A method of making a fluid flow device comprises: providing a substrate of porous material (2) impregnated with a light-sensitive substance (5) in a first state and which is configured to change from the first state to a second state when exposed to light (3), the second state being a solid state that is resistant to a solvent and the first being removable with the solvent; the substrate having a fluid flow channel (7) defined therein, the channel having a depth; exposing a beam of light (3) onto an area of the substrate surface within the fluid flow channel to deliver energy to a volume of the substrate below the area to change the light-sensitive substance to the second state; during exposure, creating a partial barrier to flow of fluid along the channel by controlling the amount of energy delivered to the volume below at least part of the area to change the light-sensitive substance to the second state in a volume of the substrate within the fluid flow channel that has a depth less than the depth of the fluid flow channel; and developing the substrate in the solvent to leave the light-sensitive substance which is in the solid state and remove the light-sensitive substance which is in the other state. The device may be a medical diagnostic device, and the substrate may be a paper substrate or may be a nitrocellulose substrate.