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

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.

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.

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.

Efficient flowless separation of mixed microbead populations on periodic ferromagnetic surface structures

The simultaneous separational control of motion of individual objects is vital to achieve high efficiency separation for biological analytes in biomedical applications. Here, we show the selective and directed movement of different populations of microbeads depending on their size in a flowless environment by means of a hexagonally structured soft-magnetic microchip platform. By adjusting strength and asymmetry of a modulated in-plane magnetic field, discrete and switchable movement patterns of two different types of beads above a magnetic surface structure are achieved. Starting from a heterogeneous mixture of bead populations and depending on the type of field sequences, directional forward transport of one type of beads is achieved, while the other bead population is immobilized. Despite significant size and magnetic content distributions within each population of microbeads, high separation efficiencies are demonstrated. The selection and movement processes are supported by full-scale magnetofluidic numerical simulations. The magnetic platform allowing multidirectional and selective microbead movement can greatly contribute to the progress of functional lab-on-chip and future diagnostics devices.