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Session: Poster session I
Session starts: Thursday 24 January, 15:00

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Zhiguang Huan (Biomaterials Technology Section, Department of BioMechanical Engineering, Faculty of Mechanical, Maritime & Materials Engineering, Delft University of Technology)
Sander Leeflang (Biomaterials Technology Section, Department of BioMechanical Engineering, Faculty of Mechanical, Maritime & Materials Engineering, Delft University of Technology)
Jie Zhou (Biomaterials Technology Section, Department of BioMechanical Engineering, Faculty of Mechanical, Maritime & Materials Engineering, Delft University of Technology)
Jurek Duszczyk (Biomaterials Technology Section, Department of BioMechanical Engineering, Faculty of Mechanical, Maritime & Materials Engineering, Delft University of Technology)

Abstract:
In recent years, research and development of advanced biomedical materials have been directed toward biodegradable materials, namely metallic, polymeric or ceramic biodegradable materials in order to avoid long-term medical complication and removal surgery. Among metallic biodegradable materials, a lot of attention has been paid to magnesium as a potential biodegradable material, especially for orthopaedic applications, because of its biodegradability in the bioenvironment as well as its relatively low Young’s modulus [1]. However, magnesium degrades too fast in physiological solutions. Alloying may effectively reduce its degradation rate, but it may also introduce cytotoxic elements. Moreover, magnesium itself lacks bioactivity. As an alternative to adding alloying elements to magnesium, adding bioactive agents to magnesium to form magensium matrix composites may offer a way to reduce the degradation rate of magnesium and enhance its bioactivity [2]. It was hypothesized that suitable bioactive agents added to magnesium might induce the formation of a surface mineral layer that could protect it from fast degradation and enhance its surface bioactivity. In this study, a composite was prepared from pure magnesium and bioactive glass powders through the powder metallurgy route. Pure magnesium prepared under the same conditions was used for comparison purposes. Fig. 1 shows a comparison in microstructure between pure magnesium and the composite, as well as the distribution of bioactive glass particles in the composite. Fig. 1 Optical micrographs of extruded Mg (a) and Mg-BG (b) rods on the transverse section; SEM micrograph of the Mg-BG composite on the transverse section of extruded rod (c); the inset shows the topography at a higher magnification. Immersion tests in the E-MEM solution were performed to determine the response of composite samples to the simulated physiological solution over a period of six days in terms of weight loss percentage and hydrogen evolution rate. Results are shown in Fig. 2. Fig. 2 Weight loss percentage (a) and hydrogen evolution (b) from Mg and Mg-BG composite samples as a function of time during the immersion tests in the E-MEM solution. Post-immersion surface morphology of Mg-BG sample in comparison with that of Mg was characterized by means of SEM and EDX (Figs. 3 and 4). For pure magnesium, the surface was covered with small particles (Fig. 3a), and exhibited a cracked corrosion layer beneath these particles. For the Mg-BG composite, however, SEM image at a higher magnification (Fig. 3b inset) did not show the presence of a cracked corrosion layer. The corrosion layer on the Mg-BG composite appeared to be more compact than that on pure magnesium. The elemental compositions of the corrosion layers on magnesium and Mg-BG samples, determined by EDX, are shown in Fig. 4. The EDX results suggested that Mg(OH)2 was the main constituent of the corrosion layer on pure magnesium (Fig. 4a), in addition to a small amount of Mg3(PO4)2 [3]. The presence of the element Ca was not confirmed. By contrast, the EDX analysis showed that apart from magnesium, oxygen and phosphorus, a small amount of calcium was present in the corrosion layer on Mg-BG sample surface after 1-day immersion (Fig. 4b). This confirmed that the surface layer of the composite contained a higher concentration of Ca than that of Mg sample. Fig. 3 SEM micrographs showing the surfaces of Mg (a) and Mg-BG (b) samples after immersion in the E-MEM solution for one day. The insets show the corrosion products at a higher magnification. The arrows point to the cracks in the oxide layer. Fig. 4 EDX analysis of the surfaces of Mg (a) and Mg-BG composite (b) samples after immersion in the E-MEM solution for one day. In summary, immersion tests of a magnesium matrix composite containing bioactive glass particles in the E-MEM solution showed that the Mg-BG composite had a lower weight loss and hydrogen evolution rate than pure magnesium, which could be attributed to Ca and P deposition on composite sample surface, induced by partial dissolution of BG into the immersion solution. The study demonstrated the feasibility of decreasing the degradation rate of magnesium and enhancing its bioactivity by adding bioactive glass particles. References 1. Staiger M.P., Pietak A.M., Huadmai J., Dias G., Magnesium and its alloys as orthopaedic biomaterials: A review, Biomater. 2006;27:1728-34. 2. Witte F., Feyerabend F., Maier P., Fisher J., et al., Biodegradable magnesium-hydroxyapatite metal matrix composites, Biomater. 2007;28:2163-74. 3. Bender S., Goellner J., Atrens A., Corrosion of AZ91 in 1N NaCl and the mechanism of magnesium corrosion, Adv. Eng. Mater. 2008;10:583-587.