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Drug-eluting stents are small, wire scaffolds that are inserted into coronary arteries that have become narrow due to the development of atherosclerosis. The stent helps to hold the artery open and also releases a drug that prevents any further blockage or obstruction occurring in the artery. The stent is made up of three parts: the stent platform, the stent coating and the drug that is eluted by the stent.
The stent’s frame is made of a metal alloy and has a mesh-like design that allows it to expand. Sometimes, the stent’s design also allows for the creation or enlargement of side openings for vessels. Different metal alloys are used, one of the most common being a cobalt chrome (L605 CoCr) alloy, which is stronger and more resilient that stainless steel.
With the cobalt chrome alloy, struts can be thinner (because the metal is so strong), which reduces the extent of restenosis. This alloy also contains less nickel than stainless steel and is therefore less likely to cause allergy. Without the drug, the stent is called a bare metal stent.
The coating is composed of a polymer that can hold and release the drug into the arterial wall. The coating lies between the drug stores and the stent and transfer of the drug takes place when contact is made with the arterial walls. The first few coatings to be approved were durable but more recent types are designed to biodegrade as the drug is released or afterwards.
In most cases, the coating is sprayed over the stent or the stent is dipped in the coating. The coating may be composed of up to three layers, one layer to act as an adhesion layer, one to hold the drug and one to slow down the release of the drug so that its effects can be extended.
The drug is the vital component of the stent, preventing smooth muscle cell proliferation and neointimal growth. This growth normally results in re-narrowing or re-stenosis of the artery In most cases, immunosuppressive and anti-proliferative agents are eluted from the stent to prevent this restenosis. The Cypher (J&J, Cordis) stent contains the immunosuppressant sirolimus, which is eluted over a thirty-day period. The Taxus (Boston Scientific) stent contains the anti-proliferative paclitaxel, which is eluted over a 90-day period.