Multimodal Miniature Surface Forces Apparatus (μSFA) for Interfacial Science Measurements

Advances in the research of intermolecular and surface interactions result from the development of new and improved measurement techniques and combinations of existing techniques. Here, we present a new miniature version of the surface forces apparatus-the μSFA-that has been designed for ease of use and multimodal capabilities with the retention of the capabilities of other SFA models including accurate measurements of the surface separation distance and physical characterization of dynamic and static physical forces (i.e., normal, shear, and friction) and interactions (e.g., van der Waals, electrostatic, hydrophobic, steric, and biospecific). The small physical size of the μSFA, compared to previous SFA models, makes it portable and suitable for integration into commercially available optical and fluorescence light microscopes, as demonstrated here. The large optical path entry and exit ports make it ideal for concurrent force measurements and spectroscopy studies. Examples of the use of the μSFA in combination with surface plasmon resonance (SPR) and Raman spectroscopy measurements are presented. Because of the short working distance constraints associated with Raman spectroscopy, an interferometric technique was developed and applied to calculate the intersurface separation distance based on Newton's rings. The introduction of the μSFA will mark a transition in SFA usage from primarily physical characterization to concurrent physical characterization with in situ chemical and biological characterization to study interfacial phenomena, including (but not limited to) molecular adsorption, fluid flow dynamics, the determination of surface species and morphology, and (bio)molecular binding kinetics.

Electrochemically Enhanced Dissolution of Silica and Alumina in Alkaline Environments

Dissolution of mineral surfaces at asymmetric solid-liquid-solid interfaces in aqueous solutions occurs in technologically relevant processes, such as chemical/mechanical polishing (CMP) for semiconductor fabrication, formation and corrosion of structural materials, and crystallization of materials relevant to heterogeneous catalysis or drug delivery. In some such processes, materials at confined interfaces exhibit dissolution rates that are orders of magnitude larger than dissolution rates of isolated surfaces. Here, the dissolution of silica and alumina in close proximity to a charged gold surface or mica in alkaline solutions of pH 10-11 is shown to depend on the difference in electrostatic potentials of the surfaces, as determined from measurements conducted using a custom-built electrochemical pressure cell and a surface forces apparatus (SFA). The enhanced dissolution is proposed to result from overlap of the electrostatic double layers between the dissimilar charged surfaces at small intersurface separation distances (<1 Debye length). A semiquantitative model shows that overlap of the electric double layers can change the magnitude and direction of the electric field at the surface with the less negative potential, which results in an increase in the rate of dissolution of that surface. When the surface electrochemical properties were changed, the dissolution rates of silica and alumina were increased by up to 2 orders of magnitude over the dissolution rates of isolated compositionally similar surfaces under otherwise identical conditions. The results provide new insights on dissolution processes that occur at solid-liquid-solid interfaces and yield design criteria for controlling dissolution through electrochemical modification, with relevance to diverse technologies.

Surface Damage Influences the JKR Contact Mechanics of Glassy Low-Molecular-Weight Polystyrene Films

Using a surface forces apparatus (SFA), we quantitatively study the influence of surface damage on the contact mechanics of self-mated glassy polystyrene (PS) films. We use the SFA to measure the contact radius, surface profile, and normal force between the films, including the adhesion force. The molecular weight (MW) of the polymer influences the repeatability of the adhesion measurements and the effective surface energy calculated using the Johnson-Kendall-Roberts (JKR) theory. For low-MW PS (MW = 2.33 kDa), the effective surface energy increases over repeated adhesion cycles as the films become progressively damaged. For high-MW PS (MW = 280 kDa), the effective surface energy is constant over repeated adhesion cycles, but hysteresis is still present, manifested in a smaller contact radius during compression of the surfaces than during separation. Our results demonstrate that while the JKR theory is appropriate for describing the contact mechanics of glassy polymer thin films on layered elastic substrates, the contact mechanics of low-MW polymer films can be complicated by surface damage to the films.

Automated Measurement of Spatially Resolved Hair-Hair Single Fiber Adhesion

The adhesion force between individual human hair fibers in a crosshair geometry was measured by observing their natural bending and adhesive jumps out of contact, using optical video microscopy. The hair fibers' natural elastic responses, calibrated by measuring their natural resonant frequencies, were used to measure the forces. Using a custom-designed, automated apparatus to measure thousands of individual hair-hair contacts along millimeter length scales of hair, it was found that a broad, yet characteristic, spatially variant distribution in adhesion force is measured on the 1 to 1000 nN scale for both clean and conditioner-treated hair fibers. Comparison between the measured adhesion forces and adhesion forces modeled from the hairs' surface topography (measured using confocal laser profilometry) shows they have a good order-of-magnitude agreement and have similar breadth and shape. The agreement between the measurements and the model suggests, perhaps unsurprisingly, that hair-hair adhesion is governed, to a first approximation, by the unique surface structure of the hairs' cuticles and, therefore, the large distribution in local mean curvature at the various individual contact points along the hairs' lengths. We posit that haircare products could best control the surface properties (or at least the adhesive properties) between hairs by directly modifying the hair surface microstructure.

Triple Function Lubricant Additives Based on Organic-Inorganic Hybrid Star Polymers: Friction Reduction, Wear Protection, and Viscosity Modification

Polymer-based lubricant additives for friction reduction, wear protection, or viscosity improvement have been widely studied. However, single additives achieving all three functions are rare. To address this need, we have explored the combination of polymer topology with organic-inorganic hybrid chemistry to simultaneously vary the temperature- and shear-dependent properties of polymer additives in solution and at solid surfaces. A topological library of lubricant additives, based on statistical copolymers of stearyl methacrylate and methyl methacrylate, ranging from linear to branched star architectures, was prepared using ruthenium-catalyzed controlled radical polymerization. Control over the polymerization yielded additives with low dispersity and comparable molecular weights, allowing evaluation of the influence of polymer architecture on friction reduction, wear protection, and bulk viscosity improvement in a commercial base oil (Yubase 4). Structure-performance relationships for these functions were assessed by a combination of a high-speed surface force apparatus (HS-SFA) experiments, wear track profilometry, quartz crystal microbalance analysis, and solution viscometry. The custom-built HS-SFA provides a unique experimental environment to measure the boundary lubrication performance under extreme shear rates (≈10⁷ s^-1) for prolonged times (24 h), mimicking the extreme conditions of automotive applications. These experiments revealed that the performance of the additives as boundary lubricants and wear protectants scales with the degree of branching. The branched architectures prohibit ordering of the additives in thin films under high-load conditions, leading to a thicker absorbed polymer brush boundary layer and therefore enhanced film fluidity and lubricity. Additionally, star polymers with increasing arm number lead to bulk viscosity modification, reflected by a significant increase in the viscosity index compared to the commercial base oil. Although outperformed by linear polymers for bulk viscosity improvement, the (hybrid) star polymers successfully combine the three distinct lubricant additive functions: friction reduction, wear protection, and bulk viscosity improvement-in a single polymeric structure. It should also be noted that, judging from HS-SFA experiments, hybrid stars carrying a silicate-based core outperform their fully organic analogues as boundary lubricants. The enhanced performance is most likely driven by attractive forces between the silicate cores and the employed metallic surfaces. Combining three function in one minimizes formulation complexity and thus opens a route to fundamentally understand and formulate key design parameters for the development of novel multifunction lubricant additives.

Time-Dependent Physicochemical Changes of Carbonate Surfaces from SmartWater (Diluted Seawater) Flooding Processes for Improved Oil Recovery

Over the past few decades, field- and laboratory-scale studies have shown enhancements in oil recovery when reservoirs, which contain high-salinity formation water (FW), are waterflooded with modified-salinity salt water (widely referred to as the low-salinity, dilution, or SmartWater effect for improved oil recovery). In this study, we investigated the time dependence of the physicochemical processes that occur during diluted seawater (i.e., SmartWater) waterflooding processes of specific relevance to carbonate oil reservoirs. We measured the changes to oil/water/rock wettability, surface roughness, and surface chemical composition during SmartWater flooding using 10-fold-diluted seawater under mimicked oil reservoir conditions with calcite and carbonate reservoir rocks. Distinct effects due to SmartWater flooding were observed and found to occur on two different timescales: (1) a rapid (<15 min) increase in the colloidal electrostatic double-layer repulsion between the rock and oil across the SmartWater, leading to a decreased oil/water/rock adhesion energy and thus increased water wetness and (2) slower (>12 h to complete) physicochemical changes of the calcite and carbonate reservoir rock surfaces, including surface roughening via the dissolution of rock and the reprecipitation of dissolved carbonate species after exchanging key ions (Ca²⁺, Mg²⁺, CO₃^2-, and SO₄^2- in carbonates) with those in the flooding SmartWater. Our experiments using crude oil from a carbonate reservoir reveal that these reservoir rock surfaces are covered with organic-ionic preadsorbed films (ad-layers), which the SmartWater removes (detaches) as flakes. Removal of the organic-ionic ad-layers by SmartWater flooding enhances oil release from the surfaces, which was found to be critical to increasing the water wetness and significantly improving oil removal from carbonates. Additionally, the increase in water wetness is further enhanced by roughening of the rock surfaces, which decreases the effective contact (interaction) area between the oil and rock interfaces. Furthermore, we found that the rate of these slower physicochemical changes to the carbonate rock surfaces increases with increasing temperature (at least up to an experimental temperature of 75 °C). Our results suggest that the effectiveness of improved oil recovery from SmartWater flooding depends strongly on the formation of the organic-ionic ad-layers. In oil reservoirs where the ad-layer is fully developed and robust, injecting SmartWater would lead to significant removal of the ad-layer and improved oil recovery.

Modulation of Hydrophobic Interaction by Mediating Surface Nanoscale Structure and Chemistry, not Monotonically by Hydrophobicity

The hydrophobic (HB) interaction plays a critical role in many colloidal and interfacial phenomena, biophysical and industrial processes. Surface hydrophobicity, characterized by the water contact angle, is generally considered the most dominant parameter determining the HB interaction. Herein, we quantified the HB interactions between air bubbles and a series of hydrophobic surfaces with different nanoscale structures and surface chemistry in aqueous media using a bubble probe atomic force microscopy (AFM). Surprisingly, it is discovered that surfaces of similar hydrophobicity can show different ranges of HB interactions, while surfaces of different hydrophobicity can have similar ranges of HB interaction. The increased heterogeneity of the surface nanoscale structure and chemistry can effectively decrease the decay length of HB interaction from 1.60 nm to 0.35 nm. Our work provides insights into the physical mechanism of HB interaction.

Toughening elastomers using mussel-inspired iron-catechol complexes

Materials often exhibit a trade-off between stiffness and extensibility; for example, strengthening elastomers by increasing their cross-link density leads to embrittlement and decreased toughness. Inspired by cuticles of marine mussel byssi, we circumvent this inherent trade-off by incorporating sacrificial, reversible iron-catechol cross-links into a dry, loosely cross-linked epoxy network. The iron-containing network exhibits two to three orders of magnitude increases in stiffness, tensile strength, and tensile toughness compared to its iron-free precursor while gaining recoverable hysteretic energy dissipation and maintaining its original extensibility. Compared to previous realizations of this chemistry in hydrogels, the dry nature of the network enables larger property enhancement owing to the cooperative effects of both the increased cross-link density given by the reversible iron-catecholate complexes and the chain-restricting ionomeric nanodomains that they form.

Simple peptide coacervates adapted for rapid pressure-sensitive wet adhesion

We report here that a dense liquid formed by spontaneous condensation, also known as simple coacervation, of a single mussel foot protein-3S-mimicking peptide exhibits properties critical for underwater adhesion. A structurally homogeneous coacervate is deposited on underwater surfaces as micrometer-thick layers, and, after compression, displays orders of magnitude higher underwater adhesion at 2 N m^-1 than that reported from thin films of the most adhesive mussel-foot-derived peptides or their synthetic mimics. The increase in adhesion efficiency does not require nor rely on post-deposition curing or chemical processing, but rather represents an intrinsic physical property of the single-component coacervate. Its wet adhesive and rheological properties correlate with significant dehydration, tight peptide packing and restriction in peptide mobility. We suggest that such dense coacervate liquids represent an essential adaptation for the initial priming stages of mussel adhesive deposition, and provide a hitherto untapped design principle for synthetic underwater adhesives.

Duplicating Dynamic Strain-Stiffening Behavior and Nanomechanics of Biological Tissues in a Synthetic Self-Healing Flexible Network Hydrogel

Biological tissues can accurately differentiate external mechanical stresses and actively select suitable strategies (e.g., reversible strain-stiffening, self-healing) to sustain or restore their integrity and related functionalities as required. Synthetic materials that can imitate the characteristics of biological tissues have a wide range of engineering and bioengineering applications. However, no success has been demonstrated to realize such strain-stiffening behavior in synthetic networks, particularly using flexible polymers, which has remained a great challenge. Here, we present one such synthetic hydrogel material prepared from two flexible polymers (polyethylene glycol and branched polyethylenimine) that exhibits both strain-stiffening and self-healing capabilities. The developed synthetic hydrogel network not only mimics the main features of biological mechanically responsive systems but also autonomously self-heals after becoming damaged, thereby recovering its full capacity to perform its normal physiological functions.

Significant Performance Enhancement of Polymer Resins by Bioinspired Dynamic Bonding

Marine mussels use catechol-rich interfacial mussel foot proteins (mfps) as primers that attach to mineral surfaces via hydrogen, metal coordination, electrostatic, ionic, or hydrophobic bonds, creating a secondary surface that promotes bonding to the bulk mfps. Inspired by this biological adhesive primer, it is shown that a ≈1 nm thick catecholic single-molecule priming layer increases the adhesion strength of crosslinked polymethacrylate resin on mineral surfaces by up to an order of magnitude when compared with conventional primers such as noncatecholic silane- and phosphate-based grafts. Molecular dynamics simulations confirm that catechol groups anchor to a variety of mineral surfaces and shed light on the binding mode of each molecule. Here, a ≈50% toughness enhancement is achieved in a stiff load-bearing polymer network, demonstrating the utility of mussel-inspired bonding for processing a wide range of polymeric interfaces, including structural, load-bearing materials.

Contact Angle and Adhesion Dynamics and Hysteresis on Molecularly Smooth Chemically Homogeneous Surfaces

Measuring truly equilibrium adhesion energies or contact angles to obtain the thermodynamic values is experimentally difficult because it requires loading/unloading or advancing/receding boundaries to be measured at rates that can be slower than 1 nm/s. We have measured advancing-receding contact angles and loading-unloading adhesion energies for various systems and geometries involving molecularly smooth and chemically homogeneous surfaces moving at different but steady velocities in both directions, ±V, focusing on the thermodynamic limit of ±V → 0. We have used the Bell Theory (1978) to derive expressions for the dynamic (velocity-dependent) adhesion energies and contact angles suitable for both (i) dynamic adhesion measurements using the classic Johnson-Kendall-Roberts (JKR, 1971) theory of "contact mechanics" and (ii) dynamic contact angle hysteresis measurements of both rolling droplets and syringe-controlled (sessile) droplets on various surfaces. We present our results for systems that exhibited both steady and varying velocities from V ≈ 10 mm/s to 1 nm/s, where in all cases but one, the advancing (V > 0) and receding (V < 0) adhesion energies and/or contact angles converged toward the same theoretical (thermodynamic) values as V → 0. Our equations for the dynamic contact angles are similar to the classic equations of Blake & Haynes (1969) and fitted the experimental adhesion data equally well over the range of velocities studied, although with somewhat different fitting parameters for the characteristic molecular length/dimension or area and characteristic bond formation/rupture lifetime or velocity. Our theoretical and experimental methods and results unify previous kinetic theories of adhesion and contact angle hysteresis and offer new experimental methods for testing kinetic models in the thermodynamic, quasi-static, limit. Our analyses are limited to kinetic effects only, and we conclude that hydrodynamic, i.e., viscous, and inertial effects do not play a role at the interfacial velocities of our experiments, i.e., V < (1-10) mm/s (for water and hexadecane, but for viscous polymers it may be different), consistent with previously reported studies.

Influence of Humidity on Grip and Release Adhesion Mechanisms for Gecko-Inspired Microfibrillar Surfaces

Geckos have developed foot pads that allow them to maintain their unique climbing ability despite vast differences of surfaces and environments, from dry desert to humid rainforest. Likewise, successful gecko-inspired mimics should exhibit adhesive and frictional performance across a similarly diverse range of climates. In this work, we focus on the effect of relative humidity (RH) on the "frictional-adhesion" behavior of gecko-inspired adhesive pads. A surface forces apparatus was used to quantitatively measure adhesion and friction forces of a microfibrillar cross-linked polydimethylsiloxane surface against a smooth hemispherical glass disk at varying relative humidity, from 0 to 100% (including fully submerged under water). Geometrically anisotropic tilted half-cylinder microfibers yield a "grip state" (high adhesion and friction forces after shearing along the tilt of the fibers, F_ad⁺ and F_∥⁺) and a "release state" (low adhesion and friction after shearing against the tilt of the fibers, F_ad^- and F_∥^-). By appropriate control of the loading path, this allows for transition between strong attachment and easy detachment. Changing the preload and shear direction gives rise to differences in the effective contact area at each fiber and the microscale and nanoscale structure of the contact while changing the relative humidity results in differences in the relative contributions of van der Waals and capillary forces. In combination, both effects lead to interesting trends in the adhesion and friction forces. At up to 75% RH, the grip state adhesion force remains constant and the ratio of grip to release adhesion force does not drop below 4.0. In addition, the friction forces F_∥⁺ and F_∥^- and the release state adhesion force F_ad^- exhibit a maximum at intermediate relative humidity between 40% and 75%.

Tuning underwater adhesion with cation-π interactions

Cation-π interactions drive the self-assembly and cohesion of many biological molecules, including the adhesion proteins of several marine organisms. Although the origin of cation-π bonds in isolated pairs has been extensively studied, the energetics of cation-π-driven self-assembly in molecular films remains uncharted. Here we use nanoscale force measurements in combination with solid-state NMR spectroscopy to show that the cohesive properties of simple aromatic- and lysine-rich peptides rival those of the strong reversible intermolecular cohesion exhibited by adhesion proteins of marine mussel. In particular, we show that peptides incorporating the amino acid phenylalanine, a functional group that is conspicuously sparing in the sequences of mussel proteins, exhibit reversible adhesion interactions significantly exceeding that of analogous mussel-mimetic peptides. More broadly, we demonstrate that interfacial confinement fundamentally alters the energetics of cation-π-mediated assembly: an insight that should prove relevant for diverse areas, which range from rationalizing biological assembly to engineering peptide-based biomaterials.

Communication: Contrasting effects of glycerol and DMSO on lipid membrane surface hydration dynamics and forces

Glycerol and dimethyl sulfoxide (DMSO) are commonly used cryoprotectants in cellular systems, but due to the challenges of measuring the properties of surface-bound solvent, fundamental questions remain regarding the concentration, interactions, and conformation of these solutes at lipid membrane surfaces. We measured the surface water diffusivity at gel-phase dipalmitoylphosphatidylcholine (DPPC) bilayer surfaces in aqueous solutions containing ≤7.5 mol. % of DMSO or glycerol using Overhauser dynamic nuclear polarization. We found that glycerol similarly affects the diffusivity of water near the bilayer surface and that in the bulk solution (within 20%), while DMSO substantially increases the diffusivity of surface water relative to bulk water. We compare these measurements of water dynamics with those of equilibrium forces between DPPC bilayers in the same solvent mixtures. DMSO greatly decreases the range and magnitude of the repulsive forces between the bilayers, whereas glycerol increases it. We propose that the differences in hydrogen bonding capability of the two solutes leads DMSO to dehydrate the lipid head groups, while glycerol affects surface hydration only as much as it affects the bulk water properties. The results suggest that the mechanism of the two most common cryoprotectants must be fundamentally different: in the case of DMSO by decoupling the solvent from the lipid surface, and in the case of glycerol by altering the hydrogen bond structure and intermolecular cohesion of the global solvent, as manifested by increased solvent viscosity.

Long range electrostatic forces in ionic liquids

Experimental evidence for long range surface forces in ionic liquids is collated and examined, key outstanding questions are identified, and possible mechanisms underpinning these long range forces are explored.

An Underwater Surface-Drying Peptide Inspired by a Mussel Adhesive Protein

Water hampers the formation of strong and durable bonds between adhesive polymers and solid surfaces, in turn hindering the development of adhesives for biomedical and marine applications. Inspired by mussel adhesion, a mussel foot protein homologue (mfp3S-pep) is designed, whose primary sequence is designed to mimic the pI, polyampholyte, and hydrophobic characteristics of the native protein. Noticeably, native protein and synthetic peptide exhibit similar abilities to self-coacervate at given pH and ionic strength. 3,4-dihydroxy-l-phenylalanine (Dopa) proves necessary for irreversible peptide adsorption to both TiO2 (anatase) and hydroxyapatite (HAP) surfaces, as confirmed by quartz crystal microbalance measurements, with the coacervate showing superior adsorption. The adsorption of Dopa-containing peptides is investigated by attenuated total reflection infrared spectroscopy, revealing initially bidentate coordinative bonds on TiO2, followed by H-bonded and eventually long-ranged electrostatic and Van der Waals interactions. On HAP, mfp3s-pep-3Dopa adsorption occurs predominantly via H-bond and outer-sphere complexes of the catechol groups. Importantly, only the Dopa-bearing compounds are able to remove interfacial water from the target surfaces, with the coacervate achieving the highest water displacement arising from its superior wetting properties. These findings provide an impetus for developing coacervated Dopa-functionalized peptides/polymers adhesive formulations for a variety of applications on wet polar surfaces.

Underwater contact adhesion and microarchitecture in polyelectrolyte complexes actuated by solvent exchange

Polyelectrolyte complexation is critical to the formation and properties of many biological and polymeric materials, and is typically initiated by aqueous mixing followed by fluid-fluid phase separation, such as coacervation. Yet little to nothing is known about how coacervates evolve into intricate solid microarchitectures. Inspired by the chemical features of the cement proteins of the sandcastle worm, here we report a versatile and strong wet-contact microporous adhesive resulting from polyelectrolyte complexation triggered by solvent exchange. After premixing a catechol-functionalized weak polyanion with a polycation in dimethyl sulphoxide (DMSO), the solution was applied underwater to various substrates whereupon electrostatic complexation, phase inversion, and rapid setting were simultaneously actuated by water-DMSO solvent exchange. Spatial and temporal coordination of complexation, inversion and setting fostered rapid (∼25 s) and robust underwater contact adhesion (Wad ≥ 2 J m(-2)) of complexed catecholic polyelectrolytes to all tested surfaces including plastics, glasses, metals and biological materials.

Time-Dependent Wetting Behavior of PDMS Surfaces with Bioinspired, Hierarchical Structures

Wetting of rough surfaces involves time-dependent effects, such as surface deformations, nonuniform filling of surface pores within or outside the contact area, and surface chemistries, but the detailed impact of these phenomena on wetting is not entirely clear. Understanding these effects is crucial for designing coatings for a wide range of applications, such as membrane-based oil-water separation and desalination, waterproof linings/windows for automobiles, aircrafts, and naval vessels, and antibiofouling. Herein, we report on time-dependent contact angles of water droplets on a rough polydimethylsiloxane (PDMS) surface that cannot be completely described by the conventional Cassie-Baxter or Wenzel models or the recently proposed Cassie-impregnated model. Shells of sand dollars (Dendraster excentricus) were used as lithography-free, robust templates to produce rough PDMS surfaces with hierarchical, periodic features ranging from 1 × 10(-7) to 1 × 10(-4) m. Under saturated vapor conditions, we found that in the short term (<1 min), the contact angle of a sessile water droplet on the templated PDMS, θ(SDT) = 140 ± 3°, was accurately described by the Cassie-Baxter model (predicted θ(SDT) = 137°); however, after 90 min, θ(SDT) fell to 110°. Fluorescent confocal microscopy confirmed that the initial reduction in θ(SDT) to 110° (the Wenzel limit) was primarily a Cassie-Baxter to Wenzel transition during which pores within the contact area filled gradually, and more rapidly for ethanol-water mixtures. After 90 min, the contact line of the water droplet became pinned, perhaps caused by viscoelastic deformation of the PDMS around the contact line, and a significant volume of water began to flow from the droplet to pores outside the contact region, causing θ(SDT) to decrease to 65° over 48 h on the rough surface. The system we present here to explore the concept of contact angle time dependence (dynamics) and modeling of natural surfaces provides insights into the design and development of long- and short-lived coatings.

Mussel-Inspired Anchoring of Polymer Loops That Provide Superior Surface Lubrication and Antifouling Properties

We describe robustly anchored triblock copolymers that adopt loop conformations on surfaces and endow them with unprecedented lubricating and antifouling properties. The triblocks have two end blocks with catechol-anchoring groups and a looping poly(ethylene oxide) (PEO) midblock. The loops mediate strong steric repulsion between two mica surfaces. When sheared at constant speeds of ∼2.5 μm/s, the surfaces exhibit an extremely low friction coefficient of ∼0.002-0.004 without any signs of damage up to pressures of ∼2-3 MPa that are close to most biological bearing systems. Moreover, the polymer loops enhance inhibition of cell adhesion and proliferation compared to polymers in the random coil or brush conformations. These results demonstrate that strongly anchored polymer loops are effective for high lubrication and low cell adhesion and represent a promising candidate for the development of specialized high-performance biomedical coatings.

α,β-Dehydro-Dopa: A Hidden Participant in Mussel Adhesion

Dopa (l-3,4-dihydroxyphenylalanine) is a key chemical signature of mussel adhesive proteins, but its susceptibility to oxidation has limited mechanistic investigations as well as practical translation to wet adhesion technology. To investigate peptidyl-Dopa oxidation, the highly diverse chemical environment of Dopa in mussel adhesive proteins was simplified to a peptidyl-Dopa analogue, N-acetyl-Dopa ethyl ester. On the basis of cyclic voltammetry and UV-vis spectroscopy, the Dopa oxidation product at neutral to alkaline pH was shown to be α,β-dehydro-Dopa (ΔD), a vinylcatecholic tautomer of Dopa-quinone. ΔD exhibited an adsorption capacity on TiO2 20-fold higher than that of the Dopa homologue in the quartz crystal microbalance. Cyclic voltammetry confirmed the spontaneity of ΔD formation in mussel foot protein 3F at neutral pH that is coupled to a change in protein secondary structure from random coil to β-sheet. A more complete characterization of ΔD reactivity adds a significant new perspective to mussel adhesive chemistry and the design of synthetic bioinspired adhesives.

Interaction Forces between Supported Lipid Bilayers in the Presence of PEGylated Polymers

Using the surface forces apparatus (SFA), interaction forces between supported lipid bilayers were measured in the presence of polyethylene glycol and two other commercially available pegylated triblock polymers, Pluronic F68 and F127. Pluronic F68 has a smaller central hydrophobic block compared to F127 and therefore is more hydrophilic. The study aimed to unravel the effects of polymer architecture and composition on the interactions between the bilayers. Our keys findings show that below the critical aggregation concentration (CAC) of the polymers, a soft, weakly anchored, polymer layer is formed on the surface of the bilayers. The anchoring strength of this physisorbed layer was found to increase significantly with the size of the hydrophobic block of the polymer, and was strongest for the more hydrophobic polymer, F127. Above the CAC, a dense polymer layer, exhibiting gel-like properties, was found to rapidly grow on the bilayers even after mechanical disruption. The cohesive interaction maintaining the gel layer structure was found to be stronger for F127, and was also found to promote the formation of highly structured aggregates on the bilayers.

High-performance mussel-inspired adhesives of reduced complexity

Despite the recent progress in and demand for wet adhesives, practical underwater adhesion remains limited or non-existent for diverse applications. Translation of mussel-inspired wet adhesion typically entails catechol functionalization of polymers and/or polyelectrolytes, and solution processing of many complex components and steps that require optimization and stabilization. Here we reduced the complexity of a wet adhesive primer to synthetic low-molecular-weight catecholic zwitterionic surfactants that show very strong adhesion (∼50 mJ m⁻²) and retain the ability to coacervate. This catecholic zwitterion adheres to diverse surfaces and self-assembles into a molecularly smooth, thin (<4 nm) and strong glue layer. The catecholic zwitterion holds particular promise as an adhesive for nanofabrication. This study significantly simplifies bio-inspired themes for wet adhesion by combining catechol with hydrophobic and electrostatic functional groups in a small molecule.

Mussels use strong filaments to adhere to rocks, preventing them from being swept away in strong currents. Here, the authors borrow and simplify chemistries from the mussel foot to create a one component adhesive system which holds potential for employment in nanofabrication protocols.

Mussel Coating Protein-Derived Complex Coacervates Mitigate Frictional Surface Damage

The role of friction in the functional performance of biomaterial interfaces is widely reckoned to be critical and complicated but poorly understood. To better understand friction forces, we investigated the natural adaptation of the holdfast or byssus of mussels that live in high-energy surf habitats. As the outermost covering of the byssus, the cuticle deserves particular attention for its adaptations to frictional wear under shear. In this study, we coacervated one of three variants of a key cuticular component, mussel foot protein 1, mfp-1 [(1) Mytilus californianus mcfp-1, (2) rmfp-1, and (3) rmfp-1-Dopa], with hyaluronic acid (HA) and investigated the wear protection capabilities of these coacervates to surfaces (mica) during shear. Native mcfp-1/HA coacervates had an intermediate coefficient of friction (μ ∼0.3) but conferred excellent wear protection to mica with no damage from applied loads, F_⊥, as high as 300 mN (pressure, P, > 2 MPa). Recombinant rmfp-1/HA coacervates exhibited a comparable coefficient of friction (μ ∼0.3); however, wear protection was significantly inferior (damage at F_⊥ > 60 mN) compared with that of native protein coacervates. Wear protection of rmfp-1/HA coacervates increased 5-fold upon addition of the surface adhesive group 3,4-dihydroxyphenylalanine, (Dopa). We propose a Dopa-dependent wear protection mechanism to explain the differences in wear protection between coacervates. Our results reveal a significant untapped potential for coacervates in applications that require adhesion, lubrication, and wear protection. These applications include artificial joints, contact lenses, dental sealants, and hair and skin conditioners.

Stick-slip friction of gecko-mimetic flaps on smooth and rough surfaces

The discovery and understanding of gecko 'frictional-adhesion' adhering and climbing mechanism has allowed researchers to mimic and create gecko-inspired adhesives. A few experimental and theoretical approaches have been taken to understand the effect of surface roughness on synthetic adhesive performance, and the implications of stick-slip friction during shearing. This work extends previous studies by using a modified surface forces apparatus to quantitatively measure and model frictional forces between arrays of polydimethylsiloxane gecko footpad-mimetic tilted microflaps against smooth and rough glass surfaces. Constant attachments and detachments occur between the surfaces during shearing, as described by an avalanche model. These detachments ultimately result in failure of the adhesion interface and have been characterized in this study. Stick-slip friction disappears with increasing velocity when the flaps are sheared against a smooth silica surface; however, stick-slip was always present at all velocities and loads tested when shearing the flaps against rough glass surfaces. These results demonstrate the significance of pre-load, shearing velocity, shearing distances, commensurability and shearing direction of gecko-mimetic adhesives and provide us a simple model for analysing and/or designing such systems.

Peptide Length and Dopa Determine Iron-Mediated Cohesion of Mussel Foot Proteins

Mussel adhesion to mineral surfaces is widely attributed to 3,4-dihydroxyphenylalanine (Dopa) functionalities in the mussel foot proteins (mfps). Several mfps, however, show a broad range (30-100%) of Tyrosine (Tyr) to Dopa conversion suggesting that Dopa is not the only desirable outcome for adhesion. Here, we used a partial recombinant construct of mussel foot protein-1 (rmfp-1) and short decapeptide dimers with and without Dopa and assessed both their cohesive and adhesive properties on mica using a surface forces apparatus (SFA). Our results demonstrate that at low pH, both the unmodified and Dopa-containing rmfp-1s show similar energies for adhesion to mica and self-self interaction. Cohesion between two Dopa-containing rmfp-1 surfaces can be doubled by Fe3+ chelation, but remains unchanged with unmodified rmfp-1. At the same low pH, the Dopa modified short decapeptide dimer did not show any change in cohesive interactions even with Fe3+. Our results suggest that the most probable intermolecular interactions are those arising from electrostatic (i.e., cation-π) and hydrophobic interactions. We also show that Dopa in a peptide sequence does not by itself mediate Fe3+ bridging interactions between peptide films: peptide length is a crucial enabling factor.

BIOLOGICAL ADHESIVES. Adaptive synergy between catechol and lysine promotes wet adhesion by surface salt displacement

In physiological fluids and seawater, adhesion of synthetic polymers to solid surfaces is severely limited by high salt, pH, and hydration, yet these conditions have not deterred the evolution of effective adhesion by mussels. Mussel foot proteins provide insights about adhesive adaptations: Notably, the abundance and proximity of catecholic Dopa (3,4-dihydroxyphenylalanine) and lysine residues hint at a synergistic interplay in adhesion. Certain siderophores—bacterial iron chelators—consist of paired catechol and lysine functionalities, thereby providing a convenient experimental platform to explore molecular synergies in bioadhesion. These siderophores and synthetic analogs exhibit robust adhesion energies (E(ad) ≥-15 millijoules per square meter) to mica in saline pH 3.5 to 7.5 and resist oxidation. The adjacent catechol-lysine placement provides a "one-two punch," whereby lysine evicts hydrated cations from the mineral surface, allowing catechol binding to underlying oxides.

Effects of Surfactants and Polyelectrolytes on the Interaction between a Negatively Charged Surface and a Hydrophobic Polymer Surface

We have measured and characterized how three classes of surface-active molecules self-assemble at, and modulate the interfacial forces between, a negatively charged mica surface and a hydrophobic end-grafted polydimethylsiloxane (PDMS) polymer surface in solution. We provide a broad overview of how chemical and structural properties of surfactant molecules result in different self-assembled structures at polymer and mineral surfaces, by studying three characteristic surfactants: (1) an anionic aliphatic surfactant, sodium dodecyl sulfate (SDS), (2) a cationic aliphatic surfactant, myristyltrimethylammonium bromide (MTAB), and (3) a silicone polyelectrolyte with a long-chain PDMS midblock and multiple cationic end groups. Through surface forces apparatus measurements, we show that the separate addition of three surfactants can result in interaction energies ranging from fully attractive to fully repulsive. Specifically, SDS adsorbs at the PDMS surface as a monolayer and modifies the monotonic electrostatic repulsion to a mica surface. MTAB adsorbs at both the PDMS (as a monolayer) and the mica surface (as a monolayer or bilayer), resulting in concentration-dependent interactions, including a long-range electrostatic repulsion, a short-range steric hydration repulsion, and a short-range hydrophobic attraction. The cationic polyelectrolyte adsorbs as a monolayer on the PDMS and causes a long-range electrostatic attraction to mica, which can be modulated to a monotonic repulsion upon further addition of SDS. Therefore, through judicious selection of surfactants, we show how to modify the magnitude and sign of the interaction energy at different separation distances between hydrophobic and hydrophilic surfaces, which govern the static and kinetic stability of colloidal dispersions. Additionally, we demonstrate how the charge density of silicone polyelectrolytes modifies both their self-assembly at polymer interfaces and the robust adhesion of thin PDMS films to target surfaces.

Microphase Behavior and Enhanced Wet-Cohesion of Synthetic Copolyampholytes Inspired by a Mussel Foot Protein

Numerous attempts have been made to translate mussel adhesion to diverse synthetic platforms. However, the translation remains largely limited to the Dopa (3,4-dihydroxyphenylalanine) or catechol functionality, which continues to raise concerns about Dopa's inherent susceptibility to oxidation. Mussels have evolved adaptations to stabilize Dopa against oxidation. For example, in mussel foot protein 3 slow (mfp-3s, one of two electrophoretically distinct interfacial adhesive proteins in mussel plaques), the high proportion of hydrophobic amino acid residues in the flanking sequence around Dopa increases Dopa's oxidation potential. In this study, copolyampholytes, which combine the catechol functionality with amphiphilic and ionic features of mfp-3s, were synthesized and formulated as coacervates for adhesive deposition on surfaces. The ratio of hydrophilic/hydrophobic as well as cationic/anionic units was varied in order to enhance coacervate formation and wet adhesion properties. Aqueous solutions of two of the four mfp-3s-inspired copolymers showed coacervate-like spherical microdroplets (ϕ ≈ 1-5 μm at pH ∼4 (salt concentration ∼15 mM). The mfp-3s-mimetic copolymer was stable to oxidation, formed coacervates that spread evenly over mica, and strongly bonded to mica surfaces (pull-off strength: ∼17.0 mJ/m(2)). Increasing pH to 7 after coacervate deposition at pH 4 doubled the bonding strength to ∼32.9 mJ/m(2) without oxidative cross-linking and is about 9 times higher than native mfp-3s cohesion. This study expands the scope of translating mussel adhesion from simple Dopa-functionalization to mimicking the context of the local environment around Dopa.

Real-time intermembrane force measurements and imaging of lipid domain morphology during hemifusion

Membrane fusion is the core process in membrane trafficking and is essential for cellular transport of proteins and other biomacromolecules. During protein-mediated membrane fusion, membrane proteins are often excluded from the membrane–membrane contact, indicating that local structural transformations in lipid domains play a major role. However, the rearrangements of lipid domains during fusion have not been thoroughly examined. Here using a newly developed Fluorescence Surface Forces Apparatus (FL-SFA), migration of liquid-disordered clusters and depletion of liquid-ordered domains at the membrane–membrane contact are imaged in real time during hemifusion of model lipid membranes, together with simultaneous force–distance and lipid membrane thickness measurements. The load and contact time-dependent hemifusion results show that the domain rearrangements decrease the energy barrier to fusion, illustrating the significance of dynamic domain transformations in membrane fusion processes. Importantly, the FL-SFA can unambiguously correlate interaction forces and in situ imaging in many dynamic interfacial systems.

During membrane fusion, lipid bilayers come into direct contact but rearrangements of lipid domains during fusion have not been thoroughly examined. Here the authors observe and correlate membrane morphology, interaction forces and domain rearrangements during hemifusion of two model membranes.

Hydrophobic, electrostatic, and dynamic polymer forces at silicone surfaces modified with long-chain bolaform surfactants

Surfactant self-assembly on surfaces is an effective way to tailor the complex forces at and between hydrophobic-water interfaces. Here, the range of structures and forces that are possible at surfactant-adsorbed hydrophobic surfaces are demonstrated: certain long-chain bolaform surfactants-containing a polydimethylsiloxane (PDMS) mid-block domain and two cationic α, ω-quarternary ammonium end-groups-readily adsorb onto thin PDMS films and form dynamically fluctuating nanostructures. Through measurements with the surface forces apparatus (SFA), it is found that these soft protruding nanostructures display polymer-like exploration behavior at the PDMS surface and give rise to a long-ranged, temperature- and rate-dependent attractive bridging force (not due to viscous forces) on approach to a hydrophilic bare mica surface. Coulombic interactions between the cationic surfactant end-groups and negatively-charged mica result in a rate-dependent polymer bridging force during separation as the hydrophobic surfactant mid-blocks are pulled out from the PDMS interface, yielding strong adhesion energies. Thus, (i) the versatile array of surfactant structures that may form at hydrophobic surfaces is highlighted, (ii) the need to consider the interaction dynamics of such self-assembled polymer layers is emphasized, and (iii) it is shown that long-chain surfactants can promote robust adhesion in aqueous solutions.

Simultaneous measurements of molecular forces and electro-optical properties of a confined 5CB liquid crystal film using a surface forces apparatus

Using a surface forces apparatus (SFA), we studied the forces associated with the reorientation of molecules of a common nematic thermotropic liquid crystal, 4'-n-pentyl-4-cyanobiphenyl (5CB), confined between two conducting (silver) surfaces and its optical behavior under the influence of electric fields with varying magnitudes and field directions. A transient attractive force was observed due to partial reorientations of the liquid crystal molecules and the flow of free ions, in addition to a stronger constant capacitance attraction between the silver surfaces. At the same time, the optical properties of the liquid crystals were observed perpendicular to the silver surfaces. Observations of shifts and fluctuations of the extraordinary wave of the (multiple beam) interference fringes measure the refractive index of the director component parallel to the surface, which is sensitive to tilt motion (or reorientation) of the liquid crystal molecules that provided details of the anisotropic orientations of the molecules and domains. Any lateral differential refractive index change is easily observed by optical microscopy. The optical microscope imaging showed that the changes in the optical properties are due to convective flow at domain boundaries of the liquid crystal molecules (and possible free ions) between the two charged surfaces. At low electric fields, propagation of domain boundaries was observed, while at higher electric fields, hexagonal patterns of flowing molecules were observed. The interplay of the force measurements and optical observations reveal a complex dynamic behavior of liquid crystals subjected to varying electric fields in confined spaces.

Adsorption mechanism of myelin basic protein on model substrates and its bridging interaction between the two surfaces

Myelin basic protein (MBP) is an intrinsically disordered (unstructured) protein known to play an important role in the stability of myelin's multilamellar membrane structure in the central nervous system. The adsorption of MBP and its capacity to interact with and bridge solid substrates has been studied using a surface forces apparatus (SFA) and a quartz crystal microbalance with dissipation (QCM-D). Adsorption experiments show that MBP molecules adsorb to the surfaces in a swollen state before undergoing a conformational change into a more compact structure with a thickness of ∼3 nm. Moreover, this compact structure is able to interact with nearby mica surfaces to form adhesive bridges. The measured adhesion force (energy) between two bridged surfaces is 1.0 ± 0.1 mN/m, (Ead = 0.21 ± 0.02 mJ/m(2)), which is slightly smaller than our previously reported adhesion force of 1.7 mN/m (Ead = 0.36 mJ/m(2)) for MBP adsorbed on two supported lipid bilayers (Lee et al., Proc. Natl. Acad. Sci. U.S.A. 2014, 111, E768-E775). The saturated surface concentration of compact MBP on a single SiO2 surface reaches a stable value of 310 ± 10 ng/cm(2) regardless of the bulk MBP concentration. A kinetic three-step adsorption model was developed that accurately fits the adsorption data. The developed model is a general model, not limited to intrinsically disordered proteins, that can be extended to the adsorption of various chemical compounds that undergo chemical reactions and/or conformational changes upon adsorbing to surfaces. Taken together with our previously published data (Lee et al., Proc. Natl. Acad. Sci. U.S.A. 2014, 111, E768-E775), the present results confirm that conformational changes of MBP upon adsorption are a key for strong adhesion, and that such conformational changes are strongly dependent on the nature of the surfaces.

Tough coating proteins: subtle sequence variation modulates cohesion

Mussel foot protein-1 (mfp-1) is an essential constituent of the protective cuticle covering all exposed portions of the byssus (plaque and the thread) that marine mussels use to attach to intertidal rocks. The reversible complexation of Fe(3+) by the 3,4-dihydroxyphenylalanine (Dopa) side chains in mfp-1 in Mytilus californianus cuticle is responsible for its high extensibility (120%) as well as its stiffness (2 GPa) due to the formation of sacrificial bonds that help to dissipate energy and avoid accumulation of stresses in the material. We have investigated the interactions between Fe(3+) and mfp-1 from two mussel species, M. californianus (Mc) and M. edulis (Me), using both surface sensitive and solution phase techniques. Our results show that although mfp-1 homologues from both species bind Fe(3+), mfp-1 (Mc) contains Dopa with two distinct Fe(3+)-binding tendencies and prefers to form intramolecular complexes with Fe(3+). In contrast, mfp-1 (Me) is better adapted to intermolecular Fe(3+) binding by Dopa. Addition of Fe(3+) did not significantly increase the cohesion energy between the mfp-1 (Mc) films at pH 5.5. However, iron appears to stabilize the cohesive bridging of mfp-1 (Mc) films at the physiologically relevant pH of 7.5, where most other mfps lose their ability to adhere reversibly. Understanding the molecular mechanisms underpinning the capacity of M. californianus cuticle to withstand twice the strain of M. edulis cuticle is important for engineering of tunable strain tolerant composite coatings for biomedical applications.

Mussel adhesive protein provides cohesive matrix for collagen type-1α

Understanding the interactions between collagen and adhesive mussel foot proteins (mfps) can lead to improved medical and dental adhesives, particularly for collagen-rich tissues. Here we investigated interactions between collagen type-1, the most abundant load-bearing animal protein, and mussel foot protein-3 (mfp-3) using a quartz crystal microbalance and surface forces apparatus (SFA). Both hydrophilic and hydrophobic variants of mfp-3 were exploited to probe the nature of the interaction between the protein and collagen. Our chief findings are: 1) mfp-3 is an effective chaperone for tropocollagen adsorption to TiO2 and mica surfaces; 2) at pH 3, collagen addition between two mfp-3 films (Wc = 5.4 ± 0.2 mJ/m(2)) increased their cohesion by nearly 35%; 3) oxidation of Dopa in mfp-3 by periodate did not abolish the adhesion between collagen and mfp-3 films, and 4) collagen bridging between both hydrophilic and hydrophobic mfp-3 variant films is equally robust, suggesting that hydrophobic interactions play a minor role. Extensive H-bonding, π-cation and electrostatic interactions are more plausible to explain the reversible bridging of mfp-3 films by collagen.

Bridging adhesion of mussel-inspired peptides: role of charge, chain length, and surface type

The 3,4-dihydroxyphenylalanine (Dopa)-containing proteins of marine mussels provide attractive design paradigms for engineering synthetic polymers that can serve as high performance wet adhesives and coatings. Although the role of Dopa in promoting adhesion between mussels and various substrates has been carefully studied, the context by which Dopa mediates a bridging or nonbridging macromolecular adhesion to surfaces is not understood. The distinction is an important one both for a mechanistic appreciation of bioadhesion and for an intelligent translation of bioadhesive concepts to engineered systems. On the basis of mussel foot protein-5 (Mfp-5; length 75 res), we designed three short, simplified peptides (15-17 res) and one relatively long peptide (30 res) into which Dopa was enzymatically incorporated. Peptide adhesion was tested using a surface forces apparatus. Our results show that the short peptides are capable of weak bridging adhesion between two mica surfaces, but this adhesion contrasts with that of full length Mfp-5, in that (1) while still dependent on Dopa, electrostatic contributions are much more prominent, and (2) whereas Dopa surface density remains similar in both, peptide adhesion is an order of magnitude weaker (adhesion energy E(ad) ∼ -0.5 mJ/m(2)) than full length Mfp-5 adhesion. Between two mica surfaces, the magnitude of bridging adhesion was approximately doubled (E(ad) ∼ -1 mJ/m(2)) upon doubling the peptide length. Notably, the short peptides mediate much stronger adhesion (E(ad) ∼ -3.0 mJ/m(2)) between mica and gold surfaces, indicating that a long chain length is less important when different interactions are involved on each of the two surfaces.

Measuring forces and spatiotemporal evolution of thin water films between an air bubble and solid surfaces of different hydrophobicity

A combination of atomic force microscopy (AFM) and reflection interference contrast microscopy (RICM) was used to measure simultaneously the interaction force and the spatiotemporal evolution of the thin water film between a bubble in water and mica surfaces with varying degrees of hydrophobicity. Stable films, supported by the repulsive van der Waals-Casimir-Lifshitz force were always observed between air bubble and hydrophilic mica surfaces (water contact angle, θ(w) < 5°) whereas bubble attachment occurred on hydrophobized mica surfaces. A theoretical model, based on the Reynolds lubrication theory and the augmented Young-Laplace equation including the effects of disjoining pressure, provided excellent agreement with experiment results, indicating the essential physics involved in the interaction between air bubble and solid surfaces can be elucidated. A hydrophobic interaction free energy per unit area of the form: WH(h) = -γ(1 - cos θ(w))exp(-h/D(H)) can be used to quantify the attraction between bubble and hydrophobized solid substrate at separation, h, with γ being the surface tension of water. For surfaces with water contact angle in the range 45° < θ(w) < 90°, the decay length DH varied between 0.8 and 1.0 nm. This study quantified the hydrophobic interaction in asymmetric system between air bubble and hydrophobic surfaces, and provided a feasible method for synchronous measurements of the interaction forces with sub-nN resolution and the drainage dynamics of thin films down to nm thickness.

Interfacial pH during mussel adhesive plaque formation

Mussel (Mytilus californianus) adhesion to marine surfaces involves an intricate and adaptive synergy of molecules and spatio-temporal processes. Although the molecules, such as mussel foot proteins (mfps), are well characterized, deposition details remain vague and speculative. Developing methods for the precise surveillance of conditions that apply during mfp deposition would aid both in understanding mussel adhesion and translating this adhesion into useful technologies. To probe the interfacial pH at which mussels buffer the local environment during mfp deposition, a lipid bilayer with tethered pH-sensitive fluorochromes was assembled on mica. The interfacial pH during foot contact with modified mica ranged from 2.2 to 3.3, which is well below the seawater pH of ~ 8. The acidic pH serves multiple functions: it limits mfp-Dopa oxidation, thereby enabling the catecholic functionalities to adsorb to surface oxides by H-bonding and metal ion coordination, and provides a solubility switch for mfps, most of which aggregate at pH ≥ 7-8.

Contact time- and pH-dependent adhesion and cohesion of low molecular weight chitosan coated surfaces

Low molecular weight chitosan (LMW chitosan, ∼5 kDa) potentially has many desirable biomedical applications such as anti-microbial, anti-tumor, and anti-diabetes. Unlike high molecular weight chitosan, LMW chitosan is easily dissolvable in aqueous solutions even at neutral and basic pH, but its dissolution mechanism is not well understood. Here, we measured adhesion and cohesion of molecularly thin LMW chitosan films in aqueous solutions in different buffer pHs (from 3.0 to 8.5) using a surface forces apparatus (SFA). Interestingly, significantly lower adhesion force was measured for LMW chitosan films compared to the high molecular weight chitosan (∼150 kDa) films. Not only the strength of adhesion is lower, but also the critical contact time where adhesion starts to increase with contact time is longer. The results from both the SFA and atomic force microscopy (AFM) indicate that, in physiological and basic conditions, the low cohesion of LMW chitosan due to the stiffness of the chain which cause strong electrostatic correlation energy penalty when they are aggregated. Here, we propose the reduction in cohesion for shorter chitosan (LMW chitosan) as an explanation of its high solubility of LMW chitosan in physiological pHs.

Influence of molecular dipole orientations on long-range exponential interaction forces at hydrophobic contacts in aqueous solutions

Strong and particularly long ranged (>100 nm) interaction forces between apposing hydrophobic lipid monolayers are now well understood in terms of a partial turnover of mobile lipid patches, giving rise to a correlated long-range electrostatic attraction. Here we describe similarly strong long-ranged attractive forces between self-assembled monolayers of carboranethiols, with dipole moments aligned either parallel or perpendicular to the surface, and hydrophobic lipid monolayers deposited on mica. We compare the interaction forces measured at very different length scales using atomic force microscope and surface forces apparatus measurements. Both systems gave a long-ranged exponential attraction with a decay length of 2.0 ± 0.2 nm for dipole alignments perpendicular to the surface. The effect of dipole alignment parallel to the surface is larger than for perpendicular dipoles, likely due to greater lateral correlation of in-plane surface dipoles. The magnitudes and range of the measured interaction forces also depend on the surface area of the probe used: At extended surfaces, dipole alignment parallel to the surface leads to a stronger attraction due to electrostatic correlations of freely rotating surface dipoles and charge patches on the apposing surfaces. In contrast, perpendicular dipoles at extended surfaces, where molecular rotation cannot lead to large dipole correlations, do not depend on the scale of the probe used. Our results may be important to a range of scale-dependent interaction phenomena related to solvent/water structuring on dipolar and hydrophobic surfaces at interfaces.

Boronate complex formation with Dopa containing mussel adhesive protein retards ph-induced oxidation and enables adhesion to mica

The biochemistry of mussel adhesion has inspired the design of surface primers, adhesives, coatings and gels for technological applications. These mussel-inspired systems often focus on incorporating the amino acid 3,4-dihydroxyphenyl-L-alanine (Dopa) or a catecholic analog into a polymer. Unfortunately, effective use of Dopa is compromised by its susceptibility to auto-oxidation at neutral pH. Oxidation can lead to loss of adhesive function and undesired covalent cross-linking. Mussel foot protein 5 (Mfp-5), which contains ∼ 30 mole % Dopa, is a superb adhesive under reducing conditions but becomes nonadhesive after pH-induced oxidation. Here we report that the bidentate complexation of borate by Dopa to form a catecholato-boronate can be exploited to retard oxidation. Although exposure of Mfp-5 to neutral pH typically oxidizes Dopa, resulting in a>95% decrease in adhesion, inclusion of borate retards oxidation at the same pH. Remarkably, this Dopa-boronate complex dissociates upon contact with mica to allow for a reversible Dopa-mediated adhesion. The borate protection strategy allows for Dopa redox stability and maintained adhesive function in an otherwise oxidizing environment.