Magnesium alloys attract attention as a biodegradable/bioabsorbable bone plates & screws and stents owing to their biodegradability and good mechanical integrity. Various surface modifications have been examined to control corrosion rate and to improve biocompatibility of the magnesium alloys. Bone plates are deformed to adjust the shape of bone and screws are rubbed with bone during screwing, which sometimes cause cracking and delamination of modified surfaces. We developed hydroxyapatite (HAp) coating, and revealed that the HAp coating improved the corrosion resistance of Mg-3mass% Al-1mass% Zn (AZ31) in cell culture medium (in vitro) and in subcutaneous tissue of mouse (in vivo) [1]. Effect of scratching HAp coated surface on the corrosion behavior of HAp-coated AZ31 was examined in cell culture medium [2]. Corrosion of the scratched surface was not accelerated in comparison with the surface without scratch. A calcium phosphate deposit layer covered the bare metal surface inside the scratch. These results suggest that it is effective to enhance calcium phosphate deposition at the defect of HAp coating to enhance the self-healing ability of the coating.

Combination of HAp coating with polymer molecules which act as a nucleus for calcium phosphate deposition is a potential way to enhance the self-healing ability. It was reported that an anionic polymer, sodium polyacrylate (SPA), acted as a nucleus for calcium phosphate deposition [3]. In this study, SPA molecule was combined with HAp coating and the corrosion behavior was examined by a static immersion test and a slow strain rate tensile (SSRT) test in a simulated body fluid [4].

HAp was coated on AZ31 at 60ºC for 1 hour in a previously reported solution [1]. SPA molecule was dissolved in a phosphate buffer at pH 8 and the HAp-coated AZ31 was dipped at room temperature for 10-30 sec. Scratches were formed on the surface of HAp- and SPA-HAp-coated AZ31 specimens with a knife. The scratched specimens were statically immersed in Hanks’ solution for 7 days. Volume of H₂ gas generated was measured. The surface after the immersion was characterized using a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). A SSRT test of HAp- and SPA-HAp-coated AZ31 tensile test specimens was performed in Hanks.

Scratched HAp- and SPA-HAp-coated AZ31 showed no significant difference in H₂ gas volume generated under the static immersion. Inside the scratches were covered with a calcium phosphate deposit layer regardless of the presence of SPA. The deposit layer with SPA was uniform and prevented the corrosion initiation; whereas, local corrosion initiated from inside the scratches without SPA. Moreover, SPA enhanced the deposition of calcium phosphate granules inside the scratches.

In the SSRT test in Hanks, HAp-coated AZ31 specimen fractured just over the yield strength with small elongation of ca. 1.5%. This result indicates that HAp-AZ31 is susceptible to stress corrosion cracking (SCC). SPA-HAp-coated AZ31 specimen showed larger elongation of ca. 2.4%. Time to failure of HAp-coated AZ31 was ca. 8.4 hours and it was elongated by 45% with SPA to ca. 12.2 hours. Some cracks of SPA-HAp coating were closed at crack mouth, suggesting that SPA enhanced the healing of cracks of HAp coating. It was revealed that the combination of SPA molecules with HAp coating enhanced the self-healing ability of HAp coating and reduced the SCC susceptibility of HAp-coated AZ31.

[1] S. Hiromoto et al., Acta Biomater., 11 (2015) 520.

[2] S. Hiromoto, Corros. Sci., 100 (2015) 284.

[3] A. Bigi et al., Biomacromolecules, 4 (2000) 752.

[4] S. Hiromoto et al., CORROSION (submitted).