Structure-dependent gas transfer performance of 3D-membranes for artificial membrane lungs

Tailored 3D Lattice Microstructures for Enhanced Functionality in Blood-Gas Exchange

Current membrane oxygenators for extracorporeal life support (ECLS) are facing their limits regarding gas exchange efficiency and long-term stability. One aspect adding to these limitations is inhomogeneous blood flow distribution inside the oxygenator's membrane structure. Triply periodic minimal surface (TPMS) lattice structures are proposed to provide increased mass transfer efficiency and local adaptability introducing heterogeneous properties. However, the adaptation of these structures for blood flow, as in ECLS, is challenging as a hemocompatible flow distribution must be established. In this study, this study proposes a novel method for the smooth, multi-scale modification of TPMS lattice structures creating a tailored flow distribution suited for blood-gas exchange. It implements this method into an automatic structure optimization within an oxygenator. After manufacturing prototypes, it experimentally evaluate the 3D flow distribution using time-resolved, contrast enhanced computed tomography comparing the optimized structure to reference geometries. The TPMS structure modification provides a significant change in flow distribution, improving homogeneity by up to 12%. The approach to creating tailored 3D TPMS lattice structures can be directly transferred to various other applications in the field of heat and mass transfer to enhance functionality, e.g., for heat exchangers or membrane contactors.

CFD Two-Phase Blood Model Predicting the Hematocrit Heterogeneity Inside Fiber Bundles of Blood Oxygenators

PURPOSE

Blood is commonly treated as single-phase homogeneous fluid in numerical simulations of blood flow within fiber bundles of blood oxygenators. However, microfluidics tests revealed the presence of hematocrit heterogeneity in blood flowing across such geometries. Given the significant role of red blood cells (RBCs) in the oxygenation process, this study aims to propose a multiphase blood model able to correctly describe the experimental evidence and computationally investigate hematocrit heterogeneities inside fiber bundles.

METHODS

The experimental results of microfluidics tests performed in a previous study were processed and based on quantitative data of image intensity, a two-phase blood model following the Eulerian-Eulerian approach was calibrated and evaluated in its predictive ability against the experimental data. The two-phase model was then used to study the RBCs distribution inside different fiber bundles at average hematocrit values of 25% and 35%, representative of hemodilution in extracorporeal blood circulation.

RESULTS

The numerical model proved to be able to describe and predict the experimental phase separation between plasma and RBCs within the microchannel geometry at different test conditions. Moreover, blood flow simulation in commercial fiber bundles revealed the presence of specific patterns in hematocrit distribution and their dependence on variations in bundle microstructure.

CONCLUSION

The two-phase blood model proposed in this study provides a tool for advanced evaluation of local fluid dynamics and identification of optimal bundle microstructure allowing further gas transfer simulations to account for a reliable heterogeneous distribution of RBCs around the oxygenating fibers.

Enhanced Hemodynamics of Anisometric TPMS Topology Reduce Blood Clotting in 3D Printed Blood Contactors

Artificial organs, such as extracorporeal membrane oxygenators, dialyzers, and hemoadsorber cartridges, face persistent challenges related to the flow distribution within the cartridge. This uneven flow distribution leads to clot formation and inefficient mass transfer over the device's functional surface. In this work, a comprehensive methodology is presented for precisely integrating triply periodic minimal surfaces (TPMS) into module housings and question whether the internal surface topology determining the flow distribution affects blood coagulation. Three module types are compared with different internal topologies: tubular, isometric, and anisometric TPMS. First, this study includes a computational fluid dynamics (CFD) simulation of the internal hemodynamics, validated through experimental residence time distributions (RTD). Blood tests using human whole blood and subsequent visualization of blood clots by computed tomography, allow the quantification of structure-induced blood clotting. The results indicate that TPMS topologies, particularly anisometric ones, serve as effective flow distributors and significantly reduce and delay blood clotting compared to conventional tubular geometries. For these novel TPMS modules, the inner surfaces can be activated chemically or functionalized to function as a selective adsorption site or biocatalytic surface or made of a permeable material to facilitate mass transfer.

Toward 3D printed microfluidic artificial lungs for respiratory support

Microfluidic artificial lungs (μALs) are a new class of membrane oxygenators. Compared to traditional hollow-fiber oxygenators, μALs closely mimic the alveolar microenvironment due to their size-scale and promise improved gas exchange efficiency, hemocompatibility, biomimetic blood flow networks, and physiologically relevant blood vessel pressures and shear stresses. Clinical translation of μALs has been stalled by restrictive microfabrication techniques that limit potential artificial lung geometries, overall device size, and throughput. To address these limitations, a high-resolution Asiga MAX X27 UV digital light processing (DLP) 3D printer and custom photopolymerizable polydimethylsiloxane (PDMS) resin were used to rapidly manufacture small-scale μALs via vat photopolymerization (VPP). Devices were designed in SOLIDWORKS with 500 blood channels and 252 gas channels, where gas and blood flow channels were oriented orthogonally and separated by membranes on the top and bottom, permitting two-sided gas exchange. Successful devices were post-processed to remove uncured resin from microchannels and assembled with external tubing in preparation for gas exchange performance testing with ovine whole blood. 3D printed channel dimensions were 172 μm-tall × 320 μm-wide, with 62 μm-thick membranes and 124 μm-wide support columns. Measured outlet blood oxygen saturation (SO2) agreed with theoretical models and rated flow of the device was 1 mL min-1. Blood side pressure drop was 1.58 mmHg at rated flow. This work presents the highest density of 3D printed microchannels in a single device, one of the highest CO2 transfer efficiencies of any artificial lung to date, and a promising approach to translate μALs one step closer to the clinic.

Mechanical Properties and Energy Absorption Abilities of Diamond TPMS Cylindrical Structures Fabricated by Selective Laser Melting with 316L Stainless Steel

Triply periodic minimal surfaces (TPMS) are structures inspired by nature with unique properties. Numerous studies confirm the possibility of using TPMS structures for heat dissipation, mass transport, and biomedical and energy absorption applications. In this study, the compressive behavior, overall deformation mode, mechanical properties, and energy absorption ability of Diamond TPMS cylindrical structures produced by selective laser melting of 316L stainless steel powder were investigated. Based on the experimental studies, it was found that tested structures exhibited different cell strut deformation mechanisms (bending-dominated and stretch-dominated) and overall deformation modes (uniform and "layer-by-layer") depending on structural parameters. Consequently, the structural parameters had an impact on the mechanical properties and the energy absorption ability. The evaluation of basic absorption parameters shows the advantage of bending-dominated Diamond TPMS cylindrical structures in comparison with stretch-dominated Diamond TPMS cylindrical structures. However, their elastic modulus and yield strength were lower. Comparative analysis with the author's previous work showed a slight advantage for bending-dominated Diamond TPMS cylindrical structures in comparison with Gyroid TPMS cylindrical structures. The results of this research can be used to design and manufacture more efficient, lightweight components for energy absorption applications in the fields of healthcare, transportation, and aerospace.

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.