Enhanced degradation resistance of metallic magnesium using biocompatible coatings for implant applications

Magnesium is an attractive metallic material for temporary implant applications. Magnesium readily dissolves in the physiological environment, and the degradation product is a non-toxic substance which can be harmlessly excreted in the urine. In fact, magnesium is essential to human metabolism and is naturally found in bone tissues, and the mechanical properties of magnesium are close to those of natural bone. However, the degradation rate of pure magnesium is unacceptably high in physiological conditions (i.e., pH level (7.4–7.6) and high chloride concentration). Consequently, a magnesium implant will lose its mechanical integrity before the tissues have sufficiently healed.\ud \ud In recent years, a significant amount of work has been carried out to improve the degradation resistance of magnesium through alloying. Even then the degradation resistance of magnesium alloys is not sufficiently high. In addition, researchers have shown that the localized degradation susceptibility of magnesium alloys could potentially affect the mechanical integrity of magnesium alloy implants during service. Consequently, there is a need for increasing the general and localized degradation resistance of magnesium-based implants during the initial stage of service.\ud \ud In the current study, two types of biocompatible materials (polymer and ceramics) were used as coating materials on pure magnesium and/or its alloy to delay general and localized degradation during service. The degradation behavior of coated samples was evaluated using electrochemical methods in simulated body fluid (SBF). Firstly, polylactic acid (PLA) was coated on a biodegradable magnesium alloy, AZ91, using a spin coating technique. PLA coating enhanced the degradation resistance of the alloy. Increasing the PLA coating thickness was found to improve the degradation resistance, but resulted in poor adhesion. Long-term EIS experiments of the PLA coated samples suggested that their degradation resistance gradually decreased with increase in SBF exposure time. In another attempt, plasma electrolytic oxidation (PEO) technique was used to coat silicate based material on pure magnesium. The PEO coating increased the polarization resistance (R(p)) of magnesium by an order of magnitude under short-term exposure to SBF, and also reduced the corrosion current (i(corr)) by 65%. However, the coating failed to perform under long-term exposure due to the porous structure of the coating. To enhance the performance of the PEO coating, biocompatible materials such as polymer/calcium phosphate were coated on top of the porous PEO layer. In vitro degradation test results showed that the dual layer coatings were very effective in reducing both the localized and general degradation of the base metal even under long-term exposure