Molecular synergy in biolubrication: The role of cartilage oligomeric matrix protein (COMP) in surface-structuring of lubricin

Frictional behaviour of plant proteins in soft contacts: unveiling nanoscale mechanisms

Despite the significance of nanotribology in the design of functional biomaterials, little is known about nanoscale friction in the presence of protein-coated soft contact surfaces. Here, we report a detailed investigation of frictional behaviour of sustainable plant proteins at the nanoscale for the first time, using deformable bio-relevant surfaces that achieve biologically relevant contact pressures. A combination of atomic force microscopy, quartz crystal microbalance with dissipation monitoring, and friction force microscopy with soft polydimethylsiloxane (PDMS, 150 kPa) surfaces was employed to elucidate the frictional properties of model plant proteins, i.e. lupine, pea, and potato proteins at the nanoscale while systematically varying the pH and ionic strength. Interactions of these plant proteins with purified mucins were also probed. We provide the much-needed direct experimental evidence that the main factor dictating the frictional properties of plant proteins is their affinity towards the surface, followed by the degree of protein film hydration. Proteins with high surface affinity, such as pea and potato protein, have better lubricating performance than lupine at the nanoscale. Other minor factors that drive lubrication are surface interactions between sliding bodies, especially at low load, whilst jamming of the contact area caused by larger protein aggregates increases friction. Novel findings reveal that interactions between plant proteins and mucins lead to superior lubricating properties, by combining high surface affinity from the plant proteins and high hydration by mucins. We anticipate that fundamental understanding gained from this work will set the stage for the design of a plethora of sustainable biomaterials and food with optimum nanolubrication performance.

Research progress of cartilage lubrication and biomimetic cartilage lubrication materials

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

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.

Phospholipids and Hyaluronan: From Molecular Interactions to Nano- and Macroscale Friction

Phospholipids and hyaluronan are two key biomolecules that contribute to the excellent lubrication of articular joints. Phospholipids alone and in combination with hyaluronan have also displayed low friction forces on smooth surfaces in micro- and nanosized tribological contacts. In an effort to develop aqueous-based lubrication systems, it is highly relevant to explore if these types of molecules also are able to provide efficient lubrication of macroscopic tribological contacts involving surfaces with roughness larger than the thickness of the lubricating layer. To this end, we investigated the lubrication performance of hyaluronan, the phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and mixtures of these two components using glass surfaces in a mini-traction machine. We compared our data with those obtained using flat silica surfaces in previous atomic force microscopy studies, and we also highlighted insights on hyaluronan–phospholipid interactions gained from recent simulations. Our data demonstrate that hyaluronan alone does not provide any lubricating benefit, but DPPC alone and in mixtures with hyaluronan reduces the friction force by an order of magnitude.

Recent Advances in Understanding the Role of Cartilage Lubrication in Osteoarthritis

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

Cellular Interactions with Lubricin and Hyaluronic Acid-Lubricin Composite Coatings on Gold Electrodes in Passive and Electrically Stimulated Environments

In the field of bionics, the long-term effectiveness of implantable bionic interfaces depends upon maintaining a "clean" and unfouled electrical interface with biological tissues. Lubricin (LUB) is an innately biocompatible glycoprotein with impressive antifouling properties. Unlike traditional antiadhesive coatings, LUB coatings do not passivate electrode surfaces, giving LUB coatings great potential for controlling surface fouling of implantable electrode interfaces. This study characterizes the antifouling properties of bovine native LUB (N-LUB), recombinant human LUB (R-LUB), hyaluronic acid (HA), and composite coatings of HA and R-LUB (HA/R-LUB) on gold electrodes against human primary fibroblasts and chondrocytes in passive and electrically stimulated environments for up to 96 h. R-LUB coatings demonstrated highly effective antifouling properties, preventing nearly all adhesion and proliferation of fibroblasts and chondrocytes even under biphasic electrical stimulation. N-LUB coatings, while showing efficacy in the short term, failed to produce sustained antifouling properties against fibroblasts or chondrocytes over longer periods of time. HA/R-LUB composite films also demonstrated highly effective antifouling performance equal to the R-LUB coatings in both passive and electrically stimulated environments. The high electrochemical stability and long-lasting antifouling properties of R-LUB and HA/R-LUB coatings in electrically stimulating environments reveal the potential of these coatings for controlling unwanted cell adhesion in implantable bionic applications.

Recent Progress in Cartilage Lubrication

Healthy articular cartilage, covering the ends of bones in major joints such as hips and knees, presents the most efficiently-lubricated surface known in nature, with friction coefficients as low as 0.001 up to physiologically high pressures. Such low friction is indeed essential for its well-being. It minimizes wear-and-tear and hence the cartilage degradation associated with osteoarthritis, the most common joint disease, and, by reducing shear stress on the mechanotransductive, cartilage-embedded chondrocytes (the only cell type in the cartilage), it regulates their function to maintain homeostasis. Understanding the origins of such low friction of the articular cartilage, therefore, is of major importance in order to alleviate disease symptoms, and slow or even reverse its breakdown. This progress report considers the relation between frictional behavior and the cellular mechanical environment in the cartilage, then reviews the mechanism of lubrication in the joints, in particular focusing on boundary lubrication. Following recent advances based on hydration lubrication, a proposed synergy between different molecular components of the synovial joints, acting together in enabling the low friction, has been proposed. Additionally, recent development of natural and bio-inspired lubricants is reviewed.

Lubricin as a tool for controlling adhesion in vivo and ex vivo

The ability to prevent or minimize the accumulation of unwanted biological materials on implantable medical devices is important in maintaining the long-term function of implants. To address this issue, there has been a focus on materials, both biological and synthetic, that have the potential to prevent device fouling. In this review, we introduce a glycoprotein called lubricin and report on its emergence as an effective antifouling coating material. We outline the versatility of lubricin coatings on different surfaces, describe the physical properties of its monolayer structures, and highlight its antifouling properties in improving implant compatibility as well as its use in treatment of ocular diseases and arthritis. This review further describes synthetic polymers mimicking the lubricin structure and function. We also discuss the potential future use of lubricin and its synthetic mimetics as antiadhesive biomaterials for therapeutic applications.

Cartilage lamina splendens inspired nanostructured coating for biomaterial lubrication

Biomaterials that are used in biological systems, such as polycarbonate urethane (PCU) knee joint implants and contact lenses, generally lack lubrication. This limits their integration with the body and impedes their function. Here, we propose a nanostructured film based on hydrophilic polysaccharide hyaluronic acid conjugated with dopamine (HADN) and zwitterionic reduced glutathione (Glu), which forms a composite coating (HADN-Glu) to enhance the lubrication between cartilage and PCU. HADN was synthesized by carbodiimide chemistry between hyaluronic acid and dopamine and deposited on PCU surface under mild oxidative conditions. Then, zwitterionic peptide-reduced glutathione was bioconjugated to HADN, forming a lubrication film. Analysis based on X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and wettability indicated that HADN and Glu had grafted successfully onto the PCU surface. Measurements of the coefficient of friction (COF), friction energy dissipation and cartilage roughness indicated that cartilage was effectively protected by the high lubrication of HADN-Glu. Both at low and high applied loads, this effect was likely due to the enhanced boundary lubrication enabled by HADN-Glu on the PCU surface. Moreover, HADN-Glu is highly biocompatible with chondrocyte cells, suggesting that this film will benefit the design of implants where lubrication is needed.

Friction between soft contacts at nanoscale on uncoated and protein-coated surfaces

The understanding of friction on soft sliding biological surfaces at the nanoscale is poorly understood as hard interfaces are frequently used as model systems. Herein, we studied the influence of elastic modulus on the frictional properties of model surfaces at the nanoscale for the first time. We prepared model silicone-based elastomer surfaces with tuneable modulus ranging from hundreds of kPa to a few MPa, similar to those found in real biological surfaces, and employed atomic force microscopy to characterize their modulus, adhesion, and surface morphology. Consequently, we used friction force microscopy to investigate nanoscale friction in hard-soft and soft-soft contacts using spherical colloidal probes covered by adsorbed protein films. Unprecedented results from this study reveal that modulus of a surface can have a significant impact on the frictional properties of protein-coated surfaces with higher deformability leading to lower contact pressure and, consequently, decreased friction. These important results pave the way forward for designing new functional surfaces for serving as models of appropriate deformability to replicate the mechanical properties of the biological structures and processes for accurate friction measurements at nanoscale.

Nanostructured Coating for Biomaterial Lubrication through Biomacromolecular Recruitment

Biomaterials employed in the articular joint cavity, such as polycarbonate urethane (PCU) for meniscus replacement, lack of lubrication ability, leading to pain and tissue degradation. We present a nanostructured adhesive coating based on dopamine-modified hyaluronan (HADN) and poly-lysine (PLL), which can reestablish boundary lubrication between the cartilage and biomaterial. Lubrication restoration takes place without the need of exogenous lubricious molecules but through a novel strategy of recruitment of native lubricious molecules present in the surrounding milieu. The biomimetic adhesive coating PLL-HADN (78 nm thickness) shows a high adhesive strength (0.51 MPa) to PCU and a high synovial fluid responsiveness. The quartz crystal microbalance with dissipation monitoring shows the formation of a thick and softer layer when these coatings are brought in contact with the synovial fluid. X-ray photoelectron spectroscopy and ConA-Alexa staining show clear signs of lubricious protein (PRG4) recruitment on the PLL-HADN surface. Effective recruitment of a lubricious protein by PLL-HADN caused it to dissipate only one-third of the frictional energy as compared to bare PCU when rubbed against the cartilage. Histology shows that this reduction makes the PLL-HADN highly chondroprotective, whereas PLL-HA coatings still show signs of cartilage wear. Shear forces in the range of 0.07-0.1 N were able to remove ∼80% of the PRG4 from the PCU-PLL-HA but only 27% from the PCU-PLL-HADN. Thus, in this study, we have shown that surface recruitment and strong adsorption of biomacromolecules from the surrounding milieu is an effective biomaterial lubrication strategy. This opens up new possibilities for lubrication system reconstruction for medical devices.

Probing the frictional properties of soft materials at the nanoscale

The understanding of friction in soft materials is of increasing importance due to the demands of industries such as healthcare, biomedical, food and personal care, the incorporation of soft materials into technology, and in the study of interacting biological interfaces. Many of these processes occur at the nanoscale, but even at micrometer length scales there are fundamental aspects of tribology that remain poorly understood. With the advent of Friction Force Microscopy (FFM), there have been many fundamental insights into tribological phenomena at the atomic scale, such as 'stick-slip' and 'super-lubricity'. This review examines the growing field of soft tribology, the experimental aspects of FFM and its underlying theory. Moving to the nanoscale changes the contact mechanics which govern adhesive forces, which in turn play a pivotal role in friction, along with the deformation of the soft interface and dissipative phenomena. We examine recent progress and future prospects in soft nanotribology.

Adhesion and Self-Assembly of Lubricin (PRG4) Brush Layers on Different Substrate Surfaces

Lubricin (LUB, aka PRG4), a mucin-like glycoprotein, is best known for the significant role it plays in the boundary lubrication, wear protection, and adhesion control systems in human joints. However, LUB exhibits a number of diverse and useful properties, including a remarkable ability to self-assemble into a telechelic brush structure onto virtually any substrate. This self-assembly behavior has spawned the emergence of numerous nontraditional applications of LUB coatings in numerous areas such as microfluidics, electrochemical sensors, contact lenses, antifouling surfaces, and bionic neural interfaces. Although LUB will readily self-assemble on most substrates, it has become apparent that the substrate has a significant influence on the LUB layer's demonstrated lubrication, antiadhesion, electrokinetic, and size-selective transport properties; however, investigations into LUB-substrate interactions and how they influence the self-assembled LUB layer structure remain a neglected aspect of LUB research. This study utilizes AFM force spectroscopy to directly assess the adhesion energy of LUB molecules adsorbed to a wide variety of different substrates which include inorganic, polymeric, and metallic materials. An analysis of the steric repulsive forces measured on approach provides a qualitative assessment of the LUB layer's mechanical modulus, related to the chain packing density, across substrates. These modulus measurements, combined with characteristic features and the dwell time dependence of the LUB adhesion forces provide insight into the organization and uniformity of the LUB brush structure. The results of these measurements indicate that LUB interactions with different substrates are highly variable and substrate-specific, resulting in a surprisingly broad spectrum of adhesion energies and layer properties (i.e., chain density, uniformity, etc.) which are not, themselves, correlated or easily predicted by substrate properties. In addition, this study finds exceptionally poor LUB adhesion to both mica and poly(methyl methacrylate) surfaces that remain widely used substrates for constructing model surfaces in fundamental tribology studies which may have significant implications for the findings of a number of foundational studies into LUB tribology and molecular synergies.

Tribological Behavior of Surface-Immobilized Novel Biomimicking Multihierarchical Polymers: The Role of Structure and Surface Attachment

The tribological properties of two novel biomimetic multihierarchical polymers, synthesized by covalently linking single bottlebrush polymers onto a hyaluronic acid (HA) backbone, were investigated in the boundary lubrication regime using the surface forces apparatus. The polymers were immobilized on flat substrates, and their lubrication properties and wear resistance were investigated in aqueous media in the absence of a polymer reservoir (i.e., no free polymer chains in the surrounding medium) in order to better reveal the underlying mechanism of surface-attached biomimetic polymers. The effects of composition, structure, and, more particularly, surface attachment (physisorbed vs chemisorbed) on the tribological properties were investigated and compared with other biomimicking systems reported in the literature. The covalently surface attached bottlebrushes allowed wear resistance between sliding surfaces to be significantly improved, compared to physisorbed bottlebrushes, with a constant coefficient of friction (10^-1) of up to few tens of MPa. The results confirm that surface-attached bottlebrushes on their own are not responsible for the extremely low friction often reported in the literature or found in articular joints. Moreover, the study confirmed that the irreversible attachment of bottlebrushes, or multihierarchical polymer layers, to surfaces is crucial to improving wear resistance between sliding surfaces in aqueous media.

Biolubrication synergy: Hyaluronan - Phospholipid interactions at interfaces

The manner in which nature has solved lubrication issues has fascinated scientists for centuries, in particular when considering that lubrication is achieved in aqueous media. The most outstanding system in this respect is likely the synovial joint, where close to frictionless motion is realized under different loads and shear rates. This review article focuses on two components present in the synovial area, hyaluronan and phospholipids. We recapitulate what has been learned about their interactions at interfaces from recent experiments, with focus on results obtained using reflectivity techniques at large scale facilities. In parallel, modelling experiments have been carried out and from these efforts new detailed knowledge about how hyaluronan and phospholipids interact has been gained. In this review we combine findings from modelling and experiments to gain deeper insight. Finally, we summarize what has been learned of the lubrication performance of mixtures of phospholipids and hyaluronan.

Lubrication by biomacromolecules: mechanisms and biomimetic strategies

Biomacromolecules play a key role in protecting human biointerfaces from friction and wear, and thus enable painless motion. Biomacromolecules give rise to remarkable tribological properties that researchers have been eager to emulate. In this review, we examine how molecules such as mucins, lubricin, hyaluronic acid and other components of biotribological interfaces provide a unique set of rheological and surface properties that leads to low friction and wear. We then highlight how researchers have used some of the features of biotribological contacts to create biomimetic systems. While the brush architecture of the glycosylated molecules present at biotribological interfaces has inspired some promising polymer brush systems, it is the recent advance in the understanding of synergistic interaction between biomacromolecules that is showing the most potential in producing surfaces with a high lubricating ability. Research currently suggests that no single biomacromolecule or artificial polymer successfully reproduces the tribological properties of biological contacts. However, by combining molecules, one can enhance their anchoring and lubricating capacity, thus enabling the design of surfaces for use in biomedical applications requiring low friction and wear.

Molecularly Engineered Biolubricants for Articular Cartilage

Lubrication within articular joints plays a crucial role in daily life, providing an extremely low coefficient of friction and preventing wear at the surface of the articular cartilage. Natural biomacromolecules responsible for lubrication are part of the synovial fluid and their degradation is associated with the onset of degenerative diseases, such as osteoarthritis (OA). The current absence of effective treatments for OA has captured the attention of chemists and material scientists over the last two decades, triggering the development of partially or fully synthetic biolubricants aimed to reduce friction within the joints and restore cartilage functions. Although there is still a long way to go before synthetic replacements of natural biolubricants can be applied clinically, this review highlights those formulations that meet the fundamental requirements for being efficient lubricants for articular cartilage.

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.