Organization structure and tribological study of hydrogel prepared by Uv light molding and casting molding methods for bionic articular cartilage

Hydrogels have similar structural and functional properties to natural articular cartilage, so they have been widely used in the research of biomimetic articular cartilage. In this research, polyvinyl alcohol (PVA) and betaine monomer were used as raw materials to prepare hydrogel on polyetheretherketone (PEEK) surface by Uv light molding and casting molding respectively. The results showed that two methods were used to prepare hydrogel film on PEEK surface successfully. The diameter of the internal hole of the hydrogel prepared by Uv light molding was larger than that prepared by casting molding. Compared with the hydrogels prepared by Uv light molding, the hydrogels prepared by casting molding have a lower friction coefficient because of their high load-carrying capacity. Different preparation methods seriously affect the formation of hydrogel structure. This study provides an important reference value for the preparation of high performance hydrogels.


Introduction
Natural articular cartilage exhibits ultra-low friction and wear properties even under high compression pressure [1]. Hydrogels have been widely studied as a potential alternative to osteoarticular cartilage [2]. PVA is a kind of long chain water-soluble polymer produced by alkali-catalyzed alcoholysis of polyvinyl acetate [3][4][5]. The disadvantages of PVA hydrogels mechanics have limited its development in biomimetic osteoarticular cartilage. Many researchers have improved the mechanical properties of PVA hydrogels by chemical modification [6], irradiation modification [7], physical blending [8][9][10], filling modification [11,12] and double-network structure [13], and a series of studies have greatly improved the toughness and strength of PVA hydrogels.
MPDSAH is a betaine zwitterion, which contains both positive and negative charge groups in its structure, and has a strong electrostatic adsorption effect on water molecules. Many researchers the lubrication of zwitterionic this special structure is studied, it is also believed that the charged components of zwitterions can absorb water to form hydration layer around, improve toughness and maintain water fluidity, exchange with water molecules in the lubricant during the process of friction, form the effect of boundary lubrication, and effectively reduce the friction coefficient and wear of hydrogel. Ruojun Mu et al [14] studied the effect of polymers carrying different charges on hydration lubrication and friction of hydrogels by swelling polymer chains with large pore size hydrogels. They found that under compression, water molecules could still adhere to the charges while maintaining shear fluidity, which effectively reduced the friction coefficient. Feichen Yang et al [15] prepared BC-PVA-PAMPS multi-network hydrogel by adding PAMPS to hydrogel. The negative charge of the sulfate group in PAMPS was used to simulate the effects of chondroitin sulfate and keratin sulfate, and the compressive strength was improved by affecting osmotic pressure. At the same time, the negative charge also increases the thickness of the water lubrication layer between the hydrogel and the relative surface, thus reducing the friction coefficient [16][17][18][19][20][21][22].
In this chapter, betaine zwitterion monomer MPDSAH was blended with polyvinyl alcohol, and the influence of Uv light molding and casting molding on the hydrogel was compared. The preparation diagram is shown in figure 1. The microstructure characteristics of hydrogels prepared by two methods were studied, and the frictional lubrication mechanism of the two kinds of hydrogels was analyzed.

Materials and methods
2.1. Experimental raw materials Raw materials and manufacturers are shown in table 1 and the solvents were deionized water.

Experimental sample preparation and performance study
Uv light molding: A mixed solution containing 15 wt% PVA and 5 wt%MPDSAH was configured, and the photoinitiator MBAA with 0.1% PVA mass fraction and crosslinker α-kA with 0.1% PVA mass fraction were Changzhou Yipintang Chemical Co., Ltd The obtained hydrogel sample was used as friction sample, and a stainless steel ball with a diameter of 6 mm was used as the upper friction pair, reciprocating linear motion was made on the sample surface, and the motion stroke length was 3 mm. The first condition: the sliding velocity of the friction pair is v = 6 mm s −1 , the sliding time is 900 s, the normal loads applied were 2 N, 4 N, 6 N, and 8 N respectively. The second condition: the load is 2 N, the sliding time is 900 s and the friction velocity is 2 mm s −1 , 4 mm s −1 and 6 mm s −1 respectively. The third condition: the load is 2 N, the friction velocity is 6 mm s −1 , the sliding time is 7000 s. The lubrication environment is deionized water.

Results and discussion
3.1. Sample composition and structure Figure 2 shows the FTIR spectra of PEEK, Uv light molding PVA-pMPDSAH and casting molding PVA-MPDSAH. As can be seen from the figure, there is a great difference between the infrared absorption spectrum of the sample surface combined with hydrogel and the characteristic peak of the infrared spectrum of the hard base surface, which indicates that the hydrogel prepared by Uv light molding and casting molding covers the surface of the PEEK. The soft hydrogel layer was successfully prepared on PEEK surface. Figure 3 shows the XRD patterns of PEEK, Uv light molding PVA-pMPDSAH and casting molding PVA-MPDSAH. Compared with PEEK substrate, the diffraction peaks of hydrogels changed, indicating that there were microcrystals inside the hydrogels. The intensity of the diffraction peak of the Uv light molding hydrogel was lower than that of the casting molding hydrogel, which indicated that the content of microcrystals in the casting molding hydrogel was higher than that of the Uv light molding hydrogel. Figure 4 shows the SEM images of the surface morphology of PEEK, Uv light molding PVA-pMPDSAH and casting molding PVA-MPDSAH. It can be seen from the figure that PEEK surface is flat, with only scratches caused by sandpaper. There are obvious micropores on the surface of hydrogel formed by Uv light molding. Under the action of ultraviolet light, PVA and MPDSAH will have network cross-linking phenomenon. The strong crosslinking force will cause the cavity phenomenon inside the sample. So, it can be seen from the macropores that there is a strong cross-linking phenomenon in its internal organization. The PVA-MPDSAH crosslinking effect of natural pouring was weak, so the surface of the hydrogel formed by casting molding was evenly distributed and the pore size was smaller than that of the hydrogel formed by Uv light molding. Figure 5 shows the three-dimensional surface topography of PEEK (a), Uv light molding PVA-pMPDSAH (b) and casting molding PVA-MPDSAH (c). The Ra and Rq of of hydrogel surface were lower than those of PEEK surface. The surface Ra and Rq of hydrogels prepared by the two methods were about 0.05 μm. Hydrogels have relatively flat surfaces.  Figure 6 shows the contact Angle of PEEK, Uv light molding PVA-pMPDSAH and casting molding PVA-MPDSAH. PEEK exhibits hydrophobic properties due to its non-polar aromatic skeleton the contact Angle is about 103 degrees. Hydrogels contain a large number of hydrophilic hydrogen bonds and charged groups, which have a strong adsorption effect on water molecules, making the sample surface extremely hydrophilic. The contact angle of Uv light molding PVA-pMPDSAH is about 51 degrees and the contact angle of casting molding PVA-MPDSAH is about 43 degrees.   coefficient under four kinds of loads is the same, which is a small increase and then tends to be stable. The friction coefficient of casting molding PVA-MPDSAH is between 0.02 ∼ 0.09 under four loads. The hydrogel has the lowest friction coefficient when the load is 2N (about 0.02). When the load is 4 N, 6 N and 8 N, the friction coefficient of hydrogels increases significantly after 500 s. When the friction load is greater than 2 N, the point pressure causes local deformation of the hydrogel, and the deformation of the hydrogel increases the resistance of the instantaneous movement of the grinding ball, and the increase of the resistance leads to the increase of the friction coefficient. The friction coefficient of Uv light molding PVA-pMPDSAH is higher than that of casting molding PVA-MPDSAH overall, mainly because the porous structure of Uv light molding PVA-pMPDSAH reduces the load bearing strength of hydrogel. Under the same load, the friction ball of Uv light molding PVA-pMPDSAH is deeper than that of casting molding PVA-MPDSAH, and the instantaneous rolling resistance of Uv light molding PVA-pMPDSAH hydrogel is greater than that of casting molding PVA-MPDSAH hydrogel. Figure 8 shows the coefficient of friction of PEEK, Uv light molding PVA-pMPDSAH and casting molding PVA-MPDSAH under 2 mm s −1 , 4 mm s −1 , 6 mm s −1 friction velocity when the load is 2 N. The friction coefficient of PEEK substrate is between 0.2 ∼ 0.32 under three friction velocity. The friction coefficient of PEEK decreased with the increase of friction velocity. During the friction process, there is no hydration lubrication phase between PEEK and deionized water, so severe wear occurs on the surface with the increase of friction speed, resulting in the increase of friction coefficient. The friction coefficient of Uv light molding PVA-pMPDSAH is between 0.1 ∼ 0.14 under three friction velocity, and the development trend of the three friction coefficients is the same. The friction coefficient of casting molding PVA-MPDSAH is between 0.01 ∼ 0.03 under three friction velocity. Casting molding PVA-MPDSAH hydrogel has excellent friction coefficient due to its good supporting effect and water boundary lubrication. Figure 9 shows the coefficient of friction and wear rate of Uv light molding PVA-pMPDSAH and casting molding PVA-MPDSAH under 2 N load, friction velocity 6 mm s −1 , friction time 7000 s. The friction coefficient of Uv light molding PVA-pMPDSAH is stable at about 0.16 in the ultra-long time. The friction coefficient of casting molding PVA-MPDSAH is lower than 0.03 in the first 3000 s, but it will rise after 3000 s and rise to the same friction coefficient as Uv light molding PVA-pMPDSAH in 7000 s. The wear rate of Uv light molding PVA-pMPDSAH is about 12.5 × 10 −3 mm 3 Nm −1 , and that of casting molding PVA-MPDSAH is about 18 × 10 −3 mm 3 Nm −1 . In the first 3000 s of the PVA-MPDSAH film, the surface hydration layer played a good lubrication role. However, with the extension of friction time, the formation of the hydration layer was destroyed by wear to a certain extent, so the friction coefficient increased to a certain extent. Figure 10 shows the two-dimensional and three-dimensional surface topography of the worn surface of Uv light molding PVA-pMPDSAH and casting  molding PVA-MPDSAH. It can be seen from the grinding spot morphology, the surface of Uv light molding PVA-pMPDSAH is network cross-linking, and good cross-linking leads to furrow wear on the surface of the grinding plate. The tissue cross-linking of casting molding PVA-MPDSAH is weak, and the surface is mainly spalling wear caused by fatigue. The flaking of material is the main reason for the higher wear rate of casting molding PVA-MPDSAH compared with Uv light molding PVA-pMPDSAH.

Conclusions
In this study, the hydrogels prepared by both methods Uv light molding and casting molding were fully formed and covered on the surface of PEEK. Hydrogels have good water absorption, good hydration lubrication makes it have a low coefficient of friction. The large cross-linking void of Uv light molding PVA-pMPDSAH leads to a larger depth of friction ball, and the increase of instantaneous movement resistance leads to a higher friction coefficient of hydrogel. The cross-linking void of casting molding PVA-MPDSAH is small and has a good bearing effect. The reduction of instantaneous movement resistance of the grinding ball makes it have a good friction coefficient. However, the good cross-linking property of Uv light molding PVA-pMPDSAH makes it have a low wear rate, while the reduction of cross-linking effect of casting molding PVA-MPDSAH makes fatigue spalling wear occur in the process of friction, so it has a high wear rate, and the friction coefficient gradually increased after 3000 s friction time.