Tunable, Sheathless Focusing of Diamagnetic Particles in Ferrofluid Microflows with a Single Set of Overhead Permanent Magnets

A High-Throughput Circular Tumor Cell Sorting Chip with Trapezoidal Cross Section

Circulating tumor cells are typically found in the peripheral blood of patients, offering a crucial pathway for the early diagnosis and prediction of cancer. Traditional methods for early cancer diagnosis are inefficient and inaccurate, making it difficult to isolate tumor cells from a large number of cells. In this paper, a new spiral microfluidic chip with asymmetric cross-section is proposed for rapid, high-throughput, label-free enrichment of CTCs in peripheral blood. A mold of the desired flow channel structure was prepared and inverted to make a trapezoidal cross-section using a micro-nanotechnology process of 3D printing. After a systematic study of how flow rate, channel width, and particle concentration affect the performance of the device, we utilized the device to simulate cell sorting of 6 μm, 15 μm, and 25 μm PS (Polystyrene) particles, and the separation efficiency and separation purity of 25 μm PS particles reached 98.3% and 96.4%. On this basis, we realize the enrichment of a large number of CTCs in diluted whole blood (5 mL). The results show that the separation efficiency of A549 was 88.9% and the separation purity was 96.4% at a high throughput of 1400 μL/min. In conclusion, we believe that the developed method is relevant for efficient recovery from whole blood and beneficial for future automated clinical analysis.

Magnetophoresis-Enhanced Elasto-Inertial Migration of Microparticles and Cells in Microfluidics

Microfluidic particle and cell manipulation techniques possess many potentials for biomedicine and healthcare. Many techniques have been developed based on active (e.g., electrical, magnetic, acoustic, and thermal) force fields and passive hydrodynamic forces (e.g., inertial and elastic lift forces). However, techniques based on a single active or passive manipulating physics cannot always meet the demands, and combining multiple physics becomes a promising strategy to promote technique flexibility and versatility. In this work, we explored the physical coupling of magnetophoresis with the elastic and inertial (i.e., elasto-inertial) lift forces for the manipulation of microparticles. Particle lateral migration was studied in a coflowing configuration of viscoelastic ferrofluid/water (sample/sheath). The particles were suspended in the viscoelastic ferrofluid and confined near the channel sidewall by a sheath flow. The coordination of magnetophoresis and elasto-inertial lift forces promoted the cross-stream migration of particles. Besides, we investigated the effect of the flow rate ratio and total flow rate on the migration of particles. Furthermore, we also investigated the effects of fluid elasticity in sample and sheath flows on particle migration using different combinations of sample and sheath flows, including Newtonian ferrofluid/water, Newtonian ferrofluid/viscoelastic fluid, and viscoelastic ferrofluid/viscoelastic coflows. Experimental results demonstrated and ascertained the promoted particle lateral migration in the PEO-based ferrofluid/water coflow. Finally, we demonstrate the proof-of-concept application of the physical coupling strategy for cell cross-stream migration and solution exchange. We envisage that this novel multiphysical coupling scheme has great potential for the flexible and versatile manipulation of microparticles and cells.

A portable microfluidic device for thermally controlled granular sample manipulation

Effective granular sample manipulation with a portable and visualizable microfluidic device is significant for lots of applications, such as point-of-care testing and cargo delivery. Herein, we report a portable microfluidic device for controlled particle focusing, migration and double-emulsion droplet release via thermal fields. The device mainly contains a microfluidic chip, a microcontroller with a DC voltage control unit, a built-in microscope with a video transmission unit and a smartphone. Five microheaters located at the bottom of the microfluidic chip are used to unevenly heat fluids and then induce thermal buoyancy flow and a thermocapillary effect, and the experiments can be conveniently visualized through a smartphone, which provides convenient sample detection in outdoor environments. To demonstrate the feasibility and multifunctionality of this device, the focusing manipulation of multiple particles is first analyzed by using silica particles and yeast cells as experimental samples. We can directly observe the particle focusing states on the screen of a smartphone, and the particle focusing efficiency can be flexibly tuned by changing the control voltage of the microheater. Then the study focus is transferred to single-particle migration. By changing the voltage combinations applied on four strip microheaters, the single particle can migrate at predetermined trajectory and speed, showing attractiveness for those applications needing sample transportation. Finally, we manipulate the special three-phase flow system of double-emulsion drops in thermal fields. Under the combined effect of the thermocapillary effect and increased instability, the shell of double-emulsion droplets gradually thins and finally breaks, resulting in the release of samples in inner cores. The core release speed can also be flexibly adjusted by changing the control voltage of the microheater. These three experiments successfully demonstrate the effectiveness and multifunctionality of this thermally actuated microfluidic device on granular manipulation. Therefore, this portable microfluidic device will be promising for lots of applications, such as analytical detection, microrobot actuation and cargo release.

Radial Magnetic Levitation and Its Application to Density Measurement, Separation, and Detection of Microplastics

This work describes the development of radial magnetic levitation (MagLev) using two radially magnetized ring magnets to solve the problem of limited operational spaces in standard MagLev and the major shortcoming of a short working distance in axial MagLev. Interestingly and importantly, we demonstrate that for the same magnet size, this new configuration of MagLev doubles the working distance over the axial MagLev without significantly sacrificing the density measurement range, whether for linear or nonlinear analysis. Meanwhile, we develop a magnetic assembly method to fabricate the magnets for the radial MagLev, where multiple magnetic tiles with single-direction magnetization are used as assembly elements. On this basis, we experimentally demonstrate that the radial MagLev has good applicability in density-based measurement, separation, and detection and show its advantages in improving separation performance compared with the axial MagLev. The open structure of two-ring magnets and good levitation characteristics make the radial MagLev have great application potential, and the performance improvement brought by adjusting the magnetization direction of magnets provides a new perspective for the magnet design in the field of MagLev.

Novel size-based design of spiral microfluidic devices with elliptic configurations and trapezoidal cross-section for ultra-fast isolation of circulating tumor cells

Investigation and analysis of circulating tumor cells (CTCs) have been valuable resources for detecting and diagnosing cancer in its early stages. Recently, enumeration and separation of CTCs via microfluidic devices have attracted significant attention due to their low cost and easy setup. In this study, novel microfluidic devices based on size-dependent cell-sorting with a trapezoidal cross-section and elliptic spiral configurations were proposed to reach label-free, ultra-fast CTCs enrichment. Firstly, the possibility and quality of separation in the devices were evaluated via a numerical simulation. Subsequently, these devices were fabricated to investigate the effects of the altering curvature and the trapezoidal cross-section on the isolation of CTCs from the peripheral blood sample at varying flow rates ranging from 0.5 mL/min to 3.5 mL/min. The experimental results indicated that the flow rate of 2.5 mL/min provided the optimal separation efficiency in the proposed devices, which was in fine agreement with the numerical analysis results. In this experiment, the purity values of CTCs between 88% and 90% were achieved, which is an indicator of the high capability of the proposed devices for the isolation and enrichment of CTCs. This strategy is hoped to overcome the limitations of classical affinity-based CTC separation approaches in the future.

Poly(ethylene oxide) Concentration Gradient-Based Microfluidic Isolation of Circulating Tumor Cells

Circulating tumor cells (CTCs) have emerged as promising circulating biomarkers for non-invasive cancer diagnosis and management. Isolation and detection of CTCs in clinical samples are challenging due to the extreme rarity and high heterogeneity of CTCs. Here, we describe a poly(ethylene oxide) (PEO) concentration gradient-based microfluidic method for rapid, label-free, highly efficient isolation of CTCs directly from whole blood samples. Stable concentration gradients of PEO were formed within the microchannel by co-injecting the side fluid (blood sample spiked with 0.025% PEO) and center fluid (0.075% PEO solution). The competition between the elastic lift force and the inertial lift force enabled size-based separation of large CTCs and small blood cells based on their distinct migration patterns. The microfluidic device could process 1 mL of blood sample in 30 min, with a separation efficiency of >90% and an enrichment ratio of >700 for tumor cells. The isolated CTCs from blood samples were enumerated by immunofluorescence staining, allowing for discrimination of breast cancer patients from healthy donors with an accuracy of 84.2%. The concentration gradient-based microfluidic separation provides a powerful tool for label-free isolation of CTCs for a wide range of clinical applications.

Negative Magnetophoresis Focusing Microchips Online-Coupled with ICP-MS for High-Throughput Single-Cell Analysis

High-throughput single-cell analysis is critical to elucidate the cell heterogeneity. Recently, droplet microchips using oil/gas phases to generate single-cell encapsulated droplets have been combined with inductively coupled plasma-mass spectrometry (ICP-MS) for determination of trace elements in single cells with a throughput of dozens of cells per min. To improve the sample throughput and avoid the oil phase introduced into ICP-MS, herein, a negative magnetophoresis focusing microchip was established and online-coupled to ICP-MS for single-cell analysis. MCF-7 cells in the paramagnetic salt solution were introduced into the designed focusing microchannel, in which they were focused into a single stream under both the magnetic repulsion force and inertial lift force, and then were introduced into ICP-MS for online single-cell analysis. The important parameters including the chip design, the concentration of the paramagnetic salt solution, flow rate, cell density, and dwell time were optimized. Under the optimal conditions, a high sample throughput of 1390 cells min^-1 was obtained. The established online analytical system was applied to study the uptake behaviors of MCF-7 cells for Zn²⁺ and ZnO nanoparticles (NPs) at a single-cell level. The single-cell analysis results indicate that MCF-7 cells displayed more remarkable heterogeneity when they were treated with ZnO NPs, and the uptake content of ZnO NPs by MCF-7 cells was less than that of Zn²⁺. Compared with other droplet microdevice-ICP-MS analysis systems, the developed system has the advantages of simple design and fabrication, no organic phase, a high throughput, and a low sample consumption (only 5 μL).

Integrated Microfluidic Handheld Cell Sorter for High-Throughput Label-Free Malignant Tumor Cell Sorting

Handheld sample preparation devices are urgently required for point-of-care diagnosis in resource-limited settings. In this paper, we develop a novel handheld sorter with a multifunction integrated microfluidic chip. The integrated microfluidic handheld sorter (μHCS) is composed of three units, including cartridges, shells, and core integrated microchip. The integrated microchip contains two flow regulators for achieving the on-chip regulation of the input flows generated by a low-cost diaphragm pump to the desired flow rates and a spiral inertial microfluidic channel for size-based cell separation. After introducing the conceptual design of our μHCS system, the performances of the separate spiral channel and flow regulator are systematically characterized and optimized, respectively. Finally, the prototype of the μHCS is successfully assembled to separate the malignant tumor cells from the clinical pleural effusions. Our μHCS is simple to use, inexpensive, portable, and compact and can be used for high-throughput label-free separation of rare cells from large volume samples in resource-limited areas.

Design and operation of magnetophoretic systems at microscale: Device and particle approaches

Combining both device and particle designs are the essential concepts to be considered in magnetophoretic system development. Researcher efforts are often dedicated to only one of these design aspects and neglecting the interplay between them. Herein, to bring out importance of the idea of integration between device and particle, we reviewed the working principle of magnetophoretic system (includes both device and particle design concepts). Since, the magnetophoretic force is influenced by both field gradient and magnetization volume, hence, accurate prediction of the magnetophoretic force is relying on the availability of information on both parameters. In device design, we focus on the different strategies used to create localized high-field gradient. For particle design, we emphasize on the scaling between hydrodynamic size and magnetization volume. Moreover, we also briefly discussed the importance of magnetoshape anisotropy related to particle design aspect of magnetophoretic systems. Next, we illustrated the need for integration between device and particle design using microscale applications of magnetophoretic systems, include magnetic tweezers and microfluidic systems, as our working example. On the basis of our discussion, we highlighted several promising examples of microscale magnetophoretic systems which greatly utilized the interplay between device and particle design. Further, we concluded the review with several factors that possibly resulted in the lack of research efforts related to device and particle design integration.

AC Insulator-Based Dielectrophoretic Focusing of Particles and Cells in an "Infinite" Microchannel

It is often necessary to prefocus particles and cells into a tight stream for subsequent separation and/or analysis in microfluidic devices. A DC electric field has been widely used for particle and cell focusing in insulator-based dielectrophoretic (iDEP) microdevices, where a large field magnitude, a high constriction ratio, and/or a long microchannel are usually required to enhance the iDEP effect. We demonstrate, in this work, an AC iDEP focusing technique, which utilizes a low-frequency AC electric field to generate both an oscillatory electrokinetic flow of the particle/cell suspension and a field direction-independent dielectrophoretic force for particle/cell focusing in a virtually "infinite" microchannel. We also develop a theoretical analysis to evaluate this focusing in terms of the AC voltage frequency, amplitude, and particle size, which are each validated through both experimental demonstration and numerical simulation. The effectiveness of AC iDEP focusing increases with the second order of electric field magnitude, superior to DC iDEP focusing with only a first-order dependence. This feature and the "infinite" channel length together remove the necessity of large electric field and/or small constriction in DC iDEP focusing of small particles.

Rapid and precise tumor cell separation using the combination of size-dependent inertial and size-independent magnetic methods

Circulating tumor cells (CTCs) play a significant role in cancer diagnosis and treatment monitoring. One of the major challenges in isolating and detecting rare CTCs from blood is that white blood cells (WBCs) have a size overlap with the target CTCs. To address this issue, we constructed a three-stage i-Mag device integrated with passive inertial microfluidics and active magnetophoresis, enabling rapid and precise separation of tumor cells from blood. The first-stage spiral inertial sorter was applied to rapidly remove small-sized red blood cells (RBCs), and then the second-stage serpentine inertial focuser and the third-stage magnetic sorter were used for removing the magnetically labeled WBCs size-independently, to significantly purify the captured tumor cells. Then, the separation performance of our i-Mag device was explored. The results indicated rapid and precise separation of breast cancer cells from diluted whole blood at a high separation efficiency of 93.84% and at a high purity of 51.47%. The purity of the collected tumor cells could be further improved to 93.60% when the blood dilution ratio was increased. We also successfully applied our i-Mag device for the isolation and detection of trace tumor cells. Our i-Mag device has numerous advantages, such as enabling high-throughput processing and high-precision separation, requiring easy manufacturing at a low cost, and providing tumor antigen-independent operation. We believe that the i-Mag device has great potential to act as a precise tool for separating various bioparticles.

A polymer-film inertial microfluidic sorter fabricated by jigsaw puzzle method for precise size-based cell separation

A polymer-film inertial microfluidic jigsaw (PIMJ) sorter with trapezoidal spiral channels using the jigsaw puzzle method was proposed to realize precise and high-throughput rare cell separation. The PIMJ sorter was fabricated by assembling laser-patterned polymer-film layers of different thicknesses. After illustrating the conceptual design and fabrication process, the effects of the cross-sectional dimension, particle size, and operational flow rate on particle focusing were systematically explored under a broad flow rate range. Then, the separation performances of the PIMJ sorter were characterized using the binary particle mixture and the blood samples spiked with four types of tumor cells. The results indicated that the complete separation of the binary particles with a minimum size difference of 2 μm was successfully realized at a high throughput up to 3000 μL/min. A high recovery ratio of 90.57%-94.14% and a high purity of 48.67%-79.05% were achieved for the separation of rare tumor cells from white blood cells (WBCs). Finally, the PIMJ sorter was successfully employed for separating rare circulating tumor cells (CTCs) from the metastatic breast and lung cancer patients with a capture ratio of 7-226 CTCs per 5 mL sample. The results proved the high sensitivity and high reliability of the PIMJ sorter. The PIMJ sorter is expected to be a potential device for precise CTC separation towards the clinical applications.

Investigation on Inertial Sorter Coupled with Magnetophoretic Effect for Nonmagnetic Microparticles

The sizes of most prokaryotic cells are several microns. It is very difficult to separate cells with similar sizes. A sorter with a contraction–expansion microchannel and applied magnetic field is designed to sort microparticles with diameters of 3, 4 and 5 microns. To evaluate the sorting efficiency of the designed sorter, numerical simulations for calculating the distributions of microparticles with similar sizes were carried out for various magnetic fields, inlet velocities, sheath flow ratios and structural parameters. The numerical results indicate that micro-particles with diameters of 3, 4 and 5 microns can be sorted efficiently in such a sorter within appropriate parameters. Furthermore, it is shown that a bigger particle size and more powerful magnetic field can result in a greater lateral migration of microparticles. The sorting efficiency of microparticles promotes a lower inlet velocity and greater sheath flow ratios. A smaller contraction–expansion ratio can induce a greater space between particle-bands. Finally, the micro particle image velocity (micro-PIV) experiments were conducted to obtain the bandwidths and spaces between particle-bands. The comparisons between the numerical and experimental results show a good agreement and make the validity of the numerical results certain.

Microfluidics for label-free sorting of rare circulating tumor cells

A review discussing the working principles and performances of label-free CTC sorting methods.

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.