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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Identifying suitable polymeric materials for use in a synthetic heart valve (MPhil)

Author: BAXTER, Samantha

Year: 2005

Supervisor(s): James Busfield

The advent of the polymeric synthetic heart valve will hopefully allow the limitations of other commonly used artificial heart valves to be overcome. Currently the valves adopted in practice include mechanical heart valves and biological heart valves. The mechanical valves are prone to thrombosis formation in the blood and require long-term anticoagulant therapy. The biological valves lose their tissue replacement function as a result of their chemical treatment which results in tissue fatigue damage to the heart valve. Polymeric synthetic heart valves that mimic the function of a real valve will overcome these difficulties, but may also have their own drawbacks as they may be susceptible to calcification. This leads to a localised stiffening which introduce stress raisers that may result in crack growth.

In this thesis elastomeric materials that have a potential use as polymeric synthetic heart valves have been assessed by measuring their strain energy release rate for a wide range of crack growth rates. This was measured under a variety of conditions of both static and cyclic loading to understand the complexity of the behaviour and to evaluate their potential performance as target materials for use in artificial heart valves.

A grade of polyurethane, which is commonly used in medical applications, appears to exhibit similar crack growth resistance behaviour to that of a strain crystallising elastomer such as natural rubber. The materials tested here exhibit cyclic dependent crack growth as a result of their viscoelastic response and potential to strain crystallise. Further attempts to clarify this behaviour were achieved through stress relaxation tests and x-ray analysis. EPDM and silicone materials show the typical crack growth behaviour of a non-crystallising elastomer such as SBR. Silicone rubber is seen to be a very weak material. These two non-crystallising elastomers exhibit time dependent crack growth behaviour under cyclic conditions. From this work, polyurethane appears to be the most suitable material for use in polymeric heart valves.