Narrow Results

The present study numerically simulated the physiological pulsatile flow in helical grafts to increase understanding of its flow mechanism which may contribute to the design of better grafts. The wall-indices like time-averaged wall shear stress (WSS) and oscillatory shear index (OSI), joint with a quantitative index for helical flow by means of Lagrangian approach, were introduced as effective instruments to classify the hemodynamic performance of helical grafts. The simulation suggests that the helical geometry created amplified WSS magnitudes as well as elevated velocities near the wall. The calculated oscillatory shear index (OSI) values were never exceeded to 0.07 which is not considered physiologically significant. In addition, the strong secondary flow in helical graft helped the flow mixing between low-momentum fluid closer to the surface and high-momentum fluid at the center which brought the high-momentum fluid to the surface. Furthermore, Helicity analysis revealed that most of the fluid particles experienced counter-clockwise rotation during the whole cardiac cycle which helps to protect the graft wall from damage by reducing the laterally directed forces and keep flow stability. It concluded that a helical graft provides guaranties for the graft wall surface to get smooth and even washing by the blood and eliminates mechanical trauma to blood cells so that atherosclerotic plaques can hardly form in the graft wall.

Intimal hyperplasia developed at the end-to-side anastomosis of artery bypass is closely related to unphysiological hemodynamics. The helical flow as a normal physiological phenomenon in arteries is beneficial to endothelial damage repair. To deeply understand the physiological flow properties in a S-type bypass (StB) graft, four end-to-side bypass models including 30°, 45°, 60° conventional bypasses and a 45° StB were compared numerically under physiological pulsatile flow. The results showed that strong helical flow was observed at the distal anastomosis of StB. The distribution of hemodynamic parameters such as helicity, average wall shear stress and oscillating shear index, etc. were significantly improved at the S-type anastomosis as compared with those of three conventional models. The area-averaged normalized helicity in StB reached maxima at the moments of maximum flow rate and systolic deceleration. The hemodynamic performance in a 45° StB was improved as compared with a 30° conventional model. It is concluded that large StB anastomosis angle can be taken to achieve good hemodynamic performance while much smaller anastomosis angle has to be adopted for conventional bypass. As such, a S-type anastomosis should be a feasible choice of clinical artery bypass grafting due to its significant improvement in hemodynamic performance.

Single-walled carbon nanotubes (SWNTs) synthesized by catalytic decomposition of an alcohol were purified by extraction. The purified SWNTs were characterized on the basis of visible-near infrared (vis-NIR) absorption, photoluminescence and Raman spectroscopic analyses, scanning electron microscopy (SEM) observation, and thermal analysis. Selective extraction of metallic nanotubes was also achieved by the extraction condition.