Articular cartilage is a highly efficacious water-based tribological system that is optimized to provide low friction and wear protection at both low and high loads (pressures) and sliding velocities that must last over a lifetime. Although many different lubrication mechanisms have been proposed, it is becoming increasingly apparent that the tribological performance of cartilage cannot be attributed to a single mechanism acting alone but on the synergistic action of multiple "modes" of lubrication that are adapted to provide optimum lubrication as the normal loads, shear stresses, and rates change. Hyaluronic acid (HA) is abundant in cartilage and synovial fluid and widely thought to play a principal role in joint lubrication although this role remains unclear. HA is also known to complex readily with the glycoprotein lubricin (LUB) to form a cross-linked network that has also been shown to be critical to the wear prevention mechanism of joints. Friction experiments on porcine cartilage using the surface forces apparatus, and enzymatic digestion, reveal an "adaptive" role for an HA-LUB complex whereby, under compression, nominally free HA diffusing out of the cartilage becomes mechanically, i.e., physically, trapped at the interface by the increasingly constricted collagen pore network. The mechanically trapped HA-LUB complex now acts as an effective (chemically bound) "boundary lubricant"-reducing the friction force slightly but, more importantly, eliminating wear damage to the rubbing/shearing surfaces. This paper focuses on the contribution of HA in cartilage lubrication; however, the system as a whole requires both HA and LUB to function optimally under all conditions. arthritis | mechanical trapping | elastohydrodynamic lubrication | biointerface | biolubrication A rticular joints are almost completely sealed from their surroundings-by the synovial membrane around the joint and by cartilage and bone above and below the joint (1, 2). These barriers restrict rapid chemical transport into and out of joints, making it difficult to replace or repair damaged internal tissue or macromolecules, particularly those molecules that are covalently attached (bound) to the internal cartilage surfaces (1-3). Thus, it is no surprise that the major molecules involved in joint lubrication [lubricin and hyaluronic acid (HA)] are noncovalently bound and yet-to function as effective "boundary lubricants" that exhibit low friction and protect surfaces from wear-they need to act as if they are chemically bound to the surfaces.Hyaluronic acid has long been considered a potential boundary lubricant for cartilage (3-6), although numerous friction experiments have shown that solutions of free HA exhibit little lubrication activity (4, 5). However, surface forces apparatus (SFA) experiments (4) on chemically grafted and cross-linked HA layers demonstrated that such HA provide excellent wear protection for surfaces shearing at high pressures (200 atm), even though high friction coefficients (μ ¼ 0.15 − 0.3) were measured. These results...

High molecular weight hyaluronic acid (HA) is present in articular joints and synovial fluid at high concentrations; yet despite numerous studies, the role of HA in joint lubrication is still not clear. Free HA in solution does not appear to be a good lubricant, being negatively charged and therefore repelled from most biological, including cartilage, surfaces. Recent enzymatic experiments suggested that mechanically or physically (rather than chemically) trapped HA could function as an "adaptive" or "emergency" boundary lubricant to eliminate wear damage in shearing cartilage surfaces. In this work, HA was chemically grafted to a layer of self-assembled amino-propyl-triethoxy-silane (APTES) on mica and then cross-linked. The boundary lubrication behavior of APTES and of chemically grafted and cross-linked HA in both electrolyte and lipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) solutions was tested with a surface forces apparatus (SFA). Despite the high coefficient of friction (COF) of μ ≈ 0.50, the chemically grafted HA gel significantly improved the lubrication behavior of HA, particularly the wear resistance, in comparison to free HA. Adding more DOPC lipid to the solution did not improve the lubrication of the chemically grafted and cross-linked HA layer. Damage of the underlying mica surface became visible at higher loads (pressure >2 MPa) after prolonged sliding times. It has generally been assumed that damage caused by or during sliding, also known as "abrasive friction", which is the main biomedical/clinical/morphological manifestation of arthritis, is due to a high friction force and, therefore, a large COF, and that to prevent surface damage or wear (abrasion) one should therefore aim to reduce the COF, which has been the traditional focus of basic research in biolubrication, particularly in cartilage and joint lubrication. Here we combine our results with previous ones on grafted and cross-linked HA on lipid bilayers, and lubricin-mediated lubrication, and conclude that for cartilage surfaces, a high COF can be associated with good wear protection, while a low COF can have poor wear resistance. Both of these properties depend on how the lubricating molecules are attached to and organized at the surfaces, as well as the structure and mechanical, viscoelastic, elastic, and physical properties of the surfaces, but the two phenomena are not directly or simply related. We also conclude that to provide both the low COF and good wear protection of joints under physiological conditions, some or all of the four major components of joints-HA, lubricin, lipids, and the cartilage fibrils-must act synergistically in ways (physisorbed, chemisorbed, grafted and/or cross-linked) that are still to be determined.

Methods of measuring friction forces in the surface forces apparatus (SFA) are presented for sliding velocities from \1 nm/s to [10 m/s. A feed-forward control (FFC) system for the piezoelectric bimorph slider attachment is introduced to allow experiments at velocities up to *4 mm/s. For still higher speeds, a motor-driven rotating mini-disk setup using a pin-on-disk geometry is presented, with modifications to enable sliding velocities in the ranges 1 cm/s-5 m/s and 1-25 m/s. Example data sets demonstrate the applicability of the approach to modeling important tribological systems including hard-disk drives. We find that mechanical system parameters such as the resonant frequencies and mutual alignments of different moving parts become increasingly important in determining the tribological response at sliding velocities above *1 cm/s (for SFA or bench top devices). Smooth or stick-slip sliding-common features of low-speed sliding-become replaced by large-amplitude oscillatory responses that depend on the load and especially the driving speed or rotational/reciprocating frequencies.Detailed recordings and modeling of these complex effects are necessary for fully understanding and controlling frictional behavior at high speeds.

McCutchen's work (1) behooves Greene et al. (2,3) to clarify their findings on fluid flow in compressed joints (2) and the role of hyaluronic acid (HA) in joint lubrication (3). In the earlier article, Greene et al. (2) used NMR to show how water (synovial fluid) was driven out of cartilage when it was compressed or loaded, much like what happens when a water-filled sponge is squeezed. Note that the compressive pressure was the interaction or lithostatic pressure experienced by a cartilage surface when pressed on (loaded) by another and was not the internal hydrostatic pressure within the fluid. Apart from the expected and well-understood elastohydrodynamic deformations of cartilage undergoing dynamic compression, Greene et al. (2) did not attribute their observations to any other particular lubrication mechanism in joints, such as weeping, boosted, biphasic, etc.; however, Greene et al. (2) noted that the fluid flow was complex and anisotropic and that it correlated this anisotropy with the changing anisotropic pore structure of cartilage. These directly measured results should not be unexpected or most unlikely.As two cartilage surfaces come closer together, they do not come into solid contact-at least not in healthy (undamaged) joints; depending on the pressure, they remain separated by a thick (>0.1 μm) biopolymer layer of adsorbed lubricin (4) and any HA bound to the cartilage surface (5), which act as boundary lubricants by reducing friction and perhaps equally important, protecting the cartilage surfaces from becoming damaged.Turning to the specific role of HA, McCutchen (1) correctly points out that, when HA is digested (with hyarulonidase) and then applied between two surfaces (including cartilage and another surface in vitro), friction is not affected, which has been interpreted as showing that HA plays no role in joint lubrication. A subsequent study (5) indeed confirmed that free HA in solution does not provide good lubricity to model surfaces, such as mica. However, the point by Greene et al. in ref. 3 was that the active HA that does the lubricating in joints was already effectively anchored at the cartilage surface-partly inside the fibrous collagen pore network and partly outside it. It is the part that protrudes into the synovial fluid that can function as a boundary lubricant, although the molecule itself is not chemically bound to the cartilage surface, which is the conventional definition of a boundary lubricant.This ability of nominally free HA to effectively function as a bound molecule arises, because physiological HA in healthy joints has a molecular mass of 2-6 MDa and a fully extended length of >10 μm. This long molecule becomes mechanically (i.e., physically rather than chemically) trapped between collagen fibrils when cartilage is compressed under a high load. We did not suggest that our mechanical trapping boundary lubrication mechanism, specifically the last line of lubrication defense in joints, replaced or was the only lubrication mechanism (3). These different mechanisms and mo...

The friction forces between various lubricated ''friction materials'' and sapphire disks were measured using a new ''high-speed'' rotating disk attachment to the surface forces apparatus (SFA). Two different clutch lubricants and two different friction materials were tested at sliding speeds and normal loads from 5 to 25 m/s, and 0.2 to 1 N (nominal pressures *1 MPa), respectively. The results show that ''resonance friction''-characterized by large amplitude oscillatory (i.e., sinusoidal) vibrations, also known as shudder or chatter-dominates dynamical considerations at high sliding speed, replacing the smooth sliding or low-amplitude stick-slip that is characteristic of low speed/low load sliding. The characteristic (rotational) speeds or frequencies at which resonance friction occurs depend only on the coupled/uncoupled mechanical resonance frequencies of the loading and friction-sensing mechanisms. In contrast, the intensity of and time to enter/ exit shudder depends strongly on the lubricating oil and, to a lesser extent, on the friction material. Physical-chemical analyses of the friction materials before and after testing showed that the samples undergo primarily structural rather than chemical changes. Our results provide new fundamental insights into the resonance friction phenomenon and suggest means for its control.