Healthy articular cartilage, covering the ends of bones in major joints such as hips and knees, presents the most efficiently‐lubricated surface known in nature, with friction coefficients as low as ...0.001 up to physiologically high pressures. Such low friction is indeed essential for its well‐being. It minimizes wear‐and‐tear and hence the cartilage degradation associated with osteoarthritis, the most common joint disease, and, by reducing shear stress on the mechanotransductive, cartilage‐embedded chondrocytes (the only cell type in the cartilage), it regulates their function to maintain homeostasis. Understanding the origins of such low friction of the articular cartilage, therefore, is of major importance in order to alleviate disease symptoms, and slow or even reverse its breakdown. This progress report considers the relation between frictional behavior and the cellular mechanical environment in the cartilage, then reviews the mechanism of lubrication in the joints, in particular focusing on boundary lubrication. Following recent advances based on hydration lubrication, a proposed synergy between different molecular components of the synovial joints, acting together in enabling the low friction, has been proposed. Additionally, recent development of natural and bio‐inspired lubricants is reviewed.
Understanding the origins of the outstanding lubricity of healthy articular cartilage is crucial both to potentially treat osteoarthritis symptoms, and for the design of better biomaterials. The mechanism of lubrication in the joints, in particular focusing on boundary lubrication is reviewed, and the recent development of natural and of bioinspired lubricants are explored.
Lubrication of Articular Cartilage Jahn, Sabrina; Seror, Jasmine; Klein, Jacob
Annual review of biomedical engineering,
07/2016, Letnik:
18, Številka:
1
Journal Article
Recenzirano
The major synovial joints such as hips and knees are uniquely efficient tribological systems, able to articulate over a wide range of shear rates with a friction coefficient between the sliding ...cartilage surfaces as low as 0.001 up to pressures of more than 100 atm. No human-made material can match this. The means by which such surfaces maintain their very low friction has been intensively studied for decades and has been attributed to fluid-film and boundary lubrication. Here, we focus especially on the latter: the reduction of friction by molecular layers at the sliding cartilage surfaces. In particular, we discuss such lubrication in the light of very recent advances in our understanding of boundary effects in aqueous media based on the paradigms of hydration lubrication and of the synergism between different molecular components of the synovial joints (namely hyaluronan, lubricin, and phospholipids) in enabling this lubrication.
•A modified lubrication model considering the influences of journal misalignment is proposed.•Surface topography and thermal balance effects are incorporated.•Influences of misalignment on ...lubrication performances are explored systematically.•Misaligned angle deteriorates the lubrication performances on the interface.
The paper investigates the influences of misalignment on the lubrication performances and lubrication regimes transition of water lubricated bearing. A modified lubrication model considering the influences of journal misalignment is proposed in the paper. Surface topography and thermal balance effects are incorporated in the theoretical model. Influences of misalignment with different surface roughness and radial clearances on lubrication performances are explored systematically. Experimental results verified the correctness of the lubrication model. Results indicate that the maximum pressure and shear stress increase, the minimum film thickness decreases and the eight dynamic coefficients increase with the misaligned angle. The region of boundary lubrication arises, the region of mixed lubrication and hydrodynamic lubrication decreases. Misaligned angle deteriorates the lubrication performances on the interface. Research results have guiding significance for further study of influences of surface topography on the structure design and optimization for such bearings.
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Due to its high molecular weight and viscosity, hyaluronic acid (HA) is widely used for viscosupplementation to provide joint pain relief in osteoarthritis. However, this benefit is ...temporary due to poor adhesion of HA on articular surfaces. In this study, we therefore conjugated HA with dopamine to form HADN, which made the HA adhesive while retaining its viscosity enhancement capacity. We hypothesized that HADN could enhance cartilage lubrication through adsorption onto the exposed collagen type II network and repair the lamina splendens. HADN was synthesized by carbodiimide chemistry between hyaluronic acid and dopamine. Analysis of Magnetic Resonance (NMR) and Ultraviolet spectrophotometry (Uv–vis) showed that HADN was successfully synthesized. Adsorption of HADN on collagen was demonstrated using Quartz crystal microbalance with dissipation (QCM-D). Ex vivo tribological tests including measurement of coefficient of friction (COF), dynamic creep, in stance (40 N) and swing (4 N) phases of gait cycle indicated adequate protection of cartilage by HADN with higher lubrication compared to HA alone. HADN solution at the cartilage-glass sliding interface not only retains the same viscosity as HA and provides fluid film lubrication, but also ensures better boundary lubrication through adsorption. To confirm the cartilage surface protection of HADN, we visualized cartilage wear using optical coherence tomography (OCT) and atomic force microscopy (AFM).
Mixed lubrication describes a lubrication regime where elastohydrodynamic lubrication and asperity contact coexist. Surface spacing is expected to be small during the interaction between nano-scale ...smooth surfaces. When alkali metal salt solution is applied as lubricant, short-range surface forces are incorporated into an existing model to simulate mixed lubricated non-conformal sliding contact. The new model is examined in a wide range of sliding velocities to demonstrate the effect of surface forces at different lubrication regime, from boundary lubrication to full film hydrodynamic lubrication. It is revealed that surface forces reduce friction coefficient particularly at mixed lubrication regime. Reduction of surface roughness improves contribution of surface forces and makes it easier to achieve superlubricity at low speed.
•Model and analyze surface force effect on water-based lubrication.•Surface force effect conduces to achievement of ultra-low friction.•Surface forces mainly take effect at mixed lubrication regime.•Contribution of surface forces is enhanced with decrease of surface roughness.
Recent experiments reveal that adhesive interactions can play a key role in causing surface instability in soft lubrication. Instances of instability include fluid entrapment in isolated pockets upon ...a soft sphere's normal contact with a hard substrate and surface wrinkling of a soft substrate as a hard sphere slides across it. These phenomena underscore a substantial distinction between hard and soft lubrication. They are of paramount importance from a fundamental standpoint, providing an entirely new explanation for the transition mechanism from elasto-hydrodynamic to the mixed lubrication regimes. Here, we introduce a new theory to elucidate these observations. Our theory modifies the Reynolds elasto-hydrodynamic equation by incorporating adhesive interaction across the fluid layer, investigating the interplay between adhesion, fluid flow and elastic instability. Our analysis proposes the addition of a new dimensionless parameter in lubrication theory, that compares the stiffness of the adhesive interaction to that of the substrate. When this parameter exceeds unity, the soft solid surface exhibits instability to small perturbations in its shape. In mathematical terms, the Reynolds equation undergoes a transition from a nonlinear diffusion equation to a nonlinear wave equation at this critical point. Post-transition, the diffusivity of the nonlinear diffusion equation turns negative, rendering the problem ill-posed. We investigate the transition using the method of characteristics and present an exact analytic solution. This solution offers insights into the occurrence of a vanishing liquid film thickness at specific locations, resulting in dry contact—initiating transition to mixed lubrication.
Highly-loaded EHL contacts in gears often operate under starved lubrication, albeit unintentionally. This is frequently correlated to mixed lubrication regimes resulting in reduced lifetime. ...Consequently, the aim of this study is to investigate the effect of starved lubrication on the operating behaviour of EHL contacts. Different amounts of initial oil volumes and surface structures were investigated by measuring the frictional behaviour at the FZG twin-disk test rig. Analytically calculated gap fill factors for initial oil volumes were assigned to the experiments. Results show that a very small amount of initial oil is sufficient for lubrication and different operating behaviours for peripherally ground, axially ground and polished surfaces. All experiments were accompanied by surface photos and roughness measurements.
•Operational behaviour of EHL contacts under starved lubrication was investigated.•Coefficient of friction was measured for different surface structures.•Very small initial oil volumes were found to be sufficient for lubrication.•Required minimal initial oil volume was found to differ with surface structures.•Highest coefficient of friction was found for peripherally ground surfaces.
Lubrication conditions have significant influences on the formation of tribofilms and then affect tribological behavior. In this work, the influence of tribofilm formation on the tribological ...behavior of textured surfaces with oval shapes was measured using a pin-on-plate tribometer. The results show that, under full lubrication, the adsorbed oil film controlled the friction and wear behavior of steel/steel tribopairs but under starved lubrication, the formation of a tribofilm significantly influenced the tribological behavior. The appropriate textured surfaces with oval-shaped dimples contribute to obtaining excellent antifriction and antiwear behavior. However, excessively high ratios of the major to the minor axis of the oval can result in high contact stresses which can destroy the tribofilm.
•Textured surfaces with oval-shaped dimples were facilely fabricated.•Influence of tribofilm formation on the surface tribological behavior was investigated.•The adsorbed-film controlled the friction and wear behavior of tribo-pairs under full lubrication.•The formation of tribofilm dominated the interface behavior under starved lubrication.
In order to improve tribological properties of AISI 4140 steel (AS) for dry/starved lubrication, AS biomimetic self-lubricating material (AS-SACT) was prepared by filling SnAgCu-TiC into biomimetic ...microtextures. Compared with AS, the friction coefficient of AS-SACT was reduced by 5.34% and wear depth was reduced by 62.39% in starved lubrication. Tribological properties of AS-SACT in starved oil are close to those of full oil lubrication. Under starved lubrication, the precipitated lubricant spreads into a 2.2 µm solid film on contact surface. In addition, the lubricant precipitation area forms a 'storage area' for storing lubricating oil and wear debris. The synergistic lubrication effect of multi-solid coating and oil lubrication makes AS-SACT have excellent anti-friction and wear-resisting effect.
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•AS-SACT was designed and prepared by filling SACT into biomimetic microtexture.•The friction properties in starved lubrication are close to those in full oil.•The precipitated lubricants spread into a 2.2 µm solid film on the contact surface.•Lubricant precipitation areas form a 'storage area' for storing oil and wear debris.
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