Double-Layered Electrospun Nanofiber Filter for the Simultaneous Removal of Urea and Ammonium from Blood

A major challenge in the development of wearable artificial kidneys (WAKs) lies in the efficient removal of urea, which is found at an extremely high concentration in the blood of patients with chronic kidney disease (CKD). Urease is an enzyme that hydrolyzes urea. While it can efficiently remove urea, toxic ammonium is produced as a byproduct. In this study, nanofibers capable of removing both urea and ammonium from the blood were fabricated. Specifically, urease was immobilized on electrospun poly(ethylene-<i>co</i>-vinyl alcohol) (EVOH)/chitosan nanofiber membranes via covalent cross-linking. Chitosan not only helped covalent immobilization via its free amino groups but also improved hemocompatibility by suppressing protein adhesion. The resulting urease-immobilized EVOH/chitosan nanofibers exhibited an outstanding urea removal performance of 690 mg/g per hour. For ammonium removal, EVOH nanofiber membranes containing sodium cobalt(II) hexacyanoferrate(II) (NaCoHCF), an ammonium adsorbent, were prepared. The fabricated EVOH/NaCoHCF membranes exhibited an ammonium adsorption capacity of 135.5 mg/g. The two types of nanofiber membranes were combined to form a double-layered nanofiber membrane that was placed in a filter holder for continuous-flow cycling experiments. Under such conditions, all urea at a concentration similar to that in the blood of CKD patients was degraded within 1 h, and ammonium production was reduced by approximately 90% of the normal level. This double-layered nanofiber membrane can achieve both urea degradation and ammonium adsorption and is expected to advance the development of WAKs, a game changer in the treatment of CKD.