The key challenges

In the vast domain of the endoprotheses the stents whatever they are, bare-metal (BMS), biodegradable, bioresorbable, covered, or drug-eluting stents (DES), must be biocompatible, stable in physiological fluids and under mechanical constraints. A long-term coating is a necessary evolution of great importance. The coating such as a film must be homogeneous, regular in its thickness, adhering perfectly at all high curvature points of the stent architecture. Moreover this film must also keep the mechanical properties of the metal alloy substrates (stainless steel, cobalt-chromium-molybdenum) and shape memory alloys such as Nitinol alloys. Long-term durability is understood twofold. First the substrate / film assembly must be mechanically stable overcoming the often encountered limitation of ultimate film delamination from the substrate immersed in liquids. Secondly, coating integrity must be achieved for biological safety, compatibility, and effectiveness preventing the in-stent restenosis.

Our Innovation 

Our innovation is based not on a chemical process but rather on the physics of the interactions at the substrate/film interface. The method is unique, original, and internationally patented (EP 09730734.2, US 12/937,201, JP 2011503460). We can deposit micrometric and nanometric thick films of polymers, especially biocompatible polymers, on any substrates (metallic, mineral, organic, composites…) whatever the shape and topology of the substrate into consideration (flat, rods, rings, curved and saddle-shaped, kinks…) whatever the surface of the substrate (smooth, rough, heterogeneous, regular or irregular textures…). The process allows us to obtain stable coating films perfectly adhering to the substrate even in a humid atmosphere or when immersed in liquids in a static or in a flow of the solvent. Furthermore, the film-coated substrate preserves the elastic and mechanical properties of the stents. The implementation, rapid and inexpensive, easily developable at large scales, allows us to design innovative advanced functional materials. By this process the new assembly makes it easier and more accessible to build an additional molecular layer for the pharmacological purpose (delivery of active drugs) concomitant with the functionality of the stent.

Our know-how

We illustrate our expertise with the FDA approved biocompatible polymer poly (ether-ether-ketone) PEEK on shape memory alloys Nitinol. To simulate the actual situation closest to the stent, the Nitinol alloy has been used as a helicoidal spring. The substrate/film assembly quality is mastered by encompassing all the spatial scales from the mesoscopic to the nanometric, both from the chemical and physical points of view. Notably any defects, such as voids, holes, cracks, dewetting areas, permanent sets, formed on the surface of films, in their thickness, or at the substrate/film interface are eliminated. Our method creates excellent film-coated substrate stable in dry, vapor, steam, and physiological liquids, preserving the stent integrity. The film responds to the specifications for physical testing:

• material analysis:

A visual assessment of the coating integrity on all appropriate surfaces was conducted to establish a baseline for comparison to coating characteristics after testing performed under other conditions. The continuity of the coating was checked by electron microscopy to track the eventual presence of any voids, holes, cracks and dewetting areas. The film appears regular, homogeneous, smooth (< 100 nm as measured by Atomic Force Microscopy). Coated film thickness is ideally close to 15 µm.

Surface of a PEEK-coated Nitinol wire as seen by Scanning Electron Microscopy (bar scale 100 µm)

Surface of a PEEK-coated Nitinol kink as seen by  Scanning Electron Microscopy (bar scale 200 µm).

• mechanical properties:

The quality and uniformity of the material preserves the thermo-mechanical properties of the stent, including stress and fatigue behavior. Radial dilation, torsion, bending, axial tension, axial and radial compression, crushing, kink resistance, were shown to be satisfactory. In particular the helicoidal coated substrates immersed in a physiological medium at 37°C were subjected to a mechanical stress in continuous cycles of axial compression / stretching equivalent to a heart rate of 60 beats per minute. Neither the film appearance, its structure, nor its adhesion were altered after more than two months.

• corrosion resistance:

The absence of corrosion byproducts, that may be toxic and cause other adverse biological and tissue responses, was checked by measuring the total quantity of particulates and ions that the coated substrate may deliver when immersed. The test labeling showed no release of sensitizing metals (< 0.0009 ppm) after more than two months in the conditions described above. In addition no surface damage after fatigue tests was noted by examination of the corrosion potential of the PEEK-coated Nitinol.

All our results indicate no delamination, breakdown or fragmentation of the coating and consequently the coated polymer film presents an excellent integrity and a potential for long-term durability addressing and keeping the functionality of the coated stent.

For further information, please contact us (Ref 01629-01)