Real-time monitoring of suspension cell-cell communication using an integrated microfluidics

In-droplet cell separation based on bipolar dielectrophoretic response to facilitate cellular droplet assays

Precise manipulation of cells within water-in-oil emulsion droplets has the potential to vastly expand the type of cellular assays that can be conducted in droplet-based microfluidics systems. However, achieving such manipulation remains challenging. Here, we present an in-droplet label-free cell separation technology by utilizing different dielectrophoretic responses of two different cell types. Two pairs of angled planar electrodes were utilized to generate positive or negative dielectrophoretic force acting on each cell type, which results in selective in-droplet movement of only one specific cell type at a time. A downstream asymmetric Y-shaped microfluidic junction splits the mother droplet into two daughter droplets, each of which contains only one cell type. The capability of this platform was successfully demonstrated by conducting in-droplet separation from a mixture of Salmonella cells and macrophages, two cell types commonly used as a bacterial pathogenicity analysis model. This technology enable the precise manipulation of cells within droplets, which can be exploited as a critical function in implementing broader ranges of droplet-based microfluidics cellular assays.

Mechanical slowing-down of cytoplasmic diffusion allows in vivo counting of proteins in individual cells

Many key regulatory proteins in bacteria are present in too low numbers to be detected with conventional methods, which poses a particular challenge for single-cell analyses because such proteins can contribute greatly to phenotypic heterogeneity. Here we develop a microfluidics-based platform that enables single-molecule counting of low-abundance proteins by mechanically slowing-down their diffusion within the cytoplasm of live Escherichia coli (E. coli) cells. Our technique also allows for automated microscopy at high throughput with minimal perturbation to native physiology, as well as viable enrichment/retrieval. We illustrate the method by analysing the control of the master regulator of the E. coli stress response, RpoS, by its adapter protein, SprE (RssB). Quantification of SprE numbers shows that though SprE is necessary for RpoS degradation, it is expressed at levels as low as 3–4 molecules per average cell cycle, and fluctuations in SprE are approximately Poisson distributed during exponential phase with no sign of bursting.

Several proteins are expressed at too low abundance in the Escherichia coli (E. coli) proteome to be detected by standard methods. Here, the authors create a microfluidics-based platform enabling single-molecule counting of low-abundance proteins by mechanically slowing-down their diffusion in live E. coli.

Using reconfigurable microfluidics to study the role of HGF in autocrine and paracrine signaling of hepatocytes

Cancer, developmental biology and tissue injury present multiple examples where groups of cells residing in close proximity communicate via paracrine factors. It is nearly impossible to dissect such cellular interactions in vivo and is quite challenging in vitro. The goal of this study is to utilize a reconfigurable microfluidic device in order to study paracrine signal exchange between groups of primary hepatocytes in vitro. Previously, we demonstrated that hepatocytes residing on protein spots containing collagen and hepatocyte growth factor (HGF) spots expressed epithelial (hepatic) phenotypes and also rescued them in neighboring hepatocytes on collagen spots that did not receive direct HGF stimulus. Herein, we designed a microfluidic device with parallel fluidic channels separated by retractable (reconfigurable) walls and employed this device to investigate interactions between groups of HGF-stimulated and unstimulated hepatocytes. Using a novel reconfigurable microfluidic device, we demonstrate that cultivation of HGF-containing protein spots upregulates the production of endogenous HGF in hepatocytes and that these HGF molecules diffuse over, causing phenotype enhancement in the recipient cells. We also show that selective treatment of the recipient hepatocytes with a c-met inhibitor (SU11274) diminishes the rescue effect, as gauged by the down-regulation of albumin and HGF expression. Our study is one of the first to demonstrate paracrine signaling via HGF in primary hepatocytes. More broadly, tools and methods described here may be used to study paracrine signaling in other types of cells and will have relevance for various fields of biomedical research from cancer to immunology.

Cell proliferation and migration inside single cell arrays

Cell proliferation and migration are fundamental processes in determining cell and tissue behaviour. In this study we show the design and fabrication of a new single cell microfluidic structure, called a "vertically integrated array" or "VIA" trap to explore quantitative functional assays including single cell attachment, proliferation and migration studies. The chip can be used in a continuous (flow-through) manner, with a continuous supply of new media, as well as in a quiescent mode. We show the fabrication of the device, together with the flow characteristics inside the network of channels and the single cell traps. The flow patterns inside the device not only facilitate cell trapping, but also protect the cells from mechanical flow-induced stress. MDA-MB-231 human breast cancer cells were used to study attachment and detachment during the cell cycle as well as explore the influences of the chemokine SDF-1 (enabling the quantification of the role of chemokine gradients both on pseudopod formation and directional cell migration).

Microfluidic platforms for the investigation of intercellular signalling mechanisms

Intercellular signalling has been identified as a highly complex process, responsible for orchestrating many physiological functions. While conventional methods of investigation have been useful, their limitations are impeding further development. Microfluidics offers an opportunity to overcome some of these limitations. Most notably, microfluidic systems can emulate the in-vivo environments. Further, they enable exceptionally precise control of the microenvironment, allowing complex mechanisms to be selectively isolated and studied in detail. There has thus been a growing adoption of microfluidic platforms for investigation of cell signalling mechanisms. This review provides an overview of the different signalling mechanisms and discusses the methods used to study them, with a focus on the microfluidic devices developed for this purpose.

A microfluidic linear node array for the study of protein-ligand interactions

We have developed a microfluidic device for the continuous separation of small molecules from a protein mixture and demonstrated its practical use in the study of protein-ligand binding, a crucial aspect in drug discovery. Our results demonstrated dose-dependent binding between bovine serum albumin (BSA) and its small-molecule site marker, Eosin Y (EY), and found that the binding reached a plateau when the BSA : EY ratio was above 1, which agreed with the eosin binding capacity of BSA reported in literature. By streamline control using a combination of two fundamental building blocks (R and L nodes) with a microdevice operated at a high flow rate (up to 1300 μL h(-1)), a solution barrier was created to "filter" off protein/protein-ligand complexes such that the small unbound molecules were isolated and quantified easily. The percentage decrease of small molecules with increasing protein concentration indicated the presence of binding events. Several fluorophores with different molecular weights were used to test the performance of the microfluidic "filter", which was tunable by 1) the total flow rate, and/or 2) the flow distribution ratio between the two device inlets; both were easily controllable by changing the syringe pump settings. Since the microdevice was operated at a relatively high flow rate, aliquots were easily recovered from the device outlets to facilitate off-chip detection. This microfluidic design is a novel and promising tool for preliminary drug screening.

Recent developments in microfluidics for cell studies

As a technique for precisely manipulating fluid at the micrometer scale, the field of microfluidics has experienced an explosive growth over the past two decades, particularly owing to the advances in device design and fabrication. With the inherent advantages associated with its scale of operation, and its flexibility in being incorporated with other microscale techniques for manipulation and detection, microfluidics has become a major enabling technology, which has introduced new paradigms in various fields involving biological cells. A microfluidic device is able to realize functions that are not easily imaginable in conventional biological analysis, such as highly parallel, sophisticated high-throughput analysis, single-cell analysis in a well-defined manner, and tissue engineering with the capability of manipulation at the single-cell level. Major advancements in microfluidic device fabrication and the growing trend of implementing microfluidics in cell studies are presented, with a focus on biological research and clinical diagnostics.

Nanoparticle-based signal generation and amplification in microfluidic devices for bioanalysis

Signal generation and amplification based on nanomaterials and microfluidic techniques have both attracted considerable attention separately due to the demands for ultrasensitive and high-throughput detection of biomolecules. This article reviews the latest development of signal amplification strategies based on nanoparticles for bioanalysis and their integration and applications in microfluidic systems. The applications of nanoparticles in bioanalysis were categorized based on the different approaches of signal amplification, and the microfluidic techniques were summarized based on cell analysis and biomolecule detection with a focus on the integration of nanoparticle-based amplification in microfluidic devices for ultrasensitive bioanalysis. The advantages and limitations of the combination of nanoparticles-based amplification with microfluidic techniques were evaluated, and the possible developments for future research were discussed.

Probing cell-cell communication with microfluidic devices

Intercellular communication is a mechanism that regulates critical events during embryogenesis and coordinates signalling within differentiated tissues, such as the nervous and cardiovascular systems. To perform specialized activities, these tissues utilize the rapid exchange of signals among networks that, while are composed of different cell types, are nevertheless functionally coupled. Errors in cellular communication can lead to varied deleterious effects such as degenerative and autoimmune diseases. However, the intercellular communication network is extremely complex in multicellular organisms making isolation of the functional unit and study of basic mechanisms technically challenging. New experimental methods to examine mechanisms of intercellular communication among cultured cells could provide insight into physiological and pathological processes alike. Recent developments in microfluidic technology allow miniaturized and integrated devices to perform intercellular communication experiments on-chip. Microfluidics have many advantages, including the ability to replicate in vitro the chemical, mechanical, and physical cellular microenvironment of tissues with precise spatial and temporal control combined with dynamic characterization, high throughput, scalability and reproducibility. In this Focus article, we highlight some of the recent work and advances in the application of microfluidics to the study of mammalian intercellular communication with particular emphasis on cell contact and soluble factor mediated communication. In addition, we provide some insights into likely direction of the future developments in this field.

Microfluidics study of intracellular calcium response to mechanical stimulation on single suspension cells

A microfluidic microdevice was developed to exert mechanical stimulation on an individual suspension cell for mechanosensation research. In this microfluidic chip, an individual cell was isolated from a population of cells, and trapped in a microchannel with a compressive component made of a deflectable membrane. The mechanosensation of HL60 cells (leukemic cells) was studied using this chip, and the results showed that mechanical stimulations could trigger extracellular calcium to flow into HL60 cells through ion channels on cell membranes. The tension on individual HL60 cells exerted by the microdevice was showed large variations in the threshold of mechanosensation activation. In contrast to previous reports using patch clamp technique, there was little influence of cytoskeleton interruption on HL60 cell mechanosensation triggered by whole-cell compression. Additionally, two functional units were integrated in one chip for carrying out mechanosensation study in parallel, where HL60 cells (leukemic cells) and Jurkat cells (lymphocytes) were shown to respond to mechanical stimulation with different kinetics. The results demonstrated that the microfluidic device provides a novel approach to investigating the mechanosensation of single suspension cells in high-throughput.

Cell signaling analysis by mass spectrometry under coculture conditions on an integrated microfluidic device

A microfluidic device was integrated in a controlled coculture system, in which the secreted proteins were qualitatively and semiquantitatively determined by a directly coupled mass spectrometer. PC12 cells and GH3 cells were cocultured under various conditions as a model of the regulation of the organism by the nervous system. A micro-solid phase extraction (SPE) column was integrated in order to remove salts from the cells secretion prior to mass spectrometry detection. A three layer polydimethylsiloxane (PDMS) microfluidic device was fabricated to integrate valves for avoiding contamination between the cells coculture zone and the pretreatment zone. Electrospray ionization (ESI)-quadrupole (Q)-time of flight (TOF)-mass spectrometry was employed to realize highly sensitive qualitative analysis and to implement semiquantitative analysis. Furthermore, cell migrations under various coculture conditions were observed and discussed. The inhibition on growth hormone secretion from GH3 cells by dopamine released from PC12 cells was investigated and demonstrated. Thus, the developed platform provides a useful tool on cell to cell signaling studies for disease monitoring and drug delivery control.

Electrochemiluminescence analysis of folate receptors on cell membrane with on-chip bipolar electrode

In this paper we report a transparent bipolar electrode based microfluidic chip-electrochemiluminescence (ECL) system for sensitive detection of folate receptors (FR) on cell membranes. This integrated system consists of a poly(dimethylsiloxane) (PDMS) layer containing a microchannel and a glass bottom sheet with indium tin oxide (ITO) strips as bipolar detectors. The ITO strips are fabricated using a PDMS micromold with carbon ink as a protective layer in place of traditional photoresist. The configuration of the bipolar electrode has great influence on the ECL intensity of Ru(bpy)(3)(2+)/tripropylamine(TPA) system. Further studies show that folic acid (FA) can strongly inhibit the ECL of the Ru(bpy)(3)(2+)/TPA system. Based on specific recognition between FA and FR on cell membrane, this microfluidic chip-ECL system is successfully applied for detecting the level of FR on human cervical tumor (HL-60) cells and MEF cells. It is found that the ECL intensity increases with the number of HL-60 cells in the range of 21 to 3.28 × 10(4) cells/mL. The average level of FR on HL-60 cells is calculated to be 8.05 ± 0.75 × 10(-18) mol/cell. While for MEF cells, it shows a much slower ECL increment than HL-60 cells due to the much lower FR level on MEF cells (5.30 ± 0.61 × 10(-19) mol/cell). Moreover, exocytosis of FA after FR mediated endocytosis was observed according to the change of the ECL signal with the incubation time of HL-60 cells in the FA- Ru(bpy)(3)(2+)/TPA system.