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We propose a simple microfluidic device for protein preconcentration based on the electrokinetic trapping principle. It comprises a narrow Nation strip that is simply cut from a commercial membrane and is integrated into a molded poly(dimethylsiloxane) (PDMS) microfluidic structure using a guiding channel. Mechanically clamping the PDMS/Nation assembly with a glass substrate results in a rapid prototypable, leak-tight, and easily disposable device. Our device preconcentrates negatively charged fluorescent proteins located at the anodic microfluidic compartment side of the Nation strip within a few minutes and up to a concentration factor of [10.sup.4]. Moreover, we present a numerical study of the preconcentration effect by solving the coupled Poisson, Nernst-Planck, and Navier Stokes equations for our type of device, which provides microscopic insight into the mechanism of preconcentration. The electrical field across the ion-permselective Nation generates concentration polarization, i.e., ion depletion at the anodic side and ion enrichment at the cathodic side for both types of ions, with a local excess of mobile positive ions in the depleted concentration polarization zone, inducing a nonequilibrium electrical double layer in close proximity to the Nation membrane. A voltage difference applied over the anodic compartment is used to generate the electrophoretic flow velocity of the negatively charged tracer biomolecules. This, in combination with the electroosmotic flow in the opposite direction, which originates from the fixed charges on the channel walls and the induced space charge near the membrane, provides the basis for the local preconcentration of the negative tracer biomolecules. 10.1021/ac102149f

We present an original concept of manipulation of magnetic microbeads in a microchannel. It is based on the dynamic motion of a self-assembled structure of ferrimagnetic beads that are retained within a microfluidic flow using a local alternating magnetic field. The latter induces a rotational motion of the magnetic particles, thereby strongly enhancing the fluid perfusion through the magnetic structure that behaves as a dynamic random porous medium. The result is a very strong particle--liquid interaction that can be controlled by adjusting the magnetic field frequency and amplitude, as well as the liquid flow rate, and is at the basis of very efficient liquid mixing. The principle is demonstrated using a microfluidic chip made of poly(methyl methacrylate) with integrated soft ferromagnetic plate structures. The latter are part of an electromagnetic circuit and serve to locally apply a magnetic field over the section of the microchannel. Starting from a laminar flow pattern of parallel fluorescein dye and nonfluorescent liquid streams, we demonstrate a 95% mixing efficiency using a mixing length of only 400 [micro]m and at liquid flows of the order of 0.5 cm/s. We anticipate that the intense interaction between the fluid and magnetic particles with functionalized surfaces holds large potential for the development of future bead-based assays.

We present a new and simple capillary electrophoresis microsystem in which the sample is injected hydrodynamically using a pressure pulse. This technique maintains the sample composition, in contrast to a classical electrokinetic injection, in which the magnitude of the electric field in the sample reservoir in combination with variations in electrophoretic mobility can lead to a biased injection. The sample is loaded using a well controlled and variable pressure pulse (0.1-1.0 s) generated by the mechanical actuation of a flexible membrane placed on the chip sample reservoir. A fluorescein/calcein-containing borate--Tris-hydroxymethylaminoethane (TRIS) sample solution is taken as a model system for CE analysis. The separation results using pressure pulse injection clearly demonstrate the advantages of our technique. In addition to the reduced bias due to the absence of an electrode in the sample well, this method allows injection of variable plug volumes by simply changing the pulse length. Moreover, a very high speed, repeatability, and sensitivity of the separation is obtained.

Research on magnetoimpedance is examined in detail, in relation to amorphous microstructured ribbons that have been chemically thinned and polished.

A test of the frequency dependent resistance noise behavior of microfabricated magnetoresistive elements based on either the anisotropic or the giant magnetoresistance effect found that thermal resistance noise was the dominant noise source above about 10 kHz. Their resistance noise behavior was investigated at room temperature in a dc magnetic field. A magnetic field-dependent 1/f contribution dominates resistance noise at low frequencies. The 1/f noise is low in high applied magnetic fields and increases in the most sensitive region of the magnetoresistance curve.

The microstructuring and fabrication of pillar-shaped structures of Fe/Cr sputtered multilayers and Co/Ca molecular beam epitaxially (MBE) evaporated multilayers enables the investigation of the CPP-magnetoresistance (MR) effect. The CPP-MR is much higher for these two samples than that of other materials due to low temperature electron-magnon scattering.

Analytical expressions for the current distribution in small pillar-like structures are obtained by a simple Ohmic model, and are used to measure the perpendicular resistance of multi-layers. Numerical electrostatic calculations, based on the finite element method, agree with these analytical expressions. This expressions facilitate the analysis of CPP-MR data of microstructure magnetic multilayers.