Mechanical aspects of blood-wall interaction : wall shear stress measurement

The predilection of atherosclerotic lesions for specific sites in the arterial tree is believed to be related t o the wall shear stress, exerted by the blood flow. Measurement of the magnitude of the wall shear stress is difficult and is frequently determined by an extrapolation of the velocity field t o find the wall shear rate and an estimation of the viscosity of the fluid in the neighborhood of the wall. In this study, it will be shown that the non-Newtonian properties of blood significantly influence the velocity field and that the extrapolation of the near wall velocity distribution t o determine the wall shear rate distribution is not very reliable. The estimation of the relevant viscosity of blood at near wall sites is not trivial due t o its complex nature. Accurate measurement of the wall shear stress in this way is not feasible and a new method to evaluate the load of blood analog fluids on the wall will be presented. Laser Doppler Anemometry measurements were performed in a three dimensional model of the human carotid artery bifurcation t o investigate the influence of non-Newtonian fluid behavior. The blood analog fluid used in these experiments resembled the viscometric properties of blood quite closely. Velocity measurements under steady flow conditions (Re = 300) were performed for a Newtonian control fluid and the blood analog fluid. Both axial and secondary velocities were measured. Evident differences between the flow fields of the Newtonian and blood analog fluids were found. The non-Newtonian axial velocity field was flattened, had lower velocity gradients at the divider wall, and higher velocity gradients at the non-divider wall. The flow separation as found with the Newtonian fluid was absent, and secondary flow was strongly decreased. It was demonstrated that the extrapolation of the near wall velocity distribution to determine the wall shear rates is not very accurate and very sensitive t o the estimated position of the wall. At the non-divider wall, the site where atherosclerotic lesions develop, the non-Newtonian properties of the blood analog fluid changed the sign of the wall shear rate and therefore the sign of the wall shear stress. It is therefore essential that the non-Newtonian properties of blood are included in a study that correlates the development of atherosclerotic lesions t o local haemodynamics. The inaccuracy of the wall shear rate measurements and the absence of a model t o predict near wall viscosity instigated an investigation for a new method to measure the wall shear stress. If a highly flexible gel layer is attached t o the inside of the model, the deformation of this gel layer is a measure for the wall shear stress. If the properties of the gel are known, the measured deformation of the gel layer can be converted t o the wall shear stress. The small deformation of the gel layer was measured with Speckle Pattern Interferometry (SPI) and the performance of the newly developed SPI apparatus was evaluated in benchmark experiments. The wall shear stress was measured in a rectangular duct under steady flow conditions using a Newtonian measuring fluid. In a physiologically relevant range, the wall shear stresses were measured very accurately. As a conclusion i t can be stated that it is essential in in-vitro studies on atherogenesis that the non-Newtonian properties of blood are taken into account because they significantly influence the velocity distribution in physiologically relevant flow geometries. Furthermore, i t is obvious that the existing methods of measuring the wall shear stress in in-vitro models are not satisfactory. A promising new method t o measure the wall shear stress is presented and can be used t o measure the mechanical load of blood analog fluid in in-vitro models.