Yoshinori Sawae

Adaptive Multimode Lubrication in Natural Synovial Joints and Artificial Joints

Abstract To examine the lubrication mechanisms in both natural synovial joints and artificial joints with artificial cartilages, pendulum tests of pig shoulder joints and simulator tests of sliding pairs of a stainless steel spherical component and natural articular cartilage or artificial cartilage have been conducted. Firstly, it was shown in pendulum tests of pig shoulder joints that both concentration of hyaluronic acid or viscosity and adsorbed film formation of proteins and phospholipids exerted a significant effect on frictional behaviour in swinging motion immediately after a loading of 100 N. Under a high load of 1 kN, low friction was observed under wide-ranging viscosity conditions, since a high load similar to body weight probably enhanced the squeeze film effect due to improved congruity. Next, frictional behaviour of sliding pairs in knee joint models, consisting of a stainless steel spherical surface and either specimens of pig tibial cartilage or polyvinylalcohol (PVA) hydrogel, was examined during walking in simulator tests. In these tests, the influences of lubricant viscosity and addition of protein on frictional behaviour were evaluated. For both compliant materials, the appropriate addition of γ-globulin to sodium hyaluronate (HA) solution maintained low friction and protected rubbing surfaces under thin film conditions. These phenomena are discussed from the viewpoint of adaptive multimode lubrication.

Effect of synovia constituents on friction and wear of ultra-high molecular weight polyethylene sliding against prosthetic joint materials

Abstract The effect of synovia constituents, such as a serum protein and hyaluronic acid, on the friction and wear of ultra-high molecular weight polyethylene (UHMWPE) was examined in a pin-on-disk test apparatus. In this study, UHMWPE pin specimens were articulated against stainless steel (SUS316) and alumina ceramic disk specimens. Both saline and diluted bovine serum were used as lubricants. Albumin, a kind of serum protein, and sodium hyaluronate were added to the saline respectively to identify their effects on the friction and wear of UHMWPE. As a result, each of the synovia constituents had a different influence on the friction and wear of UHMWPE. The wear rate in the albumin solution was not so different from that in the diluted serum, but friction coefficient was higher and the morphology of the wear surfaces observed by scanning electron microscope was quite different. On the other hand, hyaluronic acid could reduce the friction and wear of UHMWPE.

Enzymatically fabricated and degradable microcapsules for production of multicellular spheroids with well-defined diameters of less than 150 μm

Microcapsules with a single, spherical hollow core less than 150 microm in diameter were developed to obtain multicellular spheroids with well-defined sizes of less than 150 microm in diameter. An aqueous solution of phenolic hydroxyl derivative of carboxymethylcellulose (CMC-Ph) containing human hepatoma cell line (HepG2) cells and horse radish peroxidase (HRP) was injected into a coflowing stream of liquid paraffin, containing H(2)O(2), resulting in cell-enclosing CMC-Ph microparticles, 135 microm in diameter, via a peroxidase-catalyzed crosslinking reaction. The CMC-Ph microparticles were then coated with a phenolic hydroxyl derivative of alginate (Alg-Ph) gel membrane several dozen micrometers in thickness, crosslinked via the same enzymatic reaction process, followed by further crosslinking between the carboxyl groups of alginate by Sr(2+). A hollow core structure was achieved by immersing the resultant microcapsules in a medium containing cellulase, which degrades the enclosed CMC-Ph microparticles. The HepG2 cells in the microcapsules then grew and completely filled the hollow core. Multicellular spheroids the same size as the CMC-Ph microparticles, with living cells at their outer surface, were collected within 1 min by soaking them in a medium containing alginate lyase to degrade the Alg-Ph gel microcapsule membrane.

Roles of adsorbed film and gel layer in hydration lubrication for articular cartilage

Abstract To maintain low friction and low wear in natural synovial joints, adsorbed film formation on articular cartilage surface appears to play an important role in mixed or boundary lubrication regime where local direct contact occurs. Furthermore, the proteoglycan gel layer at the uppermost superficial zone in articular cartilage appears to play an important role in preserving low friction and low wear by the hydration lubrication mechanism even after removal of adsorbed film. However, the interaction or synergistic action between adsorbed film and hydrated film/surface has not yet been clarified. To examine the roles of adsorbed film and gel layer on articular cartilage surface in hydration lubrication, the changes in friction were observed in the reciprocating test of articular cartilage against a glass plate, at repeated rubbing including restarting after interrupting—unloading process. It is noticed that at restarting immediately after loading, hydration lubrication is expected to become effective. The lubricating roles of protein-adsorbed film in hydration lubrication are discussed on the basis of experimental results in both cases with and without surface gel layer. The comparison of albumin and γ -globulin is described in connection with the existence of a gel layer.

Superior lubricity in articular cartilage and artificial hydrogel cartilage

In healthy natural synovial joints, the extremely low friction and minimum wear are maintained by their superior load-carrying capacity and lubricating ability. This superior lubricating performance appears to be actualized not by single lubrication mode but by synergistic combination of multimode mechanisms such as fluid film, biphasic, hydration, gel film and/or boundary lubrication. On the contrary, in most artificial joints composed of ultra-high molecular weight polyethylene against metal or ceramic-mating material, boundary and/or mixed lubrication modes prevail and thus local direct contact brings down high friction and high-wear problems. To extend the durability of artificial joint, the reduction in friction and wear by improvement in lubrication mechanism is required as an effective design solution. In this paper, at the start, the mechanism of superior lubricity for articular cartilage is examined from the viewpoints of biphasic and boundary lubrication mechanism. Subsequently, the proposal of biomimetic artificial hydrogel cartilage is put forward to improve the lubricating modes in artificial joints. The tribological behaviours in two kinds of poly(vinyl alcohol) hydrogels are compared with that of natural cartilage. The importance in lubrication mechanism in artificial hydrogel cartilage is discussed.

Confocal analysis of local and cellular strains in chondrocyte-agarose constructs subjected to mechanical shear.

Although numerous previous studies have investigated cell deformation and mechanotransduction within isolated chondrocytes compressed in agarose gel, no published studies have examined the cellular response to shear. In the present study, a novel experimental system has been used to apply precise magnitudes of simple shear strain to isolated bovine articular chondrocytes seeded in agarose. Specimens were gelled between porous endplates which enabled the specimen to be gripped within a specially designed test rig mounted on an inverted microscope. Confocal imaging of individual chondrocytes was used to determine the local and cellular shear strains at gross static shear strains up to 15%. The central region of the specimens experienced uniform local shear strain equal to the applied gross shear strain. An image analysis technique was developed to quantify the level of cell shear strain based upon the shear-induced rotation of a best-fit ellipse. Cell deformation occurred such that the magnitude of the cellular shear strain was equal to gross shear strain. This study is the first to describe the deformation of isolated articular chondrocytes subjected to shear strain. This validated experimental system will enable future studies to examine the influence of shear on chondrocyte function and the associated mechanotransduction signalling pathways.

Effectiveness of adsorbed film and gel layer in hydration lubrication as adaptive multimode lubrication mechanism for articular cartilage

With various daily activities, the effectiveness of adsorbed film formed on a gel-like layer at the uppermost superficial articular cartilage in natural synovial joints becomes important to control the friction and wear of articular cartilage in mixed or boundary lubrication regime as an adaptive multimode lubrication mechanism. Furthermore, in the case where the adsorbed film has been removed, the proteoglycan gel layer is expected to preserve low friction and protect against the wear of bulk cartilage tissue with an effective hydration lubrication mechanism. Besides, it is indicated that the biphasic lubrication plays an important role in lowering of friction in articular cartilage containing a large amount of water. At the present stage, however, the detailed relationship between adsorbed films and hydrated gel layers has not yet been elucidated. In this article, the frictional behaviours of articular cartilage on a glass plate were observed in the reciprocating tests with the restarting process after interruption and unloading. The lubricating effectiveness of adsorbed films in hyaluronate (HA) solutions was examined using intact and partially damaged cartilage specimens. The role of albumin and γ-globulin in relation to the surface conditions of gel layer is discussed. The restarting friction immediately after reloading became lower as a result of recovery of the effective interstitial fluid pressurization and hydration and adsorbed films have a significant effect on the frictional behaviour at local contacts. To clarify the molecular phenomena taking place under rubbing condition, in situ observations of the forming adsorbed film were conducted. The measurements were performed using the fluorescent staining method for protein and HA molecules at low contact pressures.

Micro- and nanoscopic biotribological behaviours in natural synovial joints and artificial joints

Abstract To maintain low friction and low wear in natural synovial joints and joint prostheses with artificial cartilage, adsorbed film formation at micro- and nanoscopic levels appears to play an important role in mixed or boundary lubrication regime where local direct contact occurs. In natural synovial joints, the lubricating gel film is likely to preserve low friction and to protect the bulk cartilage even after the removal of the adsorbed film. For poly (vinyl alcohol) (PVA) hydrogel as artificial cartilage without lubricating gel layer, the adsorbed film is a single protective barrier. In the previous researches on the rubbing pair of PVA against itself or glass plate, the coexistence of albumin (A) and γ-globulin (G) as A:G = 1: 2 at total protein concentration of 2.1 wt% in hyaluronate solution showed the lowest wear. In this study, in situ observation of adsorption of proteins was conducted to clarify the dynamic changes in adsorptional behaviours in lubricants containing different protein constituents. In the mixed lubrication regime, the coexistence of albumin and γ-globulin as A:G = 1:2 showed the lowest friction. The role of appropriate constituents of proteins is discussed on low friction and low wear at micro- and nanoscopic levels on the basis of previous and present results.

Role of Surface Layers of Natural and Artificial Cartilage in Thin Film Lubrication

Abstract In natural synovial joint with compliant articular cartilage, the elastohydrodynamic film are likely to preserve low friction and protect the articular surfaces during walking. Under very thin film condition with local direct contacts at the start-up or in arthritis joints with low viscosity synovial fluid, other protective lubrication mechanisms such as mixed, weeping, boundary and gel film lubrication are expected to perform lubricating actions. In this paper, the role of surface layers of natural articular cartilage in thin film lubrication has been examined in reciprocating friction tests and the observation of the changes in rubbing surfaces by atomic force microscopy(AFM). Furthermore, the frictional behaviour of polyvinylalcohol (PVA) hydrogel as artificial cartilage was compared. The experimental observation for natural articular cartilage snowed that the low friction was maintained for a considerable period at the initial stage in not a lubricated but an unlubricated condition. This fact suggests that the lubricating surface layer probably composed of adsorbed molecules and gel films existed in the rubbing surfaces. With repetition of rubbing, the friction gradually increased from the initial low value. AFM images of the intact articular cartilage and the rubbed cartilage surfaces at definite sliding distances indicated that the smooth surfaces were preserved for the low friction stage and the fibrous structure appeared for the higher friction stage. In friction test, PVA showed higher friction even at the initial stage under thin film condition. This discrepancy might be derived from the deficiency of lubricating surface layer.

The Adaptive Multimode Lubrication in Knee Prostheses with Artificial Cartilage during Walking

The lubricating performance of total knee prosthesis models with compliant layer as artificial cartilage was evaluated and discussed from the viewpoint of adaptive multimode lubrication. The minimum film thickness and inclination of rubbing surface during walking were estimated by numerical analysis based on the plane inclined surface model for polyvinylalcohol (PVA) hydrogel and polyurethane. The numerical results show the effectiveness of softer material on fluid film formation during walking, although surface inclination during swing phase is larger for polyurethane than PVA hydrogel. The actual fluid film formation in knee prostheses during walking was examined by measurement of degree of separation by electric resistance method and frictional force in simulator tests. Under walking condition lubricated with lubricants of appropriate viscosity, considerable elastohydrodynamic film was formed corresponding to numerical analysis. Under thin film conditions lubricated with low viscosity lubricants where significant local direct contact occurred between rubbing surfaces due to breakdown of fluid film, the addition of proteins remarkably decreased friction and suppressed stick-slip for PVA, but increased friction for polyurethane. The addition of phospholipid liposomes had an effect in reducing of friction after repetition of rubbing process in walking motion. The adsorbed film formation of synovia constituents on stainless steel plate was observed by atomic force microscopy.