关键词： Mechanical heart valve   Valve closing dynamics   Fluid-structure interaction  

摘要： The closing velocity of the leaflets of mechanical heart valves is excessively rapid and can cause the cavitation phenomenon. Cavitation bubbles collapse and produce high pressure which then damages red blood cells and platelets. The closure mechanism of the trileaflet valve uses the vortices in the aortic sinus to help close the leaflets, which differs from that of the monoleaflet or bileaflet mechanical heart valves which mainly depends on the reverse flow. We used the commercial software program Fluent to run numerical simulations of the St. Jude Medical bileaflet valve and a new trileaflet mechanical heart valve. The results of these numerical simulations were validated with flow field experiments. The closing velocity of the trileaflet valve was clearly slower than that of the St. Jude Medical bileaflet valve, which would effectively reduce the occurrence of cavitation. The findings of this study are expected to advance the development of the trileaflet valve.

关键词： Buckypaper/Epoxy Composites   Molecular Dynamics   Graphical Processing Units (GPU)   Carbon Nanotubes (CNTs)   Failure Behavior   Elastic Modulus  

摘要： Using molecular dynamic (MD) simulation, the mechanical properties of a buckypaper/epoxy composite were calculated. In order to simulate a large model within a large enough timescale, a general MD code fully implemented on graphical processing units (GPUs) was developed. The code, which enables the simulation of large systems with about five hundreds speed up in simulation time relative to a single CPU core, is capable of running on CUDA enabled GPUs. In this study the buckpaper/epoxy composite consists of single wall carbon nanotubes (CNTs), epoxy resin (EPON-862) and the curing agent diethyltoluenediamine (DETDA). Next, the mechanical properties of buckypaper/epoxy composite, namely elastic modulus and Poisson's ratio, were studied through the simulation of a set of tension tests. For the first time directional properties of this material is calculated. A comparison with previously published data is also included. The results prove that the proposed procedure is suitable for investigating the mechanical properties of the buckypaper/epoxy composites. In addition, the results show a negative Poisson ratio and high modulus of elasticity for the buckypaper/epoxy composites, which proves the application of such material in producing artificial muscles, and advanced composites in aviation industries, respectively. Finally, a study on the failure mechanisms of the material is conducted and the integrity of the results was checked by the available experimental and analytical data.

关键词： Adhesion force   Chronoamperometry   Liposome adhesion   Mechanical model   Reaction kinetics model  

摘要： Dynamics of adhesion of single liposome at the charged mercury interface is analyzed through its amperometric signal using a reaction kinetics model and a mechanical model. We present analytical solutions of the reaction kinetics model for decoupling and identifying temporal evolution of three distinct states: i) the initial state corresponding to an intact liposome, ii) the intermediate state where the liposome is partly deformed, and iii) the final state of a lipid monolayer. The results obtained with this model indicate that all three states simultaneously evolve from the onset of the adhesion process. The new mechanical model provides a physical interpretation of the three states and emphasizes the role of the forces involved in liposome adhesion process. The main conclusion is that the water content of the liposome is released through the pores formed in the membrane rather than through the channels parallel to the electrode. Both models reproduce the measurements well in the wide potential range and offer a complementary insight into the dynamics of single adhesion event, which can find application in studies of cell adhesion and in drug delivery. (C) 2012 Elsevier B.V. All rights reserved.

关键词： Leishmania   Spermidine synthase   Inhibitors   Docking   Molecular dynamics simulations  

摘要： Protozoa Leishmania donovani (Ld) is the main cause of the endemic disease leishmaniasis. Spermidine synthase (SS), an important enzyme in the synthetic pathway of polyamines in Ld, is an essential element for the survival of this protozoan. Targeting SS may provide an important aid for the development of drugs against Ld. However, absence of tertiary structure of spermidine synthase of Leishmania donovani (LSS) limits the possibilities of structure based drug designing. Presence of the same enzyme in the host itself further challenges the drug development process. We modeled the tertiary structure of LSS using homology modeling approach making use of homologous X-ray crystallographic structure of spermidine synthase of Trypanosoma cruzi (TSS) (2.5 angstrom resolution). The modeled structure was stabilized using Molecular Dynamics simulations. Based on active site structural differences between LSS and human spermidine synthase (HSS), we screened a large dataset of compounds against modeled protein using Glide virtual screen docking and selected two best inhibitors based on their docking scores (-10.04 and -13.11 respectively) with LSS and having least/no binding with the human enzyme. Finally Molecular Dynamics simulations were used to assess the dynamic stability of the ligand bound structures and to elaborate on the binding modes. This article is part of a Special Issue entitled: Computational Methods for Protein Interaction and Structural Prediction. (C) 2012 Elsevier B.V. All rights reserved.

关键词： ADI   Ductile iron   Thin walled castings   Austenitising   Austempering  

摘要： In the present work, the potential for producing thin walled ductile iron castings with an ausferritic matrix is presented. Experimentally, thin walled iron castings of 2 mm in thickness were obtained and characterised by a nodule count of 1992 mm(-2). In addition, a reference casting was produced with a 25 mm thick wall and a nodule count of 330 mm(-2). Austenitising was carried out at 920 degrees C, whereas austempering was implemented in the 300-400 degrees C temperature range. The austenitising and austempering transformation rates were determined by dilatometry, and the results were confirmed by microstructural analyses. It was found that in thin walled castings, the austenitising and austempering times were reduced by either one-half or one-third of the ones corresponding to the reference casting. The exhibited mechanical properties of the thin walled castings were also determined as a function of austempering time and temperature. It was found that austempering at 300 degrees C for 1200 s leads to thin walled castings with a tensile strength of 1500 MPa. Accordingly, from this work, it is plausible to produce high strength thin walled castings that satisfy all the ASTM 897M grades of ausferritic ductile iron through proper heat treating.

关键词： Mechanical Properties   ZnO Nanowires   Strain Rates   Size Effects   Temperature Dependent Properties   Orientation Dependent Properties   Molecular Dynamics   Atomistic Methods  

摘要： Using molecular dynamics simulations, strain rate, temperature, size and orientation dependent mechanical properties of zinc oxide (ZnO) nanowire are carried out with tensile loading. It is found that, for the same cross-sectional areas, strain rates have almost no effect on Young's modulus, provided strain rates are within the range from 0.001 to 0.03 ps(-1). Our calculated results have also demonstrated that, as temperature ranging from 200 K to 1600 K, ZnO nanowires' Young's modulus are all monotonically decreasing with increasing temperature. Furthermore, at the temperature of 300 K, yield strength, Young's modulus, fracture strength and fracture strain increase dramatically with increasing cross sectional area. It is also demonstrated that, with increasing cross sectional area, the Young's modulus of ZnO nanowires eventually approaching that of bulk single-crystal ZnO. Finally, orientation dependent ZnO mechanical properties of nanowires have been studied at 300 K.

关键词： Kinetic parameters   NH-acid (benzhydrazide)   Stable phosphorus ylides   Transition state   Triphenylphosphine   UV spectrophotometery  

摘要： Stable crystalline phosphorus ylides were obtained in excellent yields from the 1:1:1 addition reaction between triphenylphosphine (TPP) and dialkyl acetylenedicarboxylates, in the presence of NH-acids, such as benzhydrazide. To determine the kinetic parameters of the reactions, they were monitored by UV spectrophotometery. The second order fits were automatically drawn and the values of the second order rate constant (k(2)) were calculated using standard equations within the program. At the temperature range studied the dependence of the second order rate constant (Ln k(2)) on reciprocal temperature was compatible with Arrhenius equation. This provided the relevant plots to calculate the activation energy of all reactions. Furthermore, useful information were obtained from studies of the effect of solvent, structure of reactants (different alkyl groups within the dialkyl acetylenedicarboxylates) and also concentration of reactants on the rate of reactions. On the basis of experimental data the proposed mechanism was confirmed according to the obtained results and a steady state approximation and the first step (k(2)) and third (k(3)) steps of the reactions were recognized as the rate determining steps, respectively. In addition, three speculative proposed mechanisms were theoretically investigated using quantum mechanical calculation. The results, arising from the second and third speculative mechanisms, were far from the experimental data. Nevertheless, there was a good agreement between the theoretical kinetic data, emerge from the first speculative mechanism, and experimental kinetic data of proposed mechanism.

关键词： AV   aortic valve   CABG   coronary artery bypass graft   CAD   coronary artery disease   COPD   chronic obstructive pulmonary disease   CVA   cerebral vascular accident (stroke)   GI   gastrointestinal   IABP   intraaortic balloon pump   LVEDD   left ventricular end diastolic diameter   LVEF   left ventricular ejection fraction   LVESD   left ventricular end systolic diameter   MI   myocardial infarction   NYHA   New York Heart Association class   TEE   transesophageal echocardiogram   TTE   transthoracic echocardiogram   26   35  

摘要： Background. Stentless porcine bioroots and pericardial composite roots were developed as prostheses that avoid the anticoagulation necessary for mechanical composite roots. Data on the hemodynamics of all root replacements are lacking and questions remain regarding comparative performance. Methods. This study evaluated patients receiving a stentless porcine bioroot, a pericardial, or a mechanical composite root at a single institution from January 2000 to December 2008. All patients underwent preoperative, postoperative and greater than 1-year echocardiograms. All root replacements were performed for aortic insufficiency or aneurysm. Median follow-up was 3.3 years (1 to 8 years). Results. There were no significant differences in preoperative left ventricular ejection fraction, aortic valve peak gradient, mean gradient, left ventricular end diastolic diameter, and left ventricular end systolic diameter. Postoperative peak and mean gradients in mechanical composite roots were significantly higher than stentless porcine bioroots. At greater than 1 year, stentless porcine bioroots had significantly lower peak and mean gradients than both mechanical composite roots and pericardial composite roots. The end diastolic diameter was also significantly smaller in stentless porcine bioroots at greater than 1 year than mechanical composite roots. Conclusions. All prostheses resulted in favorable hemodynamic and ventricular remodeling. Stentless porcine bioroots had the lowest gradients at greater than 1 year of all root replacements, with significantly better ventricular remodeling when compared with mechanical composite roots. Pericardial roots also had favorable hemodynamics and ventricular remodeling, suggesting that they are a viable alternative.

关键词： Polymer-matrix composites (PMCs)   Nano-composites   Mechanical properties   Thermal properties  

摘要： Polycarbonate/silica nanocomposites with different silica quantities were prepared by a melt compounding method. The effect of silica amount, in the range 1-5 wt.%, on the morphology, mechanical properties and thermal degradation kinetics of polycarbonate (PC) was investigated. Clusters of silica nanoparticles were well dispersed in the polycarbonate whose structure remained amorphous. NMR results showed intermolecular interactions involving the carbonyl groups of different polymeric chains which did not affect the intramolecular rotational motions. The presence of the lowest silica content showed a decrease in the storage and loss moduli below the glass transition temperature, probably due to a plasticization effect. However, an increase in the amount of silica increased the moduli. The presence of silica in PC slightly increased the thermal stability, except for the highest silica content which showed a decrease. The activation energies of thermal degradation for the nanocomposites depended on the amount of silica and on the degree of conversion. (C) 2012 Elsevier Ltd. All rights reserved.

关键词： Inorganic compounds   Cold working   Powder metallurgy   Deformation  

摘要： The present work aims at providing a direct experimental estimate of the amount of powder trapped at each collision during the mechanical processing by ball milling. Attention has been focused on the mechanochemical reactivity of anhydrous Ag oxalate powders, prepared by mixing Ag nitrate and Na oxalate aqueous solutions, and then filtering and drying the white precipitate formed. Suitable amounts of such powders were mechanically processed with a single milling ball. The mechanical processing induced the gradual decomposition of Ag oxalate into Ag and carbon dioxide. The process was followed by gravimetric analyses, and its kinetics related to individual collisions. It is shown that the decomposition process follows a simple exponential kinetics, which allows the identification and evaluation of an apparent rate constant. This can be regarded as a measure of the amount of powder trapped at each collision, which is roughly equal to 0.41 mg. (C) 2012 Elsevier B.V. All rights reserved.