**3. Titania nanotubes (TNTs)**

The choice of titania nanotubes as the first of the described 1D-titania nanostructures is not accidental, as the methodology of their production is well known. The electrochemical oxidation process of Ti and Ti alloys, in which TiO<sup>2</sup> nanotubes are produced, is one of the most popular methods to produce controlled and strictly defined structures on the surface of implants [99–106]. Sol-gel techniques, hydrothermal and solvothermal methods with or without templates, and atomic layer deposition (ALD) into the template are among other methods used in the production of TiO<sup>2</sup> nanotubes [107–111]. However, tailoring the process conditions and the possibility to obtain strictly defined morphology of nanotubes, caused that electrochemical anodization of titanium substrate, is particularly actively reviewed and practically used [67, 112–115]. The anodic oxidation process includes electrode reactions and metal and oxygen ions diffusion. The combination of these actions leads to the formation of oxide layer on the surface of Ti/Ti alloy, which acts as an anode (**Figure 1**).

This process is usually carried out by applying a constant voltage between 1 and 30 V in aqueous electrolyte or 5–150 V in nonaqueous electrolytes, containing in both cases approximately 1D Titania Nanoarchitecture as Bioactive and Photoactive Coatings for Modern Implants... http://dx.doi.org/10.5772/intechopen.69138 77

**Figure 1.** Scheme of the anodization setup.

different structural stability, different photo- and bioactivity, as well as different electrical and optical properties [84–86]. This fact causes that the determination of the structure of materials

In order to determine the titania structure, X-ray diffraction studies are often carried out. Characteristic set of 2Θ [°] signals: 25.33 (101), 37.80 (004), 48.08 (200), 55.12 (211) indicates the presence of anatase form, whereas the signals: 27.50 (110), 36.17 (101), 41.50 (111), 54.46 (211)

Raman spectroscopy is another very useful method to recognize and characterize the titania structures. Basing on the group analysis, there are six active Raman modes (A1g, 2B1g, 3Eg

mnm) [91, 92]. According to previous reports, the set of bands, which appear in Raman spec-

form, the base of identification is the detection of bands at 447 and 612 cm−1 [91–94]. It should

the formation of amorphic systems or amorphic systems containing small amount of crystalline phases [95, 96]. In such cases, Raman mapping of whole sample surface can help in the

The structure of thin titania films can be determined by the use of transmission electron microscopy studies, on the base of selective area electron diffraction (SAED) and the determi-

The use of mentioned in this subsection instrumental methods led to state about the structure of 1D-titania nanostructures. Even if they are not perfectly crystalline just like in case of amor-

The choice of titania nanotubes as the first of the described 1D-titania nanostructures is not accidental, as the methodology of their production is well known. The electrochemical oxida-

ular methods to produce controlled and strictly defined structures on the surface of implants [99–106]. Sol-gel techniques, hydrothermal and solvothermal methods with or without templates, and atomic layer deposition (ALD) into the template are among other methods used

the possibility to obtain strictly defined morphology of nanotubes, caused that electrochemical anodization of titanium substrate, is particularly actively reviewed and practically used [67, 112–115]. The anodic oxidation process includes electrode reactions and metal and oxygen ions diffusion. The combination of these actions leads to the formation of oxide layer on

This process is usually carried out by applying a constant voltage between 1 and 30 V in aqueous electrolyte or 5–150 V in nonaqueous electrolytes, containing in both cases approximately


nanotubes are produced, is one of the most pop-

nanotubes [107–111]. However, tailoring the process conditions and

/amd) and four active Raman modes (A1g, B1g, B2g, and Eg

is especially important for their further, for example, biomedical, applications.

) for

rutile

/

) for rutile D4h (P4<sup>2</sup>

on nanoscale causes

anatase form. For TiO<sup>2</sup>

basing on TiO<sup>2</sup>

76 Application of Titanium Dioxide

anatase D4h (I4<sup>1</sup>

prove that rutile form is our studied sample [87–90].

detailed determination of the structure.

**3. Titania nanotubes (TNTs)**

in the production of TiO<sup>2</sup>

tion process of Ti and Ti alloys, in which TiO<sup>2</sup>

the surface of Ti/Ti alloy, which acts as an anode (**Figure 1**).

nation of *d*-spacing from HRTEM images of nano-TiO<sup>2</sup>

phous samples possessing some crystalline islands in the structure.

tra at 197, 339, 519 and 639 cm−1, indicates the formation of TiO<sup>2</sup>

be pointed out that very often the fabrication of materials based on TiO<sup>2</sup>

0.1–1wt% fluoride ions. The presence of F− ions in the electrolyte is absolutely essential, as they are, in fact, responsible for the creation of nanotubes morphology. Without them, a compact oxide layer would be formed on the titanium surface. Fluorides form water-soluble [TiF<sup>6</sup> ] 2− species, both in the process of the complexation, which occurs with Ti4+ ions ejected at the oxide-electrolyte interface as well as by chemical attack of the formed TiO<sup>2</sup> . The fluoride concentration is crucial for this process, and up to it, three very different electrochemical characteristics can be obtained. In case of fluoride content ≤0.5 wt%, stable compact oxide layer is formed. If the concentration is high, approximately 1 wt%, no oxide formation is observed, as all the Ti4+ ions formed in the oxidation of Ti, immediately react with the abundant fluorides, forming soluble complex anions [TiF<sup>6</sup> ] 2−. And for intermediate fluoride concentrations, between 0.5 wt% and 1 wt%, porous oxide or nanotubes formation can be observed as a consequence of a competition between oxide formation and Ti4+ solvatization [116, 117].

By tailoring the anodization parameters, such as applied voltage, anodization time, and concentrations of chemicals, TiO<sup>2</sup> nanotubes of different diameters (from 15 nm up to 300 nm) and lengths are possible to obtain (**Figure 2**a). And in such stable conditions, regular nanoscope pores/tubes, open on the top and close at the bottom, are formed. Our studies showed that the use of very small potential (below 4 V) led to the formation of regularly nanoporous coatings. The use of the potential from the region 4–12 V led to the creation of nanotubular coatings, in which nanotubes still posses common walls. Only the use of potential higher than 12 V gave the coatings composed of independent and separated nanotubes (**Figure 2**b–d) [118].

Some modifications in the tube geometry can be achieved by changing the anodization voltage during the tube growth process. Applying voltage steps, pulsing between two appropriate voltages can be used to generate tube stacks, bamboo nanotubes, or nanolaces [119–121]. The use of organic electrolytes, such as ethylene glycol, DMSO, glycerol, or ionic liquids, leads to the formation of nanotubes of different morphology and composition. The presence of lower water content in the electrolyte, which controls tube splitting, determines the synthesis of coatings with very long tubes and smooth walls, as well as with large diameters (up to 700 nm) [122–125].

**Figure 2.** (a) The dependence between the applied voltage and diameter of nanotubes, SEM images of TNT3 (b), TNT10 (c), and TNT18 (d).

Regardless of whether an aqueous or nonaqueous electrolyte was used in the anodic oxidation process, TiO<sup>2</sup> nanotubes product is typically amorphous. It is necessary to anneal them in order to obtain crystalline product. Phase transformation to anatase takes place at 300–400°C, and from anatase to rutile at temperatures 500–700°C. Elevated temperatures (700–800°C) lead to the sintering and the collapsing of TNT [126]. The conversion temperature is strictly dependent on the several factors, such as nanotubes diameter, morphology, and impurities. It was proved that for small diameters (below 30 nm) rutile rather than anatase appears during annealing of amorphous nanotubes. Moreover, our studies showed that at very low potential (5 V), it was possible to obtain product in the rutile form, without annealing the sample. Both GAXRD and HRTEM proved this polymorphic form [118].

Taking into account the fact that titania nanotube arrays are one of the most promising candidates for coating of Ti and Ti alloys surface in implants fabrication (even for 3-D implants, as it can be seen in **Figure 3**), it is worth to consider the TNT structure and morphology impact on the cellular response.

There is a clear effect of the TNT diameter on the cell adhesion, proliferation and differentiation. Diameters of 15–20 nm are optimal and in case of such TNT presence on the surface of titanium implants, increase in adhesion and proliferation of several types of living cells, such as fibroblasts, osteoblasts, osteoclasts, mesenchymal stem cells, hematopoietic stem cells, and endothelial cells, was confirmed [127–134]. Higher tube diameters (>100 nm) have no positive influence on the increasing of adhesion or proliferation and some authors have shown that TNT of such diameters led to apoptosis, i.e., programmed cell death [135]. The size effect can be explained by the fact that integrin clustering in the cell membrane leads to a focal adhesion complex with the size of about 10 nm in diameter, perfectly fitted to nanotubes with about 15 nm diameters [134]. Gongadze et al. suggest that nanorough titanium surfaces with edges and spikes could promote the adhesion of living cells, especially osteoblasts [129]. 1D Titania Nanoarchitecture as Bioactive and Photoactive Coatings for Modern Implants... http://dx.doi.org/10.5772/intechopen.69138 79

**Figure 3.** The surface of 3-D implant (a), uncoated (b) and coated by nanotubes (c).

Regardless of whether an aqueous or nonaqueous electrolyte was used in the anodic oxida-

**Figure 2.** (a) The dependence between the applied voltage and diameter of nanotubes, SEM images of TNT3 (b), TNT10

order to obtain crystalline product. Phase transformation to anatase takes place at 300–400°C, and from anatase to rutile at temperatures 500–700°C. Elevated temperatures (700–800°C) lead to the sintering and the collapsing of TNT [126]. The conversion temperature is strictly dependent on the several factors, such as nanotubes diameter, morphology, and impurities. It was proved that for small diameters (below 30 nm) rutile rather than anatase appears during annealing of amorphous nanotubes. Moreover, our studies showed that at very low potential (5 V), it was possible to obtain product in the rutile form, without annealing the sample. Both

Taking into account the fact that titania nanotube arrays are one of the most promising candidates for coating of Ti and Ti alloys surface in implants fabrication (even for 3-D implants, as it can be seen in **Figure 3**), it is worth to consider the TNT structure and morphology impact

There is a clear effect of the TNT diameter on the cell adhesion, proliferation and differentiation. Diameters of 15–20 nm are optimal and in case of such TNT presence on the surface of titanium implants, increase in adhesion and proliferation of several types of living cells, such as fibroblasts, osteoblasts, osteoclasts, mesenchymal stem cells, hematopoietic stem cells, and endothelial cells, was confirmed [127–134]. Higher tube diameters (>100 nm) have no positive influence on the increasing of adhesion or proliferation and some authors have shown that TNT of such diameters led to apoptosis, i.e., programmed cell death [135]. The size effect can be explained by the fact that integrin clustering in the cell membrane leads to a focal adhesion complex with the size of about 10 nm in diameter, perfectly fitted to nanotubes with about 15 nm diameters [134]. Gongadze et al. suggest that nanorough titanium surfaces with edges and spikes could promote the adhesion of living cells, especially osteoblasts [129].

GAXRD and HRTEM proved this polymorphic form [118].

nanotubes product is typically amorphous. It is necessary to anneal them in

tion process, TiO<sup>2</sup>

(c), and TNT18 (d).

78 Application of Titanium Dioxide

on the cellular response.

A small diameter nanotube surface has more sharp convex edges per unit area than a large one. This fact can explain stronger cellular binding affinity on the surface of small diameter nanotubes, than on the surface of TNT with larger diameters. All studied samples mentioned above were annealed before carrying the adhesion and proliferation studies, so they were in crystalline form, mostly in anatase form. In our work, we studied the biological answer of as-obtained nanotubes, mostly amorphous, containing crystalline islands. We have noticed that even without further annealing, titania nanotubes showed better osseointegration than pure titanium. The adhesion and the proliferation of fibroblasts were different for the nanotubes of different diameters. The best biological answer was visible for nanotubes obtained at 5 V, whose diameters were ~20 nm [136]. This result is in accordance with earlier reports [127–134], however, our investigations revealed that the annealing of amorphous samples was not so indispensable [136].

Successful osseointegration is an important clinical goal but the important thing is the reduction of the bacterial biofilm formation on the surface of implant. Regardless of the type of biomaterial used, the initial inflammation response is always present and it may turn into an acute inflammation or even chronic inflammation. So, it is not surprising that the possibilities of a bacteria-repellent surface modification are investigated. Literature data and own studies showed that controlled diameter nanotubes displayed significantly changed responses to *Staphylococcus aureus* and *Staphylococcus epidermis* [137–140]. The size-effect exists for bacteria but also the structure of TNT influences the direction of the changes. According to Puckett et al., the use of larger diameter nanotubes decreased the number of live bacteria as compared to lower diameter ones and pure titanium [141]. But it is worth to know that analyzed nanotubes coatings were crystalline, in the form of anatase, as they were posttreated after anodization process by annealing. According to results of studies on amorphic nanotubes, which were not postannealed, the best antibacterial properties against *S. aureus* were seen for the nanotubes with small diameter but possessing the rutile form [118].

TiO<sup>2</sup> is one of the most photocatalytically active material used to decompose the organic pollutants and also bacteria [142–145]. The reason for this high activity is the band-edge positions in relation to typical environments, for example H<sup>2</sup> O. The energy band gap is adequate to the ultraviolet light energy and UV light promotes electrons from the valence band to the conduction band. As a result, the holes are formed in the valence band. Separated holes and electrons reach the semiconductor-environment interface, and react with appropriate redox species. Because of water presence, several highly reactive species are generated by charge exchange at the valence band (H<sup>2</sup> O + h+ → OH•) and at the conduction band (O + e− → O<sup>2</sup> - ). These radicals and peroxo ions are able to virtually oxidize all organic materials to CO<sup>2</sup> and H2 O. Furthermore, at the valence band, direct h+ transfer to adsorbed species to initiate the decomposition may also be considered [146–150].

It should be pointed out that in all of the photocatalytic applications, a higher overall reaction rate is achieved using high-surface-area geometries. Ordered nanotube arrangements offer various advantages over nanoparticulate assemblies, as their defined geometry provides strictly determined retention times in nanoscopic photoreactors. Moreover, the 1-D geometry may allow a fast carrier transport and thus less unwanted recombination losses [151, 152].

The earlier researches showed that TiO<sup>2</sup> nanotube coatings can indeed have the higher photocatalytic reactivity than a comparable nanoparticulate layer. The optimized reaction geometry for charge transfer, UV absorption characteristics over the tube, and solution diffusion effects are the main factors, which may be responsible for this effect. They allow an improved photoabsorption, longer electron lifetime, and diffusion length in TiO<sup>2</sup> nanotubes in comparison with nanoparticles [152, 153]. Photoactivity of TNT depends on the dimensions and wall thickness of the nanotubes, their crystallinity, and their packing density, because the separation and transport of charge, as well as the grain boundary effect, would greatly hinge on such factors [154]. It was shown that silver or gold particle decoration led to a significantly higher photocatalytic activity [155]. The same increasing of photoactivity was also obtained by applying an external anodic voltage [156]. These facts suggest that in case of investigated objects, a valence-band mechanism dominates, and the observed accelerating effects have a common origin in increased band bending, either by applied voltage or by the junction formation [157].

Our studies showed that amorphic titania nanotubes possessing some crystalline impurities indicated very high photoactivity in the reaction of methylene blue and acetone degradation in the presence of UV light [118]. The clear influence of the tubes diameter, and at the same time, of specific surface area, on the value of observed rate constants, was also visible. Additional information which we have obtained from our research was the fact that it was not possible to make a clear comparison of TNT photoactivity in the degradation of different organic pollutant patterns (water-soluble methylene blue and volatile aceton), as the mechanisms of their degradation were completely different and they depended on variable parameters (the size of pattern molecules which affects the reactant molecule adsorption, pH of solution). However, the surface of the implant modified by anodic oxidation can possibly be disinfected/sterilized only with the use of UV light, as it reveals higher activity than unmodified Ti surface possessing only the natural passivation oxide film. It is an important property because the same coating plays a dual role for the implants—increases osseointegration process and creates optimal conditions for carrying out the process of implant surface sterilization/disinfection with the use of UV light.

## **4. Titania nanofibrous coatings (TNFs)**

TiO<sup>2</sup>

80 Application of Titanium Dioxide

H2

 is one of the most photocatalytically active material used to decompose the organic pollutants and also bacteria [142–145]. The reason for this high activity is the band-edge posi-

to the ultraviolet light energy and UV light promotes electrons from the valence band to the conduction band. As a result, the holes are formed in the valence band. Separated holes and electrons reach the semiconductor-environment interface, and react with appropriate redox species. Because of water presence, several highly reactive species are generated by charge

These radicals and peroxo ions are able to virtually oxidize all organic materials to CO<sup>2</sup>

It should be pointed out that in all of the photocatalytic applications, a higher overall reaction rate is achieved using high-surface-area geometries. Ordered nanotube arrangements offer various advantages over nanoparticulate assemblies, as their defined geometry provides strictly determined retention times in nanoscopic photoreactors. Moreover, the 1-D geometry may allow a fast carrier transport and thus less unwanted recombination losses [151, 152].

tocatalytic reactivity than a comparable nanoparticulate layer. The optimized reaction geometry for charge transfer, UV absorption characteristics over the tube, and solution diffusion effects are the main factors, which may be responsible for this effect. They allow an improved

son with nanoparticles [152, 153]. Photoactivity of TNT depends on the dimensions and wall thickness of the nanotubes, their crystallinity, and their packing density, because the separation and transport of charge, as well as the grain boundary effect, would greatly hinge on such factors [154]. It was shown that silver or gold particle decoration led to a significantly higher photocatalytic activity [155]. The same increasing of photoactivity was also obtained by applying an external anodic voltage [156]. These facts suggest that in case of investigated objects, a valence-band mechanism dominates, and the observed accelerating effects have a common origin in increased band bending, either by applied voltage or by the junction formation [157]. Our studies showed that amorphic titania nanotubes possessing some crystalline impurities indicated very high photoactivity in the reaction of methylene blue and acetone degradation in the presence of UV light [118]. The clear influence of the tubes diameter, and at the same time, of specific surface area, on the value of observed rate constants, was also visible. Additional information which we have obtained from our research was the fact that it was not possible to make a clear comparison of TNT photoactivity in the degradation of different organic pollutant patterns (water-soluble methylene blue and volatile aceton), as the mechanisms of their degradation were completely different and they depended on variable parameters (the size of pattern molecules which affects the reactant molecule adsorption, pH of solution). However, the surface of the implant modified by anodic oxidation can possibly be disinfected/sterilized only with the use of UV light, as it reveals higher activity than unmodified Ti surface possessing only the natural passivation oxide film. It is an important property because the same coating plays a dual role for the implants—increases osseointegration process and creates optimal conditions for carrying out the process of implant surface sterilization/disinfection with the use of UV light.

photoabsorption, longer electron lifetime, and diffusion length in TiO<sup>2</sup>

O. The energy band gap is adequate

transfer to adsorbed species to initiate the

nanotube coatings can indeed have the higher pho-

nanotubes in compari-


and

O + h+ → OH•) and at the conduction band (O + e− → O<sup>2</sup>

tions in relation to typical environments, for example H<sup>2</sup>

O. Furthermore, at the valence band, direct h+

decomposition may also be considered [146–150].

The earlier researches showed that TiO<sup>2</sup>

exchange at the valence band (H<sup>2</sup>

The electrospinning is a technique mostly used in the fabrication of the TNF coatings [158–160]. It is a very simple and convenient method for the preparation of polymer fibers and ceramic fibers, which are extremely long, uniform in diameter, raging from tens nanometer to several micrometers, and diversified in compositions [161]. The electrospinning process involves a high voltage source connected to a needle and a metallic collector where the fibers are deposited. The needle, which is attached to injection pump, represents the positive electrode. The collector is connected to the negative electrode, thus creating a potential difference. Electric field created in this way stretches the drop that forms on the needle tip, which is then deformed into a conical shape (Taylor cone). When the applied electric field exceeds the surface tension of the drop, the solution is ejected in the form of an electrically charged jet, reaching the negative electrode, which is the collector. During this process, the solvent is evaporated resulting in the deposition of nanofibers over the collector. The diameter of the fibers can be adjusted by varying the rheological properties of the solution and turning the processing parameters [162, 163]. The scheme of the electrospinning setup is given in **Figure 4**. Electrospun TiO<sup>2</sup> nanoarchitecture is formed by electrospinning titania precursor (e.g., titanium(IV) alkoxides) along with adequate polymer and subsequent polymer burning in high temperature sintering process. The synthesis involves the following four steps: (1) the preparation of the titania precursor sol, (2) mixing of the sol with the polymer template to obtain the solution for electrospinning, (3) electrospinning of the solution with the use of the apparatus showed in **Figure 4**, and (4) the calcination of as-prepared TNF to obtain crystalline titania nanofibers [164]. The morphology and the diameter of the electrospun titania depend on the following parameters related to: (a) the solution, (b) the process, and (c) the ambient (**Table 2**), while the structure of formed nanofibers is strictly associated with the postcalcination process [165–173].

**Figure 4.** Scheme of electrospinning setup.


**Table 2.** Summarization of the electrospinning parameters affected the nanofibers morphology.

Another technique to produce titania nanofibrous coatings is the laser ablation, proposed by Tavangar et al. [174]. During the laser irradiation of titanium substrate, the illuminated region is heated up and vaporized, producing the plasma plume. The plume expands outwards and its temperature and pressure decreases. The next process is the condensation of plasma plume leading to the formation of liquid droplets in saturated vapor, which is responsible for the nucleation. Continuous irradiation pulses maintain the plasma plume formation, which in turn generates a continuous flow of vapor plume increasing the density of formed nucleus. Hugh amount of nuclei favors the growth of nanoparticles, which come in contact and aggregate to form interwoven nanofibrous structure [175].

Also, the anodization was used in order to obtain titania nanofibrous coatings, but only in some very special conditions. Lim and Choi reported that such fibers were obtained on the top of 20 nm in diameter nanotube array, which were more than 10 μm in length [176]. In another report, Chang et al. presented novel method to synthesize nanofibrous coatings rotating of titanium anode with as-formed nanotube arrays, with the speed of 30 rpm in the same solution, in which the anodization of TNT has taken place (ethylene glycol solution containing 0.3 wt% NH4 F and 2 wt% of H<sup>2</sup> O) for next 3 h [177].

Very interesting morphology, from the medical application point of view, has been shown for titania nanofibrous coatings obtained in the process of Ti/Ti alloy chemical oxidation with the use of hydrogen peroxide with or without simple inorganic salts: NaCl, Na<sup>2</sup> SO4 , CaCl<sup>2</sup> , in elevated temperature (80°C) [118].

Wang et al. studied the influence of titania electrospun nanofiber dimensions and microstructure pattern on the adhesion and proliferation of human osteoblasts MG63. Bioactivity of two types of nanofiber dimensions (184 ± 39 nm and 343 ± 98 nm) and two different ways of TNF alignment (flat and patterned TiO<sup>2</sup> nanofibers) were checked and the obtained results indicated that cell morphology was not sensitive to the differences in nanofiber diameter and in microscale structure [178]. These results are in contradiction to some other researchers' studies, which showed preferential cell attachment along patterned TNF. The same authors proved that the combination of microroughness and the nanotopography can be used to modify the differentiation of osteoblasts and generate an osteogenic environment [179–181].

Considering the fact that the rate of osseointegration is strictly related to the efficiency of bone-like apatite formation on the implants, Tavanger et al. used the nanofibers obtained in femtosecond laser ablation process to evaluate the apatite-inducing ability of nanofibrous titania [174]. SEM studies showed that all TNF coatings obtained by Tavanger et al. were covered by dense and homogeneous apatite precipitation layer, after soaking them in simulated body fluid (SBF) for 3 days. EDX results proved that Ca/P ratio was around 1.63, which was attributed to hydroxyapatite, possessing a composition similar to the bone. Moreover, the wettability tests of TNF were performed and very low contact angle (<9.2°), and almost complete spreading of H<sup>2</sup> O droplets was observed on all the titania nanofibrous surface samples. Conclusion of the studies was the thesis that TiO<sup>2</sup> nanofibrous structure with the rapid apatiteinducing capability is expected to improve bone formation during *in vivo* implantation [174].

Chang et al. made a bioactivity comparison of different nanomorphology titania coatings: TiO<sup>2</sup> flat, TNT, and TNF, using human osteoblasts MG63. SEM studies revealed that the cell attached to flat TiO<sup>2</sup> possessed a round morphology, whereas these ones attached to nanotubes and nanofibers, showed polygonal shape and extending filopodia. Higher ratio of cell attachment and clearly visible lamellipodia were seen on nanofibrous coatings than on nanotubular ones [177]. This suggests that titania nanofibers surface, because of its rough and porous morphology, provides optimal environment for the cell adhesion, proliferation, and differentiation. The results of bioactivity studies of nanofibrous coatings obtained during Ti/ Ti alloy chemical oxidation with the use of hydrogen peroxide with or without simple inorganic salts: NaCl, Na<sup>2</sup> SO4 , CaCl<sup>2</sup> , in elevated temperature (80°C), confirm this [118]. Results of fibroblasts adhesion and proliferation studies carried out with the use of MTT test are shown in **Figure 5**.

Another technique to produce titania nanofibrous coatings is the laser ablation, proposed by Tavangar et al. [174]. During the laser irradiation of titanium substrate, the illuminated region is heated up and vaporized, producing the plasma plume. The plume expands outwards and its temperature and pressure decreases. The next process is the condensation of plasma plume leading to the formation of liquid droplets in saturated vapor, which is responsible for the nucleation. Continuous irradiation pulses maintain the plasma plume formation, which in turn generates a continuous flow of vapor plume increasing the density of formed nucleus. Hugh amount of nuclei favors the growth of nanoparticles, which come in contact and aggre-

because of the loss of polymer and the crystallization process of

**Parameter Effect on TNF morphology Ref.**

Viscosity Low viscosity leads to bead formation, higher viscosity leads

Solvent volatility High volatility leads to formation of nanofiber in concave

because of the jet instability

Feeding rate Increase in feeding rate results in the increase of nanofiber

Calcination temperature Higher temperature results in the reduction of nanofiber size,

**Table 2.** Summarization of the electrospinning parameters affected the nanofibers morphology.

Collector geometry The geometry affects the directionality of the formed nanofibers [170]

Dielectric constant Low dielectric constant results in the formation of larger size

concentration

produced

morphology

nanofibers

diameter

titania

Polymer concentration Increase in TNF diameter with increase of polymer concentration [165–167]

Increase in TNF diameter with increase of titania precursor

to disappearance of bead but nanofiber with larger diameter is

TNF diameter decreases with increasing applied voltage; when the applied voltage is above 1.6 kV/cm, the diameter increases,

[165, 166, 168, 169]

[165,170]

[171]

[171]

[165, 166]

[165, 168, 171–173]

[165]

Also, the anodization was used in order to obtain titania nanofibrous coatings, but only in some very special conditions. Lim and Choi reported that such fibers were obtained on the top of 20 nm in diameter nanotube array, which were more than 10 μm in length [176]. In another report, Chang et al. presented novel method to synthesize nanofibrous coatings rotating of titanium anode with as-formed nanotube arrays, with the speed of 30 rpm in the same solution, in which the anodization of TNT has taken place (ethylene glycol solution

O) for next 3 h [177].

gate to form interwoven nanofibrous structure [175].

F and 2 wt% of H<sup>2</sup>

containing 0.3 wt% NH4

*Solution parameters*

82 Application of Titanium Dioxide

Titania precursor concentration

*Processing parameters* Applied voltage, electric

*Ambient parameters*

field

Taking into account the different nanotopography and the structure of TNF coatings presented in Ref. [118] (analysis of GAXRD and DRIFT data revealed that TNF obtained in the presence of H<sup>2</sup> O2 and HCl was amorphous with anatase islands; TNF/H<sup>2</sup> O2 , TNF/ H<sup>2</sup> O2 + Na<sup>2</sup> SO4 , and TNF/ H<sup>2</sup> O2 + NaCl – amorphous with anatase and rutile islands, TNF/ H<sup>2</sup> O2 + CaCl<sup>2</sup> – amorphous titania), it can be stated that both above-mentioned attributes influence the bioactivity toward osseointegration processes taking place on their surface.

**Figure 5.** Results of MTT assay carried out on TNF coatings, obtained from different oxidation mixtures, with the use of murine fibroblasts L929 (adhesion after 24 h, proliferation after 72 h and differentiation after 5 days).

Photocatalytic activity of titania nanofibers, obtained by electrospinning method and annealed in 550°C, was studied with the use of dye degradation tests (basic blue 26, basic green 4, basic violet 4) [182]. The concentration of dye solution was measured in relation to UV irradiation time, using UV-Vis spectrophotometer. Additionally, the same tests were done for TiO<sup>2</sup> nanoparticles obtained by sol-gel method and for composite coatings consisting of titania nanofibers and TiO<sup>2</sup> nanoparticles. Doh et al. showed that after 3 h of UV illumination, 25.3% of basic blue 26 was degraded by titania nanofibers. This degradation efficiency was almost the same as for TiO<sup>2</sup> nanoparticles obtained by sol-gel method, for which the value was 23.7%. In comparison, the photoactivity of composite material (TiO<sup>2</sup> nanofibers and TiO<sup>2</sup> nanoparticles) was much higher, as basic blue 26 was degraded by 78.7%. Rate constant calculated for titania nanofibers, titania nanoparticles, and composite material, on the base of simplified equation of the Langmuir-Hinshelwood kinetic model was equal adequately 15.7 × 10−4 min−1, 14.3 × 10−4 min−1, and 85.4 × 10−4 min−1. The values of kinetic rates obtained for composite material but during the degradation processes of dyes: basic green 4 and basic green violet were 81.2 × 10−4 min−1 and 67.4 × 10−4 min−1, respectively. Authors concluded that composite materials consisted of TiO<sup>2</sup> nanofibers and TiO<sup>2</sup> nanoparticles had high photocatalytic activity due to their high active surface area and due to complex pore structure. They stated that such materials could be suitable for the application to the degradation of organic dye pollutant.

The photocatalytic activity of TiO<sup>2</sup> nanofibrous coatings obtained during Ti/Ti alloy chemical oxidation with the use of hydrogen peroxide with or without simple inorganic compounds: HCl, NaCl, Na<sup>2</sup> SO4 in 80°C, was analysed also on the base of degradation of two organic pollutant patters: acetone (A) and methylene blue (MB) [118]. Based on the same simplified equation of the Langmuir-Hinshelwood kinetic model, it was possible to calculate the kinetic rates. The values of calculated kinetic rates are showed in **Table 3**.

The values obtained during these studies are very close to those, obtained by Doh et al., however, titania nanofibers obtained in the process of chemical oxidation were not post annealed and they were not enriched by the titania nanoparticles [118, 182]. What should be pointed out clearly is the fact that the observed rate constants do not inform about the real nature of the appropriate processes. However, they provide basic information about change in the


**Table 3.** The observed rate constant values for the degradation of MB (*k*obs MB) and acetone (*k*obs A) on titania nanofibers under UV light.

 degradation rate, depending on the structure, morphology, and active surface of the TiO<sup>2</sup> coatings. The different degradation rate values result from the completely different steps, which occur during the degradation processes: the multistep degradation process of methylene blue (MB) and formation and degradation of some dimeric forms like mesityl oxide, which are observed during acetone degradation.

The results of carried out photocatalytic activity studies showed the potential usefulness of TNF coatings as photoactive ones. Even if they show lower kinetic rate values than titania nanotubes for which these values were almost in all cases five to ten times higher, comparing to titania nanofibers [118].
