Microfluidic perfusion culture chip providing different strengths of shear stress for analysis of vascular endothelial function

Microfluidic artery-on-a-chip model with unidirectional gravity-driven flow for high-throughput applications

Cardiovascular disease (CVD) is the leading cause of death worldwide, with a noticeable decline in the approval of new therapeutic interventions. Currently, there is no gold standard for developing new therapies for CVDs, and preclinical models do not translate to clinical efficacy. Therefore, there is an urgent need for in vitro models that more accurately mimic human disease processes. Here we describe a model of the artery consisting of monocultures of human coronary artery endothelial cells (HCAECs) or cocultures of HCAECs with human coronary artery smooth muscle cells (HCASMCs). The model was established in the OrganoPlate® 2-lane-48 UF, a novel microfluidic device, comprised of a microtiter plate footprint with 48 chips. Fluid is circulated in a unidirectional manner by interval rocking. The creation of an air-liquid interface at the inlets at a given inclination is used to select flow paths and establish flow in one direction only, whilst capillary forces ensure the channel remains filled with fluid. We investigated the impact of unidirectional or bidirectional flow conditions. Under unidirectional flow, endothelial cells aligned with the flow direction, decreased fibronectin deposition, and smooth muscle cells presented a non-contractile phenotype, emulating the characteristics of healthy arteries. Contrarily, bidirectional flow mimicked features of early endothelial dysfunction, such as contractile morphology of vessels and increased fibronectin secretion, ICAM-1 staining, and lipid deposits. Vascular inflammation could be induced by the addition of TNFα and IL-1β in both flow conditions. Overall, the OrganoPlate® 2-lane-48 UF is a powerful platform providing both throughput and improved flow control, for creating more physiological models. Its ability to replicate key features of a healthy and diseased artery, its potential use in drug screening, and its compatibility with lab automation make it an invaluable tool for researchers aiming for more accurate and efficient therapeutic development in CVD.

Microfluidic vessel-on-chip platform for investigation of cellular defects in venous malformations and responses to various shear stress and flow conditions

A novel microfluidic platform was designed to study the cellular architecture of endothelial cells (ECs) in an environment replicating the 3D organization and flow of blood vessels. In particular, the platform was constructed to investigate EC defects in slow-flow venous malformations (VMs) under varying shear stress and flow conditions. The platform featured a standard microtiter plate footprint containing 32 microfluidic units capable of replicating wall shear stress (WSS) in normal veins and enabling precise control of shear stress and flow directionality without the need for complex pumping systems. Using genetically engineered human umbilical vein endothelial cells (HUVECs) and induced pluripotent stem cell (iPSC)-derived ECs (iECs) to express the recurrent TIE2L914F VM mutation we assessed responses on EC orientation and area, actin organization, and Golgi polarization to uni- and bidirectional flow and varying WSS. Comparison of control and TIE2L914F expressing ECs showed differential cellular responses to flow and WSS in terms of cell shape elongation, orientation of F-actin, and Golgi polarization, indicating altered mechanosensory or mechanotransduction signaling pathways in the presence of the VM causative mutation. The data also revealed significant differences in how the primary and iPSC-derived iECs responded to flow. As a conclusion, the developed microfluidic platform allowed simulation of multiple flow conditions in a scalable and pumpless format. The design made it a desirable tool for studying different EC types as well as cellular changes in vascular disease. The platform should offer new opportunities for biomechanical research by providing a controlled environment to analyze the flow-dependent mechanosensory pathways in ECs.

Fluid flow impacts endothelial-monocyte interactions in a model of vascular inflammatory fibrosis

The aberrant vascular response associated with tendon injury results in circulating immune cell infiltration and a chronic inflammatory feedback loop leading to poor healing outcomes. Studying this dysregulated tendon repair response in human pathophysiology has been historically challenging due to the reliance on animal models. To address this, our group developed the human tendon-on-a-chip (hToC) to model cellular interactions in the injured tendon microenvironment; however, this model lacked the key element of physiological flow in the vascular compartment. Here, we leveraged the modularity of our platform to create a fluidic hToC that enables the study of circulating immune cell and vascular crosstalk in a tendon injury model. Under physiological shear stress consistent with postcapillary venules, we found a significant increase in the endothelial leukocyte activation marker intercellular adhesion molecule 1 (ICAM-1), as well as enhanced adhesion and transmigration of circulating monocytes across the endothelial barrier. The addition of tissue macrophages to the tendon compartment further increased the degree of circulating monocyte infiltration into the tissue matrix. Our findings demonstrate the importance of adding physiological flow to the human tendon-on-a-chip, and more generally, the significance of flow for modeling immune cell interactions in tissue inflammation and disease.

Developing a Flow-Resistance Module for Elucidating Cell Mechanotransduction on Multiple Shear Stresses

Fluid shear stress plays a pivotal role in regulating cellular behaviors, maintaining tissue homeostasis, and driving disease progression. Cells in various tissues are specifically adapted to physiological levels of shear stress and exhibit sensitivity to variations in its magnitude, highlighting the requirement for a comprehensive understanding of cellular responses to both physiologically and pathologically relevant levels of shear stress. In this study, we developed an independent upstream flow-resistance module with high fluidic resistances comprising three microchannels. The validity of the flow-resistance module was confirmed via computational fluid dynamics (CFD) simulations and flow calibration experiments, resulting in the generation of steady wall shear stresses ranging from 0.06 to 11.57 dyn/cm2 within the interconnected cell culture chips. Gene expression profiles, cytoskeletal remodeling, and morphological changes, as well as Yes-associated protein (YAP) nuclear translocation, were investigated in response to various shear stresses to authenticate the reliability of our experimental platform, indicating an increasing trend as the shear stress increases, reaching its maximum at various shear stresses. Our findings suggest that this flow-resistance module can be readily employed for precise characterization of cellular responses under various shear stresses.

Assessment of NK cytotoxicity and interactions with porcine endothelial cells by live-cell imaging in 2D static and 3D microfluidic systems

Natural Killer (NK) cells are pivotal in immune responses to viral infections, malignancies, autoimmune diseases, and transplantation. Assessment of NK cell adhesion, migration, and cytotoxicity is fundamental for in vitro studies. We propose a novel live-cell tracking method that addresses these three major aspects of NK cell function using human NK cells and primary porcine aortic endothelial cells (PAECs) in two-dimensional (2D) static assays and an in-house cylindrical 3D microfluidic system. The results showed a significant increase of NK cytotoxicity against pTNF-activated PAECs, with apoptotic cell death observed in the majority of dead cells, while no difference was observed in the conventional Delfia assay. Computed analysis of NK cell trajectories revealed distinct migratory behaviors, including trajectory length, diameter, average speed, and arrest coefficient. In 3D microfluidic experiments, NK cell attachment to pTNF-activated PAECs substantially increased, accompanied by more dead PAECs compared to control conditions. NK cell trajectories showed versatile migration in various directions and interactions with PAECs. This study uniquely demonstrates NK attachment and killing in a 3D system that mimics blood vessel conditions. Our microscope method offers sensitive single-cell level results, addressing diverse aspects of NK functions. It is adaptable for studying other immune and target cells, providing insights into various biological questions.

Microfluidic Systems for Neural Cell Studies

Whereas the axons of the peripheral nervous system (PNS) spontaneously regenerate after an injury, the occurring regeneration is rarely successful because axons are usually directed by inappropriate cues. Therefore, finding successful ways to guide neurite outgrowth, in vitro, is essential for neurogenesis. Microfluidic systems reflect more appropriately the in vivo environment of cells in tissues such as the normal fluid flow within the body, consistent nutrient delivery, effective waste removal, and mechanical stimulation due to fluid shear forces. At the same time, it has been well reported that topography affects neuronal outgrowth, orientation, and differentiation. In this review, we demonstrate how topography and microfluidic flow affect neuronal behavior, either separately or in synergy, and highlight the efficacy of microfluidic systems in promoting neuronal outgrowth.

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.

Replicating endothelial shear stress in organ-on-a-chip for predictive hypericin photodynamic efficiency

Photodynamic therapy using Hypericin (Hy-PDT) is an alternative non-invasive treatment that enables selective tumor inhibition and angiogenesis derived from the differential recruitment of endothelial cells in the tumor microenvironment. Most PDT studies were performed on in vitro models without vascular biomechanical simulation. Our work strives to develop a microchip that generates a constant shear stress force to investigate the Hy-PDT efficiency on human umbilical vein endothelial cells (HUVECs). The microchip with a single straight microchannel was composed of the bottom layer (polystyrene), the middle layer (double-sided biocompatible adhesive tape), and the top layer (polyester film) and could produce shear stress in the range of 1.4 - 7.0 dyn cm^-2. The quantification of vascular endothelial growth factor (VEGF), cell viability, and activities of caspases 3 and 7 were assayed to validate the microchip and Hy-PDT efficacy. After the endothelization, static and dynamic cell incubations with Hy were conducted in microchips. Compared to static systems, the shear stress displayed its effect on the increasing release of VEGF and promoted more cell damage and cell death via necrosis during Hy-PDT. In conclusion, the expressive shear stress-dependent manner during PDT treatments suggests that the microchip could be an essential approach in preclinical tests to evaluate the therapeutic outcome considering the endothelial shear stress microenvironment.

Ex Vivo treatment of coronary artery endothelial cells with serum post-exercise training offers limited protection against in vitro exposure to FEC-T chemotherapy

Background: Chemotherapy treatment for breast cancer associates with well-documented cardiovascular detriments. Exercise has shown promise as a potentially protective intervention against cardiac toxicity. However, there is a paucity of evidence for the benefits of exercise on the vasculature. Objectives: This study aimed to determine the effects of chemotherapy on the vascular endothelium; and if there are protective effects of serological alterations elicited by an exercise training intervention. Methods and Results: 15 women participated in a 12-week home-based exercise intervention consisting of three high-intensity interval sessions per week. Human coronary artery endothelial cells (HCAEC) were exposed to physiological concentrations of 5-fluorouracil, epirubicin, cyclophosphamide (FEC) and docetaxel to determine a dose-response. Twenty-4 hours prior to FEC and docetaxel exposure, HCAECs were preconditioned with serum collected pre- and post-training. Annexin V binding and cleaved caspase-3 were assessed using flow cytometry and wound repair by scratch assays. Chemotherapy exposure increased HCAEC Annexin V binding, cleaved caspase-3 expression in a dose-dependent manner; and inhibited wound repair. Compared to pre-training serum, conditioning HCAECs with post-training serum, reduced Annexin V binding (42% vs. 30%, p = 0.01) when exposed to FEC. For docetaxel, there were no within-group differences (pre-vs post-exercise) for Annexin V binding or cleaved caspase-3 expression. There was a protective effect of post-training serum on wound repair for 5-flurouracil (p = 0.03) only. Conclusion: FEC-T chemotherapy drugs cause significant damage and dysfunction of endothelial cells. Preconditioning with serum collected after an exercise training intervention, elicited some protection against the usual toxicity of FEC-T, when compared to control serum.

Perfusion culture of endothelial cells under shear stress on microporous membrane in a pressure-driven microphysiological system

This paper reports perfusion culture of human umbilical vein endothelial cells (HUVECs) on a microporous membrane in a pressure-driven microphysiological system (PD-MPS), which we developed previously as a multi-throughput perfusion culture platform. We designed fluidic culture unit with microporous membrane to culture HUVECs under fluidic shear stress and constructed a perfusion culture model in the PD-MPS platform. Four fluidic culture units were arranged in the microplate-sized device, which enables four-throughput assay for characterization of HUVECs under flow. Medium flow was generated above and below the membrane by sequential pneumatic pressure to apply physiological shear stress to HUVECs. HUVECs exhibited aligned morphology to the direction of the flow with shear stress of 11.5-17.7 dyn/cm² under the flow condition, while they randomly aligned under static culture condition in a 6 well plate. We also observed 3.3- and 5.0-fold increase in the expression levels of the thrombomodulin and endothelial nitric oxide synthase mRNAs, respectively, under the flow condition in the PD-MPS compared to the static culture in 6 well plate. We also observed actin filament aligned to the direction of flow in HUVECs cultured under the flow condition.

The Modular µSiM Reconfigured: Integration of Microfluidic Capabilities to Study In Vitro Barrier Tissue Models under Flow

Microfluidic tissue barrier models have emerged to address the lack of physiological fluid flow in conventional "open-well" Transwell-like devices. However, microfluidic techniques have not achieved widespread usage in bioscience laboratories because they are not fully compatible with traditional experimental protocols. To advance barrier tissue research, there is a need for a platform that combines the key advantages of both conventional open-well and microfluidic systems. Here, a plug-and-play flow module is developed to introduce on-demand microfluidic flow capabilities to an open-well device that features a nanoporous membrane and live-cell imaging capabilities. The magnetic latching assembly of this design enables bi-directional reconfiguration and allows users to conduct an experiment in an open-well format with established protocols and then add or remove microfluidic capabilities as desired. This work also provides an experimentally-validated flow model to select flow conditions based on the experimental needs. As a proof-of-concept, flow-induced alignment of endothelial cells and the expression of shear-sensitive gene targets are demonstrated, and the different phases of neutrophil transmigration across a chemically stimulated endothelial monolayer under flow conditions are visualized. With these experimental capabilities, it is anticipated that both engineering and bioscience laboratories will adopt this reconfigurable design due to the compatibility with standard open-well protocols.

A comparative study of tumour-on-chip models with patient-derived xenografts for predicting chemotherapy efficacy in colorectal cancer patients

Inter-patient and intra-tumour heterogeneity (ITH) have prompted the need for a more personalised approach to cancer therapy. Although patient-derived xenograft (PDX) models can generate drug response specific to patients, they are not sustainable in terms of cost and time and have limited scalability. Tumour Organ-on-Chip (OoC) models are in vitro alternatives that can recapitulate some aspects of the 3D tumour microenvironment and can be scaled up for drug screening. While many tumour OoC systems have been developed to date, there have been limited validation studies to ascertain whether drug responses obtained from tumour OoCs are comparable to those predicted from patient-derived xenograft (PDX) models. In this study, we established a multiplexed tumour OoC device, that consists of an 8 × 4 array (32-plex) of culture chamber coupled to a concentration gradient generator. The device enabled perfusion culture of primary PDX-derived tumour spheroids to obtain dose-dependent response of 5 distinct standard-of-care (SOC) chemotherapeutic drugs for 3 colorectal cancer (CRC) patients. The in vitro efficacies of the chemotherapeutic drugs were rank-ordered for individual patients and compared to the in vivo efficacy obtained from matched PDX models. We show that quantitative correlation analysis between the drug efficacies predicted via the microfluidic perfusion culture is predictive of response in animal PDX models. This is a first study showing a comparative framework to quantitatively correlate the drug response predictions made by a microfluidic tumour organ-on-chip (OoC) model with that of PDX animal models.

A Review of Functional Analysis of Endothelial Cells in Flow Chambers

The vascular endothelial cells constitute the innermost layer. The cells are exposed to mechanical stress by the flow, causing them to express their functions. To elucidate the functions, methods involving seeding endothelial cells as a layer in a chamber were studied. The chambers are known as parallel plate, T-chamber, step, cone plate, and stretch. The stimulated functions or signals from endothelial cells by flows are extensively connected to other outer layers of arteries or organs. The coculture layer was developed in a chamber to investigate the interaction between smooth muscle cells in the middle layer of the blood vessel wall in vascular physiology and pathology. Additionally, the microfabrication technology used to create a chamber for a microfluidic device involves both mechanical and chemical stimulation of cells to show their dynamics in in vivo microenvironments. The purpose of this study is to summarize the blood flow (flow inducing) for the functions connecting to endothelial cells and blood vessels, and to find directions for future chamber and device developments for further understanding and application of vascular functions. The relationship between chamber design flow, cell layers, and microfluidics was studied.

Perfusion in Organ-on-Chip Models and Its Applicability to the Replication of Spermatogenesis In Vitro

Organ/organoid-on-a-chip (OoC) technologies aim to replicate aspects of the in vivo environment in vitro, at the scale of microns. Mimicking the spatial in vivo structure is important and can provide a deeper understanding of the cell-cell interactions and the mechanisms that lead to normal/abnormal function of a given organ. It is also important for disease models and drug/toxin testing. Incorporating active fluid flow in chip models enables many more possibilities. Active flow can provide physical cues, improve intercellular communication, and allow for the dynamic control of the environment, by enabling the efficient introduction of biological factors, drugs, or toxins. All of this is in addition to the fundamental role of flow in supplying nutrition and removing waste metabolites. This review presents an overview of the different types of fluid flow and how they are incorporated in various OoC models. The review then describes various methods and techniques of incorporating perfusion networks into OoC models, including self-assembly, bioprinting techniques, and utilizing sacrificial gels. The second part of the review focuses on the replication of spermatogenesis in vitro; the complex process whereby spermatogonial stem cells differentiate into mature sperm. A general overview is given of the various approaches that have been used. The few studies that incorporated microfluidics or vasculature are also described. Finally, a future perspective is given on elements from perfusion-based models that are currently used in models of other organs and can be applied to the field of in vitro spermatogenesis. For example, adopting tubular blood vessel models to mimic the morphology of the seminiferous tubules and incorporating vasculature in testis-on-a-chip models. Improving these models would improve our understanding of the process of spermatogenesis. It may also potentially provide novel therapeutic strategies for pre-pubertal cancer patients who need aggressive chemotherapy that can render them sterile, as well asfor a subset of non-obstructive azoospermic patients with maturation arrest, whose testes do not produce sperm but still contain some of the progenitor cells.

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.

Rapid Fabrication of Microfluidic Devices for Biological Mimicking: A Survey of Materials and Biocompatibility

Microfluidics is an essential technique used in the development of in vitro models for mimicking complex biological systems. The microchip with microfluidic flows offers the precise control of the microenvironment where the cells can grow and structure inside channels to resemble in vivo conditions allowing a proper cellular response investigation. Hence, this study aimed to develop low-cost, simple microchips to simulate the shear stress effect on the human umbilical vein endothelial cells (HUVEC). Differentially from other biological microfluidic devices described in the literature, we used readily available tools like heat-lamination, toner printer, laser cutter and biocompatible double-sided adhesive tapes to bind different layers of materials together, forming a designed composite with a microchannel. In addition, we screened alternative substrates, including polyester-toner, polyester-vinyl, glass, Permanox^® and polystyrene to compose the microchips for optimizing cell adhesion, then enabling these microdevices when coupled to a syringe pump, the cells can withstand the fluid shear stress range from 1 to 4 dyne cm². The cell viability was monitored by acridine orange/ethidium bromide (AO/EB) staining to detect live and dead cells. As a result, our fabrication processes were cost-effective and straightforward. The materials investigated in the assembling of the microchips exhibited good cell viability and biocompatibility, providing a dynamic microenvironment for cell proliferation. Therefore, we suggest that these microchips could be available everywhere, allowing in vitro assays for daily laboratory experiments and further developing the organ-on-a-chip concept.

Combined effect of shear stress and laser-patterned topography on Schwann cell outgrowth: synergistic or antagonistic?

Although the peripheral nervous system exhibits a higher rate of regeneration than that of the central nervous system through a spontaneous regeneration after injury, the functional recovery is fairly infrequent and misdirected. Thus, the development of successful methods to guide neuronal outgrowth, in vitro, is of great importance. In this study, a precise flow controlled microfluidic system with specific custom-designed chambers, incorporating laser-microstructured polyethylene terephthalate (PET) substrates comprising microgrooves, was fabricated to assess the combined effect of shear stress and topography on Schwann cells' behavior. The microgrooves were positioned either parallel or perpendicular to the direction of the flow inside the chambers. Additionally, the cell culture results were combined with computational flow simulations to calculate accurately the shear stress values. Our results demonstrated that wall shear stress gradients may be acting either synergistically or antagonistically depending on the substrate groove orientation relative to the flow direction. The ability to control cell alignment in vitro could potentially be used in the fields of neural tissue engineering and regenerative medicine.

Recent progress in translational engineered in vitro models of the central nervous system

The complexity of the human brain poses a substantial challenge for the development of models of the CNS. Current animal models lack many essential human characteristics (in addition to raising operational challenges and ethical concerns), and conventional in vitro models, in turn, are limited in their capacity to provide information regarding many functional and systemic responses. Indeed, these challenges may underlie the notoriously low success rates of CNS drug development efforts. During the past 5 years, there has been a leap in the complexity and functionality of in vitro systems of the CNS, which have the potential to overcome many of the limitations of traditional model systems. The availability of human-derived induced pluripotent stem cell technology has further increased the translational potential of these systems. Yet, the adoption of state-of-the-art in vitro platforms within the CNS research community is limited. This may be attributable to the high costs or the immaturity of the systems. Nevertheless, the costs of fabrication have decreased, and there are tremendous ongoing efforts to improve the quality of cell differentiation. Herein, we aim to raise awareness of the capabilities and accessibility of advanced in vitro CNS technologies. We provide an overview of some of the main recent developments (since 2015) in in vitro CNS models. In particular, we focus on engineered in vitro models based on cell culture systems combined with microfluidic platforms (e.g. 'organ-on-a-chip' systems). We delve into the fundamental principles underlying these systems and review several applications of these platforms for the study of the CNS in health and disease. Our discussion further addresses the challenges that hinder the implementation of advanced in vitro platforms in personalized medicine or in large-scale industrial settings, and outlines the existing differentiation protocols and industrial cell sources. We conclude by providing practical guidelines for laboratories that are considering adopting organ-on-a-chip technologies.

Lab-on-a-Chip for Cardiovascular Physiology and Pathology

Lab-on-a-chip technologies have allowed researchers to acquire a flexible, yet relatively inexpensive testbed to study one of the leading causes of death worldwide, cardiovascular disease. Cardiovascular diseases, such as peripheral artery disease, arteriosclerosis, and aortic stenosis, for example, have all been studied by lab-on-a-chip technologies. These technologies allow for the integration of mammalian cells into functional structures that mimic vital organs with geometries comparable to those found in vivo. For this review, we focus on microdevices that have been developed to study cardiovascular physiology and pathology. With these technologies, researchers can better understand the electrical-biomechanical properties unique to cardiomyocytes and better stimulate and understand the influence of blood flow on the human vasculature. Such studies have helped increase our understanding of many cardiovascular diseases in general; as such, we present here a review of the current state of the field and potential for the future.

Fabrication of Hollow Structures in Photodegradable Hydrogels Using a Multi-Photon Excitation Process for Blood Vessel Tissue Engineering

Engineered blood vessels generally recapitulate vascular function in vitro and can be utilized in drug discovery as a novel microphysiological system. Recently, various methods to fabricate vascular models in hydrogels have been reported to study the blood vessel functions in vitro; however, in general, it is difficult to fabricate hollow structures with a designed size and structure with a tens of micrometers scale for blood vessel tissue engineering. This study reports a method to fabricate the hollow structures in photodegradable hydrogels prepared in a microfluidic device. An infrared femtosecond pulsed laser, employed to induce photodegradation via multi-photon excitation, was scanned in the hydrogel in a program-controlled manner for fabricating the designed hollow structures. The photodegradable hydrogel was prepared by a crosslinking reaction between an azide-modified gelatin solution and a dibenzocyclooctyl-terminated photocleavable tetra-arm polyethylene glycol crosslinker solution. After assessing the composition of the photodegradable hydrogel in terms of swelling and cell adhesion, the hydrogel prepared in the microfluidic device was processed by laser scanning to fabricate linear and branched hollow structures present in it. We introduced a microsphere suspension into the fabricated structure in photodegradable hydrogels, and confirmed the fabrication of perfusable hollow structures of designed patterns via the multi-photon excitation process.

Application of microphysiological systems in biopharmaceutical research and development

Within the last 10 years, several tissue microphysiological systems (MPS) have been developed and characterized for retention of morphologic characteristics and specific gene/protein expression profiles from their natural in vivo state. Once developed, their utility is typically further tested by comparing responses to known toxic small-molecule pharmaceuticals in efforts to develop strategies for further toxicity testing of compounds under development. More recently, application of this technology in biopharmaceutical (large molecules) development is beginning to be more appreciated. In this review, we describe some of the advances made for tissue-specific MPS and outline the advantages and challenges of applying and further developing MPS technology in preclinical biopharmaceutical research.

Environmental Specification of Pluripotent Stem Cell Derived Endothelial Cells Toward Arterial and Venous Subtypes

Endothelial cells (ECs) are required for a multitude of cardiovascular clinical applications, such as revascularization of ischemic tissues or endothelialization of tissue engineered grafts. Patient derived primary ECs are limited in number, have donor variabilities and their in vitro phenotypes and functions can deteriorate over time. This necessitates the exploration of alternative EC sources. Although there has been a recent surge in the use of pluripotent stem cell derived endothelial cells (PSC-ECs) for various cardiovascular clinical applications, current differentiation protocols yield a heterogeneous EC population, where their specification into arterial or venous subtypes is undefined. Since arterial and venous ECs are phenotypically and functionally different, inappropriate matching of exogenous ECs to host sites can potentially affect clinical efficacy, as exemplified by venous graft mismatch when placed into an arterial environment. Therefore, there is a need to design and employ environmental cues that can effectively modulate PSC-ECs into a more homogeneous arterial or venous phenotype for better adaptation to the host environment, which will in turn contribute to better application efficacy. In this review, we will first give an overview of the developmental and functional differences between arterial and venous ECs. This provides the foundation for our subsequent discussion on the different bioengineering strategies that have been investigated to varying extent in providing biochemical and biophysical environmental cues to mature PSC-ECs into arterial or venous subtypes. The ability to efficiently leverage on a combination of biochemical and biophysical environmental cues to modulate intrinsic arterio-venous specification programs in ECs will greatly facilitate future translational applications of PSC-ECs. Since the development and maintenance of arterial and venous ECs in vivo occur in disparate physio-chemical microenvironments, it is conceivable that the application of these environmental factors in customized combinations or magnitudes can be used to selectively mature PSC-ECs into an arterial or venous subtype.

Determination of critical shear stress for maturation of human pluripotent stem cell-derived endothelial cells towards an arterial subtype

Human pluripotent stem cell-derived endothelial cells (hPSC-ECs) present an attractive alternative to primary EC sources for vascular grafting. However, there is a need to mature them towards either an arterial or venous subtype. A vital environmental factor involved in the arteriovenous specification of ECs during early embryonic development is fluid shear stress; therefore, there have been attempts to employ adult arterial shear stress conditions to mature hPSC-ECs. However, hPSC-ECs are naïve to fluid shear stress, and their shear responses are still not well understood. Here, we used a multiplex microfluidic platform to systematically investigate the dose-time shear responses on hPSC-EC morphology and arterial-venous phenotypes over a range of magnitudes coincidental with physiological levels of embryonic and adult vasculatures. The device comprised of six parallel cell culture chambers that were individually linked to flow-setting resistance channels, allowing us to simultaneously apply shear stress ranging from 0.4 to 15 dyne/cm ² . We found that hPSC-ECs required up to 40 hr of shear exposure to elicit a stable phenotypic change. Cell alignment was visible at shear stress <1 dyne/cm ² , which was independent of shear stress magnitude and duration of exposure. We discovered that the arterial markers NOTCH1 and EphrinB2 exhibited a dose-dependent increase in a similar manner beyond a threshold level of 3.8 dyne/cm ² , whereas the venous markers COUP-TFII and EphB4 expression remained relatively constant across different magnitudes. These findings indicated that hPSC-ECs were sensitive to relatively low magnitudes of shear stress, and a critical level of ~4 dyne/cm ² was sufficient to preferentially enhance their maturation into an arterial phenotype for future vascular tissue engineering applications.

A microfluidic chip with gravity-induced unidirectional flow for perfusion cell culture

Perfusion flow is one of the essential elements and advantages of organ-on-a-chip technology. For example, microfluidics have enabled implementation of perfusion flow and recapitulation of fluidic environment for vascular endothelial cells. The most prevalent method of implementing flow in a chip is to use a pump, which requires elaborate manipulation and complex connections, and accompanies a large amount of dead volume. Previously we devised a gravity-induced flow system which does not require tubing connections, but this method results in bidirectional flow to enable recirculation, which is somewhat different from physiological blood flow. Here, we have developed a novel microfluidic chip that enables gravity-induced, unidirectional flow by using a bypass channel with geometry different from the main channel. Human umbilical vein endothelial cells were cultured inside the chip and the effect of flow direction was examined. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2701, 2019.

3D artificial round section micro-vessels to investigate endothelial cells under physiological flow conditions

In the context of xenotransplantation, in ischemia/reperfusion injury as well as in cardiovascular research, the study of the fascinating interplay between endothelial cells (EC) and the plasma cascade systems often requires in vitro models. Blood vessels are hardly reproducible with standard flat-bed culture systems and flow-plate assays are limited in their low surface-to-volume ratio which impedes the study of the anticoagulant properties of the endothelial cells. According to the 3R regulations (reduce, replace and refine animal experimentation) we developed a closed circuit microfluidic in vitro system in which endothelial cells are cultured in 3D round section microchannels and subjected to physiological, pulsatile flow. In this study, a 3D monolayer of porcine aortic EC was perfused with human serum to mimic a xenotransplantation setting. Complement as well as EC activation was assessed in the presence or absence of complement inhibitors showing the versatility of the model for drug testing. Complement activation products as well as E-selectin expression were detected and visualized in situ by high resolution confocal microscopy. Furthermore, porcine pro-inflammatory cytokines as well as soluble complement components in the recirculating fluid phase were detected after human serum perfusion providing a better overview of the artificial vascular environment.

Fluidic Culture and Analysis of Pulmonary Artery Smooth Muscle Cells for the Study of Pulmonary Hypertension

There is an urgent need to develop novel in-vitro models to mimic the disease conditions in pulmonary hypertension (PH). We developed a microfluidic cell culture device for PH studies that withstood high shear stress. Techniques were also developed for cell recovery from the microchannel and mRNA isolation from the collected cells. Using this device, we found that shear stress caused a 7.5-fold increase in the transcription levels of a PH-related molecule, Cyclin D1.

In Vitro Microfluidic Models for Neurodegenerative Disorders

Microfluidic devices enable novel means of emulating neurodegenerative disease pathophysiology in vitro. These organ-on-a-chip systems can potentially reduce animal testing and substitute (or augment) simple 2D culture systems. Reconstituting critical features of neurodegenerative diseases in a biomimetic system using microfluidics can thereby accelerate drug discovery and improve our understanding of the mechanisms of several currently incurable diseases. This review describes latest advances in modeling neurodegenerative diseases in the central nervous system and the peripheral nervous system. First, this study summarizes fundamental advantages of microfluidic devices in the creation of compartmentalized cell culture microenvironments for the co-culture of neurons, glial cells, endothelial cells, and skeletal muscle cells and in their recapitulation of spatiotemporal chemical gradients and mechanical microenvironments. Then, this reviews neurodegenerative-disease-on-a-chip models focusing on Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Finally, this study discusses about current drawbacks of these models and strategies that may overcome them. These organ-on-chip technologies can be useful to be the first line of testing line in drug development and toxicology studies, which can contribute significantly to minimize the phase of animal testing steps.

Microfluidic Platform for the Long-Term On-Chip Cultivation of Mammalian Cells for Lab-On-A-Chip Applications

Lab-on-a-Chip (LoC) applications for the long-term analysis of mammalian cells are still very rare due to the lack of convenient cell cultivation devices. The difficulties are the integration of suitable supply structures, the need of expensive equipment like an incubator and sophisticated pumps as well as the choice of material. The presented device is made out of hard, but non-cytotoxic materials (silicon and glass) and contains two vertical arranged membranes out of hydrogel. The porous membranes are used to separate the culture chamber from two supply channels for gases and nutrients. The cells are fed continuously by diffusion through the membranes without the need of an incubator and low requirements on the supply of medium to the assembly. The diffusion of oxygen is modelled in order to find the optimal dimensions of the chamber. The chip is connected via 3D-printed holders to the macroscopic world. The holders are coated with Parlyene C to ensure that only biocompatible materials are in contact with the culture medium. The experiments with MDCK-cells show the successful seeding inside the chip, culturing and passaging. Consequently, the presented platform is a step towards Lab-on-a-Chip applications that require long-term cultivation of mammalian cells.

Vascular endothelial cell mechanosensing: New insights gained from biomimetic microfluidic models

In vivo, cells of the vascular system are subjected to various mechanical stimuli and have demonstrated the ability to adapt their behavior via mechanotransduction. Recent advances in microfluidic and "on-chip" techniques have provided the technology to study these alterations in cell behavior. Contrary to traditional in vitro assays such as transwell plates and parallel plate flow chambers, these microfluidic devices (MFDs) provide the opportunity to integrate multiple mechanical cues (e.g. shear stress, confinement, substrate stiffness, vessel geometry and topography) with in situ quantification capabilities. As such, MFDs can be used to recapitulate the in vivo mechanical setting and systematically vary microenvironmental conditions for improved mechanobiological studies of the endothelium. Additionally, adequate modelling provides for enhanced understanding of disease progression, design of cell separation and drug delivery systems, and the development of biomaterials for tissue engineering applications. Here, we will discuss the advances in knowledge about endothelial cell mechanosensing resulting from the design and application of biomimetic on-chip and microfluidic platforms.

Integrated heart/cancer on a chip to reproduce the side effects of anti-cancer drugs in vitro

Integrated Heart/Cancer on a Chip (iHCC) is a promising microfluidic platform that allows the culture of different cell types separately and application of closed-medium circulation to reproduce the side effects of doxorubicin on heart in vitro.

Self-driven perfusion culture system using a paper-based double-layered scaffold

Shear stress caused by fluid flow is known to promote tissue development from cells in vivo. Therefore, perfusion cultures have been studied to investigate the mechanisms involved and to fabricate engineered tissues in vitro, particularly those that include blood vessels. Microfluidic devices, which function with fine machinery of chambers and microsyringes for fluid flow and have small culture areas, are conventionally used for perfusion culture. In contrast, we have developed a self-driven perfusion culture system by using a paper-based double-layered scaffold as the fundamental component. Gelatin microfibers were electrospun onto a paper material to prepare the scaffold system, in which the constant perfusion of the medium and the scaffold for cell adhesion/proliferation were functionally divided into a paper and a gelatin microfiber layer, respectively. By applying both the capillary action and siphon phenomenon of the paper-based scaffold, which bridged two medium chambers at different height levels, a self-driven medium flow was achieved and the flow rate was also stable, constant, and quantitatively controllable. Moreover, the culture area was enlargeable to the cm(2) scale. The endothelial cells cultivated on this system oriented along the medium-flow direction, suggesting that the shear stress caused by medium flow was effectively applied. This perfusion culture system is expected to be useful for fabricating three-dimensional and large engineered tissues in the future.

Intrinsic FGF2 and FGF5 promotes angiogenesis of human aortic endothelial cells in 3D microfluidic angiogenesis system

The human body contains different endothelial cell types and differences in their angiogenic potential are poorly understood. We compared the functional angiogenic ability of human aortic endothelial cells (HAECs) and human umbilical vein endothelial cells (HUVECs) using a three-dimensional (3D) microfluidic cell culture system. HAECs and HUVECs exhibited similar cellular characteristics in a 2D culture system; however, in the 3D microfluidic angiogenesis system, HAECs exhibited stronger angiogenic potential than HUVECs. Interestingly, the expression level of fibroblast growth factor (FGF)2 and FGF5 under vascular endothelial growth factor (VEGF)-A stimulation was significantly higher in HAECs than in HUVECs. Moreover, small interfering RNA-mediated knockdown of FGF2 and FGF5 more significantly attenuated vascular sprouting induced from HAECs than HUVECs. Our results suggest that HAECs have greater angiogenic potential through FGF2 and FGF5 upregulation and could be a compatible endothelial cell type to achieve robust angiogenesis.

A pneumatic pressure-driven multi-throughput microfluidic circulation culture system

Here, we report a pneumatic pressure-driven microfluidic device capable of multi-throughput medium circulation culture. The circulation culture system has the following advantages for application in drug discovery: (i) simultaneous operation of multiple circulation units, (ii) use of a small amount of circulating medium (3.5 mL), (iii) pipette-friendly liquid handling, and (iv) a detachable interface with pneumatic pressure lines via sterile air-vent filters. The microfluidic device contains three independent circulation culture units, in which human umbilical vein endothelial cells (HUVECs) were cultured under physiological shear stress induced by circulation of the medium. Circulation of the medium in the three culture units was generated by programmed sequentially applied pressure from two pressure-control lines. HUVECs cultured in the microfluidic device were aligned under a one-way circulating flow with a shear stress of 10 dyn cm(-2); they exhibited a randomly ordered alignment under no shear stress and under reciprocating flow with a shear stress of 10 dyn cm(-2). We also observed 2.8- to 4.9-fold increases in expression of the mRNAs of endothelial nitric oxide synthase and thrombomodulin under one-way circulating flow with a shear stress of 10 dyn cm(-2) compared with conditions of no shear stress or reciprocating flow.

Dynamic monitoring of cellular metabolic activity in combination with live cell imaging

We present an automated analysis of the cellular dynamic metabolic activity in combination with live cell imaging, an essential factor for understanding the fundamental cellular physiological responses. For that purpose, we utilized the Intelligent Microplate Reader, a new analysis platform for marker-free cell-based assays in real-time. To demonstrate the benefit of the platform, we analyzed the relationship between various dynamic cell parameters (extracellular acidification, oxygen uptake, cell morphology, cell density and cell migration) of L929, a mouse fibroblast cell line, under the influence of sodium dodecyl sulfate. The dynamic kinetics of the monitored parameters are consistent and revealing much information about the activity occurring in the cells.

Microfluidic Organ/Body-on-a-Chip Devices at the Convergence of Biology and Microengineering

Recent advances in biomedical technologies are mostly related to the convergence of biology with microengineering. For instance, microfluidic devices are now commonly found in most research centers, clinics and hospitals, contributing to more accurate studies and therapies as powerful tools for drug delivery, monitoring of specific analytes, and medical diagnostics. Most remarkably, integration of cellularized constructs within microengineered platforms has enabled the recapitulation of the physiological and pathological conditions of complex tissues and organs. The so-called “organ-on-a-chip” technology, which represents a new avenue in the field of advanced in vitro models, with the potential to revolutionize current approaches to drug screening and toxicology studies. This review aims to highlight recent advances of microfluidic-based devices towards a body-on-a-chip concept, exploring their technology and broad applications in the biomedical field.

Flow-induced stress on adherent cells in microfluidic devices

Transduction of mechanical forces and chemical signals affect every cell in the human body. Fluid flow in systems such as the lymphatic or circulatory systems modulates not only cell morphology, but also gene expression patterns, extracellular matrix protein secretion and cell-cell and cell-matrix adhesions. Similar to the role of mechanical forces in adaptation of tissues, shear fluid flow orchestrates collective behaviours of adherent cells found at the interface between tissues and their fluidic environments. These behaviours range from alignment of endothelial cells in the direction of flow to stem cell lineage commitment. Therefore, it is important to characterize quantitatively fluid interface-dependent cell activity. Common macro-scale techniques, such as the parallel plate flow chamber and vertical-step flow methods that apply fluid-induced stress on adherent cells, offer standardization, repeatability and ease of operation. However, in order to achieve improved control over a cell's microenvironment, additional microscale-based techniques are needed. The use of microfluidics for this has been recognized, but its true potential has emerged only recently with the advent of hybrid systems, offering increased throughput, multicellular interactions, substrate functionalization on 3D geometries, and simultaneous control over chemical and mechanical stimulation. In this review, we discuss recent advances in microfluidic flow systems for adherent cells and elaborate on their suitability to mimic physiologic micromechanical environments subjected to fluid flow. We describe device design considerations in light of ongoing discoveries in mechanobiology and point to future trends of this promising technology.