Biocompatible functional surface of titanium-based implant materials

Titanium (Ti) and its alloys are widely used in orthopedic and dental implants owing to their high biocompatibility with tissues, low toxicity, and excellent mechanical properties, such as high strength, fatigue strength, and corrosion resistance. Total hip arthroplasty (THA) is predicted to rise from1.8 million in 2015 to 2.8 million in 2050, and the demand for Ti-based THA is also increasing. The biocompatibility of Ti originates from the several-nanometer-thick oxide layer present on its surface, which inhibits the redox reactions. The oxide forms spontaneously on the surface upon exposure to air and stays in thermodynamic equilibrium; however, it is easily disrupted by the interfacial shear stress owing to the low wear resistance of Ti. Ti exposed to corrosive body fluids elutes metal ions, generating wear debris in the biological fluids and tissues. This causes injury and disease, incites allergies, and promotes the formation of granulomas and even carcinomas. Furthermore, poor osseointegration due to poor adhesion with adjacent bone causes the loosening of the implant-bone interface and slows the healing process. To overcome these drawbacks of implant Ti materials, surface modifications using biocompatible TiO 2 are expected for imparting biofunctions such as osseointegration, antivirus activity, and tribocorrosion. Although various methods have been studied for the fabrication of TiO 2 on Ti alloys, anodic oxidation has attracted considerable attention owing to its advantages. This review aims to provide a comprehensive, evidence-based overview of current studies on the osseointegration, antimicrobial properties, and cytotoxicity of surface-modified implant Ti alloys, in addition to a brief introduction to different metallic biomaterials.