Hydration Lubrication Research Articles

Hydration lubrication of polyzwitterionic brushes leads to nearly friction- and adhesion-free droplet motion

Recently, there has been much progress in the design and application of oil-repellent superoleophobic surfaces. Polyzwitterionic brush surfaces are of particular interest, because of their ability to repel oil under water, even in the absence of micro-/nanostructures. The origin of this underwater superoleophobicity is attributed to the presence of a stable water film beneath the oil droplet, but this had not been demonstrated experimentally. Here, using optical interferometric techniques, we show that an oil droplet effectively hydroplanes over a water film, whose thickness is between one hundred and hundreds of nanometres. In addition, using a custom-built droplet force apparatus, we find the friction and adhesion forces to be in the nN range for millimetric-sized droplets. These forces are much lower than for other classes of well-known liquid-repellent surfaces, including the lotus-leaf effect and lubricant-infused surfaces, where the typical force is on the order of μN.

Friction properties of polyacrylamide hydrogel particle/HDPE composite under water lubrication

In order to apply the hydration lubrication of soft material in heavy load engineering fields, a series of polyacrylamide (PAAm) hydrogel particle/high-density polyethylene (HDPE) composites were prepared. Their mechanical property, soaking surface morphology, wettability, friction property and lubrication mechanism were experimentally obtained and analyzed. The results show that PAAm hydrogel particles played an important role in the lubrication and antifriction of composites, and the friction properties of 1 wt% PAAm hydrogel particle composite were superior to others of different concentrations. The present study suggests that the composite possesses both the high mechanical properties of HDPE and the excellent hydration lubrication of PAAm hydrogel particles. This finding will provide a novel approach for improving friction properties of water-lubricated materials and are conducive to practical application in engineering fields.

Lubrication: Articular Cartilage‐Inspired Surface Functionalization for Enhanced Lubrication (Adv. Mater. Interfaces 12/2019)

A charged polymer brush grafted surface, inspired by natural articular cartilage, achieves an excellent lubricating performance with a low coefficient of friction due to the mechanism of hydration lubrication. The surface charged brush forms a hydration shell which can undergo large normal load without deformation. Additionally, the rapid exchange with the water molecules nearby results in a fluidlike manner of the hydrated shells when being sheared and correspondingly the significantly reduced friction force at the sliding interface. More details can be found in article number 1900180 by Yixin Wang, Yulong Sun, Yanhong Gu, and Hongyu Zhang.

Poly-phosphocholinated Liposomes Form Stable Superlubrication Vectors.

We have prepared phosphatidylcholine (PC) vesicles (liposomes) incorporating a novel lipid/poly-phosphocholine conjugate. This both stabilizes the liposomes against aggregation (for example, during storage or when being delivered) and allows them to act as very efficient lubricating elements readily attaining superlubric performance (defined as coefficient of friction μ < 10-2) via hydration lubrication at physiological salt concentrations and pressures. In contrast, vesicles sterically protected by poly(ethylene glycol) chains (PEGylation), which is the general method of choice, while being equally stable to aggregation are much poorer lubricants under these conditions, which is attributed to the relatively poor hydration of the PEG. Our approach enables the use of PC liposomes as stable superlubrication vectors in potential biomedical applications.

Articular Cartilage‐Inspired Surface Functionalization for Enhanced Lubrication

AbstractNatural articular cartilage has excellent superlubrication property, and it is attributed to the hydration lubrication mechanism of the charged biomacromolecules which extend from the cartilage surface to form a brush‐like layer. In this study, a bioinspired brush‐like polyelectrolyte, namely poly(2‐methacryloyloxyethyl phosphorylcholine) (PMPC), is grafted onto the SiO2 wafer and polystyrene (PS) microsphere via surface‐initiated polymerization to enhance the lubrication performance. The characterization of Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, water contact angle, and scanning electron microscopy demonstrates that the PMPC polymer brushes are successfully modified onto the substrates. Furthermore, the lubrication test performed using atomic force microscope, with the PMPC‐grafted SiO2 wafer and PMPC‐grafted PS microsphere as the contact tribopair, shows that the PMPC‐functionalized surfaces significantly reduce friction coefficient under different test conditions. The tenacious water hydration shells formed surrounding the zwitterionic charges of PMPC polymer brushes are responsible for the reduced friction coefficient, which could support high pressures without being squeezed out under loading. In summary, the articular cartilage‐inspired surface functionalization method can be used to modify various substrates for enhanced lubrication.

How synergistic aqueous lubrication is mediated by natural and synthetic molecular aggregates

Nature lubricates in aqueous environment, and thus the example of a human synovial joint with its seamless function has been a fascination for scientists since the times of the birth of modern science. Here, inspired by nature, we investigate the mechanistic function of three different types of synergistic molecular aggregates. Firstly, we show how simple phospholipids lubricate hydrophilic model surfaces of silica and how this lubrication is facilitated further by the presence of an anionic polysaccharide, hyaluronan, due to the enhanced surface build-up of lubricant material. Next, we mimic natural polylectrolytesurfactant aggregation by employing a highly positively charged polyelectrolyte and anionic surfactant that strongly associate both in the bulk and at the surfaces by building structured aggregates that lubricate due to hydration lubrication. This occurs despite of the presence of strong attraction between the lubricated surfaces. This is an example of synergistic lubrication due to particular internal structural arrangement of the aggregates. Finally, we investigate the case of synergistic lubrication due to preferential surface ordering of two biological polyelectrolytes, cartilage oligomeric matrix protein and lubricin, that leads to favourable lubrication.

Bioinspired Surface Functionalization of Nanodiamonds for Enhanced Lubrication.

The addition of nanoparticles to water-based lubricants is a commonly used method to improve lubrication, but to the best of our knowledge few studies have been reported to investigate the lubrication property of surface-modified nanodiamonds (ND) with polyzwitterionic brushes. In this study, a bioinspired copolymer containing dopamine and 2-methacryloyloxyethyl phosphorylcholine (MPC) was synthesized (DMA-MPC) and then spontaneously grafted onto the ND surface (ND-MPC) through simple stirring in order to enhance lubrication. The characterization of transmission electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis indicated that the DMA-MPC was successfully modified on the ND surface. Furthermore, a series of tribological experiment were performed on a universal materials tester using glycerol, glycerol + ND, and glycerol + ND-MPC as the lubricants. It was found that the addition of ND to the lubricant (i.e., glycerol + ND and glycerol + ND-MPC) significantly reduced wear with a smaller wear scar and wear track on the tribopairs, and the coefficient of friction further decreased by about 40% when using glycerol + ND-MPC as the lubricant, which could be attributed to the hydration lubrication of the polyzwitterionic brushes modified on the ND surface and the rolling effect of nanoparticles. In conclusion, in this study a universal and versatile surface modification method was proposed on the basis of the synthesis of bioinspired copolymer DMA-MPC, which remarkably enhanced the lubrication property of ND nanoparticles when added to water-based lubricants.

Macroscale Superlubricity Enabled by Hydrated Alkali Metal Ions.

Superlubricity based on hydration lubrication provides a near-frictionless lubrication state for the extreme reduction of friction in aqueous conditions. Nevertheless, how to obtain the hydration superlubricity under macroscale conditions with higher load-carrying capacity still remains a challenge and the mechanisms governing macroscale superlubricity with hydrated ions are still not well comprehended. Here, we demonstrate that macroscale superlubricity based on hydrated alkali metal ions (Li+, Na+, K+) can be realized under high contact pressure between the Si3N4 ball and sapphire disk. The ultralow friction coefficients of 0.005 are obtained under average contact pressure up to 0.25 GPa by a universal micro-tribometer after a running-in period with acid solutions. The results reveal that running-in stage with acid solutions can not only make the worn region smoother but also generate a silica layer that is easy to shear, which provides excellent boundary lubrication. The hydration superlubricity occurs because hydration shells surrounding the alkali metal ions could generate the hydration repulsive force to sustain a large normal load and have a fluid response to shear simultaneously. These findings pave the way to the scale-up of hydration superlubricity and thus to the wide application of new water-based lubricants.

Biomimetic Lubrication and Surface Interactions of Dopamine-Assisted Zwitterionic Polyelectrolyte Coatings.

A bioinspired zwitterionic polyelectrolyte coating with excellent hydration ability has been regarded as a promising lubricating candidate for modifying artificial joint cartilage surface. In physiological fluids, the ubiquitous proteins play an important role in achieving outstanding boundary lubrication; however, a comprehensive understanding of the hydration lubrication between polyelectrolyte coatings and proteins still remains unclear. In this work, a facile fabrication of ultrasmooth polyelectrolyte coatings was developed via codeposition of synthesized poly(dopamine methacrylamide- co-2-methacryloyloxyethyl phosphorylcholine) (P(DMA- co-MPC)) and dopamine (DA) in a mild condition. Upon optimization of the feeding ratio of P(DMA- co-MPC) and DA, the as-fabricated PDA/P(DMA- co-MPC) coatings exhibit excellent lubricating properties when sliding with each other (friction coefficient μ = 0.036 ± 0.002, ∼2.8 MPa), as well as sliding with a model protein (bovine serum albumin (BSA)) layer (μ = 0.041 ± 0.005, ∼4.8 MPa) in phosphate-buffered saline (PBS, pH 7.4). Intriguingly, the lubrication in both systems shows Amontons-like behaviors: the friction is directly proportional to the applied load but independent of the shear velocity. Moreover, the PDA/P(DMA- co-MPC) coatings could resist the protein fouling (i.e., BSA) in PBS, which is crucial to prevent the surfaces from being contaminated when applied in biological media, thus maintaining their lubricating properties. Our results provide a versatile approach for facilely fabricating polyelectrolyte coatings with superior lubrication properties to both polyelectrolyte coatings and protein surfaces, with useful implications into the development of novel lubricating coatings for bioengineering applications (e.g., artificial joints).

Phospholipid-Coated Mesoporous Silica Nanoparticles Acting as Lubricating Drug Nanocarriers.

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

Label-free analysis of physiological hyaluronan size distribution with a solid-state nanopore sensor

Hyaluronan (or hyaluronic acid, HA) is a ubiquitous molecule that plays critical roles in numerous physiological functions in vivo, including tissue hydration, inflammation, and joint lubrication. Both the abundance and size distribution of HA in biological fluids are recognized as robust indicators of various pathologies and disease progressions. However, such analyses remain challenging because conventional methods are not sufficiently sensitive, have limited dynamic range, and/or are only semi-quantitative. Here we demonstrate label-free detection and molecular weight discrimination of HA with a solid-state nanopore sensor. We first employ synthetic HA polymers to validate the measurement approach and then use the platform to determine the size distribution of as little as 10 ng of HA extracted directly from synovial fluid in an equine model of osteoarthritis. Our results establish a quantitative method for assessment of a significant molecular biomarker that bridges a gap in the current state of the art.

Origin of hydration lubrication of zwitterions on graphene.

Formation of a hydration layer on charge sites can support normal pressure, and meanwhile it retains excellent fluidity to provide efficient boundary lubrication; however, it is limited to the sliding system between two similarly charged surfaces. In the present study, we report extremely low friction as the zwitterions in a lipid bilayer slide on the topmost graphene layer of graphite across pure water, with the friction coefficient falling to the level of 0.001, which provides direct evidence that hydration lubrication is effective even between such dissimilar surfaces. The origin of hydration lubrication on graphene was studied by atomic force microscopy and molecular dynamics simulation simultaneously. It reveals that a subnanometer hydration layer is confined between zwitterions and graphene, which remains as a liquid phase under normal pressure. The shear occurs between water molecules and graphene because of the extremely low shear strength of the water/graphene interface, which contributes to extremely low friction. Our finding demonstrates that the formation of a hydration layer is possible to lubricate layered materials efficiently, which has potential implications for designing efficient boundary lubrication with layered materials.

Charged polymer brushes-coated mesoporous silica nanoparticles for osteoarthritis therapy: a combination between hydration lubrication and drug delivery

Charged polymer brushes-coated mesoporous silica nanoparticles for osteoarthritis therapy: a combination between hydration lubrication and drug delivery

Ultra-low friction between boundary layers of hyaluronan-phosphatidylcholine complexes.

Can designed biomaterials emulate the unique lubrication ability of articular cartilage, and thus provide potential alleviation to friction-related joint diseases? This is the motivation behind the present study. The principles of cartilage lubrication have attracted considerable attention for decades, and several models have been proposed to elucidate it, however, the mechanism of this ultralow friction is still not clear. In this paper we explore the recent suggestion that its efficient lubrication arises from boundary layers of hyaluronan-lipid complexes at its surface, in particular exploring a range of different phosphatidylcholines (PCs) mimicking the wide range of PCs in synovial joints. The present study suggests a synergistic lubricating behavior of the different lipids in living joints, and potential treatment directions using such biomaterial complexes for widespread cartilage-friction-related diseases such as osteoarthritis.

Repulsive surfaces and lamellar lubrication of synovial joints

Surface-active phospholipid (SAPL) secreted in the synovial joint plays an important role in cartilage integrity. In healthy joints, phospholipid multibilayers coat the cartilage surface, providing boundary lamellar-repulsive hydration lubrication. Current mechanism for lubrication of synovial joints, as well as the physical and chemical nature of the cartilage surface is discussed. Friction between phospholipid (PL) bilayers attached to cartilage surfaces is considered including a discussion on the recent observation of an extreme friction reduction as a consequence of a less charged hydrophilic cartilage surface. It is proposed that the highly efficient lubrication occurring in natural joints arises from the presence of negatively charged cartilage surfaces. The lamellar-repulsive mechanisms for the reduction of friction is supported by phospholipid lamellar phases and charged macromolecules residing between contacting cartilage surfaces at pH ∼7.4.

Importance of adaptive multimode lubrication mechanism in natural synovial joints

The superior tribological performance in natural synovial joints with low friction and minimal wear appears to be actualized by not single lubrication mode but the synergistic combination of various modes from the fluid film lubrication to boundary lubrication corresponding to the severity of rubbing conditions. We have conducted the biphasic finite element analysis and some experimental studies for articular cartilage to elucidate this ingenious lubrication mechanism from the viewpoint of the adaptive multimode lubrication mechanism. In this paper, the effectiveness of different lubrication modes such as biphasic, boundary, gel-film and hydration lubrication particularly at low speed conditions is discussed.

A Trimeric Surfactant: Surface Micelles, Hydration-Lubrication, and Formation of a Stable, Charged Hydrophobic Monolayer.

The surface structure of the trimeric surfactant tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) on mica and the interactions between two such DTAD-coated surfaces were determined using atomic force microscopy and a surface force balance. In an aqueous solution of 3 mM, 5 times the critical aggregation concentration (CAC), the surfaces are coated with wormlike micelles or hemimicelles and larger (∼80 nm) bilayer vesicles. Repulsive normal interactions between the surfaces indicate a net surface charge and a solution concentration of ions close to that expected from the CAC. Moreover, this surface coating is strongly lubricating up to some tens of atmospheres, attributed to the hydration-lubrication mechanism acting at the exposed, highly hydrated surfactant headgroups. Upon replacement of the DTAD solution with surfactant-free water, the surface structures have changed on the DTAD monolayers, which then jump into adhesive contact on approach, both in water and following addition of 0.1 M NaNO3. This trimeric surfactant monolayer, which is highly hydrophobic, is found to be positively charged, which is evident from the attraction between the DTAD monolayer and negatively charged bare mica across water. These monolayers are stable over days even under a salt solution. The stability is attributed to the several stabilization pathways available to DTAD on the mica surface.

Normal and Frictional Interactions between Liposome-Bearing Biomacromolecular Bilayers.

Highly efficient lubricating boundary layers at biosurfaces such as cartilage have been proposed to comprise phospholipids complexed with biomacromolecules exposed at the surfaces. To gain insight into this, a systematic study on the normal and frictional forces between surfaces bearing a sequentially deposited model alginate-on-chitosan bilayer, bearing different adsorbed phosphatidylcholine (PC) liposomes, was carried out using a surface force balance. Structures of the resulting surface complexes were determined using atomic force microscopy (AFM) and cryo-scanning electron microscopy (cryo-SEM). The liposome/lipid-polymer complexes could maintain their integrity up to high pressures in terms of both normal and shear interactions between the surfaces, which were repeatable, reproducible, and revealed very low friction (coefficient of friction μ down to 10(-3)-10(-4), depending on the PC used) up to pressures of hundreds of atm. We attribute this remarkable lubrication capability ultimately to hydration lubrication acting at the hydrated phosphocholine headgroups of the PC lipids, either exposed at the liposome surfaces or through complexation with the polyelectrolyte bilayer. Values of μ, while low, were roughly an order of magnitude higher than for the same PC vesicles adsorbed on bare mica, a difference attributed to their lower density on the bilayer; the bilayer, however, stabilized the PC-vesicles far better than bare mica against rupture and shear at high compressions and sliding.

Lubrication of Articular Cartilage

The major synovial joints such as hips and knees are uniquely efficient tribological systems, able to articulate over a wide range of shear rates with a friction coefficient between the sliding cartilage surfaces as low as 0.001 up to pressures of more than 100 atm. No human-made material can match this. The means by which such surfaces maintain their very low friction has been intensively studied for decades and has been attributed to fluid-film and boundary lubrication. Here, we focus especially on the latter: the reduction of friction by molecular layers at the sliding cartilage surfaces. In particular, we discuss such lubrication in the light of very recent advances in our understanding of boundary effects in aqueous media based on the paradigms of hydration lubrication and of the synergism between different molecular components of the synovial joints (namely hyaluronan, lubricin, and phospholipids) in enabling this lubrication.

The amphoteric effect on friction between the bovine cartilage/cartilage surfaces under slightly sheared hydration lubrication mode

The amphoteric effect on the friction between the bovine cartilage/cartilage contacts has been found to be highly sensitive to the pH of an aqueous solution. The cartilage surface was characterized using a combination of the pH, wettability, as well as the interfacial energy and friction coefficient testing methods to support lamellar-repulsive mechanism of hydration lubrication. It has been confirmed experimentally that phospholipidic multi-bilayers are essentially described as lamellar frictionless lubricants protecting the surface of the joints against wear. At the hydrophilicity limit, the low friction would then be due to (a) lamellar slippage of bilayers and (b) a short-range (nanometer-scale) repulsion between the interfaces of negatively charged (PO4⿿) cartilage surfaces, and in addition, contribution of the extracellular matrix (ECM) collagen fibers, hyaluronate, proteoglycans aggregates (PGs), glycoprotein termed lubricin and finally, lamellar PLs phases. In this paper we demonstrate experimentally that the pH sensitivity of cartilage to friction provides a novel concept in joint lubrication on charged surfaces.