Abstract Poly (vinyl alcohol) (PVA) hydrogel with high water content is the candidate material for artificial cartilage. PVA hydrogels prepared by freeze-thawing (FT) method (PVA-FT gel), cast-drying (CD) method (PVA-CD gel) and hybrid method of FT and CD (PVA-hybrid gel) were developed and their friction and wear behaviors were evaluated. Sliding pairs of a cobalt-chromium-molybdenum alloy or alumina ceramic ball and a PVA hydrogel plate were tested in reciprocating friction test. Ultra-pure water and simulated synovial fluid were used as lubricants for friction test. PVA-FT gel showed high friction and severe wear in ultra-pure water. Friction coefficient of PVA-CD gel in ultra-pure water was quite low such as about 0.005, but scratches were observed on the surface of PVA-CD gel. PVA-hybrid gel lubricated in ultra-pure water showed low friction such as 0.004, and intact surface structure of PVA-hybrid gel remained after friction test. Simulated synovial fluid contributed to the improvement of lubrication property of all PVA hydrogels and PVA-hybrid gel showed the lowest friction coefficient such as 0.003 and minimum wear. These results indicated that PVA-hybrid gel has a great potential as the material for artificial joint. Keywords Artificial articular cartilage ; PVA hydrogel ; Artificial joint ; Wear ; Friction prs.rt("abs_end"); 1. Introduction The total joint prosthesis contributes to the recovery of joint function and relief from pain for patients suffering with rheumatoid arthritis and osteoarthritis. In total joint prosthesis, the common bearing surface for artificial joint is a combination of an ultra-high molecular weight polyethylene (UHMWPE) and a cobalt–chromium–molybdenum (Co–Cr–Mo) alloy or ceramics. However, in long-time usage, revision treatment is conducted due to the loosening of joint prostheses derived from wear particles-induced osteolysis [1] . Therefore, wear reduction of joint prosthesis materials is needed to improve the longevity of artificial joints. The lubrication mode of artificial joints composed of UHMWPE and hard materials is considered to be mixed or boundary lubrication modes. Therefore, artificial articular cartilage with high water content has been proposed for improvement of lubrication mode in artificial joints [2] and [3] for mimicking the excellent lubrication mechanism of natural synovial joint with extremely-low friction and minimum wear. When artificial articular cartilage is introduced to the frictional surface of artificial joints, fluid film formation by soft elastohydrodynamic lubrication (EHL) and micro-EHL [4] is expected. If hydrogel with high water content is used as artificial articular cartilage, biphasic lubrication mechanism [5] can be obtained by taking advantage of hydration material. Artificial articular cartilage can be used as the implant material for the focal defects of natural articular cartilage, so artificial articular cartilage has great potential for the recovery of joint function. Various elastomer and hydrogels have been proposed as artificial cartilage [6] , [7] , [8] and [9] , but our group considered that hydrogel with excellent biocompatibility, high mechanical strength and high water content is desirable to mimicking the excellent lubrication mechanism of natural articular cartilage. Therefore, our group has focused on poly (vinyl alcohol)(PVA) hydrogel with high water content as candidate material for artificial cartilage. In previous study, we evaluated the tribological behaviors of PVA hydrogel prepared by repeated freezing-thawing (FT) method [10] (PVA-FT gel) and found that the tribological behaviors of PVA-FT gel were affected by synovial fluid constituents such as proteins, phospholipids and hyaluronic acid [11] , [12] , [13] and [14] . In addition, PVA-FT gel showed excellent lubrication property in simulated synovial fluid [15] . However, wear resistance of PVA-FT gel was considered to be not enough under severe direct contact condition. Therefore, improvement of the material property of PVA hydrogel is needed to reduce both friction and wear. Suzuki et al. have newly developed the physically-cross-linked PVA hydrogel by using cast-drying (CD) method [16] (PVA-CD gel) and PVA-hybrid gel by the combination method of FT and CD [17] . Both PVA-CD and PVA-hybrid gels could show lower friction than PVA-FT gel and PVA-hydrid gel kept low friction coefficient such as 0.01 [18] rubbed against glass plate. In addition, friction and wear properties of PVA-hybrid gel have been improved and PVA-hybrid gel showed extremely low friction coefficient such as 0.006 and minimum wear [19] . However, this result was obtained by friction test with sliding pair of hydrogels and glass plate. Considering the actual use of PVA hydrogels as artificial articular cartilage, the evaluation of lubrication property of PVA hydrogels sliding against the artificial joint material is needed. Therefore, the aim of this study is to investigate the lubricati
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