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

The study addresses the need for bioresorbable, mechanically compatible implants for orthopedic and maxillofacial applications. Magnesium (Mg) is attractive due to its biocompatibility, biodegradability, osteoconductivity, and elastic modulus closer to cortical bone, which can reduce stress shielding. Prior clinical observations indicate Mg-containing constructs can enhance bone formation. However, pure Mg suffers from low strength, poor formability, limited fatigue resistance, and rapid corrosion in chloride-rich physiological environments. Reinforcing Mg with low-volume nanoparticulates (<3 vol.%) has previously improved strength, ductility, and corrosion resistance. This work investigates hollow silica (SiO₂) nanoparticles as reinforcements in Mg to enhance mechanical performance, corrosion behavior, and cytocompatibility, with the broader goal of validating Mg-based nanocomposites for craniomaxillofacial osteosynthesis and alloplastic mandibular reconstruction.

Prior studies have shown Mg-based nanocomposites reinforced with various metal oxide nanoparticles (e.g., CeO₂, Sm₂O₃, ZnO, ZrO₂, Al₂O₃, TiO₂) can enhance mechanical properties and corrosion resistance compared to commercial Mg alloys. TiO₂ additions up to 2.5 vol.% showed little effect on cytotoxicity of Mg. Silica (SiO₂), a major component of bioglass, is widely used in biomedical applications owing to high biocompatibility; SiO₂ nanoparticles have stimulated osteoblast activity and increased bone mineral density in vivo, with rapid clearance. Incorporation of bioglass into Mg increased compressive strength. Hollow SiO₂ nanoparticles offer high specific surface area, low density, and good biocompatibility, with reports of safe metabolism in vivo. Despite these advantages, effects of hollow SiO₂ nanoparticles on Mg’s microstructure, mechanical properties, degradation, and biocompatibility had not been reported prior to this study.

Materials: >99.9% purity Mg turnings (ACROS Organics) and hollow SiO₂ nanoparticles (~10–20 nm, >99.2% purity, Sigma Aldrich). Compositions: Mg reinforced with 0.5, 1.0, and 1.5 vol.% hollow SiO₂. Synthesis: Disintegrated melt deposition (DMD). Mg with weighed SiO₂ NPs heated to 750 °C under Ar; slurry stirred at 465 rpm for 5 min; bottom-poured into metallic mold with Ar at 25 L/min to disintegrate melt stream. Cast cylindrical preform (40 mm diameter) homogenized at 400 °C for 1 h; hot extruded at 350 °C at an extrusion ratio of 20.25:1 to rods of 8 mm diameter. Characterization: Microstructure via optical microscopy after light etching (acetic acid) for grain size; NP distribution by SEM (JEOL JSM-6010). XRD (Shimadzu Lab-XRD 6000, Cu Kα, λ=1.5418 Å, 2°/min) on as-extruded samples along extrusion direction. Mechanical testing: Compression tests using MTS 810 at strain rate 5×10⁻³ min⁻¹, aspect ratio l/d=1 per ASTM E9-09; five specimens per condition. Immersion corrosion: Hanks’ balanced salt solution (composition specified) at 37 °C; immersion durations 1, 2, 3, 4, 7 days with solution renewal every 24 h; sample-to-solution ratio 20 mL:1 cm². Measured pH and weight loss each cycle; corrosion products removed using 20 g CrO₃ + 1.9 g AgNO₃ in 100 mL DI water; post-7-day SEM/EDS of corroded surfaces. Corrosion rate calculated as CR = K·W/(A·T·D), K=8.76×10⁴. Wettability: Static contact angle with DI water (10 μL) at room temperature using Kruss DSA25; n=10 per sample. Cytocompatibility: Direct seeding of MC3T3-E1 pre-osteoblasts (8000 cells) on 5×2 mm discs in 96-well plates. Cell proliferation/viability by MTS (Promega) at 490 nm; cytotoxicity by LDH release (CytoTox-One, Promega; ex 560 nm/em 590 nm); n=4; P<0.05. Live/Dead staining with fluorescein diacetate (live), propidium iodide (dead), Hoechst (nuclei); fluorescence microscopy; cell morphology by SEM. Finite element analysis (FEA): Mandibular model with continuity defect reconstructed using previously developed wing-design endoprosthesis. Material properties for cortical/cancellous bone, pure Mg, and Mg/1 vol.% SiO₂ implemented. Teeth removed for modeling. Mesh: 447,669 nodes and 315,647 elements. Loading: 300 N occlusal load at incisor region (Abaqus). Outputs: von Mises stress in prosthesis and mandible; displacement magnitudes.

- Microstructure and texture: Nanocomposites exhibited equiaxed grains with significant refinement; Mg-1.5 vol.% SiO₂ average grain size ~16 μm (~55.6% finer than pure Mg). XRD showed predominantly Mg peaks; SiO₂ peaks visible at 1.5 vol.%. Texture randomization observed up to 1.0 vol.% SiO₂ (increased pyramidal plane intensity; reduced basal plane intensity), indicating reduced basal texture. - Mechanical properties (compression): Progressive increase in 0.2% compressive yield strength (CYS) and ultimate compressive strength (UCS) with SiO₂ content. Mg-1.5 vol.% SiO₂: CYS ~128 MPa; UCS ~378 MPa (highest). Fracture strain increased for 0.5 and 1.0 vol.% (max ~23.8%) vs pure Mg (~21.2%), but decreased at 1.5 vol.% (~18.1%). Properties matched/exceeded several commercial/researched Mg alloys and were comparable to bone. - Corrosion/immersion: Initial pH rose to ~9.2–9.4 at 24 h for all samples; stabilized days 2–7. Nanocomposites had lower absolute pH than pure Mg (indicative of enhanced passivation). Day 1 corrosion rate: pure Mg ~3.9 mm/y; lowest for Mg-1.5 vol.% SiO₂ ~1.6 mm/y. Over 7 days, Mg-0.5 vol.% SiO₂ showed most uniform, stable corrosion rates; 1.0 and 1.5 vol.% also decreased but with less uniform trends. SEM showed fewer pits and more-uniform passive layer in nanocomposites; Mg(OH)₂ brucite crystals were more pronounced and non-uniform in pure Mg. EDS mapping indicated Mg, O, P enrichment in corrosion layers, suggesting apatite-favorable chemistry. - Wettability: Contact angle decreased (more hydrophilic) with SiO₂ addition: pure Mg ~64°; 0.5% ~58°; 1.0% ~53°; 1.5% ~44°, favorable for cell attachment. - Cytocompatibility: MTS showed increased proliferation for 0.5 and 1.0 vol.% vs pure Mg; 1.5 vol.% showed least proliferation at days 3 and 5. LDH cytotoxicity increased with higher SiO₂ concentration; 1.5 vol.% highest but within 100% LDH reference; 0.5 and 1.0 vol.% exhibited lower cytotoxicity than pure Mg (p<0.05; n=4). Live/Dead and SEM indicated better cell attachment, spreading, and increased density over days 1–5 for 0.5 and 1.0 vol.%. - FEA (mandibular reconstruction): Prosthesis stresses: pure Mg (Group 1) ~37–110 MPa; Mg/1 vol.% SiO₂ (Group 2) ~32–93 MPa, with lower maximum stress and more even distribution. Average prosthesis displacement 0.3–0.6 mm in both groups. Mandible stresses: Group 1 defect side 22–59 MPa, unaffected 22–25 MPa; Group 2 defect side 28–69 MPa, unaffected 22–25 MPa. Maximum mandibular displacement ~1.1 mm at symphysis under 300 N load. Mg/1SiO₂ reduced stress concentration within the prosthesis wing compared to pure Mg, with similar mandibular stress levels.

Reinforcing Mg with hollow SiO₂ nanoparticles improved biomechanical performance and corrosion behavior while maintaining or improving cytocompatibility, addressing key limitations of pure Mg for implant applications. Mechanistically, grain boundary pinning by uniformly dispersed SiO₂ NPs refined grains (Hall–Petch strengthening), Orowan looping enhanced work hardening, and texture randomization up to 1 vol.% facilitated favorable plastic deformation and strength. Good interfacial bonding enabled effective load transfer. Corrosion resistance improvements stemmed from refined grains and more-uniform passive layer formation that reduced pitting and moderated pH rise, promoting an apatite-favorable surface chemistry. Enhanced hydrophilicity with increasing SiO₂ content improved protein adsorption and cell attachment. At higher NP loading (1.5 vol.%), slight agglomeration and a re-strengthened basal texture limited ductility and led to less uniform corrosion response, explaining reduced fracture strain and cytocompatibility relative to lower loadings. FEA demonstrated that Mg/1 vol.% SiO₂ prostheses experience reduced internal stress concentrations and similar mandibular stresses under functional loading, supporting their suitability for mandibular reconstruction. Overall, limiting SiO₂ to ≤1 vol.% offers an optimal balance of strength, corrosion resistance, and biocompatibility for craniomaxillofacial applications.

Mg-SiO₂ nanocomposites with low-volume hollow silica reinforcement were successfully synthesized via DMD and hot extrusion. They exhibited significant grain refinement, increased compressive strength, acceptable ductility (especially at 0.5–1.0 vol.%), improved corrosion resistance with more-uniform passivation, enhanced hydrophilicity, and favorable in vitro cytocompatibility. FEA indicated reduced prosthesis stress concentrations for Mg/1 vol.% SiO₂ under masticatory loading, with minimal displacement. These composites show promise as biodegradable implant materials for mandibular reconstruction, provided SiO₂ content is carefully controlled and limited to about 1 vol.%. Future work should validate long-term in vivo biodegradation, mechanical performance under physiological loading, and clinical translation.

- Experimental assessments were predominantly in vitro: corrosion tests up to 7 days in Hanks' solution and cell assays with MC3T3-E1 cells; no in vivo degradation or biological response data were provided. - Finite element analysis used simplified modeling assumptions (e.g., teeth removed, average 300 N load, specific material property assignments) which may not fully capture complex mandibular biomechanics. - Higher nanoparticle loading (1.5 vol.%) showed indications of agglomeration and less favorable ductility/cytocompatibility, highlighting sensitivity to dispersion and processing. - Cytotoxicity results were limited to one cell line and short incubation periods (1–5 days).