High-strength hierarchically oriented hydrogels with balanced mechanical performance

High-strength and high-toughness hydrogels have demonstrated exciting application prospects in soft robotics, artificial skin, and tissue engineering, but the development of such high-performance soft materials remains a challenging task. Here, inspired by the multiscale oriented hierarchical assembly structure of wood, this study innovatively reproduces the multilevel oriented compositional units of wood by incorporating millimeter-scale pulp fibers, nano-scale bacterial cellulose, and molecular-scale sodium lignosulfonate into pre-stretched polyvinyl alcohol hydrogels. The strong interfacial electrostatic interactions and hydrogen bonds formed between the functional groups of these components effectively stabilize and firmly bundle the hierarchical anisotropic structure. The resulting hierarchically oriented hydrogel exhibits remarkable mechanical properties, including an exceptional fracture strength exceeding 15.5 MPa, an ultimate strain of ~ 130%, and a toughness of ~ 14 MJ/m³, along with enhanced tear resistance. Notably, the hydrogel maintains satisfactory strength and toughness even in non-oriented directions, reinforcing its resistance to complex stress conditions. Furthermore, the composite hydrogel demonstrates extremely low biotoxicity, acceptable blood compatibility, and highly linear strain-sensing behavior. This study presents a natural structure-inspired hierarchical strategy, exploring a promising pathway for fabricating advanced hydrogel materials with outstanding mechanical properties. The efficient construction of hierarchically oriented structures across multiple scales endows the hydrogel with exceptional strength (15.5 MPa) and toughness (14 MJ/m3). The dense continuous phase and well-matched modulus ensure high elongation at break (130%) along the orientation direction, along with excellent breaking strength (6.3 MPa) and toughness (12.7 MJ/m3) in the non-oriented direction. These hydrogels exhibit ultralow biotoxicity, satisfactory hemocompatibility, and linear strain sensing responsiveness, making them suitable for soft robotics and tissue engineering.