ReviewDevelopment of new metallic alloys for biomedical applications
Introduction
Around 70–80% of implants are made of metallic biomaterials. Metallic biomaterials are remarkably important for the reconstruction of failed tissue, especially failed hard tissue, to improve the quality of life (QOL) of the patient. The demand for metallic biomaterials is increasing rapidly because the world population is getting increasingly older, and elderly people have a higher risk of hard tissue failure. The biological and mechanical biocompatibility of metallic biomaterials require much improvement. Furthermore, the biofunctionality of metallic biomaterials is at present inadequate, and needs to be improved.
Representative practical metallic biomaterials are stainless steels, cobalt (Co)–chromium (Cr) alloys, and titanium (Ti) and its alloys. Among these metallic biomaterials Ti alloys exhibit the highest biocompatibility, corrosion resistance, and specific strength (ratio of the tensile strength to density), compared with stainless steels and Co–Cr alloys. Co–Cr alloys exhibit the highest wear resistance and relatively higher strength compared with stainless steels and Ti alloys. Stainless steels generally exhibit higher ductility and cyclic twist strength compared with Co–Cr and Ti alloys. Stiffness is greatest for Co–Cr alloys, while it is the lowest for Ti alloys. Other metallic biomaterials, such as magnesium (Mg) alloys, iron (Fe), tantalum (Ta), and niobium (Nb) are also important, although their share of this field is small. Intensive research and development is being carried out globally on all kinds of metallic biomaterials. The elemental components of metallic biomaterials are basically non-toxic. Representative elements are Ti, Nb, Ta, molybdenum (Mo), and zirconium (Zr) [1], [2]. In addition to these, Fe, tin (Sn), Co, hafnium (Hf), manganese (Mn), and Cr [3], [4], [5], [6], [7] have also been studied. Nickel (Ni) is a popular element for addition to stainless steels, and Co–Cr alloys are often used for biomedical applications. However, nowadays, Ni is widely recognized as a high risk element from the view point of incompatability problems [8], [9], [10]. Therefore, Ni is now avoided as much as possible as an additive in metallic biomaterials. Because of this Ni-free stainless steels [11] and Co–Cr alloys [12] have recently been developed. Vanadium (V)- and aluminum (Al)-free Ti alloys [10] were developed fairly early on in the realization of Ti alloys for biomedical applications, because of the toxicity of V [13] and issues with regard to Al causing Alzheimer’s disease [14]. Al has since been proved not to be a cause of Alzheimer’s disease [15], although it has been shown to be neurotoxic [16]. Recently improvements in mechanical biocompatibility in terms of properties such as the Young’s modulus, strength/ductility balance, fatigue strength, fracture toughness, and wear resistance of metallic biomaterials have been achieved [17]. Among these properties control of the Young’s modulus in particular has been extensively investigated because the much higher Young’s moduli of metallic biomaterials compared with bone can lead to bone atrophy and poor bone remodeling [18], although implants need to exhibit structural stiffness. Therefore, low Young’s modulus Ti alloys for use in replacing failed hard tissue (bone), such as artificial hip joints, bone plates, and spinal fixation rods, are required. Nowadays new concept metallic biomaterials such as Ti alloys with self-tunable Young’s moduli for spinal fixation rods, and Ti and Zr alloys for removable implants are being investigated. Further, the creation of biofunctionalities such as bone conductivity and blood compatibility of metallic biomaterials through surface modification [19] are being widely investigated. Dental applications of metallic materials such as Ti alloys [20] and alloys of precious metals such as gold (Au) [21] and silver (Ag) [22] containing high amounts of platinum (Pt) and palladium (Pd) are also of interest. The authors hope that metallic materials for dental applications will be reviewed elsewhere in the future.
Representative topics in terms of new metallic biomaterials for implants for the reconstruction of hard tissue, as used mainly in orthopedic surgery, and for the reconstruction of soft tissues such as blood vessels are here described.
Section snippets
Titanium and its alloys
The biocompatibilities of Ti (pure Ti) and its alloys are the highest among the main metallic biomaterials, such as stainless steels and Co–Cr alloys. Ti alloys also exhibit a high specific ratio (i.e. tensile strength divided by density) and good corrosion resistance. Therefore, Ti alloys have attracted a great deal of attention for biomedical applications. Early in the development of Ti alloys for biomedical applications V-free TI alloys such as Ti–6Al–7Nb [23] and Ti–5Al–2.5Fe [24] were
Stainless steels
Stainless steels biomaterials have the longest record of practical use. SUS 316L, an austenitic stainless steel, is the only one used for biomedical applications such as bone fixation (bone plates, screw wire min-plate, etc.), spinal fixation, and cardiovascular (electric terminal, stent) applications, and as catheters. Since SUS 316L contains a large amount of Ni, there is the possibility of Ni toxicity problems, as already mentioned in the sections on titanium alloys and Co–Cr system alloys.
Cobalt alloys
Co alloys exhibit high corrosion resistance and especially excellent wear resistance [142], so they are often employed for artificial joints. Co also shows an allotropic transformation: the hcp ε and fcc γ phases are stable at room and at high temperature, respectively. The number of slip systems is larger for the γ phase than for the ε phase, so the crystal structure in the matrix phase is controlled to the γ phase in order to achieve higher ductility. In this case Ni is often employed as an
Zirconium alloys
When metallic orthopedic implants are inserted into a patient’s body and magnetic resonance imaging (MRI) is performed, defects and distortions sometimes appear in the MRI images (artifacts). These kinds of artifacts are caused by differences in the magnetic susceptibilities of the metals and living tissues [142]. The magnetic susceptibility of living tissues is −9 × 10−6 cm3 g−1, being that of water [147]. On the other hand, the magnetic susceptibility of paramagnetic Ti is 3.2 × 10−6 cm3 g−1 [148].
Tantalum and niobium alloys
Ta and Nb are non-toxic elements, as stated above, and exhibit very similar physical and chemical properties. These metals have been utilized as constituent elements of Ti alloys for biomedical applications because of their good biocompatibility and high corrosion resistance.
Ta exhibits excellent chemical stability and good biocompatibility similar to that of Ti [152], [153]. Therefore, Ta has been employed in dental and orthopedic applications such as radiographic bone markers, vascular clips,
Mg alloys
The tensile strength and elongation to fracture of femoral cortical bone under dry and wet conditions in the longitudinal direction are around 102 MPa and approximately 1.2%, and around 76 MPa and approximately 1.0%, respectively [173], and the Young’s modulus is around 10–30 GPa [17], while the tensile strength, elongation to fracture, and Young’s modulus of the magnesium (Mg) alloy AZ91D are 230 MPa, 3%, and 45 GPa, respectively. Thus the mechanical properties of Mg are closer to those of bone
Summary
Metallic biomaterials are still widely used, mainly for the reconstruction of failed hard tissue, i.e. bone. Therefore, new metallic biomaterials are continually being proposed and developed. The biofunctionalization (including mechanical biofunctionalization) of such metallic biomaterials is becoming a much more important issue in the development of high performance metallic biomaterials for the construction of implant devices. New concept metallic biomaterials such as Ti alloys with
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