ABSTRACT
What are Heart Stents? When are they Necessary?
It can be argued that the heart is the most important muscle in the human body. It is responsible for pumping roughly 7192 litres of blood around the body each and every day [1]. Like all muscle tissue, the heart requires a blood supply to bring in oxygen and remove waste. The coronary arteries supply blood to the heart, but they can become narrowed or even blocked by plaque (cholesterol and other fatty deposits)[2]; when this happens the heart muscle doesn’t receive the required amount of oxygen to function properly and may eventually lead to a heart attack.
In order to open up the blocked coronary artery, doctors will carefully guide a deflated balloon to the blockage, where it is then inflated, pushing the plaque back up against the artery walls. This process is known as an W. When the balloon is removed the vessel is supposed to remain open, this however, isn’t always the case as the walls of the vessel can be weakened by the stretch of the balloon. According to the American Heart Association 40% of arteries opened with balloon angioplasty alone close again within six months [3] by placing a stent in the artery, the amount of restenosis (or closing of the artery following an angioplasty) is reduced to 25%[4].
A heart stent is a wire mesh tube that is placed in the artery during an angioplasty after the balloon is inflated. It remains in place and acts as a scaffold pushing the plaque back against the coronary artery walls. In 1986 the first metal stent was successfully implanted into a human heart in Toulouse, France by Jacques Puel and Ulrich Sigwart, but it wasn’t until 1994 that they were approved for use in the United States[5]. Research into why arteries continued to close 25% of the time even when a stent was present revealed that smooth muscle cells were growing in response to the “controlled injury” caused by the angioplasty, much in the same way scar tissue forms in response to injury elsewhere [6]. To prevent this from happening scientists and researchers began coating the metal stents in medication that interfered with the body’s tendency to develop smooth muscle cells around the stent. In 2003 the first study was published showing that these drug eluting stents decreased the changes of restenosis to 5%-7%.
The rest of this article will examine how bare metal and drug eluting stents are manufactured and what the future holds for this product.
Important Design Considerations
To be effective, a heart stent must be both strong enough to resist the recoil from the vessel after it has been opened, and flexible enough to be threaded through to the blockage without causing damage to the vessels it passes through on its way. In the case of a drug eluting stent, the drug must be fused with the stent in such a way as to ensure its flexibility, and controlled drug release. It is especially important in the case of drug eluting stents that the stent conform to the walls of the vessel to ensure even drug distribution. A heart stent can be broken down into two components: the metal that forms it’s base and provides the required strength, and the polymer coating that carries and releases the drug.
Choice of Metallic Base
Some common material choices for metal stents are described below:
316L Stainless Steel
This is the traditional choice for bare metal and drug eluting stents [7]. 316L is a low carbon steel that is resistant to corrosion and biocompatible. Its chemical makeup is below
Fe, <0.03% C, 16-18.5% Cr, 10-14% Ni, 2-3% Mo, <2% Mn, <1% Si, <0.045% P, <0.03% S [8].
Key Characteristics: • Bio-Compatible • Strong (Tensile Strength 485MPa, Rockwell Hardness 95) • Corrosive Resistant • Relatively easy to machine because of carbon content
The optimal hot working temperature for this metal is between 1150-1260°C. 316L is subject to work hardening if processing is done too quickly. The increase in brittleness that results is not a desirable characteristic in heart stents which are required to be flexible, as a consequence A to Z Materials recommends processing 316L at low speeds and constant feed rates.
L605 Cobalt Chromium
This is a new alloy choice for the manufacturing of heart stents. Key Characteristics: • Biocompatible • Greater elasticity and strength than 316L Stainless Steel L605 is available in tubes with appropriate diameters to fill the effected vessel (from 0.8mm to 2mm). These tubes are machined into the desired geometries using high precision lasers. Laser cutting is followed by extensive chemical and mechanical cleaning. [9] <layout name="Project" />
- ↑ Kristen Forrest, Denise Schnabel, and Margaret Williams. Math by the Month. Mathematics of the heart. http://my.nctm.org/eresources/view_media.asp?article_id=7309. February 2006
- ↑ Heart Attack Multimedia. Heart Hub for Patients. http://www.hearthub.org/hc-heart-attack.htm American Heart Association. 2008
- ↑ George Dangas, MD; Frank Kuepper, MD. American Heart Association. Restenosis: Repeat Narrowing of a Coronary Artery http://circ.ahajournals.org/cgi/content/full/105/22/2586 . 2002.
- ↑ George Dangas, MD; Frank Kuepper, MD. American Heart Association. Restenosis: Repeat Narrowing of a Coronary Artery http://circ.ahajournals.org/cgi/content/full/105/22/2586 . 2002.
- ↑ Burt Cohen. Angioplasty.org. Drug Eluting Stent Overview. http://www.ptca.org/des.html. September 2008.
- ↑ The Society for Cardiovascular Angiography and Interventions. Timeline: 30 Years of Progress in Interventional Cardiology. 2008. http://www.scai.org/pr.aspx?PAGE_ID=5188
- ↑ Boston Scientific. Elements of DES Design. Chapter 1: Stent Architecture. http://www.stent.com/templatedata/imports/HTML/cstent/IC/heart-stent-products/elementsDES/elementsDES.html. 2008.
- ↑ The A to Z of Materials. Stainless Steel - Grade 316L – Properties, Fabrication and Applications. http://www.azom.com/details.asp?ArticleID=2382#_Composition. 2000-2008.
- ↑ STI Laser Industries. Stents. http://www.sti-laser.com/products.htm