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

Bioinspired Bottlebrush Polymers Effectively Alleviate Frictional Damage Both In Vitro and In Vivo

Bottlebrush polymers (BB) have emerged as compelling candidates for biosystems to face tribological challenges, including friction and wear. This study provides a comprehensive assessment of an engineered triblock BB polymer's affinity, cell toxicity, lubrication, and wear protection in both in vitro and in vivo settings, focusing on applications for conditions like osteoarthritis and dry eye syndrome. Results show that the designed polymer rapidly adheres to various surfaces (e.g., cartilage, eye, and contact lens), forming a robust, biocompatible layer for surface lubrication and protection. The tribological performance and biocompatibility are further enhanced in the presence of hyaluronic acid (HA) both in vitro and in vivo. The exceptional lubrication performance and favorable interaction with HA position the synthesized triblock polymer as a promising candidate for innovative treatments addressing deficiencies in bio-lubricant systems.

HYBID in osteoarthritis: Potential target for disease progression

HYBID is a new hyaluronan-degrading enzyme and exists in various cells of the human body. Recently, HYBID was found to over-express in the osteoarthritic chondrocytes and fibroblast-like synoviocytes. According to these researches, high level of HYBID is significantly correlated with cartilage degeneration in joints and hyaluronic acid degradation in synovial fluid. In addition, HYBID can affect inflammatory cytokine secretion, cartilage and synovium fibrosis, synovial hyperplasia via multiple signaling pathways, thereby exacerbating osteoarthritis. Based on the existing research of HYBID in osteoarthritis, HYBID can break the metabolic balance of HA in joints through the degradation ability independent of HYALs/CD44 system and furthermore affect cartilage structure and mechanotransduction of chondrocytes. In particular, in addition to HYBID itself being able to trigger some signaling pathways, we believe that low-molecular-weight hyaluronan produced by excess degradation can also stimulate some disease-promoting signaling pathways by replacing high-molecular-weight hyaluronan in joints. The specific role of HYBID in osteoarthritis is gradually revealed, and the discovery of HYBID raises the new way to treat osteoarthritis. In this review, the expression and basic functions of HYBID in joints were summarized, and reveal potential role of HYBID as a key target in treatment for osteoarthritis.

Lubricants for osteoarthritis treatment: From natural to bioinspired and alternative strategies

Osteoarthritis is the most common degenerative and highly prevalent joint disease, characterized by progressive loss and destruction of articular cartilage. The damaged cartilage surface has an increased friction, which causes patients to suffer from serious pain. Restoring the lubrication ability of the joint is central to the treatment of osteoarthritis, a key topic in medical research. A variety of lubricants have been designed to reduce friction in joints and promote cartilage tissue repair to alleviate the symptoms of osteoarthritis. Herein, we review the recent progress of lubricants from the three perspectives of natural, bioinspired, and alternative strategies for osteoarthritis treatment, as well as the structural characterization and lubrication properties of such lubricants. Specifically, natural lubricants include glycosaminoglycans, lubricin and lipids in joints, bioinspired lubricants include scaffolds mimicking hyaluronic acid or lubricin, and alternative lubricants include modified lubricants based on hyaluronic acid, lipids, nanoparticles, and peptides. We also discuss the current challenges and long-term perspectives for further research in this area.

Superlubricity of pH-responsive hydrogels in extreme environments

Poly(acrylamide-co-acrylic acid) (P(AAm-co-AA)) hydrogels are highly tunable and pH-responsive materials frequently used in biomedical applications. The swelling behavior and mechanical properties of these gels have been extensively characterized and are thought to be controlled by the protonation state of the acrylic acid (AA) through the regulation of solution pH. However, their tribological properties have been underexplored. Here, we hypothesized that electrostatics and the protonation state of AA would drive the tribological properties of these polyelectrolyte gels. P(AAm-co-AA) hydrogels were prepared with constant acrylamide (AAm) concentration (33 wt%) and varying AA concentration to control the amount of ionizable groups in the gel. The monomer:crosslinker molar ratio (200:1) was kept constant. Hydrogel swelling, stiffness, and friction behavior were studied by systematically varying the acrylic acid (AA) concentration from 0–12 wt% and controlling solution pH (0.35, 7, 13.8) and ionic strength (I = 0 or 0.25 M). The stiffness and friction coefficient of bulk hydrogels were evaluated using a microtribometer and borosilicate glass probes as countersurfaces. The swelling behavior and elastic modulus of these polyelectrolyte hydrogels were highly sensitive to solution pH and poorly predicted the friction coefficient (µ), which decreased with increasing AA concentration. P(AAm-co-AA) hydrogels with the greatest AA concentrations (12 wt%) exhibited superlubricity (µ = 0.005 ± 0.001) when swollen in unbuffered, deionized water (pH = 7, I = 0 M) and 0.5 M NaOH (pH = 13.8, I = 0.25 M) (µ = 0.005 ± 0.002). Friction coefficients generally decreased with increasing AA and increasing solution pH. We postulate that tunable lubricity in P(AAm-co-AA) gels arises from changes in the protonation state of acrylic acid and electrostatic interactions between the probe and hydrogel surface.

Walking on water: revisiting the role of water in articular cartilage biomechanics in relation to tissue engineering and regenerative medicine

The importance, and the difficulty, of generating biosynthetic articular cartilage is widely recognized. Problems arise from obtaining sufficient stiffness, toughness and longevity in the material and integration of new material into existing cartilage and bone. Much work has been done on chondrocytes and tissue macromolecular components while water, which comprises the bulk of the tissue, is largely seen as a passive component; the ‘solid matrix’ is believed to be the main load-bearing element most of the time. Water is commonly seen as an inert filler whose restricted flow through the tissue is believed to be sufficient to generate the properties measured. We propose that this model should be turned on its head. Water comprises 70–80% of the matrix and has a bulk modulus considerably greater than that of cartilage. We suggest that the macromolecular components structure the water to support the loads applied. Here, we shall examine the structure and organization of the main macromolecules, collagen, aggrecan and hyaluronan, and explore how water interacts with their polyelectrolyte nature. This may inform the biosynthetic process by identifying starting points to enable developing tissue properties to guide the cells into producing the appropriate macromolecular composition and structure.

Construction and Tribological Properties of Biomimetic Cartilage-Lubricating Hydrogels

Articular cartilage provides ultralow friction to maintain the physiological function of the knee joint, which arises from the hierarchical complex composed of hyaluronic acid, phospholipids, and lubricin, covering the cartilage surface as boundary lubrication layers. Cartilage-lubricating polymers (HA/PA and HA/PM) mimicking this complex have been demonstrated to restore the lubrication of cartilage via hydration lubrication, thus contributing to the treatment of early osteoarthritis (OA) in vivo. Here, biomimetic cartilage-lubricating hydrogels (HPX/PVA) were constructed by blending HA/PA and HA/PM (HPX) with polyvinyl alcohol (PVA) to improve the boundary lubrication and wear properties, so that the obtained hydrogels may offer a solution to the main drawbacks of PVA hydrogels used as cartilage implants. The HPX/PVA hydrogels exhibited good physicochemical and mechanical properties through hydrogen-bonding interactions, and showed lower friction and wear under the boundary lubrication and fluid film lubrication mechanisms, which remained when the hydrogels were rehydrated. Our strategy may provide new insights into exploring cartilage-inspired lubricating hydrogels.

Glycosaminoglycans and glycoproteins influence the elastic response of synovial fluid nanofilms on model oxide surfaces

Synovial fluid (SF) is the natural lubricant found in articulated joints, providing unique cartilage surface protecting films under confinement and relative motion. While it is known that the synergistic interactions of the macromolecular constituents provide its unique load-bearing and tribological performance, it is not fully understood how two of the main constituents, glycosaminoglycans (GAGs) and glycoproteins, regulate the formation and mechanics of robust load-bearing films. Here, we present evidence that the load-bearing capabilities, rather than the tribological performance, of the formed SF films depend strongly on its components' integrity. For this purpose, we used a combination of enzymatic treatments, quartz crystal microbalance with dissipation (QCM-D), and the surface forces apparatus (SFA) to characterize the formation and load-bearing capabilities of SF films on model oxide (i.e., silicates) surfaces. We find that, upon cleavage of proteins, the elasticity of the films is reduced and that cleaving GAGs results in irreversible (plastic) molecular re-arrangements of the film constituents when subjected to confinement. Understanding thin film mechanics of SF can provide insight into the progression of diseases, such as arthritis, but may also be applicable to the development of new implant surface treatments or new biomimetic lubricants.

Management of osteoarthritis: From drug molecules to nano/micromedicines

With the change in lifestyle and aging of the population, osteoarthritis (OA) is emerging as a major medical burden globally. OA is a chronic inflammatory and degenerative disease initially manifesting with joint pain and eventually leading to permanent disability. To date, there are no drugs available for the definitive treatment of osteoarthritis and most therapies have been palliative in nature by alleviating symptoms rather than curing the disease. This coupled with the vague understanding of the early symptoms and methods of diagnosis so that the disease continues as a global problem and calls for concerted research efforts. A cascade of events regulates the onset and progression of osteoarthritis starting with the production of proinflammatory cytokines, including interleukin (IL)‐1β, IL‐6, tumor necrosis factor (TNF)‐α; catabolic enzymes, such as matrix metalloproteinases (MMPs)‐1, ‐3, and ‐13, culminating into cartilage breakdown, loss of lubrication, pain, and inability to load the joint. Although intra‐articular injections of small and macromolecules are often prescribed to alleviate symptoms, low residence times within the synovial cavity severely impair their efficacy. This review will briefly describe the factors dictating the onset and progression of the disease, present the current clinically approved methods for its treatment and diagnosis, and finally elaborate on the main challenges and opportunities for the application of nano/micromedicines in the treatment of osteoarthritis. Thus, future treatment regimens will benefit from simultaneous consideration of the mechanobiological, the inflammatory, and tissue degradation aspects of the disease.

This article is categorized under:

Nanotechnology Approaches to Biology > Nanoscale Systems in Biology

Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement

Dopamine-conjugated hyaluronic acid delivered via intra-articular injection provides articular cartilage lubrication and protection

Due to its high molecular weight and viscosity, hyaluronic acid (HA) is widely used for viscosupplementation to provide joint pain relief in osteoarthritis. However, this benefit is temporary due to poor adhesion of HA on articular surfaces. In this study, we therefore conjugated HA with dopamine to form HADN, which made the HA adhesive while retaining its viscosity enhancement capacity. We hypothesized that HADN could enhance cartilage lubrication through adsorption onto the exposed collagen type II network and repair the lamina splendens. HADN was synthesized by carbodiimide chemistry between hyaluronic acid and dopamine. Analysis of Magnetic Resonance (NMR) and Ultraviolet spectrophotometry (Uv-vis) showed that HADN was successfully synthesized. Adsorption of HADN on collagen was demonstrated using Quartz crystal microbalance with dissipation (QCM-D). Ex vivo tribological tests including measurement of coefficient of friction (COF), dynamic creep, in stance (40 N) and swing (4 N) phases of gait cycle indicated adequate protection of cartilage by HADN with higher lubrication compared to HA alone. HADN solution at the cartilage-glass sliding interface not only retains the same viscosity as HA and provides fluid film lubrication, but also ensures better boundary lubrication through adsorption. To confirm the cartilage surface protection of HADN, we visualized cartilage wear using optical coherence tomography (OCT) and atomic force microscopy (AFM).

Critical analysis of in vitro stability testing of spinal implants and proposal for standardization

BACKGROUND

The American Society for Testing and Materials (ASTM), considered the gold standard worldwide, requires only testing in physiological saline solution to simulate in vivo conditions in standard testing of spinal implants.

RESEARCH DESIGN AND METHODS

We conducted an in vitro study to identify an industrial lubricant with characteristics that are most similar to those of biologically lubricating fat, blood, and tissue fluids. The use of such a material could standardize the results of in vitro mechanical tests for better clinical applications.

RESULTS

Our study has shown that the lubricity of physiological saline was well below that of human soft tissues and tissue fluids, and among the motor oils, Castrol GTX3 provided a testing environment similar to that of a living organism.

CONCLUSIONS

With the intention of standardizing and preventing a biological hazard, we have developed a reproducible mechanical testing proposal based on our experiments, which, in addition, would allow us to avoid many misunderstandings and contingencies.

Biomimetic cartilage-lubricating polymers regenerate cartilage in rats with early osteoarthritis

The early stages of progressive degeneration of cartilage in articular joints are a hallmark of osteoarthritis. Healthy cartilage is lubricated by brush-like cartilage-binding nanofibres with a hyaluronan backbone and two key side chains (lubricin and lipid). Here, we show that hyaluronan backbones grafted with lubricin-like sulfonate-rich polymers or with lipid-like phosphocholine-rich polymers together enhance cartilage regeneration in a rat model of early osteoarthritis. These biomimetic brush-like nanofibres show a high affinity for cartilage proteins, form a lubrication layer on the cartilage surface and efficiently lubricate damaged human cartilage, lowering its friction coefficient to the low levels typical of native cartilage. Intra-articular injection of the two types of nanofibre into rats with surgically induced osteoarthritic joints led to cartilage regeneration and to the abrogation of osteoarthritis within 8 weeks. Biocompatible injectable lubricants that facilitate cartilage regeneration may offer a translational strategy for the treatment of early osteoarthritis.

Enhanced In Vitro and In Vivo Efficacy of Alginate/Silk Protein/Hyaluronic Acid with Polypeptide Microsphere Delivery for Tissue Regeneration of Articular Cartilage

Alginate/Silk fibroin/hyaluronic acid (ALG/SF/HA) nanocomposites were synthesised using blending, inter-linking, and lyophilization methods. We investigated the physicochemical properties of the resulting nanocomposites, including their water retention, weight loss, porosity and cytocompatibility. The optimum ratios of the ALG/SF/HA scaffolding were 3:6.5:0.5. Nanocomposites with optimum ratios were then prepared by integrating pilose antler polypeptides (PAPS) to poly(lactic-co-glycolic acid) (PLGA) microspheres, and the performance was investigated. PAPS-ALG/SF/HA nanocomposites exhibited desirable adhesions and proliferations. Rabbit cartilage deficiencies was developed by the animal model. The cartilage repair effects deficiencies were detected and analyzed between PAPS-SF/ALG/ALG/SF/HA, and control activity classes. The deficiencies were virtually fully remedied after 13 weeks in the presence of PAPS-ALG/SF/HA class, suggesting that the PAPS-ALG/SF/HA nanocomposites had a positive effects on joint cartilage repair.

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.

Hyaluronan orders water molecules in its nanoscale extended hydration shells

Hyaluronan (HA) is an anionic, highly hydrated bio-polyelectrolyte found in the extracellular environment, like the synovial fluid between joints. We explore the extended hydration shell structure of HA in water using femtosecond elastic second-harmonic scattering (fs-ESHS). HA enhances orientational water-water correlations. Angle-resolved fs-ESHS measurements and nonlinear optical modeling show that HA behaves like a flexible chain surrounded by extended shells of orientationally correlated water. We describe several ways to determine the concentration-dependent size and shape of a polyelectrolyte in water, using the amount of water oriented by the polyelectrolyte charges as a contrast agent. The spatial extent of the hydration shell is determined via temperature-dependent measurements and can reach up to 475 nm, corresponding to a length of 1600 water molecules. A strong isotope effect, stemming from nuclear quantum effects, is observed when light water (H2O) is replaced by heavy water (D2O), amounting to a factor of 4.3 in the scattered SH intensity.

Boundary Lubrication, Hemifusion, and Self-Healing of Binary Saturated and Monounsaturated Phosphatidylcholine Mixtures ⧫

A wide range of phosphatidylcholine (PC) lipids with different degrees of unsaturation has been identified in the human synovial fluid and on the cartilage surface. The outstanding lubricity of the articular cartilage surface has been attributed to boundary layers comprising complexes of such lipids, though to date, only lubrication by single-component PC-lipid-based boundary layers has been investigated. As distinguishable lubrication behavior has been found to be related to the PC structures, we herein examined the surface morphology (on mica) and the lubrication ability of binary PC lipid mixtures, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), using atomic force microscopy (AFM) and a surface force balance (SFB). These two PC lipids are among the most abundant saturated and unsaturated PC components in synovial joints. Small unilamellar vesicles (SUVs) prepared from DPPC-POPC mixtures (8:2, 5:5, and 2:8, molar ratios) ruptured and formed bilayers on mica. The normal and shear forces between two DPPC-POPC bilayer-coated mica surfaces across the corresponding SUV dispersions show good boundary lubrication (friction coefficients ≤ ca. 10^-4) up to contact stresses of 8.3 ± 2.2 MPa for 8:2 DPPC-POPC and 5.0 ± 1.7 MPa for the others. Hemifusion induced at high normal pressures was observed, probably because of the height mismatch of two components. Reproducible successive approaches after hemifusion indicate rapid self-healing of the mica-supported bilayers in the presence of the SUVs reservoir. This work is a first step to provide insight concerning the lubrication, wear, and healing of the PC-based boundary layers, which must consist of multicomponent lipid mixtures, on the articular cartilage surface.

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.

Bioinert and Lubricious Surfaces by Macromolecular Design

The modification of a variety of biomaterials and medical devices often encompasses the generation of biopassive and lubricious layers on their exposed surfaces. This is valid when the synthetic supports are required to integrate within physiological media without altering their interfacial composition and when the minimization of shear stress prevents or reduces damage to the surrounding environment. In many of these cases, hydrophilic polymer brushes assembled from surface-interacting polymer adsorbates or directly grown by surface-initiated polymerizations (SIP) are chosen. Although growing efforts by polymer chemists have been focusing on varying the composition of polymer brushes in order to attain increasingly bioinert and lubricious surfaces, the precise modulation of polymer architecture has simultaneously enabled us to substantially broaden the tuning potential for the above-mentioned properties. This feature article concentrates on reviewing this latter strategy, comparatively analyzing how polymer brush parameters such as molecular weight and grafting density, the application of block copolymers, the introduction of branching and cross-links, or the variation of polymer topology beyond the simple, linear chains determine highly technologically relevant properties, such as biopassivity and lubrication.

Mechanism of Superlubricity Conversion with Polyalkylene Glycol Aqueous Solutions

In this study, ultralow friction coefficient (COF, μ < 0.01) was obtained through polyalkylene glycol (PAG) aqueous solutions with different molecular weights (MWs) ranging from 270 to 3930 g·mol^-1 under ambient conditions. With the increase in the MWs of PAG molecules, the threshold concentration to obtain this type of superlubric behavior gradually changed from 90 to 60 wt %. This phenomenon was closely related to the interaction between PAG chains and water molecules and the state of chemical binding. In the superlubricity system, superior load-bearing capacity was achieved at optimal threshold concentrations of all PAG aqueous solutions wherein multilayered adsorption layers that consisted of fully hydrated PAG molecules were formed on the sliding solid surfaces. With respect to the concentration below the threshold value, the existence of a shearing layer was indicated to play a significant role. Thus, the synergetic effect of sufficient adsorption of molecules and the unique shear rheology of the PAG aqueous solution were essential to achieve superlubricity.

Brushes, Graft Copolymers, or Bottlebrushes? The Effect of Polymer Architecture on the Nanotribological Properties of Grafted-from Assemblies

Surface-grafted polyzwitterions (PZW) have gained a foothold in the design of synthetic materials that closely mimic the lubricious properties of articular joints in mammals. Besides their chemical composition, the architecture of PZW brushes strongly determines their morphological, nanomechanical, and nanotribological characteristics. This emerges while comparing the properties of linear poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes with those displayed by graft copolymer and bottlebrush brushes, either featuring a low or a high content of PMPC side chains. Surface-initiated atom transfer radical polymerization (SI-ATRP) enabled the synthesis of different branched-brush architectures from multifunctional macroinitiators via multiple grafting steps, and allowed us to modulate their structure by tuning the polymerization conditions. At relatively low grafting densities (σ), long PMPC side segments extend at the interface of bottlebrush and graft copolymer brushes, providing both morphology and lubrication properties comparable to those shown by loosely grafted, linear PMPC brushes. When σ > 0.1 chains nm^-2 the effect of the branched-brush architecture on the nanotribological properties of the films became evident. Linear PMPC brushes showed the lowest friction among the studied brush structures, with a coefficient of friction (μ) that reached 1 × 10^-4, as measured by atomic force microscopy (AFM). Bottlebrush brushes showed comparatively higher friction, although the high content of hydrophilic PMPC side chains along their backbone substantially improved lubrication compared to that displayed by the more sparsely substituted graft copolymer brushes.

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.

AFM Study on Superlubricity between Ti6Al4V/Polymer Surfaces Achieved with Liposomes

Liposomes have been considered as the boundary lubricant in natural joints. They are also the main component of bionic lubricant. In this study, the tribological properties of liposomes on Ti6Al4V/polymer surface were studied by atomic force microscope (AFM) at the nanoscale. The superlubricity with a friction coefficient of 0.007 was achieved under the maximal pressure of 15 MPa, consisting with the lubrication condition of natural joints. Especially, when the AFM probe was hydrophilically modified and preadsorbed, the friction coefficient and load bearing capacity could be further improved. In addition, the probe with a large radius could maintain the stable lubrication of liposomes in the contact zone. Finally, an optimal lubrication model of liposomes was established and the critical force for superlubricity was also proposed. It was the boundary between elastic deformation and plastic deformation for vesicles. It was also the indicator of the plough effect appearing on the adsorbed layer. This work reveals the interfacial behavior of liposomes and realizes the controllable superlubricity system, providing more guidance for clinical application.

Lipid-hyaluronan synergy strongly reduces intrasynovial tissue boundary friction

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

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.

Molecular weight impact on the mechanical forces between hyaluronan and its receptor

Hyaluronan (HA) possesses manifold mechanical and signaling properties in the body. Most of these activities are largely regulated by its molecular weight, which often triggers opposing functions. However the molecular basis for such function distinction between HA size categories remains unclear. Using a combination of biophysical techniques, we measured the physical forces between HA ligand and its specific receptor CD44 in both normal and lateral directions, at different HA molecular weights and bound states. It was found that the impact of HA multivalency is more than just the sum of separate monovalent bindings. The HA-CD44 specific interaction enhances with HA molecular weight and the maximum binding occurs at ∼1000 kD, possibly due to the balance between multivalent HA zipping effect and conformational entropy. High friction patches, probably from CD44 protein clustering, was observed in friction force microscopy (FFM) upon HA shearing, which is also dependent on HA molecular weight. These results could help to understand the biophysical mechanism of HA in regulating CD44-induced physiological activities and thus facilitate the new design of HA-based material in fine tuning the receptor responses.

Modulation of Gene Expression in Infrapatellar Fat Pad-Derived Mesenchymal Stem Cells in Osteoarthritis

Aim In the osteoarthritis (OA) disease, all structures of the joint are involved. The infrapatellar Hoffa fat pad is rich in macrophages and granulocytes, which also represents a source of adipose mesenchymal progenitor cells (ASC) cells. In our study, we analyze how OA affects the ability of ASC-derived from Hoffa's fat pad to differentiate into chondrocytes. Material and methodology We took knee Hoffa's pad samples and adipose tissue from the proximal thigh from 6 patients diagnosed with severe OA and from another 6 patients with an anterior cruciate ligament (ACL) rupture without OA. From all the patients, we took subcutaneous adipose tissue from the thigh, as the control group. Samples of synovial fluid (SF) were also extracted. The gene expression was analyzed by real-time quantitative polymerase chain reaction. Results PTH1R and MMP13 expression during chondrogenic differentiation were similar between OA and ACL groups, while the expression of OPG, FGF2, TGFβ, MMP3 were significantly lower in the OA group. Exposure of differentiated ASC to OA SF induced an increase in the expression of OPG, PTH1R, and MMP13 and a decrease in the expression of FGF2 in cell culture of the ACL group. However, expression of none of these factors was altered by the OA synovial fluid in ASC cells of the OA group. Conclusion OA of the knee also affects the mesenchymal stem cells of Hoffa fat, suggesting that Hoffa fat is a new actor in the OA degenerative process that can contribute to the origin, onset, and progression of the disease.

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.

Complex solutions under shear and pressure: a rheometer setup for X-ray scattering experiments

A newly developed high-pressure rheometer for in situ X-ray scattering experiments is described. A commercial rheometer was modified in such a way that X-ray scattering experiments can be performed under different pressures and shear. First experiments were carried out on hyaluronan, a ubiquitous biopolymer that is important for different functions in the body such as articular joint lubrication. The data hint at a decreased electrostatic interaction at higher pressure, presumably due to the increase of the dielectric constant of water by 3% and the decrease of the free volume at 300 bar.

Nanoassemblies of Tissue-Reactive, Polyoxazoline Graft-Copolymers Restore the Lubrication Properties of Degraded Cartilage

Osteoarthritis leads to an alteration in the composition of the synovial fluid, which is associated with an increase in friction and the progressive and irreversible destruction of the articular cartilage. In order to tackle this degenerative disease, there has been a growing interest in the medical field to establish effective, long-term treatments to restore cartilage lubrication after damage. Here we develop a series of graft-copolymers capable of assembling selectively on the degraded cartilage, resurfacing it, and restoring the lubricating properties of the native tissue. These comprise a polyglutamic acid backbone (PGA) coupled to brush-forming, poly-2-methyl-2-oxazoline (PMOXA) side chains, which provide biopassivity and lubricity to the surface, and to aldehyde-bearing tissue-reactive groups, for the anchoring on the degenerated cartilage via Schiff bases. Optimization of the graft-copolymer architecture (i.e., density and length of side chains and amount of tissue-reactive functions) allowed a uniform passivation of the degraded cartilage surface. Graft-copolymer-treated cartilage showed very low coefficients of friction within synovial fluid, reestablishing and in some cases improving the lubricating properties of the natural cartilage. Due to these distinctive properties and their high biocompatibility and stability under physiological conditions, cartilage-reactive graft-copolymers emerge as promising injectable formulations to slow down the progression of cartilage degradation, which characterizes the early stages of osteoarthritis.

Structural behaviour of sodium hyaluronate in concentrated oppositely charged surfactant solutions

This work discusses the polyelectrolyte sodium hyaluronate (HA) and its polyelectrolyte/surfactant complexes (PESCs) with tetradecyltrimethylammonium bromide (TTAB) in the semi-dilute regime of HA and at high concentrations of TTAB. The structure and flow properties in the surfactant excess region were studied by light scattering and small angle neutron scattering (SANS) as well as by rheology. The unique behaviour of HA to maintain its high viscosity was observed even at very high TTAB concentrations of 496 mM and this effect was systematically studied in the concentration range from 1 to 25 mM HA. From the data, it could be concluded that: (1) extended rod-like structures of the PESCs prevent molecular dissolution of HA by TTAB. (2) HA and TTAB micelles interact rather weakly as seen by a low fraction of bound micelles. (3) At very high TTAB concentrations a decompaction of PESCs (fractal dimension D_f going from 2.0 to 1.2) occurs with increasing HA concentration but (4) both the entanglement of HA and the structure of the micelles are not affected.

Insight into the Tribological Behavior of Liposomes in Artificial Joints

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

Loss of bottlebrush stiffness due to free polymers

A recently introduced DNA-bottlebrush system, which is formed by the co-assembly of DNA with a genetically engineered cationic polymer-like protein, is subjected to osmotic stress conditions. We measured the inter-DNA distances by X-ray scattering. Our co-assembled DNA-bottlebrush system is one of the few bottlebrushes known to date that shows liquid crystalline behaviour. The alignment of the DNA bottlebrushes was expected to increase with imposed pressure, but interestingly this did not always happen. Molecularly detailed self-consistent field calculations targeted to complement the experiments, focused on the role of molecular crowding on the induced persistence length lp due to the side chains and the cross-sectional width D of the molecular bottlebrushes. Both the thickness as well as the backbone persistence length drop with increasing protein-polymer bulk concentrations and dramatic effects are found above the overlap threshold. The flexibilisation is more significant and therefore the bottlebrush aspect ratio, lp/D, decreases with protein-polymer concentration. This loss in aspect ratio is yet another argument why molecular bottlebrushes rarely order in anisotropic phases and may explain why bottlebrushes are excellent lubricants.

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.

Surface-Active Lipid Linings under Shear Load--A Combined in-Situ Neutron Reflectivity and ATR-FTIR Study

We study shear effects in solid-supported lipid membrane stacks by simultaneous combined in-situ neutron reflectivity (NR) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The stacks mimic the terminal surface-active phospholipid (SAPL) coatings on cartilage in mammalian joints. Piles of 11 bilayer membranes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) are immobilized at the interface of the solid silicon support and the liquid D2O backing phase. We replace the natural hyaluronic acid (HA) component of synovial fluid by a synthetic substitute, namely, poly(allylamine hydrochloride) (PAH), at identical concentration. We find the oligolamellar DMPC bilayer films strongly interacting with PAH resulting in a drastic increase of the membranes d spacing (by a factor of ∼5). Onset of shear causes a buckling-like deformation of the DMPC bilayers perpendicular to the applied shear field. With increasing shear rate we observe substantially enhanced water fractions in the membrane slabs which we attribute to increasing fragmentation caused by Kelvin-Helmholtz-like instabilities parallel to the applied shear field. Both effects are in line with recent theoretical predictions on shear-induced instabilities of lipid bilayer membranes in water (Hanasaki, I.; Walther, J. H.; Kawano, S.; Koumoutsakos, P. Phys. Rev. E 2010, 82, 051602). With the applied shear the interfacial lipid linings transform from their gel state Pβ' to their fluid state Lα. Although in chain-molten state with reduced bending rigidity the lipid layers do not detach from their solid support. We hold steric bridging of the fragmented lipid bilayer membranes by PAH molecules responsible for the unexpected mechanical stability of the DMPC linings.

Interactions of hyaluronan grafted on protein surfaces studied using a quartz crystal microbalance and a surface force balance

Vocal folds are complex and multilayer-structured where the main layer is widely composed of hyaluronan (HA). The viscoelasticity of HA is key to voice production in the vocal fold as it affects the initiation and maintenance of phonation. In this study a simple layer-structured surface model was set up to mimic the structure of the vocal folds. The interactions between two opposing surfaces bearing HA were measured and characterised to analyse HA's response to the normal and shear compression at a stress level similar to that in the vocal fold. From the measurements of the quartz crystal microbalance, atomic force microscopy and the surface force balance, the osmotic pressure, normal interactions, elasticity change, volume fraction, refractive index and friction of both HA and the supporting protein layer were obtained. These findings may shed light on the physical mechanism of HA function in the vocal fold and the specific role of HA as an important component in the effective treatment of the vocal fold disease.

Supramolecular synergy in the boundary lubrication of synovial joints

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

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

Liquid crystals of self-assembled DNA bottlebrushes

Early theories for bottlebrush polymers have suggested that the so-called main-chain stiffening effect caused by the presence of a dense corona of side chains along a central main chain should lead to an increased ratio of effective persistence length (lp,eff) over the effective thickness (Deff) and, hence, ultimately to lyotropic liquid crystalline behavior. More recent theories and simulations suggest that lp,eff ∼ Deff, such that no liquid crystalline behavior is induced by bottlebrushes. In this paper we investigate experimentally how lyotropic liquid crystalline behavior of a semiflexible polymer is affected by a dense coating of side chains. We use semiflexible DNA as the main chain. A genetically engineered diblock protein polymer C4K12 is used to physically adsorb long side chains on the DNA. The C4K12 protein polymer consists of a positively charged binding block (12 lysines, K12) and a hydrophilic random coil block of 400 amino acids (C4). From light scattering we find that, at low ionic strength (10 mM Tris-HCl), the thickness of the self-assembled DNA bottlebrushes is on the order of 30 nm and the effective grafting density is 1 side chain per 2.7 nm of DNA main chain. We find these self-assembled DNA bottlebrushes form birefringent lyotropic liquid crystalline phases at DNA concentrations as low as 8 mg/mL, roughly 1 order of magnitude lower than for bare DNA. Using small-angle X-ray scattering (SAXS) we show that, at DNA concentrations of 12 mg/mL, there is a transition to a hexagonal phase. We also show that, while the effective persistence length increases due to the bottlebrush coating, the effective thickness of the bottlebrush increases even more, such that in our case the bottlebrush coating reduces the effective aspect ratio of the DNA. This is in agreement with theoretical estimates that show that, in most cases of practical interest, a bottlebrush coating will lead to a decrease of the effective aspect ratio, whereas, only for bottlebrushes with extremely long side chains at very high grafting densities, a bottlebrush coating may be expected to lead to an increase of the effective aspect ratio.

Self-assembly and elasticity of hierarchical proteoglycan–hyaluronan brushes††Electronic supplementary information (ESI) available: Variations in areal mass density upon SLB and SAv monolayer formation determined by SE (Fig. S1). See DOI: 10.1039/c3sm51213d

We assemble aggrecan-containing hyaluronan brushes to study how the supramolecular structure and dynamics relate to material properties in hyaluronan-rich pericellular matrices.

Spatially confined yet strongly hydrated assemblies made from the proteoglycan aggrecan and the polysaccharide hyaluronan (HA) are major, functionally important components of the pericellular space around chondrocytes, and in cartilage. To better understand, how mechanical properties arise from the supramolecular structure and dynamics of such assemblies, we have studied the effect of aggrecan on the physico-chemical properties of well-defined, planar HA brushes. From interaction studies by quartz crystal microbalance with dissipation monitoring and spectroscopic ellipsometry, and compression studies by combined colloidal probe atomic force/reflection interference contrast microscopy, we find that aggrecan readily intercalates into HA brushes in a reversible manner. Aggrecan induces a drastic swelling of HA brushes, generating self-assembled films of several micrometers in thickness that are highly hydrated (>99%), elastic and very soft. The Young modulus in the linear compression regime is well below 100 Pa, and reaches several kPa at strong compression. The implications of these findings for biological function are discussed.