In one example, a flow cell includes a substrate, an electrode positioned on the substrate, and a patterned material positioned on the electrode. In this example, the patterned material includes depressions separated by interstitial regions, and a functionalized surface of the electrode is exposed at each of the depressions. In this example, the flow cell further includes a primer grafted to the functionalized surface in each of the depressions. In another example, a flow cell includes a substrate and a patterned electrode positioned on the substrate. In this other example, the patterned electrode includes depressions separated by interstitial regions, and a functionalized surface of the substrate exposed at each of the depressions. In this other example, a primer is grafted to the functionalized surface in each of the depressions.