Integrating Strength and Toughness into a Hierarchically Heterogeneous Hydrogel
The intrinsic trade-off between strength, stiffness, toughness, and fatigue resistance in hydrogels has historically precluded their use as load-bearing materials in tissue engineering, soft robotics, and artificial muscles. Conventional structural orientation strategies, including ice-templating, mechanical stretching, and hot pressing, yield insufficient hierarchical precision and fail to resolve the strength-toughness conflict. This work reports a hierarchically heterogeneous poly(vinyl alcohol)/cellulose nanofiber (PVA/CNF) hydrogel (HHPC) fabricated via directional freezing assembly followed by stretch-assisted salting out. The resulting core-sheath architecture features a compact, densified crystalline sheath and a 3D-aligned porous core with loose crystallites, seamlessly integrated through an ion-penetration gradient. The HHPC hydrogel achieves a tensile strength of 55.3 MPa, toughness of 1031 MJ m⁻³, stretchability of 3300%, and stiffness of 6.8 MPa, surpassing the fracture energy-modulus trade-off line (Γ = 234.234 kN³/² m⁻² × E⁻¹/²) for existing tough hydrogels. The material exhibits remarkable fatigue resistance and self-regenerative adhesion, with three pieces of HHPC-1.5 hydrogel (50 mm² adhesion area) bearing a 5 kg weight. This hierarchical assembly strategy establishes a new paradigm for integrating mutually exclusive mechanical properties into a single hydrogel system, enabling load-bearing applications previously inaccessible to synthetic hydrogels.