Bioresorbable stents

In medicine, a stent is any device which is inserted into a blood vessel or other internal duct in order to expand the vessel to prevent or alleviate a blockage. Traditionally, such devices are fabricated from metal mesh and remain in the body permanently or until removed through further surgical intervention. A bioresorbable stent, (also called biodegradable, or naturally-dissolving) serves the same purpose, but is manufactured from a material that may dissolve or be absorbed in the body.

The use of metal drug-eluting stents presents some potential drawbacks. These include a predisposition to late stent thrombosis, prevention of late vessel adaptive or expansive remodeling, hindrance of surgical revascularization, and impairment of imaging with multislice CT.

To overcome some of these potential drawbacks, several companies are pursuing the development of bioresorbable or bioabsorbable stents. Like metal stents, placement of a bioresorbable stent will restore blood flow and support the vessel through the healing process. However, in the case of a bioresorbable stent, the stent will gradually resorb and be benignly cleared from the body, leaving no permanent implant.

Studies have shown that the most critical period of vessel healing is largely complete by approximately three months. Therefore, the goal of a bioresorbable or “temporary” stent is to fully support the vessel during this critical period, and then resorb from the body when it is no longer needed.

Bioabsorbable scaffolds, or naturally dissolving stents, that have been investigated include base materials that are either metals or polymers. Those that have been approved in markets around the world and thus have gained the most traction are based on polymers that are similar to those used in dissolvable stitches.

Metal stent candidates are iron, magnesium, zinc and their alloys.

Iron stents were shown using an in vivo evaluation method based on the murine abdominal aorta to generate an iron oxide-filled cavity in the vascular wall. This behavior significantly narrowed the lumen and generated a potential site for rupture of the endothelium after stent degradation.

Magnesium is a relatively new biomaterial that has recently been gaining traction. While degrading harmlessly, it has been shown to possess a functional degradation time of about 30 days in vivo. This is much short of the three-to-six month window desired for bioabsorbable stents. Thus, much attention has been given to drastically reducing the rate of magnesium corrosion by alloying, coating, etc. Many novel methods have surfaced to minimize the penetration rate and hydrogen evolution rate (or, in layman's terms, the corrosion rate). One of the most successful has involved the creation of bioabsorbable metallic glasses via rapid solidification. Other, alternative solutions have included the development of magnesium-rare earth (Mg-RE) alloys which benefit from the low cytotoxicity of RE elements. Coatings and sophisticated materials processing routes are currently being developed to further decrease the corrosion rate. However a number of issues remain limiting the further development of Mg biomaterials in general.

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