Variation of corrosion behaviors of Mg and Mg-Ca alloys for bio-absorbable implant materials with impurities and additional elements

Abstract

In order to overcome the limitations of conventional bio inert metals and bio absorbable polymers as orthopedic implant materials, bio absorbable metallic materials such as Mg and Mg alloys are gaining intensive concerns due to their superior mechanical properties, bio-absorbability in the human organism, as well as inductivity of bone formation. In this study, fundamental studies were carried out in order to tailor the bio absorbing rate of the Mg and Mg-Ca alloys by variation of species and quantity of impurities and quantity of additional element of Ca. Ca was chosen as secondary additive as it is an element composing bone tissue and essential in chemical signaling inter-cells. Mg and Mg-Ca alloys with Ca content of 0.8, 5, 10, 16.2, 23, and 33wt% were molten and cast in Ar purged environment. In order to remove defects such as pores and improve uniformity of the microstructure of Mg and Mg-Ca alloy, cast billets were extruded with extrusion ratio of 25:1. Bio absorbing rate of pure Mg and Mg-Ca alloys were investigated by potentio-dynamic electrochemical corrosion test in Hank`s solution at pH 7.4 and 36.5oC. In vivo corrosion rate of cast Mg and Mg-Ca alloys were also measured by Electrochemical Impedance Spectroscopy (EIS) test after implantation of tiny test kit composed of Mg working electrode and Ti counter electrode into inter-vertebra of rat. As experimental results, it was possible to control the bio absorbing rates of Mg and Mg-Ca alloys in the range of 6x10-5A/cm2(Icorr) to 2x10-3A/cm2(Icorr) in vitro electrochemical corrosion test by controlling impurities, Ca contents, and microstructure. The corrosion rate could be decelerated down to 6x10-5A/cm2(Icorr) by reducing Fe and Ni impurities under 100ppm in pure Mg. In Mg-Ca alloys, corrosion behaviors of alloys were changed with Ca contents; 1) corrosion rate was increased up to 2x10-3A/cm2 by adding Ca contents up to 10wt%, 2) decreased down to 2.5x10-4A/cm2 by adding Ca contents up to 16.2wt%(eutectic point), 3) increased up to 1x10-3A/cm2 by increasing Ca contents up to 33wt%. Deceleration of Mg-16.2wt%Ca was found out to be mainly caused by microstructural refinement and uniformity by eutectic solidification by investigation based on microstructure observation. Contents of Mg2Ca phase and Mg/Mg2Ca ratio also played roles in changing the corrosion behavior of Mg-Ca alloys. Bio absorbing rate was definitely slowed down by extrusion of Mg-Ca alloys but no distinguishable changes in pure Mg. In comparison of corrosion rate in vitro and in vivo, half life of bio absorbing metals in vivo is lower than in vitro by 10~500 times.