A method of making a fluid flow device comprises providing a substrate of porous material, depositing a radiation-sensitive substance onto the substrate in a pattern defining one or more regions intended to receive and contain fluid during use of the device or occupying an area within such a region, such that the radiation-sensitive substance extends at least partly through the thickness of the substrate below the pattern, and exposing radiation onto the substrate thereby delivering energy to the radiation-sensitive substance in at least part of the pattern to change the radiation-sensitive substance from a first state to a second state through at least part of the thickness of the substrate. One of the first state and the second state may be less permeable than the other.