Polymethylmethacrylate (PMMA) membrane is the first synthetic polymeric hollow fiber used in dialyzers that is known to adsorb beta(2)-microglobulin. Polyester polymer alloy (PEPA), a blend of two polymers, i.e., polyarylate and polyethersulfone, is another dialysis membrane material with adsorption characteristics. In this study, the adsorption and permeation characteristics of BG-1.6PQ (PMMA) and FLX-15GW (PEPA) dialyzers were investigated by performing ultrafiltration experiments using chymotrypsinogen (molecular weight 25,000) and albumin (molecular weight 66,000) as test solutes. Although PMMA and PEPA had the same sieving coefficient for chymotrypsinogen at steady state, PMMA showed approximately 20% higher fractional adsorption than PEPA under the same initial concentrations. The fractional adsorption for albumin was approximately 20% in PEPA regardless of the ultrafiltration flow rate. The fractional adsorption for albumin in PMMA, however, increased as the ultrafiltration flow rate increased and reached 50%-60% after 10 h. Since PEPA has two skins, one inside and one outside the hollow fiber, proteins may have been adsorbed mainly by these two layers. However, since PMMA is a uniform membrane and since the higher the ultrafiltration flow rate, the higher the fractional adsorption found in PMMA, adsorption may be the result of the occlusion of the dense structure of the membrane. The amount of albumin loss is often clinically evaluated by measuring the amount of permeated albumin in the dialysate. However, when dialyzers with adsorption characteristics are examined, the loss by adsorption should also be taken into account.
The separation characteristics of aqueous albumin may be related to its blood compatibility.
Since therapeutic conditions, especially the amount of dialysate, are usually limited in continuous renal replacement therapy (CRRT), selecting an appropriate membrane is more crucial than that in chronic hemodialysis. Under such circumstances, the use of a membrane with adsorption is expected to remove a larger amount of target substances in CRRT. Five commercial dialyzers were investigated to demonstrate the importance of membrane characteristics. The adsorptive characteristics of polymethylmethacrylate (PMMA) membrane were relatively low for cytochrome c (MW 12,400), very strong for alpha-chymotrypsinogen A (MW 25,000) and relatively strong for albumin (MW 66,000), which may be understood that the adsorption in PMMA has the optimal molecular size. On the other hand, polyacrylonitrile showed relatively low affinity and polysulfone showed essentially no affinity to these protein molecules. Time- and concentration-dependent characteristics of clearance for these proteins were also demonstrated in PMMA. Then we concluded that adsorption found in PMMA may be due to the occlusion of protein molecules into pores of entirely dense membrane. Selecting membrane materials is, therefore, important not only in removing inflammatory cytokines but also in considering the loss of albumin in clinical treatments because even albumin can be adsorbed by the membrane used in blood purification therapies.
Objective: 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.Materials and Methods: 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 B 12 (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 5 200 mL/min and Q D 5 500 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.Results and Discussion: 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 B 12 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.Conclusions: 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.
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