Surfactants identified in synovial fluid and their ability to act as boundary lubricants

How Do Cartilage Lubrication Mechanisms Fail in Osteoarthritis? A Comprehensive Review

Cartilage degeneration is a characteristic of osteoarthritis (OA), which is often observed in aging populations. This degeneration is due to the breakdown of articular cartilage (AC) mechanical and tribological properties primarily attributed to lubrication failure. Understanding the reasons behind these failures and identifying potential solutions could have significant economic and societal implications, ultimately enhancing quality of life. This review provides an overview of developments in the field of AC, focusing on its mechanical and tribological properties. The emphasis is on the role of lubrication in degraded AC, offering insights into its structure and function relationship. Further, it explores the fundamental connection between AC mechano-tribological properties and the advancement of its degradation and puts forth recommendations for strategies to boost its lubrication efficiency.

Liposomal aggregates sustain the release of rapamycin and protect cartilage from friction

Liposomes show promise as biolubricants for damaged cartilage, but their small size results in low joint and cartilage retention. We developed a zinc ion-based liposomal drug delivery system for local osteoarthritis therapy, focusing on sustained release and tribological protection from phospholipid lubrication properties. Our strategy involved inducing aggregation of negatively charged liposomes with zinc ions to extend rapamycin (RAPA) release and improve cartilage lubrication. Liposomal aggregation occurred within 10 min and was irreversible, facilitating excess cation removal. The aggregates extended RAPA release beyond free liposomes and displayed irregular morphology influenced by RAPA. At nearly 100 µm, the aggregates were large enough to exceed the previously reported size threshold for increased joint retention. Tribological assessment on silicon surfaces and ex vivo porcine cartilage revealed the system's excellent protective ability against friction at both nano- and macro-scales. Moreover, RAPA was shown to attenuate the fibrotic response in human OA synovial fibroblasts. Our findings suggest the zinc ion-based liposomal drug delivery system has potential to enhance OA therapy through extended release and cartilage tribological protection, while also illustrating the impact of a hydrophobic drug like RAPA on liposome aggregation and morphology.

Effect of cholesterol on the mechanical stability of gel-phase phospholipid bilayers studied by AFM force spectroscopy

The remarkably low sliding friction of articular cartilage in the major joints such as hips and knees, which is crucial for its homeostasis and joint health, has been attributed to lipid bilayers forming lubricious boundary layers at its surface. The robustness of such layers, and thus their lubrication efficiency at joint pressures, depends on the lipids forming them, including cholesterol which is a ubiquitous component, and which may act to strengthen of weaken the bilayer. In this work, a systematic study using an atomic force microscope (AFM) was carried out to understand the effect of cholesterol on the nanomechanical stability of two saturated phospholipids, DSPC (1,2-distearoyl-sn-glycero-3-phosphatidlycholine) and DPPC (1,2-dipalmitoyl-sn-glycero- phosphatidylcholine), that differ in acyl chain lengths. Measurements were carried out both in water and in phosphate buffer solution (PBS). The nanomechanical stability of the lipid bilayers was quantitatively evaluated by measuring the breakthrough force needed to puncture the bilayer by the AFM tip. The molar fractions of cholesterol incorporated in the bilayers were 10% and 40%. We found that for both DSPC and DPPC, cholesterol significantly decreases the mechanical stability of the bilayers in solid-ordered (SO) phase. In accordance with the literature, the strengthening effect of salt on the lipid bilayers was also observed. For DPPC with 10 mol % cholesterol, the effect of tip properties and the experimental procedure parameters on the breakthrough forces were also studied. Tip radius (2-42 nm), material (Si, Si3N4, Au) and loading rate (40-1000 nm/s) were varied systematically. The values of the breakthrough forces measured were not significantly affected by any of these parameters, showing that the weakening effect of cholesterol does not result from such changes in experimental conditions. As we have previously demonstrated that mechanical robustness improves the tribological performance of lipid layers, this study helps to shed light on the mechanism of physiological lubrication. Nanoindentation of SDPC bilayers.

Research progress of cartilage lubrication and biomimetic cartilage lubrication materials

Human joints move thousands of times a day. The articular cartilage plays a vital role in joints’ protection. If there is dysfunction in cartilage lubrication, cartilage cannot maintain its normal function. Eventually, the dysfunction may bring about osteoarthritis (OA). Extensive researches have shown that fluid film lubrication, boundary lubrication, and hydration lubrication are three discovered lubrication models at cartilage surface, and analyzing and simulating the mechanism of cartilage lubrication are fundamental to the treatment of OA. This essay concludes recent researches on the progress of cartilage lubrication and biomimetic cartilage, revealing the pathophysiology of cartilage lubrication and updating bio-inspired cartilage lubrication applications.

Recent advances in superlubricity of liposomes for biomedical applications

Achieving superlubricity, a state of lubrication where friction nearly vanishes, has become one of the most promising approaches to combat friction-induced energy dissipation and medical device failure. Phospholipids are amphiphilic molecules comprising highly hydrophilic phosphatidylcholine head groups as well as hydrophobic hydrocarbon chains, When solubilized, phospholipids can readily self-assemble to form different structures such as bilayers and vesicles (liposomes). Recently, liposomes have been identified as excellent lubricants, especially in the boundary lubrication regime the most common lubrication status in the field of biotribology. In this review, we summarize recent progress in employing liposomes as key players for employing superlubricity in biomedical applications. The relationship between lipids and liposomes, manufacturing approaches, lubrication regimes, and regulation mechanisms of liposomes are discussed. Finally, we indicate possible future directions for the use of liposome-mediated superlubricity in biomedical applications.

The wettability of electron spun membranes by synovial fluid

Aseptic loosening due to periprosthetic osteolysis has been accepted as one of the leading causes of revision procedures in patients with previous joint arthroplasty. Recently, several strategies for suppression of osteolysis were proposed, mostly based on biological treatment such as mitigation of chronic inflammatory reactions. However, these biological treatments do not stop the debris migration but only reduce the inflammatory reaction. To address this shortcoming, we propose the concept of ultrahigh molecular weighted polyethylene particles filtration storage by electrospun membranes. Firstly, the surface tension of synovial fluid (SF) is obtained by use of a pendant droplet. Secondly, the contact angle of the electrospun membranes wetted by two different liquids is measured to obtain the free surface energy using of the Owens-Wendt model. Additionally, the wettability of electrospun membranes by SF as a function of technology parameters is studied.

Lipid Anchoring Improves Lubrication and Wear Resistance of the Collagen I Matrix

Osteoarthritis is a prevalent degenerative joint disease characterized by progressive articular cartilage loss and destruction. The resultant increase in friction causes severe pain. The collagen I matrix (COL I) has been used clinically for cartilage repair; however, how COL I acts at cartilage surfaces is unclear. Here, we studied adsorption and lubrication of synovial fluid components, albumin, γ-globulin, and the phospholipid DPPC, on COL I under physiological conditions using surface plasmon resonance and an in situ sensing surface force apparatus. Our results revealed COL I had poor lubrication ability, a fairly high coefficient of friction (COF, μ = 0.651 ± 0.013), and surface damage under a 7 mN load. DPPC formed an improved lubricating layer on COL I (μ = 0.072 ± 0.016). In sharp contrast, albumin and γ-globulin exhibited poor lubrication with an order of magnitude higher COF but still provided benefits by protecting COL I from wear. Hence, DPPC on COL I may help optimize COL I implantation design.

Recent Advances in Understanding the Role of Cartilage Lubrication in Osteoarthritis

The remarkable lubrication properties of normal articular cartilage play an essential role in daily life, providing almost frictionless movements of joints. Alterations of cartilage surface or degradation of biomacromolecules within synovial fluid increase the wear and tear of the cartilage and hence determining the onset of the most common joint disease, osteoarthritis (OA). The irreversible and progressive degradation of articular cartilage is the hallmark of OA. Considering the absence of effective options to treat OA, the mechanosensitivity of chondrocytes has captured attention. As the only embedded cells in cartilage, the metabolism of chondrocytes is essential in maintaining homeostasis of cartilage, which triggers motivations to understand what is behind the low friction of cartilage and develop biolubrication-based strategies to postpone or even possibly heal OA. This review firstly focuses on the mechanism of cartilage lubrication, particularly on boundary lubrication. Then the mechanotransduction (especially shear stress) of chondrocytes is discussed. The following summarizes the recent development of cartilage-inspired biolubricants to highlight the correlation between cartilage lubrication and OA. One might expect that the restoration of cartilage lubrication at the early stage of OA could potentially promote the regeneration of cartilage and reverse its pathology to cure OA.

Recent Progress in Cartilage Lubrication

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.

Lipid-Bilayer Assemblies on Polymer-Bearing Surfaces: The Nature of the Slip Plane in Asymmetric Boundary Lubrication

Phospholipid-macromolecule complexes have been proposed to form highly efficient, lubricating boundary layers at artificial soft surfaces or at biological surfaces such as articular cartilage, where the friction reduction is attributed to the hydration lubrication mechanism acting at the exposed, hydrated head groups of the lipids. Here we measure, using a surface force balance, the normal and frictional interactions between model mica substrates across several different configurations of phosphatidylcholine (PC) lipid aggregates and adsorbed polymer (PEO) layers, to provide insight into the nature of such lubricating boundary layers in both symmetric and especially asymmetric configurations. Our results reveal that, irrespective of the configuration, the slip plane between the sliding surfaces reverts wherever possible to a bilayer-bilayer interface where hydration lubrication reduces the friction strongly. Where such an interface is not available, the sliding friction remains high. These findings may account for the low friction observed between both biological and synthetic hydrogel surfaces which may be asymmetrically coated with lipid-based boundary layers and fully support the hydration lubrication mechanism attributed to act at such boundary layers.

Cartilage-inspired, lipid-based boundary-lubricated hydrogels

The lubrication of hydrogels arises from fluid or solvated surface phases. By contrast, the lubricity of articular cartilage, a complex biohydrogel, has been at least partially attributed to nonfluid, lipid-exposing boundary layers. We emulated this behavior in synthetic hydrogels by incorporating trace lipid concentrations to create a molecularly thin, lipid-based boundary layer that renews continuously. We observed a 80% to 99.3% reduction in friction and wear relative to the lipid-free gel, over a wide range of conditions. This effect persists when the gels are dried and then rehydrated. Our approach may provide a method for sustained, extreme lubrication of hydrogels in applications from tissue engineering to clinical diagnostics.

Surface forces and interaction mechanisms of soft thin films under confinement: a short review

Surface forces of soft thin films under confinement in fluids play an important role in diverse biological and technological applications, such as bio-adhesion, lubrication and micro- and nano-electromechanical systems. Understanding the involved interaction mechanisms underlying the adhesion behaviors and tribological performances (i.e., friction and lubrication) of various confined soft thin films is significant in the development of both fundamental science and practical technologies. In this review, the fundamentals of surface forces are briefly presented. The widely utilized force measurement techniques including surface forces apparatus (SFA), atomic force microscopy (AFM) and spacer layer interferometry tribometer techniques are introduced. The advances in the fundamental understanding of a wide range of adhesion and tribological phenomena have been reviewed, in terms of the intermolecular and surface interaction mechanisms involved. The influences of various factors such as confined film properties, experimental conditions (e.g., normal load, and sliding velocity) and environmental variables (e.g., salts, salinity, additives and pH) on the adhesion, friction or lubrication forces of confined soft thin films are presented. The correlation between adhesion hysteresis and friction/lubrication behaviors has been discussed. Some of the challenging issues remaining and future perspectives are also provided.

Normal and shear forces between boundary sphingomyelin layers under aqueous conditions

Sphingomyelin is one of the predominant phospholipid groups in synovial joints, where lipids have been strongly implicated in the boundary lubrication of articular cartilage; however, little attention has been paid to its lubrication behavior. In this study, we demonstrate that sphingomyelin is an excellent boundary lubricant by measuring the normal and shear forces between sphingomyelin-layer-coated surfaces with a surface force balance under aqueous conditions. Slightly negatively-charged egg sphingomyelin vesicles were adsorbed on mica either by calcium bridging or by charge screening with high concentration monovalent salt. The normal force profiles between opposing egg sphingomyelin layers (vesicles or bilayers) show long-ranged weak repulsion and short-ranged strong repulsion on approaching. Friction coefficients, calculated from the highest load, were (7.2 ± 1.7) × 10-4 at contact stresses of 9.1 ± 0.7 MPa across 0.3 mM liposome dispersion in 0.03 mM Ca2+, and (0.8-3.5) × 10-3 at contact stresses of 7.6 ± 0.8 MPa across 0.3 mM liposome dispersion in 150 mM NaNO3. Similar or slightly lower friction coefficients of (5.3 ± 0.8) × 10-4 at 9.8 ± 0.2 MPa were obtained by replacing the liposome dispersion in 0.03 mM Ca2+ by water. Such low friction coefficients, attributed to the hydration lubrication mechanism, are comparable to those of phosphatidylcholine lipids, which have been widely recognized as excellent aqueous biolubricants. Therefore, we believe that sphingomyelin, in parallel with phosphatidylcholine, contributes to the remarkably good boundary lubrication in synovial joints.

The Effect of Synovial Fluid Composition, Speed and Load on Frictional Behaviour of Articular Cartilage

Articular cartilage ensures smooth motion of natural synovial joints operating at very low friction. However, the number of patients suffering from joint diseases, usually associated with cartilage degradation, continuously increases. Therefore, an understanding of cartilage tribological behaviour is of great interest in order to minimize its degradation, preserving the reliable function of the joints. The aim of the present study is to provide a comprehensive comparison of frictional behaviour of articular cartilage, focusing on the effect of synovial fluid composition (i), speed (ii), and load (iii). The experiments were realized using a pin-on-plate tribometer with reciprocating motion. The articular cartilage pin was loaded against smooth glass plate while the tests consisted of loading and unloading phases in order to enable cartilage rehydration. Various model fluids containing albumin, γ-globulin, hyaluronic acid, and phospholipids were prepared in two different concentrations simulating physiologic and osteoarthritic synovial fluid. Two different speeds, 5 mm/s and 10 mm/s were applied, and the tests were carried out under 5 N and 10 N. It was found that protein-based solutions exhibit almost no difference in friction coefficient, independently of the concentration of the constituents. However, the behaviour is considerably changed when adding hyaluronic acid and phospholipids. Especially when interacting with γ-globulin, friction coefficient decreased substantially. In general, an important role of the interaction of fluid constituents was observed. On the other hand, a limited effect of speed was detected for most of the model fluids. Finally, it was shown that elevated load leads to lower friction, which corresponds well with previous observations. Further study should concentrate on specific explored phenomena focusing on the detailed statistical evaluation.

A Century of Cartilage Tribology Research Is Informing Lubrication Therapies

Articular cartilage is one of the most unique materials found in nature. This tissue's ability to provide low friction and low wear over decades of constant use is not surpassed, as of yet, by any synthetic materials. Lubrication of the body's joints is essential to mammalian locomotion, but breakdown and degeneration of cartilage is the leading cause of severe disability in the industrialized world. In this paper, we review how theories of cartilage lubrication have evolved over the past decades and connect how theories of cartilage lubrication have been translated to lubrication-based therapies. Here, we call upon these historical perspectives and highlight the open questions in cartilage lubrication research. Additionally, these open questions within the field's understanding of natural lubrication mechanisms reveal strategic directions for lubrication therapy.

The Amphoteric and Hydrophilic Properties of Cartilage Surface in Mammalian Joints: Interfacial Tension and Molecular Dynamics Simulation Studies

In this paper, we explain the amphoteric character of the cartilage surface by studying a lipid bilayer model built from phospholipids. We examined the interfacial tension values and molecular dynamics simulation in solutions of varying pH. The effects of negative and positive charge density (or fixed charges) on the (cartilage/cartilage) friction coefficient were investigated. In physiological (or synovial) fluid, after the isoelectric point (pI), the curve of interfacial tension decreases rapidly as it reaches pH 7.4 and then approaches a constant value at higher pH. It was shown that the curve of the interfacial tension curve exhibits a maximum value at the isoelectric point with a Gaussian shape feature. The phospholipid bilayers facilitate an almost frictionless contact in the joint. Moreover, the slippage of the bilayer and the short-range repulsion between the surfaces of the negatively charged cartilage surfaces are the main determinants of the low frictional properties of the joint.

Lipid-hyaluronan synergy strongly reduces intrasynovial tissue boundary friction

Hyaluronan (HA)-lipid layers on model (mica) surfaces massively reduce friction as the surfaces slide past each other, and have been proposed, together with lubricin, as the boundary layers accounting for the extreme lubrication of articular cartilage. The ability of such HA-lipid complexes to lubricate sliding biological tissues has not however been demonstrated. Here we show that HA-lipid layers on the surface of an intrasynovial tendon can strongly reduce the friction as the tendon slides within its sheath. We find a marked lubrication synergy when combining both HA and lipids at the tendon surface, relative to each component alone, further enhanced when the polysaccharide is functionalized to attach specifically to the tissue. Our results shed light on the lubricity of sliding biological tissues, and indicate a novel approach for lubricating surfaces such as tendons and, possibly, articular cartilage, important, respectively, for alleviating function impairment following tendon injury and repair, or in the context of osteoarthritis. STATEMENT OF SIGNIFICANCE: Lubrication breakdown between sliding biological tissues is responsible for pathologies ranging from dry eye syndrome to tendon-injury repair impairment and osteoarthritis. These are increasing with human longevity and impose a huge economic and societal burden. Here we show that synergy of hyaluronan and lipids, molecules which are central components of synovial joints and of the tendon/sheath system, can strongly reduce friction between sliding biological tissues (the extrasynovial tendon sliding in its sheath), relative to untreated tissue or to either component on its own. Our results point to the molecular origins of the very low friction in healthy tendons and synovial joints, as well as to novel treatments of lubrication breakdown in these organs.

Articular cartilage. Strong adsorption and cohesion of phospholipids with the quaternary ammonium cations providing satisfactory lubrication of natural joints

Much evidence supports the hypothesis that surface-active phospholipid (SAPL) molecules on articular cartilage (AC) adsorbed to negatively-charged proteoglycan matrix form phospholipid (membrane), are negatively charged surface (-PO₄^-) and hydrophilic. In Hills cartilage model (1984) phospholipids adsorbed to cartilage surface act as boundary lubricants making the surface extremely hydrophobic. Hydrophobic surface of AC has gained no support in all experimental facts presented in this paper and the current literature showing that AC is amphoteric and hydrophilic with the negatively charged surface (-PO₄^-). The interfacial energy of the model membrane of spherical lipid bilayers evident from phosphatidylcholine "bell-shaped curve" has amphoteric character and lowest energy in lubrication at a pH 7.4 ± 1 of the natural joint.

Phospholipid-Coated Mesoporous Silica Nanoparticles Acting as Lubricating Drug Nanocarriers

Osteoarthritis (OA) is a severe disease caused by wear and inflammation of joints. In this study, phospholipid-coated mesoporous silica nanoparticles (MSNs@lip) were prepared in order to treat OA at an early stage. The phospholipid layer has excellent lubrication capability in aqueous media due to the hydration lubrication mechanism, while mesoporous silica nanoparticles (MSNs) act as effective drug nanocarriers. The MSNs@lip were characterized by scanning electron microscope, transmission electron microscope, Fourier transform infrared spectrum, X-ray photoelectron spectrum, thermogravimetric analysis and dynamic light scattering techniques to confirm that the phospholipid layer was coated onto the surface of MSNs successfully. A series of tribological tests were performed under different experimental conditions, and the results showed that MSNs@lip with multi-layers of phospholipids greatly reduced the friction coefficient in comparison with MSNs. Additionally, MSNs@lip demonstrated sustained drug release behavior and were biocompatible based on CCK-8 assay using MC3T3-E1 cells. The MSNs@lip developed in the present study, acting as effective lubricating drug nanocarriers, may represent a promising strategy to treat early stage OA by lubrication enhancement and drug delivery therapy.

Lubrication of Articular Cartilage

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.

Ultra-low friction between boundary layers of hyaluronan-phosphatidylcholine complexes

The boundary layers coating articular cartilage in synovial joints constitute unique biomaterials, providing lubricity at levels unmatched by any human-made materials. The underlying molecular mechanism of this lubricity, essential to joint function, is not well understood. Here we study the interactions between surfaces bearing attached hyaluronan (hyaluronic acid, or HA) to which different phosphatidylcholine (PC) lipids had been added, in the form of small unilamellar vesicles (SUVs or liposomes), using a surface force balance, to shed light on possible cartilage boundary lubrication by such complexes. Surface-attached HA was complexed with different PC lipids (hydrogenated soy PC (HSPC), 1,2-dimyristoyl-sn-glycero-3-PC (DMPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-PC (POPC)), followed by rinsing. Atomic force microscopy (AFM) and cryo-scanning electron microscopy (Cryo-SEM) were used to image the HA-PC surface complexes following addition of the SUVs. HA-HSPC complexes provide very efficient lubrication, with friction coefficients as low as μ∼0.001 at physiological pressures P≈150atm, while HA-DMPC and HA-POPC complexes are efficient only at low P (up to 10-20atm). The friction reduction in all cases is attributed to hydration lubrication by highly-hydrated phosphocholine groups exposed by the PC-HA complexes. The greater robustness at high P of the HSPC (C_16(15%),C_18(85%)) complexes relative to the DMPC ((C₁₄)₂) or POPC (C₁₆, C_18:1) complexes is attributed to the stronger van der Waals attraction between the HSPC acyl tails, relative to the shorter or un-saturated tails of the other two lipids. Our results shed light on possible lubrication mechanisms at the articular cartilage surface in joints.

STATEMENT OF SIGNIFICANCE

Can designed biomaterials emulate the unique lubrication ability of articular cartilage, and thus provide potential alleviation to friction-related joint diseases? This is the motivation behind the present study. The principles of cartilage lubrication have attracted considerable attention for decades, and several models have been proposed to elucidate it, however, the mechanism of this ultralow friction is still not clear. In this paper we explore the recent suggestion that its efficient lubrication arises from boundary layers of hyaluronan-lipid complexes at its surface, in particular exploring a range of different phosphatidylcholines (PCs) mimicking the wide range of PCs in synovial joints. The present study suggests a synergistic lubricating behavior of the different lipids in living joints, and potential treatment directions using such biomaterial complexes for widespread cartilage-friction-related diseases such as osteoarthritis.

Effect of Cholesterol on the Stability and Lubrication Efficiency of Phosphatidylcholine Surface Layers

The lubrication properties of saturated PC lipid vesicles containing high cholesterol content under high loads were examined by detailed surface force balance measurements of normal and shear forces between two surface-attached lipid layers. Forces between two opposing mica surfaces bearing distearoylphosphatidylcholine (PC) (DSPC) small unilamellar vesicles (SUVs, or liposomes), or bilayers, with varying cholesterol content were measured across water, whereas dimyristoyl PC (DMPC), dipalmitoyl PC (DPPC), and DSPC SUVs containing 40% cholesterol were measured across liposome dispersions of SUVs of the same lipid composition as in the adsorbed layers. The results clearly demonstrate decreased stability and resistance to normal load with the increase in cholesterol content of DSPC SUVs. Friction coefficients between two 10% cholesterol PC-bilayers were in the same range as for 40% cholesterol bilayers (μ ≈ 10^-3), indicating that cholesterol has a more substantial effect on the mechanical properties of a bilayer than on its lubrication performance. We further find that the lubrication efficiency of DMPC and DPPC with 40% cholesterol is superior to that of DSPC 40% cholesterol, most likely because of enhanced hydration-lubrication in these systems. We previously found that when experiments are performed in the presence of a lipid reservoir, layers can self-heal and therefore their robustness is less important under such conditions. We conclude that the effect of cholesterol in decreasing the stability is more pronounced than its effect on hydration, but the stability is, in turn, less important when a lipid reservoir is present. This study complements our previous work and sheds light on the effect of cholesterol, a prominent and important physiological lipid, on the mechanical and lubrication properties of gel-phase lipid layers.

An in vitro study of cartilage-meniscus tribology to understand the changes caused by a meniscus implant

Active lifestyles increase the risk of meniscal injury. A permanent meniscus implant of polycarbonate urethane (PCU) is a promising treatment to postpone/prevent total knee arthroplasty. Study of the changes in articular cartilage tribology in the presence of PCU is essential in developing the optimum meniscus implant. Therefore, a cartilage-meniscus reciprocating, sliding model was developed in vitro, mimicking the stance and swing phases of the gait cycle. The meniscus was further replaced with PCU and surface-modified PCUs (with C18 chains, mono-functional polydimethylsiloxane groups and mono-functional polytetrafluoroethylene groups) to study the changes. The coefficient of friction (COF) was calculated, and cartilage wear was determined and quantified histologically. The cartilage-meniscus sliding resulted in low COF during both stance and swing (0.01< COF <0.12) and low wear of cartilage (scores <1). The cartilage-PCU sliding, during stance, revealed similar low COFs. But during swing, the COFs were high (average ∼1, maximum 1.6), indicating a breakdown in interstitial fluid pressurization lubrication and non-effective activation of the boundary lubrication. This may lead to wear of cartilage in long term. However, under the tested conditions the wear of cartilage against PCUs was not higher than its wear against meniscus, and the cartilage was occasionally damaged. The COF decreased with increasing the contact pressure (as-per a power equation) up to 1MPa. The changes in the surface modification of PCU did not affect PCU's tribological performance.

Insight into the Tribological Behavior of Liposomes in Artificial Joints

Liposomes are widely used in drug delivery and gene therapy, and their new role as boundary lubricant in natural/artificial joints has been found in recent years. In this study, the tribological properties of liposomes on titanium alloy (Ti6Al4 V)/UHMWPE interface were studied by a ball-on-disc tribometer. The efficient reduction of friction coefficient and wear on both surfaces under various velocities and loads is found. A multilayer structure of physically adsorbed liposomes on Ti6Al4 V surface was also observed by atomic force microscope (AFM). Except for the hydration mechanism by phosphatidylcholine (PC) groups, the well-performed tribological properties by liposomes is also attributed to the existence of adsorbed liposome layers on both surfaces, which could reduce asperities contact and show great bearing capacity. This work enriches the research on liposomes for lubrication improvement on artificial surface and shows their value in clinical application.

Evidence of a molecular boundary lubricant at snakeskin surfaces

During slithering locomotion the ventral scales at a snake's belly are in direct mechanical interaction with the environment, while the dorsal scales provide optical camouflage and thermoregulation. Recent work has demonstrated that compared to dorsal scales, ventral scales provide improved lubrication and wear protection. While biomechanic adaption of snake motion is of growing interest in the fields of material science and robotics, the mechanism for how ventral scales influence the friction between the snake and substrate, at the molecular level, is unknown. In this study, we characterize the outermost surface of snake scales using sum frequency generation (SFG) spectra and near-edge X-ray absorption fine structure (NEXAFS) images collected from recently shed California kingsnake (Lampropeltis californiae) epidermis. SFG's nonlinear optical selection rules provide information about the outermost surface of materials; NEXAFS takes advantage of the shallow escape depth of the electrons to probe the molecular structure of surfaces. Our analysis of the data revealed the existence of a previously unknown lipid coating on both the ventral and dorsal scales. Additionally, the molecular structure of this lipid coating closely aligns to the biological function: lipids on ventral scales form a highly ordered layer which provides both lubrication and wear protection at the snake's ventral surface.

Wear products of total hip arthroplasty: The case of polyethylene

Among the bearing surfaces involved in a total hip arthroplasty, ultra-high molecular weight polyethylene (UHMWPE) is the weak link. It is submitted to the friction of a harder bearing, producing wear particles, which, in turn, initiate an inflammatory reaction ultimately leading to osteolysis. This kind of bone deterioration sometimes turns out to an aggressive granuloma and may provoke implant loosening. Wear resistance of UHMWPE depends on its molecular weight and crystallinity. Some steps of the manufacturing process were improved to optimize its tribological properties and to slow down degradation resulting from mechanical (abrasion) and chemical (oxidation) phenomena. Its preparation and conservation must be performed in an inert atmosphere, i.e. without ambient oxygen. Its resistance to abrasion depends on its cross-linking degree. Its cross-linking rate was observed to increase proportionally to the irradiation doses, improving its wear resistance. However, its mechanical properties are impaired and moreover, it becomes oxidation sensitive. It is therefore necessary to submit it to a thermal treatment to eliminate free radicals that were produced during irradiation. More recently impregnation by vitamin E, a powerful anti-oxidant product, was proposed to preserve the polymer from in vivo oxidation while maintaining its mechanical properties. We raised the hypothesis that last-generation UHMWPE could offer the same wear resistance as the most performing bearings (ceramic-on-ceramic). Recent clinical results confirm the tribological performance of highly crosslinked UHMWPE in vivo. However, it remains to be seen whether this excellent wear resistance would persist under eccentric load such as edge loading, and if, in the long run, this kind of bearing proves capable of reducing the risk of osteolysis in young and active patients.

Hydration lubrication and shear-induced self-healing of lipid bilayer boundary lubricants in phosphatidylcholine dispersions

Measurements of normal and shear (frictional) forces between mica surfaces across small unilamellar vesicle (SUV) dispersions of the phosphatidylcholine (PC) lipids DMPC (14:0), DPPC (16:0) and DSPC (18:0) and POPC (16:0, 18:1), at physiologically high pressures, are reported. We have previously studied the normal and shear forces between two opposing surfaces bearing PC vesicles across pure water and showed that liposome lubrication ability improved with increasing acyl chain length, and correlated strongly with the SUV structural integrity on the substrate surface (DSPC > DPPC > DMPC). In the current study, surprisingly, we discovered that this trend is reversed when the measurements are conducted in SUV dispersions, instead of pure water. In their corresponding SUV dispersion, DMPC SUVs ruptured and formed bilayers, which were able to provide reversible and reproducible lubrication with extremely low friction (μ < 10(-4)) up to pressures of 70-90 atm. Similarly, POPC SUVs also formed bilayers which exhibited low friction (μ < 10(-4)) up to pressures as high as 160 atm. DPPC and DSPC SUVs also provided good lubrication, but with slightly higher friction coefficients (μ = 10(-3)-10(-4)). We believe these differences originate from fast self-healing of the softer surface layers (which are in their liquid disordered phase, POPC, or close to it, DMPC), which renders the robustness of the DPPC or DSPC (both in their solid ordered phase) less important in these conditions. Under these circumstances, the enhanced hydration of the less densely packed POPC and DMPC surface layers is now believed to play an important role, and allows enhanced lubrication via the hydration lubrication mechanism. Our findings may have implications for the understanding of complex biological systems such us biolubrication of synovial joints.

Future nanomedicine for the diagnosis and treatment of osteoarthritis

Current treatments for osteoarthritis (OA) are largely palliative until the joints become totally dysfunctional and prosthetic replacement becomes necessary. Effective methods are needed for diagnosing OA and monitoring its progression during its early stages, when the effects of therapeutic drugs or biological agents are most likely to be effective. Theranostic nanosomes and nanoparticles have the potential to noninvasively detect, track and treat the early stages of OA. As articular cartilage does not regenerate once it is degraded, cell-based treatments aided by superparamagnetic iron oxide nanoparticle tracking are attractive future treatment modalities for the later stages of OA progression, when significant cartilage replacement is needed. This article will describe the current and future translational approaches for the detection and noninvasive treatment of degenerative OA.

Supramolecular synergy in the boundary lubrication of synovial joints

Hyaluronan, lubricin and phospholipids, molecules ubiquitous in synovial joints, such as hips and knees, have separately been invoked as the lubricants responsible for the remarkable lubrication of articular cartilage; but alone, these molecules cannot explain the extremely low friction at the high pressures of such joints. We find that surface-anchored hyaluronan molecules complex synergistically with phosphatidylcholine lipids present in joints to form a boundary lubricating layer, which, with coefficient of friction μ≈0.001 at pressures to over 100 atm, has a frictional behaviour resembling that of articular cartilage in the major joints. Our findings point to a scenario where each of the molecules has a different role but must act together with the others: hyaluronan, anchored at the outer surface of articular cartilage by lubricin molecules, complexes with joint phosphatidylcholines to provide the extreme lubrication of synovial joints via the hydration–lubrication mechanism.

Interacting cartilage surfaces in synovial joints experience very low levels of friction, allowing unhindered mechanical motion. Here, the authors propose how a synergistic interaction of hyaluronan, lubricin and phospholipids, molecules that are ubiquitous in joints, may lead to this high level of lubrication.

The Effect of Phospholipids (Surfactant) on Adhesion and Biomechanical Properties of Tendon: A Rat Achilles Tendon Repair Model

Adhesion of the tendon is a major challenge for the orthopedic surgeon during tendon repair. Manipulation of biological environment is one of the concepts to prevent adhesion. Lots of biochemicals have been studied for this purpose. We aimed to determine the effect of phospholipids on adhesion and biomechanical properties of tendon in an animal tendon repair model. Seventy-two Wistar rats were divided into 4 groups. Achilles tendons of rats were cut and repaired. Phospholipids were applied at two different dosages. Tendon adhesion was determined histopathologically and biomechanical test was performed. At macroscopic evaluation of adhesion, there are statistically significant differences between multiple-dose phospholipid injection group and Control group and also hyaluronic acid group and Control group (p < 0.008). At microscopic evaluation of adhesion, there was no statistically significant difference (p > 0.008). Ultimate strength was highest at hyaluronic acid injection group and lowest at multiple-dose phospholipid injection group. Single-dose phospholipids (surfactant) application may have a beneficial effect on the tendon adhesion. Although multiple applications of phospholipids seem the most effective regime to reduce the tendon adhesion among groups, it deteriorated the biomechanical properties of tendon.

Liposomes tethered to a biopolymer film through the hydrophobic effect create a highly effective lubricating surface

Liposomal coatings are formed on films of a biopolymer, hydrophobically modified chitosan (hm-chitosan), containing dodecyl groups as hydrophobes along the polymer backbone. The alkyl groups insert themselves into the liposome bilayer through hydrophobic interactions and thus tether liposomes, leading to a densely packed liposome layer on the film surface. Such liposomal surfaces exhibit effective lubrication properties due to their high degree of hydration, and reduce the coefficient of friction to the biologically-relevant range. The compliancy and robustness of these tethered liposomes allow retention on the film surface upon repeated applications of shear. Such liposome coated films have potential applications in biolubrication.

Enhanced lubrication on tissue and biomaterial surfaces through peptide-mediated binding of hyaluronic acid

Lubrication is key for the efficient function of devices and tissues with moving surfaces, such as articulating joints, ocular surfaces and the lungs. Indeed, lubrication dysfunction leads to increased friction and degeneration of these systems. Here, we present a polymer-peptide surface coating platform to non-covalently bind hyaluronic acid (HA), a natural lubricant in the body. Tissue surfaces treated with the HA-binding system exhibited higher lubricity values, and in vivo were able to retain HA in the articular joint and to bind ocular tissue surfaces. Biomaterials-mediated strategies that locally bind and concentrate HA could provide physical and biological benefits when used to treat tissue-lubricating dysfunction and to coat medical devices.

Mechanical stability and lubrication by phosphatidylcholine boundary layers in the vesicular and in the extended lamellar phases

The lubrication properties of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) extended supported bilayers were studied and compared to those of surface-attached DSPC small unilamellar vesicles (liposomes) in order to elucidate the effect of phospholipid geometrical packaging on the lubrication and mechanical properties of these boundary layers. The topography and response to the nanoindentation of bilayer- and liposome-covered surfaces were studied by an atomic force microscope (AFM). In parallel, normal and shear (frictional) forces between two opposing surfaces bearing DSPC vesicles/bilayers across water were studied with the surface force balance (SFB). A correlation between nanomechanical performance in the AFM and stability and lubrication in the SFB was observed. Bilayers were readily punctured by the AFM tip and exhibited substantial hysteresis between approach and retraction curves, whereas liposomes were not punctured and exhibited purely elastic behavior. At the same time, SFB measurements showed that bilayers are less stable and less efficient lubricants compared to liposomes. Bilayers provided efficient lubrication with very low friction coefficients, 0.002-0.008 up to pressures of more then 50 atm. However, bilayers were less robust and tended to detach from the surface as a result of shear, leading to high friction for subsequent approaches at the same contact position. In contrast, liposomes showed reversible and reproducible behavior under shear and compression, exhibiting ultralow friction coefficients of μ ≈ 10(-4) for pressures as high as 180 atm. This is attributed to the increased mechanical stability of the self-closed, closely packed liposomes, which we believe results from the more defect-free nature of the finitely sized vesicles.

Some conceptual thoughts toward nanoscale oriented friction in a model of articular cartilage

This work presents a conceptual framework as to how a deficit in the synovial-fluid content, exemplified by hyaluronan or any other amphiphilic species, is capable of decisively altering the complex lubrication and wear conditions observed clinically in articular cartilage. The effect is revealed in (non)stationary regimes if the cartilage is subjected to some normal periodic load, revealing over its exploitation time increasingly dissipative, in general entropy-addressing, characteristics. It can be hypothesized that a Grotthuss-type proton transport physiology-concerning mechanism in channel-like, phospholipid-water cartilage's articulating nanospaces will be responsible for the expression of the lubrication mode. The corresponding wear involving overall change is then manifested adequately in the stationary regime, and in a viable system-parametric correlation with its lubrication counterpart. Certain analytic formulae for the nanoscale oriented coefficient of friction, involving generically H-bonds breaking mechanism, and pointing to some local-viscosity context, have been proposed for fitting the experimental data and clinical observations involving proton management at articular cartilage surfaces.

Origins of extreme boundary lubrication by phosphatidylcholine liposomes

Phosphatidylcholine (PC) vesicles have been shown to have remarkable boundary lubricating properties under physiologically-high pressures. Here we carry out a systematic study, using a surface force balance, of the normal and shear (frictional) forces between two opposing surfaces bearing different PC vesicles across water, to elucidate the origin of these properties. Small unilamellar vesicles (SUVs, diameters < 100 nm) of the symmetric saturated diacyl PCs DMPC (C(14)), DPPC (C(16)) and DSPC (C(18)) attached to mica surfaces were studied in their solid-ordered (SO) phase on the surface. Overall liposome lubrication ability improves markedly with increasing acyl chain length, and correlates strongly with the liposomes' structural integrity on the substrate surface: DSPC-SUVs were stable on the surface, and provided extremely efficient lubrication (friction coefficient μ ≈ 10(-4)) at room temperature at pressures up to at least 18 MPa. DMPC-SUVs ruptured following adsorption, providing poor high-pressure lubrication, while DPPC-SUVs behavior was intermediate between the two. These results can be well understood in terms of the hydration-lubrication paradigm, but suggest that an earlier conjecture, that highly-efficient lubrication by PC-SUVs depended simply on their being in the SO rather than in the liquid-disordered phase, should be more nuanced. Our results indicate that the resistance of the SUVs to mechanical deformation and rupture is the dominant factor in determining their overall boundary lubrication efficiency in our system.

Bio-tribology

It is now forty six years since the separate topics of friction, lubrication, wear and bearing design were integrated under the title 'Tribology' [Department of Education and Science, Lubrication (Tribology) Education and Research. A Report on the Present Position and Industry's Needs, HMSO, London, 1966]. Significant developments have been reported in many established and new aspects of tribology during this period. The subject has contributed to improved performance of much familiar equipment, such as reciprocating engines, where there have been vast improvements in engine reliability and efficiency. Nano-tribology has been central to remarkable advances in information processing and digital equipment. Shortly after widespread introduction of the term tribology, integration with biology and medicine prompted rapid and extensive interest in the fascinating sub-field now known as Bio-tribology [D. Dowson and V. Wright, Bio-tribology, in The Rheology of Lubricants, ed. T. C. Davenport, Applied Science Publishers, Barking, 1973, pp. 81-88]. An outline will be given of some of the developments in the latter field.

Normal and shear interactions between hyaluronan-aggrecan complexes mimicking possible boundary lubricants in articular cartilage in synovial joints

Using a surface force balance, normal and shear interactions have been measured between two atomically smooth surfaces coated with hyaluronan (HA), and with HA/aggrecan (Agg) complexes stabilized by cartilage link protein (LP). Such HA/Agg/LP complexes are the most abundant mobile macromolecular species permeating articular cartilage in synovial joints and have been conjectured to be present as boundary lubricants at its surface. The aim of the present study is to gain insight into the extremely efficient lubrication when two cartilage surfaces slide past each other in healthy joints, and in particular to elucidate the possible role in this of the HA/Agg/LP complexes. Within the range of our parameters, our results reveal that the HA/Agg/LP macromolecular surface complexes are much better boundary lubricants than HA alone, likely because of the higher level of hydration, due to the higher charge density, of the HA/Agg/LP layers with respect to the HA alone. However, the friction coefficients (μ) associated with the mutual interactions and sliding of opposing HA/Agg/LP layers (μ ≈ 0.01 up to pressure P of ca. 12 atm, increasing sharply at higher P) suggest that such complexes by themselves cannot account for the remarkable boundary lubrication observed in mammalian joints (up to P > 50 atm).

The classification of degenerative hip disease

It is probable that both genetic and environmental factors play some part in the aetiology of most cases of degenerative hip disease. Geneticists have identified some single gene disorders of the hip, but have had difficulty in identifying the genetics of many of the common causes of degenerative hip disease. The heterogeneity of the phenotypes studied is part of the problem. A detailed classification of phenotypes is proposed. This study is based on careful documentation of 2003 consecutive total hip replacements performed by a single surgeon between 1972 and 2000. The concept that developmental problems may initiate degenerative hip disease is supported. The influences of gender, age and body mass index are outlined. Biomechanical explanations for some of the radiological appearances encountered are suggested. The body weight lever, which is larger than the abductor lever, causes the abductor power to be more important than body weight. The possibility that a deficiency in joint lubrication is a cause of degenerative hip disease is discussed. Identifying the phenotypes may help geneticists to identify genes responsible for degenerative hip disease, and eventually lead to a definitive classification.

Effective lubrication of articular cartilage by an amphiphilic hyaluronic acid derivative

BACKGROUND

Intra-articular injection of hyaluronic acid based therapies is gaining popularity as a treatment option for non-operative management of patients with symptomatic osteoarthritis. Although there is an abundance of evidence for both biological and mechanical mechanisms of joint protection by hyaluronic acid, one clear intention of viscosupplementation is to reduce friction and wear by providing an extrinsic lubricant. We tested the in vitro friction response of a novel hyaluronic acid derivative that presents amphiphilic features to promote adhesion to the cartilage surface and thereby improve cartilage lubrication.

METHODS

Migrating Contact Area and Static Contact Area friction tests were conducted on bovine articular cartilage to assess the efficacy of two lubricants, a chemically modified amphiphilic hyaluronic acid and synovial fluid from a healthy joint, as well as a phosphate buffered saline negative control.

FINDINGS

No differences in lubrication (P=0.34) were evident between the three test articles during the Migrating Contact Area test, which represents articulation of healthy articular cartilage. The modified hyaluronic acid presented an equilibrium friction coefficient 2.8 times less than that of the synovial fluid (P ≤ 0.0005) and five times less than that of the PBS control (P ≤ 0.0001) during the Static Contact Area test, representing a mixed lubrication condition.

INTERPRETATION

The present study demonstrated that a chemically modified amphiphilic hyaluronic acid can provide equivalent lubrication to synovial fluid during articulation of loaded healthy articular cartilage and can provide superior lubrication as indicated by a lower coefficient of friction than synovial fluid under loading conditions potentially associated with cartilage wear.

Effect of functional shift of the mandible on lubrication of the temporomandibular joint

Lubrication of synovial joints reduces the coefficient of friction of the articular cartilage surface. To investigate the effect of malocclusion on the lubrication of the temporomandibular joint (TMJ), we evaluated lubricin expression in the rat TMJ immunohistochemically, under conditions of functional lateral shift of the mandible, during period of growth. Thirty 5-week-old male Wistar rats were divided into experimental, recovery, and control groups. Each rt in the experimental and recovery groups was fitted with an acrylic-plate guiding appliance. The rats in the experimental and control groups were killed at 14 and 28 days after the appliance was attached. Each rat in the recovery group was detached from the appliance at 14 days, and was killed 14 days after the appliance was removed. In the experimental group, the expression of lubricin staining in TMJ cartilage was significantly decreased during the experimental period. In the recovery group, the expression of lubricin staining in TMJ cartilage was significantly greater than in the experimental group, and there was no significant difference at 28 days between the control and recovery groups. Analysis of these data suggests that a functional lateral shift of the mandible during the growth period influences lubrication of the TMJ.

Engineering lubrication in articular cartilage

Despite continuous progress toward tissue engineering of functional articular cartilage, significant challenges still remain. Advances in morphogens, stem cells, and scaffolds have resulted in enhancement of the bulk mechanical properties of engineered constructs, but little attention has been paid to the surface mechanical properties. In the near future, engineered tissues will be able to withstand and support the physiological compressive and tensile forces in weight-bearing synovial joints such as the knee. However, there is an increasing realization that these tissue-engineered cartilage constructs will fail without the optimal frictional and wear properties present in native articular cartilage. These characteristics are critical to smooth, pain-free joint articulation and a long-lasting, durable cartilage surface. To achieve optimal tribological properties, engineered cartilage therapies will need to incorporate approaches and methods for functional lubrication. Steady progress in cartilage lubrication in native tissues has pushed the pendulum and warranted a shift in the articular cartilage tissue-engineering paradigm. Engineered tissues should be designed and developed to possess both tribological and mechanical properties mirroring natural cartilage. In this article, an overview of the biology and engineering of articular cartilage structure and cartilage lubrication will be presented. Salient progress in lubrication treatments such as tribosupplementation, pharmacological, and cell-based therapies will be covered. Finally, frictional assays such as the pin-on-disk tribometer will be addressed. Knowledge related to the elements of cartilage lubrication has progressed and, thus, an opportune moment is provided to leverage these advances at a critical step in the development of mechanically and tribologically robust, biomimetic tissue-engineered cartilage. This article is intended to serve as the first stepping stone toward future studies in functional tissue engineering of articular cartilage that begins to explore and incorporate methods of lubrication.

Liposomes as lubricants: beyond drug delivery

In this paper we review recent work (Goldberg et al., 2011a,b) on a new use for phosphatidylcholine liposomes: as ultra-efficient boundary lubricants at up to the highest physiological pressures. Using a surface force balance, we have measured the normal and shear interactions as a function of surface separation between layers of hydrogenated soy phophatidylcholine (HSPC) small unilamellar vesicles (SUVs) adsorbed from dispersion, at both pure water and physiologically high salt concentrations of 0.15 M NaNO(3). Cryo-Scanning Electron Microscopy shows each surface to be coated by a close-packed HSPC-SUV layer with an over-layer of liposomes on top. The shear forces reveal strikingly low friction coefficients down to 2×10(-5) in pure water system or 6×10(-4) in the 150 mM salt system, up to contact pressures of at least 12 MPa (pure water) or 6 MPa (high salt), comparable with those in the major joints. This low friction is attributed to the hydration lubrication mechanism arising from rubbing of the highly hydrated phosphocholine-headgroup layers exposed at the outer surface of each liposome, and provides support for the conjecture that phospholipids may play a significant role in biological lubrication.