A unified approach to dielectric single cell analysis: impedance and dielectrophoretic force spectroscopy

Microfluidic-Based Electrical Operation and Measurement Methods in Single-Cell Analysis

Cellular heterogeneity plays a significant role in understanding biological processes, such as cell cycle and disease progression. Microfluidics has emerged as a versatile tool for manipulating single cells and analyzing their heterogeneity with the merits of precise fluid control, small sample consumption, easy integration, and high throughput. Specifically, integrating microfluidics with electrical techniques provides a rapid, label-free, and non-invasive way to investigate cellular heterogeneity at the single-cell level. Here, we review the recent development of microfluidic-based electrical strategies for single-cell manipulation and analysis, including dielectrophoresis- and electroporation-based single-cell manipulation, impedance- and AC electrokinetic-based methods, and electrochemical-based single-cell detection methods. Finally, the challenges and future perspectives of the microfluidic-based electrical techniques for single-cell analysis are proposed.

Simple bioelectrical microsensor: oocyte quality prediction via membrane electrophysiological characterization

Oocyte selection is a crucial step of assisted reproductive treatment. The most common approach relies on the embryologist experience which is inevitably prone to human error. One potential approach could be the use of an electrical-based approach as an ameliorative alternative. Here, we developed a simple electrical microsensor to characterize mouse oocytes. The sensor is designed similarly to embryo culture dishes and is familiar to embryologists. Different microelectrode models were simulated for oocyte cells and a more sensitive model was determined. The final microsensor was fabricated. A differential measuring technique was proposed based on the cell presence/absence. We predicted oocyte quality by using three electrical characteristics, oocyte radii, and zona thicknesses, and also these predictions were compared with an embryologist evaluation. The evaluation of the oocyte membrane capacitance, as an electrophysiological characteristic, was found to be a more reliable method for predicting oocytes with fertilization and blastocyst formation success competence. It achieved 94% and 58% prediction accuracies, respectively, surpassing other methods and yielding lower errors. This groundbreaking research represents the first of its kind in this field and we hope that this will be a step towards improving the accuracy of the treatment.

Dielectric properties of urine in relation to bladder cancer

Many studies have investigated the dielectric properties of human and animal tissues, particularly to differentiate between normal cells and tumors. However, these studies are invasive as tissue samples have to be excised to measure the properties. This study aims to investigate the dielectric properties of urine in relation to bladder cancer, which is safe and non-invasive to patients. 30 healthy subjects and 30 bladder cancer patients were recruited. Their urine samples were subjected to urinalysis and cytology assessment. A vector network analyzer was used to measure the dielectric constant (Ɛ') and loss factor (Ɛ″) at microwave frequencies of between 0.2 and 50 GHz at 25 °C, 30 °C and 37 °C. Significant differences in Ɛ' and Ɛ″ were observed between healthy subjects and patients, especially at frequencies of between 25 and 40 GHz at 25 °C. Bladder cancer patients had significant lower Ɛ' and higher Ɛ″ compared with healthy subjects. The Ɛ' was negatively correlated with urinary exfoliated urothelial cell number, and Ɛ″ was positively correlated. The study achieved a receiver operating characteristic area under curve (ROC-AUC) score of 0.69099 and an optimum accuracy of 75% with a sensitivity of 80% and a specificity of 70%. The number of exfoliated urothelial cell had significant effect on the dielectric properties, especially in bladder cancer patients. Urinary dielectric properties could potentially be used as a tool to detect bladder cancer.

Ultrafast absorption mechanism of oil-emulsified micelles onto ferrous absorbents with dielectrophoresis force in the presence of polarization

Absorption and desorption rates were generally dependent on the concentration gradient from bulk to absorbents. A novel methodology based on a capacitor with an alternating electric field (AEF) is developed to accelerate the absorption and desorption rates with the frequency manipulation. Ferrous polystyrene microspheres (PISMs) are synthesized as absorbents, which could enhance the complex permittivities as well as dielectric properties. Theoretically, the attractive force and viscous force predominately determine the particle and micelles movement in the medium under an AEF. Oil-emulsified micelles (OEM) with various viscosities were selected as absorbates. Both the OEM and microspherical absorbents assembled through the external attractive force in the presence of the AEF. When the attractive force is equal to viscous force in the medium at the characteristic frequency, the optimal absorption rate could be obtained. The absorption rate constants of pseudo-first-order for OEMs under the polarization at 50 V and 120 kHz of frequency are ca. 10 times higher than that in absence of the polarization. The desorption rate as well as recycling efficiency could be also improved at 800 kHz. The ferrous PISMs with high complex permittivity prevented the damage from the AEF, which could be recycled 10 times of absorption and desorption with frequency manipulation under the AEF. Our methodology provides novel insights for ultrafast wastewater treatment.

Portable Bulk-Water Disinfection by Live Capture of Bacteria with Divergently Branched Porous Graphite in Electric Fields

Easy access to clean water is essential to functioning and development of modern society. However, it remains arduous to develop energy-efficient, facile, and portable water treatment systems for point-of-use (POU) applications, which is particularly imperative for the safety and resilience of society during extreme weather and critical situations. Here, we propose and validate a meritorious working scheme for water disinfection via directly capturing and removing pathogen cells from bulk water using strategically designed three-dimensional (3D) porous dendritic graphite foams (PDGFs) in a high-frequency AC field. The prototype, integrated in a 3D-printed portable water-purification module, can reproducibly remove 99.997% E. coli bacteria in bulk water at a few voltages with among the lowest energy consumption at 435.5 J·L^-1_. The PDGFs, costing $1.47 per piece, can robustly operate at least 20 times for more than 8 h in total without functional degradation. Furthermore, we successfully unravel the involved disinfection mechanism with one-dimensional Brownian dynamics simulation. The system is practically applied that brings natural water in Waller Creek at UT Austin to the safe drinking level. This research, including the working mechanism based on dendritically porous graphite and the design scheme, could inspire a future device paradigm for POU water treatment.

New Generation Dielectrophoretic-Based Microfluidic Device for Multi-Type Cell Separation

This study introduces a new generation of dielectrophoretic-based microfluidic device for the precise separation of multiple particle/cell types. The device features two sets of 3D electrodes, namely cylindrical and sidewall electrodes. The main channel of the device terminates with three outlets: one in the middle for particles that sense negative dielectrophoresis force and two others at the right and left sides for particles that sense positive dielectrophoresis force. To evaluate the device performance, we used red blood cells (RBCs), T-cells, U937-MC cells, and Clostridium difficile bacteria as our test subjects. Our results demonstrate that the proposed microfluidic device could accurately separate bioparticles in two steps, with sidewall electrodes of 200 µm proving optimal for efficient separation. Applying different voltages for each separation step, we found that the device performed most effectively at 6 Vp-p applied to the 3D electrodes, and at 20 Vp-p and 11 Vp-p applied to the sidewall electrodes for separating RBCs from bacteria and T-cells from U937-MC cells, respectively. Notably, the device's maximum electric fields remained below the cell electroporation threshold, and we achieved a separation efficiency of 95.5% for multi-type particle separation. Our findings proved the device's capacity for separating multiple particle types with high accuracy, without limitation for particle variety.

Using Impedance Flow Cytometry for Rapid Viability Classification of Heat-Treated Bacteria

In the future, rapid electrical characterization of cells with impedance flow cytometry promises to be a fast and accurate method for the evaluation of cell properties. In this paper, we investigate how the conductivity of the suspending medium along with the heat exposure time affects the viability classification of heat-treated E. coli. Using a theoretical model, we show that perforation of the bacteria membrane during heat exposure changes the impedance of the bacterial cell from effectively less conducting than the suspension medium to effectively more conducting. Consequently, this results in a shift in the differential argument of the complex electrical current that can be measured with impedance flow cytometry. We observe this shift experimentally through measurements on E. coli samples with varying medium conductivity and heat exposure times. We show that increased exposure time and lower medium conductivity results in improved classification between untreated and heat-treated bacteria. The best classification was achieved with a medium conductivity of 0.045 S/m after 30 min of heat exposure.

A microfluidic device to separate high-quality plasma from undiluted whole blood sample using an enhanced gravitational sedimentation mechanism

The growing interest in lab-on-a-chip systems for plasma separation has led to the presentation of various devices. Trench-based devices benefiting from gravitational sedimentation are efficient structures with air-locking and low speed-drawbacks. The present study introduces a fast, hemolysis-free, highly efficient blood plasma separation microfluidic device. The proposed device is based on gravitational sedimentation combined with dielectrophoresis force to promote the purity of the separated plasma, reduce the separation process time, and overcome the air-locking problem. The effect of geometrical parameters on the separation process is investigated using finite element analysis to attain optimal design specifications. A drop of whole blood (10 μl) is injected into the fabricated chip at four flow rates of 70 nl/s to 100 nl/s. It takes less than 4 min to obtain 2.2 μl plasma from undiluted blood without losing plasma proteins. Additionally, a porous Melt-Blown Polypropylene (MBPP) layer is used to eliminate the air-locking problem, which in previous trench-based microsystems led to time-consuming device preparation steps. Blood samples with various hematocrits (15%-65%) are tested with the applied voltages of 0-20 V_pp through the optimized structure. A purity of 99.98% ± 0.02% (evaluated by hemocytometry) is achieved using optimized dielectrophoresis force by the applied voltage of 20 V_pp, which is more than the previous studies. The UV-Visible spectroscopy results confirm obtaining a non-hemolyzed sample at a flow rate of 70 nl/s. The proposed device achieves a relative increase in the flow rate compared to similar previous studies while maintaining the high quality of the separated plasma. This achievement lies in using the MBPP layer and combining two separation methods.

Protein Albumin Manipulation and Electrical Quantification of Molecular Dielectrophoresis Responses for Biomedical Applications

Research relating to dielectrophoresis (DEP) has been progressing rapidly through time as it is a strong and controllable technique for manipulation, separation, preconcentration, and partitioning of protein. Extensive studies have been carried out on protein DEP, especially on Bovine Serum Albumin (BSA). However, these studies involve the usage of dye and fluorescent probes to observe DEP responses as the physical properties of protein albumin molecular structure are translucent. The use of dye and the fluorescent probe could later affect the protein's physiology. In this article, we review three methods of electrical quantification of DEP responses: electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and capacitance measurement for protein BSA DEP manipulation. The correlation of these methods with DEP responses is further discussed. Based on the observations on capacitance measurement, it can be deduced that the electrical quantifying method is reliable for identifying DEP responses. Further, the possibility of manipulating the protein and electrically quantifying DEP responses while retaining the original physiology of the protein and without the usage of dye or fluorescent probe is discussed.

CMOS-Based Electrokinetic Microfluidics With Multi-Modal Cellular and Bio-Molecular Sensing for End-to-End Point-of-Care System

The importance of point-of-care (POC) bio-molecular diagnostics capable of rapid analysis has become abundantly evident after the outbreak of the Covid-19 pandemic. While sensing interfaces for both protein and nucleic-acid based assays have been demonstrated with chip-scale systems, sample preparation in compact form factor has often been a major bottleneck in enabling end-to-end POC diagnostics. Miniaturization of an end-to-end system requires addressing the front-end sample processing, without which, the goal for low-cost POC diagnostics remain elusive. In this paper, we address bulk fluid processing with AC-osmotic based electrokinetic fluid flows that can be fully controlled, processed and automated by CMOS ICs, fabricated in TSMC 65 nm LP process. Here, we combine bulk fluid flow control with bio-molecular sensing, cell manipulation, cytometry, and separation-all of which are controlled with silicon chips for an all-in-one bio-sensing device. We show CMOS controlled pneumatic-free bulk fluid flow with fluid velocities reaching up to 160 μm/s within a microfluidic channel of 100 × 50 μm ² of cross-sectional area. We incorporate electrode arrays to allow precise control and focused cell flows ( ±2 μm precision) for robust cytometry, and for subsequent separation. We also incorporate a 16-element impedance spectroscopy receiver array for cell and label-free protein sensing. The massive scalability of CMOS-driven microfluidics, manipulation, and sensing can lead to a new design space and a new class of miniaturized sensing technologies.

Recent Advances in Electrical Impedance Sensing Technology for Single-Cell Analysis

Cellular heterogeneity is of significance in cell-based assays for life science, biomedicine and clinical diagnostics. Electrical impedance sensing technology has become a powerful tool, allowing for rapid, non-invasive, and label-free acquisition of electrical parameters of single cells. These electrical parameters, i.e., equivalent cell resistance, membrane capacitance and cytoplasm conductivity, are closely related to cellular biophysical properties and dynamic activities, such as size, morphology, membrane intactness, growth state, and proliferation. This review summarizes basic principles, analytical models and design concepts of single-cell impedance sensing devices, including impedance flow cytometry (IFC) to detect flow-through single cells and electrical impedance spectroscopy (EIS) to monitor immobilized single cells. Then, recent advances of both electrical impedance sensing systems applied in cell recognition, cell counting, viability detection, phenotypic assay, cell screening, and other cell detection are presented. Finally, prospects of impedance sensing technology in single-cell analysis are discussed.

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.

Field-Portable Microplastic Sensing in Aqueous Environments: A Perspective on Emerging Techniques

Microplastics (MPs) have been found in aqueous environments ranging from rural ponds and lakes to the deep ocean. Despite the ubiquity of MPs, our ability to characterize MPs in the environment is limited by the lack of technologies for rapidly and accurately identifying and quantifying MPs. Although standards exist for MP sample collection and preparation, methods of MP analysis vary considerably and produce data with a broad range of data content and quality. The need for extensive analysis-specific sample preparation in current technology approaches has hindered the emergence of a single technique which can operate on aqueous samples in the field, rather than on dried laboratory preparations. In this perspective, we consider MP measurement technologies with a focus on both their eventual field-deployability and their respective data products (e.g., MP particle count, size, and/or polymer type). We present preliminary demonstrations of several prospective MP measurement techniques, with an eye towards developing a solution or solutions that can transition from the laboratory to the field. Specifically, experimental results are presented from multiple prototype systems that measure various physical properties of MPs: pyrolysis-differential mobility spectroscopy, short-wave infrared imaging, aqueous Nile Red labeling and counting, acoustophoresis, ultrasound, impedance spectroscopy, and dielectrophoresis.

Closed-loop feedback control of microfluidic cell manipulation via deep-learning integrated sensor networks

Microfluidic technologies have long enabled the manipulation of flow-driven cells en masse under a variety of force fields with the goal of characterizing them or discriminating the pathogenic ones. On the other hand, a microfluidic platform is typically designed to function under optimized conditions, which rarely account for specimen heterogeneity and internal/external perturbations. In this work, we demonstrate a proof-of-principle adaptive microfluidic system that consists of an integrated network of distributed electrical sensors for on-chip tracking of cells and closed-loop feedback control that modulates chip parameters based on the sensor data. In our system, cell flow speed is measured at multiple locations throughout the device, the data is interpreted in real-time via deep learning-based algorithms, and a proportional-integral feedback controller updates a programmable pressure pump to maintain a desired cell flow speed. We validate the adaptive microfluidic system with both static and dynamic targets and also observe a fast convergence of the system under continuous external perturbations. With an ability to sustain optimal processing conditions in unsupervised settings, adaptive microfluidic systems would be less prone to artifacts and could eventually serve as reliable standardized biomedical tests at the point of care.

DEPIS: A combined dielectrophoresis and impedance spectroscopy platform for rapid cell viability and antimicrobial susceptibility analysis

Antimicrobial resistance (AMR) is caused by inappropriate or excessive antibiotic consumption. Early diagnosis of bacterial infections can greatly curb empirical treatment and thus AMR. Current diagnostic procedures are time-consuming as they rely on gene amplification and cell culture techniques that are inherently limited by the doubling rate of the involved species. Further, biochemical methods for species identification and antibiotic susceptibility testing for drug/dose effectiveness take several days and are non-scalable. We report a real-time, label-free approach called DEPIS that combines dielectrophoresis (DEP) for bacterial enrichment and impedance spectroscopy (IS) for cell viability analysis under 60 min. Target bacteria are captured on interdigitated electrodes using DEP (30 min) and their antibiotic-induced stress response is measured using IS (another 30 min). This principle is used to generate minimum bactericidal concentration (MBC) plots by measuring impedance change due to ionic release by dying bacteria in a low conductivity buffer. The results are rapid since they rely on cell death rather than cell growth which is an intrinsically slower process. The results are also highly specific and work across all bactericidal antibiotics studied, irrespective of their cellular target or drug action mechanism. More importantly, preliminary results with clinical isolates show that methicillin-susceptible Staphylococcus aureus (MSSA) can easily be differentiated from methicillin-resistant S. aureus (MRSA) under 1 h. This rapid cell analyses approach can aid in faster diagnosis of bacterial infections and benefit the clinical decision-making process for antibiotic treatment, addressing the critical issue of AMR.

Electrical properties characterization of single yeast cells by dielectrophoretic motion and electro-rotation

The electrical parameters of single cells are label-free and intrinsic properties that can reflect the physiological characteristics. In recent years, many measurement methods based on impedance spectroscopy and rotation spectrum analysis have been developed. However, most of these works need to measure the response at whole frequency range to obtain DEP spectra and estimate the electrical parameters by fitting method, which are time-consuming and limit the measurement throughput. Therefore, improving the measurement throughput for single cells is an essential problem to be solved addressed. In this paper we present a microfluidic chip that combines dielectrophoretic motion and electro-rotation technology for single-cell electrical properties characterization. Since the movement and rotation speed of single cell in mediums are related to the electrical parameters of itself, electric signals and medium, the electrical properties can be obtained by measuring and analyzing the movement trajectory and rotation speed of the cell. Numerical simulations were performed to analyze the electric field distribution of the chip under different signal configurations, which predict the movement trajectory and rotation state, and determine the values of electric field on the cells. Based on the simulation results, cell focusing, dielectrophoretic motion and electro-rotation were successfully realized. By analyzing the movement trajectory and rotation speed, the conductivity of wall and the permittivity of membrane of yeast cells were characterized. The measurement method avoids the time-consuming of the traditional rotational spectra method, and can realize rapid and efficiency and single-cell electrical characterization.

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.

Biological Living Cell in-Flow Detection Based on Microfluidic Chip and Compact Signal Processing Circuit

Detection and counting of biological living cells in continuous fluidic flows play an essential role in many applications for early diagnosis and treatment of diseases. In this regard, this study highlighted the proposal of a biochip system for detecting and enumerating human lung carcinoma cell flow in the microfluidic channel. The principle of detection was based on the change of impedance between sensing electrodes integrated in the fluidic channel, due to the presence of a biological cell in the sensing region. A compact electronic module was built to sense the unbalanced impedance between the sensing microelectrodes. It consisted of an instrumentation amplifier stage to obtain the difference between the acquired signals, and a lock-in amplifier stage to demodulate the signals at the stimulating frequency as well as to reject noise at other frequencies. The performance of the proposed system was validated through experiments of A549 cells detection as they passed over the microfluidic channel. The experimental results indicated the occurrence of large spikes (up to approximately 180 mV) over the background signal according to the passage of a single A549 cell in the continuous flow. The proposed device is simple-to-operate, inexpensive, portable, and exhibits high sensitivity, which are suitable considerations for developing point-of-care applications.

Double-peak signal features in microfluidic impedance flow cytometry enable sensitive measurement of cell membrane capacitance

The probing of individual cells at specific frequency regimes in a microfluidic impedance flow cytometer led to the observation of unusual "double peak" features in the reactive component of the resulting signal. The phenomenon was restricted to the lower frequencies (400-800 kHz) of the β-dispersion regime and its occurrence was facilitated by the co-planar microelectrode geometry in the device. To understand the reasons for this anomalous behaviour, the system was modelled using COMSOL. The simulated model agreed well with experimental observations and provided insight into the origins of this signal profile and the effect of various parameters on its behaviour. One of the most significant observations of this study was the high sensitivity of the features in the "double peak" profile to changes in cell membrane capacitance (CMC), compared to conventional "single peaks" of reactive impedance. This was consequently exploited to accurately distinguish populations of normal and glutaraldehyde treated erythrocytes based on variations in their CMC, indicating a drastic decrease in the CMC of treated cells. Additionally, we demonstrate the applicability of using this double peak effect to identify cell populations within a mixture of PBMCs. This study is an improvement over conventional approaches of measuring CMC via impedance flow cytometry by enabling the measurement of both cell size and cell membrane properties at a single frequency rather than using multiple frequencies. Using a single frequency significantly simplifies the system and reduces the associated costs. Additionally, this technique enables the measurement of CMC at relatively low frequencies.

Dielectrophoretic Manipulation of Cancer Cells and Their Electrical Characterization

Electromanipulation and electrical characterization of cancerous cells is becoming a topic of high interest as the results reported to date demonstrate a good differentiation among various types of cells from an electrical viewpoint. Dielectrophoresis and broadband dielectric spectroscopy are complementary tools for sorting, identification, and characterization of malignant cells and were successfully used on both primary tumor cells and culture cells as well. However, the literature is presenting a plethora of studies with respect to electrical evaluation of these type of cells, and this review is reporting a collection of information regarding the functioning principles of different types of dielectrophoresis setups, theory of cancer cell polarization, and electrical investigation (including here the polarization mechanisms). The interpretation of electrical characteristics against frequency is discussed with respect to interfacial/Maxwell-Wagner polarization and the parasitic influence of electrode polarization. Moreover, the electrical equivalent circuits specific to biological cells polarizations are discussed for a good understanding of the cells' morphology influence. The review also focuses on advantages of specific low-conductivity buffers employed currently for improving the efficiency of dielectrophoresis and provides a set of synthesized data from the literature highlighting clear differentiation between the crossover frequencies of different cancerous cells.

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.

Improved micro-impedance spectroscopy to determine cell barrier properties

The goal of this study was to determine whether the Tethapod system, which was designed to determine the impedance properties of lipid bilayers, could be used for cell culture in order to utilise micro-impedance spectroscopy to examine further biological applications. To that purpose we have used normal epithelial cells from kidney (RPTEC) and a kidney cancer cell model (786-O). We demonstrate that the Tethapod system is compatible with the culture of 10,000 cells seeded to grow on a small area gold measurement electrode for several days without affecting the cell viability. Furthermore, the range of frequencies for EIS measurements were tuned to examine easily the characteristics of the cell monolayer. We demonstrate significant differences in the paracellular resistance pathway between normal and cancer kidney epithelial cells. Thus, we conclude that this device has advantages for the study of cultured cells that include (i) the configuration of measurement and reference electrodes across a microfluidic channel, and (ii) the small surface area of 6 parallel measurement electrodes (2.1 mm2) integrated in a microfluidic system. These characteristics might improve micro-impedance spectroscopy measurement techniques to provide a simple tool for further studies in the field of the patho-physiology of biological barriers.

Electrorheological Sensor Encapsulating Microsphere Media for Plague Diagnosis with Rapid Visualization

Plague is a disease infected by an etiological agent, which is transmitted from fleas to a variety of wildlife rodents. Therefore, rapid diagnosis of plague on-site in the field is important. Polystyrene microspheres (SMs) of 2.2 μm diameter were synthesized by emulsion polymerization to adsorb magnetic nanoparticles (FNs), resulting in core-/shell-structured microspheres that generate a significant contrast in relative permittivities between SMs and FNs. Electrorheological displays (EDs) consisting of two indium tin oxide glasses with spacers were constructed to contain core-/shell-structured SM/FN (SM@FN) solutions for observing their transmittance change. The ED encapsulating dispersed SM@FN solution exhibited an opaque state because light was scattered significantly without the application of an alternating electric field (AEF). In the presence of an AEF, the particle chaining behavior results in enhancement of the transmittance of ED. At a specific frequency, the so-called characteristic frequency (F_c), the transmittance reaches a maximum. F_c could be used as an indicator to mark the shell materials. The antibody of Yersinia pestis (ab-Yp) was coated onto the SM@FN as a biosensing medium. The F_c of ab-Yp-modified microspheres shifted from 200 to 750 kHz with antigen coupling of Y. pestis antigen (ag-Yp). In the absence of fluorescence labeling, the large change in ED transmittance could be visualized during the Y. pestis detection. The limit of detection and the limit of quantification were ∼30 and ∼40 ng/μL, respectively, obtained within 30 s according to the highest transmittance of ED under the AEF at 750 kHz. Y. pestis detection was not affected by Escherichia coli and Staphylococcus aureus significantly. Compared with other common immunoassays, including the secondary immunochemical or enzyme-linked steps, this simple electrorheological sensor with high sensitivity and selectivity could be a candidate for on-site plague diagnosis.

Electronic classification of barcoded particles for multiplexed detection using supervised machine learning analysis

Wearable biosensors are of great interest in recent years due to their potential in health related applications. Multiplex biomarker analysis is needed in wearable devices to improve the sensitivity and reliability. Electronic barcoding of micro-particles has the possibility to enable multiplexed biomarker analysis. Compared with traditional optical and plasmonic methods for barcoding, electronically barcoded particles can be classified using ultra-compact electronic readout platforms. Nano-electronic barcoding works by depositing a thin layer of oxide on the top half of a micro-particle. The thickness and dielectric property of the oxide layer can be tuned to modulate the frequency dependent impedance signature of the particles. A one to one correspondence between a target biomarker and each barcoded particle can potentially be established using this technique. The barcoded particles could be tested with wearable devices to enable multiplex analysis for portable point-of-care diagnostics and real-time monitoring. In this work, we fabricated nine barcoded particles by forming oxide layers of different thicknesses and different dielectric materials using atomic layer deposition and assessed the ability to accurately classify particle barcodes using multi-frequency impedance cytometry in conjunction with supervised machine learning.

On-chip integrated optical stretching and electrorotation enabling single-cell biophysical analysis

Cells have different intrinsic markers such as mechanical and electrical properties, which may be used as specific characteristics. Here, we present a microfluidic chip configured with two opposing optical fibers and four 3D electrodes for multiphysical parameter measurement. The chip leverages optical fibers to capture and stretch a single cell and uses 3D electrodes to achieve rotation of the single cell. According to the stretching deformation and rotation spectrum, the mechanical and dielectric properties can be extracted. We provided proof of concept by testing five types of cells (HeLa, A549, HepaRG, MCF7 and MCF10A) and determined five biophysical parameters, namely, shear modulus, steady-state viscosity, and relaxation time from the stretching deformation and area-specific membrane capacitance and cytoplasm conductivity from the rotation spectra. We showed the potential of the chip in cancer research by observing subtle changes in the cellular properties of transforming growth factor beta 1 (TGF-β1)-induced epithelial-mesenchymal transition (EMT) A549 cells. The new chip provides a microfluidic platform capable of multiparameter characterization of single cells, which can play an important role in the field of single-cell research.

Toward Microfluidic Label-Free Isolation and Enumeration of Circulating Tumor Cells from Blood Samples

The isolation, analysis, and enumeration of circulating tumor cells (CTCs) from cancer patient blood samples are a paradigm shift for cancer patient diagnosis, prognosis, and treatment monitoring. Most methods used to isolate and enumerate these target cells rely on the expression of cell surface markers, which varies between patients, cancer types, tumors, and stages. Here, we propose a label-free high-throughput platform to isolate, enumerate, and size CTCs on two coupled microfluidic devices. Cancer cells were purified through a Vortex chip and subsequently flowed in-line to an impedance chip, where a pair of electrodes measured fluctuations of an applied electric field generated by cells passing through. A proof-of-concept of the coupling of those two devices was demonstrated with beads and cells. First, the impedance chip was tested as a stand-alone device: (1) with beads (mean counting error of 1.0%, sizing information clearly separated three clusters for 8, 15, and 20 um beads, respectively) as well as (2) with cancer cells (mean counting error of 3.5%). Second, the combined setup was tested with beads, then with cells in phosphate-buffered saline, and finally with cancer cells spiked in healthy blood. Experiments demonstrated that the Vortex HT chip enriched the cancer cells, which then could be counted and differentiated from smaller blood cells by the impedance chip based on size information. Further discrimination was shown with dual high-frequency measurements using electric opacity, highlighting the potential application of this combined setup for a fully integrated label-free isolation and enumeration of CTCs from cancer patient samples. © 2019 International Society for Advancement of Cytometry.

Crossing constriction channel-based microfluidic cytometry capable of electrically phenotyping large populations of single cells

This paper presents a crossing constriction channel-based microfluidic system for high-throughput characterization of specific membrane capacitance (Csm) and cytoplasm conductivity (σcy) of single cells. In operations, cells in suspension were forced through the major constriction channel and instead of invading the side constriction channel, they effectively sealed the side constriction channel, which led to variations in impedance data. Based on an equivalent circuit model, these raw impedance data were translated into Csm and σcy. As a demonstration, the developed microfluidic system quantified Csm (3.01 ± 0.92 μF cm-2) and σcy (0.36 ± 0.08 S m-1) of 100 000 A549 cells, which could generate reliable results by properly controlling cell positions during their traveling in the crossing constriction channels. Furthermore, the developed microfluidic impedance cytometry was used to distinguish paired low- and high-metastatic carcinoma cell types of SACC-83 (ncell = ∼100 000) and SACC-LM cells (ncell = ∼100 000), distinguishing significant differences in both Csm (3.16 ± 0.90 vs. 2.79 ± 0.67 μF cm-2) and σcy (0.36 ± 0.06 vs.0.41 ± 0.08 S m-1). As high-throughput microfluidic impedance cytometry, this technique may add a new marker-free dimension to flow cytometry in single-cell analysis.

Impedance Study of Dopamine Effects after Application on 2D and 3D Neuroblastoma Cell Cultures Developed on a 3D-Printed Well

In this work, the assessment of the interactions of a bioactive substance applied to immobilized cells in either a two-dimensional (2D) or three-dimensional (3D) arrangement mimicking in vivo tissue conditions is presented. In particular, dopamine (DA) was selected as a stimulant for the implementation of an impedance analysis with a specific type of neural cells (murine neuroblastoma). The aim of this study was the extraction of calibration curves at various frequencies with different known dopamine concentrations for the description of the behavior of dopamine applied to 2D and 3D cell cultures. The results present the evaluation of the mean impedance value for each immobilization technique in each frequency. The differential responses showed the importance of the impedance when frequency is applied in both 2D and 3D immobilization cases. More specifically, in 2D immobilization matrix impedance shows higher values in comparison with the 3D cell culture. Additionally, in the 3D case, the impedance decreases with increasing concentration, while in the 2D case, an opposite behavior was observed.

Developments in label-free microfluidic methods for single-cell analysis and sorting

Advancements in microfluidic technologies have led to the development of many new tools for both the characterization and sorting of single cells without the need for exogenous labels. Label-free microfluidics reduce the preparation time, reagents needed, and cost of conventional methods based on fluorescent or magnetic labels. Furthermore, these devices enable analysis of cell properties such as mechanical phenotype and dielectric parameters that cannot be characterized with traditional labels. Some of the most promising technologies for current and future development toward label-free, single-cell analysis and sorting include electronic sensors such as Coulter counters and electrical impedance cytometry; deformation analysis using optical traps and deformation cytometry; hydrodynamic sorting such as deterministic lateral displacement, inertial focusing, and microvortex trapping; and acoustic sorting using traveling or standing surface acoustic waves. These label-free microfluidic methods have been used to screen, sort, and analyze cells for a wide range of biomedical and clinical applications, including cell cycle monitoring, rapid complete blood counts, cancer diagnosis, metastatic progression monitoring, HIV and parasite detection, circulating tumor cell isolation, and point-of-care diagnostics. Because of the versatility of label-free methods for characterization and sorting, the low-cost nature of microfluidics, and the rapid prototyping capabilities of modern microfabrication, we expect this class of technology to continue to be an area of high research interest going forward. New developments in this field will contribute to the ongoing paradigm shift in cell analysis and sorting technologies toward label-free microfluidic devices, enabling new capabilities in biomedical research tools as well as clinical diagnostics. This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > Diagnostic Nanodevices.

An anti-clogging method for improving the performance and lifespan of blood plasma separation devices in real-time and continuous microfluidic systems

On-chip blood plasma separators using microfluidic channels are typically developed as disposable devices for short-term use only because blood cells tend to clog the microchannels, limiting their application in real-time and continuous systems. In this study, we propose an anti-clogging method. We applied dielectrophoresis to prevent microchannel clogging in a plasma separator that can be used over long periods for real-time and continuous monitoring. Prior to applying the anti-clogging method, the blood plasma separator stopped working after 4 h. In contrast, by manipulating the separator with the new anti-clogging method at a voltage of 20 V, it continued working in a long-term experiment for 12 h without performance deterioration or an increase in cell loss. Two critical performance parameters of the manipulated separator, the purity efficiency and the plasma yield, were 97.23 ± 5.43% and 38.95 ± 9.34%, respectively, at 20 V after 15 min. Interestingly, the two performance parameters did not decrease during the long-term experiment. Hence, the blood plasma separator with the anti-clogging method is an interesting device for use in real-time and continuous blood plasma separation systems because of its consistent performance and improved lifespan.

Non-Linear Cellular Dielectrophoretic Behavior Characterization Using Dielectrophoretic Tweezers-Based Force Spectroscopy inside a Microfluidic Device

Characterization of cellular dielectrophoretic (DEP) behaviors, when cells are exposed to an alternating current (AC) electric field of varying frequency, is fundamentally important to many applications using dielectrophoresis. However, to date, that characterization has been performed with monotonically increasing or decreasing frequency, not with successive increases and decreases, even though cells might behave differently with those frequency modulations due to the nonlinear cellular electrodynamic responses reported in previous works. In this report, we present a method to trace the behaviors of numerous cells simultaneously at the single-cell level in a simple, robust manner using dielectrophoretic tweezers-based force spectroscopy. Using this method, the behaviors of more than 150 cells were traced in a single environment at the same time, while a modulated DEP force acted upon them, resulting in characterization of nonlinear DEP cellular behaviors and generation of different cross-over frequencies in living cells by modulating the DEP force. This study demonstrated that living cells can have non-linear di-polarized responses depending on the modulation direction of the applied frequency as well as providing a simple and reliable platform from which to measure a cellular cross-over frequency and characterize its nonlinear property.

Frequency-Modulated Wave Dielectrophoresis of Vesicles And Cells: Periodic U-Turns at the Crossover Frequency

We have formulated the dielectrophoretic force exerted on micro/nanoparticles upon the application of frequency-modulated (FM) electric fields. By adjusting the frequency range of an FM wave to cover the crossover frequency f _X in the real part of the Clausius-Mossotti factor, our theory predicts the reversal of the dielectrophoretic force each time the instantaneous frequency periodically traverses f _X . In fact, we observed periodic U-turns of vesicles, leukemia cells, and red blood cells that undergo FM wave dielectrophoresis (FM-DEP). It is also suggested by our theory that the video tracking of the U-turns due to FM-DEP is available for the agile and accurate measurement of f _X . The FM-DEP method requires a short duration, less than 30 s, while applying the FM wave to observe several U-turns, and the agility in measuring f _X is of much use for not only salty cell suspensions but also nanoparticles because the electric-field-induced solvent flow is suppressed as much as possible. The accuracy of f _X has been verified using two types of experiment. First, we measured the attractive force exerted on a single vesicle experiencing alternating-current dielectrophoresis (AC-DEP) at various frequencies of sinusoidal electric fields. The frequency dependence of the dielectrophoretic force yields f _X as a characteristic frequency at which the force vanishes. Comparing the AC-DEP result of f _X with that obtained from the FM-DEP method, both results of f _X were found to coincide with each other. Second, we investigated the conductivity dependencies of f _X for three kinds of cell by changing the surrounding electrolytes. From the experimental results, we evaluated simultaneously both of the cytoplasmic conductivities and the membrane capacitances using an elaborate theory on the single-shell model of biological cells. While the cytoplasmic conductivities, similar for these cells, were slightly lower than the range of previous reports, the membrane capacitances obtained were in good agreement with those previously reported in the literature.

Development of microfluidic impedance cytometry enabling the quantification of specific membrane capacitance and cytoplasm conductivity from 100,000 single cells

This paper presents a new microfluidic impedance cytometry with crossing constriction microchannels, enabling the characterization of cellular electrical markers (e.g., specific membrane capacitance (C_sm) and cytoplasm conductivity (σ_cy)) in large cell populations (~ 100,000 cells) at a rate greater than 100 cells/s. Single cells were aspirated continuously through the major constriction channel with a proper sealing of the side constriction channel. An equivalent circuit model was developed and the measured impedance values were translated to C_sm and σ_cy. Neural network was used to classify different cell populations where classification success rates were calculated. To evaluate the developed technique, different tumour cell lines, and the effects of epithelial-mesenchymal transitions on tumour cells were examined. Significant differences in both C_sm and σ_cy were found for H1299 and HeLa cell lines with a classification success rate of 90.9% in combination of the two parameters. Meanwhile, tumour cells A549 showed significant decreases in both C_sm and σ_cy after epithelial-mesenchymal transitions with a classification success rate of 76.5%. As a high-throughput microfluidic impedance cytometry, this technique can add a new marker-free dimension to flow cytometry in single-cell analysis.

Combined AC-electrokinetic effects: Theoretical considerations on a three-axial ellipsoidal model

AC fields induce charges at the structural interfaces of particles or biological cells. The interaction of these charges with the field generates frequency‐dependent forces that are the basis for AC‐electrokinetic effects such as dielectrophoresis (DEP), electrorotation (ROT), electro‐orientation, and electro‐deformation. The effects can be used for the manipulation or dielectric single‐particle spectroscopy. The observation of a particular effect depends on the spatial and temporal field distributions, as well as on the shape and the dielectric and viscoelastic properties of the object. Because the effects are not mutually independent, combined frequency spectra are obtained, for example, discontinuous DEP and ROT spectra with ranges separated by the reorientation of nonspherical objects in the linearly and circularly polarized DEP and ROT fields, respectively. As an example, the AC electrokinetic behavior of a three‐axial ellipsoidal single‐shell model with the geometry of chicken‐red blood cells is considered. The geometric and electric problems were separated using the influential‐radius approach. The obtained finite‐element model can be electrically interpreted by an RC model leading to an expression for the Clausius–Mossotti factor, which permits the derivation of force, torque, and orientation spectra, as well as of equations for the critical frequencies and force plateaus in DEP and of the characteristic frequencies and peak heights in ROT. Expressions for the orientation in linearly and circularly polarized fields, as well as for the reorientation frequencies were also derived. The considerations suggested that the simultaneous registration of various AC‐electrokinetic spectra is a step towards the dielectric fingerprinting of single objects.

Electrical impedance microflow cytometry with oxygen control for detection of sickle cells

Polymerization of intracellular sickle hemoglobin induced by low oxygen tension has been recognized as a primary determinant of the pathophysiologic manifestations in sickle cell disease. Existing flow cytometry techniques for detection of sickle cells are typically based on fluorescence markers or cellular morphological analysis. Using microfluidics and electrical impedance spectroscopy, we develop a new, label-free flow cytometry for non-invasive measurement of single cells under controlled oxygen level. We demonstrate the capability of this new technique by determining the electrical impedance differential of normal red blood cells obtained from a healthy donor and sickle cells obtained from three sickle cell patients, under normoxic and hypoxic conditions and at three different electrical frequencies, 156 kHz, 500 kHz and 3 MHz. Under normoxia, normal cells and sickle cells can be separated completely using electrical impedance at 156 kHz and 500 kHz but not at 3 MHz. Sickle cells, intra-patient and inter-patient show significantly different electrical impedance between normoxia and hypoxia at all three frequencies. This study shows a proof of concept that electrical impedance signal can be used as an indicator of the disease state of a red blood cell as well as the cell sickling events in sickle cell disease. Electrical impedance-based microflow cytometry with oxygen control is a new method that can be potentially used for sickle cell disease diagnosis and monitoring.

Microwave measurement of giant unilamellar vesicles in aqueous solution

A microwave technique is demonstrated to measure floating giant unilamellar vesicle (GUV) membranes in a 25 μm wide and 18.8 μm high microfluidic channel. The measurement is conducted at 2.7 and 7.9 GHz, at which a split-ring resonator (SRR) operates at odd modes. A 500 nm wide and 100 μm long SRR split gap is used to scan GUVs that are slightly larger than 25 μm in diameter. The smaller fluidic channel induces flattened GUV membrane sections, which make close contact with the SRR gap surface. The used GUVs are synthesized with POPC (16:0–18:1 PC 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine), SM (16:0 Egg Sphingomyelin) and cholesterol at different molecular compositions. It is shown that SM and POPC bilayers have different dielectric permittivity values, which also change with measurement frequencies. The obtained membrane permittivity values, e.g. 73.64-j6.13 for POPC at 2.7 GHz, are more than 10 times larger than previously reported results. The discrepancy is likely due to the measurement of dielectric polarization parallel with, other than perpendicular to, the membrane surface. POPC and SM-rich GUV surface sections are also clearly identified. Further work is needed to verify the obtained large permittivity values and enable accurate analysis of membrane composition.

Dielectrophoresis-based microfluidic platforms for cancer diagnostics

The recent advancement of dielectrophoresis (DEP)-enabled microfluidic platforms is opening new opportunities for potential use in cancer disease diagnostics. DEP is advantageous because of its specificity, low cost, small sample volume requirement, and tuneable property for microfluidic platforms. These intrinsic advantages have made it especially suitable for developing microfluidic cancer diagnostic platforms. This review focuses on a comprehensive analysis of the recent developments of DEP enabled microfluidic platforms sorted according to the target cancer cell. Each study is critically analyzed, and the features of each platform, the performance, added functionality for clinical use, and the types of samples, used are discussed. We address the novelty of the techniques, strategies, and design configuration used in improving on existing technologies or previous studies. A summary of comparing the developmental extent of each study is made, and we conclude with a treatment of future trends and a brief summary.

Miniaturized Impedance Flow Cytometer: Design Rules and Integrated Readout

A dual-channel credit-card-sized impedance cell counter featuring a throughput of 2000 cell/s and detection of single yeast cells (5 μm) with a signal-to-noise ratio of 20 dB is presented. Its compactness is achieved by a CMOS ASIC combining a lock-in impedance demodulator with an oversampling 20-bit ΣΔ ADC and real-time peak detection embedded in field-programmable gate array. The module is coupled to a dielectrophoretic cell-sorting microfluidic device, offering compact and label-free electrical readout that replaces the need for a fluorescence microscope and, thus, is suitable for point-of-care diagnostics. The independent role of each dimension of the planar sensing microelectrodes is demonstrated, with simulations and experiments, along with its relevant effect on the spectrum of thin channels, deriving useful design guidelines.

Dielectrophoretic Separation of Live and Dead Monocytes Using 3D Carbon-Electrodes

Blood has been the most reliable body fluid commonly used for the diagnosis of diseases. Although there have been promising investigations for the development of novel lab-on-a-chip devices to utilize other body fluids such as urine and sweat samples in diagnosis, their stability remains a problem that limits the reliability and accuracy of readouts. Hence, accurate and quantitative separation and characterization of blood cells are still crucial. The first step in achieving high-resolution characteristics for specific cell subpopulations from the whole blood is the isolation of pure cell populations from a mixture of cell suspensions. Second, live cells need to be purified from dead cells; otherwise, dead cells might introduce biases in the measurements. In addition, the separation and characterization methods being used must preserve the genetic and phenotypic properties of the cells. Among the characterization and separation approaches, dielectrophoresis (DEP) is one of the oldest and most efficient label-free quantification methods, which directly purifies and characterizes cells using their intrinsic, physical properties. In this study, we present the dielectrophoretic separation and characterization of live and dead monocytes using 3D carbon-electrodes. Our approach successfully removed the dead monocytes while preserving the viability of the live monocytes. Therefore, when blood analyses and disease diagnosis are performed with enriched, live monocyte populations, this approach will reduce the dead-cell contamination risk and achieve more reliable and accurate test results.

Label-Free Detection of Bacillus anthracis Spore Uptake in Macrophage Cells Using Analytical Optical Force Measurements

Understanding the interaction between macrophage cells and Bacillus anthracis spores is of significant importance with respect to both anthrax disease progression, spore detection for biodefense, as well as understanding cell clearance in general. While most detection systems rely on specific molecules, such as nucleic acids or proteins and fluorescent labels to identify the target(s) of interest, label-free methods probe changes in intrinsic properties, such as size, refractive index, and morphology, for correlation with a particular biological event. Optical chromatography is a label free technique that uses the balance between optical and fluidic drag forces within a microfluidic channel to determine the optical force on cells or particles. Here we show an increase in the optical force experienced by RAW264.7 macrophage cells upon the uptake of both microparticles and B. anthracis Sterne 34F2 spores. In the case of spores, the exposure was detected in as little as 1 h without the use of antibodies or fluorescent labels of any kind. An increase in the optical force was also seen in macrophage cells treated with cytochalasin D, both with and without a subsequent exposure to spores, indicating that a portion of the increase in the optical force arises independent of phagocytosis. These results demonstrate the capability of optical chromatography to detect subtle biological differences in a rapid and sensitive manner and suggest future potential in a range of applications, including the detection of biological threat agents for biodefense and pathogens for the prevention of sepsis and other diseases.

Dielectrophoresis assisted loading and unloading of microwells for impedance spectroscopy

Dielectric spectroscopy (DS) is a noninvasive, label-free, fast, and promising technique for measuring dielectric properties of biological cells in real time. We demonstrate a microchip that consists of electro-activated microwell arrays for positive dielectrophoresis assisted cell capture, DS measurements, and negative dielectrophoresis driven cell unloading; thus, providing a high-throughput cell analysis platform. To the best of our knowledge, this is the first microfluidic chip that combines electro-activated microwells and DS to analyze biological cells. Device performance is tested using Saccharomyces cerevisiae (yeast) cells. DEP response of yeast cells is determined by measuring their Clausius-Mossotti factor using biophysical models in parallel plate microelectrode geometry. This information is used to determine the excitation frequency to load and unload wells. Effect of yeast cells on the measured impedance spectrum was examined both experimentally and numerically. Good match between the numerical and experimental results establishes the potential use of the microchip device for extracting subcellular properties of biological cells in a rapid and nonexpensive manner.

Top-down fabrication meets bottom-up synthesis for nanoelectronic barcoding of microparticles

Traditional optical and plasmonic techniques for barcoding of micro-particles for multiplexed bioassays are generally high in throughput, however bulky instrumentation is often required for performing readout. Electrical impedance based detection allows for ultra-compact instrumentation footprint necessary for wearable devices, however to date, the lack of ability to electronically barcode micro-particles has been a long standing bottleneck towards enabling multiplexed electronic biomarker assays. Nanoelectronic barcoding, which to the best of our knowledge is the first impedance based solution for micro-particle barcoding, works by forming tunable nano-capacitors on the surface of micro-spheres, effectively modulating the frequency dependent dielectric properties of the spheres allowing one bead barcode to be distinguished from another. Nanoelectronic barcoding uses a well-known, but unexplored electromagnetic phenomenon of micro-particles: the Clausius-Mossotti (CM) factor spectrum of a Janus particle (JP) shifts depending on the zeta (wall) potential of the metallic half of the microsphere, and the fact that the complex impedance spectrum of a particle directly corresponds to the CM factor spectrum. A one-to-one correspondence will be established between each biomarker and the corresponding engineered microsphere. This transformative new method for barcoding will enable a new class of handheld and wearable biosensors capable of multiplexed continuous temporal bio-monitoring. The proposed nano-electronically barcoded particles utilize both bottom-up synthesis and top-down fabrication to enable precisely engineered frequency dependent dielectric signatures. Multi-frequency lock-in measurements of the complex impedance, in conjunction with multi-variate analysis of impedance data, allows for particle differentiation using a fully functional ultra-compact electronic detector.

In situ single cell detection via microfluidic magnetic bead assay

We present a single cell detection device based on magnetic bead assay and micro Coulter counters. This device consists of two successive micro Coulter counters, coupled with a high gradient magnetic field generated by an external magnet. The device can identify single cells in terms of the transit time difference of the cell through the two micro Coulter counters. Target cells are conjugated with magnetic beads via specific antibody and antigen binding. A target cell traveling through the two Coulter counters interacts with the magnetic field, and have a longer transit time at the 1st counter than that at the 2nd counter. In comparison, a non-target cell has no interaction with the magnetic field, and hence has nearly the same transit times through the two counters. Each cell passing through the two counters generates two consecutive voltage pulses one after the other; the pulse widths and magnitudes indicating the cell’s transit times through the counters and the cell’s size respectively. Thus, by measuring the pulse widths (transit times) of each cell through the two counters, each single target cell can be differentiated from non-target cells even if they have similar sizes. We experimentally proved that the target human umbilical vein endothelial cells (HUVECs) and non-target rat adipose-derived stem cells (rASCs) have significant different transit time distribution, from which we can determine the recognition regions for both cell groups quantitatively. We further demonstrated that within a mixed cell population of rASCs and HUVECs, HUVECs can be detected in situ and the measured HUVECs ratios agree well with the pre-set ratios. With the simple device structure and easy sample preparation, this method is expected to enable single cell detection in a continuous flow and can be applied to facilitate general cell detection applications such as stem cell identification and enumeration.

Specific membrane capacitance, cytoplasm conductivity and instantaneous Young's modulus of single tumour cells

As label-free biomarkers, biophysical properties of cells are widely used for cell type classification. However, intrinsic biophysical markers, e.g., specific membrane capacitance (C_{specific membrane}), cytoplasm conductivity (σ_conductivity) and instantaneous Young’s modulus (E_{instantaneous}) measured for hundreds of single cells were not yet reported. In this study, single cells in suspension (adherent cells treated with trypsin) were aspirated through a microfluidic constriction channel at 25 °C, and the entry processes and impedance profiles were recorded and translated to C_{specific membrane}, σ_conductivity and E_{instantaneous}. C_{specific membrane}, σ_conductivity and E_{instantaneous} of five cell types were quantified as 2.10±0.38 μF cm⁻², 0.91±0.15 S m⁻¹ and 5.52±0.95 kPa for H460 cells (n_cell=437); 2.52±0.54 μF cm⁻², 0.83±0.12 S m⁻¹ and 5.54±1.04 kPa for H446 cells (n_cell=410); 2.45±0.57 μF cm⁻², 0.99±0.18 S m⁻¹ and 5.16±1.68 kPa for A549 cells (n_cell=442); 1.86±0.31 μF cm⁻², 1.07±0.18 S m⁻¹ and 3.86±0.81 kPa for 95D cells (n_cell=415); 2.03±0.35 μF cm⁻², 0.99±0.16 S m⁻¹ and 3.49±0.70 kPa for 95C cells (n_cell=290). The database of C_{specific membrane}, σ_conductivity and E_{instantaneous} may serve as a reference for future studies of cellular biophysical properties.