Local flow alterations created by a metallic stent in a simulated coronary artery were studied to compare the hemodynamic effects of two different stent geometries. Dye injection flow visualization and computational fluid dynamics were used. Resting and exercise conditions were studied. Flow visualization using the dye injection method provided a qualitative picture of stent hemodynamics while the computational approach provided detailed quantitative information on the flow next to the vessel wall near the intersections of stent wires. Dye injection visualization revealed that more dye became entrapped between the wires where the wire spacing was smallest. The dye washout times were shorter under exercise conditions for both wire spacings tested. The computational results showed that stagnation zones were continuous from one wire to the next when the wire spacing was small. Results from greater wire spacing (more than six wire diameters) showed that the stagnation zones were separate for at least part of the cardiac cycle. The sizes of the stagnation zones were larger under exercise conditions, and the largest stagnation zones were observed distal to the stent. These studies demonstrate that stent geometry has a significant effect on local hemodynamics. The observation that fluid stagnation is continuous in stents with wire spacings of less than six wire diameters may provide a criterion for future stent design.

Void formation in polymeric materials is of critical importance in a wide range of process technologies such as composites manufacturing, polymer devolatilization and foam molding. A predictive model for volatile-induced void formation and growth during the processing of thermoplastic polymeric materials is presented. This model is comprehensive, incorporating the effect of material properties, such as surface tension and moisture solubility and diffusivity in these materials, and processing conditions and part thickness on the spatial nucleation and growth of voids. Fickian diffusion of volatiles during polymer processing provides concentration profiles that are coupled with classical nucleation theory to predict nucleation density through the part thickness as a function of process temperature, heating rate and applied pressure. Void growth is predicted at each material point based on local vapor pressure, material properties and process conditions. Parametric studies to elucidate the effect of various processing parameters and material properties on moisture-driven Void Formation in Amorphous Thermoplastic Polymeric Materials: I 341 void formation are conducted for amorphous thermoplastic engineering polymers such as polyetherimide (PEI). Further, the role of material and processing parameters in the generation of a spatial distribution of void content within the part being processed is also demonstrated.

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