bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic Simulation of a Stationary Proton Exchange Membrane Fuel Cell System
Authors: Min Kyoungdoug , Kang Sanggyu , Mueller Fabian , Auckland John , Brouwer Jacob
Published: September 2009

A dynamic model of a stationary proton exchange membrane (PEM) fuel cell system has been developed in MATLAB-SIMULINK®. The system model accounts for the fuel processing system, PEM stack with coolant, humidifier with anode tail-gas oxidizer, and an enthalpy wheel for cathode air. Four reactors are modeled for the fuel processing system: (1) an autothermal reformation (ATR) reactor, (2) a high temperature shift (HTS) reactor, (3) a low temperature shift (LTS) reactor, and (4) a preferential oxidation reactor. Chemical kinetics for ATR that describe steam reformation of methane and partial oxidation of methane were simultaneously solved to accurately predict the reaction dynamics. The chemical equilibrium of CO with H2O was assumed at HTS and LTS reactor exits to calculate CO conversion corresponding to the temperature of each reactor. A quasi-one-dimensional PEM unit cell was modeled with five control volumes for solving the dynamic species and mass conservation equations and seven control volumes to solve the dynamic energy balance. The quasi-one-dimensional cell model is able to capture the details of membrane electrode assembly behavior, such as water transport, which is critical to accurately determine polarization losses. The dynamic conservation equations, primary heat transfer equations and equations of state are solved in each bulk component, and each component is linked together to represent the complete system. The model predictions well matched the observed experimental dynamic voltage, stack coolant outlet temperature, and catalytic partial oxidation (CPO) temperature responses to perturbations. The dynamic response characteristics of the current system are representative of a typical stationary PEM fuel cell system. The dynamic model is used to develop and test a proportional-integral (PI) fuel flow controller that determines the fuel flow rate to maintain the uniform system efficiency. The dynamic model is shown to be a useful tool for investigating the effects of inlet conditions, load, and fuel flow perturbations and for the development of control strategies for enhancing system performance.
Modeling and Verification of Steady State Operational Changes on the Performance of a Solid Oxide Fuel Cell
Authors: Greene Eric S. , Chiu Wilson K. S. , Burke A. Alan , Medeiros Maria G. , Carreiro Louis G.
Published: September 2009

Solid oxide fuel cells (SOFCs) offer many potential benefits as an energy conversion device. This paper addresses experimental validation of a numerical SOFC model that has been developed. Results are compared at steady state operation for temperatures ranging from 1073 K to 1173 K and for H2 gas concentrations fuel supplies of 10–90% with a balance of N2. The results agree well with a maximum of 13.3% difference seen between the numerical and experimental results, which is within the limit of the experimental uncertainties and the material constants that are measured, with most comparisons well below this level. It is concluded that since the model is very sensitive to material properties and temperature that for the best results they should be as specific as possible to the experiment. These specific properties were demonstrated in this paper and a validation of a full fuel cell model, with a concentration on the anode, was presented.
Simulation of the Postcombustor for the Treatment of Toxic and Flammable Exhaust Gases of a Micro-Solid Oxide Fuel Cell
Authors: Raberger N. B. , Stutz M. J. , Hotz N. , Poulikakos D.
Published: September 2009

This work investigates numerically a catalytic postcombustor for a micro-solid oxide fuel cell (SOFC) system. The postcombustor oxidizes toxic and explosive carbon monoxide (CO) and hydrogen exiting a solid oxide fuel cell to carbon dioxide and water. A single 1 mm diameter monolith reactor channel coated with platinum catalyst is modeled in this work. The inlet stream composition is provided by a semi-analytical 2D model of a detailed SOFC system. The model of the postcombustor includes the 2D axisymmetric Navier–Stokes equations, heat conduction in the channel wall, and a multistep finite-rate mechanism for the surface reactions. It is shown that under the operation conditions considered, the influence of homogeneous (gas phase) reactions can be neglected. The model predicts the expected adiabatic temperatures at the postcombustor outlet correctly and can be used for dimensioning and optimization. Postcombustor performance varies significantly with the choice of the operating parameters of the fuel cell. The most critical molecule at the SOFC outlet is shown to be CO because its depletion is slower than that of H2 for the entire operating range of the SOFC. It can be shown that the postcombustor is able to reduce the level of CO below the toxicity threshold of 25 ppm. Although higher voltages of the fuel cell lead to faster CO conversion in the postcombustor, they also result in a significant increase in wall temperature of the catalyst device. Furthermore, the percentage of SOFC power output used for pump work is lowest for the voltage where the maximum power is reached. For postcombustion the optimal operation point of the SOFC is at the voltage for maximum power of the SOFC system.
Multiphysics Modeling of Assembly Pressure Effects on Proton Exchange Membrane Fuel Cell Performance
Authors: Zhou Y. , Lin G. , Shih A. J. , Hu S. J.
Published: September 2009

The clamping pressure used in assembling a proton exchange membrane (PEM) fuel cell stack can have significant effects on the overall cell performance. The pressure causes stack deformation, particularly in the gas diffusion layer (GDL), and impacts gas mass transfer and electrical contact resistance. Existing research for analyzing the assembly pressure effects is mostly experimental. This paper develops a sequential approach to study the pressure effects by combining the mechanical and electrochemical phenomena in fuel cells. The model integrates gas mass transfer analysis based on the deformed GDL geometry and modified parameters with the microscale electrical contact resistance analysis. The modeling results reveal that higher assembly pressure increases cell resistance to gas mass transfer, causes an uneven current density distribution, and reduces electrical contact resistance. These combined effects show that as the assembly pressure increases, the PEM fuel cell power output increases first to a maximum and then decreases over a wide range of pressures. An optimum assembly pressure is observed. The model is validated against published experimental data with good agreements. This study provides a basis for determining the assembly pressure required for optimizing PEM fuel cell performance.
Control Oriented Analysis of a Hybrid Solid Oxide Fuel Cell and Gas Turbine System
Authors: Tsourapas Vasilis , Sun Jing , Stefanopoulou Anna
Published: September 2009

The goal of this work is to investigate the feasibility of a hybrid solid oxide fuel cell (SOFC) and gas turbine (GT) system for mobile power production. A system consisting of a gas turbine, a burner, and an SOFC is examined to gain fundamental understanding of the system dynamics. A control oriented dynamic model is developed to provide the critically needed tool for system feasibility analysis and control strategy design. System optimization and transient analysis are performed based on the system model to determine the desired operating conditions and load following limitations. It is shown that the open loop system will shut down in the case of a large load step. Based on the insights learned from the open loop analysis, a feedback control scheme is proposed. The feedback scheme is based on a reference governor, which modifies the load applied to the generator to guarantee stability and fast tracking during transients.
Control Oriented Modeling of Solid Oxide Fuel Cell Auxiliary Power Unit for Transportation Applications
Authors: Sorrentino Marco , Pianese Cesare
Published: September 2009

This paper reports on the development of a control-oriented model for simulating a hybrid auxiliary power unit (APU) equipped with a solid oxide fuel cell (SOFC) stack. Such a work is motivated by the strong interest devoted to SOFC technology due to its highly appealing potentialities in terms of fuel savings, fuel flexibility, cogeneration, low-pollution and low-noise operation. In this context, the availability of a model with acceptable computational burden and satisfactory accuracy can significantly enhance both system and control strategy design phases for APUs destined to a wide application area (e.g., mild-hybrid cars, trains, ships, and airplanes). The core part of the model is the SOFC stack, surrounded by a number of ancillary devices: air compressor/blower, regulating pressure valves, heat exchangers, prereformer, and postburner. Since the thermal dynamics is clearly the slowest one, a lumped-capacity model is proposed to describe the response of SOFC and heat exchangers to load (i.e., operating current) variation. The stack model takes into account the dependence of stack voltage on operating temperature, thus adequately describing the typical voltage undershoot following a decrease in load demand. On the other hand, due to their faster dynamics the mass transfer and electrochemistry processes are assumed instantaneous. The hybridizing device, whose main purpose is to assist the SOFC system (i.e., stack and ancillaries) during transient conditions, consists of a lead-acid battery pack. Battery power dependence on current is modeled, taking into account the influence of actual state of charge on open circuit voltage and internal resistance. The developed APU model was tested by simulating typical auxiliary power demand profiles for a heavy-duty truck in parked-idling phases. Suited control strategies also were developed to avoid operating the SOFC stack under severe thermal transients and, at the same time, to guarantee a charge sustaining operation of the battery pack. In order to assess the benefits achievable by introducing the SOFC-APU on board of a commercial truck, the simulated fuel consumption was compared with the fuel consumed by idling the thermal engine. From the simulation carried out, it emerges how the SOFC-APU allows achieving a potential reduction in fuel consumption of up to 70%.
Product Development Task Planning Using Worth Flow Analysis
Authors: Miwa Toshiharu , Ishii Kosuke
Published: September 2009

The acceleration of the product development cycle continues to be a significant challenge for manufacturing enterprises around the world. This paper describes a task planning method that minimizes the number of trial and error to reduce the development time for large-scale and complex products at the early stage of product development. The proposed method matches groups of product components and determines the development sequence for each component to minimize the amount of feedback information required across task groups. The method provides, as evaluation indices for task prioritization, the product-sum of engineering interaction among components and worth of each component, which the authors define as the “worth flow.” A generic hair dryer with simple mechanical structure serves as an example, illustrating that the proposed method contributes to the reduction in the amount of information required for setting the interface links by 65% compared with the conventional planning methods.
Algebraic Dimensional Reduction for Microfluidic Simulation
Authors: Danczyk Josh , Suresh Krishnan
Published: September 2009

Microfluidic devices exhibit a high-aspect ratio in that their channel-widths are much smaller than their overall lengths. High-aspect geometry leads to an unduly large finite element mesh, making the (otherwise popular) finite element method (FEM) a poor choice for modeling microfluidic devices. An alternate computational strategy is to exploit well-known analytical solutions for fluid flow over the narrow channels of a device, and then either (a) assume the same analytical solutions for the cross-flow regions, or (b) exploit these solutions to set-up artificial boundary conditions over the cross-flow regions. Such simplified models are computationally far superior to brute-force FEM, but do not support the generality or flexibility of FEM. In this paper, we propose a third strategy for exploiting the analytical solutions: (c) directly incorporate them into standard FE-based analysis via algebraic reduction techniques. The advantages of the proposed strategy are (1) designers can use standard computer-aided design/computer-aided engineering (CAD/CAE) environments to model, analyze, and postprocess microfluidic simulation; (2) well-established dual-weighted residuals can be used to estimate modeling errors; and (3), if desired, one can eliminate the dependency on analytical solutions over selected regions, and instead revert to brute-force FEM. The simplicity and generality of the proposed method is inherited from the model reduction process, so are its theoretical properties, while simultaneously its computational efficiency is inherited from the use of analytical solutions.
A Simplified Systematic Method of Acquiring Design Specifications From Customer Requirements
Authors: Sun Nuogang , Mei Xuesong , Zhang Youyun
Published: September 2009

Faithfully obtaining design specifications from customer requirements is essential for successful designs. The natural lingual, inexact, incomplete, and vague attributes of customer requirements make it very difficult to map customer requirements to design specifications. In general design process, the design specifications are determined by designers based on their experience and intuition, and often a certain target value is set for a specification. However, it is on one hand very difficult, on the other hand unreasonable, so a suitable limit range rather than a certain value is preferred at the beginning of design, especially at the concept design process. In this paper, a simplified systematic approach of transforming customer requirements to design specifications is proposed. First, a two-stepped clustering approach for grouping customer requirements and design specifications based on the house of quality matrix is presented, by which the mapping is limited to within each group. To further simplify the inference mapping rules of customer requirements and design specifications, the minimal condition inference mapping rules for each design specification are extracted based on rough set theory. In the end, a suitable value range is determined for a specification by applying the fuzzy rule matrix.
Interoperability Between PLM and RoHS Compliance Management Based on XML and Smart Client
Authors: Zhou Chuan Hong , Eynard Benoît , Roucoules Lionel
Published: September 2009

Nowadays, more and more companies are going to adapt themselves to restriction on the use of certain hazardous substances (RoHS) compliance and enforcement by leveraging product lifecycle management (PLM) technology. The paper presents a general framework that integrates the bill of material extensible markup language (XML) files from heterogeneous PLM systems to RoHS compliance management system. This framework is based on the use of Smart Client technology to obtain flexibility required for complex and distributed environments. It also makes use of XML technology and data structure to obtain reuse and robustness.
Constraints Based Nonrigid Registration for 2D Blade Profile Reconstruction in Reverse Engineering
Authors: Li Yongqing , Ni Jun
Published: September 2009

In reverse engineering, 2D profile curve reconstruction based on cross-sectional points is a very crucial step for surface reconstruction such as lofting surface, swept surface, translational surface, and rotational surface. Unlike the traditional constrained fitting method that assumes that the cross-sectional points have been segmented in advance, and that the initial fitted curves are very close to the points, we propose a nonrigid registration method, through which a template curve can be automatically transformed and deformed to best fit the cross-sectional points. Compared with constrained fitting, nonrigid registration does not need any data preprocessing such as sorting, segmentation, and parametrization. The simulated and real examples have demonstrated the effectiveness and superiority of nonrigid registration for 2D blade profile curve reconstruction.
Exergetic Performance Analysis of a Gas Turbine Cycle Integrated With Solid Oxide Fuel Cells
Authors: Dincer Ibrahim , Rosen Marc A. , Zamfirescu Calin
Published: September 2009

Energy and exergy assessments are reported of integrated power generation using solid oxide fuel cells (SOFCs) with internal reforming and a gas turbine cycle. The gas turbine inlet temperature is fixed at 1573 K and the high-temperature turbine exhaust heats the natural gas and air inputs, and generates pressurized steam. The steam mixes at the SOFC stack inlet with natural gas to facilitate the reformation process. The integration of solid oxide fuel cells with gas turbines increases significantly the power generation efficiency relative to separate processes and reduces greatly the exergy loss due to combustion, which is the most irreversible process in the system. The other main exergy destruction is attributable to electrochemical fuel oxidation in the SOFC. The energy and exergy efficiencies of the integrated system reach 70–80%, which compares well to the efficiencies of approximately 55% typical of conventional combined-cycle power generation systems. Variations in the energy and exergy efficiencies of the integrated system with operating conditions are provided, showing, for example, that SOFC efficiency is enhanced if the fuel cell active area is augmented. The SOFC stack efficiency can be maximized by reducing the steam generation while increasing the stack size, although such measures imply a significant and nonproportional cost rise. Such measures must be implemented cautiously, as a reduction in steam generation decreases the steam/methane ratio at the anode inlet, which may increase the risk of catalyst coking. A detailed assessment of an illustrative example highlights the main results.
Frequency-Domain Identification of Linear Time-Periodic Systems Using LTI Techniques
Authors: Allen Matthew S.
Published: September 2009

A variety of systems can be faithfully modeled as linear with coefficients that vary periodically with time or linear time-periodic (LTP). Examples include anisotropic rotor-bearing systems, wind turbines, and nonlinear systems linearized about a periodic trajectory. Many of these have been treated analytically in the literature, yet few methods exist for experimentally characterizing LTP systems. This paper presents a set of tools that can be used to identify a parametric model of a LTP system, using a frequency-domain approach and employing existing algorithms to perform parameter identification. One of the approaches is based on lifting the response to obtain an equivalent linear time-invariant (LTI) form and the other based is on Fourier series expansion. The development focuses on the preprocessing steps needed to apply LTI identification to the measurements, the postprocessing needed to reconstruct the LTP model from the identification results, and the interpretation of the measurements. This elucidates the similarities between LTP and LTI identification, allowing the experimentalist to transfer insight between the two. The approach determines the model order of the system and the postprocessing reveals the shapes of the time-periodic functions comprising the LTP model. Further postprocessing is also presented, which allows one to generate the state transition and time-varying state matrices of the system from the output of the LTI identification routine, so long as the measurement set is adequate. The experimental techniques are demonstrated on simulated measurements from a Jeffcott rotor mounted on an anisotropic flexible shaft supported by anisotropic bearings.
A High Precision Direct Integration Scheme for Nonlinear Dynamic Systems
Authors: Li Kuinian , Darby Antony P.
Published: September 2009

Based on the high precision direct (HPD) integration scheme for linear systems, a high precision direct integration scheme for nonlinear (HPD-NL) dynamic systems is developed. The method retains all the advantages of the standard HPD scheme (high precision with large time-steps and computational efficiency) while allowing nonlinearities to be introduced with little additional computational effort. In addition, limitations on minimum time step resulting from the approximation that load varies linearly between time-steps are reduced by introducing a polynomial approximation of the load. This means that, in situations where a rapidly varying or transient dynamic load occurs, a larger time-step can still be used while maintaining a good approximation of the forcing function and, hence, the accuracy of the solution. Numerical examples of the HPD-NL scheme compared with Newmark’s method and the fourth-order Runge–Kutta (Kutta 4) method are presented. The examples demonstrate the high accuracy and numerical efficiency of the proposed method.
A Recursive Hybrid Time-Stepping Scheme for Intermittent Contact in Multi-Rigid-Body Dynamics
Authors: Bhalerao Kishor D. , Anderson Kurt S. , Trinkle Jeffrey C.
Published: September 2009

This paper describes a novel method for the modeling of intermittent contact in multi-rigid-body problems. We use a complementarity based time-stepping scheme in Featherstone’s divide and conquer framework to efficiently model the unilateral and bilateral constraints in the system. The time-stepping scheme relies on impulse-based equations and does not require explicit collision detection. A set of complementarity conditions is used to model the interpenetration constraint and a linearized friction cone is used to yield a linear complementarity problem. The divide and conquer framework ensures that the size of the resulting mixed linear complementarity problem is independent of the number of bilateral constraints in the system. This makes the proposed method especially efficient for systems where the number of bilateral constraints is much greater than the number of unilateral constraints. The method is demonstrated by applying it to a falling 3D double pendulum.
A Methodology to Analyze Changes in Lipid Core and Calcification Onto Fibrous Cap Vulnerability: The Human Atherosclerotic Carotid Bifurcation as an Illustratory Example
Authors: Kiousis Dimitrios E. , Rubinigg Stephan F. , Auer Martin , Holzapfel Gerhard A.
Published: September 2009

A lipid core that occupies a high proportion of the plaque volume in addition to a thin fibrous cap is a predominant indicator of plaque vulnerability. Nowadays, noninvasive imaging modalities can identify such structural components, however, morphological criteria alone cannot reliably identify high-risk plaques. Information, such as stresses in the lesion’s components, seems to be essential. This work presents a methodology able to analyze the effect of changes in the lipid core and calcification on the wall stresses, in particular, on the fibrous cap vulnerability. Using high-resolution magnetic resonance imaging and histology of an ex vivo human atherosclerotic carotid bifurcation, a patient-specific three-dimensional geometric model, consisting of four tissue components, is generated. The adopted constitutive model accounts for the nonlinear and anisotropic tissue behavior incorporating the collagen fiber orientation by means of a novel and robust algorithm. The material parameters are identified from experimental data. A novel stress-based computational cap vulnerability index is proposed to assess quantitatively the rupture-risk of fibrous caps. Nonlinear finite element analyses identify that the highest stress regions are located at the vicinity of the shoulders of the fibrous cap and in the stiff calcified tissue. A parametric analysis reveals a positive correlation between the increase in lipid core portion and the mechanical stress in the fibrous cap and, hence, the risk for cap rupture. The highest values of the vulnerability index, which correlate to more vulnerable caps, are obtained for morphologies for which the lipid cores were severe; heavily loaded fibrous caps were thus detected. The proposed multidisciplinary methodology is able to investigate quantitatively the mechanical behavior of atherosclerotic plaques in patient-specific stenoses. The introduced vulnerability index may serve as a more quantitative tool for diagnosis, treatment and prevention.
High Strain Rate Testing of Bovine Trabecular Bone
Authors: Pilcher A. , Wang X. , Kaltz Z. , Garrison J. G. , Niebur G. L. , Mason J. , Song B. , Cheng M. , Chen W.
Published: September 2009

In spinal vertebral burst fractures, the dynamic properties of the trabecular centrum, which is the central region of porous bone inside the vertebra, can play an important role in determining the failure mode. If the failure occurs in the posterior portion of the vertebral body, spinal canal occlusion can occur and ejected trabecular bone can impact the spinal cord resulting in serious injury. About 15% of all spinal cord injuries are caused by such burst fractures. Unfortunately, due to the uniqueness of burst fracture injuries, postinjury investigation cannot always accurately assess the degree of damage caused by these fractures. This research makes an effort to begin understanding the governing effects in this important bone fracture event. Measurements of the dynamic deformation response of bovine trabecular bone with the marrow intact and marrow removed using a modified split-Hopkinson pressure bar apparatus are reported and compared with quasistatic deformation response results. Because trabecular bone is more compliant and lower in strength than cortical bone, typical Hopkinson pressure bar experimental techniques used for high strain rate testing of harder materials cannot be applied. Instead, a quartz-crystal-embedded, split-Hopkinson pressure bar developed for testing compliant, low strength materials is used. Care is taken into account for the orthotropic properties in the bone by testing only along the principle material axes, determined through microcomputed tomography. In addition, shaping of the stress wave pulse is used to ensure a constant strain rate and homogeneous specimen deformation. Results indicate that the strength of trabecular bone increases by a factor of approximately 2–3 when the strain rate increases from 10−3 s−1 to 500 s−1 and that the bone fractures beyond a critical strain.
A New Observation on the Stress Distribution in the Coronary Artery Wall
Authors: Wang Chong , Guo Xiaomei , Kassab Ghassan S.
Published: September 2009

The stress distribution in the vessel wall has an important bearing on vascular function in health and disease. We studied the relationship between the transmural stress distribution and the opening angle (OA) to determine the stress gradient. The simulation of wall stress was based on transmural measurements of strain and material properties of coronary arteries in reference to the zero-stress state. A one-layer model with material constants of the intact vessel was used to calculate the circumferential stress distribution. A sensitivity analysis using both one- and two-layer models (intima-media and adventitia layers) was carried out to study the effect of the OA on the circumferential stress distribution and average circumferential stress. A larger OA always shifts the circumferential stress from the intima-media to the adventitia layer. We report a new observation that the circumferential stress at the adventitia may exceed that at the intima at physiological loading due to the larger OA in the porcine coronary artery. This has important implications for growth and remodeling, where an increase in opening angle may shift excessive stress from the inner layer to the outer layer.
Mitral Valve Finite Element Modeling: Implications of Tissues’ Nonlinear Response and Annular Motion
Authors: Stevanella Marco , Votta Emiliano , Redaelli Alberto
Published: September 2009

Finite element modeling represents an established method for the comprehension of the mitral function and for the simulation of interesting clinical scenarios. However, current models still do not include all the key aspects of the real system. We implemented a new structural finite element model that considers (i) an accurate morphological description of the valve, (ii) a description of the tissues’ mechanical properties that accounts for anisotropy and nonlinearity, and (iii) dynamic boundary conditions that mimic annulus and papillary muscles’ contraction. The influence of such contraction on valve biomechanics was assessed by comparing the computed results with the ones obtained through an auxiliary model with fixed annulus and papillary muscles. At the systolic peak, the leaflets’ maximum principal stress contour showed peak values in the anterior leaflet at the strut chordae insertion zone (300 kPa) and near the annulus (200–250 kPa), while much lower values were detected in the posterior leaflet. Both leaflets underwent larger tensile strains in the longitudinal direction, while in the circumferential one the anterior leaflet experienced nominal tensile strains up to 18% and the posterior one experienced compressive strains up to 23% associated with the folding of commissures and paracommissures, consistently with tissue redundancy. The force exerted by papillary muscles at the systolic peak was equal to 4.11 N, mainly borne by marginal chordae (76% of the force). Local reaction forces up to 45 mN were calculated on the annulus, leading to tensions of 89 N/m and 54 N/m for its anterior and posterior tracts, respectively. The comparison with the results of the auxiliary model showed that annular contraction mainly affects the leaflets’ circumferential strains. When it was suppressed, no more compressive strains could be observed and peak strain values were located in the belly of the anterior leaflet. Computational results agree to a great extent with experimental data from literature. They provided insight into some of the features characterizing normal mitral function, such as annular contraction and leaflets’ tissue anisotropy and nonlinearity. Some of the computed results may be useful in the design of surgical devices and techniques. In particular, forces applied on the annulus by the surrounding tissues could be considered as an indication for annular prostheses design.
Characterization of the Fatigue Behavior of the Medial Collateral Ligament Utilizing Traditional and Novel Mechanical Variables for the Assessment of Damage Accumulation
Authors: Zec Michelle L. , Thistlethwaite Paul , Frank Cyril B. , Shrive Nigel G.
Published: September 2009

Ligaments are regularly subjected to repetitive loading in vivo. Typically, mechanical studies focus on repetitive loading protocols of short duration, while those characterizing damage accumulation over a longer duration (i.e., fatigue studies) are lacking. The aims of this study were as follows: (a) to demonstrate that damage does accumulate in ligament tissue subjected to repetitive loading and (b) to evaluate existing and new methods for characterizing fatigue damage accumulation. It was hypothesized that ligaments would accumulate damage with repetitive loading as evidenced by failure at stresses well below ultimate tensile strength, creep curve discontinuities, and by reductions in stiffness during loading. Eight normal medial collateral ligaments from female New Zealand white rabbits were cycled in tension, between 0 MPa and 28 MPa, to failure or until 259,200 cycles, whichever came first. Medial collateral ligaments that did not fail were subsequently loaded to failure. Displacement rates (dlmax/dt) as well as primary, secondary, and tertiary creeps were monitored as indices of damage accumulation and impending mechanical failure. Additionally, the relative utilities of tangent, secant, and chord stiffness parameters were critically evaluated. Finally, new uses for the second derivative of force-displacement data were explored. Three out of eight ligaments failed during testing, demonstrating that ligaments can fail in fatigue under moderate tensile stress in vitro. The evaluation of displacement rates (dlmax/dt), as well as primary through tertiary creep patterns, were not well suited to predicting failure in normal ligaments until rupture was all but imminent. Tangent stiffness, which was calculated from a mathematically defined start of the “linear region,” was surprisingly constant throughout testing. Secant stiffness dropped in a predictable fashion, providing a global indicator of tissue stiffness, but did not provide any insight into fiber mechanics. Chord stiffness, on the other hand, appeared to be sensitive to fiber recruitment patterns. The second derivative of force-displacement data proved to be a useful means of (a) objectively defining the start of the linear region and (b) inferring changes in fiber recruitment patterns within ligament tissue. Tangent, secant, and chord stiffnesses highlight different attributes of ligament responses to loading; hence these parameters cannot be used interchangeably. Additionally, the second derivative of the force-displacement curve was introduced as a useful descriptive and analytical tool.
A Phenomenological Model for Mechanically Mediated Growth, Remodeling, Damage, and Plasticity of Gel-Derived Tissue Engineered Blood Vessels
Authors: Raykin Julia , Rachev Alexander I. , Gleason Rudolph L.
Published: September 2009

Mechanical stimulation has been shown to dramatically improve mechanical and functional properties of gel-derived tissue engineered blood vessels (TEBVs). Adjusting factors such as cell source, type of extracellular matrix, cross-linking, magnitude, frequency, and time course of mechanical stimuli (among many other factors) make interpretation of experimental results challenging. Interpretation of data from such multifactor experiments requires modeling. We present a modeling framework and simulations for mechanically mediated growth, remodeling, plasticity, and damage of gel-derived TEBVs that merge ideas from classical plasticity, volumetric growth, and continuum damage mechanics. Our results are compared with published data and suggest that this model framework can predict the evolution of geometry and material behavior under common experimental loading scenarios.
Highly conformal film growth by chemical vapor deposition. II. Conformality enhancement through growth inhibition
Authors: Yanguas-Gil A. , Kumar N. , Yang Y. , Abelson J. R.
Published: September 2009

The authors present a novel strategy for enhancing conformality in chemical vapor deposition (CVD) based on the concept of growth inhibition. In Part I, they showed how surface site blocking was responsible for the increase in conformality observed at higher pressures in high vapor pressure precursors. In this work, they apply this concept to enhance conformality by considering a secondary species that acts as an inhibitor of the precursor. The experimental results obtained for the growth of TiB2 from Ti(BH4)3dme (dme=dimethoxyethane), with dme acting as the growth inhibitor, agree well with the models, based on a first order adsorption/desorption kinetics, and show how this strategy greatly enhances the conformality of nonconformal precursors. Finally, they show how multiple surface mechanisms including surface site blocking, associative desorption, long surface residence time of by-products, or coadsorption of two reactants can induce growth inhibition. Consequently, this strategy can be potentially applied to many existing CVD systems, provided suitable molecular inhibitors are identified.
Lack of Effect of Efavirenz on the Pharmacokinetics of Tipranavir-Ritonavir in Healthy Volunteers
Authors: la Porte C. J. L. , Sabo J. P. , Béïque L. , Cameron D. W.
Published: September 2009

Previously it has been shown that tipranavir-ritonavir (TPV/r) does not affect efavirenz (EFV) plasma concentrations. This study investigates the effect of steady-state EFV on steady-state TPV/r pharmacokinetics. This was a single-center, open-label, multiple-dose study of healthy adult female and male volunteers. TPV/r 500/200 mg twice a day (BID) was given with food for 24 days. After dosing with TPV/r for 10 days, EFV 600 mg once a day was added to the regimen. Intensive pharmacokinetic (PK) sampling was done on days 10 and 24. Validated bioanalytical high-pressure liquid chromatography-tandem mass spectrometry methods were used to determine plasma tipranavir (TPV), ritonavir (RTV), and EFV concentrations. Thirty-four subjects were entered into the study, and 16 subjects completed it. The geometric mean ratios (90% confidence intervals) for TPV and RTV area under the curves,Cmaxs, andCmins comparing TPV/r alone and in combination with EFV were 0.97 (0.87 to 1.09), 0.92 (0.81 to 1.03), and 1.19 (0.93 to 1.54) for TPV and 1.03 (0.78 to 1.38), 0.92 (0.65 to 1.30), and 1.04 (0.72 to 1.48) for RTV. Frequently observed adverse events were diarrhea, headache, dizziness, abnormal dreams, and rash. EFV had no effect on the steady-state PK of TPV or RTV, with the exception of a 19% increase in the TPVCmin, which is not clinically relevant. TPV/r can be safely coadministered with EFV and without the need for a dose adjustment.
Rapid Detection of a Point Mutation in the
parC
Gene Associated with Decreased Susceptibility to Fluoroquinolones in
Mycoplasma bovis
Authors: Lysnyansky I. , Mikula I. , Gerchman I. , Levisohn S.
Published: September 2009

Comparison of the quinolone resistance-determining regions (QRDRs) in 42
Mycoplasma bovis
clinical isolates revealed amino acid substitutions at both GyrA (position 83) and ParC (position 84) in 10/11 enrofloxacin-resistant strains. The mutation present in the
parC
QRDR was discriminative for enrofloxacin resistance by
parC
PCR-restriction fragment length polymorphism. Comparison of molecular profiles by insertion sequence typing suggests that the currently prevalent enrofloxacin-resistant
M. bovis
strain evolved by selection under field conditions from one of the susceptible strains.
Characterization of Two Seryl-tRNA Synthetases in Albomycin-Producing
Streptomyces
sp. Strain ATCC 700974
Authors: Zeng Yu , Roy Hervé , Patil Preeti B. , Ibba Michael , Chen Shawn
Published: September 2009

The Trojan horse antibiotic albomycin, produced by
Streptomyces
sp. strain ATCC 700974, contains a thioribosyl nucleoside moiety linked to a hydroxamate siderophore through a serine residue. The seryl nucleoside structure (SB-217452) is a potent inhibitor of seryl-tRNA synthetase (SerRS) in the pathogenic bacterium
Staphylococcus aureus
, with a 50% inhibitory concentration (IC
50
) of ∼8 nM. In the albomycin-producing
Streptomyces
sp., a bacterial SerRS homolog (Alb10) was found to be encoded in a biosynthetic gene cluster in addition to another
s
erRS
gene (
serS1
) at a different genetic locus. Alb10, named SerRS2 herein, is significantly divergent from SerRS1, which shows high homology to the housekeeping SerRS found in other
Streptomyces
species. We genetically and biochemically characterized the two genes and the proteins encoded. Both genes were able to complement a temperature-sensitive
serS
mutant of
Escherichia coli
and allowed growth at a nonpermissive temperature.
serS2
was shown to confer albomycin resistance, with specific amino acid residues in the motif 2 signature sequences of SerRS2 playing key roles. SerRS1 and SerRS2 are comparably efficient in vitro, but the

K
m

of serine for SerRS2 measured during tRNA aminoacylation is more than 20-fold higher than that for SerRS1. SB-217452 was also enzymatically generated and purified by two-step chromatography. Its IC
50
against SerRS1 was estimated to be 10-fold lower than that against SerRS2. In contrast, both SerRSs displayed comparable inhibition kinetics for serine hydroxamate, indicating that SerRS2 was specifically resistant to SB-217452. These data suggest that mining
Streptomyces
genomes for duplicated aminoacyl-tRNA synthetase genes could provide a novel approach for the identification of natural products targeting aminoacyl-tRNA synthetases.
Patients with Multidrug-Resistant Tuberculosis Display Impaired Th1 Responses and Enhanced Regulatory T-Cell Levels in Response to an Outbreak of Multidrug-ResistantMycobacterium tuberculosisM and Ra Strains
Authors: Geffner Laura , Yokobori Noemí , Basile Juan , Schierloh Pablo , Balboa Luciana , Romero María Mercedes , Ritacco Viviana , Vescovo Marisa , González Montaner Pablo , Lopez Beatriz , Barrera Lucía , Alemán Mercedes , Abatte Eduardo , Sasiain María C. , de la Barrera Silvia
Published: September 2009

In Argentina, multidrug-resistant tuberculosis (MDR-TB) outbreaks emerged among hospitalized patients with AIDS in the early 1990s and thereafter disseminated to the immunocompetent community. Epidemiological, bacteriological, and genotyping data allowed the identification of certain MDRMycobacterium tuberculosisoutbreak strains, such as the so-called strain M of the Haarlem lineage and strain Ra of the Latin America and Mediterranean lineage. In the current study, we evaluated the immune responses induced by strains M and Ra in peripheral blood mononuclear cells from patients with active MDR-TB or fully drug-susceptible tuberculosis (S-TB) and in purified protein derivative-positive healthy controls (group N). Our results demonstrated that strain M was a weaker gamma interferon (IFN-γ) inducer than H37Rv for group N. Strain M induced the highest interleukin-4 expression in CD4+and CD8+T cells from MDR- and S-TB patients, along with the lowest cytotoxic T-lymphocyte (CTL) activity in patients and controls. Hence, impairment of CTL activity is a hallmark of strain M and could be an evasion mechanism employed by this strain to avoid the killing of macrophages by M-specific CTL effectors. In addition, MDR-TB patients had an increased proportion of circulating regulatory T cells (Treg cells), and these cells were further expanded upon in vitroM. tuberculosisstimulation. Experimental Treg cell depletion increased IFN-γ expression and CTL activity in TB patients, with M- and Ra-induced CTL responses remaining low in MDR-TB patients. Altogether, these results suggest that immunity to MDR strains might depend upon a balance between the individual host response and the ability of differentM. tuberculosisgenotypes to drive Th1 or Th2 profiles.
TREATING EPILEPSY VIA ADAPTIVE NEUROSTIMULATION: A REINFORCEMENT LEARNING APPROACH
Authors: PINEAU JOELLE , GUEZ ARTHUR , VINCENT ROBERT , PANUCCIO GABRIELLA , AVOLI MASSIMO
Published: September 2009

This paper presents a new methodology for automatically learning an optimal neurostimulation strategy for the treatment of epilepsy. The technical challenge is to automatically modulate neurostimulation parameters, as a function of the observed EEG signal, so as to minimize the frequency and duration of seizures. The methodology leverages recent techniques from the machine learning literature, in particular the reinforcement learning paradigm, to formalize this optimization problem. We present an algorithm which is able to automatically learn an adaptive neurostimulation strategy directly from labeled training data acquired from animal brain tissues. Our results suggest that this methodology can be used to automatically find a stimulation strategy which effectively reduces the incidence of seizures, while also minimizing the amount of stimulation applied. This work highlights the crucial role that modern machine learning techniques can play in the optimization of treatment strategies for patients with chronic disorders such as epilepsy.
SPIKING NEURAL NETWORKS
Authors: GHOSH-DASTIDAR SAMANWOY , ADELI HOJJAT
Published: September 2009

Most current Artificial Neural Network (ANN) models are based on highly simplified brain dynamics. They have been used as powerful computational tools to solve complex pattern recognition, function estimation, and classification problems. ANNs have been evolving towards more powerful and more biologically realistic models. In the past decade, Spiking Neural Networks (SNNs) have been developed which comprise of spiking neurons. Information transfer in these neurons mimics the information transfer in biological neurons, i.e., via the precise timing of spikes or a sequence of spikes. To facilitate learning in such networks, new learning algorithms based on varying degrees of biological plausibility have also been developed recently. Addition of the temporal dimension for information encoding in SNNs yields new insight into the dynamics of the human brain and could result in compact representations of large neural networks. As such, SNNs have great potential for solving complicated time-dependent pattern recognition problems because of their inherent dynamic representation. This article presents a state-of-the-art review of the development of spiking neurons and SNNs, and provides insight into their evolution as the third generation neural networks.
JEANS INSTABILITY IN THE PRESENCE OF VISCOUS EFFECTS
Authors: CARLEVARO NAKIA , MONTANI GIOVANNI
Published: September 2009

An analysis of gravitational instability in the presence of dissipative effects made. In particular, the standard Jeans mechanism and the generalization in treating the universe expansion are both analyzed when bulk viscosity affects the first-order Newtonian dynamics. As results, the perturbation evolution is found to be damped by dissipative processes and the top-down mechanism of structure fragmentation is suppressed. In such a scheme, the value of the Jeans mass remains unchanged also in the presence of viscosity.
TOWARDS IMMUNIZING MANET'S SOURCE ROUTING PROTOCOLS AGAINST PACKET DROPPERS
Authors: DJENOURI DJAMEL , MAHMOUDI OTHMANE , BOUAMAMA MOHAMED
Published: September 2009

This paper deals with security of routing protocols of Mobile Ad hoc Networks (MANETs), and proposes a solution to immunize such protocols against packet dropping misbehavior. Most of the current secure protocols are vulnerable to packet dropping misbehavior, which can be exploited by selfish nodes and malicious ones as well. For example, simply by dropping RREQ (Route Request) packets a selfish node can exclude itself from routes and thereby avoid receiving data packets to forward. On the other hand, a malicious node can drop RERR (Route Error) packets to keep the use of failed routes, possibly resulting in a denial of service. To mitigate this vulnerability we propose a hybrid solution that secures routing protocols against the dropping of both directed and broadcast control packets, in which a different approach is adapted for each kind of packets. Dealing with control packets represents the main contribution in this manuscript, as all the current proposals in the context of selfish nodes only consider data packets. Our solution can be integrated with any source routing protocol. In this work it was implemented with one of the most secure protocols, namely ENDAIR. The resulting new extended secure protocol was assessed and analyzed through an extensive simulation study.
ECONOMIC AND SOCIAL CHARACTERISTICS OF ALBANIAN IMMIGRANT ENTREPRENEURS IN GREECE
Authors: HALKIAS DAPHNE , HARKIOLAKIS NICHOLAS , THURMAN PAUL , RISHI MEENAKSHI , EKONOMOU LAMBROS , CARACATSANIS SYLVA M. , AKRIVOS PATRICK DIMITRIS
Published: September 2009

Greece has experienced rapid growth in immigrant and refugee populations since 1990. Although most are immigrants from Albania and throughout the Balkan region, some immigrant and refugee groups arriving in Greece also come from the former Soviet Union, Southeast Asia and Africa. Some of these newcomers have started small businesses in their quest to become economically self-sufficient, serve the consumer needs of fellow newcomers, and integrate into community life. The purpose of this research is two-fold: to review the extant literature on social and economic factors influencing immigrant entrepreneurship in Greece, and to determine characteristics and business profiles of Albanian immigrant-owned small businesses within the municipality of Attiki — the location of Athens, Greece's capital city and largest urban center.