Clinically relevant microfluidic magnetophoretic isolation of rare-cell populations for diagnostic and therapeutic monitoring applications

Microfluidic technology for cell biology-related applications: a review

Fluid flow at the microscale level exhibits a unique phenomenon that can be explored to fabricate microfluidic devices integrated with components that can perform various biological functions. In this manuscript, the importance of physics for microscale fluid dynamics using microfluidic devices has been reviewed. Microfluidic devices provide new opportunities with regard to spatial and temporal control over cell growth. Furthermore, the manuscript presents an overview of cellular stimuli observed by combining surfaces that mimic the complex biochemistries and different geometries of the extracellular matrix, with microfluidic channels regulating the transport of fluids, soluble factors, etc. We have also explained the concept of mechanotransduction, which defines the relation between mechanical force and biological response. Furthermore, the manipulation of cellular microenvironments by the use of microfluidic systems has been highlighted as a useful device for basic cell biology research activities. Finally, the article focuses on highly integrated microfluidic platforms that exhibit immense potential for biomedical and pharmaceutical research as robust and portable point-of-care diagnostic devices for the assessment of clinical samples.

Scaling up stem cell production: harnessing the potential of microfluidic devices

Stem cells are specialised cells characterised by their unique ability to both self-renew and transform into a wide array of specialised cell types. The widespread interest in stem cells for regenerative medicine and cultivated meat has led to a significant demand for these cells in both research and practical applications. Despite the growing need for stem cell manufacturing, the industry faces significant obstacles, including high costs for equipment and maintenance, complicated operation, and low product quality and yield. Microfluidic technology presents a promising solution to the abovementioned challenges. As an innovative approach for manipulating liquids and cells within microchannels, microfluidics offers a plethora of advantages at an industrial scale. These benefits encompass low setup costs, ease of operation and multiplexing, minimal energy consumption, and the added advantage of being labour-free. This review presents a thorough examination of the prominent microfluidic technologies employed in stem cell research and explores their promising applications in the burgeoning stem cell industry. It thoroughly examines how microfluidics can enhance cell harvesting from tissue samples, facilitate mixing and cryopreservation, streamline microcarrier production, and efficiently conduct cell separation, purification, washing, and final cell formulation post-culture.

A Review of Single-Cell Microrobots: Classification, Driving Methods and Applications

Single-cell microrobots are new microartificial devices that use a combination of single cells and artificial devices, with the advantages of small size, easy degradation and ease of manufacture. With externally driven strategies such as light fields, sound fields and magnetic fields, microrobots are able to carry out precise micromanipulations and movements in complex microenvironments. Therefore, single-cell microrobots have received more and more attention and have been greatly developed in recent years. In this paper, we review the main classifications, control methods and recent advances in the field of single-cell microrobot applications. First, different types of robots, such as cell-based microrobots, bacteria-based microrobots, algae-based microrobots, etc., and their design strategies and fabrication processes are discussed separately. Next, three types of external field-driven technologies, optical, acoustic and magnetic, are presented and operations realized in vivo and in vitro by applying these three technologies are described. Subsequently, the results achieved by these robots in the fields of precise delivery, minimally invasive therapy are analyzed. Finally, a short summary is given and current challenges and future work on microbial-based robotics are discussed.

Recent advances for cancer detection and treatment by microfluidic technology, review and update

Numerous cancer-associated deaths are owing to a lack of effective diagnostic and therapeutic approaches. Microfluidic systems for analyzing a low volume of samples offer a precise, quick, and user-friendly technique for cancer diagnosis and treatment. Microfluidic devices can detect many cancer-diagnostic factors from biological fluids and also generate appropriate nanoparticles for drug delivery. Thus, microfluidics may be valuable in the cancer field due to its high sensitivity, high throughput, and low cost. In the present article, we aim to review recent achievements in the application of microfluidic systems for the diagnosis and treatment of various cancers. Although microfluidic platforms are not yet used in the clinic, they are expected to become the main technology for cancer diagnosis and treatment. Microfluidic systems are proving to be more sensitive and accurate for the detection of cancer biomarkers and therapeutic strategies than common assays. Microfluidic lab-on-a-chip platforms have shown remarkable potential in the designing of novel procedures for cancer detection, therapy, and disease follow-up as well as the development of new drug delivery systems for cancer treatment.

Multiphysics microfluidics for cell manipulation and separation: a review

Multiphysics microfluidics, which combines multiple functional physical processes in a microfluidics platform, is an emerging research area that has attracted increasing interest for diverse biomedical applications. Multiphysics microfluidics is expected to overcome the limitations of individual physical phenomena through combining their advantages. Furthermore, multiphysics microfluidics is superior for cell manipulation due to its high precision, better sensitivity, real-time tunability, and multi-target sorting capabilities. These exciting features motivate us to review this state-of-the-art field and reassess the feasibility of coupling multiple physical processes. To confine the scope of this paper, we mainly focus on five common forces in microfluidics: inertial lift, elastic, dielectrophoresis (DEP), magnetophoresis (MP), and acoustic forces. This review first explains the working mechanisms of single physical phenomena. Next, we classify multiphysics techniques in terms of cascaded connections and physical coupling, and we elaborate on combinations of designs and working mechanisms in systems reported in the literature to date. Finally, we discuss the possibility of combining multiple physical processes and associated design schemes and propose several promising future directions.

The Origins and the Current Applications of Microfluidics-Based Magnetic Cell Separation Technologies

The magnetic separation of cells based on certain traits has a wide range of applications in microbiology, immunology, oncology, and hematology. Compared to bulk separation, performing magnetophoresis at micro scale presents advantages such as precise control of the environment, larger magnetic gradients in miniaturized dimensions, operational simplicity, system portability, high-throughput analysis, and lower costs. Since the first integration of magnetophoresis and microfluidics, many different approaches have been proposed to magnetically separate cells from suspensions at the micro scale. This review paper aims to provide an overview of the origins of microfluidic devices for magnetic cell separation and the recent technologies and applications grouped by the targeted cell types. For each application, exemplary experimental methods and results are discussed.

Simulation and fabrication of an integrating well-aligned silicon nanowires substrate for trapping circulating tumor cells labeled with Fe3O4 nanoparticles in a microfluidic device

Introduction: Circulating tumor cells (CTCs) are the transformed tumor cells that can penetrate into the bloodstream and are available at concentrations as low as 1-100 cells per milliliter. To trap CTCs in the blood, one valid and mature technique that has been developed is the magnetophoresis-based separation in a microfluidic channel. Recently, nanostructured platforms have also been developed to trap specific targeted and marker cells in the blood. We aimed to integrate both in one platform to improve trapping. Methods: Here, we developed a numerical scheme and an integrated device that considered the interaction between drag and magnetic forces on paramagnetic labeled cells in the fluid as well as interaction of these two forces with the adhesive force and the surface friction of the nanowires substrate. We aimed on developing a more advanced technique that integrated the magnetophoretic property of some Fe₃O₄ paramagnetic nanoparticles (PMNPs) with a silicon nanowires (SiNWs) substrate in a microfluidic device to trap MDA-MB231 cell lines as CTCs in the blood. Results: Simulation indicated assuming that the nanoparticles adhere perfectly to the white blood cells (WBCs) and the CTCs, the magnetic moment of the CTCs was almost one order of magnitude larger than that of the WBCs, so its attraction by the magnetic field was much higher. In general with significant statistics, the integrated device can trap almost all of the CTCs on the SiNWs substrate. In the experimental section, we took advantage of the integrated trapping techniques, including micropost barriers, magnetophoresis, and nanowires-based substrate to more effectively isolate the CTCs. Conclusion: The simulation indicated that the proposed device could almost trap all of the CTCs onto the SiNWs substrate, whereas trapping in flat substrates with magnetophoretic force was very low. As a result of the magnetic field gradient, magnetophoretic force was applied to the cells through the nanoparticles, which would efficiently drive down the nanoparticle-tagged cells. For the experimental validation, anti-EpCAM antibodies for specific binding to tumor cells were used. Using this specific targeting method and by statistically counting, it was shown that the proposed technique has excellent performance and results in the trapping efficiency of above 90%.

Sheathless acoustic based flow cell sorter for enrichment of rare cells

Cell enrichment is a powerful tool in many kinds of cell research, especially in applications with low abundance cell types. In this work, we developed a microfluidic fluorescence activated cell sorting device that was able to perform on-demand, low loss cell detection, and sorting. The chip utilizes three-dimensional acoustic standing waves to position all cells in the same fluid velocity regime without sheath. When the cells pass through a laser interrogation region, the scattering and fluorescent signals are detected, translated and transported to software. The target cells are then identified by gating on the plots. Short bursts of standing acoustic waves are triggered by order from PC to sort target cells within predefined gating region. For very low abundance and rare labeled lymphocytes mixed with high concentration unlabeled white blood cells (WBCs), (1-100 labeled lymphocytes are diluted in 10⁶ WBCs in 1 ml volume fluid), the device is able to remove more than 98% WBCs and recover labeled lymphocytes with efficiency of 80%. We further demonstrated that this device worked with real clinical samples by successfully isolating fetal nucleated red blood cells (FNRBCs) in the blood samples from pregnant women with male fetus. The obtained cells were sequenced and the expressions of (sex determining region Y) SRY genes were tested to determine fetal cell proportion. In genetic analysis, the proportion of fetal cells in the final picked sample is up to 40.64%. With this ability, the device proposed could be valuable for biomedical applications involving fetal cells, circulating tumor cells, and stem cells.

Surface acoustic wave (SAW) techniques in tissue engineering

Recently, the introduction of surface acoustic wave (SAW) technique for microfluidics has drawn a lot of attention. The pattern and mutual communication in cell layers, tissues, and organs play a critical role in tissue homeostasis and regeneration and may contribute to disease occurrence and progression. Tissue engineering aims to repair and regenerate damaged organs, depending on biomimetic scaffolds and advanced fabrication technology. However, traditional bioengineering synthesis approaches are time-consuming, heterogeneous, and unmanageable. It is hard to pattern cells in scaffolds effectively with no impact on cell viability and function. Here, we summarize a biocompatible, easily available, label-free, and non-invasive tool, surface acoustic wave (SAW) technique, which is getting a lot of attention in tissue engineering. SAW technique can realize accurate sorting, manipulation, and cells' pattern and rapid formation of spheroids. By integrating several SAW devices onto lab-on-a-chip platforms, tissue engineering lab-on-a-chip system was proposed. To the best of our knowledge, this is the first report to summarize the application of this novel technique in the field of tissue engineering.

Design of a Hybrid Inertial and Magnetophoretic Microfluidic Device for CTCs Separation from Blood

Circulating tumor cells (CTCs) isolation from a blood sample plays an important role in cancer diagnosis and treatment. Microfluidics offers a great potential for cancer cell separation from the blood. Among the microfluidic-based methods for CTC separation, the inertial method as a passive method and magnetic method as an active method are two efficient well-established methods. Here, we investigated the combination of these two methods to separate CTCs from a blood sample in a single chip. Firstly, numerical simulations were performed to analyze the fluid flow within the proposed channel, and the particle trajectories within the inertial cell separation unit were investigated to determine/predict the particle trajectories within the inertial channel in the presence of fluid dynamic forces. Then, the designed device was fabricated using the soft-lithography technique. Later, the CTCs were conjugated with magnetic nanoparticles and Ep-CAM antibodies to improve the magnetic susceptibility of the cells in the presence of a magnetic field by using neodymium permanent magnets of 0.51 T. A diluted blood sample containing nanoparticle-conjugated CTCs was injected into the device at different flow rates to analyze its performance. It was found that the flow rate of 1000 µL/min resulted in the highest recovery rate and purity of ~95% and ~93% for CTCs, respectively.

Automated fluorescence quantification of extracellular vesicles collected from blood plasma using dielectrophoresis

Tumor-secreted exosomes and other extracellular vesicles (EVs) in circulation contain valuable biomarkers for early cancer detection and screening. We have previously demonstrated collection of cancer-derived nanoparticles (NPs) directly from whole blood and plasma with a chip-based technique that uses a microelectrode array to generate dielectrophoretic (DEP) forces. This technique enables direct recovery of NPs from whole blood and plasma. The biomarker payloads associated with collected particles can be detected and quantified with immunostaining. Accurately separating the fluorescence intensity of stained biomarkers from background (BG) levels becomes a challenge when analyzing the blood from early-stage cancer patients in which biomarker concentrations are low. To address this challenge, we developed two complementary techniques to standardize the quantification of fluorescently immunolabeled biomarkers collected and concentrated at predictable locations within microfluidic chips. The first technique was an automated algorithm for the quantitative analysis of fluorescence intensity at collection regions within the chip compared to levels at adjacent regions. The algorithm used predictable locations of particle collection within the chip geometry to differentiate regions of collection and BG. We successfully automated the identification and removal of optical artifacts from quantitative calculations. We demonstrated that the automated system performs nearly the same as a human user following a standard protocol for manual artifact removal with Pearson's r-values of 0.999 and 0.998 for two different biomarkers (n = 36 patients). We defined a usable dynamic range of fluorescence intensities corresponding to 1 to 2000 arbitrary units (a.u.). Fluorescence intensities within the dynamic range increased linearly with respect to exposure time and particle concentration. The second technique was the implementation of an internal standard to adjust levels of biomarker fluorescence based on the relative collection efficiency of the chip. Use of the internal standard reduced variability in measured biomarker levels due to differences in chip-to-chip collection efficiency, especially at low biomarker concentrations. The internal standard did not affect linear trends between fluorescence intensity and exposure time. Adjustments using the internal standard improved linear trends between fluorescence intensity and particle concentration. The optical quantification techniques described in this paper can be easily adapted for other lab-on-a-chip platforms that have predefined regions of biomarker or particle collection and that rely on fluorescence detection.

Enrichment and ratiometric detection of circulating tumor cells using PSMA- and folate receptor-targeted magnetic and surface-enhanced Raman scattering nanoparticles

The presence of circulating tumor cells (CTCs) in a patient's bloodstream is a hallmark of metastatic cancer. The detection and analysis of CTCs is a promising diagnostic and prognostic strategy as they may carry useful genetic information from their derived primary tumor, and the enumeration of CTCs in the bloodstream has been known to scale with disease progression. However, the detection of CTCs is a highly challenging task owing to their sparse numbers in a background of billions of background blood cells. To effectively utilize CTCs, there is a need for an assay that can detect CTCs with high specificity and can locally enrich CTCs from a liquid biopsy. We demonstrate a versatile methodology that addresses these needs by utilizing a combination of nanoparticles. Enrichment is achieved using targeted magnetic nanoparticles and high specificity detection is achieved using a ratiometric detection approach utilizing multiplexed targeted and non-targeted surface-enhanced Raman Scattering Nanoparticles (SERS-NPs). We demonstrate this approach with model prostate and cervical circulating tumor cells and show the ex vivo utility of our methodology for the detection of PSMA or folate receptor over-expressing CTCs. Our approach allows for the mitigation of interference caused by the non-specific uptake of nanoparticles by other cells present in the bloodstream and our results from magnetically trapped CTCs reveal over a 2000% increase in targeted SERS-NP signal over non-specifically bound SERS-NPs.

On-chip analysis of magnetically labeled cells with integrated cell sorting and counting techniques

In studies on cell therapies, cells often need to be magnetically labeled and then tracked using magnetic resonance imaging (MRI) techniques. To achieve good imaging performance on infused cells, the analysis of the sorted, labeled cells before infusion is necessary. Herein, we developed a microfluidic chip to quantitatively analyze magnetically labeled cells. The chip was equipped with a magnetophoresis-based cell sorting function and an impedance-based cell counting function. Using RAW264.7 macrophages, we confirmed the two functions of the chip, obtained the number and the magnetic loading distribution of the sorted, labeled cells, and ultimately demonstrated the broad applications of the chip in rapidly selecting a proper flow rate for the buffer solution in the cell sorting process, determining the total average magnetic loading of the labeled cells for the cell labeling process, and offering a necessary reference for the processing of the sorted cells for high performance in vivo imaging. This work provides an integrated lab-on-a-chip design for quantitatively analyzing magnetically labeled cells and thus can promote MRI-based cell-tracking studies.

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.

Dynamic Halbach array magnet integrated microfluidic system for the continuous-flow separation of rare tumor cells

Circulating tumor cells (CTCs), the most representative rare cells in peripheral blood, have received great attention due to their clinical utility in liquid biopsy. The downstream analysis of intact CTCs isolated from peripheral blood provides important clinical information for personalized medicine. However, current CTC isolation and detection methods have been challenged by their extreme rarity and heterogeneity. In this study, we developed a novel microfluidic system with a continuously moving Halbach array magnet (dHAMI microfluidic system) for negative isolation CTCs from whole blood, which aimed to capture non-target white blood cells (WBCs) and elute target CTCs. The dynamic and continuous movement of the Halbach array magnet generated a continuous magnetic force acting on the magnetic bead-labelled WBCs in the continuous-flow fluid to negatively exclude the WBCs from the CTCs. Furthermore, the continuously moving magnetic field effectively eliminated the effect of magnetic bead aggregation on the fluid flow to realize the continuous-flow separation of the CTCs without a sample loading volume limitation. The experimental procedure for CTC negative isolation using the dHAMI microfluidic system could be completed within 40 min. Under the optimized experimental conditions of the dHAMI microfluidic system, including the flow rate and concentration of the immunomagnetic bead, the average CTC capture rate over a range of spiked cell numbers (50–1000 cancer cells per mL) was up to 91.6% at a flow rate of 100 μL min⁻¹. Finally, the CTCs were successfully detected in 10 of 10 (100%) blood samples from patients with cancer. Therefore, the dHAMI microfluidic system could effectively isolate intact and heterogeneous CTCs for downstream cellular and molecular analyses, and this robust microfluidic platform with an excellent magnetic manipulation performance also has great application potential for the separation of other rare cells.

We develop a dynamic Halbach array magnet integrated microfluidic system for continuous-flow separation of circulating tumor cells from whole blood.

Microfluidic technologies for circulating tumor cell isolation

Metastasis is the main cause of tumor-related death, and the dispersal of tumor cells through the circulatory system is a critical step in the metastatic process. Early detection and analysis of circulating tumor cells (CTCs) is therefore important for early diagnosis, prognosis, and effective treatment of cancer, enabling favorable clinical outcomes in cancer patients. Accurate and reliable methods for isolating and detecting CTCs are necessary to obtain this clinical information. Over the past two decades, microfluidic technologies have demonstrated great potential for isolating and detecting CTCs from blood. The present paper reviews current advanced microfluidic technologies for isolating CTCs based on various biological and physical principles, and discusses their fundamental advantages and drawbacks for subsequent cellular and molecular assays. Owing to significant genetic heterogeneity among CTCs, microfluidic technologies for isolating individual CTCs have recently been developed. We discuss these single-cell isolation methods, as well as approaches to overcoming the limitations of current microfluidic CTC isolation technologies. Finally, we provide an overview of future innovative microfluidic platforms.

Magnetic Force-Based Microfluidic Techniques for Cellular and Tissue Bioengineering

Live cell manipulation is an important biotechnological tool for cellular and tissue level bioengineering applications due to its capacity for guiding cells for separation, isolation, concentration, and patterning. Magnetic force-based cell manipulation methods offer several advantages, such as low adverse effects on cell viability and low interference with the cellular environment. Furthermore, magnetic-based operations can be readily combined with microfluidic principles by precisely allowing control over the spatiotemporal distribution of physical and chemical factors for cell manipulation. In this review, we present recent applications of magnetic force-based cell manipulation in cellular and tissue bioengineering with an emphasis on applications with microfluidic components. Following an introduction of the theoretical background of magnetic manipulation, components of magnetic force-based cell manipulation systems are described. Thereafter, different applications, including separation of certain cell fractions, enrichment of rare cells, and guidance of cells into specific macro- or micro-arrangements to mimic natural cell organization and function, are explained. Finally, we discuss the current challenges and limitations of magnetic cell manipulation technologies in microfluidic devices with an outlook on future developments in the field.

Acoustic impedance-based size-independent isolation of circulating tumour cells from blood using acoustophoresis

Label-free isolation of CTCs from blood is critical for the development of diagnostic and prognostic tools for cancer. Here, we report a label-free method based on acoustic impedance contrast for the isolation of CTCs from peripheral blood mononuclear cells (PBMCs) in a microchannel using acoustophoresis. We describe a method in which the acoustophoretic migration of PBMCs is arrested by matching their acoustic impedance with that of the sample medium, and CTCs that have different acoustic impedance compared to PBMCs migrate toward the pressure node or antinode and thus become isolated. We show that acoustic streaming which can adversely affect the CTC isolation is suppressed owing to the inhomogeneous liquid flow configuration. We establish a method for isolation of CTCs that have higher or lower acoustic impedance compared to PBMCs by controlling the acoustic impedance contrast of the liquids across the channel. Applying this method, we demonstrate label-free isolation of HeLa and MDA-MB-231 cells from PBMCs (collected from 2.0 mL of blood) within one hour yielding a recovery of >86% and >50-fold enrichment. Combined impedance and size-based sorting is proposed as a promising tool for the effective isolation of CTCs from blood.

Recent advances and current challenges in magnetophoresis based micro magnetofluidics

The combination of magnetism and microscale fluid flow has opened up a new era for handling and manipulation of samples in microfluidics. In particular, magnetophoresis, the migration of particles in a magnetic field, is extremely attractive for microfluidic handling due to its contactless nature, independence of ionic concentration, and lack of induced heating. The present paper focuses on recent advances and current challenges of magnetophoresis and highlights the key parameters affecting the manipulation of particles by magnetophoresis. The magnetic field is discussed according to their relative motion to the sample as stationary and dynamic fields. The migration of particles is categorized as positive and negative magnetophoresis. The applications of magnetophoresis are discussed according to the basic manipulation tasks such as mixing, separation, and trapping of particles or cells. Finally, the paper highlights the limitations of current approaches and provides the future perspective for this research area.

Dual-Selective and Dual-Enhanced SERS Nanoprobes Strategy for Circulating Hepatocellular Carcinoma Cells Detection

The detection of hepatocellular carcinoma (HCC) circulating tumor cells (CTCs) from a blood sample can be a very powerful noninvasive approach for the early detection and therapy of liver cancer. However, the extreme rarity of tumor cells in blood containing billions of other cells makes the capture and identification of CTCs with sufficient sensitivity and specificity a real challenge. Here, a magnetically assisted surface-enhanced Raman scattering (SERS) biosensor for HCC CTC detection is reported for the first time. The biosensor consists of two basic elements: anti-ASGPR antibody-Fe₃ O₄ @Ag magnetic nanoparticles and anti-GPC3 antibody-Au@Ag@DTNB nanorods. According to the dual-selectivity of the anti-ASGPR and anti-GPC3 antibodies and the dual-enhancement SERS signal of the MNPs silver shell and the Au@Ag NRs SERS tags, a limit of detection of 1 cell mL^-1 for HCC CTC in human peripheral blood samples with a linear relationship from 1 to 100 cells mL^-1 can be obtained. The system shows good performance in real serum, which suggests it may be a promising tool for HCC clinical diagnosis.

Hydrodynamics in Cell Studies

Hydrodynamic phenomena are ubiquitous in living organisms and can be used to manipulate cells or emulate physiological microenvironments experienced in vivo. Hydrodynamic effects influence multiple cellular properties and processes, including cell morphology, intracellular processes, cell–cell signaling cascades and reaction kinetics, and play an important role at the single-cell, multicellular, and organ level. Selected hydrodynamic effects can also be leveraged to control mechanical stresses, analyte transport, as well as local temperature within cellular microenvironments. With a better understanding of fluid mechanics at the micrometer-length scale and the advent of microfluidic technologies, a new generation of experimental tools that provide control over cellular microenvironments and emulate physiological conditions with exquisite accuracy is now emerging. Accordingly, we believe that it is timely to assess the concepts underlying hydrodynamic control of cellular microenvironments and their applications and provide some perspective on the future of such tools in in vitro cell-culture models. Generally, we describe the interplay between living cells, hydrodynamic stressors, and fluid flow-induced effects imposed on the cells. This interplay results in a broad range of chemical, biological, and physical phenomena in and around cells. More specifically, we describe and formulate the underlying physics of hydrodynamic phenomena affecting both adhered and suspended cells. Moreover, we provide an overview of representative studies that leverage hydrodynamic effects in the context of single-cell studies within microfluidic systems.

The power of solid supports in multiphase and droplet-based microfluidics: towards clinical applications

Multiphase and droplet microfluidic systems are growing in relevance in bioanalytical-related fields, especially due to the increased sensitivity, faster reaction times and lower sample/reagent consumption of many of its derived bioassays. Often applied to homogeneous (liquid/liquid) reactions, innovative strategies for the implementation of heterogeneous (typically solid/liquid) processes have recently been proposed. These involve, for example, the extraction and purification of target analytes from complex matrices or the implementation of multi-step protocols requiring efficient washing steps. To achieve this, solid supports such as functionalized particles (micro or nanometric) presenting different physical properties (e.g. magnetic, optical or others) are used for the binding of specific entities. The manipulation of such supports with different microfluidic principles has both led to the miniaturization of existing biomedical protocols and the development of completely new strategies for diagnostics and research. In this review, multiphase and droplet-based microfluidic systems using solid suspensions are presented and discussed with a particular focus on: i) working principles and technological developments of the manipulation strategies and ii) applications, critically discussing the level of maturity of these systems, which can range from initial proofs of concept to real clinical validations.

Fundamentals of affinity cell separations

Cell separations using affinity methods continue to be an enabling science for a wide variety of applications. In this review, we discuss the fundamental aspects of affinity separation, including the competing forces for cell capture and elution, cell-surface interactions, and models for cell adhesion. Factors affecting separation performance such as bond affinity, contact area, and temperature are presented. We also discuss and demonstrate the effects of nonspecific binding on separation performance. Metrics for evaluating cell separations are presented, along with methods of comparing separation techniques for cell isolation using affinity capture.

Label-free ferrohydrodynamic cell separation of circulating tumor cells

Circulating tumor cells (CTCs) have significant implications in both basic cancer research and clinical applications. To address the limited availability of viable CTCs for fundamental and clinical investigations, effective separation of extremely rare CTCs from blood is critical. Ferrohydrodynamic cell separation (FCS), a label-free method that conducted cell sorting based on cell size difference in biocompatible ferrofluids, has thus far not been able to enrich low-concentration CTCs from cancer patients' blood because of technical challenges associated with processing clinical samples. In this study, we demonstrated the development of a laminar-flow microfluidic FCS device that was capable of enriching rare CTCs from patients' blood in a biocompatible manner with a high throughput (6 mL h^-1) and a high rate of recovery (92.9%). Systematic optimization of the FCS devices through a validated analytical model was performed to determine optimal magnetic field and its gradient, ferrofluid properties, and cell throughput that could process clinically relevant amount of blood. We first validated the capability of the FCS devices by successfully separating low-concentration (∼100 cells per mL) cancer cells using six cultured cell lines from undiluted white blood cells (WBCs), with an average 92.9% cancer cell recovery rate and an average 11.7% purity of separated cancer cells, at a throughput of 6 mL per hour. Specifically, at ∼100 cancer cells per mL spike ratio, the recovery rates of cancer cells were 92.3 ± 3.6% (H1299 lung cancer), 88.3 ± 5.5% (A549 lung cancer), 93.7 ± 5.5% (H3122 lung cancer), 95.3 ± 6.0% (PC-3 prostate cancer), 94.7 ± 4.0% (MCF-7 breast cancer), and 93.0 ± 5.3% (HCC1806 breast cancer), and the corresponding purities of separated cancer cells were 11.1 ± 1.2% (H1299 lung cancer), 10.1 ± 1.7% (A549 lung cancer), 12.1 ± 2.1% (H3122 lung cancer), 12.8 ± 1.6% (PC-3 prostate cancer), 11.9 ± 1.8% (MCF-7 breast cancer), and 12.2 ± 1.6% (HCC1806 breast cancer). Biocompatibility study on H1299 cell line and HCC1806 cell line showed that separated cancer cells had excellent short-term viability, normal proliferation and unaffected key biomarker expressions. We then demonstrated the enrichment of CTCs in blood samples obtained from two patients with newly diagnosed advanced non-small cell lung cancer (NSCLC). While still at its early stage of development, FCS could become a complementary tool for CTC separation for its high recovery rate and excellent biocompatibility, as well as its potential for further optimization and integration with other separation methods.

Transferrin-navigation Nano Artificial Antibody Fluorescence Recognition of Circulating Tumor Cells

Specific recognition of circulating tumor cells (CTCs) is of great significance for cancer diagnosis and personalized therapy. The antibodies and aptamer are commonly used for recognition of CTCs, but they often suffer from low stability and high cost. Therefore, chemically stable and low-cost artificial recognition elements are still highly demanded. Herein, we prepared nano artificial antibody based on molecular imprinting and applied for fluorescence recognition of CTCs. Surface imprinting was employed to construct a transferrin (TRA)-imprinted layer on the surface of rhodamine doped silica nanoparticles. Take advantage of the specific interaction between TRA and TRA receptor (overexpressed on cancer cells), the as-prepared TRA-imprinted artificial antibody was allowed for specific targeting cancer cells mediated by TRA. And the average recognition efficiency of the artificial antibody for the cancer cells was 88% through flow cytometry. Finally, the nano artificial antibody was successfully applied to specific identify mimetic CTCs, under the same conditions, the recognition ability of artificial antibody for CTCs was 8 times higher than the white blood cells.

An integrated experimental and economic evaluation of cell therapy affinity purification technologies

Aim: To present an integrated techno-economic analysis assessing the feasibility of affinity purification technologies using the manufacture of induced pluripotent stem cell-derived progenitor photoreceptors for retinal dystrophies as a case study. Materials & methods: Sort purity, progenitor yield and viable cell recovery were investigated for three cell sorting techniques: fluorescent-activated cell sorting (FACS); magnetic-activated cell sorting (MACS); and a novel technology SpheriTech beads. Experimentally derived metrics were incorporated into an advanced bioprocess economics tool to determine cost of goods per dose for each technology. Results & conclusion: Technical and bioprocess benefits were noted with SpheriTech beads which, unlike FACS and MACS, require no cell labeling. This simplifies the bioprocess, reduces cell loss and leaves target cells label free. The economic tool predicted cost drivers and a critical dose (7 × 107 cells per dose) shifting the most cost-effective technology from FACS to MACS. Process optimization is required for SpheriTech to compete economically.

Microfluidic approaches for isolation, detection, and characterization of extracellular vesicles: Current status and future directions

Extracellular vesicles (EVs) are cell-derived vesicles present in body fluids that play an essential role in various cellular processes, such as intercellular communication, inflammation, cellular homeostasis, survival, transport, and regeneration. Their isolation and analysis from body fluids have a great clinical potential to provide information on a variety of disease states such as cancer, cardiovascular complications and inflammatory disorders. Despite increasing scientific and clinical interest in this field, there are still no standardized procedures available for the purification, detection, and characterization of EVs. Advances in microfluidics allow for chemical sampling with increasingly high spatial resolution and under precise manipulation down to single molecule level. In this review, our objective is to give a brief overview on the working principle and examples of the isolation and detection methods with the potential to be used for extracellular vesicles. This review will also highlight the integrated on-chip systems for isolation and characterization of EVs.

Label-free single-cell separation and imaging of cancer cells using an integrated microfluidic system

The incidence of cancer is increasing worldwide and metastatic disease, through the spread of circulating tumor cells (CTCs), is responsible for the majority of the cancer deaths. Accurate monitoring of CTC levels in blood provides clinical information supporting therapeutic decision making, and improved methods for CTC enumeration are asked for. Microfluidics has been extensively used for this purpose but most methods require several post-separation processing steps including concentration of the sample before analysis. This induces a high risk of sample loss of the collected rare cells. Here, an integrated system is presented that efficiently eliminates this risk by integrating label-free separation with single cell arraying of the target cell population, enabling direct on-chip tumor cell identification and enumeration. Prostate cancer cells (DU145) spiked into a sample with whole blood concentration of the peripheral blood mononuclear cell (PBMC) fraction were efficiently separated and trapped at a recovery of 76.2 ± 5.9% of the cancer cells and a minute contamination of 0.12 ± 0.04% PBMCs while simultaneously enabling a 20x volumetric concentration. This constitutes a first step towards a fully integrated system for rapid label-free separation and on-chip phenotypic characterization of circulating tumor cells from peripheral venous blood in clinical practice.

In situ single cell detection via microfluidic magnetic bead assay

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

Microfluidic Techniques for Analytes Concentration

Microfluidics has been undergoing fast development in the past two decades due to its promising applications in biotechnology, medicine, and chemistry. Towards these applications, enhancing concentration sensitivity and detection resolution are indispensable to meet the detection limits because of the dilute sample concentrations, ultra-small sample volumes and short detection lengths in microfluidic devices. A variety of microfluidic techniques for concentrating analytes have been developed. This article presents an overview of analyte concentration techniques in microfluidics. We focus on discussing the physical mechanism of each concentration technique with its representative advancements and applications. Finally, the article is concluded by highlighting and discussing advantages and disadvantages of the reviewed techniques.

Biomedical applications of high gradient magnetic separation: progress towards therapeutic haeomofiltration

High gradient magnetic separation is a well-established technology in the mineral processing industry, and has been used for decades in the bioprocessing industry. Less well known is the increasing role that high gradient magnetic separation is playing in biomedical applications, for both diagnostic and therapeutic purposes. We review here the state of the art in this emerging field, with a focus on therapeutic haemofiltration, the key enabling technologies relating to the functionalisation of magnetic nanoparticles with target-specific binding agents, and the development of extra-corporeal circuits to enable the in situ filtering of human blood.

Skeletal stem cell isolation: A review on the state-of-the-art microfluidic label-free sorting techniques

Skeletal stem cells (SSC) are a sub-population of bone marrow stromal cells that reside in postnatal bone marrow with osteogenic, chondrogenic and adipogenic differentiation potential. SSCs reside only in the bone marrow and have organisational and regulatory functions in the bone marrow microenvironment and give rise to the haematopoiesis-supportive stroma. Their differentiation capacity is restricted to skeletal lineages and therefore the term SSC should be clearly distinguished from mesenchymal stem cells which are reported to exist in extra-skeletal tissues and, critically, do not contribute to skeletal development. SSCs are responsible for the unique regeneration capacity of bone and offer unlimited potential for application in bone regenerative therapies. A current unmet challenge is the isolation of homogeneous populations of SSCs, in vitro, with homogeneous regeneration and differentiation capacities. Challenges that limit SSC isolation include a) the scarcity of SSCs in bone marrow aspirates, estimated at between 1 in 10-100,000 mononuclear cells; b) the absence of specific markers and thus the phenotypic ambiguity of the SSC and c) the complexity of bone marrow tissue. Microfluidics provides innovative approaches for cell separation based on bio-physical features of single cells. Here we review the physical principles underlying label-free microfluidic sorting techniques and review their capacity for stem cell selection/sorting from complex (heterogeneous) samples.

A chip assisted immunomagnetic separation system for the efficient capture and in situ identification of circulating tumor cells

The detection of circulating tumor cells (CTCs), a kind of "liquid biopsy", represents a potential alternative to noninvasive detection, characterization and monitoring of carcinoma. Many previous studies have shown that the number of CTCs has a significant relationship with the stage of cancer. However, CTC enrichment and detection remain notoriously difficult because they are extremely rare in the bloodstream. Herein, aided by a microfluidic device, an immunomagnetic separation system was applied to efficiently capture and in situ identify circulating tumor cells. Magnetic nanospheres (MNs) were modified with an anti-epithelial-cell-adhesion-molecule (anti-EpCAM) antibody to fabricate immunomagnetic nanospheres (IMNs). IMNs were then loaded into the magnetic field controllable microfluidic chip to form uniform IMN patterns. The IMN patterns maintained good stability during the whole processes including enrichment, washing and identification. Apart from its simple manufacture process, the obtained microfluidic device was capable of capturing CTCs from the bloodstream with an efficiency higher than 94%. The captured cells could be directly visualized with an inverted fluorescence microscope in situ by immunocytochemistry (ICC) identification, which decreased cell loss effectively. Besides that, the CTCs could be recovered completely just by PBS washing after removal of the permanent magnets. It was observed that all the processes showed negligible influence on cell viability (viability up to 93%) and that the captured cells could be re-cultured for more than 5 passages after release without disassociating IMNs. In addition, the device was applied to clinical samples and almost all the samples from patients showed positive results, which suggests it could serve as a valuable tool for CTC enrichment and detection in the clinic.

Innovative Tools and Technology for Analysis of Single Cells and Cell-Cell Interaction

Heterogeneity in single-cell responses and intercellular interactions results from complex regulation of cell-intrinsic and environmental factors. Single-cell analysis allows not only detection of individual cellular characteristics but also correlation of genetic content with phenotypic traits in the same cell. Technological advances in micro- and nanofabrication have benefited single-cell analysis by allowing precise control of the localized microenvironment, cell manipulation, and sensitive detection capabilities. Additionally, microscale techniques permit rapid, high-throughput, multiparametric screening that has become essential for -omics research. This review highlights innovative applications of microscale platforms in genetic, proteomic, and metabolic detection in single cells; cell sorting strategies; and heterotypic cell-cell interaction. We discuss key design aspects of single-cell localization and isolation in microfluidic systems, dynamic and endpoint analyses, and approaches that integrate highly multiplexed detection of various intracellular species.

A high-throughput acoustic cell sorter

Acoustic-based fluorescence activated cell sorters (FACS) have drawn increased attention in recent years due to their versatility, high biocompatibility, high controllability, and simple design. However, the sorting throughput for existing acoustic cell sorters is far from optimum for practical applications. Here we report a high-throughput cell sorting method based on standing surface acoustic waves (SSAWs). We utilized a pair of focused interdigital transducers (FIDTs) to generate SSAW with high resolution and high energy efficiency. As a result, the sorting throughput is improved significantly from conventional acoustic-based cell sorting methods. We demonstrated the successful sorting of 10 μm polystyrene particles with a minimum actuation time of 72 μs, which translates to a potential sorting rate of more than 13,800 events per second. Without using a cell-detection unit, we were able to demonstrate an actual sorting throughput of 3300 events per second. Our sorting method can be conveniently integrated with upstream detection units, and it represents an important development towards a functional acoustic-based FACS system.

Photoresponsive immunomagnetic nanocarrier for capture and release of rare circulating tumor cells

7-Aminocoumarin compound was synthesized and used as phototrigger to cage EpCAM-antibody to construct a photocontrolled CTCs capture and release system.

Isolation, release and culture of rare circulating tumor cells (CTCs) may, if implemented, promote the progress of individualized anti-tumor therapies. To realize the release of CTCs without disruption of their viability for further culture and analysis, we designed an effective photocontrolled CTC capture/release system by combination of photochemistry and immunomagnetic separation. 7-Aminocoumarin was synthesized as the phototrigger to bridge the connection between the anti-EpCAM antibody and the magnetic beads. The coumarin moieties produced cleavage of a C–O bond under both ultraviolet (UV) and near-infrared (NIR) light illumination, breaking the bridge and releasing CTCs from the immunomagnetic beads. Compared with conventional immunomagnetic separation systems, the negative influence of absorbed immunomagnetic beads on further CTCs culture and analysis was effectively eliminated. The system can specifically recognize 10² MCF-7 cells in 1 mL of human whole blood samples with 90% efficiency and 85% purity. Under the irradiation of UV and NIR light, 73 ± 4% and 52 ± 6% of captured cells were released with a viability of 90% and 97%, respectively. Furthermore, this technique has been used to detect CTCs from whole blood of cancer patients with high purity. This study demonstrates that the photochemical-based immunomagnetic separation method for isolating, releasing and culturing CTCs from clinic patients may provide new opportunities for cancer diagnosis and personalized therapy.

A single inlet two-stage acoustophoresis chip enabling tumor cell enrichment from white blood cells

Metastatic disease is responsible for most cancer deaths, and hematogenous spread through circulating tumor cells (CTC) is a prerequisite for tumor dissemination. CTCs may undergo epithelial-mesenchymal transition where many epithelial cell characteristics are lost. Therefore, CTC isolation systems relying on epithelial cell markers are at risk of losing important subpopulations of cells. Here, a simple acoustophoresis-based cell separation instrument is presented. Cells are uniquely separated while maintained in their initial suspending medium, thus eliminating the need for a secondary cell-free medium to hydrodynamically pre-position them before the separation. When characterizing the system using polystyrene particles, 99.6 ± 0.2% of 7 μm diameter particles were collected through one outlet while 98.8 ± 0.5% of 5 μm particles were recovered through a second outlet. Prostate cancer cells (DU145) spiked into blood were enriched from white blood cells at a sample flow rate of 100 μL min(-1) providing 86.5 ± 6.7% recovery of the cancer cells with 1.1 ± 0.2% contamination of white blood cells. By increasing the acoustic intensity a recovery of 94.8 ± 2.8% of cancer cells was achieved with 2.2 ± 0.6% contamination of white blood cells. The single inlet approach makes this instrument insensitive to acoustic impedance mismatch; a phenomenon reported to importantly affect accuracy in multi-laminar flow stream acoustophoresis. It also offers a possibility of concentrating the recovered cells in the chip, as opposed to systems relying on hydrodynamic pre-positioning which commonly dilute the target cells.

An Integrated Platform for Isolation, Processing, and Mass Spectrometry-based Proteomic Profiling of Rare Cells in Whole Blood

Isolation and molecular characterization of rare cells (e.g. circulating tumor and stem cells) within biological fluids and tissues has significant potential in clinical diagnostics and personalized medicine. The present work describes an integrated platform of sample procurement, preparation, and analysis for deep proteomic profiling of rare cells in blood. Microfluidic magnetophoretic isolation of target cells spiked into 1 ml of blood at the level of 1000-2000 cells/ml, followed by focused acoustics-assisted sample preparation has been coupled with one-dimensional PLOT-LC-MS methodology. The resulting zeptomole detection sensitivity enabled identification of ∼4000 proteins with injection of the equivalent of only 100-200 cells per analysis. The characterization of rare cells in limited volumes of physiological fluids is shown by the isolation and quantitative proteomic profiling of first MCF-7 cells spiked into whole blood as a model system and then two CD133+ endothelial progenitor and hematopoietic cells in whole blood from volunteers.