A blood vessel model constructed from collagen and cultured vascular cells

Bioengineered human blood vessels

Since the advent of the vascular anastomosis by Alexis Carrel in the early 20th century, the repair and replacement of blood vessels have been key to treating acute injuries, as well as chronic atherosclerotic disease. Arteries serve diverse mechanical and biological functions, such as conducting blood to tissues, interacting with the coagulation system, and modulating resistance to blood flow. Early approaches for arterial replacement used artificial materials, which were supplanted by polymer fabrics in recent decades. With recent advances in the engineering of connective tissues, including arteries, we are on the cusp of seeing engineered human arteries become mainstays of surgical therapy for vascular disease. Progress in our understanding of physiology, cell biology, and biomanufacturing over the past several decades has made these advances possible.

3D Bioprinting-Tunable Small-Diameter Blood Vessels with Biomimetic Biphasic Cell Layers

Blood vessel damage resulting from trauma or diseases presents a serious risk of morbidity and mortality. Although synthetic vascular grafts have been successfully commercialized for clinical use, they are currently only readily available for large-diameter vessels (>6 mm). Small-diameter vessel (<6 mm) replacements, however, still present significant clinical challenges worldwide. The primary objective of this study is to create novel, tunable, small-diameter blood vessels with biomimetic two distinct cell layers [vascular endothelial cell (VEC) and vascular smooth muscle cell (VSMC)] using an advanced coaxial 3D-bioplotter platform. Specifically, the VSMCs were laden in the vessel wall and VECs grew in the lumen to mimic the natural composition of the blood vessel. First, a novel bioink consisting of VSMCs laden in gelatin methacryloyl (GelMA)/polyethylene(glycol)diacrylate/alginate and lyase was designed. This specific design is favorable for nutrient exchange in an ambient environment and simultaneously improves laden cell proliferation in the matrix pore without the space restriction inherent with substance encapsulation. In the vessel wall, the laden VSMCs steadily grew as the alginate was gradually degraded by lyase leaving more space for cell proliferation in matrices. Through computational fluid dynamics simulation, the vessel demonstrated significantly perfusable and mechanical properties under various flow velocities, flow viscosities, and temperature conditions. Moreover, both VSMCs in the scaffold matrix and VECs in the lumen steadily proliferated over time creating a significant two-cell-layered structure. Cell proliferation was confirmed visually through staining the markers of alpha-smooth muscle actin and cluster of differentiation 31, commonly tied to angiogenesis phenomena, in the vessel matrices and lumen, respectively. Furthermore, the results were confirmed quantitatively through gene analysis which suggested good angiogenesis expression in the blood vessels. This study demonstrated that the printed blood vessels with two distinct cell layers of VECs and VSMCs could be potential candidates for clinical small-diameter blood vessel replacement applications.

Engineered biomaterials for heart disease

Ischemic heart disease is the most common type of heart disease, responsible for roughly 10 million deaths worldwide annually. While standard clinical interventions have resulted in improved patient outcomes, access to small diameter vessels required for cardiovascular interventions, and long-term patient mortality rates associated with eventual heart failure, remain critical challenges. In this current opinion piece we discuss novel methodologies for the advancement of vascular grafts, cardiac patches, and injectable drug delivery depot technologies as they relate to treatment of ischemic heart disease, including bilayered conduits, acellular bioactive extracellular matrix (ECM) scaffolds, and protease-responsive hydrogel delivery platforms. We address the motivation for innovation and current limitations in the field of engineered biomaterials for myocardial ischemia therapeutics and interventions.

COLLAGEN: STRUCTURE, METABOLISM, PRODUCTION AND INDUSTRIAL APPLICATION

This review presents the current scientific literature data about structure, properties, and functions of collagen, which is known as one of the most abundant human and animal proteins. The building of collagen molecule from the primary structure to submolecular formations, the main stages of its synthesis and biodegradation are briefly described. The information about collagen diversity, its features and metabolic ways in various tissues, including skin, tendons, bones, etc. is presented. The problems of pathologies caused by collagen synthesis and breakdown disorders as well as age-related changes in collagen properties and their causes are discussed. A comparative analysis of the advantages and disadvantages of collagen and its derivatives obtaining from various sources (animals, marine, and recombinant) is given. The most productive methods for collagen extraction from various tissues are shown. The concept of collagen hydrolysis conditions influence on the physicochemical properties and biological activity of the obtained products is described. The applications of collagen and its products in various fields of industrial activity, such as pharmaceutical, cosmetic industry and medicine, are discussed. Further prospective directions of fundamental and applied investigations in this area of research are outlined.

Tissue engineering: from the bedside to the bench and back to the bedside

The field of Tissue Engineering and Regenerative Medicine has evolved rapidly over the past thirty years. This review will summarize its history, current status and direction through the lens of clinical need, its progress through science in the laboratory and application back into patients. We can take pride in the fact that much effort and progress began with the surgical problems of children and that many surgeons in the pediatric surgical specialties have become pioneers and investigators in this new field of science, engineering, and medicine. Although the field has yet to fulfill its great promise, there have been several examples where a therapy has progressed from the first idea to human application within a short span of time and, in many cases, it has been applied in the surgical care of children.

Considerations in the Development of Small-Diameter Vascular Graft as an Alternative for Bypass and Reconstructive Surgeries: A Review

BACKGROUND

Current design strategies for small diameter vascular grafts (< 6 mm internal diameter; ID) are focused on mimicking native vascular tissue because the commercially available grafts still fail at small diameters, notably due to development of intimal hyperplasia and thrombosis. To overcome these challenges, various design approaches, material selection, and surface modification strategies have been employed to improve the patency of small-diameter grafts.

REVIEW

The purpose of this review is to outline various considerations in the development of small-diameter vascular grafts, including material choice, surface modifications to enhance biocompatibility/endothelialization, and mechanical properties of the graft, that are currently being implanted. Additionally, we have taken into account the general vascular physiology, tissue engineering approaches, and collective achievements of the authors in this area. We reviewed both commercially available synthetic grafts (e-PTFE and PET), elastic polymers such as polyurethane and biodegradable and bioresorbable materials. We included naturally occurring materials by focusing on their potential application in the development of future vascular alternatives.

CONCLUSION

Until now, there are few comprehensive reviews regarding considerations in the design of small-diameter vascular grafts in the literature. Here-in, we have discussed in-depth the various strategies employed to generate engineered vascular graft due to their high demand for vascular surgeries. While some TEVG design strategies have shown greater potential in contrast to autologous or synthetic ePTFE conduits, many are still hindered by high production cost which prevents their widespread adoption. Nonetheless, as tissue engineers continue to develop on their strategies and procedures for improved TEVGs, soon, a reliable engineered graft will be available in the market. Hence, we anticipate a viable TEVG with resorbable property, fabricated via electrospinning approach to hold a greater potential that can overcome the challenges observed in both autologous and allogenic grafts. This is because they can be mechanically tuned, incorporated/surface-functionalized with bioactive molecules and mass-manufactured in a reproducible manner. It is also found that most of the success in engineered vascular graft approaching commercialization is for large vessels rather than small-diameter grafts used as cardiovascular bypass grafts. Consequently, the field of vascular engineering is still available for future innovators that can take up the challenge to create a functional arterial substitute.

Characterising Vascular Cell Monolayers Using Electrochemical Impedance Spectroscopy and a Novel Electroanalytical Plot

INTRODUCTION

Biological research relies on the culture of mammalian cells, which are prone to changes in phenotype during experiments involving several passages of cells. In regenerative medicine, specifically, there is an increasing need to expand the characterisation landscape for stem cells by identifying novel stable markers. This paper reports on a novel electric cell-substrate impedance sensing-based electroanalytical diagram which can be used for the "electrical characterisation" of cell monolayers consisting of smooth muscle cells, endothelial cells or co-culture.

MATERIALS AND METHODS

Interdigitated electrodes were microfabricated using standard cleanroom procedures and integrated into cell chambers. Electrochemical impedance spectroscopy data were acquired for 2 vascular cell types after they formed monolayers on the electrodes.

RESULTS AND DISCUSSION

A Mean impedance per unit area vs Mean phase plots provided a reproducible, visually obvious and statistically significant method of characterising cell monolayers. This electroanalytic diagram has never been used in previous papers, but it confirms findings by other research groups using similar approaches that the complex impedance spectra of different cell type are different. Further work is required to determine whether this method could be extended to other cell types, and if this is the case, a library of "signature spectra" could be generated for "electrical characterisation" of cells.

From arteries to capillaries: approaches to engineering human vasculature

From micro-scaled capillaries to millimeter-sized arteries and veins, human vasculature spans multiple scales and cell types. The convergence of bioengineering, materials science, and stem cell biology has enabled tissue engineers to recreate the structure and function of different hierarchical levels of the vascular tree. Engineering large-scale vessels has been pursued over the past thirty years to replace or bypass damaged arteries, arterioles, and venules, and their routine application in the clinic may become a reality in the near future. Strategies to engineer meso- and microvasculature have been extensively explored to generate models to study vascular biology, drug transport, and disease progression, as well as for vascularizing engineered tissues for regenerative medicine. However, bioengineering of large-scale tissues and whole organs for transplantation, have failed to result in clinical translation due to the lack of proper integrated vasculature for effective oxygen and nutrient delivery. The development of strategies to generate multi-scale vascular networks and their direct anastomosis to host vasculature would greatly benefit this formidable goal. In this review, we discuss design considerations and technologies for engineering millimeter-, meso-, and micro-scale vessels. We further provide examples of recent state-of-the-art strategies to engineer multi-scale vasculature. Finally, we identify key challenges limiting the translation of vascularized tissues and offer our perspective on future directions for exploration.

Advancing tissue-engineered vascular grafts via their endothelialization and mechanical conditioning

Tissue engineering has garnered significant attention for its potential to address the predominant modes of failure of small diameter vascular prostheses, namely mid-graft thrombosis and anastomotic intimal hyperplasia. In this review, we described two main features underpinning the promise of tissue-engineered vascular grafts: the incorporation of an antithrombogenic endothelium, and the generation of a structurally and biomechanically mimetic extracellular matrix. From the early attempts at the in-vitro endothelialization of vascular prostheses in the 1970s through to the ongoing clinical trials of fully tissue-engineered vascular grafts, the historical advancements and unresolved challenges that characterize the current state-of-the-art are summarized in a manner that establishes a guide for the development of an effective vascular prosthesis for small diameter arterial reconstruction. The importance of endothelial cell purity and their arterial specification for the prevention of both diffuse neointimal hyperplasia and the accelerated development of atherosclerotic lesions is delineated. Additionally, the need for an extracellular matrix that recapitulates both the composition and structure of native elastic arteries to facilitate the protracted stability and patency of an engineered vasoactive conduit is described. Finally, the capacity of alternative sources of cells and mechanical conditioning to overcome these technical barriers to the clinical translation of an effective small diameter vascular prosthesis is discussed. In conclusion, this review provides an overview of the historical development of tissue-engineered vascular grafts, highlighting specific areas warranting further research, and commentating on the outlook of a clinically feasible and therapeutically efficacious vascular prosthesis for small diameter arterial reconstruction.

Tissue-engineered vessel derived from human fibroblasts with an electrospun scaffold

Advanced cardiovascular disease often requires surgical revascularization for small diameter arterial bypass procedures, and there is a need for alternative grafts in those patients lacking autologous vein. A decellularized biological vessel with the characteristics of a small artery and the ability to remodel in vivo could replace currently available bypass grafts. In this study, a biodegradable electrospun scaffold was specifically designed to be placed in a biomimetic perfusion system to generate a tissue-engineered vessel from human dermal fibroblasts. The polyglycolic acid electrospun scaffold was co-electrosprayed with a sacrificial porogen microparticle, polyethylene oxide, to increase porosity and pore size. After a 10-week culture period in the biomimetic system, the tissue-engineered vessel derived from human fibroblasts was further processed with decellularization to form an allogeneic tissue-engineered vessel. The tissue-engineered vessel had a similar morphology by histological staining for collagen and elastin before and after decellularization. The mechanical properties (burst pressure, ultimate tensile strength, and elastic modulus) remained stable after decellularization and were on the same magnitude as a human saphenous vein. The decellularization processing demonstrated no loss of collagen, near complete removal of DNA, and no presence of intracellular proteins. The decellularized tissue-engineered vessel supported the growth of endothelial cells on the surface, and fibroblasts were able to migrate into the midportion of the matrix. Therefore, an electrospun scaffold provides a versatile biomaterial to create a decellularized tissue-engineered vessel derived from human dermal fibroblasts with morphological and mechanical properties for use as a small diameter vascular graft.

Collagen/Chitosan Functionalization of Complex 3D Structures Fabricated by Laser Direct Writing via Two-Photon Polymerization for Enhanced Osteogenesis

The fabrication of 3D microstructures is under continuous development for engineering bone substitutes. Collagen/chitosan (Col/CT) blends emerge as biomaterials that meet the mechanical and biological requirements associated with bone tissue. In this work, we optimize the osteogenic effect of 3D microstructures by their functionalization with Col/CT blends with different blending ratios. The structures were fabricated by laser direct writing via two-photons polymerization of IP-L780 photopolymer. They comprised of hexagonal and ellipsoidal units 80 µm in length, 40 µm in width and 14 µm height, separated by 20 µm pillars. Structures’ functionalization was achieved via dip coating in Col/CT blends with specific blending ratios. The osteogenic role of Col/CT functionalization of the 3D structures was confirmed by biological assays concerning the expression of alkaline phosphatase (ALP) and osteocalcin secretion as osteogenic markers and Alizarin Red (AR) as dye for mineral deposits in osteoblast-like cells seeded on the structures. The structures having ellipsoidal units showed the best results, but the trends were similar for both ellipsoidal and hexagonal units. The strongest osteogenic effect was obtained for Col/CT blending ratio of 20/80, as demonstrated by the highest ALP activity, osteocalcin secretion and AR staining intensity in the seeded cells compared to all the other samples.

Evaluating platelet activation related to the degradation products of biomaterials using molecular markers

Increasing numbers of biodegradable medical devices may be used in the circulatory system. The effects of the released degradation products from these medical devices on the blood may be gradual and cumulative. When they reach critical levels, they may cause thrombosis and other complications. For this reason, it is important to evaluate the blood compatibility of degradation products for quality control and development of these devices. In the present study, we evaluated the degradation products of four biodegradable materials (collagen, polylactic acid, calcium phosphate ceramics, and magnesium) using platelet activation molecular markers that are associated with thrombosis. We found that the degradation products activate platelets to a certain extent, and that the degradation products produced during various degradation time periods activate platelets to varying degrees. This platelet activation occurs via several mechanisms, most of which are associated with the physicochemical properties of the degradation products, including ion concentration, pH, molecular microstructure, and molecular weight. Our findings not only provide a clearer understanding of the effects of degradation products from blood-contacting biodegradable devices, but also provide material for screening of degradation behavior so as to improve quality control for these devices.

In Vitro Alternatives for Ocular Safety Testing: An Outline of Assays and Possible Future Developments

An increasing number of in vitro tests are being proposed as alternatives for animals in ocular safety testing. These in vitro systems use a variety of approaches and measure a broad range of endpoints, including cellular cytotoxicity and metabolism. We will briefly review the development and validation of these model systems. Several tests appear promising, and may prove useful as adjuncts in our current ocular safety assessment schemes. We will also discuss how these tests can most rapidly be brought to use in ocular safety assessment, and where efforts should be directed to develop more-highly predictive in vitro model systems for the future.

Nanocomposite hydrogels for tissue engineering applications

Tissue engineering is an important field of regenerative medicine, which combines scaffolds and cell transplantation to develop substitute tissues and/or promote tissue regeneration. Hydrogels, a three-dimensional network with high water content and biocompatibility, have been widely used as scaffolds to mimic the structure and properties of tissues. However, the low mechanical strength and limited functions of traditional hydrogels greatly limited their applications in tissue engineering. Recently, nanocomposite hydrogels, with its advantages of high mechanical property and some unique properties (such as electrical conductivity, antibacterial, antioxidation, magnetic responsiveness), have emerged as the most versatile and innovative technology, which provides a new opportunity as a unique tool for fabricating hydrogels with excellent properties. In this review, we summarize the recent advances in fabricating nanocomposite hydrogels and their applications in tissue engineering. In addition, the future and prospects of nanocomposite hydrogels for tissue engineering are also discussed.

Continuous Formation of Ultrathin, Strong Collagen Sheets with Tunable Anisotropy and Compaction

The multiscale organization of protein-based fibrillar materials is a hallmark of many organs, but the recapitulation of hierarchal structures down to fibrillar scales, which is a requirement for withstanding physiological loading forces, has been challenging. We present a microfluidic strategy for the continuous, large-scale formation of strong, handleable, free-standing, multicentimeter-wide collagen sheets of unprecedented thinness through the application of hydrodynamic focusing with the simultaneous imposition of strain. Sheets as thin as 1.9 μm displayed tensile strengths of 0.5-2.7 MPa, Young's moduli of 3-36 MPa, and modulated the diffusion of molecules as a function of collagen nanoscale structure. Smooth muscle cells cultured on engineered sheets oriented in the direction of aligned collagen fibrils and generated coordinated vasomotor responses. The described biofabrication approach enables rapid formation of ultrathin collagen sheets that withstand physiologically relevant loads for applications in tissue engineering and regenerative medicine, as well as in organ-on-chip and biohybrid devices.

Application of tissue-engineered interventions for coronary artery bypass grafts

Coronary artery bypass graft is one of the extensively conducted procedures to release occlusion in the coronary vessel. Various biological grafts are used for this purpose, superiorly, saphenous vein graft, if unavailable, other vessels in the body, with likewise characteristics are exploited for the purpose. The choice of graft is yet under discovery that could impeccably meet all the requirements. Variation in perioperative and postoperative results have given uneven clinical inferences of these conduits. Alternatively, tissue-engineering is also being applied in this area for clinical improvements. This review underlines some of the commonly used grafts for coronary artery bypass graft and advancements in tissue engineering for this purpose.

Elastin-Based Materials: Promising Candidates for Cardiac Tissue Regeneration

Stroke and cardiovascular episodes are still some of the most common diseases worldwide, causing millions of deaths and costing billions of Euros to healthcare systems. The use of new biomaterials with enhanced biological and physical properties has opened the door to new approaches in cardiovascular applications. Elastin-based materials are biomaterials with some of the most promising properties. Indeed, these biomaterials have started to yield good results in cardiovascular and angiogenesis applications. In this review, we explore the latest trends in elastin-derived materials for cardiac regeneration and the different possibilities that are being explored by researchers to regenerate an infarcted muscle and restore its normal function. Elastin-based materials can be processed in different manners to create injectable systems or hydrogel scaffolds that can be applied by simple injection or as patches to cover the damaged area and regenerate it. Such materials have been applied to directly regenerate the damaged cardiac muscle and to create complex structures, such as heart valves or new bio-stents that could help to restore the normal function of the heart or to minimize damage after a stroke. We will discuss the possibilities that elastin-based materials offer in cardiac tissue engineering, either alone or in combination with other biomaterials, in order to illustrate the wide range of options that are being explored. Moreover, although tremendous advances have been achieved with such elastin-based materials, there is still room for new approaches that could trigger advances in cardiac tissue regeneration.

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.

Rapid endothelialization of small diameter vascular grafts by a bioactive integrin-binding ligand specifically targeting endothelial progenitor cells and endothelial cells

Establishing and maintaining a healthy endothelium on vascular and intravascular devices is crucial for the prevention of thrombosis and stenosis. Generating a biofunctional surface on vascular devices to recruit endothelial progenitor cells (EPCs) and endothelial cells (ECs) has proven efficient in promoting in situ endothelialization. However, molecules conventionally used for EPC/EC capturing generally lack structural stability, capturing specificity, and biological functionalities, which have limited their applications. Discovery of effective, specific, and structurally stable EPC/EC capturing ligands is desperately needed. Using the high-throughput One-Bead One-Compound combinatorial library screening technology, we recently identified a disulfide cyclic octa-peptide LXW7 (cGRGDdvc), which possesses strong binding affinity and functionality to EPCs/ECs, weak binding to platelets, and no binding to inflammatory cells. Because LXW7 is cyclic and 4 out of the 8 amino acids are unnatural D-amino acids, LXW7 is highly proteolytically stable. In this study, we applied LXW7 to modify small diameter vascular grafts using a Click chemistry approach. In vitro studies demonstrated that LXW7-modified grafts significantly improved EPC attachment, proliferation and endothelial differentiation and suppressed platelet attachment. In a rat carotid artery bypass model, LXW7 modification of the small diameter vascular grafts significantly promoted EPC/EC recruitment and rapidly achieved endothelialization. At 6 weeks after implantation, LXW7-modified grafts retained a high patency of 83%, while the untreated grafts had a low patency of 17%. Our results demonstrate that LXW7 is a potent EPC/EC capturing and platelet suppressing ligand and LXW7-modified vascular grafts rapidly generate a healthy and stable endothelial interface between the graft surface and the circulation to reduce thrombosis and improve patency. STATEMENT OF SIGNIFICANCE: In this study, One-Bead One-Compound (OBOC) technology has been applied for the first time in discovering bioactive ligands for tissue regeneration applications. Current molecules used to modify artificial vascular grafts generally lack EPC/EC capturing specificity, biological functionalities and structural stability. Using OBOC technology, we identified LXW7, a constitutionally stable disulfide cyclic octa-peptide with strong binding affinity and biological functionality to EPCs/ECs, very weak binding to platelets and no binding to inflammatory cells. These characteristics are crucial for promoting rapid endothelialization to prevent thrombosis and improve patency of vascular grafts. LXW7 coating technology could be applied to a wide range of vascular and intravascular devices, including grafts, stents, cardiac valves, and catheters, where a "living" endothelium and healthy blood interface are needed.

Thoracic aortic surgery: status and upcoming novelties

Several novel technological developments and surgical approaches have characterized the field of aortic surgery in the recent decade. The progressive introduction of endovascular procedures, minimally invasive surgical techniques and hybrid approaches have changed the practice in aortic surgery and generated new trends and questions. Also, the advancements in the manufacturing of tissue engineered vascular grafts as substitutes for aortic replacements are enlightening new avenues in the treatment of aortic disease. This review will provide an overview of the current novel perspectives, debates and trends in major thoracic aortic surgery.

Development of Poly(1,8-octanediol-co-citrate-co-ascorbate) Elastomers with Enhanced Ascorbate Performance for Use as a Graft Coating to Prevent Neointimal Hyperplasia

Small-diameter expanded polytetrafluoroethylene (ePTFE) graft surfaces have poor long-term patency due to limited endothelial cell (EC) coverage and anastomotic intimal hyperplasia. Multifunctional elastomers that coat the ePTFE graft surface to promote EC adhesion while simultaneously inhibiting intimal hyperplasia are highly desirable. Poly(diol-co-citrate) (PDC), a thermoset elastomer, is biodegradable, biocompatible, and mimics vascular mechanical properties. Engineering antioxidant components into PDC polymeric structures improves biocompatibility by attenuating oxidative stress yet is limited by bioavailability. Herein, we develop a new ascorbate protection and deprotection strategy (APDS) for loading bioactive ascorbic acid into the structure of PDC elastomers to improve poly(1,8-octanediol-co-citrate-co-ascorbate) (POCA) prepolymer ascorbate activity. Elastomers cured from APDS POCA prepolymers provide twice the active ascorbate sites on the elastomer surface (35.19 ± 1.64 ng mg-1 cm-2) versus unprotected POCA (Un.POCA, 18.31 ± 0.97 ng mg-1 cm-2). APDS POCA elastomers displayed suitable mechanical properties for vascular graft coating [Young's modulus (2.15-2.61 MPa), elongation (189.5-214.6%) and ultimate tensile strength (2.73-3.61 MPa)], and superior surface antioxidant performance through 1,1-diphenyl-2-picrylhydrazyl free radical scavenging and lipid peroxidation inhibition as compared to poly(1,8-octanediol-co-citrate) (POC) and Un.POCA. Hydrolytic degradation of APDS POCA occurred within 12 weeks under physiological conditions with a mass loss of 25.8 ± 3.4% and the degradation product retaining ascorbate activity. APDS POCA elastomer surfaces supported human aortic endothelial cell proliferation while inhibiting human aortic smooth muscle cell proliferation in vitro. APDS POCA elastomer surfaces displayed superior decomposition of S-nitrosothiols compared to POC and Un.POCA. Taken together, these findings indicate the potential of APDS POCA elastomers to serve as bioactive, therapeutic coatings that enhance the long-term patency of small diameter ePTFE grafts.

Engineering of Hydrogel Materials with Perfusable Microchannels for Building Vascularized Tissues

Vascular systems are responsible for various physiological and pathological processes related to all organs in vivo, and the survival of engineered tissues for enough nutrient supply in vitro. Thus, biomimetic vascularization is highly needed for constructing both a biomimetic organ model and a reliable engineered tissue. However, many challenges remain in constructing vascularized tissues, requiring the combination of suitable biomaterials and engineering techniques. In this review, the advantages of hydrogels on building engineered vascularized tissues are discussed and recent engineering techniques for building perfusable microchannels in hydrogels are summarized, including micromolding, 3D printing, and microfluidic spinning. Furthermore, the applications of these perfusable hydrogels in manufacturing organ-on-a-chip devices and transplantable engineered tissues are highlighted. Finally, current challenges in recapitulating the complexity of native vascular systems are discussed and future development of vascularized tissues is prospected.

3D Bioprinting Strategies for the Regeneration of Functional Tubular Tissues and Organs

It is difficult to fabricate tubular-shaped tissues and organs (e.g., trachea, blood vessel, and esophagus tissue) with traditional biofabrication techniques (e.g., electrospinning, cell-sheet engineering, and mold-casting) because these have complicated multiple processes. In addition, the tubular-shaped tissues and organs have their own design with target-specific mechanical and biological properties. Therefore, the customized geometrical and physiological environment is required as one of the most critical factors for functional tissue regeneration. 3D bioprinting technology has been receiving attention for the fabrication of patient-tailored and complex-shaped free-form architecture with high reproducibility and versatility. Printable biocomposite inks that can facilitate to build tissue constructs with polymeric frameworks and biochemical microenvironmental cues are also being actively developed for the reconstruction of functional tissue. In this review, we delineated the state-of-the-art of 3D bioprinting techniques specifically for tubular tissue and organ regeneration. In addition, this review described biocomposite inks, such as natural and synthetic polymers. Several described engineering approaches using 3D bioprinting techniques and biocomposite inks may offer beneficial characteristics for the physiological mimicry of human tubular tissues and organs.

J, McAllister TN, L'Heureux N. Human textiles: A cell-synthesized yarn as a truly "bio" material for tissue engineering applications

In the field of tissue engineering, many groups have come to rely on the extracellular matrix produced by cells as the scaffold that provides structure and strength to the engineered tissue. We have previously shown that sheets of Cell-Assembled extracellular Matrix (CAM), which are entirely biological yet robust, can be mass-produced for clinical applications using normal, adult, human fibroblasts. In this article, we demonstrate that CAM yarns can be generated with a range of physical and mechanical properties. We show that this material can be used as a simple suture to close a wound or can be assembled into fully biological, human, tissue-engineered vascular grafts (TEVGs) that have high mechanical strength and are implantable. By combining this truly "bio" material with a textile-based assembly, this original tissue engineering approach is highly versatile and can produce a variety of strong human textiles that can be readily integrated in the body. STATEMENT OF SIGNIFICANCE: Yarn of synthetic biomaterials have been turned into textiles for decades because braiding, knitting and weaving machines can mass-produce medical devices with a wide range of shapes and mechanical properties. Here, we show that robust, completely biological, and human yarn can be produced by normal cells in vitro. This yarn can be used as a simple suture material or to produce the first human textiles. For example, we produced a woven tissue-engineered vascular grafts with burst pressure, suture retention strength and transmural permeability that surpassed clinical requirements. This novel strategy holds the promise of a next generation of medical textiles that will be mechanically strong without any foreign scaffolding, and will have the ability to truly integrate into the host's body.

Bioresorbable silk grafts for small diameter vascular tissue engineering applications: In vitro and in vivo functional analysis

The success of tissue-engineered vascular graft (TEVG) predominantly relies on the selection of a suitable biomaterial and graft design. Natural biopolymer silk has shown great promise for various tissue-engineering applications. This study is the first to investigate Indian endemic non-mulberry silk (Antheraea assama-AA) - which inherits naturally superior mechanical and biological traits (e.g., RGD motifs) compared to Bombyx mori-BM silk, for TEVG applications. We designed bi-layered biomimetic small diameter AA-BM silk TEVGs adopting a new fabrication methodology. The inner layer showed ideally sized (~40 µm) pores with interconnectivity to allow cellular infiltration, and an outer dense electrospun layer that confers mechanical resilience. Biodegradation of silk TEVGs into amino acids as resorbable byproducts corroborates their in vivo remodeling ability. Following our previous reports, we surgically implanted human adipose tissue-derived stromal vascular fraction (SVF) seeded silk TEVGs in Lewis rats as abdominal aortic interposition grafts for 8 weeks. Adequate suture retention strength (0.45 ± 0.1 N) without any blood seepage post-implantation substantiate the grafts' viability. AA silk-based TEVGs showed superior animal survival and graft patency compared to BM silk TEVGs. Histological analysis revealed neo-tissue formation, host cell infiltration and graft remodeling in terms of extracellular matrix turnover. Altogether, this study demonstrates promising aspects of AA silk TEVGs for vascular tissue engineering applications. STATEMENT OF SIGNIFICANCE: Clinical 'off the shelf' implementation of tissue-engineered vascular grafts (TEVGs) remains a challenge. Achieving optimal blood vessel regeneration requires the use of bioresorbable materials having suitable degradation rates while producing minimal or no toxic byproducts. Host cell recruitment and preventing acute thrombosis are other pre-requisites for successful graft remodeling. In this study, for the first time we explored the use of naturally derived Indian endemic non-mulberry Antheraea assama silk in combination with Bombyx mori silk for TEVG applications by adopting a new biomimetic approach. Our bi-layered silk TEVGs were optimally porous, mechanically resilient and biodegradable. In vivo implantation in rat aorta showed long-term patency and graft remodeling by host cell infiltration and extracellular matrix deposition corroborating their clinical feasibility.

Artificial small-diameter blood vessels: materials, fabrication, surface modification, mechanical properties, and bioactive functionalities

Cardiovascular diseases, especially ones involving narrowed or blocked blood vessels with diameters smaller than 6 millimeters, are the leading cause of death globally. Vascular grafts have been used in bypass surgery to replace damaged native blood vessels for treating severe cardio- and peripheral vascular diseases. However, autologous replacement grafts are not often available due to prior harvesting or the patient's health. Furthermore, autologous harvesting causes secondary injury to the patient at the harvest site. Therefore, artificial blood vessels have been widely investigated in the last several decades. In this review, the progress and potential outlook of small-diameter blood vessels (SDBVs) engineered in vitro are highlighted and summarized, including material selection and development, fabrication techniques, surface modification, mechanical properties, and bioactive functionalities. Several kinds of natural and synthetic polymers for artificial SDBVs are presented here. Commonly used fabrication techniques, such as extrusion and expansion, electrospinning, thermally induced phase separation (TIPS), braiding, 3D printing, hydrogel tubing, gas foaming, and a combination of these methods, are analyzed and compared. Different surface modification methods, such as physical immobilization, surface adsorption, plasma treatment, and chemical immobilization, are investigated and are compared here as well. Mechanical requirements of SDBVs are also reviewed for long-term service. In vitro biological functions of artificial blood vessels, including oxygen consumption, nitric oxide (NO) production, shear stress response, leukocyte adhesion, and anticoagulation, are also discussed. Finally, we draw conclusions regarding current challenges and attempts to identify future directions for the optimal combination of materials, fabrication methods, surface modifications, and biofunctionalities. We hope that this review can assist with the design, fabrication, and application of SDBVs engineered in vitro and promote future advancements in this emerging research field.

Construction of vascular graft with circumferentially oriented microchannels for improving artery regeneration

Design and fabrication of scaffolds with three-dimensional (3D) topological cues inducing regeneration of the neo-tissue comparable to native one remains a major challenge in both scientific and clinical fields. Here, we developed a well-designed vascular graft with 3D highly interconnected and circumferentially oriented microchannels by using the sacrificial sugar microfiber leaching method. The microchannels structure was capable of promoting the migration, oriented arrangement, elongation, and the contractile phenotype expression of vascular smooth muscle cells (VSMCs) in vitro. After implantation into the rat aorta defect model, the microchannels in vascular grafts simultaneously improved the infiltration and aligned arrangement of VSMCs and the oriented deposition of extracellular matrix (ECM), as well as the recruitment and polarization of macrophages. These positive results also provided protection and support for ECs growth, and ultimately accelerated the endothelialization. Our research provides a new strategy for the fabrication of grafts with the capability of inducing arterial regeneration, which could be further extended to apply in preparing other kinds of oriented scaffolds aiming to guide oriented tissue in situ regeneration.

Vascular Microphysiological Systems to Model Diseases

Human vascular microphysiological systems (MPS) represent promising three-dimensional in vitro models of normal and diseased vascular tissue. These systems build upon advances in tissue engineering, microfluidics, and stem cell differentiation and replicate key functional units of organs and tissues. Vascular models have been developed for the microvasculature as well as medium-size arterioles. Key functions of the vascular system have been reproduced and stem cells offer the potential to model genetic diseases and population variation in genes that may increase individual risk for cardiovascular disease. Such systems can be used to evaluate new therapeutics options.

Potential of mesenchymal stem cells for bioengineered blood vessels in comparison with other eligible cell sources

Application of stem cells in tissue engineering has proved to be effective in many cases due to great proliferation and differentiation potentials as well as possible paracrine effects of these cells. Human mesenchymal stem cells (MSCs) are recognized as a valuable source for vascular tissue engineering, which requires endothelial and perivascular cells. The goal of this review is to survey the potential of MSCs for engineering functional blood vessels in comparison with other cell types including bone marrow mononuclear cells, endothelial precursor cells, differentiated adult autologous smooth muscle cells, autologous endothelial cells, embryonic stem cells, and induced pluripotent stem cells. In conclusion, MSCs represent a preference in making autologous tissue-engineered vascular grafts (TEVGs) as well as off-the-shelf TEVGs for emergency vascular surgery cases.

Proanthocyanidin-crosslinked collagen/konjac glucomannan hydrogel with improved mechanical properties and MRI trackable biodegradation for potential tissue engineering scaffolds

Collagen (Col) has been intensively exploited as a biomaterial for its excellent biocompatibility, biodegradation and bioactivity. However, the poor mechanical properties and rapid biodegradation of reconstituted collagen hydrogels have always been the bottlenecks for their further development especially for vascular tissue engineering. Herein, based on the self-assembly characteristics of collagen, a ternary hydrogel scaffold, comprising rigid collagen molecules, flexible konjac glucomannan (KGM) chains and biocompatible crosslinkers of proanthocyanidin (PA), has been designed to achieve a synergistic interaction for essentially optimizing the mechanical properties of the so-obtained Col/KGM/PA hydrogel, which possesses not only substantially improved strength but also good elasticity. PA endows these scaffolds with controllable biodegradation and anti-calcification and antioxidant activities. TEM discovered the co-existence of two types of fibrils with distinctly different arrangement patterns, explaining the contribution of KGM macromolecules to elasticity generation. The in vivo variations of Col/KGM/PA implants are visualized in real-time by magnetic resonance imaging (MRI). Moreover, a quantitative technique of MRI T₂-mapping combined with histology is designed to visualize the in vivo biodegradation mechanism of layer-by-layer erosion for these hydrogels. Simultaneously, three different relationships between the respective processes of in vivo degradation and in vivo dehydration of these controlled hydrogel implants were clearly revealed by this technique. Such a designed Col/KGM/PA composite hydrogel realizes the essential integration of good biocompatibility, controllable biodegradation and improved mechanical properties for developing a desired scaffold material for tissue engineering applications.

Application of Human Induced Pluripotent Stem Cells in Generating Tissue-Engineered Blood Vessels as Vascular Grafts

In pace with the advancement of tissue engineering during recent decades, tissue-engineered blood vessels (TEBVs) have been generated using primary seed cells, and their impressive success in clinical trials have demonstrated the great potential of these TEBVs as implantable vascular grafts in human regenerative medicine. However, the production, therapeutic efficacy, and readiness in emergencies of current TEBVs could be hindered by the accessibility, expandability, and donor-donor variation of patient-specific primary seed cells. Alternatively, using human induced pluripotent stem cells (hiPSCs) to derive seed vascular cells for vascular tissue engineering could fundamentally address this current dilemma in TEBV production. As an emerging research field with a promising future, the generation of hiPSC-based TEBVs has been reported recently with significant progress. Simultaneously, to further promote hiPSC-based TEBVs into vascular grafts for clinical use, several challenges related to the safety, readiness, and structural integrity of vascular tissue need to be addressed. Herein, this review will focus on the evolution and role of hiPSCs in vascular tissue engineering technology and summarize the current progress, challenges, and future directions of research on hiPSC-based TEBVs.

Electrospun nanofibers for the fabrication of engineered vascular grafts

Attention has recently increased in the application of electrospun fibers because of their putative capability to create nanoscale platforms toward tissue engineering. To some extent, electrospun fibers are applicable to the extracellular matrix by providing a three-dimensional microenvironment in which cells could easily acquire definite functional shape and maintain the cell-to-cell connection. It is noteworthy to declare that placement in different electrospun substrates with appropriate physicochemical properties enables cells to promote their bioactivities, dynamics growth and differentiation, leading to suitable restorative effects. This review paper aims to highlight the application of biomaterials in engineered vascular grafts by using electrospun nanofibers to promote angiogenesis and neovascularization.

Disease-inspired tissue engineering: Investigation of cardiovascular pathologies

Once focused exclusively on the creation of tissues to repair or replace diseased or damaged organs, the field of tissue engineering has undergone an important evolution in recent years. Namely, tissue engineering techniques are increasingly being applied to intentionally generate pathological conditions. Motivated in part by the wide gap between 2D cultures and animal models in the current disease modeling continuum, disease-inspired tissue-engineered platforms have numerous potential applications, and may serve to advance our understanding and clinical treatment of various diseases. This review will focus on recent progress toward generating tissue-engineered models of cardiovascular diseases, including cardiac hypertrophy, fibrosis, and ischemia reperfusion injury, atherosclerosis, and calcific aortic valve disease, with an emphasis on how these disease-inspired platforms can be used to decipher disease etiology. Each pathology is discussed in the context of generating both disease-specific cells as well as disease-specific extracellular environments, with an eye toward future opportunities to integrate different tools to yield more complex and physiologically relevant culture platforms. Ultimately, the development of effective disease treatments relies upon our ability to develop appropriate experimental models; as cardiovascular diseases are the leading cause of death worldwide, the insights yielded by improved in vitro disease modeling could have substantial ramifications for public health and clinical care.

Controlling magnesium corrosion and degradation-regulating mineralization using matrix GLA protein

Magnesium (Mg) alloys are embraced for their biodegradability and biocompatibility. However, Mg degrades spontaneously in the biological environment in vivo and in vitro, triggering deposition of calcium phosphate on the metal. Upon complete metal absorption, minerals remain in the tissue, which could lead to pathogenic calcification. Hence, our aims are to test the feasibility of matrix GLA protein (MGP) to locally inhibit Mg mineralization that is induced by metal alloy degradation. MGP is a small secretory protein that has been shown to inhibit soft tissue calcification. We exposed Mg to MGP, stably transfected into mammalian cells. Results showed that less calcium and phosphorous deposition on the Mg surface when MGP was present relative to when it was not. In the in vivo mouse intramuscular model conducted for 4 and 6 weeks, Mg rods were embedded in collagen scaffolds, seeded with cells overexpressing MGP. Microtomography, electron dispersive x-ray spectroscopy, and histology assessments revealed lower deposited mineral volume around Mg rods from the MGP group. Compared to other groups, higher volume loss after implantation was observed from the MGP group at both time points, indicating a higher corrosion rate without the protective mineral layer. This study is the first to our knowledge to demonstrate that local exposure to a biomolecule, such as MGP, can modulate the corrosion of Mg-based implants. These findings may have important implications for the future design of endovascular stents and orthopedic devices.

Fabrication Techniques for Vascular and Vascularized Tissue Engineering

Impaired or damaged blood vessels can occur at all levels in the hierarchy of vascular systems from large vasculatures such as arteries and veins to meso- and microvasculatures such as arterioles, venules, and capillary networks. Vascular tissue engineering has become a promising approach for fabricating small-diameter vascular grafts for occlusive arteries. Vascularized tissue engineering aims to fabricate meso- and microvasculatures for the prevascularization of engineered tissues and organs. The ideal small-diameter vascular graft is biocompatible, bridgeable, and mechanically robust to maintain patency while promoting tissue remodeling. The desirable fabricated meso- and microvasculatures should rapidly integrate with the host blood vessels and allow nutrient and waste exchange throughout the construct after implantation. A number of techniques used, including engineering-based and cell-based approaches, to fabricate these synthetic vasculatures are herein explored, as well as the techniques developed to fabricate hierarchical structures that comprise multiple levels of vasculature.

Evaluating platelet activation related to the degradation of biomaterials using molecular markers

The effective assessment of platelet activation is an important component of the evaluation of cardiovascular implants. Currently, most evaluation is performed based on the ISO 10993-4 international standard. However, the methods specified in this standard were originally designed for non-degradable materials, and the applicability of these methods to evaluate degradable materials has not been carefully assessed. Here, the platelet activation response was evaluated (using blood from health rabbits) for three typical degradable materials (collagen, polylactic acid, and hydroxyapatite) by measuring the widely used molecular markers CD62 P, CD63, and CD40 L and the three molecular markers PF4, β-TG, and TXB2 that are referenced in the ISO 10993-4 standard. The variations of these six markers were compared in the simulated degradation of the three test materials. The results showed differences in platelet activation with degradation that were strongly related to the surface physicochemical properties. Changes in the surface roughness and contact angle of the materials correlated with changes in the degree of platelet activation. The six tested platelet activation molecular markers show promise for assessment of platelet function in degradable medical devices, providing guidance for quality control strategies and the design and improvement of safe medical devices.

The early days of vascular and heart valve prostheses: a historical review

This surgical heritage article provides a historical overview of the most important early advances of vascular- and valvular surgery, that lead to the development of currently used vascular- and valvular prostheses and materials. The first writings describing techniques in vascular surgery mainly focussed on hemorrhage control and date from around 1600 B.C. The strategy of vessel ligation was first mentioned in Western literature around 200 B.C. In the 18^th century, techniques of ligation were expanded towards attempts of vessel restoration. The first artificial vascular prosthesis was made in 1894. From this time on, vascular prostheses were used in animal experiments and around 1900 for the first time in humans. More than 60 years later, in 1952, the first mechanical heart valve prosthesis was implanted. Four years later, the first successful biological heart valve implantation followed. In 2000, a transcatheter heart valve was successfully implanted in a human for the first time. Over time, procedures and techniques became more efficient and effective. This led to new developments, such as the manufacturing of a tissue engineered blood vessel in 1986. Nowadays, dozens of different valve prostheses have been devised, both mechanical and biological. Still, no ideal model of vascular and heart valve prosthesis exists.

Bioprinting of freestanding vascular grafts and the regulatory considerations for additively manufactured vascular prostheses

Vasculature is the network of blood vessels of an organ or body part that allow for the exchange of nutrients and waste to and from every cell, thus establishing a circulatory equilibrium. Vascular health is at risk from a variety of conditions that includes disease and trauma. In some cases, medical therapy can alleviate the impacts of the condition. Intervention is needed in other instances to restore the health of abnormal vasculature. The main approaches to treat vascular conditions are endovascular procedures and open vascular reconstruction that often requires a graft to accomplish. However, current vascular prostheses have limitations that include size mismatch with the native vessel, risk of immunogenicity from allografts and xenografts, and unavailability of autografts. In this review, we discuss efforts in bioprinting, an emerging method for vascular reconstruction. This includes an overview of 3D printing processes and materials, graft characterization strategies and the regulatory aspects to consider for the commercialization of 3D bioprinted vascular prostheses.