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===Two Polymer Layers===
===Two Polymer Layers===
A drug free layer of polybutyl methacrylate forms a barrier between the drug containing polyethylene vinyl acetate layer (PEVA) and the vessel wall in the Cypher drug eluting stent.  The outerlayer serves as a barrier to drug diffusion slowing the release of the drug to the surrounding tissue, and protects the vessel tissue from the non-biocompatible PEVA.  Even with this barrier present, the two layer polymer stent releases more anti-restenonic drug into the surrounding tissues than does the single polymer layer.
A drug free layer of polybutyl methacrylate forms a barrier between the drug containing polyethylene vinyl acetate layer (PEVA) and the vessel wall in the Cypher drug eluting stent.  The outerlayer serves as a barrier to drug diffusion slowing the release of the drug to the surrounding tissue, and protects the vessel tissue from the non-biocompatible PEVA.  Even with this barrier present, the two layer polymer stent releases more anti-restenonic drug into the surrounding tissues than does the single polymer layer.
== Problems with the polymer layer ==
Although initial results showed drug eluting stents to be more effective in reducing restenosis, further research has revealed that the polymer coating is linked to stent {{WP|thrombosis}} that occurs anywhere from a few days to three years after implantation of the stent.<ref> Cutlip, Donald.  Coronary Artery Stent Thrombosis.  Uptodate for Patients. http://www.uptodate.com/patients/content/topic.do?topicKey=chd/71579. June 12 2008.
4</ref>  This phenomenon is reduced to the same level as with bare metal stents as long as patients continue to take anti platelet therapy. <ref>Ge Junbo, Limus-eluting Stents with Poly-L-lactic Acid Coating.  Asia-Pacific Cardiology Volume 1 - Issue I. Published: December 2007. http://www.touchcardiology.com/articles/limus-eluting-stents-with-poly-l-lactic-acid-coating</ref>
Scanning electron microscopes have been used to examine the polymer coating following balloon expansion, scientist believe that these defects in the polymer layer may contribute to the increased level of thrombosis[4].
== Currently Used Manufacturing Practices ==
American Patent No. 2008/0114449 A1 describes a method for manufacturing stents that allows different grain sizes to be achieved in different areas of the stent, to provide different values of yield strength <ref>Daniel J. Gregorich et al. United States Patent Application. US 2008/0114449 A1: MEDICAL DEVICES AND METHODS OF MAKING THE SAME.  May 15, 2008. http://www.google.com/patents?id=e5WqAAAAEBAJ&printsec=abstract&zoom=4&dq=making+heart+stents#PPA1,M1</ref>.  Grain size α 1/yield strength. Areas of the stent with different widths to optimize drug delivery are subjected to the same amount of force during deployment.  By changing the yield strength of these areas some of the defects described above by Ge Junbo regarding defects in the polymer from expansion could be avoided.  The following image is taken from the patent application and details where the different sized grains are located.  The area with the smaller grain size has the larger yield strength.  The connector areas (Wc) can sustain the strain of expanding the stent, the high yield strength in this area also allows the connectors to conform to the vessel walls more easily.  By employing multiple microstructures the stent can have both radial strength (from the large grains) and flexibility (from the high yield strength of the smaller grains)


== References ==
== References ==

Revision as of 04:07, 19 November 2008

ABSTRACT

Template:MECH370

This article will review whats new and exciting where the worlds of cardiology and materials science meet. I hope to look at ways to improve energy efficiency in manufacturing, bio-compatibility and function

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

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: • Biocompatible • 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]

MP35N Cobalt Nickel

MP35 Cobalt Nickel is composed of: 35% Ni, 35% Co, 20% Cr and 10% Mo. Besides being used in the manufacturing of heart stents this alloy is often used in marine environments because of its resistance to corrosion in harsh environments. Key Characteristics: • Biocompatibility • High Strength • Toughness • Ductility • Corrosion resistance [10]

Drug Eluting Coating

In early experiments the drugs designed to prevent restenosis were fused directly with the stents. The drastic shape change that resulted when the stent was expanded to fill the blocked vessel caused the drug coating to crack and flake off. Consequently, the drug was not delivered evenly over the entire surface of the stent. Scientists realized that by impregnating a polymer with the anti-restenosis drugs and fusing this polymer to the metal stent they not only avoided cracking and uneven drug delivery, but that they were also able to control the rate and timing of drug release. As with the selection of metal used, the polymer selected must also be bio-compatible. The drugs are imbedded in a polymer matrix. Factors that control the release of drug from the matrix include the concentration of drug loaded into the matrix, and the permeability of the polymer. Selection of polymer characteristics and drug concentration is determined based on the method of action and properties of the anti-restenotic drug selected[11] Different approaches for controlling the release of the drug are described below.

Single Polymer Layer

The Taxus stent is an example of a coronary artery stent coated in a single polymer layer (chemical name: poly(styrene-b-isobutylene-b-styrene). Initially higher levels of drug are released from the polymers surface, after a few days this rate is decreased and a low level of drug release is sustained.

Two Polymer Layers

A drug free layer of polybutyl methacrylate forms a barrier between the drug containing polyethylene vinyl acetate layer (PEVA) and the vessel wall in the Cypher drug eluting stent. The outerlayer serves as a barrier to drug diffusion slowing the release of the drug to the surrounding tissue, and protects the vessel tissue from the non-biocompatible PEVA. Even with this barrier present, the two layer polymer stent releases more anti-restenonic drug into the surrounding tissues than does the single polymer layer.

Problems with the polymer layer

Although initial results showed drug eluting stents to be more effective in reducing restenosis, further research has revealed that the polymer coating is linked to stent W that occurs anywhere from a few days to three years after implantation of the stent.[12] This phenomenon is reduced to the same level as with bare metal stents as long as patients continue to take anti platelet therapy. [13] Scanning electron microscopes have been used to examine the polymer coating following balloon expansion, scientist believe that these defects in the polymer layer may contribute to the increased level of thrombosis[4].

Currently Used Manufacturing Practices

American Patent No. 2008/0114449 A1 describes a method for manufacturing stents that allows different grain sizes to be achieved in different areas of the stent, to provide different values of yield strength [14]. Grain size α 1/yield strength. Areas of the stent with different widths to optimize drug delivery are subjected to the same amount of force during deployment. By changing the yield strength of these areas some of the defects described above by Ge Junbo regarding defects in the polymer from expansion could be avoided. The following image is taken from the patent application and details where the different sized grains are located. The area with the smaller grain size has the larger yield strength. The connector areas (Wc) can sustain the strain of expanding the stent, the high yield strength in this area also allows the connectors to conform to the vessel walls more easily. By employing multiple microstructures the stent can have both radial strength (from the large grains) and flexibility (from the high yield strength of the smaller grains)

References

  1. 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
  2. Heart Attack Multimedia. Heart Hub for Patients. http://www.hearthub.org/hc-heart-attack.htm American Heart Association. 2008
  3. 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.
  4. 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.
  5. Burt Cohen. Angioplasty.org. Drug Eluting Stent Overview. http://www.ptca.org/des.html. September 2008.
  6. 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
  7. 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.
  8. 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.
  9. STI Laser Industries. Stents. http://www.sti-laser.com/products.htm
  10. The A to Z of Materials. MP35N – Composition, Properties and Applications of MP35N Nickel-Cobalt Alloy by Alloy Wire International. http://www.azom.com/details.asp?ArticleID=4200. 2000-2008.
  11. Boston Scientific. Elements of DES Design. Chapter 2: Polymer Carrier. http://www.bostonscientific.com/templatedata/imports/HTML/coronaryflash/ElementsDES/Chp2/US_CH2.html. 2008.
  12. Cutlip, Donald. Coronary Artery Stent Thrombosis. Uptodate for Patients. http://www.uptodate.com/patients/content/topic.do?topicKey=chd/71579. June 12 2008. 4
  13. Ge Junbo, Limus-eluting Stents with Poly-L-lactic Acid Coating. Asia-Pacific Cardiology Volume 1 - Issue I. Published: December 2007. http://www.touchcardiology.com/articles/limus-eluting-stents-with-poly-l-lactic-acid-coating
  14. Daniel J. Gregorich et al. United States Patent Application. US 2008/0114449 A1: MEDICAL DEVICES AND METHODS OF MAKING THE SAME. May 15, 2008. http://www.google.com/patents?id=e5WqAAAAEBAJ&printsec=abstract&zoom=4&dq=making+heart+stents#PPA1,M1

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