Gallol-Rich Hyaluronic Acid Hydrogels: Shear-Thinning, Protein Accumulation against Concentration Gradients, and Degradation-Resistant Properties

We report the multifunctionality of a small adhesive functional group called gallol (three hydroxyls attached to benzene), which is the ubiquitous moiety found in many vegetables and fruits. First, the chemical tethering of gallols to a polysaccharide backbone and the addition of another gallol-rich compound, oligo-epigallocatechin gallate, result in the spontaneous gelation of the hyaluronic acid-gallol, and the cross-linking is due to the extensive level of hydrogen bond formations from both gallol-to-gallol and gallol-to-hyaluronic acid. Second, we found that the gallol-involved cross-linking is reversible, resulting in a shear-thinning effect of the hyaluronic acid-gallol hydrogels, allowing this hydrogel system to be injectable. Third, due to gallol's superior ability to bind proteins via noncovalent interactions, the hyaluronic acid-gallol hydrogels exhibit spontaneous loading of proteins from a buffer solution to the hydrogel inside against the concentration gradient (i.e., active entrapment phenomenon). By simply dipping the gels into a protein-containing solution (270 μg/mL), approximately 93% of the total proteins is actively entrapped into the gels. Furthermore, the protein affinity of the gallols is useful for physically immobilizing the degradation enzyme, hyaluronidase, to prevent the rapid, uncontrolled degradation of the gallol-rich hyaluronic acid gels.