New insights into nickel-free superelastic titanium alloys for biomedical applications

doi:10.1016/j.cossms.2019.100783

Current Opinion in Solid State and Materials Science

Volume 23, Issue 6, December 2019, 100783

https://doi.org/10.1016/j.cossms.2019.100783 Get rights and content

Highlights

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A concise review of recent advances in Ni-free superelastic Ti alloys.
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A detailed discussion of architectural design enabled superelasticity without resort to reversible phase transformation.
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An in-depth analysis of additive manufacturing for Ni-free superelastic Ti alloys.
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A comprehensive assessment of the properties of both dense and porous Ni-free superelastic Ti alloys vs. human bones.
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An overview of representative biomedical applications of superelastic Ti alloys.

Abstract

Superelastic titanium (Ti) alloys are a group of unique functional metallic materials capable of recovering a substantial amount of mechanical strain thereby offering superior resilience. Such strain recovery is significantly greater than that exhibited by conventional elasticity and has been demonstrated to be clearly beneficial and necessary for a vast range of biomedical and dental applications. For example, the age-related physiological deterioration of bones signifies the necessity of employing superelastic implants. Currently, NiTi alloy remains to be the premier choice of superelastic alloys in the broad biomedical sector. However, recently reinforced views on the toxic, carcinogenic and allergenic properties of nickel have resulted in intensified concerns. This has encouraged the design and fabrication of Ni-free superelastic Ti alloys. In addition, enabled by additive manufacturing (AM) or 3D printing, hierarchical micro-architectured lattice meta-materials can exhibit exceptional superelasticity without undergoing phase transformations, upending the conventional perception and unlocking brand-new pathways to exploiting metal superelasticity. This article discusses recent developments in Ni-free superelastic Ti alloys and the determining factors affecting their superelastic recoverable strain. The importance of implant superelasticity relative to the elastic and “superelastic” properties of human bones is examined. Also discussed are the advances in Ni-free Ti-based superelastic alloy design and superelasticity-demanding medical and dental applications. The impact of the AM-enabled micro-architectural design on the development of superelastic structures or superelastic meta-materials is deliberated. Future research priorities are suggested.

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 ( $A_{f}$ ) [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 ( $σ_{SIM}$ ), 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 ${N i}^{2 +}$ , ${C o}^{2 +}$ , ${C u}^{2 +}$ or ${C r}^{2 +}$ 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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New insights into nickel-free superelastic titanium alloys for biomedical applications

Highlights

Abstract

Introduction

Section snippets

Due to reversible stress-induced martensitic transformation

Potential health hazards associated with Ni-containing superelastic alloys

Elastic properties of human bones

Additive manufacturing of Ni-free superelastic titanium alloys

Applications of superelasticity in medical and dental industries

Concluding remarks and future perspectives

Acknowledgements

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Mater. Sci. Eng., A

Mater. Sci. Eng., C

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Mater. Sci. Eng., A

Mater. Sci. Eng., C

J. Mech. Behav. Biomed. Mater.

A“ super-elastic” single crystal calibration bar

Br. J. Appl. Phys.

History and current situation of shape memory alloys devices for minimally invasive surgery

Open Med. Devices J.

Effect of low-temperature phase changes on the mechanical properties of alloys near composition TiNi

J. Appl. Phys.

Superelastic organic crystals

Angew. Chem. Int. Ed.

An electrochemical investigation of solid cadmium-gold alloys

J. Am. Chem. Soc.

Plastic deformation and diffusionless phase changes in metals-the gold-cadmium beta-phase

Trans. Am. Instit. Min. Metall. Engineers

Metall. Mater. Trans.

Mechanism of shape memory effect and superelasticity

Shape Mem. Mater.

An overview of nickel–titanium alloys used in dentistry

Int. Endod. J.

Shape Memory Effects in Alloys

Sentalloy, the Story of Superelasticity

A Proposed Medical Application of the Shape Memory Effect: A NiTi Harrington Rod for the Treatment of Scoliosis, Shape Memory Effects in Alloys

A vena cava filter using thermal shape memory alloy: experimental aspects 1

Radiology

Present and future applications of shape memory and superelastic materials

MRS Proceedings

Studies and applications of NiTi shape memory alloys in the medical field in China

Bio-Med. Mater. Eng.

Medical and dental applications of shape memory alloys

Shape Mem. Mater.

Transluminal expandable nitinol coil stent grafting: preliminary report

Radiology

Variable curvature shape-memory spatula for laparoscopic surgery

Surg. Endosc.

Laparoscopic inguinal hernioplasty

Surg. Endosc.

Percutaneous femoropopliteal graft placement

Radiology