Hydration Lubrication Research Articles

Supramolecular repair of hydration lubrication surfaces

<h2>Summary</h2> Although advances in coating technologies have allowed us to match—or even exceed—the lubricity of Nature's low-friction surfaces, the performance of synthetic materials inevitably diminishes over time as the surfaces are worn and damaged by irreversible breakage of covalent bonds. Synthetic systems lack the bespoke repair mechanisms that replenish hydration lubrication surfaces in Nature. Here, we demonstrate dynamic repair of low-friction surfaces prepared through a surface-selective self-assembly strategy. Monolayers of lubricating polymers associate with functionalized surfaces through strong and specific host–guest interactions, leading to hydration lubrication surfaces with low coefficients of friction (0.024–0.028). Following friction-induced dissociation of the polymers, the polymer-to-surface interaction is restored by the reformation of host–guest complexes, thus repairing the monolayer, renewing the lubricity, and reducing the effects of wear. Such dynamically restored low-friction materials will be an essential tool in decreasing global energy use—a fifth of which is expended overcoming friction.

Near-Infrared-Light-Modulated Lubricating Coating Enabled by Photothermal Microgels.

As a micro-/nano-sized hydrogel, polymeric microgel not only has three-dimensional (3D) molecular networks but also displays the small-size effect, which has been widely used in various fields, such as drug nanocarrier, photonic crystal, functional coating, and aqueous lubrication. In this work, a photothermal lubricating coating was prepared using polymeric/inorganic hybrid microgels and its surficial friction was deliberately modulated by the remote irradiation of near-infrared (NIR) light. Specifically, a photothermal hybrid microgel, Fe3O4@poly(N-isopropylacrylamide-co-polyacrylic acid) (Fe3O4@PNA), was first fabricated and then sprayed onto poly(ethyleneimine) (PEI)-modified substrate to form a lubricating microgel coating. At room temperature, this microgel coating was hydrophilic and achieved good hydration lubrication with relatively low friction. After the introduction of NIR light, the photothermal microgel coating converted light energy into heat energy for increasing its own temperature rapidly. Due to the thermosensitive PNA shell, the wettability of the coating was transformed to hydrophobicity above the lower critical solution temperature (LCST), resulting in a remarkable increase in friction. In other words, the surficial friction of this microgel coating could be reversibly modulated using NIR light. This work expands the application scope of microgels in the field of aqueous lubrication and introduces the functional microgels into making the smart lubricating coating for the first time. This basic research in the field of friction control may provide an efficient strategy for the design of interfacial sensing, controlled transmission, and intelligent manipulator.

Polymer-based lubricating materials for functional hydration lubrication

The hydration lubrication was proposed by Jacob Klein, a famous tribologist, in 2001. The core of this mechanism is the formation of a strong hydrated layer surrounding polar groups. Over the past 20 years, various lubricating materials have been developed according to this mechanism. Among them, polymer-based lubricating materials have attracted much attention because of their facile tunability of structures and functions. Especially, they have played a tremendous role in functional hydration lubrication, which means that other interesting functions can be integrated into hydration lubrication during water-based friction, such as stimuli-responsive property, biomedical performance and biomimetic design. This review outlines the recent progress of functional polymeric materials employed for realizing versatile hydration lubrication, which are categorized as: (i) stimuli-responsive lubricating materials, through external stimuli to control the interfacial friction; (ii) biomedical lubricating materials, integrating lubricating ability into biomedical materials; (iii) biomimetic lubricating materials, designed by learning from living organisms and nature.

Structural Evidence for a Reinforcing Response and Retention of Hydration During Confinement of Cartilage Lipids

Lipids have an important role in the complex lubrication of articulating joints, however changes in lipid phase behavior that occur owing to mechanical confinement are not well understood. Here, a surface force-type apparatus has been combined with neutron reflectometry to measure confinement-induced changes in the structure of lipids, the major surface-active component of the lubricant in articulating joints. The same incompressible state was accessed under low uniaxial stress (1 bar), irrespective of whether the lipids had started out unconfined above or below the Lα phase transition, and irrespective of whether they were fully or partially hydrated. In this incompressible state, the lipid component had thickened indicating extension and rearrangement of the lipid chains in response to the applied stress. The small amount of water remaining between each lipid bilayer was found to be similar for all chain lengths and starting phases. This represents the first structural evidence of the tightly bound water layer at the headgroups, which is required for hydration lubrication under load.

Creating polysaccharide-protein complexes to control aqueous lubrication

An emerging understanding of natural biolubricants such as saliva and synovial fluid reveals that they rely on synergistic interactions between biomacromolecules in aqueous media, wherein one polymeric component promotes adsorption to biological substrates while another facilitates hydration lubrication. We hypothesize that this phenomenon can be achieved by combining the strong adsorption characteristics of proteins with the hydration of polysaccharides. To confirm this hypothesis, hydrated soluble complexes were formed through electrostatic interactions between positively-charged lysozyme molecules and anionic residues on pectic polysaccharides extracted from Plantago ovata seed mucilage. Boundary friction was measured between smooth hydrophobic polydimethylsiloxane (PDMS) surfaces using a ball-and-disk tribometer, while adsorption was assessed using a combination of the quartz-crystal microbalance with dissipation monitoring technique and ellipsometry. The results indicate that complexation between polysaccharides and proteins leads to improved adsorption on hydrophobic PDMS surfaces, which in turn decreases boundary friction in sliding contact. By comparing the adsorption behaviour of the complexes and individual components, we conclude that pectic polymers maintain the film's hydration while lysozyme improves surface adsorption, thereby providing new evidence that physical complexes can be utilised to design bi-functional surface layers. Our findings open up a range of opportunities for application in food and oral healthcare products, particularly in the use of food-compatible ingredients to control aqueous lubrication.

Recent Progress in Cartilage Lubrication.

Healthy articular cartilage, covering the ends of bones in major joints suchas 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.

An experimental and molecular dynamics study of the superlubricity enabled by hydration lubrication

Recently, hydration lubrication has emerged as a promising approach to macroscale liquid superlubricity. However, the reported methods till now exhibit a long running-in process that features high and fluctuating friction evolution. Herein, we have employed glycerol aqueous solutions as the model to reveal the mechanism governing the achievement of superlow friction by hydration lubrication, aiming to erase the long and disturbing running-in. Through experimental investigations and molecular dynamics simulations, it is found that the initial high friction of dilute glycerol aqueous solution with the water/glycerol weight ratio of 2.0 is due to the ineffective film formation between the tribopairs, and the superlow low friction after the long running-in is attributed to the synergetic effect of increased viscosity due to water evaporation, increased film thickness due to higher speeds, reduced contact pressure due to the wear of friction materials, and the adsorbed molecular layers on steel surfaces. To improve the initial lubricating condition that is responsible for the high coefficient of friction at the start of the test, the formula has been modified with the water/glycerol weight ratio reduced to 0.2, and its friction performances have been further investigated. Surprisingly, remarkable and robust superlow friction appears almost as soon as the test starts. Molecular dynamics sliding simulation reveals the formation of shearing layers or hydration layers near the surfaces and the load-supporting layers between them. The results in this work are helpful to expand the understanding of the superlubricity mechanism enabled by hydration lubrication, and it also provides a novel strategy to liquid superlubricity using aqueous media that features vanished running-in.

Friction and wear behaviours of polyacrylamide hydrogel microsphere/UHMWPE composite under water lubrication

The insufficient lubrication caused by the low viscosity of water has greatly limited the application of water-lubricated bearing, especially under the conditions of heavy load and low speed. In order to solve this problem, a composited strategy is proposed by using the hydration lubrication of polyacrylamide (PAAm) hydrogel to improve the tribological performances of UHMWPE. The PAAm hydrogel microspheres were synthesized by emulsion polymerization, and the PAAm hydrogel microsphere/UHMWPE composites were prepared by melt blending method. The effects of particle size, crosslinking density and filling content of PAAm microspheres on the tribological performances of the composites were tested and analyzed. The results show that the PAAm fragments can form a hydration lubrication layer and improve the tribological performances of the composite. The design of PAAm microsphere/UHMWPE composite provides a novel method to develop high-performance materials for water-lubricated stern tube bearing.

A new insight into more effective viscosupplementation based on the synergy of hyaluronic acid and phospholipids for cartilage friction reduction

Osteoarthritis is a joint disease characterized by progressive damage to the cartilage. For a long time, viscosupplements have been based on hyaluronic acid (HA), which is known to relieve the pain of osteoarthritic patients. Recently, however, it has been hypothesized that phospholipids and HA may be collectively responsible for the extremely low friction observed in joint models (mica tape), suggesting that such a combination might lead to more effective viscosupplements. Using a pin-on-plate tribometer, we have tested this hypothesis by analyzing the coefficient of friction of conventional HA viscosupplements compared to the new viscosupplement formula of phospholipids and HA. Our findings show that phospholipids and HA do indeed lead to the lowest friction in the joints at both cartilage-mica tape and cartilage-glass interfaces (joint models). Furthermore, these results also hold true when both viscosupplements are mixed with synovial fluid simulating the real conditions after viscosupplement injection into joints. Therefore, we have validated the synergic effect of phospholipids and HA on lowering the cartilage friction and we further hypothesized that this effect is due to hydration lubrication. We believe these findings might contribute to development of new viscosupplement which would significantly improve the lives of person suffering from osteoarthrosis.

Self-adhesive lubricated coating for enhanced bacterial resistance

Limited surface lubrication and bacterial biofilm formation pose great challenges to biomedical implants. Although hydrophilic lubricated coatings and bacterial resistance coatings have been reported, the harsh and tedious synthesis greatly compromises their application, and more importantly, the bacterial resistance property has seldom been investigated in combination with the lubrication property. In this study, bioinspired by the performances of mussel and articular cartilage, we successfully synthesized self-adhesive lubricated coating and simultaneously achieved optimal lubrication and bacterial resistance properties. Additionally, we reported the mechanism of bacterial resistance on the nanoscale by studying the adhesion interactions between biomimetic coating and hydrophilic/hydrophobic tip or living bacteria via atomic force microscopy. In summary, the self-adhesive lubricated coating can effectively enhance lubrication and bacterial resistance performances based on hydration lubrication and hydration repulsion, and represent a universal and facial strategy for surface functionalization of biomedical implants.

Modulating Interfacial Energy Dissipation via Potential-Controlled Ion Trapping.

As a metal (gold) surface at a given, but variable potential slides past a dielectric (mica) surface at a fixed charge, across aqueous salt solutions, two distinct dissipation regimes may be identified. In regime I, when the gold potential is such that counterions are expelled from between the surfaces, which then come to adhesive contact, the frictional dissipation is high, with coefficient of friction μ ≈ 0.8–0.9. In regime II, when hydrated counterions are trapped between the compressed surfaces, hydration lubrication is active and friction is much lower, μ = 0.05 ± 0.03. Moreover, the dissipation regime as the surfaces contact is largely retained even when the metal potential changes to the other regime, attributed to the slow kinetics of counterion expulsion from or penetration into the subnanometer intersurface gap. Our results indicate how frictional dissipation between such a conducting/nonconducting couple may be modulated by the potential applied to the metal.

Gelatin-based composite hydrogels with biomimetic lubrication and sustained drug release

The occurrence of osteoarthritis is closely related to progressive and irreversible destruction of the articular cartilage, which increases the friction significantly and causes further inflammation of the joint. Thus, a scaffold for articular cartilage defects should be developed via lubrication restoration and drug intervention. In this study, we successfully synthesized gelatin-based composite hydrogels, namely GelMA-PAM-PMPC, with the properties of biomimetic lubrication and sustained drug release by photopolymerization of methacrylic anhydride modified gelatin (GelMA), acrylamide (AM), and 2-methacryloyloxyethyl phosphorylcholine (MPC). Tribological test showed that the composite hydrogels remarkably enhanced lubrication due to the hydration lubrication mechanism, where a tenacious hydration shell was formed around the zwitterionic phosphocholine headgroups. In addition, drug release test indicated that the composite hydrogels efficiently encapsulated an anti-inflammatory drug (diclofenac sodium) and achieved sustained release. Furthermore, the in vitro test revealed that the composite hydrogels were biocompatible, and the mRNA expression of both anabolic and catabolic genes of the articular cartilage was suitably regulated. This indicated that the composite hydrogels could effectively protect chondrocytes from inflammatory cytokine-induced degeneration. In summary, the composite hydrogels that provide biomimetic hydration lubrication and sustained local drug release represent a promising scaffold for cartilage defects in the treatment of osteoarthritis.

Bioinspired double self-adhesion coating based on dopamine, coating resin and phosphorylcholine for surface lubrication and antifouling functionalization

ABSTRACT Implanted medical devices that have poor friction property or biofilm formation can limit their service life and cause discomfort in patients. Recently, some zwitterionic coatings have been studied to modify the biomaterials surface for lubricating function, but the grafting methods of coatings are complicated and also seldom take the bacterial antiadhesion property into account at the same time. In our studies, motivated by the properties of nature mussels and human articular, we firstly successfully synthesized double adhesion protection of self-adhesive ternary polymer coating and achieved the excellent lubrication and antifouling functionalization of the medical devices surface. In details, the X-ray photoelectron spectroscopy, scanning electron microscope and the water contact angles could characterize the successful modification on the surface of titanium substrate. Additionally, the tribological tests carried out by atomic force microscope verified the ternary polymer could enhance the lubrication property owing to the hydration lubrication mechanism. Meanwhile, it also possessed the bacterial antiadhesion property for the initial 24 h attributed to the hydration repulsive force. We believe that, as a simple and universal preparation method, the ternary polymer could make a great significance for improving the surface function of biomaterials and alleviating patients’ discomfort.

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.

Interactions Between Bilayers of Phospholipids Extracted from Human Osteoarthritic Synovial Fluid

Duncan Dowson, whom this issue commemorates, was a world leader in the field of biotribology, with prolific contributions both in fluid-based and boundary lubrication of biological tissues, in particular articular cartilage, a central issue in biotribology due to its importance for joint homeostasis. Here we explore further the issue of cartilage boundary lubrication, which has been attributed to phospholipid (PL)-exposing layers at the cartilage surface in part. A surface force balance (SFB) with unique sensitivity is used to investigate the normal and frictional interactions of the boundary layers formed by PLs extracted from osteoarthritic (OA) human synovial fluid (hSF). Our results reveal that vesicles of the OA-hSF lipids rupture spontaneously to form bilayers on the mica substrate (which, like the in-vivo articular cartilage surface in synovial joints, is negatively-charged) which then undergo hemifusion at quite low pressures in the SFB, attributed to the large heterogeneity of the hSF lipids. Nanometric friction measurements reveal friction coefficients μ ≈ 0.03 across the hemifused bilayer of these lipids, indicating residual hydration lubrication at the lipid-headgroup/substrate interface. Addition of calcium ions causes an increase in friction to μ ≈ 0.2, attributed either to calcium-bridging attraction of lipid headgroups to the negatively-charged substrate, or a shift of the slip plane to the more dissipative hydrophobic-tail/hydrophobic-tail interface. Our results suggest that the heterogeneity and composition of the OA-hSF lipids may be associated with higher friction at the cartilage boundary layers – and thus a connection with greater wear and degradation – due to hemifusion of the exposed lipid bilayers.

Synergistic Microgel-Reinforced Hydrogels as High-Performance Lubricants.

The ability to create a superlubricious aqueous lubricant is important for various biological and technological applications. Here, a nonlipid biolubricant with strikingly low friction coefficients is fabricated (patented) by reinforcing a fluid-like hydrogel composed of biopolymeric nanofibrils with proteinaceous microgels, which synergistically provide superlubricity on elastomeric surfaces in comparison to any of the sole components. This two-component lubricant composed of positively charged lactoferrin microgels and negatively charged κ-carrageenan hydrogels is capable of exceeding the high lubricating performance of real human saliva in tribo tests using both smooth and textured surfaces, latter mimicking the human tongue’s wettability, topography, and compliance. The favorable electrostatic attraction between mutually oppositely charged microgels and the hydrogel reinforces the mechanical properties of the hydrogel, allowing friction reduction by combining the benefits of both viscous and hydration lubrication. The superlubricity of these microgel-reinforced hydrogels offers a unique prospect for the fabrication of biocompatible aqueous lubricants for dry-mouth therapy and/or designing of nonobesogenic nutritional technologies.

Advances in Understanding Hydrogel Lubrication

Since their inception, hydrogels have gained popularity among multiple fields, most significantly in biomedical research and industry. Due to their resemblance to biological tribosystems, a significant amount of research has been conducted on hydrogels to elucidate biolubrication mechanisms and their possible applications as replacement materials. This review is focused on lubrication mechanisms and covers friction models that have attempted to quantify the complex frictional characteristics of hydrogels. From models developed on the basis of polymer physics to the concept of hydration lubrication, assumptions and conditions for their applicability are discussed. Based on previous models and our own experimental findings, we propose the viscous-adhesive model for hydrogel friction. This model accounts for the effects of confinement of the polymer network provided by a solid surface and poroelastic relaxation as well as the (non) Newtonian shear of a complex fluid on the frictional force and quantifies the frictional response of hydrogels-solid interfaces. Finally, the review delineates potential areas of future research based on the current knowledge.

Dopamine/Phosphorylcholine Copolymer as an Efficient Joint Lubricant and ROS Scavenger for the Treatment of Osteoarthritis.

Osteoarthritis (OA) is a chronic joint disease and its progression and pathogenesis are highly associated with the significant increase of joint friction and overproduction of reactive oxygen species (ROS) in inflammation. Combination of ROS elimination and lubrication enhancement may provide a novel strategy for the treatment of OA. In the present study, a pure biomaterial and nondrug system P(DMA-co-MPC), synthesized via free radical copolymerization, was designed and developed for the first time using 2-methacryloxyethyl phosphorylcholine (MPC) as a bioinspired lubricant and N-(3,4-dihydroxyphenethyl)methacrylamide (DMA) as an ROS scavenger. Our results showed that the P(DMA-co-MPC) aggregates could efficiently eliminate the ROS radicals and provide good lubrication property by adjusting the molar ratio of DMA and MPC in the copolymer. It is attributed to the antioxidant function of the hydroquinone moiety in DMA and the hydration lubrication effect of the zwitterionic phosphocholine group in MPC. Furthermore, the in vitro experiments demonstrated that the P(DMA-co-MPC) showed good biocompatibility with MC3T3-E1 cells and intracellular anti-inflammatory property by inhibiting the production of ROS and regulating the expression levels of pro-inflammatory cytokines, pain-related gene, anabolic genes, and catabolic genes. In conclusion, the drug-free P(DMA-co-MPC) aggregates developed herein can achieve dual functions of lubrication enhancement and anti-inflammatory effect and thus they may be representative as promising candidates for the treatment of OA.

Designing soft/hard double network hydrogel microsphere/UHMWPE composites to promote water lubrication performance

Several soft tissues residing in the living body have excellent hydration lubrication properties and can provide effective protection during relative motion. In order to apply this advantage of soft matters in practical applications and try to avoid its disadvantage, such as swelling and weakening in water, a design strategy of a soft/hard double network (DN) hydrogel microsphere modified ultrahigh molecular weight polyethylene (UHMWPE) composite is proposed in this study. A series of microspheres of urea-formaldehyde (UF), polyacrylamide (PAAm) hydrogel, UF/PAAm double network, and their composites were prepared. The mechanical properties, swelling, wettability, friction properties, and the lubrication mechanisms of the composites were investigated. The results show that DN microspheres can have an excellent stability and provide hydration lubrication. The performance of 75 DN-1 composite was superior to others. This finding will provide a novel strategy for the development of water-lubricated materials and have wide application in engineering fields.

Synthesis of articular cartilage‐inspired branched polyelectrolyte polymer for enhanced lubrication

The superlubrication property of articular cartilage is attributed to the hydration lubrication mechanism. This involves the complexation of biomacromolecules with phosphatidylcholine lipids to form a lubricating boundary layer on the surface. Based on this mechanism, a branched polyelectrolyte polymer – PEI-PMPC was synthesised in this study to enhance lubrication via tert-butyl hydroperoxide-initiated grafting polymerisation of 2-methacryloyloxyethyl phosphorylcholine (MPC) onto polyethyleneimine (PEI) with various molecular weights. Following the characterisation of the polymer using nuclear magnetic resonance, a series of tribological tests were performed under different normal loads and different scan rates. The results showed that the PEI-PMPC polyelectrolyte polymer in aqueous solution could effectively reduce the friction coefficient and wear generation between the polyethylene ball and the silica wafer. Additionally, an improvement in the lubrication performance was detected for the PEI-PMPC polyelectrolyte polymer when high molecular weight PEI was used during the reaction, which was attributed to the formation of the hydration shells surrounding the zwitterionic charges of PMPC and the viscosity of the polymer. In conclusion, the PEI-PMPC polyelectrolyte polymer developed herein was characterised by enhanced lubrication, and may be potentially used for biomedical applications such as intra-articular injection to restore joint lubrication.