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 â€œNickel 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%â€“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.Â Â 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.Â 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.”