bluuebunny/crossref_metadata_2025 huggingface dataset:
Suppression assays with human T regulatory cells: A technical guide
Authors: McMurchy Alicia N. , Levings Megan K.
Published: December 2011

The suppression of inappropriate immune responses by Treg cells is one of the major ways that the body maintains immune tolerance and homeostasis. Since defects in the suppressive capacity of Treg cells underlie many different immune‐mediated diseases, there is great interest in developing ways to track the number and function of Treg cells as biomarkers of tolerance and in devising ways to enhance their function therapeutically. However, the methods of studying human Treg cells are fraught with technical challenges that can often lead to misinterpretation. The most common way to determine the suppressive capacity of human Treg cells is to measure their ability to suppress the proliferation of responding CD4+ T cells. Here, we discuss the technical considerations that must be taken into account when performing suppression of T‐cell proliferation assays with human Treg cells. We also consider how T cells may falsely appear suppressive because of dying cells in the system, improper resting of T‐cell lines prior to the assay, or insufficient proliferation of the responding T cells. We propose that, in the future, classification of a population of cells as “regulatory” should rely on more than a simple test for blockade of CD4+ T‐cell proliferation.
Vegetation index‐based crop coefficients to estimate evapotranspiration by remote sensing in agricultural and natural ecosystems
Authors: Glenn Edward P. , Neale Christopher M. U. , Hunsaker Doug J. , Nagler Pamela L.
Published: December 2011

Crop coefficients were developed to determine crop water needs based on the evapotranspiration (ET) of a reference crop under a given set of meteorological conditions. Starting in the 1980s, crop coefficients developed through lysimeter studies or set by expert opinion began to be supplemented by remotely sensed vegetation indices (VI) that measured the actual status of the crop on a field‐by‐field basis. VIs measure the density of green foliage based on the reflectance of visible and near infrared (NIR) light from the canopy, and are highly correlated with plant physiological processes that depend on light absorption by a canopy such as ET and photosynthesis. Reflectance‐based crop coefficients have now been developed for numerous individual crops, including corn, wheat, alfalfa, cotton, potato, sugar beet, vegetables, grapes and orchard crops. Other research has shown that VIs can be used to predict ET over fields of mixed crops, allowing them to be used to monitor ET over entire irrigation districts. VI‐based crop coefficients can help reduce agricultural water use by matching irrigation rates to the actual water needs of a crop as it grows instead of to a modeled crop growing under optimal conditions. Recently, the concept has been applied to natural ecosystems at the local, regional and continental scales of measurement, using time‐series satellite data from the MODIS sensors on the Terra satellite. VIs or other visible‐NIR band algorithms are combined with meteorological data to predict ET in numerous biome types, from deserts, to arctic tundra, to tropical rainforests. These methods often closely match ET measured on the ground at the global FluxNet array of eddy covariance moisture and carbon flux towers. The primary advantage of VI methods for estimating ET is that transpiration is closely related to radiation absorbed by the plant canopy, which is closely related to VIs. The primary disadvantage is that they cannot capture stress effects or soil evaporation. Copyright © 2011 John Wiley & Sons, Ltd.
APC as a mobile scaffold: Regulation and function at the nucleus, centrosomes, and mitochondria
Authors: Lui Christina , Mills Kate , Brocardo Mariana G. , Sharma Manisha , Henderson Beric R.
Published: December 2011

Genetic mutations of adenomatous polyposis coli (APC) predispose to high risk of human colon cancer. APC is a large tumor suppressor protein and truncating mutations disrupt its normal roles in regulating cell migration, DNA replication/repair, mitosis, apoptosis, and turnover of oncogenic β‐catenin. APC is targeted to multiple subcellular sites, and here we discuss recent evidence implicating novel protein interactions and functions of APC in the nucleus and at centrosomes and mitochondria. The ability of APC to shuttle between these and other cell locations is hypothesized to be integral to its cellular function. © 2011 IUBMB IUBMB Life, 2011
Growth factor release from a chemically modified elastomeric poly(1,8‐octanediol‐co‐citrate) thin film promotes angiogenesisin vivo
Authors: Sharma Arun K. , Bury Matthew I. , Fuller Natalie J. , Rozkiewicz Dorota I. , Hota Partha V. , Kollhoff David M. , Webber Matthew J. , Tapaskar Natalie , Meisner Jay W. , Lariviere Patrick J. , DeStefano Samantha , Wang Deli , Ameer Guillermo A. , Cheng Earl Y.
Published: December 2011

The ultimate success ofin vivoorgan formation utilizingex vivoexpanded “starter” tissues relies heavily upon the level of vascularization provided by either endogenous or artificial induction of angiogenic or vasculogenic events. To facilitate proangiogenic outcomes and promote tissue growth, an elastomeric scaffold previously shown to be instrumental in the urinary bladder regenerative process was modified to release proangiogenic growth factors. Carboxylic acid groups on poly(1,8‐octanediol‐co‐citrate) films (POCfs) were modified with heparan sulfate creating a heparan binding POCf (HBPOCf). Release of proangiogenic growth factors vascular endothelial growth factor (VEGF), fibroblast growth factor 2 (FGF2), and insulin‐like growth factor 1 (IGF‐1) from HBPOCfs demonstrated an approximate threefold increase over controls during a 30‐day time coursein vitro. Atomic force microscopy demonstrated significant topological differences between films. Subcutaneous implantation of POCf alone, HBPOCf, POCf‐VEGF, and HBPOCf‐VEGF within the dorsa of nude rats yielded increased vascular growth in HBPOCf‐VEGF constructs. Vessel quantification studies revealed that POCfs alone contained 41.1 ± 4.1 vessels/mm2, while HBPOCf, POCf‐VEGF, and HBPOCF‐VEGF contained 41.7 ± 2.6, 76.3 ± 9.4, and 167.72 ± 15.3 vessels/mm2, respectively. Presence of increased vessel growth was demonstrated by CD31 and vWF immunostaining in HBPOCf‐VEGF implanted areas. Data demonstrate that elastomeric POCfs can be chemically modified and possess the ability to promote angiogenesisin vivo. © 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part A, 2012.
Protective effect of selenium against mercury‐induced toxicity on hematological and biochemical parameters of Oreochromis niloticus
Authors: Cogun Hikmet Y. , Fırat Özgür , Fırat Özge , Yüzereroǧlu Tüzin A. , Gök Gülbin , Kargin Ferit , Kötemen Yasemin
Published: December 2011

In this study, to identify mercury (Hg) toxicity and whether selenium (Se) has any role in alleviation of this toxicity, it was investigated the changes in hematological and serum biochemical parameters of Oreochromis niloticus. Fish were exposed to 0.01 and 0.1 mg/L Hg and 0.01 mg/L Hg + 0.1 mg/L Se and 0.1 mg/L Hg + 1.0 mg/L Se for 7 and 14 days. The exposure of O. niloticus to Hg alone resulted in decreases in red blood cell, white blood cell, hemoglobin, hematocrit values, and cholinesterase activity while it increased in alanine aminotransferase and aspartate aminotransferase activities and cortisol and glucose levels. Se, in combination with Hg, partially or totally caused an alleviation for the toxic effect of Hg on the above mentioned hematological and biochemical parameters. The results of our study showed that Se has a protective effect against toxicity induced by Hg. © 2012 Wiley Periodicals, Inc. J Biochem Mol Toxicol 26:117–122, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/jbt.20417
Beyond (the ABCs): education, community, and feminism in Afghanistan
Authors: Brodsky Anne E. , Portnoy Galina A. , Scheibler Jill E. , Welsh Elena A. , Talwar Gitika , Carrillo Amy
Published: December 2011

This article examines the meaning, operation, and outcomes of education and related formation of feminist identity development within an Afghan women's humanitarian and political organization. Qualitative data, including 110 interviews, archival review, and participant observations, were collected using a feminist, community, strengths‐based approach and were re‐analyzed here with a focus on educational processes. Findings revealed multiple educational mechanisms, both similar to and different from many Western assumptions. Within these educational mechanisms, themes of critical consciousness and feminist identity also arose. Outcomes were mapped against Downing and Roush's (1985) feminist identity development model. Similarities, differences, limitations, and lessons in the application of a Western model to an Afghan context are discussed. Findings have implications for understanding indigenous educational methods, the development of critical consciousness and “feminist” identity in global perspective, and cross‐cultural, feminist, community psychology research and application. © 2012 Wiley Periodicals, Inc.
AN EXPLORATORY CROSS‐COUNTRY ANALYSIS OF GENDERED INSTITUTIONS
Authors: van Staveren Irene
Published: December 2011

The standard empowerment model underlying gender policies by international organisations emphasises women's access to resources. This paper presents an exploratory analysis of the relative importance of access to resources as compared with women's agency, recognising that this agency may be limited by gendered institutional constraints. It presents a cross‐country analysis with a variety of formal and informal gendered institutions, access to resources and well‐being achievements. The regression analysis suggests that women's empowerment depends both on access to resources (positively) and on gendered institutions (negatively), with different institutions affecting different dimensions of empowerment. Copyright © 2011 John Wiley & Sons, Ltd.
Postmortem role of calpains in Pekin duck skeletal muscles
Authors: Chang Ya‐Shiou , Chou Rong‐Ghi R
Published: December 2011

BACKGROUD: It is generally agreed that calpains are involved in postmortem proteolysis of skeletal muscle and improve meat tenderness. However, little information regarding the postmortem role of calpains in duck skeletal muscle is known. Therefore, the purpose of this study was to examine the role of calpains in Pekin duck postmortem breast muscles (BM) and leg and thigh muscles (LM) muscles at 5 °C.

RESULTS: The postmortem pH was lower (P < 0.05) in BM than in LM. Western blots indicated that postmortem desmin degradation and the 30/32 kDa troponin‐T degradation product accumulation were more rapid in BM than in LM. Casein zymograms showed that at‐death µ‐calpain activity was higher in BM than in LM. As time post mortem increased, µ‐calpain was activated and autolyzed more rapidly and extensively in BM than in LM, but µ/m‐calpain was activated at a relative slower rate compared with µ‐calpain. Correlation results showed that µ‐calpain activity, rather than µ/m‐calpain activity, in BM samples was highly correlated with the abundance of desmin and the 30/32 kDa troponin‐T degradation components across the postmortem period. However, no such correlations were found with LM µ‐ and µ/m‐calpains.

CONCLUSION: Therefore, our results suggest that BM µ‐calpain with a faster and more extensive activation and autolysis would play a relatively dominant role in dictating degradation of desmin and troponin‐T in postmortem duck muscle. Copyright © 2011 Society of Chemical Industry
Irresolvable complex mixture of hydrocarbons in soybean oil deodorizer distillate
Authors: Ju Yi‐Hsu , Huynh Lien‐Huong , Gunawan Setiyo , Chern Yaw‐Terng , Kasim Novy S.
Published: December 2011

Aliphatic hydrocarbons (HCs) can be used as a fingerprint of a given seed oil. Only by characterization of aliphatic HCs could contamination by mineral oil in that seed oil be confirmed. During the isolation of squalene from soybean oil deodorizer distillate, a significant amount of unknown HCs, ca. 44 wt%, was obtained. These seemingly‐easy‐to‐identify HCs turned out to be much more difficult to elucidate due to the presence of an irresolvable complex mixture (ICM). The objective of this study was to purify and identify the unknown ICM of aliphatic HCs from soybean oil deodorizer distillate. Purification of the ICM was successfully achieved by using modified Soxhlet extraction, followed by modified preparative column chromatography, and finally by classical preparative column chromatography. FT‐IR, TLC, elemental analysis, GC/FID, NMR and GC‐MS analyses were then performed on the purified HCs. The GC chromatogram detected the presence of ICM peaks comprising two major peaks and a number of minor peaks. Validation methods such as IR and NMR justified that the unknowns are saturated HCs. This work succeeded in tentatively identifying the two major peaks in the ICM as cycloalkane derivatives.
Current Trends in the Field of Self‐Healing Materials
Authors: Guimard Nathalie K. , Oehlenschlaeger Kim K. , Zhou Jiawen , Hilf Stefan , Schmidt Friedrich Georg , Barner‐Kowollik Christopher
Published: December 2011

The evolution of material design has mirrored advancements in the understanding of materials, nature, and the requirements of target applications. Originally, materials were only intended to play a passive role, but with a deepened understanding of material properties and design has come an improved ability to harness these properties to create materials with predetermined response mechanisms. This article has three aims: i) to briefly discuss the origin of and motivation for having materials that are capable of undergoing healing either extrinsically or intrinsically; ii) to present the most recent and promising advancements in the field of self‐healing materials; and iii) to discuss important material design and property specifications that should be considered in order to promote the development of optimized self‐healing materials.
Diffusion tensor imaging of freezing of gait in patients with white matter changes
Authors: Youn Jinyoung , Cho Jin Whan , Lee Won Yong , Kim Gyeong‐Moon , Kim Sung Tae , Kim Hee‐Tae
Published: December 2011

Background:

Freezing of gait is a common and disabling symptom of parkinsonism. However, the corresponding anatomic structures have yet to be clearly elucidated.

Methods:

We performed diffusion tensor imaging on 40 subjects with white matter changes. We compared apparent diffusion coefficient values and fraction anisotropy values of 7 candidate anatomic structures between 14 patients with freezing of gait (freezing of gait group) and 26 without freezing of gait (control group).

Results:

Fraction anisotropy values of the bilateral pedunculopontine nucleus, bilateral superior premotor cortex, right orbitofrontal area, and left supplement motor area were significantly lower in the freezing of gait group than in the control group. In contrast, there were no significant differences in apparent diffusion coefficient values between freezing of gait and control groups.

Conclusions:

Our findings suggest that the bilateral pedunculopontine nucleus, bilateral superior premotor cortex, right orbitofrontal area, and left supplement motor area are closely related to freezing of gait. © 2011 Movement Disorder Society
Parkinson's disease motor subtypes and mood
Authors: Burn David J. , Landau Sabine , Hindle John V. , Samuel Michael , Wilson Kenneth C. , Hurt Catherine S. , Brown Richard G. ,   for the PROMS‐PD Study Group
Published: December 2011

Parkinson's disease is heterogeneous, both in terms of motor symptoms and mood. Identifying associations between phenotypic variants of motor and mood subtypes may provide clues to understand mechanisms underlying mood disorder and symptoms in Parkinson's disease. A total of 513 patients were assessed using the Hospital Anxiety and Depression Scale, and separately classified into anxious, depressed, and anxious‐depressed mood classes based on latent class analysis of a semistructured interview. Motor subtypes assessed related to age‐of‐onset, rate of progression, presence of motor fluctuations, lateralization of motor symptoms, tremor dominance, and the presence of postural instability and gait symptoms and falls. The directions of observed associations tended to support previous findings with the exception of lateralization of symptoms, for which there were no consistent or significant results. Regression models examining a range of motor subtypes together indicated increased risk of anxiety in patients with younger age‐of‐onset and motor fluctuations. In contrast, depression was most strongly related to axial motor symptoms. Different risk factors were observed for depressed patients with and without anxiety, suggesting heterogeneity within Parkinson's disease depression. Such association data may suggest possible underlying common risk factors for motor subtype and mood. Combined with convergent evidence from other sources, possible mechanisms may include cholinergic system damage and white matter changes contributing to non‐anxious depression in Parkinson's disease, while situational factors related to threat and unpredictability may contribute to the exacerbation and maintenance of anxiety in susceptible individuals. © 2011 Movement Disorder Society
Simultaneous variable flip angle–actual flip angle imaging method for improved accuracy and precision of three‐dimensional T1 and B1 measurements
Authors: Hurley Samuel A. , Yarnykh Vasily L. , Johnson Kevin M. , Field Aaron S. , Alexander Andrew L. , Samsonov Alexey A.
Published: December 2011

A new time‐efficient and accurate technique for simultaneous mapping of T1 and B1 is proposed based on a combination of the actual flip angle (FA) imaging and variable FA methods. Variable FA–actual FA imaging utilizes a single actual FA imaging and one or more spoiled gradient‐echo acquisitions with a simultaneous nonlinear fitting procedure to yield accurate T1/B1 maps. The advantage of variable FA–actual FA imaging is high accuracy at either short T1 times or long repetition times in the actual FA imaging sequence. Simulations show this method is accurate to 0.03% in FA and 0.07% in T1 for ratios of repetition time to T1 time over the range of 0.01–0.45. We show for the case of brain imaging that it is sufficient to use only one small FA spoiled gradient‐echo acquisition, which results in reduced spoiling requirements and a significant scan time reduction compared to the original variable FA method. In vivo validation yielded high‐quality 3D T1 maps and T1 measurements within 10% of previously published values and within a clinically acceptable scan time. The variable FA–actual FA imaging method will increase the accuracy and clinical feasibility of many quantitative MRI methods requiring T1/B1 mapping such as dynamic contrast enhanced perfusion and quantitative magnetization transfer imaging. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Prospective motion correction with continuous gradient updates in diffusion weighted imaging
Authors: Herbst Michael , Maclaren Julian , Weigel Matthias , Korvink Jan , Hennig Juergen , Zaitsev Maxim
Published: December 2011

Despite the existence of numerous motion correction methods, head motion during MRI continues to be a major source of artifacts and can greatly reduce image quality. This applies particularly to diffusion weighted imaging, where strong gradients are applied during long encoding periods. These are necessary to encode microscopic movements. However, they also make the technique highly sensitive to bulk motion. In this work, we present a prospective motion correction method where all applied gradients are adjusted continuously to compensate for changes of the object position and ensure the desired phase evolution in the image coordinate frame. Additionally, in phantom experiments this new technique is used to reproduce motion artifacts with high accuracy by changing the position of the imaging frame relative to the measured object. In vivo measurements demonstrate the validity of the new correction method. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Six is enough? Comparison of diffusion parameters measured using six or more diffusion‐encoding gradient directions with deterministic tractography
Authors: Lebel Catherine , Benner Thomas , Beaulieu Christian
Published: December 2011

Diffusion tensor imaging tractography is commonly used to quantify white matter tracts in the human brain via parameters such as fractional anisotropy and mean diffusivity. Simulation studies recommend the use of more than six directions for robust parameter estimates; however, no study has examined the impact of the number of gradient directions on deterministic tractography‐derived diffusion parameters in human brain. Here, for 10 major white matter tracts in 11 healthy volunteers at 1.5 T, six‐direction diffusion tensor imaging data were compared to 30‐ or 60‐direction data, keeping scan time and number of b = 0 images constant within each test. Mean diffusivity was systematically lower for six‐direction protocols (20/40 comparisons); six‐direction data had higher fractional anisotropy in the superior longitudinal fasciculus and smaller tract volume for the genu of the corpus callosum. In general, parameter differences due to the number of directions were smaller than those from intersubject variation or signal‐to‐noise ratio. Despite some absolute differences, standard deviations were significantly different for only one of 160 comparisons. Thus, six‐direction data provide diffusion measures with comparable robustness to 30‐ or 60‐direction data and yield appropriate parameter values for most white matter tracts, although there are clear advantages in acquiring higher angular resolution data. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Comparison of NMR lipid profiles in mitotic arrest and apoptosis as indicators of paclitaxel resistance in cervical cell lines
Authors: Zietkowski Dominik. , Payne Geoffrey S. , Nagy Eszter , Mobberley Margaret A. , Ryder Timothy A. , deSouza Nandita M.
Published: December 2011

This study aimed to characterize changes in lipid saturation using magnetic resonance spectroscopy of sensitive (HeLa) and resistant (C33A; Me180) cervical cancer cell lines following exposure to paclitaxel to explore lipid profiles as biomarkers of drug resistance. Spectra were acquired at 11.74 T. Flow cytometry, electron, and confocal microscopy assessed cellular morphology. Western blots assessed cytoplasmic phospholipase A2, fatty acid synthase, and acyl‐CoA synthetase1 expression. After 24 h of paclitaxel exposure, >60% of cells showed mitotic arrest. At 48 h, HeLa cells showed apoptosis while C33A/Me180 cells showed normal morphology indicating resistance. MR‐visible lipids increased significantly in all lines at 24 h with further increases at 48 h; resistant lines showed smaller increases than HeLa. Cytoplasmic phospholipase A2 and fatty acid synthase levels were unchanged at 24 h and dropped at 48 h in HeLa; acyl‐CoA synthetase1 was higher in Me180/C33A than in HeLa controls but did not increase significantly. The percentage of cells displaying lipid droplets increased significantly at 24 and 48 h in all lines; droplet size increased only in HeLa cells. Droplet number was >3–4× greater in apoptotic compared with mitotic‐arrested cells. Apoptotic cells accumulate unsaturated fatty acids in large (relative to control) droplets; resistant lines accumulated smaller droplets with less triglycerides. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Nonuniform and multidimensional Shinnar‐Le Roux RF pulse design method
Authors: Grissom William A. , McKinnon Graeme C. , Vogel Mika W.
Published: December 2011

The Shinnar‐Le Roux (SLR) radiofrequency (RF) pulse design algorithm is widely used for designing slice‐selective RF pulses due to its intuitiveness, optimality, and speed. SLR is limited, however, in that it is only capable of designing one‐dimensional pulses played along constant gradients. We present a nonuniform SLR RF pulse design framework that extends most of the capabilities of classical SLR to nonuniform gradient trajectories and multiple dimensions. Specifically, like classical SLR, the new method is a hard pulse approximation‐based technique that uses filter design relationships to produce the lowest power RF pulse that satisfies target magnetization ripple levels. The new method is validated and compared with methods conventionally used for nonuniform and multidimensional large‐tip‐angle RF pulse design. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
R mapping in the presence of macroscopic B0 field variations
Authors: Hernando Diego , Vigen Karl K. , Shimakawa Ann , Reeder Scott B.
Published: December 2011

$R_2^{\ast ab}$
mapping has important applications in MRI, including functional imaging, tracking of super‐paramagnetic particles, and measurement of tissue iron levels. However, $R_2^{\ast ab}$
measurements can be confounded by several effects, particularly the presence of fat and macroscopic $B_0$
field variations. Fat introduces additional modulations in the signal. Macroscopic field variations introduce additional dephasing that results in accelerated signal decay. These effects produce systematic errors in the resulting $R_2^{\ast ab}$
maps and make the estimated $R_2^{\ast ab}$
values dependent on the acquisition parameters. In this study, we develop a complex‐reconstruction, confounder‐corrected $R_2^{\ast ab}$
mapping technique, which addresses the presence of fat and macroscopic field variations for both 2D and 3D acquisitions. This technique extends previous chemical shift‐encoded methods for $R_2^{\ast ab}$, fat and water mapping by measuring and correcting for the effect of macroscopic field variations in the acquired signal. The proposed method is tested on several 2D and 3D phantom and in vivo liver, cardiac, and brain datasets. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Generalized stochastic approach for constitutive equation in linear elasticity: a random matrix model
Authors: Guilleminot J. , Soize C.
Published: December 2011

This work is concerned with the construction of stochastic models for random elasticity matrices, allowing either for the generation of elasticity tensors exhibiting some material symmetry properties almost surely (integrating the statistical dependence between the random stiffness components) or for the modeling of random media that requires the mean of a stochastic anisotropy measure to be controlled apart from the level of statistical fluctuations. To this aim, we first introduced a decomposition of the stochastic elasticity tensor on a deterministic tensor basis and considered the probabilistic modeling of the random components, having recourse to the MaxEnt principle. Strategies for random generation and estimation were further reviewed, and the approach was exemplified in the case of a material that was transversely isotropic almost surely. In a second stage, we made use of such derivations to propose a generalized model for random elasticity matrices that took into account, almost separately, constraints on both the level of stochastic anisotropy and the level of statistical fluctuations. An example was finally provided and showed the efficiency of the approach. Copyright © 2011 John Wiley & Sons, Ltd.
When can it be not optimal to adopt a new technology? A viability theory solution to a two‐stage optimal control problem of new technology adoption
Authors: Krawczyk Jacek B. , Serea Oana‐Silvia
Published: December 2011

Politicians often deplore economic agents’ behaviour when they do not accept new technologies. For a new technology to be adopted, the new technology value function needs to dominate the old technology value function. If this is the case, a technology switch will occur. We characterise the value functions, without computing them, using the fact that their hypographs are viability kernels of some auxiliary control problems and study whether the graphs intersect. If they do not, the corresponding value functions do not dominate each other, and the switch cannot occur at a positive time. Using this characterisation, we analyse a technology adoption problem and show how to recognise the models, for which the switch will occur at time zero or never, without solving an optimal control problem. We conclude that the current control regime may not change if the economic agents’ preferences are modelled as an integral of discounted differences between a reward from the flow variable (control) and a penalty from the stock variable (state).Copyright © 2011 John Wiley & Sons, Ltd.
Two‐Grid hp‐Version DGFEMs for Strongly Monotone Second‐Order Quasilinear Elliptic PDEs
Authors: Congreve Scott , Houston Paul , Wihler Thomas P.
Published: December 2011

In this article we develop the a priori error analysis of so‐called two‐grid hp‐version discontinuous Galerkin finite element methods for the numerical approximation of strongly monotone second‐order quasilinear partial differential equations. In this setting, the fully nonlinear problem is first approximated on a coarse finite element space V(𝒯H,P). The resulting ‘coarse’ numerical solution is then exploited to provide the necessary data needed to linearize the underlying discretization on the finer space V(𝒯h,p); thereby, only a linear system of equations is solved on the richer space V(𝒯h,p). Numerical experiments confirming the theoretical results are presented. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Automated parameter selection for rational Arnoldi approximation of Markov functions
Authors: Güttel Stefan , Knizhnerman Leonid
Published: December 2011

Rational Arnoldi is a powerful method for approximating functions of large sparse matrices times a vector. The selection of asymptotically optimal parameters for this method is crucial for its fast convergence. We present a heuristic for the automated pole selection when the function to be approximated is of Markov type, such as the matrix square root. The performance of this approach is demonstrated at several numerical examples. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Advanced finite element formulations for modeling thin piezoelectric structures
Authors: Klinkel Sven , Legner Dieter , Wagner Werner
Published: December 2011

This contribution is concerned with mixed finite element formulations for modeling piezoelectric beam and shell structures. Due to the electromechanical coupling, specific deformation modes are joined with electric field components. In bending dominated problems incompatible approximation functions of these fields cause incorrect results. These effects occur in standard finite element formulations, where interpolation functions of lowest order are used. A mixed variational approach is introduced to overcome these problems. The mixed formulation allows for a consistent approximation of the electromechanical coupled problem. It utilizes six independent fields and could be derived from a Hu‐Washizu variational principle. Displacements, rotations and the electric potential are employed as nodal degrees of freedom. According to the Timoshenko theory (beam) and the Reissner‐Mindlin theory (shell), the formulations account for constant transversal shear strains. To incorporate three dimensional constitutive relations all transversal components of the electric field and the strain field are enriched by mixed finite element interpolations. Thus the complete piezoelectric coupling is appropriately captured. The common assumption of vanishing transversal stress and dielectric displacement components is enforced in an integral sense. Some numerical examples will demonstrate the capability of the presented finite element formulation. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modelling and remodelling of biological tissue in the framework of continuum biomechanics
Authors: Ehlers Wolfgang , Krause Robert , Markert Bernd
Published: December 2011

A biological tissue in general is formed by cells, extracellular matrix (ECM) and fluids. Consequently, its overall material behaviour results from its components and their interaction among each other. Furthermore, in case of living tissues, the material properties do not remain constant but naturally change due to adaptation processes or diseases.

In the context of the Theory of Porous Media (TPM), a continuum‐mechanical model is introduced to describe the complex fluid‐structure interaction in biological tissue on a macroscopic scale. The tissue is treated as an aggregate of two immiscible constituents, where the cells and the ECM are summarised to a solid phase, whereas the fluid phase represents the extracellular and interstitial liquids as well as necrotic debris and cell or matrix precursors in solution.

The growth and remodelling processes are described by a distinct mass exchange between the fluid and solid phase, which also results in a change of the constituent material behaviour. To furthermore guarantee the compliance with the entropy principle, the growth energy is introduced as an additional quantity. It measures the average of chemical energy available for cell metabolism, and thus, controls the growth and remodelling processes.

To set an example, the presented model is applied for the simulation of the early stages of avascular tumour growth in the framework of the finite element method (FEM). (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Comparing exact inversion and singular perturbation approaches for a serial flexible manipulator
Authors: Gorius Thomas , Seifried Robert , Eberhard Peter
Published: December 2011

This paper reviews the singular perturbation control theory in the context of flexible multibody systems. The theory is motivated and explained by a simple, but sufficiently complex model. It is explained that based on the singular perturbed model an end‐effector trajectory tracking can be achieved by an integral manifold controller. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Synthesis of Adequate Mathematical Description of Rolling Mill Motion
Authors: Menshikov Yuri
Published: December 2011

Synthesis of adequate mathematical description of the main mechanical line of rolling mill are considered. The four‐mass model with weightless elastic connections is chosen as mathematical model of dynamic system of the main mechanical line of the rolling mill. The problem was reduced to solution of integral equation of the first kind (to unsteady problem). The methods of obtaining of the steady solutions are suggested. Synthesis of the adequate mathematical descriptions with unitary model of external load are suggested. The metal rolling was executed with using of real experimental measurements as an example. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the description and measurement of the friction coefficient in models of multibody dynamics
Authors: Perzborn Nils , Ostermeyer Georg‐Peter
Published: December 2011

In many fields of the multi‐body world friction must be taken into account. Friction is usually reduced to a ratio: the coefficient of friction. Newer theories also consider dynamic friction laws. For the use in multi‐body models, a stepped complexity and measurably hedged description of the friction is necessary. Therefore, different experimental designs that allow dynamic measurements are discussed. An example friction pair is investigated by measurements and the quality of the results is pointed out. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A consistent direct transcription method for optimal control problems in multibody dynamics
Authors: Siebert Ralf , Betsch Peter
Published: December 2011

The present work deals with optimal control problems governed by differential‐algebraic equations (DAEs). In particular, the control effort, which is necessary for moving a multibody system from one configuration to another, will be minimized. The orientation of the rigid bodies will be described using directors, which facilitates the integration of the equations of motion with an energy‐momentum consistent time‐stepping scheme [1]. This type of structure‐preserving integrators offer outstanding numerical stability and robustness properties in comparison to the often applied generalized coordinates formulation. In the context of optimal control, other kinds of consistent integrators have been applied previously in [2] and [3]. We will test the different formulations with two numerical examples, a 3‐link manipulator and a satellite. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Piezoelectric control of flexible vibrations in rotating beams: An experimental study
Authors: Zehetner Christian , Zenz Georg , Gerstmayr Johannes
Published: December 2011

Active control of flexible vibrations by distributed piezoelectric actuators and sensors plays an increasing role in engineering, especially in light‐weight structures. Exemplarily, in this contribution a rotating beam is studied which can be found in many practical applications, e.g. as robot arms or flexible manipulators in production processes. It has been intensively shown in the literature that it is possible to completely suppress the flexible vibrations by an appropriate distribution of piezoelectric actuation strains. In order to compensate the inertial forces in the considered rotating beam, a complex distribution is obtained, such that a practical realisation would be very extensive. To overcome the problem, a discrete approximation by piezoelectric patches is applied. In order to find an optimal configuration for an experimental setup, and to investigate several control strategies, a numerical simulation model has been implemented based on Bernoulli‐Euler beam theory. The numerical results are verified by an experimental set‐up, in which 48 piezoelectric patches have been attached on a beam with rectangular hollow cross‐section. Each patch can be used either as an actuator or a sensor. Additionally, strain gauges can be used as sensors. For monitoring, acceleration sensors are used. The control system is implemented within a dSpace environment. The results show a significant reduction of the flexible vibrations. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Experimental and numerical investigations on engineering tissue enhanced by mechanical stimulation
Authors: Yi Jeong‐Hun , Stoffel Marcus , Weichert Dieter , Nebelung Sven , Rath Björn
Published: December 2011

The aim of the paper is to investigate the remodeling phenomenon of a cell‐seeded material (collagen type‐I) due to collagen type‐II newly synthesized by the cells. For the experiments, a cell‐seeded condensed collagen gel is mechanically stimulated in a bioreactor for four weeks. The remodeled stiffness of the cell‐seeded gel is measured by a compression test and is explained with an evolution law. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Homogenisation method to capture the non‐linear behaviour of intervertebral discs in multi‐body systems
Authors: Karajan Nils , Ehlers Wolfgang , Röhrle Oliver , Schmitt Syn
Published: December 2011

The present contribution is motivated by the desire to compute physiological loads on the intervertebral discs (IVD) of a human lumbar spine during activities like standing, bending and falling. Following this, a mechanical multi‐body system (MBS) is utilised to capture the overall mechanical behaviour of a human, whereas an inhomogeneous, anisotropic, multi‐phasic finite‐element model (FEM) is applied to resolve the resulting field quantities inside an IVD. In order to couple the FEM of the IVD with the numerically diverse MBS, a homogenisation procedure has to be applied such that field quantities can be converted into discrete quantities. In particular, the MBS captures the mechanical behaviour of an IVD using a bushing element, which provides discrete force‐displacement and moment‐rotation relations.

The goal of this contribution is to present a homogenisation method for the IVD as well as a possibility to include the homogenised results in the MBS without the need for embedded FE computations in the MBS. Instead, certain deformation modes of the IVD are pre‐computed and represented using a non‐linear constitutive equations. This task becomes even more challenging, as the resulting discrete DOF of a motion segment appear in a coupled fashion due to the structure of the IVD, i. e., a rotation in the sagittal plane triggers a resulting moment and a resulting force. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Forward dynamics applied to a three‐dimensional continuum‐mechanical model of the upper limb
Authors: Röhrle Oliver , Ramasamy Ellankavi , Schmitt Syn
Published: December 2011

This paper introduces, for the first time, a methodology to achieve a forward dynamics simulation of the musculoskeletal system using three‐dimensional continuum‐mechanical skeletal muscle models. This is achieved by coupling one‐ and three‐dimensional skeletal muscle models. The feasibility of this methodology is demonstrated through a forward dynamics simulation of the upper limb involving the biceps and triceps muscle. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Active strain and activation models in cardiac electromechanics
Authors: Rossi Simone , Ruiz‐Baier Ricardo , Pavarino Luca F. , Quarteroni Alfio
Published: December 2011

We present a model for mechanical activation of the cardiac tissue depending on the evolution of the transmembrane electrical potential and certain gating/ionic variables that are available in most of electrophysiological descriptions of the cardiac membrane. The basic idea consists in adding to the chosen ionic model one ordinary differential equation for the kinetics of the mechanical activation function. A relevant example illustrates the desired properties of the proposed model, such as delayed muscle contraction and correct magnitude of the muscle fibers' shortening. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the Mathematical Simulation of the Measuring of the Intraocular Pressure by Applanation Method after Refractive Surgery
Authors: Bauer Svetlana , Semenov Boris , Kornikov Vladimir , Kachanov Andrey
Published: December 2011

Applanation tonometry estimates intraocular pressure (IOP) by quantifying the force needed to create a defined amount of deformation of the cornea (Goldmann tonometer) or by estimating the diameter of the circular contact area of the cornea and flat tonometer of defined load (Maklakov tonometer). The first simplest models of the applanation method for measurement of the IOP were based on approach, in which an eyeball is modelled as a thin‐walled spherical liquid‐filled soft shell with corneal biomechanical properties. It was usually supposed that these properties were the same for all patients. In this work numerical simulation have been carried out using finite element code ANSYS. The eye shell is modeled as two joint shells (cornea and sclera) with different mechanical properties. The results are obtained for numerous sets of parameters and were compared to clinical data. For statistics the measurements of IOP for both eyes of 120 patients before and one month after refractive surgery are used. All parameter of refractive surgery (depth, the width and the place of ablation ‐ refractive surgery for myopia or hypermetropia) have effect on IOP reading obtained with both Goldmann and Maklakov tonometry. The results obtained by Goldmann tonometer are significantly more sensitive to all parameters of refractive surgery than those found with Maklakov tonometer with load 10 g. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Influence of micro‐structure on fibre push‐out tests
Authors: Brylka Barthel , Fritzen Felix , Böhlke Thomas , Weidenmann Kay André
Published: December 2011

In fibre reinforced materials, the interface toughness has a significant influence on strength and damage resistance. Especially in discontinuous fibre reinforced composites, where high densities of fibre ends are apparent, it has been shown that additives which improve the interfacial toughness can increase the effective strength of the materials [1]. Due to the fact that interface properties are strongly dependent on the manufacturing process, only experimental techniques providing the possibility to take these influences into account, are promising for an integrated material characterization. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Damage and fracture behavior in dynamic tension tests: Modelling and numerical simulations
Authors: Gerke Steffen , Brünig Michael
Published: December 2011

The continuum damage model is based on a general thermodynamic framework for the modeling of rate and temperature dependent behavior of anisotropically damaged elastic‐plastic materials subjected to fast deformation. The introduction of damaged and fictitious undamaged configurations allows the definition of damage tensors and the corresponding free energy functions lead to material laws affected by damage and temperature. The damage condition and the corresponding damage rule strongly depend on stress triaxiality. Furthermore, the rate and temperature dependence is reflected in a multiplicative decomposition of the plastic hardening and damage softening functions. The macro crack behavior is characterized by a triaxiality dependent fracture criterion.

The continuum damage model is implemented into LS‐DYNA as user defined material model. Corresponding numerical simulations of unnotched and notched tension tests with high strain rates demonstrate the plastic and damage processes during the deformation leading to final fracture numerically predicted by an element erosion technique. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On an Energetic Interpretation of a Phase Field Model for Fracture
Authors: Kuhn Charlotte , Müller Ralf
Published: December 2011

In the pioneering work by Griffith, it is assumed that a crack propagates, if this is energetically favorable. However, this original formulation requires a pre‐existing initial crack. In order to bypass this deficiency of classical Griffith theory, Francfort and Marigo advocate a global variational criterion, where the total energy is minimized with respect to any admissible displacement field and crack set. Bourdin's regularized approximation of this variational formulation makes use of a continuous scalar field to indicate cracks. Based on this regularization a phase field fracture model is formulated. The crack field is assumed to follow a Ginzburg‐Landau type evolution equation, and cracking is addressed as a phase transition problem. The coupled problem of mechanical balance equations and the evolution equation is solved using the finite element method combined with an implicit time integration scheme. The numerical solution naturally yields the crack evolution including crack propagation, kinking, branching and initiation without any additional criteria. In this work we study the driving mechanisms behind the crack evolution in the phase field fracture model and compare to the purely energetic considerations of the underlying variational formulation. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A tensorial framework for strain induced damage of plastically deformed metals
Authors: Zapara Maksim , Tutyshkin Nikolai , H. Müller Wolfgang , Wille Ralf
Published: December 2011

A tensorial framework for strain induced ductile damage of plastically deformed metals is developed in terms of both plastic flow theory and continuum damage mechanics. A symmetric second order damage rate tensor is used in order to study various processes with large finite deformations in combination with damage analysis. The definition of this tensor is physically meaningful since its volumetric and deviatoric parts describe the damage increments caused by an increase in the void volume and by a change in the shape of the void, respectively. Such a view on damage kinetics leads to the introduction of two measures for damage assessment which allow predicting not only a risk of macroscopic failure but also the onset of void coalescence. Material functions appearing in the constitutive equations for damage are determined both by own experiments and by known results from literature. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)