Total hip and knee joint prostheses composed of ultra-high molecular weight polyethylene (UHMWPE) and metal or ceramics have been widely applied. Efficacious treatments such as crosslinking, addition of vitamin E and phospholipid coating to UHMWPE have reduced wear and extended the life of joint prostheses. However, wear problems have not yet been completely solved for cases involving severe conditions, where direct contact can occur in mixed or boundary lubrication. In contrast, extremely low friction and minimum wear are maintained for a lifetime in healthy natural synovial joints containing articular cartilage with superior lubricity. Accordingly, joint prostheses containing artificial hydrogel cartilage with properties similar to those of articular cartilage are expected to show superior tribological functions. In establishing the function of artificial hydrogel cartilage as a novel material for joint prostheses, the tribological properties of hydrogel materials used and synergistic performance with synovia constituents are both important. In this study, the lubrication ability and wear resistance properties of poly(vinyl alcohol) (PVA) hydrogels were evaluated by differences in friction and wear properties in reciprocating tests lubricated with saline and simulated synovial fluid. Biphasic finite element (FE) analysis was applied to elucidate the role of biphasic lubrication mechanism in hydrogels.

As biocompatible artificial hydrogel cartilage materials, three PVA hydrogels were prepared using the repeated freeze-thawing (FT) method, the cast-drying (CD) method and the hybrid method for laminated gel of FT on CD, which are physically crosslinked with hydrogen bonding but differ in terms of structure and mechanical properties. First the frictional behavior of the ellipsoidal PVA hydrogel specimens was examined in reciprocating tests against a glass plate, which corresponds to simplified knee prosthesis model (Fig.1), with a sliding speed of 20 mm/s under constant continuous loading. As shown in Fig.1, the three hydrogels exhibited different frictional behaviors in a saline solution. It is noteworthy that the hybrid gel maintained very low friction until the end of test. The CD gel showed slightly higher friction and a gradual increase. Meanwhile, the FT gel showed initial medium friction and a gradual increase. Time-dependent frictional behavior was clarified with biphasic lubrication mechanism via biphasic FE analysis. Contact surface observation showed minimal wear without scratches for hybrid gel in saline.

Next, simulated synovial fluid composed of 0.5 wt% hyaluronic acid (HA, molecular weight: 920,000 Da), 1.4 wt% albumin, 0.7 wt% gamma-globulin and 0.01 wt% L-alpha dipalmitoylphosphatidylcholine (DPPC), was used to evaluate tribological performance of these gels in physiological condition. As shown in Fig.2, PVA hydrogels in simulated synovial fluid exhibited very low friction, with hybrid gel showing an extremely low friction coefficient of 0.003 in the test. These friction differences were sustained by biphasic FE analysis. Hybrid gel further showed very little wear (Fig.3), which is favorable in terms of hydrogel durability.

These results indicate the importance of superior lubricity and wear resistance of PVA hybrid gel for the clinical application of artificial hydrogel cartilage in joint prostheses.

In joint prostheses where ultra-high molecular weight polyethylene (UHMWPE) is used as bearing material, efficacious treatments such as crosslinking, addition of vitamin E and the grafting of phospholipid polymer are known to improve wear resistance. Under severe conditions of various daily activities, however, friction and wear problems in such prostheses have not yet been completely solved. In contrast, extremely low friction and minimum wear have been maintained for a lifetime in healthy natural synovial joints containing articular cartilage with superior lubricity. Accordingly, joint prostheses containing artificial hydrogel cartilage with properties similar to those of articular cartilage are expected to show superior tribological functions. In establishing the function of artificial hydrogel cartilage as a novel material for joint prostheses, the tribological properties of hydrogel materials used and synergistic performance with synovia constituents are both important. In this study, the influence of synovia constituents on friction and wear in artificial hydrogels was examined in reciprocating test and compared with that for articular cartilage.

As biocompatible artificial hydrogel cartilage materials, three poly(vinyl alcohol) (PVA) hydrogels were prepared using the repeated freeze-thawing (FT) method, the cast-drying (CD) method and hybrid method for CD on FT, which are physically crosslinked with hydrogen bonding but differ in terms of structure and mechanical properties. First the frictional behavior of the PVA hydrogels and articular cartilage as ellipsoidal specimens was examined in reciprocating tests against a glass plate with a sliding speed of 20 mm/s under constant continuous loading. As shown in Fig.1, the three hydrogels exhibited different frictional behaviors in a saline solution. It is noteworthy that the hybrid gel maintained very low friction until the end of test. The CD gel showed slightly higher friction and a gradual increase. Meanwhile, the FT gel showed initial medium friction and a gradual increase echoing the time-dependent behavior of natural articular cartilage. Based on these observations, focus was placed on FT gel and articular cartilage to examine how synovia constituents influence friction and wear in these hydrogel materials.

In human body, lubricating constituents in synovial fluids such as hyaluronic acid, proteins, glycoproteins and phospholipids are considered to reduce the coefficient of friction in solid-to-solid interaction. Here, the effects of hyaluronic acid (HA, molecular weight: 9.2×10⁵), serum proteins and phospholipid were examined. Dipalmitoylphosphatidylcholine (DPPC) was used as a typical phospholipid. As indicated in Fig.2 for repeated reciprocating tests, addition of HA alone was effective particularly for PVA-FT hydrogel. The combination of HA and DPPC was more effective in reduction of friction. The simulated synovial fluid (composed of HA 0.5 wt%, DPPC 0.01 wt%, albumin(Alb) 1.4 wt% and gamma-globulin (g-glob) 0.7 wt%) exhibited both low friction and minimum wear. The rubbing surfaces of articular cartilage and FT gel after tests are shown in Fig.3. On the articular cartilage surface, gel-like surface layer existed. On the FT gel surface, the original texture was observed without damage.

These results indicate the importance of synovia constituents for the clinical application of artificial hydrogel cartilage in joint prostheses.

In joint prostheses using ultra-high molecular weight polyethylene (UHMWPE) as bearing material, wear problems are not yet completely solved under severe conditions in various daily activities, although efficacious treatments such as crosslinking, addition of vitamin E and the grafting of phospholipid polymer improved the wear properties. In contrast, in healthy natural synovial joints possessing articular cartilage as biphasic bearing material lubricated with synovial fluid, minimal wear with extremely low friction has been maintained for a whole life. Therefore, the joint prosthesis with artificial hydrogel cartilage with similar properties to articular cartilage is expected to show superior tribological functions with very low friction and infinitesimal wear if the appropriate lubrication mechanism is actualized. In this study, the effectiveness of biphasic lubrication mechanism in hydrogel through significant load support by fluid phase is evaluated in finite element (FE) analysis for reciprocating motion.

As biocompatible artificial hydrogel cartilage materials, two kinds of poly (vinyl alcohol) (PVA) hydrogels were prepared by the repeated freezing-thawing method and the cast-drying method, which are physically crosslinked with hydrogen bonding but different in structure and mechanical properties. To evaluate these time dependent behaviors of load-support ratio of fluid/solid phases and friction, two-dimensional biphasic FE analysis for cylindrical PVA hydrogel cartilage as 1.5 mm thick soft layer and radius of 5 mm was conducted under continuous loading of 0.2 N/mm by impermeable rigid plate in reciprocating motion in Fig. 1. The sliding speed is 4 mm/s for stroke of 8 mm at period of 4 s. A commercial package ABAQUS (6.8–4), which was appropriately evaluated for the biphasic FE analyses, was used in this study. The biphasic tissue was modeled by CPE4RP (four-node bilinear displacement and pore pressure, reduced integration with hour glass control) elements. The mechanical properties such as permeability, Young's modulus and Poisson ratio were estimated by curve fitting to stress relaxation behaviors in compression test.

As indicated in Fig. 2, it is worth noting that the cast-drying PVA shows significant interstitial fluid pressurization compared with a repeated freezing-thawing PVA hydrogel at 292 s after start-up, where coefficient of friction for solid-to-solid was assumed as 0.2. Changes in friction for PVA hydrogels in reciprocating motion were estimated as shown in Fig. 3. In spite of high friction (0.2) for solid-to-solid, cast-drying PVA brought the gradual decreasing in friction, probably due to rising of load-support ratio by fluid phase from initial 74% to 80%.

In human body, lubricating constituents in synovial fluids such as hyaluronic acid, proteins, glycoproteins and phospholipids can reduce the coefficient of friction for solid-to-solid. As suggested for low coefficient of friction for solid-to-solid as 0.01 in Fig. 3, rubbing friction is expected to be reduced to significantly low level.

As described above, the effective biphasic lubrication can sustain low friction level and minimal wear in synergistic action with soft-elastohydrodynamic lubrication, hydration lubrication and boundary lubrication as a similar mechanism to natural cartilage in various daily activities. These results indicate the usefulness of artificial hydrogel cartilage for longer durability in joint prostheses for clinical application.

Various treatments for ultra-high molecular polyethylene (UHMWPE) such as cross-linking, addition of vitamin E and the grafting of phospholipid polymer improved the wear properties. However, wear problems still occur in joint prostheses in mixed or boundary lubrication modes under severe conditions. As an alternate method, the joint prosthesis with artificial hydrogel cartilage with similar properties to articular cartilage is expected to show superior tribological functions with very low friction and low wear if the adaptive multimode lubrication mechanism is actualized. In this study, the effectiveness of hydrogel structure and adsorbed film formed on artificial cartilage surfaces is examined in reciprocating tests in related to biphasic, hydration and boundary lubrication modes.

The frictional behaviors of artificial cartilage materials against flat glass plate in the reciprocating test were observed. As upper specimens, poly(vinyl alcohol) (PVA) hydrogel ellipsoidal specimen as 2 mm soft layer were prepared. PVA hydrogel specimens were prepared by the repeated freezing-thawing method and the cast-drying method. The sliding speed and stroke length were 20 mm/s and 35 mm, respectively. Applied load was 2.94 N or 9.8 N. The lubricants are saline or saline solutions containing L-α-Dipalmitoyl phosphatidyl-choline (DPPC), serum protein and/or hyaluronate(HA).

As shown in Fig. 1, the repeated freezing-thawing PVA shows gradual increase in friction from initial medium value immediately after loading of 2.94 N to high level. For the same test condition, the articular cartilage exhibited similar time-dependent frictional behavior from initial lower friction to high level as estimated by biphasic lubrication theory. On the contrary, it is noticed that a low friction is maintained for cast-drying PVA hydrogel, particularly two-layer laminated PVA hydrogel until 140 m sliding. The improvement of frictional behaviors in cast-drying PVA hydrogel is considered to have been brought about by the improvement of water retention ability of the hydrogel with uniform microstructure controlled by hydrogen bond.

Next, the influence of lubricant constituents on tribological behaviors of freezing-thawing PVA hydrogel was examined in repeated reciprocating test including unloading-restarting process at each 36 m sliding at 9.8 N. The frictional behavior for the freezing-thawing PVA hydrogel could be improved with supplying appropriate lubricant constituents as shown in Fig. 2. In lubricated condition with HA solution containing 0.01 wt% DPPC, 1.4 wt% albumin and 0.7 wt% γ-globulin, low friction was maintained and very little visible wear was confirmed in micrograph. Adsorbed films appear to contribute to the effective synergistic lubrication even under high load of 9.8 N in reciprocating test.

As described above, the effectiveness of synergistic lubrication for PVA hydrogel specimens is shown for improvement of tribological behaviors of artificial cartilage as a superior mechanism to natural cartilage. These results indicate the possibility of artificial hydrogel cartilage for longer durability in joint prostheses for clinical application.

The reduction of both friction and wear is required in existing joint prostheses composed of ultra-high molecular weight polyethylene (UHMWPE) and metallic or ceramic components, or even in Hard-on-Hard joint prostheses. In contrast, the healthy natural synovial joints with rubbing surfaces of articular cartilage are likely to operate at very low friction and low wear for the entire lifetime in the adaptive multimode lubrication mechanism, in which various lubrication modes become effective in various daily activities. Therefore, to establish a similar lubrication mechanism in joint prostheses by the application of compliant artificial cartilage, we conduct various researches to improve lubrication modes resulting in reduction in both friction and wear. In this paper, the effectiveness of the hydrogel artificial cartilage of high water content is discussed from the viewpoint of bionic design to mimic natural synovial joints.

The aim of this paper is to facilitate a function based on multimode lubrication mechanism in joint prostheses similar to natural synovial joints. Firstly, the possibility of full elastohydrodynamic lubrication was evaluated by experimental methods in friction tester and joint simulator. The joint prostheses with compliant rubbing materials or polymer-on-hard joint with better geometrical congruity showed siginificant fluid film formation, but some local intimate contact occurred. Therefore, as the second viewpoint, the effectiveness of adsorbed film formation was examined. The noteworthy phenomena are remarkable reduction in friction for artificial joint with poly(vinyl alcohol) (PVA) hydrogel articular surfaces and a notable increase in friction for artificial joint with polyurethane surface in hyaluronate solutions containing serum proteins. These results indicated that adsorbed protein films can reduce or increase friction and wear depending on probably fluid film thickness.

Other findings of effectiveness of layered adsorbed film and negative effect of heterogeneous adsorbed film are described on the basis of various observation in friction tests.

As the third viewpoint, the importance of biphasic lubrication and hydration lubrication for hydrogel surface with high water content is discussed. In friction tests of natural articular cartilage against glass plate, it was observed that the unloading for 5 min after continuous 30 min rubbing reduced the friction at restarting probably due to biphasic lubrication and/or hydration lubrication after rehydration, where adsorbed films have some influences on friction and wear. For joint prostheses with compliant hydrogel artificial cartilage, similar mechanism is required for surface and bulk structure of artificial cartilage.

In this paper, several important essential points from the bionic design are indicated for development of the next generation for joint prostheses with higher function and better longevity.