Velocities and stress levels of axisymmetric, azimuthal flow within the toroidal rotary seal of the IBM 2997 continuous flow cell separator and the implications

Continuous flow blood fraction separators are used to facilitate the removal of specific blood components for donation or for certain medical procedures. Problems with one such device, the IBM 2997 Blood Processor, have been noted in a number of independent investigations. A key feature of this particular unit is a ceramic rotary seal that allows the continuous separation by centrifugation to take place. The equation of motion for flow inside a split toroid cavity within the rotary seal has been solved numerically; velocities and shear stresses found numerically compare favorably with limiting case, analytical solutions. Predicted torque values as a function of rotation rate and fluid viscosity also served as an experimental check on the validity of the mathematical findings. Comparison of calculated shear stress levels and exposure times with known thresholds for cell damage shows that platelet and leukocyte losses may indeed be caused by the seal. Suggestions are made to improve performance of the IBM Blood Processor.