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.