a separatory device is disclosed wherein a bundle of hollow fibers have their end portions potted in a polyurethane tube - sheet , said polyurethane consisting of an isocyanate - containing prepolymer which is non - catalytically crosslinked with a polyol . the resulting tube - sheet has demonstrated an unexpected degree of adherence to the fibers . in addition , essentially no adverse toxicity effects are encountered in biomedical applications of said device .