of Mg-Zn/2.0%CNF with average roughness (Ra) of 19.16 ± 3.4 nm, high hydrophobicity (> 100°) and good oxidation behaviour. Moreover, the controlled releases of Mg2+ and Zn2+ ions were achieved too. The XRD analysis verified the presence of Mg (35 - 80 9), Mg-Zn alloy (35 - 40 9) and CNF (53 9) in the composite. The Raman spectroscopy analysis confirmed the presence of CNF in the Mg composites for all specimens. Besides, biocompatibility test confirmed the improvement of osteoblast cells viability and the composites were found non-toxic to the cells (> 70% viability). Further study on the optimisation using BBD showed that all factors significantly contributed towards high mechanical strength (5409.7 MPa of elastic modulus and 60.7 Hv of hardness) and corrosion resistance (up to 52%). The presence of Mg-Zn solid solution has improved the nanomechanical properties of the composites when 1.8% of CNF was compacted using 425 MPa at 500°C sintering temperature that resulted in the records of 832 MPa elastic modulus, 549.7 MPa hardness and 605 N/m stiffness. Hydrophobicity and Ra were the major contributing factors that produced high corrosion resistance and controlled ions release. The Mg-Zn/1.8%CNF has also successfully stimulated cell growth with nontoxic properties towards osteoblast cells. This work concludes that the optimum conditions and processing techniques for the fabrication Mg composite were found at 1.8% of CNF, 425 MPa of compaction pressure, and 500°C of sintering temperature.