HA composite hydrogel as artificial cartilage

Wear 376-377 (2017) 329–336

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Wear journal homepage: www.elsevier.com/locate/wear

Biotribology behavior and fluid load support of PVA/HA composite hydrogel as artificial cartilage Kai Chen n, Xuehui Yang, Dekun Zhang n, Linmin Xu, Xin Zhang, Qingliang Wang School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, China

art ic l e i nf o

a b s t r a c t

Article history: Received 27 August 2016 Received in revised form 18 November 2016 Accepted 22 November 2016

The human body joint motion is very complicated, which mainly includes sliding, swing, rotation. Therefore, cartilage covering the joint surface bears the repeated friction which is caused by the different movements during the whole life. So sliding, swing and torsion friction behavior of hydrogels need to be researched as synthetic articular cartilage. In this paper, PVA/HA composite hydrogel is cross-linked on the UHMWPE surface through chemical grafting and freezing-thawing method. Biotribology behavior and fluid load support are researched. The results show that swing and torsion friction coefficients are negligibly small, while sliding friction coefficient is largest. There is a negative linear relationship between fluid load support and friction coefficient. Fluid load supports are relative high under swing and torsion friction, so the swing and torsion friction coefficients are relative low. Hydrogel can be replenished by re-swelling to sustain the fluid pressurization during friction under lubrication condition. Both fluid load support and biphasic lubrication due to its porous structure with large amount of water contribute to the low friction coefficient. & 2016 Elsevier B.V. All rights reserved.

Keywords: PVA/HA composite hydrogel Fluid load support Biotribology Lubrication

1. Introduction The hip and knee joint are the main load-bearing joints in human body, and they are the main movement parts of human body [1–3]. The basic daily movements of hip joint are flexionextension, adduction-abduction, internal and external rotation and circumduction. The basic daily movements of knee joint are flexion-extension, adduction-abduction, internal and external rotation and horizontal migration of front and back. So human can complete all kinds of action because of the existing movement of hip and knee joints. The joints are the largest load bearing biological friction pairs. Therefore, cartilage covering the joint surface bears the repeated friction which is caused by the different movements during the whole life. Hence, the articular cartilage wears and fails, which causes the symptom of pain, swelling and bone crepitus. Finally, osteoarthritis is generated [4–9]. Nowadays, the osteoarthritis which is caused by the wear of articular cartilage has become the first disabling disease around the world. About 400 million people suffer the joint disease. Due to limit of the selfrepair ability, it cannot repair along once the damage or disease is caused. Therefore, the replace of artificial joint is set to rebuild the joint characteristic [10]. n

Corresponding authors. E-mail addresses: [email protected] (K. Chen), [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.wear.2016.11.033 0043-1648/& 2016 Elsevier B.V. All rights reserved.

The artificial joint replacement has become the effective way to treat the joint disease or trauma. However, due to the lack of the nature metabolism of artificial implant materials, the compatibility of prosthesis interface and life medium is poor. Moreover, the contact interface of total joint replacement implants is hard-face to hard-face, and the contact surface exists wear. A large number of clinical medical research has confirmed that the wear of artificial joints is the main reason of the aseptic loosening of joint replacement. The local osteolysis which is caused by wear particles leads to the aseptic loosening. This is the main reason of failure of artificial joint replacement [11–13]. Polyvinyl alcohol (PVA) hydrogel has three-dimensional network structure, which is similar to the natural articular cartilage. Meanwhile, it possesses the properties of solid and liquid and it also has a good biocompatibility, mechanical property and biological tribology performance. Recent years, PVA hydrogel is expected to become the cartilage repair material and replace the biological materials [14]. As the human body joints have many movement modes, the research on the friction properties under different movement modes is very important. Researchers researches mainly on tangential sliding friction and wear behavior of the hydrogel [15–18]. Besides, few researches study the flexion-extension (swing) and adduction-abduction (rotation) of the hip and knee joints. Therefore, it is important to study and understand the biological tribology behavior and lubrication mechanism under different movement modes. This paper studies the friction properties and

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fluid load support of PVA/HA composite hydrogel under different movement modes (sliding friction, swing friction and rotating friction). Also, the effects of three movement modes on the friction lubrication of PVA/HA composite hydrogel are explored. PVA/HA composite hydrogel was bonded with the acetabulum of ultrahigh-molecular-weight polyethylene (UHMWPE) by the chemical bonding method, and the friction and wear properties are tested by hip simulation experiments. The bearing and damage mechanism of PVA/HA composite hydrogel is studied, which provides theoretical basis for the study of bionic joint repair materials.

2. Experiment material and method 2.1. Preparation method of PVA/HA composite hydrogel Potassium dichromate and concentrated sulfuric acid were weighed with mass percent of 1:4. It was put into the thermostat water bath with 70°C. It was stirred to uniformity and dichromate oxidation solution formed. The UHMWPE sample was immerged in the 70°C dichromate oxidation solution for 10 min. PVA, HA and deionized water were weighed with mass percent (15% PVA, 3% HA and 82% deionized water). After swelling 24 h under room temperature, it was immerged in the thermostat water bath with 95°C. The PVA/HA composite hydrogel solution was produced. PVA and deionized water were weighed with mass percent of 7% and 91.5%. The former steps were repeated and PVA solution was produced. Concentrated sulphuric acid with 1.5% were put into as catalyst and grafting solution was produced. The cleaned UHMWPE sample was immerged in the grafting solution and put into the thermostat water bath with 90°C for 2 h. The UHMWPE sample was cleaned with 90°C deionized water to clean the unreacted PVA. The produced 15%PVA and 3%HA composite hydrogel was put into the surface UHMWPE sample to make PVA/HA composite hydrogel reach to 2mm. The sample was put into the cooled storage incubator with 20°C for 6–10 h and thawed for 2– 3 h under the room temperature. This process was repeated for 9 times and PVA/HA composite hydrogel artificial cartilage was produced [19]. 2.2. Acetabulum grafting PVA/HA composite hydrogel The UHMWPE was processed into acetabulum with roughing treatment to increase the reacting area for oxidative esterification and the fixed area with hydrogel. Its diameter is 32 mm. Stainless steel metal ball joint was processed to match the acetabulum, and its diameter is 28 mm. Using the methods of chemical grafting which is shown in the Section 2.1 to prepare acetabulum grafting PVA/HA composite hydrogel. It was put into the cooled storage incubator with 20°C for 6–10 h, and thawed at room temperature for 2–3 h. This process was repeated for 9 times and acetabulum grafting PVA/HA composite hydrogel was produced (Fig. 1).

Fig. 1. UHMWPE acetabulum with PVA/HA composite hydrogel.

2.3. Experiment device and parameter The sliding, swing, rotation friction tests was carried on the UMT multi-functional micro friction testing machine to test the cobalt chromium molybdenum (CoCrMo) alloy balls with PVA/HA composite hydrogel (Test diagram is shown in Fig. 2). The diameter of CoCrMo ball is 28 mm, and The UHMWPE sample of sliding friction is 20 mm  20 mm  30 mm. The thickness of the PVA/HA composite hydrogel is 2 mm, and lubricating media is 25% bovine serum. The sliding test loads are chosen as 10 N, 20 N, 30 N and 40 N and the speeds are chosen as 1 mm/s, 5 mm/s, 10 mm/s, 20 mm/s, 30 mm/s, 40 mm/s and 60 mm/s. The sliding distance of CoCrMo ball is 10 mm, and the experimental parameters are shown in Table 1. The swing test load are 10 N, 20 N and 30 N and the swing angle are 5°, 10° and 15°, which is shown in Table 2. The rotation test load of are 10 N, 30 N and 50 N and the rotation angle are 5°, 10° and 15°, which is shown in Table 3. The UHMWPE sample of swing friction and rotation friction is φ30 mm. 2.4. Friction finite element model of PVA/HA composite hydrogel 2.4.1. Sliding friction finite element model of PVA/HA composite hydrogel According to the size of PVA/HA composite hydrogel and CoCrMo alloy ball, the friction finite element model of CoCrMo alloy ball and PVA/HA composite hydrogel was established. The thickness of the PVA/HA composite hydrogel is 2 mm, and the thickness of UHMWPE is 2 mm. The diameter of CoCrMo alloy ball is 28 mm. Saint-Venant method is used to simplify this model. The CoCrMo ball was divided into 14608 entity unit with 8 node reducing integration (C3D8R), and PVA/HA composite hydrogel was divided into 22560 entity unit with 8 node pore pressure integration (C3D8RP). The UHMWPE was divided into 22560 entity unit with 8 node reducing integration (C3D8R), and the model of CoCrMo ball and PVA/HA composite hydrogel was established (Fig. 3). The face-to-face contact effect was used to simulation analysis, molybdenum cobalt chromium ball surface was set as the main plane and PVA/HA composite hydrogel as minor plane. PVA/ HA composite hydrogel was fixed with UHMWPE to restrain the movement of UHMWPE on the direction of horizon, vertical and rotation. The pore pressure of PVA/HA composite hydrogel layer surrounding was set as 0 [20–25]. The sliding speed and load are shown in Table 1, and the material properties are shown in Table 4. 2.4.2. Swing and rotation finite element model of PVA/HA composite hydrogel According to the size of PVA/HA composite hydrogel and CoCrMo alloy ball, the swing and rotation friction finite element model of CoCrMo alloy ball and PVA/HA composite hydrogel was established. The thickness of the PVA/HA composite hydrogel is 2 mm, and the thickness of UHMWPE is 2 mm. The diameter of CoCrMo alloy ball is 28 mm. Saint-Venant method is used to simplify this model. The CoCrMo ball was divided into 14608 entity unit with 8 node reducing integration (C3D8R), and PVA/HA composite hydrogel was divided into 22560 entity unit with 8 node pore pressure integration (C3D8RP). The UHMWPE was divided into 22560 entity unit with 8 node reducing integration (C3D8R), and the model of CoCrMo ball and PVA/HA composite hydrogel was established (Fig. 4). The material properties are shown in Table 4. The face-to-face contact effect was used to simulation analysis of swing. The CoCrMo ball surface was set as the main plane and PVA/HA composite hydrogel as minor plane. PVA/HA composite hydrogel was fixed with UHMWPE to restrain the movement of

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Fig. 2. Sliding, swing and rotation friction experiment device of PVA/HA composite hydrogel.

Table 1 Experimental parameters for sliding friction testing. Influencing factor

Experimental parameters

Load/N Sliding speed/ mm/s

10, 20, 30, 40 1, 5, 10, 20, 30, 40, 60

Table 2 Experimental parameters for swing friction testing. Influencing factor

Experimental parameters

Load/N Swing angle/deg

10, 20, 30 5, 10, 15 Fig. 4. Swing and rotation friction FEM model.

Table 3 Experimental parameters for rotation friction testing. Influencing factor

Experimental parameters

Load/N Torsional angle/deg

10, 30, 50 5, 10, 15

UHMWPE on the direction of horizon, vertical and rotation. The pore pressure of PVA/HA composite hydrogel layer surrounding was set as 0, and the swing angle of CoCrMo ball was set as 5°, 10°, 15° [20–25]. The face-to-face contact effect was used to simulation analysis of rotation. The CoCrMo ball surface was set as the main plane and PVA/HA composite hydrogel as minor plane. PVA/HA composite hydrogel was fixed with UHMWPE to restrain the movement of UHMWPE on the direction of horizon, vertical and rotation. The pore pressure of PVA/HA composite hydrogel layer surrounding was set as 0, and the swing angle of CoCrMo ball was set as 5°, 10°, 15° [20–25]. 2.5. Wear test of acetabulum with PVA/HA composite hydrogel

Fig. 3. Sliding friction FEM model.

Table 4 Material properties of PVA/HA composite hydrogel, UHMWPE and CoCrMo ball. Material

Elastic modulus

Poisson's ratio

Void ratio

Permeability

PVA/HA composite hydrogel UHMWPE Co–Cr–Mo ball

1.07 MPa

0.45

4.56

2.8  10

1258 MPa 210 GPa

0.2 0.3

– –

– –

14

m4/Ns

The wear test of acetabulum with PVA/HA composite hydrogel was carried on hip joint simulation tester, which is shown in Fig. 5. The experiment standard of the hip joint friction test is ISO14242. The load, displacement and environment parameter of the hip wear testing machine are shown in Fig. 6. CoCrMo alloy femoral head that was used in the experiment was provided by Beijing Chunlizhengda Medical Instruments Co., Ltd. The diameter is 28 mm, and the lubricating media is 25% calf serum. UHMWPE acetabulum was set as the contrast specimen, which was carried out the bio-tribology test at meantime.

3. Results and discussion 3.1. Influence on PVA/HA composite hydrogel by load under different movement modes Fig. 7 shows the relationship between friction coefficient of

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Fig. 5. Hip joint simulator.

3000

30

Load

2500

Swing Tosion

Angle/°

Load/N

20 2000 1500 1000

10 0

500

-10

0

0.0

0.2

0.4

0.6

0.8

1.0

-20

0.0

Time/s

0.2

0.4

0.6

0.8

1.0

Time/s

Fig. 6. Test condition of hip joint simulator.

increase of P, and the friction coefficient increases gradually with the load [26]. The relationships between three movement modes and the load are as follows:

0.25

Friction Coefficient

0.20 Sliding Swing Torsion

0.15

Sliding Fitting Swing Fitting Torsion Fitting

0.10

0.05

0.00

10

20

30

40

50

Load/N Fig. 7. Relationship of friction coefficient and load under different movement modes.

PVA/HA composite hydrogel and load under different movement modes. The sliding friction coefficient, swing friction coefficient and the rotation friction coefficient shows an increase trend with the increase of load. Besides, the sliding friction coefficient of PVA/ HA composite hydrogel is larger than that of swing friction coefficient and rotation friction coefficient. With the increase of load, the depth of deformation increases gradually. The deformation force of PVA/HA composite hydrogel which is overcame by the friction increases. On the other hand, the contact area of CoCrMo ball and PVA/HA composite hydrogel increases, and the number of Blob (m) which is adsorbed by the PVA/HA composite hydrogel increase with the increase of positive pressure P. Under the low sliding speed, F ∝ mvτ , and m ∝ P . Therefore, F increases with the

Sliding friction: μ = 0.077P 0.28

(1)

Swing friction: μ = 0.0061P 0.79

(2)

Rotation friction: μ = 0.014P 0.43

(3)

According to the relationship equation between friction coefficient and the load, under the small load, the swing and rotation friction coefficient of PVA/HA composite hydrogel is very small, and the sliding friction coefficient is largest. With the increase of load, the growth rate of the rotation friction coefficient is low, so the rotation friction coefficient of PVA/HA composite hydrogel stay in a very low level; The growth rates of sliding friction coefficient and rotation friction coefficient are close, because the initial friction coefficient of swing is small and the swing friction coefficient of PVA/HA composite hydrogel is small with the increase of load. Fig. 8 shows the relationship between friction coefficient of PVA/HA composite hydrogel and fluid load support under different movement modes. With the fluid load support of PVA/HA composite hydrogel decrease, the sliding friction coefficient, swing friction coefficient and rotation friction coefficients show an increase trend. As the fluid load support decreases, the growth rate of sliding friction coefficient is maximum. For the fluid load support of PVA/HA composite hydrogel is low during swing friction, the sliding friction coefficient is relative large all the time. When the PVA/HA composite hydrogel are in swing friction and rotation friction, fluid load support is high. Therefore, swing friction coefficient and rotation friction coefficient are low. The relationships

K. Chen et al. / Wear 376-377 (2017) 329–336

0.25

Rotation friction: μ = 0.291 − 0.294W f /W Sliding Swing Torsion

Friction Coefficient

0.20

Sliding Fit Swing Fit Torsion Fit

0.15

0.10

0.05

0.00

0.65

0.70

0.75

0.80

0.85

Fluild Load Support Fig. 8. Relationship of friction coefficient and fluid load support under different loads and movement modes.

between three movement modes and the fluid load support are as follows:

Sliding friction: μ = 1.4 − 1.83W f /W

(4)

Swing friction: μ = 0.43 − 0.47W f /W

(5)

333

(6)

According to the relationship between three movement modes and fluid liquid support, when fluid load support decreases to 0.1, sliding friction coefficient, swing friction coefficient and rotation friction coefficient increase to 0.183, 0.047, and 0.0294 respectively. Therefore, the friction performance is influenced by the fluid load support of PVA/HA composite hydrogel. Fig. 9 shows the flow velocity of three movement modes with the load of 30N. The flow velocity of PVA/HA composite hydrogel in sliding friction, swing friction and rotation friction are 2.9  10 2 mm/s, 7.6  10 3 mm/s and 5.0  10 3 mm/s, respectively. Because of the liquid flow on the contact surface of PVA/HA composite hydrogel, the liquid loss of PVA/HA composite hydrogel occurs on the contact surface. Besides, when PVA/HA composite hydrogel is in the sliding friction, the flow velocity is maximum and the fluid loss is most serious, which lead to the decrease of fluid load support and increase of the solid substrate support. Hence, the sliding friction coefficient is the largest. When PVA/HA composite hydrogel is in swing friction and rotation friction, the flow velocity of PVA/HA composite hydrogel is 1/4 and 1/6 of that in sliding friction. So, the fluid load support is relative stronger, and the swing friction coefficient and rotation friction coefficient is relative small.

Fig. 9. Pore flow velocity clouds under different loads and movement modes (30 N).

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Sliding Swing Torsion

0.16

Sliding Fit Swing Fit Torsion Fit

0.14

0.12

Friction Coefficient

Friction Coefficient

0.14

0.10 0.08 0.06

Sliding Swing Torsion

0.12

Sliding Fit Swing Fit Torsion Fit

0.10 0.08 0.06 0.04 0.02

0.04 0

2

4

6

8

10

12

0.00

14

Speed/mm/s

0.66 0.68 0.70 0.72 0.74 0.76 0.78 0.80 0.82

Fluild Load Support

Fig. 10. Relationship of friction coefficient and speed under different movement modes.

3.2. Influence on PVA/HA composite hydrogel by speed under different movement modes Fig. 10 shows the relationship between friction coefficient and speed under different movement modes. With the increase of speed, the sliding, swing and rotation friction coefficient of PVA/ HA composite hydrogel shows an increase of trend, the relationship between speed and friction coefficient under three movement modes is as follows:

Sliding friction:μ = 0.07v 0.26

(7)

Swing friction: μ = 0.018v 0.45

(8)

Rotation friction: μ = 0.092v 0.6

(9)

According to the relationship between friction coefficient of PVA/HA composite hydrogel and speed, rotation friction speed affects the friction coefficient most severely, the sliding friction take the second place and the swing friction coefficient is minimum. When the sliding friction speed increase from 1 mm/s to 10 mm/s, the sliding friction coefficient of PVA/HA composite hydrogel increases from 0.072 to 0.13. When the swing friction speed increases from 4.88 mm/s to 14.64 mm/s, the friction coefficient increases from 0.037 to 0.06. Similarly, when rotation friction speed increases from 0.38 mm/s to 1.14 mm/s, the rotation friction coefficient increases from 0.052 to 0.1. Therefore, sliding velocity and rotation velocity have great influence on the friction coefficient of PVA/HA composite hydrogel. Especially, during the rotation friction, as the contact area of CoCrMo ball and PVA/HA composite hydrogel remains the same, which causes the liquid continuous loss in the contact surface. The liquid phase cannot flow back to contact surface on time, and thus increase the solid substrate bearing. Besides, the flow velocity increases with the increase of rotation friction velocity. The fluid loss is more serious, which leads to the increase of rotation friction coefficient. Fig. 11 shows the relationship between friction coefficient and fluid load support under different movement modes. With the decrease of fluid load support of PVA/HA composite hydrogel, sliding friction coefficient, swing friction coefficient and rotation friction coefficient show an increase trend. As the fluid load support decreases, the growth rate of sliding friction coefficient is maximum. Besides, the fluid load support of PVA/HA composite hydrogel is low during sliding friction, so the sliding friction coefficient is relative large all the time. During swing friction and shear friction, the fluid load support is high. Therefore, swing

Fig. 11. Relationship of friction coefficient and fluid load support under different speeds and movement modes.

friction coefficient and rotation friction coefficient is smaller than that of sliding friction. The relationship between fluid load support and friction coefficient under three movement modes is as follows:

Sliding friction: μ = 1.22 − 1.59W f /W

(10)

Swing friction: μ = 0.46 − 0.52W f /W

(11)

Rotation friction: μ = 0.44 − 0.49W f /W

(12)

During sliding friction, the fluid load support of PVA/HA composite hydrogel reduces from 0.722 to 0.674, and sliding friction coefficient increases by 0.058. During swing friction, the fluid load support of PVA/HA composite hydrogel reduces from 0.82 to 0.775, and swing friction coefficient increases by 0.023. Similarly, during rotation friction, fluid load support of PVA/HA composite hydrogel reduces from 0.786 to 0.688, which reduces by 0.1, and rotation friction coefficient increases by 0.48. Therefore, the fluid load support is relative high during rotation friction and sliding friction with the increase of speed under three movement modes. Hence, rotation friction coefficient and sliding friction coefficient is large. Fig. 12 shows the flow velocity of PVA/HA composite hydrogel during sliding friction and swing friction under the same load. When the sliding friction speed of PVA/HA composite hydrogel is 5mm/s and swing friction speed is 4.88 mm/s, the flow velocities are 1.98  10 2 mm/s and 1.05  10 2 mm/s, respectively. Due to the fluid flow in the contact surface of PVA/HA composite hydrogel, the fluid loss of PVA/HA composite hydrogel is generated. The fluid velocity is relative large, and liquid loss is more serious, which leads to fluid load support of PVA/HA composite hydrogel decreases. Therefore, with the increase of the friction coefficient, the fluid velocity of PVA/HA composite hydrogel is about 1/2 of that of sliding friction. Hence, the fluid support is strong, and the swing friction coefficient is less than the sliding friction coefficient. Fig. 13 shows the fluid velocity of PVA/HA composite hydrogel during sliding friction and rotation friction under the same load. The sliding friction speed of PVA/HA composite hydrogel is 1 mm/ s, and the rotation friction speed is 1.14 mm/s. The fluid velocities are 3.79  10 3 mm/s and 2.72  10 3 mm/s. During rotation friction, the fluid velocity of PVA/HA composite hydrogel is about 1/6 of that of sliding friction. The fluid loss is small, and the fluid load support of PVA/HA composite hydrogel is strong. Therefore, the rotation friction coefficient is less than the sliding friction coefficient.

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Fig. 12. Pore flow velocity clouds under different speeds in sliding friction and swing friction.

Fig. 13. Pore flow velocity clouds under different speeds in sliding friction and rotation friction.

1th

2500

2500

1000th

Load/N

1500

1500

1000

1000

500

500

0 0.0

0.5

1.0 0.0

100th 1000th 3000th 5000th

2000

UHMWPE UHMWPE-PVA/HA

2000

Load/N

100th

10th

0.5

1.0 0.0

0.5

1.0 0.0

0.5

1.0

Time/s Fig. 14. Load responses curve of acetabulum on hip joint simulator.

0

0.0

0.2

0.4

0.6

0.8

1.0

Time/s Fig. 15. Load response curve of acetabulum with PVA/HA composite hydrogel.

3.3. The bio-tribology damage experiment of UHMWPE acetabulum with PVA/HA composite hydrogel Fig. 14 shows the load response of UHMWPE acetabulum and UHMWPE acetabulum with PVA/HA composite hydrogel layer on the hip joint simulation test. The load support value of UHMWPE acetabulum with PVA/HA composite hydrogel layer is smaller than

that of UHMWPE acetabulum, which shows that acetabulum with the PVA/HA composite hydrogel layer has excellent load buffering ability. Therefore, acetabulum with PVA/HA composite hydrogel layer has excellent mechanical characteristics. Fig. 15 shows the load response of acetabulum with PVA/HA composite hydrogel layer on the hip joint simulation test. With the increase of cycle times, the load response of PVA/HA composite

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Acknowledgements This research is supported by National Natural Science Foundation of China (Grant No. 51641510, 51505478), Natural Science Foundation of Jiangsu Province (Grant No. BK20160257), China Postdoctoral Science Found (Grant No. 2015M580487) and National Key Research and Development Program of China (Grant No. 2016YFC1101803).

References

Fig. 16. The morphology of acetabulum with PVA/HA composite hydrogel.

hydrogel layer increases gradually. It shows that mechanical buffering ability of PVA/HA composite hydrogel layer is small, which is caused by the high fluid loss of PVA/HA composite hydrogel under the high load cycles. The PVA/HA composite hydrogel cartilage layer is worn, which lead to the decrease of the fluid load support of bionic cartilage with PVA/HA composite hydrogel layer. The mechanical buffer ability decrease, which causes the broken of PVA/HA composite hydrogel. Finally, the acetabulum fails (Fig. 16), and the wear resistance of acetabulum could be up to 3000 times.

4. Conclusion

(1) As the load increases, the deformation depth of PVA/HA composite hydrogel increases gradually and the deformation force of friction increases. Besides, the contact area of CoCrMo ball and PVA/HA composite hydrogel increases, and the number of Blob which is adsorbed by the bionic joint increases. Therefore, friction coefficient increases gradually with the increase of load at low sliding speed. (2) Under the same load, the fluid load support of PVA/HA composite hydrogel has great influence on the friction property. When the fluid load support decrease to 0.1, the sliding friction coefficient, swing friction coefficient and rotation friction coefficient increases by 0.183, 0.047 and 0.0294, respectively. During swing and rotation friction, the liquid velocity of PVA/HA composite hydrogel is 1/4 and 1/6 of that of sliding friction. Therefore, the fluid load support is high, and the friction coefficient is relative small. (3) The sliding and rotation speed has great influence on the friction coefficient of PVA/HA composite hydrogel. With the increase of speed, the fluid load support of PVA/HA composite hydrogel decreases sharply during rotation and sliding friction. Thus, the friction coefficient of PVA/HA composite hydrogel is relative large during rotation and sliding friction. (4) The acetabulum with PVA/HA composite hydrogel layer has excellent load buffering ability. Under high load, the wear resistance of acetabulum could up to 3000 times.

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