VISUALIZATION OF WALL SHEAR RATE IN ARTIFICIAL ORGANS

Abstract

The proper function of an artificial organ in the cardiovascular system depends very much on the blood flow adjacent to the artificial walls. These walls usually are made of artificial material and to a larger or lesser degree are thrombogenic. A blood flow with no flow separations and stagnation zones is required to avoid the generation of thrombi. A precondition for solving this problem is the assessment of the wall shear rate. A new method to visualize the latter is proposed. This new method permits to look selectively at the flow close to the wall. The selection is made by using a fluid, which does not permit the light to penetrate deeply into the flow. The transparent flow model is illuminated by a diffusive light source. The light illuminates only the viscous sub layer, so only the particles moving close to the wall are lighted. The flow adjacent to the wall can thus be visualized. Water is used as a model fluid. Black Indian ink is used to obscure the bulk flow. The concentration of the ink is such, that the light penetrates only 10 micrometers. Hollow glass spheres with a mean diameter of 16 micrometers are used. They are covered with aluminum for good reflectance. The size of the sphere is in the same order as the depth of light penetration. The flow is recorded with a digital camera with a resolution of 1024 ± 1024 pixel. The resulting images are processed using the cross-correlation method. A vector field of the wall shear flow is the result. The method has been used to investigate the flow of a cardiac assist device. Flow separations are clearly shown and give an indication for an improvement of the design.