1. journal of the mechanical behavior of biomedical materials 20 (2013) 407–415
Available online at www.sciencedirect.com
www.elsevier.com/locate/jmbbm
Opinion Piece
Biocompatibility of Ti-alloys for long-term implantation
Mohamed Abdel-Hady Gepreela,n, Mitsuo Niinomib
a
Department of Materials Science and Engineering, Egypt-Japan University of Science and Technology (E-JUST), Alexandria, Borgelarab
21934, Egypt
b
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
art i cle i nfo
ab st rac t
Article history:
The design of new low-cost Ti-alloys with high biocompatibility for implant applications,
Received 17 July 2012
using ubiquitous alloying elements in order to establish the strategic method for suppres-
Received in revised form
sing utilization of rare metals, is a challenge. To meet the demands of longer human
6 November 2012
life and implantation in younger patients, the development of novel metallic alloys
Accepted 17 November 2012
for biomedical applications is aiming at providing structural materials with excellent
Available online 6 December 2012
chemical, mechanical and biological biocompatibility. It is, therefore, likely that the next
Keywords:
generation of structural materials for replacing hard human tissue would be of those
Implants
Ti-alloys that do not contain any of the cytotoxic elements, elements suspected of causing
Compatibility
neurological disorders or elements that have allergic effect. Among the other mechanical
Long-term implantation
properties, the low Young’s modulus alloys have been given a special attention recently, in
Ti-alloys
order to avoid the occurrence of stress shielding after implantation. Therefore, many
Low cost implants
Ti-alloys were developed consisting of biocompatible elements such as Ti, Zr, Nb, Mo, and
Ta, and showed excellent mechanical properties including low Young’s modulus. However,
a recent attention was directed towards the development of low cost-alloys that have a
minimum amount of the high melting point and high cost rare-earth elements such as Ta,
Nb, Mo, and W. This comes with substituting these metals with the common low cost, low
melting point and biocompatible metals such as Fe, Mn, Sn, and Si, while keeping excellent
mechanical properties without deterioration. Therefore, the investigation of mechanical and
biological biocompatibility of those low-cost Ti-alloys is highly recommended now lead
towards commercial alloys with excellent biocompatibility for long-term implantation.
& 2012 Elsevier Ltd. All rights reserved.
1.
Background
The continual growth of the world population and the
increase in traffic accidents especially for young people, more
pronounced in the developing countries (WHO, 2012), have
brought an ever-increasing need for materials specially suited
for bio-implant applications. Up till now, over 7 million
n
Corresponding author. Tel.: þ20 11 47375539; fax: þ20 304599520.
E-mail address: geprell@yahoo.com (M. Abdel-Hady Gepreel).
1751-6161/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jmbbm.2012.11.014
˚
Branemark System implants have been placed in human
bodies (Nabeel, 2012), over 1,000,000 spinal rod implantations
have been done between 1980–2000, and 250,000 total hip
replacements are performed annually in United States only
(Christian, 2004). Not only the replacement surgeries have
increased, but also the revision surgeries of hip and knee
implants. These revision surgeries which cause pain for the

2. 408
journal of the mechanical behavior of biomedical materials 20 (2013) 407 –415
patient are very expensive, besides, their success rate is
rather small. The total number of hip revision surgeries is
expected to increase by 137% and that of knee revision
surgeries by 607% between the years 2005 and 2030 (Kurtz
et al., 2007). Nowadays, researchers are working hard to
develop materials for long life implantation in human body.
This is because the commercial biomaterials have exhibited
tendencies to fail after long-term use due to various reasons
such as low fatigue strength, high modulus compared to that
of bone, low wear and corrosion resistance and lack of
biocompatibility. The various causes for revision surgery
and the key solutions are schematically illustrated in Fig. 1.
Another acceptable reason for the increase in the number of
revision surgeries is the higher life expectancy. The implants
are now expected to serve for much longer period or until
lifetime without failure or revision surgery. The development
of appropriate material with high longevity and excellent
biocompatibility is highly essential.
Generally, the most common materials used in orthopedic
implants are metals and a type of plastic called polyethylene.
These two material types are combined in most joint implants,
that is, one component is made from metal, and one from
polyethylene. When properly designed and implanted, the two
components can rub together smoothly while minimizing
wear. Although some pure metals have excellent characteristics for use as implants, most metallic implants are made
from alloys, namely, stainless steels, cobalt–chromium alloys,
and titanium alloys (Geetha et al., 2009).
Various metallic materials have been used for total hip
replacements as well as other joint replacement surgeries,
i.e., knees, shoulders, bone plates. Additional applications
include trauma and spinal fixation devices, cardiovascular
stents, and most recently replacement of spinal discs (Rack
and Qazi, 2006; Semlitisch, 1987).
Stainless steel shows moderate mechanical properties and
good corrosion resistance in human body fluid environment;
therefore, it is most often used in implants that are intended
to help in fractures repair, such as bone plates, bone screws,
pins, and rods. Cobalt–chromium alloys are also strong, hard,
biocompatible, and corrosion resistant; hence, they are used
in a variety of joint replacement implants, as well as some
fracture repair implants, that require a long service life.
In recent years, titanium and titanium alloys are extensively
used as bone replacement implants due to their excellent
mechanical properties, corrosion resistance and biocompatibility as compared to the other metallic materials (Liu et al.,
2004; Tian et al., 2010).
Below is a systematic discussion on the main concepts
driving the progress in metallic implants research in the last
two decades ended with results of newly developed alloys.
This discussion will focus on the importance of both the
biological and mechanical biocompatibility for the long-life
Fig. 1 – Various causes for failure of implants that leads to revision surgery, footed with a proposed system for better
performance.

3. journal of the mechanical behavior of biomedical materials 20 (2013) 407 –415
implants. In order to prevent failure after implantation, the
mechanical biocompatibility emphasizing the importance of
low Young’s modulus and how to achieve it, are also presented.
Finally, some economical considerations for the production of
new metallic implants will be discussed. This review will help
materials researchers to develop competitive materials for
implant applications, as well as its importance for surgeons to
choose the most proper materials for specific application.
2.
Biological biocompatibility of implants
As explained above, the most common implants used long time
ago are made from stainless steel, titanium and Ti-alloys
(mainly due to their high corrosion resistance) and Co–Cr-based
alloys (mainly due to their high wear and corrosion resistance.)
In other words, these alloys are considered chemically stable
with respect to the internal chemistry of the human body
(i.e., good chemical biocompatibility). Even though the metals
used in implants are quite corrosion-resistant, there are still
some interchanges of metal ions into the tissues or tissue fluids
(Orden et al., 1982). The amount of metal ions released is related
to the corrosion resistance of the metal, the environmental
409
conditions (i.e., pH, chloride ion concentration, temperature,
etc.), mechanical factors (i.e., pre-existing cracks, surface abrasion, and film adhesion), electrochemical effects (i.e., applied
potential, galvanic effects, pitting, or crevices), and the dense
cell concentrations around implants (Oshida, 2006).
Reported in Table 1 are the common metals and alloys that
are used in implant applications, their microstructure and
their mechanical properties. As shown in this table, the
majority of implants contain; vanadium, aluminum, cobalt,
copper, chromium, molybdenum, nickel, titanium and various elements. It is well known that any metal surrounded by
biological systems will suffer corrosion to some extent
(Hallab et al., 2001). The biological biocompatibility of any
implant, which is defined by its toxicity, carcinogenicity, and
metal sensitivity from the release of metal ions, must be
quantified to decrease the patient’s risk and failure of
implants. Corrosion and the release of metal ions due to
the wear of the implant inside the human body are the source
of many adverse pathophysiological effects (Gotman, 1997).
That is why, the biological effect of elements, metals, and
alloys are being extensively studied.
For example, the cytotoxicity of typical surgical implant
alloys and pure metals have been studied by many
Table 1 – Selected orthopedic alloys developed and/or utilized as biomedical implants and their mechanical properties
(E ¼elastic modulus, YS ¼ yield stress, UTS¼ultimate tensile strength).
Alloy designation (mass%)
Microstructure
E (GPa)
Bone
nn
Stainless steel 316L
Annealed [1]
Hot forged [1]
nn
CoCrMo
Cast [1]
Wrought [1]
nn
cp Ti (grade 4)
Annealed [2]
nn
Ti–6Al–4V
Annealed [1]
Hot forged [1]
nn
Ti–6Al–7Nb
Annealed [3]
nn
Ti–5Al–2.5Fe
Cast [3]
Annealed [3]
nn
Ti–13Nb–13Zr
annealed[2]
nn
Ti-11.5Mo–6Zr–4.5Sn (BIII)
Annealed [3]
nn
Ti–15Mo–5Zr–3Al
Annealed [2]
Ti–15Mo–3Nb–0.3O
Annealed [4]
nn
Ti–35Nb–5Ta–7Zr (TNZT)
Annealed [4]
nn
Ti–35Nb–5Ta–7Zr–0.4O (TNZTO)
Annealed [4]
Ti–29Nb–13Ta–4.5Zr
(TNTZ) annealed[5]
Viscoelastic composite
Austenite
YS (MPa)
10–30
200
UTS (MPa)
Fatigue limit (MPa)a
90–140
170
140
145
295
450
860
565
1200
400
500
480
550
350
680
900
780
1000
400
600
800
900
500
820
780
900
860
425
725
900
1030
500
620
690
525
900
930
540
1020
1020
490
530
Austenite
480
585
590
265
976
1010
450
400
420
325
200–230
a
105
aþb
110
aþb
105
aþb
110
aþb
79
b
b
79
80
b
82
b
55
b
b
66
65
[1] Ref. (Semlitsch and Willert, 1980) [2] Ref. (Li, 2000) [3] Ref. (Boyer et al., 2007) [4] Ref. (Narayan, 2012) [5] Ref. (Niinomi and Nakai, 2011).
At 107 cycles.
nn
Commercially used in biomedical applications.
a

4. 410
journal of the mechanical behavior of biomedical materials 20 (2013) 407 –415
researchers as reported in Biesiekierski et al. (2012); Davidson
and Kovacs (1989); Kuroda et al. (1998); Okazaki et al. (1996),
Steinemann (1980). Vanadium is classified in the sterile
abscess (toxic) group, and aluminium in the capsule (scar
tissue) group. Ti, Zr, Nb and Ta exhibit excellent biocompatibility and are in the loose connective vascularized (vital)
group regarding tissue reaction. Kawahara reported that Ti,
Zr, Ta and Pd are low cytotoxic elements (Kawahara et al.,
1963). From the above information it is concluded that the
ideal biomaterial should possess good biological biocompatibility by being free of toxic elements. Therefore, the stainless
steel, Co–Cr-based and Ti–6Al–4V alloys, the most common
implants alloys, are not the ideal alloys to be used for long
term implantation in human body from the biological point
of view, due to their high content of high cytotoxic elements
such as (V, Ni, Coy). Nickel is also known as allergenic
carcinogen element that exhibits one of the highest sensitivities in metallic allergen tests (Koster et al., 2000).
Therefore, the research on development of Ni-free
Co-based (Yamanaka et al., 2011) and Ti-based (Oak et al., 2009)
alloys are being done. In the same way, intensive efforts are
being done to substitute Ti–6Al–4V alloy with V-free titanium
alloys for biomedical applications. For this reason Ti–6Al–7Nb
and Ti–5Al–2.5Fe have been developed (Semlitsch et al., 1985;
Zwicker et al., 1980). However, it was reported that Al is an
element involved in severe neurological, e.g., Alzheimer’s
disease and metabolic-bone disease, e.g., osteomalacia
(Boyce et al., 1992). So, V- and Al-free Ti-alloys are being
developed too. One of the important V- and Al-free Ti-alloys
is Ti–13Nb–13Zr alloy (Steinemann et al., 1993) being free of
toxic elements and showing improved bone biocompatibility
and corrosion resistance compared to that of Co–Cr-based
and Ti–6Al–4V alloys (Davidson et al., 1994). Due to other
concerns such as mechanical biocompatibility, as will be
discussed below, many other b-type Ti alloys composed of
the high biocompatible elements (i.e., Ta, Nb, Zr, Mo, W, Sn, ..)
were developed such as Ti–29Nb–13Ta–4.6Zr (TNTZ) (Kuroda
et al., 1998), Ti–35Nb–5Ta–7Zr (TNZT), Ti–12Mo–6Zr–2Fe
(Steinemann, 1980), Ti–Mo and many others.
3.
Mechanical biocompatibility of implants
The metallic implants, in many cases, should not only avoid
short-term rejection and infection, but should also provide
long-term biocompatibility and avoid long-term materials
limitations. Besides the biological biocompatibility discussed
above, the mechanical biocompatibility is vital for long term
implantation (He and Hagiwara, 2006). In this section, the
mechanical biocompatibility (i.e., high strength, long lifetime,
high-wear resistance and low Young’s modulus) is discussed.
3.1.
Fatigue and wear resistance
The cyclic loading is applied to orthopedic implants during
body motion, resulting in alternating plastic deformation of
microscopically small zones of stress concentration produced
by notches or microstructural inhomogeneities. Therefore,
the long lifetime of implant, which is related to its fatigue
resistance, is a crucial property of implant materials. Shown
in Table 1 are the strength and fatigue strength of common
alloys used in implant manufacture. The strength and so
fatigue strength of alloys are related to the alloy composition
and prior thermo-mechanical processing history. Fatigue
strength is also highly affected by surface processing, finishing and treatments. Hence, the alloys show a range of such
important mechanical properties and can be controlled with
proper processing and heat treatments. It is well known that
the higher the fatigue strength of an alloy is, the longer
lifetime for an implant made of it is in service. Generally,
Co–Cr alloys and (aþb)-type Ti-alloys show high fatigue
resistance when compared to other metallic biomaterials.
Recently, TNTZ (a b-type Ti-alloy) showed high fatigue
strength too with proper thermomechanical treatments
(Niinomi and Nakai, 2011). It is worthy to highlight here that
the notch sensitivity, which is changing with microstructure
control, is a very important aspect, since it can lead to poor
fatigue performance in some materials which have high
strength and fatigue strength (Li, 2000).
In addition, the other mechanical properties (such as
Young’s modulus and wear resistance) should be also considered, because they may limit the usage of the alloys in
manufacturing implants even if the strength and fatigue
strength are mechanically biocompatible.
Stainless steel and Co–Cr alloys show good wear resistance
and relatively high strength compared to that of bone, as
shown in Table 1. In addition, good fatigue resistance is
achievable, through microstructure control. However, these
materials still suffer from a large degree of biomechanical
incompatibility, due to their high elastic modulus (about
200 GPa), compared to that of the bone (max. 30 GPa).
3.2.
Stiffness of implants
As mentioned above, when these alloys with low stiffness
mismatch with bone are used as a hip implant, e.g., a femoral
stem, the implant takes over a considerable part of body
loading, which shields the bone from the necessary stressing
required to maintain its strength, density, and healthy structure. Such an effect, usually termed ‘‘stress shielding’’,
eventually causes bone loss, implant loosening, and premature failure of the artificial hip (Mansour et al., 1995).
Therefore, these alloys are not recommended in general in
manufacturing implants that transfer loads to bone for long
term implantation (more than 10 years) (Oshida, 2006).
The stiffness of titanium and its alloys is substantially
lower than that of other conventional metallic implant
materials such as stainless steel or Co–Cr–Mo alloys, as
shown in Table 1. Therefore, compared to stainless steel
and Co–Cr alloys, Ti-based alloys are excellent biomaterials
for long-term implantation due to their relatively low Young’s
modulus, good fatigue resistance and excellent biological
passivity (Song et al., 1999a). However, the most common Ti
alloys used in bio-implantation are of a and aþb type alloys
that still show relatively high elastic modulus (about 120 GPa)
when compared with that of bone (max. 30 GPa), these
materials still suffer from a considerable degree of biomechanical incompatibility.

5. journal of the mechanical behavior of biomedical materials 20 (2013) 407 –415
One of the examples is the application of Ti–6Al–4V as a
femoral stems in total hip replacements. This alloy has a
relatively high elastic modulus (about 110 GPa) and stress
shielding is reported when it is used as femoral stem (Oshida,
2006). Moreover, the existing alloy can release toxic ions (e.g.,
V and Al) into the body, leading to undesirable long-term
effects (Cui and Guo, 2009; Kuroda et al., 1998; Lopez et al.,
2001; Oshida, 2006).
Therefore, the decrease of the Young’s modulus of implants
was an important target for the researchers in the last two
decades. It is well known that the Young’s modulus changes
according to the type of the phases existing in the alloy
(Matlakhova et al., 2005; Zhou et al., 2004a). For example, it
has been reported that the o-phase has the highest Young’s
modulus, and the martensite a00 -phase has a lower modulus
than the martensite a0 -phase, and the b-phase has the lowest
modulus among these phases in most Ti alloys (Matlakhova
et al., 2005; Zhou et al., 2004b). Thus, extensive investigations
have been carried out to develop b-type alloys with a low
Young’s modulus, superelasticity, shape memory effect and
satisfactory biocompatibility for the replacement of human
bone (Ikehata et al., 2004; Matlakhova et al., 2005; Niinomi,
2003; Saito et al., 2003).
The research of biomedical titanium alloy focused on
b-type titanium alloys which contain non-toxic elements
such as Nb, Ta, Zr, Mo and Sn in order to obtain lower elastic
modulus, higher corrosion resistance and improved tissue
response (Hallab et al., 2001; Oshida, 2006). Therefore,
b-titanium alloys can now replace the Ti–6Al–4V alloy which
is considered the most important biomedical titanium alloy.
Various b-type Ti-alloys have been developed and meet the
above mentioned needs of showing low Young’s modulus and
being free of toxic elements or elements that cause allergic
effect. Namely, Ti–15Mo–5Zr–3Al (Semlitsch et al., 1985),
Ti–12Mo–6Zr–2Fe (Okazaki et al., 1996), Ti–15Mo, Ti–29Nb–
13Ta–4.6Zr (Kuroda et al., 1998), Ti–35Nb–5Ta–7Zr and Ti–
13Zr–13Nb (Steinemann et al., 1993), have been developed
for medical implant applications. All these alloys show low
Young’s modulus as compared to that of Ti–6Al–4V alloy.
Moreover, the superelastic and shape memory behavior
observed in Ti–Ni alloy have made it widely applied to
biomedical uses. But Ni is a toxic element, as explained
above, that is why the development of Ni-free superelastic
and shape memory alloys was a recent target of many
researchers too. For example, Ti–Nb–X (X¼ Zr, Ta, Mo, Au,
Pd, Pt, Al, Ga, Ge, Sn, Sc, O), Ti–Mo–Y (Y¼ Ta, Nb, Zr, Au, Pd, Pt,
Al, Ga, Ag) and Ti–V–Z (Z¼ Nb, Sn, Al) alloys were designed to
improve the superelastic and the shape memory property of
the biomedical Ti-alloy (Duerig et al., 1982; Hosoda et al.,
2003; Kim et al., 2004; 2005; Kuramoto et al., 2006; Song et al.,
1999a; 1999b; Zhou et al., 2004a; 2004b).
However, most of these compositions were formulated principally by trial and error, which by no means represents the
optimum choices. There has been little theoretical investigation
to guide alloy development for high strength and low modulus
biomedical applications using, for example, the d-electrons
concept (Kuroda et al., 1998; Matsugi et al., 2010), and first
principles electronic calculations (Song et al., 1999a) and others.
In a recent study (Kuroda et al., 1998), some bÀtype
titanium alloys composed of non-toxic elements Nb, Ta, Zr,
411
Mo and Sn were designed based on molecular orbital calculations of electronic structures. Niimomi et.al. has developed
Ti–29Nb–13Ta–4.6Zr (TNTZ) alloy that shows Young’s modulus as low as 60 GPa. However, TNTZ and other recently
developed alloys with relatively low Young’s modulus, such
as Ti–35Nb–5Ta–7Zr (TNZT), Ti–15Mo–2.8Nb–3Al and others,
show relatively low ultimate tensile strength and fatigue
strength, as shown in Table 1. It is important to stress again
here that the strength and fatigue strength of an alloy can be
improved through the proper post treatments. For example,
the strength and fatigue strength of TNTZ alloy raised
significantly from 420 and 325 MPa in the solution treatment
condition to 1100 and 775 MPa after thermomechanical treatments, respectively (Niinomi and Nakai, 2011). However, this
is on the expense of an increase in the Young’s modulus from
65 to 85 GPa after the treatments.
Therefore, considerable efforts have been devoted by materials engineers and researchers to develop new b-titanium
with high strength and low modulus.
3.3.
Ti-alloys with low Young’s modulus
The Young’s modulus changes with bÀphase stability as was
discussed in details in previous studies (Abdel-Hady et al.,
2006, 2007, 2008, 2009). The least stable single bÀphase alloys
show minimum values in Young’s modulus in the bÀtype
alloys (Abdel-Hady et al., 2006). Also, it was reported that the
Zr addition (Abdel-Hady et al., 2007; 2009) as well as small
addition of oxygen enhanced the elastic properties of the
Ti-alloys. Also, both Zr and O worked as bÀstabilizers in the
bÀtype Ti-alloys (Abdel-Hady et al., 2006, 2009). With the aid
of BoÀMd diagram, the present author has developed new
high Zr-content alloys free of toxic elements. These alloys
showed high strength (more than 1200 MPa) and low Young’s
modulus (less than 50 GPa) under different treatments, as
shown in Fig. 2. Detailed discussion of the mechanical and
physical properties of these alloys is presented elsewhere
(Abdel-Hady and Morinaga, 2009a; 2009b)). Here, Bo is the
average bond order between atoms, and Md is the average dorbital energy level (eV) of the elements in the alloy. In the
same way, Ti–24Nb–4Zr–7.9Sn alloy was developed and
showed high strength (850 MPa) and low Young’s modulus
(42 GPa) (Hao et al., 2007). Many researchers are concerned
with increasing the strength and decreasing the Young’s
modulus of biocompatible b-type Ti-alloys through alloy
design, thermomechanical treatments and manufacturing
methods.
It is important to note that cold deformation of b-type
Ti-alloys contributes in controlling the Young’s modulus of
the alloy depending on the deformation technique and the
final microstructure, since deformation and/or recrystallization textures are developed under some thermomechanical
schemes, as observed here in Fig. 2. Controlling the grain size
and introducing texture in the alloy through the proper
thermomechanical treatments have been reported by many
authors (Hosoda et al., 2006; Kim et al., 2006; Kuramoto et al.,
2006) and considered as very effective tool to reach the target
in developing more mechanical-biocompatible implants.

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journal of the mechanical behavior of biomedical materials 20 (2013) 407 –415
ST
1200
CR
60
900
UTS (MPa)
Young'smodulus (GPa)
80
40
600
20
300
0
0
Z00
Z01
Z11
Z00
Z01
Z11
Fig. 2 – Effect of phase stability and thermomechanical treatment on the Young’s modulus (a), and the ultimate tensile
strength (UTS) (b), of theTi67Zr20Nb10Ta3, Z00, Ti66Zr20Nb10Ta3O1, Z01, and Ti65Zr20Nb10Ta3Fe1O1, Z11, alloys after solution
treatment (ST) and after 90%CR (CR).
Fig. 3 – Elastic admissible strain plotted against Young’s
modulus of bone compared to the commercial biomedical
alloys (namely; stainless steel, SUS-316L, Co–Cr based alloy,
CoCrMo, Ti–6Al–4V, Ti–64ELI, commercial pure Ti, Cp–Ti,
Ti–35Nb–5Ta–7Zr–0.4O, TNZTO, and Ti–13Zr–13Nb) and the
recently developed alloys (namely; Ti–29Nb–13Ta–4.6Zr, TNTZ,
Ti–30Zr–8Mo, Ti-8Mo Ti65Zr20Nb10Ta3Fe1O1, Z11, and Ti–5Fe–
3Nb–3Zr, TFNZ), all in the annealing condition, are promising
for future long-term implant applications as they show elastic
admissible strain higher than the commercial alloys.
A useful relation between strength and Young’s modulus
that guides materials selection for bio-implant applications is
presented as the elastic admissible strain of an alloy. The
elastic admissible strain, defined as the yield stress-tomodulus ratio, is a quite important parameter considered in
orthopedic applications. The higher the elastic admissible
strain is, calculated from this relation, the more suitable the
materials for such applications are (Song et al., 1999a; 1999b).
It is important to mention here that the Young’s modulus of
alloys is measured at loads close to zero. However, some of
the recently b-type Ti-alloys show nonlinear elasticity (Saito
et al., 2003; Abdel-Hady et.al., 2008). Hence, these alloys
are showing higher elastic strain than calculated from
this relation and become more suitable for orthopedic
applications. Fig. 3 shows the elastic admissible strain of
the most common bio-implant alloys in addition to the most
promising b-type Ti-alloys developed recently for implant
applications. The recently designed alloys Z11 (ST and CR)
showed elastic admissible strain higher than other low
Young’s modulus Ti-alloys including the high Zr-content
alloys (i.e., Ti–30Zr–8Mo) (Niinomi and Nakai, 2011). Interesting is Z11 alloy show nonlinear elasticity and the actual
elastic strain of this alloy is more than 2%. Z11 alloy seems
promising for future long-life implant applications since it
shows elastic admissible strain even higher than that of bone
itself.
Also the production technique and the implant structure
became recently tools to reduce the Young’s modulus of
metallic implants. Much research is being carried out to
produce metallic implants with cellular structure that show
very low Young’s modulus. For example, Ti–6Al–4V alloy with
cellular structure showed low Young’s modulus as low as
50 MPa (Cansizoglu et al., 2008; Li et al., 2006). The strength
and Young’s modulus of cellular structures are well
controlled through struts width, angles and relative density
of the structure (Cansizoglu et al., 2008; Li et al., 2006;
Schwerdtfeger et al., 2010).
4.
Low cost implants
As discussed above, the future long term metallic implants
should be made of those alloys that show high mechanical
compatibility (i.e., high strength, high wear resistance and
low Young’s modulus) and are composed of non-toxic elements. The most promising alloys for implant applications
are bÀtype titanium alloys. That is why, many bÀtype
titanium alloys composed of non-toxic elements Nb, Ta, Zr,
Mo, Hf, Au, Pd, Pt, Ag, Ga, Ge, Sc, and Sn were developed in
the last two decades (Cui and Guo, 2009; Kuroda et al., 1998;
Lopez et al., 2001; Niinomi, 2003).
However, most of these developed b-titanium alloys contain considerable amounts of the expensive, high melting
point, and high density metals (such as; Nb, Ta, Zr, and Mo).
These elements are also rare ones due to their low abundances in the earth’s crust. In contrast, titanium is considered to be a ubiquitous element since it has the tenth highest
Clarke number of all the elements. This leads to high cost of
the raw materials and difficulty in the alloy preparation due
to the high melting points of the constituent elements that
leads to macro- and micro-segregations (Narita et al., 2012;
Zhou et al., 2006). Generally, the production cost and/or
difficulty of any alloy limit its range of applications and are

7. journal of the mechanical behavior of biomedical materials 20 (2013) 407 –415
considered the main reasons behind the ease of commercialization of any Ti-alloys. Many of the recently developed
alloys failed to compete with the commercial alloys due to
the difficulty in its production and their high content of
expensive rare earth metals (i.e., Nb, Ta, Zr, and Mo).
In addition, it is important for the strategy of developing
titanium alloys for high performance is using ubiquitous
elements of the alloying.
Consequently, there is a great need to develop new bÀtype
Ti alloys for biomedical applications, composed of non-toxic
and low cost common metals, such as Mn, Fe, Si and Sn
(Helsen and Bremr, 1998), that show high strength, high
corrosion resistance and low Young’s modulus. However,
the minimum Young’s modulus reported for the binary
Ti–M (M¼Mn and Fe) alloys is 95 GPa which, in some sense,
is still high compared to that of bone, while Ti–N (N¼Si and
Sn) binary alloys would intrinsically show high Young’s
modulus. This is because Si and Sn are aÀstabilizing
elements and they cannot maintain bÀphase as the predominant phase in the alloy when added alone to the alloys.
Therefore, the co-addition of b-stabilizing elements with
these elements is essential to have b-type alloys.
The author reported in a previous work that, in b-type
Ti-alloys, the Young’s modulus is decreasing with increasing
the Bo value of the alloy in the BoÀMddiagram (Abdel-Hady
et al., 2006; Kuroda et al., 1998). In BoÀMddiagram, the
alloying vectors of Fe, Mn, Si and Sn are going to lower
Bo values (Abdel-Hady et al., 2007). Therefore, to design
b-titanium alloys with low Young’s modulus, it is still important
to co-add the elements with high Bo values (i.e., Mo, Nb, Ta,
Zr and Hf) (Abdel-Hady et al., 2006, 2007) even with small
quantities. Using the BoÀMddiagram would be very useful in
achieving this aim. For example, Ti–Fe–Ta and Ti–Fe–Ta–Zr
alloys were developed for bio-implant applications with the
aid of Bo-Mddiagram (Kuroda et al., 2005). Also, with the aid
of BoÀMddiagram, the present author is developing new low
cost Ti–Fe–Nb–Zr alloys (TFNZ) that show Young’s modulus of
75 GPa and UTS of 1169 MPa, detailed explanation will be
presented elsewhere. The TFNZ alloy shows elastic admissible strain higher than TNTZ, Cp Ti, Ti-64 ELI and SUS-316L,
and comparable to the alloys with high content of the
expensive rare earth metals. This means that the proposed
low cost and biocompatible Ti-alloys for long-time implantation can compete with the other commercial alloys and even
the recently developed b-type Ti-alloys from mechanical
biocompatibility point of view at least.
Another very important advantage of b-titanium alloys to
be commercialized is its high cold workability. This is
because the production cost of the implants is highly
concerned with the easiness of formation or manufacturing.
Due to many technical considerations (namely; surface finishing,
dimensional accuracy, sub-deformation treatments and heating
process) the cold forming ability is a cost effective process if
compared to the cost of hot forming of Ti-alloys.
5.
Conclusion
Considering both the mechanical and biological biocompatibility of implants, the production cost, and using ubiquitous
413
alloying elements in order to establish the strategic method
for suppressing utilization of rare metals, it seemed that the
future metallic implants for long-term usage would be those
bÀtype Ti alloys, composing mainly of low cost common
metals such as Mn, Sn or Fe, that show high strength, low
Young’s modulus and good cold workability.
Acknowledgments
Part of the experimental work presented in this paper
was done at the laboratory and under the supervision of
Prof. Masahiko Morinaga, Nagoya University, Japan. This
study was supported partially by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan, the Japan Society for the
Promotion of Science, by the 21st centaury Global Center of
Excellence of Japan (G-COE), and by Science and Technology
Research Fund (STDF), Egypt.
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