Eduardo Bock

Computational Fluid Dynamics Investigation of a Centrifugal Blood Pump

In the development of a ventricular assist device, computational fluid dynamics (CFD) analysis is an efficient tool to obtain the best design before making the final prototype. In this study, different designs of a centrifugal blood pump were developed to investigate flow characteristics and performance. This study assumed the blood flow as being an incompressible homogeneous Newtonian fluid. A constant velocity was applied at the inlet; no slip boundary conditions were applied at device wall; and pressure boundary conditions were applied at the outlet. The CFD code used in this work was based on the finite volume method. In the future, the results of CFD analysis can be compared with flow visualization and hemolysis tests.

Cardiovascular Simulator Improvement: Pressure Versus Volume Loop Assessment

This article presents improvement on a physical cardiovascular simulator (PCS) system. Intraventricular pressure versus intraventricular volume (PxV) loop was obtained to evaluate performance of a pulsatile chamber mimicking the human left ventricle. PxV loop shows heart contractility and is normally used to evaluate heart performance. In many heart diseases, the stroke volume decreases because of low heart contractility. This pathological situation must be simulated by the PCS in order to evaluate the assistance provided by a ventricular assist device (VAD). The PCS system is automatically controlled by a computer and is an auxiliary tool for VAD control strategies development. This PCS system is according to a Windkessel model where lumped parameters are used for cardiovascular system analysis. Peripheral resistance, arteries compliance, and fluid inertance are simulated. The simulator has an actuator with a roller screw and brushless direct current motor, and the stroke volume is regulated by the actuator displacement. Internal pressure and volume measurements are monitored to obtain the PxV loop. Left chamber internal pressure is directly obtained by pressure transducer; however, internal volume has been obtained indirectly by using a linear variable differential transformer, which senses the diaphragm displacement. Correlations between the internal volume and diaphragm position are made. LabVIEW integrates these signals and shows the pressure versus internal volume loop. The results that have been obtained from the PCS system show PxV loops at different ventricle elastances, making possible the simulation of pathological situations. A preliminary test with a pulsatile VAD attached to PCS system was made.

Magnetic suspension of the rotor of a ventricular assist device of mixed flow type.

This work presents results of preliminary studies concerning application of magnetic bearing in a ventricular assist device (VAD) being developed by Dante Pazzanese Institute of Cardiology-IDPC (São Paulo, Brazil). The VAD-IDPC has a novel architecture that distinguishes from other known VADs. In this, the rotor has a conical geometry with spiral impellers, showing characteristics that are intermediate between a centrifugal VAD and an axial VAD. The effectiveness of this new type of blood pumping principle was showed by tests and by using it in heart surgery for external blood circulation. However, the developed VAD uses a combination of ball bearings and mechanical seals, limiting the life for some 10 h, making impossible its long-term use or its use as an implantable VAD. As a part of development of an implantable VAD, this work aims at the replacement of ball bearings by a magnetic bearing. The most important magnetic bearing principles are studied and the magnetic bearing developed by Escola Politécnica of São Paulo University (EPUSP-MB) is elected because of its very simple architecture. Besides presenting the principle of the EPUSP-MB, this work presents one possible alternative for applying the EPUSP-MB in the IDPC-VAD.

Implantable centrifugal blood pump with dual impeller and double pivot bearing system: electromechanical actuator, prototyping, and anatomical studies.

An implantable centrifugal blood pump has been developed with original features for a left ventricular assist device. This pump is part of a multicenter and international study with the objective to offer simple, affordable, and reliable devices to developing countries. Previous computational fluid dynamics investigations and wear evaluation in bearing system were performed followed by prototyping and in vitro tests. In addition, previous blood tests for assessment of normalized index of hemolysis show results of 0.0054±2.46 × 10⁻³ mg/100 L. An electromechanical actuator was tested in order to define the best motor topology and controller configuration. Three different topologies of brushless direct current motor (BLDCM) were analyzed. An electronic driver was tested in different situations, and the BLDCM had its mechanical properties tested in a dynamometer. Prior to evaluation of performance during in vivo animal studies, anatomical studies were necessary to achieve the best configuration and cannulation for left ventricular assistance. The results were considered satisfactory, and the next step is to test the performance of the device in vivo.

A New Technique to Control Brushless Motor for Blood Pump Application

This article presents a back-electromotive force (BEMF)-based technique of detection for sensorless brushless direct current motor (BLDCM) drivers. The BLDCM has been chosen as the energy converter in rotary or pulsatile blood pumps that use electrical motors for pumping. However, in order to operate properly, the BLDCM driver needs to know the shaft position. Usually, that information is obtained through a set of Hall sensors assembled close to the rotor and connected to the electronic controller by wires. Sometimes, a large distance between the motor and controller makes the system susceptible to interference on the sensor signal because of winding current switching. Thus, the goal of the sensorless technique presented in this study is to avoid this problem. First, the operation of BLDCM was evaluated on the electronic simulator PSpice. Then, a BEMF detector circuitry was assembled in our laboratories. For the tests, a sensor-dependent system was assembled where the direct comparison between the Hall sensors signals and the detected signals was performed. The obtained results showed that the output sensorless detector signals are very similar to the Hall signals at speeds of more than 2500 rpm. Therefore, the sensorless technique is recommended as a responsible or redundant system to be used in rotary blood pumps.

A New Model of Centrifugal Blood Pump for Cardiopulmonary Bypass: Design Improvement, Performance, and Hemolysis Tests

A new model of blood pump for cardiopulmonary bypass (CPB) application has been developed and evaluated in our laboratories. Inside the pump housing is a spiral impeller that is conically shaped and has threads on its surface. Worm gears provide an axial motion of the blood column. Rotational motion of the conical shape generates a centrifugal pumping effect and improves pumping performance. One annular magnet with six poles is inside the impeller, providing magnetic coupling to a brushless direct current motor. In order to study the pumping performance, a mock loop system was assembled. Mock loop was composed of Tygon tubes (Saint-Gobain Corporation, Courbevoie, France), oxygenator, digital flowmeter, pressure monitor, electronic driver, and adjustable clamp for flow control. Experiments were performed on six prototypes with small differences in their design. Each prototype was tested and flow and pressure data were obtained for rotational speed of 1000, 1500, 2000, 2500, and 3000 rpm. Hemolysis was studied using pumps with different internal gap sizes (1.35, 1.45, 1.55, and 1.7 mm). Hemolysis tests simulated CPB application with flow rate of 5 L/min against total pressure head of 350 mm Hg. The results from six prototypes were satisfactory, compared to the results from the literature. However, prototype #6 showed the best results. Best hemolysis results were observed with a gap of 1.45 mm, and showed a normalized index of hemolysis of 0.013 g/100 L. When combined, axial and centrifugal pumping principles produce better hydrodynamic performance without increasing hemolysis.

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.

Speed control of the Implantable Centrifugal Blood Pump to avoid aortic valve stenosis: Simulation and implementation

This paper presents a computational simulation and implementation of electromechanical actuator performance of the Implantable Centrifugal Blood Pump (ICBP) as part of a speed controller study to avoid aortic valve stenosis. The ICBP as Left Ventricular Assist Device (LVAD) is an electromechanical device designed for long-term assist left heart in performing its functions. The centrifugal pumps are controlled by varying the rotor (impeller) speed. ICBP successful operation depends on an appropriate rotational speed control system, ensuring: 1) no reverse flow through the pump during left ventricle diastolic phase, and 2) aortic valve correct opening, avoiding later valve stenosis. A computational model of the actuator done in Matlab / Simulink (R2010b, Mathworks, Massachusetts, USA) was used in the simulations. Control and signal processing was used Labview (National Instruments, Austin, USA). Signals equivalent to intraventricular pressure and a variable rotational reference were used to evaluate motor and speed controller performance. Speed values were chosen so that pressure pump exceeds intraventricular pressure only after the opening of aortic valve. The proposed controller is Proportional-Integral (PI) type. The simulation results were satisfactory, no steady error in response speed. Practical tests showed satisfactory results to follow speed reference signal, as simulated. Future studies will evaluate the bands control.

Modeling study of an Implantable Centrifugal Blood Pump actuator with redundant sensorless control

The permanent magnet brushless direct current motor (BLDC), have been the main component in most of the Ventricular Assist Devices (VAD) development. An Implantable Centrifugal Blood Pump is being developed at the Institute Dante Pazzanese of Cardiology (IDPC) as VAD to assist patients with cardiovascular diseases. To develop a high performance controller is necessary to have a reliable virtual model of the BLDC. Toolbox SimPowerSystems software package was used to study the dynamic system. Permant Magnet Synchronous Machine (PMSM)block implements the differential equations for the motor through a state-space model, this is main contribution of this work. Sensorless control was used as redundant position rotor control. The model showed satisfactory results when compared with data given by the manufacturer in its catalog. In future works, the results presented in this work will be used to improve the motor model. Thus, it will allow reliable simulations of new proposed controllers.

Design of Supervisory Control System for Ventricular Assist Device

When a patient have severe heart diseases, Ventricular Assist Device (VAD) implantation may be necessary. However, the improvement of the interaction between the device and the patient’s behavior is crucial. Currently, the control of these pumps does not follow changes in patient behavior and the devices are no safe. Therefore, if VAD has no faults tolerance and no dynamic behavior according to the cardiovascular system performance, there is a serious limitation on expected results. This research investigates a mechatronic approach for this class of devices based on advanced techniques for control, instrumentation and automation to define a method for developing a hierarchical supervisory control system to control a VAD dynamically and securely. To apply this method, concepts based on Petri nets and Safety Instrumented Systems are used. This innovation reduces the interventions and unnecessary drugs, enabling a reduction of deposable material and patient hospitalization, and contributes to sustainability concept.