Zinc Corrosion Behavior Favors Bioabsorbable Stents

Researchers at Michigan Technological University (MTU; Houghton, USA) examined Zinc as a candidate for a bioabsorbable cardiac stent material since it is necessary in a myriad of biological processes, exhibits antiatherogenic properties, and possesses outstanding ductility. Besides serving as physiologically as a promoter of healthy vessels, it also combines the best behaviors of iron and magnesium (the current bioabsorbable stent materials), and could supplant them as the favored base metal for bioabsorbable cardiac stents.

To examine critical aspects of biocorrosion, the researchers cut Zinc wires of 99.99%+ purity into 15 mm segments, and smoothed them to avoid any burrs that would hinder implantation. The wires were also subject to UV illumination to eradicate organic contamination at the molecular level, and placed in the abdominal aorta of rats by puncturing the arterial adventitia. After a period of 1.5, 3, 4.5, or 6 months, the rats were euthanized and the wires were then harvested for analysis and the state of the artery documented during necropsy. To preserve any corrosion layer on the wires’ surfaces, the attached tissue was dehydrated by immersing the samples for a short time in absolute ethanol.

The results showed that the wires that remained in the biological milieu for 1.5 and 3 months showed signs of relatively uniform corrosion. The remaining metallic cores of both explants had ragged edges in cross section, which are posited to correspond to the removal of material by semi-localized dissolution. After 4.5 and 6 months in vivo, the shallow, evenly distributed features yielded to relatively severe, localized corrosion. In all cases, tissue adhered preferentially to points of localized attack. A limited amount of microscale corrosion particulate was firmly embedded in the still-attached arterial tissue surrounding the metallic zinc core. The study was published early online on March 14, 2013, in Advanced Materials.

“Metallic zinc appears to be one of the few physiologically acceptable metals that have not yet been considered for application in a bioabsorbable stent,” said lead author Jeremy Goldman, PhD, of the MTU department of biomedical engineering. “Zinc is widely acknowledged as an essential element for basic biological function, as it participates in nucleic acid metabolism, signal transduction, apoptosis regulation, and gene expression, in addition to interacting with a variety of organic ligands.”

The researchers also stressed that the toxic potential of a zinc stent should be negligible, since assuming complete degradation within one year of the stent, the expected daily dose of zinc would be 150 μg per day, which is just a fraction of the recommended daily value for infants, and a mere 4% of the recommended daily intake for adult males, making the systemic toxicity of a zinc stent nonexistent.

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