The potential of a dielectrophoresis activated cell sorter (DACS) as a next generation cell sorter

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.

Microfluidic device combining hydrodynamic and dielectrophoretic trapping for the controlled contact between single micro-sized objects and application to adhesion assays

The understanding of cell-cell and cell-matrix interactions via receptor and ligand binding relies on our ability to study the very first events of their contact. Of particular interest is the interaction between a T cell receptor and its cognate peptide-major histocompatibility complex. Indeed, analyzing their binding kinetics and cellular avidity in large-scale low-cost and fast cell sorting would largely facilitate the access to cell-based cancer immunotherapies. We thus propose a microfluidic tool able to independently control two types of micro-sized objects, put them in contact for a defined time and probe their adhesion state. The device consists of hydrodynamic traps holding the first type of cell from below against the fluid flow, and a dielectrophoretic system to force the second type of object to remain in contact with the first one. First, the device is validated by performing an adhesion frequency assay between fibroblasts and fibronectin coated beads. Then, a study is conducted on the modification of the cellular environment to match the dielectrophoretic technology requirements without modifying the cell viability and interaction functionalities. Finally, we demonstrate the capability of the developed device to put cancer cells and a population of T cells in contact and show the discrimination between specific and non-specific interactions based on the pair lifetime. This proof-of-concept device lays the foundations for the development of next generation fast cell-cell interaction technologies.

High-precision, low-complexity, high-resolution microscopy-based cell sorting

Continuous flow cell sorting based on image analysis is a powerful concept that exploits spatially-resolved features in cells, such as subcellular protein localisation or cell and organelle morphology, to isolate highly specialised cell types that were previously inaccessible to biomedical research, biotechnology, and medicine. Recently, sorting protocols have been proposed that achieve impressive throughput by combining ultra-high flow rates with sophisticated imaging and data processing protocols. However, moderate image quality and high complex experimental setups still prevent the full potential of image-activated cell sorting from being a general-purpose tool. Here, we present a new low-complexity microfluidic approach based on high numerical aperture wide-field microscopy and precise dielectrophoretic cell handling. It provides high-quality images with unprecedented resolution in image-activated cell sorting (i.e., 216 nm). In addition, it also allows long image processing times of several hundred milliseconds for thorough image analysis, while ensuring reliable and low-loss cell processing. Using our approach, we sorted live T cells based on subcellular localisation of fluorescence signals and demonstrated that purities above 80% are possible while targeting maximum yields and sample volume throughputs in the range of μl min^-1. We were able to recover 85% of the target cells analysed. Finally, we ensure and quantify the full vitality of the sorted cells cultivating the cells for a period of time and through colorimetric viability tests.

Microfluidics for the Isolation and Detection of Circulating Tumor Cells

Nowadays, liquid biopsy represents one of the most promising techniques for early diagnosis, monitoring, and therapy screening of cancer. This novel methodology includes, among other techniques, the isolation, capture, and analysis of circulating tumor cells (CTCs). Nonetheless, the identification of CTC from whole blood is challenging due to their extremely low concentration (1-100 per ml of whole blood), and traditional methods result insufficient in terms of purity, recovery, throughput and/or viability of the processed sample. In this context, the development of microfluidic devices for detecting and isolating CTCs offers a wide range of new opportunities due to their excellent properties for cell manipulation and the advantages to integrate and bring different laboratory processes into the microscale improving the sensitivity, portability, reducing cost and time. This chapter explores current and recent microfluidic approaches that have been developed for the analysis and detection of CTCs, which involve cell capture methods based on affinity binding and label-free methods and detection based on electrical, chemical, and optical sensors. All the exposed technologies seek to overcome the limitations of commercial systems for the analysis and isolation of CTCs, as well as to provide extended analysis that will allow the development of novel and more efficient diagnostic tools.

Foundation of the Manipulation Technology for Tiny Objects Based on the Control of the Heterogeneity of Electric Fields

Effective sorting and extraction of tiny plastic objects is becoming increasingly important for manufacturing high-quality recycled plastics. Herein, we designed a manipulation device for tiny objects that can drive multiple target objects individually. This type of device has a potential to sort tiny pieces of a wide variety of materials, not strongly depending on their physical properties, by combining different detection meanings. In this study, two types of devices were tested as the basic components of the proposed device. One of them had a single object-holding point and the other had two of them. These holding points consisted of strip-shaped electrodes facing each other. The high voltage applied to the facing electrodes created forces heading toward the object-holding points caused by the heterogeneity of the electric field in the devices. The forces created in these devices were determined from the motion analysis of a glass sphere, which is a model for target objects, and a numerical simulation. The results indicate that dielectrophoretic forces are dominant at locations that are sufficiently remote from the holding point, and the Coulombic force caused by dielectric barrier discharge is dominant near the high-voltage electrodes with the holding point. Moreover, the transfer of a glass sphere from one holding point to an adjacent point was successfully demonstrated.

Selective Single-Cell Sorting Using a Multisectorial Electroactive Nanowell Platform

Current approaches in targeted patient treatments often require the rapid isolation of specific rare target cells. Stream-based dielectrophoresis (DEP) based cell sorters have the limitation that the maximum number of sortable cell types is equivalent to the number of output channels, which makes upscaling to a higher number of different cell types technically challenging. Here, we present a microfluidic platform for selective single-cell sorting that bypasses this limitation. The platform consists of 10 000 nanoliter wells which are placed on top of interdigitated electrodes (IDEs) that facilitate dielectrophoresis-driven capture of cells. By use of a multisectorial design formed by 10 individually addressable IDE structures, our platform can capture a large number of different cell types. The sectorial approach allows for fast and straightforward modification to sort complex samples as different cell types are captured in different sectors and therefore removes the need for individual output channels per cell type. Experimental results obtained with a mixed sample of benign (MCF-10A) and malignant (MDA-MB-231) breast cells showed a target to nontarget sorting accuracy of over 95%. We envision that the high accuracy of our platform, in addition to its versatility and simplicity, will aid clinical environments where reliable sorting of varying complex samples is essential.

A flow-through microfluidic chip for continuous dielectrophoretic separation of viable and non-viable human T-cells

Effective methods for rapid sorting of cells according to their viability are critical in T cells based therapies to prevent any risk to patients. In this context, we present a novel microfluidic device that continuously separates viable and non-viable T-cells according to their dielectric properties. A dielectrophoresis (DEP) force is generated by an array of castellated microelectrodes embedded into a microfluidic channel with a single inlet and two outlets; cells subjected to positive DEP forces are drawn toward the electrodes array and leave from the top outlet, those subjected to negative DEP forces are repelled away from the electrodes and leave from the bottom outlet. Computational fluid dynamics is used to predict the device separation efficacy, according to the applied alternative current (AC) frequency, at which the cells move from/to a negative/positive DEP region and the ionic strength of the suspension medium. The model is used to support the design of the operational conditions, confirming a separation efficiency, in terms of purity, of 96% under an applied AC frequency of 1.5 × 10⁶  Hz and a flow rate of 20 μl/h. This work represents the first example of effective continuous sorting of viable and non-viable human T-cells in a single-inlet microfluidic chip, paving the way for lab-on-a-chip applications at the point of need.

A technology of a different sort: microraft arrays

A common procedure performed throughout biomedical research is the selection and isolation of biological entities such as organelles, cells and organoids from a mixed population. In this review, we describe the development and application of microraft arrays, an analysis and isolation platform which enables a vast range of criteria and strategies to be used when separating biological entities. The microraft arrays are comprised of elastomeric microwells with detachable polymer bases (microrafts) that act as capture and culture sites as well as supporting carriers during cell isolation. The technology is elegant in its simplicity and can be implemented for samples possessing tens to millions of objects yielding a flexible platform for applications such as single-cell RNA sequencing, subcellular organelle capture and assay, high-throughput screening and development of CRISPR gene-edited cell lines, and organoid manipulation and selection. The transparent arrays are compatible with a multitude of imaging modalities enabling selection based on 2D or 3D spatial phenotypes or temporal properties. Each microraft can be individually isolated on demand with retention of high viability due to the near zero hydrodynamic stress imposed upon the cells during microraft release, capture and deposition. The platform has been utilized as a simple manual add-on to a standard microscope or incorporated into fully automated instruments that implement state-of-the-art imaging algorithms and machine learning. The vast array of selection criteria enables separations not possible with conventional sorting methods, thus garnering widespread interest in the biological and pharmaceutical sciences.

Game-Changing Approaches in Sperm Sex-Sorting: Microfluidics and Nanotechnology

Simple Summary

Sexing of sperm cells, including the capacity to preselect the sex of offspring prior to reproduction, has been a major target of reproductive biotechnology for a very long time. The advances in molecular biology, biophysics, and computer science over the past few decades, as well as the groundbreaking new methods introduced by scientists, have contributed to some major breakthroughs in a variety of branches of medicine. In particular, assisted reproduction is one of the areas in which emerging technologies such as nanotechnology and microfluidics may enhance the fertility potential of samples of sex-sorted semen, thus improving the reproductive management of farm animals and conservation programs. In human medicine, embryo sex-selection using in vitro fertilization (IVF) and preimplantation genetic testing (PGT) is accepted only for medical reasons. Using sex-sorting before IVF would enable specialists to prevent sex-linked genetic diseases and prevent the discharge of embryos which are not suitable for transfer due to their sex.

Abstract

The utilization of sex-sorted sperm for artificial insemination and in-vitro fertilization is considered a valuable tool for improving production efficiency and optimizing reproductive management in farm animals, subsequently ensuring sufficient food resource for the growing human population. Despite the fact that sperm sex-sorting is one of the most intense studied technologies and notable progress have been made in the past three decades to optimize it, the conception rates when using sex-sorted semen are still under expectations. Assisted reproduction programs may benefit from the use of emergent nano and microfluidic-based technologies. This article addresses the currently used methods for sperm sex-sorting, as well as the emerging ones, based on nanotechnology and microfluidics emphasizing on their practical and economic applicability.

On the design, functions, and biomedical applications of high-throughput dielectrophoretic micro-/nanoplatforms: a review

As an efficient, rapid and label-free micro-/nanoparticle separation technique, dielectrophoresis (DEP) has attracted widespread attention in recent years, especially in the field of biomedicine, which exhibits huge potential in biomedically relevant applications such as disease diagnosis, cancer cell screening, biosensing, and others. DEP technology has been greatly developed recently from the low-flux laboratory level to high-throughput practical applications. In this review, we summarize the recent progress of DEP technology in biomedical applications, including firstly the design of various types and materials of DEP electrode and flow channel, design of input signals, and other improved designs. Then, functional tailoring of DEP systems with endowed specific functions including separation, purification, capture, enrichment and connection of biosamples, as well as the integration of multifunctions, are demonstrated. After that, representative DEP biomedical application examples in aspects of disease detection, drug synthesis and screening, biosensing and cell positioning are presented. Finally, limitations of existing DEP platforms on biomedical application are discussed, in which emphasis is given to the impact of other electrodynamic effects such as electrophoresis (EP), electroosmosis (EO) and electrothermal (ET) effects on DEP efficiency. This article aims to provide new ideas for the design of novel DEP micro-/nanoplatforms with desirable high throughput toward application in the biomedical community.

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.

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.

Separation, Characterization, and Handling of Microalgae by Dielectrophoresis

Microalgae biotechnology has a high potential for sustainable bioproduction of diverse high-value biomolecules. Some of the main bottlenecks in cell-based bioproduction, and more specifically in microalgae-based bioproduction, are due to insufficient methods for rapid and efficient cell characterization, which contributes to having only a few industrially established microalgal species in commercial use. Dielectrophoresis-based microfluidic devices have been long established as promising tools for label-free handling, characterization, and separation of broad ranges of cells. The technique is based on differences in dielectric properties and sizes, which results in different degrees of cell movement under an applied inhomogeneous electrical field. The method has also earned interest for separating microalgae based on their intrinsic properties, since their dielectric properties may significantly change during bioproduction, in particular for lipid-producing species. Here, we provide a comprehensive review of dielectrophoresis-based microfluidic devices that are used for handling, characterization, and separation of microalgae. Additionally, we provide a perspective on related areas of research in cell-based bioproduction that can benefit from dielectrophoresis-based microdevices. This work provides key information that will be useful for microalgae researchers to decide whether dielectrophoresis and which method is most suitable for their particular application.

Continuous dielectrophoretic particle separation via isomotive dielectrophoresis with bifurcating stagnation flow

We present a novel technique for continuous label-free separation of particles based on their dielectrophoretic crossover frequencies. Our technique relies on our unique microfluidic geometry which performs hydrodynamic focusing, generates a stagnation flow with two outlets, and simultaneously produces an isomotive dielectrophoretic field via wall-situated electrodes. To perform particle separation, we hydrodynamically focus particles onto stagnation streamlines and use isomotive dielectrophoretic force to nudge the particles off these streamlines and direct them into appropriate outlets. Focusing particles onto stagnation streamlines obviates the need for large forces to be applied to the particles and therefore increases system throughput. The use of isomotive (spatially uniform) dielectrophoretic force increases system reliability. To guide designers, we develop and describe a simple scaling model for the particle separation dynamics of our technique. The model predicts the range of particle sizes that can be separated as well as the processing rate that can be achieved as a function of system design parameters: channel size, flow rate, and applied potential. Finally, as a proof-of-principle, we use this technique to separate polystyrene bead and cell mixtures of the same diameters as well as mixtures of both particles with varying diameters.