A microfluidic device for measuring cell migration towards substrate-bound and soluble chemokine gradients

Integration of secreted signaling molecule sensing on cell monitoring platforms: a critical review

Monitoring cell secretion in complex microenvironments is crucial for understanding cellular behavior and advancing physiological and pathological research. While traditional cell culture methods, including organoids and spheroids, provide valuable models, real-time monitoring of cell secretion of signaling molecules remains challenging. Integrating advanced monitoring technologies into these systems often disrupts the delicate balance of the microenvironment, making it difficult to achieve sensitivity and specificity. This review explored recent strategies for integrating the monitoring of cell secretion of signaling molecules, crucial for understanding and replicating cell microenvironments, within cell culture platforms, addressing challenges such as non-adherent cell models and the focus on single-cell methodologies. We highlight advancements in biosensors, microfluidics, and three-dimensional culture methods, and discuss their potential to enhance real-time, multiplexed cell monitoring. By examining the advantages, limitations, and future prospects of these technologies, we aim to contribute to the development of integrated systems that facilitate comprehensive cell monitoring, ultimately advancing biological research and pharmaceutical development.

Lamellipodia-Mediated Osteoblast Haptotaxis Guided by Fibronectin Ligand Concentrations on a Multiplex Chip

Skull morphogenesis is a complex, dynamic process involving two different germ layers and progressing to the coordinated, directional growth of individual bones. The mechanisms underlying directional growth toward the apex are not completely understood. Here, a microfluidic chip-based approach is utilized to test whether calvarial osteoblast progenitors undergo haptotaxis on a gradient of Fibronectin1 (FN1) via lamellipodia. Mimicking the embryonic cranial mesenchyme's FN1 pattern, FN1 gradients is established in the chip using computer modeling and fluorescent labeling. Primary mouse calvarial osteoblast progenitors are plated in the chip along an array of segmented gradients of adsorbed FN1. The study performs single-cell tracking and measures protrusive activity. Haptotaxis is observed at an intermediate FN1 concentration, with an average directional migration index (yFMI) of 0.07, showing a significant increase compared to the control average yFMI of -0.01. A significant increase in protrusive activity is observed during haptotaxis. Haptotaxis is an Arp2/3-dependent, lamellipodia-mediated process. Calvarial osteoblast progenitors treated with the Arp2/3 (Actin Related Protein 2/3 complex) inhibitor CK666 show significantly diminished haptotaxis, with an average yFMI of 0.01. Together, these results demonstrate haptotaxis on an FN1 gradient as a new mechanism in the apical expansion of calvarial osteoblast progenitors during development and shed light on the etiology of calvarial defects.

Tetralol derivative NNC-55-0396 targets hypoxic cells in the glioblastoma microenvironment: an organ-on-chip approach

Glioblastoma (GBM) is a highly malignant brain tumour characterised by limited treatment options and poor prognosis. The tumour microenvironment, particularly the central hypoxic region of the tumour, is known to play a pivotal role in GBM progression. Cells within this region adapt to hypoxia by stabilising transcription factor HIF1-α, which promotes cell proliferation, dedifferentiation and chemoresistance. In this study we sought to examine the effects of NNC-55-0396, a tetralol compound which overactivates the unfolded protein response inducing apoptosis, using the organ-on-chip technology. We identified an increased sensitivity of the hypoxic core of the chip to NNC, which correlates with decreasing levels of HIF1-α in vitro. Moreover, NNC blocks the macroautophagic process that is unleashed by hypoxia as revealed by increased levels of autophagosomal constituent LC3-II and autophagy chaperone p62/SQSTM1. The specific effects of NNC in the hypoxic microenvironment unveil additional anti-cancer abilities of this compound and further support investigations on its use in combined therapies against GBM.

Microengineered In Vitro Assays for Screening and Sorting Manufactured Therapeutic T Cells

Adoptively transferred T cells constitute a major class of current and emergent cellular immunotherapies for the treatment of disease, including but not limited to cancer. Although key advancements in molecular recognition, genetic engineering, and manufacturing have dramatically enhanced their translational potential, therapeutic potency remains limited by poor homing and infiltration of transferred cells within target host tissues. In vitro microengineered homing assays with precise control over micromechanical and biological cues can address these shortcomings by enabling interrogation, screening, sorting, and optimization of therapeutic T cells based on their homing capacity. In this article, the working principles, application, and integration of microengineered homing assays for the mechanistic study of biophysical and biomolecular cues relevant to homing of therapeutic T cells are reviewed. The potential for these platforms to enable scalable enrichment and screening of next-generation manufactured T cell therapies for cancer is also discussed.

Adaptive pathfinding by nucleokinesis during amoeboid migration

Motile cells encounter microenvironments with locally heterogeneous mechanochemical composition. Individual compositional parameters, such as chemokines and extracellular matrix pore sizes, are well known to provide guidance cues for pathfinding. However, motile cells face diverse cues at the same time, raising the question of how they respond to multiple and potentially competing signals on their paths. Here, we reveal that amoeboid cells require nuclear repositioning, termed nucleokinesis, for adaptive pathfinding in heterogeneous mechanochemical micro-environments. Using mammalian immune cells and the amoeba Dictyostelium discoideum, we discover that frequent, rapid and long-distance nucleokinesis is a basic component of amoeboid pathfinding, enabling cells to reorientate quickly between locally competing cues. Amoeboid nucleokinesis comprises a two-step polarity switch and is driven by myosin-II forces that readjust the nuclear to the cellular path. Impaired nucleokinesis distorts path adaptions and causes cellular arrest in the microenvironment. Our findings establish that nucleokinesis is required for amoeboid cell navigation. Given that many immune cells, amoebae, and some cancer cells utilize an amoeboid migration strategy, these results suggest that nucleokinesis underlies cellular navigation during unicellular biology, immunity, and disease.

Immunity-on-a-Chip: Integration of Immune Components into the Scheme of Organ-on-a-Chip Systems

Studying the immune system in vitro aims to understand how, when, and where the immune cells migrate/differentiate and respond to the various triggering events and the decision points along the immune response journey. It becomes evident that organ-on-a-chip (OOC) technology has a superior capability to recapitulate the cell-cell and tissue-tissue interaction in the body, with a great potential to provide tools for tracking the paracrine signaling with high spatial-temporal precision and implementing in situ real-time, non-destructive detection assays, therefore, enabling extraction of mechanistic information rather than phenotypic information. However, despite the rapid development in this technology, integration of the immune system into OOC devices stays among the least navigated tasks, with immune cells still the major missing components in the developed models. This is mainly due to the complexity of the immune system and the reductionist methodology of the OOC modules. Dedicated research in this field is demanded to establish the understanding of mechanism-based disease endotypes rather than phenotypes. Herein, we systemically present a synthesis of the state-of-the-art of immune-cantered OOC technology. We comprehensively outlined what is achieved and identified the technology gaps emphasizing the missing components required to establish immune-competent OOCs and bridge these gaps.

Modeling immunity in microphysiological systems

There is a need for better predictive models of the human immune system to evaluate safety and efficacy of immunomodulatory drugs and biologics for successful product development and regulatory approvals. Current in vitro models, which are often tested in two-dimensional (2D) tissue culture polystyrene, and preclinical animal models fail to fully recapitulate the function and physiology of the human immune system. Microphysiological systems (MPSs) that can model key microenvironment cues of the human immune system, as well as of specific organs and tissues, may be able to recapitulate specific features of the in vivo inflammatory response. This minireview provides an overview of MPS for modeling lymphatic tissues, immunity at tissue interfaces, inflammatory diseases, and the inflammatory tumor microenvironment in vitro and ex vivo. Broadly, these systems have utility in modeling how certain immunotherapies function in vivo, how dysfunctional immune responses can propagate diseases, and how our immune system can combat pathogens.

3D printed inserts for reproducible high throughput screening of cell migration

Cell migration is a fundamental and complex phenomenon that occurs in normal physiology and in diseases like cancer. Hence, understanding cell migration is very important in the fields of developmental biology and biomedical sciences. Cell migration occurs in 3 dimensions (3D) and involves an interplay of migrating cell(s), neighboring cells, extracellular matrix, and signaling molecules. To understand this phenomenon, most of the currently available techniques still rely on 2-dimensional (2D) cell migration assay, also known as the scratch assay or the wound healing assay. These methods suffer from limited reproducibility in creating a cell-free region (a scratch or a wound). Mechanical/heat related stress to cells is another issue which hampers the applicability of these methods. To tackle these problems, we developed an alternative method based on 3D printed biocompatible cell inserts, for quantifying cell migration in 24-well plates. The inserts were successfully validated via a high throughput assay for following migration of lung cancer cell line (A549 cell line) in the presence of standard cell migration promoters and inhibitors. We also developed an accompanying image analysis pipeline which demonstrated that our method outperforms the state-of-the-art methodologies for assessing the cell migration in terms of reproducibility and simplicity.

A Bidirectional Single-Cell Migration and Retrieval Chip for Quantitative Study of Dendritic Cell Migration

Dendritic cell (DC) migration is a fundamental step during execution of its adaptive immunity functions. Studying DC migration characteristics is critical for development of DC-dependent allergy treatments, vaccines, and cancer immunotherapies. Here, a microfluidics-based single-cell migration platform is described that enables high-throughput and precise bidirectional cell migration assays. It also allows selective retrieval of cell subpopulations that have different migratory potentials. Using this microfluidic platform, DC migration is investigated in response to different chemoattractants and inhibitors, quantitatively describe DC migration patterns and retrieve DC subpopulations of different migratory potentials for differential gene expression analysis. This platform opens an avenue for precise characterization of cell migration and potential discovery of therapeutic modulators.

Laser-assisted protein micropatterning in a thermoplastic device for multiplexed prostate cancer biomarker detection

Immunoassays are frequently used for analysis of protein biomarkers. The specificity of antibodies enables parallel analysis of several target proteins, at the same time. However, the implementation of such multiplexed assays into cost-efficient and mass-producible thermoplastic microfluidic platforms remains difficult due to the lack of suitable immobilization strategies for different capture antibodies. Here, we introduce and characterize a method to functionalize the surfaces of microfluidic devices manufactured in the thermoplastic material cyclic olefin copolymer (COC) by a rapid prototyping process. A laser-induced immobilization process enables the surface patterning of anchor biomolecules at a spatial resolution of 5 μm. We employ the method for the analysis of prostate cancer associated biomarkers by competitive immunoassays in a microchannel with a total volume of 320 nL, and successfully detected the proteins PSA, CRP, CEA and IGF-1 at clinically relevant concentrations. Finally, we also demonstrate the simultaneous analysis of three markers spiked into undiluted human plasma. In conclusion, this method opens the way to transfer multiplexed immunoassays into mass-producible microfluidic platforms that are suitable for point of care applications.

T Cells Chemotaxis Migration Studies with a Multi-Channel Microfluidic Device

Immune surveillance is dependent on lymphocyte migration and targeted recruitment. This can involve different modes of cell motility ranging from random walk to highly directional environment-guided migration driven by chemotaxis. This study protocol describes a flow-based microfluidic device to perform quantitative multiplex cell migration assays with the potential to investigate in real time the migratory response of T cells at the population or single-cell level. The device also allows for subsequent in situ fixation and direct fluorescence analysis of the cells in the microchannel.

Distinct Fates of Chemokine and Surrogate Molecule Gradients: Consequences for CCR7-Guided Dendritic Cell Migration

Chemokine-guided leukocyte migration is a hallmark of the immune system to cope with invading pathogens. Intruder confronted dendritic cells (DCs) induce the expression of the chemokine receptor CCR7, which enables them to sense and migrate along chemokine gradients to home to draining lymph nodes, where they launch an adaptive immune response. Chemokine-mediated DC migration is recapitulated and intensively studied in 3D matrix migration chambers. A major caveat in the field is that chemokine gradient formation and maintenance in such 3D environments is generally not assessed. Instead, fluorescent probes, mostly labelled dextran, are used as surrogate molecules, thereby neglecting important electrochemical properties of the chemokines. Here, we used site-specifically, fluorescently labelled CCL19 and CCL21 to study the establishment and shape of the chemokine gradients over time in the 3D collagen matrix. We demonstrate that CCL19 and particularly CCL21 establish stable, but short-distance spanning gradients with an exponential decay-like shape. By contrast, dextran with its neutral surface charge forms a nearly linear gradient across the entire matrix. We show that the charged C-terminal tail of CCL21, known to interact with extracellular matrix proteins, is determinant for shaping the chemokine gradient. Importantly, DCs sense differences in the shape of CCL19 and CCL21 gradients, resulting in distinct spatial migratory responses.

Multifactorial assessment of neutrophil chemotaxis efficiency from a drop of blood

Following injury and infection, neutrophils are guided to the affected site by chemoattractants released from injured tissues and invading microbes. During this process (chemotaxis), neutrophils must integrate multiple chemical signals, while also responding to physical constraints and prioritizing their directional decisions to generate an efficient immune response. In some clinical conditions, human neutrophils appear to lose the ability to chemotax efficiently, which may contribute both directly and indirectly to disease pathology. Here, a range of microfluidic designs is utilized to test the sensitivity of chemotaxing neutrophils to various perturbations, including binary decision-making in the context of channels with different chemoattractant gradients, hydraulic resistance, and angle of approach. Neutrophil migration in long narrow channels and planar environments is measured. Conditions in which neutrophils are significantly more likely to choose paths with the steepest chemoattractant gradient and the most direct approach angle, and find that migration efficiency across planar chambers is inversely correlated with chamber diameter. By sequential measurement of neutrophil binary decision-making to different chemoattractant gradients, or chemotactic index in sequential planar environments, data supporting a model of biased random walk for neutrophil chemotaxis are presented.

Spatial determinates of effector and memory CD8+ T cell fates

The lymph node plays a critical role in mounting an adaptive immune response to infection, clearance of foreign pathogens, and cancer immunosurveillance. Within this complex structure, intranodal migration is vital for CD8⁺ T cell activation and differentiation. Combining tissue clearing and volumetric light sheet fluorescent microscopy of intact lymph nodes has allowed us to explore the spatial regulation of T cell fates. This has determined that short-lived effector (T_SLEC ) are imprinted in peripheral lymph node interfollicular regions, due to CXCR3 migration. In contrast, stem-like memory cell (T_SCM ) differentiation is determined in the T cell paracortex. Here, we detail the inflammatory and chemokine regulators of spatially restricted T cell differentiation, with a focus on how to promote T_SCM . We propose a default pathway for T_SCM differentiation due to CCR7-directed segregation of precursors away from the inflammatory effector niche. Although volumetric imaging has revealed the consequences of intranodal migration, we still lack knowledge of how this is orchestrated within a complex chemokine environment. Toward this goal, we highlight the potential of combining microfluidic chambers with pre-determined complexity and subcellular resolution microscopy.

The Critical Importance of Spatial and Temporal Scales in Designing and Interpreting Immune Cell Migration Assays

Intravital microscopy and other direct-imaging techniques have allowed for a characterisation of leukocyte migration that has revolutionised the field of immunology, resulting in an unprecedented understanding of the mechanisms of immune response and adaptive immunity. However, there is an assumption within the field that modern imaging techniques permit imaging parameters where the resulting cell track accurately captures a cell's motion. This notion is almost entirely untested, and the relationship between what could be observed at a given scale and the underlying cell behaviour is undefined. Insufficient spatial and temporal resolutions within migration assays can result in misrepresentation of important physiologic processes or cause subtle changes in critical cell behaviour to be missed. In this review, we contextualise how scale can affect the perceived migratory behaviour of cells, summarise the limited approaches to mitigate this effect, and establish the need for a widely implemented framework to account for scale and correct observations of cell motion. We then extend the concept of scale to new approaches that seek to bridge the current "black box" between single-cell behaviour and systemic response.

Dendritic Cell Migration Is Tuned by Mechanical Stiffness of the Confining Space

The coordination of cell migration of immune cells is a critical aspect of the immune response to pathogens. Dendritic cells (DCs), the sentinels of the immune system, are exposed to complex tissue microenvironments with a wide range of stiffnesses. Recent studies have revealed the importance of mechanical cues in immune cell trafficking in confined 3D environments. However, the mechanism by which stiffness modulates the intrinsic motility of immature DCs remains poorly understood. Here, immature DCs were found to navigate confined spaces in a rapid and persistent manner, surveying a wide range when covered with compliant gels mimicking soft tissues. However, the speed and persistence time of random motility were both decreased by confinement in gels with higher stiffness, mimicking skin or diseased, fibrotic tissue. The impact of stiffness of surrounding tissue is crucial because most in vitro studies to date have been based on cellular locomotion when confined by microfabricated polydimethylsiloxane structures. Our study provides evidence for a role for environmental mechanical stiffness in the surveillance strategy of immature DCs in tissues.

Simulating CXCR5 Dynamics in Complex Tissue Microenvironments

To effectively navigate complex tissue microenvironments, immune cells sense molecular concentration gradients using G-protein coupled receptors. However, due to the complexity of receptor activity, and the multimodal nature of chemokine gradients in vivo, chemokine receptor activity in situ is poorly understood. To address this issue, we apply a modelling and simulation approach that permits analysis of the spatiotemporal dynamics of CXCR5 expression within an in silico B-follicle with single-cell resolution. Using this approach, we show that that in silico B-cell scanning is robust to changes in receptor numbers and changes in individual kinetic rates of receptor activity, but sensitive to global perturbations where multiple parameters are altered simultaneously. Through multi-objective optimization analysis we find that the rapid modulation of CXCR5 activity through receptor binding, desensitization and recycling is required for optimal antigen scanning rates. From these analyses we predict that chemokine receptor signaling dynamics regulate migration in complex tissue microenvironments to a greater extent than the total numbers of receptors on the cell surface.

High-throughput injection molded microfluidic device for single-cell analysis of spatiotemporal dynamics

Single-cell level analysis of various cellular behaviors has been aided by recent developments in microfluidic technology. Polydimethylsiloxane (PDMS)-based microfluidic devices have been widely used to elucidate cell differentiation and migration under spatiotemporal stimulation. However, microfluidic devices fabricated with PDMS have inherent limitations due to material issues and non-scalable fabrication process. In this study, we designed and fabricated an injection molded microfluidic device that enables real-time chemical profile control. This device is made of polystyrene (PS), engineered with channel dimensions optimized for injection molding to achieve functionality and compatibility with single cell observation. We demonstrated the spatiotemporal dynamics in the device with computational simulation and experiments. In temporal dynamics, we observed extracellular signal-regulated kinase (ERK) activation of PC12 cells by stimulating the cells with growth factors (GFs). Also, we confirmed yes-associated protein (YAP) phase separation of HEK293 cells under stimulation using sorbitol. In spatial dynamics, we observed the migration of NIH 3T3 cells (transfected with Lifeact-GFP) under different spatiotemporal stimulations of PDGF. Using the injection molded plastic devices, we obtained comprehensive data more easily than before while using less time compared to previous PDMS models. This easy-to-use plastic microfluidic device promises to open a new approach for investigating the mechanisms of cell behavior at the single-cell level.

Recapitulating monocyte extravasation to the synovium in an organotypic microfluidic model of the articular joint

The synovium of osteoarthritis (OA) patients can be characterized by an abnormal accumulation of macrophages originating from extravasated monocytes. Since targeting monocyte extravasation may represent a promising therapeutic strategy, our aim was to develop an organotypic microfluidic model recapitulating this process. Synovium and cartilage were modeled by hydrogel-embedded OA synovial fibroblasts and articular chondrocytes separated by a synovial fluid channel. The synovium compartment included a perfusable endothelialized channel dedicated to monocyte injection. Monocyte extravasation in response to chemokines and OA synovial fluid was quantified. The efficacy of chemokine receptor antagonists, RS-504393 (CCR2 antagonist) and Cenicriviroc (CCR2/CCR5 antagonist) in inhibiting extravasation was tested pre-incubating monocytes with the antagonists before injection. After designing and fabricating the chip, culture conditions were optimized to achieve an organotypic model including synovial fibroblasts, articular chondrocytes, and a continuous endothelial monolayer expressing intercellular adhesion molecule-1 and vascular cell adhesion molecule-1. A significantly higher number of monocytes extravasated in response to the chemokine mix (p< 0.01) and OA synovial fluid (p< 0.01), compared to a control condition. In both cases, endothelium pre-activation enhanced monocyte extravasation. The simultaneous blocking of CCR2 and CCR5 proved to be more effective (p< 0.001) in inhibiting monocyte extravasation in response to OA synovial fluid than blocking of CCR2 only (p< 0.01). The study of extravasation in the model provided direct evidence that OA synovial fluid induces monocytes to cross the endothelium and invade the synovial compartment. The model can be exploited either to test molecules antagonizing this process or to investigate the effect of extravasated monocytes on synovium and cartilage cells.

Modeling the Mechanobiology of Cancer Cell Migration Using 3D Biomimetic Hydrogels

Understanding how cancer cells migrate, and how this migration is affected by the mechanical and chemical composition of the extracellular matrix (ECM) is critical to investigate and possibly interfere with the metastatic process, which is responsible for most cancer-related deaths. In this article we review the state of the art about the use of hydrogel-based three-dimensional (3D) scaffolds as artificial platforms to model the mechanobiology of cancer cell migration. We start by briefly reviewing the concept and composition of the extracellular matrix (ECM) and the materials commonly used to recreate the cancerous ECM. Then we summarize the most relevant knowledge about the mechanobiology of cancer cell migration that has been obtained using 3D hydrogel scaffolds, and relate those discoveries to what has been observed in the clinical management of solid tumors. Finally, we review some recent methodological developments, specifically the use of novel bioprinting techniques and microfluidics to create realistic hydrogel-based models of the cancer ECM, and some of their applications in the context of the study of cancer cell migration.

Immobilisation of CXCL8 gradients in microfluidic devices for migration experiments

The release of inflammatory chemokines leads to the formation of chemokine gradients that result in the directed migration of immune cells to the site of injury. In this process, cells respond to soluble gradients (chemotaxis) as well as to immobilised gradients (haptotaxis). Surface-bound chemokine gradients are mostly presented by endothelial cells and supported by glycosaminoglycans (GAGs), such as heparan sulfate, involving the GAG binding site of chemokines. Microfluidic devices have been used to analyse cell migration along soluble chemokine gradients, as these devices allow the generation of stable gradients with resolutions in the range of microns. To immobilise well-controlled soluble gradients of interleukin-8 (CXCL8), an inflammatory chemokine, we developed a simple procedure using a heparin-coated PDMS-microfluidic device. We used these immobilised gradients for migration experiments with CXCL8-responsive THP-1 cells and confirmed directed cell migration. This setup might be useful for the examination of factors that may alter chemotaxis and haptotaxis as well as synergistic and antagonistic effects of other soluble and immobilised chemokines.

How Have Leukocyte In Vitro Chemotaxis Assays Shaped Our Ideas about Macrophage Migration?

Simple Summary

The migration of immune cells is vital during inflammatory responses. Macrophages, which are a subset of immune cells, are unique in the ways they migrate because they can switch between different mechanism of migration. This crucial feature of macrophage migration has been underappreciated in the literature because technologies used to study macrophage migration were not able to efficiently detect those subtle differences between macrophages and other immune cells. This review article describes popular technologies used to study macrophage migration and critically assesses their advantages and disadvantages in macrophage migration studies.

Abstract

Macrophage chemotaxis is crucial during both onset and resolution of inflammation and unique among all leukocytes. Macrophages are able to switch between amoeboid and mesenchymal migration to optimise their migration through 3D environments. This subtle migration phenotype has been underappreciated in the literature, with macrophages often being grouped and discussed together with other leukocytes, possibly due to the limitations of current chemotaxis assays. Transwell assays were originally designed in the 1960s but despite their long-known limitations, they are still one of the most popular methods of studying macrophage migration. This review aims to critically evaluate transwell assays, and other popular chemotaxis assays, comparing their advantages and limitations in macrophage migration studies.

Mechanical Forces Orchestrate Brain Development

During brain development, progenitors generate successive waves of neurons that populate distinct cerebral regions, where they settle and differentiate within layers or nuclei. While migrating and differentiating, neurons are subjected to mechanical forces arising from the extracellular matrix, and their interaction with neighboring cells. Changes in brain biomechanical properties, during its formation or aging, are converted in neural cells by mechanotransduction into intracellular signals that control key neurobiological processes. Here, we summarize recent findings that support the contribution of mechanobiology to neurodevelopment, with focus on the cerebral cortex. Also discussed are the existing toolbox and emerging technologies made available to assess and manipulate the physical properties of neurons and their environment.

Advances in single cell technologies in immunology

The immune system is composed of heterogeneous populations of immune cells that regulate physiological processes and protect organisms against diseases. Single cell technologies have been used to assess immune cell responses at the single cell level, which are crucial for identifying the causes of diseases and elucidating underlying biological mechanisms to facilitate medical therapy. In the present review we first discuss the most recent advances in the development of single cell technologies to investigate cell signaling, cell-cell interactions and cell migration. Each technology's advantages and limitations and its applications in immunology are subsequently reviewed. The latest progress toward commercialization, the remaining challenges and future perspectives for single cell technologies in immunology are also briefly discussed.

How single-cell immunology is benefiting from microfluidic technologies

The immune system is a complex network of specialized cells that work in concert to protect against invading pathogens and tissue damage. Imbalances in this network often result in excessive or absent immune responses leading to allergies, autoimmune diseases, and cancer. Many of the mechanisms and their regulation remain poorly understood. Immune cells are highly diverse, and an immune response is the result of a large number of molecular and cellular interactions both in time and space. Conventional bulk methods are often prone to miss important details by returning population-averaged results. There is a need in immunology to measure single cells and to study the dynamic interplay of immune cells with their environment. Advances in the fields of microsystems and microengineering gave rise to the field of microfluidics and its application to biology. Microfluidic systems enable the precise control of small volumes in the femto- to nanoliter range. By controlling device geometries, surface chemistry, and flow behavior, microfluidics can create a precisely defined microenvironment for single-cell studies with spatio-temporal control. These features are highly desirable for single-cell analysis and have made microfluidic devices useful tools for studying complex immune systems. In addition, microfluidic devices can achieve high-throughput measurements, enabling in-depth studies of complex systems. Microfluidics has been used in a large panel of biological applications, ranging from single-cell genomics, cell signaling and dynamics to cell-cell interaction and cell migration studies. In this review, we give an overview of state-of-the-art microfluidic techniques, their application to single-cell immunology, their advantages and drawbacks, and provide an outlook for the future of single-cell technologies in research and medicine.

Cellular locomotion using environmental topography

Eukaryotic cells migrate by coupling the intracellular force of the actin cytoskeleton to the environment. While force coupling is usually mediated by transmembrane adhesion receptors, especially those of the integrin family, amoeboid cells such as leukocytes can migrate extremely fast despite very low adhesive forces¹. Here we show that leukocytes cannot only migrate under low adhesion but can also transmit forces in the complete absence of transmembrane force coupling. When confined within three-dimensional environments, they use the topographical features of the substrate to propel themselves. Here the retrograde flow of the actin cytoskeleton follows the texture of the substrate, creating retrograde shear forces that are sufficient to drive the cell body forwards. Notably, adhesion-dependent and adhesion-independent migration are not mutually exclusive, but rather are variants of the same principle of coupling retrograde actin flow to the environment and thus can potentially operate interchangeably and simultaneously. As adhesion-free migration is independent of the chemical composition of the environment, it renders cells completely autonomous in their locomotive behaviour.

Microtubules control cellular shape and coherence in amoeboid migrating cells

Cells navigating through complex tissues face a fundamental challenge: while multiple protrusions explore different paths, the cell needs to avoid entanglement. How a cell surveys and then corrects its own shape is poorly understood. Here, we demonstrate that spatially distinct microtubule dynamics regulate amoeboid cell migration by locally promoting the retraction of protrusions. In migrating dendritic cells, local microtubule depolymerization within protrusions remote from the microtubule organizing center triggers actomyosin contractility controlled by RhoA and its exchange factor Lfc. Depletion of Lfc leads to aberrant myosin localization, thereby causing two effects that rate-limit locomotion: (1) impaired cell edge coordination during path finding and (2) defective adhesion resolution. Compromised shape control is particularly hindering in geometrically complex microenvironments, where it leads to entanglement and ultimately fragmentation of the cell body. We thus demonstrate that microtubules can act as a proprioceptive device: they sense cell shape and control actomyosin retraction to sustain cellular coherence.

Unique Cancer Migratory Behaviors in Confined Spaces of Microgroove Topography with Acute Wall Angles

In recent years, many types of micro-engineered platform have been fabricated to investigate the influences of surrounding microenvironments on cell migration. Previous researches demonstrated that microgroove-based topographies can influence cell motilities of normal and cancerous cells differently. In this study, the microgroove wall angle was altered from obtuse to acute angles and the resulting differences in the responses of normal and cancer cells were investigated to explore the geometrical characteristics that can efficiently distinguish normal and cancer cells. Interestingly, different trends in cell motilities of normal and cancer cells were observed as the wall angles were varied between 60–120°, and in particular, invasive cancer cells exhibited a unique, oscillatory migratory behavior. Results from the immunostaining of cell mechanotransduction components suggested that this difference stemmed from directional extensions and adhesion behaviors of each cell type. In addition, the specific behaviors of invasive cancer cells were found to be dependent on the myosin II activity, and modulating the activity could revert cancerous behaviors to normal ones. These novel findings on the interactions of acute angle walls and cancer cell migration provide a new perspective on cancer metastasis and additional strategies via microstructure geometries for the manipulations of cell behaviors in microscale biodevices.

Chemotactic Responses of Jurkat Cells in Microfluidic Flow-Free Gradient Chambers

Gradients of soluble molecules coordinate cellular communication in a diverse range of multicellular systems. Chemokine-driven chemotaxis is a key orchestrator of cell movement during organ development, immune response and cancer progression. Chemotaxis assays capable of examining cell responses to different chemokines in the context of various extracellular matrices will be crucial to characterize directed cell motion in conditions which mimic whole tissue conditions. Here, a microfluidic device which can generate different chemokine patterns in flow-free gradient chambers while controlling surface extracellular matrix (ECM) to study chemotaxis either at the population level or at the single cell level with high resolution imaging is presented. The device is produced by combining additive manufacturing (AM) and soft lithography. Generation of concentration gradients in the device were simulated and experimentally validated. Then, stable gradients were applied to modulate chemotaxis and chemokinetic response of Jurkat cells as a model for T lymphocyte motility. Live imaging of the gradient chambers allowed to track and quantify Jurkat cell migration patterns. Using this system, it has been found that the strength of the chemotactic response of Jurkat cells to CXCL12 gradient was reduced by increasing surface fibronectin in a dose-dependent manner. The chemotaxis of the Jurkat cells was also found to be governed not only by the CXCL12 gradient but also by the average CXCL12 concentration. Distinct migratory behaviors in response to chemokine gradients in different contexts may be physiologically relevant for shaping the host immune response and may serve to optimize the targeting and accumulation of immune cells to the inflammation site. Our approach demonstrates the feasibility of using a flow-free gradient chamber for evaluating cross-regulation of cell motility by multiple factors in different biologic processes.

The multilayered complexity of the chemokine receptor system

The chemokines receptor family are membrane-expressed class A-specific seven-transmembrane receptors linked to G proteins. Through interaction with the corresponding ligands, the chemokines, they induce a wide variety of cellular responses including cell polarization, movement, immune and inflammatory responses, as well as the prevention of HIV-1 infection. Like a Russian matryoshka doll, the chemokine receptor system is more complex than initially envisaged. This review focuses on the mechanisms that contribute to this dazzling complexity and how they modulate the signaling events triggered by chemokines. The chemokines and their receptors exist as monomers, dimers and oligomers, their expression pattern is highly regulated, and the ligands can bind distinct receptors with similar affinities. The use of novel imaging-based technologies, particularly real-time imaging modalities, has shed new light on the very dynamic conformations that chemokine receptors adopt depending on the cellular context, and that affect chemokine-mediated responses. This complex scenario presents both challenging and exciting opportunities for drug discovery.

Concepts of GPCR-controlled navigation in the immune system

G‐protein–coupled receptor (GPCR) signaling is essential for the spatiotemporal control of leukocyte dynamics during immune responses. For efficient navigation through mammalian tissues, most leukocyte types express more than one GPCR on their surface and sense a wide range of chemokines and chemoattractants, leading to basic forms of leukocyte movement (chemokinesis, haptokinesis, chemotaxis, haptotaxis, and chemorepulsion). How leukocytes integrate multiple GPCR signals and make directional decisions in lymphoid and inflamed tissues is still subject of intense research. Many of our concepts on GPCR‐controlled leukocyte navigation in the presence of multiple GPCR signals derive from in vitro chemotaxis studies and lower vertebrates. In this review, we refer to these concepts and critically contemplate their relevance for the directional movement of several leukocyte subsets (neutrophils, T cells, and dendritic cells) in the complexity of mouse tissues. We discuss how leukocyte navigation can be regulated at the level of only a single GPCR (surface expression, competitive antagonism, oligomerization, homologous desensitization, and receptor internalization) or multiple GPCRs (synergy, hierarchical and non‐hierarchical competition, sequential signaling, heterologous desensitization, and agonist scavenging). In particular, we will highlight recent advances in understanding GPCR‐controlled leukocyte navigation by intravital microscopy of immune cells in mice.

Lymphocyte perform reverse adhesive haptotaxis mediated by integrins LFA-1

Cell Guidance by anchored molecules, or haptotaxis, is crucial in development, immunology and cancer. Adhesive haptotaxis, or guidance by adhesion molecules, is well established for mesenchymal cells like fibroblasts, whereas its existence remains unreported for amoeboid cells that require less or no adhesion to migrate. We show here in vitro that amoeboid human T lymphocytes develop adhesive haptotaxis versus densities of integrin ligands expressed by high endothelial venules. Moreover, lymphocytes orient towards increasing adhesion with VLA-4 integrins, like all mesenchymal cells, but towards decreasing adhesion with LFA-1 integrins, which has never been observed. This counterintuitive ‘reverse haptotaxis’ cannot be explained with the existing mesenchymal mechanisms of competition between cells’ pulling edges or of lamellipodia growth activated by integrins, which favor orientation towards increasing adhesion. Mechanisms and functions of amoeboid adhesive haptotaxis remain unclear, however multidirectional integrin-mediated haptotaxis may operate around transmigration ports on endothelium, stromal cells in lymph nodes, and inflamed tissue where integrin ligands are spatially modulated.

Frontiers in Microfluidics, a Teaching Resource Review

This is a literature teaching resource review for biologically inspired microfluidics courses or exploring the diverse applications of microfluidics. The structure is around key papers and model organisms. While courses gradually change over time, a focus remains on understanding how microfluidics has developed as well as what it can and cannot do for researchers. As a primary starting point, we cover micro-fluid mechanics principles and microfabrication of devices. A variety of applications are discussed using model prokaryotic and eukaryotic organisms from the set of bacteria (Escherichia coli), trypanosomes (Trypanosoma brucei), yeast (Saccharomyces cerevisiae), slime molds (Physarum polycephalum), worms (Caenorhabditis elegans), flies (Drosophila melangoster), plants (Arabidopsis thaliana), and mouse immune cells (Mus musculus). Other engineering and biochemical methods discussed include biomimetics, organ on a chip, inkjet, droplet microfluidics, biotic games, and diagnostics. While we have not yet reached the end-all lab on a chip, microfluidics can still be used effectively for specific applications.

An Integrated Pipeline for Combining in vitro Data and Mathematical Models Using a Bayesian Parameter Inference Approach to Characterize Spatio-temporal Chemokine Gradient Formation

All protective and pathogenic immune and inflammatory responses rely heavily on leukocyte migration and localization. Chemokines are secreted chemoattractants that orchestrate the positioning and migration of leukocytes through concentration gradients. The mechanisms underlying chemokine gradient establishment and control include physical as well as biological phenomena. Mathematical models offer the potential to both understand this complexity and suggest interventions to modulate immune function. Constructing models that have powerful predictive capability relies on experimental data to estimate model parameters accurately, but even with a reductionist approach most experiments include multiple cell types, competing interdependent processes and considerable uncertainty. Therefore, we propose the use of reduced modeling and experimental frameworks in complement, to minimize the number of parameters to be estimated. We present a Bayesian optimization framework that accounts for advection and diffusion of a chemokine surrogate and the chemokine CCL19, transport processes that are known to contribute to the establishment of spatio-temporal chemokine gradients. Three examples are provided that demonstrate the estimation of the governing parameters as well as the underlying uncertainty. This study demonstrates how a synergistic approach between experimental and computational modeling benefits from the Bayesian approach to provide a robust analysis of chemokine transport. It provides a building block for a larger research effort to gain holistic insight and generate novel and testable hypotheses in chemokine biology and leukocyte trafficking.

Microfluidics as an Emerging Precision Tool in Developmental Biology

Microfluidics has become a precision tool in modern biology. It enables omics data to be obtained from individual cells, as compared to averaged signals from cell populations, and it allows manipulation of biological specimens in entirely new ways. Cells and organisms can be perturbed at extraordinary spatiotemporal resolution, revealing mechanistic insights that would otherwise remain hidden. In this perspective article, we discuss the current and future impact of microfluidic technology in the field of developmental biology. In addition, we provide detailed information on how to start using this technology even without prior experience.

Human Bone Marrow-Derived Mesenchymal Stem Cells Home via the PI3K-Akt, MAPK, and Jak/Stat Signaling Pathways in Response to Platelet-Derived Growth Factor

Mesenchymal stem cells (MSCs) have great potential to improve clinical outcomes for many inflammatory and degenerative diseases either through intravenously delivered MSCs or through mobilization and migration of endogenous MSCs to injury sites, termed "stem cell homing." Stem cell homing involves the processes of attachment to and transmigration through endothelial cells lining the vasculature and migration through the tissue stroma to a site of injury or inflammation. Although the process of leukocyte transendothelial migration (TEM) is well understood, far less is known about stem cell homing. In this study, a transwell-based model was developed to monitor adherence and TEM of human MSCs in response to chemokine exposure. Specifically, transwell membranes lined with human synovial microvascular endothelial cells were partitioned from the tissue injury-mimetic site containing chemokine stromal cell-derived factor-1 (SDF-1). Two population subsets of MSCs were studied: migratory cells that initiated transmigration on the endothelial lining and nonmigratory cells. We hypothesized that cells would adhere to and migrate through the endothelial lining in response to SDF-1 exposure and that gene and protein expression changes would be observed between migratory and nonmigratory cells. We validated a vasculature model for MSC transmigration that showed increased expression of several genes and activation of proteins of the PI3K-Akt, MAPK, and Jak/Stat signaling pathways. These findings showed that MSC homing may be driven by activation of PDGFRA/PI3K/Akt, PDGFRA/MAPK/Grb2, and PDGFRA/Jak2/Stat signaling, as a result of SDF-1-stimulated endothelial cell production of platelet-derived growth factor. This model can be used to further investigate these key regulatory molecules toward the development of targeted therapies.

Increasing access to microfluidics for studying fungi and other branched biological structures

BACKGROUND

Microfluidic systems are well-suited for studying mixed biological communities for improving industrial processes of fermentation, biofuel production, and pharmaceutical production. The results of which have the potential to resolve the underlying mechanisms of growth and transport in these complex branched living systems. Microfluidics provide controlled environments and improved optical access for real-time and high-resolution imaging studies that allow high-content and quantitative analyses. Studying growing branched structures and the dynamics of cellular interactions with both biotic and abiotic cues provides context for molecule production and genetic manipulations. To make progress in this arena, technical and logistical barriers must be overcome to more effectively deploy microfluidics in biological disciplines. A principle technical barrier is the process of assembling, sterilizing, and hydrating the microfluidic system; the lack of the necessary equipment for the preparatory process is a contributing factor to this barrier. To improve access to microfluidic systems, we present the development, characterization, and implementation of a microfluidics assembly and packaging process that builds on self-priming point-of-care principles to achieve "ready-to-use microfluidics."

RESULTS

We present results from domestic and international collaborations using novel microfluidic architectures prepared with a unique packaging protocol. We implement this approach by focusing primarily on filamentous fungi; we also demonstrate the utility of this approach for collaborations on plants and neurons. In this work we (1) determine the shelf-life of ready-to-use microfluidics, (2) demonstrate biofilm-like colonization on fungi, (3) describe bacterial motility on fungal hyphae (fungal highway), (4) report material-dependent bacterial-fungal colonization, (5) demonstrate germination of vacuum-sealed Arabidopsis seeds in microfluidics stored for up to 2 weeks, and (6) observe bidirectional cytoplasmic streaming in fungi.

CONCLUSIONS

This pre-packaging approach provides a simple, one step process to initiate microfluidics in any setting for fungal studies, bacteria-fungal interactions, and other biological inquiries. This process improves access to microfluidics for controlling biological microenvironments, and further enabling visual and quantitative analysis of fungal cultures.

Colorectal tumor-on-a-chip system: A 3D tool for precision onco-nanomedicine

Awareness that traditional two-dimensional (2D) in vitro and nonrepresentative animal models may not completely emulate the 3D hierarchical complexity of tissues and organs is on the rise. Therefore, posterior translation into successful clinical application is compromised. To address this dearth, on-chip biomimetic microenvironments powered by microfluidic technologies are being developed to better capture the complexity of in vivo pathophysiology. Here, we describe a "tumor-on-a-chip" model for assessment of precision nanomedicine delivery on which we validate the efficacy of drug-loaded nanoparticles in a gradient fashion. The model validation was performed by viability studies integrated with live imaging to confirm the dose-response effect of cells exposed to the CMCht/PAMAM nanoparticle gradient. This platform also enables the analysis at the gene expression level, where a down-regulation of all the studied genes (MMP-1, Caspase-3, and Ki-67) was observed. This tumor-on-chip model represents an important development in the use of precision nanomedicine toward personalized treatment.

Immature dendritic cells navigate microscopic mazes to find tumor cells

Dendritic cells (DCs) are potent antigen-presenting cells with high sentinel ability to scan their neighborhood and to initiate an adaptive immune response. Whereas chemotactic migration of mature DCs (mDCs) towards lymph nodes is relatively well documented, the migratory behavior of immature DCs (imDCs) in tumor microenvironments is still poorly understood. Here, microfluidic systems of various geometries, including mazes, are used to investigate how the physical and chemical microenvironment influences the migration pattern of imDCs. Under proper degree of confinement, the imDCs are preferentially recruited towards cancer vs. normal cells, accompanied by increased cell speed and persistence. Furthermore, a systematic screen of cytokines, reveals that Gas6 is a major chemokine responsible for the chemotactic preference. These results and the accompanying theoretical model suggest that imDC migration in complex tissue environments is tuned by a proper balance between the strength of the chemical gradients and the degree of spatial confinement.

Nuclear positioning facilitates amoeboid migration along the path of least resistance

During metazoan development, immune surveillance and cancer dissemination, cells migrate in complex three-dimensional microenvironments^1-3. These spaces are crowded by cells and extracellular matrix, generating mazes with differently sized gaps that are typically smaller than the diameter of the migrating cell^4,5. Most mesenchymal and epithelial cells and some-but not all-cancer cells actively generate their migratory path using pericellular tissue proteolysis⁶. By contrast, amoeboid cells such as leukocytes use non-destructive strategies of locomotion⁷, raising the question how these extremely fast cells navigate through dense tissues. Here we reveal that leukocytes sample their immediate vicinity for large pore sizes, and are thereby able to choose the path of least resistance. This allows them to circumnavigate local obstacles while effectively following global directional cues such as chemotactic gradients. Pore-size discrimination is facilitated by frontward positioning of the nucleus, which enables the cells to use their bulkiest compartment as a mechanical gauge. Once the nucleus and the closely associated microtubule organizing centre pass the largest pore, cytoplasmic protrusions still lingering in smaller pores are retracted. These retractions are coordinated by dynamic microtubules; when microtubules are disrupted, migrating cells lose coherence and frequently fragment into migratory cytoplasmic pieces. As nuclear positioning in front of the microtubule organizing centre is a typical feature of amoeboid migration, our findings link the fundamental organization of cellular polarity to the strategy of locomotion.

Deconstructing Immune Microenvironments of Lymphoid Tissues for Reverse Engineering

The immune microenvironment presents a diverse panel of cues that impacts immune cell migration, organization, differentiation, and the immune response. Uniquely, both the liquid and solid phases of every specific immune niche within the body play an important role in defining cellular functions in immunity at that particular location. The in vivo immune microenvironment consists of biomechanical and biochemical signals including their gradients, surface topography, dimensionality, modes of ligand presentation, and cell-cell interactions, and the ability to recreate these immune biointerfaces in vitro can provide valuable insights into the immune system. This manuscript reviews the critical roles played by different immune cells and surveys the current progress of model systems for reverse engineering of immune microenvironments with a focus on lymphoid tissues.

Microfluidic Platform to Quantify Neutrophil Migratory Decision-Making

Neutrophils are the most abundant leukocytes in blood, serving as the first line of host defense in tissue damage and infections. Upon activation by chemokines released from pathogens or injured tissues, neutrophils migrate through complex tissue microenvironments toward sites of infections along the chemokine gradients, in a process named chemotaxis. However, current methods for measuring neutrophil chemotaxis require large volumes of blood and are often bulk, endpoint measurements. To address the need for rapid and robust assays, we engineered a novel dual gradient microfluidic platform that precisely quantifies neutrophil migratory decision-making with high temporal resolution. Here, we present a protocol to measure neutrophil migratory phenotypes (velocity, directionality) with single-cell resolution.

Tunable DNA Hybridization Enables Spatially and Temporally Controlled Surface-Anchoring of Biomolecular Cargo

The controlled immobilization of biomolecules onto surfaces is relevant in biosensing and cell biological research. Spatial control is achieved by surface-tethering molecules in micro- or nanoscale patterns. Yet, there is an increasing demand for temporal control over how long biomolecular cargo stays immobilized until released into the medium. Here, we present a DNA hybridization-based approach to reversibly anchor biomolecular cargo onto micropatterned surfaces. Cargo is linked to a DNA oligonucleotide that hybridizes to a sequence-complementary, surface-tethered strand. The cargo is released from the substrate by the addition of an oligonucleotide that disrupts the duplex interaction via toehold-mediated strand displacement. The unbound tether strand can be reloaded. The generic strategy is implemented with small-molecule or protein cargo, varying DNA sequences, and multiple surface patterning routes. The approach may be used as a tool in biological research to switch membrane proteins from a locally fixed to a free state, or in biosensing to shed biomolecular receptors to regenerate the sensor surface.

Fluorescently Tagged CCL19 and CCL21 to Monitor CCR7 and ACKR4 Functions

Chemokines are essential guidance cues orchestrating cell migration in health and disease. Cognate chemokine receptors sense chemokine gradients over short distances to coordinate directional cell locomotion. The chemokines CCL19 and CCL21 are essential for recruiting CCR7-expressing dendritic cells bearing pathogen-derived antigens and lymphocytes to lymph nodes, where the two cell types meet to launch an adaptive immune response against the invading pathogen. CCR7-expressing cancer cells are also recruited by CCL19 and CCL21 to metastasize in lymphoid organs. In contrast, atypical chemokine receptors (ACKRs) do not transmit signals required for cell locomotion but scavenge chemokines. ACKR4 is crucial for internalizing and degrading CCL19 and CCL21 to establish local gradients, which are sensed by CCR7-expressing cells. Here, we describe the production of fluorescently tagged chemokines by fusing CCL19 and CCL21 to monomeric red fluorescent protein (mRFP). We show that purified CCL19-mRFP and CCL21-mRFP are versatile and powerful tools to study CCR7 and ACKR4 functions, such as receptor trafficking and chemokine scavenging, in a spatiotemporal fashion. We demonstrate that fluorescently tagged CCL19 and CCL21 permit the visualization and quantification of chemokine gradients in real time, while CCR7-expressing leukocytes and cancer cells sense the guidance cues and migrate along the chemokine gradients.

Viable cell culture in PDMS-based microfluidic devices

Microfluidics has played a vital role in developing novel methods to investigate biological phenomena at the molecular and cellular level during the last two decades. Microscale engineering of cellular systems is nevertheless a nascent field marked inherently by frequent disruptive advancements in technology such as PDMS-based soft lithography. Viable culture and manipulation of cells in microfluidic devices requires knowledge across multiple disciplines including molecular and cellular biology, chemistry, physics, and engineering. There has been numerous excellent reviews in the past 15 years on applications of microfluidics for molecular and cellular biology including microfluidic cell culture (Berthier et al., 2012; El-Ali, Sorger, & Jensen, 2006; Halldorsson et al., 2015; Kim et al., 2007; Mehling & Tay, 2014; Sackmann et al., 2014; Whitesides, 2006; Young & Beebe, 2010), cell culture models (Gupta et al., 2016; Inamdar & Borenstein, 2011; Meyvantsson & Beebe, 2008), cell secretion (Schrell et al., 2016), chemotaxis (Kim & Wu, 2012; Wu et al., 2013), neuron culture (Millet & Gillette, 2012a, 2012b), drug screening (Dittrich & Manz, 2006; Eribol, Uguz, & Ulgen, 2016; Wu, Huang, & Lee, 2010), cell sorting (Autebert et al., 2012; Bhagat et al., 2010; Gossett et al., 2010; Wyatt Shields Iv, Reyes, & López, 2015), single cell studies (Lecault et al., 2012; Reece et al., 2016; Yin & Marshall, 2012), stem cell biology (Burdick & Vunjak-Novakovic, 2009; Wu et al., 2011; Zhang & Austin, 2012), cell differentiation (Zhang et al., 2017a), systems biology (Breslauer, Lee, & Lee, 2006), 3D cell culture (Huh et al., 2011; Li et al., 2012; van Duinen et al., 2015), spheroids and organoids (Lee et al., 2016; Montanez-Sauri, Beebe, & Sung, 2015; Morimoto & Takeuchi, 2013; Skardal et al., 2016; Young, 2013), organ-on-chip (Bhatia & Ingber, 2014; Esch, Bahinski, & Huh, 2015; Huh et al., 2011; van der Meer & van den Berg, 2012), and tissue engineering (Andersson & Van Den Berg, 2004; Choi et al., 2007; Hasan et al., 2014). In this chapter, we provide an overview of PDMS-based microdevices for microfluidic cell culture. We discuss the advantages and challenges of using PDMS-based soft lithography for microfluidic cell culture and highlight recent progress and future directions in this area.

Integrating Immunology and Microfluidics for Single Immune Cell Analysis

The field of immunoengineering aims to develop novel therapies and modern vaccines to manipulate and modulate the immune system and applies innovative technologies toward improved understanding of the immune system in health and disease. Microfluidics has proven to be an excellent technology for analytics in biology and chemistry. From simple microsystem chips to complex microfluidic designs, these platforms have witnessed an immense growth over the last decades with frequent emergence of new designs. Microfluidics provides a highly robust and precise tool which led to its widespread application in single-cell analysis of immune cells. Single-cell analysis allows scientists to account for the heterogeneous behavior of immune cells which often gets overshadowed when conventional bulk study methods are used. Application of single-cell analysis using microfluidics has facilitated the identification of several novel functional immune cell subsets, quantification of signaling molecules, and understanding of cellular communication and signaling pathways. Single-cell analysis research in combination with microfluidics has paved the way for the development of novel therapies, point-of-care diagnostics, and even more complex microfluidic platforms that aid in creating in vitro cellular microenvironments for applications in drug and toxicity screening. In this review, we provide a comprehensive overview on the integration of microsystems and microfluidics with immunology and focus on different designs developed to decode single immune cell behavior and cellular communication. We have categorized the microfluidic designs in three specific categories: microfluidic chips with cell traps, valve-based microfluidics, and droplet microfluidics that have facilitated the ongoing research in the field of immunology at single-cell level.

Angiogenic Self-Assembling Peptide Scaffolds for Functional Tissue Regeneration

Implantation of acellular biomimetic scaffolds with proangiogenic motifs may have exciting clinical utility for the treatment of ischemic pathologies such as myocardial infarction. Although direct delivery of angiogenic proteins is a possible treatment option, smaller synthetic peptide-based nanostructured alternatives are being investigated due to favorable factors, such as sustained efficacy and high-density epitope presentation of functional moieties. These peptides may be implanted in vivo at the site of ischemia, bypassing the first-pass metabolism and enabling long-term retention and sustained efficacy. Mimics of angiogenic proteins show tremendous potential for clinical use. We discuss possible approaches to integrate the functionality of such angiogenic peptide mimics into self-assembled peptide scaffolds for application in functional tissue regeneration.