Fluid mechanics and crossfiltration in hollow-fiber hemodialyzers.

The efficiency of a hemodialyzer is largely dependent on its ability to facilitate diffusion between blood and dialysis solution. The diffusion process can be impaired if there is a mismatch between blood and dialysate flow distribution in the dialyzer. For this reason it is important that average and regional blood and dialysate flow velocities do not differ significantly. Single-fiber flow velocity should be similar in the center and the periphery of the bundle. Similarly, dialysate flow in the central region of the dialyzer and in the peripheral areas should be similar. In this way the best blood-to-dialysate flow countercurrent configuration is obtained, and the diffusive process is optimized. Unfortunately this optimal situation is hard to achieve, and frequently a significant blood-to-dialysate flow mismatch may occur in hollow-fiber hemodialyzers either due to uneven blood flow distribution or to a dialysate channeling phenomenon external to the fiber bundle. Attempts to optimize flows has been made in the blood compartment designing specific blood ports while, in the dialysate, different options have been proposed such as space yarns (spacing filaments preventing contact between fibers) or the moiré structure (waved shape of fibers to prevent contact between adjacent fibers). Furthermore, the process of transmembrane crossfiltration along the length of the dialyzer can be very different in quantity and direction, thus interfering significantly with the diffusion process. In particular, maximal rates of direct filtration (blood to dialysate) are achieved in the proximal part of the dialyzer, while in the distal part ultrafiltration is minimal and it can also change direction producing significant amounts of backfiltration.