Intrigued by this phenomenon, we examined the possible deposition of polyelectrolytes from the solution onto the surface using Quartz Crystal Microbalance (QCM, see Methods). Fig. shows that the frequency continued to decrease when a mixture of PAA and PAH was flown over the PEM-coated surface (solid line). PEM-coated surfaces appeared to enable a continuous build-up of new but surfacebound polyelectrolyte multilayers. This experiment was validated by comparing the obtained results with an uncoated surface (dashed line). In both cases, a Langmuir-shape response did not establish , suggesting that a steady state could not be reached within the timescales of our experiments (30 min is the longest time interval used in this study). Instead, a continuous deposition of polyelectrolytes on the surfaces occurred. Thus, both initially uncoated and PEM-coated surfaces can enable a continuous buildup of new surface-bound polyelectrolyte multilayers (or, perhaps, surface-bound polyelectrolyte complexes), with a rate that is higher when the surface was coated with PEMs. The build-up can be prevented using an oligo ethyleneglycol-functionalized PLL (PLL-OEG25%, with the 25% indicating the percentage of lysine groups that are functionalized) (dotted line). Fig. summarizes our findings and shows that the disassembly of PEC occurred under the condition with optimal charge density (pH values between pKa1 and pKa2, see Supplementary Information 3.2). The assembly of PECs, instead, took place at pH values that increasingly deviated from this region, unless a PLL-coated surface was used. We validated this effect by two additional experiments wherein we changed the third layer of the PEM coating. First, we used a fluorescein 5 isothiocyanate-labeled PLL (PLL-FITC0.3-1%) as the third layer to examine if PLL could desorb from the surfaces. 30 Supplementary Information, Fig. shows that the collected output is fluorescently active, confirming that (a trace amount of) PLL-FITC appears to be stripped of the surfaces as charged species, travel through the microfluidic channel. Second, we repeated the experiment with a pH gradient of 4 h -1 with PLL-OEG25% as the third layer to show that the history-dependency in PEC formation can be established on a coated but effectively uncharged surface. Supplementary Information, Fig. shows that the complexation of PAH and PAA and the disassembly of PAH-PAA occur at different pH values (5.5, and 9.0), and, thus, confirms that history dependency of the functionalized but uncharged surface is similar to the observed behavior for unfunctionalized surfaces. How exactly PEM-coated surfaces could provide a mechanism to temporarily retain polyelectrolyte complexes in the microfluidic channel and thereby support the formation of PECs is beyond the scope of this work. A thorough investigation of the mechanisms underlying our observations would require more details on the types of PECs in the solution and their affinities with the surface-bound PEM. Overall, our observations show that PLL-coated surfaces can allow the polyelectrolytes to adsorb onto, and PLL to desorb from, the surfaces, thereby supporting rather than perturbing the formation of PEC.