Introduction: The functional requirements to the load-bearing implant materials imply the proximity of their mechanical behavior to that of bone, “tissue-friendly” surface properties and adequate bulk biocompatibility. Ti-based metastable beta-type superelastic alloys are prospective materials for such applications, since they offer an excellent alternative to the highly biomechanically-compatible Ti-Ni shape memory alloys, being exempt of toxic nickel. The present study is aimed at assessing the potential of Ti-Nb- and Ti-Zr-based superelastic alloys as load-bearing implant materials.
Materials and Methods: Ti-22Nb-6Zr(6Ta) and Ti-18Zr-14Nb (at.%) alloys were produced by induction skull melting and vacuum arc melting. The obtained TNZ(T) and TZN ingots were subjected to cold rolling, post-deformation annealing and age hardening (optional). Then they were characterized using microstructural analysis (TEM and X-ray diffraction), mechanical (static and multicycle testing), surface (tribology and scratch testing, nano-indentation and wettability measurements), and corrosion resistance evaluation techniques (open-circuit potential, corrosion rate measurements, and corrosion fatigue testing). For biocompatibility evaluation, in vitro cytotoxicity tests were carried out.
Results and Discussion: All thermomechanically-treated TNZ(T) and TZN (Fig. 1) superelastic alloys exhibit low elastic modulus (~40 GPa) and stable superelastic behavior at human body temperature[1]. The difference between them resides in the maximum superelastic strain they can sustain: it is significantly higher for the TZN than for the TNZ(T) alloys: ~5% vs ~3%. Corrosion resistance of TNZ(T) superelastic alloys is comparable to that of biomedical titanium[2]. Local martensitic transformation and superelasticity effect are shown to improve the corrosion fracture resistance of these materials[3]. Surface of both the as-cast and thermomechanically-treated TNZ(T) alloys is potentially friendly to surrounding body tissues, as supported by the preliminary results of in vitro cytotoxicity tests (Fig. 2).
Fig. 1. Typical loading-unloading cycling diagrams of TZN alloy quenched from 700°C: SE – superelasticity, SME – shape memory effect upon heating
Fig.2. Optical image of human bone marrow mesenchymal stem cell culture on the TNZ surface after 7-day incubation in (DMEM + 10% FBS) media at 37°C, 5 % CO2
Conclusion: The Ti-Nb-based superelastic alloys (TNZ(T)) are adequate implant materials in terms of their biomechanical compatibility, surface characteristics, corrosion resistance and absence of cytotoxicity. Their maximum superelastic strains are however relatively low, which may impact negatively fatigue life of an implant. The Ti-Zr-based alloy (TZN) combines all the advantages of its Ti-Nb-based counterparts, but its exhibits almost twice as large superelastic strain, which makes it exceptionally promising as load-bearing implant material.
Fonds de recherche du Québec – Nature et Technologies (FRQ-NT).; Natural Sciences and Engineering Research Council of Canada (NSERC).; Ministry of Education and Science of the Russian Federation (Grant No. К4-2014-018) and project № 3055 of the basic part of State task № 2014/113.
References:
[1] K. Inaekyan, V. Brailovski, S. Prokoshkin, V. Pushin, S.Dubinskiy, V.Sheremetyev, Comparative study of structure formation and mechanical behavior of age-hardened Ti-Nb-Zr and Ti-Nb-Ta shape memory alloys, Mater. Charact. (2015), v 103, p 65-74.
[2] Yu. Zhukova, Yu. Pustov, A. Konopatsky, M. Filonov, Characterization of electrochemical behavior and surface oxide films on superelastic biomedical Ti–Nb–Ta alloy in simulated physiological solutions, J. Alloys Comp. (2014), v 586, p S535-S538.
[3] Yu.S. Zhukova, Yu.A. Pustov, A.S. Konopatsky, M.R. Filonov, S.D. Prokoshkin, Electrochemical behavior of novel superelastic biomedical alloys in simulated physiological media under cyclic load, J. Mater. Eng. Perform. 23 (2014) 2677–2681.