Origins of hydration lubrication

The non-monotonic effect of sucrose on interactions between lipid-bearing surfaces

The extremely low sliding friction of articular cartilage in synovial joints has been attributed to phospholipid boundary layers, lubricating via the hydration lubrication mechanism at their exposed, highly hydrated polar-head-groups, in a medium - the synovial fluid - where osmolytes, which may modify the hydration layer, are ubiquitous. Here, using a surface force balance (SFB), we carried out a systematic study to elucidate the effect of sucrose, a known osmotic regulator solute, with concentrations csucrose, ranging from 5 to 20 wt%, on the normal and shear forces between interacting phosphatidylcholine (PC) bilayers, both in the gel (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) and liquid (1,2-dimyristoyl-sn-glycero-3-phosphocholine, DMPC) phases, supported on atomically-smooth mica substrates. Several additional approaches including cryo-transmission electron microscope, atomic force microscopy, small- and wide-angle X-ray scattering, differential scanning calorimetry, dynamic light scattering and zeta potential measurements are exploited to get additional insight into the nature of the sucrose-dependent interactions. As csucrose is varied, a remarkable variation in the friction is observed: a marked reduction in friction is seen at low csucrose, but at higher sucrose levels the friction increases, for both gel and liquid phase lipids. This challenges the expectation that hydration lubrication is degraded by osmotic solutes, due to their competing for water of hydration, and reveals for the first time a non-monotonic effect of a sugar on the interactions, particularly frictional forces, between lipid bilayers. This non-monotonic effect correlates with the bilayer potential, and is attributed to a concentration-dependent affinity of the sugar to the PC headgroups.

Local mapping of the nanoscale viscoelastic properties of fluid membranes by AFM nanorheology

Biological membranes are intrinsically dynamic entities that continually adapt their biophysical properties and molecular organisation to support cellular function. Current microscopy techniques can derive high-resolution structural information of labelled molecules but quantifying the associated viscoelastic behaviour with nanometre precision remains challenging. Here, we develop an approach based on atomic force microscopy in conjunction with fast nano-actuators to map the viscoelastic response of unlabelled supported membranes with nanometre spatial resolution. On fluid membranes, we show that the method can quantify local variations in the molecular mobility of the lipids and derive a diffusion coefficient. We confirm our experimental approach with molecular dynamics simulations, also highlighting the role played by the water at the interface with the membrane on the measurement. Probing ternary model bilayers reveals spatial correlations in the local diffusion over distances of ≈20 nm within liquid disordered domains. This lateral correlation is enhanced in native bovine lens membranes, where the inclusion of protein-rich domains induces four-fold variations in the diffusion coefficient across < 100 nm of the fluid regions, consistent with biological function. Our findings suggest that diffusion is highly localised in fluid biomembranes.

Nanoparticle Lubricant and Imaging Agent: Preventing and Assessing Cartilage Tissue Damage

Introducing additives to industrial lubricants reduces friction and wear between articulating metal surfaces by mechanistically impeding interfacial adhesion, heat dissipation, and abrasion. With this inspiration, we report the synthesis and use of tantalum oxide (Ta2O5) nanoparticles as a nanolubricant and tribosupplement (i.e., tribology-augmenting agent) for articular cartilage. Further, as tantalum oxide absorbs X-rays, the nanolubricant is also a contrast-agent for computed tomography (CT). These dual purpose nanoparticles, decorated with a short poly (ethylene glycol) and cationic trimethylammonium silane coating, suspend in aqueous fluid to form a CT active nanolubricant. In an ex vivo cartilage-on-cartilage model, the nanolubricant outperforms the clinical standard, Synvisc-One, as a viscosupplement during high load, low velocity sliding associated with low Hersey numbers and high static friction. Differential diffusion of the nanolubricants into healthy and degraded cartilage demonstrates the diagnostic capability of the nanolubricant to also distinguish disease state by μCT.

Cartilage Lubrication from the Perspective of Wettability

Cartilage exhibits an extremely low friction and very low wearability within the liquid environment of the joint. It is also capable of switching wettability between superhydrophilicity and hydrophobicity in both wetting and dry conditions (specific experimental operations or open wounds). Therefore, the understanding of cartilage lubrication from the perspective of wettability provides inspiration for the design of artificial cartilage and sections with motion of soft actuators with extremely low coefficients of friction (COF). In this review, the lubrication of articular cartilage is introduced and discussed from the view of wettability. First, basic principles of articular cartilage lubrication and wettability are described with a focus on compositions and wettability of articular cartilage, and in particular the relationship between the phospholipid layers and wettability on articular cartilage, and the supramolecular synergy of synovial fluid on the lubrication of articular cartilage. Subsequently, the wettability and lubrication of articular cartilage under different stimuli (such as shear, pH, temperature, and electric field) is introduced for insights into cartilage lubrication. Finally, we present a comprehensive summary and delineate the challenges within the domain of cartilage lubrication and wettability for assisting researchers in formulating viable concepts for the design of efficient cartilage substitution or smart soft lubricating devices.

Liposomic lubricants suppress acute inflammatory gene regulation in the joint in vivo

Osteoarthritis (OA) is a widespread, debilitating joint disease associated with articular cartilage degradation. It is driven via mechano-inflammatory pathways, whereby catabolic genes in the cartilage-embedded chondrocytes are presumed up-regulated due to increased shear stress arising from friction at the cartilage surface as joints articulate. The enhanced expression of these cartilage-degrading and inflammatory genes leads to tissue degeneration. However, the nature of the stress, and how the cells within the joint respond to it, are poorly understood. Here we show, in a proof of concept study on a mouse model where surgical joint destabilisation has been carried out to induce OA, that the early up-regulation of the matrix metalloproteinase 3 (Mmp3) gene, a member of the matrix-degrading MMP family, and of the interleukin-1 beta (Il1b) gene, a key mediator of inflammatory response, are significantly suppressed when lipid-based lubricants are injected into the joints. We attribute this to the reduction in frictional stress on the chondrocytes due to the lubricant at the cartilage surface. At the same time, Timp1, a compression but not shear-stress sensitive gene, is unaffected by lubricant. Our results demonstrate that cartilage lubrication modulates catabolic gene regulation in OA, shed strong light on the nature of the chondrocytes' response to shear stress, and have clear implications for novel OA treatments. STATEMENT OF SIGNIFICANCE: Osteoarthritis (OA) is a widespread, debilitating joint disease associated with degradation of the articular cartilage, the tissue that covers and protects the joint surfaces as they rotate. Such degradation is due to catabolic enzymes expressed by cartilage-embedded chondrocytes (the only cell type in cartilage) in response to mechanical stress. In this proof-of-concept study in a mouse OA model, we show that reduction of cartilage friction by liposome-based lubricants suppresses the production of the catabolic, OA-related genes in chondrocytes. Our findings provide direct evidence in an animal model that catabolic genes are induced in chondrocytes in a mechanosensitive manner, related to the friction at the cartilage surface, and identify putative novel OA treatments through efficient cartilage lubrication.

Toward low-friction and high-adhesion solutions: Emerging strategies for nanofibrous scaffolds in articular cartilage engineering

Aging, trauma, pathology, and poor natural tissue regeneration are the leading causes of osteoarthritis (OA), an articular cartilage disease. Electrospun scaffolds have gained attention as potential matrices for the treatment of OA because of their high degree of ECM mimicry, which suits chondrocyte migration, adhesion, and proliferation. However, none of the products recently introduced in the market are nanofiber-based. This study aimed to review the scope and tribology of nanofibrous articular cartilage scaffolds. Herein, we briefly discuss cartilage lubrication and strategies for promoting cell adhesion in electrospun materials. Next, we discuss the emerging need to study the biotribological properties of scaffolds. Finally, we review new perspectives on surface functionalization, surface segregation, Janus membranes, layer-by-layer fabrication, and nanofibrous composites. We conclude that cell adhesion and low-friction conciliation remain poorly explored in the recent literature. The topic intersection might create novelties in the field.

Liposome-Based Interventions in Knee Osteoarthritis

Osteoarthritis (OA) is the most common degenerative disease of the joints, causing significant disability and socio-economic burden in the aging population. Simultaneously, however, it is a common occurrence in younger individuals, initiated by joint injuries or obesity alongside other factors. Intravenous and oral pharmaceutical OA management have both been associated with systemic adverse effects, thereby resulting in a growing interest in intra-articular (IA) treatment. IA-administered drugs circumvent the requirement for high dosage, offering immediate access to the site of interest while minimizing any unfavorable effects. Nonetheless, IA-injected drugs, administered in their free form, present low retention time in the knee joint raising the need for multiple injection dosage regimens, while their capability to target the cartilage or specific cell populations is limited. Liposomes, due to their unique characteristics and tunable nature, have proven to be excellent candidates for the management of knee OA. This review explores the last decade's research on the efficacy of various IA liposomal formulations, investigating their multifaceted properties as pharmaceutical carriers, lubricating agents, and a basis for combinatorial approaches paving the way to novel treatment solutions for OA.

Superwetting Gels: Wetting Principles, Applications, and Challenges

Along with the in-depth understanding of wetting behaviors in nature, superwetting gels have received a lot of attention in the past decade. The viscoelasticity of gel materials makes wetting characteristics different from those of rigid materials and brings diverse functionality. In this Review, we summarize the current progress in principles of gel wettability from two aspects: wetting on gels and wetting of gels. Distinct from rigid substrates, the viscoelasticity and solid-liquid coexistence of gel materials introduce additional factors, including surface tension and deformation, resulting in various wetting phenomena. Besides, the similarity between gels and tissues broadens its applications in biomedical devices and smart interfacial regulation. We further conclude the current application that utilizes superwetting gels. Finally, we provide our perspective for future research directions.

Unfolding of von Willebrand Factor Type D Like Domains Promotes Mucin Adhesion

Mucins are the macromolecular key components of mucus. On wet epithelia of mammals, mucin solutions and gels act as powerful biolubricants and reduce friction and wear by generating a sacrificial layer and establishing hydration lubrication. Yet the structure-function relationship of mucin adhesion and lubrication remains elusive. We study the adhesion behavior of mucin using atomic force microscopy-based single molecule force spectroscopy with covalently attached, lab-purified salivary MUC5B and gastric MUC5AC. We can resolve the structural motifs mediating adhesion on chemically distinct substrates, such as highly oriented pyrolytic graphite and steel. We report on force-induced partial unfolding of the von Willebrand factor type D like domains and deliver their unfolding rates and free energy barriers. These domains serve to dissipate energy during the desorption process of mucins. Partial mucin unfolding might significantly contribute to the stability of a sacrificial mucin layer during shearing processes, enhancing the lubrication potential of mucin solutions.

A motion-responsive injectable lubricative hydrogel for efficient Achilles tendon adhesion prevention

Achilles tendon is a motor organ that is prone to tissue adhesion during its repair process after rupture. Therefore, developing motion-responsive and anti-adhesive biomaterials is an important need for the repair of Achilles tendon rupture. Here, we report an injectable lubricative hydrogel (ILH) based on hydration lubrication mechanism, which is also motion-responsive based on sol-gel reversible transmission. The lubrication performance is achieved by zwitterionic polymers as we previously proved, and the sol-gel reversible transmission is enabled by dynamic disulfide bonds. Firstly, ILH was proved to be successfully prepared and lubricated as well as sol-gel reversible via FTIR characterization, rheological measurement and tribological tests. Then, in vitro cell experiments and coagulation tests demonstrated the optimal cytocompatibility and hemocompatibility of ILH. To evaluate the potential of ILH's biofunction in vivo, SD rats' Achilles tendon rupture & repair model was established. The animal experiments' results showed that ILH significantly prevented tendon adhesion and thus promote tendon healing by inhibiting TGFβ1-Smad2/3 pathway. We believe this work will open a new horizon for tendon adhesion-free repair.

Potential-dependent superlubricity of stainless steel and Au(111) using a water-in-surface-active ionic liquid mixture

HYPOTHESIS

The friction and interfacial nanostructure of a water-in-surface-active ionic liquid mixture, 1.6 M 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate ([BMIm][AOT]), can be tuned by applying potential on Au(111) and stainless steel.

EXPERIMENTAL

Atomic force microscopy (AFM) was used to examine the friction and interfacial nanostructure of 1.6 M [BMIm][AOT] on Au(111) and stainless steel at different potentials.

FINDINGS

Superlubricity (vanishing friction) is observed for both surfaces at OCP+1.0 V up to a surface-dependent critical normal force due to [AOT]- bilayers adsorbing strongly to the positively charged surface thus allowing AFM tip to slide over solution-facing hydrated anion charged groups. High-resolution AFM imaging reveals ripple-like features within near-surface layers, with the smallest amplitudes at OCP+1 V, indicating the highest structural stability and resistance to thermal fluctuations due to highly ordered boundary [AOT]- bilayers templating robust near-surface layers. Exceeding the critical normal force at OCP+1.0 V causes the AFM tip to penetrate the hydrated [AOT]- layer and slide over alkyl chains, increasing friction. At OCP and OCP-1.0 V, higher friction correlates with more pronounced ripples, attributed to the rougher templating [BMIm]+ boundary layer. Kinetic experiments show that switching from OCP-1.0 V to OCP+1.0 V achieves superlubricity within 15 s, enabling real-time friction control.

Earthworm inspired lubricant self-pumping hydrogel with sustained lubricity at high loading

The development of mechanically robust super-lubrication hydrogel materials with sustained lubricity at high contact pressures is challenging. In this work, inspired by the durable lubricity feature of the earthworm epidermis, a multilevel structural super-lubrication hydrogel (MS-SLH) system, the so-called lubricant self-pumping hydrogel, is developed. The MS-SLH system is manufactured by chemically dissociating a double network hydrogel to generate robust and wrinkled lubrication layer, and then laser etching was used to generate cylindrical texture pores as gland-like pockets for storing lubricants. The surface of MS-SLH system shows ultrafast hydration characteristics and reversible pore-closing and pore-opening behavior. The current MS-SLH system shows excellent SL features, as follows: a very low COF (~0.0079) at high contact pressure condition (P: 11.32 MPa); a stable and robust SL lifespan (COF: ~0.0028, P: 8.48 MPa, 100k cylces) without surface wear; and a sustained lubricity period (3700 cycles) with limited lubricant volume (5 μL) in air. The robust and sustained lubricity of the MS-SLH system is likely attributed to the synergy from the strong electrostatic repulsion effect at the sliding interface, the robust but compliant modulus of the dissociation lubrication layer, and the self-pumping lubricant release from the gland-like pocket of the texture pores during the dynamic shearing process. The demonstration experiments based on self-built equipments intuitively exhibit durable SL behavior of MS-SLH system. This work provides an easy strategy for the large-scale manufacture of high-performance water-lubrication coatings suitable for high-end medical devices or moving parts.

Peculiar Effect of Water on Tribological Properties of Natural Deep Eutectic Solvent

Deep eutectic solvents (DESs) have emerged as promising green liquid lubricants for solving tribological problems due to their economic and excellent physicochemical and lubrication properties. However, a trace amount of water would affect their structure, physicochemical properties, and tribological properties. The effect of water on the tribological properties of DES is still unclear and needs further investigate. Herein, we carried out a systematic investigation into the chemical structure, rheological properties, and tribological performance of DES-water (DES-W) binary systems constructed by combining DES with varying contents of water. The results revealed that low levels of water in DES had a minimal impact on its chemical structure but affected its fluidity and viscosity. Frictional experiments demonstrated that DES-W binary systems displayed a reduced coefficient of friction from 0.094 to 0.025 compared to pure DESs and manifested outstanding antiwear properties under a high-load condition. This was attributed to the formation of hydration layers, adsorption layers, and tribochemical films at the tribointerface through physicochemical adsorption and tribochemical reactions. Our findings not only foster the design and development of green lubricating materials but also expand the engineering applications of DESs to solve wear-related mechanical failures in practical application.

Macroscale Superlubrication Achieved with Shear-Thinning Semisolid Lubricants

Macrosuperlubric materials are pivotal for reducing friction and wear in engineering applications. However, current solid superlubricants require intricate fabrication and specific conditions (e.g., vacuum or inert atmospheres), while liquid superlubricants are prone to creep, leakage, and corrosion. Here, a novel semisolid subnanometer nanowire (SNW) superlubrication material based on the shear-thinning effect is introduced to overcome these challenges. The SNWs achieve an exceptionally low friction coefficient (0.008-0.009) with silicon nitride (Si3N4) and polytetrafluoroethylene (PTFE) tribo-pairs, demonstrating a brief running-in period (≈39 s) and stable superlubrication over extended friction (12 h, >120 000 cycles). The combination of the shear-thinning network structure mechanism, the adsorption membrane mechanism, and hydrodynamic effects provides a synergistic effect, playing a crucial role in achieving superlubricity. Additionally, SNWs can be combined with various base oils to create semisolid gel lubricants with superlubricating properties. This innovative approach addresses the limitations of current superlubrication systems and introduces a new category of semisolid gel lubricants, significantly expanding the applications of superlubrication materials.

Cell-inspired, massive electromodulation of friction via transmembrane fields across lipid bilayers

Transient electric fields across cell bilayer membranes can lead to electroporation and cell fusion, effects crucial to cell viability whose biological implications have been extensively studied. However, little is known about these behaviours in a materials context. Here we find that transmembrane electric fields can lead to a massive, reversible modulation of the sliding friction between surfaces coated with lipid-bilayer membranes-a 200-fold variation, up to two orders of magnitude greater than that achieved to date. Atomistic simulations reveal that the transverse fields, resembling those at cell membranes, lead to fully reversible electroporation of the confined bilayers and the formation of inter-bilayer bridges analogous to the stalks preceding intermembrane fusion. These increase the interfacial dissipation through reduced hydration at the slip plane, forcing it to revert in part from the low-dissipation, hydrated lipid-headgroup plane to the intra-bilayer, high-dissipation acyl tail interface. Our results demonstrate that lipid bilayers under transmembrane electric fields can have striking materials modification properties.

Ion-specific ice provides a facile approach for reducing ice friction

HYPOTHESIS

Ice friction plays a crucial role in both basic study and practical use. Various strategies for controlling ice friction have been developed. However, one unsolved puzzle regarding ice friction is the effect of ion-ice interplay on its tribological properties.

EXPERIMENTS AND SIMULATIONS

Here, we conducted ice friction experiments and summarized the specific effects of hydrated ions on ice friction. By selecting cations and anions, the coefficient of ice friction can be reduced by more than 70 percent. Experimental spectra, low-field nuclear magnetic resonance (LF-NMR), density functional theory (DFT) calculations, and Molecular dynamics (MD) simulations demonstrated that the addition of ions could break the H-bonds in water.

FINDINGS

The link between the charge density of ions and the coefficients of ice friction was revealed. A part of the ice structure was changed from an ice-like to a liquid-like interfacial water structure with the addition of ions. Lower charge density ions led to weaker ionic forces with the water molecules in the immobilized water layer, resulting in free water molecules increasing in the lubricating layer. This study provides guidance for preparing ice-making solutions with low friction coefficients and a fuller understanding of the interfacial water structure at low temperatures.

Network interactions simultaneously enhance stiffness and lubricity of triple-network hydrogels

Synthetic hydrogels displaying cartilage-mimetic bulk and surface properties may serve as cartilage substitutes. Multi-network, electrostatic hydrogels that leverage intra- and inter-network repulsive and attractive forces represent a promising approach. Herein, triple network (TN) hydrogels were prepared to obtain a combination of desired characteristics (i.e., hydration, stiffness, shear stress, and friction properties). The TN hydrogels were comprised of a negatively charged 1st network and a neutral 2nd network possessing hydrophobic associations. Presumed to significantly influence surface properties, the 3rd network charge was systematically varied as cationic, anionic, and zwitterionic. A double-network (DN) hydrogel, comprised of the same 1st and 2nd network as for the TN hydrogels, was included as a control as well as native cartilage specimens. Micro-indentation was performed with a steel ball, yielding stiffness values as well as the contact area during sliding. The lubrication in both deionized (DI) water and fetal bovine serum (FBS) was evaluated with the micro-indenter wherein the stage reciprocated in a range of speeds. All the TN hydrogels exhibited greater Youngs modulus than the DN hydrogel control. The TN bearing a cationic 3rd network exhibited an exceptionally high Youngs modulus of ≈1.4 MPa, which was even higher than that of the cartilage samples. In both DI water and FBS, for most testing speeds, the TN hydrogels exhibited lower friction coefficient (COF) values and lower shear stresses than DN hydrogel as well as the native cartilage specimens.

Inspired by lubricin: a tailored cartilage-armor with durable lubricity and autophagy-activated antioxidation for targeted therapy of osteoarthritis

Osteoarthritis (OA), which disables articular cartilage, affects millions of people. The self-healing capacity is inhibited by internal oxidative stress and external lubrication deficiency and enzymatic degradation. To overcome these challenges, a tailored cartilage-armor is designed to ameliorate the inflamed cartilage, which is implemented by a novel collagen type II (Col II)-binding peptide conjugated zwitterionic polymer (PSB-b-PColBP, PSP). By mimicking natural lubricin, PSP specifically targets the cartilage surface and forms an in situ hydration armor. This engineered cartilage-armor can prevent enzymatic cartilage degradation (nearly 100% resistance to catabolic enzymes) and provide durable lubrication properties (COF < 0.013 for 500 cycles). An autophagy-activation process, absent in previous biomimetic lubricants, enhances the enzymatic activity of the tailored cartilage-armor, offering effective anti-oxidant properties to suppress oxidative stress. By inhibiting the PI3K-Akt/NF-κB signaling pathway, chondrocytes protected by the tailored armor can secrete a cartilage matrix even in inflammatory microenvironments. In OA rat models, osteophyte formation and the inflammatory response have been inhibited by the cartilage-armor, demonstrating a therapeutic effect comparable to most drug-loaded systems. This study underscores the potential of tailoring cartilage-armor with the cartilage targeting and autophagy-activating properties in integrating offensive-defensive mechanisms for cartilage remodeling. This represents an alternative strategy for clinical OA therapy.

An injectable self-lubricating supramolecular polymer hydrogel loaded with platelet lysate to boost osteoarthritis treatment

Globally, osteoarthritis (OA) is the most prevalent joint disease and is characterized by infiltration of M1 macrophages in the synovium, anabolic-catabolic imbalance of the extracellular matrix (ECM), increased articular shear force and overproduction of reactive oxygen species (ROS). Disease-modifying OA drugs are not yet available, and treatments for OA focus solely on reducing pain and inflammation and have limited therapeutic effect. Herein, we developed an injectable self-lubricating poly(N-acryloyl alaninamide) (PNAAA) hydrogel loaded with platelet lysate (PL) (termed "PNAAA@PL") for treating OA. Tribological and drug release tests revealed suitable lubrication properties and sustained release of bioactive factors in PNAAA@PL. In vitro experiments showed that PNAAA@PL alleviated interleukin-1β (IL-1β)-induced anabolic-catabolic imbalance of chondrocytes and repolarized pro-inflammatory M1 macrophages to the anti-inflammatory M2 phenotype via intracellular ROS scavenging. Additionally, the PNAAA@PL hydrogel enhanced the migratory capacity and chemotaxis ability of stem cells, which are essential for chondrogenesis. In vivo, the functionalized PNAAA@PL hydrogel acted like synovial fluid following intra-articular injection into a rat OA model with anterior cruciate ligament transection, ultimately attenuating cartilage degeneration and synovitis. According to molecular mechanism studies, PNAAA@PL repairs cartilage in the OA model by inhibiting the NF-ĸB pathway. Overall, this self-lubricating PNAAA@PL hydrogel offers a comprehensive strategy for preventing OA progression by engineering a biophysiochemical microenvironment to generate high-quality hyaline cartilage.

Cell volume regulation modulates macrophage-related inflammatory responses via JAK/STAT signaling pathways

Cell volume as a characteristic of changes in response to external environmental cues has been shown to control the fate of stem cells. However, its influence on macrophage behavior and macrophage-mediated inflammatory responses have rarely been explored. Herein, through mediating the volume of macrophages by adding polyethylene glycol (PEG), we demonstrated the feasibility of fine-tuning cell volume to regulate macrophage polarization towards anti-inflammatory phenotypes, thereby enabling to reverse macrophage-mediated inflammation response. Specifically, lower the volume of primary macrophages can induce both resting macrophages (M0) and stimulated pro-inflammatory macrophages (M1) to up-regulate the expression of anti-inflammatory factors and down-regulate pro-inflammatory factors. Further mechanistic investigation revealed that macrophage polarization resulting from changing cell volume might be mediated by JAK/STAT signaling pathway evidenced by the transcription sequencing analysis. We further propose to apply this strategy for the treatment of arthritis via direct introduction of PEG into the joint cavity to modulate synovial macrophage-related inflammation. Our preliminary results verified the credibility and effectiveness of this treatment evidenced by the significant inhibition of cartilage destruction and synovitis at early stage. In general, our results suggest that cell volume can be a biophysical regulatory factor to control macrophage polarization and potentially medicate inflammatory response, thereby providing a potential facile and effective therapy for modulating macrophage mediated inflammatory responses. STATEMENT OF SIGNIFICANCE: Cell volume has recently been recognized as a significantly important biophysical signal in regulating cellular functionalities and even steering cell fate. Herein, through mediating the volume of macrophages by adding polyethylene glycol (PEG), we demonstrated the feasibility of fine-tuning cell volume to induce M1 pro-inflammatory macrophages to polarize towards anti-inflammatory M2 phenotype, and this immunomodulatory effect may be mediated by the JAK/STAT signaling pathway. We also proposed the feasible applications of this PEG-induced volume regulation approach towards the treatment of osteoarthritis (OA), wherein our preliminary results implied an effective alleviation of early synovitis. Our study on macrophage polarization mediated by cell volume may open up new pathways for immune regulation through microenvironmental biophysical clues.

Bridging Microscopic Dynamics and Hydraulic Permeability in Mechanically-Deformed Nanoporous Materials

In the field of nanoconfined fluids, there are striking examples of deformation/transport coupling in which mechanical solicitation of the confining solid and dynamics of the confined fluid impact each other. While this intriguing behavior can be harnessed for applications (e.g., energy storage, phase separation, catalysis), the underlying mechanisms remain to be understood. Here, using molecular simulations, we investigate fluid flow in deformable nanoporous materials subjected to external mechanical stresses. We show that the pore mechanical properties significantly affect fluid flow as they lead to significant pore deformations and different fluid organization at the solid surface. Despite such mechanical effects, we show that the fluid thermodynamic properties (i.e., adsorption) can be linked consistently to Darcy's law for the permeability by invoking a pore size definition based on the concept of Gibbs' dividing surface. In particular, regardless of the solid stiffness and applied external stress, all data can be rationalized by accounting for the fluid viscosity and slippage at the solid surface (independently of a specific pore size definition). Using such a formalism, we establish that the intimate relation─derived using the linear response theory─between collective diffusivity and hydraulic permeability remains valid. This allows linking consistently microscopic dynamics experiments and macroscopic permeability experiments on fluid flow in deformable nanoporous materials.

Self-assembly of sustainable plant protein protofilaments into a hydrogel for ultra-low friction across length scales

Designing plant protein-based aqueous lubricants can be of great potential to achieve sustainability objectives by capitalising on inherent functional groups without using synthetic chemicals; however, such a concept remains in its infancy. Here, we engineer a class of self-assembled sustainable materials by using plant-based protofilaments and their assembly within a biopolymeric hydrogel giving rise to a distinct patchy architecture. By leveraging physical interactions, this material offers superlubricity with friction coefficients of 0.004-to-0.00007 achieved under moderate-to-high (102-to-103 kPa) contact pressures. Multiscale experimental measurements combined with molecular dynamics simulations reveal an intriguing synergistic mechanism behind such ultra-low friction - where the uncoated areas of the protofilaments glue to the surface by hydrophobic interactions, whilst the hydrogel offers the hydration lubrication. The current approach establishes a robust platform towards unlocking an untapped potential of using plant protein-based building blocks across diverse applications where achieving superlubricity and environmental sustainability are key performance indicators.

Counterion Distribution in the Stern Layer on Charged Surfaces

Counterion adsorption at the solid-liquid interface affects numerous applications. However, the counterion adsorption density in the Stern layer has remained poorly evaluated. Here we report the direct determination of surface charge density at the shear plane between the Stern layer and the diffuse layer. By the Grahame equation extension and streaming current measurements for different solid surfaces in different aqueous electrolytes, we are able to obtain the counterion adsorption density in the Stern layer, which is mainly related to the surface charge density but is less affected by the bulk ion concentration. The charge inversion concentration is further found to be sensitive to the ion type and ion valence rather than to the charged surface, which is attributed to the ionic competitive adsorption and ion-ion correlations. Our findings offer a framework for understanding ion distribution in many physical and chemical processes where the Stern layer is ubiquitous.

Theory of electrotuneable mechanical force of solid-liquid interfaces: A self-consistent treatment of short-range van der Waals forces and long-range electrostatic forces

Elucidating the mechanical forces between two solid surfaces immersed in a communal liquid environment is crucial for understanding and controlling adhesion, friction, and electrochemistry in many technologies. Although traditional models can adequately describe long-range mechanical forces, they require substantial modifications in the nanometric region where electronic effects become important. A hybrid quantum-classical model is employed herein to investigate the separation-dependent disjoining pressure between two metal surfaces immersed in an electrolyte solution under potential control. We find that the pressure between surfaces transits from a long-range electrostatic interaction, attractive or repulsive depending on the charging conditions of surfaces, to a strong short-range van der Waals attraction and then an even strong Pauli repulsion due to the redistribution of electrons. The underlying mechanism of the transition, especially the attractive-repulsive one in the short-range region, is elucidated. This work contributes to the understanding of electrotunable friction and lubrication in a liquid environment.

Electrochemical Investigation of the Stability of Poly-Phosphocholinated Liposomes

Poly[2-(methacryloyloxy)ethyl phosphorylcholine] liposomes (pMPC liposomes) gained attention during the last few years because of their potential use in treating osteoarthritis. pMPC liposomes that serve as boundary lubricants are intended to restore the natural lubrication properties of articular cartilage. For this purpose, it is important that the liposomes remain intact and do not fuse and spread as a lipid film on the cartilage surface. Here, we investigate the stability of the liposomes and their interaction with two types of solid surfaces, gold and carbon, by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). With the aid of a hydrophilic species used as an electroactive probe in the solution, the charge transfer characteristics of the electrode surfaces are obtained. Additionally, from EIS, the capacitance characteristics of the surfaces are derived. No decrease of the peak currents and no displacement of the peak potentials to greater overpotentials are observed in the CV experiments. No decrease in the apparent capacitance and increase in the charge transfer resistance is observed in the EIS experiments. On the contrary, all parameters in both CV and EIS do change in the opposite direction. The obtained results confirm that there is only physical adsorption without fusion and spreading of the pMPC liposomes and without the formation of lipid films on the surfaces of both gold and carbon electrodes.

Disparate External Electric Field Effect on the Adsorption and Shear Behavior of Monovalent and Trivalent Ions in Electrolyte Solution

Reducing friction is of great interest, and an external potential applied to the friction pair can regulate lubricity. Electrochemical atomic force microscopy (EC-AFM) is used to study the tribological and adsorption behavior of monovalent and trivalent ionic solutions between charged surfaces. An opposite trend of coefficient of friction (COF) and normal force that varies with the applied electric potential is witnessed. Direct force measurements and theoretical models have disclosed that, for the NaCl solution, the negative electric field reduces the COF by increasing cation adsorption. As for LaCl3 solution, the positive electric field promotes the primary adsorption of anions on HOPG, resulting in the disappearance of the attractive ion-ion correlation between the trivalent ions, thereby reducing the COF. The shear behavior of adsorbed ions in electrolyte solution is sensitive to their valence, because of their different surface force contribution. The study further provides a framework to optimize the design of hydration lubrication.

Lubricating Polymer Gels/Coatings: Syntheses and Measurement Strategies

Straightforward design and long-term functionality for tribological considerations has prompted an extensive substitution of polymers for metals across various applications, from industrial machinery to medical devices. Lubrication of and by polymer gels/coatings, essential for ensuring the cost-effective operation and reliability of applications, has gained strong momentum by benefiting from the structural characteristics of natural lubrication systems (such as articular cartilage). The optimal synthetic strategy for lubricating polymer gels/coatings would be a holistic approach, wherein the lubrication mechanism in relation to the structural properties offers a pathway to design tailor-made materials. This review considers recent synthesis strategies for creating lubricating polymer gels/coatings from the molecular level (including polymer brushes, loops, microgels, and hydrogels), and assessing their frictional properties, as well as considering the underlying mechanism of their lubrication.

Novel biomimetic macromolecules system for highly efficient lubrication, ROS scavenging and osteoarthritis treatment

The early stages of osteoarthritis (OA) in the joints are typically characterized by two key factors: the dysfunction of articular cartilage lubrication and inflammation resulting from the excessive production of reactive oxygen species (ROS). Synthetic injectable macromolecular materials present great potential for preventing the progression of early OA. In this study, to mimic the excellent lubricity of brush-like aggregates found in natural synovial fluid, we develop a novel macromolecular biolubricant (CS-PS-DA) by integrating adhesion and hydration groups onto backbone of natural biomacromolecules. CS-PS-DA exhibits a strong affinity for cartilage surfaces, enabling the formation of a stable lubrication layer at the sliding interface of degraded cartilages to restore joint lubrication performance. In vitro results from ROS scavenging and anti-inflammatory experiments indicate the great advantage of CS-PS-DA to decrease the levels of proinflammatory cytokines by inhibiting ROS overproduction. Finally, in vivo rats OA model demonstrates that intra-cavitary injection of CS-PS-DA could effectively resist cartilage wear and mitigated inflammation in the joints. This novel biolubricant provides a new and timely strategy for the treatment of OA.

Fundamental, mechanism and development of hydration lubrication: From bio-inspiration to artificial manufacturing

Friction and lubrication are ubiquitous in all kinds of movements and play a vital role in the smooth operation of production machinery. Water is indispensable both in the lubrication systems of natural organisms and in hydration lubrication systems. There exists a high degree of similarity between these systems, which has driven the development of hydration lubrication from biomimetic to artificial manufacturing. In particular, significant advancements have been made in the understanding of the mechanisms of hydration lubrication over the past 30 years. This enhanced understanding has further stimulated the exploration of biomimetic inspiration from natural hydration lubrication systems, to develop novel artificial hydration lubrication systems that are cost-effective, easily transportable, and possess excellent capability. This review summarizes the recent experimental and theoretical advances in the understanding of hydration-lubrication processes. The entire paper is divided into three parts. Firstly, surface interactions relevant to hydration lubrication are discussed, encompassing topics such as hydrogen bonding, hydration layer, electric double layer force, hydration force, and Stribeck curve. The second part begins with an introduction to articular cartilage in biomaterial lubrication, discussing its compositional structure and lubrication mechanisms. Subsequently, three major categories of bio-inspired artificial manufacturing lubricating material systems are presented, including hydrogels, polymer brushes (e.g., neutral, positive, negative and zwitterionic brushes), hydration lubricant additives (e.g., nano-particles, polymers, ionic liquids), and their related lubrication mechanism is also described. Finally, the challenges and perspectives for hydration lubrication research and materials development are presented.

Mucoadhesive, antioxidant, and lubricant catechol-functionalized poly(phosphobetaine) as biomaterial nanotherapeutics for treating ocular dryness

Dry eye disease (DED) is associated with ocular hyperosmolarity and inflammation. The marketed topical eye drops for DED treatment often lack bioavailability and precorneal residence time. In this study, we investigated catechol-functionalized polyzwitterion p(MPC-co-DMA), composed of 2-methacryloyloxyethyl phosphorylcholine (MPC) and dopamine methacrylamide (DMA) monomers, as potential topical nanotherapeutics for DED. The copolymers were synthesized via random free-radical copolymerization, producing different proportions of catecholic functionalization. All as-prepared polymer compositions displayed good ocular biocompatibility. At a feeding ratio of 1:1, p(MPC1-co-DMA1) can facilitate a robust mucoadhesion via Michael addition and/or Schiff base reaction, thus prolonging ocular residence time after 4 days of topical instillation. The hydration lubrication of MPC and radical-scavenging DMA endow the nano-agent to ease tear-film hyperosmolarity and corneal inflammation. A single dose of p(MPC1-co-DMA1) (1 mg/mL) after 4 days post-instillation can protect the cornea against reactive oxygen species, inhibiting cell apoptosis and the over-expression of pro-inflammatory factors (IL-6 and TNF-α). In clinical assessment, DED-induced rabbit eyes receiving p(MPC1-co-DMA1) could increase lacrimal fluid secretion by 5-fold higher than cyclosporine A. The catechol-functionalized polyzwitterion with enhanced lubricity, mucoadhesion, and anti-oxidation/anti-inflammation properties has shown high promise as a bioactive eye drop formulation for treating DED.

Unraveling Water-Based Lubrication with Carbon Dots of Asphaltene Origin

Despite the lower toxicity of water-based lubricants over nonrenewable petroleum-based analogues, they face challenges in achieving widespread adoption due to low stability and inadequate friction-reduction performance. To address this, a cost-effective nanoadditive is synthesized by expansive oxidation of asphaltenes to create biocompatible asphaltene-derived carbon dots [(ACDs); 5 nm]. These ACDs exhibit excellent water redispersibility, promoting long-term friction reduction and marking the first use of an asphaltene-based system for friction reduction in water or oil. Even at low loadings (0.2-4.0 wt %), ACDs significantly reduce friction on steel surfaces (>54%) with tribofilm stability surpassing pristine carbon dots, typical carbon-based graphene quantum dots, and inorganic nanomaterials (commercial 5 and 20 nm silica). The ACDs' attributes include high negative zeta potential, considerable water uptake, varied functional groups, biocompatibility, and a nanodisc shape conducive to stable tribofilm formation through effective particle stacking. The scalable synthesis, high yield, and impressive water redispersibility of ACDs position them favorably for commercial water-based lubrication.

Injectable Microgels with Hybrid Exosomes of Chondrocyte-Targeted FGF18 Gene-Editing and Self-Renewable Lubrication for Osteoarthritis Therapy

Abnormal silencing of fibroblast growth factor (FGF) signaling significantly contributes to joint dysplasia and osteoarthritis (OA); However, the clinical translation of FGF18-based protein drugs is hindered by their short half-life, low delivery efficiency and the need for repeated articular injections. This study proposes a CRISPR/Cas9-based approach to effectively activate the FGF18 gene of OA chondrocytes at the genome level in vivo, using chondrocyte-affinity peptide (CAP) incorporated hybrid exosomes (CAP/FGF18-hyEXO) loaded with an FGF18-targeted gene-editing tool. Furthermore, CAP/FGF18-hyEXO are encapsulated in methacrylic anhydride-modified hyaluronic (HAMA) hydrogel microspheres via microfluidics and photopolymerization to create an injectable microgel system (CAP/FGF18-hyEXO@HMs) with self-renewable hydration layers to provide persistent lubrication in response to frictional wear. Together, the injectable CAP/FGF18-hyEXO@HMs, combined with in vivo FGF18 gene editing and continuous lubrication, have demonstrated their capacity to synergistically promote cartilage regeneration, decrease inflammation, and prevent ECM degradation both in vitro and in vivo, holding great potential for clinical translation.

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.

The surface force balance: direct measurement of interactions in fluids and soft matter

Over the last half-century, direct measurements of surface forces have been instrumental in the exploration of a multitude of phenomena in liquid, soft, and biological matter. Measurements of van der Waals interactions, electrostatic interactions, hydrophobic interactions, structural forces, depletion forces, and many other effects have checked and challenged theoretical predictions and motivated new models and understanding. The gold-standard instrument for these measurements is thesurface force balance(SFB), orsurface forces apparatus, where interferometry is used to detect the interaction force and distance between two atomically smooth planes, with 0.1 nm resolution, over separations from about 1 µm down to contact. The measured interaction forcevs.distance gives access to the free energy of interaction across the fluid film; a fundamental quantity whose general form and subtle features reveal the underlying molecular and surface interactions and their variation. Motivated by new challenges in emerging fields of research, such as energy storage, biomaterials, non-equilibrium and driven systems, innovations to the apparatus are now clearing the way for new discoveries. It is now possible to measure interaction forces (and free energies) with control of electric field, surface potential, surface chemistry; to measure time-dependent effects; and to determine structurein situ. Here, we provide an overview the operating principles and capabilities of the SFB with particular focus on the recent developments and future possibilities of this remarkable technique.

Highly Concentrated Electrolyte Superlubricants Enhanced by Interfacial Water Competition Around Chemically Active MgO Additives

The low concentration of water-based lubricants and the high chemical inertness of the additives involved are often regarded as basic norms in the design of liquid lubricants. Herein, a novel liquid superlubricant of an aqueous solution containing a relatively high concentration of salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), is reported for the first time, and the superlubricity stability and load-bearing capacity of the optimized system (MgO0.10/LiTFSI10) are effectively strengthened by the addition of only trace (0.10 wt %) water-chemically active MgO additives. It demonstrates higher applicable loads, lower COF (∼0.004), and stability relative to the base solution. Only a trace amount of MgO additive is needed for the superlubricity, which makes up for the cost and environmental deficiencies of LiTFSI10. The weak interaction region between free water and the outer-layer water of Li+ hydration shells becomes a possible ultralow shear resistance sliding interface; the Mg(OH)2 layer, generated by the reaction of MgO with water, further creates additional weakly interacting interfaces, leading to the formation of an asymmetric contact between the clusters/particles within the hydrodynamic film by moderating the competition between interfacial water and free water, thus achieving high load-bearing macroscopic superlubricity. This study deepens the contribution of electrolyte concentration to ionic hydration and superlubricity due to the low shear slip layer formed by interfacial water competition with water-activated solid additives, providing new insights into the next generation of high load-bearing water-based liquid superlubricity systems.

Reprogramming Macrophage Polarization, Depleting ROS by Astaxanthin and Thioketal-Containing Polymers Delivering Rapamycin for Osteoarthritis Treatment

Osteoarthritis (OA) is a chronic joint disease characterized by synovitis and joint cartilage destruction. The severity of OA is highly associated with the imbalance between M1 and M2 synovial macrophages. In this study, a novel strategy is designed to modulate macrophage polarization by reducing intracellular reactive oxygen species (ROS) levels and regulating mitochondrial function. A ROS-responsive polymer is synthesized to self-assemble with astaxanthin and autophagy activator rapamycin to form nanoparticles (NP@PolyRHAPM ). In vitro experiments show that NP@PolyRHAPM significantly reduced intracellular ROS levels. Furthermore, NP@PolyRHAPM restored mitochondrial membrane potential, increased glutathione (GSH) levels, and promoted intracellular autophagy, hence successfully repolarizing M1 macrophages into the M2 phenotype. This repolarization enhanced chondrocyte proliferation and vitality while inhibiting apoptosis. In vivo experiments utilizing an anterior cruciate ligament transection (ACLT)-induced OA mouse model revealed the anti-inflammatory and cartilage-protective effects of NP@PolyRHAPM , effectively mitigating OA progression. Consequently, the findings suggest that intra-articular delivery of ROS-responsive nanocarrier systems holds significant promise as a potential and effective therapeutic strategy for OA treatment.

Correlation between viscoelastic response and frictional properties of hydrated zwitterionic polymer brush film in narrowing shear gap

HYPOTHESIS

A hydrated 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer brush exhibits exceptional lubricity. This lubrication mechanism has traditionally been attributed to either the inherent fluidity of the brush or the water film that forms owing to its hydrophilic nature. Given previous findings that the frictional properties of the MPC polymer brush film show load dependence, we hypothesize that the lubrication mechanism can be elucidated by examining the shear gap (varies owing to the load) dependence of the brush's viscoelastic response.

EXPERIMENTS

MPC polymer brush films with different thicknesses were prepared. Their viscoelastic responses were evaluated across different shear gap widths, and the frictional properties were subsequently compared across states with distinct viscoelastic behaviors.

FINDINGS

The observed shear viscoelasticity demonstrated a clear gap dependence that correlated with frictional attributes. Our data suggests that the lubrication mechanism shifts based on the shear gap. Specifically, two states exhibited low coefficients of friction: one where the osmotic pressure supports the load while allowing flexible deformation of the brush film, and the other where the brush film undergoes compression and transitions to a fully elastic state.

Insight into the hydration friction of lipid bilayers

Hydration layers formed on charged sites play crucial roles in many hydration lubrication systems in aqueous media. However, the underlying molecular mechanism is not well understood. Herein, we explored the hydration friction of lipid bilayers with different charged headgroups at the nanoscale through a combination of frequency-modulation atomic force microscopy and friction force microscopy. The nanoscale friction experiments showed that the hydration friction coefficient and frictional energy dissipation of a cationic lipid (DPTAP) were much lower than those of zwitterionic (DPPE) and anionic (DPPG) lipids. The hydration layer probing at the surfaces of different lipid bilayers clearly revealed the relationship between the charged lipid headgroups and hydration layer structures. Our detailed analysis demonstrated that the cationic lipid had the largest hydration force in comparison with zwitterionic and anionic lipids. These friction and hydration force results indicated that the difference of the lipid headgroup charge resulted in different hydration strengths which led to the difference of hydration friction behaviors. The findings in this study provide molecular insights into the hydration friction of lipid bilayers, which has potential implications for the development of efficient hydration lubrication systems with boundary lipid bilayers in aqueous media.

Thermosensitive Triblock Copolymer for Slow-Release Lubricants under Ocular Conditions

The ocular environment is crucial for a biological lubrication system. An unstable condition of tear film may cause a series of ocular diseases due to serious friction, such as dry eye syndrome, which has drawn extensive attention nowadays. In this study, an in vitro biocompatible superlubricity system, containing thermogelling copolymers (PCGA-PEG-PCGA) and slow-release lubricant (PEG 300/Tween 80), was constructed. First, the sol-gel transition temperature and gel strength of PCGA-PEG-PCGA were adjusted based on the ocular environment by regulating the length of PCGA blocks. Furthermore, the copolymer hydrogel exhibited a reliable slow-release property within 10 days and showed low cytotoxicity. Then, the superlubricity (coefficient of friction of approximately 0.005) was achieved with its released PEG 300/Tween 80 aqueous solution at the sliding velocity range of 1-100 mm s-1 and pressure range of 10-22 kPa. However, the lubrication behaviors varied, while PEG 300 chains and Tween 80 micelles were demonstrated to form a multilayer and a single layer adsorption structure on the sliding surface, respectively. On the whole, the composite lubrication systems, especially the one composed of Tween 80, showed excellent tribological properties owing to the stable slow-release and full hydration effects under ocular conditions, which hold great potential for improving ocular lubrication and maintaining human visual health.

Fabrication and Characterization of Mucin Nanoparticles for Drug Delivery Applications

Mucin glycoproteins are ideal biomacromolecules for drug delivery applications since they naturally offer a plethora of different functional groups that can engage in specific and unspecific binding interactions with cargo molecules. However, to fabricate drug carrier objects from mucins, suitable stabilization mechanisms have to be implemented into the nanoparticle preparation procedure that allow for drug release profiles that match the requirements of the selected cargo molecule and its particular mode of action. Here, we describe two different methods to prepare crosslinked mucin nanoparticles that can release their cargo either on-demand or in a sustained manner. This method chapter includes a description of the preparation and characterization of mucin nanoparticles (stabilized either with synthetic DNA strands or with covalent crosslinks generated by free radical polymerization), as well as protocols to quantify the release of a model drug from those nanoparticles.

Surface forces and friction between Langmuir-Blodgett polymer layers in a nonpolar solvent

Optimization of boundary lubrication by tuning the confined molecular structures formed by surface-active additives such as surfactants and polymers is of key importance to improving energy efficiency in mechanical processes. Here, using the surface forces apparatus (SFA), we have directly measured the normal and shear forces between surface layers of a functionalised olefin copolymer (FOCP) in n-dodecane, deposited onto mica using the Langmuir-Blodgett (LB) technique. The FOCP has an olefin backbone decorated with a statistical distribution of polar-aromatic groups, with a structure that we term as "centipede". The effect of lateral confinement, characterised by the surface pressure, Πdep, at the air-water interface at which the LB films are transferred, was examined. Normal force profiles revealed that the thickness of the LB films increased significantly with Πdep, with the film thickness (t > 20 nm) inferring a multi-layered film structure, consistent with the interfacial characterisation results from synchrotron X-ray reflectivity (XRR) measurements. The coefficient of friction, µ, between the LB films spanned two orders of magnitude from superlubricity (µ ∼ 0.002) to much higher friction (µ > 0.1) depending nonlinearly on Πdep, with the lowest friction observed at the intermediate Πdep. Molecular arrangement upon LB compression leads to the multilayer film with a structure akin to an interfacial gel, with transient crosslinking facilitated by the intra- and inter-molecular interactions between the functional groups. We attribute the differences in frictional behaviour to the different prevalence of the FOCP functional groups at the lubricating interface, which depends sensitively on the degree of compression at the air-water interface prior to the LB deposition. The LB films remain intact after repeated compression (up to pressures of 10 MPa) and shear cycles, indicating strong surface anchorage and structural robustness as a load-bearing and shear-mediating boundary layer. These unprecedented results from the friction measurements between LB films of a statistical copolymer in oil point towards new strategies for tailoring macromolecular architecture for mediating efficient energy dissipation in oil-based tribological applications.

The effect of cations and epigallocatechin gallate on in vitro salivary lubrication

Ionic valency influences oral processing by changing salivary behavior and merits more attention since little is known. In this study, the influence of three ionic valences (monovalent, divalent and trivalent), ionic strength and epigallocatechin gallate (EGCG) on lubricating properties of saliva were investigated. Tribological measurements were used to characterize the lubrication response of KCl, MgCl2, FeCl3, and AlCl3 in combination with EGCG to the ex vivo salivary pellicle. KCl at 150 mM ionic strength provided extra lubrication via hydration lubrication. Contrarily, trivalent salts aggregated together with the salivary mucins via ionic cross-link interactions, which led to a decrease in salivary lubrication. FeCl3 and AlCl3 affected the salivary lubrication differently, which was attributed to changes in the pH. Finally, in presence of EGCG, FeCl3 interacted with EGCG via chelating interactions, preventing salivary protein aggregation. This resulted in less desorption of the salivary film, retaining the lubrication ability of salivary proteins.

Physically cross-linked organo-hydrogels for friction interfaces in joint replacements: design, evaluation and potential clinical applications

This paper investigates physically crosslinked organo-hydrogels for total hip replacement surgery. Current materials in artificial joints have limitations in mechanical performance and biocompatibility. To overcome these issues, a new approach based on hydrogen bonds between polyvinyl alcohol, poly(2-hydroxyethyl methacrylate), and glycerin is proposed to develop bioactive organo-hydrogels with improved mechanical properties and biocompatibility. This study analyzes local pathological characteristics, systemic toxicity, and mechanical properties of the gels. The results show that the gels possess excellent biocompatibility and mechanical strength, suggesting their potential as an alternative material for total hip replacement surgery. These findings contribute to improving patient outcomes in joint replacement procedures.

A polymer network architecture provides superior cushioning and lubrication of soft tissue compared to a linear architecture

We report the relationships between linear vs. network polymer architecture and biomechanical outcomes including lubrication and cushioning when the polymers are applied to the surface of articulating knee cartilage. Aqueous formulations of the bioinspired polymer poly(2-methacryloyloxylethyl phosphorylcholine) (pMPC) exhibit tuneable rheological properties, with network pMPC exhibiting increased elasticity and viscosity compared to linear pMPC. Application of a polymer network, compared to a linear one, to articulating tissue surfaces reduces friction, lessens tissue strain, minimizes wear, and protects tissue - thereby improving overall tissue performance. Administration of the network pMPC to the middle carpal joint of skeletally mature horses elicits a safe response similar to saline as monitored over a 70 day period.

Preparation and Properties of PDMS Surface Coating for Ultra-Low Friction Characteristics

Polydimethylsiloxane (PDMS) has excellent physical-chemical properties and good biocompatibility. Thus, PDMS has been widely applied in biomedical applications. However, the low surface free energy and surface hydrophobicity of PDMS can easily lead to adverse symptoms, such as tissue damage and ulceration, during medical treatment. Therefore, the construction of a hydrophilic low-friction surface on the PDMS surface could be helpful for alleviating patient discomfort and would be of great significance for broadening the application of PDMS in the field of interventional medical catheters. Existing surface modification methods such as hydrogel coatings and chemical grafting suffer from several deficiencies including uncontrollable thickness, surface fragility, and low surface strength. In this study, a hydrophilic surface with ultra-low friction properties was prepared on the surface of PDMS by an ultraviolet light (UV) curing method. The monomer acrylamide (AM) was induced by a photoinitiator to form a coating on the surface of the silicone rubber by in situ polymerization. The surface roughness of the as-prepared coatings was regulated by adding different concentrations of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) to the monomer solution, and the coating properties were systematically characterized. The results indicated that the roughness and thickness of the as-prepared coatings decreased with increasing AMPS concentration and the as-prepared coatings had good hydrophilicity and low-friction properties. The Coefficient of Friction (CoF) was as low as 0.0075 in the deionized water solution, which was 99.7% lower than that of the unmodified PDMS surface. Moreover, the coating with a lower surface roughness exhibited better low-friction properties. The results reported herein provide new insight into the preparation of hydrophilic, low-friction coatings on polymer surfaces.

Ion adsorption and hydration forces: a comparison of crystalline mica vs. amorphous silica surfaces

Hydration forces are ubiquitous in nature and technology. Yet, the characterization of interfacial hydration structures and their dependence on the nature of the substrate and the presence of ions have remained challenging and controversial. We present a systematic study using dynamic Atomic Force Microscopy of hydration forces on mica surfaces and amorphous silica surfaces in aqueous electrolytes containing chloride salts of various alkali and earth alkaline cations of variable concentrations at pH values between 3 and 9. Our measurements with ultra-sharp AFM tips demonstrate the presence of both oscillatory and monotonically decaying hydration forces of very similar strength on both atomically smooth mica and amorphous silica surfaces with a roughness comparable to the size of a water molecule. The characteristic range of the forces is approximately 1 nm, independent of the fluid composition. Force oscillations are consistent with the size of water molecules for all conditions investigated. Weakly hydrated Cs+ ions are the only exception: they disrupt the oscillatory hydration structure and induce attractive monotonic hydration forces. On silica, force oscillations are also smeared out if the size of the AFM tip exceeds the characteristic lateral scale of the surface roughness. The observation of attractive monotonic hydration forces for asymmetric systems suggests opportunities to probe water polarization.

Macroscale Superlubricity of Hydrated Anions in the Boundary Lubrication Regime

Cations can achieve excellent hydration lubrication at smooth interfaces under both microscale and macroscale conditions due to the boundary layer composed of hydration shells surrounding charges, but what about anions? Commonly used friction pairs are negatively charged at the solid/solution interface. Achieving anionic adsorption through constructing positively charged surfaces is a prerequisite for studying the hydration lubrication of anions. Here we report the hydration layer composed of anions adsorbed on the positively charged polymer/sapphire interface at acidic electrolyte solutions with pH below the isoelectric point, which contributes to the hydration lubrication of anions. Strongly hydrated anions (for the case of SO42-) exhibit stable superlubricity comparable to cations, with strikingly low boundary friction coefficient of 0.003-0.007 under contact pressures above 15 MPa without a running-in period. The hydration lubrication performance of anions is determined by both the ionic hydration strength and ion adsorption density based on the surface potential and tribological experiments. The results shed light on the role of anions in superlubricity and hydration lubrication, which may be relevant for understanding the lubrication mechanism and improving lubrication performance in acidic environments, for example, in acid pumps, sealing rings of compressors for handling acidic media, and processing devices of nuclear waste.

A Facile Strategy for the Fabrication of Cell-laden Porous Alginate Hydrogels Based on Two-phase Aqueous Emulsions

Porous alginate hydrogels possess many advantages as cell carriers. However, current pore generation methods require either complex or harsh fabrication processes, toxic components, or extra purification steps, limiting the feasibility and affecting the cellular survival and function. In this study, a simple and cell-friendly approach to generate highly porous cell-laden alginate hydrogels based on two-phase aqueous emulsions is reported. The pre-gel solutions, which contain two immiscible aqueous phases of alginate and caseinate, are crosslinked by calcium ions. The porous structure of the hydrogel construct is formed by subsequently removing the caseinate phase from the ion-crosslinked alginate hydrogel. Those porous alginate hydrogels possess heterogeneous pores around 100 μm and interconnected paths. Human white adipose progenitors (WAPs) encapsulated in these hydrogels self-organize into spheroids and show enhanced viability, proliferation, and adipogenic differentiation, compared to non-porous constructs. As a proof of concept, this porous alginate hydrogel platform is employed to prepare core-shell spheres for coculture of WAPs and colon cancer cells, with WAP clusters distributed around cancer cell aggregates, to investigate cellular crosstalk. This efficacious approach is believed to provide a robust and versatile platform for engineering porous-structured alginate hydrogels for applications as cell carriers and in disease modeling.

Transforming sustainable plant proteins into high performance lubricating microgels

With the resource-intensive meat industry accounting for over 50% of food-linked emissions, plant protein consumption is an inevitable need of the hour. Despite its significance, the key barrier to adoption of plant proteins is their astringent off-sensation, typically associated with high friction and consequently poor lubrication performance. Herein, we demonstrate that by transforming plant proteins into physically cross-linked microgels, it is possible to improve their lubricity remarkably, dependent on their volume fractions, as evidenced by combining tribology using biomimetic tongue-like surface with atomic force microscopy, dynamic light scattering, rheology and adsorption measurements. Experimental findings which are fully supported by numerical modelling reveal that these non-lipidic microgels not only decrease boundary friction by an order of magnitude as compared to native protein but also replicate the lubrication performance of a 20:80 oil/water emulsion. These plant protein microgels offer a much-needed platform to design the next-generation of healthy, palatable and sustainable foods.

Contrasting Changes in Strongly and Weakly Bound Hydration Water of a Protein upon Denaturation

Water is considered integral for the stabilization and function of proteins, which has recently attracted significant attention. However, the microscopic aspects of water ranging up to the second hydration shell, including strongly and weakly bound water at the sub-nanometer scale, are not yet well understood. Here, we combined terahertz spectroscopy, thermal measurements, and infrared spectroscopy to clarify how the strongly and weakly bound hydration water changes upon protein denaturation. With denaturation, that is, the exposure of hydrophobic groups in water and entanglement of hydrophilic groups, the number of strongly bound hydration water decreased, while the number of weakly bound hydration water increased. Even though the constraint of water due to hydrophobic hydration is weak, it extends to the second hydration shell as it is caused by the strengthening of hydrogen bonds between water molecules, which is likely the key microscopic mechanism for the destabilization of the native state due to hydration.