Covered stents and scaffold-based medical applications are used to treat a variety of medical conditions in cardiology, gastroenterology, urology, Pulmonology, etc. The reasons for covering a medical device are vast and may vary from sealing and fluid redirection to preventing migration or atraumatic coating to prevent damage to tissue. Stents can be covered with porous or non-porous covers depending on the cover requirements, that depend on the function, target tissue, duration of use, etc.
Years back, the medical industry has shifted to the use of minimally invasive procedures using self-expandable and balloon-expandable stents. This shift presented a dramatic change in technologies and materials used for covering. This change is due to the fact that stent grafts have to be crimped to be inserted into narrow delivery systems, and thus, properties such as crimping force and loading force became essential. This trend has increased the use of polymers and technologies enabling low cover thickness (wall thickness of 15-100 µm) that are homogenous and accurate and are suitable for a large variety of geometries.
Polyurethane is a family of copolymers formed by a reaction between polyol and diisocyanate in the presence of a suitable catalyst. Since a variety of diisocyanates and polyols can be used, a broad spectrum of material properties can be produced and there are hundreds of different types of polyurethanes. Polyurethanes are highly versatile, they can be used for coating and can be formulated to provide good biocompatibility and hemocompatibility. They have high endurance, strength, elasticity, and abrasion resistance, they can be tailored to resist body fluids, and have been used in the medical device industry for many years.
Thermoplastic Polyurethanes (TPU’s)
Thermoplastic polyurethane (TPU) belongs to the thermoplastic elastomer family. It’s high strength, elasticity, and resistance to abrasion provides an overall enhanced durability to the end product. TPU’s are dissolved in a solvent and then applied onto stent-based devices using dipping or spraying technologies. These techniques result in the creation of an impermeable (non-pours) cover, with high mechanical strength and a relatively low thickness. For that, TPU dip covers are among the most common stent covers and are used in diverse medical devices such as; non-vascular stents, clot retrievers, filtration devices, etc.
Dip & Spray Covering Technologies
Dip coating is a very common technology due to the ability to achieve a fast and cost-effective manufacturing process. Spray coating uses a nozzle that breaks the polymer into tiny droplets that deposit onto a horizontal revolving mandrel connected to a stent. This process eliminates gravitational forces and increases the vertical uniformity of the cover’s thickness. This technology is often used with longer stents or hypo tubes requiring thickness uniformity. Both dip and spray covering provide impermeable covers with very low cover thickness (wall thickness of 15-100 µm) and high thickness homogeneity for a vast variety of stent geometries.
TPU Lamination Technology
Medibrane’s engineers have vast experience in finding ‘out of the box’ solutions to stent covering challenges. One of the techniques they use is TPU lamination. TPU can be dissolved in solution and then molded into a tube or a membrane that can be laminated using heat and pressure onto the stent-based device. This enables the covering of very complex geometries with very thin and accurate covers. It also enables covering with only one layer of very thin cover, on the inner diameter or the outer diameter, farther reducing the overall cover thickness and crimping profile. For this reason, it can be used in cases where the cover thickness and crimping profile are crucial. Another advantage of this technique is the ability to generate a selective covering whereas the TPU membrane can be attached to the stent only in specific regions. This technique is usually implemented when ultra-low crimping profile and radial forces are required.
Porous TPU Stent Covers
Medibrane’s engineers can drill laser holes in the medical device’s TPU cover to create a porous cover enabling tissue ingrowth to enhance clinical outcomes such as prevent migration. This technique is useful in cases where stent grafts require a very thin porous cover and TPU is the material of choice.
Polyurethane Covering Technologies;
Polyurethane Covering Is Suitable For:
Peripheral stent grafts
Intra vascular pump
Intra bronchial valve
Wall thickness range: 15-100 µm
Coating Tolerance: ± 15% of nominal thickness
Coating weight tolerance: ± 15% of nominal weight