Functionalization of an experimental Ti-Nb-Zr-Ta alloy with a biomimetic coating produced by plasma electrolytic oxidation

doi:10.1016/j.jallcom.2018.08.154

Journal of Alloys and Compounds

Volume 770, 5 January 2019, Pages 1038-1048

https://doi.org/10.1016/j.jallcom.2018.08.154 Get rights and content

Highlights

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A β experimental Ti-Nb-Zr-Ta alloy with low elastic modulus was developed and functionalized.
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The Ti-Nb-Zr-Ta alloy exhibited good electrochemical stability and surface properties.
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Plasma electrolytic oxidation (PEO) was a feasible surface treatment for the experimental alloy.
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PEO-treated surfaces exhibited remarkable electrochemical stability.

Abstract

This study developed an experimental quaternary titanium (Ti) alloy and evaluated its surface properties and electrochemical stability. The viability for a biofunctional surface treatment was also tested. Ti-35Nb-7Zr-5Ta (wt%) alloy was developed from pure metals. Commercially pure titanium (cpTi) and Ti-6Al-4V were used as controls. All groups had two surface conditions: untreated (machined surface) and modified by plasma electrolytic oxidation (PEO) (treated surface). The experimental alloy was successfully synthesized and exhibited β microstructure. PEO treatment created a porous surface with increased roughness, surface free energy, hardness and electrochemical stability (p < 0.05). For the machined surfaces, the Ti-Nb-Zr-Ta alloy presented the lowest hardness and elastic modulus (p < 0.05) and displayed greater polarization resistance relative to cpTi. Only PEO-treated cpTi and Ti-Al-V alloys exhibited anatase and rutile as crystalline structures. The β experimental Ti-Nb-Zr-Ta alloy seems to be a good alternative for the manufacture of dental implants, since it presents elastic modulus closer to that of bone, feasibility for surface treatment, electrochemical stability and absence of toxic elements.

Introduction

Commercially pure titanium (cpTi) has been widely used as a biomaterial for medical and dental implants and presents long-term longevity for clinical outcomes [1]. Nevertheless, one of the disadvantages of pure titanium (Ti) is its low deformability and wear resistance, which could increase the risk of implant fractures [[2], [3], [4]]. Therefore, the addition of alloying elements to Ti has emerged as an alternative to replace this material and enhance its properties [5].

The α/β-phase Ti-6Al-4V alloy is one of the first Ti-based biomaterials fabricated for implant applications [6], and it is still one of the most-used materials for this purpose, owing to its better corrosion resistance and mechanical properties compared with those of cpTi [7]. However, there is a significant concern about the potential cytotoxic effects that the release of aluminum (Al) and vanadium (V) ions might cause to long-term human health [8,9]. Also, another disadvantage of the commercially used Ti-based alloy is its higher elastic modulus when compared with that of bone tissue, leading to a stress-shielding phenomenon that can cause resorption of the bone adjacent to the implant and osteoporosis, with consequent eventual implant loosening [2,10,11].

Recently, experimental Ti alloys have been developed to overcome these biological and mechanical shortcomings, combining superelasticity with biocompatibility. For these reasons, many studies have focused on β-Ti alloys with non-toxic elements, such as niobium (Nb), zirconium (Zr) and tantalum (Ta) [6,7,11,12]. The system combining these three elements with Ti, forming the Ti-Nb-Zr-Ta alloys, has been highlighted as a novel potential implant material for clinical application due to its low Young's modulus, close to that of bone tissue [[12], [13], [14]]. Further, this system is optimized due to its adequate cytocompatibility and good corrosion resistance [11,12,15,16].

These excellent biological and corrosion characteristics are achieved mainly due to an oxide film (TiO₂, Nb₂O₅, ZrO₂ or Ta₂O₅) spontaneously formed on Ti and its alloys when exposed to atmospheric air [7,17,18]. Sometimes, the oxide films are not capable of bearing the continuous degradation to which dental implants are exposed in the oral environment (e.g. saliva, pH, biofilm and fluoride). Such degradation can trigger an inflammatory reaction, with peri-implant bone loss induced by the release of corrosive products and wear debris [18,19]. Thus, it is necessary to enhance the bioactivity and corrosion resistance of Ti-based materials used for dental implant applications.

Different surface modification methods have been carried out to overcome this limitation [17,20,21]. Plasma electrolytic oxidation (PEO) is an efficient technique that consists of an electrochemical process capable of creating a strong porous film that adheres to the substrate [22], allowing for the incorporation of bioactive elements, such as calcium (Ca) and phosphorus (P) [21], which plays an important role in improving the osseointegration process [20,23]. Some in vitro and in vivo studies [20,24] have reported the excellent bioactivity of PEO treatment on β-type Ti-Nb-Ta-Zr alloys. The results showed that the incorporation of Ca, P and magnesium (Mg) into the oxide layer is a promising technique to improve the bioactivity and hard-tissue compatibility of the Ti-29Nb-13Ta-4.6Zr alloy [24]. Also, the TiO₂ layer doped with Ca, P and strontium (Sr) on the surface of the Ti-35Nb-2Ta-3Zr alloy displayed improved cell function and osseointegration ability [20].

Although some improvement has been made in the bioactivity properties, no study was found in the literature to provide an in-depth understanding of the electrochemical behavior of the Ti-Nb-Zr-Ta alloy when treated with PEO in physiological conditions. Therefore, to extend the clinical application of this alloy in the implant dentistry field, we aimed to evaluate the role of PEO treatment in the microstructural, mechanical, chemical and electrochemical properties of the experimental Ti-35Nb-7Zr-5Ta (in wt%) alloy and compare it with those materials clinically used for the manufacture of dental implants (cpTi and Ti-6Al-4V).

Section snippets

Experimental design

The experimental design of the present study is illustrated in Fig. 1. CpTi and Ti-6Al-4V discs (Mac-Master Carr, Elmhurst, IL, USA) with 10 mm diameter and 2 mm thickness were used and compared with Ti-35Nb-7Zr-5Ta (in wt%). The microstructural, mechanical, chemical and corrosion behaviors of the Ti-based materials were evaluated with and without PEO treatment.

Fabrication of the experimental alloy

The Ti-35Nb-7Zr-5Ta alloy (in wt%) was fabricated by melting Ti, Nb, Zr and Ta pure metals (Sigma–Aldrich, St. Louis, MO, USA) in an

Results and discussion

Fig. 2 shows the surface topography obtained by AFM. The surface morphology was evidently changed by the PEO treatment, creating volcano-like structures with micropores over the entire surface as a consequence of the micro-sparks that occurred during the process [29]. Such characteristic was more evident in the cpTi and TiAlV materials, whereas a flatter surface was observed for the TiNbZrTa alloy. Indeed, in the cross-section view, the TiNbZrTa PEO surface showed a smaller discrepancy in the

Conclusions

The Ti-35Nb-7Zr-5Ta alloy was developed and functionalized with PEO for the comparison of its physical, mechanical, chemical and electrochemical properties with those of commercially available implant materials (cpTi and Ti-6Al-V). Based on the results of this in vitro study, the TiNbZrTa alloy is a suitable option for the manufacture of dental implants. This β experimental alloy presented electrochemical behavior similar to that of TiAlV, but with little tendency to transport ions across the

Acknowledgments

This work was supported by the São Paulo State Research Foundation (FAPESP), Brazil (grant numbers 2016/11470-6 and 2017/01320-0). The authors express their gratitude to Jamille Altheman for her contribution and support at the Laboratory of Technological Plasmas at Univ. Estadual Paulista (UNESP), to Dr Richard Landers and Rita Vinhas from the University of Campinas (Institute of Physics Gleb Wataghin) for providing the XPS facility, to Dr Mathew T Mathew from the University of Illinois at

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Functionalization of an experimental Ti-Nb-Zr-Ta alloy with a biomimetic coating produced by plasma electrolytic oxidation

Highlights

Abstract

Introduction

Section snippets

Experimental design

Fabrication of the experimental alloy

Results and discussion

Conclusions

Acknowledgments

Mater. Sci. Eng. C

Mater. Sci. Eng. A

Dent. Mater.

Toxicol. Appl. Pharmacol.

Biomaterials

Mater. Sci. Eng. C

Acta Biomater.

J. Mech. Behav. Biomed. Mater.

Acta Biomater.

Appl. Surf. Sci.

Corrosion Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

J. Prosthet. Dent.

Acta Biomater.

Surf. Coatings Technol.

Mater. Sci. Eng. C

J. Alloys Compd.

Biomaterials

Mater. Char.

Mater. Des.

Surf. Coatings Technol.

Min. Sci. Technol.