Preparation of ultrahigh-molecular-weight polyethylene grafted with polyvinyl alcohol hydrogel as an artificial joint

Abstract

A chemical grafting method was used to graft UHMWPE with PVA hydrogel to be used as an artificial joint. The effects of temperature, grafting time, catalyst amount and grafting solution concentration were studied by orthogonal experiments. The results showed that active groups were formed on the UHMWPE surface on using dichromate for oxidation, and then a chemical graft occurred using the catalyst to catalyse hydroxyls in the PVA hydrogel molecules and surface active groups of UHMWPE. The optimum reaction conditions were as follows: 1% concentrated sulphuric acid as catalyst, 2.5 h reaction time and 85 °C temperature. The shear strength of the artificial joint can reach 1 MPa. The contact angles of UHMWPE are decreased from 104° to 39° by grafting and the surface wettability is effectively improved.

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1 Introduction

Articular cartilage, possessing excellent viscoelasticity and self-lubrication properties, can alleviate or reduce the impact and friction generated by human movements, and perform a key function in daily activities and sports.¹ Studies have shown that the mending capacity of adult articular cartilage is highly limited; moreover, partial or full self-repair can be achieved if the cartilage diameter is <3 mm,^2–4 beyond which self-repair cannot be achieved. The occurrence of articular cartilage lesions significantly increases with increase in age, and the incidence of articular cartilage injury is reportedly 5%. In specific individuals, such as athletes, the incidence may be as high as 22% to 50%.⁵ Once the injury or disease affects the cartilage, self-repair is difficult.^6,7 Considerable efforts have been exerted to identify and develop ideal prosthetic cartilage materials since a considerable period of time.⁸ Artificial joint replacement is one of the most effective treatments in the clinical demand. Ultrahigh-molecular-weight polyethylene (UHMWPE) has also been extensively researched as an articular material. This polymer possesses biocompatibility and mechanical properties that can meet implanting material requirements. However, wear particles are produced by the worn of UHMWPE, which will be phagocytosed by macrophages; moreover, a large number of bone-resorptive factors are released.⁹ For the purpose of prolonging the working life of artificial joints, it is necessary to modify the artificial joint so that it mimics the characteristics of natural cartilage.

In recent years, polyvinyl alcohol (PVA) hydrogel artificial cartilage implant materials have attracted the attention of researchers.^10–14 PVA hydrogels contain a considerable amount of water, and possess mechanical properties as well as biocompatibility similar to those of human articular cartilage; furthermore, liquid can penetrate and extrude the hydrogels after the load is applied.¹⁵ PVA hydrogels are a very promising articular cartilage replacement material in clinical settings.¹⁶ Masanori¹⁷ implanted PVA hydrogel into animal cartilage and found that after two years of replacing the articular cartilage with PVA hydrogel, it had a good effect on implantation; moreover, bone and joint disease do not develop, and the PVA hydrogel degrades but does not crack. However, PVA hydrogels still present several problems, such as poor performance in combination with basal bone and inability to bear the severe load conditions applied to the surface of human joints.^18–20 Gu²¹ achieved a microscopic mechanical interlocking connection with PVA hydrogel and metal fibre network, and then used bone cement (PMMA) to bond the fibre mesh with the underlying bone (or metal). This method can achieve a mechanical–chemical connection between the articular cartilage and underlying bone (or metal), with the shear strength of the connected biomimetic cartilage and bone ranging between 0.2 and 0.5 MPa. Meng²² used BG to connect PVA hydrogel and pig femur cancellous bone and obtained bionic layered artificial cartilage/bone composite implants.

However, given that the combination methods to bond PVA hydrogel and UHMWPE are physical in nature, the bonding strength is not high, and thus overall hydrogel shedding may occur. PVA hydrogel and UHMWPE bonded by chemical grafting methods show high strength, which can resolve the problem of hydrogel shedding. However, active groups do not exist on the surface of UHMWPE. Therefore, to form a strong connection between the PVA hydrogel and the UHMWPE surface by chemical methods, the UHMWPE surface must first be activated. Activation results in the formation of active functional groups on the polymer surface. Then, through a chemical catalytic method that attaches hydroxyl groups onto the molecular chains of PVA and forms reactive functional groups on the surface, a chemical reaction occurs to form a chemical graft. This grafting enables PVA molecules to be grafted onto the UHMWPE surface. In the present study, UHMWPE and PVA are used as raw materials. The oxidization of UHMWPE and the grafting impact factors (temperature, catalyst, grafting time, amount of catalyst and grafting concentration) are further examined. The effects of the grafting impact factor on the bonding strength of UHMWPE and PVA hydrogel are also researched to optimize the UHMWPE grafting conditions.

2 Materials and methods

2.1 Materials

The substrate used was UHMWPE (molecular weight = 3 × 10⁶). The oxidising reagent was dichromate oxidation solution (potassium dichromate: concentrated sulphuric acid mass ratio = 1 : 4). The catalyst was concentrated sulphuric acid. The grafting solution was 5% to 12% PVA solution.

2.2 Orthogonal experiment design

The amounts of temperature, time, concentration of the grafting solution and concentration of the catalyst were used as impact factors. Table 1 shows the orthogonal experiment design, which was used to optimise the grafting experiment. Ultimately, the best combination of conditions was selected by measuring the bonding strength.

Table 1 H₂SO₄-catalyzed grafting reaction orthogonal table

Number	Temperature/°C	Time/h	Grafted solution concentration/%	Amount of the catalyst/%
1	80	0.5	5	0.5
2	80	1.5	7	1
3	80	2.5	9	1.5
4	85	0.5	7	1.5
5	85	1.5	9	0.5
6	85	2.5	5	1
7	90	0.5	9	1
8	90	1.5	5	1.5
9	90	2.5	7	0.5

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2.3 Sample preparation

2.3.1 UHMWPE surface oxidation and grafting. Ultrahigh-molecular-weight polyethylene surface modification was accomplished in two steps. The first step involved the use of dichromate oxidation solution to oxidize the UHMWPE surface, and the second step involved the catalytic reaction to graft PVA molecules onto the UHMWPE surface after oxidation.
(1) Preparing PVA solution. Given that PVA consists of solid particles, the process of solution preparation included foregoing swelling and posterior dissolution. According to the experimental requirements, different concentrations of the PVA solution were formulated. First, the mass of the beaker was weighed and recorded, and then a certain mass of PVA (1750 ± 50 particles) and deionised water were weighed using an electronic analytical balance with a sensitivity of 0.01 mg. The weighed PVA was added to a beaker with deionised water, the total weight of the solution and the beaker was weighed and recorded, and then a glass rod was used for gentle stirring for a considerable time period. The polymers were sealed in plastic wrap and allowed to spontaneously swell for 2 h to 3 h. After swelling, the mixture was placed in an electronic thermostat water bath at 95 °C to be heated until dissolved. In the process of heating, a glass rod was used to continuously stir the mixture to accelerate dissolution. Given that heating and stirring during the dissolution facilitates continuous water evaporation, the concentration of the PVA solution eventually became significantly higher than the initial concentration. Thus, water was added to maintain the concentration of the PVA solution. The beaker and solution were weighed at regular intervals during dissolution, and deionised water at 95 °C was supplied based on weight loss. After the complete dissolution of PVA, stirring was stopped, the beaker was removed from the water bath and plastic wrap was used to cover the beaker, which was then ultrasonicated for 2 h to 3 h to discharge the bubbles.
(2) Oxidation of UHMWPE with dichromate oxidation solution. Potassium dichromate and concentrated sulphuric acid at a mass ratio of 1 : 4 were mixed to yield a dichromate oxidation solution, which was placed in a 75 °C water bath. UHMWPE blocks were soaked in the dichromate oxidation solution for 10 min and were washed with abundant deionised water. The blocks were then placed in nitric acid solution at 50 °C for 10 min. Small molecules that were formed by surface oxidation were removed. Finally, the blocks were removed and again washed with copious amounts of deionised water.
(3) UHMWPE and PVA grafting. PVA solution was adjusted to yield the respective grafting solution concentrations: the catalyst was added as required in Table 1 with constant stirring, and then the mixture was placed in a water bath at the corresponding temperature as mentioned in Table 1. Oxidised UHMWPE blocks were placed in the PVA solution, which was stirred and allowed to react for the period of time corresponding to Table 1. After the reaction, the UHMWPE blocks were removed and cleaned with deionised water to remove unbound PVA molecules on the surface.

2.3.2 Preparation of the shear sample. Before applying the hydrogel on the UHMWPE surface after PVA grafting, the UHMWPE samples were treated with adhesive tape (Fig. 1) so that the hydrogel could be sandwiched between overlapping structures composed of two parallel UHMWPE plate-like samples. The bonding length of the sample was 15 mm. The length, width and thickness of each plate-shaped UHMWPE sample were 35, 20 and 4 mm, respectively, whereas the length, width and thickness of the intermediate hydrogel sample were 12.5, 20 and 2 mm, respectively. The thickness error of the hydrogel of the same sample did not exceed 0.1 mm.

Fig. 1 Overlapping structure of the shear force measurement sample.

After discharging the bubbles, 15% PVA hydrogel permeated into the UHMWPE pores, as shown in Fig. 1. Bubbles were discharged again via an ultrasound to avoid the effects of their presence on the mechanical properties. Each sample was wrapped and sealed in plastic wrap, and then placed in a cold storage box at a temperature of −20 °C to freeze for 6 h to 12 h. Furthermore, the sample was removed, thawed at room temperature for 3 h to 4 h, and then replaced in a cold storage box for freezing. The freeze–thaw cycle was repeated nine times, and then the combined UHMWPE with PVA hydrogel sample was obtained. The prepared sample was placed in a sealed bag filled with deionised water.

An electronic universal tensile tester was used to test the shear bond force. The average shear force of each parallel sample group was used as the shear force. The sample bonding shear strength is expressed as follows:

where P is the tensile shear strength of UHMWPE and hydrogel (MPa), F is the maximum load of shear strength destruction (N) and is the shear bond force, L is the length of the sample adhesive surface (mm), and M is the width of the sample adhesive surface (mm). When L = 12.5 mm and M = 20 mm, the shear strength can be determined according to the above formula.

2.4 FT-IR

The functional groups of untreated UHMWPE, oxidized UHMWPE and grafted UHMWPE were analyzed by a Fourier-transform infrared (FT-IR) spectrometer (Model: VERTEX 80v). All measurements were performed from 500 to 4000 cm⁻¹ in transmission mode, with a scan resolution of 4 cm⁻¹.

2.5 Surface morphology

Scanning electron microscopy (SEM, Model: Quanta 250) was used to observe the surface morphology of the powders.

2.6 Differential scanning calorimetry analysis

To examine the effects of irradiation on crystallinity, differential scanning calorimetry (DSC, Model: NETZSCH STA 449 C) was performed on at least three specimens for every type of material. Approximately 5 mg of the specimen was used in the measurement. An alumina pan was placed in the sample cavity as a reference before the tests. All specimens were heated from 25 to 250 °C at 10 °C min⁻¹.

2.7 Contact angle measurements

The static-water contact angles of all the sample surfaces were measured by a contact angle instrument (Model: JC2000B). Each sample was tested three times at different regions of its surface, and the mean values were taken.

3 Results and discussion

3.1 The results of orthogonal experiments

Table 2 shows the effects of temperature, time, grafting solution concentration and catalyst concentration on grafting with concentrated sulphuric acid as the catalyst. Among the four factors, the effect range of grafting time on the shear bond strength between UHMWPE and PVA showed the widest range at 0.157. The shear bond strength obtained from the three reaction times indicated that the shear strength of the binding interface increased with increasing reaction time. Composite front test results further revealed that when the reaction time was 3 h, the increase in interfacial shear strength was not significant; thus, we selected 2.5 h as the grafting reaction time.

Table 2 H₂SO₄-catalyzed orthogonal test results

Number	Temperature	Time	PVA	Amount of catalyst	Bonding force/MPa
1	80 °C	0.5 h	5%	0.5%	0.6840
2	80 °C	1.5 h	7%	1.0%	0.8909
3	80 °C	2.5 h	9%	1.5%	1.0080
4	85 °C	0.5 h	7%	1.5%	0.9220
5	85 °C	1.5 h	9%	0.5%	0.9080
6	85 °C	2.5 h	5%	1.0%	0.9852
7	90 °C	0.5 h	9%	1.0%	0.9400
8	90 °C	1.5 h	5%	1.5%	0.9000
9	90 °C	2.5 h	7%	0.5%	1.0240
I	0.8610	0.8487	0.8564	0.8720	—
II	0.9384	0.8996	0.9456	0.9387	—
III	0.9547	1.0057	0.9520	0.9433	—
Range	0.0937	0.1570	0.0956	0.0713	—

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The effect of PVA solution concentration on the shear strength of the grafted interface reflected the competitive reaction between the hydroxyl groups of PVA and other molecules for the functional groups on the UHMWPE surface. High-concentration PVA showed poor liquidity. However, the reaction is reversible, and increasing the concentration of PVA is equivalent to increasing the concentration of the reactant, which can promote a positive reaction. Thus, the effect of PVA concentration is affected by both the factors. From the relationship between the shear strength of the grafted interface and the PVA concentration, the increasing trend of shear strength slowed down when the concentration reached 7%. Therefore, the PVA concentration of the grafting reaction was 7%.

The effect of grafting temperature on the shear strength of the grafted interface lies in the promoting role of temperature increase on the movement of PVA molecules. Thus, the shear strength of the grafted interface increased with increasing temperature. However, the heat deformation temperature of UHMWPE should be considered. The material properties were affected when the temperature was too high; therefore, the temperature should not be excessively high. The experimental results showed that at temperatures higher than 85 °C, the change in shear strength between the UHMWPE and PVA interface was insignificant. Thus, at 85 °C, temperature had little effect on the grafting reaction. Thus, the optimal grafting temperature of 85 °C was selected.

Catalyst concentration had the least effect on catalytic grafting. Studies on the influence of catalyst concentration on the shear strength of the UHMWPE and PVA hydrogel interface showed that when the amount of catalyst reached 1%, the catalytic effect nearly reached equilibrium. Further increases in catalyst concentration led to limited increases in shear strength. Therefore, the optimal catalyst concentration of the reaction was selected as 1%. Range analysis revealed that the effect of grafting time on the grafting reaction was the most significant, followed by the concentration of the grafting solution (PVA concentration), and then temperature. Catalyst concentration had the lowest impact on the grafting effect.

The optimum reaction conditions based on the interfacial shear strength between UHMWPE and PVA were identified as follows: UHMWPE as substrate, concentrated sulphuric acid as catalyst, reaction time of 2.5 h, temperature of 85 °C, grafting liquid concentration of 7% and catalyst concentration of 1%. It can be seen that the grafted interface of UHMWPE and PVA hydrogel does not contain crevices, which shows that the grafting of UHMWPE and PVA hydrogel is effective (Fig. 2).

Fig. 2 Grafted interface morphology image of UHMWPE and PVA hydrogel.

3.2 FT-IR

Fig. 3 shows the IR spectra of untreated, oxidised and grafted UHMWPE. A comparison of Fig. 3(a) and (b) showed that an evident IR absorption peak did not appear at 1714 cm⁻¹ in Fig. 3(a). However, after dichromate treatment, an apparent absorption peak was found in the IR spectrum. After dichromate oxidising solution treatment, the methyl groups at the edge of the UHMWPE surface were oxidised to form carboxylic acid, aldehyde, or carbonyl groups. Moreover, given the strong oxidising power of dichromate, the methylene chains revealed oxidative fractures and formed carboxylic acid, aldehyde, or carbonyl groups. Therefore, a C=O absorption peak was found at 1714 cm⁻¹ in the IR spectrum.

Fig. 3 FTIR spectra of UHMWPE, oxidized UHMWPE and grafted UHMWPE.

To distinguish unsaturated ketones, aldehydes and carboxylic acids, we refer to the IR absorption spectrum of –H. In the aldehyde groups, strong absorption peaks appeared between 2700 and 2800 cm⁻¹ for the –CH in RCHO. For the carboxylic acid groups, a broad absorption peak appeared between 2500 and 3500 cm⁻¹ for the O–H in RCOOH, as shown in Fig. 3(b). The presence of characteristic aldehyde absorption peaks between 2700 and 2800 cm⁻¹ cannot be ascertained, and only weak characteristic carboxylic acid absorption peaks between 3000 and 3500 cm⁻¹ can be seen. According to theoretical research, aldehyde groups are oxidised to carboxylic acids under strong dichromate oxidising conditions. The presence of carboxylic acid and saturated carbonyl groups on the UHMWPE surface can be detected after the reaction.

A comparison of Fig. 3(b) and (c) indicated that when UHMWPE was combined with PVA after oxidation, the IR absorption peak at 1714 cm⁻¹ basically disappeared. The C=O groups that were formed on the surface of the oxidised UHMWPE chemically reacted with the C–OH groups on the PVA molecular chains under the action of the concentrated sulphuric acid catalyst. The C=O bonds disappeared and C–O bonds were formed (the C=O bonds would not disappear if fatty esters were formed; if an acetal or ketal reaction occurred, the C=O bond would disappear and an ether bond would be formed). Upon the introduction of PVA, the C–O stretching vibration absorption peak of a secondary alcoholic hydroxyl group appeared in the vicinity of 1100 cm⁻¹.

Fig. 3(c) shows a strong absorption peak at 1000 cm⁻¹ to 1200 cm⁻¹. The peak amplitude can predict the formation of aliphatic polyethers in the reaction, caused by acetal or ketal reactions under the catalytic action of concentrated sulphuric acid in the reaction between oxidised UHMWPE and PVA. A strong, broad absorption peak attributed to the –OH absorption peak appeared between 3200 and 3550 cm⁻¹. After the catalytic reaction with sulphuric acid, a chemical reaction occurred between PVA and oxidised UHMWPE, thus causing the PVA molecules to be chemically bonded onto the surface of the chemically oxidised UHMWPE. Thus, after the reaction completion and the removal of non-bonded PVA by washing, an IR microscope was used to measure the presence of hydroxyl groups in the PVA molecules.

3.3 SEM

Fig. 4 shows the surface morphologies of UHMWPE, oxidized UHMWPE and grafted UHMWPE. It can be seen that the surfaces of oxidized UHMWPE and grafted UHMWPE were more microporous than that of UHMWPE, which was mainly caused by the dissolution of the non-crystalline region of the surface. At the same time the surface became rough; the macromolecular chains of the UHMWPE surface got fractured and formed polar functional groups, resulting in pores appearing on the UHMWPE surface.

Fig. 4 SEM images of UHMWPE, oxidized UHMWPE and grafted UHMWPE (500×).

3.4 DSC

Fig. 5 shows the DSC curves of UHMWPE, oxidized UHMWPE and grafted UHMWPE. When the temperature was higher than 160 °C, the DSC results of untreated UHMWPE were significantly higher than those of the oxidized and grafted UHMWPE, indicating that at this temperature, the treated UHMWPE released a greater quantity of heat than the untreated UHMWPE. The DSC curve of the grafted UHMWPE declined faster than that of the oxidized UHMWPE, which was because the PVA present on the grafted UHMWPE surface was in a heat absorbant state at that temperature. Therefore, the grafted UHMWPE released a lower quantity of heat than the oxidized UHMWPE.

Fig. 5 DSC curves of UHMWPE, oxidized UHMWPE and grafted UHMWPE.

3.5 Surface wettability

Fig. 6 shows the contact angles of UHMWPE, oxidized UHMWPE and grafted UHMWPE. The contact angles of UHMWPE, oxidized UHMWPE and grafted UHMWPE were 104°, 88.5°, and 39° (Table 3). The hydrophilicity was enhanced when UHMWPE was oxidized, which results from the formation of hydrophilic functional groups on the surface, such as carbonyl groups, aldehyde groups and other hydrophilic groups. However, the hydrophilic groups on the oxidized surface were limited, thus the decrease of the hydrophilic angle was not significant. The hydrophilicity of grafted UHMWPE was further improved due to the large amount of hydroxyl groups in PVA. The combination of PVA with oxidized UHMWPE resulted in a good enhancement of the surface wettability after grafting.

Fig. 6 Hydrophilic angle figures of UHMWPE, oxidized UHMWPE and graft UHMWPE.

Table 3 Contact angles of UHMWPE, oxidized and grafted UHMWPE

Material	Contact angle
UHMWPE	104°
Oxidized UHMWPE	88.5°
Grafted UHMWPE	39°

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3.6 Chemical grafting mechanism

The chemical grafting mechanisms of interfacial grafting can be divided into two types. One involves the enhancement of the combination between the body and matrix groups of the chemical graft, which does not produce new compounds; the other produces a chemical reaction and new compounds. In the chemical grafting mechanism, the chemical groups on the material surface and another compatible chemical group formed a new chemical graft (Fig. 7). The oxidation of the UHMWPE surface and the catalytic reaction of PVA solution belong to this type of interface grafting mechanism.

Fig. 7 Chemical reaction between the enhanced group A and the basement surface group B.

The oxidation reaction can be significantly accelerated by adding different divalent metal salts. The transition metal salt catalyst can form a redox system with the reactant to promote hydroperoxide decomposition and produce a large number of active centres. Dichromate oxidation treatment resulted in the generation of oxygen-containing groups (C=O, COH, COOH) on the UHMWPE surface. Introducing polar groups can improve the wettability of the UHMWPE surface, slight oxidation of the surface and fracturing of the macromolecular chains produced polar groups.

Fig. 8 shows that dichromate oxidation altered the functional groups on the UHMWPE surface. The process of dichromate oxidation first involved replacing a C atom bonded to an H atom. After O directly bonded with C in chromic acid, the chromic acid disappeared, and then hydroxyl group formed in that position and was oxidised. Aldehyde groups and carbonyl groups were formed under the dichromate conditions. Depending on the oxidation time and oxidant amount, aldehyde groups were oxidised to form carboxyl groups.

Fig. 8 Changes in the functional groups of UHMWPE after dichromate oxidation.

The mechanism of the grafting reaction between the oxidised surface of UHMWPE and PVA is shown in Fig. 9. The first step involved a typical acid-catalysed carbonyl addition. The acid catalyst protonated the carbonyl groups, and alcohol groups (weak nucleophile) then attacked the protonated, activated carbonyl groups. The positively charged intermediate lost a proton to generate a hemiacetal. When alcohol is added to a hemiacetal molecule, a ‘complete’ acetal is formed. Like ketone and aldehyde hydrates, most hemiacetals are unstable; therefore, separation and purification was impossible. The second step involved converting the hemiacetal into a stable species. First, a hydroxy group was protonated, yielding water and generating a resonance-stabilised carbenium ion. An alcohol group then attacked the carbenium ion, followed by the loss of a proton to generate an acetal.

Fig. 9 Grafting mechanisms of UHMWPE and PVA.

The two important parts of the mechanism are as follows:

(1) Acid-catalysed nucleophilic addition to the carbonyl;

(2) The protonation and loss of –OH functional groups, the occurrence of an SN1 reaction and the subsequent alcohol attack.

Hydration can be catalysed by either acids or bases, but acetal formation can be catalysed only with an acid. The first step (hemi-acetal formation) can be base-catalysed, which involves the attack of alkoxy anions and the protonation of alkoxy groups. The second step substitutes the –OR of the alcohol with a hemiacetal –OH. The hydroxyl ion is a poor leaving group in SN2 reactions, thus the ion cannot replace the –OH alkoxy group. However, this substitution can occur under acidic conditions because the protonation of –OH alkoxy groups and the loss of water can generate a resonance-stabilised cation.

4 Conclusion

After oxidation treatment, carbonyl and other oxygen-containing groups were formed on the surface of UHMWPE, and then PVA molecules were observed on the UHMWPE surface after catalytic action. The chemical grafting mechanism of UHMWPE and PVA involved an acetal (ketal) reaction. The reversible reaction had two important parts: (1) acid-catalysed nucleophilic addition to the carbonyl group; and (2) an SN1 reaction through protonation, the loss of an OH functional group and subsequent alcohol attack. The optimal reaction conditions for UHMWPE and PVA hydrogel, which were selected based on shear strength, were as follows: reaction time, 2.5 h; temperature, 85 °C; PVA hydrogel concentration, 7%; and amount of catalyst, 1%. The shear strength of the artificial joint can reach 1 MPa. The contact angles of UHMWPE are decreased from 104° to 39° by grafting, and the surface wettability is effectively improved.

Acknowledgements

This research is supported by the Specialized Research Fund for the Doctoral Program of Higher Education (Grant no. 20110095110001), the National Natural Science Foundations of China (Grant no. 51275514 and 51405489), the Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF12A06) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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