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Protein-based underwater adhesives of marine organisms exhibit extraordinary binding strength in high salinity based on utilizing a variety of molecular interaction mechanisms. These include acid-base interactions, bidentate bindings or complex hydrogen bonding interactions, and electrochemical manipulation of interfacial bonding. In this Perspective, we briefly review recent progress in the field, and we discuss how interfacial electrochemistry can vary interfacial forces by concerted tuning of surface charging, hydration forces, and tuning of the interfacial ion concentration. We further discuss open questions, controversial findings, and new paths into understanding and utilizing redox-proteins and derived polymers for enhancing underwater adhesion in a complex salt environment.

Optical fields in metal-dielectric multilayers display typical features of quantum systems, such as energy level quantization and avoided crossing, underpinned by an isomorphism between the Helmholtz and Schrödinger wave equations. This article builds on the fundamental concepts and methods of quantum theory to facilitate the understanding and design of multicavity resonators. It also introduces the surface forces apparatus (SFA) as a powerful tool for rapid, continuous, and extensive characterization of mode dispersion and hybridization. Instead of fabricating many different resonators, two equal metal-dielectric-metal microcavities were created on glass lenses and displaced relative to each other in a transparent silicone oil using the SFA. The fluid thickness was controlled in real time with nanometer accuracy from more than 50 μm to less than 20 nm, reaching mechanical contact between the outer cavities in a few minutes. The fluid gap acted as a third microcavity providing optical coupling and producing a complex pattern of resonance splitting as a function of the variable thickness. An optical wave in this symmetric three-cavity resonator emulated a quantum particle with nonzero mass in a potential comprising three square wells. Interference between the wells produced a 3-fold splitting of degenerate energy levels due to hybridization. The experimental results could be explained using the standard methods and formalism of quantum mechanics, including symmetry operators and the variational method. Notably, the interaction between square wells produced bonding, antibonding, and nonbonding states that are analogous to hybridized molecular orbitals and are relevant to the design of "epsilon-near-zero" devices with vanishing dielectric permittivity.

Common fluids cannot sustain static mechanical stresses at the macroscopic scale because they lack molecular order. Conversely, crystalline solids exhibit long-range order and mechanical strength at the macroscopic scale. Combining the properties of fluids and solids, liquid crystal films respond to mechanical confinement by both flowing and generating static forces. The elastic response, however, is very weak for film thicknesses exceeding 10 nm. In this study, the mechanical strength of a fluid film was enhanced by introducing topological defects in a cholesteric liquid crystal, producing unique viscoelastic and optomechanical properties. The cholesteric was confined under strong planar anchoring conditions between two curved surfaces with sphere-sphere contact geometry similar to that of large colloidal particles, creating concentric dislocation loops. During surface retraction, the loops shrank and periodically disappeared at the surface contact point, where the cholesteric helix underwent discontinuous twist transitions, producing weak oscillatory surface forces. On the other hand, new loop nucleation was frustrated by a topological barrier during fluid compression, creating a metastable state. This generated exceptionally large forces with a range exceeding 100 nm as well as extended blueshifts of the photonic bandgap. The metastable cholesteric helix eventually collapsed under a high compressive load, triggering a stick-slip-like cascade of defect nucleation and twist reconstruction events. These findings were explained using a simple theoretical model and suggest a general approach to enhance the mechanical strength of one-dimensional periodic materials, particularly cholesteric colloid mixtures.

Graphene produced by chemical vapor deposition (CVD) is a promising candidate for implementing graphene in a range of technologies. In most device configurations, one side of the graphene is supported by a solid substrate, wheras the other side is in contact with a medium of interest, such as a liquid or other two-dimensional material within a van der Waals stack. In such devices, graphene interacts on both faces via noncovalent interactions and therefore surface energies are key parameters for device fabrication and operation. In this work, we directly measured adhesive forces and surface energies of CVD-grown graphene in dry nitrogen, water, and sodium cholate using a modified surface force balance. For this, we fabricated large (∼1 cm(2)) and clean graphene-coated surfaces with smooth topography at both macro- and nanoscales. By bringing two such surfaces into contact and measuring the force required to separate them, we measured the surface energy of single-layer graphene in dry nitrogen to be 115 ± 4 mJ/m(2), which was similar to that of few-layer graphene (119 ± 3 mJ/m(2)). In water and sodium cholate, we measured interfacial energies of 83 ± 7 and 29 ± 6 mJ/m(2), respectively. Our work provides the first direct measurement of graphene surface energy and is expected to have an impact both on the development of graphene-based devices and contribute to the fundamental understanding of surface interactions.

Using a surface forces apparatus and an atomic force microscope, we characterized the adhesive properties of adsorbed layers of two recombinant variants of Perna viridis foot protein 5 (PVFP-5), the main surface-binding protein in the adhesive plaque of the Asian green mussel. In one variant, all tyrosine residues were modified into 3,4-dihydroxy-l-phenylalanine (DOPA) during expression using a residue-specific incorporation strategy. DOPA is a key molecular moiety underlying underwater mussel adhesion. In the other variant, all tyrosine residues were preserved. The layer was adsorbed on a mica substrate and pressed against an uncoated surface. While DOPA produced a stronger adhesion than tyrosine in contact with the nanoscopic Si3N4 probe of the atomic force microscope, the two variants produced comparable adhesion on the curved macroscopic mica surfaces of the surface forces apparatus. These findings show that the presence of DOPA is not a sufficient condition to generate strong underwater adhesion. Surface chemistry and contact geometry affect the strength and abundance of protein-surface bonds created during adsorption and surface contact. Importantly, the adsorbed protein layer has a random and dynamic polymer-network structure that should be optimized to transmit the tensile stress generated during surface separation to DOPA surface bonds rather than other weaker bonds. National Research Foundation (NRF) We acknowledge financial support from the Marine Science Research and Development Program (MSRDP) of the Singapore National Research Foundation (NRF), Grant No. MSRDP-P29.

Lubrication of soft polydimethylsiloxane (PDMS) elastomer interfaces was studied in aqueous mixtures of porcine gastric mucin (PGM) and branched polyethyleneimine (b-PEI) at neutral pH and various ionic strengths (0.1-1.0 M). While neither PGM nor b-PEI improved lubrication compared to polymer-free buffer solution, their mixtures produced a synergistic lubricating effect by reducing friction coefficients by nearly two orders of magnitude, especially at slow sliding speed in the boundary lubrication regime. An array of spectroscopic studies revealed that small cationic b-PEI molecules were able to strongly bind and penetrate into large anionic PGM molecules, producing an overall contraction of the randomly coiled PGM conformation. The interaction also affected the structure of the folded PGM protein terminals, decreased the surface potential and increased light absorbance in PGM:b-PEI mixtures. Adding an electrolyte (NaCl) weakened the aggregation between PGM and b-PEI, and degraded the lubrication synergy, indicating that electrostatic interactions drive PGM:b-PEI complexation.

By combining dynamic light scattering, circular dichroism spectroscopy, atomic force microscopy, and surface force apparatus, the conformation of bovine submaxillary mucin in dilute solution and nanomechanical properties of mucin layers adsorbed on mica have been investigated. The samples were prepared by additional chromatographic purification of commercially available products. The mucin molecule was found to have a z-average hydrodynamic diameter of ca. 35 nm in phosphate buffered solution, without any particular secondary or tertiary structure. The contour length of the mucin is larger than, yet of the same order of magnitude as the diameter, indicating that the molecule can be modeled as a relatively rigid polymeric chain due to the large persistence length of the central glycosylated domain. Mucin molecules adsorbed abundantly onto mica from saline buffer, generating polymer-like, long-ranged, repulsive, and nonhysteretic forces upon compression of the adsorbed layers. Detailed analysis of such forces suggests that adsorbed mucins had an elongated conformation favored by the stiffness of the central domain. Acidification of aqueous media was chosen as means to reduce mucin-mucin and mucin-substrate electrostatic interactions. The hydrodynamic diameter in solution did not significantly change when the pH was lowered, showing that the large persistence length of the mucin molecule is due to steric hindrance between sugar chains, rather than electrostatic interactions. Remarkably, the force generated by an adsorbed layer with a fixed surface coverage also remained unaltered upon acidification. This observation can be linked to the surface-protective, pH-resistant role of bovine submaxillary mucin in the variable environmental conditions of the oral cavity.

The response of a cholesteric liquid crystal film to mechanical confinement is theoretically investigated considering planar anchoring conditions on the limiting surfaces and pure twist distortion of the liquid crystal director. We evaluate the total twist angle and normal force acting on the surfaces as a function of the film thickness. Assuming the Rapini-Papoular functional form for the surface anchoring energy, we show that the surface twist angle undergoes discontinuous jumps or continuous transitions as a function of the film thickness and anchoring strength. The transitions take place at well-defined film thicknesses, related to the intrinsic periodicity of the cholesteric liquid crystal, and produce oscillations in the normal force and position of the optical band-gap as the film thickness is varied.

Application of charged polymer chains as additives for lubricating neutral surfaces in aqueous environment, especially via polymer physisorption, is generally impeded by the electrostatic repulsion between adjacent polymers on the surface. In this study, we have investigated the adsorption and aqueous lubricating properties of an amphiphilic triblock copolymer, comprised of a neutral poly(ethylene glycol) (PEG) block, a hydrophobic poly(2-methoxyethyl acrylate) (PMEA) block, and a charged poly(methacrylic acid) (PMAA) block, namely PEG-b-PMEA-b-PMAA. After adsorption onto a nonpolar hydrophobic surface from aqueous solution, an equal and homogeneous mixture of neutral PEG and charged PMAA chains is formed on the surface, with an adsorbed polymer mass comparable to its fully neutral counterpart, PEG-b-PMEA-b-PEG. The lubricity of PEG-b-PMEA-b-PMAA showed significant improvement compared to fully charged polymer chains, e.g. poly(acrylic acid)-block-poly(2-methoxyethyl acrylate) (PAA-b-PMEA), which is attributed to dilution of charged moieties on the surface and subsequent improvement of the lubricating film stability.

International audience; Large-area ordered arrays can be created by the self-assembly of linear defects in thin Smectic-A films deposited in air on crystalline substrates. Such structures could find applications in nanoparticle assembly and soft lithography. The smectic layers are bent in concentric cylinders 10 under the effect of conflicting strong anchoring conditions at the substrate surface and free interface, and neighboring cylindrical domains are separated by curvature walls. We have studied the internal structure of such domains near the surface of muscovite mica using synchrotron x-ray diffraction. Our findings show that the domains are centered on virtual singularities, running below the substrate plane, and rest upon a surface region of submicrometric thickness where layers are 15 flat and vertical, satisfying the planar anchoring condition imposed by the substrate.

We demonstrated by means of interferometry that optical vortices can be generated by diffraction of a laser beam from a birefringent nematic liquid crystal that spontaneously creates a periodic array of electro-convective domains and edge dislocations under an applied electric field. The diffracted beam of order m produced by an elementary dislocation comprises a number |m| of distinct optical vortices, each with unit topological charge. Birefringent liquid crystal arrays provide a fast, convenient, and promising way of generating and studying optical vortices. The used materials are inexpensive, fabrication processes are simple, and both input polarization and applied field can be used as external controls to switch the optical vortices on and off.

Using electron scanning microscopy, we have studied the protein deposit left on silicon and mica substrates by dried droplets of aqueous solutions of bovine beta-lactoglobulin at low concentration and pH = 2-7. We have observed different self-assembled structures: homogeneous layers, hexagonal platelets and flower-shaped patterns laying flat on the surface, and rods formed by columns. Homogeneous layers covered the largest area of the droplet deposit. The other structures were found in small isolated regions, where the protein solution dried in the form of microdroplets. The presence of hexagonal platelets, flower-shaped patterns and columnar rods shows that beta-lactoglobulin self-assembles at the surface in a hexagonal columnar phase, which has never been observed in solution. A comparison with proteins showing similar aggregates suggests that beta-lactoglobulin structures grow from hexagonal germs composed of discotic nanometric building blocks, possibly possessing an octameric structure. We propose that discotic building blocks of beta-lactoglobulin may be produced by the anisotropic interaction with the solid surface.

8 pages; International audience; We studied the anisotropic aggregation of spherical latex particles dispersed in a lyotropic liquid crystal presenting three nematic phases: calamitic, biaxial, and discotic. We observed that in the nematic calamitic phase aggregates of latex particles are formed, which become larger and anisotropic in the vicinity of the transition to the discotic phase, due to a coalescence process. Such aggregates are weakly anisotropic and up to 50 μm long and tend to align parallel to the director field. At the transition to the discotic phase, the aggregates dissociated and re-formed when the system was brought back to the calamitic phase. This shows that the aggregation is due to attractive and repulsive forces generated by the particular structure of the nematic phase. The surface-induced positional order was investigated by surface force apparatus experiments with the lyotropic system confined between mica surfaces, revealing the existence of a presmectic wetting layer around the surfaces and oscillating forces of increasing amplitude as the confinement thickness was decreased. We discuss the possible mechanisms responsible for the reversible aggregation of latex particles, and we propose that capillary condensation of theNC phase, induced by the confinement between the particles, could reduce or remove the gradient of order parameter, driving the transition of aggregates from solidlike to liquidlike and gaslike.

Actin protein is a major component of the cell cytoskeleton, and its ability to respond to external forces and generate propulsive forces through the polymerization of filaments is central to many cellular processes. The mechanisms governing actin's abilities are still not fully understood because of the difficulty in observing these processes at a molecular level. Here, we describe a technique for studying actin–surface interactions by using a surface forces apparatus that is able to directly visualize and quantify the collective forces generated when layers of noninterconnected, end-tethered actin filaments are confined between 2 (mica) surfaces. We also identify a force-response mechanism in which filaments not only stiffen under compression, which increases the bending modulus, but more importantly generates opposing forces that are larger than the compressive force. This elastic stiffening mechanism appears to require the presence of confining surfaces, enabling actin filaments to both sense and respond to compressive forces without additional mediating proteins, providing insight into the potential role compressive forces play in many actin and other motor protein-based phenomena.

Using a surface force apparatus, we have measured the normal and friction forces between layers of the human glycoprotein lubricin, the major boundary lubricant in articular joints, adsorbed from buffered saline solution on various hydrophilic and hydrophobic surfaces: i), negatively charged mica, ii), positively charged poly-lysine and aminothiol, and iii), hydrophobic alkanethiol monolayers. On all these surfaces lubricin forms dense adsorbed layers of thickness 60-100 nm. The normal force between two surfaces is always repulsive and resembles the steric entropic force measured between layers of end-grafted polymer brushes. This is the microscopic mechanism behind the antiadhesive properties showed by lubricin in clinical tests. For pressures up to similar to 6 atm, lubricin lubricates hydrophilic surfaces, in particular negatively charged mica (friction coefficient mu = 0.02-0.04), much better than hydrophobic surfaces (mu > 0.3). At higher pressures, the friction coefficient is higher (mu > 0.2) for all surfaces considered and the lubricin layers rearrange under shear. However, the glycoprotein still protects the underlying substrate from damage up to much higher pressures. These results support recent suggestions that boundary lubrication and wear protection in articular joints are due to the presence of a biological polyelectrolyte on the cartilage surfaces.

The energetics of amyloid fibrillar aggregation of β-lactoglobulin (βLG) following incubation at high temperature and acid pH was studied by differential scanning calorimetry in the presence of Cu(2+) or Fe(3+) cations, and without any metal. Cu(2+) and metal-free protein solutions showed a distinct exothermic response that disappeared almost completely when the Fe(3+) molar concentration was ten times greater than the βLG concentration. Thioflavin T fluorescence studies in solution and atomic force microscopy analysis of the deposit left on flat mica substrates by heat-incubated βLG solutions correlated the absence of exothermic response of Fe(3+)-βLG solutions with a lack of fibril production. In contrast, abundant fibril deposits were observed for Cu(2+)-βLG solutions, with a rich polymorphism of multistrand fibrillar structures. Electron paramagnetic resonance revealed that Fe(3+) permanently binds to βLG in the aggregate state whereas Cu(2+) plays a catalytic role without binding to the protein. We propose that Fe(3+) inhibits fibril production after binding to a key region of the protein sequence, possibly interfering with the nucleation step of the fibrillation process and opening a nonfibrillar aggregation pathway. These findings suggest that transition metal ions can be utilized to effectively modulate protein self-assembly into a variety of structures with distinct morphologies at the nanoscale level.

Mucus is a viscous slime that plays a vital role in protecting and lubricating biological tissues, in particular, soft epithelium interfaces such as in the stomach, intestines, and esophagus. Previous attempts to generate mucus models that mimick or simulate its characteristics have been predominantly focused on the rheological properties. This study investigates both rheological and tribological shear properties of thin films of gastric mucus from a porcine source and its mimics at compliant soft interfaces. The lubricating efficacy of biological mucus and its mimics was observed to be superior at hydrophilic tribological interfaces compared to hydrophobic ones. Facile spreading of all mucus samples at hydrophilic steel-polydimethylsiloxane (PDMS) interfaces allowed for the retainment of the lubricating films over a wide range of speed, slide/roll ratio, and external load. In contrast, poor wetting at hydrophobic PDMS-PDMS interfaces led to depletion of the mucus samples from the interface with increasing speed. Among the different mucus models investigated in this study, fluid mixtures of commercially available porcine gastric mucin (PGM) and polyacrylic acid (PAA) displayed the most persistent lubricating effects under various tribological experimental conditions. A mixture of PGM and PAA holds a high potential as mucus mimic, not only for its rheological similarity, but also for its excellent lubricity in soft compliant and hydrophilic contacts.

Normal (e.g., adhesion) and lateral (friction) forces were measured between physisorbed and chemically grafted layers of hyaluronic acid (HA), an anionic polyelectrolyte in the presence of lubricin (Lub), a mucinous glycoprotein, on mica surfaces using a surface forces apparatus (SFA). This work demonstrates that high friction coefficients between the surfaces do not necessarily correlate with surface damage and that chemically grafted HA acts synergistically with Lub to provide friction reduction and enhanced wear protection to the surfaces. Surface immobilization of HA by grafting is necessary for such wear protection. Increasing the concentration of Lub enhances the threshold load that a chemically grafted HA surface can be subjected to before the onset of wear. Addition of Lub does not have any beneficial effect if HA is physisorbed to the mica surfaces. Damage occurs at loads less than 1 mN regardless of the amount of Lub, indicating that the molecules in the bulk play little or no role in protecting the surfaces from damage. Lub penetrates into the chemically bound HA to form a visco-elastic gel that reduces the coefficient of friction as well as boosts the strength of the surface against abrasive wear (damage).

Using an atomic force microscope (AFM) we confined a smectic-A liquid crystal (LC) between a flat glass plate and a 10-μm glass sphere attached to the free end of the AFM cantilever. Both surfaces were treated with a surfactant that induces normal alignment of the LC molecules. We measured the force F acting on the cantilever while varying the plate-sphere distance D with subnanometer precision. For D < 50 nm, the force was periodically oscillating and decayed as D was increased. Analyzing the force in the framework of a simple model of elastic deformation of the smectic layers, we have evaluated the undeformed layer thickness a(0) and compressibility modulus B. Compared to other techniques used to determine a(0) and B, AFM measurements are faster and require a much smaller amount (microliters) of LC. Moreover, they are based on purely mechanical deformations of the LC structure and do not require any static or radiative electromagnetic field.

In the present work the effect of the nanoscale morphology on the transport of ethanol/water mixtures through SBS block copolymer membranes is reported. In particular, the ethanol selectivity and overall flux can be optimized by varying the nanostructure of the SBS membranes. Membranes with well controlled nanostructure, characterized by AFM and TEM, have been prepared by selecting the casting solvent on the basis of interaction parameters ( χ ) with polybutadiene (PB) and polystyrene (PS) blocks and the amount of PS in the copolymer.

International audience; Using polarizing optical microscopy we studied thin films and droplets of smectic-A 4-cyano-4(')-n-octylbiphenyl (8CB) liquid crystal deposited in air on crystalline surfaces of muscovite mica that induce monostable planar anchoring. The competition with the homeotropic anchoring at the 8CB-air interface leads to the formation of one-dimensional (1D) patterns composed of straight, parallel defect domains that are organized in periodic arrays over areas as large as several mm(2). We have developed a simple model which identifies the arrays with self-assembled ``oily streaks,'' comprising straight disclination lines and curvature walls. The model reproduces the observed monotonic increase of the period p with the film thickness h in the range p=1-4 mu m and h=0.8-17 mu m. For higher values of h we observed a sharp transition to a 2D lattice of fragmented focal conic domains. Despite the apparent generality of our model for hybrid planar-homeotropic anchoring conditions, periodic arrays of straight oily streaks have been observed so far only for 8CB on crystalline surfaces such as mica or MoS(2). Our model indicates that this specificity is due to a particularly strong anchoring of the liquid crystal on such surfaces.

Cartilage sections were cut from the middle zone of pig knee articular cartilage and attached to substrates in two different kinds of newly designed 'pressure cells', one for fluorescence the other for NMR measurements. The fluorescence cell was filled with buffer solution containing fluorescently marked 70 kDa dextran which was allowed to diffuse into the cartilage pores. A second glass surface was then pressed down onto the thin cartilage sample under different loads (pressures), and the resulting compression (strain) and change in pore volume were measured as a function of time, simultaneously with measurements of the lateral diffusion and flow pattern of the dextran molecules using Fluorescence Recovery After Photobleaching (FRAP). Complementary experiments were made on the normal diffusion coefficients of pure electrolyte solutions (no dextran) in thicker cartilage sections with pulse-gradient NMR using a new pressure cell suitable for such measurements. Taken together our results show that the highly anisotropic structure of cartilage has a strong effect on the way fluid diffuses laterally and normally at different stages of compression. Our results also show how geometric constraints on a cartilage network and trapped high MW polymer such as HA during normal compressions are likely to affect both the normal and the lateral mobilities of polyelectrolytes and water.

"Lubricin (LUB) is a glycoprotein of the synovial cavity of human articular joints, where it serves as an antiadhesive, boundary lubricant, and regulating factor for the cartilage surface. It has been proposed that these properties are related to the presence of a long, extended, heavily glycosylated and highly hydrated mucinous domain in the central part of the LUB molecule. In this work, we show that LUB has a contour length of 220 +/- 30 nm and a persistence length of

Friction and adhesion measurements between surfaces of cross-linked, stiff polymers of varying roughness against smooth, bare mica surfaces were carried out in dry air as well as in the presence of lubricating oil. The nominal (macroscopic) contact area varies with the applied load according to the Johnson, Kendall and Roberts (JKR) theory, yet shows significant hysteresis due to the irreversibility arising from the loading/unloading curves of multiple asperities. Upon introducing the oil between the surfaces, the critical shear stress is reduced to zero due to the elimination of the adhesion force. However, the effect is less noticeable on the friction coefficient. Lastly, the effect of increasing the (RMS) roughness was greatest over the first few nanometers due to the diminution of the adhesion-controlled contribution to the friction, after which a further increase in roughness had less dramatic effects. A model is presented to account for the observed adhesion hysteresis during repeated loading/unloading cycles of purely elastically deforming rough surfaces.

Using a surface forces apparatus we have studied two thermotropic nematic liquid crystals (5CB and ME10.5) subjected to hybrid (homeotropic/planar) anchoring conditions. A film of nematic material is constrained between two curved smooth surfaces separated by less than 2500 angstrom. The intersurface force is nonmonotonic with the separation, being repulsive for thicknesses larger than similar or equal to 100 angstrom and strongly adhesive at a shorter scale. While the repulsion can be qualitatively explained by an elastic model of director deformation, including anchoring deviation at the boundaries, the attraction cannot be explained either by elasticity or by dispersive forces. The expected confinement-induced anchoring transition has not been observed for a thickness as small as 200 angstrom.

The self-assembly of proteins and polypeptides in amyloid fibrillar aggregates is rapidly emerging as a promising route towards the fabrication of nano-objects with controlled morphologies and properties. Transition metal ions are known to play an important but elusive role in the amyloid fibrillation associated with neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. We have considered the effect of copper ions Cu2+ on the nanoscale morphology and fibrillation kinetics of beta-lactoglobulin (beta LG), a model protein not related to amyloid diseases, which denatures and self-assembles in nanofibrils upon heating at low pH and ionic strength. We found that an increasing level of Cu2+ decreases the enthalpy of denaturation and significantly increases the rate of fibril nucleation, also producing a small increase of the fibril elongation rate. Cu2+ acts as a catalytic agent during protein denaturation and fibrillation, without binding to beta LG before or after heating, and produces only minor changes in fibril morphology. Beside possible implications for amyloid pathologies in vivo, our results suggest that transition metal ions can be used to control the self-assembly of protein-based nano-objects in vitro.

Forces generated by polymerizing/de-polymerizing actin filaments confined between two mica surfaces were measured using the Surface Forces Apparatus. The measurements show that confined actin filaments exhibit complex force-generation dynamics involving multiple “modes”, the predominance of which is determined by the confinement gap and the applied force (confining pressure).

A compression cell designed to fit inside an NMR spectrometer was used to investigate (i) the in situ dynamic strain response and structural changes of the internal pore network, and (ii) the diffusion and flow of interstitial water, in full thickness cartilage samples as they were mechanically deformed under a constant compressive load (pressure) and then allowed to recover (swell again) when the load was removed. Selective enzymatic digestion of the collagen fibril network and the glycopolysaccharide hyaluronic acid (HA) was performed to mimic some of the structural and compositional changes associated with osteoarthritis. Digestion of collagen gave rise to mechanical ‘dynamic softening’ and—perhaps more importantly—nearly complete loss in the ability to recover through swelling, both effects due to the disruption of the hierarchical structure and fibril interconnectivity in the collagen network which adversely affects its ability to deform reversibly and to properly regulate the pressurization and resulting rate and direction of interstitial fluid flow. In contrast, digestion of HA inside the collagen pore network caused the cartilage to ‘dynamically stiffen’ which is attributed to the decrease in the osmotic (entropic) pressure of the digested HA molecules confined in the cartilage pores that causes the network to contract and thereby become less permeable to flow. These digestion-induced changes in cartilage's properties reveal a complex relationship between the molecular weight and concentration of the HA in the interstitial fluid, and the structure and properties of the collagen fibril pore network, and provide new insights into how changes in either could influence the onset and progression of osteoarthritis.

We have implemented an apertureless Scanning Near-Field Optical Microscope (SNOM) setup on a commercial Atomic Force Microscope (AFM), using a suitable optical apparatus and preserving AFM performances. This approach is promising for extending SNOM analysis to delicate samples, such as liquid or polymer films. Preliminary measurements on glass-metal nanometric steps show a lateral resolution better than 10 nm both in topography and in optical signal.