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Biomedical subjects

P E Allaire

Publications and source records attributed to P E Allaire.

At least 19 recordsLinked to original sources
PubMed2003

Design of a continuous flow centrifugal pediatric ventricular assist device.

Thousands of pediatric patients suffering from cardiomyopathy or single ventricular physiologies secondary to debilitating heart defects may benefit from long-term mechanical circulatory support due to the limited number of donor hearts available. This article presents the initial design of a fully implantable centrifugal pediatric ventricular assist device (PVAD) for 2 to 12 year olds. Conventional pump design equations, including a nondimensional scaling approach, enabled performance estimations of smaller scale versions (25 mm and 35 mm impeller diameters) of our adult support VAD. Based on this estimated performance, a computational model of the PVAD with a 35 mm impeller diameter was generated. Employing computational fluid dynamics (CFD) software, the flow paths through the PVAD and overall performance were analyzed for steady state flow conditions. The numerical simulations involved flow rates of 2 to 5 LPM for rotational speeds of 2750 to 3250 RPM and incorporated a k-epsilon fluid turbulence model with a logarithmic wall function to characterize near-wall flow conditions. The CFD results indicated best efficiency points ranging from 25% to 28%, which correlate well with typical values of blood pumps. The results further demonstrated that the pump could deliver 2 to 5 LPM at 70 to 95 mmHg for desired physiologic conditions in resting 2 to 12 year olds. Scalar stress levels remained below 300 Pa, thereby signifying potentially low levels of hemolysis. Several flow regions in the pump exhibited signs of vortices, retrograde flow, and stagnation points, which require optimization and further study. This CFD model represents a reasonable starting point for future model enhancements, leading to prototype manufacturing and experimental validation.

Child↗
PubMed2002

The application of quantitative oil streaking to the HeartQuest left ventricular assist device.

Methods of flow visualization using oil streaking are established techniques for investigating surface shear and near wall flow patterns. Recent studies have used an array of oil dots on a surface which form streaks when exposed to shear forces. This method is generally qualitative, but it is possible to make quantitative measurements of the shear if the oil streaks have been calibrated. This paper presents the application of a quantitative oil streak method to the HeartQuest left ventricular assist device (LVAD). An array of dots was applied to the top housing of the pump, yielding quantitative values for the shear and qualitative patterns of the near wall flow in that region. The results were used to locate regions likely to promote thrombosis, such as stagnation points or recirculation regions. Regions of high shear, where hemolysis might occur, also can be identified with this method. In addition to being an important design technique, quantitative oil streaking assisted in the verification of computational fluid dynamics results within the HeartQuest LVAD.

Cardiovascular Diseases↗
PubMed2000

Computational flow study of the continuous flow ventricular assist device, prototype number 3 blood pump.

A computational fluid dynamics study of blood flow in the continuous flow ventricular assist device, Prototype No. 3 (CFVAD3), which consists of a 4 blade shrouded impeller fully supported in magnetic bearings, was performed. This study focused on the regions within the pump where return flow occurs to the pump inlet, and where potentially damaging shear stresses and flow stagnation might occur: the impeller blade passages and the narrow gap clearance regions between the impeller-rotor and pump housing. Two separate geometry models define the spacing between the pump housing and the impeller's hub and shroud, and a third geometry model defines the pump's impeller and curved blades. The flow fields in these regions were calculated for various operating conditions of the pump. Pump performance curves were calculated, which compare well with experimentally obtained data. For all pump operating conditions, the flow rates within the gap regions were predicted to be toward the inlet of the pump, thus recirculating a portion of the impeller flow. Two smaller gap clearance regions were numerically examined to reduce the recirculation and to improve pump efficiency. The computational and geometry models will be used in future studies of a smaller pump to determine increased pump efficiency and the risk of hemolysis due to shear stress, and to insure the washing of blood through the clearance regions to prevent thrombosis.

Computer Simulation↗
PubMed2000

Numerical analysis of blood flow in the clearance regions of a continuous flow artificial heart pump.

The CFVAD3 is the third prototype of a continuous flow ventricular assist device being developed for implantation in humans. The pump consists of a fully shrouded 4-blade impeller supported by magnetic bearings. On either side of this suspended rotating impeller is a small clearance region through which the blood flows. The spacing and geometry of these clearance regions are very important to the successful operation of this blood pump. Computational fluid dynamics (CFD) solutions for this flow were obtained using TascFlow, a software package available from AEA Technology, U.K. Flow in these clearance regions was studied parametrically by varying the size of the clearance, the blood flow rate into the pump, and the rotational speed of the pump. The numerical solutions yield the direction and magnitude of the flow and the dynamic pressure. Experimentally measured pump flow rates are compared to the numerical study. The results of the study provide guidance for improving pump efficiency. It is determined that current clearances can be significantly reduced to improve pump efficiency without negative impacts.

Computer Simulation↗
PubMed1999

Blood flow in a continuous flow ventricular assist device.

A numerical analysis was performed to predict the shear stresses, flow rates, and the velocity profiles in a continuous flow ventricular assist device, the CFVAD3. The problem was modeled as a rotating disk over a stationary disk. A variety of clearances was tested for the CFVAD3 coupled with a range of rotational speeds and pressure gradients. Velocity fields were generated using solutions obtained with FLOW3D software (AEA Technology, Pittsburgh, PA, U.S.A.) Analysis of these solutions shows that the pressure differential effect has a stronger influence on the flow than the rotational effect of the impeller Ekman layer. The predicted shear stresses reflect these changes in the volume flow rates and the speeds shown in the velocity profiles. Based on the predictions of the software, the optimum clearance and rotational speed were chosen. The conclusion is that a speed in the range of 2,200-2,400 rpm should be chosen depending on the efficiency of the pump.

Blood Flow Velocity↗
PubMed1999

Test controller design, implementation, and performance for a magnetic suspension continuous flow ventricular assist device.

A new continuous flow ventricular assist device using full magnetic suspension has been designed, constructed, and tested. The magnetic suspension centers the centrifugal pump impeller within the clearance passages in the pump, thus avoiding any form of contact. The noncontact operation is designed to give very high expected mechanical reliability, large clearances, low hemolysis, and a relatively small size compared to current pulsatile devices. A unique configuration of magnetic actuators on the inlet side and exit sides of the impeller provides full 5 axis control and suspension of the impeller. The bearing system is divided into segments which allow for 3 displacement axes and 2 angular control axes. The controller chosen for the first suspension tests consists of a decentralized set of 5 proportional integral derivative (PID) controllers. This document describes both the controller and an overview of some results pertaining to the magnetic bearing performance. The pump has been successfully operated in both water and blood under design conditions suitable for use as a ventricular assist device.

Equipment Design↗
PubMed1999

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.

Equipment Design↗
PubMed1999

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.

Blood Flow Velocity↗
PubMed1996

Prototype continuous flow ventricular assist device supported on magnetic bearings.

This article describes a prototype continuous flow pump (CFVAD2) fully supported in magnetic bearings. The pump performance was measured in a simulated adult human circulation system. The pump delivered 6 L/min of flow at 100 mm Hg of differential pressure head operating at 2,400 rpm in water. The pump is totally supported in 4 magnetic bearings: 2 radial and 2 thrust. Magnetic bearings offer the advantages of no required lubrication and large operating clearances. The geometry and other properties of the bearings are described. Bearing parameters such as load capacity and current gains are discussed. Bearing coil currents were measured during operation in air and water. The rotor was operated in various orientations to determine the actuator current gains. These values were then used to estimate the radial and thrust forces acting on the rotor in both air and water. Much lower levels of force were found than were expected, allowing for a very significant reduction in the size of the next prototype. Hemolysis levels were measured in the prototype pump and found not to indicate damage to the blood cells.

Animals↗
PubMed1980

Squeeze forces in contact lenses with a steep base curve radius.

Hydrodynamic forces occur on lenses due to pressures created in tear films during squeeze motions. Pressures and squeeze forces are calculated for lenses with both flat and steepened base curve radii. A parabolic two-dimensional tear film tickness is assumed for calculation purposes. A tear film with a peripheral film thickness of one-half of the central film thickness produces over three times the squeeze force produced by a constant tear film thickness with the same central clearance. Also, the distance moved by the lens toward the corneal surface is determined.

Contact Lenses↗
PubMed1977

Tear-film dynamics and oxygen tension under a circular contact lens.

The cornea requires a minimum level of oxygenation; this may be prevented by a contact lens, and corneal swelling and other undesirable effects may result. By employing thin-film lubrication theory, this study derives analytical solutions for tear-film velocity components and pressure under a circular contact lens. Oxygen-tension contours are then obtained for various values of squeeze action and parallel motion of the lens.

Contact Lenses↗
PubMed1977

Finite deformation theory for in vivo human skin.

A finite deformation mathematical model of in vivo human skin has been developed for the normal physiological load range. Uniaxial load-deformation measurements were carried out with a non-invasive extensometer and utilized in formulating the model. The in vivo strain energy function was found to be a linear function of the first two strain invariants and a quadratic function of the third strain invariant. Only three independent constants were necessary to specify the strain energy function completely for the upper extremities of human volunteers.

Adult↗
PubMed1977

Practical applications of skin biomechanics.

The biomechanical properties of skin have an important influence on plastic surgical decisions. They aid the surgeon in planning elective incisions, excisions, or scar revisions. They provide insight into the most appropriate method of coverage of skin defects as well as the design of an artificial skin substitute. Skin biomechanics can, in part, be characterized in vivo by either a skin extensiometer or by studying the deformation of skin defects. These methods indicate the magnitude and directional orientation of skin tensions which are dependent partly on the mechanical characteristics of the dermal fibers and partly on the pattern in which they are woven. The tensions to which the skin are subjected can be classified as either static or dynamic in origin. Static skin tensions are the natural tensions existing in skin. The magnitude of static tensions varies between individuals, at different sites in the same person, and in different directions in many sites. The dynamic tensions are caused by a combination of forces which are associated with joint movement, mimetic and other voluntary muscle activity, and gravity. Knowledge of these tensions allows the surgeon to align the operative site in the direction of maximal tension and to approximate the wound with the least amount of tension. As a consequence of this, the scar healing between the cut edges of the wound should be narrow and inconspicuous.

Biomechanical Phenomena↗
PubMedDate not supplied

In vitro characterization of a magnetically suspended continuous flow ventricular assist device.

A magnetically suspended continuous flow ventricular assist device using magnetic bearings was developed aiming at an implantable ventricular assist device. The main advantage of this device includes no mechanical wear and minimal chance of blood trauma such, as thrombosis and hemolysis, because there is no mechanical contact between the stationary and rotating parts. The total system consists of two subsystems: the centrifugal pump and the magnetic bearing. The centrifugal pump is comprised of a 4 vane logarithmic spiral radial flow impeller and a brushless DC motor with slotless stator, driven by the back emf commutation scheme. Two radial and one thrust magnetic bearing that dynamically controls the position of the rotor in a radial and axial direction, respectively, contains magnetic coils, the rotor's position sensors, and feedback electronic control system. The magnetic bearing system was able to successfully suspend a 365.5g rotating part in space and sustain it for up to 5000 rpm of rotation. Average force-current square factor of the magnetic bearing was measured as 0.48 and 0.44 (kg-f/Amp2) for radial and thrust bearing, respectively. The integrated system demonstrated adequate performance in mock circulation tests by providing a 6 L/min flow rate against 100 mmHg differential pressure at 2300 rpm. Based on these in vitro performance test results, long-term clinical application of the magnetically suspended continuous flow ventricular assist device is very promising after system optimization with a hybrid system using both active (electromagnet) and passive (permanent magnets) magnet bearings.

Biomechanical Phenomena↗
PubMedDate not supplied

Pulsatile operation of a centrifugal ventricular assist device with magnetic bearings.

A prototype bench top model of a continuous flow ventricular assist device using an impeller suspended by magnetic bearings has been developed. Generation of a pulsatile pressure was studied using both a computer model and in vitro loop tests of the prototype. The motivation for developing a computer model for a blood pump in the natural circulation is two-fold. First, it allows simulation of the pump under a large variety of operating conditions. Second, it provides insight into what parameters of the system design are important for achieving a specific result. For example, in one case, an aortic pressure of 118/87 mmHg was generated by varying the speed from 2,000 to 2,600 rpm. The computer model was verified by coupling the centrifugal pump prototype to a mock circulatory system. The results of the model were verified by generating an aortic pressure of 113/78 mmHg while varying the speed from 2,000 to 2,600 rpm. These experiments have shown that it is possible to generate pulsatile pressure similar to that of native physiology using a centrifugal left ventricular assist device. Further tests will be required to quantify the effects on hemolysis.

Biomedical Engineering↗
PubMedDate not supplied

Implantable centrifugal pump with hybrid magnetic bearings.

Test methods and results of in vitro assessment of a centrifugal pump with a magnetically suspended impeller are provided. In vitro blood tests have been completed with a resulting normalized milligram index of hemolysis (NmIH) of 12.4 +/- 4.1, indicating that hemolysis is not a problem. Hydraulic characterization of the system with water has shown that a nominal pumping condition of 6 L/min at 100 mmHg was met at 2,200 rpm. Maximum clinically usable cardiac output is predicted be 10 L/min. The magnetic bearing supported impeller did not contact the housing and was shown to be stable under a variety of pumping conditions. The driving motor efficiency is 75% at the nominal condition. Finally, a description of the clinical version of the pump under development is provided.

Heart-Assist Devices↗