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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5846_Библиотеки_им_академика_М_И_Перельмана
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34 Surface Modification of Magnesium and its Alloys for Biomedical Applications
(a)
5 um
(c)
5 um
(b)
(d)
Counts
O
Mg
Counts
O
Mg
P
246
Energy (KeV)
P
246
Energy (KeV)
8100
8100
Figure 2.4 Surface morphology and EDS analysis of Mg-Mn-Zn alloy after acid washing and
phosphating treated for 10 s. (a) Surface morphology after acid activation. (b) EDS spectrum of
the framed area in (a). (c) Surface morphology after phosphating treatment for 10 s. (d) EDS
spectrum of the framed area in (c).
(a)
5 um
(b)
Mg
Counts
O
0
P
Ca
2
468
Energy (KeV)
Zn
10
Figure 2.5 Surface morphology and EDS analysis of the extruded Mg-Mn-Zn alloy after
immersion in phosphate-treating solution for 3 min. (a) Surface morphology. (b) EDS spectrum of
the framed area in (a).
Phosphate treatment of magnesium alloy implants for biomedical applications 35
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(a) (b)
5 um
(c)
Figure 2.6 Surface morphology and EDS analysis of extruded Mg-Mn-Zn alloy afte r
immersion in phosphate-treating solution for 5 min. (a) Surface morphology. (b) Morphology
of particle. (c) EDS spectrum of the framed area in (a).
Mg
Counts
O
0
P
Ca
2
46810
Energy, KeV
2 um
phosphating for 3 min. A fine network structure was observed on the surface. EDS
results proved that a phosphate film was formed on the surface. After 5 min of
treatment, the surface was covered by a relatively dense layer, as shown in
Figure 2.6(a). Several small particles were found on the surface, as shown in
Figure 2.6(b). EDS in Figure 2.6(c) also confirms the formation of phosphate. At
the end of this stage, the magnesium sample was covered by a phosphate film. Kouisni
et al. (2005) reported that two types of particles were observed, light and dark, on the
surface of magnesium after 30 s immersion in a zinc phosphating bath. After 2 min
only dark particles were visible, and after 5 min immersion the phospha te coating
coved about 90% of the surface. The dark and compact phosphating layer was considered to be the growth site of the following crystal phosphate (Kouisni et al., 2005). In
our study, only a few light particles were observed on the surface.
The Third Stage (from C to D in the curve). In this stage, the potential increases
much more slowly with the increase of phosphating duration in comparison with that
in the second stage. After the second stage, the magnesium sample has been
completely covered by a compact phosphate layer. The reaction between magnesium
substrate and the bath solution was reduced by the formation of the phosphate layer.
Further deposition of phosphate has to nucleate and grow on the surface of the phosphate layer. As shown in Figure 2.7, lots of phosphate particles were found on the
36 Surface Modification of Magnesium and its Alloys for Biomedical Applications
Figure 2.7 Surface morphology of the extruded Mg-Mn-Zn alloy after immersion in phosphatetreating solution for 7 min.
surface of magnesium alloy a fter phosphating treatment for 7 min. This stage was also
reported in other studies (Kouisni et al., 2005).
The Fourth Stage (from D to E in the curve). In this stage, the potential changes
very slowly with the treatment time, corresponding to a stable growth of phosphate on
the substrate. Figure 2.8 shows the surface morphologies of the phosphate coating after
phosphating treatment for 10 min to 50 min. The magnesium sample was covered by a
(a) (b)
(c) (d)
Figure 2.8 Scanning electron microscope (SEM) surface morphologies of the Mg-Mn-Zn alloy
after phosphating treatment for different durations. (a) 10 min. (b) 20 min. (c) 30 min. (d)
50 min.
Phosphate treatment of magnesium alloy implants for biomedical applications 37
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rough layer with a regular and prismatic shape crystal. Nearly no difference can be
found in the surface morphology between the samples treated for 10e50 min, indicating that the growth rate is very low at this stage. Small-angle X-ray diffraction
(SAXD) results indicate that the phosphate is brushite.
2.5.2 Surface microstructure and composition
The phosphating coating can be roughly divided into two layers depending on the processing parameters (Chen et al., 2010; Zhang, Huang, Yang, Zhang, & AI, 2007). The
lower layer is a flat and dense amorphous phosphate layer with low roughness, and the
top layer is a porous large crystal layer with high roughness (Chen et al., 2010; Li et al.,
2010). As shown in Figure 2.6(a), after a short immersion time, a dense and flat phos-
phating c oating was formed on the surface and the extension of phosphating duration
results in a porous and rough surface. The two-layer structure coating provides good
protection to the substance. But on the other hand, the surface roughness will also influence the cell adhesion and spreading on the surface. Research on hydroxaptite
(HAp) formed on magnesium and AZ31 in a solution also reported that the HAp
coatings consisted of an inner dense layer and an outer coarse layer (Hiromoto &
Tomozawa, 2011). The inner layer was composed of dome-shape precipitates densely
packed and the outer layer was composed of rod-like crystals growing from each dome
in the radial direction.
It is possible to change the surface morphology by further treatment. Kannan (2012)
reported that the surface morphology of the calcium phosphate coating was changed
from a loosely packed and grain-like surface structure to a flat and compact surface
by using a pulse-potential method instead of constant potential. This closely packed
morphology exhibited w3 times higher polarization resistance and 65% lower corrosion current than the loosely packed structure.
The two-layer structure displays different characteristics. Besides the difference in
the chemical composition and structure, the adhesive strength is also different. Chen’s
results on barium phospha te coating on AZ31 (Chen et al., 2010) indicated that
the adhesion strength between the top layer and the lower layer (0.94 MPa) was
lower than the value between the lower layer and the substitute (1.45 MPa), but
the two-layer structure provided a higher corrosion resistance than the lower-layer
coating.
2.6 Anticorrosion resistance
2.6.1 Electrochemical test
Nearly all available papers on phosphating coating on magnesium alloy report that the
phosphating treatment improved the anticorrosion resistance of magnesium alloy not
only in a biological environment (Ye et al., 2013) but also other solutions (Kouisni
et al., 2005). Results indicate that the phosphating process changes the coating struc-
ture and also changes the surface morphology. The coating structure in turn might
38 Surface Modification of Magnesium and its Alloys for Biomedical Applications
alternate the anticorrosion resistance. Figure 2.9 shows typical Tafle curves of phosphated magnesium and magne sium substrate measured in a SBF. From the curves, it can
be found that 10 min phosphate treatment moves the curve to a noble direction significantly. Further extension in the phosphate duration moves the curve to a more noble
direction. Calculated electrochemical parameters from these curves are summarized in
Table 2.2. From these data, it can be clearly demonstrated that the phosphating treat-
ment increases the corrosion potential, reduces the corrosion current density, and
increases the corrosion resistance. Also with the increase of the phosphating duration,
the corrosion potential becomes more noble, the current density decreases, and the
corrosion resistance increases. The best results were obtained after phosphating treatment for 30 min. However, the improvement in the anticorrosion resistance by the
extension in the phosphate duration from 10 to 50 min is lower than the improvement
caused by the phosphate treatment for 10 min.
–1
–2
–3
–4
–5
Log (current/A)
–6
–7
–8
–1.65 –1.60 –1.55 –1.50 –1.45 –1.40 –1.35 –1.30 –1.25 –1.20 –1.15
Bare alloy
Treated for 10 min
Treated for 20 min
Treated for 30 min
Treated for 50 min
Potential (V)
Figure 2.9 Electrochemical polarization curves in SBF solution of magnesium alloy samples
with and without phosphating treatment. SBF, simulated body fluid.
Table 2.2 The parameters of the electrochemical polarization tests
in SBF solution
Samples i
(mA/cm2) E
corr
corr(V(SCE)
)Rp(U)
Bare alloy
Phosphated for 10 min
Phosphated for 20 min
Phosphated for 30 min
Phosphated for 50 min
6.666
4.435
2.960
1.384
0.914
1.552
1.446
1.430
1.408
1.391
2665
3812
6590
7770
9975
Phosphate treatment of magnesium alloy implants for biomedical applications 39
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2.6.2 Immersion test
Electrochemical tests reveal the corrosion behavior of magnesium in a short time. To
assess the long-term corrosion properties, the magnesium samples before and after
phosphating treatment were also immersed into SBF for several days and the pH
values were recorded.
The pH values of the SBF solutions in which the bare and the phosphated magnesium samples were immersed (for 30 min) are shown in Figure 2.10 against the immersion time. For both the bare samples and the phosphated samples, pH values increase
with the immersion time. However, the increase of the pH value of the SBF solution
containing the phosphated samples is slower than that of the solutions containing the
bare samples. After 1 day of immersion, the pH value of the solutions containing the
bare samples was about 8.17 while the pH value of the solution containing the phosphated samples was about 7.95. After 2 days, 4 days, and 9 days of immersion, the difference in pH between the solutions containing the phosphated samples and the bare
samples was as high as 0.5 unit. The low pH value demonstrates that dissolution or
the degradation of the phosphated magnesium is lower than the bare alloy.
2.6.3 Surface morphology
Figure 2.11 shows the surface morphologies of the bare samples after immersion in
SBF for different durations. After 1 day of immersion, the surface of the bare sample
was covered by a corrosion layer with many cracks on the surface. Some white particles were also deposited on the corroded surface. With the increase of the immersion
time, more and more particles were observed. EDS analysis was conducted on the
9.2
9.0
8.8
8.6
8.4
8.2
pH
8.0
7.8
7.6
7.4
Figure 2.10 Change in the pH value of SBF solutions containing the bare samples and the
phosphated samples with the immersion time (Xu, Zhang, & Yang, 2009). SBF, simulated body
fluid.
Bare samples
Phosphated samples
0123
456789
Time (day)
10
40 Surface Modification of Magnesium and its Alloys for Biomedical Applications
corrosion layer and the particles after 9 days of immersion. The results indicate that
both the corrosion product and the deposition particles are mainly composed of O,
P, Mg, Ca, and a small amount of Na. The ratios of Ca to P (Ca/P) are less than
0.8, which is much lower than that of HA.
The surface morphologies of the phosphated samples after immersion in SBF solu-
tion for different durations are shown in Figure 2.12 . After 1 day of immersion, partial
brushite crystals lost their sharp edges and corners, as marked by the arrow in
Figure 2.12(a), indicating that the brushite crystals on the surface of magnesium sub-
strate began to dissolve in SBF. Also, some white particles were found on the surface.
After 2 days of immersion, no obviou s difference in the surface morphology was
observed by SEM compared to that immersed for 1 day. After 4 days of immersion,
more and more brushite crystals were dissolved, as shown in Figure 2.12(c). Nine
days later (Figure 2.12(d)), there were cracks on the surface of the sample, displaying
the brushite dissolved and became many smaller parts. Meanwhile, more particles
were found on the surface. EDS analysis results on the dissolving brushite and the
deposited particles disclose that the dissolving brushite layer is mainly composed of
P, O, Ca, and a tiny amount of Mg, and the Ca/P ratio is 1.467, close to that of calcium
phosphates. The particles deposited on the surface are mainly composed of O, P, Mg,
Ca, and Na, and the Ca/P ratio is 1.292. In order to identify the phase transformation of
(a) (b)
(c) (d)
Figure 2.11 Surface morphology of the bare samples immersed in SBF solution for (a) 1 day,
(b) 2 days, (c) 4 days, and (d) 9 days (Xu, Zhang, et al., 2009). SBF, simulated body fluid.
Phosphate treatment of magnesium alloy implants for biomedical applications 41
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(a) (b)
(c)
Figure 2.12 Surface morphology of the phosphated samples immersed in SBF solution for (a)
1 day, (b) 2 days, (c) 4 days, and (d) 9 days (Xu, Zhang, et al., 2009). SBF, simulated body fluid.
Mg
HA
(d)
(a)
(b)
Intensity
(c)
(d)
10 20 30 40 50 60 70 80
2 theta
Figure 2.13 X-ray diffraction patterns of the phosphated Mg-Mn-Zn alloy immersed in SBF for
9 days. (a) The phosphated Mg alloy sample, (b) Mg-Mn-Zn substrate, (c) HA (JCPDS09-0432),
and (d) brushite (JCPDS09-0077) (Xu, Zhang, et al., 2009). SBF, simulated body fluid.
42 Surface Modification of Magnesium and its Alloys for Biomedical Applications
the brushite layer during the immersion, XRD was conducted on the surface of the
phosphate sample after immersion in SBF solution for 9 days, as shown in Figure 2.13.
It can be found that HA phase as well as magnesium matrix were detected, but the
brushite phase completely disappeared, suggesting that the brushite was transformed
to HA during the immersion.
In the research on DCPD coating on magnesium (Wang, Wei, & Gao, 2009), it was
reported that after 3 days of immersion in SBF, the DCPD coating was still intact and
completely covered the substrate. DCPD dissolution was observed on the third day and
became noticeable on the fifth day, but it did not completely disappear until the end of
the e xperiment. The DCPD phase was not observed by XRD from the seventh day, and
no new phases were found in the entire immersion period.
In another study, it was reported that the surface morphology of DCPD changed
with the hydrothermal temperature and duration. High temperature and long duration
resulted in a dissolution of DCPD and disappearance of the platy structure (Chang
et al., 2013).
2.7 In vitro biocompatibility
The previous section revealed that the surface morphology of the phosphated magnesium changes from a porous but flat surface with Ca and P elements to a rough brushite
surface with the extension of the phosphating process. Cell adhesion is influenced by
lots of surface properties of a substrate, including the chemical composition, physical
topology, and whether it is hydrophilic or hydrophobic. Brushite with a long prismatic
shape provides a substrate with strong anticorrosion properties but also a rough surface. However, the rough surface might resist the adhesion.
Table 2.3 lists the toxicity test results by MTT assay on the bare magnesium alloy
and the phosphated magnesium alloy. The L929 cell line was used to assess the cytotoxicity. The extract was prepared according to Standard ISO10993-12. Relative
Table 2.3 MTT assay of the bare Mg alloys and the phosphated Mg
alloys
Incubation
Materials
Extract of bare magnesium 1 87.0 1
Extract of the phosphated
magnesium alloy
duration (day) RGR (%) Toxicity grade
2 99.7 1
3 109.3 0
1 89.6 1
2 100.8 0
3 110.4 0
Phosphate treatment of magnesium alloy implants for biomedical applications 43
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growth rate (RGR) of the cells on the bare magnesium alloys ranges between 87.0 and
109.3%, responding to a toxicity Grade 1. Meanwhile, RGR on the phosphated magnesium alloy is about 89.6e110.4%, similar to that of the bare magnesium alloy.
Toxicity of grade one was tested both for the bare magne sium alloy and the phosphate
magnesium alloy; however, the toxicity meets the standard requirement, which means
that these materials are safe to the L929 cell line.
Another study also reported that the calcium phosphate-coated WE43 alloy showed
a cell (L929) viability of 83e96% up to 4 days of incubation while the cell viability on
the WE43 substrate was only 25e34.8% (Ye et al., 2013).
These results demonstrate that phosphating treatment can improve the cell toxicity
of magnesium alloys. However, the phosphating system has to be selected carefully to
avoid side effects.
Cell nontoxicity is a basic requirement for biomaterials. For bone implant, it is also
desired that bone cell can adhere to the surface, spread on the surface, and proliferate
significantly. The cell adhesion is affected by the surface chemical composition, surface
toughness, and surface hydrophilic and hydrophobic properties. Figure 2.14 shows the
cell morphologies (L929 cell line) on the bare magnesium alloy, the phosphated magnesium alloy, and cp-Ti titanium as a control sample. Several cells with a nearly round
shape were observed on the bare magnesium sample. On the control sample, also only
several cells with a round shape were observed on the surface. On the contrary, many
cells were observed on the phosphated magnesium sample, and the cells nearly covered
the whole sample. In addition, the cells spread out and contacted each other, displaying
good living conditions. Even after 3 and 5 days of incubation, there were still only several
cells on the surface of the bare magnesium alloy, as shown in Figures 2.15 and 2.16.
However, the cells covered the whole surface of the phosphated magnesium and cp-Ti
samples after 3 and 5 days of incubation. It is believed that the phosphate group and
Ca element on the phosphated magnesium sample greatly contributed to the good cell
compatibility.
Figure 2.17 displays the cell number on the samples after 1 day to 5 days culture. At
all culture periods, cells on the phosphated magnesium sample show a significantly
higher proliferation rate than the bare magnesium alloy (p < 0.05).
2.8 In vivo investigation
In vivo degradation of magnesium and magnesium alloy is quite different from the in
vitro behavior. An in vivo study on the corrosion resistanc e of the Ca-P coated AZ60
magnesium alloy indicated that severe degradation of uncoated Mg alloy was observed
by micro-CT image 3 months postimplantation (Xiao et al., 2013). The calculated results displayed that the corrosion rate of the Ca-P coated Mg was only one-third of that
of the uncoat ed Mg alloy.
Newly formed osteoid tissue was observed around both the naked Mg alloy implant
and the Ca-P coated Mg alloy implant after 4 weeks implantatio n by fluoroscopic microscopy (Xu, Pan, et al., 2009). Compared with the naked Mg alloy implant, the
newly formed osteoid tissue around the phosphated Mg alloy implant was compact
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