3D curvature-instructed endothelial flow response and tissue vascularization

A nanoporous hydrogel-based model to study chemokine gradient-driven angiogenesis under luminal flow

The growth of new blood vessels through angiogenesis is a highly coordinated process, which is initiated by chemokine gradients that activate endothelial cells within a perfused parent vessel to sprout into the surrounding 3D tissue matrix. While both biochemical signals from pro-angiogenic factors, as well as mechanical cues originating from luminal fluid flow that exerts shear stress on the vessel wall, have individually been identified as major regulators of endothelial cell sprouting, it remains unclear whether and how both types of cues synergize. To fill this knowledge gap, here, we created a 3D biomimetic model of chemokine gradient-driven angiogenic sprouting, in which a micromolded tube inside a hydrogel matrix is seeded with endothelial cells and connected to a perfusion system to control fluid flow rates and resulting shear forces on the vessel wall. To allow for the formation of chemokine gradients despite the presence of luminal flow, a nanoporous synthetic hydrogel that supports angiogenesis but limits the interstitial flow proved crucial. Using this system, we find that luminal flow and resulting shear stress is a major regulator of the speed and morphogenesis of angiogenic sprouting, whose action is mediated through changes in vascular permeability.

A three-dimensional vessel-on-chip model to study Puumala orthohantavirus pathogenesis

Puumala orthohantavirus (PUUV) infection in humans can result in hemorrhagic fever with renal syndrome. Endothelial cells (ECs) are primarily infected with increased vascular permeability as a central aspect of pathogenesis. Historically, most studies included ECs cultured under static two-dimensional (2D) conditions, thereby not recapitulating the physiological environment due to their lack of flow and inherent pro-inflammatory state. Here, we present a high-throughput model for culturing primary human umbilical vein ECs in 3D vessels-on-chip in which we compared host responses of these ECs to those of static 2D-cultured ECs on a transcriptional level. The phenotype of ECs in vessels-on-chip more closely resembled the in vivo situation due to higher similarity in expression of genes encoding described markers for disease severity and coagulopathy, including IDO1, LGALS3BP, IL6 and PLAT, and more diverse endothelial-leukocyte interactions in the context of PUUV infection. In these vessels-on-chip, PUUV infection did not directly increase vascular permeability, but increased monocyte adhesion. This platform can be used for studying pathogenesis and assessment of possible therapeutics for other endotheliotropic viruses even in high biocontainment facilities.

Droplet-based microfluidics for engineering shape-controlled hydrogels with stiffness gradient

Current biofabrication strategies are limited in their ability to replicate native shape-to-function relationships, that are dependent on adequate biomimicry of macroscale shape as well as size and microscale spatial heterogeneity, within cell-laden hydrogels. In this study, a novel diffusion-based microfluidics platform is presented that meets these needs in a two-step process. In the first step, a hydrogel-precursor solution is dispersed into a continuous oil phase within the microfluidics tubing. By adjusting the dispersed and oil phase flow rates, the physical architecture of hydrogel-precursor phases can be adjusted to generate spherical and plug-like structures, as well as continuous meter-long hydrogel-precursor phases (up to 1.75 m). The second step involves the controlled introduction a small molecule-containing aqueous phase through a T-shaped tube connector to enable controlled small molecule diffusion across the interface of the aqueous phase and hydrogel-precursor. Application of this system is demonstrated by diffusing co-initiator sodium persulfate (SPS) into hydrogel-precursor solutions, where the controlled SPS diffusion into the hydrogel-precursor and subsequent photo-polymerization allows for the formation of unique radial stiffness patterns across the shape- and size-controlled hydrogels, as well as allowing the formation of hollow hydrogels with controllable internal architectures. Mesenchymal stromal cells are successfully encapsulated within hollow hydrogels and hydrogels containing radial stiffness gradient and found to respond to the heterogeneity in stiffness through the yes-associated protein mechano-regulator. Finally, breast cancer cells are found to phenotypically switch in response to stiffness gradients, causing a shift in their ability to aggregate, which may have implications for metastasis. The diffusion-based microfluidics thus finds application mimicking native shape-to-function relationship in the context of tissue engineering and provides a platform to further study the roles of micro- and macroscale architectural features that exist within native tissues.

Tissue-Engineered Microvessels: A Review of Current Engineering Strategies and Applications

Microvessels, including arterioles, capillaries, and venules, play an important role in regulating blood flow, enabling nutrient and waste exchange, and facilitating immune surveillance. Due to their important roles in maintaining normal function in human tissues, a substantial effort has been devoted to developing tissue-engineered models to study endothelium-related biology and pathology. Various engineering strategies have been developed to recapitulate the structural, cellular, and molecular hallmarks of native human microvessels in vitro. In this review, recent progress in engineering approaches, key components, and culture platforms for tissue-engineered human microvessel models is summarized. Then, tissue-specific models, and the major applications of tissue-engineered microvessels in development, disease modeling, drug screening and delivery, and vascularization in tissue engineering, are reviewed. Finally, future research directions for the field are discussed.

The Role of Biophysical Factors in Organ Development: Insights from Current Organoid Models

Biophysical factors play a fundamental role in human embryonic development. Traditional in vitro models of organogenesis focused on the biochemical environment and did not consider the effects of mechanical forces on developing tissue. While most human tissue has a Young's modulus in the low kilopascal range, the standard cell culture substrate, plasma-treated polystyrene, has a Young's modulus of 3 gigapascals, making it 10,000-100,000 times stiffer than native tissues. Modern in vitro approaches attempt to recapitulate the biophysical niche of native organs and have yielded more clinically relevant models of human tissues. Since Clevers' conception of intestinal organoids in 2009, the field has expanded rapidly, generating stem-cell derived structures, which are transcriptionally similar to fetal tissues, for nearly every organ system in the human body. For this reason, we conjecture that organoids will make their first clinical impact in fetal regenerative medicine as the structures generated ex vivo will better match native fetal tissues. Moreover, autologously sourced transplanted tissues would be able to grow with the developing embryo in a dynamic, fetal environment. As organoid technologies evolve, the resultant tissues will approach the structure and function of adult human organs and may help bridge the gap between preclinical drug candidates and clinically approved therapeutics. In this review, we discuss roles of tissue stiffness, viscoelasticity, and shear forces in organ formation and disease development, suggesting that these physical parameters should be further integrated into organoid models to improve their physiological relevance and therapeutic applicability. It also points to the mechanotransductive Hippo-YAP/TAZ signaling pathway as a key player in the interplay between extracellular matrix stiffness, cellular mechanics, and biochemical pathways. We conclude by highlighting how frontiers in physics can be applied to biology, for example, how quantum entanglement may be applied to better predict spontaneous DNA mutations. In the future, contemporary physical theories may be leveraged to better understand seemingly stochastic events during organogenesis.

The Edifice of Vasculature-On-Chips: A Focused Review on the Key Elements and Assembly of Angiogenesis Models

The conception of vascularized organ-on-a-chip models provides researchers with the ability to supply controlled biological and physical cues that simulate the in vivo dynamic microphysiological environment of native blood vessels. The intention of this niche research area is to improve our understanding of the role of the vasculature in health or disease progression in vitro by allowing researchers to monitor angiogenic responses and cell-cell or cell-matrix interactions in real time. This review offers a comprehensive overview of the essential elements, including cells, biomaterials, microenvironmental factors, microfluidic chip design, and standard validation procedures that currently govern angiogenesis-on-a-chip assemblies. In addition, we emphasize the importance of incorporating a microvasculature component into organ-on-chip devices in critical biomedical research areas, such as tissue engineering, drug discovery, and disease modeling. Ultimately, advances in this area of research could provide innovative solutions and a personalized approach to ongoing medical challenges.

Distinct cellular microenvironment with cytotypic effects regulates orderly regeneration of vascular tissues

Regeneration of the architecturally complex blood vascular system requires precise temporal and spatial control of cell behaviours. Additional components must be integrated into the structure to achieve clinical success for in situ tissue engineering. Consequently, this study proposed a universal method for including any substrate type in vascular cell extracellular matrices (VCEM) via regulating selective adhesion to promote vascular tissue regeneration. The results uncovered that the VCEM worked as cell adhesion substrates, exhibited cell type specificity, and functioned as an address signal for recognition by vascular cells, which resulted in matching with the determined cells. The qPCR and immunofluorescence results revealed that a cell type-specific VCEM could be designed to promote or inhibit cell adhesion, consistenting with the expression patterns of eyes absent 3 (Eya3). In addition, a 3D vascular graft combined with VCEM which could recapitulate the vascular cell-like microenvironment was fabricated. The vascular graft revealed a prospective role for cellular microenvironment in the establishment of vascular cell distribution and tissue architecture, and potentiated the orderly regeneration and functional recovery of vascular tissues in vivo. The findings demonstrate that differential adhesion between cell types due to the cellular microenvironment is sufficient to drive the complex assembly of engineered blood vessel functional units, and underlies hierarchical organization during vascular regeneration.

Tiny Organs, Big Impact: How Microfluidic Organ-on-Chip Technology Is Revolutionizing Mucosal Tissues and Vasculature

Organ-on-chip (OOC) technology has gained importance for biomedical studies and drug development. This technology involves microfluidic devices that mimic the structure and function of specific human organs or tissues. OOCs are a promising alternative to traditional cell-based models and animals, as they provide a more representative experimental model of human physiology. By creating a microenvironment that closely resembles in vivo conditions, OOC platforms enable the study of intricate interactions between different cells as well as a better understanding of the underlying mechanisms pertaining to diseases. OOCs can be integrated with other technologies, such as sensors and imaging systems to monitor real-time responses and gather extensive data on tissue behavior. Despite these advances, OOCs for many organs are in their initial stages of development, with several challenges yet to be overcome. These include improving the complexity and maturity of these cellular models, enhancing their reproducibility, standardization, and scaling them up for high-throughput uses. Nonetheless, OOCs hold great promise in advancing biomedical research, drug discovery, and personalized medicine, benefiting human health and well-being. Here, we review several recent OOCs that attempt to overcome some of these challenges. These OOCs with unique applications can be engineered to model organ systems such as the stomach, cornea, blood vessels, and mouth, allowing for analyses and investigations under more realistic conditions. With this, these models can lead to the discovery of potential therapeutic interventions. In this review, we express the significance of the relationship between mucosal tissues and vasculature in organ-on-chip (OOC) systems. This interconnection mirrors the intricate physiological interactions observed in the human body, making it crucial for achieving accurate and meaningful representations of biological processes within OOC models. Vasculature delivers essential nutrients and oxygen to mucosal tissues, ensuring their proper function and survival. This exchange is critical for maintaining the health and integrity of mucosal barriers. This review will discuss the OOCs used to represent the mucosal architecture and vasculature, and it can encourage us to think of ways in which the integration of both can better mimic the complexities of biological systems and gain deeper insights into various physiological and pathological processes. This will help to facilitate the development of more accurate predictive models, which are invaluable for advancing our understanding of disease mechanisms and developing novel therapeutic interventions.

Engineering vascularised organoid-on-a-chip: strategies, advances and future perspectives

In recent years, advances in microfabrication technology and tissue engineering have propelled the development of a novel drug screening and disease modelling platform known as organoid-on-a-chip. This platform integrates organoids and organ-on-a-chip technologies, emerging as a promising approach for in vitro modelling of human organ physiology. Organoid-on-a-chip devices leverage microfluidic systems to simulate the physiological microenvironment of specific organs, offering a more dynamic and flexible setting that can mimic a more comprehensive human biological context. However, the lack of functional vasculature has remained a significant challenge in this technology. Vascularisation is crucial for the long-term culture and in vitro modelling of organoids, holding important implications for drug development and personalised medical approaches. This review provides an overview of research progress in developing vascularised organoid-on-a-chip models, addressing methods for in vitro vascularisation and advancements in vascularised organoids. The aim is to serve as a reference for future endeavors in constructing fully functional vascularised organoid-on-a-chip platforms.

Analyzing the effects of helical flow in blood vessels using acoustofluidic-based dynamic flow generator

The effects of helical flow in a blood vessel are investigated in a dynamic flow generator using surface acoustic wave (SAW) in the microfluidic device. The SAW, generated by an interdigital transducer (IDT), induces acoustic streaming, resulting in a stable and consistent helical flow pattern in microscale channels. This approach allows rapid development of helical flow within the channel without directly contacting the medium. The precise design of the window enables the creation of distinct unidirectional vortices, which can be controlled by adjusting the amplitude of the SAW. Within this device, optimal operational parameters of the dynamic flow generator to preserve the integrity of endothelial cells are found, and in such settings, the actin filaments within the cells are aligned to the desired state. Our findings reveal that intracellular Ca2+ concentrations vary in response to flow conditions. Specifically, comparable maximum intensity and graphical patterns were observed between low-flow rate helical flow and high-flow rate Hagen-Poiseuille flow. These suggest that the cells respond to the helical flow through mechanosensitive ion channels. Finally, adherence of monocytes is effectively reduced under helical flow conditions in an inflammatory environment, highlighting the atheroprotective role of helical flow. STATEMENT OF SIGNIFICANCE: Helical flow in blood vessels is well known to prevent atherosclerosis. However, despite efforts to replicate helical flow in microscale channels, there is still a lack of in vitro models which can generate helical flow for analyzing its effects on the vascular system. In this study, we developed a method for generating steady and constant helical flow in microfluidic channel using acoustofluidic techniques. By utilizing this dynamic flow generator, we were able to observe the atheroprotective aspects of helical flow in vitro, including the enhancement of calcium ion flux and reduction of monocyte adhesion. This study paves the way for an in vitro model of dynamic cell culture and offers advanced investigation into helical flow in our circulatory system.

Flow in fetoplacental-like microvessels in vitro enhances perfusion, barrier function, and matrix stability

Proper placental vascularization is vital for pregnancy outcomes, but assessing it with animal models and human explants has limitations. We introduce a 3D in vitro model of human placenta terminal villi including fetal mesenchyme and vascular endothelium. By coculturing HUVEC, placental fibroblasts, and pericytes in a macrofluidic chip with a flow reservoir, we generate fully perfusable fetal microvessels. Pressure-driven flow facilitates microvessel growth and remodeling, resulting in early formation of interconnected and lasting placental-like vascular networks. Computational fluid dynamics simulations predict shear forces, which increase microtissue stiffness, decrease diffusivity, and enhance barrier function as shear stress rises. Mass spectrometry analysis reveals enhanced protein expression with flow, including matrix stability regulators, proteins associated with actin dynamics, and cytoskeleton organization. Our model provides a powerful tool for deducing complex in vivo parameters, such as shear stress on developing vascularized placental tissue, and holds promise for unraveling gestational disorders related to the vasculature.

Manufacturing the multiscale vascular hierarchy: progress toward solving the grand challenge of tissue engineering

In human vascular anatomy, blood flows from the heart to organs and tissues through a hierarchical vascular tree, comprising large arteries that branch into arterioles and further into capillaries, where gas and nutrient exchange occur. Engineering a complete, integrated vascular hierarchy with vessels large enough to suture, strong enough to withstand hemodynamic forces, and a branching structure to permit immediate perfusion of a fluidic circuit across scales would be transformative for regenerative medicine (RM), enabling the translation of engineered tissues of clinically relevant size, and perhaps whole organs. How close are we to solving this biological plumbing problem? In this review, we highlight advances in engineered vasculature at individual scales and focus on recent strategies to integrate across scales.

Orthopedic Scaffolds: Evaluation of Structural Strength and Permeability of Fluid Flow via an Open Cell Neovius Structure for Bone Tissue Engineering

The ability of bone to regenerate itself through mechanobiological responses is its dynamic property. Mechanical cues from a neighboring environment produce the structural strain to promote blood flow and bone marrow mobility that in turn aids the bone regeneration process. Occurrences of these phenomena are crucial for the success of metallic scaffolds implanted in the host bone tissue. Thus, permeability and fluid flow-induced wall shear stress (WSS) are two parameters that directly influence cell bioactivities inside a scaffold and are crucial for effective bone tissue regeneration. Given that the scaffolds shall be implanted in the body, permeability assessment was carried out using non-Newtonian fluid. In this work, the triply periodic minimal surface scaffolds with Neovius architectures were fabricated by using selective laser melting technology. The estimation of fluid flow was carried out using computational fluid dynamics (CFD) analysis with a non-Newtonian blood fluid model. Further, the structural strength of various open cell Neovius lattices was evaluated using a static compression test, and in vitro cell culture using Alamar blue assay was evaluated. Results revealed that the values of intrinsic blood flow permeability of the three-dimensional (3D)-printed open cell porous scaffold with Neovius architecture were of the same order of magnitude as those of human bone, ranging from 0.0025 × 10-9 to 0.0152 × 10-9 m2. The structural elastic modulus and compressive strength of NOCL40, NOCL50, and NOCL60 lattices range from 3.27 to 3.71 GPa and 194 to 205 MPa, respectively. All of the values are comparable to the human bone, thus making these lattices a suitable alternative for orthopedic applications.

Bio-inspired microfluidics: A review

Biomicrofluidics, a subdomain of microfluidics, has been inspired by several ideas from nature. However, while the basic inspiration for the same may be drawn from the living world, the translation of all relevant essential functionalities to an artificially engineered framework does not remain trivial. Here, we review the recent progress in bio-inspired microfluidic systems via harnessing the integration of experimental and simulation tools delving into the interface of engineering and biology. Development of "on-chip" technologies as well as their multifarious applications is subsequently discussed, accompanying the relevant advancements in materials and fabrication technology. Pointers toward new directions in research, including an amalgamated fusion of data-driven modeling (such as artificial intelligence and machine learning) and physics-based paradigm, to come up with a human physiological replica on a synthetic bio-chip with due accounting of personalized features, are suggested. These are likely to facilitate physiologically replicating disease modeling on an artificially engineered biochip as well as advance drug development and screening in an expedited route with the minimization of animal and human trials.

3D micropattern force triggers YAP nuclear entry by transport across nuclear pores and modulates stem cells paracrine

Biophysical cues of the cellular microenvironment tremendously influence cell behavior by mechanotransduction. However, it is still unclear how cells sense and transduce the mechanical signals from 3D geometry to regulate cell function. Here, the mechanotransduction of human mesenchymal stem cells (MSCs) triggered by 3D micropatterns and its effect on the paracrine of MSCs are systematically investigated. Our findings show that 3D micropattern force could influence the spatial reorganization of the cytoskeleton, leading to different local forces which mediate nucleus alteration such as orientation, morphology, expression of Lamin A/C and chromatin condensation. Specifically, in the triangular prism and cuboid micropatterns, the ordered F-actin fibers are distributed over and fully transmit compressive forces to the nucleus, which results in nuclear flattening and stretching of nuclear pores, thus enhancing the nuclear import of YES-associated protein (YAP). Furthermore, the activation of YAP significantly enhances the paracrine of MSCs and upregulates the secretion of angiogenic growth factors. In contrast, the fewer compressive forces on the nucleus in cylinder and cube micropatterns cause less YAP entering the nucleus. The skin repair experiment provides the first in vivo evidence that enhanced MSCs paracrine by 3D geometry significantly promotes tissue regeneration. The current study contributes to understanding the in-depth mechanisms of mechanical signals affecting cell function and provides inspiration for innovative design of biomaterials.

Dynamic Stimulations with Bioengineered Extracellular Matrix-Mimicking Hydrogels for Mechano Cell Reprogramming and Therapy

Cells interact with their surrounding environment through a combination of static and dynamic mechanical signals that vary over stimulus types, intensity, space, and time. Compared to static mechanical signals such as stiffness, porosity, and topography, the current understanding on the effects of dynamic mechanical stimulations on cells remains limited, attributing to a lack of access to devices, the complexity of experimental set-up, and data interpretation. Yet, in the pursuit of emerging translational applications (e.g., cell manufacturing for clinical treatment), it is crucial to understand how cells respond to a variety of dynamic forces that are omnipresent in vivo so that they can be exploited to enhance manufacturing and therapeutic outcomes. With a rising appreciation of the extracellular matrix (ECM) as a key regulator of biofunctions, researchers have bioengineered a suite of ECM-mimicking hydrogels, which can be fine-tuned with spatiotemporal mechanical cues to model complex static and dynamic mechanical profiles. This review first discusses how mechanical stimuli may impact different cellular components and the various mechanobiology pathways involved. Then, how hydrogels can be designed to incorporate static and dynamic mechanical parameters to influence cell behaviors are described. The Scopus database is also used to analyze the relative strength in evidence, ranging from strong to weak, based on number of published literatures, associated citations, and treatment significance. Additionally, the impacts of static and dynamic mechanical stimulations on clinically relevant cell types including mesenchymal stem cells, fibroblasts, and immune cells, are evaluated. The aim is to draw attention to the paucity of studies on the effects of dynamic mechanical stimuli on cells, as well as to highlight the potential of using a cocktail of various types and intensities of mechanical stimulations to influence cell fates (similar to the concept of biochemical cocktail to direct cell fate). It is envisioned that this progress report will inspire more exciting translational development of mechanoresponsive hydrogels for biomedical applications.

Covalently Grafted Biomimetic Matrix Reconstructs the Regenerative Microenvironment of the Porous Gradient Polycaprolactone Scaffold to Accelerate Bone Remodeling

Integrating a biomimetic extracellular matrix to improve the microenvironment of 3D printing scaffolds is an emerging strategy for bone substitute design. Here, a "soft-hard" bone implant (BM-g-DPCL) consisting of a bioactive matrix chemically integrated on a polydopamine (PDA)-coated porous gradient scaffold by polyphenol groups is constructed. The PDA-coated "hard" scaffolds promoted Ca²⁺ chelation and mineral deposition; the "soft" bioactive matrix is beneficial to the migration, proliferation, and osteogenic differentiation of stem cells in vitro, accelerated endogenous stem cell recruitment, and initiated rapid angiogenesis in vivo. The results of the rabbit cranial defect model (Φ = 10 mm) confirmed that BM-g-DPCL promoted the integration between bone tissue and implant and induced the deposition of bone matrix. Proteomics confirmed that cytokine adhesion, biomineralization, rapid vascularization, and extracellular matrix formation are major factors that accelerate bone defect healing. This strategy of highly chemically bonded soft-hard components guided the construction of the bioactive regenerative scaffold.

Realizations of vascularized tissues: From in vitro platforms to in vivo grafts

Vascularization is essential for realizing thick and functional tissue constructs that can be utilized for in vitro study platforms and in vivo grafts. The vasculature enables the transport of nutrients, oxygen, and wastes and is also indispensable to organ functional units such as the nephron filtration unit, the blood-air barrier, and the blood-brain barrier. This review aims to discuss the latest progress of organ-like vascularized constructs with specific functionalities and realizations even though they are not yet ready to be used as organ substitutes. First, the human vascular system is briefly introduced and related design considerations for engineering vascularized tissues are discussed. Second, up-to-date creation technologies for vascularized tissues are summarized and classified into the engineering and cellular self-assembly approaches. Third, recent applications ranging from in vitro tissue models, including generic vessel models, tumor models, and different human organ models such as heart, kidneys, liver, lungs, and brain, to prevascularized in vivo grafts for implantation and anastomosis are discussed in detail. The specific design considerations for the aforementioned applications are summarized and future perspectives regarding future clinical applications and commercialization are provided.

High cell density and high-resolution 3D bioprinting for fabricating vascularized tissues

Three-dimensional (3D) bioprinting techniques have emerged as the most popular methods to fabricate 3D-engineered tissues; however, there are challenges in simultaneously satisfying the requirements of high cell density (HCD), high cell viability, and fine fabrication resolution. In particular, bioprinting resolution of digital light processing-based 3D bioprinting suffers with increasing bioink cell density due to light scattering. We developed a novel approach to mitigate this scattering-induced deterioration of bioprinting resolution. The inclusion of iodixanol in the bioink enables a 10-fold reduction in light scattering and a substantial improvement in fabrication resolution for bioinks with an HCD. Fifty-micrometer fabrication resolution was achieved for a bioink with 0.1 billion per milliliter cell density. To showcase the potential application in tissue/organ 3D bioprinting, HCD thick tissues with fine vascular networks were fabricated. The tissues were viable in a perfusion culture system, with endothelialization and angiogenesis observed after 14 days of culture.

Engineered Biomimetic Membranes for Organ-on-a-Chip

Organ-on-a-chip (OOC) systems are engineered nanobiosystems to mimic the physiochemical environment of a specific organ in the body. Among various components of OOC systems, biomimetic membranes have been regarded as one of the most important key components to develop controllable biomimetic bioanalysis systems. Here, we review the preparation and characterization of biomimetic membranes in comparison with the features of the extracellular matrix. After that, we review and discuss the latest applications of engineered biomimetic membranes to fabricate various organs on a chip, such as liver, kidney, intestine, lung, skin, heart, vasculature and blood vessels, brain, and multiorgans with perspectives for further biomedical applications.

Mechanical regulation of the early stages of angiogenesis

Favouring or thwarting the development of a vascular network is essential in fields as diverse as oncology, cardiovascular disease or tissue engineering. As a result, understanding and controlling angiogenesis has become a major scientific challenge. Mechanical factors play a fundamental role in angiogenesis and can potentially be exploited for optimizing the architecture of the resulting vascular network. Largely focusing on in vitro systems but also supported by some in vivo evidence, the aim of this Highlight Review is dual. First, we describe the current knowledge with particular focus on the effects of fluid and solid mechanical stimuli on the early stages of the angiogenic process, most notably the destabilization of existing vessels and the initiation and elongation of new vessels. Second, we explore inherent difficulties in the field and propose future perspectives on the use of in vitro and physics-based modelling to overcome these difficulties.

Recent advancements and future requirements in vascularization of cortical organoids

The fields of tissue engineering and disease modeling have become increasingly cognizant of the need to create complex and mature structures in vitro to adequately mimic the in vivo niche. Specifically for neural applications, human brain cortical organoids (COs) require highly stratified neurons and glial cells to generate synaptic functions, and to date, most efforts achieve only fetal functionality at best. Moreover, COs are usually avascular, inducing the development of necrotic cores, which can limit growth, development, and maturation. Recent efforts have attempted to vascularize cortical and other organoid types. In this review, we will outline the components of a fully vascularized CO as they relate to neocortical development in vivo. These components address challenges in recapitulating neurovascular tissue patterning, biomechanical properties, and functionality with the goal of mirroring the quality of organoid vascularization only achieved with an in vivo host. We will provide a comprehensive summary of the current progress made in each one of these categories, highlighting advances in vascularization technologies and areas still under investigation.

Adaptable Microfluidic Vessel-on-a-Chip Platform for Investigating Tumor Metastatic Transport in Bloodstream

Cancer metastasis counts for 90% of cancer fatalities, and its development process is still a mystery. The dynamic process of tumor metastatic transport in the blood vessel is not well understood, in which some biomechanical factors, such as shear stress and various flow patterns, may have significant impacts. Here, we report a microfluidic vessel-on-a-chip platform for recapitulating several key metastatic steps of tumor cells in blood vessels on the same chip, including intravasation, circulating tumor cell (CTC) vascular adhesion, and extravasation. Due to its excellent adaptability, our system can reproduce various microenvironments to investigate the specific interactions between CTCs and blood vessels. On the basis of this platform, effects of important biomechanical factors on CTC adhesion such as vascular surface properties and vessel geometry-dependent hemodynamics were specifically inspected. We demonstrated that CTC adhesion is more likely to occur under certain mechano-physiological situations, such as vessels with vascular glycocalyx (VGCX) shedding and hemodynamic disturbances. Finally, computational models of both the fluidic dynamics in vessels and CTC adhesion were established based on the confocal scanned 3D images. The modeling results are believed to provide insights into exploring tumor metastasis progression and inspire new ideas for anticancer therapy development.

Mechanical regulation of signal transduction in angiogenesis

Biophysical and biochemical cues work in concert to regulate angiogenesis. These cues guide angiogenesis during development and wound healing. Abnormal cues contribute to pathological angiogenesis during tumor progression. In this review, we summarize the known signaling pathways involved in mechanotransduction important to angiogenesis. We discuss how variation in the mechanical microenvironment, in terms of stiffness, ligand availability, and topography, can modulate the angiogenesis process. We also present an integrated view on how mechanical perturbations, such as stretching and fluid shearing, alter angiogenesis-related signal transduction acutely, leading to downstream gene expression. Tissue engineering-based approaches to study angiogenesis are reviewed too. Future directions to aid the efforts in unveiling the comprehensive picture of angiogenesis are proposed.

Metallic Scaffold with Micron-Scale Geometrical Cues Promotes Osteogenesis and Angiogenesis via the ROCK/Myosin/YAP Pathway

The advent of precision manufacturing has enabled the creation of pores in metallic scaffolds with feature size in the range of single microns. In orthopedic implants, pore geometries at the micron scale could regulate bone formation by stimulating osteogenic differentiation and the coupling of osteogenesis and angiogenesis. However, the biological response to pore geometry at the cellular level is not clear. As cells are sensitive to curvature of the pore boundary, this study aimed to investigate osteogenesis in high- vs low-curvature environments by utilizing computer numerical control laser cutting to generate triangular and circular precision manufactured micropores (PMpores). We fabricated PMpores on 100 μm-thick stainless-steel discs. Triangular PMpores had a 30° vertex angle and a 300 μm base, and circular PMpores had a 300 μm diameter. We found triangular PMpores significantly enhanced the elastic modulus, proliferation, migration, and osteogenic differentiation of MC3T3-E1 preosteoblasts through Yes-associated protein (YAP) nuclear translocation. Inhibition of Rho-associated kinase (ROCK) and Myosin II abolished YAP translocation in all pore types and controls. Inhibition of YAP transcriptional activity reduced the proliferation, pore closure, collagen secretion, alkaline phosphatase (ALP), and Alizarin Red staining in MC3T3-E1 cultures. In C166 vascular endothelial cells, PMpores increased the VEGFA mRNA expression even without an angiogenic differentiation medium and induced tubule formation and maintenance. In terms of osteogenesis-angiogenesis coupling, a conditioned medium from MC3T3-E1 cells in PMpores promoted the expression of angiogenic genes in C166 cells. A coculture with MC3T3-E1 induced tubule formation and maintenance in C166 cells and tubule alignment along the edges of pores. Together, curvature cues in micropores are important stimuli to regulate osteogenic differentiation and osteogenesis-angiogenesis coupling. This study uncovered key mechanotransduction signaling components activated by curvature differences in a metallic scaffold and contributed to the understanding of the interaction between orthopedic implants and cells.

Engineering the multiscale complexity of vascular networks

The survival of vertebrate organisms depends on highly regulated delivery of oxygen and nutrients through vascular networks that pervade nearly all tissues in the body. Dysregulation of these vascular networks is implicated in many common human diseases such as hypertension, coronary artery disease, diabetes and cancer. Therefore, engineers have sought to create vascular networks within engineered tissues for applications such as regenerative therapies, human disease modelling and pharmacological testing. Yet engineering vascular networks has historically remained difficult, owing to both incomplete understanding of vascular structure and technical limitations for vascular fabrication. This Review highlights the materials advances that have enabled transformative progress in vascular engineering by ushering in new tools for both visualizing and building vasculature. New methods such as bioprinting, organoids and microfluidic systems are discussed, which have enabled the fabrication of 3D vascular topologies at a cellular scale with lumen perfusion. These approaches to vascular engineering are categorized into technology-driven and nature-driven approaches. Finally, the remaining knowledge gaps, emerging frontiers and opportunities for this field are highlighted, including the steps required to replicate the multiscale complexity of vascular networks found in nature.

Engineering multiscale structural orders for high-fidelity embryoids and organoids

Embryoids and organoids hold great promise for human biology and medicine. Herein, we discuss conceptual and technological frameworks useful for developing high-fidelity embryoids and organoids that display tissue- and organ-level phenotypes and functions, which are critically needed for decoding developmental programs and improving translational applications. Through dissecting the layers of inputs controlling mammalian embryogenesis, we review recent progress in reconstructing multiscale structural orders in embryoids and organoids. Bioengineering tools useful for multiscale, multimodal structural engineering of tissue- and organ-level cellular organization and microenvironment are also discussed to present integrative, bioengineering-directed approaches to achieve next-generation, high-fidelity embryoids and organoids.

3D Microvascularized Tissue Models by Laser-Based Cavitation Molding of Collagen

3D tissue models recapitulating human physiology are important for fundamental biomedical research, and they hold promise to become a new tool in drug development. An integrated and defined microvasculature in 3D tissue models is necessary for optimal cell functions. However, conventional bioprinting only allows the fabrication of hydrogel scaffolds containing vessel-like structures with large diameters (>100 µm) and simple geometries. Recent developments in laser photoablation enable the generation of this type of structure with higher resolution and complexity, but the photo-thermal process can compromise cell viability and hydrogel integrity. To address these limitations, the present work reports in situ 3D patterning of collagen hydrogels by femtosecond laser irradiation to create channels and cavities with diameters ranging from 20 to 60 µm. In this process, laser irradiation of the hydrogel generates cavitation gas bubbles that rearrange the collagen fibers, thereby creating stable microchannels. Such 3D channels can be formed in cell- and organoid-laden hydrogel without affecting the viability outside the lumen and can enable the formation of artificial microvasculature by the culture of endothelial cells and cell media perfusion. Thus, this method enables organs-on-a-chip and 3D tissue models featuring complex microvasculature.

Lymphangion-chip: a microphysiological system which supports co-culture and bidirectional signaling of lymphatic endothelial and muscle cells

The pathophysiology of several lymphatic diseases, such as lymphedema, depends on the function of lymphangions that drive lymph flow. Even though the signaling between the two main cellular components of a lymphangion, endothelial cells (LECs) and muscle cells (LMCs), is responsible for crucial lymphatic functions, there are no in vitro models that have included both cell types. Here, a fabrication technique (gravitational lumen patterning or GLP) is developed to create a lymphangion-chip. This organ-on-chip consists of co-culture of a monolayer of endothelial lumen surrounded by multiple and uniformly thick layers of muscle cells. The platform allows construction of a wide range of luminal diameters and muscular layer thicknesses, thus providing a toolbox to create variable anatomy. In this device, lymphatic muscle cells align circumferentially while endothelial cells aligned axially under flow, as only observed in vivo in the past. This system successfully characterizes the dynamics of cell size, density, growth, alignment, and intercellular gap due to co-culture and shear. Finally, exposure to pro-inflammatory cytokines reveals that the device could facilitate the regulation of endothelial barrier function through the lymphatic muscle cells. Therefore, this bioengineered platform is suitable for use in preclinical research of lymphatic and blood mechanobiology, inflammation, and translational outcomes.

Physiologic flow-conditioning limits vascular dysfunction in engineered human capillaries

Hemodynamics play a central role in the health and disease of the coronary and peripheral vascular systems. Vessel-lining endothelial cells are known mechanosensors, responding to disturbances in flow - with mechanosensitivity hypothesized to change in response to metabolic demands. The health of our smallest microvessels have been lauded as a prognostic marker for cardiovascular health. Yet, despite numerous animal models, studying these small vessels has proved difficult. Microfluidic technologies have allowed a number of 3D vascular models to be developed and used to investigate human vessels. Here, two such systems are employed for examining 1) interstitial flow effects on neo-vessel formation, and 2) the effects of flow-conditioning on vascular remodeling following sustained static culture. Interstitial flow is shown to enhance early vessel formation via significant remodeling of vessels and interconnected tight junctions of the endothelium. In formed vessels, continuous flow maintains a stable vascular diameter and causes significant remodeling, contrasting the continued anti-angiogenic decline of statically cultured vessels. This study is the first to couple complex 3D computational flow distributions and microvessel remodeling from microvessels grown on-chip (exposed to flow or no-flow conditions). Flow-conditioned vessels (WSS < 1Pa for 30 μm vessels) increase endothelial barrier function, result in significant changes in gene expression and reduce reactive oxygen species and anti-angiogenic cytokines. Taken together, these results demonstrate microvessel mechanosensitivity to flow-conditioning, which limits deleterious vessel regression in vitro, and could have implications for future modeling of reperfusion/no-flow conditions.

Organs-on-a-chip models for biological research

We explore the utility of bioengineered human tissues-individually or connected into physiological units-for biological research. While much smaller and simpler than their native counterparts, these tissues are complex enough to approximate distinct tissue phenotypes: molecular, structural, and functional. Unlike organoids, which form spontaneously and recapitulate development, "organs-on-a-chip" are engineered to display some specific functions of whole organs. Looking back, we discuss the key developments of this emerging technology. Thinking forward, we focus on the challenges faced to fully establish, validate, and utilize the fidelity of these models for biological research.

Advanced human-relevant in vitro pulmonary platforms for respiratory therapeutics

Over the past years, advanced in vitro pulmonary platforms have witnessed exciting developments that are pushing beyond traditional preclinical cell culture methods. Here, we discuss ongoing efforts in bridging the gap between in vivo and in vitro interfaces and identify some of the bioengineering challenges that lie ahead in delivering new generations of human-relevant in vitro pulmonary platforms. Notably, in vitro strategies using foremost lung-on-chips and biocompatible "soft" membranes have focused on platforms that emphasize phenotypical endpoints recapitulating key physiological and cellular functions. We review some of the most recent in vitro studies underlining seminal therapeutic screens and translational applications and open our discussion to promising avenues of pulmonary therapeutic exploration focusing on liposomes. Undeniably, there still remains a recognized trade-off between the physiological and biological complexity of these in vitro lung models and their ability to deliver assays with throughput capabilities. The upcoming years are thus anticipated to see further developments in broadening the applicability of such in vitro systems and accelerating therapeutic exploration for drug discovery and translational medicine in treating respiratory disorders.

New Technologies With Increased Precision Improve Understanding of Endothelial Cell Heterogeneity in Cardiovascular Health and Disease

Endothelial cells (ECs) are vital for blood vessel integrity and have roles in maintaining normal vascular function, healing after injury, and vascular dysfunction. Extensive phenotypic heterogeneity has been observed among ECs of different types of blood vessels in the normal and diseased vascular wall. Although ECs with different phenotypes can share common functions, each has unique features that may dictate a fine-tuned role in vascular health and disease. Recent studies performed with single-cell technology have generated powerful information that has significantly improved our understanding of EC biology. Here, we summarize a variety of EC types, states, and phenotypes recently identified by using new, increasingly precise techniques in transcriptome analysis.

Integration of substrate- and flow-derived stresses in endothelial cell mechanobiology

Endothelial cells (ECs) lining all blood vessels are subjected to large mechanical stresses that regulate their structure and function in health and disease. Here, we review EC responses to substrate-derived biophysical cues, namely topography, curvature, and stiffness, as well as to flow-derived stresses, notably shear stress, pressure, and tensile stresses. Because these mechanical cues in vivo are coupled and are exerted simultaneously on ECs, we also review the effects of multiple cues and describe burgeoning in vitro approaches for elucidating how ECs integrate and interpret various mechanical stimuli. We conclude by highlighting key open questions and upcoming challenges in the field of EC mechanobiology.

Versatile Vessel-on-a-Chip Platform for Studying Key Features of Blood Vascular Tumors

Tumor vessel-on-a-chip systems have attracted the interest of the cancer research community due to their ability to accurately recapitulate the multiple dynamic events of the metastatic cascade. Vessel-on-a-chip microfluidic platforms have been less utilized for investigating the distinctive features and functional heterogeneities of tumor-derived vascular networks. In particular, vascular tumors are characterized by the massive formation of thrombi and severe bleeding, a rare and life-threatening situation for which there are yet no clear therapeutic guidelines. This is mainly due to the lack of technological platforms capable of reproducing these characteristic traits of the pathology in a simple and well-controlled manner. Herein, we report the fabrication of a versatile tumor vessel-on-a-chip platform to reproduce, investigate, and characterize the massive formation of thrombi and hemorrhage on-chip in a fast and easy manner. Despite its simplicity, this method offers multiple advantages to recapitulate the pathophysiological events of vascular tumors, and therefore, may find useful applications in the field of vascular-related diseases, while at the same time being an alternative to more complex approaches.

Organ-on-a-chip systems for vascular biology

Organ-on-a-chip (OOC) platforms involve the miniaturization of cell culture systems and enable a variety of novel experimental approaches. These range from modeling the independent effects of biophysical forces on cells to screening novel drugs in multi-organ microphysiological systems, all within microscale devices. As in living systems, the incorporation of vascular structure is a key feature common to almost all organ-on-a-chip systems. In this review we highlight recent advances in organ-on-a-chip technologies with a focus on the vasculature. We first present the developmental process of the blood vessels through which vascular cells assemble into networks and remodel to form complex vascular beds under flow. We then review self-assembled vascular models and flow systems for the study of vascular development and biology as well as pre-patterned vascular models for the generation of perfusable microvessels for modeling vascular and tissue function. We finally conclude with a perspective on developing future OOC approaches for studying different aspects of vascular biology. We highlight the fit for purpose selection of OOC models towards either simple but powerful testbeds for therapeutic development, or complex vasculature to accurately replicate human physiology for specific disease modeling and tissue regeneration.

Perfusion and endothelialization of engineered tissues with patterned vascular networks

As engineered tissues progress toward therapeutically relevant length scales and cell densities, it is critical to deliver oxygen and nutrients throughout the tissue volume via perfusion through vascular networks. Furthermore, seeding of endothelial cells within these networks can recapitulate the barrier function and vascular physiology of native blood vessels. In this protocol, we describe how to fabricate and assemble customizable open-source tissue perfusion chambers and catheterize tissue constructs inside them. Human endothelial cells are seeded along the lumenal surfaces of the tissue constructs, which are subsequently connected to fluid pumping equipment. The protocol is agnostic with respect to biofabrication methodology as well as cell and material composition, and thus can enable a wide variety of experimental designs. It takes ~14 h over the course of 3 d to prepare perfusion chambers and begin a perfusion experiment. We envision that this protocol will facilitate the adoption and standardization of perfusion tissue culture methods across the fields of biomaterials and tissue engineering.

Go with the flow: modeling unique biological flows in engineered in vitro platforms

Interest in recapitulating in vivo phenomena in vitro using organ-on-a-chip technology has grown rapidly and with it, attention to the types of fluid flow experienced in the body has followed suit. These platforms offer distinct advantages over in vivo models with regards to human relevance, cost, and control of inputs (e.g., controlled manipulation of biomechanical cues from fluid perfusion). Given the critical role biophysical forces play in several tissues and organs, it is therefore imperative that engineered in vitro platforms capture the complex, unique flow profiles experienced in the body that are intimately tied with organ function. In this review, we outline the complex and unique flow regimes experienced by three different organ systems: blood vasculature, lymphatic vasculature, and the intestinal system. We highlight current state-of-the-art platforms that strive to replicate physiological flows within engineered tissues while introducing potential limitations in current approaches.

Vessel-on-a-chip models for studying microvascular physiology, transport, and function in vitro

To understand how the microvasculature grows and remodels, researchers require reproducible systems that emulate the function of living tissue. Innovative contributions toward fulfilling this important need have been made by engineered microvessels assembled in vitro with microfabrication techniques. Microfabricated vessels, commonly referred to as "vessels-on-a-chip," are from a class of cell culture technologies that uniquely integrate microscale flow phenomena, tissue-level biomolecular transport, cell-cell interactions, and proper three-dimensional (3-D) extracellular matrix environments under well-defined culture conditions. Here, we discuss the enabling attributes of microfabricated vessels that make these models more physiological compared with established cell culture techniques and the potential of these models for advancing microvascular research. This review highlights the key features of microvascular transport and physiology, critically discusses the strengths and limitations of different microfabrication strategies for studying the microvasculature, and provides a perspective on current challenges and future opportunities for vessel-on-a-chip models.