Via a general diffusion-based method we term ‘Diffusion of Reactive Agent into PErmeable media’, we form micrometer-scale lateral chemical gradients in polymer brushes via selective alkylation. This microfluidic brush gradient generation method enables formation of 2-D chemical potential gradients over length scales of a few hundred micrometers. The source channel for the reactive chemicals can be seen in the far left. The reactants diffuse into the material surrounding the source channel and react with the underlying polymer brush, forming the chemical gradient shown on the right. DRAPE enables elaborate patterns which may facilitate precise control of molecular transport, e.g., directed translation, concentration, or separation, driven by chemical potential gradients