A device for assay of a liquid sample. The device comprises: a support composed of an organic macromolecule, said support having a surface divided into two areas located adjacent to each other; a divider in the surface, defining the border of both areas to separate a first area from a second area; a detection layer affixed to the first area and containing a reagent; and a water-swelling layer affixed to the second area, said water-swelling layer expanding by absorbing water.