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At least 1,027 records · Page 57Linked to original sources
PubMed1996

Cardiovascular and hormonal changes induced by isolation and confinement.

The cardiovascular changes induced by microgravity are in general described as the result of the loss of hydrostatic pressure. Other factors are also important: restricted environment with 1) elimination of mobility, action and 2) isolation always found in space environment or during simulation studies. Several studies indicate an analogy between microgravity and confinement. The results of simulation studies could be misinterpreted without a control confinement study present in the protocol.

Cardiovascular Physiological Phenomena↗
PubMed2005

A comparison of torque required to fracture rotary files with tips bound in simulated curved canal.

The purpose of this study was to compare torque force and rotation needed to fracture three types of nickel titanium alloy rotary instruments in a simulated curved root canal space that were bound at the file tip. Files of similar size tips were studied. The files studied were ProFiles with 0.04 taper diameters of 15, 20, 25, 30, 35, 40, 45; 0.04 ProFile GT sizes 20, 30, 40; and ProTaper files sizes S1, S2, F1, F2, and F3. All files were 25 mm in length. Unwinding was defined as the rotation in degrees it took for a file to fracture after the first evidence of permanent deformation. All files exhibited permanent deformation before breaking, with the ProFile GT files demonstrating the greatest unwinding. The #45 0.04 ProFile withstood the most force while the #20 ProFile GT required the least amount of force before beginning to exhibit permanent deformation. The S1 and S2 ProTaper files fractured with so little rotation that no extended data were recorded. Generally, as the file diameter increased, the force needed to begin unwinding also increased. Also, as the file diameter increased, the force needed to fracture also increased.

Analysis of Variance↗
PubMed1994

Quantum dynamics as a stochastic process.

The quantum Liouville equation is solved in the Wigner representation using generalized Monte Carlo techniques. For small increments of time, the solution is represented as a sequential classical evolution in phase space followed by a quantum "jump" distribution in momentum space, with the latter simulated via a stochastic method. Extending the work initiated by John and Remler [Ann Phys. (N.Y.) 180, 152 (1987)] the technique is developed and validated for higher dimensions. Also, an alternative algorithm is developed and applied to study motion of a quantum system in an anharmonic quartic potential well, with significantly improved results.

Algorithms↗
PubMed2001

Superluminal pulse reflection in asymmetric one-dimensional photonic band gaps.

Superluminal pulse reflection is shown to occur in a class of one-dimensional asymmetric photonic band gaps in which a spectral window inside the gap is opened. By means of a coupled-mode equation analysis, we describe in detail two possible realizations of superluminal pulse reflection that can be achieved using fiber Bragg gratings. The former method is based on the introduction of a defect into the otherwise periodic dielectric structure, whereas the latter one exploits the interference of two closely-spaced resonance modes and simulates the dispersion properties of an inverted medium possessing a doublet line.

Journal Article↗
PubMed2002

High-power regime of femtosecond-laser pulse propagation in silica: multiple-cone formation.

We present a numerical study of the (2+1)-dimensional propagation dynamics of femtosecond-laser pulses in silica. In particular, considered are pulses, whose power is tens to hundreds of times higher than the threshold for self-focusing. We solve the axially symmetric, extended, nonlinear Schrödinger equation for the laser electric field, including group velocity dispersion, Kerr nonlinearity, plasma formation and defocusing, self-steepening, and space-time focusing. Our simulation results reveal that the high-power pulses are split spatially, as well as temporally, several times into multiple cones during its propagation. This new structure is formed as a result of the interplay of strong Kerr self-focusing and plasma defocusing. The number of cones and their angle with respect to the propagation axis increase with incident pulse energy. The uncertainty, which may be contained in the evaluation of plasma response and band-to-band transition rate, and the pulse disturbance by modulation instability are also analyzed. Although these influence the details of the pulse propagation, they do not affect the essence of our results: the multiple-cone formation.

Journal Article↗
PubMed2004

Transverse fluctuations of grafted polymers.

We study the statistical mechanics of grafted polymers of arbitrary stiffness in a two-dimensional embedding space with Monte Carlo simulations. The probability distribution function of the free end is found to be highly anisotropic and non-Gaussian for typical semiflexible polymers. The reduced distribution in the transverse direction, a Gaussian in the stiff and flexible limits, shows a double-peak structure at intermediate stiffnesses. We also explore the response to a transverse force applied at the polymer free end. We identify F-Actin as an ideal benchmark for the effects discussed.

Journal Article↗
PubMed2004

Solution of the two-star model of a network.

The p-star model or exponential random graph is among the oldest and best known of network models. Here we give an analytic solution for the particular case of the two-star model, which is one of the most fundamental of exponential random graphs. We derive expressions for a number of quantities of interest in the model and show that the degenerate region of the parameter space observed in computer simulations is a spontaneously symmetry-broken phase separated from the normal phase of the model by a conventional continuous phase transition.

Journal Article↗
PubMed2003

Observation of nonlinear frequency-sweeping suppression with rf diffusion.

Nonlinear frequency sweeping of unstable waves in a laboratory plasma is suppressed upon application of rf fields. Frequency sweeping is driven by a population of energetic electrons trapped in a magnetic dipole field that excite drift-resonant potential fluctuations and create coherent structures in phase space. Self-consistent numerical simulation reproduces the suppression and suggests an explanation due to rf scattering of energetic electrons that destroys the phase-space structures.

Journal Article↗
PubMed2002

Structure prediction as a tool for solution of the crystal structures of metallo-organic complexes using powder X-ray diffraction data.

A simulated-annealing direct-space approach has been applied to predict the crystal structures of a series of metallo-organic complexes containing Zn, Cu and Ni. The prediction methodology generates a set of energetically reasonable crystal structures among which the actual structure is present, but it is not always possible to specify unambiguously the known crystal structure solely on the basis of energy. In each case, however, the ambiguity may be resolved by recourse to laboratory powder X-ray diffraction (PXRD) data. In this manner, structure prediction is shown to be a powerful tool for structure solution using PXRD data, with the additional advantage that indexing of the PXRD profile is not required at the outset.

Journal Article↗
PubMed2005

Determination of the structure of the violet pigment C22H12Cl2N6O4 from a non-indexed X-ray powder diagram.

The violet pigment methylbenzimidazolonodioxazine, C22H12Cl2N6O4 (systematic name: 6,14-dichloro-3,11-dimethyl-1,3,9,11-tetrahydro-5,13-dioxa-7,15-diazadiimidazo-[4,5-b:4',5'-m]pentacene-2,10-dione), shows an X-ray powder diagram consisting of only ca 12 broad peaks. Indexing was not possible. The structure was solved by global lattice energy minimizations. The program CRYSCA [Schmidt & Kalkhof (1999), CRYSCA. Clariant GmbH, Pigments Research, Frankfurt am Main, Germany] was used to predict the possible crystal structures in different space groups. By comparing simulated and experimental powder diagrams, the correct structure was identified among the predicted structures. Owing to the low quality of the experimental powder diagram the Rietveld refinements gave no distinctive results and it was difficult to prove the correctness of the crystal structure. Finally, the structure was confirmed to be correct by refining the crystal structure of an isostructural mixed crystal having a better X-ray powder diagram. The compound crystallizes in P1, Z=1. The crystal structure consists of a very dense packing of molecules, which are connected by hydrogen bridges of the type N-H...O=C. This packing explains the observed insolubility. The work shows that crystal structures of molecular compounds may be solved by lattice energy minimization from diffraction data of limited quality, even when indexing is not possible.

Heterocyclic Compounds, 4 or More Rings↗
PubMed2000

Intercellular coupling mediated by potassium accumulation in peg-and-socket junctions.

Coupling of smooth muscle cells is important for coordination of gastrointestinal motility. Small structures called peg-and-socket junctions (PSJs) have been found between muscle cells and may play a role in electrical coupling due to extracellular potassium accumulation in the narrow cleft between the muscle cells. A model was developed in which an electrical boundary element model of the cell morphology is used in conjunction with a finite difference model which described ionic fluxes and diffusion of extracellular potassium in the PSJ. The boundary element model used a combination of triangular and cylindrical elements to reduce computational demand while ensuring accuracy. Barrier kinetics were used to model the underlying ionic transport mechanisms. Seven ionic transport mechanisms were used to create the transmembrane voltage waveform. Results indicate that PSJs may produce significant coupling between smooth muscle cells under appropriate conditions. Coupling increased exponentially with increasing length and with decreasing intercellular gap.

Adherens Junctions↗
PubMed2003

Free-energy landscape of a chameleon sequence in explicit water and its inherent alpha/beta bifacial property.

A sequence in yeast MATalpha2/MCM1/DNA complex that folds into alpha-helix or beta-hairpin depending on the surroundings has been known as "chameleon" sequence. We obtained the free-energy landscape of this sequence by using a generalized-ensemble method, multicanonical molecular dynamics simulation, to sample the conformational space. The system was expressed with an all-atom model in explicit water, and the initial conformation for the simulation was a random one. The free-energy landscape demonstrated that this sequence inherently has an ability to form either alpha or beta structure: The conformational distribution in the landscape consisted of two alpha-helical clusters with different packing patterns of hydrophobic residues, and four beta-hairpin clusters with different strand-strand interaction patterns. Narrow pathways connecting the clusters were found, and analysis on the pathways showed that a compact structure formed at the N-terminal root of the chameleon sequence controls the cluster-cluster transitions. The free-energy landscape indicates that a small conditional change induces alpha-beta transitions. Additional unfolding simulations done with replacing amino acids showed that the chameleon sequence has an advantage to form an alpha-helix. Current study may be useful to understand the mechanism of diseases resulting from abnormal chain folding, such as amyloid disease.

Amino Acid Sequence↗
PubMed2003

A model of fluid flow in solid tumors.

Solid tumors consist of a porous interstitium and a neoplastic vasculature composed of a network of capillaries with highly permeable walls. Blood flows across the vasculature from the arterial entrance point to the venous exit point, and enters the tumor by convective and diffusive extravasation through the permeable capillary walls. In this paper, an integrated theoretical model of the flow through the tumor is developed. The flow through the interstitium is described by Darcy's law for an isotropic porous medium, the flow along the capillaries is described by Poiseuille's law, and the extravasation flux is described by Starling's law involving the pressure on either side of the capillaries. Given the arterial, the venous, and the ambient pressure, the problem is formulated in terms of a coupled system of integral and differential equations for the vascular and interstitial pressures. The overall hydrodynamics is described in terms of hydraulic conductivity coefficients for the arterial and venous flow rates whose functional form provides an explanation for the singular behavior of the vascular resistance observed in experiments. Numerical solutions are computed for an idealized case where the vasculature is modeled as a single tube, and charts of the hydraulic conductivities are presented for a broad range of tissue and capillary wall conductivities. The results in the physiological range of conditions are found to be in good agreement with laboratory observations. It is shown that the assumption of uniform interstitial pressure is not generally appropriate, and predictions of the extravasation rate based on it may carry a significant amount of error.

Arteries↗
PubMedDate not supplied

A priori identifiability of distributed models of blood-tissue exchange.

A priori identifiability deals with the uniqueness of the solution for the unknown parameters of a dynamic model from a given input-output experiment, and is a prerequisite for well posedness of parameter estimation from the data. Identifiability has been extensively investigated for lumped parameter, linear, and nonlinear dynamic models, in particular, compartmental models of biological systems. Much less attention has been devoted to distributed parameter model, in particular, those describing blood-tissue exchange, which are normally used to interpret regional multiple tracer dilution experiments. In this paper, we study a priori identifiability of distributed parameter models of transcapillary exchange, focusing first on a single capillary (one-region) model, then moving on to a single capillary-interstitial fluid (two-region) model, and finally to an organ model also describing flow heterogeneity.

Capillaries↗
PubMed1984

Comparison of azlocillin, ceftizoxime, cefoxitin, and amikacin alone and in combination against Pseudomonas aeruginosa in a neutropenic-site rabbit model.

The efficacy of beta-lactam antibiotics and amikacin alone and in various combinations against Pseudomonas aeruginosa was studied in a rabbit model simulating a closed-space infection in a locally neutropenic site. Six strains of P. aeruginosa were studied in semipermeable chambers placed subcutaneously in rabbits. Therapy was begun 4 h after inoculation of 5 X 10(4) CFU of bacteria per ml of pooled rabbit serum into the chambers. Antibiotics were administered intramuscularly every 6 h for 16 doses. Quantitative bacteriology was measured at the start of therapy and at 20, 44, and 92 h thereafter. Antibiotic concentrations were measured in blood and chamber fluid. Results were compared with in vitro tests of susceptibility and synergy. No single-agent therapy eradicated any of the six test organisms. Azlocillin (100 mg/kg per dose) plus amikacin (20 mg/kg per dose) eliminated five of six organisms by 92 h, and ceftizoxime (100 mg/kg per dose) plus amikacin (20 mg/kg per dose) eliminated three of six test strains. Azlocillin plus ceftizoxime (each 100 mg/kg per dose) failed to eliminate any of the six strains. To eliminate P. aeruginosa in this model, two drugs were required, with one being an aminoglycoside. In vitro susceptibility tests of synergy were predictive of successful therapy whenever the antibiotic concentrations (free and total) at the infection site exceeded the MBC for both the aminoglycoside alone and the beta-lactam when tested in combination with amikacin.

Agranulocytosis↗
PubMed1989

Influence of protein binding on therapeutic efficacy of cefoperazone.

The effect of protein binding of cefoperazone (89.3% bound to rabbit serum) on antibacterial activity in serum was tested in a model that simulated a closed-space infection in a neutropenic host. Four gram-negative bacilli were tested in the model with cefoperazone doses of 20 and 200 mg/kg administered intramuscularly every 6 h for 16 doses. Cefoperazone efficacy was measured at 92 h by determining the log10 decrease in bacterial count from that of the control for five paired studies with three isolates. A significantly better response was demonstrated when the free (non-protein-bound) drug concentration exceeded the MICs and MBCs for the infecting microorganisms at the infection site at all times (P less than 0.005). This supports the concept that free (unbound) drug is the active component in treating bacterial infections.

Animals↗
PubMed1978

Effect of hydrostatic tensile stress on the growth of Escherichia coli and Bacillus cereus.

The specific growth rates of Escherichia coli and Bacillus cereus were measured for growth media in a flask, a lens-plate arrangement simulating an isolated capillary space, and a lens-plate arrangement under hydrostatic tensile stress. The specific growth rates of the bacteria were the same for the flask and lens-plate arrangement without hydrostatic tensile stress, but were enhanced when the growth media were subjected to hydrostatic tensile stress. The enhanced specific growth rates reached steady values at a tensile stress of 40 pascals. The effect was observed up to tensile stresses of around 100 pascals. The maximum increase in specific growth rate was 25% for E. coli and 22% for B. cereus.

Bacillus cereus↗