Covered stents (both for short or long-term) are used to treat a variety of medical conditions in neurology, cardiology, gastroenterology, urology, pulmonology, etc. Stent-based medical applications are covered by biocompatible polymers when either fluids redirection, sealing, leak prevention, tissue ingrowth or tissue restriction, are required.
Biocompatible Polymers Used for Stent Covering
Biocompatibility is defined as “the ability of a material to perform with an appropriate host response in a specific application” (Williams, 2008). However, the biocompatibility is determined according to the device and not only according to the material, and thus, the implant site, duration and intended use should be carefully considered. Medibrane’s engineers specialize in matching the right polymer cover for each medical application, after considering the designated site or tissue, mechanical, functional specification and the application’s unique geometry.
Silicone and Polyurethane
To restrict tissue ingrowth, prevent leaks, redirect fluids and enable fluid passage, stents are usually covered with non-porous, impermeable covers. Silicone and Polyurethane are the most used polymers for such stent covers.
Polyurethane is a family of co-polymers formed by a reaction between a polyol with a diisocyanate . Since a variety of diisocyanates and polyols can be used, a broad spectrum of material properties can be produced. Properties such as biocompatibility, hemocompatibility, mechanical properties (elasticity, strength and abrasion resistance) etc., are the reason polyurethanes covered stents are used in the medical device industry for many years.
Silicone is a polymer composed of repeating units of siloxane. Silicones can be synthesized with a wide range of compositions and properties. Since the 1950s silicone has been used in the medical device industry due to its low chemical reactivity, long term mechanical properties, stability, and, biocompatibility.
Medibrane’s Unique Adhesion Platform
Medibrane’s engineers improved adhesion forces as the first step for all covering options, since then it is known as Medibrane’s unique adhesion platform. The first step of the platform is surface activation that prepares the metal surface for bonding. The second step is encapsulation coating that encircles the stent’s struts to form a closed loop, or what we call, a thin tie layer. This generates a very strong connection between the cover and the encapsulated polymer.
Dip and Spray Covering Technologies
Polyurethanes and Silicones match the requirements for covering many medical devices such as; neurovascular stents, heart valves, clot retrievals, ureteral stents, gastrointestinal stents, etc. The polymer is applied onto the metal frame using dipping or spraying technology, resulting in the formation of an impermeable, non-pours cover. Dip coating is a very common technology due to the ability to achieve faster and cost-effective manufacturing process. Spray coating uses a nozzle that breaks the polymer into tiny droplets and deposit onto a horizontal revolving mandrel connected to a stent. This process eliminates gravitational forces and increases the vertical uniformity of the cover thickness. this technology is often used with long stents or hypo tubes requiring thickness homogeneity.
Advantages of Dip & Spray Covering Technologies
Create an impermeable, non-porous cover
Suitable for a variety of geometries
Can reach high accuracy with very low cover thickness
Enables faster and cost-effective process manufacturing with high yield
Wall thickness range 15-100 µm
Coating Tolerance ± 15% of nominal thickness
Coating weight tolerance ± 15% of nominal weight