Self-supporting biocatalytic polyelectrolyte complex hollow fiber membranes via salt-dilution induced phase separation

The potential use of biocatalytic membranes in industries such as water treatment, pharmaceuticals, and medicine has attracted much interest in recent years. However, the use of organic solvents in traditional membrane fabrication techniques presents a challenge in the direct integration of enzymes during the membrane production. Thus, enzyme immobilization relies on consecutive modification steps (coating, binding, activation). Yet, recent progress in membrane fabrication using all-aqueous phase inversion of polyelectrolyte complexes (PEC) enables the direct incorporation of enzymes in the polymer solution. We present the fabrication of free-standing, defect-free and mechanically stable biocatalytic PEC hollow fiber membranes via the salt-dilution induced phase separation method. A polymer solution containing poly(sodium-4-styrenesulfonate) (PSS), poly(diallyldimethylammonium chloride) (PDADMAC), KBr, and the enzyme alkaline phosphatase (ALP) was extruded through a spinneret with a mixture of water and glycerol as lumen fluid into a water coagulation bath at 0 °C. The mechanical stability of the fibers is tuned by subsequent photo-induced cross-linking using 4,4’-diazidostilbene-2,2’-difulfonate tetrahydrate (DAS) and UV light. The resulting membranes have a molecular weight cut-off of 400 Da, a pure water permeance of 1.5 LMH/bar, and a burst pressure of 7 bar. The immobilization of ALP proves successful, as the immobilized shows similar catalytic activity to that of the free enzyme, and both storage and thermal stability are improved. Moreover, the enzymes retain 70 % of their initial activity after UV cross-linking with DAS and maintain their catalytic activity after repeated wash and filtration cycles. This work highlights the versatility of the fabrication of polyelectrolyte complex hollow fiber membranes via the salt-dilution induced phase separation, serving as a platform technology for the fabrication of porous, selective and bioactive membranes.