A microfluidic chip for real-time studies of the volume of single cells

Concepts, electrode configuration, characterization, and data analytics of electric and electrochemical microfluidic platforms: a review

Microfluidic cytometry (MC) and electrical impedance spectroscopy (EIS) are two important techniques in biomedical engineering. Microfluidic cytometry has been utilized in various fields such as stem cell differentiation and cancer metastasis studies, and provides a simple, label-free, real-time method for characterizing and monitoring cellular fates. The impedance microdevice, including impedance flow cytometry (IFC) and electrical impedance spectroscopy (EIS), is integrated into MC systems. IFC measures the impedance of individual cells as they flow through a microfluidic device, while EIS measures impedance changes during binding events on electrode regions. There have been significant efforts to improve and optimize these devices for both basic research and clinical applications, based on the concepts, electrode configurations, and cell fates. This review outlines the theoretical concepts, electrode engineering, and data analytics of these devices, and highlights future directions for development.

Real-time computer assisted measurement of oocyte and embryo volume for assessment of transport parameters

Cryopreservation of gametes has revolutionized both animal agriculture and human reproductive medicine. Although many new technologies have tremendously improved the cryopreservation of oocytes and embryos, osmotic stress encountered during the equilibration process can cause their loss of function. Rational cryoprotective agent (CPA) equilibration strategies can be used to minimize this stress but require trained personnel to monitor the process in individual oocytes or embryos or require the use of suboptimal average transport parameter values in mathematically guided protocols. To enable individually optimized equilibration of CPAs in individual cells, here we establish experimental and computational techniques to track the osmotic behavior of individual bovine oocytes and embryos during CPA equilibration in real time. We designed a microfluidic device to provide a controlled flow of CPA and modified standard image analysis techniques to estimate real-time cell volume changes. In particular, we used a level-set method to define a boundary within a contour plot which could automate the image analysis process. A colour based level set algorithm coupled with contour smoothing not only provided the best fit but also reduced the segmentation time to well under a second per image. The accuracy of the automated method was comparable to human segmented images for both oocytes and embryos. This technology should enable both rapid evaluation of key biophysical parameters in oocytes and embryos undergoing CPA equilibration and the development of real-time feedback-control of CPA equilibration, enabling individual oocyte- and embryo-specific optimal protocols.

Microfluidic Time-Division Multiplexing Accessing Resistive Pulse Sensor for Particle Analysis

Due to its simplicity and robustness, pore-based resistive pulse sensors have been widely used to detect, measure, and analyze particles at length scales ranging from nanometers to micrometers. While multiple pore-based resistive pulse sensors are preferred to increase the analysis throughput and to overcome the clogging issues, the scalability is often limited. In response, by combining the time-division multiple access technique in the telecommunication field with the microfluidics, we reported a microfluidic time-division multiplexing accessing (TDMA) single-end resistive pulse sensor, in which particles can be analyzed through a scalable number of microfluidic channels. With an eight-channel microfluidic device and polystyrene particles as proof-of-principle, we successfully demonstrated this multiplexed technology is effective in measuring the particle size and concentration, in analyzing the particle arriving dynamics, and in discriminating mixed populations. Importantly, the availability of multiple sensing pores provides a robust mechanism to overcome the clogging issue, allowing the analysis to continue even when some of the pores are clogged. We anticipate this TDMA approach could find wide applications and facilitate future development of multiplexed resistive pulse sensing from the microscale to nanoscale.

High-throughput Screening of Erratic Cell Volume Regulation Using a Hydrogel-based Single-cell Microwell Array

Here, we report that a single-cell microwell array based on photocrosslinked hydrogel can be used to screen cells exhibiting a defective regulatory volume decrease (RVD) in high-throughput. The RVD is a regulatory function of cells that maintains cell volume homeostasis in a hypotonic medium. Single Madin-Darby canine kidney (MDCK) cells grown in the microwells were loaded with a volume-sensitive fluorescence dye. Changes in the volume of discrete single cells were traced for 20 min in a hypotonic solution using a wide-field fluorescence microscopy. The volume changes of more than 100 single cells were analyzed simultaneously using time-lapse fluorescence micrographs. Cells showing erratic RVD could be easily screened from the image analysis. Nearly 40% of the MDCK single cells exhibited weak, or no, RVD. Since other previously reported methods could not detect as many changes in the volume of discrete singles cells as the method used in this report, we anticipate that our reported method will provide an efficient way of elucidating the RVD mechanisms of cells that have not yet been completely understood.

Droplet-based microfluidic platform for measurement of rapid erythrocyte water transport

Cell membrane water permeability is an important determinant of epithelial fluid secretion, tissue swelling, angiogenesis, tumor spread and other biological processes. Cellular water channels, aquaporins, are important drug targets. Water permeability is generally measured from the kinetics of cell volume change in response to an osmotic gradient. Here, we developed a microfluidic platform in which cells expressing a cytoplasmic, volume-sensing fluorescent dye are rapidly subjected to an osmotic gradient by solution mixing inside a ~0.1 nL droplet surrounded by oil. The solution mixing time was <10 ms. Osmotic water permeability was deduced from a single, time-integrated fluorescence image of an observation area in which the time after mixing was determined through spatial position. Water permeability was accurately measured in aquaporin-expressing erythrocytes with half-times for osmotic equilibration down to <50 ms. Compared with conventional water permeability measurements using costly stopped-flow instrumentation, the microfluidic platform here utilizes sub-microliter blood sample volume, does not suffer from mixing artifacts, and replaces challenging kinetic measurements by single image capture using a standard laboratory fluorescence microscope.

Microfluidic impedance flow cytometry enabling high-throughput single-cell electrical property characterization

This article reviews recent developments in microfluidic impedance flow cytometry for high-throughput electrical property characterization of single cells. Four major perspectives of microfluidic impedance flow cytometry for single-cell characterization are included in this review: (1) early developments of microfluidic impedance flow cytometry for single-cell electrical property characterization; (2) microfluidic impedance flow cytometry with enhanced sensitivity; (3) microfluidic impedance and optical flow cytometry for single-cell analysis and (4) integrated point of care system based on microfluidic impedance flow cytometry. We examine the advantages and limitations of each technique and discuss future research opportunities from the perspectives of both technical innovation and clinical applications.

A microchip integrating cell array positioning with in situ single-cell impedance measurement

A micro-chip integrating cell positioning with in situ, real-time and long-time impedance measurement on a single cell using nano-modified measuring electrodes.

Review of methods to probe single cell metabolism and bioenergetics

Single cell investigations have enabled unexpected discoveries, such as the existence of biological noise and phenotypic switching in infection, metabolism and treatment. Herein, we review methods that enable such single cell investigations specific to metabolism and bioenergetics. Firstly, we discuss how to isolate and immobilize individuals from a cell suspension, including both permanent and reversible approaches. We also highlight specific advances in microbiology for its implications in metabolic engineering. Methods for probing single cell physiology and metabolism are subsequently reviewed. The primary focus therein is on dynamic and high-content profiling strategies based on label-free and fluorescence microspectroscopy and microscopy. Non-dynamic approaches, such as mass spectrometry and nuclear magnetic resonance, are also briefly discussed.

Quantifying the volume of single cells continuously using a microfluidic pressure-driven trap with media exchange

We demonstrate a microfluidic device capable of tracking the volume of individual cells by integrating an on-chip volume sensor with pressure-activated cell trapping capabilities. The device creates a dynamic trap by operating in feedback; a cell is periodically redirected back and forth through a microfluidic volume sensor (Coulter principle). Sieve valves are positioned on both ends of the sensing channel, creating a physical barrier which enables media to be quickly exchanged while keeping a cell firmly in place. The volume of individual Saccharomyces cerevisiae cells was tracked over entire growth cycles, and the ability to quickly exchange media was demonstrated.

Evaluation of single cell electrical parameters from bioimpedance of a cell suspension

The present study introduces a simple and detailed analysis technique to extract the electrical properties of a single cell from impedance spectroscopy data from a group of cells in suspension, leading to a more reliable and cost effective diagnosis process for disease detection.

Microfluidic device for trapping and monitoring three dimensional multicell spheroids using electrical impedance spectroscopy

In this paper, we report the design, fabrication, and testing of a lab-on-a-chip based microfluidic device for application of trapping and measuring the dielectric properties of microtumors over time using electrical impedance spectroscopy (EIS). Microelectromechanical system (MEMS) techniques were used to embed opposing electrodes onto the top and bottom surfaces of a microfluidic channel fabricated using Pyrex substrate, chrome gold, SU-8, and polydimethylsiloxane. Differing concentrations of cell culture medium, differing sized polystyrene beads, and MCF-7 microtumor spheroids were used to validate the designs ability to detect background conductivity changes and dielectric particle diameter changes between electrodes. The observed changes in cell medium concentrations demonstrated a linear relation to extracted solution resistance (Rs), while polystyrene beads and multicell spheroids induced changes in magnitude consistent with diameter increase. This design permits optical correlation between electrical measurements and EIS spectra.

Microcapillary-assisted dielectrophoresis for single-particle positioning

Here, we demonstrate microcapillary-assisted dielectrophoresis (μC-DEP), a new capability for precise positioning of particles or biological cells in applications such as dynamic assays. The method largely derives from a need to evade the challenges faced with hydrodynamic trapping of particles or cells at microcapillaries typically realized through brief application of suction. Microcapillaries here serve a dual purpose by firstly squeezing field lines to define localized positive DEP traps and then establishing an exclusive access to the trapped cell for probing. Strength of the traps is presented through numerical results at various excitation frequencies. Their effectiveness is shown experimentally against relevant solution conductivities using 10 μm polystyrene microspheres. Usefulness of the method for positioning individual cells is demonstrated via experimental results on cell viability and single-cell impedance spectroscopy.

Microfluidic impedance spectroscopy as a tool for quantitative biology and biotechnology

A microfluidic device that is able to perform dielectric spectroscopy is developed. The device consists of a measurement chamber that is 250 μm thick and 750 μm in radius. Around 1000 cells fit inside the chamber assuming average quantities for cell radius and volume fraction. This number is about 1000 folds lower than the capacity of conventional fixtures. A T-cell leukemia cell line Jurkat is tested using the microfluidic device. Measurements of deionized water and salt solutions are utilized to determine parasitic effects and geometric capacitance of the device. Physical models, including Maxwell-Wagner mixture and double shell models, are used to derive quantities for sub-cellular units. Clausius-Mossotti factor of Jurkat cells is extracted from the impedance spectrum. Effects of cellular heterogeneity are discussed and parameterized. Jurkat cells are also tested with a time domain reflectometry system for verification of the microfluidic device. Results indicate good agreement of values obtained with both techniques. The device can be used as a unique cell diagnostic tool to yield information on sub-cellular units.

Visualization of irrigation fluid flow and calculation of its velocity distribution in the anterior chamber by particle image velocimetry

PURPOSE

To visualize irrigation fluid flow and calculate its velocity distribution in the anterior chamber during phacoemulsification by particle image velocimetry.

METHODS

Porcine eyes were fixed in a glass chamber filled with balanced salt solution. An ultrasound handpiece was fixed to the glass chamber, and its tip was inserted into the anterior chamber through a corneal incision. Irrigation fluid was mixed with fluorescein-labeled liposomes as tracer particles. During phacoemulsification without ultrasound, a sheet-like Nd-YAG pulsed laser beam was emitted and moved from the iris plane to the top of the cornea continuously. Images of illuminated liposomes in the anterior chamber were captured at short intervals with a CCD camera, and the velocity distribution of irrigation fluid flow was calculated by particle image velocimetry.

RESULTS

By particle image velocimetry, the flow velocity distribution could be calculated in any plane of the anterior chamber. Dynamic flow of the irrigation fluid, ejected from the tip of the ultrasound handpiece and returned to an aspiration port, was visualized clearly in the anterior chamber. The maximum flow velocity in the anterior chamber was 342 ± 131 mm/s.

CONCLUSIONS

Particle image velocimetry enabled the visualization of irrigation fluid flow and quantification of its velocity distribution in different planes of the anterior chamber during cataract surgery. These data are essential for evaluating the safety and efficacy of new surgical settings and devices during phacoemulsification.

Continuous and long-term volume measurements with a commercial Coulter counter

We demonstrate a method to enhance the time resolution of a commercial Coulter counter and enable continuous and long-term cell size measurements for growth rate analyses essential to understanding basic cellular processes, such as cell size regulation and cell cycle progression. Our simple modifications to a commercial Coulter counter create controllable cell culture conditions within the sample compartment and combine temperature control with necessary adaptations to achieve measurement stability over several hours. We also wrote custom software, detailed here, to analyze instrument data files collected by either this continuous method or standard, periodic sampling. We use the continuous method to measure the growth rate of yeast in G1 during a prolonged arrest and, in different samples, the dependency of growth rate on cell size and cell cycle position in arrested and proliferating cells. We also quantify with high time resolution the response of mouse lymphoblast cell culture to drug treatment. This method provides a technique for continuous measurement of cell size that is applicable to a large variety of cell types and greatly expands the set of analysis tools available for the Coulter counter.

Dynamic measurement of the height and volume of migrating cells by a novel fluorescence microscopy technique

We propose a new technique to measure the volume of adherent migrating cells. The method is based on a negative staining where a fluorescent, non-cell-permeant dye is added to the extracellular medium. The specimen is observed with a conventional fluorescence microscope in a chamber of uniform height. Given that the fluorescence signal depends on the thickness of the emitting layer, the objects excluding the fluorescent dye (i.e., cells) appear dark, and the decrease of the fluorescent signal with respect to the background is expected to give information about the height and the volume of the object. Using a glass microfabricated pattern with steps of defined heights, we show that the drop in fluorescence intensity is indeed proportional to the height of the step and obtain calibration curves relating fluorescence intensity to height. The technique, termed the fluorescence displacement method, is further validated by comparing our measurements with the ones obtained by atomic force microscopy (AFM). We apply our method to measure the real-time volume dynamics of migrating fish epidermal keratocytes subjected to osmotic stress. The fluorescence displacement technique allows fast and precise monitoring of cell height and volume, thus providing a valuable tool for characterizing the three-dimensional behaviour of migrating cells.

A new floating electrode structure for generating homogeneous electrical fields in microfluidic channels

In this article a new parallel electrode structure in a microfluidic channel is described that makes use of a floating electrode to get a homogeneous electrical field. Compared to existing parallel electrode structures, the new structure has an easier production process and there is no need for an electrical connection to both sides of the microfluidic chip. With the new chip design, polystyrene beads suspended in background electrolyte have been detected using electrical impedance measurements. The results of electrical impedance changes caused by beads passing the electrodes are compared with results in a similar planar electrode configuration. It is shown that in the new configuration the coefficient of variation of the impedance changes is lower compared to the planar configuration (0.39 versus 0.56) and less dependent on the position of the beads passage in the channel as a result of the homogeneous electrical field. To our knowledge this is the first time that a floating electrode is used for the realization of a parallel electrode structure. The proposed production method for parallel electrodes in microfluidic channels can easily be applied to other applications.

A microfluidic device for simultaneous electrical and mechanical measurements on single cells

This paper presents a microfluidic device for simultaneous mechanical and electrical characterization of single cells. The device performs two types of cellular characterization (impedance spectroscopy and micropipette aspiration) on a single chip to enable cell electrical and mechanical characterization. To investigate the performance of the device design, electrical and mechanical properties of MC-3T3 osteoblast cells were measured. Based on electrical models, membrane capacitance of MC-3T3 cells was determined to be 3.39±1.23 and 2.99±0.82 pF at the aspiration pressure of 50 and 100 Pa, respectively. Cytoplasm resistance values were 110.1±37.7 kΩ (50 Pa) and 145.2±44.3 kΩ (100 Pa). Aspiration length of cells was found to be 0.813±0.351 μm at 50 Pa and 1.771±0.623 μm at 100 Pa. Quantified Young's modulus values were 377±189 Pa at 50 Pa and 344±156 Pa at 100 Pa. Experimental results demonstrate the device's capability for characterizing both electrical and mechanical properties of single cells.

Single-cell bioelectrical impedance platform for monitoring cellular response to drug treatment

The response of cells to a chemical or biological agent in terms of their impedance changes in real-time is a useful mechanism that can be utilized for a wide variety of biomedical and environmental applications. The use of a single-cell-based analytical platform could be an effective approach to acquiring more sensitive cell impedance measurements, particularly in applications where only diminutive changes in impedance are expected. Here, we report the development of an on-chip cell impedance biosensor with two types of electrodes that host individual cells and cell populations, respectively, to study its efficacy in detecting cellular response. Human glioblastoma (U87MG) cells were patterned on single- and multi-cell electrodes through ligand-mediated natural cell adhesion. We comparatively investigated how these cancer cells on both types of electrodes respond to an ion channel inhibitor, chlorotoxin (CTX), in terms of their shape alternations and impedance changes to exploit the fine detectability of the single-cell-based system. The detecting electrodes hosting single cells exhibited a significant reduction in the real impedance signal, while electrodes hosting confluent monolayer of cells showed little to no impedance change. When single-cell electrodes were treated with CTX of different doses, a dose-dependent impedance change was observed. This enables us to identify the effective dose needed for this particular treatment. Our study demonstrated that this single-cell impedance system may potentially serve as a useful analytical tool for biomedical applications such as environmental toxin detection and drug evaluation.

Measurement of the volume growth rate of single budding yeast with the MOSFET-based microfluidic Coulter counter

We report on measurements of the volume growth rate of ten individual budding yeast cells using a recently developed MOSFET-based microfluidic Coulter counter. The MOSFET-based microfluidic Coulter counter is very sensitive, provides signals that are immune from the baseline drift, and can work with cell culture media of complex composition. These desirable features allow us to directly measure the volume growth rate of single cells of Saccharomyces cerevisiae LYH3865 strain budding yeast in YNB culture media over a whole cell cycle. Results indicate that all budding yeast follow a sigmoid volume growth profile with reduced growth rates at the initial stage before the bud emerges and the final stage after the daughter gets mature. Analysis of the data indicates that even though all piecewise linear, Gomperitz, and Hill's function models can fit the global growth profile equally well, the data strongly support local exponential growth phenomenon. Accurate volume growth measurements are important for applications in systems biology where quantitative parameters are required for modeling and simulation.

Effect of diffusion on impedance measurements in a hydrodynamic flow focusing sensor

This paper investigated the effects of diffusion between non-conductive sheath and conductive sample fluids in an impedance-based biosensor. Impedance measurements were made with 2- and 4-electrode configurations. The 4-electrode design offers the advantage of impedance measurements at low frequencies (<1 kHz) without the deleterious effects of double layer impedance which are present in the 2-electrode design. Hydrodynamic flow focusing was achieved with a modified T-junction design with a smaller cross-section for the sample channel than for the focusing channel, which resulted in 2D focusing of the sample stream with just one sheath stream. By choosing a non-conductive sheath fluid and a conductive sample fluid, the electric field was confined to the focused stream. In order to utilize this system for biosensing applications, we characterized it for electrical and flow parameters. In particular, we investigated the effects of varying flow velocities and flow-rate ratios on the focused stream. Increasing flow-rate ratios reduced the cross-sectional area of the focused streams as was verified by finite element modeling and confocal microscopy. Antibody mediated binding of Escherichia coli to the electrode surface caused an increase in solution resistance at low frequencies. The results also showed that the diffusion mass transport at the interface of the two streams limited the benefits of increased flow focusing. Increasing flow velocities could be used to offset the diffusion effect. To optimize detection sensitivity, flow parameters and mass transport must be considered in conjunction, with the goal of reducing diffusion of conducting species out of the focused stream while simultaneously minimizing its cross-sectional area.

An integrated microfluidic system for studying cell-microenvironmental interactions versatilely and dynamically

We presented an integrated microfluidic system for dynamical study of cell-microenvironmental interactions. We demonstrated its precisely spatio-temporal control in the flow direction and the multi-site staying of the fluids by groups of monolithic microfabricated valves through digital operation, aside from the regulated communication between two loci based on real-time microenvironment transition. Using this system, a series of functional manipulations, including specific delivery, addressable surface treatment, positional cell loading and co-culture were performed quickly and efficiently for biological applications. Sequentially, we carried out the potential utility of this system in the research of dynamic microenvironmental influence to cells using a patho-physiological interaction during cancer initiation and progression. Our results exhibit the passive role but collaborative response of NIH 3T3 fibroblasts to the soluble signals from hepatocellular carcinoma cells, and also the variable behaviors of carcinoma cells under different environmental stimulation. This system can facilitate the in vitro investigation of cell-microenvironmental interactions occurred in numerous biological and pathogenic processes.

An overview of recent strategies in pathogen sensing

Pathogenic bacteria are one of the major concerns in food industries and water treatment facilities because of their rapid growth and deleterious effects on human health. The development of fast and accurate detection and identification systems for bacterial strains has long been an important issue to researchers. Although confirmative for the identification of bacteria, conventional methods require time-consuming process involving either the test of characteristic metabolites or cellular reproductive cycles. In this paper, we review recent sensing strategies based on micro- and nano-fabrication technology. These technologies allow for a great improvement of detection limit, therefore, reduce the time required for sample preparation. The paper will be focused on newly developed nano- and micro-scaled biosensors, novel sensing modalities utilizing microfluidic lab-on-a-chip, and array technology for the detection of pathogenic bacteria.