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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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Biodegradable polymeric coatings for surface modification of magnesium-based biomaterials 365
5.0
4.5
4.0
3.5
3.0
Glue
Mg
Film
2.5
2.0
1.5
Adhesion strength, MPa
1.0
0.5
0.0
PLLA (LMW)
PLLA (HMW)
PCL (LMW) PCL (HMW)
Figure 13.6 Adhesion strength of polymer coatings on magnesium metal.
From Xu & Yamamoto (2012a).
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
Corrosion rate, μm/day
1.5
1.0
0.5
0.0
Uncoated Mg PLLA (LMW) PLLA (HMW) PCL (LMW) PCL (HMW)
Figure 13.7 Corrosion rate of uncoated magnesium and magnesium coated with different
polymers after 10-day immersion in cell culture conditions.
From Xu & Yamamoto (2012b).
366 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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A surfacedegradation polymer,poly(1,3-trimethylene carbonate) (PTMC), was coated
on a magnesium alloy (Mg-Zn-Mn) using an evaporation method by Wang et al. (2013).
The degradation test results suggested that the PTMC coating protected the alloy from
degradation. The corrosion current density of the PTMC-coated alloy was reduced by
three orders and one order of magnitude as compared to that of bare and PCL-coated magnesium alloy, respectively. It was suggested that the homogeneous surface degradation of
the PTMC coating, i.e. exterior to interior (surface degradation behavior), and also its
charge neutral degradation products, contributed to its excellent performance. As shown
in Figure 13.8, the degradation rate of the PTMC-coated magnesium alloy was signifi-
cantly lower than that of the uncoated and PCL-coated magnesium alloy. They observed
that the PTMC-coated alloy exhibited the least increase of pH, Mg
2þ
release, and weight
loss during the degradation, as compared to the uncoated and PCL-coated alloy. Interestingly, even after 30 days of testing,the pH value of PTMC-coated alloy solution remained
at w7.4, whereas for the uncoated and PCL-coated alloy it increased sharply. Their postdegradation analysis revealed that the PTMC-coated alloy exhibited a smooth and homogeneoussurface with the coating intact, whereas the PCL-coated alloy exhibited a number
of areas of localised corrosion. They reportedthat PTMC on magnesium alloy was homogeneously thinned from the surface to the interior, which is a surface degradation mode,
and hence advantageous for long-term degradation control of magnesium alloy.
Adbal-hay, Barakat, et al. (2013) investigated the performance of electrospinning
coating of PLA on a magnesium alloy (AM50). They compared the electrospinning
method with a dip-coating method of coating PLA on the alloy. It was reported that
both the techniques significantly enhance the short-term degradation resistance of
the alloy. The corrosion currents decreased by 87.5% and 78.09%, for dip-coated
and electrospin-coated alloy, respectively, as compared to the bare alloy. However, under long-term testing the degradation resistance of the electrospinning-coated alloy
was superior to that of dip-coated alloy, which was porous in nature (Figure 13.9 ).
It was reported that in the dip-coating process, a thick layer could generate three polymer films: (1) a primer, to ensure good adhesion and protect the substrate; (2) an
Figure 13.8 Potentiodynamic
polarisation curves of magnesium
alloy (Mg-Zn-Mn) uncoated and
coated with two different polymers.
From Wang et al. (2013).
–0.9
–1.0
–1.1
–1.2
–1.3
–1.4
–1.5
–1.6
Potential (V)
–1.7
–1.8
–1.9
–2.0
–2.1
MgZnMn
MgZnMn-PCL
MgZnMn-PTMC
–10 –9 –8 –7 –6 –5 –4 –3 –2
lgi (A.cm
–2
)
Biodegradable polymeric coatings for surface modification of magnesium-based biomaterials 367
5.5
Uncoated
4.5
Nanofibers layer
3.5
2.5
1.5
0.5
–0.5
0
Dip-coating layer
48 96 144 192 240
)
2
Weight loss (mg/cm
288 336 384
Exposure time (h)
Figure 13.9 Weight loss of
uncoated and polymer-coated
(with different methods)
magnesium alloy (AM50).
From Abdal-hay, Barakat, et al.
(2013).
intermediate film, impeding the transport of aggressive species; and (3) a top-coating
film, on the substrate. With the electrospinning-coated alloy, in contrast, the polymer
was deposited layer-by-layer on the substrates, and thereby a fully interconnected
porous 3-D structure was generated. The authors suggested that the adhesion in electrospinning coating was via many points of interaction between the nonwoven polymer
layer and magnesium substrate and hence did not produce any delamination or/and
blistering on the polymer layer.
Based on the literature, it can be suggested that polymer-alone coating provides
magnesium-based materials only short-term resistance to degradation. Under longterm testing, it appears that the coating fails due to fluid penetr ation. Compared to
bulk degradation polymer, surface degradation polymer appears to be superior for
these applications.
13.6 Hybrid coatings
Due to the long-term ineffectiveness of polymer-alone coatings in controlling the
degradation of magnesium-based materials, hybrid coatings, i.e. a combination of
ceramics and biodegradable polymers, have been investigated in recent years. The
advantage of this type of coating is that the degradation properties of the base material
can be tailored effectively to produce implant materials with specific properties. Typically, the material is coated with a ceramic and then a top-coat of polymer is appli ed.
However, composite coatings have also been investigated.
Calcium phosphate (CaP) is a biocompatible ceramic, which is naturally present in
bone. Although CaP coatings on magnesium-based materials have shown some
improvement in terms of degradation resistance (Kannan, 2012a,b; Kannan & Orr,
2011; Kannan & Wallipa, 2012; Shadanbaz & Dias, 2011), one of the major issues
is the porosity in the CaP coatings. However, the porosity in the CaP coating can be
effectively sealed using biodegradable polymers for enhanced performance.
368 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Oosterbeek, Seal, Se itz, and Hyland (2013) coated CaP and other bioceramics
(A e MgCO
CaCO
3
, Mg(OH)2and NaHCO3;Be Ca10(PO4)6(OH)2, Mg(H2PO4)2and
3
) in combination with PLA on pure magnesium. They used an immersion
method to coat the bioceramics on the metal and then coated PLA onto the bioceramic
surface. The morphology of the two bioceramics was different, i.e. bioceramic A
exhibited a thin, plate-like structure, and the bioceramic coating B was found to be
a much thicker layer of elongated plate-like crystals. Interestingly, it was found that
both the composite coatings exhibited poor adhesion strength as compared to PLA
alone coating (Figure 13.10). The authors attributed this to the poor bonding between
the bioceramic layer and the magnesium substrate. All of the coatings provided some
resistance to degradation; however, the coatings blistere d and cracked due to gas evolution, a by-product of magnesium degradation. A correlation was drawn between the
maximum adhesion strength of the coating and the time taken for the cracking during
the degradation tests (Figure 13.11). It was suggested that as gas accumulates within
the blister, the pressure gradually increases, which leads to a gradually increasing
stress between the coating and magnesium substrate, and hence a coating with lower
adhesion strength will fail earlier.
Wang, Zhao, Chen, Li, and Zhang (2012) used an electrodeposition method to coat
dicalcium phosphate dihydrate (DCPD) on a magnesium alloy (Mg-Zn) and then coated
PCL by immersing the DCPD-coated sample in a 2 wt.% PCL chloroform solution.
The corrosion current of the hybrid coating was one-third of the DCPD-coated alloy.
Long-term tests also confirmed that the hybrid coating exhibited higher degradation
resistance as compared to the alloy coated with DCPD alone. Similarly, Kannan and
Liyanaarachchi (2013) coated CaP on a magnesium alloy (AZ91) using the electrode-
position method and further coated PLA using a spin-coating method. Their electrochemical degradation test results showed that the hybrid coating enhanced the
14
Adhesion
Shear
12
10
8
6
4
Max. adhesion strength (MPa)
2
00
A+PLA B+PLA PLA
7
6
5
4
3
2
1
Figure 13.10 Maximum adhesion and shear strength of PLA and two composite coatings on
pure magnesium.
From Oosterbeek et al. (2013).
Max. shear strength (MPa)
Biodegradable polymeric coatings for surface modification of magnesium-based biomaterials 369
300
250
200
Time to crack (h)
150
0
Figure 13.11 Correlation between adhesion strength of a polymer coating and time taken for the
coating to crack.
From Oosterbeek et al. (2013).
6 9 12 153
Adhesion strength (MPa)
degradation resistance of the alloy by more than two orders of magnitude as compared
to the bare alloy and one order of magnitude higher than that of the CaP-coated alloy,
after 1 h exposure in simulated body fluid (SBF). Long-term (48 h) experimental results
also confirmed that the hybrid coating performed better than the bare alloy and the
CaP-coated alloy. Importantly, the hybrid coating improved the localised degradation
resistance of the alloy significantly, which is critical for better in-service mechanical
integrity. The Rp of the alloy with hybrid coating decreased with increase in SBF exposure period; however, the Rp was appreciably high even after 48 h exposure to SBF.
While the bare alloy and CaP-coated alloy exhibited patches of localised degradation,
there was no evidence of localised attack on the alloy with the hybrid coating
(Figure 13.12).
Plasma electrolytic oxidation (PEO), also called micro-arc oxidation (MAO), is a
relatively new technique to produce a hard ceramic coating on metallic materials such
as magnesium and titanium alloys (Alabbasi, Kannan, Walter, Stomer, & Blawert,
2013; Sreekanth & Rameshbabu, 2011; Tang, Yu, Luo, & Wang, 2012). A major draw-
back of this technique is that it produces a coating with a large number of pores.
Research has been carried out to reduce the porosity of the coating for enhanced performance. Recently, Narayanan et al. (2014) have published a comprehensive review
article on MAO coating for magnesium-based biomaterials. Guo et al. (2011) developed
a composite micro-arc oxidation (MAO)-PLLA on a magnesium alloy (WE42) and
examined its degradation behaviour. It was reported that the PLLA coating effectively
sealed the microcracks and micropores on the surface of the MAO coating by physical
interlocking to interfere with the corrosion ions. The MAO-PLLA coated alloy exhibited
lower i
than the alloy with MAO-alone coating or the bare alloy. The hybrid coating
corr
on the alloy served as an effective barrier against the corrosive electrolyte. The authors
suggested that the PLLA sealing layer was connected with the outer layer by physical
interlocking and by penetrating into most of the pores and microcracks. Recently, Alab-
basi, Mehjabeen, Kannan, Ye, and Blawert (2014) produced a porous silicate-based
PEO layer on pure magnesium and then attempted to seal the pores using PLLA. The
370 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Figure 13.12 Photographs of
Bare alloy
CaP coated
uncoated and CaP-PLAecoated
magnesium alloy (AZ91) after
immersion in SBF for 48 h.
From Kannan & Liyanaarachchi
(2013).
1mm
—
CaP + PLA coated
PEO coating was developed using a pulsed constant current method, and PLLA was
coated using a two-step spin-coating method. Electrochemical test results showed that
the polarisation resistance of the PEO-PLLAecoated magnesium was close to two orders of magnitude higher than that of the PEO-coated magnesium. The polarisation
resistance of the PEO-PLLAecoated magnesium decreased with increase in exposure
time; however, it was higher even after 100 h, i.e. 6 times higher than that of the
PEO-coated magnesium after 48 h exposure (Figure 13.13). There was also no evidence
Figure 13.13 Polarisation
resistance of uncoated,
PEO-coated and
PEO-PLLAecoated magnesium in
SBF.
From Alabbasi et al. (2014).
)
2
(Ω.cm
R
6
10
5
10
4
10
3
p
10
2
10
0
24 48
Pure magnesium
PEO
PEO-PLLA
72
Time (h)
96
Biodegradable polymeric coatings for surface modification of magnesium-based biomaterials 371
Severe localized corrosion
Pure Mg
MgO / Mg(OH)
Localized corrosion
Polymer dissolution
Figure 13.14 A schematic representation of the degradation mechanism of PEO-PLLAecoated
magnesium as compared to uncoated and PEO-only coated magnesium.
From Alabbasi et al. (2014).
MgO
PEO
PLLA
2
of localised degradation attack on the PEO-PLLAecoated magnesium. A schematic representation of the degradation mechanism of the hybrid coating in comparison with uncoated and PEO-only coated magnesium was presented by the authors (Figure 13.14).
A few researchers have also investigated the performance of bioceramic dispersed
polymer coatings on magnesium-based materials. Abdal-hay, Amna, et al. (2013) produced a coating of PCL containing nano-hydroxyapatite (nHAP) on a magnesium
alloy (AM50). They used a dip-coating method to form this composite coating
(nHAP-PCL). The degradation results showed that both the PCL and nHAp/PCL composites showed improvement in the degradation resistance as compared to the bare
metal in SBF. Under long-term testing, the composite coatings exhibited more uniform
and superior resistance to degradation attack than PCL. The composite coatings displayed better mechanical integrity than the bare metal. Both the coated samples did
not show any significant weight loss during the initial immersion time, which suggests
that these coatings can provide protection during the initial service period. The tensile
strength of the coated and uncoated samples decreas ed by w13% and 34%, respectively, in comparison to unexposed magnesium alloy. Hahn et al. (2011) deposited
HAPechitosan using aerosol deposition method on a magnesium alloy (AZ31). The
produced coatings were dense and well-adherent to the alloy. The coatings exhibited
high adhesion strength, i.e. 24.6e27.7 MPa, and improved the degradation resistance
as compared to the bare alloy.
A recent study by Abdal-hay, Dewidar, Lim, and Lim (2014) reported the performance of Ti-O thin films and PLA on a magnesium alloy (AM50). The authors
used the EB-PVD method for coating Ti-O film, followed by dip-coating with PLA.
372 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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14
12
)
–2
10
8
6
4
2
0
evolution rate (ml cm
2
H
–2
–4
024681012141618
Immersion time/day
Ti-O/PLA composite sample
Mg bar
Figure 13.15 Hydrogen evolution rate of uncoated and Ti-O/PLA-coated magnesium alloy
(AM50) with different immersion periods.
From Abdal-hay et al. (2014).
It was found that the use of the composite coating layer decreased the hydrogen
evolution rate (Figure 13.15), which led the authors to suggest that this method may
be useful for magnesium-based implants during the initial stages of healing, since
this limits the formation of hydrogen gas around the host tissue.
It is evident from the literature that hybrid coatings have significantly improved the
general and localised degradation resistance of magnesium-based materials. The
research in the area of biodegradable magnesium-based materials for the next few
years will be primarily focused on hybrid coatings. However, it is important to understand the degradation tendency of the hard ceramics, i.e. PEO/MAO coatings, after the
bone healing process. In vivo studies should be carried out on these coated implants for
successful orthopaedic implant applications.
13.7 Conclusions
Biodegradable polymer coatings have proven to be beneficial for enhancing the degradation resistance of magnesium-based materials for biodegradable mini-implant applications. However, polymer coating alone might not solve the problem, since body fluid
can permeate through the polymer coatings, especially in bulk degradation polymers.
In this regard, surface degradation polymers could be more effective than bulk degradation polymers. However, poor adhesive strength of polymer-alone coatings has also
been reported for the poor long-term performance. On the other hand, hybrid coatings,
i.e. combinations of ceramics and polymers, have shown promising results and hence
could potentially delay the localised degradation of magnesium-based implants during
the initial service period, which is critical for better mechanical integrity particularly in
load-bearing orthopaedic implant applications.
Biodegradable polymeric coatings for surface modification of magnesium-based biomaterials 373
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