Performance of a continuous flow ventricular assist device: magnetic bearing design, construction, and testing

Performance Enhancement of a Magnetic System in a Ultra Compact 5-DOF-Controlled Self-Bearing Motor for a Rotary Pediatric Ventricular-Assist Device to Diminish Energy Input

Research interests of compact magnetically levitated motors have been strongly increased in development of durable and biocompatible mechanical circulatory support (MCS) devices for pediatric heart disease patients. In this study, an ultra-compact axial gap type self-bearing motor with 5-degrees of freedom (DOF) active control for use in pediatric MCS devices has been developed. The motor consists of two identical motor stators and a centrifugal levitated rotor. This paper investigated a design improvement of the magnetic circuit for the self-bearing motor undergoing development in order to diminish energy input by enhancing magnetic suspension and rotation performances. Geometries of the motor were refined based on numerical calculation and three-dimensional (3D) magnetic field analysis. The modified motor can achieve higher suspension force and torque characteristics than that of a previously developed prototype motor. Oscillation of the levitated rotor was significantly suppressed by 5-DOF control over rotating speeds up to 7000 rpm with lower energy input, indicating efficacy of the design refinement of the motor.

Numerical Solution for Blood Flow in a Centrifugal Ventricular Assist Device

A very small centrifugal pump, fully supported by magnetic bearings, is being developed for use as a ventricular assist device to be implanted in humans. In this paper, we apply computational fluid dynamics to model the blood flow to aid in the design of the ventricular assist device. The flow of blood through the pump has been modeled using computational fluid dynamics (CFD) software that is commercially available from AEA Technology, UK. The flow regions modeled in version 3 of the Continuous Flow Ventricular Assist Device (CF3) are the fully shrouded four bladed impeller and the two clearance regions around the impeller that are bounded by the pump hub and shroud. This paper describes the geometry and computational grids developed for the flow regions, and the equations of motion for the blood flow are developed. The overall numerically-evaluated flow rates and head rise have similar trends to the flow parameters experimentally measured, indicating that future pump designs can be effectively modeled numerically before being constructed and tested. Numerical solutions are presented and compared with experimentally-obtained overall pump performance results. These solutions are used to predict shear stress levels to be experienced by the blood flowing through the pump, and it is predicted that hemolysis will be insignificant. The solutions also indicate no regions of flow stagnation that can be a source of thrombosis in pumps. The calculations provide a viable design method to achieve improved efficiency in future versions of this pump.

Optimization of a Hybrid Magnetic Bearing for a Magnetically Levitated Blood Pump via 3-D FEA

In order to improve the performance of a magnetically levitated (maglev) axial flow blood pump, three-dimensional (3-D) finite element analysis (FEA) was used to optimize the design of a hybrid magnetic bearing (HMB). Radial, axial, and current stiffness of multiple design variations of the HMB were calculated using a 3-D FEA package and verified by experimental results. As compared with the original design, the optimized HMB had twice the axial stiffness with the resulting increase of negative radial stiffness partially compensated for by increased current stiffness. Accordingly, the performance of the maglev axial flow blood pump with the optimized HMBs was improved: the maximum pump speed was increased from 6000 rpm to 9000 rpm (50%). The radial, axial and current stiffness of the HMB was found to be linear at nominal operational position from both 3-D FEA and empirical measurements. Stiffness values determined by FEA and empirical measurements agreed well with one another. The magnetic flux density distribution and flux loop of the HMB were also visualized via 3-D FEA which confirms the designers' initial assumption about the function of this HMB.

Fluid force predictions and experimental measurements for a magnetically levitated pediatric ventricular assist device

The latest generation of artificial blood pumps incorporates the use of magnetic bearings to levitate the rotating component of the pump, the impeller. A magnetic suspension prevents the rotating impeller from contacting the internal surfaces of the pump and reduces regions of stagnant and high shear flow that surround fluid or mechanical bearings. Applying this third-generation technology, the Virginia Artificial Heart Institute has developed a ventricular assist device (VAD) to support infants and children. In consideration of the suspension design, the axial and radial fluid forces exerted on the rotor of the pediatric VAD were estimated using computational fluid dynamics (CFD) such that fluid perturbations would be counterbalanced. In addition, a prototype was built for experimental measurements of the axial fluid forces and estimations of the radial fluid forces during operation using a blood analog mixture. The axial fluid forces for a centered impeller position were found to range from 0.5 +/- 0.01 to 1 +/- 0.02 N in magnitude for 0.5 +/- 0.095 to 3.5 +/- 0.164 Lpm over rotational speeds of 6110 +/- 0.39 to 8030 +/- 0.57% rpm. The CFD predictions for the axial forces deviated from the experimental data by approximately 8.5% with a maximum difference of 18% at higher flow rates. Similarly for the off-centered impeller conditions, the maximum radial fluid force along the y-axis was found to be -0.57 +/- 0.17 N. The maximum cross-coupling force in the x direction was found to be larger with a maximum value of 0.74 +/- 0.22 N. This resulted in a 25-35% overestimate of the radial fluid force as compared to the CFD predictions; this overestimation will lead to a far more robust magnetic suspension design. The axial and radial forces estimated from the computational results are well within a range over which a compact magnetic suspension can compensate for flow perturbations. This study also serves as an effective and novel design methodology for blood pump developers employing magnetic suspensions. Following a final design evaluation, a magnetically suspended pediatric VAD will be constructed for extensive hydraulic and animal testing as well as additional validation of this design methodology.

A compact highly efficient and low hemolytic centrifugal blood pump with a magnetically levitated impeller

A magnetically levitated (maglev) centrifugal blood pump (CBP), intended for use as a ventricular assist device, needs to be highly durable and reliable for long-term use without any mechanical failure. Furthermore, maglev CBPs should be small enough to be implanted into patients of various size and weight. We have developed a compact maglev CBP employing a two-degree-of-freedom controlled magnetic bearing, with a magnetically suspended impeller directly driven by an internal brushless direct current (DC) motor. The magnetic bearing actively controls the radial motion of the impeller and passively supports axial and angular motions using a permanent magnet embedded in the impeller. The overall dimensions of the maglev CBP are 65 mm in diameter and 40 mm in height. The total power consumption and pump efficiency for pumping 6 L/min against a head pressure of 105 mm Hg were 6.5 W and 21%, respectively. To evaluate the characteristics of the maglev CBP when subjected to a disturbance, excitation of the base, simulating the movement of the patient in various directions, and the sudden interception of the outlet tube connected with the pump in a mock circulatory loop, simulating an unexpected kink and emergent clamp during a heart surgery, were tested by monitoring the five-degree-of-freedom motion of the impeller. Furthermore, the hemolytic characteristics of the maglev CBP were compared with those of the Medtronic Biomedicus BPX-80, which demonstrated the superiority of the maglev CBP.

Computational Design and Experimental Performance Testing of an Axial-Flow Pediatric Ventricular Assist Device

The Virginia Artificial Heart Institute continues to design and develop an axial-flow pediatric ventricular assist device (PVAD) for infants and children in the United States. Our research team has created a database to track potential PVAD candidates at the University of Virginia Children's Hospital. The findings of this database aided with need assessment and design optimization of the PVAD. A numerical analysis of the optimized PVAD1 design (PVAD2 model) was also completed using computational fluid dynamics (CFD) to predict pressure-flow performance, fluid force estimations, and blood damage levels in the flow domain. Based on the PVAD2 model and after alterations to accommodate manufacturing, a plastic prototype for experimental flow testing was constructed via rapid prototyping techniques or stereolithography. CFD predictions demonstrated a pressure rise range of 36-118 mm Hg and axial fluid forces of 0.8-1.7 N for flows of 0.5-3 l/min over 7000-9000 rpm. Blood damage indices per CFD ranged from 0.24% to 0.35% for 200 massless and inert particles analyzed. Approximately 187 (93.5%) of the particles took less than 0.14 seconds to travel completely through the PVAD. The mean residence time was 0.105 seconds with a maximum time of 0.224 seconds. Additionally, in a water/glycerin blood analog solution, the plastic prototype produced pressure rises of 20-160 mm Hg for rotational speeds of 5960 +/- 18 rpm to 9975 +/- 31 rpm over flows from 0.5 to 4.5 l/min. The numerical results for the PVAD2 and the prototype hydraulic testing indicate an acceptable design for the pump, represent a significant step in the development phase of this device, and encourage manufacturing of a magnetically levitated prototype for animal experiments.

Novel maglev pump with a combined magnetic bearing

The newly developed pump is a magnetically levitated centrifugal blood pump in which active and passive magnetic bearings are integrated to construct a durable ventricular assist device. The developed maglev centrifugal pump consists of an active magnetic bearing, a passive magnetic bearing, a levitated impeller, and a motor stator. The impeller is set between the active magnetic bearing and the motor stator. The active magnetic bearing uses four electromagnets to control the tilt and the axial position of the impeller. The radial movement of the levitated impeller is restricted with the passive stability dependent upon the top stator and the passive permanent magnetic bearing to reduce the energy consumption and the control system complexity. The top stator was designed based upon a magnetic field analysis to develop the maglev pump with sufficient passive stability in the radial direction. By implementing this analysis design, the oscillating amplitude of the impeller in the radial direction was cut in half when compared with the simple shape stator. This study concluded that the newly developed maglev centrifugal pump displayed excellent levitation performance and sufficient pump performance as a ventricular assist device.

Design and transient computational fluid dynamics study of a continuous axial flow ventricular assist device

A ventricular assist device (VAD), which is a miniaturized axial flow pump from the point of view of mechanism, has been designed and studied in this report. It consists of an inducer, an impeller, and a diffuser. The main design objective of this VAD is to produce an axial pump with a streamlined, idealized, and nonobstructing blood flow path. The magnetic bearings are adapted so that the impeller is completely magnetically levitated. The VAD operates under transient conditions because of the spinning movement of the impeller and the pulsatile inlet flow rate. The design method, procedure, and iterations are presented. The VAD's performance under transient conditions is investigated by means of computational fluid dynamics (CFD). Two reference frames, rotational and stationary, are implemented in the CFD simulations. The inlet and outlet surfaces of the impeller, which are connected to the inducer and diffuser respectively, are allowed to rotate and slide during the calculation to simulate the realistic spinning motion of the impeller. The flow head curves are determined, and the variation of pressure distribution during a cardiac cycle (including systole and diastole) is given. The axial oscillation of impeller is also estimated for the magnetic bearing design. The transient CFD simulation, which requires more computer resources and calculation efforts than the steady simulation, provides a range rather than only a point for the VAD's performance. Because of pulsatile flow phenomena and virtual spinning movement of the impeller, the transient simulation, which is realistically correlated with the in vivo implant scenarios of a VAD, is essential to ensure an effective and reliable VAD design.

Experimental Determination of Dynamic Characteristics of the VentrAssist Implantable Rotary Blood Pump

The VentrAssist implantable rotary blood pump, intended for long-term ventricular assist, is under development and is currently being tested for its rotor-dynamic stability. The pump consists of a shaftless impeller, which also acts as the rotor of the brushless DC motor. The impeller remains passively suspended in the pump cavity by hydrodynamic forces, which result from the small clearances between the outside surfaces of the impeller and the pump cavity. These small clearances range from approximately 50 microm to 230 microm in size in the version of pump reported here. This article presents experimental investigation into the dynamic characteristics of the impeller-bearing-pump housing system of the rotary blood pump for increasing pump speeds at different flow rates. The pump was mounted on a suspension system consisting of a platform and springs, where the natural frequency and damping ratio for the suspension system were determined. Real-time measurements of the impeller's displacement were performed using Hall effect sensors. A vertical disturbance force was exerted onto the pump housing, causing the impeller to be displaced in vertical direction from its dynamic equilibrium position within the pump cavity. The impeller displacement was represented by a decaying sine wave, which indicated the impeller restoring to its equilibrium position. From the decaying sine wave the natural frequency and stiffness coefficient of the system were determined. Furthermore, the logarithmic decrement method was used to determine the damping ratio and eventually the damping coefficient of the system. Results indicate that stiffness and damping coefficients increased as flow rate and pump speed increased, representing an increase in stability with these changing conditions. However, pump speed had a greater influence on the stiffness and damping coefficients than flow rate did, which was evident through dynamic analysis. Overall the experimental method presented in this article was successful in determining the dynamic characteristics of the system.

Testing of a centrifugal blood pump with a high efficiency hybrid magnetic bearing

The purpose of this article is to present test results for a second generation, high efficiency, nonpulsatile centrifugal blood pump that is being developed for use as a left ventricular assist device (LVAD). The LVAD pump uses a hybrid passive-active magnetic bearing support system that exhibits extremely low power loss, low vibration, and high reliability under transient conditions and varying pump orientations. A unique feature of the second generation design configuration is the very simple and direct flow path for both main and washing blood flows. The pump was tested in both vertical and horizontal orientations using a standard flow loop to demonstrate the performance and durability of the second generation LVAD. Steady state and transient orientation pump operating characteristics including pressure, flow, speed, temperatures, vibration, and rotor orientation were measured. During the tests, pump performance was mapped at several operating conditions including points above and below the nominal design of 5 L/min at 100 mm Hg pressure rise. Flow rates from 2 to 7 L/min and pressure rises from 50 to 150 mm Hg were measured. Pump speeds were varied during these tests from 2,500 to 3,500 rpm. The nominal design flow of 5 L/min at 100 mm Hg pressure rise was successfully achieved at the design speed of 3,000 rpm. After LVAD performance testing, both 28 day continuous duty and 5 day transient orientation durability tests were completed without incident. A hydrodynamic backup bearing design feasibility study was also conducted. Results from this design study indicate that an integral hydrodynamic backup bearing may be readily incorporated into the second generation LVAD and other magnetically levitated pump rotors.

Impeller Behavior and Displacement of the VentrAssist Implantable Rotary Blood Pump

The VentrAssist implantable rotary blood pump, intended for long-term ventricular assist, is under development and is currently being tested for its rotor-dynamic stability. The pump is of the centrifugal type and consists of a shaftless impeller, also acting as the rotor of the brushless DC motor. The impeller remains passively suspended in the pump cavity by hydrodynamic forces, resulting from the small clearances between the impeller outside surfaces and the pump cavity. In the older version of the pump tested, these small clearances range from approximately 50 microm to 230 microm; the displacement of the impeller relative to the pump cavity is unknown in use. This article presents two experiments: the first measured displacement of the impeller using eddy-current proximity sensors and laser proximity sensors. The second experiment used Hall-effect proximity sensors to measure the displacement of the impeller relative to the pump cavity. All transducers were calibrated prior to commencement of the experiments. Voltage output from the transducers was converted into impeller movement in five degrees of freedom (x, y, z, theta(x), and theta(y)). The sixth degree of freedom, the rotation about the impeller axis (theta(z)), was determined by the commutation performed by the motor controller. The impeller displacement was found to be within the acceptable range of 8 micro m to 222 microm, avoiding blood damage and contact between the impeller and cavity walls. Thus the impeller was hydrodynamically suspended within the pump cavity and results were typical of centrifugal pump behavior. This research will be the basis for further investigation into the stiffness and damping coefficient of the pump's hydrodynamic bearing.

Magnetically suspended centrifugal blood pump with an axially levitated motor

The longevity of a rotary blood pump is mainly determined by the durability of its wearing mechanical parts such as bearings and seals. Magnetic suspension techniques can be used to eliminate these mechanical parts altogether. This article describes a magnetically suspended centrifugal blood pump using an axially levitated motor. The motor comprises an upper stator, a bottom stator, and a levitated rotor-impeller between the stators. The upper stator has permanent magnets to generate an attractive axial bias force on the rotor and electric magnets to control the inclination of the rotor. The bottom stator has electric magnets to generate attractive forces and rotating torque to control the axial displacement and rotation of the rotor. The radial displacement of the rotor is restricted by passive stability. A shrouded impeller is integrated within the rotor. The performance of the magnetic suspension and pump were evaluated in a closed mock loop circuit filled with water. The maximum amplitude of the rotor displacement in the axial direction was only 0.06 mm. The maximum possible rotational speed during levitation was 1,600 rpm. The maximum pressure head and flow rate were 120 mm Hg and 7 L/min, respectively. The pump shows promise as a ventricular assist device.

Magnetically suspended centrifugal blood pump with a self bearing motor

A magnetically suspended centrifugal blood pump with a self bearing motor has been developed for long-term ventricular assistance. A rotor of the self bearing motor is actively suspended and rotated by an electromagnetic field without mechanical bearings. Radial position of the rotor is controlled actively, and axial position of the rotor is passively stable within the thin rotor structure. An open impeller and a semiopened impeller were examined to determine the best impeller structure. The outer diameter and height of the impeller are 63 and 34 mm, respectively. Both the impellers indicated similar pump performance. Single volute and double volute structures were also tested to confirm the performance of the double volute. Power consumption for levitation and radial displacement of the impeller with a rotational speed of 1,500 rpm were 0.7 W and 0.04 mm in the double volute, while those in the single volute were 1.3 W and 0.07 mm, respectively. The stator of the self bearing motor was redesigned to avoid magnetic saturation and improve motor performance. Maximum flow rate and pressure head were 9 L/min and 250 mm Hg, respectively. The developed magnetically suspended centrifugal blood pump is a candidate for an implantable left ventricular assist device.

An ultradurable and compact rotary blood pump with a magnetically suspended impeller in the radial direction

A magnetically suspended centrifugal blood pump has been developed with a self-bearing motor for long-term ventricular assist systems. The rotor of the self-bearing motor is not only actively suspended in the radial direction, but also is rotated by an electromagnetic field. The pump has a long lifetime because there are no mechanical parts such as seals and motor bearings. An outer rotor mechanism was adopted for the self-bearing motor. The stator was constructed in the central space of the motor. The rotor shaped thin ring was set at the circumferential space of the stator. Six vanes were extended from the upper surface of the rotor toward the center of the pump to construct an open-type impeller. The outer diameter and the height of the impeller are 63 mm and 34 mm, respectively. The magnetic bearing method and the servomotor mechanism were adopted to levitate and rotate the rotor. Radial movements of the rotor and rotation are controlled actively by using electromagnets in the stator. Axial movement and tilt of the rotor are restricted by passive stability to simplify the control. The radial gap between the rotor and the stator is 1 mm. A closed-loop circuit filled with water was used to examine basic performance of the pump. Maximum flow rate and pressure head were 8 L/min and 200 mm Hg, respectively. Maximum amplitude of radial displacement of the impeller was 0.15 mm. The impeller could be suspended completely without touching the casing wall during the entire pumping process. Power consumption of the pump was only 9.5 W to produce a flow rate of 5 L/min against a pressure head of 100 mm Hg. We conclude that the pump has sufficient performance for the implantable ventricular assist system.

New strategies for the management of acute decompensated heart failure

Acute heart failure in adults is the unfolding of heart failure in minutes, hours or a few days. Low output heart failure describes a form of heart failure in which the heart pumps blood at a rate at rest or with exertion that is below the physiological range and the metabolizing tissues extract their required oxygen from blood at a lower rate, causing a proportionately smaller oxygen amount remaining in the blood. Therefore, a widened arterial-venous oxygen difference occurs. High output heart failure is characterized by pumping blood with a rate above the physiological range at rest or during exertion, resulting in an arterial-venous oxygen difference, which is normal or low. This may be caused by peripheral vasodilatation during sepsis or thyrotoxicosis, blood shunting, or reduced blood oxygen content/viscosity (Fig. 1). The differentiation between low output heart failure versus high output heart failure is of highest importance for the choice of therapy and therefore the information and the monitoring of the systemic vascular resistance. Patients who present with acute heart failure suffer from a severe complication of different cardiac disorders. Most often they have an acute injury that affects their myocardial performance (eg, myocardial infarction) or valvular/chamber integrity (mitral regurgitation, ventricular septal rupture), which leads to an acute rise in left-ventricular filling pressures resulting in pulmonary edema.

Magnetically suspended rotary blood pump with radial type combined motor-bearing

A magnetically suspended centrifugal blood pump is being developed with a combined motor-bearing for long-term ventricular assist systems. The combined motor-bearing actively suspends a rotor in a radial direction to deal with radial force unbalance in the pump and rotates the rotor by using the electric magnetic field. Therefore, the pump has no mechanical parts such as bearings of the motor and has a long lifetime. The developed pump consists of a thin rotor with a semi open-type 6 vane impeller and a stator to suspend and rotate the rotor. The rotor has 4-pole permanent magnets on the circumferential surface. The outer diameter and the thickness of the rotor are 60 mm and 8 mm, respectively. Axial movement and tilt of the rotor are restricted by passive stability based on the thin rotor structure. Radial movements of the rotor, such as levitation in radial direction and rotation, are controlled actively by using electric magnets of the stator. The electric magnet coils to produce levitation and rotation forces are constructed on the periphery stator. The p +/- 2-pole algorithm and the synchronous motor mechanism are adopted to levitate and rotate the rotor. The radial gap between the rotor and the stator is 1 mm. A closed-loop circuit filled with water was connected to the developed pump to examine the basic performance of the pump and the magnetic suspension system. Maximum rotational speed, flow rate, and head were 2,800 rpm, 11 L/min, and 270 mm Hg, respectively. The rotor with the impeller could be suspended completely during the entire pumping process. We conclude the pump with the combined motor-bearing has sufficient performance for the blood pump.

Paradoxical effects of viscosity on the VentrAssist rotary blood pump

The ability of the VentrAssist blood pump to perform at its optimum design point is determined by a number of factors such as geometry of the pump, surface roughness, and fluid properties. Once the fluid properties are known, the performance characteristics of the pump can be optimized for that fluid. It is important to understand the effects of dynamic viscosity mu (called simply viscosity hereafter) on the performance characteristics and stability of the pump. The performance envelope of the pump and the needs of the patient must be matched. The VentrAssist pump has no shaft, seals, or fixed bearings and relies on the fluid-dynamic forces to maintain its effective performance. A number of different fluids have been tested to determine the effects of viscosity and density on pump performance. These include aqueous glycerol, red blood cells (RBCs) suspended in phosphate buffered saline solution (PBS), and Haemaccel. The effects of viscosity on the bearing stiffness, stage efficiency, and the pressure-flow rate (HQ) are characterized. The experimental results show a slight increase in the pressure rise across the pump shown as a positive upward shift of the H-Q curves with a decrease in viscosity; however, this is relatively small. A paradox in system efficiency exists: for a given fluid asymptotic viscosity, the system efficiency (product of magnetic and stage efficiency) using Haemaccel or PBS is greater than for the same viscosity of aqueous glycerol.

Characterization of a magnetic bearing system and fluid properties for a continuous flow ventricular assist device

This article presents the performance test results of the CFVAD3 continuous flow blood pump in an artificial human circulation system. The CFVAD3 utilizes magnetic bearings that support a thin pancake impeller, the shape of which allows for a very compact pump whose total axial length is less than 5 cm with a radial length of about 10 cm. This gives a total volume of about 275 cc. The impeller itself has 4 vanes with a designed operating point of 6 L/min at 100 mm Hg of differential pressure and 2,000 rpm. The advantages of magnetic bearings, such as large clearance spaces and no mechanical wear, are elaborated upon. Furthermore, bearing model parameters such as load capacity and current gains are described. These parameters in conjunction with the operating conditions during testing are then used to estimate the fluid forces, stiffness, and damping properties while pumping. Knowledge of these parameters is desirable because of their effects on pump behavior. In addition, a better plant model will allow more robust control algorithms to be devised that can boost pump performance and reliability.