While cardiovascular device-induced thrombosis is associated with negative patient outcomes, the convoluted nature of the processes resulting in a thrombus makes the full thrombotic network too computationally expensive to simulate in the complex geometries and flow fields associated with devices. A macroscopic, continuum computational model is developed based on a simplified network, which includes terms for platelet activation (chemical and mechanical) and thrombus deposition and growth in regions of low wall shear stress (WSS). Laminar simulations are performed in a two-dimensional asymmetric sudden expansion geometry and compared with in vitro thrombus size data collected using whole bovine blood. Additionally, the predictive power of the model is tested in a flow cell containing a series of symmetric sudden expansions and contractions. Thrombi form in the low WSS area downstream of the asymmetric expansion and grow into the nearby recirculation region, and thrombus height and length largely remain within 95 % confidence intervals calculated from the in vitro data for 30 min of blood flow. After 30 min, predicted thrombus height and length are 0.94 and 4.32 (normalized by the 2.5 mm step height). Importantly, the model also correctly predicts locations of thrombus deposition observed in the in vitro flow cell of expansions and contractions. As the simulation results, which rely on a greatly reduced model of the thrombotic network, are still able to capture the macroscopic behavior of the full network, the model shows promise for timely predictions of device-induced thrombosis toward optimizing and expediting the device development process.

Thrombosis and thromboembolization remain large obstacles in the design of cardiovascular devices. In this study, the temporal behavior of thrombus size within a backward-facing step (BFS) model is investigated, as this geometry can mimic the flow separation which has been found to contribute to thrombosis in cardiac devices. Magnetic resonance imaging (MRI) is used to quantify thrombus size and collect topographic data of thrombi formed by circulating bovine blood through a BFS model for times ranging between 10 and 90 min at a constant upstream Reynolds number of 490. Thrombus height, length, exposed surface area, and volume are measured, and asymptotic behavior is observed for each as the blood circulation time is increased. Velocity patterns near, and wall shear stress (WSS) distributions on, the exposed thrombus surfaces are calculated using computational fluid dynamics (CFD). Both the mean and maximum WSS on the exposed thrombus surfaces are much more dependent on thrombus topography than thrombus size, and the best predictors for asymptotic thrombus length and volume are the reattachment length and volume of reversed flow, respectively, from the region of separated flow downstream of the BFS.

Commercial mayonnaise and refrigerated ranch salad dressing were inoculated at two levels with two strains of Escherichia coli O157:H7, a non-pathogenic E. coli, and the non-fecal coliform Enterobacter aerogenes. Results showed that at the high inoculation level (>106 colony forming units [CFU]/g) in mayonnaise stored at room temperature (ca. 22°C) both strains of O157:H7 were undetected at 96 h. At the high inoculation level, all strains of coliform bacteria tested survived longer in salad dressing stored at 4°C than in mayonnaise stored at 22°C. The O157:H7 strains were still present at low levels after 17 days. The survival time in the low-level inoculum (104CFU/g) study decreased, but the survival pattern in the two products was similar to that observed in the high-level inoculum study. Slight differences in survival among strains were observed. The greater antimicrobial effect of mayonnaise may be attributable to differences in pH, water activity (aw), nutrients, storage temperature, and the presence of lysozyme in the whole eggs used in the production of commercial mayonnaise. Coliform bacteria survived longer in refrigerated salad dressing than in mayonnaise particularly at the high-level inoculum. Both mayonnaise (pH 3.91) and salad dressing (pH 4.51) did not support the growth of any of the microorganisms even though survival was observed.

Traveling-bubble cavitation inception tests were conducted in a 30.48 cm water tunnel with a Schiebe headform. A computer code was developed to statistically model cavitation inception on a Schiebe headform, consisting of a numerical solution to the Rayleigh-Plesset equation coupled to a set of trajectory equations. Using this code, trajectories and growths were computed for bubbles of varying initial sizes. An initial off-body distance was specified and the bubble was free to follow an offbody trajectory. A Monte Carlo cavitation simulation was performed in which a variety of random processes were modeled. Three different nuclei distributions were specified including one similar to that measured in the water tunnel experiment. The results compared favorably to the experiment. Cavitation inception was shown to be sensitive to nuclei distribution. Off-body effect was also found to be a significant factor in determining whether or not a bubble would cavitate. The effect of offbody trajectories on the critical bubble diameter was examined. The traditional definition of critical diameter based on the minimum pressure coefficient of the body or the measurement of liquid tension was found to be inadequate in defining cavitation inception. 10 12 14 16 Water Speed (m/sec) Fig. 1 Cavitation inception results on ITTC standard headform from various facilities (from Lindgren and Johnson, 1966)

Three-dimensional laser Doppler velocimetry measurements were acquired in a mock-circulatory loop proximal to a Björk-Shiley monostrut valve in the mitral position, and synchronous ensemble-averaging was applied to form an "average" beat. Two axial locations in the regurgitant flow region of the valve (in the minor orifice) were mapped, and maximum Reynolds shear stresses were calculated. A large spike in regurgitant flow was noted at the beginning of systole, which may be the squeeze flow phenomenon computed by other researchers. A region of sustained regurgitant flow 50 msec later was the focus of this study. Maximum velocities of approximately 3.7 mps were noted, and maximum Reynolds shear stresses of approximately 10,000 dyne/cm2 were calculated. Comparisons were made of two-dimensional (ignoring tangential component) versus three-dimensional shear stresses, and, in this case, in regions of high stress, the differences were insignificant. This suggests that the tangential component of velocity can probably be ignored in similar measurements where the tangential velocity is likely to be small.

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Laser Doppler velocimetry, with a high temporal resolution (1 ms time windows), was used to measure the flow field in two regions (major and minor orifices) near the aortic and mitral valves (Bjork Shiley monostrut Nos. 25 and 27, respectively) of the Penn State artificial heart. The motion of each valve was also investigated using a 1000 frame/s video camera in order to estimate the valve's closing velocity. Fluid velocities in excess of and opposite to valve closing velocity were detected near the valve, providing evidence of "squeeze flow." Maximum Reynolds shear stresses of approximately 20,000 dyn/cm2 and time-averaged Reynolds shear stresses of approximately 2000 dyn/cm2 were observed during the regurgitant flow phase. These elevated Reynolds shear stresses suggest that regurgitant jets play a role in the hemolysis and thrombosis associated with tilting disk heart valves in an artificial heart environment.

Streak line flow visualization and laser Doppler velocimetry (LDV) were conducted in the regurgitant leakage flow region of 3 mechanical heart valve types: CarboMedics, Medtronic Hall, and St. Jude Medical. Streak line flow visualization identified regions of high regurgitant flow, and LDV measurements were focused on those locations. Maximum regurgitant flow velocities after valve closure ranged from 0.7 to 2.6 m/s, and maximum Reynolds shear stress after valve closure ranged from 450 to 3,600 dyne/cm2. These data indicate that leakage flows can generate turbulent jets with elevated Reynolds stresses even in bileaflet valves.