Electrical Impedance Spectroscopy for Detection of Cells in Suspensions Using Microfluidic Device with Integrated Microneedles

Concurrently Probing the Mechanical and Electrical Characteristics of Living Cells via an Integrated Microdevice

The mechanical and electrical properties of cells serve as critical indicators of their physiological and pathological state. Currently, distinct setups are required to measure the electrical and mechanical responses of cells. In addition, most existing methods such as optical trapping (OT) and atomic force microscopy (AFM) are labor-intensive, expensive, and low-throughput. Here, we developed a microdevice that integrates automated cell trapping, deformation, and electric impedance spectroscopy to overcome these limitations. Our device enables parallel aspiration of tens of trapped cells in a highly scalable manner by simply adjusting the applied pressures, allowing for rapid probing of the dynamic viscoelastic properties of cells. Furthermore, embedded microelectrodes enable concurrent investigations of the electrical impedance of the cells. Through testing on different cell types, our platform demonstrated superior capabilities in comprehensive cell characterization and phenotyping, highlighting its great potential as a versatile tool for single cell analysis, drug screening, and disease detection.

Detection of Gram-positive and Gram-negative bacteria membrane permeabilization induced by pulsed electric field using electrochemical admittance spectroscopy

Electrochemical impedance or admittance spectroscopy (EIS or EAS) has been widely used for decades, offering a label-free, rapid, real-time, and non-destructive assay for optically opaque and turbid bacterial solutions. However, PEF-induced changes in the bacterial envelope can present challenges in detecting the extent of membrane permeabilization in both Gram-positive and Gram-negative bacteria due to their distinct morphological properties. Here, we used a new approach for detecting bacterial membrane permeabilization induced by PEF using electrochemical admittance spectroscopy (EAS). The metabolic activity results have shown that the larger L. d. bulgaricus bacteria was found to be significantly more resistant to PEF strengths ranging from 4 to 16 kV/cm than the smaller E. coli bacteria at shorter PEF treatment durations (10 × 10 µs pulses). Interestingly, the difference in the increase of the admittance magnitude and a decrease in phase angle between the PEF treatment times of 10 × 10 µs and 10 × 100 µs pulses at different PEF strengths was more pronounced for E. coli bacteria samples. Our results demonstrate that EAS is more effective in comparing the degree of membrane permeabilization of Gram-positive and Gram-negative bacteria when longer PEF treatment durations are applied.

An impedance flow cytometry with integrated dual microneedle for electrical properties characterization of single cell

Electrical characteristics of living cells have been proven to reveal important details about their internal structure, charge distribution and composition changes in the cell membrane, as well as the extracellular context. An impedance flow cytometry is a common approach to determine the electrical properties of a cell, having the advantage of label-free and high throughput. However, the current techniques are complex and costly for the fabrication process. For that reason, we introduce an integrated dual microneedle-microchannel for single-cell detection and electrical properties extraction. The dual microneedles utilized a commercially available tungsten needle coated with parylene. When a single cell flows through the parallel-facing electrode configuration of the dual microneedle, the electrical impedance at multiple frequencies is measured. The impedance measurement demonstrated the differential of normal red blood cells (RBCs) with three different sizes of microbeads at low and high frequencies, 100 kHz and 2 MHz, respectively. An electrical equivalent circuit model (ECM) was used to determine the unique membrane capacitance of individual cells. The proposed technique demonstrated that the specific membrane capacitance of an RBC is 9.42 mF/m-2, with the regression coefficients, ρ at 0.9895. As a result, this device may potentially be used in developing countries for low-cost single-cell screening and detection.

Projection Micro-Stereolithography to Manufacture a Biocompatible Micro-Optofluidic Device for Cell Concentration Monitoring

In this work, a 3D printed biocompatible micro-optofluidic (MoF) device for two-phase flow monitoring is presented. Both an air-water bi-phase flow and a two-phase mixture composed of micrometric cells suspended on a liquid solution were successfully controlled and monitored through its use. To manufacture the MoF device, a highly innovative microprecision 3D printing technique was used named Projection Microstereolithography (PμSL) in combination with the use of a novel 3D printable photocurable resin suitable for biological and biomedical applications. The concentration monitoring of biological fluids relies on the absorption phenomenon. More precisely, the nature of the transmission of the light strictly depends on the cell concentration: the higher the cell concentration, the lower the optical acquired signal. To achieve this, the microfluidic T-junction device was designed with two micrometric slots for the optical fibers' insertion, needed to acquire the light signal. In fact, both the micro-optical and the microfluidic components were integrated within the developed device. To assess the suitability of the selected biocompatible transparent resin for optical detection relying on the selected working principle (absorption phenomenon), a comparison between a two-phase flow process detected inside a previously fully characterized micro-optofluidic device made of a nonbiocompatible high-performance resin (HTL resin) and the same made of the biocompatible one (BIO resin) was carried out. In this way, it was possible to highlight the main differences between the two different resin grades, which were further justified with proper chemical analysis of the used resins and their hydrophilic/hydrophobic nature via static water contact angle measurements. A wide experimental campaign was performed for the biocompatible device manufactured through the PμSL technique in different operative conditions, i.e., different concentrations of eukaryotic yeast cells of Saccharomyces cerevisiae (with a diameter of 5 μm) suspended on a PBS (phosphate-buffered saline) solution. The performed analyses revealed that the selected photocurable transparent biocompatible resin for the manufactured device can be used for cell concentration monitoring by using ad hoc 3D printed micro-optofluidic devices. In fact, by means of an optical detection system and using the optimized operating conditions, i.e., the optimal values of the flow rate FR=0.1 mL/min and laser input power P∈{1,3} mW, we were able to discriminate between biological fluids with different concentrations of suspended cells with a robust working ability R2=0.9874 and Radj2=0.9811.

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

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

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 Review on Microfluidics-Based Impedance Biosensors

Electrical impedance biosensors are powerful and continuously being developed for various biological sensing applications. In this line, the sensitivity of impedance biosensors embedded with microfluidic technologies, such as sheath flow focusing, dielectrophoretic focusing, and interdigitated electrode arrays, can still be greatly improved. In particular, reagent consumption reduction and analysis time-shortening features can highly increase the analytical capabilities of such biosensors. Moreover, the reliability and efficiency of analyses are benefited by microfluidics-enabled automation. Through the use of mature microfluidic technology, complicated biological processes can be shrunk and integrated into a single microfluidic system (e.g., lab-on-a-chip or micro-total analysis systems). By incorporating electrical impedance biosensors, hand-held and bench-top microfluidic systems can be easily developed and operated by personnel without professional training. Furthermore, the impedance spectrum provides broad information regarding cell size, membrane capacitance, cytoplasmic conductivity, and cytoplasmic permittivity without the need for fluorescent labeling, magnetic modifications, or other cellular treatments. In this review article, a comprehensive summary of microfluidics-based impedance biosensors is presented. The structure of this article is based on the different substrate material categorizations. Moreover, the development trend of microfluidics-based impedance biosensors is discussed, along with difficulties and challenges that may be encountered in the future.

A Review of Advanced Impedance Biosensors with Microfluidic Chips for Single-Cell Analysis

Electrical impedance biosensors combined with microfluidic devices can be used to analyze fundamental biological processes for high-throughput analysis at the single-cell scale. These specialized analytical tools can determine the effectiveness and toxicity of drugs with high sensitivity and demonstrate biological functions on a single-cell scale. Because the various parameters of the cells can be measured depending on methods of single-cell trapping, technological development ultimately determine the efficiency and performance of the sensors. Identifying the latest trends in single-cell trapping technologies afford opportunities such as new structural design and combination with other technologies. This will lead to more advanced applications towards improving measurement sensitivity to the desired target. In this review, we examined the basic principles of impedance sensors and their applications in various biological fields. In the next step, we introduced the latest trend of microfluidic chip technology for trapping single cells and summarized the important findings on the characteristics of single cells in impedance biosensor systems that successfully trapped single cells. This is expected to be used as a leading technology in cell biology, pathology, and pharmacological fields, promoting the further understanding of complex functions and mechanisms within individual cells with numerous data sampling and accurate analysis capabilities.

Zwitterions for impedance spectroscopy: The new buffers in town

Studying the role of buffers in impedance spectroscopy is a relatively unexplored area. We demonstrate a special class of biologically relevant buffers known as Good's zwitterionic buffers that show improved performance over standard electrolyte buffers (e.g. PBS) currently widely used in impedance spectroscopy measurements of bacterial suspensions. Our theoretical and experimental comparisons of conductivity of classical and zwitterionic buffers at various different concentrations show that ion-ion interaction effects are significantly higher in zwitterionic buffers as compared to classical buffers at the concentrations at which they are used. This and the fact that zwitterions have larger sizes leads to the lowering of their conductivity which significantly improves their impedance sensing ability. We illustrate through an example of heat-induced ionic release in model S. typhi and S. aureus bacteria that having a low conductivity buffer is indeed beneficial for biological impedance measurements. In fact, the best buffer for impedance studies can be chosen solely based on their electrical properties as long as they are also biologically compatible. This gives Good's zwitterionic buffers an edge over conventional media as they satisfy both these criteria.

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.

Dielectrophoretic ultra-high-frequency characterization and in silico sorting on uptake of rare earth elements by Cupriavidus necator

Rare earth elements (REEs) are widely used across different industries due to their exceptional magnetic and electrical properties. In this work, Cupriavidus necator is characterized using dielectrophoretic ultra-high-frequency measurements, typically in MHz range to quantify the properties of cytoplasm in C. necator for its metal uptake/bioaccumulation capacity. Cupriavidus necator, a Gram-negative bacteria strain is exposed to REEs like europium, samarium, and neodymium in this study. Dielectrophoretic crossover frequency experiments were performed on the native C. necator species pre- and post-exposure to the REEs at MHz frequency range. The net conductivity of native C. necator, Cupriavidus europium, Cupriavidus samarium, and Cupriavidus neodymium are 15.95 ± 0.029 μS/cm, 16.15 ± 0.028 μS/cm, 16.05 ± 0.029 μS/cm, 15.61 ± 0.005 μS/cm respectively. The estimated properties of the membrane published by our group are used to develop a microfluidic sorter by modeling and simulation to separate REE absorbed C. necator from the unabsorbed native C. necator species using COMSOL Multiphysics commercial software package v5.5.

Bioimpedance Sensor and Methodology for Acute Pain Monitoring

The paper aims to revive the interest in bioimpedance analysis for pain studies in communicating and non-communicating (anesthetized) individuals for monitoring purpose. The plea for exploitation of full potential offered by the complex (bio)impedance measurement is emphasized through theoretical and experimental analysis. A non-invasive, low-cost reliable sensor to measure skin impedance is designed with off-the-shelf components. This is a second generation prototype for pain detection, quantification, and modeling, with the objective to be used in fully anesthetized patients undergoing surgery. The 2D and 3D time-frequency, multi-frequency evaluation of impedance data is based on broadly available signal processing tools. Furthermore, fractional-order impedance models are implied to provide an indication of change in tissue dynamics correlated with absence/presence of nociceptor stimulation. The unique features of the proposed sensor enhancements are described and illustrated here based on mechanical and thermal tests and further reinforced with previous studies from our first generation prototype.

An impedimetric assay for the identification of abnormal mitochondrial dynamics in living cells

Mitochondrial dynamics (fission and fusion) plays an important role in cell functions. Disruption in mitochondrial dynamics has been associated with diseases such as neurobiological disorders and cardiovascular diseases. Analysis of mitochondrial fission/fusion has been mostly achieved through direct visualization of the fission/fusion events in live-cell imaging of fluorescently labeled mitochondria. In this study, we demonstrated a label-free, non-invasive Electrical Impedance Spectroscopy (EIS) approach to analyze mitochondrial dynamics in a genetically modified human neuroblastoma SH-SY5Y cell line with no huntingtin protein expression. Huntingtin protein has been shown to regulate mitochondria dynamics. We performed EIS studies on normal SH-SY5Y cells and two independent clones of huntingtin-null cells. The impedance data was used to determine the suspension conductivity and further cytoplasmic conductivity and relate to the abnormal mitochondrial dynamics. For instance, the cytoplasm conductivity value was increased by 11% from huntingtin-null cells to normal cells. Results of this study demonstrated that EIS is sensitive to characterize the abnormal mitochondrial dynamics that can be difficult to quantify by the conventional microscopic method.

A Comparative Study of Two Fractional-Order Equivalent Electrical Circuits for Modeling the Electrical Impedance of Dental Tissues

Background: Electrical impedance spectroscopy (EIS) is a fast, non-invasive, and safe approach for electrical impedance measurement of biomedical tissues. Applied to dental research, EIS has been used to detect tooth cracks and caries with higher accuracy than visual or radiographic methods. Recent studies have reported age-related differences in human dental tissue impedance and utilized fractional-order equivalent circuit model parameters to represent these measurements. Objective: We aimed to highlight that fractional-order equivalent circuit models with different topologies (but same number of components) can equally well model the electrical impedance of dental tissues. Additionally, this work presents an equivalent circuit network that can be realized using Electronic Industries Alliance (EIA) standard compliant RC component values to emulate the electrical impedance characteristics of dental tissues. Results: To validate the results, the goodness of fits of electrical impedance models were evaluated visually and statistically in terms of relative error, mean absolute error (MAE), root mean squared error (RMSE), coefficient of determination (R2), Nash–Sutcliffe’s efficiency (NSE), Willmott’s index of agreement (WIA), or Legates’s coefficient of efficiency (LCE). The fit accuracy of proposed recurrent electrical impedance models for data representative of different age groups teeth dentin supports that both models can represent the same impedance data near perfectly. Significance: With the continued exploration of fractional-order equivalent circuit models to represent biological tissue data, it is important to investigate which models and model parameters are most closely associated with clinically relevant markers and physiological structures of the tissues/materials being measured and not just “fit” with experimental data. This exploration highlights that two different fractional-order models can fit experimental dental tissue data equally well, which should be considered during studies aimed at investigating different topologies to represent biological tissue impedance and their interpretation.

Detection and differentiation of bacteria by electrical bioimpedance spectroscopy

Detecting bacteria in samples and differentiating between Gram-negative and Gram-positive species is an important challenge, and the most common method, Gram staining, is very time consuming. The aim of this study was to evaluate the electrical bioimpedance spectroscopy (EBIS) technique as an inexpensive and practical tool for real-time detection of bacteria and differentiation between Gram-positive and Gram-negative species. The relevant sensitivity for differentiating between species was found in the magnitude and phase at frequencies of 158,489 and 5248 Hz, respectively, at a bacterial concentration of 1 μg/μl. Subsequently, the sensitivity was estimated as a function of bacterial concentration. Our results demonstrated that EBIS can potentially distinguish between presence and absence of bacteria as well as between different types of bacteria.

Low-Cost, Compact, and Rapid Bio-Impedance Spectrometer with Real-Time Bode and Nyquist Plots

Bioelectric impedance spectroscopy (BIS) has been widely used to study the electrical properties of biological tissue based on the characteristics of the complex electrical impedance dispersions. One of the problems in using the BIS method is the length of time required for the data acquisition process and possibly data analysis as well. In this research, a compact and work rapidly BIS instrumentation system has been developed at a low cost. It is designed to work in the frequency range of 100 Hz to 100 kHz, which is generally used in the fields of biophysics and medical physics. The BIS instrumentation system is built using several integrated modules. The modules are an AC current source to produce a selectable injection current; a data acquisition system to measure voltage, current, and phase difference rapidly and simultaneously; and software to calculate and display measurement results in the form of Bode and Nyquist plots in real time. The developed BIS system has been validated using a simple RC circuit as the sample being tested. The average time needed in the process of data acquisition and analysis until the formation of impedance dispersion curves in the form of Bode and Nyquist plots, for 54 sample frequencies, is less than one minute. The system is able to identify R and C values of the sample with a maximum error of 1.5%. In addition, some simple application examples are also presented in this paper.

Cell Cytometry: Review and Perspective on Biotechnological Advances

Cell identification and enumeration are essential procedures within clinical and research laboratories. For over 150 years, quantitative investigation of body fluids such as counts of various blood cells has been an important tool for diagnostic analysis. With the current evolution of point-of-care diagnostics and precision medicine, cheap and precise cell counting technologies are in demand. This article reviews the timeline and recent notable advancements in cell counting that have occurred as a result of improvements in sensing including optical and electrical technology, enhancements in image processing capabilities, and contributions of micro and nanotechnologies. Cell enumeration methods have evolved from the use of manual counting using a hemocytometer to automated cell counters capable of providing reliable counts with high precision and throughput. These developments have been enabled by the use of precision engineering, micro and nanotechnology approaches, automation and multivariate data analysis. Commercially available automated cell counters can be broadly classified into three categories based on the principle of detection namely, electrical impedance, optical analysis and image analysis. These technologies have many common scientific uses, such as hematological analysis, urine analysis and bacterial enumeration. In addition to commercially available technologies, future technological trends using lab-on-a-chip devices have been discussed in detail. Lab-on-a-chip platforms utilize the existing three detection technologies with innovative design changes utilizing advanced nano/microfabrication to produce customized devices suited to specific applications.

Multichannel Electrical Impedance Spectroscopy Analyzer with Microfluidic Sensors

Impedance spectroscopy is a common approach in assessing passive electrical properties of biological matter. However, several problems appear in microfluidic devices in connection with the requirement for high sensitivity of signal acquisition from small volume sensors. The developed compact and inexpensive analyzer provides impedance spectroscopy measurement from three sensors, both connected in direct and differential modes. Measurement deficiencies are reduced with a novel design of sensors, measurement method, optimized electronics, signal processing, and mechanical design of the analyzer. Proposed solutions are targeted to the creation of reliable point-of-care (POC) diagnostic and monitoring appliances, including lab-on-a-chip type devices in the next steps of development. The test results show the good working ability of the developed analyzer; however, also limitations and problems that require attention and further improvement are appointed.

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.

Design, Preparation and Performance Study of On-Chip Flow-Through Amperometric Sensors with an Integrated Ag/AgCl Reference Electrode

To improve the reference potential stability of on-chip amperometric sensors, we propose a novel integrated Ag/AgCl reference electrode structure. This structure can refresh the saturated potassium chloride filling solution surrounding the Ag/AgCl electrode. We then designed a flow-through amperometric sensor and a multilayer microfluidic chip based on the integrated reference electrode. In order to improve the detection signal strength of the flow-through sensor, a numerical simulation model was established. The simulation results showed that a combination of (1) using a step-type detection cell structure that maintains micro-channel width while reducing micro-channel height, and (2) controlling the sample flow rate to limit the mass transfer of the sensor surface effectively, improves the detection signal strength. The step-type detection cell structure had dimensions of 200 μm × 200 μm × 100 μm (length × width × height), and the electroosmotic flow driving voltage was 120 V/cm. Finally, successful trace detection of Mg²⁺ and Pb²⁺ in the water was achieved using the amperometric sensor and microfluidic chip: detection limits were 5 μmol/L and 84 μmol/L. The preparation of an on-chip flow-through amperometric sensor with an integrated Ag/AgCl reference electrode will facilitate improved portability of microfluidic detection technology.