The Effects of SurfaceMechanical Deformation andBovine Serum Albumin on the Tribological Properties of Polyvinyl Alcohol Hydrogel as an Artificial Cartilage

+e mechanical and tribological properties of polyvinyl alcohol hydrogel as an arti/cial cartilage were studied under water and bovine serum albumin-lubricated sliding conditions. +e frictional properties of the polyvinyl alcohol hydrogel were investigated via reciprocating frictional tests. +e e1ect of surface mechanical deformation on the tribological properties of the polyvinyl alcohol hydrogel as an arti/cial cartilage was studied by concurrently recording the z-axis displacement and friction coe3cient time.+ree di1erent factors were chosen including load, speed, and lubrication.+e results showed that the albumin solution could reverse the trend in the coe3cient of friction in tests at di1erent loading levels.+ere was no improvement in the friction condition in albumin at low speeds. However, when the speed was increased to 2Hz, the coe3cient of friction was signi/cantly reduced. Wear testing was also conducted, and wear tracks were found on the sample surface. +e results also showed that even though the surface deformation could recover as the water phase of the porous structure recovered, the coe3cient of friction continued to increase simultaneously. +is relationship between mechanical and frictional tests indicated that biphasic lubrication e1ects may not be the only dominant factor underlying the excellent friction properties of polyvinyl alcohol hydrogel.


Introduction
Many patients with osteoarthritis or advanced rheumatoid disease su er from severe joint pain and immobility.Mobility and workplace e ciency will decrease when the injuries resulting from joint disease are severe.e most common therapy is to perform total joint arthroplasty or hemiarthroplasty that replaces the entire joint or parts of the natural joints with di erent arti cial components. is treatment is a practical treatment for patients with severe joint disease.Unfortunately, the complications of joint replacement are always related to tribological failure [1].Failures in joint replacement include pain and wear of the joint material including the natural articular cartilage and the arti cial joint material [2][3][4][5].Histological examination of the peri-implant tissue revealed some wear in the polyethylene particles.is is because the damaged articular cartilage of the joint surface has been surgically replaced by arti cial cartilage, and the surface material of the joint has been worn out.
Articular cartilage covers the end of the joint surface and is a living material with a porous structure containing roughly 70%-85% of its weight in water [6].Articular cartilage has a particularly complex structure and complicated chemical composition.
e frictional properties of articular cartilage are of considerable interest to scientists and engineers in mechanical engineering or materials science for nearly a century.To reduce the severe wear phenomenon of the material in the arti cial joint surface, some new types of arti cial joint materials that could replace natural cartilage have recently been developed.e synthetic articular cartilage from the polymer hydrogel was developed and studied [7].Hydrogels are porous materials with a three-dimensional network structure.Hydrogels have similar properties as articular cartilage including compliance, hydrophilicity, and swelling capacity [8][9][10].e uid content in the solid matrix of articular cartilage is mainly water, and it helps the synovial joint to markedly reduce the friction and wear between cartilage counterparts.
e FDA (Food and Drug Administration, USA) has approved the biomedical application of polyvinyl alcohol (PVA) hydrogel in various instruments including medical membranes, drug delivery vehicles, and contact lenses [11,12].
ere is a fairly similar structure pattern in the porous tissue of hydrogels [13], and broken cartilage might be able to be replaced by hydrogels in surgery [14][15][16].Recently, the arti cial meniscus has been developed via PVA hydrogels to replace damaged rabbit joints [17].e results showed that the meniscus without any break was in good condition.is proved that the polyvinyl alcohol hydrogel could be used in synovial joints for medical purposes.
e aim of this study was to investigate the frictional and mechanical properties of PVA hydrogels as arti cial cartilage when the synovial component was used as a lubricant.To achieve our research aims, experiments were performed to examine the tribological behavior between PVA hydrogels and stainless steel in an albumin solution and deionized water via a reciprocating motion.e coe cient of friction was recorded and evaluated in each test.In addition, the surface morphology and interior structure were tested by scanning electron microscopy.
e PVA had a saponi cation degree of 99%.e PVA is a water-soluble and nontoxic polymer.It has excellent chemical stability and is widely used in medical products.A stainless steel ball (316; 4 mm diameter) was used as the counterpart in the friction tests.Bovine serum albumin (BSA) was supplied by Beijing HMK Biotechnology Co., Ltd., China, with a molecular weight of 68 kDa.

Preparation of Specimen.
PVA hydrogel was prepared via freezing and unfreezing cycles [18].We used an aqueous mixture of PVA and dimethyl sulfoxide (DMSO) solution to obtain a PVA hydrogel via freezing.e ratio of DMSO and water was 80 wt.% and 20 wt.%, respectively.e weight proportion of PVA was 15 wt.%. e preparation of the PVA hydrogel includes several procedures.First, the mixture of PVA and DMSO solution was stirred at 100 °C for more than 3 hours.Next, the solution was rapidly added to the stainless steel mold, which was put in a medical refrigerator for 24 hours at −20 °C.Finally, the mold was warmed to room temperature, and the PVA hydrogel was removed from the mold and put into distilled water for more than 20 days to exchange the DMSO solvent.PVA hydrogel samples were stored in distilled water and immersed in a lubricant for 15 minutes before each test.e specimens were 20 mm long, 15 mm wide, and 3 mm thick.

Instrument and Methodology.
e coe cient of friction between arti cial articular cartilage and stainless steel ball was measured with a UMT-3 Micro-Tribometer (CETR Inc., USA) using a ball-plate reciprocating movement (Figure 1).e stainless steel ball was 4 mm and was placed on a xture on the tribometer.e PVA hydrogel sample was stuck on a stainless steel plate that could be xed on a reciprocating table dragged by a back-actuating motor.e reciprocating distance was 5 mm.In the beginning of the experiment, the PVA hydrogel samples were prepared and submerged in the container lled with lubricant for more than ten minutes.e thickness of the specimen was measured with calipers.ree factors were chosen including load, speed, and lubrication.ere were two levels for the lubricant and the speed.e load has three levels.Each test was conducted three times for 6 minutes.In addition, a long-term wear test of 60 minutes was conducted to examine the surface changes in the frictional surface.
In the reciprocating frictional tests, the displacement of the z-axis was also measured during each test.e reciprocating movement was used in the tester and was designed to   2 Advances in Materials Science and Engineering accurately simulate the joint activities in daily life.Although the mode of motion was simple, it accurately recapitulated the frictional performance of natural or arti cial materials [19].e frictional data and surface change were obtained through these experiments.e coe cient of friction was acquired from the whole reciprocating cycles of the tests.e value of the friction coe cient was calculated by u F/N, where u, N, and F are the friction coe cient, load, and friction force, respectively.e value of the coe cient of friction is the mean value.
e factors and levels for the designed experiments are shown in Table 1.In earlier studies, the range of natural normal joint contact stresses was about 0.1-2.0MPa [20,21].Katta et al. [22] used the contact stress ranging from 0.2 to 0.4 MPa to evaluate the friction properties of the articular cartilage.Pan et al. [7] used a contact stress of 0.37-1.32MPa to study the frictional properties of the PVA hydrogel.Here, we used Hertzian contact theory and contact stresses of 0.4-0.6MPa, which is the range of normal contact stress.e reciprocating sliding speed was from 10 mm/s to 20 mm/s to model human motions such as walking (normal knee joints) [23].
e lubricants used here were deionized water and albumin (1 g/dl).Synovial uid is an e cient lubricant that can reduce cartilage friction in natural joints.It is necessary to study the role of proteins in synovial uids including albumin.Albumin had been studied in lm thickness experiments, and its concentration is 0.7-1.8g/dl in healthy synovial uid [24,25].
e sample topography and worn surface in the friction tests were observed via scanning electron microscopy.e sample was freeze-dried to remove water and coated for scanning electron microscopy [26] (HITACHI S-3500N).
Surface roughness was measured by a TIME3200 prolometer (Time Group Inc., China) to evaluate the surface change in the tests.e sampling length used in the experiments was 0.8 mm. is method could not break or destroy the sample surface.e results were reported as the average value in the measurements.

Measurement of Mechanical Properties.
Figure 2 shows uncon ned compression tests to evaluate the samples.e hydrogel samples were cut with a circular knife, and then cylinder-shaped samples for mechanical measurements were obtained.e diameter of the cylindrical samples was 5 mm.e samples were compressed between two stainless steel plates at 0.6 mm/min.e compression modulus of the PVA hydrogel sample was measured using where E′ is the compression modulus, σ is the stress, and ε is the strain.e UMT-3 Micro-Tribometer (CETR Inc., USA) was also used in the mechanical measurements.It has a force  e displacement in the z-axis was also measured in the reciprocating frictional test.3 and 4 show friction changes for samples lubricated with deionized water or albumin under three loading levels.e coe cient of fraction data (Figure 3) indicates that the coe cient increased at 1 Hz and decreased when the speed was higher.

E ects of Load. Figures
e test was conducted at the lowest loading level (1 N), and the highest coe cient of friction was 0.14 at 1 Hz.
When the samples were tested in albumin solution, the state of friction shifted abruptly in disparate trends at different speeds.
is result re ected the diverse trend of friction coe cient with the lowest value of 0.58 at the three loading levels.is indicates that when the albumin solution was used as the lubricant, the in uence of loading on the coe cient of friction reversed.

E ects of Speed.
e bar graph shows the e ects of speed on the coe cient of friction when the PVA hydrogel was tested with the stainless steel ball.Figures 5-7 indicate that the coe cient of friction increased as the speed increased from 1 Hz to 2 Hz under the lubrication of deionized water.
is situation is unchanged across the three loading levels.Albumin and water had similar trends.e coe cient of friction increased and was slightly altered while the load was in the lowest level of 1 N.As the load increased from 1 N to 2 N, the coe cient of friction markedly dropped from 0.076 to 0.065.When the load was 3 N and the speed increased, the friction condition shifted dramatically from high to low (fallen by one-third).Figure 6 shows a similar phenomenon when the load was 2 N with albumin.e results indicated that the speed had an opposing e ect on the friction with di erent lubricants at high loading levels.

E ects of Lubrication.
e bar charts of Figures 8 and 9 show the impact of lubricant on the tribological properties of the PVA hydrogel as an arti cial cartilage.e bar graph in Figure 8 shows a similar growth trend of the friction state lubricated by deionized water as that with albumin.e absolute value of the coe cient of friction did not indicate signi cant di erences between these two lubricants.As the reciprocating motion frequency increased from 1 Hz to 2 Hz, a divergent view was seen (Figure 9).e coe cient of friction tested in deionized water had di erent values.ey were twice as high as those tested in albumin. is result means that the maximum value of friction coe cient was reduced by lubrication of the albumin solution.Advances in Materials Science and Engineering

Surface Morphology. A long-term test was conducted to examine the wear properties of the sample in albumin.
According to the scanning electron micrograph of the sample (Figure 10), the surface had wear with two obvious wear tracks.e porous surface of PVA hydrogel samples changed.

Mechanical Properties.
Figure 11 shows the relationship between stress and strain as the PVA hydrogel samples were compressed.e contact force was measured as strain moved from 50% to 70%. e changes in compression modulus are also shown in Figure 11.When the strain was 60% and the stress was 0.68 MPa, the compression modulus was 1.14 ± 0.15 MPa.

e Relationship between Frictional and Mechanical Properties.
e displacement of the z-axis could be measured during each reciprocating frictional test.e data were processed by data viewer 2.16 of UMT-3 Micro-Tribometer (CETR Inc., USA).Figures 12 and 13 show that the coe cient of friction and z-axis displacement changed during the 6-minute test (2 Hz, 3 N, and albumin solution).e value variation of z-axis displacement implies deformation at the surface of the sample.At the beginning, a load of 3 N was applied to the surface.It started to deform slightly.On the contrary, the coe cient of starting friction rose sharply and reached a peak value of 0.16.e sample surface then deformed and reached a peak value of 48.31 at 32 seconds.

Surface Roughness.
After the frictional tests were completed, the surface morphology of the sample surface was examined to reveal the broken surface.
e surface roughness was tested before and after these tests under albumin solution (AS) and deionized water (DW).Figure 14 shows that the surface roughness changed slightly in all tests.
e albumin solution could reduce the surface change for its better lubricating properties.

Discussion
Substantial attention has been given to the potential in uence of diverse loading levels on the friction condition of natural and arti cial cartilages [27,28].In some studies, the coe cient of friction declines with increasing load [22].Some people speculated that this is related to the testing conditions or testing con gurations on which the samples were measured [29].e decline in the friction coe cient reached a turning point if the testing condition of these experiments could be changed.Due to the measurement Advances in Materials Science and Engineering range of the force transducer, three loads were used.e increasing load gradually decreased the coe cient of friction at high speed.However, the increasing load also led to an increase in the coe cient of friction while the speed was low.When the speed was low, the PVA hydrogel easily deformed because the load could be applied longer than at high speed in the same sample surface.e high deformation would also result in a large contact area in the frictional pair, and the coe cient of friction would increase concurrently.
Another important parameter was the turning point mentioned above.
e turning point could change the prevailing trend of the friction, and it might be related to the reciprocating speed.Moreover, the phenomenon of hydration and dehydration of the articular cartilage had signi cant e ects on the declining trend of friction [30].In this study, there was no single trend of the coe cient of friction with the stainless steel ball against the arti cial cartilage.When deionized water was used as the lubricant and the load was increased, the variation in friction showed the opposite trend as albumin: the coe cient increased at 1 Hz and reduced at 2 Hz.
In the conditions with di erent stages of osteoarthritis (OA), the damaged state of natural cartilage of human and the use of hyaluronic acid and dipalmitoyl phosphatidylcholine a ect the start-up friction in the initial sample movement [31].When a lubricant solution of hyaluronic acid was used, the coe cient of friction indicated an obvious decline compared to Ringer's solution.Forsey et al. found a friction reduction as high as 0.146 when the lubricant was 6 Advances in Materials Science and Engineering changed from Ringer's to hyaluronic acid solution.Hyaluronic acid is a part of the synovial uid in the natural joint.Dipalmitoyl phosphatidylcholine is a good boundary lubricant on natural cartilage surfaces [32].Here, if the speed was high, there was also a reduction in the coe cient of friction, while the lubricant of deionized water was replaced with albumin solution.However, this trend was dissimilar to that tested under deionized water.Human joints move at a variety of speeds.e results indicated that albumin in synovial uid might not reduce friction when people move slowly.ese other constituents of synovial uid should be examined by more sophisticated methods to determine how speed, load, and lubrication can reduce joint friction.Hills [33] showed that the bene ts of hyaluronic acid as an e ective lubricant are attributed to its wetting properties.It can attract a large amount of water to the synovial uid.Our data showed that albumin is a regular protein with no wetting properties in the lubricating liquid.Albumin is easily dissolved in water or saline.e nonwetting action of albumin means that albumin could be used as a boundary lubricant instead of changing the surface wettability, which is related to the surface energy.Previous research [31] has shown that hyaluronic acid and lipids are a better lubricant for the joint.Albumin could be used in such situations to reduce the friction and wear while clarifying its main function.
In the hydrodynamic area, the role of albumin and bovine serum in lubrication has been studied in lm thickness measurements conducted on a ball-on-disc device with a cobalt-chromium metal femoral head [25].ey found that the aggregation and absorbance of the protein occur at the surface with a thin protein lm to reduce the friction.e friction pair would have low solid contact between their surfaces with resulting low friction.is is because the lm thicknesses are higher at low speeds than at high speeds.It is interesting to compare their results with our results.Figure 3 shows that the coe cient of friction is lower at 1 Hz when the albumin solution was used as the lubricant and the load was 1 N.However, when the load increased to 2 N or 3 N, the coe cient of friction was no longer low at 1 Hz.
ese results can partially be explained by the lm thickness changes mentioned previously [25].In addition, the load factor should be considered in future studies of lm thickness measurements.
e evaluation of the worn PVA hydrogel sample showed that the top layer of the sample failed.e porous structure of the surface changed.
e scanning electron microscopy results revealed that the main wear mechanisms were surface fatigue wear.As a result, the uid support that can greatly reduce this friction would decrease during reciprocating motion [34].
In this study, the variation in the z-axis displacement indicated that the surface deformation can evaluate the relationship between surface depression and frictional properties.e results of friction coe cient and the z-axis displacement showed that the surface deformation changed but lagged behind the transformation of the friction coe cient.When the coe cient of friction reached a peak value of 0.16, the surface sample began to deform.When the  Advances in Materials Science and Engineering coe cient of friction changed steadily, the deformation of the sample had its largest (48.31 mm at 32 seconds).
In the previous study, the PVA hydrogel is regarded as a kind of porous material with many water components that can be used in biomedical areas [35].While the load was applied, the water in its 3D space was pushed out and deformation increased.As a result, friction conditions got worse.e interstitial uid pressurization of the biphasic lubrication mechanism in natural articular cartilage has been tested [36,37] and can also be applied to this arti cial cartilage material.If the water phase of the cartilage takes most proportion of the load, then the solid phase of the cartilage can be protected with lower pressure and the friction coe cient would be reduced.
Here, the relationship between the surface deformation and the friction condition was unclear.When the test duration was more than 32 seconds, a very interesting phenomenon was seen: the sample's surface had recovered, and the friction coe cient has no distinct change in Figure 12.
e data show that the coe cient of friction increased gradually from 0.056 at 32 seconds to 0.06 at 6 minutes, but the sample's recovery from deformation occurred steadily over the same period.ere may be other important factors that greatly a ect the frictional properties of the sample other than biphasic lubrication.
According to the biphasic lubrication mechanism, the friction coe cient of the samples decreases if the surface is recovered with the water phase.However, in our research, when the deformation decreased 32 seconds later, the friction coe cient increased smoothly from 0.056 to 0.060.
is indicates that the surface damage might have a greater in uence than the water phase support induced from surface deformation.e destruction of the hydrogel surface might explain the increasing coe cient of friction during the testing period.Whether the surface change or the surface deformation or other factors dominate this observation requires further study.

Conclusions
e e ects of surface mechanical deformation and bovine serum albumin on the tribological properties of the PVA hydrogel as arti cial cartilage were studied.
e results showed that the lubrication of albumin had a crucial e ect on the tribological properties of arti cial cartilage.e inuence of the load on coe cient of friction was reversed in albumin.When the speed was high (2 Hz), the friction reduced, and the changing trend of the friction coe cient was the same as that lubricated by deionized water.Surface wear appeared, and the sample surface had tracks after wear testing.e mechanical and frictional results showed that even though the surface deformation could recover in the water phase, the porous structure returned, and the coe cient of friction continued to increase simultaneously.
e biphasic lubrication e ects may not be the only factor dominating the excellent friction properties of the PVA hydrogel; the surface change, or other factors, might also have more important e ects.

Figure 4 :
Figure 4: E ect of load under albumin solution lubrication.

Figure 5 :Figure 3 :
Figure 5: E ect of speed under 1 N load condition.

Figure 8 :Figure 7 :Figure 6 :
Figure 8: E ect of lubrication condition at a speed of 1 Hz.

Figure 10 :Figure 9 :
Figure 10: Surface morphology examination by scanning electron microscopy.(a) Unworn surface of PVA hydrogel.(b) Scanning electron micrograph of the wear surface, two wear tracks had formed.(c) e enlarged view of a wear track.

Figure 14 :
Figure 14: Surface roughness of the samples.

Table 1 :
Factors and levels for the design of experiments.