Hierarchically reversible crosslinking polymeric hydrogels with highly efficient self-healing, robust mechanical properties, and double-driven shape memory behavior

Designing hydrogels with both excellent mechanical properties and self-healing ability has attracted significant attention because the applications of hydrogels are restricted in many fields due to their poor mechanical properties and short service life. Herein, a novel type of hierarchically reversible crosslinked hydrogel was designed and fabricated using a poly(vinyl alcohol)-borax (PVA/borax) hydrogel and poly(1-allyl-2-thiourea-co-acrylamide) (P(ATU-co-AM)), where P(ATU-co-AM) was interpenetrated in the PVA/borax network to form a semi-interpenetrating network (semi-IPN). The PVA crystalline domains generated from the freezing-thawing process served as second crosslinking points in the semi-IPN structure, further enhancing the mechanical properties of hydrogels. The obtained hydrogels exhibited excellent mechanical properties (tensile strength of 3.6 MPa), rapid self-healing procedure (10 min), high healing efficiency (over 90%), agile pH-driven shape memory behavior (recovered within 260 s) and Ca2+-driven shape memory behavior, which are attributed to the reversible backbones imposed by the dynamic borate bonds, abundant hydrogen bonds from PVA and P(ATU-co-AM) chains, and crystallinity of PVA through freezing-thawing cycles. Taking advantage of these fabulous performances and inspired by the bending behavior of Mimosa, a soft catcher actuator was constructed as an example to illustrate the practical applications of this hydrogel, which could catch a plastic fish in Ca2+ solution. Thus, due to the self-healing property, actuators with complicated architectures and excellent shape memory behavior can be fabricated by assembling different double layer hydrogels. This study provides a novel strategy to prepare robust hydrogels without sacrificing their self-healing ability and mechanical properties, hopefully promoting the research on robust self-healing hydrogels.