N. Haandbæk, K. Mathwig, R. Streichan, N. Goedecke, S. C. Bürgel, F. Heer and A. Hierlemann
Bandwidth Compensation for High Resolution Impedance Spectroscopy
Proceedings Eurosensors XXV, Sept. 4 – 7, 2011, Athens, Greece
Procedia Engineering 25 (2011) 1209.
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This paper reports on a microfluidic differential impedance cytometer, which uses a bandwidth compensation technique together with a small detection volume and multi-frequency analysis to achieve an increased sensitivity. The bandwidth compensation technique allows for measurements within small bandwidths by accounting for the increased signal amplitude dependence on the particle speed. We demonstrate detection and clear baseline discrimination of polystyrene beads with diameters of 1 μm and 2 μm and the discrimination of 5 μm beads from yeast cells of similar size. We show that using multiple frequencies in parallel significantly improves the discrimination performance of the cytometer.

K. Mathwig, F. Müller and U. Gösele
Particle transport in asymmetrically modulated pores
New Journal of Physics 13 (2011) 033038.
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Brownian motion plays an important role in the separation of small particles and molecules, but generally leads to undirected motion or intermixing by diffusion. Matthias and Müller (2003 Nature 424 53–7) reported on the experimental realization of a drift ratchet, a microfluidic particle transport mechanism that utilizes random fluctuations instead, i.e. a Brownian motor. Here, we offer a new interpretation of this previously published work on the drift ratchet. New experiments, which allow us to distinguish between particles of different sizes, as well as a re-examination of the original work, lead to the conclusion that the measured particle transport does not result from a ratchet effect. We demonstrate that the transport is caused by convection instead. While our result challenges one specific type of experiment, we do not assess the feasibility of a drift ratchet in principle. Instead, we identify the experimental conditions that need to be fulfilled for the successful separation of particles.

K. Mathwig, M. Geilhufe, F. Müller and U. Gösele
Bias-assisted KOH etching of macroporous silicon membranes
Journal of Micromechanics and Microengineering 21 (2011) 035015.
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This paper presents an improved technique to fabricate porous membranes from macroporous silicon as a starting material. A crucial step in the fabrication process is the dissolution of silicon from the backside of the porous wafer by aqueous hydroxide to open up the pores. We improved this step by biasing the silicon wafer electrically against the KOH. By monitoring the current–time characteristics a good control of the process is achieved and the yield is improved. Also, the etching can be stopped instantaneously and automatically by short-circuiting Si and KOH. Moreover, the bias-assisted etching allows for the controlled fabrication of silicon dioxide tube arrays when the silicon pore walls are oxidized and inverted pores are released.

Y. Qin, Y. Kim, L. Zhang, S.-M. Lee, R. B. Yang, A. Pan, K. Mathwig, M. Alexe, U. Gösele and M. Knez
Preparation and Elastic Properties of Helical Nanotubes Obtained by Atomic Layer Deposition with Carbon Nanocoils as Templates
small 6 (2010) 910.
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Helical oxide nanotubes prepared by atomic layer deposition (ALD) replicate the helical morphology of the carbonnanocoil templates. ALD is superior to other coating technologies for the high-curvature surfaces of the nanostructures in terms of conformality and thickness control. The helical nanotubes have much better elasticity than straight nanotubes.

S. Grimm, J. Martín, G. Rodriguez, M. Fernández-Gutierrez, K. Mathwig, R. B. Wehrspohn, U. Gösele, J. San Roman, C. Mijangos and M. Steinhart
Cellular interactions of biodegradable nanorod arrays prepared by nondestructive extraction from nanoporous alumina
Journal of Material Chemistry 20 (2010) 3171.
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Biodegradable extracellular matrices (ECMs) consisting of mechanically stable arrays of poly(lactide) nanorods (nanorod diameter ≈ 200 nm; lattice constant ≈ 500 nm) with areas up to 9 cm² were prepared by nondestructive extraction from recyclable self-ordered nanoporous alumina hard templates. Fibroblasts formed dense tissue layers on the heparin/gelatin activated nanorod arrays, showed excellent adhesion and exhibited a highly elongated morphology such as in natural tissue. The synthetic approach reported here combining advantages of top-down lithography (well-defined topography) and self-assembly (low costs, high throughput, feature size in the 100 nm range) may yield ECMs for biomedical applications. Remarkably, on microrod arrays with four times larger feature sizes the fibroblasts were significantly less elongated and their proliferation was strongly reduced.

K. Mathwig
Transport von Teilchen in asymmetrisch modulierten Poren
Dissertation, Martin-Luther-Universität Halle-Wittenberg (2009).
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The separation of mesoscopic particles by their size has numerous applications in industry, medicine and biology. Brownian motion plays an important role in the filtration or separation of small particles, but generally leads to an undirected motion or intermixing by diffusion. The present work deals with the microfluidic transport of particles by the drift ratchet effect, which utilizes Brownian motion instead to aid in the separation. In the experiment, suspended particles are microfluidically driven through an array of asymmetrically modulated microchannels fabricated out of macroporous silicon. A new interpretation of the first report on experimental demonstration of the drift ratchet effect (Matthias & Müller Nature 424, 53–57, 2003) is offered: New experiments, which allow to distinguish between particles of different sizes, as well as a reexamination of the original work leads to the conclusion that the measured particle transport does not result from a drift ratchet effect. It is demonstrated that convective mixing instead causes the transport.

R. Hillebrand, S. Grimm, R. Giesa, H.-W. Schmidt, K. Mathwig, U. Gösele and M. Steinhart
Characterization of microrod arrays by image analysis
Applied Physics Letters 94 (2009) 164103.
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The uniformity of the properties of array elements was evaluated by statistical analysis of microscopic images of array structures, assuming that the brightness of the array elements correlates quantitatively or qualitatively with a microscopically probed quantity. Derivatives and autocorrelation functions of cumulative frequency distributions of the object brightnesses were used to quantify variations in object properties throughout arrays. Thus, different specimens, the same specimen at different stages of its fabrication or use, and different imaging conditions can be compared systematically. As an example, we analyzed scanning electron micrographs of microrod arrays and calculated the percentage of broken microrods.

R. Hillebrand, S. Grimm, R. Giesa, H.-W. Schmidt, K. Mathwig, U. Gösele and M. Steinhart
Polymer microrod arrays prepared by non-destructive molding evaluated by real space image analysis
Polymer Preprints 50 (2009) 365.
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Ensembles of aligned polymer nanorpds and microrods are promising materials for self-cleaning surfaces, bioinspired adhesive structures, tissue engineering or sensor arrays. We demonstrare the nondestructive mechanical extraction of arrays of crosslinked polyacrylate nanorods and microrods from thus recyclabe nanoporous anodic aluminum oxide and macroporous silicon templates without necking yielding or cohering. Furthermore, we have developed procedures based on real space image analysis of scanning probe microscopy images of arrays thus released to determine the number of fractured rods and to quantify the uniformity of the array properties.

K. Mathwig, W. Kaiser, A. Somers, J. P. Reithmaier, A. Forchel, K. Ohira, S. M. Ullah and S. Arai
DFB Lasers With Deeply Etched Vertical Grating Based on InAs–InP Quantum-Dash Structures
IEEE Photonics Technology Letters 19 (2007) 264.
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Distributed feedback lasers with first-order vertical grating based on AlInGaAs-InAs-InP quantum-dash lasers were fabricated by electron beam lithography and Cl₂-Ar reactive ion etching with an electron cyclotron resonance source. Low threshold currents and single-mode operation with sidemode suppression ratios of 48 dB and a direct modulation bandwidth of 5.5 GHz were demonstrated.

K. Mathwig
Monomodige InP-basierende Quantenpunktlaser: Herstellung, statische und dynamische Eigenschaften
Diplomarbeit, Universität Würzburg (2005).
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Ziel dieser Arbeit war die Herstellung von InP-Quantendash-DFB-Lasern mit einer Wellenläge um 1,55 μm für Telekommunikationsanwendungen. Der Schwerpunkt der Diplomarbeit lag in der Entwicklung von auf die speziellen Materialeigenschaften von Quantendash-Laserstrukturen abgestimmten Trockenätzprozessen für die laterale Nanostrukturierung. Damit wurden monomodig emittierende Laserbauelemente hergestellt, und deren spezielle Eigenschaften wurden charakterisiert.