499 research outputs found
Molecular-dynamics simulations of stacking-fault-induced dislocation annihilation in pre-strained ultrathin single-crystalline copper films
We report results of large-scale molecular-dynamics (MD) simulations of
dynamic deformation under biaxial tensile strain of pre-strained
single-crystalline nanometer-scale-thick face-centered cubic (fcc) copper
films. Our results show that stacking faults, which are abundantly present in
fcc metals, may play a significant role in the dissociation, cross-slip, and
eventual annihilation of dislocations in small-volume structures of fcc metals.
The underlying mechanisms are mediated by interactions within and between
extended dislocations that lead to annihilation of Shockley partial
dislocations or formation of perfect dislocations. Our findings demonstrate
dislocation starvation in small-volume structures with ultra-thin film
geometry, governed by a mechanism other than dislocation escape to free
surfaces, and underline the significant role of geometry in determining the
mechanical response of metallic small-volume structures.Comment: 28 pages, 3 figure
Sine-Wave Electrical Stimulation Initiates a Voltage-Gated Potassium Channel-Dependent Soft Tissue Response Characterized by Induction of Hemocyte Recruitment and Collagen Deposition
Soft tissue repair is a complex process that requires specific communication between multiple cell types to orchestrate effective restoration of physiological functions. Macrophages play a critical role in this wound healing process beginning at the onset of tissue injury. Understanding the signaling mechanisms involved in macrophage recruitment to the wound site is an essential step for developing more effective clinical therapies. Macrophages are known to respond to electrical fields, but the underlying cellular mechanisms mediating this response is unknown. This study demonstrated that lowâamplitude sineâwave electrical stimulation (ES) initiates a soft tissue response in the absence of injury in Procambarus clarkii. This cellular response was characterized by recruitment of macrophageâlike hemocytes to the stimulation site indicated by increased hemocyte density at the site. ES also increased tissue collagen deposition compared to sham treatment (P \u3c 0.05). Voltageâgated potassium (KV) channel inhibition with either 4âaminopyridine or astemizole decreased both hemocyte recruitment and collagen deposition compared to saline infusion (P \u3c 0.05), whereas inhibition of calciumâpermeable channels with ruthenium red did not affect either response to ES. Thus, macrophageâlike hemocytes in P. clarkii elicit a woundâlike response to exogenous ES and this is accompanied by collagen deposition. This response is mediated by KV channels but independent of Ca2+ channels. We propose a significant role for KV channels that extends beyond facilitating Ca2+ transport via regulation of cellular membrane potentials during ES of soft tissue
Approximate Analytical Model for the Squeeze-Film Lubrication of the Human Ankle Joint with Synovial Fluid Filtrated by Articular Cartilage
The aim of this article is to propose an analytical approximate squeeze-film lubrication model of the human ankle joint for a quick assessment of the synovial pressure field and the load carrying due to the squeeze motion. The model starts from the theory of boosted lubrication for the human articular joints lubrication (Walker et al., Rheum Dis 27:512â520, 1968; Maroudas, Lubrication and wear in joints. Sector, London, 1969) and takes into account the fluid transport across the articular cartilage using Darcyâs equation to depict the synovial fluid motion through a porous cartilage matrix. The human ankle joint is assumed to be cylindrical enabling motion in the sagittal plane only. The proposed model is based on a modified Reynolds equation; its integration allows to obtain a quick assessment on the synovial pressure field showing a good agreement with those obtained numerically (Hlavacek, J Biomech 33:1415â1422, 2000). The analytical integration allows the closed form description of the synovial fluid film force and the calculation of the unsteady gap thickness
Search for axions in streaming dark matter
A new search strategy for the detection of the elusive dark matter (DM) axion
is proposed. The idea is based on streaming DM axions, whose flux might get
temporally enormously enhanced due to gravitational lensing. This can happen if
the Sun or some planet (including the Moon) is found along the direction of a
DM stream propagating towards the Earth location. The experimental requirements
to the axion haloscope are a wide-band performance combined with a fast axion
rest mass scanning mode, which are feasible. Once both conditions have been
implemented in a haloscope, the axion search can continue parasitically almost
as before. Interestingly, some new DM axion detectors are operating wide-band
by default. In order not to miss the actually unpredictable timing of a
potential short duration signal, a network of co-ordinated axion antennae is
required, preferentially distributed world-wide. The reasoning presented here
for the axions applies to some degree also to any other DM candidates like the
WIMPs.Comment: 5 page
Effect of osmotic stress on the expression of TRPV4 and BKCa channels and possible interaction with ERK1/2 and p38 in cultured equine chondrocytes
The metabolic activity of articular chondrocytes is influenced by osmotic alterations that occur in articular cartilage secondary to mechanical load. The mechanisms that sense and transduce mechanical signals from cell swelling and initiate volume regulation are poorly understood. The purpose of this study was to investigate how the expression of two putative osmolyte channels [transient receptor potential vanilloid 4 (TRPV4) and large-conductance Ca2+-activated K+ (BKCa)] in chondrocytes is modulated in different osmotic conditions and to examine a potential role for MAPKs in this process. Isolated equine articular chondrocytes were subjected to anisosmotic conditions, and TRPV4 and BKCa channel expression and ERK1/2 and p38 MAPK protein phosphorylation were investigated using Western blotting. Results indicate that the TRPV4 channel contributes to the early stages of hypo-osmotic stress, while the BKCa channel is involved in responding to elevated intracellular Ca2+ and mediating regulatory volume decrease. ERK1/2 is phosphorylated by hypo-osmotic stress (P < 0.001), and p38 MAPK is phosphorylated by hyperosmotic stress (P < 0.001). In addition, this study demonstrates the importance of endogenous ERK1/2 phosphorylation in TRPV4 channel expression, where blocking ERK1/2 by a specific inhibitor (PD98059) prevented increased levels of the TRPV4 channel in cells exposed to hypo-osmotic stress and decreased TRPV4 channel expression to below control levels in iso-osmotic conditions (P < 0.001)
Surface diffusion coefficients by thermodynamic integration: Cu on Cu(100)
The rate of diffusion of a Cu adatom on the Cu(100) surface is calculated
using thermodynamic integration within the transition state theory. The results
are found to be in excellent agreement with the essentially exact values from
molecular-dynamics simulations. The activation energy and related entropy are
shown to be effectively independent of temperature, thus establishing the
validity of the Arrhenius law over a wide range of temperatures. Our study
demonstrates the equivalence of diffusion rates calculated using thermodynamic
integration within the transition state theory and direct molecular-dynamics
simulations.Comment: 4 pages (revtex), two figures (postscript
Sampling the diffusion paths of a neutral vacancy in Silicon with quantum mechanical calculations
We report a first-principles study of vacancy-induced self-diffusion in
crystalline silicon. Starting form a fully relaxed configuration with a neutral
vacancy, we proceed to search for local diffusion paths. The diffusion of the
vacancy proceeds by hops to first nearest neighbor with an energy barrier of
0.40 eV in agreement with experimental results. Competing mechanisms are
identified, like the reorientation, and the recombination of dangling bonds by
Wooten-Winer-Weaire process.Comment: 10 pages, 5 figures, accepted for publication in Pysical review
An Arthroscopic Device to Assess Articular Cartilage Defects and Treatment with a Hydrogel
The hydraulic resistance R across osteochondral tissue, especially articular cartilage, decreases with degeneration and erosion. Clinically useful measures to quantify and diagnose the extent of cartilage degeneration and efficacy of repair strategies, especially with regard to pressure maintenance, are still developing. The hypothesis of this study was that hydraulic resistance provides a quantitative measure of osteochondral tissue that could be used to evaluate the state of cartilage damage and repair. The aims were to (1) develop a device to measure R in an arthroscopic setting, (2) determine whether the device could detect differences in R for cartilage, an osteochondral defect, and cartilage treated using a hydrogel ex vivo, and (3) determine how quickly such differences could be discerned. The apparent hydraulic resistance of defect samples was ~35% less than intact cartilage controls, while the resistance of hydrogel-filled groups was not statistically different than controls, suggesting some restoration of fluid pressurization in the defect region by the hydrogel. Differences in hydraulic resistance between control and defect groups were apparent after 4Â s. The results indicate that the measurement of R is feasible for rapid and quantitative functional assessment of the extent of osteochondral defects and repair. The arthroscopic compatibility of the device demonstrates the potential for this measurement to be made in a clinical setting
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