state of the art research report in Mechanical engineering field

John Doe

Russ Engineering Center

Wright State University

Dear Ms. Alysoun Taylor-Hall:

I am submitting my formal report, “Stents: The Designs, Materials, and Machinery,” to satisfy the EGR 3350/5350 course requirements for the spring semester of 2014.

For any question, you can contact me at [email protected].

Sincerely,

John Doe

BME undergraduate

Technical Communications for Engineers and Computer Scientists

EGR 3350:03

Formal Report

State-of-the-art

Stents: The Designs, Materials, and Machinery

John Doe

Ms. Alysoun Taylor-Hall

4/18/2014

Table of Contents

Page #

Transmittal Letter 1

Title Page 2

Abstract 4

Introduction 5-6

Discussion 6-13

Conclusion 14

Work Cited 15

Abstract

This formal report provides state-of-the-art research on stents.

Plaque buildup can block blood flow within vessels. The lack of blood supply in the body can cause severe pain or even death. A stent, which is a medical device, is inserted by doctors through arteries. Once the stent reaches the plaque, it expands outwards, forcing the plaque to break apart. Once the stent fully expands, blood flow regulates back to normal.

Design is a major factor on how well stents perform after implants. A stent must be structured to be anatomically and physiologically compatible with the body. Different modeled stents are able to perform different functions. Material is another factor that affects the overall performance of a stent. This report will mostly cover “Nitinol” since results have shown it is the most biocompatible material. Studies have also shown that Nitinol is one of the most corrosion resistant metals due to its protective surface layer. General information on how temperature and stress affect the shape of a Nitinol stent will be discussed.

Stent manufacturing discussion is also provided in this report. Laser processing has become the predominant stent cutting method in the market. Technology has a major effect on how stents perform and with modern laser processing, stents are able to have precise and accurate cuts. Specific advantages of laser processing will be discussed in the machinery section.

The formal report provides state-of-the-art research on how designs, materials, and machinery collaborate to make a lifesaving medical device known as a stent.

Stents: The Designs, Materials, and Machinery

Introduction

Purpose

The purpose of this report is to provide state-of-the-art research on stents. This research satisfies the EGR 3350/5350 course requirements for the spring semester of 2014.

Problem

In the United States, approximately 1.5 million stents are placed in coronary arteries every year [1]. Reduced blood flow due to the buildup of plaque around vessel walls causes heart attacks and can lead to death. A stent, which is an expandable mesh tube, is used to reopen a damaged blood vessels.

Scope

This report will discuss topics concerning the designs, materials (specifically Nitinol), and machinery used on modern stents. A historical research about stents will not be included in the formal report.

Background

Build-up of plaque is known as atherosclerosis and can lead to a reduced blood flow in the arteries. Over time, plaque gets attached to the blood vessel walls and often can lead to an increase in the blood pressure. Plaque buildup could be caused by multiple of factors including smoking, unhealthy diet, or cardiovascular disease. A decrease in blood flow will reduce the bodies’ blood circulation, which causes severe pain; this restriction of blood supply is known as ischemia.

A stent is used by doctors to reopen blood vessels. By reopening the blood vessel (which increases vessel’s radius), blood flow will eventually regulate back to normal. Stents are inserted and transported through arteries, specifically arteries from the upper leg or near the elbow. When the stent reaches the damaged portion of the vessel, a balloon within the stent is inflated, causing the stent to expand, which increases the arterial radius. Once the stent attaches to the arterial wall, the balloon is then deflated and withdrawn out from the artery. The stent pushes outward on the artery, preventing any future blockages. Modern stents are coated with medicine to prevent restenosis, which is the reformation of the plaque buildup [2].

Discussion

Design

Overview

Stents are designs based on the anatomical structures of the blood vessels. A well designed model will enhance the stent’s performance. The ideal stent design will allow the blood vessel and the stent to collaborate as if they were one. The geometrical properties of a stent allow it to maintain its position by sticking to the inner-arterial tissue. The mechanical behavior of a stent must be flexible and compatible in order for it to be placed within a blood vessel. An illustration of a stent can be seen in Figure 1.

Rings

Connectors

Source: preview.turbosquid.com

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Figure 1

Stent models contain a mesh structure. Size and stiffness of a stent is strictly dependent on the region where the stent is placed within the blood vessel. The rings provide radial support and the connectors provide stability [3]. Radial support is a crucial factor in stent modeling; the stent exerts an outward force, keeping the arterial walls open so circulating blood is able to flow through. If the stent’s radial support does not function properly, plaque is then able to rebuild which can cause the walls to collapse once again. If the radial force of the stent fails, the force of the arterial walls can alternate the stent’s original shape. A stent implant is expected to last a life time, so the stability of the stent should not worsen.

The connectors within the stent provide stability by forcing each ring to be dependent on the one next to it. The connectors cause the radial force that is exerted by the stent to be equilibrated throughout all of the tube; this maintains a consistent stent/artery size. The mesh structure of the stent allows the circulating blood and the inner-walls of the arteries to interact. The mesh structure is a highly important factor of the design for keeping the arterial walls nourished.

Balloon inflation

Most stents designs contain a balloon in the center that gets inflated. When the balloon is inflated, the stent expands outward, forcing the stent to stick the arterial walls. The inflation of the balloon creates an opening (or widens the radius) within the plaque clog. Once clog is destroyed, circulating blood is then able to flow through. After the balloon has been fully inflated, the stent then exerts a slight force that pushes outward; this radial force prevents the blood vessel from collapsing. The balloon is then deflated and withdrawn; the stent is left within the vessel for a life time.

Coil vs. Cell

Stent designs will influence the performance of the implant. Different geometrical/mechanical aspects will affect how well the blood vessel behaves towards a certain stent.

An illustration of the structural differences between a coil and cell stent designs is seen in Figure 2.

Cell

Coil

Figure 2

Source: nchc.org

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Coil

· The coil stent design does not contain connectors which increases the flexibility of the stent.

· The lack of connectors in this design creates wider openings, which allow portions of the arterial tissue to droop.

· The lack of connectors also decreases the stability of the stent which causes the outward radial force of the entire stent to decrease [4].

Cell

· The closed-cell stent design contains an increase amount of connectors (connects each zigzag corner).

· The increase count connectors in this design increases the radial strength, which prevent the vessel from closing back up (restenosis).

· The increased amount of connectors causes this design to be less flexible than the coil design [4].

Material

1

Materials used on stents must be flexible, expandable, and biocompatible. According to Patrick Driscoll of MedMarket Diligence, incompatible materials (foreign substances to the body) will influence the immune system to trigger inflammation and trauma. Stainless-steel is the most common material used on stents because it is the least-expensive (compared to gold and Nitinol). Stainless-steel is not completely biocompatible, causing corrosion and restenosis. Results have shown that Nitinol has proven to be the most affective material when placed inside the human body [2].

Nitinol

Nitinol contains rare properties that cannot be found in other metals. Nitionl’s structure is highly elastic and biocompatible. The crystal structures within the material alter (due to temperature or stress) its shape. Nitinol is also corrosion resistant. Its super elastic feature allows it to be crush recoverable [5].

According to the National Drug Code, 55% of Nitinol is composed out of nickel. Nickel allows the Nitinol stents to have a transformable structure. Usually, an increase in the amount nickel within a substance causes corrosion, but what causes Nitinol to be highly biocompatible inside the body? The surface of a Nitinol stent is covered by an oxide layer, making the stent to be highly corrosion resistant. The oxide layer protects the inner material of Nitinol from disintegrating, allowing the stent to maintain its unique texture. This oxide layer is also found on dental archwires, so no matter what the pH level inside the mouth is, the oxide layer will protect the archwires from corroding [5].

An illustration of two stents can be seen in Figure 3 (Left: Stent with no protective layer) (Right: Oxide layer on the surface).

Corroded Stent

Nitinol stent (Oxide layer)

Source: [5]

Figure 3

Temperature and Stress

The radial force of a Nitinol stent is dependent on the change of temperature and stress that is acting on it. In low or high temperatures, Nitinol changes its crystal structure. This allows the stent to collaborate with the artery; if the artery expands (for ex: due to an increase in temperature when exercising), the stent expands along with it. Due to this factor, the stent works along with the arterial wall as if they were one, allowing the right amount of blood flow to circulate in the vessels.

Another factor that alters the shape of a Nitinol stent is stress. When stress is applied, the crystal structures within are altered (same with temperature). This allows the stent to change shapes due to the opposing force caused by the blood vessel walls. For an example, if the blood vessel’s radius decreases, the radius of the stent decreases along with it. Unlike other materials, Nitinol is also crush- recoverable. When stress is removed, the crystal structures recover back their normal shape [5]. This factor allows the stent to mimic the bodies anatomical and physiological functions.

Nitinol stents are manufactured in such a way that their radius is slightly larger than the blood vessels they are inserted into. This allows the stent to exert an outward force to prevent the blood vessel form collapsing (reformation of plaque build-up) [5].

Since Nitinol is easily altered by temperature change, the stents are specially packaged for protection. The stents are taken out of the package when they are needed for the implants [5]. Once it is inserted to the damaged portion of the vessel, the stent expands towards the vessel walls.

Medication

An illustration of a medicine coated stent can be seen below in Figure 4.

Figure 4 Source: nature .com

Many stents are coated with medicine. The purpose of having medicine on the stent is to locally release drugs [6]. The coated medicine gets degraded and released towards tissue walls. This process inhibits the reformation of restenosis (plaque buildup). Eventually, these drug molecules fully dissipate, but the structure of stent is remained. The development of drug eluting stents has decreased the rate of restenosis to below 10% [6]; making stent implants a reliable treatment for atherosclerosis.

Machinery

Overview

Over the years, technology has drastically advanced. Machines are now quicker and more precise than ever. Medical devices have evolved to be biocompatible with the human body. As the study of medicine improved, manufacturing did as well. Modern machinery is able to reach submicron levels [7]. Precise and accurate cuts are extremely crucial in machinery, especially when these devices are being inserted into the body. The slightest defects on medical devices could lead to death.

Laser Processing

Laser processing has become the leading technique for stent development. Laser processing is able to produce accurate and precise cuts; producing the same results every time, which makes laser processing cost effective. Laser processing results are 200% to 500% better than regular manufacturing processes [7]. A bulleted list of laser processing information can be seen in Figure 5. According to Aerotech.com, the diameter of a hair is close to 100 μm. The thickness of a stent wall is about 25% of a human hair. These measurements are extremely small in size so it is clear to see why laser processing has become the favored method for stent cutting. The Slightest error could cause the stent to not function properly. If the cuts on the stents are not accurate and precise, plaque buildup can occur once again.

Figure 5 Source: [7]

Aerotech Automation 3200 (A3200)

According the aerotech.com, the A3200 is a laser processing system (software) that offers 32 axes of synchronous motion. The A3200 is used in medical stent processing. The A3200 is able to program in circumferential units; this means that the machine will adjust to the inserted the cylinder (stent) size. This allows the A3200 laser to produce cuts for all different sizes of stents. The A3200 also provides contoured motion. Contoured motion is cutting of circular shapes and arcs (which are available on different stent models). The optimizing cutting velocity can be also controlled in A3200, this allows the stent to have a proportional surface texture [7]. An illustration of the Aerotech’s VascuLathe can be seen in Figure 6.

Figure 6

Source: aerotech.com

The slowed acceleration of cutting in the circular regions are calculated beforehand by the laser machine. The acceleration on the cutting laser can be set, but durring the cutting process, the machine is able to manipulate set values to produce the best performance [7]. In summary, laser procesing is the predominiat maufacturing method for stent production. Product results are consistent and accurate, which lead it to be more cost effective.

Conclusion

Plaque is able to build within blood vessel walls, this process is known as athersclerosis. The increase in the amout of plaque within a vessel can decrease blood flow; since there is less space for the blood to flow through, the pressure within the veseel increases causing high blood pressure. A stent, which is a medical device, is instered through arteries. Once the stent reaches the damaged portion of the vessel, the stent expands outwards. In order for a stent to be reliable, its design must be able to collaborate with the body. Stent desings contain a mesh structure, this allows the stent to be flexible. A mesh sturcture also maitains intercation between the blood and tissue of vessel walls. Most stent models contain a balloon in the center. When the balloom is inflated, it expands the stent out, which then widens the plaque clog. Stent materials must be flexible, expandable, and biocompatible. Results have proved that Nitinol is preferred since it is more corrosion resistant than stainless-steel. The oxide layer on the surface of Nitinol protects it from corroding. Nitniol is also able to change the shape of its crystals, this allows it to have a transformable structure. Modern stents are coated with medicine; the medicine prevents the restenosis process. To obtain the best stent performance, manufacturing must be reliable. Technology has drastically advanced over the years. Laser processing has become the predominant method for stent manufacturing. Laser processing is able to produce the finest cuts, making compatible within the body. Laser processing is able to produce the same stent results, which makes it more cost-effective. Lasers are able to produce cuts stents that are smaller than the thickness of a human hair. Cutting velocity and acceleration can be set manually by the user. Overall, modern stents are able to save millions of lives by simply reopening blood vessels to keep the body’s blood circulating.

Work cited

[1] Josip Tambaca, Suncica Canic, Mate Kosor, R. David Fish, David Paniagua. (2011). Mechanical Behavior of Fully Expanded Commercially Available Endovascular Coronary Stents. Retrieved from http://www.math.uh.edu/~canic/hemopapers/THIstents.pdf

[2] About P. Driscoll (2009). Materials used in stent construction. Retrieved from http://blog.mediligence.com/2009/06/11/materials-used-in-stent-construction/

[3] Nicolas Foin (2012). Drug Eluting Stent Designs and Bifurcation Bench Stenting. Retrieved from http://www.icimeeting.com/2012/images/stories/PDF/1424_Foin_Tue_A.pdf

[4] Vincent Hoang (2004). Stent Design and Engineer. Vimage. Retrieved from http://www.me.ucr.edu/sendesign/0405/team3/Stent_Design.doc

[5] Dieter Stoeckel, Alan Pelton, Tom Duerig (2003). Self-Expanding Nitinol Stents: Material and Design Considerations. Retrieved from http://www.nitinol.com/media/reference-lib rary/005.pdf

[6] Julie Shabto (2014). Bioabsorbable Coronary Stents. Retrieved from http://dujs.dartmouth.edu/wp-content/uploads/2011/03/14_pdfsam_11w_final.pdf

[7] Ken Hetrick (2009). LASER PROCESSING IN STENT PRODUCTION. Retrieved from http://www.aerotech.com/media/245989/TMDLaserProcessing.pdf

http://www.nchc.org.tw/tw/e_paper/sub_subject/index.php?EPAPER_ID=51&SUB_SUBJECT_ID=47

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