Direct AFM measurements of adhesion forces between calcium oxalate monohydrate and kidney epithelial cells in the presence of Ca2+ and Mg2+ ions

Adhesion forces between the calcium oxalate monohydrate (COM, whewellite) crystal and the layer of the epithelial kidney cells have been directly measured under buffer solutions by using atomic force microscope (AFM). Two renal epithelial lines, MDCK (a collecting duct line) and LLC-PK1 (a proximal tubular line), were used. All experiments were conducted in buffer solutions containing additional Ca(2+) and Mg(2+) ions in the various concentrations. For MDCK-cells, the obtained values of the adhesion force were in the range 0.12-0.51 nN and 0.12-0.20 nN for Ca(2+) and Mg(2+), respectively. No adhesion force (larger than 0.05 nN) has been found for LLC-PK1 cells. The "critical" concentrations of ions, near which the adhesion force (for MDCK-cells) was maximal, were found to be 100 mM. The "critical" concentration of ions and the tendency of the adhesion forces with the changing ions concentration, confirm earlier results of Lieske et al. [J.C. Lieske, G. Farell, S. Deganello, Urol. Res. 32 (2004) 117-123], in which the affinity (rather than the adhesion force) between the COM micro-crystals and the layer of the MDCK-cells were measured, calculating the radioactive signal of radioactive (14)C COM-crystals stuck to the cells. We believe that the aggregation of the COM crystals does not occur in the bulk urine due to short travel time through the nephron. If so, the kidney stone formation is determined by COM-seeding on the tubules walls. The further growth of the stone on the seed can take practically unlimited time because the COM crystal is practically is not soluble in water or urine solutions. The value of the adhesion force can be useful for evaluation of the adhesion energy or probability of the COM-aggregates to stick to the kidney epithelium under the urine flow. This probability is calculated taking into account the adhesion force, F(ad), and hydrodynamic driving force of the flow. This probability reflects the opportunity of the small aggregates to grow and form the kidney stones.     

Characterization of the physical properties of model biomembranes at the nanometer scale with the atomic force microscope

Interaction forces and topography of mixed phospholipid-glycolipid bilayers were investigated by atomic force microscopy (AFM) in aqueous conditions with probes functionalized with self-assembled monolayers terminating in hydroxy groups. Short-range repulsive forces were measured between the hydroxy-terminated probe and the surface of the two-dimensional (2-D) solid-like domains of distearoyl-phosphatidylethanolamine (DSPE) and digalactosyldiglyceride (DGDG). The form and range of the short-range repulsive force indicated that repulsive hydration/steric forces dominate the interaction at separation distances of 0.3-1.0 nm after which the probe makes mechanical contact with the bilayers. At loads < 5 nN the bilayer was elastically deformed by the probe, while at higher loads plastic deformation of the bilayer was observed. Surprisingly, a short-range repulsive force was not observed at the surface of the 2-D liquid-like dioleoylphosphatidylethanolamine (DOPE) film, despite the identical head groups of DOPE and DSPE. This provides direct evidence for the influence of the structure and mechanical properties of lipid bilayers on their interaction forces, an effect which may be a major importance in the control of biological processes such as cell adhesion and membrane fusion. The step height measured between lipid domains in the AFM topographic images was larger than could be accounted for by the thickness and mechanical properties of the molecules. A direct correlation was observed between the repulsive force range over the lipid domains and the topographic contrast, which provides direct insight into the fundamental mechanisms of AFM imaging in aqueous solutions. This study demonstrates that chemically modified AFM probes can be used in combination with patterned lipid bilayers as a novel and powerful approach to characterize the nanometer scale chemical and physical properties of heterogeneous biosurfaces such as cell membranes.     

AFM measurements of forces between silica surfaces

Interaction forces and adhesion between a silica sphere and a flat silica surface in aqueous electrolyte solutions were investigated by atomic force microscopy. The forces were measured as a function of surface separation, pH and NaCl concentration as the surfaces were approaching each other. The adhesion force was determined upon retraction with respect to pH, NaCl concentration and contact time. The magnitude of the long range repulsive force was decreasing with decreasing pH. A short range repulsive force was observed at pH = 2, but no long range repulsive forces were observed at this pH. Force measurements showed that adhesion of silica surfaces in water was obstructed by short and long range repulsive forces. Adhesion was enhanced when both the long and the short range repulsive force was mitigated. A maximum adhesion force of 7.8 mN/m was measured at pH = 12.5 when the short range force vanished and the long range repulsive force was reduced by increasing the NaCl concentration. At pH = 12.5, the work of adhesion was calculated to be 1.2 mJ/m² according to the Derjaguin–Muller–Toporov (DMT) model. Adhesion energy was much less at pH = 2 (0.3 mJ/m²) due to persistive short range repulsion.     

Physical interpretation of the maximum receptor-ligand bond spacing to ensure cell adhesion in ligand-coated substrates

Recent experiments by Arnold et al. (Arnold, M.; Cavalcanti-Adam, E. A.; Glass, R.; Blummel, J.; Eck, W.; Kantlehner, M.; Kessler, H.; Spatz, J. P. ChemPhysChem 2004, 5, 383) revealed that a distance of less than 58-73 nm between receptor-ligand bonds is necessary to ensure focal adhesion in integrin-mediated cell adhesion on ligand-coated substrates. In this letter, we consider focal adhesion growth to be a process assisted by thermal fluctuations and receptor-ligand binding and resisted by repulsive "bulge pressure" and membrane deformation. By applying balance between these forces, we obtain a critical spacing of receptor-ligand bonds given as 2h[alpha kBT/beta Eh3)1/3(E/p)5]1/14, above which the growth of focal adhesion becomes difficult. Here h and E are the in-plane modulus and thickness of a cell membrane, respectively, p is a repulsive "bulge pressure" between the cell membrane and substrate, and alpha and beta are constants on the order of 1. We use typical values of E and h for cell membranes and obtain the critical spacing of receptor-ligand bonds of around 39-89 nm for a wide range of repulsive bulge pressure.     

AFM interaction study of alpha-alumina particle and c-sapphire surfaces at high-ionic-strength electrolyte solutions

Ionic strength dependence of interaction and friction forces between hydrophilic alpha-alumina particles and c-sapphire surfaces (0001) were investigated under basic pH conditions using the colloidal probe method. The compression of the double layer could be seen from force-distance curves as the ionic strength of the solution increased. The forces were repulsive at all ionic strengths measured, even though the interaction distance changed drastically. No jump to contact occurred. The interaction distance decreased from about 20 nm in 10(-3) M KCl solution to about 7 nm in the 1 M KCl case. The lubricating effect of hydrated cations on the lateral friction force was demonstrated at high electrolyte concentrations. This was attributed to more hydrated cations being present in the solution. The friction behavior was closely related to the short-range repulsive forces between the alpha-alumina surfaces at pH 11.     

Temperature dependent nano indentation of thin polymer films with the scanning force microscope

The scanning force microscope (SFM) was used to investigate the temperature dependent micro mechanical properties of polymethylmethacrylate (PMMA) films with a thickness of 35 nm in the range of the radius of gyration. Force-distance curves were performed in the glass transition range to create permanent nanometric indentations with maximal forces up to 4 μN. Quantitative measurements of the indentation depth during and after application of the force, hysteresis energy and slope of the loading part are carried out as function of sample temperature and applied force. The glass transition of the polymer film can be clearly identified by the change of the mechanical properties of the polymer. Surprisingly, only a small change of elasticity at the glass transition is observed.     

Nanoscale repulsive forces between mica and silica surfaces in aqueous solutions

Nanoscale repulsive forces between mineral surfaces in aqueous solutions were measured for the asymmetric mica-silica system. The force measured with an atomic force microscope (AFM) has universal character in the short range, less than ～1 nm or about 3-4 water molecules, independent of solution conditions, that is, electrolyte ion (Na, Ca, Al), concentration (10(-6)-10(-2)M), and pH (3.9-8.2). Notably, the force is essentially the same as for the glass-silica system. Single force curves for a mica-silica system in a 10(-4)M aqueous NaCl solution at pH ～ 5.1 show oscillations with a period of about 0.25 nm, roughly the diameter of a water molecule, a consequence of a layer-by-layer dehydration of the surfaces when pushed together. This result provides additional support to the idea that nanoscale repulsive forces between mineral surfaces in aqueous solutions arise from a surface-induced water effect; the water between two mineral plates that are pushed together becomes structured and increasingly anchored to the surface of the plates by the creation of a hydrogen-bonding network that prevents dehydration of the surfaces.     

Colloidal forces between bitumen surfaces in aqueous solutions measured with atomic force microscope

Colloidal forces between bitumen surfaces in aqueous solutions were measured with an atomic force microscope (AFM). The results showed a significant impact of solution pH, salinity, calcium and montmorillonite clay addition on both long-range (non-contact) and adhesion (pull-off) forces. Weaker long-range repulsive forces were observed under conditions of lower solution pH, higher salinity and higher calcium concentration. Lower solution pH, salinity and calcium concentration resulted in a stronger adhesion forces. The addition of montmorillonite clays increased long-range repulsive forces and decreased adhesion forces, particularly when co-added with calcium ions. The measured force profiles were fitted with extended DLVO theory to show the repulsive electrostatic double layer and attractive hydrophobic forces being the dominant components in the long-range forces between the bitumen surfaces. At a very short separation distance (less than 4–6 nm), a strong repulsion of steric origin was observed. The findings provide a fundamental understanding of bitumen emulsion stability and a mechanism of bitumen “aeration” in bitumen recovery processes from oil sands.     

Analysis of particle-wall interactions during particle free fall

In this study, the vertical motion of a particle in a quiescent fluid falling toward a horizontal plane wall is analyzed, based on simplified models. Using the distance between the particle and wall as a parameter, the effects of various forces acting on the particle and the particle motion are examined. Without the colloidal and Brownian forces being included, the velocity of small particles is found to be approximately equal to the inverse of the drag force correction function used in this study as the particle approaches the near-wall region. Colloidal force is added to the particle equation of motion as the particle moves a distance comparable to its size. It is found that the particle might become suspended above or deposited onto the wall, depending on the Hamaker constant, the surface potentials of the particle and wall, and the thickness of the electrical double layer (EDL). For strong EDL repulsive force and weaker van der Waals (VDW) attractive force, the particle will become suspended above the wall at a distance at which the particle velocity is zero. This location is referred to as the equilibrium distance. The equilibrium distance is found to increase with increased in EDL thickness when a repulsive force barrier appears in the colloidal force interaction. For the weak EDL repulsive force and strong VDW attractive force case, the particle can become deposited onto the wall without the Brownian motion effect. The Brownian jump length was found to be very small. Many Brownian jumps would be required in a direction toward the wall for a suspended particle to become deposited.     

Dielectrophoretic levitation in the presence of shear flow: implications for colloidal fouling of filtration membranes

The ability of dielectrophoretic (DEP) forces created using a microelectrode array to levitate particles in a colloidal suspension is studied experimentally and theoretically. The experimental system employs microfabricated electrode arrays on a glass substrate to apply repulsive DEP forces on polystyrene latex particles suspended in an aqueous medium. A numerical model based on the convection-diffusion-migration equation is presented to calculate the concentration distribution of colloidal particles in shear flow under the influence of a repulsive DEP force field. The results obtained from the numerical simulations are compared against trajectory analysis results and experimental data. The results indicate that by incorporating ac electric field-induced DEP forces in a shear flow, particle accumulation and deposition on the flow channel surfaces can be significantly reduced or even completely averted. The mathematical model is then used to indicate how the deposition behavior is modified in the presence of a permeable substrate, representative of tangential flow membrane filtration operations. The results indicate that the repulsive dielectrophoretic (DEP) forces imparted to the particles suspended in the feed can be employed to mitigate membrane fouling in a cross-flow filtration process.     

Direct nanomechanical measurement of an anchoring transition in a nematic liquid crystal subject to hybrid anchoring conditions

We have used a surface forces apparatus to measure the normal force between two solid curved surfaces confining a film of nematic liquid crystal (5CB, 4'-n-pentyl-4-cyanobiphenyl) under hybrid planar-homeotropic anchoring conditions. Upon reduction of the surface separation D, we measured an increasingly repulsive force in the range D = 35-80 nm, reaching a plateau in the range D = 10-35 nm, followed by a short-range oscillatory force at D < 5 nm. The oscillation period was comparable to the cross-sectional diameter of the liquid crystal molecule and characteristic of a configuration with the molecules parallel to the surfaces. These results show that the director field underwent a confinement-induced transition from a splay-bend distorted configuration at large D, which produces elastic repulsive forces, to a uniform planar nondegenerate configuration with broken homeotropic anchoring, which does not produce additional elastic forces as D is decreased. These findings, supported by measurements of the birefringence of the confined film at different film thicknesses, provide the first direct observation of an anchoring transition on the nanometer scale.     

Direct evaluation of DLVO theory for predicting long-range forces between a yeast cell and a surface

Using a combined gradient optical trap and evanescent wave light-scattering force-measurement technique, long-range colloidal forces were measured between a single Candida albicans yeast cell and a flat, bare glass surface in electrolyte concentrations ranging from 0.1 to 100 mM NaCl. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was compared to experimentally measured equilibrium force curves and found to provide a close approximation to the decay length of the measured forces for electrolyte concentrations up to about 0.23 mM NaCl. At higher electrolyte concentrations (>/=0.5 mM NaCl), decay lengths of force curves in experimental measurements were consistently longer than Debye lengths calculated from the electrolyte concentrations. In electrolyte concentrations of 10 and 100 mM NaCl, most cells attached rapidly, which prevented measurements of long-range forces. The small fraction of cells remaining unattached in these higher electrolyte concentrations displayed purely repulsive forces. These results show that the DLVO theory accurately describes cell-surface interactions when the Debye length is in the range of 20-30 nm but underpredicts the decay length of the interactions at higher electrolyte concentrations.     

Forces between polyethylene surfaces in oxyethylene dodecyl ether solutions as influenced by the number of oxyethylene groups

The atomic force microscopy (AFM) colloidal probe technique was used to study the effect of oxyethylene dodecyl ethers, C12En (n = 1-7), on interactions between hydrophobic polyethylene (PE) surfaces in aqueous solutions. Long-range (colloidal) and contact (pull-off) forces were measured between 10 to 20 microm PE spheres and a flat PE surface at concentrations of surfactant of 1 x 10(-6) and 1 x 10(-4) M. The surface tension of the surfactant solutions and contact angles at PE surfaces were also studied. The influence of the number of oxyethylene groups in the surfactant molecule was examined. Initially, long-range attractive (hydrophobic) forces between the PE surfaces were observed that decreased in range and magnitude with an increase in the number of oxyethylene groups in 1 x 10(-4) M solutions. Above four oxyethylene groups per molecule, repulsive forces were observed. The measured pull-off force between PE surfaces decreased monotonically from approximately 500 mJ/m2 for C12E1 to 150 mJ/m2 for C12E7. The interfacial energy was calculated on the basis of the JKR model, taking into account long-range forces operating outside the contact area. The interfacial energies decreased from 43-47 mJ/m2 for PE-water and PE-C12E1 (1 x 10(-4) M) interfaces to approximately 18 mJ/m2 for PE-C12E7 (1 x 10(-4) M). The interfacial energy was also calculated from measured contact angles and surface tensions using Neumann's equation of state and Young's equation. A similar relationship between interfacial energy and the number of oxyethylene groups was observed on the basis of contact and surface tension measurements. However, interfacial energy values were smaller, within 15-20 mJ/m2, than those calculated from AFM pull-off force measurements.     

Hydration forces between silica surfaces: experimental data and predictions from different theories

Silica is a very interesting system that has been thoroughly studied in the last decades. One of the most outstanding characteristics of silica suspensions is their stability in solutions at high salt concentrations. In addition to that, measurements of direct-interaction forces between silica surfaces, obtained by different authors by means of surface force apparatus or atomic force microscope (AFM), reveal the existence of a strong repulsive interaction at short distances (below 2 nm) that decays exponentially. These results cannot be explained in terms of the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, which only considers two types of forces: the electrical double-layer repulsion and the London-van der Waals attraction. Although there is a controversy about the origin of the short-range repulsive force, the existence of a structured layer of water molecules at the silica surface is the most accepted explanation for it. The overlap of structured water layers of different surfaces leads to repulsive forces, which are known as hydration forces. This assumption is based on the very hydrophilic nature of silica. Different theories have been developed in order to reproduce the exponentially decaying behavior (as a function of the separation distance) of the hydration forces. Different mechanisms for the formation of the structured water layer around the silica surfaces are considered by each theory. By the aid of an AFM and the colloid probe technique, the interaction forces between silica surfaces have been measured directly at different pH values and salt concentrations. The results confirm the presence of the short-range repulsion at any experimental condition (even at high salt concentration). A comparison between the experimental data and theoretical fits obtained from different theories has been performed in order to elucidate the nature of this non-DLVO repulsive force.     

Penetration of living cell membranes with fortified carbon nanotube tips

We have fabricated robust nanosurgical needles suitable for single cell operations by modifying multiwalled carbon nanotube (MCNT)-terminated atomic force microscopy (AFM) tips. Extra-long MCNT AFM tips were prepared and fortified with molecular layers of carbon to overcome mechanical instabilities and then coated with an outer shell of gold to promote chemical versatility. The terminal diameters of the final fabricated tips were approximately 30-40 nm, and the MCNT probes were several micrometers in length. We illustrate the capability of these modified MCNT tips to carry nanoparticulate payloads and to penetrate the plasma membrane of living pleural mesothelial cells at the smallest indentation depths (100-200 nm) and lowest penetration forces (100-200 pN) currently reported for these procedures.     

Gradient theory for structural forces in thin fluid films near the critical point

We consider a general form of the local gradient theory for structural forces in thin fluid films near the critical point. A complete analytical solution of this problem at separation distances substantially larger than the molecular size is presented. That is, pi= -[330Psi(-4)H(8.8923-Psi) + 384exp(-Psi)H(Psi-8.8923)]H(-Gamma)+[1134.36Psi(-4)H(8.8923-Psi + 192exp(-Psi)H(Psi-8.8923)]H(Gamma) where Gamma <0 for similar (both attractive or both repulsive) and Gamma >0 for dissimilar (one attractive, another repulsive) bounding surfaces. Here pi and Psi are the structural force and the separation distance scaled in a certain way, and H(.) is a unit step function. For binary polymer films, the results are further elaborated to include a scaling dependence of the structural force on polymerization degrees of components.     

Friction and normal interaction forces between irreversibly attached weakly charged polymer brushes

Polyelectrolyte brushes were built on mica by anchoring polystyrene-poly(acrylic acid) (PS-b-PAA) diblock copolymers at a controlled surface density in a polystyrene monolayer covalently attached to OH-activated mica surfaces. Compared to physisorbed polymer brushes, these irreversibly attached charged brushes allow the polymer grafting density to remain constant upon changes in environmental conditions (e.g., pH, salt concentration, compression, and shear). The normal interaction and friction forces as a function of surface separation distance and at different concentrations of added salt (NaCl) were investigated using a surface forces apparatus. The interaction force profiles were completely reversible both on loading and receding and were purely repulsive. For a constant polymer grafting density, the influence of the polyelectrolyte charges and the Debye screening effect on the overall interaction forces was investigated. The experimental interaction force profiles agree very well with scaling models developed for neutral and charged polymer brushes. The variation of the friction force between two PAA brushes in motion with respect to each other as a function of surface separation distance appeared to be similar to that observed with neutral brushes. This similarity suggests that the increase in friction is associated with an increase in mutual interpenetration upon compression as observed with neutral polymers. The effect of the PAA charges and added ions was more significant on the repulsive normal forces than on the friction forces. The reversible characteristics of the normal force profiles and friction measurements confirmed the strong attachment of the PAA brushes to the mica substrate. High friction coefficients (ca 0.3) were measured at relatively high pressures (40 atm) with no surface damage or polymer removal.     

Effect of water on the interfacial structures of room-temperature ionic liquids

The interfacial structures of cyano-based room-temperature ionic liquids play a very important role in reducing friction. However, the presence of water impairs their tribological performance. The interfacial structures and friction forces of 1-ethyl-3-methylimidazolium dicyanamide, [EMIM][DCN], and the effect of water on these structures and forces were investigated using atomic force microscopy. In addition, the interaction of water and [EMIM][DCN] was evaluated using Fourier-transform infrared (FT-IR) spectroscopy. Multiple repulsive layers were observed in the [EMIM][DCN] solution. This solution showed low friction force because these repulsive layers worked as protective layers against friction. On the other hand, the specific repulsive layer characteristics of [EMIM][DCN] could not be observed in a [EMIM][DCN] + 2 wt% H₂O solution. FT-IR results indicated that the layer structure of [EMIM][DCN] was disturbed by the addition of H₂O. Therefore, the solution containing water exhibited a high friction force.     

Long-Range Interaction Forces between Polymer-Supported Lipid Bilayer Membranes

Much of the short-range forces and structures of softly supported DMPC bilayers has been described previously. However, one interesting feature of the measured force-distance profile that remained unexplained is the presence of a long-range exponentially decaying repulsive force that is not observed between rigidly supported bilayers on solid mica substrate surfaces. This observation is discussed in detail here based on recent static and dynamic surface force experiments. The repulsive forces in the intermediate distance regime (mica-mica separations from 15 to 40 nm) are shown to be due not to an electrostatic force between the bilayers but to compression (deswelling) of the underlying soft polyelectrolyte layer, which may be thought of as a model cytoskeleton. The experimental data can be fit by simple theoretical models of polymer interactions from which the elastic properties of the polymer layer can be deduced.     

表面力仪的改进及溶液中DLVO理论的验证

改进了现有表面力仪,使之具有更低的造价和更高的实验精度.利用改进后的表面力仪测量了0.1mol/L的NaCl溶液中两云母表面间的作用力-距离曲线;通过与计算得到的曲线对比验证了DLVO理论.结果表明:在0.1mol/L的NaCl溶液中,两云母表面间在距离较大时的作用力测量值与DLVO理论值相符合,但在距离较小时测量值出现附加的短程斥力,且此斥力呈现指数衰减规律.