The dynamic nature of natural and fatty acid modified calcite surfaces.

Calcium carbonate, particularly in the form of calcite, is an abundant mineral widely used in both human-made products and biological systems. The calcite surface possesses a high surface energy, making it susceptible to the adsorption of organic contaminants. Moreover, the surface is also reactive towards a range of chemicals, including water. Consequently, studying and maintaining a clean and stable calcite surface is only possible under ultrahigh vacuum conditions and for limited amounts of time. When exposed to air or solution, the calcite surface undergoes rapid transformations, demanding a comprehensive understanding of the properties of calcite surfaces in different environments. Similarly, attention must also be directed towards the kinetics of changes, whether induced by fluctuating environments or at constant condition. All these aspects are encompassed in the expression "dynamic nature", and are of crucial importance in the context of the diverse applications of calcite. In many instances, the calcite surface is modified by adsorption of fatty acids to impart a desired nonpolar character. Although the binding between carboxylic acid groups and calcite surfaces is strong, the fatty acid layer used for surface modification undergoes significant alterations when exposed to water vapour and liquid water droplets. Therefore, it is also crucial to understand the dynamic nature of the adsorbed layer. This review article provides a comprehensive overview of the current understanding of both the dynamics of the calcite surface as well as when modified by fatty acid surface treatments.

Albumin-Hyaluronan Interactions: Influence of Ionic Composition Probed by Molecular Dynamics.

The lubrication mechanism in synovial fluid and joints is not yet fully understood. Nevertheless, intermolecular interactions between various neutral and ionic species including large macromolecular systems and simple inorganic ions are the key to understanding the excellent lubrication performance. An important tool for characterizing the intermolecular forces and their structural consequences is molecular dynamics. Albumin is one of the major components in synovial fluid. Its electrostatic properties, including the ability to form molecular complexes, are closely related to pH, solvation, and the presence of ions. In the context of synovial fluid, it is relevant to describe the possible interactions between albumin and hyaluronate, taking into account solution composition effects. In this study, the influence of Na+, Mg2+, and Ca2+ ions on human serum albumin-hyaluronan interactions were examined using molecular dynamics tools. It was established that the presence of divalent cations, and especially Ca2+, contributes mostly to the increase of the affinity between hyaluronan and albumin, which is associated with charge compensation in negatively charged hyaluronan and albumin. Furthermore, the most probable binding sites were structurally and energetically characterized. The indicated moieties exhibit a locally positive charge which enables hyaluronate binding (direct and water mediated).

Influence of the Molecular Weight and the Presence of Calcium Ions on the Molecular Interaction of Hyaluronan and DPPC.

Hyaluronan is an essential physiological bio macromolecule with different functions. One prominent area is the synovial fluid which exhibits remarkable lubrication properties. However, the synovial fluid is a multi-component system where different macromolecules interact in a synergetic fashion. Within this study we focus on the interaction of hyaluronan and phospholipids, which are thought to play a key role for lubrication. We investigate how the interactions and the association structures formed by hyaluronan (HA) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) are influenced by the molecular weight of the bio polymer and the ionic composition of the solution. We combine techniques allowing us to investigate the phase behavior of lipids (differential scanning calorimetry, zeta potential and electrophoretic mobility) with structural investigation (dynamic light scattering, small angle scattering) and theoretical simulations (molecular dynamics). The interaction of hyaluronan and phospholipids depends on the molecular weight, where hyaluronan with lower molecular weight has the strongest interaction. Furthermore, the interaction is increased by the presence of calcium ions. Our simulations show that calcium ions are located close to the carboxylate groups of HA and, by this, reduce the number of formed hydrogen bonds between HA and DPPC. The observed change in the DPPC phase behavior can be attributed to a local charge inversion by calcium ions binding to the carboxylate groups as the binding distribution of hyaluronan and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine is not changed.

Bioinspired Adhesion Polymers: Wear Resistance of Adsorption Layers.

Mussel adhesive polymers owe their ability to strongly bind to a large variety of surfaces under water to their high content of 3,4-dihydroxy-l-phenylalanine (DOPA) groups and high positive charge. In this work, we use a set of statistical copolymers that contain medium-length poly(ethylene oxide) side chains that are anchored to the surface in three different ways: by means of (i) electrostatic forces, (ii) catechol groups (as in DOPA), and (iii) the combination of electrostatic forces and catechol groups. A nanotribological scanning probe method was utilized to evaluate the wear resistance of the formed layers as a function of normal load. It was found that the combined measurement of surface topography and stiffness provided an accurate assessment of the wear resistance of such thin layers. In particular, surface stiffness maps allowed us to identify the initiation of wear before a clear topographical wear scar was developed. Our data demonstrate that the molecular and abrasive wear resistance on silica surfaces depends on the anchoring mode and follows the order catechol groups combined with electrostatic forces > catechol groups alone > electrostatic forces alone. The devised methodology should be generally applicable for evaluating wear resistance or "robustness" of thin adsorbed layers on a variety of surfaces.

Thermoresponsive Pentablock Copolymer on Silica: Temperature Effects on Adsorption, Surface Forces, and Friction.

The adsorption of hydrophilic or amphiphilic multiblock copolymers provides a powerful means to produce well-defined "smart" surfaces, especially if one or several blocks are sensitive to external stimuli. We focus here on an A-B-A-B-A copolymer, where A is a cationic poly((3-acrylamido-propyl)-trimethylammonium chloride) (PAMPTMA) block containing 15 (end blocks) or 30 (middle block) repeat units and B is a neutral thermosensitive water-soluble poly(2-isopropyl-2-oxazoline) (PIPOZ) block with 50 repeat units. X-ray reflectivity and quartz crystal microbalance with dissipation monitoring were employed to study the adsorption of PAMPTMA15-PIPOZ50-PAMPTMA30-PIPOZ50-PAMPTMA15 on silica surfaces. The latter technique was employed at different temperatures up to 50 °C. Surface forces and friction between the two silica surfaces across aqueous pentablock copolymer solutions at different temperatures were determined with the atomic force microscopy colloidal probe force and friction measurements. The cationic pentablock copolymer was found to have a high affinity to the negatively charged silica surface, leading to a thin (2 nm) and rigid adsorbed layer. A steric force was encountered at a separation of around 3 nm from hard wall contact. A capillary condensation of a polymer-rich phase was observed at the cloud point of the solution. The friction forces were evaluated using Amontons' rule modified with an adhesion term.

Influence of high hydrostatic pressure on solid supported DPPC bilayers with hyaluronan in the presence of Ca2+ ions.

The molecular mechanisms responsible for outstanding lubrication of natural systems, like articular joints, have been the focus of scientific research for several decades. One essential aspect is the lubrication under pressure, where it is important to understand how the lubricating entities adapt under dynamic working conditions in order to fulfill their function. We made a structural investigation of a model system consisting of two of the molecules present at the cartilage interface, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and hyaluronan, at high hydrostatic pressure. Phospholipid layers are found at the cartilage surfaces and are able to considerably reduce friction. Their behavior under load and varied solution conditions is important as pressures of 180 bar are encountered during daily life activities. We focus on how divalent ions, like Ca2+, affect the interaction between DPPC and hyaluronan, as other investigations have indicated that calcium ions influence their interaction. It could be shown that already low amounts of Ca2+ strongly influence the interaction of hyaluronan with DPPC. Our results suggest that the calcium ions increase the amount of adsorbed hyaluronan indicating an increased electrostatic interaction. Most importantly, we observe a modification of the DPPC phase diagram as hyaluronan absorbs to the bilayer which results in an Lα-like structure at low temperatures and a decoupling of the leaflets forming an asymmetric bilayer structure.

Physical crosslinking of hyaluronic acid in the presence of phospholipids in an aqueous nano-environment.

Hyaluronic acid and phospholipids are two components in the synovial joint cavity that contribute to joint lubrication synergistically. Molecular dynamics simulations were performed and hydrogen bonds in hyaluronic acid were analyzed to identify specific sites that are responsible for its physical cross-linking. Two molecular masses of hyaluronic acid, 10 kDa and 160 kDa, were considered. We use molecular dynamics simulations and the small world network approach to investigate dynamic couplings using a distance map applied to oxygen atoms in a chain of hyaluronic acid in the presence of phospholipids and water. The distance characterizing the coupling can be defined in various ways to bring out the most evident differences between various scenarios of the polymer chain conformation We show herein a physical distance understood as H-bond length and classes of these distances which are defined in a coarse-grained picture of the molecule. Simulation results indicate that addition of phospholipids has little influence on hyaluronic acid crosslinking. However, longer chains and addition of lipids promote appreciably long lasting (resilient) networks that may be of importance in biological systems. Specific sites for hydrogen bonding of phospholipids to hyaluronic acid have also been identified.

Correction: Physical crosslinking of hyaluronic acid in the presence of phospholipids in an aqueous nano-environment.

Correction for 'Physical crosslinking of hyaluronic acid in the presence of phospholipids in an aqueous nano-environment' by Piotr Beldowski et al., Soft Matter, 2018, DOI: 10.1039/c8sm01388h.

Influence of Glycosylation on Interfacial Properties of Recombinant Mucins: Adsorption, Surface Forces, and Friction.

Interfacial properties of two brush-with-anchor mucins, C-P55 and C-PSLex, have been investigated at the aqueous solution/poly(methyl methacrylate) (PMMA) interface. Both are recombinant mucin-type fusion proteins, produced by fusing the glycosylated mucin part of P-selectin glycoprotein ligand-1 (PSLG-1) to the Fc part of a mouse immunoglobulin in two different cells. They are mainly expressed as dimers upon production. Analysis of the O-glycans shows that the C-PSLex mucin has the longer and more branched side chains, but C-P55 has slightly higher sialic acid content. The adsorption of the mucins to PMMA surfaces was studied by quartz crystal microbalance with dissipation. The sensed mass, including the adsorbed mucin and water trapped in the layer, was found to be similar for these two mucin layers. Atomic force microscopy with colloidal probe was employed to study surface and friction forces between mucin-coated PMMA surfaces. Purely repulsive forces of steric origin were observed between mucin layers on compression, whereas a small adhesion was detected between both mucin layers on decompression. This was attributed to chain entanglement. The friction force between C-PSLex-coated PMMA is lower than that between C-P55-coated PMMA at low loads, but vice versa at high loads. We discuss our results in terms of the differences in the glycosylation composition of these two mucins.

Synergies in lubrication.

To slide surfaces against each other with application of a minimum force and minimum wear has been important since ancient times, and it remains equally important today. The use of oil-soluble lubricants is widely spread in technology, whereas living organisms have developed water-soluble lubricants to facilitate sliding motions. In this perspective article we focus on water-based lubrication in the boundary lubrication regime, and particularly lubrication synergies. This focus has, of course, found inspiration from the outstanding lubrication properties of synovial joints. It has ignited significant amount of research, mostly aimed at answering the question: Which molecule is the magic biolubricant? Different research groups have advocated different answers, and the debate has been intensive. In this article we argue that the question in itself is inappropriate. The relevant question is rather the following: How do molecules work in synergy to provide superior lubrication?

Automated coating procedures to produce poly(ethylene glycol) brushes in fused-silica capillaries.

Many bioanalytical methods rely on electrophoretic separation of structurally labile and surface active biomolecules such as proteins and peptides. Often poor separation efficiency is due to surface adsorption processes leading to protein denaturation and surface fouling in the separation channel. Flexible and reliable approaches for preventing unwanted protein adsorption in separation science are thus in high demand. We therefore present new coating approaches based on an automated in-capillary surface-initiated atom transfer radical polymerization process (covalent coating) as well as by electrostatically adsorbing a presynthesized polymer leading to functionalized molecular brushes. The electroosmotic flow was measured following each step of the covalent coating procedure providing a detailed characterization and quality control. Both approaches resulted in good fouling resistance against the four model proteins cytochrome c, myoglobin, ovalbumin, and human serum albumin in the pH range 3.4-8.4. Further, even samples containing 10% v/v plasma derived from human blood did not show signs of adsorbing to the coated capillaries. The covalent as well as the electrostatically adsorbed coating were both found to be stable and provided almost complete suppression of the electroosmotic flow in the pH range 3.4-8.4. The coating procedures may easily be integrated in fully automated capillary electrophoresis methodologies.

Structure of DPPC-hyaluronan interfacial layers - effects of molecular weight and ion composition.

Hyaluronan and phospholipids play an important role in lubrication in articular joints and provide in combination with glycoproteins exceptionally low friction coefficients. We have investigated the structural organization of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) Langmuir layers at the solution-air interface at different length scales with respect to the adsorption of hyaluronan (HA). This allows us to assemble a comprehensive picture of the adsorption and the resulting structures, and how they are affected by the molecular weight of HA and the presence of calcium ions. Brewster angle microscopy and grazing incident diffraction were used to determine the lateral structure at the micro- and macro scale. The data reveals an influence of HA on both the macro and micro structure of the DPPC Langmuir layer, and that the strength of this effect increases with decreasing molecular weight of HA and in presence of calcium ions. Furthermore, from X-ray reflectivity measurements we conclude that HA adsorbs to the hydrophilic part of DPPC, but data also suggest that two types of interfacial structures are formed at the interface. We argue that hydrophobic forces and electrostatic interactions play important rules for the association between DPPC and HA. Surface pressure area isotherms were used to determine the influence of HA on the phase behavior of DPPC while electrophoretic mobility measurements were used to gain insight into the binding of calcium ions to DPPC vesicles and hyaluronan.

Tethered poly(2-isopropyl-2-oxazoline) chains: temperature effects on layer structure and interactions probed by AFM experiments and modeling.

Thermoresponsive polymer layers on silica surfaces have been obtained by utilizing electrostatically driven adsorption of a cationic-nonionic diblock copolymer. The cationic block provides strong anchoring to the surface for the nonionic block of poly(2-isopropyl-2-oxazoline), referred to as PIPOZ. The PIPOZ chain interacts favorably with water at low temperatures, but above 46 °C aqueous solutions of PIPOZ phase separate as water becomes a poor solvent for the polymer. We explore how a change in solvent condition affects interactions between such adsorbed layers and report temperature effects on both normal forces and friction forces. To gain further insight, we utilize self-consistent lattice mean-field theory to follow how changes in temperature affect the polymer segment density distributions and to calculate surface force curves. We find that with worsening of the solvent condition an attraction develops between the adsorbed PIPOZ layers, and this observation is in good agreement with predictions of the mean-field theory. The modeling also demonstrates that the segment density profile and the degree of chain interpenetration under a given load between two PIPOZ-coated surfaces rise significantly with increasing temperature.

Temperature-dependent adsorption and adsorption hysteresis of a thermoresponsive diblock copolymer.

A nonionic-cationic diblock copolymer, poly(2-isopropyl-2-oxazoline)60-b-poly((3-acrylamidopropyl)trimethylammonium chloride)17, (PIPOZ60-b-PAMPTMA17), was utilized to electrostatically tether temperature-responsive PIPOZ chains to silica surfaces by physisorption. The effects of polymer concentration, pH, and temperature on adsorption were investigated using quartz crystal microbalance with dissipation monitoring and ellipsometry. The combination of these two techniques allows thorough characterization of the adsorbed layer in terms of surface excess, thickness, and water content. The high affinity of the cationic PAMPTMA17 block to the negatively charged silica surface gives rise to a high affinity adsorption isotherm, leading to (nearly) irreversible adsorption with respect to dilution. An increase in solution pH lowers the affinity of PIPOZ to silica but enhances the adsorption of the cationic block due to increasing silica surface charge density, which leads to higher adsorption of the cationic diblock copolymer. Higher surface excess is also achieved at higher temperatures due to the worsening of the solvent quality of water for the PIPOZ block. Interestingly, a large hysteresis in adsorbed mass and other layer properties was observed when the temperature was cycled from 25 to 45 °C and then back to 25 °C. Possible causes for this temperature hysteresis are discussed.

Comparison of a brush-with-anchor and a train-of-brushes mucin on poly(methyl methacrylate) surfaces: adsorption, surface forces, and friction.

Interfacial properties of two types of mucins have been investigated at the aqueous solution/poly(methyl methacrylate) (PMMA) interface. One is commercially available bovine submaxillary mucin, BSM, which consists of alternating glycosylated and nonglycosylated regions. The other one is a recombinant mucin-type fusion protein, PSGL-1/mIgG2b, consisting of a glycosylated mucin part fused to the Fc part of an immunoglobulin. PSGL-1/mIgG2b is mainly expressed as a dimer upon production. A quartz crystal microbalance with dissipation was used to study the adsorption of the mucins to PMMA surfaces. The mass of the adsorbed mucin layers, including the adsorbed mucin and water trapped in the layer, was found to be significantly higher for PSGL-1/mIgG2b than for BSM. Atomic force microscopy with colloidal probe was employed to study interactions and frictional forces between mucin-coated PMMA surfaces. Purely repulsive forces of steric origin were observed between PSGL-1/mIgG2b mucin layers, whereas a small adhesion was detected between BSM layers and attributed to bridging. Both mucin layers reduced the friction force between PMMA surfaces in aqueous solution. The reduction was, however, significantly more pronounced for PSGL-1/mIgG2b. The effective friction coefficient between PSGL-1/mIgG2b-coated PMMA surfaces is as low as 0.02 at low loads, increasing to 0.24 at the highest load explored, 50 nN. In contrast, a friction coefficient of around 0.7 was obtained between BSM-coated PMMA surfaces. The large differences in interfacial properties for the two mucins are discussed in relation to their structural differences.

Nanostructured composite layers of mussel adhesive protein and ceria nanoparticles.

Mussel adhesive proteins are known for their high affinity to a range of different surfaces, and they therefore appear as ideal candidates for producing thin inorganic-organic composite films with high robustness. In this work we explore the possibility of making cohesive films utilizing layer-by-layer deposition of the highly positively charged mussel adhesive protein, Mefp-1, and negatively charged ceria nanoparticles. This particular material combination was chosen due to recent findings that such films provide good corrosion protection. Quartz crystal microbalance with dissipation monitoring (QCM-D) was used for following the film formation process in situ on silica surfaces. A close to linear growth of the film with number of deposited layers was found for up to 18 deposition steps, the highest number of depositions investigated in this work. The Mefp-1 concentration during film deposition affected the film properties, where a higher protein concentration resulted in a stiffer film. It was also found that the added mass could be amplified by using a Mefp-1 solution containing small aggregates. The surface nanomechanical properties of dried multilayer films were investigated using peak force QNM (quantitative nanomechanical mapping) in air. Homogeneous surface coverage was found under all conditions explored, and the Young's modulus of the outer region of the coating increased when a higher Mefp-1 concentration was used during film deposition. The nature of the outermost surface layer was found to significantly affect the surface nanomechanical properties. The abrasion resistance of the coating was measured by using controlled-force contact mode AFM.

Kinetic and equilibrium aspects of adsorption and desorption of class II hydrophobins HFBI and HFBII at silicon oxynitride/water and air/water interfaces.

Hydrophobins are relatively small globular proteins produced by filamentous fungi. They display unusual high surface activity and are implied as mediators of attachment to surfaces, which has resulted in high scientific and technological interest. In this work we focus on kinetic and equilibrium aspects of adsorption and desorption properties of two representatives of class II hydrophobins, namely HFBI and HFBII, at a negatively charged hydrophilic solid/water interface and at the air/water interface. The layers formed at the air/liquid interface were examined in a Langmuir trough, whereas layers formed at the solid/liquid interface were studied using dual polarization interferometry (DPI) under different flow conditions. For comparison, another globular protein, lysozyme, was also investigated. It was found that both the adsorbed amount and the adsorption kinetics were different for HFBI and HFBII, and the adsorption behavior of both hydrophobins on the negatively charged surface displayed some unusual features. For instance, even though the adsorption rate for HFBI was slowed down with increasing adsorbed amount as expected from packing constraints at the interface, the adsorption kinetics curves for HFBII displayed a region indicating adsorption cooperativity. Further, it was found that hydrophobin layers formed under flow partly desorbed when the flow was stopped, and the desorption rate for HFBII was enhanced in the presence of hydrophobins in solution.

Hyaluronan and phospholipid association in biolubrication.

It is becoming increasingly clear that the outstanding lubrication of synovial joints is achieved by a sophisticated hierarchical structure of cartilage combined with synergistic actions of surface-active components present in the synovial fluid. In this work we focus on the association of two components of the synovial fluid, hyaluronan and dipalmitoyl phosphatidyl choline (DPPC), in bulk solution and at interfaces. We demonstrate that hyaluronan associates with DPPC vesicles and adsorbs to supported DPPC bilayers. The association structures formed at the interface are sufficiently stable to allow sequential adsorption of DPPC and hyaluronan, whereby promoting the formation of thick composite layers of these two components. The lubricating ability of such composite layers was probed by the AFM colloidal probe technique and found to be very favorable with low friction coefficients and high load bearing capacity. With DPPC as the last adsorbed component, a friction coefficient of 0.01 was found up to pressures significantly above what is encountered in healthy synovial joints. Hyaluronan as the last added component increases the friction coefficient to 0.03 and decreases the load bearing capacity somewhat (but still above what is needed in the synovial joint). Our data demonstrate that self-assembly structures formed by hyaluronan and phospholipids at interfaces are efficient aqueous lubricants, and it seems plausible that such self-assembly structures contribute to the exceptional lubrication of synovial joints.

Electrostatically anchored branched brush layers.

A novel type of block copolymer has been synthesized. It consists of a linear cationic block and an uncharged bottle-brush block. The nonionic bottle-brush block contains 45 units long poly(ethylene oxide) side chains. This polymer was synthesized with the intention of creating branched brush layers firmly physisorbed to negatively charged surfaces via the cationic block, mimicking the architecture (but not the chemistry) of bottle-brush molecules suggested to be present on the cartilage surface, and contributing to the efficient lubrication of synovial joints. The adsorption properties of the diblock copolymer as well as of the two blocks separately were studied on silica surfaces using quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry. The adsorption kinetics data highlight that the diblock copolymers initially adsorb preferentially parallel to the surface with both the cationic block and the uncharged bottle-brush block in contact with the surface. However, as the adsorption proceeds, a structural change occurs within the layer, and the PEO bottle-brush block extends toward solution, forming a surface-anchored branched brush layer. As the adsorption plateau is reached, the diblock copolymer layer is 46-48 nm thick, and the water content in the layer is above 90 wt %. The combination of strong electrostatic anchoring and highly hydrated branched brush structures provide strong steric repulsion, low friction forces, and high load bearing capacity. The strong electrostatic anchoring also provides high stability of preadsorbed layers under different ionic strength conditions.

Cationic poly(N-isopropylacrylamide) block copolymer adsorption investigated by dual polarization interferometry and lattice mean-field theory.

A series of cationic diblock copolymers, poly(N-isopropylacrylamide)(48)-block-poly((3-acrylamidopropyl)trimethylammonium chloride)(X), abbreviated as PNIPAAM(48)-b-PAMPTMA(+)(X) (X = 0, 6, 10, 14, and 20), has been synthesized, and their adsorption onto silicon oxynitride from aqueous solution has been investigated using dual polarization interferometry. The polymer adsorption was modeled by using a lattice mean-field theory, and a satisfactory consistency between theory and experiments was found in terms of surface excess and layer thickness. Both theory and experiments show that the adsorption is limited by steric repulsion for X < X(max) and by electrostatic interactions for X > X(max). Modeling demonstrates that significant surface charge regulation occurs due to adsorption. Both the nonionic and cationic block exhibit nonelectrostatic affinity to silicon oxynitride and thus contribute to the driving force for adsorption, and modeling is used for clarifying how changes in the nonelectrostatic affinity affects the surface excess. The segments of the nonionic and cationic blocks seem less segregated when both have a nonelectrostatic affinity for the surface compared to the case where the segments had no surface affinity. Adsorption kinetics was investigated experimentally. Two kinetic regimes were observed: the adsorption rate is initially controlled by the mass transfer rate to the surface and at higher coverage is limited by the attachment rate.