Shape retaining self-healing metal-coordinated hydrogels

Abstract

Metal-coordinated hydrogels are physical hydrogels entirely crosslinked by complexes between ligand decorated polymers and metal ions. The mechanical properties of these hydrogels strongly depend on the density and dynamics of metal-coordinated interactions. Most commonly, telechelic metal-coordinated hydrogels contain catechol or histidine ligands, although hydrogels containing a stronger complexation agent, nitrocatechol, have been reported. Here, we introduce a pyrogallol end-functionalized polymer that can be crosslinked with di- and trivalent ions, in contrast to previously reported metal-coordinated hydrogels. We can tune the mechanical properties of the hydrogels with the types of ions used and the density of crosslinking sites. Ions form nm-sized precipitates that bind to pyrogallols and impart distinct properties to the hydrogels: strong ion–pyrogallol interactions that form in the presence of Al³⁺, V³⁺, Mn²⁺, Fe³⁺, Co₂⁺, Ni²⁺ and Cu²⁺ result in long relaxation times. The resulting hydrogels display solid-like yet reversible mechanical properties, such that they can be processed into macroscopic 3D structures that retain their shapes. Weak ion–pyrogallol interactions that form in the presence of Ca²⁺ or Zn²⁺ result in short relaxation times. The resulting hydrogels display a fast self-healing behavior, suited for underwater glues, for example. The flexibility of tuning the mechanical properties of hydrogels simply by selecting the adequate ion–pyrogallol pair broadens the mechanical properties of metal-coordinated hydrogels to suit a wide range of applications that require them to retain their shape for a given time to act as dampers.