Corrosion Resistance and Biocompatibility Assessment of a Biodegradable Hydrothermal-Coated Mg-Zn-Ca Alloy: An in Vitro and in Vivo Study

Abstract

A hydrothermal (HT) coating was applied to the biomedical Mg-Zn-Ca alloy surface by microarc oxidation (MAO) and heat treatment. Then, the corrosion resistance and biocompatibility of the coated alloy was evaluated in vitro and in vivo. The corrosion rate (CR) of HT-coated implants was significantly lower in experiment. In addition, this CR increased over time in vivo but was stable, albeit higher, in vitro. The proliferation, adhesion, and live activity of bone marrow stem cells (BMSCs) were significantly greater on the surface of the HT-coated Mg alloy in vitro. Serum Mg²⁺ was always within the normal range in rabbits with implants, although Ca²⁺ was higher than normal for both uncoated and coated scaffolds. There were no significant pathological effects on the main organs of alloy-implanted rabbits compared with healthy animals. Thus, the HT coating significantly improved the corrosion resistance and biocompatibility of the Mg-Zn-Ca alloy.

Figure 1

Uncoated and coated Mg–Zn–Ca alloy scaffolds used in this study. (a) Uncoated scaffolds are shown. (b) Hydrothermal (HT)-coated scaffolds are shown. (c) Positioning of a scaffold containing autogenous morselized bone at a bone defect in the rabbit ulna.

Figure 2

Gross appearance of the uncoated and HT-coated Mg–Zn–Ca alloy after being immersed in SBF solution for 1, 3, 7, 14, and 21 days. The uncoated alloy had completely dissolved by day 14.

Figure 3

Cell proliferation of BMSCs in medium preincubated with the Mg alloy. (a, b) BMSCs were cultured in the control medium or in the medium collected after a HT-coated or uncoated Mg–Zn–Ca alloy sample was first incubated in the medium for (a) 3 days and (b) 7 days. Cell proliferation was assessed with the CCK-8 assay. Higher OD450 values indicate a higher number of cells and thus greater cell proliferation. *p < 0.05 and **p < 0.01.

Figure 4

Live/dead staining of BMSCs cultured on uncoated and HT-coated Mg–Zn–Ca alloy samples. (a–d) Cells were cultured on uncoated Mg alloy (a, b) and HT-coated Mg alloy (c, d) for 3 days (a, c) and 7 days (b, d). Green fluorescent cells are alive, whereas red fluorescent cells are dead.

Figure 5

SEM of the cell-seeded surface of uncoated and HT-coated Mg–Zn–Ca alloy samples. (a–d) Samples of the uncoated Mg alloy (a, b) and HT-coated Mg alloy (c, d) were seeded with BMSCs and cultured for 3 days (a, c) and 7 days (b, d) before analysis by SEM.

Figure 6

X-ray images of uncoated and HT-coated Mg–Zn–Ca scaffolds in rabbit ulnae 4, 8, and 12 weeks after implantation. The red arrow indicates the position of the scaffold, the black arrow indicates hydrogen produced by degradation, and the yellow arrow indicates bone regeneration.

Figure 7

Micro-CT images and volume determination of the uncoated and HT-coated scaffolds in a rabbit model of bone defect. (a–f) Implanted Mg alloy scaffolds that were either uncoated (a–c) or HT coated (d–f) were examined by three-dimensional micro-CT at week 4 (a, d), week 8 (b, e), and week 12 (c, f) after surgery. In each panel, the lower-right image shows the remaining volume of the implant as calculated by micro-CT.

Figure 8

Extracted volumes of hydrogen gas generated by the uncoated and HT-coated Mg alloy scaffolds in a rabbit model of bone defect. Gas was extracted at weeks 2, 4, 8, and 12 after surgery. **p < 0.01.

Figure 9

Serum Mg²⁺ and Ca²⁺ concentrations. (a, b) Serum Mg²⁺ (a) and Ca²⁺ (b) levels were determined in rabbits that received Mg alloy scaffold implants that were either uncoated or HT coated. Serum samples were analyzed at weeks 4, 8, and 12 after surgery.

Figure 10

Histochemical analysis of organs in rabbits receiving Mg alloy scaffolds. Images of the liver (a), brain (b), kidney (c), and spleen (d) tissues taken from experimental animals at 4, 8, and 12 weeks after surgery and stained with hematoxylin and eosin.