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.

3D-Printed silicone anatomic patient simulator to enhance training on cardiopulmonary bypass.

BACKGROUND: Simulator training is important for teaching perfusion students fundamental skills associated with CBP before they start working in the clinic. Currently available high-fidelity simulators lack anatomic features that would help students visually understand the connection between hemodynamic parameters and anatomic structure. Therefore, a 3D-printed silicone cardiovascular system was developed at our institution. This study aimed to determine whether using this anatomic perfusion simulator instead of a traditional "bucket" simulator would better improve perfusion students' understanding of cannulation sites, blood flow, and anatomy. METHODS: Sixteen students were tested to establish their baseline knowledge. They were randomly divided into two groups to witness a simulated bypass pump run on one of two simulators - anatomic or bucket - then retested. To better analyze the data, we defined "true learning" as characterized by an incorrect answer on the pre-simulation assessment being corrected on the post-simulation assessment. RESULTS: The group that witnessed the simulated pump run on the anatomic simulator showed a larger increase in mean test score, more instances of true learning, and a larger gain in the acuity confidence interval. CONCLUSIONS: Despite the small sample size, the results suggest that the anatomic simulator is a valuable instrument for teaching new perfusion students.

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.

The Evolution of Durable, Implantable Axial-Flow Rotary Blood Pumps.

Left ventricular assist devices (LVADs) are increasingly used to treat patients with end-stage heart failure. Implantable LVADs were initially developed in the 1960s and 1970s. Because of technological constraints, early LVADs had limited durability (eg, membrane or valve failure) and poor biocompatibility (eg, driveline infections and high rates of hemolysis caused by high shear rates). As the technology has improved over the past 50 years, contemporary rotary LVADs have become smaller, more durable, and less likely to result in infection. A better understanding of hemodynamics and end-organ perfusion also has driven research into the enhanced functionality of rotary LVADs. This paper reviews from a historical perspective some of the most influential axial-flow rotary blood pumps to date, from benchtop conception to clinical implementation. The history of mechanical circulatory support devices includes improvements related to the mechanical, anatomical, and physiologic aspects of these devices. In addition, areas for further improvement are discussed, as are important future directions-such as the development of miniature and partial-support LVADs, which are less invasive because of their compact size. The ongoing development and optimization of these pumps may increase long-term LVAD use and promote early intervention in the treatment of patients with heart failure.

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.

Construction and Evaluation of a Bio-Engineered Pump to Enable Subpulmonary Support of the Fontan Circulation: A Proof-of-Concept Study.

Our objective was to create a bio-engineered pump (BEP) for subpulmonary Fontan circulation support capable of luminal endothelialization and producing a 2-6 mmHg pressure gradient across the device without flow obstruction. To accomplish this, porcine urinary bladder submucosa was decellularized to produce a urinary bladder matrix (UBM) which produced acellular sheets of UBM. The UBM was cultured with human umbilical vein endothelial cells producing a nearly confluent monolayer of cells with the maintenance of typical histologic features demonstrating UBM to be a suitable substrate for endothelial cells. A lamination process created bilayer UBM sheets which were formed into biologic reservoirs. BEPs were constructed by securing the biologic reservoir between inlet and outlet valves and compressed with a polyurethane balloon. BEP function was evaluated in a simple flow loop representative of a modified subpulmonary Fontan circulation. A BEP with a 92-mL biologic reservoir operating at 60 cycles per minute produced pulsatile downstream flows without flow obstruction and generated a favorable pressure gradient across the device, maintaining upstream pressure of 6 mm Hg and producing downstream pressure of 13 mm Hg. The BEP represents potential long-term assistance for the Fontan circulation to relieve venous hypertension, provide pulsatile pulmonary blood flow and maintain cardiac preload.

Hemocompatibility Assessment Platform Drive System Design: Trade-off between Motor Performance and Hemolysis.

Left ventricular assist devices (LVADs) have long been used to treat adults with heart failure, but LVAD options for pediatric patients with heart failure are lacking. Despite the urgent need for long-term, implantable pediatric LVADs, design challenges such as hemolysis, pump thrombosis, and bleeding persist. We have developed a Hemocompatibility Assessment Platform (HAP) to identify blood trauma from individual LVAD components. A HAP would aid in refining pump components before in vivo testing, thereby preventing unnecessary animal sacrifice and reducing development time and cost. So that the HAP does not confound hemolysis data, the HAP drive system consists of an enlarged air-gap motor coupled to a magnetic levitation system. Although it is known that an enlarged air gap motor will have diminished performance, while the larger gap in the motor will cause less blood damage, the trade-offs are not fully characterized. Therefore, in this study we evaluated these trade-offs to determine an optimal rotor diameter for the HAP drive motor. The motor performance was characterized with an experimental method by determining the torque constant for the HAP drive motor with varied rotor diameters. The torque threshold was set as 10 mNm to achieve a nominal current of 3.5A. Hemolysis in the HAP drive motor gap was estimated by calculating scalar shear stress generated in the HAP motor gap analytically and numerically. A design criterion of 30 Pa was selected for scalar shear stress to achieve minimal hemolysis and platelet activation in the HAP drive system.Clinical Relevance- We evaluated a Hemocompatibility Assessment Platform for developing LVAD prototypes that can best balance motor performance and hemocompatibility. This design method can assist with optimizing the drive system during the research stage and illustrates how motor geometry can be tuned to reduce blood trauma.

Preparing end-stage heart failure patients and care providers in the era of climate change-driven hurricanes.

Patients with end-stage congestive heart failure are at elevated risk for harm when extreme storms threaten and strike their communities. Individuals with compromised heart function require customized hurricane protection and preparedness approaches. We provide mitigation strategies for providers and their teams, as well as the patients themselves to ensure their safety and uninterrupted access to healthcare resources and quality care during hurricane impact and in the aftermath.

In vivo Hemodynamic Evaluation of an Implantable Left Ventricular Assist Device in a Long-term Anti-coagulation Regimen.

Left ventricular assist devices (LVADs) are used to treat patients with severe (New York Heart Association class IV) heart failure. Thrombosis and bleeding are severe LVAD-related complications; thus, an effective anticoagulation regimen is crucial for successful postoperative management. The CH-VAD™ (CH Biomedical, Inc.) is a small, implantable, full-support (>5 L/min) LVAD with a centrifugal flow pump that has a fully magnetically levitated rotor, which confers superior hemocompatibility. In this study, the CH-VAD™ was implanted in two calves to evaluate its hemocompatibility and to establish an anticoagulation regimen for future GLP (good laboratory practice) studies. Heparin infusion was used during the surgery, and during postoperative management, the proper dosage of warfarin was given orally to maintain an international normalized ratio (INR) between 2.0 and 3.0. Pump performance, animal condition, and hematology results were recorded throughout the study (approximately 60 days). The results show that under the established anticoagulation regimen, the CH-VAD™ was well tolerated in the bovine model, with no significant thrombus or thromboembolic lesion formation in distal end organs. Low plasma free hemoglobin levels suggest that the device did not cause hemolysis. These results and the experience gained pave the way for future GLP studies.

Numerical and Experimental Approach to Characterize a BLDC Motor with Different Radial-gap to Improve Hemocompatibility Performance.

Left ventricular assist devices (LVADs) have increasingly been used clinically to treat heart failure patients. However, hemolysis, pump thrombosis, infection and bleeding still persist as major limitations of LVAD technology. Assessing LVAD hemocompatibility using a blood shear stress device (BSSD) has clear advantages, as the BSSD could provide a better experimental platform to develop reliable, quantifiable blood trauma assays to perform iterative testing of LVAD designs. In this study, a BSSD was proposed with short blood exposure time and no seals or contact bearings to reduce blood trauma caused by the test platform. Enlarged air-gap drive motor in BSSD is essential to avoid high shear stress; however, it would significantly reduce the motor torque, which may result in inadequate force to drive the entire system. In order to evaluate and optimize the drive motor air-gap to ensure adequate motor torque as well as acceptable range for blood exposure time and shear stress, a numerical brushless DC (BLDC) motor model was established using finite element method (FEM) in numerical simulation software COMSOL. The model was first validated by the experimental results. Then numerical model with different air-gap was evaluated on the torque and speed constant changes. In the end, two equations were generated based on the curves derived from the torque and speed constant calculations. Determining these relationships between motor performance and motor air-gap will facilitate the development of an appropriate BLDC motor size for the BSSD, considering the design limitations in our future work.

Can We Improve Vaginal Tissue Healing Using Customized Devices: 3D Printing and Biomechanical Changes in Vaginal Tissue.

BACKGROUND: Determining biomechanical changes in vaginal tissue with tissue stretch is critical for understanding the role of mechanotransduction on vaginal tissue healing. Noncontact dynamic optical coherence elastography (OCE) can quantify biomechanical changes in vaginal tissues noninvasively. Improved vaginal tissue healing will reduce postoperative complications from vaginal surgery. AIMS: (1) To complete dimensional assessments (DAs) of the vaginal tract. (2) To elucidate biomechanical properties (BMP) of porcine vaginal tissues (PVT). (3) Compare BMPs of piglet and adult PVTs after placement of customized vaginal dilators (VD) by OCE and uniaxial mechanical testing (MT). METHODS: Pilot study using adult nulliparous pig and piglet PVTs (n = 20 each). DA of PVTs was performed using silicone molding. 3D-printed VDs were used to achieve different Relative Diameter Change (RDC) of the PVTs (no dilatation, and -50%, 0%, 50% RDC). Elastographic testing using OCE and MT. RESULTS: Using OCE, no significant differences (SD) were noted between adult and piglet PVT (p = 0.74) or by stretch direction (p = 0.300). SD was noted with increasing RDC (p = 0.023). Using MT, there were SD in tissue stiffness between adult and piglet PVT (p = 0.048), but no SD as a function of RDC (p = 0.750) or stretch direction (p = 0.592). CONCLUSIONS: This study quantified biomechanical changes in PVT with customized stretching by 3D printed VD using both OCE and MT. This work has implications for the mechanotransduction of vaginal wound healing and noninvasive assessment of vaginal diseases.

Design Method Using Statistical Models for Miniature Left Ventricular Assist Device Hydraulics.

Left ventricular assist devices (LVADs) are increasingly used to treat heart failure patients. These devices' impeller blades and diffuser vanes must be designed for hydraulic performance and hemocompatibility. The traditional design method, applying mean-line theory, is not applicable to the design of small-scale pumps such as miniature LVADs. Furthermore, iterative experimental testing to determine how each geometric variable affects hydraulic performance is time and labor intensive. In this study, we tested a design method wherein empirical hydraulic results are used to establish a statistical model to predict pump hydraulic performance. This method was used to design an intra-atrial blood pump. Five geometric variables were chosen, and each was assigned two values to define the variable space. The experimental results were then analyzed with both correlation analysis and linear regression modeling. To validate the linear regression models, 2 test pumps were designed: mean value of each geometric variable within the boundaries, and random value of each geometric variable within the boundaries. The statistical model accurately predicted the hydraulic performance of both pump designs within the boundary space. This method could be expanded to include more geometric variables and broader boundary conditions, thus accelerating the design process for miniature LVADs.

Hemodynamic Evaluation of an Intra-Atrial Blood Pump on a Pulsatile Mock Circulatory Loop.

An intra-atrial pump (IAP) was proposed that would be affixed to the atrial septum to support the compromised left ventricle (LV) without harming the ventricular tissue in patients with early-stage heart failure. The IAP is designed to operate in parallel with the LV, drawing blood from the left atrium and unloading the LV. In previous hydraulic studies, different blade geometries were tested for the IAP; however, it is important to know how the blade geometry affects the IAP's hemodynamic performance in the human cardiovascular system. In this study, a mock circulatory loop (MCL) with physiological response was used to evaluate the hemodynamic effects of IAP blade geometry and connection configuration in the human cardiovascular system. In a $2 \times 2$ study, two different blade geometries (with steep vs flat pressure/flow curves) were tested in two different connection configurations: the proposed configuration (left atrium to aorta) and the conventional configuration for LVADs (LV to aorta). We found that atrial cannulation is feasible and creates a beneficial hemodynamic environment, although it is inferior to the one created by ventricular cannulation. The steepgradient pump performed better than the flat-gradient pump in atrial insertion.

In Vivo Hemodynamic Evaluation of CH-VAD in a Bovine Model for 14 Days.

The CH-VAD is a centrifugal-flow magnetically levitated (maglev) left ventricular assist device (LVAD) used to treat end-stage heart failure. It is implanted in the chest cavity; the inflow cannula is inserted into the apex of the left ventricle, and the outflow graft is anastomosed to the aorta. Among several key VAD system improvements, the CH-VAD has a smaller body size than other LVADs and its maglev system offers a large-gap design that makes it superior in terms of hemocompatibility. In this study, we implanted the CH-VAD in a calf and evaluated the hemodynamic and hemocompatibility characteristics over a 14-day period. The hemodynamic parameters, the pump data, and blood test results were recorded throughout the study. The results showed that the CH-VAD provided hemodynamic stability. Hemocompatibility testing indicated negligible hemolysis throughout the study, and no signs of infection were seen. On necropsy, the results showed only expected focal mild-to-moderate adhesions between the pericardial sac (along the pump) and the adjacent rib cage, and between the pericardial sac and the heart. Gross examination of internal organs was unremarkable. Examination of the CH-VAD after explantation revealed no evidence of thrombus formation internally or around the inflow or outflow cannulas.

In Vivo Feasibility Study of an Intra-Atrial Blood Pump for Partial Support of the Left Ventricle.

We are designing an intra-atrial pump (IAP) that will be affixed to the atrial septum and support the compromised left ventricle (LV) in patients with early-stage heart failure without harming the ventricular tissue. It will operate in parallel with the LV, drawing blood from the left atrium and unloading the LV. In previous hydraulic and hemodynamic studies, different blade geometries were tested for the IAP, and the hemodynamic results obtained using a mock circulatory loop showed that the IAP can successfully reduce end-diastolic volume and increase the total systemic flow rate. In the current study, we used a bovine model to validate the in vitro hemodynamic results and better understand how the IAP interacts with the cardiovascular system in vivo. Because this was the first study assessing the complete device in a living system, it was also necessary to determine the best manufacturing techniques and ideal sensor placements. In the bovine model, we were able to successfully implant the IAP across the atrial septum with the outflow graft connected to a peripheral artery. The implanted IAP was capable of providing partial support (1-3 L/min) in vivo. These results indicate that atrial cannulation is feasible and creates a beneficial hemodynamic environment.

Preliminary design of the internal geometry in a minimally invasive left ventricular assist device under pulsatile-flow conditions.

PURPOSE: A minimally invasive, partial-assist, intra-atrial blood pump has been proposed, which would unload the left ventricle with a flow path from the left atrium to the arterial system. Flow modulation is a common strategy for ensuring washout in the pump, but it can increase power consumption because it is typically achieved through motor-speed variation. However, if a pump's performance curve had the proper gradient, flow modulation could be realized passively. To achieve this goal, we propose a pump performance operating curve as an alternative to the more standard operating point. METHODS AND RESULTS: Mean-line theory was employed to generate an initial set of geometries that were then tested on a hydraulic test rig at ~20,000 r/min. Experimental results show that the intra-atrial blood pump performed below the operating region; however, it was determined that smaller hub diameter and longer chord length bring the performance of the intra-atrial blood pump device closer to the operating curve. CONCLUSION: We found that it is possible to shape the pump performance curve for specifically targeted gradients over the operating region through geometric variations inside the pump.

Replication of pressure-volume loop with controllable ESPVR and EDPVR curves on a personalized mock circulatory loop based on elastance function.

In the development of a left ventricular assist device (LVAD), it is important to evaluate the LVAD's hemodynamic effect on the compromised left ventricle (LV) before surgical implantation. The mock circulatory loop (MCL) is widely accepted as an in vitro test platform to evaluate LVADs across a wide range of operational conditions as a way to examine how the device and the cardiovascular system interact. Unfortunately, most MCLs represent an oversimplified model of cardiac function, with disease states simulated through generalized changes in heart rate and stroke volume. Because heart failure (HF) severity varies substantially among patients, an MCL is needed that can mimic the pressure-volume loop of an individual patient. In this work, two numerical elastance models, derived from a specific pressure volume loop template, were used to control the LV simulator of the MCL to simulate different degrees of HF. The numerical elastance model was then scaled to change the slopes of the end-systolic (ESPVR) and end-diastolic (EDPVR) pressure volume relationship curves to simulate systolic and diastolic dysfunction. The resulting experimental pressure volume loops are consistent with theoretical loops, demonstrating the feasibility of creating an MCL that can be customized for the patient.

Systematic Design of a Magnetically Levitated Brushless DC Motor for a Reversible Rotary Intra-Aortic Blood Pump.

The IntraVAD is a miniature intra-aortic ventricular assist device (VAD) designed to work in series with the compromised left ventricle. A reverse-rotation control (RRc) mode has been developed to increase myocardial perfusion and reduce ventricular volume. The RRc mode includes forward rotation in systole and reverse rotation in diastole, which requires the IntraVAD to periodically reverse its rotational direction in synchrony with the cardiac cycle. This periodic reversal leads to changes in pressure force over the impeller, which makes the entire system less stable. To eliminate the mechanical wear of a contact bearing and provide active control over the axial position of the rotor, a miniature magnetically levitated bearing (i.e., the PM-Coil module) composed of two concentric permanent magnetic (PM) rings and a pair of coils-one on each side-was proposed to provide passive radial and active axial rotor stabilization. In the early design stage, the numerical finite element method (FEM) was used to optimize the geometry of the brushless DC (BLDC) motor and the maglev module, but constructing a new model each time certain design parameters were adjusted required substantial computation time. Because the design criteria for the module had to be modified to account for the magnetic force produced by the motor and for the hemodynamic changes associated with pump operation, a simplified analytic expression was derived for the expected magnetic forces. Suitable bearings could then be designed capable of overcoming these forces without repeating the complicated FEM simulation for the motor. Using this method at the initial design stage can inform the design of the miniature maglev BLDC motor for the proposed pulsatile axial-flow VAD.