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Comprehensive Summary: With high water content, excellent biocompatibility and lubricating properties, and a microstructure similar to that of the extracellular matrix, hydrogel is becoming one of the most promising materials as a substitute for articular cartilage. However, it is a challenge for hydrogel materials to simultaneously satisfy high loading and low friction. Most hydrogels are brittle, with fracture energies of around 10 J·m−2, as compared with ∼1000 J·m−2 for cartilage. A great deal of effort has been devoted to the synthesis of hydrogels with improved mechanical properties, such as increasing the compactness of the polymer network, introducing dynamic non‐covalent bonds, and increasing the hydrophobicity of the polymer, all at the expense of the lubricating properties of the hydrogel. Herein, we develop a hydrogel material with anisotropic tubular structures where the compactness gradually decreases and eventually disappears from the surface to the subsurface, achieving a balance between lubrication and load‐bearing. The porous layer with hydrophilic carboxyl groups on the surface exhibits extremely low friction (coefficient of friction (COF) ∼0.003, 1 N; COF ∼0.08, 20 N) against the hard steel ball, while the bottom layer acts as an excellent load‐bearing function. What is more, the gradual transition of the tubular structures between the surface and the subsurface ensures the uniform distribution of friction stress between a lubricating and bearing layers, which endows the material with long‐lasting and smooth friction properties. The extraordinary lubricious performance of the hydrogels with anisotropic tubular structure has potential applications in tissue engineering and medical devices. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: With high water content, excellent biocompatibility and lubricating properties, and a microstructure similar to that of the extracellular matrix, hydrogel is becoming one of the most promising materials as a substitute for articular cartilage. However, it is a challenge for hydrogel materials to simultaneously satisfy high loading and low friction. Most hydrogels are brittle, with fracture energies of around 10 J·m−2, as compared with ∼1000 J·m−2 for cartilage. A great deal of effort has been devoted to the synthesis of hydrogels with improved mechanical properties, such as increasing the compactness of the polymer network, introducing dynamic non‐covalent bonds, and increasing the hydrophobicity of the polymer, all at the expense of the lubricating properties of the hydrogel. Herein, we develop a hydrogel material with anisotropic tubular structures where the compactness gradually decreases and eventually disappears from the surface to the subsurface, achieving a balance between lubrication and load‐bearing. The porous layer with hydrophilic carboxyl groups on the surface exhibits extremely low friction (coefficient of friction (COF) ∼0.003, 1 N; COF ∼0.08, 20 N) against the hard steel ball, while the bottom layer acts as an excellent load‐bearing function. What is more, the gradual transition of the tubular structures between the surface and the subsurface ensures the uniform distribution of friction stress between a lubricating and bearing layers, which endows the material with long‐lasting and smooth friction properties. The extraordinary lubricious performance of the hydrogels with anisotropic tubular structure has potential applications in tissue engineering and medical devices. [ABSTRACT FROM AUTHOR]