Shear Stress Levels Research Articles

Flow characteristics and hemolytic performance of the new Breethe centrifugal blood pump in comparison with the CentriMag and Rotaflow pumps.

Blood pumps have been increasingly used in mechanically assisted circulation for ventricular assistance and extracorporeal membrane oxygenation support or during cardiopulmonary bypass for cardiac surgery. However, there have always been common complications such as thrombosis, hemolysis, bleeding, and infection associated with current blood pumps in patients. The development of more biocompatible blood pumps still prevails during the past decades. As one of those newly developed pumps, the Breethe pump is a novel extracorporeal centrifugal blood pump with a hybrid magnetic and mechanical bearing with attempt to reduce device-induced blood trauma. To characterize the hydrodynamic and hemolytic performances of this novel pump and demonstrate its superior biocompatibility, we use a combined computational and experimental approach to compare the Breethe pump with the CentriMag and Rotaflow pumps in terms of flow features and hemolysis under an operating condition relevant to ECMO support (flow: 5 L/min, pressure head: ~350 mmHg). The computational results showed that the Breethe pump has a smaller area-averaged wall shear stress (WSS), a smaller volume with a scalar shear stress (SSS) level greater than 100 Pa and a lower device-generated hemolysis index compared to the CentriMag and Rotaflow pumps. The comparison of the calculated residence times among the three pumps indicated that the Breethe pump might have better washout. The experimental data from the in vitro hemolysis testing demonstrated that the Breethe pump has the lowest normalized hemolysis index (NIH) than the CentriMag and Rotaflow pumps. It can be concluded based on both the computational and experimental data that the Breethe pump is a viable pump for clinical use and it has better biocompatibility compared to the clinically accepted pumps.

Stress and input energy analyses of shearing a particle bed under a centrifugal field

Effective shearing of fine powders for surface modification and dry coating requires a compressed powder bed to prevent aeration. A device known as Mechanofusion® provides the necessary conditions using a centrifugal field, under which submicron particles are coated onto coarser host particles by shearing under great compressive forces at very high strain rates. This creates composite powders with enhanced properties, such as good flowability, surface functional effects, etc. Understanding the dynamics of particle motion, required level of shear stresses and input energy is a prerequisite for process optimisation. Here, numerical simulation by the Discrete Element Method is carried out, analysing features which are otherwise difficult to obtain by experimental work, e.g. the determination of the thickness of the zone in which particles experience shearing strains, as necessary for functional effect, compressive forces on particles, and input energy. Correlations are proposed for the energy input requirement and stresses experienced by the device as a function of rotational speed.

The Role of Elevated Wall Shear Stress in Progression of Pulmonary Vein Stenosis: Evidence from Two Case Studies

Pulmonary vein stenosis is a serious condition characterized by restriction or blockage due to fibrotic tissue ingrowth that develops in the pulmonary veins of infants or children. It is often progressive and can lead to severe pulmonary hypertension and death. Efforts to halt or reverse disease progression include surgery and catheter-based balloon dilation and stent implantation. Its cause and mechanism of progression are unknown. In this pilot study, we propose and explore the hypothesis that elevated wall shear stress at discrete pulmonary venous sites triggers stenosis. To assess this theory, we retrospectively analyzed cardiac catheterization, lung scan, and X-ray computed tomography data to estimate wall shear stress in the pulmonary veins at multiple time points during disease progression in two patients. Results are consistent with the existence of a level of elevated wall shear stress above which the disease is progressive and below which progression is halted. The analysis also suggests the possibility of predicting the target lumen size necessary in a given vein to reduce wall shear stress to normal levels and remove the trigger for stenosis progression.

Undrained Cyclic Response and Resistance of Saturated Calcareous Sand considering Initial Static Shear Effect

Sand elements in the natural or manmade field have often undergone initial static shear stresses before suffering cyclic loading. To explore the effect of static shear stress, a series of undrained cyclic triaxial tests were performed on dense and loose calcareous sand under different initial and cyclic shear stresses. The triaxial test results are used to describe the effect of static shear stress on the cyclic response of the calcareous sand with different relative density. Cyclic mobility, flow deformation, and residual deformation accumulation are the three main failure modes under varying static and cyclic shear stress levels. The cyclic resistance of dense sand is greater than that of loose sand, but the initial static stress has different effects on the cyclic resistance of the two kinds of sand. The dense sand owns a higher cyclic resistance with SSR increasing, while for the loose sand, 0.12 is the critical SSR corresponding to the lowest value of the cyclic resistance. The dense sand has more fast accumulation of dissipated energy, compared with loose sand. Additionally, an exponential relationship is established between static shear stress, relative density, and normalized energy density.

The effect of particle shape on the deformation and stress reduction of a gravel soil due to wetting

This paper investigates the effect of particle shape on the stress reduction and collapse deformation of gravelly soil using a medium-scale direct shear test apparatus under different relative densities, normal stress, and shear stress levels. A new method based on the Micro-Deval test was introduced to produce sub-angular particles from angular particles. Therefore, two series of soil specimens were obtained with the same rock origin, particle size distribution, and relative density but different particle shapes. In addition to traditional direct shear tests on dry and wet specimens, a specific test procedure was applied to explore the stress reduction and collapse of soil specimens due to wetting. The results of the tests, including shear stress–shear displacement and vertical displacement-shear displacement, were compared. The results showed that the stress reduction and settlement due to wetting increased with vertical and shear stress levels in both types of particle shapes, with higher values in angular particle shapes. The particle breakage of the soil specimens was also studied quantitatively using the change in the particle size distribution before and after the test. It was shown that the wetting of the samples had more impact on the particle breakage in angular gravel than sub-angular gravel, which increased linearly with the normal stress.

Stress distribution in the walls of major arteries: implications for atherogenesis.

There is a correlation between the sites of atheroma development and stress points in the arterial system. Generally, pulse pressure results in stresses acting on the vascular vessel, including longitudinal stress, radial or normal stress, tangential stress or hoop stress and shear stress. This paper explores the relationship between arterial wall shear stress and pulsatile blood pressure with the aim of furthering the understanding of atherogenesis and plaque progression. We computed the magnitude of the shear stresses within the carotid bifurcation geometry of a patient and calculated the increase in shear stress levels that would occur when the blood pressure and pulse pressures rise during exertion. We also determined in which layer of the artery wall the maximum shear stress is located, and computed the shear stress at different levels within the media. We used the theory of laminate analysis, (Classical Laminate Plate Theory), to analyse the stress distribution on the carotid artery wall. Computational Fluid Dynamics (CFD) analysis was used on anatomy based on a CT angiogram of the carotid bifurcation of a patient with a 90% stenosis on the right side and 10% on the left. The pulsatile non-Newtonian blood flow with a resting blood pressure of 120/80 mmHg and an exertion pressure of 200/100 mmHg was simulated and the resultant forces were transferred to an ANSYS Composite PrepPost (ACP) model for wall shear stress analysis. A multilayer elastic, anisotropic, and inhomogeneous arterial wall (intima, internal elastic lamina, media, external elastic lamina, and adventitial layers) was modelled and the shear stress magnitudes and change over time between the layers was calculated. Shear stress in the individual composite layers is far greater than that acting on the endothelium (less than 5 Pa). At rest, the maximum variation of shear stress in the arterial wall occurs in the intima (138 Pa) and adventitia (135 Pa). The medial layer has the lowest variation of shear stress. Under severe exertion, the maximum shear stress magnitude in the intimal layer and the adjacent medial layer is near the ultimate stress level. The maximum/minimum shear stress ratios during the cardiac cycle vary most widely in the innermost part of the media, adjacent to the intima, with a four-fold ratio increase. This compares with a less than two-fold increase in all the other layers including the intima and adventitia, making the inner media the most vulnerable layer to mechanical injury. This study showed that the magnitude of exertion-induced shear stress approaches the ultimate stress limit in the intima and the immediate adjacent medial layer. The variation in stress is maximal in the inner layer of the media. These findings correlate the site of atheroma development with the most vulnerable site for injury in the media and emphasise the impact of pulse pressure. Further biological studies are required to ascertain whether this leads to injury that initiates atheroma that then precipitates an injury/healing cycle.

Prediction Models of Shear Parameters and Dynamic Creep Instability for Asphalt Mixture under Different High Temperatures.

This study mainly investigates the prediction models of shear parameters and dynamic creep instability for asphalt mixture under different high temperatures to reveal the instability mechanism of the rutting for asphalt pavement. Cohesive force c and internal friction angle φ in the shear strength parameters for asphalt mixture were obtained by the triaxial compressive strength test. Then, through analyzing the influence of different temperatures on parameters c and φ, the prediction models of shear strength parameters related to temperature were developed. Meanwhile, the corresponding forecast model related to confining pressure and shear strength parameters was obtained by simplifying the calculation method of shear stress level on the failure surface under cyclic loading. Thus, the relationship of shear stress level with temperature was established. Furthermore, the cyclic time FN of dynamic creep instability at 60 °C was obtained by the triaxial dynamic creep test, and the effects of confining pressure and shear stress level were considered. Results showed that FN decreases exponentially with the increase in stress levels under the same confining pressure and increases with the increase in confining pressure. The ratio between shear stress level and corresponding shear strength under the same confining pressure was introduced; thus, the relationship curve of FN with shear stress level can eliminate the effect of different confining pressures. The instability prediction model of FN for asphalt mixture was established using exponential model fitting analysis, and the rationality of the model was verified. Finally, the change rule of the parameters in the instability prediction model was investigated by further changing the temperature, and the instability forecast model in the range of high temperature for the same gradation mixture was established by the interpolation calculation.

Transition between Hall-Petch and inverse Hall-Petch behavior in nanocrystalline silicon carbide

Despite much experimental and simulation effort, the existence of a Hall-Petch to inverse Hall-Petch transition in nanocrystalline ceramics remains elusive. By employing molecular dynamics simulations, we unambiguously reveal a transition from strengthening to softening in the shear deformation of nanocrystalline silicon carbide ceramics as a function of grain size. Results show a well-defined maximum in the shear strength for grain sizes in the range 6.2 to 7.7 nm. Further decrease in grain size leads to diminishing strength, consistent with an inverse Hall-Petch behavior. As grain size is reduced the increasing grain boundary (GB) regions lead to homogenization of shear stresses across the microstructure, allowing for lower local shear stress levels at higher macroscopically applied stresses. This delays shear localization within GB regions, preventing cavitation, nanocracking, and premature failure, and is responsible for the observed Hall-Petch behavior. In contrast, at grain sizes 6.2 nm, the rather compliant nature of the structurally disordered GB regions dominates the mechanical response, reducing the shear strength and triggering a transition into the inverse Hall-Petch behavior. A composite model delineating the transition between Hall-Petch and inverse Hall-Petch behavior is successful at describing the mechanical behavior of nanocrystalline silicon carbide as a function of grain size.

QS2: DIY Bioreactors for Tissue Engineering - An Ultra-low-cost Platform for Perfusion Cell Culture and Live Imaging

Purpose:While tissue engineering offers the promise of revolutionary innovation, scalable three-dimensional tissue culture is limited by the diffusion of nutrients and oxygen making media perfusion obligatory. Unfortunately, the cost of bioreactors for large construct tissue culture can be prohibitive, with a typical perfusion chamber costing several thousand dollars, and even small petri-dish-sized devices costing hundreds of dollars each. We have developed a low-cost perfusion setup that seals collagen-based perfusable cellular constructs within a sterile PDMS well between coverslips, allowing for repeated live-imaging of perfused 3D engineered tissues. Herein we describe fabrication of this novel system and validate its utility.Methods:Molds and frames were designed on 3D-modeling software (Fusion 360) and printed on a Prusa i3 MK3S 3D printer in poly(lactic acid) (PLA). Molds were filled with poly(dimethyl siloxane) (PDMS), which was cured to form chambers, bubble traps, mason jar lid chambers, and media reservoir lid adapters. In total, the tissue culture chamber device, mason jar lid inset, media reservoir lid, and bubble trap require 4, 1, 2, and 4 unique printed components, respectively.Results:Each perfusion chamber can be assembled for under 8 USD per device and reused repeatedly. The current model has a tissue chamber custom-built with 18x10x4 (LxWxH)mm3 dimensions, but this chamber can be readily customized to experiment-specific dimensions. These devices allow cellular hydrogel constructs to be maintained in a sterile environment after assembly, perfused at varying rates to expose cells to different levels of shear stress, and the cells can be intermittently imaged with light, fluorescent or confocal microscopy - an unparalleled benefit for monitoring of experiments and collection of timepoint imaging data. The perfusion circuit consists of autoclavable glass and PDMS components, including a bubble trap, a crucial component of the circuit for preventing air bubbles that can damage cells and block microchannels, and a lid adapter, which allows 50 mL conical tubes to serve as self-oxygenating media reservoirs. Media changes can be performed via peristaltic pump perfusion or with syringe-based cell culture techniques for static culture. Constructs have been perfused within standard incubators for up to 14 days demonstrating normal cell viability without contamination or evidence of infection, with longer perfusion culture intervals (>1 month) currently being tested.Conclusion:The increasing accessibility of 3D-modeling and 3D-printing has enabled rapid prototyping of devices to address the problems that we face as surgeon-scientists. We have developed a low-cost tissue-engineering perfusion circuit that facilitates 3D-tissue-culture while allowing for repeated live-imaging as the cultured tissue develops. The instructions for our setup can be utilized to replicate our devices in other labs, and these designs can be readily customized to meet the needs of specific experimental aims.

Studying dynamic stress effects on the behaviour of THP-1 cells by microfluidic channels

Atherosclerosis is a long-term disease process of the vascular system that is characterized by the formation of atherosclerotic plaques, which are inflammatory regions on medium and large-sized arteries. There are many factors contributing to plaque formation, such as changes in shear stress levels, rupture of endothelial cells, accumulation of lipids, and recruitment of leukocytes. Shear stress is one of the main factors that regulates the homeostasis of the circulatory system; therefore, sudden and chronic changes in shear stress may cause severe pathological conditions. In this study, microfluidic channels with cavitations were designed to mimic the shape of the atherosclerotic blood vessel, where the shear stress and pressure difference depend on design of the microchannels. Changes in the inflammatory-related molecules ICAM-1 and IL-8 were investigated in THP-1 cells in response to applied shear stresses in an continuous cycling system through microfluidic channels with periodic cavitations. ICAM-1 mRNA expression and IL-8 release were analyzed by qRT-PCR and ELISA, respectively. Additionally, the adhesion behavior of sheared THP-1 cells to endothelial cells was examined by fluorescence microscopy. The results showed that 15 Pa shear stress significantly increases expression of ICAM-1 gene and IL-8 release in THP-1 cells, whereas it decreases the adhesion between THP-1 cells and endothelial cells.

Optimisation of an ultrasonic torsion fatigue system for high strength materials

The bearable shear stress amplitude in very high cycle fatigue regime (VHCF) is of interest for many applications in mechanics (springs, crankshafts, ball bearings, etc.) when a very high repetition of load cycles occur. This paper proposes to optimise an existing torsion gigacycle fatigue testing system, developed fifteen years ago by the Pr C. Bathias’ research team. The aim is to increase the shear stress amplitude applied to the investigated specimen and, consequently, meet the expectations from industries about fatigue life for high strength materials under shear loading. The system differs from other ultrasonic torsion fatigue machines that can be found in the literature, by the combination of two amplification horns, transforming a translation movement into a rotational one. In this work, there are two optimisation objective functions: (i) maximize the shear stress level in the specimen and (ii) minimize the stress level in the fatigue system and mainly in the pin connecting the two horns. The development of the optimised device is essentially carried out by a parametric study and numerical simulations through modal and harmonic analysis. Numerical results are compared with the analytical 1D solution and with experimental results obtained from the new real experimental set-up. Finally, new results in the gigacycle domain are presented concerning the torsion fatigue strength of high strength steels (50CrV4 and 16MnCr5) and discussed.

Stress fields around magma chambers influenced by elastic thermo-mechanical deformation: implications for forecasting chamber failure

Defining the conditions that lead to the rupture of a magma chamber is essential to forecast eruptions. So far, models simulating magma chamber dynamics have neglected the effects of elastic thermal expansion in the host rocks surrounding a new injection of magma, focusing instead primarily on elastic-plastic deformation and more recently, on visco-elastic deformation. Here we fill this gap by building a suite of elastic thermo-mechanical models to determine the stress field around a variably heated crustal magma chamber. We first consider linear elastic mechanical models with only the effect of magma pressure. We then present purely thermal models simulating heat distribution around a heated chamber. Finally, we present coupled linear elastic thermo-mechanical models that highlight the influence of temperature on the distribution of crustal stresses. Results show that thermal expansion–induced stresses generate two competing consequences: (1) they increase the level of shear stress around the magma chamber and (2) they partially suppress the level of tensile stress generated by the magmatic pressure. These competing effects influence the short-timescale conditions required for the failure of immature magmatic systems and hence the nucleation of dikes which may ultimately feed eruptions during unrest. Therefore, soon after a new magmatic recharge event, the contribution of temperature increase in the host rocks, following the new influx of magma into a crustal magma chamber, should be considered.

Single-Lap Joints Bonded with Epoxy Nanocomposite Adhesives: Effect of Organoclay Reinforcement on Adhesion and Fatigue Behaviors

Organoclay, such as organo-modified montmorillonite (MMT), is one of the potential filler candidates in the preparation of epoxy nanocomposite adhesives for automotive and civil engineering industries. However, there is no systematic research on the effect of organoclay reinforcement on its adhesion and fatigue behaviors in shear loading. Here, single-lap joints (SLJs) of stainless steel substrates bonded with epoxy/MMT nanocomposite adhesives, consisting of diglycidyl ether of bisphenol A and poly(oxypropylene) diamine with various weight contents of a commercialized MMT filler, were prepared. The dispersity of MMT and bulk mechanical properties were investigated by wide-angle X-ray scattering/diffraction, the uniaxial tensile test, and dynamic mechanical analysis. The lap shear strengths and failure modes were investigated based on the single-lap shear test, and the joint durability was examined by using the fatigue test with various shear stress levels. Through the incorporation of the MMT fillers in epoxy resins, the lap shear strengths increased from 25 to 40 MPa, the failure modes changed from adhesive failure mode to cohesive one, and the fatigue lifetimes prolonged with the addition of a small amount of MMT.

Experimental and numerical investigation on crack mechanism of folded flawed rock-like material under uniaxial compression

Strength, deformability and failure behaviors of rock masses are significantly affected by the widespread natural fissures that are often distributed in overlapped patterns. To investigate the failure mechanism of rock masses with overlapped fissures, uniaxial compressive tests and numerical studies were conducted on Portland cement samples containing folded flaws of different dip angles and undulation angles. The folded flaw was overlapped by several fractures at the ends. Crack initiation, propagation and coalescence were systematically investigated. Results show that there are two patterns in terms of the deformational curves of specimens containing folded flaws, i.e., the bimodal type and the single peak type. Five different failure modes were observed, including compaction, dilation, shear off crush and slip. In all specimens crack coalescence is caused by the linkage of shear cracks with the folded flaw although the failure surfaces are different because of the different failure modes of the folded flaw. After coalescence, the folded flaw can still resist a certain level of shear stress, indicating that it has shear strength, which contributes to the residual peak strength showing in the stress-displacement curves. It is also found that the failure characteristics of specimens containing folded flaws integrate the failure characteristics of rock joint under compressive shear and smooth fracture under compression, which is significantly correlated with the undulation and dip angles of the folded flaw. The findings can provide a better understanding of the failure mechanism of rock masses with folded fractures.

Natural Killer Cell Mobilization in Breast and Prostate Cancer Survivors: The Implications of Altered Stress Hormones Following Acute Exercise

Natural killer (NK) cells from the innate immune system are integral to overall immunity and also in managing the tumor burden during cancer. Breast (BCa) and prostate cancer (PCa) are the most common tumors in U.S. adults. Both BCa and PCa are frequently treated with hormone suppression therapies that are associated with numerous adverse effects including direct effects on the immune system. Regular exercise is recommended for cancer survivors to reduce side effects and improve quality of life. Acute exercise is a potent stimulus for NK cells in healthy individuals with current evidence indicating that NK mobilization in individuals with BCa and PCa is comparable. NK cell mobilization results from elevations in shear stress and catecholamine levels. Despite a normal NK cell response to exercise, increases in epinephrine are attenuated in BCa and PCa. The significance of this potential discrepancy still needs to be determined. However, alterations in adrenal hormone signaling are hypothesized to be due to chronic stress during cancer treatment. Additional compensatory factors induced by exercise are reviewed along with recommendations on standardized approaches to be used in exercise immunology studies involving oncology populations.

Effect of low wall shear stress on the morphology of endothelial cells and its evaluation indicators

ObjectiveThis study is designed to explore the morphological changes of endothelial cells (ECs) under different levels of shear stress and find the effective evaluation indicators with in vivo and in vitro experiments. MethodsHuman umbilical vein endothelial cells (HUVECs) and Sprague–Dawley rats were used to study the effect of different shear stress which applied by means of parallel plate-flow chamber and abdominal aorta banding model on the morphology of endothelial cells. Then, fluorescence images were acquired by means of a confocal laser-scanning microscope. Finally, Cell morphological indicators were extracted by SRAD-MCW computer image processing algorithm for quantitative analysis. Results1) The morphological changes of HUVECs were observed after exposure to shear stress for 6 h, the HUVECs were elongated and spindle-shaped. And the degree of cell deformability was different while the exposure time was different, then it became stable after 18 h. The HUVECs exposure to high shear stress (HSS) exhibited an ordered cell arrangement, while the HUVECs exposure to low shear stress (LSS) showed a disordered cell arrangement. 2) Traditional cell morphological indicators such as area, perimeter, long axis diameter, short axis diameter and orientation angle were not significantly different between the normal shear stress (NSS) group and the LSS group (P > 0.05), but the intercellular space characteristics such as the junction length per unit area and the triple points per unit area were significantly different (P < 0.05). ConclusionThese results demonstrate that the size and duration of shear stress can affect the morphology and arrangement of endothelial cells. The commonly used evaluation indicators for studying the effect of shear stress on the morphology of endothelial cells, including area, perimeter, long axis diameter, short axis diameter and orientation angle, etc., had no significant statistical significance, while the intercellular space characteristics including the junction length per unit area and the triple points per unit area can be used as effective indicator to study the effect of shear stress on the morphology of endothelial cells.

The biomechanical basis of biased epithelial tube elongation in lung and kidney development.

ABSTRACTDuring lung development, epithelial branches expand preferentially in a longitudinal direction. This bias in outgrowth has been linked to a bias in cell shape and in the cell division plane. How this bias arises is unknown. Here, we show that biased epithelial outgrowth occurs independent of the surrounding mesenchyme, of preferential turnover of the extracellular matrix at the bud tips and of FGF signalling. There is also no evidence for actin-rich filopodia at the bud tips. Rather, we find epithelial tubes to be collapsed during early lung and kidney development, and we observe fluid flow in the narrow tubes. By simulating the measured fluid flow inside segmented narrow epithelial tubes, we show that the shear stress levels on the apical surface are sufficient to explain the reported bias in cell shape and outgrowth. We use a cell-based vertex model to confirm that apical shear forces, unlike constricting forces, can give rise to both the observed bias in cell shapes and tube elongation. We conclude that shear stress may be a more general driver of biased tube elongation beyond its established role in angiogenesis.This article has an associated ‘The people behind the papers’ interview.

Changes in Energy Metabolism and Cell Damage in RPTEC‐TERT1 Cells Exposed to Hypoxia or Ischemia after Shear Stress Conditioning

Study Objective Renal proximal tubules are exposed in vivo to physiological levels of shear stress. We aim to determine how preconditioning human proximal tubule cells with shear stress affects their sensibility to hypoxia or ischemia. Hypothesis Physiological levels of shear stress would induce further differentiation and metabolic specialization resulting in changes in their response to low oxygen or nutrient conditions. Methods Immortalized human proximal tubule cells (RPTEC-TERT1 from Evercyte) were grown to confluence in Ibidi µSlides and then exposed for three days to two flow regimes providing negligible (0.01 dyn/cm2) or physiological shear stress (0.3 dyn/cm2). Hypoxia or ischemic conditions were applied for 16h, by keeping cells under 1%O2 alone or together with glucose deprivation. Results RPTEC-TERT1 cells were first adapted to a culturing regime under 5 mM glucose culture medium, instead of 17 mM glucose present in the medium recommended by this cell line vendor. Proliferation rates and expression of phenotypic markers were unchanged. RPTEC-TERT1 cells switched from proliferative-glycolytic metabolism to non-proliferative-oxidative metabolism upon reaching confluency, approximately in 6-7 days. Seven days confluent, not-perfused RPTEC-TERT1 cells exposed to hypoxia or ischemia for 16 hours exhibited 58% and 75% mortality, respectively. In cells perfused for 3 days, preconditioning with shear stress reduced their sensitivity to hypoxia (mortality = 51% no-SS vs. 35% SS) and ischemia (mortality = 83% no-SS vs. 75% SS). Shear stress conditioning increased expression of genes related to energy metabolism and cells stress (AMPK alpha-2, HIF-1alpha, KIM-1) Conclusions Preconditioning of RPTEC-TERT1 by exposing them to physiological shear-stress modifies expression of genes related to mechanisms used to cope with cellular stress and this may explain the reduction in cell mortality when exposed to hypoxia or oxygen-glucose deprivation.

Integrating Microfluidic Models with Hemoglobin‐Based Oxygen Carriers for Investigating Oxygen‐Mediated Endothelial Function

Inadequate oxygen delivery to tissue, or hypoxia, is caused by extensive abnormalities in the structure and function of the vascular network. Hypoxia also has a significant role in vascular diseases such as stroke, pregnancy disorders, and cancer. Despite its prevalence, there is no established method for alleviating hypoxia and its deleterious effects to vascular function. Existing experimental models are incapable of enabling cellular and intercellular microcirculatory measurements while also controlling the local oxygen environment, impeding our ability to mechanistically study these effects. Hemoglobin-based oxygen carriers (HBOCs) are a promising class of red blood cell substitute which have been well characterized for transfusion medicine applications, yet little has been done to study the effects of HBOC administration on vascular function at the cellular level. In this work, we put forth a novel integration of microfluidic vascular models with HBOCs to investigate the effects of oxygen transport in microcirculation in vitro. We hypothesize that alleviating hypoxia with HBOCs restores normal blood vessel structure and function. We fabricated fully-lumenized microvessels in vitro which mimic post-capillary venules. We assessed vessel stability by immunofluorescence staining, and permeability under environmentally-controlled normoxic and hypoxic conditions. We then synthesized polymerized hemoglobin (PolyHb), an HBOC with favorable molecular weight and oxygen-carrying properties to alleviate hypoxia while minimizing extravasation and subsequent toxicity. We validated that PolyHb may be synthesized under varying conditions to selectively control the HBOC size and oxygen affinity, demonstrating that these properties can be modified to enable favorable oxygen transport and may be varied based on the desired application. As compared to previous generation HBOCs, PolyHb can attain molecular weights which will enable intravascular circulation to minimize toxicity, making PolyHb an ideal candidate for microfluidic investigation into the physiological effects of rescuing hypoxia. In future work, we will perfuse PolyHb through the engineered microvessels at constant flow and physiological shear stress levels to assess the ability of PolyHb to relieve induced hypoxia under imposed circumferential and longitudinally oriented hypoxia gradients.

Effects of inflow boundary layer on the wake of a radially non-uniform porous disk

This study presents the results of an experimental investigation focusing on the effects of the inflow boundary layer on the wake characteristics of a 0.12 m diameter porous disk with radially non-uniform porosity in terms of mean flow, turbulence, and wake scaling. Two-dimensional two-component particle image velocimetry measurements within the wake are performed up to 7.5 diameters downstream as the disk is lowered deeper into a boundary layer that is representative of a neutral atmospheric boundary layer over a flat terrain. Results show that otherwise symmetrical wake velocity profiles that exist outside the boundary layer get skewed and sheared around the disk centerline in the boundary layer due to the inflow wind shear. The turbulent kinetic energy, its production, and Reynolds shear stress levels in the wake get asymmetrical around the centerline of the disk such that the production of turbulent kinetic energy is observed to be higher above centerline. Due to the inflow shear, the wake centerline gets shifted downwards (i.e., toward the wind tunnel wall), which is in contrast to the observations on real wind turbine wakes in the literature where the wake actually lifts up. The asymmetrical and skewed velocity profiles both in the streamwise and cross-stream directions can be collapsed onto a single function by using proper wake scaling parameters based on the ratio of local strain to average strain within the velocity profile calculated separately for either side of the wake.