A Nitinol structure with functionally gradient pure titanium layers and hydroxyapatite over-coating for orthopedic implant applications

In this study, functionally gradient multi-layer coatings are developed on the Nitinol surface for orthopedic implant applications to eliminate potentially toxic Ni ion release and to improve biocompatibility. Using the direct energy deposition (DED) technique, both the core Nitinol and titanium gradient layers are additively manufactured with high purity. The completely alloyed microstructure without microstructural defects provides superior bonding strength compared to other metal coating techniques that rely on particle-to-particle bonding (e.g., thermal spray and physical/chemical vapor deposition). An additional biomedical over-coating of hydroxyapatite (HA) was deposited on the outer surface of the gradient layer structure using the cold spray technique to enhance biocompatibility without compromising high crystallinity. The resultant bonding strength of HA coating was determined to be 26 MPa, which exceeded the minimum adhesion strength requirement (15 MPa) by the ISO-13779 standard (implants for surgery-HA). The in vitro test on the nickel (Ni) ion release revealed that the Ni release rate from the entire gradient Nitinol structure is very low (0.036 μg/ cm^2 ), which is an order of magnitude smaller than commercial Nitinol orthodontic wires, thereby suggesting a promising potential of the novel Nitinol structure in orthopedic and other implantable applications.