Understanding Complex Tribofilms by Means of H3BO3-B2O3 Model Glasses.

The discovery of the spontaneous reaction of boric oxides with moisture in the air to form lubricious H3BO3 films has led to great interest in the tribology of boron compounds in general. Despite this, a study of the growth kinetics of H3BO3 on a B2O3 substrate under controlled relative humidity (RH) has not yet been reported in the literature. Here, we describe the tribological properties of H3BO3-B2O3 glass systems after aging under controlled RH over different lengths of time. A series of tribological tests has been performed applying a normal load of 15 N, at both room temperature and 100 °C in YUBASE 4 oil. In addition, the cause of H3BO3 film failure under high-pressure and high-temperature conditions has been studied to find out whether the temperature, the tribostress, or both influence the removal of the lubricious film from the contact points. The following techniques were exploited: confocal Raman spectroscopy to characterize the structure and chemical nature of the glass systems, environmental scanning electron microscopy to examine the morphology of the H3BO3 films developed, atomic force microscopy to monitor changes in roughness as a consequence of the air exposure, focused-ion-beam scanning electron microscopy to measure the average thickness of the H3BO3 films grown over various times on B2O3 glass substrates and to reveal the morphology of the sample in the vertical section, tribological tests to shed light on the system's lubricating properties, and finally small-area X-ray photoelectron spectroscopy to investigate the composition of the transfer film formed on the steel ball while tribotesting.

The relationship between skin function, barrier properties, and body-dependent factors.

BACKGROUND: Skin is a multilayer interface between the body and the environment, responsible for many important functions, such as temperature regulation, water transport, sensation, and protection from external triggers. OBJECTIVES: This paper provides an overview of principal factors that influence human skin and describes the diversity of skin characteristics, its causes and possible consequences. It also discusses limitations in the barrier function of the skin, describing mechanisms of absorption. METHODS: There are a number of in vivo investigations focusing on the diversity of human skin characteristics with reference to barrier properties and body-dependent factors. RESULTS: Skin properties vary among individuals of different age, gender, ethnicity, and skin types. In addition, skin characteristics differ depending on the body site and can be influenced by the body-mass index and lifestyle. Although one of the main functions of the skin is to act as a barrier, absorption of some substances remains possible. CONCLUSIONS: Various factors can alter human skin properties, which can be reflected in skin function and the quality of everyday life. Skin properties and function are strongly interlinked.

Collective dehydration of ions in nano-pores.

Hydrated ions can enter nanometre pores that are smaller than the hydrated ion diameter - the associated dehydration mechanism is still poorly understood. Using an adjustable model slit pore between negatively charged mica surfaces, we have followed the dehydration of highly confined Na+ counter-ions as a function of salt concentration. We applied external load to the slit pore and resolved the induced sub-nanometre film-thickness transitions, in order to gain information about any structural elements present. At a given concentration, the pull-off force required to reopen the collapsed pore is a sensitive measure for the final hydration state of the confined ions at the interface. Remarkably, we observe a two-step evolution of pull-off force, suggesting two-stage collective ion dehydration. There is a notable coincidence between this process and the occurrence of hydrated-ion layering, as previously observed for K+ ions, suggesting that a similar mechanism is at work. The gained insights into equilibrium collective ion dehydration in nano pores add to our fundamental understanding of confined electrical double layers. This may be ultimately translated into design criteria for future nano-porous electrode materials and nanofiltration membranes used for water treatment, or electrical-double-layer capacitors.

Stepwise collapse of highly overlapping electrical double layers.

When two charged surfaces and their accompanying electrical double layers (EDLs) approach each other in an electrolyte solution, the EDLs first begin to overlap and finally collapse under confinement. During this collapse we can observe repulsive forces and film-thickness transitions, which contain valuable information about different structural elements present at the interface. Sensing and discriminating these transitions by size and frequency of occurrence is possible via direct force measurements. Changing salt concentration or pH provide additional means to shift chemical potentials and interfacial populations, and therefore also to shift the relative stability of these structural elements. We provide new evidence that the previously observed oscillatory surface force appearing at the final stages of collapse of the EDL is initially due to layering transitions between hydrated ions, which then develop into smaller transitions between highly confined adsorbed ion states.

Microslips to "Avalanches" in Confined, Molecular Layers of Ionic Liquids.

We have measured forces between mica surfaces across two hydrophobic ionic liquids with a surface forces apparatus. Both surface-adsorbed water and alkyl-chain length on the imidazolium cation influence the structure of the nanoconfined film and the dynamics of film-thickness transitions. Friction shows accumulative microslips as precursors to collective "avalanches" that abruptly reduce friction momentarily. This behavior is interpreted as a consequence of interlayer ion correlations within the 1 to 2 nm thick film; we identify this to be analogous to the friction response of crackling noise systems over a broad range of sizes.

Skin-textile friction and skin elasticity in young and aged persons.

BACKGROUND/PURPOSE: The mechanical properties of human skin are known to change with ageing, rendering skin less resistant to friction and shear forces, as well as more vulnerable to wounds. Until now, only few and contradictory results on the age-dependent friction properties of skin have been reported. This study has investigated in detail the influence of age on the friction of human skin against textiles. METHODS: In vivo skin-friction measurements on a force plate were combined with skin analyses concerning elasticity, hydration, pH value and sebum content. Thirty-two young and 28 aged persons rubbed their volar forearm in a reciprocating motion against various textiles on the force plate, using defined normal loads and sliding velocities, representing clinically relevant contact conditions. RESULTS: Mean friction coefficients ranged from 0.30 +/- 0.04 (polytetrafluoroethylene) to 0.43 +/- 0.04 (cotton/polyester). No significant differences in the friction properties of skin were found between the age groups despite skin elasticity being significantly lower in the aged persons. Skin hydration was significantly higher in the elderly, whereas no significant differences were observed in either skin pH value or sebum content. CONCLUSION: Adhesion is usually assumed to be the dominant factor in skin friction, but our observations imply that deformation is also an important factor in the friction of aged skin. In the elderly, lower skin elasticity and skin turgor are associated with more pronounced skin tissue displacements and greater shear forces during frictional contact, emphasizing the importance of friction reduction in wound-prevention programmes.

Influence of epidermal hydration on the friction of human skin against textiles.

Friction and shear forces, as well as moisture between the human skin and textiles are critical factors in the formation of skin injuries such as blisters, abrasions and decubitus. This study investigated how epidermal hydration affects the friction between skin and textiles.The friction between the inner forearm and a hospital fabric was measured in the natural skin condition and in different hydration states using a force plate. Eleven males and eleven females rubbed their forearm against the textile on the force plate using defined normal loads and friction movements. Skin hydration and viscoelasticity were assessed by corneometry and the suction chamber method, respectively.In each individual, a highly positive linear correlation was found between skin moisture and friction coefficient (COF). No correlation was observed between moisture and elasticity, as well as between elasticity and friction. Skin viscoelasticity was comparable for women and men. The friction of female skin showed significantly higher moisture sensitivity. COFs increased typically by 43% (women) and 26% (men) when skin hydration varied between very dry and normally moist skin. The COFs between skin and completely wet fabric were more than twofold higher than the values for natural skin rubbed on a dry textile surface.Increasing skin hydration seems to cause gender-specific changes in the mechanical properties and/or surface topography of human skin, leading to skin softening and increased real contact area and adhesion.

Study of skin-fabric interactions of relevance to decubitus: friction and contact-pressure measurements.

BACKGROUND/PURPOSE: Prolonged pressure as well as friction and shear forces at the skin-textile interface are decisive physical parameters in the development of decubitus. The present article describes the contact phenomena at the skin-textile interface and the development of a purpose-built textile friction analyser (TFA) for the tribological assessment of skin-fabric interactions, in connection with decubitus prevention. METHODS: Interface pressure distributions were recorded in the pelvic and femoral regions between supine persons and a foam mattress. Fabrics made of various natural and synthetic yarns were investigated using the TFA. A vertical load of 7.7 kPa was applied to the swatches, simulating high interface pressures at the skin-fabric interface and clinical conditions of bedridden persons. Fabrics were rubbed in reciprocating motions against a validated skin-simulating material to determine static as well as dynamic friction coefficients (COFs). RESULTS: Maximum contact pressures ranged from 5.2 to 7.7 kPa (39-58 mmHg) and exceeded the capillary closure pressure (32 mmHg) in all investigated bedding positions. For both COFs, a factor of 2.5 was found between the samples with the lowest and highest values. Our results were in a similar range to COFs found in measurements on human skin in vivo. The results showed that our test method can detect differences of 0.01 in friction coefficients. CONCLUSION: TFA measurements allow the objective and reliable study of the tribology of the skin-textile biointerface and will be used to develop medical textiles with improved performance and greater efficacy for decubitus prevention.

The effect of surface ions on water adsorption to mica.

We have measured the adsorption isotherms of water on a single surface of freshly cleaved mica with K+ on the surface, and on mica where the K+ has been exchanged for H+. Using a very sensitive interferometric technique, we have found a significant difference between the two isotherms at submonolayer coverage, for relative vapor pressures p/p0 < 0.5. The K+-mica isotherm shows a pronounced convexity, suggesting distinct adsorption sites, whereas the H+-mica isotherm is flatter. The two isotherms converge above monolayer coverage. The results give a graphic demonstration of the importance of nanoscale surface heterogeneities for vapor adsorption at submonolayer coverage.

Capabilities of femtosecond laser ablation inductively coupled plasma mass spectrometry for depth profiling of thin metal coatings.

The capabilities of ultraviolet femtosecond laser ablation inductively coupled plasma mass spectrometry (UV-fs-LA-ICPMS) for depth profile analysis of thin metal coatings were evaluated. A standard sample consisting of a single Cr thin layer of 500 nm +/- 5% on a Ni substrate was used. A fast washout was obtained by a high-efficiency aerosol dispersion ablation cell (V approximately 1 cm3), which allowed single-shot analysis with increased depth resolution. Laser ablation was performed in helium at atmospheric pressure conditions. A laser repetition rate of 1 Hz and low laser fluence (<0.5 J/cm2) were used. Very low ablation rates (<10 nm/pulse) were determined by atomic force microscopy (AFM). Information about the crater geometry and morphology was investigated using scanning electron microscopy and AFM. The depth resolution, calculated via the maximum slope of the tangent in the layer interface region, was smaller than 300 nm. Our data indicate that UV-fs-LA-ICPMS represents a powerful combination of high lateral and depth resolution for the analysis of thin metal coatings. Moreover, an overall ion yield, defined as the ratio of detected ions and ablated atoms, of approximately 5 x 10-5 was estimated for the chromium layer under the operating conditions chosen. The absolute amount of ablated material per laser pulse was approximately 1 pg, which corresponds to a detection limit of 180 microg/g.

Interaction forces and morphology of a protein-resistant poly(ethylene glycol) layer.

The molecular interactions on a protein-resistant surface coated with low-molecular-weight poly(ethylene glycol) (PEG) copolymer brushes are investigated using the extended surface forces apparatus. The observed interaction force is predominantly repulsive and nearly elastic. The chains are extended with respect to the Flory radius, which is in agreement with qualitative predictions of scaling theory. Comparison with theory allows the determination of relevant quantities such as brush length and adsorbed mass. Based on these results, we propose a molecular model for the adsorbed copolymer morphology. Surface-force isotherms measured at high resolution allow distinctive structural forces to be detected, suggesting the existence of a weak equilibrium network between poly(ethylene glycol) and water--a finding in accordance with the remarkable solution properties of PEG. The occurrence of a fine structure is interpreted as a water-induced restriction of the polymer's conformational space. This restriction is highly relevant for the phenomenon of PEG protein resistance. Protein adsorption requires conformational transitions, both in the protein as well as in the PEG layer, which are energetically and kinetically unfavorable.

Protein-mediated boundary lubrication in arthroplasty.

Wear of articulated surfaces can be a major lifetime-limiting factor in arthroplasty. In the natural joint, lubrication is effected by the body's natural synovial fluid. Following arthroplasty, and the subsequent reformation of the synovial membrane, a fluid of similar composition surrounds the artificial joint. Synovial fluid contains, among many other constituents, a substantial concentration of the readily adsorbing protein albumin. The ability of human serum albumin to act as a boundary lubricant in joint prostheses has been investigated using a pin-on-disc tribometer. Circular dichroism spectroscopy was employed to follow the temperature- and time-dependent conformational changes of human serum albumin in the model lubricant solution. Effects of protein conformation and polymer surface hydrophilicity on protein adsorption and the resulting friction in the boundary lubrication regime have been investigated. Unfolded proteins preferentially adsorb onto hydrophobic polymer surfaces, where they form a compact, passivating layer and increase sliding friction-an effect that can be largely suppressed by rendering the substrate more hydrophilic. A molecular model for protein-mediated boundary friction is proposed to consolidate the observations. The relevance of the results for in vivo performance and ex vivo hip-joint testing are discussed.

Surface modification of PLGA microspheres.

Microspheres made of poly(lactic-co-glycolic acid) (PLGA) are biocompatible and biodegradable, rendering them a promising tool in the context of drug delivery. However, nonspecific adsorption of plasma proteins on PLGA micro- and nanospheres is a main limitation of drug targeting. Poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG), physisorbed on flat metal oxide surfaces, has previously been shown to suppress protein adsorption drastically. The goal of our work was to characterize the efficiency of the protein repellent character of PLL-g-PEG on PLGA microspheres and to show the feasibility of introducing functional groups on the PLGA microspheres via functionalized PLL-g-PEG. To quantify the adsorbed amount of protein, a semiquantitative method that uses confocal laser scanning microscopy (CLSM) was applied. The first part of the experiment confirms the feasibility of introducing specific functional groups on PLL-g-PEG-coated PLGA microspheres. In the second part of the experiment, PLL-g-PEG-coated PLGA microspheres show a drastic decrease of adsorbed proteins by two orders of magnitude in comparison to uncoated PLGA microspheres. Low protein-binding, functionalizable microspheres provide a fundamental basis for the design of drug delivery and biosensor systems.

Chemically patterned, metal oxide based surfaces produced by photolithographic techniques for studying protein- and cell-surface interactions I: Microfabrication and surface characterization.

Chemical patterns on smooth wafer substrates comprising areas with two different metals have been produced by vacuum metal deposition and photolithographic techniques. The combination of metals has been chosen from the series titanium (Ti), aluminium (Al), vanadium (V) and niobium (Nb), producing patterns (dots and stripes with dimensions of 50, 100 and 150 micrometer) with one of the metals as the background and with the second metal (foreground pattern) deposited on the background metal. The structure and chemical composition of the patterned surfaces were evaluated by scanning electron microscopy, X-ray photoelectron spectroscopy and imaging time-of-flight secondary-ion mass spectrometry. The surfaces proved to be geometrically well defined with the expected surface-chemical composition, i.e. a surface oxide (passive) film essentially composed of TiO(2),Al(2)O(3),V(2)O(5), or Nb(2)O(5). Ti/Ti patterned surfaces were produced as controls and found to show no chemical composition contrast. The surface roughness of the pattern was greater than that of the background by a factor of 2-3, but was still extremely smooth with Ra<2nm. The patterns serve as model surfaces for studying in vitro the behaviour of cells as well as the adsorption of serum proteins on different metal oxides, which will be reported in a companion paper. These surfaces can be used to compare and contrast the response of osteoblasts to Ti and other alloy components, such as Al, V, or Nb, which are used in load-bearing medical implants.

Optical grating coupler biosensors.

By incorporating a grating in a planar optical waveguide one creates a device with which the spectrum of guided lightmodes can he measured. When the surface of the waveguide is exposed to different solutions, the peaks in the spectrum shift due to molecular interactions with the surface. Optical waveguide lightmode spectroscopy (OWLS) is a highly sensitive technique that is capable of real-time monitoring of these interactions. Since this integrated optical method is based on the measurement of the polarizability density (i.e., refractive index) in the vicinity of the waveguide surface, radioactive, fluorescent or other kinds of labeling are not required. In addition, measurement of at least two guided modes enables the absolute mass of adsorbed molecules to be determined. In this article, the technique will be described in some detail, and applications from different areas will be discussed. Selected examples will be presented to demonstrate how monitoring the modification of different metal oxides with polymers and the response of the coated oxides to biofluids help in the design of novel biomaterials; how OWLS is useful for accurate bioaffinity sensing, which is a key issue in the development of new drugs; and how the quantitative study of protein-DNA/RNA and cell surface interactions can enhance the understanding of processes in molecular and cellular biology.

Comparative investigation of the surface properties of commercial titanium dental implants. Part I: chemical composition.

The surfaces of five commercially available titanium implants (Brånemark Nobel Biocare, 3i ICE, 3i OSSEOTITE, ITI-TPS, and ITI-SLA) were compared by scanning electron microscopy, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectroscopy. All five implant types were screw-shaped and fabricated from commercially pure (cp) titanium, but their surface properties differed both as regards surface morphology and surface chemical composition. The macro- and microstructure of the implant surfaces were investigated by scanning electron microscopy. The surfaces chemical composition was determined using the surface-sensitive analytical techniques of X-ray photoelectron spectroscopy and time-of-flight secondary ion spectrometry. Surface topographies were found to reflect the type of mechanical/chemical fabrication procedures applied by the manufacturers. The titanium oxide (passive) layer thickness was similar (5-6 nm) and typical for oxide films grown at or near room temperature. A variety of elements and chemical compounds not related to the metal composition were found on some implant types. They ranged from inorganic material such as sodium chloride to specific organic compounds believed to be due to contamination during fabrication or storage. The experimental findings are believed to make a contribution to a better understanding of the interplay between industrial fabrication procedure and physico-chemical implant surface properties.

Density fluctuations under confinement: when is a fluid not a fluid?

Knowing the behavior of a fluid in small volumes is essential for the understanding of a vast array of common problems in science, such as biological interactions, fracture propagation, and molecular tribology and adhesion, as well as pressure solvation and other geophysical processes. When a fluid is confined, its phase behavior is altered and excluded-volume effects become apparent. Pioneering measurements performed with the surface forces apparatus have revealed so-called structural or oscillatory solvation forces as well as the occurrence of a finite shear stress, which was interpreted as a solidification transition. Here, we report measurements obtained with an extended surface forces apparatus, which makes use of fast spectral correlation to gain insight into the behavior of a thin film of cyclohexane confined within attoliter volumes, with simultaneous measurement of film thickness and refractive index. With decreasing pore width, cyclohexane is found to undergo a drastic transition from a three-dimensional bulk fluid to a two-dimensional adsorbate with strikingly different properties. Long-range density fluctuations of unexpected magnitude are observed.

Wavelength-dependent roughness: a quantitative approach to characterizing the topography of rough titanium surfaces.

Topographies of grit-blasted, etched, grit-blasted and etched, and microfabricated and etched surfaces of commercially pure titanium have been investigated. Such surface topographies vary across the scale range of interest for dental implants, extending from nanometers to millimeters. The complete characterization of topography requires the use of complementary methods. This study compared the topographic characterization methods of non-contact laser profilometry, interference microscopy, stereo-scanning electron microscopy (stereo-SEM), and atomic force microscopy. Non-contact laser profilometry was shown to be a useful method to characterize topographic features in the micron to millimeter range, whereas interference microscopy and stereo-SEM can be employed down to the submicron range. Stereo-SEM is particularly useful for quantifying topographies with complex, strongly corrugated ("sharp"), and high-aspect-ratio features and was shown to be complementary to non-contact laser profilometry and interference microscopy. Because of tip-related envelope problems, atomic force microscopy was not found to be suitable for the type of surfaces investigated in this study. Independent of the method used, the commonly used "integral" amplitude roughness parameters, such as Ra, Rq, or Rt, were often of limited value in the description of actual implant surfaces. The application of the wavelength-dependent roughness approach was shown to be an effective method for the description of surface topographies in the complete range of characteristic roughness and is also a useful means of examining the effects of surface treatment processes.

Feasibility study of an online toxicological sensor based on the optical waveguide technique.

Morphological properties of the cells often change as an early response to the presence of a pharmacologically acting toxic substance [Etcheverry, S.B., Crans, D.C., Keramidas, A.D., Cortizo, A.M., Arch. Biochem. Biophys. 338 (1997) 7-14]. Recently it has been shown that living animal cell adhesion and spreading can be monitored online and quantitatively via the interaction of the cells with the evanescent electromagnetic field present at the surface of an optical waveguide [Ramsden, J.J., Li, S.Y., Heinzle, E., Prinosil, J.E. Cytometry 19 (1995) 97-102]. In the present study, optical waveguide lightmode spectroscopy (OWLS) and confocal laser scanning microscopy (CLSM), which provides information about the shape of the cells at the surface, were compared under identical experimental conditions. This allowed for the correlation between the cell-shape information from CLSM and the cell-surface interaction measurements from OWLS. The proposed design of the microsystem sensor involves the establishment of a cell layer on the surface of the waveguide and the subsequent online measurement of the morphological response of the cells to various toxic substances. In the present study, the setup was evaluated using cells from an osteoblastic MC 3T3-E1 cell line, and sodium hypochlorite was used as model toxic substance. Comparing the OWLS signal to the morphological response measured by CLSM reveals that OWLS is effective in monitoring not only cell attachment and spreading but also the cellular response to toxic compounds (i.e. by means of change in cell morphology). For the model toxin, the OWLS measurements indicate that, at concentrations above 0.01%, the cells exhibit a clearly discernable morphological effect (i.e. a decrease in average cell contact area). Thus, the potential of an on-line sensor based on OWLS to applications in toxicology, pharmacy and biocompatibility was demonstrated.

Surface characterization of implant materials c.p. Ti, Ti-6Al-7Nb and Ti-6Al-4V with different pretreatments.

The biocompatibility of commercially pure titanium and its alloys is closely related to their surface properties, with both the composition of the protecting oxide film and the surface topography playing an important role. Surfaces of commercially pure titanium and of the two alloys Ti-6Al-7Nb and Ti-6Al-4V (wt %) have been investigated following three different pretreatments: polishing, nitric acid passivation and pickling in nitric acid-hydrogen fluoride. Nitric acid treatment is found to substantially reduce the concentration of surface contaminants present after polishing. The natural 4-6 nm thick oxide layer on commercially pure titanium is composed of titanium oxide in different oxidation states (TiO2, Ti2O3 and TiO), while for the alloys, aluminium and niobium or vanadium are additionally present in oxidized form (Al2O3, Nb2O5 or V-oxides). The concentrations of the alloying elements at the surface are shown to be strongly dependent on the pretreatment process. While pickling increases the surface roughness of both commercially pure titanium and the alloys, different mechanisms appear to be involved. In the case of commercially pure titanium, the dissolution rate depends on grain orientation, whereas in the case of the two alloys, selective alpha-phase dissolution and enrichment of the beta-phase appears to occur.