Interactions between adsorbed hydrogenated soy phosphatidylcholine (HSPC) vesicles at physiologically high pressures and salt concentrations

Cell-inspired, massive electromodulation of friction via transmembrane fields across lipid bilayers

Transient electric fields across cell bilayer membranes can lead to electroporation and cell fusion, effects crucial to cell viability whose biological implications have been extensively studied. However, little is known about these behaviours in a materials context. Here we find that transmembrane electric fields can lead to a massive, reversible modulation of the sliding friction between surfaces coated with lipid-bilayer membranes-a 200-fold variation, up to two orders of magnitude greater than that achieved to date. Atomistic simulations reveal that the transverse fields, resembling those at cell membranes, lead to fully reversible electroporation of the confined bilayers and the formation of inter-bilayer bridges analogous to the stalks preceding intermembrane fusion. These increase the interfacial dissipation through reduced hydration at the slip plane, forcing it to revert in part from the low-dissipation, hydrated lipid-headgroup plane to the intra-bilayer, high-dissipation acyl tail interface. Our results demonstrate that lipid bilayers under transmembrane electric fields can have striking materials modification properties.

Dehydration does not affect lipid-based hydration lubrication

Phosphatidylcholine (PC) lipid bilayers at surfaces massively reduce sliding friction, via the hydration lubrication mechanism acting at their highly-hydrated phosphocholine headgroups, a central paradigm of biological lubrication, particularly at articular cartilage surfaces where low friction is crucial for joint well-being. Nanotribological measurements probed the effect on such lubrication of dehydration by dimethyl sulfoxide (DMSO), known to strongly dehydrate the phosphocholine headgroups of such PC bilayers, i.e. reduce the thickness of the inter-bilayer water layer, and thus expected to substantially degrade the hydration lubrication. Remarkably, and unexpectedly, we found that the dehydration has little effect on the friction. We used several approaches, including atomic force microscopy, small- and wide-angle X-ray scattering and all-atom molecular dynamics simulations to elucidate this. Our results show that while DMSO clearly removes hydration water from the lipid head-groups, this is offset by both higher areal head-group density and by rigidity-enhancement of the lipid bilayers, both of which act to reduce frictional dissipation. This sheds strong light on the robustness of lipid-based hydration lubrication in biological systems, despite the ubiquitous presence of bio-osmolytes which compete for hydration water.

Morphological and Mechanical Characterization of Extracellular Vesicles and Parent Human Synoviocytes under Physiological and Inflammatory Conditions

The morphology of fibroblast-like synoviocytes (FLS) issued from the synovial fluid (SF) of patients suffering from osteoarthritis (OA), rheumatoid arthritis (RA), or from healthy subjects (H), as well as the ultrastructure and mechanical properties of the FLS-secreted extracellular vesicles (EV), were analyzed by confocal microscopy, transmission electron microscopy, atomic force microscopy, and tribological tests. EV released under healthy conditions were constituted of several lipid bilayers surrounding a viscous inner core. This "gel-in" vesicular structure ensured high mechanical resistance of single vesicles and good tribological properties of the lubricant. RA, and to a lesser extent OA, synovial vesicles had altered morphology, corresponding to a "gel-out" situation with vesicles surrounded by a viscous gel, poor mechanical resistance, and poor lubricating qualities. When subjected to inflammatory conditions, healthy cells developed phenotypes similar to that of RA samples, which reinforces the importance of inflammatory processes in the loss of lubricating properties of SF.

Synovial Extracellular Vesicles: Structure and Role in Synovial Fluid Tribological Performances

The quality of the lubricant between cartilaginous joint surfaces impacts the joint’s mechanistic properties. In this study, we define the biochemical, ultrastructural, and tribological signatures of synovial fluids (SF) from patients with degenerative (osteoarthritis-OA) or inflammatory (rheumatoid arthritis-RA) joint pathologies in comparison with SF from healthy subjects. Phospholipid (PL) concentration in SF increased in pathological contexts, but the proportion PL relative to the overall lipids decreased. Subtle changes in PL chain composition were attributed to the inflammatory state. Transmission electron microscopy showed the occurrence of large multilamellar synovial extracellular vesicles (EV) filled with glycoprotein gel in healthy subjects. Synovial extracellular vesicle structure was altered in SF from OA and RA patients. RA samples systematically showed lower viscosity than healthy samples under a hydrodynamic lubricating regimen whereas OA samples showed higher viscosity. In turn, under a boundary regimen, cartilage surfaces in both pathological situations showed high wear and friction coefficients. Thus, we found a difference in the biochemical, tribological, and ultrastructural properties of synovial fluid in healthy people and patients with osteoarthritis and arthritis of the joints, and that large, multilamellar vesicles are essential for good boundary lubrication by ensuring a ball-bearing effect and limiting the destruction of lipid layers at the cartilage surface.

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.

Bioinspired Bottlebrush Polymers for Aqueous Boundary Lubrication

An extremely efficient lubrication system is achieved in synovial joints by means of bio-lubricants and sophisticated nanostructured surfaces that work together. Molecular bottlebrush structures play crucial roles for this superior tribosystem. For example, lubricin is an important bio-lubricant, and aggrecan associated with hyaluronan is important for the mechanical response of cartilage. Inspired by nature, synthetic bottlebrush polymers have been developed and excellent aqueous boundary lubrication has been achieved. In this review, we summarize recent experimental investigations of the interfacial lubrication properties of surfaces coated with bottlebrush bio-lubricants and bioinspired bottlebrush polymers. We also discuss recent advances in understanding intermolecular synergy in aqueous lubrication including natural and synthetic polymers. Finally, opportunities and challenges in developing efficient aqueous boundary lubrication systems are outlined.

Thermal Friction Enhancement in Zwitterionic Monolayers

We introduce a model for zwitterionic monolayers and investigate its tribological response to changes in applied load, sliding velocity, and temperature by means of molecular-dynamics simulations. The proposed model exhibits different regimes of motion depending on temperature and sliding velocity. We find a remarkable increase of friction with temperature, which we attribute to the formation and rupture of transient bonds between individual molecules of opposite sliding layers, triggered by the out-of-plane thermal fluctuations of the molecules' orientations. To highlight the effect of the molecular charges, we compare these results with analogous simulations for the charge-free system. These findings are expected to be relevant to nanoscale rheology and tribology experiments of locally-charged lubricated systems such as, e.g., experiments performed on zwitterionic monolayers, phospholipid micelles, or confined polymeric brushes in a surface force apparatus.

Insight into the Lubrication Behavior of Phospholipids Pre-adsorbed on Silica Surfaces at Different Adsorption Temperatures

Phospholipids, as essential components in joint synovial fluid, play a dominant role in joint lubrication. In this study, atomic force microscopy was used to evaluate the normal and shear forces between two surfaces bearing three types of phospholipids with different acyl chain lengths, which were pre-adsorbed onto silica surfaces at different temperatures (25, 45, and 60 °C). When the pre-adsorption temperature was below the phospholipid phase transition temperature (T_m), a super-low friction coefficient [1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC): 0.002; 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC): 0.007] between two opposing silica surfaces in water was achieved because of the super-low shear strength of the hydration shell and robustness of the vesicle when the load was less than the critical value (DSPC: 500 nN; DPPC: 85 nN). However, when the pre-adsorption temperature exceeded T_m, the silica surface was covered by a bilayer structure with many defects, which exhibited poor adsorption density and low bearing capacity, resulting in a relatively high friction coefficient. This study gains insights into the influence of structure and temperature on the lubrication mechanism of phospholipids as biolubricants, providing guidance for the application of artificial joint synovial fluid.

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.

Surface Interactions between Boundary Layers of Poly(ethylene oxide)-Liposome Complexes: Lubrication, Bridging, and Selective Ligation †

Poly(ethylene oxide), PEO, is widely exploited in biomedical applications, while phosphatidylcholine (PC) lipids (in the form of bilayers or liposomes) have been identified as very efficient boundary lubricants in aqueous media. Here we examine, using a surface force balance (SFB), the interactions between surface-adsorbed layers of PEO complexed with small unilamellar vesicles (SUVs, i.e. liposomes) or with bilayers of PC lipids, both well below and a little above their main gel-to-liquid phase-transition temperatures T_M. The morphology of PEO layers (adsorbed onto mica), to which liposomes were added, was examined using atomic force microscopy (AFM) and cryo-scanning electron microscopy (cryo-SEM). Our results reveal that the PC lipids could attach to the PEO either as vesicles or as bilayers, depending on whether they were above or below T_M. Under water (no added salt), excellent lubrication, with friction coefficients down to 10^-3-10^-4, up to contact stresses of 6.5 MPa (comparable to those in the major joints) was observed between two surfaces bearing such PEO-PC complexes. At 0.1 M KNO₃ salt concentration (comparable to physiological salt levels), the friction between such surfaces was considerably higher, attributed to bridging by the polymer chains. Remarkably, such bridging could be suppressed and the friction could be restored to its previous low value if the KNO₃ was replaced with NaNO₃, as a result of the different PEO-mica ligation properties of Na⁺ compared to those of K⁺. Our results provide insight into the properties of PEO-PC complexes in potential applications, and large interfacial effects that can result from the seemingly innocuous replacement of K⁺ by Na⁺ ions.

Superlubricity of Polyalkylene Glycol Aqueous Solutions Enabled by Ultrathin Layered Double Hydroxide Nanosheets

It was previously proved that the existence of a large amount of hydrogen ions in water-based lubricants can easily lead to a superlubric state; however, it was also shown that these hydrogen ions could cause severe corrosion. As part of a large family of attractive clays, layered double hydroxides (LDHs) possess excellent tribological properties in water-based lubrication systems. In the present work, two different kinds of LDHs are dispersed in polyalkylene glycol (PAG) aqueous solutions, in two distinct forms: ultrathin nanosheets (ULDH-NS) of ca. 60 nm wide and ca. 1 nm thick (single or double layer) and nanoparticles (LDH-NP) of ca. 19.73 nm wide and ca. 8.68 nm thick. We find that the addition of ULDH-NS greatly shortens (as much as 85%) the running-in period prior to reaching the superlubricity regime and increases the ultimate load-bearing capacity by about four times. As compared to the fluid film thickness of the lubricating PAG solution, their ultrathin longitudinal dimension will not impair or influence the fluid film coverage in the contact zone. The analysis of sliding solid surfaces and the atomic force microscope microscale friction test demonstrate that the adsorption of ULDH-NS enables the sliding solid surfaces to be polished and protected because of their relatively weak interlayer interaction and increased adhesion effect. Owing to their superior tribological properties as lubricant additives, ultrathin LDH nanosheets hold great potential for enabling liquid superlubricity in industrial applications in the future.

Investigation of the lubrication properties and synergistic interaction of biocompatible liposome-polymer complexes applicable to artificial joints

Achievement of efficient biolubrication is essential for the design of artificial joints with long lifetimes. This study examines the frictional behaviors and adsorption structures of liposomes and liposome complexes with biocompatible polymers to reveal the underlying lubrication mechanisms between biomimetic bearing surfaces of polyetheretherketone (PEEK) and silicon nitride (Si₃N₄). The liposomes with increasing carbon chain lengths exhibit the remarkable lubrication capabilities that correlate strongly with the structural integrity of small unilamellar vesicles adsorbed on the Si₃N₄ surfaces, while the bilayer structures weaken the stability of vesicles against rupture and cause the increase of friction. The synergistic interaction of liposomes and biocompatible negative-charged polymer leads to the formation of a boundary-lubricating layer with high-density liposome-polymer complex structures that can efficiently improve the lubrication properties of liposomes. Our findings might have implications for future biolubrication investigations on biocompatible liposome-polymer complexes applicable to artificial joints at the specified macroscale conditions.

Preparation and Characterization of Solid-Supported Lipid Bilayers Formed by Langmuir-Blodgett Deposition: A Tutorial

The structure, phase behavior, and properties of cellular membranes are derived from their composition, which includes phospholipids, sphingolipids, sterols, and proteins with various levels of glycosylation. Because of the intricate nature of cellular membranes, a plethora of in vitro studies have been carried out with model membrane systems that capture particular properties such as fluidity, permeability, and protein binding but vastly simplify the membrane composition in order to focus in detail on a specialized property or function. Supported lipid bilayers (SLB) are widely used as archetypes for cellular membranes, and this instructional review primarily focuses on the preparation and characterization of SLB systems formed by Langmuir deposition methods. Typical characterization methods, which take advantage of the planar orientation of SLBs, are illustrated, and references that go into more depth are included. This invited instructional review is written so that nonexperts can quickly gain in-depth knowledge regarding the preparation and characterization of SLBs. In addition, this work goes beyond traditional instructional reviews to provide expert readers with new results that cover a wider range of SLB systems than those previously reported in the literature. The quality of an SLB is frequently not well described, and details such as topological defects can influence the results and conclusions of an individual study. This article quantifies and compares the quality of SLBs fabricated from a variety of gel and fluid compositions, in correlation with preparation techniques and parameters, to generate general rules of thumb to guide the construction of designed SLB systems.

Structure and Functionality of Polyelectrolyte Brushes: A Surface Force Perspective

The unique functionality of polyelectrolyte brushes depends on several types of specific interactions, including solvent structure effects, hydrophobic forces, electrostatic interactions, and specific ion interactions. Subtle variations in the solution environment can lead to conformational and surface structural changes of the polyelectrolyte brushes, which are mainly discussed from a surface-interaction perspective in this Focus Review. A brief overview is given of recent theoretical and experimental progress in the structure of polyelectrolyte brushes in various environments. Two important techniques for surface-force measurements are described, the surface forces apparatus (SFA) and atomic force microscopy (AFM), and some recent results on polyelectrolyte brushes are shown. Lastly, this Focus Review highlights the use of these surface-grafted polyelectrolyte brushes in the creation of functional surfaces for various applications, including nonfouling surfaces, boundary lubricants, and stimuli-responsive surfaces.

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.

Synergies in lubrication

In living organisms the aqueous medium is used for providing low friction forces. This is achieved by synergistic actions of different biomolecules that together accomplish a high load bearing capacity and sustain an easily sheared water layer.

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.

Interleaflet sliding in lipidic bilayers under shear flow: comparison of the gel and fluid phases using reversed non-equilibrium molecular dynamics simulations

The friction between two rubbing surfaces lubricated by water can be diminished if they are coated with phospholipidic bilayers or brushes of polyelectrolytes. In the case of a coating by lipid membranes, the friction is lower when the lipids are in the gel phase rather than in the liquid phase. We investigated the response of fluid or gel bilayers to a mechanical load or under shear using non-equilibrium molecular dynamics simulations (NEMD) to understand whether this difference could come from intermonolayer sliding. The system is composed of a single fully hydrated bilayer of coarse grained phospholipids under a parallel shear with vorticity parallel to the bilayer. In both the liquid and the gel phases, an intermonolayer slip was measured in the velocity profile. In the liquid phase this slip is proportional to the shear stress. In the tilted gel phase of our model the stress is not systematically linear and relaxes differently when the shear is in the direction of the tilt or perpendicular to it. The impact of surface tension (or load) on the friction is different for the liquid and gel phases, but grossly the slip remains of the same order of magnitude.

Ultrastructural analysis of healthy synovial fluids in three mammalian species

A better knowledge of synovial fluid (SF) ultrastructure is required to further understand normal joint lubrication and metabolism. The aim of the present study was to elucidate SF structural features in healthy joints from three mammalian species of different size compared with features in biomimetic SF. High-resolution structural analysis was performed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and environmental SEM/wet scanning transmission electron microscopy mode complemented by TEM and SEM cryogenic methods. Laser-scanning confocal microscopy (LCM) was used to locate the main components of SF with respect to its ultrastructural organization. The present study showed that the ultrastructure of healthy SF is built from a network of vesicles with a size range from 100 to a few hundred nanometers. A multilayered organization of the vesicle membranes was observed with a thickness of about 5 nm. LCM study of biological SF compared with synthetic SF showed that the microvesicles consist of a lipid-based membrane enveloping a glycoprotein gel. Thus, healthy SF has a discontinuous ultrastructure based on a complex network of microvesicles. This finding offers novel perspectives for the diagnosis and treatment of synovial joint diseases.

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.

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.

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).

Nanofluids mediating surface forces

Fluids containing nanostructures, known as nanofluids, are increasingly found in a wide array of applications due to their unique physical properties as compared with their base fluids and larger colloidal suspensions. With several tuneable parameters such as the size, shape and surface chemistry of nanostructures, as well as numerous base fluids available, nanofluids also offer a new paradigm for mediating surface forces. Other properties such as local surface plasmon resonance and size dependent magnetism of nanostructures also present novel mechanisms for imparting tuneable surface interactions. However, our fundamental understanding, experimentally and theoretically, of how these parameters might affect surface forces remains incomplete. Here we review recent results on equilibrium and dynamic surface forces between macroscopic surfaces in nanofluids, highlighting the overriding trends in the correlation between the physical parameters that characterise nanofluids and the surface forces they mediate. We also discuss the challenges that confront existing surface force knowledge as a result of this new paradigm.

Biomimetic hydration lubrication with various polyelectrolyte layers on cross-linked polyethylene orthopedic bearing materials

Natural joints rely on fluid thin-film lubrication by the hydrated polyelectrolyte layer of cartilage. However, current artificial joints with polyethylene (PE) surfaces have considerably less efficient lubrication and thus much greater wear, leading to osteolysis and aseptic loosening. This is considered a common factor limiting prosthetic longevity in total hip arthroplasty (THA). However, such wear could be mitigated by surface modification to mimic the role of cartilage. Here we report the development of nanometer-scale hydrophilic layers with varying charge (nonionic, cationic, anionic, or zwitterionic) on cross-linked PE (CLPE) surfaces, which could fully mimic the hydrophilicity and lubricity of the natural joint surface. We present evidence to support two lubrication mechanisms: the primary mechanism is due to the high level of hydration in the grafted layer, where water molecules act as very efficient lubricants; and the secondary mechanism is repulsion of protein molecules and positively charged inorganic ions by the grafted polyelectrolyte layer. Thus, such nanometer-scaled hydrophilic polymers or polyelectrolyte layers on the CLPE surface of acetabular cup bearings could confer high durability to THA prosthetics.

The boundary lubrication of chemically grafted and cross-linked hyaluronic acid in phosphate buffered saline and lipid solutions measured by the surface forces apparatus

High molecular weight hyaluronic acid (HA) is present in articular joints and synovial fluid at high concentrations; yet despite numerous studies, the role of HA in joint lubrication is still not clear. Free HA in solution does not appear to be a good lubricant, being negatively charged and therefore repelled from most biological, including cartilage, surfaces. Recent enzymatic experiments suggested that mechanically or physically (rather than chemically) trapped HA could function as an "adaptive" or "emergency" boundary lubricant to eliminate wear damage in shearing cartilage surfaces. In this work, HA was chemically grafted to a layer of self-assembled amino-propyl-triethoxy-silane (APTES) on mica and then cross-linked. The boundary lubrication behavior of APTES and of chemically grafted and cross-linked HA in both electrolyte and lipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) solutions was tested with a surface forces apparatus (SFA). Despite the high coefficient of friction (COF) of μ ≈ 0.50, the chemically grafted HA gel significantly improved the lubrication behavior of HA, particularly the wear resistance, in comparison to free HA. Adding more DOPC lipid to the solution did not improve the lubrication of the chemically grafted and cross-linked HA layer. Damage of the underlying mica surface became visible at higher loads (pressure >2 MPa) after prolonged sliding times. It has generally been assumed that damage caused by or during sliding, also known as "abrasive friction", which is the main biomedical/clinical/morphological manifestation of arthritis, is due to a high friction force and, therefore, a large COF, and that to prevent surface damage or wear (abrasion) one should therefore aim to reduce the COF, which has been the traditional focus of basic research in biolubrication, particularly in cartilage and joint lubrication. Here we combine our results with previous ones on grafted and cross-linked HA on lipid bilayers, and lubricin-mediated lubrication, and conclude that for cartilage surfaces, a high COF can be associated with good wear protection, while a low COF can have poor wear resistance. Both of these properties depend on how the lubricating molecules are attached to and organized at the surfaces, as well as the structure and mechanical, viscoelastic, elastic, and physical properties of the surfaces, but the two phenomena are not directly or simply related. We also conclude that to provide both the low COF and good wear protection of joints under physiological conditions, some or all of the four major components of joints-HA, lubricin, lipids, and the cartilage fibrils-must act synergistically in ways (physisorbed, chemisorbed, grafted and/or cross-linked) that are still to be determined.

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.