A heart stent is a small, mesh-like tube placed into a narrowed artery to restore proper blood flow, commonly used to treat coronary artery disease. These devices act as internal scaffolds, physically holding the vessel open to prevent re-closure following a balloon angioplasty. The success of a heart stent relies on specialized materials that must be compatible with the human body while providing necessary mechanical support. The selection of materials, ranging from metal alloys to biodegradable polymers, dictates the stent’s performance and biological interaction once implanted.
Permanent Metallic and Polymer Components
The structural core of most modern heart stents, including both bare-metal stents and the framework of drug-eluting stents, consists of specific metal alloys designed for strength and biocompatibility. First-generation permanent stents utilized 316L stainless steel, known for its good mechanical properties and corrosion resistance. While stainless steel provided adequate structural support, it often required a thicker strut design, which was associated with higher rates of tissue re-narrowing.
Today, cobalt-chromium (CoCr) and platinum-chromium (PtCr) alloys are the predominant materials for permanent stent platforms. CoCr alloys, such as L605, offer superior radial strength and a higher elastic modulus, enabling the use of ultra-thin struts (typically 70 to 90 micrometers). PtCr alloys further enhance the stent’s visibility under X-ray imaging (radiopacity) due to the addition of platinum, a significant advantage during implantation.
Some permanent stents also incorporate non-drug-eluting polymers, which act as inert coatings to improve hemocompatibility. For instance, certain second-generation stents have used polymers like phosphorylcholine, found naturally in cell membranes, to reduce platelet adhesion and minimize the risk of blood clot formation. These permanent polymers are designed to remain on the metallic scaffold indefinitely.
The Composition of Drug-Eluting Coatings
Drug-eluting stents (DES) utilize a specialized coating system to prevent the excessive tissue growth that causes the artery to narrow again (restenosis). This coating is composed of two main elements: an active antiproliferative drug and a polymer carrier matrix. The drug is typically a “limus” agent (e.g., sirolimus, everolimus, or zotarolimus), which slows the proliferation of smooth muscle cells in the artery wall.
The polymer carrier is the delivery vehicle for the drug, controlling the rate at which the medication is released into the vessel wall over time. Early DES coatings used durable, non-degradable polymers like poly(ethylene-co-vinyl acetate) (PEVA) or poly(styrene-b-isobutylene-b-styrene) (SIBS) that remain permanently after the drug is fully eluted. However, the long-term persistence of these durable polymers has been linked to chronic inflammation and a risk of late-stage blood clots.
Newer generations of DES often employ biodegradable polymers, such as Poly(lactic-co-glycolic acid) (PLGA) or Poly-L-lactic acid (PLLA), for the coating. These polymers release the drug and then harmlessly break down into simple compounds like carbon dioxide and water over a period of months. This temporary presence provides drug support during the initial healing phase, after which the polymer resorbs completely, potentially reducing the risk of long-term inflammatory issues.
Bioresorbable Scaffold Materials
Bioresorbable scaffolds (BRS) are designed to provide temporary support to the artery before dissolving completely over one to three years. This temporary nature allows the artery to return to a more natural, uncaged state after the initial injury and remodeling are complete. The most common material used for BRS scaffolding is the bioresorbable polymer Poly-L-Lactic Acid (PLLA).
PLLA is a semi-crystalline polymer that offers the high mechanical strength necessary to support the artery immediately after implantation. Once the scaffolding’s role is complete, PLLA slowly degrades through hydrolysis and is metabolized by the body. Another approach uses absorbable metallic alloys, primarily those based on magnesium.
Magnesium-based scaffolds, like the Magmaris, provide a metallic backbone with higher initial radial strength compared to polymer scaffolds. These metallic scaffolds degrade into magnesium ions, which are naturally present in the body, over about a year. Both PLLA and magnesium-based scaffolds are typically coated with a biodegradable polymer and an antiproliferative drug to control the healing response.
Why Material Choice Matters
The selection of stent materials is a complex engineering challenge governed by a strict set of biological and mechanical performance requirements. One primary consideration is radial strength, the ability of the stent to withstand the inward pressure of the vessel wall and maintain an open lumen. Cobalt-chromium and platinum-chromium alloys are favored in permanent stents because they offer high radial strength even with very thin struts, minimizing disruption to blood flow.
Biocompatibility is another requirement, ensuring the material does not provoke an inflammatory or immune response in the body. Materials must also possess adequate flexibility to be maneuvered through the arteries and expand without fracturing during the procedure. Furthermore, the material must be hemocompatible, minimizing the risk of blood clot formation on its surface, often assisted by specialized coatings or polished metallic surfaces.
The trade-off between material properties is evident when comparing permanent metal to absorbable scaffolds. Permanent metals provide the highest long-term structural support, but their permanence can be a drawback for vessel flexibility over time. Conversely, bioresorbable polymers like PLLA offer the advantage of temporary presence, but they typically have lower initial radial strength and require thicker struts to compensate.