Branch Mode Selection during Early Lung Development

Many organs of higher organisms, such as the vascular system, lung, kidney, pancreas, liver and glands, are heavily branched structures. The branching process during lung development has been studied in great detail and is remarkably stereotyped. The branched tree is generated by the sequential, non-random use of three geometrically simple modes of branching (domain branching, planar and orthogonal bifurcation). While many regulatory components and local interactions have been defined an integrated understanding of the regulatory network that controls the branching process is lacking. We have developed a deterministic, spatio-temporal differential-equation based model of the core signaling network that governs lung branching morphogenesis. The model focuses on the two key signaling factors that have been identified in experiments, fibroblast growth factor (FGF10) and sonic hedgehog (SHH) as well as the SHH receptor patched (Ptc). We show that the reported biochemical interactions give rise to a Schnakenberg-type Turing patterning mechanisms that allows us to reproduce experimental observations in wildtype and mutant mice. The kinetic parameters as well as the domain shape are based on experimental data where available. The developed model is robust to small absolute and large relative changes in the parameter values. At the same time there is a strong regulatory potential in that the switching between branching modes can be achieved by targeted changes in the parameter values. We note that the sequence of different branching events may also be the result of different growth speeds: fast growth triggers lateral branching while slow growth favours bifurcations in our model. We conclude that the FGF10-SHH-Ptc1 module is sufficient to generate pattern that correspond to the observed branching modesComment: Initially published at PLoS Comput Bio

Direct imaging of the structural change generated by dielectric breakdown in MgO based magnetic tunnel junctions

MgO based magnetic tunnel junctions are prepared to investigate the dielectric breakdown of the tunnel barrier. The breakdown is directly visualized by transmission electron microscopy measurements. The broken tunnel junctions are prepared for the microscopy measurements by focussed ion beam out of the junctions characterized by transport investigations. Consequently, a direct comparison of transport behavior and structure of the intact and broken junctions is obtained. Compared to earlier findings in Alumina based junctions, the MgO barrier shows much more microscopic pinholes after breakdown. This can be explained within a simple model assuming a relationship between the current density at the breakdown and the rate of pinhole formation

Hybrid FEM-BEM approach for two-and three-dimensional open boundary magnetostatic problems

ABSTRACT In order to eliminate additional degrees of freedom in the surrounding domain of a charged or polarized object, we implement an open boundary method based on a hybrid FEM-BEM approach which is tested for magnetostatic problems. The underlying functional dependency of domain and boundary variables entails a sparsity decrease of the system matrix with an increasing surface area to volume ratio. Such a case is commonly at hand if systems with a high aspect ratio are considered. Therefore, we also propose an effective way which allows for the treatment of such systems in a simplified two-dimensional form without neglecting the threedimensional characteristics of the external field. The approach is tested for the cases of a three-dimensional homogeneously magnetized sphere and a thin magnetic sheet

A hydrodynamic switch: Microfluidic separation system for magnetic beads

Weddemann A, Wittbracht F, Auge A, Hütten A. A hydrodynamic switch: Microfluidic separation system for magnetic beads. APPLIED PHYSICS LETTERS. 2009;94(17):173501.In this work a device for separating small magnetic particles in continuous flow is introduced, consisting of two microfluidic channels that are connected by a junction channel. Applying two different flow rates, particles can be separated combining hydrodynamic and magnetophoretic effects. The two different flow rates introduce an additional degree of freedom that enables the microfluidic geometry to act as a hydrodynamic switch that can overcome diffusive contributions making the device applicable for particles of the size scale below 100 nm. Theoretical predictions based on finite element methods are compared to experimental observations

Magnetic ratchet for biotechnological applications

Auge A, Weddemann A, Wittbracht F, Hütten A. Magnetic ratchet for biotechnological applications. APPLIED PHYSICS LETTERS. 2009;94(18):183507.Transport and separation of magnetic beads are important in "lab on a chip" environments for biotechnological applications. One possible solution for this is the on-off ratchet concept. An asymmetric magnetic potential and Brownian motion of magnetic beads are required for such a ratchet. The asymmetric magnetic potential is achieved by combining an external magnetic field with a spatially periodic array of conducting lines. In this work finite element method simulations are carried out to design this asymmetric potential and to evaluate transport rates. Furthermore, experiments are carried out so as to compare to the simulation results

Magnetic Nanoparticles for Novel Granular Spintronic Devices -the gGMR sensor

ABSTRACT Superparamagnetic nanoparticles have a wide range of applications in modern electric devices. Recent developments have identified them as components for a new type of magnetoresistance sensor based on highly ordered monolayers of such nanocrystallites. In this work, we propose a model for the numeric evaluation of the sensor properties. Based on the solutions of the LandauLifshitz-Gilbert equation for a set of homogeneously magnetized spheres arranged in highly symmetric monolayers, we analyze how different device properties may be adjusted to specific demands by modifications of the microstructure. We characterize sensor properties and identify different measurement regimes which correspond to specific dominating energy contributions. In particular, we find a novel measuring mode where increased field sensitivity is bought at the cost of an inherent device noise