In vivo performance of a rare earth free Mg-Zn-Ca alloy manufactured using twin roll casting for potential applications in the cranial and maxillofacial fixation devices

Abstract

A magnesium alloy containing essential, non-toxic, biodegradable elements such as Ca and Zn has been fabricated using a novel twin-roll casting process (TRC). Microstructure, mechanical properties, in vivo corrosion and biocompatibility have been assessed and compared to the properties of the rare earth (RE) element containing WE43 alloy. TRC Mg-0.5 wt% Zn- 0.5 wt% Ca exhibited fine grains with an average grain size ranging from 70 to 150 μm. Mechanical properties of a TRC Mg-0.5Zn-0.5Ca alloy showed an ultimate tensile strength of 220 MPa and ductility of 9.3%. The TRC Mg-0.5Zn-0.5Ca alloy showed a degradation rate of 0.51 ± 0.07 mm/y similar to that of the WE43 alloy (0.47 ± 0.09 mm/y) in the rat model after 1 week of implantation. By week 4 the biodegradation rates of both alloys studied were lowered and stabilized with fewer gas pockets around the implant. The histological analysis shows that both WE43 and TRC Mg-0.5Zn-0.5Ca alloy triggered comparable tissue healing responses at respective times of implantation. The presence of more organized scarring tissue around the TRC Mg-0.5Zn-0.5Ca alloys suggests that the biodegradation of the RE-free alloy may be more conducive to the tissue proliferation and remodelling process.

Keywords: Biocompatibility; Biodegradable implants; In vivo degradation; Mg–Zn–Ca alloy; Twin-roll strip casting.

Graphical abstract

Fig. 1

Polarised light microscopic images of (a–c) TRC Mg–Zn–Ca alloy and (d) WE43 alloy [56].

Fig. 2

Backscattered electron (BSE) images of (a, b) TRC Mg–Zn–Ca alloy and (c, d) WE43 alloy at (a, c) low and (b, d) high magnifications respectively.

Fig. 3

(a) Bright-field optical image of the deformed region with recrystallized grains along the parallel direction. (b, c) Inverse pole figures at two random locations where (b₁, c₁) are the ODFs and (b₂, c₂) are the distribution of misorientation angles.

Fig. 4

(a) Tensile load curves and (b) summary of the mechanical properties of TRC Mg–Zn–Ca alloy and WE43 alloy.

Fig. 5

In vivo corrosion rate of TRC Mg–Zn–Ca and WE43 alloys after 1 week and 4 weeks of implantation (asterisk represent statistical significance: *p < 0.05 and NS represent no significance).

Fig. 6

Representative 3D reconstructed μ-CT images showing the presence of gas around (a, b) TRC Mg–Zn–Ca and (c, d) WE43 implant within rat model after 1 week and 4 weeks. (e, h) Histograms of the gas pocket size in (e, f) TRC Mg–Zn–Ca alloy and (g, h) WE43 alloy implant group after 1 week and 4 weeks of implantation.

Fig. 7

Histological appearance of the H&E stained tissue after (a, b) 1 week and (c, d) 4 weeks of in vivo testing. The inserted images in (a) and (b) show macrophages lining the pocket wall for TRC Mg-0.5Zn-0.5Ca alloy and multinucleate giant cells and macrophages for WE43 alloy respectively (indicated by *). Dashed lines indicate the tissue in direct contact with the implant.