JP3936180B2 - Polymer gel lubrication method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lubrication method in which a polymer gel as a lubricant is disposed between relative sliding members in water, and preferably a water lubrication technique in a sliding portion of an artificial joint used in a living body. Applies to
[0002]
[Prior art]
Water-containing gels such as hydrogels have been widely studied as artificial joint materials since they have low toxicity to living bodies. Clinical application of gel lubrication characteristics under high loads such as artificial joints is being investigated. On the other hand, in the medical device field, micromachines (micromachines) have been developed that provide a manipulating function at the tip of a catheter used during surgery. In consideration of lubricity as a material used for living bodies, water-containing hydrogels are considered suitable.
[0003]
[Problems to be solved by the invention]
Although the load applied to the friction field of such a micromachine is extremely low, there has been little research on the lubrication characteristics of hydrogel under low load conditions. Also, in the micromechanical friction field, the manifestation of the effect of surface force on frictional force cannot be ignored, and in particular, the presence of water at the frictional interface as in the living body causes hydrogen bonding and meniscus force at the interface. Etc. are factors that increase friction. In such a case, the friction is generally reduced by a treatment such as hydrophobization of the surface. However, it is not clear whether surface hydrophobization is effective in materials that swell and swell and contain water like this type of hydrogel. Therefore, it has also been proposed to obtain a lubricating effect by the exuded water by using polyvinyl alcohol having a porous property with high water impregnation ability. However, the water retention capacity of the gel surface is not sufficient, and a lower coefficient of friction could not be obtained.
[0004]
Therefore, in the present invention, in the low load friction of the hydrogel, the chemical properties of the gel surface are controlled to clarify the relationship with the lubrication characteristics, thereby improving the lubrication performance of the friction part in water such as in vivo. An object is to provide a molecular gel lubrication method.
[0005]
[Means for Solving the Problems]
For this reason, the technical solution means adopted by the present invention is:
In a lubricating method in which a polymer gel as a lubricant is disposed between relative sliding members in water, the polymer gel is a hydrophilic polymer gel, and the functional group of the hydrophilic polymer gel is hydrophilic. Further, the hydrophilic polymer gel is selected from polyvinyl alcohol, polyacrylamide, polyacrylic acid and copolymers thereof, and the functional group is a carboxylic acid group added to an amide group. This is a polymer gel lubrication method.
Further, the polymer gel lubrication method is characterized in that irregularities are formed on the lubrication surface of the hydrophilic polymer gel.
The polymer gel lubrication method is characterized in that the ratio of the contact area to the lubrication area by the formation of the irregularities is about 40 to 60%.
[0006]
[Embodiment]
Hereinafter, embodiments of the polymer gel lubrication method of the present invention will be described in detail with reference to the drawings. FIG. 1A is a model diagram of an embodiment of a hydrophilic polymer gel used in the polymer gel lubrication method of the present invention, and FIG. 1B is a model diagram of a hydrophobic polymer gel as a comparative example. 2 is a schematic diagram of the friction tester, FIG. 3 is a time-dependent change diagram of the friction force of the test gel, FIG. 4 is a relationship diagram between the physical properties (polymerization degree) of the test gel and the friction coefficient, and FIG. 5 is a test gel. FIG. 6 is a comparison diagram of the test gel slider (relative sliding member as a friction object) type and the friction coefficient for each gel type, and FIG. 7 is a test diagram. Fig. 8 is a graph showing the relationship between the storage days of the gel and the coefficient of friction, Fig. 8 is a graph showing the relationship between the acrylic acid concentration of the test gel and the coefficient of friction, Fig. 9 is a surface profile of the test gel, and Fig. 10 is a surface profile of the test gel and in the atmosphere. FIG. 11 is a chart showing the relationship between the contact area ratio of the test gel in air and the friction coefficient. FIG. 12 is a relationship diagram between the contact area ratio of the test gel in water and the friction coefficient, FIG. 13 is a relationship diagram between the recess depth of the test gel in air and the friction coefficient, and FIG. 14 is a recess in the test gel in water. It is a relationship figure of depth and a friction coefficient.
[0007]
As shown in FIG. 1 (A), the polymer gel lubrication method of the present invention is a lubrication method in which a polymer gel as a lubricant is disposed between relative sliding members in water. And a hydrophilic polymer gel such as polyvinyl alcohol (PVA), polyacrylamide (PAAm), polyacrylic acid and copolymers thereof, and the functional groups of the hydrophilic polymer gel are a hydroxyl group, an amide group, a carboxyl group A hydrophilic group selected from one or more of the above.
[0008]
In order to guide the polymer gel lubrication method of the present invention, the friction test conducted in relation to the lubrication characteristics by controlling the chemical properties of the gel surface in the low load friction of the hydrogel will be described in detail below.
<Lubrication test>
<Sample>
The properties of the monomer solution for preparing various gels are summarized below.
-Polyvinyl alcohol (PVA) gel In an ultrasonic disperser, PVA was dissolved in distilled water (10 wt%), and freeze-thawed (-10C to room temperature) was repeated 10 times in a cycle of about 24 hours. Obtained.
-Polyacrylamide (PAAm) and N-isopropylacrylamide (NiPAAm) gel An aqueous monomer solution (PAAm is 10 wt%, NiPAAm is 15 wt%) and the cross-linking agent N, N'-methylenebisacrylamide (BIS) at 1 mol%. After dissolution, N, N, N, N-tetramethyleneethylenediamine (TMED) was added to the aqueous solution, and nitrogen or argon substitution was performed in an ice bath. Thereafter, a polymerization initiator ammonium peroxodisulfide (APS) was added at 40 g / l, and the mixture was stirred and molded.
-Acrylamide-acrylic acid copolymerization (Poly (AAm-co-AAc)) gel 10 wt% AAm aqueous solution, TMED was added to an aqueous solution of AAc dissolved at ~ 20 mol%, and then replaced with nitrogen or argon in an ice bath. It was. Thereafter, the polymerization initiator APS was added at 40 g / l, and the mixture was stirred and molded.
-Polyvinyl methyl ether (PVME) gel PVME aqueous solution crosslinked with γ rays.
[0009]
FIG. 2 is a schematic diagram of a low-load friction tester, and each test gel is formed by sandwiching between two glass slides with a spacer having a thickness of 0.5 mm, and thus basically has a smooth surface. With respect to the PVME, pores on the order of μm were formed on the surface and in the bulk. All friction tests were performed in ultrapure water at room temperature. A friction test was performed with the test gel as a sample sample held in water, and the load was monitored with a strain gauge attached to a leaf spring supporting the sample. On the other hand, the obtained frictional force was calculated by detecting the displacement of the leaf spring supporting the slider with an eddy current sensor and multiplying this by the spring constant. The slider (curvature radius: 7.8 mm) made of optical glass (BK-7) and steel (SUJ-2) is used, and the coefficient of static friction accompanying the increase in the holding time from the start of load to the start of friction As a result, it has been reported that the load time until the start of friction is 20-30 s. As other friction conditions, the load was 3 to 25 mN, and the friction speed was 20 μm / s.
[0010]
<Results and discussion>
<Underwater friction test of PVA gel>
FIG. 3 shows a frictional force curve by a typical time change obtained during a friction test in water using PVA gel as a sample. The slider (SUJ-2) was pressed against the sample (PVA gel) and held at a constant load for about 20 seconds, and then the sample stage was moved at a constant speed to start friction. For about 6 s after the start of friction, the apparent friction force increased rapidly to about 1 mN, accompanied by flow resistance due to the elastic deformation of the gel. After that, a stick-slip phenomenon occurred and the frictional state was reached. In steady friction, the average friction coefficient was a very low value of about 0.05. A soft gel as a sample is deformed by being pressed by a hard slider. That is, the contact area depends on the physical properties of the gel.
[0011]
Therefore, the average degree of polymerization in the test gel was changed and the influence on friction was examined. FIG. 4 is a graph showing the relationship between the physical properties (polymerization degree) of the test gel and the coefficient of friction, and PVA gels having an average polymerization degree of 800 to 2500 (both used are those having a monomer aqueous solution concentration of 10 wt%). . The average coefficient of friction tended to decrease with an increase in the degree of polymerization of the PVA gel. The change in the average friction coefficient at this time was as low as 0.1 to 0.05, and in all cases, the friction coefficient was low. PVA gel has been reported to exhibit excellent lubrication characteristics as a soft lubricating film for artificial joints. This is because of the elastohydrodynamic lubrication and the water that exudes from the gel when a load is applied. It is considered a thing.
[0012]
Therefore, a friction test was conducted by changing the moisture content of the gel to 82 to 95%. FIG. 5 is a graph showing the relationship between the moisture content (solution concentration) of the test gel and the friction coefficient. The test gel was PVA gel, and the average degree of polymerization was unified to 3500. At any moisture content, the friction coefficient was 0.1 or less, and the moisture content dependence of the average friction coefficient was small. As a tendency, the higher moisture content gel had a lower coefficient of friction. On the other hand, in the friction test in the atmosphere, the surface was dry even when water was present inside the gel, and the average friction coefficient at that time was as high as 0.9. From these results, it is understood that it is important that sufficient water is present at the slider-gel interface in order to realize low friction in the lubrication by the PVA gel.
[0013]
<In-water friction test of various gels>
In addition to PVA gel, four types of gels of PAAm, NiPAAm and PVME having different molecular structures were used, and a friction test was performed in water. The result is shown in FIG. FIG. 6 is a comparison diagram of a test gel slider (relative sliding member which is a friction object) type and a friction coefficient for each gel type. The friction conditions were the same as in the above-described PVA gel friction test. BK-7 (Glass slider) was used for the slider. In all gels, the concentration of the monomer aqueous solution before gel preparation was adjusted to 10 wt%. The average coefficient of friction of each was 0.1 for the lowest PVA gel, 0.2 for PAAm gel, 0.9 for NiPAAm and PVME gel. Since NiPAAm and PVME gel have the same coefficient of friction as PVA gel with a dry surface, it was found that these materials are poor in lubricity even in water.
[0014]
Next, the PVA gel, PAAm, and NiPAAm gel were subjected to a friction test by changing the slider from BK-7 to SUJ-2 (Steel slider). The PVA gel showed almost no change in the coefficient of friction due to the difference in the slider material, and both showed low values of 0.1, indicating excellent lubricity. The PAAm gel has an extremely high friction coefficient of 1 or more when the slider is SUJ-2. On the other hand, for NiPAAm gel, the friction coefficient decreased from 0.9 to 0.4 by setting the slider to SUJ-2. In this test, since the counterpart material is gel and soft, it may be considered that the contact area of the slider depends only on the gel and not on the slider. That is, the fact that the coefficient of friction strongly depends on the slider material is thought to be due to the chemical properties of the slider-gel interface being involved in the friction.
[0015]
The reason why the average friction coefficient of the PVA gel is the lowest and the PVME gel is high is considered from the viewpoint of the molecular structure of the gel. The PVA gel has a hydrophilic hydroxyl group (OH group) in the molecular side chain. Has a hydrophobic OCH ₃ group. That is, it is considered that the PVA gel having a hydrophilic group has a high ability to retain water acting as a lubricant at the slider-gel interface. Further, when the PAAm gel and the NiPAAm gel were compared, when the glass slider was used, the PAAm gel having a hydrophilic amide group showed a lower friction coefficient than the NiPAAm gel, similarly to the PVA gel and the PVME gel. The NiPAAm gel also has an amide bond in the molecular side chain, but the isopropyl group, which is a hydrophobic group, is a steric hindrance and is hydrophobic. However, when SUJ-2 is used as the slider material, further examination is necessary for the result that the influence on the friction coefficient is reversed.
[0016]
<Change in friction coefficient of PAAm gel over time>
FIG. 7 is a graph showing the relationship between the storage days of the test gel and the friction coefficient, and shows the change in the friction coefficient when the storage period from the preparation of the PAAm gel to the friction coefficient test is changed. The slider used was BK-7. Those that exceeded 100 days after the gel was prepared had an average coefficient of friction of 0.1 or less, and those that passed one year decreased to 0.04. This has the same lubricity as PVA gel. A study of the PAAm-acetone system shows a correlation between storage time and gel swelling, which is reported to be due to hydrolysis of the amide bond moiety. That is, in this test, the PAAm gel was hydrolyzed to have a COOH (carboxyl) group in the side chain, and like the OH group of the PVA gel, it became easy to interact with water, so the friction coefficient was PVA gel. It is thought that it fell to the value equivalent to.
[0017]
<Lubrication with copolymer gel>
In the discussion so far, it has been considered that the PVA gel showed high lubricity regardless of the slider material because the OH group of the gel molecule side chain retains water acting as a lubricant. Further, the result of the decrease in the average friction coefficient of the PAAm gel with the storage period was considered to be the influence of the increase in COOH groups due to the hydrolysis of the gel molecules in the above paragraph. Therefore, the influence of COOH groups on the friction coefficient was examined using a copolymer of PAAm and PAAc. SUJ-2 was used for the slider. The addition concentration of AAc (acrylic acid) was 0 to 20% by weight ratio with AAm, and the load was 5 to 20 mN.
[0018]
The friction test results are shown in FIG. FIG. 8 is a graph showing the relationship between the acrylic acid concentration of the test gel and the friction coefficient. When AAc was not added, the average friction coefficient was as high as about 1. However, the average friction coefficient decreased to 0.1 or less by adding 0.2% of AAc. Thereafter, when AAc was increased to 5 and 10%, the friction coefficient increased slightly, and reached 20% when it was 20%. This tendency did not depend on the load. Regarding the result that the friction coefficient increases with the increase in the additive concentration, it is considered that the change in the physical properties of the gel is also involved, but since the additive concentration of AAc when the friction coefficient is low is extremely low, The effect of physical properties is considered to be small. Further, it was found from this result that the addition of a small amount of AAc is important for lowering the friction coefficient.
[0019]
Furthermore, the improvement of the hydrophilicity by a carboxyl (COOH) group can be considered as the reason. An interesting result was obtained that the coefficient of friction again increased as the additive concentration increased. The reason will be described with reference to the model shown in FIG. In a state where the addition concentration is low, as shown in FIG. 1 (A), in addition to the amide (CONH ₂ ) group of AAm, the hydrophilicity of the gel surface is increased by introducing the COOH group of AAc, and water acting as a lubricant is retained. It is thought that the ability to keep increased. However, when the concentration of AAc increases, as shown in the comparative example of FIG. 1B, the CONH ₂ group and the COOH group are coupled by hydrogen bonds, and the lubricating CONH ₂ group and the COOH group are attached to the gel surface. It is thought that it does not exist and becomes incorporated into the bulk of the gel. For this reason, it is considered that the hydrophobic gel molecule main chain is exposed on the gel surface and the lubricity is lowered.
[0020]
The fact that the friction coefficient was low in the PVME or NiPAAm gel having a hydrophobic functional group as opposed to the low friction coefficient in the PVA gel or PAAm gel having a hydrophilic group also supports this idea. This result shows that there is almost no load dependence, and further, the amount of change in the friction coefficient is larger than when the gel hardness or moisture content is changed (see FIGS. 4 and 5). It was found that the introduction of the group was effective in reducing friction. In other words, it was revealed that the chemical properties (molecular structure, hydrophilicity) of the gel have a great influence on the friction.
[0021]
FIG. 9 is a surface shape diagram of the test gel. The surface shape of the test gel was considered, where b is the contact surface length that forms a friction surface with a planar shape of a tortoiseshell pattern, a is the recess length with little contribution to friction, and h is the recess depth. FIG. 10 is a table showing the relationship between the surface shape of the test gel showing the test results and the coefficient of friction in air and water. It can be understood from the relationship between the contact area ratio and the friction coefficient in FIGS. 10 and 11 (in the air test) and FIG. 12 (underwater test) that the friction coefficient is the highest when the contact area ratio is 40 to 60%. It is small. Regarding the recess depth h, the friction coefficient is high when flat and relatively shallow at 1.92 μm in both the atmospheric and underwater tests. The relationship is not clear. If the recess has a certain depth, it is considered that water for low friction can be secured at the slider-gel interface which is the lubrication surface.
[0022]
As described above, as a result of the friction test of various gels under water at a low load (5 to 20 mN), the following has been clarified. PVA gel showed an excellent lubricity with an average friction coefficient as low as about 0.1 regardless of the slider material. PVA gel with high water content and high polymerization degree has the lowest friction coefficient, so even under such low load conditions, the contact area is small and the friction interface of water after applying load It was suggested that the exudation of spills affected the friction. The lubricity of the gel was strongly dependent on its molecular structure and when compared with similar gels, the friction coefficients were PVA gel <PVME gel and PAAm <NiPAAm. That is, it became clear that the hydrophilicity of the gel is deeply related to the lubricity in water. Furthermore, the friction coefficient was remarkably lowered by adding a small amount of AAc to PAAm and introducing a hydrophilic functional group (COOH group) into the gel. From the result that the PVA gel has the lowest friction coefficient, it was concluded that in the lubrication with hydrogel, it is important that water is retained at the slider-gel interface and acts as a lubricant.
[0023]
As described above, the embodiment of the present invention has been described, but within the scope of the present invention, the kind of hydrophilic polymer gel as a lubricant (polyvinyl alcohol, polyacrylamide, polyacrylic acid, and copolymers thereof) In addition, an appropriate hydrophilic polymer gel such as polystyrene sulfonic acid), a target site where the polymer gel is used as a lubricant, the type of hydrophilic group in the functional group (hydroxyl group, amide group, carboxyl group, amino Group, ketone group, ester group, sulfonic acid group, ether group, etc.), addition mol% of carboxyl group to amide group in functional group, uneven shape of lubricating surface of hydrophilic polymer gel, contact area in the uneven shape About a ratio, a recessed part depth, etc., it can select suitably.
[0024]
【The invention's effect】
As described above in detail, according to the present invention, in the lubricating method in which the polymer gel that is a lubricant is disposed between the relative sliding members in water, the polymer gel is used as a hydrophilic polymer gel. In addition, by making the functional group of the hydrophilic polymer gel a hydrophilic group, the polymer gel is excellent in water due to the presence of the hydrophilic functional group that easily interacts with the lubricant water. It became possible to use as.
[0025]
In addition, when the hydrophilic polymer gel is selected from polyvinyl alcohol, polyacrylamide, polyacrylic acid and copolymers thereof, a material having low toxicity is required by adopting an optimal hydrophilic polymer gel. It can contribute to the improvement of innocuous lubricity due to water in vivo.
Furthermore, when the functional group is selected from one or more of a hydroxyl group, an amide group, a carboxyl group, etc., the lubricity is greatly improved by a combination of a suitable hydrophilic polymer gel and the functional group. Was realized.
[0026]
Furthermore, when several mol% of a carboxyl group is added to the amide group as the functional group, it is optimized most effectively without forming a hydrophobic surface due to the generation of intramolecular hydrogen bonds, and the friction coefficient is reduced ( About 1/10 of the conventional level). Moreover, when unevenness is formed on the lubrication surface of the hydrophilic polymer gel, the lubrication performance is further improved by the water retention function of the recess.
[0027]
Furthermore, when the ratio of the contact area to the lubrication area due to the unevenness formation is about 40 to 60%, the friction reduction effect is further improved by an appropriate balance between the friction area as the convex part and the water retaining part as the concave part. .
Thus, according to the present invention, there is provided a polymer gel lubrication method for improving the lubrication performance of a friction part in water such as in vivo.
[Brief description of the drawings]
FIG. 1 shows an embodiment of a polymer gel lubrication method according to the present invention, and FIG. 1 (A) shows an embodiment of a hydrophilic polymer gel used in the polymer gel lubrication method of the present invention. FIG. 1B is a model diagram of a hydrophobic polymer gel as a comparative example.
FIG. 2 is a schematic view of the friction tester.
FIG. 3 is a time-dependent change diagram of the friction force of the test gel.
FIG. 4 is a graph showing the relationship between the physical properties (degree of polymerization) of the test gel and the friction coefficient.
FIG. 5 is a graph showing the relationship between the moisture content (solution concentration) of the test gel and the friction coefficient.
FIG. 6 is a comparison diagram of the type of slider (relative sliding member that is a friction object) of the test gel and the coefficient of friction for each type of gel.
FIG. 7 is a graph showing the relationship between the storage days of the test gel and the friction coefficient.
FIG. 8 is a graph showing the relationship between the acrylic acid concentration of the test gel and the friction coefficient.
FIG. 9 is a surface shape diagram of the test gel.
FIG. 10 is a table showing the relationship between the surface shape of the test gel and the friction coefficient in air and water.
FIG. 11 is a graph showing the relationship between the contact area ratio of the test gel in air and the friction coefficient.
FIG. 12 is a graph showing the relationship between the contact area ratio of the test gel in water and the friction coefficient.
FIG. 13 is a graph showing the relationship between the depth of the recess of the test gel and the coefficient of friction in the atmosphere.
FIG. 14 is a graph showing the relationship between the depth of the concave portion of the test gel and the coefficient of friction in water.

Claims (3)

In a lubricating method in which a polymer gel as a lubricant is disposed between relative sliding members in water, the polymer gel is a hydrophilic polymer gel, and the functional group of the hydrophilic polymer gel is hydrophilic. Further, the hydrophilic polymer gel is selected from polyvinyl alcohol, polyacrylamide, polyacrylic acid and copolymers thereof, and the functional group has a carboxyl group added to an amide group. Polymer gel lubrication method. 2. The polymer gel lubrication method according to claim 1, wherein irregularities are formed on the lubrication surface of the hydrophilic polymer gel. The method of lubricating a polymer gel according to claim 2, wherein a ratio of a contact area to a lubrication area by the unevenness formation is approximately 40 to 60%.

Images