Next, we examined if the surface-bound poly-L-lysine (PLL) could provide the microfluidic channel with the capacity to further perturb the assembly of PECs. That is, we repeated the experiment with a pH gradient of 4 h -1 but using a microfluidic channel with its surface coated with a PLL layer (or, more specifically, a polyelectrolyte multilayer, PEM). We used a three-layered system (PLL-PSS-PLL) as we previously reported that such a polyelectrolyte multilayer (PEM) allows for higher density of PLL on the surface. Fig. shows that the transition for the negative gradient did not change and remained at a pH value between 6.0 and 6.5. The transition for the positive gradient (which occurred at a pH value between 6.5 and 7.0), however, was significantly reduced in comparison to the values when the channel was uncoated. Surprisingly, the addition of positively charged polyelectrolytes on the surface did not lead to a perturbed output but instead strengthened the PEC formation and it enabled the PAH-PAA complex to form at a pH closer to the expected optimum.