Microfluidic Magnetic Bead Assay for Cell Detection

Resistive Pulse Sensing on a Capillary-Assisted Microfluidic Platform for On-Site Single-Particle Analyses

Capillary-assisted flow is valuable for utilizing microfluidics-based electrical sensing platforms at on-site locations by simplifying microfluidic operations and system construction; however, incorporating capillary-assisted flow in platforms requires easy microfluidic modification and stability over time for capillary-assisted flow generation and sensing performance. Herein, we report a capillary-assisted microfluidics-based electrical sensing platform using a one-step modification of polydimethylsiloxane (PDMS) with polyethylene glycol (PEG). As a model of electrical sensing platforms, this work focused on resistive pulse sensing (RPS) using a micropore in a microfluidic chip for label-free electrical detection of single analytes, and filling the micropore with an electrolyte is the first step to perform this RPS. The PEG-PDMS surfaces remained hydrophilic after ambient storage for 30 d and assisted in generating an electrolyte flow for filling the micropore with the electrolyte. We demonstrated the successful detection and size analysis of micrometer particles and bacterial cells based on RPS using the microfluidic chip stored in a dry state for 30 d. Combining this capillary-assisted microfluidic platform with a portable RPS system makes on-site detection and analysis of single pathogens possible.

Portable and sensitive detection of cancer cells via a handheld luminometer

A simple and sensitive chemiluminescent method for portable detection of cancer cells via a handheld luminometer.

The Potential Application of Magnetic Nanoparticles for Liver Fibrosis Theranostics

Liver fibrosis is a major cause of morbidity and mortality worldwide due to chronic liver damage and leading to cirrhosis, liver cancer, and liver failure. To date, there is no effective and specific therapy for patients with hepatic fibrosis. As a result of their various advantages such as biocompatibility, imaging contrast ability, improved tissue penetration, and superparamagnetic properties, magnetic nanoparticles have a great potential for diagnosis and therapy in various liver diseases including fibrosis. In this review, we focus on the molecular mechanisms and important factors for hepatic fibrosis and on potential magnetic nanoparticles-based therapeutics. New strategies for the diagnosis of liver fibrosis are also discussed, with a summary of the challenges and perspectives in the translational application of magnetic nanoparticles from bench to bedside.

Label-free counting of affinity-enriched circulating tumor cells (CTCs) using a thermoplastic micro-Coulter counter (μCC)

Coulter counters are used for counting particles and biological cells. Most Coulter counters are designed to analyze a sample without the ability to pre-process the sample prior to counting. For the analysis of rare cells, such as circulating tumor cells (CTCs), it is not uncommon to require enrichment before counting due to the modest throughput of μCCs and the high abundance of interfering cells, such as blood cells. We report a microfluidic-based Coulter Counter (μCC) fabricated using simple, low-cost techniques for counting rare cells that can be interfaced to sample pre- and/or post-processing units. In the current work, a microfluidic device for the affinity-based enrichment of CTCs from whole blood into a relatively small volume of ∼10 μL was interfaced to the μCC to allow for exhaustive counting of single CTCs following release of the CTCs from the enrichment chip. When integrated to the CTC affinity enrichment chip, the μCC could count the CTCs without loss and the cells could be collected for downstream molecular profiling or culturing if required. The μCC sensor counting efficiency was >93% and inter-chip variability was ∼1%.

Microfluidics-Implemented Biochemical Assays: From the Perspective of Readout

Over the past decades, microfluidics has emerged as an increasingly important tool to perform biochemical assays for diagnosis and healthcare. The precise fluid control and molecule manipulation within microfluidics greatly contribute to developing assays with simplicity and convenience. The advantages of microfluidics, including decreased consumption of reagents and samples, lower operating and analysis time, much lower cost, and higher integration and automation over traditional systems, offer a great platform to meet the needs of point-of-care applications. In this Review, versatile strategies are outlined and recent advances in microfluidics-implemented assays are discussed from the perspective of readout, because a convenient and straightforward readout is what a biochemical assay requires and the end user desires. Functions and properties arising from each readout are reviewed and the advantages and limitations of each readout are discussed together with current challenges and future perspectives.

A Microfluidic Sensor for Continuous, in Situ Surface Charge Measurement of Single Cells

Cell surface charge has been recognized as an important cellular property. We developed a microfluidic sensor based on resistive pulse sensing to assess surface charge and sizes of single cells suspended in a continuous flow. The device consists of two consecutive resistive pulse sensors (RPSs) with identical dimensions. Opposite electric fields were applied on the two RPSs. A charged cell in the RPSs was accelerated or decelerated by the electric fields and thus exhibited different transit times passing through the two RPSs. The cell surface charge is measured with zeta potential that can be quantified with the transit time difference. The transit time of each cell can be accurately detected with the width of pulses generated by the RPS, while the cell size can be calculated with the pulse magnitude at the same time. This device has the ability to detect surface charges and sizes of individual cells with high tolerance in cell types and testing solutions compared with traditional electrophoretic light scattering methods. Three different types of cells including HeLa cancer cells, human dermal fibroblast cells, and human umbilical vein endothelial cells (HUVECs) were tested with the sensor. Results showed a significant difference of zeta potentials between HeLa cells and fibroblasts or HUVECs. In addition, when HeLa cells were treated with various concentrations of glutamine, the effects on cancer cell surface charge were detected. Our results demonstrated the great potential of using our sensor for cell type sorting, cancer cell detection, and cell status analysis.

Magnetic-Assisted Treatment of Liver Fibrosis

Chronic liver injury can be induced by viruses, toxins, cellular activation, and metabolic dysregulation and can lead to liver fibrosis. Hepatic fibrosis still remains a major burden on the global health systems. Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) are considered the main cause of liver fibrosis. Hepatic stellate cells are key targets in antifibrotic treatment, but selective engagement of these cells is an unresolved issue. Current strategies for antifibrotic drugs, which are at the critical stage 3 clinical trials, target metabolic regulation, immune cell activation, and cell death. Here, we report on the critical factors for liver fibrosis, and on prospective novel drugs, which might soon enter the market. Apart from the current clinical trials, novel perspectives for anti-fibrotic treatment may arise from magnetic particles and controlled magnetic forces in various different fields. Magnetic-assisted techniques can, for instance, enable cell engineering and cell therapy to fight cancer, might enable to control the shape or orientation of single cells or tissues mechanically. Furthermore, magnetic forces may improve localized drug delivery mediated by magnetism-induced conformational changes, and they may also enhance non-invasive imaging applications.

Raman spectroscopy to unravel the magnetic properties of iron oxide nanocrystals for bio-related applications

Iron oxide nanocrystals have become a versatile tool in biomedicine because of their low cytotoxicity while offering a wide range of tuneable magnetic properties that may be implemented in magnetic separation, drug and heat delivery and bioimaging. These capabilities rely on the unique magnetic features obtained when combining different iron oxide phases, so that an important portfolio of magnetic properties can be attained by the rational design of multicomponent nanocrystals. In this context, Raman spectroscopy is an invaluable and fast-performance tool to gain insight into the different phases forming part of the nanocrystals to be used, allowing correlation of the magnetic properties with the envisaged bio-related applications.

Resistive pulse sensing as particle counting and sizing method in microfluidic systems: Designs and applications review

Resistive pulse sensing is a well-known and established method for counting and sizing particles in ionic solutions. Throughout its development the technique has been expanded from detection of biological cells to counting nanoparticles and viruses, and even registering individual molecules, e.g., nucleotides in nucleic acids. This technique combined with microfluidic or nanofluidic systems shows great potential for various bioanalytical applications, which were hardly possible before microfabrication gained the present broad adoption. In this review, we provide a comprehensive overview of microfluidic designs along with electrode arrangements with emphasis on applications focusing on bioanalysis and analysis of single cells that were reported within the past five years.

A Numerical Model of Blood Flow Velocity Measurement Based on Finger Ring

Aiming to measure the blood flow velocity in a finger, a novel noninvasive method, i.e., a ring with a heat source chip and a temperature sensor, is designed in this paper. The heat source chip is used to heat the finger and generate heat diffusion between the chip and the temperature sensor. And the temperature sensor is designed to measure the temperature difference. Since the blood flow is the main medium of heat diffusion in bodies, part from the heat energy in the tissue will be taken away by the flowing blood. Therefore, the blood flow velocity can be acquired via its relationship with the temperature difference. Compared to the ultrasound Doppler method and the laser Doppler method, the proposed method guarantees a more convenient operation in more flexible work sites. We also analyze the theory between heat transfer and laminar flow. Finally, several simulations are conducted, and the influence of the relevant factors (i.e., the number of blood vessels, the radius, etc.) corresponding to the simulation results is also discussed.

Shaping iron oxide nanocrystals for magnetic separation applications

Iron oxide nanostructures are attractive for a variety of bio-related applications given their wide range of magnetic properties. Here, we report on the study of the magnetophoretic mobility of octapod-shaped nanocrystals, which we relate to stoichiometry, quality and elongation in the 111 direction of these cubic structures. This special morphology combines magnetocrystalline anisotropies, increases shape anisotropy and hinders the formation of an epitaxial wüstite-magnetite interface. As a result, one obtains nanocrystals with large magnetic susceptibility and small coercivity, that is, with optimum characteristics for magnetic guidance, separation, and drug delivery, due to the increased magnetophoretic mobility displayed.

An Inertia-Deformability Hybrid Circulating Tumor Cell Chip: Design, Clinical Test, and Numerical Analysis

Detection and capture of circulating tumor cells (CTCs) with microfluidic chips hold significance in cancer prognosis, diagnosis, and anti-cancer treatment. The counting of CTCs provides potential tools to evaluate cancer stages as well as treatment progress. However, facing the challenge of rareness in blood, the precise enumeration of CTCs is challenging. In the present research, we designed an inertial-deformability hybrid microfluidic chip using a long spiral channel with trapezoid-circular pillars and a capture zone. To clinically validate the device, the microfluidic chip has been tested for the whole blood and lysed blood with a small number of CTCs (colorectal and nonsmall-cell lung cancer) spiked in. The capture efficiency reaches over 90% for three types of cancer cell lines at the flow rate of 1.5 mL/h. Following numerical modeling was conducted to explain the working principle and working condition (Reynolds number below 10 and Dean number around 1). This design extended the effective capture length, improved the capture efficiency, and made the CTC enumeration much easier. We believe that this hybrid chip is promising clinically in the CTCs enumeration, evaluation of cancer therapy, and pharmacological responses.

Highly Chemiluminescent Magnetic Beads for Label-Free Sensing of 2,4,6-Trinitrotoluene

Until now, despite the great success acquired in scientific research and commercial applications, magnetic beads (MBs) have been used for nothing more than a carrier in most cases in bioassays. In this work, highly chemiluminescent magnetic beads containing N-(4-aminobutyl)-N-ethyl isoluminol (ABEI) and Co²⁺ (Co²⁺/ABEI/MBs) were first synthesized via a facile strategy. ABEI and Co²⁺ were grafted onto the surface of carboxylated MBs by virtue of a carboxyl group and electrostatic interaction. The as-prepared Co²⁺/ABEI/MBs exhibited good paramagnetic properties, satisfactory stability, and intense chemiluminescence (CL) emission when reacted with H₂O₂, which was more than 150 times that of ABEI functionalized MBs. Furthermore, it was found that 2,4,6-trinitrotoluene (TNT) aptamer could attach to the surface of Co²⁺/ABEI/MBs via electrostatic interaction and coordination interaction between TNT aptamer and Co²⁺, leading to a decrease in CL intensity due to the catalytic site Co²⁺ being blocked by the aptamer. In the presence of TNT, TNT would bind strongly with TNT aptamer and detach from the surface of Co²⁺/ABEI/MBs, resulting in partial restoration of the CL signal. Accordingly, label-free aptasensor was developed for the determination of TNT in the range of 0.05-25 ng/mL with a detection limit of 17 pg/mL. This work demonstrates that Co²⁺/ABEI/MBs are easily connected with recognition biomolecules, which are not only magnetic carriers but also direct sensing interfaces with excellent CL activity. It provides a novel CL interface with a magnetic property which easily separates analytes from the sample matrix to construct label-free bioassays.

Asymmetric bead aggregation for microfluidic immunodetection

We report an asymmetric immunoaggregation assay for rapid, label-free, and sub-picomolar protein detection. Asymmetric immunoaggregated beads (AIBs) are formed when binding occurs between 1 μm magnetic (MG) and 2.8 μm polystyrene (PS) beads coated with specific antibodies for a target antigen. Detection of such aggregation is achieved by optical monitoring of AIBs in a flow under an external magnetic field. AIBs are attracted to the upper surface of the microchannel by a magnetic field and made to slide along the surface by a flow drag force. This sliding behavior is in contrast with other particles such as MG and PS beads; while attracted MG beads hardly slide due to their small size, PS beads quickly move with the flow due to the lack of magnetism. Sliding AIBs are optically monitored in a designated sensing area in the microchannel. A custom-built program code is used for counting the AIBs and further analysis of parameters such as velocity and number distributions that are correlated with target concentrations. The detection range from 54 pg mL^-1 to 54 ng mL^-1 is demonstrated for the influenza type A H1N1 nucleoprotein (NP). This immunosensing system is simple, highly sensitive, and capable of quantitative detection of antigens in a single test without fluorescent or enzyme labeling, hence is useful for the rapid detection of biomarkers in clinical and biomedical applications.

Efficient Enrichment and Analyses of Bacteria at Ultralow Concentration with Quick-Response Magnetic Nanospheres

Enrichment and purification of bacteria from complex matrices are crucial for their detection and investigation, in which magnetic separation techniques have recently show great application advantages. However, currently used magnetic particles all have their own limitations: Magnetic microparticles exhibit poor binding capacity with targets, while magnetic nanoparticles suffer slow magnetic response and high loss rate during treatment process. Herein, we used a highly controllable layer-by-layer assembly method to fabricate quick-response magnetic nanospheres (MNs), and with Salmonella typhimurium as a model, we successfully achieve their rapid and efficient enrichment. The MNs combined the advantages of magnetic microparticles and nanoparticles. On the one hand, the MNs had a fast magnetic response, and almost 100% of the MNs could be recovered by 1 min attraction with a simple magnetic scaffold. Hence, using antibody conjugated MNs (immunomagnetic nanospheres, IMNs) to capture bacteria hardly generated loss and did not need complex separation tools or techniques. On the other hand, the IMNs showed much excellent capture capacity. With 20 min interaction, almost all of the target bacteria could be captured, and even only one bacterium existing in the samples was not missed, comparing with the immunomagnetic microparticles which could only capture less than 50% of the bacteria. Besides, the IMNs could achieve the same efficient enrichment in complex matrices, such as milk, fetal bovine serum, and urine, demonstrating their good stability, strong anti-interference ability, and low nonspecific adsorption. In addition, the isolated bacteria could be directly used for culture, polymerase chain reaction (PCR) analyses, and fluorescence immunoassay without a release process, which suggested our IMNs-based enrichment strategy could be conveniently coupled with the downstream identification and analysis techniques. Thus, the MNs provided by this work showed great superiority in bacteria enrichment, which would be a promising tool for bacteria detection and investigation.

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.

Acoustofluidic bacteria separation

Bacterial separation from human blood samples can help with the identification of pathogenic bacteria for sepsis diagnosis. In this work, we report an acoustofluidic device for label-free bacterial separation from human blood samples. In particular, we exploit the acoustic radiation force generated from a tilted-angle standing surface acoustic wave (taSSAW) field to separate E. coli from human blood cells based on their size difference. Flow cytometry analysis of the E. coli separated from red blood cells (RBCs) shows a purity of more than 96%. Moreover, the label-free electrochemical detection of the separated E. coli displays reduced non-specific signals due to the removal of blood cells. Our acoustofluidic bacterial separation platform has advantages such as label-free separation, high biocompatibility, flexibility, low cost, miniaturization, automation, and ease of in-line integration. The platform can be incorporated with an on-chip sensor to realize a point-of-care (POC) sepsis diagnostic device.