In Vitro Testing and Comparison of Additively Manufactured Polymer Impellers for the CentriMag Blood Pump

Incorporating Unresolved Stresses in Blood Damage Modeling: Energy Dissipation More Accurate Than Reynolds Stress Formulation

OBJECTIVE

Reynolds Averaged Navier Stokes (RANS) models are often used as the basis for modeling blood damage in turbulent flows. To predict blood damage by turbulence stresses that are not resolved in RANS, a stress formulation that represents the corresponding scales is required. Here, we compare two commonly employed stress formulations: a scalar stress representation that uses Reynolds stresses as a surrogate for unresolved fluid stresses, and an effective stress formulation based on energy dissipation.

METHODS

We conducted unsteady RANS simulations of the CentriMag blood pump with three different closure models and a Large Eddy Simulation (LES) for reference. We implemented both stress representations in all models and compared the resulting total stress distributions in Eulerian and Lagrangian frameworks.

RESULTS

The Reynolds-stress-based approach overestimated the contribution of unresolved stresses in RANS, with differences between closure models of up to several orders of magnitude. With the dissipation-based approach, the total stresses predicted with RANS deviated by about 50% from the LES reference, which was more accurate than only considering resolved stresses.

CONCLUSION

The Reynolds-stress-based formulation proved unreliable for estimating scalar stresses in our RANS simulations, while the dissipation-based approach provided an accuracy improvement over simply neglecting unresolved stresses.

SIGNIFICANCE

Our results suggest that dissipation-based inclusion of unresolved stresses should be the preferred choice for blood damage modeling in RANS.

In vitro comparative study of red blood cell and VWF damage on 3D printing biomaterials under different blood-contacting conditions

Mechanical circulatory support devices (MCSDs) are often associated with hemocompatible complications such as hemolysis and gastrointestinal bleeding when treating patients with end-stage heart failure. Shear stress and exposure time have been identified as the two most important mechanical factors causing blood damage. However, the materials of MCSDs may also induce blood damage when contacting with blood. In this study, the red blood cell and von Willebrand Factor (VWF) damage caused by four 3D printing biomaterials were investigated, including acrylic, PCISO, Somos EvoLVe 128, and stainless steel. A roller pump circulation experimental platform and a rotor blood-shearing experimental platform were constructed to mimic static and dynamic blood-contacting conditions of materials in MCSDs, respectively. Free hemoglobin assay and VWF molecular weight analysis were performed on the experimental blood samples. It indicated that different 3D printing materials and technology could induce different levels of damage to red blood cells and VWF, with acrylic causing the least damage under both static and dynamic conditions. In addition, it was found that blood damage measured for the same material differed on the two platforms. Therefore, a combination of static and dynamic experiments should be used to comprehensively investigate the effects of blood damage caused by the material. It can provide a reference for the design and evaluation of materials in different components of MCSDs.

In-vitro investigation of endothelial monolayer retention on an inflow VAD cannula inside a beating heart phantom

Ventricular assist devices (VADs) provide an alternative solution to heart transplantation for patients with end-stage heart failure. Insufficient hemocompatibility of VAD components can result in severe adverse events, such as thromboembolic stroke, and readmissions. To enhance VAD hemocompatibility, and avoid thrombus formation, surface modification techniques and endothelialization strategies are employed. In this work, a free form patterning topography is selected to facilitate the endothelialization of the outer surface of the inflow cannula (IC) of a commercial VAD. An endothelialization protocol for convoluted surfaces such as the IC is produced, and the retainment of the endothelial cell (EC) monolayer is evaluated. To allow this evaluation, a dedicated experimental setup is developed to simulate realistic flow phenomena inside an artificial, beating heart phantom with a VAD implanted on its apex. The procedural steps of mounting the system result to the impairment of the EC monolayer, which is further compromised by the developed flow and pressure conditions, as well as by the contact with the moving inner structures of the heart phantom. Importantly, the EC monolayer is better maintained in the lower part of the IC, which is more susceptible to thrombus formation and may therefore aid in minimizing the hemocompatibility related adverse events after the VAD implantation.

Electrostatic Repulsive Features of Free-Standing Titanium Dioxide Nanotube-Based Membranes in Biofiltration Applications

This study presents the electrostatic repulsive features of electrochemically fabricated titanium dioxide nanotube (NT)-based membranes with different surface nanomorphologies in cross-flow biofiltration applications while maintaining a creatinine clearance above 90%. Although membranes exhibit antifouling behavior, their blood protein rejection can still be improved. Due to the electrostatically negative charge of the hexafluorotitanate moiety, the fabricated biocompatible, superhydrophilic, free-standing, and amorphous ceramic nanomembranes showed that about 20% of negatively charged 66 kDa blood albumin was rejected by the membrane with ∼100 nm pores. As the nanomorphology of the membrane was shifted from NTs to nanowires by varying fabrication parameters, pure water flux and bovine serum albumin (BSA) rejection performance were reduced, and the membrane did not lose its antifouling behavior. Herein, nanomembranes with different surface nanomorphologies were fabricated by a multi-step anodic oxidation process and characterized by scanning electron microscopy, atomic force microscopy, water contact angle analysis, X-ray diffraction, and energy-dispersive X-ray spectroscopy. The membrane performance of samples was measured in 3D printed polyethylene terephthalate glycol flow cells replicating implantable artificial kidney models to determine their blood toxin removal and protein loss features. In collected urine mimicking samples, creatinine clearances and BSA rejections were measured by the spectrophotometric Jaffe method and high-performance liquid chromatography.

Mechanical stress-induced hemolysis of bovine blood is donor-dependent

INTRODUCTION

In vitro hemolysis testing is an essential method for assessing the hemolytic potential of blood pumps, but has poor reproducibility. Further investigations are needed to determine the sources and extent of variability and to find a practical way to reduce the variation.

METHODS

A small volume blood circulating loop driven by a Centrimag pump was established to provide relatively higher hemolysis readouts within a short run time and to be able to sequentially perform multiple repeated hemolysis tests in a working day.

RESULTS

The repeatability with this system was demonstrated as the %RSD at 4.3% for the NIH or MIH from three repeated tests using the same blood. The bovine blood from different randomly selected donors was tested and gave more than a two-fold difference in NIH results (0.077 vs. 0.032 g/100 L) under the same testing conditions and same pump. This wide variation in hemolysis using bovine blood from different donors happened repeatedly. More importantly, it was observed that the difference in hemolysis test results using the blood drawn from the same donor on multiple days was narrow although the native hematocrits varied. The %RSD of NIH values obtained on five different days were 6.8%, 8.4%, 11.5%, and 7.8% for donor-specific blood from donors 1 to 4, respectively.

CONCLUSION

The study results indicate that the mechanical stress-induced hemolysis behavior is donor-dependent. It has been also demonstrated that the reproducibility of in vitro hemolysis testing can be improved when the blood drawn from same donor is used.

Hemodynamic investigation and in vitro evaluation of a novel mixed flow blood pump

INTRODUCTION

Ventricular assist devices (VADs) are considered as an effective treatment for patients with advanced heart failure, while complications associated with blood damage remain a burden. Structure design innovation has potential to reduce hemolysis and improve hemocompatibility.

METHODS

In this research, a novel mixed flow blood pump that integrates structural features of the axial and centrifugal VADs was proposed. The pump consists of an inducer, a mixed impeller supported by two ceramic pivot bearings and a volute. The flow field and laminar viscous shear stress were analyzed by the in-silico simulation. The hydraulic and hemolytic performance were evaluated in vitro by using a 3D printed pump.

RESULTS

The flow field distribution showed that streamlines in the connection area were smoothly transitioned through structural integration and no irregular flow occurred in the entire flow channel. The axial blades work as a fluid accelerator (generating 18.56% of the energy), and the centrifugal blades provide the main pressure head. The proportion of fluid inside the pump exposed to low laminar viscous shear stress (<50 Pa) and high laminar viscous shear stress (>150 Pa) was 99.02% and 0.03%, respectively. The in vitro hemolysis test results showed that the NIH (Normalized Index of Hemolysis) value of the mixed pump is 0.0079 ± 0.0039 g/100 L (n=6).

CONCLUSION

It can be concluded that the mixed flow structure is effective to improve hydraulic performance, eliminate flow disturbance and minimize shear stresses. This novel pump design is expected to provide a new direction for the development of next-generation VADs.

In Vitro and In Vivo Feasibility Study for a Portable VV-ECMO and ECCO2R System

Extracorporeal membrane oxygenation (ECMO) is an established rescue therapy for patients with chronic respiratory failure waiting for lung transplantation (LTx). The therapy inherent immobilization may result in fatigue, consecutive deteriorated prognosis, and even lost eligibility for transplantation. We conducted a feasibility study on a novel system designed for the deployment of a portable ECMO device, enabling the physical exercise of awake patients prior to LTx. The system comprises a novel oxygenator with a directly connected blood pump, a double-lumen cannula, gas blender and supply, as well as control and energy management. In vitro experiments included tests regarding performance, efficiency, and blood damage. A reduced system was tested in vivo for feasibility using a novel large animal model. Six anesthetized pigs were first positioned in supine position, followed by a 45° angle, simulating an upright position of the patients. We monitored performance and vital parameters. All in vitro experiments showed good performance for the respective subsystems and the integrated system. The acute in vivo trials of 8 h duration confirmed the results. The novel portable ECMO-system enables adequate oxygenation and decarboxylation sufficient for, e.g., the physical exercise of designated LTx-recipients. These results are promising and suggest further preclinical studies on safety and efficacy to facilitate translation into clinical application.

Hemolysis Testing In Vitro: A Review of Challenges and Potential Improvements

Many medical devices such as cardiopulmonary bypass systems, mechanical heart valves, or ventricular assist devices are intended to come into contact with blood flow during use. In vitro hemolysis testing can provide valuable information about the hemocompatibility of prototypes and thus help reduce the number of animal experiments required. Such tests play an important role as research and development tools for objective comparisons of prototypes and devices as well as for the extrapolation of their results to clinical outcomes. Therefore, it is important to explore and provide new ways to improve current practices. In this article, the main challenges of hemolysis testing are described, namely the difficult blood sourcing, the high experimental workload, and the low reproducibility of test results. Several approaches to address the challenges identified are proposed and the respective literature is reviewed. These include the replacement of blood as the "shear-sensitive fluid" by alternative test fluids, the replacement of sparse, manual sampling and blood damage assessment by a continuous and automated monitoring, as well as an analysis of categories and causes of variability in hemolysis test results that may serve as a structural template for future studies.

Validation of Numerically Predicted Shear Stress-dependent Dissipative Losses Within a Rotary Blood Pump

Computational fluid dynamics find widespread application in the development of rotary blood pumps (RBPs). Yet, corresponding simulations rely on shear stress computations that are afflicted with limited resolution while lacking validation. This study aimed at the experimental validation of integral hydraulic properties to analyze global shear stress resolution across the operational range of a novel RBP. Pressure head and impeller torque were numerically predicted based on Unsteady Reynolds-averaged Navier-Stokes (URANS) simulations and validated on a testbench with integrated sensor modalities (flow, pressure, and torque). Validation was performed by linear regression and Bland-Altman analysis across nine operating conditions. In power loss analysis (PLA), in silico hydraulic power losses were derived based on the validated hydraulic quantities and balanced with in silico shear-dependent dissipative power losses. Discrepancies among both terms provided a measure of in silico shear stress resolution. In silico and in vitro data correlated with low discordance in pressure (r = 0.992, RMSE = 1.02 mmHg), torque (r = 0.999, RMSE = 0.034 mNm), and hydraulic power losses (r = 0.990, RMSE = 0.015W). PLA revealed numerically predicted dissipative losses to be up to 34.4% smaller than validated computations of hydraulic losses. This study confirmed the suitability of URANS settings to predict integral hydraulic properties. However, numerical credibility was hampered by lacking resolution of shear-dependent dissipative losses.

Potential Factors for Poor Reproducibility of In Vitro Hemolysis Testing

In vitro testing of hemolysis is essential for the validation and development of ventricular assist devices. However, as many factors influence hemolysis, such tests' inter- and intralaboratory reproducibility is poor. In this work, CentriMag blood pumps were used to conduct a hemolysis study according to ASTM F1841 with blood from 23 bovine donors. Complementary blood analysis, including cell count, plasma composition, and viscosity, was performed to identify factors relevant to the variability of hemolysis testing results. Three strategies were tested to improve reproducibility: albumin supplementation, maintaining glucose concentration, and replacement of plasma with plasma-like buffer. Differences in red blood cell stability among donors were responsible for the largest portion of the total variance. Hematocrit varied widely among donors, and its adjustment to a standard value led to the artificial introduction of between-donor differences, especially in viscosity. It seems likely, that a more careful selection of donors with similar characteristics or repeated blood collection from the same donor could improve reproducibility. However, no direct correlations were found between the susceptibility to hemolysis and individual donor or blood characteristics in this study. The addition of albumin and glucose had a negligible effect while washing blood samples with artificial plasma significantly reduced mean hemolysis, although not its variation. The findings contribute to the understanding of variability in hemolysis experiments and give reason to question the common standard operating procedures, such as hemodilution or maintaining glucose concentration. To confirm the factors identified here, additional studies isolating the effects of individual factors are necessary.

Systems of conductive skin for power transfer in clinical applications

The primary aim of this article is to review the clinical challenges related to the supply of power in implanted left ventricular assist devices (LVADs) by means of transcutaneous drivelines. In effect of that, we present the preventive measures and post-operative protocols that are regularly employed to address the leading problem of driveline infections. Due to the lack of reliable wireless solutions for power transfer in LVADs, the development of new driveline configurations remains at the forefront of different strategies that aim to power LVADs in a less destructive manner. To this end, skin damage and breach formation around transcutaneous LVAD drivelines represent key challenges before improving the current standard of care. For this reason, we assess recent strategies on the surface functionalization of LVAD drivelines, which aim to limit the incidence of driveline infection by directing the responses of the skin tissue. Moreover, we propose a class of power transfer systems that could leverage the ability of skin tissue to effectively heal short diameter wounds. In this direction, we employed a novel method to generate thin conductive wires of controllable surface topography with the potential to minimize skin disruption and eliminate the problem of driveline infections. Our initial results suggest the viability of the small diameter wires for the investigation of new power transfer systems for LVADs. Overall, this review uniquely compiles a diverse number of topics with the aim to instigate new research ventures on the design of power transfer systems for IMDs, and specifically LVADs.

Characterization of the competing role of surface-contact and shear stress on platelet activation in the setting of blood contacting devices

Supraphysiological shear stress and surface-contact are recognized as driving mechanisms of platelet activation (PA) in blood contacting devices (BCDs). However, the competing role of these mechanisms in triggering thrombogenic events is poorly understood. Here, we characterized the dynamics of PA in response to the combined effect of shear stress and material exposure. Human platelets were stimulated with different levels of shear stress (500, 750, 1000 dynes/cm²) over a range of exposure times (10, 20, and 30 min) within capillary tubes made of various polymeric materials. Polyethylene (PE), polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), and polyether ether ketone (PEEK), used for BCDs fabrication, were investigated as compared to glass and thromboresistant Sigma™-coated glass. PA was quantified using the Platelet Activity State assay. Our results indicate that mechanical stimulation and polymer surface-contact both significantly contribute to PA. Notably, the contribution of the mechanical stimulus ranges between +36% and +43%, while that associated with polymer surface-contact ranges from +48% to +59%, depending on the exposure time. In more detail, our results indicate that: (i) PA increases with increasing shear stress magnitude; (ii) PA has a non-linear, time-dependent relationship to exposure time; (iii) PA is largely influenced by biomaterials, with PE and PEEK having respectively the lowest and highest prothrombotic potential; (iv) the effects of polymer surface-contact and shear stress are not correlated and can be studied separately. Our results suggest the importance of incorporating the evaluation of platelet activation driven by the combined effect of shear stress and polymer surface-contact for the comprehensive assessment, and eventually minimization, of BCDs thrombogenic potential.