A High-Order\nDouble Network Hydrogel

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