Fig 4. In native cartilage, lubricin bound to the surface facilitates HA aggregation near the surface (A). Lubricin likely entraps HA through entanglements, causing a local increase in viscosity near the tissue surface (A, inset). When lubricin is removed from the surface, HA does not aggregate (B) and surface viscosity is likely similar to the bulk (B, inset). Boundary lubrication by lubricin shifts the boundary regime down and increased viscosity near the surface shifts the elastoviscous transition such that the low friction regime occurs at lower sliding speeds consistent with viscous boundary lubrication (C). These phenomena of lubricin and HA replicate the lubrication by synovial fluid which transition to low friction at low speeds. By treating synovial fluid with hyaluronidase, friction is shifted back towards the boundary mode plateau as the fluid viscosity decreases, and trypsin treatment disrupts the synergy between lubricin and HA by digesting lubricin (n = 4; data points represent mean±SEM).