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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5326_Библиотеки_им_академика_М_И_Перельмана
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Surface modification by natural biopolymer coatings on magnesium alloys 303
(a)
(b)
(c)
O
132
105
253
203
152
101
492
394
295
197
Mg
79
52
26
C
0
0.00 1.00 2.00 3.00
Mg
O
50
C
0
1.00
2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
Mg
O
92
C
0
0.00
1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00
Element Wt % At %
C K
O K
MgK
P K
P
4.00 5.00 6.00 7.00 8.00
Element Wt % At %
C K
O K
MgK
P K
P
Element Wt % At %
C K
O K
MgK
P K
P
07.70
40.88
32.82
18.60
03.90
40.02
38.40
17.69
07.71
31.62
49.46
11. 21
12.45
49.65
26.23
11. 67
06.52
50.26
31.74
11. 47
12.80
39.42
40.57
07.22
(d)
423
338
253
169
84
Mg
O
C
0
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00
Element Wt % At %
C K
O K
MgK
P K
P
08.58
25.17
57.28
08.96
14.48
31.89
47.76
05.87
Figure 11.2 SEM micrographs and the EDS (Energy Dispersive Spectrometer) spectrum of
the phytic acid-modified WE43 magnesium alloy with a solution of (a) pH 3, (b) pH 5, (c) pH 8,
and (d) pH 10.
Adapted from Ye et al. (2012).
304 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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5.87%, correspondingly. However, much narrower microcracks and smoother films
were found, which suggested that less phytic acid but smoother and more compact
films were coated on the sample surfaces as the pH value increased.
The reaction between PA and WE43 alloy can be demonstrated by the following
equation (Crea, De Robertis, De Stefano, & Sammartano, 2006) and Figure 11.3(a):
iMg
2þ
þ HjPhy
ð12jÞ
¼ MgiPhyH
ð122ijÞ
j
(11.1)
in which Phy refers to phytic acid ion and i and j are the stoichiometric ratios.
In low pH environments, the matrix released more Mg
2þ
, which could produce
more PhyeMg c hel ate. However, PhyeMg chelate was par tly soluble in pH ¼ 3
solution, so there would be less chelate comp lex on the surface of the WE43 Mg
alloy than in the pH 5. At a high pH, the corrosion procedure was inhibited, so
less Mg
2þ
could react with PA. Different from other conditions, the structure of
PA was 1eq/5ax (Figure 11.3(b)) in pH 10; when the samples were put into
SBF, the structure of PA changed to 1ax/5eq. (Here, ax is the axial bond between
carbon and substituent that is projected vertically up or down on the carbon ring
conformation, whereas eq is the equatorial band between a carbon and a substituent
that extends out of a ring of carbons.) During this change, the chelate bond between
2þ
Mg
with phosphate groups would break down, resulting in the migration of
phytic acid to the SBF solution. Meanwhile, because the stability of Phy-Ca was
O
P
OH
O
OH
O
P
OH
Mg
–
O
OH)
2+
–
O
R
R
(a)
R
R
(b)
R
R
1ax / 5eq
R
R
R
R
R
R
+ Mg
2+
R
R
R
1eq / 5ax
R
O
O
P
–
O
R
R
R
R
O
–
O
2+
Mg
R
(R = P
O
Figure 11.3 (a) The mechanism of the PhyeMg complex formation. (b) The conformation of
the phytic acid in different solution conditions.
Adapted from Ye et al. (2012).
Surface modification by natural biopolymer coatings on magnesium alloys 305
higher than Phy-Mg and there were more Ca2þthan Mg2þ(2.5 mmol/L vs
1.5 mmol/L) in the SBF, the Phy after the bond cleavage would combine with calcium ions to form CaePhy chelate. The dissolve of PhyeMg film accelerated Mg
alloy corrosion.
The corrosion rate of PA-treated WE43 alloy samples follows the order: pH 5 < pH
3 < pH 8 < pH 10 in SBF solution, as indicated by the hydrogen evolution method
(Figure 11.4) and potentiodynamic polarization parameters (Table 11.1).
20
2
18
16
14
12
10
8
6
4
2
Volume of evoled hydrogen / ml / cm
0
–2
0204060
Figure 11.4 Hydrogen evolution volumes of the phytic acid-modified WE43 Mg with different
pH and the control samples as a function of the immersion time in SBF.
pH = 3
pH = 5
pH = 8
pH = 10
Control
Immersion time / h
80 100 120
Table 11.1 Potentiodynamic polarization parameters of the phytic
acid-modified WE43 Mg samples and the control in SBF solutions
at 37
Coating condition
pH 3 1.84 5.26 10
pH 5 1.79 8.08 10
pH 8 1.85 5.08 10
pH 10 2.08 3.07 10
Control 1.85 5.09 10
Adapted from Ye et al. (2012).
C
Corrosion potential:
Ecorr (V)
Corrosion current
density: Icorr (A cmL2)
4
4
4
3
4
306 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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As shown in Figure 11.5(a), all the modified groups exhibited better cell viabilities
of L929 than the unmodified control after 1, 2, and 4 days of culture, and the hemolysis rate of PA-coated WE43 magnesium alloys were lower than 5% (Figure 11.5(b)),
indicating that the modified WE43 magnesium alloy met the requirements of the
hemolysis standard (ISO, 2006) for biomedical materials.
To conclude, the PA-treated WE43 alloys possess superior corrosion properties and
biocompatibility compared with untreated WE43 alloy. The solution converted group
(a)
100%
90%
80%
70%
60%
50%
40%
Cell viability (%)
30%
20%
10%
0%
(b)
9%
pH = 5
pH = 3
9.27%
pH = 10
pH = 8
1
pH = 5
pH = 3
Control
Time in culture / day
pH = 10
pH = 8
2
Control
pH = 5
pH = 3
pH = 10
pH = 8
4
Control
6%
3.60%
Hemolysis rate(%)
3%
0%
Control
2.14%
pH = 3 pH = 5 pH = 8 pH = 10
2.02%
Different samples
2.52%
Figure 11.5 Biocompatibility. (a) L929 cell viability expressed as a percentage of the viability
of cells in the control and phytic acid-modified WE43 Mg extraction solution after 1, 2, and
4 days of culture. (b) Hemolysis percentage of the WE43 Mg alloy and the phytic acidemodified
WE43 Mg alloy.
Surface modification by natural biopolymer coatings on magnesium alloys 307
with a pH of 5 displayed the best properties among the modified samples. Phytic acid
modification is an industrial method to improve the corrosion property of metal alloys.
This work successfully adopted the traditional method from industry to biomedical
applications and achieved some ideal results.
11.3 Chitosan modification
11.3.1 Introduction to chitosan
Chitosan, the deacetylated product of chitin, is soluble in dilute acids such as acetic
acid and formic acid. Commercially, chitin and chitosan can be obtained from shellfish
sources, such as crabs and shrimp (Khor & Lim, 2003). The investigation on chitosan
mainly focuses on its preparation with varied molecular weights and degrees of acetylation (DA) from chitin, the relationship between its solution properties and DA, and
the preparation of derivatives and applications (Rinaudo, 2006).
Most of the naturally existing polysaccharides, such as cellulose, dextran, pectin,
alginic acid, agar, agarose, and carrageenans, are neutral or acidic, whereas chitin
and chitosan are examples of highly basic polysaccharides. Their unique properties
include polyoxysalt ions and optical structural characteristics (Majeti, 2000). Meanwhile, chitosan is the only pseudonatural cationic polymer, and it could be used in
many fields for its unique characters (flocculants for protein recovery, depollution,
etc.). Being soluble in aqueous solutions, it is commonly used in different applications,
such as solutions, gels, or films and fibers (Rinaudo, 2006). In addition, it has been used
for the development of ocular bandage lenses (Rinaudo, 2006), the treatment of wastewater, as substratum for skin replacement (Ravi Kumar, 2000), and as a drug carrier.
11.3.2 Chitosan coating on Mge1.4 wt% Ca alloys
Gu et al. (2009b) coated chitosan on the surface of extruded Mge1.4 wt% Ca alloy for
two main considerations: (1) the insolubility of chitosan in alkaline aqueous solutions
may protect the Mg alloy substrate from corrosion; and (2) after being degraded by
enzyme action and hydrolysis, the degradation products of chitosan are saccharides
and glucos amines, which may be incorporated into glycoproteins or excreted as carbon
dioxide gas during respiration.
Chitosan solutions were prepared with four types of chitosan (type 1: DD ¼ 87%,
h ¼ 15 mPa,s, Mw ¼ 1.0 10
type 3: DD ¼ 90%, h ¼ 370 mPa$s, Mw ¼ 27 10
h ¼ 725 mPa$s, Mw ¼ 60 10
Here DD, h, and Mw represented the degree of deacetylation, the intrinsic viscosity,
and the molecular weight, respectively. After passivation in 10 g/L C
and activation in 200 ml/L H
ples were dip-coated in chitosan solution with one, three, six, and nine cycles at a withdraw speed of 1 cm/min. Each time after coating, the samples would be evaporated at
60
C for 20 min.
4
; type 2: DD ¼ 85%, h ¼ 200 mPa$s, Mw ¼ 15 104;
4
) at one weight percentage in 0.2% acetic acid.
(85%) and 100 g/L NH4HF2, magnesium alloy sam-
3PO4
4
; type 4: DD ¼ 83%,
2H2O4
$2H2O
308 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Chitosan coating prepared using type 3 chitosan after six cycles of dip coating
showed the smoothest and flattest surfaces (Figure 11.6) among the four types. The
surfaces after one, three, and nine cycles of type 3 chitosan coating showed the appearance of pores with different sizes, in comparison with the flat and compact surface of
six cycles of type 3 chitosan coating.
The hydrogen evolution volume and the variation in the pH values were
detected during an immersion test in SBF (Figure 11.7), which il lustrated that
six cycles of ty pe 3 chitosan coating had the highest corrosion resistance. After
10 days of immersion, the surface of the uncoated MgeCa alloy sample suffered
severe corrosion (Figure 11.8), while the surface of six cycles of type 3 chitosan
coating sample presented a r el at ively smooth and regular morphology without any
cracks.
(a)
(d)
(b) (c)
(e) (f)
(g) (h) (i)
Figure 11.6 Plane-view SEM images of the surfaces of (a) uncoated MgeCa alloy and chitosan
coatings with molecular weights of approximately (b) 1.0 10
and (e) 6.0 10
of the surfaces of the chitosan coatings with molecular weight of approximately 2.7 10
one cycle, (h) three cycles, and (i) nine cycles.
Adapted from Gu, Zheng, et al. (2009b).
5
for six cycles. (f) Cross-sectional SEM image of (d). Plane-view SEM images
4
, (c) 1.5 105, (d) 2.7 105,
5
for (g)
Surface modification by natural biopolymer coatings on magnesium alloys 309
15
(a)
)
2
10
5
Volume of evolved hydrogen (ml / cm
0
Uncoated
Type 1–6 layers
Type 2–6 layers
Type 3–6 layers
Type 4–6 layers
Type 3–1 layer
Type 3–3 layers
Type 3–9 layers
500 100
Immersion time (h)
10.5
(b)
10.0
pH value
9.5
9.0
8.5
8.0
7.5
0 50 100
Uncoated
Type 1–6 layers
Type 2–6 layers
Type 3–6 layers
Type 4–6 layers
Type 3–1 layer
Type 3–3 layers
Type 3–9 layers
Immersion time (h)
Figure 11.7 The hydrogen evolution volume (a) and the variation in the pH value of SBF
(b) for samples coated by chitosan with different molecular weights (1.0 10
2.7 10
weight (w2.7 10
5
, and 6.0 105) and different cycles of chitosan coating with the same molecular
5
).
150 200 250
150 200 250
4
, 1.5 105,
310 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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(a)
(d)
Figure 11.8 SEM images of the surfaces of chitosan coatings with a molecular weight of
approximately 2.7 10
5
(b)
(e)
for (a) zero cycle (uncoated), (b) one cycle, (c) three cycles, (d) six
(c)
(f)
Element Wt% At%
C K
O K
MgK
P K
CaK
cycles, and (e) nine cycles after immersion in SBF for 10 days. (f) Typical EDS spectra of the
chitosan-coated MgeCa alloy after 10 days immersion in SBF.
Adapted from Gu, et al. (2009b).
Results showed that th e dip coating of chitosan had effectively improved the
biocorrosion resistance of the MgeCa alloy in the simulated body fluid,
especially the coating with six cycles produced by 2.7 10
5
molecular weight
chitosan.
In this work, a suitable type of chitosan and an appropriate dip-coating procedure
are screened out as the best coating to improve the corrosion property of MgeCa
alloy. However, the attachment force between the substrate and the chitosan coating
has not been tested, and the morphology of the cross-section has been ignored. Apart
from that, much more work to test the coating properties, such as biocompatibility,
has to be done.
11.3.3 Phosphate/chitosan composite films by electrophoretic
deposition
Hydroxyapatite (HA, Ca10(PO4)6(OH)2) constitutes 65% of the mineral fraction of
human bone and tooth, which is commonly used as coating of the implants due to
its promotion of new bone ingrowth (Petit, 1999). However, the bonding strength
between HA and magnesium alloy substrates does not meet the request. In order to
improve the poor HA-Mg interfacial strength in EPD (Electrophoretic Deposition
Fabrication) fabrication, calcium phosphate/chitosan composite films were fabricated
on the surface of micro-arc oxidized (MAO)-AZ91D alloy through EPD followed by a
conversion process of the coatings in phosphate-buffered saline (PBS) (Wu, Wen, Dai,
Surface modification by natural biopolymer coatings on magnesium alloys 311
Lu, & Yang, 2010). The coating phases changed from HA/chitosan to Ca(OH)2during
EPD process and then transferred into HA immersed in PBS.
The degree of deacetylation of chitosan (Mw ¼ 200,000) was about 85%. All the
electrolytes were prepared by mixing an n-HA/ethanol suspension with an n-HA/chitosaneacetic acid aqueous solution, according to Table 11.2, where n-HA stands for
stoichiometric HA for EPD precipitated through a wet chemical technique by Wu
et al. (2010).
The coating conditions and composition of deposited coatings are shown in Table
11.3 and Figure 11.9. As the content of chitosan increased, the achievement ratio
improved. This may be due to the reaction or electrostatic attraction between the
charge of chitosan and the electrode, which is needed to prove. It revealed that
flake-like crystals in different diameters wer e formed on the surface of samples.
Also, with the increase of n-HA/chitosaneacetic acid aqueous solution, less flakelike crystals and more pores were produced, which is mainly caused by gas evolution
in the deposited coatings.
As revealed by x-ray diffraction (XRD) patterns (Figure 11.10(a)), when the n-HA
particles were dissolved into the chitosaneacetic acid aqueous solution, the intensities
of HA peaks gradually decreased, while those of Ca(OH)
gradually increased as the
2
volume percentage of n-HA/chitosaneacetic acid aqueous solution increased in the
electrolyte. Furthermore, the adhesion of the coatings to the substrate was enhanced
gradually and the cracks in the layers became less. Chitosan reduced the surface energy
of HA powders and stronger adhesion between the substrate and Ca(OH)
could be
2
produced, leading to dispersed particles and crack-free coatings. The reaction might
occur as follows (Wu et al., 2010):
CHIT NH
Ca
ðPO4Þ6ðOHÞ2þ 2Hþ/10 Ca2þþ 6PO
10
2H
O þ 2e/H2þ 2OH
2
CHIT NH
2þ
þ 2OH/CaðOHÞ
Ca
þ H3Oþ/CHIT NH
2
þ
þ OH/CHIT NH2þ H2O (11.5)
3
2
þ
þ H2O (11.2)
3
3
þ 2H2O (11.3)
4
(11.4)
(11.6)
After immersion for 3, 10, and 15 days in PBS solution, HA and dicalcium phosphate dihydrate (DCPD CaHPO
$2H2O) were converted from Ca(OH)2, as revealed
4
by the XRD patterns (Figure 11.10(b) and (c)) of coatings. It is thought that
Ca(OH)
(DCPD), as shown in Eqn (11.7e11.10). With the portion of Ca(OH)
might be gradually converted to HA directly or experience a precursor phase
2
gradually
2
increasing in the EPD layer, the appearance of the samples after conversion became
more homogeneous and the size of the particles became smaller (Figure 11.11).
This research successfully produced dense and uniform HA layers by converting
HA/chitosan to Ca(OH)
during EPD, and then transferred into HA in PBS. As the
2
Table 11.2 Compositions of six electrolytes for EPD (volume of electrolyte: 500 mL)
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312 Surface Modification of Magnesium and its Alloys for Biomedical Applications
n-HA/chitosan-acetic acid solution (ml) n-HA/ethanol suspension (ml)
Volume of chitosanacetic acid solution
Electrolyte
(ml) Weight of n-HA (g)
E-I 0 0 2.5 500 0
E-II 0.5 100 2.0 400 20
E-III 1.0 200 1.5 300 40
E-IV 1.5 300 1.0 200 60
E-V 2.0 400 0.5 100 80
E-VI 2.5 500 0 0 100
Adapted from Wu et al. (2010).
Volume of chitosanacetic acid solution
(ml)
Volume percentage
of n-HA/chitosan
acetic acid solution
(vol%)Weight of n-HA (g)
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