Investigation of Tumor Cell Behaviors on a Vascular Microenvironment-Mimicking Microfluidic Chip

Bridging the gap between tumor-on-chip and clinics: a systematic review of 15 years of studies

Over the past 15 years, the field of oncology research has witnessed significant progress in the development of new cell culture models, such as tumor-on-chip (ToC) systems. In this comprehensive overview, we present a multidisciplinary perspective by bringing together physicists, biologists, clinicians, and experts from pharmaceutical companies to highlight the current state of ToC research, its unique features, and the challenges it faces. To offer readers a clear and quantitative understanding of the ToC field, we conducted an extensive systematic analysis of more than 300 publications related to ToC from 2005 to 2022. ToC offer key advantages over other in vitro models by enabling precise control over various parameters. These parameters include the properties of the extracellular matrix, mechanical forces exerted on cells, the physico-chemical environment, cell composition, and the architecture of the tumor microenvironment. Such fine control allows ToC to closely replicate the complex microenvironment and interactions within tumors, facilitating the study of cancer progression and therapeutic responses in a highly representative manner. Importantly, by incorporating patient-derived cells or tumor xenografts, ToC models have demonstrated promising results in terms of clinical validation. We also examined the potential of ToC for pharmaceutical industries in which ToC adoption is expected to occur gradually. Looking ahead, given the high failure rate of clinical trials and the increasing emphasis on the 3Rs principles (replacement, reduction, refinement of animal experimentation), ToC models hold immense potential for cancer research. In the next decade, data generated from ToC models could potentially be employed for discovering new therapeutic targets, contributing to regulatory purposes, refining preclinical drug testing and reducing reliance on animal models.

Bio-inspired microfluidics: A review

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

Unraveling Cancer Metastatic Cascade Using Microfluidics-based Technologies

Cancer has long been a leading cause of death. The primary tumor, however, is not the main cause of death in more than 90% of cases. It is the complex process of metastasis that makes cancer deadly. The invasion metastasis cascade is the multi-step biological process of cancer cell dissemination to distant organ sites and adaptation to the new microenvironment site. Unraveling the metastasis process can provide great insight into cancer death prevention or even treatment. Microfluidics is a promising platform, that provides a wide range of applications in metastasis-related investigations. Cell culture microfluidic technologies for in vitro modeling of cancer tissues with fluid flow and the presence of mechanical factors have led to the organ-on-a-chip platforms. Moreover, microfluidic systems have also been exploited for capturing and characterization of circulating tumor cells (CTCs) that provide crucial information on the metastatic behavior of a tumor. We present a comprehensive review of the recent developments in the application of microfluidics-based systems for analysis and understanding of the metastasis cascade from a wider perspective.

Generation of programmable dynamic flow patterns in microfluidics using audio signals

Customised audio signals, such as musical notes, can be readily generated by audio software on smartphones and played over audio speakers. Audio speakers translate electrical signals into the mechanical motion of the speaker cone. Coupling the inlet tube to the speaker cone causes the harmonic oscillation of the tube, which in turn changes the velocity profile and flow rate. We employ this strategy for generating programmable dynamic flow patterns in microfluidics. We show the generation of customised rib and vortex patterns through the application of multi-tone audio signals in water-based and whole blood samples. We demonstrate the precise capability to control the number and extent of the ribs and vortices by simply setting the frequency ratio of two- and three-tone audio signals. We exemplify potential applications of tube oscillation for studying the functional responses of circulating immune cells under pathophysiological shear rates. The system is programmable, compact, low-cost, biocompatible, and durable. These features make it suitable for a variety of applications across chemistry, biology, and physics.

Dual Channel Microfluidics for Mimicking the Blood-Brain Barrier

High-resolution imaging is essential for analysis of the steps and way stations of cargo transport in in vitro models of the endothelium. In this study, we demonstrate a microfluidic system consisting of two channels horizontally separated by a cell-growth-promoting membrane. Its design allows for high-resolution (down to single-molecule level) imaging using a high numerical aperture objective with a short working distance. To reduce optical aberrations and enable single-molecule-sensitive imaging, an observation window was constructed in the membrane via laser cutting with subsequent structuring using 3D multiphoton lithography for improved cell growth. The upper channel was loaded with endothelial cells under flow conditions, which showed polarization and junction formation. A coculture of human vascular endothelial cells with pericytes was developed that mimics the blood-brain barrier. Finally, this dual channel microfluidics system enabled 3D localization microscopy of the cytoskeleton and 3D single-molecule-sensitive tracing of lipoprotein particles.

Micropatterned Coculture Platform for Screening Nerve-Related Anticancer Drugs

Accumulating evidence suggests that the neural microenvironment plays a vital role in the development and metastasis of cancers. The development of drug candidates or drug combinations targeting the neural microenvironment is thus becoming increasingly urgent. However, the low content of conventional drug screening platforms is a bottleneck that limits the drug evaluation process. In this study, we present a micropatterned coculture-based high-content (μCHC) platform by integrating a micropatterned coculture chip with the high-content analysis (HCA) system, for studying the neuron-cancer cell interactions and drug screening (simultaneously detecting 96 kinds of post-drug-treated conditions). We investigate the contribution of neurons on the migration of cancer cells from different tissues and validate the capability of the μCHC system to study the interaction between neurons and cancer cells. Moreover, we test the effects of individual or combinatory agents targeting the neuron or cancer cell on the neuron-cancer cell interactions, which proposes an optimized therapy regime for targeting both nervous and cancerous factors. Our study suggests that the μCHC system is a facile platform for screening drug candidates or drug combinations for clinical cancer therapy with high efficiency and fidelity.

3D In Vitro Human Organ Mimicry Devices for Drug Discovery, Development, and Assessment

The past few decades have shown significant advancement as complex in vitro humanized systems have substituted animal trials and 2D in vitro studies. 3D humanized platforms mimic the organs of interest with their stimulations (physical, electrical, chemical, and mechanical). Organ-on-chip devices, including in vitro modelling of 3D organoids, 3D microfabrication, and 3D bioprinted platforms, play an essential role in drug discovery, testing, and assessment. In this article, a thorough review is provided of the latest advancements in the area of organ-on-chip devices targeting liver, kidney, lung, gut, heart, skin, and brain mimicry devices for drug discovery, development, and/or assessment. The current strategies, fabrication methods, and the specific application of each device, as well as the advantages and disadvantages, are presented for each reported platform. This comprehensive review also provides some insights on the challenges and future perspectives for the further advancement of each organ-on-chip device.

A new microfluidic model that allows monitoring of complex vascular structures and cell interactions in a 3D biological matrix

Microfluidic organ-on-a-chip designs are used to mimic human tissues, including the vasculature. Here we present a novel microfluidic device that allows the interaction of endothelial cells (ECs) with pericytes and the extracellular matrix (ECM) in full bio-matrix encased 3D vessel structures (neovessels) that can be subjected to continuous, unidirectional flow and perfusion with circulating immune cells. We designed a polydimethylsiloxane (PDMS) device with a reservoir for a 3D fibrinogen gel with pericytes. Open channels were created for ECs to form a monolayer. Controlled, continuous, and unidirectional flow was introduced via a pump system while the design facilitated 3D confocal imaging. In this vessel-on-a-chip system, ECs interact with pericytes to create a human cell derived blood vessel which maintains a perfusable lumen for up to 7 days. Dextran diffusion verified endothelial barrier function while demonstrating the beneficial role of supporting pericytes. Increased permeability after thrombin stimulation showed the capacity of the neovessels to show natural vascular response. Perfusion of neovessels with circulating THP-1 cells demonstrated this system as a valuable platform for assessing interaction between the endothelium and immune cells in response to TNFα. In conclusion: we created a novel vascular microfluidic device that facilitates the fabrication of an array of parallel soft-channel structures in ECM gel that develop into biologically functional neovessels without hard-scaffold support. This model provides a unique tool to conduct live in vitro imaging of the human vasculature during perfusion with circulating cells to mimic (disease) environments in a highly systematic but freely configurable manner.

Engineering of Hydrogel Materials with Perfusable Microchannels for Building Vascularized Tissues

Vascular systems are responsible for various physiological and pathological processes related to all organs in vivo, and the survival of engineered tissues for enough nutrient supply in vitro. Thus, biomimetic vascularization is highly needed for constructing both a biomimetic organ model and a reliable engineered tissue. However, many challenges remain in constructing vascularized tissues, requiring the combination of suitable biomaterials and engineering techniques. In this review, the advantages of hydrogels on building engineered vascularized tissues are discussed and recent engineering techniques for building perfusable microchannels in hydrogels are summarized, including micromolding, 3D printing, and microfluidic spinning. Furthermore, the applications of these perfusable hydrogels in manufacturing organ-on-a-chip devices and transplantable engineered tissues are highlighted. Finally, current challenges in recapitulating the complexity of native vascular systems are discussed and future development of vascularized tissues is prospected.

Microfluidic applications on circulating tumor cell isolation and biomimicking of cancer metastasis

The prognosis of malignant tumors is challenged by insufficient means to effectively detect tumors at early stage. Liquid biopsy using circulating tumor cells (CTCs) as biomarkers demonstrates a promising solution to tackle the challenge, because CTCs play a critical role in cancer metastatic process via intravasation, circulation, extravasation, and formation of secondary tumor. However, the effectiveness of the solution is compromised by rarity, heterogeneity, and vulnerability associated with CTCs. Among a plethora of novel approaches for CTC isolation and enrichment, microfluidics leads to isolation and detection of CTCs in a cost-effective and operation-friendly way. Development of microfluidics also makes it feasible to model the cancer metastasis in vitro using a microfluidic system to mimick the in vivo microenvironment, thereby enabling analysis and monitor of tumor metastasis. This paper aims to review the latest advances for exploring the dual-roles microfluidics has played in early cancer diagnosis via CTC isolation and investigating the role of CTCs in cancer metastasis; the merits and drawbacks for dominating microfluidics-based CTC isolation methods are discussed; biomimicking cancer metastasis using microfluidics are presented with example applications on modelling of tumor microenvironment, tumor cell dissemination, tumor migration, and tumor angiogenesis. The future perspectives and challenges are discussed.

Recent Advances in Microfluidic Platforms Applied in Cancer Metastasis: Circulating Tumor Cells' (CTCs) Isolation and Tumor-On-A-Chip

Cancer remains the leading cause of death worldwide despite the enormous efforts that are made in the development of cancer biology and anticancer therapeutic treatment. Furthermore, recent studies in oncology have focused on the complex cancer metastatic process as metastatic disease contributes to more than 90% of tumor-related death. In the metastatic process, isolation and analysis of circulating tumor cells (CTCs) play a vital role in diagnosis and prognosis of cancer patients at an early stage. To obtain relevant information on cancer metastasis and progression from CTCs, reliable approaches are required for CTC detection and isolation. Additionally, experimental platforms mimicking the tumor microenvironment in vitro give a better understanding of the metastatic microenvironment and antimetastatic drugs' screening. With the advancement of microfabrication and rapid prototyping, microfluidic techniques are now increasingly being exploited to study cancer metastasis as they allow precise control of fluids in small volume and rapid sample processing at relatively low cost and with high sensitivity. Recent advancements in microfluidic platforms utilized in various methods for CTCs' isolation and tumor models recapitulating the metastatic microenvironment (tumor-on-a-chip) are comprehensively reviewed. Future perspectives on microfluidics for cancer metastasis are proposed.

Cancer Modeling-on-a-Chip with Future Artificial Intelligence Integration

Cancer is one of the leading causes of death worldwide, despite the large efforts to improve the understanding of cancer biology and development of treatments. The attempts to improve cancer treatment are limited by the complexity of the local milieu in which cancer cells exist. The tumor microenvironment (TME) consists of a diverse population of tumor cells and stromal cells with immune constituents, microvasculature, extracellular matrix components, and gradients of oxygen, nutrients, and growth factors. The TME is not recapitulated in traditional models used in cancer investigation, limiting the translation of preliminary findings to clinical practice. Advances in 3D cell culture, tissue engineering, and microfluidics have led to the development of "cancer-on-a-chip" platforms that expand the ability to model the TME in vitro and allow for high-throughput analysis. The advances in the development of cancer-on-a-chip platforms, implications for drug development, challenges to leveraging this technology for improved cancer treatment, and future integration with artificial intelligence for improved predictive drug screening models are discussed.

Microfluidics-Based Biomaterials and Biodevices

The rapid development of microfluidics technology has promoted new innovations in materials science, particularly by interacting with biological systems, based on precise manipulation of fluids and cells within microscale confinements. This article reviews the latest advances in microfluidics-based biomaterials and biodevices, highlighting some burgeoning areas such as functional biomaterials, cell manipulations, and flexible biodevices. These areas are interconnected not only in their basic principles, in that they all employ microfluidics to control the makeup and morphology of materials, but also unify at the ultimate goals in human healthcare. The challenges and future development trends in biological application are also presented.

A microfluidic mammary gland coculture model using parallel 3D lumens for studying epithelial-endothelial migration in breast cancer

In breast cancer development, crosstalk between mammary epithelial cells and neighboring vascular endothelial cells is critical to understanding tumor progression and metastasis, but the mechanisms of this dynamic interplay are not fully understood. Current cell culture platforms do not accurately recapitulate the 3D luminal architecture of mammary gland elements. Here, we present the development of an accessible and scalable microfluidic coculture system that incorporates two parallel 3D luminal structures that mimic vascular endothelial and mammary epithelial cell layers, respectively. This parallel 3D lumen configuration allows investigation of endothelial-epithelial crosstalk and its effects of the comigration of endothelial and epithelial cells into microscale migration ports located between the parallel lumens. We describe the development and application of our platform, demonstrate generation of 3D luminal cell layers for endothelial cells and three different breast cancer cell lines, and quantify their migration profiles based on number of migrated cells, area coverage by migrated cells, and distance traveled by individual migrating cells into the migration ports. Our system enables analysis at the single-cell level, allows simultaneous monitoring of endothelial and epithelial cell migration within a 3D extracellular matrix, and has potential for applications in basic research on cellular crosstalk as well as drug development.

Responses of Cellular Adhesion Strength and Stiffness to Fluid Shear Stress during Tumor Cell Rolling Motion

Biochemical and physical factors affect the rolling of tumor cells across the blood vessel. The biochemical factors have been well studied, while the influence of physical factors such as fluid shear stress (FSS) remains poorly understood. Here, human glioma cells (U87 cells) in a straight microfluidic channel were exposed to FSS (0.12, 1.2, and 1.8 dyn/cm²); and their locomotion behaviors from crawling-to-rolling and changes in cellular morphology (concave, elongated, less elongated, and round) were observed. The adhesion strength and stiffness of the cells of different morphologies were analyzed using a live single-cell extractor and atomic force microscopy, respectively. In general, the FSS stimulated cells showed stronger adhesion strength than the cells not exposed to FSS. The cell not exposed to FSS always exhibited greater nuclear stiffness than cortex stiffness, while after FSS treatment the cortex hardened and nucleus softened, where the round-shaped cell had a cortex that was more rigid than its nucleus. These results indicated that FSS influenced the biomechanics of circulating tumor cells, and elucidation of the mechanical responses to FSS might provide a deeper insight for cancer metastasis.

Short-term 3D culture systems of various complexity for treatment optimization of colorectal carcinoma

Three-dimensional (3D) cultures have the potential to increase the predictive value of pre-clinical drug research and bridge the gap towards anticipating clinical outcome of proposed treatments. However, their implementation in more advanced drug-discovery programs is still in its infancy due to the lack of reproducibility and low time- and cost effectiveness. HCT116, SW620 and DLD1 cells, cell lines with distinct mutations, grade and origin, were co-cultured with fibroblasts and endothelial cells (EC) in 3D spheroids. Clinically relevant drugs, i.e. 5-fluorouracil (5−FU), regorafenib and erlotinib, were administered individually to in CRC cell cultures. In this study, we established a robust, low-cost and reproducible short-term 3D culture system addressing the various complexities of the colorectal carcinoma (CRC) microenvironment. We observed a dose-dependent increase of erlotinib sensitivity in 3D (co-)cultures compared to 2D cultures. Furthermore, we compared the drug combination efficacy and drug-drug interactions administered in 2D, 3D and 3D co-cultures. We observed that synergistic/additive drug-drug interactions for drug combinations administered at low doses shifted towards additive and antagonistic when applied at higher doses in metastatic CRC cells. The addition of fibroblasts at various ratios and EC increased the resistance to some drug combinations in SW620 and DLD1 cells, but not in HCT116. Retreatment of SW620 3D co-cultures with a low-dose 3-drug combination was as active (88% inhibition, relative to control) as 5-FU treatment at high dose (100 μM). Moreover, 3D and 3D co-cultures responded variably to the drug combination treatments, and also signalling pathways were differently regulated, probably due to the influence of fibroblasts and ECs on cancer cells. The short-term 3D co-culture system developed here is a powerful platform for screening (combination) therapies. Understanding of signalling in 3D co-cultures versus 3D cultures and the responses in the 3D models upon drug treatment might be beneficial for designing anti-cancer therapies.

A Microfluidic Cell Stretch Device to Investigate the Effects of Stretching Stress on Artery Smooth Muscle Cell Proliferation in Pulmonary Arterial Hypertension

A microfluidic cell stretch device was developed to investigate the effects of stretching stress on pulmonary artery smooth muscle cell (PASMC) proliferation in pulmonary arterial hypertension (PAH). The microfluidic device harbors upper cell culture and lower control channels, separated by a stretchable poly(dimethylsiloxane) membrane that acts as a cell culture substrate. The lower channel inlet was connected to a vacuum pump via a digital switch-controlled solenoid valve. For cyclic stretch at heartbeat frequency (80 bpm), the open or close time for each valve was set to 0.38 s. Proliferation of normal PASMCs and those obtained from patients was enhanced by the circumferential stretching stimulation. This is the first report showing patient cells increased in number by stretching stress. These results are consistent with the abnormal proliferation observed in PAH. Circumferential stretch stress was applied to the cells without increasing the pressure inside the microchannel. Our data may suggest that the stretch stress itself promotes cell proliferation in PAH.

Tumor-on-a-chip platform to investigate progression and drug sensitivity in cell lines and patient-derived organoids

Most cancer treatment strategies target cell proliferation, angiogenesis, migration, and intravasation of tumor cells in an attempt to limit tumor growth and metastasis. An in vitro platform to assess tumor progression and drug sensitivity could provide avenues to enhance our understanding of tumor metastasis as well as precision medicine. We present a microfluidic platform that mimics biological mass transport near the arterial end of a capillary in the tumor microenvironment. A central feature is a quiescent perfused 3D microvascular network created prior to loading tumor cells or patient-derived tumor organoids in an adjacent compartment. The physiological delivery of nutrients and/or drugs to the tumor then occurs through the vascular network. We demonstrate the culture, growth, and treatment of tumor cell lines and patient-derived breast cancer organoids. The platform provides the opportunity to simultaneously and dynamically observe hallmark features of tumor progression including cell proliferation, angiogenesis, cell migration, and tumor cell intravasation. Additionally, primary breast tumor organoids are viable in the device for several weeks and induce robust sprouting angiogenesis. Finally, we demonstrate the feasibility of our platform for drug discovery and personalized medicine by analyzing the response to chemo- and anti-angiogenic therapy. Precision medicine-based cancer treatments can only be realized if individual tumors can be rapidly assessed for therapeutic sensitivity in a clinically relevant timeframe (⪅14 days). Our platform indicates that this goal can be achieved and provides compelling opportunities to advance precision medicine for cancer.

Recent Progress in the Development of Microfluidic Vascular Models

The blood vessel is part of the circulatory system, and systemic circulation provides the blood supply to all tissues. Arteries are pathways through which the blood is carried, and the capillaries have a key role in material exchange to maintain the tissue environment. Blood vessels have structures appropriate for their functions, and their sizes and cell types are different. In this review, we introduced recent studies of the microfluidic vascular models. The model structures are classified mainly as poly(dimethylsiloxane) and hydrogel microchannels and self-assembled networks. Basic phenomena and functions were realized in vascular models, including fluid shear stress, cell strain, interstitial flow, endothelial permeation, angiogenesis, and thrombosis. In some models, endothelial cells were co-cultured with smooth muscle cells, pericytes, and fibroblasts in an extracellular matrix. Examples of vascular models involving the brain, lung, liver, kidney, placenta, and cancer were also introduced.

Jianpi Jiedu decoction, a traditional Chinese medicine formula, inhibits tumorigenesis, metastasis, and angiogenesis through the mTOR/HIF-1α/VEGF pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Traditional Chinese medicine has been utilized for the treatment of cancer. Jianpi Jiedu decoction (JPJD), a traditional Chinese medicine formula, has been used for the treatment of colorectal cancer for decades. However, the underlying molecular mechanistic basis for the effect of JPJD on colorectal cancer is poorly understood.

AIM OF THE STUDY

The aim of this study was to identify the effects of JPJD on human colon cancer cells in vitro as well as in vivo and to investigate the mechanistic basis for the anticancer effect of JPJD.

MATERIALS AND METHODS

The in vitro antitumor activity of JPJD was assessed by MTT assay, flow cytometric analysis, wound-healing assay, transwell assays, and tube formation assays in order to assess cell activity, apoptosis, migration, invasion, and angiogenesis, respectively. The anticancer properties of JPJD in vivo were assessed by immunohistochemistry in a nude mouse xenograft model of HCT116 cells. In addition, the level of mTOR/HIF-1α/VEGF signaling pathway proteins in HCT116 cells and tumor tissue was evaluated by immunoblotting.

RESULTS

In vitro, JPJD significantly inhibited colorectal cancer cell lines viability and proliferation. Flow cytometry analysis demonstrated JPJD to induce HCT116 cell apoptosis. Additionally, JPJD effectively suppressed tumor cell migration, invasion, and angiogenesis by inhibiting the mTOR/HIF-1α/VEGF signaling pathway. In vivo, JPJD significantly inhibited HCT116 tumor growth in athymic nude mice, decreased the levels of CD34 as well as VEGF, and downregulated the mTOR/HIF-1α/VEGF pathway.

CONCLUSIONS

JPJD treatment produced anti-colorectal tumor effects by inhibiting tumorigenesis, metastasis, as well as angiogenesis through the mTOR/HIF-1α/VEGF pathway. Thus, these results provide a strong rationale for the therapeutic use of JPJD in cancer treatment. Further studies are required to investigate the mechanisms underlying anti-CRC effect of JPJD.

Advances in tumor-endothelial cells co-culture and interaction on microfluidics

The metastasis in which the cancer cells degrade the extracellular matrix (ECM) and invade to the surrounding and far tissues of the body is the leading cause of mortality in cancer patients. With a lot of advancement in the field, yet the biological cause of metastasis are poorly understood. The microfluidic system provides advanced technology to reconstruct a variety of in vivo-like environment for studying the interactions between tumor cells (TCs) and endothelial cells (ECs). This review gives a brief account of both two-dimensional models and three-dimensional microfluidic systems for the analysis of TCs-ECs co-culture as well as their applications to anti-cancer drug screening. Furthermore, the advanced methods for analyzing cell-to-cell interactions at single-cell level were also discussed.

Deciphering the relative contribution of vascular inflammation and blood rheology in metastatic spreading

Vascular adhesion of circulating tumor cells (CTCs) is a key step in cancer spreading. If inflammation is recognized to favor the formation of vascular "metastatic niches," little is known about the contribution of blood rheology to CTC deposition. Herein, a microfluidic chip, covered by a confluent monolayer of endothelial cells, is used for analyzing the adhesion and rolling of colorectal (HCT-15) and breast (MDA-MB-231) cancer cells under different biophysical conditions. These include the analysis of cell transport in a physiological solution and whole blood over a healthy and a TNF-α inflamed endothelium with a flow rate of 50 and 100 nl/min. Upon stimulation of the endothelial monolayer with TNF-α (25 ng/ml), CTC adhesion increases from 2 to 4 times whilst cell rolling velocity only slightly reduces. Notably, whole blood also enhances cancer cell deposition from 2 to 3 times, but only on the unstimulated vasculature. For all tested conditions, no statistically significant difference is observed between the two cancer cell types. Finally, a computational model for CTC transport demonstrates that a rigid cell approximation reasonably predicts rolling velocities while cell deformability is needed to model adhesion. These results would suggest that, within microvascular networks, blood rheology and inflammation contribute similarly to CTC deposition, thereby facilitating the formation of metastatic niches along the entire network, including the healthy endothelium. In microfluidic-based assays, neglecting blood rheology would significantly underestimate the metastatic potential of cancer cells.

A Microvascularized Tumor-mimetic Platform for Assessing Anti-cancer Drug Efficacy

Assessment of anti-cancer drug efficacy in in vitro three-dimensional (3D) bioengineered cancer models provides important contextual and relevant information towards pre-clinical translation of potential drug candidates. However, currently established models fail to sufficiently recapitulate complex tumor heterogeneity. Here we present a chip-based tumor-mimetic platform incorporating a 3D in vitro breast cancer model with a tumor-mimetic microvascular network, replicating the pathophysiological architecture of native vascularized breast tumors. The microfluidic platform facilitated formation of mature, lumenized and flow-aligned endothelium under physiological flow recapitulating both high and low perfused tumor regions. Metastatic and non-metastatic breast cancer cells were maintained in long-term 3D co-culture with stromal fibroblasts in a poly(ethylene glycol)-fibrinogen hydrogel matrix within adjoining tissue chambers. The interstitial space between the chambers and endothelium contained pores to mimic the “leaky” vasculature found in vivo and facilitate cancer cell-endothelial cell communication. Microvascular pattern-dependent flow variations induced concentration gradients within the 3D tumor mass, leading to morphological tumor heterogeneity. Anti-cancer drugs displayed cell type- and flow pattern-dependent effects on cancer cell viability, viable tumor area and associated endothelial cytotoxicity. Overall, the developed microfluidic tumor-mimetic platform facilitates investigation of cancer-stromal-endothelial interactions and highlights the role of a fluidic, tumor-mimetic vascular network on anti-cancer drug delivery and efficacy for improved translation towards pre-clinical studies.

Cell migration in microengineered tumor environments

Recent advances in microengineered cell migration platforms are discussed critically with a focus on how cell migration is influenced by engineered tumor microenvironments, the medical relevance being to understand how tumor microenvironments may promote or suppress the progression of cancer. We first introduce key findings in cancer cell migration under the influence of the physical environment, which is systematically controlled by microengineering technology, followed by multi-cues of physico-chemical factors, which represent the complexity of the tumor environment. Recognizing that cancer cells constantly communicate not only with each other but also with tumor-associated cells such as vascular, fibroblast, and immune cells, and also with non-cellular components, it follows that cell motility in tumor microenvironments, especially metastasis via the invasion of cancer cells into the extracellular matrix and other tissues, is closely related to the malignancy of cancer-related mortality. Medical relevance of forefront research realized in microfabricated devices, such as single cell sorting based on the analysis of cell migration behavior, may assist personalized theragnostics based on the cell migration phenotype. Furthermore, we urge development of theory and numerical understanding of single or collective cell migration in microengineered platforms to gain new insights in cancer metastasis and in therapeutic strategies.

A Biomimetic Microfluidic Tumor Microenvironment Platform Mimicking the EPR Effect for Rapid Screening of Drug Delivery Systems

Real-time monitoring of tumor drug delivery in vivo is a daunting challenge due to the heterogeneity and complexity of the tumor microenvironment. In this study, we developed a biomimetic microfluidic tumor microenvironment (bMTM) comprising co-culture of tumor and endothelial cells in a 3D environment. The platform consists of a vascular compartment featuring a network of vessels cultured with endothelial cells forming a complete lumen under shear flow in communication with 3D solid tumors cultured in a tumor compartment. Endothelial cell permeability to both small dye molecules and large liposomal drug carriers were quantified using fluorescence microscopy. Endothelial cell intercellular junction formation was characterized by immunostaining. Endothelial cell permeability significantly increased in the presence of either tumor cell conditioned media (TCM) or tumor cells. The magnitude of this increase in permeability was significantly higher in the presence of metastatic breast tumor cells as compared to non-metastatic ones. Immunostaining revealed impaired endothelial cell-cell junctions in the presence of either metastatic TCM or metastatic tumor cells. Our findings indicate that the bMTM platform mimics the tumor microenvironment including the EPR effect. This platform has a significant potential in applications such as cell-cell/cell-drug carrier interaction studies and rapid screening of cancer drug therapeutics/carriers.

Streptavidin-biotin-peroxidase nanocomplex-amplified microfluidics immunoassays for simultaneous detection of inflammatory biomarkers

Simultaneous, sensitive and quantitative detection of biomarkers in infectious disease is crucial for guiding antimicrobial treatment and predicting prognosis. This work reported an ultrasensitive and quantitative microfluidic immunoassay combined with the streptavidin-biotin-peroxidase (SA-B-HRP) nanocomplex-signal amplification system (MIS) to detect two inflammatory biomarkers, procalcitonin (PCT, for discriminating bacterial infections from nonbacterial infections) and interleukin-6 (IL-6, for monitoring the kinetics of infectious disease) simultaneously. The amplification system was based on the one step self-assembly of SA and B-HRP to form the SA-B-HRP nanocomplex, which effectively amplified the chemiluminescent signals. The linear ranges for PCT and IL-6 detections by MIS were 250-1.28 × 10⁵ pg mL^-1 and 5-1280 pg mL^-1, and the limit of detection (LOD) were 48.9 pg mL^-1 and 1.0 pg mL^-1, respectively, both of which were significantly improved compared with microfluidic immunoassays without amplification system (MI). More importantly, PCT and IL-6 in human serum could be simultaneously detected in the same run by MIS, which could greatly improve the detection efficiency and reduce the cost. Given the advantages of high sensitivity, multiplex and quantitative detection, MIS could be potentially applied for detection of biomarkers at low concentration in clinical diagnosis.

Biophysical Regulation of Cancer Stem/Initiating Cells: Implications for Disease Mechanisms and Translation

Cancer stem/initiating cells (CSCs) are a subset of tumor cells proposed to play privileged roles in seeding tumors and driving metastasis. CSCs have emerged as an increasingly important target of interest in cancer biology and therapy. Recent work has suggested that CSC maintenance and metastatic potential may be modulated by physical inputs within the tissue microenvironment, including interstitial pressure and extracellular matrix stiffness. Here we review recent progress in our understanding of CSC regulation by biophysical signals within the tumor microenvironment. While the mechanistic basis of this signaling remains incompletely understood, we discuss emerging evidence that mechanical inputs can epigenetically regulate CSC behavior and that some CSCs can evade mechanotransductive signals to more efficiently infiltrate tissue. We also describe efforts to leverage these findings to engineer culture platforms for the characterization of CSC mechanics for discovery and screening.

Advances in on-chip vascularization

Microfluidics is invaluable for studying microvasculature, development of organ-on-chip models and engineering microtissues. Microfluidic design can cleverly control geometry, biochemical gradients and mechanical stimuli, such as shear and interstitial flow, to more closely mimic in vivo conditions. In vitro vascular networks are generated by two distinct approaches: via endothelial-lined patterned channels, or by self-assembled networks. Each system has its own benefits and is amenable to the study of angiogenesis, vasculogenesis and cancer metastasis. Various techniques are employed in order to generate rapid perfusion of these networks within a variety of tissue and organ-mimicking models, some of which have shown recent success following implantation in vivo. Combined with tuneable hydrogels, microfluidics holds great promise for drug screening as well as in the development of prevascularized tissues for regenerative medicine.

Microengineered cancer-on-a-chip platforms to study the metastatic microenvironment

More than 90% of cancer-related deaths can be attributed to the occurrence of metastatic diseases. Recent studies have highlighted the importance of the multicellular, biochemical and biophysical stimuli from the tumor microenvironment during carcinogenesis, treatment failure, and metastasis. Therefore, there is a need for experimental platforms that are able to recapitulate the complex pathophysiological features of the metastatic microenvironment. Recent advancements in biomaterials, microfluidics, and tissue engineering have led to the development of living multicellular microculture systems, which are maintained in controllable microenvironments and function with organ level complexity. The applications of these "on-chip" technologies for detection, separation, characterization and three dimensional (3D) propagation of cancer cells have been extensively reviewed in previous works. In this contribution, we focus on integrative microengineered platforms that allow the study of multiple aspects of the metastatic microenvironment, including the physicochemical cues from the tumor associated stroma, the heterocellular interactions that drive trans-endothelial migration and angiogenesis, the environmental stresses that metastatic cancer cells encounter during migration, and the physicochemical gradients that direct cell motility and invasion. We discuss the application of these systems as in vitro assays to elucidate fundamental mechanisms of cancer metastasis, as well as their use as human relevant platforms for drug screening in biomimetic microenvironments. We then conclude with our commentaries on current progress and future perspectives of microengineered systems for fundamental and translational cancer research.