PTFE (Polytetrafluoroethylene) is a linear polymer consisting of fluorine and carbon molecules. ePTFE is the expanded version of PTFE that is responsible for its microporous structure. Due to its high strength-to-weight ratio, biocompatibility, high thermal resistance, low friction, long term stability, chemically inert qualities, etc., ePTFE is most suitable for the medical device industry. ePTFE can be stretched and deformed and shaped into multiple geometries such as; tapes, membranes, films, tubes, fibers, and sheets. ePTFE, having a microporous structure, is ideal for porous covering allowing both sealing and tissue in-growth, and thus, may prevent migration.
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). The medical device industry quickly adopted ePTFE and today it is commonly used for stent covering in many medical markets; from gastric self-expandable stents to vascular abdominal aortic aneurysm devices. ePTFE is also commonly used in cardiology, in structural heart applications such as; heart valves, septal occluders, left atrial appendage implants, mitral valve clips, etc.
ePTFE Covering Technologies
One of the disadvantages of using ePTFE is the technology in which it is usually implemented onto the stent; suturing. Suturing is a manual, time-consuming procedure, less repetitive and harder to scale up, compared to an automated process. Although suturing enables covering with only 1 layer of ePTFE, the sutures themselves create another “mini-layer” and generate stress in the area of the sutures. This is a disadvantage when it comes to crimping profile.
Another covering technology using ePTFE is lamination. ePTFE can be laminated onto the inner diameter of the stent, the outer diameter, or on both inner and outer diameter. Lamination technology uses temperature and pressure to connect the cover to the metal scaffold. Lamination of 2 layers generates a “sandwich-like” cover, as the 2 layers adhere with a strong connection in the overlap area. This works great in situations when the increase in thickness and crimping profile does not pose a problem.
Medibrane Sintering Technique
Another form of 2-layer lamination is the sintering process in which 2 layers of ePTFE sleeves are sintered onto each other and then adhered to the metal stent. This process requires higher temperatures than the usual lamination process; between 340⁰C and 400⁰C. The effect on the thermal properties of the nitinol stent should be carefully considered before using this process. Compared to the single layer sutureless lamination this technique involves high temperature and requires two layers that increase the crimping profile.
Medibrane’s Innovative Sutureless Lamination Technology
Medibrane’s engineers overcame the challenge of covering with only 1 layer of ePTFE without sutures while maintaining strong adhesion forces between the polymeric cover and the metal stent. This unique sutureless lamination technology is based on Medibrane’s adhesion platform. This platform was developed to improve adhesion between the metal stent and the polymer cover, two materials that do not normally bond together. The adhesion platform activates the metal’s surface and then an encapsulation coating encircles the stent’s struts, thereby generating strong adhesion between the stent and the covering. The strong bond generated between the stent and the cover enables our engineers to replace the previous suturing technique with a new, automated, repetitive, high yield technology, that enables a fast and easy scale up with high yield. The elimination of the need for sutures enables reduced cover thickness and crimping profile since the “mini-layer” of the sutures is no longer needed.
ePTFE Covering Technologies
- 1-layer sutureless lamination
- 2-layer “sandwich” lamination
- 2-layer sintering lamination
ePTFE Covering Is Suitable For:
- Heart Valves
- Left Atrial Appendage Implants
- Mitral Valve Clips
- Septal Occluders
- Peripheral Covered Stents
- Single or double layer (Inner diameter, outer diameter, or both inner and outer layers)
- Wall-Thickness range: 30 – 100 [µm]