USE OF PROTON INDUCED X-RAY EMISSION TO EVALUATE
THE INCORPORATION OF BIOACTIVE ELEMENTS BY A PVDCOATED BIOMEDICAL STAINLESS STEEL IN HANKS’
SOLUTION
Renato Altobelli Antunes1, Isolda Costa2
1
Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas, Universidade Federal do ABC, Santo
André (SP), Brasil
2
Centro de Ciência e Tecnologia de Materiais, Instituto de Pesquisas Energéticas e Nucleares, São Paulo
(SP), Brasil
E-mail: renato.antunes@ufabc.edu.br
Abstract. The potential use of proton induced x-ray emission (PIXE) to identify the presence of bioactive
elements such as calcium and phosphorus in the surface of bare or coated biomedical metallic alloys is
hardly investigated in the literature. The aim of this work was to assess the sensitivity of PIXE to detect
calcium and phosphorus on TiN, TiCN and DLC films deposited by physical vapor deposition (PVD)
methods on the surface of the surgical stainless steel ASTM F-138. The results evidenced the sensitivity of
PIXE to detect potentially bioactive elements in the surface of the PVD-coated alloy. The incorporation
depends on the morphology of the film.
Keywords: PIXE, PVD films, Bioactive elements, DLC, TiN
1.
INTRODUCTION
Proton induced x-ray emission (PIXE) is a non-destructive and high sensitivity
physical method of multi-elemental quantitative analysis. The method consists of
irradiating the specimen to be analyzed with an ion beam (H+, He+), aiming the
emission of characteristic x-ray spectrum whose energy is detected by a Si(Li) detector.
The method is limited by the absorption in the detector window, identifying elements
with atomic number Z>10, being more sensitive in the ranges 20<Z<40 and Z>70. The
method allows a chemical analysis with sensitivity of several ppm [Jallot et al., 2010].
Owing to this powerful detection capability the technique is often employed to identify
trace elements that contaminate tissues surrounding metallic prostheses [Guibert et al.,
2006; Barbotteau et al., 2004]. Other application in the biomedical field is the study of
biomaterial/biological fluids interfaces, following the formation of calcium phosphate
compounds on the surface of implanted biomaterials [Ektessabi and Hamdi, 2002]. This
approach is particularly frequent for the study of bioactivity in bioactive glasses [Lao et
al., 2008; Oudadesse et al., 2002].
Physical vapor deposition (PVD) technology has emerged as an important
method of producing thin films to protect metallic implants against wear, corrosion and
fatigue during their service life [Antunes and De Oliveira, 2009]. Different ceramic hard
coatings are employed with this purpose such as diamond like carbon (DLC), titanium
nitride (TiN) and titanium carbonitride (TiCN) [Chu, 2006; Harman et al., 1997; Yang et
al., 2007]. The overall performance of the films strongly depends on their morphology
which is determined by the deposition method and parameters [Panjan et al., 2010]. The
intrinsic biocompatibility of these films is well-established in the literature [Hasebe et
al., 2007; Serro et al., 2009]. Conversely, few studies are devoted to the investigation of
the bioactivity of PVD-coated metallic biomaterials. Piscanec et al. (2004) assessed the
bioactivity of a TiN-coated titanium alloy. According to the results, the TiN layer was
oxidized forming a TiOxNy phase. Ca2+ ions were, then, spontaneously adsorbed on the
surface of this oxinitride, giving rise to the nucleation of a calcium phosphate
compound. This study motivates similar investigations for other intrinsic biocompatible
PVD layers such as TiCN and DLC. In this regard, the aim of this work was to use
PIXE to evaluate the incorporation of bioactive elements, namely Ca and P by TiN,
TiCN and DLC films deposited on the ASTM F-138 biomedical stainless steel after
immersion in Hanks’ solution.
2.
MATERIALS AND METHODS
2.1 Material
The chemical composition of the ASTM F-138 stainless steel used in this work
is shown in Table 1.
Table 1. Chemical composition of the ASTM F-138 alloy used in this work.
C
Mass
Si
S
P
Cr
Mn
Cu
Ni
Mo
N
Fe
0,007 0,037 0,002 0,007 17,40 1,780 0,030 13,50 2,120 0,070 Bal.
(%)
2.2 Deposition process
TiN and TiCN coatings were deposited using a HTC 1200 PVD unit
manufactured by Hauzer Techno Coating Europe BV Venlo, The Netherlands. This
machine uses four orthogonally mounted cathodes (1000×170×14 mm), which surround
a three fold rotation substrate holder turntable. The cathodes are equipped with a
cathodic arc deposition technique. Before deposition, specimens were cleaned in
phosphoric acid, alkaline and detergent solution and deionized water in an ultrasonic
cleaner system. Metallic ion etching was also performed to improve adhesion. Ionized
metallic ions were accelerated from cathode to tolls biased in 1200 V. The deposition
occurs in four steps: pump down and heating, metal ion etching, reactive deposition and
cool down. The detailed process parameters are show in Table 2. The deposition rate
was 0.8–1.0 μm/h. The resulting film thickness was approximately 2 μm. DLC coating
was deposited through a magnetron sputtering method. This film consists of a tungsten
carbide containing DLC (W-DLC) with a thickness of approximately 2 m. The
deposition system comprises a sputtering device with a tungsten carbide target (99.99%
purity). The pressure of the process was kept constant at 0.8 Pa. The temperature of the
substrate was held at 180 ºC during deposition. The concentration of acetylene during in
the deposition chamber was 70 sccm and that of argon was 200 sccm.
Table 2. Deposition parameters for the TiN and TiCN films.
Parameter
Pressure
Substrate bias
Arc current
Substrate temperature
Deposition time
Cooling time
Value
1 Pa
200 V
50 A
450 – 500 ºC
5400 s
7200 s
2.3 PIXE
A detailed description of the PIXE measurement method is given elsewhere
[Aburaya et al., 2006]. The analyses were conducted with the ASTM F-138 stainless
steel coated by the TiN, TiCN and DLC layers deposited through the conditions
described in the previous sub-section. The bare substrate was also tested for comparison
purposes. The measurements were performed with specimens that were not immersed in
physiological solution and with specimens immersed in Hanks’ solution at 37 ºC for 28
days.
2.4 Film morphology
The morphology of the TiN, TiCN and DLC films was observed through
scanning electron microscopy (microscope Philips XL30 SEM).
3. RESULTS AND DISCUSSION
3.1 SEM micrographs
SEM micrographs of the PVD layers are shown in Fig. 1. As expected, the
presence of microdefects was observed in the films. Pinholes and macroparticles are
intrinsically produced during the PVD processes. Macroparticles are originated from
molten particles generated during the evaporation of the metallic target during the PVD
process which can solidify on the surface of the substrate. They can give rise to
porosity, forming new pathways to the penetration of electrolyte, thus harming the
corrosion resistance of the coated substrate. Chenglong et al. (2005) and Yang et al.
(2005) have found similar characteristics for TiN layers produced by PVD processes.
Liu et al. (1995) reported the formation of pinholes whose diameter reached at to 3 m
in PVD thin films. As seen in Fig. 1, the presence of macroparticles is more accentuated
on the DLC surface than on the TiN and TiCN films. If, in one hand, the intrinsic
defects can be deleterious to the corrosion resistance of the base material, in the other
hand, they can facilitate the incorporation of potentially bioactive elements from the
solution.
a)
Pinholes
Macroparticles
Pinholes
b)
Pinholes
Macroparticles
c)
Figure 1. SEM micrographs of the PVD films: a) TiN; b) TiCN; c) DLC.
3.2 PIXE measurements
Table 3 shows PIXE results for the concentration of elements on the surface of
the ASTM F-138 substrate non-immersed and immersed in Hanks’ solution at 37 ºC for
28 days. Only the main alloying elements of the steel (Fe, Cr, Ni and Mo) and the
potentially bioactive elements (Ca and P) are displayed in the table.
Table 3. PIXE results for the concentration of elements on the surface of the ASTM
F-138 substrate non-immersed and immersed in Hanks’ solution at 37 ºC for 28 days.
Concentration (mass %)
Element
Non-immersed
Immersed
P
-----
0.09
Ca
0.02
0.04
Fe
60.7
60.5
Cr
21.0
20.9
Ni
13.2
13.2
Mo
1.86
1.95
As shown in Table 3 phosphorus was identified only after immersion in Hanks’
solution for the bare ASTM F-138 specimen where calcium appeared as an impurity
even in the non-immersed condition. The high sensitivity of the PIXE analysis is
evident, as very few amounts of the potentially bioactive elements could be
unequivocally identified. Bioactivity of stainless steel implants has never been reported.
Indeed, this result should not be envisaged as an indication of bioactivity for the bare
ASTM F-138 but rather as an evidence of the suitability of the PIXE method to analyze
trace elements on the surface of biomaterials. It is interesting to note that the
concentration of the most abundant elements in the alloy remained relatively unchanged
after immersion, suggesting that the material was stable in the testing conditions.
Tables 4 to 6 show PIXE results for the concentration of elements on the surface
of the TiN-coated, TiCN-coated and DLC-coated ASTM F-138 stainless steel nonimmersed and immersed in Hanks’ solution at 37 ºC for 28 days. Only the main alloying
elements of the steel (Fe, Cr, Ni and Mo), the potentially bioactive elements (Ca and P)
and the main constituent of each film (Ti for the TiN and TiCN films and W for the
DLC film) are displayed in the table. Nitrogen and carbon are not displayed due to the
limitation of PIXE to detect elements with Z<15.
The TiN-coated and TiCN-coated specimens did not present evidence of
incorporation of the bioactive elements P and Ca. These elements have been found in
both PVD layers even before immersion in Hanks’ solution. After immersion, the
relative mass of both Ca and P was not significantly altered and any variation can be
ascribed to inherent measurement errors associated with the PIXE technique.
For the DLC-coated steel the amount of Ca and P increased more significantly
after immersion in Hanks’ solution. This result does not confirm the formation of
bioactive compounds, as the PIXE method as only used to identify each element
separately. However, it is an indication that the DLC film is apparently more prone to
incorporate the bioactive elements than the TiN and TiCN layers.
Table 4. PIXE results for the concentration of elements on the surface of the TiN film
non-immersed and immersed in Hanks’ solution at 37 ºC for 28 days.
Concentration (mass %)
Element
Non-immersed
Immersed
P
0.03
0.05
Ca
0.02
0.01
Fe
55.8
55.0
Cr
18.0
17.6
Ni
12.8
12.5
Mo
1.68
1.66
Ti
8.89
10.3
Table 5. PIXE results for the concentration of elements on the surface of the TiCN film
non-immersed and immersed in Hanks’ solution at 37 ºC for 28 days.
Concentration (mass %)
Element
Non-immersed
Immersed
P
0.04
0.04
Ca
0.01
0.01
Fe
58.1
58.3
Cr
19.4
19.5
Ni
13.0
12.9
Mo
1.77
1.76
Ti
4.54
4.35
Table 6. PIXE results for the concentration of elements on the surface of the DLC film
non-immersed and immersed in Hanks’ solution at 37 ºC for 28 days.
Concentration (mass %)
Element
Non-immersed
Immersed
P
0.03
0.07
Ca
0.01
0.08
Fe
54.1
52.7
Cr
18.2
17.8
Ni
10.4
11.5
Mo
1.42
1.39
W
5.59
5.81
Indeed, there are some reports in the literature investigating the osteoinductive
properties of DLC films [Olivares et al., 2007; Cui et al.; 2005). It is generally agreed
that some degree of porosity favors the osseointegration of implants. In this regard,
coated prostheses should encompass this characteristic in order to facilitate the
incorporation of bioactive elements. This seems to be the case of the DLC films
evaluated in this work. As observed in the SEM micrographs of Fig. 1, DLC film was
more defective than the TiN and TiCN layers, favoring the development of porosity
during immersion in Hanks’ solution. PIXE analysis evidenced the incorporation of Ca
and P from the solution in the DLC film, but not in the TiN and TiCN layers.
4. CONCLUSIONS
SEM micrographs revealed the presence of pinholes and macroparticles on the
surface of the TiN, TiCN and DLC films. The DLC layer was more defective than the
TiN and TiCN films. PIXE analysis proved to be very sensitive, successfully identifying
the bioactive trace elements Ca and P. Moreover, the technique evidenced that the DLC
film incorporated Ca and P from the Hanks’ solution. Conversely, this incorporation was
not confirmed for the TiN and TiCN layers. The more defective nature of the DLC film
seems to be important to favor the incorporation of the bioactive elements. PIXE is a
suitable method to detect this phenomenon in biomedical metallic alloys.
ACKNOWLEDGEMENTS
The authors are grateful to CNPq for the financial support to this work.
Bodycote Brasimet, especially Dr. Ronaldo Ruas, is acknowledged for kindly
performing the deposition of the PVD layers employed in this work. Especial thanks are
given to Dr. Márcia de Almeida Rizzutto from the Institute of Physics (University of
São Paulo) for her invaluable help with the PIXE measurements and discussion of the
corresponding results.
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