Production and characterization of TiO2 nanotubes on Ti-Nb-Mo-Sn system for biomedical applications
Introduction
Titanium (Ti) and its alloys have been widely used in the biomedical field, mainly in artificial joints and orthopedic and dental implants, because of their low elastic modulus, high fatigue strength, excellent corrosion resistance and greater biocompatibility than that of stainless steels and Co-Cr alloys materials. In this field, an in-depth understanding of the bone-implant interface is fundamental. It is known that considerable differences in the elastic moduli of bone and implant materials may lead to bone resorption, stress shielding, and eventually implant failure [1], [2]. Within this context, the β-Ti alloys are promising because their elastic modulus is lower than that of α and α + β Ti alloys. Moreover, βTi alloys can be processed at lower temperatures than α + β alloys and some heavily stabilized alloys can even be cold deformed [3]. Previous studies have achieved outstanding results when using Nb, Zr, Mo, Ta and Sn as β stabilizers in the manufacture of Ti alloys [2], [4], [5], [6]. Sn is used as an alloying element not only because of its potential to improve the alloy's corrosion resistance [5], [7] but also because of its ability to inhibit excessive ω phase precipitation in metastable β Ti alloys [5], [8]. Studies have shown that the addition of Mo to Ti alloys provides good cytocompatibility [9], [10] and a considerable gain in corrosion resistance [11], thus making it a very suitable candidate in the design of innovative Ti alloys for biomedical applications. Previous studies [12], [13], [14] have reported the high cytocompatibility of Nb and Sn applied as alloy elements, proving them to be nontoxic and nonallergenic. According to Cremasco and co-workers [15], Nb and Sn caused no toxic effects and show good cell adhesion, as indicated by in vitro cytocompatibility. Moreover, the classification mentioned by Banerjee and Williams [16] reveals that Nb and Sn are nontoxic regarding several biological aspects.
The domain of surface mechanical properties is crucial to bone-implant materials meant for long-term applications. According to several authors [17], [18], TiO2 nanotubes show lower elastic modulus when compared to the substrate (Ti). Santos et al. [19] revealed that it was possible to obtain an enhanced surface due to a homogeneous TiO2 nanotube layer with elastic modulus values suitable to improve osseointegration. An interesting work conducted by Campanelli et al. [20] demonstrated that the nanoscale thickness of the nanotubes does not affect the fatigue performance of the substrate. An ex vivo equine implantation was made by Shivaram and co-workers [21] to evaluate the mechanical degradation of TiO2 nanotubes and the results showed no significant changes in the morphology of the nanotubes, indicating that their damage resistance withstands implantation and explantation.
However, the success of a bone implant depends not only on a low elastic modulus but also on good osseointegration at the bone-implant interface. Given that the surface of Ti alloy is not adequate to promote new bone formation in the early stages of the implantation [22], osseointegration may be improved by modifying the surface of the implant, e.g., through the controlled growth of a self-organized TiO2 nanotube layer via electrochemical anodization [23]. The enormous interest in the nanotubes for biological applications is due to their high surface area and size dependent effects [24], [25]. Also, the correlation between the adhesion and growing of mesenchymal stem cells and the nanolayers is noteworthy [26], as well as the propagation of the osteoblast cells in these nanotubes [27] and the remarkable use of these layers as drug delivery capsules [28], [29], [30]
As-anodized TiO2 nanotubes are usually amorphous, and depending on the heat treatment that is applied, TiO2 can show two polymorphic structures: anatase and rutile. These polymorphic structures display different physical and chemical properties, such as distinct wettability characteristics, which strongly affect cell response [31]. In recent years, metallic titanium containing self-organized TiO2 nanotube layers has attracted increasing attention due to the unique combination of titanium oxide properties, such as non-toxicity and high stability in adverse conditions of moisture and pH, with the benefits provided by nanostructured materials. Implant materials generally remain in contact with living cells for long periods of time, while reduced cell viability may give rise to inflammatory reactions, resulting in enormous medical costs and even an increase in morbidity. A remarkable advantage of working with TiO2 nanotubes is the tremendous gain in surface area and their dimensional similarities with cells and tissues, which is extremely useful for biomedical applications because it can enhance cell proliferation and adhesion [23]. Moreover, these nanotubes stimulate the growth of osteoblast cells, improving osseointegration [1], [32].
This paper evaluates the effect of the addition of Mo on the Ti-Nb-Sn system. The alloys were subjected to an optimization process by varying the electrolytic concentration, aiming the formation of a self-organized layer of TiO2 nanotubes. The influence of substrate surface roughness on the process of TiO2 nanotube formation was also investigated, as were the crystallization parameters and wettability of the grown nanotubes layers.
Section snippets
Experimental procedure
Ti alloy samples were arc melted in a furnace under an inert argon atmosphere, using non-consumable tungsten electrode. The composition of samples of Ti-30Nb-4Sn-xMo (x = 0, 1 and 2 wt%) alloy was characterized by X-ray fluorescence spectroscopy (XRF) (Shimadzu EDX7000 spectrometer). The interstitial contents of O and N were analyzed in a Leco TC400 analyzer, using the inert gas fusion method (ASTM E1569). The samples were encapsulated in quartz tubes with an argon atmosphere and the chemical
Morphological and mechanical characterization of the substrate
Since the nano-features of implants strongly interfere in cell adherence, the preparation parameters of the nanotubes were investigated. The nanotubes were produced on the substrates of Ti-30Nb-4Sn, Ti-30Nb-4Sn-1Mo, and Ti-30Nb-4Sn-2Mo alloys. The compositions of the substrates were assessed by XRF and the results are in accordance with the alloy's nominal composition, as shown in Table 1, which also lists the measured interstitial O and N contents. According to the ASTM E 1409-13 standard, 0.5
Electrochemical anodization to produce TiO2 nanotubes and evaluation of their morphology
Titanium oxide nanotube arrays can be obtained by electrochemical anodization, which is a low cost process that allows for the easy control of parameters that affect the dimensional and morphological characteristics of nanotubes. According to the literature, the anodization time and etching rate controlled by fluoride content affect nanotube length, while voltage can interfere in their diameter [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]]. Our
Structural evaluation of TiO2 nanotubes
The TiO2 nanotube layer formed during anodization presents an amorphous atomic arrangement. The crystallization of this amorphous oxide layer during heat treatment depends on the treatment temperature and on the use of dopants. When this layer is heat-treated, the amorphous arrangement first crystallizes into anatase structure. As the temperature increases, the anatase structure may turn into rutile structure, which is the most stable form of this oxide. In attempt to evaluate the effect of
Conclusions
In summary, the Ti-Nb-Sn system with a gradual addition of Mo content showed effective behavior as substrate for TiO2 nanotube growth. This study led to the following conclusions:
The addition of Mo to Ti-Nb-Sn alloys suppresses martensite precipitation after solution heat treatment followed by water quenching. Consequently, the elastic modulus of the sample containing 2% of Mo was slightly higher, although all the samples presented relatively low elastic moduli (between 60 and 75 GPa), which is
Acknowledgements
The authors gratefully acknowledge the LNNano/CNPEM for access to the AFM facilities; the Brazilian research funding agencies FAPESP (State of São Paulo Research Foundation) Grants #2014/06099-1 and 2014/00159-2 and CNPq (National Council for Scientific and Technological Development) Grant # 484379/2012-7 for their financial support. G. Rabello was funded by the FAPEAM (State of Amazonas Research Foundation).
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