Laser-doppler anemometer to study velocity fields in the vicinity of prosthetic heart valves

Laser-Doppler anemometry is relatively new technique which is used for measuring velocity fields. It has major applications in the field ofin vitro biofluid mechanics. The laser-Doppler anemometers have many advantages compared with the traditional hot-wire or hot-film anemometers which are still mainly used in studies of biofluid mechanics. A laser-Doppler anemometer (I.d.a) system which can be used to measurein vitro velocity and shear-stress profiles in the vicinity of prosthetic heart valves is described. Accurate velocity measurements in the vicinity of prosthetic heart valves are very scarce, and the use of I.d.a systems will facilitate acquisition of these data.     

In vitro velocity measurements down strem from the lonescu-Shiley aortic bioprosthesis in steady and pulsatile flow

Velocity measurements were made in vitro using laser Doppler anemometry (LDA) downstream from an lonescu-Shiley (IS) bioprosthetic aortic heart valve. Velocity measurements were made in both steady and pulsatile flow. A systematic, flow mapping approach to the measurement methodology showed that the IS valve generated a large jetlike flow constriction. The acceleration ratio, defined as the maximum mean velocity for the IS valve divided by that for no valve obstructing the flow, was as high as 2·4 for steady flow and 2·6 for pulsatile flow. It was concluded that the IS valve generated a flow quite unlike that observed by other in vestigators for the natural human aortic valve, after which the leaflet design of the IS valve was modelled. In addition, a comparative analysis of steady and pulsatile results was undertaken. It was found that the pulsatile flow results for the systolic ejection interval could be divided into three phases, denoted early, mid, and late systole, as defined by the flow structure at the data plane location. Only during midsystole were the pulsatile flow results approximated by the steady flow results. Also, it was found that the magnitude of the flow disturbance measured in steady flow tended to be an upper bound on that measured for pulsatile flow.     

Anin vitro traumatic injury model to examine the response of neurons to a hydrodynamically-induced deformation

A novelin vitro system was developed to examine the effects of traumatic mechanical loading on individual cells. The cell shearing injury device (CSID) is a parallel disk viscometer that applies fluid shear stress with variable onset rate. The CSID was used in conjunction with microscopy and biochemical techniques to obtain a quantitative expression of the deformation and functional response of neurons to injury. Analytical and numerical approximations of the shear stress at the bottom disk were compared to determine the contribution of secondary flows. A significant portion of the shear stress was directed in ther-direction during start-up, and therefore the full Navier-Stokes equation was necessary to accurately describe the transient shear stress. When shear stress was applied at a high rate (800 dyne cm⁻² sec⁻¹) to cultured neurons, a range of cell membrane strains (0.01 to 0.53) was obtained, suggesting inhomogeneity in cellular response. Functionally, cytosolic calcium and extracellular lactate dehydrogenase levels increased in response to high strain rate (>1 sec⁻¹) loading, compared with quasistatic (<1 sec⁻¹) loading. In addition, a subpopulation of the culture subjected to rapid deformation subsequently died. These strain rates are relevant to those shown to occur in traumatic injury, and as such, the CSID is an appropriate model for studying the biomechanics and pathophysiology of neuronal injury.     

In Vitro Cell Shearing Device to Investigate the Dynamic Response of Cells in a Controlled Hydrodynamic Environment

Mechanical stresses and strains play important roles in the normal growth and development of biological tissues, yet the cellular mechanisms of mechanotransduction have not been identified. A variety of in vitro systems for applying mechanical loads to cell populations have been developed to gain insight into these mechanisms. However, limitations in the ability to control precisely relevant aspects of the mechanical stimuli have obscured the physical relationships between mechanical loading and the biochemical signals that mediate the cellular response. We present a novel in vitro cell shearing device based on the principles of a cone and plate viscometer that utilizes microstepper motor technology to control independently the dynamic and steady components of a hydrodynamic shear-stress environment. Physical measurements of the cone velocity demonstrated faithful reproduction of user-defined input wave forms. Computational modeling of the fluid environment for the unsteady startup confirmed small inertial contributions and negligible secondary flows. Finally, we present experimental results demonstrating the onset rate dependence of functional and structural responses of endothelial cell cultures to dynamically applied shear stress. The controlled cell shearing device is a novel tool for elucidating mechanisms by which mechanical forces give rise to the biological signals that modulate cellular morphology and metabolism. © 2000 Biomedical Engineering Society. PAC00: 8780Rb, 8717-d