Competing activation as a result of a three-stage process. Our experimental setup consists of low-pressure pumps that con�nuously feed the PLL-coated channel with an acid, a base, and a pH indicator (phenol red, I) (Fig. ). We first developed a mathema�cal model (Supplementary Informa�on S4) to simulate how the compe��on between the bulk ac�va�on and the local ac�va�on could occur as part of a three-stage process (Fig. ): i) an acid (indicated as H + ) is used to protonate, to 'charge', the free amine groups of the lysine residues that are immobilized on the surface. The state of phenol red (protonated form, I1, or deprotonated form I2) depends on the pH. In this stage, only phenol red in state I1 should be present in the channel; ii), a base (indicated as '-') is used to change the pH which forces the transi�on from I1 into I2. The reac�ve layer, however, causes the opposite transi�on by either protona�ng phenol red or neutralizing the base locally; iii) the capacity of the reac�ve surface is effec�vely limited by the number of lysine residues available in the reac�ve layer and the system establishes a steady state concentra�on of I2, or more specifically, the ra�o [I2]/[I1].