Ultra-low Friction and High Load-Bearing Hydrogel with Tubular Structure Based on Controllable Light-Induced Dissociation

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<middle dot>m(-2), as compared with similar to 1000 J<middle dot>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) similar to 0.003, 1 N; COF similar to 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.