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  • 1
    ISSN: 0021-9304
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine , Technology
    Notes: The permeability characteristics of a water-segmented polyurethane (Biomer) system under the conditions encountered in circulatory-assist devices were investigated. A diffusion cell and permeability system providing precise control of membrane boundary conditions and allowing continuous measurement of water vapor transmission was designed. Liquid water at 37°C was used as the donor fluid and the system incorporated a constant-flow nitrogen carrier gas and an optical dew point sensor downstream to determine the water vapor mass flow rate as a function of time. The mass flow rate was then numerically integrated and plotted against time to allow calculation of effective diffusion coefficient (D) by the dynamic time lag method. Steady-state permeabilities were found to be insensitive to donor chamber hydrostatic pressure (50-200 mm Hg) indicating that bulk flow is not a transport mechanism in these membranes. The permeability coefficient (P) was independent of membrane thickness (H) over the four samples tested (0.0102, 0.0148, 0.0269, and 0.0366 cm), with an average value of 3.29 × 10-4cm2/s. Thus, diffusion was Fickian with negligible boundary layers. A plot of lag time versus H2 was linear (R = 0.98) yielding a value for D of 2.18 × 10-7cm2/s. A water-Biomer partition coefficient was determined for each sample with an average value of 1525, indicating a moderately hydrophilic membrane with a water sorption of 6.3% at 37°C. Since water transport is by Fickian diffusion in the absence of bulk flow, liquid water cannot be expected to accumulate in circulatory-assist devices unless a condensing surface is maintained within the system.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0066-4189
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The fluid mechanics of artificial blood pumps has been studied since the early 1970s in an attempt to understand and mitigate hemolysis and thrombus formation by the device. Pulsatile pumps are characterized by inlet jets that set up a rotational "washing" pattern during filling. Strong regurgitant jets through the closed artificial heart valves have Reynolds stresses on the order of 10,000 dynes/cm2 and are the most likely cause of red blood cell damage and platelet activation. Although the flow in the pump chamber appears benign, low wall shear stresses throughout the pump cycle can lead to thrombus formation at the wall of the smaller pumps (10Đ??50 cc). The local fluid mechanics is critical. There is a need to rapidly measure or calculate the wall shear stress throughout the device so that the results may be easily incorporated into the design process.
    Type of Medium: Electronic Resource
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