Development of Biomedical Beta-Titanium Alloys with Deformation-induced Changeable Elastic Modulus
AbstractElastic moduli of bones are generally much lower than those of metals. For example, Young’s modulus of bone is approximately 10–30 GPa, while Young’s moduli of most metals used for biomedical applications are much higher; a stainless steel (SUS316L) and a titanium alloy (Ti-6Al-4V ELI), both of which are used practically, exhibit Young’s moduli of approximately 200 and 110 GPa, respectively. Accordingly, the load transfer between a metallic implant and the surrounding bone is nonhomogeneous, resulting in a reduction of stress stimulation to the bone (stress shielding effect). Under such conditions, bone atrophy is likely to occur, leading to the loosening of the metallic implant and re- fracturing of the bone. In order to mitigate the stress shielding effect, metals with Young’s moduli equal to that of the bone are considered ideal for fabricating metallic implants. Therefore, many beta-type titanium alloys with low Young’s modulus have been developed in the last two decades. However, it was recently discovered that low Young’s modulus implants can sometimes be inconvenient for surgeons, depending on the implant type. For example, in spinal fixation devices, the amount of springback for the rod should be small enough to improve its handling ability during operations. The amount of springback of an implant is inversely associated with Young’s modulus: a low Young’s modulus leads to a large amount of springback. In other words, a high Young’s modulus is desired by surgeons, whereas a low Young’s modulus is desirable for patients. Thus, it is necessary to satisfy the contradictory requirements of surgeons and patients when developing spinal fixation devices. Under such the situations, we have proposed a novel concept, deformation- induced changeable Young’s modulus, in titanium alloys for biomedical applications as shown in Fig. 1 [1].