Efficiency Evaluation of a Magnetically Driven Multiple Disk Centrifugal Blood Pump

Heart failure is expected to ail over 8 million people in America by 2030 leaving many in need of cardiac replacement. To accommodate this large volume of people, ventricular assist devices (VADs) are necessary to provide mechanical circulatory support. Current VADs exhibit issues such as thrombosis and hemolysis caused by large local pressure drops and turbulent flow within the pump. Multiple disk centrifugal pumps (MDCPs) use shearing and centrifugal forces to produce laminar flow patterns and eliminate large pressure drops within the pump which greatly reduce risks that are in current VADs. The MDCP has a shaft drive system (SDS) that causes leakage between the motor and housing that when implanted can cause blood loss, infection, thrombosis and hemolysis. To eliminate these adverse effects, a magnetic external motor-driven system (MEMDS) was implemented. An efficiency study was performed to examine the efficacy of the MEMDS by comparing the hydraulic work of the MDCP to the power required to run the pump. This was done by measuring inlet and outlet pressures, outlet flow rate and input current at various input voltages and resistances. The results showed the MDCP could produce physiologic flow characteristics with a flow rate of 4.90 L/min and outlet pressure of 61.33 mmHg at an impeller speed of 989.79 rpm. Other VADs generate flow rates around 5 L/min at rotational speeds of 2400 rpm for centrifugal pumps and 12000 rpm for axial pumps. When compared to the SDS, the MEMDS exhibited similar efficiencies of 3.89% and 3.50% respectively. This study shows promise in the advancement of MDCP

Study of a bi-directional axial flow blood pump

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.A common treatment for circulatory disorders is the application of rotary blood pumps to locally increase blood flow to required levels. Existing devices tend to support flow from inlet to outlet and in that direction only. This thesis presents a bi-directional pump that may enable ventricle assist devices (VAD) to support blood flow to the organs during systole, when rotating in one direction, and to increase coronary perfusion during diastole, when rotating in the other. For each flow direction blade profiles were designed and tested for performance. Both designs were merged to obtain a symmetric profile to provide flow support in both directions. This initial bi-directional design was optimised using computational fluid dynamics modelling. The model was set to accelerate to a maximum forward rotational speed of 8,000 rpm, change rotational direction after 300 ms and accelerate to 2,400 rpm whilst rotating backwards. Experimental testing was carried out to validate the computational results. In the forward direction, the pump was predicted to deliver 39 cm3 compared to 19 cm3 in the backward direction. Pressure heads reached maxima of 2.2 kPa in forward and 0.16 kPa in backward direction. Analysis of wall shear stress profiles at the blades’ surface showed that the maximum was 140 Pa lasting less than 300 ms in the forward direction, whilst in the backward direction this was approximately 23 Pa lasting for 700 ms. A design for the bi-directional blades is established and characterised computationally and experimentally. Analysis of the blade pressure profiles confirmed generation of pressure rise in both directions. The computational results for wall shear stress were predicted to be below the accepted limits of haemolysis. Recirculation zones were found at the outlet in the backward rotating direction. Future work may reduce those by using guide vanes at either side of the rotor

Towards patient-specific modelling as a pre-operative planning strategy and follow up assessment for the treatment of advanced heart failure with rotary blood pumps

Background: Ventricular Assist Devices (VADs) insertion is an established treatment for patients with end-stage heart failure waiting for a heart transplant or in need for long-term circulatory support (destination therapy). Rotary blood pumps (RBP) are the most popular devices in view of their size and performance. Pre-operative planning strategy for the insertion of a left ventricular assist device (LVAD) requires a timely discussion at a Multi-Disciplinary Team Meeting (MDT). Clinical-decision making is based according to the needs of the patient and must be processed without delays. Nevertheless, thrombus formation remains a feared complication which affects outcome. VADs operate in a flow regime which is difficult to simulate: the transitional region at the boundary of laminar and turbulent flow (low Reynolds number). Different methods have been used but the best approach remains debatable. Computational Fluid Dynamics (CFD) is an attractive and invaluable tool for the study of the interactions between VADs and the cardiovascular system. The aim of this thesis is three-fold: a) to investigate the use of pressure-volume analysis in a clinical setting through the review of six heart failure patients previously discussed at a MDT meeting with a view to predict or guide further management; b) to review the theory behind modelling approaches to VADs and their interactions with the cardiovascular system for better understanding of their clinical use. Then, an overview of computational fluid dynamics (CFD) is considered as a prelude to its application to the analysis of VADs performance. Additionally, the development of a simplified model of centrifugal pump will be used in initial simulations as preliminary analysis; c) to examine an example of a proof-of-concept pilot patient-specific model of an axial flow pump (HeartMate II) as pre-operative planning strategy in a patient-specific model with a view to identify potential critical areas that may affect pump function and outcome in a clinical setting. Material and Methods: 3D reconstruction from CT-scan images of patients who underwent the insertion of rotary blood pumps, namely HeartWare HVAD and HeartMate II. Ansys Fluent has been used for CFD analysis based on the fundamental governing equations of motion. Blood has been modelled as incompressible, Newtonian fluid with density = 1060 and viscosity = 0.0035 kg/m-s. The laminar and SST models have been used for comparison purposes. The rotational motion of the impeller has been implemented using the moving reference frame (MRF) approach. The sliding mesh method has also been used to account for unsteady interaction between stationary and moving part. The no-slip condition has been applied to all walls, which were assumed to be rigid. Boundary conditions consisting of velocity inlet and pressure outlet of the pump based on different settings and constant rotational speed for the impeller. Pressure-velocity coupling has been based on the coupled scheme. Spatial discretisation consisted of the “least square cell based” gradient for velocity and “PRESTO” or second order for pressure. Second order upwind has been set for the momentum, turbulent kinetic energy and specific dissipation rate. First order implicit has been set for transient formulation. The pseudo transient algorithm (steady state), the high order relaxation term and the warped-face gradient correction have been used to add an unsteady term to the solution equations with the aim to improve stability and enhance convergence. Specific settings have been considered for comparison purposes. Results: Pressure-volume simulation analysis in six advanced heart failure patients showed that an integrated model of the cardiovascular system based on lumped-parameter representation, modified time-varying elastance and pressure-volume analysis of ventricular function seems a feasible and suitable approach yielding a sufficiently accurate quantitative analysis in real time, therefore applicable within the time-constraints of a clinical setting. Lumped-parameter models consist of simultaneous ordinary differential equations complemented by an algebraic balance equation and are suitable for examination of global distribution of pressure, flow and volume over a range of physiological conditions with inclusion of the interaction between modelled components. Higher level lumped-parameter modelling is needed to address the interaction between the circulation and other systems based on a compromise between complexity and ability to set the required parameters to personalise an integrated lumped-parameter model for a patient-specific approach. CARDIOSIM© fulfils these requirements and does address the systems interaction with its modular approach and assembly of models with varying degree of complexity although 0-D and 1-D coupling may be required for the evaluation of long-term VAD support. The challenge remains the ability to predict outcome over a longer period of time. The preliminary CFD simulations with the HeartWare HVAD centrifugal pump demonstrated that it is possible to obtain an accurate analysis in a timely manner to complement the clinical review process. The simulations with the pilot patient-specific model of the HeartMate II axial flow pump revealed that a complex 3D reconstruction is feasible in a timely manner and can be used to generate sufficiently accurate results to be used in the context of a MDT meeting for the purposes of clinical decision-making. Overall, these three studies demonstrate that the time frame of the simulations was within hours which may fit the time constraints of the clinical environment in the context of a MDT meeting. More specifically, it was shown that the laminar model may be used for an initial evaluation of the flow development within the pump. Nonetheless, the k- model offers higher accuracy if the timeline of the clinical setting allows for a longer simulation. Conclusion: This thesis aimed at the understanding of the use of computational modelling as a pre-operative planning strategy and follow up assessment for the treatment of advanced heart failure with rotary blood pumps. The novelty lays in the use of both pressure-volume simulation analysis and 3D flow dynamics studies in VADs with a view to treatment optimisation and outcome prediction within the time constraints of a clinical setting in the context of a MDT meeting. The clinical significance and the contribution to the field is a more targeted approach for different groups of patients and a more quantitative evaluation in the clinical decision process based on a pro-active co-operation between clinicians and scientists reducing the potential for “guess work”. The results of this thesis are a proof-of-concept as a prelude to a potential future implementation of patient-specific modelling within a clinical setting on a daily basis demonstrating a clear clinical significance and contribution to the field. The proposed approach does not consider modelling and simulation as a substitute for clinical experience but an additional tool to guide therapeutic intervention and complement the clinical decision process in which the clinician remains the ultimate decision-maker. Such an approach may well add a different dimension to the problem of heart failure with potential for high return in terms of patient’s outcome and long-term surveillance. The same principles would be applicable to other cardiovascular problems in line with the current concept of “Team Approach” such as the Heart Team, the Structural Heart Team or the Aortic Team. The present work has taken this concept closer to clinical delivery and has highlighted its potential but further work remains to be done in refining the technique.Background: Ventricular Assist Devices (VADs) insertion is an established treatment for patients with end-stage heart failure waiting for a heart transplant or in need for long-term circulatory support (destination therapy). Rotary blood pumps (RBP) are the most popular devices in view of their size and performance. Pre-operative planning strategy for the insertion of a left ventricular assist device (LVAD) requires a timely discussion at a Multi-Disciplinary Team Meeting (MDT). Clinical-decision making is based according to the needs of the patient and must be processed without delays. Nevertheless, thrombus formation remains a feared complication which affects outcome. VADs operate in a flow regime which is difficult to simulate: the transitional region at the boundary of laminar and turbulent flow (low Reynolds number). Different methods have been used but the best approach remains debatable. Computational Fluid Dynamics (CFD) is an attractive and invaluable tool for the study of the interactions between VADs and the cardiovascular system. The aim of this thesis is three-fold: a) to investigate the use of pressure-volume analysis in a clinical setting through the review of six heart failure patients previously discussed at a MDT meeting with a view to predict or guide further management; b) to review the theory behind modelling approaches to VADs and their interactions with the cardiovascular system for better understanding of their clinical use. Then, an overview of computational fluid dynamics (CFD) is considered as a prelude to its application to the analysis of VADs performance. Additionally, the development of a simplified model of centrifugal pump will be used in initial simulations as preliminary analysis; c) to examine an example of a proof-of-concept pilot patient-specific model of an axial flow pump (HeartMate II) as pre-operative planning strategy in a patient-specific model with a view to identify potential critical areas that may affect pump function and outcome in a clinical setting. Material and Methods: 3D reconstruction from CT-scan images of patients who underwent the insertion of rotary blood pumps, namely HeartWare HVAD and HeartMate II. Ansys Fluent has been used for CFD analysis based on the fundamental governing equations of motion. Blood has been modelled as incompressible, Newtonian fluid with density = 1060 and viscosity = 0.0035 kg/m-s. The laminar and SST models have been used for comparison purposes. The rotational motion of the impeller has been implemented using the moving reference frame (MRF) approach. The sliding mesh method has also been used to account for unsteady interaction between stationary and moving part. The no-slip condition has been applied to all walls, which were assumed to be rigid. Boundary conditions consisting of velocity inlet and pressure outlet of the pump based on different settings and constant rotational speed for the impeller. Pressure-velocity coupling has been based on the coupled scheme. Spatial discretisation consisted of the “least square cell based” gradient for velocity and “PRESTO” or second order for pressure. Second order upwind has been set for the momentum, turbulent kinetic energy and specific dissipation rate. First order implicit has been set for transient formulation. The pseudo transient algorithm (steady state), the high order relaxation term and the warped-face gradient correction have been used to add an unsteady term to the solution equations with the aim to improve stability and enhance convergence. Specific settings have been considered for comparison purposes. Results: Pressure-volume simulation analysis in six advanced heart failure patients showed that an integrated model of the cardiovascular system based on lumped-parameter representation, modified time-varying elastance and pressure-volume analysis of ventricular function seems a feasible and suitable approach yielding a sufficiently accurate quantitative analysis in real time, therefore applicable within the time-constraints of a clinical setting. Lumped-parameter models consist of simultaneous ordinary differential equations complemented by an algebraic balance equation and are suitable for examination of global distribution of pressure, flow and volume over a range of physiological conditions with inclusion of the interaction between modelled components. Higher level lumped-parameter modelling is needed to address the interaction between the circulation and other systems based on a compromise between complexity and ability to set the required parameters to personalise an integrated lumped-parameter model for a patient-specific approach. CARDIOSIM© fulfils these requirements and does address the systems interaction with its modular approach and assembly of models with varying degree of complexity although 0-D and 1-D coupling may be required for the evaluation of long-term VAD support. The challenge remains the ability to predict outcome over a longer period of time. The preliminary CFD simulations with the HeartWare HVAD centrifugal pump demonstrated that it is possible to obtain an accurate analysis in a timely manner to complement the clinical review process. The simulations with the pilot patient-specific model of the HeartMate II axial flow pump revealed that a complex 3D reconstruction is feasible in a timely manner and can be used to generate sufficiently accurate results to be used in the context of a MDT meeting for the purposes of clinical decision-making. Overall, these three studies demonstrate that the time frame of the simulations was within hours which may fit the time constraints of the clinical environment in the context of a MDT meeting. More specifically, it was shown that the laminar model may be used for an initial evaluation of the flow development within the pump. Nonetheless, the k- model offers higher accuracy if the timeline of the clinical setting allows for a longer simulation. Conclusion: This thesis aimed at the understanding of the use of computational modelling as a pre-operative planning strategy and follow up assessment for the treatment of advanced heart failure with rotary blood pumps. The novelty lays in the use of both pressure-volume simulation analysis and 3D flow dynamics studies in VADs with a view to treatment optimisation and outcome prediction within the time constraints of a clinical setting in the context of a MDT meeting. The clinical significance and the contribution to the field is a more targeted approach for different groups of patients and a more quantitative evaluation in the clinical decision process based on a pro-active co-operation between clinicians and scientists reducing the potential for “guess work”. The results of this thesis are a proof-of-concept as a prelude to a potential future implementation of patient-specific modelling within a clinical setting on a daily basis demonstrating a clear clinical significance and contribution to the field. The proposed approach does not consider modelling and simulation as a substitute for clinical experience but an additional tool to guide therapeutic intervention and complement the clinical decision process in which the clinician remains the ultimate decision-maker. Such an approach may well add a different dimension to the problem of heart failure with potential for high return in terms of patient’s outcome and long-term surveillance. The same principles would be applicable to other cardiovascular problems in line with the current concept of “Team Approach” such as the Heart Team, the Structural Heart Team or the Aortic Team. The present work has taken this concept closer to clinical delivery and has highlighted its potential but further work remains to be done in refining the technique

Investigation of flow through a semi axial centrifugal pump

Thesis (Master)--Izmir Institute of Technology, Mechanical Engineering, Izmir, 2006Includes bibliographical references (leaves: 88-89)Text in English; Abstract:Turkish and Englishxiii, 89 leavesThe aim of present study is to perform a numerical work to investigate flow inside a semi axial centrifugal pump. The results of the study can be used to improve the design of the pump.The Navier Stokes equations with appropriate boundary conditions are solved for a 3 dimensional rotating geometry. To solve the governing equations, Fluent program is used. Fluent is a commercial CFD program, which has been developed based on the finite volume method. The mesh for the flow volume is created by Gambit.The k -e turbulence model is used to handle the turbulence inside the flow. Obtained results are compared with experimental test results. An acceptable agreement between the numerical and experimental results is observed.Based on the obtained results, the velocity vectors, pressure distributions on impeller and diffuser blades and flow patterns are plotted for three different flow rates.The necessary discussions are performed for these results. It is observed that the design of the impeller provides a regular flow inside the channel; however some vortexes are observed in the channel between the diffuser blades.The effect of the surface roughness is also investigated and the problem is solved for four different roughness values (0, 50, 100 and 250 m). It is found that the roughness of the surface affects the characteristic curves of the pump. By increasing the roughness, the head and efficiency of the pump decrease however the consumed power is almost constant

A centrifugal microfluidic platform for capturing, assaying and manipulation of beads and biological cells

Microfluidics is deemed a field with great opportunities, especially for applications in medical diagnostics. The vision is to miniaturize processes typically performed in a central clinical lab into small, simple to use devices - so called lab-on-a-chip (LOC) systems. A wide variety of concepts for liquid actuation have been developed, including pressure driven flow, electro-osmotic actuation or capillary driven methods. This work is based on the centrifugal platform (lab-on-a-disc). Fluid actuation is performed by the forces induced due to the rotation of the disc, thus eliminating the need for external pumps since only a spindle motor is necessary to rotate the disc and propel the liquids inside of the micro structures. Lab-on-a-disc systems are especially promising for point-of-care applications involving particles or cells due to the centrifugal force present in a rotating system. Capturing, assaying and identification of biological cells and microparticles are important operations for lab-on-a-disc platforms, and the focus of this work is to provide novel building blocks towards an integrated system for cell and particle based assays. As a main outcome of my work, a novel particle capturing and manipulation scheme on a centrifugal microfluidic platform has been developed. To capture particles (biological cells or micro-beads) I designed an array of V-shaped micro cups and characterized it. Particles sediment under stagnant flow conditions into the array where they are then mechanically trapped in spatially well-defined locations. Due to the absence of flow during the capturing process, i.e. particle sedimentation is driven by the artificial gravity field on the centrifugal platform, the capture efficiency of this approach is close to 100% which is notably higher than values reported for typical pressure driven systems. After capturing the particles, the surrounding medium can easily be exchanged to expose them to various conditions such as staining solutions or washing buffers, and thus perform assays on the captured particles. By scale matching the size of the capturing elements to the size of the particles, sharply peaked single occupancy can be achieved. Since all particles are arrayed in the same focal plane in spatially well defined locations, operations such as counting or fluorescent detection can be performed easily. The application of this platform to perform multiplexed bead-based immunoassays as well as the discrimination of various cell types based on intra cellular and membrane based markers using fluorescently tagged antibodies is demonstrated. Additionally, methods to manipulate captured particles either in batch mode or on an individual particle level have been developed and characterized. Batch release of captured particles is performed by a novel magnetic actuator which is solely controlled by the rotation frequency of the disc. Furthermore, the application of this actuator to rapidly mix liquids is shown. Manipulation of individual particles is performed using an optical tweezers setup which has been developed as part of this work. Additionally, this optical module also provides fluorescence detection capabilities. This is the first time that optical tweezers have been combined with a centrifugal microfluidic system. This work presents the core technology for an integrated centrifugal platform to perform cell and particle based assays for fundamental research as well as for point-of- care applications. The key outputs of my specific work are: 1. Design, fabrication and characterization of a novel particle capturing scheme on a centrifugal microfluidic platform (V-cups) with very high capture efficiency (close to 100%) and sharply peaked single occupancy (up to 99.7% single occupancy). 2. A novel rotation frequency controlled magnetic actuator for releasing captured particles as well as for rapidly mixing liquids has been developed, manufactured and characterized. 3. The V-cup platform has successfully been employed to capture cells and perform multi-step antibody staining assays for cell discrimination. 4. An optical tweezers setup has been built and integrated into a centrifugal teststand, and successful manipulation of individual particles trapped in the V-cup array is demonstrated

DESIGN OPTIMIZATION AND PRECLINICAL TESTING OF PEDIATRIC ROTARY BLOOD PUMPS AND COMPONENTS: TOWARDS THE PEDIAFLOW® VAD

Limited options exist for children (BSA<1.5 m2) requiring long-term mechanical circulatory support (MCS). Unlike adults where compact, 3rd generation, continuous-flow, implantable rotary blood pumps (RBPs) are now the standard for ventricular assist device (VAD)-indications, the only pediatric-approved chronic MCS device is the Berlin Heart® EXCOR®: a 1st generation pulsatile, pneumatically-driven, paracorporeal life-saving technology albeit with a substantial risk profile associated with frequent neurological and coagulation-related serious adverse events. In support of the smallest and most vulnerable patients, the goal of this research is to facilitate the development and translation of next-generation pediatric RBPs, including the University of Pittsburgh-led Consortium’s PediaFlow®: a miniature, implantable, rotodynamic, fully magnetically levitated, continuous-flow pediatric VAD intended to support patients between 3 to 15 kg at a flow rates of 0.3-1.5 L/min for up to six months. Presented here is the i) development of a standardized method for in vitro mechanical blood trauma testing of pediatric MCS devices; ii) design and ex vivo evaluation of a novel, pediatric-appropriate, suction resistant, placement insensitive, left ventricular drainage cannula; iii) creation of an MCS-tailored monitoring software for preclinical testing; iv) development of a PediaFlow®-specific flow estimation algorithm; and v) hemocompatibility findings in vitro and in vivo of the 4th generation PediaFlow® (PF4) VAD. The PF4, comparable in size to an AA battery, is the embodiment of more than a decade of extensive computational and experimental efforts over the span of four device iterations to minimize size, optimize performance, and maximize safety. This dissertation represents the work and results to date of PediaFlow® PF4 on the path to preclinical testing to submit an Investigation Device Exception (IDE) application in anticipation of eventual clinical trials

Evaluation of CFD based hemolysis prediction methods

Accurate quantitative evaluation of shear stress-related hemolysis (destruction of red blood cells) could be used to improve blood handling devices, including left ventricular assist devices (LVAD). Computational Fluid Dynamics (CFD) predicts the fluid dynamics of complex pump geometry and has been used to track the shear stress history of red blood cells as they travel through these devices. Several models that predict the relationship between hemolysis, shear stress and exposure time have been used to evaluate the hemolysis in the pumps. However, the prediction accuracy has not reached the satisfactory level. The goal of my thesis is to investigate the application of CFD in determining hemolysis using different hemolysis prediction methods. • This approach is two-fold. First it is done on a simplified geometry designed to produce known and controllable shear stresses. This device is known as the mag-lev shearing device and was designed using CFD in order to study erythrocyte damage in terms of the effects of shear stress. This mathematical solution for annular shearing device will be used to verify computational data. • Secondly, I applied the same methods to the LEV-VAD pump, currently under development at RIT. The grid independent mesh was obtained for RIT axial pump and was utilized for further studies. In Characteristic curve (Pressure vs. Flow), the experimental pressure rise data was compared with the pressure difference data from CFD simulation of the RIT mini pump. • Hemolysis was estimated for both geometries using four different hemolysis analysis methods, referred to as: Threshold Value, Mass-Weighted Average, Eulerian and Lagrangian approaches. The pump numerical hemolysis predictions are compared with the previous in vitro hemolysis data using bovine blood. The numerical simulation of flow field for mag-lev shearing device was compared with the analytical solution of the fluid dynamics inside the gap regions of the device. In the future, the mag-lev shearing device will be used with animal and human blood to empirically evaluate the hemolysis and this empirical data may be used to validate the numerical methods presented here

The potential of a tesla type device as a non pulsatile blood pump

A review of the published work on pumps designed to assist a failing biological heart has been made and discusses the different types of pump presently on the market with the characteristics of each. The materials used to make these pumps are also discussed, together with some of the methods of output control. The application to the patient is described together with the advantages and disadvantages. A Tesla type pump appeared to offer an alternative solution to those problems listed above. This is not a new design but appeared to offer advantages if applied to the application of pumping blood from outside the body. One of these was that at a constant speed, the pump supplied constant fluid pressure irrespective of the delivery. It also appears that the pump can give fluid shear stress levels that are less than the amount that will seriously damage blood components. A prototype pump has been built and tested. The pump achieved the performance target delivery of 10 I/min at a differential pressure of 200 mm Hg. This was considered to be greater than the average performance produced by existing blood pumps but the maximum performance that could be produced by the human heart under extreme conditions. The pump reached a maximum speed of about 4000 rev/min with a maximum power consumption of about 120 Watts. The results indicate that this type of pump is a potential blood pump in terms of the delivery and pressures achieved. The characteristic performance figures are within the envelope of published theoretical results. The pump tested here needs further development to improve the hydraulic performance. Recommendations are made for the direction of future work to improve the pump efficiency and flow patterns, biocompatibility and methods of production. Controls and power supply also need improvement