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Frictional properties of physically cross-linked PVA hydrogels as artificial cartilage
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Biosurface and Biotribology 2 (2016) 11–17 www.elsevier.com/locate/bsbt
Frictional properties of physically cross-linked PVA hydrogels as artificial cartilage S. Sasakia,n, T. Murakamia, A. Suzukib b
a Research Center for Advanced Biomechanics, Kyushu University, Fukuoka 819-0395, Japan Research Institute of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan
Received 20 November 2015; received in revised form 16 February 2016; accepted 17 February 2016
Abstract Physically cross-linked PVA gels (freeze-thawed PVA gel and cast-dried PVA gel) are used as biocompatible materials since special chemicals were not used in their preparation. Cast-dried PVA gels are therefore promising as candidate materials for artificial cartilage because of their excellent frictional property. In this study, three types of gels-freeze-thawed gels, cast-dried gels and hybrid gels which are laminated gels consisting of both cast-dried layer and freeze-thawed layer were prepared and analyzed. The frictional behaviors of these three PVA hydrogels were examined in reciprocating test. Further, the effects of sterilization with ethanol on frictional behaviors of these PVA hydrogels were evaluated. & 2016 Southwest Jiaotong University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Poly(vinyl alcohol) hydrogel; Physically cross-linked gel; Frictional property; Swelling property; Atomic force microscope
1. Introduction Poly(vinyl alcohol) (PVA) is a synthetic, water-soluble polymer, which has low toxicity, excellent mechanical strength, and high biocompatibility. Physically cross-linked PVA gels have been prepared by two method. One is repeated Freeze-thawing method [1], which is a conventional method. The gel so prepared is called here as FT gel. The second is a cast-drying method, and the gel is called as CD gel [2,3]. In this method, gelation occurs during the drying process after casting a PVA solution into a mold. Cast-drying method is well-known for making a transparent film, but newly recognized method to prepare PVA hydrogels [2]. Both FT and CD gels have three-dimensional amorphous network, physically cross-linked by microcrystallites, and the nanometer-scale network structures are similar. However, the network structures on a micrometer-scale differ from each other. This is because the distribution of microcrystallite is quite different. n
Corresponding author. E-mail address: [email protected] (S. Sasaki). Peer review under responsibility of Southwest Jiaotong University.
There are many reports for use of FT gels as candidate materials for artificial cartilage. For example, FT gel hip prosthesis showed good frictional behavior similar to that of the natural joint [4,5]. Furthermore, in case of the knee prosthesis model using FT gel and simulated synovial fluid under walking conditions there was very low frictional property [6]. Recently, it has been reported that CD gels showed lower coefficient of friction than FT gels and natural cartilage in reciprocating friction tests in saline solution [7,8], and CD gels exhibited lower coefficient of friction with marginal increase with increase in sliding distance. These results show that frictional properties of physically crosslinked gels were determined by both surface and bulk properties including permeability. More recently, a new type of physically cross-linked PVA gel, a laminated hybrid gel of CD on FT gel (hybrid PVA hydrogel), was successfully developed [9]. The hybrid PVA hydrogel showed a very low coefficient of friction of about 0.01 in reciprocating friction test in saline solution and appeared undamaged under microscopic observation. Hydrogels similar to natural articular cartilage are required for the development of artificial joints with lubrication mechanism of natural synovial joints. Therefore, PVA gels
http://dx.doi.org/10.1016/j.bsbt.2016.02.002 2405-4518/& 2016 Southwest Jiaotong University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
0.20 After EtOH treatment (in simulated synovial fluid)
µk (-)
0.15
As prepared (in water)
0.10 After EtOH treatment (in water)
0.05 As prepared (in simulated synovial fluid)
0.00 0
50
100 150 200 Sliding Distance (m)
250
300
6
Wt / Wd (-)
5 4 3 2 1 0
Before After EtOH treatment
Fig. 1. (a) Coefficient of friction and (b) swelling ratio of FT gels with/without germicidal treatment with EtOH.
show promise for medical applications, specifically, as candidate materials for artificial cartilage. However, sterilization of PVA gel is essential if they have to be used as candidate material for articular cartilage. There are many reports about sterilization of gel [10,11], for example gamma-ray sterilization, but this method is difficult to make the equipment available. And autoclave sterilization cannot be used for physically cross-linked PVA gels as microcrystallites are completely destroyed above 90 1C in water. Chemical sterilization by ethanol (EtOH) treatment is another option, but the effect of EtOH on frictional property of PVA gel needs to be investigated. In this study, the effect of sterilization by EtOH treatment on frictional properties of physically crosslinked PVA gels for use as artificial cartilage is discussed. 2. Materials and methods 2.1. Sample preparation 2.1.1. FT gel PVA pre-gel solution (15 wt%) was prepared by dissolving PVA powder (PVA117; Kuraray Co., Ltd.) in pure water at a temperature around 90 1C over a period of 2 h and the degree
of polymerization was 1700, and the degree of hydrolysis was between 98 and 99 mol%. The PVA pre-gel solution (30.0 g) was then decanted into a polystyrene dish (with an inner diameter of 85 mm). The pre-gel solution was frozen for 8 h at –20 1C and then thawed for 9 h at 4 1C, and this process was repeated 4 times. The FT gel thus obtained were soaked in 1 L pure water for 48 h. 2.1.2. CD gel PVA pre-gel solution (30.0 g) was decanted into a polystyrene dish (with an inner diameter of 85 mm) and left to dry at 8 1C at 50%RH conditions for 7 days and then at 20 1C and 40%RH [12] in a temperature and humidity controlled chamber (SU-242, ESPEC) until the weight became almost constant. The CD film thus obtained was soaked in 1 L pure water for 48 h. 2.1.3. Hybrid PVA hydrogel The PVA pre-gel solution (15.0 g) was decanted into a polystyrene dish (with an inner diameter of 85 mm). The first layer, FT gel layer, was obtained by repeating 4 times the freezing (for 8 h at –20 1C) and thawing (for 9 h at 4 1C) process. After freezing-thawing process for FT gel formation the second CDgel layer was prepared by adding 15 g of the PVA-pre gel solution on the FT gel layer. This was then dried at 8 1C and 50%RH conditions for 7 days and then at 20 1C and 40%RH [12]. The drying conditions were controlled for temperature and humidity using a controlled chamber. The hybrid PVA hydrogel film thus obtained was soaked in 1 L pure water for 48 h. 2.2. Germicidal treatment with ethanol (EtOH) All three gels FT, CD and hybrid PVA hydrogels were soaked in 70% Ethanol for 24 h for sterilization. After EtOH sterilization, the gels were soaked in sterile pure water for 48 h. 2.3. Swelling ratio To analyze the swelling property, about 10 10 mm2 samples were cut out from the swollen gels, and then the weight of swollen sample, Wt, was measured. After the measurement, the gels were dried at 60 1C for 24 h, and the weight of dried sample, Wd, was measured. From these measurements, swelling ratio, Wt/Wd, was calculated. 2.4. Reciprocating friction test We cut 40 10 mm2 fully swollen gel plate as friction test sample. The ball-on-plate reciprocating friction test was conducted at room temperature (about 23–25 1C), at a sliding speed of 20 mm s 1 and at a sliding distance of 300 m (stroke: 25 mm and total 6000 cycles) at a constant load of 5.88 N, using friction tester (TriboGear TYPE:38, HEIDON). Test conditions such as sliding speed, stroke, sliding distance and constant load were selected to properly evaluate friction and wear properties of PVA hydrogels in mixed or boundary
S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
13
400 nm As prepared
After treatment
500 nm
500 nm 400 nm
3.0 2.5
Frequency (%)
Before treatment
2.0
After treatment
1.5 1.0 0.5 0.0
0
2
4 Young's Modulus (MPa)
6
8
Fig. 2. AFM topography images of FT gel at swollen state (a) before and (b) after EtOH treatment. (c) Young’s modulus distribution of surface of FT gel.
lubrication mode in simplified reciprocating test under similar conditions to the previous studies [9,12], although simulator tests are required before clinical application. Polycrystalline alumina ceramic ball of 26 mm diameter was used as an upper ball specimen. The lubricant was pure water or simulated synovial fluid. From the measured value of tangential force, coefficient of friction, μk, was calculated. Simulated synovial fluid was saline solution containing 0.15 M NaCl (Otsuka Pharmaceutical Factory Inc.), 0.5 wt% sodium hyaluronate (molecular weight: 9.2 105), 0.01 wt% Lα-dipalmitoyl phosphatidylcholine, 1.4 wt% bovine serum albumin (Wako Pure Chemical Industries Ltd.) and 0.7 wt% human γ-globulin (Wako Pure Chemical Industries Ltd.). 2.5. Atomic force microscope measurement The surface images of swollen gels were obtained by using an atomic force microscope (AFM) system (Dimension Icon AFM, Bruker). The probe was silicon nitride (SNL-10A, Bruker) and
the measurement mode was peakforce tapping mode. Young’s modulus of hydrogel was estimated by the indentation test. 3. Results and discussions 3.1. FT gel Fig. 1(a) shows effect of EtOH treatment on coefficient of friction, μk, of FT gels in different lubricants, and Fig. 1(b) shows swelling ratio of FT gels under different conditions. μk of FT gels before EtOH treatment increased gradually in water, whereas after treatment FT gels showed constant friction. In simulated synovial fluid, both before and after EtOH treatment FT gels showed constant friction, but μk of after EtOH treatment FT gels was higher than that of FT gel before treatment. In addition, swelling ratio decreased after treatment. The decrease of swelling ratio was caused by increase in physically cross-linked points (microcrystallites) during EtOH treatment, as EtOH is a poor solvent for PVA gel. It was noticed that EtOH treatment reduced
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S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
0.20 After EtOH treatment (in water)
µk (-)
0.15
0.10 After EtOH treatment (in simulated synovial fluid) As prepared (in water)
0.05
As prepared (in simulated synovial fluid)
0.00 0
50
100 150 200 Sliding Distance (m)
250
300
6
Wt / Wd (-)
5 4 3 2 1
Before After EtOH treatment
Fig. 3. (a) Coefficient of friction and (b) swelling ratio of CD gels with/without germicidal treatment with EtOH.
reciprocating friction to 300 m in the sliding test in pure water. It appeared that increase in microcrystallites (decrease in swelling ratio), improved wear resistance. Fig. 2(a) and (b) shows AFM image in pure water of FT gel surface before and after EtOH treatment. FT gels had a rough and dendritic surface [11]. As shown in this figure, the surface of swollen gels did not change before and after EtOH treatment. However as shown in Fig. 2(c), Young’s modulus of FT gel increased after EtOH treatment. It is considered that treatment with EtOH affected the gel bulk properties rather than gel surface properties and that the decrease in swelling ratio and the increase of Young’s modulus were caused by formation additional microcrystallites in bulk largely high hydraulic permeability. Thus, surface properties of FT gels was not affected by EtOH treatment. 3.2. CD gel Fig. 3(a) shows frictional behavior of CD gel before and after treatment with EtOH. μk of CD gel maintained initial value during reciprocating test in pure water. However absolute μk value dramatically increased after EtOH treatment. Furthermore, it was noticed that μk decreased in simulated synovial fluid in comparison to water. It is believed that
components of lubricant improved frictional property on the CD gel surface. As shown in Fig. 3(b), swelling ratio apparently decreased on EtOH treatment. This would be due to the formation of additional microcrystallites during shrinking in poor solvent. Increase of μk are considered to be related to the hardening of gel surface. Fig. 4 shows the AFM image and it can be seen that EtOH treatment had changed the surface properties due to increase in microcrystallites formation. CD gel had a smooth surface about 10–20 nm height [13]. After EtOH treatment, CD gel surface became dendritic and similar to FT gel. It would appear that the gel surface shrank inhomogeneously. Furthermore Young’s modulus increased after EtOH treatment in Fig. 4(c). The increment of μk after EtOH treatment was mainly caused by change in surface condition of gel and it shows that friction of hydrogel was controlled by adsorbed film formation on gel surface in simulated synovial fluid. 3.3. Hybrid PVA gel Fig. 5(a) shows the effect of EtOH sterilization on frictional behavior of hybrid PVA gels in different lubricants, and Fig. 5(b) shows swelling ratio of hybrid PVA hydrogels in different conditions. As shown in Fig. 5(a), untreated hybrid
S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
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400 nm As prepared
After treatment
500 nm
500 nm 400 nm
5
Frequency (%)
4
After treatment
3
2
Before treatment
1
0
0
2
4 6 Young's Modulus (MPa)
8
Fig. 4. AFM topography images of CD gel at swollen state (a) before and (b) after EtOH treatment. (c) Young’s modulus distribution of surface of CD gel.
PVA gel maintained superior low friction during test in simulated synovial fluid. However, μk dramatically increased after EtOH treatment. It was due to surface hardening caused by decrease of swelling ratio of hybrid PVA hydrogels in both lubricants after treatment with EtOH, as shown in Fig. 5(b). It was believed that additional hydrogen bonds were formed during shrinking caused by treatment of EtOH. It was difficult to maintain surface condition and frictional property after sterilization with EtOH. In simulated synovial fluid, μk was lower than in pure water (Fig. 5(a)). However swelling ratio decreased in simulated synovial fluid (Fig. 5(b)). The decrease in swelling ratio indicated that this fluid was poor solvent for hybrid PVA hydrogel, therefore the gel became hard. From these results, it can be concluded that the lubricative property was improved by effective constituents in lubricant. Fig. 6 shows change in AFM image of swollen hybrid gels in pure water before and after EtOH treatment. This figure, shows that surface image did not change with treatment with EtOH. However, as shown in Fig. 7, Young’s modulus of hybrid gel surface increased after EtOH treatment. Therefore it is considered that increase of μk and decrease of swelling ratio
were caused by hardening of gel surface due to formation additional microcrystallites during EtOH treatment. And it is showed that the decrease of μk in simulated synovial fluid depends to a large extent on constituents in lubricant.
3.4. Effect of EtOH treatment on frictional property of PVA gel The increase of Young’s modulus were observed in all type of PVA gels because EtOH is poor solvent for PVA gels. However, FT gel showed less increase of μk. It seems that the shrinking of FT gel occurred throughout the gel, and that the shrinking of CD gel and Hybrid PVA gel occurred in surface mainly, as shown in AFM image. It is because FT gel has high permeability. As CD gel and hybrid PVA gel have low permeability, surface hardening was noticeable. From these results, germicidal treatment that penetrate through gel at short time was required. As an alternate method in our next study, we are evaluating the influence of gamma-ray radiation on tribological, mechanical and swelling properties of PVA hydrogels.
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S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
0.20
0.15 µk (-)
After EtOH Treatment (in pure water)
0.10 After EtOH Treatment (in simulated synovial fluid)
0.05
As prepared (in pure water) As prepared (in simulated synovial fluid)
0.00 0
50
100 150 200 250 Sliding Distance (m)
300
6
Wt / Wd (-)
5 4
EtOH treatment before after
3 2 1
Pure water
Simulated synovial fluid Lubricant
Fig. 5. (a) Coefficient of friction and (b) swelling ratio of hybrid PVA gels with/without germicidal treatment with EtOH.
400 nm
After EtOH treatment
As prepared
1 µm
1 µm −400 nm
Fig. 6. AFM topography images of hybrid PVA gels before/after germicidal treatment with EtOH.
4. Conclusions Effect of chemical sterilization with EtOH treatment on frictional properties of three types of physically cross-linked PVA gels were compered. It was shown that hybrid PVA hydrogels had excellent frictional properties in comparison to than FT gel and CD gel in cases without germicidal treatment before sterilization with ethanol. But, except FT gel in water,
EtOH treatment increased the μk value due to formation of additional hydrogen bonds. It was also found that μk decreased in simulated synovial fluid because the lubricating property was improved by adsorbed film formation with constituents in the lubricant. Thus for use of such candidate material for development of articular cartilage the sterilization method and its effect has to be carefully studied and the lubricant should also be appropriately selected or it should be evaluated using
S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
1.0 Before Treatment
After Treatment
Frequency (%)
0.8 0.6 0.4 0.2 0.0
0
2 4 6 Young's Modulus (MPa)
8
Fig. 7. Young’s modulus distribution of surface of hybrid PVA gel.
simulated synovial fluid to get results as close to the in vivo situation as possible. And it was shown that germicidal treatment that penetrate through gel at short time was required, thus gamma-ray sterilization should be considered. Acknowledgments The authors would like to thank Kuraray Co., Ltd. for kindly supplying PVA powders. This work was supported in part by the Grant-in-Aid for Specially Promoted Research of Japan Society for the Promotion of Science (KAKENHI No. 23000011). References [1] K. Tamura, et al., A new hydrogel and its medical application, ASAIO J. 32 (1) (1986) 605–608.
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[2] E. Otsuka, et al., A simple method to obtain a swollen PVA gel crosslinked by hydrogen bonds, J. Appl. Polym. Sci. 114 (1) (2009) 10–16. [3] E. Otsuka, et al., Swelling properties of physically cross-linked PVA gels prepared by a cast-drying method, Prog. Coll. Polym. Sci. 136 (2009) 121–126. [4] K. Mabuchi, et al., Lubrication property and mechanical durability of the joint prostheses containing cartilage-like polyvinyl alcohol gel, J. Soc. Orthop. Biomech. 8 (1986) 101–105. [5] Sasada, T., Biomechanics and biomaterials – friction behavior of an artificial articular cartilage. In: Proceedings of the 3rd world biomaterials congress, 2, 6, 1988. [6] T. Murakami, et al., The adaptive multimode lubrication in knee prostheses with artificial cartilage during walking, Tribol. Ser. 32 (1997) 371–382. [7] T. Murakami, et al. Time-dependent frictional behaviors in hydorogel artificial cartilage materials, in: Proceedings of the 6th International Biotribology Forum: BIOTRIBOLOGY FUKUOKA, 65–69, 2011. [8] T. Murakami, Superior lubricity in articular cartilage and artificial hydrogel cartilage, Proc. IMechE Part J: J. Eng. Tribol. 228 (2014) 1099–1111. [9] A. Suzuki, S. Sasaki, S. Sasaki, T. Noh, K. Nakashima, S. Yarimitsu, et al. Elution and wear of PVA hydrogels by reciprocating friction, In: Proceedings of the world tribology congress, extended abstracts, Torino, 2013. [10] M. Tohfafarosh, et al., Effects of gamma and e-beam sterilization on the chemical, mechanical and tribological properties of a novel hydrogel, J. Mech. Behav. Biomed. Mater. 53 (2016) 250–256. [11] D. Kanjickal, et al., Effects of sterilization on poly(ethylene glycol) hydrogels, J. Biomed. Mater. Res. Part A 87A (3) (2008) 608–617. [12] S. Yarimitsu, et al. Evaluation Of Friction And Wear Properties Of Poly (vinyl Alcohol) Hydrogels As Artificial Articular Cartilage. [13] T. Murakami, et al., Evaluation of a superior lubrication mechanism with biphasic hydrogels for artificial cartilage, Tribol. Int. 89 (2015) 19–26.
Available online at www.sciencedirect.com
Biosurface and Biotribology 2 (2016) 11–17 www.elsevier.com/locate/bsbt
Frictional properties of physically cross-linked PVA hydrogels as artificial cartilage S. Sasakia,n, T. Murakamia, A. Suzukib b
a Research Center for Advanced Biomechanics, Kyushu University, Fukuoka 819-0395, Japan Research Institute of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan
Received 20 November 2015; received in revised form 16 February 2016; accepted 17 February 2016
Abstract Physically cross-linked PVA gels (freeze-thawed PVA gel and cast-dried PVA gel) are used as biocompatible materials since special chemicals were not used in their preparation. Cast-dried PVA gels are therefore promising as candidate materials for artificial cartilage because of their excellent frictional property. In this study, three types of gels-freeze-thawed gels, cast-dried gels and hybrid gels which are laminated gels consisting of both cast-dried layer and freeze-thawed layer were prepared and analyzed. The frictional behaviors of these three PVA hydrogels were examined in reciprocating test. Further, the effects of sterilization with ethanol on frictional behaviors of these PVA hydrogels were evaluated. & 2016 Southwest Jiaotong University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Poly(vinyl alcohol) hydrogel; Physically cross-linked gel; Frictional property; Swelling property; Atomic force microscope
1. Introduction Poly(vinyl alcohol) (PVA) is a synthetic, water-soluble polymer, which has low toxicity, excellent mechanical strength, and high biocompatibility. Physically cross-linked PVA gels have been prepared by two method. One is repeated Freeze-thawing method [1], which is a conventional method. The gel so prepared is called here as FT gel. The second is a cast-drying method, and the gel is called as CD gel [2,3]. In this method, gelation occurs during the drying process after casting a PVA solution into a mold. Cast-drying method is well-known for making a transparent film, but newly recognized method to prepare PVA hydrogels [2]. Both FT and CD gels have three-dimensional amorphous network, physically cross-linked by microcrystallites, and the nanometer-scale network structures are similar. However, the network structures on a micrometer-scale differ from each other. This is because the distribution of microcrystallite is quite different. n
Corresponding author. E-mail address: [email protected] (S. Sasaki). Peer review under responsibility of Southwest Jiaotong University.
There are many reports for use of FT gels as candidate materials for artificial cartilage. For example, FT gel hip prosthesis showed good frictional behavior similar to that of the natural joint [4,5]. Furthermore, in case of the knee prosthesis model using FT gel and simulated synovial fluid under walking conditions there was very low frictional property [6]. Recently, it has been reported that CD gels showed lower coefficient of friction than FT gels and natural cartilage in reciprocating friction tests in saline solution [7,8], and CD gels exhibited lower coefficient of friction with marginal increase with increase in sliding distance. These results show that frictional properties of physically crosslinked gels were determined by both surface and bulk properties including permeability. More recently, a new type of physically cross-linked PVA gel, a laminated hybrid gel of CD on FT gel (hybrid PVA hydrogel), was successfully developed [9]. The hybrid PVA hydrogel showed a very low coefficient of friction of about 0.01 in reciprocating friction test in saline solution and appeared undamaged under microscopic observation. Hydrogels similar to natural articular cartilage are required for the development of artificial joints with lubrication mechanism of natural synovial joints. Therefore, PVA gels
http://dx.doi.org/10.1016/j.bsbt.2016.02.002 2405-4518/& 2016 Southwest Jiaotong University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
12
S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
0.20 After EtOH treatment (in simulated synovial fluid)
µk (-)
0.15
As prepared (in water)
0.10 After EtOH treatment (in water)
0.05 As prepared (in simulated synovial fluid)
0.00 0
50
100 150 200 Sliding Distance (m)
250
300
6
Wt / Wd (-)
5 4 3 2 1 0
Before After EtOH treatment
Fig. 1. (a) Coefficient of friction and (b) swelling ratio of FT gels with/without germicidal treatment with EtOH.
show promise for medical applications, specifically, as candidate materials for artificial cartilage. However, sterilization of PVA gel is essential if they have to be used as candidate material for articular cartilage. There are many reports about sterilization of gel [10,11], for example gamma-ray sterilization, but this method is difficult to make the equipment available. And autoclave sterilization cannot be used for physically cross-linked PVA gels as microcrystallites are completely destroyed above 90 1C in water. Chemical sterilization by ethanol (EtOH) treatment is another option, but the effect of EtOH on frictional property of PVA gel needs to be investigated. In this study, the effect of sterilization by EtOH treatment on frictional properties of physically crosslinked PVA gels for use as artificial cartilage is discussed. 2. Materials and methods 2.1. Sample preparation 2.1.1. FT gel PVA pre-gel solution (15 wt%) was prepared by dissolving PVA powder (PVA117; Kuraray Co., Ltd.) in pure water at a temperature around 90 1C over a period of 2 h and the degree
of polymerization was 1700, and the degree of hydrolysis was between 98 and 99 mol%. The PVA pre-gel solution (30.0 g) was then decanted into a polystyrene dish (with an inner diameter of 85 mm). The pre-gel solution was frozen for 8 h at –20 1C and then thawed for 9 h at 4 1C, and this process was repeated 4 times. The FT gel thus obtained were soaked in 1 L pure water for 48 h. 2.1.2. CD gel PVA pre-gel solution (30.0 g) was decanted into a polystyrene dish (with an inner diameter of 85 mm) and left to dry at 8 1C at 50%RH conditions for 7 days and then at 20 1C and 40%RH [12] in a temperature and humidity controlled chamber (SU-242, ESPEC) until the weight became almost constant. The CD film thus obtained was soaked in 1 L pure water for 48 h. 2.1.3. Hybrid PVA hydrogel The PVA pre-gel solution (15.0 g) was decanted into a polystyrene dish (with an inner diameter of 85 mm). The first layer, FT gel layer, was obtained by repeating 4 times the freezing (for 8 h at –20 1C) and thawing (for 9 h at 4 1C) process. After freezing-thawing process for FT gel formation the second CDgel layer was prepared by adding 15 g of the PVA-pre gel solution on the FT gel layer. This was then dried at 8 1C and 50%RH conditions for 7 days and then at 20 1C and 40%RH [12]. The drying conditions were controlled for temperature and humidity using a controlled chamber. The hybrid PVA hydrogel film thus obtained was soaked in 1 L pure water for 48 h. 2.2. Germicidal treatment with ethanol (EtOH) All three gels FT, CD and hybrid PVA hydrogels were soaked in 70% Ethanol for 24 h for sterilization. After EtOH sterilization, the gels were soaked in sterile pure water for 48 h. 2.3. Swelling ratio To analyze the swelling property, about 10 10 mm2 samples were cut out from the swollen gels, and then the weight of swollen sample, Wt, was measured. After the measurement, the gels were dried at 60 1C for 24 h, and the weight of dried sample, Wd, was measured. From these measurements, swelling ratio, Wt/Wd, was calculated. 2.4. Reciprocating friction test We cut 40 10 mm2 fully swollen gel plate as friction test sample. The ball-on-plate reciprocating friction test was conducted at room temperature (about 23–25 1C), at a sliding speed of 20 mm s 1 and at a sliding distance of 300 m (stroke: 25 mm and total 6000 cycles) at a constant load of 5.88 N, using friction tester (TriboGear TYPE:38, HEIDON). Test conditions such as sliding speed, stroke, sliding distance and constant load were selected to properly evaluate friction and wear properties of PVA hydrogels in mixed or boundary
S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
13
400 nm As prepared
After treatment
500 nm
500 nm 400 nm
3.0 2.5
Frequency (%)
Before treatment
2.0
After treatment
1.5 1.0 0.5 0.0
0
2
4 Young's Modulus (MPa)
6
8
Fig. 2. AFM topography images of FT gel at swollen state (a) before and (b) after EtOH treatment. (c) Young’s modulus distribution of surface of FT gel.
lubrication mode in simplified reciprocating test under similar conditions to the previous studies [9,12], although simulator tests are required before clinical application. Polycrystalline alumina ceramic ball of 26 mm diameter was used as an upper ball specimen. The lubricant was pure water or simulated synovial fluid. From the measured value of tangential force, coefficient of friction, μk, was calculated. Simulated synovial fluid was saline solution containing 0.15 M NaCl (Otsuka Pharmaceutical Factory Inc.), 0.5 wt% sodium hyaluronate (molecular weight: 9.2 105), 0.01 wt% Lα-dipalmitoyl phosphatidylcholine, 1.4 wt% bovine serum albumin (Wako Pure Chemical Industries Ltd.) and 0.7 wt% human γ-globulin (Wako Pure Chemical Industries Ltd.). 2.5. Atomic force microscope measurement The surface images of swollen gels were obtained by using an atomic force microscope (AFM) system (Dimension Icon AFM, Bruker). The probe was silicon nitride (SNL-10A, Bruker) and
the measurement mode was peakforce tapping mode. Young’s modulus of hydrogel was estimated by the indentation test. 3. Results and discussions 3.1. FT gel Fig. 1(a) shows effect of EtOH treatment on coefficient of friction, μk, of FT gels in different lubricants, and Fig. 1(b) shows swelling ratio of FT gels under different conditions. μk of FT gels before EtOH treatment increased gradually in water, whereas after treatment FT gels showed constant friction. In simulated synovial fluid, both before and after EtOH treatment FT gels showed constant friction, but μk of after EtOH treatment FT gels was higher than that of FT gel before treatment. In addition, swelling ratio decreased after treatment. The decrease of swelling ratio was caused by increase in physically cross-linked points (microcrystallites) during EtOH treatment, as EtOH is a poor solvent for PVA gel. It was noticed that EtOH treatment reduced
14
S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
0.20 After EtOH treatment (in water)
µk (-)
0.15
0.10 After EtOH treatment (in simulated synovial fluid) As prepared (in water)
0.05
As prepared (in simulated synovial fluid)
0.00 0
50
100 150 200 Sliding Distance (m)
250
300
6
Wt / Wd (-)
5 4 3 2 1
Before After EtOH treatment
Fig. 3. (a) Coefficient of friction and (b) swelling ratio of CD gels with/without germicidal treatment with EtOH.
reciprocating friction to 300 m in the sliding test in pure water. It appeared that increase in microcrystallites (decrease in swelling ratio), improved wear resistance. Fig. 2(a) and (b) shows AFM image in pure water of FT gel surface before and after EtOH treatment. FT gels had a rough and dendritic surface [11]. As shown in this figure, the surface of swollen gels did not change before and after EtOH treatment. However as shown in Fig. 2(c), Young’s modulus of FT gel increased after EtOH treatment. It is considered that treatment with EtOH affected the gel bulk properties rather than gel surface properties and that the decrease in swelling ratio and the increase of Young’s modulus were caused by formation additional microcrystallites in bulk largely high hydraulic permeability. Thus, surface properties of FT gels was not affected by EtOH treatment. 3.2. CD gel Fig. 3(a) shows frictional behavior of CD gel before and after treatment with EtOH. μk of CD gel maintained initial value during reciprocating test in pure water. However absolute μk value dramatically increased after EtOH treatment. Furthermore, it was noticed that μk decreased in simulated synovial fluid in comparison to water. It is believed that
components of lubricant improved frictional property on the CD gel surface. As shown in Fig. 3(b), swelling ratio apparently decreased on EtOH treatment. This would be due to the formation of additional microcrystallites during shrinking in poor solvent. Increase of μk are considered to be related to the hardening of gel surface. Fig. 4 shows the AFM image and it can be seen that EtOH treatment had changed the surface properties due to increase in microcrystallites formation. CD gel had a smooth surface about 10–20 nm height [13]. After EtOH treatment, CD gel surface became dendritic and similar to FT gel. It would appear that the gel surface shrank inhomogeneously. Furthermore Young’s modulus increased after EtOH treatment in Fig. 4(c). The increment of μk after EtOH treatment was mainly caused by change in surface condition of gel and it shows that friction of hydrogel was controlled by adsorbed film formation on gel surface in simulated synovial fluid. 3.3. Hybrid PVA gel Fig. 5(a) shows the effect of EtOH sterilization on frictional behavior of hybrid PVA gels in different lubricants, and Fig. 5(b) shows swelling ratio of hybrid PVA hydrogels in different conditions. As shown in Fig. 5(a), untreated hybrid
S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
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400 nm As prepared
After treatment
500 nm
500 nm 400 nm
5
Frequency (%)
4
After treatment
3
2
Before treatment
1
0
0
2
4 6 Young's Modulus (MPa)
8
Fig. 4. AFM topography images of CD gel at swollen state (a) before and (b) after EtOH treatment. (c) Young’s modulus distribution of surface of CD gel.
PVA gel maintained superior low friction during test in simulated synovial fluid. However, μk dramatically increased after EtOH treatment. It was due to surface hardening caused by decrease of swelling ratio of hybrid PVA hydrogels in both lubricants after treatment with EtOH, as shown in Fig. 5(b). It was believed that additional hydrogen bonds were formed during shrinking caused by treatment of EtOH. It was difficult to maintain surface condition and frictional property after sterilization with EtOH. In simulated synovial fluid, μk was lower than in pure water (Fig. 5(a)). However swelling ratio decreased in simulated synovial fluid (Fig. 5(b)). The decrease in swelling ratio indicated that this fluid was poor solvent for hybrid PVA hydrogel, therefore the gel became hard. From these results, it can be concluded that the lubricative property was improved by effective constituents in lubricant. Fig. 6 shows change in AFM image of swollen hybrid gels in pure water before and after EtOH treatment. This figure, shows that surface image did not change with treatment with EtOH. However, as shown in Fig. 7, Young’s modulus of hybrid gel surface increased after EtOH treatment. Therefore it is considered that increase of μk and decrease of swelling ratio
were caused by hardening of gel surface due to formation additional microcrystallites during EtOH treatment. And it is showed that the decrease of μk in simulated synovial fluid depends to a large extent on constituents in lubricant.
3.4. Effect of EtOH treatment on frictional property of PVA gel The increase of Young’s modulus were observed in all type of PVA gels because EtOH is poor solvent for PVA gels. However, FT gel showed less increase of μk. It seems that the shrinking of FT gel occurred throughout the gel, and that the shrinking of CD gel and Hybrid PVA gel occurred in surface mainly, as shown in AFM image. It is because FT gel has high permeability. As CD gel and hybrid PVA gel have low permeability, surface hardening was noticeable. From these results, germicidal treatment that penetrate through gel at short time was required. As an alternate method in our next study, we are evaluating the influence of gamma-ray radiation on tribological, mechanical and swelling properties of PVA hydrogels.
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S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
0.20
0.15 µk (-)
After EtOH Treatment (in pure water)
0.10 After EtOH Treatment (in simulated synovial fluid)
0.05
As prepared (in pure water) As prepared (in simulated synovial fluid)
0.00 0
50
100 150 200 250 Sliding Distance (m)
300
6
Wt / Wd (-)
5 4
EtOH treatment before after
3 2 1
Pure water
Simulated synovial fluid Lubricant
Fig. 5. (a) Coefficient of friction and (b) swelling ratio of hybrid PVA gels with/without germicidal treatment with EtOH.
400 nm
After EtOH treatment
As prepared
1 µm
1 µm −400 nm
Fig. 6. AFM topography images of hybrid PVA gels before/after germicidal treatment with EtOH.
4. Conclusions Effect of chemical sterilization with EtOH treatment on frictional properties of three types of physically cross-linked PVA gels were compered. It was shown that hybrid PVA hydrogels had excellent frictional properties in comparison to than FT gel and CD gel in cases without germicidal treatment before sterilization with ethanol. But, except FT gel in water,
EtOH treatment increased the μk value due to formation of additional hydrogen bonds. It was also found that μk decreased in simulated synovial fluid because the lubricating property was improved by adsorbed film formation with constituents in the lubricant. Thus for use of such candidate material for development of articular cartilage the sterilization method and its effect has to be carefully studied and the lubricant should also be appropriately selected or it should be evaluated using
S. Sasaki et al. / Biosurface and Biotribology 2 (2016) 11–17
1.0 Before Treatment
After Treatment
Frequency (%)
0.8 0.6 0.4 0.2 0.0
0
2 4 6 Young's Modulus (MPa)
8
Fig. 7. Young’s modulus distribution of surface of hybrid PVA gel.
simulated synovial fluid to get results as close to the in vivo situation as possible. And it was shown that germicidal treatment that penetrate through gel at short time was required, thus gamma-ray sterilization should be considered. Acknowledgments The authors would like to thank Kuraray Co., Ltd. for kindly supplying PVA powders. This work was supported in part by the Grant-in-Aid for Specially Promoted Research of Japan Society for the Promotion of Science (KAKENHI No. 23000011). References [1] K. Tamura, et al., A new hydrogel and its medical application, ASAIO J. 32 (1) (1986) 605–608.
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[2] E. Otsuka, et al., A simple method to obtain a swollen PVA gel crosslinked by hydrogen bonds, J. Appl. Polym. Sci. 114 (1) (2009) 10–16. [3] E. Otsuka, et al., Swelling properties of physically cross-linked PVA gels prepared by a cast-drying method, Prog. Coll. Polym. Sci. 136 (2009) 121–126. [4] K. Mabuchi, et al., Lubrication property and mechanical durability of the joint prostheses containing cartilage-like polyvinyl alcohol gel, J. Soc. Orthop. Biomech. 8 (1986) 101–105. [5] Sasada, T., Biomechanics and biomaterials – friction behavior of an artificial articular cartilage. In: Proceedings of the 3rd world biomaterials congress, 2, 6, 1988. [6] T. Murakami, et al., The adaptive multimode lubrication in knee prostheses with artificial cartilage during walking, Tribol. Ser. 32 (1997) 371–382. [7] T. Murakami, et al. Time-dependent frictional behaviors in hydorogel artificial cartilage materials, in: Proceedings of the 6th International Biotribology Forum: BIOTRIBOLOGY FUKUOKA, 65–69, 2011. [8] T. Murakami, Superior lubricity in articular cartilage and artificial hydrogel cartilage, Proc. IMechE Part J: J. Eng. Tribol. 228 (2014) 1099–1111. [9] A. Suzuki, S. Sasaki, S. Sasaki, T. Noh, K. Nakashima, S. Yarimitsu, et al. Elution and wear of PVA hydrogels by reciprocating friction, In: Proceedings of the world tribology congress, extended abstracts, Torino, 2013. [10] M. Tohfafarosh, et al., Effects of gamma and e-beam sterilization on the chemical, mechanical and tribological properties of a novel hydrogel, J. Mech. Behav. Biomed. Mater. 53 (2016) 250–256. [11] D. Kanjickal, et al., Effects of sterilization on poly(ethylene glycol) hydrogels, J. Biomed. Mater. Res. Part A 87A (3) (2008) 608–617. [12] S. Yarimitsu, et al. Evaluation Of Friction And Wear Properties Of Poly (vinyl Alcohol) Hydrogels As Artificial Articular Cartilage. [13] T. Murakami, et al., Evaluation of a superior lubrication mechanism with biphasic hydrogels for artificial cartilage, Tribol. Int. 89 (2015) 19–26.