Blood flowsthrough medical assistance devices arediscussed with a focus on the reinfusion part of the circuit.The main medical problematics related to thebloodflow dynamics are presentedand experimental results are discussed.Mechanical stresses induced by blood flow overartificial surfaces in Extracorporeal Membrane Oxygenationcircuit enhance the risk for hemolysis and platelet activation.The return cannulaisa particularly sensitive component due to the interaction between the cannula blood flow and the blood flow in the punctured vessel .Mixing processes are affected by cannulation strategy and theassociated cannula position, orientation and design.Co-axialtubes have been used to produce a co-flowing confined jetsimilar to that of the Extracorporeal Membrane Oxygenationreturn cannulaas described Fig 1. Particle Image Velocimetry and Planar LaserInduced Fluorescence have been used to investigate the flowrate ratio between the jet and the co-flow,the cannulaposition and the cannula design.
The jet is turbulent and its mixing is controlled bythree main structures: lateral flow entrainment, shear layerinduced vortices and backflow along the wall. The establishmentof a fully-developed flow requires a longer distance ascompared to a full mixing. Cannula position significantly affectsthe recirculation size on each side of the jet. However,the mixing efficiency is not impacted by the cannula positionor its tip design but iscontrolled by the flowrate ratioand the cannula diameter.The shear stress isan important mechanical parameter that may trigger plateletactivation and hemolysis. The strongest shear stresses arefound in the shear layer and increase proportionally to theflow rate ratio. Shifting the cannula laterally breaks the symmetryof the flow resulting in a thinner shear layer on thenarrow side that displays a lower viscous shear stress comparedto the wide side. The maximum shear stress found inthis experiment is comparable to the shear stress thresholdinducing platelet activation defined by previous studies.The talk concludes with an outlook onnumerical simulation possibilitiesforbiological flows.
 FuchsG, Berg N, Broman LM, Prahl WittbergL (2018)Scientific Reports, 8(1), 13985.DOI10.1038/s41598-018-32247-y
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