Facile biomimetic self-coacervation of tannic acid and polycation: Tough and wide pH range of underwater adhesives

Bioinspired underwater adhesives with tough and stable performance are increasingly in demand for biomedical and engineering applications. However, the current methods of synthesizing them usually require sophisticated chemical conjugation or modification and expensive adhesive building blocks. Here, by taking advantage of the catecholic and polyelectrolyte features of mussel foot proteins, we report a facile yet powerful strategy to the development of a strong and cost-effective self-coacervating adhesive based on a coacervation-induced adhesion mechanism. The adhesive comprises a low-cost, commercially available cationic polyelectrolyte-polyamidoamine-epichlorohydrin (PAE)-that is crosslinked in situ by naturally occurring dendritic molecules of tannic acid (TA). A complex coacervate gel (TAPA) is formed after directly mixing pyrogallol-containing TA with PAE polycation. With the gel matrix serving as a robust adhesive when applied underwater to various substrates owing to its synergistic azetidinium-phenolic electrostatic and hydrogen bonding interactions. Compared to previously reported catechol-based adhesives, the polyelectrolyte coacervate gel has a widely tunable underwater adhesive strength (50.8 +/- 6.8 to 604.8 +/- 9.5 kPa) in wide ranges of pH (3-11) and ionic strength (0-1 M NaCl) and displays reusable adhesiveness (<10 cycles). This adhesive strength increases substantially in basic conditions (pH > 9), which trigger the self-crosslinking of PAE chains. By synergistically combining electrostatic adsorption and macroscopic-scale interactions, the incorporated nanocellulose fillers further contribute to the strong cohesion of coacervate adhesives. The easy-to-prepare TAPA adhesive also has excellent antibacterial activity. This in-situ formation strategy opens an innovative and facile mute to mimic the self-coacervation and adhesion stability of biomimetic source, leading to a multifunctional bonding solution for biological/engineering adhesive applications.