The mechanical design aspects of a small diameter vascular prosthesis

Master Thesis


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University of Cape Town

Failure of medium to small diameter vascular grafts is believed to be in part due to the compliance mismatch between the native artery and the implanted graft. Consequently, designers are examining the use of more compliant materials for their manufacture. Ether free polyurethanes are currently amongst the most popular materials for use in biological implants although these materials are inherently too stiff for use in vascular prostheses. These materials can be made more compliant by introducing porosity. Apart from creating a more compliant overall material, under optimal biological conditions, the porosity may lead to cell in growth through the thickness of the graft allowing an endothelial cell layer to form on the inner flow surface. Compliance and cell ingrowth are both important characteristics that determine the successful functioning of the graft. The current work is part of a collaborative venture with the Cardiovascular Research Unit (CVRU) at the University of Cape Town to design and develop a new polyurethane graft. Finite element models are used to facilitate stress analyses and to evaluate the long-term behaviour and compliance of various graft designs made from a bio-inert thermoplastic polyurethane. Material properties of the polyurethane are determined from uniaxial tension tests, simple-shear tests and viscoelastic shear tests. The constitutive equations for a compressible, large strain hyper elastic material model with viscoelasticity are implemented in the finite element code using material constants calculated from the test data. The behaviour of the finite element model is verified by using a single element test and comparing results to the material data. The finite element model is validated for use m more sophisticated problems by comparing axi-symmetric models with in vitro experiments. An artery/graft anastomosis is then analysed by modelling the artery as an incompressible hyperplastic material. Further more complex graft designs are analysed with internal growth channels and spiral reinforcing winds. Viscoelastic effects are also examined. The modelling method is discussed and important results are noted.

Bibliography: pages 81-86.