Elsevier

Materials Letters

Volume 264, 1 April 2020, 127285
Materials Letters

Mechanical properties, degradation behaviors and biocompatibility of micro-alloyed Mg-Sr-RE alloys for stent applications

https://doi.org/10.1016/j.matlet.2019.127285Get rights and content

Highlights

  • Microalloyed Mg-Sr-RE alloys are prepared with the total alloying addition restricted to 0.5 wt%.

  • Mg-Sr-RE alloys show 0.5–2 times increment of ductility than Mg-Sr.

  • Mg-Sr-RE alloys exhibit 45–66% reduction of corrosion rates compared with Mg-Sr.

  • Mg-Sr-RE alloys stimulate the endothelial cells proliferation and endothelialization related gene expression.

Abstract

This article describes the development of Mg-Sr-RE (RE = La, Nd, Gd) alloys based on a microalloying strategy for stent applications, which aims to control the biosafety and improve the degradation simultaneously. The results reveal that the designed Mg-Sr-RE alloys are close to a single-phase solid solution with the total alloying addition restricted to 0.5 wt%. Mg-Sr-RE alloy exhibits 0.5–2 times increment of ductility while 45–66% slower corrosion rates than binary Mg-Sr. Both the indirect and direct cell assays indicate that Mg-Sr-RE alloys reduce HAVSMCs proliferation, stimulate the HUVECs growth and endothelialization related gene expression, which may benefit in the healing of stent-assisted vascular injuries.

Introduction

Magnesium alloys have shown considerable potential in the case of biodegradable cardiovascular stents. However, available magnesium alloys are confronted with challenges in passing regulatory biosafety evaluations for official approvals. One negative concern is the incorporation of different alloying elements, especially non-essential metals, in magnesium, as these metals are not necessary for life and may have a tendency to accumulate in organs [1], [2]. Many magnesium alloys with essential nutrients have been developed for biomedical application, including Mg-Ca [3], Mg-Sr [4], Mg-Zn [5] based alloys.

Metals usualy have a dose-effect relationship in toxicology. In trace amounts, although some alloying elements are non-essential, they will not cause adverse effect on human health [6]. Microalloying could also prevent the formation of large amount of intermetallic precipitates and thus generate an optimized degradation behavior. Hence, the development of magnesium alloys with trace addition of alloying elements might be a potential strategy to control the biosafety and improve the materials degradation simultaneously. According to the microalloying concept, rare element (RE) and strontium (Sr) were added into magnesium in trace level. From the view point of biosafety, RE and Sr exhibit dose depedent cell behavior and low concentration of them is harmless to human body. Metallurgically, RE elements could weaken the texture and thus improve the ductility of magnesium alloys with the alloying concentration as low as 0.2 wt% [7]. Take into account the beneficial effects of RE on magnesium corrosion, they could be suitable microalloying candidates for biomedical magnesium alloys. Sr was also chosen with respect to its beneficial effect on the mechanical, corrosion and biocompatibility of magnesium alloys according to our previous study [8]. Furthermore, Mg-0.5Sr alloy increased the HUVECs viability and showed lower risk of thrombosis than WE43 stent [9]. Consequently, three micro-alloyed Mg-Sr-RE alloys were designed and prepared. The total alloying addition was restricted to no more than 0.5 wt% and the designed alloys are close to a single phase solid solution. The mechanical properties, corrosion and cytocompatibility were systematically investigated.

Section snippets

Materials and methods

Mg-Sr-La, Mg-Sr-Nd and Mg-Sr-Gd alloys were casted, homogenized at 550 °C for 6 h and then extruded at 395 ℃. Binary Mg-Sr alloy was prepared as control. The chemical compositions were analyzed and the results are listed in Table S1. The microstructure was observed using an optical microscope (Nikon, LV100ND) and an environmental scanning electron microscope (ESEM, Quanta 200 FEG) equipped with an energy-dispersive spectrometer (EDS). X-ray diffraction (XRD, Bruker, D8 focus) was employed to

Results and discussion

Fig. 1a shows the microstructures of the as-extruded Mg-Sr-RE alloys. The average grain size of Mg-Sr alloy is decreased from 39 ± 4.0 μm to 29 ± 3.6 μm, 15 ± 2.8 μm and 12 ± 1 μm with adding trace amount of La, Nd and Gd, respectively. Chemical analysis indicates that the content of Sr and three RE elements in the alloys is below their solid solubility in Mg (Table S1). XRD patterns demonstrate that there exist no second phases (Fig. 1b). The present designed Mg-Sr-RE alloys are therefore

Conclusions

The microalloying Mg-Sr-RE alloys reveal fine-grained microstructure and high ductility (EL: 20–40%). The corrosion rates are reduced by 45–66% than that of Mg-Sr control. Surprisingly, RE microalloying could significantly improve endothelial cell attachment, spreading and proliferation on the surface of Mg-Sr-RE alloys during 10 days’ culture, while halt the smooth muscle cell proliferation. Taken together, microalloying Mg-Sr-RE alloys are demonstrated with promising features for stent

CRediT authorship contribution statement

Hongyan Tang: Investigation and Writing - original draft. Fan Wang: Investigation. Dan Li: Investigation. Xuenan Gu: Writing - review & editing, Supervision. Yubo Fan: Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key R&D Program of China (2018YFC1106600), A Foundation for the Author of National Excellent Doctoral Dissertation of PR China (201463), Young Elite Scientists Sponsorship Program By CAST (2017QNRC001), Beijing Natural Science Foundation (2192027).

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