An immobilizing device for biological material comprises a rigid support (12) carrying a substrate layer (20, 20′) of polymer having biological immobilizing properties, e.g. for amino and nucleic acids. Substantially solid ultra-thin substrate layers (20′) having a thickness less than about 5 micron, preferably between about 0.1 and 0.5 micron, and micro-porous, ultra-thin substrate layers (20′) having a thickness less than about 5 micron, preferably less than 3 micron, 2 or 1 micron are shown, which may be segmented by isolating moats M. The substrate layer is on a microscope slide (302), round disc (122), bio-cassette, at the bottom of a well of a multiwell plate, and as a coating inside a tube.