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Blood triggered corrosion of magnesium alloys

https://doi.org/10.1016/j.mseb.2011.06.006Get rights and content

Abstract

Intravascular stents manufactured out of bioabsorbable magnesium (Mg) or Mg-alloys are considered as auspicious candidates for the next stent generation. However, before clinical application numerous physical and biological tests, especially to predict the clinically highly important degradation kinetics in vivo, have to be performed. In a Chandler-Loop model, the initial degradation of eight different magnesium alloys during 6 h in contact with human whole blood was investigated. The magnesium release varied between 0.91 ± 0.33 mg/cm2 (MgAl9Zn1) and 2.57 ± 0.38 mg/cm2 (MgZn1). No correlation could be found with Mg release data obtained after immersion in simulated body fluid (SBF). This pilot study showed that Mg corrosion is highly influenced by the biological test environment (SBF or blood, etc.) and that a modified Chandler-Loop model with human whole blood may be superior to predict corrosion of Mg alloys under clinical conditions than the SBF models presently used.

Introduction

During the last decade considerable advances were achieved in intravascular stent techniques for the treatment of arterial disease. In particular, drug eluting stents partly replaced bare metal stents [1]. Meanwhile, several more biological stent types, like the CD34 antibody coated endothelial progenitor cell (EPC) capture stent, are under verification in patient studies [2]. However, results of several clinical studies indicate that the ideal stent is not yet available, and therefore, industries as well as the scientific community keep on working hard on the development of more sophisticated solutions for the next stent era. At the present time, two principle priorities in stent development can be identified, (a) developing long lifetime and fully hemocompatible stents [3] and, (b) designing fully degradable stents made from polymers or metals [4]. The advantage of the latter stents is that the mechanical support for the stenosed artery is only temporary, and that after the required healing process of the artery, the stents get resorbed, avoiding potential disadvantages using a permanent metallic stent, e.g. risk of late thrombosis. First clinical trials support the safety profile of magnesium stents, however, after 4 months the stents were completely degraded [5].

Therefore, slower degradation rates are requested to provide sufficient radial force to improve long-term patency rates. Before launching a new stent material to clinical application, numerous in vitro tests have to be performed. However, at present there is a lack of appropriate short and long-term models that mimic in vivo conditions for reliable screening of potential Mg alloys [6], [7]. Very few studies have been done in a physiologically accurate system such as fresh human whole blood under flow conditions [8], [9], [10].

The aim of this in vitro study was to investigate the corrosion behavior of eight different experimental magnesium alloys after 6 h immersion in a modified Chandler-Loop model with fresh human whole blood and to compare the results with the corrosion data in PBS as SBF.

Section snippets

Materials

Eight experimental magnesium alloys without Mn content were investigated [11]. The alloys tested and their composition can be seen in Table 1. From each alloy 18 samples with a diameter of 10 mm and height of 1 mm were polished with SiC 1200 in ethanol on both sides and ultrasonically cleaned in ethanol for 3 min.

Chandler-Loop experiments in heparinized human whole blood

Experiments were performed using a well established in vitro closed loop model (modified Chandler-Loop) using 3 IU high-molecular-weight heparin (Heparin-Natrium-25000-ratiopharm

Analytical determination of Mg release

The results of mean magnesium release after 6 h in blood and PBS are given in Fig. 1 for all Mg-alloys. Due to an immense corrosion attack in PBS, accompanied by loss of sample shape and the occurrence of precipitates in the tubes the electrolytes of MgAl9 could not be analyzed. On the contrary, the lowest values in human whole blood were found with MgAl9 (1.23 ± 0.17 mg/cm2) and MgAl9Zn1 (0.91 ± 0.23 mg/cm2). The corrosion rates of the other Mg-alloys were nearly twice as high with values between

Discussion

The introduction of intravascular stents was a major breakthrough in interventional cardiology and radiology. However, numerous clinical studies with both bare metal stents and drug eluting stents showed several limitations. Especially, in-stent stenosis, late stent thrombosis and problems with repeat revascularization are still unsolved major critical issues. Therefore, the ideal stent should support a fast healing of the intima, prevent hyperplasia, possibly by an antiproliferative drug, and

Conclusion

This pilot study showed that the appraisal of the in vitro corrosion results of magnesium alloys is highly influenced by the biological test environment (SBF, etc.). In human whole blood, the lowest corrosion values (Magnesium release) were found for the MgAl9 (1.23 ± 0.17 mg/cm2) and MgAl9Zn1 (0.91 ± 0.23 mg/cm2) alloys. The corrosion rates of the other Mg-alloys were nearly twice as high with values between 2.16 ± 0.33 and 2.57 ± 0.38 mg/cm2, respectively. In contrast, the corrosion of MgAl9, the alloy

Acknowledgement

This study was partly supported by AiF (project no: KF0548101PK7).

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