Calcium and phosphorous enrichment of porous niobium and titanium oxides for biomaterial applications
Graphical abstract
Introduction
The good biocompatibility and mechanical properties of transition metals have made them the material of choice for the development of orthopedic and dental implants [1]. Their biocompatibility is mainly due to the thin native oxide layer that spontaneously forms on their surface upon exposure to air and passivates the surface reducing ion release. Among transition metals, titanium and its alloys (mainly Ti-6Al-4V) have been widely used because of their low Young's modulus that helps in reducing the shear stress between bone and implant and therefore can lead to healthier and faster bone regeneration [2].
In the last years, concerns on possible cytotoxic effects due to aluminum or vanadium [3] release prompted the search for new materials to replace Al and V in the alloy. Niobium is a good candidate because of its biocompatibility; it has been used as a substitute of vanadium in the Ti-6Al-4V alloy (Ti-6Al-7Nb [4]), in other alloys such as Ti-13Nb-13Zr [5], Ti-35Nb-7Zr-5Ta [6], or in binary alloy with titanium [7]. Recent studies have also shown good biocompatibility of pure niobium used as a coating [8,9] and a better bone tissue adhesion on niobium than on titanium [10].
Together with the nature of the bulk material (which determines the implant mechanical performances), the implant surface properties (surface morphology and chemistry) can strongly influence the fate of the implant since they are crucial for the interaction of the implant with the surrounding tissues. Among the different mechanical and chemical surface modification strategies [[11], [12], [13], [14]], anodic oxidation has been reported in literature [11,15] as a very versatile method to tune both the morphology and the chemistry of the metal surface. By tailoring the anodizing parameters (such as applied potential, anodic current, electrolyte, anodizing time or temperature [16,17]), the process can be tuned and the so-called Anodic Spark Deposition (ASD) regime can be achieved, with the formation of pores and the inclusion of chemical species from the electrolyte into the oxide layer. The possibility to control the surface morphology by growing porous oxide layers and to tune the surface chemical composition make ASD a well-suited oxide growth method in view of biomedical applications. In particular, porous surfaces which increase the effective surface area and mimic the trabecular bone structure are expected to promote the implant osseointegration [18]. Moreover, the inclusion of Ca and P into the oxide with a Ca/P ratio close to 1.67 (the hydroxyapatite stoichiometry) is a sought-after result since it can improve the implant osteoconduction [19,20].
ASD oxide growth has been reported on different transition metals, from the most studied titanium and its alloys [11,[21], [22], [23]] to tantalum [24] or zirconium [25] up to other metals like aluminum [26] or magnesium [27]. Some works dealt with the ASD of pure niobium using different combinations of electrolytes: silicate solutions [28], solutions containing phosphoric acid [29], phosphoric acid and copper nitrate [21] or nickel phosphate complexes [30]. In view of biomedical applications, attention has been focused on the amount of Ca and P that can be included in the oxide layers using electrolytes solutions containing calcium hypophosphite with either magnesium acetate or calcium formate [31,32], calcium acetate and phosphoric acid with a single or a two-step anodization [33,34].
In this work, we apply ASD to niobium with the target to obtain a biomimetic surface, i.e. a porous surface with a high amount of Ca and P inclusions and a Ca/P ratio close to 1.67, the Ca/P ratio of hydroxyapatite. In this perspective, an interesting paper by Frauchiger et al. [23] applied ASD to titanium using an electrolyte containing calcium phosphate, calcium acetate and the chelating agent di‑sodium ethylenediamine tetraacetate (Na2-EDTA). EDTA is used to increase the solubility of calcium through complex formation. Moreover, the negative charge of the Ca-EDTA complex at basic pH facilitates the incorporation of calcium in the oxide layer at the positively charged niobium electrode [23]. Here we have adopted the electrolyte introduced in [23] to investigate the ASD on niobium in order to explore the influence of anodizing potential on the morphology and surface chemical composition of the oxide layers. The effectiveness of this electrolyte has been evaluated through comparison with anodizing experiments using phosphoric and sulphuric acid solutions. As mentioned before, since the implant surface plays a key role in the implant/tissue interaction, a careful morphological study has been carried out with AFM, obtaining a quantitative characterization of surface roughness, pores diameter and pores depth. Moreover, SEM cross section analysis was used to evaluate the thickness of the oxide layer. To investigate the surface chemical composition of the oxide layers and to evaluate the amount of electrolyte inclusions as a function of the anodizing conditions, we exploited the high surface sensitivity of X-ray Photoemission Spectroscopy and acquired high-resolution XPS spectra using an electron neutralizer to avoid sample charging. ASD experiments on titanium have been performed under the same experimental conditions used for niobium to evaluate peculiar features of niobium anodization with respect to the more widely investigated titanium.
Section snippets
Substrate preparation
Commercially pure 2 mm thick niobium sheets (kindly provided by Dr. Musenich, INFN Genova) and 1 mm thick titanium foil (purchased from Advent Research Materials Ltd.) were used as substrates. All samples were mechanically cut to 1 cm × 1 cm squares and polished using a Struers Labopol-5 machine with grinding plane rotating at 200 rpm (SiC papers with P1000 ÷ P2500 grain size of Fepa-P scale). After polishing, samples were ultrasonically rinsed for 5 min in Milli-Q water, 5 min in ethanol,
Niobium substrate
The characterization of polished samples before anodizing is a key step to correctly evaluate the effects of the oxidation process on sample morphology and chemical composition. Moreover, the substrate morphology (e.g. the presence of edges) can influence the anodizing process, in particular the development of sparks [36]. To have a reference starting point, all substrates underwent the same polishing and cleaning treatment before anodizing. The morphological and chemical characterization of
Conclusions
The anodic oxidation of niobium was carried out in an electrolyte containing Ca and P, previously used for the oxidation of titanium [23]. AFM analysis showed the formation of microporous oxide layers on the niobium surface. Pores with diameters and depths up to 2 μm form when the limiting potential is raised up to 230 V. A marked increase in surface inhomogeneity is observed for anodization at 250 V. The surface morphological changes observed when the potential increases are accompanied by the
CRediT authorship contribution statement
P. Canepa: Investigation, Formal analysis, Visualization, Writing - original draft. G. Firpo: Investigation. L. Mattera: Methodology. M. Canepa: Conceptualization, Writing - review & editing. O. Cavalleri: Supervision, Conceptualization, Writing - original draft, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thank F. Gatti for granting the access to the polishing machine. This work has been performed under the project DIFILAB of the Università degli Studi di Genova. Financial support from “Fondo Sociale Europeo Regione Liguria” (project n. DPU12UNIGE82/3600) and from Università degli Studi di Genova is acknowledged. P.C. and M.C. are not relatives.
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