Blood-contacting magnetic levitation bearing design using computational fluid dynamics for haemocompatibility.

The Hemocompatibility Assessment Platform (HAP) is a testing rig that will allow for the evaluation of blood trauma caused by individual components of rotary blood pumps including the NeoVAD - a proposed paediatric Left Ventricular Assist Device (LVAD). It is important that the HAP itself is only minimally haemolytic such that the plasma free haemoglobin measured can be assumed to come from the test component. In this study, Computational Fluid Dynamics simulations have been carried out to inform the design of a magnetically levitated motor bearing gap. Simulations show that issues with the original design, namely stagnation regions and large recirculation zones can be mitigated with the introduction of a pipe that introduces blood-flow to the centre of the bearing and disrupts the secondary flow patterns that cause these issues.Clinical relevance- The consequent reduction in shear exposure time will reduce heamolsyis from the HAP. The redesign of the bearing will result in reduced baseline blood trauma from the HAP, thus allowing quantification of test component haemolysis and will therefore aid the design of future paediatric LVADs.

Dissipated energy and efficiency as objective functions for the design of the NeoVAD rotary blood pump.

Minimising haemolytic blood damage is an important objective when designing rotary blood pumps, however, calculating haemolysis can be computationally expensive and inaccurate. Efficiency and dissipated energy are much more easily calculable hydraulic parameters in the design and analysis of rotary blood pumps and although there is work to suggest that efficiency is not a good indicator of haemocompatibility, i.e. more efficient pumps do not necessarily cause less damage, there is recent speculation that dissipated energy can act as an easily calculable haemolysis analogue.This study shows that for design purposes, optimising for maximum efficiency and minimum dissipated energy are functionally the same as they are inherently and closely linked. Moreover a demonstration of rotary blood pump design has been completed using the NeoVAD paediatric left ventricular assist device optimising for both objective functions. The resulting designs appear similar in rotor blade shape and are similar in hydraulic performance.Clinical relevance- This reinforces the direct link between efficiency and dissipated energy when analysing rotary blood pumps at a given design operating point. This raises questions either of the claim that efficiency cannot be used as an easily calculable analogue for haemolysis or the validity of dissipated energy to act in this same manner.

Machine learning based on computational fluid dynamics enables geometric design optimisation of the NeoVAD blades.

The NeoVAD is a proposed paediatric axial-flow Left Ventricular Assist Device (LVAD), small enough to be implanted in infants. The design of the impeller and diffuser blades is important for hydrodynamic performance and haemocompatibility of the pump. This study aimed to optimise the blades for pump efficiency using Computational Fluid Dynamics (CFD), machine learning and global optimisation. Meshing of each design typically included 6 million hexahedral elements and a Shear Stress Transport turbulence model was used to close the Reynolds Averaged Navier-Stokes equations. CFD models of 32 base geometries, operating at 8 flow rates between 0.5 and 4 L/min, were created to match experimental studies. These were validated by comparison of the pressure-flow and efficiency-flow curves with those experimentally measured for all base prototype pumps. A surrogate model was required to allow the optimisation routine to conduct an efficient search; a multi-linear regression, Gaussian Process Regression and a Bayesian Regularised Artificial Neural Network predicted the optimisation objective at design points not explicitly simulated. A Genetic Algorithm was used to search for an optimal design. The optimised design offered a 5.51% increase in efficiency at design point (a 20.9% performance increase) as compared to the best performing pump from the 32 base designs. An optimisation method for the blade design of LVADs has been shown to work for a single objective function and future work will consider multi-objective optimisation.

Dynamic Evaluation of an Active Axial Magnetic Levitated Bearing System in a Hemocompatibility Assessment Platform.

To evaluate the hemocompatibility of individual components of our pediatric left ventricular assist device (LVAD), we proposed a hemocompatibility assessment platform (HAP) with a magnetic levitated bearing system. The HAP consists of a drive system utilizing a brushless direct current (BLDC) motor, passive magnetic bearings (PMB), and an active magnetically levitated bearing (AMB) to reduce the hemolysis generated by HAP itself. In this study, we designed and evaluated the performance of the AMB by measuring radial and axial displacements of the rotor resulting from radially destabilizing forces as well as the performance of the drive system when rotated at increasing speeds to 1,200 rotations per minute (rpm). The results show that, with radial disturbance, the AMB is capable of maintaining axial stability for the BLDC motor system. The AMB can control up to 1,200 rpm without any contact between the rotor and stator. Future work includes geometry optimization for the AMB structure and increase the capability to control stable high-speed rotation for the entire system. Clinical Relevance- This work furthers the development of the magnetic levitated bearing system for a hemocompatibility assessment platform that will be used to enhance and accelerate the development of adult and pediatric LVADs.

Numerical and Experimental Analysis for a Magnetic Levitation System in a Hemocompatibility Assessment Platform.

Development of pediatric left ventricular assist devices (LVADs) has lagged behind that of adult LVADs, primarily due to the size and hemocompatibility constraints of pediatric anatomy. To quantify sources of blood trauma during LVAD development, we proposed a hemocompatibility assessment platform (HAP) that can evaluate the hemocompatibility of individual components of LVADs. To eliminate the hemolysis induced by the HAP itself, we incorporated passive magnetic (PM) bearings to suspend the rotor radially and an active magnetic bearing (AMB) to control the axial position. In this study, we numerically evaluated AMB forces of 2 geometries and validated the model by comparing its predictions with experimental results. The magnetic forces generated by the AMB were evaluated by increasing the rotor-stator gap from 0.1 mm to 0.5 mm with a 0.1 mm increment and by varying the coil current from -2 A to 2 A with a 1 A increment. The average error of the numerical models was 8.8% and 7.0% for the two geometries, respectively. Higher errors were found at smaller (<0.2mm) rotor-stator gaps. For both biasing ring sizes, the AMB exhibits high magnetic stiffness from -1 A to 1 A, though it saturates for currents of -2 A and 2 A. This region of high current stiffness was identified as the optimal control region. In future work, this function will be used to tune a control algorithm to modulate current supplied to the AMB, ultimately stabilizing the rotor axially. Clinical Relevance- This work furthers the development of a hemocompatibility assessment platform that will enhance and accelerate the development of adult and pediatric LVADs.

Physiological Control Algorithm for a Pulsatile-flow 3D Printed Circulatory Model to Simulate Human Cardiovascular System.

The human heart is responsible for maintaining constant, pulsatile blood flow in the human body. Mock circulatory loops (MCLs) have long been used as the mechanical representations of the human cardiovascular system and as test beds for mechanical circulatory support (MCS) devices and other interventional medical devices. This technology could also be used as a training and educational tool for surgeons/clinicians. To ensure the MCL can accurately simulate the pulsatile human cardiovascular system, it is essential that the MCL can reproduce human physiological responses, e.g., the Frank-Starling Mechanism, in a controllable operating environment. In this study, by using an elastance function template to control the simulated left ventricle, we created controllable pulsatile physiological flow in a 3D printed silicone vascular structure to successfully simulate the hemodynamic environment of the human cardiovascular system. Clinical Relevance- This work will provide an in vitro test platform to simulate the human cardiovascular system. The accurate simulation of human cardiovascular anatomy and hemodynamic environment will allow this device to be an ideal training/educational tool for surgeons/clinicians to recreate various physiological conditions that cannot be created in vivo in animal or cadaver models.