The impact of the Impeller's hub design on the performance and blood damage in a microaxial mechanical circulatory support device - A numerical study.

This study uses CFD methods to investigate the effects of the impeller's geometry on the hemodynamic characteristics, pump performance, and blood damage parameters, in a percutaneous microaxial Mechanical Circulatory Support (MCS) device. The numerical simulations employ the steady state Reynolds-Averaged Navier-Stokes approximation using the SST k-ω turbulent model. Three different impeller models are examined with different hub conversion angles (α = 0○, 3○ and 5○). The analysis includes 23 cases for different pressure heads (Δp = 60-80 mmHg) and angular velocities (ω = 30-52 kRPM). The obtained flow rate is compared between the cases to assess the effect of the impeller's design and working conditions on the pump performance. The comparative risk of shear-induced platelet activation is estimated using the statistical median of the stress-accumulation values calculated along streamlines. The risk of hemolysis is estimated using the average exposure time to shear stress above a threshold (τ > 425 Pa). The results reveal that the shape of the impeller's hub has a great impact on the flow patterns, performance, and risk of blood damage, as well as the angular velocity. The highest flow rate (Q = 3.7 L/min) and efficiency (η = 11.3 %) were achieved using a straight hub (α = 0○). Similarly, for the same condition of flow and pressure, the straight hub impeller has the lowest blood damage risk parameters. This study shed light on the effect of pump design on the performance and risk of blood damage, indicating the roles of the hub shape and angular velocity as dominant parameters.

Impedance Pumping and Resonance in a Multi-Vessel System.

Impedance pumping is a mechanism that generates flow in a compliant vessel by repeatedly actuating the vessel asymmetrically, without employing any internal valves, blades, or other mechanisms. The net flow is obtained by establishing a constructive wave pattern. Elaborate studies of impedance pumping in a single vessel have shown that the flow rate strongly depends on the actuation frequency, as well as on other parameters, such as actuator location and amplitude, and that it operates best in the resonance mode. The present study extends these principles to a network of multiple compliant vessels, representing a cardiovascular system. The flow is modeled numerically by the one-dimensional approximation of the Navier-Stokes equations. Two configurations were examined, systems consisting of three and five compliant vessels. First, the natural frequencies of these configurations were identified. Then, the dependence of the net flow rate (NFR) on the actuating frequency was explored, showing that impedance pumping operates best in the resonance mode in the case of a network of vessels as well. The impact of other parameters were studied as well, such as the location of one or two actuators, actuation amplitude, actuator width, the duty cycle, and the phase lag between the actuators. The results show that impedance pumps can generate significant NFR and the obtained NFR can be manipulated by properly setting up one or more of the governing parameters. These findings indicate that impedance pumping principles may be applied to flow control of the cardiovascular system.

Toroidal plasmoid generation via extreme hydrodynamic shear.

Saint Elmo's fire and lightning are two known forms of naturally occurring atmospheric pressure plasmas. As a technology, nonthermal plasmas are induced from artificially created electromagnetic or electrostatic fields. Here we report the observation of arguably a unique case of a naturally formed such plasma, created in air at room temperature without external electromagnetic action, by impinging a high-speed microjet of deionized water on a dielectric solid surface. We demonstrate that tribo-electrification from extreme and focused hydrodynamic shear is the driving mechanism for the generation of energetic free electrons. Air ionization results in a plasma that, unlike the general family, is topologically well defined in the form of a coherent toroidal structure. Possibly confined through its self-induced electromagnetic field, this plasmoid is shown to emit strong luminescence and discrete-frequency radio waves. Our experimental study suggests the discovery of a unique platform to support experimentation in low-temperature plasma science.

The Driving Mechanism for Unidirectional Blood Flow in the Tubular Embryonic Heart.

The embryonic heart of vertebrate embryos, including humans, has a tubular thick-wall structure when it first starts to beat. The tubular embryonic heart (TEH) does not have valves, and yet, it produces an effective unidirectional blood flow. The actual pumping mechanism of the TEH is still controversial with pros and cons for either peristaltic pumping (PP) or impedance pumping (IP). On the other hand, observation of movies of the contractile TEH of the quail revealed a propagating wave from the venous end towards the arterial end that occludes the lumen behind the leading edge. This pattern of contraction represents a complex PP with a duty cycle, and was defined here as biological pumping (BP). In this work we developed a heart-like model that represents the main features of the chick TEH and allows for numerical analysis of all the three pumping mechanisms (i.e., IP, PP, and BP) as well as a comprehensive sensitivity evaluation of the structural, operating, and mechanical parameters. The physical model also included components representing the whole circulatory system of the TEH. The simulations results revealed that the BP mechanism yielded the level and time-dependent pattern of blood flow and blood pressure, as well as contractility that were observed in experiments.

Dimensionless analysis of valveless pumping in a thick-wall elastic tube: Application to the tubular embryonic heart.

The physical mechanism that drives blood flow in the valveless tubular embryonic heart is still debatable whether it is peristaltic flow or valveless dynamic suction. Previous studies of valveless pumping were concerned with either the role of the excitation parameters or the mechanisms that generate the unidirectional outflow. In this study, a dimensionless one-dimensional (1D) analysis of the valveless pumping due to local excitation at an asymmetric longitudinal location was performed for non-uniform thick-wall elastic tubes, including tubes with local bulging and tapering. A general tube law that accounts for wall thicknesses was implemented for describing the physically realistic dynamics of the tube and the two-step MacCormack algorithm was utilized for the numerical analysis. A comprehensive analysis was conducted to explore the affecting roles of the system (e.g., tube geometry) and the working (e.g., Strouhal number and flow friction parameter) parameters on the net outflow of the pump. The maximal positive net outflow in all the tested cases always occurred when the natural Strouhal number was about π. Flow reversals were observed only for relatively low friction parameters. A local bulging at the site of excitation and thick walls contributed to larger outflows, while tube tapering reduced the net outflow.

Fluid-structure interaction model of aortic valve with porcine-specific collagen fiber alignment in the cusps.

Native aortic valve cusps are composed of collagen fibers embedded in their layers. Each valve cusp has its own distinctive fiber alignment with varying orientations and sizes of its fiber bundles. However, prior mechanical behavior models have not been able to account for the valve-specific collagen fiber networks (CFN) or for their differences between the cusps. This study investigates the influence of this asymmetry on the hemodynamics by employing two fully coupled fluid-structure interaction (FSI) models, one with asymmetric-mapped CFN from measurements of porcine valve and the other with simplified-symmetric CFN. The FSI models are based on coupled structural and fluid dynamic solvers. The partitioned solver has nonconformal meshes and the flow is modeled by employing the Eulerian approach. The collagen in the CFNs, the surrounding elastin matrix, and the aortic sinus tissues have hyperelastic mechanical behavior. The coaptation is modeled with a master-slave contact algorithm. A full cardiac cycle is simulated by imposing the same physiological blood pressure at the upstream and downstream boundaries for both models. The mapped case showed highly asymmetric valve kinematics and hemodynamics even though there were only small differences between the opening areas and cardiac outputs of the two cases. The regions with a less dense fiber network are more prone to damage since they are subjected to higher principal stress in the tissues and a higher level of flow shear stress. This asymmetric flow leeward of the valve might damage not only the valve itself but also the ascending aorta.

Fully coupled fluid-structure interaction model of congenital bicuspid aortic valves: effect of asymmetry on hemodynamics.

A bicuspid aortic valve (BAV) is a congenital cardiac disorder where the valve consists of only two cusps instead of three, as in a normal tricuspid valve (TAV). Although 97 % of BAVs include asymmetric cusps, little or no prior studies have investigated the blood flow through a three-dimensional BAV and root. The aim of the present study was to characterize the effect of asymmetric BAV on the blood flow using fully coupled fluid-structure interaction (FSI) models with improved boundary conditions and tissue properties. This study presents four FSI models, including a native TAV, asymmetric BAVs with or without a raphe, and an almost symmetric BAV. Cusp tissue is composed of hyperelastic finite elements with collagen fibres embedded in the elastin matrix. A full cardiac cycle is simulated by imposing the same physiological blood pressures for all the TAV and BAV models. The latter have significantly smaller opening areas compared with the TAV. Larger stress values were found in the cusps of BAVs with fused cusps, at both the systolic and diastolic phases. The asymmetric geometry caused asymmetric vortices and much larger flow shear stress on the cusps which could be a potential initiator for early valvular calcification of BAVs.

Numerical model of the aortic root and valve: optimization of graft size and sinotubular junction to annulus ratio.

OBJECTIVE: The aim of this study was to determine the influence of aortic annulus (AA) diameter and the ratio of the sinotubular junction (STJ) diameter to the AA diameter on aortic valve hemodynamics and tissue mechanics and to suggest optimal values. METHODS: Sixteen cases of aortic roots with AA diameters between 22 and 28 mm and an STJ/AA diameter ratio between 0.8 and 1.4 were numerically modeled. Average coaptation height and mechanical stresses were calculated from 3-dimensional finite element analysis of the aortic valve and root. Five additional fluid structure interaction (FSI) models with an AA diameter of 24 mm and an STJ/AA ratio between 0.6 and 1.4 were also constructed. The material properties of the tissues were from porcine valves and boundary conditions were physiologic and normal blood pressures. RESULTS: In most cases, average coaptation height decreased with an increase in the STJ/AA ratio. Those cases with AA diameters between 24 and 26 mm and an STJ/AA ratio between 0.8 and 1.0 had a relatively large average coaptation height (>3 mm) and similar stress distribution during diastole. The flow shear stress values on the cusps at peak systole increased at the same time as the STJ/AA ratio decreased, similar to the opening area. CONCLUSIONS: Relatively large coaptation, low stress in the tissues during diastole, and low flow shear stress during systole is the best combination for cases of AA diameter between 24 and 26 mm with identical STJ diameter. Valve-sparing procedures that prevent AA expansion are preferable.

Aortic root numeric model: annulus diameter prediction of effective height and coaptation in post-aortic valve repair.

OBJECTIVE: The aim of the present study was to determine the influence of the aortic annulus (AA) diameter in order to examine the performance metrics, such as maximum principal stress, strain energy density, coaptation area, and effective height in the aortic valve. METHODS: Six cases of aortic roots with an AA diameter of 20 and 30 mm were numerically modeled. The coaptation height and area were calculated from 3-dimensional fluid structure interaction models of the aortic valve and root. The structural model included flexible cusps in a compliant aortic root with material properties similar to the physiologic values. The fluid dynamics model included blood hemodynamics under physiologic diastolic pressures of the left ventricle and ascending aorta. Furthermore, zero flow was assumed for effective height calculations, similar to clinical measurements. In these no-flow models, the cusps were loaded with a transvalvular pressure decrease. All other parameters were identical to the fluid structure interaction models. RESULTS: The aortic valve models with an AA diameter range of 20 to 26 mm were fully closed, and those with an AA diameter range of 28 to 30 mm were only partially closed. Increasing the AA diameter from 20 to 30 mm decreased the averaged coaptation height and normalized cusp coaptation area from 3.3 to 0.3 mm and from 27% to 2.8%, respectively. Increasing the AA diameter from 20 to 30 mm decreased the effective height from 10.9 to 8.0 mm. CONCLUSIONS: A decreased AA diameter increased the coaptation height and area, thereby improving the effective height during procedures, which could lead to increased coaptation and better valve performance.

A general three-dimensional parametric geometry of the native aortic valve and root for biomechanical modeling.

The complex three-dimensional (3D) geometry of the native tricuspid aortic valve (AV) is represented by select parametric curves allowing for a general construction and representation of the 3D-AV structure including the cusps, commissures and sinuses. The proposed general mathematical description is performed by using three independent parametric curves, two for the cusp and one for the sinuses. These curves are used to generate different surfaces that form the structure of the AV. Additional dependent curves are also generated and utilized in this process, such as the joint curve between the cusps and the sinuses. The model's feasibility to generate patient-specific parametric geometry is examined against 3D-transesophageal echocardiogram (3D-TEE) measurements from a non-pathological AV. Computational finite-element (FE) mesh can then be easily constructed from these surfaces. Examples are given for constructing several 3D-AV geometries by estimating the needed parameters from echocardiographic measurements. The average distance (error) between the calculated geometry and the 3D-TEE measurements was only 0.78±0.63mm. The proposed general 3D parametric method is very effective in quantitatively representing a wide range of native AV structures, with and without pathology. It can also facilitate a methodical quantitative investigation over the effect of pathology and mechanical loading on these major AV parameters.

Effect of asymmetry on hemodynamics in fluid-structure interaction model of congenital bicuspid aortic valves.

A bicuspid aortic valve (BAV) is a congenital cardiac disorder where the valve consists of only two cusps instead of three in a normal tricuspid valve (TAV). Although 97% of BAVs include asymmetric cusps, little or no prior studies investigated the blood flow through physiological three-dimensional BAV and root. This study presents four fully coupled fluid-structure interaction (FSI) models, including native TAV, asymmetric BAV with or without a raphe and an almost symmetric BAV. The FSI simulations are based on coupled structural and fluid dynamics solvers that allow accurate modeling of the pressure load on both the root and the cusps. The partitioned solver has non-conformal meshes and the flow is modeled employing an Eulerian approach. The cusps tissue in the structural model is composed of hyperelastic finite elements with collagen fiber network embedded in the elastin matrix. The tissues behavior of the aortic sinuses is also hyperelastic. The coaptation is modeled with master-slave contact algorithm. A full cardiac cycle is simulated by imposing the same physiological blood pressure at the upstream and downstream boundaries, for all the TAV and BAV models. The latter have significantly smaller opening area compared to the TAV. Larger stress values were also found in the cusps of the BAV models with fused cusps, both at the systolic and diastolic phases. The asymmetric geometry cause asymmetric vortices and much larger wall shear stress on the cusps, which is a potential cause for early valvular calcification in BAVs.

A fluid-structure interaction model of the aortic valve with coaptation and compliant aortic root.

While aortic valve root compliance and leaflet coaptation have significant influence on valve closure, their implications have not yet been fully evaluated. The present study developed a full fluid-structure interaction (FSI) model that is able to cope with arbitrary coaptation between the leaflets of the aortic valve during the closing phase. Two simplifications were also evaluated for the simulation of the closing phase only. One employs an FSI model with a rigid root and the other uses a "dry" (without flow) model. Numerical tests were performed to verify the model. New metrics were defined to process the results in terms of leaflet coaptation area and contact pressure. The axial displacement of the leaflets, closure time and coaptation parameters were similar in the two FSI models, whereas the dry model, with imposed uniform load on the leaflets, produced larger coaptation area and contact pressure, larger axial displacement and faster closure time compared with the FSI model. The differences were up to 30% in the coaptation area, 55% in the contact pressure and 170% in the closure time. Consequently, an FSI model should be used to accurately resolve the kinematics of the aortic valve and leaflet coaptation details during the end-closing stage.

An experimental and numerical study of the flow and mass transfer in a model of the wearable artificial kidney dialyzer.

BACKGROUND: Published studies of the past decades have established that mass transfer across the dialyzer membrane is governed by diffusion, convection and osmosis. While the former is independent of the pressure in the liquids, the latter two are pressure dependent and are enhanced when the pressure difference across the membrane is increased. The goal of the present study is to examine the impact of pulsatile flow on the transport phenomena across the membrane of a high-flux dialyzer in a wearable artificial kidney (WAK) with a novel single small battery-operated pulsatile pump that drives both the blood and dialysate in a counter-phased manner, maximizing the trans-membrane pressure. METHODS: Both in-vitro experimental and numerical tools are employed to compare the performance of the pulsatile WAK dialyzer with a traditional design of a single-channel roller blood pump together with a centrifugal pump that drives the dialysate flow. The numerical methods utilize the axisymmetric Navier-Stokes and mass transfer equations to model the flow in the fibers of the dialyzer. RESULTS: While diffusion is still the dominating transport regime, the WAK pump enhances substantially the trans-membrane pressure and thus increases mass convection that might be as high as 30% of the overall transfer. This increase is obtained due to the design of the pulsatile WAK pump that increases ultrafiltration by increasing the trans-membrane pressure. CONCLUSIONS: The experimental and numerical results revealed that when pumping at similar flow rates, a small battery-operated pulsatile pump provides clearances of urea and creatinine similar as or better than a large heavy AC-powered roller pump.

Numerical model of flow in a sac-type ventricular assist device.

This article addresses the growing need for comprehensive tools to investigate the hemodynamics of ventricular assist devices (VADs) in general and sac-type VADs in particular. Numerical simulations can be very helpful in these efforts. However, full simulation of flow inside sac-type VADs poses several key problems, among them simulation of the mechanical heart valves and calculation of the motion of flexible walls. We present a simplified three-dimensional (3D) numerical model of a sac-VAD chamber. The walls in the simplified model are defined to move according to experimental measurements, and the valves are modeled in fully open or fully closed positions. The model is validated by comparison to a fully coupled fluid-structure interaction numerical simulation and to experimental measurements using continuous digital particle image velocimetry. Our results demonstrate that the flexible wall motion is sensitive to changes in pressure distribution inside the chamber. However, small variations in wall motion do not significantly affect the global features of flow inside the chamber. Therefore, the simplified model can be used to predict the 3D time-dependent flow field in the VAD.

The hemodynamics of the Berlin pulsatile VAD and the role of its MHV configuration.

The 3D flow in a model of the Berlin ventricular assist device (VAD) chamber with monoleaflet valves placed in S-shape conduits was simulated numerically. The blood flow dynamics were described in terms of flow patterns, velocity, pressure, and shear stress. The hemodynamic properties and the VAD's potential risk for thrombosis were evaluated in terms of mixing and washout properties, and global estimations of platelet level of activation (LOA). In order to evaluate the role of valves on the flow in the chamber, the flow in a model with bileaflet valves in straight conduits was simulated and compared with the original case. The results showed that in both models a large rotating flow was developed in the chamber during filling. This vortex filled the entire chamber and moved constantly up to the peak ejection phase, resulting in relatively low shear stress (up to 0.4 Pa) and no lasting stagnation regions. Significant shear stresses were found near the valves with higher values near the outlet valve in both models. The configuration of valves and conduits had a large effect on VAD washout and mixing properties, with advantage to the bileaflet model. However, since the bileaflet valves exhibited higher shear stresses, higher LOA were found for the bileaflet model.

Physical stresses at the air-wall interface of the human nasal cavity during breathing.

The nose is the front line defender of the respiratory system and is rich with mechanoreceptors, thermoreceptors, and nerve endings. A time-dependent computational model of transport through nasal models of a healthy human has been used to analyze the fields of physical stresses that may develop at the air-wall interface of the nasal mucosa. Simulations during quiet breathing revealed wall shear stresses as high as 0.3 Pa in the noselike model and 1.5 Pa in the anatomical model. These values are of the same order of those known to exist in uniform large arteries. The distribution of temperature near the nasal wall at peak inspiration is similar to that of wall shear stresses. The lowest temperatures occur in the vicinity of high stresses due to the narrow passageway in these locations. Time and spatial gradients of these stresses may have functional effects on nasal sensation of airflow and may play a role in the well-being of nasal breathing.

Induced current bio-impedance technique for monitoring cryosurgery procedure in a two-dimensional head model using generalized coordinate systems.

In the noninvasive bio-impedance technique, small amplitude currents are applied to the body and the developing potentials on its surface are measured. This noninvasive technique is used to monitor physiological and pathological processes, which alter the values or the spatial distribution of the electrical impedance inside the human body. A possible application of the bio-impedance technique is monitoring brain cryosurgery procedure--a surgical technique that employs freezing to destroy undesirable tissues. A numerical solver was developed to evaluate the ability of an induced-current bio-impedance system to monitor the growth of the frozen tissue inside the head in simulation. The forward-problem bio-impedance solver, which is based on the finite volume method in generalized two-dimensional (2-D) coordinate systems, was validated by a comparison to a known analytical solution for body-fitted and Cartesian meshing grids. The sensitivity of the developed surface potential to the ice-ball area was examined using a 2-D head model geometry, and was found to range between 0.8 x 10(-2) and 1.68 x 10(-2) (relative potential difference/mm2), depending on the relative positioning of the excitation coil and the head. The maximal sensitivity was achieved when the coil was located at the geometrical center of the model.

Contactless bio-impedance monitoring technique for brain cryosurgery in a 3D head model.

A contactless induced-current bio-impedance system for monitoring brain cryosurgery procedure was modeled and numerically simulated, where the excitation coil was also performing as the measuring, or pick-up coil. A segmented three-dimensional (3D) MRI database was used for building the volume conductor geometry, and the numerical finite-volume method was employed for solving the forward problem for calculating the scalar potential distribution and the second-order voltage change on the pick-up coil. Several coil configurations were considered, varying in their relative positioning to the 3D head model. For each case, the sensitivity of the measured voltage change on the excitation coil to the volume of a frozen lesion was calculated. The highest sensitivity (1.1 x 10(-5) relative voltage change per mm3 of frozen tissue) was obtained for a coil arrangement where its closest segment to the volume conductor is at the maximum distance away from the frozen region position. The simulated system signal-to-carrier ratio was O(10(-8)).

The air-conditioning capacity of the human nose.

The nose is the front line defender of the respiratory system. Unsteady simulations in three-dimensional models have been developed to study transport patterns in the human nose and its overall air-conditioning capacity. The results suggested that the healthy nose can efficiently provide about 90% of the heat and the water fluxes required to condition the ambient inspired air to near alveolar conditions in a variety of environmental conditions and independent of variations in internal structural components. The anatomical replica of the human nose showed the best performance and was able to provide 92% of the heating and 96% of the moisture needed to condition the inspired air to alveolar conditions. A detailed analysis explored the relative contribution of endonasal structural components to the air-conditioning process. During a moderate breathing effort, about 11% reduction in the efficacy of nasal air-conditioning capacity was observed.