Fabrication and Characterization of Hydroxyapatite-Based Coatings on Titanium or Its Alloys

The surfaces of biomaterials are subjected to a variety of modifications, including ion implantation, etching, anodizing, or deposition of hydroxyapatite (HAp) and nanohydroxyapatite (nanoHAp) coatings. HAp and nanoHAp coatings are deposited on the surface of implants to provide corrosion resistance, prevent migration of alloying elements into surrounding tissues (known as the metallosis phenomenon), and ensure a permanent connection between the implant and bone by allowing bone to grow into the coating. In addition, HAp is a naturally occurring material in the human body (mainly in bones and dental enamel), further enhancing the implant&s;s biocompatibility. The main methods for producing coatings based on HAp and nanoHAp are sol–gel, biomimetic, plasma spraying, thermal spraying, and electrophoretic deposition (EPD) methods. The last of these allows the coatings to be obtained with a wide range of thicknesses on components of different shapes at a low cost and uncomplicated process. Unfortunately, the HAp coatings used today are characterized by high thickness and hardness, which significantly reduces their mechanical properties and poses a risk of damage during implantation. The crushing of HAp crystals can initiate inflammatory processes and cause bone loss in the vicinity of the implant. In addition, mainly applied by plasma spraying, they have high compressive intrinsic stresses, which reduce adhesion to the substrate. To improve the properties of HAps, methods have been developed to produce nanoHAps. Nanocrystalline HAp exhibits better sinterability, higher density (resulting from the much larger specific surface area of nanomaterials), and greater resistance to brittle fracture during heat treatment. It has also been demonstrated that nanoHAp, compared to its microscale counterpart, has greater bioactivity. This chapter describes the properties of nanoHAp-based coatings, including those with biological protection, deposited on the surface of titanium and its alloys using various techniques such as electrophoretic deposition, the sol–gel method, or the biomimetic method.