Refurbishment of Extracorporeal Life Support Oxygenators in the Context of In Vitro Testing

Refurbishing single use extracorporeal membrane oxygenation (ECMO) oxygenators for in vitro research applications is common. However, the refurbishment protocols that are established in respective laboratories have never been evaluated. In the present study, we aim at proving the relevance of a well-designed refurbishing protocol by quantifying the burden of repeatedly reused oxygenators. We used the same three oxygenators in 5 days of 6 hours whole blood experiments. During each experiment day, the performance of the oxygenators was measured through the evaluation of gas transfer. Between experiment days, each oxygenator was refurbished applying three alternative refurbishment protocols based on purified water, pepsin and citric acid, and hydrogen peroxide solutions, respectively. After the last experiment day, we disassembled the oxygenators for visual inspection of the fiber mats. The refurbishment protocol based on purified water showed strong degeneration with a 40-50 %-performance drop and clearly visible debris on the fiber mats. Hydrogen peroxide performed better; nevertheless, it suffered a 20% decrease in gas transfer as well as clearly visible debris. Pepsin/citric acid performed best in the field, but also suffered from 10% performance loss and very few, but visible debris. The study showed the relevance of a well-suited and well-designed refurbishment protocol. The distinct debris on the fiber mats also suggests that reusing oxygenators is ill-advised for many experiment series, especially regarding hemocompatibility and in vivo testing. Most of all, this study revealed the relevance of stating the status of test oxygenators and, if refurbished, comment on the implemented refurbishment protocol in detail.

Novel Size-Variable Dedicated Rodent Oxygenator for ECLS Animal Models-Introduction of the "RatOx" Oxygenator and Preliminary In Vitro Results

The overall survival rate of extracorporeal life support (ECLS) remains at 60%. Research and development has been slow, in part due to the lack of sophisticated experimental models. This publication introduces a dedicated rodent oxygenator ("RatOx") and presents preliminary in vitro classification tests. The RatOx has an adaptable fiber module size for various rodent models. Gas transfer performances over the fiber module for different blood flows and fiber module sizes were tested according to DIN EN ISO 7199. At the maximum possible amount of effective fiber surface area and a blood flow of 100 mL/min, the oxygenator performance was tested to a maximum of 6.27 mL O₂/min and 8.2 mL CO₂/min, respectively. The priming volume for the largest fiber module is 5.4 mL, while the smallest possible configuration with a single fiber mat layer has a priming volume of 1.1 mL. The novel RatOx ECLS system has been evaluated in vitro and has demonstrated a high degree of compliance with all pre-defined functional criteria for rodent-sized animal models. We intend for the RatOx to become a standard testing platform for scientific studies on ECLS therapy and technology.

TPMS-based membrane lung with locally-modified permeabilities for optimal flow distribution

Membrane lungs consist of thousands of hollow fiber membranes packed together as a bundle. The devices often suffer from complications because of non-uniform flow through the membrane bundle, including regions of both excessively high flow and stagnant flow. Here, we present a proof-of-concept design for a membrane lung containing a membrane module based on triply periodic minimal surfaces (TPMS). By warping the original TPMS geometries, the local permeability within any region of the module could be raised or lowered, allowing for the tailoring of the blood flow distribution through the device. By creating an iterative optimization scheme for determining the distribution of streamwise permeability inside a computational porous domain, the desired form of a lattice of TPMS elements was determined via simulation. This desired form was translated into a computer-aided design (CAD) model for a prototype device. The device was then produced via additive manufacturing in order to test the novel design against an industry-standard predicate device. Flow distribution was verifiably homogenized and residence time reduced, promising a more efficient performance and increased resistance to thrombosis. This work shows the promising extent to which TPMS can serve as a new building block for exchange processes in medical devices.

Three-dimensional membranes for artificial lungs: Comparison of flow-induced hemolysis

BACKGROUND

Membranes based on triply periodic minimal surfaces (TPMS) have proven a superior gas transfer compared to the contemporary hollow fiber membrane (HFM) design in artificial lungs. The improved oxygen transfer is attributed to disrupting the laminar boundary layer adjacent to the membrane surface known as main limiting factor to mass transport. However, it requires experimental proof that this improvement is not at the expense of greater damage to the blood. Hence, the aim of this work is a valid statement regarding the structure-dependent hemolytic behavior of TPMS structures compared to the current HFM design.

METHODS

Hemolysis tests were performed on structure samples of three different kind of TPMS-based designs (Schwarz-P, Schwarz-D and Schoen's Gyroid) in direct comparison to a hollow fiber structure as reference.

RESULTS

The results of this study suggest that the difference in hemolysis between TPMS membranes compared to HFMs is small although slightly increased for the TPMS membranes. There is no significant difference between the TPMS structures and the hollow fiber design. Nevertheless, the ratio between the achieved additional oxygen transfer and the additional hemolysis favors the TPMS-based membrane shapes.

CONCLUSION

TPMS-shaped membranes offer a safe way to improve gas transfer in artificial lungs.

In-Vitro Visualization of Thrombus Growth in Artificial Lungs Using Real-Time X-Ray Imaging: A Feasibility Study

PURPOSE

Extracorporeal membrane oxygenation has gained increasing attention in the treatment of patients with acute and chronic cardiopulmonary and respiratory failure. However, clotting within the oxygenators or other components of the extracorporeal circuit remains a major complication that necessitates at least a device exchange and bears risks of adverse events for the patients. In order to better predict thrombus growth within oxygenators, we present an approach for in-vitro visualization of thrombus growth using real-time X-ray imaging.

METHODS

An in-vitro test setup was developed using low-dose anticoagulated ovine blood and allowing for thrombus growth within 4 h. The setup was installed in a custom-made X-ray setup that uses phase-contrast for imaging, thus providing enhanced soft-tissue contrast, which improves the differentiation between blood and potential thrombus growth. During experimentation, blood samples were drawn for the analysis of blood count, activated partial thromboplastin time and activated clotting time. Additionally, pressure and flow data was monitored and a full 360° X-ray scan was performed every 15 min.

RESULTS

Thrombus formation indicated by a pressure drop and changing blood parameters was monitored in all three test devices. Red and white thrombi (higher/lower attenuation, respectively) were successfully segmented in one set of X-ray images.

CONCLUSION

We showed the feasibility of a new in-vitro method for real-time thrombus growth visualization by means of phase contrast X-ray imaging. In addition, with more blood parameters that are clinically relevant, this approach might contribute to improved oxygenator exchange protocols in the clinical routine.

Introducing 3D-potting: a novel production process for artificial membrane lungs with superior blood flow design

Currently, artificial-membrane lungs consist of thousands of hollow fiber membranes where blood flows around the fibers and gas flows inside the fibers, achieving diffusive gas exchange. At both ends of the fibers, the interspaces between the hollow fiber membranes and the plastic housing are filled with glue to separate the gas from the blood phase. During a uniaxial centrifugation process, the glue forms the “potting.” The shape of the cured potting is then determined by the centrifugation process, limiting design possibilities and leading to unfavorable stagnation zones associated with blood clotting. In this study, a new multiaxial centrifugation process was developed, expanding the possible shapes of the potting and allowing for completely new module designs with potentially superior blood flow guidance within the potting margins. Two-phase simulations of the process in conceptual artificial lungs were performed to explore the possibilities of a biaxial centrifugation process and determine suitable parameter sets. A corresponding biaxial centrifugation setup was built to prove feasibility and experimentally validate four conceptual designs, resulting in good agreement with the simulations. In summary, this study shows the feasibility of a multiaxial centrifugation process allowing greater variety in potting shapes, eliminating inefficient stagnation zones and more favorable blood flow conditions in artificial lungs.

Graphic abstract

Biologically Inspired, Open, Helicoid Impeller Design for Mechanical Circulatory Assist

Rotating impeller actuated by electromagnet has been a key technological innovation which surpassed earlier limitations of pulsatile pumps. Current impeller design, however, is alien to the functional unit of the human circulatory system and remains a potential cause of adverse prothrombotic events such as hemolysis or pump thrombosis by forcing blood cells to pass over a narrow space available within the rapidly alternating blades attached along its central hub, creating fundamentally a nonphysiologic flow, especially for miniaturized percutaneous blood pumps. Here, we present a biologically inspired, open, helicoid (BiO-H) impeller design for a circulatory assist device that has a fundamentally different footprint from the conventional Archimedean screw-based impeller designs by implementing new design features inspired by an avian right atrioventricular valve. Design parameters including an inner diameter, helix height, overall height, helix revolutions/pitch, blade length, blade thickness, introductory blade angle, number of blades, and blade shape were optimized for maximum output volumetric flow rate through the parametric analysis in computational fluid dynamics simulation. BiO-H shows an improved flow path with 2.25-fold less cross-sectional area loss than the conventional impeller designs. BiO-H with a diameter of 15 mm resulted in a maximum flow rate of 25 L/min at 15,000 revolutions per minute in simulation and showed further improved pressure-flow relationship in benchtop experiments. The design shows promise in increasing flow and could serve as a new impeller design for future blood pumps.

EndOxy: Dynamic Long-Term Evaluation of Endothelialized Gas Exchange Membranes for a Biohybrid Lung

In the concept of a biohybrid lung, endothelial cells seeded on gas exchange membranes form a non-thrombogenic an anti-inflammatory surface to overcome the lacking hemocompatibility of today’s oxygenators during extracorporeal membrane oxygenation. To evaluate this concept, the long-term stability and gas exchange performance of endothelialized RGD-conjugated polydimethylsiloxane (RGD-PDMS) membranes was evaluated. Human umbilical vein endothelial cells (ECs) were cultured on RGD-PDMS in a model system under physiological wall shear stress (WSS) of 0.5 Pa for up to 33 days. Gas exchange performance was tested with three biological replicates under elevated WSS of 2.5 Pa using porcine blood adjusted to venous values following ISO 7199 and blood gas analysis. EC morphology was assessed by immunocytochemistry (n = 3). RGD-PDMS promoted endothelialization and stability of endothelialized membranes was shown for at least 33 days and for a maximal WSS of 2.5 Pa. Short-term exposure to porcine blood did not affect EC integrity. The gas transfer tests provided evidence for the oxygenation and decarboxylation of the blood across endothelialized membranes with a decrease of transfer rates over time that needs to be addressed in further studies with larger sample sizes. Our results demonstrate the general suitability of RGD-PDMS for biohybrid lung applications, which might enable long-term support of patients with chronic lung failure in the future.

Low-flow assessment of current ECMO/ECCO2R rotary blood pumps and the potential effect on hemocompatibility

Background

Extracorporeal carbon dioxide removal (ECCO₂R) uses an extracorporeal circuit to directly remove carbon dioxide from the blood either in lieu of mechanical ventilation or in combination with it. While the potential benefits of the technology are leading to increasing use, there are very real risks associated with it. Several studies demonstrated major bleeding and clotting complications, often associated with hemolysis and poorer outcomes in patients receiving ECCO₂R. A better understanding of the risks originating specifically from the rotary blood pump component of the circuit is urgently needed.

Methods

High-resolution computational fluid dynamics was used to calculate the hemodynamics and hemocompatibility of three current rotary blood pumps for various pump flow rates.

Results

The hydraulic efficiency dramatically decreases to 5–10% if operating at blood flow rates below 1 L/min, the pump internal flow recirculation rate increases 6–12-fold in these flow ranges, and adverse effects are increased due to multiple exposures to high shear stress. The deleterious consequences include a steep increase in hemolysis and destruction of platelets.

Conclusions

The role of blood pumps in contributing to adverse effects at the lower blood flow rates used during ECCO₂R is shown here to be significant. Current rotary blood pumps should be used with caution if operated at blood flow rates below 2 L/min, because of significant and high recirculation, shear stress, and hemolysis. There is a clear and urgent need to design dedicated blood pumps which are optimized for blood flow rates in the range of 0.5–1.5 L/min.

Physiological and Technical Considerations of Extracorporeal CO2 Removal

This article is one of ten reviews selected from the Annual Update in Intensive Care and Emergency Medicine 2019. Other selected articles can be found online at https://www.biomedcentral.com/collections/annualupdate2019. Further information about the Annual Update in Intensive Care and Emergency Medicine is available from http://www.springer.com/series/8901.

Towards a Biohybrid Lung Assist Device: N-Acetylcysteine Reduces Oxygen Toxicity and Changes Endothelial Cells' Morphology

The development of an endothelialized membrane oxygenator requires solution strategies combining the knowledge of oxygenators with endothelial cells' biology. Since it is well known that exposing cells towards pure oxygen causes oxidative stress, this aspect has to be taken into account in the development of a biohybrid oxygenator system. N-Acetylcysteine (NAC) is known for its antioxidant properties in cells. We tested its applicability for the development of an endothelialized oxygenator model. Cultivating human umbilical vein derived endothelial cells (HUVEC) up to 6 days with increasing concentrations of NAC from 1 to 30 mM revealed NAC toxicity at concentrations from 20 mM. Cell density clearly decreased after radical oxygen species exposure in non-NAC pretreated cells compared to 20 mM NAC precultured HUVEC after 3 and 6 days. Also the survival rate after ROS treatment could be restored by incubation with NAC from 15 to 25 mM for all time points. NAC treated cells changed their morphology from typical endothelial cells' cobblestone pattern to a fusiform, elongated configuration. Transformed cells were still positive for typical endothelial cell markers. Our present results show the potential of NAC for the protection of an endothelial cell layer in an endothelialized membrane oxygenator due to its antioxidative properties. Moreover, NAC induces a morphological change in HUVEC similar to dynamic cultivation procedures.