fabrication of biopolymers reinforced tnt/ha coatings on ti: evaluation of its corrosion resistance...
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Accepted Manuscript
Title: Fabrication of biopolymers reinforced TNT/HAcoatings on Ti: Evaluation of its Corrosion resistance andBiocompatibility
Author: Vairamuthu Raj Mohamed Sirajudeen Mumjitha
PII: S0013-4686(14)02061-1DOI: http://dx.doi.org/doi:10.1016/j.electacta.2014.10.055Reference: EA 23569
To appear in: Electrochimica Acta
Received date: 10-6-2014Revised date: 13-10-2014Accepted date: 13-10-2014
Please cite this article as: Vairamuthu Raj, Mohamed Sirajudeen Mumjitha,Fabrication of biopolymers reinforced TNT/HA coatings on Ti: Evaluationof its Corrosion resistance and Biocompatibility, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2014.10.055
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Fabrication of biopolymers reinforced TNT/HA coatings on Ti: Evaluation of its
Corrosion resistance and Biocompatibility
Vairamuthu Raj* and Mohamed Sirajudeen Mumjitha
Advanced Materials Research Laboratory, Department of Chemistry, Periyar University,
Salem 11, Tamil Nadu, India
* Corresponding author E-mail address: [email protected]
Tel: +919790694972, +919789703632
Graphical abstract
Abstract
Titania nanotube (TNT) arrays were fabricated on Ti alloy by rapid breakdown
anodization (RBA) method and chitosan (CS) - polyvinylpyrrolidone (PVP) biopolymers
were codeposited on TNT followed by subsequent electrodeposition of hydroxyapatite (HA,
Ca10(PO4)6(OH)2,). The coatings were characterized by FE-SEM, EDX, XRD and AT-FTIR
techniques. The mechanical, anticorrosion properties and biocompatibility of the coatings
were evaluated. SEM analysis shows that among all coatings, the TNT/CS-PVP/HA coating
possesses homogeneous surface and the incorporation of PVP into CS resulted in better
morphology. The tentative mechanism for the deposition of CS-PVP biopolymers on TNT
was proposed. Studies on the mechanical properties indicate that the addition of PVP into CS
increase the hardness and adhesion strength of the coatings and the deposition of HA on
TNT/CS-PVP coating further increase the mechanical strength. Similarly TNT/CS-PVP/HA
coating shows good corrosion resistance, better fibroblast cell adhesion and low cytotoxicity
behaviour than TNT and TNT/CS-PVP coatings.
Keywords: Rapid breakdown Anodization, TNT, biopolymers, anti-corrosion properties,
biocompatibility
1. Introduction
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Titanium and its alloys have become excellent materials of choice for the production
of long term prosthetic, cardiovascular and orthodontic implants compared to stainless steel
and magnesium alloys due to their low toxicity, biocompatibility and good mechanical
properties [1-3]. Even though these materials exhibit high corrosion resistance and high
specific strength, the body fluid environment inevitably results in the release of
noncompatible metal ions by the implants into the body. These released ions are found to
cause allergic sand toxic reactions. To overcome the problem of ion release, many surface
treatment techniques like anodic oxidation [4], plasma spraying [5], sol-gel method [6],
electrophoretic deposition [7] and ion implantation [8] have been attempted to modify the
topography and chemistry of titanium.
Another major problem is that the titanium and its alloys are bioinert materials, which
can be covered by the host organism without being integrated with bone [9, 10]. To overcome
this shortage, HA has been applied as a coating on the metallic implants due to its similarity
in chemical compositions, high biocompatibility and osteoconductivity to bone tissue of
human [11, 12]. To enhance the adherence of the HA coating on titanium alloy by reducing
the thermal expansion mismatch of HA coating and metal substrate, several recent studies
have introduced intermediate layers like single oxide coatings such as alumina [13] titania
[14, 15] composites [16, 17], polymers [18, 19] carbon nanotubes [20] between bioactive HA
coating and metal substrate and provided some encouraging results. Among all the materials
so far implanted into the body, polymers for bone generation must be biocompatible. In
addition, they should be mutable, shapeable, or polymerizable in situ to ensure good
integration in the defective area. As natural bone is an inorganic/organic composite, to mimic
nature considerable research interests have been developed to fabricate composites such as
CS/Ag/PVP [21], CNT/HA/CS [22], CS/graphene oxide [23], CS/silk fibroin [24] and
CS/MgO [25] as synthetic bone materials. Such organic/inorganic composites have the
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potential for greater functionality and performance than pure organic and inorganic materials.
CS has long been marked as a one of the most promising natural polymers exhibiting
properties such as chemical inertness, biodegradability, biocompatibility, high quality film
forming properties and low cost [26, 27]. To improve its mechanical strength, a rigid polymer
like pyrrole [28], polyethylene glycol [29] and PVP [30] is incorporated into the CS flexible
matrix. PVP has attracted considerable interest due to its hydrophilicity, lubricity, anti-
adhesive property and excellent biocompatibility and it has the capability to form bonding
with CS [30]. A possible route to harness the excellent properties of TNT, CS, PVP and HA
for applications is through incorporating them into composite materials. Recently, a unique
electrode material comprised of highly ordered TNT achieved within a few minutes by RBA
method, has been shown to possesses large surface area, good uniformity, conformability and
high porosity. Compared to other methods, RBA method produces a thick TNT layer with a
chemical bond between the oxide and the substrate that likely results in the enhanced
adhesion strength. Its potent nanostructuring offers a microenvironment so that high quantity
of polymer can infiltrate into the oxide TNT framework effectively.
So, an attempt was made in the present work to fabricate TNT/CS-PVP/HA
multilayer film on Ti alloy through RBA and eletrodeposition methods. On this TNT, CS-
PVP biopolymers and HA films were deposited layer by layer through electrodeposition
method. The corrosion behaviour of this processed alloy in Ringer’s solution was studied by
potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques.
The morphology and composition of the composite film existing on the Ti alloy surface were
determined by SEM, EDX, XRD and AT-FTIR techniques. It is fair and reasonable to
speculate that the integration of TNT, CS, PVP and HA would pave an ideal platform for the
fabrication of bioimplant materials.
2. Experimental
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2.1 Materials
Ti alloy sheet (purity 99%) of 0.5 mm thickness was purchased from Sigma-Aldrich
Chemie GmbH, Schnelldorf, Germany. HF, HNO3, NH4Cl, NaOH, NaCl, CaCl2, KCl,
Ca(NO3)2, (NH4)3PO4, CH3COOH, PVP and CS (produced from shrimp shell), having more
than 85% degree of acetylation were supplied by Aldrich Chemie, GmbH.
2.2 Synthesis of TNT coating by RBA
The sample material was Ti alloy with size of 1.0 cm × 1.0 cm and a thickness of 0.5
mm. Prior to anodization, the samples were ground with sandpaper up to 1,200 grit and then
chemically etched in a mixture of HF, HNO3 and H2O in the ratio 1:4:5 for 30s followed by
rinsing with deionised water and ethanol for 20 min sequentially and then dried. The
anodization was performed in a two-electrode configuration connected to a DC power supply
with Ti alloy as the anode and graphite foil (1cm × 1cm) as cathode at a potential of 20V for
10 min in an electrolyte containing 0.3M NH4Cl in a thermostatically controlled manner [31,
32].
2.3 Synthesis of TNT/CS-PVP composite films
To the CS solution (1g in 1% acetic acid), various concentrations (2 - 6 wt %) of PVP
were gradually added and the solution was ultrasonicated for 30 min before electrodeposition
for uniform distribution. The deposition was performed in an individual cell using a three
electrode configuration in which anodized titanium served as the cathode and platinum
electrode acts as an anode. A saturated calomel electrode (SCE) was used as the reference
electrode and it was carried out in potentiostatic mode by applying a potential of -2.5 V vs.
SCE for 45 min using an electrochemical system CHI 760C (CH instruments, USA). The
inter-electrode distance and deposition area were maintained at 10 mm and 1cm2.
2.4 Electrodeposition of HA coating
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Stoichiometric HA nanoparticles for electrodeposition were prepared by wet chemical
method. Briefly, 0.6-M ammonium phosphate solution was added slowly into 1.0-M calcium
nitrate solution at 70 ºC. The pH of the solution was adjusted to 6 by adding NaOH. Stirring
was performed for 8h at 70 ºC and then 24h at room temperature and this suspension was
used for deposition of HA [33]. The electrodeposition of HA was carried out using the same
set up as mentioned for the deposition of biopolymers. Here TNT/CS-4%PVP coated
substrate was used as working electrode. 0.1 M NaCl was added in order to improve the
conductivity of the electrolyte. Before electrodeposition, the suspensions were ultrasonicated
for 30 min to achieve a homogeneous dispersion. HA nanoparticles were electrodeposited for
30 min under constant potential at – 1.5 V vs Ag/AgCl [34].
2.5 Characterization of the fabricated coatings
The microstructural and morphological characterization of coated samples was carried
out using Scanning Electron Microscope (Hitachi, Japan, S-3400N) and subsequently the
elemental composition of the coatings were analyzed by Energy dispersive spectroscopy
(EDS). The phase analysis was done using XRD (Philips PAN analytical Xpertpro,
PW3040/60). Fourier transform infrared - attenuated total reflectance (AT-FTIR)
spectroscopy 400 PerkinElmer infrared spectrometer in the wavenumber of 400−4000 cm−1
was used to investigate the functional groups present in the deposited coatings.
Microhardness was measured by Vickers microhardness method on automatic microhardness
tester LM 247 ATLECO (LECO Corporation, St. Joseph, MI) at 25 g load and 10 s dwell
time. Thickness of the coating was evaluated using Dermitron thickness tester. For assessing
the adhesion of the composite coating qualitatively on Ti substrate, a standard test method
(SCOTCH Tape method, ASTM D 3359-02) was used. The corrosion studies were carried
out in Ringer’s simulated body fluid (SBF-the aqueous solution containing 8.60 g/l NaCl,
0.33 g/l CaCl2 and 0.3 g/l KCl) at 37 ± 0.1°C using Electrochemical workstation (CHI
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instruments, 760model). A conventional three electrode setup was used for electrochemical
measurement, with Pt as counter electrode, standard Ag/AgCl as reference electrode and
anodized Ti/Ti as the working electrode. The exposed area of working electrode was
maintained at 1 cm2. OCPT measurements were monitored for all coatings with the exposure
time of 1hr. Similarly tafel measurements were performed by applying potential in the range
of +2 V to -2 V with a scan rate of 1 mV/s. Likewise, the impedance measurements were
performed by applying a sinusoidal wave of 10 mV to the working electrode at a frequency
range of 0.1 MHz – 10 MHz.
2.6 In vitro biocompatibility studies
2.6.1 Cell culture
Mouse fibroblasts (L929) cells were obtained from National Centre for Cell Science
(NCCS), Pune, India and were cultured in minimal essential media (Hi Media Laboratories)
supplemented with 10% Fetal Bovine Serum (FBS), Streptomycin (100 U/mL) and Penicillin
(100 U/mL) (Cistron laboratories). The cell culture was then incubated under the humidified
atmosphere (CO2) at 37ºC. The samples under examinations were sterilized in an autoclave at
120 ºC for 2 h and placed in 24 well cell culture plates. After the stipulated time period (2
days), the coated samples were washed twice with phosphate buffer saline (1X PBS, pH 7.4).
The cells were detected by live/dead staining. The amount of 200 μL of dye mixture (100
μL/mg Acridine Orange (green fluorescence in live cells) and 100 μL/mg ethidium bromide
(red fluorescence in dead cells) in distilled water) were mixed with 2 ml cell suspension
(30,000 cells/ml) in 6-well plate. The suspension was immediately examined and viewed
under olympus inverted fluorescence microscope (Ti-Eclipse) at 200 × and 400 x
magnification.
2.6.2 Cell viability test
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Cell viability was determined using MTT (3-(4,5-dimethyl-2-yl)-2,5-
diphenyltetrozolium bromide) assay. Cells with the density of 1 × 105/wells were seeded on
the samples in 24 well culture plates. The culture medium was replaced with new medium
every day. After the incubation period (1 and 3 days), the samples were removed from the
respective wells and the wells were washed with phosphate buffered saline (pH = 7.4). Only
those cells that are adherent to the well walls were found viable and incubated with 0.5%
MTT solution. The viable cells reduce the MTT into insoluble formazan precipitate by
mitochondrial succinic dehydrogenase. After 4 h incubation, 0.1% dimethyl sulfoxide
(DMSO) was added to dissolve the formazan crystals. After these procedures, the absorbance
of the content of each well was determined at 570 nm with a UV-spectrophotometer. Cell
viability (%) related to the control wells containing cell culture medium without the samples
was calculated based on the average of five replicates using the following equation:
% Cell Viability = [A570test / A570control] × 100
Graphs were plotted using the % of Cell Viability at Y-axis and various samples in X-
axis.
3. Results and Discussion
3.1 Microstructure and composition of the coatings
Fig. 1a and b shows the SEM images of bundles of closely packed TNT arrays
prepared in NH4Cl solution. The tubes have an average diameter of 30 nm, a length of 40 µm,
and a wall thickness of about 10 nm. It is also noted that the nanotubes grow rather rapidly
within 10 min and have a highly ordered in bundles. Fig. 1 (c–f) shows the SEM images of
biopolymers deposited on TNT taken at lower magnification; 100µm. Fig. 1c reveals the
surface morphology of the CS deposited TNT coating. The microstructure exhibited
significant porosity (8%) with a typical pore size of about 100μm. The porosity can also be a
result of the hydrogen evolution at the cathode surface during electrodeposition and also
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many cracks are seen which can be attributed to drying shrinkage. When the concentration of
PVP was further augmented to 4%, the coating becomes smoother and denser with non-
porous morphology as depicted in Fig. 1e. The surface of the film is flat and smooth which is
beneficial to resist bacteria adhesion. When the concentration of PVP was further increased to
6%, the morphology of the coating remains the same, as depicted in Fig. 1f.
Fig. 2 (a-d) represents the SEM images of the above mentioned samples taken at
higher magnification, 200 nm. Fig. 2a depicts the distribution of small amount of CS
nanoparticles on TNT arrays indicating the low deposition rate of CS. This is because, CS
possess a high charge density in 1% acetic acid solution and so the electrostatic repulsion
between monomeric units is high which stiffen the chain leading to a rigid rod confirmation.
Due to the electrostatic repulsion of charged CS macromolecules at the cathode surface, the
deposition mass is low. On seeing Fig. 2b, it can be observed that there is an increase in the
quantity of the CS-PVP polymer nanoparticles when 2% PVP was added into the CS
solution. At 4% concentration of PVP in CS-PVP deposited TNT, the quantities of the
deposited particles are high and they are almost uniformly distributed on the TNT surface
which means that, increase in the PVP concentration increases the deposition mass (Fig. 2c).
While adding PVP into the CS solution hydrogen bond formed between them reduce the
positive charge density and form a homogeneous phase thereby increase the deposition mass.
Due to these interactions, stability of the film increases with high anticorrosion property as
shown in corrosion studies. On the contrary, when the concentration of PVP was further
increased to 6%, quantity of the deposited particles decreases due to increase in suspension
viscosity as shown in Fig. 2d. Thus the PVP has the capability to change the microstructure
and the properties of the material.
Fig. 2e shows the SEM image of HA deposited on TNT/CS-4%PVP taken at lower
magnification. It shows a net-like matrix, containing morphologically amorphous flakes
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connected each other. Such a net-like matrix formed by HA was also reported by Okido et al
[35]. On viewing the morphology of HA, magnification 200 nm depicted in Fig. 2f, it can be
seen that the assemblies of small flakes and plate-like particles interweave with each other.
The thickness of an individual HA flake is less than 20 nm and they are grown perpendicular
to the coating surface. However, at a higher magnification, a nearly needle-like phase that
was characteristic of HA was observed as seen in the insert in Fig 2f. The length and
thickness of the needle is approximately 500 nm and 30 nm. The visual porosity level seems
to be dramatically lower and beneath the HAP layer, agglomerated CS-4% PVP polymer
particles are seen. This open interconnected porous structure facilitates the penetration of the
surrounding bone tissue and hence leads to better biointegration and mechanical stability.
Fig. 2g displays the EDS spectrum of the TNT/CS-4%PVP/HAp composite coating. It
shows well defined peaks for Ti, O, Ca and P elements at 4.508 KeV, 0.525 KeV, 3.69 KeV
and 2.013 keV respectively revealing the presence of CS, PVP and HA coatings on TNT.
This is also evident from X-Ray diffraction studies.
3.2 Deposition mechanism
Fig. 3a exhibits the comparative curves for the current behaviour of CS-2-6%PVP
codeposited onto TNT substrate during potentiostatic deposition at -2.5 V for 45 min.
Initially, a quick drop of current indicates the charging of double layer or the fast formation
of barrier CS-PVP polymer layer onto TNT surface. Finally, a constant value of the current is
reached corresponding to the growth of the polymer layer at the TNT surface under the same
deposition rate. By comparing these curves, it can be identified that, the drop of the current is
very low for the deposition of the CS alone due to the low amount of CS deposit as shown in
Fig 2a. It is suggested that electrostatic repulsion of charged CS macromolecules at the
cathode surface prevented deposit formation. Meanwhile, the current drop increases with an
increase in the PVP concentration up to 4% indicating that the addition of PVP increases the
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deposition mass. Thus the addition of PVP resulted in partial charge compensation, which
promoted deposit formation at the electrode surface and resulted in an increasing deposition
yield [36]. But when the concentration of PVP was further increased to 6%, the current
increases as the electrophoretic mobility of the ions decreases due to the increase in the
viscosity of the solution which in turn decreases the deposition mass.
Electrodeposition is an important technique used to fabricate composite coatings. The
important point to be discussed is the electrochemical mechanism of the codeposition of CS
and PVP as shown in Fig 3b. CS can be protonated and dissolved in acidic solutions as
shown in Eqn. [1]. At low pH, protonated CS becomes a cationic polyelectrolyte [36]:
CS-NH2 + H3O+ → CS-NH3+ + H2O ---------- Eqn. [1]
When PVP was gradually blended into the CS solution, some positive charges in the
backbone of CS were balanced by the negative charges of PVP. After voltage was applied
between the two electrodes for a certain time, positively charged CS having electrostatic
interaction with PVP move towards the cathode and interact with the hydroxyl groups present
on the surface of TNT arrays. CS experienced a higher pH (6.5) near the cathode using the
following cathodic reaction, Eqn. [2]
2H2O + 2e- → H2 + 2OH− ---------- Eqn. [2]
Here CS amino groups were deprotonated and became insoluble. Then, chitosan and
PVP as shown in Eqn. [3] were co-deposited onto the surface of TNT which acted as cathode.
Thus a visible gel formed at the negative electrode.
CS–NH3+ + -OC-PVP → CS–NH3
+--OC-PVP ---------- Eqn. [3]
CS when dissolved in 1% acetic acid it is hydrophilic in nature but after deposition
the CS coating slightly changed to hydrophobic. While PVP was incorporated into CS
coating it changes the hydrophobic nature of the coating to hydrophilic which results in the
increase in surface energy leads to the better cell attachment (shown in sec 3.7).
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Likewise, the electrochemical and chemical reactions taking place on the surface of
TNT/CS-4%PVP during electrodeposition of HA are suggested as follows:
O2 + 2H2O + 4e- → 4OH- ---------- Eqn. [4]
2H2PO4- + 2e- → 2HPO4
2− + H2↑---------- Eqn. [5]
2HPO42- + 2e- → 2PO4
3− + H2↑---------- Eqn. [6]
These reactions resulted in production of OH-, besides; the pH of the electrolyte had
been adjusted to 6.0 before electrochemical deposition, so the pH around the surface of
TNT/CS-4%PVP increased when a voltage was applied. The high concentration hydroxyl
ions could promote the formation of HA, as follows:
10Ca2+ + 6PO43− + 2OH- → Ca10(PO4)6(OH)2 ---------- Eqn. [7]
3.3 AT-FTIR spectral analysis
Fig 4 (a-c) depicts the AT-FTIR spectral results of (a) TNT, (b) TNT/CS-4%PVP and
(c) TNT/CS-4%PVP/HA respectively. From Fig 4a, it was observed that the peaks at 3601.45
cm-1 and 2990 cm-1 were attributed to the stretching vibration of the OH band from surface
adsorbed water and bending vibrations of OH groups respectively. This indicates that a
significant fraction of H2O and OH groups exist in as anodized sample. The bands at
625.02 cm-1 and 493.47 cm-1 was assigned to the vibration of Ti-O and Ti-O-Ti bond in TNT.
AT-FTIR spectra of Fig 4b shows the CS characteristic band observed at 1559 cm-1 and 3367
cm-1 assigned to the stretching vibration of amino and NH group of CS [37] shifted to
1599.31 cm -1 and 3652.62 cm-1 indicating the electrostatic interaction between CS and PVP.
The CO vibrational peak of PVP observed at 1663cm-1 [38] shifted to 1640.77cm-1 supports
the presence of interaction between PVP and CS. The peaks at 933.39 cm-1, 1320.5 cm-1 and
1095.29 cm-1 were attributed to the saccharide structure, CH and C-O stretching of CS
respectively. In the case of Fig 4c, a new peak arise at 601.22 cm-1 is the characteristic
absorption peak of phosphate group of HA [39].
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3.4 XRD analysis
Fig.5 (a-c) corresponds to the XRD patterns of TNT, TNT/CS-PVP, TNT/CS-
PVP/HAp coatings fabricated on Ti alloy. Fig. 5a depicts the XRD pattern of TNT and the
diffraction peaks at 2θ values of 25.2º, 37.9º, 48.3º, 53.8º, 62.7º, 75.3º are assigned to (101),
(103), (200), (105), (213), (107) planes of anatase form of TiO2, respectively (JCPDS card
no: 21-1272). The peak at 2θ position 36º with plane (101) is assigned to the rutile form of
TiO2 (JCPDS no: 21-1276). Thus a mixture of anatase and rutile phases of titania was formed
during RBA of Ti alloy. The anatase and the rutile diffraction peaks looks very sharp
indicating the high crystalline nature of the coating. XRD pattern of TNT/CS-PVP composite
coating depicted in Fig 5b shows new diffraction peaks at 2θ values of 12.6º and 20.32º are
assigned to CS and PVP composite (JCPDS card no: 39-1984) respectively and these results
are in accordance with the earlier reports [40]. The broad diffraction peaks of the polymer
composite indicate its amorphous nature [41]. XRD pattern of Fig 5c show peaks with planes
at 2θ positions 31.7º (211) and 43.2º (222) correspond to the HA fabricated on TNT/CS-PVP
composite coatings. These results are in good agreement with JCPDS card No: 09-0432. The
HA diffraction peak also looks very sharp revealing its crystalline nature. When CS-PVP and
HA were coated on TNT, the intensity of the anatase diffraction peaks gets decreases.
3.5. Thickness, Hardness and adhesion strength
Table 1 lists the thickness, hardness and adhesion strength values of Ti alloy, TNT,
TNT/CS-2-6%PVP and TNT/CS-4%PVP/HAp composite coatings. The thickness and
mechanical properties of TNT such as hardness and adhesion strength were almost same
compared to the substrate and these values increased when CS-PVP was coated on TNT and
it is further increased for TNT/CS-4%PVP/HA composite coating. Thus TNT/CS-
4%PVP/HA coating shows maximum hardness, thickness and adhesion strength than any
other coatings. Usually, poor intermixing of HA within the TNT led to peeling of HA layer.
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In this particular case, presence of CS-PVP allowed good mixing of TNT, CS-PVP and HA
layers by modifying their respective surface charges, thereby permitting the formation of
uniform multi layers on Ti substrate enabling good mechanical strength to the substrate. Due
to the presence of these multi layers, the overall thickness of the coating is higher which
obviously increase the mechanical properties of the coatings. In the same manner, the
thickness and mechanical properties of the coating increases from 2-4% concentration of PVP
in TNT/CS-PVP coating and at 6% it decreases but adhesion strength remains the same. This
could be due to decrease in the porosity of the coatings as the concentration of PVP increases.
3.6 Corrosion studies
3.6.1 Tafel polarization technique
Fig. 6 (a, b) represents the potentiodynamic polarization behaviour of various samples
immersed in Ringer's solution. The parameters obtained by fitting the potentiodynamic
curves are summarized in Tables 2 and 3 along with the OCP values. On seeing these values,
it can be observed that TNT/CS-4%PVP/HA coating has higher OCP value compared to
other coatings. Likewise the OCP value increases with increase in the percentage of PVP
from 2% to 4% in the composite and at 6% PVP it decreases. The tafel polarization curves
indicate that coated Ti alloy showed a drastic shift of corrosion potential in the anodic region
compared to bare suggesting the improved corrosion resistance of the coated substrate. Fig 6a
and Table 2 illustrate that the corrosion resistance of the TNT/CS-4%PVP coating (5.35×102
KΩ cm-2) is high compared to the CS deposited TNT (4.15×102 KΩ cm-2) and also the
anticorrosion properties of the coating increases with an increase in the PVP concentration up
to 4% and after that, decreases. These results are in accordance with the OCP values. This
shows that the blending of PVP into the CS imparts mechanical strength to the layer. When
CS alone was electrodeposited on TNT, a porous coating with much smaller thickness was
obtained (Fig. 1c), thus producing a detrimental effect on its corrosion resistance. When PVP
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was codeposited with CS, the size and the number of pores decreases and thickness of the
coating increases. With increase in the concentration of PVP, a nonporous morphology with
better thickness was obtained (shown in Fig 1e) which can effectively prevent the corrosion
solution from penetrating into the coating and reacting with the Ti substrate. When the
concentration of PVP exceed to 6%, deposition rate of CS-PVP and the thickness of the
coating decreases and obviously the corrosion potential decreases.
From Fig 6b and Table 3, it can be inferred that among all samples, TNT/CS-
4%PVP/HA shows higher corrosion potential (0.866 V) and lower cathodic current density
(1.04×10-8 µA cm-2) than any other coatings. It proves the effective inhibition of water
reduction which is the main driving force for Ti corrosion. These results can be mainly
ascribed to the thickness, hardness and surface morphology of the coatings. Thus the
presence of CS, PVP and HA components along with TNT acted as a barrier to electrons and
ions transport between the substrate and the ringer electrolyte, thereby, preventing corrosion.
Based on the polarisation results, the bio-corrosion resistance of the samples is ranked as
follows: TNT/CS-4%PVP/HAp > TNT/CS-4%PVP > TNT/CS-6%PVP > TNT/CS-2%PVP >
TNT/CS > TNT > uncoated titanium.
3.6.2 Electrochemical impedance studies
In order to further understand the corrosion process, electrochemical impedance
spectroscopy for various samples were measured and the relevant curves, charge transfer
resistance values of the coatings are presented in Fig. 7 (a, b) and Tables 2, 3 respectively.
The circuit depicted in Fig 7c was used to fit the impedance curves [42, 43]. As seen from the
equivalent circuit, Rs refers to the resistance of the solution, Cdl is the electric double layer
capacitance and Rct is the charge transfer resistance that is related to the electrochemical
corrosion rate. Fig 7a and Table 2 reveals that the Rct value of CS coating (6.84×101 kΩ cm-2)
deposited on TNT is less compared to the CS-PVP coated TNT. The Rct value increases from
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2% to 4% PVP in TNT/CS-4%PVP coating and then decreases. This may be due to the
drastic change in the surface morphology of the coatings from porous to nonporous and high
deposition rate. From Fig 7b and Table 3, it can be identified that the Rct value of TNT/CS-
4%PVP/HA coating (2.31×103 kΩ cm-2) was found to be much higher than the Rct value of
TNT/CS-4%PVP coating (3.22×102 kΩ cm-2). Thus the presence of HA coating enhances the
corrosion resistance of the specimen. The Rct values obtained from the Nyquist plots are
almost same compared to the Rp values obtained from the polarization studies but for
TNT/CS and TNT/CS-4%PVP/HA coatings it was different. This may be due to the ohmic
drop or else the OCP modifies the oxygen content near the working electrode which modifies
the surface condition. Thus “we can tune the anticorrosion properties of the coatings by
adjusting the components of the electrolyte used for synthesizing composite coating.”
3.7 Cell culture studies
3.7.1 Cytocompatibility
Cytocompatibility is one of the important biological evaluations of biomaterials which
can reflect the cell/biomaterial interactions. The cell viability was conducted for the TNT,
TNT/CS, TNT/CS-4%PVP and TNT/CS-4%PVP/HAp samples with respect to control (Fig.
8a) and the respective optical microscopic morphologies of L929 cell line after 3 days of
culture are displayed in Fig. 8 (b-e). To verify this hypothesis, MTT data for 3 days of
incubation are presented in Fig. 8f. After 3 day culture, attached cells can be found on all
samples. Fig 8a represents the control used to measure cytotoxicity. A small number of round
cells randomly distribute on TNT sample indicating its toxic nature (Fig. 8b), while CS
coated TNT sample shows mediocre cell viability (Fig. 8c). No obvious change in the cell
morphology can be found on both samples. In the case of CS-PVP coated TNT sample, the
number of cells increases and they become big (Fig. 8d). This indicate that the incorporation
of PVP into CS coating achieved better cell response as compared to pure CS coating [44].
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This is because of the decrease in the charge density of the coatings due to the electrostatic
interaction of PVP and CS (shown in Fig. 3b). As a consequence of this, cell proliferation
was enhanced by the resultant weakening electrostatic effects between cell membranes and
the coatings. Another plausible reason may be the addition of PVP increases the hydrophilic
nature of the coatings which results in the increase in surface energy leads to better cell
attachment. Similarly, in the TNT/CS-PVP/HA sample, the number and size of the cells are
very high compared to those of TNT/CS-PVP sample, shown in Fig. 8e and the cell
morphology similar to that of control group (Fig. 8a) proving that this sample is better for cell
attachment, growth and proliferation. The higher cellular responses of HA coating is due to
its chemical composition which can provide lots of Ca2+ which do good to absorbance of
protein like fibronectin and vimentin. Moreover, HA coating can protect the Ti alloy from
corrosion which reduces the rapid increase of pH value of solutions, thus alkalescent
conditions suitable for cell growth are kept. Hence, the finding of good compatibility in vitro
makes TNT/CS-PVP/HAp advanced composite coating of Ti alloy worthy of further studies
for development of surgical implants.
3.7.2 Cell adhesion
Fig. 9 (a-c) shows fluorescence images of the morphology of L929 cells cultured on
the TNT, TNT/CS-4%PVP and TNT/CS-PVP/HAp samples. Round cells were observed (Fig.
9a) on TNT sample. While at the same time point, cells cultured on TNT/CS-PVP coating
(Fig. 9b) were elongated and exhibited a spindle-shaped configuration. While cells on
TNT/CS-4%PVP/HAp coating displayed a more spread-out morphology (Fig. 9c) as
compared to TNT and TNT/CS-PVP coating. The reason is the same as explained in sec
3.7.1. These results demonstrate that spreading of L929 cells on TNT/CS-4%PVP/HAp
coating is more pronounced than that on TNT and TNT/CS-PVP coating. Poor cell adhesion
of TNT sample may be due to the steep increase in pH or the release of ions into the medium.
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Surface chemistry is also an important factor affecting cell spreading on biomaterials. It has
been demonstrated that L929 cell spreading was promoted by HAp coated samples having
flake morphology. The results obtained in this work are to some extent in accordance with
observations reported already [45, 46]. Similarly the biopolymers CS-PVP coating also
increases the cell adhesion and proliferation. Thus the use of a biocompatible material such as
HAp in combination with a biodegradable polymer gives a ‘bioartificial advanced composite’
with enhanced biocompatibility and other biological functions.
4. Conclusion
In this paper, TNT, various percentages of PVP (2% - 6%) in the TNT/CS-PVP,
TNT/CS-4%PVP/HA coatings were fabricated on Ti alloy via RBA and electrodeposition
methods. From SEM analysis, it was found that with increase in PVP concentration its
incorporation into the CS coating increases and also the surface of the coating become
smoother when the concentration of PVP is 4%. The addition of 4% PVP into CS coating not
only make the surface smoother but also increase the thickness, hardness, adhesion strength
and corrosion resistance of the coatings. The presence of TNT, CS, PVP and HA coatings on
Ti alloy was confirmed by AT-FTIR and XRD analysis. Similarly HA coating on TNT/CS-
4%PVP increase the mechanical properties and corrosion resistance of the coating effectively
compared to TNT, TNT/CS-4%PVP coatings. The biological studies clarified that the PVP
plays an active role in the enhancement of cell viability of the TNT/CS-PVP composite
coating on Ti alloy. HA coated TNT/CS-4%PVP shows superior cell attachment and very
low cytotoxicity compared to TNT, TNT/CS-4%PVP coatings. Thus the obtained results
pave the way for the electrochemical fabrication of novel coatings for biomedical implants.
Acknowledgement
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One of the authors (M. Mumjitha) would like to acknowledge the DST-INSPIRE
division for providing the INSPIRE fellowship (IF110352).
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Fig 1 SEM micrographs of i) (a, b) TNT, ii) TNT/CS-PVP coatings taken at lower
magnification, 100µm: (c) TNT/CS, (d) TNT/CS-2%PVP, (e) TNT/CS-4%PVP, (f)
TNT/CS-6%PVP.
Fig 2 SEM micrographs of i) TNT/CS-PVP coatings taken at higher magnification, 200 nm:
(a) TNT/CS, (b) TNT/CS-2%PVP, (c) TNT/CS-4%PVP, (d) TNT/CS-6%PVP and
ii) TNT/CS-4%PVP/HA coating of magnification (e) 1µm and (f) 200 nm and (g)
EDX spectra of TNT/CS-4%PVP/HA coating.
Fig 3 Amperometry plot of electrodeposition process of (a) CS and various % of PVP on
TNT and (b) schematic illustration for the electrodeposition of CS-PVP on TNT.
Fig 4 FTIR spectra of (a) TNT, (b) TNT/CS-4%PVP and (c) TNT/CS-4%PVP/HA
coatings.
Fig 5 XRD patterns of (a) TNT, (b) TNT/CS-4%PVP and (c) TNT/CS-4%PVP/HA
coatings.
Fig 6 Comparative Potentiodynamic polarization curves for (a) various % of PVP in CS-
PVP coating on TNT and (b) various coatings on TNT along with bare Ti.
.Fig 7 Comparative Nyquist plots for (a) various % of PVP in CS-PVP coating on TNT and
(b) Various coatings on TNT along with bare Ti.
Fig 8 Optical images of L929 cells cultured on various samples for a period of 3 days: (a)
control, (b) TNT, (c) TNT/CS, (d) TNT/CS-PVP, (e)TNT/CS-4%PVP/HA and (f)
represents the cell viability of L-929 cells expressed as a percentage after 3 days
incubation.
Fig 9 Fluorescence images of L929 cells adhered on (a) TNT, (b) TNT/CS-4%PVP and (c)
TNT/CS-4%PVP/HA coatings.
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Table 1 Thickness, Hardness and Adhesion strength of Ti alloy and various coatings
fabricated on Ti alloy.
Table 2 Corrosion parameters obtained from Tafel and Nyquist plots for various % of PVP in
CS-PVP coatings fabricated on TNT.
Various
samples
OCP
(V)
Corrosion
potential
(V)
Corrosion current
density
(µA cm-2)
Polarization
resistance
(KΩ cm-2)
Charge transfer
resistance
(KΩ cm-2)
TNT/CS 0.189 0.208 6.29×10-7 4.15×102 6.84×101
TNT/CS-2%PVP 0.430 0.429 5.55×10-7 4.84×102 1.70×102
TNT/CS-4%PVP 0.761 0.753 8.18×10-8 5.35×102 3.22×102
TNT/CS-6%PVP 0.673 0.668 9.97×10-8 4.91×102 2.21×102
Various Samples Thickness
(μm)
Hardness
(GPa)
Adhesion
strength (MPa)
Ti alloy 9.1 ± 0.9 7.32 ± 0.22 -
TNT 9.3 ± 1.2 7.34 ± 0.22 20.1 ± 0.6
TNT/CS 12.22 ± 1.1 7.56 ± 0.25 21.4 ± 1
TNT/CS-2%PVP 14.03 ± 0.8 7.61 ± 0.24 22.5 ± 0.8
TNT/CS-4%PVP 17.54 ± 1.6 7.69 ± 0.22 24.4 ± 0.6
TNT/CS-6%PVP 15.62 ± 1.2 7.63 ± 0.23 23.9 ± 0.9
TNT/CS-4%PVP/HA 22.08 ± 1.5 8.02± 0.22 23.2 ± 0.8
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Table 3 Corrosion parameters obtained from Tafel and Nyquist plots of various coatings
fabricated on Ti substrate.
Various
samples
OCP
(V)
Corrosion
potential
(V)
Corrosion
current
(μA cm-2)
Polarization
resistance
(KΩ cm-2)
Charge transfer
resistance
(KΩ cm-2)
Bare -0.699 -0.698 7.59×10-5 4.29×101 1.37×101
TNT -0.570 -0.571 5.87×10-6 7.47×101 3.92×101
TNT/CS 0.189 0.208 6.29×10-7 4.15×102 6.84×101
TNT/CS-4%PVP 0.761 0.753 8.18×10-8 5.35×102 3.22×102
TNT/CS-4%PVP/HA 0.857 0.866 1.04×10-8 7.03×102 2.31×103
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Fig 1
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Fig 2
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Fig 3
0 600 1200 1800 2400 3000
0.000
0.002
0.004
0.006
0.008 aC
urre
nt (A
)
Time (Sec)
CS CS - 2% PVP CS - 4% PVP CS - 6% PVP
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Fig 4
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Fig 5
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Fig 6
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Fig 7
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Fig 8
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Fig 9
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