Dynamic bonds enable high toughness and multifunctionality in gelatin/tannic acid-based hydrogels with tunable mechanical properties

Biopolymer-based functional hydrogels with excellent mechanical properties are desired, but their fabrication remains a challenge. Learning from the tofu-making process, we developed a freely formable hydrogel with high toughness and stiffness from the hydrogen bond-rich coacervation of tannic acid and gelatin through a simple hot-pressing process that transforms the coacervate particles into a bulk hydrogel. The mechanical properties of the obtained gelatin/tannic acid hydrogel (G/T gel) can be controlled by tuning the weight ratio of tannic acid to gelatin in the gel. The G/T gel with optimum mechanical properties possesses high Young's modulus, fracture strain, and fracture energy of ∼60 MPa, ∼10, and ∼24 kJ m-2, respectively. These properties arise from the phase-separated structure and high concentration of dynamic hydrogen bonds with widely distributed bond strengths. These dynamic hydrogen bonds also enable multifunctional properties of the gel, such as self-recovery, self-healing, rebuildability and shape memory. The combination of excellent mechanical properties, good biocompatibility, and useful functionalities into one hydrogel that comes from renewable sources demonstrates the great potential of G/T gels.