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 alpha + beta 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 degrees), 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.