Tannic Acid-Based Multifunctional Hydrogels with Facile Adjustable Adhesion and Cohesion Contributed by Polyphenol Supramolecular Chemistry

Adhesiveness of hydrogels depends on the balance and synergy of their cohesion and adhesion. However, it is a challenge to fabricate catechol-based hydrogels with high adhesiveness because the required condition for cohesion and adhesion of these hydrogels is in conflict with each other: strong cohesion (gelation) requires a weak basic condition, whereas strong adhesion requires an acidic condition. Here, we demonstrated that by utilizing polyphenol supramolecular chemistry, the coexistence of strong cohesion and adhesion can be achieved in a hydrogel via the one-pot method. Poly(dimethyl diallyl ammonium chloride)/tannic acid (PDDA/TA) hydrogel has been studied as a proof of concept. Compared with catechol moieties that covalently grafted on polymer chains, TA can bring high density of pyrogallol/catechol functional groups for polymers via a noncovalent pathway, as well as high acidity in the system. As a result, the cohesion of the hydrogel is enhanced significantly, the highest storage moduli can reach up to ca. 0.15 MPa; besides, the high acidity of the hydrogel prevents pyrogallol/catechol groups from oxidation and guarantees strong adhesion; thus, the hydrogel can adhere to diverse substrates steadily, including tissues, glass, metals, and plastic. Moreover, because of the adjustable adhesiveness via changing the pH, the PDDA/TA hydrogel becomes a unique system with patternable adhesiveness. In addition, the hydrogel has rapid self-healing and high ionic conductivity (∼4.3 S m^-1). This study demonstrates that utilizing polyphenol chemistry in the construction of hydrogels opens a new path toward multifunctional hydrogels with improved properties.