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228 Surface Modication of Magnesium and its Alloys for Biomedical Applications
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Biocompatibility of surface-
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modied magnesium and
7
magnesium alloys
Ke Yang, Xiao Lin
Chinese Academy of Sciences, Shenyang, China
7.1 Introduction
Biocompatibility is a critical factor for successful applications of biom edical materials. During the years between 1940 and 1980, biomaterials were thought to be bioinert. Therefore, it was considered that biocompatibility would be achieved by designing material to be extremely chemically stable. However, with the development of biomaterials, it is gradually being recognized that it will be better that a biomaterial can properly react with the surrounding tissues rather than being totally inert. More recently, it has become widely accepted that, for some purposes, biomaterials that can nally degrade in the physiological environment are better than those that exist permanently. In 2008, David F. Williams proposed a concept of biocompatibility as follows: Biocompatibility refers to the ability of a biomaterial to perform its desired function with respect to a medical therapy, without eliciting any undesirable local or systemic effects in the recipient or beneciary of that therapy, but generating the most appropriate benecial cellular or tissue response in that specic situation, and optimizing the clinically relevant performance of that therapy (Williams, 2008).
Magnesium (Mg)-based metals include pure Mg and Mg alloys. They are degrad­able in the body environment, which is an advantage compared with permanently implanted metallic biomaterials. For surface-modied Mg-based metals, not only the surface should generate the most appropriate benecial cellular or tissue response,but also the degradation products of the alloys and the surface modication layers should not elicit any undesirable local or systemic effects. In this chapter, we will analyze the medical suitability of the materials introduced by the surface modi­cations on Mg-based metals. Then, we will introduce both the in vitro and in vivo biocompatibility studies of surface-modied Mg-based metals and discuss the inu­encing factors on biocompatibility. Lastly, we will give a likely trend of the future development of the biocompatible surface-modied Mg-based metals.
7.2 Biocompatibility of the materials introduced
by surface modication
Since the surface-modied Mg-based metals are biodegradable in the body environ­ment, the alloying elements in the alloys and the materials introduced onto alloy
Surface Modication of Magnesium and its Alloys for Biomedical Applications. http://dx.doi.org/10.1016/B978-1-78242-077-4.00007-3
Copyright © 2015 Elsevier Ltd. All rights reserved.
232 Surface Modication of Magnesium and its Alloys for Biomedical Applications
surfaces by different surface modications should be controlled to ensure that the degradation products from the Mg implants will not elicit any undesirable local or systemic effects. Information on the biocompatibility of alloying elements can be found elsewhere (Habibovic & Barralet, 2011; Li & Zheng, 2013; Witte et al.,
2008). Here we give several recommended alloy elements from the biocompatibility
consideration in Table 7.1 . In this section, we mainly focus on the biocompatibility of the materials introduced by surface modication on Mg-based metals. The general properties of commonly adopted coati ng materials, which are introduced in detail in the following sections, are listed in Table 7.2.
7.2.1 Calcium phosphates
Many works have studied calcium phosphate-containing coatings on the biodegrad­able Mg-based metals (Chang, Tian, Liu, & Duan, 2013; Niu et al., 2013; Shadanbaz
& Dias, 2012; Wang, Guan, Wang, Ren, & Wang, .; Wen et al., 2009). The main inor-
ganic component in bones is exactly a biological apatite (CaP). Thus, CaP has always been considered to have an excellent biocompatibility, especially for the bone tissues. There are many kinds of CaPs, including calcium phosphate dehydrate (DCP), octacal­cium phosphate (OCP), tricalcium phosphate (TCP), and hydroxyapatite (HA). The biocompatibilities of CaPs have been clearly revealed, and they are all biodegradable, although they have different degradation rates (Shadanbaz & Dias, 2012). CaPs are suggested as the coating materials on biodegradable Mg-based metals.
7.2.2 Magnesium phosphate
Magnesium phosphate-containing coatings are mainly fabricated by the substrate­involving chemical reactions through either chemical conversion (Ishizaki,
Shigematsu, & Saito, 2009; Zhou, Shan, Han, & Ke, 2008) or microarc oxidation
(MAO) (Bai et al., 2012; Bala Srinivasan, Liang, Blawert, St Rmer, & Dietzel,
2009; Bala Srinivasan, Liang, Blawert, St Rmer, & Dietzel, 2010; Gao, Yerokhin, & Matthews, 2013; Lin et al., 2014) methods. Recently, magnesium phosphate cement
(MPC) has attracted much attention as a potential biodegradable bone-implant material (Mestres & Ginebra, 2011; Waselau, Samii, Weisbrode, Litsky, & Bertone, 2007;
Wu et al., 2008; Yu et al., 2010). The in vitro and in vivo studies showed that MPC
has excellent biocompatibility. It is believed that magnesium phosphate can be trans­formed into CaP in vivo, which has higher chemical stability.
7.2.3 Magnesium silicate
Magnesium silicate-containing coatings on the biodegradable Mg-based metals have been fabricated through anodization (Wu et al., 2007; Xue, Yun, Schulz, & Shanov,
2011) or MAO (Gu et al., 2011; Lin, Tan, Zhang et al., 2013; Wan et al., 2013; Yao, Li, & Jiang, 2009) methods. Magnesium silicate was mainly in the form of Mg
which has been widely studied as a biodegradable material for the scaffold in bone­tissue engineering and/or load-bearing devices due to its biodegradability and good
SiO4,
2
Biocompatibility of surface-modied magnesium and magnesium alloys 233
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Table 7.1 Recommended alloy elements for biodegradable
magnesium alloy
Elements Biofunctionality References
Calcium Major component in human bone;
essential in chemical signaling with cells; elevated calcium has a number of stimulating effects on osteoblasts
Zinc Plays an important role in various
physiological processes; one of the constituents of the antioxidant system; involved in the synthesis of a large number of proteins and is required for their stability; important to the bone tissues health
Silicon An essential trace element in the
skeletal development; associated with calcium in an early stage of calcication; induce proliferation of osteoblast-like cells and angiogenesis during bone regeneration
Strontium About 98% in the human body is
in the skeleton; strontium renalate is used clinically in the treatment of osteoporotic patients in Europe; positive effects on ostegenic differentiation and mineral formation; inhibitory effects on osteoclastic differentiation and resorption; dose-dependent effect
Copper Osteoporosis associates with
copper deciency, while a supplement of copper may have a potential therapeutic application in the treatment and prevention of involutional osteoporosis; essential for blood vessel formation
(Adams, Manseld, Perlot, &
Shapiro, 2001; Dvorak et al., 2004; Ilich & Kerstetter, 2000)
(Prasad, Bao, Beck, Kucuk, &
Sarkar, 2004; Tapiero & Tew, 2003; Yamaguchi, Oishi, & Suketa, 1988)
(Carlisle, 1970; Carlisle, 1982;
Shie, Ding, Chang et al., 2011; Zhai et al., 2012)
(Bonnelye, Chabadel, Saltel, &
Jurdic, 2008; Grynpas & Marie, 1990; Marie, Ammann, Boivin, & Rey, 2001; Skoryna, 1984)
(Rico et al., 2000; Yee, Kubena,
Walker, Champney, & Sampson, 1995; De Lima et al., 2008)
234 Surface Modication of Magnesium and its Alloys for Biomedical Applications
Table 7.2 Biocompatibility and biodegradability of coating materials
on biodegradable magnesium alloy
Materials Biocompatibility Biodegradability
Calcium phosphates Excellent biocompatibility Degradable; degradation
rate varies signicantly between different calcium phosphates
Magnesium phosphate Biocompatible Degradable
Magnesium silicate Biocompatible Degradable; degradation
rate is very low
Magnesium uoride Dose-dependent
biocompatibility
Titania Questionable; depend on its
size and existing state
Alumina Biocompatible Not degradable
Degradable polymers Biocompatible Degradable; degradation
Degradable
Not degradable
products may elicit slight inammatory reaction
biocompatibility (Kharaziha & Fathi, 2009; Kharaziha & Fathi, 2010; Ni & Chang,
2009; Ni, Chou, & Chang, 2007). It has been reported that silicon (Si) is an essential trace
element in skeletal development (Carlisle, 1982; Jugdaohsingh et al., 2004; Yamada
et al., 2003). It was also found that an appropriate Si concentration was effective in
supporting the proliferation of osteoblast-like cells (Shie et al., 2011). Thus, magnesium silicate is a biocompatible coating material. However, it should be noted that its degra­dation rate is very low (Ni & Chang, 2009; Tavangarian & Emadi, 2011).
7.2.4 Magnesium uoride
Magnesium uoride (MgF2)-containing coatings were prepared on biodegradable Mg-based metals through hydrouoric acid-involving chemical conversion (Chiu,
Wong, Cheng, & Man, 2007; Lin, Tan, Wan et al., 2013; Pereda et al., 2010; Thomann et al., 2010; Yan et al., 2010) or MAO (Pan, Chen, Wang, & Lin, 2013; Pan, Chen, Wang, & Zhao, 2013; Seyfoori, Mirdamadi, Khavandi, & Rau, 2012) methods.
Fluorine (F) is a natural component in human bones and teeth (Zheng, Wu, Ng,
Wang, & Lian, 2002). A proper release of F will not cause harm to the organisms
(Thomann et al., 2010). The bone response to uoride is dose-dependent: a low-dose release of F will facilitate the bone formation; however, a high dose of it will cause the formation of poorly mineralized osteoid (Ellingsen, 1995). Thus, MgF able coating material, although the degradation rate should be controlled.
is an accept-
2
Biocompatibility of surface-modied magnesium and magnesium alloys 235
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7.2.5 Titania
Some researchers fabricated titania-containing coatings on biodegradable Mg-based metals through one of the methods of solegel (Hu et al., 2011; Li, Chen, Zhang,
Hu, & Su, 2010), direct current reactive magnetron sputtering (Chen et al., 2011),
and ion implantation (Liu, Xin, Tian, & Chu, 2007; Zhao et al., 2013). The titania coat­ings have been prepared on the permanently implanted metallic materials, showing excellent biocompatibility and bioactivity (Chen et al., 2004; Huang et al., 2003;
Jokinen et al., 1998). A porous titania ceramic has been studied as a bone graft substi-
tute, which demonstrated excellent biocompatibility (Sabetrasekh, Tiainen,
Lyngstadaas, Reseland, & Haugen, 2011). However, titania is an inert and poorly
soluble material (Wang et al., 2007). The use of titania as coating material on biodegradable Mg-based metals is controversial. In addition, the titania particles from the coating might elicit some undesirable local or systemic effect s (Wang
et al., 2007).
7.2.6 Alumina
Alumina-containing coatings have been prepared on biodegradable Mg-based metals through plasma immersion ion implantation (Liu et al., 2007; Wong et al., 2013; Zhao,
Wu, Pan, Yeung, & Chu, 2012)orfiltered cathodic arc deposition (Xin et al., 2008)
methods. Alumina is frequently used for medical implants and prostheses because of its excellent biocompatibility (Fischer et al., 2005; Thomas, Barnstorf, Summer,
Willmann, & Przybilla, 2003). However, alumina is a bioinert ceramic with high
chemical stability, and its degradability could be a concern when applied on biode­gradable Mg-based metals.
7.2.7 Organic materials
Many researchers have fabricated organic materials containing coatings on biodegrad­able Mg-based metals (Abdal-hay, Dewidar, & Lim, 2012; Abdal-hay, Amna, & Lim,
2013a; Abdal-hay, Barakat, & Lim, 2013b; Abdal-hay, Barakat, & Lim, 2013c; Alabbasi, Liyanaarachchi, & Kannan, 2012; Bai et al., 2012; Cui, Beniash, Gawalt, Xu, & Sfeir, 2013; Degner, Singer, Cordero, Boccaccini, & Virtanen, 2013; Fekry, Ghoneim, & Ameer, 2014; Gray-Munro, Seguin, & Strong, 2009; Killian, Wagener, Schmuki, & Virtanen, 2010; Ko, Choe, Jung, & Kim, 2013; Kunjukunju et al., 2013; Li, Cao, Zhang, Zhang, & He, 2010b; Liu et al., 2013; Oosterbeek, Seal, Seitz, & Hyland, 2013; Ostrowski et al., 2013; Shanshan, Lili, Yingxue, Bingchun, & Ke, 2013; Srinivasan, Ranjani, & Rajendran, 2013; Wagener, Killian, Turhan, & Virtanen, 2013; Wang, Zhao, Chen, Li, & Zhang, 2012; Wang et al., 2013; Wong et al., 2010; Xu & Yamamoto, 2012; Xue et al., 2012; Zomorodian et al., 2013). The coating
materials include the well-known biodegradable polymers, such as poly( acid) (PLA) (Abdal-hay et al., 2013c; Alabbasi et al., 2012; Gray-Munro et al.,
2009; Xu & Yamamoto, 2012), poly (lactide-co-glycolide) (PLGA) (Li, Cao et al., 2010; Shanshan et al., 2013), poly (e-caprolactone) (PCL) (Degner et al.,
L-lactic
236 Surface Modication of Magnesium and its Alloys for Biomedical Applications
2013; Wang et al., 2012; Wong et al., 2010), polyether imide (PEI) (Zomorodian et al., 2013), poly(1,3-trimethylene carbonate) (PTMC) (Wang et al., 2013), and
silane (Ko et al., 2013; Liu et al., 2013; Xue et al., 2012). Protein (Killian et al.,
2010; Wagener et al., 2013) and peptide (Cui et al., 2013) were also used as the
coating materials. The biocompatibility of the above materials has been well addressed (Francis Suh & Matthew, 2000; Gunatillake & Adhikari, 2003; Mohanty,
Misra, & Hinrichsen, 2000; Rezwan, Chen, Blaker, & Boccaccini, 2006). However,
the degradation of some polymers usually produces a high quantity of acidic degra­dation products in a short time, which may induce a slight inammatory response (Bostman & Pihlajamaki, 2000; Taylor, Daniels, Andriano, & Heller, 1994).
7.3 Biocompatibility of surface-modied magnesium-based metals e in vitro results
Various surface modications have been applied to Mg-based metals to control their degradation rates. The coatings can be divided into three classes according to their compositions: inorganic coatings, organic coatings, and inorg anic/organic com­posite coatings. In this section, in vitro biocompatibility studies of these three classes of coatings are introduced.
7.3.1 Inorganic coating
A number of methods have been used to modify the surfaces of biodegradable Mg­based metals by forming inorganic coatings, as illustrated in Figure 7.1. The coating mechanisms and formed coating materials are summarized in Table 7.3. Although the compositions of coatings prepared by the same method could vary from each other, their physicochemical properties were similar. To a great extent, the surface biocom­patibility of the surface-modied Mg-based metals should depend on the physico­chemical properties of the surface layers. Thus in the following part, in vitro biocompatibility of the inorganic coatings is introduced according to the modication methods.
Methods for fabricating inorganic coatings
Alkaline
treatment
Heat
treatment
Biomimetic
treatment
Hydrofluoric acid
treatment
Acid
treatment
Cathodic
electrodeposition
Phytic acid
treatment
Constant
current mode
Micro-arc oxidation
voltage mode
Anodization
Constant
Figure 7.1 Main inorganic coating methods for biodegradable magnesium alloy. Sometimes more than one method is used to fabricate inorganiceinorganic composite coatings.
Ion
implantation
Biocompatibility of surface-modied magnesium and magnesium alloys 237
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Table 7.3 Coating mechanisms and formed coating materials of
several inorganic coating methods for biodegradable magnesium alloy
Method Coating mechanism Coating materials
Alkaline treatment Substrate-involved
chemical reaction
Biomimetic treatment Substrate-involved
chemical reaction; pH variation-induced chemical deposition
Acid treatment Substrate-involved
chemical reaction
Cathodic electrodeposition Chemical reaction;
electrochemical reaction; pH variation-induced chemical deposition
Micro-arc oxidation Substrate-involved
chemical reaction; electrochemical reaction
MgO; Mg(OH)
MgO; calcium phosphates;
magnesium phosphates
Phytic-Mg phytate
complex; MgF Mg(OH)
Calcium phosphates,
especially HA; bioactive element doped calcium phosphates
MgO, calcium phosphates;
magnesium phosphates; CaO, MgF silicates
2
; MgO;
2
2
, magnesium
2
7.3.1.1 Alkaline treatment
In this treatment, Mg-based metals were immersed in an alkaline solution. A coating will be formed through a chemical reaction. A heat treatment is always followed to densify the coating. Gu et al. (Gu, Zheng, Cheng, & Zheng, 2009) soaked a Mg1.4 wt% Ca alloy in three different alkaline solutions (Na NaHCO
) for 24 h and subsequently annealed the alloy at 773 K for 12 h. Magnesium
3
oxide layers were formed on the surface of the Mg-Ca alloy after different alkaline treatments. All the heat-treated Mg alloy samples showed no cell toxicity to L929 cells during 7 d of co-culture. Lorenz et al. (Lorenz et al., 2009) passivated a pure Mg (99.9% purity) by soaking it in 1 M NaOH for 24 h. The formed passive lm, which was mainly composed of MgO/Mg(OH)
, was in the order of nanometers in size. The
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initial dissolution rate of the pure Mg in the cell culture medium was decreased by approximately a factor of 20 due to the passivation. The human HeLa cell density on the treated sample was signicantly higher than the polished one, where only single isolated cells could be found and the cell spreading was poor. For the passivated sample, however, strong formation of the actin cytoskeleton could be observed, and the cells were well spread, covering most of the surface.
HPO4,Na2CO3, and
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