Carcinoma-associated fibroblasts promoted tumor spheroid invasion on a microfluidic 3D co-culture device

Cells, tissues, and organs on chips: challenges and opportunities for the cancer tumor microenvironment

The transition to increasingly sophisticated microfluidic systems has led to the emergence of "organ-on-chip" technology that can faithfully recapitulate organ-level function. Given the rapid progress at the interface between microfluidics and cell biology, there is need to provide a focused evaluation of the state-of-the-art in microfluidic systems for cancer research to advance development, accelerate discovery of novel insights, and facilitate cooperation between engineers, biologists and oncologists in the clinic. Here, we provide a focused review of microfluidics technology from cells- and tissues- to organs-on-chips with application toward studying the tumor microenvironment. Key aspects of the tumor microenvironment including angiogenesis, hypoxia, biochemical gradients, tumor-stromal interactions, and the extracellular matrix are summarized for both solid tumors and non-solid hematologic malignancies. An overview of microfluidic systems designed specifically to answer questions related to different aspects of the tumor microenvironment is provided, followed by an examination of how these systems offer new opportunities to study outstanding challenges related to the major cancer hallmarks. Challenges also remain for microfluidics engineers, but it is hoped that cooperation between engineers and biologists at the intersection of their respective fields will lead to significant impact on the utility of organs-on-chips in cancer research.

An integrated microfludic device for culturing and screening of Giardia lamblia

In vitro culturing of trophozoites was important for research of Giardia lamblia (G. lamblia), especially in discovery of anti-Giardia agents. The current culture methods mainly suffer from lab-intension or the obstacle in standardizing the gas condition. Thus, it could benefit from a more streamlined and integrated approach. Microfluidics offers a way to accomplish this goal. Here we presented an integrated microfluidic device for culturing and screening of G. lamblia. The device consisted of a polydimethylsiloxane (PDMS) microchip with an aerobic culture system. In the microchip, the functionality of integrated concentration gradient generator (CGG) with micro-scale cell culture enables dose-response experiment to be performed in a simple and reagent-saving way. The diffusion-based culture chambers allowed growing G. lamblia at the in vivo like environment. It notable that the highly air permeable material of parallel chambers maintain uniform anaerobic environment in different chambers easily. Using this device, G. lamblia were successfully cultured and stressed on-chip. In all cases, a dose-related inhibitory response was detected. The application of this device for these purposes represents the first step in developing a completely integrated microfluidic platform for high-throughput screening and might be expanded to other assays based on in vitro culture of G. lamblia with further tests.

Induction of epithelial-to-mesenchymal transition in proximal tubular epithelial cells on microfluidic devices

In proteinuric nephropathy, epithelial-to-mesenchymal transition (EMT) is an important mechanism that causes renal interstitial fibrosis. The precise role of EMT in the pathogenesis of fibrosis remains controversial, partly due to the absence of suitable in vitro or in vivo models. We developed two microfluidic and compartmental chips that reproduced the fluidic and three-dimensional microenvironment of proximal tubular epithelial cells in vivo. Using one microfluidic device, we stimulated epithelial cells with a flow of healthy human serum, heat-inactivated serum and complement C3a, which mimicked the flow of urine within the proximal tubule. We observed that epithelial cells exposed to serum proteins became apoptotic or developed a mesenchymal phenotype. Incubating cells with C3a induced similar features. However, cells exposed to heat-inactivated serum did not adopt the mesenchymal phenotype. Furthermore, we successfully recorded the cellular morphological changes and the process of transmigration into basement membrane extract during EMT in real-time using another three-dimensional microdevice. In conclusion, we have established a cell-culture system that mimics the native microenvironment of the proximal tubule to a certain extent. Our data indicates that EMT did occur in epithelial cells that were exposed to serum proteins, and C3a plays an essential role in this pathological process.

A bioengineered heterotypic stroma-cancer microenvironment model to study pancreatic ductal adenocarcinoma

Interactions between neoplastic epithelial cells and components of a reactive stroma in pancreatic ductal adenocarcinoma (PDAC) are of key significance behind the disease's dismal prognosis. Despite extensive published research in the importance of stroma-cancer interactions in other cancers and experimental evidence supporting the importance of the microenvironment in PDAC progression, a reproducible three-dimensional (3D) in vitro model for exploring stroma-cancer interplay and evaluating therapeutics in a physiologically relevant context has been lacking. We introduce a humanized microfluidic model of the PDAC microenvironment incorporating multicellularity, extracellular matrix (ECM) components, and a spatially defined 3D microarchitecture. Pancreatic stellate cells (PSCs) isolated from clinically-evaluated human tissue specimens were co-cultured with pancreatic ductal adenocarcinoma cells as an accessible 3D construct that maintained important tissue features and disease behavior. Multiphoton excitation (MPE) and Second Harmonic Generation (SHG) imaging techniques were utilized to image the intrinsic signal of stromal collagen in human pancreatic tissues and live cell-collagen interactions within the optically-accessible microfluidic tissue model. We further evaluated the dose-response of the model with the anticancer agent paclitaxel. This bioengineered model of the PDAC stroma-cancer microenvironment provides a complementary platform to elucidate the complex stroma-cancer interrelationship and to evaluate the efficacy of potential therapeutics in a humanized system that closely recapitulates key PDAC microenvironment characteristics.

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.

An integrated microfluidic device in marine microalgae culture for toxicity screening application

Algal assay using marine microalgae has emerged as an important method to evaluate the toxicity of chemicals, which is currently undertaken using conventional culture and additional detection of physiological cellular endpoints. While effective, this approach can be labor-intensive and thus could benefit from a more streamlined, integrated approach. Microfluidics offers a way to accomplish this goal. Here, we demonstrate a microfluidic device which consists of a concentration gradient generator (CGG), diffusible culturing module and power-free valve system. It allows the processes of chemical liquid dilution and diffusion, micro-scale microalgal culture (in batch or chemostatic conditions), cell stimulation and on-lined screening to be integrated into a single device. Using the device, marine microalgae were successfully cultured and stressed on-chip. The simple assay provides multi-biological response measurements of cell division rate, autofluorescence and esterase activity. This work showed promising in developing a microfluidic platform for toxicity screening based on marine microalgal culture.

Microdevices for nanomedicine

This review surveys selected methods of manufacture and applications of microdevices-miniaturized functional devices capable of handling cell and tissue cultures or producing particles-and discusses their potential relevance to nanomedicine. Many characteristics of microdevices such as miniaturization, increased throughput, and the ability to mimic organ-specific microenvironments are promising for the rapid, low-cost evaluation of the efficacy and toxicity of nanomaterials. Their potential to accurately reproduce the physiological environments that occur in vivo could reduce dependence on animal models in pharmacological testing. Technologies in microfabrications and microfluidics are widely applicable for nanomaterial synthesis and for the development of diagnostic devices. Although the use of microdevices in nanomedicine is still in its infancy, these technologies show promise for enhancing fundamental and applied research in nanomedicine.

Microfluidics for manipulating cells

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

Biomimetic tumor microenvironment on a microfluidic platform

Tumor microenvironment is a highly complex system consisting of non-cancerous cells, soluble factors, signaling molecules, extracellular matrix, and mechanical cues, which provides tumor cells with integrated biochemical and biophysical cues. It has been recognized as a significant regulator in cancer initiation, progression, metastasis, and drug resistance, which is becoming a crucial component of cancer biology. Modeling microenvironmental conditions of such complexity in vitro are particularly difficult and technically challenging. Significant advances in microfluidic technologies have offered an unprecedented opportunity to closely mimic the physiological microenvironment that is normally encountered by cancer cells in vivo. This review highlights the recent advances of microfluidic platform in recapitulating many aspects of tumor microenvironment from biochemical and biophysical regulations. The major events relevant in tumorigenesis, angiogenesis, and spread of cancer cells dependent on specific combinations of cell types and soluble factors present in microenvironmental niche are summarized. The questions and challenges that lie ahead if this field is expected to transform the future cancer research are addressed as well.

High-content single-cell analysis on-chip using a laser microarray scanner

High-content cellomic analysis is a powerful tool for rapid screening of cellular responses to extracellular cues and examination of intracellular signal transduction pathways at the single-cell level. In conjunction with microfluidics technology that provides unique advantages in sample processing and precise control of fluid delivery, it holds great potential to transform lab-on-a-chip systems for high-throughput cellular analysis. However, high-content imaging instruments are expensive, sophisticated, and not readily accessible. Herein, we report on a laser scanning cytometry approach that exploits a bench-top microarray scanner as an end-point reader to perform rapid and automated fluorescence imaging of cells cultured on a chip. Using high-content imaging analysis algorithms, we demonstrated multiplexed measurements of morphometric and proteomic parameters from all single cells. Our approach shows the improvement of both sensitivity and dynamic range by two orders of magnitude as compared to conventional epifluorescence microscopy. We applied this technology to high-throughput analysis of mesenchymal stem cells on an extracellular matrix protein array and characterization of heterotypic cell populations. This work demonstrates the feasibility of a laser microarray scanner for high-content cellomic analysis and opens up new opportunities to conduct informative cellular analysis and cell-based screening in the lab-on-a-chip systems.

Microfluidic 3D cell culture: potential application for tissue-based bioassays

Current fundamental investigations of human biology and the development of therapeutic drugs commonly rely on 2D monolayer cell culture systems. However, 2D cell culture systems do not accurately recapitulate the structure, function or physiology of living tissues, nor the highly complex and dynamic 3D environments in vivo. Microfluidic technology can provide microscale complex structures and well-controlled parameters to mimic the in vivo environment of cells. The combination of microfluidic technology with 3D cell culture offers great potential for in vivo-like tissue-based applications, such as the emerging organ-on-a-chip system. This article will review recent advances in the microfluidic technology for 3D cell culture and their biological applications.

On-chip anticancer drug test of regular tumor spheroids formed in microwells by a distributive microchannel network

This paper proposes a new cytotoxicity assay in a microfluidic device with microwells and a distributive microfluidic channel network for the formation of cancer cell spheroids. The assay can generate rapid and uniform cell clusters in microwells and test in situ cytotoxicity of anticancer drugs including sequential drug treatments, long term culture of spheroids and cell viability assays. Inlet ports are connected to the microwells by a hydraulic resistance network. This uniform distribution of cell suspensions results in regular spheroid dimensions. Injected cancer cells were trapped in microwells, and aggregated into tumor spheroids within 3 days. A cytotoxicity test of the spheroids in microwells was subsequently processed in the same device without the extraction of cells. The in situ cytotoxicity assay of tumor spheroids in microwells was comparable with the MTT assay on hanging drop spheroids using a conventional 96-well plate. It was observed that the inhibition rate of the spheroids was less than that in the 2D culture dish and the effect on tumor spheroids was different depending on the anticancer drug. This device could provide a convenient in situ assay tool to assess the cytotoxicity of anticancer drugs on tumor spheroids, offering more information than the conventional 2D culture plate.

Cytotoxicity of quantum dots assay on a microfluidic 3D-culture device based on modeling diffusion process between blood vessels and tissues

In this work, a novel quantum dot (QD) cytotoxicity assay platform on a microfluidic three-dimensional (3D) culture device via imitating the diffusion process between blood vessels and tissues was developed. The device is composed of a main channel and two sets of cell culture chambers. The cell culture chambers were located at different distances from the main channel and were divided into "close chambers" and "far chambers". HepG2 cells were cultured in an agarose matrix under 3D conditions and kept at high viability for at least three days. Fluorescein sodium and fluorescein isothiocyanate conjugated to bovine serum albumin (FITC-BSA) were used as models to demonstrate the diffusion process between main channel and cell culture chambers. QD cytotoxicity was evaluated by determining cell apoptosis, intracellular reactive oxygen species (ROS) and glutathione (GSH) with specific fluorescence probes. Cell autophagy inhibitor 3-methyladenine (3-MA) could reduce cell apoptosis at low concentrations of QDs, which proves that cell autophagy plays a key role in QD cytotoxicity. The effect of a series of 3-MA solutions on cell apoptosis at QD concentration of 40 μg mL(-1) was investigated, which showed that the percentage of cell apoptosis decreased ∼15% from 0 to 12 mM 3-MA. The device shows potential as a high-throughput, low-cost and time-saving platform and constructs a more vivid biomimetic microenvironment for the QD cytotoxicity study.

Tumor cell migration in complex microenvironments

Tumor cell migration is essential for invasion and dissemination from primary solid tumors and for the establishment of lethal secondary metastases at distant organs. In vivo and in vitro models enabled identification of different factors in the tumor microenvironment that regulate tumor progression and metastasis. However, the mechanisms by which tumor cells integrate these chemical and mechanical signals from multiple sources to navigate the complex microenvironment remain poorly understood. In this review, we discuss the factors that influence tumor cell migration with a focus on the migration of transformed carcinoma cells. We provide an overview of the experimental and computational methods that allow the investigation of tumor cell migration, and we highlight the benefits and shortcomings of the various assays. We emphasize that the chemical and mechanical stimulus paradigms are not independent and that crosstalk between them motivates the development of new assays capable of applying multiple, simultaneous stimuli and imaging the cellular migratory response in real-time. These next-generation assays will more closely mimic the in vivo microenvironment to provide new insights into tumor progression, inform techniques to control tumor cell migration, and render cancer more treatable.

A microfluidic-based device for study of transendothelial invasion of tumor aggregates in realtime

Circulating tumor aggregates exhibit a high metastatic potential and could potentially serve as an important target for cancer therapies. In this study, we developed a microfluidic model that reconstitutes and is representative of the principal components of biological blood vessels, including vessel cavity, endothelium, and perivascular matrix containing chemokines. Using this model, the transendothelial invasion of tumor aggregates can be observed and recorded in realtime. In this study we analyzed the extravasation process of salivary gland adenoid cystic carcinoma (ACC) cell aggregates. ACC aggregates transmigrated across the endothelium under the stimulation of chemokine CXCL12. The endothelial integrity was irreversibly damaged at the site of transendothelial invasion. The transendothelial invasion of ACC aggregates was inhibited by AMD3100, but the adhesion of ACC aggregates to the endothelium was not affected by the CXCR4 antagonist. This model allows for detailed study of the attachment and transendothelial invasion of tumor aggregates; thus, it would be a useful tool for analysis of the underlying mechanisms of metastasis and for testing novel anti-metastasis agents.

Isolation of mammary epithelial cells from three-dimensional mixed-cell spheroid co-culture

While enormous efforts have gone into identifying signaling pathways and molecules involved in normal and malignant cell behaviors(1-2), much of this work has been done using classical two-dimensional cell culture models, which allow for easy cell manipulation. It has become clear that intracellular signaling pathways are affected by extracellular forces, including dimensionality and cell surface tension(3-4). Multiple approaches have been taken to develop three-dimensional models that more accurately represent biologic tissue architecture(3). While these models incorporate multi-dimensionality and architectural stresses, study of the consequent effects on cells is less facile than in two-dimensional tissue culture due to the limitations of the models and the difficulty in extracting cells for subsequent analysis. The important role of the microenvironment around tumors in tumorigenesis and tumor behavior is becoming increasingly recognized(4). Tumor stroma is composed of multiple cell types and extracellular molecules. During tumor development there are bidirectional signals between tumor cells and stromal cells(5). Although some factors participating in tumor-stroma co-evolution have been identified, there is still a need to develop simple techniques to systematically identify and study the full array of these signals(6). Fibroblasts are the most abundant cell type in normal or tumor-associated stromal tissues, and contribute to deposition and maintenance of basement membrane and paracrine growth factors(7). Many groups have used three dimensional culture systems to study the role of fibroblasts on various cellular functions, including tumor response to therapies, recruitment of immune cells, signaling molecules, proliferation, apoptosis, angiogenesis, and invasion(8-15). We have optimized a simple method for assessing the effects of mammary fibroblasts on mammary epithelial cells using a commercially available extracellular matrix model to create three-dimensional cultures of mixed cell populations (co-cultures)(16-22). With continued co-culture the cells form spheroids with the fibroblasts clustering in the interior and the epithelial cells largely on the exterior of the spheroids and forming multi-cellular projections into the matrix. Manipulation of the fibroblasts that leads to altered epithelial cell invasiveness can be readily quantified by changes in numbers and length of epithelial projections(23). Furthermore, we have devised a method for isolating epithelial cells out of three-dimensional co-culture that facilitates analysis of the effects of fibroblast exposure on epithelial behavior. We have found that the effects of co-culture persist for weeks after epithelial cell isolation, permitting ample time to perform multiple assays. This method is adaptable to cells of varying malignant potential and requires no specialized equipment. This technique allows for rapid evaluation of in vitro cell models under multiple conditions, and the corresponding results can be compared to in vivo animal tissue models as well as human tissue samples.

Probing the role of mesenchymal stem cells in salivary gland cancer on biomimetic microdevices

Mesenchymal stem cells (MSCs) are the progenitors of stromal cells, which have been found to interact with cancer cells and represent an important target for cancer therapies. Salivary gland cancer is an aggressive malignant epithelial tumor and can easily disseminate to distant sites in vivo. In this work, we probed the role of MSCs in salivary gland cancer in an in vivo like tumor microenvironment by using a series of functional microfluidic devices for 2D and 3D assays. The results demonstrated that MSCs could be recruited by salivary gland cancer cells (ACC-M) and this effect was potentially mediated by TGF-β secreted by cancer cells. In addition, MSCs exhibited the ability to squeeze into ACC-M spheroids by dispersing the cell-cell connection and reducing the expression of E-cadherin in cancer cells. In particular, MSCs exhibited the ability to enhance the invasion of salivary cancer under a chemokine CXCL12 gradient, indicating the involvement of a CXCL12-CXCR4 pathway and the tumor-promoting properties of MSCs in cancer progression. This study recreated an in vivo-like tumor microenvironment by constructing a series of functional microdevices and explored the role of MSCs in ACC invasion for the first time. It provides a unique platform to open up new options to target stromal cells for cancer therapy and also analyzed the potential risk of using MSCs as drug delivery carriers for therapeutic purposes in carcinoma treatment.

Multi-function microsystem for cells migration analysis and evaluation of photodynamic therapy procedure in coculture

Cell migration is an important physiological process, which is involved in cancer metastasis. Therefore, the investigation of cell migration may lead to the development of novel therapeutic approaches. In this study, we have successfully developed a microsystem for culture of two cell types (non-malignant and carcinoma) and for analysis of cell migration dependence on distance between them. Finally, we studied quantitatively the influence of photodynamic therapy (PDT) procedures on the viability of pairs of non-malignant (MRC5 or Balb/3T3) and carcinoma (A549) cells coculture. The proposed geometry of the microsystem allowed for separate introduction of two cell lines and analysis of cells migration dependence on distance between the cells. We found that a length of connecting microchannel has an influence on cell migration and viability of non-malignant cells after PDT procedure. Summarizing, the developed microsystem can constitute a new tool for carrying out experiments, which offers a few functions: cell migration analysis, carcinoma and non-malignant cells coculture, and evaluation of PDT procedure in the various steps of cell migration.

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.

Organs-on-a-chip: a focus on compartmentalized microdevices

Advances in microengineering technologies have enabled a variety of insights into biomedical sciences that would not have been possible with conventional techniques. Engineering microenvironments that simulate in vivo organ systems may provide critical insight into the cellular basis for pathophysiologies, development, and homeostasis in various organs, while curtailing the high experimental costs and complexities associated with in vivo studies. In this article, we aim to survey recent attempts to extend tissue-engineered platforms toward simulating organ structure and function, and discuss the various approaches and technologies utilized in these systems. We specifically focus on microtechnologies that exploit phenomena associated with compartmentalization to create model culture systems that better represent the in vivo organ microenvironment.

Stem Cell Transplantation for Hematological Malignancies: Prospects for Personalized Medicine and Co-therapy with Mesenchymal Stem Cells

Bone marrow transplantation is a form of cell therapy that has been in practice for decades for the treatment of hematological disorders and solid tumors. Immunosuppressive therapy has been a mainstay for treatment, but the severity of the adverse effects has made it an undesirable choice. Mesenchymal stem cells (MSCs), which reside in the vascular regions of the bone marrow, have been shown to serve as cellular support for the hematopoietic stem cell (HSC) niche. Furthermore, the immune suppressive properties of MSCs have been explored in the treatment of inflammatory and autoimmune disorders. Thus, co-therapy with MSCs has been shown to facilitate engraftment of hematopoietic cells by suppressive graft versus host disease (GvHD). Although the mechanism by which MSCs suppress GvHD is unclear, the experimental evidence suggests that this partly occurs by modulation of immune response such as the induction of regulatory T cells. This paper discusses the role of MSCs as co-therapy for the future of stem cell transplantation, with the overarching theme of personalized medicine for cell-based health interventions.

Microfluidic devices for studying heterotypic cell-cell interactions and tissue specimen cultures under controlled microenvironments

Microfluidic devices allow for precise control of the cellular and noncellular microenvironment at physiologically relevant length- and time-scales. These devices have been shown to mimic the complex in vivo microenvironment better than conventional in vitro assays, and allow real-time monitoring of homotypic or heterotypic cellular interactions. Microfluidic culture platforms enable new assay designs for culturing multiple different cell populations and∕or tissue specimens under controlled user-defined conditions. Applications include fundamental studies of cell population behaviors, high-throughput drug screening, and tissue engineering. In this review, we summarize recent developments in this field along with studies of heterotypic cell-cell interactions and tissue specimen culture in microfluidic devices from our own laboratory.

Electrokinetic gated injection-based microfluidic system for quantitative analysis of hydrogen peroxide in individual HepG2 cells

A microfluidic system to determine hydrogen peroxide (H(2)O(2)) in individual HepG2 cells based on the electrokinetic gated injection was developed for the first time. A home-synthesized fluorescent probe, bis(p-methylbenzenesulfonate)dichlorofluorescein (FS), was employed to label intracellular H(2)O(2) in the intact cells. On a simple cross microchip, multiple single-cell operations, including single cell injection, cytolysis, electrophoresis separation and detection of H(2)O(2), were automatically carried out within 60 s using the electrokinetic gated injection and laser-induced fluorescence detection (LIFD). The performance of the method was evaluated under the optimal conditions. The linear calibration curve was over a range of 4.39-610 amol (R(2)=0.9994). The detection limit was 0.55 amol or 9.0×10(-10) M (S/N=3). The relative standard deviations (RSDs, n=6) of migration time and peak area were 1.4% and 4.8%, respectively. With the use of this method, the average content of H(2)O(2) in single HepG2 cells was found to be 16.09±9.84 amol (n=15). Separation efficiencies in excess of 17,000 theoretical plates for the cells were achieved. These results demonstrated that the efficient integration and automation of these single-cell operations enabled the sensitive, reproducible, and quantitative examination of intracellular H(2)O(2) at single-cell level. Owing to the advantages of simple microchip structure, controllable single-cell manipulation and ease in building, this platform provides a universal way to automatically determine other intracellular constituents within single cells.

Controlling mass transport in microfluidic devices

Microfluidic platforms offer exquisite capabilities in controlling mass transport for biological studies. In this review, we focus on recent developments in manipulating chemical concentrations at the microscale. Some techniques prevent or accelerate mixing, whereas others shape the concentration gradients of chemical and biological molecules. We also highlight several in vitro biological studies in the areas of organ engineering, cancer, and blood coagulation that have benefited from accurate control of mass transfer.

Cell-based assays: fuelling drug discovery

It has been estimated that over a billion dollars in resources can be consumed to obtain clinical approval, and only a few new chemical entities are approved by the US Food and Drug Administration (FDA) each year. Therefore it is of utmost importance to obtain the maximum amount of information about biological activity, toxicological profile, biochemical mechanisms, and off-target interactions of drug-candidate leads in the earliest stages of drug discovery. Cell-based assays, because of their peculiar advantages of predictability, possibility of automation, multiplexing, and miniaturization, seem the most appealing tool for the high demands of the early stages of the drug-discovery process. Nevertheless, cellular screening, relying on different strategies ranging from reporter gene technology to protein fragment complementation assays, still presents a variety of challenges. This review focuses on main advantages and limitations of different cell-based approaches, and future directions and trends in this fascinating field.