Corrosion Behavior of Calcium Phosphate-Coated Biomedical Magnesium Alloy Under in Vitro and In Vivo Environments
Authors: Hiromoto Sachiko , Inoue Motoki , Taguchi Tetsushi 
Published: February 2020

Magnesium alloys attract attention as a biodegradable/bioabsorbable medical device because of the biodegradability and good mechanical integrity. Corrosion rate of the magnesium alloys should be controlled depending on the implantation part of body as well as the biocompatibility should be improved. Hydroxyapatite (HAp) and octacalcium phosphate (OCP) coatings were developed to control the corrosion rate and to improve the hard tissue compatibility. Corrosion behavior of HAp- and OCP-coated Mg-3mass% Al-1mass% Zn (AZ31) was examined in cell culture medium (in vitro) or in subcutaneous tissue of mouse (in vivo). Effect of medium-replacement and addition of gelatin in medium on the corrosion behavior was also examined. Chemically polished AZ31 was used as a reference. Concentration of Mg ions in medium was quantified.
Apparent corrosion initiation of substrate AZ31 was delayed for ca. 1 week with HAp and OCP coatings in medium while Mg ion release from as-polished AZ31 initiated from the beginning. Filiform corrosion took place, but visible growth of filiform corrosion stopped within several days. Afterwards, Mg ion release continued for 1 year with almost a constant corrosion rate. The corrosion rate of AZ31 alloy in medium was reduced by more than 50% with HAp and OCP coatings. HAp coating showed superior corrosion protection effect than OCP coating. After 1 year of immersion, cross section of the immersed specimens showed that uniform corrosion took place underneath the coatings. The wall of filiform corrosion pits was covered with thick corrosion products layer. These results indicate that corrosion continued for long time under the coating and corrosion products layer in medium.
The periodical medium replacement during immersion test did not significantly change the initial corrosion rate for 4 weeks. However, the medium-replacement rather reduced the corrosion rate of HAp-coated AZ31 comparing to the result without medium-replacement. Deposition of calcium phosphate from medium was enhanced with the medium-replacement, leading to the improvement of corrosion protection effect of HAp coating.
As-polished AZ31 caused the formation of gas cavity in subcutaneous tissue of mouse due to H2 gas generated in corrosion reaction. Such a gas cavity formation was not observed around the implanted HAp- and OCP-coated specimens, indicating that corrosion of substrate AZ31 was also reduced by HAp and OCP coating in subcutaneous tissue. After 16 weeks of implantation, the specimens were retrieved from the mouse and surface and cross section was observed using SEM. Macro- and micro-pits were observed on the surface and underneath the coatings. Unlike in medium, corrosion pits appeared to grow in depth direction rather than in lateral direction in subcutaneous tissue. Because body fluid circulation in the subcutaneous tissue is limited, to simulate the soft tissue environment, gelatin was added in medium and HAp-coated AZ31 was immersed for 4 weeks. Then, the similar corrosion morphology with macro- and micro-pits was obtained in the gelatin-added medium. These results indicate that corrosion of HAp- and OCP-coated AZ31 is localized in diffusion limited environment.
The developed HAp and OCP coatings reduced the corrosion rate of Mg alloy in medium and in soft tissue. The corrosion morphology, especially shape of local corrosion, depended on the corrosion environment. The decrease of mass transfer in the environment caused the localization of corrosion of Mg alloy.