Coatings of bioactive glasses and hydroxyapatite and their properties
Andrea Cattini
Coatings of bioactive glasses and hydroxyapatite and their properties
Introduction
In last decades, biomedical research has acquired a growing importance. Moreover, the progressive
aging of the population is driving the development of materials for bone implants field.
Despite the production of prosthetic implants is a practice already mature and well-established,
research is constantly evolving with the aim of increase the compatibility of the prosthesis with host
tissue and extend their life expectancy.
In particular, a problem still affecting the metal prosthesis is the of the formation of a fibrous
capsule at their interface with the host tissues. In some cases the capsule allows micro movements,
which may induces inflammatory reactions and, in turn, detachment and failure of the implant [1].
The problem is due to the almost inert behavior, respect to biological systems, of used materials.
Different solutions to the problem have been proposed such as the modification of the implants’
superficial morphology or the use of special cements [2].
Another applied solution is to cover prosthesis with bioactive materials, ie special materials able to
promote the formation of bone tissues and to bond with them. Most of bioactive materials are
ceramics, then they are inherently brittle; thus their use in load bearing applications is severely
limited [2]. The production of bioactive coatings on metallic components is therefore an optimal
solution since it allows to combine toughness of substrate with bone-binding ability of the coating
[2]. In particular, this study is focused on two types of bioactive materials: hydroxyapatite and a
bioactive glass.
Among bioactive materials hydroxyapatite is the most widely used since its composition and its
structure are very similar to those of the mineral component of bones. Moreover hydroxyapatite is
highly stable in biological environment. Bioactive glasses are special glasses which have a high
bone-bonding ability; some of them is also able to bond to soft tissue. Among bioactive materials
bioactive glasses heve the highest index of bioactivity [1].
Plasma spray is the most used technique for the production of bioactive coatings. In fact most of
commercial bioactive coatings are produced in hydroxyapatite via plasma spray [2]. This technique
is based on the production of a thermal plasma from a mixture of gases by means of an electric arc.
The plasma jet can reach temperatures higher than 15000 °C and speed greater than 1000 m/s. The
feedstock materials are injected into the plasma, which melts and drags the particles until the
substrate [3]. When the molten or semi-molten particles impinge the substrate, they flatten and
solidify. Then, layer by layer coating forms. In traditiona plasma spray feedstock materials are
coarse powder. Suspension Plasma Spray (SPS) is an evolution of plasma spray, in which the
feedstock materials are suspensions [4].The use of a liquid carrier allows spraying sub-micrometric
particles, then coatings with a really fine structure can be produced via SPS.
The aim of this study is to produce composite coatings of hydroxyapatite and bioactive glass via
suspension plasma spray. The purpose of composite structures is to combine the high bioactivity of
bioactive glasses with the superior in vivo stability of hydroxyapatite.
Different composite microstructures were produced: mixture (Composite), double layered (Duplex)
and with graded composition (Graded). Since bioactive glass has the higher bioattivity and
hydroxyapatite is more stable, the Duplex and Graded coatings were produced with surfaces of
bioactive glass and deepest layer of hydroxyapatite. In this way the higly bioactive glass interfaces
with the biological environment while the stable hydroxyapatite protects the substrate and assures a
good adhesion to coatings. The coatings were in vitro testes and characterized.
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Coatings of bioactive glasses and hydroxyapatite and their properties
Andrea Cattini
Methods
Feedstock suspensions:
Bioactive glass: BG_Ca glass (wt. %: 46.9 SiO2, 42.3 CaO, 4.7 Na2O, 6.1 P2O5) was used in this
study. The glasses was produced via conventional melt-quenching method. The raw materials were
previously mixed, and then they were melted in platinum crucibles at 1450° C. The melt was
splashed in room-temperature water. The obtained frit was dried at 110°C for 12 hours and then
dry grounded in agate mortar. Powder was sieved at 63 µm. Then it was attrition milled in ethanol
with using 0.8 mm zirconia balls. Beycostat C213 was added as a dispersant (2 wt. % of dry
powder). Final powder had a mean diameter of 4.7 μm. The suspension was obtained mixing 20
wt.% of solid in 80 wt.% of ethanol.
Hydroxyapatite: hydroxyapatite was in form of spray dried powder commercializad by Tomita. The
powder had a mean diameter of 120 μm, then it was attrition milled using 0.8 mm zirconia balls.
Ethanol was the milling fluid and Beycostat C213 was the dispersant (2 wt. % of dry powder). The
mean diameter of final powder was of 4.6 μm. The suspension was obtained mixing 20 wt.% of
solid in 40 wt.% of water and 40 wt.% of ethanol. The water was added since it may reduce the
decomposition of hydroxyapatite during plasma spray.
Coatings production:
Praxair SG-100 configured in subsonic mode was
34
the used torch. The torch had a radial internal Power [kW]:
injection. In order to obtain the different
45
Ar [slpm]:
microstructures, a feeding system composed by Gases
7.5
H2 [slpm]:
two independent peristaltic pumps was used.
55
Regulating the flow rates of the suspensions the Distance [mm]:
750
composite microstructures were built up (Fig. 1). Torch speed [mm/s]:
Sand-blasted discs of 316 l stainless steel were Scan step [mm]:
10
used as substrates. Their roughness was Ra=3.5 Suspensions flowrate [g/min]: 30
µm. After an initial screening, a sets of parameters was chosen to produce all the coatings. The used
4 xbioactive
8 sessions glass
passages [n°]
parameters are listed in the Tables 1. In addition to Torch
the composite
coatings, pure
Table 1: Spray
(BGC) and pure hydroxyapatite (HA) coatings were produced
withparameters
the same parameters and used as
reference.
Composite
Duplex
Graded
Figure 1: Flow rates of suspensions used during coatings deposition [g/min]
Coatings characterization:
In vitro tests: in vitro tests were performed by soaking samples in acellular simulated body fluid
(SBF) and mantainig the system at a controlled temperature of 37°C. Before in vitro test, the
samples were cut to obtain 1 cm2 area specimens. They were soaked in 20 ml of SBF in a sealed
container. Samples were extracted from SBF after periods of 1, 7, and 14 days.
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Coatings of bioactive glasses and hydroxyapatite and their properties
Andrea Cattini
X ray diffraction: X ray Diffraction (XRD) was performed on coatings before and after in vitro
tests. The used diffractometer was a PANalytical X’Pert Pro (Cu Kα radiation λ=1.54060 Å, Nifilter) with “Mirror” incident slit. XRD spectra were collected with a step size of 0.017° and a time
step of 91.17s (X’Celerator detector), in the 2θ range 10-70°.
Scanning Electron Microscopy: coatings were observed by SEM before and after in vitro tests, both
on surfaces and cross-sections. The SEM used in this study was an environmental-SEM (ESEM
Quanta-200, Fei) and during the surface observation it was operated in low vacuum mode with a
water pressure of 0.53 Torr. The ESEM was equipped with X-EDS microanalysis (Oxford INCA350).
Scratch test: scratch critical loads of coatings were measured by means of Open platform (CSM
Instruments) equipped with 100 µm Rockwell diamond tip (linear load from 20 mN to 30 N, scratch
length 3mm, load rate 10 N/min). Critical loads were determined by acoustic emission and optical
analysis. Three scratch were performed on each sample.
Results and Discussion
Composite
Duplex
Graded
Figure 2: EDS compositional maps of coatings cross section
Regardless of the composition, the composite coatings a)
appeared continuous and homogeneous. To verify if the
obtained microstructures corresponded to the expected
ones, the polished cross-sections were analyzed through
EDS analysis. Compositional maps were acquired (Fig.
2). The silicon was selected as the marker of BG_Ca,
while the phosphorous was chosen to indicate the
hydroxyapatite. The microstructures, as is shown in
Figure 2, were similar to those expected (Fig. 1). The b)
microstructure of the Composite sample appears as a glass
matrix within which zones of HA are dispersed. The
cross-section of Duplex sample showed the two layers
division and, in the Graded sample, the composition
varies along the thickness of the coating. The main
difference between expected microstructures and obtained
ones is the greater presence of BG_Ca in the latters. The
higher deposition efficiency of the bioactive glass respect Figure 3: ESEM images of coatings surface:
a) Composite, b) Duplex
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Coatings of bioactive glasses and hydroxyapatite and their properties
Andrea Cattini
to hydroxyapatite is the reason of the
difference. In fact, the BGC coating was thick
approximately the double of HA, although
both were produced using the same
parameters of composite coatings. The values
of the critical load measured by scratch test
were: 21.2 N for the Duplex sample, and 27.4
N for the Composite sample. During the
scratch test of the Graded sample instead, the
maximum load (30 N) was reached without
the the detachment of the coating. Then, the
graded microstructure provided the greater
adhesion between coating and substrate.
a)
b)
The surfaces of the samples showed a high
roughness mainly due to semi-spherical
strucutres (Fig. 3a). The structures were
probably due to sintering and swelling
phenomena that occur at the BGC on the
Figure
4:
X
ray
diffraction
pattern:
a) As produced coatings,
b) after 14 days in SBF surfaces [5]. In fact, the phenomenon was
X: TTCP, O: TCP, C: CaO
more prominent in BGC, Duplex and Graded
samples. The HA coatings didn't show the
semi-spherical strucutres. At higher magnification, crystallized zones were observed (Fig. 3b). .
The crystals were very thin with aciculat morphology.
Comparing the X-ray diffraction spectra (Fig. 4a), a similarity between Duplex, Graded and BGC
ones was observed. In the spectra, the broad band between 25 and 35 2θ, due to the glassy phase,
and the peaks of the pseudowollastonite (CaSiO3) were present. Instead, the pattern of the
Composite sample was more similar to that of the HA coating. This is not surprising since, on
Composite sample, the hydroxyapatite was not only in the deepest layers but also on the superficial
ones.
Finally, the HA coating spectrum showed the hydroxyapatite pattern and weak peaks of the
decomposition phases TTCP, TCP and CaO. It is
therefore expected that even the HA composing the
composite coatings suffered low decomposition. After
two weeks of immersion in SBF (Fig. 4b) the spectra
of all the samples were similar: only the peaks of
hydroxyapatite were present. In most of the
samplesthe peaks were broad. The wide shape of the
peaks was caused by the microcrystalline nature of the
precipitated hydroxyapatite.
Observing the surface of samples after in vitro test, a
change in the morphology occurred starting from the Figure 5: ESEM images of coatings surface after
one day in SBF (sample Graded)
first day of immersion. In fact, on all samples except
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Coatings of bioactive glasses and hydroxyapatite and their properties
Andrea Cattini
the HA coating, the classical "cauliflower-like" morphology [6] of in vitro grown hydroxyaptite
formed after one day in SBF (Fig. 5). The development of hydroxyapatite was confirmed also by
EDS microanalysis on cross sections. For example, on the section of the Graded sample (Fig. 6) a
superficial layer rich in calcium and phosphorus formed after one day of immersed in SBF.
Figure 6: EDS analysis of Graded sample after one day soaking in SBF
Future perspectives
In this study, composite coatings on hydroxyapatite and bioglass were evaluated. Regardless of the
microstructure, all coatings showed in vitro bioactivity. The Graded sample was the most promising
since it exhibited the better adhesion to the substrate. Therefore the future developments of the
study should be focused on Graded sample. A continuation of the work may be further analysis
such as in vitro cellular tests, and a detailed mechanical characterization both before and after in
vitro tests. Future developments of this study are the further optimization of the deposition
parameters. In particular, the bioactivity and the mechanical properties may be increased by a finer
microstructure, then the deposition of finer powder can be a way to improve coatings properties.
Finally, the production of more complex coatings, such as three component gradient, may be a
further evolution.
References
[1] L. L. Hench. Bioceramics: from concept to clinic. Journal of the American Ceramic Society. 74 (1991) pp.1487
[2] A. Sola, D. Bellucci, V. Cannillo and A. Cattini. Bioactive glass coatings: a review. Surface Engineering. 27 (2011)
pp.560
[3] L. Pawłowski, The Science and Engineering of Thermal Spray Coatings, (2008) Wiley, Chichester
[4] L. Pawlowski. Suspension and solution thermal spray coatings. Surface and Coatings Technology. 203 (2009)
pp.2807
[5] A. Cattini, L. Łatka, D. Bellucci, G. Bolelli, A. Sola, L. Lusvarghi, L. Pawłowski, and V. Cannillo. Suspension
plasma sprayed bioactive glass coatings: Effects of processing on microstructure, mechanical properties and in-vitro
behaviour. Surface and Coatings Technology. In Press
[6] D.K. Pattanayak. Apatite wollastonite–poly methyl methacrylate bio-composites. Materials Science and
Engineering: C. 29 (2009) pp. 1709
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