Human Vascular Microphysiological System for in vitro Drug Screening

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.

Strategies for improved endothelial cell adhesion in microphysiological vascular model systems

Human tissue-engineered blood vessels (TEBVs) have been applied as model systems to study a wide range of vascular diseases including Hutchinson-Gilford Progeria Syndrome and early atherosclerosis. Central to the utility of TEBVs as an in vitro blood vessel model is the maintenance of a functional endothelium under physiologically relevant shear stresses. Establishing and maintaining a confluent endothelial monolayer is challenging. In this protocol, we outline an optimized procedure for the endothelialization of TEBVs. We optimized the following key conditions affecting endothelial cell (EC) adherence in the vessel: EC seeding density, rotation time, and the application of perfusion. This protocol results in TEBVs with sustained EC luminal coverage that demonstrate alignment in the direction of applied flow and responsiveness to inflammatory stimuli. To facilitate rapid screening of EC coverage during the fabrication and perfusion steps, we re-designed TEBV chambers to include a viewing window that allows for efficient monitoring and assessment of the endothelialization process using fluorescence microscopy. By identifying key factors that affect EC attachment in TEBVs, this protocol may serve as a valuable resource for researchers seeking to achieve successful endothelialization of engineered blood vessel constructs.

Latest developments of microphysiological systems (MPS) in aging-related and geriatric diseases research: A review

Aging is a gradual and irreversible process accompanied by the decline in tissue function and a significantly increased risk of various aging-related and geriatric diseases. Especially in the paradoxical context of accelerated global aging and the widespread emergence of pandemics, aging-related and geriatric diseases have become leading causes of individual mortality and disability, drawing increasing attention from researchers and investors alike. Despite the utility of current in vitro systems and in vivo animal models for studying aging, these approaches are limited by insurmountable inherent constraints. In response, microphysiological systems (MPS), leveraging advances in tissue engineering and microfluidics, have emerged as highly promising platforms. MPS are capable of replicating key features of the tissue microenvironment within microfabricated devices, offering biomimetic tissue culture conditions that enhance the in vitro simulation of intact or precise human body structure and function. This capability improves the predictability of clinical trial outcomes while reducing time and cost. In this review, we focus on recent advancements in MPS used to study age-related and geriatric diseases, with particular emphasis on the application of organoids and organ-on-a-chip technologies in understanding cardiovascular diseases, cerebrovascular diseases, neurodegenerative diseases, fibrotic diseases, locomotor and sensory degenerative disorders, and rare diseases. And we aim to provide readers with critical guidelines and an overview of examples for modeling age-related and geriatric diseases using MPS, exploring mechanisms, treatments, drug screening, and other subsequent applications, from a physiopathological perspective, emphasizing the characteristic of age-related and geriatric diseases and their established correlations with the aging process. We also discuss the limitations of current models and propose future directions for MPS in aging research, highlighting the potential of interdisciplinary approaches to address unresolved challenges in the field.

Programmable embedded bioprinting for one-step manufacturing of arterial models with customized contractile and metabolic functions

Replicating the contractile function of arterial tissues in vitro requires precise control of cell alignment within 3D structures, a challenge that existing bioprinting techniques struggle to meet. In this study, we introduce the voxel-based embedded construction for tailored orientational replication (VECTOR) method, a voxel-based approach that controls cellular orientation and collective behavior within bioprinted filaments. By fine-tuning voxel vector magnitude and using an omnidirectional printing trajectory, we achieve structural mimicry at both the macroscale and the cellular alignment level. This dual-scale approach enhances vascular smooth muscle cell (VSMC) function by regulating contractile and synthetic pathways. The VECTOR method facilitates the construction of 3D arterial structures that closely replicate natural coronary architectures, significantly improving contractility and metabolic function. Moreover, the resulting multilayered arterial models (AMs) exhibit precise responses to pharmacological stimuli, similar to native arteries. This work highlights the critical role of structural mimicry in tissue functionality and advances the replication of complex tissues in vitro.

Modeling human atherosclerotic lesions in the test tube: Are we there yet?

Atherosclerotic cardiovascular diseases remain a leading cause of morbidity and mortality worldwide. Atherogenesis is a slow and life-long process characterized by the accumulation of lipoproteins and immune cells within the arterial wall. Atherosclerosis has been successfully modeled in animals: However, there are economic, ethical, and translational concerns when using these models. There is also growing recognition of the need for robust human-based in vitro systems that can faithfully recapitulate key aspects of human atherosclerosis. Such systems may offer advantages in terms of scalability, reproducibility, and ability to manipulate specific variables, thereby facilitating a deeper understanding of disease mechanisms and accelerating the development of targeted therapeutics. Leveraging innovative in vitro platforms holds promise in complementing traditional animal models of atherosclerosis. In the present review, we discuss the advantages and disadvantages of recently developed models of atherosclerosis and propose ideas to be considered when developing future generations of models.

Endothelial dysfunction and cardiovascular diseases: The role of human induced pluripotent stem cells and tissue engineering

Cardiovascular disease (CVD) remains to be the leading cause of death globally today and therefore the need for the development of novel therapies has become increasingly important in the cardiovascular field. The mechanism(s) behind the pathophysiology of CVD have been laboriously investigated in both stem cell and bioengineering laboratories. Scientific breakthroughs have paved the way to better mimic cell types of interest in recent years, with the ability to generate any cell type from reprogrammed human pluripotent stem cells. Mimicking the native extracellular matrix using both organic and inorganic biomaterials has allowed full organs to be recapitulated in vitro. In this paper, we will review techniques from both stem cell biology and bioengineering which have been fruitfully combined and have fueled advances in the cardiovascular disease field. We will provide a brief introduction to CVD, reviewing some of the recent studies as related to the role of endothelial cells and endothelial cell dysfunction. Recent advances and the techniques widely used in both bioengineering and stem cell biology will be discussed, providing a broad overview of the collaboration between these two fields and their overall impact on tissue engineering in the cardiovascular devices and implications for treatment of cardiovascular disease.

Manufacturing and validation of small-diameter vascular grafts: A mini review

The field of small-diameter vascular grafts remains a challenge for biomaterials scientists. While decades of research have brought us much closer to developing biomimetic materials for regenerating tissues and organs, the physiological challenges involved in manufacturing small conduits that can transport blood while not inducing an immune response or promoting blood clots continue to limit progress in this area. In this short review, we present some of the most recent methods and advancements made by researchers working in the field of small-diameter vascular grafts. We also discuss some of the most critical aspects biomaterials scientists should consider when developing lab-made small-diameter vascular grafts.

Challenging the status quo: a framework for mechanistic and human-relevant cardiovascular safety screening

Traditional approaches to preclinical drug safety assessment have generally protected human patients from unintended adverse effects. However, these assessments typically occur too late to make changes in the formulation or in phase 1 and beyond, are highly dependent on animal studies and have the potential to lead to the termination of useful drugs due to liabilities in animals that are not applicable in patients. Collectively, these elements come at great detriment to both patients and the drug development sector. This phenomenon is particularly problematic in the area of cardiovascular safety assessment where preclinical attrition is high. We believe that a more efficient and translational approach can be defined. A multi-tiered assessment that leverages our understanding of human cardiovascular biology, applies human cell-based in vitro characterizations of cardiovascular responses to insult, and incorporates computational models of pharmacokinetic relationships would enable earlier and more translational identification of human-relevant liabilities. While this will take time to develop, the ultimate goal would be to implement such assays both in the lead selection phase as well as through regulatory phases.

Engineering pre-vascularized 3D tissue and rapid vascular integration with host blood vessels via co-cultured spheroids-laden hydrogel

Recent advances in regenerative medicine and tissue engineering have enabled the biofabrication of three-dimensional (3D) tissue analogues with the potential for use in transplants and disease modeling. However, the practical use of these biomimetic tissues has been hindered by the challenge posed by reconstructing anatomical-scale micro-vasculature tissues. In this study, we suggest that co-cultured spheroids within hydrogels hold promise for regenerating highly vascularized and innervated tissues, bothin vitroandin vivo. Human adipose-derived stem cells (hADSCs) and human umbilical vein cells (HUVECs) were prepared as spheroids, which were encapsulated in gelatin methacryloyl hydrogels to fabricate a 3D pre-vascularized tissue. The vasculogenic responses, extracellular matrix production, and remodeling depending on parameters like co-culture ratio, hydrogel strength, and pre-vascularization time forin vivointegration with native vessels were then delicately characterized. The co-cultured spheroids with 3:1 ratio (hADSCs/HUVECs) within the hydrogel and with a pliable storage modulus showed the greatest vasculogenic potential, and ultimately formedin vitroarteriole-scale vasculature with a longitudinal lumen structure and a complex vascular network after long-term culturing. Importantly, the pre-vascularized tissue also showed anastomotic vascular integration with host blood vessels after transplantation, and successful vascularization that was positive for both CD31 and alpha-smooth muscle actin covering 18.6 ± 3.6μm2of the luminal area. The described co-cultured spheroids-laden hydrogel can therefore serve as effective platform for engineering 3D vascularized complex tissues.

Vascular organs-on-chip made with patient-derived endothelial cells: technologies to transform drug discovery and disease modeling

INTRODUCTION

Vascular diseases impart a tremendous burden on healthcare systems in the United States and across the world. Efforts to improve therapeutic interventions are hindered by limitations of current experimental models. The integration of patient-derived cells with organ-on-chip (OoC) technology is a promising avenue for preclinical drug screening that improves upon traditional cell culture and animal models.

AREAS COVERED

The authors review induced pluripotent stem cells (iPSC) and blood outgrowth endothelial cells (BOEC) as two sources for patient-derived endothelial cells (EC). They summarize several studies that leverage patient-derived EC and OoC for precision disease modeling of the vasculature, with a focus on applications for drug discovery. They also highlight the utility of patient-derived EC in other translational endeavors, including ex vivo organogenesis and multi-organ-chip integration.

EXPERT OPINION

Precision disease modeling continues to mature in the academic space, but end-use by pharmaceutical companies is currently limited. To fully realize their transformative potential, OoC systems must balance their complexity with their ability to integrate with the highly standardized and high-throughput experimentation required for drug discovery and development.

Atherosclerotic three-layer nanomatrix vascular sheets for high-throughput therapeutic evaluation

In vitro atherosclerosis models are essential to evaluate therapeutics before in vivo and clinical studies, but significant limitations remain, such as the lack of three-layer vascular architecture and limited atherosclerotic features. Moreover, no scalable 3D atherosclerosis model is available for making high-throughput assays for therapeutic evaluation. Herein, we report an in vitro 3D three-layer nanomatrix vascular sheet with critical atherosclerosis multi-features (VSA), including endothelial dysfunction, monocyte recruitment, macrophages, extracellular matrix remodeling, smooth muscle cell phenotype transition, inflammatory cytokine secretion, foam cells, and calcification initiation. Notably, we present the creation of high-throughput functional assays with VSAs and the use of these assays for evaluating therapeutics for atherosclerosis treatment. The therapeutics include conventional drugs (statin and sirolimus), candidates for treating atherosclerosis (curcumin and colchicine), and potential gene therapy (miR-146a-loaded liposomes). The high efficiency and flexibility of the scalable VSA functional assays should facilitate drug discovery and development for atherosclerosis.

Cardiovascular Tissue Engineering Models for Atherosclerosis Treatment Development

In the early years of tissue engineering, scientists focused on the generation of healthy-like tissues and organs to replace diseased tissue areas with the aim of filling the gap between organ demands and actual organ donations. Over time, the realization has set in that there is an additional large unmet need for suitable disease models to study their progression and to test and refine different treatment approaches. Increasingly, researchers have turned to tissue engineering to address this need for controllable translational disease models. We review existing and potential uses of tissue-engineered disease models in cardiovascular research and suggest guidelines for generating adequate disease models, aimed both at studying disease progression mechanisms and supporting the development of dedicated drug-delivery therapies. This involves the discussion of different requirements for disease models to test drugs, nanoparticles, and drug-eluting devices. In addition to realistic cellular composition, the different mechanical and structural properties that are needed to simulate pathological reality are addressed.

Microphysiological Conditions Do Not Affect MDR1-Mediated Transport of Rhodamine 123 above an Artificial Proximal Tubule

Despite disadvantages, such as high cost and their poor predictive value, animal experiments are still the state of the art for pharmaceutical substance testing. One reason for this problem is the inability of standard cell culture methods to emulate the physiological environment necessary to recapitulate in vivo processes. Microphysiological systems offer the opportunity to close this gap. In this study, we utilize a previously employed microphysiological system to examine the impact of pressure and flow on the transportation of substances mediated by multidrug resistance protein 1 (MDR1) across an artificial cell-based tubular barrier. By using a miniaturized fluorescence measurement device, we could continuously track the MDR1-mediated transport of rhodamine 123 above the artificial barrier over 48 h. We proved that applying pressure and flow affects both active and passive transport of rhodamine 123. Using experimental results and curve fittings, the kinetics of MDR1-mediated transport as well as passive transport were investigated; thus, a kinetic model that explains this transport above an artificial tubular barrier was identified. This kinetic model demonstrates that the simple Michaelis-Menten model is not an appropriate model to explain the MDR1-mediated transport; instead, Hill kinetics, with Hill slope of n = 2, is a better fit. The kinetic values, K_m, V_max, and apparent permeability (P_app), obtained in this study are comparable with other in vivo and in vitro studies. Finally, the presented proximal tubule-on-a-chip can be used for pharmaceutical substance testing and to investigate pharmacokinetics of the renal transporter MDR1.

Building blocks of microphysiological system to model physiology and pathophysiology of human heart

Microphysiological systems (MPS) are drawing increasing interest from academia and from biomedical industry due to their improved capability to capture human physiology. MPS offer an advanced in vitro platform that can be used to study human organ and tissue level functions in health and in diseased states more accurately than traditional single cell cultures or even animal models. Key features in MPS include microenvironmental control and monitoring as well as high biological complexity of the target tissue. To reach these qualities, cross-disciplinary collaboration from multiple fields of science is required to build MPS. Here, we review different areas of expertise and describe essential building blocks of heart MPS including relevant cardiac cell types, supporting matrix, mechanical stimulation, functional measurements, and computational modelling. The review presents current methods in cardiac MPS and provides insights for future MPS development with improved recapitulation of human physiology.

A quantitative meta-analysis comparing cell models in perfused organ on a chip with static cell cultures

As many consider organ on a chip for better in vitro models, it is timely to extract quantitative data from the literature to compare responses of cells under flow in chips to corresponding static incubations. Of 2828 screened articles, 464 articles described flow for cell culture and 146 contained correct controls and quantified data. Analysis of 1718 ratios between biomarkers measured in cells under flow and static cultures showed that the in all cell types, many biomarkers were unregulated by flow and only some specific biomarkers responded strongly to flow. Biomarkers in cells from the blood vessels walls, the intestine, tumours, pancreatic island, and the liver reacted most strongly to flow. Only 26 biomarkers were analysed in at least two different articles for a given cell type. Of these, the CYP3A4 activity in CaCo2 cells and PXR mRNA levels in hepatocytes were induced more than two-fold by flow. Furthermore, the reproducibility between articles was low as 52 of 95 articles did not show the same response to flow for a given biomarker. Flow showed overall very little improvements in 2D cultures but a slight improvement in 3D cultures suggesting that high density cell culture may benefit from flow. In conclusion, the gains of perfusion are relatively modest, larger gains are linked to specific biomarkers in certain cell types.

Freestanding hydrogel lumens for modeling blood vessels and vasodilation

Lumen structures exist throughout the human body, and the vessels of the circulatory system are essential for carrying nutrients and oxygen and regulating inflammation. Vasodilation, the widening of the blood vessel lumen, is important to the immune response as it increases blood flow to a site of inflammation, raises local temperature, and enables optimal immune system function. A common method for studying vasodilation uses excised vessels from animals; major drawbacks include heterogeneity in vessel shape and size, time-consuming procedures, sacrificing animals, and differences between animal and human biology. We have developed a simple, user-friendly in vitro method to form freestanding cell-laden hydrogel rings from collagen and quantitatively measure the effects of vasodilators on ring size. The hydrogel rings are composed of collagen I and can be laden with human vascular smooth muscle cells, a major cellular and structural component of blood vessels, or lined with endothelial cells in the lumen. The methods presented include a 3D printed device (which is amenable to future fabrication by injection molding) and commercially available components (e.g., Teflon tubing or a syringe) to form hydrogel rings between 2.6-4.6 mm outer diameter and 0.79-1.0 mm inner diameter. Here we demonstrate a significant difference in ring area in the presence of a known vasodilator, fasudil (p < 0.0001). Our method is easy to implement and provides a foundation for a medium-throughput solution to generating vessel model structures for future investigations of the fundamental mechanisms of vasodilation (e.g., studying uncharacterized endogenous molecules that may have vasoactivity) and testing vasoactive drugs.

Human endothelial cells form an endothelium in freestanding collagen hollow filaments fabricated by direct extrusion printing

Fiber-shaped materials have great potential for tissue engineering applications as they provide structural support and spatial patterns within a three-dimensional construct. Here we demonstrate the fabrication of mechanically stable, meter-long collagen hollow filaments by a direct extrusion printing process. The fibres are permeable for oxygen and proteins and allow cultivation of primary human endothelial cells (ECs) at the inner surface under perfused conditions. The cells show typical characteristics of a well-organized EC lining including VE-cadherin expression, cellular response to flow and ECM production. The results demonstrate that the collagen tubes are capable of creating robust soft tissue filaments. The mechanical properties and the biofunctionality of these collagen hollow filaments facilitate the engineering of prevascularised tissue engineering constructs.

Biofabrication of Collagen Tissue-Engineered Blood Vessels with Direct Co-Axial Extrusion

Cardiovascular diseases are considered one of the worldwide causes of death, with atherosclerosis being the most predominant. Nowadays, the gold standard treatment is blood vessel replacement by bypass surgery; however, autologous source is not always possible. Thereby, tissue-engineered blood vessels (TEBVs) are emerging as a potential alternative source. In terms of composition, collagen has been selected in many occasions to develop TEBVs as it is one of the main extracellular matrix components of arteries. However, it requires specific support or additional processing to maintain the tubular structure and appropriate mechanical properties. Here, we present a method to develop support-free collagen TEBVs with co-axial extrusion in a one-step procedure with high concentrated collagen. The highest concentration of collagen of 20 mg/mL presented a burst pressure of 619.55 ± 48.77 mmHg, being able to withstand perfusion of 10 dynes/cm². Viability results showed a high percentage of viability (86.1 and 85.8% with 10 and 20 mg/mL, respectively) of human aortic smooth muscle cells (HASMCs) and human umbilical vein endothelial cells (HUVEC) after 24 h extrusion. Additionally, HUVEC and HASMCs were mainly localized in their respective layers, mimicking the native distribution. All in all, this approach allows the direct extrusion of collagen TEBVs in a one-step procedure with enough mechanical properties to be perfused.

3D Tissue-Engineered Vascular Drug Screening Platforms: Promise and Considerations

Despite the efforts devoted to drug discovery and development, the number of new drug approvals have been decreasing. Specifically, cardiovascular developments have been showing amongst the lowest levels of approvals. In addition, concerns over the adverse effects of drugs to the cardiovascular system have been increasing and resulting in failure at the preclinical level as well as withdrawal of drugs post-marketing. Besides factors such as the increased cost of clinical trials and increases in the requirements and the complexity of the regulatory processes, there is also a gap between the currently existing pre-clinical screening methods and the clinical studies in humans. This gap is mainly caused by the lack of complexity in the currently used 2D cell culture-based screening systems, which do not accurately reflect human physiological conditions. Cell-based drug screening is widely accepted and extensively used and can provide an initial indication of the drugs' therapeutic efficacy and potential cytotoxicity. However, in vitro cell-based evaluation could in many instances provide contradictory findings to the in vivo testing in animal models and clinical trials. This drawback is related to the failure of these 2D cell culture systems to recapitulate the human physiological microenvironment in which the cells reside. In the body, cells reside within a complex physiological setting, where they interact with and respond to neighboring cells, extracellular matrix, mechanical stress, blood shear stress, and many other factors. These factors in sum affect the cellular response and the specific pathways that regulate variable vital functions such as proliferation, apoptosis, and differentiation. Although pre-clinical in vivo animal models provide this level of complexity, cross species differences can also cause contradictory results from that seen when the drug enters clinical trials. Thus, there is a need to better mimic human physiological conditions in pre-clinical studies to improve the efficiency of drug screening. A novel approach is to develop 3D tissue engineered miniaturized constructs in vitro that are based on human cells. In this review, we discuss the factors that should be considered to produce a successful vascular construct that is derived from human cells and is both reliable and reproducible.

Human Vascular Wall Microfluidic Model for Preclinical Evaluation of Drug-Induced Vascular Injury

Drug-induced vascular injury (DIVI) in preclinical animal models often leads to candidate compound termination during drug development. DIVI has not been documented in human clinical trials with drugs that cause DIVI in preclinical animals. A robust human preclinical assay for DIVI is needed as an early vascular injury screen. A human vascular wall microfluidic tissue chip was developed with a human umbilical vein endothelial cell (HUVEC)-umbilical artery smooth muscle cell (vascular smooth muscle cell, VSMC) bilayer matured under physiological shear stress. Optimized temporal flow profiles produced HUVEC-VSMC bilayers with quiescent endothelial cell (EC) monolayers, EC tight junctions, and contractile VSMC morphology. Dose-response testing (3-30 μM concentration) was conducted with minoxidil and tadalafil vasodilators. Both drugs have demonstrated preclinical DIVI but lack clinical evidence. The permeability of severely damaged engineered bilayers (30 μM tadalafil) was 4.1 times that of the untreated controls. Immunohistochemical protein assays revealed contrasting perspectives on tadalafil and minoxidil-induced damage. Tadalafil impacted the endothelial monolayer with minor injury to the contractile VSMCs, whereas minoxidil demonstrated minor EC barrier injury but damaged VSMCs and activated ECs in a dose-response manner. This proof-of-concept human vascular wall bilayer model of DIVI is a critical step toward developing a preclinical human screening assay for drug development. Impact statement More than 90% of drug candidates fail during clinical trials due to human efficacy and toxicity concerns. Preclinical studies rely heavily on animal models, although animal toxicity and drug metabolism responses often differ from humans. During the drug development process, perfused in vitro human tissue chips could model the clinical drug response and potential toxicity of candidate compounds. Our long-term objective is to develop a human vascular wall tissue chip to screen for drug-induced vascular injury. Its application could ultimately reduce drug development delays and costs, and improve patient safety.

Recent Progress in in vitro Models for Atherosclerosis Studies

Atherosclerosis is the primary cause of hardening and narrowing arteries, leading to cardiovascular disease accounting for the high mortality in the United States. For developing effective treatments for atherosclerosis, considerable efforts have been devoted to developing in vitro models. Compared to animal models, in vitro models can provide great opportunities to obtain data more efficiently, economically. Therefore, this review discusses the recent progress in in vitro models for atherosclerosis studies, including traditional two-dimensional (2D) systems cultured on the tissue culture plate, 2D cell sheets, and recently emerged microfluidic chip models with 2D culture. In addition, advanced in vitro three-dimensional models such as spheroids, cell-laden hydrogel constructs, tissue-engineered blood vessels, and vessel-on-a-chip will also be covered. Moreover, the functions of these models are also summarized along with model discussion. Lastly, the future perspectives of this field are discussed.

In Situ Fabrication and Perfusion of Tissue-Engineered Blood Vessel Microphysiological System

Human tissue-engineered blood vessels (TEBVs) that exhibit vasoactivity can be used to test drug toxicity, modulate pro-inflammatory cytokines, and model disease states in vitro. We developed a novel device to fabricate arteriole-scale human endothelialized TEBVs in situ with smaller volumes and higher throughput than previously reported. Both primary and induced pluripotent stem cell (iPSC)-derived cells can be used. Four collagen TEBVs with 600μm inner diameter and 2.9 mm outer diameter are fabricated by pipetting a solution of collagen and medial cells into a three-layer acrylic mold. After gelation, the TEBVs are released from the mold and dehydrated. After suturing the TEBVs in place and changing the mold parts to form a perfusion chamber, the TEBVs are endothelialized in situ, and then media is perfused through the lumen. By removing 90% of the water after gelation, the TEBVs become mechanically strong enough for perfusion at the physiological shear stress of 0.4 Pa within 24 h of fabrication and maintain function for at least 5 weeks.

The vascular niche in next generation microphysiological systems

In recent years, microphysiological system (MPS, also known as, organ-on-a-chip or tissue chip) platforms have emerged with great promise to improve the predictive capacity of preclinical modeling thereby reducing the high attrition rates when drugs move into trials. While their designs can vary quite significantly, in general MPS are bioengineered in vitro microenvironments that recapitulate key functional units of human organs, and that have broad applications in human physiology, pathophysiology, and clinical pharmacology. A critical next step in the evolution of MPS devices is the widespread incorporation of functional vasculature within tissues. The vasculature itself is a major organ that carries nutrients, immune cells, signaling molecules and therapeutics to all other organs. It also plays critical roles in inducing and maintaining tissue identity through expression of angiocrine factors, and in providing tissue-specific milieus (i.e., the vascular niche) that can support the survival and function of stem cells. Thus, organs are patterned, maintained and supported by the vasculature, which in turn receives signals that drive tissue specific gene expression. In this review, we will discuss published vascularized MPS platforms and present considerations for next-generation devices looking to incorporate this critical constituent. Finally, we will highlight the organ-patterning processes governed by the vasculature, and how the incorporation of a vascular niche within MPS platforms will establish a unique opportunity to study stem cell development.

Effects of statins on the biological features of mesenchymal stem cells and therapeutic implications

Statins are well-known lipid-lowering drugs. The pleiotropic effects of statins have brought about some beneficial effects on improving the therapeutic outcomes of cell therapy and tissue engineering approaches. In this review, the impact of statins on mesenchymal stem cell behaviors including differentiation, apoptosis, proliferation, migration, and angiogenesis, as well as molecular pathways which are responsible for such phenomena, are discussed. A better understanding of pathways and mechanisms of statin-mediated effects on mesenchymal stem cells will pave the way for the expansion of statin applications. Furthermore, since designing a suitable carrier for statins is required to maintain a sufficient dose of active statins at the desired site of the body, different systems for local delivery of statins are also reviewed.

A high-throughput microfluidic bilayer co-culture platform to study endothelial-pericyte interactions

Microphysiological organ-on-chip models offer the potential to improve the prediction of drug safety and efficacy through recapitulation of human physiological responses. The importance of including multiple cell types within tissue models has been well documented. However, the study of cell interactions in vitro can be limited by complexity of the tissue model and throughput of current culture systems. Here, we describe the development of a co-culture microvascular model and relevant assays in a high-throughput thermoplastic organ-on-chip platform, PREDICT96. The system consists of 96 arrayed bilayer microfluidic devices containing retinal microvascular endothelial cells and pericytes cultured on opposing sides of a microporous membrane. Compatibility of the PREDICT96 platform with a variety of quantifiable and scalable assays, including macromolecular permeability, image-based screening, Luminex, and qPCR, is demonstrated. In addition, the bilayer design of the devices allows for channel- or cell type-specific readouts, such as cytokine profiles and gene expression. The microvascular model was responsive to perturbations including barrier disruption, inflammatory stimulation, and fluid shear stress, and our results corroborated the improved robustness of co-culture over endothelial mono-cultures. We anticipate the PREDICT96 platform and adapted assays will be suitable for other complex tissues, including applications to disease models and drug discovery.

3D bioprinting and microscale organization of vascularized tissue constructs using collagen-based bioink

Bioprinting three-dimensional (3D) tissue equivalents have progressed tremendously over the last decade. 3D bioprinting is currently being employed to develop larger and more physiologic tissues, and it is of particular interest to generate vasculature in biofabricated tissues to aid better perfusion and transport of nutrition. Having an advantage over manual culture systems by bringing together biological scaffold materials and cells in precise 3D spatial orientation, bioprinting could assist in placing endothelial cells in specific spatial locations within a 3D matrix to promote vessel formation at these predefined areas. Hence, in the present study, we investigated the use of bioprinting to generate tissue-level capillary-like networks in biofabricated tissue constructs. First, we developed a bioink using collagen type-1 supplemented with xanthan gum (XG) as a thickening agent. Using a commercial extrusion-based multi-head bioprinter and collagen-XG bioink, the component cells were spatially assembled, wherein the endothelial cells were bioprinted in a lattice pattern and sandwiched between bioprinted fibroblasts layers. 3D bioprinted constructs thus generated were stable, and maintained structural shape and form. Post-print culture of the bioprinted tissues resulted in endothelial sprouting and formation of interconnected capillary-like networks within the lattice pattern and between the fibroblast layers. Bioprinter-assisted spatial placement of endothelial cells resulted in fabrication of patterned prevascularized constructs that enable potential regenerative applications in the future.

Biofabrication of tissue engineering vascular systems

Cardiovascular disease (CVD) is the leading cause of death among persons aged 65 and older in the United States and many other developed countries. Tissue engineered vascular systems (TEVS) can serve as grafts for CVD treatment and be used as in vitro model systems to examine the role of various genetic factors during the CVD progressions. Current focus in the field is to fabricate TEVS that more closely resembles the mechanical properties and extracellular matrix environment of native vessels, which depends heavily on the advance in biofabrication techniques and discovery of novel biomaterials. In this review, we outline the mechanical and biological design requirements of TEVS and explore the history and recent advances in biofabrication methods and biomaterials for tissue engineered blood vessels and microvascular systems with special focus on in vitro applications. In vitro applications of TEVS for disease modeling are discussed.

Rebuilding the Vascular Network: In vivo and in vitro Approaches

As the material transportation system of the human body, the vascular network carries the transportation of materials and nutrients. Currently, the construction of functional microvascular networks is an urgent requirement for the development of regenerative medicine and in vitro drug screening systems. How to construct organs with functional blood vessels is the focus and challenge of tissue engineering research. Here in this review article, we first introduced the basic characteristics of blood vessels in the body and the mechanism of angiogenesis in vivo, summarized the current methods of constructing tissue blood vessels in vitro and in vivo, and focused on comparing the functions, applications and advantages of constructing different types of vascular chips to generate blood vessels. Finally, the challenges and opportunities faced by the development of this field were discussed.

Emulating Early Atherosclerosis in a Vascular Microphysiological System Using Branched Tissue-Engineered Blood Vessels

Atherosclerosis begins with the accumulation of cholesterol-carrying lipoproteins on blood vessel walls and progresses to endothelial cell dysfunction, monocyte adhesion, and foam cell formation. Endothelialized tissue-engineered blood vessels (TEBVs) have previously been fabricated to recapitulate artery functionalities, including vasoconstriction, vasodilation, and endothelium activation. Here, the initiation of atherosclerosis is emulated by designing branched TEBVs (brTEBVs) of various geometries treated with enzyme-modified low-density-lipoprotein (eLDL) and TNF-α to induce endothelial cell dysfunction and adhesion of perfused human monocytes. Locations of monocyte adhesion under pulsatile flow are identified, and the hemodynamics in the brTEBVs are characterized using particle image velocimetry (PIV) and computational fluid dynamics (CFD). Monocyte adhesion is greater at the side outlets than at the main outlets or inlets, and is greatest at larger side outlet branching angles (60° or 80° vs 45°). In PIV experiments, the branched side outlets are identified as atherosclerosis-prone areas where fluorescent particles show a transient swirling motion following flow pulses; in CFD simulations, side outlets with larger branching angles show higher vorticity magnitude and greater flow disturbance than other areas. These results suggest that the branched TEBVs with eLDL/TNF-α treatment provide a physiologically relevant model of early atherosclerosis for preclinical studies.

One-Step Formation of Protein-Based Tubular Structures for Functional Devices and Tissues

Tubular biological structures consisting of extracellular matrix (ECM) proteins and cells are basic functional units of all organs in animals and humans. ECM protein solutions at low concentrations (5-10 milligrams per milliliter) are abundantly used in 3D cell culture. However, their poor "printability" and minute-long gelation time have made the direct extrusion of tubular structures in bioprinting applications challenging. Here, this limitation is overcome and the continuous, template-free conversion of low-concentration collagen, elastin, and fibrinogen solutions into tubular structures of tailored size and radial, circumferential and axial organization is demonstrated. The approach is enabled by a microfabricated printhead for the consistent circumferential distribution of ECM protein solutions and lends itself to scalable manufacture. The attached confinement accommodates minute-long residence times for pH, temperature, light, ionic and enzymatic gelation. Chip hosted ECM tubular structures are amenable to perfusion with aqueous solutions and air, and cyclic stretching. Predictive collapse and reopening in a crossed-tube configuration promote all-ECM valves and pumps. Tissue level function is demonstrated by factors secreted from cells embedded within the tube wall, as well as endothelial or epithelial barriers lining the lumen. The described approaches are anticipated to find applications in ECM-based organ-on-chip and biohybrid structures, hydraulic actuators, and soft machines.

Generation of Vascular Smooth Muscle Cells From Induced Pluripotent Stem Cells: Methods, Applications, and Considerations

The developmental origin of vascular smooth muscle cells (VSMCs) has been increasingly recognized as a major determinant for regional susceptibility or resistance to vascular diseases. As a human material-based complement to animal models and human primary cultures, patient induced pluripotent stem cell iPSC-derived VSMCs have been leveraged to conduct basic research and develop therapeutic applications in vascular diseases. However, iPSC-VSMCs (induced pluripotent stem cell VSMCs) derived by most existing induction protocols are heterogeneous in developmental origins. In this review, we summarize signaling networks that govern in vivo cell fate decisions and in vitro derivation of distinct VSMC progenitors, as well as key regulators that terminally specify lineage-specific VSMCs. We then highlight the significance of leveraging patient-derived iPSC-VSMCs for vascular disease modeling, drug discovery, and vascular tissue engineering and discuss several obstacles that need to be circumvented to fully unleash the potential of induced pluripotent stem cells for precision vascular medicine.

Healthy and diseased in vitro models of vascular systems

Irregular hemodynamics affects the progression of various vascular diseases, such atherosclerosis or aneurysms. Despite the extensive hemodynamics studies on animal models, the inter-species differences between humans and animals hamper the translation of such findings. Recent advances in vascular tissue engineering and the suitability of in vitro models for interim analysis have increased the use of in vitro human vascular tissue models. Although the effect of flow on endothelial cell (EC) pathophysiology and EC-flow interactions have been vastly studied in two-dimensional systems, they cannot be used to understand the effect of other micro- and macro-environmental parameters associated with vessel wall diseases. To generate an ideal in vitro model of the vascular system, essential criteria should be included: 1) the presence of smooth muscle cells or perivascular cells underneath an EC monolayer, 2) an elastic mechanical response of tissue to pulsatile flow pressure, 3) flow conditions that accurately mimic the hemodynamics of diseases, and 4) geometrical features required for pathophysiological flow. In this paper, we review currently available in vitro models that include flow dynamics and discuss studies that have tried to address the criteria mentioned above. Finally, we critically review in vitro fluidic models of atherosclerosis, aneurysm, and thrombosis.

Symbiotic Photosynthetic Oxygenation within 3D-Bioprinted Vascularized Tissues

In this study, we present the photosynthetic oxygen (O2) supply to mammalian cells within a volumetric extracellular matrix-like construct, whereby a three-dimensional (3D)-bioprinted fugitive pattern encapsulating unicellular green algae, Chlamydomonas reinhardtii (C. reinhardtii), served as a natural photosynthetic O2-generator. The presence of bioprinted C. reinhardtii enhanced the viability and functionality of mammalian cells while reducing the hypoxic conditions within the tissues. We were able to subsequently endothelialize the hollow perfusable microchannels formed after enzymatic removal of the bioprinted C. reinhardtii-laden patterns from the matrices following the initial oxygenation period, to obtain biologically relevant vascularized mammalian tissue constructs. The feasibility of co-culture of C. reinhardtii with human cells, the printability and the enzymatic degradability of the fugitive bioink, as well as the exploration of C. reinhardtii as a natural, eco-friendly, cost-effective, and sustainable source of O2 would likely promote the development of engineered tissues, tissue models, and food for various applications.

Human iPS Cell-derived Tissue Engineered Vascular Graft: Recent Advances and Future Directions

Tissue engineered vascular grafts (TEVGs) generated from human primary cells represent a promising vascular interventional therapy. However, generation and application of these TEVGs may be significantly hindered by the limited accessibility, finite expandability, donor-donor functional variation and immune-incompatibility of primary seed cells from donors. Alternatively, human induced pluripotent stem cells (hiPSCs) offer an infinite source to obtain functional vascular cells in large quantity and comparable quality for TEVG construction. To date, TEVGs (hiPSC-TEVGs) with significant mechanical strength and implantability have been generated using hiPSC-derived seed cells. Despite being in its incipient stage, this emerging field of hiPSC-TEVG research has achieved significant progress and presented promising future potential. Meanwhile, a series of challenges pertaining hiPSC differentiation, vascular tissue engineering technologies and future production and application await to be addressed. Herein, we have composed this review to introduce progress in TEVG generation using hiPSCs, summarize the current major challenges, and encapsulate the future directions of research on hiPSC-based TEVGs. Graphical abstract.

Modeling early stage atherosclerosis in a primary human vascular microphysiological system

Novel atherosclerosis models are needed to guide clinical therapy. Here, we report an in vitro model of early atherosclerosis by fabricating and perfusing multi-layer arteriole-scale human tissue-engineered blood vessels (TEBVs) by plastic compression. TEBVs maintain mechanical strength, vasoactivity, and nitric oxide (NO) production for at least 4 weeks. Perfusion of TEBVs at a physiological shear stress with enzyme-modified low-density-lipoprotein (eLDL) with or without TNFα promotes monocyte accumulation, reduces vasoactivity, alters NO production, which leads to endothelial cell activation, monocyte accumulation, foam cell formation and expression of pro-inflammatory cytokines. Removing eLDL leads to recovery of vasoactivity, but not loss of foam cells or recovery of permeability, while pretreatment with lovastatin or the P2Y11 inhibitor NF157 reduces monocyte accumulation and blocks foam cell formation. Perfusion with blood leads to increased monocyte adhesion. This atherosclerosis model can identify the role of drugs on specific vascular functions that cannot be assessed in vivo.

Direct extrusion of individually encapsulated endothelial and smooth muscle cells mimicking blood vessel structures and vascular native cell alignment

Cardiovascular diseases (CVDs) are considered the principal cause of worldwide death, being atherosclerosis the main etiology. Up to now, the predominant treatment for CVDs has been bypass surgery from autologous source. However, due to previous harvest or the type of disease, this is not always an option. For this reason, tissue engineering blood vessels (TEBV) emerged as an alternative graft source for blood vessel replacement. In order to develop a TEBV, it should mimic the architecture of a native blood vessel encapsulating the specific vascular cells in their respective layers with native alignment, and with appropriate mechanical stability. Here, we propose the extrusion of two different cell encapsulating hydrogels, mainly alginate and collagen, and a sacrificial polymer, through a triple coaxial nozzle, which in contact with a crosslinking solution allows the formation of bilayered hollow fibers, mimicking the architecture of native blood vessels. Prior to extrusion, the innermost cell encapsulating hydrogel was loaded with human umbilical vein endothelial cells (HUVECs), whereas the outer hydrogel was loaded with human aortic smooth muscle cells (HASMCs). The size of the TEVB could be controlled by changing the injection speed, presenting homogeneity between the constructs. The obtained structures were robust, allowing its manipulation as well as the perfusion of liquids. Both cell types presented high rates of survival after the extrusion process as well as after 20 days in culture (over 90%). Additionally, a high percentage of HASMC and HUVEC were aligned perpendicular and parallel to the TEBV, respectively, in their own layers, resembling the physiological arrangement found in vivo. Our approach enables the rapid formation of TEBV-like structures presenting high cell viability and allowing proliferation and natural alignment of vascular cells.

Microphysiological systems for the modeling of wound healing and evaluation of pro-healing therapies

Wound healing is a multivariate process involving the coordinated response of numerous proteins and cell types. Accordingly, biomedical research has seen an increased adoption of the use of in vitro wound healing assays with complexity beyond that offered by traditional well-plate constructs. These microphysiological systems (MPS) seek to recapitulate one or more physiological features of the in vivo microenvironment, while retaining the analytical capacity of more reductionist assays. Design efforts to achieve relevant wound healing physiology include the use of dynamic perfusion over static culture, the incorporation of multiple cell types, the arrangement of cells in three dimensions, the addition of biomechanically and biochemically relevant hydrogels, and combinations thereof. This review provides a brief overview of the wound healing process and in vivo assays, and we critically review the current state of MPS and supporting technologies for modelling and studying wound healing. We distinguish between MPS that seek to inform a particular phase of wound healing, and constructs that have the potential to inform multiple phases of wound healing. This distinction is a product of whether analysis of a particular process is prioritized, or a particular physiology is prioritized, during design. Material selection is emphasized throughout, and relevant fabrication techniques discussed.

Application of Oxidative Stress to a Tissue-Engineered Vascular Aging Model Induces Endothelial Cell Senescence and Activation

Clinical studies have established a connection between oxidative stress, aging, and atherogenesis. These factors contribute to senescence and inflammation in the endothelium and significant reductions in endothelium-dependent vasoreactivity in aged patients. Tissue-engineered blood vessels (TEBVs) recapitulate the structure and function of arteries and arterioles in vitro. We developed a TEBV model for vascular senescence and examined the relative influence of endothelial cell and smooth muscle cell senescence on vasoreactivity. Senescence was induced in 2D endothelial cell cultures and TEBVs by exposure to 100 µM H₂O₂ for one week to model chronic oxidative stress. H₂O₂ treatment significantly increased senescence in endothelial cells and mural cells, human neonatal dermal fibroblasts (hNDFs), as measured by increased p21 levels and reduced NOS3 expression. Although H₂O₂ treatment induced senescence in both the endothelial cells (ECs) and hNDFs, the functional effects on the vasculature were endothelium specific. Expression of the leukocyte adhesion molecule vascular cell adhesion molecule 1 (VCAM-1) was increased in the ECs, and endothelium-dependent vasodilation decreased. Vasoconstriction and endothelium-independent vasodilation were preserved despite mural cell senescence. The results suggest that the functional effects of vascular cell senescence are dominated by the endothelium.

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.

Vascular Microphysiological Systems to Model Diseases

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

iPSC-Derived Endothelial Cells Affect Vascular Function in a Tissue-Engineered Blood Vessel Model of Hutchinson-Gilford Progeria Syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is a rare disorder caused by a point mutation in the Lamin A gene that produces the protein progerin. Progerin toxicity leads to accelerated aging and death from cardiovascular disease. To elucidate the effects of progerin on endothelial cells, we prepared tissue-engineered blood vessels (viTEBVs) using induced pluripotent stem cell-derived smooth muscle cells (viSMCs) and endothelial cells (viECs) from HGPS patients. HGPS viECs aligned with flow but exhibited reduced flow-responsive gene expression and altered NOS3 levels. Relative to viTEBVs with healthy cells, HGPS viTEBVs showed reduced function and exhibited markers of cardiovascular disease associated with endothelium. HGPS viTEBVs exhibited a reduction in both vasoconstriction and vasodilation. Preparing viTEBVs with HGPS viECs and healthy viSMCs only reduced vasodilation. Furthermore, HGPS viECs produced VCAM1 and E-selectin protein in TEBVs with healthy or HGPS viSMCs. In summary, the viTEBV model has identified a role of the endothelium in HGPS.

Organs-on-a-Chip

Organs-on-chips, also known as "tissue chips" or microphysiological systems (MPS), are bioengineered microsystems capable of recreating aspects of human organ physiology and function and are in vitro tools with multiple applications in drug discovery and development. The ability to recapitulate human and animal tissues in physiologically relevant three-dimensional, multi-cellular environments allows applications in the drug development field, including; (1) use in assessing the safety and toxicity testing of potential therapeutics during early-stage preclinical drug development; (2) confirmation of drug/therapeutic efficacy in vitro; and (3) disease modeling of human tissues to recapitulate pathophysiology within specific subpopulations and even individuals, thereby advancing precision medicine efforts. This chapter will discuss the development and evolution of three-dimensional organ models over the past decade, and some of the opportunities offered by MPS technology that are not available through current standard two-dimensional cell cultures, or three-dimensional organoid systems. This chapter will outline future avenues of research in the MPS field, how cutting-edge biotechnology advances are expanding the applications for these systems, and discuss the current and future potential and challenges remaining for the field to address.

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.

In vitro and in vivo evaluation of 3D bioprinted small-diameter vasculature with smooth muscle and endothelium

The ability to fabricate perfusable, small-diameter vasculature is a foundational step toward generating human tissues/organs for clinical applications. Currently, it is highly challenging to generate vasculature integrated with smooth muscle and endothelium that replicates the complexity and functionality of natural vessels. Here, a novel method for directly printing self-standing, small-diameter vasculature with smooth muscle and endothelium is presented through combining tailored mussel-inspired bioink and unique 'fugitive-migration' tactics, and its effectiveness and advantages over other methods (i.e. traditional alginate/calcium hydrogel, post-perfusion of endothelial cells) are demonstrated. The biologically inspired, catechol-functionalized, gelatin methacrylate (GelMA/C) undergoes rapid oxidative crosslinking in situ to form an elastic hydrogel, which can be engineered with controllable mechanical strength, high cell/tissue adhesion, and excellent bio-functionalization. The results demonstrate the bioprinted vascular construct possessed numerous favorable, biomimetic characteristics such as proper biomechanics, higher tissue affinity, vascularized tissue manufacturing ability, beneficial perfusability and permeability, excellent vasculoactivity, and in vivo autonomous connection (∼2 weeks) as well as vascular remodeling (∼6 weeks). The advanced achievements in creating biomimetic, functional vasculature illustrate significant potential toward generating a complicated vascularized tissue/organ for clinical transplantation.

Vascular Cells and Tissue Constructs Derived from Human Pluripotent Stem Cells for Toxicological Screening

The ability of human stem cells to generate somatic cell lineages makes them ideal candidates for use in toxicological testing and eventually, preclinical drug development. Such resources would support an evolution away from human primary cells or research animal models, which suffer from variability and poor predictability, toward off-the-shelf assays of chemical toxicity and drug efficacy using human cells and tissues. To this end, we generated vascular cell populations (smooth muscle cells and endothelial cells) from human pluripotent stem cells (hPSCs), arranged them into 3D co-cultures within supportive gel matrices, and directed their propensity for self-organization resembling microvasculature. The resulting vascular cell populations and co-cultured constructs were then arrayed in high throughput and used for screening a library of environmental and clinical chemical agents for immunological and toxicological responses. The screen effectively stratified the chemicals into various levels of toxicity, with both cell type-specific and co-culture-dependent responses observed. Thus, hPSC-derived vascular cells and constructs could be progressed further toward use in toxicant and drug screening.

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.

Advanced drug delivery systems and artificial skin grafts for skin wound healing

Cutaneous injuries, especially chronic wounds, burns, and skin wound infection, require painstakingly long-term treatment with an immense financial burden to healthcare systems worldwide. However, clinical management of chronic wounds remains unsatisfactory in many cases. Various strategies including growth factor and gene delivery as well as cell therapy have been used to enhance the healing of non-healing wounds. Drug delivery systems across the nano, micro, and macroscales can extend half-life, improve bioavailability, optimize pharmacokinetics, and decrease dosing frequency of drugs and genes. Replacement of the damaged skin tissue with substitutes comprising cell-laden scaffold can also restore the barrier and regulatory functions of skin at the wound site. This review covers comprehensively the advanced treatment strategies to improve the quality of wound healing.

Cardiovascular disease models: A game changing paradigm in drug discovery and screening

Cardiovascular disease is the leading cause of death worldwide. Although investment in drug discovery and development has been sky-rocketing, the number of approved drugs has been declining. Cardiovascular toxicity due to therapeutic drug use claims the highest incidence and severity of adverse drug reactions in late-stage clinical development. Therefore, to address this issue, new, additional, replacement and combinatorial approaches are needed to fill the gap in effective drug discovery and screening. The motivation for developing accurate, predictive models is twofold: first, to study and discover new treatments for cardiac pathologies which are leading in worldwide morbidity and mortality rates; and second, to screen for adverse drug reactions on the heart, a primary risk in drug development. In addition to in vivo animal models, in vitro and in silico models have been recently proposed to mimic the physiological conditions of heart and vasculature. Here, we describe current in vitro, in vivo, and in silico platforms for modelling healthy and pathological cardiac tissues and their advantages and disadvantages for drug screening and discovery applications. We review the pathophysiology and the underlying pathways of different cardiac diseases, as well as the new tools being developed to facilitate their study. We finally suggest a roadmap for employing these non-animal platforms in assessing drug cardiotoxicity and safety.

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.