Hydration Lubrication Research Articles

Sliding-Induced Rehydration in Hydrogels for Restoring Lubrication and Anticreeping Capability.

Fluid exudation in cartilage under normal loading can be counteracted by a sliding-induced rehydration phenomenon, which has a hydrodynamic origin related to a wedge effect at the contact inlet. Similar to cartilage, hydrogels also exhibit tribological rehydration properties, and we mimic this phenomenon to restore hydration lubrication and overcome creeping. It occurs within a specific velocity range and is mainly dependent on the applied load and hydrogel network structures. Crucially, a certain velocity in the mixed lubrication regime can produce a hydrodynamic pressure peak at the wedge and drive the rehydration inflow to overcome the extrusion. At lower sliding velocities in the boundary lubrication regime, inflows are insufficient to counteract fluid exudation, whereas at higher velocities in the hydrodynamic lubrication regime, the inlet wedge effect would diminish. These results suggest that tribological rehydration offers a novel approach to enhancing load-bearing capacity and maintaining lubrication in the hydrogels.

Enhanced creaminess of plant-based milk via enrichment of papain hydrolyzed oleosomes

There is an increased consumer demand for plant-based milk in substituting dairy milk due to the ethical, health concerns and environmentally-friendly choice. However, perceived creaminess as dominant attributes present a big challenge in consumer acceptance for those milk alternatives. In this study, we developed a novel and easily scalable strategy to enhance the creaminess of soy milk via enrichment of oleosomes. The soybean oleosome creams were extracted and hydrolyzed with papain, resulting in formation of oil droplets with more phospholipid and less protein at the surface, which significantly reduce friction coefficient in the presence of saliva (from 0.15 to 0.03 at a speed around 50 mm/s). Moreover, blending papain-hydrolyzed oleosome creams with raw soy milk enables the creation of a plant-based milk that matches the nutritional profile, lubrication properties, and creaminess of full-fat dairy milk. QCM-D and passive microrheology were employed to characterize hydration of oleosomes into the mucin layer and relevant viscosity change, suggesting that papain hydrolyzed oleosome might decrease friction coefficient via hydration lubrication mechanism. This approach could be applied to enhance the creaminess mouthfeel and nutritional profile of plant-based milk.

Carbon fiber/nano SiO2 reinforced polyelectrolyte-graft UHMWPE for water lubricated superlubricity

In this study, a new kind of UHMWPE composite with ultralow coefficient of friction (COF) and high mechanical properties was developed. An efficient method was established to graft hydrophilic monomer 3-sulfopropyl methacrylate potassium salt (SPMK) onto UHMWPE powder by thermal initiation radical polymerization. The COF of UHMWPE grafted SPMK sample was significantly reduced by 50 %. After filling carbon fiber and nano SiO2, the COF of UHMWPE composites can be reduced as low as 0.009 at 0.1 m/s. Besides, a 76.1 % decrease in wear rate of single UHMWPE was achieved. The outstanding tribological properties of UHMWPE composite was attributed to the synergistic effect of hydration lubrication and particle reinforcement. The prepared UHMWPE composite has great potential to realize water lubricated bearing.

A bio-inspired Janus hydrogel patch facilitates oral ulcer repair by combining prolonged wet adhesion and lubrication

Oral ulcers, the most common type of mucosal lesion, are both highly prevalent and prone to recurrence. In the persistently moist environment of the oral cavity, current therapeutic patches face challenges such as short adhesion time, disruption by food particles and bacteria, and oral movements. To address these challenges, we develop a Janus patch, named ANSB, inspired by the multi-layered and asymmetric structure of natural mucosa, featuring a long-lasting adhesive layer and a lubricating layer. By eliminating the salivary barrier and leveraging covalent crosslinking between tissue surface amine groups and N-hydroxysuccinimide ester (NHS), the adhesive layer—composed of gelatin and acrylic acid—achieves rapid (≤ 30 s), strong (≥ 45 kPa), and durable (≥ 8 h) adhesion to wet buccal tissues. Furthermore, the efficient lubricating effect (COF = 0.02 ± 0.003) provided by zwitterions renders the lubricating layer of ANSB highly similar to natural mucosal tissue, effectively preventing bacterial invasion, secondary damage, and unintended adhesion. Additionally, the strong interlayer bonding and complementary mechanical properties are confirmed, resulting in a unified performance characterized by rapid wet adhesion, hydration lubrication, and enhanced mechanical strength. Importantly, ANSB treatment demonstrates a long-term protective barrier and superior therapeutic effects in rat oral ulcers, inhibiting pseudomembrane formation and accelerating tissue regeneration without causing secondary damage. Consequently, this distinctive Janus patch, characterized by prolonged adhesion, efficient lubrication, and simple preparation, holds significant potential for clinical oral ulcer treatment. Statement of SignificanceOral ulcers, with healing impeded by secondary injury and bacterial invasion due to the absence of protective barriers, are highly prevalent and recurrent. However, sustaining therapeutic materials and physical barriers in the highly dynamic and moist oral environment poses a considerable challenge. In this study, a Janus patch (ANSB) that integrated a soft wet adhesive layer and a tough lubricating layer was developed to reconstruct a new protective barrier thus preventing external stimuli such as secondary damage and bacterial infiltration. This innovative patch with high therapeutic potential for oral ulcers may offer a new way for ulcer treatment based on barrier protection.

Thirst and the influence of ionic concentration; an investigation into the effect on salivary lubrication and the role of MUC5B

Saliva plays a crucial role in lubricating the oral cavity and maintaining a normal hydrated mouthfeel, with the main macromolecule in saliva, MUC5B, playing a significant role in its lubricating properties. Mucins form a hydrated biogel that covers the oral epithelium and forms part of the mucosal pellicle, maintaining proper hydration. This study investigates the relationship between ionic concentration, saliva lubrication, and the perception of oral dryness. At baseline, sodium was found to be ten times more concentrated in the mucosal pellicle compared to bulk saliva, in contrast to potassium which was only 1.5 times more concentrated in the pellicle. Increasing concentrations of mouthwashes containing sodium or potassium indicated a preferential sequestering of sodium in the mucosal pellicle with sodium chloride rinses being associated with an increased thirst perception. We therefore propose that sodium binds to MUC5B, reducing the electrostatic repulsions between its negatively charged moieties. This interaction subsequently leads to a compaction of the mucin structure, which decreases the hydration of MUC5B and reduces its lubricating properties. We determined that increased sodium concentrations compact the mucin structure as indicated by transmission electron microscopy. However, we found that increased sodium concentrations were associated with improved the lubrication of salivary and MUC5B films in an oral tribology mimic, decreasing the coefficient of friction from 0.044 to 0.018, and 0.416 to 0.100 with the addition of 200 mM sodium chloride for saliva and MUC5B, respectively. MUC5B was also demonstrated to become more hydrated with increasing ionic concentrations, as indicated by Quartz Crystal Microbalance with Dissipation. We therefore propose that increased hydration shells surrounding the sodium counterions are responsible for this improved lubrication via the hydration lubrication mechanism. Lastly, it was observed that the spinnbarkeit of saliva decreased with increasing ionic concentrations, indicating a reduced stretchiness in its rheological properties. However, the concentration of MUC5B alone did not appear to affect spinnbarkeit, highlighting that many factors influence the biophysical properties of saliva.

Unravelling molecular origins of improved tribological properties of amino acid ionic liquid water-based lubricants

In the quest for sustainable lubrication, this study explores the potential of water-based lubricants (WBLs) with amino acid ionic liquid (AAIL) additives. The research combines experimental and reactive molecular dynamics (MD) simulations to assess the tribological and rheological enhancements provided by AAILs. Specifically, tetrabutylphosphonium (P4444) cations paired with Serine (Ser) and Tryptophan (Trp) anions were mixed into water to create AAIL WBLs. The introduction of AAILs significantly improved the lubricants’ rheological properties and reduced the coefficient of friction by up to 34 %. This reduction is attributed to the formation of hydration shells around the amino acid anions, which create a chemically adsorbed tribofilm on iron oxide surfaces, and P4444 cations that establish a load-bearing layer in the middle regions of the lubricant. This dual-layer structure, supported by hydration lubrication, effectively maintains lubricant fluidity while bearing substantial loads. Among the AAIL WBLs tested, those with a thicker, more stable, hydrated amino acid tribofilm demonstrated more significant friction reduction. These insights into the molecular mechanisms behind AAIL WBLs’ tribological behaviour underscore their promise as eco-friendly lubricants, contributing to advancing a circular economy in industries reliant on lubrication.

Excellent lubricating hydrogels with rapid photothermal sterilization for medical catheters coating

Bacterial infection and tissue damage caused by friction are two major threats to patients’ health in medical catheter implantation. Hydrogels with antibacterial and lubrication effects are competitive candidates for catheter coating materials. Photothermal therapy (PTT) is a highly efficient bactericidal method. Here, a composite hydrogel containing MXene nanosheets and hydrophilic 3-sulfopropyl methacrylate potassium salt (SPMK) is reported, which is synthesized through the one-pot method and heat-initiated polymerization. The hydrogel shows excellent antibacterial performance against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) in 3 min in the air or 20 min in the water environment under near-infrared light (NIR; 808 nm) irradiation. The friction coefficient of the hydrogel is about 0.11, which is 48% lower than that without SPMK. The rapid photothermal sterilization is attributed to the outstanding antibacterial ability and thermal effect of photoactivated MXene. The ultra-low friction is the result of the hydration lubrication mechanism. This study provides a potential strategy for the surface coatings of biomedical catheters, which enables rapid sterilization and extremely low interface resistance between catheters and biological tissues.

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.

A Common Hydration Lubrication Based on Hydrophobically Modified Sulfobetaine Polyurethane and Synovial Components in the Application of Artificial Meniscus

A Common Hydration Lubrication Based on Hydrophobically Modified Sulfobetaine Polyurethane and Synovial Components in the Application of Artificial Meniscus

Synovial Lubrication Factors‐Recruiting Biomimetic Polyphenolized Hyaluronic Acid for Promoting Cartilage Repair

AbstractOsteoarthritis is a common degenerative disease characterized by continuous detachment of lubrication factors from damaged cartilage matrix. This study proposes a paradigm that combines natural polyanionic polysaccharides (hyaluronic acid, HA) with dopamine (DA) to design a synovial lubrication factors‐recruiting biomimetic polyphenolized lubrication system. The biomimetic lubricants can use DA's polarity and hydrophilic groups as specific binding sites of type II collagen's polarity and hydrophilic groups to stably bind cartilage surface, and further utilize molecular specific interaction to recruit detached, joint endogenous or chondrocytes expressed synovial lubrication factors, continuously forming supramolecular lubrication complex onto cartilage matrix to induce joint lubrication. Simultaneously, the biolubricants can also utilize catechol groups to eliminate anionic free radicals and resist reactive oxygen species invasion in the early stages of joint lubrication failure. The prepared biolubricants exhibit low coefficient of friction (µ < 0.08), which is attributed to fluid and hydration lubrication provided by HA and DA side chains through fluid dynamic effect and interfacial hydration adsorption. Cell and animal experiments also demonstrate polyphenolic biolubricants have good biocompatibility and promote cartilage regeneration. Therefore, the developed synovial lubricants‐recruiting biomimetic polyphenolized biolubricants have potential for treating osteoarthritis, and also provide a theoretical basis for other biological tissues with lubrication problems.

Lubricin-Inspired Nanozymes Reconstruct Cartilage Lubrication System with an "In-Out" Strategy.

Lubricin, secreted primarily by chondrocytes, plays a critical role in maintaining the function of the cartilage lubrication system. However, both external factors such as friction and internal factors like oxidative stress can disrupt this system, leading to osteoarthritis. Inspired by lubricin, a lubricating nanozyme, that is, Poly-2-acrylamide-2-methylpropanesulfonic acid sodium salt-grafted aminofullerene, is developed to restore the cartilage lubrication system using an "In-Out" strategy. The "Out" aspect involves reducing friction through a combination of hydration lubrication and ball-bearing lubrication. Simultaneously, the "In" aspect aims to mitigate oxidative stress by reducing free radical, increasing autophagy, and improving the mitochondrial respiratory chain. This results in reduced chondrocyte senescence and increased lubricin production, enhancing the natural lubrication ability of cartilage. Transcriptome sequencing and Western blot results demonstrate that it enhances the functionality of mitochondrial respiratory chain complexes I, III, and V, thereby improving mitochondrial function in chondrocytes. In vitro and in vivo experiments show that the lubricating nanozymes reduce cartilage wear, improve chondrocyte senescence, and mitigate oxidative stress damage, thereby mitigating the progression of osteoarthritis. These findings provide novel insights into treating diseases associated with oxidative stress and frictional damage, such as osteoarthritis, and set the stage for future research and development of therapeutic interventions.

A bilayer composite hydrogel with simultaneous high load bearing and superior lubrication by dopamine modified nano-hydroxyapatite

Zwitterionic polymer is an ideal material with excellent biocompatibility and hydrophilicity for biomimetic soft tissue. Inspired by the structure of natural cartilage, a bilayer structural model was designed. By using dopamine to modify nano-hydroxyapatite as an additive, we prepared a bilayer nanocomposite hydrogel with excellent surface lubrication and high bottom load-bearing. The porous structure on the top layer could interact with water molecules to form a lubrication film for lower friction (CoF∼0.01) while the bottom layer provides sufficient compressive strength, arriving at approximately 14.55 MPa. The outstanding tribological behavior was mainly attributed to the biphasic and hydration lubrication of the hydrogel. This study provides a new design idea for the optimization of hydrogel lubrication performance.

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.

Robust and Lubricating Interface Semi-Interpenetrating Network on Inert Polymer Substrates Enabled by Subsurface-Initiated Polymerization.

Lubricating hydrogel coatings on inert rubber and plastic surfaces significantly reduce friction and wear, thus enhancing material durability and lifespan. However, achieving optimal hydration lubrication typically requires a porous polymer network, which unfortunately reduces their mechanical strength and limits their applicability where robust durability and wear-resistance are essential. In the research, a hydrogel coating with remarkable wear resistance and surface stability is developed by forming a semi-interpenetrating polymer network with polymer substrate at the interface. By employing a good solvent swelling method, monomers, and photoinitiators are embedded within the substrates' subsurface, followed by in situ polymerization under ultraviolet light, creating a robust semi-interpenetrating and entangled network structure. This approach, offering a thicker energy-dissipating layer, outperforms traditional surface modifications in wear resistance while preserving anti-fatigue, hydrophilicity, oleophobicity, and other properties. Adaptable to various rubber and plastic substrates by using suitable solvents, this method provides an efficient solution for creating durable, lubricating surfaces, broadening the potential applications in multiple industries.

Enzyme initiated in situ gelation as an oxygen-tolerant, high adhesive, and versatile strategy for hydrogel coating preparation

Hydrogel coatings have great potential for applications in antifouling, biomedical, and electronic fields, but the lack of preparation strategies with mild gel-forming conditions, high adhesion, and versatility hinders their widespread use. Herein, a method called glucose oxidase (GOD)-initiated in situ gelation (GOD-IIG) is developed for preparing hydrogel coatings. The procedure includes immobilizing Fe[Gly]2 and GOD on the surface of the substrates by forming an intermediate layer using epoxy resin, and then immersing the pre-treated substrates into a monomer solution to initiate polymerization. In situ gelation on the surface of the substrates is triggered by free radicals generated through the redox reaction between ferrous glycine (Fe[Gly]2) and hydrogen peroxide (H2O2) produced by the GOD-catalyzed glucose oxidation process, which can be carried out in open-air environments. Using the GOD-IIG method, hydrogel coatings constructed from common low-adhesive polymers displayed an adhesive strength of up to 686 kPa and stable underwater adhesion for at least 7 days. Moreover, this method shows controllable hydrogel thickness and recyclable monomer solution for at least 25 times at room temperature. It is also applicable to a variety of hydrogel structures and substrates, such as plastic, metal, and glass, regardless of flatness or geometry. In order to demonstrate the applications of hydrogel coatings prepared by GOD-IIG method, the lubrication and antifouling abilities of the coatings were assessed, and the results indicated that the coatings had good hydration lubrication as well as oil and biofouling resistance. This method holds promise for advancing the preparation of hydrogel coatings and has potential applications in the fields of electrical, biological, and environmental engineering.

Supramolecular self‐assembled nanoparticles for targeted therapy of myocardial infarction by enhancing cardiomyocyte mitophagy

AbstractMyocardial infarction accompanied by diabetes mellitus is accepted as the most serious type of coronary heart disease, and among the current treatment strategies, the precise delivery of protective drugs for inhibiting cardiomyocyte apoptosis is still a challenge. In this study, we developed a biodegradable nanoparticles‐based delivery system with excellent macrophage escape, cardiac targeting, and drug release properties to achieve targeted therapy of myocardial infarction. Specifically, a copolymer of p(DMA–MPC–CD) combining self‐adhesion, hydration lubrication, and targeting peptide binding site was successfully prepared by free radical copolymerization, and it was self‐assembled on the surface of melatonin‐loaded dendritic mesoporous silica nanoparticles (bMSNs) following the integration of adamantane‐modified cardiac homing peptide (CHP) based on supramolecular host–guest interaction. Importantly, a hydration layer formed around the zwitterionic phosphorylcholine groups of the multifunctional nanoparticles, which was confirmed by the enhancement in hydration lubrication and reduction in coefficient of friction, prevented the nanoparticles from phagocytosis by the macrophages. The in vivo bioluminescence imaging test indicated that the nanoparticles were endowed with satisfied cardiac targeting capability, and the in vivo mice study demonstrated that the intravenous injection of drug‐loaded nanoparticles (namely bMSNs–Mel@PDMC–CHP) effectively reduced cardiomyocyte apoptosis, alleviated myocardial interstitial fibrosis, and enhanced cardiac function.

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) (1mg/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.

Exploring the relevance between load-bearing capacity and surface friction behavior based on a layered hydrogel cartilage prototype

Cartilage is well lubricated over a lifetime and this phenomenon is attributed to both of the surface hydration lubrication and the matrix load-bearing capacity. Lubricious hydrogels with a layered structure are designed to mimic cartilage as potential replacements. While many studies have concentrated on improving surface hydration to reduce friction, few have experimentally detected the relationship between load-bearing capacity of hydrogels and their interface friction behavior. In this work, a bilayer hydrogel, serving as a cartilage prototype consisted of a top thick hydrated polymer brush layer and a bottom hydrogel matrix with tunable modulus was designed to investigate this relationship. The coefficient of friction (COF, μ) is defined as the sum of interfacial component (μInt) and deformation/hysteresis component (μHyst). The presence of the top hydration layer effectively dissipates contact stress and reduces the interface interaction (μInt), leading to a stable and low COF. The contribution of mechanical deformation (μHyst) during the sliding shearing process to COF can be significantly reduced by increasing the local mechanical modulus, thereby enhancing the load-bearing capacity. These results show that the strategy of coupling surface hydration layer with a high load-bearing matrix can indeed enhance the lubrication performance of hydrogel cartilage prototypes, and implies a promising routine for designing robust soft matter lubrication system and friction-control devices.

Tribological Behavior of Sulfonated Polyether Ether Ketone with Three Different Chemical Structures under Water Lubrication.

With the development of the shipbuilding industry, it is necessary to improve tribological properties of polyether ether ketone (PEEK) as a water-lubricated bearing material. In this study, the sulfonated PEEK (SPEEK) with three distinct chemical structures was synthesized through direct sulfonated polymerization, and high fault tolerance and a controllable sulfonation degree ensured the batch stability. The tribological and mechanical properties of SPEEK with varying side groups (methyl and tert-butyl) and rigid segments (biphenyl) were compared after sintering in a vacuum furnace. Compared to the as-made PEEK, as the highly electronegative sulfonic acid group enhanced the hydration lubrication, the friction coefficient and wear rate of SPEEK were significantly reduced by 30% and 50% at least without affecting the mechanical properties. And lower steric hindrance and entanglement between molecular chains were proposed to be partially responsible for the lowest friction behavior of SPEEK with methyl side groups, making it a promising and competitive option for water-lubricated bearings.

The Effects of Splayed Lipid Molecules on Lubrication by Lipid Bilayers

The outstanding lubrication of articular cartilage in the major synovial joints such as hips and knees, essential for the joint well-being, has been attributed to boundary layers of lipids at the outer cartilage surfaces, which have very low friction mediated by the hydration lubrication mechanism at their highly hydrated exposed headgroups. However, the role of spontaneously present lipid splays—lipids with an acyl tail in each of the opposing bilayers—in modulating the frictional force between lipid bilayers has not, to date, been considered. In this study, we perform all-atom molecular dynamics simulations to quantitatively assess the significance of splayed molecules within the framework of lubricating lipid bilayers. We demonstrate that, although transient, splayed molecules significantly increase the inter-membrane friction until their retraction back into the lamellar phase, with this effect more steadily occurring at lower sliding velocities that are comparable to the physiological velocities of sliding articular cartilage.