Computational Analysis of the Effects of Fiber Deformation on the Microstructure and Permeability of Blood Oxygenator Bundles

Mechanical loads on the polymeric fibers of oxygenating bundles are commonly present due to bundle press-fitting during device assembly and blood pressure load. However, computational fluid dynamics (CFD) simulations for fiber bundle optimization neglect possible changes in microstructure due to such deformations. The aim of this study is to investigate the impact of fiber deformability on bundle microstructure and fluid dynamics mainly in terms of permeability. Fibers from commercial mats typically used for blood oxygenators were mechanically tested and based on these experimental data, a material model was developed to simulate the structural deformations the fibers undergo under press-fitting and blood pressure loads. Then, CFD simulations were performed on deformed bundle repetitive units to investigate permeability under varying loading conditions. The effects of different bundle geometric parameters on the variation of bundle permeability due to press-fitting were evaluated. Bundle press-fitting results in significant changes in microstructure that are reflected in a bundle permeability more than halved for a 15% press-fitting. This impact on permeability is present in all the simulated fiber bundles and becomes more pronounced as the pitch between fibers and thus bundle porosity decreases. Instead, the analyses on pressurized bundle show only small deformations caused by pressure load, with permeability changes below 1%. While blood pressure effects could be neglected, bundle press-fitting turns out to have a significant impact on bundle microstructure and permeability. Neglecting such microstructure variations during CFD simulations could also lead to incorrect assessment of the local fluid dynamics within the bundle.

Extended Cellular Deposits on Gas Exchange Capillaries are Not an Indicator of Clot Formation: Analysis of Different Membrane Oxygenators

Antithrombogenic coatings of artificial surfaces within extracorporeal membrane oxygenation (ECMO) circuits improved its bio- and hemocompatibility. However, there is still a risk of thrombus formation in particular within the membrane oxygenator (MO). Since inflammatory cells are essential components within clots, the aim was to identify the extent of cellular accumulations on gas exchange capillaries from different ECMO systems. Thirty-four MOs (PLS, n = 27, Getinge; Hilite 7000 LT, n = 7, Fresenius Medical Care, Germany) were collected from adult patients. The extent of cellular deposits on gas exchange capillaries was classified using nuclear 4',6-diamidino-2-phenylindole staining and fluorescence microscopy. All Hilite oxygenators exhibited small cellular deposits. In contrast, the cellular distribution was heterogeneous on capillaries from PLS oxygenators: small deposits (34%), clusters (44%) and membrane-spanning cell structures (pseudomembranes) (22%). Overall, the median fluorescence intensity was significantly higher in the PLS group. Nevertheless, within 3 days before MO removal, there was no alteration in critical parameters (d-dimer and fibrinogen levels, platelet counts, and pressure drop across the MO). In conclusion, despite the histological differences on the gas capillaries from different types of oxygenators, there was no further evidence of increased inflammation and coagulation parameters that indicate clot formation within oxygenators.

Numerical modeling of pulsatile blood flow through a mini-oxygenator in artificial lungs

While previous in vitro studies showed divergent results concerning the influence of pulsatile blood flow on oxygen advection in oxygenators, no study was done to investigate the uncertainty affected by blood flow dynamics. The aim of this study is to utilize a computational fluid dynamics model to clarify the debate concerning the influence of pulsatile blood flow on the oxygen transport. The computer model is based on a validated 2D finite volume approach that predicts oxygen transfer in pulsatile blood flow passing through a 300-micron hollow-fiber membrane bundle with a length of 254 mm, a building block for an artificial lung device. In this study, the flow parameters include the steady Reynolds number (Re = 2, 5, 10 and 20), Womersley parameter (Wo = 0.29, 0.38 and 0.53) and sinusoidal amplitude (A = 0.25, 0.5 and 0.75). Specifically, the computer model is extended to verify, for the first time, the previously measured O₂ transport that was observed to be hindered by pulsating flow in the Biolung, developed by Michigan Critical Care Consultants. A comprehensive analysis is carried out on computed profiles and fields of oxygen partial pressure (P_O2) and oxygen saturation (S_O2) as a function of Re, Wo and A. Based on the present results, we observe the positive and negative effects of pulsatile flow on P_O2 at different blood flow rates. Besides, the S_O2 variation is not much influenced by the pulsatile flow conditions investigated. While being consistent with a recent experimental study, the computed O₂ volume flow rate is found to be increased at high blood flow rates operated with low frequency and high amplitude. Furthermore, the present study qualitatively explains that divergent outcomes reported in previous in vitro experimental studies could be owing to the different blood flow rates adopted. Finally, the contour analysis reveals how the spatial distributions of P_O2 and S_O2 vary over time.

Animal blood in translational research: How to adjust animal blood viscosity to the human standard

Animal blood is used in mock circulations or in forensic bloodstain pattern analysis. Blood viscosity is important in these settings as it determines the driving pressure through biomedical devices and the shape of the bloodstain. However, animal blood can never exactly mimic human blood due to erythrocyte properties differing among species. This results in the species-specific shear thinning behavior of blood suspensions, and it is therefore not enough to adjust the hematocrit of an animal blood sample to mimic the behavior of human blood over the entire range of shear rates that are present in the body. In order to optimize experiments that require animal blood, we need models to adapt the blood samples. We here offer mathematical models derived for each species using a multi linear regression approach to describe the influence of shear rate, hematocrit, and temperature on blood viscosity. Results show that pig blood cannot be recommended for experiments at low flow conditions (<200 s-1 ) even though erythrocyte properties are similar in pigs and humans. However, pig blood mimics human blood excellently at high flow condition. Horse blood is unsuitable as experimental model in this regard. For several studied conditions, sheep blood was the closest match to human blood viscosity among the tested species.