Zung-Hang Wei

Cell patterning using microstructured ferromagnetic thin films

Magnetic cell patterning is demonstrated through controlling the micromagnetic states in microstructured ferromagneticthin films. The number of magnetic nanoparticles entering the cells by endocytosis can be determined by magnetophoresis experiment and is found to be dependent of the cocultured extracellular magnetic nanoparticles concentrations. In zigzag magnetic films the effects of cell patterning differ for magnetic films at as-deposited state and at remanent states after applying fields in different directions. Remanent states of concentric rings are proposed for cell patterning. Cells can be arranged at any positions in sequence by selectively changing the magnetic field directions.

Nonuniform magnetization structures in thin soft type ferromagnetic elements of elliptical shape

Stable micromagnetic configurations are investigated numerically in thin soft type ferromagnetic elements of elliptical shape with typical thickness of Lz=30 nm and with aspect ratios 1⩽Lx/Ly⩽3.5. In this range of aspect ratios, a complete description of stable magnetization patterns is obtained including the critical sizes for stability of one-vortex and quasiuniform micromagnetic configurations. Due to the absence of sharp corners, the behavior of the quasiuniform state in an external magnetic field resembles that of a single-domain ellipsoidal particle of similar shape. Nevertheless, the calculated hysteresis loops of elliptical elements show a considerable reduction of the switching fields in comparison with the Stoner–Wohlfarth model due to the development of a new mode of instability. The magnetization curves for elliptical elements, initially in one-, two- or three- vortex states, show significant dependence on the direction of external magnetic field with respect to the longest particle’s axis.

Mechanisms of magnetization reversals in elliptical thin films

The size dependence of reversal mechanisms and the corresponding switching fields of elliptical Permalloy thin films are studied by micromagnetic simulation. From the simulated spin configurations, it is shown that for ellipses with different dimensions there exist three reversal mechanisms: the buckling-instability mode, the vortex-propagation mode, and the mixed mode of buckling-instability and vortex-propagation modes.

A permalloy zigzag structure based magnetic bio-sensor

A magnetic fluid consisting of Fe3O4 magnetic nanoparticles is embedded inside cells by intracellular uptake. A micro-fabricated magnetic zigzag-shaped surface structure is studied for use as a biosensor. We have developed a MOKE magnetometer based methodology to measure the different hysteresis loop signals between cells with and without being placed on zigzag sensors. Adding the magnetic cells on the structure decreases the coercivity from the magneto-optical Kerr effect (MOKE) signal of zigzag magnetic thin films because of the magnetic properties of superparamagnetic nanoparticles. The magnetoresistance measurement observed is that the switching fields of the zigzag structure with magnetic cells are significantly increased compared to the case without cells in the hard axis of the external field applied.

Charged droplet transportation under direct current electric fields as a cell carrier

This study verifies the reliability of the cell transport through chargeddroplets under DC electrical fields. Over 90% cell viability can be obtained after charging and discharging for thousands of times. The high cell viability indicates that the electrical charging of a droplet barely influences the cells inside over a long working period. DC field induced AC electrical signals via droplet motion are also observed and analyzed quantitatively. The measured AC signals help us understand the charge transfer of a droplet during charging and discharging processes.

Magnetic fluid micromixer with tapered magnets

A magnetic fluid micromixer with high mixing efficiency using nonuniform magnetic field produced by tapered magnets is proposed. From the numerical results, the mixing efficiency increases with the saturation magnetization of the magnets. The larger the saturation magnetization is, the stronger the magnetic force acted on the magnetic fluid will be. On the other hand, asymmetric magnetic stray field that induces circulation between two fluids leads to higher mixing efficiency than a symmetric one.

Evolution of vortex states under external magnetic field

Abstract The behavior of one-, two-, and three-vortex states of submicron-sized elliptical elements under the influence of external magnetic field is investigated here by means of micromagnetic simulations. The total magnetization curves show abrupt changes at the nucleation or annihilation of vortices. The magnetization curves and critical field show significant dependence on the field direction with respect to the particle’s long axis.

Influences of the aspect ratio and film thickness on switching properties of elliptical Permalloy elements

The size dependence on the switching properties of microstructured Permalloy (Ni/sub 80/Fe/sub 20/) ellipses were investigated by magnetoresistance measurements and magnetic force microscopy. Elements with fixed short axes of 1 /spl mu/m, long axes varying from 2 to 10 /spl mu/m, and film thickness varying from 8 to 55 nm were fabricated by electron beam lithography through a lift-off technique. A single-domain configuration was observed in the elements with the range of aspect ratios (long/short axis) from 5 to 10. More complex domain structures appear in the lower aspect ratio and thicker samples. The switching properties show a strong dependence on the film thickness as well as the aspect ratio. The switching fields of uniform magnetization reversal increase with increasing thickness up to a critical value (24<t/sub c/<40 nm), whereas they decrease with increasing thickness above t/sub c/. Nevertheless, the switching fields only show weak dependency on aspect ratio.

Comparison of cell behavior on pva/pva-gelatin electrospun nanofibers with random and aligned configuration

Electrospinning technique is able to create nanofibers with specific orientation. Poly(vinyl alcohol) (PVA) have good mechanical stability but poor cell adhesion property due to the low affinity of protein. In this paper, extracellular matrix, gelatin is incorporated into PVA solution to form electrospun PVA-gelatin nanofibers membrane. Both randomly oriented and aligned nanofibers are used to investigate the topography-induced behavior of fibroblasts. Surface morphology of the fibers is studied by optical microscopy and scanning electron microscopy (SEM) coupled with image analysis. Functional group composition in PVA or PVA-gelatin is investigated by Fourier Transform Infrared (FTIR). The morphological changes, surface coverage, viability and proliferation of fibroblasts influenced by PVA and PVA-gelatin nanofibers with randomly orientated or aligned configuration are systematically compared. Fibroblasts growing on PVA-gelatin fibers show significantly larger projected areas as compared with those cultivated on PVA fibers which p-value is smaller than 0.005. Cells on PVA-gelatin aligned fibers stretch out extensively and their intracellular stress fiber pull nucleus to deform. Results suggest that instead of the anisotropic topology within the scaffold trigger the preferential orientation of cells, the adhesion of cell membrane to gelatin have substantial influence on cellular behavior.

Magnetization reversal in nanostructured permalloy rings

Summary form only given. Recently, nanostructured rings have attracted much attention in the application of MRAM design because the shape anisotropy makes it easier to generate a closure flux loop spin configuration, which avoids the field leakage and therefore enhances the data storage density. A Permalloy ring with the size of 187.5 nm outer diameter, 112.5 nm inner diameter, and 30nm thickness has been studied by means of micromagnetic simulation. The authors show the hysteresis loop of the ring from our simulation result, with schematic pictures used to represent the stable states I, II, and III of the plateaus.