On computational homogenization of microscale crack propagation

The effective response of microstructures undergoing crack propagation is studied by homogenizing the response of statistical volume elements (SVEs). Because conventional boundary conditions (Dirichlet, Neumann and strong periodic) all are inaccurate when cracks intersect the SVE boundary, we herein use first order homogenization to compare the performance of these boundary conditions during the initial stage of crack propagation in the microstructure, prior to macroscopic localization. Using weakly periodic boundary conditions that lead to a mixed formulation with displacements and boundary tractions as unknowns, we can adapt the traction approximation to the problem at hand to obtain better convergence with increasing SVE size. In particular, we show that a piecewise constant traction approximation, which has previously been shown to be efficient for stationary cracks, is more efficient than the conventional boundary conditions in terms of convergence also when crack propagation occurs on the microscale. The performance of the method is demonstrated by examples involving grain boundary crack propagation modelled by conventional cohesive interface elements as well as crack propagation modelled by means of the extended finite element method in combination with the concept of material forces. © 2016 The Authors. International Journal for Numerical Methods in Engineering Published by John Wiley & Sons Ltd.     

Linear spectral unmixing using endmember coexistence rules and spatial correlation

Mixed pixels are often formed when surface materials are smaller than the spatial resolution of a sensor, or two or more ground features fall within a pixel. Spectral unmixing, decomposing a mixed pixel into a set of endmembers and their corresponding abundance fractions, is an important method for extracting the underlying spectral and spatial information from remote sensing images. Recent studies have shown that it is difficult to increase the accuracy of unmixing using single pixel processing. Here, we suggest combining information on the fundamental interrelations of ground components and a priori knowledge on how ground components co-exist or exclude each other according to general geographic and geomorphic relations with spectral information may allow improved unmixing. Therefore, we propose a novel spectral unmixing method to estimate endmember abundances based on linear spectral mixing model with endmember coexistence rules and spatial correlation (LSMM-R&C). This method was implemented by incorporating endmember coexistence rules along with spatial correlation into a weighted least square method. Experiments with both synthetic and real satellite images were carried out to verify the proposed method, and its performance was also evaluated in comparison to the commonly used LSMM (linear spectral mixture method), LAU (local adaptive unmixing), ISU (iterative spectral unmixing) and ISMA (iterative spectral mixture analysis) methods. LSMM-R&C showed the smallest error, and was more effective at revealing the detailed spatial distribution of endmembers’ abundance, showing high potential for solving the problem of spatial heterogeneity among neighbouring pixels.     

Two-color DNA nanoprobe of intracellular dynamics

We have developed a correlation microscopy technique to follow the dynamics of quantum dot labeled DNA within living cells. The temporal correlation functions of the labels reflect the fluctuations of the DNA nanoprobe as a result of its interactions with the cellular environment. They provide a sensitive measure for the length of the probe on the scale of a persistence length (～50 nm) and reveal strong nonthermal dynamics of the cell. These results pave the way for dynamic observations of DNA conformational changes in vivo.     

Electromechanical properties of pressure-actuated poly(dimethylsiloxane) microfluidic push-down valves

Pressure-actuated poly(dimethylsiloxane) (PDMS) valves have been characterized with respect to their electromechanical properties. Measurements of the valve opening and closing times, threshold pressures, and impedance spectra for closed valves can be used to assess the quality of the devices in general, determine their suitability for specialized applications, such as providing electrical isolated fluidic compartments for planar patch clamping, and specify ideal operating conditions. For our particular valve designs, we report valve opening times of the order of 10-100 micros, making them suitable for rapid buffer exchange applications. They can effectively provide reversible electrical isolation between adjacent fluidic compartments with typical resistances of 5 Gohms in the closed state, which meets the gigaohm requirement for patch clamping applications.     

Towards harnessing theories through tool support for hard real-time Java programming

We present a rationale for a selection of tools that assist developers of hard real-time applications to verify that programs conform to a Java real-time profile and that platform-specific resource constraints are satisfied. These tools are specialised instances of more generic static analysis and model checking frameworks. The concepts are illustrated by two case studies, and the strengths and the limitations of the tools are discussed.     

Localization aligned weakly periodic boundary conditions

When computing the homogenized response of a representative volume element (RVE), a popular choice is to impose periodic boundary conditions on the RVE. Despite their popularity, it is well known that standard periodic boundary conditions lead to inaccurate results if cracks or localization bands in the RVE are not aligned with the periodicity directions. A previously proposed remedy is to use modified strong periodic boundary conditions that are aligned with the dominating localization direction in the RVE. In the present work, we show that alignment of periodic boundary conditions can also conveniently be performed on weak form. Starting from a previously proposed format for weak micro‐periodicity that does not require a periodic mesh, we show that aligned weakly periodic boundary conditions may be constructed by only modifying the mapping (mirror function) between the associated parts of the RVE boundary. In particular, we propose a modified mirror function that allows alignment with an identified localization direction. This modified mirror function corresponds to a shifted stacking of RVEs, and thereby ensures compatibility of the dominating discontinuity over the RVE boundaries. The proposed method leads to more accurate results compared to using unaligned periodic boundary conditions, as demonstrated by the numerical examples. Copyright © 2016 John Wiley & Sons, Ltd.     

Microfluidic chip for low-flow push-pull perfusion sampling in vivo with on-line analysis of amino acids

Multilayer soft lithography was used to prepare a poly(dimethylsiloxane) microfluidic chip that allows for in vivo sampling of amino acid neurotransmitters by low-flow push-pull perfusion. The chip incorporates a pneumatically actuated peristaltic pump to deliver artificial cerebrospinal fluid to a push-pull perfusion probe, pull sample from the probe, perform on-line derivatization with o-phthaldialdehyde, and push derivatized amino acids into the flow-gated injector of a high-speed capillary electrophoresis-laser-induced fluorescence instrument. Peristalsis was achieved by sequential actuation of six, 200 microm wide by 15 microm high control valves that drove fluid through three fluidic channels of equal dimensions. Electropherograms with 100,000 theoretical plates were acquired at approximately 20-s intervals. Relative standard deviations of peak heights were 4% in vitro, and detection limits for the excitatory amino acids were approximately 60 nM. For in vivo measurements, push-pull probes were implanted in the striatum of anesthetized rats and amino acid concentrations were monitored while sampling at 50 nL/min. o-Phosphorylethanolamine, glutamate, aspartate, taurine, glutamine, serine, and glycine were all detected with stable peak heights observed for over 4 h with relative standard deviations of 10% in vivo. Basal concentrations of glutamate were 1.9 +/- 0.6 microM (n = 4) in good agreement with similar methods. Monitoring of dynamic changes of neurotransmitters resulting from 10-min applications of 70 mM K(+) through the push channel of the pump was demonstrated. The combined system allows temporal resolution for multianalyte monitoring of approximately 45 s with spatial resolution 65-fold better than conventional microdialysis probe with 4-mm length. The system demonstrates the feasibility of sampling from a complex microenvironment with transfer to a microfluidic device for on-line analysis.