Abstract
Biomaterials that are used in biological systems, such as polycarbonate urethane (PCU) knee joint implants and contact lenses, generally lack lubrication. This limits their integration with the body and impedes their function. Here, we propose a nanostructured film based on hydrophilic polysaccharide hyaluronic acid conjugated with dopamine (HADN) and zwitterionic reduced glutathione (Glu), which forms a composite coating (HADN-Glu) to enhance the lubrication between cartilage and PCU. HADN was synthesized by carbodiimide chemistry between hyaluronic acid and dopamine and deposited on PCU surface under mild oxidative conditions. Then, zwitterionic peptide-reduced glutathione was bioconjugated to HADN, forming a lubrication film. Analysis based on X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and wettability indicated that HADN and Glu had grafted successfully onto the PCU surface. Measurements of the coefficient of friction (COF), friction energy dissipation and cartilage roughness indicated that cartilage was effectively protected by the high lubrication of HADN-Glu. Both at low and high applied loads, this effect was likely due to the enhanced boundary lubrication enabled by HADN-Glu on the PCU surface. Moreover, HADN-Glu is highly biocompatible with chondrocyte cells, suggesting that this film will benefit the design of implants where lubrication is needed.
Highlights
Biolubrication in living systems is based on maintaining a fluid film between articulating surfaces, whose thickness dictates the lubrication mechanism [1,2]
Boundary lubrication is attributed to Lamina splendens, which is an acellular and non-fibrous layer [7] adsorbed on parallelly-oriented collagen fibrils at the surface of the cartilage
Caligaris et al has shown that shifting from fluid film lubrication to boundary lubrication with high friction could happen in the fiction system of cartilage-glass [22]
Summary
Biolubrication in living systems is based on maintaining a fluid film between articulating surfaces, whose thickness dictates the lubrication mechanism [1,2]. For instance, is known to be lubricated through hydrostatic weeping lubrication [3,4] at high loads due to its biphasic structure [5,6] and through boundary lubrication at low loads during the gait cycle. The extremely low kinetic coefficient of friction (COF m~0.005) [10,17] observed in the knee joint is due to lubricating molecules, whereby HA increases the viscosity of the synovial fluid and works synergistically with glycoprotein and zwitterionic lipid to enhance boundary lubrication [18]. A coating of poly (2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate) [poly(MPC-co-BMA)] on polyurethane (PU) surface yields low friction due to sufficient hydration and boundary lubrication [23]. Its lubrication properties against cartilage are suboptimal [24]; during the swing phase of the gait cycle the PCU-cartilage friction is an order of magnitude higher than meniscus-cartilage friction [4], which can potentially cause cartilage damage [4] in the long run
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