Current membrane technologies are important in many industrial separation processes, especially in water purification and desalination. However, the large operating pressure gradient that is required for conventional reverse osmosis filtration method makes water purification a very expensive process. Aquaporins are transmembrane water channel proteins that exist in biological plasma membranes. Aquaporins aid in biological water filtration processes by transporting water molecules at high speed, while selectively blocking out other kinds of solutes. Innovative aquaporin-reconstituted biomimetic membranes are envisaged to overcome the problem of high operating pressures needed, and hold great potential for the use in water purification processes, giving high flux while keeping the energy consumption low. It has been reported that the osmotic permeability of a single aquaporin is in the range from 6×10-14 to 24×10-14 m3 s-1, and the permeability of an aquaporin-reconstituted biomimetic membrane is 167 mm/s/bar, which is by two orders of magnitude greater than commercial polymeric membranes. Therefore, aquaporin-based biomimetic membranes have attracted worldwide attention among those attempting to design a suitable biomimetic device for water treatment.
Three critical factors should be considered in designing an ideal separation membrane using aquaporins. The first one is that aquaporins must retain their water selectivity and transportation capability in the biomimetic membrane matrix. The second factor is that salt leakage should be minimum for the barrier layer. Last but not least, the mechanical strength of the membrane matrix must be sufficiently strong in the final application. Considering these three factors, we designed and fabricated novel aquaporin-based biomimetic membranes for forward osmosis and nanofiltration respectively. We functionally reconstituted E. coli protein aquaporin Z into vesicles that made from an amphiphilic triblock copolymer poly(2-methyloxazoline)-b-poly(dimethylsiloxane)-b-poly(2-methyloxazoline). Vesicles reconstituted with aquaporin Z were then coated onto a porous support membrane (cellulose acetate or alumina membrane). The structure and permeability of vesicles were confirmed by confocal laser scanning microscopy and a stop-flow apparatus. Membrane performance was characterized by nanofiltration and forward osmosis test. Both water flux and salt rejection were greatly increased for the Aquaporin Z embedded membrane compared with a membrane without Aquaporin Z, indicating that the water channel protein made a significant contribution to the water treatment performance.