Potential Load-Bearing Bone Substitution Material: Carbon-Fiber-Reinforced Magnesium-Doped Hydroxyapatite Composites with Excellent Mechanical Performance and Tailored Biological Properties

A potential load-bearing bone substitution and repair material, that is, carbon fiber (CF)-reinforced magnesium-doped hydroxyapatite (CF/Mg-HAs) composites with excellent mechanical performance and tailored biological properties, was constructed via the hydrothermal method and spark plasma sintering. A high-resolution transmission electron microscopy (TEM) was employed to characterize the nanostructure of magnesium-doped hydroxyapatite (Mg-HA). TEM images showed that the doping of Mg-induced distortions and dislocations in the hydroxyapatite lattice, resulting in decreased crystallinity and enhanced dissolution. Compressive strengths of 10% magnesium-doped hydroxyapatite (1Mg-HAs) and CF-reinforced 1Mg-HAs (CF/1Mg-HAs) were within the range of that of cortical bone. Compared with 1Mg-HAs, the fracture toughness of CF/1Mg-HAs increased by approximately 38%. The bioactivity, biocompatibility, and osteogenic induction properties of Mg-HAs and CF/Mg-HAs composites were evaluated in vitro using simulated body fluid (SBF) immersion, cell culture, osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs), and expression of genes associated with osteogenesis. When Mg-HAs were immersed in SBF, Mg2+ continued to release for up to 21 days. Mg-HAs demonstrated a satisfactory ability to induce apatite formation in comparison with HAs. The cell proliferation and morphology on CF/1Mg-HAs were similar to those of 1Mg-HAs, suggesting that adding CF had no adverse effect on cellular activity. The expression levels of osteogenesis-related genes [osteocalcin (OPN), osteopontin (OCN), and runt-related transcription factor 2 (Runx2)] on 1Mg-HAs were significantly higher at days 3 and 7 than those on HAs and 0.5Mg-HAs groups. This finding suggests that a certain amount of Mg doping had beneficial influences in the different stages of osteogenic differentiation and could induce osteogenic differentiation of BMSCs. The new bone volume to total volume ratio of implanted 1Mg-HAs (30.9% ± 4.1%) and CF/1Mg-HAs (25.4% ± 5.4%) was remarkably higher than that of HAs (21.6% ± 3.9%). 1Mg-HAs and CF/1Mg-HAs tailored an ideal effect of new bone information and implant osseointegration. The excellent mechanical performance and tailored biological properties of CF/Mg-HAs were attributed to nano Mg-doped HA, CF reinforcing, refined microstructure, and controlled composition.