Reagents used to fabricate gels. Alginate-PEG-N3 (1) can form gels via divalent ion cross-linking (e.g. Ba2+), as shown in A, or via incubation with MDT-PEG-MDT (2), where spontaneous covalent cross-links between 1 and 2 are formed via Staudinger ligation, as shown in B. Following gel formation with Ba2+, gels may be further functionalized with MDT-labeled agents, such as MDT-PEG-carboxyfluorescein (3), as illustrated in C (fluorescence image, excited with blue light).