Research article
Received: 27 June 2012
Revised: 31 December 2012
Accepted: 3 February 2013
Published online in Wiley Online Library: 2 April 2013
(wileyonlinelibrary.com) DOI 10.1002/sia.5257
Corrosion resistance and cytotoxicity of a
MgF2 coating on biomedical Mg–1Ca alloy
via vacuum evaporation deposition method
N. Li,a Y. D. Li,b Y. B. Wang,a M. Li,c Y. Cheng,c Y. H. Wuc and Y. F. Zhenga,c*
To reduce the biocorrosion rate and enhance the biocompatibility by surface modification, MgF2 coatings were prepared on
Mg–1Ca alloy using vacuum evaporation deposition method. The average thickness of the coating was about 0.95 mm. The
results of immersion test and electrochemical test indicated that the corrosion rate of Mg–1Ca alloy was effectively decreased
after coating with MgF2. The MgF2 coating induced calcium phosphate deposition on Mg–1Ca alloy. After 72 h culture, MG63
cells and MC3T3-E1 cells were well spread on the surface of the MgF2-coated Mg–1Ca alloy, while few cells were observed on
uncoated Mg–1Ca alloy samples. In summary, MgF2 coating showed beneficial effects on the corrosion resistance and thus
improved cell response of the Mg–1Ca alloy effectively and should be a good surface modification method for other biomedical
magnesium alloys. Copyright © 2013 John Wiley & Sons, Ltd.
Keywords: Mg–1Ca alloy; magnesium fluoride; surface modification; corrosion resistance; biocompatibility
Introduction
Surf. Interface Anal. 2013, 45, 1217–1222
* Correspondence to: Y. F. Zheng, Department of Materials Science and Engineering,
College of Engineering, Peking University, Beijing 100871, China E-mail:
[email protected]
a State Key Laboratory for Turbulence and Complex System and Department of
Materials Science and Engineering, College of Engineering, Peking University,
Beijing 100871, China
b DongGuan EONTEC Co., Ltd, Yin Quan Industrial District, Qing Xi, DongGuan
523662, China
c Center for Biomedical Materials and Tissue Engineering, Academy for
Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China
Copyright © 2013 John Wiley & Sons, Ltd.
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As a lightweight metal with mechanical properties similar to
natural bone, a natural ionic presence with significant functional
roles in biological systems, and a material that can degrade in the
electrolytic environment of the body, magnesium-based implants
have the potential to serve as biocompatible, osteoconductive,
degradable implants for load-bearing applications.[1] However,
the medical applications of Mg alloys had been hindered owing
to the fast corrosion rate mismatched with the tissue healing rate.[2]
In order to slow down the corrosion rate of magnesium alloys,
as well as to maintain their mechanical integrate and to improve
their biocompatibility, various kinds of surface modification techniques have been developed, such as alkaline heat treatment,[3,4]
electrodeposition,[5–7] polymer coating,[8] phosphating treatment [9]
and microarc oxidation.[10,11]
Fluorine is a natural component of the human skeleton and
teeth. This is a rationale for using F supplements for treatment
of osteoporosis, as F incorporation into bone increases the size
of the apatite crystals and larger crystals are more resistant to
osteoclastic attack.[12] In addition, fluoride is one of the few
known agents that can stimulate osteoblast proliferation and
increase new mineral deposition in cancellous bones.[12,13]
Fluoride treatment of the implant surface was employed to
degrade cytotoxicity of titanium implants [14] and promote
osseointegration in the early phase of healing following dental
implant installation.[15]
Fluoride treatments also have been extensively reported to
improve the corrosion resistance of Mg and its alloys in industrial
environments [16–18] and in biomedical field.[19–24] Witte et al. [20]
found MgF2 coating could reduce in vivo corrosion rate of
LAE442 magnesium alloy with no elevated fluoride concentration
in the adjacent bone. Drynda et al. [23] developed fluoride-coated
magnesium-calcium alloys that exhibit improved mechanical features, decreased degradation kinetics and a good biocompatibility
toward vascular cells in vitro. Thomann et al. [21] reported fluoridecoated Mg0.8Ca implants possess good clinical tolerance in a rabbit
model and show a slight reduction of the degradation velocity in
comparison to uncoated Mg0.8Ca implants. Yan et al. [24] successfully prepared a flourinated coating on AZ31B magnesium alloy,
which could maintain the mechanical property of the alloy for as
long as 45 days in SBF.
Fluoride treatment of magnesium alloys is usually conducted
by immersing the samples in hydrofluoric acid. This method is
time-consuming (several hours to several days), and the treatment should be very carefully performed since hydrofluoric acid
is very toxic. Vacuum evaporation deposition is a time-efficient
surface modification technique. MgF2 coatings on magnesium
alloys prepared by vacuum evaporation deposition method have
not been reported.
In the present study, MgF2 coatings were prepared on Mg–1Ca
alloy using vacuum evaporation deposition method. The
microstructure and in vitro corrosion properties of MgF2-coated
Mg–Ca alloy were assessed. The surface biocompatibility of the
MgF2 coatings was evaluated by the direct assays.
N. Li et al.
Material and methods
Preparation of samples
The Mg–1Ca (1wt.%) alloy ingot was cast by commercial pure
Mg (99.98%) and Ca (99.95%) in a crucible under a mixed gas
atmosphere of SF6 and CO2, then was extruded into rod at
210 C with a reduction ratio of 17. The experimental Mg–1Ca
alloy samples were machined from the extruded rod with 12 mm
in diameter and 2 mm in thickness, mechanically polished up to
2000 grit, ultrasonically cleaned in acetone, absolute ethanol and
distilled water.
For the deposition of the MgF2 film, a custom-made multifunction coating machine (Technol Science Co., Ltd., China) was
employed. MgF2 powder was placed in a molybdenum boat
and evaporated at a base pressure of 0.1 Pa. The deposition time
was 3 min.
The surface and cross-sectional morphologies of MgF2 coatings
were characterized by environmental scanning electron
microscopy (ESEM, Quanta 200FEG), equipped with an energydispersive spectroscopy (EDS) attachment. The grazing incident
X-ray diffraction (GIXRD) was conducted on a Rigaku Dmax2500-V
X-ray diffractometer with Cu Ka radiation to identify constituent
phases of the coating. The incident angle was 1 and the scanning
rate was 4 /min.
hydrogen evolution rate were monitored during the immersion
test. An average of three measurements was taken. After 20 d
immersion, the samples were removed from of the solution,
rinsed with distilled water and dried in air. Changes on the
surface microstructure of coated and bare samples before and
after immersion were checked by ESEM and an X-ray diffractometer
(Rigaku DMAX 2400) using Cu Ka radiation.
Direct cell adhesion assay
Human osteosarcoma cells (MG63) and mouse osteoblast-like
cells (MC3T3-E1) were used in the direct cell adhesion assay.
MG63 cells were cultured in MEM, while MC3T3-E1 cells were cultured in a-MEM. Both culture media contained 10%FBS, 100 U ml 1
penicillin and 100 mg ml 1 streptomycin. Coated and uncoated
Mg–1Ca alloy samples were settled in the bottom of each well of
24-well cell culture plates, and 1 ml cell suspensions were seeded
in each well at a cell density of 6 104. After 72 h culture in a
humidified atmosphere with 5% CO2 at 37 C, the samples were
rinsed with PBS for three times and then fixed in 2.5% glutaraldehyde solution for 2 h at room temperature. The samples were
then dehydrated in a gradient ethanol/distilled water mixture
(50, 60, 70, 80, 90 and 100%) for 10 min each and dried in a
hexamethyldisilazane solution. The morphologies of the cells
adhered to the surfaces of samples were observed using ESEM.
Electrochemical test
A three-electrode cell was used for electrochemical measurements with a platinum counter-electrode and a saturated
calomel electrode as the reference electrode. The tests were
performed in Hank’s solution (NaCl 8.00 g/L, KCl 0.40 g/L, CaCl2
0.14 g/L, NaHCO3 0.35 g/L, MgSO47H2O 0.20 g/L, Na2HPO412H2O
0.12 g/L, KH2PO4 0.06 g/L, pH = 7.4) at 37 0.5 C using a CHI660C
electrochemistry workstation. The exposed area of the working
electrode to the electrolyte was 1.13 cm2. Potentiodynamic
polarization scans were performed at a scanning rate of 1 mV/s,
and the initial potential was 300 mV below the open circuit
potential (OCP). The electrochemical impedance spectroscopy
measurement was carried out from 100 mHz to 100 kHz at
OCP values. An average of three measurements was taken for
each group.
Results and discussion
Characterization of the MgF2 coating
The top-view surface and cross-sectional morphologies of
MgF2 coatings on the Mg–1Ca alloy samples are shown in Fig. 1,
revealing an average coating thickness of about 0.95 mm. The line
scanning of elements distribution across the coating and the
substrate depicted in Fig. 1(b) demonstrated elements existed
in the coating are mainly Mg and F. GIXRD patterns in Fig. 2 prove
the coating was MgF2. The MgF2 coatings were transparent and
could not be identified with the naked eye. The crack formed
while cooling due to the different thermal expansion coefficient
of the film and substrate.
Electrochemical corrosion
Immersion test
The immersion test was carried out in Hank’s solution at 37 0.5 C
according to ASTM-G31-72. The pH value of the solution and the
Figure 3(a) shows the OCP–time curves of the bare and MgF2coated Mg–1Ca alloy samples exposed to Hank’s solution for 1 h.
The OCP curve recorded for the untreated sample was more
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Figure 1. SEM micrographs of (a) surface morphology and (b) cross-sectional morphology with EDS line scan of the MgF2-coated Mg–1Ca alloy.
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Copyright © 2013 John Wiley & Sons, Ltd.
Surf. Interface Anal. 2013, 45, 1217–1222
Corrosion resistance and cytotoxicity of MgF2 coating on Mg–Ca alloy
which was related to the corrosion rate. The MgF2-coated
Mg–1Ca alloy exhibited a much higher polarization resistance than
the bare Mg–1Ca alloy.
Immersion corrosion
Figure 2. GIXRD patterns of the MgF2-coated Mg–1Ca alloy.
positive than the MgF2-coated one, and both samples exhibited a
similar variation trend with the extension of immersion time.
Figure 3(b) shows typical potentiodynamic polarization curves of
the bare and MgF2-coated samples after OCP measurements. The
MgF2-coated Mg–1Ca alloy sample showed reduced kinetics of
anodic and cathodic reactions than that of untreated Mg–1Ca
sample. The corrosion current density of the MgF2-coated
Mg–1Ca alloy was half ((6.06 1.00) mAcm 2) of that of the bare
Mg–Ca alloy ((11.58 2.31) mAcm 2), and the corrosion rates
calculated according to ASTM-G102-89 were (0.14 0.02)
mmyr 1 and (0.26 0.05) mmyr 1, respectively. Two time
constants can be seen in the Nyquist plots in Fig. 3(c): one capacitance loop at high frequency and the other at low frequency,
which are related to the electric double layer and the surface film,
respectively. The polarization resistance of the samples can be
obtained from the diameter of the near semi-circular spectra
Figure 4 presents the variation of the pH value of the immersion
solution and the hydrogen evolution volume during the immersion
period. As shown in Fig. 4(a), hydrogen evolution volume of both
kinds of samples displayed approximately linear relationship with
the immersion time, and the bare Mg–1Ca alloy released much
more hydrogen than the MgF2-coated one. Smaller increase in pH
value of the immersion solution also indicated the MgF2 coating
protected the Mg–1Ca alloy from fast degradation (Fig. 4(b)).
Figure 5 displays the surface morphology of the untreated
Mg–1Ca alloy sample and MgF2-coated Mg–1Ca alloy samples
after 20 d immersion in Hank’s solution. It can be seen that
untreated Mg–1Ca alloy sample exhibited a severe corrosion
attack with some deep pits visible (Fig. 5(a)). The EDS analysis in
Fig. 5(c) and Fig. 5(d) reveals the existed elements on the corroded
sample surface were mainly C, O and Mg, with Cl detected at the
corrosion pit position 1 and trace Ca and P detected at area 2. The
surface morphology of the MgF2-coated sample didn’t change
much after 20 d immersion (Fig. 5(b)), and the EDS analysis in Fig. 5
(e) and Fig. 5(f) reveals compounds that rich in Ca and P had
deposited on the MgF2 film. This enhanced deposition of calcium
phosphate might suggest an accelerated mineral deposition rate
on the Mg–1Ca alloy sample when implanted into bone.
The XRD patterns of the untreated Mg–1Ca alloy sample and
MgF2-coated Mg–1Ca alloy samples after 20 d immersion in
Hank’s are shown in Fig. 6. The resulting corrosion products for
untreated Mg–1Ca alloy sample after 20 d immersion are mainly
Mg(OH)2, with the peak intensity of a-Mg was remarkably
decreased. In contrast, only small amount of Mg(OH)2 can be
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Figure 3. (a) Open circuit potential, (b) potentiodynamic polarization curves and (c) Nyquist plots of bare and MgF2-coated Mg–1Ca alloy samples in
Hank’s solution.
N. Li et al.
Figure 4. (a) The hydrogen evolution volume and (b) the change of pH value of Hank’s solution incubating bare and MgF2-coated Mg–1Ca alloy
samples as a function of the immersion time.
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Figure 5. The surface morphologies of (a) bare Mg–1Ca alloy, (b) MgF2-coated Mg–1Ca alloy samples after 20 d immersion in Hank’s solution and EDS
spectrum of (c) area 1 in (a), (d) area 2 in (a), (e) area 3 in (b) and (f) area 4 in (b).
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Surf. Interface Anal. 2013, 45, 1217–1222
Corrosion resistance and cytotoxicity of MgF2 coating on Mg–Ca alloy
implants should maintain their mechanical integrity before the
reparative phase for strong healing union of fracture bone
approximately 3–4 months [26] avoiding the second fracture. If
this MgF2 layer alone cannot guarantee the maintenance of the
mechanical strength for that long, double layer could be
prepared on magnesium alloys, and the MgF2 layer acts as an
interlayer between the substrate and the outer layer, sucn as
hydroxyapatite.[27]
Cell experiments
Figure 6. The XRD patterns of bare and MgF2-coated Mg–1Ca alloy samples after 20 d immersion in Hank’s solution.
detected on the surface of the MgF2-coated Mg–1Ca alloy
samples.
The MgF2 coating insulates the Mg–1Ca alloy bulk from the
electrolyte and thereby decreases the corrosive attack of chloride
ions. Therefore, the corrosion of the Mg–1Ca alloy bulk cannot
start before the protective coating has dissolved or peeled. The
valid life of the surface coating guarantees not only the reduced
corrosion rate but also the mechanical integrity of magnesium
alloy implants.[11] The results of immersion test indicates the
in vitro valid life of the MgF2 coating is over 20 days, which is
better than that of the alkaline-heated and chitosan coatings
on Mg–1Ca alloy sample (about 10 days’ validity).[4,25] Normally,
The morphologies of MG63 and MC3T3-E1 cells cultured on the
bare Mg–1Ca alloy sample and MgF2-coated Mg–1Ca alloy
samples for 72 h are shown in Fig. 7. Difference in the response
of the cells to bare Mg–1Ca alloy sample and MgF2-coated
Mg–1Ca alloy samples is obvious. The untreated Mg–1Ca alloy
sample suffered pitting corrosion attack, and cell adhesion
processes were hindered. Only a few MG63 cells with unhealthy
round shape and hardly any MC3T3-E1 cells can be seen on the
bare Mg–1Ca surface. The MgF2 film effectively retard corrosion
of the Mg–1Ca substrate and prevent alkalization of the culture
medium, so both MG63 and MC3T3-E1 cells exhibited healthy
configuration with a cytoplastic and extended morphology and
several pseudopodium contacting the sample surface.
It is reported that F only showed cytotoxicity to UMR-106 cells
at high concentration (10 3M).[28] The F released from MgF2
coating could not reach that level since the low solubility of
MgF2 (0.13 g/l).
Surf. Interface Anal. 2013, 45, 1217–1222
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Figure 7. Morphologies of MG63 cells on (a) bare Mg–1Ca alloy, (b) MgF2-coated Mg–1Ca alloy and MC3T3 cells on (c) bare Mg–1Ca alloy, (d) MgF2coated Mg–1Ca alloy.
N. Li et al.
Conclusions
MgF2 film was successfully deposited on the surface of Mg–1Ca
alloy by vacuum evaporation deposition method. The results of
immersion test and electrochemical test indicated that the
MgF2-coated samples exhibited much lower degradation rate
than the uncoated ones. The MgF2 coating induced calcium
phosphate deposition on Mg–1Ca alloy. In the direct cell assay,
MG63 cells and MC3T3-E1 cells adhered and spread well on the
surface of the MgF2-coated samples after 72 h culture.
Acknowledgements
This work was supported by the National Basic Research Program
of China (973 Program) (Grant No. 2012CB619102), the National
High Technology Research and Development Program of China
(863 Program) under grant number 2011AA030101 and
2011AA030103, the National Science Fund for Distinguished
Young Scholars (Grant No. 51225101), the National Natural
Science Foundation of China (No. 31170909), the Research Fund
for the Doctoral Program of Higher Education under Grant No.
20100001110011, theNatural Science Foundation of Heilongjiang
Province (ZD201012) and Guangdong Province innovation R&D
team project (no. 201001C0104669453).
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