Materials used in stent construction

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The materials used in coronary stents must be flexible, supportive, capable of expansion, and biocompatible. Typically, foreign materials implanted into the human body results in trauma, inflammation, immune response and eventual healing and/or scarring. Materials that are not biocompatible can induce many complications, including cytotoxic chemical buildup and chronic inflammation.

Most stents are built on a stainless steel platform, the least-expensive stent material available. Unfortunately, stainless steel is not fully compatible with the human body and implantation usually is followed closely by restenosis and thrombosis. In addition, stainless steel can pose difficulties related to some types of imaging, such as magnetic resonance. As such, researchers are working to develop alternative platform materials such as gold, titanium, cobalt-chromium alloy, tantalum alloy, nitinol and several types of polymer.

Materials that are not biocompatible can cause one of any number of complications. The ideal coronary stent surface does not cause a reaction in the human body.

For quite some time, it has been known that gold is biocompatible and usually inert, as well as highly visible. Cobalt-chromium was first developed for use in watch springs. Newer variations have proven to be effective stent materials. Tantalum is a shiny, flexible, and highly radio-opaque metal. While it is more brittle than stainless steel, tantalum has proven to be quite resistant to corrosion.

Nitinol (55% nickel and 45% titanium and named from the “Nickel Titanium Naval Ordinance Laboratory, sometimes called NiTi) is highly biocompatible, decreases the rate of corrosion, is very flexible and has excellent shape memory when heated to a certain temperature. Unfortunately, nitinol can be difficult to manufacture; as little as a 0.01% change in composition can drastically alter the temperature at which the alloy is transformed. In addition, the allow must be created in a vacuum as the titanium component is highly reactive to air-borne oxygen and nitrogen particles.

Certain polymers have found use as stent materials. Silicone (a condensation polymer) induces low rates of tissue trauma, but it also presents challenges in terms of biodurability, tensile and coil strength, and inner-to-outer diameter. Polyethylene and polyurethane have been used as stent materials, however, they have been found to induce sludge in 20%–30% of patients. These materials also encourage protein adherence and biofilm formation. While polyurethane is one of the most reactive of stent materials used, it does have good tensile and coil strength.  The number and type of polymers developed for use in medical devices is expanding as different polymer types, chemistries and manufacturing processes are used to produce devices or device coatings with a wide variety of functional characteristics.

Some polymers are biodegradable, bioabsorbable, or bioerodible. Biodegradable or bioabsorbable stents contain a major component (such as an enzyme or microbe) that degrades quickly enough to make them appropriate for short-term uses. A bioerodible polymer is a water-insoluble polymer that has been converted into a water-soluble material. Biodegradable materials can form an effective stent coating because they can be mixed with an antirestinotic drug and will degrade within a few weeks, thus releasing the drug into the surrounding tissue and reducing the risk of restenosis. Examples of biodegradable polymers are: polyesters, polyorthoesters and polyanhydrides. Collagen is also very biocompatible and reduces the rate of restenosis and thrombosis. In addition, anticoagulants and fibrinolytic agents bound to the collagen can aid in drug delivery.

However, studies have shown that the stent surface after biodegradation can be very uneven and, as such, can induce various cells to adhere to the surface. This in turn produces an increased risk of complications.

Shape-memory polymers can be used to produce a device that will transition from a temporary state to a different (permanent) state through the inducement of a stimulus of heat or cold.


See the MedMarket Diligence report #C245, “Worldwide Drug-Eluting, Bare Metal and Other Coronary Stents, 2008-2017.”

13 thoughts on “Materials used in stent construction

  1. Fernando Pastrana

    Very good recopilation of different material used in stents fabrication and also a not deep review about biocompatibility is a good start to began ti research

  2. Angie Firmalino

    Thank you for this article. I am one of thousands who are suffering from a similarly made implant called Essure. It is a combination of nitonol, stainless steel, and PET fibers. We invite you to read some of our stories on http://www.essureprocedure.net. We also have a support group on Facebook called Essure Problems. Today there are four women from our group having hysterectomies because of Essure. The pregnancy being reported on that site is alarming as well! It’s a dangerous product without proper labeling or warnings! Interested in your opinion on this device! Thank you!

  3. Dan Downey

    Hi Pat:

    thanks for the article. our company manufactures state-of-the-art Vacuum Deposition Coating equipment and the processes (recipes) for coating formulations. Medical is a key market. Do you know any major stent manufacturers looking for coatings partners to advance innovation? our company: mustangvac.com for your viewing pleasure.
    Regards,
    Dan Downey

    ps: do you do any consulting work?

  4. P. Driscoll Post author

    Dan:

    Thanks for your post. I am not aware of any stent manufacturers who are looking for partners in this area, and would likely only know if they came to me directly, since that would likely be proprietary information.

    I do consulting, but it is highly dependent on there being a virtual perfect fit between what the client wants/needs and what my capabilities are, available resources, current workload, etc. Is your interest centered on the question in this post, or another?

    Email me at patrick [at] mediligence [dot] com with the specifics and I can see whether there is a fit.

    Patrick

  5. Benard

    This is a very well researched and articulated summary on stents. It can easily inspire one to venture into bio-materials
    Thank you.

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