The use of the traditional growing-from approach to prepare surface-initiated polymer brushes is widespread as it produces polymer brushes with higher grafting densities than grafting-to methods. In this article, we present an investigation of a passing-though approach that supplies the monomer from below the initiator-functionalized surface, inverting the concentration gradient found in the traditional growing-from technique that has been shown to increase the D. of brushes. Using Fourier transform infrared (FTIR) spectroscopy mapping combined with substrate masking, we show that the brushes incorporate only monomer diffusing from below and not from the surrounding solution. Further, we characterize these brushes with contact angle analysis, FTIR, and atomic force microscopy and compare them to brushes synthesized by the traditional growing-from approach. Finally, we demonstrate that several properties of the zwitterionic polymer brush prepared by our passing-through method, for example, wettability, grafting density, uniformity, salt permeation retardation, and fouling resistance, are superior to those of brushes prepared by the growingfrom technique.