Investigation Of Surface Modification On Degradable Magnesium Alloy JDBM Used As Bone Implants

Magnesium and its alloys have potential to be biomaterial because of its low density, high specific strength and good biocompatibility.The low corrosion resistance of Mg,which can make Mg degrade in human body , as well as good strength properties, can be outstanding advantages for Mg and its alloys used as degradable orthopedic implant materials.The main problem which restricts its development is that the fast corrosion rate of the existing magnesium alloy does not match the concrescence rate of the hard tissue. Alloying and surface modification are assumed to solve this problem properly.In the present work, a patent magnesium alloy Mg-3.0Nd-0.2Zn-0.4Zr (wt%, hereafter, denoted as JDBM) which is supposed to be used as degradable orthopedic implants was researched systematically, with commercial magnesium alloy AZ31 as a reference. The corrosion rates of AZ31 and JDBM were 1.00mm/y and 1.25mm/year respectively, and both of them were higher than the criterion 0.5mm/y. However, the modes of their corrosion behavior were different. The surface of JDBM was smooth after corrosion products were removed; while the surface of AZ31 had many deep pits on it by SEM observation. The further study on electrochemical tests manifested that pitting corrosion didn't occurred on the surface of JDBM at the early period of immersion time and a more protective and compact film layer has been found upon the surfaces of JDBM. The corrosion product mainly contained Mg(OH)₂, HA(Ca₁₀(PO₄)₆(OH)₂) and (Ca, Mg)₃(PO₄)₂, which can provide an good combination with growing cells and bone tissue and improve biocompatibility greatly. In addition, an interesting result was found that the corrosion rate of magnesium alloys would slow down in dynamic corrosion in comparison with the static corrosion, which may relate to the fact that the flowing Hank's solution prevented the absorption for Cl- on the surface of corroded layer and dissolution of the protective film. Although JDBM showed best hemocompatibility of the tested magnesium alloys both in platelet adhesion test and hemolysis test, the high hemolysis of JDBM does not meet the criterion and surface modification should be used for JDBM further application.A fine acicular structure coating less than 100 nm in diameter approximately 10μm in thickness covers JDBM surface, and the coating mainly composed of Ca10(PO4)6(OH)2 (abbreviated to be HA). The HA coating with the coarse morphology can provide the growing path for new cells and tissue and improve matrix biocompatibility. JDBM alloy shows better corrosion resistance after surface modification, and coating maintains a good integrity after immersion in static and dynamic Hank's solution for 10 days as well as electrochemical tests. Hydrogen test manifests that HA-coating has good protective effect on the matrix in bio-corrosive environment. And JDBM alloy with HA coating doesn't cause a great variation of SBF's pH value. Hemolysis test also suggested that hemolysis of JDBM coated with HA had greatly decreased due to the improvement of corrosion resistance and could meet the requirements then.A coarse and porous coating approximately 10μm in thickness covers JDBM surface after micro-arc oxidation, and the coating mainly composed of MgO and Mg3(P04)2. After immersion in Hank's solution for 10 days, the apatite corrosion products were observed on the surface of the samples, which can also improve sample's biocompatibility. Hydrogen test demonstrated better corrosion resistance after micro-arc oxidation. However,the pores could not separate the matrix and corrosion solution effectively, and only after soaked in SBF could improve the matrix's biocompatibility with the closure of the pores.The present results suggests that the new patent magnesium alloy JDBM is a promising candidate alloy to be applied as degradable biomaterials and is worthwhile for further investigation in vivo corrosive environment.