Towards a biocompatible artificial lung: Covalent functionalization of poly(4-methylpent-1-ene) (TPX) with cRGD pentapeptide

Biohybrid lung Development: Towards Complete Endothelialization of an Assembled Extracorporeal Membrane Oxygenator

Towards the establishment of a long-term lung-assist device to be used both as a bridge and as an alternative to lung transplantation according to final destination therapy, we develop the biohybrid lung (BHL) on the technical basis of contemporary extracorporeal membrane oxygenation (ECMO). Here, to overcome the significant drawbacks of ECMO, in particular the missing hemocompatibility of the artificial surfaces, all blood-contacting areas need to be endothelialized sufficiently. In continuation of our recent accomplishments, demonstrating the feasibility of establishing a physiological acting endothelial cell (EC) monolayer on the hollow fiber membranes (HFMs) of the ECMO in vitro, the next step towards BHL translation is the endothelialization of the complete oxygenator, consisting of HFMs and the surrounding housing. Therefore, we assessed EC seeding inside our model oxygenator (MOx), which simulated the conditions in the assembled HFM oxygenators in order to identify the most important factors influencing efficient endothelialization, such as cell seeding density, cell distribution, incubation time and culture medium consumption. Overall, upon adjusting the concentration of infused ECs to 15.2 × 10⁴/cm² and ensuring optimal dispersion of cells in the MOx, viable and confluent EC monolayers formed on all relevant surfaces within 24 h, even though they comprised different polymers, i.e., the fibronectin-coated HFMs and the polysulfone MOx housing. Periodic medium change ensured monolayer survival and negligible apoptosis rates comparable to the reference within the assembled system. By means of these results, revealing essential implications for BHL development, their clinical translation is coming one step closer to reality.

Improving hemocompatibility of artificial lungs by click conjugation of glycoengineered endothelial cells onto blood-contacting surfaces

Artificial lungs, also known as oxygenators, allow adequate oxygenation of the blood in patients with severe respiratory failure and enable patient survival. However, the insufficient hemocompatibility of the current of artificial lungs hampers their long-term use. Therefore, in this study, a novel strategy was developed to efficiently endothelialize blood-contacting surfaces to improve their hemocompatibility. Hollow fiber membranes (HFMs) were functionalized with dibenzylcyclooctyne (DBCO), and endothelial cells were glycoengineered for covalent conjugation to DBCO by a copper-free click reaction. Metabolic glycoengineering using azidoacetylmannosamine-tetraacylated (Ac₄ManNAz) resulted in highly efficient functionalization of endothelial cells with azide (N₃) molecules on the cell surface without negative impact on cell viability. After 48 h, significantly improved endothelialization was detected on the HFM surfaces functionalized with DBCO compared to unmodified HFMs. Endothelial cells were responsive to inflammatory stimulus and expressed adhesion-promoting molecules (E-selectin, VCAM-1, and ICAM-1). Furthermore, the hemocompatibility of HFMs was analyzed by dynamic incubation with fresh human blood. DBCO-coated and uncoated HFMs showed a comparable hemocompatibility, but the endothelialization of HFMs significantly reduced the activation of blood coagulation and platelets. Interestingly, the incubation of endothelialized HFMs with human blood further reduced the expression of E-selectin and VCAM-1 in endothelial cells. In this study, a highly efficient, cell-compatible method for endothelialization of artificial lungs was established. This click chemistry-based method can be also applied for the endothelialization of other artificial surfaces for tissue engineering and regenerative medicine applications.

Towards Biohybrid Lung Development-Fibronectin-Coating Bestows Hemocompatibility of Gas Exchange Hollow Fiber Membranes by Improving Flow-Resistant Endothelialization

To provide an alternative treatment option for patients with end-stage lung disease, we aim for biohybrid lung development (BHL) based on hollow fiber membrane (HFM) technology used in extracorporeal membrane oxygenators. For long-term BHL application, complete hemocompatibility of all blood-contacting surfaces is indispensable and can be achieved by their endothelialization. Indeed, albumin/heparin (AH) coated HFM enables initial endothelialization, but as inexplicable cell loss under flow conditions was seen, we assessed an alternative HFM coating using fibronectin (FN). Therefore, endothelial cell (EC) adherence and viability on both coated HFM were analyzed by fluorescence-based staining. Functional leukocyte and thrombocyte adhesion assays were performed to evaluate hemocompatibility, also in comparison to blood plasma coated HFM as a clinically relevant control. To assess monolayer resistance and EC behavior under clinically relevant flow conditions, a mock circulation setup was established, which also facilitates imitation of lung-disease specific blood gas settings. Besides quantification of flow-associated cell loss, endothelial responses towards external stimuli, like flow exposure or TNFα stimulation, were analyzed by qRT-PCR, focusing on inflammation, thrombus formation and extracellular matrix production. Under static conditions, both coated HFM enabled the generation of a viable, confluent, non-inflammatory and anti-thrombogenic monolayer. However, by means of homogenous FN coating, cell retention and physiologic gene regulation towards an improved hemocompatible-and extracellular matrix producing phenotype, was significantly superior compared to the inhomogeneous AH coating. In summary, our adaptable in-house FN coating secures the endothelial requirements for long-term BHL application and may promote monolayer establishment on all other blood contacting surfaces of the BHL (e.g., cannulae).

Towards Biohybrid Lung: Induced Pluripotent Stem Cell Derived Endothelial Cells as Clinically Relevant Cell Source for Biologization

In order to provide an alternative treatment option to lung transplantation for patients with end-stage lung disease, we aim for the development of an implantable biohybrid lung (BHL), based on hollow fiber membrane (HFM) technology used in extracorporeal membrane oxygenators. Complete hemocompatibility of all blood contacting surfaces is crucial for long-lasting BHL durability and can be achieved by their endothelialization. Autologous endothelial cells (ECs) would be the ideal cell source, but their limited proliferation potential excludes them for this purpose. As induced pluripotent stem cell-derived ECs enable the generation of a large number of ECs, we assessed and compared their capacity to form a viable and confluent monolayer on HFM, while indicating physiologic EC-specific anti-thrombogenic and anti-inflammatory properties. ECs were generated from three different human iPSC lines, and seeded onto fibronectin-coated poly-4-methyl-1-pentene (PMP) HFM. Following phenotypical characterization, ECs were analyzed for their thrombogenic and inflammatory behavior with or without TNFα induction, using FACS and qRT-PCR. Complementary, leukocyte- and platelet adhesion assays were carried out. The capacity of the iPSC-ECs to reendothelialize cell-free monolayer areas was assessed in a scratch assay. ECs sourced from umbilical cord blood (hCBECs) were used as control. iPSC-derived ECs formed confluent monolayers on the HFM and showed the typical EC-phenotype by expression of VE-cadherin and collagen-IV. A low protein and gene expression level of E-selectin and tissue factor was detected for all iPSC-ECs and the hCBECs, while a strong upregulation of these markers was noted upon stimulation with TNFα. This was in line with the physiological and strong induction of leukocyte adhesion detected after treatment with TNFα, iPSC-EC and hCBEC monolayers were capable of reducing thrombocyte adhesion and repopulating scratched areas. iPSCs offer the possibility to provide patient-specific ECs in abundant numbers needed to cover all blood contacting surfaces of the BHL with a viable, non-thrombogenic and non-inflammatory monolayer. iPSC-EC clones can differ in terms of their reendothelialization rate, and pro-inflammatory response. However, a less profound inflammatory response may even be advantageous for BHL application. With the proven ability of the seeded iPSC-ECs to reduce thrombocyte adhesion, we expect that thrombotic events that could lead to BHL occlusion can be avoided, and thus, justifies further studies on enabling BHL long-term application.

Toward a Long-Term Artificial Lung

Only a very small portion of end-stage organ failures can be treated by transplantation because of the shortage of donor organs. Although artificial long-term organ support such as ventricular assist devices provide therapeutic options serving as a bridge-to-transplantation or destination therapy for end-stage heart failure, suitable long-term artificial lung systems are still at an early stage of development. Although a short-term use of an extracorporeal lung support is feasible today, the currently available technical solutions do not permit the long-term use of lung replacement systems in terms of an implantable artificial lung. This is currently limited by a variety of factors: biocompatibility problems lead to clot formation within the system, especially in areas with unphysiological flow conditions. In addition, proteins, cells, and fibrin are deposited on the membranes, decreasing gas exchange performance and thus, limiting long-term use. Coordinated basic and translational scientific research to solve these problems is therefore necessary to enable the long-term use and implantation of an artificial lung. Strategies for improving the biocompatibility of foreign surfaces, for new anticoagulation regimes, for optimization of gas and blood flow, and for miniaturization of these systems must be found. These strategies must be validated by in vitro and in vivo tests, which remain to be developed. In addition, the influence of long-term support on the pathophysiology must be considered. These challenges require well-connected interdisciplinary teams from the natural and material sciences, engineering, and medicine, which take the necessary steps toward the development of an artificial implantable lung.

Regioselectivity of the 1,3-Dipolar Cycloaddition of Organic Azides to 7-Heteronorbornadienes. Synthesis of β-Substituted Furans/Pyrroles

An efficient procedure for the preparation of β-substituted furans/pyrroles is presented. The methodology is based on the use of 7-oxa/azanorbornadienes as dipolarophiles in 1,3-dipolar cycloaddition with benzyl azide. The triazoline cycloadduct thus formed spontaneously decomposes via a retro-Diels-Alder (rDA) reaction to afford a β-substituted furan/pyrrole derivative and a stable triazole. The scope of this tandem 1,3-dipolar cycloaddition/rDA reaction was studied with thirteen 7-heteronorbornadienes. This study allowed a deep knowledge of the regioselectivity of the reaction, which can be tuned through the substituents of the heteronorbornadienic systems.

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.

Endothelialization and characterization of titanium dioxide-coated gas-exchange membranes for application in the bioartificial lung

Fouling on the gas-exchange hollow-fiber membrane (HFM) of extracorporeal membrane oxygenation (ECMO) devices by blood components and pathogens represents the major hurdle to their long-term application in patients with lung deficiency or unstable hemodynamics. Although patients are treated with anticoagulants, deposition of blood proteins onto the membrane surface may still occur after few days, leading to insufficient gas transfer and, consequently, to device failure. The aim of this study was to establish an endothelial cell (EC) monolayer onto the gas-exchange membrane of an ECMO device with a view to developing a hemocompatible bioartificial lung. Poly(4-methyl-1-pentene) (PMP) gas-exchange membranes were coated with titanium dioxide (TiO₂), using the pulsed vacuum cathodic arc plasma deposition (PVCAPD) technique, in order to generate a stable interlayer, enabling cell adhesion onto the strongly hydrophobic PMP membrane. The TiO₂ coating reduced the oxygen transfer rate (OTR) of the membrane by 22%, and it successfully mediated EC attachment. The adhered ECs formed a confluent monolayer, which retained a non-thrombogenic state and showed cell-to-cell, as well as cell-to-substrate contacts. The established monolayer was able to withstand physiological shear stress and possessed a "self-healing" capacity at areas of induced monolayer disruption. The study demonstrated that the TiO₂ coating mediated EC attachment and the establishment of a functional EC monolayer.

STATEMENT OF SIGNIFICANCE

Surface endothelialization is considered an effective approach to achieve complete hamocompatibility of blood-contacting devices. Several strategies to enable endothelial cell adhesion onto stents and vascular prostheses have already been described in the literature. However, only few studies investigated the feasibility of establishing an endothelial monolayer onto the gas exchange membrane of ECMO devices, using peptides or proteins that were weakly adsorbed via dip coating techniques. This study demonstrated the effectiveness of an alternative and stable titanium dioxide coating for gas-exchange membranes, which enabled the establishment of a confluent, functional and non-activated endothelial monolayer, while maintaining oxygen permeability.

Adding Functions to Biomaterial Surfaces through Protein Incorporation

The concept of biomaterials has evolved from one of inert mechanical supports with a long-term, biologically inactive role in the body into complex matrices that exhibit selective cell binding, promote proliferation and matrix production, and may ultimately become replaced by newly generated tissues in vivo. Functionalization of material surfaces with biomolecules is critical to their ability to evade immunorecognition, interact productively with surrounding tissues and extracellular matrix, and avoid bacterial colonization. Antibody molecules and their derived fragments are commonly immobilized on materials to mediate coating with specific cell types in fields such as stent endothelialization and drug delivery. The incorporation of growth factors into biomaterials has found application in promoting and accelerating bone formation in osteogenerative and related applications. Peptides and extracellular matrix proteins can impart biomolecule- and cell-specificities to materials while antimicrobial peptides have found roles in preventing biofilm formation on devices and implants. In this progress report, we detail developments in the use of diverse proteins and peptides to modify the surfaces of hard biomaterials in vivo and in vitro. Chemical approaches to immobilizing active biomolecules are presented, as well as platform technologies for isolation or generation of natural or synthetic molecules suitable for biomaterial functionalization.

Synthesis and evaluation of the biostability and cell compatibility of novel conjugates of nucleobase, peptidic epitope, and saccharide

This article reports the synthesis of a new class of conjugates containing a nucleobase, a peptidic epitope, and a saccharide and the evalution of their gelation, biostability, and cell compatibility. We demonstrate a facile synthetic process, based on solid-phase peptide synthesis of nucleopeptides, to connect a saccharide with the nucleopeptides for producing the target conjugates. All the conjugates themselves (1-8) display excellent solubility in water without forming hydrogels. However, a mixture of 5 and 8 self-assembles to form nanofibers and results in a supramolecular hydrogel. The proteolytic stabilities of the conjugates depend on the functional peptidic epitopes. We found that TTPV is proteolytic resistant and LGFNI is susceptible to proteolysis. In addition, all the conjugates are compatible to the mammalian cells tested.