(A-36) Mechanical and biocompatibility investigation of magnesium boron nitride nanocomposites as a potential orthopedic implant material

Introduction:: The most prevalent inert metals in orthopedic implants such as titanium alloys can cause stress shielding, possible infection, and future fracture of the fixation device, which often require a second revision surgery. Magnesium (Mg) has been under investigation as a promising alternative material to inert metal alloys for orthopedic fixation devices due to a mechanical behavior highly similar to bone, a favorable biocompatibility, and a high level of biodegradability in vivo. The biodegradability of Mg is a particular advantage as it would permit the implant to degrade at a controlled rate such that bone regeneration can occur unimpeded, eliminating risks of revision surgeries. However, the low mechanical strength and low corrosion resistance of Mg in physiological environments present a significant challenge for the development of Mg-based devices for orthopedic applications. A potential solution to these limitations is the incorporation of nanoparticles to reinforce the Mg matrix (Mg-BN), increasing its strength. We selected boron nitride as a candidate for Mg reinforcement due to the physiological role of boron as a micronutrient with an essential role in osteogenesis and bone maintenance. Boron has also been previously demonstrated to stimulate a pro-angiogenic effect in wound healing by upregulating angiogenesis markers, upregulating growth factors and cytokines, and facilitating pre-osteoblast differentiation by activating genes associated with osteogenesis when incorporated into other metals such as titanium. A comprehensive of evaluation of Mg-BN as an orthopedic implant candidate is needed to facilitate the discovery of biodegradable implant materials.


Materials and Methods:: In this work, the effect of adding boron nitride (BN) nanoparticles (0.5 and 1.5 Vol.%) on the mechanical properties, corrosion behavior, and biocompatibility of Mg-based nanocomposites was investigated compared to 0%BN Mg. Nanocomposites were fabricated using Mg powder with >98.5% purity and 60-300 µm particle size and BN nanoparticles produced by microwave sintering. To evaluate material synthesis, the microstructure was visualized using scanning electron microscopy (SEM) of etched and sonicated coupons. The microhardness was tested using the Micro Vickers hardness tester and compression tests were performed to assess the mechanical properties of the material. The porosity was also measured using a density meter and compared with the grain sizes computationally derived using the microstructure results. To characterize the corrosion resistance of the nanocomposites, electrochemical tests were performed in a simulated body fluid (SBF) using potentiodynamic curves and immersion tests were used to measure mass loss over time in SBF. Further, the hydrophilicity of the surface of the nanocomposites was measured using a contact angle goniometer for SBF and water. Because the biocompatibility and biodegradability of Mg is the primary advantage of this material, biocompatibility tests were performed in MC3t3 mouse pre-osteoblasts using coupon-leached media and the Prestoblue cytotoxicity assay. SEM was also used to examine the microstructure of the nanocomposites before and after corrosion along with element-dispersive X-ray spectroscopy (EDS) to detect elements present on the surface. To further characterize corrosive behavior, X-ray diffraction (XRD) analyses were performed, and compounds present on the material surface post-corrosion were determined.


Results, Conclusions, and Discussions:: Nanocomposites were comprehensively assessed using microstructure analyses, microhardness, compression tests, in vitro corrosion, contact angle, and cytotoxicity tests. The microhardness and strength determined using a compression test increased in nanocomposites compared to 0% BN Mg, indicating that particle reinforcement was successful. A significant increase in the electrochemical corrosion rates of Mg-BN nanocomposites was also detected while, interestingly, immersion tests demonstrated an increase of corrosion in Mg-BN nanocomposites (Figure A). Analysis of the microstructure indicated no significant difference in particle grain sizes between nanocomposites although porosity increased with an increased vol% of BN (Figure B). Crystalline surface byproducts post-corrosion was detected and identified via SEM, EDX, and XRD, and the results indicated that oxidation occurred at an increased rate in nanocomposites compared to 0% BN Mg. Further, BN appeared to remain on the surface of the nanocomposites post-corrosion as detected using XRD. Biocompatibility assessments illustrated that the incorporation of BN nanoparticles had no significant impact on the cytotoxicity of Mg and that samples were hydrophilic based on contact angle in both SBF and water (Figure C). These results confirm that the addition of BN nanoparticles to the Mg matrix increases strength without influencing cytotoxicity in vitro. Further investigation into the chemical behavior of nanocomposites in physiological environments is needed to determine the potential impact of corrosive byproducts. We are additionally proceeding to test surface treatments and formulation methods that would increase the viability of these materials in vivo. Further, qPCR analysis of genes related to osteogenesis will be performed to ascertain whether osteogenic differentiation is occurring due to the presence of boron. These data provide significant considerations for the usage of Mg-BN nanocomposites as an implant material.


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