M. Keith Sharp

Effects of Biaxial Oscillatory Shear Stress on Endothelial Cell Proliferation and Morphology

Wall shear stress (WSS) on anchored cells affects their responses, including cell proliferation and morphology. In this study, the effects of the directionality of pulsatile WSS on endothelial cell proliferation and morphology were investigated for cells grown in a Petri dish orbiting on a shaker platform. Time and location dependent WSS was determined by computational fluid dynamics (CFD). At low orbital speed (50 rpm), WSS was shown to be uniform (0–1 dyne/cm2) across the bottom of the dish, while at higher orbital speed (100 and 150 rpm), WSS remained fairly uniform near the center and fluctuated significantly (0–9 dyne/cm2) near the side walls of the dish. Since WSS on the bottom of the dish is two‐dimensional, a new directional oscillatory shear index (DOSI) was developed to quantify the directionality of oscillating shear. DOSI approached zero for biaxial oscillatory shear of equal magnitudes near the center and approached one for uniaxial pulsatile shear near the wall, where large tangential WSS dominated a much smaller radial component. Near the center (low DOSI), more, smaller and less elongated cells grew, whereas larger cells with greater elongation were observed in the more uniaxial oscillatory shear (high DOSI) near the periphery of the dish. Further, cells aligned with the direction of the largest component of shear but were randomly oriented in low magnitude biaxial shear. Statistical analyses of the individual and interacting effects of multiple factors (DOSI, shear magnitudes and orbital speeds) showed that DOSI significantly affected all the responses, indicating that directionality is an important determinant of cellular responses. Biotechnol. Bioeng. 2012; 109:695–707.

Computationally Determined Shear on Cells Grown in Orbiting Culture Dishes

A new computational model, using computational fluid dynamics (CFD), is presented that describes fluid behavior in cylindrical cell culture dishes resulting from motion imparted by an orbital shaker apparatus. This model allows for the determination of wall shear stresses over the entire area of the bottom surface of a dish (representing the growth surface for cells in culture) which was previously too complex for accurate quantitative analysis. Two preliminary cases are presented that show the complete spatial resolution of the shear on the bottom of the dishes. The maximum shear stress determined from the model is compared to an existing simplified point function that provides only the maximum value. Furthermore, this new model incorporates seven parameters versus the four in the previous technique, providing improved accuracy. Optimization of computational parameters is also discussed.

Spatial and temporal resolution of shear in an orbiting petri dish

It is well documented that physiological and morphological properties of anchored cells are influenced by fluid shear stress. Common orbital shakers provide a means of simultaneously applying shear stress to cells for tens to hundreds of cases by loading the shaker with multiple dishes. However, the complex flow in orbiting dishes is amenable to analytical solution for resolving shear created by the fluid motion only for simplified conditions. The only existing quantification of shear in this flow is an equation that estimates a constant scalar value of shear for the entire surface of the dish. In practice, wall shear stress (WSS) will be oscillatory rather than steady due to the travelling waveform and will vary across the surface of the dish at any instant in time. This article presents a computational model that provides complete spatial and temporal resolution of WSS over the bottom surface of a dish throughout the orbital cycle. The model is reasonably well validated by the analytical solution, with resultant WSS magnitudes that are within 0.99 ± 0.42 dyne/cm2. The model results were compared to tangential WSS magnitudes obtained using one‐dimensional optical velocimetry at discreet locations on the bottom of an orbiting dish. The experimental minimum and maximum WSS at 1 mm from the center of the dish were 6 and 7 dyne/cm2, respectively, whereas WSS generated from the computational model ranged from 0.5 to 8.5 dyne/cm2. The experimental minimum and maximum WSS at 12 mm from the center of the dish were 6 and 16 dyne/cm2, respectively, whereas WSS generated from the computational model ranged from 0.5 to 14 dyne/cm2. Discrepancies between the experimental and computational data may be attributed to a sparse sampling rate for the experimental probe, a sharp gradient at the sample area which could cause the unidirectional probe to be inaccurate if its location were not precise, and too few particles to track and a scattering of the signal by the free surface when the liquid is shallow.

Numerical simulation of the influence of gravity and posture on cardiac performance

AbstractA numerical model of the cardiovascular system was used to quantify the influences on cardiac function of intrathoracic pressure and intravascular and intraventricular hydrostatic pressure, which are fundamental biomechanical stimuli for orthostatic response. The model included a detailed arterial circulation with lumped parameter models of the atria, ventricles, pulmonary circulation, and venous circulation. The venous circulation was divided into cranial, central, and caudal regions with nonlinear compliance. Changes in intrathoracic pressure and the effects of hydrostatic pressure were simulated in supine, launch, sitting, and standing postures for 0, 1, and 1.8 G. Increasing intrathoracic pressure experienced with increasing gravity caused 12% and 14% decreases in cardiac output for 1 and 1.8 G supine, respectively, compared to 0 G. Similar results were obtained for launch posture, in which the effects of changing intrathoracic pressure dominated those of hydrostatic pressure. Compared to 0 G, cardiac output decreased 0.9% for 1 G launch and 15% for 1.8 G launch. In sitting and standing, the position of the heart above the hydrostatic indifference level caused the effects of changing hydrostatic pressure to dominate those of intrathoracic pressure. Compared to 0 G, cardiac output decreased 13% for 1 G sitting and 23% for 1.8 G sitting, and decreased 17% for 1 G standing and 31% for 1.8 G standing. For a posture change from supine to standing in 1 G, cardiac output decreased, consistent with the trend necessary to explain orthostatic intolerance in some astronauts during postflight stand tests. Simulated lower body negative pressure (LBNP) in 0 G reduced cardiac output and mean aortic pressure similar to 1 G standing, suggesting that LBNP provides at least some cardiovascular stimuli that may be useful in preventing postflight orthostatic intolerance. A unifying concept, consistent with the Frank–Starling mechanism of the heart, was that cardiac output was proportional to cardiac diastolic transmural pressure for all postures and gravitational accelerations.

A strain-based flow-induced hemolysis prediction model calibrated by in vitro erythrocyte deformation measurements.

Hemolysis is caused by fluid stresses in flows within hypodermic needles, blood pumps, artificial hearts, and other cardiovascular devices. Developers of cardiovascular devices may expend considerable time and effort in testing of prototypes, because there is currently insufficient understanding of how flow-induced cell damage occurs to accurately predict hemolysis. The objective of this project was to measure cell deformation in response to a range of flow conditions, and to develop a constitutive model correlating cell damage to fluid stresses. An experimental system was constructed to create Poiseuille flow under a microscope with velocities up to 4 m/s, Reynolds number to 200, and fluid stresses to 5000 dyn/cm(2). Pulsed laser illumination and a digital camera captured images of cells deformed by the flow. Equilibrium equations were developed to relate fluid stresses to cell membrane tension, and a viscoelastic membrane model was used to predict cell strain. Measurements of aspect ratio as a function of shear stress and duration of shear were used to calibrate the cell deformation model. Hemolysis prediction was incorporated with a threshold strain value for cell rupture. The new model provides an improved match to experimentally observed hemolytic stress thresholds, particularly at long exposure times, and may reduce the empiricism of hemolysis prediction.

Maximizing information from space data resources: a case for expanding integration across research disciplines

Regulatory systems are affected in space by exposure to weightlessness, high-energy radiation or other spaceflight-induced changes. The impact of spaceflight occurs across multiple scales and systems. Exploring such interactions and interdependencies via an integrative approach provides new opportunities for elucidating these complex responses. This paper argues the case for increased emphasis on integration, systematically archiving, and the coordination of past, present and future space and ground-based analogue experiments. We also discuss possible mechanisms for such integration across disciplines and missions. This article then introduces several discipline-specific reviews that show how such integration can be implemented. Areas explored include: adaptation of the central nervous system to space; cerebral autoregulation and weightlessness; modelling of the cardiovascular system in space exploration; human metabolic response to spaceflight; and exercise, artificial gravity, and physiologic countermeasures for spaceflight. In summary, spaceflight physiology research needs a conceptual framework that extends problem solving beyond disciplinary barriers. Administrative commitment and a high degree of cooperation among investigators are needed to further such a process. Well-designed interdisciplinary research can expand opportunities for broad interpretation of results across multiple physiological systems, which may have applications on Earth.

Peristalsis with Oscillating Flow Resistance: A Mechanism for Periarterial Clearance of Amyloid Beta from the Brain

Alzheimer’s disease is characterized by accumulation of amyloid-β (Aβ) in the brain and in the walls of cerebral arteries. The focus of this work is on clearance of Aβ along artery walls, the failure of which may explain the accumulation of Aβ in Alzheimer’s disease. Periarterial basement membranes form continuous channels from cerebral capillaries to major arteries on the surface of the brain. Arterial pressure pulses drive peristaltic flow in the basement membranes in the same direction as blood flow. Here we forward the hypothesis that flexible structures within the basement membrane, if oriented such they present greater resistance to forward than retrograde flow, may cause net reverse flow, advecting Aβ along with it. A solution was obtained for peristaltic flow with low Reynolds number, long wavelength compared to channel height and small channel height compared to vessel radius in a Darcy–Brinkman medium representing a square array of cylinders. Results show that retrograde flow is promoted by high cylinder volume fraction and low peristaltic amplitude. A decrease in cylinder concentration and/or an increase in amplitude, both of which may occur during ageing, can reduce retrograde flow or even cause a transition from retrograde to forward flow. Such changes may explain the accumulation of Aβ in the brain and in artery walls in Alzheimer’s disease.

In vitro validation of flow measurement with phase contrast MRI at 3 tesla using stereoscopic particle image velocimetry and stereoscopic particle image velocimetry-based computational fluid dynamics.

To validate conventional phase‐contrast MRI (PC‐MRI) measurements of steady and pulsatile flows through stenotic phantoms with various degrees of narrowing at Reynolds numbers mimicking flows in the human iliac artery using stereoscopic particle image velocimetry (SPIV) as gold standard.

Modelling of cardiovascular response to graded orthostatic stress: role of capillary filtration

Eur J Clin Invest 2011; 41 (8): 807–819

Using PIV to determine relative pressures in a stenotic phantom under steady flow based on the Pressure-Poisson equation

Pressure gradient across a Gaussian-shaped 87% area stenosis phantom was estimated by solving the pressure Poisson equation (PPE) for a steady flow mimicking the blood flow through the human iliac artery. The velocity field needed to solve the pressure equation was obtained using particle image velocimetry (PIV). A steady flow rate of 46.9 ml/s was used, which corresponds to a Reynolds number of 188 and 595 at the inlet and stenosis throat, respectively (in the range of mean Reynolds number encountered in-vivo). In addition, computational fluid dynamics (CFD) simulation of the same flow was performed. Pressure drops across the stenosis predicted by PPE/PIV and CFD were compared with those measured by a pressure catheter transducer. RMS errors relative to the measurements were 17% and 10% for PPE/PIV and CFD, respectively.