Biocompatible and label-free separation of cancer cells from cell culture lines from white blood cells in ferrofluids

The Shape Effect of Acoustic Micropillar Array Chips in Flexible Label-Free Separation of Cancer Cells

The precise isolation of circulating tumor cells (CTCs) from blood samples is a potent tool for cancer diagnosis and clinical prognosis. However, CTCs are present in extremely low quantities in the bloodstream, posing a significant challenge to their isolation. In this study, we propose a non-contact acoustic micropillar array (AMPA) chip based on acoustic streaming for the flexible, label-free capture of cancer cells. Three shapes of micropillar array chips (circular, rhombus, and square) were fabricated. The acoustic streaming characteristics generated by the vibration of microstructures of different shapes are studied in depth by combining simulation and experiment. The critical parameters (voltage and flow rate) of the device were systematically investigated using microparticle experiments to optimize capture performance. Subsequently, the capture efficiencies of the three micropillar structures were experimentally evaluated using mouse whole blood samples containing cancer cells. The experimental results revealed that the rhombus microstructure was selected as the optimal shape, demonstrating high capture efficiency (93%) and cell activity (96%). Moreover, the reversibility of the acoustic streaming was harnessed for the flexible release and capture of cancer cells, facilitating optical detection and analysis. This work holds promise for applications in monitoring cancer metastasis, bio-detection, and beyond.

Viscoelastic microfluidics for enhanced separation resolution of submicron particles and extracellular vesicles

Manipulation, focusing, and separation of submicron- and nanoparticles such as extracellular vesicles (EVs), viruses and bacteria have broad applications in disease diagnostics and therapeutics. Viscoelastic microfluidic technology emerges as a promising technique, and it shows an unparalleled capacity to manipulate and separate submicron particles in a high resolution based on the elastic effects of non-Newtonian mediums. The maximum particle separation resolution for the reported state-of-the-art viscoelastic microfluidics is around 200 nm. To further enhance the reseparation resolution, this work develops a viscoelastic microfluidic device that can achieve a finer separation resolution up to 100 nm, by optimising the operating conditions such as flow rate, flow rate ratio and polyethylene oxide (PEO) concentration. With these optimised conditions, we separated a ternary mixture of 100 nm, 200 nm and 500 nm polystyrene particles, with purities above 90%, 70% and 82%, respectively. Furthermore, we also applied the developed viscoelastic microfluidic device for the separation of cancer cell-secreted extracellular vesicles (EVs) into three different size groups. After single processing, the separation efficiencies for small EVs (sEVs, <150 nm), medium EVs (mEVs, 150-300 nm), and large EVs (>300 nm) were 86%, 80% and 50%, respectively. The enrichment factors for the three EV groups were 2.4, 1.1 and 1.3, respectively. Moreover, we observed an unexpected effect of high PEO concentrations (2000-5000 ppm) on the lateral migration of nanoparticles where nanoparticles of up to 50 nm surprisingly can migrate and concentrate at the middle of the microchannel. This simple and label-free viscoelastic microfluidic device possesses excellent potential for sorting submicron particles for various chemical, biological, medical and environmental applications.

Measuring cell deformation by microfluidics

Microfluidics is an increasingly popular method for studying cell deformation, with various applications in fields such as cell biology, biophysics, and medical research. Characterizing cell deformation offers insights into fundamental cell processes, such as migration, division, and signaling. This review summarizes recent advances in microfluidic techniques for measuring cellular deformation, including the different types of microfluidic devices and methods used to induce cell deformation. Recent applications of microfluidics-based approaches for studying cell deformation are highlighted. Compared to traditional methods, microfluidic chips can control the direction and velocity of cell flow by establishing microfluidic channels and microcolumn arrays, enabling the measurement of cell shape changes. Overall, microfluidics-based approaches provide a powerful platform for studying cell deformation. It is expected that future developments will lead to more intelligent and diverse microfluidic chips, further promoting the application of microfluidics-based methods in biomedical research, providing more effective tools for disease diagnosis, drug screening, and treatment.

Lab-on-a-chip systems for cancer biomarker diagnosis

Lab-on-a-chip (LOC) or micro total analysis system is one of the microfluidic technologies defined as the adaptation, miniaturization, integration, and automation of analytical laboratory procedures into a single instrument or "chip". In this article, we review developments over the past five years in the application of LOC biosensors for the detection of different types of cancer. Microfluidics encompasses chemistry and biotechnology skills and has revolutionized healthcare diagnosis. Superior to traditional cell culture or animal models, microfluidic technology has made it possible to reconstruct functional units of organs on chips to study human diseases such as cancer. LOCs have found numerous biomedical applications over the past five years, including integrated bioassays, cell analysis, metabolomics, drug discovery and delivery systems, tissue and organ physiology and disease modeling, and personalized medicine. This review provides an overview of the latest developments in microfluidic-based cancer research, with pros, cons, and prospects.

Label-free microfluidic cell sorting and detection for rapid blood analysis

Blood tests are considered as standard clinical procedures to screen for markers of diseases and health conditions. However, the complex cellular background (>99.9% RBCs) and biomolecular composition often pose significant technical challenges for accurate blood analysis. An emerging approach for point-of-care blood diagnostics is utilizing "label-free" microfluidic technologies that rely on intrinsic cell properties for blood fractionation and disease detection without any antibody binding. A growing body of clinical evidence has also reported that cellular dysfunction and their biophysical phenotypes are complementary to standard hematoanalyzer analysis (complete blood count) and can provide a more comprehensive health profiling. In this review, we will summarize recent advances in microfluidic label-free separation of different blood cell components including circulating tumor cells, leukocytes, platelets and nanoscale extracellular vesicles. Label-free single cell analysis of intrinsic cell morphology, spectrochemical properties, dielectric parameters and biophysical characteristics as novel blood-based biomarkers will also be presented. Next, we will highlight research efforts that combine label-free microfluidics with machine learning approaches to enhance detection sensitivity and specificity in clinical studies, as well as innovative microfluidic solutions which are capable of fully integrated and label-free blood cell sorting and analysis. Lastly, we will envisage the current challenges and future outlook of label-free microfluidics platforms for high throughput multi-dimensional blood cell analysis to identify non-traditional circulating biomarkers for clinical diagnostics.

Simultaneous and Multimodal Antigen-Binding Profiling and Isolation of Rare Cells via Quantitative Ferrohydrodynamic Cell Separation

Simultaneous cell profiling and isolation based on cellular antigen-binding capacity plays an important role in understanding and treating diseases. However, fluorescence-activated cell sorting (FACS) and magnetic-activated cell sorting (MACS) are not able to meet this need, due to their requirement for a large quantity of target cells and the limitation stemming from bimodal separation. Here we report a microfluidic method, termed quantitative ferrohydrodynamic cell separation (qFCS), that achieved multimodal rare cell sorting and simultaneous antigen profiling at a ∼30,000 cell min^-1 throughput with a 96.49% recovery rate and a 98.72% purity of recovered cells. qFCS profiles and sorts cells via cellular magnetic content of the magnetically labeled cells, which correlates to cellular antigen-binding capacity. By integrating cellular magnetophoresis and diamagnetophoresis in biocompatible ferrofluids, we demonstrate that the resulting qFCS device can accurately profile and isolate rare cells even when present at ∼1:50,000 target to background cells frequency. We show that the qFCS device could accurately profile and isolate T lymphocytes based on a low-expression CD154 antigen and allow on-device analysis of cells after processing. This method could address the need for simultaneous and multimodal rare cell isolation and profiling in disease diagnostics, prognostics, and treatment.

Magnetic Nanoparticles for Protein Separation and Purification

Proteins are essential for various functions such as brain activity and muscle contraction in humans. Even though food is a source of proteins, the bioavailability of proteins in most foods is usually limited due to matrix interaction with other biomolecules. Thus, it is essential to extract these proteins and provide them as a nutraceutical supplement to maintain protein levels and avoid protein deficiency. Hence, protein purification and extraction from natural sources are highly significant in biomedical applications. Chromatography, crude mechanical disruption, use of extractive chemicals, and electrophoresis are some of the methods applied to isolate specific proteins. Even though these methods possess several advantages, they are unable to extract specific proteins with high purity. A suitable alternative is the use of nanoparticles, which can be beneficial in protein purification and extraction. Notably, magnetic iron and iron-based nanoparticles have been employed in protein extraction processes and can be reused via demagnetization due to their magnetic property, smaller size, morphology, high surface-to-volume ratio, and surface charge-mediated property. This chapter is a summary of various magnetic nanoparticles (MNPs) that can be used for the biomolecular separation of proteins.

Extraction of small extracellular vesicles by label-free and biocompatible on-chip magnetic separation

Small vesicles (sEVs) are closely related to many diseases as they carry various bio-markers. Efficient separation of sEVs from complex biological samples is essential and prerequisite for the following treatment and further disease diagnosis. Here we propose a label-free and biocompatible on-chip magnetic separation system for efficient extraction of sEVs from cell culture supernatant. Through an on-chip ultra-high gradient magnetic field module, a magnetic field gradient close to 100 000 T m^-1 is generated inside the separation microchannel. By using fluorescent particles of 200 nm and 1000 nm to simulate sEVs and other bioparticles in a complex sample, the system design and the experimental parameters are optimized. Flow cytometry and a proposed fluorescence intensity analysis method both verify that the recovery rate and purity of 200 nm particles can reach 84.91% and 98.02%, respectively. Then, a biocompatible ferrofluid is utilized in the separation system to separate sEVs from the cell culture supernatant. The results tested by nanoparticle tracking analysis show that the recovery rate and purity of sEVs are 85.80% and 80.45%, superiorly exceeding the performance that the ultracentrifugation method can provide.

Recent Advances in Microfluidic Platform for Physical and Immunological Detection and Capture of Circulating Tumor Cells

CTCs (circulating tumor cells) are well-known for their use in clinical trials for tumor diagnosis. Capturing and isolating these CTCs from whole blood samples has enormous benefits in cancer diagnosis and treatment. In general, various approaches are being used to separate malignant cells, including immunomagnets, macroscale filters, centrifuges, dielectrophoresis, and immunological approaches. These procedures, on the other hand, are time-consuming and necessitate multiple high-level operational protocols. In addition, considering their low efficiency and throughput, the processes of capturing and isolating CTCs face tremendous challenges. Meanwhile, recent advances in microfluidic devices promise unprecedented advantages for capturing and isolating CTCs with greater efficiency, sensitivity, selectivity and accuracy. In this regard, this review article focuses primarily on the various fabrication methodologies involved in microfluidic devices and techniques specifically used to capture and isolate CTCs using various physical and biological methods as well as their conceptual ideas, advantages and disadvantages.

High-Resolution Separation of Nanoparticles Using a Negative Magnetophoretic Microfluidic System

The separation and purification of a sample of interest is essential for subsequent detection and analysis procedures, but there is a lack of effective separation methods with which to purify nano-sized particles from the sample media. In this paper, a microfluidic system based on negative magnetophoresis is presented for the high-resolution separation of nanoparticles. The system includes on-chip magnetic pole arrays and permalloys that symmetrically distribute on both sides of the separation channel and four permanent magnets that provide strong magnetic fields. The microfluidic system can separate 200 nm particles with a high purity from the mixture (1000 nm and 200 nm particles) due to a magnetic field gradient as high as 10,000 T/m being generated inside the separation channel, which can provide a negative magnetophoretic force of up to 10 pN to the 1000 nm particle. The overall recovery rate of the particles reaches 99%, the recovery rate of 200 nm particles is 84.2%, and the purity reaches 98.2%. Compared with the existing negative magnetophoretic separation methods, our system not only exhibits high resolution on particle sizes (800 nm), but also improves the sample processing throughput, which reaches 2.5 μL/min. The microfluidic system is expected to provide a new solution for the high-purity separation of nanoparticles, as well as nanobiological samples.

Basic Principles and Recent Advances in Magnetic Cell Separation

Magnetic cell separation has become a key methodology for the isolation of target cell populations from biological suspensions, covering a wide spectrum of applications from diagnosis and therapy in biomedicine to environmental applications or fundamental research in biology. There now exists a great variety of commercially available separation instruments and reagents, which has permitted rapid dissemination of the technology. However, there is still an increasing demand for new tools and protocols which provide improved selectivity, yield and sensitivity of the separation process while reducing cost and providing a faster response. This review aims to introduce basic principles of magnetic cell separation for the neophyte, while giving an overview of recent research in the field, from the development of new cell labeling strategies to the design of integrated microfluidic cell sorters and of point-of-care platforms combining cell selection, capture, and downstream detection. Finally, we focus on clinical, industrial and environmental applications where magnetic cell separation strategies are amongst the most promising techniques to address the challenges of isolating rare cells.

Droplet-based microfluidics in biomedical applications

Droplet-based microfluidic systems have been employed to manipulate discrete fluid volumes with immiscible phases. Creating the fluid droplets at microscale has led to a paradigm shift in mixing, sorting, encapsulation, sensing, and designing high throughput devices for biomedical applications. Droplet microfluidics has opened many opportunities in microparticle synthesis, molecular detection, diagnostics, drug delivery, and cell biology. In the present review, we first introduce standard methods for droplet generation (i.e., passive and active methods) and discuss the latest examples of emulsification and particle synthesis approaches enabled by microfluidic platforms. Then, the applications of droplet-based microfluidics in different biomedical applications are detailed. Finally, a general overview of the latest trends along with the perspectives and future potentials in the field are provided.

Simultaneous biochemical and functional phenotyping of single circulating tumor cells using ultrahigh throughput and recovery microfluidic devices

Profiling circulating tumour cells (CTCs) in cancer patients' blood samples is critical to understand the complex and dynamic nature of metastasis. This task is challenged by the fact that CTCs are not only extremely rare in circulation but also highly heterogeneous in their molecular programs and cellular functions. Here we report a combinational approach for the simultaneous biochemical and functional phenotyping of patient-derived CTCs, using an integrated inertial ferrohydrodynamic cell separation (i²FCS) method and a single-cell microfluidic migration assay. This combinatorial approach offers unique capability to profile CTCs on the basis of their surface expression and migratory characteristics. We achieve this using the i²FCS method that successfully processes whole blood samples in a tumor cell marker and size agnostic manner. The i²FCS method enables an ultrahigh blood sample processing throughput of up to 2 × 10⁵ cells s^-1 with a blood sample flow rate of 60 mL h^-1. Its short processing time (10 minutes for a 10 mL sample), together with a close-to-complete CTC recovery (99.70% recovery rate) and a low WBC contamination (4.07-log depletion rate by removing 99.992% of leukocytes), results in adequate and functional CTCs for subsequent studies in the single-cell migration device. For the first time, we employ this new approach to query CTCs with single-cell resolution in accordance with their expression of phenotypic surface markers and migration properties, revealing the dynamic phenotypes and the existence of a high-motility subpopulation of CTCs in blood samples from metastatic lung cancer patients. This method could be adopted to study the biological and clinical value of invasive CTC phenotypes.

Advanced biomedical applications of iron oxide nanostructures based ferrofluids

Ferrofluids or magnetic nanofluids are highly stable colloidal suspensions of magnetic nanoparticles (NPs) dispersed into various base fluids. These stable ferrofluids possess high thermal conductivity, improved thermo-physical properties, higher colloidal stability, good magnetic properties, and biocompatibility, which are the primary driving forces behind their excellent performance, and thus enable them to be used for a wide range of practical applications. The most studied and advanced ferrofluids are based on iron oxide nanostructures especially NPs, because of their easy and large-scale synthesis at low costs. Although in the last decade, several review articles are available on ferrofluids but mainly focused on preparations, properties, and a specific application. Hence, a collective and comprehensive review article on the recent progress of iron oxide nanostructures based ferrofluids for advanced biomedical applications is undeniably required. In this review, the state of the art of biomedical applications is presented and critically analyzed with a special focus on hyperthermia, drug delivery/nanomedicine, magnetic resonance imaging, and magnetic separation of cells. This review article provides up-to-date information related to the technological advancements and emerging trends in iron oxide nanostructures based ferrofluids research focused on advanced biomedical applications. Finally, conclusions and outlook of iron oxide nanostructures based ferrofluids research for biomedical applications are presented.

Label-free inertial-ferrohydrodynamic cell separation with high throughput and resolution

Rapid and label-free separation of target cells from biological samples provided unique opportunity for disease diagnostics and treatment. However, even with advanced technologies for cell separation, the limited throughput, high cost and low separation resolution still prevented their utility in separating cells with well-defined physical features from a large volume of biological samples. Here we described an ultrahigh-throughput microfluidic technology, termed as inertial-ferrohydrodynamic cell separation (inertial-FCS), that rapidly sorted through over 60 milliliters of samples at a throughput of 100 000 cells per second in a label-free manner, differentiating the cells based on their physical diameter difference with ∼1-2 μm separation resolution. Through the integration of inertial focusing and ferrohydrodynamic separation, we demonstrated that the resulting inertial-FCS devices could separate viable and expandable circulating tumor cells from cancer patients' blood with a high recovery rate and high purity. We also showed that the devices could enrich lymphocytes directly from white blood cells based on their physical morphology without any labeling steps. This label-free method could address the needs of high throughput and high resolution cell separation in circulating tumor cell research and adoptive cell transfer immunotherapy.

Magnetic Polymers for Magnetophoretic Separation in Microfluidic Devices

Magnetophoresis offers many advantages for manipulating magnetic targets in microsystems. The integration of micro-flux concentrators and micro-magnets allows achieving large field gradients and therefore large reachable magnetic forces. However, the associated fabrication techniques are often complex and costly, and besides, they put specific constraints on the geometries. Magnetic composite polymers provide a promising alternative in terms of simplicity and fabrication costs, and they open new perspectives for the microstructuring, design, and integration of magnetic functions. In this review, we propose a state of the art of research works implementing magnetic polymers to trap or sort magnetic micro-beads or magnetically labeled cells in microfluidic devices.

Magnetic nanoparticles in microfluidics-based diagnostics: an appraisal

The use of magnetic nanoparticles (MNPs) in microfluidics based diagnostics is a classic case of micro-, nano- and bio-technology coming together to design extremely controllable, reproducible, and scalable nano and micro 'on-chip bio sensing systems.' In this review, applications of MNPs in microfluidics ranging from molecular diagnostics and immunodiagnostics to clinical uses have been examined. In addition, microfluidic mixing and capture of analytes using MNPs, and MNPs as carriers in microfluidic devices has been investigated. Finally, the challenges and future directions of this upcoming field have been summarized. The use of MNP-based microfluidic devices, will help in developing decentralized or 'point of care' testing globally, contributing to affordable healthcare, particularly, for middle- and low-income developing countries.

Fundamentals of integrated ferrohydrodynamic cell separation in circulating tumor cell isolation

Methods to separate circulating tumor cells (CTCs) from blood samples were intensively researched in order to understand the metastatic process and develop corresponding clinical assays. However current methods faced challenges that stemmed from CTCs' heterogeneity in their biological markers and physical morphologies. To this end, we developed integrated ferrohydrodynamic cell separation (iFCS), a scheme that separated CTCs independent of their surface antigen expression and physical characteristics. iFCS integrated both diamagnetophoresis of CTCs and magnetophoresis of blood cells together via a magnetic liquid medium, ferrofluid, whose magnetization could be tuned by adjusting its magnetic volume concentration. In this paper, we presented the fundamental theory of iFCS and its specific application in CTC separation. Governing equations of iFCS were developed to guide its optimization process. Three critical parameters that affected iFCS's cell separation performance were determined and validated theoretically and experimentally. These parameters included the sample flow rate, the volumetric concentration of magnetic materials in the ferrofluid, and the gradient of the magnetic flux density. We determined these optimized parameters in an iFCS device that led to a high recovery CTC separation in both spiked and clinical samples.

Self-Assembled Hexagonal Superparamagnetic Cone Structures for Fabrication of Cell Cluster Arrays

In this study, we demonstrated that arrays of cell clusters can be fabricated by self-assembled hexagonal superparamagnetic cone structures. When a strong out-of-plane magnetic field was applied to the ferrofluid on a glass substrate, it will induce the magnetic poles on the upper/lower surfaces of the continuous ferrofluid to increase the magnetostatic energy. The ferrofluid will then experience hydrodynamic instability and be split into small droplets with cone structures because of the compromising surface tension energy and magnetostatic energy to minimize the system's total energy. Furthermore, the ferrofluid cones were orderly self-assembled into hexagonal arrays to reach the lowest energy state. After dehydration of these liquid cones to form solid cones, polydimethylsiloxane was cast to fix the arrangement of hexagonal superparamagnetic cone structures and prevent the leakage of magnetic nanoparticles. The U-343 human neuronal glioblastoma cells were labeled with magnetic nanoparticles through endocytosis in co-culture with a ferrofluid. The number of magnetic nanoparticles internalized was (4.2 ± 0.84) × 10⁶ per cell by the cell magnetophoresis analysis. These magnetically labeled cells were attracted and captured by hexagonal superparamagnetic cone structures to form cell cluster arrays. As a function of the solid cone size, the number of cells captured by each hexagonal superparamagnetic cone structure was increased from 48 to 126 under a 2000 G out-of-plane magnetic field. The local magnetic field gradient of the hexagonal superparamagnetic cone was 117.0-140.9 G/mm from the cell magnetophoresis. When an external magnetic field was applied, we observed that the number of protrusions of the cell edge decreased from the fluorescence images. It showed that the local magnetic field gradient caused by the hexagonal superparamagnetic cones restricted the cell growth and migration.

Label-free separation of nanoscale particles by an ultrahigh gradient magnetic field in a microfluidic device

The need for fast and accurate analysis of low-concentration species is ubiquitous nowadays. The separation and purification techniques restrict the highly sensitive detection of low-abundance nanoparticles. On the other hand, the commonly used separation techniques of labeling procedures limit their implementation in various applications. We report a microfluidic system with ultrahigh magnetic field for the label-free separation of nanoscale particles. Using high-permeability alloys and on-chip integrated magnetic micro-pole arrays, the external strong magnetic field can be conducted into the microfluidic device to form a magnetic field of high intensity and gradient, therefore separating particles of nanometer size with high efficiency. An ultrahigh gradient magnetic field greater than 105 T m-1 can be generated in the separation channel. Moreover, a negative magnetophoretic technique to separate nanoparticles is established in this device. Then, the label-free separation of nanoparticles is achieved in this microfluidic system perfused by a ferrofluid with an extremely low concentration (0.01%). A mixture of 0.2 μm and 1 μm particles is used to verify the performance of the device, where the recovery rate of 0.2 μm particles is 88.79%, and the purity reaches 94.72%. Experimental results show that the device can efficiently separate nanoscale particles with ultrahigh resolution, and in future, it may develop into a versatile and robust tool for the separation and purification of the biological samples of nanometer size.

Label-free ferrohydrodynamic separation of exosome-like nanoparticles

Isolation of exosomes from biological samples provides a minimally-invasive alternative for basic understanding, diagnosis, and prognosis of metastatic cancers. The biology and clinical values of exosomes are under intensive investigation, yet most studies are limited by technical challenges in recovering these exosomes with heterogeneous sizes and cargos from biological samples. We report a novel method based on "particle ferrohydrodynamics" and its associated microfluidic device, termed as the FerroChip, which can separate exosome-like nanoparticles from microliters of cell culture media and human serum in a label-free, continuous-flow and size-dependent manner, and achieves a high recovery rate (94.3%) and a high purity (87.9%). Separated exosome-like nanoparticles had diameters, morphology, and protein expressions that were consistent with other reports. This method, upon further molecular characterization, could potentially facilitate basic understanding of exosomes and its clinical application in blood liquid biopsy.

Magnetically driven microfluidics for isolation of circulating tumor cells

Circulating tumor cells (CTCs) largely contribute to cancer metastasis and show potential prognostic significance in cancer isolation and detection. Miniaturization has progressed significantly in the last decade which in turn enabled the development of several microfluidic systems. The microfluidic systems offer a controlled microenvironment for studies of fundamental cell biology, resulting in the rapid development of microfluidic isolation of CTCs. Due to the inherent ability of magnets to provide forces at a distance, the technology of CTCs isolation based on the magnetophoresis mechanism has become a routine methodology. This historical review aims to introduce two principles of magnetic isolation and recent techniques, facilitating research in this field and providing alternatives for researchers in their study of magnetic isolation. Researchers intend to promote effective CTC isolation and analysis as well as active development of next-generation cancer treatment. The first part of this review summarizes the primary principles based on positive and negative magnetophoretic isolation and describes the metrics for isolation performance. The second part presents a detailed overview of the factors that affect the performance of CTC magnetic isolation, including the magnetic field sources, functionalized magnetic nanoparticles, magnetic fluids, and magnetically driven microfluidic systems.

The Fabrication and Application Mechanism of Microfluidic Systems for High Throughput Biomedical Screening: A Review

Microfluidic systems have been widely explored based on microfluidic technology, and it has been widely used for biomedical screening. The key parts are the fabrication of the base scaffold, the construction of the matrix environment in the 3D system, and the application mechanism. In recent years, a variety of new materials have emerged, meanwhile, some new technologies have been developed. In this review, we highlight the properties of high throughput and the biomedical application of the microfluidic chip and focus on the recent progress of the fabrication and application mechanism. The emergence of various biocompatible materials has provided more available raw materials for microfluidic chips. The material is not confined to polydimethylsiloxane (PDMS) and the extracellular microenvironment is not limited by a natural matrix. The mechanism is also developed in diverse ways, including its special physical structure and external field effects, such as dielectrophoresis, magnetophoresis, and acoustophoresis. Furthermore, the cell/organ-based microfluidic system provides a new platform for drug screening due to imitating the anatomic and physiologic properties in vivo. Although microfluidic technology is currently mostly in the laboratory stage, it has great potential for commercial applications in the future.

Opto-magnetic Selection and Isolation of Single Cells

Capturing single cells from large heterogenous populations based solely on observable traits is necessary for many cell biology applications and remains a major technical challenge. The protocol we present allows the isolation of viable and metabolically active cells selected for their shape, migration speed, contact to other cells, or intracellular protein localization. We previously introduced a method termed Cell Labeling via Photobleaching (CLaP) for the efficient tagging of cells chosen for visual criteria. Here we describe a new protocol for capturing such cells using ferromagnetic beads termed single-cell magneto-optical capture (scMOCa). This technology is especially useful when the number of target cells represents an extremely low fraction of the total population (potentially one single cell), a situation in which conventional sorting techniques like fluorescent or magnetic activated cell sorting (F/MACS) cannot provide satisfactory results in terms of capture efficiency and specificity. scMOCa uses the lasers of a confocal microscope to photobleach and crosslink biotin-4-fluorecein molecules to cell membranes. Streptavidin coated magnetic beads then adhere to biotin moieties and a magnet allows the capture of illuminated cells. By precisely controlling liquid volumes and spacing between the different parts of a simple setup, high cell selectivity and capture efficacy can be achieved. scMOCA allows visual selection and isolation of any number of cells in a microscopy field and captured cells remain viable to generate new colonies of chosen phenotypes for downstream analyses.

Recent Advances in Continuous-Flow Particle Manipulations Using Magnetic Fluids

Magnetic field-induced particle manipulation is simple and economic as compared to other techniques (e.g., electric, acoustic, and optical) for lab-on-a-chip applications. However, traditional magnetic controls require the particles to be manipulated being magnetizable, which renders it necessary to magnetically label particles that are almost exclusively diamagnetic in nature. In the past decade, magnetic fluids including paramagnetic solutions and ferrofluids have been increasingly used in microfluidic devices to implement label-free manipulations of various types of particles (both synthetic and biological). We review herein the recent advances in this field with focus upon the continuous-flow particle manipulations. Specifically, we review the reported studies on the negative magnetophoresis-induced deflection, focusing, enrichment, separation, and medium exchange of diamagnetic particles in the continuous flow of magnetic fluids through microchannels.

Tumor antigen-independent and cell size variation-inclusive enrichment of viable circulating tumor cells

Isolation of circulating tumor cells (CTCs) from blood provides a minimally-invasive alternative for basic understanding, diagnosis, and prognosis of metastatic cancer. The roles and clinical values of CTCs are under intensive investigation, yet most studies are limited by technical challenges in the comprehensive enrichment of intact and viable CTCs with minimal white blood cell (WBC) contamination. Here, we report a novel method based on contrast of cell magnetization in biocompatible ferrofluids (a colloidal magnetic nanoparticle suspension), termed as integrated ferrohydrodynamic cell separation (iFCS), that enriches CTCs in a tumor antigen-independent and cell size variation-inclusive manner, achieves a high throughput (12 mL h-1), high recovery rate (99.08% at down to ∼10 cells per mL spike ratio), and low WBC contamination (533 cells for every one milliliter blood processed) and is biocompatible. This method will enable large cohort research to define the clinical and diagnostic value of CTC subtypes.

Opto-magnetic capture of individual cells based on visual phenotypes

The ability to isolate rare live cells within a heterogeneous population based solely on visual criteria remains technically challenging, due largely to limitations imposed by existing sorting technologies. Here, we present a new method that permits labeling cells of interest by attaching streptavidin-coated magnetic beads to their membranes using the lasers of a confocal microscope. A simple magnet allows highly specific isolation of the labeled cells, which then remain viable and proliferate normally. As proof of principle, we tagged, isolated, and expanded individual cells based on three biologically relevant visual characteristics: i) presence of multiple nuclei, ii) accumulation of lipid vesicles, and iii) ability to resolve ionizing radiation-induced DNA damage foci. Our method constitutes a rapid, efficient, and cost-effective approach for isolation and subsequent characterization of rare cells based on observable traits such as movement, shape, or location, which in turn can generate novel mechanistic insights into important biological processes.

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.

Computational modeling and fluorescence microscopy characterization of a two-phase magnetophoretic microsystem for continuous-flow blood detoxification

Magnetic beads can be functionalized to capture and separate target pathogens from blood for extracorporeal detoxification. The beads can be magnetically separated from a blood stream and collected into a coflowing buffer solution using a two-phase liquid-liquid continuous-flow microfluidic device in the presence of an external field. However, device design and process optimization, i.e. high bead recovery with minimum blood loss or dilution remain a substantial technological challenge. We introduce a CFD-based Eulerian-Lagrangian computational model that enables the rational design and optimization of such systems. The model takes into account dominant magnetic and hydrodynamic forces on the beads as well as coupled bead-fluid interactions. Fluid flow (Navier-Stokes equations) and mass transfer (Fick's law) between the coflowing fluids are solved numerically, while the magnetic force on the beads is predicted using analytical methods. The model is demonstrated via application to a prototype device and used to predict key performance metrics; degree of bead separation, flow patterns, and mass transfer, i.e. blood diffusion to the buffer phase. The impact of different process variables and parameters - flow rates, bead and magnet dimensions and fluid viscosities - on both bead recovery and blood loss or dilution is quantified for the first time. The performance of the prototype device is characterized using fluorescence microscopy and the experimental results are found to match theoretical predictions within an absolute error of 15%. While the model is demonstrated here for analysis of a detoxification device, it can be readily adapted to a broad range of magnetically-enabled microfluidic applications, e.g. bioseparation, sorting and sensing.

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.

Yeast cell fractionation by morphology in dilute ferrofluids

Morphology is an important particle (both biological and synthetic) property and potentially a useful marker for label-free particle separation. We present in this work a continuous-flow morphology-based fractionation of a heterogeneous mixture of drug-treated yeast cells in dilute ferrofluids. Such a diamagnetic cell separation technique utilizes the negative magnetophoretic motion to direct pre-focused yeast cells to morphology-dependent streamlines in a laminar flow. The separation performance is evaluated by comparing the exiting positions of the four classified groups of yeast cells: Singles, Doubles, Triples, and Others. We also develop a three-dimensional numerical model to simulate the separation process by the use of the experimentally determined correction factor for each group of non-spherical cells. The determining factors in this separation are studied both experimentally and numerically, the results of which show a reasonable agreement.

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.

Magnetofluidic micromixer based on a complex rotating magnetic field

We report a magnetically actuated micromixer for mixing non-magnetic microparticles in a microfluidic system.