Functional arteries grown in vitro

Synthetic vascular graft that heals and regenerates

Millions of synthetic vascular grafts (sVG) are needed annually to address vascular diseases (a leading cause of death in humans) and kidney failure (as vascular access). However, in 70+ years since the first sVG in humans, we still do not have sVGs that fully endothelialize (the "holy grail" for truly successful grafts). The lack of healthy endothelium is believed to be a main cause for thrombosis, stenosis, and infection (the major reasons for graft failure). The immune-mediated foreign body response to traditional sVG materials encapsulates the materials in fibrotic scar suppressing vascularized healing. Here, we describe the first sVG optimized for vessel wall vascularization via uniform, spherical 40 μm pores. This sVG induced unprecedented rapid healing of luminal endothelium in a demanding and clinically relevant sheep model, probably by attracting and modulating macrophages and foreign body giant cells towards diverse, pro-healing phenotypes. Both this sVG and the control (PTFE grafts) remained 100 % patent during the implantation period. This advancement has broad implications beyond sVGs in tissue engineering and biocompatibility.

My Struggles and Dreams as a Chemical Engineer

My career has not been straightforward. Although I am a chemical engineer, and I'm proud of that, I took a path from chemistry and engineering to one that also involved experimental biology and medicine. This was very unusual many decades ago. In so doing, I met with rejection and ridicule early in my career. However, by going down that path, I was able to make discoveries and inventions that I hope have saved and improved lives, and I've been able to train a great number of people who are going down the road I began traveling over many years ago.

Development of small tissue engineered blood vessels and their clinical and research applications

Since the first tissue engineered blood vessel (TEBV) was developed, different approaches, biomaterial scaffolds and cell sources have been used to obtain an engineered vessel as much similar as native vessels in terms of structure, functionality and mechanical properties. At the same time, diverse needs to obtain a functional TEBV have emerged, such as for blood vessel replacement for cardiovascular diseases (CVDs) to be used as artery bypass, to vascularize tissue engineered constructs, or even to model vascular diseases or drug testing. In this review, after briefly describing the native structure and function of arteries, we will give an overview of different biomaterials, cells and methods that have been used during the last years for the development of small TEBV (1-6 mm diameter). The importance of perfusing the TEBV to acquire functionality and maturation will be also discussed. Finally, we will center the review on TEBV applications beyond their use as vascular graft for CVDs.

Artificial blood vessels-clinical development of TEVG

The market for small-diameter vascular grafts (< 6 mm) used in cardiac and vascular surgery has not yet been fully established, as stable long-term patency has not been achieved. This paper focuses on the clinical development of tissue-engineered vascular grafts (TEVGs), especially those that have progressed to clinical trials, and introduces their current status with historical background and future directions. This review was created based on a translation of the Japanese review first reported in the Japanese Journal of Artificial Organs in 2023 (vol. 52, no. 3, pp. 161-166), with some modifications.

Modeling and Optimization of Nonlinear Viscoelastic Behavior for Tissue-Engineered Blood Vessels

Vascular tissue engineering technology uses tubular viscoelastic materials as intermediaries to transmit the mechanical stimuli required for the construction of vascular grafts. However, most existing studies rely on elastic models, which fail to capture the time-dependent nature of viscoelastic materials. Moreover, the long fabrication cycles, high costs, and complex parameter measurements in tissue engineering pose significant challenges to experimental approaches. There is thus an urgent need to develop a viscoelastic mechanical model that combines physical interpretability, computational efficiency, and predictive accuracy, enabling precise characterization of material responses and unified quantification across experimental platforms. Here, we propose an error-corrected linear solid (ECLS) model with an embedded correction term to address the predictive deviations of conventional models in nonlinear viscoelastic scenarios. Instead of expanding the traditional model structure, the ECLS incorporates an error correction method that improves predictive performance while maintaining structural simplicity. Experiments were conducted on three representative viscoelastic materials-silicone rubber, polyurethane, and polytetrafluoroethylene-to acquire time-resolved response data through stress relaxation and creep tests. The fitting performance was quantitatively evaluated using the Euclidean norm and the Akaike information criterion, enabling a systematic comparison between the ECLS model and three classical models (Kelvin-Voigt, Maxwell, and standard linear solid [SLS]). The results show that the ECLS model exhibits higher predictive accuracy over a wide time range, with an average goodness of fit (R2) of 0.99, representing an improvement of ∼6% compared to the SLS model. Furthermore, the Root Mean Square Error (RMSE) and Mean Absolute Error (MAE) of the ECLS model are at least one order of magnitude lower than those of the traditional models, significantly improving the description of nonlinear viscoelastic behavior and providing more accurate predictions of material viscoelastic mechanical behavior. Therefore, the ECLS model not only improves the modeling accuracy of viscoelastic behavior but also establishes a unified and scalable framework for predicting and optimizing the mechanical performance of tissue-engineered vessels, expanding the application potential of mechanical modeling in bioreactor design and biomaterials development.

Tissue Engineering and Regenerative Medicine: Perspectives and Challenges

From the pioneering days of cell therapy to the achievement of bioprinting organs, tissue engineering, and regenerative medicine have seen tremendous technological advancements, offering solutions for restoring damaged tissues and organs. However, only a few products and technologies have received United States Food and Drug Administration approval. This review highlights significant progress in cell therapy, extracellular vesicle-based therapy, and tissue engineering. Hematopoietic stem cell transplantation is a powerful tool for treating many diseases, especially hematological malignancies. Mesenchymal stem cells have been extensively studied. The discovery of induced pluripotent stem cells has revolutionized disease modeling and regenerative applications, paving the way for personalized medicine. Gene therapy represents an innovative approach to the treatment of genetic disorders. Additionally, extracellular vesicle-based therapies have emerged as rising stars, offering promising solutions in diagnostics, cell-free therapeutics, drug delivery, and targeted therapy. Advances in tissue engineering enable complex tissue constructs, further transforming the field. Despite these advancements, many technical, ethical, and regulatory challenges remain. This review addresses the current bottlenecks, emphasizing novel technologies and interdisciplinary research to overcome these hurdles. Standardizing practices and conducting clinical trials will balance innovation and regulation, improving patient outcomes and quality of life.

Modulating biomechanical and integrating biochemical cues to foster adaptive remodeling of tissue engineered matrices for cardiovascular implants

Cardiovascular disease remains one of the leading causes of mortality in the Western world. Congenital heart disease affects nearly 1 % of newborns, with approximately one-fourth requiring reconstructive surgery during their lifetime. Current cardiovascular replacement options have significant limitations. Their inability to grow poses particular challenges for pediatric patients. Tissue Engineered Matrix (TEM)-based in situ constructs, with their self-repair and growth potential, offer a promising solution to overcome the limitations of current clinically used replacement options. Various functionalization strategies, involving the integration of biomechanical or biochemical components to enhance biocompatibility, have been developed for Tissue Engineered Vascular Grafts (TEVG) and Tissue Engineered Heart Valves (TEHV) to foster their capacity for in vivo remodeling. In this review, we present the current state of clinical translation for TEVG and TEHV, and provide a comprehensive overview of biomechanical and biochemical functionalization strategies for TEVG and TEHV. We discuss the rationale for functionalization, the implementation of functionalization cues in TEM-based TEVG and TEHV, and the interrelatedness of biomechanical and biochemical cues in the in vivo response. Finally, we address the challenges associated with functionalization and discuss how interdisciplinary research, especially when combined with in silico models, could enhance the translation of these strategies into clinical applications. STATEMENT OF SIGNIFICANCE: Cardiovascular disease remains one of the leading causes of mortality, with current replacements being unable to grow and regenerate. In this review, we present the current state of clinical translation for tissue engineered vascular grafts (TEVG) and heart valves (TEHV). Particularly, we discuss the rationale and implementation for functionalization cues in tissue engineered matrix-based TEVGs and TEHVs, and for the first time we introduce the interrelatedness of biomechanical and biochemical cues in the in-vivo response. These insights pave the way for next-generation cardiovascular implants that promise better durability, biocompatibility, and growth potential. Finally, we address the challenges associated with functionalization and discuss how interdisciplinary research, especially when combined with in silico models, could enhance the translation of these strategies into clinical applications .

Immune-mediated regeneration of cell-free vascular grafts in an ovine model

We developed acellular tissue engineered vessels (ATEV) using small intestine submucosa (SIS) incorporating heparin and a novel protein named H2R5. ATEVs were implanted into the arterial circulation of an ovine animal model, demonstrating high primary patency rates over a period of three months. Implanted grafts were infiltrated by host cells, the majority of which were monocytes/macrophages (MC/MΦ), as demonstrated by scRNA sequencing and immunostaining. They also developed functional endothelial and medial layers that deposited new extracellular matrix leading to matrix remodeling and acquisition of mechanical properties that were similar to those of native arteries. Notably, during this short implantation time, ATEVs turned into functional neo-arteries, as evidenced by the development of the vascular contractile function. Our findings underscore the potential of H2R5-functionalized ATEVs as promising candidates for tissue replacement grafts in a large pre-clinical animal model and highlight the contribution of macrophages in vascular regeneration.

Unravelling molecular mechanisms in atherosclerosis using cellular models and omics technologies

Despite the discovery and prevalent clinical use of potent lipid-lowering therapies, including statins and PCSK9 inhibitors, cardiovascular diseases (CVD) caused by atherosclerosis remain a large unmet clinical need, accounting for frequent deaths worldwide. The pathogenesis of atherosclerosis is a complex process underlying the presence of modifiable and non-modifiable risk factors affecting several cell types including endothelial cells (ECs), monocytes/macrophages, smooth muscle cells (SMCs) and T cells. Heterogeneous composition of the plaque and its morphology could lead to rupture or erosion causing thrombosis, even a sudden death. To decipher this complexity, various cell model systems have been developed. With recent advances in systems biology approaches and single or multi-omics methods researchers can elucidate specific cell types, molecules and signalling pathways contributing to certain stages of disease progression. Compared with animals, in vitro models are economical, easily adjusted for high-throughput work, offering mechanistic insights. Hereby, we review the latest work performed employing the cellular models of atherosclerosis to generate a variety of omics data. We summarize their outputs and the impact they had in the field. Challenges in the translatability of the omics data obtained from the cell models will be discussed along with future perspectives.

Vascular endothelial cells derived from transgene-free pig induced pluripotent stem cells for vascular tissue engineering

Induced pluripotent stem cells (iPSCs) hold great promise for the treatment of cardiovascular diseases through cell-based therapies, but these therapies require extensive preclinical testing that is best done in species-in-species experiments. Pigs are a good large animal model for these tests due to the similarity of their cardiovascular system to humans. However, a lack of adequate pig iPSCs (piPSCs) that are analogous to human iPSCs has greatly limited the potential usefulness of this model system. Herein, transgene-free piPSCs with true pluripotency were generated by using reprogramming factors in an optimized pig pluripotency medium. Using an effective differentiation protocol, piPSCs were used to derive endothelial cells (ECs) which displayed EC markers and functionality comparable to native pig ECs. Further, piPSC-ECs demonstrated suitability for vascular tissue engineering, producing a tissue engineered vascular conduit (TEVC) that displayed the upregulation of flow responding markers. In an in vivo functional study, these piPSC-EC-TEVCs maintained the expression of endothelial markers and prevented thrombosis as interposition inferior vena cava grafts in immunodeficient rats. The piPSCs described in this study open up the possibility of unique preclinical species-in-species large animal modeling for the furtherance of modeling of cell-based cardiovascular tissue engineering therapies. STATEMENT OF SIGNIFICANCE: While there has been significant progress in the development of cellularized cardiovascular tissue engineered therapeutics using stem cells, few of them have moved into clinical trials. This is due to the lack of a robust preclinical large animal model to address the high safety and efficacy standards for transplanted therapeutics. In this study, pig stem cells that are analagous to human's were created to address this bottleneck. They demonstrated the ability to differentiate into functional endothelial cells and were able to create a tissue engineered therapeutic that is analogous to a human therapy. With these cells, future experiments testing the safety and efficacy of tissue engineered constructs are possible, bringing these crucial therapeutics closer to the patients that need them.

Design, fabrication, and application of bioengineering vascular networks based on microfluidic strategies

Vascularization is a critical component of tissue engineering research and is essential for enhancing the success rate of tissue construction and function. Over the past decade, researchers have explored various methods to construct in vitro vascular networks, including 3D printing, cell sphere technology, and microfluidics. Microfluidic technology has garnered significant attention due to its notable advantages in precision, controllability, flexibility, and applicability. It can be primarily classified into two modes: (i) the pre-designed mode, which involves creating vascular networks by pre-designing vascular channels and seeding endothelial cells, encompassing microfluidic chips and microfluidic spinning technologies; and (ii) the self-assembly mode, where cell spheres are fabricated using microfluidic technology and subsequently self-assemble into vascular networks. In this review, we first provide a brief overview of the normal physiological and pathological characteristics of vascular networks, followed by a discussion of the factors to be considered in designing in vitro vascular networks, and conclude with an examination of the classification of technologies for the preparation of microfluidic vascular networks and recent advancements. It is anticipated that in vitro vascular network models will soon be successfully applied in regenerative medicine and drug development.

Vascular smooth muscle cells can be circumferentially aligned inside a channel using tunable gelatin microribbons

The gold standard to measure arterial health is vasodilation in response to nitric oxide. Vasodilation is generally measured via pressure myography of arteries isolated from animal models. However, animal arteries can be difficult to obtain and may have limited relevance to human physiology. It is, therefore, critical to engineer human cell-based arterial models capable of contraction. Vascular smooth muscle cells (SMCs) must be circumferentially aligned around the vessel lumen to contract the vessel, which is challenging to achieve in a soft blood vessel model. In this study, we used gelatin microribbons to circumferentially align SMCs inside a hydrogel channel. To accomplish this, we created tunable gelatin microribbons of varying stiffnesses and thicknesses and assessed how SMCs aligned along them. We then wrapped soft, thick microribbons around a needle and encapsulated them in a gelatin methacryloyl hydrogel, forming a microribbon-lined channel. Finally, we seeded SMCs inside the channel and showed that they adhered best to fibronectin and circumferentially aligned in response to the microribbons. Together, these data show that tunable gelatin microribbons can be used to circumferentially align SMCs inside a channel. This technique can be used to create a human artery-on-a-chip to assess vasodilation via pressure myography, as well as to align other cell types for 3Din vitromodels.

Advancements in Omics and Breakthrough Gene Therapies: A Glimpse into the Future of Peripheral Artery Disease

Peripheral artery disease (PAD), a highly prevalent global disease, associates with significant morbidity and mortality in affected patients. Despite progress in endovascular and open revascularization techniques for advanced PAD, these interventions grapple with elevated rates of arterial restenosis and vein graft failure attributed to intimal hyperplasia (IH). Novel multiomics technologies, coupled with sophisticated analyses tools recently powered by advances in artificial intelligence, have enabled the study of atherosclerosis and IH with unprecedented single-cell and spatial precision. Numerous studies have pinpointed gene hubs regulating pivotal atherogenic and atheroprotective signaling pathways as potential therapeutic candidates. Leveraging advancements in viral and nonviral gene therapy (GT) platforms, gene editing technologies, and cutting-edge biomaterial reservoirs for delivery uniquely positions us to develop safe, efficient, and targeted GTs for PAD-related diseases. Gene therapies appear particularly fitting for ex vivo genetic engineering of IH-resistant vein grafts. This manuscript highlights currently available state-of-the-art multiomics approaches, explores promising GT-based candidates, and details GT delivery modalities employed by our laboratory and others to thwart mid-term vein graft failure caused by IH, as well as other PAD-related conditions. The potential clinical translation of these targeted GTs holds the promise to revolutionize PAD treatment, thereby enhancing patients' quality of life and life expectancy.

Insulin-transferrin-selenium promote formation of tissue-engineered vascular grafts in early stage of culture

To create tissue-engineered vascular grafts (TEVGs) in vitro, vascular smooth muscle cells (VSMCs) must function effectively and produce sufficient extracellular matrix (ECM) in a three-dimensional space. In this study, we investigated whether the addition of insulin-transferrin-selenium (ITS), a medium supplement, could enhance TEVG formation. PGA fabric was used as the scaffold, and 1% ITS was added to the medium. After two weeks, the tissues were examined using electron microscopy and staining. The ITS group exhibited a denser structure and increased collagen production. VSMCs were cultured in two dimensions with ITS and assessed for collagen production, cell growth, and glucose metabolism. The results showed that ITS supplementation increased collagen production, cell growth, glucose utilization, lactate production, and ATP levels. Furthermore, reducing the amount of fetal bovine serum (FBS) in the medium did not affect the TEVGs or VSMCs when ITS was present. In conclusion, ITS improves TEVG construction by promoting VSMCs growth and reducing the need for FBS.

Mesenchymal Stem Cell-Conditioned Media-Loaded Microparticles Enhance Acute Patency in Silk-Based Vascular Grafts

Coronary artery disease leads to over 360,000 deaths annually in the United States, and off-the-shelf bypass graft options are currently limited and/or have high failure rates. Tissue-engineered vascular grafts (TEVGs) present an attractive option, though the promising mesenchymal stem cell (MSC)-based implants face uncertain regulatory pathways. In this study, "artificial MSCs" (ArtMSCs) were fabricated by encapsulating MSC-conditioned media (CM) in poly(lactic-co-glycolic acid) microparticles. ArtMSCs and control microparticles (Blank-MPs) were incubated over 7 days to assess the release of total protein and the vascular endothelial growth factor (VEGF-A); releasates were also assessed for cytotoxicity and promotion of smooth muscle cell (SMC) proliferation. Each MP type was loaded in previously published "lyogel" silk scaffolds and implanted as interposition grafts in Lewis rats for 1 or 8 weeks. Explanted grafts were assessed for patency and cell content. ArtMSCs had a burst release of protein and VEGF-A. CM increased proliferation in SMCs, but not after encapsulation. TEVG explants after 1 week had significantly higher patency rates with ArtMSCs compared to Blank-MPs, but similar to unseeded lyogel grafts. ArtMSC explants had lower numbers of infiltrating macrophages compared to Blank-MP explants, suggesting a modulation of inflammatory response by the ArtMSCs. TEVG explants after 8 weeks showed no significant difference in patency among the three groups. The ArtMSC explants showed higher numbers of SMCs and endothelial cells within the neotissue layer of the graft compared to Blank-MP explants. In sum, while the ArtMSCs had positive effects acutely, efficacy was lost in the longer term; therefore, further optimization is needed.

Low concentrations of TNF-α in vitro transform the phenotype of vascular smooth muscle cells and enhance their survival in a three-dimensional culture system

BACKGROUND

Vascular smooth muscle cells (VSMCs) are commonly used as seed cells in tissue-engineered vascular constructions. However, their variable phenotypes and difficult to control functions pose challenges. This study aimed to overcome these obstacles using a three-dimensional culture system.

METHODS

Calf VSMCs were administered tumor necrosis factor-alpha (TNF-α) before culturing in two- and three-dimensional well plates and polyglycolic acid (PGA) scaffolds, respectively. The phenotypic markers of VSMCs were detected by immunofluorescence staining and western blotting, and the proliferation and migration abilities of VSMCs were detected by CCK-8, EDU, cell counting, scratch, and Transwell assays.

RESULTS

TNF-α rapidly decreased the contractile phenotypic markers and elevated the synthetic phenotypic markers of VSMCs, as well as markedly increasing the proliferation and migration ability of VSMCs under two- and three-dimensional culture conditions.

CONCLUSIONS

TNF-α can rapidly induce a phenotypic shift in VSMCs and change their viability on PGA scaffolds.

Modular formation of in vitro tumor models for oncological research/therapeutic drug screening

In recognition of the lethal nature of cancer, extensive efforts have been made to understand the mechanistic causation while identifying the effective therapy modality in hope to eradicate cancerous cells with minimal damage to healthy cells. In search of such effective therapeutics, establishing pathophysiologically relevant in vitro models would be of importance in empowering our capabilities of truly identifying those potent ones with significantly reduction of the preclinical periods for rapid translation. In this regard, wealthy progresses have been achieved over past decades in establishing various in vitro and in vivo tumor models. Ideally, the tumor models should maximally recapture the key pathophysiological attributes of their native counterparts. Many of the current models have demonstrated their utilities but also showed some noticeable limitations. This book chapter will briefly review some of the mainstream platforms for in vitro tumor models followed by detailed elaboration on the modular strategies to form in vitro tumor models with complex structures and spatial organization of cellular components. Clearly, with the ability to modulate the building modules it becomes a new trend to form in vitro tumor models following a bottom-up approach, which offers a high flexibility to satisfy the needs for pathophysiological study, anticancer drug screening or design of personalized treatment.

Biomimetic Approaches in Scaffold-Based Blood Vessel Tissue Engineering

Cardiovascular diseases remain a leading cause of mortality globally, with atherosclerosis representing a significant pathological means, often leading to myocardial infarction. Coronary artery bypass surgery, a common procedure used to treat coronary artery disease, presents challenges due to the limited autologous tissue availability or the shortcomings of synthetic grafts. Consequently, there is a growing interest in tissue engineering approaches to develop vascular substitutes. This review offers an updated picture of the state of the art in vascular tissue engineering, emphasising the design of scaffolds and dynamic culture conditions following a biomimetic approach. By emulating native vessel properties and, in particular, by mimicking the three-layer structure of the vascular wall, tissue-engineered grafts can improve long-term patency and clinical outcomes. Furthermore, ongoing research focuses on enhancing biomimicry through innovative scaffold materials, surface functionalisation strategies, and the use of bioreactors mimicking the physiological microenvironment. Through a multidisciplinary lens, this review provides insight into the latest advancements and future directions of vascular tissue engineering, with particular reference to employing biomimicry to create systems capable of reproducing the structure-function relationships present in the arterial wall. Despite the existence of a gap between benchtop innovation and clinical translation, it appears that the biomimetic technologies developed to date demonstrate promising results in preventing vascular occlusion due to blood clotting under laboratory conditions and in preclinical studies. Therefore, a multifaceted biomimetic approach could represent a winning strategy to ensure the translation of vascular tissue engineering into clinical practice.

Harnessing the Regenerative Potential of Fetal Mesenchymal Stem Cells and Endothelial Colony-Forming Cells in the Biofabrication of Tissue-Engineered Vascular Grafts (TEVGs)

Tissue engineering is a promising approach for the production of small-diameter vascular grafts; however, there are limited data directly comparing the suitability of applicable cell types for vessel biofabrication. Here, we investigated the potential of adult smooth muscle cells (SMCs), placental mesenchymal stem cells (MSCs), placental endothelial colony-forming cells (ECFCs), and a combination of MSCs and ECFCs on highly porous biocompatible poly(ɛ-caprolactone) (PCL) scaffolds produced via melt electrowriting (MEW) for the biofabrication of tissue-engineered vascular grafts (TEVGs). Cellular attachment, proliferation, and deposition of essential extracellular matrix (ECM) components were analysed in vitro over four weeks. TEVGs cultured with MSCs accumulated the highest levels of collagenous components within a dense ECM, while SMCs and the coculture were more sparsely populated, ascertained via histological and immunofluorescence imaging, and biochemical assessment. Scanning electron microscopy (SEM) enabled visualisation of morphological differences in cell attachment and growth, with MSCs and SMCs infiltrating and covering scaffolds completely within the 28-day culture period. Coverage and matrix deposition by ECFCs was limited. However, ECFCs lined the ECM formed by MSCs in coculture, visualised via immunostaining. Thus, of cells investigated, placental MSCs were identified as the preferred cell source for the fabrication of tissue-engineered constructs, exhibiting extensive population of porous polymer scaffolds and production of ECM components; with the inclusion of ECFCs for luminal endothelialisation, an encouraging outcome warranting further consideration in future studies. In combination, these findings represent a substantial step toward the development of the next generation of small-diameter vascular grafts in the management of cardiovascular disease.

Nonlinear Elasticity of Blood Vessels and Vascular Grafts

The transplantation of vascular grafts has emerged as a prevailing approach to address vascular disorders. However, the development of small-diameter vascular grafts is still in progress, as they serve in a more complicated mechanical environment than their counterparts with larger diameters. The biocompatibility and functional characteristics of small-diameter vascular grafts have been well developed; however, mismatch in mechanical properties between the vascular grafts and native arteries has not been accomplished, which might facilitate the long-term patency of small-diameter vascular grafts. From a point of view in mechanics, mimicking the nonlinear elastic mechanical behavior exhibited by natural blood vessels might be the state-of-the-art in designing vascular grafts. This review centers on elucidating the nonlinear elastic behavior of natural blood vessels and vascular grafts. The biological functionality and limitations associated with as-reported vascular grafts are meticulously reviewed and the future trajectory for fabricating biomimetic small-diameter grafts is discussed. This review might provide a different insight from the traditional design and fabrication of artificial vascular grafts.

Bioengineered vascular grafts with a pathogenic TGFBR1 variant model aneurysm formation in vivo and reveal underlying collagen defects

Thoracic aortic aneurysm (TAA) is a life-threatening vascular disease frequently associated with underlying genetic causes. An inadequate understanding of human TAA pathogenesis highlights the need for better disease models. Here, we established a functional human TAA model in an animal host by combining human induced pluripotent stem cells (hiPSCs), bioengineered vascular grafts (BVGs), and gene editing. We generated BVGs from isogenic control hiPSC-derived vascular smooth muscle cells (SMCs) and mutant SMCs gene-edited to carry a Loeys-Dietz syndrome (LDS)-associated pathogenic variant (TGFBR1A230T). We also generated hiPSC-derived BVGs using cells from a patient with LDS (PatientA230T/+) and using genetically corrected cells (Patient+/+). Control and experimental BVGs were then implanted into the common carotid arteries of nude rats. The TGFBR1A230T variant led to impaired mechanical properties of BVGs, resulting in lower burst pressure and suture retention strength. BVGs carrying the variant dilated over time in vivo, resembling human TAA formation. Spatial transcriptomics profiling revealed defective expression of extracellular matrix (ECM) formation genes in PatientA230T/+ BVGs compared with Patient+/+ BVGs. Histological analysis and protein assays validated quantitative and qualitative ECM defects in PatientA230T/+ BVGs and patient tissue, including decreased collagen hydroxylation. SMC organization was also impaired in PatientA230T/+ BVGs as confirmed by vascular contraction testing. Silencing of collagen-modifying enzymes with small interfering RNAs reduced collagen proline hydroxylation in SMC-derived tissue constructs. These studies demonstrated the utility of BVGs to model human TAA formation in an animal host and highlighted the role of reduced collagen modifying enzyme activity in human TAA formation.

Development of a Tri-Layered Vascular Construct and In Vitro Evaluation of Endothelization

Advances in the development of vascular substitutes for small-sized arteries are ongoing because the present grafts do not entirely meet the requirements of native equivalents and are suboptimal in clinical performance. This study aims to develop a tri-layered vascular construct mimicking natural tissue using polyester blends and to investigate its endothelization through in vitro studies as a potential small-caliber vascular graft. The innermost layer is obtained by dip coating as a tubular porous film with a lumen diameter of 3 mm and a pore size of ≤8 µm. Circumferentially aligned electrospun fiber (diameter 100-800 nm) with a deviation angle of 15° are deposited over the porous film forming the intermediate layer. The random electrospun fibers (diameter 100-1100 nm) deviating at different angles are wrapped as the outermost layer. The mechanical properties of the tri-layered vascular construct are determined to be 44.80 ± 14.80 MPa for Young's modulus and 4.25 ± 0.75 MPa for ultimate tensile strength. MTS and cell behavior studies show that the isolated human umbilical cord vein endothelial cells proliferate and line the lumen of the vascular substitute. The vascular construct developed, with its biomimetic architecture, mechanical features, size, and endothelization, can be tested with in vivo studies.

Hemodynamics and Wall Mechanics of Vascular Graft Failure

Blood vessels are subjected to complex biomechanical loads, primarily from pressure-driven blood flow. Abnormal loading associated with vascular grafts, arising from altered hemodynamics or wall mechanics, can cause acute and progressive vascular failure and end-organ dysfunction. Perturbations to mechanobiological stimuli experienced by vascular cells contribute to remodeling of the vascular wall via activation of mechanosensitive signaling pathways and subsequent changes in gene expression and associated turnover of cells and extracellular matrix. In this review, we outline experimental and computational tools used to quantify metrics of biomechanical loading in vascular grafts and highlight those that show potential in predicting graft failure for diverse disease contexts. We include metrics derived from both fluid and solid mechanics that drive feedback loops between mechanobiological processes and changes in the biomechanical state that govern the natural history of vascular grafts. As illustrative examples, we consider application-specific coronary artery bypass grafts, peripheral vascular grafts, and tissue-engineered vascular grafts for congenital heart surgery as each of these involves unique circulatory environments, loading magnitudes, and graft materials.

Graph Network-Based Simulation of Multicellular Dynamics Driven by Concentrated Polymer Brush-Modified Cellulose Nanofibers

Manipulating the three-dimensional (3D) structures of cells is important for facilitating to repair or regenerate tissues. A self-assembly system of cells with cellulose nanofibers (CNFs) and concentrated polymer brushes (CPBs) has been developed to fabricate various cell 3D structures. To further generate tissues at an implantable level, it is necessary to carry out a large number of experiments using different cell culture conditions and material properties; however this is practically intractable. To address this issue, we present a graph-neural network-based simulator (GNS) that can be trained by using assembly process images to predict the assembly status of future time steps. A total of 24 (25 steps) time-series images were recorded (four repeats for each of six different conditions), and each image was transformed into a graph by regarding the cells as nodes and the connecting neighboring cells as edges. Using the obtained data, the performances of the GNS were examined under three scenarios (i.e., changing a pair of the training and testing data) to verify the possibility of using the GNS as a predictor for further time steps. It was confirmed that the GNS could reasonably reproduce the assembly process, even under the toughest scenario, in which the experimental conditions differed between the training and testing data. Practically, this means that the GNS trained by the first 24 h images could predict the cell types obtained 3 weeks later. This result could reduce the number of experiments required to find the optimal conditions for generating cells with desired 3D structures. Ultimately, our approach could accelerate progress in regenerative medicine.

Extracellular Matrix Interactome in Modulating Vascular Homeostasis and Remodeling

The ECM (extracellular matrix) is a major component of the vascular microenvironment that modulates vascular homeostasis. ECM proteins include collagens, elastin, noncollagen glycoproteins, and proteoglycans/glycosaminoglycans. ECM proteins form complex matrix structures, such as the basal lamina and collagen and elastin fibers, through direct interactions or lysyl oxidase-mediated cross-linking. Moreover, ECM proteins directly interact with cell surface receptors or extracellular secreted molecules, exerting matricellular and matricrine modulation, respectively. In addition, extracellular proteases degrade or cleave matrix proteins, thereby contributing to ECM turnover. These interactions constitute the ECM interactome network, which is essential for maintaining vascular homeostasis and preventing pathological vascular remodeling. The current review mainly focuses on endogenous matrix proteins in blood vessels and discusses the interaction of these matrix proteins with other ECM proteins, cell surface receptors, cytokines, complement and coagulation factors, and their potential roles in maintaining vascular homeostasis and preventing pathological remodeling.

Biofabrication of engineered blood vessels for biomedical applications

To successfully engineer large-sized tissues, establishing vascular structures is essential for providing oxygen, nutrients, growth factors and cells to prevent necrosis at the core of the tissue. The diameter scale of the biofabricated vasculatures should range from 100 to 1,000 µm to support the mm-size tissue while being controllably aligned and spaced within the diffusion limit of oxygen. In this review, insights regarding biofabrication considerations and techniques for engineered blood vessels will be presented. Initially, polymers of natural and synthetic origins can be selected, modified, and combined with each other to support maturation of vascular tissue while also being biocompatible. After they are shaped into scaffold structures by different fabrication techniques, surface properties such as physical topography, stiffness, and surface chemistry play a major role in the endothelialization process after transplantation. Furthermore, biological cues such as growth factors (GFs) and endothelial cells (ECs) can be incorporated into the fabricated structures. As variously reported, fabrication techniques, especially 3D printing by extrusion and 3D printing by photopolymerization, allow the construction of vessels at a high resolution with diameters in the desired range. Strategies to fabricate of stable tubular structures with defined channels will also be discussed. This paper provides an overview of the many advances in blood vessel engineering and combinations of different fabrication techniques up to the present time.

Antioxidant and Trilayered Electrospun Small-Diameter Vascular Grafts Maintain Patency and Promote Endothelialisation in Rat Femoral Artery

Vascular grafts with a small diameter encounter inadequate patency as a result of intimal hyperplasia development. In the current study, trilayered electrospun small-diameter vascular grafts (PU-PGACL + GA) were fabricated using a poly(glycolic acid) and poly(caprolactone) blend as the middle layer and antioxidant polyurethane with gallic acid as the innermost and outermost layers. The scaffolds exhibited good biocompatibility and mechanical properties, as evidenced by their 6 MPa elastic modulus, 4 N suture retention strength, and 2500 mmHg burst pressure. Additionally, these electrospun grafts attenuated cellular oxidative stress and demonstrated minimal hemolysis (less than 1%). As a proof-of-concept, the preclinical evaluation of the grafts was carried out in the femoral artery of rodents, where the conduits demonstrated satisfactory patency. After 35 days of implantation, ultrasound imaging depicted adequate blood flow through the grafts, and the computed vessel diameter and histological staining showed no significant stenosis issue. Immunohistochemical analysis confirmed matrix deposition (38% collagen I and 16% elastin) and cell infiltration (42% for endothelial cells and 55% for smooth muscle cells) in the explanted grafts. Therefore, PU-PGACL + GA showed characteristics of a clinically relevant small-diameter vascular graft, facilitating re-endothelialization while preserving the anticoagulant properties of the synthetic blood vessels.

The Future of Microsurgery: Vascularized Composite Allotransplantation and Engineering Vascularized Tissue

Background

Microsurgical techniques have revolutionized the field of reconstructive surgery and are the mainstay for complex soft tissue reconstruction. However, their limitations have promoted the development of viable alternatives. This article seeks to explore technologies that have the potential of revolutionizing microsurgical reconstruction as it is currently known, reflect on current and future vascularized composite allotransplantation (VCA) practices, as well as describe the basic science within emerging technologies and their potential translational applications.

Methods

A literature review was performed of the technologies that may represent the future of microsurgery: vascularized tissue engineering (VCA) and flap-specific tissue engineering.

Results

VCA has shown great promise and has already been employed in the clinical setting (especially in face and limb transplantation). Immunosuppression, logistics, cost, and regulatory pathways remain barriers to overcome to make it freely available. Vascularized and flap-specific tissue engineering remain a laboratory reality but have the potential to supersede VCA. The capability of creating an off-the-shelf free flap matching the required tissue, size, and shape is a significant advantage. However, these technologies are still at the early stage and require significant advancement before they can be translated into the clinical setting.

Conclusion

VCA, vascularized tissue engineering, and flap-specific bioengineering represent possible avenues for the evolution of current microsurgical techniques. The next decade will elucidate which of these three strategies will evolve into a tangible translational option and hopefully bring a paradigm shift of reconstructive surgery.

Development of a Novel Hierarchically Biofabricated Blood Vessel Mimic Decorated with Three Vascular Cell Populations for the Reconstruction of Small-Diameter Arteries

The availability of grafts to replace small-diameter arteries remains an unmet clinical need. Here, the validated methodology is reported for a novel hybrid tissue-engineered vascular graft that aims to match the natural structure of small-size arteries. The blood vessel mimic (BVM) comprises an internal conduit of co-electrospun gelatin and polycaprolactone (PCL) nanofibers (corresponding to the tunica intima of an artery), reinforced by an additional layer of PCL aligned fibers (the internal elastic membrane). Endothelial cells are deposited onto the luminal surface using a rotative bioreactor. A bioprinting system extrudes two concentric cell-laden hydrogel layers containing respectively vascular smooth muscle cells and pericytes to create the tunica media and adventitia. The semi-automated cellularization process reduces the production and maturation time to 6 days. After the evaluation of mechanical properties, cellular viability, hemocompatibility, and suturability, the BVM is successfully implanted in the left pulmonary artery of swine. Here, the BVM showed good hemostatic properties, capability to withstand blood pressure, and patency at 5 weeks post-implantation. These promising data open a new avenue to developing an artery-like product for reconstructing small-diameter blood vessels.

Outcomes of Arterial Bypass With the Human Acellular Vessel for Chronic Limb-Threatening Ischemia Performed Under the FDA Expanded Access Program

OBJECTIVE

To report outcomes of the human acellular vessel (HAV) implanted for limb salvage through the Food and Drug Administration (FDA) Expanded Access Program for patients with chronic limb-threatening ischemia with no autologous conduit.

METHODS

The HAV is a bioengineered vascular conduit designed with human vascular smooth muscle cells. The product is under regulatory study. From April 2019 to November 2021, the HAV was implanted in 14 patients (12 men; mean age, 62±14 years) at 3 US centers. Each case was performed with a single-use investigational new drug Expanded Access Program issued by the FDA. Institutional review board approval was obtained; technical and clinical outcomes were analyzed.

RESULTS

A single 6-mm-diameter (40-cm-long) HAV was implanted in 9 patients; 5 patients required 2 HAVs sewn together as a composite. Technical success was 100%. Median follow-up was 12 (range, 1 to 41) months. Primary and secondary patency rates were 72% and 81% at 12 months; assisted primary patency was attained in 4 patients. Amputation-free survival was 93% at 6 months and 77% at 12 months. All patients with a patent HAV experienced clinical improvement with no HAV-related infections or adverse events. There were 4 deaths in the cohort, late mortality unrelated to the HAV.

CONCLUSION

The HAV is a safe and effective "off-the-shelf" biologic conduit. This experience from the FDA Expanded Access Program in this population with few alternative limb salvage options will help guide regulatory deliberations for patients with lower extremity ischemia and no autologous bypass conduit options.

Integrating Fused Deposition Modeling and Melt Electrowriting for Engineering Branched Vasculature

We demonstrate for the first time the combination of two additive manufacturing technologies used in tandem, fused deposition modelling (FDM) and melt electrowriting (MEW), to increase the range of possible MEW structures, with a focus on creating branched, hollow scaffolds for vascularization. First, computer-aided design (CAD) was used to design branched mold halves which were then used to FDM print conductive polylactic acid (cPLA) molds. Next, MEW was performed over the top of these FDM cPLA molds using polycaprolactone (PCL), an FDA-approved biomaterial. After the removal of the newly constructed MEW scaffolds from the FDM molds, complementary MEW scaffold halves were heat-melded together by placing the flat surfaces of each half onto a temperature-controlled platform, then pressing the heated halves together, and finally allowing them to cool to create branched, hollow constructs. This hybrid technique permitted the direct fabrication of hollow MEW structures that would otherwise not be possible to achieve using MEW alone. The scaffolds then underwent in vitro physical and biological testing. Specifically, dynamic mechanical analysis showed the scaffolds had an anisotropic stiffness of 1 MPa or 5 MPa, depending on the direction of the applied stress. After a month of incubation, normal human dermal fibroblasts (NHDFs) were seen growing on the scaffolds, which demonstrated that no deleterious effects were exerted by the MEW scaffolds constructed using FDM cPLA molds. The significant potential of our hybrid additive manufacturing approach to fabricate complex MEW scaffolds could be applied to a variety of tissue engineering applications, particularly in the field of vascularization.

Modulation of Smooth Muscle Cell Phenotype for Translation of Tissue-Engineered Vascular Grafts

Translation of small-diameter tissue-engineered vascular grafts (TEVGs) for the treatment of coronary artery disease (CAD) remains an unfulfilled promise. This is largely due to the limited integration of TEVGs into the native vascular wall-a process hampered by the insufficient smooth muscle cell (SMC) infiltration and extracellular matrix deposition, and low vasoactivity. These processes can be promoted through the judicious modulation of the SMC toward a synthetic phenotype to promote remodeling and vascular integration; however, the expression of synthetic markers is often accompanied by a decrease in the expression of contractile proteins. Therefore, techniques that can precisely modulate the SMC phenotypical behavior could have the potential to advance the translation of TEVGs. In this review, we describe the phenotypic diversity of SMCs and the different environmental cues that allow the modulation of SMC gene expression. Furthermore, we describe the emerging biomaterial approaches to modulate the SMC phenotype in TEVG design and discuss the limitations of current techniques. In addition, we found that current studies in tissue engineering limit the analysis of the SMC phenotype to a few markers, which are often the characteristic of early differentiation only. This limited scope has reduced the potential of tissue engineering to modulate the SMC toward specific behaviors and applications. Therefore, we recommend using the techniques presented in this review, in addition to modern single-cell proteomics analysis techniques to comprehensively characterize the phenotypic modulation of SMCs. Expanding the holistic potential of SMC modulation presents a great opportunity to advance the translation of living conduits for CAD therapeutics.

Small diameter expanded polytetrafluoroethylene vascular graft with differentiated inner and outer biomacromolecules for collaborative endothelialization, anti-thrombogenicity and anti-inflammation

Without differentiated inner and outer biological function, expanded polytetrafluoroethylene (ePTFE) small-diameter (<6 mm) artificial blood vessels would fail in vivo due to foreign body rejection, thrombosis, and hyperplasia. In order to synergistically promote endothelialization, anti-thrombogenicity, and anti-inflammatory function, we modified the inner and outer surface of ePTFE, respectively, by grafting functional biomolecules, such as heparin and epigallocatechin gallate (EGCG), into the inner surface and polyethyleneimine and rapamycin into the outer surface via layer-by-layer self-assembly. Fourier-transform infrared spectroscopy showed the successful incorporation of EGCG, heparin, and rapamycin. The collaborative release profile of heparin and rapamycin lasted for 42 days, respectively. The inner surface promoted human umbilical vein endothelial cells (HUVECs) adhesion and growth and that the outer surface inhibited smooth muscle cells growth and proliferation. The modified ePTFE effectively regulated the differentiation behavior of RAW264.7, inhibited the expression of proinflammatory mediator TNF-α, and up-regulated the expression of anti-inflammatory genes Arg1 and Tgfb-1. The ex vivo circulation results indicated that the occlusions and total thrombus weight of modified ePTFE was much lower than that of the thrombus formed on the ePTFE, presenting good anti-thrombogenic properties. Hence, the straightforward yet efficient synergistic surface functionalization approach presented a potential resolution for the prospective clinical application of small-diameter ePTFE blood vessel grafts.

Flexible, Suturable, and Leak-free Scaffolds for Vascular Tissue Engineering Using Melt Spinning

Coronary artery disease affects millions worldwide. Bypass surgery remains the gold standard; however, autologous tissue is not always available. Hence, the need for an off-the-shelf graft to treat these patients remains extremely high. Using melt spinning, we describe here the fabrication of tubular scaffolds composed of microfibers aligned in the circumferential orientation mimicking the organized extracellular matrix in the tunica media of arteries. By variation of the translational extruder speed, the angle between fibers ranged from 0 to ∼30°. Scaffolds with the highest angle showed the best performance in a three-point bending test. These constructs could be bent up to 160% strain without kinking or breakage. Furthermore, when liquid was passed through the scaffolds, no leakage was observed. Suturing of native arteries was successful. Mesenchymal stromal cells were seeded on the scaffolds and differentiated into vascular smooth muscle-like cells (vSMCs) by reduction of serum and addition of transforming growth factor beta 1 and ascorbic acid. The scaffolds with a higher angle between fibers showed increased expression of vSMC markers alpha smooth muscle actin, calponin, and smooth muscle protein 22-alpha, whereas a decrease in collagen 1 expression was observed, indicating a positive contractile phenotype. Endothelial cells were seeded on the repopulated scaffolds and formed a tightly packed monolayer on the luminal side. Our study shows a one-step fabrication for ECM-mimicking scaffolds with good handleability, leak-free property, and suturability; the excellent biocompatibility allowed the growth of a bilayered construct. Future work will explore the possibility of using these scaffolds as vascular conduits in in vivo settings.

Research trends in cardiovascular tissue engineering from 1992 to 2022: a bibliometric analysis

Background

Cardiovascular tissue engineering (CTE) is a promising technique to treat incurable cardiovascular diseases, such as myocardial infarction and ischemic cardiomyopathy. Plenty of studies related to CTE have been published in the last 30 years. However, an analysis of the research status, trends, and potential directions in this field is still lacking. The present study applies a bibliometric analysis to reveal CTE research trends and potential directions.

Methods

On 5 August 2022, research articles and review papers on CTE were searched from the Web of Science Core Collection with inclusion and exclusion criteria. Publication trends, research directions, and visual maps in this field were obtained using Excel (Microsoft 2009), VOSviewer, and Citespace software.

Results

A total of 2,273 documents from 1992 to 2022 were included in the final analysis. Publications on CTE showed an upward trend from 1992 [number of publications (Np):1] to 2021 (Np:165). The United States (Np: 916, number of citations: 152,377, H-index: 124) contributed the most publications and citations in this field. Research on CTE has a wide distribution of disciplines, led by engineering (Np: 788, number of citations: 40,563, H-index: 105). "Functional maturation" [red cluster, average published year (APY): 2018.63, 30 times], "cell-derived cardiomyocytes" (red cluster, APY: 2018.43, 46 times), "composite scaffolds" (green cluster, APY: 2018.54, 41 times), and "maturation" (red cluster, APY: 2018.17, 84 times) are the main emerging keywords in this area.

Conclusion

Research on CTE is a hot research topic. The United States is a dominant player in CTE research. Interdisciplinary collaboration has played a critical role in the progress of CTE. Studies on functional maturation and the development of novel biologically relevant materials and related applications will be the potential research directions in this field.

Horizon Scanning in Tissue Engineering Using Citation Network Analysis

BACKGROUND

Establishing a horizon scanning method is critical for identifying technologies that require new guidelines or regulations. We studied the application of bibliographic citation network analysis to horizon scanning.

OBJECTIVE

The possibility of applying the proposed method to interdisciplinary fields was investigated with the emphasis on tissue engineering and its example, three-dimensional bio-printing.

METHODOLOGY AND RESULTS

In all, 233,968 articles on tissue engineering, regenerative medicine, biofabrication, and additive manufacturing published between January 1, 1900 and November 3, 2021 were obtained from the Web of Science Core Collection. The citation network of the articles was analyzed for confirmation that the evolution of 3D bio-printing is reflected by tracking the key articles in the field. However, the results revealed that the major articles on the clinical application of 3D bio-printed products are located in clusters other than that of 3D bio-printers. We investigated the research trends in this field by analyzing the articles published between 2019 and 2021 and detected various basic technologies constituting tissue engineering, including microfluidics and scaffolds such as electrospinning and conductive polymers. The results suggested that the research trend of technologies required for product development and future clinical applications of the product are sometimes detected independently by bibliographic citation network analysis, particularly for interdisciplinary fields.

CONCLUSION

This method can be applied to the horizon scanning of an interdisciplinary field. However, identifying basic technologies of the targeted field and following the progress of research and the integration process of each component of technology are critical.

Current Strategies for Engineered Vascular Grafts and Vascularized Tissue Engineering

Blood vessels not only transport oxygen and nutrients to each organ, but also play an important role in the regulation of tissue regeneration. Impaired or occluded vessels can result in ischemia, tissue necrosis, or even life-threatening events. Bioengineered vascular grafts have become a promising alternative treatment for damaged or occlusive vessels. Large-scale tubular grafts, which can match arteries, arterioles, and venules, as well as meso- and microscale vasculature to alleviate ischemia or prevascularized engineered tissues, have been developed. In this review, materials and techniques for engineering tubular scaffolds and vasculature at all levels are discussed. Examples of vascularized tissue engineering in bone, peripheral nerves, and the heart are also provided. Finally, the current challenges are discussed and the perspectives on future developments in biofunctional engineered vessels are delineated.

Current biofabrication methods for vascular tissue engineering and an introduction to biological textiles

Cardiovascular diseases are the leading cause of mortality in the world and encompass several important pathologies, including atherosclerosis. In the cases of severe vessel occlusion, surgical intervention using bypass grafts may be required. Synthetic vascular grafts provide poor patency for small-diameter applications (< 6 mm) but are widely used for hemodialysis access and, with success, larger vessel repairs. In very small vessels, such as coronary arteries, synthetics outcomes are unacceptable, leading to the exclusive use of autologous (native) vessels despite their limited availability and, sometimes, quality. Consequently, there is a clear clinical need for a small-diameter vascular graft that can provide outcomes similar to native vessels. Many tissue-engineering approaches have been developed to offer native-like tissues with the appropriate mechanical and biological properties in order to overcome the limitations of synthetic and autologous grafts. This review overviews current scaffold-based and scaffold-free approaches developed to biofabricate tissue-engineered vascular grafts (TEVGs) with an introduction to the biological textile approaches. Indeed, these assembly methods show a reduced production time compared to processes that require long bioreactor-based maturation steps. Another advantage of the textile-inspired approaches is that they can provide better directional and regional control of the TEVG mechanical properties.

Tissue Engineering of Vascular Grafts: A Case Report From Bench to Bedside and Back

For over 25 years, our group has used regenerative medicine strategies to develop improved biomaterials for use in congenital heart surgery. Among other applications, we developed a tissue-engineered vascular graft (TEVG) by seeding tubular biodegradable polymeric scaffolds with autologous bone marrow-derived mononuclear cells. Results of our first-in-human study demonstrated feasibility as the TEVG transformed into a living vascular graft having an ability to grow, making it the first engineered graft with growth potential. Yet, outcomes of this first Food and Drug Administration-approved clinical trial evaluating safety revealed a prohibitively high incidence of early TEVG stenosis, preventing the widespread use of this promising technology. Mechanistic studies in mouse models provided important insight into the development of stenosis and enabled advanced computational models. Computational simulations suggested both a novel inflammation-driven, mechano-mediated process of in vivo TEVG development and an unexpected natural history, including spontaneous reversal of the stenosis. Based on these in vivo and in silico discoveries, we have been able to rationally design strategies for inhibiting TEVG stenosis that have been validated in preclinical large animal studies and translated to the clinic via a new Food and Drug Administration-approved clinical trial. This progress would not have been possible without the multidisciplinary approach, ranging from small to large animal models and computational simulations. This same process is expected to lead to further advances in scaffold design, and thus next generation TEVGs.

Construction of Rapid Extracellular Matrix-Deposited Small-Diameter Vascular Grafts Induced by Hypoxia in a Bioreactor

Cardiovascular disease has become one of the most globally prevalent diseases, and autologous or vascular graft transplantation has been the main treatment for the end stage of the disease. However, there are no commercialized small-diameter vascular graft (SDVG) products available. The design of SDVGs is promising in the future, and SDVG preparation using an in vitro bioreactor is a favorable method, but it faces the problem of long-term culture of >8 weeks. Herein, we used different oxygen (O₂) concentrations and mechanical stimulation to induce greater secretion of extracellular matrix (ECM) from cells in vitro to rapidly prepare SDVGs. Culturing with 2% O₂ significantly increased the production of the ECM components and growth factors of human dermal fibroblasts (hDFs). To accelerate the formation of ECM, hDFs were seeded on a polycaprolactone (PCL) scaffold and cultured in a flow culture bioreactor with 2% O₂ for only 3 weeks. After orthotopic transplantation in rat abdominal aorta, the cultured SDVGs (PCL-decellularized ECM) showed excellent endothelialization and smooth muscle regeneration. The vascular grafts cultured with hypoxia and mechanical stimulation could accelerate the reconstruction speed and obtain an improved therapeutic effect and thereby provide a new research direction for improving the production and supply of SDVGs.

Application of decellularized vascular matrix in small-diameter vascular grafts

Coronary artery bypass grafting (CABG) remains the most common procedure used in cardiovascular surgery for the treatment of severe coronary atherosclerotic heart disease. In coronary artery bypass grafting, small-diameter vascular grafts can potentially replace the vessels of the patient. The complete retention of the extracellular matrix, superior biocompatibility, and non-immunogenicity of the decellularized vascular matrix are unique advantages of small-diameter tissue-engineered vascular grafts. However, after vascular implantation, the decellularized vascular matrix is also subject to thrombosis and neoplastic endothelial hyperplasia, the two major problems that hinder its clinical application. The keys to improving the long-term patency of the decellularized matrix as vascular grafts include facilitating early endothelialization and avoiding intravascular thrombosis. This review article sequentially introduces six aspects of the decellularized vascular matrix as follows: design criteria of vascular grafts, components of the decellularized vascular matrix, the changing sources of the decellularized vascular matrix, the advantages and shortcomings of decellularization technologies, modification methods and the commercialization progress as well as the application prospects in small-diameter vascular grafts.

Computer Model-Driven Design in Cardiovascular Regenerative Medicine

Continuing advances in genomics, molecular and cellular mechanobiology and immunobiology, including transcriptomics and proteomics, and biomechanics increasingly reveal the complexity underlying native tissue and organ structure and function. Identifying methods to repair, regenerate, or replace vital tissues and organs remains one of the greatest challenges of modern biomedical engineering, one that deserves our very best effort. Notwithstanding the continuing need for improving standard methods of investigation, including cell, organoid, and tissue culture, biomaterials development and fabrication, animal models, and clinical research, it is increasingly evident that modern computational methods should play increasingly greater roles in advancing the basic science, bioengineering, and clinical application of regenerative medicine. This brief review focuses on the development and application of computational models of tissue and organ mechanobiology and mechanics for purposes of designing tissue engineered constructs and understanding their development in vitro and in situ. Although the basic approaches are general, for illustrative purposes we describe two recent examples from cardiovascular medicine-tissue engineered heart valves (TEHVs) and tissue engineered vascular grafts (TEVGs)-to highlight current methods of approach as well as continuing needs.

Six-year outcomes of a phase II study of human-tissue engineered blood vessels for peripheral arterial bypass

Objective

The human acellular vessel (HAV) was evaluated for surgical bypass in a phase II study. The primary results at 24 months after implantation have been reported, and the patients will be evaluated for ≤10 years.

Methods

In the present report, we have described the 6-year results of a prospective, open-label, single-treatment arm, multicenter study. Patients with advanced peripheral artery disease (PAD) requiring above-the-knee femoropopliteal bypass surgery without available autologous graft options had undergone implantation with the HAV, a bioengineered human tissue replacement blood vessel. The patients who completed the 24-month primary portion of the study will be evaluated for ≤10 years after implantation. The present mid-term analysis was performed at the 6-year milestone (72 months) for patients followed up for 24 to 72 months.

Results

HAVs were implanted in 20 patients at three sites in Poland. Seven patients had discontinued the study before completing the 2-year portion of the study: four after graft occlusion had occurred and three who had died of causes deemed unrelated to the conduit, with the HAV reported as functional at their last visit. The primary results at 24 months showed primary, primary assisted, and secondary patency rates of 58%, 58%, and 74%, respectively. One vessel had developed a pseudoaneurysm deemed possibly iatrogenic; no other signs of structural failure were reported. No rejections or infections of the HAV occurred, and no patient had required amputation of the implanted limb. Of the 20 patients, 13 had completed the primary portion of the study; however, 1 patient had died shortly after 24 months. Of the remaining 12 patients, 3 died of causes unrelated to the HAV. One patient had required thrombectomy twice, with secondary patency achieved. No other interventions were recorded between 24 and 72 months. At 72 months, five patients had a patent HAV, including four patients with primary patency. For the entire study population from day 1 to month 72, the overall primary, primary assisted, and secondary patency rate estimated using Kaplan-Meier analysis was 44%, 45%, and 60% respectively, with censoring for death. No patient had experienced rejection or infection of the HAV, and no patient had required amputation of the implanted limb.

Conclusions

The infection-resistant, off-the-shelf HAV could provide a durable alternative conduit in the arterial circuit setting to restore the lower extremity blood supply in patients with PAD, with remodeling into the recipient's own vessel over time. The HAV is currently being evaluated in seven clinical trials to treat PAD, vascular trauma, and as a hemodialysis access conduit.

Construction of spider silk protein small-caliber tissue engineering vascular grafts based on dynamic culture and its performance evaluation

Tissue engineering is an alternative method for preparing small-caliber (<6 mm) vascular grafts. Dynamic mechanical conditioning is being researched as a method to improve mechanical properties of tissue engineered blood vessels. This method attempts to induce unique reaction in implanted cells that regenerate the matrix around them, thereby improving the overall mechanical stability of the grafts. In this study, we used a bioreactor to seed endothelial cells and smooth muscle cells into the inner and outer layers of the electrospun spider silk protein scaffold respectively to construct vascular grafts. The cell proliferation, mechanical properties, blood compatibility and other indicators of the vascular grafts were characterized in vitro. Furthermore, the vascular grafts were implanted in Sprague Dawley rats, and the vascular grafts' patency, extracellular matrix formation, and inflammatory response were evaluated in vivo. We aimed to construct spider silk protein vascular grafts with the potential for in vivo implantation by using a pulsating flow bioreactor. The results showed that, when compared with the static culture condition, the dynamic culture condition improved cell proliferation on vascular scaffolds and enhanced mechanical function of vascular scaffolds. In vivo experiments also showed that the dynamic culture of vascular grafts was more beneficial for the extracellular matrix deposition and anti-thrombogenesis, as well as reducing the inflammatory response of vascular grafts. In conclusion, dynamic mechanical conditioning aid in the resolution of challenges impeding the application of electrospun scaffolds and have the potential to construct small-caliber blood vessels with regenerative function for cardiovascular tissue repair.

Investigation of Cell Adhesion and Cell Viability of the Endothelial and Fibroblast Cells on Electrospun PCL, PLGA and Coaxial Scaffolds for Production of Tissue Engineered Blood Vessel

Endothelialization of artificial scaffolds is considered an effective strategy for increasing the efficiency of vascular transplantation. This study aimed to compare the biophysical/biocompatible properties of three different biodegradable fibrous scaffolds: Poly (ɛ-caprolactone) (PCL) alone, Poly Lactic-co-Glycolic Acid (PLGA) alone (both processed using Spraybase^® electrospinning machine), and Coaxial scaffold where the fiber core and sheath was made of PCL and PLGA, respectively. Scaffold structural morphology was assessed by scanning electron microscope and tensile testing was used to investigate the scaffold tension resistance over time. Biocompatibility studies were carried out with human umbilical vein endothelial cells (HUVEC) and human vascular fibroblasts (HVF) for which cell viability (and cell proliferation over a 4-day period) and cell adhesion to the scaffolds were assessed by cytotoxicity assays and confocal microscopy, respectively. Our results showed that all biodegradable polymeric scaffolds are a reliable host to adhere and promote proliferation in HUVEC and HVF cells. In particular, PLGA membranes performed much better adhesion and enhanced cell proliferation compared to control in the absence of polymers. In addition, we demonstrate here that these biodegradable membranes present improved mechanical properties to construct potential tissue-engineered vascular graft.

Advances in the development of tubular structures using extrusion-based 3D cell-printing technology for vascular tissue regenerative applications

Until recent, there are no ideal small diameter vascular grafts available on the market. Most of the commercialized vascular grafts are used for medium to large-sized blood vessels. As a solution, vascular tissue engineering has been introduced and shown promising outcomes. Despite these optimistic results, there are limitations to commercialization. This review will cover the need for extrusion-based 3D cell-printing technique capable of mimicking the natural structure of the blood vessel. First, we will highlight the physiological structure of the blood vessel as well as the requirements for an ideal vascular graft. Then, the essential factors of 3D cell-printing including bioink, and cell-printing system will be discussed. Afterwards, we will mention their applications in the fabrication of tissue engineered vascular grafts. Finally, conclusions and future perspectives will be discussed.

In Vitro Differentiation of Myoblast Cell Lines on Spider Silk Scaffolds in a Rotating Bioreactor for Vascular Tissue Engineering

Functional construction of tissue-engineered vessels as an alternative to autologous vascular grafts has been shown to be feasible, however the proliferation of seeded smooth-muscle cells remains a limiting factor. We employed a rotating bioreactor system to improve myoblast cell differentiation on a spider silk scaffold for tissue-engineered vessel construction. C2C12 myofibroblast cells were seeded on the surface of spider silk scaffold constructs and cultivated in a rotating bioreactor system with a continuous rotation speed (1 rpm). Cell function, cell growth and morphological structure and expression of biomarkers were analyzed using scanning electron microscopy, the LIVE/DEAD^® assay, Western blot and quantitative real-time PCR analyses. A dense myofibroblast cell sheet could be developed which resembled native blood vessel muscular tissue in morphological structure and in function. Bioreactor perfusion positively affected cell morphology, and increased cell viability and cell differentiation. The expression of desmin, MYF5 and MEF2D surged as an indication of myoblast differentiation. Cell-seeded scaffolds showed a tear-down at 18 N when strained at a set speed (20 mm min^-1). Spider silk scaffolds appear to offer a reliable basis for engineered vascular constructs and rotating bioreactor cultivation may be considered an effective alternative to complex bioreactor setups to improve cell viability and biology.

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

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

The regenerative potential of Pax3/Pax7 on skeletal muscle injury

Background

Skeletal muscle mishaps are the most well-known incidents in society, especially among athletes and the military population. From the various urgency, this accident needs to be cured more quickly. However, the current treatment still has some shortcomings and is less effective. In this case, Paired box 3 and Paired box 7 (Pax3/Pax7) proteins that induce stem cells could potentially be an alternative treatment for skeletal muscle injuries. This paper aimed to analyse the potential treatment of Pax3/Pax7 proteins inducing the stem cell for skeletal muscle injuries.

The main body of the abstract

We did a narrative review by gathering several scientific journals from several leading platforms like PubMed and Scopus. As common accidents, skeletal muscle disease could be due to workplace and non-workplace causes. The highest risk occurs in the athlete and military environment. The treatment of current skeletal muscle injuries is protection, rest, ice, compression, and elevation (PRICE), non-steroidal anti-inflammatory drugs (NSAIDs), and mechanical stimulation. However, it is considered less effective, especially in NSAIDs, inhibiting myogenic cell proliferation. The current finding indicates that the stem cells have markers known as Pax3/Pax7. The role of both markers in muscle injury, Pax3/Pax7, as transcription factors will induce cell division by H3K4 methylation mechanisms and chromatin modifications that stimulate gene activation.

Conclusion

Regulation by Pax3/Pax7 factors that affect stem cells and stem cell proliferation is one of the alternative treatments. This regulation can accelerate the healing of injury victims, especially injuries to the skeletal muscles. Finally, after being compared, Pax3/Pax7 induces stem cells to have the potential to be one of the skeletal muscle injury treatments.

Keywords

Pax3 and Pax7, Pax3/Pax7, Skeletal muscle, Athlete, Stem cells, Cell proliferation, Injuries.