Influence of synovia constituents on tribological behaviors of articular cartilage

The extremely low friction and minimal wear in natural synovial joints appear to be established by effective lubrication mechanisms based on appropriate combination of articular cartilage and synovial fluid.The complex structure of cartilage composed of collagen and proteoglycan with high water content contributes to high load-carrying capacity as biphasic materials and the various constituents of synovial fluid play important roles in various lubrication mechanisms.However,the detailed differences in functions of the intact and damaged cartilage tissues,and the interaction or synergistic action of synovia constituents with articular cartilage have not yet been clarified.In this study,to examine the roles of synovia constituents and the importance of cartilage surface conditions,the changes in friction were observed in the reciprocating tests of intact and damaged articular cartilage specimens against glass plate lubricated with lubricants containing phospholipid,protein and/or hyaluronic acid as main constituents in synovial fluid.The effectiveness of lubricant constituents and the influence of cartilage surface conditions on friction are discussed.In addition,the protectiveness by synovia constituents for intact articular cartilage surfaces is evaluated.

Running-in friction of hip joint replacements can be significantly reduced:The effect of surface-textured acetabular cup

Hip joint replacements represent the most effective way of treatment for patients suffering from joint diseases.Despite the rapid improvement of implant materials over the last few decades,limited longevity associated with wear-related complications persists as the main drawback.Therefore,improved tribological perfor-mance is required in order to extend the service life of replacements.The effect of surface texturing of ultra-high molecular weight polyethylene(UHMWPE)acetabular cup was investigated in the present study.Unique tilling method was utilized for manufacturing the dimples with controlled diameter and depths on the contact surface of the cup.The experiments with four commercial femoral components and two model lubricants were realized.The main attention was paid to a coefficient of friction considering the differences between the original and the dimpled cups.Results showed remarkable lowering of friction,in general.Focusing on the simulated human synovial fluid,friction was reduced by 40%(alumina ceramic),38.8%(zirconia toughened ceramic),25.5%(metal),and 9.9%(oxinium).In addition,the dimples helped to keep the friction stable without fluctuations.To conclude,the paper brings a new insight into frictional behaviour of the hip replacements during running-in phase which is essential for overall implant lifespan.It is believed that proper surface texturing may rapidly improve the life quality of millions of patients and may lead to considerable financial savings.

Biomimetic artificial cartilage:fibre-reinforcement of PVA hydrogel to promote biphasic lubrication mechanism

The fibre-reinforced structure in synovial articular cartilage plays an important role in enhancing the fluid load support in the biphasic lubrication mechanism.Poly(vinyl alcohol)(PVA)hydrogel is a potential biomimetic articular cartilage material.In this study,PVA hydrogel was reinforced with PVA fibres to improve its frictional properties.By computational analysis,the position of the PVA fibre layer was examined with a migrating contact condition to reduce the friction coefficient.To improve the fibre reinforcement,a method for fibre placement was developed to retain the initial strain of the fibre in the hydrogel matrix.The experimental results showed that the fibre-reinforced PVA hydrogel with a surface fibre layer had a low friction coefficient of 0.031 in pure water.The fibre-reinforced PVA hydrogel successfully reduced friction coefficient.

Transitional behaviour between biphasic lubrication and soft elastohydrodynamic lubrication of poly(vinyl alcohol)hydrogel using microelectromechanical system pressure sensor

The soft hydrogel material is expected for a candidate material as biomimetic artificial cartilage with synergisticfunctionalities of adaptive multimode lubrication.In boundary lubrication mode of hydrogel material,the biphasiclubrication mechanism cooperatively exerts its functionality.In hydrodynamic lubrication mode,it is preferable that thelubricating surfaces be impermeable to trap the fluid pressure in contact surfaces,whereas the actual biphasic materiallike a hydrogel is a permeable material with surface porosity.lt is indicated that the interstitial fluid pressurisation inthe permeable biphasic material can contribute to significant fluid load support under lower sliding speed condition.So,the authors examined how the contrary fluid pressure effect appears in the transition from the boundarylubrication mode to soft elastohydrodynamic lubrication mode.In the experiment,a small pressure sensor was utilisedto measure the in-situ fluid pressure in sliding condition.Although the experimental condition of this study wasselective,the result showed a possibility of the negative effect of the biphasic surface,in which the permeable surfacediminished the hydrodynamic fluid pressure.This means that one should manage and enhance the biphasic lubricationabilities in wide operation range when the hydrogel material was used as a load bearing material.

Erratum to: Influence of synovia constituents on tribological behaviors of articular cartilage

Erratum to Friction 1(2):150-162(2013)DOI 10.1007/s40544‐013‐0010‐6 The original version of this article unfortunately contained the incorrect lengend“HA 0.5 wt%+DPPC 0.01 wt%+albumin 1.4 wt%”in Fig.7 on page 155,instead of.

On/off switching of adhesion in gecko-inspired adhesives

In this study,the adhesion-detachment behaviour of a gecko-inspired adhesive pad was investigated to understand the on/off switching mechanisms of adhesion in gecko feet.A macroscopic spatula model was fabricated using silicone rubber,and adhesion tests combining lateral sliding and vertical debonding were conducted.It was observed that the contact state and the adhesion force of the pad vary considerably with the direction of lateral sliding prior to debonding,and that the pad achieves adhesion during debonding even when it loses contact due to excess lateral sliding.These results explain the mech-anisms behind the on/off switching and stable adhesion of gecko feet,and suggest the possibility of developing new-generation adhesives capable of switchable adhesion.

Experimental study on boundary lubricity of superficial area of articular cartilage and synovial fluid

The boundary lubrication mechanism at the articulating surface of natural synovial joints has been the subject of much discussion in tribology.In this study,to elucidate the lubricating function of the superficial area of articular cartilage and synovial fluid(SF),cartilage specimens were processed with four different treatments:gentle and severe washing with detergent,incubation in NaCl solution,and trypsin digestion to selectively remove certain constituents from the cartilage surface.Subsequently,the frictional characteristics were examined in phosphate-buffered saline(PBS)and SF against glass.Angularly reciprocating sliding tests with a spherical glass probe and square articular cartilage specimens were performed at low contact loads in the mN range to extract the frictional behavior in the superficial area of the cartilage specimens.Meanwhile,the cartilage surface was observed to confirm the effects of treatments on the morphology of the cartilage surface using a fluorescence microscope and water-immersion methods.The coefficient of friction(COF)of the prepared cartilage specimens was varied from 0.05 to over 0.3 in PBS.However,a certain group of cartilage specimens exhibited a low COF of less than 0.1 with limited variation.For the low COF group of specimens,all four treatments increased the COF in PBS to different extents,and fluorescence microscopy revealed that the integrity of the cartilage surface was deteriorated by treatments.This means that the intact cartilage surface had lubricating constituents to maintain low friction,and the removal of such constituents resulted in the loss of the intrinsic boundary lubricity of the cartilage surface.The variation in the COF of the cartilage specimens was suppressed in SF because it had a clear boundary lubrication effect on the cartilage surface.The lubricating effect of SF could be confirmed even after degenerative treatment.