Di(2-picolyl)amine-functionalized poly(ethylene glycol) hydrogels with tailorable metal–ligand coordination crosslinking

A new class of metallo-hydrogels has been developed using di(2-picolyl)amine (DPA)-functionalized 4-arm polyethylene glycol (4A-PEG-DPAn) polymers crosslinked by metal–ligand coordination. The number of the DPA group per polymer core was controlled to prepare 4A-PEG-DPAn polymers with n = 4 or 8. The 4A-PEG-DPAn polymers were crosslinked using four metal ions (i.e., Ni2+, Co2+, Cu2+, and Zn2+) to form 4A-PEG-DPAn (M2+) hydrogels. The 4A-PEG-DPA8 (M2+) hydrogels presented denser networks and longer relaxation times than the 4A-PEG-DPA4 (M2+) hydrogels, indicating the number of the DPA group per polymer core played a dominant role in determining the crosslinked network and the properties. The 4A-PEG-DPA8 (Ni2+) hydrogels possessed the most highly crosslinked network among the four types of 4A-PEG-DPA8 (M2+) hydrogels due to the strong binding affinity between DPA and Ni2+ ions. Given the dynamic DPA–M2+ coordination crosslinks in the network, 4A-PEG-DPAn (M2+) hydrogels were capable of stimuli-responsiveness (i.e., pH, temperature, and redox), self-healing, shear-thinning, and injectability. Taken together, we have demonstrated that the microstructures and properties of 4A-PEG-DPAn (M2+) hydrogels can be tailored by controlling the number of the DPA group per polymer core as well as the choice of metal ions, showing great potential for specific applications.