Abstract: A method for making on-flow cell three-dimensional polymer structures includes loading a polymer precursor solution onto a flow cell. The polymer precursor solution includes a monomer, a crosslinker, and a photoinitiator. The flow cell includes at least one channel for receiving the polymer precursor solution. The at least one channel has an upper interior surface and a lower interior surface. The method further includes illuminating the polymer precursor solution through a patterned photomask using a light at a wavelength sufficient to activate the photoinitiator. Activation of the photoinitiator polymerizes at least some of the polymer precursor solution underneath apertures in the patterned photomask and forms three-dimensional polymer structures that extend from the upper interior surface to the lower interior surface of the at least one channel.