Enhanced Endothelial Cell Retention on Shear-Stressed Synthetic Vascular Grafts Precoated with RGD-Cross-Linked Fibrin

Re-endothelialization of Decellularized Scaffolds With Endothelial Progenitor Cell Capturing Aptamer: A New Strategy for Tissue-Engineered Heart Valve

Tissue-engineered heart valve (TEHV) is a promising alternative to current heart valve substitute. Decellularized porcine aortic heart valves (DAVs) are the most common scaffolds of TEHV. Hard to endothelialization is one of the disadvantages of DAVs. Therefore, we aimed to immobilize endothelial progenitor cell (EPC)-aptamer onto DAVs for accelerating endothelialization. In this study, three groups of scaffolds were constructed: DAVs, aptamer-immobilized DAVs (aptamer-DAVs), and glutaraldehyde crosslinked DAVs (GA-DAVs). The results of flow cytometry revealed that EPC-aptamer was specific to EPCs and was immobilized onto DAVs. Cells adhesion experiments demonstrated that EPCs adhered more tightly onto aptamer-DAVs group than other two groups of scaffolds. And cell proliferation assay indicated that EPCs seeded onto aptamer-DAVs group grew faster than DAVs group and GA-DAVs group. Moreover, dynamic capture experiment in flow conditions revealed that the number of EPCs captured by aptamer-DAVs group was more than other two groups. In conclusion, aptamer-DAVs could specifically promote adhesion and proliferation of EPCs and had ability to capture EPCs in simulated flow condition. This could promote re-endothelialization of scaffolds.

Improving Bioactive Characteristics of Small Diameter Polytetrafluoroethylene Stent Grafts by Electrospinning: A Comparative Hemocompatibility Study

Polytetrafluoroethylene (PTFE) is a commonly used biomaterial for the manufacturing of vascular grafts and several strategies, such as coatings, have been explored to improve the hemocompatibility of small-diameter prostheses. In this study, the hemocompatibility properties of novel stent grafts covered with electrospun PTFE (LimFlow Gen-1 and LimFlow Gen-2) were compared with uncoated and heparin-coated PTFE grafts (Gore Viabahn®) using fresh human blood in a Chandler closed-loop system. After 60 min of incubation, the blood samples were examined hematologically and activation of coagulation, platelets, and the complement system were analyzed. In addition, the adsorbed fibrinogen on the stent grafts was measured and the thrombogenicity was assessed by SEM. Significantly lower adsorption of fibrinogen was measured on the surface of heparin-coated Viabahn than on the surface of the uncoated Viabahn. Furthermore, LimFlow Gen-1 stent grafts showed lower fibrinogen adsorption than the uncoated Viabahn®, and the LimFlow Gen-2 stent grafts showed comparable fibrinogen adsorption as the heparin-coated Viabahn®. SEM analysis revealed no sign of thrombus formation on any of the stent surfaces. LimFlow Gen-2 stent grafts covered with electrospun PTFE exhibited bioactive characteristics and revealed improved hemocompatibility in terms of reduced adhesion of fibrinogen, activation of platelets, and coagulation (assessed by β-TG and TAT levels) similar to heparin-coated ePTFE prostheses. Thus, this study demonstrated improved hemocompatibility of electrospun PTFE. The next step is to conduct in vivo studies to confirm whether electrospinning-induced changes to the PTFE surface can reduce the risk of thrombus formation and provide clinical benefits.

Acellular Tissue-Engineered Vascular Grafts from Polymers: Methods, Achievements, Characterization, and Challenges

Extensive and permanent damage to the vasculature leading to different pathogenesis calls for developing innovative therapeutics, including drugs, medical devices, and cell therapies. Innovative strategies to engineer bioartificial/biomimetic vessels have been extensively exploited as an effective replacement for vessels that have seriously malfunctioned. However, further studies in polymer chemistry, additive manufacturing, and rapid prototyping are required to generate highly engineered vascular segments that can be effectively integrated into the existing vasculature of patients. One recently developed approach involves designing and fabricating acellular vessel equivalents from novel polymeric materials. This review aims to assess the design criteria, engineering factors, and innovative approaches for the fabrication and characterization of biomimetic macro- and micro-scale vessels. At the same time, the engineering correlation between the physical properties of the polymer and biological functionalities of multiscale acellular vascular segments are thoroughly elucidated. Moreover, several emerging characterization techniques for probing the mechanical properties of tissue-engineered vascular grafts are revealed. Finally, significant challenges to the clinical transformation of the highly promising engineered vessels derived from polymers are identified, and unique perspectives on future research directions are presented.

Bio-inspired hemocompatible surface modifications for biomedical applications

When blood first encounters the artificial surface of a medical device, a complex series of biochemical reactions is triggered, potentially resulting in clinical complications such as embolism/occlusion, inflammation, or device failure. Preventing thrombus formation on the surface of blood-contacting devices is crucial for maintaining device functionality and patient safety. As the number of patients reliant on blood-contacting devices continues to grow, minimizing the risk associated with these devices is vital towards lowering healthcare-associated morbidity and mortality. The current standard clinical practice primarily requires the systemic administration of anticoagulants such as heparin, which can result in serious complications such as post-operative bleeding and heparin-induced thrombocytopenia (HIT). Due to these complications, the administration of antithrombotic agents remains one of the leading causes of clinical drug-related deaths. To reduce the side effects spurred by systemic anticoagulation, researchers have been inspired by the hemocompatibility exhibited by natural phenomena, and thus have begun developing medical-grade surfaces which aim to exhibit total hemocompatibility via biomimicry. This review paper aims to address different bio-inspired surface modifications that increase hemocompatibility, discuss the limitations of each method, and explore the future direction for hemocompatible surface research.

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.

Progressive Reinvention or Destination Lost? Half a Century of Cardiovascular Tissue Engineering

The concept of tissue engineering evolved long before the phrase was forged, driven by the thromboembolic complications associated with the early total artificial heart programs of the 1960s. Yet more than half a century of dedicated research has not fulfilled the promise of successful broad clinical implementation. A historical account outlines reasons for this scientific impasse. For one, there was a disconnect between distinct eras each characterized by different clinical needs and different advocates. Initiated by the pioneers of cardiac surgery attempting to create neointimas on total artificial hearts, tissue engineering became fashionable when vascular surgeons pursued the endothelialisation of vascular grafts in the late 1970s. A decade later, it were cardiac surgeons again who strived to improve the longevity of tissue heart valves, and lastly, cardiologists entered the fray pursuing myocardial regeneration. Each of these disciplines and eras started with immense enthusiasm but were only remotely aware of the preceding efforts. Over the decades, the growing complexity of cellular and molecular biology as well as polymer sciences have led to surgeons gradually being replaced by scientists as the champions of tissue engineering. Together with a widening chasm between clinical purpose, human pathobiology and laboratory-based solutions, clinical implementation increasingly faded away as the singular endpoint of all strategies. Moreover, a loss of insight into the healing of cardiovascular prostheses in humans resulted in the acceptance of misleading animal models compromising the translation from laboratory to clinical reality. This was most evident in vascular graft healing, where the two main impediments to the in-situ generation of functional tissue in humans remained unheeded–the trans-anastomotic outgrowth stoppage of endothelium and the build-up of an impenetrable surface thrombus. To overcome this dead-lock, research focus needs to shift from a biologically possible tissue regeneration response to one that is feasible at the intended site and in the intended host environment of patients. Equipped with an impressive toolbox of modern biomaterials and deep insight into cues for facilitated healing, reconnecting to the “user needs” of patients would bring one of the most exciting concepts of cardiovascular medicine closer to clinical reality.

Current challenges and future trends in manufacturing small diameter artificial vascular grafts in bioreactors

Cardiovascular diseases are a leading cause of death. Vascular surgery is mainly used to solve this problem. However, the generation of a functional and suitable substitute for small diameter (< 6 mm) displacement is challengeable. Moreover, synthetic prostheses, made of polyethylene terephthalate and extended polytetrafluoroethylene show have shown insufficient performance. Therefore, the challenges dominating the use of autografts have prevented their efficient use. Tissue engineering is highlighted in regenerative medicine perhaps in aiming to address the issue of end-stage organ failure. While organs and complex tissues require the vascular supply to support the graft survival and render the bioartificial organ role, vascular tissue engineering has shown to be a hopeful method for cell implantation by the production of tissues in vitro. Bioreactors are a salient point in vascular tissue engineering due to the capability for reproducible and controlled variations showing a new horizon in blood vessel substitution. This review strives to display the overview of current concepts in the development of small-diameter by using bioreactors. In this work, we show a critical look at different factors for developing small-diameter and give suggestions for future studies.

Specifications for Innovative, Enabling Biomaterials Based on the Principles of Biocompatibility Mechanisms

In any engineering discipline, whenever products are designed, manufactured and ultimately utilized for the benefits of society, a series of specifications for the product are defined, and maybe refined, in order that they perform as effectively as possible, with due attention being paid to the safety, and economic aspects. These specifications are established with respect to all of the relevant properties, including those determined by mechanical, physical, chemical, manufacturing and environmental conditions, and the resulting design and materials selection reflects the optimal balance. In areas of medical technology, these specifications should be based on both functionality, which determines whether a device can actually perform as intended, and biocompatibility, which determines how the device interacts, both acutely and chronically, with the body. Unfortunately, whilst so much progress has been made with the development of superior functionality for the treatment and diagnosis of so many disease states, this is not the same for biocompatibility, where the single most-important currently adopted specification is that the device should do no harm, which falls far short of the ideal requirement. This paper addresses the profound need for biomaterials specifications to be based on the mechanisms of biocompatibility.

Targeted Delivery of Bioactive Molecules for Vascular Intervention and Tissue Engineering

Cardiovascular diseases are the leading cause of death in the United States. Treatment often requires surgical interventions to re-open occluded vessels, bypass severe occlusions, or stabilize aneurysms. Despite the short-term success of such interventions, many ultimately fail due to thrombosis or restenosis (following stent placement), or incomplete healing (such as after aneurysm coil placement). Bioactive molecules capable of modulating host tissue responses and preventing these complications have been identified, but systemic delivery is often harmful or ineffective. This review discusses the use of localized bioactive molecule delivery methods to enhance the long-term success of vascular interventions, such as drug-eluting stents and aneurysm coils, as well as nanoparticles for targeted molecule delivery. Vascular grafts in particular have poor patency in small diameter, high flow applications, such as coronary artery bypass grafting (CABG). Grafts fabricated from a variety of approaches may benefit from bioactive molecule incorporation to improve patency. Tissue engineering is an especially promising approach for vascular graft fabrication that may be conducive to incorporation of drugs or growth factors. Overall, localized and targeted delivery of bioactive molecules has shown promise for improving the outcomes of vascular interventions, with technologies such as drug-eluting stents showing excellent clinical success. However, many targeted vascular drug delivery systems have yet to reach the clinic. There is still a need to better optimize bioactive molecule release kinetics and identify synergistic biomolecule combinations before the clinical impact of these technologies can be realized.

Interplay of Geometric Cues and RGD/BMP-2 Crosstalk in Directing Stem Cell Fate

Within the native microenvironment, extracellular matrix (ECM) components are thought to display a complex and heterogeneous distribution, spanning several length scales. Herein, the objective is to mimic, in vitro, the hierarchical organization of proteins and growth factors as well as their crosstalk. Photolithography technique was used to adjacently pattern geometrically defined regions of RGD and BMP-2 mimetic peptides onto glass substrates. These ECM-derived ligands are known to jointly regulate mesenchymal stem cells (MSCs) osteogenic differentiation. By manipulating the spatial distribution of dually grafted peptides, the extent of human MSCs osteogenic differentiation was significantly affected, depending on the shape of peptide micropatterns. Our data highlight the existence of a strong interplay between geometric cues and biochemical signals. Such in vitro systems provide a valuable tool to investigate mechanisms by which multiple ECM cues overlap to regulate stem cell fate, thereby contributing to the design of bioinspired biomaterials for bone tissue engineering applications.

Fibrin nanostructures for biomedical applications

Fibrin is a versatile biopolymer that has been extensively used in tissue engineering. In this paper fibrin nanostructures prepared using a technique based on the catalytic effect of fibrin-bound thrombin are presented. This technique enables surface-attached thin fibrin networks to form with precisely regulated morphology without the development of fibrin gel in bulk solution. Moreover, the influence of changing the polymerization time, along with the antithrombin III and heparin concentrations on the morphology of fibrin nanostructures was explored. The binding of bioactive molecules (fibronectin, laminin, collagen, VEGF, bFGF, and heparin) to fibrin nanostructures was confirmed. These nanostructures can be used for the surface modification of artificial biomaterials designed for different biomedical applications (e.g. artificial vessels, stents, heart valves, bone and cartilage constructs, skin grafts, etc.) in order to promote the therapeutic outcome.

The Quest for Nonthrombotic Surface Modifications to Achieve Hemocompatibility of Implantable Devices

The use of blood-contacting implantable devices is limited by surface-induced thrombosis, which has led to the development of thromboresistant surfaces. Multidisciplinary efforts have promoted the development of surface modifications to minimize thrombosis by targeting surface-induced coagulation. To this date, no material has been identified that remains irrevocably hemocompatible with time but many options are now available with their own limitations. Essential to this review is the understanding of some of the challenges in this field and newer opportunities for hemocompatibility research. This report will also briefly review many of the achievements in the development of hemocompatible biomaterial coating, including surface modifications against protein adsorption and platelet adhesion, biomimetism, and endothelialization.

Co-expression of fibulin-5 and VEGF165 increases long-term patency of synthetic vascular grafts seeded with autologous endothelial cells

Small caliber synthetic vascular grafts are commonly used for bypass surgery and dialysis access sites but have high failure rates because of neointima formation and thrombosis. Seeding synthetic grafts with endothelial cells (ECs) provides a biocompatible surface that may prevent graft failure. However, EC detachment following exposure to blood flow still remains a major obstacle in the development of biosynthetic grafts. We tested the hypothesis that induced expression by the seeded EC, of vascular endothelial growth factor165 (VEGF165) and of fibulin-5, an extracellular matrix glycoprotein that has a crucial role in elastin fiber organization and increase EC adherence to surfaces, may improve long-term graft patency. Autologous ECs were isolated from venous segments, and were transduced with retroviral vectors expressing fibulin-5 and VEGF165. The modified cells were seeded on expanded polytetrafluoroethylene (ePTFE) grafts and implanted in a large animal model. Three months after transplantation, all grafts seeded with modified EC were patent on a selective angiography, whereas only a third of the control grafts were patent. Similar results were shown at 6 months. Thus, seeding ePTFE vascular grafts with genetically modified EC improved long-term small caliber graft patency. The biosynthetic grafts may provide a novel therapeutic modality for patients with peripheral vascular disease and patients requiring vascular access for hemodialysis.

Tailoring biomaterial surface properties to modulate host-implant interactions: implication in cardiovascular and bone therapy

Host body response to a foreign medical device plays a critical role in defining its fate post implantation. It is thus important to control host-material interactions by designing innovative implant surfaces. In the recent years, biochemical and topographical features have been explored as main target to produce this new type of bioinert or bioresponsive implants. The review discusses specific biofunctional materials and strategies to achieve a precise control over implant surface properties and presents possible solutions to develop next generation of implants, particularly in the fields of bone and cardiovascular therapy.

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.

Nylon-3 polymers that enable selective culture of endothelial cells

Substrates that selectively encourage the growth of specific cell types are valuable for the engineering of complex tissues. Some cell-selective peptides have been identified from extracellular matrix proteins; these peptides have proven useful for biomaterials-based approaches to tissue repair or regeneration. However, there are very few examples of synthetic materials that display selectivity in supporting cell growth. We describe nylon-3 polymers that support in vitro culture of endothelial cells but do not support the culture of smooth muscle cells or fibroblasts. These materials may be promising for vascular biomaterials applications.

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.

A fluorophore-tagged RGD peptide to control endothelial cell adhesion to micropatterned surfaces

The long-term patency rates of vascular grafts and stents are limited by the lack of surface endothelialisation of the implanted materials. We have previously reported that GRGDS and WQPPRARI peptide micropatterns increase the endothelialisation of prosthetic materials in vitro. To investigate the mechanisms by which the peptide micropatterns affect endothelial cell adhesion and proliferation, a TAMRA fluorophore-tagged RGD peptide was designed. Live cell imaging revealed that the micropatterned surfaces led to directional cell spreading dependent on the location of the RGD-TAMRA spots. Focal adhesions formed within 3 h on the micropatterned surfaces near RGD-TAMRA spot edges, as expected for cell regions experiencing high tension. Similar levels of focal adhesion kinase phosphorylation were observed after 3 h on the micropatterned surfaces and on surfaces treated with RGD-TAMRA alone, suggesting that partial RGD surface coverage is sufficient to elicit integrin signaling. Lastly, endothelial cell expansion was achieved in serum-free conditions on gelatin-coated, RGD-TAMRA treated or micropatterned surfaces. These results show that these peptide micropatterns mainly impacted cell adhesion kinetics rather than cell proliferation. This insight will be useful for the optimization of micropatterning strategies to improve vascular biomaterials.

Improved adhesion and differentiation of endothelial cells on surface-attached fibrin structures containing extracellular matrix proteins

Currently used vascular prostheses are hydrophobic and do not allow endothelial cell (EC) adhesion and growth. The aim of this study was to prepare fibrin (Fb)-based two-dimensional (2D) and three-dimensional (3D) assemblies coated with extracellular matrix (ECM) proteins and to evaluate the EC adhesion, proliferation and differentiation on these assemblies in vitro. Coating of Fb with collagen, laminin (LM), and fibronectin (FN) was proved using the surface plasmon resonance technique. On all Fb assemblies, ECs reached higher cell densities than on polystyrene after 3 and 7 days of culture. Immunoflurescence staining showed better assembly of talin and vinculin into focal adhesion plaques, and also more apparent staining of vascular endothelial cadherin on surface-attached 3D Fb and protein-coated Fb assemblies. On these samples, ECs also contained a lower concentration of intercellular adhesion molecule-1, measured by enzyme-linked immunosorbent assay. Higher concentrations of CD31 (platelet-endothelial cell adhesion molecule-1) were found on 3D Fb coated with LM, and higher concentrations of von Willebrand factor were found on 3D Fb coated with type I collagen or LM in comparison to 2D Fb layers. The results indicate that ECM protein-coated 2D and 3D Fb assemblies can be used for versatile applications in various tissue replacements where endothelialization is desirable, for example, vascular prostheses and heart valves.

A prototype tissue engineered blood vessel using amniotic membrane as scaffold

In this study, we used amniotic membrane (AM), a natural extracellular matrix, as a scaffold for the fabrication of tissue engineered blood vessels (TEBVs). The inner surface of the denuded glutaraldehyde cross-linked AM tube was endothelialized with porcine vascular endothelial cells (ECs) and subjected to a physiological (12 dynecm(-2)) shear stress (SS) for 2 and 4 days. The results showed that after applying SS, an intact EC monolayer was maintained in the lumen surface of the TEBV. The ECs were aligned with their long axis parallel to the blood flow. The immunofluorescent microscopy showed that the intercellular junctional proteins, PECAM-1 and VE-cadherin, were surrounding the EC periphery and were better developed and more abundant in SS-treated TEBVs than the static controls. The Western blot indicated that the expressions of PECAM-1 and VE-cadherin were increased by 72 ± 9% and 67 ± 7%, respectively, after shear stress treatment. The distribution pattern of integrin β1 was mainly at the interface of ECs and AM in static TEBVs but it was extended to the cell-cell junctions after SS treatment. The SS promoted the expression of integrin α(v)β(3) without altering its distribution in TEBV. The results suggest that glutaraldehyde cross-linked AM tube can potentially be used as a scaffold biomaterial for TEBV fabrication. Most importantly, the use of an AM tube shortened the TEBV fabrication.

Experimental and computational analysis of cellular interactions with nylon-3-bearing substrates

The ability to design biomaterials that interact with biological environments in a predictable manner necessitates an improved understanding of how surface chemistry influences events such as protein adsorption and cell adhesion. In this work, we examined mechanisms governing the interactions between 3T3 fibroblasts and nylon-3 polymers, which have a protein-like polyamide backbone and are highly amenable to tuning of chemical and physical properties. Protein adsorption and cell adhesion to a library of nylon-3 polymers were characterized and analyzed by partial least squares regression. This analysis revealed that specific chemical features of the nylon-3 polymers correlated with the extent of protein adsorption, which, in turn, correlated with cell adhesion in a serum-containing environment. In contrast, in a serum-free environment, cell adhesion could be predicted solely from chemical properties. Enzymatic treatments of 3T3 cells before plating indicated that proteins bound to the cell surface mediated cell-nylon-3 polymer interactions under serum-free conditions, with additional analysis suggesting that cell-associated fibronectin played a dominant role in adhesion in the absence of serum. The mechanistic insight gained from these studies can be used to inform the design of new polymer structures in addition to providing a basis for continued development of nylon-3 copolymers for tissue engineering applications.

Using polymeric materials to control stem cell behavior for tissue regeneration

Patients with organ failure often suffer from increased morbidity and decreased quality of life. Current strategies of treating organ failure have limitations, including shortage of donor organs, low efficiency of grafts, and immunological problems. Tissue engineering emerged about two decades ago as a strategy to restore organ function with a living, functional engineered substitute. However, the ability to engineer a functional organ is limited by a limited understanding of the interactions between materials and cells that are required to yield functional tissue equivalents. Polymeric materials are one of the most promising classes of materials for use in tissue engineering, due to their biodegradability, flexibility in processing and property design, and the potential to use polymer properties to control cell function. Stem cells offer potential in tissue engineering because of their unique capacity to self-renew and differentiate into neurogenic, osteogenic, chondrogenic, and myogenic lineages under appropriate stimuli from extracellular components. This review examines recent advances in stem cell-polymer interactions for tissue regeneration, specifically highlighting control of polymer properties to direct adhesion, proliferation, and differentiation of stem cells, and how biomaterials can be designed to provide some of the stimuli to cells that the natural extracellular matrix does.

Maleimide-thiol coupling of a bioactive peptide to an elastin-like protein polymer

Recombinant elastin-like protein (ELP) polymers display several favorable characteristics for tissue repair and replacement as well as drug delivery applications. However, these materials are derived from peptide sequences that do not lend themselves to cell adhesion, migration, or proliferation. This report describes the chemoselective ligation of peptide linkers bearing the bioactive RGD sequence to the surface of ELP hydrogels. Initially, cystamine is conjugated to ELP, followed by the temperature-driven formation of elastomeric ELP hydrogels. Cystamine reduction produces reactive thiols that are coupled to the RGD peptide linker via a terminal maleimide group. Investigations into the behavior of endothelial cells and mesenchymal stem cells on the RGD-modified ELP hydrogel surface reveal significantly enhanced attachment, spreading, migration and proliferation. Attached endothelial cells display a quiescent phenotype.

In vivo tissue engineering of heart valves: evolution of a novel concept

Current tissue-engineering principles of heart valves include tissue- or stem cell-derived cells with subsequent in vitro incubation on various scaffolds prior to implantation. Limitations of this approach include a long in vitro culture, an accompanied risk of infection and sophisticated, cost-intensive infrastructures. An 'off-the-shelf' heart valve with in vivo endothelialization and tissue-regeneration potential would overcome these limitations. Additionally, the development of a heart valve with growth potential would be a huge improvement for pediatric patients. This article discusses different starter matrices, homing and immobilization strategies of host cells and masking approaches of inflammatory structures for in vivo surface and tissue engineering of heart valves. Novel concepts will be presented based on highly specific DNA-aptamers immobilized on the heart valve surface as capture molecules for endothelial progenitor cells circulating in the bloodstream.

Engineering high-density endothelial cell monolayers on soft substrates

This study demonstrates that a confluent monolayer of endothelial cells (ECs) can be tissue engineered on a soft substrate with a cell density and morphology that approximates in vivo conditions. We achieved formation of a confluent EC monolayer on polydimethylsiloxane (PDMS) elastomer by microcontact printing of fibronectin (FN) in a square lattice array of 3microm diameter circular islands at a 6microm pitch. Uniform coatings of FN or serum proteins on PDMS or on tissue-culture-treated polystyrene failed to support the equivalent EC density and/or confluence. The ECs on the FN micropatterned PDMS achieved a density of 1,536+/-247cellsmm(-2), close to the 3,215+/-336cellsmm(-2) observed in vivo from porcine pulmonary artery and significantly higher (2- to 5-fold) than EC density on other materials. The probable mechanism for enhanced EC adhesion, growth and density is increased focal adhesion (FA) formation between the ECs and the substrate. After 14days culture, the micropatterned FN surface increased the average number of FAs per cell to 35+/-10, compared to 7+/-6 for ECs on PDMS uniformly coated with FN. Thus, microscale patterning of FN into FA-sized, circular islands on PDMS elastomer promotes the formation of EC monolayers with in vivo-like cell density and morphology.

Photo-initiated grafting of gelatin/N-maleic acyl-chitosan to enhance endothelial cell adhesion, proliferation and function on PLA surface

Vascular graft surface properties significantly affect adhesion, growth and function of endothelial cells (ECs). The bulk degradation property of poly(lactic acid) (PLA) makes it possible for it to be replaced by cellular materials and PLA is desirable as a scaffold material for vascular grafts. However, PLA has an unfavorable surface property for EC adhesion and proliferation due to the lack of a selective cell adhesion motif. Photo-initiated surface-grafting polymerization is a promising method for immobilizing certain biomacromolecules on material surfaces without compromising bulk properties. N-Maleic acyl-chitosan (NMCS) is a novel biocompatible amphiphilic derivative of chitosan with double bonds and can be initiated by ultraviolet light. In this study, gelatin was complexed with NMCS via hydrophobic interaction, and gel/NMCS complex thus formed was then grafted on the PLA surface to improve EC biocompatibility. X-ray photoelectron and Fourier transform infrared spectroscopy, and water contact angle measurement confirmed immobilization of the gel/NMCS complex on PLA surface. Moreover, the gel/NMCS modified PLA enhanced human umbilical vein endothelial cell (HUVEC) spreading and flattening, and promoted the expression of more structured CD31 and vWF compared to unmodified PLA film. Compared to the unmodified PLA surface, the HUVECs on the modified PLA surface had elevated uptake of acetylated low-density lipoprotein, and maintained the ability to modulate metabolic activity upon exposure to shear stress at 5dyncm(-2) by up-regulating nitric oxide and prostacyclin production. Cell retention was 1.6 times higher on the gel/NMCS-PLA surface, demonstrating its improved potential for hemocompatibility. These results indicate that photo-initiated surface-grafting of the biomimetic gel/NMCS complex is an effective method to modify material surfaces as vascular grafts.

Polymeric materials for tissue engineering of arterial substitutes

Cardiovascular disease is the leading cause of mortality in the United States. The limited availability of healthy autologous vessels for bypass grafting procedures has led to the fabrication of prosthetic vascular conduits. Synthetic polymeric materials, while providing the appropriate mechanical strength, lack the compliance and biocompatibility that bioresorbable and naturally occurring protein polymers offer. Vascular tissue engineering approaches have emerged in order to meet the challenges of designing a vascular graft with long-term patency. In vitro culture techniques that have been explored with vascular cell seeding of polymeric scaffolds and the use of bioactive polymers for in situ arterial regeneration have yielded promising results. This review describes the development of polymeric materials in various tissue engineering strategies for the improvement in the mechanical and biological performance of an arterial substitute.

Biofunctionalization of biomaterials for accelerated in situ endothelialization: a review

The higher patency rates of cardiovascular implants, including vascular bypass grafts, stents, and heart valves are related to their ability to inhibit thrombosis, intimal hyperplasia, and calcification. In native tissue, the endothelium plays a major role in inhibiting these processes. Various bioengineering research strategies thereby aspire to induce endothelialization of graft surfaces either prior to implantation or by accelerating in situ graft endothelialization. This article reviews potential bioresponsive molecular components that can be incorporated into (and/or released from) biomaterial surfaces to obtain accelerated in situ endothelialization of vascular grafts. These molecules could promote in situ endothelialization by the mobilization of endothelial progenitor cells (EPC) from the bone marrow, encouraging cell-specific adhesion (endothelial cells (EC) and/or EPC) to the graft and, once attached, by controlling the proliferation and differentiation of these cells. EC and EPC interactions with the extracellular matrix continue to be a principal source of inspiration for material biofunctionalization, and therefore, the latest developments in understanding these interactions will be discussed.