MXene possesses great potential in enriching the functionalities of hydrogels due to its unique metallic conductivity, high aspect ratio, near-infrared light (NIR light) responsiveness, and wide tunability, however, the poor compatibility of MXene with hydrogels limits further applications. In this work, we report a uniformly dispersed MXene-functionalized poly-N-isopropylacrylamide (PNIPAM)/poly-2-acrylamido-2-methyl-1-propanesulfonic acid (PAMPS) double network hydrogel (M–DN hydrogel) that can achieve switchable friction regulation by using the NIR light. The dispersity of MXene in hydrogels was significantly improved by incorporating the chitosan (CS) polymer. This M–DN hydrogel showed much low coefficient of friction (COF) at 25 °C due to the presence of hydration layer on hydrogel surface. After illuminating with the NIR light, M–DN hydrogel with good photothermal effect rapidly raised the temperature to above the lower critical solution temperature (LCST), which led to an obvious increase of surface COF owing to the destruction of the hydration layer. In addition, M–DN friction control hydrogel showed good recyclability and controllability by tuning "on-off" of the NIR light. This work highlights the construction of functional MXene hydrogels for intelligent lubrication, which provides insight for interface sensing, controlled transmission, and flexible robotic arms.

Synovial fluid is made up of various biomacromolecules, including hyaluronic acid, aggrecans, lubricins, and phosphatidylcholine lipid, which are assembled onto the surface of articular cartilage in a gel state. Among them, brush-like biomacromolecules or assemblies have a vital effect on human joint lubrication. Inspired by this, the combination of brush-like molecular structures and gel-like assembly may be an efficient approach for the synthesis of biomimetic lubricating matters. Learning from the lubrication system of human joints, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes grafted poly(N-isopropylacrylamide-co-acrylic acid) (poly(NIPAAm-co-AA)) microgels, abbreviated as MBs-g-MGs, were synthesized as one kind of biomimetic lubricating additives. It is worth noting that this bionic strategy considered both molecular structure and assembled form, which enabled this hairy microgel to achieve low friction in aqueous medium. Meanwhile, the effective lubrication was still achieved when using MBs-g-MGs at high temperature, indicating that this microgel maintains a good lubricating effect over a wide range of temperature. In addition, this kind of microgel possessed good biocompatibility, which laid the foundation for potential biomedical applications. Looking beyond, these biomimetic microgels may provide an effective lubricating effect for water-based sliding interfaces, especially in biomedical systems.