Interfacial Adhesion of Fully Transient, Mussel-Inspired Hydrogels with Different Network Crosslink Modalities

In fully transient, mussel-inspired hydrogels, metal-coordinate complexes form supramolecular crosslinks, which offer tunable viscoelastic properties and mechanical reversibility. The metal-coordination complexation that comprises the crosslinks can take on tris-, bis-, mono-, and free-state modalities (3, 2, 1, or 0 ligands per ion, respectively). Although prior work has established relationships between network crosslinking and mechanical properties, the effect of crosslink and ligand modalities on gel-surface adhesion is not well understood for fully transient hydrogels. Using glass and nickel-coated spherical probes, the effect of network crosslinking modalities on the adhesive strength of hydrogels based on histidine-Ni²⁺ and nitrodopamine-Fe³⁺ ion crosslinks is investigated. Since crosslink modalities have a strong impact on the mechanical properties of the bulk network, it is first determined how adhesion relates to the mechanical properties, regardless of the distribution of crosslinking modalities and ligand type. It is ultimately found that the peak adhesive stress increases with decreasing percentage of ligands in tris-crosslinks.