Antithrombotic and fibrinolytic system of human endothelial cells seeded on PTFE: the effects of surface modification of PTFE by ammonia plasma treatment and ECM protein coatings

Small-Caliber Vascular Grafts Engineered from Decellularized Leaves and Cross-Linked Gelatin

Despite advances in vascular replacement and repair, fabricating small-diameter vascular grafts with low thrombogenicity and appropriate tissue mechanics remains a challenge. A wide range of platforms have been developed to use plant-derived scaffolds for various applications. Unlike animal tissue, plants are primarily composed of cellulose which can offer a promising, nonthrombogenic alternative capable of promoting cell attachment and redirecting blood flow. By taking advantage of the biocompatibility and mechanical properties of cellulose, we developed small-diameter vascular grafts using decellularized leatherleaf viburnum and cross-linked gelatin. Different terrestrial plant leaves (leatherleaf, spinach, and parsley) were decellularized with sodium dodecyl sulfate, egtazic acid and/or Tergitol, followed by a bleach and Triton X-100 clearing solution, and then evaluated for decellularization efficiency, mechanical integrity, and recellularization potential. Hematoxylin and eosin staining and DNA quantification revealed successful removal of cells in all leatherleaf conditions. Methods of 3D graft fabrication were evaluated, and leatherleaf scaffolds maintained suitable tensile and rupture strength properties. 2D scaffolds and 3D grafts were seeded with rat endothelial cells. Cells remained viable for over 14 days with cell densities comparable to other natural and synthetic scaffolds. This study demonstrates the potential of cost effective and readily available decellularized plants to generate small-diameter vascular grafts capable of recellularization and with suitable mechanical properties. Impact statement Due to the prevalence of coronary heart disease in the United States, small-caliber vascular grafts for coronary bypass surgery are in high demand. We evaluate decellularized plant leaves as potential candidates for small-diameter vascular grafts with appropriate mechanical properties and recellularization potential.

Unexpected exfoliation and activity of nano poly(tetrafluoroethylene) particles from magnetic stir bars: Discovery and implication

Magnetic stir bars are routinely used by most of researchers in the fields of chemistry, biology and environment etc. An incredible phenomenon, in which the magnetic stirring increased reaction rate by tens of times under ultrasound irradiation, impelled us to explore roles of magnetic stirring. Unexpectedly, the thimbleful nano PTFE particles, from shell of magnetic stir bar, were exfoliated during magnetic stirring and account for ultrahigh tribocatalytic and piezocatalytic activities under ultrasonic irradiation. Reactive oxygen species (ROS), such as hydroxyl radical (OH), superoxide radicals (O₂^-) and singlet oxygen (¹O₂) were generated in the present of PTFE under ultrasound irradiation, which is desired in the pollution control. The newly discovered PTFE activity, against the conventional wisdom that PTFE is inert, which also reminds the researchers that the trace amount of PTFE ground during magnetic stirring may inadvertently botch our experiments and introduce false positive results, especially involving routine magnetic stirring and ultrasound irradiation operation in laboratory. In addition, the safety and inertness of PTFE may require further review in PTFE-based commercial, industrial and biomedical settings.

Surface Zwitterionization of Expanded Poly(tetrafluoroethylene) via Dopamine-Assisted Consecutive Immersion Coating

Expanded polytetrafluoroethylene (ePTFE) is one of the materials widely used in the biomedical field, yet its application is being limited by adverse reactions such as thrombosis when it comes in contact with blood. Thus, a simple and robust way to modify ePTFE to be biologically inert is sought after. Modification of ePTFE without high-energy pretreatment, such as immersion coating, has been of interest to researchers for its straightforward process and ease in scaling up. In this study, we utilized a two-step immersion coating to zwitterionize ePTFE membranes. The first coating consists of the co-deposition of polyethylenimine (PEI) and polydopamine (PDA) to produce amine groups in the surface of the ePTFE for further functionalization. These amine groups from PEI will be coupled with the epoxide group of the zwitterionic copolymer, poly(GMA-co-SBMA) (PGS), via a ring-opening reaction in the second coating. The coated ePTFE membranes were physically and chemically characterized to ensure that each step of the coating is successful. The membranes were also tested for their thrombogenicity via quantification of the blood cells attached to it during contact with biological solutions. The coated membranes exhibited around 90% reduction in attachment with respect to the uncoated ePTFE for both Gram-positive and Gram-negative strains of bacteria (Staphylococcus aureus and Escherichia coli). The coating was also able to resist blood cell attachment from human whole blood by 81.57% and resist red blood cell attachment from red blood cell concentrate by 93.4%. These ePTFE membranes, which are coated by a simple immersion coating, show significant enhancement of the biocompatibility of the membranes, which shows promise for future use in biological devices.

Elucidation of Endothelial Cell Hemostatic Regulation with Integrin-Targeting Hydrogels

Despite advances in the development of materials for cardiovascular devices, current strategies generally lack the thromboresistance of the native endothelium both in terms of efficacy and longevity. To harness this innate hemostatic regulation and improve long-term hemocompatibility, biohybrid devices are designed to promote endothelialization. Much of the research effort to date has focused on the use of extracellular matrix (ECM)-mimics and coatings to promote endothelial cell adhesion and migration with less attention given to the effect of the supported ECM binding events on hemostatic regulation. In this study, we developed integrin-targeted hydrogels to investigate the individual and combined effects of integrin binding events supported by many ECM-based coatings (α1β1, α2β1, α5β1, αvβ3). Targeted endothelial cell integrin interactions were first confirmed with antibody blocking studies and then correlated with gene expression of hemostatic regulators and a functional assay of platelet attachment and activation. Surfaces that targeted integrins α1β1 and α2β1 resulted in an endothelial cell layer that exhibited a thromboresistant phenotype with an associated reduction in platelet attachment and activation. It is anticipated that identification of specific integrins that promote endothelial cell adhesion as well as thromboresistance will enable the design of cardiovascular materials with improved long-term hemocompatibility.

A Review of Integrin-Mediated Endothelial Cell Phenotype in the Design of Cardiovascular Devices

Sustained biomaterial thromboresistance has long been a goal and challenge in blood-contacting device design. Endothelialization is one of the most successful strategies to achieve long-term thromboresistance of blood-contacting devices, with the endothelial cell layer providing dynamic hemostatic regulation. It is well established that endothelial cell behavior is influenced by interactions with the underlying extracellular matrix (ECM). Numerous researchers have sought to exploit these interactions to generate improved blood-contacting devices by investigating the expression of hemostatic regulators in endothelial cells on various ECM coatings. The ability to select substrates that promote endothelial cell-mediated thromboresistance is crucial to advancing material design strategies to improve cardiovascular device outcomes. This review provides an overview of endothelial cell regulation of hemostasis, the major components found within the cardiovascular basal lamina, and the interactions of endothelial cells with prominent ECM components of the basement membrane. A summary of ECM-mimetic strategies used in cardiovascular devices is provided with a focus on the effects of key adhesion modalities on endothelial cell regulators of hemostasis.

The formulation and characterization of 3D printed grafts as vascular access for potential use in hemodialysis

Arteriovenous graft (AVG) failure continues to be a life-threatening problem in haemodialysis. Graft failure can occur if the implanted graft is not well-matched to the vasculature of the patient. Likewise, stenosis often develops at the vein-graft anastomosis, contributing to thrombosis and early graft failure. To address this clinical need, a novel ink formulation comprised of ACMO/TMPTA/TMETA for 3D printing a AVG was developed (ACMO-AVG), in which the printed AVG was biocompatible and did not induce cytotoxicity. The ease of customizing the ACMO-AVG according to different requirements was demonstrated. Furthermore, the AVG displayed similar mechanical properties to the commercially available arteriovenous ePTFE graft (ePTFE-AVG). Unlike ePTFE-AVG, the ACMO-AVG displayed excellent anti-fouling characteristics because no plasma protein adsorption and platelet adhesion were detected on the luminal surfaces after 2 h of incubation. Similarly, exposure to human endothelial cells and human vascular smooth muscle cells did not result in any cell detection on the surfaces of the ACMO-AVG. Thus, the present study demonstrates a newly developed 3D printing ink formulation that can be successfully 3D printed into a clinically applicable vascular access used for haemodialysis.

Effect of tissue specificity on the performance of extracellular matrix in improving endothelialization of cardiovascular implants

Natural extracellular matrix (ECM) deposited in situ by cultured endothelial cells (ECs) has been proven effective in accelerating endothelialization of titanium (Ti) cardiovascular implants (CVIs) in our previous studies. In this study, the ECM deposited by smooth muscle cells (SMCs) was used in comparison to investigate the effects of tissue specificity of the ECM on the ability to accelerate endothelialization of CVIs. The results demonstrated that the ECM deposited by ECs and SMCs (EC-ECM, SMC-ECM, respectively) differed considerably in components and fibril morphology. Surface modification of Ti CVIs with both types of natural ECM was effective in improving their in vitro hemocompatibility and cytocompatibility simultaneously. However, the endothelialization of ECM-modified Ti CVIs in a canine model demonstrated a high tissue specificity of the ECM. Although the ECM deposited by SMCs (SMC-ECM) induced fewer platelet adhesion and sustained better growth and viability of ECs in vitro, its performance in accelerating in vivo endothelialization of Ti CVIs was extremely poor. In contrast, the ECM deposited by ECs (EC-ECM) led to complete endothelium formation in vivo.

In vitro and in vivo studies of ePTFE vascular grafts treated with P15 peptide

The purpose of this study is to evaluate the effectiveness of P15 cell-binding peptide treated ePTFE vascular grafts in vitro and in vivo. The P15 peptide was covalently immobilized onto ePTFE vascular grafts by an atmospheric plasma coating method. In vitro cell growth properties were studied using primary human umbilical vein endothelial cells (HUVECs) and primary human umbilical artery smooth muscle cells (HUASMCs). X-ray photoelectron spectroscopy and amino-acid analysis were used to analyze the surface characteristics of the peptide treated and untreated grafts. The cell growth study showed that the P15 peptide effectively promoted the adhesion and proliferation of endothelial cells. 700% more endothelial cells were proliferated on the P15-treated ePTFE grafts compared to the untreated ePTFE controls. In contrast, the P15 peptide was significantly less effective for promoting the adhesion and proliferation of smooth muscle cells than endothelial cells; only about 100% more smooth muscle cells proliferated on the P15-treated samples compared to the untreated control samples. The sheep model was used in the in vivo study. The amount of neointimal hyperplasia present at the arterial and venous sides of the anastomosis and the degree of endothelialization on the luminal surface of the grafts were assessed. Four P15-treated grafts and two control grafts were implanted as arteriovenous grafts between the femoral artery and vein or the carotid artery and jugular vein in two sheep (n = 6). The in vivo study showed that the thickness of the neointimal hyperplasia of untreated grafts was 3-times thicker than that of P15-treated grafts (P < 0.05) at the venous side of the anastomosis. P15-treated grafts also had a higher degree of endothelialization on the graft lumen.

Seeding density matters: extensive intercellular contact masks the surface dependence of endothelial cell-biomaterial interactions

The effects of seeding density have often been overlooked in evaluating endothelial cell-biomaterial interactions. This study compared the cell attachment and proliferation characteristics of endothelial cells on modified poly (L: -lactic acid) (PLLA) films conjugated to gelatin and chitosan at low and high seeding densities (5,000 and 50,000 cells/cm(2)). During the early stage (2 h) of cell-biomaterial interaction, a low seeding density enabled us to observe the intrinsic surface-dependent differences in cell attachment capacity and morphogenesis, whereas extensive intercellular interactions at high seeding density masked differences between substrates and improved cell attachment on low-affinity substrates. During the later stage of cell-biomaterial interaction over 7-days of culture, the proliferation rate was found to be surface-dependent at low seeding density, whereas this surface-dependent difference was not apparent at high seeding density. It is recommended that low seeding density should be utilized for evaluating biomaterial applications where EC density is likely to be low, such as in situ endothelialization of blood-contacting devices.

Human blood vessel-derived endothelial progenitors for endothelialization of small diameter vascular prosthesis

BACKGROUND

Coronary bypass graft failure as a result of acute thrombosis and intimal hyperplasia has been the major challenge in surgical procedures involving small-diameter vascular prosthesis. Coating synthetic grafts with patients' own endothelial cells has been suggested to improve the patency rate and overall success of bypass surgeries.

METHODOLOGY/PRINCIPAL FINDINGS

We isolated endothelial progenitor cells (EPCs) from leftover pieces of human saphenous vein/mammary artery. We demonstrate that EPCs can be expanded to generate millions of cells under low-density culture conditions. Exposure to high-density conditions induces differentiation to endothelial cell phenotype. EPC-derived endothelial cells show expression of CD144high, CD31, and vWF. We then assessed the ability of differentiated endothelial cells to adhere and grow on small diameter expanded polytetrafluoroethylene (ePTFE) tubings. Since ePTFE tubings are highly hydrophobic, we optimized protocols to introduce hydrophilic groups on luminal surface of ePTFE tubings. We demonstrate here a stepwise protocol that involves introduction of hydrophilic moieties and coating with defined ECM components that support adhesion of endothelial cells, but not of blood platelets.

CONCLUSION/SIGNIFICANCE

Our data confirms that endothelial progenitors obtained from adult human blood vessels can be expanded in vitro under xenoprotein-free conditions, for potential use in endothelialization of small diameter ePTFE grafts. These endothelialized grafts may represent a promising treatment strategy for improving the clinical outcome of small-caliber vascular grafts in cardiac bypass surgeries.

Tubular nanofiber scaffolds for tissue engineered small-diameter vascular grafts

Quick establishment of a confluent and stable endothelial cells (ECs) layer in the lumen of vascular grafts is critical for long-term patency of small-diameter vascular grafts. The objective of the study was to fabricate tubular nanofiber scaffolds, incorporate ECs onto the lumen of the scaffolds, and establish an animal model to prove the basic concept of using the scaffolds as vascular grafts. Poly(L-lactic acid)-co-poly(epsilon-caprolactone) P(LLA-CL 70:30) tubular nanofiber scaffolds were fabricated by electrospinning onto a rotating mandrel. Collagen was coated onto the scaffolds after air plasma treatment. Structure and mechanical property of the scaffolds were studied by scanning electron microscopy and tensile stress measurement, respectively. Human coronary artery endothelial cells (HCAECs) were rotationally seeded onto the lumen of the scaffolds at the speed of 6 rpm for 4 h through a customized seeding device, followed with static culture. Results showed evenly distributed and well-spread HCAECs throughout the lumen of the scaffold from 1 day onward to 10 days after seeding. Further, HCAECs maintained phenotypic expression of PECAM-1. To prove the basic concept of using the scaffolds as vascular grafts, acellular tubular P(LLA-CL) nanofiber scaffolds (inner diameter 1 mm) were implanted into rabbits to replace the inferior superficial epigastric veins. Results showed the scaffolds sustained the surgical process, kept the structure integrity, and showed the patency for 7 weeks.

The influence of biomaterials on endothelial cell thrombogenicity

Driven by tissue engineering and regenerative medicine, endothelial cells are being used in combination with biomaterials in a number of applications for the purpose of improving blood compatibility and host integration. Endothelialized vascular grafts are beginning to be used clinically with some success in some centers, while endothelial seeding is being explored as a means of creating a vasculature within engineered tissues. The underlying assumption of this strategy is that when cultured on artificial biomaterials, a confluent layer of endothelial cells maintain their non-thrombogenic phenotype. In this review the existing knowledge base of endothelial cell thrombogenicity cultured on a number of different biomaterials is summarized. The importance of selecting appropriate endpoint measures that are most reflective of overall surface thrombogenicity is the focus of this review. Endothelial cells inhibit thrombosis through three interconnected regulatory systems (1) the coagulation cascade, (2) the cellular components of the blood such as leukocytes and platelets and (3) the complement cascade, and also through effects on fibrinolysis and vascular tone, the latter which influences blood flow. Thus, in order to demonstrate the thrombogenic benefit of seeding a biomaterial with EC, the conditions under which EC surfaces are more likely to exhibit lower thrombogenicity than unseeded biomaterial surfaces need to be consistent with the experimental context. The endpoints selected should be appropriate for the dominant thrombotic process that occurs under the given experimental conditions.

Tissue factor and thrombomodulin expression on endothelial cell-seeded collagen modules for tissue engineering

The creation of functional tissue engineering constructs to repair or replace diseased tissues requires a well-formed vasculature network within the construct and the endothelial cells lining that vascular bed must display a nonthrombogenic phenotype. A new approach to tissue engineering involves the assembly of smaller components (modules fabricated at the hundred micron scale) into larger constructs. The modules, collagen gel containing the particular tissue cell of interest, are covered with endothelial cells prior to assembly so that the interconnected channels that are formed are lined with endothelial cells, creating a mimic of a vascular network. Here, we confirmed (using confocal microscopy primarily) that the human umbilical vein endothelial cells, seeded on collagen gel modules without a second embedded cell and without flow, bore the molecular markers of low thrombogenicity. Two days, after seeding on the modules, endothelial cells displayed the typical cobblestone morphology, formed tight cell-cell junctions and covered the whole module surface. Immunofluorescence staining showed that at both 2 days and 7 days after seeding, only a few cells expressed tissue factor while this number was dramatically increased after TNFalpha stimulation. On the other hand, thrombomodulin was expressed by the majority of seeded cells and expression was reduced after TNFalpha stimulation.

Review: Endothelial Cells Cultured on Engineered Vascular Grafts Are Able to Transduce Shear Stress

In vitro endothelialization of small-diameter vascular prostheses confluently lined with cultured autologous endothelial cells (ECs) before clinical implantation improves their patency. Many authors have studied the effects of shear stress on ECs seeded on various substrates showing activation of mitogen-activated protein (MAP) kinases. Very few studies have reported any functional EC response to shear stress when they are seeded on vascular grafts. The purpose of this in vitro study was to investigate whether ECs were able to transduce shear stress. Human saphenous vein ECs were seeded on 6 mm fibrin-glue-coated grafts, then submitted to 15 dyn/cm(2) for 10, 30, and 120 min. Cell lysates were submitted to Western blot analysis to detect phosphorylated ERK 1/2 and p38. ERK 1/2 activation was observed at 10 min (1.6 fold) followed by a lower activation than under static conditions at 30 and 120 min. Shear stress induced a significant increase in p38 phosphorylation (2.5 fold) at 10 and 30 min, decreasing at 120 min. Thus, ECs are able to transduce shear stress in an in vitro model in closed clinical conditions, but the ERK 1/2 and p38 temporal activation profile is different. We provide new insights into the validity of the vascular tissue engineering approach.

Improving arterial prosthesis neo-endothelialization: application of a proactive VEGF construct onto PTFE surfaces

The formation of a confluent endothelium on expanded polytetrafluoroethylene (PTFE) vascular prostheses has never been observed. This lack of endothelialization is known to be one of the main reasons leading to the development of thromboses and/or intimal hyperplasia. In this context, several efforts were put forward to promote endothelial cell coverage on the internal surface of synthetic vascular prostheses. The goal of the present study was to immobilize the vascular endothelial growth factor (VEGF) onto Teflon PTFE surfaces to generate a proactive polymer construct favoring interaction with endothelial cells. An ammonia plasma treatment was first used to graft amino groups on PTFE films. Subsequent reactions were performed to covalently bind human serum albumin (HSA) on the polymer surface and to load this protein with negative charges, which allows adsorbtion of VEGF onto HSA via strong electrostatic interactions. X-ray photoelectron spectroscopy (XPS) experiments along with surface derivatization strategies were performed between each synthesis step to ascertain the occurrence of the various molecules surface immobilization. Finally, the electrostatic binding of VEGF to the negatively charged HSA matrix was performed and validated by ELISA. Endothelial cell adhesion and migration experiments were carried out to validate the potential of this VEGF-containing biological construct to act as a proactive media toward the development of endothelial cells.

Fabrication of thromboresistant multilayer thin film on plasma treated poly (vinyl chloride) surface

Layer-by-layer deposited anticoagulant multilayer films were prepared on ammonia plasma treated poly (vinyl chloride) (PVC). Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) and contact angle results revealed the presence of -NH2 on the ammonia plasma treated PVC surfaces and the layer-by-layer self-assembly process. The stability of multilayer film was studied with the radio labeled method. The remainder bovine serum albumin (BSA) in cross-linked 5(heparin/BSA) multilayer films dipped in phosphate buffered saline (PBS, pH 7.4) was more than 90% in 40 days. The static platelet adhesion result indicated the anticoagulant multilayer films deposited on the plasma treated PVC reduced platelet adhesion drastically and no thrombus forming. The plasma recalcification time revealed that the multilayer modified surfaces greatly prolonged the plasma recalcification time. Such an easy processing and shape-independent method may have good potential for surface modification of cardiovascular devices.

In vitro study of cell-promoting multiple-armed peptides

The purpose of this study was to compare the effectiveness of several linear and branch cell-binding peptides to promote cell growth in prosthetic vascular grafts. In this in vitro study, the peptides were covalently immobilized onto expanded polytetrafluoroethylene (ePTFE) vascular grafts. Cell-growth properties were studied using primary human umbilical vein endothelial cells (HUVECs) and primary human umbilical artery smooth muscle cells (HUASMCs). Linear peptides (P15 and P15') and multiple-armed peptides (MAP4-I and MAP4-II) were covalently bonded onto ePTFE grafts by an atmospheric plasma coating method. X-ray photoelectron spectroscopy and amino acid analysis were used to analyze the surface characteristics of the peptide-coated samples. Cell adhesion, proliferation, and morphology were evaluated by culturing HUVECs and HUASMCs onto the surfaces of different samples: ePTFE control, chemically activated ePTFE, P15-coated ePTFE, and MAP4-coated ePTFE. The cell culture experiments were repeated several times to obtain statistically reliable cell-growth data. Cell-growth data were statistically analyzed by the two-way statistical analysis of variance. The study showed that multiple-armed MAP4 peptides were significantly more effective in promoting endothelial cells than the structurally similar linear P15 peptides. There were 800% more HUVECs proliferated on the MAP4-coated ePTFE samples compared with the ePTFE control. MAP4 peptides were 80% more effective for promoting HUVECs than P15 peptides. In contrast, MAP4 peptides were significantly less effective for promoting HUASMCs than HUVECs. There were only about 100% more HUASMCs proliferated on the MAP4-coated ePTFE samples compared with the ePTFE control. MAP4 and P15 peptides had similar cell-promoting characteristics for SMCs.