A Novel Radially Graded Hydroxyapatite-Based Composite for Bioactive Implant Interfaces

The combination of superior mechanical properties, corrosion resistance, and biological characteristics makes Ti-6Al-4V a widely used biomaterial. However, its clinical application as an orthopedic implant is limited by its hardness and weak osseointegration capacity. This study focuses on the development of a functionally graded biomaterial that has increased bioactivity and decreased mechanical mismatch with living tissue. In this scope, a novel radially functionally graded Ti-5Mo/hydroxyapatite (HA) biocomposite was successfully fabricated via pressure-assisted sintering using a specially designed mold that enabled directional HA enrichment toward the outer surface. This architectural design addresses the persistent challenge of combining mechanical reliability with biological functionality in load-bearing implants. Comprehensive characterization was performed, including x-ray diffraction (XRD), Rietveld refinement, optical and scanning electron microscopy (SEM/EDX), atomic force microscopy (AFM), contact angle measurements, and in vitro cytotoxicity assays using L929 fibroblast cells. Mechanical behavior was assessed through Brazilian splitting, three-point bending, microhardness, and tribological testing under both dry and corrosive conditions. The biocomposite exhibited a dual-phase α + β titanium matrix and HA-derived oxides in the surface layers, with a graded increase in porosity and hardness from the core to the periphery. Mechanical tests revealed a bending modulus of 22.4 GPa, close to that of cortical bone, while the surface showed enhanced roughness, hydrophilicity (contact angle 35.3°), and corrosion resistance. In vitro results confirmed the material's biocompatibility and superior cell viability in HA-rich regions. These findings demonstrate that the developed Ti-5Mo/HA FGM offers a structurally and biologically optimized solution for orthopedic and dental implant applications.