Functional arteries grown in vitro

3D Bioprinting and Translation of Beta Cell Replacement Therapies for Type 1 Diabetes

Type 1 diabetes (T1D) is an autoimmune disorder in which the body's own immune system selectively attacks beta cells within pancreatic islets resulting in insufficient insulin production and loss of the ability to regulate blood glucose (BG) levels. Currently, the standard of care consists of BG level monitoring and insulin administration, which are essential to avoid the consequences of dysglycemia and long-term complications. Although recent advances in continuous glucose monitoring and automated insulin delivery systems have resulted in improved clinical outcomes for users, nearly 80% of people with T1D fail to achieve their target hemoglobin A1c (HbA1c) levels defined by the American Diabetes Association. Intraportal islet transplantation into immunosuppressed individuals with T1D suffering from impaired awareness of hypoglycemia has resulted in lower HbA1c, elimination of severe hypoglycemic events, and insulin independence, demonstrating the unique potential of beta cell replacement therapy (BCRT) in providing optimal glycemic control and a functional cure for T1D. BCRTs need to maximize cell engraftment, long-term survival, and function in the absence of immunosuppression to provide meaningful clinical outcomes to all people living with T1D. One innovative technology that could enable widespread translation of this approach into the clinic is three-dimensional (3D) bioprinting. Herein, we review how bioprinting could facilitate translation of BCRTs as well as the current and forthcoming techniques used for bioprinting of a BCRT product. We discuss the strengths and weaknesses of 3D bioprinting in this context in addition to the road ahead for the development of BCRTs. Impact statement Significant research developments in beta cell replacement therapies show its promise in providing a functional cure for type 1 diabetes (T1D); yet, their widespread clinical use has been difficult to achieve. This review provides a brief overview of the requirements for a beta cell replacement product followed by a discussion on both the promise and limitations of three-dimensional bioprinting in facilitating the fabrication of such products to enable translation into the clinic. Advancements in this area could be a key component to unlocking the safety and effectiveness of beta cell therapy for T1D.

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.

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.

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.

Formation of oriented fishbone-like pores in biodegradable polymer scaffolds using directional phase-separation processing

The scaffold is a dreamed biomaterial of tissue engineers which can culture cells three-dimensionally outgrowing the two-dimensional cell culture in a petri dish to repair or regenerate tissues and organs. To maximize the performance of this dreamed material, complex three-dimensional (3D) structures should be generated with a simple technique and nontoxic ingredients. Many tissues have tubular or fibrous bundle architectures such as nerve, muscle, tendon, ligament, blood vessel, bone and teeth. The concept of mimicking the extracellualr matrix in real tissue has recently been applied to scaffold development. In this study, a novel method for preparing the poly(L-lactic acid) (PLLA) scaffold with a tubular architecture is presented. Solid-liquid phase-separation was applied to form tubular pores in the scaffold using the directional freezing apparatus. Pores formed in this manner exhibited a fishbone like morphology due to the two crystalline phases of 1,4-dioxane. A tubular diameter of ca. 60-250 μm was achieved by regulating the PLLA concentration and the cooling rate. The compressive modulus of the fishbone-like porous scaffold showed higher values than that of non-directional porous scaffold.

Human iPSCs Stretch to Improve Tissue-Engineered Vascular Grafts

Human induced pluripotent stem cells (hiPSCs) provide a potentially unlimited cell source for producing autologous tissue-engineered vascular grafts (TEVGs), which currently suffer from low mechanical strength. In this issue of Cell Stem Cell, Luo et al. (2020) describe optimized culture media and a mechanical stretching regimen to produce hiPSC-derived TEVGs with mechanical behavior similar to that of natural vessels.

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.

Modification of decellularized vascular xenografts with 8-arm polyethylene glycol suppresses macrophage infiltration but maintains graft degradability

Because acellular vascular xenografts induce an immunological reaction through macrophage infiltration, they are conventionally crosslinked with glutaraldehyde (GA). However, the GA crosslinking reaction inhibits not only the host immune reaction around the graft but also the graft's enzymatic degradability, which is one of the key characteristics of acellular grafts that allow them to be replaced by host tissue. In this study, we used an 8-arm polyethylene glycol (PEG) to successfully suppress macrophage infiltration, without eliminating graft degradation. Decellularized ostrich carotid arteries were modified with GA or N-hydroxysuccinimide-activated 8-arm PEG (8-arm PEG-NHS), which has a molecular weight of 17 kDa. To evaluate the enzymatic degradation in vitro, the graft was immersed in a collagenase solution for 12 hr. The 8-arm PEG-modified graft was degraded to the same extent as the unmodified graft, but the GA-modified graft was not degraded. The graft was transplanted into rat subcutaneous tissue for up to 8 weeks. Although CD68-positive cells accumulated in the unmodified graft, they did not infiltrate into either modified graft. However, the GA-modified grafts calcified, but the 8-arm PEG-modified graft did not calcify after transplantation. These data suggested that 8-arm PEG-NHS is a promising modification agent for biodegradable vascular xenografts, to suppress acute macrophage infiltration only.

Bioengineered human acellular vessels recellularize and evolve into living blood vessels after human implantation

Traditional vascular grafts constructed from synthetic polymers or cadaveric human or animal tissues support the clinical need for readily available blood vessels, but often come with associated risks. Histopathological evaluation of these materials has shown adverse host cellular reactions and/or mechanical degradation due to insufficient or inappropriate matrix remodeling. We developed an investigational bioengineered human acellular vessel (HAV), which is currently being studied as a hemodialysis conduit in patients with end-stage renal disease. In rare cases, small samples of HAV were recovered during routine surgical interventions and used to examine the temporal and spatial pattern of the host cell response to the HAV after implantation, from 16 to 200 weeks. We observed a substantial influx of alpha smooth muscle actin (αSMA)-expressing cells into the HAV that progressively matured and circumferentially aligned in the HAV wall. These cells were supported by microvasculature initially formed by CD34⁺/CD31⁺ cells in the neoadventitia and later maintained by CD34^-/CD31⁺ endothelial cells in the media and lumen of the HAV. Nestin⁺ progenitor cells differentiated into either αSMA⁺ or CD31⁺ cells and may contribute to early recellularization and self-repair of the HAV. A mesenchymal stem cell-like CD90⁺ progenitor cell population increased in number with duration of implantation. Our results suggest that host myogenic, endothelial, and progenitor cell repopulation of HAVs transforms these previously acellular vessels into functional multilayered living tissues that maintain blood transport and exhibit self-healing after cannulation injury, effectively rendering these vessels like the patient's own blood vessel.

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.

3D printing for tissue engineering in otolaryngology

Airway and other head and neck disorders affect hundreds of thousands of patients each year and most require surgical intervention. Among these, congenital deformity that affects newborns is particularly serious and can be life-threatening. In these cases, reconstructive surgery is resolutive but bears significant limitations, including the donor site morbidity and limited available tissue. In this context, tissue engineering represents a promising alternative approach for the surgical treatment of otolaryngologic disorders. In particular, 3D printing coupled with advanced imaging technologies offers the unique opportunity to reproduce the complex anatomy of native ear, nose, and throat, with its import in terms of functionality as well as aesthetics and the associated patient well-being. In this review, we provide a general overview of the main ear, nose and throat disorders and focus on the most recent scientific literature on 3D printing and bioprinting for their treatment.

Engineering Biomaterials with Micro/Nanotechnologies for Cell Reprogramming

Cell reprogramming is a revolutionized biotechnology that offers a powerful tool to engineer cell fate and function for regenerative medicine, disease modeling, drug discovery, and beyond. Leveraging advances in biomaterials and micro/nanotechnologies can enhance the reprogramming performance in vitro and in vivo through the development of delivery strategies and the control of biophysical and biochemical cues. In this review, we present an overview of the state-of-the-art technologies for cell reprogramming and highlight the recent breakthroughs in engineering biomaterials with micro/nanotechnologies to improve reprogramming efficiency and quality. Finally, we discuss future directions and challenges for reprogramming technologies and clinical translation.

Sequential adaptation of perfusion and transport conditions significantly improves vascular construct recellularization and biomechanics

Recellularization of ex vivo-derived scaffolds remains a significant hurdle primarily due to the scaffolds subcellular pore size that restricts initial cell seeding to the scaffolds periphery and inhibits migration over time. With the aim to improve cell migration, repopulation, and graft mechanics, the effects of a four-step culture approach were assessed. Using an ex vivo-derived vein as a model scaffold, human smooth muscle cells were first seeded onto its ablumen (Step 1: 3 hr) and an aggressive 0-100% nutrient gradient (lumenal flow under hypotensive pressure) was created to initiate cell migration across the scaffold (Step 2: Day 0 to 19). The effects of a prolonged aggressive nutrient gradient created by this single lumenal flow was then compared with a dual flow (lumenal and ablumenal) in Step 3 (Day 20 to 30). Analyses showed that a single lumenal flow maintained for 30 days resulted in a higher proportion of cells migrating across the scaffold toward the vessel lumen (nutrient source), with improved distribution. In Step 4 (Day 31 to 45), the transition from hypotensive pressure (12/8 mmHg) to normotensive (arterial-like) pressure (120/80 mmHg) was assessed. It demonstrated that recellularized scaffolds exposed to arterial pressures have increased glycosaminoglycan deposition, physiological modulus, and Young's modulus. By using this stepwise conditioning, the challenging recellularization of a vein-based scaffold and its positive remodeling toward arterial biomechanics were obtained.

Tissue-Engineered Vascular Grafts with Advanced Mechanical Strength from Human iPSCs

Vascular smooth muscle cells (VSMCs) can be derived in large numbers from human induced pluripotent stem cells (hiPSCs) for producing tissue-engineered vascular grafts (TEVGs). However, hiPSC-derived TEVGs are hampered by low mechanical strength and significant radial dilation after implantation. Here, we report generation of hiPSC-derived TEVGs with mechanical strength comparable to native vessels used in arterial bypass grafts by utilizing biodegradable scaffolds, incremental pulsatile stretching, and optimal culture conditions. Following implantation into a rat aortic model, hiPSC-derived TEVGs show excellent patency without luminal dilation and effectively maintain mechanical and contractile function. This study provides a foundation for future production of non-immunogenic, cellularized hiPSC-derived TEVGs composed of allogenic vascular cells, potentially serving needs to a considerable number of patients whose dysfunctional vascular cells preclude TEVG generation via other methods.

Modular Fabrication of Intelligent Material-Tissue Interfaces for Bioinspired and Biomimetic Devices

One of the goals of biomaterials science is to reverse engineer aspects of human and nonhuman physiology. Similar to the body's regulatory mechanisms, such devices must transduce changes in the physiological environment or the presence of an external stimulus into a detectable or therapeutic response. This review is a comprehensive evaluation and critical analysis of the design and fabrication of environmentally responsive cell-material constructs for bioinspired machinery and biomimetic devices. In a bottom-up analysis, we begin by reviewing fundamental principles that explain materials' responses to chemical gradients, biomarkers, electromagnetic fields, light, and temperature. Strategies for fabricating highly ordered assemblies of material components at the nano to macro-scales via directed assembly, lithography, 3D printing and 4D printing are also presented. We conclude with an account of contemporary material-tissue interfaces within bioinspired and biomimetic devices for peptide delivery, cancer theranostics, biomonitoring, neuroprosthetics, soft robotics, and biological machines.

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.

Layer-specific cell differentiation in bi-layered vascular grafts under flow perfusion

Bioengineered grafts have the potential to overcome the limitations of autologous and non-resorbable synthetic vessels as vascular substitutes. However, one of the challenges in creating these living grafts is to induce and maintain multiple cell phenotypes with a biomimetic organization. Our biomimetic grafts with heterotypic design hold promises for functional neovessel regeneration by guiding the layered cellular and tissue organization into a native-like structure. In this study, a perfusable two-compartment bioreactor chamber was designed for the further maturation of these vascular grafts, with a compartmentalized exposure of the graft's luminal and outer layer to cell-specific media. We used the system for a co-culture of endothelial colony forming cells and multipotent mesenchymal stromal cells (MSCs) in the vascular grafts, produced by combining electrospinning and melt electrowriting. It was demonstrated that the targeted cell phenotypes (i.e. endothelial cells (ECs) and vascular smooth muscle cells (vSMCs), respectively) could be induced and maintained during flow perfusion. The confluent luminal layer of ECs showed flow responsiveness, as indicated by the upregulation of COX-2, KLF2, and eNOS, as well as through stress fiber remodeling and cell elongation. In the outer layer, the circumferentially oriented, multi-layered structure of MSCs could be successfully differentiated into vSM-like cells using TGFβ, as indicated by the upregulation of αSMA, calponin, collagen IV, and (tropo)elastin, without affecting the endothelial monolayer. The cellular layers inhibited diffusion between the outer and the inner medium reservoirs. This implies tightly sealed cellular layers in the constructs, resulting in truly separated bioreactor compartments, ensuring the exposure of the inner endothelium and the outer smooth muscle-like layer to cell-specific media. In conclusion, using this system, we successfully induced layer-specific cell differentiation with a native-like cell organization. This co-culture system enables the creation of biomimetic neovessels, and as such can be exploited to investigate and improve bioengineered vascular grafts.

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.

Autologous endothelialized small-caliber vascular grafts engineered from blood-derived induced pluripotent stem cells

An ideal cell source for human therapeutic and disease modeling applications should be easily accessible and possess unlimited differentiation and expansion potential. Human induced pluripotent stem cells (hiPSCs) derived from peripheral blood mononuclear cells (PBMCs) represent a promising source given their ease of harvest and their pluripotent nature. Previous studies have demonstrated the feasibility of using PBMC-derived hiPSCs for vascular tissue engineering. However, so far, no endothelialization of hiPSC-derived tissue engineered vascular grafts (TEVGs) based on fully biodegradable polymers without xenogenic matrix components has been shown. In this study, we have generated hiPSCs from PBMCs and differentiated them into αSMA- and calponin-positive smooth muscle cells (SMCs) as well as endothelial cells (ECs) positive for CD31, vWF and eNOS. Both cell types were co-seeded on PGA-P4HB starter matrices and cultured under static or dynamic conditions to induce tissue formation in vitro. The resulting small diameter vascular grafts showed abundant amounts of extracellular matrix, containing a thin luminal layer of vWF-positive cells and a subendothelial αSMA-positive layer approximating the architecture of native vessels. Our results demonstrate the successful generation of TEVGs based on SMCs and ECs differentiated from PBMC-derived hiPSC combined with a biodegradable polymer. These results pave the way for developing autologous PBMC-derived hiPSC-based vascular constructs for therapeutic applications or disease modeling. STATEMENT OF SIGNIFICANCE: We report for the first time the possibility to employ human peripheral blood mononuclear cell (PBMC)-derived iPSCs to generate biodegradable polymer-based tissue engineered vascular grafts (TEVG), which mimic the native layered architecture of blood vessels. hiPSCs from PBMCs were differentiated into smooth muscle cells as well as endothelial cells. These cells were co-seeded on a biodegradable PGA/P4HB scaffold and cultured in a bioreactor to induce tissue formation in vitro. The resulting small diameter TEVG showed abundant amounts of extracellular matrix, containing a αSMA-positive layer in the interstitium and a thin luminal layer of vWF-positive endothelial cells approximating the architecture of native vessels. Our findings improving the generation of autologous vascular replacements using blood as an easily accessible cell source.

Application of in-body tissue architecture-induced Biotube vascular grafts for vascular access: Proof of concept in a beagle dog model

INTRODUCTION

The first choice of vascular access for hemodialysis is an autogenous arteriovenous fistula, because prosthetic arteriovenous grafts have a high probability of failure. In this study, Biotubes, in-body tissue architecture-induced autologous collagenous tubes, were evaluated for their potential use as vascular access grafts. Three animal implantation models were developed using beagle dogs, and the in vivo performance of Biotubes was observed after implantation in the acute phase as a pilot study.

METHODS

Biotubes (internal diameter ca. 4.0 mm, length ca. 5.0 cm, and wall thickness ca. 0.7 mm) were prepared through subcutaneous embedding of specially designed molds in beagle dogs for 8 weeks. The Biotubes were then implanted between the common carotid artery and the jugular vein of beagles via three methods, including side-to-side (in) -end-to-end (out) as type 1 (n = 4), side-to-side (both) as type 2 (n = 4), and side-to-end (in) -end-to-side (out) as type 3 (n = 1 using a composite Biotube).

RESULTS

Although two cases in type 1 and 2 resulted in Biotube deformation, all cases were patent for 4 weeks and maintained a continuous turbulent flow. At 4 weeks after implantation, percutaneous puncture could be performed repeatedly without aneurysm formation or hemorrhage.

CONCLUSION

Within a short implantation period, with limited animal numbers, this proof-of-concept study showed that Biotubes may have a high potential for use in vascular access.

Generation of a Purified iPSC-Derived Smooth Muscle-like Population for Cell Sheet Engineering

Induced pluripotent stem cells (iPSCs) provide a potential source for the derivation of smooth muscle cells (SMCs); however, current approaches are limited by the production of heterogeneous cell types and a paucity of tools or markers for tracking and purifying candidate SMCs. Here, we develop murine and human iPSC lines carrying fluorochrome reporters (Acta2hrGFP and ACTA2eGFP, respectively) that identify Acta2+/ACTA2+ cells as they emerge in vitro in real time during iPSC-directed differentiation. We find that Acta2hrGFP+ and ACTA2eGFP+ cells can be sorted to purity and are enriched in markers characteristic of an immature or synthetic SMC. We characterize the resulting GFP+ populations through global transcriptomic profiling and functional studies, including the capacity to form engineered cell sheets. We conclude that these reporter lines allow for generation of sortable, live iPSC-derived Acta2+/ACTA2+ cells highly enriched in smooth muscle lineages for basic developmental studies, tissue engineering, or future clinical regenerative applications.

3D bioprinting of vascular conduits for pediatric congenital heart repairs

In children with congenital heart defects, surgical correction often involves the use of valves, patches or vascular conduits to establish anatomic continuity. Due to the differences between the pediatric and adult populations, tissue reconstruction in pediatric patients requires a substantially different approach from those in adults. Cardiovascular anatomy of children with congenital heart defect vary, which requires tailored surgical operations for each patient. Since grafts used in these palliative surgeries are sensitive to the local hemodynamic environments, their geometries need to be precisely designed to ensure long-term performance. Tissue engineered vascular grafts (TEVGs) have made tremendous progress over the past decade, but it remains difficult to fabricate patient- and operation-specific vascular grafts. This review summarizes historical milestones of TEVG development for repairing pediatric congenital defects and current clinical outcomes. We also highlight ongoing works on 3D bioprinting of TEVGs with complex geometries and address the current limitations of each technique. Although 3D bioprinted vascular grafts with appropriate functions are yet to be developed, some of the current researches are promising to create better patient specific tissue engineered vascular grafts in the future.

Chemical and Biological Approaches to Regenerative Medicine and Tissue Engineering

The loss or failure of an organ or tissue is one of the most frequent, devastating, and costly problems in human healthcare. The areas of regenerative medicine and tissue engineering apply the principles of chemistry, biology, and engineering to create new tissues and organs. Here we discuss some of the early work in this field and, in particular, review our studies combining chemistry, materials science, biology, and engineering to create new tissues and organs.

Tuning the biomimetic behavior of scaffolds for regenerative medicine through surface modifications

Tissue engineering and regenerative medicine rely extensively on biomaterial scaffolds to support cell adhesion, proliferation, and differentiation physically and chemically in vitro and in vivo. Changes to the surface characteristics of the scaffolds have the greatest impact on cell response. Here, we discuss five dominant surface modification approaches used to biomimetically improve the most common scaffolds for tissue engineering, those based on aliphatic polyesters. Scaffolds of aliphatic polyesters such as poly(l-lactic acid), poly(l-lactic-co-glycolic acid), and poly(ε-caprolactone) are often used in tissue engineering because they provide desirable, tunable properties such as ease of manufacturing, good mechanical properties, and nontoxic degradation products. However, cell-surface interactions necessary for tissue engineering are limited on these materials by their smooth postfabrication surfaces, hydrophobicity, and lack of recognizable biochemical binding sites. The surface modification techniques that have been developed for synthetic polymer scaffolds reduce initial barriers to cell adhesion, proliferation, and differentiation. Topographical modification, protein adsorption, mineral coating, functional group incorporation, and biomacromolecule immobilization each contribute through varying mechanisms to improving cell interactions with aliphatic polyester scaffolds. Furthermore, rational combination of methods from these categories can provide nuanced, specific environments for targeted tissue development.

Differential outcomes of venous and arterial tissue engineered vascular grafts highlight the importance of coupling long-term implantation studies with computational modeling

Electrospinning is commonly used to generate polymeric scaffolds for tissue engineering. Using this approach, we developed a small-diameter tissue engineered vascular graft (TEVG) composed of poly-ε-caprolactone-co-l-lactic acid (PCLA) fibers and longitudinally assessed its performance within both the venous and arterial circulations of immunodeficient (SCID/bg) mice. Based on in vitro analysis demonstrating complete loss of graft strength by 12 weeks, we evaluated neovessel formation in vivo over 6-, 12- and 24-week periods. Mid-term observations indicated physiologic graft function, characterized by 100% patency and luminal matching with adjoining native vessel in both the venous and arterial circulations. An active and robust remodeling process was characterized by a confluent endothelial cell monolayer, macrophage infiltrate, and extracellular matrix deposition and remodeling. Long-term follow-up of venous TEVGs at 24 weeks revealed viable neovessel formation beyond graft degradation when implanted in this high flow, low-pressure environment. Arterial TEVGs experienced catastrophic graft failure due to aneurysmal dilatation and rupture after 14 weeks. Scaffold parameters such as porosity, fiber diameter, and degradation rate informed a previously described computational model of vascular growth and remodeling, and simulations predicted the gross differential performance of the venous and arterial TEVGs over the 24-week time course. Taken together, these results highlight the requirement for in vivo implantation studies to extend past the critical time period of polymer degradation, the importance of differential neotissue deposition relative to the mechanical (pressure) environment, and further support the utility of predictive modeling in the design, use, and evaluation of TEVGs in vivo. STATEMENT OF SIGNIFICANCE: Herein, we apply a biodegradable electrospun vascular graft to the arterial and venous circulations of the mouse and follow recipients beyond the point of polymer degradation. While venous implants formed viable neovessels, arterial grafts experienced catastrophic rupture due to aneurysmal dilation. We then inform a previously developed computational model of tissue engineered vascular graft growth and remodeling with parameters specific to the electrospun scaffolds utilized in this study. Remarkably, model simulations predict the differential performance of the venous and arterial constructs over 24 weeks. We conclude that computational simulations should inform the rational selection of scaffold parameters to fabricate tissue engineered vascular grafts that must be followed in vivo over time courses extending beyond polymer degradation.

Rapid fabrication of reinforced and cell-laden vascular grafts structurally inspired by human coronary arteries

Design strategies for small diameter vascular grafts are converging toward native-inspired tissue engineered grafts. A new automated technology is presented that combines a dip-spinning methodology for depositioning concentric cell-laden hydrogel layers, with an adapted solution blow spinning (SBS) device for intercalated placement of aligned reinforcement nanofibres. This additive manufacture approach allows the assembly of bio-inspired structural configurations of concentric cell patterns with fibres at specific angles and wavy arrangements. The middle and outer layers were tuned to structurally mimic the media and adventitia layers of native arteries, enabling the fabrication of small bore grafts that exhibit the J-shape mechanical response and compliance of human coronary arteries. This scalable automated system can fabricate cellularized multilayer grafts within 30 min. Grafts were evaluated by hemocompatibility studies and a preliminary in vivo carotid rabbit model. The dip-spinning-SBS technology generates constructs with native mechanical properties and cell-derived biological activities, critical for clinical bypass applications.

Collagen-Based Tissue Engineering Strategies for Vascular Medicine

Cardiovascular diseases (CVDs) account for the 31% of total death per year, making them the first cause of death in the world. Atherosclerosis is at the root of the most life-threatening CVDs. Vascular bypass/replacement surgery is the primary therapy for patients with atherosclerosis. The use of polymeric grafts for this application is still burdened by high-rate failure, mostly caused by thrombosis and neointima hyperplasia at the implantation site. As a solution for these problems, the fast re-establishment of a functional endothelial cell (EC) layer has been proposed, representing a strategy of crucial importance to reduce these adverse outcomes. Implant modifications using molecules and growth factors with the aim of speeding up the re-endothelialization process has been proposed over the last years. Collagen, by virtue of several favorable properties, has been widely studied for its application in vascular graft enrichment, mainly as a coating for vascular graft luminal surface and as a drug delivery system for the release of pro-endothelialization factors. Collagen coatings provide receptor-ligand binding sites for ECs on the graft surface and, at the same time, act as biological sealants, effectively reducing graft porosity. The development of collagen-based drug delivery systems, in which small-molecule and protein-based drugs are immobilized within a collagen scaffold in order to control their release for biomedical applications, has been widely explored. These systems help in protecting the biological activity of the loaded molecules while slowing their diffusion from collagen scaffolds, providing optimal effects on the targeted vascular cells. Moreover, collagen-based vascular tissue engineering substitutes, despite not showing yet optimal mechanical properties for their use in the therapy, have shown a high potential as physiologically relevant models for the study of cardiovascular therapeutic drugs and diseases. In this review, the current state of the art about the use of collagen-based strategies, mainly as a coating material for the functionalization of vascular graft luminal surface, as a drug delivery system for the release of pro-endothelialization factors, and as physiologically relevant in vitro vascular models, and the future trend in this field of research will be presented and discussed.

Captopril treatment during development alleviates mechanically induced aortic remodeling in newborn elastin knockout mice

Deposition of elastin and collagen in the aorta correlates with increases in blood pressure and flow during development, suggesting that the aorta adjusts its mechanical properties in response to hemodynamic stresses. Elastin knockout (Eln^-/-) mice have high blood pressure and pathological remodeling of the aorta and die soon after birth. We hypothesized that decreasing blood pressure in Eln^-/- mice during development may reduce hemodynamic stresses and alleviate pathological remodeling of the aorta. We treated Eln^+/+ and Eln^-/- mice with the anti-hypertensive medication captopril throughout embryonic development and then evaluated left ventricular (LV) pressure and aortic remodeling at birth. We found that captopril treatment decreased Eln^-/- LV pressure to values near Eln^+/+ mice and alleviated the wall thickening and changes in mechanical behavior observed in untreated Eln^-/- aorta. The changes in thickness and mechanical behavior in captopril-treated Eln^-/- aorta were not due to alterations in measured elastin or collagen amounts, but may have been caused by alterations in smooth muscle cell (SMC) properties. We used a constitutive model to understand how changes in stress contributions of each wall component could explain the observed changes in composite mechanical behavior. Our modeling results show that alterations in the collagen natural configuration and SMC properties in the absence of elastin may explain untreated Eln^-/- aortic behavior and that partial rescue of the SMC properties may account for captopril-treated Eln^-/- aortic behavior.

Regenerative and durable small-diameter graft as an arterial conduit

Despite significant research efforts, clinical practice for arterial bypass surgery has been stagnant, and engineered grafts continue to face postimplantation challenges. Here, we describe the development and application of a durable small-diameter vascular graft with tailored regenerative capacity. We fabricated small-diameter vascular grafts by electrospinning fibrin tubes and poly(ε-caprolactone) fibrous sheaths, which improved suture retention strength and enabled long-term survival. Using surface topography in a hollow fibrin microfiber tube, we enable immediate, controlled perfusion and formation of a confluent endothelium within 3-4 days in vitro with human endothelial colony-forming cells, but a stable endothelium is noticeable at 4 weeks in vivo. Implantation of acellular or endothelialized fibrin grafts with an external ultrathin poly(ε-caprolactone) sheath as an interposition graft in the abdominal aorta of a severe combined immunodeficient Beige mouse model supports normal blood flow and vessel patency for 24 weeks. Mechanical properties of the implanted grafts closely approximate the native abdominal aorta properties after just 1 week in vivo. Fibrin mediated cellular remodeling, stable tunica intima and media formation, and abundant matrix deposition with organized collagen layers and wavy elastin lamellae. Endothelialized grafts evidenced controlled healthy remodeling with delayed and reduced macrophage infiltration alongside neo vasa vasorum-like structure formation, reduced calcification, and accelerated tunica media formation. Our studies establish a small-diameter graft that is fabricated in less than 1 week, mediates neotissue formation and incorporation into the native tissue, and matches the native vessel size and mechanical properties, overcoming main challenges in arterial bypass surgery.

A model of guided cell self-organization for rapid and spontaneous formation of functional vessels

Most achievements to engineer blood vessels are based on multiple-step manipulations such as manual sheet rolling or sequential cell seeding followed by scaffold degradation. Here, we propose a one-step strategy using a microfluidic coextrusion device to produce mature functional blood vessels. A hollow alginate hydrogel tube is internally coated with extracellular matrix to direct the self-assembly of a mixture of endothelial cells (ECs) and smooth muscle cells (SMCs). The resulting vascular structure has the correct configuration of lumen, an inner lining of ECs, and outer sheath of SMCs. These "vesseloids" reach homeostasis within a day and exhibit the following properties expected for functional vessels (i) quiescence, (ii) perfusability, and (iii) contractility in response to vasoconstrictor agents. Together, these findings provide an original and simple strategy to generate functional artificial vessels and pave the way for further developments in vascular graft and tissue engineering and for deciphering the angiogenesis process.

Development of In Vitro Bioengineered Vascular Grafts for Microsurgery and Vascular Surgery Applications

Introduction

The use of vascular grafts is continuing to rise due to the increasing prevalence of coronary artery bypass grafting and microvascular flap-based tissue reconstructions. The current options of using native vessels (saphenous vein) or the synthetic grafts (Dacron) have been unable to manage current needs. In this study, we employed an original tissue engineering approach to develop a multi-layered vascular graft that has the potential to address some of the limitations of the existing grafts.

Materials and Methods

Biomaterials, gelatin and fibrin, were used to develop a two-layered vascular graft. The graft was seeded with endothelial cells and imaged using confocal microscopy. The graft's architecture and its mechanical properties were also characterized using histology, Scanning Electron Microscopy and rheological studies.

Results

Our methodology resulted in the development of a vascular graft with precise spatial localization of the two layers. The endothelial cells fully covered the lumen of the developed vascular graft, thus providing a non-thrombogenic surface. The elastic modulus of the biomaterials employed in this graft was found to be 5.186 KPa, paralleling that of internal mammary artery. The burst pressure of this graft was also measured and was found close to that of the saphenous vein (~2000 mm Hg).

Conclusions

We were successfully able to employ a unique method to synthesize a multi-layered vascularized graft having adequate biological and mechanical properties. Studies are ongoing involving implantation of this developed vascular graft in the rat femoral artery and characterization of parameters such as vascular remodeling and patency.

Bioreactors Design, Types, Influencing Factors and Potential Application in Dentistry. A Literature Review

Objectives:

A variety of bioreactors and related approaches have been applied to dental tissues as their use has become more essential in the field of regenerative dentistry and dental tissue engineering. The review discusses the various types of bioreactors and their potential application in dentistry.

Methods:

Review of the literature was conducted using keywords (and MeSH) like Bioreactor, Regenerative Dentistry, Fourth Factor, Stem Cells, etc., from the journals published in English. All the searched abstracts, published in indexed journals were read and reviewed to further refine the list of included articles. Based on the relevance of abstracts pertaining to the manuscript, full-text articles were assessed.

Results:

Bioreactors provide a prerequisite platform to create, test, and validate the biomaterials and techniques proposed for dental tissue regeneration. Flow perfusion, rotational, spinner-flask, strain and customize-combined bioreactors have been applied for the regeneration of bone, periodontal ligament, gingiva, cementum, oral mucosa, temporomandibular joint and vascular tissues. Customized bioreactors can support cellular/biofilm growth as well as apply cyclic loading. Center of disease control & dip-flow biofilm-reactors and micro-bioreactor have been used to evaluate the biological properties of dental biomaterials, their performance assessment and interaction with biofilms. Few case reports have also applied the concept of in vivo bioreactor for the repair of musculoskeletal defects and used customdesigned bioreactor (Aastrom) to repair the defects of cleft-palate.

Conclusions:

Bioreactors provide a sterile simulated environment to support cellular differentiation for oro-dental regenerative applications. Also, bioreactors like, customized bioreactors for cyclic loading, biofilm reactors (CDC & drip-flow), and micro-bioreactor, can assess biological responses of dental biomaterials by simultaneously supporting cellular or biofilm growth and application of cyclic stresses.

Engineering blood vessels and vascularized tissues: technology trends and potential clinical applications

Vascular tissue engineering has the potential to make a significant impact on the treatment of a wide variety of medical conditions, including providing in vitro generated vascularized tissue and organ constructs for transplantation. Since the first report on the construction of a biological blood vessel, significant research and technological advances have led to the generation of clinically relevant large and small diameter tissue engineered vascular grafts (TEVGs). However, developing a biocompatible blood-contacting surface is still a major challenge. Researchers are using biomimicry to generate functional vascular grafts and vascular networks. A multi-disciplinary approach is being used that includes biomaterials, cells, pro-angiogenic factors and microfabrication technologies. Techniques to achieve spatiotemporal control of vascularization include use of topographical engineering and controlled-release of growth/pro-angiogenic factors. Use of decellularized natural scaffolds has gained popularity for engineering complex vascularized organs for potential clinical use. Pre-vascularization of constructs prior to implantation has also been shown to enhance its anastomosis after implantation. Host-implant anastomosis is a phenomenon that is still not fully understood. However, it will be a critical factor in determining the in vivo success of a TEVGs or bioengineered organ. Many clinical studies have been conducted using TEVGs, but vascularized tissue/organ constructs are still in the research & development stage. In addition to technical challenges, there are commercialization and regulatory challenges that need to be addressed. In this review we examine recent advances in the field of vascular tissue engineering, with a focus on technology trends, challenges and potential clinical applications.

Recent advances in three-dimensional bioprinting of stem cells

In spite of being a new field, three-dimensional (3D) bioprinting has undergone rapid growth in the recent years. Bioprinting methods offer a unique opportunity for stem cell distribution, positioning, and differentiation at the microscale to make the differentiated architecture of any tissue while maintaining precision and control over the cellular microenvironment. Bioprinting introduces a wide array of approaches to modify stem cell fate. This review discusses these methodologies of 3D bioprinting stem cells. Fabricating a fully operational tissue or organ construct with a long life will be the most significant challenge of 3D bioprinting. Once this is achieved, a whole human organ can be fabricated for the defect place at the site of surgery.

Carboxymethyl kappa carrageenan-modified decellularized small-diameter vascular grafts improving thromboresistance properties

The development of decellularized small-diameter vascular grafts is a potential solution for patients requiring vascular reconstructive procedures. However, there is a limitation for acellular scaffolds due to incomplete recellularization and exposure of extracellular matrix components to whole blood resulting in platelet adhesion. To address this issue, a perfusion decellularization method was developed using a custom-designed set up which completely removed cell nuclei and preserved three-dimensional structure and mechanical properties of native tissue (sheep carotid arteries). Afterwards, carboxymethyl kappa carrageenan (CKC) was introduced as a novel anticoagulant in vascular tissue engineering which can inhibit thrombosis formation. The method enabled uniform immobilization of CKC on decellularized arteries as a result of interaction between amine functional groups of decellularized arteries and carboxyl groups of CKC. The CKC modified graft significantly reduced platelet adhesion from 44.53 ± 2.05% (control) to 19.57 ± 1.37% (modified) and supported endothelial cells viability, proliferation, and nitric oxide production. Overall, the novel CKC modified scaffold provides a promising solution for thrombosis formation of small-diameter vessels and could be a potent graft for future in vivo applications in vascular bypass procedures. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1690-1701, 2019.

Mechanical properties of l-lysine based segmented polyurethane vascular grafts and their shape memory potential

Segmented polyurethanes based on polycaprolactone, 4,4 (metylene-bis-cyclohexyl) isocyanate, and l-lysine were synthesized, manufactured as small vascular grafts and characterized according to ISO 7198 standard for cardiovascular implants-tubular vascular prosthesis. In terms of mechanical properties, the newly synthesized polyurethane films exhibited lower secant modulus than Tecoflex™ SG 80A, a well-known medical grade polyurethane. Similarly, when tested as grafts, the l-lysine-based polyurethane exhibited lower longitudinal failure load (11.5 N vs. 116 N), lower circumferential failure load per unit length (5.67 N/mm vs. 14.0 N/mm) and lower suture forces for both nylon (13.3 N vs. 24.0 N) and silk (14.0 N vs. 19.3 N) when compared to Tecoflex™ SG 80A grafts. l-Lysine-based graft exhibited a burst strength of 3620 mmHg (482.6 kPa) and a compliance of 0.16%/mmHg. The cell adhesion was demonstrated with NIH/3T3 fibroblasts where cell adhesion was observed on both films and grafts, while cell alignment was observed only on the grafts. The mechanical properties of this polyurethane and the possibility of strain-induced PCL crystals as the switching phase for shape memory materials, allowed a strain recovery ratio and a strain fixity ratio with values higher than 95% and 90%, respectively, with a repeatability of the shape-memory properties up to 4 thermo-mechanical cycles. Overall, the properties of lysine-based polyurethanes are suitable for large diameter vascular grafts where cell alignment can be controlled by their shape memory potential.

The Evolution of Tissue Engineered Vascular Graft Technologies: From Preclinical Trials to Advancing Patient Care

Currently available synthetic grafts have contributed to improved outcomes in cardiovascular surgery. However, the implementation of these graft materials at small diameters have demonstrated poor patency, inhibiting their use for coronary artery bypass surgery in adults. Additionally, when applied to a pediatric patient population, they are handicapped by their lack of growth ability. Tissue engineered alternatives could possibly address these limitations by producing biocompatible implants with the ability to repair, remodel, grow, and regenerate. A tissue engineered vascular graft (TEVG) generally consists of a scaffold, seeded cells, and the appropriate environmental cues (i.e., growth factors, physical stimulation) to induce tissue formation. This review critically appraises current state-of-the-art techniques for vascular graft production. We additionally examine current graft shortcomings and future prospects, as they relate to cardiovascular surgery, from two major clinical trials.

Concise Review: Patency of Small-Diameter Tissue-Engineered Vascular Grafts: A Meta-Analysis of Preclinical Trials

Several patient groups undergoing small‐diameter (<6 mm) vessel bypass surgery have limited autologous vessels for use as grafts. Tissue‐engineered vascular grafts (TEVG) have been suggested as an alternative, but the ideal TEVG remains to be generated, and a systematic overview and meta‐analysis of clinically relevant studies is lacking. We systematically searched PubMed and Embase databases for (pre)clinical trials and identified three clinical and 68 preclinical trials ([>rabbit]; 873 TEVGs) meeting the inclusion criteria. Preclinical trials represented low to medium risk of bias, and binary logistic regression revealed that patency was significantly affected by recellularization, TEVG length, TEVG diameter, surface modification, and preconditioning. In contrast, scaffold types were less important. The patency was 63.5%, 89%, and 100% for TEVGs with a median diameter of 3 mm, 4 mm, and 5 mm, respectively. In the group of recellularized TEVGs, patency was not improved by using smooth muscle cells in addition to endothelial cells nor affected by the endothelial origin, but seems to benefit from a long‐term (46–240 hours) recellularization time. Finally, data showed that median TEVG length (5 cm) and median follow‐up (56 days) used in preclinical settings are relatively inadequate for direct clinical translation. In conclusion, our data imply that future studies should consider a TEVG design that at least includes endothelial recellularization and bioreactor preconditioning, and we suggest that more standard guidelines for testing and reporting TEVGs in large animals should be considered to enable interstudy comparisons and favor a robust and reproducible outcome as well as clinical translation.

Decoupling the Effect of Shear Stress and Stretch on Tissue Growth and Remodeling in a Vascular Graft

The success of cardiovascular tissue engineering (TE) strategies largely depends on the mechanical environment in which cells develop a neotissue through growth and remodeling processes. This mechanical environment is defined by the local scaffold architecture to which cells adhere, that is, the microenvironment, and by external mechanical cues to which cells respond, that is, hemodynamic loading. The hemodynamic environment of early developing blood vessels consists of both shear stress (due to blood flow) and circumferential stretch (due to blood pressure). Experimental platforms that recapitulate this mechanical environment in a controlled and tunable manner are thus critical for investigating cardiovascular TE. In traditional perfusion bioreactors, however, shear stress and stretch are coupled, hampering a clear delineation of their effects on cell and tissue response. In this study, we uniquely designed a bioreactor that independently combines these two types of mechanical cues in eight parallel vascular grafts. The system is computationally and experimentally validated, through finite element analysis and culture of tissue constructs, respectively, to distinguish various levels of shear stress (up to 5 Pa) and cyclic stretch (up to 1.10). To illustrate the usefulness of the system, we investigated the relative contribution of cyclic stretch (1.05 at 0.5 Hz) and shear stress (1 Pa) to tissue development. Both types of hemodynamic loading contributed to cell alignment, but the contribution of shear stress overruled stretch-induced cell proliferation and matrix (i.e., collagen and glycosaminoglycan) production. At a macroscopic level, cyclic stretching led to the most linear stress-stretch response, which was not related to the presence of shear stress. In conclusion, we have developed a bioreactor that is particularly suited to further unravel the interplay between hemodynamics and in situ TE processes. Using the new system, this work highlights the importance of hemodynamic loading to the study of developing vascular tissues.

Modeling elastin-associated vasculopathy with patient induced pluripotent stem cells and tissue engineering

Elastin-associated vasculopathies are life-threatening conditions of blood vessel dysfunction. The extracellular matrix protein elastin endows the recoil and compliance required for physiologic arterial function, while disruption of function can lead to aberrant vascular smooth muscle cell proliferation manifesting through stenosis, aneurysm, or vessel dissection. Although research efforts have been informative, they remain incomplete as no viable therapies exist outside of a heart transplant. Induced pluripotent stem cell technology may be uniquely suited to address current obstacles as these present a replenishable supply of patient-specific material with which to study disease. The following review will cover the cutting edge in vascular smooth muscle cell modeling of elastin-associated vasculopathy, and aid in the development of human disease modeling and drug screening approaches to identify potential treatments. Vascular proliferative disease can affect up to 50% of the population throughout the world, making this a relevant and critical area of research for therapeutic development.