NOVELTY - An assay device comprises a microchannel (1), and greater than or equal to 2 sets of individually encoded microcarriers having a shape relative to the cross-section of the microchannel that allows having, over the entire length of the microchannel. At least two of any of the microcarriers are arranged side by side without touching each other and without touching the perimeter of the microchannel. The device comprises a filter structure (4) to restrict the movement of the microcarriers in the longitudinal direction of the microchannel while letting the fluids flow through. USE - An assay device used for a chip for multiplexed assays (claimed). ADVANTAGE - The device improves the mass transfer in biological multiplexing assays based on microcarriers and simplifies the overall procedure for preparing the setup, executing the biological assay and performing the necessary readouts.