Strontium-doped Hydroxyapatite Coatings Deposited on Mg-4Zn Alloy: Physical-chemical Properties and in vitro Cell Response

Biodegradable magnesium (Mg)-based biomaterials have draw extensively attention, due to the high strength-to-weight ratio, low elastic modulus and good biocompatibility. However, the high corrosion rate is still a major obstacle for the potential clinical applications. Therefore, the highly biocompatible hydroxyapatite (HA) coatings are usually introduced to restrain the interactions between Mg-based substrate and the body fluid environment. In the present paper, HA and strontium (Sr)-doped HA coatings were prepared on Mg-4Zn substrates by electrochemical deposition. The surface properties of the samples were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), three-dimensional laser scanning microscopy (3D LSM) and a contact angle video system. The dynamic ion release, protein adsorption, cell adhesion, proliferation and differentiation behavior of the samples were also evaluated. The results reveal that the incorporation of Sr in the HA coatings leads to lattice distortion and decreased crystallinity. The smaller amount of Mg ion release of the Sr-doped HA coated samples suggests a better corrosion resistance. The improved protein adsorption and initial adhesion of mesenchymal stem cells (MSCs) of the Sr-doped samples should be due to their higher surface roughness and wettability. The introduction of Sr leads to comparable cell proliferation behavior, but significantly improved osteogenic differentiation. It is concluded that the Sr-doped HA coatings are promising candidates for the protective biocompatible coating on Mg-based implants.