Differential electronic detector to monitor apoptosis using dielectrophoresis-induced translation of flowing cells (dielectrophoresis cytometry)

On-chip dielectrophoretic single-cell manipulation

Bioanalysis at a single-cell level has yielded unparalleled insight into the heterogeneity of complex biological samples. Combined with Lab-on-a-Chip concepts, various simultaneous and high-frequency techniques and microfluidic platforms have led to the development of high-throughput platforms for single-cell analysis. Dielectrophoresis (DEP), an electrical approach based on the dielectric property of target cells, makes it possible to efficiently manipulate individual cells without labeling. This review focusses on the engineering designs of recent advanced microfluidic designs that utilize DEP techniques for multiple single-cell analyses. On-chip DEP is primarily effectuated by the induced dipole of dielectric particles, (i.e., cells) in a non-uniform electric field. In addition to simply capturing and releasing particles, DEP can also aid in more complex manipulations, such as rotation and moving along arbitrary predefined routes for numerous applications. Correspondingly, DEP electrodes can be designed with different patterns to achieve different geometric boundaries of the electric fields. Since many single-cell analyses require isolation and compartmentalization of individual cells, specific microstructures can also be incorporated into DEP devices. This article discusses common electrical and physical designs of single-cell DEP microfluidic devices as well as different categories of electrodes and microstructures. In addition, an up-to-date summary of achievements and challenges in current designs, together with prospects for future design direction, is provided.

AI-Powered Microfluidics: Shaping the Future of Phenotypic Drug Discovery

Phenotypic drug discovery (PDD), which involves harnessing biological systems directly to uncover effective drugs, has undergone a resurgence in recent years. The rapid advancement of artificial intelligence (AI) over the past few years presents numerous opportunities for augmenting phenotypic drug screening on microfluidic platforms, leveraging its predictive capabilities, data analysis, efficient data processing, etc. Microfluidics coupled with AI is poised to revolutionize the landscape of phenotypic drug discovery. By integrating advanced microfluidic platforms with AI algorithms, researchers can rapidly screen large libraries of compounds, identify novel drug candidates, and elucidate complex biological pathways with unprecedented speed and efficiency. This review provides an overview of recent advances and challenges in AI-based microfluidics and their applications in drug discovery. We discuss the synergistic combination of microfluidic systems for high-throughput screening and AI-driven analysis for phenotype characterization, drug-target interactions, and predictive modeling. In addition, we highlight the potential of AI-powered microfluidics to achieve an automated drug screening system. Overall, AI-powered microfluidics represents a promising approach to shaping the future of phenotypic drug discovery by enabling rapid, cost-effective, and accurate identification of therapeutically relevant compounds.

Apoptotic Bodies in the Pancreatic Tumor Cell Culture Media Enable Label-Free Drug Sensitivity Assessment by Impedance Cytometry

The ability to rapidly and sensitively predict drug response and toxicity using in vitro models of patient-derived tumors is essential for assessing chemotherapy efficacy. Currently, drug sensitivity assessment for solid tumors relies on imaging adherent cells or by flow cytometry of cells lifted from drug-treated cultures after fluorescent staining for apoptotic markers. Subcellular apoptotic bodies (ABs), including microvesicles that are secreted into the culture media under drug treatment can potentially serve as markers for drug sensitivity, without the need to lift cells under culture. However, their stratification to quantify cell disassembly is challenging due to their compositional diversity, with tailored labeling strategies currently needed for the recognition and cytometry of each AB type. It is shown that the high frequency impedance phase versus size distribution of ABs determined by high-throughput single-particle impedance cytometry of supernatants in the media of gemcitabine-treated pancreatic tumor cultures exhibits phenotypic resemblance to lifted apoptotic cells and enables shape-based stratification within distinct size ranges, which is not possible by flow cytometry. It is envisioned that this tool can be applied in conjunction with the appropriate pancreatic tumor microenvironment model to assess drug sensitivity and toxicity of patient-derived tumors, without the need to lift cells from cultures.

Dynamically controlled dielectrophoresis using resonant tuning

Electrically polarizable micro- and nanoparticles and droplets can be trapped using the gradient electric field of electrodes. But the spatial profile of the resultant dielectrophoretic force is fixed once the electrode structure is defined. To change the force profile, entire complex lab-on-a-chip systems must be re-fabricated with modified electrode structures. To overcome this problem, we propose an approach for the dynamic control of the spatial profile of the dielectrophoretic force by interfacing the trap electrodes with a resistor and an inductor to form a resonant resistor-inductor-capacitor (RLC) circuit. Using a dielectrophoretically trapped water droplet suspended in silicone oil, we show that the resonator amplitude, detuning, and linewidth can be continuously varied by changing the supply voltage, supply frequency, and the circuit resistance to obtain the desired trap depth, range, and stiffness. We show that by proper tuning of the resonator, the trap range can be extended without increasing the supply voltage, thus preventing sensitive samples from exposure to high electric fields at the stable trapping position. Such unprecedented dynamic control of dielectrophoretic forces opens avenues for the tunable active manipulation of sensitive biological and biochemical specimen in droplet microfluidic devices used for single-cell and biochemical reaction analysis.

Microfluidic measurement of individual cell membrane water permeability

This paper reports a microfluidic lab-on-chip for dynamic particle sizing and real time individual cell membrane permeability measurements. To achieve this, the device measures the impedance change of individual cells or particles at up to ten time points after mixing with different media, e.g. dimethyl sulfoxide or DI water, from separate inlets. These measurements are enabled by ten gold electrode pairs spread across a 20 mm long microchannel. The device measures impedance values within 0.26 s after mixing with other media, has a detection throughput of 150 samples/second, measures impedance values at all ten electrodes at this rate, and allows tracking of individual cell volume changes caused by cell osmosis in anisosmotic fluids over a 1.3 s postmixing timespan, facilitating accurate individual cell estimates of water permeability. The design and testing were performed using yeast cells (Saccharomyces cerevisiae). The relationship between volume and impedance in both polystyrene calibration beads as well as the volume-osmolality relationship in yeast were demonstrated. Moreover, we present the first noninvasive and non-optically-based water permeability measurements in individual cells.

A one-step molded microfluidic chip featuring a two-layer silver-PDMS microelectrode for dielectrophoretic cell separation

Dielectrophoresis (DEP) is a powerful technique for label-free cell separation in microfluidics. Easily-fabricated DEP separators with low cost and short turnaround time are in extremely high demand in practical applications, especially clinical usage where disposable devices are needed. DEP separators exploiting microelectrodes made of conducting polydimethylsiloxane (PDMS) composites enable the construction of advantageous 3D volumetric electrodes with a simple soft-lithography process. Yet, existing devices incorporating microelectrodes in conducting PDMS generally have their fluidic sidewalls constructed using a different material, and consequently require extra lithography of a sacrificial layer on the semi-finished master for molding the electrode and fluidic sidewalls in separate steps. Here we demonstrate a novel microfluidic DEP separator with a 3D electrode and fluidic structure entirely integrated within silver-PDMS composites. We develop a further simplified one-step molding process with lower cost using a readily-available and reusable SU8 master, eliminating the need for the additional lithography step in existing techniques. The uniquely designed two-layer electrode exhibits a spatially non-uniform electric field that enables cell migration in the vertical direction. The electrode upper layer then offers a harbor-like region for the trapping of the target cells that have drifted upwards, which shelters them from being dragged away by the main flow streams in the lower layer, and thus allows higher operation flow rate. We also optimize the upper layer thickness as a critical dimension for protecting the trapped cells from high drag and show easy widening of our device by elongation of the digits. We demonstrate that the elongated digits involving more parallel flow paths maintain a high capture efficiency of 95.4% for live cells with 85.6% purity in the separation of live/dead HeLa cells. We also investigate the device feasibility in a viability assay for cells post anti-cancer drug treatment.

Human Medulloblastoma Cell Lines: Investigating on Cancer Stem Cell-Like Phenotype

Medulloblastoma (MB) is the most common malignant pediatric brain tumor. Despite the progress of new treatments, the risk of recurrence, morbidity, and death remains significant and the long-term adverse effects in survivors are substantial. The fraction of cancer stem-like cells (CSCs) because of their self-renewal ability and multi-lineage differentiation potential is critical for tumor initiation, growth, and resistance to therapies. For the development of new CSC-targeted therapies, further in-depth studies are needed using enriched and stable MB-CSCs populations. This work, aimed at identifying the amount of CSCs in three available human cell lines (DAOY, D341, and D283), describes different approaches based on the expression of stemness markers. First, we explored potential differences in gene and protein expression patterns of specific stem cell markers. Then, in order to identify and discriminate undifferentiated from differentiated cells, MB cells were characterized using a physical characterization method based on a high-frequency dielectrophoresis approach. Finally, we compared their tumorigenic potential in vivo, through engrafting in nude mice. Concordantly, our findings identified the D283 human cell line as an ideal model of CSCs, providing important evidence on the use of a commercial human MB cell line for the development of new strategic CSC-targeting therapies.

High-throughput label-free characterization of viable, necrotic and apoptotic human lymphoma cells in a coplanar-electrode microfluidic impedance chip

The study and the characterization of cell death mechanisms are fundamental in cell biology research. Traditional death/viability assays usually involve laborious sample preparation and expensive equipment or reagents. In this work, we use electrical impedance spectroscopy as a label-free methodology to characterize viable, necrotic and apoptotic human lymphoma U937 cells. A simple three-electrode coplanar layout is used in a differential measurement scheme and thousands of cells are measured at high-throughput (≈200 cell/s). Tailored signal processing enables accurate and robust cell characterization without the need for cell focusing systems. The results suggest that, at low frequency (0.5 MHz), signal magnitude enables the discrimination between viable/necrotic cells and cell fragments, whereas phase information allows discriminating between viable cells and necrotic cells. At higher frequency (10 MHz) two subpopulations of cell fragments are distinguished. This work substantiates the prominent role of electrical impedance spectroscopy for the development of next-generation cell viability assays.

Quantitative Model for Ion Transport and Cytoplasm Conductivity of Chinese Hamster Ovary Cells

In mammalian cells cytoplasm ion concentrations and hence cytoplasm conductivity is an important indicator of their physiological state. Changes in the cytoplasm conductivity has been associated with physiological changes such as progression of cancer and apoptosis. In this work, a model that predicts the effects of physiological changes in ion transport on the cytoplasm conductivity of Chinese hamster ovary (CHO) cells is demonstrated. We determined CHO-specific model parameters, Na⁺/K⁺ ATPase pumps and ion channels densities, using a flux assay approach. The obtained sodium (P_Na), potassium (P_K) and chloride (P_Cl) permeability and Na⁺/K⁺ ATPase pump density were estimated to be 5.6 × 10^-8 cm/s, 5.6 × 10^-8 cm/s, 3.2 × 10^-7 cm/s and 2.56 × 10^-11 mol/cm², respectively. The model was tested by comparing the model predictions with the experimentally determined temporal changes in the cytoplasm conductivity of Na⁺/K⁺ ATPase pump inhibited CHO cells. Cells' Na⁺/K⁺ ATPase pumps were inhibited using 5 mM Ouabain and the temporal behavior of their cytoplasm conductivity was measured using dielectrophoresis cytometry. The measured results are in close agreement with the model-calculated values. This model will provide insight on the effects of processes such as apoptosis or external media ion concentration on the cytoplasm conductivity of mammalian cells.

Accurate quantification of apoptosis progression and toxicity using a dielectrophoretic approach

A loss of ability of cells to undergo apoptosis (programmed cell death, whereby the cell ceases to function and destroys itself) is commonly associated with cancer, and many anti-cancer interventions aim to restart the process. Consequently, the accurate quantification of apoptosis is essential in understanding the function and performance of new anti-cancer drugs. Dielectrophoresis has previously been demonstrated to detect apoptosis more rapidly than other methods, and is low-cost, label-free and rapid, but has previously been unable to accurately quantify cells through the apoptotic process because cells in late apoptosis disintegrate, making cell tracking impossible. In this paper we use a novel method based on light absorbance and multi-population tracking to quantify the progress of apoptosis, benchmarking against conventional assays including MTT, trypan blue and Annexin-V. Analyses are performed on suspension and adherent cells, and using two apoptosis-inducing agents. IC₅₀ measurements compared favourably to MTT and were superior to trypan blue, whilst also detecting apoptotic progression faster than Annexin-V.

Single cell dielectrophoresis study of apoptosis progression induced by controlled starvation

Nutrient depletion in fed-batch cultures and at the end of batch cultures is among the main causes of stress on cells and a trigger of apoptosis. In this study, we investigated changes in the cytoplasm conductivity of Chinese hamster ovary (CHO) cells under controlled starvation. Employing a single-cell dielectrophoresis (DEP) cytometer, we measured the DEP response of CHO cells incubated in a medium without glucose and glutamine over a 48-h period. Using the measured data in conjunction with numerical simulations, we determined the cytoplasm conductivity of viable and apoptotic cell subpopulations. The results show that a small subpopulation of apoptotic cells emerges after 24 to 36 h of starvation and increases rapidly over a short period of time, <12 h. The apoptotic cells have a dramatically lower cytoplasm conductivity, ∼0.05 S/m, than viable cells, ∼0.45 S/m. Viability of starvation cultures was measured by fluorescent cytometry, DEP cytometry, and trypan blue exclusion assays. DEP, Annexin V, caspase-8, and 7-AAD assays show a similar decline in viability after 36 h of starvation and indicate a very low viability after 48 h. Trypan blue exclusion assay fails to detect early-stage viability decline and estimates a much higher viability after 48 h.

Differential Ring Oscillator Based Capacitance Sensor for Microfluidic Applications

A simple high frequency capacitance sensor with 180 aF sensitivity is designed for a wide range of microfluidic applications. The sensor is implemented utilizing differential ring oscillators operating at [Formula: see text] MHz with a differential signal at [Formula: see text] MHz. The sensor occupies [Formula: see text] cm × 2 cm on a printed circuit board. The sensor is tuned using two precision variable capacitors and has a full scale range of [Formula: see text] pF. The sensor was able to detect less than 1% Isopropyl Alcohol in DI water and to detect 15 μm polystyrene spheres flowing over 25 μm lines and spaces coplanar electrodes in a microfluidic channel. The compact differential ring oscillator based architecture of the design makes it suitable to be integrated into microprocessor based systems for detection in Lab on Chip or Lab on Board applications.

Dielectrophoresis study of temporal change in internal conductivity of single CHO cells after electroporation by pulsed electric fields

Applying sufficiently strong pulsed electric fields to a cell can permeabilize the membrane and subsequently affect its dielectric properties. In this study, we employ a microfluidic dielectrophoresis cytometry technique to simultaneously electroporate and measure the time-dependent dielectric response of single Chinese hamster ovary cells. Using experimental measurements along with numerical simulations, we present quantitative results for the changes in the cytoplasm conductivity of single cells within seconds after exposure to 100 μs duration pulsed electric fields with various intensities. It is shown that, for electroporation in a medium with conductivity lower than that of the cell's cytoplasm, the internal conductivity of the cell decreases after the electroporation on a time scale of seconds and stronger pulses cause a larger and more rapid decrease. We also observe that, after the electroporation, the cell's internal conductivity is constrained to a threshold. This implies that the cell prevents some of the ions in its cytoplasm from diffusing through the created pores to the external medium. The temporal change in the dielectric response of each individual cell is continuously monitored over minutes after exposure to pulsed electric fields. A time constant associated with the cell's internal conductivity change is observed, which ranges from seconds to tens of seconds depending on the applied pulse intensity. This experimental observation supports the results of numerical models reported in the literature.

Dielectric model for Chinese hamster ovary cells obtained by dielectrophoresis cytometry

We present a dielectric model and its parameters for Chinese hamster ovary (CHO) cells based on a double-shell structure which includes the cell membrane, cytoplasm, nuclear envelope, and nucleoplasm. Employing a dielectrophoresis (DEP) based technique and a microfluidic system, the DEP response of many single CHO cells is measured and the spectrum of the Clausius-Mossotti factor is obtained. The dielectric parameters of the model are then extracted by curve-fitting to the measured spectral data. Using this approach over the 0.6-10 MHz frequency range, we report the values for CHO cells' membrane permittivity, membrane thickness, cytoplasm conductivity, nuclear envelope permittivity, and nucleoplasm conductivity. The size of the cell and its nuclei are obtained using optical techniques.

Effect of a dual inlet channel on cell loading in microfluidics

Unwanted sedimentation and attachment of a number of cells onto the bottom channel often occur on relatively large-scale inlets of conventional microfluidic channels as a result of gravity and fluid shear. Phenomena such as sedimentation have become recognized problems that can be overcome by performing microfluidic experiments properly, such as by calculating a meaningful output efficiency with respect to real input. Here, we present a dual-inlet design method for reducing cell loss at the inlet of channels by adding a new " upstream inlet " to a single main inlet design. The simple addition of an upstream inlet can create a vertically layered sheath flow prior to the main inlet for cell loading. The bottom layer flow plays a critical role in preventing the cells from attaching to the bottom of the channel entrance, resulting in a low possibility of cell sedimentation at the main channel entrance. To provide proof-of-concept validation, we applied our design to a microfabricated flow cytometer system (μFCS) and compared the cell counting efficiency of the proposed μFCS with that of the previous single-inlet μFCS and conventional FCS. We used human white blood cells and fluorescent microspheres to quantitatively evaluate the rate of cell sedimentation in the main inlet and to measure fluorescence sensitivity at the detection zone of the flow cytometer microchip. Generating a sheath flow as the bottom layer was meaningfully used to reduce the depth of field as well as the relative deviation of targets in the z-direction (compared to the x-y flow plane), leading to an increased counting sensitivity of fluorescent detection signals. Counting results using fluorescent microspheres showed both a 40% reduction in the rate of sedimentation and a 2-fold higher sensitivity in comparison with the single-inlet μFCS. The results of CD4(+) T-cell counting also showed that the proposed design results in a 25% decrease in the rate of cell sedimentation and a 28% increase in sensitivity when compared to the single-inlet μFCS. This method is simple and easy to use in design, yet requires no additional time or cost in fabrication. Furthermore, we expect that this approach could potentially be helpful for calculating exact cell loading and counting efficiency for a small input number of cells, such as primary cells and rare cells, in microfluidic channel applications.

Monitoring the dielectric response of single cells following mitochondrial adenosine triphosphate synthase inhibition by oligomycin using a dielectrophoretic cytometer

One of the main uses of adenosine triphosphate (ATP) within mammalian cells is powering the Na(+)/K(+) ATPase pumps used to maintain ion concentrations within the cell. Since ion concentrations determine the cytoplasm conductivity, ATP concentration is expected to play a key role in controlling the cytoplasm conductivity. The two major ATP production pathways within cells are via glycolysis within the cytoplasm and via the electron transport chain within the mitochondria. In this work, a differential detector combined with dielectrophoretic (DEP) translation in a microfluidic channel was employed to observe single cell changes in the cytoplasm conductivity. The DEP response was made sensitive to changes in cytoplasm conductivity by measuring DEP response versus media conductivity and using double shell models to choose appropriate frequencies and media conductivity. Dielectric response of Chinese hamster ovary (CHO) cells was monitored following inhibition of the mitochondria ATP production by treatment with oligomycin. We show that in CHO cells following exposure to oligomycin (8 μg/ml) the cytoplasm conductivity drops, with the majority of the change occurring within 50 min. This work demonstrates that dielectric effects due to changes in ATP production can be observed at the single cell level.

On-chip microelectrode impedance analysis of mammalian cell viability during biomanufacturing

The characterization of cell viability is a challenging task in applied biotechnology, as no clear definition of cell death exists. Cell death is accompanied with a change in the electrical properties of the membrane as well as the cell interior. Therefore, changes in the physiology of cells can be characterized by monitoring of their dielectric properties. We correlated the dielectric properties of industrially used mammalian cells, sedimented over interdigitated microelectrodes, to the AC signal response across the chip. The voltage waveforms across the electrodes were processed to obtain the circuit impedance, which was used to quantify the changes in cell viability. We observed an initial decrease in impedance, after which it remained nearly constant. The results were compared with data from the dye exclusion viability test, the cell specific oxygen uptake rate, and the online viable cell density data from capacitance probes. The microelectrode technique was found to be sensitive to physiological changes taking place inside the cells before their membrane integrity is compromised. Such accurate determination of the metabolic status during this initial period, which turned out to be less well captured in the dye exclusion tests, may be essential for several biotechnology operations.

Microarray dot electrodes utilizing dielectrophoresis for cell characterization

During the last three decades; dielectrophoresis (DEP) has become a vital tool for cell manipulation and characterization due to its non-invasiveness. It is very useful in the trend towards point-of-care systems. Currently, most efforts are focused on using DEP in biomedical applications, such as the spatial manipulation of cells, the selective separation or enrichment of target cells, high-throughput molecular screening, biosensors and immunoassays. A significant amount of research on DEP has produced a wide range of microelectrode configurations. In this paper; we describe the microarray dot electrode, a promising electrode geometry to characterize and manipulate cells via DEP. The advantages offered by this type of microelectrode are also reviewed. The protocol for fabricating planar microelectrodes using photolithography is documented to demonstrate the fast and cost-effective fabrication process. Additionally; different state-of-the-art Lab-on-a-Chip (LOC) devices that have been proposed for DEP applications in the literature are reviewed. We also present our recently designed LOC device, which uses an improved microarray dot electrode configuration to address the challenges facing other devices. This type of LOC system has the capability to boost the implementation of DEP technology in practical settings such as clinical cell sorting, infection diagnosis, and enrichment of particle populations for drug development.

The changing dielectric properties of CHO cells can be used to determine early apoptotic events in a bioprocess

To ensure maximum productivity of recombinant proteins it is desirable to prolong cell viability during a mammalian cell bioprocess, and therefore important to carefully monitor cell density and viability. In this study, five different and independent methods of monitoring were applied to Chinese hamster ovary (CHO) cells grown in a batch culture in a controlled bioreactor to determine cell density and/or cell viability. They included: a particle counter, trypan blue exclusion (Cedex), an in situ bulk capacitance probe, an off-line fluorescent flow cytometer, and a prototype dielectrophoretic (DEP) cytometer. These various techniques gave similar values during the exponential growth phase. However, beyond the exponential growth phase the viability measurements diverged. Fluorescent flow cytometry with a range of fluorescent markers was used to investigate this divergence and to establish the progress of cell apoptosis: the cell density estimates by the intermediate stage apoptosis assay agreed with those obtained by the bulk capacitance probe and the early stage apoptosis assay viability measurements correlated well with the DEP cytometer. The trypan blue assay showed higher estimates of viable cell density and viability compared to the capacitance probe or the DEP cytometer. The DEP cytometer measures the dielectric properties of individual cells and identified at least two populations of cells, each with a distinct polarizability. As verified by comparison with the Nexin assay, one population was associated with viable (non-apoptotic) cells and the other with apoptotic cells. From the end of the exponential through the stationary and decline stages there was a gradual shift of cell count from the viable into the apoptotic population. However, the two populations maintained their individual dielectric properties throughout this shift. This leads to the conclusion that changes in bulk dielectric properties of cultures might be better modeled as shifts in cells between different dielectric sub-populations, rather than assuming a homogeneous dielectric population. This shows that bulk dielectric probes are sensitive to the early apoptotic changes in cells. DEP cytometry offers a novel and unique technology for analyzing and characterizing mammalian cells based on their dielectric properties, and suggests a potential application of the device as a low-cost, label-free, electronic monitor of physiological changes in cells.