Programmable Hydrogels by Combining Persistent and Transient Dynamic Bonds

Out-of-equilibrium chemistry has been applied to polymer systems to mimic the autonomous behavior of biological materials. In this study, hydrogels with self-healing properties were developed by integrating crosslinks from dynamic and persistent metal-ligand coordination with transient anhydride bonds. Polymers containing terpyridine ligands and carboxylic acid groups were synthesized and crosslinked with divalent metal ions (Fe2+, Ni²⁺, Co2+, Zn²⁺, Cu²⁺). The coordination bonds impart persistent stability, while transient anhydrides formed on treatment with1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) temporarily enhance crosslink density. Distinct behaviors are observed based on the choice of metal, with Ni²⁺ forming robust, stable networks; Zn²⁺ creating moderately dynamic gels; and Cu²⁺ yielding highly dynamic, soft materials. EDC fueling increased storage moduli significantly, with transient effects lasting up to 280 minutes depending on the metal ion. Self-healing experiments demonstrated orthogonal contributions from metal coordination and transient anhydrides, enabling recovery of stress and strain under varying conditions. Additionally, complex and reversible 2D stiffness patterns were generated by spatially controlled EDC treatment of Zn²⁺ and Cu²⁺ hydrogel films, demonstrating programmability and reusability.