Biomedical subjects

B Kasemo

Publications and source records attributed to B Kasemo.

At least 19 recordsLinked to original sources

PubMed2001

Variations in coupled water, viscoelastic properties, and film thickness of a Mefp-1 protein film during adsorption and cross-linking: a quartz crystal microbalance with dissipation monitoring, ellipsometry, and surface plasmon resonance study.

We have measured the time-resolved adsorption kinetics of the mussel adhesive protein (Mefp-1) on a nonpolar, methyl-terminated (thiolated) gold surface, using three independent techniques: quartz crystal microbalance with dissipation monitoring (QCM-D), surface plasmon resonance, and ellipsometry. The QCM-D and ellipsometry data shows that, after adsorption to saturation of Mefp-1, cross-linking of the protein layer using NaIO4 transforms it from an extended (approximately 20 nm), water-rich, and hydrogel-like state to a much thinner (approximately 5 nm), compact, and less water-rich state. Furthermore, we show how quantitative data about the thickness, shear elastic modulus, and shear viscosity of the protein film can be obtained with the QCM-D technique, even beyond the Sauerbrey regime, if frequency (f) and energy dissipation (D) measurements measured at multiple harmonics are combined with theoretical simulations using a Voight-based viscoelastic model. The modeling result was confirmed by substituting H2O for D2O. As expected, the D2O substitution does not influence the actual adsorption behavior, but resulted in expected differences in the estimated effective density and shear viscosity. These results provide new insight and understanding about the adsorption kinetics and crosslinking behavior of Mefp-1. They also demonstrate how the above three techniques complement each other for biomolecule adsorption studies.

Adsorption↗

PubMed2001

Simulation of two-dimensional streptavidin crystallization.

We present lattice Monte Carlo simulations of the growth of streptavidin islands at a biotinylated lipid layer. The model employed takes into account attractive anisotropic lateral interactions between streptavidin tetramers. With a minimal set of interactions, we reproduce the formation of rectangular islands experimentally observed at pH > or = 9.0. Specifically, we analyze two scenarios of the island growth. First, if streptavidin is rapidly adsorbed at t = 0 (stepwise coverage change without ongoing adsorption), the average linear island size is found to grow according to the Lifshitz-Slyozov law, R proportional to t(1/3). Second, if the island growth occurs in parallel with streptavidin adsorption limited by diffusion in the solution, the Lifshitz-Slyozov law is also applicable, but only at the late stage, when the streptavidin coverage is appreciable.

Adsorption↗

PubMed2001

Folding of bundles of alpha-helices in solution, membranes, and adsorbed overlayers.

We propose a coarse-grained lattice model for Monte Carlo simulations of folding of proteins consisting of several alpha-helices. A chain representing a protein is considered to contain A and B monomers forming relatively stiff A subchains, mimicking helices, and flexible B links between these subchains, respectively. Using this model, we simulate (1) folding of four-helix proteins in solution; (2) folding of membrane proteins containing one, two, or four helices; and (3) refolding of four-helix proteins adsorbed at the liquid-solid interface. For these cases, we show typical scenarios of protein folding and refolding and study the dependence of the folding time on the chain length. Combining the latter results with those already available in the literature, we discuss the relative rates of folding of proteins belonging to different classes.

Algorithms↗

PubMed2000

Ordering of adsorbed proteins.

Adapting a hard hexagon model to describing protein adsorption, we show by using Monte Carlo simulations that the ordering of adsorbed proteins may strongly depend on the relative location of the hydrophilic and hydrophobic patches on their surface. Specifically, proteins may form dimers, trimers, small rings, and zig-zag or straight chains or islands consisting of such fragments.

Adsorption↗

PubMed2000

Formation of supported membranes from vesicles.

Using a combination of the quartz crystal microbalance and surface plasmon resonance techniques, we have studied the spontaneous formation of supported lipid bilayers from small (approximately 25 nm) unilamellar vesicles. Together these experimental methods measure the amount of lipid adsorbed on the surface and the amount of water trapped by the lipid. With this approach, we have, for the first time, been able to observe in detail the progression from the adsorption of intact vesicles to rupture and bilayer formation. Monte Carlo simulations reproduce the data.

Adsorption↗

PubMed2000

Monte Carlo simulation of diffusion of adsorbed proteins.

We present the results of three-dimensional lattice Monte Carlo simulations of protein diffusion on the liquid-solid interface in a wide temperature range including the most interesting temperatures (from slightly below T(f) and up to T(c), where T(f) and T(c) are the folding and collapse temperatures). For the model under consideration (27 monomers of two types), the temperature dependence of the diffusion coefficient is found to obey the Arrhenius law with the normal value (approximately 10(-2)-10(-3) cm(2)/s) of the preexponential factor. Proteins 2000;39:76-81.

Adsorption↗

PubMed1999

Adsorption and coadsorption of water and glycine on TiO2.

Adsorption of water, ions, and biomolecules constitutes the first events occurring at biomaterial-biosystem interfaces. In this work, the adsorption and coadsorption of water and glycine on TiO2 were studied by thermal desorption spectroscopy (TDS). The first water monolayer desorbs in three peaks around 180K, 300K, and 400K, which are assigned to water molecularly adsorbed at oxygen sites, at Ti4+ sites, and to recombination of dissociated water, respectively. A fourth desorption peak (160K), appearing at coverages > 0.8 monolayer, is attributed to water clusters and multilayers. The water-TiO2 interaction is changed if the surface is annealed in vacuum, which leads to increased hydroxylation. Desorption spectra from glycine overlayers evaporated on TiO2 in situ show that around 40% of the first monolayer desorbs as intact molecules ( approximately 300-450 K) and the remainder as dissociation fragments and surface reaction products around 600 K. At coverages > 0.6 monolayers, intact molecules desorbing from cluster multilayers at 310 K are detected. The glycine desorption spectra are unaffected by coadsorbed water. In contrast, coadsorption of glycine displaces water from more strongly bound states in the monolayer to more weakly bound states and clusters, making the surface more hydrophobic. The study shows that TDS is a powerful method for characterizing biomaterial surfaces with regard to their interaction with biologically relevant molecules.

Adsorption↗

PubMed1999

Implant surfaces and interface processes.

The past decades and current R&D of biomaterials and medical implants show some general trends. One major trend is an increased degree of functionalization of the material surface, better to meet the demands of the biological host system. While the biomaterials of the past and those in current use are essentially bulk materials (metals, ceramics, polymers) or special compounds (bioglasses), possibly with some additional coating (e.g., hydroxyapatite), the current R&D on surface modifications points toward much more complex and multifunctional surfaces for the future. Such surface modifications can be divided into three classes, one aiming toward an optimized three-dimensional physical microarchitecture of the surface (pore size distributions, "roughness", etc.), the second one focusing on the (bio) chemical properties of surface coatings and impregnations (ion release, multi-layer coatings, coatings with biomolecules, controlled drug release, etc.), and the third one dealing with the viscoelastic properties (or more generally the micromechanical properties) of material surfaces. These properties are expected to affect the interfacial processes cooperatively, i.e., there are likely synergistic effects between and among them: The surface is "recognized" by the biological system through the combined chemical and topographic pattern of the surface, and the viscoelastic properties. In this presentation, the development indicated above is discussed briefly, and current R&D in this area is illustrated with a number of examples from our own research. The latter include micro- and nanofabrication of surface patterns and topographies by the use of laser machining, photolithographic techniques, and electron beam and colloidal lithographies to produce controlled structures on implant surfaces in the size range 10 nm to 100 microns. Examples of biochemical modifications include mono- or lipid membranes and protein coatings on different surfaces. A new method to evaluate, e.g., biomaterial-protein and biomaterial-cell interactions--the Quartz Crystal Microbalance--is described briefly.

Biocompatible Materials↗

PubMed1998

Measurements Using the Quartz Crystal Microbalance Technique of Ferritin Monolayers on Methyl-Thiolated Gold: Dependence of Energy Dissipation and Saturation Coverage on Salt Concentration.

The adsorption kinetics of ferritin as a function of ionic strength has been studied with a new quartz crystal microbalance technique, allowing simultaneous measurement of the frequency shift (proportional to the mass uptake under certain conditions) and of changes in the energy dissipation caused by the adlayer. The measurements were performed with methyl-terminated (hydrophobic) thiol-covered gold surfaces, at pH 7.0 and ionic strengths in the range 1-200 mM KCl. The saturation uptake increases rapidly with increasing ionic strength in the range 20-50 mM and is then independent of ionic strength at >100 mM. The dissipation factor reveals, in the low coverage regime, distinct differences in the adlayer properties at low and high ionic strength, respectively. These results are briefly discussed in terms of the screening properties of the solvent and its influence on the protein-protein interaction in solution and on the surface. Copyright 1998 Academic Press.

Journal Article↗

PubMed1998

Structural changes in hemoglobin during adsorption to solid surfaces: effects of pH, ionic strength, and ligand binding.

We have studied the adsorption of two structurally similar forms of hemoglobin (met-Hb and HbCO) to a hydrophobic self-assembled methyl-terminated thiol monolayer on a gold surface, by using a Quartz Crystal Microbalance (QCM) technique. This technique allows time-resolved simultaneous measurements of changes in frequency (f) (c.f. mass) and energy dissipation (D) (c.f. rigidity/viscoelastic properties) of the QCM during the adsorption process, which makes it possible to investigate the viscoelastic properties of the different protein layers during the adsorption process. Below the isoelectric points of both met-Hb and HbCO, the DeltaD vs. Deltaf graphs displayed two phases with significantly different slopes, which indicates two states of the adsorbed proteins with different visco-elastic properties. The slope of the first phase was smaller than that of the second phase, which indicates that the first phase was associated with binding of a more rigidly attached, presumably denatured protein layer, whereas the second phase was associated with formation of a second layer of more loosely bound proteins. This second layer desorbed, e.g., upon reduction of Fe3+ of adsorbed met-Hb and subsequent binding of carbon monoxide (CO) forming HbCO. Thus, the results suggest that the adsorbed proteins in the second layer were in a native-like state. This information could only be obtained from simultaneous, time-resolved measurements of changes in both D and f, demonstrating that the QCM technique provides unique information about the mechanisms of protein adsorption to solid surfaces.

Adsorption↗

PubMed1998

Monte Carlo simulation of denaturation of adsorbed proteins.

Denaturation of model proteinlike molecules at the liquid-solid interface is simulated over a wide temperature range by employing the lattice Monte Carlo technique. Initially, the molecule containing 27 monomers of two types (A and B) is assumed to be adsorbed in the native folded state (a 3 x 3 x 3 cube) so that one of its sides is in contact with the surface. The details of the denaturation kinetics are found to be slightly dependent on the choice of the side, but the main qualitative conclusions hold for all the sides. In particular, the kinetics obey approximately the conventional first-order law at T > Tc (Tc is the collapse temperature for solution). With decreasing temperature, below Tc but above Tf (Tf is the folding temperature for solution), deviations appear from the first-order kinetics. For the most interesting temperatures, that is, below Tf, the denaturation kinetics are shown to be qualitatively different from the conventional ones. In particular, the denaturation process occurs via several intermediate steps due to trapping in metastable states. Mathematically, this means that (i) the transition to the denatured state of a given molecule is nonexponential, and (ii) the denaturation process cannot be described by a single rate constant kappar. One should rather introduce a distribution of values of this rate constant (different values of kappar correspond to the transitions to the altered state via different metastable states).

Adsorption↗

PubMed1998

Monte Carlo simulation of the kinetics of protein adsorption.

Adsorption of proteins occurs via diffusion toward the interface, actual adsorption, and subsequent irreversible conformational changes resulting in denaturation of the native protein structure. The conventional kinetic models describing these steps are based on the assumption that the denaturation transitions obey the first-order law with a single value of the denaturation rate constant kappar. Meanwhile, recent Monte Carlo simulations indicate that, in general, the denaturation process cannot be described by a single rate constant kappar. One should rather introduce a distribution of this rate constant (physically, different values of kappar correspond to the transitions to the altered state via different metastable states). We have calculated the kinetics of irreversible adsorption of proteins with and without distribution of the denaturation rate constant kappar in the limits when protein diffusion in the solution is, respectively, rapid or slow. In both cases, the adsorption kinetics with distribution of kappar are found to be close to those with a single-valued rate constant kappar provided that the average value of kappar in the former case is equal to kappar for the latter case. This conclusion holds even for wide distributions of kappar. The consequences of this finding for the fitting of global experimental kinetics on the basis of phenomenological equations are briefly discussed.

Adsorption↗

PubMed1998

Surface specific kinetics of lipid vesicle adsorption measured with a quartz crystal microbalance.

We have measured the kinetics of adsorption of small (12.5-nm radius) unilamellar vesicles onto SiO2, oxidized gold, and a self-assembled monolayer of methyl-terminated thiols, using a quartz crystal microbalance (QCM). Simultaneous measurements of the shift in resonant frequency and the change in energy dissipation as a function of time provide a simple way of characterizing the adsorption process. The measured parameters correspond, respectively, to adsorbed mass and to the mechanical properties of the adsorbed layer as it is formed. The adsorption kinetics are surface specific; different surfaces cause monolayer, bilayer, and intact vesicle adsorption. The formation of a lipid bilayer on SiO2 is a two-phase process in which adsorption of a layer of intact vesicles precedes the formation of the bilayer. This is, to our knowledge, the first direct evidence of intact vesicles as a precursor to bilayer formation on a planar substrate. On an oxidized gold surface, the vesicles adsorb intact. The intact adsorption of such small vesicles has not previously been demonstrated. Based on these results, we discuss the capacity of QCM measurements to provide information about the kinetics of formation and the properties of adsorbed layers.

Adsorption↗

PubMed1997

Monte Carlo simulation of protein folding with orientation-dependent monomer-monomer interactions.

We present the results of lattice Monte Carlo simulations of protein folding in the framework of a model taking into account (i) the dependence of the energy of interaction of amino-acid residues on their orientation and (ii) the rigidity of the polypeptide chain with respect to the formation of kinks. If the chain is flexible, the final protein structures are predicted to be compact. Increasing the energy cost of creation of kinks is found to favor the formation of flat structures mimicking an ideal antiparallel beta sheet. For compact structures, the kinetics of folding exhibit the standard two-phase regime (a rapid collapse to one of the metastable stable, followed by slow reconfiguration of the chain to the native structure). For flat structures, the transition to the native state is often gradual.

Algorithms↗

PubMed1997

Glow discharge plasma treatment for surface cleaning and modification of metallic biomaterials.

Glow discharge plasma treatment is a frequently used method for cleaning, preparation, and modification of biomaterial and implant surfaces. The merits of such treatments are, however, strongly dependent on the process parameters. In the present work the possibilities, limitations, and risks of plasma treatment for surface preparation of metallic materials are investigated experimentally using titanium as a model system, and also discussed in more general terms. Samples were treated by different low-pressure direct current plasmas and analyzed using Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), atomic force microscopy, scanning electron microscopy, and light microscopy. The plasma system is a home-built, ultra-high vacuum-compatible system that allows sample introduction via a load-lock, and precise control of pressure, gas composition and flow rate, etc. This system allows uniform treatment of cylindrical and screw-shaped samples. With appropriate plasma parameters, argon plasma remove all chemical traces from former treatments (adsorbed contaminants and other impurities, and native oxide layers), in effect producing cleaner and more well-controlled surfaces than with conventional preparation methods. Removal (sputtering) rates up to 30 nm/min are possible. However, when inappropriate plasma parameters are used, the result may be increased contamination and formation of unintentional or undesired surface layers (e.g., carbides and nitrides). Plasma-cleaned surfaces provide a clean and reproducible starting condition for further plasma treatments to form well-controlled surface layers. Oxidation in pure O2 (thermally or in oxygen plasmas) results in uniform and stoichiometric TiO2 surface oxide layers of reproducible composition and thicknesses in the range 0.5-150 nm, as revealed by AES and XPS analyses. Titanium nitride layers were prepared by using N2 plasmas. While mild plasma treatments leave the surface microstructure unaffected, heavy plasma treatment can give rise to dramatic morphologic changes. Comparison of these results with corresponding analyses of commercial implants and electropolished and/or anodically oxidized samples shows that the plasma treatment offers superior control of the surface status. However, it is also shown that improper control of the plasma process can produce unwanted and irreproducible results.

Argon↗

PubMed1997

Simultaneous frequency and dissipation factor QCM measurements of biomolecular adsorption and cell adhesion.

We have measured the energy dissipation of the quartz crystal microbalance (QCM), operating in the liquid phase, when mono- or multi-layers of biomolecules and biofilms form on the QCM electrode (with a time resolution of ca. 1 s). Examples are taken from protein adsorption, lipid vesicle adsorption and cell adhesion studies. Our results show that even very thin (a few nm) biofilms dissipate a significant amount of energy owing to the QCM oscillation. Various mechanisms for this energy dissipation are discussed. Three main contributions to the measured increase in energy dissipation are considered. (i) A viscoelastic porous structure (the biofilm) that is strained during oscillation, (ii) trapped liquid that moves between or in and out of the pores due to the deformation of the film and (iii) the load from the bulk liquid which increases the strain of the film. These mechanisms are, in reality, not entirely separable, rather, they constitute an effective viscoelastic load. The biofilms can therefore not be considered rigidly coupled to the QCM oscillation. It is further shown theoretically that viscoelastic layers with thicknesses comparable to the biofilms studied in this work can induce energy dissipation of the same magnitude as the measured ones.

Adsorption↗

PubMed1997

Chemical characterization and reactivity of iron chelator-treated amphibole asbestos.

Iron in amphibole asbestos is implicated in the pathogenicity of inhaled fibers. Evidence includes the observation that iron chelators can suppress fiber-induced tissue damage. This is believed to occur via the diminished production of fiber-associated reactive oxygen species. The purpose of this study was to explore possible mechanisms for the reduction of fiber toxicity by iron chelator treatments. We studied changes in the amount and the oxidation states of bulk and surface iron in crocidolite and amosite asbestos that were treated with iron-chelating desferrioxamine, ferrozine, sodium ascorbate, and phosphate buffer solutions. The results have been compared with the ability of the fibers to produce free radicals and decompose hydrogen peroxide in a cell-free system in vitro. We found that chelators can affect the amount of iron at the surface of the asbestos fibers and its valence, and that they can modify the chemical reactivity of these surfaces. However, we found no obvious or direct correlations between fiber reactivity and the amount of iron removed, the amount of iron at the fiber surface, or the oxidation state of surface iron. Our results suggest that surface Fe3+ ions may play a role in fiber-related carboxylate radical formation, and that desferrioxamine and phosphate groups detected at treated fiber surfaces may play a role in diminishing and enhancing, respectively, fiber redox activity. It is proposed that iron mobility in the silicate structure may play a larger role in the chemical reactivity of asbestos than previously assumed.

Asbestos, Amphibole↗

PubMed1996

Bone response to surface-modified titanium implants: studies on the early tissue response to machined and electropolished implants with different oxide thicknesses.

The bone formation around titanium implants with varied surface properties is investigated. Machined and electropolished samples with and without thick, anodically formed surface oxides were prepared, surface characterized and inserted in the cortical bone of rabbits (1, 3 and 6 weeks). Scanning electron microscopy, scanning Auger electron spectroscopy and atomic force microscopy revealed marked differences in oxide thickness, surface topography and roughness, but no significant differences in surface chemical composition, between the different groups of implants. Light microscopic morphology and morphometry showed that all implants were in contact with bone and had a large proportion of bone within the threads at 6 weeks. The smooth, electropolished implants, irrespective of anodic oxidation, were surrounded by less bone than the machined implants after 1 week. After 6 weeks the bone volume as well as the bone-implant contact were lower for the merely electropolished implants than for the other three groups. Our study shows that a high degree of bone contact and bone formation are achieved with titanium implants which are modified with respect to oxide thickness and surface topography. However, the result with the smooth (electropolished) implants indicates that a reduction of surface roughness, in the initial phase, decreases the rate of bone formation in rabbit cortical bone.

Animals↗