Surface modification and corrosion properties of Mg based alloys for bio-implant application
Sara Kaabi Falahieh Asl
Date of Issue2016
School of Mechanical and Aerospace Engineering
A*STAR Singapore Institute of Manufacturing Technology
The use of implants for bone repair has a considerable and successful history. Traditionally, metallic implants have been used in such a way that the implant would be surgically removed after sufficient bone healing has been achieved. During the last decade, interest in biodegradable magnesium (Mg) implants has increased dramatically. The use of Mg is based on the fact that Mg is one of the essential elements for human metabolism, and the density and elastic modulus of Mg are close to the human bone, thus, avoiding stress shielding effect. However, Mg alloys corrodes too rapidly, resulting in hydrogen evolution and consequently, local alkalization close to the surgery region as well as premature degradation in the implant’s mechanical integrity before bone healing occurred. All these factors impede the practical applications of Mg implants. In this study, a hydrothermal coating process was used to provide, uniform and biodegradable inorganic calcium-phosphate (Ca-P) and calcium-phosphate/polymer (Ca-P/Polymer) composite coatings on AZ31 magnesium substrate that slow the corrosion of Mg and to meet different requirements for implant application. In the current study two different types of novel Ca-P/Polymer composites coatings were successfully deposited for the first time on AZ31 magnesium alloy to reduce Ca-P material brittleness by using polyacrylic acid and carboxymethyl cellulose polymers. The results showed, crystal phase and coating’s morphology could be successfully controlled by the type and concentration of polymer used which could affect the coating’s degradation rate as well. Incorporation of polymer in the Ca-P coatings reduced the coating elastic modulus bringing it close to that of Mg and that of human bone. Apart from mechanical properties, cell proliferation studies indicated that composite coatings induced better cell attachment compared to the purely inorganic Ca-P coating. Moreover, in order to inhibit bacterial adhesion and prevent implant-associated infection, anti-infective coatings with local drug delivery ability were successfully deposited on AZ31 magnesium substrate. The corrosion performance of coatings were comprehensively studied with potentiodynamic, electrochemical impedance spectroscopy (EIS) as well as long-term immersion tests. The obtained coating could decrease the corrosion rate of AZ31 substrate by 100-10,000 fold. A detailed investigation was performed in order to understand the deposition mechanism of coatings on different metallic substrates during hydrothermal process to determine the various steps in the coating formation and growth. It was found that successful coating deposition is strongly dependent on substrate-solution interface and resulted from substrate corrosion in the first step of deposition process. The obtained coatings could be promising candidates for surface protection of Mg for implant application with the multiple functions of corrosion protection and cell attachment/cell growth promotion.