Boundary film–liquid mixed lubrication of biomimetic lubricants on cartilage interfaces

Lubricin in synovial fluid is the primary boundary lubricant for articular cartilage. Its excellent boundary lubricating capability comes from its unique bottlebrush structure. In this study, a biomimetic polymer containing 2-hydroxyethyl acrylate and 2-methacryloyloxyethyl phosphorylcholine (MPC) was synthesized. This polymer is structurally similar to lubricin and achieves a water-trapping capacity similar to that of lubricin through the design of the pMPC moiety. Atomic force microscopy (AFM) imaging indicates that changes in the molecular chain length and concentration lead to different self-assembled polymer structures. The lateral force between the AFM probe and silicon surface covered by the polymer demonstrates that the self-assembled structure of polymer molecules on the surface affects its boundary lubrication ability. When polymers are adsorbed onto the surface of articular cartilage to replace natural lubricin, the system exhibits extremely low friction (~0.028) under boundary film–liquid mixed lubrication conditions, which is close to the coefficient of friction (COF, ~0.024) of natural lubricin under the same conditions. The study of this biomimetic polymer provides a strategy for lubricating damaged cartilage during the initial stages of osteoarthritis development.