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    • 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
    • 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.
    • 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
    • 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.
    • 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.
    • 412 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
    • 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. r e f e r e nc e s Abdel-Hady, M., Fuwa, H., Henoshita, K., Morinaga, M., 2008. Change in anisotropy of mechanical properties with b-phase stability in high Zr-containing Ti-based alloys. Materials Science and Engineering A 480, 167–174. Abdel-Hady, M., Fuwa, H., Henoshita, K., Shinzato, Y., Morinaga, M., 2007. Phase stability change with Zr content in b-type Ti–Nb alloys. Scripta Materialia 57 (11), 1000–1003. Abdel-Hady, M., Hinoshita, K., Morinaga, M., 2006. General approach to phase stability and elastic properties of beta-type Ti-alloys using electronic parameters. Scripta Materialia 55 (5), 477–480. Abdel-Hady, M., Morinaga, M., 2009a. 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