All 4-PEG-histidine networks were made by a combination of 200 mg/mL 4-PEG-histidine, 0.2 M solutions of either NiCl2, CuCl2 or ZnCl2 and 1.0 M MOPS buffer. The ratio was adjusted such that final viscoelastic fluid consists of 10 wt. % 4-PEG-histidine with a stoichiometric amount of the transition metal to the ligand (ML2 or ML3) and with a final buffer concentration of 0.2 M. All 4-PEG-imidazole networks were made similarly to 4-PEG-histidine. The ratio was adjusted such that final viscoelastic fluid consists of 20 wt. % 4-PEG-imidazole with a stoichiometric amount of the transition metal to the ligand (ML2 or ML4) and with a final buffer concentration of 0.2 M.