Topic: Biologically and Mechanically Biocompatible Titanium Alloys

Abstract

Metallic biomaterials such as stainless steels, Co-Cr alloys, and titanium and its alloys are currently utilized as structural materials in artificial hip joints, bone plates and screws, and artificial dental roots; they are mainly used in implants that replace hard tissue. Some metallic biomaterials known as stents are also used for reconstructing blood vessels. Among biometals, titanium alloys have high biocompatibility, specific strength, and corrosion resistance, and exhibit the most suitable characteristics for biomedical applications. Recently, the titanium alloys that are composed of nontoxic and allergyfree elements have been developed by taking their biological biocompatibility account into. These alloys must also have high strength and a long fatigue life, that is, high fatigue strength. A low Young’s modulus equivalent to that of the cortical bone is a simultaneously required in order to inhibit bone absorption. Porous titanium and its alloys that have a low Young’s modulus have also been developed. Titanium alloys for biomedical applications must have various functionalities such as superelasticity and shape memory characteristics. However, the mechanism of the superelastic behavior of some titanium alloys in biomedical applications is still unknown. It has been observed that wear loss occurs, indifferent areas such as the area between the stem and the bone, and loosening may also occur in such cases. Further, there is a possibility fretting fatigue will occur in the contact area of two bodies, for example, between like the bone plate and the screw. Therefore, the wear and fretting fatigue characteristics of titanium alloy used in biomedical applications are also very important. The Young’s modulus and tensile strength, ductility, fatigue life, fretting fatigue life, wear properties, functionalities, etc., should be controlled such that their a manner that their levels are suitable for structural biomaterials used in implants that replace hard tissue. These factors may be collectively referred to as mechanical biocompatibilities in the broad sense. For the long term usage of metallic implants, mechanical biocompatibilities should be enhanced.

Research and development of bioactive ceramic surface modifications for improving the biocompatibility of titanium alloys is also increasing because titanium alloys are classified as bioinert from the view point of the patterns of osteogenesis.

Further, biopolymer surface modifications are carried out on titanium alloys for achieving excellent blood compatibility. It is difficult to bond biopolymers to metallic biomaterials via chemical bonding. The first step in this task is to coat a polymer on the surface of titanium alloys, or to press the polymer into porous titanium alloys. At present, the challenging task of bonding blood-compatible polymers to titanium alloys via chemical bonding is being attempted. If this surface modification of metallic biomaterials via chemical bonding is successfully performed, high-endurance long-life scaffolds for artificial organs may be developed for incorporation with tissue engineering.

In the future, new biomaterials and biomaterials science will be developed when the harmonization of titanium (metallic), ceramic, and polymer biomaterials via chemical bonding is satisfactorily achieved. Simultaneously, the harmonization of titanium (metallic)-ceramic-polymer biomaterials and living tissues will also be achieved.

Furthermore, the direct or indirect evaluation of biocompatibility using animals, or cells, and the evaluation of mechanical performance parameters such as fatigue, fretting fatigue, and fracture toughness is also being actively undertaken.

Since new developments in titanium alloys for biomedical applications are taking place, the selected topics on research and development in titanium alloys for biomedical applications mainly in our laboratory will be presented.

Curriculum Vitae -- Mitsuo NIINOMI