The Electrochemical Behavior of Titanium Improved by Nanotubular Oxide Formed by Anodization for Biomaterial Applications : A Review

Titanium oxide nanotube is gaining prominence in many applications like solar energy, sensors, catalyst and biomaterials. In this paper, we briefly review the electrochemical behavior of titanium oxide nanotube prepared by anodization in simulated body fluids. The electrochemical behavior of TiO2 nanotube depends on its morphology and surface properties. When titanium oxide nanotube formed by anodization, these surface properties are depending strongly on two factors, the parameters of anodization process and the conditions of employed electrolyte. These factors must be chosen in correct manner to optimized titanium oxide nanotube with desired properties for specific application.

The variety of the applications of Ti is attributed to its excellent corrosion resistant, light weight 4 , low elastic modulus and biocompatibility 1 .The high and remarkable corrosion resistance is attributed to a very stable passive oxide film formed on its surface when exposed to the air or water 2,5 .The self-healing nature and compactness of the titanium oxide film make it suitable for many hostile environments 1 .

Titanium in biomaterials
In biomedical devices, selecting materials for different components depends especially on several factors.First, the implant materials must possess an excellent biocompatibility, excellent corrosion resistance and appropriate mechanical properties 6 .low elastic modulus is required to be close to that of a human bone, in order to transfer the adequate mechanical stress to the surrounding bone 7 .
Moreover, the surface of biomaterials play an extremely important role in the response of artificial medical devices to the biological environment.When they have been implanted into the human body, a series of interactions occur between the surface of biomaterials and the biological environment.Thus, their surface characteristics such as surface morphology, microstructure, composition, and properties can determine the efficiency of artificial implants 8 .
In the field of medicine, titanium is well established as a biomaterial for bone anchorage and in soft tissues, e.g. in pacemakers, neurological implants, and metabolic sensors [9][10][11] (fig.1).The biological, mechanical and physical properties of titanium and its alloys play significant roles in the longetivity of the prostheses and implants 13 .
The layer of titanium dioxide (TiO 2 ) covered titanium surface is believed contribute to its overall biocompatibility [14][15][16] .This layer with approximately 5 nm thickness forms spontaneously in air or water and exhibits favorable thermodynamic properties, and low ion formation and electrical conductivity in the physiological environment 5 .The stability of the titanium oxide film depends strongly on its composition, structure and thickness 17 .
Moreover, titanium oxide exhibits unique semiconductor-like properties that allow for the surface to change from Ti 4+ to Ti 3+ , which make it ability to inhibit reactive oxygen species involved in the inflammatory response [18][19][20] .Due to the semiconductor-like properties of TiO 2 , it can acts as an electron donor or acceptor, and thereby actively participate in catalytic reactions with biologically relevant radicals 4 .

Importance of titanium oxide nanostructure
As bone is alive, and in order to prevent loosening and inflammation when an implant inserted close to it, an implant must be accepted by the bone cells, and must be able to bond to and grow on it.The surface of an implant can be modified to optimize its properties and to maximize its bioactivity when interfacing with natural tissue.The surface treatments or modifications methods can enhance the biological characteristics of implants by adding material with desired properties, changing the composition or removing unwanted material from the implant surface.A number of techniques have been specifically introduced for titanium and titanium alloys, such as silanized titania, photochemistry, self-assembled monolayers, protein resistance and protein immobilization [21][22][23][24] .
Another suggestion to improve the biocompatibility of titanium and its alloys, is to use two scale surface modification, the first is nano-scale for bioactivity of the implant, and the second is microscale for osteoblast adhesion 25 .
A nanometre (nm) is defined as the millionth part of a millimetre ( 10 -9 m).The nanotechnology based on the fact that new functionalities or properties result from the nanoscale character of the components.New properties and/or functionalities can be related to scratch resistance, colourfulness, transparency, conductivity, strength etc, which result from the altered ratio of surface atoms to volume atoms and from the quantum-mechanical behaviour 26 .
Recently, many researchers found that the formation of nanotube oxide surfaces on the native oxide resulted in very strong reinforcement of the bone response 25,[27][28][29][30][31] .When titanium oxide surface tubes with nanoscale dimensions formed, the bioactivity of the implant improved and enhanced osteoblast adhesion on the surface 32 .The high surface area of titanium oxide nanotube and its ability for cell interlocking promote bonding to bone 21,33,34 .
Nanotubes fabricated on implant material surfaces not only provide great potential in promoting cell adhesion, but also in proliferation and differentiation.Moreover, nanotubes can offer the possibility of bacterial infection control by loading the tubes with antibacterial agents.However, studies were conducted to establish the optimum nanotopography for favourable cell response, and the optimum sizes of nanotubes for recognition and adherence by the sensing element of a bone cell 21 .
In vitro studies on the titanium oxide nanotube showed that nanotubular surface provides a favorable template for the growth of bone cells.Moreover, the cells cultured tests on the nanotubular surfaces showed higher adhesion, proliferation, alkaline phosphate activity and bone matrix deposition compared to those grown on smooth titanium surfaces.From in vivo studies, the biocompatibility results suggested that nanotubular titania does not cause chronic inflammation or fibrosis 35 .Titanium oxide nanotube have attracted extensive attention due to their large surface area, low electron recombination and charge-transport properties, and are considered to be superior chemical materials because they form onedimensional channels for carrier transport.They are of considerable interest for applications in the field of photocatalysis 36 , water photolysis 37 , remediation of organic pollutants 38 , gas sensors 39 , solar energy cells 40 , a promising role for biomaterials 2 and corrosion resistance 41 .

Anodization as preferred method to forming nanostructure
There are a variety of preparation methods that have been used to fabricate titanium oxide nanotubes such as template synthesis 42 , sol-gel methods 43 , hydrothermal synthesis 44 , evaporation 45 , ion-beam synthesis[ 46 and anodic oxidation 47 .Among these, electrochemical anodization is widely used, due to its simplicity, cost effective technique, controllable and reproducible.It is also possible to control the size and the shape of nanotubes to the desired dimensions.In addition, titanium oxide nanotubes layer prepared by this method is strongly adherent on the substrate, which is very important for many practical applications 2,48 .
A comparison of three methods, the assisted-template method, electrochemical anodic oxidation and hydrothermal treatment, along with their advantages and disadvantages was made in previous review 12 .Titanium oxide nanotubes with ordered alignment and high aspect ratio had been produced by using anodization 12,21,49 .
The anodization process can be conducted by using an electrochemical cell consisting of two electrodes, one is platinum ( or graphite) as a counter electrode and the second is the working electrode representing the anode of the cell, as shown in fig, 2 .A DC power source was employed for the purpose.Before anodization, the sample surface must be prepared by appropriate methods, i.e. employing polishing papers of different grades and ultrasonic cleaning.The anodization was performed by increasing the potential of the sample from 0 V to the desired potential with a suitable scan rate, followed by holding the sample in the potential for specific time at chosen temperature 27,51,52 .For example, fig. 3 presented titanium oxide nanotubes formed on titanium metal by anodization in 1M Na 2 SO 4 + 0.5 wt.% NaF at 20 V for 30 min 53 .
It should be possible to control the nanotube size and morphology for the desired application by controlling the electrochemical conditions, i.e. applied voltage, anodization time, and the solution conditions, i.e. composition of the electrolyte and fluoride ions concentration, pH, water content in the electrolyte and its temperature 12,[51][52][53][54][55][56][57] The first time of the titanium oxide nanotube arrays generation was grown in HF electrolytes or acidic HF mixtures [58][59][60] .These layers have a limited thickness about 500-600 nm.By taking into account the importance of the pH gradient within the tube 7 , and using buffered neutral electrolytes that containing NaF or NH4F instead of HF, the thickness of the self-organized nanotube titanium oxide layers will increase more than 2 micrometer [61][62][63][64] .The third generation of the nanotubes were grown in (almost) water free electrolytes.In glycerol electrolytes, the grown tubes have extremely smooth walls and a tube length exceeding 7 micrometer 65 .When the anodization applied using CH 3 COOH electrolytes remarkably small tube diameters could be obtained 66 .
Over the past few years, the application of anodization allow the tube diameters to be adjusted over a wide range from 10 to 250 nm, and the length of nanotube from 100 nm to several hundred nm can be grown 67,68 .In aqueous electrolytes, applying anodic potentials in the range of 1-20 V will produce tubes with diameters between 15 and 100 nm 67,69 .However, the thickness of the nanotube layers can be influenced by varying the anodization time and the pH of the electrolyte 52,62 .

Some studies on the titanium oxide nanotube
Titanium oxide nanotube grown from a Ti metal and its alloys have attracted wide scientific and technological interest.The electrochemical growth of titanium oxide nanotubes on titanium and its alloys has a history of about 14 years 70 .
Zwilling et al 32 first produced nanotubular titanium dioxide on titanium, and the fabrication of titanium oxide nanotube arrays via anodic oxidation of titanium foil in a fluoride-based solution were first reported in 2001 by Grimes and co-workers 59 .After that, many workers applied anodization process to fabrication titanium oxide nanotube in several electrolytes, and characterized the microstructure of the surface by many techniques, such as scanning electron microscopy (SEM), X-ray diffraction (XRD) patterns, and Transmission electron microscope (TEM) 1,13,40,48 .Further studies were focused on the effect of the anodization process parameters on the formation and the properties of titanium oxide nanotubes, and how to control and optimize the desired dimension of the oxide nanotube for the desired applications 2,56,57,[71][72][73][74][75] .The mechanism of titanium oxide nanotube formation by anodization was discussed by many researchers 30,[76][77][78] .
In the literature, there are many reviews available about titanium nanotubes formation and properties 21,56,77,99 , and its special properties for implant applications 12,21,100,101 .But the information about how nanotube oxide can improve the electrochemical properties of titanium as biomaterial is very little, and there are fluctuations in the research results about it.Hence, this project will focus on the titanium oxide nanotube layer prepared by anodization and its role in the change of titanium behavior for biomaterial applications.

The electrochemical behavior of titanium oxide nanotube The conditions used to study the electrochemical behavior
The electrochemical corrosion behavior of metals is governed by the interaction between the materials surface and the electrolyte 102 .In the case of nanotube oxide, the effective surface area, determined by the nanotube size 103 , is expected to play an important role in the electrochemical performance of nanotubular titanium and its alloys 74 .
To explain the role of nanotube oxide in changing the electrochemical behavior of titanium, the studies were also conducted in titanium without nanotube oxide, which has a stable, dense oxide layers, consists mainly of TiO 2 104 , thin (about 3-8 nm in thickness), amorphous, and stoichiometrically defective 93 .

Fig. 1: Biomedical application of titanium and its alloys 12
Fig. 2 : The electrochemical cell used for anodization process 30 The electrochemical behavior of titanium oxide nanotube in simulated biological environment was studied using many electrochemical techniques, like open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS), potentiodynamic (PD) and potentiostatic polarization (PS) tests.
The open circuit potential ( or corrosion potential (E corr )) reflects the composite results of the electrochemical reactions taking place at the electrode and solution interface.Therefore, the variation in the OCP with immersion time can be employed to study the electrochemical processes 94 .From potentiodynamic poalization measuremants, the corrosion potential (E corr ) and current (I corr ) can be determined, as well as the passivation current (I pass ).The electrochemical impedance spectroscopy (EIS) is a powerful technique to study the corrosion of metals or alloys.It can provide quantitative evaluation of the corrosion properties of the studied system, which may be difficult to assess using conventional electrochemical measurements such as potentiostatic or potentiodynamic techniques.By appropriate interpretation of the EIS data in conjunction with an equivalent circuit (EC), detailed information on the electrochemical process at the film/solution interface can be disclosed 94,53 .

Results about electrochemical behavior of titanium oxide nanotube
Table 1.summaries some recently studies on the electrochemical behavior of titanium and some of its alloys with titanium oxide nanotube prepared by anodization.From the table, the results of most studies indicate that titanium materials with titanium oxide nanotube have a better corrosion resistance in simulated body fluid solutions than that of bare titanium.A smaller current density and higher OCP was observed for titanium oxide nanotube, which means an increase in the stability of titanium surface.
On the other hand, three studies found that the titanium with titanium oxide nanotube exhibited significantly higher corrosion current density and lower corrosion resistance than that of the bare titanium, meaning a lower corrosion resistance than that of titanium.

discussion of the results
e diameter and length of the nanotubes and the thickness of the barrier layer can strongly change the electrochemical corrosion behavior of nanotubular titanium 52,74 .The differences in the methods used for preparation of the sample surface before anodization may have some effects on the properties of the oxide nanotube formed.Moreover, an important factor is the properties of the electrolyte employed in the anodization process 1,54,96 .
The EIS of titanium oxide nanotubes indicate the presence of two time constants corresponding to the presence of two interfaces, a highly corrosion resistant barrier layer and the outer nanotubular layer.When Ti exposed to air, a thin and stable oxide several nanometers thick is formed on its surface 113 .In contrast, the anodization process in fluoride solutions results in the growth of uniform arrays atop the titanium surface and a barrier layer separating the nanotubes from the conducting titanium.While the diameter and length of TiO 2 nanotube depends on the anodization voltage 52,69 , the thickness of the barrier layer increases with anodizing voltages and approximately equal to the pore radius 12 .In previous work, researchers found that a higher anodizing voltage leads to not only larger diameter of TiO 2 nanotubes, but also thicker barrier layers 12,91 .The thicker inner barrier layer of nanotubes may play a more important role in corrosion resistance of titanium which agrees with the result of EIS 105 .
Many previous studies have shown that the size of titanium oxide nanotube is a key factor to determine its corrosion behavior.When a small diameter TiO 2 nanotube grows on pure Ti, a lower corrosion rate is reported, but large nanotubes have an opposite effect 106 .When the diameter of the nanotube is increased, the electrochemical stability increases first and then diminishes when its diameter more than 86 nm 74 .Improved corrosion resistance of pure titanium when a small nanotube covered it may be ascribed to relatively thick oxide layer formed during anodization at the substrate/nanotube interface.However, the large oxide nanotubes possess a high specific surface area to contact with electrolyte thus beneficial to the diffusion of corrosive ions and corrosion products.The larger nanotubes provide more channels for the electrolyte to reach the barrier layer consequently increasing the chance of the reaction between corrosive ions and titanium oxide 91,102 .This phenomenon was reported by many workers who used nanotubes approximately 150-220 nm in size in the corrosion tests 74,91,93 .Some studies reported that the corrosion attack of the nanotubular Ti is attributed to the collapse of TiO 2 nanotubue layer 74 .There is a suggestion that the nanotubes may act as effective channels for the electrolyte to reach the interface.Moreover, Saji and Choe suggested that the lower corrosion resistance of the nanotubular alloy can be associated with the distinctly separated barrier oxide/ concave shaped tube bottom interface 27 .
Another important factor, is the wall between TiO 2 nanotubes, which may be influencing its electrochemical stability.The morphology of the corroded titanium oxide nanotubes after the potentiodynamic polarization test showing that the nanotubular structure collapses at the localized corrosion site 114 .The nanotubes forming by anodizing titanium oxide have a dual-layer wall.The inner layer is composed from titanium oxide whereas the outer layer is composed of some form of titanium hydroxide with a relatively low density.Between adjacent nanotubes there is a space, and when corrosive ions in the artificial saliva penetrate, the quick dissolution of the outer layer of TiO 2 nanotubes will leading to deposition of phosphate compounds 115 .When the phosphate compounds present in the solution, it will block the small space and retard diffusion of oxygen and/or other dissolved products to and away from the surface of the outer layer, respectively.With reduced oxygen, rebuilding of titanium oxide is slowed, causing further dissolution of the wall between adjacent nanotubes and eventually collapse of the wall 74 .
Thus, optimized parameters of the anodization process to form nanotube oxide, such as anodization potential, nature of the electrolyte, concentration of the electrolyte, duration of anodization, temperature and the potential sweep rate is critical in achieving the desirable size and shape of the self-ordered nanotubes 27,51,55 .
For more improvement in the electrochemical corrosion behavior of titanium oxide nanotube, many workers suggested that suitable heat treatment processes of oxide nanotube can provide the required bioactivity and chemical stability which is important in biocompatibility point of view for potential implant applications 27,82 .
The structure of the grown oxide nanotube can be amorphous or crystalline, strongly dependent on the electrochemical parameters of the anodization process, such as the applied potential, the time of anodization, or the sweep rate of the potential.For example, at low voltages (below 20 V), the structure of the oxide films on Ti has typically been reported to be amorphous, and crystallization to take place at higher voltages 116 .Moreover, depending on the anodizing conditions the crystal structure can be anatase, a mixture of anatase and rutile, or rutile 117 .
Many of the studies confirm that the thermal treatments resulting in an improvement of the adhesion of nanotubes to the titanium substrate require temperatures above 400 °C118,119 .At such temperatures, structural changes lead to a crystalline titanium dioxide 118 .These structural changes can enhance the mechanical stability of the porous anodic layers without changing the diameter of the oxide nanotubes.Thus, an appropriate heat treatment can render titanium oxide nanotubes .
After nanotubes were annealed, there was a significant decrease in the passive current density and corrosion current density 27,120,121 , because the anatase nanotubes had a stable phase compared to the amorphous nanotubes 120,121 .Furthermore, the standard potential E o of anatase nanotubes was lower than that of amorphous one, which can be attributed to the surface area decrease because the pores became smaller or were closed after annealing 120 .

The role of titanium oxide nanotube on the improve the osseointegration of biomaterials
Recently, many of the researches focus on the biomaterials surface, aimed to understanding the fundamental processes at the interface between the implant surfaces and surrounding living tissues, that the human biology and nature should be considered in the design of functionalized biomaterials 122 .
When the biomaterial implanted in soft tissue, it induces a cell-mediated inflammatory response and fibrotic encapsulation 123 , which means an inflammatory processes immediately following the implantation, that are inherently present at biomaterials surfaces during wound healing events 18 .
Dur ing ear ly stage wound healing produce nitric oxide (NO), which is a key indicator of pro-inflammatory signal transduction in the inflammatory response and antimicrobial defense 124 .The inflammatory cells generate superoxide anion and reactive oxygen intermediates 125 .Further degradation may yield hydroxyl radicals, singlet oxygen, and hydrogen peroxide.Thus, the ability of a biomaterial to interact with inflammatory cells, and reactive oxygen and nitrogen species may be relevant for the host response.Results of titanium oxide nanotubes were shown to reduce the levels of the pro-inflammatory compared with a planar TiO 2 control surface, which suggests that the titanium oxide nanotubes elicit a favorable response in soft tissues.This may be due to the increased catalytic oxide surface area of the TiO 2 nanotubes 4,48 .The TiO 2 nanotubes possess an empty volume for filling with bioactivating species and provide an interface suitable for anchoring connective tissue 84 .
The results of many studies conducted to evaluate the nanomaterials chemistry suggest that increased bone cell functions may be independent of the bulk materials chemistry but rather rely on the degree of the nanostructured surface roughness [126][127][128] .Titanium oxide nanotubes offer an empty volume that can be filled with bioactive species and provide an interface suitable for anchoring connective tissue.Results indicated that the TiO 2 nanotubes enhance the process of apatite formation as compared to that on a common native oxide layer on titanium 84,129 , which considered to be an essential step for the bone-binding ability of biomaterials to the living bone.The incorporating of calcium phosphate crystals into the nanotubes can result in better adhesion of the coating to the substrate, and more osseointegration 17 .
To further improve the bioactivity and biocompatibility of titanium, various types of surface modification methods have been explored to enhance hydroxyapatite formation 130 .The nanotubular structure increased the surface area and also more 'in-between-nanotube path-ways for fluid' 129 .The faster and better nucleation on the nanotubes might be caused by the fact that the ions have better access to the surface when it is nanotubular, they can diffuse into the channels and form nuclei very homogeneously all over the walls, whereas the apatite growth on flat TiO 2 proceeds in a more heterogeneous, mushroom-like manner 90 .Thus, the increased surface area of the titanium oxide nanotubes is useful for accelerated bone growth in orthopedic/dental applications 129 .Balasundaram and Webster suggest that nanophase has ability to mimic the dimensions of the constituents and components in natural bone-like proteins and hydroxyapatite 8 .
The annealing of the amorphous nanotube layers to anatase or a mixture of anatase and rutile will enhance the apatite formation 84 .The proper heat treatment introduction of anatase phase form of titanium oxide in the nanotubes can further improve the biological activity 130 .

CONCLUSION
When titanium and its alloys subject to anodization in suitable manner, the formation of TiO 2 nanotube will occure.The electrochemical behavior of titanium oxide nanotube in simulated body fluids depends on its morphology and surface properties.When titanium oxide nanotube formed by anodization, these surface properties are depending strongly on two factors, The first is the parameters of anodization process, i.e. applied potential, duration of anodization and the method employed to prepare the sample surface.The second is the conditions of the electrolyte employed in the anodization cell, i.e. composition, fluoride ion concentration, temperature, pH and water content in the electrolyte.Thus, all these factors must be taking into account to produce titanium oxide nanotube with desired properties that needed for special application.

Fig. 3 :
Fig. 3: The SEM images and EdX of titanium oxide nanotubes formed on titanium by anodization in 1M Na 2 SO 4 + 0.5 wt.% NaF at 20 V for 30 min53

Table 1 :
The conditions of anodization process employed to produce titanium oxide nanotube on the surface of titanium and some of its alloys and its electrochemical behavior in simulated body