Dual ionically cross-linked hydrogels with ultra-tough, stable, and self-healing properties

Excellent mechanical and self-healing features could make hydrogels an ideal candidate for the application of load-bearing soft tissue replacements such as cartilage. In this study, a dual ionically cross-linked 2-hydroxypropyltrimethyl ammonium chloride chitosan (HACC)/poly(acrylic acid) (PAAc)-Fe3+ hydrogel was constructed using a one-pot method (in situ polymerization of AAc in the presence of HACC and Fe3+). Both macromolecular positively charged HACC and Fe3+ metal ions acted as cross-linkers to form ionic bonds with negatively charged PAAc. The HACC/PAAc-Fe3+ hydrogels demonstrated ultra-high mechanical strengths (tensile strength of ca. 9.86 MPa and compressive stresses greater than 95 MPa at 99% strain), excellent self-recoverability (ca. over 90% toughness recovery within 5 h without any external stimuli), outstanding self-healing properties (ca. 74% self-healing efficiency at 70 °C for 48 h), transparency, and high stabilities in aqueous environments. The mechanical properties of the hydrogels could be adjusted by varying the concentration of HACC and Fe3+. This work provides a new approach for the construction of novel tough and transparent hydrogels with a fully ionically cross-linked network.