Hollow silica reinforced magnesium nanocomposites with enhanced mechanical and biological properties with computational modeling analysis for mandibular reconstruction

Bioresorbable metals, particularly magnesium (Mg) alloys, are promising biomaterials for orthopedic and maxillofacial applications due to their biocompatibility and bioresorbability. Mg's elastic modulus is similar to cortical bone, reducing the stress-shielding effect. Previous studies have demonstrated Mg's osteoconductive properties and its ability to accelerate bone regeneration. However, pure Mg suffers from low strength, poor formability, low fatigue resistance, and rapid degradation in physiological environments. The addition of nano-reinforcements can overcome these limitations. Various metal oxide nanoparticles (NPs), including cerium oxide (CeO₂), have been successfully incorporated into Mg matrices to improve mechanical and corrosion properties. Silica (SiO₂), known for its biocompatibility and use in bioglass, is another potential reinforcement. Hollow SiO₂ NPs offer a high surface area and are extensively researched for biomedical applications. This study aims to investigate the effect of adding hollow SiO₂ NPs on the microstructural, mechanical, degradation, and biocompatibility properties of pure Mg, addressing the lack of previous research on this specific combination.

The introduction provides a comprehensive review of the literature on bioresorbable metals, particularly magnesium alloys, and their applications in orthopedic and maxillofacial surgery. It highlights the advantages of magnesium, such as its biocompatibility and elastic modulus similar to bone, while acknowledging its limitations, including low strength and rapid degradation. The literature review cites numerous studies showcasing the benefits of nano-reinforcement in magnesium alloys, using various metal oxide nanoparticles. The review specifically focuses on silica (SiO₂) and its biocompatibility, highlighting its use in bioglass and its potential as a reinforcement material. The unique properties of hollow SiO₂ nanoparticles are also discussed, emphasizing their high surface area and suitability for biomedical applications. The absence of prior research on Mg-hollow SiO₂ nanocomposites is emphasized, justifying the current study's focus.

Mg-SiO₂ nanocomposites were synthesized using the disintegrated melt disposition (DMD) method. Pure Mg turnings and hollow SiO₂ NPs (10–20 nm) were used. The mixture was heated to 750°C under argon, stirred, and bottom-poured into a mold. Homogenization and hot extrusion were performed to create cylindrical rods. Microstructure characterization involved optical microscopy and scanning electron microscopy (SEM). X-ray diffraction (XRD) was used to analyze phase composition and texture. Compressive mechanical testing was conducted using an MTS 810 machine. Immersion testing in Hanks' balanced salt solution was performed to evaluate corrosion behavior, measuring weight loss and pH changes. Wettability was assessed using contact angle measurements. Cytotoxicity and cell proliferation were evaluated using MC3T3-E1 cells with MTS and LDH assays. Live/dead cell staining and SEM were used to observe cell morphology and adhesion. Finally, finite element analysis (FEA) was performed to simulate the stress and displacement in a customized alloplastic mandibular reconstruction model using both pure Mg and Mg/1SiO₂.

The addition of hollow SiO₂ NPs resulted in grain refinement in the Mg matrix. The compressive yield strength and ultimate compressive strength increased progressively with increasing SiO₂ content, reaching the highest values with Mg-1.5 vol.% SiO₂ (~128 MPa and ~378 MPa, respectively). Mg-0.5 vol.% and Mg-1.0 vol.% SiO₂ nanocomposites showed simultaneous improvements in fracture strain, while Mg-1.5 vol.% SiO₂ showed a slight decrease. The corrosion rates of the nanocomposites were significantly lower than that of pure Mg, with Mg-0.5 vol.% SiO₂ showing the best corrosion resistance. Cell proliferation of osteoblast cells was significantly higher for Mg-0.5 vol.% and Mg-1.0 vol.% SiO₂ nanocomposites compared to pure Mg. Cytotoxicity tests indicated that Mg-0.5 vol.% and Mg-1.0 vol.% SiO₂ nanocomposites exhibited lower cytotoxicity than pure Mg. FEA revealed that Mg/1SiO₂ showed more even stress distribution and less stress concentration in the mandibular prosthesis compared to pure Mg. The magnitude of displacement was less than 1 mm for both groups.

The improved mechanical properties of the Mg-SiO₂ nanocomposites are attributed to grain refinement, Orowan strengthening, and texture randomization. The enhanced corrosion resistance is due to the formation of a more uniform passive layer and the reduced grain boundary attack. The superior biocompatibility of the Mg-0.5 vol.% and Mg-1.0 vol.% SiO₂ nanocomposites is linked to their improved corrosion resistance and the enhanced hydrophilicity of their surfaces. The FEA results highlight the potential of Mg/1SiO₂ as a mandibular reconstruction material, due to its superior stress distribution compared to pure Mg. However, the optimal SiO₂ concentration seems to be around 1 vol.%, as higher concentrations (1.5 vol.%) showed decreased cell proliferation and slightly increased corrosion.

This study successfully demonstrated the potential of Mg-SiO₂ nanocomposites as biodegradable implants for mandibular reconstruction. The optimal SiO₂ concentration appears to be around 1 vol.%, offering a good balance between enhanced mechanical properties, corrosion resistance, and biocompatibility. Further research could focus on optimizing the processing parameters to further reduce agglomeration at higher SiO₂ concentrations and investigate the long-term in vivo behavior of these nanocomposites.

The study primarily focused on in vitro evaluation. Long-term in vivo studies are needed to fully assess the biodegradation and bone integration of these nanocomposites. The FEA model, while useful, is a simplification of the complex in vivo conditions. Further investigations are warranted to explore the influence of other factors like loading conditions and bone quality on the performance of these implants.