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Ricardo Pereira Nogueira,1,2 Jose Deuzimar Uchoa,3,4 Fanny Hilario,2 Gabriela de Fátima Santana-Melo,5 Luana Marotta Reis de Vasconcellos,5 Fernanda Roberta Marciano,6 Virginie Roche,2 Alberto Moreira Jorge Junior,2,7 Anderson Oliveira Lobo4 1Chemical Engineering Department, Khalifa University of Science and Technology, Abu Dhabi 127788, United Arab Emirates; 2Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, GrenobleINP*LEPMI, Grenoble 38000, France; 3Federal Institute of Education, Science and Technology of Piauí, Teresina 64053-390, Brazil; 4Interdisciplinary Laboratory for Advanced Materials, BioMatLab Group, Materials Science and Engineering Graduate Program, UFPI – Federal University of Piaui, Teresina 64049-550 Brazil; 5Department of Bioscience and Oral Diagnosis, Institute of Science and Technology, Sao Paulo State University, Sao José dos Campos 12245-000, Brazil; 6Department of Physics, Federal University of Piaui, Teresina PI 64049-550 Brazil; 7Department of Materials Engineering, Federal University of São Carlos, São Carlos 13565-905, BrazilCorrespondence: Anderson Oliveira LoboCampus Universitário Ministro Petrônio Portella, Av. Universitária, s/n, Centro de Tecnologia, Laboratório Interdisciplinar de Materiais Avançados, Ininga, Teresina, PI 64049-550, BrazilTel +55 86 32371057Email lobo@ufpi.edu.brBackground: Nanostructured surface modifications of Ti-based biomaterials are moving up from a highly-promising to a successfully-implemented approach to developing safe and reliable implants.Methods: The study’s main objective is to help consolidate the knowledge and identify the more suitable experimental strategies related to TiO2 nanotubes-modified surfaces. In this sense, it proposes the thorough investigation of two optimized nanotubes morphologies in terms of their biological activity (cell cytotoxicity, alkaline phosphatase activity, alizarin red mineralization test, and cellular adhesion) and their electrochemical behavior in simulated body fluid (SBF) electrolyte. Layers of small-short and large-long nanotubes were prepared and investigated in their amorphous and crystallized states and compared to non-anodized samples.Results: Results show that much more than the surface area development associated with the nanotubes’ growth; it is the heat treatment-induced change from amorphous to crystalline anatase-rutile structures that ensure enhanced biological activity coupled to high corrosion resistance.Conclusion: Compared to both non-anodized and amorphous nanotubes layers, the crystallized nano-structures’ outstanding bioactivity was related to the remarkable increase in their hydrophilic behavior, while the enhanced electrochemical stability was ascribed to the thickening of the dense rutile barrier layer at the Ti surface beneath the nanotubes.Keywords: surface modification, TiO2 nanotubes, commercially pure titanium, bioactivity

TiO 2 nanotubes constitute a relevant surface modification technique to improve the surface properties of orthopedic implants. However, the interpretation of their electrochemical characterization by impedance spectroscopy in physiological environments remains unclear. The present study proposes a “Two-channel transmission line model” based on the de Levie’s theory for porous electrodes. This approach considers cylindrical pores of finite length where the electrical signal is distributed in various directions along the pores walls. Transport phenomena occur mostly in the nanotubes' solid channels and are considered as “anomalous”, i.e. , frequency-dependent due to the semi-conductive properties of TiO 2 . In these conditions, the interface cannot be modeled by usual parallel or series arrangements of R-L-C elements. The impedance of the almost non-reactive interface is regarded as purely pseudo-capacitive and modeled by a distributed Constant-Phase-Element (CPE). It is shown that such transmission line model applies to the nanotubes system since it permits to fit successfully (with significant fitting parameters values) EIS data measurements of two very different morphologies of nanotubes in a physiological environment. From a practical point of view, results indicate that the huge increase in the effective surface thanks to the self-structured nanotube layer might improve the biocompatibility without any concomitant decrease of the corrosion resistance.

To study the synergetic influence of TiO 2 nanotubes (NTs) morphology and crystalline structure on their electrochemical performances and apatite-forming ability, various sizes of nanotubes were synthesized via anodic oxidation of Ti and then annealed at different temperatures. XRD analysis and SEM observations confirmed that as-anodized amorphous nanotubes crystallize into anatase phase when annealed at 450 °C and into a mixture of anatase and rutile when annealed at 550 °C, without significant morphological modifications. Corrosion resistance was assessed by Open Circuit Potential measurements (OCP) and by potentiodynamic polarization curves while apatite-forming ability was evaluated by measuring the amount of Hydroxyapatite (HAp) precipitated on samples surfaces when soaked in Simulated Body Fluid (SBF) solution. Experiments confirmed that anodized titanium possesses much better corrosion resistance and bioactivity than flat Ti substrate and that annealed nanotubes are more suitable for biomedical applications than amorphous ones. Additionally, this study highlights paradoxical features such as plain anatase structure showed high bioactivity, but a mixed structure was preferable because of its synergistically better chemical stability and mechanical properties. Longer nanotubes had high corrosion resistance, but their apatite-forming ability after 14 days was poor; shorter nanotubes were less corrosion resistant, but induced thicker layer of HAp when immersed in SBF. Finally, the best compromise for implants surfaces was discussed regarding thermal, mechanical, electrochemical, chemical and bioactive properties.