Some examples herein provide a hydrogel on a substrate. The hydrogel includes a three-dimensional network of polymer chains; first functional groups coupled to the polymer chains; amplification primers coupled to the polymer chains via the first functional groups; and second functional groups coupled to the polymer chains and reversibly cross-linking the polymer chains to one another. Some examples herein provide a method of using a hydrogel. The method includes hybridizing a target polynucleotide to an amplification primer coupled to a hydrogel; cleaving cross-linkages within the hydrogel within which the target polynucleotide is hybridized to the amplification primer; and amplifying the target polynucleotide using additional amplification primers within the hydrogel within which the cross-linkages have been cleaved.