Cell-inspired, massive electromodulation of friction via transmembrane fields across lipid bilayers.

Transient electric fields across cell bilayer membranes can lead to electroporation and cell fusion, effects crucial to cell viability whose biological implications have been extensively studied. However, little is known about these behaviours in a materials context. Here we find that transmembrane electric fields can lead to a massive, reversible modulation of the sliding friction between surfaces coated with lipid-bilayer membranes-a 200-fold variation, up to two orders of magnitude greater than that achieved to date. Atomistic simulations reveal that the transverse fields, resembling those at cell membranes, lead to fully reversible electroporation of the confined bilayers and the formation of inter-bilayer bridges analogous to the stalks preceding intermembrane fusion. These increase the interfacial dissipation through reduced hydration at the slip plane, forcing it to revert in part from the low-dissipation, hydrated lipid-headgroup plane to the intra-bilayer, high-dissipation acyl tail interface. Our results demonstrate that lipid bilayers under transmembrane electric fields can have striking materials modification properties.

Direct measurement of the viscoelectric effect in water.

The viscoelectric effect concerns the increase in viscosity of a polar liquid in an electric field due to its interaction with the dipolar molecules and was first determined for polar organic liquids more than 80 y ago. For the case of water, however, the most common polar liquid, direct measurement of the viscoelectric effect is challenging and has not to date been carried out, despite its importance in a wide range of electrokinetic and flow effects. In consequence, estimates of its magnitude for water vary by more than three orders of magnitude. Here, we measure the viscoelectric effect in water directly using a surface force balance by measuring the dynamic approach of two molecularly smooth surfaces with a controlled, uniform electric field between them across highly purified water. As the water is squeezed out of the gap between the approaching surfaces, viscous damping dominates the approach dynamics; this is modulated by the viscoelectric effect under the uniform transverse electric field across the water, enabling its magnitude to be directly determined as a function of the field. We measured a value for this magnitude, which differs by one and by two orders of magnitude, respectively, from its highest and lowest previously estimated values.

Lipid-Bilayer Assemblies on Polymer-Bearing Surfaces: The Nature of the Slip Plane in Asymmetric Boundary Lubrication.

Phospholipid-macromolecule complexes have been proposed to form highly efficient, lubricating boundary layers at artificial soft surfaces or at biological surfaces such as articular cartilage, where the friction reduction is attributed to the hydration lubrication mechanism acting at the exposed, hydrated head groups of the lipids. Here we measure, using a surface force balance, the normal and frictional interactions between model mica substrates across several different configurations of phosphatidylcholine (PC) lipid aggregates and adsorbed polymer (PEO) layers, to provide insight into the nature of such lubricating boundary layers in both symmetric and especially asymmetric configurations. Our results reveal that, irrespective of the configuration, the slip plane between the sliding surfaces reverts wherever possible to a bilayer-bilayer interface where hydration lubrication reduces the friction strongly. Where such an interface is not available, the sliding friction remains high. These findings may account for the low friction observed between both biological and synthetic hydrogel surfaces which may be asymmetrically coated with lipid-based boundary layers and fully support the hydration lubrication mechanism attributed to act at such boundary layers.

Effects of Hyaluronan Molecular Weight on the Lubrication of Cartilage-Emulating Boundary Layers.

Osteoarthritic joints contain lower-molecular-weight (MW) hyaluronan (hyaluronic acid, HA) than healthy joints. To understand the relevance of this HA size effect for joint lubrication, the friction and surface structure of cartilage-emulating surfaces with HA of different MWs were studied using a surface force balance (SFB) and atomic force microscopy (AFM). Gelatin (gel)-covered mica surfaces were coated with high-MW HA (HHA), medium-MW HA (MHA), or low-MW HA (LHA), and lipids of hydrogenated soy l-α-phosphatidylcholine (HSPC) in the form of small unilamellar vesicles, using a layer-by-layer assembly method. SFB results indicate that the gel-HHA-HSPC boundary layer provides very efficient lubrication, attributed to hydration lubrication at the phosphocholine headgroups exposed by the HA-attached lipids, with friction coefficients (COF) as low as 10-3-10-4 at contact stresses at least up to P = 120 atm. However, for the gel-MHA-HSPC and gel-LHA-HSPC surfaces, the friction, initially low, increases sharply at much lower pressures (up to 30-60 atm at most). This higher friction with the shorter chains may be due to their weaker total adhesion energy to the gelatin, where the attraction between the negatively charged HA and the weakly positively charged gelatin is attributed largely to counterion-release entropy. Thus, the complexes of LHA and MHA with the lubricating HSPC lipids are more easily removed by shear during sliding, especially at high stresses, than the HHA-HSPC complex, which is strongly adhered to gelatin. This is ultimately the reason for lower-pressure lubrication breakdown with the shorter polysaccharides. Our results provide molecular-level insight into why the decrease in HA molecular weight in osteoarthritic joints may be associated with higher friction at the articular cartilage surface, and may have relevance for treatments of osteoarthritis involving intra-articular HA injections.

Normal and shear forces between boundary sphingomyelin layers under aqueous conditions.

Sphingomyelin is one of the predominant phospholipid groups in synovial joints, where lipids have been strongly implicated in the boundary lubrication of articular cartilage; however, little attention has been paid to its lubrication behavior. In this study, we demonstrate that sphingomyelin is an excellent boundary lubricant by measuring the normal and shear forces between sphingomyelin-layer-coated surfaces with a surface force balance under aqueous conditions. Slightly negatively-charged egg sphingomyelin vesicles were adsorbed on mica either by calcium bridging or by charge screening with high concentration monovalent salt. The normal force profiles between opposing egg sphingomyelin layers (vesicles or bilayers) show long-ranged weak repulsion and short-ranged strong repulsion on approaching. Friction coefficients, calculated from the highest load, were (7.2 ± 1.7) × 10-4 at contact stresses of 9.1 ± 0.7 MPa across 0.3 mM liposome dispersion in 0.03 mM Ca2+, and (0.8-3.5) × 10-3 at contact stresses of 7.6 ± 0.8 MPa across 0.3 mM liposome dispersion in 150 mM NaNO3. Similar or slightly lower friction coefficients of (5.3 ± 0.8) × 10-4 at 9.8 ± 0.2 MPa were obtained by replacing the liposome dispersion in 0.03 mM Ca2+ by water. Such low friction coefficients, attributed to the hydration lubrication mechanism, are comparable to those of phosphatidylcholine lipids, which have been widely recognized as excellent aqueous biolubricants. Therefore, we believe that sphingomyelin, in parallel with phosphatidylcholine, contributes to the remarkably good boundary lubrication in synovial joints.

Surface Interactions between Boundary Layers of Poly(ethylene oxide)-Liposome Complexes: Lubrication, Bridging, and Selective Ligation †.

Poly(ethylene oxide), PEO, is widely exploited in biomedical applications, while phosphatidylcholine (PC) lipids (in the form of bilayers or liposomes) have been identified as very efficient boundary lubricants in aqueous media. Here we examine, using a surface force balance (SFB), the interactions between surface-adsorbed layers of PEO complexed with small unilamellar vesicles (SUVs, i.e. liposomes) or with bilayers of PC lipids, both well below and a little above their main gel-to-liquid phase-transition temperatures TM. The morphology of PEO layers (adsorbed onto mica), to which liposomes were added, was examined using atomic force microscopy (AFM) and cryo-scanning electron microscopy (cryo-SEM). Our results reveal that the PC lipids could attach to the PEO either as vesicles or as bilayers, depending on whether they were above or below TM. Under water (no added salt), excellent lubrication, with friction coefficients down to 10-3-10-4, up to contact stresses of 6.5 MPa (comparable to those in the major joints) was observed between two surfaces bearing such PEO-PC complexes. At 0.1 M KNO3 salt concentration (comparable to physiological salt levels), the friction between such surfaces was considerably higher, attributed to bridging by the polymer chains. Remarkably, such bridging could be suppressed and the friction could be restored to its previous low value if the KNO3 was replaced with NaNO3, as a result of the different PEO-mica ligation properties of Na+ compared to those of K+. Our results provide insight into the properties of PEO-PC complexes in potential applications, and large interfacial effects that can result from the seemingly innocuous replacement of K+ by Na+ ions.

Boundary Lubrication, Hemifusion, and Self-Healing of Binary Saturated and Monounsaturated Phosphatidylcholine Mixtures ⧫.

A wide range of phosphatidylcholine (PC) lipids with different degrees of unsaturation has been identified in the human synovial fluid and on the cartilage surface. The outstanding lubricity of the articular cartilage surface has been attributed to boundary layers comprising complexes of such lipids, though to date, only lubrication by single-component PC-lipid-based boundary layers has been investigated. As distinguishable lubrication behavior has been found to be related to the PC structures, we herein examined the surface morphology (on mica) and the lubrication ability of binary PC lipid mixtures, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), using atomic force microscopy (AFM) and a surface force balance (SFB). These two PC lipids are among the most abundant saturated and unsaturated PC components in synovial joints. Small unilamellar vesicles (SUVs) prepared from DPPC-POPC mixtures (8:2, 5:5, and 2:8, molar ratios) ruptured and formed bilayers on mica. The normal and shear forces between two DPPC-POPC bilayer-coated mica surfaces across the corresponding SUV dispersions show good boundary lubrication (friction coefficients ≤ ca. 10-4) up to contact stresses of 8.3 ± 2.2 MPa for 8:2 DPPC-POPC and 5.0 ± 1.7 MPa for the others. Hemifusion induced at high normal pressures was observed, probably because of the height mismatch of two components. Reproducible successive approaches after hemifusion indicate rapid self-healing of the mica-supported bilayers in the presence of the SUVs reservoir. This work is a first step to provide insight concerning the lubrication, wear, and healing of the PC-based boundary layers, which must consist of multicomponent lipid mixtures, on the articular cartilage surface.

Poly-phosphocholinated Liposomes Form Stable Superlubrication Vectors.

We have prepared phosphatidylcholine (PC) vesicles (liposomes) incorporating a novel lipid/poly-phosphocholine conjugate. This both stabilizes the liposomes against aggregation (for example, during storage or when being delivered) and allows them to act as very efficient lubricating elements readily attaining superlubric performance (defined as coefficient of friction µ < 10-2) via hydration lubrication at physiological salt concentrations and pressures. In contrast, vesicles sterically protected by poly(ethylene glycol) chains (PEGylation), which is the general method of choice, while being equally stable to aggregation are much poorer lubricants under these conditions, which is attributed to the relatively poor hydration of the PEG. Our approach enables the use of PC liposomes as stable superlubrication vectors in potential biomedical applications.

Distinct biological events generated by ECM proteolysis by two homologous collagenases.

It is well established that the expression profiles of multiple and possibly redundant matrix-remodeling proteases (e.g., collagenases) differ strongly in health, disease, and development. Although enzymatic redundancy might be inferred from their close similarity in structure, their in vivo activity can lead to extremely diverse tissue-remodeling outcomes. We observed that proteolysis of collagen-rich natural extracellular matrix (ECM), performed uniquely by individual homologous proteases, leads to distinct events that eventually affect overall ECM morphology, viscoelastic properties, and molecular composition. We revealed striking differences in the motility and signaling patterns, morphology, and gene-expression profiles of cells interacting with natural collagen-rich ECM degraded by different collagenases. Thus, in contrast to previous notions, matrix-remodeling systems are not redundant and give rise to precise ECM-cell crosstalk. Because ECM proteolysis is an abundant biochemical process that is critical for tissue homoeostasis, these results improve our fundamental understanding its complexity and its impact on cell behavior.

Anomalous viscosity-time behavior of polysaccharide dispersions.

Using viscosity and dynamic light scattering (DLS) measurements, we monitored the changes in the properties of dispersions of chitosan (a cationic polysaccharide) in acidic solution over a period of up to 700 h. Different polymer concentrations, weight average molecular weights, and degrees of deacetylation were examined. We found that the solution rheology and chitosan aggregates continue to change even up to 700 h. It was observed, remarkably, using both capillary and cone and plate viscometry that the viscosity decreased significantly during the storage period of the chitosan dispersions, with a rapid initial decrease and a slow approach to the steady state value. DLS measurements over this period could be interpreted in terms of a gradual decrease in the size of the chitosan aggregates in the dispersion. This behavior is puzzling, insofar as one expects the dissolution of compact polymer aggregates with time into individual polymer chains to increase the viscosity rather than decrease it as observed: We attribute this apparently anomalous behavior to the fact that the chitosan aggregates are rigid crystalline rod-like entities, which dissolved with time from dispersion of overlapping rods (with high viscosity) into solution of individual random coils (with lower viscosity). A detailed model comparing the hydrodynamic behavior of the initial overlapping rod-like aggregates with the subsequent free coils in solution is in semi-quantitative agreement with our observation.

On the question of whether lubricants fluidize in stick-slip friction.

Intermittent sliding (stick-slip motion) between solids is commonplace (e.g., squeaking hinges), even in the presence of lubricants, and is believed to occur by shear-induced fluidization of the lubricant film (slip), followed by its resolidification (stick). Using a surface force balance, we measure how the thickness of molecularly thin, model lubricant films (octamethylcyclotetrasiloxane) varies in stick-slip sliding between atomically smooth surfaces during the fleeting (ca. 20 ms) individual slip events. Shear fluidization of a film of five to six molecular layers during an individual slip event should result in film dilation of 0.4-0.5 nm, but our results show that, within our resolution of ca. 0.1 nm, slip of the surfaces is not correlated with any dilation of the intersurface gap. This reveals that, unlike what is commonly supposed, slip does not occur by such shear melting, and indicates that other mechanisms, such as intralayer slip within the lubricant film, or at its interface with the confining surfaces, may be the dominant dissipation modes.

Effect of Cholesterol on the Stability and Lubrication Efficiency of Phosphatidylcholine Surface Layers.

The lubrication properties of saturated PC lipid vesicles containing high cholesterol content under high loads were examined by detailed surface force balance measurements of normal and shear forces between two surface-attached lipid layers. Forces between two opposing mica surfaces bearing distearoylphosphatidylcholine (PC) (DSPC) small unilamellar vesicles (SUVs, or liposomes), or bilayers, with varying cholesterol content were measured across water, whereas dimyristoyl PC (DMPC), dipalmitoyl PC (DPPC), and DSPC SUVs containing 40% cholesterol were measured across liposome dispersions of SUVs of the same lipid composition as in the adsorbed layers. The results clearly demonstrate decreased stability and resistance to normal load with the increase in cholesterol content of DSPC SUVs. Friction coefficients between two 10% cholesterol PC-bilayers were in the same range as for 40% cholesterol bilayers (µ ≈ 10-3), indicating that cholesterol has a more substantial effect on the mechanical properties of a bilayer than on its lubrication performance. We further find that the lubrication efficiency of DMPC and DPPC with 40% cholesterol is superior to that of DSPC 40% cholesterol, most likely because of enhanced hydration-lubrication in these systems. We previously found that when experiments are performed in the presence of a lipid reservoir, layers can self-heal and therefore their robustness is less important under such conditions. We conclude that the effect of cholesterol in decreasing the stability is more pronounced than its effect on hydration, but the stability is, in turn, less important when a lipid reservoir is present. This study complements our previous work and sheds light on the effect of cholesterol, a prominent and important physiological lipid, on the mechanical and lubrication properties of gel-phase lipid layers.

Frictional Dissipation Pathways Mediated by Hydrated Alkali Metal Ions.

Frictional energy dissipation between sliding solid surfaces in aqueous media may proceed by different pathways. Using a surface force balance (SFB), we have examined systematically how such dissipation is mediated by the series of hydrated cations M(+) = Li(+), Na(+), and K(+) that are trapped between two atomically smooth, negatively charged, mica surfaces sliding across the ionic solutions over many orders of magnitude loading. By working at local contact pressures up to ca. 30 MPa (∼300 atm), up to 2 orders of magnitude higher than earlier studies, we could show that the frictional dissipation at constant sliding velocity, represented by the coefficient of sliding friction µM+, decreased as µLi+ > µNa+ ≳ µK+. This result contrasts with the expectation (in conceptual analogy with the Hofmeister series) that the lubrication would improve with the extent of ionic hydration, since that would have led to the opposite µM+ sequence. It suggests, rather, that frictional forces, even in such simple systems, can be dominated by rate-activated pathways where the size of the hydration shell becomes a dissipative liability, rather than by the hydration-shell dissipation expected via the hydration lubrication mechanism.

A Trimeric Surfactant: Surface Micelles, Hydration-Lubrication, and Formation of a Stable, Charged Hydrophobic Monolayer.

The surface structure of the trimeric surfactant tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) on mica and the interactions between two such DTAD-coated surfaces were determined using atomic force microscopy and a surface force balance. In an aqueous solution of 3 mM, 5 times the critical aggregation concentration (CAC), the surfaces are coated with wormlike micelles or hemimicelles and larger (∼80 nm) bilayer vesicles. Repulsive normal interactions between the surfaces indicate a net surface charge and a solution concentration of ions close to that expected from the CAC. Moreover, this surface coating is strongly lubricating up to some tens of atmospheres, attributed to the hydration-lubrication mechanism acting at the exposed, highly hydrated surfactant headgroups. Upon replacement of the DTAD solution with surfactant-free water, the surface structures have changed on the DTAD monolayers, which then jump into adhesive contact on approach, both in water and following addition of 0.1 M NaNO3. This trimeric surfactant monolayer, which is highly hydrophobic, is found to be positively charged, which is evident from the attraction between the DTAD monolayer and negatively charged bare mica across water. These monolayers are stable over days even under a salt solution. The stability is attributed to the several stabilization pathways available to DTAD on the mica surface.

Development of Long-Circulating Zwitterionic Cross-Linked Micelles for Active-Targeted Drug Delivery.

Blood stability, active targeting, and controlled drug release are the most important features to design desirable drug carriers. Here, we demonstrate a zwitterionic biodegradable cross-linked micelle based on a penta-block copolymer, which utilizes poly(carboxybetaine methacrylate) as hydrophilic segment, poly(ε-caprolactone) as biodegradable hydrophobic segment, poly(S-2-hydroxyethyl-O-ethyl dithiocarbonate methacrylate) (PSODMA) block as thiol protecting segment for cross-linking, and cyclic Arg-Gly-Asp-d-Tyr-Lys [c(RGDyK)] as targeting ligand. As a result, this micelle possessed excellent colloidal stability at high dilution and in 50% fetal bovine serum. In vitro drug release experiment showed no burst release under physiological conditions but accelerated drug release in mimicking tumor tissue environment. In vivo tests showed that the drug-loaded micelles had prolonged half-life in bloodstream, enhanced therapeutic efficiency, and reduced cardiac toxicity and biotoxicity compared with free drug formulation. Taken together, the reported c(RGDyK)-modified zwitterionic interfacially cross-linked micelle has emerged as an appealing platform for cancer therapy.

Hydration lubrication and shear-induced self-healing of lipid bilayer boundary lubricants in phosphatidylcholine dispersions.

Measurements of normal and shear (frictional) forces between mica surfaces across small unilamellar vesicle (SUV) dispersions of the phosphatidylcholine (PC) lipids DMPC (14:0), DPPC (16:0) and DSPC (18:0) and POPC (16:0, 18:1), at physiologically high pressures, are reported. We have previously studied the normal and shear forces between two opposing surfaces bearing PC vesicles across pure water and showed that liposome lubrication ability improved with increasing acyl chain length, and correlated strongly with the SUV structural integrity on the substrate surface (DSPC > DPPC > DMPC). In the current study, surprisingly, we discovered that this trend is reversed when the measurements are conducted in SUV dispersions, instead of pure water. In their corresponding SUV dispersion, DMPC SUVs ruptured and formed bilayers, which were able to provide reversible and reproducible lubrication with extremely low friction (µ < 10(-4)) up to pressures of 70-90 atm. Similarly, POPC SUVs also formed bilayers which exhibited low friction (µ < 10(-4)) up to pressures as high as 160 atm. DPPC and DSPC SUVs also provided good lubrication, but with slightly higher friction coefficients (µ = 10(-3)-10(-4)). We believe these differences originate from fast self-healing of the softer surface layers (which are in their liquid disordered phase, POPC, or close to it, DMPC), which renders the robustness of the DPPC or DSPC (both in their solid ordered phase) less important in these conditions. Under these circumstances, the enhanced hydration of the less densely packed POPC and DMPC surface layers is now believed to play an important role, and allows enhanced lubrication via the hydration lubrication mechanism. Our findings may have implications for the understanding of complex biological systems such us biolubrication of synovial joints.

Normal and shear forces between charged solid surfaces immersed in cationic surfactant solution: the role of the alkyl chain length.

Using a surface force balance (SFB), we measured the boundary friction and the normal forces between mica surfaces immersed in a series of alkyltrimethylammonium chloride (TAC) surfactant solutions well above the critical micelle concentration (CMC). The surfactants that were used--C14TAC, C16TAC, and C18TAC--varied by the length of the alkyl chain. The structures of the adsorbed layers on the mica were obtained using AFM imaging and ranged from flat bilayers to rodlike micelles. Despite the difference in alkyl chain, all the surfactant solutions reduce the friction between the two mica surfaces enormously relative to immersion in water, and have similar friction coefficients (µ ≈ 0.001). The pressure at which such lubrication breaks down is higher for the surfactants with longer chain lengths and indicates that an important role of the chain length is to provide a more robust structure of the adsorbed layers which maintains its integrity to higher pressures.

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.

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.