'University of Babylon - Department of Mechanical Engineering, Faculty of Engineering'
Abstract
The leaflet material in heart valve prostheses is required to be both flexible and durable to eliminate the need for chronic anticoagulation medication and accommodate younger patients with longer life expectancies. This investigation aims to provide two of the necessary tools to design and test suitably flexible and durable materials for heart valve replacement. These tools address the question of how to model the stress-strain behaviour of polymer networks and thermoplastic polyurethanes in particular, as well as how to practically evaluate the durability of the proposed material. A model for polyurethane stress-strain behaviour is proposed, whereby the number of monomers between crosslinks is suggested to evolve with macroscopic strain. Following the polymer chain entanglement theory, the increase in the number of monomers between crosslinks is further extended to be a function of strain rate, incorporating the viscous effect observed in polyurethanes. A multistation, micro-tensile specimen fatigue tester was developed to evaluate material durability. The proposed equilibrium polyurethane model accurately predicts the experimental data across the full material strain range. The proposed model extension sufficiently captures the rate dependence of polyurethane, however, fails to account for the raised specimen temperatures at high strain rates. The developed fatigue tester is verified to successfully feature selectable variables including test frequency (1 - 20 Hz), amplitude (1 - 6 mm), waveform (Triangular, Sinusoidal, Square and Custom) and environmental temperature control (23 - 50 oC). Less than 10% error in measured force is observed when compared to a commercial tensile tester. The proposed model successfully provides a platform to aid the design of flexible materials suitable for heart valve leaflets. The developed fatigue tester enables the assessment of material durability across a range of test conditions, successfully providing a tool for leaflet material durability analysis and verification
