Covered Stents: Engineering Overview of Membrane Function and Modern Manufacturing Improvements

Covered stents differ from bare-metal and drug‑eluting stents because they include a membrane layer, commonly ePTFE, polyurethane, or Fabric. This layer changes the device from a simple scaffold into a sealing structure. For engineers, the membrane is the defining element that enables perforation sealing, aneurysm exclusion, and containment of unstable plaque or thrombus.

What the Membrane Does Clinically

In a coronary perforation, the membrane creates an immediate physical barrier that stops extravasation and prevents tamponade.In aneurysms, it blocks flow into the sac, allowing it to thrombose and reducing wall stress.

In fistulas, it mechanically closes the abnormal connection and restores physiologic flow.In saphenous vein grafts, it holds friable plaque against the vessel wall and reduces distal embolization.In thrombus‑rich lesions, it contains the material behind a single sealed lumen and restores patency when other tools fail.

Engineering Advantages Provided by the Membrane

The membrane enables sealing, isolation, and containment functions that standard stents cannot perform. It allows rapid hemodynamic stabilization and provides a percutaneous alternative to surgery in complex anatomies.

Engineering Limitations Introduced by the Membrane

Because the membrane adds bulk and stiffness, deliverability through tortuous or calcified pathways becomes more difficult.

Many covered stents require high deployment pressures, which increase the risk of tearing and vessel trauma.Membrane shortening during expansion remains a challenge, as does side‑branch occlusion due to complete surface coverage.PTFE‑based coverings endothelialize slowly, increasing thrombotic risk. Restenosis, particularly at the edges, is common and often relates to mechanical injury or the structure of legacy membrane assemblies.

Typical Failure Modes

Thrombosis can occur due to delayed healing and altered flow at the membrane edges.

Restenosis results from inflammation or incomplete coverage.

Tearing can eliminate the sealing effect.

Malapposition stems from stiffness and high profile.

In vein grafts, bulky designs may disturb plaque and trigger embolization.Side‑branch occlusion appears whenever the membrane spans an ostium.

How Medibrane Technologies Reduce These Limitations

Microlayer Polymer Deposition

This process produces extremely thin, uniform polymer layers that keep mechanical strength but greatly improve flexibility.

Because the layers are thinner and smoother, they decrease overall stent profile and ease delivery through tortuous anatomy.

The improved crossing performance directly addresses one of the major weaknesses of traditional thick PTFE membranes.

ADHERA Surface Activation

ADHERA increases chemical bonding strength between the stent surface and the polymer membrane.

With stronger adhesion, the membrane no longer requires a “sandwich” configuration (inner and outer layers).

ADHERA allows the membrane to be placed only on the internal diameter or only on the external diameter, depending on the clinical and mechanical requirement.

This reduces material usage, improves flexibility, and avoids unnecessary stiffening.

Sutureless Lamination of ePTFE and Fabric

Much of the bulk described above comes from how the membrane is attached, not from the membrane itself. ePTFE bonds to almost nothing, so classic covered stents sinter two ePTFE layers to each other through the open cells at 337 to 390 °C. The weld holds the layers together, not the frame: the strut sits trapped in a pocket, never bonded, with voids at its shoulders. The second layer exists because it is the adhesive, which is why these constructions cannot go thin or single-sided. Fabric covers carry a different penalty, since every stitch of a sewn cover is a needle hole, a stress riser, and a potential leak path.

Medibrane bonds the membrane instead. For ePTFE, ADHERA surface activation forms a Chemical bond between the bare metal and the membrane, so one layer bonded at every strut is enough, placed on the internal diameter only or the external diameter only. Lamination runs at roughly 100 to 200 °C, far below the sintering window, and the extruded ePTFE sleeve elongates over 300 percent against 50 to 70 percent for sintered ePTFE, so it crimps and foreshortens with the frame rather than shifting against it. No shifting, no voids.

For fabric, Fusicloth Fabric carries a biocompatible binding polymer layer on an implantable fabric such as woven polyester, and laminates onto nitinol at 180 °C (266 °F) with no sutures, conforming to complex frame shapes. The fabric keeps its strength, sealing, and tissue ingrowth, without the needle holes. In either material, a bonded single layer can go down to 20 to 30 microns, against roughly 225 microns for a sintered double-layer encapsulation.

The gains map directly onto the limitations above: lower profile and stiffness, better crossing and apposition, no needle hole or weld terminus for a tear to start from, foreshortening absorbed by an elastic membrane, no strut void for thrombus to form in, and a pore structure never taken near the PTFE melt. Because the bond is placed by design rather than by a furnace, it can also be partial or confined to selected zones, which is the basis of the selective bonding described next.

Combination of ADHERA and Selective Bonding

When surface activation is combined with selective bonding, the membrane can be attached only in specific zones rather than across the entire surface. This offers several engineering benefits:

• Reduced overall stiffness of the construct

• Improved stent maneuverability during navigation

• Less stiffening of the delivery catheter

• Lower loading forces during crimping

• Ability to use smaller‑diameter delivery systems

• Better handling and trackability in complex anatomy

Selective bonding with strong adhesion allows membrane fixation only where it is needed for mechanical stability, without forcing a full‑surface lamination that increases rigidity.

Phosphorylcholine (PC) Coating

PC mimics natural cell membranes and increases hemocompatibility. Faster endothelialization lowers thrombosis risk and improves long‑term patency, addressing one of the historical disadvantages of covered stents.