Development of Endothelial Cell Networks in 3D Tissues by Combination of Melt Electrospinning Writing with Cell‐Accumulation Technology

A remaining challenge in tissue engineering approaches is the in vitro vascularization of engineered constructs or tissues. Current approaches in engineered vascularized constructs are often limited in the control of initial vascular network geometry, which is crucial to ensure full functionality of these constructs with regard to cell survival, metabolic activity, and potential differentiation ability. Herein, the combination of 3D‐printed poly‐ε‐caprolactone scaffolds via melt electrospinning writing with the cell‐accumulation technique to enable the formation and control of capillary‐like network structures is reported. The cell‐accumulation technique is already proven itself to be a powerful tool in obtaining thick (50 µm) tissues and its main advantage is the rapid production of tissues and its ease of performance. However, the applied combination yields tissue thicknesses that are doubled, which is of outstanding importance for an improved handling of the scaffolds and the generation of clinically relevant sample volumes. Moreover, a correlation of increasing vascular endothelial growth factor secretion to hypoxic conditions with increasing pore sizes and an assessment of the formation of neovascular like structures are included.     

Thermophysical Properties of a Chromium–Nickel–Molybdenum Steel in the Solid and Liquid Phases

Numerical simulation of vacuum arc re-melting, pressurized or protective electro-slag re-melting, and ingot casting have become quite important in the metal industry. However, a major drawback of these simulation techniques is the lack of accurate thermophysical properties for temperatures above 1,500 K. Heat capacity, heat of fusion, density, and thermal conductivity are important input parameters for the heat transfer equation. Since, direct measurements of thermal conductivity of alloys in the liquid state are almost impossible, its estimation from electrical conductivity using the Wiedemann–Franz law is very useful. The afore-mentioned thermophysical properties of several steels are investigated within the context of an ongoing project. Here, we present a full set of thermophysical data for the chromium–nickel–molybdenum steel meeting the standard DIN 1.4435 (X2CrNiMo18-14-3); these values will be used by our partner to simulate various re-melting and solidification processes. Wire-shaped samples of the steel are resistively volume-heated, as part of a fast capacitor discharge circuit. Time-resolved measurements with sub-μs resolution of current through the specimen are performed with a Pearson probe. The voltage drop across the specimen is measured with knife-edge contacts and ohmic voltage dividers, the temperature of the sample with a pyrometer, and the volumetric expansion of the wire with a fast acting CCD camera. These measurements enable the heat of fusion, the heat capacity, and the electrical resistivity to be determined as a function of temperature in the solid and liquid phases. The thermal conductivity and thermal diffusivity are estimated via the Wiedemann–Franz law.     

Reduced-order preconditioning for bidomain simulations

Simulations of the bidomain equations involve solving large, sparse, linear systems of the form Ax = b. Being an initial value problems, it is solved at every time step. Therefore, efficient solvers are essential to keep simulations tractable. Iterative solvers, especially the preconditioned conjugate gradient (PCG) method, are attractive since memory demands are minimized compared to direct methods, albeit at the cost of solution speed. However, a proper preconditioner can drastically speed up the solution process by reducing the number of iterations. In this paper, a novel preconditioner for the PCG method based on system order reduction using the Arnoldi method (A-PCG) is proposed. Large order systems, generated during cardiac bidomain simulations employing a finite element method formulation, are solved with the A-PCG method. Its performance is compared with incomplete LU (ILU) preconditioning. Results indicate that the A-PCG estimates an approximate solution considerably faster than the ILU, often within a single iteration. To reduce the computational demands in terms of memory and run time, the use of a cascaded preconditioner was suggested. The A-PCG was applied to quickly obtain an approximate solution, and subsequently a cheap iterative method such as successive overrelaxation (SOR) is applied to further refine the solution to arrive at a desired accuracy. The memory requirements are less than those of direct LU but more than ILU method. The proposed scheme is shown to yield significant speedups when solving time evolving systems.     

Interfacial charges and electric field distribution in organic hetero-layer light-emitting devices

The electric field distribution in organic hetero-layer light-emitting devices based on N,N^′-diphenyl-N,N^′-bis(1-naphtyl)-1,1^′-biphenyl-4,4^′-diamine (NPB) and 8-tris-hydroxyquinoline aluminium (Alq₃) has been investigated under different bias conditions using capacitance–voltage measurements. Although this method yields primarily information on the differential capacitance, the data give clear evidence for the presence of negative interfacial charges with a density of 6.8×10¹¹e cm⁻² at the NPB/Alq₃ interface at large reverse bias. This leads to a jump of the electric field at the interface and a non-uniform field distribution in the hetero-layer device.     

Energy content of world trade

This paper constructs a comprehensive dataset of oil and total energy embedded in world trade of manufacturing goods for 73 countries from 1978 to 2000. Applying the data to debates on the dependency on foreign energy sources makes clear that achieving complete energy independence in the foreseeable future is unlikely to be feasible and may not be desirable. Applying it to the discussion of environmental Kuznets curves (EKCs) highlights an important distinction between production and consumption of energy. Richer countries use relatively less energy in their industrial production yet still consume relatively large amounts of energy indirectly. A further investigation largely excludes structural shifts of production in and out of the manufacturing sector as an explanation for the downward-sloping portion of the EKC. Country-level analyses add caveats but show tentative support for the cross-country conclusions.     

Fracture of austenitic steel subject to a wide range of stress triaxiality ratios and crack deformation modes

The stress triaxiality ratio (hydrostatic pressure divided by von Mises equivalent stress) strongly affects the fracture behaviour of materials. Various fracture criteria take this effect into consideration in their effort to predict failure. The dependency of the fracture locus on the stress triaxiality ratio has to be investigated in order to evaluate these criteria and improve the understanding of ductile fracture.This was done by comparing the experimental results of austenitic steel specimens with a strong variation in their stress triaxiality ratios. The specimens had cracks with varying depths and crack tip deformation modes; tension, in-plane shear, and out-of-plane shear. The crack growth in fracture mechanics specimens was compared with the failure of standard testing specimens for tension, upsetting and torsion. By associating the experimental results with finite element simulations it was possible to compare the critical plastic equivalent strain and stress triaxiality ratio values at fracture. In the investigated triaxiality regime an exponential dependency of the fracture locus on the stress triaxiality ratio was found.     

Verification of cardiac tissue electrophysiology simulators using an N-version benchmark

Ongoing developments in cardiac modelling have resulted, in particular, in the development of advanced and increasingly complex computational frameworks for simulating cardiac tissue electrophysiology. The goal of these simulations is often to represent the detailed physiology and pathologies of the heart using codes that exploit the computational potential of high-performance computing architectures. These developments have rapidly progressed the simulation capacity of cardiac virtual physiological human style models; however, they have also made it increasingly challenging to verify that a given code provides a faithful representation of the purported governing equations and corresponding solution techniques. This study provides the first cardiac tissue electrophysiology simulation benchmark to allow these codes to be verified. The benchmark was successfully evaluated on 11 simulation platforms to generate a consensus gold-standard converged solution. The benchmark definition in combination with the gold-standard solution can now be used to verify new simulation codes and numerical methods in the future.     

A nonlinear filtered‐x prediction error method algorithm for digital predistortion in digital subscriber line systems

Digital predistortion of nonlinear systems is an important topic in many practical applications. This paper considers direct predistortion of a Volterra system by connecting in tandem an adaptive Volterra predistorter. The coefficients of the predistorter can be recursively estimated using the nonlinear filtered‐x least mean squares (NFxLMS) algorithm. In this paper, the prediction error method (PEM) is used to derive a novel nonlinear filtered‐x PEM (NFxPEM) algorithm. A simulation study on Volterra systems shows that the NFxPEM algorithm more significantly suppresses spectral regrowth and converges much faster than the NFxLMS algorithm. Also, the NFxPEM algorithm is used in this paper to design more efficient digital predistorter—as compared with the NFxLMS algorithm—for digital subscriber line systems. Copyright © 2012 John Wiley & Sons, Ltd.