Accepted Manuscript
Title: Surface Modification on Biodegradable Magnesium Alloys as
Orthopedic Implant Materials to Improve the Bio-Adaptability: A Review
Author: Peng Wan, Lili Tan, Ke Yang
PII: S1005-0302(16)30057-3
DOI: http://dx.doi.org/doi: 10.1016/j.jmst.2016.05.003
Reference: JMST 710
To appear in: Journal of Materials Science & Technology
Received date: 2-11-2015
Revised date: 17-12-2015
Accepted date: 18-12-2015
Please cite this article as: Peng Wan, Lili Tan, Ke Yang, Surface Modification on Biodegradable
Magnesium Alloys as Orthopedic Implant Materials to Improve the Bio-Adaptability: A Review,
Journal of Materials Science & Technology (2016), http://dx.doi.org/doi:
10.1016/j.jmst.2016.05.003.
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Surface Modification on Biodegradable Magnesium Alloys
as Orthopedic Implant Materials to Improve the
Bio-adaptability: A Review
Peng Wan, Lili Tan, Ke Yang*
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
[Received 2 November 2015; Received in revised form 17 December 2015; Accepted 18
December 2015]
*Corresponding author. Tel.: +86 24 23971628; Fax: +86 24 23971628.
E-mail address: kyang@imr.ac.cn (K. Yang).
Magnesium (Mg) and its alloys as a novel kind of biodegradable material have
attracted much fundamental research and valuable exploration to develop its clinical
application. Mg alloys degrade too fast at the early stage after implantation, thus
commonly leading to some problems such as osteolysis, early fast mechanical loss,
hydric bubble aggregation, gap formation between the implants and the tissue.
Surface modification is one of the effective methods to control the degradation
property of Mg alloys to adapt to the need of organism. Some coatings with bioactive
elements have been developed, especially for the micro-arc oxidation coating which
has high adhesion strength and can be added with Ca, P, Sr elements. Chemical
deposition coating including bio-mimetic deposition coating, electro-deposition
coating and chemical conversion coating can provide good anticorrosion property as
well as better bioactivity with higher Ca and P content in the coating. From the
biodegradation study, it can be seen that surface coating protected the Mg alloys at the
early stage providing the Mg alloy substrate with lower degradation rate. The
biocompatibility study showed that the surface modification could provide the cell
and tissue stable and weak alkaline surface micro-environment adapting to the cell
adhesion and tissue growth. The surface modification also decreased the mechanical
loss at the early stage adapting to the load-bearing requirement at this stage. From the
interface strength between Mg alloys implants and the surrounding tissue study, it can
be seen that the surface modification improved the bio-adhesion of Mg alloys with the
surrounding tissue which is believed to be contributed to the tissue adaptability of the
surface modification. Therefore, the surface modification adapts the biodegradable
magnesium alloys to the need of biodegradation, biocompatibility and mechanical
Page 1 of 18
loss property. For the different clinical application, different surface modification
methods can be provided to adapt to the clinical requirements for the Mg alloy
implants.
Key words: Bio-adaptability; Coating; Biodegradable; Magnesium alloys; Orthopedic
implants
1. Introduction
Metallic materials play an important role as biomaterials to assist with the repair
or replacement of bone tissue that has become diseased or damaged due to their
combination of high mechanical strength and fracture toughness, which are more
suitable for load-bearing applications compared with ceramics or polymeric
materials[1]. However, the limitations of current metallic biomaterials are the
unmatched elastic moduli with that of natural bone tissue and possible release of toxic
metallic ions and/or particles through corrosion or wear processes[2,3]. Moreover
metallic biomaterials remain as permanent fixtures which must be removed by a
second surgical procedure after healing[4].
Magnesium-based metals can readily dissolve or corrode in aqueous solutions,
which inspire the biomaterial researchers to develop a new concept of degradable
implants. Over the last decade, research interest is rapidly growing in fundamental
research and valuable exploration to develop its clinical application. The rapid
degradation of magnesium however is a double edged sword, and after implantation it
will lead to some problems such as osteolysis, early fast mechanical loss, hydric
bubble aggregation, gap formation between the implants and the tissue[5]. Thus it is
necessary to control the corrosion rate of the materials to satisfy their clinical
demands. In response, surface modification has been suggested as the effective
methods to control the degradation of Mg alloys.
Moreover, there is a need for this new generation of biodegradable implants
which should be able to stimulate the healing responses of injured tissues at the
molecular level. For example in bone graft strategies, it should provide
osteoinductivity, osteoconductivity, suitable degradation/resorption and replacement
by new bone tissue[6]. Moreover, it is also expected to be avoided from infection by
bacterial invasion.
There are lots of technologies available for coating magnesium and its alloys.
These include the micro-arc oxidation coating, chemical conversion coating,
electrodeposition coating, bio-mimetic deposition coating, etc.[7‒11]. Each of these will
Page 2 of 18
be described in detail in the following sections. Considering the demands of various
clinical applications, especially in orthopedics, different modification methods are
applied in view of their specific performances. The focus of this review concerns the
development of biocompatible and biodegradable coatings for Mg and its alloys, with
the intent of improving the bio-adaptability among degradation, mechanical integrity
and biocompatibility for orthopedic application.
2. Current Coating Technologies
2.1. Micro-arc oxidation coating
Micro-arc oxidation (MAO) treatment, also known as plasma electrolytic
oxidation, is a common technique for corrosion protection of magnesium alloys in the
industrial sector. In micro-arc oxidation, higher potentials are applied which locally
exceeds the dielectric breakdown potential of the growing oxide film and discharges
occur. These discharges result in localized plasma reactions, with conditions of high
temperature and pressure which modify the growing oxide. Processes include melting,
melt-flow, re-solidification, sintering and densification of the growing oxide[12]. By
the MAO process a relatively thick, dense and hard oxide coating can be produced on
the surface of magnesium alloys[13]. The coating is a chemical conversion of the
substrate metal into its oxide, and grows both inwards and outwards from the original
metal surface, which has excellent adhesion to the substrate and offers protection
against wear and corrosion.
2.2. Chemical conversion coating
Conversion coatings arise in a complex interaction of metal dissolution and
precipitation, usually during treatments in aqueous solutions. The chemical
conversion layers are obtained by immersion of substrates in a bath and show, besides
magnesium oxide and magnesium hydroxide, mixtures of other metal oxides and
hydroxides, which arise from the dissolved ions in the bath[5]. Such conversion
coatings represent an effective way to increase the corrosion resistance of magnesium
alloys or, as a pre-treatment, to improve the adhesion of a final deposited coating[14].
2.3. Electrophoretic deposition coating
Electrophoretic deposition (EPD) is a term for a broad range of processes which
include cathodic electrodeposition, anodic electrodeposition, and electrophoretic
coating. A characteristic feature of this process is that colloidal particles suspended in
a liquid medium migrate under the influence of an electric field (electrophoresis) and
are deposited onto an electrode. Fig. 1 presents a schematic illustration of the two
electrophoretic deposition processes[15]. This method mainly concerns the deposition
Page 3 of 18
of inorganic phases. The relevant literature reveals that cathodic electrodeposition
leads to better results in the production of HA layers than chemical conversion layers.
However, careful adjustment of the parameters is necessary. Sometimes there are
traces of the substrate mixed into the coating forming new phases, such that the
coating is not purely by deposition but also to some extent by conversion at the
interface.
3. Adaption to the Need of Biodegradation Property of Biodegradable
Magnesium Alloys by Surface Modification
The idea of biodegradable Mg implants was discarded a century ago because of
their rapid degradation. Recent advances in the design and processing of metal alloys
has revived interest in Mg-based materials and devices. Most researches have focused
on decreasing Mg degradation through alloying and surface treatment. However, the
potential long-term toxicity induced by addition of alloying elements (e.g., rare earth
elements) is a concern[16]. Coating is proved to be an effective method to make Mg
degradation tunable and biocompatible. It can be seen that surface coating protected
the Mg alloys at the early stage providing the Mg alloy substrate with lower
degradation rate[5]. From the viewpoint of corrosion resistance, different coatings
showed varied degradation behaviors, which is dependent on the formation
mechanism. Chemical conversion including MAO can provide better corrosion
resistant coating by the chemical reaction or plasma discharging oxidation.
The degradation behavior of MAO-coated Mg alloys in simulated body fluids
solution was widely studied by Lin et al.[17], Xu et al.[18] and Wan et al.[19]. In all
studies MAO showed a lower corrosion current density in polarization studies than
uncoated samples. Lin et al.[17] fabricated a forsterite-containing MAO coating on
ZK60 magnesium alloy and demonstrated the voltage influence on the morphology
and subsequent degradation property of the MAO layer. The corrosion resistance of
MAO coating was increased with the elevation of the preparation voltage. Besides,
the effect of various electrolytes on the electrochemical corrosion behavior of MAO
coating has previously been studied[20]. Liang et al.[21] also reported the influence of
pH on the deterioration of MAO coated AM50 alloy in NaCl solutions. Moreover, it is
known that the coating with typical porous structures is featured by MAO technique,
where the solution could penetrate at the pore places and corrode the matrix. Thus the
composite coating was fabricated outside the MAO coating to improve the corrosion
resistance[22]. In particular, a MAO coating with self-sealing structure was reported by
Gan et al.[23]. The Ca‒P compounds can be simultaneous deposited in the pore during
Page 4 of 18
MAO process (as shown in Fig. 2). Wang et al.[24] studied the in vitro and in vivo
degradation of this MAO coating with self-sealing structure. The results (Fig. 3)
showed the coating significantly retarded the in vivo corrosion after implantation for
12 weeks compared with the uncoated sample.
Fig. 4 summarized the percentage reduction on the corrosion rates of magnesium
alloys with different coatings reported in the literature compared with uncoated alloys.
It can be seen that polymer coatings (chitosan coatings[25], polycaprolactone and
dichloromethane coatings[26]), electrodeposition coatings (dicyclopentadiene (DCPD),
hydroxyapatite (HA) and fluoridated hydroxyapatite (FHA) coatings[27]), alkaline heat
treatment[28] and fluoride treatment[29] can reduce the corrosion rate of the magnesium
alloy substrate by approximately 50%‒80%. In comparison, a more than 90%
reduction in corrosion rate can be obtained for the MAO coatings[30]. Overall, the
MAO coating is very stable, hard and corrosion resistant. For orthopaedic implants,
MAO coating could be supposed to desired coating which can effectively slow down
the corrosion rate and also supply better adhesion and abrasion resistance.
4. Adaption to the Need of Biocompatibility Property of Biodegradable
Magnesium Alloys by Surface Modification
For biomedical applications coatings should possess, besides corrosion
protection, other functions, such as an enhancement of biocompatibility or
osseointegration in the case of orthopaedic applications, bioactivity, antibiotic ability,
or local drug delivery ability. As for orthopedic applications, bioactive coatings such
as calcium phosphate and fluoride-containing layers are the most interesting in this
category. Fig. 5 shows the model explaining the improvements due to the presence of
bioactive calcium orthophosphate coatings on Mg and its alloys[31].
One approach to obtain calcium phosphate-containing coatings is immersion in
simulated body fluids (SBF); this process is often termed bio-mimetic if carried out at
37 °C and a pH of 7.4. Various compositions of surface layers depending on the bath
solution were reported by Rettig and Virtanen[32], including amorphous carbonated
calcium/magnesium phosphate layers which formed after immersion in SBF solution
for 5 days. Moreover the adjustment of the phases during processing is a most
important requirement. The layers are often mainly amorphous, but they contain some
crystallized HA and also other calcium phosphate phases[5]. Chen et al.[33] used a
calculated equilibrium diagram to obtain a stable HA coating using a calcium nitrate
and sodium phosphate solution. Nevertheless, a post-treatment in alkaline solution
was necessary to develop a HA component within the coating. Wang et al.[34] obtained
Page 5 of 18
a Ca‒P coating in the AZ31 alloys (as shown in Fig. 6) and assessed the
biocompatibility via in vitro and in vivo tests. The results showed that the coating
could significantly decrease the happening of the hemolysis and showed better
osseointegration after implantation.
Electrodeposition (ED) has been suggested as another means of depositing CaP
coatings on Mg based on the extensive research and application of ED on clinical
titanium implants. Privious investigations reported ED as a successful means of
establishing single crystalline phases such as HA[27], brushite[35] or octacalcium[36].
Most authors described the obtained CaP coatings as porous or microporous, whilst
high-temperature sintering/annealing was mentioned and shown to create a more
dense and uniform CaP coating post-ED[37], with greater adhesion properties[38]. The
electrophoresis process has also been modified to include pulse currents as opposed to
a constant current in recent protocols. Adjustment of pulse current parameters and
electrolyte solution has been suggested as an effective means of controlling the
coating structure deposited on the substrate[35]. Qiu et al. fabricated a Si doped Ca‒P
coating by pulse ED and found that the double layer structure was formed due to
pulse parameters[39]. The coating with composition of DCPD showed excellent
biocompatibility for the orthopedic application. Moreover, to improve the bioactivity
and biofunction, many nutrient elements in the human body were doped into the HA
coating with formation of Si‒HA, Sr‒HA, FHA and Ag‒HA[40‒43]. The
biocompatibility study showed that the coating could provide the cell and tissue stable
and weak alkaline surface micro-environment which adapting to the cell adhesion and
tissue growth.
On the other hand, the rapid degradations of magnesium alloys could induce
some problems such as inflammation and hydrogen evolution of Mg implants[43],
which will affect their future clinical applications. The in vivo corrosion study by
Witte[44] shows that all magnesium implants exhibited clinically and radiographically
visible subcutaneous gas bubbles, which appeared within one week after surgery.
Song et al.[45] studied and compared the corrosion rate of different Mg alloys and
postulated hydrogen evolution rate 0.01 ml/cm2/day as a tolerated level in the human
body. Thus, the harm effects by hydrogen gas accumulation could be avoided, if a
favorable magnesium alloy with a suitable coating is applied as an implant material.
It was known that the processing and deformation will influence the corrosion of
magnesium alloys. Thus the degradation was not only dependent on the coating
protection, but also relevant to the processing status of the Mg alloy substrate.
Page 6 of 18
Carboneras et al.[46] studied the in vitro performance of magnesium processed by
different routes. It is found that the metallurgical route used to produce magnesium
has more significant consequences on biodegradation and biocompatibility than the
effects of the surface coating. Han et al.[47] studied the degradation behavior of
different processing status and also biological response of Mg‒Sr alloy for bone
substitutes. The results showed that the as-cast Mg‒Sr alloy exhibited a rapid
degradation rate compared to the as-extruded alloy due to the intergranular
distribution of second phase and micro-galvanic corrosion. However, the initial
degradation could be tailored by the coating protection, which was proved to be
cytocompatible and also suitable for bone repair observed by in vivo implantation.
The integrated fracture calluses were formed and bridged the fracture gap without gas
bubble accumulation (Fig. 7), meanwhile the substitutes simultaneously degraded[47].
Thereby, it is potential to obtain the tailored degradation according to the clinical
demands by regulation of microstructure of Mg alloys and combined appropriate
coating.
5. Adaption to the Need of Mechanical Integrity of Biodegradable Magnesium
Alloys by Surface Modification
Basically, the magnesium-based implants are expected to be used with a
temporary function, such as bone fixation and bone substitute. The implants should
provide a mechanical supporting especially in the load-bearing sites and promote
bone healing[48]. Thus an ideal implant for orthopedic application should be able to
compromise its degradation and mechanical integrity during implantation, as
illustrated in Fig. 8. Theoretically, degradation occurs in initial with a very slow rate
to maintain enough mechanical strength for the vessel forming and bone remodeling.
After a period of 3‒6 months, the bone reconstruction is expected to be completed[49].
Bakhsheshi-Rad et al.[50] developed a new surface treatment for the Mg‒Ca‒Bi
alloy, combining physical vapor deposition (PVD) and dip coating techniques. The
results showed that this coating has significantly higher compressive strength, thus
can sufficiently protect the alloy and enhance the mechanical properties. Shen et al.[51]
designed a bio-glass coated magnesium alloys with a combination of suitable
mechanical strength and adjustable corrosion resistance. This coating combined with
mild interfacial stress could improve the cohesion/adhesion strength.
Tan et al.[52] studied the loss of mechanical properties in vivo and bone-implant
interface strength of AZ31B magnesium alloy screws with Si-containing coating. The
interface strength was evaluated in terms of the extraction torque required to back out
Page 7 of 18
the screws. The loss of mechanical properties over time was indicated by one-point
bending load loss of the screws after these were extracted at different times. The
results showed that the extraction torque of coated AZ31B increased with
implantation time, and was higher than that of poly-L-lactic-acid (PLLA) after
4 weeks of implantation (as shown in Fig. 9). The bending loads of non-coated
AZ31B and PLLA screws degraded sharply after implantation, and that of coated
AZ31B degraded more slowly (as shown in Fig. 10).
From the interface strength between Mg alloys implants and the surrounding
tissue study, it can be seen that the surface modification improved the bio-adhesion of
Mg alloys with the surrounding tissue which is believed to be contributed to the tissue
adaptability of the surface modification.
The valid periods of the coating is significant to ensure good corrosion resistance
and the mechanical integrity of magnesium alloy implants. It was reported that the in
vitro degradation of MAO coating on Mg‒Ca alloys is over 50 days[30], which is
significantly greater than that of an alkaline heated and chitosan-coated sample, which
showed 10 days validity[25,28]. The studies on the in vivo degradation of MAO coating
showed that the implants could maintain most integrity at 12‒18 weeks[53].
Comparatively, the failure periods of DCPD, HA and FHA coatings were about
1 week, 2‒3 weeks and 4‒6 weeks[27], respectively, whereas a MgF2 coating was
degraded within 4 weeks after implantation[54].
Thus different coatings should be chosen according to the clinical application,
such as bone fixation, bone grafting, ACL reconstruction, and varied for different
tissues, e.g. 6‒12 weeks for upper limbs[55], 12‒24 weeks for lower limbs[56]. In
general, micro-arc oxidation (MAO) coating was preferentially used in bone fixation,
like screw (as shown in Fig. 11(a)), which was attributed to its abrasive and corrosion
resistance. Yuan et al. developed DCPD coating on JDBM alloys for bone fixation
application (Fig. 11(b))[57]. Also Chemical conversion coating[34] was employed for
plates and screws (provided by Trauson Holdings Company Ltd, as shown in Fig. 11(c,
d)). Regarding bone substitutes, Han et al.[47] developed Sr, Ca, P contained MAO
coating to tailor degradation and biological response for bone repair and
reconstruction.
Biodegradable magnesium-based metals are considered as the next generation of
metallic biomaterials and their efficiency and efficacy are proved by more animal
tests and clinical trials[58], which give us great confidence in the future development
on Mg-based implants and devices. Accompanied more devices and products
Page 8 of 18
developing and being approved by CE and other drug regulation administration,
coating will receive more attentions and exert impacts to ensure bio-safety, clinical
efficacy and bio-functions.
6. Conclusion
The previous literature have revealed that a wide range of coatings on Mg and
Mg alloys can increase the corrosion resistance of these materials. More functions
achieved by the coatings, besides increasing the corrosion resistance of the substrates,
are controllable degradability, mechanical maintainability and improved
osseointegration, as demonstrated by in vitro and in vivo testing. In this review, the
relevant adaptability between these performances was discussed. From the above
results, it can be concluded that the surface modification could adapt the
biodegradable Mg alloys to the need of biodegradation, biocompatibility and
mechanical loss property. For the different clinical application, different surface
modification methods can be provided to adapt to the clinical requirements for the Mg
alloy implants.
Acknowledgement
This work was financially supported by the National Basic Research Program of
China (973 Program, No. 2012CB619101).
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[57] J. Niu, G. Yuan, Y. Liao, L. Mao, J. Zhang, Y. Wang, F. Huang, Y. Jiang, Y. He, W.
Ding, Mater. Sci. Eng. C 33 (2013) 4833‒4841.
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Figure captions list:
Fig. 1. Schematic illustration of electrophoretic deposition process: (a) Cathodic EPD,
(b) anodic EPD[15].
Fig. 2. Morphologies of MAO coatings with self-sealing structure[23].
Fig. 3. 2D slice and 3D reconstruction morphologies of (a) the uncoated and (b)
coated sample at implantation periods of 4, 8, 12 weeks by HRTXRT[24].
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Fig. 4. Reduction in corrosion rate for magnesium alloys with different coatings
deduced from the literature[25‒30].
Fig. 5. A model explaining the improvements due to the presence of bioactive calcium
orthophosphate coatings on Mg and its alloys. (A) A relatively rapid degradation rate
of Mg might lead to formation of gaps at the interface. (B) A typical tetracycline label
taken 14 weeks post-operation. (C) Protective calcium orthophosphate coatings can
reduce the degradation rate and simultaneously ameliorate biocompatibility. (D)
Corrosion-protective effects of calcium orthophosphate coatings measured via the H2
release rate and the change in pH value[31].
Fig. 6. Morphology of Ca‒P coating fabricated in AZ31B alloy by conversion
method[34].
Fig. 7. Post-op X-ray images after implantation of (a) 4 and (b) 8 weeks for (1) as-cast
alloy, (2) as-extruded alloy and (3) as-cast alloy with coating[47].
Fig. 8. Illustration of an ideal model between mechanical integrity and degradation for
orthopedic application
Fig. 9. Results of extraction torque measurements on non-coated AZ31B, coated
AZ31B, PLLA and Ti6Al4V implanted in vivo for 1, 4, 12 and 21 weeks,
respectively[52].
Fig. 10. Variations of the one-point bending loads of non-coated AZ31B, coated
AZ31B and PLLA screws when extracted from the bones after 1, 4, 12 and 21 weeks
of implantation[52].
Fig. 11. The magnesium-based metallic devices with different coatings application, (a)
MAO coating on pure magnesium srew (Courtesy EON Co., Ltd), (b) DCPD coating
on JDBM alloys[57], (c) AZ31B screw with Si-containing coating[34], (d, e) chemical
conversion coated screw and plate for animal test (Courtesy Trauson Holdings
Company Ltd).
Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
Fig.5
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Fig.6
Fig.7
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Fig.8
Fig.9
Fig.10
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Fig.11
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