Impedance spectroscopy-based cell/particle position detection in microfluidic systems

Optimizing Microfluidic Impedance Cytometry by Bypass Electrode Layout Design

Microfluidic impedance cytometry (MIC) has emerged as a popular technique for single-cell analysis. Traditional MIC electrode designs consist of a pair of (or three) working electrodes, and their detection performance needs further improvements for microorganisms. In this study, we designed an 8-electrode MIC device in which the center pair was defined as the working electrode, and the connection status of bypass electrodes could be changed. This allowed us to compare the performance of layouts with no bypasses and those with floating or grounding electrodes by simulation and experiment. The results of detecting Φ 5 μm beads revealed that both the grounding and the floating electrode outperformed the no bypass electrode, and the grounding electrode demonstrated the best signal-to-noise ratio (SNR), coefficient of variation (CV), and detection sensitivity. Furthermore, the effects of different bypass grounding areas (numbers of grounding electrodes) were investigated. Finally, particles passing at high horizontal positions can be detected, and Φ 1 μm beads can be measured in a wide channel (150 μm) using a fully grounding electrode, with the sensitivity of bead volume detection reaching 0.00097%. This provides a general MIC electrode optimization technology for detecting smaller particles, even macromolecular proteins, viruses, and exosomes in the future.

Investigation of geometry-dependent sensing characteristics of microfluidic for single-cell 3D localization

Flow cytometry-based measurement techniques have been widely used for single-cell characterizations, such as impedance, size, and dielectric properties. However, in the measurement process, the reliability of the output measurement signal directly affects the ability of the microsystem to judge the characteristics of single cells. Here, we designed a multiple nonparallel electrode structure for single-cell 3D localization. The performance of the structures was studied by analyzing the changes in electric field strength and the output differential current. The effects of microchannel height, sensing electrode distance, electrode inclination angle, and electrode width on output signals are investigated by analyzing the current change and electric field strength of single cells passing from the center of the microchannel. The numerical simulation results indicate that, when the microchannel height is 20 µm, the distance of the sensing electrodes is 100 µm, the inclination angle is 30°, the electrode width is 20 µm, and the optimal signal quality can be obtained. Reducing the height of the flow channel and shortening the sensing electrode spacing can significantly improve the signal amplitude. When the channel height is 20 µm, the signal intensity increases by 80% than that of 30 µm. The signal intensity of induced current with the sensing electrode spacing of 100 µm is 42% higher than that with the spacing of 120 µm. We analyzed the presence of multiple independent cells and adherent cells in the detection area and demonstrated through simulation that the signal changes caused by multi-cells can be superimposed by multiple single-cell signals. The induced current signal intensity of the same volume of cells with an ellipticity of 1 is 49% lower than that of cells with an ellipticity of 4. Based on the numerical investigation, we expect that the optimal geometry structure design will aid in the development of better performance signal cell impedance cytometry microsystems.

Machine learning implementation strategy in imaging and impedance flow cytometry

Imaging and impedance flow cytometry is a label-free technique that has shown promise as a potential replacement for standard flow cytometry. This is due to its ability to provide rich information and archive high-throughput analysis. Recently, significant efforts have been made to leverage machine learning for processing the abundant data generated by those techniques, enabling rapid and accurate analysis. Harnessing the power of machine learning, imaging and impedance flow cytometry has demonstrated its capability to address various complex phenotyping scenarios. Herein, we present a comprehensive overview of the detailed strategies for implementing machine learning in imaging and impedance flow cytometry. We initiate the discussion by outlining the commonly employed setup to acquire the data (i.e., image or signal) from the cell. Subsequently, we delve into the necessary processes for extracting features from the acquired image or signal data. Finally, we discuss how these features can be utilized for cell phenotyping through the application of machine learning algorithms. Furthermore, we discuss the existing challenges and provide insights for future perspectives of intelligent imaging and impedance flow cytometry.

Particle Counting Methods Based on Microfluidic Devices

Particle counting serves as a pivotal constituent in diverse analytical domains, encompassing a broad spectrum of entities, ranging from blood cells and bacteria to viruses, droplets, bubbles, wear debris, and magnetic beads. Recent epochs have witnessed remarkable progressions in microfluidic chip technology, culminating in the proliferation and maturation of microfluidic chip-based particle counting methodologies. This paper undertakes a taxonomical elucidation of microfluidic chip-based particle counters based on the physical parameters they detect. These particle counters are classified into three categories: optical-based counters, electrical-based particle counters, and other counters. Within each category, subcategories are established to consider structural differences. Each type of counter is described not only in terms of its working principle but also the methods employed to enhance sensitivity and throughput. Additionally, an analysis of future trends related to each counter type is provided.

A review on intelligent impedance cytometry systems: Development, applications and advances

Impedance cytometry is a well-established technique for counting and analyzing single cells, with several advantages, such as convenience, high throughput, and no labeling required. A typical experiment consists of the following steps: single-cell measurement, signal processing, data calibration, and particle subtype identification. At the beginning of this article, we compared commercial and self-developed options extensively and provided references for developing reliable detection systems, which are necessary for cell measurement. Then, a number of typical impedance metrics and their relationships to biophysical properties of cells were analyzed with respect to the impedance signal analysis. Given the rapid advances of intelligent impedance cytometry in the past decade, this article also discussed the development of representative machine learning-based approaches and systems, and their applications in data calibration and particle identification. Finally, the remaining challenges facing the field were summarized, and potential future directions for each step of impedance detection were discussed.

Floating-Electrode-Enabled Impedance Cytometry for Single-Cell 3D Localization

As a label-free, low-cost, and noninvasive tool, impedance measurement has been widely used in single-cell characterization analysis. However, due to the tiny volume of cells, the uncertainty of the spatial position in the microchannel will bring measurement errors in single-cell electrical parameters. To overcome the issue, we designed a novel microdevice configured with a coplanar differential electrode structure to accurately resolve the spatial position of single cells without constraining techniques such as additional sheath fluids or narrow microchannels. The device precisely localizes single cells by measuring the induced current generated by the combined action of the floating electrode and the differential electrodes when single cells flow through the electrode-sensing area. The device was experimentally validated by measuring 6 μm yeast cells and 10 μm particles, achieving spatial localization with a resolution down to 2.1 μm (about 5.3% of the channel width) in lateral direction and 1.2 μm (about 5.9% of the channel height) in the vertical direction at a flow rate of 1.2 μL/min. In addition, by comparing measurement of yeast cells and particles, it was demonstrated that the device not only localizes the single cells or particles but also simultaneously characterizes their status properties such as velocity and size. The device offers a competitive electrode configuration in impedance cytometry with the advantages of simple structure, low cost, and high throughput, promising cell localization and thus electrical characterization.

Recent advances in non-optical microfluidic platforms for bioparticle detection

The effective analysis of the basic structure and functional information of bioparticles are of great significance for the early diagnosis of diseases. The synergism between microfluidics and particle manipulation/detection technologies offers enhanced system integration capability and test accuracy for the detection of various bioparticles. Most microfluidic detection platforms are based on optical strategies such as fluorescence, absorbance, and image recognition. Although optical microfluidic platforms have proven their capabilities in the practical clinical detection of bioparticles, shortcomings such as expensive components and whole bulky devices have limited their practicality in the development of point-of-care testing (POCT) systems to be used in remote and underdeveloped areas. Therefore, there is an urgent need to develop cost-effective non-optical microfluidic platforms for bioparticle detection that can act as alternatives to optical counterparts. In this review, we first briefly summarise passive and active methods for bioparticle manipulation in microfluidics. Then, we survey the latest progress in non-optical microfluidic strategies based on electrical, magnetic, and acoustic techniques for bioparticle detection. Finally, a perspective is offered, clarifying challenges faced by current non-optical platforms in developing practical POCT devices and clinical applications.

A Simple Micromilled Microfluidic Impedance Cytometer with Vertical Parallel Electrodes for Cell Viability Analysis

Microfluidic impedance cytometry has been demonstrated as an effective platform for single cell analysis, taking advantage of microfabricated features and dielectric cell sensing methods. In this study, we present a simple microfluidic device to improve the sensitivity, accuracy, and throughput of single suspension cell viability analysis using vertical sidewall electrodes fabricated by a widely accessible negative manufacturing method. A microchannel milled through a 75 µm platinum wire, which was embedded into poly-methyl-methacrylate (PMMA), created a pair of parallel vertical sidewall platinum electrodes. Jurkat cells were interrogated in a custom low-conductivity buffer (1.2 ± 0.04 mS/cm) to reduce current leakage and increase device sensitivity. Confirmed by live/dead staining and electron microscopy, a single optimum excitation frequency of 2 MHz was identified at which live and dead cells were discriminated based on the disruption in the cell membrane associated with cell death. At this frequency, live cells were found to exhibit changes in the impedance phase with no appreciable change in magnitude, while dead cells displayed the opposite behavior. Correlated with video microscopy, a computational algorithm was created that could identify cell detection events and determine cell viability status by application of a mathematical correlation method.

Droplet Detection and Sorting System in Microfluidics: A Review

Since being invented, droplet microfluidic technologies have been proven to be perfect tools for high-throughput chemical and biological functional screening applications, and they have been heavily studied and improved through the past two decades. Each droplet can be used as one single bioreactor to compartmentalize a big material or biological population, so millions of droplets can be individually screened based on demand, while the sorting function could extract the droplets of interest to a separate pool from the main droplet library. In this paper, we reviewed droplet detection and active sorting methods that are currently still being widely used for high-through screening applications in microfluidic systems, including the latest updates regarding each technology. We analyze and summarize the merits and drawbacks of each presented technology and conclude, with our perspectives, on future direction of development.

Microfluidic channel sensory system for electro-addressing cell location, determining confluency, and quantifying a general number of cells

Microfluidics is a highly useful platform for culturing, monitoring, and testing biological cells. The integration of electrodes into microfluidic channels extends the functionality, sensing, and testing capabilities of microfluidic systems. By employing an electrochemical impedance spectroscopy (EIS) technique, the non-invasive, label-free detection of the activities of cells in real-time can be achieved. To address the movement toward spatially resolving cells in cell culture, we developed a sensory system capable of electro-addressing cell location within a microfluidic channel. This simple system allows for real-time cell location, integrity monitoring (of barrier producing cells), and confluency sensing without the need for frequent optical evaluation-saving time. EIS results demonstrate that cells within microfluidic channels can be located between various pairs of electrodes at different positions along the length of the device. Impedance spectra clearly differentiates between empty, sparse, and confluent microfluidic channels. The system also senses the level of cell confluence between electrode pairs-allowing for the relative quantification of cells in different areas of the microfluidic channel. The system's electrode layout can easily be incorporated into other devices. Namely, organ-on-a-chip devices, that require the monitoring of precise cell location and confluency levels for understanding tissue function, modeling diseases, and for testing therapeutics.

Time-domain signal averaging to improve microparticles detection and enumeration accuracy in a microfluidic impedance cytometer

Microfluidic impedance cytometry is a powerful system to measure micro and nano-sized particles and is routinely used in point-of-care disease diagnostics and other biomedical applications. However, small objects near a sensor's detection limit are plagued with relatively significant background noise and are difficult to identify for every case. While many data processing techniques can be utilized to reduce noise and improve signal quality, frequently they are still inadequate to push sensor detection limits. Here, we report the first demonstration of a novel signal averaging algorithm effective in noise reduction of microfluidic impedance cytometry data, improving enumeration accuracy, and reducing detection limits. Our device uses a 22 µm tall × 100 µm wide (with 30 µm wide focused aperture) microchannel and gold coplanar microelectrodes that generate an electric field, recording bipolar pulses from polystyrene microparticles flowing through the channel. In addition to outlining a modified moving signal averaging technique theoretically and with a model data set, we also performed a compendium of characterization experiments including variations in flow rate, input voltage, and particle size. Multivariate metrics from each experiment are compared including signal amplitude, pulse width, background noise, and signal-to-noise ratio (SNR). Incorporating our technique resulted in improved SNR and counting accuracy across all experiments conducted, and the limit of detection improved from 5 to 1 µm particles without modifying microchannel dimensions. Succeeding this, we envision implementing our modified moving average technique to develop next-generation microfluidic impedance cytometry devices with an expanded dynamic range and improved enumeration accuracy. This can be exceedingly useful for many biomedical applications, such as infectious disease diagnostics where devices may enumerate larger-scale immune cells alongside sub-micron bacterium in the same sample.

Inertial microfluidics for high-throughput cell analysis and detection: a review

Since it was first proposed in 2007, inertial microfluidics has been extensively studied in terms of theory, design, fabrication, and application. In recent years, with the rapid development of microfabrication technologies, a variety of channel structures that can focus, concentrate, separate, and capture bioparticles or fluids have been designed and manufactured to extend the range of potential biomedical applications of inertial microfluidics. Due to the advantages of high throughput, simplicity, and low device cost, inertial microfluidics is a promising candidate for rapid sample processing, especially for large-volume samples with low-abundance targets. As an approach to cellular sample pretreatment, inertial microfluidics has been widely employed to ensure downstream cell analysis and detection. In this review, a comprehensive summary of the application of inertial microfluidics for high-throughput cell analysis and detection is presented. According to application areas, the recent advances can be sorted into label-free cell mechanical phenotyping, sheathless flow cytometric counting, electrical impedance cytometer, high-throughput cellular image analysis, and other methods. Finally, the challenges and prospects of inertial microfluidics for cell analysis and detection are summarized.

Microscopic impedance cytometry for quantifying single cell shape

In this work, we investigated the ability of impedance flow cytometry to measure the shape of single cells/particles. We found that the impedance pulses triggered by micro-objects that are asymmetric in morphology show a tilting trend, and there is no such a tilting trend for symmetric ones. Therefore, we proposed a new metric, tilt index, to quantify the tilt level of the impedance pulses. Through simulation, we found that the value of tilt index tends to be zero for perfectly symmetrical objects, while the value is greater than zero for asymmetrical ones. Also, this metric was found to be independent on the trajectories (i.e., lateral, and z-direction shift) of the target micro-object. In experiments, we adopted a home-made lock-in amplifier and performed experiments on 10 μm polystyrene beads and Euglena gracilis (E. gracilis) cells with varying shapes. The experimental results coincided with the simulation results and demonstrated that the new metric (tilt index) enables the impedance cytometry to characterize the shape single cells/particles without microscopy or other optical setups.

Microfluidic impedance cytometry for single-cell sensing: Review on electrode configurations

Single-cell analysis has gained considerable attention for disease diagnosis, drug screening, and differentiation monitoring. Compared to the well-established flow cytometry, which uses fluorescent-labeled antibodies, microfluidic impedance cytometry (MIC) offers a simple, label-free, and noninvasive method for counting, classifying, and monitoring cells. Superior features including a small footprint, low reagent consumption, and ease of use have also been reported. The MIC device detects changes in the impedance signal caused by cells passing through the sensing/electric field zone, which can extract information regarding the size, shape, and dielectric properties of these cells. According to recent studies, electrode configuration has a remarkable effect on detection accuracy, sensitivity, and throughput. With the improvement in microfabrication technology, various electrode configurations have been reported for improving detection accuracy and throughput. However, the various electrode configurations of MIC devices have not been reviewed. In this review, the theoretical background of the impedance technique for single-cell analysis is introduced. Then, two-dimensional, three-dimensional, and liquid electrode configurations are discussed separately; their sensing mechanisms, fabrication processes, advantages, disadvantages, and applications are also described in detail. Finally, the current limitations and future perspectives of these electrode configurations are summarized. The main aim of this review is to offer a guide for researchers on the ongoing advancement in electrode configuration designs.

Single-cell microfluidic impedance cytometry: from raw signals to cell phenotypes using data analytics

The biophysical analysis of single-cells by microfluidic impedance cytometry is emerging as a label-free and high-throughput means to stratify the heterogeneity of cellular systems based on their electrophysiology. Emerging applications range from fundamental life-science and drug assessment research to point-of-care diagnostics and precision medicine. Recently, novel chip designs and data analytic strategies are laying the foundation for multiparametric cell characterization and subpopulation distinction, which are essential to understand biological function, follow disease progression and monitor cell behaviour in microsystems. In this tutorial review, we present a comparative survey of the approaches to elucidate cellular and subcellular features from impedance cytometry data, covering the related subjects of device design, data analytics (i.e., signal processing, dielectric modelling, population clustering), and phenotyping applications. We give special emphasis to the exciting recent developments of the technique (timeframe 2017-2020) and provide our perspective on future challenges and directions. Its synergistic application with microfluidic separation, sensor science and machine learning can form an essential toolkit for label-free quantification and isolation of subpopulations to stratify heterogeneous biosystems.

Positional dependence of particles and cells in microfluidic electrical impedance flow cytometry: origin, challenges and opportunities

Microfluidic electrical impedance flow cytometry is now a well-known and established method for single-cell analysis. Given the richness of the information provided by impedance measurements, this non-invasive and label-free approach can be used in a wide field of applications ranging from simple cell counting to disease diagnostics. One of its major limitations is the variation of the impedance signal with the position of the cell in the sensing area. Indeed, identical particles traveling along different trajectories do not result in the same data. The positional dependence can be considered as a challenge for the accuracy of microfluidic impedance cytometers. On the other hand, it has recently been regarded by several groups as an opportunity to estimate the position of particles in the microchannel and thus take a further step in the logic of integrating sensors in so-called "Lab-on-a-chip" devices. This review provides a comprehensive overview of the physical grounds of the positional dependence of impedance measurements. Then, both the developed strategies to reduce position influence in impedance-based assays and the recent reported technologies exploiting that dependence for the integration of position detection in microfluidic devices are reviewed.

PRESCIENT: platform for the rapid evaluation of antibody success using integrated microfluidics enabled technology

Identifying antibodies (Abs) that neutralize infectious agents is the first step for developing therapeutics, vaccines, and diagnostic tools for these infectious agents. However, current approaches for identifying neutralizing Abs (nAbs) typically rely on dilution-based assays that are costly, inefficient, and only survey a small subset of the entire repertoire. There are also intrinsic biases in many steps of conventional nAb identification processes. More importantly, conventional assays rely on simple Ab-antigen binding assays, which may not result in identifying the most potent nAbs, as the strongest binder may not be the most potent nAb. Droplet microfluidic systems have the capability to overcome such limitations by conducting complex multi-step assays with high reliability, resolution, and throughput in a pico-liter volume water-in-oil emulsion droplet format. Here, we describe the development of PRESCIENT (Platform for the Rapid Evaluation of antibody SucCess using Integrated microfluidics ENabled Technology), a droplet microfluidic system that can enable high-throughput single-cell resolution identification of nAb repertoires elicited in response to viral infection. We demonstrate PRESCIENT's ability to identify Abs that neutralize a model viral agent, Murine coronavirus (murine hepatitis virus), which causes high mortality rates in experimentally infected mice. In-droplet infection of host cells by the virus was first demonstrated, followed by demonstration of in-droplet neutralization by nAbs produced from a single Ab-producing hybridoma cell. Finally, fluorescence intensity analyses of two populations of hybridoma cell lines (nAb-producing and non-nAb-producing hybridoma cell lines) successfully discriminated between the two populations. The presented strategy and platform have the potential to identify and investigate neutralizing activities against a broad range of potential infectious agents for which nAbs have yet to be discovered, significantly advancing the nAb identification process as well as reinvigorating the field of Ab discovery, characterization, and development.

Digital quantification and selection of high-lipid-producing microalgae through a lateral dielectrophoresis-based microfluidic platform

Microalgae are promising alternatives to petroleum as renewable biofuel sources, however not sufficiently economically competitive yet. Here, a label-free lateral dielectrophoresis-based microfluidic sorting platform that can digitally quantify and separate microalgae into six outlets based on the degree of their intracellular lipid content is presented. In this microfluidic system, the degree of cellular lateral displacement is inversely proportional to the intracellular lipid level, which was successfully demonstrated using Chlamydomonas reinhardtii cells. Using this functionality, a quick digital quantification of sub-populations that contain different intracellular lipid level in a given population was achieved. In addition, the degree of lateral displacement of microalgae could be readily controlled by simply changing the applied DEP voltage, where the level of gating in the intracellular lipid-based sorting decision could be easily adjusted. This allowed for selecting only a very small percentage of a given population that showed the highest degree of intracellular lipid content. In addition, this approach was utilized through an iterative selection process on natural and chemically mutated microalgal populations, successfully resulting in enrichment of high-lipid-accumulating microalgae. In summary, the developed platform can be exploited to quickly quantify microalgae lipid distribution in a given population in real-time and label-free, as well as to enrich a cell population with high-lipid-producing cells, or to select high-lipid-accumulating microalgal variants from a microalgal library.

Microfluidic impedance cytometry device with N-shaped electrodes for lateral position measurement of single cells/particles

Tracking the lateral position of single cells and particles plays an important role in evaluating the efficiency of microfluidic cell focusing, separation and sorting. In this work, we present an N-shaped electrode-based microfluidic impedance cytometry device for the measurement of the lateral position of single cells and particles in continuous flows. Specifically, a simple analytical expression for determining the particle lateral position is derived from the measured electrical signal and geometry relationship among the positions of the flowing particles, electrodes and microchannel. This microfluidic system is experimentally validated by measuring the lateral positions of 5, 7 and 10 μm diameter beads and human red blood cells (RBCs) flowing in a 200 μm wide channel at varying flow rates up to 59.3 μl min^-1. Statistical analyses show a good correlation (R² = 0.99) and agreement (Bland-Altman analysis) between our results and those obtained by a microscopy imaging method. The resolution of our system reflected by the root-mean-square deviation (RMSD) is 10.3 μm (5.15% of the channel width) for 5 and 10 μm beads, and 11.4 μm (5.7% of the channel width) for RBCs at a flow rate of 42.4 μl min^-1. Compared to the existing impedance-based methods for measuring the particle lateral position, we achieve the highest resolution, highest flow rate and smallest measured particle size (3.6 μm beads). The experimental results of the mixture with 5 and 10 μm beads demonstrate that our device does not merely measure the lateral position of single particles or cells, but also can characterize their physical properties (e.g., size) simultaneously. Furthermore, we demonstrate the position monitoring of sheath flow-induced particle focusing, which is in quantitative agreement with the results by imaging quantification. With the advantages of rapid and accurate processing of electrical signal and high throughput of the impedance flow cytometry, this novel N-shaped electrode-based system can be easily integrated with other microfluidic platforms as a downstream approach for the real-time measurement of the lateral position and physical properties of single cells and particles.

High-throughput electrical position detection of single flowing particles/cells with non-spherical shape

We present an innovative impedance cytometer for the measurement of the cross-sectional position of single particles or cells flowing in a microchannel. As predicted by numerical simulations and experimentally validated, the proposed approach is applicable to particles/cells with either spherical or non-spherical shape. In particular, the optics-free high-throughput position detection of individual flowing red blood cells (RBCs) is demonstrated and applied to monitor RBCs hydrodynamic focusing under different sheath flow conditions. Moreover, the device provides multiparametric information useful for lab-on-a-chip applications, including particle inter-arrival times and velocity profile, as well as RBCs mean corpuscular volume, distribution width and electrical opacity.

Gradient in the electric field for particle position detection in microfluidic channels

In this work, a new method to track particles in microfluidic channels is presented. Particle position tracking in microfluidic systems is crucial to characterize sorting systems or to improve the analysis of cells in impedance flow cytometry studies. By developing an electric field gradient in a two parallel electrode array the position of the particles can be tracked in one axis by impedance analysis. This method can track the particle's position at lower frequencies and measure the conductivity of the system at higher frequencies. A 3-D simulation was performed showing particle position detection and conductivity analysis. To experimentally validate the technique, a microfluidic chip that develops a gradient in the electric field was fabricated and used to detect the position of polystyrene particles in one axis and measure their conductivity at low and high frequencies, respectively.

A simple electrical approach to monitor dielectrophoretic focusing of particles flowing in a microchannel

This paper reports an impedance-based system for the quantitative assessment of dielectrophoretic (DEP) focusing of single particles flowing in a microchannel. Particle lateral positions are detected in two electrical sensing zones placed before and after a DEP-focusing region, respectively. In each sensing zone, particle lateral positions are estimated using the unbalance between the opposite pulses of a differential current signal obtained with a straightforward coplanar electrode configuration. The system is used to monitor the focusing of polystyrene beads of 7 or 10 μm diameter, under various conditions of DEP field intensities and flow rates that produce different degrees of focusing. This electrical approach represents a simple and valuable alternative to optical methods for monitoring of particle focusing systems.

An Aptamer-Based Capacitive Sensing Platform for Specific Detection of Lung Carcinoma Cells in the Microfluidic Chip

Improvement of methods for reliable and early diagnosis of the cellular diseases is necessary. A biological selectivity probe, such as an aptamer, is one of the candidate recognition layers that can be used to detect important biomolecules. Lung cancer is currently a typical cause of cancer-related deaths. In this work, an electrical sensing platform is built based on amine-terminated aptamer modified-gold electrodes for the specific, label-free detection of a human lung carcinoma cell line (A549). The microdevice, that includes a coplanar electrodes configuration and a simple microfluidic channel on a glass substrate, is fabricated using standard photolithography and cast molding techniques. A procedure of self-assembly onto the gold surface is proposed. Optical microscope observations and electrical impedance spectroscopy measurements confirm that the fabricated microchip can specifically and effectively identify A549 cells. In the experiments, the capacitance element that is dominant in the change of the impedance is calculated at the appropriate frequency for evaluation of the sensitivity of the biosensor. Therefore, a simple, inexpensive, biocompatible, and selective biosensor that has the potential to detect early-stage lung cancer would be developed.

A microfluidic device for motility and osmolality analysis of zebrafish sperm

A microfluidic chip is described that facilitates research and quality control analysis of zebrafish sperm which, due to its miniscule (i.e., 2-5 μl) sample volume and short duration of motility (i.e., <1 min), present a challenge for traditional manual assessment methods. A micromixer molded in polydimethylsiloxane (PDMS) bonded to a glass substrate was used to activate sperm samples by mixing with water, initiated by the user depressing a transfer pipette connected to the chip. Sample flow in the microfluidic viewing chamber was able to be halted within 1 s, allowing for rapid analysis of the sample using established computer-assisted sperm analysis (CASA) methods. Zebrafish sperm cell activation was consistent with manual hand mixing and yielded higher values of motility at earlier time points, as well as more subtle time-dependent trends in motility, than those processed by hand. Sperm activation curves, which indicate sample quality by evaluating percentage and duration of motility at various solution osmolalities, were generated with on-chip microfabricated gold floor electrodes interrogated by impedance spectroscopy. The magnitude of admittance was linearly proportional to osmolality and was not affected by the presence of sperm cells in the vicinity of the electrodes. This device represents a pivotal step in streamlining methods for consistent, rapid assessment of sperm quality for aquatic species. The capability to rapidly activate sperm and consistently measure motility with CASA using the microfluidic device described herein will help improve the reproducibility of studies on sperm and assist development of germplasm repositories.

Continuous detection of micro-particles by fiber Bragg grating Fabry-Pérot flow cytometer

A novel method to detect different sizes of micro-particles using a fiber Bragg grating Fabry-Pérot (FBG-FP) flow cytometer is presented. The chip is composed of a FBG-FP cavity integrated in a microfluidic channel. Solution with three different sizes of polystyrene particles flowing through the channel induces variations in the transmission spectrum of the FBG-FP cavity. Theoretical and experimental data show that different sizes of particles reveal different resonant wavelengths with a good resonance shift sensitivity of 10^-5. Additionally, the chip is easy to fabricate and features with non-contact and label-free operation. This study demonstrates a promising potential of the FBG-FP flow cytometer in medical and biological sensing.

Applications of Bioimpedance Measurement Techniques in Tissue Engineering

Rapid development in the field of tissue engineering necessitates implementation of monitoring methods for evaluation of the viability and characteristics of the cell cultures in a real-time, non-invasive and non-destructive manner. Current monitoring techniques are mainly histological and require labeling and involve destructive tests to characterize cell cultures. Bioimpedance measurement technique which benefits from measurement of electrical properties of the biological tissues, offers a non-invasive, label-free and real-time solution for monitoring tissue engineered constructs. This review outlines the fundamentals of bioimpedance, as well as electrical properties of the biological tissues, different types of cell culture constructs and possible electrode configuration set ups for performing bioimpedance measurements on these cell cultures. In addition, various bioimpedance measurement techniques and their applications in the field of tissue engineering are discussed.