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198 Surface Modification of Magnesium and its Alloys for Biomedical Applications
was characterized as beach marks running from the circumference. SCC region mainly
consisted of transgranular cracking and a few cleavage marks, which may be indicative
of hydrogen-assisted cracking. Apart from transgranular cracking, a few localized pits
were also visible. Stress corrosion crack propagated transgranularly until the critical
stress intensity (i.e., fracture toughness) was attained. At this stage, the mechanical
overloading takes over, and the central part of the fracture surface revealed ductile failure characterized by dimple formation at a higher magnification (Figure 5.14(c)).
Biodegradable temporary implants are required to serve at least for a definite period
depending on their applications. For the implant devices with sharp corners and
protrusions, it is essential to determine the K
and stress corrosion crack growth
ISCC
rate for predicting the service life of the component in the physiological environment.
The implant devices generally have locations of stress concentrations such as the
fastener holes, etc. that often show crack-like behaviour. The in-service inspection
of the crack growth of such implants will be a nontrivial task. However, the flawtolerant design assessment of the implant devices (that may have a pre-existing fine
crack), by the CNT testing as demonstrated in the study by Choudhary and Singh
Raman (2012), will provide crucial information on whether the existing crack will
be tolerable for the given service time of the device (James & Sire, 2010).
5.5 Prevention of SCC
There are a few recommendations to avoid SCC in practices, which are discussed
below.
1. Selection of SCC resistant alloy
The SCC resistant alloys can be produced by judicious selection of alloying additions. For example, Mg alloys with higher concentration of Al are more susceptible to
SCC, while the addition of Zr and rare earths as alloying elements have been reported
to increase the SCC resistance (Baker & Avedesian, 1999).
2. Control of stress
SCC propagates when the stress intensities are above a critical level, which has
been reported to be attained at 30e50% of the tensile yield strength of the material
(Winzer et al., 2005). Hence, it is important to design Mg alloys having the threshold
stress intensities reasonably lower than those presented by the service conditions. In
this vein, a great importance should be given to the use of simple shapes that will
provide fewer local high stress generating points (Winzer et al., 2005).
3. Surface modifications and coatings
Coatings may be an appropriate measure for preventing the SCC to some extent.
As discussed, localized corrosion such as pitting is a common cause for initiation
of SCC in Mg alloys. Accordingly, the coating, which decreases the pitting susceptibility of Mg alloys in human body fluid, could help in avoiding/retarding the SCC
(Baker & Avedesian, 1999). Kannan and Lynnley (2011) recently studied the
Mechanical integrity of magnesium alloys for biomedical applications 199
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mechanical integrity of the hydroxyapatite (HA)-coated Mg alloy while exposed to
body fluid and observed a signi ficant improvement in mechanical integrity due to
the coating. The enhanced mechanical integrity of the HA-coated specimen was attributed to the decrease in susceptibility of the coated alloy to localized corrosion/pitting.
An effective modificati on of stresses or environment is not practical in biomedical
applications because implants are expected to work under given stress conditions in the
corrosive human body fluid. So, the development of SCC-resistant Mg alloys appears
to be one of the viable solutions. But the alloying elements must be nontoxic, which
may limit the choices of alloying elements. Therefore, there is a value in exploring
SCC mitigation with the use of biocompatible coatings on Mg alloys (Chen, Birbilis,
& Abbott, 2011; Ng, Wong, & Cheng, 2010; Yu, Pan, & Uan, 2010; Zho u, Shan, Han,
& Ke, 2008).
5.6 Conclusions
This chapter emphasizes the need for characterization of SCC of biodegradable Mg
alloys for temporary implant applications.
Mg alloys have been consistently shown to be susceptible to SCC in SBF (that
contains considerable amount of chloride) and as a result may suffer premature
cracking during service. The localized corrosion is the common cause of pit initiation.
The role of pits is primarily to provide a film-free and active surface through which
atomic hydrogen can enter into the matrix and can subsequently cause embrittlement
and cracking. The predominant mode for SCC crack propagation in Mg alloys was
transgranular cracking. The slow strain rate testing experiments under different electrochemical conditions have suggested the SCC mechanism for a common Mg alloy
in m-SBF to be the combined effect of HASCC and localized dissolution.
To prevent SCC initiation, the most promising recommendation is the use of appropriate biocompatible coatings that retard the localized corrosion/pitting.
The chapter also describes the importance of determination of threshold stress
intensity for stress corrosion cracking (i.e., K
for quantitative characterization of SCC before their use as implants and provides
some typical data that were generated using a simple and inexpensive testing
approach.
) as the critical design parameter
ISCC
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Effect of amino acids and proteins
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on the in vitro performance
6
of coated magnesium for
biomedical applications
Nicholas Travis Kirkland1, Jay Waterman
1
Nagasaki University, Nagasaki, Japan;2University of Canterbury, Christchurch,
New Zealand
6.1 Introduction
Magnesium corrosion rates and mechanisms depend heavily on the corrosion environment. The complexity of the in vivo environment precludes that the corrosion
may be influenced in many different ways. In vi tro corrosion tests, ideally, si mplify
the complicated environm en t o f the body to m ak e measuring corros io n r ates and performance easier and cheaper to perform. To use an in vitro approximation that will
accurately predict in vivo performance, it is necessary to understand the effect
different components have on the corrosion mechanisms. Unfortunately, these effects are often interdependent on other aspects of the system, making clear relationships difficult to obtain. Alloys, coatings, and choice of buffe r are just a few of the
many system facets that can interact with other parts of the solut ion, affecting results
in difficult-to-predict ways. Consequently, the final corrosion mechanisms in a body
environment are not necessarily the same as the sum of the individual components of
an in vitro test. To that end, it is critical to understand the role and effects of in vivo
fluid components on a wide spectrum of test cases/conditions to effectively choose an
in vitro test.
The organic components of the in vivo environment are characteristic of the
complex interactions that can affect corrosion rates of Mg in unexpected ways.
Although large molecules found in the body, such as amino acids and proteins, may
not react directly with the electrochemical corrosion reactions of Mg, they have
been shown to affect the ultimate corrosion rates in vitro (Virtanen, 2011). These
molecules, too often neglected in more simple in vitro experiments, can affect the rates
of adsorption to the surface of corroding Mg, affecting properties such as buffer capacity and interacting with the corrosion products. These secondary processes in turn
affect the corrosion rates and performance of the material in vitro. Before any in vitro
test may be accurately used to determine the correct degradation rate of Mg-based
implants in vivo, it is necessary to understand and fully characterize these effects
and understand how these molecules affect corrosion behavior.
2
Surface Modification of Magnesium and its Alloys for Biomedical Applications. http://dx.doi.org/10.1016/B978-1-78242-077-4.00006-1
Copyright © 2015 Elsevier Ltd. All rights reserved.
206 Surface Modification of Magnesium and its Alloys for Biomedical Applications
The use of Mg and Mg alloys as biomedical implant materials is primarily limited
by the corrosion rate requirements of the implant. The corrosive nature of the body
means that, for many implant applications, corrosion rates of pure Mg and Mg compatible alloys are sufficiently high that additional corrosion protection must be applied.
Biocompatible coatings are one of the most promising solutions to this problem
(Zeng, Dietzel, Witte, Hort, & Blawert, 2008). Calcium phosphate (CaP) coatings
have been widely used to improve the biocompatibility of traditional permanent metal
implants (Leon & Jansen, 2008). CaP coatings are insoluble enough in physiological
systems to provide corrosion protection as well as biocompatibility. However, due to
the complex nature of the in vivo environment, it is necessary to understand how the
addition of these coatings will interact with more complex molecules in solution.
Therefore, an investigation into the effects of such a coating on the corrosion layers,
behavior, and ultimate corrosion rate in both amino acids and protein solutions are
presented here.
6.2 The role of amino acids and proteins in biocorrosion
6.2.1 Amino acidsebridging the physiological gap
Amino acids (AA), molecules containing C, H, O, and N, are essentially the building
blocks of proteins and perform countless functions in the body’s metabolism (Puleo &
Bizios, 2009). In a biocorrosion sense, this includes acting as a pH regulator, affecting
the buffering capacity of in vitro solutions, although not as effectively as carbonic
buffering (Malda et al., 2008). Further, they may influence corrosion behavior of
Mg alloys by adsorbing onto the sample surface, following the Langmuir isotherm
(Ashassi-Sorkhabi, Ghasemi, & Seifzadeh, 2005; Ashassi-Sorkhabi, Majidi, &
Seyyedi, 2004). They have also been found to form a complex with metal cations,
potentially encouraging the dissolution of metal (Bruneel & Helsen, 1988; William
& William, 2004). They may chelate (metal atom attached to large molecule) with
Mg and inhibit the formation of the insoluble Mg compounds that form the passive
layer, decreasing the corrosion resistance (Yamamoto & Hiromoto, 2009). It has
been suggested that other organi c compounds may have a simil ar effect to reduce
the corrosion resistance of any insoluble salt layer on Mg (Yamamoto & Hiromoto,
2009).
Conversely, it has also been found that AA absorbed onto the surface of Mg alloys
increased the resistance to polarization and reduced the corrosion current density
(Gu, Zheng, & Chen, 2009). A similar increase in corrosion resistance has also
been reported for steel and aluminum (Ashassi-Sorkhabi et al., 2004, 2005;
El-Shafei, Moussa, & El-Far, 1997). To date it appears the specific role of amino acids
on Mg biomaterials is unclear, and it certainly warrants further investigation.
The addition of the amino acids to measure corrosion is essential to bridge the gap
between simple ionic solutions and more complicated protein solutions used in
biocorrosion experiments. Perhaps the most commonly used solution used in the
literature is Eagle’s Minimum Essential Medium (MEM), a formula containing ionic
Effect of amino acids and proteins on the in vitro performance of coated magnesium 207
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compounds and amino acids in amounts similar to those found in the body, providing a
suitable environment for cell cultures. However, the number of Mg-based experiments
that have used an AA solution is still relatively small (Gu et al., 2009; Mueller, Lucia
Nascimento, & Lorenzo De Mele, 2010; Witte et al., 2007; Xu et al., 2009; Yamamoto
& Hiromoto, 2009; Yang & Zhang, 2009; Yun et al., 2009; Zheng, Gu, Xi, & Chai,
2010). Others have used a variation of MEM, namely, Dulbecco’s Modi fied Eagle
Medium (DMEM), a version of MEM with additional amino acids present
(Carboneras, Garc~a-A-Alonso, & Escudero, 2011). Others have also tried McCoy’s
medium (Yun et al., 2009), PRMI1640 (Xu et al., 2009), and a simulated blood plasma
(Yang & Zhang, 2009).
The reason for the lack of widespread use of MEM in the bio-Mg literature may
stem from its traditional function as a base media for protein of cell cultures rather
than corrosion tests. However, MEM and similar solutions provide a link between a
completely inorganic salt solution and one containing proteins and is a necessary
solution in the understanding of both coated and uncoated Mg corrosion in vivo.
6.2.2 Proteinsean important biological addition
The importance of proteins in the body is well understood. For the sake of conciseness,
their salient properties will not be discussed in great detail herei n, and instead the
reader is directed toward some of the many excellent tomes on the subject (Horbett,
2004; Latour, 2008). In short, they are known to be a vital part to the success of
any implant in the body, and their connection to the surface is crucial because they provide attachment sites on which cells bind (Miller, Fainerman, Leser, & Michel, 2004).
In a biocorrosion sense, proteins may directly influence the corrosion properties of
metals (Clark & Williams, 1982), as well as being important to their perceived biocompatibility (Latour, 2008). Built of chains of amino acids, the structures of proteins
typically have functional groups on the outside that can be polar, nonpolar, hydrophilic, or hydrophobic, thus creating a complex surface. For a given protein with a set
isoelectric point, the overall charge depends on the pH of the environment (Latour,
2008). This charge and surface composition will affect the adsorption rates of the
proteins onto the surface. It is the adsorption of these proteins that can affect the corrosion surface, and any interactions are crucial to the overall corrosion behavior.
Fetal bovine serum (FBS) is most commonly used to provide proteins for in vitro
SBFs for corrosion tests (Eliezer & Witte, 2010; Keim, Brunner, Fabry, & Virtanen,
2011; Kirkland & Birbilis, 2013; Salunke, Shanov, & Witte, 2011). Taken from the
blood plasma of a calf fetus, it is often used for cell culture work due to the amount
of growth factors it contains as well as the low number of antibodies (Willmer,
1965). FBS typically contains proteins amounting to 30e45 g/L (Equitech-Bio Inc.,
2010), with the majority of the protein composition made of bovine serum albumin
(BSA) (Lenter, 1981). BSA is effectively the bovine equivalent of human serum
albumin (HAS), which accounts for about half of the blood serum protein in humans.
The biocorrosion properties of Mg and Mg alloys have been shown to be strongly
affected by the addition and concentration of proteins in solution. Several studies have
shown that proteins decreased the corrosion of various Mg alloys (Eliezer & Witte,
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