Poly(vinyl alcohol)-Tannic Acid Hydrogels with Excellent Mechanical Properties and Shape Memory Behaviors.

Shape memory hydrogels have promising applications in a wide variety of fields. Here we report the facile fabrication of a novel type of shape memory hydrogels physically crosslinked with both stronger and weaker hydrogen bonding (H-bonding). Strong multiple H-bonding formed between PVA and TA leads to their coagulation when they are physically mixed at an elevated temperature and easy gelation at room temperature. The amorphous structure and strong H-bonding endow the PVA-TA hydrogels with excellent mechanical properties, as indicated by their high tensile strengths (up to 2.88 MPa) and high elongations (up to 1100%). The stronger H-bonding between PVA and TA functions as the "permanent" crosslinks and the weaker H-bonding between PVA chains as the "temporary" crosslinks. The reversible breakage and formation of the weaker H-bonding imparts the PVA-TA hydrogels with excellent temperature-responsive shape memory. Wet and dried hydrogel samples with a deformed or elongated shape can recover to their original shapes when immersed in 60°C water in a few seconds or at 125°C in about 2.5 min, respectively.