Tribological characterization of various lubricin-mimetic polymers for biolubrication applications

Abstract

Healthy articular joints exhibit highly efficient lubrication and wear resistance. However, joints do not heal easily after injury and they are subjected to severe diseases, such as osteoarthritis (OA). Lubricin, an important component of synovial fluid, is recognized to have a major lubricating role in cartilage. The loss of lubricin is considered to be a factor in the pathology of OA. Lubricin has a bottle-brush architecture, which provides excellent lubrication because it prevents interpenetration between brushes bound to shearing surfaces. Inspired by this bottle-brush structure, a series of lubricin-mimetic polymers (mimLUBs) has been synthesized by the Putnam Group at Cornell. In this study, we used the Surface Forces Apparatus (SFA) to characterize the tribological properties of three different types of synthetic mimLUBs. All polymers shared the same lubricating domain but possessed binding units either (i) at one terminus (ii) at both termini or (iii) randomly distributed throughout their backbone. First, SFA compressive normal force measurements were performed on mimLUB-coated mica surfaces to assess both the uncompressed film thickness and the ability of each mimetic to resist to compression without being squeezed out of the junction. Second, the SFA was used to monitor both the friction forces and the onset of damage between sheared mimLUB-coated mica surfaces. Our data indicate that the two mimLUB polymers with binding units located either at one terminus or at both termini could achieve friction coefficients below the friction coefficient of lubricin sheared between mica surfaces, while the polymer with randomly distributed binding units failed to lubricate. Overall, two out of the three mimLUBs tested had good lubricating and wear protecting abilities, at relatively high shearing velocities and pressures. Collectively, our findings suggest that, by tuning their molecular structure, engineered lubricin-mimetics can achieve good binding affinity and lubricating ability.