Biomedical Engineering Reference
In-Depth Information
A recent report highlited the contrasting requirements of patients versus surgeons for
metallic implants. The rquirements of the patients dictate that the implants have a Young's
modulus similar to that of the bone, whereas while the surgeons require a high Young's
modulus for inhibiting springback both during and after the operation procedure (Nakai,
2011a). Titanium alloys that simultaneously satisfy the demands of both patients and
surgeons are thus necessiated.
There is a demand to remove the implant when the bone fracture is healed, in which case,
the adhesion of the implant to the bone must be weak enough to inhibit the refracture of the
bone. This requires titanium alloys having poor bone conductivity, but excellent
biocompatibility (Zhao, 2011).
This chapter introduces low modulus -type titanium alloys and various methods of
lowering the Young's modulus of the -type titanium alloy, improving the strength and
fatigue strength of the -type titanium alloy while maintaining a low Young's modulus, the
evaluation of the effect of the Young's modulus on bone atrophy using rabbits, titanium
alloys with variable Young's moduli, and removable titanium alloys are described.
2. Low modulus β-type titanium alloys for biomedical applications
A number of -type titanium alloys with low Young's modulus have been developed for use
in the human body. The titanium alloys composed of safe alloying elements developed to
date include the following; Ti-13Nb-13Zr, Ti-12Mo-6Zr-2Fe, Ti-15Mo, Ti-15Mo-5Zr-5Sn, Ti-
15Mo-5Zr-3Al, Ti-16Nb-10Hf, Ti-15Mo-2.8Nb-0.2Si, Ti-30Ta, Ti-35Zr-10Nb, Ti-8Fe-8Ta, Ti-
8Fe-8Ta-4Zr, Ti-35Nb-7Zr-5Ta, Ti-29Nb-13Ta-4.6Zr (TNTZ), and Ti-Nb-Sn system alloys
(Niinomi, 2011a).
Shape memory and super-elastic Ti-Nb based alloys have been also been
developed; Ti-Nb, Ti-Nb-O, Ti-Nb-Sn, Ti-Nb-Al, Ti-22Nb-(0.5-2.0)O (at%),Ti-Nb-Zr, Ti-Nb-
Zr-Ta, Ti-Nb-Zr-Ta-O, Ti-Nb-Ta-Zr-N, Ti-Nb-Mo, Ti-22Nb-6Ta(at%), Ti-Nb-Au, Ti-Nb-Pt,
Ti-Nb-Ta, and Ti-Nb-Pd system alloys. Ti-Mo based alloys have been developed; Ti-Mo-Ga,
Ti-Mo-Ge, Ti-Mo-Sn, Ti-Mo-Ag, Ti-5Mo-(2-5)Ag (at%), Ti-5Mo-(1-3)Sn (at%), in addition to
Ti-Sc-Mo system alloys. The Ti-Ta based alloys are Ti-50Ta, Ti-50Ta-4Sn, and Ti-50Ta-10Zr.
Other alloys such as Ti-7Cr-(1.5, 3.0, 4.5)Al super elastic and shape memory alloys, Gum
Metal (Ti-25at% (Ta, Nb, V) + (Zr, Hf, O)), and Ti-9.7Mo-4Nb-2V-3Al super elastic alloys
have also been developed (Niinomi, 2011b).
3. Further decreases in Young's modulus
Improvements in the static strength of biomaterials such as the tensile strength can be
achieved by employing strengthening mechanisms including work hardening, grain
refinement strengthening, precipitation strengthening, and dispersion strengthening. One of
the best ways to increase tensile strength while maintaining a low Young's modulus is to
introduce a number of dislocations into the alloy system by conventional severe cold
working techniques such as severe cold rolling and swaging, and by special severe cold
working techniques such as high pressure torsion (HPT), accumulated roll-bonding (ARB)
and equal channel angular pressing (ECAP) (Yilmazer, 2009).
Figure
1 (Niinomi, 2010a) shows the relationships between the tensile properties and
working ratio of Ti-29Nb-13Ta-4.6Zr (TNTZ) while Fig. 2 (Niinomi, 2010a) shows the
relationship between the Young's modulus and working ratio of TNTZ after subjecting
TNTZ to severe cold working by general severe cold rolling or swaging in both cases.
