Massachusetts Institute of Technology materials scientist Niels Holten-Andersen has long wondered how he could steal strategies that certain organisms have evolved to build biopolymer materials and use them to tune the behavior of synthetic polymers. The effort has paid off, as Holten-Andersen and his colleagues have now hatched a simple but general approach to tuning mechanical and optical properties of polymers by binding their building blocks to metals. The researchers created a class of polymers, which they call metallogels, made from polyethylene glycol units linked together with metal-ligand chemistry. By altering the metal complex composition, the team can tailor a polymer’s behavior. For instance, the researchers controlled the mechanical properties of one set of polymers by cross-linking polyethylene glycol units with histidine bound to different transition metals, including zinc, nickel, and copper (Nat. Mater. 2015, DOI: 10.1038/nmat4401). By swapping out the transition metal-histidine cross-linkers for lanthanide-terpyridyl complexes, the researchers created metallogels with tunable optical properties (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b07394). Like other luminescent materials, such as quantum dots, the metallogels emit characteristic colors when exposed to ultraviolet light. But these polymers also reversibly change color in response to shifting temperatures, mechanical strains, and pH values, meaning they could be used as sensitive “smart” paints and coatings, Holten-Andersen says.