We then used fluorescein-labelled PLL (Mw=15-30kDa, and 0.03-0.1% fluorescein) to confirm that PLL is indeed immobilized on, and homogeneously distributed across the channel (Supplementary Informa�on, Fig. ). We set the pumps to change the input pH in the channel in a sequence that alternates between pH 3 and 10, and determined the output of [I2]/[I1] using an absorbance detector connected to the ou�low of the channel. The output signal follows the patern of the input pH, and in accordance with the predic�ons from the model, the output signal is slightly delayed compared to the input pH. The observa�on that this readout remains consistent throughout mul�ple cycles demonstrates that our experimental pla�orm can establish a robust readout of the intended compe�ng ac�va�on network. To ensure that the coa�ng remained anchored on the surface, we also demonstrated that the fluorescence intensity in the channels is unchanged over the course of the experiment (Supplementary Informa�on, Fig. ).