A stent coating is a surface material placed on a metal or polymer mesh stent that is implanted in the body to hold open previously blocked arteries, and the coating usually serves one of three purposes. Passive stent coatings are inert materials that are meant to prevent rejection responses that can encourage the build-up of tissue on the stent itself, which is facilitated by circulating blood or cellular growth into the stent's openings from the endothelial wall. The endothelial wall is a thin layer of living cells or epithelium on the inside of blood vessels that has a tendency to grow into the holes in stent mesh and reestablish blockage of the artery. Drug-eluting stents also generally have two types of coatings, which are pharmacological substances designed to prevent restenosis or thrombosis as frequent side effects of stent implants.
Stent material that is designed to provide a passive surface that doesn't chemically or biologically interact with the body ranges from metals like gold to naturally-occurring plastic compounds like phosphorylcholine. Gold is one of the earliest types of stent coating to be tried and, due to the metal's inert nature in the human body, has shown both antibacterial effects as well as a natural ability to reduce thrombosis, a coagulation of blood around the stent surface. Stents coated with gold, however, have shown an increase in restenosis, or a reoccurring narrowing of the artery over time. Phosphorylcholine (PC) is similar chemically to the exterior layer of red blood cells, so it has also shown an ability to reduce thrombosis as a stent coating. One advantage with the PC stent coating is that it has shown minimal side effects past the six-month point for implantation, and this is attributed to the fact that the substance biologically mimics the exterior wall of red blood cells.
Other types of passive stent coating materials include heparin to reduce thrombosis, which may also be used in combination with drug-eluting coatings, and diamond-like carbon (DLC) nanometer scale films that have been shown to reduce instances of hyperplasia. Hyperplasia is an abnormal enlargement of the blood vessel due to cellular growth in the region. Both silicon carbide and titanium-nitride-oxide have also been tried as a form of passive stent coating. Silicon carbide can be bound to hydrogen, giving it an ability to reduce thrombosis, and titanium-nitride-oxide is used to coat stainless steel stents to reduce both hyperplasia and blood platelet binding to the stent wall.
Drug-eluting stent coatings are usually of a cytostatic nature. This means that the drug suppresses cell division in the region of the stent instead of actually killing off living cells as cytotoxic drugs do. While drug-eluting stents are much more expensive than types of stents with inert coatings, studies as of 2010 have shown that they can reduce restenosis rates after nine months from 36.3% with uncoated stents down to 8.9% with drug-coated versions. Repeat procedures and heart attacks after stent implantation were also considerably lower, at 8.8% for the average patient with a drug-eluting stent coating versus 21% for uncoated versions. Hirudin is one form of natural drug being studied as a drug-eluting stent coating as well, which is an anticoagulant compound obtained from the glands of leeches.