Surface-treatment-induced three-dimensional capillary morphogenesis in a microfluidic platform

Surface modifications of COP-based microfluidic devices for improved immobilisation of hydrogel proteins: long-term 3D culture with contractile cell types and ischaemia model

The tissue microenvironment plays a crucial role in tissue homeostasis and disease progression. However, the in vitro simulation has been limited by the lack of adequate biomimetic models in the last decades. Thanks to the advent of microfluidic technology for cell culture applications, these complex microenvironments can be recreated by combining hydrogels, cells and microfluidic devices. Nevertheless, this advance has several limitations. When cultured in three-dimensional (3D) hydrogels inside microfluidic devices, contractile cells may exert forces that eventually collapse the 3D structure. Disrupting the compartmentalisation creates an obstacle to long-term or highly cell-concentrated assays, which are extremely relevant for multiple applications such as fibrosis or ischaemia. Therefore, we tested surface treatments on cyclic-olefin polymer-based microfluidic devices (COP-MD) to promote the immobilisation of collagen as a 3D matrix protein. Thus, we compared three surface treatments in COP devices for culturing human cardiac fibroblasts (HCF) embedded in collagen hydrogels. We determined the immobilisation efficiency of collagen hydrogel by quantifying the hydrogel transversal area within the devices at the studied time points. Altogether, our results indicated that surface modification with polyacrylic acid photografting (PAA-PG) of COP-MD is the most effective treatment to avoid the quick collapse of collagen hydrogels. As a proof-of-concept experiment, and taking advantage of the low-gas permeability properties of COP-MD, we studied the application of PAA-PG pre-treatment to generate a self-induced ischaemia model. Different necrotic core sizes were developed depending on initial HCF density seeding with no noticeable gel collapse. We conclude that PAA-PG allows long-term culture, gradient generation and necrotic core formation of contractile cell types such as myofibroblasts. This novel approach will pave the way for new relevant in vitro co-culture models where fibroblasts play a key role such as wound healing, tumour microenvironment and ischaemia within microfluidic devices.

Reversible adhesives with controlled wrinkling patterns for programmable integration and discharging

Switchable and minimally invasive tissue adhesives have great potential for medical applications. However, on-demand adherence to and detachment from tissue surfaces remain difficult. We fabricated a switchable hydrogel film adhesive by designing pattern-tunable wrinkles to control adhesion. When adhered to a substrate, the compressive stress generated from the bilayer system leads to self-similar wrinkling patterns at short and long wavelengths, regulating the interfacial adhesion. To verify the concept and explore its application, we established a random skin flap model, which is a crucial strategy for repairing severe or large-scale wounds. Our hydrogel adhesive provides sufficient adhesion for tissue sealing and promotes neovascularization at the first stage, and then gradually detaches from the tissue while a dynamic wrinkling pattern transition happens. The gel film can be progressively ejected out from the side margins after host-guest integration. Our findings provide insights into tunable bioadhesion by manipulating the wrinkling pattern transition.

The interface stiffness and topographic feature dictate interfacial invasiveness of cancer spheroids

During cancer metastasis, tumor cells likely navigate, in a collective manner, discrete tissue spaces comprising inherently heterogeneous extracellular matrix microstructures where interfaces may be frequently encountered. Studies have shown that cell migration modes can be determined by adaptation to mechanical/topographic cues from interfacial microenvironments. However, less attention has been paid to exploring the impact of interfacial mechnochemical attributes on invasive and metastatic behaviors of tumor aggregates. Here, we excogitated a collagen matrix-solid substrate interface platform to investigate the afore-stated interesting issue. Our data revealed that stiffer interfaces stimulated spheroid outgrowth by motivating detachment of single cells and boosting their motility and velocity. However, stronger interfacial adhesive strength between matrix and substrate led to the opposite outcomes. Besides, this interfacial parameter also affected the morphological switch between migration modes of the detached cells and their directionality. Mechanistically, myosin II-mediated cell contraction, compared to matrix metalloproteinases-driven collagen degradation, was shown to play a more crucial role in the invasive outgrowth of tumor spheroids in interfacial microenvironments. Thus, our findings highlight the importance of heterogeneous interfaces in addressing and combating cancer metastasis.

A Transwell-Based Vascularized Model to Investigate the Effect of Interstitial Flow on Vasculogenesis

Interstitial flow plays a significant role in vascular system development, mainly including angiogenesis and vasculogenesis. However, compared to angiogenesis, the effect of interstitial flow on vasculogenesis is less explored. Current in vitro models for investigating the effect of interstitial flow on vasculogenesis heavily rely on microfluidic chips, which require microfluidic expertise and facilities, and may not be accessible to biological labs. Here, we proposed a facile approach to building perfusable vascular networks through the self-assembly of endothelial cells in a modified transwell format and investigated the effect of interstitial flow on vasculogenesis. We found that the effect of interstitial flow on vasculogenesis was closely related to the existence of VEGF and fibroblasts in the developed model: (1) In the presence of fibroblasts, interstitial flow (within the range of 0.1-0.6 μm/s) facilitated the perfusability of the engineered vasculatures. Additional VEGF in the culture medium further worked synergically with interstitial flow to develop longer, wider, denser, and more perfusable vasculatures than static counterparts; (2) In the absence of fibroblasts, vasculatures underwent severe regression within 7 days under static conditions. However, interstitial flow greatly inhibited vessel regression and enhanced vascular perfusability and morphogenesis without the need for additional VEGF. These results revealed that the effect of interstitial flow might vary depending on the existence of VEGF and fibroblasts, and would provide some guidelines for constructing in vitro self-assembled vasculatures. The established transwell-based vascularized model provides a simple method to build perfusable vasculatures and could also be utilized for creating functional tissues in regenerative medicine.

Stepwise molecular mechanisms responsible for chemoresistance in bladder cancer cells

Chemotherapy resistance is an obstacle to cancer therapy and is considered a major cause of recurrence. Thus, understanding the mechanisms of chemoresistance is critical to improving the prognosis of patients. Here, we have established a stepwise gemcitabine-resistant T24 bladder cancer cell line to understand the molecular mechanisms of chemoresistance within cancer cells. The characteristics of the stepwise chemoresistance cell line were divided into 4 phases (parental, early, intermediate, and late phases). These four phase cells showed increasingly aggressive phenotypes in vitro and in vivo experiments with increasing phases and revealed the molecular properties of the biological process from parent cells to phased gemcitabine-resistant cell line (GRC). Taken together, through the analysis of gene expression profile data, we have characterized gene set of each phase indicating the response to anticancer drug treatment. Specifically, we identified a multigene signature (23 genes including GATA3, APOBEC3G, NT5E, MYC, STC1, FOXD1, SMAD9) and developed a chemoresistance score consisting of that could predict eventual responsiveness to gemcitabine treatment. Our data will contribute to predicting chemoresistance and improving the prognosis of bladder cancer patients.

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.

Methods for Forming Human Lymphatic Microvessels In Vitro and Assessing their Drainage Function

This chapter describes methods to engineer human lymphatic microvessels in vitro and to assess their fluid and solute drainage capacities. The lymphatics are formed within micropatterned type I collagen gels that contain a blind-ended channel for the growth of lymphatic endothelial cells. Because the vessels have one blind end and one open end each, they mimic the terminal structure of the native lymphatic microvascular tree. The solute drainage rates that are measured from the engineered lymphatics in vitro can be directly compared with published results from intact vessels in vivo. Practical considerations to increase the accuracy of the drainage assays are discussed.

Stem cell-based vascularization of microphysiological systems

Microphysiological systems (MPSs) (i.e., tissue or organ chips) exploit microfluidics and 3D cell culture to mimic tissue and organ-level physiology. The advent of human induced pluripotent stem cell (hiPSC) technology has accelerated the use of MPSs to study human disease in a range of organ systems. However, in the reduction of system complexity, the intricacies of vasculature are an often-overlooked aspect of MPS design. The growing library of pluripotent stem cell-derived endothelial cell and perivascular cell protocols have great potential to improve the physiological relevance of vasculature within MPS, specifically for in vitro disease modeling. Three strategic categories of vascular MPS are outlined: self-assembled, interface focused, and 3D biofabricated. This review discusses key features and development of the native vasculature, linking that to how hiPSC-derived vascular cells have been generated, the state of the art in vascular MPSs, and opportunities arising from interdisciplinary thinking.

Engineering new microvascular networks on-chip: ingredients, assembly, and best practices

Tissue engineered grafts show great potential as regenerative implants for diseased or injured tissues within the human body. However, these grafts suffer from poor nutrient perfusion and waste transport, thus decreasing their viability post-transplantation. Graft vascularization is therefore a major area of focus within tissue engineering because biologically relevant conduits for nutrient and oxygen perfusion can improve viability post-implantation. Many researchers utilize microphysiological systems as testing platforms for potential grafts due to an ability to integrate vascular networks as well as biological characteristics such as fluid perfusion, 3D architecture, compartmentalization of tissue-specific materials, and biophysical and biochemical cues. While many methods of vascularizing these systems exist, microvascular self-assembly has great potential for bench-to-clinic translation as it relies on naturally occurring physiological events. In this review, we highlight the past decade of literature and critically discuss the most important and tunable components yielding a self-assembled vascular network on chip: endothelial cell source, tissue-specific supporting cells, biomaterial scaffolds, biochemical cues, and biophysical forces. This article discusses the bioengineered systems of angiogenesis, vasculogenesis, and lymphangiogenesis, and includes a brief overview of multicellular systems. We conclude with future avenues of research to guide the next generation of vascularized microfluidic models and future tissue engineered grafts.

Hydrogels as Drug Delivery Systems: A Review of Current Characterization and Evaluation Techniques

Owing to their tunable properties, controllable degradation, and ability to protect labile drugs, hydrogels are increasingly investigated as local drug delivery systems. However, a lack of standardized methodologies used to characterize and evaluate drug release poses significant difficulties when comparing findings from different investigations, preventing an accurate assessment of systems. Here, we review the commonly used analytical techniques for drug detection and quantification from hydrogel delivery systems. The experimental conditions of drug release in saline solutions and their impact are discussed, along with the main mathematical and statistical approaches to characterize drug release profiles. We also review methods to determine drug diffusion coefficients and in vitro and in vivo models used to assess drug release and efficacy with the goal to provide guidelines and harmonized practices when investigating novel hydrogel drug delivery systems.

Interstitial Hypertension Suppresses Escape of Human Breast Tumor Cells Via Convection of Interstitial Fluid

Introduction

Interstitial hypertension, a rise in interstitial fluid pressure, is a common feature of many solid tumors as they progress to an invasive state. It is currently unclear whether this elevated pressure alters the probability that tumor cells eventually escape into a neighboring blood or lymphatic vessel.

Methods

In this study, we analyze the escape of MDA-MB-231 human breast tumor cells from a ~3-mm-long preformed aggregate into a 120-μm-diameter empty cavity in a micromolded type I collagen gel. The "micro-tumors" were located within ~300 μm of one or two cavities. Pressures of ~0.65 cm H₂O were applied only to the tumor ("interstitial hypertension") or to its adjacent cavity.

Results

This work shows that interstitial hypertension suppresses escape into the adjacent cavity, but not because tumor cells respond directly to the pressure profile. Instead, hypertension alters the chemical microenvironment at the tumor margin to one that hampers escape. Administration of tumor interstitial fluid phenocopies the effects of hypertension.

Conclusions

This work uncovers a link between tumor pressure, interstitial flow, and tumor cell escape in MDA-MB-231 cells, and suggests that interstitial hypertension serves to hinder further progression to metastatic escape.

Electronic Supplementary Material

The online version of this article (10.1007/s12195-020-00661-w) contains supplementary material, which is available to authorized users.

Microfluidics for Angiogenesis Research

Angiogenesis is a natural and vital phenomenon of neovascularization that occurs from pre-existing vasculature, being present in many physiological processes, namely in development, reproduction and regeneration. Being a highly dynamic and tightly regulated process, its abnormal expression can be on the basis of several pathologies. For that reason, angiogenesis has been a subject of major interest among the scientific community, being transverse to different areas and founding particular attention in tissue engineering and cancer research fields. Microfluidics has emerged as a powerful tool for modelling this phenomenon, thereby surpassing the limitations associated to conventional angiogenic models. Holding a tremendous flexibility in terms of experimental design towards a specific goal, microfluidic systems can offer an unlimited number of opportunities for investigating angiogenesis in many relevant scenarios, namely from its fundamental comprehension in normal physiological processes to the identification and testing of new therapeutic targets involved on pathological angiogenesis. Additionally, microvascular 3D in vitro models are now opening up new prospects in different fields, being used for investigating and establishing guidelines for the development of next generation of 3D functional vascularized grafts. The promising applications of this emerging technology in angiogenesis studies are herein overviewed, encompassing fundamental and applied research.

Strategies for elevating hematopoietic stem cells expansion and engraftment capacity

Hematopoietic stem cells (HSCs) are a rare cell population in adult bone marrow, mobilized peripheral blood, and umbilical cord blood possessing self-renewal and differentiation capability into a full spectrum of blood cells. Bone marrow HSC transplantation has been considered as an ideal option for certain disorders treatment including hematologic diseases, leukemia, immunodeficiency, bone marrow failure syndrome, genetic defects such as thalassemia, sickle cell anemia, autoimmune disease, and certain solid cancers. Ex vivo proliferation of these cells prior to transplantation has been proposed as a potential solution against limited number of stem cells. In such culture process, MSCs have also been shown to exhibit high capacity for secretion of soluble mediators contributing to the principle biological and therapeutic activities of HSCs. In addition, endothelial cells have been introduced to bridge the blood and sub tissues in the bone marrow, as well as, HSCs regeneration induction and survival. Cell culture in the laboratory environment requires cell growth strict control to protect against contamination, symmetrical cell division and optimal conditions for maximum yield. In this regard, microfluidic systems provide culture and analysis capabilities in micro volume scales. Moreover, two-dimensional cultures cannot fully demonstrate extracellular matrix found in different tissues and organs as an abstract representation of three dimensional cell structure. Microfluidic systems can also strongly describe the effects of physical factors such as temperature and pressure on cell behavior.

Tissue Engineering of the Microvasculature

The ability to generate new microvessels in desired numbers and at desired locations has been a long-sought goal in vascular medicine, engineering, and biology. Historically, the need to revascularize ischemic tissues nonsurgically (so-called therapeutic vascularization) served as the main driving force for the development of new methods of vascular growth. More recently, vascularization of engineered tissues and the generation of vascularized microphysiological systems have provided additional targets for these methods, and have required adaptation of therapeutic vascularization to biomaterial scaffolds and to microscale devices. Three complementary strategies have been investigated to engineer microvasculature: angiogenesis (the sprouting of existing vessels), vasculogenesis (the coalescence of adult or progenitor cells into vessels), and microfluidics (the vascularization of scaffolds that possess the open geometry of microvascular networks). Over the past several decades, vascularization techniques have grown tremendously in sophistication, from the crude implantation of arteries into myocardial tunnels by Vineberg in the 1940s, to the current use of micropatterning techniques to control the exact shape and placement of vessels within a scaffold. This review provides a broad historical view of methods to engineer the microvasculature, and offers a common framework for organizing and analyzing the numerous studies in this area of tissue engineering and regenerative medicine. © 2019 American Physiological Society. Compr Physiol 9:1155-1212, 2019.

Reconstruction of Hepatic Tissue Structures Using Interstitial Flow in a Microfluidic Device

Construction of three-dimensional (3D) hepatic tissue structures is important for in vitro tissue engineering of the liver, because 3D culture of hepatocytes is critical for the maintenance of liver-specific functions. Although conventional 3D culture methods are useful for constructing 3D hepatic tissue structures, the precise control of culture microenvironments is required to construct more physiological tissues in vitro. Recent advances in microfluidics technologies have allowed us to utilize microfluidic devices for hepatic cell culture, which opened the door for creating more physiological 3D culture models of the liver. Here, we describe the method for the construction of hepatic tissue structures using a microfluidic device which has a 3D gel region with adjacent microchannels. Primary rat hepatocytes are seeded into a microchannel in a microfluidic device. The cells are then cultured in interstitial flow conditions, which leads to the construction of 3D tissue structures.

A 3D microvascular network model to study the impact of hypoxia on the extravasation potential of breast cell lines

Hypoxia is a common feature of the tumor microenvironment. Accumulating evidence has demonstrated hypoxia to be an important trigger of tumor cell invasion or metastasizes via hypoxia-signaling cascades, including hypoxia-inducible factors (HIFs). Microfluidic model can be a reliable in vitro tool for systematically interrogating individual factors and their accompanying downstream effects, which may otherwise be difficult to study in complex tumor tissues. Here, we used an in vitro model of microvascular networks in a microfluidic chip to measure the extravasation potential of breast cell lines subjected to different oxygen conditions. Through the use of HIF-1α knock-down cell lines, we also validated the importance of HIF-1α in the transmigration ability of human breast cell lines. Three human breast cell lines derived from human breast tissues (MCF10A, MCF-7 and MDA-MB-231) were used in this study to evaluate the role of hypoxia in promoting metastasis at different stages of cancer progression. Under hypoxic conditions, HIF-1α protein level was increased, and coincided with changes in cell morphology, viability and an elevated metastatic potential. These changes were accompanied by an increase in the rate of extravasation compared to normoxia (21% O₂). siRNA knockdown of HIF-1α in hypoxic tumors significantly decreased the extravasation rates of all the cell lines tested and may have an effect on the function of metastatic and apoptotic-related cellular processes.

Construction of Continuous Capillary Networks Stabilized by Pericyte-like Perivascular Cells

IMPACT STATEMENT

Construction of capillary networks is a fundamental challenge for the development of three-dimensional (3D) tissue engineering. However, it is not well understood how to construct stable capillary networks that maintain a luminal size similar to that of capillary structures in vivo (i.e., <10 μm diameter). In this study, we demonstrated the construction of stable capillary networks covered by pericyte-like perivascular cells using an in vitro 3D angiogenesis model by optimizing interactions between endothelial cells and perivascular cells. Our 3D angiogenesis model can be combined with 3D culture of epithelial cells in the context of vascularization of 3D tissue-engineered constructs.

Wet-AMD on a Chip: Modeling Outer Blood-Retinal Barrier In Vitro

Choroidal neovascularization (CNV) in the retinal pigment epithelium (RPE)-choroid complex constituting outer blood retinal barrier (oBRB) is a critical pathological step in various ophthalmic diseases, which results in blindness, such as wet type age-related macula degeneration. Current in vitro experimental models using petri dishes or transwell are unable to study CNV morphogenesis. Here, a unique organotypic eye-on-a-chip model is described that mimics the RPE-choroid complex in vitro. This model consists of an RPE monolayer and adjacent perfusable blood vessel network, which is supporting barrier function of oBRB. The intact barrier function of the RPE-choroid complex is reconstituted while maintaining important structural features. Further, this model can successfully mimic the pathogenesis of CNV especially in terms of morphogenesis, which is penetrating angiogenic sprouts from pre-existing choroidal vessels that result in breakdown of RPE monolayer. The alleviation of the pathological angiogenesis can be modeled with bevacizumab, a clinical drug for CNV treatment. It is believed that this model can be used to aid in the development of advanced in vitro eye drug evaluation in conjunction with animal models.

A tuneable microfluidic system for long duration chemotaxis experiments in a 3D collagen matrix

In many cell types, migration can be oriented towards a chemical stimulus. In mammals, for example, embryonic cells migrate to follow developmental cues, immune cells migrate toward sites of inflammation, and cancer cells migrate away from the primary tumour and toward blood vessels during metastasis. Understanding how cells migrate in 3D environments in response to chemical cues is thus crucial to understanding directed migration in normal and disease states. To date, chemotaxis in mammalian cells has been primarily studied using 2D migration models. However, it is becoming increasingly clear that the mechanisms by which cells migrate in 2D and 3D environments dramatically differ, and cells in their native environments are confronted with a complex chemical milieu. To address these issues, we developed a microfluidic device to monitor the behaviour of cells embedded in a 3D collagen matrix in the presence of complex concentration fields of chemoattractants. This tuneable microsystem enables the generation of (1) homogeneous, stationary gradients set by a purely diffusive mechanism, or (2) spatially evolving, stationary gradients, set by a convection-diffusion mechanism. The device allows for stable gradients over several days and is large enough to study the behaviour of large cell aggregates. We observe that primary mature dendritic cells respond uniformly to homogeneous diffusion gradients, while cell behaviour is highly position-dependent in spatially variable convection-diffusion gradients. In addition, we demonstrate a directed response of cancer cells migrating away from tumour-like aggregates in the presence of soluble chemokine gradients. Together, this microfluidic device is a powerful system to observe the response of different cells and aggregates to tuneable chemical gradients.

Directional migration of mesenchymal stem cells under an SDF-1α gradient on a microfluidic device

Homing of peripheral stem cells is regulated by one of the most representative homing factors, stromal cell-derived factor 1 alpha (SDF-1α), which specifically binds to the plasma membrane receptor CXCR4 of mesenchymal stem cells (MSCs) in order to initiate the signaling pathways that lead to directional migration and homing of stem cells. This complex homing process and directional migration of stem cells have been mimicked on a microfluidic device that is capable of generating a chemokine gradient within the collagen matrix and embedding endothelial cell (EC) monolayers to mimic blood vessels. On the microfluidic device, stem cells showed directional migration toward the higher concentration of SDF-1α, whereas treatment with the CXCR4 antagonist AMD3100 caused loss of directionality of stem cells. Furthermore, inhibition of stem cell's main migratory signaling pathways, Rho-ROCK and Rac pathways, caused blockage of actomyosin and lamellipodia formation, decreasing the migration distance but maintaining directionality. Stem cell homing regulated by SDF-1α caused directional migration of stem cells, while the migratory ability was affected by the activation of migration-related signaling pathways.

A perforated microhole-based microfluidic device for improving sprouting angiogenesis in vitro

Microfluidic technology is an important research tool for investigating angiogenesis in vitro. Here, we fabricated a polydimethylsiloxane (PDMS) microfluidic device with five cross-shaped chambers using a coverslip molding method. Then, the perforated PDMS microhole arrays prepared by soft lithography were assembled in the device as barriers; a single microhole had a diameter of 100 μm. After injecting type I collagen into the middle gel chamber, we added a culture medium containing a vascular endothelial growth factor (VEGF) into the middle chamber. It would generate a linear concentration gradient of VEGF across the gel region from the middle chamber to the four peripheral chambers. Human umbilical vein endothelial cells (HUVECs) were then seeded on the microhole barrier. With VEGF stimulation, cells migrated along the inner walls of the microholes, formed annularly distributed cell clusters at the gel-barrier interface, and then three-dimensionally (3D) sprouted into the collagen scaffold. After 4 days of culture, we quantitatively analyzed the sprouting morphogenesis. HUVECs cultured on the microhole barrier had longer sprouts than HUVECs cultured without the barrier (controls). Furthermore, the initial distribution of sprouts was more regular and more connections of tube-like structures were generated when the microhole barrier was used. This study introduces a novel microfluidic device containing both microtopographic structures and 3D collagen. HUVECs cultured with the microhole barrier could form well-interconnected tube-like structures and are thus an ideal in vitro angiogenesis model.

Micro-engineered perfusable 3D vasculatures for cardiovascular diseases

Vessel geometries in microengineered in vitro vascular models are important to recapitulate a pathophysiological microenvironment for the study of flow-induced endothelial dysfunction and inflammation in cardiovascular diseases. Herein, we present a simple and novel extracellular matrix (ECM) hydrogel patterning method to create perfusable vascularized microchannels of different geometries based on the concept of capillary burst valve (CBV). No surface modification is necessary and the method is suitable for different ECM types including collagen, matrigel and fibrin. We first created collagen-patterned, endothelialized microchannels to study barrier permeability and neutrophil transendothelial migration, followed by the development of a biomimetic 3D endothelial-smooth muscle cell (EC-SMC) vascular model. We observed a significant decrease in barrier permeability in the co-culture model during inflammation, which indicates the importance of perivascular cells in ECM remodeling. Finally, we engineered collagen-patterned constricted vascular microchannels to mimic stenosis in atherosclerosis. Whole blood was perfused (1-10 dyne cm^-2) into the microdevices and distinct platelet and leukocyte adherence patterns were observed due to increased shear stresses at the constriction, and an additional convective flow through the collagen. Taken together, the developed hydrogel patterning technique enables the formation of unique pathophysiological architectures in organ-on-chip microsystems for real-time study of hemodynamics and cellular interactions in cardiovascular diseases.

Beyond organoids: In vitro vasculogenesis and angiogenesis using cells from mammals and zebrafish

The ability to culture complex organs is currently an important goal in biomedical research. It is possible to grow organoids (3D organ-like structures) in vitro; however, a major limitation of organoids, and other 3D culture systems, is the lack of a vascular network. Protocols developed for establishing in vitro vascular networks typically use human or rodent cells. A major technical challenge is the culture of functional (perfused) networks. In this rapidly advancing field, some microfluidic devices are now getting close to the goal of an artificially perfused vascular network. Another development is the emergence of the zebrafish as a complementary model to mammals. In this review, we discuss the culture of endothelial cells and vascular networks from mammalian cells, and examine the prospects for using zebrafish cells for this objective. We also look into the future and consider how vascular networks in vitro might be successfully perfused using microfluidic technology.

Construction of stable capillary networks using a microfluidic device

Construction of stable capillary networks is required to provide sufficient oxygen and nutrients to the deep region of thick tissues, which is important in the context of 3D tissue engineering. Although conventional in vitro culture models have been used to investigate the mechanism of capillary formation, recent advances in microfluidics technologies allowed us to control biophysical and biochemical culture environments more precisely, which led to the construction of functional and stable capillary networks. In this study, endothelial cells and mesenchymal stem cells were co-cultured in microfluidic devices to construct stable capillary networks, which resulted in the construction of luminal structures covered by pericytes. Interactions between endothelial cells and mesenchymal stem cells are also discussed in the context of capillary formation.

In vitro nasal mucosa gland-like structure formation on a chip

The emergence of microfluidic epithelial models using diverse types of cells within a physiologically relevant microenvironment has the potential to be a powerful tool for preclinical drug screening and pathophysiological studies. However, to date, few studies have reported the development of a complicated in vitro human nasal epithelial model. The aim of this study was to produce an in vitro human nasal mucosa model for reliable drug screening and clinical applications. Here, we integrated and optimized several culture conditions such as cell type, airway culture conditions, and hydrogel scaffolds into a microfluidic chip to construct an advanced in vitro human nasal mucosa model. We observed that the inducing factors for nasal gland-like structures were secreted from activated human dermal microvascular endothelial cells. Furthermore, our in vitro nasal mucosa presented different appearance and characteristics under hypoxic conditions. Morphological and functional similarities between in vivo nasal mucosa and our model indicated its utilization as a reliable research model for nasal diseases including allergic rhinitis, chronic sinusitis, and nasal polyposis.

Generating Multicompartmental 3D Biological Constructs Interfaced through Sequential Injections in Microfluidic Devices

A novel technique is presented for molding and culturing composite 3D cellular constructs within microfluidic channels. The method is based on the use of removable molding polydimethylsiloxane (PDMS) inserts, which allow to selectively and incrementally generate composite 3D constructs featuring different cell types and/or biomaterials, with a high spatial control. The authors generate constructs made of either stacked hydrogels, with uniform horizontal interfaces, or flanked hydrogels with vertical interfaces. The authors also show how this technique can be employed to create custom-shaped endothelial barriers and monolayers directly interfaced with 3D cellular constructs. This method dramatically improves the significance of in vitro 3D biological models, enhancing mimicry and enabling for controlled studies of complex biological districts.

Integrating perfusable vascular networks with a three-dimensional tissue in a microfluidic device

Creating vascular networks in tissues is crucial for tissue engineering.

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.

Angiogenic Type I Collagen Extracellular Matrix Integrated with Recombinant Bacteriophages Displaying Vascular Endothelial Growth Factors

Here, a growth-factor-integrated natural extracellular matrix of type I collagen is presented that induces angiogenesis. The developed matrix adapts type I collagen nanofibers integrated with synthetic colloidal particles of recombinant bacteriophages that display vascular endothelial growth factor (VEGF). The integration is achieved during or after gelation of the type I collagen and the matrix enables spatial delivery of VEGF into a desired region. Endothelial cells that contact the VEGF are found to invade into the matrix to form tube-like structures both in vitro and in vivo, proving the angiogenic potential of the matrix.

Constructive remodeling of a synthetic endothelial extracellular matrix

The construction of well-controllable in vitro models of physiological and pathological vascular endothelium remains a fundamental challenge in tissue engineering and drug development. Here, we present an approach for forming a synthetic endothelial extracellular matrix (ECM) that closely resembles that of the native structure by locally depositing basement membrane materials onto type 1 collagen nanofibers only in a region adjacent to the endothelial cell (EC) monolayer. Culturing the EC monolayer on this synthetic endothelial ECM remarkably enhanced its physiological properties, reducing its vascular permeability, and promoting a stabilized, quiescent phenotype. We demonstrated that the EC monolayer on the synthetic endothelial ECM neither creates non-physiological barriers to cell-cell or cell-ECM interactions, nor hinders molecular diffusion of growth factors and other molecules. The synthetic endothelial ECM and vascular endothelium on it may help us enter in a new phase of research in which various models of the biological barrier behavior can be tested experimentally.

Recapitulating the Tumor Ecosystem Along the Metastatic Cascade Using 3D Culture Models

Advances in cancer research have shown that a tumor can be likened to a foreign species that disrupts delicately balanced ecological interactions, compromising the survival of normal tissue ecosystems. In efforts to mitigate tumor expansion and metastasis, experimental approaches from ecology are becoming more frequently and successfully applied by researchers from diverse disciplines to reverse engineer and re-engineer biological systems in order to normalize the tumor ecosystem. We present a review on the use of 3D biomimetic platforms to recapitulate biotic and abiotic components of the tumor ecosystem, in efforts to delineate the underlying mechanisms that drive evolution of tumor heterogeneity, tumor dissemination, and acquisition of drug resistance.

A quantitative microfluidic angiogenesis screen for studying anti-angiogenic therapeutic drugs

Anti-angiogenic therapy, which suppresses tumor growth by disrupting oxygen and nutrient supply from blood to the tumor, is now widely accepted as a treatment for cancer. To investigate the mechanisms of action of these anti-angiogenesis drugs, new three dimensional (3D) cell culture-based drug screening models are increasingly employed. However, there is no in vitro high-throughput screening (HTS) angiogenesis assay that can provide uniform culture conditions for the quantitative assessment of physiological responses to chemoattractant reagents under various concentrations of anti-angiogenesis drugs. Here we describe a method for screening and quantifying the vascular endothelial growth factor (VEGF)-induced chemotactic response on human umbilical vein endothelial cells (HUVECs) cultured with different concentrations of bortezomib, a selective 26S proteasome inhibitor. With this quantitative microfluidic angiogenesis screen (QMAS), we demonstrate that bortezomib-induced endothelial cell death is preceded by a series of morphological changes that develop over several days. We also explore the mechanisms by which bortezomib can inhibit angiogenesis.

Optimizing design and fabrication of microfluidic devices for cell cultures: An effective approach to control cell microenvironment in three dimensions

The effects of gradients of bioactive molecules on the cell microenvironment are crucial in several biological processes, such as chemotaxis, angiogenesis, and tumor progression. The elucidation of the basic mechanisms regulating cell responses to gradients requires a tight control of the spatio-temporal features of such gradients. Microfluidics integrating 3D gels are useful tools to fulfill this requirement. However, even tiny flaws in the design or in the fabrication process may severely impair microenvironmental control, thus leading to inconsistent results. Here, we report a sequence of actions aimed at the design and fabrication of a reliable and robust microfluidic device integrated with collagen gel for cell culturing in 3D, subjected to a predetermined gradient of biomolecular signals. In particular, we developed a simple and effective solution to the frequently occurring technical problems of gas bubble formation and 3D matrix collapsing or detaching from the walls. The device here proposed, in Polydimethylsiloxane, was designed to improve the stability of the cell-laden hydrogel, where bubble deprived conditioning media flow laterally to the gel. We report the correct procedure to fill the device with the cell populated gel avoiding the entrapment of gas bubbles, yet maintaining cell viability. Numerical simulations and experiments with fluorescent probes demonstrated the establishment and stability of a concentration gradient across the gel. Finally, chemotaxis experiments of human Mesenchymal Stem Cells under the effects of Bone Morphogenetic Protein-2 gradients were performed in order to demonstrate the efficacy of the system in controlling cell microenvironment. The proposed procedure is sufficiently versatile and simple to be used also for different device geometries or experimental setups.

Microfluidics for nano-pathophysiology

Nanotechnology-based drug delivery systems hold promise for innovative medical treatment of cancers. While drug materials are constantly under development, there are no practical cell-based models to assess whether these materials can reach the target tissue. Recently developed microfluidic systems have revolutionized cell-based experiments. In these systems, vascular endothelial cells and interstitium are set in microchannels that mimic microvessels. Drug permeability can be assayed in these blood vessel models under fluidic conditions that mimic blood flow. In this review, we describe device fabrication, disease model development, nanoparticle permeability assays, and the potential utility of these systems in the future.

Strategies affording prevascularized cell-based constructs for myocardial tissue engineering

The production of a functional cardiac tissue to be transplanted in the injured area of the infarcted myocardium represents a challenge for regenerative medicine. Most cell-based grafts are unviable because of inadequate perfusion; therefore, prevascularization might be a suitable approach for myocardial tissue engineering. To this aim, cells with a differentiation potential towards vascular and cardiac muscle phenotypes have been cocultured in 2D or 3D appropriate scaffolds. In addition to these basic approaches, more sophisticated strategies have been followed employing mixed-cell sheets, microvascular modules, and inosculation from vascular explants. Technologies exerting spatial control of vascular cells, such as topographical surface roughening and ordered patterning, represent other ways to drive scaffold vascularization. Finally, microfluidic devices and bioreactors exerting mechanical stress have also been employed for high-throughput scaling-up production in order to accelerate muscle differentiation and speeding the endothelialization process. Future research should address issues such as how to optimize cells, biomaterials, and biochemical components to improve the vascular integration of the construct within the cardiac wall, satisfying the metabolic and functional needs of the myocardial tissue.

Physiologically relevant organs on chips

Recent advances in integrating microengineering and tissue engineering have generated promising microengineered physiological models for experimental medicine and pharmaceutical research. Here we review the recent development of microengineered physiological systems, or also known as "ogans-on-chips", that reconstitute the physiologically critical features of specific human tissues and organs and their interactions. This technology uses microengineering approaches to construct organ-specific microenvironments, reconstituting tissue structures, tissue-tissue interactions and interfaces, and dynamic mechanical and biochemical stimuli found in specific organs, to direct cells to assemble into functional tissues. We first discuss microengineering approaches to reproduce the key elements of physiologically important, dynamic mechanical microenvironments, biochemical microenvironments, and microarchitectures of specific tissues and organs in microfluidic cell culture systems. This is followed by examples of microengineered individual organ models that incorporate the key elements of physiological microenvironments into single microfluidic cell culture systems to reproduce organ-level functions. Finally, microengineered multiple organ systems that simulate multiple organ interactions to better represent human physiology, including human responses to drugs, is covered in this review. This emerging organs-on-chips technology has the potential to become an alternative to 2D and 3D cell culture and animal models for experimental medicine, human disease modeling, drug development, and toxicology.

Regeneration-on-a-chip? The perspectives on use of microfluidics in regenerative medicine

The aim of regenerative medicine is to restore or establish normal function of damaged tissues or organs. Tremendous efforts are placed into development of novel regenerative strategies, involving (stem) cells, soluble factors, biomaterials or combinations thereof, as a result of the growing need caused by continuous population aging. To satisfy this need, fast and reliable assessment of (biological) performance is sought, not only to select the potentially interesting candidates, but also to rule out poor ones at an early stage of development. Microfluidics may provide a new avenue to accelerate research and development in the field of regenerative medicine as it has proven its maturity for the realization of high-throughput screening platforms. In addition, microfluidic systems offer other advantages such as the possibility to create in vivo-like microenvironments. Besides the complexity of organs or tissues that need to be regenerated, regenerative medicine brings additional challenges of complex regeneration processes and strategies. The question therefore arises whether so much complexity can be integrated into microfluidic systems without compromising reliability and throughput of assays. With this review, we aim to investigate whether microfluidics can become widely applied in regenerative medicine research and/or strategies.

Vascular endothelial growth factor (VEGF) and platelet (PF-4) factor 4 inputs modulate human microvascular endothelial signaling in a three-dimensional matrix migration context

The process of angiogenesis is under complex regulation in adult organisms, particularly as it often occurs in an inflammatory post-wound environment. As such, there are many impacting factors that will regulate the generation of new blood vessels which include not only pro-angiogenic growth factors such as vascular endothelial growth factor, but also angiostatic factors. During initial postwound hemostasis, a large initial bolus of platelet factor 4 is released into localized areas of damage before progression of wound healing toward tissue homeostasis. Because of its early presence and high concentration, the angiostatic chemokine platelet factor 4, which can induce endothelial anoikis, can strongly affect angiogenesis. In our work, we explored signaling crosstalk interactions between vascular endothelial growth factor and platelet factor 4 using phosphotyrosine-enriched mass spectrometry methods on human dermal microvascular endothelial cells cultured under conditions facilitating migratory sprouting into collagen gel matrices. We developed new methods to enable mass spectrometry-based phosphorylation analysis of primary cells cultured on collagen gels, and quantified signaling pathways over the first 48 h of treatment with vascular endothelial growth factor in the presence or absence of platelet factor 4. By observing early and late signaling dynamics in tandem with correlation network modeling, we found that platelet factor 4 has significant crosstalk with vascular endothelial growth factor by modulating cell migration and polarization pathways, centered around P38α MAPK, Src family kinases Fyn and Lyn, along with FAK. Interestingly, we found EphA2 correlational topology to strongly involve key migration-related signaling nodes after introduction of platelet factor 4, indicating an influence of the angiostatic factor on this ambiguous but generally angiogenic signal in this complex environment.

A microfluidic platform for generating large-scale nearly identical human microphysiological vascularized tissue arrays

This paper reports a polydimethylsiloxane microfluidic model system that can develop an array of nearly identical human microtissues with interconnected vascular networks. The microfluidic system design is based on an analogy with an electric circuit, applying resistive circuit concepts to design pressure dividers in serially-connected microtissue chambers. A long microchannel (550, 620 and 775 mm) creates a resistive circuit with a large hydraulic resistance. Two media reservoirs with a large cross-sectional area and of different heights are connected to the entrance and exit of the long microchannel to serve as a pressure source, and create a near constant pressure drop along the long microchannel. Microtissue chambers (0.12 μl) serve as a two-terminal resistive component with an input impedance >50-fold larger than the long microchannel. Connecting each microtissue chamber to two different positions along the long microchannel creates a series of pressure dividers. Each microtissue chamber enables a controlled pressure drop of a segment of the microchannel without altering the hydrodynamic behaviour of the microchannel. The result is a controlled and predictable microphysiological environment within the microchamber. Interstitial flow, a mechanical cue for stimulating vasculogenesis, was verified by finite element simulation and experiments. The simplicity of this design enabled the development of multiple microtissue arrays (5, 12, and 30 microtissues) by co-culturing endothelial cells, stromal cells, and fibrin within the microchambers over two and three week periods. This methodology enables the culturing of a large array of microtissues with interconnected vascular networks for biological studies and applications such as drug development.

Pharmacology on microfluidics: multimodal analysis for studying cell-cell interaction

Understanding the mechanisms of cell-cell interaction is a key unanswered question in modern pharmacology, given crosstalk defects are at the basis of many pathologies. Microfluidics represents a valuable tool for analyzing intercellular communication mediated by transmission of soluble signals, as occurring for example between neurons and glial cells in neuroinflammation, or between tumor and surrounding cells in cancer. However, the use of microfluidics for studying cell behavior still encompasses many technical and biological challenges. In this review, a state of the art of successes, potentials and limitations of microfluidics applied to key biological questions in modern pharmacology is analyzed and commented.

Screening therapeutic EMT blocking agents in a three-dimensional microenvironment

Epithelial-mesenchymal transition (EMT) plays a critical role in the early stages of dissemination of carcinoma leading to metastatic tumors, which are responsible for over 90% of all cancer-related deaths. Current therapeutic regimens, however, have been ineffective in the cure of metastatic cancer, thus an urgent need exists to revisit existing protocols and to improve the efficacy of newly developed therapeutics. Strategies based on preventing EMT could potentially contribute to improving the outcome of advanced stage cancers. To achieve this goal new assays are needed to identify targeted drugs capable of interfering with EMT or to revert the mesenchymal-like phenotype of carcinoma to an epithelial-like state. Current assays are limited to examining the dispersion of carcinoma cells in isolation in conventional 2-dimensional (2D) microwell systems, an approach that fails to account for the 3-dimensional (3D) environment of the tumor or the essential interactions that occur with other nearby cell types in the tumor microenvironment. Here we present a microfluidic system that integrates tumor cell spheroids in a 3D hydrogel scaffold, in close co-culture with an endothelial monolayer. Drug candidates inhibiting receptor activation or signal transduction pathways implicated in EMT have been tested using dispersion of A549 lung adenocarcinoma cell spheroids as a metric of effectiveness. We demonstrate significant differences in response to drugs between 2D and 3D, and between monoculture and co-culture.

Extracellular matrix heterogeneity regulates three-dimensional morphologies of breast adenocarcinoma cell invasion

Plasticity and reciprocity of breast cancer cells to various extracellular matrice (ECMs) are three-dimensionally analyzed in quantitative way in a novel and powerful microfluidic in vitro platform. This successfully demonstrates the metastatic potential of cancer cells and their effective strategies of ECM proteolytic remodeling and morphological change, while interacting with other cells and invading into heterogeneous ECMs.

In vitro model of tumor cell extravasation

Tumor cells that disseminate from the primary tumor and survive the vascular system can eventually extravasate across the endothelium to metastasize at a secondary site. In this study, we developed a microfluidic system to mimic tumor cell extravasation where cancer cells can transmigrate across an endothelial monolayer into a hydrogel that models the extracellular space. The experimental protocol is optimized to ensure the formation of an intact endothelium prior to the introduction of tumor cells and also to observe tumor cell extravasation by having a suitable tumor seeding density. Extravasation is observed for 38.8% of the tumor cells in contact with the endothelium within 1 day after their introduction. Permeability of the EC monolayer as measured by the diffusion of fluorescently-labeled dextran across the monolayer increased 3.8 fold 24 hours after introducing tumor cells, suggesting that the presence of tumor cells increases endothelial permeability. The percent of tumor cells extravasated remained nearly constant from1 to 3 days after tumor seeding, indicating extravasation in our system generally occurs within the first 24 hours of tumor cell contact with the endothelium.

Microfluidics for manipulating cells

Microfluidics, a toolbox comprising methods for precise manipulation of fluids at small length scales (micrometers to millimeters), has become useful for manipulating cells. Its uses range from dynamic management of cellular interactions to high-throughput screening of cells, and to precise analysis of chemical contents in single cells. Microfluidics demonstrates a completely new perspective and an excellent practical way to manipulate cells for solving various needs in biology and medicine. This review introduces and comments on recent achievements and challenges of using microfluidics to manipulate and analyze cells. It is believed that microfluidics will assume an even greater role in the mechanistic understanding of cell biology and, eventually, in clinical applications.