Molecular mechanisms of aqueous boundary lubrication by mucinous glycoproteins

Mucins have long been recognized as instrumental to biolubrication but the molecular details of their lubrication mechanisms have only been explored relatively recently. The glycoprotein PRG4, also known as lubricin, shares many features with mucins and appears to lubricate through similar mechanisms. A number of studies have contributed to a more in-depth understanding of mucin adsorption and layer formation on surfaces and the mechanisms by which these layers lubricate. Although mucinous glycoproteins differ in their aggregation properties, their adsorption behaviors on surfaces, and in their ability to reduce friction, they share important similarities favorable for lubrication. They are highly hydrated, they adsorb strongly to a broad range of surfaces, and the layers they form are both sterically and electrostatically repulsive, all attributes thought to contribute to boundary lubrication. They also hydrophilize hydrophobic surfaces, promoting the formation of aqueous fluid films that can lower friction at already relatively low sliding speeds. In this paper we briefly review current knowledge of mucin adsorption and lubrication, with a focus on recent advances.     

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.     

Macro- to nanoscale wear prevention via molecular adsorption

As the size of mechanical systems shrinks from macro- to nanoscales, surface phenomena such as adhesion, friction, and wear become increasingly significant. This paper demonstrates the use of alcohol adsorption as a means of continuously replenishing the lubricating layer on the working device surfaces and elucidates the tribochemical reaction products formed in the sliding contact region. Friction and wear of native silicon oxide were studied over a wide range of length scales from macro- to nanoscales using a ball-on-flat tribometer (millimeter scale), sidewall microelectromechanical system (MEMS) tribometer (micrometer scale), and atomic force microscopy (nanometer scale). In all cases, the alcohol vapor adsorption successfully lubricated and prevented wear. Imaging time-of-flight secondary ion mass spectrometry analysis of the sliding contact region revealed that high molecular weight oligomeric species were formed via tribochemical reactions of the adsorbed linear alcohol molecules. These tribochemical products seemed to enhance the lubrication and wear prevention. In the case of sidewall MEMS tests, the lifetime of the MEMS device was radically increased via vapor-phase lubrication with alcohol.     

Lipids and lipid mixtures in boundary layers: From hydration lubrication to osteoarthritis

The hydration layer surrounding the phosphocholine headgroups of single-component phosphatidylcholine lipids, or of lipid-mixtures, assembled at an interface greatly modifies the interfacial properties and interactions. As water molecules within the hydration layer are held tightly by the headgroup but are nonetheless very fluid on shear, the boundary lipid layers, exposing the highly hydrated headgroup arrays, can provide efficient boundary lubrication when sliding against an opposing surface, at physiologically high contact pressures. In addition, any free lipids in the surrounding liquid can heal defects which may form during sliding on the boundary phosphatidylcholine layer. Similar boundary lipid layers contribute to the lubricating, pressure-bearing, and wear-protection functions of healthy articular joints. This review presents a survey of the relationship between the molecular composition of the interfacial complex and the lubrication behavior of the lipid-based boundary layers, which could be beneficial for designing boundary lubricants for intra-articular injection for the treatment of early osteoarthritis.     

Synthesis and interfacial properties of novel cationic polyelectrolytes with brush-on-brush structure of poly(ethylene oxide) side chains

Novel cationic polyelectrolytes with a brush-on-brush structure of poly(ethylene oxide) (PEO) side chains and a charge-containing polyacrylate backbone were synthesized. The PEO side chains were not directly attached to the backbone but via polymethacrylate spacers, thus locating the PEO chains a distance away from the charged units of the backbone. The cationic brush-on-brush polyelectrolytes with high density of PEO chains showed a strong affinity to silica surfaces, provided the backbone charge density was high enough. The adsorption of these polymers was studied by QCM-D giving very high sensed mass, 20 mg/m². It was shown by direct force measurements that protective surface layers were formed by the novel polyelectrolytes, generating strongly repulsive steric forces, which provided an effective barrier against flocculation. The adsorbed layer was sufficiently robust to withstand sliding experiments under a pressure of up to 35 MPa. The friction force in water was very low, and the lubrication was characterized by a friction coefficient in the range of 0.02-0.06.     

Sliding friction and wear behaviors of plasma sprayed aluminum–bronze coating in artificial seawater

The friction and wear behaviors of plasma sprayed aluminum–bronze (CuAl) coating sliding against silicon nitride (Si₃N₄) in artificial seawater were investigated and compared with those in pure water and dry sliding. The morphologies of the worn surfaces were analyzed by three‐dimensional non‐contact surface mapping and scanning electron microscopy. Moreover, chemical states of the tribochemical products of CuAl/Si₃N₄ in seawater were characterized by X‐ray photoelectron spectroscopy. Results show that the plasma sprayed CuAl coating possessed a specific wear rate (in order of 10^?7 mm³/Nm) in seawater more than 600 times smaller than that in dry sliding due to the great alleviation in abrasion wear and splats delamination. Besides, the CuAl/Si₃N₄ had a friction coefficient of 0.06 in seawater, significantly lower and more stable than those in pure water and dry sliding. The tribochemical products of CuAl/Si₃N₄ in seawater, which were proved to be silica, alumina, and their hydrates, transformed into a loosened wear‐debris layer under the coagulation effect of the seawater and dominated the excellent lubrication state in artificial seawater. Copyright © 2014 John Wiley & Sons, Ltd.     

The influence of electrostatic forces on protein adsorption

In this paper we investigate the importance of electrostatic double layer forces on the adsorption of human serum albumin by UV-ozone modified polystyrene. Electrostatic forces were measured between oxidized polystyrene surfaces and gold-coated atomic force microscope (AFM) probes in phosphate buffered saline (PBS) solutions. The variation in surface potential with surface oxygen concentration was measured. The observed force characteristics were found to agree with the theory of electrical double layer interaction under the assumption of constant potential. Chemically patterned polystyrene surfaces with adjacent 5 microm x 5 microm polar and non-polar domains have been studied by AFM before and after human serum albumin adsorption. A topographically flat surface is observed before protein adsorption indicating that the patterning process does not physically modify the surface. Friction force imaging clearly reveals the oxidation pattern with the polar domains being characterised by a higher relative friction compared to the non-polar, untreated domains. Far-field force imaging was performed on the patterned surface using the interleave AFM mode to produce two-dimensional plots of the distribution of electrostatic double-layer forces formed when the patterned polystyrene surfaces is immersed in PBS. Imaging of protein layers adsorbed onto the chemically patterned surfaces indicates that the electrostatic double-layer force was a significant driving force in the interaction of protein with the surface.     

Friction laws for lubricated nanocontacts

We have used friction force microscopy to probe friction laws for nanoasperities sliding on atomically flat substrates under controlled atmosphere and liquid environment, respectively. A power law relates friction force and normal load in dry air, whereas a linear relationship, i.e., Amontons' law, is observed for junctions fully immersed in model lubricants, namely, octamethylciclotetrasiloxane and squalane. Lubricated contacts display a remarkable friction reduction, with liquid and substrate specific friction coefficients. Comparison with molecular dynamics simulations suggests that load-bearing boundary layers at junction entrance cause the appearance of Amontons' law and impart atomic-scale character to the sliding process; continuum friction models are on the contrary of limited predictive power when applied to lubrication effects. An attempt is done to define general working conditions leading to the manifestation of nanoscale lubricity due to adsorbed boundary layers.     

Adsorbed Layers of Ferritin at Solid and Fluid Interfaces Studied by Atomic Force Microscopy

The adsorption of the iron storage protein ferritin was studied by liquid tapping mode atomic force microscopy in order to obtain molecular resolution in the adsorbed layer within the aqueous environment in which the adsorption was carried out. The surface coverage and the structure of the adsorbed layer were investigated as functions of ionic strength and pH on two different charged surfaces, namely chemically modified glass slides and mixed surfactant films at the air-water interface, which were transferred to graphite substrates after adsorption. Surface coverage trends with both ionic strength and pH indicate the dominance of electrostatic effects, with the balance shifting between intermolecular repulsion and protein-surface attraction. The resulting behavior is more complex than that seen for larger colloidal particles, which appear to follow a modified random sequential adsorption model monotonically. The structure of the adsorbed layers at the solid surfaces is random, but some indication of long-range order is apparent at fluid interfaces, presumably due to the higher protein mobility at the fluid interface. Copyright 2000 Academic Press.     

Adsorption properties of poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) at a hydrophobic interface: influence of tribological stress, pH, salt concentration, and polymer molecular weight

The adsorption properties of a graft copolymer of poly(ethylene glycol) (PEG) with a polycationic backbone, namely, poly( l-lysine)- graft-poly(ethylene glycol) (PLL- g-PEG), onto nonpolar, hydrophobic PDMS surfaces from aqueous solution and the lubrication properties of the self-mated sliding contacts of PDMS surfaces modified with PLL- g-PEG have been investigated. Whereas PLL- g-PEG is spontaneously attracted to negatively charged surfaces as a result of the polycationic PLL backbone, the collective interaction of (CH 2) 4 hydrocarbon moieties on the lysine units in the PLL backbone with nonpolar, hydrophobic surfaces also enables the adsorption of PLL- g-PEG onto hydrophobic surfaces such as PDMS. The adsorption and lubrication properties of PLL- g-PEG have been investigated by varying the aqueous solution parameters, such as pH (2, 7, and 12) and KCl concentration (0, 0.01, 0.1, and 1 M) as well as the length of the PLL backbone of the copolymer (20 vs 375 kDa). In the absence of tribological stress, the adsorption of PLL- g-PEG onto PDMS surfaces was mainly governed by the KCl concentration, whereas the role of pH or the molecular weight of the copolymer was of relatively minor importance; for all pH values, the adsorbed mass decreased with increasing KCl concentration. Under tribological stress, however, a clear dependence of the lubrication properties of PLL- g-PEG on all of the studied parameters, including pH, KCl concentration, and backbone molecular weight, was observed. The adsorption strength of PLL- g-PEG on PDMS surfaces, rather than the adsorbed mass itself, appeared to be the most critical parameter in determining the lubrication properties.     

Friction and adsorption of aqueous polyoxyethylene (Tween) surfactants at hydrophobic surfaces

The nanotribological responses of a series of nonionic polyoxyethylene surfactants (Tween 20, Tween 40, Tween 60, and Tween 80) were investigated after they were adsorbed from aqueous solution onto atomically smooth hydrophobic substrates. The hydrophobic surfaces were composed of a condensed monolayer of octadecyltriethoxysilane (OTE; contact angle theta>110 degrees ). The nanorheological measurements were performed using a modified surface forces apparatus after coating atomically smooth mica with these OTE monolayers, while adsorption measurements were performed using phase-modulated ellipsometry on silicon wafers coated with these same monolayers. The minimum surface-surface separation observed under high load in friction studies agreed quantitatively with the thickness obtained from ellipsometry. For Tweens 20, 40, and 60, the thickness of the adsorbed film increases with increasing alkyl chain length. Systematic investigations of the nanorheological response showed that there is a "solid-like" elastic response from confined surfactant layers, which is the case for the smallest separations to separations up to slightly larger than twice the adsorbed film thickness. In kinetic friction, these confined layers are characterized by a shear stress of approximately 3 MPa with minimal dependence on shear rate. The magnitude of the sliding shear stress is the same as the apparent yield stress at approximately 3 MPa; it is independent of alkyl chain length within the Tween family of surfactants and corresponds to a nominal friction coefficient of mu approximately 1. A similar friction coefficient is observed for boundary lubrication on the macroscopic scale in a tribometer utilizing hydrophobic surfaces and mu approximately 1.1 for Tweens 20, 40, and 60. These results suggest that while Tween molecules adsorb onto hydrophobic surfaces to form a robust separating layer, the lubricating properties of these layers are dominated by a highly dissipative slip plane, the same for all alkyl chain lengths.     

Surface sliding friction of negatively charged polyelectrolyte gels

The friction between two polyelectrolyte gels carrying the same or opposite sign of charges has been investigated using a rheometer. It is found that the friction was strongly dependent on the interfacial interaction between two gel surfaces. In the repulsive interaction case, especially, the friction was extremely low. The friction behavior is attempted to be described in terms of the hydrodynamic lubrication of the solvent layer between two like-charged gel surfaces, which is formed due to the electrostatic repulsion of the two gel surfaces. From the theoretical analysis (hydrodynamic mechanism), the friction behaviors were explained qualitatively, all of the experimental results, nevertheless, could not be understood well. The viscoelastic feature of the gel and the non-Newtonian behavior of water at the friction interface are considered to be important to elucidate the gel friction.     

Silica surfaces lubrication by hydrated cations adsorption from electrolyte solutions

Adsorption of hydrated cations on hydrophilic surfaces has been related to a variety of phenomena associated with the short-range interaction forces and mechanisms of the adhesive contact between the surfaces. Here we have investigated the effect of the adsorption of cations on the lateral interaction. Using lateral force microscopy (LFM), we have measured the friction force between a silica particle and silica wafer in pure water and in electrolyte solutions of LiCl, NaCl, and CsCl salts. A significant lubrication effect was demonstrated for solutions of high electrolyte concentrations. It was found that the adsorbed layers of smaller and more hydrated cations have a higher lubrication capacity than the layers of larger and less hydrated cations. Additionally, we have demonstrated a characteristic dependence of the friction force on the sliding velocity of surfaces. A mechanism for the observed phenomena based on the microstructures of the adsorbed layers is proposed.     

Structure of Protein Layers during Competitive Adsorption

The formation of protein layers during competitive adsorption was studied with ellipsometry. Single, binary, and ternary protein solutions of human serum albumin (HSA), IgG, and fibrinogen (Fgn) were investigated at concentrations corresponding to blood plasma diluted 1/100. As a model surface, hydrophobic hexamethyldisiloxane (HMDSO) plasma polymer modified silica was used. By using multiambient media measurements of the bare substrate prior to protein adsorption the adsorbed amount as well as the thickness and refractive index of the adsorbed protein layer could be followedin situand in real time. Under conditions used in these experiments neither IgG nor fibrinogen could fully displace serum albumin from the interface. The buildup of the protein layer occurred via different mechanisms for the different protein systems. Fgn adsorbed in a rather flat orientation at low adsorbed amounts, while at higher surface coverage the protein reoriented to a more upright orientation in order to accommodate more molecules in the adsorbed layer. IgG adsorption proceeded mainly end-on with little reorientation or conformational change on adsorption. Finally, for HSA an adsorbed layer thickness greater than the molecular dimensions was observed at high concentrations (although not at low), indicating that aggregates or multilayers formed on HMDSO plasma polymer surfaces. For all protein mixtures the adsorbed layer structure and buildup indicated that Fgn was the protein dominating the adsorbed layer, although HSA partially blocked the adsorption of this protein. At high surface concentration, HSA/Fgn mixtures show an abrupt change in both adsorbed layer thickness and refractive index suggesting, e.g., an interfacial phase transition of the mixed protein layer. A similar but less pronounced behavior was observed for HSA/IgG. For IgG/Fgn and HSA/IgG/Fgn a buildup of the adsorbed layer similar to that displayed by Fgn alone was observed.     

Viscoelastic signature of physisorbed macromolecules at the solid-liquid interface

Viscoelastic behavior of a solution boundary layer at a solid-liquid interface could differ from that of bulk solution due to molecular adsorption at the interface. Such a property can be used as a characteristic signature to indicate the molecular adsorption at the interface. In this work, we systematically measured the viscoelastic properties of polyethylene glycol (PEG) solution boundary layers in contact with a gold surface using a quartz crystal resonator technique. The results show that viscosity and shear modulus of the PEG boundary layer increase with the PEG concentration in the solution; the increasing rate depends on the molecular weight. For relatively small PEG molecules, the viscoelastic property of the PEG solution boundary layer is almost indistinguishable from that of the bulk solution of the same concentration, indicating no adsorption at the interface. For larger PEG polymers (with molecular weights above a few thousands grams per mole), the viscoelastic property of the solution boundary layer differs distinctively from that of the corresponding bulk solution. The difference can be attributed to physisorption of PEG molecules on the Au surface, which alters the viscoelastic behaviors of the boundary layer. The results suggest that adsorption behaviors of macromolecules at a solid-liquid interface might be inferred from the changes of the viscoelastic properties of a solution boundary layer.     

Adhesion and stable low friction provided by a subnanometer-thick monolayer of a natural polysaccharide

Using a surface forces apparatus, we have investigated the adhesive and lubrication forces of mica surfaces separated by a molecularly thin, subnanometer film of a high-molecular-weight (2.3 MDa) anionic polysaccharide from the algae Porphyridium sp. adsorbed from aqueous solution. The adhesion and friction forces of the confined biopolymer were monitored as a function of time, shearing distance, and driving velocity under a large range of compressive loads (pressures). Although the thickness of the dilute polysaccharide was <1 nm, the friction was low (coefficient of friction = 0.015), and no wear was ever observed even at a pressure of 110 atm over 3 decades of velocity, so long as the shearing distances were less than twice the contact diameter. Atomic force microscopy in solution shows that the biopolymer is able to adsorb to the mica surface but remains mobile and easily dragged upon shearing. The adhesion (adsorption) of this polysaccharide even to negatively charged surfaces, its stable low friction, its robustness (high-load carrying capacity and good wear protection), and the weak (logarithmic) dependence of the friction force on the sliding velocity make this class of polyelectrolytes excellent candidates for use in water-based lubricant fluids and as potential additives to synovial fluid in joints and other biolubricating fluids. The physical reasons for the remarkable tribological properties of the ultrathin polysaccharide monolayer are discussed and appear to be quite different from those of other polyelectrolytes and proteins that act as thick "polymer brush" layers.     

Negative velocity dependence of friction for poly(2‐Acrylamido‐2‐methyl propanesulfonic acid) hydrogel sliding against a glass surface in the low‐velocity region

The frictional behavior of poly(2‐acrylamido‐2‐methylpropanesulfonic acid) (PAMPS) hydrogel sliding against a glass substrate in water over a wide sliding velocity (v) region has been investigated. The results showed that the frictional behavior of PAMPS gel conformed to a hydrodynamic lubrication mechanism only at relatively high sliding velocities. At low sliding velocities, a “negative” velocity dependence of friction was observed, which we believe not to be attributable to the experimental friction‐measuring mode used. This wider and weak mixed region at low sliding velocities is in contrast to the extremely narrow mixed region in the case of solid friction with a lubricant. The area of the PAMPS hydrogel surface subject to shearing decreased with increasing sliding velocity, and this would seem to be responsible for the weakly negative dependence of friction on velocity. In addition, the friction was found to increase with increasing the compressive modulus (E) of the gels attributing to the shearing exerted on the gel surface, in which the shear stress increased with E. The hydration layer between the sliding surfaces also contributes to the friction and weakens the dependence of friction on E to some extent. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 765–772     

Selective adsorption of protein molecules on phase-separated sapphire surfaces

Site-selective adsorption of protein molecules was found on sapphire surfaces that exhibit a phase separation into two domains: weakly charged hydrophobic domain and negatively charged hydrophilic one. Ferritin and bovine serum albumin molecules, which are negatively charged in a buffer solution, are adsorbed to the hydrophobic domains. Avidin molecules, which are positively charged, are adsorbed to the other domain. Fibrinogen molecules, which consist of both negative and positive modules, are adsorbed to the whole sapphire surface. Hemoglobin molecules, whose net charge is almost zero, are also adsorbed to the whole surfaces. These results indicate that electrostatic double layer interaction is the primary origin of the observed selectivity. Dependence of protein adsorption or desorption behaviors on the pH value can also be interpreted by the proposed model.     

Importance of the catalytic effect of the substrate in the functionality of lubricant additives: the case of molybdenum dithiocarbamates

Molybdenum dithiocarbamates (MoDTCs) are lubricant additives very efficient in reducing the friction of steel, and they are used in a number of industrial applications. The functionality of these additives is ruled by the chemical interactions occurring at the buried sliding interface, which are of key importance for the improvement of the lubrication performance. Yet, these tribochemical processes are very difficult to monitor in real time. Ab initio molecular dynamics simulations are the ideal tool to shed light on such a complicated reactivity. In this work, we perform ab initio simulations, both in static and tribological conditions, to understand the effect of surface oxidation on the tribochemical reactivity of MoDTC, and we find that when the surfaces are covered by oxygen, the first dissociative steps of the additives are significantly hindered. Our preliminary tribological tests on oxidized steel discs support these results. Bare metallic surfaces are necessary for a stable adsorption of the additives, their quick decomposition, and the formation of a durable MoS₂ tribolayer. This work demonstrates the importance of the catalytic role of the substrate and confirms the full capability of the computational protocol in the pursuit of materials and compounds more efficient in reducing friction.     

Polyzwitterionic brushes: Extreme lubrication by design

Polymers offer the advantage that they may independently combine desirable supramolecular structure with useful local monomeric properties to yield optimal performance of different tasks. Here we utilise the remarkable lubricating properties both of dense polymer brushes, and of hydration sheaths about charges via the emerging paradigm of hydration lubrication, to design a grafted-from polyzwitterionic brush system, where each of the monomers has a structure similar to the highly-hydrated phosphorylcholine headgroups of phosphatidylcholine lipids. Such polyzwitterions are grown from a macroinitiator coating the substrate (mica) surface using atom transfer radical polymerisation (ATRP) of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) to form exceptionally robust poly(MPC) brushes. We have characterized these brush layers via X-ray reflectometry, X-ray photoelectron spectroscopy, surface forces measurements and atomic force microscopy. Such brushes, designed to optimise their lubrication properties, are indeed found to provide state of the art boundary lubrication, achieving friction coefficients as low as 0.0004 at pressures up to 75 atmospheres over a wide range of sliding velocities. Such low friction is comparable with that of articular cartilage in healthy mammalian joints, which represents nature’s benchmark for boundary lubrication in living organisms, and suggests that hydration lubrication plays a major role in reducing friction in biological systems.