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Biomedical subjects

Jacob Israelachvili

Publications and source records attributed to Jacob Israelachvili.

18 recordsLinked to original sources
PubMed2007

Properties of confined and sheared rhodamine B films studied by SFA-FECO spectroscopy.

We have used a surface forces apparatus and multiple beam interferometry to measure the absorbance of thin films of rhodamine B in water/ethylene glycol solutions while applying and measuring normal and lateral (shear) forces. Both normal and shear forces induced changes in the absorption spectra indicating a change in molecular alignment, and rhodamine-rhodamine and rhodamine-surface interactions. We also measured differences in the absorbance spectra in different regions of the contact indicating, as expected, that the stresses are not uniform throughout the contact area. We also observed crystallization (solidification) parallel to the shearing direction.

Journal Article↗
PubMed2006

Adhesion and detachment mechanisms of sugar surfaces from the solid (glassy) to liquid (viscous) states.

The viscosity of some sugars varies continuously by approximately 10 orders of magnitude over a temperature range of 50 degrees C. Sugars are therefore ideal model materials for studying the failure mechanism of materials as they change from the solid to the liquid state. We have used a surface forces apparatus coupled to interference and optical microscopy imaging to study the way two sugar surfaces adhere and detach from adhesive contact. The sugar-coated layers on mica displayed significant loading-unloading hysteresis, and the adhesive strength and failure mechanism during a "loading-unloading cycle" depended on the loading rate, contact time, and unloading rate. The failure of two glassy sugar surfaces was manifested by the nucleation of many sharp microcracks at the external boundary that rapidly propagate along the original contact interface. At the other extreme, when the material is a liquid, "failure" occurs through the nucleation and inward growth of large rounded ripples, characteristic of a Saffman-Taylor fingering instability. In the transition from glassy to viscous failure, sharp crack tips and smooth rounded fingers coexist during the crack propagation. In addition, whereas the detachment geometry of two glassy surfaces was characterized by a peeling-like mechanism along a plane, the fingers associated with the "snapping" of a liquid neck extended in all directions. These findings provide insights into the adhesion and failure mechanisms of materials at the micro/nanoscales and are also relevant to the action of interparticle forces between sugar particles, such as those used in inhalation drug delivery.

Adhesiveness↗
PubMed2006

Adhesion and friction in gecko toe attachment and detachment.

Geckos can run rapidly on walls and ceilings, requiring high friction forces (on walls) and adhesion forces (on ceilings), with typical step intervals of approximately 20 ms. The rapid switching between gecko foot attachment and detachment is analyzed theoretically based on a tape model that incorporates the adhesion and friction forces originating from the van der Waals forces between the submicron-sized spatulae and the substrate, which are controlled by the (macroscopic) actions of the gecko toes. The pulling force of a spatula along its shaft with an angle between theta 0 and 90 degrees to the substrate, has a "normal adhesion force" contribution, produced at the spatula-substrate bifurcation zone, and a "lateral friction force" contribution from the part of spatula still in contact with the substrate. High net friction and adhesion forces on the whole gecko are obtained by rolling down and gripping the toes inward to realize small pulling angles between the large number of spatulae in contact with the substrate. To detach, the high adhesion/friction is rapidly reduced to a very low value by rolling the toes upward and backward, which, mediated by the lever function of the setal shaft, peels the spatulae off perpendicularly from the substrates. By these mechanisms, both the adhesion and friction forces of geckos can be changed over three orders of magnitude, allowing for the swift attachment and detachment during gecko motion. The results have obvious implications for the fabrication of dry adhesives and robotic systems inspired by the gecko's locomotion mechanism.

Adhesiveness↗
PubMed2006

Triboelectrification between smooth metal surfaces coated with self-assembled monolayers (SAMs).

Using a modified surface forces apparatus, we have simultaneously measured the friction and triboelectrification between both similar and dissimilar molecularly smooth hexadecanethiol-coated metal surfaces on mica substrates. On shearing dissimilar surfaces, the tribocurrent increases dramatically as the load or pressure is increased, with large fluctuations about the mean. Neither charge transfer nor fluctuations are observed when the symmetric surfaces are sheared against each other. We also find that the type of friction, i.e., stick-slip or smooth sliding, the load and friction force, the sliding distance, and recent previous history have additional fine influences on the triboelectrification. Our results suggest that frictional dissipation induces electron-hole formation and charge transfer between two shearing surfaces due to molecular-level roughness and defects and local dielectric constant changes, giving rise to the observed tribocurrents.

Journal Article↗
PubMed2006

Recent progress in understanding hydrophobic interactions.

We present here a brief review of direct force measurements between hydrophobic surfaces in aqueous solutions. For almost 70 years, researchers have attempted to understand the hydrophobic effect (the low solubility of hydrophobic solutes in water) and the hydrophobic interaction or force (the unusually strong attraction of hydrophobic surfaces and groups in water). After many years of research into how hydrophobic interactions affect the thermodynamic properties of processes such as micelle formation (self-assembly) and protein folding, the results of direct force measurements between macroscopic surfaces began to appear in the 1980s. Reported ranges of the attraction between variously prepared hydrophobic surfaces in water grew from the initially reported value of 80-100 Angstrom to values as large as 3,000 Angstrom. Recent improved surface preparation techniques and the combination of surface force apparatus measurements with atomic force microscopy imaging have made it possible to explain the long-range part of this interaction (at separations >200 Angstrom) that is observed between certain surfaces. We tentatively conclude that only the short-range part of the attraction (<100 Angstrom) represents the true hydrophobic interaction, although a quantitative explanation for this interaction will require additional research. Although our force-measuring technique did not allow collection of reliable data at separations <10 Angstrom, it is clear that some stronger force must act in this regime if the measured interaction energy curve is to extrapolate to the measured adhesion energy as the surface separation approaches zero (i.e., as the surfaces come into molecular contact).

Hydrophobic and Hydrophilic Interactions↗
PubMed2006

Differences between non-specific and bio-specific, and between equilibrium and non-equilibrium, interactions in biological systems.

The interaction forces between biological molecules and surfaces are much more complex than those between non-biological molecules or surfaces, such as colloidal particle surfaces. This complexity is due to a number of factors: (i) the simultaneous involvement of many different molecules and different non-covalent forces - van der Waals, electrostatic, solvation (hydration, hydrophobic), steric, entropic and 'specific', and (ii) the flexibility of biological macromolecules and fluidity of membranes. Biological interactions are better thought of as 'processes' that evolve in space and time and, under physiological conditions, involve a continuous input of energy. Such systems are, therefore, not at thermodynamic equilibrium, or even tending towards equilibrium. Recent surface forces apparatus (SFA) and atomic force microscopy (AFM) measurements on supported model membrane systems (protein-containing lipid bilayers) illustrate these effects. It is suggested that the major theoretical challenge is to establish manageable theories or models that can describe the spatial and time evolution of systems consisting of different molecules subject to certain starting conditions or energy inputs.

Biophysical Phenomena↗
PubMed2006

Friction and tribochemical reactions occurring at shearing interfaces of nanothin silver films on various substrates.

The tribological and tribochemical properties of 5-10 nm thick Ag films sliding on various metal and inorganic substrates were measured using a surface forces apparatus coupled with ex situ x-ray photoelectron spectroscopy. We observed enhanced chemical reactivity in the sheared regions compared to the unsheared regions, which we attribute to significant frictional heating in agreement with two recent simulations. It is also found that the initial topography (roughness) of the films plays a significant role in determining the friction, wear, and tribochemical reactions. The larger the surface roughness, the larger are the friction coefficients. Initially smooth surfaces, forming large continuous junctions, create large wear debris particles; whereas initially rough surfaces, forming many small junctions, create many small particles. Even though initially smooth surfaces are chemically less reactive than initially rough surfaces, they are tribochemically more reactive, forming two to three times more oxides of silver during shearing than under static conditions and organometals of silver that under normal (static) conditions do not form. The chemical reactions observed cannot be explained without including the tribological processes, such as the local stresses, temperature rises, and type of wear produced by the shearing surfaces.

Journal Article↗
PubMed2006

Thermodynamic equilibrium of domains in a two-component Langmuir monolayer.

This [corrected] article outlines the results from a combined experimental and theoretical study on the properties of circular domains in a mixed Langmuir monolayer at thermodynamic equilibrium. The mixed monolayer consisted of a binary mixture of dimyristoyl-phosphatidyl-choline and dihydrocholesterol. A long-term fluorescence microscopy study of these domains was carried out over the course of approximately 60 h. Image analysis of the domains over time revealed that the domains ripened slowly with an [corrected] increase in mean domain radius and a [corrected] decrease in domain number density. At the end of the measurement, the domains remained polydisperse, and true thermodynamic equilibrium was not reached. Theoretically, collective thermodynamic equilibrium properties such as mean domain size and size distribution were calculated by combining micelle self-assembly theory and the "equivalent dipole" model for the self-energy of two-dimensional domains. The calculations predicted existence of finite-sized circular domains at equilibrium. This suggests that equilibrium circular monolayer domains of single- or multicomponent lipids with a finite size distribution should form only at very limited experimental conditions. Both the predicted mean domain size and size distribution are strongly affected by line tension and dipole moment density difference. A comparison between the theoretical and experimental results is made.

Cholestanol↗
PubMed2005

Crystallization in thin liquid films induced by shear.

It is known that the thin-film structure of confined fluids and solids can be changed when the confining surfaces are sheared. Positional and orientational short- or long-range reordering can occur that often have no bulk counterparts. These multilayer, monolayer, or even sub-monolayer effects are important for understanding adhesion and friction processes, but they have proved difficult to measure, partly due to a lack of experimental techniques and partly to their apparent subtle dependence on many experimental parameters. Here we report the use of shear measurements and "optical absorption spectroscopy" in the surface forces apparatus to measure a shear-induced phase transition of an anisotropic (dye) molecule confined between two shearing mica surfaces in aqueous solution. Our studies on the shear-induced ordering and friction forces of highly anisotropic cyanine dye molecules in thin water films show only a weak effect of molecular anisotropy on shear-induced ordering, friction forces, and the onset of shear-induced crystallization, although dramatic changes do occur when the confined molecules ultimately crystallize.

Journal Article↗
PubMed2005

Static forces, structure and flow properties of complex fluids in highly confined geometries.

The Surface Forces Apparatus has been successfully used to measure the static and dynamic forces between surfaces across ultra-thin films of water and aqueous electrolyte solutions, and--more recently--polyelectrolyte-coated and articular cartilage surfaces in various solutions including hyaluronan, lubricin, and synovial fluid. The results give new insights into the lubricating action of biological lubricants such as synovial fluid and hyaluronan (a polysaccharide in synovial fluid), and biological surfaces such as phospholipid bilayers and cartilage surfaces. Contrary to earlier indications of long-range water-structuring at biological surfaces, more recent measurements clearly show that the viscosity of physiologically concentrated water (saline) is bulk-like beyond the first 1 or 2 layers from a single surface, and beyond 4-6 layers in thin films between two surfaces (the structure and forces may, however, be affected to larger distances). This implies that most structural, interaction force, and viscosity-related phenomena are determined--not only by the properties of the solvent (water)per se--but also by the surfaces and the water, ions, solutes, and macromolecules (proteins, polymers) exposed or adsorbed at the surfaces and, to a lesser degree, dissolved in the solvent. However, sometimes it is difficult to make a clear differentiation, e.g., one could consider hydration or surface-bound 'structured' water as part of the surface or as part of the intervening water between the two surfaces.

Animals↗
PubMed2004

Lubrication and wear properties of grafted polyelectrolytes, hyaluronan and hylan, measured in the surface forces apparatus.

Hyaluronan is believed to have an important function in the boundary biolubrication of articular cartilage. Using a Surface Forces Apparatus, we tested the tribological properties of surface bound, rather than "free" hyaluronan. The grafting process of the polyelectrolyte included either a biological route via an HA-binding protein or a chemical reaction to covalently bind the polymer to a lipid bilayer coated surface. In another reaction, we constructed a surface with covalently grafted hylan (crosslinked hyaluronan). We studied the normal and shear forces between these surfaces. None of the systems demonstrated comparable lubrication to that found between cartilage surfaces except at very low loads. Both grafted hyaluronan and hylan generated coefficients of friction between 0.15 and 0.3. Thus, the polysaccharide, which is a constituent of the lamina splendens (outermost cartilage layer), is not expected to be the responsible molecule for the great lubricity of cartilage; however, it may contribute to the load bearing and wear protection of these surfaces. This was concluded from the results with hylan, where a thin gel layer was sufficient to shield the underlying surfaces from damage even at applied pressures of over 200 atmospheres during shear. Our study shows that a low coefficient of friction is not a requirement for, or necessarily a measure of, wear protection.

Aluminum Silicates↗
PubMed2004

Synergistic interactions of lipids and myelin basic protein.

This report describes force measurements and atomic force microscope imaging of lipid-protein interactions that determine the structure of a model membrane system that closely mimics the myelin sheath. Our results suggest that noncovalent, mainly electrostatic and hydrophobic, interactions are responsible for the multilamellar structure and stability of myelin. We find that myelin basic protein acts as a lipid coupler between two apposed bilayers and as a lipid "hole-filler," effectively preventing defect holes from developing. From our protein-mediated-adhesion and force-distance measurements, we develop a simple quantitative model that gives a reasonably accurate picture of the molecular mechanism and adhesion of bilayer-bridging proteins by means of noncovalent interactions. The results and model indicate that optimum myelin adhesion and stability depend on the difference between, rather than the product of, the opposite charges on the lipid bilayers and myelin basic protein, as well as on the repulsive forces associated with membrane fluidity, and that small changes in any of these parameters away from the synergistically optimum values can lead to large changes in the adhesion or even its total elimination. Our results also show that the often-asked question of which membrane species, the lipids or the proteins, are the "important ones" may be misplaced. Both components work synergistically to provide the adhesion and overall structure. A better appreciation of the mechanism of this synergy may allow for a better understanding of stacked and especially myelin membrane structures and may lead to better treatments for demyelinating diseases such as multiple sclerosis.

Hydrophobic and Hydrophilic Interactions↗
PubMed2004

The nonlinear nature of friction.

Tribology is the study of adhesion, friction, lubrication and wear of surfaces in relative motion. It remains as important today as it was in ancient times, arising in the fields of physics, chemistry, geology, biology and engineering. The more we learn about tribology the more complex it appears. Nevertheless, recent experiments coupled to theoretical modelling have made great advances in unifying apparently diverse phenomena and revealed many subtle and often non-intuitive aspects of matter in motion, which stem from the nonlinear nature of the problem.

Animals↗
PubMed2004

Role of lipid interactions in autoimmune demyelination.

A morphological transformation involving loss of adhesion between myelin lamellae and formation of myelin vesicles has been described as a mechanism for demyelination in multiple sclerosis and marmoset experimental allergic encephalomyelitis (EAE). Although protein interactions are involved in maintaining normal myelin structure, we describe here how lipids contribute to myelin stability and how lipid changes in EAE, including increases in lipid polyunsaturation and negatively charged phosphatidylserine (PS), promote demyelination. Three physico-chemical techniques were used to identify these changes: (1) Langmuir monolayer isotherms indicated that EAE white matter lipids were significantly more "expanded" (fluid) than controls. (2) NMR spectroscopy indicated that EAE myelin lipids were more polyunsaturated than controls. (3) High-performance liquid chromatography (HPLC) with an evaporative light scattering detector indicated increased PS in EAE compared to controls, while sphingomyelin (SM), sulfatides and phosphatidylcholine (PC) were decreased. We present a physical model considering electrostatic, van der Waals and undulation forces to quantify the effect of these changes on myelin adhesion at the extracellular interface. Taken together, the isotherm, NMR, HPLC and modeling results support a mechanism for autoimmune demyelination whereby the composition of myelin lipids is altered in a manner that increases myelin fluidity, decreases myelin adhesion, increases membrane curvature, and promotes vesiculation.

Animals↗
PubMed2004

Large deformations during the coalescence of fluid interfaces.

Surface forces and shape changes were simultaneously measured during the approach and coalescence of two liquid-liquid and liquid-air interfaces. Large normal and lateral deformations were observed that are nevertheless consistent with a simple theoretical analysis of the long-range effects of short-range attractive van der Waals forces. The results imply that two fluidlike structures such as liquid droplets and soft biological cells can sense each other at much larger separations than previously assumed based on criteria taken from the interactions of hard particles.

Journal Article↗
PubMed2004

Correlation of AFM and SFA measurements concerning the stability of supported lipid bilayers.

Phospholipid bilayers were studied by means of atomic force microscopy (AFM) and a surface force apparatus (SFA). The stability of the supported bilayers was described by the amount of irregularities in the topography of the membrane by means of AFM and by the occurrence of hemifusion in the SFA, which is an indicator of defective bilayers. The bilayers, composed of lipids having the same headgroup but different chain lengths in the two leaflets, were prepared by Langmuir-Blodgett deposition and transferred at different surface pressures. The topography of the supported bilayers in aqueous solution, as imaged by AFM, revealed an increasing number of defects in the supported lipid membranes with decreased deposition pressure of the outer lipid layer. These defects, which appeared in the form of monolayer and bilayer (self-assembled) thick holes within the membrane, were energetically favorable over an evenly depleted bilayer. We found that the quantity of these defects (holes of </=0.5 micro m diameter and covering up to 30% of the surface area) correlated well with the stability of the bilayers as measured by SFA, a truly complementary instrument.

Elasticity↗
PubMed2002

Shear alignment of confined hydrocarbon liquid films.

Shear-induced structural reordering in thin liquid films of the linear saturated alkane n-eicosane (C20H42) was investigated using a surface forces apparatus and freeze-fracture (atomic force) microscopy (AFM). By rapidly freezing a shearing film followed by splitting (cleaving) the films from the confining mica substrate surfaces, it was possible to obtain AFM images of the structures of the films during steady-state sliding, revealing striped domains approximately 2 A in height and a few nanometer wide whose structure depends on the sliding velocity and, most likely, also on the sliding distance and time. In contrast, confined but unsheared films yielded completely featureless images. To the best of our knowledge, the results are the first direct experimental measurement of shear-induced ordering in nano-confined films resulting in layering and domain formation, but any molecular-level alignment, if present, could not be established.

Journal Article↗