Elastoviscous Transitions of Articular Cartilage Reveal a Mechanism of Synergy between Lubricin and Hyaluronic Acid

Collagen-based hydrogels induce stem cell chondrogenesis and hyaline cartilage regeneration: an in vivo study

Injectable, self-crosslinking collagen-based hydrogels are beneficial for chondrocytes to secrete matrix, positioning them as promising candidates for cartilage tissue engineering. However, previous studies lacked insight into the ability of cell-free collagen-based hydrogels to regenerate hyaline cartilage defect. Therefore, this study aimed to evaluate the potential of collagen-based hydrogels (Col and ColHA) to induce chondrogenic differentiation of stem cells and in situ hyaline cartilage regeneration. Both Col and ColHA hydrogels self-crosslinked in situ and exhibited similar physical properties. In vitro experiments showed they supported the survival, adhesion, spreading, and proliferation of bone marrow stem cells (BMSCs). Moreover, both hydrogels induced ectopic differentiation of BMSCs into chondrocytes when implanted subcutaneously into the back of nude mice. ColHA hydrogel notably enhanced type II collagen secretion. The results of repairing cartilage defects in situ revealed both hydrogels facilitated hyaline cartilage regeneration and maintained cartilage phenotype without exogenous BMSCs. Hydrogels encapsulating BMSCs expedited cartilage repair, and ColHA/BMSC constructs showed better mechanical properties, suggesting their potential for cartilage repair applications. This study implies that collagen-based hydrogels are good candidates for hyaline cartilage regeneration.

Degradation of lubricating molecules in synovial fluid alters chondrocyte sensitivity to shear strain

Articular joints facilitate motion and transfer loads to underlying bone through a combination of cartilage tissue and synovial fluid, which together generate a low-friction contact surface. Traumatic injury delivered to cartilage and the surrounding joint capsule causes secretion of proinflammatory cytokines by chondrocytes and the synovium, triggering cartilage matrix breakdown and impairing the ability of synovial fluid to lubricate the joint. Once these inflammatory processes become chronic, posttraumatic osteoarthritis (PTOA) development begins. However, the exact mechanism by which negative alterations to synovial fluid leads to PTOA pathogenesis is not fully understood. We hypothesize that removing the lubricating macromolecules from synovial fluid alters the relationship between mechanical loads and subsequent chondrocyte behavior in injured cartilage. To test this hypothesis, we utilized an ex vivo model of PTOA that involves subjecting cartilage explants to a single rapid impact followed by continuous articulation within a lubricating bath of either healthy synovial fluid, phosphate-buffered saline (PBS), synovial fluid treated with hyaluronidase, or synovial fluid treated with trypsin. These treatments degrade the main macromolecules attributed with providing synovial fluid with its lubricating properties; hyaluronic acid and lubricin. Explants were then bisected and fluorescently stained to assess global and depth-dependent cell death, caspase activity, and mitochondrial depolarization. Explants were tested via confocal elastography to determine the local shear strain profile generated in each lubricant. These results show that degrading hyaluronic acid or lubricin in synovial fluid significantly increases middle zone chondrocyte damage and shear strain loading magnitudes, while also altering chondrocyte sensitivity to loading.

Impact of arthroscopy on post-procedure intra-articular elbow injections: A cadaveric study

OBJECTIVE

To determine the influence of arthroscopy and injection volume on post-procedure intra-articular (IA) injection extravasation.

STUDY DESIGN

Ex vivo prospective study.

SAMPLE POPULATION

A total of 40 paired canine cadaver forelimbs.

METHODS

After radiographs and computed tomography (CT) scans with three-dimensional (3D) digital bone model reconstructions, elbows were randomly assigned to the arthroscopy or control group and randomly assigned to receive an IA injection of 2 or 4 mL of contrast. Elbow arthroscopy was performed on assigned specimens, followed by IA injections of contrast in all elbows, and imaging was repeated. 3D digital model volumes were compared. Images were interpreted and scored for extravasation by a radiologist unaware of treatment and volume assignments.

RESULTS

Based on CT images and regardless of treatment group, IA injections of 4 mL resulted in a mean extravasation score of 2.25 (SD 0.97) versus 1.55 (SD 1.05) (p = .02) for 2 mL IA injections. The change in 3D model volumes after IA injections was a mean of 13.2 cm3 (SD 5.85) after 4 mL injections, compared to 6.97 cm3 (SD 6.28) (p = .003) after 2 mL injections. On radiographic evaluation, but not CT, the mean extravasation scores were 2.45 (SD 1.15) for the arthroscopy group and 1.25 (SD 0.79) for the control group (p < .001).

CONCLUSION

A larger volume of IA injection resulted in higher CT extravasation scores and larger 3D volumes regardless of arthroscopic treatment.

CLINICAL SIGNIFICANCE

IA injections performed immediately after arthroscopy resulted in 50% or less extravasation, especially with a smaller IA injection volume.

O-glycans Expand Lubricin and Attenuate Its Viscosity and Shear Thinning

Lubricin, an intrinsically disordered glycoprotein, plays a pivotal role in facilitating smooth movement and ensuring the enduring functionality of synovial joints. The central domain of this protein serves as a source of this excellent lubrication and is characterized by its highly glycosylated, negatively charged, and disordered structure. However, the influence of O-glycans on the viscosity of lubricin remains unclear. In this study, we employ molecular dynamics simulations in the absence and presence of shear, along with continuum simulations, to elucidate the intricate interplay between O-glycans and lubricin and the impact of O-glycans on lubricin's conformational properties and viscosity. We found the presence of O-glycans to induce a more extended conformation in fragments of the disordered region of lubricin. These O-glycans contribute to a reduction in solution viscosity but at the same time weaken shear thinning at high shear rates, compared to nonglycosylated systems with the same density. This effect is attributed to the steric and electrostatic repulsion between the fragments, which prevents their conglomeration and structuring. Our computational study yields a mechanistic mechanism underlying previous experimental observations of lubricin and paves the way to a more rational understanding of its function in the synovial fluid.

Mechanobiology of Hyaluronan: Connecting Biomechanics and Bioactivity in Musculoskeletal Tissues

Hyaluronan (HA) plays well-recognized mechanical and biological roles in articular cartilage and synovial fluid, where it contributes to tissue structure and lubrication. An understanding of how HA contributes to the structure of other musculoskeletal tissues, including muscle, bone, tendon, and intervertebral discs, is growing. In addition, the use of HA-based therapies to restore damaged tissue is becoming more prevalent. Nevertheless, the relationship between biomechanical stimuli and HA synthesis, degradation, and signaling in musculoskeletal tissues remains understudied, limiting the utility of HA in regenerative medicine. In this review, we discuss the various roles and significance of endogenous HA in musculoskeletal tissues. We use what is known and unknown to motivate new lines of inquiry into HA biology within musculoskeletal tissues and in the mechanobiology governing HA metabolism by suggesting questions that remain regarding the relationship and interaction between biological and mechanical roles of HA in musculoskeletal health and disease.

Advances in viscosupplementation and tribosupplementation for early-stage osteoarthritis therapy

Joint kinematic instability, arising from congenital or acquired musculoskeletal pathoanatomy or from imbalances in anabolism and catabolism induced by pathophysiological factors, leads to deterioration of the composition, structure and function of cartilage and, ultimately, progression to osteoarthritis (OA). Alongside articular cartilage degeneration, synovial fluid lubricity decreases in OA owing to a reduction in the concentration and molecular weight of hyaluronic acid and surface-active mucinous glycoproteins that form a lubricating film over the articulating joint surfaces. Minimizing friction between articulating joint surfaces by lubrication is fundamental for decreasing hyaline cartilage wear and for maintaining the function of synovial joints. Augmentation with highly viscous supplements (that is, viscosupplementation) offers one approach to re-establishing the rheological and tribological properties of synovial fluid in OA. However, this approach has varied clinical outcomes owing to limited intra-articular residence time and ineffective mechanisms of chondroprotection. This Review discusses normal hyaline cartilage function and lubrication and examines the advantages and disadvantages of various strategies for restoring normal joint lubrication. These strategies include contemporary viscosupplements that contain antioxidants, anti-inflammatory drugs or platelet-rich plasma and new synthetic synovial fluid additives and cartilage matrix enhancers. Advanced biomimetic tribosupplements offer promise for mitigating cartilage wear, restoring joint function and, ultimately, improving patient care.

Heterogeneous distribution of viscosupplements in vivo is correlated to ex vivo frictional properties of equine cartilage

Intra-articular injections of hyaluronic acid (HA) are the cornerstone of osteoarthritis (OA) treatments. However, the mechanism of action and efficacy of HA viscosupplementation are debated. As such, there has been recent interest in developing synthetic viscosupplements. Recently, a synthetic 4 wt% polyacrylamide (pAAm) hydrogel was shown to effectively lubricate and bind to the surface of cartilage in vitro. However, its ability to localize to cartilage and alter the tribological properties of the tissue in a live articulating large animal joint is not known. The goal of this study was to quantify the distribution and extent of localization of pAAm in the equine metacarpophalangeal or metatarsophalangeal joint (fetlock joint), and determine whether preferential localization of pAAm influences the tribological properties of the tissue. An established planar fluorescence imaging technique was used to visualize and quantify the distribution of fluorescently labeled pAAm within the joint. While the pAAm hydrogel was present on all surfaces, it was not uniformly distributed, with more material present near the site of the injection. The lubricating ability of the cartilage in the joint was then assessed using a custom tribometer across two orders of magnitude of sliding speed in healthy synovial fluid. Cartilage regions with a greater coverage of pAAm, that is, higher fluorescent intensities, exhibited friction coefficients nearly 2-fold lower than regions with lesser pAAm (Rrm = -0.59, p < 0.001). Collectively, the findings from this study indicate that intra-articular viscosupplement injections are not evenly distributed inside a joint, and the tribological outcomes of these materials is strongly determined by the ability of the material to localize to the articulating surfaces in the joint.

Recombinant manufacturing of multispecies biolubricants

Lubricin, a lubricating glycoprotein abundant in synovial fluid, forms a low-friction brush polymer interface in tissues exposed to sliding motion including joints, tendon sheaths, and the surface of the eye. Despite its therapeutic potential in diseases such as osteoarthritis and dry eye disease, there are few sources available. Through rational design, we developed a series of recombinant lubricin analogs that utilize the species-specific tissue-binding domains at the N- and C-termini to increase biocompatibility while replacing the central mucin domain with an engineered variant that retains the lubricating properties of native lubricin. In this study, we demonstrate the tissue binding capacity of our engineered lubricin product and its retention in the joint space of rats. Next, we present a new bioprocess chain that utilizes a human-derived cell line to produce O-glycosylation consistent with that of native lubricin and a purification strategy that capitalizes on the positively charged, hydrophobic N- and C-terminal domains. The bioprocess chain is demonstrated at 10 L scale in industry-standard equipment utilizing commonly available ion exchange, hydrophobic interaction and size exclusion chromatography resins. Finally, we confirmed the purity and lubricating properties of the recombinant biolubricant. The biomolecular engineering and bioprocessing strategies presented here are an effective means of lubricin production and could have broad applications to the study of mucins in general.

Scale-Dependent Rheology of Synovial Fluid Lubricating Macromolecules

Synovial fluid (SF) is the complex biofluid that facilitates the exceptional lubrication of articular cartilage in joints. Its primary lubricating macromolecules, the linear polysaccharide hyaluronic acid (HA) and the mucin-like glycoprotein proteoglycan 4 (PRG4 or lubricin), interact synergistically to reduce boundary friction. However, the precise manner in which these molecules influence the rheological properties of SF remains unclear. This study aimed to elucidate this by employing confocal microscopy and multiscale rheometry to examine the microstructure and rheology of solutions containing recombinant human PRG4 (rhPRG4) and HA. Contrary to previous assumptions of an extensive HA-rhPRG4 network, it is discovered that rhPRG4 primarily forms stiff, gel-like aggregates. The properties of these aggregates, including their size and stiffness, are found to be influenced by the viscoelastic characteristics of the surrounding HA matrix. Consequently, the rheology of this system is not governed by a single length scale, but instead responds as a disordered, hierarchical network with solid-like rhPRG4 aggregates distributed throughout the continuous HA phase. These findings provide new insights into the biomechanical function of PRG4 in cartilage lubrication and may have implications in the development of HA-based therapies for joint diseases like osteoarthritis.

Loss of effective lubricating viscosity is the primary mechanical marker of joint inflammation in equine synovitis

Inflammation of the synovium, known as synovitis, plays an important role in the pathogenesis of osteoarthritis (OA). Synovitis involves the release of a wide variety of pro-inflammatory mediators in synovial fluid (SF) that damage the articular cartilage extracellular matrix and induce death and apoptosis in chondrocytes. The composition of synovial fluid is dramatically altered by inflammation in OA, with changes to both hyaluronic acid and lubricin, the primary lubricating molecules in SF. However, the relationship between key biochemical markers of joint inflammation and mechanical function of SF is not well understood. Here, we demonstrate the application of a novel analytical framework to measure the effective viscosity for SF lubrication of cartilage, which is distinct from conventional rheological viscosity. Notably, in a well-established equine model of synovitis, this effective lubricating viscosity decreased by up to 10,000-fold for synovitis SF compared to a ~4 fold change in conventional viscosity measurements. Further, the effective lubricating viscosity was strongly inversely correlated (r = -0.6 to -0.8) to multiple established biochemical markers of SF inflammation, including white blood cell count, prostaglandin E2 (PGE2 ), and chemokine ligand (CCLs) concentrations, while conventional measurements of viscosity were poorly correlated to these markers. These findings demonstrate the importance of experimental and analytical approaches to characterize functional lubricating properties of synovial fluid and their relationships to soluble biomarkers to better understand the progression of OA.

The Actin Cytoskeleton as a Regulator of Proteoglycan 4

OBJECTIVE

The superficial zone (SZ) of articular cartilage is responsible for distributing shear forces for optimal cartilage loading and contributes to joint lubrication through the production of proteoglycan 4 (PRG4). PRG4 plays a critical role in joint homeostasis and is chondroprotective. Normal PRG4 production is affected by inflammation and irregular mechanical loading in post-traumatic osteoarthritis (PTOA). THe SZ chondrocyte (SZC) phenotype, including PRG4 expression, is regulated by the actin cytoskeleton in vitro. There remains a limited understanding of the regulation of PRG4 by the actin cytoskeleton in native articular chondrocytes. The filamentous (F)-actin cytoskeleton is a potential node in crosstalk between mechanical stimulation and cytokine activation and the regulation of PRG4 in SZCs, therefore developing insights in the regulation of PRG4 by actin may identify molecular targets for novel PTOA therapies.

MATERIALS AND METHODS

A comprehensive literature search on PRG4 and the regulation of the SZC phenotype by actin organization was performed.

RESULTS

PRG4 is strongly regulated by the actin cytoskeleton in isolated SZCs in vitro. Biochemical and mechanical stimuli have been characterized to regulate PRG4 and may converge upon actin cytoskeleton signaling.

CONCLUSION

Actin-based regulation of PRG4 in native SZCs is not fully understood and requires further elucidation. Understanding the regulation of PRG4 by actin in SZCs requires an in vivo context to further potential of leveraging actin arrangement to arthritic therapeutics.

Modulating mechanobiology as a therapeutic target for synovial fibrosis to restore joint lubrication

OBJECTIVES

Fibroses are disorders linked to persistence of myofibroblasts due to biochemical (e.g., Transforming growth factor-β) and biophysical cues (e.g., a stiff microenvironment). In the context of osteoarthritis, fibrotic changes in the joint-lining synovium have been linked with disease progression. The objective of this study was to probe synovial fibroblast mechanobiology and how essential functions (i.e., lubrication) are altered in fibrotic environments.

DESIGN

Both ex vivo and in vitro synovium models were assessed for fibrotic and lubrication biomarkers to better understand the role of mechanobiology and lubrication. Additionally, in vitro, work on small molecules targeting mechanobiology was assessed.

RESULTS

Our results indicated that modulating mechanobiology could rescue the fibrotic phenotype instigated by stiffening microenvironment that resulted in altered lubricant expression. A small molecule therapeutic, fasudil, blocked ROCK-mediated contractility and this inhibition of the fibrotic mechano-response of synovial fibroblasts restored proper lubrication function, providing insight into mechanisms of disease progression as well as a new avenue for therapeutic development.

CONCLUSION

This study identifies synovial fibrosis as a condition that potentially has joint-wide deficits through inhibiting lubrication. Additionally, modulating mechanobiology (i.e., ROCK-mediated contractility) may pose a potential target for small molecule therapies that can be delivered to the joint space.

CLASSIFICATION

Applied Biological Sciences.

Soluble and EV-Associated Diagnostic and Prognostic Biomarkers in Knee Osteoarthritis Pathology and Detection

Osteoarthritis (OA) is the most common degenerative disease of the connective tissue of the human musculoskeletal system. Despite its widespread prevalence, there are many limitations in its diagnosis and treatment. OA diagnosis currently relies on the presence of clinical symptoms, sometimes accompanied by changes in joint X-rays or MRIs. Biomarkers help not only to diagnose early disease progression but also to understand the process of OA in many ways. In this article, we briefly summarize information on articular joints and joint tissues, the pathogenesis of OA and review the literature about biomarkers in the field of OA, specifically inflammatory cytokines/chemokines, proteins, miRNA, and metabolic biomarkers found in the blood, synovial fluid and in extracellular vesicles.

Polyacrylamide hydrogel lubricates cartilage after biochemical degradation and mechanical injury

Intra-articular injections of hyaluronic acid have been a mainstay of osteoarthritis treatment for decades. However, controversy surrounds the mechanism of action and efficacy of this therapy. As such, there has been recent interest in developing synthetic lubricants that lubricate cartilage. Recently, a synthetic 4 wt% polyacrylamide (pAAm) hydrogel was shown to effectively decrease lameness in horses. However, its mechanism of action and ability to lubricate cartilage is unknown. The goal of this study was to characterize the lubricating ability of this hydrogel and determine its efficacy for healthy and degraded cartilage. The study utilized previously established IL-1β-induced biochemical degradation and mechanical impact injury models to degrade cartilage. The lubricating ability of the hydrogel was then characterized using a custom-built tribometer using a glass counterface and friction was evaluated using the Stribeck framework for articular cartilage. pAAm hydrogel was shown to significantly lower the friction coefficient of cartilage explants from both degradation models (30%-40% reduction in friction relative to controls). A striking finding from this study was the aggregation of the pAAm hydrogel at the articulating surface. The surface aggregation was observed in the histological sections of explants from all treatment groups after tribological evaluation. Using the Stribeck framework, the hydrogel was mapped to higher Sommerfeld numbers and was characterized as a viscous lubricant predominantly in the minimum friction mode. In summary, this study revealed that pAAm hydrogel lubricates native and degraded cartilage explants effectively and may have an affinity for the articulating surface of the cartilage.

Coefficient of Friction and Height Loss: Two Criteria Used to Determine the Mechanical Property and Stability of Regenerated Versus Natural Articular Cartilage

Background: The coefficient of friction (CoF) serves as an indicator for the mechanical properties of natural and regenerated articular cartilage (AC). After tribological exposure, a height loss (HL) of the cartilage pair specimens can be measured. Our aim was to determine the CoF and HL of regenerated AC tissue and compare them with those of natural AC from non-operated joints and AC from joints where the regenerated tissues had been created after different treatments. Methods: In partial-thickness defects of the trochleae of the stifle joints of 60 Göttingen Minipigs, regenerated AC was created. In total, 40 animals received a Col I matrix, 20 laden with autologous chondrocytes, and 20 without. The defects of 20 animals were left empty. The healing periods were 24 and 48 weeks. A total of 10 not-operated animals, delivered the “external” control specimens. Osteochondral pins were harvested from defect and non-defect areas, the latter serving as “internal” controls. Using a pin-on-plate tribometer, we measured the CoF and the HL. Results: The CoF of the regenerated AC ranged from 0.0393 to 0.0688, and the HL, from 0.22 mm to 0.3 mm. The differences between the regenerated AC of the six groups and the “external” controls were significant. The comparison with the “internal” controls revealed four significant differences for the CoF and one for the HL in the operated groups. No differences were seen within the operated groups. Conclusions: The mechanical quality of the regenerated AC tissue showed inferior behavior with regard to the CoF and HL in comparison with natural AC. The comparison of regenerated AC tissue with AC from untreated joints was more promising than with AC from the treated joints.

Translating musculoskeletal bioengineering into tissue regeneration therapies

Musculoskeletal injuries and disorders are the leading cause of physical disability worldwide and a considerable socioeconomic burden. The lack of effective therapies has driven the development of novel bioengineering approaches that have recently started to gain clinical approvals. In this review, we first discuss the self-repair capacity of the musculoskeletal tissues and describe causes of musculoskeletal dysfunction. We then review the development of novel biomaterial, immunomodulatory, cellular, and gene therapies to treat musculoskeletal disorders. Last, we consider the recent regulatory changes and future areas of technological progress that can accelerate translation of these therapies to clinical practice.

Shear-responsive boundary-lubricated hydrogels attenuate osteoarthritis

Lipid-based boundary layers formed on liposome-containing hydrogels can facilitate lubrication. However, these boundary layers can be damaged by shear, resulting in decreased lubrication. Here, a shear-responsive boundary-lubricated drug-loaded hydrogel is created by incorporating celecoxib (CLX)-loaded liposomes within dynamic covalent bond-based hyaluronic acid (HA) hydrogels (CLX@Lipo@HA-gel). The dynamic cross-linked network enables the hydrogel to get restructured in response to shear, and the HA matrix allows the accumulation of internal liposome microreservoirs on the sliding surfaces, which results in the formation of boundary layers to provide stable lubrication. Moreover, hydration shells formed surrounding the hydrogel can retard the degradation process, thus helping in sustaining lubrication. Furthermore, in vitro and in vivo experiments found that CLX@Lipo@HA-gels can maintain anabolic-catabolic balance, alleviate cartilage wear, and attenuate osteoarthritis progression by delivering CLX and shear-responsive boundary lubrication. Overall, CLX@Lipo@HA-gels can serve as shear-responsive boundary lubricants and drug-delivery vehicles to alleviate friction-related diseases like osteoarthritis.

Articulation inspired by nature: a review of biomimetic and biologically active 3D printed scaffolds for cartilage tissue engineering

In the human body, articular cartilage facilitates the frictionless movement of synovial joints. However, due to its avascular and aneural nature, it has a limited ability to self-repair when damaged due to injury or wear and tear over time. Current surgical treatment options for cartilage defects often lead to the formation of fibrous, non-durable tissue and thus a new solution is required. Nature is the best innovator and so recent advances in the field of tissue engineering have aimed to recreate the microenvironment of native articular cartilage using biomaterial scaffolds. However, the inability to mirror the complexity of native tissue has hindered the clinical translation of many products thus far. Fortunately, the advent of 3D printing has provided a potential solution. 3D printed scaffolds, fabricated using biomimetic biomaterials, can be designed to mimic the complex zonal architecture and composition of articular cartilage. The bioinks used to fabricate these scaffolds can also be further functionalised with cells and/or bioactive factors or gene therapeutics to mirror the cellular composition of the native tissue. Thus, this review investigates how the architecture and composition of native articular cartilage is inspiring the design of biomimetic bioinks for 3D printing of scaffolds for cartilage repair. Subsequently, we discuss how these 3D printed scaffolds can be further functionalised with cells and bioactive factors, as well as looking at future prospects in this field.

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.

Biomaterial Integration in the Joint: Pathological Considerations, Immunomodulation, and the Extracellular Matrix

Defects of articular joints are becoming an increasing societal burden due to a persistent increase in obesity and aging. For some patients suffering from cartilage erosion, joint replacement is the final option to regain proper motion and limit pain. Extensive research has been undertaken to identify novel strategies enabling earlier intervention to promote regeneration and cartilage healing. With the introduction of decellularized extracellular matrix (dECM), researchers have tapped into the potential for increased tissue regeneration by designing biomaterials with inherent biochemical and immunomodulatory signals. Compared to conventional and synthetic materials, dECM-based materials invoke a reduced foreign body response. It is therefore highly beneficial to understand the interplay of how these native tissue-based materials initiate a favorable remodeling process by the immune system. Yet, such an understanding also demands increasing considerations of the pathological environment and remodeling processes, especially for materials designed for early disease intervention. This knowledge would avoid rejection and help predict complications in conditions with inflammatory components such as arthritides. This review outlines general issues facing biomaterial integration and emphasizes the importance of tissue-derived macromolecular components in regulating essential homeostatic, immunological, and pathological processes to increase biomaterial integration for patients suffering from joint degenerative diseases. This article is protected by copyright. All rights reserved.

A Hyaluronic Acid-Based Formulation with Simultaneous Local Drug Delivery and Antioxidant Ability for Active Viscosupplementation

Hyaluronic acid (HA) and its derivatives are widely used for intra-articular injection to augment compromised viscoelastic properties of damaged synovial fluid. Combining HA-based devices with anti-inflammatory drugs or bioactive principles in order to provide an additional benefit to the viscosupplementation is emerging as a new promising approach to improve the clinical outcome. Here, we aim to design a novel active viscosupplementation agent that can load and release hydrophobic drugs and at the same time possessing antioxidant properties. Optimized ternary systems named HCV based on HA, (2-hydroxypropyl)-β-cyclodextrin (CD), and vitamin E (VE), without being engaged in formal chemical bonding with each other, showed the best viscoelastic and lubrication properties along with antioxidant capabilities, able to solubilize and release DF. The physical-chemical characterization suggested that the HCV system displayed rheological synergism and higher thermal stability because of the presence of VE and its antioxidant activity, and the loading of hydrophobic drugs was improved by the presence of CD and VE. Cell morphology and viability tests on L929 cells exhibited high biocompatibility of the HCV system with higher level expression of anti-inflammatory interleukin-10.

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).

A Feasibility Study on Tribological Origins of Knee Acoustic Emissions

OBJECTIVE

Considering the knee as a fluid-lubricated system, articulating surfaces undergo different lubrication modes and generate joint acoustic emissions (JAEs) while moving. The goal of this study is to compare knee biomechanical signals against synchronously recorded joint sounds and assess the hypothesis that these JAEs are attributed to tribological origins.

METHODS

JAE, electromyography, ground reaction force signals, and motion capture markers were synchronously recorded from 10 healthy subjects while performing two-leg and one-leg squat exercises. The biomechanical signals were processed with standard inverse dynamic analysis through musculoskeletal modeling, and a tribological parameter, lubrication coefficient, was calculated from these signals. Besides, JAEs were divided into short windows, and 64 time-frequency features were extracted. The lubrication coefficients and joint sound features of the two-leg squats were used to label the windows and train a classifier that discriminates the knee lubrication modes only based on JAE features. Then, the classifier was used to predict the label of one-leg squat JAE windows. To evaluate these results, the predicted joint sound labels were directly compared against the associated lubrication coefficients.

RESULTS

The trained classifier achieves a high test-accuracy of 84% distinguishing lubrication modes of the one-leg squat JAE windows. The Pearson correlation coefficient between the estimated friction coefficient and the predicted JAE scores was 0.830.08. Furthermore, the lubrication coefficient threshold, separating two lubrication modes, was calculated from joint sound labels, and it decreased by half from two-leg to one-leg squats. This result was consistent with the tribological changes in the knee load as it was inversely doubled in one-leg squats. These results verify that JAEs contain salient information on knee tribology.

SIGNIFICANCE

This study supports the potential use of JAEs as a quantitative digital biomarker to extract tribological information about joint lubrication modes and loading conditions. Since arthritis and many other conditions impact the roughness of cartilage and other surfaces within the knee, the use of JAEs in clinical applications could thereby have broad implications for studying joint frictions and monitoring joint structural changes with wearable devices.

Non-clinical assessment of lubrication and free radical scavenging of an innovative non-animal carboxymethyl chitosan biomaterial for viscosupplementation: An in-vitro and ex-vivo study

OBJECTIVE

Lubrication and free radical scavenging are key features of biomaterials used for viscosupplementation (VS) of joints affected by osteoarthritis (OA). The objective of this study was to describe the non-clinical performance characterization of KiOmedine® CM-Chitosan, a non-animal carboxymethyl chitosan, in order to assess its intended action in VS and to compare it to existing viscosupplements based on crosslinked hyaluronan (HA) formulations.

METHOD

The lubrication capacity of the tested viscosupplements (VS) was evaluated in-vitro and ex-vivo. In-vitro, the coefficient of friction (COF) was measured using a novel tribological system. Meanwhile, an ex-vivo biomechanical model in ovine hindlimbs was developed to assess the recovery of join mobility after an intra-articular (IA) injection. Free radical scavenging capacity of HA and KiOmedine® CM-Chitosan formulations was evaluated using the Trolox Equivalent Antioxidant Capacity (TEAC) assay.

RESULTS

In the in-vitro tribological model, KiOmedine® CM-Chitosan showed high lubrication capacity with a significant COF reduction than crosslinked HA formulations. In the ex-vivo model, the lubrication effect of KiOmedine® CM-Chitosan following an IA injection in the injured knee was proven again by a COF reduction. The recovery of joint motion was optimal with an IA injection of 3 ml of KiOmedine® CM-Chitosan, which was significantly better than the crosslinked HA formulation at the same volume. In the in-vitro TEAC assay, KiOmedine® CM-Chitosan showed a significantly higher free radical scavenging capacity than HA formulations.

CONCLUSION

Overall, the results provide a first insight into the mechanism of action in terms of lubrication and free radical scavenging for the use of KiOmedine® CM-Chitosan as a VS treatment of OA. KiOmedine® CM-Chitosan demonstrated a higher capacity to scavenge free radicals, and it showed a higher recovery of mobility after a knee lesion than crosslinked HA formulations. This difference could be explained by the difference in chemical structure between KiOmedine® CM-Chitosan and HA and their formulations.

Cartilage articulation exacerbates chondrocyte damage and death after impact injury

Posttraumatic osteoarthritis (PTOA) is typically initiated by momentary supraphysiologic shear and compressive forces delivered to articular cartilage during acute joint injury and develops through subsequent degradation of cartilage matrix components and tissue remodeling. PTOA affects 12% of the population who experience osteoarthritis and is attributed to over $3 billion dollars annually in healthcare costs. It is currently unknown whether articulation of the joint post-injury helps tissue healing or exacerbates cellular dysfunction and eventual death. We hypothesize that post-injury cartilage articulation will lead to increased cartilage damage. Our objective was to test this hypothesis by mimicking the mechanical environment of the joint during and post-injury and determining if subsequent joint articulation exacerbates damage produced by initial injury. We use a model of PTOA that combines impact injury and repetitive sliding with confocal microscopy to quantify and track chondrocyte viability, apoptosis, and mitochondrial depolarization in a depth-dependent manner. Cartilage explants were harvested from neonatal bovine knee joints and subjected to either rapid impact injury (17.34 ± 0.99 MPa, 21.6 ± 2.45 GPa/s), sliding (60 min at 1 mm/s, under 15% axial compression), or rapid impact injury followed by sliding. Explants were then bisected and fluorescently stained for cell viability, caspase activity (apoptosis), and mitochondria polarization. Results show that compared to either impact or sliding alone, explants that were both impacted and slid experienced higher magnitudes of damage spanning greater tissue depths.

Squeeze-film properties of synovial fluid and hyaluronate-based viscosupplements

The rheological properties of synovial fluid and hyaluronate (HA) solutions have been studied using a variety of viscometers and rheometers. These devices measure the viscosity of the fluid's resistance to shearing forces, which is useful when studying the lubrication and frictional properties of movable joints. Less commonly used is a squeeze-film fluid test, mechanistically similar to when two joint surfaces squeeze interposed fluid. In our study, we used squeeze-film tests to determine the rheological response of normal bovine synovial fluid and 10 mg/ml HA-based solutions, Hyalgan/Hyalovet, commercially available 500-700 kDa HA viscosupplements, and a 1000 kDa sodium hyaluronate (NaHy) solution. We found similar rheological responses (fluid thickness, viscosity, viscosity-pressure relationship) for all three fluids, though synovial fluid's minimum squeeze-film thickness was slightly thicker. Squeeze-film loading speed did not affect these results. Different HA concentrations and molecular weights also did not have a significant or consistent effect on the squeeze-film responses. An unexpected result for the HA-solutions was a linear increase in minimum fluid-film thickness with increasing initial fluid-film thickness. This result was attributed to faster gelling of thicker HA-solutions, which formed at a lower squeeze-film strain and higher squeeze-film strain rate compared to thinner layers. Also included is a review of the literature on viscosity measurements of synovial fluid and HA solutions.

Articular Cartilage Friction, Strain, and Viability Under Physiological to Pathological Benchtop Sliding Conditions

In vivo, articular cartilage is exceptionally resistant to wear, damage, and dysfunction. However, replicating cartilage's phenomenal in vivo tribomechanics (i.e., high fluid load support, low frictions and strains) and mechanobiology on the benchtop has been difficult, because classical testing approaches tend to minimize hydrodynamic contributors to tissue function. Our convergent stationary contact area (cSCA) configuration retains the ability for hydrodynamically-mediated processes to contribute to interstitial hydration recovery and tribomechanical function via 'tribological rehydration'. Using the cSCA, we investigated how in situ chondrocyte survival is impacted by the presence of tribological rehydration during the reciprocal sliding of a glass counterface against a compressively loaded equine cSCA cartilage explant. When tribological rehydration was compromised during testing, by slow-speed sliding, 'pathophysiological' tribomechanical environments and high surface cell death were observed. When tribological rehydration was preserved, by high-speed sliding, 'semi-physiological' sliding environments and suppressed cell death were realized. Inclusion of synovial fluid during testing fostered 'truly physiological' sliding outcomes consistent with the in vivo environment but had limited influence on cell death compared to high-speed sliding in PBS. Subsequently, path analysis identified friction as a primary driver of cell death, with strain an indirect driver, supporting the contention that articulation mediated rehydration can benefit both the biomechanical properties and biological homeostasis of cartilage.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-021-00671-2.

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.

The role of synovial fluid constituents in the lubrication of collagen-glycosaminoglycan scaffolds for cartilage repair

Extracellular matrix (ECM)-derived scaffolds have shown promise as tissue-engineered grafts for promoting cartilage repair. However, there has been a lack of focus on fine-tuning the frictional properties of scaffolds for cartilage tissue engineering as well as understanding their interactions with synovial fluid constituents. Proteoglycan-4 (PRG4) and hyaluronan (HA) are macromolecules within synovial fluid that play key roles as boundary mode lubricants during cartilage surface interactions. The overall objective of this study was to characterize the role PRG4 and HA play in the lubricating function of collagen-glycosaminoglycan (GAG) scaffolds for cartilage repair. As a first step towards this goal, we aimed to develop a suitable in vitro friction test to establish the boundary mode lubrication parameters for collagen-GAG scaffolds articulated against glass in a phosphate buffered saline (PBS) bath. Subsequently, we sought to leverage this system to determine the effect of physiological synovial fluid lubricants, PRG4 and HA, on the frictional properties of collagen-GAG scaffolds, with scaffolds hydrated in PBS and bovine synovial fluid (bSF) serving as negative and positive controls, respectively. At all compressive strains examined (ε = 0.1-0.5), fluid depressurization within hydrated collagen-GAG scaffolds was >99% complete at ½ minute. The coefficient of friction was stable at all compressive strains (ranging from a low 0.103 ± 0.010 at ε = 0.3 up to 0.121 ± 0.015 at ε = 0.4) and indicative of boundary-mode conditions. Immunohistochemistry demonstrated that PRG4 from recombinant human (rh) and bovine sources adsorbed to collagen-GAG scaffolds and the coefficient of friction for scaffolds immersed in rhPRG4 (0.067 ± 0.027) and normal bSF (0.056 ± 0.020) solution decreased compared to PBS (0.118 ± 0.21, both p < 0.05, at ε = 0.2). The ability of the adsorbed rhPRG4 to reduce friction on the scaffolds indicates that its incorporation within collagen-GAG biomaterials may enhance their lubricating ability as potential tissue-engineered cartilage replacements. To conclude, this study reports the development of an in vitro friction test capable of characterizing the coefficient of friction of ECM-derived scaffolds tested in a range of synovial fluid lubricants and demonstrates frictional properties as a potential design parameter for implants and materials for soft tissue replacement.

Comparative Tribology: Articulation-induced rehydration of cartilage across species

Articular cartilage is a robust tissue that facilitates load distribution and wear-free articulation in diarthrodial joints. These biomechanical capabilities are fundamentally tied to tissue hydration, whereby high interstitial fluid pressures and fluid load support facilitate the maintenance of low tissue strains and frictions. Our recent ex vivo studies of cartilage sliding biomechanics using the convergent stationary contact area (cSCA) configuration, first introduced by Dowson and colleagues, unexpectedly demonstrated that sliding alone can promote recovery of interstitial pressure and lubrication lost to static compression through a mechanism termed 'tribological rehydration.' Although exclusively examined in bovine stifle cartilage to date, we hypothesized that tribological rehydration, i.e., the ability to recover/modulate tissue strains and lubrication through sliding, is a universal behavior of articular cartilage. This study aimed to establish if, and to what extent, sliding-induced tribological rehydration is conserved in articular cartilage across a number of preclinical animal species/models and diarthrodial joints. Using a comparative approach, we found that articular cartilage from equine, bovine, ovine, and caprine stifles, and porcine stifle, hip, and tarsal joints all exhibited remarkably consistent sliding speed-dependent compression/strain recovery and lubrication behaviors under matched contact stresses (0.25 MPa). All cartilage specimens tested supported robust, tribological rehydration during high-speed sliding (>30 mm/s), which as a result of competitive recovery of interstitial lubrication, promoted remarkable decreases in kinetic friction during continuous sliding. The conservation of tribological rehydration across mammalian quadruped articular cartilage suggests that sliding-induced recovery of interstitial hydration represents an important tissue adaptation and largely understudied contributor to the biomechanics of cartilage and joints.

Inflammatory and Noninflammatory Synovial Fluids Exhibit New and Distinct Tribological Endotypes

Inferior synovial lubrication is a hallmark of osteoarthritis (OA), and synovial fluid (SF) lubrication and composition are variable among OA patients. Hyaluronic acid (HA) viscosupplementation is a widely used therapy for improving SF viscoelasticity and lubrication, but it is unclear how the effectiveness of HA viscosupplements varies with arthritic endotype. The objective of this study was to investigate the effects of the HA viscosupplement, Hymovis®, on the lubricating properties of diseased SF from patients with noninflammatory OA and inflammatory arthritis (IA). The composition (cytokine, HA, and lubricin concentrations) of the SF was measured as well as the mechanical properties (rheology, tribology) of the SF alone and in a 1:1 mixture with the HA viscosupplement. Using rotational rheometry, no difference in SF viscosity was detected between disease types, and the addition of HA significantly increased all fluids' viscosities. In noninflammatory OA SF, friction coefficients followed a typical Stribeck pattern, and their magnitude was decreased by the addition of HA. While some of the IA SF also showed typical Stribeck behavior, a subset showed more erratic behavior with highly variable and larger friction coefficients. Interestingly, this aberrant behavior was not eliminated by the addition of HA, and it was associated with low concentrations of lubricin. Aberrant SF exhibited significantly lower effective viscosities compared to noninflammatory OA and IA SF with typical tribological behavior. Collectively, these results suggest that different endotypes of arthritis exist with respect to lubrication, which may impact the effectiveness of HA viscosupplements in reducing friction.

Rheological properties of synovial fluid due to viscosupplements: A review for osteoarthritis remedy

The synovial fluid is a transparent electrolyte solution included in joints to provide lubrication helping the proper movement. It exhibits complex rheological properties due to the interaction among its constituents i.e. hyaluronic acid, albumin, lubricin and phospholipids. In degenerative osteoarthritis and inflammatory rheumatoid arthritis diseases, the quantity of synovial fluid and lubrication efficiency significantly deteriorates. In that case, viscosupplementation with intra-articular hyaluronic acid may be prescribed to replenish the concentration, the molecular weight and the rheological properties of natural synovial fluid. The present review concentrates on the recent advancements in viscosupplementation with emphasis into their rheological properties, its effects on the rheological behavior of synovial fluid, and finally its clinical effectiveness. Initially, the properties of synovial fluid are summarized, and then a discussion on commercial viscosupplements, the role of polymeric properties and their rheological properties are reviewed. Moreover, a detailed discussion on the clinical effectiveness and challenges of viscosupplements are provided.

Topological Polymer Chemistry Enters Materials Science: Expanding the Applicability of Cyclic Polymers

Polymer-topology effects can alter technologically relevant properties when cyclic macromolecules are applied within diverse materials formulations. These include coatings, polymer networks, or nanostructures for delivering therapeutics. While substituting linear building blocks with cyclic analogues in commonly studied materials is itself of fundamental interest, an even more fascinating observation has been that the introduction of physical or chemical boundaries (e.g., a grafting surface or cross-links) can amplify the topology-related effects observed when employing cyclic polymer-based precursors for assembling multidimensional objects. Hence, the application of cyclic polymers has enabled the fabrication of coatings with enhanced biorepellency and superior lubricity, broadened the tuning potential for mechanical properties of polymer networks, increased the thermodynamic stability, and altered the capability of loading and releasing drugs within polymeric micelles.

Force Transmission Between the Gastrocnemius and Soleus Sub-Tendons of the Achilles Tendon in Rat

The Achilles tendon (AT) is comprised of three distinct sub-tendons bound together by the inter-subtendon matrix (ISTM). The interactions between sub-tendons will have important implications for AT function. The aim of this study was to investigate the extent to which the ISTM facilitates relative sliding between sub-tendons, and serves as a pathway for force transmission between the gastrocnemius (GAS) and soleus (SOL) sub-tendons of the rat AT. In this study, ATs were harvested from Wistar rats, and the mechanical behavior and composition of the ISTM were explored. To determine force transmission between sub-tendons, the proximal and distal ends of the GAS and SOL sub-tendons were secured, and the forces at each of these locations were measured during proximal loading of the GAS. To determine the ISTM mechanical behavior, only the proximal GAS and distal SOL were secured, and the ISTM was loaded in shear. Finally, for compositional analysis, histological examination assessed the distribution of matrix proteins throughout sub-tendons and the ISTM. The results revealed distinct differences between the forces at the proximal and distal ends of both sub-tendons when proximal loading was applied to the GAS, indicating force transmission between GAS and SOL sub-tendons. Inter-subtendon matrix tests demonstrated an extended initial low stiffness toe region to enable some sub-tendon sliding, coupled with high stiffness linear region such that force transmission between sub-tendons is ensured. Histological data demonstrate an enrichment of collagen III, elastin, lubricin and hyaluronic acid in the ISTM. We conclude that ISTM composition and mechanical behavior are specialized to allow some independent sub-tendon movement, whilst still ensuring capacity for force transmission between the sub-tendons of the AT.

On the Dependence of Rheology of Hyaluronic Acid Solutions and Frictional Behavior of Articular Cartilage

Hyaluronic acid (HA) injections represent one of the most common methods for the treatment of osteoarthritis. However, the clinical results of this method are unambiguous mainly because the mechanism of action has not been clearly clarified yet. Viscosupplementation consists, inter alia, of the improvement of synovial fluid rheological properties by injected solution. The present paper deals with the effect of HA molecular weight on the rheological properties of its solutions and also on friction in the articular cartilage model. Viscosity and viscoelastic properties of HA solutions were analyzed with a rotational rheometer in a cone-plate and plate-plate configuration. In total, four HA solutions with molecular weights between 77 kDa and 2010 kDa were tested. The frictional measurements were realized on a commercial tribometer Bruker UMT TriboLab, while the coefficient of friction (CoF) dependency on time was measured. The contact couple consisted of the articular cartilage pin and the plate made from optical glass. The contact was fully flooded with tested HA solutions. Results showed a strong dependency between HA molecular weight and its rheological properties. However, no clear dependence between HA molecular weight and CoF was revealed from the frictional measurements. This study presents new insight into the dependence between rheological and frictional behavior of the articular cartilage, while such an extensive investigation has not been presented before.

Interaction with Cartilage Increases the Viscosity of Hyaluronic Acid Solutions

Injection of hyaluronic acid (HA) viscosupplements is a prevalent treatment for patients suffering from mild to moderate osteoarthritis. The efficacy of these supplements is attributed to increased synovial fluid viscosity, which leads to improved lubrication and reduced pain. Therefore, viscosity is a key parameter to consider in the development of HA supplements. HA localizes near the cartilage surface, resulting in a viscosity gradient with heightened viscosity near the surface. Traditional rheological measurements confine HA between metal fixtures and therefore do not capture the effect of HA localization that occurs on cartilage. In these experiments, we investigate the effect of modifying rheometer fixtures with cartilage surface coatings on the effective viscosity of HA solutions. Our results demonstrate up to a 20-fold increase in effective viscosity when HA was confined between cartilage surfaces compared to steel surfaces. For low-molecular-weight HA, the effective viscosity was dependent on the gap height between the rheometer plates, which is consistent with the formation of a viscous boundary film. Together, these results indicate that this method for assessing HA viscosity may be more relevant to lubrication than traditional methods and may provide a more accurate method for predicting the viscosity of HA viscosupplements in vivo where HA is able to interact with the cartilage surface.

Uncovering friction dynamics using hydrogel particles as soft ball bearings

Rolling ball bearings are widely known and applied to decrease friction between two surfaces. More recently, hydrogel-hydrogel tribopairs have also revealed good but rather complex lubrication properties. Here, we use hydrogels as ball bearings to elucidate that soft spherical particles have nontrivial rate-dependent lubrication behavior. Unlike Newtonian lubrication or dry solid friction, hydrogel particles in suspension transition through four frictional regimes as a function of sliding velocity. We relate the different regimes to the deformation of the particles at different gap sizes, which changes the effective contact area between the sliding surfaces. By systematically varying the particle characteristics and the surface properties of the sliding surfaces, we assign potential mechanisms for each of the different lubricating regimes as a function of velocity: (I) relatively high friction due to particle flattening and direct contact between interacting bodies (II) decrease of friction owing to the presence of rolling particles (III) large inflow of particles in a confined space leading to compressed particles and (IV) the formation of a thick lubricating layer. Using these suspensions with soft, deformable particles as a ball bearing system, we provide new insights into soft material friction with applications in emulsions, powders, pastes or other granular materials.

Distinct tribological endotypes of pathological human synovial fluid reveal characteristic biomarkers and variation in efficacy of viscosupplementation at reducing local strains in articular cartilage

OBJECTIVE

Viscosupplementation has been used for decades to treat mild to moderate osteoarthritis, yet it is unknown if the lubricating function of different pathological synovial fluids (SF) vary, or if they respond differentially to viscosupplementation. The objectives of this study were to (i) evaluate the friction coefficients and induced shear strains in articular cartilage when lubricated with pathological SF, (ii) identify the effect of hyaluronic acid (HA) supplementation on friction coefficients and shear strains, and (iii) identify SF biomarkers that correlate with lubricating function.

METHOD

Human pathological SF was grouped by white blood cell count (inflammatory: >2000 cells/mm³, n = 6; non-inflammatory: <2000 cells/mm³, n = 6). Compositional analyses for lubricin and cytokines were performed. Friction coefficients and local tissue shear strain measurements were coupled using new, microscale rheological analyses by lubricating neonatal bovine cartilage explants with SF alone and in a 1:1 ratio with HA (Hymovis®).

RESULTS

Friction coefficients were not significantly different between the inflammatory and non-inflammatory pathologies (p = 0.09), and were poorly correlated with peak tissue strains at the cartilage articular surface (R² = 0.34). A subset of inflammatory SF samples induced higher tissue strains, and HA supplementation was most effective at lowering friction and tissue strains in this inflammatory subset. Across all pathologies there were clear relationships between polymorphonuclear neutrophil (PMN), IL-8, and lubricin concentrations with cartilage tissue strains.

CONCLUSION

These results suggest that pathological SF is characterized by distinct tribological endotypes where SF lubricating behaviors are differentially modified by viscosupplementation and are identifiable by biomarkers.

A Century of Cartilage Tribology Research Is Informing Lubrication Therapies

Articular cartilage is one of the most unique materials found in nature. This tissue's ability to provide low friction and low wear over decades of constant use is not surpassed, as of yet, by any synthetic materials. Lubrication of the body's joints is essential to mammalian locomotion, but breakdown and degeneration of cartilage is the leading cause of severe disability in the industrialized world. In this paper, we review how theories of cartilage lubrication have evolved over the past decades and connect how theories of cartilage lubrication have been translated to lubrication-based therapies. Here, we call upon these historical perspectives and highlight the open questions in cartilage lubrication research. Additionally, these open questions within the field's understanding of natural lubrication mechanisms reveal strategic directions for lubrication therapy.

Effect of Lubricin Mimetics on the Inhibition of Osteoarthritis in a Rat Anterior Cruciate Ligament Transection Model

BACKGROUND

Lubricin, a mucinous glycoprotein, plays a chondroprotective role as a constituent of synovial fluid. Structural analogs have been synthesized to mimic the structure and function of native lubricin in an effort to recapitulate this effect with the goal of delaying progression of osteoarthritis (OA).

PURPOSE

To investigate the efficacy of intra-articular injections of lubricin mimetics in slowing or preventing the progression of posttraumatic OA by using a rat anterior cruciate ligament transection model.

STUDY DESIGN

Controlled laboratory design.

METHODS

Four lubricin mimetics were investigated, differing from one another in their binding orientations and steric interactions. Eighty skeletally mature Sprague-Dawley rats underwent bilateral anterior cruciate ligament transections and were randomly allocated to receive intra-articular injections (50 µL/injection) of 1 of the 4 mimetics in the right knee and equal volumes of saline injection in the contralateral knee (control). All rats were euthanized 8 weeks postoperatively and assessed via biomechanical analysis, which evaluated comparative friction coefficients across the 4 groups, and histological evaluation of articular cartilage, osteophytes, and synovitis. The Osteoarthritis Research Society International (OARSI) histopathological assessment system was used to evaluate the degree of articular cartilage degeneration and osteophytes, while synovitis was assessed through a semiquantitative scoring system. Binding efficacy of the 4 mimetics was assessed in vitro and in vivo through the immunohistochemical localization of polyethylene glycol. Articular cartilage degeneration and synovitis scoring data analyses were performed with generalized estimating equation modeling.

RESULTS

Injection of the group 3 mimetic (random 24 + 400 + 30) directly correlated with improved OARSI scores for femoral articular cartilage degeneration when compared with saline-injected contralateral control knees (P = .0410). No lubricin mimetic group demonstrated statistically significant differences in OARSI scores for tibial articular cartilage degeneration. Injection of the group 4 mimetic (AB 24 + 400 + 30) led to a statistically significant difference in osteophyte OARSI score (P = .0019). None of the 4 lubricin mimetics injections incited an additive synovial inflammatory response. Immunohistochemical staining substantiated the binding capacity of all 4 mimetics, while in vivo experimentation revealed that the group 1 and 3 mimetics were still retained within the joint 4 weeks after injection. There were no differences in friction coefficients between any pair of groups and no significant trends based on lubricin mimetic structure.

CONCLUSION

We demonstrated that the tribosupplementation of a traumatically injured knee with a specific lubricin structural analog may attenuate the natural progression of OA.

CLINICAL RELEVANCE

The current lack of efficacious clinical options to counter the onset and subsequent development of OA suggests that further investigation into the synthesis and behavior of lubricin analogs could yield novel translational applications.

Influence of Block Length on Articular Cartilage Lubrication with a Diblock Bottle-Brush Copolymer

We report how the tribological properties of a class of diblock copolymers with architecture and function inspired by the lubricating glycoprotein lubricin correlate to chemical composition. This class of diblock copolymers, consisting of a cationic cartilage-binding block and a brush-lubricating block, demonstrates that boundary lubrication of articular cartilage more strongly depends on the cartilage-binding block than the lubrication block. Specifically, the cartilage-binding functional groups (tertiary or quaternary amines) and cartilage-binding block length significantly influence the degree of lubrication under boundary mode experimental conditions. An optimal number (∼24 in this case) of cartilage-binding groups led to the lowest coefficient of friction, and an increase or decrease in the number of cations in the binding block led to partial (>24, and between 12 and 24) or complete (=12) loss of lubricating ability. The length of the lubricating block (DP = 200 or 400) chosen in this study had no effect on the degree of lubrication. These results are put into context in terms of binding affinity to the cartilage and the spatial packing density of the polymer on the cartilage surface and can serve as a useful guide for future designs of synthetic lubricants that rival the efficacy of natural lubricants.

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.

Dynamics of Synovial Fluid Aggregation under Shear

The synovial fluid (SF) that lubricates articular joints exhibits complex rheological and tribological properties due to the interactions and behaviors of its various molecular components. Under shear, SF films abruptly thicken by more than 300% and large, dense aggregates form within the fluid. In this study, we used the Surface Force Apparatus to elucidate which SF components are involved in this shear-induced transformation by (i) determining which (if any) of all major SF components replicate the behavior of SF under shear and (ii) observing the effect of removing implicated components from SF by enzymatic digestion. While most previous studies of SF have focused on the tribological roles of lubricin or hyaluronic acid, our results indicate that albumin is a key contributor to the formation of aggregates in SF under shear. Our results also suggest that SF aggregation is associated with efficient surface protection against wear. As our findings are based on experiments involving rigid, nonporous surfaces, they may be used to investigate shear-mediated aggregation mechanisms occurring during the lubrication of artificial joints, ultimately advancing our current vision of implant design.

Synergy between Zwitterionic Polymers and Hyaluronic Acid Enhances Antifouling Performance

Challenges associated with nonspecific adsorption of proteins on sensor surfaces have steered the development of novel antifouling materials and strategies. Inspired by human synovial fluid composition and structure, we designed synergistic antifouling coatings with mixtures of hyaluronic acid (HA) and a zwitterionic bottlebrush polymer (BB). Using a fast and convenient online surface modification method, the polymers were immobilized on the Au surface, significantly increasing its hydrophilicity. Using surface plasmon resonance (SPR), a 10:1 ratio of HA to BB was found optimal to provide the best antifouling performance. Bovine serum albumin (BSA) adsorption on HA-BB coated surfaces was 0.2 ng/cm², which was 60 times lower than BB or HA alone and 25 times lower than the commonly accepted ultralow adsorption limit (<5 ng/cm²), demonstrating the synergistic effect of HA and BB against nonspecific protein adsorption. This was found to be independent of BSA concentration up to physiological concentrations. Furthermore, the antifouling performance of HA-BB coated surfaces was tested against milk and serum, showing almost 92% lower protein adsorption than that on bare surfaces, suggesting the potential efficacy of this antifouling coating in real life settings.

The clot thickens: Autologous and allogeneic fibrin sealants are mechanically equivalent in an ex vivo model of cartilage repair

Fibrin sealants are commonly used in cartilage repair surgeries to adhere cells or grafts into a cartilage defect. Both autologous and commercial allogeneic fibrin sealants are used in cartilage repair surgeries, yet there are no studies characterizing and comparing the mechanical properties of fibrin sealants from all-autologous sources. The objectives of this study were to investigate (i) the effect of fibrinogen and thrombin sources on failure mechanics of sealants, and (ii) how sealants affect the adhesion of particulated cartilage graft material (BioCartilage) to surrounding cartilage under physiological loading. Allogeneic thrombin and fibrinogen were purchased (Tisseel), and autologous sources were prepared from platelet-rich plasma (PRP) and platelet-poor plasma (PPP) generated from human blood. To compare failure characteristics, sealants were sandwiched between cartilage explants and pulled to failure. The effect of sealant on the adhesion of BioCartilage graft to cartilage was determined by quantifying microscale strains at the graft-cartilage interface using an in vitro cartilage defect model subjected to shear loading at physiological strains well below failure thresholds. Fibrinogen sources were not equivalent; PRP fibrinogen created sealants that were more brittle, failed at lower strains, and resulted in sustained higher strains through the graft-cartilage interface depth compared to PPP and allogeneic sources. PPP clotted slower compared to PRP, suggesting PPP may percolate deeper into the repair to provide more stability through the tissue depth. There was no difference in bulk failure properties or microscale strains at the graft-cartilage interface between the purely autologous sealant (autologous thrombin + PPP fibrinogen) and the commercial allogeneic sealant. Clinical Significance: All-autologous fibrin sealants fabricated with PPP have comparable adhesion strength as commercial allogeneic sealants in vitro, whereas PRP creates an inferior all-autologous sealant that sustains higher strains through the graft-cartilage interface depth.

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.

A Systematic Review and Guide to Mechanical Testing for Articular Cartilage Tissue Engineering

Articular cartilage is integral to the mechanical function of many joints in the body. When injured, cartilage lacks the capacity to self-heal, and thus, therapies and replacements have been developed in recent decades to treat damaged cartilage. Given that the primary function of articular cartilage is mechanical in nature, rigorous physical evaluation of cartilage tissues undergoing treatment and cartilage constructs intended for replacement is an absolute necessity. With the large number of groups developing cartilage tissue engineering strategies, however, a variety of mechanical testing protocols have been reported in the literature. This lack of consensus in testing methods makes comparison between studies difficult at times, and can lead to misinterpretation of data relative to native tissue. Therefore, the purpose of this study was to systematically review mechanical testing of articular cartilage and cartilage repair constructs over the past 10 years (January 2009-December 2018), to highlight the most common testing configurations, and to identify key testing parameters. For the most common tests, key parameters identified in this systematic review were validated by characterizing both cartilage tissue and hydrogels commonly used in cartilage tissue engineering. Our findings show that compression testing was the most common test performed (80.2%; 158/197), followed by evaluation of frictional properties (18.8%; 37/197). Upon further review of those studies performing compression testing, the various modes (ramp, stress relaxation, creep, dynamic) and testing configurations (unconfined, confined, in situ) are described and systematically reviewed for parameters, including strain rate, equilibrium time, and maximum strain. This systematic analysis revealed considerable variability in testing methods. Our validation testing studies showed that such variations in testing criteria could have large implications on reported outcome parameters (e.g., modulus) and the interpretation of findings from these studies. This analysis is carried out for all common testing methods, followed by a discussion of less common trends and directions in the mechanical evaluation of cartilage tissues and constructs. Overall, this work may serve as a guide for cartilage tissue engineers seeking to rigorously evaluate the physical properties of their novel treatment strategies. Impact Statement Articular cartilage tissue engineering has made significant strides with regard to treatments and replacements for injured tissue. The evaluation of these approaches typically involves mechanical testing, yet the plethora of testing techniques makes comparisons between studies difficult, and often leads to misinterpretation of data compared with native tissue. This study serves as a guide for the mechanical testing of cartilage tissues and constructs, highlighting recent trends in test conditions and validating these common procedures. Cartilage tissue engineers, especially those unfamiliar with mechanical testing protocols, will benefit from this study in their quest to physically evaluate novel treatment and regeneration approaches.

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.

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.