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54 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Chemical solution deposition
of hydroxyapatite and
3
octacalcium phosphate coatings
for magnesium and its alloys
to improve biocompatibility
Sachiko Hiromoto
National Institute for Materials Science, Tsukuba-shi, Ibaraki, Japan
3.1 Introduction
Improvement and control of corrosion resistance are an essential issue for the practical
application of bioabsorbable Mg/Mg alloys to biomedical devices. Here, the control of
corrosion resistance implies the retardation of corrosion initiation and/or reduction of
the corrosion rate; the former and the latter correspond to the retention of original
strength until the init iation of corrosion and the gradual degradation of strength,
respectively. Implant materials should show good biocompatibility, depending on
the implantation sites. Surface coating is an effective method to improve both corrosion resistance and biocompatibility of metallic materials; other methods include the
alloying and control of microstructure.
In the case of conventional orthopaedic and dental metallic biomaterials such as
Ti alloys, calcium phosphate compounds, especially hydroxyapatite (HAp), are often
coated on the surface to improve biocompatibility, mainly bone conductivity, because
HAp is a main component of bones (Kasuga, 2010; Leon & Jansen, 2009). The bone
conductivity of HAp coatings depends on the crystallinity and how different the composition is from the stoichiometric composition (Layrolle & Daculsi, 2009). As an artificial
bone, b-tricalcium phosphate (b-TCP) and octacalcium phosphate (OCP), in addition to
HAp, are examined to control the dissolution rate and bone conduction behaviours, since
b-TCP and OCP are soluble in physiological environments in which stoichiometric HAp
is stable, and OCP is a precursor of HAp (Ioku, 1996; Layrolle & Daculsi, 2009). Additionally, the prior study revealed that the deposition of calcium phosphate on pure Mg
improves the corrosion resistance in a simulated body fluid (Kuwahara, Al-Abdullat,
Mazaki, Tsutsumi, & Aizawa, 2001). Therefore, calcium phosphate compounds, such
as HAp, b-TCP and OCP, are promising coating materials for bioabsorbable Mg/Mg
alloys to retard corrosion initiation and control corrosion rate thereafter.
It is important to develop a coating method that can control the coating thickness
and the crystal structure, crystallinity and composition of the calcium phosphate compounds on the surface of Mg/Mg alloys. Preferably, the coating method would be
Surface Modification of Magnesium and its Alloys for Biomedical Applications. http://dx.doi.org/10.1016/B978-1-78242-078-1.00003-7
Copyright © 2015 Elsevier Ltd. All rights reserved.
60 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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simple and applicable to any shape of device. The chemical solution deposition (CSD)
method satisfies these requirements. However, the HAp formation on Mg/Mg alloys in
aqueous solutions is difficult because Mg/Mg alloys easily corrode under such conditions, and dissolved Mg ions prevent the crystallisation of HAp (Bigi et al., 1993). In
some cases, amorphous calcium phosphate or a precursor of HAp such as dicalcium
phosphate anhydrous, dicalcium phosphate dihydrate or b-TCP, is electrochemically
deposited in the first step; subsequently, they are crystallised or transformed to HAp
in an alkaline solution in the second step (Song, Shan, & Han, 2008; Song, Zhang,
Li, Zhao, & Zhang, 2010). In other cases, dissolution of Mg ions, i.e. corrosion of a
Mg substrate, is prevented with the addition of a fluoride (NaF, HF, etc.) in the pretreatment solution or the calcium phosphate solution (Bakhsheshi-Rad, Idris, &
Abdul-Kadir, 2013; Jo et al., 2011; Song et al., 2010; Wen et al., 2009). Then HAp
and/or other calcium phosphate compounds including fluoridated hydroxyapatite are
electrochemically deposited concurrently or in the second step. Since fluorides are
not environmentally friendly, a fluoride-free process is preferable.
We developed a novel CSD method for the single-step formation of HAp and OCP
coatings on Mg/Mg alloys (Hiromoto, 2009; Hiromoto & Tomozawa, 2010; Hiromoto &
Tomozawa, 2011; Tomozawa, Hiromoto, & Harada, 2010; Tomozawa & Hiromoto,
2011a, 2011b). Formation of HAp and OCP can be controlled with the pH of the treat-
ment solution and the treatment period (Hiromoto & Tomozawa, 2011; Tomozawa &
Hiromoto, 2011b). The thickness of HAp and OCP coatings can be controlled with
the treatment period (Tomozawa & Hiromoto, 2011a, 2011c). The formation mecha-
nism and chemical and mechanical properties of an OCP- and HAp-coated Mg/Mg alloy
will be introduced in this chapter.
3.2 Hydroxyapatite and octacalcium phosphate coatings
formed by a chemical solution deposition technique
3.2.1 Chemical solution deposition
CSD is a coating method in which the coating is formed with the reaction between the
substrate surface and components of the solution (Yoshimura, 2009). This method is
named following the name ‘chemical vapour deposition’. CSD is a kind of soft
solution-processing performed using a simple experimental setup under atmospheric
pressure and a temperature below 373 K. While the CSD reaction can be accelerated
(activated) with hydrothermal and electrochemical reactions, the simple CSD process
without extreme heating or electrochemical power is more beneficial for industries.
The simple CSD can be applied to low heat-resistant materials, complicated shape substrates and porous materials. The refore, CSD attracts attention as a low-cost and environmentally friendly coating process. On the other hand, CSD is not often employed in
industries because it is difficult to form a coating with CSD in many cases. Dots or
islands, rather than a coating, are easily formed on the substrate due to the preferential
growth of the nuclei over the formation of new nuclei. To form a uniform coating, precise control of the deposition conditions is necessary.
Chemical solution deposition of hydroxyapatite and octacalcium phosphate coatings 61
The biomimetic coating method (a kind of CSD) is used for the calcium phosphate
coating of conventional metallic biomaterials (Kokubo, 1996; Liu & Hunziker, 2009)
because the biomimetically formed coating is expected to show better biocompatibility
than coatings formed under extreme conditions, such as high temperature. In a typical
biomimetic process to form bone-like hydroxyapatite, pre-treated substrate metals and
alloys are immersed in a Kokubo’s simulated body fluid (SBF) or Hank’s solution
supersaturated with calcium phosphate under physiological conditions of 310 K and
pH 7.4. When the biomimetic method is applied to an Mg/Mg alloy, HAp coatings
with very low crystallinity are formed (Al-Abdullat et al., 2001; Keim, Brunner, Fabry,
& Virtanen, 2011; Kuwahara et al., 2001). This is because the SBF and Hank’s solu-
tion contain chloride ions, which cause the severe corrosion of Mg/M g alloys; in addition, the dissolved Mg ions prevent the crystallisation of HAp (Bigi et al., 1993).
Therefore, a new simple CSD method is needed for Mg/Mg alloys to form a coating
consisting of well-crystallised HAp and OCP. The treatment solution should not
contain chloride ions or environmentally hazardous or biologically toxic substances.
The treatment temperature should be below 373 K. It was supposed that the prevention
of HAp crystallisation by Mg ions can be overcome by rapid formation of a coating to
stop the corrosion of Mg/Mg alloys as soon as possible. An increase in the concentration of Ca and phosphate ions is one way to achieve that goal; however, the solubility
of inorganic Ca salts, except for CaCl
acetic acid calcium disodium salt hydrate (Ca-EDTA: C
, is not sufficiently high. Ethylenediaminetetra-
2
10H12N2O8Na2
Ca), a
Ca-chelate compound, was then employed because its solubility in aqueous solution
is higher than that of inorganic Ca salts. Additionally, it has been used for the synthesis
of HAp on titanium, aluminium, copper and iron substrates (Fujishiro, Sato, & Oku-
waki, 1995; Fujishiro, Yabuki, Kawamura, Sato, & Okuwaki, 1993). Ca-EDTA
showed a wide pH range for the formation of OCP and HAp coatings comparable
to other calcium chelate compounds such as N-(2-Hydroxyethyl)ethylenediamine-
0,N0
N,N
-triacetic acid calcium salt (Ca-EDTA-OH: C10H16N2O7Ca), calcium gluco-
nate (C
CaO14) and calcium lactate (C6H10CaO6).
12H18
Deposition of calcium phosphate compounds on Mg/Mg alloys is suggested to
initiate from the corrosion of the Mg/Mg alloy substrate, as illustrated in Figure 3.1
(Hiromoto, 2009). Generation of OH
ions in the cathode reaction raises the surface
pH, leading to the nucleation of calcium phosphate compounds on the surface. The
pH increase is limited to the surface, which is the key to forming a uniform coating
rather than dots and islands.
3.2.2 Influence of deposition conditions
According to the proposed deposition mechanism shown in Figure 3.1, temperature,
pH and concentration of Ca-E DTA and H
coating behaviour. The influence of these factors on the coating behaviour was then
examined.
Treatment temperature needs to be higher than a certain level to dissociate Ca-EDTA
and to initiate the calcium phosphate deposition reaction. The calcium phosphate compound did not deposit at a temperature below 313 K (Figure 3.2). At a temperature
n
ions influence the calcium phosphate
xPO4
(deg
)
62 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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dissociation
Ca(EDTA)
Mg / Mg alloy
2–
Mg(EDTA)
reaction
Calcium phosphate formation reaction
Thermal
2+
Ca
EDTA
2–
2+
Mg
Anode
Mg
H
2PO4
4–
Mg(OH)
–
–
e
, (OH–)
2
Ca
(PO4)6(OH)
10
Ca8(HPO4)2(PO4)4·5H2O
OH–, H
2
H2O
Cathode
reaction
2
Figure 3.1 The proposed mechanism of calcium phosphate deposition on Mg/Mg alloys using
Ca-EDTA by chemical solution deposition method.
(a)
HAp
Mg(OH)2
Mg
373 K
363 K
Intensity
353 K
(d)
Pure
Mg
(c)
(b)
333 K
313 K
0 102030405060
2θ
ree
Figure 3.2 (a) X-ray diffraction (XRD) patterns of pure Mg treated with 50 mmol/L of
Ca-EDTA and KH
at pH 8.9 and various treatment temperatures for 8 h. Scanning electron
2PO4
microscopy (SEM) images of pure Mg treated at (b) 313 K, (c) 333 K and (d) 363 K.
From Tomozawa et al. (2010), with permission.
above 313 K, deposition of HAp could be observed. The amount of deposition and crystallinity of HAp increased with an increase in temperature, as evidenced by the increase
in intensity and sharpness of the diffraction peaks originating from HAp (Figure 3.2)
(Tomozawa et al., 2010). The HAp deposition was also enhanced by an increase in
Ca-EDTA concentration (Figure 3.3)(Tomozawa et al., 2010). Concurrently, micro-
pitting (localised corrosion) of substrate Mg/Mg alloys was accelerated with an increase
(deg
)
Chemical solution deposition of hydroxyapatite and octacalcium phosphate coatings 63
HAp
Mg
Pure Mg
(c)
250 mmol / L
Intensity
(b)
50 mmol / L
(a)
10 mmol / L
0 102030405060
Figure 3.3 X-ray diffraction (XRD) patterns of pure Mg treated at pH 8.9 and 363 K for 2 h
with (a) 10 mmol/L, (b) 50 mmol/L and (c) 250 mmol/L of Ca-EDTA and KH
From Tomozawa et al. (2010), with permission.
2θ
ree
2PO4
.
in temperature and Ca-EDTA concentration. The pitting density of the substrate Mg
alloy treated at 363 K was higher than that at 333 K (Figure 3.4). Since fatigue
behaviour of Mg/Mg alloys is sensitive to micro-pits, as mentioned later (Hiromoto,
Tomozawa, & Maruyama, 2013; Khan, Miyashita, Mutoh, & Koike, 2008), the treat-
ment temperature and concentration of Ca-EDTA should be decided to achieve a
balance between the coating speed and the magnitude of substrate pitting.
The crystal structure of calcium phosphate compo unds varied with the pH of the
treatment solution. OCP and HAp were formed roughly below and above pH 7, respectively, on pure Mg and AZ31 alloy (Figure 3.5)(Tomozawa et al., 2010). At a pH
below 6, monetite (CaPO
(OH) and CaHPO4) was apt to be formed over the OCP layer
3
as a by-product (Hiromoto, Inoue, Taguchi, Yamane, & Ohtsu, in press). In the pH
range from neutral to weak alkaline, OCP transformed to HAp with an increase in
treatment period, and a mixture of OCP and HAp can be formed. Depending on the
kind of Mg alloy, b-TCP was deposited at a relatively low pH, while b-TCP did not
form a uniform coating (Figure 3.6). The deposition of b-TCP at 363 K was surprising
because b-TCP is formed only at more than 1073 K according to thermodynamic stability (Layrolle & Daculsi, 2009). In this case, it is suggested that the Mg alloy released
a large amount of Mg ions that were incorporated into the deposited calcium phosphate
compound and caused the formation of b-TCP.
3.2.3 Influence of Mg/Mg alloy substrate
The morphology and structure of calcium phosphate coatings depends on the composition and microstructure of Mg alloys, since the corrosion behaviour of Mg/Mg alloys
depends on these factors (Shaw, 2003; Song, 2011). Pure Mg, AZ31, AZ61, AZ91 and
64 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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(a)
(b)
(c) (d)
Figure 3.4 Scanning electron microscopy (SEM) images of WE43 treated with 250 mmol/L of
Ca-EDTA and KH
substrate WE43 treated at (c) 363 K and (d) 333 K after chemically removing the hydroxyapatite
(HAp) coatings.
at pH 8.3 and (a) 363 K and (b) 333 K for 1 h. SEM images of the
2PO4
WE43 alloys were treated under the same condition as that for HAp coating. The
amount of HAp deposited varied depending on Mg and Mg alloys, as indicated by
the intensity of the X-ray diffraction (XRD) peak at around 26 degrees from HAp
(Figure 3.7(a)). The amount of HAp deposited on pure Mg was larger than that on
AZ series alloys. This fact is attributed to the lower corrosion resistance of pure Mg
compared to that of the AZ alloys. The low corrosion resistance causes a rapid increase
in the surface pH, leading to an enhanced nucleation of HAp. The amount of HAp
deposited increased with an increase of Al content because of the enhancement of
micro-galvanic corrosion between the matrix and b-phase (second phase), which is
apt to precipitate with an increase of Al content of AZ alloys. In the case of WE43,
Y preferentially precipitated in grain boundaries, and an Mg-Y intermetallic compound sometimes precipitated inside grains as a second phase (Sato, 2007). Microgalvanic coupling between matrix and Mg-Y intermetallic compound might accelerate
the corrosion in the treatment solution, leading to the deposition of a relatively large
amount of HAp on WE43. The acceleration of corrosion on WE43 compared to the
AZ alloys and pure Mg was indicated by the deposition of b-TCP only on WE43 under
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