Thin-film nitinol (NiTi): A feasibility study for a novel aortic stent graft material

Abstract

Although technological improvements continue to advance the designs of aortic stent grafts, miniaturization of the required delivery systems would allow their application to be available to a wider range of patients and potentially decrease the access difficulties that are encountered. We performed this feasibility study to determine if thin-film NiTi (Nitinol) could be used as a covering for stent grafts ranging from 16 mm to 40 mm in diameter. Specifically, we wished to determine the profile reduction attainable and improve the flexibility of our design. Using a novel hot-sputter deposition technique, we created sheets of thin-film NiTi (TFN) with a tensile strength of >500 Megapascal (MPa) and thickness of 5-10 microns. TFN was used to cover stents, which were then deployed in vitro. Patterned thin film was fabricated via a lift-off technique; grafts were constructed with stents ranging from 16-40 mm and deployed in a pulsatile flow system from the smallest diameter polymer tubing into which the stent and TFN would fit. The bending/stiffness ratio vs similar sized expanded polytetrafluoroethylene (ePTFE)-covered stents was also determined. TFN was created in both non-patterned and patterned forms, with a tensile strength of >100 MPa for the latter. We created devices that were successfully deployed via delivery systems half the size of fabric-covered stent grafts (ie, the 16 mm stent graft that originally was delivered via a 16French (F) system was reduced to 8F, and the 40 mm stent graft delivered via a 24F system was reduced to 12F). No migration of the devices was observed with deployment in both straight and curved tubing, which was sized so that the stent grafts were oversized by 20%. Both forms of the thin-film were noted to be more flexible than the same sized ePTFE stent graft, and the patterned graft had an additional 15-30% flexibility vs the non-patterned film. These in vitro results demonstrate the feasibility of TFN for covering stent grafts designed for placement in the aorta. The delivery profile can be significantly reduced across a wide range of sizes, while the material remained more flexible than ePTFE.