E. McCummiskey, W. M. Dempster, D. H. Nash, University of Strathclyde, Glasgow, United Kingdom; T. Ashton, D. Stevenson, Vascutek Terumo, Inchinnan, United Kingdom

Summary: The Mechanical Engineering Department at the University of Strathclyde and Vascutek Terumo have been fostering close links for several years to assist in product development. Currently the research team is investigating a more conclusive method of design and life prediction of their ‘Anaconda’ nitinol endovascular device using a finite element model to establish the product life in vivo. Abdominal aortic aneurysms are areas of localised swelling in the abdominal aorta due to a weakening of the arterial wall. Being asymptomatic in most cases, many go unnoticed until rupture, at which point high fatality rates are observed. For those which are detected, often other ailments prohibit the patient from open surgery and so an endovascular method of repair, using a device manufactured from polyester fabric and superelastic nitinol wire, wound into coils and sutured to the fabric, is now in production. This device is then implanted through small incisions in the femoral arteries. The main focus of the research at this time is to ensure that the superelastic nitinol material model entered into the finite element model is accurate and to identify the influence of uncertainty in the material model, particularly due to the effects of asymmetric tension-compression stress- strain characteristics and the changes to these properties during low cycle loading. To this end, a programme of testing has been carried out on the material to establish its specific stress-strain characteristic for these conditions Firstly, the behaviour of superelastic SE508 has been investigated in tension and compression. As the nitinol used in the device is in wire form, this proves difficult to test in compression. To this end, two sizes of tubing were obtained from Euroflex with properties matching the SE508 wire as closely as possible. The tubing was tested in tension and compression and wire was tested in tension and from these results the compression characteristic of the wire was extrapolated out. Secondly, an investigation was carried out into the low magnitude cycling effects to determine when the material properties remain steady. Wires were cycled through 400 cycles and the curves at each cycle recorded to ascertain not only the overall change, but also rate of change of this transformation. It was deemed necessary to complete these tests due to the wide variability in nitinol characteristics, including its process dependency and temperature dependency. Finite element analysis is used to model the proximal ring of the device in operation and impart the strain behaviour of the device in vivo. As it proves difficult to tests these devices in real conditions the model must be heavily relied upon to ensure accuracy. To this end it is important to understand the accuracy limitations and any short comings of the finite element model. The findings from these investigations are discussed in the context of the finite element model together with the effects of these material characteristics on calculation accuracy.