Friction Of Hydrogels Research Articles

Hyaluronan incorporation into model contact lens hydrogels as a built-in lubricant: Effect of hydrogel composition and proteoglycan 4 as a lubricant in solution.

Contact lens friction significantly correlates with subjective comfort. Hyaluronan (HA) and proteoglycan 4 (PRG4) are natural boundary lubricants present in the body. The objective of this study was to assess the effect of crosslinked HA into the bulk of model contact lens materials pHEMA, pHEMA/TRIS, and DMAA/TRIS on surface wettability, protein sorption, and boundary lubricating properties at a material-cornea biointerface, both alone and synergistically with PRG4 in solution. Surface wettability was assessed by water contact angle measurement, protein sorption by lysozyme sorption assay, and boundary lubricating properties using an in vitro friction test method. HA incorporation (HAinc ) increased the surface wettability of all materials, and reduced protein sorption for pHEMA and DMAA/TRIS. HAinc increased friction for pHEMA, and DMAA/TRIS, whereas a decrease was observed for pHEMA/TRIS. A combination of HAinc and PRG4sol had a synergistic effect of reducing friction only for pHEMA/TRIS. This combination had similar friction reduction compared with PRG4sol alone for DMAA/TRIS. These results indicate HA incorporation could be an effective internal wetting agent, antiadhesive, and boundary lubricant for pHEMA/TRIS silicone hydrogels. In conclusion, HA incorporation can reduce friction of hydrogels alone and in combination with PRG4 in solution, though in a hydrogel composition-dependent (e.g., TRIS) manner. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1818-1826, 2018.

共振ずり測定によるナノ閉じ込め液体の特性・摩擦解析

The resonance shear measurement (RSM), which we developed, can evaluate the rheological and tribological properties of confined liquids at surface separation distances (D) from μm (practically liquids are in the bulk state) to nm thicknesses. Thus, it is especially useful for studying the boundary lubrication for which the lubricant layer becomes in the nm level thickness and the solid surfaces are supposed to be partially in contact. In this article, we describe the principle and advantages of RSM, and review our recent RSM studies on tribology of nano-confined liquids. Four phenyl ether lubricant oils with different bulk viscosity confined between mica surfaces, and two ionic liquids ([C4mim] [NTf2]) and [C4mim] [BF4]) showed significant increase in viscosity and the their magnitude relation became reverse when the gap became in nanometer thick. Friction of hydrogel (double network gel) and silica sphere was dominated by the elasticity of the deformed gel-silica interface.

Effect of Surface Geometric Patterns on Sliding Friction of Hydrogels

Effect of Surface Geometric Patterns on Sliding Friction of Hydrogels

Development of Instrument for Simultaneous Measurement of Rotating and Sliding Friction of Hydrogels

Gels have superior features such as high water content, materials permeability, extremely low friction, and biocompatibility, which are not found in the hard and dry materials, because these features are due to their soft and wet features. Now artificial gels have become similar to natural gels of the human body: the articular cartilage of our knee has both several MPa of mechanical strength and about 0.01 of surface frictional coefficient. Above all, we paid attention to high functionality of shape memory gel (SMG). The frictional behavior of SMG was studied. SMG has some characteristics such as shape-memory characteristics; low friction characteristics wear resistance than the conventional gels. SMG has a shape-memory characteristic by temperature responsiveness namely the temperature change. SMG is expected practical use of to medical materials and machine materials. As for the reason, a shape-memory function does not need chemical fixation, and what I can fix mechanically is given. The machine fixation enables the solvent nonuse in the burden decrease of the patient by the medical care, machine assembling and the design and the reduction of the part mark, the miniaturization of the apparatus. In this study, Implication water rate of different SMG of the composition is measured, and power by changing implication water rate reports the characteristic, and we performed the friction behavior of SMG for the purpose of the use to the underwater apparatus. We made the gel that was low friction with high strength for the practical use of SMG and evaluated it in a Young's modulus, water content ratio, a coefficient of friction quantitatively. The dynamic frictional behavior was observed with Young's modulus and compared with frictional measurement data. This result shows that surface action peculiar to soft and wet materials except the friction behavior based on a contact theory works. This friction behavior means adaptation range expansion of the gel. We developed a new device to inspect the conformity of the range. As for it, a vertical direction and rotatory direction can measure seal materials made with gel materials. As for appearing, the low friction evaluation that gel seal materials were able to be superior to in comparison with the commercial seal materials was provided as a result of in this study. New utility value of the high-performance gel is found by elucidating friction behavior of gel changing a concept of the conventional friction and believes it when it is with the clue to practical use.

Friction of Zwitterionic Hydrogel by Dynamic Polymer Adsorption

A simplified model describing the sliding friction of hydrogel on solid surface by dynamic adsorption of the polymer chains is proposed on the basis of polymer adsorption–repulsion theory. This dynamic adsorption model is used to analyze the friction results of zwitterionic hydrogels sliding over glass substrates with different substrate wettability, hydrogel swelling degree, ionic strength, and pH of bath solution. The adsorption time τb of polymer strands is found to decrease with the increase in sliding velocity or the Weissenberg number as a result of stretching. The adsorption time τb0 and the adsorption energy Uads at stress-free condition, which are characteristic for each friction system, are also estimated. Roughly, a master curve is observed for the normalized adsorption lifetime τb/τb0 and the Weissenberg number, with less dependence on the adsorption energy and the bulk properties of the gels in the observed experimental conditions. Thus, the dynamic adsorption model successfully correlates th...

In situ observation of a hydrogel-glass interface during sliding friction.

Direct observation of hydrogel contact with a solid surface in water is indispensable for understanding the friction, lubrication, and adhesion of hydrogels under water. However, this is a difficult task since the refractive index of hydrogels is very close to that of water. In this paper, we present a novel method to in situ observe the macroscopic contact of hydrogels with a solid surface based on the principle of critical refraction. This method was applied to investigate the sliding friction of a polyacrylamide (PAAm) hydrogel with glass by using a strain-controlled parallel-plate rheometer. The study revealed that when the compressive pressure is not very high, the hydrogel forms a heterogeneous contact with the glass, and a macro-scale water drop is trapped at the soft interface. The pre-trapped water spreads over the interface to decrease the contact area with the increase in sliding velocity, which dramatically reduces the friction of the hydrogel. The study also revealed that this heterogeneous contact is the reason for the poor reproducibility of hydrogel friction that has been often observed in previous studies. Under the condition of homogeneous full contact, the molecular origin of hydrogel friction in water is discussed. This study highlights the importance of direct interfacial observation to reveal the friction mechanism of hydrogels.

Friction of hydrogels with controlled surface roughness on solid flat substrates

This study investigated the effect of hydrogel surface roughness on its sliding friction against a solid substrate having modestly adhesive interaction with hydrogels under small normal pressure in water. The friction test was performed between bulk polyacrylamide hydrogels of varied surface roughness and a smooth glass substrate by using a strain-controlled rheometer with parallel-plates geometry. At small pressure (normal strain 1.4-3.6%), the flat surface gel showed a poor reproducibility in friction. In contrast, the gels with a surface roughness of 1-10 μm order showed well reproducible friction behaviors and their frictional stress was larger than that of the flat surface hydrogel. Furthermore, the flat gel showed an elasto-hydrodynamic transition while the rough gels showed a monotonous decrease of friction with velocity. The difference between the flat surface and the rough surface diminished with the increase of the normal pressure. These phenomena are associated with the different contact behaviors of these soft hydrogels in liquid, as revealed by the observation of the interface using a confocal laser microscope.

Frictional Property of Hydrogels Prepared under Electric Fields

We studied frictional property of hydrogels prepared under electric fields. Pre-gel solution was filled between two electrodes and voltage was applied during crosslinking reactions. After removing the gel from the electrodes, frictional testing for the gel surfaces prepared on both cathode and anode was performed against a silicon dioxide coated surface. For the gels containing two types of monomers, an electrolyte and a neutral, friction coefficient for the gel surface prepared on the anode was much smaller than that for the cathode as well as that without electric voltage. In contrast, no significant difference was seen between the two surfaces for the single-component gels. Our results suggest that this simple technique of applying electric voltage during gelation is effective for controlling and reducing friction of hydrogels.

Effect of substrate adhesion and hydrophobicity on hydrogel friction.

In this paper, the frictional behavior of a neutral hydrogel, polyvinyl alcohol (PVA), on smooth solid substrates with various levels of hydrophobicity have been investigated in water using a strain-controlled parallel-plate rheometer. For the sliding velocity dependence of friction, we detected a distinct friction transition on hydrophobic substrates that are strongly adhesive to the gel, while no clear transition was observed on hydrophilic substrates that are weakly adhesive to the gel. Even on the most hydrophobic substrate, the maximum frictional stress is approximately 1/10-1/5 of the gel's elastic modulus under a large normal strain of 26%. Furthermore, the frictional stress on hydrophobic substrates in the high velocity region, larger than the transition, is much lower than that on hydrophilic ones. We attempted to explain these phenomena with the help of two models: a molecular model based on the thermal fluctuations occurring during adsorption-desorption of polymers and a continuum mechanics model based on elastic dewetting and forced wetting.

A study on the friction properties of poly(vinyl alcohol) hydrogel as articular cartilage against titanium alloy

Many biomaterials are being developed to repair or replace articular cartilage. One of these materials, poly(vinyl alcohol) (PVA) hydrogel prepared from aqueous solution of the polymer by freezing and thawing method may exhibit the mechanical properties required to withstand the harsh environment of diarthrodial joints. To better understand how PVA hydrogel friction is affected by different variable factors, a three-factor, three-level designed orthogonal experiment was developed. Factors include lubricant, sliding speed, and normal load. Friction coefficient of the PVA hydrogel was found to depend significantly on load and sliding speed. Lubricant had little effects on the friction coefficient. Friction coefficient of the PVA hydrogel decreased with the increase of sliding speed and the friction coefficient approximately increased linearly with the increasing load. Average friction coefficient decreased from 0.0447 to 0.0379 while the sliding speed increased from 0.06 to 0.22 m/s. Average friction coefficient increased from 0.0276 to 0.0546, almost increasing one time, while the load increased from 5 to 15 N.

Role of water in the lubrication of hydrogel

In previous reports, the frictional property of hydrogel was evaluated by using the pin-on-disk type of friction tester. As a result, the hydrogel showed the change in a friction coefficient depending on the speed. The friction property of hydrogel is similar to the Stribeck curve. From this result, it was confirmed that the hydrogel was operated by the mode of hydrodynamic lubrication. In order to investigate the effect of mechanical property and chemical characteristic of hydrogel for hydrodynamic lubrication, the hydrogel was prepared by using two kinds of monomers and by changing the gel concentration. The frictional property of hydrogel was evaluated by using the pin-on-disk type of friction tester. The friction coefficient does not depend on the hardness. It depends on the speed. The friction properties of hydrogel at the speed/load showed the Stribeck curve. It was confirmed that the hydrogel showed the behavior of the hydrodynamic lubrication at low load and high velocity. Although the hardness of both hydrogels is the same, the Stribeck curve of the DMAA hydrogel showed the friction coefficient which was lower than that of the AAm hydrogel. NOE spectrum of both hydrogels with the lowest friction and with the same hardness was measured in NMR. With the comparison of the NOE phenomenon of two hydrogels, DMAA hydrogel showed NOE phenomenon, which was clearer than AAm hydrogel. It was indicated that the low friction was caused by the attractive force of the water molecule in the hydrophilic macromolecule chain. It is important to realize that the low friction of hydrogel is caused by the hydrophilic macromolecule chain attracting the water molecule.

ゲル‐サイエンスとテクノロジー 高強度低摩擦ゲル‐機能性素材としての新展開

The author's research group have systematically studied the surface sliding friction of hydrogels, and various unique features in gel friction have been discovered. The frictional force and its dependencies on the load are quite different depending on the chemical structures of the gels, surface properties of the opposing substrates, and the measurement condition, which are completely different from those of solids. Most importantly, the friction coefficient of gels exhibits a value as low as 0.001, which cannot be obtained from the friction between two solid materials.Therefore, gels have a high potential as a low friction material, such as an artificial cartilage. However, conventional gels are mechanically too weak to be used as load bearing materials. The author's research group recently developed a general method to obtain very strong gels by introducing a double network structure of the gel. These high strength hydrogels, containing 90% water, sustain a compressive pressure as high as decades of MPa and show a high wear resistance due to their extremely low friction. These gels might open new era of soft and wet materials for substituting articular cartilage and other tissues.

Surface friction of hydrogels with well-defined polyelectrolyte brushes.

Hydrogels of poly(2-hydroxyethyl methacrylate) (PHEMA) with well-defined polyelectrolyte brushes of poly(sodium 4-styrenesulfonate) (PNaSS) of various molecular weights were synthesized, keeping the distance between the polymer brushes constant at ca. 20 nm. The effect of polyelectrolyte brush length on the sliding friction against a glass plate, an electrorepulsive solid substrate, was investigated in water in a velocity range of 7.5 x 10(-5) to 7.5 x 10(-2) m/s. It is found that the presence of polymer brush can dramatically reduce the friction when the polymer brushes are short. With an increase in the length of the polymer brush, this drag reduction effect only works at a low sliding velocity, and the gel with long polymer brushes even shows a higher friction than that of a normal network gel at a high sliding velocity. The strong polymer length and sliding velocity dependence indicate a dynamic mechanism of the polymer brush effect.

Surface friction of hydrogels

The sliding friction of various kinds of hydrogels has been studied and it was found that the frictional behaviors of the hydrogels do not conform to Amonton's law F=μW, which well describes the friction of solids. The frictional force and its dependencies on the load are quite different depending on the chemical structures of the gels, surface properties of the opposing substrates, and the measurement condition. The gel friction is explained in terms of interfacial interaction, either attractive or repulsive, between the polymer chain and the solid surface. According to this model, the frictional force is ascribed to the viscous flow of solvent at the interface in the repulsive case. In the attractive case, the force to detach the adsorbing chain from the substrate appears as friction. Surface adhesion between glass particles and gels measured by AFM showed a good correlation with the friction, which support the repulsion-adsorption model proposed by authors.

Intelligent Gels

AbstractWe reported an electro-driven chemomechanical hydrogel showing quick responses with worm-like motility. The principle of the motion is based on the molecular assembly reaction of cationic surfactant and negatively charged hydrogel. And direction of complexation accompanying gel contraction is controlled by changing the polarity of the applied electric field. Both thermodynamics and kinetics of surfactant binding and diffusion are investigated experimentally and theoretically. We also reported shape memory hydrogel by order-disorder transition of alkyl side chain, and some examples od friction of hydrogels showing that frictional behaviors of hydorgels do not conform to Amonton's law.

Friction of hydrogel and polyurethane elastic layers when sliding against each other under a mixed lubrication regime

The use of a compliant material to form the articulating surfaces of a replacement joint has been shown to encourage fluid film lubrication in the joint in conditions of cyclic loading and continuous joint motion. When the joint is heavily loaded with little or no movement, however, joints consisting of a metal component articulating on a compliant layer have been shown to give high friction. This study investigates the friction when two compliant layered components slide against each other in conditions of mixed lubrication, which are present in the joint during slow heavily loaded motion and compares the results with a hard metallic surface sliding on the compliant polyurethane and hydrogel materials. Start up friction on polyurethane-polyurethane contacts was extremely high (coefficient of friction up to 4) and was significantly greater than start up friction for metal on polyurethane, or polyurethane on hydrogel. Metal on hydrogel gave the lowest value for start up coefficient of friction of 0.25. The polyurethane-polyurethane combination gave stick slip motion during low velocity sliding, with peak coefficients of friction rising to 4 during the stick phase, causing damage to the surfaces. The steady state sliding coefficients of friction for the other three material combinations were similar and in the range 0.07 to 0.1. The most appropriate material combination for cushion form joints is a hard surface sliding on a compliant layer and the compliant layer should preferably be porous.

Friction of hydrogel and polyurethane elastic layers when sliding against each other under a mixed lubrication regime

The use of a compliant material to form the articulating surfaces of a replacement joint has been shown to encourage fluid film lubrication in the joint in conditions of cyclic loading and continuous joint motion. When the joint is heavily loaded with little or no movement, however, joints consisting of a metal component articulating on a compliant layer have been shown to give high friction. This study investigates the friction when two compliant layered components slide against each other in conditions of mixed lubrication, which are present in the joint during slow heavily loaded motion and compares the results with a hard metallic surface sliding on the compliant polyurethane and hydrogel materials. Start up friction on polyurethane-polyurethane contacts was extremely high (coefficient of friction up to 4) and was significantly greater than start up friction for metal on polyurethane, or polyurethane on hydrogel. Metal on hydrogel gave the lowest value for start up coefficient of friction of 0.25. The polyurethane-polyurethane combination gave stick slip motion during low velocity sliding, with peak coefficients of friction rising to 4 during the stick phase, causing damage to the surfaces. The steady state sliding coefficients of friction for the other three material combinations were similar and in the range 0.07 to 0.1. The most appropriate material combination for cushion form joints is a hard surface sliding on a compliant layer and the compliant layer should preferably be porous.