Biofluid Flow and Heat Transfer

2nd EditionInternational audienceMajor moving biological fluids, or biofluids, are blood and air that cooperate to bring oxygento the body’s cells and eliminate carbon dioxide produced by these cells. Blood is conveyed ina closed network composed of 2 circuits in series — the systemic and pulmonary circulation—, each constituted by arteries, capillaries, and veins, under the synchronized action of theleft and right cardiac pumps, respectively. Air is successively inhaled from and exhaled to theatmosphere through the airway openings (nose and/or mouth). In the head, blood generatesthe cerebrospinal fluid in choroid plexi of all compartments of the ventricular system andreceives it in arachnoid villi. Other biofluids are either secreted, such as bile from the liverand breast milk that both transport released substances with specific tasks, or excreted,such as urine from kidneys or sweat from skin glands that both convey useless materials andwaste produced by the cell metabolism. In addition to the convective transport, peristalsis,which results from the radial contraction and relaxation of mural smooth muscles, propelsthe content of the lumen of the muscular bioconduit (e.g., digestive tract) in an anterogradedirection.Blood circulation and air flow in the respiratory tract are widely explored because oftheir vital functions. Note that inhaled air is transported through the respiratory tract bytwo processes: convection down to bronchioles and diffusion down to pulmonary alveoli.1Furthermore, investigations of these physiological flows in deformable bioconduits give riseto models such as the Starling resistance that themselves become object of new study fieldsin physics and mechanics (e.g., collapsible tubes) as well as of new developments in math-ematical modeling and scientific computing. Whereas fluid–structure interaction problemsin aeronautics and civil engineering deal with materials of distinct properties, blood streamand vessel wall correspond to two domains of nearly equal physical properties, as both bloodand vessels have densities close to that of water. New processing strategies must then beconceived

Comparison Between Bench-Top and Computational Modelling of Cerebral Thromboembolism in Ventricular Assist Device Circulation

Despite improvements in ventricular assist devices (VAD) design, VAD-induced stroke rates remain remarkably high at 14–47%. We previously employed computational fluid dynamics (CFD) to propose adjustment of VAD outflow graft (VAD-OG) implantation to reduce stoke. Herein, we present an in-vitro model of cerebral vessel embolization in VAD-assisted circulation, and compare benchtop results to CFD predictions. The benchtop flow-loop consists of a 3D printed aortic bed using Accura 60 polymer driven by a continuous-flow pump. Three hundred spherical particles simulating thrombi of 2, 3.5, and 5 mm diameters were injected at the mock VAD-OG inlet. A water and glycerin mixture (3.8 cP viscosity) synthetically mimicked blood. The flowrate was adjusted to match the CFD Reynolds number. Catch cans were used to capture and count particles reaching cerebral vessels. VAD-OG geometries were evaluated using comparison of means Z-score range of −1.96 ≤ Z ≤ 1.96 to demonstrate overall agreement between computational and in-vitro techniques. Z-scores were: (i) Z = −1.05 for perpendicular (0°), (ii) Z = 0.32 for intermediate (30°), and (iii) Z = −0.52 for shallow (60°) anastomosis and confirmed agreement for all geometries. This study confirmed added benefits of using a left carotid artery bypass-graft with percent embolization reduction: 22.6% for perpendicular, 21.2% for intermediate, and 11.9% for shallow anastomoses. The shallow anastomosis demonstrated lower degrees of aortic arch flow recirculation, consistent with steady-flow computations. Quantitatively and qualitatively, contemporary steady-flow computational models for predicting VAD-induced cerebral embolization can be achieved in-vitro to validate the CFD equivalent