A small metal tube, roughly the size of the tip of a little finger, can be permanently implanted inside a blood vessel. Its job is simple: support the arterial wall so that it does not collapse and allow blood to flow freely. But one question has long lingered in the biomaterials research community: Does the metal really remain inert once it has been implanted?
The answer, apparently, is no.
drg. Bonifasius Primario Wicaksono, M.Biotech., a researcher from the Biomedical Engineering Program at the Graduate School of Universitas Gadjah Mada, published a scientific review examining the phenomenon of ion release from two of the world's most widely used intravascular stent materials: 316L stainless steel (SS 316L) and cobalt-chromium L605 (CoCr L605). The research was presented at the 5th ICICI-BME international forum in Bandung in November 2017 and formed part of UGM's intravascular stent research program funded by Indonesia's Ministry of Higher Education.
When Arteries Narrow and Metal Becomes the Answer
Cardiovascular disease is the leading cause of death in Indonesia. Data from the Indonesian Ministry of Health's Basic Health Research indicate that 26.4 percent of deaths in Indonesia are attributable to cardiovascular disease, with a prevalence of 0.5 percent of the population. One of the major contributing conditions is atherosclerosis, in which plaque accumulates along arterial walls and obstructs blood flow.
One of the most commonly selected treatments is the implantation of an intravascular stent, a mesh-like metal tube inserted into a narrowed blood vessel through a minimally invasive procedure. SS 316L and CoCr L605 dominate the global stent market because of their high mechanical strength. SS 316L offers a favorable balance between strength and elongation, making it relatively easy to shape when balloon expansion deploys the stent inside an artery. CoCr L605, meanwhile, has a higher elastic modulus—210 GPa compared with 190 GPa for SS 316L—allowing it to be manufactured with substantially thinner struts without sacrificing radial strength.
On paper, both materials appear ideal. The problem emerges after the stent is implanted, when the metal begins interacting with a complex biological environment.
Nickel Ions, Acidity, and Unexpected Reactions
The surfaces of SS 316L and CoCr L605 are covered by a thin oxide layer that provides protection. In SS 316L, this layer contains Fe, Cr, Ni, Mo, and Mn. In CoCr L605, it consists primarily of Co and Cr. Both layers contain OH⁻ ions, which represent a point of vulnerability.
H⁺ ions in bodily fluids, particularly when pH decreases, bind to OH⁻ in the oxide layer and disrupt the stability of the ionic bonds. As a result, metal ions can be released into the surrounding tissue. Studies cited in the review demonstrate a direct correlation: the lower the fluid pH, the higher the concentration of released nickel ions. At a pH of 4.3, ion release was substantially greater than at pH 6.3. For CoCr L605, chromium ion release approached zero at pH values above 6 but increased sharply when the pH fell below that level.
In addition to chemical factors, the stent's design also plays a role. Stents with numerous sharp bends are more susceptible to microscopic gaps in areas of high curvature, which can trigger crevice corrosion and further increase ion release. Scanning electron microscopy revealed microfractures in curved regions of both materials.
“Nickel and chromium ions released from stainless steel and cobalt chromium as intravascular biomaterial may cause damage to human tissues, where the stent was placed. Other investigation and research to reduce the amount of nickel and chromium ion release is needed, to make a safer biomaterial for intravascular stents.” — drg. Bonifasius Primario Wicaksono, M.Biotech., et al.
From Allergies to Restenosis: A Lifelong Risk
Released nickel ions are not merely passive contaminants. Clinically, nickel exposure can trigger dermatitis, allergic reactions, and even asthma. Within blood vessels, the consequences may be more serious, including local immune reactions, acute inflammation, and, in certain cases, DNA mutations. Nickel and molybdenum ions from stainless steel and cobalt-chromium alloys have also been directly associated with in-stent restenosis, the re-narrowing of an artery within the stent itself. This risk may be particularly concerning in patients with a history of metal allergies.
What makes the situation particularly complex is the fact that stents are generally implanted for life. There is no routine “replacement” procedure. This means that patients may potentially be exposed to ion release for decades.
However, the review also provides an important qualification: as long as the rate of ion release remains controlled, the amount of released nickel generally remains below allergy thresholds and below daily dietary intake levels. The key factors are maintaining the stability of bodily fluid pH and selecting an appropriate stent design—two factors that can potentially be addressed through intervention.
This research is not an endpoint but rather a roadmap. Questions about how to design more inert stents or coat their surfaces to make them more resistant to H⁺ ion attack remain open. In biomedical engineering laboratories, research continues in pursuit of a small metal tube that can truly be trusted to remain safe throughout a patient's lifetime.
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Author: Anny Anggraini
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