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84 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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2
Surface design of biodegradable
magnesium alloys for biomedical
3
applications
P.K. Chu, G.S. Wu
City University of Hong Kong, Hong Kong, China
3.1 Introduction
Surface is crucial to biomaterials because it not only provides a platform for cell/
biomaterial interactions and associated chemical reactions, but also acts as a shield
to resist corrosion or wear in the physiological environment. Magnesium (Mg), as
the fourth most abundant cation in the human body, is essential to human metabolism
and naturally found in bone tissues (Staiger, Pietak, Huadmai, & Dias, 2006).
However, compared to traditional biometals such as Ti-based alloys, biodegradable
Mg alloys have a more reactive surface and normally degrade too quickly, especially
in the initial stage under physiological conditions (Jame sh, Kumar, & Narayanan,
2011; Song, 2007; Witte et al., 2006; Wu, Zhao, Zhang, Jamesh, & Chu, 2013).
Adverse effects can occur to the patients if excessive Mg ions and other corrosion
products cannot be effectively absorbed by the surrounding tissues or excreted properly. Moreover, rapid degradation will bring about premature failure, thereby inducing
Mg-based implants to lose the desirable ability of osteosynthesis and counteracting the
capability of Mg alloys to mitigate the stress-shielding effect as a result of their
Young’s modulus (E ¼ 41e45 GPa) being similar to that of bones (E ¼ 3e20 GPa)
(Wu, Zhao, et al., 2013; Zberg, Uggowitzer, & L€offler, 2009).
Surface modification is one of the desirable methods to overcome the drawbacks and
expedite the development of new-generation biodegradable metals (Narayanan, Park, &
Lee, 2014; Wu, Jamesh, & Chu, 2013). A temporary surface fabricated on Mg-based
materials can be used to tailor the mechanical performance, corrosion behavior, and
biological properties to meet clinical requirements. In this chapter, the common surface
treatment techniques suitable for Mg alloys and the design principles in the development of degradable Mg alloys to address clinical needs are discussed. The role of the
various coating techniques and ion implantation are described with examples.
3.2 Surface modification techniques
Several coating technologies are applicable to Mg and its alloys, including electrochemical plating, conversion coatings, anodizing, organic coatings, and vapor-phase
processes. Each of them has special advantages and limitations, and they are brie fly
described in the following sections.
Surface Modification of Magnesium and its Alloys for Biomedical Applications. http://dx.doi.org/10.1016/B978-1-78242-077-4.00003-6
Copyright © 2015 Elsevier Ltd. All rights reserved.
90 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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3.2.1 Electrochemical deposition
Electrochemical deposition or plating is an old but practical technique to deposit
metallic coatings. It is usually subdivided into two types: electroplating and electroless
plating. In both cases, the metal salt in a solution is reduced to the metallic form on the
surface of the substrate. The distinction between electroplating and electroless plating
is that the electrons for reduction in electroplating are supplied by an external source,
whereas the reducing electrons in electroless plating are supplied by a chemical
reducing ag ent in the solution or, in the case of immersion plating, the substrate itself
(Gray & Luan, 2002).
3.2.1.1 Electrodeposition
In electrodeposition or electroplating as schematically illustrated in Figure 3.1, the surface of a substrate is modified in an aqueous or nonaqueous electrolytic environment
by applying external power. In the electrolysis cell, the sample being plated is the cathode and immersed in a solut ion containing the requi red metal in an oxidized form. The
anode is made of the metal to be deposited on the substrate. When power is turned on,
the met al atoms are oxidized and dissolved in the solution according to the following
reaction: Me(s) / Me
electrolyte are reduced at the interface between the solution and the cathode according
to the reaction: Me
solved is equal to the rate at which the cathode is plated. Hence, the ions in the bath are
continuously replenished by the anode. In addition, a nonconsumable anode such as
lead or carbon can be used in electrodeposition. In this case, ions of the metal to be
plated must be periodically replenished in the bath after they have been extracted
from the solution. At present, electroplating is widely used in the industry to coat
nþ
(aq) þ ne. On the cathode, the dissolved metal ions in the
nþ
(aq) þ ne/ Me(s). Usually, the rate at which the anode is dis-
Figure 3.1 Schematic
illustrating the setup in
electrodeposition.
Source: Sudagar et al. (2013)
and Carraro et al. (2007)
with permission from Elsevier.
e
Anode
+–
–
Power
n+
Me
Cathode
–
e
Surface design of biodegradable magnesium alloys for biomedical applications 91
metallic products. The technique can provide protection against species in aggressive
environments and even render the products’ special surface properties such as decorative effects (Carraro, Maboudian, & Magagnin, 2007; Kanani, 2005; Schlesinger
& Paunovic, 2010; Sudagar, Lian, & Sha, 2013).
3.2.1.2 Electroless deposition
In comparison to electrodeposition, electroless deposition or plating uses only one
electrode and no external power source. The electroless deposition process can be
divided into two types: autocatalytic deposition and galvanic displacement. In autocatalytic deposition, reduction of metallic ions in the solution and film deposition can be
carried out by oxidation of the chemical compound in the solution itself, that is, a
reducing agent. This reducing agent at a defined temperature that depends on the
reducing agent and bath composition can spontaneously oxidize and free electrons
for the reduction of metallic ions. Thus, it is named autocatalytic because oxidation
of the reducing agent can start or become self-sustained only on the deposited metal
surface. Figure 3.2 shows a schematic of electrole ss deposition with the reducing agent
as the source of electrons. Galvanic displacement or immersion plating has a mechanism different from that of autocatalytic deposition. In immersion plating, reducing
agents are not required because the base materials can behave as the reducing agent.
Galvanic displacement takes place when the base material is displaced by a metallic
ion in the solution having a lower oxidation potential than the displaced metal ion.
As a sequence, the base material is dissolved in the solution and the metallic ions in
R
R
R
R
R
R
–
e
n+
M
M
n+
M
ElectrolyteSubstrate
Figure 3.2 Electroless deposition with
reducing agent (R) as the source of electrons.
Source: Sudagar et al. (2013) and Carraro et al.
(2007) with permission from Elsevier.
n+
M
92 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Specimen rotator
Thermometer
(90 °C)
Electroless nickel
bath
Water area
Figure 3.3 Experimental apparatus of electroless nickel plating.
Source: Li et al. (2006) with permission from Elsevier.
Thermostat
Teflon cap
Samples
Temperature control
the solution are reduced on the surface of the base material (Carraro et al., 2007;
Kanani, 2005; Schlesinger & Paunovic, 2010; Sudagar et al., 2013).
Figure 3.3 depicts a simplified schematic of the apparatus used in electroless nickel
plating. This apparatus consists of a plating tank, thermost at for temperature control,
and specimen rotator. The samples are placed in a glass tank covered with a Teflon
cap with the thermostat (Li, An, & Wu, 2006). Electroless plating has advantages
over electroplating because power sources are not needed. Autocatalytic deposition
can also avoid the effects of current distribution, thereby improving the thickness uniformity. Therefore, it is more suitable for plating components with a complex shape.
The typical disadvantages are that the plating process is usually slower and cannot
create thick coatings. Consequently, electroless deposition is commonly used for
decorative purposes under mild working conditions.
3.2.2 Chemical conversion coatings
3.2.2.1 General aspects
Chemical conversion is one of the important coating techniques to prepare coatings on
metals by converting the part of the surface into the coating by means of a chemical or
electrochemical process. The produced surface layer can be composed of metal oxides,
chromates, phosphates, or other compounds that are chemically bonded to the surface.
Because conversion coatings are formed in situ, adhesion to the subst rate is generally
very good, and so conversion as a pretreatment is effective in improving adhesion of
the final coating. Several different types of conversion coatings have been developed
by, for instance, chromate, phosphate/permanganate, and fluorozirconate treatments.
One of the main disadvantages of conversion coatings is the toxicity of the treatment
solutions. The conventional conversion coatings are based on chromium compounds
that have been shown to be toxic and carcinogenic, and it is imperative to develop
environmentally friendly processes. Besides, conversion coatings suffer from the
nonuniform surface composition. If a conversion coating with uniform composition
is needed, all the elements should be present in the alloy uniformly. At present,
Surface design of biodegradable magnesium alloys for biomedical applications 93
conversion coatings are mainly used for corrosion protection, hardness improvement,
and color change as well as paint primers (Gray & Luan, 2002; Hornberger, Virtanen,
& Boccaccini, 2012).
3.2.2.2 Anodization and microarc oxidation
Anodization is a type of classical electrochemical conversion. It encompasses electrode reactions in combination with an electric field-driven metal and oxygen ion diffusion, leading to the formation of an oxide film on the anode surface. The structural and
chemical properties of the anodic oxides can be varied over a wide range by altering
the process parameters, such as anode potential, electrolyte composition, temperature,
and current. Anodic oxidation is a well-established method and can produce different
types of protective oxide films on metals with excellent adhesion and bonding
(Liu, Chu, & Ding, 2004).
Microarc oxidation (MAO), also known as plasma electrolytic oxidation (PEO), is
an electrochemical surface treatment process based on anodizing. Compared to conventional anodizing, it uses much higher potentials. Figure 3.4 presents the schematic
of microarc oxidation and the related electrical circuit. The sample is immersed in a
bath containing the special electrolyte and forms one of the electrodes in the electrochemical cell, with the other counter-electrode being made from an inert material such
as stainless steel. Potent ials of more than 200 V are applied between these two
electrodes. They may be in the forms of continuous or pulsed direct current (DC) or
alternating pulses in which the stainless steel counter electrode may be grounded.
When the potential exceeds the dielectric breakdown potential of the oxide film,
discharges occur, resulting in localized plasma reactions to modify the oxide. Similar
to conventional anodic oxidation coating, the coating adheres better to the substrate
Copper anode bar
Sample
Electrolyte
Stainless steel counter electrode
Figure 3.4 Schematic of microarc oxidation.
Source: Dunleavy et al. (2009) with permission from Elsevier.
Variable number
of capacitors 2–150 μF
100 × V probe
1 × V probe
1Ω
Scope ground
Single phase
50 Hz AC
PC
Oscilloscope
94 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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metal than that formed by plasma spraying, because the MAO coating is a chemical
conversion of the substrate metal into its oxide growing both inward and outward
from the original metal surface. This process can produce thick (tens or hundreds of
micrometers) and largely crystalline oxide coatings on metals such as titanium (Ti),
aluminum (Al), and Mg. The coating provides high hardness, a continuous barrier
against wear, corrosion, and heat, and electrical insulation. Usually the coating properties depend on the substrate as well as composition of the electrolyte and electrical
regime (Curran & Clyne, 2005a, 2005b, 2006; Dunleavy, Golosnoy, Curran, & Clyne,
2009).
3.2.3 Physical vapor deposition
In physical vapor deposition (PVD), atoms or molecules are deposited onto a substrate
from a vapor phase. The process can be roughly divided into three steps: generation of
particles from the target materials, transportation, and film growth. To obtain the desirable films, substrate temperature, particle energy, reactive gas properties, and film
density need to be tailored in the PVD process. Compared to wet coating methods
such as electroplating and microarc oxidation, PVD is a dry coating method and boasts
unique merits such as low deposition temperature, strong adhesion, multicomponent
layers, and high coating density. Evaporation, sputter deposition, and ion plating are
the three main types of PVD (Gray & Luan, 2002; Liu et al., 2004).
3.2.3.1 Evaporation
Evaporation is one of the most widely used thin film deposition techniques. A solid
material is heated in a vacuum chamber to a temperature that generates some vapor
from the material. In vacuum, the vaporized particles can travel directly to the target
or substrate without encountering too many collisions and condense to the solid state,
forming a film. Generally speaking, an evaporation system requires a main chamber,
vacuum pump, and energy source that evaporates the materials. The source is normally
placed on the bottom of the chamber, often in an upright crucible because it becomes
liquid during heating in most cases. The substrates are held inverted by suitable
fixtures on the top of the chamber with surfaces to be coated facing down toward
the vapor source. A filament or electron beam source is typically used to vaporize
the materials (Harsha, 2006; Mattox, 2010a; Wasa, Kanno, & Kotera, 2012).
3.2.3.2 Sputtering
Sputtering is another widely used thin film fabrication technique. Sputtering is a
process in which ionized atoms are accelerated to a target surface to eject atoms
from the surface. The ejected atoms are then condensed onto a sample to be plated,
forming a thin film composed of sputtered materials. Sputter deposition has many advantages over other deposition methods such as evaporation, electroplating, and chemical vapor deposition (CVD). For example, sputter deposition can form smooth, dense,
conformal, and continuous films more easily than evaporation because it produces a
high-energy flux that leads to high surface mobility on the substrate surface. Usually,
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