Fundamental Study for the Prediction of Fatigue Lives of Elastomeric Polymers Used in Artificial Heart Pump

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For cardiovascular applications, mechanical properties of a series of segmented polyether polyurethanes with soft segment component of different molecular weights ( <M_W> =850, 1350, and 2000) were studied by tensile and fatigue tests. The soft segment is composed of polytetramethylene glycol and 4,4'-diphenylmethane diiso-cyanate (MDI); the hard segment contains propylene diamine and MDI. With the increase in <M_W> , the elastic modulus and tensile strength decreased, but the ultimate elongation increased. The designated TM5 polyurethane ( <M_W> =2000) showed the lowest elastic modulus and fairly high strength and ductility. This material was considered to be suitable for prosthetic components which need high flexibility and toughness. TM3 polyurethane ( <M_W> =1350) indicated favorable blood compatibility and surface characteristics, which might be useful for the blood-contacting surfaces. For structural components like cannulae and pump housings, TM1 ( <M_W> =850) was considered to … More be most suited because of its high strength and low ductility.Effects of test temperature (24゜C, 37゜C, and 50゜C) and cyclic rate (0.8Hz, 2Hz, and 5Hz) on the fatigue properties were studied on TM5 polyurethane and Biomer (Ethicon) aiming to obtain basic data for the development of an accelerated endurance test method and the establishment of a criterion for the fatigue-life prediction. With the increase in the ambient temperature, the dynamic stress relaxation and the decrease in the dynamic elastic modulus became larger. The higher the cyclic rate was, the less the change in the mechanical properties was, if compared at the same repetition number. Preliminary durability tests in the saline solution kept at 37゜C indicated that TM5 polyurethane endures 200 million cycles of deformation under conditions of 50% mean strain, 15% strain amplitude, and 5Hz cyclic rate. The reduction of the mean stress was observed throughout the period. This stress relaxation phenomenon might be a serious problem for the long-term application of this kind of material for blood pumps. Less