Verification of physicochemical structures of dialysis membrane using reversal dialysis technique

Solute removal performance of the dialyzer is closely related to physicochemical structures of the membrane. The objective of this study is to devise a new in vitro evaluation technique to directly correlate the physicochemical structures of the membrane to the dialysis performance.

Commercial 11 dialyzers with cellulose triacetate (CTA), ethylene vinylalcohol co-polymer (EVAL), asymmetry CTA (termed ATA), 4 polysulfone (PSf), polyether sulfone (PES), and 3 polyester polymer alloy (PEPA) membranes were employed for investigation. Normal dialysis experiments were performed with aqueous creatinine (MW 113), bromophenol blue (MW 670), vitamin B₁₂ (MW 1355), chymotrypsin (MW 25,000) test solutions. Reversal dialysis experiments were devised introducing the test solution outside the hollow fiber (HF) and the dialysate inside the HF, respectively. Clearances were measured under Q_B  = 200 mL/min and Q_D  = 500 mL/min at 310 K. The ratio of clearance in the reversal dialysis to that in the normal dialysis termed the yielding factor of asymmetry, f_YA , was defined for evaluation.

According to the classic mass transfer theory, clearance in the normal dialysis and that in the reversal dialysis are expected to be identical; however, the average f_YA was 0.96 for creatinine and was always slightly lower than unity in 10 dialyzers out of 11. This may be caused by un-uniform distribution of the test solution outside the HF, which was more likely to occur than when it flowed inside the HF. Unlike creatinine, the average f_YA for vitamin B₁₂ was 1.0 and that for chymotrypsin was 1.06 to 1.45 in membranes with heterogeneous structures (ATA, PSf, PEPA, and PES), but was almost unity (1.02) in homogeneous membrane (CTA). This must be based on the fact that the membrane permeability in the reversal dialysis is much larger than that in the normal dialysis due to the physicochemical structures of the heterogeneous membrane that has a wedge-like pore size distribution in the radial direction.

A wedge-like pore size distribution in the radial direction in heterogeneous membrane was semiquantitatively evaluated by introducing a reversal dialysis technique with a new index.