Role of shear stress and stretch in vascular mechanobiology

Blood vessels are under constant mechanical loading from blood pressure and flow which cause internal stresses (endothelial shear stress and circumferential wall stress, respectively). The mechanical forces not only cause morphological changes of endothelium and blood vessel wall, but also trigger biochemical and biological events. There is considerable evidence that physiologic stresses and strains (stretch) exert vasoprotective roles via nitric oxide and provide a homeostatic oxidative balance. A perturbation of tissue stresses and strains can disturb biochemical homeostasis and lead to vascular remodelling and possible dysfunction (e.g. altered vasorelaxation, tone, stiffness, etc.). These distinct biological endpoints are caused by some common biochemical pathways. The focus of this brief review is to point out some possible commonalities in the molecular pathways in response to endothelial shear stress and circumferential wall stretch.     

冷弯薄壁型钢组合墙体抗剪性能试验研究

为推动冷弯薄壁型钢结构体系由低层向多层发展,解决传统剪力墙抗剪承载力不足、端柱易发生屈曲和局部承压破坏的技术难题,提出了冷弯薄壁型钢钢管端柱蒙皮钢板剪力墙的新型墙体构造形式,以实现“强端柱、弱墙板”的设计原则。通过水平低周往复加载试验对剪力墙的抗剪性能进行研究,试验结果表明：钢管端柱蒙皮钢板剪力墙的破坏形式为钢板周边螺钉连接破坏导致蒙皮作用失效,钢管端柱未见压屈。减小剪力墙周边自攻螺钉的间距,使钢板拉力带得到充分发展,可提高剪力墙的抗剪承载力、抗侧刚度、延性以及耗能能力。对钢管—钢板自攻螺钉连接试件的抗剪性能进行试验研究,结果表明：螺钉连接试件的破坏特征为钢板的孔壁承压破坏,自攻螺钉群的抗剪承载力具有“群体折减效应”。基于剪力墙和自攻螺钉连接试件的抗剪试验结果,提出了钢管端柱蒙皮钢板剪力墙的抗剪承载力和抗侧刚度的计算方法。研究成果为多层冷弯薄壁型钢结构体系的理论研究和工程应用提供可靠依据。

     

Experimental study on shear resistance of reactive powder concrete beams without stirrups

Abstract

The tests results of 18 longitudinally reinforced reactive powder concrete (RPC) beams without stirrups and subjected to combined flexural and shear are presented in this paper. The main test variables were the ratio of the shear span-to-effective depth (a/d), the ratio of the longitudinal reinforcement (ρ_w), the percentage of steel fibers volume fractions (V_f) and the percentage of silica fume (SF) powder. The findings of this paper reveal that addition of steel fibers into the RPC mixture does not considerably affect the initial diagonal cracking load but it influences the ultimate load capacity. For all tested fibrous RPC beams, the ratio of the ultimate shear force to diagonal cracking force (V_u∕V_cr) have an average ratio of about 2.5. The shear design equations recommended by the (ACI 318-14) code, (CSA A23.3-04) code, (NZS 3101-06) code and (BS 8110-97) code have been modified to predict the ultimate shear strength of RPC beams without stirrups. The suggested equations gave satisfied predictions for the shear strength of the tested RPC beams with coefficients of variation (COV) ranging from 0.09049 to 0.1817.     

Experimental and theoretical investigation of the shear resistance of steel fibre reinforced prestressed concrete X-beams—Part I: Experimental work

This is the first part of two papers on the experimental (Part I) and theoretical (Part II) resistance of steel fibre reinforced precast concrete beams. Short steel fibres have been introduced into prestressed concrete X beams in order to study their behaviour under shear loads. The X beams, which have circular web profiles, were chosen to represent longitudinal sections from 215 mm deep prestressed precast hollow cored floor units, which are known to fail in shear in a brittle manner. No shear links were used. Round hooked end high strength steel (HS), and thin amorphous metal (AM) fibres were used in volume fractions up to 2.0%. The maximum flexural strength of fibre reinforced concrete (FRC) was 10.28 N/mm², some 50% greater than plain concrete. In the plain concrete beams the ratio η of the ultimate shear resistance to the cracking resistance was 1.0, as expected. For the fibre beams η=1.43 to 1.52 for the HS fibres and η-1.23 for the AM fibres. Theoretical and empirical equations were developed using modified FRC principal tensile stress methods to predict ultimate shear strength and are given in Part II.

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Résumé Il s'agit de la première partie d'un article sur la résistance des poutres en béton préfabriqué renforcé de fibres d'acier, présentant les expériences (partie I) et la théorie (partie II). De courtes fibres d'acier ont été utilisées dans des poutres X précontraintes pour étudier leur comportement sous l'action de forces de cisaillement. Les poutres X, qui ont des ames de profil circulaire, ont été choisies pour représenter des sections longitudinales des dalles alvéolées précontraintes de 215 mm d'épaisseur, qui sont connues pour leur mode de rupture casant sous l'action des efforts tranchants. L'armature de cisaillement n'a pas été utilisée. Des crochets ronds en acier à haute adhérence et de minces fibres métalliques amonrphes ont été utilisés jusqu'à des fractions de 2% du volume. La résistance à la tension maximale sous l'effet de flexion du béton de fibres d'acier était de 10,28 N/mm², soit 50% de plus que pour le béton normal. Pour les poutres en béton normal, le rapport η entre la résistance ultime au cisaillement et la résistance à la fissuration était de 1,0, comme prévu. Pour les poutres en fibres d'acier ce rapport était entre 1,43 et 1,52 pour l'acier à haute adhérence et 1,23 pour les fibres métalliques amorphes. Des équatins théoriques et empiriques ont été développées à l'aide de méthodes de contrainte de tension principlale du béton de fibres d'acier modifiées pour la prédiction de la résistance ultime au cisaillement; celles-ci sont données dans la deuxième partie.

     

An in vivo comparative study of the e-polytetrafluoroethylene vascular prostheses: Vitaflon and Gore-Tex

A comparative study was performed in order to validate new Russian e-PTFE vascular prostheses Vitaflon (St. Petersburg, Russia). The Gore-Tex prostheses were chosen as a referential model. The prostheses were implanted in the venous and arterial positions in 13 dog experiments. After the implantation time was over a comprehensive histological and histochemical examination of excized specimens was performed. It was demonstrated that there is no difference in healing and functional properties between the two studied prostheses.     

Experimental studies on CO2 corrosion of rubber materials for packer under compressive stress in gas wells

The corrosion performance of rubber materials exposed to high temperature high pressure CO₂ environment has been investigated under a compressive state. The dissimilarity of attack was compared in two exposure conditions i.e. gaseous CO₂ and liquid CO₂ under different compressive stress by mechanical properties testing and morphology analysis. Experimental comparisons showed that rubber samples exposed to liquid CO₂ exhibited greater reactivity as compared to gaseous CO₂. The existence of compressive stress would aggravate the corrosion of CO₂ to rubber materials. Factors that affect the corrosion process have been discussed to evaluate corrosion behavior of the packer rubber materials.     

Effect of particle size and boundary conditions on the shear stress in an annular shear cell

The effect of particle size and boundary geometry in granular shear flows is investigated. The measured shear stress of glass spheres in an annular shear cell experiment is reported. In order to explore the particle size effect, the experiments are run using four different particle diameters, d = 2, 3, 4, and 5 mm. It is found that the shear stress follows the Bagnold scaling with respect to the apparent shear rate, but deviates from it with respect to particle size. For high solids concentration the results deviate qualitatively from the kinetic theory for bounded granular shear flows, where the non-dimensional shear stress measured with large particles exceeds that measured for small particles by as much as one order of magnitude. The effect of the boundary geometry is explored by using three different boundary types; type 1 employs aluminum radial half-cylinders, type 2 employs aluminum hemispheres arranged in a polar hexagonal closed packed configuration, and type 3 employs sandpaper. It is shown that the geometry of the boundary has an insignificant effect on dilute flows of small particles. For denser flows and/or larger particles the difference is evident. The sandpaper boundary, which is different from the aluminum ones both in geometry and in its material properties, yields the lowest stress. These results imply that in granular materials-structure interaction, the structure’s properties are just as important as the properties of the granular material. Their interaction may also depend on the relative size between the structure and the grain size.     

Thermochemical studies of surface-induced activation in human granulocytesin vitro

The technique of microcalorimetry has been extensively applied in studies of metabolic processes in human blood cells in suspension. In contrast to other types of blood cells, human granulocytes and other phagocytic cells are activated by contact with the stainless steel vessel in which the sample under study is enclosed. The activation is characterized by the production of increased quantities of heat energy with variable time-dependent profiles. The present study demonstrates that lining the stainless steel vessels with fluoroethylene-polypropylene will completely eliminate the contact activation of granulocytes. It is shown that this modified procedure enhances the quality of microcalorimetry as a technique for the measurement of granulocyte metabolism and function under physiological conditionsin vitro.     

Experimental studies on shear failure of freeze-bonds in saline ice: Part I. Set-up, failure mode and freeze-bond strength

The strength of freeze-bonds in thin saline ice has been investigated through two series (in 2008 and 2009) of experiments in the Hamburg Ship Model Basin (HSVA) as a function of the normal confinement (σ), the submersion time (Δt) and the initial ice temperature (T_i). The freeze-bonds were mostly formed in a submerged state, but some were also formed in air. The experimental set-up was improved in the 2009 experiments. In 2008 a ductile-like failure mode dominated (78%), whereas in 2009 the brittle-like dominated (93%). We suggest that this is a combined ice and test set-up effect. The 2009 experimental procedures allowed for careful sample handling giving higher strength and it was softer. Both these things should provoke a more brittle-like force-time response. The average freeze-bond strength in brittle-like samples was around 9 kPa while in ductile-like samples was around 2 kPa. The maximum freeze-bonds strength were measured for short submersion times, from 1 to 20 min, and reached a maximum value of 30 kPa.A Mohr-Coulomb like failure model was found appropriate to represent the freeze-bond shear strength as function of the normal confinement. Saline freeze-bonds in saline water had cohesion/friction angle around 4 and 1.4 kPa/25° for the brittle- and ductile-like samples respectively, which fitted well with previously published data.A bell-shape dependence for τ_c vs. Δt was found, which agreed with the predictions by Shafrova and Høyland (2007). We suggest that this is essentially a freeze-bond porosity effect and propose three phases in time with subsequent cooling, heating and equilibrium to account for this trend. Qualitative experiments showed that the submersion time and the initial ice temperature were strongly coupled.To account for the connection between contact time, block dimensions and ice properties and the freeze-bond strength, dimensionless number were used. Fourier scaling was more appropriate than Froude scaling to scale freeze-bonds.The freeze-bonding made in air developed fast (in less than 30 s) when the ice was cold and dry, but no freeze-bonding occurred for the same contact times when the ice was warm and wet.     

Collagen- and gelatine-based films sealing vascular prostheses: evaluation of the degree of crosslinking for optimal blood impermeability

The stiffness as well as the biodegradation rate of collagen and gelatine products can be modulated by performing a number of crosslinking treatments. In many biomedical applications, an optimal degree of crosslinking seems to exist, depending on the mechanical and/or biosynthesis properties of the host site. The aim of this study was to evaluate the optimal degree of crosslinking of collagen and gelatine films, to be used as sealants for vascular prostheses. Various crosslinking treatments, including exposure to aldehydes, dehydrothemal treatment, carbodiimide crosslinking and combinations of them, were performed on collagen and gelatine films, and the resulting increases in stiffness, degree of crosslinking and denaturation temperature were evaluated. Analogue crosslinking treatments were also performed on sealed prostheses, which were then tested for blood leakage. The experimental results showed that a good blood impermeability of both collagen and gelatine films was obtained for crosslinking density of about 1.2–1.3 × 10⁻⁵ mol/cm³, which could be yielded by a dehydrothermal crosslinking treatment (DHT). In particular, dehydrothermally treated gelatine-coated prostheses were found to perform better than analogue collagen-coated ones. The presence of glycerol in crosslinked collagen films was found to have plasticizing effects, which are likely to facilitate blood impermeability, and to increase the thermal stability of collagen.     

Integrated microdevice for long-term automated perfusion culture without shear stress and real-time electrochemical monitoring of cells

Electrochemical techniques based on ultramicroelectrodes (UMEs) play a significant role in real-time monitoring of chemical messengers' release from single cells. Conversely, precise monitoring of cells in vitro strongly depends on the adequate construction of cellular physiological microenvironment. In this paper, we developed a multilayer microdevice which integrated high aspect ratio poly(dimethylsiloxane) (PDMS) microfluidic device for long-term automated perfusion culture of cells without shear stress and an independently addressable microelectrodes array (IAMEA) for electrochemical monitoring of the cultured cells in real time. Novel design using high aspect ratio between circular "moat" and ring-shaped micropillar array surrounding cell culture chamber combined with automated "circular-centre" and "bottom-up" perfusion model successfully provided continuous fresh medium and a stable and uniform microenvironment for cells. Two weeks automated culture of human umbilical endothelial cell line (ECV304) and neuronal differentiation of rat pheochromocytoma (PC12) cells have been realized using this device. Furthermore, the quantal release of dopamine from individual PC12 cells during their culture or propagation process was amperometrically monitored in real time. The multifunctional microdevice developed in this paper integrated cellular microenvironment construction and real-time monitoring of cells during their physiological process, and would possibly provide a versatile platform for cell-based biomedical analysis.     

The effect of remodelling and contractility of the actin cytoskeleton on the shear resistance of single cells: a computational and experimental investigation

The biomechanisms that govern the response of chondrocytes to mechanical stimuli are poorly understood. In this study, a series of in vitro tests are performed, in which single chondrocytes are subjected to shear deformation by a horizontally moving probe. Dramatically different probe force–indentation curves are obtained for untreated cells and for cells in which the actin cytoskeleton has been disrupted. Untreated cells exhibit a rapid increase in force upon probe contact followed by yielding behaviour. Cells in which the contractile actin cytoskeleton was removed exhibit a linear force–indentation response. In order to investigate the mechanisms underlying this behaviour, a three-dimensional active modelling framework incorporating stress fibre (SF) remodelling and contractility is used to simulate the in vitro tests. Simulations reveal that the characteristic force–indentation curve observed for untreated chondrocytes occurs as a result of two factors: (i) yielding of SFs due to stretching of the cytoplasm near the probe and (ii) dissociation of SFs due to reduced cytoplasm tension at the front of the cell. In contrast, a passive hyperelastic model predicts a linear force–indentation curve similar to that observed for cells in which the actin cytoskeleton has been disrupted. This combined modelling–experimental study offers a novel insight into the role of the active contractility and remodelling of the actin cytoskeleton in the response of chondrocytes to mechanical loading.     

Interfacial shear stress distribution in model composites: the effect of fibre modulus

The micromechanics of reinforcement have been investigated for a continuous intermediate-modulus (IM) carbon fibre embedded in an epoxy resin (MY-750). The embedded single-fibre (fragmentation) geometry was employed as the loading configuration. A laser Raman spectroscopic method was used to obtain the fibre strain distribution along the embedded fibre fragments, at various levels of applied strain. The interfacial shear stress distribution along the fibre was derived through a balance of forces analysis.A number of parameters, such as the maximum interfacial shear stress at each level of applied strain and the fibre debonded length, were evaluated. The maximum interfacial shear stress of the IM fibre system was found to increase by 80%, compared with the high-modulus fibre system examined previously, while the distance from the fibre end where the interfacial shear stress maximizes was significantly shorter. The debonded length was found to increase only marginally up to an applied strain of 1.8%, followed by a dramatic rate of increase between 1.8% and 2.5% of applied strain.     

State of the art: interface shear resistance of asphalt surface layers

Interface shear resistance is a measure of the bonding between two layers under shear loading. Adequate interface shear resistance and long-term bonding of the surface to the underlying pavement are critical to the performance of pavement structures. Interface shear strength is a function of adhesion, friction and aggregate embedment or interlock and is commonly modelled as a Mohr–Coulomb type envelope. Measurement of interface shear resistance can be performed in the field on full-scale pavements, in the laboratory on cores recovered from the surface or in the laboratory using samples prepared in the laboratory. However, laboratory testing of cores recovered from the field is likely to be more reliable and repeatable than field testing. There is a large range of test methods and procedures for the measurement of interface bond. These test methods are generally grouped into three main loading mechanisms; axial tension, torsional shear and direct shear. Direct shear tests offer a more comprehensive assessment of the full interface strength. The interface’s resistance to shear can be characterised by its strength, modulus/stiffness or work/energy. The results are affected by the test protocol, tack coat type and application rate, test temperature, applied normal stress and rate of loading, interface condition and post-construction trafficking. Of these, the test temperature is the most influential factor. A number of studies have reported contradictory and conflicting conclusions with regard to the importance of various factors and conditions on the different measures of interface shear resistance. Such inconsistent findings likely stem from the complicated interaction between the various interface conditions and testing protocols. The fundamental factors affecting monotonic interface strength are now reasonably well understood. The focus of future research is expected to be on shear fatigue performance of interfaces.     

方钢管混凝土框架-薄钢板剪力墙边框柱刚度研究

为研究大跨高比的对角槽钢加劲钢板墙结构,该文对3个1/3缩尺的钢板剪力墙试件进行了拟静力试验研究,包括一个拼接式钢板剪力墙和2个拼接式-对角槽钢加劲钢板剪力墙。试验结果表明钢板剪力墙有良好的耗能能力,对角加劲钢板墙滞回曲线饱满呈梭形。槽钢的两个翼缘与钢板连接,形成具有更大抗扭刚度闭口截面,在加载过程中避免了加劲肋的扭转而导致加劲效果降低。对角布置的槽钢加劲肋具有较大的抗弯刚度,在弹性阶段提高钢板的弹性屈曲荷载,限制钢板平面外变形;在弹塑性阶段能起到增大拉力带的作用,提高结构承载力。推导了框架柱的剪力、轴力和弯矩计算公式,结果表明对角槽钢加劲形式对边缘构件的附加轴力和剪力作用较大,因此在设计时应考虑加劲肋的支撑作用。     

Endothelial cell culture model for replication of physiological profiles of pressure, flow, stretch, and shear stress in vitro

The phenotype and function of vascular cells in vivo are influenced by complex mechanical signals generated by pulsatile hemodynamic loading. Physiologically relevant in vitro studies of vascular cells therefore require realistic environments where in vivo mechanical loading conditions can be accurately reproduced. To accomplish a realistic in vivo-like loading environment, we designed and fabricated an Endothelial Cell Culture Model (ECCM) to generate physiological pressure, stretch, and shear stress profiles associated with normal and pathological cardiac flow states. Cells within this system were cultured on a stretchable, thin (～500 μm) planar membrane within a rectangular flow channel and subject to constant fluid flow. Under pressure, the thin planar membrane assumed a concave shape, representing a segment of the blood vessel wall. Pulsatility was introduced using a programmable pneumatically controlled collapsible chamber. Human aortic endothelial cells (HAECs) were cultured within this system under normal conditions and compared to HAECs cultured under static and "flow only" (13 dyn/cm(2)) control conditions using microscopy. Cells cultured within the ECCM were larger than both controls and assumed an ellipsoidal shape. In contrast to static control control cells, ECCM-cultured cells exhibited alignment of cytoskeletal actin filaments and high and continuous expression levels of β-catenin indicating an in vivo-like phenotype. In conclusion, design, fabrication, testing, and validation of the ECCM for culture of ECs under realistic pressure, flow, strain, and shear loading seen in normal and pathological conditions was accomplished. The ECCM therefore is an enabling technology that allows for study of ECs under physiologically relevant biomechanical loading conditions in vitro.