Many medical applications require covering for a variety of clinical outcomes such as; leak prevention, tissue ingrowth of tissue restriction, managing blood flow, etc. Stent polymeric covers can be either porous or impermeable, depending on the biocompatible polymer and covering technology selected. Biocompatibility is defined as “the ability of a material to perform with an appropriate host response in a specific application” (Williams, 2008). Biocompatibility is tested and determined by the final device, rather than the by material alone, and thus, the implant site, duration, and intended use should be considered. We use polymers that were investigated clinically and have a long history of use in the medical device industry, such as; medical grade Silicone, Dacron, ePTFE, and a variety of thermoplastic Polyurethanes. The selected polymer influences the cover parameters together with the covering technology, as they both determine the thickness, radial strength, crimping profile, as well as porous or non-porous nature of the cover. Our engineers have vast experience with dipping, spraying, and lamination technologies and can assist you in choosing the best technology and polymer according to the product’s cover specification.
First Step for All Medibrane’s Covering Solutions; Our Unique Adhesion Platform
Medibrane’s engineers improve adhesion forces as the first step for all covering options using Medibrane’s unique adhesion platform. The platform’s first step is surface activation used to prepare the metal surface for bonding with a polymeric material. This treatment changes the chemistry of the metal surface to provide better mechanical attachment between the cover and the stent. The second step is the encapsulation coating. The coating encircles the stent’s struts to form a closed-loop; this is very similar to the suturing technique. The polymer cover is then implemented onto this closed-loop, or what we call a thin tie layer. This generates a very strong connection between the cover and the encapsulated polymer.
Impermeable, Non-Porous Covers
To restrict tissue ingrowth, prevent leaks, redirect fluids, and enable fluid passage, stents are covered with non-porous, impermeable covers. Silicone and Polyurethane are the most used polymers to generate such covers. The polymer is dissolved and then applied onto the metal frame using dipping or spraying technology. Dip and spray covering technologies are very common due to the ability to achieve a fast, repetitive, and cost-effective manufacturing process. Polyurethanes can also be implemented using lamination technology. TPU can be dissolved in solution and then molded into a membrane that can be laminated using heat and pressure onto the stent-based device. It 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. These covering technologies are suitable for a variety of geometries and can reach high accuracy with very low cover thickness. These covers match the requirements of many medical stents such as; neurovascular stent grafts, clot retrievals, ureteral stent grafts, gastrointestinal stent grafts, etc.
To enable tissue ingrowth, prevent migration, and redirect fluids, stents are covered with porous covers. Dacron and ePTFE are the most used polymers to generate porous stent covers. The polymer is applied onto the metal frame using suturing or lamination technologies.
Fabrics are historically sutured and not laminated since lamination of 1-layer without surface activation of the metal will normally result in delamination, and 2-layer lamination with fabric will result in a very thick cover with a high crimping profile, or constrain the stent completely. Suturing is a manual, time-consuming procedure, non-repetitive, and harder to scale up, compared to an automated process. Although suturing enables covering with only 1-layer, 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. ePTFE, on the other hand, can be laminated with 2-layers. Lamination technology uses temperature and pressure to connect the cover to the metal scaffold. The lamination of 2 layers generates a “sandwich-like” cover, as the 2 layers adhere with a strong connection at the overlapping area. 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 compared to the usual lamination process, of up to 400⁰C. The effect on the thermal properties of the nitinol stent should be carefully considered before using this process. These techniques work great only in situations when the increase in thickness and crimping profile do not pose a problem.
Medibrane’s engineers overcame this challenge by developing an innovative technology in which 1-layer of dacron or ePTFE can be laminated onto the stent without the need for sutures while maintaining strong adhesion. The ability to maintain strong adhesion forces with only 1-layer of cover is achieved due to Medibarne’s unique adhesion platform. ePTFE and dacron are commonly used in structural heart devices such as; heart valves, septal occluders, left atrial appendage implants, mitral valve clips, in stents for blood vessels larger than 10mm, to prevent blood clots, and a variety of additional applications.