Single axis controlled hybrid magnetic bearing for left ventricular assist device: hybrid core and closed magnetic circuit

In previous studies, we presented main strategies for suspending the rotor of a mixed‐flow type (centrifugal and axial) ventricular assist device (VAD), originally presented by the Institute Dante Pazzanese of Cardiology (IDPC), Brazil. Magnetic suspension is achieved by the use of a magnetic bearing architecture in which the active control is executed in only one degree of freedom, in the axial direction of the rotor. Remaining degrees of freedom, excepting the rotation, are restricted only by the attraction force between pairs of permanent magnets. This study is part of a joint project in development by IDPC and Escola Politecnica of São Paulo University, Brazil. This article shows advances in that project, presenting two promising solutions for magnetic bearings. One solution uses hybrid cores as electromagnetic actuators, that is, cores that combine iron and permanent magnets. The other solution uses actuators, also of hybrid type, but with the magnetic circuit closed by an iron core. After preliminary analysis, a pump prototype has been developed for each solution and has been tested. For each prototype, a brushless DC motor has been developed as the rotor driver. Each solution was evaluated by in vitro experiments and guidelines are extracted for future improvements. Tests have shown good results and demonstrated that one solution is not isolated from the other. One complements the other for the development of a single‐axis‐controlled, hybrid‐type magnetic bearing for a mixed‐flow type VAD.     

Concept for a new hydrodynamic blood bearing for miniature blood pumps

The most crucial element of a long-term implantable rotary blood pump is the rotor bearing. Because of heat generation and power loss resulting from friction, seals within the devices have to be avoided. Actively controlled magnetic bearings, although maintenance-free, increase the degree of complexity. Hydrodynamic bearings for magnetically coupled rotors may offer an alternative solution to this problem. Additionally, for miniature pumps, the load capacity of hydrodynamic bearings scales slower than that of, for example, magnetic bearings because of the cube-square-law. A special kind of hydrodynamic bearing is a spiral groove bearing (SGB), which features an excellent load capacity. Mock-loop tests showed that SGBs do not influence the hydraulic performance of the tested pumps. Although, as of now, the power consumption of the SBG is higher than for a mechanical pivot bearing, it is absolutely contact-free and has an unlimited lifetime. The liftoff of the rotor occurs already at 10% of design speed. Further tests and flow visualization studies on scaled-up models must demonstrate its overall blood compatibility.     

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 Sealless Centrifugal Blood Pump with Passive Magnetic and Hydrodynamic Bearings

We are developing a permanently implantable ventricular assist system based on a sealless centrifugal blood pump. The impeller of the pump is supported by a passive radial magnetic bearing acting in synergy with hydrodynamic bearings. Torque is transmitted to the impeller by electromagnetic coupling via an integrated axial flux gap motor. Computer modeling has been used extensively to guide the hydraulic and electromagnetic design of the pump. As part of the development effort, a prototype system was built, which consisted of a radial magnetic bearing, an axial air gap motor, and a pivot bearing to constrain the axial motion. The following testing has been completed to validate the design. First, hydraulic tests have demonstrated sufficient hydraulic performance. Second, preliminary in vitro evaluation of hemolysis was low compared to that of a BioPump control. Third, a 6 h in vivo experiment was successfully completed.     

Optimization of an axial flow heart pump with active and passive magnetic bearings

Optimization of a magnetically suspended left ventricular assist device (LVAD) is crucial. We desire a totally implantable, long-life LVAD that delivers the necessary flow rate, pressure rise, and blood compatibility. By using a novel combination of passive and active magnetic bearings (AMBs), we have developed an axial flow LVAD prototype, the LEV-VAD, which provides an unobstructed blood flow path, preventing stagnation regions for the blood. Our current effort is focused on the optimization of the magnetic suspension system to allow for control of the AMB, minimizing its size and power consumption. The properties of the passive magnetic bearings and AMBs serve as parameter space, over which a cost function is minimized, subject to constraints such as suspension stability and sufficient disturbance rejection capabilities. The design process is expected to lead to the construction of a small prototype pump along with the necessary robust controller for the AMB. Sensitivity of the LVAD performance with respect to various design parameters is examined in-depth and an optimized, more compact LVAD prototype is designed.     

In Vitro Hydrodynamic Analysis of Pin and Cone Bearing Designs of the Jarvik 2000 Adult Ventricular Assist Device

The Jarvik 2000 adult ventricular assist device (VAD) is a second‐generation blood pump with mechanical contact bearings. The original configuration of the pump employed a pin bearing and a more recent configuration uses a cone bearing. We compare the hydrodynamic performance of the two designs under steady‐state and pulsatile flow conditions in vitro. Furthermore, we employ the Intermittent Low Speed (ILS) Flowmaker Controller to demonstrate the effect on pulsatility index (PI) performance of both device configurations. We use an open‐loop flow system in both steady‐state and pulsatile arrangements, complete with pressure transducers and flow probes. Working fluid was a 3.6 cP blood‐analog, glycerin‐water solution. Steady‐state flow tests were carried out to determine pressure‐flow (H‐Q) performance curves. Pulsatile tests under normotensive, hypertensive, and hypotensive conditions were executed with controller speed 3 (10 710 ± 250 rpm) at 100 beats per minute. Steady‐state tests show greater capacity for pressure and flow with the cone bearing, compared with pin bearing, with best efficiency point (BEP) 68% greater for cone bearing. Pulsatile tests show the cone bearing design to yield a 20% increase in Q_avg, a 17% decrease in pulsatility index (PI_Q), and a qualitative increase in pressure responsivity. The ILS mode (for both bearing designs) decreases Q_avg by 68% and likewise increases PI_Q by 360% and pulsatility ratio (R_pul) by 200%. The ILS controller regularly reduces the flow, increasing pulsatility index during device operation. The Jarvik 2000 continuous‐flow VAD can sustain pulsatile flow under pulsating pressure conditions. The new cone bearing design yields increased flow rates over the earlier pin bearing design.     

The progressive wave pump: numerical multiphysics investigation of a novel pump concept with potential to ventricular assist device application

This article describes the numerical fluid-structure interaction (FSI) validation of a new pumping concept and the possibility for application of a further developed type, as an implantable ventricular assist device (VAD). The novel principle of the so-called progressive wave pump is based on the interaction of an elastic membrane actuated by forced excitation with a surrounding fluid and the pump housing. By applying forced vibrations to one end of the membrane, a transversal wave builds up and progresses to the far end generating both a positive pressure gradient and flow rate. Among others, two axisymmetric geometrical configurations are possible, namely the discoidal and the tubular design. The first one has been built as a physical prototype and is experimentally investigated. In addition, a corresponding numerical FSI model is set up and validated against the experimental findings. Based on this validated numerical method, further numerical investigations are conducted focusing on the development of a tubular progressive wave pump concept with regard to its potential for application as a VAD in the future. To address VAD-relevant issues such as size, hydraulic performance, and blood trauma, corresponding numerical simulations involving macroscopic blood trauma models have been performed. Although being still in an early phase of development, the results are promising and indicate that the wave pump concept in its present state is feasible and can be further developed and investigated as a new type of blood pump.     

Development of a Compact, Highly Efficient, Totally Implantable Motor–Driven Assist Pump System

Abstract: We have developed a compact, highly efficient, totally implantable assist pump system, which consists of a motor–driven assist pump and a transcutaneous energy and optical information transmission system. The motor–driven assist pump consists of ad. c. brushless motor and a specially designed miniature ball screw. A magnetic coupling mechanism between the blood pump and an actuator provides active blood filling via mild suction force. The controller consists of a PID follow–up controller using an 8–bit one–chip microcomputer. The volume of the pump is 350 ml, and its controller is 210 ml. Pump outflow of 5. 8 L/min was obtained against a mean afterload of 100 mm Hg. The pump showed a high efficiency rate and good durability. An efficiency rate of 19–21% (pump output/motor input) was obtained during 87 days of continuous pumping. No mechanical trouble occurred for an accumulated period of 6 months.     

Use of a ventricular assist device as a bridge to transplantation in a patient with single ventricle physiology and total cavopulmonary anastomosis

Mechanical circulatory support can be used to manage acute and chronic cardiac failure in both adult and pediatric patients. Traditionally, extracorporeal membrane oxygenation (ECMO) has been the most common form of mechanical circulatory support in children. However, more recently, in cases of pure ventricular dysfunction, ventricular assist devices (VADs) have offered specific advantages over ECMO, including better ventricular recovery, reduced anticoagulation requirements, decreased use of blood products and decreased cost. We present the use of a VAD in an adolescent with single-ventricle physiology, who could not be weaned from cardiopulmonary bypass (CPB) after undergoing a revision of a modified Fontan operation. Gas exchange was provided by the patient's lungs while the centrifugal VAD was used successfully to support the circulation as a bridge, first to a totally implantable pulsatile VAD and subsequently to heart transplantation.     

Peripheral Vascular Resistances During Total Left Heart Bypass with an Oscillated Blood Flow

For development aimed at a totally implantable type ventricular assist device (VAD), the vibrating flow pump (VFP) has been developed at Tohoku University. A transcutaneous energy transmission system (TETS) using amorphous fibers was developed to power the totally implantable VAD system. The VFP works at a high frequency compared to that of a natural heart of a biological system. It is a frequency of 10-50 Hz. In this research, animal experiments with left heart bypass were carried out with healthy adult goats. For comparison between nonpulsatile flow and oscillated flow, a rotary pump (RP) and the VFP were used in the experiments. For the achievement of total left heart bypass, left ventricular approaches were carried out, and blood was pumped from the left ventricle to the descending aorta. Adequate support of the left heart was provided by both pumps. In terms of the results, the vascular resistances tended to decrease during the use of both pumps during 100% bypass driving. When we compared these pumps at the same flow rate, the resistances during RP driving were significantly smaller than those during VFP driving. These results may suggest that the influences of the VFP upon the peripheral vessels may be relatively small compared to those of the RP. This may be an important result when a stable hemodynamic condition is required during artificial circulation. The VFP was considered as a candidate for a totally implantable VAD as a result.     

Development of rear‐impeller axial flow blood pump for realization of axial flow blood pump installed at aortic valve position

In this study, rear‐impeller axial flow blood pumps (RIAFBP) were developed to realize a trans‐valve axial ventricular assist device (VAD) which consists of the latter blood pump and a polymer monomembrane aortic valve, such as the jellyfish valve. The motor of the RIAFBP is installed in the left ventricle, and its impeller is placed at the aortic valve position. In the prototype RIAFBP, the rotation of the motor is sustained by polyethylene bushings. The RIAFBP has a length of 50 mm and diameter of 19.6 mm. The miniature RIAFBP has the same construction as that of the prototype; however, it employs a ceramic bearing and fin bearing to improve endurance and to reduce blood stagnation. The miniature RIAFBP has a length of 63 mm and diameter of 12 mm. Both RIAFBPs were examined by an in vitro experiment using a 33% glycerin solution. The prototype RIAFBP achieved a maximum pump outflow of 8.5 L/min against a pump head of 100 mm Hg at a rotational speed of 12 000 rpm. The miniature RIAFBP achieved 7 L/min against a pump head of 70 mm Hg at a rotational speed of 21 600 rpm. In conclusion, the miniature RIAFBP has enough pump performance to realize the trans‐valve axial VAD.     

Left Heart Bypass Using the Oscillated Blood Flow with Totally Implantable Vibrating Flow Pump

Abstract Aiming at a totally implantable ventricular assist device (VAD), a vibrating flow pump (VFP) was developed in Tohoku University. A transcutaneous energy transmission system (TETS) using an amorphous fiber was developed for the totally implantable VAD system. The VFP works with a higher frequency than the natural heart of a biological system, a frequency of 10–50 Hz. In this research, animal experiments on left heart bypass were performed with healthy goats. Blood from the apex of the left ventricle was received and was sent to the aorta so that an adequate supporting effect of the left heart was provided. In particular, the depression effect of the left ventricle was obvious. As a result, sufficient artificial heart flow was provided. For a totally implantable type VAD, left heart bypass of almost 100% may become necessary in some situations. Therefore, apex approaches of left heart bypass may be desirable. From an anatomical consideration, an apex of the heart is suitable for the VFP of this totally implantable type. In the left heart bypass for which the apex of the heart was used, an almost 100% bypass was possible. This is a requirement that is important when waiting for recovery of sufficient cardiac function. It is also important that left heart circulation is maintained fully by an artificial heart of the complete implantation type. The VFP was considered to be useful as a totally implantable type artificial heart from the results.     

Evaluation of an implantable motor-driven left ventricular assist device

A console based implantable motor-driven left ventricular assist device (LVAD) was developed and tested. Ten sheep weighing 42-73 kg (mean, 54.4 kg) were used as the experimental animals. Four animals survived 5-12 h (mean, 9.5 h). The mean pump flow was 1.63 L/min, ranging from 0.8 to 2.5 L/min. The cause of termination was respiratory failure in 3 animals, bleeding in 2, ventricular fibrillation in 2, vent tube obstruction in 1, thrombus formation in 1, and mechanical failure of the driving console in 1. Following the in vivo studies, the computer regulated controller was tested in a mock circulatory system. The LVAD provided 5.34 L/min of maximum output against a mean afterload of 80 mm Hg with a filling pressure of 15 mm Hg when the pump rate was 80 bpm in the fixed rate mode. With an increase in the pump afterload from 80 to 140 mm Hg, the total system efficiency varied from 7.81 to 8.34% when the pump preload was 15 mm Hg. An ultracompact, completely implantable electromechanical VAD has been under development. This device should fit in a 60 kg adult. As the next step, we are preparing to implant this ultracompact implantable VAD with an electronic controller in an animal model with better results being expected.     

A strategy for designing of customized electromechanical actuators of blood pumps

Congestive heart failure is a pathology of global incidence that affects millions of people worldwide. When the heart weakens and fails to pump blood at physiological rates commensurate with the requirements of tissues, two main alternatives are cardiac transplant and ventricular assist devices (VADs). This article presents the design strategy for development of a customized VAD electromagnetic actuator. Electromagnetic actuator is a brushless direct current motor customized to drive the pump impeller by permanent magnets located in rotor–stator coupling. In this case, ceramic pivot bearings support the VAD impeller. Electronic circuitry controls rotation switching current in stator coils. The proposed methodology consisted of analytical numerical design, tridimensional computational modeling, numerical simulations using Maxwell software, actuator prototyping, and validation in the dynamometer. The axial flow actuator was chosen by its size and high power density compared to the radial flow type. First step consisted of estimating the required torque to drive the pump. Torque was estimated at 2100 rpm and mean current of 0.5 A. Numerical analysis using finite element method mapped vectors and fields to build stator coils and actuator assemblage. After tests in the dynamometer, experimental results were compared with numerical simulation and validated the proposed model. In conclusion, the proposed methodology for designing of VAD electromechanical actuator was considered satisfactory in terms of data consistency, feasibility, and reliability.     

Potential mechanisms for muscle-powered cardiac support

Biomechanical actuation of an implanted ventricular assist device (VAD) is an attractive means of providing long-term circulatory support. Studies show that energy from electrically stimulated skeletal muscle can, in principle, be used to provide tether-free cardiac assistance without the need for percutaneous drivelines or bulky energy transmission hardware. A mechanical prosthesis designed to harness the contractile power of in situ skeletal muscle has been developed in this laboratory that collects energy from the latissimus dorsi muscle and transmits it in the form of hydraulic power. In order to use this technique to pump blood however, a practical means to deliver this energy to the bloodstream must be devised. Presented here are six prospective mechanisms designed to accomplish this task, five of which also eliminate blood contacting surfaces that often lead to thromboembolic complications in chronic VAD patients.     

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.     

HeartQuest ventricular assist device magnetically levitated centrifugal blood pump

Improvements in implantable ventricular assist device (VAD) performance will be required to obtain patient outcomes that are comparable with those of heart transplantation. The HeartQuest VAD (WorldHeart, Oakland, CA, U.S.A.) is an advanced device, with full magnetic suspension of the rotor, designed to address specific clinical shortcomings in existing devices and to maximize margins of safety and performance for an implantable assist device. The device dimensions are 35 x 75 mm, with a total weight of 440 g. The system was designed using extensive computer modeling of device function; a total of two iterations of device prototypes were built before building the clinical version. Animal study results have been very promising, with over 30 calf studies completed. Plasma-free hemoglobin levels returned to preoperative levels, and other hematology results were in the normal ranges. Highlights include clean surfaces seen in a 116-day experiment with no anticoagulation after day 43. Feasibility clinical trials are planned to start in 2006.     

Feasibility of a Tiny Gyro Centrifugal Pump as an Implantable Ventricular Assist Device

The Gyro pumps were developed for long-term circulatory support. The first generation Gyro pump (C1E3) achieved 1 month paracorporeal circulatory support in chronic animal experiments; the second generation (PI702) implantable ventricular assist device (VAD) was successful for over 6 months. The objective of the next generation Gyro pump is for use as a long-term totally implantable VAD and for pediatric circulatory support. This tiny Gyro pump (KP101) was fabricated with the same design concept as the other Gyro pumps. The possibility of an implantable VAD was determined after performance and hemolysis test results were compared to those of the other Gyro pumps. The pump housing and impeller were fabricated from polycarbonate with an impeller diameter of 35 mm. The diameter and height of the pump housings are 52.3 mm and 29.9 mm, respectively. At this time, a DC brushless motor drives the KP101, which is the same as that for the C1E3. The pump performance was measured in 37% glycerin water at 37 degrees C. Hemolysis tests were performed utilizing a compact mock loop filled with fresh bovine blood in a left ventricular assist device (LVAD) condition at 37 degrees C. The KP101 achieved the LVAD conditions of 5 L/min and 100 mm Hg at 2,900 rpm; generated 10 L/min against 100 mm Hg at 3,200 rpm; 3 L/min against 90 mm Hg at 2,600 rpm; and 2 L/min against 80 mm Hg at 2,400 rpm. In addition, the pump efficiency during this experiment was 12.5%. The other Gyro pumps. that is, the C1E3, PI601, and PI701, in an LVAD condition require 1,600, 2,000, and 2,000 rpm, respectively. The KP101 produced a normalized index of hemolysis (NIH) value of 0.005 g/100 L. With regard to the NIH, the other Gyro pumps, namely the C1E3, PI601, and PI701 demonstrated 0.0007, 0.0028, and 0.004 g/100 L, respectively. The KP101 produced an acceptable pressure flow curve for a VAD. The NIH value was higher than that of other Gyro pumps, but is in an acceptable range.     

Motor Feedback Physiological Control for a Continuous Flow Ventricular Assist Device

The response of a continuous flow magnetic bearing supported ventricular assist device, the CFVAD3 (CF3) to human physiologic pressure and flow needs is varied by adjustment of the motor speed. This paper discusses a model of the automatic feedback controller designed to develop the required pump performance. The major human circulatory, mechanical, and electrical systems were evaluated using experimental data from the CF3 and linearized models developed. An open-loop model of the human circulatory system was constructed with a human heart and a VAD included. A feedback loop was then closed to maintain a desired reference differential pressure across the system. A proportional-integral (PI) controller was developed to adjust the motor speed and maintain the system reference differential pressure when changes occur in the natural heart. The effects of natural heart pulsatility on the control system show that the reference blood differential pressure is maintained without requiring CF3 motor pulsatility.     

Development of magnetic bearing system for a new third-generation blood pump

A magnetic bearing system is a crucial component in a third-generation blood pump, particularly when we consider aspects such as system durability and blood compatibility. Many factors such as efficiency, occupying volume, hemodynamic stability in the flow path, mechanical stability, and stiffness need to be considered for the use of a magnetic bearing system in a third-generation blood pump, and a number of studies have been conducted to develop novel magnetic bearing design for better handling of these factors. In this study, we developed and evaluated a new magnetic bearing system having a motor for a new third-generation blood pump. This magnetic bearing system consists of a magnetic levitation compartment and a brushless direct current (BLDC) motor compartment. The active-control degree of freedom is one; this control is used for controlling the levitation in the axial direction. The levitation in the radial direction has a passive magnetic levitation structure. In order to improve the system efficiency, we separated the magnetic circuit for axial levitation by using a magnetic circuit for motor drive. Each magnetic circuit in the bearing system was designed to have a minimum gap by placing mechanical parts, such as the impeller blades, outside the circuit. A custom-designed noncontact gap sensor was used for minimizing the system volume. We fabricated an experimental prototype of the proposed magnetic bearing system and evaluated its performance by a control system using the Matlab xPC Target system. The noncontact gap sensor was an eddy current gap sensor with an outer diameter of 2.38 mm, thickness of 0.88 mm, and resolution of 5 μm. The BLDC motor compartment was designed to have an outer diameter of 20 mm, length of 28.75 mm, and power of 4.5 W. It exhibited a torque of 8.6 mNm at 5000 rpm. The entire bearing system, including the motor and the sensor, had an outer diameter of 22 mm and a length of 97 mm. The prototype exhibited sufficient levitation performance in the stop state and the rotation state with a gap of 0.2 mm between the rotor and the stator. The system had a steady position error of 0.01 μm in the stop state and a position error of 0.02 μm at a rotational speed of 5000 rpm; the current consumption rates were 0.15 A and 0.17 A in the stop state and the rotation state, respectively. In summary, we developed and evaluated a unique magnetic bearing system with an integrated motor. We believe that our design will be an important basis for the further development of the design of an entire third-generation blood pump system.