Dual-functional coatings on magnesium alloys: Enhancing corrosion behavior under stress and osteogenic effect in osteoporotic rats

Abstract

Magnesium-based implants are subjected to complicated stresses during implantation in the human body. In this case, they undergo stress-induced corrosion and usually lead to early breakage or disintegration of implants. Calcium phosphate compounds have been used as coatings, bone cement and drug carrier in orthopedic application. Their success has been expanded to Mg alloys. Calcium phosphate-based coatings succeeded in corrosion control and osteointegration of Mg alloys in a big extent. However, their protective effect under stress condition is questionable considering the brittle nature. With the assistance of high pH microenvironment near Mg alloy surface, calcium phosphate coatings have the self-repair potential via the dissolution-precipitation mechanism. Inspired by this phenomenon, strontium doped hydroxyapatite (Sr-HA) coatings with higher solubility have been prepared on the surface of Mg alloys. Sr-HA coatings served as not only a physical barrier effect to postpone pitting formation, but also provide ion supplement to seal the cracks initiated during tensile deformation. The results obtained in slow strain rate tensile test (SSRT) demonstrated improved corrosion and mechanical behaviors of Sr-HA coated Mg alloys. Specifically, the elongation of Sr-HA coated alloy increased by 56.7 and 35.2% as compared to the bare and HA coated one in DMEM. In ovariectomized rats, the implants exhibited only 2.7–5.6% reduction of implant volume after 12 weeks implantation. Sr-HA coating further increased early new bone formation than HA coated implants. 12 weeks post-implantation, a large amount of bony tissue with higher trabecular thickness and hardness was formed adjacent to the Sr-HA coated implants. This study demonstrated the efficiency of a rapid dissolved Sr-HA coating on the corrosion behavior of Mg alloys under mechanical loading and the osteogenesis in ovariectomized rats. It therefore holds a promising practical significance for the future coating design strategy for Mg alloys when trying to expand their application range to locations with higher loading conditions.