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Surface modification by natural biopolymer coatings on magnesium alloys 313
Table 11.3 Coating conditions and phase proportion after coating
in different electrolytes
Deposit
coatings Conditions and phase (wt%) of deposited coatings
D-I D-II D-III D-IV D-V D-VI
Electrolyte E-I E-II E-III E-IV E-V E-VI
Deposition
current
density
2
(A/cm
Coating
condition
Achievement
ratio (%)
HA 100 67 45 35 0
Ca(OH)
2
Adapted from Wu et al. (2010).
<0.01 <0.01 0.01 0.04 0.12 0.22
)
Thin Detached Poorly
Uniform Uniform Uniform
uniform
72 0 12 888692
0 32 5464100
percentage of chitosan increased, the HA coating became more homogeneous, indicating higher biocompatibility.
6x
ðOHÞ
0 x 1
2x
3
þð2 xÞOH/
4
(11.7)
(11.8)
CaðOHÞ
2þ
þ HPO
Ca
Ca
10x
2þ
þ HPO
Ca
6CaHPO
/Ca2þþ 2OH
2
2
þ 2H2O þð6 xÞPO
4
ðHPO4ÞxðPO4Þ
2
þ 2H2O/CaHPO4,2H2O (11.9)
4
,2H2O þð4 xÞCaðOHÞ2/
4
(11.10)
Ca
10x
ðHPO4Þ
6x
ðOHÞ
þð18 xÞH2O 0 x 1
2x
EPD is a frequently used method in modification, and some electrochemical reaction may occur during the procedure. In this work, it is upset when bioactive HA particles are converted into unbioactive Ca(OH)
in EPD, which decreases the
2
biocompatibility of the coatings, but the authors are sapiential to transform Ca(OH)
into HA by a subsequent immersion in PBS solution. Although the reaction procedure
in the article is not definitive, especially the changes of chitosan in the formulas, it is an
encourage work to apply a conversion thought transfering a bio-inert coating to a biomimetic or even bioactive surface by a low cost and easy method.
2
314 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Figure 11.9 SEM images of the coatings prepared from five electrolytes on the MAO-AZ91D
substrate by EPD. (a, b) D-I prepared from the E-I electrolyte. (c, d) D-III prepared from the E-III
electrolyte. (e, f) D-IV prepared from the E-IV electrolyte. (g, h) D-V prepared from the E-V
electrolyte. (i, j) D-VI prepared from the E-VI electrolyte.
Adapted from Wu et al. (2010).
Surface modification by natural biopolymer coatings on magnesium alloys 315
Figure 11.9 Continued.
11.3.4 Phosphate/chitosan composite films by aerosol
deposition
Highly dense and adherent HAepolymer composite coatings onto metallic substrates
were difficult to fabricate by conventional low-temperature processes, such as electrochemical and EPD. Hahn et al. (2011) chose the aerosol deposition (AD) method to
improve the properties of the coatings, which can deposit dense and well-adherent
ceramic coatings on metal substrates at room temperature.
HA nanocrystalline powder (volumetric mean diameter of 15 nm) was heated at
1050
C for 2 h. The chitosan powder (DD ¼ 85%) was dry ball-milled for 12 h in
a planetary mill using ZrO
powders containing 5, 10, and 20 wt% of chitosan were prepared by a simple drypowder mixing process and then used for coating by AD over the entire surface of
hot-rolled AZ31 Mg alloy plates.
The coatings showed different microstructures depending on the amount of chitosan added (Figure 11.12). The rough surface with a network structure of HA coating
became smoother, with the amount of chitosan increasing. In addition, the coatings
balls and a jar. Thereafter, the HA e chitosan composite
2
316 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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o
o
o
o
o
o
Intensity / a.u.
o
o
o
o
o
o
20 30 40 50 60 70 80 90
o
o
o
o
o
o
o
o
o
o
o
2 theta / deg.
HA
o
Ca(OH)
Mg
o
o
o
o
(b)(a)
2
D-VI
D-V
o
o
o
o
o
o
o
o
o
o
HA
Ca(OH)
o
DCPD
o
o
o
K-VI 3 days
o
o
o
K-V 3 days
(c)
HA
Ca(OH)
2
o
o
2
DCPD
K-VI 15 days
o
o
o
K-VI 10 days
D-IV
Intensity / a.u.
K-IV 3 days
Intensity / a.u.
K-V 10 days
D-III
D-I
10
20
30 40 50
2 theta / deg.
60 70 80 90 20 40 60 80
K-III 3 days
2 theta / deg.
K-IV 10 days
Figure 11.10 XRD patterns of the coatings (a) prepared from five electrolytes on the MAO-AZ91D Mg alloy substrate by EPD. (b) After immersion in
PBS for 3 days. (b) After immersion in PBS for 10 and 15 days. D-I, K-I coatings were prepared from the E-I electrolyte; D-III, K-III coatings were
prepared from the E-III electrolyte; D-IV, K-IV coatings were prepared from the E-IV electrolyte; D-V, K-V coatings were prepared from the E-V
electrolyte; and D-VI, K-VI coatings were prepared from the E-VI electrolyte.
Surface modification by natural biopolymer coatings on magnesium alloys 317
Figure 11.11 SEM images of four coatings after immersion into PBS for 3 days. (a) K-III
converted from the D-III coatings. (b) K-IV converted from the D-IV samples. (c) K-V converted from the D-V coatings. (d) K-VI converted from the D-VI coatings.
Adapted from Wu et al. (2010).
showed fairly dense cross-sectional microstructures (Figure 11.12) without obvious
pores or cracks.
The adhesion strengths between the AD coatings and substrates were 27.7e24.6
MPa, which is higher than those of electrochemically deposited HA coatings and
EPD HA coatings. The results of the protective property of the coatings investigated
by electrochemical measurem ents at 37
C in SBF are listed in Table 11.4. It was
found that the coating formed on the AZ31 alloy significantly promoted its resistance
to corrosive degradation, even though the addition of chitosan slightly lowered the
corrosion resistance of the HA coating.
The MC3T3-E1 preosteoblast cell line attached on the samples after 5 h of incubation was used to characterize the cell attachment behaviors on the HAechitosan composite coatings. The field-emission gun (FEG) scanning electron microscopy (SE M)
morphologies of the cells presented in Figure 11.13 showed that more multiple filopodia of the cells were found to spread on the surface of the HAechitosan composite
318 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Figure 11.12 SEM micrographs of the surfaces of the (a) pure HA, (b) HA-5 wt% chitosan, (c)
HA-10 wt% chitosan, and (d) HA-20 wt% chitosan composite coatings deposited on the AZ31
Mg alloy substrates. Cross-sectional SEM micrographs of (e) the pure HA and (f) HA-20 wt%
chitosan composite coatings deposited on AZ31 Mg alloy substrates.
Adapted from Hahn et al. (2011).
Table 11.4 Electrochemical corrosion parameters derived from
potentiodynamic polarization
Corrosion
potential: E
corr
(V)
AZ31 1.733 3.839 10
HA 1.569 4.765 10
HA-5 wt% chitosan 1.601 1.515 10
HA-10 wt% chitosan 1.581 3.144 10
HA-20 wt% chitosan 1.586 5.097 10
Adapted from Hahn et al. (2011).
Corrosion current
density: I
corr
(A/cm2)
4
6
5
5
5
coating with increasing content of chitosan, implying that the biocompatibility was
improved by the incorporation of chitosan.
This AD method used gas as a solvent to avoid an aqueous solution, which
reduced the corrosion probability of the active magnesium substrate. Meanwhile,
superior to the electrochemical deposition and the EPD coating method of HA/chitosan, this method does not require MAO preprocessing or later stage conversion,
which makes it eas ier and lower in cost.
Surface modification by natural biopolymer coatings on magnesium alloys 319
Figure 11.13 Filed-emission gun SEM micrographs of the MC3T3-E1 cells attached to the
samples: (a) uncoated AZ31 substrate, (b) HA coating, and HAechitosan composite coatings
with (c) 5 wt% and (d) 20 wt% chitosan.
Adapted from Hahn et al. (2011).
11.4 Stearic acid modification
11.4.1 Introduction to stearic acid
Stearic acid, another name for octadecanoic acid CH3(CH2)16COOH, is one of the
most common fatty acids. I t exists as a glycerol ester in most animal and plant fats
(Beare-Rogers, Dieffenbacher, & Holm, 2001). Stearic acid is more abundant in animal fat (up to 30%) than vegetable fat (typically <5%). The important exceptions
are cocoa butter and shea butter, in which the stearic acid content ( as a triglyceride)
is 28e45%. Unlike the other long-chain saturated fatty acids, stearic acid has no
effect on lipoprotein cholesterol concentrations in men or women (Yu, Derr,
Etherton, & Kris-Etherton, 1995). Results from the study by Kelly et al. (2001)
indicate that stearic acid (19 g/day) in the diet has favorable effects on thrombogenic and atherogenic risk factors in males; the authors recommend that the food
industry consider enriching foods with stearic acid instead of palmitic acid and
trans fatty acids. Thus, stearic acid is nontoxic and biocompatible with the human
body. With a polar head group that can bind with metal cations and a nonpolar
chain that confers solubility in orga ni c solv ent s, ste ar ic aci d is c ommo nly us ed
in the production of detergents, soaps, and cosmetics, such as shampoos and
shaving cream products.
320 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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11.4.2 Stearic acid coating on magnesium
A study by Ng et al. (2010) attempted to employ stearic acid (SA, CH3(CH2)16COOH)
as an organic coating for enhancing the corrosion resistance of Mg. The organic
coating was prepared by a two-step process using hyd rothermal treatment to form a
Mg(OH)
For comparison, three different coating conditions were used (Table 11.5). It was
concluded that higher viscosity resulted in a thicker coating film.
of Mg samples. When the samples were soaked in SA, the SA diffused easily into the
hydroxide layer due to the open structure of the latter, and the reaction took place
according to Eqn (11.11)
the structure of the coating. As the SA filled up the space between the hydroxide
branches, it reacted with the Mg(OH)
contributed to the strong adhesion of SA coating to the substrate.
different preparation conditions. The calibration curve was obtained from Fourier
transform infrared spectroscopy for spectra of different mixtures of MgSt and SA
using the peaks at 1702 cm
MgSt to SA (by weight) were 49.7:50.3, 33.8:66.2, and 39.6:60.1 for SA1eMg,
SA2eMg, and SA3eMg, respectively. Among these samples, for the thickest
coating layer (230 mm), the surface of SA2Mg was relatively smooth and compact
(Figure 11.15(aec)). The differences in composition could be attributed to the
different soaking temperatures, leading to different degrees of completion of the reaction and differences in viscosity as well.
Hanks’ solution. The results of electrochemical test (Table 11.5) showed that among
the coated samples, where Rt (Rt ¼ Rpo þ Rp) stands for the total corrosion resistance of a potymer-coated metal sample. Rpo and Rp are the “pore resistance” of
the coating and the polarization at the coating/substrate interface, respectively.
SA2-Mg exhibited the best corrosion resistance due to its thicker and better quality
film. After immersion in Hanks’ solution (Figure 11.16) bare Mg corroded severely
after 6 days of immersion. For SA1eMg, the surface also underwent heavy corrosion
attack after 6 days of immersion, consistent with the results of EIS (Electrochemical
Impedance Spectroscopy) measurements. However, SA2eMg still kept almost its
original appearance, even after 80 days of immersion.
Figure 11.17. In the initial period, the corrosion resistance of SA2-coated Mg was four
orders of magnitude greater than bare Mg and gradually dropped to about 40 times in the
long run, which facilitated the maintenance of mechanical strength of implants in the
layer at 120C for 24 h, then immersion in SA to form an organic coating.
2
After hydrothermal treatment, a loose layer of Mg(OH)
was formed on the surface
2
MgðOHÞ
þ 2CH3ðCH2Þ16COOH/CH3ðCH2Þ16COO
2
Mg þ H2O
2
(11.11)
The schemati c diagram of Figure 11.14 proposed the mechanism of formation and
to form magnesium stearate (MgSt), which
2
Figure 11.15 showed different surface morphologies of the coating layers in
1
for SA and at 1573 cm1for MgSt. The ratios of
Electrochemical measurements and long term immersion test were conducted in
The variation of the corrosion resistance between SA2eMg and bare Mg is given in
Table 11.5 Summary of coating conditions and corrosion parameters
Surface modification by natural biopolymer coatings on magnesium alloys 321
Viscosities
Polarization test Day 5 (pH 5.5) Day 30 (pH 7.4)
of SA or SA
Sample
designation
solution
(centipoise)
Coating
thickness(mm)
Bare Mg ee 1.80 0.25 10
HT-Mg e 170 1.71 4.0 10
SA1-Mg 0.5 180 1.49 0.12 10
SA2-Mg 5.2 230 1.45 11.2 10
SA3-Mg 2.3 210 1.45 0.14 10
Rt(Rt¼ Rpoþ Rp) is the total corrosion resistance of a polymer-coated metal sample. Rpoand Rpare the pore resistance of the coating and the polarization resistance at the coating/substrate
interface, respectively.
E
corr
(VSCE)
I
corr
(A/cm2)
E
oc
(VSCE)
3
1.94 0.03 1.78 25
6
Not tested eee
6
1.86 0.7 ee
9
1.48 700 1.54 107
6
1.69 43 1.62 102
R
t
(kUcm2)
E
oc
(VSCE)
R
t
(kUcm2)
322 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Figure 11.14 Schematic diagram showing the structure of organic coating on Mg.
Adapted from Ng et al. (2010).
(a)
(b)
(c) (d)
Figure 11.15 Optical micrographs showing the cross-section of different Mg samples (coating
thickness shown in parentheses): (a) HTeMg (170 mm), (b) SA1eMg (180 mm), (c) SA2eMg
(230 mm), and (d) SA3eMg (210 mm).
Adapted from Ng et al. (2010).
bone healing period. Along with the degradation of the coating, apatite phases formed
on the sample surface (Figure 11.18), showing the potential excellent biocompatibility.
To sum up, MgSt worked as a bridge between Mg substrates with SA, and the
higher viscosities of SA could produce a thicker and more compact coating. The corrosion resistance had been significantly improved by SA-coated HT-Mg.
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