The Use of Fluid Mechanics to Predict Regions of Microscopic Thrombus Formation in Pulsatile VADs

Fluid Dynamic Study of the Penn State Pediatric Total Artificial Heart

Penn State University is developing a pediatric total artificial heart (TAH) as a bridge-to-transplant device that supports infants and small children with single ventricle anomalies or biventricular heart failure to address high waitlist mortality rates for pediatric patients with severe congenital heart disease (CHD). Two issues with mechanical circulatory support devices are thrombus formation and thromboembolic events. This in vitro study characterizes flow within Penn State's pediatric total artificial heart under physiological operating conditions. Particle image velocimetry (PIV) is used to quantify flow within the pump and to calculate wall shear rates (WSRs) along the internal pump surface to identify potential thrombogenic regions. Results show that the diastolic inflow jets produce sufficient wall shear rates to reduce thrombus deposition potential along the inlet side of the left and right pumps. The inlet jet transitions to rotational flow, which promotes wall washing along the apex of the pumps, prevents flow stasis, and aligns flow with the outlet valve prior to systolic ejection. However, inconsistent high wall shear rates near the pump apex cause increased thrombogenic potential. Strong systolic outflow jets produce high wall shear rates near the outlet valve to reduce thrombus deposition risk. The right pump, which has a modified outlet port angle to improve anatomical fit, produces lower wall shear rates and higher thrombus susceptibility potential (TSP) compared to the left pump. In summary, this study provides a fluid dynamic understanding of a new pediatric total artificial heart and indicates thrombus susceptibility is primarily confined to the apex, consistent with similar pulsatile heart pumps.

Effect of Hematocrit and Elevated Beat Rate on the 12cc Penn State Pediatric Ventricular Assist Device

Congenital heart disease affects approximately 40,000 infants annually in the United States with 25% requiring invasive treatment. Due to limited number of donor hearts and treatment options available for children, pediatric ventricular assist devices (PVADs) are used as a bridge to transplant. The 12cc pneumatic Penn State PVAD is optimized to prevent platelet adhesion and thrombus formation at patient nominal conditions; however, children demonstrate variable blood hematocrit and elevated heart rates. Therefore, with pediatric patients exhibiting greater variability, particle image velocimetry is used to evaluate the PVAD with three non-Newtonian hematocrit blood analogs (20%, 40%, and 60%) and at two beat rates (75 and 120 bpm) to understand the device's performance. The flow fields demonstrate a strong inlet jet that transitions to a solid body rotation during diastole. During systole, the rotation dissipates and reorganizes into an outlet jet. This flow field is consistent across all hematocrits and beat rates but at a higher velocity magnitude during 120 bpm. There are also minor differences in flow field timing and surface washing due to hematocrit. Therefore, despite patient differences in hematocrit or required pumping output, thorough surface washing can be achieved in the PVAD by altering operating conditions, thus reducing platelet adhesion potential.

Feasibility of an implantable bioreactor for renal cell therapy using silicon nanopore membranes

The definitive treatment for end-stage renal disease is kidney transplantation, which remains limited by organ availability and post-transplant complications. Alternatively, an implantable bioartificial kidney could address both problems while enhancing the quality and length of patient life. An implantable bioartificial kidney requires a bioreactor containing renal cells to replicate key native cell functions, such as water and solute reabsorption, and metabolic and endocrinologic functions. Here, we report a proof-of-concept implantable bioreactor containing silicon nanopore membranes to offer a level of immunoprotection to human renal epithelial cells. After implantation into pigs without systemic anticoagulation or immunosuppression therapy for 7 days, we show that cells maintain >90% viability and functionality, with normal or elevated transporter gene expression and vitamin D activation. Despite implantation into a xenograft model, we find that cells exhibit minimal damage, and recipient cytokine levels are not suggestive of hyperacute rejection. These initial data confirm the potential feasibility of an implantable bioreactor for renal cell therapy utilizing silicon nanopore membranes.

Performance of a Hydrogel Coated Nitinol with Oligonucleotide-Modified Nanoparticles Within Turbulent Conditions of Blood-Contacting Devices

INTRODUCTION

Hydrogels offer a wide range of applications in the antithrombotic modification of biomedical devices. The functionalization of these hydrogels with potentially drug-laden nanoparticles in the context of deviceassociated turbulence is critically under-studied. Thus, the purpose of this study was to use a hydrogel-coating nitinol surface as a model to understand the functions of hydrogels and the capture of nanoparticles under clinically relevant flow conditions.

METHODS

Nitinol was coated by an oligonucleotide (ON) functionalized hydrogel. Nanoparticles were functionalized with complementary oligonucleotides (CONs). The capture of CONfunctionalized nanoparticles by the ON-functionalized hydrogel surfaces was studied under both static and dynamic attachment conditions. Fluorescent-labelling of nanoparticles was utilized to assess capture efficacy and resistance to removal by device-relevant flow conditions.

RESULTS

The specificity of the ON-CON bond was verified, exhibiting a dose-dependent attachment response. The hydrogel coating was resistant to stripping by flow, retaining >95% after exposure to one hour of turbulent flow. Attachment of nanoparticles to the hydrogel was higher in the static condition than under laminar flow (p < 0.01), but comparable to that of attachment under turbulent flow. Modified nitinol samples underwent one hour of flow treatment under both laminar and turbulent regimes and demonstrated decreased nanoparticle loss following static conjugation rather than turbulent conjugation (36.1% vs 53.8%, p < 0.05). There was no significant difference in nanoparticle functionalization by upstream injection between laminar and turbulent flow.

CONCLUSION

The results demonstrate promising potential of hydrogelfunctionalized nitinol for capturing nanoparticles using nucleic acid hybridization. The hydrogel structure and ONCON bond integrity both demonstrated a resistance to mechanical damage and loss of biomolecular functionalization by exposure to turbulence. Further investigation is warranted to highlight drug delivery and antithrombogenic modification applications of nanoparticle-functionalized hydrogels.

Experimental Hemodynamics within the Penn State Fontan Circulatory Assist Device

For children born with a single functional ventricle, the Fontan operation bypasses the right ventricle by forming a four-way total cavopulmonary connection adapting the existing ventricle for the systemic circulation. However, upon adulthood, many Fontan patients exhibit low cardiac output and elevated venous pressure, eventually requiring a heart transplantation. Despite efforts to develop a Fontan pump or use an existing ventricular assist device for failing Fontan support, there is still no device designed or tested for subpulmonary support. Penn State University is developing a hydrodynamically levitated Fontan circulatory assist device (FCAD) for bridge-to-transplant or destination therapy. The FCAD hemodynamics, at both steady and pulsatile conditions for three pump operating conditions, were quantified using particle image velocimetry to determine the velocity magnitudes and Reynolds normal and shear stresses. Data were acquired at three planes (0 mm and ±25% of the radius) for the inferior and superior vena cavae inlets and the pulmonary artery outlet. The inlets had a blunt velocity profile that became skewed towards the collecting volute as fluid approached the rotor. At the outlet, regardless of the flow condition, a high-velocity jet exited the volute and moved downstream in a helical pattern. Turbulent stresses observed at the volute exit were influenced by the rotor's rotation. Regardless of inlet conditions, the pump demonstrated advantageous behavior for clinical use with a predictable flow field and a low risk of platelet adhesion and hemolysis based on calculated wall shear rates and turbulent stresses, respectively.

Computational Fluid-Structure Interaction Study of a New Wave Membrane Blood Pump

PURPOSE

Wave membrane blood pumps (WMBP) are novel pump designs in which blood is propelled by means of wave propagation by an undulating membrane. In this paper, we computationally studied the performance of a new WMBP design (J-shaped) for different working conditions, in view of potential applications in human patients.

METHODS

Fluid-structure interaction (FSI) simulations were conducted in 3D pump geometries and numerically discretized by means of the extended finite element method (XFEM). A contact model was introduced to capture membrane-wall collisions in the pump head. Mean flow rate and membrane envelope were determined to evaluate hydraulic performance. A preliminary hemocompatibility analysis was performed via calculation of fluid shear stress.

RESULTS

Numerical results, validated against in vitro experimental data, showed that the hydraulic output increases when either the frequency or the amplitude of membrane oscillations were higher, with limited increase in the fluid stresses, suggesting good hemocompatibility properties. Also, we showed better performance in terms of hydraulic power with respect to a previous design of the pump. We finally studied an operating point which achieves physiologic flow rate target at diastolic head pressure of 80 mmHg.

CONCLUSION

A new design of WMBP was computationally studied. The proposed FSI model with contact was employed to predict the new pump hydraulic performance and it could help to properly select an operating point for the upcoming first-in-human trials.

In vitro real-time magnetic resonance imaging for quantification of thrombosis

OBJECTIVES

Thrombosis is a leading cause of failure for cardiovascular devices. While computational simulations are a powerful tool to predict thrombosis and evaluate the risk for medical devices, limited experimental data are available to validate the simulations. The aim of the current study is to provide experimental data of a growing thrombus for device-induced thrombosis.

MATERIALS AND METHODS

Thrombosis within a backward-facing step (BFS), or sudden expansion was investigated, using bovine and human blood circulated through the BFS model for 30 min, with a constant inflow rate of 0.76 L/min. Real-time three-dimensional flow-compensated magnetic resonance imaging (MRI), supported with Magnevist, a contrast agent improving thrombus delineation, was applied to quantify thrombus deposition and growth within the model.

RESULTS

The study showed that the BFS model induced a flow recirculation region, which facilitated thrombosis. By 30 min, in comparison to bovine blood, human blood resulted in smaller thrombus formation, in terms of the length (13.3 ± 0.6 vs. 18.1 ± 1.3 mm), height (2.3 ± 0.1 vs. 2.6 ± 0.04 mm), surface area exposed to blood (0.67 ± 0.03 vs 1.05 ± 0.08 cm²), and volume (0.069 ± 0.004 vs. 0.093 ± 0.007 cm³), with p < 0.01. Normalization of the thrombus measurements, which excluded the flow recirculation effects, suggested that the thrombus sizes increased during the first 15 min and stabilized after 20 min. Blood properties, including viscosity, hematocrit, and platelet count affected thrombosis.

CONCLUSION

For the first time, contrast agent-supported real-time MRI was performed to investigate thrombus deposition and growth within a sudden expansion. This study provides experimental data for device-induced thrombosis, which is valuable for validation of computational thrombosis simulations.

Use of electron microscopy to study platelets and thrombi

Electron microscopy has been a valuable tool for the study of platelet biology and thrombosis for more than 70 years. Early studies using conventional transmission and scanning electron microscopy (EM) provided a foundation for our initial understanding of platelet structure and how it changes upon platelet activation. EM approaches have since been utilized to study platelets and thrombi in the context of basic, translational and clinical research, and they are instrumental in the diagnosis of multiple platelet function disorders. In this brief review, we provide a sampling of the many contributions EM based studies have made to the field, including both historical highlights and contemporary applications. We will also discuss exciting new imaging modalities based on EM and their utility for the study of platelets, hemostasis and thrombosis into the future.

Combined In Silico and In Vitro Approach Predicts Low Wall Shear Stress Regions in a Hemofilter that Correlate with Thrombus Formation In Vivo

A major challenge in developing blood-contacting medical devices is mitigating thrombogenicity of an intravascular device. Thrombi may interfere with device function or embolize from the device to occlude distant vascular beds with catastrophic consequences. Chemical interactions between plasma proteins and bioengineered surface occur at the nanometer scale; however, continuum models of blood predict local shear stresses that lead to platelet activation or aggregation and thrombosis. Here, an iterative approach to blood flow path design incorporating in silico, in vitro, and in vivo experiments predicted the occurrence and location of thrombi in an implantable hemofilter. Low wall shear stress (WSS) regions identified by computational fluid dynamics (CFD) predicted clot formation in vivo. Revised designs based on CFD demonstrated superior performance, illustrating the importance of a multipronged approach for a successful design process.

Methods for determination of stagnation in pneumatic ventricular assist devices

Background:

A pneumatic paediatric ventricular assist device developed at the Foundation of Cardiac Surgery Development, Zabrze, equipped with valves based on J. Moll’s design, with later modifications introduced at the Institute of Turbomachinery, Lodz University of Technology, was tested numerically and experimentally. The main aim of those investigations was to detect stagnation zones within the ventricular assist device and indicate advantages and limitations of both approaches.

Methods:

In the numerical transient test, a motion of the diaphragm and discs was simulated. Two different methods were used to illustrate stagnation zones in the ventricular assist device. The flow pattern inside the chamber was represented by velocity contours and vectors to validate the results using images obtained in the laser particle image velocimetry experiment.

Results:

The experimental light-based method implied problems with proper illumination of regions in the wall vicinity. High-resolution flow data and other important parameters as stagnation regions or flow patterns in regions not accessible for light in the particle image velocimetry method are derived in the numerical solution. However, computations of a single case are much more time-consuming if compared to a single experiment conducted on a well-calibrated stand.

Conclusion:

The resulting main vortexes in the central part of the pump chamber and the velocity magnitudes are correlated in both methods, which are complementary and when used together offer better insight into the flow structure inside the ventricular assist device and enable a deeper analysis of the results.

Original article submission: Platelet stress accumulation analysis to predict thrombogenicity of an artificial kidney

An implantable artificial kidney using a hemofilter constructed from an array of silicon membranes to provide ultrafiltration requires a suitable blood flow path to ensure stable operation in vivo. Two types of flow paths distributing blood to the array of membranes were evaluated: parallel and serpentine. Computational fluid dynamics (CFD) simulations were used to guide the development of the blood flow paths. Pressure data from animal tests were used to obtain pulsatile flow conditions imposed in the transient simulations. A key consideration for stable operation in vivo is limiting platelet stress accumulation to avoid platelet activation and thrombus formation. Platelet stress exposure was evaluated by CFD particle tracking methods through the devices to provide distributions of platelet stress accumulation. The distributions of stress accumulation over the duration of a platelet lifetime for each device revealed that stress accumulation for the serpentine flow path exceeded levels expected to cause platelet activation while the accumulated stress for the parallel flow path was below expected activation levels.

Device thrombosis and pre-clinical blood flow models for assessing antithrombogenic efficacy of drug-device combinations

Thrombosis associated with blood-contacting devices is a complex process involving several component interactions that have eluded precise definition. Extensive investigations of individual biological modules such as protein adsorption, coagulation cascade activation and platelet activation/adhesion/aggregation have provided an initial foundation for developing biomaterials for blood-contacting devices, but a material that is intrinsically non-thrombogenic is yet to be developed. The well-recognized association between fluid dynamics parameters such as shear stress, vortices, stagnation and thrombotic processes such as platelet aggregation and coagulation aggravate thrombosis on most device geometries that elicit these flow disturbances. Thus, antithrombotic drugs that were developed to treat thrombosis associated with vascular diseases such as atherosclerosis have also been adapted to mitigate the risk of device thrombosis. However, balancing the risk of bleeding with the antithrombotic efficacy of these drugs continues to be a challenge, and surface modification of devices with these drug molecules to mitigate device thrombosis locally has been explored. Pre-clinical blood flow models to test the effectiveness of these drug-device combinations have also evolved and several in-vitro, ex-vivo, and in-vivo test configurations are available with their attendant merits and limitations. Despite considerable efforts toward iterative design and testing of blood contacting devices and antithrombogenic surface modifications, device thrombosis remains an unsolved problem.

Development of a platelet adhesion transport equation for a computational thrombosis model

Thrombosis is a significant issue for cardiovascular device development and use. While thrombosis models are available, very few are device-related and none have been thoroughly validated experimentally. Here, we introduce a surface adherent platelet transport equation into a continuum model to account for the biomaterial interface/blood interaction. Using a rotating disc system and polyurethane-urea material, we characterize steady and pulsatile flow fields using laser Doppler velocimetry. In vitro measurements of platelet adhesion are used in combination with the LDV data to provide further experimental validation. The rotating disc system is computationally studied using the device-induced thrombosis model with the surface platelet adherent transport equation. The results indicate that the flow field is in excellent agreement to the experimental LDV data and that the platelet adhesion simulations are in good agreement with the in vitro platelet data. These results provide good evidence that this transport equation can be used to express the relationship between blood and a biomaterial if the correct platelet adhesion characteristics are known for the biomaterial. Further validation is necessary with other materials.

Development of a computational model for macroscopic predictions of device-induced thrombosis

While cardiovascular device-induced thrombosis is associated with negative patient outcomes, the convoluted nature of the processes resulting in a thrombus makes the full thrombotic network too computationally expensive to simulate in the complex geometries and flow fields associated with devices. A macroscopic, continuum computational model is developed based on a simplified network, which includes terms for platelet activation (chemical and mechanical) and thrombus deposition and growth in regions of low wall shear stress (WSS). Laminar simulations are performed in a two-dimensional asymmetric sudden expansion geometry and compared with in vitro thrombus size data collected using whole bovine blood. Additionally, the predictive power of the model is tested in a flow cell containing a series of symmetric sudden expansions and contractions. Thrombi form in the low WSS area downstream of the asymmetric expansion and grow into the nearby recirculation region, and thrombus height and length largely remain within 95 % confidence intervals calculated from the in vitro data for 30 min of blood flow. After 30 min, predicted thrombus height and length are 0.94 and 4.32 (normalized by the 2.5 mm step height). Importantly, the model also correctly predicts locations of thrombus deposition observed in the in vitro flow cell of expansions and contractions. As the simulation results, which rely on a greatly reduced model of the thrombotic network, are still able to capture the macroscopic behavior of the full network, the model shows promise for timely predictions of device-induced thrombosis toward optimizing and expediting the device development process.

Multi-objective optimization of pulsatile ventricular assist device hemocompatibility based on neural networks and a genetic algorithm

PURPOSE

Given the benefit of pulsatile blood flow for perfusion of coronary arteries and end organs, pulsatile ventricular assist devices (VADs) are still widely used as paracorporeal mechanical circulatory support devices in clinical applications. However, poor hemocompatibility limits the service period of the VADs. Most previous improvements on VAD hemocompatibility were conducted by trial-and-error CFD analysis, which does not easily arrive at the best solution.

METHODS

In this paper, a multi-objective optimization method integrating neural networks and NSGA-II (Non-dominated Sorted Genetic Algorithm-II) based on FSI simulation was developed and applied to a pulsatile VAD to optimize its hemocompatibility. First, the VAD blood chamber was parameterized with the principal geometrical parameters. Three hemocompatibility indices including hemolysis, platelet activation, and platelet deposition were chosen as goal functions. The neural networks were built to fit the nonlinear relationship between goal functions and geometrical parameters. Next, a multi-objective optimization algorithm (NSGA-II) was used to search out the Pareto optimal solutions in the built neural networks. Finally, the best compromise solution was selected from the Pareto optimal solutions by a fuzzy membership approach and validated by FSI simulation.

RESULTS

The best compromise solution simultaneously possesses an acceptable hemolysis index, platelet activation index, and platelet deposition index, and the corresponding relative errors between the indices predicted by optimization algorithm and the one calculated by FSI simulations are all less than 5%.

CONCLUSIONS

The results suggest that the proposed multi-objective optimization method has the potential for application in optimizing pulsatile VAD hemocompatibility, and may also be applied to other blood-wetted devices.

Design of a pulsatile flow facility to evaluate thrombogenic potential of implantable cardiac devices

Due to expensive nature of clinical trials, implantable cardiac devices should first be extensively characterized in vitro. Prosthetic heart valves (PHVs), an important class of these devices, have been shown to be associated with thromboembolic complications. Although various in vitro systems have been designed to quantify blood-cell damage and platelet activation caused by nonphysiological hemodynamic shear stresses in these PHVs, very few systems attempt to characterize both blood damage and fluid dynamics aspects of PHVs in the same test system. Various numerical modeling methodologies are also evolving to simulate the structural mechanics, fluid mechanics, and blood damage aspects of these devices. This article presents a completely hemocompatible small-volume test-platform that can be used for thrombogenicity studies and experimental fluid mechanics characterization. Using a programmable piston pump to drive freshly drawn human blood inside a cylindrical column, the presented system can simulate various physiological and pathophysiological conditions in testing PHVs. The system includes a modular device-mounting chamber, and in this presented case, a 23 mm St. Jude Medical (SJM) Regents® mechanical heart valve (MHV) in aortic position was used as the test device. The system was validated for its capability to quantify blood damage by measuring blood damage induced by the tester itself (using freshly drawn whole human blood). Blood damage levels were ascertained through clinically relevant assays on human blood while fluid dynamics were characterized using time-resolved particle image velocimetry (PIV) using a blood-mimicking fluid. Blood damage induced by the tester itself, assessed through Thrombin-anti-Thrombin (TAT), Prothrombin factor 1.2 (PF1.2), and hemolysis (Drabkins assay), was within clinically accepted levels. The hydrodynamic performance of the tester showed consistent, repeatable physiological pressure and flow conditions. In addition, the system contains proximity sensors to accurately capture leaflet motion during the entire cardiac cycle. The PIV results showed skewing of the leakage jet, caused by the asymmetric closing of the two leaflets. All these results are critical to characterizing the blood damage and fluid dynamics characteristics of the SJM Regents® MHV, proving the utility of this tester as a precise system for assessing the hemodynamics and thrombogenicity for various PHVs.

Numerical model of full-cardiac cycle hemodynamics in a total artificial heart and the effect of its size on platelet activation

The SynCardia total artificial heart (TAH) is the only Food and Drug Administration (FDA) approved device for replacing hearts in patients with congestive heart failure. It pumps blood via pneumatically driven diaphragms and controls the flow with mechanical valves. While it has been successfully implanted in more than 1300 patients, its size precludes implantation in smaller patients. This study's aim was to evaluate the viability of scaled-down TAHs by quantifying thrombogenic potentials from flow patterns. Simulations of systole were first conducted with stationary valves, followed by an advanced full-cardiac cycle model with moving valves. All the models included deforming diaphragms and platelet suspension in the blood flow. Flow stress accumulations were computed for the platelet trajectories and thrombogenic potentials were assessed. The simulations successfully captured complex flow patterns during various phases of the cardiac cycle. Increased stress accumulations, but within the safety margin of acceptable thrombogenicity, were found in smaller TAHs, indicating that they are clinically viable.