Evaluation of Single-Cell Cytokine Secretion and Cell-Cell Interactions with a Hierarchical Loading Microwell Chip

CellMag-CARWash: A High Throughput Droplet Microfluidic Device for Live Cell Isolation and Single Cell Applications

The recent push toward understanding an individual cell's behavior and identifying cellular heterogeneity has created an unmet need for technologies that can probe live cells at the single-cell level. Cells within a population are known to exhibit heterogeneous responses to environmental cues. These differences can lead to varied cellular states, behavior, and responses to therapeutics. Techniques are needed that are not only capable of processing and analyzing cellular populations at the single cell level, but also have the ability to isolate specific cell populations from a complex sample at high throughputs. The new CellMag-Coalesce-Attract-Resegment Wash (CellMag-CARWash) system combines positive magnetic selection with droplet microfluidic devices to isolate cells of interest from a mixture with >93% purity and incorporate treatments within individual droplets to observe single cell biological responses. This workflow is shown to be capable of probing the single cell extracellular vesicle (EV) secretion of MCF7 GFP cells. This article reports the first measurement of β-Estradiol's effect on EV secretion from MCF7 cells at the single cell level. Single cell processing revealed that MCF7 GFP cells possess a heterogeneous response to β-Estradiol stimulation with a 1.8-fold increase relative to the control.

A Capillary-Force-Driven, Single-Cell Transfer Method for Studying Rare Cells

The characterization of individual cells within heterogeneous populations (e.g., rare tumor cells in healthy blood cells) has a great impact on biomedical research. To investigate the properties of these specific cells, such as genetic biomarkers and/or phenotypic characteristics, methods are often developed for isolating rare cells among a large number of background cells before studying their genetic makeup and others. Prior to using real-world samples, these methods are often evaluated and validated by spiking cells of interest (e.g., tumor cells) into a sample matrix (e.g., healthy blood) as model samples. However, spiking tumor cells at extremely low concentrations is challenging in a standard laboratory setting. People often circumvent the problem by diluting a solution of high-concentration cells, but the concentration becomes inaccurate after series dilution due to the fact that a cell suspension solution can be inhomogeneous, especially when the cell concentration is very low. We report on an alternative method for low-cost, accurate, and reproducible low-concentration cell spiking without the use of external pumping systems. By inducing a capillary force from sudden pressure drops, a small portion of the cellular membrane was aspirated into the reservoir tip, allowing for non-destructive single-cell transfer. We investigated the surface membrane tensions induced by cellular aspiration and studied a range of tip/tumor cell diameter combinations, ensuring that our method does not affect cell viability. In addition, we performed single-cell capture and transfer control experiments using human acute lymphoblastic leukemia cells (CCRF-CEM) to develop calibrated data for the general production of low-concentration samples. Finally, we performed affinity-based tumor cell isolation using this method to generate accurate concentrations ranging from 1 to 15 cells/mL.

Understanding Macrophage-Tumor Interactions: Insights from Single-Cell Behavior Monitoring in a Sessile Microdroplet System

Interaction between tumor-associated macrophages and tumor cells is crucial for tumor development, metastasis, and the related immune process. However, the macrophages are highly heterogeneous spanning from anti-tumorigenic to pro-tumorigenic, which needs to be understood at the single-cell level. Herein, a sessile microdroplet system designed for monitoring cellular behavior and analyzing intercellular interaction, demonstrated with macrophage-tumor cell pairs is presented. An automatic procedure based on the inkjet printing method is utilized for the precise pairing and co-encapsulation of heterotypic cells within picoliter droplets. The sessile nature of microdroplets ensures controlled fusion and provides stable environments conducive to adherent cell culture. The nitric oxide generation and morphological changes over incubation are explored to reveal the complicated interactions from a single-cell perspective. The immune response of macrophages under distinct cellular microenvironments is recorded. The results demonstrate that the tumor microenvironment displays a modulating role in polarizing macrophages from anti-tumorigenic into pro-tumorigenic phenotype. The approach provides a versatile and compatible platform to investigate intercellular interaction at the single-cell level, showing promising potential for advancing single-cell behavior studies.

Microarray Platforms Based on 3D Printing

This paper introduces an innovative method for the fabrication and infusion of microwell arrays based on digital light processing (DLP) 3D printing. A low-cost DLP 3D printer is employed to fabricate microstructures rapidly with a broad dynamic range while maintaining high precision and fidelity. We constructed microwell arrays with varying diameters, from 200 to 2000 μm and multiple aspect ratios, in addition to microchannels with widths ranging from 45 to 1000 μm, proving the potential and flexibility of this fabrication method. The superimposition of parallel microchannels onto the microwell array, facilitated by positive or negative pressure, enabled the transfer of liquid to the microwells. Upon removal of the microchannel chip, a dispensed microdroplet array was obtained. This array can be modulated by adjusting the volume of the microwells and the inflow fluid. The filled microwell array allows chip-to-chip dispensing to the microreactor array through binding and centrifugation, facilitating multistep and multireagent assays. The 3D printing approach also enables the fabrication of intricate cavity designs, such as micropyramid arrays, which can be integrated with parallel microchannels to generate spheroid flowcells. This device demonstrated the ability to generate spheroids and manipulate their environment. We have successfully utilized precise modulation of spheroids size and performed parallel drug dose-response assays to evaluate its effectiveness. Furthermore, we managed to execute dynamic drug combinations based on a compact spheroids array, utilizing two orthogonal parallel microchannels. Our findings suggest that both the combination and temporal sequence of drug administration have a significant impact on therapeutic outcomes.

Label-free separation of peripheral blood mononuclear cells from whole blood by gradient acoustic focusing

Efficient techniques for separating target cells from undiluted blood are necessary for various diagnostic and research applications. This paper presents acoustic focusing in dense media containing iodixanol to purify peripheral blood mononuclear cells (PBMCs) from whole blood in a label-free and flow-through format. If the blood is laminated or mixed with iodixanol solutions while passing through the resonant microchannel, all the components (fluids and cells) rearrange according to their acoustic impedances. Red blood cells (RBCs) have higher effective acoustic impedance than PBMCs. Therefore, they relocate to the pressure node despite the dense medium, while PBMCs stay near the channel walls due to their negative contrast factor relative to their surrounding medium. By modifying the medium and thus tuning the contrast factor of the cells, we enriched PBMCs relative to RBCs by a factor of 3600 to 11,000 and with a separation efficiency of 85%. That level of RBC depletion is higher than most other microfluidic methods and similar to that of density gradient centrifugation. The current acoustophoretic chip runs up to 20 µl/min undiluted whole blood and can be integrated with downstream analysis.

Identification of druggable regulators of cell secretion via a kinome-wide screen and high-throughput immunomagnetic cell sorting

The identification of genetic regulators of cell secretions is challenging because it requires the sorting of a large number of cells according to their secretion patterns. Here we report the development and applicability of a high-throughput microfluidic method for the analysis of the secretion levels of large populations of immune cells. The method is linked with a kinome-wide loss-of-function CRISPR screen, immunomagnetically sorting the cells according to their secretion levels, and the sequencing of their genomes to identify key genetic modifiers of cell secretion. We used the method, which we validated against flow cytometry for cytokines secreted from primary mouse CD4+ (cluster of differentiation 4-positive) T cells, to discover a subgroup of highly co-expressed kinase-coding genes that regulate interferon-gamma secretion by these cells. We validated the function of the kinases identified using RNA interference, CRISPR knockouts and kinase inhibitors and confirmed the druggability of selected kinases via the administration of a kinase inhibitor in an animal model of colitis. The technique may facilitate the discovery of regulatory mechanisms for immune-cell activation and of therapeutic targets for autoimmune diseases.

Expanding CAR-T cell immunotherapy horizons through microfluidics

Chimeric antigen receptor (CAR)-T cell therapies have revolutionized cancer treatment, particularly in hematological malignancies. However, their application to solid tumors is limited, and they face challenges in safety, scalability, and cost. To enhance current CAR-T cell therapies, the integration of microfluidic technologies, harnessing their inherent advantages, such as reduced sample consumption, simplicity in operation, cost-effectiveness, automation, and high scalability, has emerged as a powerful solution. This review provides a comprehensive overview of the step-by-step manufacturing process of CAR-T cells, identifies existing difficulties at each production stage, and discusses the successful implementation of microfluidics and related technologies in addressing these challenges. Furthermore, this review investigates the potential of microfluidics-based methodologies in advancing cell-based therapy across various applications, including solid tumors, next-generation CAR constructs, T-cell receptors, and the development of allogeneic "off-the-shelf" CAR products.

Antibodies, repertoires and microdevices in antibody discovery and characterization

Therapeutic antibodies are paramount in treating a wide range of diseases, particularly in auto-immunity, inflammation and cancer, and novel antibody candidates recognizing a vast array of novel antigens are needed to expand the usefulness and applications of these powerful molecules. Microdevices play an essential role in this challenging endeavor at various stages since many general requirements of the overall process overlap nicely with the general advantages of microfluidics. Therefore, microfluidic devices are rapidly taking over various steps in the process of new candidate isolation, such as antibody characterization and discovery workflows. Such technologies can allow for vast improvements in time-lines and incorporate conservative antibody stability and characterization assays, but most prominently screenings and functional characterization within integrated workflows due to high throughput and standardized workflows. First, we aim to provide an overview of the challenges of developing new therapeutic candidates, their repertoires and requirements. Afterward, this review focuses on the discovery of antibodies using microfluidic systems, technological aspects of micro devices and small-scale antibody protein characterization and selection, as well as their integration and implementation into antibody discovery workflows. We close with future developments in microfluidic detection and antibody isolation principles and the field in general.

Microfluidic-based platforms for cell-to-cell communication studies

Intercellular communication is critical to the understanding of human health and disease progression. However, compared to traditional methods with inefficient analysis, microfluidic co-culture technologies developed for cell-cell communication research can reliably analyze crucial biological processes, such as cell signaling, and monitor dynamic intercellular interactions under reproducible physiological cell co-culture conditions. Moreover, microfluidic-based technologies can achieve precise spatial control of two cell types at the single-cell level with high throughput. Herein, this review focuses on recent advances in microfluidic-based 2D and 3D devices developed to confine two or more heterogeneous cells in the study of intercellular communication and decipher the advantages and limitations of these models in specific cellular research scenarios. This review will stimulate the development of more functionalized microfluidic platforms for biomedical research, inspiring broader interests across various disciplines to better comprehend cell-cell communication and other fields, such as tumor heterogeneity and drug screening.

Microfluidic device combining hydrodynamic and dielectrophoretic trapping for the controlled contact between single micro-sized objects and application to adhesion assays

The understanding of cell-cell and cell-matrix interactions via receptor and ligand binding relies on our ability to study the very first events of their contact. Of particular interest is the interaction between a T cell receptor and its cognate peptide-major histocompatibility complex. Indeed, analyzing their binding kinetics and cellular avidity in large-scale low-cost and fast cell sorting would largely facilitate the access to cell-based cancer immunotherapies. We thus propose a microfluidic tool able to independently control two types of micro-sized objects, put them in contact for a defined time and probe their adhesion state. The device consists of hydrodynamic traps holding the first type of cell from below against the fluid flow, and a dielectrophoretic system to force the second type of object to remain in contact with the first one. First, the device is validated by performing an adhesion frequency assay between fibroblasts and fibronectin coated beads. Then, a study is conducted on the modification of the cellular environment to match the dielectrophoretic technology requirements without modifying the cell viability and interaction functionalities. Finally, we demonstrate the capability of the developed device to put cancer cells and a population of T cells in contact and show the discrimination between specific and non-specific interactions based on the pair lifetime. This proof-of-concept device lays the foundations for the development of next generation fast cell-cell interaction technologies.

Highly efficient cell-microbead encapsulation using dielectrophoresis-assisted dual-nanowell array

Recent advancements in micro/nanofabrication techniques have led to the development of portable devices for high-throughput single-cell analysis through the isolation of individual target cells, which are then paired with functionalized microbeads. Compared with commercially available benchtop instruments, portable microfluidic devices can be more widely and cost-effectively adopted in single-cell transcriptome and proteome analysis. The sample utilization and cell pairing rate (∼33%) of current stochastic-based cell-bead pairing approaches are fundamentally limited by Poisson statistics. Despite versatile technologies having been proposed to reduce randomness during the cell-bead pairing process in order to statistically beat the Poisson limit, improvement of the overall pairing rate of a single cell to a single bead is typically based on increased operational complexity and extra instability. In this article, we present a dielectrophoresis (DEP)-assisted dual-nanowell array (ddNA) device, which employs an innovative microstructure design and operating process that decouples the bead- and cell-loading processes. Our ddNA design contains thousands of subnanoliter microwell pairs specifically tailored to fit both beads and cells. Interdigitated electrodes (IDEs) are placed below the microwell structure to introduce a DEP force on cells, yielding high single-cell capture and pairing rates. Experimental results with human embryonic kidney cells confirmed the suitability and reproducibility of our design. We achieved a single-bead capture rate of >97% and a cell-bead pairing rate of >75%. We anticipate that our device will enhance the application of single-cell analysis in practical clinical use and academic research.

FLUIDOT: A Modular Microfluidic Platform for Single-Cell Study and Retrieval, with Applications in Drug Tolerance Screening and Antibody Mining

Advancements in lab-on-a-chip technologies have revolutionized the single-cell analysis field. However, an accessible platform for in-depth screening and specific retrieval of single cells, which moreover enables studying diverse cell types and performing various downstream analyses, is still lacking. As a solution, FLUIDOT is introduced, a versatile microfluidic platform incorporating customizable microwells, optical tweezers and an interchangeable cell-retrieval system. Thanks to its smart microfluidic design, FLUIDOT is straightforward to fabricate and operate, rendering the technology widely accessible. The performance of FLUIDOT is validated and its versatility is subsequently demonstrated in two applications. First, drug tolerance in yeast cells is studied, resulting in the discovery of two treatment-tolerant populations. Second, B cells from convalescent COVID-19 patients are screened, leading to the discovery of highly affine, in vitro neutralizing monoclonal antibodies against SARS-CoV-2. Owing to its performance, flexibility, and accessibility, it is foreseen that FLUIDOT will enable phenotypic and genotypic analysis of diverse cell samples and thus elucidate unexplored biological questions.

Microwell array chip-based single-cell analysis

Single-cell profiling is key to uncover the cellular heterogeneity and drives deep understanding of cell fate. In recent years, microfluidics has become an ideal tool for single-cell profiling owing to its benefits of high throughput and automation. Among various microfluidic platforms, microwell has the advantages of simple operation and easy integration with in situ analysis ability, making it an ideal technique for single-cell studies. Herein, recent advances of single-cell analysis based on microwell array chips are summarized. We first introduce the design and preparation of different microwell chips. Then microwell-based cell capture and lysis strategies are discussed. We finally focus on advanced microwell-based analysis of single-cell proteins, nucleic acids, and metabolites. The challenges and opportunities for the development of microwell-based single-cell analysis are also presented.

High precision, high throughput generation of droplets containing single cells

The Poisson limit is a major problem for the isolation of single cells in different single-cell technologies and applications. In droplet-based single-cell assays, a scheme that is increasingly popular, the intrinsic randomness during single-cell encapsulation in droplets requires most of the created droplets to be empty, which has a profound impact on the efficiency and throughput of such techniques, and on the predictability of the combinatory droplet assays. Here we present a simple passive microfluidic system overcoming this limitation with unprecedented efficacy, allowing the generation of single-cell droplets for a wide range of operating conditions, with extremely high throughput (more than 22 000 single-cell loaded droplets per minute) and with an extremely low fault ratio (doublets or empty droplets), applicable to any cells and deformable particles. This versatile technique will shift the paradigm of single-cell encapsulation and will impact single-cell sequencing, rare cell isolation, multicellular/bead studies in immunology or cancer biology, etc.

Cell pairing for biological analysis in microfluidic devices

Cell pairing at the single-cell level usually allows a few cells to contact or seal in a single chamber and provides high-resolution imaging. It is pivotal for biological research, including understanding basic cell functions, creating cancer treatment technologies, developing drugs, and more. Laboratory chips based on microfluidics have been widely used to trap, immobilize, and analyze cells due to their high efficiency, high throughput, and good biocompatibility properties. Cell pairing technology in microfluidic devices provides spatiotemporal research on cellular interactions and a highly controlled approach for cell heterogeneity studies. In the last few decades, many researchers have emphasized cell pairing research based on microfluidics. They designed various microfluidic device structures for different biological applications. Herein, we describe the current physical methods of microfluidic devices to trap cell pairs. We emphatically summarize the practical applications of cell pairing in microfluidic devices, including cell fusion, cell immunity, gap junction intercellular communication, cell co-culture, and other applications. Finally, we review the advances and existing challenges of the presented devices and then discuss the possible development directions to promote medical and biological research.

A Microfluidic Approach for Probing Heterogeneity in Cytotoxic T-Cells by Cell Pairing in Hydrogel Droplets

Cytotoxic T-cells (CTLs) exhibit strong effector functions to leverage antigen-specific anti-tumoral and anti-viral immunity. When naïve CTLs are activated by antigen-presenting cells (APCs) they display various levels of functional heterogeneity. To investigate this, we developed a single-cell droplet microfluidics platform that allows for deciphering single CTL activation profiles by multi-parameter analysis. We identified and correlated functional heterogeneity based on secretion profiles of IFNγ, TNFα, IL-2, and CD69 and CD25 surface marker expression levels. Furthermore, we strengthened our approach by incorporating low-melting agarose to encapsulate pairs of single CTLs and artificial APCs in hydrogel droplets, thereby preserving spatial information over cell pairs. This approach provides a robust tool for high-throughput and single-cell analysis of CTLs compatible with flow cytometry for subsequent analysis and sorting. The ability to score CTL quality, combined with various potential downstream analyses, could pave the way for the selection of potent CTLs for cell-based therapeutic strategies.

Effects of extracellular adhesion molecules on immune cell mediated solid tumor cell killing

Adoptive cell therapy (ACT) using ex vivo engineered/expanded immune cells demonstrated poor efficacy against solid tumors, despite its great success in treating various hematopoietic malignancies. To improve ACT for solid tumors, it is crucial to comprehend how the numerous components of the tumor microenvironment (TME) surrounding solid tumor cells influence killing ability of immune cells. In this study, we sought to determine the effects of extracellular adhesion provided by extracellular matrix (ECM) of TME on immune cell cytotoxicity by devising microwell arrays coated with proteins either preventing or promoting cell adhesion. Solid tumor cells in bovine serum albumin (BSA)-coated microwells did not attach to the surfaces and exhibited a round morphology, but solid tumor cells in fibronectin (FN)-coated microwells adhered firmed to the substrates with a flat shape. The seeding densities of solid tumor cells and immune cells were tuned to maximize one-to-one pairing within a single microwell, and live cell imaging was performed to examine dynamic cell-cell interactions and immune cell cytotoxicity at a single cell level. Both natural killer (NK) cells and T cells showed higher cytotoxicity against round tumor cells in BSA-coated microwells compared to flat tumor cells in FN-coated microwells, suggesting that extracellular adhesion-mediated firm adhesion of tumor cells made them more resistant to immune cell-mediated killing. Additionally, NK cells and T cells in FN-coated microwells exhibited divergent dynamic behaviors, indicating that two distinct subsets of cytotoxic lymphocytes respond differentially to extracellular adhesion cues during target cell recognition.

Screening T-Cell Activity via a Photodetachable DNA-Copolymer Nanocage and Its Therapeutic Application

Screening T-cell activity and selecting active ones from large ex vivo-expanded populations before reinfusion is important for the success of T-cell therapy. Cytokine secretion is the evaluation criterion of cell immune activity. Cell membrane-anchored probes and microchamber-based techniques have been used to screen cytokine secretion at the single-cell level. However, they are either easily affected by nearby cells' secretion or lack of single-cell encapsulation efficiency. Here, we design a photodetachable DNA-copolymer nanocage on the cell membrane for screening the activities of ex vivo-expanded T cells by in-situ monitoring cytokine interferon-gamma (IFN-γ) secretion. The ones with good immune activity are selected for therapeutic application. DNA-copolymer nanocage is self-assembled on a cell membrane to encapsulate a single T cell. A self-quenched IFN-γ recognition aptamer is contained in the DNA-copolymer nanocage, which recovers fluorescence in response to IFN-γ secretion to indicate individual T-cell activity. The active T cells are collected after fluorescence-activated cell sorting, irradiated with 5 min UV light to detach nanocage from the cell membrane, and continuously cocultured with downstream cells. The selected Jurkat cells and CD19 CAR-T cells showed improved capabilities for downstream cell activation and cancer cell killing. The cell membrane-detachable DNA-copolymer nanocage-based T-cell activity screening and selection would have promising applications in T-cell therapy.

High-resolution imaging of protein secretion at the single-cell level using plasmon-enhanced FluoroDOT assay

Secreted proteins mediate essential physiological processes. With conventional assays, it is challenging to map the spatial distribution of proteins secreted by single cells, to study cell-to-cell heterogeneity in secretion, or to detect proteins of low abundance or incipient secretion. Here, we introduce the "FluoroDOT assay," which uses an ultrabright nanoparticle plasmonic-fluor that enables high-resolution imaging of protein secretion. We find that plasmonic-fluors are 16,000-fold brighter, with nearly 30-fold higher signal-to-noise compared with conventional fluorescence labels. We demonstrate high-resolution imaging of different secreted cytokines in the single-plexed and spectrally multiplexed FluoroDOT assay that revealed cellular heterogeneity in secretion of multiple proteins simultaneously. Using diverse biochemical stimuli, including Mycobacterium tuberculosis infection, and a variety of immune cells such as macrophages, dendritic cells (DCs), and DC-T cell co-culture, we demonstrate that the assay is versatile, facile, and widely adaptable for enhancing biological understanding of spatial and temporal dynamics of single-cell secretome.

Miniaturized and multiplexed high-content screening of drug and immune sensitivity in a multichambered microwell chip

Here, we present a methodology based on multiplexed fluorescence screening of two- or three-dimensional cell cultures in a newly designed multichambered microwell chip, allowing direct assessment of drug or immune cell cytotoxic efficacy. We establish a framework for cell culture, formation of tumor spheroids, fluorescence labeling, and imaging of fixed or live cells at various magnifications directly in the chip together with data analysis and interpretation. The methodology is demonstrated by drug cytotoxicity screening using ovarian and non-small cell lung cancer cells and by cellular cytotoxicity screening targeting tumor spheroids of renal carcinoma and ovarian carcinoma with natural killer cells from healthy donors. The miniaturized format allowing long-term cell culture, efficient screening, and high-quality imaging of small sample volumes makes this methodology promising for individualized cytotoxicity tests for precision medicine.

High-throughput single-cell antibody secretion quantification and enrichment using droplet microfluidics-based FRET assay

High-throughput screening and enrichment of antibody-producing cells have many important applications. Herein, we present a droplet microfluidic approach for high-throughput screening and sorting of antibody-secreting cells using a Förster resonance electron transfer (FRET)-based assay. The FRET signal is mediated by the specific binding of the secreted antibody to two fluorescently labeled probes supplied within a droplet. Functional hybridoma cells expressing either membrane-bound or secreted monoclonal antibodies (mAbs), or both, were efficiently differentiated in less than 30 min. The antibody secretion rate by individual hybridoma cells was recorded in the range of 14,000 Abs/min, while the density of membrane-bound fraction was approximately 100 Abs/μm². Combining the FRET assay with droplet-based single-cell sorting, an 800-fold enrichment of antigen-specific cells was achieved after one round of sorting. The presented system overcomes several key limitations observed in conventional FACS-based screening methods and should be applicable to assaying various other secreted proteins.

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FRET-based screening assay of antibody-secreting cells in microfluidic droplets

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Membrane-bound and secreted antibodies of the same cell are efficiently differentiated

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Using mouse hybridoma cells antibody secretion assay is completed in 30 min

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FRET-based droplet sorting enables over 800-fold enrichment in one round of sorting

Hydrogels for Single-Cell Microgel Production: Recent Advances and Applications

Single-cell techniques have become more and more incorporated in cell biological research over the past decades. Various approaches have been proposed to isolate, culture, sort, and analyze individual cells to understand cellular heterogeneity, which is at the foundation of every systematic cellular response in the human body. Microfluidics is undoubtedly the most suitable method of manipulating cells, due to its small scale, high degree of control, and gentle nature toward vulnerable cells. More specifically, the technique of microfluidic droplet production has proven to provide reproducible single-cell encapsulation with high throughput. Various in-droplet applications have been explored, ranging from immunoassays, cytotoxicity assays, and single-cell sequencing. All rely on the theoretically unlimited throughput that can be achieved and the monodispersity of each individual droplet. To make these platforms more suitable for adherent cells or to maintain spatial control after de-emulsification, hydrogels can be included during droplet production to obtain “microgels.” Over the past years, a multitude of research has focused on the possibilities these can provide. Also, as the technique matures, it is becoming clear that it will result in advantages over conventional droplet approaches. In this review, we provide a comprehensive overview on how various types of hydrogels can be incorporated into different droplet-based approaches and provide novel and more robust analytic and screening applications. We will further focus on a wide range of recently published applications for microgels and how these can be applied in cell biological research at the single- to multicell scale.

Cycle Performance of Aerated Lightweight Concrete Windowed and Windowless Wall Panel from the Perspective of Lightweight Deep Learning

This paper aims to explore the seismic mechanical properties of newly developed fabricated aerated lightweight concrete (ALC) wall panels to clarify the interaction mechanism between wall panels and structures. It first introduces the lightweight deep learning object detection algorithm and constructs a network model with faster operation speed based on the convolutional neural network. Secondly, combined with the deep learning object detection algorithm, the quasi-static loading system is adopted to conduct the repeated loading test on two fabricated ALC wall panels. Finally, the hysteresis load-displacement curve of each test is recorded. The experimental results show that the proposed deep learning algorithm greatly improves the operation speed and compresses the model size without reducing the accuracy. The lightweight deep learning algorithm is applied to the study of the slip performance of the wall plate. The pretightening force of the connecting screw characterizes the slip performance between the wall plate and the structural beam, thereby affecting the deformation response of the wall plate when the interstory displacement increases. The hysteresis curve of the ALC wall panel has obvious squeezing effect, indicating that the slip of the connector can unload part of the external load and delay the damage of the wall panel. The skeleton curve suggests that the fabricated windowless ALC wall panel has higher positive and negative initial stiffness and bearing capacity than the fabricated windowed wall panel. However, the degradation analysis of the stiffness curve reveals that the lateral stiffness deviation of the fabricated windowless ALC wall panel is more obvious. It confirms that the proposed connection method based on the lightweight deep learning model can improve the seismic performance of ALC wall panels and provide reference for the structural analysis of embedding fabricated ALC wall panels. This work shows the important practical value for exploring the application effect of embedded ALC wall panels.

Live single cell imaging assays in glass microwells produced by laser-induced deep etching

Miniaturization of cell culture substrates enables controlled analysis of living cells in confined micro-scale environments. This is particularly suitable for imaging individual cells over time, as they can be monitored without escaping the imaging field-of-view (FoV). Glass materials are ideal for most microscopy applications. However, with current methods used in life sciences, glass microfabrication is limited in terms of either freedom of design, quality, or throughput. In this work, we introduce laser-induced deep etching (LIDE) as a method for producing glass microwell arrays for live single cell imaging assays. We demonstrate novel microwell arrays with deep, high-aspect ratio wells that have rounded, dimpled or flat bottom profiles in either single-layer or double-layer glass chips. The microwells are evaluated for microscopy-based analysis of long-term cell culture, clonal expansion, laterally organized cell seeding, subcellular mechanics during migration and immune cell cytotoxicity assays of both adherent and suspension cells. It is shown that all types of microwells can support viable cell cultures and imaging with single cell resolution, and we highlight specific benefits of each microwell design for different applications. We believe that high-quality glass microwell arrays enabled by LIDE provide a great option for high-content and high-resolution imaging-based live cell assays with a broad range of potential applications within life sciences.

Pairing cells of different sizes in a microfluidic device for immunological synapse monitoring

Analyzing cell-cell interaction is essential to investigate how immune cells function. Elegant designs have been demonstrated to study lymphocytes and their interaction partners. However, these devices have been targeting cells of similar dimensions. T lymphocytes are smaller, more deformable, and more sensitive to pressure than many cells. This work aims to fill the gap of a method for pairing cells with different dimensions. The developed method uses hydrodynamic flow focusing in the z-direction for on-site modulation of effective channel height to capture smaller cells as single cells. Due to immune cells' sensitivity to pressure, the proposed method provides a stable system without any change in flow conditions at the analysis area throughout experiments. Paired live cells have their activities analyzed with calcium imaging at the immunological synapse formed under a controlled environment. The method is demonstrated with primary human T lymphocytes, acute myeloid leukemia (AML) cell lines, and primary AML blasts.

Magnetophoretic Micro-Distributor for Controlled Clustering of Cells

Cell clustering techniques are important to produce artificial cell clusters for in vitro models of intercellular mechanisms at the single-cell level. The analyses considering physical variables such as the shape and size of cells have been very limited. In addition, the precise manipulation of cells and control of the physical variables are still challenging. In this paper, a magnetophoretic device consisting of a trampoline micromagnet and active elements that enable the control of individual selective jumping motion and positioning of a micro-object is proposed. Based on a numerical simulation under various conditions, automatic separation or selective clustering of micro-objects according to their sizes is performed by parallel control and programmable manipulation. This method provides efficient control of the physical variables of cells and grouping of cells with the desired size and number, which can be a milestone for a better understanding of the intercellular dynamics between clustered cells at the single-cell level for future cell-on-chip applications.

Efficient recovery of potent tumour-infiltrating lymphocytes through quantitative immunomagnetic cell sorting

Adoptive cell therapies require the recovery and expansion of highly potent tumour-infiltrating lymphocytes (TILs). However, TILs in tumours are rare and difficult to isolate efficiently, which hinders the optimization of therapeutic potency and dose. Here we show that a configurable microfluidic device can efficiently recover potent TILs from solid tumours by leveraging specific expression levels of target cell-surface markers. The device, which is sandwiched by permanent magnets, balances magnetic forces and fluidic drag forces to sort cells labelled with magnetic nanoparticles conjugated with antibodies for the target markers. Compared with conventional cell sorting, immunomagnetic cell sorting recovered up to 30-fold higher numbers of TILs, and the higher levels and diversity of the recovered TILs accelerated TIL expansion and enhanced their therapeutic potency. Immunomagnetic cell sorting also allowed us to identify and isolate potent TIL subpopulations, in particular TILs with moderate levels of CD39 (a marker of T-cell reactivity to tumours and T-cell exhaustion), which we found are tumour-specific, self-renewable and essential for the long-term success of adoptive cell therapies.

Microfluidic technology for multiple single-cell capture

Microfluidic devices are widely used in single-cell capture and for pairing single cells or groups of cells for cell-cell interaction analysis; these advances have improved drug screening and cell signal transduction analysis. The complex in vivo environment involves interactions between two cells and among multiple cells of the same or different phenotypes. This study reviewed the core principles and performance of several microfluidic multiple- and single-cell capture methods, namely, the microwell, valve, trap, and droplet methods. The advantages and disadvantages of the methods were compared, and suggestions regarding their application to multiple-cell capture were provided. The results may serve as a reference for research on microfluidic multiple single-cell coculture technology.

Recent Advances in Microfluidic Platforms for Programming Cell-Based Living Materials

Cell-based living materials, including single cells, cell-laden fibers, cell sheets, organoids, and organs, have attracted intensive interests owing to their widespread applications in cancer therapy, regenerative medicine, drug development, and so on. Significant progress in materials, microfabrication, and cell biology have promoted the development of numerous promising microfluidic platforms for programming these cell-based living materials with a high-throughput, scalable, and efficient manner. In this review, the recent progress of novel microfluidic platforms for programming cell-based living materials is presented. First, the unique features, categories, and materials and related fabrication methods of microfluidic platforms are briefly introduced. From the viewpoint of the design principles of the microfluidic platforms, the recent significant advances of programming single cells, cell-laden fibers, cell sheets, organoids, and organs in turns are then highlighted. Last, by providing personal perspectives on challenges and future trends, this review aims to motivate researchers from the fields of materials and engineering to work together with biologists and physicians to promote the development of cell-based living materials for human healthcare-related applications.

An automated real-time microfluidic platform to probe single NK cell heterogeneity and cytotoxicity on-chip

Cytotoxicity is a vital effector mechanism used by immune cells to combat pathogens and cancer cells. While conventional cytotoxicity assays rely on averaged end-point measures, crucial insights on the dynamics and heterogeneity of effector and target cell interactions cannot be extracted, emphasizing the need for dynamic single-cell analysis. Here, we present a fully automated droplet-based microfluidic platform that allowed the real-time monitoring of effector-target cell interactions and killing, allowing the screening of over 60,000 droplets identifying 2000 individual cellular interactions monitored over 10 h. During the course of incubation, we observed that the dynamics of cytotoxicity within the Natural Killer (NK) cell population varies significantly over the time. Around 20% of the total NK cells in droplets showed positive cytotoxicity against paired K562 cells, most of which was exhibited within first 4 h of cellular interaction. Using our single cell analysis platform, we demonstrated that the population of NK cells is composed of individual cells with different strength in their effector functions, a behavior masked in conventional studies. Moreover, the versatility of our platform will allow the dynamic and resolved study of interactions between immune cell types and the finding and characterization of functional sub-populations, opening novel ways towards both fundamental and translational research.

Cytokines: From Clinical Significance to Quantification

Cytokines are critical mediators that oversee and regulate immune and inflammatory responses via complex networks and serve as biomarkers for many diseases. Quantification of cytokines has significant value in both clinical medicine and biology as the levels provide insights into physiological and pathological processes and can be used to aid diagnosis and treatment. Cytokines and their clinical significance are introduced from the perspective of their pro- and anti-inflammatory effects. Factors affecting cytokines quantification in biological fluids, native levels in different body fluids, sample processing and storage conditions, sensitivity to freeze-thaw, and soluble cytokine receptors are discussed. In addition, recent advances in in vitro and in vivo assays, biosensors based on different signal outputs and intracellular to extracellular protein expression are summarized. Various quantification platforms for high-sensitivity and reliable measurement of cytokines in different scenarios are discussed, and commercially available cytokine assays are compared. A discussion of challenges in the development and advancement of technologies for cytokine quantification that aim to achieve real-time multiplex cytokine analysis for point-of-care situations applicable for both biomedical research and clinical practice are discussed.

Edge-Enhanced Microwell Immunoassay for Highly Sensitive Protein Detection

Highly sensitive biosensors that can detect low concentrations of protein biomarkers at the early stages of diseases or proteins secreted from single cells are of importance for disease diagnosis and treatment assessment. This work reports a new signal amplification mechanism, that is, edge enhancement based on the vertical sidewalls of microwells for ultra-sensitive protein detection. The fluorescence emission at the edge of the microwells is highly amplified due to the microscopic axial resolution (depth of field) and demonstrates a microring effect. The enhanced fluorescence intensity from microrings is calibrated for bovine serum albumin detection, which shows a 6-fold sensitivity enhancement and a lower limit of detection at the microwell edge, compared to those obtained on a flat surface. The microwell chip is used to separate single cells, and the wall of each microwell is used to detect interferon-γ secretion from T cells stimulated with a peptide and whole cancer cells. Given its edge-enhancement ability, the microwell technique can be a highly sensitive biosensing platform for disease diagnosis at an early stage and for assessing potential treatments at the single-cell level.

Latest Updates on the Advancement of Polymer-Based Biomicroelectromechanical Systems for Animal Cell Studies

Biological sciences have reached the fundamental unit of life: the cell. Ever-growing field of Biological Microelectromechanical Systems (BioMEMSs) is providing new frontiers in both fundamental cell research and various practical applications in cell-related studies. Among various functions of BioMEMS devices, some of the most fundamental processes that can be carried out in such platforms include cell sorting, cell separation, cell isolation or trapping, cell pairing, cell-cell communication, cell differentiation, cell identification, and cell culture. In this article, we review each mentioned application in great details highlighting the latest advancements in fabrication strategy, mechanism of operation, and application of these tools. Moreover, the review article covers the shortcomings of each specific application which can open windows of opportunity for improvement of these devices.

Single-cell Analysis Technologies for Immuno-oncology Research: from Mechanistic Delineation to Biomarker Discovery

The successes with immune checkpoint blockade (ICB) and chimeric antigen receptor (CAR)-T-cell therapy in treating multiple cancer types have established immunotherapy as a powerful curative option for patients with advanced cancers. Unfortunately, many patients do not derive benefit or long-term responses, highlighting a pressing need to perform complete investigation of the underlying mechanisms and the immunotherapy-induced tumor regression or rejection. In recent years, a large number of single-cell technologies have leveraged advances in characterizing immune system, profiling tumor microenvironment, and identifying cellular heterogeneity, which establish the foundations for lifting the veil on the comprehensive crosstalk between cancer and immune system during immunotherapies. In this review, we introduce the applications of the most widely used single-cell technologies in furthering our understanding of immunotherapies in terms of underlying mechanisms and their association with therapeutic outcomes. We also discuss how single-cell analyses help to deliver new insights into biomarker discovery to predict patient response rate, monitor acquired resistance, and support prophylactic strategy development for toxicity management. Finally, we provide an overview of applying cutting-edge single-cell spatial-omics to point out the heterogeneity of tumor-immune interactions at higher level that can ultimately guide to the rational design of next-generation immunotherapies.

Measuring single-cell protein secretion in immunology: Technologies, advances, and applications

The dynamics, nature, strength, and ultimately protective capabilities of an active immune response are determined by the extracellular constitution and concentration of various soluble factors. Generated effector cells secrete such mediators, including antibodies, chemo‐ and cytokines to achieve functionality. These secreted factors organize the individual immune cells into functional tissues, initiate, orchestrate, and regulate the immune response. Therefore, a single‐cell resolved analysis of protein secretion is a valuable tool for studying the heterogeneity and functionality of immune cells. This review aims to provide a comparative overview of various methods to characterize immune reactions by measuring single‐cell protein secretion. Spot‐based and cytometry‐based assays, such as ELISpot and flow cytometry, respectively, are well‐established methods applied in basic research and clinical settings. Emerging novel technologies, such as microfluidic platforms, offer new ways to measure and exploit protein secretion in immune reactions. Further technological advances will allow the deciphering of protein secretion in immunological responses with unprecedented detail, linking secretion to functionality. Here, we summarize the development and recent advances of tools that allow the analysis of protein secretion at the single‐cell level, and discuss and contrast their applications within immunology.

Multiplexed Single-Cell Leukocyte Enzymatic Secretion Profiling from Whole Blood Reveals Patient-Specific Immune Signature

Enzymatic secretion of immune cells (leukocytes) plays a dominant role in host immune responses to a myriad of biological triggers, including infections, cancers, and cardiovascular diseases. Current tools to probe these leukocytes inadequately profile these vital biomarkers; the need for sample preprocessing steps of cell lysis, labeling, washing, and pipetting inevitably triggers the cells, changes its basal state, and dilutes the individual cell secretion in bulk assays. Using a fully integrated system for multiplexed profiling of native immune single-cell enzyme secretion from 50 μL of undiluted blood, we eliminate sample handling. With a total analysis time of 60 min, the integrated platform performs six tasks of leukocyte extraction, cell washing, fluorescent enzyme substrate mixing, single-cell droplet making, droplet incubation, and real-time readout for leukocyte secretion profiling of neutrophil elastase, granzyme B, and metalloproteinase. We calibrated the device, optimized the protocols, and tested the leukocyte secretion of acute heart failure (AHF) patients at admission and predischarge. This paper highlights the presence of single-cell enzymatic immune phenotypes independent of CD marker labeling, which could potentially elucidate the innate immune response states. We found that patients recovering from AHF showed a corresponding reduction in immune-cell enzymatic secretion levels and donor-specific enzymatic signatures were observed, which suggests patient-to-patient heterogeneous immune response. This platform presents opportunities to elucidate the complexities of the immune response from a single drop of blood and bridge the current technological, biological, and medical gap in understanding immune response and biological triggers.

Single-Cell Secretion Analysis in the Engineered Tumor Microenvironment Reveals Differential Modulation of Macrophage Immune Responses

It is increasingly recognized that the cellular microenvironment plays critical roles in regulating the fate and physiology of cells. Despite recent advancements in single-cell analysis technologies, engineering and integration of the microenvironment for single-cell analysis platforms remain limited. Here, we report a single-cell cytokine secretion analysis platform that integrated both the three-dimensional cell culture and the primary oral squamous cell carcinoma tumor cell co-culture to provide both physical and physiological cues for single cells to be analyzed. We apply the platform to investigate the immune responses of human macrophages stimulated with the ligand of toll-like receptor 4 lipopolysaccharide. Notably, we observe the differential modulation effect in cytokine secretions by the tumor microenvironment, in which antitumor cytokine TNF-a secretion was attenuated, and protumor cytokine IL-6 would increase. The differential modulation effect is conserved from cell line-derived macrophages to primary macrophages derived from healthy donors. Immunofluorescence staining further reveals that ∼50% of macrophage cells could be polarized from M1 to the M2 phenotype within 12 h in the engineered tumor microenvironment. This work demonstrates the significance of the cell microenvironment toward single-cell analysis, which could help to evaluate how immune cells will respond in the complex microenvironment more accurately.

Deciphering cell-cell interactions and communication from gene expression

Cell-cell interactions orchestrate organismal development, homeostasis and single-cell functions. When cells do not properly interact or improperly decode molecular messages, disease ensues. Thus, the identification and quantification of intercellular signalling pathways has become a common analysis performed across diverse disciplines. The expansion of protein-protein interaction databases and recent advances in RNA sequencing technologies have enabled routine analyses of intercellular signalling from gene expression measurements of bulk and single-cell data sets. In particular, ligand-receptor pairs can be used to infer intercellular communication from the coordinated expression of their cognate genes. In this Review, we highlight discoveries enabled by analyses of cell-cell interactions from transcriptomic data and review the methods and tools used in this context.

Methods and platforms for analysis of nucleic acids from single-cell based on microfluidics

Single-cell nucleic acid analysis aims at discovering the genetic differences between individual cells which is well known as the cellular heterogeneity. This technology facilitates cancer diagnosis, stem cell research, immune system analysis, and other life science applications. The conventional platforms for single-cell nucleic acid analysis more rely on manual operation or bulky devices. Recently, the emerging microfluidic technology has provided a perfect platform for single-cell nucleic acid analysis with the characteristic of accurate and automatic single-cell manipulation. In this review, we briefly summarized the procedure of single-cell nucleic acid analysis including single-cell isolation, single-cell lysis, nucleic acid amplification, and genetic analysis. And then, three representative microfluidic platforms for single-cell nucleic acid analysis are concluded as valve-, microwell-, and droplet-based platforms. Furthermore, we described the state-of-the-art integrated single-cell nucleic acid analysis systems based on the three platforms. Finally, the future development and challenges of microfluidics-based single-cell nucleic acid analysis are discussed as well.

Time-resolved microwell cell-pairing array reveals multiple T cell activation profiles

The differences in behaviour between individual cells in a large population are often important, yet are masked in bulk analyses where only average parameters are measured. One unresolved question in the field of immunology is the extent to which important immunological phenomena such as immunodominance to cancer antigens correlates with the average activity of a population of antigen-specific T lymphocytes, or with the activity of individual "outlier" cells. Despite progress in single cell technologies, few platforms are available that can deliver time-resolved, functional analysis at single cell resolution, for these investigations. We have developed an accessible high-throughput platform to measure single T cell signalling in real time following time-controlled stimulation by live antigen presenting cells. The cell-trap array consists of thousands of individual microwells cast in an agarose block, which is biocompatible and permeable to nutrients. Single T cells are isolated in wells via passive sedimentation and size exclusion, achieving up to 90% occupancy. The device enables simultaneous activation of thousands of single CD8⁺ cells. Stimulation with soluble reagents (ionomycin, anti-CD3 antibodies) or antigen presenting cells leads to changes in intracellular calcium concentrations which were measured using calcium-chelating fluorophore dyes. The platform was used to demonstrate a range of activation profiles among individual cells of a cloned, antigen specific CD8⁺ T cell hybridoma in response to both nonspecific stimuli and specific, physiologically relevant antigen stimulation. The presence of two different activation profiles was demonstrated, together with rare outlier behaviour among cells that are essentially clonal.

Localized Cell-Surface Sampling of a Secreted Factor Using Cell-Targeting Beads

Intercellular communication through the secretion of soluble factors plays a vital role in a wide range of biological processes (e.g., homeostasis, immune response), yet identification and quantification of many of these factors can be challenging due to their degradation or sequestration in cell culture media prior to analysis. Here, we present a customizable bead-based system capable of simultaneously binding to live cells (through antibody-mediated cell tethering) and capturing cell-secreted molecules. Our functionalized beads capture secreted molecules (e.g., hepatocyte growth factor secreted by fibroblasts) that are diminished when sampled via traditional supernatant analysis techniques (p < 0.05), effectively rescuing a reduced signal in the presence of neutralizing components in the cell culture media. Our system enables capture and analysis of molecules integral to chemical communication that would otherwise be markedly decreased prior to analysis.