A stent is a small, tube-like device placed inside a body passage to keep it open. These devices are commonly used to support narrowed or blocked structures, such as arteries. Stents provide a scaffold to maintain the patency of these vessels. The procedure of placing a stent is generally minimally invasive.
Primary Stent Materials
Stents are primarily manufactured from two broad categories: metals and polymers. Metallic stents, often called bare metal stents, were among the first types developed and remain widely used. Common metallic alloys include 316L stainless steel, which offers strength and corrosion resistance. Cobalt-chromium alloys are also frequently utilized, providing higher strength that allows for thinner stent struts compared to stainless steel. Nitinol, a nickel-titanium alloy, is known for its shape-memory and superelastic properties.
Polymeric materials represent another class, distinguishing between durable and biodegradable types. Durable polymers, such as certain polyurethanes, remain in the body long-term. Biodegradable, or bioresorbable, polymers are designed to naturally break down and be absorbed by the body over time. Examples include poly-L-lactic acid (PLLA) and poly(lactic-co-glycolic acid) (PLGA).
Material Properties and Suitability
The selection of stent materials depends on their specific properties, which influence their suitability for implantation. Biocompatibility is a primary consideration, meaning the material must not provoke an adverse reaction or be toxic to the body’s tissues. Materials are chosen to minimize irritation and allow for proper healing around the implanted device.
Mechanical properties are important, ensuring the stent can perform its function effectively. Radial strength is needed to resist compressive forces from the vessel wall and maintain the vessel’s open diameter. Flexibility is necessary for the stent to navigate anatomy during insertion and conform to the vessel’s natural movements. Stents must exhibit fatigue resistance to withstand continuous pulsatile forces of blood flow over many years.
Radiopacity, or visibility under X-ray imaging, is another property that allows clinicians to accurately place and monitor the stent. Materials like platinum or iridium enhance this visibility. For drug-eluting stents, the material, often a polymer, is designed to release medication in a controlled manner into the surrounding tissue to prevent re-narrowing of the vessel.
Material Interaction with the Body
Once a stent is implanted, its materials interact with the body’s biological systems. An initial inflammatory response is a natural reaction to the presence of a foreign object and mechanical injury during placement. This response involves the activation of cells like platelets and leukocytes at the site of implantation. Material choices and stent designs aim to minimize this inflammation to promote proper healing.
Tissue integration and healing are important processes that occur over time. Endothelial cells, which form the inner lining of blood vessels, ideally grow over the stent struts, a process called endothelialization. This re-covering of the stent surface is important for long-term function and to reduce the risk of complications. For biodegradable stents, the material undergoes biodegradation or resorption. This means the stent gradually breaks down into harmless components absorbed by the body, leaving no permanent implant. This process typically occurs over several months to a few years, allowing the vessel to regain its natural flexibility.
Long-term considerations related to material-body interaction include the potential for late-stent thrombosis, which is the formation of blood clots within the stent, and restenosis, which is the re-narrowing of the vessel. The specific material properties and how they influence the biological response, such as inflammation and tissue healing, can affect the likelihood of these long-term issues. For instance, some polymer coatings on drug-eluting stents can delay complete healing, potentially influencing these outcomes.