Initial Stretch Research Articles

A Biomechanical Model for Fluidization of Cells under Dynamic Strain

Recent experiments have investigated the response of smooth muscle cells to transient stretch-compress (SC) and compress-stretch (CS) maneuvers. The results indicate that the transient SC maneuver causes a sudden fluidization of the cell while the CS maneuver does not. To understand this asymmetric behavior, we have built a biomechanical model to probe the response of stress fibers to the two maneuvers. The model couples the cross-bridge cycle of myosin motors with a viscoelastic Kelvin-Voigt element that represents the stress fiber. Simulation results point to the sensitivity of the myosin detachment rate to tension as the cause for the asymmetric response of the stress fiber to the CS and SC maneuvers. For the SC maneuver, the initial stretch increases the tension in the stress fiber and suppresses myosin detachment. The subsequent compression then causes a large proportion of the myosin population to disengage rapidly from actin filaments. This leads to the disassembly of the stress fibers and the observed fluidization. In contrast, the CS maneuver only produces a mild loss of myosin motors and no fluidization.

The role of human ankle plantar flexor muscle-tendon interaction and architecture in maximal vertical jumping examined in vivo.

Humans utilise elastic tendons of lower limb muscles to store and return energy during walking, running and jumping. Anuran and insect species use skeletal structures and/or dynamics in conjunction with similarly compliant structures to amplify muscle power output during jumping. We sought to examine whether human jumpers use similar mechanisms to aid elastic energy usage in the plantar flexor muscles during maximal vertical jumping. Ten male athletes performed maximal vertical squat jumps. Three-dimensional motion capture and a musculoskeletal model were used to determine lower limb kinematics that were combined with ground reaction force data in an inverse dynamics analysis. B-mode ultrasound imaging of the lateral gastrocnemius (GAS) and soleus (SOL) muscles was used to measure muscle fascicle lengths and pennation angles during jumping. Our results highlighted that both GAS and SOL utilised stretch and recoil of their series elastic elements (SEEs) in a catapult-like fashion, which likely serves to maximise ankle joint power. The resistance of supporting of body weight allowed initial stretch of both GAS and SOL SEEs. A proximal-to-distal sequence of joint moments and decreasing effective mechanical advantage early in the extension phase of the jumping movement were observed. This facilitated a further stretch of the SEE of the biarticular GAS and delayed recoil of the SOL SEE. However, effective mechanical advantage did not increase late in the jump to aid recoil of elastic tissues.

Mechanical characterization of microcapsules with membrane permeability by using indentation analysis.

Mechanical modeling of the deformation of a liquid-filled spherical microcapsule indented by a sharp truncated-cone indenter was proposed, in which membrane permeability was taken into account. The change in the internal volume of the microcapsule due to fluid permeation was calculated on the basis of Kedem and Katchalsky equations (1958, "Thermodynamic Analysis of the Permeability of Biological Membranes to Non-electrolytes," Biochim. Biophys. Acta, 27, pp. 229-246). The membrane hydraulic permeability, membrane initial stretch, and effective osmotic pressure difference across the membrane of an alginate-poly(l)lysine-alginate (APA) microcapsule were identified by fitting calculated and measured force-displacement curves. The difference between deformed shapes with and without membrane permeability was shown, suggesting the spatial resolution of image analysis performed to measure the membrane permeability from the volume loss. The influences of changes in permeability, initial stretch, and a parameter β, used for determining the effective osmotic pressure difference, on the force-displacement relationship were examined, and mechanisms causing changes in the force-displacement relationship were discussed.

The Determination of Optimal Initial Tension in Rat Coronary Artery Using Wire Myography

The aim of the present study was to determine the optimal initial tension, i.e. initial stretch for rat coronary artery when using the multi-wire myograph system. We used the normalization procedure to mimic physiological conditions and to stretch the coronary arterial segments to normalized internal circumference (IC(1)). It is determined the internal circumference when the vessel relaxed under a transmural pressure of 100 mm Hg (IC(100)), and the IC(1) is calculated by multiplying the IC(100) by a factor k. The impact of different factor k on the initial stretch and agonist-induced tension of coronary arteries were investigated. The results showed that the maximal agonist-induced tension was achieved at the factor k value of 0.90 and the initial stretch tension was given 1.16+/-0.04 mN/mm. The most appropriate factor k value was 0.90-0.95 and the most appropriate initial tension was 1.16-1.52 mN/mm. The equilibration time of the coronary artery segments should be at least 1.0 h. In the same optimal initial tension, the agonist-induced tension increased as equilibration time lengthened.

Characteristics analysis of a novel centralized-driving flip-flow screen

During screening operation, blinding or clogging of screen perforations generally occurs to reduce the sieving capacity and efficiency. Recently, the flip-flow screening has been widely recognized as a feasible method to deal with the problem. In this paper, a novel centralized-driving flip-flow screen (CFS) was developed for the separation of fine and moist coal, and the key structures, namely, a centralized-driving mechanism and a quasi-circle beam mounted with the mat were designed for high reliability and stability. By means of a test on an experimental prototype, the effect of some factors, i.e., initial stretch and hardness of the polyurethane panel, respectively, and the rotation speed of the driving motor on the kinematic characteristic of the screen surface was investigated. Results show that without an initial stretch, the sieve mat generates the largest vibratory amplitude while the slacker the sieve mat initially is, the smaller amplitude it will accomplish. And an increase in the rotation speed could cause a rise in the vibratory amplitude. Unlike the two factors, the hardness does not have a definite effect on the kinematic performance, on which a further study is required. Finally, screening processing on a laboratory prototype was conducted to draw the conclusion that the developed CFS also has a high sieving efficiency for the fine and moist coal.

Mechanical and Geometrical Characterization of the Conveyors Belts from Mineral Resources Exploitation

Conveyor belt is one of the most common types of continuous handling equipment used for moving rocks of various sizes horizontally or inclined. From researches done in the exploitation of mineral resources and rocks and from the Strength of Materials calculations in this paper is observed above the conveyor sizing and other elements of the transporter requires knowing how belt traction forces (tensions thereof) varies on the conveyor length. In all cases the operation of the conveyor belt must be provided without engaging the slip phenomenon and limiting its displacement (between sets of rollers) so conveyor can operating at optimum parameters. The materials that make up a conveyor belt can be divided into two categories: materials for coating plates or outer shell and a resistance part, areas with inserts that can be textile and steel cables. Because the bands are subjected to tensile efforts and wear phenomena we determine the maximum strength they can operate without going into permanent deformation regime. Conveyor belt dimensioning calculations and other elements of a conveyor varies depending on its length and the carrier operating situations. We can see, also, that sizing conveyor belt and other conveyor components require knowledge of how belt traction forces (tensions thereof) varies along the length of the conveyor. The tensions from the conveyor belt will be study for the climb section and for the descend section. It mentions that the specific resistance to motion of the belt on full branch, consisting of resistances due: current material deformation, rotation rollers camps because pressing rubber layer and roll belt and belt bending. Since current material strength due to deformation and bending belt represents a large percentage of total specific resistance to motion, it follows that resistance decreases with decreasing displacement belt between rollers and increase with the effort in belt, respectively. This explains that resistance decreases with increasing length of conveyor (especially when its initial stretch is great), and with increasing conveyor tilt too. Also, we observe, from the values of forces calculated in the paper, that the resistance to rotation of the roller bearings decreases with increasing load on the roll, hence the values adopted for specific resistance to movement can be more larger on the empty branch than on the full. Knowing how to calculate resistances to motion of the belt, you can fix that occur belt efforts through the points made on the contour method, starting point of the drum motor development and ending point of winding it. These movement resistances calculated for empty and full branches are needed to precisely determine the installed power of the conveyor belt.

Axial prestretch and circumferential distensibility in biomechanics of abdominal aorta

Elastic arteries are significantly prestretched in an axial direction. This property minimises axial deformations during pressure cycle. Ageing-induced changes in arterial biomechanics, among others, are manifested via a marked decrease in the prestretch. Although this fact is well known, little attention has been paid to the effect of decreased prestretch on mechanical response. Our study presents the results of an analytical simulation of the inflation-extension behaviour of the human abdominal aorta treated as nonlinear, anisotropic, prestrained thin-walled as well as thick-walled tube with closed ends. The constitutive parameters and geometries for 17 aortas adopted from the literature were supplemented with initial axial prestretches obtained from the statistics of 365 autopsy measurements. For each aorta, the inflation-extension response was calculated three times, with the expected value of the initial prestretch and with the upper and lower confidence limit of the initial prestretch derived from the statistics. This approach enabled age-related trends to be evaluated bearing in mind the uncertainty in the prestretch. Despite significantly decreased longitudinal prestretch with age, the biomechanical response of human abdominal aorta changes substantially depending on the initial axial stretch was used. In particular, substituting the upper limit of initial prestretch gave mechanical responses which can be characterised by (1) low variation in axial stretch and (2) high circumferential distensibility during pressurisation, in contrast to the responses obtained for their weakly prestretched counterparts. The simulation also suggested the significant effect of the axial prestretch on the variation of axial stress in the pressure cycle. Finally, the obtained results are in accordance with the hypothesis that circumferential-to-axial stiffness ratio is the quantity relatively constant within this cycle.

On the contact problem of an inflated spherical hyperelastic membrane

In this paper, the mechanics of contact of an inflated spherical non-linear hyperelastic membrane pressed between two rigid plates has been studied. We have considered the membrane material to be a homogeneous and isotropic Mooney–Rivlin hyperelastic solid. All three cases, namely frictionless, no-slip and stick–slip conditions have been considered separately in the plate-membrane contact region. The stretch of the membrane, and the surface traction (for no-slip contact) has been determined. For the stick–slip case, the sliding front is observed to be initiated at the contact periphery which moves towards the pole. The state at which the impending wrinkling condition occurs has been determined analytically. It is observed that the impending wrinkling state occurs at the periphery of the contact. Based on this, the minimum initial stretch (inflation) required to prevent wrinkling at any point in the membrane has been determined.

Scouts behave as streakers in honeybee swarms

Harmonic radar tracking was used to record the flights of scout bees during takeoff and initial flight path of two honeybee swarms. One swarm remained intact and performed a full flight to a destination beyond the range of the harmonic radar, while a second swarm disintegrated within the range of the radar and most of the bees returned to the queen. The initial stretch of the full flight is characterized by accelerating speed, whereas the disintegrating swarm flew steadily at low speed. The two scouts in the swarm displaying full flight performed characteristic flight maneuvers. They flew at high speed when traveling in the direction of their destination and slowed down or returned over short stretches at low speed. Scouts in the disintegrating swarm did not exhibit the same kind of characteristic flight performance. Our data support the streaker bee hypothesis proposing that scout bees guide the swarm by traveling at high speed in the direction of the new nest site for short stretches of flight and slowing down when reversing flight direction.

Influence of Damped Propagation of Dopant on the Excitation Kinetics of Doped Quantum Dots

Excitation in quantum dots is a hotly pursued phenomenon. Realizing it, we investigate the excitation kinetics of a repulsive impurity doped quantum dot as the dopant is propagating. Such a propagation is assumed to be important because of its close connection with impurity drift in nanodevices. The problem has been made more realistic by considering the dopant propagation to be damped. For simplicity, we have considered an inherently linear propagation of the dopant, and the impurity potential has been assumed to have a Gaussian nature. The damping strength and the initial spatial stretch of the dopant have been found to fabricate the said kinetics in a delicate way. However, in the overdamped region, we find attainment of stabilization in the excitation rate invariably. Determination of average accelerating force imparted onto the propagating dopant seems to consolidate the findings. The present investigation is believed to provide some useful insight into the phenomenon of damping that has potential im...

Finite inflation of an initially stretched hyperelastic circular membrane

Abstract In this paper, finite axisymmetric inflation of an initially stretched flat circular hyperelastic membrane has been analyzed. The membrane material has been assumed to be a homogeneous and isotropic Mooney–Rivlin solid. The inflation problem has been reduced to a set of three first order ordinary differential equations using a set of appropriately defined variables. An interesting method based on the invariance of these equations to scaling has been used to solve the two point boundary value problem without much effort. This method does not require any special technique for negotiating the limit points in the pressure–stretch relations of the membrane. Several inflation results of an initially unstretched and pre-stretched circular membrane for various material parameters are obtained. The roles of pre-stretch and internal pressure on the inflation mechanics are clearly delineated. The initial stretch is observed to have some interesting counter-intuitive effects on the inflation of the membrane.

Hydrogeological characterization and assessment of groundwater quality in shallow aquifers in vicinity of Najafgarh drain of NCT Delhi

Najafgarh drain is the biggest drain in Delhi and contributes about 60% of the total wastewater that gets discharged from Delhi into river Yamuna. The drain traverses a length of 51 km before joining river Yamuna, and is unlined for about 31 km along its initial stretch. In recent times, efforts have been made for limited withdrawal of groundwater from shallow aquifers in close vicinity of Najafgarh drain coupled with artificial recharge of groundwater. In this perspective, assessment of groundwater quality in shallow aquifers in vicinity of the Najafgarh drain of Delhi and hydrogeological characterization of adjacent areas were done. The groundwater quality was examined in perspective of Indian as well as World Health Organization’s drinking water standards. The spatial variation in groundwater quality was studied. The linkages between trace element occurrence and hydrochemical facies variation were also established. The shallow groundwater along Najafgarh drain is contaminated in stretches and the area is not suitable for large-scale groundwater development for drinking water purposes.

Indentation Analysis of Microcapsule with Initial Stretch

Indentation Analysis of Microcapsule with Initial Stretch

Passive Viscosity Decreases with Deletion of PEVK Region of Cardiac Titin

Diastolic dysfunction is a poorly understood but clinically pervasive syndrome that is characterized by increased diastolic stiffness. Diastolic stiffness needs to be better understood, especially the mechanisms that give rise to viscous stiffness. Viscous stiffness is strain-rate sensitive stiffness giving rise to hysteresis and stress relaxation. Here we focused on passive viscosity by measuring hysteresis from the area between the stretch and release passive force-sarcomere length curves and used both wild-type (WT) mice and PEVK knockout (KO) mice in which the PEVK element has been deleted. The PEVK element is a proline-glutamate-valine-lysine (PEVK) rich segment that is part of titin's spring region and that has previously been shown to be an important source of viscosity in the cardiac sarcomere. A physiological stretch protocol which mimics the cardiac cycle was performed on both KO and WT skinned fiber bundles. This protocol consists of a series of triangles stretches which take the prep from slack sarcomere length to a sarcomere length of 2.2 μm at 300%/s (i.e. ∼600 beats/min). In both KO and WT, hysteresis during the first stretch and release cycle was highest and quickly decreased to about ∼45% of the initial value during steady state. Hysteresis was significantly lower in the KO during both the initial stretch and steady state but did not completely disappear. After 100 cycles, hysteresis was 225±53 pJ/mm2/sarcomere in the WT and 120±32 pJ/mm2/sarcomere in the KO. Because hysteresis was not completely absent in the PEVK KO tissues, we are currently studying the possibility that cMyBP-C contributes to viscosity as well, using both cMyBP-C extraction and post-translational modifications of cMyBP-C.

Asymptotic solutions and new insights for cylinder and core–shell polymer gels in a solvent

A polymer gel in a solvent can swell to a large extent, and the large deformation caused by the swelling makes studies on the behaviors of gels difficult due to the high nonlinearity. In this paper, we study three homogeneous deformations of a cylinder gel in the cases of free-swelling, swellings under uniaxial constraint and under equal biaxial constraint and the inhomogeneous deformation of a spherical core–shell gel. The aim is to deduce new insights from the asymptotic solutions. In particular, the roles of the chemical potential are fully explored through appropriate parameters. For homogeneous deformations, the governing nonlinear algebraic equations are solved using perturbation methods. Analytical formulae for the stretch and stress are obtained and the key controlling parameters are found, which lead to some interesting insightful information. Also, according to some critical values of this parameter, the plane of the polymer network parameter and the solvent parameter can be divided into five domains, in which the gel has different behaviors. The inhomogeneous deformation of the core–shell gel (which can be used as a drug-carrier in applications) is governed by a complicated variable-coefficient nonlinear differential equation. It is not possible to obtain the exact solution. However, when the above-mentioned parameter is small or large, we manage to construct asymptotic solutions. In the first case, two key transformations are introduced, which make the leading-order equation twice integrable. In the second case, we use an extended method of matched asymptotics to construct the leading-order solution. For a core–shell gel, debonding can happen due to the swelling-induced stress. Our analytical solutions suggest that the best strategy for preventing this is to increase the initial stretch, since such an increase has no influence on the volume ratio but leads to the reduction of the stress. Also, how the geometry of the shell reduces the volume ratio is explicitly revealed. All the analytical results are also validated by numerical solutions.

Effect of bud scars on the mechanical properties of Saccharomyces cerevisiae cell walls

To determine the effect of bud scars on the mechanical properties of the walls of Saccharomyces cerevisiae cells, freshly cultivated stationary phase cells stained with Alexa Fluor 488 conjugated wheat germ agglutinin were sorted according to the number of bud scars using fluorescence-activated cell sorting (FACS). The groups were daughter cells with no bud scars, and mother cells separated further by number of scars (one, two and more than two). Cells with more than three scars were very rare.Compression testing by micromanipulation was used to determine key mechanical properties of the sorted cells. For all cells the force and fractional deformation at bursting could be determined. For 69% of cells overall but only 32% of daughter cells, a large strain mathematical model using a linear elastic constitutive equation for the wall material could be fitted to force deformation data up to cell wall failure. For these cells, the wall surface modulus, elastic modulus, initial stretch ratio and strain energy per unit volume at bursting could be estimated. For the remainder of the cells, the lack of permanent deformation on repeated compression and release (at deformations not causing bursting) suggested the cell wall material was non-linear elastic but with no observable plastic behaviour.This is the first report to show directly that bud scars affect the global mechanical properties of yeast cells and that the important distinction with respect to scars is between daughter and mother cells. The former were smaller with more elastic walls and a higher mean initial stretch ratio. For cells for which the model could be fitted, the mean circumferential strain at bursting decreased with scarring (consistent with stiffer walls) whilst the stress increased. This may be due to the reported absence of chitin in the walls of daughter cells.

Intramolecular signaling in tandem-GAF domains from PDE5 and PDE10 studied with a cyanobacterial adenylyl cyclase reporter

The dimeric mammalian phosphodiesterases (PDEs) are regulated by N-terminal domains. In PDE5, the GAF-A subdomain of a GAF-tandem (GAF-A and -B) binds the activator cGMP and in PDE10 GAF-B binds cAMP. GAF-tandem chimeras of PDE5 and 10 in which the 36 aa linker helix between GAF-A and -B was swapped lost allosteric regulation of a reporter adenylyl cyclase. In 16 consecutive constructs we substituted the PDE10 linker with that from PDE5. An initial stretch of 10 amino acids coded for isoform specificity. A C240Y substitution uncoupled cyclase activity from regulation, whereas C240F, L or G did not. The C240Y substitution increased basal activity to stimulated levels. Notably, over the next 12 substitutions basal cyclase activity decreased linearly.Further targeted substitutions were based on homology modeling using the PDE2 structure. No combination of substitutions within the initial 10 linker residues caused loss of regulation. The full 10 aa stretch was required. Modeling indicated a potential interaction of the linker with a loop from GAF-A. To interrupt H-bonding a glycine substitution of the loop segment was generated. Despite reduction of basal activity, loss of regulation was maintained. Possibly, the orientation of the linker helix is determined by formation of the dimer at the initial linker segment. Downstream deflections of the linker helix may have caused loss of regulation.

Florística e estrutura da comunidade arbórea de uma floresta estacional semidecidual secundária em Viçosa, MG

Este trabalho teve como objetivo estudar a composição florística e a estrutura da comunidade arbórea, bem como a distribuição das espécies em diferentes grupos ecológicos e os solos dos estádios inicial e avançado, de uma Floresta Estacional Semidecidual localizada no campus da Universidade Federal de Viçosa, em Viçosa, MG (20°46' S e 42°52' W). O levantamento foi realizado em meio hectare, onde foram alocadas 10 parcelas de 25 x 10 m em cada trecho, sendo amostrados todos os indivíduos com circunferência do tronco a 1,30 m do solo (CAP) > 15 cm. Registraram-se 820 indivíduos, sendo 440 no trecho de floresta inicial e 380 no de floresta avançada. No levantamento florístico da floresta inicial foram amostradas 76 espécies, pertencentes a 28 famílias, destacando-se como as de maior número de indivíduos Fabaceae (137), Urticaceae (45) e Sapindaceae (41) e, as com os maiores valores de importância, Cecropia glaziovii Snethl., Anadenanthera peregrina (L.) Speng., Piptadenia gonoacantha (Mart.) Macbr., Allophylus sericeus Radlk., Siparuna guianensis Aubl. e Maclura tinctoria (L.) Don ex Steud. O índice de diversidade de Shannon (H') foi de 3,57 nat.ind.-1 e a equabilidade (J'), de 0,82. Já na floresta avançada foram amostradas 59 espécies, distribuídas em 26 famílias, das quais Fabaceae (103), Meliaceae (49) e Flacourtiaceae (34) sobressaíram com maior número de indivíduos. Por sua vez, as espécies com os maiores valores de importância foram A. peregrina, Trichilia pallida Swartz, Cariniana estrellensis (Raddi) Kuntze., P. gonoacantha, Rollinia silvatica Mart. e S. guianensi. O índice de diversidade de Shannon (H') foi de 3,49 nat.ind.-1 e a equabilidade (J'), de 0,85. No contínuo, o índice de Shannon (H') e a equabilidade (J') foram de 3,82 nat.ind.-1 e 0,84, respectivamente. Cada floresta apresentou distinta identidade florística e estrutural, tendendo a ser minimizada com o avanço da sucessão.

Folding of Small Proteins Using Constrained Molecular Dynamics

The focus of this paper is to examine whether conformational search using constrained molecular dynamics (MD) method is more enhanced and enriched toward "native-like" structures compared to all-atom MD for the protein folding as a model problem. Constrained MD methods provide an alternate MD tool for protein structure prediction and structure refinement. It is computationally expensive to perform all-atom simulations of protein folding because the processes occur on a time scale of microseconds. Compared to the all-atom MD simulation, constrained MD methods have the advantage that stable dynamics can be achieved for larger time steps and the number of degrees of freedom is an order of magnitude smaller, leading to a decrease in computational cost. We have developed a generalized constrained MD method that allows the user to "freeze and thaw" torsional degrees of freedom as fit for the problem studied. We have used this method to perform all-torsion constrained MD in implicit solvent coupled with the replica exchange method to study folding of small proteins with various secondary structural motifs such as, α-helix (polyalanine, WALP16), β-turn (1E0Q), and a mixed motif protein (Trp-cage). We demonstrate that constrained MD replica exchange method exhibits a wider conformational search than all-atom MD with increased enrichment of near-native structures. "Hierarchical" constrained MD simulations, where the partially formed helical regions in the initial stretch of the all-torsion folding simulation trajectory of Trp-cage were frozen, showed a better sampling of near-native structures than all-torsion constrained MD simulations. This is in agreement with the zipping-and-assembly folding model put forth by Dill and co-workers for folding proteins. The use of hierarchical "freeze and thaw" clustering schemes in constrained MD simulation can be used to sample conformations that contribute significantly to folding of proteins.

Experimental study of cellular instability and extinction of non-premixed opposed-flow tubular flames

An experimental study of cellular instability of non-premixed opposed-flow tubular flames was conducted burning H 2 diluted with CO 2 flowing against air. The transitions to cellularity, cellular structures, and extinction conditions were determined as a function of the initial mixture strength, stretch rate, and curvature. The progression of cellular structures from the onset of cells through extinction was analyzed by flame imaging using an intensified CCD camera. Three different procedures of decreasing the Damköhler number (forward process), as well as using those same procedures in the opposite progression of increasing the Damköhler number (backward process) were completed. Significant flame hysteresis was seen and the forward transition occurred at a lower Damköhler number than the backward transition. Mechanical perturbations were conducted to show that the onset of cellularity could be realized at a higher Damköhler number than without perturbations. Once cellular instability was induced, it was possible to perturb the flame into multiple stable cellular states and extinguish the flame at a much higher Damköhler number than without perturbations. Images are shown of rotating cellular flames and a cellular instability regime at an initial mixture strength greater than unity and away from extinction conditions. A qualitative explanation of flame rotation and a general categorizing of three distinct flame regimes is given.