A High-Order Double Network Hydrogel

Natural hydrogels, such as cartilage, have a multiscale hierarchical structure composed of multiple modulus-contrasting building blocks, bringing about their extraordinary mechanical properties. Conventional tough engineering hydrogels, such as Double Network (DN) hydrogel, lack cartilage’s high-order aggregated structure across multiple length scales, although they have a molecular composition similar to cartilage. This work focuses on high-order architecture in DN hydrogels through superimposing microphase separation and nanocrystalline domains in interpenetrating molecular networks of stiff chitosan and soft poly(vinyl alcohol) by using freezing-thawing-assisted alkali out. The constructed High-order Double Network (HDN) architecture has a molecular composition identical to initial interpenetrating molecular networks but exhibits hierarchical multiscale discrepancies, including bicontinuous phase at microscale, nanocrystalline domains at nanoscale, and polymer chain packing at subnanoscale. We find that these structural differences are strongly correlated with the macroscopic properties of the hydrogel, such as turbidity, stiffness, strength, and toughness. We reveal the stepwise multiscale fracture mechanism of the HDN architecture that leads to a highly synergistic toughening effect. The HDN hydrogel also exhibits excellent multifunctional properties, including antiswelling, durability, biocompatibility, antibacterial activity, degradability, and plasticity. We believe that the high-order architecture presented in this work would shed new light on the future development of high-performance DN hydrogels that approximate natural hydrogels.