Influence Of Mechanical Loading Research Articles

Behaviour of metallic seals in CASTOR casks under normal and accident conditions of transport: qualification requirements

AbstractThis paper presents several BAM requirements for a sealing qualification process for metallic seals in transport casks. On qualification tests performed by GNS, advice will be given for the implementation of necessary investigations. This paper shows the results of special research by GNS which were used as proof concerning tightness behaviour of seals with aluminium and silver outer jackets under the influence of mechanical loading, particles and corrosion.

Sexual dimorphism and variations in mechanical loading influence humeral bone volume in the young adult skeleton

Sexual dimorphism and variations in mechanical loading influence humeral bone volume in the young adult skeleton

Cell–matrix interactions and dynamic mechanical loading influence chondrocyte gene expression and bioactivity in PEG-RGD hydrogels

The pericellular matrix (PCM) surrounding chondrocytes is thought to play an important role in transmitting biochemical and biomechanical signals to the cells, which regulates many cellular functions including tissue homeostasis. To better understand chondrocytes interactions with their PCM, three-dimensional poly(ethylene glycol) (PEG) hydrogels containing Arg–Gly–Asp (RGD), the cell-adhesion sequence found in fibronectin and which is present in the PCM of cartilage, were employed. RGD was incorporated into PEG hydrogels via tethers at 0.1, 0.4 and 0.8 mM concentrations. Bovine chondrocytes were encapsulated in the hydrogels and subjected to dynamic compressive strains (0.3 Hz, 18% amplitude strain) for 48 h, and their response assessed by cell morphology, ECM gene expression, cell proliferation and matrix synthesis. Incorporation of RGD did not influence cell morphology under free swelling conditions. However, the level of cell deformation upon an applied strain was greater in the presence of RGD. In the absence of dynamic loading, RGD appears to have a negative effect on chondrocyte phenotype, as seen by a 4.7-fold decrease in collagen II/collagen I expressions in 0.8 mM RGD constructs. However, RGD had little effect on early responses of chondrocytes (i.e. cell proliferation and matrix synthesis/deposition). When isolating RGD as a biomechanical cue, cellular response was very different. Chondrocyte phenotype (collagen II/collagen I ratio) and proteoglycan synthesis were enhanced with higher concentrations of RGD. Overall, our findings demonstrate that RGD ligands enhance cartilage-specific gene expression and matrix synthesis, but only when mechanically stimulated, suggesting that cell–matrix interactions mediate chondrocyte response to mechanical stimulation.

The influence of mechanical loading on osseointegration: an animal study

Osseointegration of implant provides a stable support for the prosthesis under functional loads. The timing of loading is a critical parameter that can govern the success of the osseointegration of implant. However, it is not clear whether the early loading can affect the success of osseointegration, or whether the no-loading healing period can be shortened. This paper presents an animal study conducted to investigate how external loads influence the osseointegration at the initial stage of healing. Titanium implants were inserted into the goat tibia laterally, and different axial loadings were applied to the implants in 4 weeks after surgery. After the 2 weeks period of early loading, animals were sacrificed and the tibia bones with the implants were cut off from the bodies. Then mechanical test was employed to find out the differences in the pull-out force, and shear strength at the bone-implant interface between the non-loaded and the loaded implants. The implant-bone interfaces were analyzed by histomorphometric method, SEM (scanning electron micrograph) and EDS (energy density spectrum). The results indicated that the bone-implant interface did not well integrate 4 weeks after surgery, and the fibrous tissue could be found at the interfaces of the specimens without loadings. While the results of loaded specimens with 10 N axial force showed that that parts of the interface were well integrated, indicating that the early mild loading may play a positive role in the process of the osseointegration. The results support that a certain range of external loading would influence the process of osseointegration, and appropriate mechanical loading can be applied to shorten the osseointegration period after surgery.

Novel epigenetic regulation of skeletal muscle myosin heavy chain genes. Focus on “Differential epigenetic modifications of histones at the myosin heavy chain genes in fast and slow skeletal muscle fibers and in response to muscle unloading”

skeletal muscle is a highly plastic tissue, capable of responding and adapting to a variety of physiological stimuli, e.g., muscle loading and unloading. Multiple functional phenotypes of skeletal muscle exist throughout the body at the same time. For example, muscles required for antigravity support of the skeleton or sustained locomotion are generally slow-contracting, low force-generating, and relatively resistant to fatigue, while other muscles required for quick, powerful movements are generally fast-contracting, high force-generating, and easily fatigued. In fact, skeletal muscle phenotypes have been characterized using combinations of muscle properties, including metabolic, fatigability, color, innervation, or predominant contractile protein expression. Myosin heavy chain (MHC) is the most abundant protein in skeletal muscle, comprising ∼25% of the total protein content, and is a major component of the complex responsible for generating contractile force in skeletal muscle. At least nine MHC isoforms, each transcribed from their own gene, exist in mammalian striated muscle; but only slow type I MHC and various isoforms of fast type II (IIa, IIx/d, and IIb MHC isoforms) are present in adult limb skeletal muscle that is not undergoing regeneration (16). It is well established that skeletal muscle phenotype, particularly MHC gene expression, can be altered by different states of muscular activity or inactivity, as well as various hormonal and metabolic factors (1).

Mechanical Stimulation Alters Pleiotrophin and Aggrecan Expression by Human Intervertebral Disc Cells and Influences Their Capacity to Stimulate Endothelial Cell Migration

The influence of mechanical load on pleiotrophin (PTM) and aggrecan expression by intervertebral disc (IVD) cells, and the effects of disc cell conditioned medium on endothelial cell migration was investigated. To examine possible interactions of mechanical loads and known pro- and antiangiogenic factors, which may regulate disc angiogenesis during degeneration. Pleiotrophin expression can be influenced by mechanical stimulation and has been associated with disc vascularization. Disc aggrecan inhibits endothelial cell migration, suggesting an antiangiogenic role. A possible interplay between these factors is unknown. The influence of the respective predominant load (cyclic strain for anulus fibrosus and hydrostatic pressure for nucleus pulposus cells) on PTN and aggrecan expression by IVD cells was determined by real-time RT-PCR and Western blotting (PTN only). The effects of IVD cell conditioned medium on endothelial cell migration were analyzed in a bioassay using human microvascular endothelial (HMEC-1) cells. Application of both mechanical loads resulted in significant alterations of gene expression of PTN (+67%, P = 0.004 in anulus cells; +29%, P = 0.03 in nucleus cells) and aggrecan (+42%, P = 0.03 in anulus cells, -25%, P = 0.03 in nucleus cells). These effects depended on the cell type, the applied load, and timescale. Conditioned media of nucleus pulposus cells enhanced HMEC-1 migration, but this effect was diminished after 2.5 MPa hydrostatic pressure, when aggrecan expression was diminished, but not 0.25 MPa, when expression levels were unchanged. Mechanical loading influences PTN expression by human IVD cells. Conditioned media from nucleus pulposus cell cultures stimulated HMEC-1 endothelial cell migration. This study demonstrates that the influence of mechanical loads on vascularization of the human IVD is likely to be complex and does not correlate simply with altered expression of known pro- and antiangiogenic factors.

Influence of load regime change on railway vehicle solid wheel reliability

This paper analyses the influence of mechanical loads on railway vehicle solid wheel reliability. Reliability calculations is carried out according to the specifically set methodology — reliability is calculated according to the data about working loads and critical stress distribution. The influences on solid wheel reliability that are analysed are the following: standard deviation of lateral force distribution and mathematical expectation and standard deviation of the vertical force.

Chloride Penetration into Concrete

Reinforced concrete structures may be exposed to an aggressive environment. In this case combined mechanical and environmental actions may act simultaneously and their synergetic influences have to be taken into consideration. In this contribution results of tests on two different types of concrete and mortars are presented and discussed. Capillary absorption of not carbonated and carbonated concrete has been determined. The influence of mechanical load on capillary suction has been studied separately. A moderate compressive load reduces the coefficient of capillary suction, while capillary suction is enhanced by higher mechanical loads as micro-cracks are formed. Chloride diffusion has been measured by means of the diffusion cell test. The diffusion coefficient observed in carbonated concrete is roughly speaking one order of magnitude higher than the value measured on not carbonated concrete.

Mechanical regulation of osteoclastic genes in human osteoblasts

Bone adaptation to mechanical load is accompanied by changes in gene expression of bone-forming cells. Less is known about mechanical effects on factors controlling bone resorption by osteoclasts. Therefore, we studied the influence of mechanical loading on several key genes modulating osteoclastogenesis. Human osteoblasts were subjected to various cell stretching protocols. Quantitative RT-PCR was used to evaluate gene expression. Cell stretching resulted in a significant up-regulation of receptor activator of nuclear factor-κB ligand (RANKL) immediate after intermittent loading (3×3h, 3×6h, magnitude 1%). Continuous loading, however, had no effect on RANKL expression. The expression of osteoprotegerin (OPG), macrophage-colony stimulating factor (M-CSF), and osteoclast inhibitory lectin (OCIL) was not significantly altered. The data suggested that mechanical loading could influence osteoclasts recruitment by modulating RANKL expression in human osteoblasts and that the effects might be strictly dependent on the quality of loading.

Parametric estimation study of interstrand conductance in multi-strand superconducting cables

Interstrand conductance is a key parameter to understand the current distribution and stability events in multi-strand superconducting cables. In this paper, a new approach employing the parameter estimation method from system identification theory is applied to estimate the interstrand conductance from existing current distribution model based on experimental data of voltage differences at cable ends. Based on transient voltage measurements at cable ends this method estimates interstrand conductance conveniently and accurately under different conditions (temperature, cable length, cable compaction, etc.). The details of interstrand conductance between all combinations of sub-cables at different cabling stages were obtained. The influence of mechanical load on interstrand conductance was also studied. The experimental data sheds new light on how the mechanics of cable compaction and movement under simulated Lorentz load affects the electrical parameters, namely the interstrand conductance. The data are useful input for cable stability simulations and AC loss estimation, and the experimental method can be used to better characterize cables prior to magnet winding.

Viscoelastic properties of human mesenchymally-derived stem cells and primary osteoblasts, chondrocytes, and adipocytes

The mechanical properties of single cells play important roles in regulating cell-matrix interactions, potentially influencing the process of mechanotransduction. Recent studies also suggest that cellular mechanical properties may provide novel biological markers, or “biomarkers,” of cell phenotype, reflecting specific changes that occur with disease, differentiation, or cellular transformation. Of particular interest in recent years has been the identification of such biomarkers that can be used to determine specific phenotypic characteristics of stem cells that separate them from primary, differentiated cells. The goal of this study was to determine the elastic and viscoelastic properties of three primary cell types of mesenchymal lineage (chondrocytes, osteoblasts, and adipocytes) and to test the hypothesis that primary differentiated cells exhibit distinct mechanical properties compared to adult stem cells (adipose-derived or bone marrow-derived mesenchymal stem cells). In an adherent, spread configuration, chondrocytes, osteoblasts, and adipocytes all exhibited significantly different mechanical properties, with osteoblasts being stiffer than chondrocytes and both being stiffer than adipocytes. Adipose-derived and mesenchymal stem cells exhibited similar properties to each other, but were mechanically distinct from primary cells, particularly when comparing a ratio of elastic to relaxed moduli. These findings will help more accurately model the cellular mechanical environment in mesenchymal tissues, which could assist in describing injury thresholds and disease progression or even determining the influence of mechanical loading for tissue engineering efforts. Furthermore, the identification of mechanical properties distinct to stem cells could result in more successful sorting procedures to enrich multipotent progenitor cell populations.

Influence of Mechanical Loading on Resonance Frequency Analysis and Trabecular Structure of Peri-implant Bone

Purpose: The objective of this study was to clarify the effect of mechanical loading on the trabecular structure of peri-implant bone and determine its correlation with the implant stability quotient (ISQ) value.Methods: This study used 3 immunologically mature male dogs. Three months after bilateral extraction of the mandibular premolars of the dogs, 4 implants were placed in these sites for each dog. After 3 months to allow healing of the implant sites, superstructures were prepared on the right side (loaded side). The implants on the left side were covered with mucosa (non-loaded side). The ISQ values of all implants were measured 3 months after placing the superstructures; subsequently, the animals were sacrificed. After the mandible was excised and fixed, micro-CT images were acquired and three-dimensional analysis, performed.Results: The means and S.D. of the ISQ value for the 12 implants were 69.6±6.2 and 62.9±13.8 for the loaded and non-loaded sides, respectively. No significant difference was noted between the 2 sides. Mean and S.D. of bone volume fraction (BV/TV) were 13.0±6.6 and 9.3±3.8; trabecular thickness (Tb.Th), 97.7±27.7 µm and 91.0 ± 24.0 µm; and trabecular number (Tb.N), 1.24±0.30/mm and 0.99±0.20/mm for the loaded and non-loaded sides, respectively. BV/TV and Tb.N values were higher for the loaded side.Conclusion: Morphological changes occurred in response to functional loading. However, lamellar bone was observed around the implants on both sides. These findings revealed that changes in the bone structure due to functional loading of implants had negligible or no effect on the ISQ value.

Artificial Ground Freezing of Fully Saturated Soil: Thermal Problem

Freezing of water in porous media such as soil results in a change of mechanical and thermal properties. This change of properties is exploited during artificial ground freezing (AGF) employed in tunneling, during open excavations, and for retaining structures. In this paper, a phase-change model for the simulation of freezing of fully saturated soil is presented. Disregarding the influence of mechanical loading on the freezing process, only the solution of the thermal problem, i.e., determination of the temperature field, is addressed. The (macroscopic) thermal properties are related to the material parameters of the different material phases, such as soil particles, water, and ice. This approach allows us to minimize the number of input parameters of the phase-change model and to account for the change of thermal properties in the course of the freezing process. The performance of the proposed model is illustrated by the reanalysis of freezing experiments and of an AGF construction site in Vienna, Austria.

Mechanical regulation of HB-GAM expression in bone cells

Bone adaption upon mechanical stimulation is accompanied by changes in gene expression. In this context we investigated the influence of mechanical loading on heparin binding growth associated molecule (HB-GAM) expression, an extracellular matrix molecule which in cell culture has been shown to stimulate the differentiation of osteoblasts. We obtained information on the participating signal transduction pathways using a mitogenic loading regimen. Specific inhibitors of various signal transduction pathways were added to loaded cells and to unloaded controls. By semi-quantitative PCR studies we demonstrated a rapid decrease of HB-GAM expression in primary osteoblasts and SaOs-2 cells by 20–30% upon mechanical loading within 30min. We showed that the RGD–integrin interaction is involved in the regulation of HB-GAM expression. Furthermore, integrity of the cytoskeleton, stretch-activated, and voltage-sensitive Ca2+ channels as well as gap junctional communication are necessary for the downregulation of HB-GAM expression by mechanical loading.

A thermodynamically motivated implicit gradient damage framework and its application to brick masonry cracking

A new gradient damage model is derived starting from thermodynamic statements. Nonstandard terms are added to the free energy potential for this purpose. In contrast with existing approaches, these terms are related to the kinematics of the problem and therefore come into play already in the elastic regime. Following the argument that the second law of thermodynamics need only be satisfied globally for a nonlocal continuum, an energy balance equation is derived which must be solved together with the usual equilibrium equations. Numerical solutions of this coupled set of equations have been generated for a one-dimensional problem in order to study the localisation behaviour of the new model. In the limit where the elastic modulus related to the energy balance goes to 0, an earlier gradient damage model is retrieved. This simplified model is used to study cracking of brick masonry under the influence of mechanical loading.

Crosslinking density influences the morphology of chondrocytes photoencapsulated in PEG hydrogels during the application of compressive strain

Chondrocyte deformation, which occurs during mechanical loading, is thought to play an important role in the mechanotransduction pathway. In designing a scaffold that can be gelled in situ for cartilage tissue engineering, an important consideration is the influence of mechanical loading. This study tested the hypothesis that changes in the crosslinking density of a hydrogel scaffold influence the morphology of encapsulated chondrocytes in response to an applied load. Chondrocytes were entrapped in photocrosslinkable hydrogel scaffolds based on poly(ethylene glycol) (PEG) with two crosslinking densities, 0.119 and 0.376 mol/l, with the higher density having a 11-fold higher compressive modulus. The cell-embedded hydrogels were subjected to static compressive strains between 0% and 20% after 1 and 6 days of culture. Using confocal laser scanning microscopy, chondrocytes in the highly crosslinked gel at day 1 deformed more than gels in the more loosely crosslinked gel. By day 6, this finding was reversed. When single cells within a region were followed, heterogeneities in cell deformation were observed on both a macroscopic and microscopic scale. These heterogeneities were greater in the highly crosslinked gel. These findings demonstrate that different levels of cell deformation and heterogeneity may be obtained by varying the crosslinking density in PEG hydrogels.

Mechanobiological predictions of femoral anteversion in cerebral palsy.

Many morphological changes occur during development of the proximal femur. The anteversion angle is a measure of the rotation of the neck of the femur around the diaphysis. In normal development anteversion is 30 degrees at birth and decreases to 15 degrees by skeletal maturity. In children with cerebral palsy (CP) anteversion often increases slightly and remains high throughout development. Previous models have proposed that cyclic hydrostatic stress decreases the growth rate while cyclic octahedral shear stress increases the growth rate. In this study we also examine changes in the growth direction caused by deformation of the developing cartilage. Using these mechanobiological principles we considered the influence of mechanical loads on the formation of the anteversion angle in normal and CP development. Loads were applied to a three-dimensional finite element model of the proximal femur. From the resulting stresses and deformations at the growth front we calculated the growth rate and growth direction and simulated the progression of the growth front over 6 months. The model predicted a decrease in anteversion angle (-2 degrees over 6 months) under normal-loading conditions, and an increase in anteversion (+ 1 degrees over 6 months) under CP-loading conditions. These results compare well with observations during skeletalgenesis, in which the anteversion angle decreases rapidly in the first few years of normal growth and may increase in children with CP.

Analysis and Experiment of Gas Leakage Through Composite Laminates for Propellant Tanks

The mechanism of the through-thickness gas leakage of carbon fiber-reinforced plastics (CFRP) laminates is investigated in view of propellant leaks for composite tanks of reusable launch vehicles. In this study analysis of leakage caused by the existence of matrix cracks acting as the chain of leakage paths is developed under the simple assumption that conductance for leakage is a function of crack-opening displacements. The analytical results in consideration of mechanical and thermal loads are compared with experimental results, which are measured as helium gas leaks through carbon fiber-reinforced plastics laminates containing matrix cracks at room temperature. Good agreement between the analytical and experimental data is confirmed. Numerical analysis based on the proposed method can be used to evaluate the influence of mechanical loads on propellant leak through CFRP cross-ply laminates. The analytical calculations show that the increase of propellant leakage can be caused by the enlargement of crack-opening displacements caused by mechanical and thermal loads, increase of crack density, and decrease of temperature.

The Influence Of Deterioration On The Lifetime Of Timber Structures

Decay and drying cracks affect the strength of historic timber structures. Because of the traditional timber dimensions used, drying cracks are often large, decreasing the effective cross section of members. Deterioration can also be caused by biological decay, depending on wood species, climatic conditions, structural detailing, etc. Decay can affect the load carrying capacity, having considerable economic and cultural consequences. However, when the decay is no longer active and the damage is limited, repair is not always necessary. When the reliability of the structure is above a certain minimum acceptance level, it is still able to fulfil its task and can be maintained in the original state as much as possible. A method to determine the safety of deteriorated structures is lifetime modelling using damage accumulation models. It allows for the assessment of structures, combining both the influence of mechanical loads (live, wind and snow loads) as well as reduced strength values caused by cracks or decay. Some of the parameters of the model have been determined on the basis of time to failure tests on timber and timber joints, already lasting 40 years. The influence of cracks and decay on the strength of timber members is presented and the consequences for the lifetime expectancy of structures are shown. The model accounts for the level of decay, which is generally determined via in-situ measurements, the rate of decay, as well as the residual strength of decayed timber. The latter two are generally determined in laboratory or field experiments. Different safety factors can be used in the model and future maintenance schemes can be developed. Practical examples are given dealing with cracked beams and with the residual lifetime of decayed piles.

Mechanosensitive induction of apoptosis in fibroblasts is regulated by thrombospondin-1 and integrin associated protein (CD47).

Fibroblasts are cultured in three-dimensional collagen matrices to investigate the effect of mechanical tension on the regulation of apoptosis. Under the influence of mechanical loading, the cells show little apoptosis whereas releasing of tension leads to an increase up to tenfold during the first 24 h and remains constant for further 48 h. An autocrine loop of the integrin alpha(V)beta(3)/CD47 receptor complex and thrombospondin-1 is identified as the molecular coupling device between mechanical loading and apoptosis: The integrin alpha(V)beta(3) is expressed under mechanical loading as well as unloading whereas the CD47 could only be identified after the release of tension. The secreted thrombospondin binds to the active receptor and induces apoptosis. The presented mechanosensitive regulation of apoptosis in fibroblast cultures could be an essential mechanism for the regression of the granulation tissue by apoptosis in the process of wound healing.