A release of Ti-ions from nanostructured titanium oxide surfaces

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Abstract

A release of Ti-ions from four distinct surface oxides into a liquid Dulbecco Modified Eagle Medium is investigated. Four oxide layers grown on bulk titanium samples were characterized in detail with respect to their morphology, wetting parameters and phase and chemical compositions and subsequently exposed to the medium. In spite of fundamental differences, which existed between the four individual layers in terms of their thickness, nanoscale structure and contact area with the liquid, a concentration of Ti-ions transferred into the medium was markedly similar. These results suggest that a key thermodynamic parameter governing the transfer of Ti-ions is their solubility limit in the medium. For the investigated oxide-liquid systems, the solubility limit was estimated as 10–20 μg/l at 37 °C. Interestingly, many less systematic data reported so far in the literature can be rationalized based on a similar saturation scenario.

Introduction

Over last decades, titanium and its alloys have become a prime research topic in the field of biomaterials. When exposed to atmospheric environments or solutions, a thin titanium oxide layer (2 to 5 nm [1]) spontaneously forms on the metal surface, acting as a barrier to progressive corrosion reactions [2], [3], [4]. This oxide layer also limits interface reactions between Ti alloys and physiological environments [1,5]. Moreover, the oxide layers covering titanium can be modified in a controlled way by forming titanium oxide nanotube structures (TNT). These modifications are employed in many practical situations like fuel cells, biosensors, environmental control, photocatalytic systems and solar cell technologies [6,7]. In the medical field, a particular attention is given to the orthopedic implant therapy [2,[8], [9], [10], [11], [12], [13]]. The surfaces of implants underwent a development from bio-inert porous Ti to bio-active and nano-structured surfaces covered with TNTs [2,9]. In this respect, the electrochemical anodization (EA) is widely used technique to form TNT layers during anodic etching processes [7,8]. The EA is a cost-effective method which results in well-aligned TiO2 hollow tubular structures growing perpendicularly to the surface of the substrate [13], [14], [15]. Morphology and properties of TNTs can be controlled by the composition and pH of the electrolyte, time and voltage applied during the EA process [1,13]. The TNT surface layers exhibit some positive effects like improved biocompatibility [12], promotion of biological activity [13], support of adhesion, cell proliferation and differentiation [5]. On the other hand, the TNTs expose a considerably larger surface area to the body fluids and tissues [12,13] which may result in an enhanced flux of toxic Ti-ions into the bio-environments. Ti-ions released from TNTs could be toxic to cells at concentrations exceeding 10 μg/l/24 h. Their increased levels may also stimulate a production of cytokines and thus trigger an overall inflammatory process with adverse effects on the cell metabolism [16,17]. While large body of research has already tackled the individual aspects of the TNTs biocompatibility, a systematic study which would establish a quantitative link between parameters of the individual oxide layers and the flux of Ti-ions into the liquid bio-environments is still missing. This is why the present study focuses on the Ti-ion release from four distinctly different titanium surfaces into a Dulbecco Eagle Medium/Ham Nutrient Mixture F12 modified by 10% fetal bovine serum and 1% penicillin/streptomycin (DMEM-F12). Prior to exposures in the DMEM-F12, the oxide surfaces were characterized in detail with respect to their morphology, wetting parameters and phase and chemical composition.

Section snippets

Material states

Four material states interacting with a liquid medium were investigated: (i) as received titanium foils with unmodified surfaces (foil thickness 0.25 mm, purity 99.7% from Sigma-Aldrich), further referred to as Ti), (ii) the foil after anodization with thicker but homogeneous oxide layer (material state AQ-ox) and (iii and iv) two foils after anodization treatment which resulted in TNT formation. Two anodization times, namely 75 and 120 min, were used to vary geometrical parameters of the TNTs.

SEM and STEM investigations

Oxide surfaces of the material states Ti, AQ-ox, AQ-75 and AQ-120 were investigated with respect to their morphology, crystal structure and chemical composition. The representative STEM micrographs are shown in Fig. 1. From a brief inspection of the four micrographs it is clear that the parameters of the four oxide layers are substantially different. Thus, while the oxide film is only about 12 nm thick in the case of untreated Ti in Fig. 1a, the anodization treatment increased the oxide layer

Discussion

Obviously, the number of Ti-ions transferred from the surfaces into the DMEM-F12 media may scale with the total oxide area which is wetted during the pickling experiment. In the present study, the wetting angles β were measured experimentally for all the four investigated material states. Moreover, we have formulated a simple model which helps to estimate the oxide area in contact with the liquid. Both, experimental and numerical results are summarized in Tables 4 and 5. The contact angles

Summary and conclusions

The transfer of Ti-ions from four distinctly different titanium oxide surfaces into the DMEM-F12 bio-fluid was investigated. Two compact oxides and two TNT-structured surfaces covering bulk titanium foils were considered. The quantitative SEM and STEM analyses yielded data on chemical composition and microstructural parameters of the oxide layers. Their wetting behavior was characterized experimentally and the contact area with the bio-fluid estimated using a simplified model. Finally, the

CRediT authorship contribution statement

Kateřina Vrchovecká: Investigation, Formal analysis, Writing – original draft. Adam Weiser: Investigation, Resources. Jan Přibyl: Investigation, Formal analysis. Jan Kuta: Investigation, Formal analysis. Jakub Holzer: Resources. Monika Pávková-Goldbergová: Conceptualization, Project administration. Dinara Sobola: Investigation, Formal analysis. Antonín Dlouhý: Conceptualization, Investigation, 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

Financial support was obtained from the Czech Science Foundation under the project no. 20-11321S. Additional resources were provided by the Technology Agency of the Czech Republic, project no. MUNI/11/63905/2018, the MEYES CR (LM2015043). Authors took advantage of the Research Infrastructure RECETOX RI (No LM2018121) financed by the Ministry of Education, Youth and Sports, and the Operational Programme Research, Development and Innovation - project CETOCOEN EXCELLENCE (No

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