Manipulation of magnetically labeled and unlabeled cells with mobile magnetic traps

Stray Magnetic Field Variations and Micromagnetic Simulations: Models for Ni0.8Fe0.2 Disks Used for Microparticle Trapping

Patterned micro-scale thin-film magnetic structures, in conjunction with weak (~few tens of Oe) applied magnetic fields, can create energy landscapes capable of trapping and transporting fluid-borne magnetic microparticles. These energy landscapes arise from magnetic field magnitude variations that arise in the vicinity of the magnetic structures. In this study, we examine means of calculating magnetic fields in the local vicinity of permalloy (Ni0.8Fe0.2) microdisks in weak (~tens of Oe) external magnetic fields. To do this, we employ micromagnetic simulations and the resulting calculations of fields. Because field calculation from micromagnetic simulations is computationally time-intensive, we discuss a method for fitting simulated results to improve calculation speed. Resulting stray fields vary dramatically based on variations in micromagnetic simulations-vortex vs. non-vortex micromagnetic results-which can each appear despite identical simulation final conditions, resulting in field strengths that differ by about a factor of two.

Enhanced acoustic streaming effects via sharp-edged 3D microstructures

Acoustofluidic micromanipulation is an important tool for biomedical research, where acoustic forces offer the ability to manipulate fluids, cells, and particles in a rapid, biocompatible, and contact-free manner. Of particular interest is the investigation of acoustically driven sharp edges, where high tip velocity magnitudes and strong acoustic potential gradients drive rapid motion. Whereas prior devices utilizing 2D sharp edges have demonstrated promise for micromanipulation activities, taking advantage of 3D structures has the potential to increase their performance and the range of manipulation activities. In this work, we investigate high-magnitude acoustic streaming fields in the vicinity of sharp-edged, sub-wavelength 3D microstructures. We numerically model and experimentally demonstrate this in fabricating parametrically configured 3D microstructures whose tip-angle and geometry influence acoustic streaming velocities and the complexity of streaming vortices, finding that the simulated and realized velocities and streaming patterns are both tunable and a function of microstructure shape. These sharp-edge interfaces hold promise for biomedical studies benefiting from precise and targeted micromanipulation.

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.

Short- and Long-Range Microparticle Transport on Permalloy Disk Arrays in Time-Varying Magnetic Fields

We investigate maneuvering superparamagnetic microparticles, or beads, in a remotely-controlled, automated way across arrays of few-micron-diameter permalloy disks. This technique is potentially useful for applying tunable forces to or for sorting biological structures that can be attached to magnetic beads, for example nucleic acids, proteins, or cells. The particle manipulation method being investigated relies on a combination of stray fields emanating from permalloy disks as well as time-varying externally applied magnetic fields. Unlike previous work, we closely examine particle motion during a capture, rotate, and controlled repulsion mechanism for particle transport. We measure particle velocities during short-range motion—the controlled repulsion of a bead from one disk toward another—and compare this motion to a simulation based on stray fields from disk edges. We also observe the phase-slipping and phase-locked motion of particles engaging in long-range transport in this manipulation scheme.

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.

Recent advances in microfluidic technologies for separation of biological cells

Cell separation has always been a key topic in academic research, especially in the fields of medicine and biology, due to its significance in diagnosis and treatment. Accurate, high-throughput and non-invasive separation of individual cells is key to driving the development of biomedicine and cellular biology. In recent years, a series of researches on the use of microfluidic technologies for cell separation have been conducted to solve bio-related problems. Hence, we present here a comprehensive review on the recent developments of microfluidic technologies for cell separation. In this review, we discuss several cell separation methods, mainly including: physical and biochemical method, their working principles as well as their practical applications. We also analyze the advantages and disadvantages of each method in detail. In addition, the current challenges and future prospects of microfluidic-based cell separation were discussed.

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.

Characterizing the Process Physics of Ultrasound-Assisted Bioprinting

3D bioprinting has been evolving as an important strategy for the fabrication of engineered tissues for clinical, diagnostic, and research applications. A major advantage of bioprinting is the ability to recapitulate the patient-specific tissue macro-architecture using cellular bioinks. The effectiveness of bioprinting can be significantly enhanced by incorporating the ability to preferentially organize cellular constituents within 3D constructs to mimic the intrinsic micro-architectural characteristics of native tissues. Accordingly, this work focuses on a new non-contact and label-free approach called ultrasound-assisted bioprinting (UAB) that utilizes acoustophoresis principle to align cells within bioprinted constructs. We describe the underlying process physics and develop and validate computational models to determine the effects of ultrasound process parameters (excitation mode, excitation time, frequency, voltage amplitude) on the relevant temperature, pressure distribution, and alignment time characteristics. Using knowledge from the computational models, we experimentally investigate the effect of selected process parameters (frequency, voltage amplitude) on the critical quality attributes (cellular strand width, inter-strand spacing, and viability) of MG63 cells in alginate as a model bioink system. Finally, we demonstrate the UAB of bilayered constructs with parallel (0°-0°) and orthogonal (0°-90°) cellular alignment across layers. Results of this work highlight the key interplay between the UAB process design and characteristics of aligned cellular constructs, and represent an important next step in our ability to create biomimetic engineered tissues.

3D Nanochannel Array for High-Throughput Cell Manipulation and Electroporation

Electroporation has been one of the most commonly used physical methods for gene/drug delivery. Compared to other nonviral counterparts, electroporation enables optimization of delivery efficiency by tuning the electric field applied on cells. Commercial electroporation, however, results in stochastic transfection and significant cellular damage mostly due to its "bulk" environment. In this chapter, we introduce nanoelectroporation (NEP) which has demonstrated living cell transfection in a highly controllable manner. In NEP, the electric field can be precisely focused on a single cell positioned on nanochannels. Safe single-cell electroporation as well as "electrophoretic" molecular delivery can be achieved on the same device. This system achieves significantly higher transfection efficiency and cellular viability than commercial systems. This device is unique in that it can efficiently deliver genetic molecules (e.g., DNAs, RNAs) that exceed 10 kbp in size. The NEP device based on a 3D nanochannel array prototype was fabricated using cleanroom techniques. For achieving precise cell to nanochannel pairing, three on-chip high-throughput manipulation technologies were developed, that is, magnetic tweezers (MT), dielectrophoresis (DEP), and thin-film microfluidics.

Magnetic Field Changes Macrophage Phenotype

Macrophages play a crucial role in homeostasis, regeneration, and innate and adaptive immune responses. Functionally different macrophages have different shapes and molecular phenotypes that depend on the actin cytoskeleton, which is regulated by the small GTPase RhoA. The naive M0 macrophages are slightly elongated, proinflammatory M1 are round, and M2 antiinflammatory macrophages are elongated. We have recently shown in the rodent model system that genetic or pharmacologic interference with the RhoA pathway deregulates the macrophage actin cytoskeleton, causes extreme macrophage elongation, and prevents macrophage migration. Here, we report that an exposure of macrophages to a nonuniform magnetic field causes extreme elongation of macrophages and has a profound effect on their molecular components and organelles. Using immunostaining and Western blotting, we observed that magnetic force rearranges the macrophage actin cytoskeleton, the Golgi complex, and the cation channel receptor TRPM2, and modifies the expression of macrophage molecular markers. We have found that the magnetic-field-induced alterations are very similar to changes caused by RhoA interference. We also analyzed magnetic-field-induced forces acting on macrophages and found that the location and alignment of magnetic-field-elongated macrophages correlate very well with the simulated distribution and orientation of such magnetic force lines.

Real-time magnetic actuation of DNA nanodevices via modular integration with stiff micro-levers

DNA nanotechnology has enabled complex nanodevices, but the ability to directly manipulate systems with fast response times remains a key challenge. Current methods of actuation are relatively slow and only direct devices into one or two target configurations. Here we report an approach to control DNA origami assemblies via externally applied magnetic fields using a low-cost platform that enables actuation into many distinct configurations with sub-second response times. The nanodevices in these assemblies are manipulated via mechanically stiff micron-scale lever arms, which rigidly couple movement of a micron size magnetic bead to reconfiguration of the nanodevice while also enabling direct visualization of the conformation. We demonstrate control of three assemblies—a rod, rotor, and hinge—at frequencies up to several Hz and the ability to actuate into many conformations. This level of spatiotemporal control over DNA devices can serve as a foundation for real-time manipulation of molecular and atomic systems.

DNA molecular machines hold promise for biological nanotechnology, but how to actuate them in a fast and programmable manner remains challenging. Here, Lauback et al. demonstrate direct manipulation of DNA origami assemblies via a micrometer-long stiff mechanical lever controlled by a magnetic field.

Architecture for Directed Transport of Superparamagnetic Microbeads in a Magnetic Domain Wall Routing Network

Directed transport of biological species across the surface of a substrate is essential for realizing lab-on-chip technologies. Approaches that utilize localized magnetic fields to manipulate magnetic particles carrying biological entities are attractive owing to their sensitivity, selectivity, and minimally disruptive impact on biomaterials. Magnetic domain walls in magnetic tracks produce strong localized fields and can be used to capture, transport, and detect individual superparamagnetic microbeads. The dynamics of magnetic microbead transport by domain walls has been well studied. However, demonstration of more complex functions such as selective motion and sorting using continuously driven domain walls in contiguous magnetic tracks is lacking. Here, a junction architecture is introduced that allows for branching networks in which superparamagnetic microbeads can be routed along dynamically-selected paths by a combination of rotating in-plane field for translation, and a pulsed out-of-plane field for path selection. Moreover, experiments and modeling show that the select-field amplitude is bead-size dependent, which allows for digital sorting of multiple bead populations using automated field sequences. This work provides a simple means to implement complex routing networks and selective transport functionalities in chip-based devices using magnetic domain wall conduits.

Tuning bacterial hydrodynamics with magnetic fields

Magnetotactic bacteria are a group of motile prokaryotes that synthesize chains of lipid-bound, magnetic nanoparticles called magnetosomes. This study exploits their innate magnetism to investigate previously unexplored facets of bacterial hydrodynamics at surfaces. Through use of weak, uniform, external magnetic fields and local, micromagnetic surface patterns, the relative strength of hydrodynamic, magnetic, and flagellar force components is tuned through magnetic control of the bacteria's orientation. The resulting swimming behaviors provide a means to experimentally determine hydrodynamic parameters and offer a high degree of control over large numbers of living microscopic entities. The implications of this controlled motion for studies of bacterial motility near surfaces and for micro- and nanotechnology are discussed.

Automatic and Selective Single Cell Manipulation in a Pressure-Driven Microfluidic Lab-On-Chip Device

A microfluidic lab-on-chip device was developed to automatically and selectively manipulate target cells at the single cell level. The device is composed of a microfluidic chip, mini solenoid valves with negative-pressurized soft tubes, and a LabView^®-based data acquisition device. Once a target cell passes the resistive pulse sensing gate of the microfluidic chip, the solenoid valves are automatically actuated and open the negative-pressurized tubes placed at the ends of the collecting channels. As a result, the cell is transported to that collecting well. Numerical simulation shows that a 0.14 mm³ volume change of the soft tube can result in a 1.58 mm/s moving velocity of the sample solution. Experiments with single polystyrene particles and cancer cells samples were carried out to demonstrate the effectiveness of this method. Selectively manipulating a certain size of particles from a mixture solution was also achieved. Due to the very high pressure-driven flow switching, as many as 300 target cells per minute can be isolated from the sample solution and thus is particularly suitable for manipulating very rare target cells. The device is simple, automatic, and label-free and particularly suitable for isolating single cells off the chip one by one for downstream analysis.

Multifunctional, inexpensive, and reusable nanoparticle-printed biochip for cell manipulation and diagnosis

Isolation and characterization of rare cells and molecules from a heterogeneous population is of critical importance in diagnosis of common lethal diseases such as malaria, tuberculosis, HIV, and cancer. For the developing world, point-of-care (POC) diagnostics design must account for limited funds, modest public health infrastructure, and low power availability. To address these challenges, here we integrate microfluidics, electronics, and inkjet printing to build an ultra-low-cost, rapid, and miniaturized lab-on-a-chip (LOC) platform. This platform can perform label-free and rapid single-cell capture, efficient cellular manipulation, rare-cell isolation, selective analytical separation of biological species, sorting, concentration, positioning, enumeration, and characterization. The miniaturized format allows for small sample and reagent volumes. By keeping the electronics separate from microfluidic chips, the former can be reused and device lifetime is extended. Perhaps most notably, the device manufacturing is significantly less expensive, time-consuming, and complex than traditional LOC platforms, requiring only an inkjet printer rather than skilled personnel and clean-room facilities. Production only takes 20 min (vs. up to weeks) and $0.01-an unprecedented cost in clinical diagnostics. The platform works based on intrinsic physical characteristics of biomolecules (e.g., size and polarizability). We demonstrate biomedical applications and verify cell viability in our platform, whose multiplexing and integration of numerous steps and external analyses enhance its application in the clinic, including by nonspecialists. Through its massive cost reduction and usability we anticipate that our platform will enable greater access to diagnostic facilities in developed countries as well as POC diagnostics in resource-poor and developing countries.

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

In this article, we review lab on a chip (LOC) devices that have been developed for processing magnetically labelled biological analytes, e.g., proteins, nucleic acids, viruses and cells, based on micromagnetic structures and a time-varying magnetic field. We describe the methods that have been developed for fabricating micromagnetic arrays and the bioprocessing operations that have been demonstrated using superparamagnetic (SPM) beads, i.e., programmed transport, switching, separation of specific analytes, and pumping and mixing of fluids in microchannels. The primary advantage of micromagnet devices is that they make it possible to develop systems that control individual SPM beads, enabling high-efficiency separation and analysis. These devices do not require hydrodynamic control and lend themselves to parallel processing of large arrays of SPM beads with modest levels of power consumption. Micromagnet devices are well suited for bioanalytical applications that require high-resolution separation, e.g., detection of rare cell types such as circulating tumour cells, or biosensor applications that require multiple magnetic bioprocessing operations on a single chip.

Dynamic trajectory analysis of superparamagnetic beads driven by on-chip micromagnets

We investigate the non-linear dynamics of superparamagnetic beads moving around the periphery of patterned magnetic disks in the presence of an in-plane rotating magnetic field. Three different dynamical regimes are observed in experiments, including (1) phase-locked motion at low driving frequencies, (2) phase-slipping motion above the first critical frequency f_c1, and (3) phase-insulated motion above the second critical frequency f_c2. Experiments with Janus particles were used to confirm that the beads move by sliding rather than rolling. The rest of the experiments were conducted on spherical, isotropic magnetic beads, in which automated particle position tracking algorithms were used to analyze the bead dynamics. Experimental results in the phase-locked and phase-slipping regimes correlate well with numerical simulations. Additional assumptions are required to predict the onset of the phase-insulated regime, in which the beads are trapped in closed orbits; however, the origin of the phase-insulated state appears to result from local magnetization defects. These results indicate that these three dynamical states are universal properties of bead motion in non-uniform oscillators.

Micromagnet arrays for on-chip focusing, switching, and separation of superparamagnetic beads and single cells

Nonlinear magnetophoresis (NLM) is a novel approach for on-chip transport and separation of superparamagnetic (SPM) beads, based on a travelling magnetic field wave generated by the combination of a micromagnet array (MMA) and an applied rotating magnetic field. Here, we present two novel MMA designs that allow SPM beads to be focused, sorted, and separated on-chip. Converging MMAs were used to rapidly collect the SPM beads from a large region of the chip and focus them into synchronised lines. We characterise the collection efficiency of the devices and demonstrate that they can facilitate on-chip analysis of populations of SPM beads using a single-point optical detector. The diverging MMAs were used to control the transport of the beads and to separate them based on their size. The separation efficiency of these devices was determined by the orientation of the magnetisation of the micromagnets relative to the external magnetic field and the size of the beads and relative to that of micromagnets. By controlling these parameters and the rotation of the external magnetic field we demonstrated the controlled transport of SPM bead-labelled single MDA-MB-231 cells. The use of these novel MMAs promises to allow magnetically-labelled cells to be efficiently isolated and then manipulated on-chip for analysis with high-resolution chemical and physical techniques.

Magnetic tweezers-based 3D microchannel electroporation for high-throughput gene transfection in living cells

A novel high-throughput magnetic tweezers-based 3D microchannel electroporation system capable of transfecting 40 000 cells/cm(2) on a single chip for gene therapy, regenerative medicine, and intracellular detection of target mRNA for screening cellular heterogeneity is reported. A single cell or an ordered array of individual cells are remotely guided by programmable magnetic fields to poration sites with high (>90%) cell alignment efficiency to enable various transfection reagents to be delivered simultaneously into the cells. The present technique, in contrast to the conventional vacuum-based approach, is significantly gentler on the cellular membrane yielding >90% cell viability and, moreover, allows transfected cells to be transported for further analysis. Illustrating the versatility of the system, the GATA2 molecular beacon is delivered into leukemia cells to detect the regulation level of the GATA2 gene that is associated with the initiation of leukemia. The uniform delivery and a sharp contrast of fluorescence intensity between GATA2 positive and negative cells demonstrate key aspects of the platform for gene transfer, screening and detection of targeted intracellular markers in living cells.

On-chip magnetic separation and encapsulation of cells in droplets

Single cell study is gaining importance because of the cell-to-cell variation that exists within cell population, even after significant initial sorting. Analysis of such variation at the gene expression level could impact single cell functional genomics, cancer, stem-cell research, and drug screening. The on-chip monitoring of individual cells in an isolated environment would prevent cross-contamination, provide high recovery yield, and enable study of biological traits at a single cell level. These advantages of on-chip biological experiments is a significant improvement for a myriad of cell analyses methods, compared to conventional methods, which require bulk samples and provide only averaged information on cell structure and function. We report on a device that integrates a mobile magnetic trap array with microfluidic technology to provide the possibility of separation of immunomagnetically labeled cells and their encapsulation with reagents into picoliter droplets for single cell analysis. The simultaneous reagent delivery and compartmentalization of the cells immediately following sorting are all performed seamlessly within the same chip. These steps offer unique advantages such as the ability to capture cell traits as originated from its native environment, reduced chance of contamination, minimal use of the reagents, and tunable encapsulation characteristics independent of the input flow. Preliminary assay on cell viability demonstrates the potential for the device to be integrated with other up- or downstream on-chip modules to become a powerful single-cell analysis tool.

Ultra-localized single cell electroporation using silicon nanowires

Analysis of cell-to-cell variation can further the understanding of intracellular processes and the role of individual cell function within a larger cell population. The ability to precisely lyse single cells can be used to release cellular components to resolve cellular heterogeneity that might be obscured when whole populations are examined. We report a method to position and lyse individual cells on silicon nanowire and nanoribbon biological field effect transistors. In this study, HT-29 cancer cells were positioned on top of transistors by manipulating magnetic beads using external magnetic fields. Ultra-rapid cell lysis was subsequently performed by applying 600-900 mV(pp) at 10 MHz for as little as 2 ms across the transistor channel and the bulk substrate. We show that the fringing electric field at the device surface disrupts the cell membrane, leading to lysis from irreversible electroporation. This methodology allows rapid and simple single cell lysis and analysis with potential applications in medical diagnostics, proteome analysis and developmental biology studies.

A MagDot-Nanoconveyor Assay Detects and Isolates Molecular Biomarkers

The ability to quickly analyze, separate, and manipulate multiple types of biomarkers from small sample volumes is a significant step toward personalized medicine.

A microfabricated magnetic force transducer-microaspiration system for studying membrane mechanics

The application of forces to cell membranes is a powerful method for studying membrane mechanics. To apply controlled dynamic forces on the piconewton scale, we designed and characterized a microfabricated magnetic force transducer (MMFT) consisting of current-carrying gold wires patterned on a sapphire substrate. The experimentally measured forces applied to paramagnetic and ferromagnetic beads as a function of applied current agree well with theoretical models. We used this device to pull tethers from microaspirated giant unilamellar vesicles and measure the threshold force for tether formation. In addition, the interlayer drag coefficient of the membrane was determined from the tether-return velocity under magnetic force-free conditions. At high levels of current, vesicles expanded as a result of local temperature changes. A finite element thermal model of the MMFT provided absolute temperature calibration, allowing determination of the thermal expansivity coefficient of stearoyl-oleoyl-phosphatidycholine vesicles (1.7 ± 0.4 × 10(-3) K(-1)) and characterization of the Joule heating associated with current passing through the device. This effect can be used as a sensitive probe of temperature changes on the microscale. These studies establish the MMFT as an effective tool for applying precise forces to membranes at controlled rates and quantitatively studying membrane mechanical and thermo-mechanical properties.

Magnetophoresis of nanoparticles

Iron oxide cores of 35 nm are coated with gold nanoparticles so that individual particle motion can be tracked in real time through the plasmonic response using dark field optical microscopy. Although Brownian and viscous drag forces are pronounced for nanoparticles, we show that magnetic manipulation is possible using large magnetic field gradients. The trajectories are analyzed to separate contributions from the different types of forces. With field gradients up to 3000 T/m, forces as small as 1.5 fN are detected.

Spin re-orientation in magnetostatically coupled Ni(80)Fe(20) ellipsoidal nanomagnets

We investigate the influence of magnetostatic coupling on the spin configurations and magnetization reversal mechanism in a one-dimensional linear chain of densely packed Ni(80)Fe(20) ellipsoidal nanomagnets arranged in two basic configurations (elements coupled along the major or minor axes). Using magnetic force microscopy (MFM) we observed that for geometrically identical ellipsoidal nanomagnets the magnetic states at remanence are strongly dependent on the arrangement of the ellipsoid due to competition between the inherent shape and configuration anisotropies. When the elements are coupled along the major axis, the individual elements adopt a single domain magnetic state at remanence for field applied along the linear chain. This is in contrast with a wide range of magnetic states (single vortex states, double vortex states and modified single domain states) observed for elements coupled along the minor axis and also isolated elements. We have conducted a detailed investigation on the magnetization reversal mechanisms for both configurations and have correlated our experimental results with micromagnetic simulations.

Simultaneous magnetic manipulation and fluorescent tracking of multiple individual hybrid nanostructures

Controlled transport of multiple individual nanostructures is crucial for nanoassembly and nanodelivery but is challenging because of small particle size. Although atomic force microscopy and optical and magnetic tweezers can control single particles, it is extremely difficult to scale these technologies for multiple structures. Here, we demonstrate a "nano-conveyer-belt" technology that permits simultaneous transport and tracking of multiple individual nanospecies in a selected direction. The technology consists of two components: nanocontainers, which encapsulate the nanomaterials transported, and nanoconveyer arrays, which use magnetic force to manipulate individual or aggregate nanocontainers. This technology is extremely versatile. For example, nanocontainers encapsulate quantum dots or rods and superparamagnetic iron oxide nanoparticles in <100 nm nanocontainers, the smallest magnetic composites to have been simultaneously moved and optically tracked. Similarly, the nanoconveyers consist of patterned microdisks or zigzag nanowires, whose dimensions can be controlled through micropatterning. The nanoconveyer belt technology could impact multiple fields, including nanoassembly, biomechanics, nanomedicine, and nanofluidics.