Transverse and Longitudinal Hydraulic Conductivities in Hollow Fiber Dialyzer Evaluated Using Capillary Flow Analysis

A cylindrical blood dialyzer houses a hollow fiber bundle, where the dialysate inlet and outlet are located at both surface ends. The transverse and longitudinal dialysate flow paths perpendicular and parallel to the hollow fibers, respectively, could be regarded as narrow capillaries. The angle of the dialysate flow toward the central axis measured from the surface of the hollow fiber bundle after injection perpendicular to the bundle is given by the transverse-to-longitudinal hydraulic conductivity ratio κtr/κln. Because the dialysate flow paths parallel to the hollow fibers are always larger than those perpendicular, i.e., κtr/κln < 1, the dialysate fluid can only move easily along the surface of the hollow fiber bundle towards the dialysate outlet without reaching the central axis of this bundle. The transverse-to-longitudinal flow path diameter ratio was evaluated to be 0.3. Because the flow rate through the capillary was proportional to the diameter squared, κtr/κln was estimated to be 0.1, which is about the radius-to-length ratio R/L of the hollow fiber bundle. Current dialyzers were considered to allow the dialysate flow to reach the central axis of the hollow fiber bundle because they satisfy κtr/κln ≥ R/L. Slight decreases in the hollow fiber density were expected to provide distinct increases in κtr/κln and dialysis efficiency by allowing the dialysate flow to reach the central axis while maintaining a comparatively low increase in the fiber bundle's diameter.