Simultaneous diamagnetic and magnetic particle trapping in ferrofluid microflows via a single permanent magnet

Enhanced microfluidic multi-target separation by positive and negative magnetophoresis

We introduce magnetophoresis-based microfluidics for sorting biological targets using positive Magnetophoresis (pM) for magnetically labeled particles and negative Magnetophoresis (nM) for label-free particles. A single, externally magnetized ferromagnetic wire induces repulsive forces and is positioned across the focused sample flow near the main channel's closed end. We analyze magnetic attributes and separation performance under two transverse dual-mode magnetic configurations, examining magnetic fields, hydrodynamics, and forces on microparticles of varying sizes and properties. In pM, the dual-magnet arrangement (DMA) for sorting three distinct particles shows higher magnetic gradient generation and throughput than the single-magnet arrangement (SMA). In nM, the numerical results for SMA sorting of red blood cells (RBCs), white blood cells (WBCs), and prostate cancer cells (PC3-9) demonstrate superior magnetic properties and throughput compared to DMA. Magnetized wire linear movement is a key design parameter, allowing device customization. An automated device for handling more targets can be created by manipulating magnetophoretic repulsion forces. The transverse wire and magnet arrangement accommodate increased channel depth without sacrificing efficiency, yielding higher throughput than other devices. Experimental validation using soft lithography and 3D printing confirms successful sorting and separation, aligning well with numerical results. This demonstrates the successful sorting and separating of injected particles within a hydrodynamically focused sample in all systems. Both numerical and experimental findings indicate a separation accuracy of 100% across various Reynolds numbers. The primary channel dimensions measure 100 µm in height and 200 µm in width. N52 permanent magnets were employed in both numerical simulations and experiments. For numerical simulations, a remanent flux density of 1.48 T was utilized. In the experimental setup, magnets measuring 0.5 × 0.5 × 0.125 inches and 0.5 × 0.5 × 1 inch were employed. The experimental data confirm the device's capability to achieve 100% separation accuracy at a Reynolds number of 3. However, this study did not explore the potential impact of increased flow rates on separation accuracy.

Creating multiple ultra-long longitudinal magnetization textures by strongly focusing azimuthally polarized circular Airy vortex beams

We come up with a simple feasible scheme for the creation of multiple ultra-long longitudinal magnetization textures. This is realized by directly strongly focusing azimuthally polarized circular Airy vortex beams onto an isotropic magneto-optical medium based on the vectorial diffraction theory and the inverse Faraday effect. It is found that, by jointly tuning the intrinsic parameters (i. e. the radius of main ring, the scaling factor, and the exponential decay factor) of the incoming Airy beams and the topological charges of the optical vortices, we are able to garner not only super-resolved scalable magnetization needles as usual, but also steerable magnetization oscillations and nested magnetization tubes with opposite polarities for the first time. These exotic magnetic behaviors depend on the extended interplay between the polarization singularity of multi-ring structured vectorial light fields and the additional vortex phase. The findings demonstrated are of great interest in opto-magnetism and emerging classical or quantum opto-magnetic applications.

High-gradient magnetophoretic bead trapping for enhanced electrochemical sensing and particle manipulation

Magnetic particles are routinely used in many biochemical techniques. As such, the manipulation of these particles is of paramount importance for proper detection and assay preparation. This paper describes a magnetic manipulation and detection paradigm that allows sensing and handling highly sensitive magnetic bead-based assays. The simple manufacturing process presented in this manuscript employs a CNC machining technique and an iron microparticle-doped PDMS (Fe-PDMS) compound to create magnetic microstructures that enhance magnetic forces for magnetic bead confinement. Said confinement, generates increases in local concentrations at the detection site. Higher local concentrations increase the magnitude of the detection signal, leading to higher assay sensitivity and lower limit of detection (LOD). Furthermore, we demonstrate this characteristic signal enhancement in both fluorescence and electrochemical detection techniques. We expect this new technique to allow users to design fully integrated magnetic bead-based microfluidic devices with the goal of preventing sample losses and enhancing signal magnitudes in biological experiments and assays.

Tunable hydrodynamic focusing with dual-neodymium magnet-based microfluidic separation device

Microfluidic separation technologies are the focus of various biological applications, such as disease diagnostics, single-cell analysis, and therapeutics. Different methods and devices were proposed in the micro-separation field, focusing on minimizing the chemical deformation and physical damage to the particles throughout the separation process; however, it is still a challenge. This paper proposes a hydrodynamic focusing-based microfluidic separation device equipped with a dual-neodymium magnet for positive magnetophoretic microparticles and cell separation. Hydrodynamic focusing is used to help to sort the particles and minimize the damage to the microparticles through the proposed different inlet flow rates between the two focusing channels. The dual magnets help to separate the particles in two stages. The system's novelty is integrating the hydrodynamic focusing with the dual magnetics system, where the hydrodynamic focusing is with variable inlet flow rates. The performance of the proposed microfluidic particle separator is numerically assessed under various operating parameters, including the concentration of the particle in the injected solution and flow rate ratios of high to the low focusing flows on the efficiency of the separation. Following the proposed separation method, it was possible to separate the 16 and 10 [Formula: see text] microparticles with the first-round efficiency of 21% with a quality of 92%, respectively. The developed particle separation system can significantly broaden its applications in a variety of biomedical research studies.

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.

Recent Development of Microfluidic Technology for Cell Trapping in Single Cell Analysis: A Review

Microfluidic technology has emerged from the MEMS (Micro-Electro-Mechanical System)-technology as an important research field. During the last decade, various microfluidic technologies have been developed to open up a new era for biological studies. To understand the function of single cells, it is very important to monitor the dynamic behavior of a single cell in a living environment. Cell trapping in single cell analysis is urgently demanded There have been some review papers focusing on drug screen and cell analysis. However, cell trapping in single cell analysis has rarely been covered in the previous reviews. The present paper focuses on recent developments of cell trapping and highlights the mechanisms, governing equations and key parameters affecting the cell trapping efficiency by contact-based and contactless approach. The applications of the cell trapping method are discussed according to their basic research areas, such as biology and tissue engineering. Finally, the paper highlights the most promising cell trapping method for this research area.

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.

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.

Dual-neodymium magnet-based microfluidic separation device

Microfluidic-based separation methods have been highlighted for a number of biological applications, such as single cell analysis, disease diagnostics, and therapeutics. Although a number of previous studies have been carried out to minimize the physical damage and chemical deformations of the sample during the separation process, it still remains a challenge. In this paper, we developed a microfluidic device with dual-neodymium magnet-based negative magnetophoresis for the separation of the microparticles and cells. The poly(ethylene oxide) (PEO) was added to the solution to increase the viscoelasticity of the medium which could assist the sorting of the microparticles in the microfluidic device even at low flow rates, while minimizing damage to the cells and microparticles. Following this method, it was possible to separate 10 and 16 μm microparticles with high efficiency of 99 ± 0.1%, and 97 ± 0.8%, respectively. We also demonstrated the separation of glioblastoma cancer cells and neural stem cells (NSCs) in the microfluidic device.

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.

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.

Recent Advances and Future Perspectives on Microfluidic Liquid Handling

The interdisciplinary research field of microfluidics has the potential to revolutionize current technologies that require the handling of a small amount of fluid, a fast response, low costs and automation. Microfluidic platforms that handle small amounts of liquid have been categorised as continuous-flow microfluidics and digital microfluidics. The first part of this paper discusses the recent advances of the two main and opposing applications of liquid handling in continuous-flow microfluidics: mixing and separation. Mixing and separation are essential steps in most lab-on-a-chip platforms, as sample preparation and detection are required for a variety of biological and chemical assays. The second part discusses the various digital microfluidic strategies, based on droplets and liquid marbles, for the manipulation of discrete microdroplets. More advanced digital microfluidic devices combining electrowetting with other techniques are also introduced. The applications of the emerging field of liquid-marble-based digital microfluidics are also highlighted. Finally, future perspectives on microfluidic liquid handling are discussed.

Magnetofluidic concentration and separation of non-magnetic particles using two magnet arrays

The present paper reports the use of diluted ferrofluid and two arrays of permanent magnets for the size-selective concentration of non-magnetic particles. The micro magnetofluidic device consists of a straight channels sandwiched between two arrays of permanent magnets. The permanent magnets create multiple capture zones with minimum magnetic field strength along the channel. The complex interaction between magnetic forces and hydrodynamic force allows the device to operate in different regimes suitable for concentration of non-magnetic particles with small difference in size. Our experimental results show that non-magnetic particles with diameters of 3.1 μm and 4.8 μm can be discriminated and separated with this method. The results from this study could be used as a guide for the design of size-sensitive separation devices for particle and cell based on negative magnetophoresis.

Label-Free Microfluidic Manipulation of Particles and Cells in Magnetic Liquids

Manipulating particles and cells in magnetic liquids through so-called "negative magnetophoresis" is a new research field. It has resulted in label-free and low-cost manipulation techniques in microfluidic systems and many exciting applications. It is the goal of this review to introduce the fundamental principles of negative magnetophoresis and its recent applications in microfluidic manipulation of particles and cells. We will first discuss the theoretical background of three commonly used specificities of manipulation in magnetic liquids, which include the size, density and magnetic property of particles and cells. We will then review and compare the media used in negative magnetophoresis, which include paramagnetic salt solutions and ferrofluids. Afterwards, we will focus on reviewing existing microfluidic applications of negative magnetophoresis, including separation, focusing, trapping and concentration of particles and cells, determination of cell density, measurement of particles' magnetic susceptibility, and others. We will also examine the need for developing biocompatible magnetic liquids for live cell manipulation and analysis, and its recent progress. Finally, we will conclude this review with a brief outlook for this exciting research field.