Endothelial cell attachment and shear response on biomimetic polymer-coated vascular grafts

Strategies for improving endothelial cell adhesion to blood-contacting medical devices

The endothelium is a critical mediator of homeostasis on blood-contacting surfaces in the body, serving as a selective barrier to regulate processes such as clotting, immune cell adhesion, and cellular response to fluid shear stress. Implantable cardiovascular devices including stents, vascular grafts, heart valves, and left ventricular assist devices are in direct contact with circulating blood and carry a high risk for platelet activation and thrombosis without a stable endothelial cell (EC) monolayer. Development of a healthy endothelium on the blood-contacting surface of these devices would help ameliorate risks associated with thrombus formation and eliminate the need for long-term anti-platelet or anti-coagulation therapy. Although ECs have been seeded onto or recruited to these blood-contacting surfaces, most ECs are lost upon exposure to shear stress due to circulating blood. Many investigators have attempted to generate a stable EC monolayer by improving EC adhesion using surface modifications, material coatings, nanofiber topology, and modifications to the cells. Despite some success with enhanced EC retention in vitro and in animal models, no studies to date have proven efficacious for routinely creating a stable endothelium in the clinical setting. This review summarizes past and present techniques directed at improving the adhesion of ECs to blood-contacting devices.

Evaluation of human umbilical vein endothelial cells growth onto heparin-modified electrospun vascular grafts

One of the main challenges of cardiovascular tissue engineering is the development of bioresorbable and compliant small-diameter vascular grafts (SDVG) for patients where autologous grafts are not an option. In this work, electrospun bilayered bioresorbable SDVG based on blends of poly(L-lactic acid) (PLLA) and segmented polyurethane (PHD) were prepared and evaluated. The inner layer of these SDVG was surface-modified with heparin, following a methodology involving PHD urethane functional groups. Heparin was selected as anticoagulant agent, and also due to its ability to promote human umbilical vein endothelial cells (HUVECs) growth and to inhibit smooth muscle cells over-proliferation, main cause of neointimal hyperplasia and restenosis. Immobilized heparin was quantified and changes in SDVG microstructure were investigated through SEM. Tensile properties of the heparin-functionalized SDVG resembled those of saphenous vein. Vascular grafts were seeded with HUVECs and cultured on a flow-perfusion bioreactor to analyze the effect of heparin on graft endothelization under simulated physiological-like conditions. The analysis of endothelial cells attachment and gene expression (Real-Time PCR) pointed out that the surface functionalization with heparin successfully promoted a stable and functional endothelial cell layer.

An in Vitro Flow Model for Cardiovascular Inflammation

Cardiovascular disease is modern-day plague with a vast number of lives claimed, and an enormous socio-economic cost incurred. Hemodynamics of the cardiovascular system play an important mechanistic role in disease development. For instance, atherosclerotic plaque depositions are often correlated with regions of turbulent flow patterns and disturbed hemodynamic shear stress. A simplified, rigid, in vitro, flow model of a real-size aortic arch is described. The flow in the arched vessel is attached and healthy at the outer curvature, while it is separated and disturbed at the inner curvature wall, which is an ideal setting to study cardiovascular disease. Endothelial cells can be cultured on the lumen of the aortic arch model under controlled flow conditions and extracted from the inner and outer curvature walls for biochemical signaling studies. The flow velocity field in the model is characterized using particle image velocimetry PIV which allows for the estimation of the wall shear stress. This helps in correlating the underlying hemodynamics to the biomechanical response of the endothelium.

Shear-resistant hydrogels to control permeability of porous tubular scaffolds in vascular tissue engineering

Aiming to perfuse porous tubular scaffolds for vascular tissue engineering (VTE) with controlled flow rate, prevention of leakage through the scaffold lumen is required. A gel coating made of 8% w/v alginate and 6% w/v gelatin functionalized with fibronectin was produced using a custom-made bioreactor-based method. Different volumetric proportions of alginate and gelatin were tested (50/50, 70/30, and 90/10). Gel swelling and stability, and rheological, and uniaxial tensile tests reveal superior resistance to the aggressive biochemical microenvironment, and their ability to withstand physiological deformations (~10%) and wall shear stresses (5-20 dyne/cm²). These are prerequisites to maintain the physiologic phenotypes of vascular smooth muscle cells and endothelial cells (ECs), mimicking blood vessels microenvironment. Gels can induce ECs proliferation and colonization, especially in the presence of fibronectin and higher percentages of gelatin. The custom-designed bioreactor enables the development of reproducible and homogeneous tubular gel coating. The permeability tests show the effectiveness of tubular scaffolds coated with 70/30 alginate/gelatin gel to occlude wadding pores, and therefore prevent leakages. The synthesized double-layered tubular scaffolds coated with alginate/gelatin gel and fibronectin represent both promising substrate for ECs and effective leakproof scaffolds, when subjected to pulsatile perfusion, for VTE applications.

A Novel Pulse Damper for Endothelial Cell Flow Bioreactors

PURPOSE

Peristaltic pumps (PP) are favored in flow bioreactors for their non-contact sterile design. But they produce pulsatile flow, which is consequential for the cultured cells. A novel pulse damper (PD) is reported for pulsatility elimination.

METHODS

The PD design was implemented to target static pressure pulsatility and flow rate (velocity) pulsatility from a PP. Damping effectiveness was tested in a macro-scale, closed-loop recirculating bioreactor mimicking the aortic arch at flow rates up to (4 L/min). Time-resolved particle image velocimetry was used to characterize the velocity field. Endothelial cells (EC) were grown in the bioreactor, and subjected to continuous flow for 15 min with or without PD.

RESULTS

The PD was found to be nearly 90% effective at reducing pulsatility. The EC exposed to low PP flow without PD exhibited distress signaling in the form of increased ERK1/2 phosphorylation (2.5 folds) when compared to those exposed to the same flow with PD. At high pump flow without PD, the cells detached and did not survive, while they were perfectly healthy with PD.

CONCLUSIONS

Flow pulsatility from PP causes EC distress at low flow and cell detachment at high flow. Elevated temporal shear stress gradient combined with elevated shear stress magnitude at high flow are believed to be the cause of cell detachment and death. The proposed PD design was effective at minimizing the hemodynamic stressors in the pump's output, demonstrably reducing cell distress. Adoption of the proposed PD design in flow bioreactors should improve experimental protocols.

Development and in vivo validation of tissue-engineered, small-diameter vascular grafts from decellularized aortae of fetal pigs and canine vascular endothelial cells

Background

Tissue engineering has emerged as a promising alternative for small-diameter vascular grafts. The aim of this study was to determine the feasibility of using decellularized aortae of fetal pigs (DAFPs) to construct tissue-engineered, small-diameter vascular grafts and to test the performance and application of DAFPs as vascular tissue-engineered scaffolds in the canine arterial system.

Methods

DAFPs were prepared by continuous enzymatic digestion. Canine vascular endothelial cells (ECs) were seeded onto DAFPs in vitro and then the vascular grafts were cultured in a custom-designed vascular bioreactor system for 7 days of dynamic culture following 3 days of static culture. The grafts were then transplanted into the common carotid artery of the same seven dogs from which ECs had been derived (two grafts were prepared for each dog with one as a backup; therefore, a total of 14 tissue-engineered blood vessels were prepared). At 1, 3, and 6 months post-transplantation, ultrasonography and contrast-enhanced computed tomography (CT) were used to check the patency of the grafts. Additionally, vascular grafts were sampled for histological and electron microscopic examination.

Results

Tissue-engineered, small-diameter vascular grafts can be successfully constructed using DAFPs and canine vascular ECs. Ultrasonographic and CT test results confirmed that implanted vascular grafts displayed good patency with no obvious thrombi. Six months after implantation, the grafts had been remodeled and exhibited a similar structure to normal arteries. Immunohistochemical staining showed that cells had evenly infiltrated the tunica media and were identified as muscular fibroblasts. Scanning electron microscopy showed that the graft possessed a complete cell layer, and the internal cells of the graft were confirmed to be ECs by transmission electron microscopy.

Conclusions

Tissue-engineered, small-diameter vascular grafts constructed using DAFPs and canine vascular ECs can be successfully transplanted to replace the canine common carotid artery. This investigation potentially paves the way for solving a problem of considerable clinical need, i.e., the requirement for small-diameter vascular grafts.

Blood-Vessel Mimicking Structures by Stereolithographic Fabrication of Small Porous Tubes Using Cytocompatible Polyacrylate Elastomers, Biofunctionalization and Endothelialization

Blood vessel reconstruction is still an elusive goal for the development of in vitro models as well as artificial vascular grafts. In this study, we used a novel photo-curable cytocompatible polyacrylate material (PA) for freeform generation of synthetic vessels. We applied stereolithography for the fabrication of arbitrary 3D tubular structures with total dimensions in the centimeter range, 300 µm wall thickness, inner diameters of 1 to 2 mm and defined pores with a constant diameter of approximately 100 µm or 200 µm. We established a rinsing protocol to remove remaining cytotoxic substances from the photo-cured PA and applied thio-modified heparin and RGDC-peptides to functionalize the PA surface for enhanced endothelial cell adhesion. A rotating seeding procedure was introduced to ensure homogenous endothelial monolayer formation at the inner luminal tube wall. We showed that endothelial cells stayed viable and adherent and aligned along the medium flow under fluid-flow conditions comparable to native capillaries. The combined technology approach comprising of freeform additive manufacturing (AM), biomimetic design, cytocompatible materials which are applicable to AM, and biofunctionalization of AM constructs has been introduced as BioRap(®) technology by the authors.

In vivo evaluation of biomimetic fluorosurfactant polymer-coated expanded polytetrafluoroethylene vascular grafts in a porcine carotid artery bypass model

OBJECTIVE

The objective of this study was to evaluate the potential for biomimetic self-assembling fluorosurfactant polymer (FSP) coatings incorporating heptamaltose (M7-FSP) to block nonspecific protein adsorption, the cell adhesive RGD peptide (RGD-FSP), or the endothelial cell-selective CRRETAWAC peptide (cRRE-FSP) to improve patency and endothelialization in small-diameter expanded polytetrafluoroethylene (ePTFE) vascular graft implants.

METHODS

ePTFE vascular grafts (4 mm in diameter, 5 cm in length) were coated with M7-FSP, RGD-FSP, or cRRE-FSP by dissolving FSPs in distilled water and flowing solution through the graft lumen for 24 hours. Coatings were confirmed by receding water contact angle measurements on the lumen surface. RGD-FSP and cRRE-FSP grafts were presodded in vitro with porcine pulmonary artery endothelial cells (PPAECs) using a custom-designed flow system. PPAEC coverage on the lumen surface was visualized with epifluorescent microscopy and quantified. Grafts were implanted as carotid artery interposition bypass grafts in seven pigs for 33 ± 2 days (ePTFE, n = 3; M7-FSP, n = 4; RGD-FSP, n = 3; cRRE-FSP, n = 4). Patency was confirmed immediately after implantation with duplex color flow ultrasound and at explantation with contrast-enhanced angiography. Grafts were sectioned for histology and stained: Movat pentachrome stain to outline vascular layers, immunofluorescent staining to identify endothelial cells (anti-von Willebrand factor antibody), and immunohistochemical staining to identify smooth muscle cells (anti-smooth muscle α-actin antibody). Neointima to lumen area ratio was determined to evaluate neointimal hyperplasia.

RESULTS

Receding water contact angle measurements on graft luminal surfaces were significantly lower (P < .05) on FSP-coated ePTFE surfaces (M7-FSP, 40 ± 16 degrees; RGD-FSP, 25 ± 10 degrees; cRRE-FSP, 33 ± 16 degrees) compared with uncoated ePTFE (126 ± 2 degrees), confirming presence of the FSP layer. In vitro sodding of PPAECs on RGD-FSP and cRRE-FSP grafts resulted in a confluent monolayer of PPAECs on the luminal surface, with a similar cell population on RGD-FSP (1200 ± 187 cells/mm(2)) and cRRE-FSP (1134 ± 153 cells/mm(2)) grafts. All grafts were patent immediately after implantation, and one of three uncoated, two of three RGD-FSP, two of four M7-FSP, and two of four cRRE-FSP grafts remained patent after 1 month. PPAEC coverage of the lumen surface was seen in all patent grafts. RGD-FSP grafts had a slightly higher neointima to lumen area ratio (0.53 ± 0.06) compared with uncoated (0.29 ± 0.15), M7-FSP (0.20 ± 0.15), or cRRE-FSP (0.17 ± 0.09) grafts.

CONCLUSIONS

Biomimetic FSP-coated ePTFE grafts can be used successfully in vivo and have potential to support endothelialization. Grafts modified with the M7-FSP and cRRE-FSP showed lower intimal hyperplasia compared with RGD-FSP grafts.

Thrombotic responses of endothelial outgrowth cells to protein-coated surfaces

There is significant clinical need for viable small-diameter vascular grafts. While there are many graft biomaterials in development, few have been clinically successful. Evaluation of grafts with a clinically relevant model is needed to drive development. This work examined extracellular matrix coatings on the thrombotic phenotype of endothelial outgrowth cells (EOCs). EOCs were tested on flat plates and tubular grafts. Flat plate studies examined collagen I, collagen IV, fibronectin and α-elastin coatings. EOCs attached or proliferated more readily on collagen I and fibronectin surfaces as determined by total DNA. The production of activated protein C (APC) by EOCs was also dependent on the surface coating, with collagen I and fibronectin displaying a higher activity than both collagen IV and α-elastin on flat plate studies. Based on these results, only collagen I and fibronectin coatings were tested on expanded polytetrafluoroethylene (ePTFE) in the ex vivo model. Tubular samples showed significantly greater tissue factor pathway inhibitor gene expression on collagen I than on fibronectin. Platelet adhesion was not significantly different, but EOCs on collagen I produced significantly lower APC than on fibronectin, suggesting that differences exist between the flat plate and tubular cultures. Overall, while the hemostatic phenotype of EOCs displayed some differences, cell responses were largely independent of the matrix coating. EOCs adhered strongly to both fibronectin- and collagen-I-coated ePTFE grafts under ex vivo (100 ml/min) flow conditions suggesting the usefulness of this clinically relevant cell source, testing modality, and shunt model for future work examining biomaterials and cell conditioning before implantation.

Oxygen control with microfluidics

Cellular function and behavior are affected by the partial pressure of O2, or oxygen tension, in the microenvironment. The level of oxygenation is important, as it is a balance of oxygen availability and oxygen consumption that is necessary to maintain normoxia. Changes in oxygen tension, from above physiological oxygen tension (hyperoxia) to below physiological levels (hypoxia) or even complete absence of oxygen (anoxia), trigger potent biological responses. For instance, hypoxia has been shown to support the maintenance and promote proliferation of regenerative stem and progenitor cells. Paradoxically, hypoxia also contributes to the development of pathological conditions including systemic inflammatory response, tumorigenesis, and cardiovascular disease, such as ischemic heart disease and pulmonary hypertension. Current methods to study cellular behavior in low levels of oxygen tension include hypoxia workstations and hypoxia chambers. These culture systems do not provide oxygen gradients that are found in vivo or precise control at the microscale. Microfluidic platforms have been developed to overcome the inherent limits of these current methods, including lack of spatial control, slow equilibration, and unachievable or difficult coupling to live-cell microscopy. The various applications made possible by microfluidic systems are the topic of this review. In order to understand how the microscale can be leveraged for oxygen control of cells and tissues within microfluidic systems, some background understanding of diffusion, solubility, and transport at the microscale will be presented in addition to a discussion on the methods for measuring the oxygen tension in microfluidic channels. Finally the various methods for oxygen control within microfluidic platforms will be discussed including devices that rely on diffusion from liquid or gas, utilizing on-or-off-chip mixers, leveraging cellular oxygen uptake to deplete the oxygen, relying on chemical reactions in channels to generate oxygen gradients in a device, and electrolytic reactions to produce oxygen directly on chip.

Chemical activation and changes in surface morphology of poly(ε-caprolactone) modulate VEGF responsiveness of human endothelial cells

The high degree of clinical routine in percutaneous transluminal coronary angioplasty (PTCA) with and without stenting has not changed the fact that a large number of coronary heart disease patients are still affected by post-operative complications such as restenosis and thrombosis. Because re-endothelialization is the crucial aspect of wound healing after cardiovascular implant surgery, there is a need for modern biomaterials to aid endothelial cells in their adhesion and functional recovery post-stenting. This study systematically examines the potential of numerous chemical polymer modifications with regard to endothelialization. Poly(ε-caprolactone) (PCL) and its chemically activated forms are investigated in detail, as well as the impact of polymer surface morphology and precoating with matrix protein. Human umbilical vein endothelial cells (HUVECs) are used to characterize endothelial cell responses in terms of in vitro viability and adhesion. As a potential component in drug eluting implants, VEGF is applied as stimulus to boost endothelial cell proliferation on the polymer. In conclusion, plasma chemical activation of PCL combined with VEGF stimulation best enhances in vitro endothelialization. Examining the impact of morphological, chemical and biological modifications of PCL, this study makes an important new contribution towards the existing body of work on polymer endothelialization.

Late-outgrowth endothelial progenitors from patients with coronary artery disease: endothelialization of confluent stromal cell layers

Patients with coronary artery disease (CAD) are the primary candidates to receive small-diameter tissue-engineered blood vessels (TEBVs). Peripheral blood derived endothelial progenitor cells (EPCs) from CAD patients (CAD EPCs) represent a minimally invasive source of autologous cells for TEBV endothelialization. We have previously shown that human CAD EPCs are highly proliferative and express many of the hallmarks of mature and healthy endothelial cells; however, their behavior on stromal cells that comprise the media of TEBVs has not yet been evaluated. Primary CAD EPCs or control human aortic endothelial cells (HAECs) were seeded over confluent, quiescent layers of human smooth muscle cells (SMCs) using a direct co-culture model. The percent coverage, adhesion strength, alignment under flow and generation of flow-induced nitric oxide of the seeded CAD EPCs were compared to that of HAECs. The integrin-binding profile of CAD EPCs was also evaluated over a layer of confluent, quiescent SMCs. Direct comparison of our CAD EPC results to analogous co-culture studies with cord blood EPCs show that both types of blood-derived EPCs are viable options for endothelialization of TEBVs.

Synthetic ePTFE grafts coated with an anti-CD133 antibody-functionalized heparin/collagen multilayer with rapid in vivo endothelialization properties

An anti-CD133 antibody multilayer functionalized by heparin/collagen on an expanded polytetrafluoroethylene (ePTFE) graft was developed to accelerate early endothelialization. The surface modification of ePTFE grafts demonstrated that the multilayer is stable in static incubation and shaking conditions and that the anti-CD133 antibodies were successfully cross-linked onto the surface. Blood compatibility tests revealed that the coimmobilized heparin/collagen films in the presence or absence of anti-CD133 antibodies prolonged the blood coagulation time and that there was less platelet activation and aggregation, whereas the hemolysis rate was comparable with the bare ePTFE grafts. Cellular proliferation was not inhibited, as the heparin/collagen synthetic vascular grafts coated with CD133 antibody showed little cytotoxicity. The endothelial cells adhered well to the modified ePTFE grafts during a cell adhesion assay. A porcine carotid artery transplantation model was used to evaluate the modified ePTFE grafts in vivo. The results of histopathological staining and scanning electron microscopy indicated that the anti-CD133 antibody was able to accelerate the attachment of vascular endothelial cells onto the ePTFE grafts, resulting in early rapid endothelialization. The success of the anti-CD133 antibody-functionalized heparin/collagen multilayer will provide an effective selection system for the surface modification of synthetic vascular grafts and improve their use in clinical applications.

The "artificial artery" as in vitro perfusion model

Metabolic stimuli, pressure, and fluid shear stress (FSS) are major mediators of vascular plasticity. The exposure of the vessel wall to increased laminar FSS is the main trigger of arteriogenesis, the remodelling of pre-existent arterio-arteriolar anastomoses to functional conductance arteries. In this study, we have used an in vitro bioreactor to investigate cell-specific interactions, molecular mechanisms as well as time-dependent effects under laminar FSS conditions. This bioreactor termed “artificial artery” can be used for screening potential arterio-protective substances, pro-arteriogenic factors, and for investigating biomarkers of cardiovascular diseases such as cardiac diseases. The bioreactor is built up out of 14 hollow fiber membranes colonized with endothelial cells (HUVECs) on the inside and smooth muscle cells (HUASMCs) on the outside. By means of Hoechst 33342 staining as well as immunocytochemistry of ß-catenin and α-smooth-muscle-actin, a microporous polypropylene membrane was characterized as being the appropriate polymer for co-colonization. Defined arterial flow conditions (0.1 N/m2 and 3 N/m2), metabolic exchange, and cross-talk of HUVECs and HUASMCs through hollow fibers mimic physiological in vivo conditions of the vasculature. Analysing mono- and co-culture secretomes by MALDI-TOF-TOF mass spectrometry, we could show that HUVECs secreted Up4A upon 3 N/m2. A constant cellular secretion of randomly chosen peptides verified viability of the “artificial artery” for a cultivation period up to five days. qRT-PCR analyses revealed an up-regulation of KLF2 and TIMP1 as mechano-regulated genes and demonstrated arterio-protective, homeostatic FSS conditions by a down-regulation of EDN1. Expression analyses of VWF and EDN1 furthermore confirmed that RNA of both cell types could separately be isolated without cross-contamination. CCND1 mRNA expression in HUVECs did not change upon FSS indicating a quiescent endothelial phenotype. Taken together, the “artificial artery” provides a solid in vitro model to test pharmacological active compounds for their impact on arterio-damaging or arterio-protective properties on vascular response.