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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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418 Surface Modification of Magnesium and its Alloys for Biomedical Applications
TOT
]
2+
[Mg
0.010
(a)
0.0
0.1
0.2
0.3
0.4
0.5
(b)
Mg2+MgHPO
2
O(s) Mg(OH)2(c)
4.3H2
(PO4)2(s)
Mg
3
410128
6
pH
[Mn2+]
[PO
[Mg
[Mn
3–
4
2+
]
2+
= 10.00 mM
TOT
]
TOT = 10.00 mM
= 500.00 mM
TOT
]
= 10.00 mM
TOT
0.008
TOT
0.006
]
3–
[PO
4
H3PO
4
MgH2PO
H2PO
MgHPO
4
+
4
–
4.3H2
O(s)
Mg3(PO4)2(c)
Mg
(PO4)2(s)
3
0.004
0.002
0.000
24
6
810
12
pH
Figure 15.5 Thermo-equilibrium predominance area diagram for the Mn2þ,Mg2þand PO
3
4
ions calculated using the MEDUSA software package (a and b). It can be seen that Mg(OH)
(exists as intermediate layer) is the dominate deposition when bath pH is high, and then MgPO
and MnPO4precipitate orderly along with pH decrease (outer layer).
2
4
PO
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MgO
OH
Biocompatible strontium-phosphate and manganese-phosphate conversion coatings 419
–
2+
Mn
3–
+
H
4
pH 4.0
(b) (c) (d)(a)
2H+ + 2e– H
H2OOH–+ H
Mg Mg2++ 2e
pH 4.9
2
+
–
2+
Mg
pH 10.5
2+
+ PO3– Mg3(PO4)
Mg
4
Mg + 2OH– Mg(OH)
2
2
pH 5.2
Mg2++ PO3– Mn3(PO4)
pH 5.6
4
2
T = 5 min , t = 80 oC, [Mn2+] = 0.01 M, [PO3–] = 0.01 M
Figure 15.6 Schematic presentation of the formation process of MnPO4coating on Mg alloy AZ91D.
4
420 Surface Modification of Magnesium and its Alloys for Biomedical Applications
2.5x10
Survey o
2.0x10
1.5x10
10
Intensity / counts
Mg
5.0x10
0
1200 1000 800 600 400 200 0
Mn
C
P
Mg
1.4x10
O1s
1.2x10
10
8.0x10
6.0x10
Intensity / counts
4.0x10
2.0x10
0
540 538 536 534
542544
PO
4
532 528 526
O-Mn
530
Binding energy / eV Binding energy / eV
700
Mg2p
600
500
400
300
Intensity / counts
200
100
Mg-PO
4
56 54 52 50 48 46 44
MgO/Mg(OH)
2
3800
Mn2p
3600
3400
3200
3000
2800
2600
Intensity / counts
2400
2200
2000
Mn 2p
1/2
660 658 656 654 652 650 648 646 644 642 640 638 636
Mn 2p
3/2
Binding energy / eVBinding energy / eV
2000
P2p
1800
1600
1400
1200
1000
800
Intensity / counts
600
400
200
142 140 138 136 134 132 130 128 126
PO
4
Binding energy / eV
70
Depth profile
60
50
40
30
20
10
Atomic concentration / %
0
0 500 1000 1500 2000 2500
Sputter time / sec
O 1s
Mg 2p
AI 2p
Zn 2p
P 2p
Mn 2p
Figure 15.7 XPS analysis of the MgPO4conversion coating, survey, high resolution of O1s,
Mg2p, Mn2p, P2p, and depth profile of the coating formed after 5 min of immersion at pH4-
80
C, revealing the existence of elements of O, Mg, P and Mn, and their distribution over the
coating system.
Reprinted from Chen et al. (2013), with permission.
the corrosion kinetics. It was noted that the cathodic reactions were profoundly
inhibited by the MnPO
Mg(OH)
, which acts as an inhibitor of oxygen and consequent cathodic kinetics,
2
coating, attributed to the formation of intermediate insoluble
4
similar to that of chromate coatings on Al alloys as Kendig and Buchheit presented
(Kendig & Buchheit, 2003). A breakdown potential point and pseudo-passivation
region are evident in all anodic branches, revealing that the anodic dissolution reaction
has also been retarded (Figure 15.8(a) and (b)).
Biocompatible strontium-phosphate and manganese-phosphate conversion coatings 421
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–
.4
(a)
SCE
/ V
corr
E
(b)
SCE
/ V
corr
E
(c)
–2
–Z” / ohms cm
pH = 4.0
–
.6
–
.8
–1.0
–1.2
–1.4
–1.6
–1.8
–2.0
–
8–
–
.4
T = 80 º C
.6
–
.8
–
–1.0
–1.2
–1.4
–1.6
–1.8
–2.0
8– 7–
–
16000
pH = 4.0
14000
12000
10000
8000
6000
4000
2000
0
–2000
–2000
0
Bare AZ91D
RT
40 ºC
50 ºC
60 ºC
70 ºC
80 ºC
7–
6– 5– 4– 3– 2
6– 5– 4– 3–
2000
4000
Z’ / ohms cm
i
i
corr
corr
/ A cm
/ A cm
6000
–2
–2
8000
–2
10000
Bare AZ91D
pH 2.0
pH 4.0
pH 6.0
Bare AZ91D
RT
40 ºC
50 ºC
60 ºC
70 ºC
80 ºC
12000
14000
2
16000
Figure 15.8 Potentiodynamic polarisation curves of various AZ91D obtained in 0.1 M NaCl at
a sweep rate of 1 mV/s (a, b). Nyquist plots of the various AZ91D obtained in 0.1 M NaCl with a
frequency range from 100 kHz to 10 mHz (c, d). (e) 3D plot of the corrosion current density i
corr
(z axis) against treating temperature (x axis) and pH (y axis). (f) The equivalent circuit used for
analysis of the EIS data of various AZ91D. (R
phase element of a film on the sample surface. R
, solution resistance. Rc, resistance and constant-
s
,Q,Qdl, the charge transfer resistance of a
ct
coating, constant phase element of the charge transfer and charge transfer capacitance of a
double layer, respectively. W, Warburg element accounting the diffusion of species.)
Reprinted from Chen et al. (2013), with permission.
422 Surface Modification of Magnesium and its Alloys for Biomedical Applications
16000
(d)
–2
T = 80 º C
14000
12000
10000
8000
6000
4000
–Z” / ohms cm
2000
0
– 2000
–
2000 2000 4000 6000 8000 10000 1200014000 160000
Z’ / ohms cm
Bare AZ91D
pH 2.0
pH 4.0
pH 6.0
–2
(e)
0
2
4
6
8
10
12
14
6.0
5.5
pH
–2
–6
14
12
10
A cm
/ 10
corr
i
8
6
4
2
0
70
60
50
Temperature / º C
5.0
4.5
4.0
3.5
3.0
40
2.5
2.0
30
(f)
R
s
Q
Q
R
c
dl
R
ct
W
Figure 15.8 Continued.
In terms of EIS characterisation (Figure 15.8(c) and (d)), only one capacitive loop
and a significant increase in diameter of the capacitive loop were observed for the
MnPO
coatings, correlating with a passive nature (Forsyth et al., 2009). The EIS
4
data were simulated using the equivalent circuit presented in Figure 15.8(f) (EC-Lab
package, version 10.2). The parameters R
element representing a non-ideal capacitance related to the coating), R
resistance), Q
to the double layer), R
(constant phase element representing a non-ideal capacitance related
dl
(the charge transfer resistance) and W (representing the War-
ct
(the solution resistance), Q (constant phase
s
(the coating
c
burg element, which represents the diffusion of species) calculated. This equivalent
circuit was chosen due to being the most simple representation of a filmed surface
that can accommodate local breakdown/defects. Long-term salt spray evaluation
(Figure 15.9) demonstrated that the optimal MnPO
conversion coating, generated
4
Biocompatible strontium-phosphate and manganese-phosphate conversion coatings 423
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Salt Spray for 72 h
Bare AZ91D pH4-RT
pH2-80ºCpH4-80ºC pH6-80ºC
pH4-40ºC
pH4-70ºCpH4-60ºCpH4-50ºC
Figure 15.9 Various AZ91D alloys after salt spray for 48 h according to ASTM B117 standard.
(Scale bar, 20 cm.)
Reprinted from Chen et al. (2013), with permission.
at the mild pH and high temperature, offered a corrosion protection similar to that of
the DOW chromate conversion coating (Ger, Yang, Sung, Hwu, & Liu, 2004; Yang,
Tsai, Huang, & Lin, 2012).
15.3.2 Advantages and disadvantages over other coating
technologies
MnPO4possesses some unique advantages and disadvantages over other current
coating technologies. Bracci et al. stated that Mn ions hinder precipitation of highly
crystalline HA and lead to formation of amorphous Mn-CaPO
have a higher solubility in aqueous solution and a greater amount of Mn ions are
released into the incubation medium, though no such data has been disclosed. Such
a high content of Mn in the CaPO
coating demonstrates a greater promotion of oste-
4
oblast differentiation and mineralisation, compared with incorporation of Sr and Mg
(Bracci et al., 2009). Beneficial impacts on osteoblast proliferation, activation of their
that is supposed to
4
424 Surface Modification of Magnesium and its Alloys for Biomedical Applications
metabolism and differentiation have also been de monstrated (Bigi et al., 2005; Gy€orgy
et al., 2004). Addition of divalent Mn ions into HA not only alters its morphology but
also reduces its crystallinity, which is favourable for adsorption and disappearance
during bone remodeling (Mayer et al., 2008; Sopyan et al., 2011).
Apart from Ca, a major component of bone, Mn is one of the trace elements, and thus
it is critical to determine the tolerable level of daily intake when MnPO
is used as a
4
coating on an implant. The research results associated with this aspect originated from
Mn-doped coatings and are not consistent, as discussed in the introduction section.
Thus, it is an urgent task to undertake relevant investigations of acceptable release rates
of Mn (II) cations from implants into the human body. The characteristic cracked structure of MnPO
conversion coating is an issue, as these cracks may be vulnerable sites for
4
corrosive medium to penetrate and initiate galvanic corrosion, which would incur
cracking, which will jeopardise the desirable mechanical properties.
15.4 Applications and future development
Biocompatible SrPO4-based conversion coatings are one of the promising candidates
to protect Mg orthopaedic implants from rapid in vivo degradation and promote bone
fracture recovery. The newly developed SrPO
stage of immersion in MEM and thus effectively prolongs the lifespan of the underlying Mg as supporting materials with desirable mechanical properties. Meanwhile,
degradation products of SrPO
new tissue formation. Therefore, SrPO
contain Sr cations, which promote cell growth and
4
4
biomaterial perspective. Such coatings have a promising future to control the degradation rate of Mg implants and achieve a faster healing process. In addition to its biomedical functions, its superior protectiveness against corrosion and low cost of production
may make it applicable where conventional steel and Al components are deemed to be
replaced by light-weight Mg, such as transportation, electronic and household appliances. A good example for such industrial applications of SrPO
can be seen in Figure 15.3, which demonstrates the outstanding corrosion protection
of SrPO
on a mobile phone chassis made of die-cast Mg alloy AZ91D. After exposure
4
to salt spray for 72 h, no evident corrosion spots emerge on the surface covered by
SrPO
, compared with bare AZ91D, CaPO4and MnPO4coatings.
4
The research works on biocompatible Sr-based coatings are still rare compared with
other divalent metal-phosphate coatings. Chen and his colleagues’ work has identified
the way forward, but intensive and in-depth exploration is desired to pave a way for
clinical trials. For instance, the tolerable level of Sr cations releasing into the human
body has not yet been identified. In vitro cell culture trials are necessary and should
be expanded to cover more cell categories to obtain reliable results on cytotoxicity.
Afterwards, in vivo implantation should be conducted to evaluate the biocompatibility
and bioactivity of SrPO
dation of SrPO
-coated Mg, only some preliminary investigations have been per-
4
coatings in real biomedical scenarios. In terms of biodegra-
4
formed by Chen et al.; thus, in-depth, systematic and long-term evaluations are still
needed.
coating degrades slowly at the initial
4
plays a dual role that is favourable from a
conversion coating
4
Biocompatible strontium-phosphate and manganese-phosphate conversion coatings 425
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MnPO4-based conversion coatings could play a positive role in bone formation
andnewtissuegrowthoverMgimplantsurfaces as long as their degrada tion
process can be controlled at a biocompatible level; however, more data are required
and current data are unsatisfactory. Currently, MnPO
coatings have been specif-
4
ically used to reduce the corrosion rate of Mg engineering components rather
than for the biomedical field. A few Mn-doped ceramic coatings, such as HA,
TCP and TiO
, have been attempted, to perform its beneficial function to acti vate
2
cell proliferation and growth. B ut there are disputes about what is the maximum
amount of Mn ions released from implant that can b e safely consumed by the
human body. In vitro and in vivo investigations ar e necessary prior to performing
clinical trials.
In addition to biological considerations, some other characteristic aspects are also
essential when the end-users make the decision on whether or not to adapt a new
coating technology for Mg-based implant materials. Wear resistance is critical when
the coating is applied onto a component that will be subject to continuous abrasion
or impact. The influence of wear debris on the surrounding tissues should also be
monitored. High bonding strength between coating and implant is desirable to achieve
satisfactory bone-implant integration, in particular in the early post-implantation
period. Since cells are sensitive to surface features ranging from the nanoscale to
the mesoscale, a key coating design criterion will be to provide appropriate topography
to facilitate cell differentiation even without the requirement for an appropriate surface
chemistry.
15.5 General discussion
It is of significant importance to reduce the high biodegradation rate of Mg-based
implant materials to get full fracture recovery prior to the degradation of their mechanical strength. Thus, a large number of biocompatible and protective coatings have been
developed to address this challenge. Of these, Sr- and Mn-based coatings have
emerged as new candidate materials for Ti implants in the past few years, although little work relevant to Mg biomaterials has been reported. One of the major reasons may
be the toxicity concern regarding release of excessive Sr and Mn ions, which are nominally trace elements in the human body. Nowadays, even the tolerable threshold of
daily intake of such elements, in particular the long-term safety, is still not certain.
It is therefore important to carry out further in vitro and in vivo trials regarding biocompatibility of Sr- and Mn-based protective coatings prior to commercialisation or clinical assessments, given their superior performance in biodegradation reduction for Mg
alloys.
The SrPO
bio-literature, as the corrosion protection herein is high (even very high in respect to
coatings for non-bio or structural Mg alloys). Three key factors e the high crystallinity, 100% surface coverage and defect-free morphology e lead to the superior
performance. Meanwhile, the high crystallinity imparts a low solubility and longtime stability to the SrPO
conversion coating described herein is differentiated from others in the
4
coating, which degrades slowly when immersed in
4
426 Surface Modification of Magnesium and its Alloys for Biomedical Applications
Table 15.2 Summary of the details of SrPO
and MnPO4coatings
4
(Chen et al., 2013, 2014)
SrPO
4
Colour Light grey Dark grey Vision inspection
Uniformity Homogenous Homogenous SEM observation
Chemical
composition
Adhesion 23.5 2.8 MPa 19.4 3.1 MPa Modified ASTM
Thickness 1.9 0.2 mm 1.1 0.3 mm SEM cross-
Roughness 8.5 0.6 mm 12.3 1.9 mm 3D profilometry
Mg(OH)
MgHPO
Mg
3
Sr apatite
,
2
,
4
(PO4)2and
biological environments. The details of the SrPO4and MnPO4coatings developed by
Chen et al. have been listed in Table 15.2.
The highly protective coating can delay the initial burst of degradation products of
Mg implants on exposure to the human body, which is often a concern in biomedical
applications. Excessive release of corrosion products, including fragments of coating
substances, will incur an advers e effect on the surrounding cells. The results presented
by Chen et al. (2014) also demonstrate that the SrPO
ation of hMSC, thus providing a potential solution to avoid delayed or non-uniform
fracture. This is important in the context of biodegradable Mg, as rapid initial
in vivo degradation results in serious physiological effects. Long-term biocompatibility
investigation of the coating material after the entire degradation process is essential to
clarify how it would perform after biomedical implantation. The preliminary data set
presented herein is encouraging, and future work will aim to assess the biocompatible
mechanism of the SrPO
MnPO
coatings have been investigated for decades to perform a protective
4
conversion coatings using in vivo animal models.
4
function on iron, steel, Zn and now Mg alloys. But none of these coatings has ever
been used for biomedical applications, because highly toxic Mn (VII) and Mn (III)
are common impurities in the coatings. As a result, biocompatible ceramic coatings
have been doped with Mn (II) cations only to facilitate reaction between implant
and bone tissue via promoting ligand-binding affinity of integrins and activating cell
adhesion. Whilst the MnPO
coating described in this chapter was designed for the
4
general purpose of corrosion protection, it is worth conducting in vitro and in vivo
trials on biocompatibility since there were no present toxic impurities.
Phosphatation is an endothermic reaction; thus, coating procedures conducted at
low temperature do not provide sufficient energy to generate phosphates satisfactorily.
MnPO
Mg(OH)
(Mn/Mg)
4
and
2
(PO4)
3
coating promotes the prolifer-
4
Methodology
XRD and XPS
2
C-633 (Chen
et al. (2014))
sectional
observation
Biocompatible strontium-phosphate and manganese-phosphate conversion coatings 427
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Under such circumstances, the phosphating rate is either slow or does not take place at
all. Consequently, long immersion times, up to a few days, may be required to give rise
to a complete phosphate film e with extremely thin coatings being undesirable for
corrosion resistance. In contrast, high processing temperatures can offer enough activation energy and eventually accelerate the phosphating rate. A thick phosphate film
can be achieved in a relatively short time and render corrosion resistance to Mg
substrates.
Bath pH is another crucial impact that significantly influences MnPO
coating
4
morphology and thickness. A low pH (2.0) bath results in a coating with higher Mn
and P content and greater thickness but evident network structure. The characteristic
network structure of MnPO
-based conversion coatings can be correlated to the
4
hydrogen evolution process rather than dehydration effects. Corrosive medium will
readily penetrate this thick barrier coating and contact the substrate through the defect
sites in the network structure and eventually deteriorate corrosion resistance. On the
contrary, higher pH (6.0) induces a rough but thin coating with a lower Mn and
P weight fraction. This can be attributed to the low acidity that inhibits the dissolution
of Mg subst rate and consequently moderates the nuclei formation rate of MnPO4
coating. Thus, processing parameters have a profound influence on the final coating
properties and should be optimised carefully.
15.6 Summary
SrPO4-andMnPO4-based conversion coatings are emerging technologies for the
modification of the morphology and composition of the surface of Mg implant materials for tailoring the biodegradation/corrosion rate. Chemical conversion coating
is the simplest technique to achieve stable, compact and corrosion-resistant coatings.
However, processing parameters, such as bath pH and concentration, temperature
and time, need to be optimised for premium coating properties. In the present chapter, it was attempted to review the recent developments and highlight the important
issues associated with Sr- and Mn-based conversion coatings. Reported research on
biocompatible Sr and Mn coatings for Mg alloys is limited. Little work so far has
been focused on their in vitro or in vivo performance, in particular for long-term
biocompatibility.
CaPO
conversion coatings have been fabricated to reduce corrosion of Mg
4
components in some commercial applications, including orthopaedic implants. Of
these, HA plays a superior role in corrosion inhibition owing to its high crystallinity
(low solubility) and stability compared to its counterparts, such as DCPD and TCP.
However, according to the complex equilibrium states of CaPO
components, it is
4
difficult to achieve HA coatings without impurities, which will deteriorate its protective function for Mg alloys. Thus, SrPO
chemical nature to Ca. The results demonstrate that the SrPO
is proposed to replace HA owing to its similar
4
coating, fabricated via a
4
simple chemical conversion process, provides superior protection for Mg against
biodegradation following initial immersion in MEM.
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