Induction Plasma Sprayed Biological-like Apatite Coatings for Biomedical Applications
Max Loszach, François Gitzhofer
CREPE, Department of Chemical and Biotechnological Engineering,
Université de Sherbrooke
Sherbrooke, Québec, CANADA
Induction plasma sprayed coatings were developed based on a sol-gel suspension, including
different ionic substitutions such as K, Mg, F, Cl and Na in order to enhance the biological
compatibility. Calcium phosphates coatings are well-known for their use as bones substitutes
considering their excellent biocompatibility and bioactivity [1-2]. Although the crystallographic
structure of the synthetic hydroxyapatite (HAP) is similar to natural bone, biological apatites are
always non-stoechiometric and presents some incorporated elements on trace level in its lattice
[3]. Several studies [4-7] have shown that these elements, even in trace levels, play an important
role on the biological process as well during implantation as during the life-time of the bone.
A suspension based on nitrates and ammonium [8] was prepared by a sol-gel way including the
different ionic substitutions at different concentration rate. The coating was then realized using
induction plasma spray under in Ar/O2 gas mixture. The deposited coatings were characterized
using X-ray diffraction (XRD) and scanning electron microscope (SEM). The concentration of
the different substitution as well as the Ca/P ratio was analysed using neutron activation.
The reactivity of the coatings was also studied by an immersion for different time (1, 3, 7, 14 and
28 days) in a simulated body fluid (SBF) based on Kokubo method [9]. Samples were
characterized after immersion using SEM and XRD and Ion Chromatography/Mass Spectrometry
(IC/MS). Analysis were performed on the SBF in order to see the dissolution rate as a function of
the different ions added to the HA suspension.
Key words: Inductive plasma, sol-gel suspension, biomaterials, biological-like apatite
2 Experimental procedures
1 Introduction
Hydroxyapatite [HA: Ca10 (PO4)6(OH)2] [10]
is a bio-ceramic material which belongs to
the calcium phosphate family which is wellknown for its use in biological applications.
Having similarity with the crystallographic
structure of natural bone, it is often applied
clinically as a coating on an inert metallic
implant such as Ti-6Al-4V. Despite of the
structural and crystallographic similarities
with synthetic Hydroxyapatite (HAP),
natural apatites always presents a
nonstoichiometric nature incorporating
several elements on trace level in its lattice
[11, 12]. It is generally accepted that HAP
crystallizes in the hexagonal space group
P63/m with 2 formula units Ca5(PO4)3(OH)
per unit cell [13, 14]. Geometrically, it is an
hexagonal stack of isolated PO43tetrahedrous creating two kinds of tunnels
parallel to the c-axis. The first kind of tunnel
is filled with Ca+ ions which forms CaO9polyhedral while the second is lined by
oxygen and other Ca ions is occupied by
OH- anions [10, 13]. The presence of such
tunnels gives appatites structure ion
exchange properties. Although these
elements are at trace level, several studies
have shown their importance on the different
level of biological process upon
implantation [15, 16]. Numerous studies
have been trying to incorporate elements
such as Mg, K, Na, Cl, F… in order to
obtain a mono or pluri-substitued HAP using
conventional sintering approach. The aim of
the present work is to produce an HAP
coating with combined substituted element
present in biological apatite on a titaniumbased substrate using induction plasma
spraying of a suspension and to characterize
it.
2-1 Preparation
The suspension injected in the induction
plasma system was prepared using the
Tanahashi’s method [17] and a protocol
developed by Kannan et al. [18]. Calcium
nitrate tetrahydrate [Ca(NO3)2.4H2O, Sigma
Aldrich], diammonium hydrogen phosphate
[(NH4)2HPO4, Fisher], sodium nitrate
[NaNO3, Sigma Aldrich], magnesium nitrate
hexahydrate [Mg(NO3)2.6H2O, Sigma
Aldrich], potassium nitrate [KNO3, Sigma
Aldrich], ammonium chloride [NH4Cl,
Sigma Aldrich], and ammonium fluoride
[NH4F, Sigma Aldrich] were used as
precursors for the synthesis. A mixture
containing nitrates of Ca, Na, K and Mg was
stirred at a rate of 2000 rpm, while a mixture
of (NH4)2HPO4, NH4F and NH4Cl was
slowly added at a rate of 40 mL/min. The
concentration of the different precursors can
be found on table 1. After this step, the pH
value was around 4. It was then increased to
a value of 10 by adding an ammonium
hydroxide (NH4OH) solution. After the
equilibration of the pH value, the mixture
was heated at 90°C for 2h under a constant
stirring of 2000 rpm. The precipitated
suspension was then settled down for 24h in
order to maturate the precipitate. After 24h,
the suspension was centrifuged for 10 min at
2000 rpm in order to eliminate the last trace
of liquid and obtain a concentrate paste. It
was then mixed with deionised water in
order to obtain the suspension.
Table 1.Concentrations of the different precursors
Precursor
Ca(NO3)2.4H2O
(NH4)2HPO4
KNO3
NaNO3
Mg(NO3)2.6H2O
NH4Cl
NH4F
Ca/P ratio
Molarity
Weight(g) in
1000 mL
472,2
158,5
0,35
9,64
13,03
0,55
0,1
1,0
0,6
0,0017
0,057
0,025
0,005
0,001
1,67
Table 2 : process parameters during coating depositions
2-2 Plasma process
The system used to generate the plasma is
the PL-50 supplied by Tekna Plasma
Systems Inc (sherbrooke, Quebec, Canada)
connected to a 3 MHz LEPEL HF power
generator. The system including the
deposition chamber is described in figure 1.
The sample holder was passing under the
plasma with a speed of 50 cm/s and a
spraying distance of 16 cm. In the meantime,
a suspension was axially injected by a
peristaltic pump with a feed rate of 10
mL/min directly into the plasma chamber
through the probe. The process parameters
are summarized in table 2.
Figure 1 : RF plasma torch PL-50 by Tekna Plasma
Systems Inc [Boulos M.I, 1992]
Plasma Power
30 kW
Central gas (Ar)
Sheath gas (Oxygen)
23 slpm
63 slpm
Atomizing gas (Argon)
12 slpm
Working pressure
100 mmTorr
2-3 Characterization
Phase compositions were determined using
X-ray diffraction (XRD) on a Philips X’pert
Pro MPD X-ray diffractometer (Eindhoven,
Netherlands). Microstructures of the coating
were observed on a Hitachi VPSEM
S3000N Scanning Electron Microscope
(Tokyo, Japan) by secondary electron
microscopy (SE) and backscattered electron
microscopy (BSE). Elemental analysis for
the presence of all elements were performed
using neutron activation. Rietveld analysis
have been performed using the MAUD
software [19].
3 Results and Discussion
The XRD pattern of the coating is presented
in figure 2. Both hydroxyapatite and β-TCP
phases are present on the pattern. The
concentrations of the different substituted
elements are not significantly high enough
to affect the XRD pattern. It is also
interesting to notice the low intensity of the
CaO peak which corresponds to a low
concentration of this phase which is
normally a problem in HA plasma deposited
coatings since CaO is cytotoxic for the
different bones cells [20]. Quantitative
analysis on the phase concentration using
the Rietveld method shows a ratio close to
80/20 respectively for HA and β-TCP which
at the same time is beneficial for the stability
(HA) and for the resorbability (β-TCP) [21,
22].
important role in dental caries prevention
[30].
Table 3 Neutron activation results for the s-HAP coating
Sample 1
Sample 2
Sample 3
Ca (%wt)
36,8
39,6
39,9
K (ppm)
264
418
391
Mg (ppm)
5192
6122
5350
Na (ppm)
6272
8606
8696
F (ppm)
173
237
97
Figure 2 XRD pattern for the s-HAP Coating
Cl (ppm)
793
1864
1165
The elemental analyses of the s-HAP
coating are presented in table 3. The
characterization has been performed on three
different series of coatings, in order to
confirm the reproducibility of both of the
suspension and deposition protocols. The
results confirm the presence of all different
substituted elements in the coating structure.
The concentration of each element is very
close to the biological apatite found in bones
structure [2]. Although the existing scientific
reports [24, 25] have well studied the
distribution of the single or dual elements
substitution in the apatite structure, there are
no studies concerning multi-substitutions in
the synthetic apatite. However, several
studies have documented the importance of
the influence of each of these elements,
although they are only existing in trace
levels in biological apatite. Magnesium,
already well-known in substituted synthetic
apatite, plays an important role in the
calcification process, as well on the mineral
metabolism [26]; sodium intercedes on bone
metabolism and osteoporosis [27]; chlorine
plays a significant role in the development
of an acidic environment on the surface of
the bone which activates osteoclastes in
bone resorption process [28]; potassium has
an active part in mineralization and
biochemical processes [29]; fluorine is
already well-known for its ability to stabilize
the apatite structure which plays an
P (%)
18,3
19,1
19,5
Bone
sHAP
Ca
24,5
38,8
Element concentration (%wt)
P
Na
Mg
K
Cl
11,5
0,7
0,55 0,03
0,1
19
0,86 0,55 0,04 0,15
F
0,02
0,015
The SEM micrographs of the microstructure
are presented in figure 3 and 4. The coating
presents a high open microporosity around
10 µm, which results from the long
deposition distance as well as the from high
concentration of the suspension in
comparison with the low power used. It
presents a cauliflower microstructure with
the presence of a nano-structured
organisation showing an important surface
rugosity. Although we have been able to
reproduce the bone composition using the
SPS technology, there is still some
engineering to be done on the coating
microstructure as there should be the
presence of micropores in the range of 10
µm to allow for biological fluids circulation
for bone cells in-growth and larger pores in
the range of 100 -500 µm to allow for cell
development. [31, 32]
4 Conclusions
To conclude, a bi-phasic mixture of βTCP/HA, enable to enhance the resorbability
of the material in order to favour the bones
remodelling process has been developed by
inductive plasma using a SPS method. The
coatings possess a microstructure enables to
enhance the fluid circulation trough the
material, which present an important open
microporosity around 10 µm. The elemental
analysis performed by neutron activation
shows that the elements substituted inside
the s-HAP structure are at concentrations
really close from natural bone.
5 Reference
Figure 3 SEM micrographs of the microstructure of the
coating (x250 and x5,0k)
Figure 4 SEM micrograph of the cross section of the
coating (x700 and x1,0k)
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