An Investigation on the Effect of Morphologies on Corrosion Behaviour of Nanostructured Hydroxyapatite-Titania Scaffolds

Article (PDF Available)inJournal of Bionanoscience Vol-4(1):1-5 · January 2011with44 Reads
DOI: 10.1166/jbns.2010.1038
Abstract
The viable application of hydroxyapatite (HAp) scaffolds requires to posses the unison of properties: porosity, bioactivity, mechanical toughness etc. Such properties strongly depend on the geometric factors such as the size, morphology/microstructure of HAp. We have developed a hydrothermal based approach to synthesize HAp-titania scaffold with different morphologies ranging from smooth film to cauliflower to urchin like structures. The structural characterization by XRD reveals the formation of HAp phase. The SEM analysis suggests the formation of HAp nanosheets or their subsequent assembly when reaction carried out under basic conditions without and with the oxidizing agent H2O2, respectively. The detailed investigation of corrosion behaviour of all HAp-titania scaffold samples was undertaken by potentiodynamic technique in Ringer’s simulated body fluid solution at close to human body temperature i.e., 37 �C. The shift in the OCP values of HAp-titania scaffold samples towards nobler side and the relatively more posivite Ecorr values observed for these samples than that of bare Ti-foil, suggesting superior corrosion resistance in case of HAp-titania scaffold samples than that of bare Ti-foil. The detailed results on structural characterization and discussion on corrosion behaviour of HAp-titania scaffold samples with different morphologies/microstructures are presented.
RESEARCH ARTICLE
Copyright © 2011 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Bionanoscience
Vol. 4, 1–5, 2011
An Investigation on the Effect of Morphologies on
Corrosion Behaviour of Nanostructured
Hydroxyapatite-Titania Scaffolds
Simantini Nayak
1 2
, Kamala Kanta Nanda
1
, Purna C. Rath
1
,
Sarama Bhattacharjee
1
, and Yatendra S. Chaudhary
1 3
1
Colloids and Materials Chemistry Department, Institute of Minerals and Materials Technology (CSIR),
Bhubaneswar 751013, India
2
Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
3
Department of Chemistry, University of Oxford, South Parks Road, OX1 3QR, U.K.
The viable application of hydroxyapatite (HAp) scaffolds requires to posses the unison of properties:
porosity, bioactivity, mechanical toughness etc. Such properties strongly depend on the geometric
factors such as the size, morphology/microstr ucture of HAp. We have developed a hydrothermal
based approach to synthesize HAp-titania scaffold with different morphologies ranging from smooth
film to cauliflower to urchin like structures. The structural characterization by XRD reveals the for-
mation of HAp phase. The SEM analysis suggests the formation of HAp nanosheets or their sub-
sequent assembly when reaction carried out under basic conditions without and with the oxidizing
agent H
2
O
2
, respectively. The detailed investigation of corrosion behaviour of all HAp-titania scaffold
samples was undertaken by potentiodynamic technique in Ringer’s simulated body fluid solution at
close to human body temperature i.e., 37
C. The shift in the OCP values of HAp-titania scaffold
samples towards nobler side and the relatively more posivite E
corr
values observed for these sam-
ples than that of bare Ti-foil, suggesting superior corrosion resistance in case of HAp-titania scaffold
samples than that of bare Ti-foil. The detailed results on structural characterization and discussion
on corrosion behaviour of HAp-titania scaffold samples with different morphologies/microstructures
are presented.
Keywords: Hydroxyapatite, Nanostructure, Simulated Body Fluid, Corrosion, Scaffold.
1. INTRODUCTION
The Titanium and its alloys have found extensive appli-
cations as orthopedic implants in human body because
of their higher strength than that of polymeric implants
and high toughness than that of ceramic implants.
1
How-
ever these metallic materials are susceptible to corrosion
by body fluids, which may lead to infection, local pain,
swelling and loosening and consequently the in vivo fail-
ure of implant.
2
The human body shows natural reaction
against prosthetic devices causing the osteolysis and has
the tendency to isolate from the surrounding live tissues.
Moreover, metallic surfaces are not adequately bioactive,
in general, and surface modification is usually required
to improve the bioactivity so as to improve osteointegra-
tion with bone tissues. To overcome these problems, Ti
Author to whom correspondence should be addressed.
implants are generally coated with bio-active hydroxya-
patite [(Ca
10
(PO
4
6
(OH)
2
] (HAp)—a bio-ceramic which
exhibit physico-chemical resemblance with mineral con-
stituents of human bones and teeth.
3
The coating passi-
vates the body fluid to come in direct contact with metal
and hence minimizes the corrosion of implants. The ability
of such HAp coatings helps to integrate implants to bone
and support new bone generation.
4
Further, some in vitro
studies have revealed that the surface parameters of bio
implant scaffold such as topography may play an impor-
tant role in growth of tissue, retaining their integrity, cell
mobility and control over cellular activity.
5
The physico
chemical and bio-physiological activity of HAp coating
strongly depends upon geometrical factors such as particle
size, dimensional anisotropy, morphology and microstruc-
tures etc. Therefore, apart from the mechanical toughness,
the unison nanocrystalline nature, porosity and bioactiv-
ity, such that it may allow the growth of tissues within
J. Bionanosci. 2011, Vol. 4, No. 1 1557-7910/2011/4/001/005 doi:10.1166/jbns.2011.1038 1
RESEARCH ARTICLE
An Investigation on the Effect of Morphologies on Corrosion Behaviour of Nanostructured HAp-Titania Scaffolds Nayak et al.
and around the scaffold while maintaining the integrity of
bone tissue are highly desirable for its viable application as
bio-implant. The synthesis of phase-pure HAp crystal with
desired morphology and their uniform smooth coating on
metal substrates has been the focus of intensive research
over the last decades.
6
Various synthetic approaches such
as molten salt synthesis, sol–gel, electrochemical deposi-
tion, template mediated synthesis and plasma spray tech-
niques have been explored for HAp synthesis
7–18
and HAp
composite.
19–20
But, either due their poor crystalinity/phase
impurity or the poor corrosion resistance has limited their
practical applications. Very recently, we have developed
a low temperature hydrothermal based approach to fab-
ricate nanostructured porous HAp-titania with different
morphologies which meets the desired properties such as
mechanical toughness, biocompatibility for their possible
bio-implant scaffold. Here, it is worth mentioning that the
HAp with different morphologies tends to exhibit different
degree of adhesion with titania or in other words the corro-
sion behaviour. The detailed investigation of the corrosion
behaviour of HAp-titania samples with respect to different
morphologies is essentially crucial for fundamental under-
standing and their practical application.
In this paper, the structural characterization and detailed
results on the corrosion behaviour of HAp-titania scaffolds
with different morphologies undertaken in simulated body
fluid at the temperature close to human body temperature
(37
C) are presented and the corrosion mechanism is
discussed in detail.
2. EXPERIMENTAL AND METHODS
Titanium foils (1 × 1cm
2
used as substrate and cal-
cium nitrate [Ca(NO
3
2
· 4H
2
O] and diammonium hydro-
gen phosphate [(NH
4
2
HPO
4
] used as precursor were
obtained from Aldrich, USA. Sodium hydroxide, hydrogen
peroxide, calcium chloride, sodium chloride and potassium
chloride were obtained from Merck, India. All chemicals
were used as obtained without any further purification.
2.1. Synthesis of Hydroxyapatite
A detailed synthesis of HAp-titania scaffolds exploiting
hydrothermal approach is explained elsewhere.
21
Briefly,
the HAp sol was prepared by dropwise addition of 5 ml
aqueous solution of 0.1 M diammonium hydrogen phos-
phate into 8.35 ml aqueous solution of 0.1 M of calcium
nitrate under vigorous stirring.
The prepared HAp sol was then poured into 45 ml
Teflon-lined vessel. A titanium foil was placed in this ves-
sel. Then, 10 ml of 1 M aqueous NaOH solution were
added to facilitate in-situ formation of sodium titanate
nuclei to subsequently control the HAp recrystallization
and its morphology.
This was then allowed for hydrothermal treatment at
240
C(5
C min
1
heating rate) for 4 h. After the
completion of hydrothermal reactions, reaction vessel was
allowed to cool down to RT. The HAp–titania sample was
then recovered and washed with deionised water to remove
the loosely adsorbed HAp and allowed to dry at RT. This
HAp–titania sample is referred to as HAP in the following
text. The control experiment was also performed without
introducing NaOH, and following the similar steps as men-
tioned above. This sample is referred as C-HAP in the
following text.
Another set of reaction was also carried out in the pres-
ence of oxidizing agent i.e., by adding 1.5 ml of 50% H
2
O
2
solution to HAp sol and 10 ml of 1 M with NaOH sepa-
rately, followed by same reaction steps as stated for HAP
sample and is referred to as HAP–H
2
O
2
in the following
text.
2.2. Structural Characterization
The formation of HAp phase and the presence of any other
phase/impurity, if any, were examined using X-ray diffrac-
tometer (XPERT PANalytical). The morphology of all
HAp-titania samples was examined by scanning electron
microscope (S-3400N, Hitachi). The compositional anal-
ysis was performed by an energy dispersive spectroscopy
(EDS, EDAX Instruments) attachment on the FEI Technai
G
2
20 TEM.
2.3. HAp-Titania Scaffold Corrosion Behaviour
The corrosion resistance behaviour of HAp is one of the
crucial parameter for its bio-implant application. There-
fore, to study the corrosion behaviour of HAp-titania
scaffold, the Potentiodynamic polarization measurements
were carried out using a potentiostat (VersaSTAT-3). The
Ringer’s simulated body fluid (SBF—the aqueous solution
containing 8.60 g l
1
NaCl, 0.33 g l
1
CaCl
2
and 0.30 g l
1
KCl) was used as the corrosion medium. The pH of this
solution was maintained at 7.4. The corrosion behaviour
of all samples was examined at the temperature close to
human body temperature i.e., 37
C.
Polarization curves were measured by a standard three
electrode system using saturated calomel (SCE) and plat-
inum wire as reference and counter electrode, respectively.
The polarization curves were recorded at the scan rate of
5mVs
1
. All the potentials mentioned here are referred
against reference (SCE) electrode. The morphologies of
representative samples after potentiodynamic polarization
measurements were also examined by scanning electron
microscope.
3. RESULTS AND DISCUSSION
The as synthesized HAp-titania samples were white in
color. The X-ray diffraction patterns recorded exhibit well
resolved peaks [(102), (112), (301), (212), (310), (312),
2
J. Bionanosci. 4, 1–5, 2011
RESEARCH ARTICLE
Nayak et al. An Investigation on the Effect of Morphologies on Corrosion Behaviour of Nanostructured HAp-Titania Scaffolds
(004)] and [502]), which suggest the formation of crys-
talline HAp phase, Figure 1. To further explorethe mor-
phology and size, these samples were subjected to SEM
analysis. The SEM images of HAp-titania samples are
shown in Figure 2. The hydrothermal reaction of HAp sol
onto titanium substrate, in case of C-HAP sample, leads
to the formation of smooth film consisting of nanoparti-
cle aggregates and does not show any distinct features,
Figure 2(a). Whereas, the introduction of NaOH, in case
of HAP sample, leads to the formation of cauliflower
like structures, Figure 2(b). It appears that under high pH
values (when NaOH used in the case of HAP) the con-
centration of OH-ions increases which may preferentially
adsorb onto the surface of Ca
5
(PO
4
3
OH nuclei and hence
may lead to the anisotropic growth of HAp nanosheets,
which subsequently aggregates to form cauliflower like
structures. Further, the addition of oxidizing agent H
2
O
2
(HAP–H
2
O
2
led to the assembly of nanofibres into urchin
like structures, Figure 2(c). Upon the addition of H
2
O
2
,
the enhanced oxidation of titanium substrate (in the case
of sample HAP–H
2
O
2
is probably leading to the densifi-
cation of HAp nanosheets and possibly these nanosheets
are subsequently undergoing self-assembly to form urchin
kind of morphology via rolling behaviour under high tem-
perature conditions.
22–25
Further the elemental analysis was performed to exam-
ine whether the HAp films grown onto titania are solely
composed of HAp or not. EDS revealed the presence of
Ca, P and O elements in the case of all samples (inset of
Fig. 2). Additional peaks corresponding to the presence of
Ti were also observed in case of HAp–titania scaffold sam-
ples. Whereas the Cu signal in all EDS is due to Cu-grid
used for TEM analysis. These results further suggest the
formation of HAp.
25 30
C-HAP
HAP
HAP-H
2
O
2
35 40 45
2θ (degree)
50 55 60 65
(102)
(112)
(301)
(212)
(310)
(312)
(004)
(502)
Intensity (a.u.)
Fig. 1. The XRD pattern of as-synthesized HAp-titania scaffold
samples.
500 nm
(a)
Energy (eV)
Counts (a.u.)
0 200 400 600 800 1000
CKa
OKa
NaKa
CuKb
CuKa
TiKb
TiKa
CaKa
PKa
TiLa
Energy (eV)
Counts (a.u.)
0 200 400 600 800 1000
OKa
CuKb
CuKa
CaKa
TiLa
PKa
NaKa
CKa
TiKa
TiKa
(b)
(c)
500 nm
500 nm
Fig. 2. SEM images of as-synthesized (a) C-HAP, (b) HAP, (c) HAP–
H
2
O
2
and inset shows their EDS pattern.
To explore the corrosion behaviour of HAp-titania sam-
ples, the potentiodynamic polarization tests were under-
taken in Ringer’s simulated body fluid solution. It can be
seen in Figure 3 that at all potential values, the anodic
current densities for all HAp-titania samples are lower
than that of observed for bare Ti foil, suggesting bet-
ter corrosion resistance for HAp-titania samples than that
of bare Ti-foil. According to Galvele,
24
corrosion current
is the intersection between the anodic and cathodic lin-
ear extrapolations at E
corr
. The value is directly related
to electrode potential, and it can provide more realistic
information related to electrochemical behaviour of the
materials. The parameters such as open circuit potential,
corrosion potential (E
corr
and corrosion current density
(I
corr
values determined from these curves are summa-
rized in Table-I. The OCP value moves towards noble side
for HAp-titania scaffold samples than that of observed
for Ti-foil suggesting their higher thermodynamic stabil-
ity. The more negative E
corr
values observed for Ti-foil
J. Bionanosci 4, 1–5, 2011 3
RESEARCH ARTICLE
An Investigation on the Effect of Morphologies on Corrosion Behaviour of Nanostructured HAp-Titania Scaffolds Nayak et al.
10n 100n
1μ 10μ 100μ
1m 10m 100m 1
4
2
0
2
4
6
C-HAP
HA-H
2
O
2
HAP
Ti foil
Log current density (A cm
–2
)
Potential (V vs SCE)
Fig. 3. The potentiodynamic potential versus log current density curves
for HAp-titania scaffold and bare Ti-foil samples.
than that of HAp-titania samples suggest the improved cor-
rosion resistance than that of Ti-foil. It appears that the
presence of HAp layer acts as a barrier to electrons and
ions transport between the substrate (Ti) and the SBF elec-
trolyte, thus reducing electrochemical reaction rate. More-
over, the improved corrosion resistance of HAp-titania
scaffold samples by many folds than that of bare Ti foil
is due to the combined effect of the semi–insulating HAp
coating and TiO
2
interlayer which act as barrier for corro-
sion reaction. Among the three HAp-titania scaffold sam-
ples, C-HAP shows the higher corrosion resistance than
that of HAP sample. The relative lower corrosion resis-
tance of HAP–H
2
O
2
samples appears due to ease of the
penetration of simulated body fluid in to the urchin like
structures (as shown by SEM analysis) which may subse-
quently lead to the electrochemical reaction at the HAp-
titania interface. The difference in the relative corrosion
resistance of samples HAP and HAP–H
2
O
2
can be under-
stood by the difference in the degree of porosity and
thus the access of simulated body fluid to the titania-
hydroxyapatite interface. The representative SEM images
of HAp-titania scaffold samples after the corrosion exper-
iments shown in Figures 4(a) and (b) suggest a slight
densification of the HAp coating. However, the overall
morphology of HAp-titain samples is retained even after
the corrosion experiments. To evaluate the hydroxyapatite
Table I. The OCP, E
corr
and I
corr
values of bare Ti foil and HAp-titania
scaffold samples.
Sample OCP (mV) E
corr
(mV) I
corr
(Acm
2
)
Bare-Ti 28662 25700 3466
HAP 14500 15330 280
HAP–H
2
O
2
12300 25200 1390
C-HAP 10400 4763 000829
(a)
(b)
500 nm
500 nm
Fig. 4. Representative SEM image of HAp-titania scaffolds after corro-
sion experiments (a) HAP, (b) HAP–H
2
O
2
.
phase stability after corrosion experiments, the XRD pat-
tern were recorded for HAp-titania scaffold samples after
corrosion experiments. The representative XRD pattern of
hydroxyapatite-titania samples shown in Figure 5 exhibit
the appearance of some additional peaks, which predom-
inantly correspond to hydroxyapatite phase. These results
suggest the stability of hydroxyapatite phase even after
corrosion experiments. The possible chemical reactions
involved in the electrochemical corrosion process seem to
undergo through two steps:
26
First, hydrogen ions (H
+
are produced at the interface:
Ti + 2H
2
O TiO
2
+ 4H
+
+ 4e
(1)
25 30 35 40 45 50 55 60 65
(002)
(102)
(112)
(301)
(212)
(310)
(203)
(216)
(321)
HAP
HAP-H
2
O
2
Intensity (a.u.)
2θ (degree)
Fig. 5. The XRD pattern of representative HAp-titania scaffold samples
after corrosion experiments.
4 J. Bionanosci. 4, 1–5, 2011
RESEARCH ARTICLE
Nayak et al. An Investigation on the Effect of Morphologies on Corrosion Behaviour of Nanostructured HAp-Titania Scaffolds
Then followed by the localized dissolution of HAp in high
H
+
concentration area:
Ca
10
PO
4
6
OH
2
+ 2H
+
10Ca
2+
+ 6PO
3
4
+ 2H
2
O(2)
In brief, the difference in the degree of corrosion resis-
tance of different HAp-titania scaffold samples is because
of the different degree of porosity/ease of access to the
HAp-titania interface. Such corrosion behaviour observed
and the SEM analysis results indicate the growth of HAp
onto titania substrates, in case of HAP–H
2
O
2
, is taking
place by rolling growth mechanism, which possess differ-
ent degrees of porosity, is consistent with this corrosion
mechanism.
The adhesiveness of these HAp-titania scaffolds coat-
ings onto Ti was studied by scotch-tape method before and
after corrosion experiments. Interestingly, these HAp scaf-
fold coating were found strongly adhered with the Ti sub-
strate as did not peel off in case of all samples HAp-titania
samples even after the corrosion experiments performed
onto these samples.
4. CONCLUSION
The simple hydrothermal approach allowed the control
over the growth of HAp with different morphology by
varying the reaction conditions. The growth of HAp onto
titania substrate-titania scaffold samples act as a barrier
and diminishes the corrosion by many folds in simulated
body fluid. Though the variation of the extent of corrosion
resistant is dependent on the morphology/microstructure of
the HAp scaffolds grown onto titania and infact limited by
the access of simulated body fluid to the HAp-titania inter-
face. Although the maximum corrosion resistance exhib-
ited by C-HAP, the HAp-titania scaffold with the urchin
kind of morphology meets the properties such as enhanced
mechanical strength, porosity (to allow the growth of
bone within and around the scaffold) and improved cor-
rosion resistance as required for a viable HAp scaffolds
application.
Acknowledgments: The authors are grateful to CSIR,
New Delhi for financial assistance, Dr. B. Satpati, IMMT
Bhubaneswar for EDS analysis and Professor B. K.
Mishra, Director, IMMT Bhubaneswar for providing sup-
port to undertake this work.
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J. Bionanosci 4, 1–5, 2011 5
  • [Show abstract] [Hide abstract] ABSTRACT: NiTi alloy is used as biomaterial due to its unique properties, but the high content of Ni (about 50 at.%) in biomedical NiTi is of concern. Hydroxyapatite/titania composite coating was directly electrodeposited on the surface of NiTi alloy. The coated samples were characterized using X-ray diffraction, scanning electron microscopy, infrared spectroscopy, bonding strength test, polarization and electrochemical impedance spectroscopy (EIS). Results showed that addition of TiO2 to the electrolyte changed the morphology of hydroxyapatite from thin flake-flower-like crystals to needle-flower-like crystals, and the coating was much denser. Besides, hydroxyapatite crystal grains in the coating were preferentially arranged in the [001] direction, which was perpendicular to the surface of NiTi alloy. The addition of TiO2 improved the bonding strength between the coating and the substrate. Corrosion resistance of NiTi in the simulated body fluid at 37 °C was significantly improved by more than 50 times by electrodeposition of the hydroxyapatite/titania composite coating.Research Highlights► HAP/TiO2 coating was directly fabricated on NiTi by electrodeposition. ► Corrosion behavior of NiTi coated with HAP/TiO2 coating was studied. ► HAP/TiO2 coating can protect the NiTi from corrosion availably in the SBF.
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