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J Rühe

Publications and source records attributed to J Rühe.

8 recordsLinked to original sources
PubMed2004

Electrolyte-induced collapse of a polyelectrolyte brush.

We have investigated the electrolyte-induced collapse of a polyelectrolyte brush covalently attached to a planar solid surface. Positively charged poly-4-vinyl [N-methyl-pyridinium] (MePVP) brushes were prepared in situ at the surface by free radical chain polymerization using a surface-immobilized initiator monolayer ("grafting from" technique) and 4-vinylpyridine as the monomer, followed by a polymer-analogous quaternization reaction. The height of the brushes was measured as a function of the external salt concentration via multiple-angle null ellipsometry. As predicted by mean-field theory, the height of the MePVP brushes remains unaffected by the addition of low amounts of external salt. At higher salt concentrations the brush height decreases. The extent to which the brush shrinks strongly depends on the nature of the salt present in the environment. MePVP brushes collapse to almost the dry layer thickness upon the addition of potassium iodide to a contacting aqueous medium. In contrast, the collapse of MePVP brushes having bromide or chloride counterions is much less pronounced. These brushes remain in a highly swollen state even after large amounts of salt have been added to the solution.

Journal Article↗
PubMed2004

Collective dynamics of an end-grafted polymer brush in solvents of varying quality.

The dynamic structure of a chemically end-grafted polystyrene brush bathed in solvents of varying interactions was studied by evanescent wave dynamic light scattering. It reveals distinct behavior under good and poor solvent conditions. The cooperative diffusion is a generic feature of a good solvent environment, whereas a second slow relaxation mode appears in the theta solvent regime. Its characteristics resemble self-diffusion of clusters in a gel while weak concentration fluctuations in the polymer brush decay similarly to a semidilute polymer solution.

Journal Article↗
PubMed2001

Surface attached ultrathin polymer monolayers for control of cell adhesion.

BACKGROUND: Calcific degeneration is the major drawback of bioprostheses. None of the numerous preventive approaches omitted calcification. Previous studies showed that cellular surface seeding decreases calcium uptake in vitro but achievement of coverage remains problematic. A new approach is presented masking glutaraldehyde residues with a polymer layer allowing cell seeding. The aim of this study was to evaluate different polymers for suitability. METHODS: Ten polymers--covalently bound to glass--were tested for their ability to seed animal and human cells. Quality of coverage was evaluated by light and scanning electron microscopy, and polymers were characterized physicochemically. RESULTS: Quality of cellular growth was similar for canine and human cells. Five polymers allowed excellent surface coverage, two led to a decrease of cell adherence, and four to poor cellular growth. No correlation between molecular weight, thickness, hydrophilicity, or charge of the polymer and cell growth was found. CONCLUSIONS: Polymer monolayers can promote cellular growth but without correlation to physicochemical characteristics. Polymers covalently bound to biologic tissue appear to be a promising approach for achieving cellular coverage of biomaterials.

Animals↗
PubMed2000

Functional tethered lipid bilayers.

Our strategy to provide the structural basis for the build-up of functional tethered membranes focuses on three approaches: the first one is based on the pre-organization of a monomolecular layer of a lipopolymer at the water/air interface which is then transferred to a solid support. Prior to deposition, the substrate is coated with a layer of benzophenone-derivatized silane molecules that allow for a stable covalent attachment by photo-cross-linking of some of the monomer units of the lipopolymer to the support. An alternative concept realizes a layer-by-layer deposition of the various structural elements: (1) the attachment layer with the reactive sites for the chemical stabilization; (2) a polymer 'cushion' prepared by adsorption and simultaneous or subsequent partial covalent binding to the reactive sites; and (3) a lipid monolayer transferred from the water/air interface, that contains a certain amount of lipids with reactive headgroups which, upon binding to the polymer tether, act as anchor lipids stabilizing the whole monolayer/cushion-composite. And finally, we build peptide-supported monolayers by first (self-) assembling amino acid sequences of various lengths via a SH-group near their N-terminus onto Au substances and use then their COO(-)-terminus to chemically attach phosphatidyl-ethanolamine lipids to form a stable monolayer of lipid-peptide conjugates. All the individual preparation steps and the various resulting (multi-) layers are characterized by surface plasmon spectroscopy, X-ray and neutron-reflectometry, contact angle measurements, IR spectroscopy, fluorescence microscopy, scanning probe microscopies, as well as, electrochemical techniques. For all tethering systems, the final membranes' architecture is obtained by fusing lipid vesicles onto the lipid monolayer. Proteins can be incorporated by either fusing vesicles that are loaded with the respective receptors, pores, or ion pumps via a reconstitution procedure, or via a transfer directly from a micellar solution to the pre-formed lipid bilayer at the solid support by a dialysis step. Two structural/dynamical features of tethered membranes which are considered to be of particular functional relevance, i.e. the degree of water uptake and, hence, the degree of swelling of the polymer support, as well as the lateral mobility of the lipid molecules in the membrane, are tested by surface plasmon optics and by measurements of the fluorescence recovery after photobleaching (FRAP), respectively. The results confirm that the presented preparation protocols yield fluid bilayers that mimic certain relevant properties of biological membranes. The functional characterization of tethered membranes, which is briefly summarized, is based on various electrochemical techniques, in particular, impedance spectroscopy, cyclic voltammetry, and chronoamperometric studies. The results obtained for reconstituted H(+)-ATPase from chloroplasts and E. coli and for cytochrome oxidase (with and without cytochrome c) confirm the incorporation of the proteins in an active form, thus, opening opportunities for novel sensor formats or offering a completely new model membrane system.

Biopolymers↗
PubMed1999

Tailoring of surfaces with ultrathin polymer films for survival and growth of neurons in culture.

We have studied the adhesion, growth, and survival of dissociated cells of rat cerebellum onto glass surfaces derivatized with covalently bound ultrathin polymer layers. The surface coatings were prepared by growing polymer chains directly at the surfaces of solid substrates by using self-assembled monolayers of radical chain initiators. Methacrylate and acrylamide polymers with different polarities were covalently attached to the surfaces and the adhesion behaviour of the neuronal cells to these surfaces was studied. Best adhesion and neurite outgrowth properties were found using a positively charged polymer (poly(methacryloyl oxypropyl (trimethyl ammonium) bromide)). This 'grafting from' method for the generation of surface modifications can be used for tailoring the composition of the surface of the substrates. Our experiments demonstrate the potential of these layers for the design of polymeric surface coatings for long term stability of neuronal culture.

Animals↗
PubMedDate not supplied

The polymer-supported phospholipid bilayer: tethering as a new approach to substrate-membrane stabilization.

We present a new molecular engineering approach in which a polymer-supported phospholipid bilayer is vertically stabilized by controlled covalent tethering at both the polymer-substrate and polymer-bilayer interfaces. This approach is based on lipopolymer molecules, which not only form a polymer cushion between the phospholipid bilayer and a solid glass substrate but also act as covalent connections (tethers) between the bilayer and cushion. Our approach involves Langmuir-Blodgett transfer of a phospholipid-lipopolymer monolayer followed by Schaefer transfer of a pure phospholipid monolayer and is capable of varying the tethering density between the polymer layer and the phospholipid bilayer in a very controlled manner. Further stabilization is achieved if the glass substrate is surface-functionalized with a benzophenone silane. In this case, a photocross-linking reaction between the polymer and benzophenone group allows for the covalent attachment of the polymer cushion to the glass substrate. This approach is similar to that recently reported by Wagner and Tamm in which double tethering is achieved via lipopolymer silanes (Wagner, M. L.; Tamm, L. K. Biophys. J. 2000, 79, 1400). To obtain a deeper understanding of how the covalent tethering affects the lateral mobility of the bilayer, we performed fluorescence recovery after photobleaching (FRAP) experiments on polymer-tethered bilayers at different tethering densities (lipopolymer/phospholipid molar ratios). The FRAP data clearly indicate that the hydrophobic lipopolymer moieties act as rather immobile obstacles within the phospholipid bilayer, thereby leading to hindered diffusion of phospholipids. Whereas the high lateral diffusion coefficient of D = 17.7 mum(2)/s measured at low tethering density (5 mol % lipopolymer) indicates rather unrestricted motion within the bilayer, corresponding values at moderate (10 mol % lipopolymer) and high (30 mol % lipopolymer) tethering densities of D = 9.7 mum(2)/s and D = 1.1 mum(2)/s, respectively, show significant hindered diffusion. These results are contrary to the recent findings on similar membrane systems reported by Wagner and Tamm in which no significant change in phospholipid diffusion was found between 0 and 10 mol % lipopolymer. Our experimental report leads to a deeper understanding of the complex problem of interlayer coupling and offers a path toward a compromise between stability of the whole system and lateral mobility within the bilayer. Furthermore, the FRAP measurements show that polymer-tethered membranes are very interesting model systems for studying problems of restricted diffusion within two-dimensional fluids.

Diffusion↗