Assessing the properties of biodegradable magnesium alloy AZ31 protected by a polymer layer on a plasma electrolytic oxidized (PEO) surface

Abstract

In the present work we consider the AZ31 Mg alloy for applications as medical implants with bone contact. The alloy has the advantages that it combines a high strength with light weight. Most importantly, it eventually dissolves in the body environment and there is therefore no need to remove implants by additional surgery. However, this dissolution should not happen too fast in order to guarantee mechanical integrity during the healing process. This requires the application of appropriate composite coatings. In a first step, an inorganic protective layer is formed by plasma electrolytic oxidation (PEO) using an electrolyte which contains calcium glycerophosphate (CaGP). Special emphasis is placed on the effect of varying CaGP concentrations in the electrolyte, considering molar concentrations between 0.01 and 0.03 mol/l. In a second step, polymer coatings were applied on composite layers to find out whether the system properties can be further improved. Two types of polymer layers were developed using a spin coating method, based on the biodegradable polymer systems polycaprolactone and polyethylene glycol. The results obtained in the present investigation suggest that introducing CaGP into the electrolyte of plasma electrolyte oxidation induces a composite layer consisting of MgO and CaP compounds. The coatings developed in 0.02 mol/l CaGP electrolyte exhibit superior adhesion strength. A significant tenfold increase in corrosion resistance and the highest cell viability are observed on coatings developed in 0.02 mol/l CaGP electrolyte and then superimposed with PCL. These results suggest that a coating on AZ31, after PEO in an electrolyte with 0.02 mol/l CaGP and then superimposed with PCL, represents a promising candidate for applications as bio-absorbable bone implant material. The coated material by far outperforms the material with no surface treatment.