Hollow fiber membranes are self-supporting tubes with a porous wall, in which the lumen carries one stream and the outer surface the other, so the two are separated by a thin selective skin ^[1]. The wall is asymmetric: a dense or finely porous layer sits on a sponge-like support with finger voids, and the support carries the pressure load while the skin sets selectivity ^[1]. Fibers are spun from a polymer dope by phase inversion, and the bore fluid composition, the air gap and the coagulation bath fix the wall structure and the skin thickness ^[1]. Because the wall is thin, the permeance per unit driving force is high, and a module packs a large area into a small volume ^[2]. The same geometry is reproduced in chitosan and in conductive diamond, where the wall chemistry rather than the polymer sets the surface interaction ^[3].