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. Keywords Poly(vinyl alcohol) hydrogel ; Physically cross-linked gel ; Frictional property ; Swelling property ; Atomic force microscope prs.rt("abs_end"); 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] and [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. 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] and [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] and [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 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] and [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?°C 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 cross-linked PVA gels for use as artificial cartilage is discussed. 2. Materials and methods 2.1. Sample preparation 2.1.1. FT gelPVA pre-gel solution (15?wt%) was prepared by dissolving PVA powder (PVA117; Kuraray Co., Ltd.) in pure water at a temperature around 90?°C 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?°C and then thawed for 9?h at 4?°C, 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 gelPVA 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?°C at 50%RH conditions for 7 days and then at 20?°C 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
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