New insights into nickel-free superelastic titanium alloys for biomedical applications
Introduction
The superelasticity of a metallic material refers to the significant recoverable strain exhibited by the material on unloading. This phenomenon, generally driven by stress-induced reversible martensitic transformation, was first reported in 1958 [1], followed by the milestone development of the nickel-based NiTi in 1962 [2], [3]. Superelasticity is closely related to the shape memory effect in terms of the driving mechanism. To date, Ti-rich NiTi (i.e. Martensitic Nitinol) and Ni-rich NiTi (i.e. Austenitic Nitinol) are the premier shape memory and superelastic alloys in use, respectively. Table 1 summarises the important developments of superelasticity with emphasis on medical applications. Fig. 1, on the other hand, reveals the increasing trend in superelasticity-related patents filed since early 1990s. The last three decades have seen an impressive rise in the demand for superelastic properties across various technological fields. The drop after 2017 is caused by data lags.
Recently, research in superelastic bio-materials has shifted towards the design and development of Ni-free superelastic Ti alloys. This is due to the rising concerns over the potential health hazards corresponding to the release of Ni ions from the Ni-containing implants after implantation.
This paper discusses the most recent advances in biomedical Ni-free superelastic Ti alloys including new alloy design principles, novel manufacturing processes, and emerging applications, along with additive manufacturing (AM) enabled hierarchical micro-architectural superelastic lattice structures.
Section snippets
Due to reversible stress-induced martensitic transformation
Superelasticity typically arises from the reversible stress-induced martensitic transformation (Fig. 2). Unlike the shape memory effect where the material is deformed while in the martensitic state, superelasticity is achieved above the austenite finish temperature () [43] (i.e. in the Austenitic state). The body-centred cubic (BCC) β-phase in Ti-based superelastic alloys is referred to as the “austenite” phase. When loaded up to a certain critical stress (), β phase transforms into
Potential health hazards associated with Ni-containing superelastic alloys
Ni-rich NiTi alloy is the most commonly used superelastic alloy as implants, orthodontic wires and dental and surgical tools. This is ongoing while the serious health hazards of nickel are clearly established [66]. The primary potential risk associated with the use of NiTi alloy is Ni hypersensitivity and contact dermatitis. Metal ions such as , , or are haptens with high immunogenic potential. Particularly, Ni is classified as carcinogenic, genotoxic, mutagenic, allergenic
Elastic properties of human bones
Superelastic materials have great advantages for orthopaedic load-bearing implants due to their superior damping properties [125]. These properties minimise local stress intensity experienced by bone tissue and the bone-implant elastic mismatch. Here we limit our attention to the superelastic-like properties of human bones and their elastic properties as a function of age, bone types and anatomic sites.
Additive manufacturing of Ni-free superelastic titanium alloys
Ni-free superelastic Ti alloys have been fabricated by vacuum arc remelting, vacuum induction melting and electron beam melting primarily for research purposes. For instance, the superelastic Ti-25Nb-25Ta (wt.%) [100] and Ti-6Mo-4Sn (at.%) [170] alloys were synthesised using cold crucible induction melting to achieve the desired homogeneity. Chemical homogeneity is particularly important in the case of superelastic alloys due to the high sensitivity of martensitic transformation temperatures to
Applications of superelasticity in medical and dental industries
Superelasticity has been in practical use in the biomedical and dental industries for over 20 years. In addition, it has found niche applications in other sectors. Selected examples are discussed below.
Concluding remarks and future perspectives
The following insights can be gained from this critical review:
- (1)
In addition to the classical superelasticity driven by stress-induced reversible martensitic transformations, hierarchical micro-architectured designs enabled by AM can lead to more significant superelastic behaviour, without undergoing phase transformations. Such superelastic micro-architectures or meta-materials offer an entirely new pathway to the utilisation of superelasticity. It upends the conventional
Acknowledgements
This work was supported by the Australian Research Council (ARC), Australia through the Linkage Program (LP) under LP140100608.
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