A stent is a small, specialized mesh tube deployed inside a narrowed body lumen, most commonly a blood vessel, to restore and maintain proper blood flow. This medical device acts as an internal scaffold, providing mechanical support to compromised vessel walls following a procedure like balloon angioplasty. Stents rely on specialized materials that must be biocompatible and capable of withstanding the dynamic forces within the body. These materials are engineered to provide structural integrity, controlled drug delivery, and sometimes, a temporary presence.
The Structural Foundation: Metals and Alloys
The foundational structure of most stents is a permanent metallic scaffold, chosen for its strength and ability to resist the vessel’s tendency to narrow again, a process known as recoil. Early stents were made from 316L stainless steel, an alloy of iron, chromium, and nickel, offering excellent mechanical properties and corrosion resistance. However, stainless steel’s density requires the stent struts—the tiny beams forming the mesh—to be relatively thick to achieve adequate radial strength.
Modern permanent stents frequently utilize cobalt-chromium (CoCr) alloys. CoCr is approximately 45% stronger than stainless steel, allowing manufacturers to reduce the thickness of the stent struts without compromising radial support. Thinner struts improve the flexibility and maneuverability of the device during placement. They also minimize the amount of foreign material exposed to the vessel wall, potentially reducing the risk of an adverse biological reaction.
Another specialized metallic material is Nitinol, a nickel-titanium alloy, predominantly used for stents placed in peripheral arteries, such as the legs. Unlike the balloon-expandable stainless steel or CoCr stents used in the coronary arteries, Nitinol devices are self-expanding. This alloy exhibits superelasticity and shape memory, allowing it to be compressed into a small catheter for delivery. Once released, it automatically springs back to its pre-set, expanded shape. This flexibility is crucial for arteries in the limbs that are subject to constant movement, bending, and compression, allowing the stent to adapt to the dynamic environment.
The Functional Layers: Polymers and Drug Coatings
While the metal frame provides mechanical support, a functional coating is often applied to the surface to address restenosis, the re-narrowing of the vessel. Drug-eluting stents (DES) incorporate a thin layer of specialized polymers that carry anti-proliferative medications. The polymer acts as a carrier matrix, binding the therapeutic drug—often a “limus” agent like sirolimus—to the metal scaffold.
This polymer layer is engineered to control the drug’s elution, or release, into the surrounding vessel wall over a specific time frame, typically several weeks to months. Controlling the elution rate is important to prevent the excessive growth of scar tissue, the main cause of restenosis. The polymer’s composition dictates this release kinetic, with the drug diffusing out of the matrix as the polymer slowly degrades or dissolves.
Historically, some stents used durable polymers that remained on the scaffold indefinitely after drug release. Newer generations of DES increasingly employ biodegradable or bio-erodable polymers, such as poly(lactic-co-glycolic acid). These temporary polymers deliver the drug and then completely break down into inert, biocompatible substances like water and carbon dioxide. This approach avoids the long-term presence of a foreign polymer, which has been linked to chronic inflammation and delayed healing of the vessel lining.
Material Lifespan: Permanent Versus Bioresorbable Stents
The decision to use a permanent metal scaffold or a temporary structure depends on the long-term goal of the intervention. Permanent stents, made of stainless steel or cobalt-chromium, remain in the body for the patient’s lifetime, providing continuous support to prevent vessel collapse. While they fulfill the immediate need for mechanical stability, their enduring presence is a limitation because the vessel cannot fully return to its natural, flexible state.
This limitation led to the development of Bioresorbable Scaffolds (BRS), which provide temporary support before completely disappearing. These scaffolds are made from materials the body can metabolize and absorb over time, serving as a transient structure. The goal of BRS is to provide mechanical scaffolding for a few months—the period when the vessel is most prone to recoil—and then vanish, allowing the vessel to restore its natural function and flexibility.
Polymer-Based BRS
One class of BRS utilizes high-grade polymers, such as poly-L-lactic acid (PLLA), which maintains structural strength for approximately six to twelve months post-implantation. After the support phase, the PLLA scaffold begins to break down, with complete absorption often taking about eighteen to twenty-four months.
Metal-Based BRS
Another approach uses absorbable metal alloys, primarily based on magnesium, which offers a faster degradation profile. Magnesium-based scaffolds lose their mechanical integrity within a few months and are fully absorbed into the bloodstream as inorganic ions within nine to twelve months. This material strategy focuses on temporary intervention and complete vessel restoration.