118
P. Hameed et al.
with several surface modification processes. However, the most common, simple,
and cost-effective surface modification, which results in nano features is anodization.
Anodization is a chemical surface modification process in which etching of the
native oxide layer formed on the metal surface is performed, especially titanium
which tends to form a strong oxide layer which can be tailored to develop nanotubular
structures with the help of an electrolyte in the presence of an external electric field.
This surface modification process is a straightforward and robust technique that
helps develop nanotubes of different diameters and height by tailoring the following
parameters: 1. Electrolyte composition, 2. Voltage and 3. exposure time. The setup
consists of two electrodes: the working electrode being the anode and the platinum
or graphite electrode being employed as a cathode. The electrolyte mostly contains
solvents that have better dielectric potential and, at the same time, do not result in any
residual impurities on the substrate. The most fascinating advantage of this method
is not only the development of nanotubes but nanopillars, nanoflowers, and nanodots
with resultant 3D morphology. The nanotubes are post-processed with the help of
sintering.
As mentioned earlier, the concentration of electrolyte, voltage and anodization
time [80, 81] that govern the nanotube growth can help in tailoring the tube diameter
with the size range of 15–300 nm and different tube length can also be developed
[80].
The development of self-organized nanotube layers has been attempted for the
past two decades with the help of fluoride-containing electrolyte solutions [82, 83].
Apart from the fluoride-based electrolyte, an attempt to utilize organic solvents with
low water content has been made in developing the nanotube structures by Wei et al.
[84]. The amount of water utilized in the electrolyte is the key factor in determining
uniform nanotube growth. This finding resulted in the development of a mechanism
termed “pore-wall-splitting” in which the nanopores oxide layer forms as the first
step, following which the tubes tend to originate from the ordered porous oxide [84].
The mechanisms involved in anodization and the process involved in developing different nanostructured oxides like nanotubes pores with hole morphology,
nanochannels and microcones on the surface of titanium and titanium alloys has been
critically reviewed by Roy et al. [81] and Kowalski et al. [85].
The nanotubes were formed on cp-Ti initially; however, this can be achieved in
titanium alloys as well if the alloying elements are from the transition group. Such
alloy combinations which tend to result with uniform nanotube formation are Ti6Al-4 V [86], Ti-6Al-7Nb [86, 87], Ti-xZr alloy with varied Zr concentrations [88,
89] to name a few.
3.2 Nanotube Diameter the Key for the Cellular Response
Cellular attachment on nanotubular structures has been demonstrated in the in vitro
level [90, 91], resulting in higher adhesion, proliferation, and ALP activity. Several
researchers attempted to influence the cellular adhesion to the titania nanotubes, and
P. Hameed et al.
with several surface modification processes. However, the most common, simple,
and cost-effective surface modification, which results in nano features is anodization.
Anodization is a chemical surface modification process in which etching of the
native oxide layer formed on the metal surface is performed, especially titanium
which tends to form a strong oxide layer which can be tailored to develop nanotubular
structures with the help of an electrolyte in the presence of an external electric field.
This surface modification process is a straightforward and robust technique that
helps develop nanotubes of different diameters and height by tailoring the following
parameters: 1. Electrolyte composition, 2. Voltage and 3. exposure time. The setup
consists of two electrodes: the working electrode being the anode and the platinum
or graphite electrode being employed as a cathode. The electrolyte mostly contains
solvents that have better dielectric potential and, at the same time, do not result in any
residual impurities on the substrate. The most fascinating advantage of this method
is not only the development of nanotubes but nanopillars, nanoflowers, and nanodots
with resultant 3D morphology. The nanotubes are post-processed with the help of
sintering.
As mentioned earlier, the concentration of electrolyte, voltage and anodization
time [80, 81] that govern the nanotube growth can help in tailoring the tube diameter
with the size range of 15–300 nm and different tube length can also be developed
[80].
The development of self-organized nanotube layers has been attempted for the
past two decades with the help of fluoride-containing electrolyte solutions [82, 83].
Apart from the fluoride-based electrolyte, an attempt to utilize organic solvents with
low water content has been made in developing the nanotube structures by Wei et al.
[84]. The amount of water utilized in the electrolyte is the key factor in determining
uniform nanotube growth. This finding resulted in the development of a mechanism
termed “pore-wall-splitting” in which the nanopores oxide layer forms as the first
step, following which the tubes tend to originate from the ordered porous oxide [84].
The mechanisms involved in anodization and the process involved in developing different nanostructured oxides like nanotubes pores with hole morphology,
nanochannels and microcones on the surface of titanium and titanium alloys has been
critically reviewed by Roy et al. [81] and Kowalski et al. [85].
The nanotubes were formed on cp-Ti initially; however, this can be achieved in
titanium alloys as well if the alloying elements are from the transition group. Such
alloy combinations which tend to result with uniform nanotube formation are Ti6Al-4 V [86], Ti-6Al-7Nb [86, 87], Ti-xZr alloy with varied Zr concentrations [88,
89] to name a few.
3.2 Nanotube Diameter the Key for the Cellular Response
Cellular attachment on nanotubular structures has been demonstrated in the in vitro
level [90, 91], resulting in higher adhesion, proliferation, and ALP activity. Several
researchers attempted to influence the cellular adhesion to the titania nanotubes, and