Separation of neural stem cells by whole cell membrane capacitance using dielectrophoresis

Electrical signature of heterogeneous human mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) have gained traction in transplantation therapy due to their immunomodulatory, paracrine, immune-evasive, and multipotent differentiation potential. The inherent heterogeneity of hMSCs poses a challenge for therapeutic treatments and necessitates the identification of robust biomarkers to ensure reproducibility in both in vivo and in vitro experiments. In this study, we utilized dielectrophoresis (DEP), a label-free electrokinetic phenomenon, to investigate the heterogeneity of hMSCs derived from bone marrow (BM) and adipose tissue (AD). The electrical properties of BM-hMSCs were compared to homogeneous mouse fibroblasts (NIH-3T3), human fibroblasts (WS1), and human embryonic kidney cells (HEK-293). The DEP profile of BM-hMSCs differed most from HEK-293 cells. We compared the DEP profiles of BM-hMSCs and AD-hMSCs and found that they have similar membrane capacitances, differing cytoplasm conductivity, and transient slopes. Inducing both populations to differentiate into adipocyte and osteoblast cells revealed that they behave differently in response to differentiation-inducing cytokines. Histology and reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analyses of the differentiation-related genes revealed differences in heterogeneity between BM-hMSCs and AD-hMSCs. The differentiation profiles correlate well with the DEP profiles developed and indicate differences in the heterogeneity of BM-hMSCs and AD-hMSCs. Our results demonstrate that using DEP, membrane capacitance, cytoplasm conductivity, and transient slope can uniquely characterize the inherent heterogeneity of hMSCs to guide robust and reproducible stem cell transplantation therapies.

Bio-inspired microfluidics: A review

Biomicrofluidics, a subdomain of microfluidics, has been inspired by several ideas from nature. However, while the basic inspiration for the same may be drawn from the living world, the translation of all relevant essential functionalities to an artificially engineered framework does not remain trivial. Here, we review the recent progress in bio-inspired microfluidic systems via harnessing the integration of experimental and simulation tools delving into the interface of engineering and biology. Development of "on-chip" technologies as well as their multifarious applications is subsequently discussed, accompanying the relevant advancements in materials and fabrication technology. Pointers toward new directions in research, including an amalgamated fusion of data-driven modeling (such as artificial intelligence and machine learning) and physics-based paradigm, to come up with a human physiological replica on a synthetic bio-chip with due accounting of personalized features, are suggested. These are likely to facilitate physiologically replicating disease modeling on an artificially engineered biochip as well as advance drug development and screening in an expedited route with the minimization of animal and human trials.

A Magnetically and Electrically Powered Hybrid Micromotor in Conductive Solutions: Synergistic Propulsion Effects and Label-Free Cargo Transport and Sensing

Electrically powered micro- and nanomotors are promising tools for in vitro single-cell analysis. In particular, single cells can be trapped, transported, and electroporated by a Janus particle (JP) using an externally applied electric field. However, while dielectrophoretic (DEP)-based cargo manipulation can be achieved at high-solution conductivity, electrical propulsion of these micromotors becomes ineffective at solution conductivities exceeding ≈0.3 mS cm^-1 . Here, JP cargo manipulation and transport capabilities to conductive near-physiological (<6 mS cm^-1 ) solutions are extended successfully by combining magnetic field-based micromotor propulsion and navigation with DEP-based manipulation of various synthetic and biological cargos. Combination of a rotating magnetic field and electric field results in enhanced micromotor mobility and steering control through tuning of the electric field frequency. In addition, the micromotor's ability of identifying apoptotic cell among viable and necrotic cells based on their dielectrophoretic difference is demonstrated, thus, enabling to analyze the apoptotic status in the single-cell samples for drug discovery, cell therapeutics, and immunotherapy. The ability to trap and transport live cells towards regions containing doxorubicin-loaded liposomes is also demonstrated. This hybrid micromotor approach for label-free trapping, transporting, and sensing of selected cells within conductive solutions opens new opportunities in drug delivery and single-cell analysis, where close-to-physiological media conditions are necessary.

A correlation of conductivity medium and bioparticle viability on dielectrophoresis-based biomedical applications

Dielectrophoresis (DEP) bioparticle research has progressed from micro to nano levels. It has proven to be a promising and powerful cell manipulation method with an accurate, quick, inexpensive, and label-free technique for therapeutic purposes. DEP, an electrokinetic phenomenon, induces particle movement as a result of polarization effects in a nonuniform electrical field. This review focuses on current research in the biomedical field that demonstrates a practical approach to DEP in terms of cell separation, trapping, discrimination, and enrichment under the influence of the conductive medium in correlation with bioparticle viability. The current review aims to provide readers with an in-depth knowledge of the fundamental theory and principles of the DEP technique, which is influenced by conductive medium and to identify and demonstrate the biomedical application areas. The high conductivity of physiological fluids presents obstacles and opportunities, followed by bioparticle viability in an electric field elaborated in detail. Finally, the drawbacks of DEP-based systems and the outlook for the future are addressed. This article will aid in advancing technology by bridging the gap between bioscience and engineering. We hope the insights presented in this review will improve cell suspension medium and promote DEP-viable bioparticle manipulation for health-care diagnostics and therapeutics.

Microfluidics for Neuronal Cell and Circuit Engineering

The widespread adoption of microfluidic devices among the neuroscience and neurobiology communities has enabled addressing a broad range of questions at the molecular, cellular, circuit, and system levels. Here, we review biomedical engineering approaches that harness the power of microfluidics for bottom-up generation of neuronal cell types and for the assembly and analysis of neural circuits. Microfluidics-based approaches are instrumental to generate the knowledge necessary for the derivation of diverse neuronal cell types from human pluripotent stem cells, as they enable the isolation and subsequent examination of individual neurons of interest. Moreover, microfluidic devices allow to engineer neural circuits with specific orientations and directionality by providing control over neuronal cell polarity and permitting the isolation of axons in individual microchannels. Similarly, the use of microfluidic chips enables the construction not only of 2D but also of 3D brain, retinal, and peripheral nervous system model circuits. Such brain-on-a-chip and organoid-on-a-chip technologies are promising platforms for studying these organs as they closely recapitulate some aspects of in vivo biological processes. Microfluidic 3D neuronal models, together with 2D in vitro systems, are widely used in many applications ranging from drug development and toxicology studies to neurological disease modeling and personalized medicine. Altogether, microfluidics provide researchers with powerful systems that complement and partially replace animal models.

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.

Engineering Tissues of the Central Nervous System: Interfacing Conductive Biomaterials with Neural Stem/Progenitor Cells

Conductive biomaterials provide an important control for engineering neural tissues, where electrical stimulation can potentially direct neural stem/progenitor cell (NS/PC) maturation into functional neuronal networks. It is anticipated that stem cell-based therapies to repair damaged central nervous system (CNS) tissues and ex vivo, "tissue chip" models of the CNS and its pathologies will each benefit from the development of biocompatible, biodegradable, and conductive biomaterials. Here, technological advances in conductive biomaterials are reviewed over the past two decades that may facilitate the development of engineered tissues with integrated physiological and electrical functionalities. First, one briefly introduces NS/PCs of the CNS. Then, the significance of incorporating microenvironmental cues, to which NS/PCs are naturally programmed to respond, into biomaterial scaffolds is discussed with a focus on electrical cues. Next, practical design considerations for conductive biomaterials are discussed followed by a review of studies evaluating how conductive biomaterials can be engineered to control NS/PC behavior by mimicking specific functionalities in the CNS microenvironment. Finally, steps researchers can take to move NS/PC-interfacing, conductive materials closer to clinical translation are discussed.

Continuous Particle Separation Driven by 3D Ag-PDMS Electrodes with Dielectric Electrophoretic Force Coupled with Inertia Force

Cell separation has become @important in biological and medical applications. Dielectrophoresis (DEP) is widely used due to the advantages it offers, such as the lack of a requirement for biological markers and the fact that it involves no damage to cells or particles. This study aimed to report a novel approach combining 3D sidewall electrodes and contraction/expansion (CEA) structures to separate three kinds of particles with different sizes or dielectric properties continuously. The separation was achieved through the interaction between electrophoretic forces and inertia forces. The CEA channel was capable of sorting particles with different sizes due to inertial forces, and also enhanced the nonuniformity of the electric field. The 3D electrodes generated a non-uniform electric field at the same height as the channels, which increased the action range of the DEP force. Finite element simulations using the commercial software, COMSOL Multiphysics 5.4, were performed to determine the flow field distributions, electric field distributions, and particle trajectories. The separation experiments were assessed by separating 4 µm polystyrene (PS) particles from 20 µm PS particles at different flow rates by experiencing positive and negative DEP. Subsequently, the sorting performances of the 4 µm PS particles, 20 µm PS particles, and 4 µm silica particles with different solution conductivities were observed. Both the numerical simulations and the practical particle separation displayed high separating efficiency (separation of 4 µm PS particles, 94.2%; separation of 20 µm PS particles, 92.1%; separation of 4 µm Silica particles, 95.3%). The proposed approach is expected to open a new approach to cell sorting and separating.

Dielectrophoresis applications in biomedical field and future perspectives in biomedical technology

Dielectrophoresis (DEP) is a technique to manipulate trajectories of polarisable particles in non-uniform electric fields by utilising unique dielectric properties. The manipulation of a cell using DEP has been demonstrated in various modes, thereby indicating potential applications in the biomedical field. In this review, recent DEP applications in the biomedical field are discussed. This review is intended to highlight research work that shows significant approach related to dielectrophoresis application in biomedical field reported between 2016 and 2020. Firstly, single-shell model and multiple-shell model of cells are introduced. Current device structures and recently introduced electrode patterns for DEP applications are discussed. Secondly, the biomedical uses of DEP in liquid biopsies, stem cell therapies, and diagnosis of infectious diseases due to bacteria and viruses are presented. Finally, the challenges in DEP research are discussed, and the reported solutions are explained. DEP's potential research directions are mentioned. This article is protected by copyright. All rights reserved.

Methods of Generating Dielectrophoretic Force for Microfluidic Manipulation of Bioparticles

Manipulation of microscale bioparticles including living cells is of great significance to the broad bioengineering and biotechnology fields. Dielectrophoresis (DEP), which is defined as the interactions between dielectric particles and the electric field, is one of the most widely used techniques for the manipulation of bioparticles including cell separation, sorting, and trapping. Bioparticles experience a DEP force if they have a different polarization from the surrounding media in an electric field that is nonuniform in terms of the intensity and/or phase of the electric field. A comprehensive literature survey shows that the DEP-based microfluidic devices for manipulating bioparticles can be categorized according to the methods of creating the nonuniformity via patterned microchannels, electrodes, and media to generate the DEP force. These methods together with the theory of DEP force generation are described in this review, to provide a summary of the methods and materials that have been used to manipulate various bioparticles for various specific biological outcomes. Further developments of DEP-based technologies include identifying materials that better integrate with electrodes than current popular materials (silicone/glass) and improving the performance of DEP manipulation of bioparticles by combining it with other methods of handling bioparticles. Collectively, DEP-based microfluidic manipulation of bioparticles holds great potential for various biomedical applications.

Microscale nonlinear electrokinetics for the analysis of cellular materials in clinical applications: a review

This review article presents a discussion of some of the latest advancements in the field of microscale electrokinetics for the analysis of cells and subcellular materials in clinical applications. The introduction presents an overview on the use of electric fields, i.e., electrokinetics, in microfluidics devices and discusses the potential of electrokinetic-based methods for the analysis of liquid biopsies in clinical and point-of-care applications. This is followed by four comprehensive sections that present some of the newest findings on the analysis of circulating tumor cells, blood (red blood cells, white blood cells, and platelets), stem cells, and subcellular particles (extracellular vesicles and mitochondria). The valuable contributions discussed here (with 131 references) were mainly published during the last 3 to 4 years, providing the reader with an overview of the state-of-the-art in the use of microscale electrokinetic methods in clinical analysis. Finally, the conclusions summarize the main advancements and discuss the future prospects.

Dielectrophoretic Characterization of Tenogenically Differentiating Mesenchymal Stem Cells

Tendons are collagenous musculoskeletal tissues that connect muscles to bones and transfer the forces necessary for movement. Tendons are susceptible to injury and heal poorly, with long-term loss of function. Mesenchymal stem cell (MSC)-based therapies are a promising approach for treating tendon injuries but are challenged by the difficulties of controlling stem cell fate and of generating homogenous populations of stem cells optimized for tenogenesis (differentiation toward tendon). To address this issue, we aim to explore methods that can be used to identify and ultimately separate tenogenically differentiated MSCs from non-tenogenically differentiated MSCs. In this study, baseline and tenogenically differentiating murine MSCs were characterized for dielectric properties (conductivity and permittivity) of their outer membrane and cytoplasm using a dielectrophoretic (DEP) crossover technique. Experimental results showed that unique dielectric properties distinguished tenogenically differentiating MSCs from controls after three days of tenogenic induction. A single shell model was used to quantify the dielectric properties and determine membrane and cytoplasm conductivity and permittivity. Together, cell responses at the crossover frequency, cell morphology, and shell models showed that changes potentially indicative of early tenogenesis could be detected in the dielectric properties of MSCs as early as three days into differentiation. Differences in dielectric properties with tenogenesis indicate that the DEP-based label-free separation of tenogenically differentiating cells is possible and avoids the complications of current label-dependent flow cytometry-based separation techniques. Overall, this work illustrates the potential of DEP to generate homogeneous populations of differentiated stem cells for applications in tissue engineering and regenerative medicine.

Dielectrophoresis as a tool for electrophysiological characterization of stem cells

Dielectrophoresis (DEP), a nonlinear electrokinetic technique caused by Maxwell-Wagner interfacial polarization of neutral particles in an electrolyte solution, is a powerful cell manipulation method used widely for various applications such as enrichment, trapping, and sorting of heterogeneous cell populations. While conventional cell characterization and sorting methods require tagging or labeling of cells, DEP has the potential to manipulate cells in a label-free way. Due to its unique ability to characterize and sort cells without the need of labeling, there is renewed interest in using DEP for stem cell research and regenerative medicine. Stem cells have the potential to differentiate into various lineages, but achieving homogeneous cell phenotypes from an initially heterogeneous cell population is a challenge. Using DEP to efficiently and affordably identify, sort, and enrich either undifferentiated or differentiated stem cell populations in a label-free way would advance their potential uses for applications in tissue engineering and regenerative medicine. This review summarizes recent, significant research findings regarding the electrophysiological characterization of stem cells, with a focus on cellular dielectric properties, i.e., permittivity and conductivity, and on studies that have obtained these measurements using techniques that preserve cell viability, such as crossover frequency. Potential applications for DEP in regenerative medicine are also discussed. Overall, DEP is a promising technique and, when used to characterize, sort, and enrich stem cells, will advance stem cell-based regenerative therapies.

A Prominent Cell Manipulation Technique in BioMEMS: Dielectrophoresis

BioMEMS, the biological and biomedical applications of micro-electro-mechanical systems (MEMS), has attracted considerable attention in recent years and has found widespread applications in disease detection, advanced diagnosis, therapy, drug delivery, implantable devices, and tissue engineering. One of the most essential and leading goals of the BioMEMS and biosensor technologies is to develop point-of-care (POC) testing systems to perform rapid prognostic or diagnostic tests at a patient site with high accuracy. Manipulation of particles in the analyte of interest is a vital task for POC and biosensor platforms. Dielectrophoresis (DEP), the induced movement of particles in a non-uniform electrical field due to polarization effects, is an accurate, fast, low-cost, and marker-free manipulation technique. It has been indicated as a promising method to characterize, isolate, transport, and trap various particles. The aim of this review is to provide fundamental theory and principles of DEP technique, to explain its importance for the BioMEMS and biosensor fields with detailed references to readers, and to identify and exemplify the application areas in biosensors and POC devices. Finally, the challenges faced in DEP-based systems and the future prospects are discussed.

Electrical Impedance Spectroscopy for Monitoring Chemoresistance of Cancer Cells

Electrical impedance spectroscopy (EIS) is an electrokinetic method that allows for the characterization of intrinsic dielectric properties of cells. EIS has emerged in the last decade as a promising method for the characterization of cancerous cells, providing information on inductance, capacitance, and impedance of cells. The individual cell behavior can be quantified using its characteristic phase angle, amplitude, and frequency measurements obtained by fitting the input frequency-dependent cellular response to a resistor-capacitor circuit model. These electrical properties will provide important information about unique biomarkers related to the behavior of these cancerous cells, especially monitoring their chemoresistivity and sensitivity to chemotherapeutics. There are currently few methods to assess drug resistant cancer cells, and therefore it is difficult to identify and eliminate drug-resistant cancer cells found in static and metastatic tumors. Establishing techniques for the real-time monitoring of changes in cancer cell phenotypes is, therefore, important for understanding cancer cell dynamics and their plastic properties. EIS can be used to monitor these changes. In this review, we will cover the theory behind EIS, other impedance techniques, and how EIS can be used to monitor cell behavior and phenotype changes within cancerous cells.

Continuous ES/Feeder Cell-Sorting Device Using Dielectrophoresis and Controlled Fluid Flow

Pluripotent stem cells (PSCs) are considered as being an important cell source for regenerative medicine. The culture of PSCs usually requires a feeder cell layer or cell adhesive matrix coating such as Matrigel, laminin, and gelatin. Although a feeder-free culture using a matrix coating has been popular, the on-feeder culture is still an effective method for the fundamental study of regenerative medicine and stem cell biology. To culture PSCs on feeder cell layers, the elimination of feeder cells is required for biological or gene analysis and for cell passage. Therefore, a simple and cost-effective cell sorting technology is required. There are several commercialized cell-sorting methods, such as FACS or MACS. However, these methods require cell labeling by fluorescent dye or magnetic antibodies with complicated processes. To resolve these problems, we focused on dielectrophoresis (DEP) phenomena for cell separation because these do not require any fluorescent or magnetic dyes or antibodies. DEP imposes an electric force on living cells under a non-uniform AC electric field. The direction and magnitude of the DEP force depend on the electric property and size of the cell. Therefore, DEP is considered as a promising approach for sorting PSCs from feeder cells. In this study, we developed a simple continuous cell-sorting device using the DEP force and fluid-induced shear force. As a result, mouse embryonic stem cells (mESCs) were purified from a mixed-cell suspension containing mESCs and mouse embryonic fibroblasts (MEFs) using our DEP cell-sorting device.

Microfluidic-Based Approaches in Targeted Cell/Particle Separation Based on Physical Properties: Fundamentals and Applications

Cell separation is a key step in many biomedical research areas including biotechnology, cancer research, regenerative medicine, and drug discovery. While conventional cell sorting approaches have led to high-efficiency sorting by exploiting the cell's specific properties, microfluidics has shown great promise in cell separation by exploiting different physical principles and using different properties of the cells. In particular, label-free cell separation techniques are highly recommended to minimize cell damage and avoid costly and labor-intensive steps of labeling molecular signatures of cells. In general, microfluidic-based cell sorting approaches can separate cells using "intrinsic" (e.g., fluid dynamic forces) versus "extrinsic" external forces (e.g., magnetic, electric field, etc.) and by using different properties of cells including size, density, deformability, shape, as well as electrical, magnetic, and compressibility/acoustic properties to select target cells from a heterogeneous cell population. In this work, principles and applications of the most commonly used label-free microfluidic-based cell separation methods are described. In particular, applications of microfluidic methods for the separation of circulating tumor cells, blood cells, immune cells, stem cells, and other biological cells are summarized. Computational approaches complementing such microfluidic methods are also explained. Finally, challenges and perspectives to further develop microfluidic-based cell separation methods are discussed.

Self-aligned microfluidic contactless dielectrophoresis device fabricated by single-layer imprinting on cyclic olefin copolymer

The trapping and deflection of biological cells by dielectrophoresis (DEP) at field non-uniformities in a microfluidic device is often conducted in a contactless dielectrophoresis (cDEP) mode, wherein the electrode channel is in a different layer than the sample channel, so that field penetration through the interceding barrier causes DEP above critical cut-off frequencies. In this manner, through physical separation of the electrode and sample channels, it is possible to spatially modulate electric fields with no electrode-induced damage to biological cells in the sample channel. However, since this device requires interlayer alignment of the electrode to sample channel and needs to maintain a thin interceding barrier (~ 15 μm) over the entire length over which DEP is needed (~ 1 cm), variations in alignment and microstructure fidelity cause wide variations in cDEP trapping level and frequency response across devices. We present a strategy to eliminate interlayer alignment by fabricating self-aligned electrode and sample channels, simultaneously with the interceding barrier layer (14-μm width and 50-μm depth), using a single-layer imprint and bond process on cyclic olefin copolymer. Specifically, by designing support structures, we preserve fidelity of the high aspect ratio insulating posts in the sample channel and the interceding barrier between the sample and electrode channels over the entire device footprint (~ 1 cm). The device operation is validated based on impedance measurements to quantify field penetration through the interceding barrier and by DEP trapping measurements. The presented fabrication strategy can eventually improve cDEP device manufacturing protocols to enable more reproducible DEP performance. Graphical abstract.

Biophysical differentiation of susceptibility and chemical differences in Staphylococcus aureus

Differentiating bacteria strains using biophysical forces has been the focus of recent studies using dielectrophoresis (DEP). The refinement of these studies has created high-resolution separations such that very subtle properties of the cells are enough to induce significant differences in measurable biophysical properties. These high-resolution capabilities build upon the advantages of DEP which include small sample sizes and fast analysis times. Studies focusing on differentiating antimicrobial resistant and susceptible bacteria potentially have significant impact on human health and medical care. A prime example is Staphylococcus aureus, which commonly colonizes adults without ill effects. However, the pathogen is an important cause of infections, including surgical site infections. Treatment of S. aureus infections is generally possible with antimicrobials, but antimicrobial resistance has emerged. Of special importance is resistance to methicillin, an antimicrobial created in response to resistance to penicillin. Here, dielectrophoresis is used to study methicillin-resistant (MRSA) and -susceptible S. aureus (MSSA) strains, both with and without the addition of a fluorescent label. The capture onset potential of fluorescently-labeled MRSA (865 ± 71 V) and thus the ratio of electrokinetic to dielectrophoretic mobility, was found to be higher than that of fluorescently-labeled MSSA (685 ± 61 V). This may be attributable to the PBP2a enzyme present in the MRSA strain and not in the MSSA bacteria. Further, unlabeled MRSA was found to have a capture onset potential of 732 ± 44 V, while unlabeled MSSA was found to have a capture onset potential of 562 ± 59 V. This shows that the fluorescently-labeled bacteria require a higher applied potential, and thus ratio of mobilities, to capture than the unlabeled bacteria.

Detection of Rare Objects by Flow Cytometry: Imaging, Cell Sorting, and Deep Learning Approaches

Flow cytometry nowadays is among the main working instruments in modern biology paving the way for clinics to provide early, quick, and reliable diagnostics of many blood-related diseases. The major problem for clinical applications is the detection of rare pathogenic objects in patient blood. These objects can be circulating tumor cells, very rare during the early stages of cancer development, various microorganisms and parasites in the blood during acute blood infections. All of these rare diagnostic objects can be detected and identified very rapidly to save a patient's life. This review outlines the main techniques of visualization of rare objects in the blood flow, methods for extraction of such objects from the blood flow for further investigations and new approaches to identify the objects automatically with the modern deep learning methods.

Label-free enrichment of fate-biased human neural stem and progenitor cells

Human neural stem and progenitor cells (hNSPCs) have therapeutic potential to treat neural diseases and injuries since they provide neuroprotection and differentiate into astrocytes, neurons, and oligodendrocytes. However, cultures of hNSPCs are heterogeneous, containing cells linked to distinct differentiated cell fates. HNSPCs that differentiate into astrocytes are of interest for specific neurological diseases, creating a need for approaches that can detect and isolate these cells. Astrocyte-biased hNSPCs differ from other cell types in electrophysiological properties, namely membrane capacitance, and we hypothesized that this could be used to enrich these cells using dielectrophoresis (DEP). We implemented a two-step DEP sorting scheme, consisting of analysis to define the optimal sorting frequency followed by separation of cells at that frequency, to test whether astrocyte-biased cells could be separated from the other cell types present in hNSPC cultures. We developed a novel device that increased sorting reproducibility and provided both enriched and depleted cell populations in a single sort. Astrocyte-biased cells were successfully enriched from hNSPC cultures by DEP sorting, making this the first study to use electrophysiological properties for label-free enrichment of human astrocyte-biased cells. Enriched astrocyte-biased human cells enable future experiments to determine the specific properties of these important cells and test their therapeutic efficacy in animal models of neurological diseases.

High-throughput continuous dielectrophoretic separation of neural stem cells

We created an integrated microfluidic cell separation system that incorporates hydrophoresis and dielectrophoresis modules to facilitate high-throughput continuous cell separation. The hydrophoresis module consists of a serpentine channel with ridges and trenches to generate a diverging fluid flow that focuses cells into two streams along the channel edges. The dielectrophoresis module is composed of a chevron-shaped electrode array. Separation in the dielectrophoresis module is driven by inherent cell electrophysiological properties and does not require cell-type-specific labels. The chevron shape of the electrode array couples with fluid flow in the channel to enable continuous sorting of cells to increase throughput. We tested the new system with mouse neural stem cells since their electrophysiological properties reflect their differentiation capacity (e.g., whether they will differentiate into astrocytes or neurons). The goal of our experiments was to enrich astrocyte-biased cells. Sorting parameters were optimized for each batch of neural stem cells to ensure effective and consistent separations. The continuous sorting design of the device significantly improved sorting throughput and reproducibility. Sorting yielded two cell fractions, and we found that astrocyte-biased cells were enriched in one fraction and depleted from the other. This is an advantage of the new continuous sorting device over traditional dielectrophoresis-based sorting platforms that target a subset of cells for enrichment but do not provide a corresponding depleted population. The new microfluidic dielectrophoresis cell separation system improves label-free cell sorting by increasing throughput and delivering enriched and depleted cell subpopulations in a single sort.

On-Chip Impedance for Quantifying Parasitic Voltages During AC Electrokinetic Trapping

OBJECTIVE

Assessing the effectiveness of microfluidic device structures for enabling electrokinetic or acoustic trapping requires imaging of model particles within each device under the requisite force fields. To avoid the need for extensive microscopy, the use of valuable biological samples, and reliance on a trained operator to assess efficacy of trapping, we explore electrical means to identify device geometry variations that are responsible for the poor trapping.

RESULTS

Using the example of AC electrokinetic trapping over an insulated channel in a contact-less dielectrophoresis mode, we present an on-chip method to acquire impedance spectra on the microfluidic device for quantifying the parasitic voltage drops.

CONCLUSION

Based on the parasitic voltage drops, device geometries can be designed to maximize fraction of the applied voltage that is available for dielectrophoretic manipulation and the measured on-chip impedance can rapidly inform downstream decisions on particle manipulation.

Impacts of low concentration surfactant on red blood cell dielectrophoretic responses

Cell dielectrophoretic responses have been extensively studied for biomarker expression, blood typing, sepsis, circulating tumor cell separations, and others. Surfactants are often added to the analytical buffer in electrokinetic cellular microfluidic systems to lower surface/interfacial tensions. In nonelectrokinetic systems, surfactants influence cell size, shape, and agglomeration; this has not been systematically documented in electrokinetic systems. In the present work, the impacts of the Triton X-100 surfactant on human red blood cells (RBCs) were explored via ultraviolet-visible spectroscopy (UV-Vis) and dielectrophoresis (DEP) to compare nonelectrokinetic and electrokinetic responses, respectively. The UV-Vis spectra of Triton X-100 treated RBCs were dramatically different from that of native RBCs. DEP responses of RBCs were compared to RBCs treated with low concentrations of Triton X-100 (0.07-0.17 mM) to ascertain surfactant effects on dielectric properties. A star-shaped electrode design was used to quantify RBC dielectric properties by fitting a single-shell oblate cell model to experimentally-derived DEP spectra. The presence of 0.07 and 0.11 mM of Triton X-100 shifted the RBC's DEP spectra yielding lower crossover frequencies ( f C O ) . The single-shell oblate model revealed that cell radius and membrane permittivity are the dominant influencers of DEP spectral shifts. The trends observed were similar for 0.11 mM and 0.07 mM Triton X-100 treated cells. However, a further increase of Triton X-100 to 0.17 mM caused cells to only exhibit negative DEP. The magnitude of the DEP force increased with Triton X-100 concentration. This work indicates that dynamic surfactant interactions with cell membranes alter cell dielectric responses and properties.

Neural responses to electrical stimulation in 2D and 3D in vitro environments

Electrical stimulation (ES) to manipulate the central (CNS) and peripheral nervous system (PNS) has been explored for decades, recently gaining momentum as bioelectronic medicine advances. The application of ES in vitro to modulate a variety of cellular functions, including regenerative potential, migration, and stem cell fate, are being explored to aid neural degeneration, dysfunction, and injury. This review describes the materials and approaches for the application of ES to the PNS and CNS microenvironments, towards an improved understanding of how ES can be harnessed for beneficial clinical applications. Emphasized are some recent advances in ES, including conductive polymers, methods of charge transfer, impact on neural cells, and a brief overview of alternative methodologies for cellular targeting including magneto, ultrasonic, and optogenetic stimulation. This review will examine how heterogenous cell populations, including neurons, glia, and neural stem cells respond to a wide range of conductive 2D and 3D substrates, stimulation regimes, known mechanisms of response, and how cellular sources impact the response to ES.

Identification of neural stem and progenitor cell subpopulations using DC insulator-based dielectrophoresis

Neural stem and progenitor cells (NSPCs) are an extremely important group of cells that form the central nervous system during development and have the potential to repair damage in conditions such as stroke impairment, spinal cord injury and Parkinson's disease degradation. Current schemes for separation of NSPCs are inadequate due to the complexity and diversity of cells in the population and lack sufficient markers to distinguish diverse cell types. This study presents an unbiased high-resolution separation and characterization of NSPC subpopulations using direct current insulator-based dielectrophoresis (DC-iDEP). The properties of the cells were identified by the ratio of electrokinetic (EK) to dielectrophoretic (DEP) mobilities. The ratio factor of NSPCs showed more heterogeneity variance (SD = 3.4-3.9) than the controlled more homogeneous human embryonic kidney cells (SD = 1.1), supporting the presence of distinct subpopulations of cells in NSPC cultures. This measure reflected NSPC fate potential since the ratio factor distribution of more neurogenic populations of NSPCs was distinct from the distribution of astrogenic NSPC populations (confidence level >99.9%). The abundance of NSPCs captured with different ranges of ratio of EK to DEP mobilities also exhibit final fate trends consistent with established final fates of the chosen samples. DC-iDEP is a novel, label-free and non-destructive method for differentiating and characterizing, and potentially separating, neural stem cell subpopulations that differ in fate.

DEP-on-a-Chip: Dielectrophoresis Applied to Microfluidic Platforms

Dielectric particles in a non-uniform electric field are subject to a force caused by a phenomenon called dielectrophoresis (DEP). DEP is a commonly used technique in microfluidics for particle or cell separation. In comparison with other separation methods, DEP has the unique advantage of being label-free, fast, and accurate. It has been widely applied in microfluidics for bio-molecular diagnostics and medical and polymer research. This review introduces the basic theory of DEP, its advantages compared with other separation methods, and its applications in recent years, in particular, focusing on the different electrode types integrated into microfluidic chips, fabrication techniques, and operation principles.

A mathematical model of dielectrophoretic data to connect measurements with cell properties

Dielectrophoresis (DEP) brings about the high-resolution separations of cells and other bioparticles arising from very subtle differences in their properties. However, an unanticipated limitation has arisen: difficulty in assignment of specific biological features which vary between two cell populations. This hampers the ability to interpret the significance of the variations. To realize the opportunities made possible by dielectrophoresis, the data and the diversity of structures found in cells and bioparticles must be linked. While the crossover frequency in DEP has been studied in-depth and exploited in applications using AC fields, less attention has been given when a DC field is present. Here, a new mathematical model of dielectrophoretic data is introduced which connects the physical properties of cells to specific elements of the data from potential- or time-varied DEP experiments. The slope of the data in either analysis is related to the electrokinetic mobility, while the potential at which capture initiates in potential-based analysis is related to both the electrokinetic and dielectrophoretic mobilities. These mobilities can be assigned to cellular properties for which values appear in the literature. Representative examples of high and low values of properties such as conductivity, zeta potential, and surface charge density for bacteria including Streptococcus mutans, Rhodococcus erythropolis, Pasteurella multocida, Escherichia coli, and Staphylococcus aureus are considered. While the many properties of a cell collapse into one or two features of data, for a well-vetted system the model can indicate the extent of dissimilarity. The influence of individual properties on the features of dielectrophoretic data is summarized, allowing for further interpretation of data. Graphical abstract.

Label-free enrichment of primary human skeletal progenitor cells using deterministic lateral displacement

Skeletal stem cells (SSCs) are present in bone marrow (BM) and offer great potential for bone regenerative therapies. However, in the absence of a unique marker, current sorting approaches remain challenging in the quest for simple strategies to deliver SSCs with consistent regeneration and differentiation capacities. Microfluidics offers the possibility to sort cells marker-free, based on intrinsic biophysical properties. Recent studies indicate that SSCs are stiffer than leukocytes and are contained within the larger cell fraction in BM. This paper describes the use of deterministic lateral displacement (DLD) to sort SSCs based on cell size and stiffness. DLD is a technology that uses arrays of micropillars to sort cells based on their diameter. Cell deformation within the device can change the cell size and affect sorting - here evidenced using human cell lines and by fractionation of expanded SSCs. Following sorting, SSCs remained viable and retained their capacity to form clonogenic cultures (CFU-F), indicative of stem cell potential. Additionally, larger BM cells showed enhanced capacity to form CFU-F. These findings support the theory that SSCs are more abundant within the larger BM cell fraction and that DLD, or other size-based approaches, could be used to provide enriched SSC populations with significant implications for stem cell research and translation to the clinic.