Micromagnet arrays enable precise manipulation of individual biological analyte-superparamagnetic bead complexes for separation and sensing

Combined Funnel, Concentrator, and Particle Valve Functional Element for Magnetophoretic Bead Transport Based on Engineered Magnetic Domain Patterns

Controlled actuation of superparamagnetic beads (SPBs) within a microfluidic environment using tailored dynamic magnetic field landscapes (MFLs) is a potent approach for the realization of point-of-care diagnostics within Lab-on-a-chip (LOC) systems. Making use of an engineered magnetic domain pattern as the MFL source, a functional LOC-element with combined magnetophoretic "funnel", concentrator, and "valve" functions for micron-sized SPBs is presented. A parallel-stripe domain pattern design with periodically decreasing/increasing stripe lengths is fabricated in a topographically flat continuous exchange biased (EB) thin film system by ion bombardment induced magnetic patterning (IBMP). It is demonstrated that, upon application of external magnetic field pulses, a fully reversible concentration of SPBs at the domain pattern's focal point occurs. In addition, it is shown that this functionality may be used as an SPB "funnel", allowing only a maximum number of particles to pass through the focal point. Adjusting the pulse time length, the focal point can be clogged up for incoming SPBs, resembling an on/off switchable particle "valve". The observations are supported by quantitative theoretical force considerations.

Numerical Studies on the Motions of Magnetically Tagged Cells Driven by a Micromagnetic Matrix

Precisely controlling magnetically tagged cells in a complex environment is crucial to constructing a magneto-microfluidic platform. We propose a two-dimensional model for capturing magnetic beads from non-magnetic fluids under a micromagnetic matrix. A qualitative description of the relationship between the capture trajectory and the micromagnetic matrix with an alternating polarity configuration was obtained by computing the force curve of the magnetic particles. Three stages comprise the capture process: the first, where motion is a parabolic fall in weak fields; the second, where the motion becomes unpredictable due to the competition between gravity and magnetic force; and the third, where the micromagnetic matrix finally captures cells. Since it is not always obvious how many particles are adhered to the surface, attachment density is utilized to illustrate how the quantity of particles influences the capture path. The longitudinal magnetic load is calculated to measure the acquisition efficiency. The optimal adhesion density is 13%, and the maximum adhesion density is 18%. It has been demonstrated that a magnetic ring model with 100% adhesion density can impede the capture process. The results offer a theoretical foundation for enhancing the effectiveness of rare cell capture in practical applications.

Ionic Liquid-Based Magnetic Needle Headspace Single-Drop Microextraction Combined with HPLC/UV for the Determination of Chlorophenols in Wastewater

A magnetic needle headspace single-drop microextraction (MN-HS-SDME) method coupled to HPLC/UV has been developed. Trihexyl(tetradecyl)phosphonium chloride was employed as an ionic liquid (IL) solvent for the headspace extraction of some chlorophenol (CP) compounds from wastewater samples. Despite of the nonmagnetic character of the IL, a significant improvement in the extraction efficiency was obtained by the magnetization of the single-drop microextraction needle using a pair of permanent disk magnets. A simplex method for the fast optimization of the experimental conditions (e.g., stirring speed, ionic strength, pH, extraction time and temperature) was used. The coefficients of determination (R2) varied between 0.9932 and 0.9989, the limits of detection were from 0.004 to 0.007 μg mL-1 and the relative recoveries were in the range of 88-120% for the studied analytes. The developed MN-HS-SDME HPLC/UV method was successfully applied to the determination of CPs in industrial wastewater.

Magnetophoretic circuits: A review of device designs and implementation for precise single-cell manipulation

Lab-on-a-chip tools have played a pivotal role in advancing modern biology and medicine. A key goal in this field is to precisely transport single particles and cells to specific locations on a chip for quantitative analysis. To address this large and growing need, magnetophoretic circuits have been developed in the last decade to manipulate a large number of single bioparticles in a parallel and highly controlled manner. Inspired by electrical circuits, magnetophoretic circuits are composed of passive and active circuit elements to offer commensurate levels of control and automation for transporting individual bioparticles. These specifications make them unique compared to other technologies in addressing crucial bioanalytical applications and answering fundamental questions buried in highly heterogeneous cell populations. In this comprehensive review, we describe key theoretical considerations for manufacturing and simulating magnetophoretic circuits. We provide a detailed tutorial for operating magnetophoretic devices containing different circuit elements (e.g., conductors, diodes, capacitors, and transistors). Finally, we provide a critical comparison of the utility of these devices to other microchip-based platforms for cellular manipulation, and discuss how they may address unmet needs in single-cell biology and medicine.

Three-dimensional close-to-substrate trajectories of magnetic microparticles in dynamically changing magnetic field landscapes

The transport of magnetic particles (MPs) by dynamic magnetic field landscapes (MFLs) using magnetically patterned substrates is promising for the development of Lab-on-a-chip (LOC) systems. The inherent close-to-substrate MP motion is sensitive to changing particle-substrate interactions. Thus, the detection of a modified particle-substrate separation distance caused by surface binding of an analyte is expected to be a promising probe in analytics and diagnostics. Here, we present an essential prerequisite for such an application, namely the label-free quantitative experimental determination of the three-dimensional trajectories of superparamagnetic particles (SPPs) transported by a dynamically changing MFL. The evaluation of defocused SPP images from optical bright-field microscopy revealed a "hopping"-like motion of the magnetic particles, previously predicted by theory, additionally allowing a quantification of maximum jump heights. As our findings pave the way towards precise determination of particle-substrate separations, they bear deep implications for future LOC detection schemes using only optical microscopy.

Directed Assembly of Nanomaterials for Making Nanoscale Devices and Structures: Mechanisms and Applications

Nanofabrication has been utilized to manufacture one-, two-, and three-dimensional functional nanostructures for applications such as electronics, sensors, and photonic devices. Although conventional silicon-based nanofabrication (top-down approach) has developed into a technique with extremely high precision and integration density, nanofabrication based on directed assembly (bottom-up approach) is attracting more interest recently owing to its low cost and the advantages of additive manufacturing. Directed assembly is a process that utilizes external fields to directly interact with nanoelements (nanoparticles, 2D nanomaterials, nanotubes, nanowires, etc.) and drive the nanoelements to site-selectively assemble in patterned areas on substrates to form functional structures. Directed assembly processes can be divided into four different categories depending on the external fields: electric field-directed assembly, fluidic flow-directed assembly, magnetic field-directed assembly, and optical field-directed assembly. In this review, we summarize recent progress utilizing these four processes and address how these directed assembly processes harness the external fields, the underlying mechanism of how the external fields interact with the nanoelements, and the advantages and drawbacks of utilizing each method. Finally, we discuss applications made using directed assembly and provide a perspective on the future developments and challenges.

Magnetic-Field-Driven Reconfigurable Microsphere Arrays for Laser Display Pixels

Reconfigurable microlaser arrays are essential to the construction of display panels where the individual pixel should be highly tunable in resonance mode, optical polarization, and lasing wavelength upon external control signals. Here we demonstrate a facile yet reliable approach to fabrication of organic microlaser pixels, in which the assembly of microsphere arrays on each pixel is controlled according to the near-field magnetostatic confinement. The geometrical configuration of diamagnetic microspheres could be readily modulated with the near-field potential traps by using the external field to alternate the saturation magnetization of the underneath micromagnet. The motion of microspheres can be modulated among several states upon applied field, and the reconfigurable microsphere array is thus achieved with high spatial precision and rapid temporal response. Moreover, both isolated and coupled spheres serve as low-threshold microlasers with tunable optical resonance modes, whereas the switching between the vertical and horizontal alignments of coupled spheres manipulates the polarization of lasing outputs. By repeating the magnetostatic confinement on the same substrate, the full-color laser display pixels with magnetically tunable color expression capability are successfully achieved.

High-throughput precise particle transport at single-particle resolution in a three-dimensional magnetic field for highly sensitive bio-detection

Precise manipulation of microparticles have fundamental applications in the fields of lab-on-a-chip and biomedical engineering. Here, for the first time, we propose a fully operational microfluidic chip equipped with thin magnetic films composed of straight tracks and bends which precisely transports numerous single-particles in the size range of ~ 2.8–20 µm simultaneously, to certain points, synced with the general external three-axial magnetic field. The uniqueness of this design arises from the introduced vertical bias field that provides a repulsion force between the particles and prevents unwanted particle cluster formation, which is a challenge in devices operating in two-dimensional fields. Furthermore, the chip operates as an accurate sensor and detects low levels of proteins and DNA fragments, being captured by the ligand-functionalized magnetic beads, while lowering the background noise by excluding the unwanted bead pairs seen in the previous works. The image-processing detection method in this work allows detection at the single-pair resolution, increasing the sensitivity. The proposed device offers high-throughput particle transport and ultra-sensitive bio-detection in a highly parallel manner at single-particle resolution. It can also operate as a robust single-cell analysis platform for manipulating magnetized single-cells and assembling them in large arrays, with important applications in biology.

Solid-phase extraction methods for nucleic acid separation. A review

The separation and purification of biomacromolecules such as nucleic acid is a perpetual topic in separation processes and bioengineering (fine chemicals, biopharmaceutical engineering, diagnostics, and biological characterization). In principle, the solid-phase extraction for nucleic acid exhibits efficient phase separation, low pollution risk, and small sample demand, compared to the conventional liquid-phase extraction. Herein, solid-phase extraction methods are systematically reviewed to outline research progress and explore additional solid-phase sorbents and devices for novel, flexible, and high-efficiency nucleic acid separation processes. The functional materials capture nucleic acid, magnetic and magnetic-free solid-phase extraction methods, separation device design and optimization, and high-throughput automatable applications based on high-performance solid-phase extraction are summarized. Finally, the current challenges and promising topics are discussed.

Transport Efficiency of Biofunctionalized Magnetic Particles Tailored by Surfactant Concentration

Controlled transport of surface-functionalized magnetic beads in a liquid medium is a central requirement for the handling of captured biomolecular targets in microfluidic lab-on-chip biosensors. Here, the influence of the physiological liquid medium on the transport characteristics of functionalized magnetic particles and on the functionality of the coupled protein is studied. These aspects are theoretically modeled and experimentally investigated for prototype superparamagnetic beads, surface-functionalized with green fluorescent protein immersed in buffer solution with different concentrations of a surfactant. The model reports on the tunability of the steady-state particle substrate separation distance to prevent their surface sticking via the choice of surfactant concentration. Experimental and theoretical average velocities are discussed for a ratchet-like particle motion induced by a dynamic external field superposed on a static locally varying magnetic field landscape. The developed model and experiment may serve as a basis for quantitative forecasts on the functionality of magnetic particle transport-based lab-on-chip devices.

Mattertronics for programmable manipulation and multiplex storage of pseudo-diamagnetic holes and label-free cells

Manipulating and separating single label-free cells without biomarker conjugation have attracted significant interest in the field of single-cell research, but digital circuitry control and multiplexed individual storage of single label-free cells remain a challenge. Herein, by analogy with the electrical circuitry elements and electronical holes, we develop a pseudo-diamagnetophoresis (PsD) mattertronic approach in the presence of biocompatible ferrofluids for programmable manipulation and local storage of single PsD holes and label-free cells. The PsD holes conduct along linear negative micro-magnetic patterns. Further, eclipse diode patterns similar to the electrical diode can implement directional and selective switching of different PsD holes and label-free cells based on the diode geometry. Different eclipse heights and junction gaps influence the switching efficiency of PsD holes for mattertronic circuitry manipulation and separation. Moreover, single PsD holes are stored at each potential well as in an electrical storage capacitor, preventing multiple occupancies of PsD holes in the array of individual compartments due to magnetic Coulomb-like interaction. This approach may enable the development of large programmable arrays of label-free matters with high throughput, efficiency, and reliability as multiplex cell research platforms.

Design of micromagnetic arrays for on-chip separation of superparamagnetic bead aggregates and detection of a model protein and double-stranded DNA analytes

Magnetically actuated lab-on-a-chip (LOC) technologies have enabled rapid, highly efficient separation of specific biomarkers and cells from complex biological samples. Nonlinear magnetophoresis (NLM) is a technique that uses a microfabricated magnet array (MMA) and a time varying external magnetic field to precisely control the transport of superparamagnetic (SPM) beads on the surface of a chip based on their size and magnetization. We analyze the transport and separation behavior of SPM monomers and dimers on four MMA geometries, i.e., circular, triangular, square and rectangular shaped micromagnets, across a range of external magnetic field rotation frequencies. The measured critical frequency of the SPM beads on an MMA, i.e., the velocity for which the hydrodynamic drag on a bead exceeds the magnetic force, is closely related to the local magnetic flux density landscape on a micromagnet in the presence of an external magnetic field. A set of design criteria has been established for the optimization of MMAs for NLM separation, with particular focus on the shape of the micromagnets forming the array. The square MMA was used to detect a model protein biomarker and gene fragment based on a magnetic bead assembly (MBA) assay. This assay uses ligand functionalized SPM beads to capture and directly detect an analyte through the formation of SPM bead aggregates. These beads aggregates were detected through NLM separation and microscopic analysis resulting in a highly sensitive assay that did not use carrier fluid.

An integrated actuating and sensing system for light-addressable potentiometric sensor (LAPS) and light-actuated AC electroosmosis (LACE) operation

To develop a lab on a chip (LOC) integrated with both sensor and actuator functions, a novel two-in-one system based on optical-driven manipulation and sensing in a microfluidics setup based on a hydrogenated amorphous silicon (a-Si:H) layer on an indium tin oxide/glass is first realized. A high-intensity discharge xenon lamp functioned as the light source, a chopper functioned as the modulated illumination for a certain frequency, and a self-designed optical path projected on the digital micromirror device controlled by the digital light processing module was established as the illumination input signal with the ability of dynamic movement of projected patterns. For light-addressable potentiometric sensor (LAPS) operation, alternating current (AC)-modulated illumination with a frequency of 800 Hz can be generated by the rotation speed of the chopper for photocurrent vs bias voltage characterization. The pH sensitivity, drift coefficient, and hysteresis width of the Si3N4 LAPS are 52.8 mV/pH, -3.2 mV/h, and 10.5 mV, respectively, which are comparable to the results from the conventional setup. With an identical two-in-one system, direct current illumination without chopper rotation and an AC bias voltage can be provided to an a-Si:H chip with a manipulation speed of 20 μm/s for magnetic beads with a diameter of 1 μm. The collection of magnetic beads by this light-actuated AC electroosmosis (LACE) operation at a frequency of 10 kHz can be easily realized. A fully customized design of an illumination path with less decay can be suggested to obtain a high efficiency of manipulation and a high signal-to-noise ratio of sensing. With this proposed setup, a potential LOC system based on LACE and LAPS is verified with the integration of a sensor and an actuator in a microfluidics setup for future point-of-care testing applications.

Magnetic microparticle concentration and collection using a mechatronic magnetic ratcheting system

Magnetic ratcheting cytometry is a promising approach to separate magnetically-labeled cells and magnetic particles based on the quantity of magnetic material. We have previously reported on the ability of this technique to separate magnetically-labeled cells. Here, with a new chip design, containing high aspect ratio permalloy micropillar arrays, we demonstrate the ability of this technique to rapidly concentrate and collect superparamagnetic iron oxide particles. The platform consists of a mechatronic wheel used to generate and control a cycling external magnetic field that impinges on a “ratcheting chip.” The ratcheting chip is created by electroplating a 2D array of high aspect ratio permalloy micropillars onto a glass slide, which is embedded in a thin polymer layer to create a planar surface above the micropillars. By varying magnetic field frequency and direction through wheel rotation rate and angle, we direct particle movement on chip. We explore the operating conditions for this system, identifying the effects of varying ratcheting frequency, along with time, on the dynamics and resulting concentration of these magnetic particles. We also demonstrate the ability of the system to rapidly direct the movement of superparamagnetic iron oxide particles of varying sizes. Using this technique, 2.8 μm, 500 nm, and 100 nm diameter superparamagnetic iron oxide particles, suspended within an aqueous fluid, were concentrated. We further define the ability of the system to concentrate 2.8 μm superparamagnetic iron oxide particles, present in a liquid suspension, into a small chip surface area footprint, achieving a 100-fold surface area concentration, and achieving a concentration factor greater than 200%. The achieved concentration factor of greater than 200% could be greatly increased by reducing the amount of liquid extracted at the chip outlet, which would increase the ability of achieving highly sensitive downstream analytical techniques. Magnetic ratcheting-based enrichment may be useful in isolating and concentrating subsets of magnetically-labeled cells for diagnostic automation.

Nanonewton Magnetophoretic Microtrap Array for Microsystems

Here we report on the development of a lab-on-chip that integrates a dense array of micrometer-sized magnetic traps, with each individual trap generating a magnetic force as high as a few nN on standard superparamagnetic beads. The composite materials embedding traps are prepared from the microstructural engineering of a mixture between iron microparticles and polydimethylsiloxane. This approach breaks with standard microfabrication technologies: it is inexpensive, relatively easy to implement, and offers the ability to modulate the magnetic properties of the composites on a customized basis. The magnetic forces acting on the superparamagnetic beads have been measured following two approaches: first, on-chip through the hydrodynamic determination of the holding magnetic force, simultaneously on a large population of traps; and second, ex situ, by atomic force microscopy equipped with a colloidal probe, on individual traps. The experimental results have been compared with calculations from finite element modeling. Despite the geometrical simplification of the modeled system, both experiments and calculations give consistent values of force, ranging from 0.5 to 5 nN. These findings show that in operando determination of forces is a robust method that gives a high throughput overview of the forces acting in the device. It further demonstrates that the use of such functional composite materials can be a relevant alternative to standard microfabrication technologies, as it leads to competitive magnetophoretic performances.

Optical detection of the magnetophoretic transport of superparamagnetic beads on a micromagnetic array

Micromagnetic arrays (MMAs) have proven to be powerful tools for controlling the transport and separation of bioanalytes, i.e., they allow bioanalyte-superparamagnetic (SPM) bead complexes of specific size and magnetization to be moved in a synchronized manner that is precisely controlled with the orientation of an external magnetic field. This article presents a laser-photodetector system for the simple detection of individual SPM beads moving on a specific region of an MMA. This system detects the SPM beads through the change in intensity of reflective light as they move from the highly reflective micromagnetics to the supporting substrate. We demonstrate that this opti-MMA system allowed the size, number, and magnetic and optical properties of the SPM beads to be rapidly determined for regions > 49 µm² in size. The response of the opti-MMA system was characterized in several optical configurations to develop a theoretical description of its sensitivity and dynamic range. The speed, low-cost, and sensitivity of this system promises to allow MMAs to be readily applied in in vitro diagnostics and biosensing.

Evaluating and forecasting movement patterns of magnetically driven microbeads in complex geometries

The manipulation of superparamagnetic microbeads for lab-on-a-chip applications relies on the steering of microbeads across an altering stray field landscape on top of soft magnetic parent structures. Using ab initio principles, we show three-dimensional simulations forecasting the controlled movement of microbeads. Simulated aspects of microbead behaviour include the looping and lifting of microbeads around a magnetic circular structure, the flexible bead movement along symmetrically distributed triangular structures, and the dragging of magnetic beads across an array of exchange biased magnetic microstripes. The unidirectional motion of microbeads across a string of oval elements is predicted by simulations and validated experimentally. Each of the simulations matches the experimental results, proving the robustness and accuracy of the applied numerical method. The computer experiments provide details on the particle motion not accessible by experiments. The simulation capabilities prove to be an essential part for the estimation of future lab-on-chip designs.

Direct identification of the herpes simplex virus UL27 gene through single particle manipulation and optical detection using a micromagnetic array

Magnetophoretic lab on a chip technologies are rapidly evolving into integrated systems for the identification of biomarkers and cells with ultra-high sensitivity. We demonstrate the highly efficient detection of the Human herpes simplex virus type 1 (HSV) UL27 gene through the programmed assembly of superparamagnetic (SPM) nanoparticles based on oligonucleotide hybridization. The state of assembly of the SPM nanoparticles was determined by optical signature of the synchronized motion on the beads on a micromagnetic array (MMA). This technique has been used to identify <200 copies of the HSV UL27 gene without amplification in less than 20 minutes. The MAA can also be used to separate gene-SPM bead aggregates from millions of unreacted SPM beads based on nonlinear magnetophoresis (NLM). The MMA-optical detection system promises to enable highly sensitive, nucleic acid analysis to be performed without amplification and with the consumption of minimal amounts of reagent.

Experimental and theoretical study on the microparticle trapping and release in a deformable nano-sieve channel

Deformable microfluidics may be potentially used in cell manipulation, optical sensing, and imaging applications, and have drawn considerable scientific interests in the recent past. The excellent tunability of deformable microfluidic devices can provide controllable capture, deposition, and target release. We demonstrated a one-dimensional nano-sieve device to capture microparticles from suspensions. Size-selective capturing and release of micro- and nanoparticles was achieved by simply adjusting the flow rate. Almost all the microparticles were trapped in the nano-sieve device at a flow rate of 20 μl min^-1. Increasing the flow rate induces a hydrodynamic deformation of the roof of the compliant device and allows most of the microparticles to pass through the channel. We also established a theoretical model based on computational fluid dynamics to reveal the relationship of the hydrodynamically induced deformation, channel dimensions, and capture efficiency that supports and rationalizes the experimental data. We have predicted the capture efficiency of micro-and nanoparticles in a nano-sieve device with various geometries and flow rates. This study may be important to the optimization of next generation deformable microfluidics for efficient micro- and nanostructure manipulations.

Optical Magnetometry of Single Biocompatible Micromagnets for Quantitative Magnetogenetic and Magnetomechanical Assays

The mechanical manipulation of magnetic nanoparticles is a powerful approach to probing and actuating biological processes in living systems. Implementing this technique in high-throughput assays can be achieved using biocompatible micromagnet arrays. However, the magnetic properties of these arrays are usually indirectly inferred from simulations or Stokes drag measurements, leaving unresolved questions about the actual profile of the magnetic fields at the micrometer scale and the exact magnetic forces that are applied. Here, we exploit the magnetic field sensitivity of nitrogen-vacancy color centers in diamond to map the 3D stray magnetic field produced by a single soft ferromagnetic microstructure. By combining this wide-field optical magnetometry technique with magneto-optic Kerr effect microscopy, we fully analyze the properties of the micromagnets, including their magnetization saturation and their size-dependent magnetic susceptibility. We further show that the high magnetic field gradients produced by the micromagnets, greater than 10⁴ T·m^-1 under an applied magnetic field of about 100 mT, enables the manipulation of magnetic nanoparticles smaller than 10 nm inside living cells. This work paves the way for quantitative and parallelized experiments in magnetogenetics and magnetomechanics in cell biology.

Single-domain antibodies as promising experimental tools in imaging and isolation of porcine epidemic diarrhea virus

Single-domain antibody (sdAb) or nanobody possesses specific features non-accessible for conventional antibodies that make them suitable for research and biotechnological applications. Porcine epidemic diarrhea virus (PEDV) causes lethal diarrhea in piglets, resulting in great economic losses all over the world. To detect and isolate PEDV rapidly and accurately is important for the control and further research of the clinical PEDV strains. In this study, four sdAb fragments (sdAb-Mc19/29/30/37) targeting the membrane (M) protein of PEDV were selected from sdAb library that was constructed through M protein-immunized Camelus bactrianus. The selected sdAb-Mcs were solubly expressed in Escherichia coli. The functional characteristics analysis revealed that the recombinant sdAb-Mcs have excellent binding activity and specificity to M protein but have no neutralizing activity to PEDV. For further application, sdAb-Mc37 was conjugated with quantum dots to synthesize a nanoprobe for imaging PEDV in vero cells. The observed fluorescence in vero cells clearly reflects that PEDV virions can be reliably recognized and labeled by the nanoprobe. Furthermore, the sdAb-Mc29 was conjugated with superparamagnetic nanobeads to construct immunomagnetic nanobeads (IMNBs) used to isolate PEDV. One PEDV strain was successfully isolated from clinical fecal sample, suggesting IMNBs as a novel and efficient tool suitable for PEDV isolation from clinical samples. This study provided a novel application and substantiated the suitability of sdAb as a specific binder for the isolation of viruses.

Magnetophoretic lensing by concentric topographic cylinders of perpendicular magnetic anisotropy multilayers

Colloidal magnetophoretic lensing of water suspended micrometer-sized superparamagnetic beads (SPBs) above a topographically patterned magnetic thin film system with perpendicular magnetic anisotropy is demonstrated. The magnetic pattern consisting of concentric annuli of micron-sized widths has been superimposed with a rotating external magnetic field, and it is shown that the trajectories of the SPBs above this structure are similar to light rays in an optical focusing lens. SPB trajectories converge towards the central region and have divergent trajectories while passing the center. The experimental findings are corroborated by a quantitative model for the SPB trajectories. The magnetophoretic lensing effect leads to a high SPB concentration in the center of the pattern and may be useful for applications where SPBs have to approach each other in a controlled way.

Magnetophoretic induced convective capture of highly diffusive superparamagnetic nanoparticles

Micro-magnets producing magnetic field gradients as high as 106 T m-1 have been used to efficiently trap nanoparticles with a magnetic core of just 12 nm in diameter. Particle capture efficiency increases with increasing particle concentration. Comparison of measured capture kinetics with numerical modelling reveals that a threshold concentration exists below which capture is diffusion-driven and above which it is convectively-driven. This comparison also shows that two-way fluid-particle coupling is responsible for the formation of convective cells, the size of which is governed by the height of the droplet. Our results indicate that for a suspension with a nanoparticle concentration suitable for bioassays (around 0.25 mg ml-1), all particles can be captured in less than 10 minutes. Since nanoparticles have a significantly higher surface-to-volume ratio than the more widely used microparticles, their efficient capture should contribute to the development of next generation digital microfluidic lab-on-chip immunoassays.

Nanophotonic trapping: precise manipulation and measurement of biomolecular arrays

Optical trapping is a powerful and widely used laboratory technique in the biological and materials sciences that enables rapid manipulation and measurement at the nanometer scale. However, expanding the analytical throughput of this technique beyond the serial capabilities of established single-trap microscope-based optical tweezers remains a current goal in the field. In recent years, advances in nanotechnology have been leveraged to create innovative optical trapping methods that increase the number of available optical traps and permit parallel manipulation and measurement of arrays of optically trapped targets. In particular, nanophotonic trapping holds significant promise for integration with other lab-on-a-chip technologies to yield compact, robust analytical devices. In this review, we highlight progress in nanophotonic manipulation and measurement, as well as the potential for implementing these on-chip functionalities in biological research and biomedical applications. WIREs Nanomed Nanobiotechnol 2018, 10:e1477. doi: 10.1002/wnan.1477 This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Microfluidics cell sample preparation for analysis: Advances in efficient cell enrichment and precise single cell capture

Single cell analysis has received increasing attention recently in both academia and clinics, and there is an urgent need for effective upstream cell sample preparation. Two extremely challenging tasks in cell sample preparation-high-efficiency cell enrichment and precise single cell capture-have now entered into an era full of exciting technological advances, which are mostly enabled by microfluidics. In this review, we summarize the category of technologies that provide new solutions and creative insights into the two tasks of cell manipulation, with a focus on the latest development in the recent five years by highlighting the representative works. By doing so, we aim both to outline the framework and to showcase example applications of each task. In most cases for cell enrichment, we take circulating tumor cells (CTCs) as the target cells because of their research and clinical importance in cancer. For single cell capture, we review related technologies for many kinds of target cells because the technologies are supposed to be more universal to all cells rather than CTCs. Most of the mentioned technologies can be used for both cell enrichment and precise single cell capture. Each technology has its own advantages and specific challenges, which provide opportunities for researchers in their own area. Overall, these technologies have shown great promise and now evolve into real clinical applications.