Nir Kampf

Lubrication by charged polymers

Long-ranged forces between surfaces in a liquid control effects from colloid stability to biolubrication, and can be modified either by steric factors due to flexible polymers, or by surface charge effects. In particular, neutral polymer ‘brushes’ may lead to a massive reduction in sliding friction between the surfaces to which they are attached, whereas hydrated ions can act as extremely efficient lubricants between sliding charged surfaces. Here we show that brushes of charged polymers (polyelectrolytes) attached to surfaces rubbing across an aqueous medium result in superior lubrication compared to other polymeric surfactants. Effective friction coefficients with polyelectrolyte brushes in water are lower than about 0.0006–0.001 even at low sliding velocities and at pressures of up to several atmospheres (typical of those in living systems). We attribute this to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments. Our findings may have implications for biolubrication effects, which are important in the design of lubricated surfaces in artificial implants, and in understanding frictional processes in biological systems.

Normal and frictional forces between surfaces bearing polyelectrolyte brushes

Normal and shear forces were measured as a function of surface separation, D, between hydrophobized mica surfaces bearing layers of a hydrophobic-polyelectrolytic diblock copolymer, poly(methyl methacrylate)- block-poly(sodium sulfonated glycidyl methacrylate) copolymer (PMMA- b-PSGMA). The copolymers were attached to each hydrophobized surface by their hydrophobic PMMA moieties with the nonadsorbing polyelectrolytic PSGMA tails extending into the aqueous medium to form a polyelectrolyte brush. Following overnight incubation in 10 (-4) w/v aqueous solution of the copolymer, the strong hydrophobic attraction between the hydrophobized mica surfaces across water was replaced by strongly repulsive normal forces between them. These were attributed to the osmotic repulsion arising from the confined counterions at long-range, together with steric repulsion between the compressed brush layers at shorter range. The corresponding shear forces on sliding the surfaces were extremely low and below our detection limit (+/-20-30 nN), even when compressed down to a volume fraction close to unity. On further compression, very weak shear forces (130 +/- 30 nN) were measured due to the increase in the effective viscous drag experienced by the compressed, sliding layers. At separations corresponding to pressures of a few atmospheres, the shearing motion led to abrupt removal of most of the chains out of the gap, and the surfaces jumped into adhesive contact. The extremely low frictional forces between the charged brushes (prior to their removal) is attributed to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments.

Articular Cartilage Proteoglycans As Boundary Lubricants: Structure and Frictional Interaction of Surface-Attached Hyaluronan and Hyaluronan–Aggrecan Complexes

Mammalian synovial joints are extremely efficient lubrication systems reaching friction coefficient μ as low as 0.001 at high pressures (up to 100 atm) and shear rates (up to 10(6) to 10(7) Hz); however, despite much previous work, the exact mechanism responsible for this behavior is still unknown. In this work, we study the molecular mechanism of synovial joint lubrication by emulating the articular cartilage superficial zone structure. Macromolecules extracted and purified from bovine hip joints using well-known biochemical techniques and characterized with atomic force microscope (AFM) have been used to reconstruct a hyaluronan (HA)--aggrecan layer on the surface of molecularly smooth mica. Aggrecan forms, with the help of link protein, supramolecular complexes with the surface-attached HA similar to those at the cartilage/synovial fluid interface. Using a surface force balance (SFB), normal and shear interactions between a HA--aggrecan-coated mica surface and bare mica have been examined, focusing, in particular, on the frictional forces. In each stage, control studies have been performed to ensure careful monitoring of the macromolecular surface layers. We found the aggrecan--HA complex to be a much better boundary lubricant than the HA alone, an effect attributed largely to the fluid hydration sheath bound to the highly charged glycosaminoglycan (GAG) segments on the aggrecan core protein. A semiquantitative model of the osmotic pressure is used to describe the normal force profiles between the surfaces and interpret the boundary lubrication mechanism of such layers.

Fluidity of water and of hydrated ions confined between solid surfaces to molecularly thin films

In contrast to non-associating liquids such as oils or organic solvents, whose viscosity diverges when they are confined by solid surfaces to films thinner than about ten molecular diameters, recent studies reveal that salt-free water remains fluid, with a viscosity close to its bulk value, even when confined to films down to only one or two monolayers thick. For the case of high concentration aqueous salt solutions compressed down to subnanometre films between confining planar surfaces, the hydration sheaths about the ions (trapped between the oppositely charged surfaces) also remain extremely fluid: this behaviour is attributed to the tenacity of water molecules in the hydration layers together with their rapid relaxationlexchange time. Related experiments on highly compressed, polyelectrolyte brushes in aqueous media reveal a remarkable lubricity which is in large measure attributed to similar hydration layers about the charged segments: this water of hydration strongly resists being squeezed out, but at the same time it may rapidly exchange with adjacent water molecules, thereby remaining quite fluid and acting as a molecular lubricant.

Origins of hydration lubrication

Why is friction in healthy hips and knees so low? Hydration lubrication, according to which hydration shells surrounding charges act as lubricating elements in boundary layers (including those coating cartilage in joints), has been invoked to account for the extremely low sliding friction between surfaces in aqueous media, but not well understood. Here we report the direct determination of energy dissipation within such sheared hydration shells. By trapping hydrated ions in a 0.4-1 nm gap between atomically smooth charged surfaces as they slide past each other, we are able to separate the dissipation modes of the friction and, in particular, identify the viscous losses in the subnanometre hydration shells. Our results shed light on the origins of hydration lubrication, with potential implications both for aqueous boundary lubricants and for biolubrication.

Normal and Shear Interactions between Hyaluronan−Aggrecan Complexes Mimicking Possible Boundary Lubricants in Articular Cartilage in Synovial Joints

Using a surface force balance, normal and shear interactions have been measured between two atomically smooth surfaces coated with hyaluronan (HA), and with HA/aggrecan (Agg) complexes stabilized by cartilage link protein (LP). Such HA/Agg/LP complexes are the most abundant mobile macromolecular species permeating articular cartilage in synovial joints and have been conjectured to be present as boundary lubricants at its surface. The aim of the present study is to gain insight into the extremely efficient lubrication when two cartilage surfaces slide past each other in healthy joints, and in particular to elucidate the possible role in this of the HA/Agg/LP complexes. Within the range of our parameters, our results reveal that the HA/Agg/LP macromolecular surface complexes are much better boundary lubricants than HA alone, likely because of the higher level of hydration, due to the higher charge density, of the HA/Agg/LP layers with respect to the HA alone. However, the friction coefficients (μ) associated with the mutual interactions and sliding of opposing HA/Agg/LP layers (μ ≈ 0.01 up to pressure P of ca. 12 atm, increasing sharply at higher P) suggest that such complexes by themselves cannot account for the remarkable boundary lubrication observed in mammalian joints (up to P > 50 atm).

Hydration lubrication: exploring a new paradigm

Lubrication by hydration shells that surround, and are firmly attached to, charges in water, and yet are highly fluid, provide a new mode for the extreme reduction of friction in aqueous media. We report new measurements, using a mica surface-force balance, on several different systems which exhibit hydration lubrication, extending earlier studies significantly to shed new light on the nature and limits of this mechanism. These include lubrication by hydrated ions trapped between charged surfaces, and boundary lubrication by surfactants, by polyzwitterionic brushes and by close-packed layers of phosphatidylcholine vesicles. Sliding friction coefficients as low as 10(-4) or even lower, and mean contact pressures of up to 17 MPa or higher are indicated. This suggests that the hydration lubrication mechanism may underlie low-friction sliding in biological systems, in which such pressures are rarely exceeded.

Long-ranged attraction between disordered heterogeneous surfaces.

Interactions in aqueous media between uniformly charged surfaces are well understood, but most real surfaces are heterogeneous and disordered. Here we show that two such heterogeneous surfaces covered with random charge domains experience a long-range attraction across water that is orders of magnitude stronger than van der Waals forces, even in the complete absence of any charge correlations between the opposing surfaces. We demonstrate that such strong attraction may arise generally, even for overall neutral surfaces, from the inherent interaction asymmetry between equally and between oppositely charged domains.

Rheological characterization of κ-carrageenan soy milk gels

Abstract Soy-milk gels were characterized by their uniaxial compression, stress relaxation and degree of elasticity. Their stress at failure was 19.3–23.1 kPa in the pH range of 5.4–6.4. Significant differences were found for gels at the lower pH values of 4.4–5.0. Maximum stress at failure was achieved for soy milk gels in the pH range of 5.2–6.2. At pH 5.2 there was a significant increase in the gel’s stress at failure compared with lower pH. The gel deformability modulus increasedfrom 17.5 kPa at pH6.4 to 54 kPa at pH 5.2, then decreased to 11.2 kPa at pH 4.4. Henckys strain at failure was also dependent on pH and decreasedfrom 0.65–0.67 at pH values of 6.2–6.4 to 0.28 at pH 4.4. Soymilk gels became more brittle when the pH of the system decreased. Results (strength, strain at failure and deformability modulus) were compared with systems of cows milk and water gels. Gels produced at pH 5.2 were chosen for the stress relaxation and elasticity tests. Relaxation curves at five strain levels were normalized and linearized. k 1 and k 2 , the constants of Pelegs model, and were E A calculated. From the calculated recoverable work and results of mechanical testing, conclusions on the character of the gel were derived. A hypothesis on the relationships between recoverable work and the asymptotic modulus is also proposed. Weight losses of soy milk gels during compression, stress relaxation and elasticity determinations were recorded and an explanation for this phenomenon is proposed. A description of soy milk κ-carrageenan gel formation is proposed and the possible role of aspargyl and glutamyl residues is described.

Mechanical Stability and Lubrication by Phosphatidylcholine Boundary Layers in the Vesicular and in the Extended Lamellar Phases

The lubrication properties of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) extended supported bilayers were studied and compared to those of surface-attached DSPC small unilamellar vesicles (liposomes) in order to elucidate the effect of phospholipid geometrical packaging on the lubrication and mechanical properties of these boundary layers. The topography and response to the nanoindentation of bilayer- and liposome-covered surfaces were studied by an atomic force microscope (AFM). In parallel, normal and shear (frictional) forces between two opposing surfaces bearing DSPC vesicles/bilayers across water were studied with the surface force balance (SFB). A correlation between nanomechanical performance in the AFM and stability and lubrication in the SFB was observed. Bilayers were readily punctured by the AFM tip and exhibited substantial hysteresis between approach and retraction curves, whereas liposomes were not punctured and exhibited purely elastic behavior. At the same time, SFB measurements showed that bilayers are less stable and less efficient lubricants compared to liposomes. Bilayers provided efficient lubrication with very low friction coefficients, 0.002-0.008 up to pressures of more then 50 atm. However, bilayers were less robust and tended to detach from the surface as a result of shear, leading to high friction for subsequent approaches at the same contact position. In contrast, liposomes showed reversible and reproducible behavior under shear and compression, exhibiting ultralow friction coefficients of μ ≈ 10(-4) for pressures as high as 180 atm. This is attributed to the increased mechanical stability of the self-closed, closely packed liposomes, which we believe results from the more defect-free nature of the finitely sized vesicles.