a membrane for hemodialysis is prepared by casting , extruding or spinning a solution of a polyether - polycarbonate block copolymer and a swelling agent having a molecular weight of about 1 , 000 to about 20 , 000 daltons , then partially evaporating the solvent , gelling and washing . the membrane has a top cut - off value less than about 68 , 000 daltons , and hence is substantially impermeable to albumin , but nonetheless provides high permeability to water and to other small molecules .