Insight into the Tribological Behavior of Liposomes in Artificial Joints

Bioinspired Interfacial Friction Control: From Chemistry to Structures to Mechanics

Organisms in nature have evolved a variety of surfaces with different tribological properties to adapt to the environment. By studying, understanding, and summarizing the friction and lubrication regulation phenomena of typical surfaces in nature, researchers have proposed various biomimetic friction regulation theories and methods to guide the development of new lubrication materials and lubrication systems. The design strategies for biomimetic friction/lubrication materials and systems mainly include the chemistry, surface structure, and mechanics. With the deepening understanding of the mechanism of biomimetic lubrication and the increasing application requirements, the design strategy of multi-strategy coupling has gradually become the center of attention for researchers. This paper focuses on the interfacial chemistry, surface structure, and surface mechanics of a single regulatory strategy and multi-strategy coupling approach. Based on the common biological friction regulation mechanism in nature, this paper reviews the research progress on biomimetic friction/lubrication materials in recent years, discusses and analyzes the single and coupled design strategies as well as their advantages and disadvantages, and describes the design concepts, working mechanisms, application prospects, and current problems of such materials. Finally, the development direction of biomimetic friction lubrication materials is prospected.

Insight into the assembly of lipid-hyaluronan complexes in osteoarthritic conditions

Interactions between molecules in the synovial fluid and the cartilage surface may play a vital role in the formation of adsorbed films that contribute to the low friction of cartilage boundary lubrication. Osteoarthritis (OA) is the most common degenerative joint disease. Previous studies have shown that in OA-diseased joints, hyaluronan (HA) not only breaks down resulting in a much lower molecular weight (MW), but also its concentration is reduced ten times. Here, we have investigated the structural changes of lipid-HA complexes as a function of HA concentration and MW to simulate the physiologically relevant conditions that exist in healthy and diseased joints. Small angle neutron scattering and dynamic light scattering were used to determine the structure of HA-lipid vesicles in bulk solution, while a combination of atomic force microscopy and quartz crystal microbalance was applied to study their assembly on a gold surface. We infer a significant influence of both MW and HA concentrations on the structure of HA-lipid complexes in bulk and assembled on a gold surface. Our results suggest that low MW HA cannot form an amorphous layer on the gold surface, which is expected to negatively impact the mechanical integrity and longevity of the boundary layer and could contribute to the increased wear of the cartilage that has been reported in joints diseased with OA.

Recent advances in superlubricity of liposomes for biomedical applications

Achieving superlubricity, a state of lubrication where friction nearly vanishes, has become one of the most promising approaches to combat friction-induced energy dissipation and medical device failure. Phospholipids are amphiphilic molecules comprising highly hydrophilic phosphatidylcholine head groups as well as hydrophobic hydrocarbon chains, When solubilized, phospholipids can readily self-assemble to form different structures such as bilayers and vesicles (liposomes). Recently, liposomes have been identified as excellent lubricants, especially in the boundary lubrication regime the most common lubrication status in the field of biotribology. In this review, we summarize recent progress in employing liposomes as key players for employing superlubricity in biomedical applications. The relationship between lipids and liposomes, manufacturing approaches, lubrication regimes, and regulation mechanisms of liposomes are discussed. Finally, we indicate possible future directions for the use of liposome-mediated superlubricity in biomedical applications.

Engineering Hyaluronic Acid for the Development of New Treatment Strategies for Osteoarthritis

Osteoarthritis (OA) is a degenerative joint disease that is characterized by inflammation of the joints, degradation of cartilage, and the remodeling of other joint tissues. Due to the absence of disease-modifying drugs for OA, current clinical treatment options are often only effective at slowing down disease progression and focus mainly on pain management. The field of tissue engineering has therefore been focusing on developing strategies that could be used not only to alleviate symptoms of OA but also to regenerate the damaged tissue. Hyaluronic acid (HA), an integral component of both the synovial fluid and articular cartilage, has gained widespread usage in developing hydrogels that deliver cells and biomolecules to the OA joint thanks to its biocompatibility and ability to support cell growth and the chondrogenic differentiation of encapsulated stem cells, providing binding sites for growth factors. Tissue-engineering strategies have further attempted to improve the role of HA as an OA therapeutic by developing diverse modified HA delivery platforms for enhanced joint retention and controlled drug release. This review summarizes recent advances in developing HA-based hydrogels for OA treatment and provides additional insights into how HA-based therapeutics could be further improved to maximize their potential as a viable treatment option for OA.

The wormlike micelles formed using an ionic liquid surfactant and polar organic solvents at low temperature without additives and their lubricant properties

Wormlike micelles (or reverse wormlike micelles) are flexible cylindrical chains that are normally formed in water (or a nonpolar organic solvent) at 25.0 °C or above; the formation of wormlike micelles at lower temperatures is rare. Here, we have reported wormlike micelles formed at low temperature using an ionic liquid surfactant (1-octadecyl-3-nonyl imidazolium bromide) in polar organic solvents (including 1,3-propanediol, 1,2-propylene glycol, N,N-dimethylformamide, and glycerol/1,2-propylene glycol mixture) in the absence of any additives. The viscoelasticity and morphology of the wormlike micelles were studied using rheology, small-angle X-ray scattering, and cryo-transmission electron microscopy. The viscoelastic properties of the wormlike micelles in polar solvents are affected by the solvent type (or the weight ratio of glycerol to 1,2-propylene glycol), surfactant concentration, and temperature. Moreover, the G' and G'' crossover twice in the dynamic curves, which is different from the case in water. The first crossover (at low frequency) corresponds to the relaxation time for the alkyl chains to disentangle from the transient network, and the second crossover (at high frequency) is related to the segmental motion of the chains. Furthermore, the tribological performance of these wormlike micelles is investigated at low temperature. It is found that the protective film (formed by the physical adhesion of the wormlike micelles on the surface of friction disk pair) and the tribochemical reaction together lead to good antifriction and antiwear performance, which indicates the application prospects of these wormlike micelles in low-temperature lubrication.

Lipid-Bilayer Assemblies on Polymer-Bearing Surfaces: The Nature of the Slip Plane in Asymmetric Boundary Lubrication

Phospholipid-macromolecule complexes have been proposed to form highly efficient, lubricating boundary layers at artificial soft surfaces or at biological surfaces such as articular cartilage, where the friction reduction is attributed to the hydration lubrication mechanism acting at the exposed, hydrated head groups of the lipids. Here we measure, using a surface force balance, the normal and frictional interactions between model mica substrates across several different configurations of phosphatidylcholine (PC) lipid aggregates and adsorbed polymer (PEO) layers, to provide insight into the nature of such lubricating boundary layers in both symmetric and especially asymmetric configurations. Our results reveal that, irrespective of the configuration, the slip plane between the sliding surfaces reverts wherever possible to a bilayer-bilayer interface where hydration lubrication reduces the friction strongly. Where such an interface is not available, the sliding friction remains high. These findings may account for the low friction observed between both biological and synthetic hydrogel surfaces which may be asymmetrically coated with lipid-based boundary layers and fully support the hydration lubrication mechanism attributed to act at such boundary layers.

Designer Nanoparticles as Robust Superlubrication Vectors

Phosphatidylcholine lipid bilayers or liposomes at interfaces in aqueous environments can provide extremely efficient lubrication. This is attributed to the hydration lubrication mechanism acting at the highly hydrated phosphocholine-headgroup layers exposed at the outer surface of each bilayer. Micelles exposing such phosphocholine groups could be an attractive alternative to liposomes due to their much easier preparation and structure control, but all studies to date of surfactant micelles have revealed that at relatively low normal stresses the surface layers rupture and friction increases abruptly. Here, we examine surface interactions between three kinds of phosphocholine-exposing micelles with different designed structures: single-tail surfactant micelles, homo-oligomeric micelles, and block copolymer micelles. Normal and shear forces between mica surfaces immersed in solutions of these micelles were measured using a surface force balance. The adsorbed layers on the mica were imaged using atomic force microscope, revealing surface structures ranging from wormlike to spherical micelles. The block copolymer micelles showed relatively low coverage arising from their stabilizing corona and consequently poor lubrication (μ ∼ 10-1). In contrast, the surfactant and homo-oligomeric micelles fully covered the mica surface and demonstrated excellent lubrication (μ ∼ O(10-3)). However, while the boundary layer of single-tailed surfactant micelles degraded under moderate pressure, the homo-oligomeric micellar boundary layer was robust at all applied contact pressures in our study (up to about 5 MPa). We attribute the difference to the much greater energy required to remove a homo-oligomeric molecule from its micelle, resulting in far greater stability under pressure and shear.

Injectable Lipid-Based Depot Formulations: Where Do We Stand?

The remarkable number of new molecular entities approved per year as parenteral drugs, such as biologics and complex active pharmaceutical ingredients, calls for innovative and tunable drug delivery systems. Besides making these classes of drugs available in the body, injectable depot formulations offer the unique advantage in the parenteral world of reducing the number of required injections, thus increasing effectiveness as well as patient compliance. To date, a plethora of excipients has been proposed to formulate depot systems, and among those, lipids stand out due to their unique biocompatibility properties and safety profile. Looking at the several long-acting drug delivery systems based on lipids designed so far, a legitimate question may arise: How far away are we from an ideal depot formulation? Here, we review sustained release lipid-based platforms developed in the last 5 years, namely oil-based solutions, liposomal systems, in situ forming systems, solid particles, and implants, and we critically discuss the requirements for an ideal depot formulation with respect to the used excipients, biocompatibility, and the challenges presented by the manufacturing process. Finally, we delve into lights and shadows originating from the current setups of in vitro release assays developed with the aim of assessing the translational potential of depot injectables.

Friction reduction mechanism of the cuticle surface in the sandhopper talitrus saltator (Amphipoda, talitridae)

In many cases, strong friction reduction is critical for success of both living organisms and engineering systems. Some arthropods exhibit good antifriction abilities in their specific living environments and have inspired many innovations for solving industry challenges. However, the current literature mainly focused on terrestrial insects, such as beetles, grasshoppers and katydids. The antifriction mechanisms in amphibious arthropods are still unknown, even if their surfaces are optimized for both air and water environments. Herein the tribological properties of the cuticle surface of the sandhopper Talitrus saltator were studied using a universal microtribometer. Further investigations were developed to identify the microstructural, compositional, wettability, and mechanical properties of the sandhopper shell cuticles. It was found that increasing normal force can significantly reduce the coefficient of friction of the shell cuticle, especially for the alive and rewet sandhopper shells. The shell consists of bottle-like nano-caves in its exocuticle, nano-tubes in its mesocuticle, and gauze-like multilayers in its endocuticle. Under physiological conditions, glycoprotein-like fluid fillings exist in both the bottle-like caves and the nano-tubes below and cover on the shell surface. More importantly, a new antifriction mechanism of lubricant-squeezing nano-porous system was established for the sandhopper shell. This work can deepen our understanding in antifriction surfaces of amphibiotic crustaceans, and provide a potential approach to resolve the friction challenge in micro-machines, especially for the applications under aqueous condition. STATEMENT OF SIGNIFICANCE: Friction regulation is one of the critical mechanisms for animal locomotion in natural environments. However, not much is known about the mechanism of amphibious arthropods to reduce friction between their body and diverse environments, particularly achieving adaption under both air and aqueous conditions. We quantitatively study the microstructural, compositional and mechanical properties of the sandhopper (Talitrus saltator) shell cuticle and tribological behaviors under different conditions. Our results reveal the nano-porous system with fluid fillings for the sandhopper's shell and demonstrate the potential antifriction mechanism of this amphibious animal. We anticipate this work will inspire some effective antifriction designs for micro-machines, especially for their applications in complex environment like human body.

Glucosamine sulphate-loaded distearoyl phosphocholine liposomes for osteoarthritis treatment: combination of sustained drug release and improved lubrication

Osteoarthritis (OA) is a chronic joint disease resulting from joint inflammation and damage. In this study, we employed a boundary lubricant known as a 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) liposome for loading of an anti-inflammatory drug d-glucosamine sulphate (GAS) to construct a treatment strategy allowing for sustained anti-inflammation and reduced damage. This kind of drug-loaded nanocarrier integrates the anti-inflammatory effect of the GAS and the lubrication ability of DSPC liposomes without the involvement of complex synthesis processes leading to easier popularization. Our experimental results indicated that the GAS-loaded DSPC liposomes could release GAS in a sustained manner while providing good lubrication in pure water (H2O) and phosphate buffered saline (PBS). Moreover, the GAS-loaded DSPC liposomes prepared at a 2 : 8 molar ratio in PBS exhibited a greater entrapment efficiency, lower GAS release rate and smaller friction coefficient as compared to those prepared in H2O. The superiority of the drug release and lubrication ability achieved with the GAS-loaded DSPC liposomes in PBS were elucidated on the basis of salt-induced enhancement in liposomal stability and hydration lubrication by the hydrated salt ions. Such GAS release accelerated the viability and proliferation of primary mouse chondrocytes while also providing the anti-inflammatory and chondroprotective potential for tumor necrosis factor (TNF-α) induced chondrocyte degeneration through the down-regulation of pro-inflammatory cytokines, pain related gene and catabolic proteases, as well as the up-regulation of anabolic components. We envision that the GAS-loaded DSPC liposomes could represent a promising new strategy for clinical treatment of OA in the future.

Surface Interactions between Boundary Layers of Poly(ethylene oxide)-Liposome Complexes: Lubrication, Bridging, and Selective Ligation †

Poly(ethylene oxide), PEO, is widely exploited in biomedical applications, while phosphatidylcholine (PC) lipids (in the form of bilayers or liposomes) have been identified as very efficient boundary lubricants in aqueous media. Here we examine, using a surface force balance (SFB), the interactions between surface-adsorbed layers of PEO complexed with small unilamellar vesicles (SUVs, i.e. liposomes) or with bilayers of PC lipids, both well below and a little above their main gel-to-liquid phase-transition temperatures T_M. The morphology of PEO layers (adsorbed onto mica), to which liposomes were added, was examined using atomic force microscopy (AFM) and cryo-scanning electron microscopy (cryo-SEM). Our results reveal that the PC lipids could attach to the PEO either as vesicles or as bilayers, depending on whether they were above or below T_M. Under water (no added salt), excellent lubrication, with friction coefficients down to 10^-3-10^-4, up to contact stresses of 6.5 MPa (comparable to those in the major joints) was observed between two surfaces bearing such PEO-PC complexes. At 0.1 M KNO₃ salt concentration (comparable to physiological salt levels), the friction between such surfaces was considerably higher, attributed to bridging by the polymer chains. Remarkably, such bridging could be suppressed and the friction could be restored to its previous low value if the KNO₃ was replaced with NaNO₃, as a result of the different PEO-mica ligation properties of Na⁺ compared to those of K⁺. Our results provide insight into the properties of PEO-PC complexes in potential applications, and large interfacial effects that can result from the seemingly innocuous replacement of K⁺ by Na⁺ ions.

Zwitterionic Hydrogel Incorporated Graphene Oxide Nanosheets with Improved Strength and Lubricity

Graphene oxide (GO) has been evaluated as a multifunctional cross-linker or reinforcement agent in composite hydrogels. In this study, a nanocomposite hydrogel consisting of GO nanosheets and zwitterionic poly(sulfobetaine methacrylate) (PSBMA) was synthesized in an aqueous system via chemical and physical cross-linking effects. GO nanosheets were well dispersed in the hydrogels and effectively cross-linked into the sulfobetaine methacrylate (SBMA) polymer chains through the electrostatic interactions. The PSBMA hydrogel exhibited a significant enhancement in the compressive stress (close to a 5-fold increase) and a remarkable reduction in the coefficient of friction (COF) (corresponding to a decline of 52-76%) after the embedding of GO nanosheets. These improvements indicate the existence of synergetic interaction and good compatibility between GO nanosheets and the PSBMA hydrogel matrix, which results in an intertwined network structure with higher load-bearing capacity and better lubrication properties. This study provides potential in the development of new graphene-polymer composites, which is beneficial for cartilage replacement with high mechanical properties and excellent lubrication characteristics.

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.

Phospholipid-Coated Mesoporous Silica Nanoparticles Acting as Lubricating Drug Nanocarriers

Osteoarthritis (OA) is a severe disease caused by wear and inflammation of joints. In this study, phospholipid-coated mesoporous silica nanoparticles (MSNs@lip) were prepared in order to treat OA at an early stage. The phospholipid layer has excellent lubrication capability in aqueous media due to the hydration lubrication mechanism, while mesoporous silica nanoparticles (MSNs) act as effective drug nanocarriers. The MSNs@lip were characterized by scanning electron microscope, transmission electron microscope, Fourier transform infrared spectrum, X-ray photoelectron spectrum, thermogravimetric analysis and dynamic light scattering techniques to confirm that the phospholipid layer was coated onto the surface of MSNs successfully. A series of tribological tests were performed under different experimental conditions, and the results showed that MSNs@lip with multi-layers of phospholipids greatly reduced the friction coefficient in comparison with MSNs. Additionally, MSNs@lip demonstrated sustained drug release behavior and were biocompatible based on CCK-8 assay using MC3T3-E1 cells. The MSNs@lip developed in the present study, acting as effective lubricating drug nanocarriers, may represent a promising strategy to treat early stage OA by lubrication enhancement and drug delivery therapy.

Investigation on the Nanomechanics of Liposome Adsorption on Titanium Alloys: Temperature and Loading Effects

The mechanical properties of liposomes, determined by the lipid phase state at ambient temperature, have a close relationship with their physiological activities. Here, atomic force microscopy (AFM) was used to produce images and perform force measurements on titanium alloys at two adsorbed temperatures. The mechanical properties were evaluated under repeated loading and unloading, suggesting a better reversibility and resistance of gel phase liposomes. The liquid phase liposomes were irreversibly damaged during the first approach while the gel phase liposomes could bear more iterations, resulting from water flow reversibly going across the membranes. The statistical data offered strong evidence that the lipid membranes in the gel phase are robust enough to resist the tip penetration, mainly due to their orderly organization and strong hydrophobic interactions between lipid molecules. This work regarding the mechanical properties of liposomes with different phases provides guidance for future clinical applications, such as artificial joints.

Regulation Mechanism of Salt Ions for Superlubricity of Hydrophilic Polymer Cross-Linked Networks on Ti6Al4V

Poly(vinylphosphonic acid) (PVPA) cross-linked networks on Ti₆Al₄V show superlubricity behavior when sliding against polytetrafluoroethylene in water-based lubricants. The superlubricity can occur but only with the existence of salt ions in the polymer cross-linked networks. This is different from the phenomenon in most polymer brushes. An investigation into the mechanism revealed that cations and anions in the lubricants worked together to yield the superlubricity even under harsh conditions. It is proposed that the preferential interactions of cations with PVPA molecules rather than water molecules are the main reason for the superlubricity in water-based lubricants. The interaction of anions with water molecules regulates the properties of the tribological interfaces, which influences the magnitude of the friction coefficient. Owing to the novel cross-linked networks and the interactions between cations and polymer molecules, their superlubricity can be maintained even at a high salt ion concentration of 5 M. These excellent properties make PVPA-modified Ti₆Al₄V a potential candidate for application in artificial implants.