Surface Treatment on Mg Alloys for Controlling Biofunctionality and Biodegradation

The surface of Mg alloys can be treated with bioresorbable ceramics or polymers along with novel signaling biomolecules (such as growth factors, DNA, and proteins) or pharmaceutical agents (such as antibiotics, anti-inflammatory drugs) to control their biodegradation and biofunctionality.

3.4.1 Bioresorbable Ceramic Coatings on Mg Alloys

Surface treatment can improve the biocompatibility of Mg alloys by reducing the degradation rate and inducing better tissue-implant integration. Calcium phosphates or its derivatives (such as hydroxyapatite and TCP) have been coated on Mg alloys to control alloy degradation and tissue integration properties since Ca and P are the major components of natural bone (Song et al. 2010; Wang et al. 2010b; Cui et al. 2008; Hiromoto and Yamamoto 2009; Wen et al. 2009; Yang et al. 2008, 2009; Zhang and Wei 2009). If osteoconductive minerals quickly form on the Mg-Zn alloy at the early stage of degradation after surface modification, and the coatings remain intact for the initial 3-6 weeks, the chances for an osteointegrated interface increase after implantation. In addition, pitting corrosion of Mg is delayed in the presence of phosphate ions (Xin et al. 2008). Therefore, calcium phosphate-based coatings, especially osteoconductive minerals such as hydroxyapatite (HA) and TCP, are very useful in further promoting osteointegra-tion around Mg implants and constructing new bone (Cao et al. 2010; Lu et al. 2004; Stewart et al. 2004; de Lavos-Valereto et al. 2002; Sun et al. 2002; Lavos-Valereto et al. 2001; Labat et al. 1999; Tsui et al. 1998). For example, Song et al. deposited three kinds of calcium phosphate coatings, including brushite (DCPD, CaHPO4 • 2H2O), hydroxyapatite (HA, Ca10(P04)6(0H)2), and fluoridated hydroxyapatite (FHA, Ca5(P04)3(0H)1-xFx) using electrodesposition on a biodegradable Mg-Zn alloy (Song et al. 2010). The results showed that these coatings decreased the degradation rate of Mg-Zn alloy, and the precipitates formed in modified, simulated biological fluid showed higher Ca/P molar ratios on the HA- and FHA-coated Mg-Zn alloys than the uncoated and DCPD-coated Mg-Zn alloy, which facilitated bone-like apatite formation. Both the HA and FHA coating could promote the nucleation of osteoconductive minerals (bone-like apatite) for 1 month. Hiromoto et al. developed a new method of direct synthesis of HAp on Mg and AZ series alloys (AZ31, AZ61, and AZ91) by immersion in a Ca-EDTA/KH2P04 solution, which can maintain a sufficiently high concentration of Ca ions on the Mg surface. Highly crystallized HAp coatings were successfully produced, and HAp-coated specimens showed 103-104 times lower anodic current density than as-polished specimens in a polarization test. The results provided strong evidence that the HAp coating could minimize the corrosion of Mg alloys (Hiromoto and Yamamoto 2009). An adherent Ca-deficient HAp coating on Mg-Zn-Ca alloy was successfully deposited by a pulse electrodeposition process without any post-treatment (Wang et al. 2010b). The slow strain rate tensile (SSRT) test results showed that the ultimate tensile strength and time of fracture for the coated Mg-Zn-Ca alloy are greater than those of the uncoated one, which is beneficial in supporting fractured bone for a longer time. Thus, HAp coatings on Mg alloys are promising for orthopedic applications. MgF2 coatings have also been reported to reduce Mg alloy corrosion in a rabbit model (Witte et al. 2010).

3.4.2 Polymer Coating and Surface Modification

A new approach to control the corrosion rate of Mg alloys is to fabricate a polymeric membrane or coating on the surface. Wong et al. sprayed a mixture of polycaprolactone (PCL) and dichloromethane (DCM) onto AZ91 Mg alloys. The pore size was controlled during the layer-by-layer spraying process. The results demonstrated that the deposition of the polymeric membrane on Mg alloys reduced the degradation rate of AZ91 and maintained the bulk mechanical properties for a longer period of time. The in vitro results indicated that these polymeric coatings have good cytocompatibility properties with osteoblasts compared to uncoated AZ91 alloys. The in vivo study suggested that the uncoated Mg alloys degrade more rapidly than that of the polymer-coated Mg alloys. Although new bone formation was found on both coated and uncoated Mg alloys, as determined by Micro-CT, higher volumes of new bone were observed on the polymer-coated Mg alloys. Histological analysis did not show any inflammation, necrosis, or hydrogen gas accumulation on the coated Mg alloys. Collectively, these data provided evidence that polymeric coatings on Mg alloys may mediate the degradation of Mg alloys (Wong et al. 2010).

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