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ABSTRACT: The fluid transport produced by rectangular shaped, magnetically actuated artificial cilia of 70 μm length and 20 μm width was determined by means of phase-locked Micro Particle Image Velocimetry (μPIV) measurements in a closed microfluidic chamber. The phase-averaged flow produced by the artificial cilia reached up to 130 μm s(-1) with an actuation cycle frequency of 10 Hz. Analysis of the measured flow data indicate that the present system is capable of achieving volume flow rates of V[combining dot above](cilia) = 14 ± 4 μl min(-1) in a micro channel of 0.5 × 5 mm(2) cross-sectional area when no back pressure is built up. This corresponds to an effective pressure gradient of 6 ± 1 Pa m(-1), which equals a pressure difference of 0.6 ± 0.1 mPa over a distance of 100 μm between two rows of cilia. These results were derived analytically from the measured velocity profile by treating the cilia as a thin boundary layer. While the cilia produce phase-averaged velocities of the order of O(10(2)μm s(-1)), time-resolved measurements showed that the flow field reverses two times during one actuation cycle inducing instantaneous velocities of up to approximately 2 mm s(-1). This shows that the flow field is dominated by fluid oscillations and flow rates are expected to increase if the beating motion of the cilia is further improved.
Lab on a Chip 06/2011; 11(12):2017-22. · 5.67 Impact Factor
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ABSTRACT: In this paper we quantitatively analyse the performance of magnetically-driven artificial cilia for lab-on-a-chip applications. The artificial cilia are fabricated using thin polymer films with embedded magnetic nano-particles and their deformation is studied under different external magnetic fields and flows. A coupled magneto-mechanical solid-fluid model that accurately captures the interaction between the magnetic field, cilia and fluid is used to simulate the cilia motion. The elastic and magnetic properties of the cilia are obtained by fitting the results of the computational model to the experimental data. The performance of the artificial cilia with a non-uniform cross-section is characterised using the numerical model for two channel configurations that are of practical importance: an open-loop and a closed-loop channel. We predict that the flow and pressure head generated by the artificial cilia can be as high as 18 microlitres per minute and 3 mm of water, respectively. We also study the effect of metachronal waves on the flow generated and show that the fluid propelled increases drastically compared to synchronously beating cilia, and is unidirectional. This increase is significant even when the phase difference between adjacent cilia is small. The obtained results provide guidelines for the optimal design of magnetically-driven artificial cilia for microfluidic propulsion.
Lab on a Chip 02/2011; 11(12):2002-10. · 5.67 Impact Factor
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ABSTRACT: Portable microacoustic sensors capable of detection on the molecular level have recently become of increasing interest. Thin-film
acoustic sensors based on GaN substrates are of particular merit because of their high chemical stability, high acoustic wave
velocity and their ability to detect molecules in liquids. In this paper the use of sensitive layers for selective particle
detection, as well as an investigation into the binding affinity of GaN, are presented. A sensitive layer for detection of
diols, such as sugars or neurotransmitters, has been realized by attaching a polymer layer to the surface. A final cytotoxicity
test has also been conducted on all of the presented substrates. In this work layers were successfully attached to a substrate
and characterized, exhibiting long term chemical stability as well as good homogeneity and roughness making them adequate
for future applications in microacoustic sensors. The cytotoxicity test yielded negative results, indicating that toxins are
not liberated to the medium, thereby contaminating the sample. These types of sensors find their application in the quantification
of samples and analysis of their chemical properties.
KeywordsSensitive layers–microacoustic sensors–gallium nitride–photochemistry
01/2010: pages 339-342;
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ABSTRACT: Human papillomavirus (HPV) plays a key role in the development of cervical and laryngeal cancers. The aim of our study was to compare the performance of a new hydrogel-based HPV genotyping biochip assay (Biochip) to a commercially available and CE-marked conventional PCR followed by reverse hybridization (GenID-PCR). One hundred twenty-three samples were available for the study. Of these samples, 101/123 were gynecological swabs, 8/123 were swabs or biopsy samples of genital warts, 7/123 were biopsy samples of otorhinolaryngeal lesions, 5/123 were samples of skin warts, and 2/123 were samples of orolabial abnormalities. These molecular methods for HPV genotyping showed comparable sensitivity and specificity. However, 19/123 of the results were discrepant. Specifically, Biochip showed better performance in the detection of multiple infections, especially when more than one high-risk genotype was present. Due to the different probe configurations used in the two assays, GenID-PCR achieves only group-specific detection of many HPV genotypes, whereas Biochip allows for specific identification. Overall, the newly developed HPV chip system (Biochip) proved to be a suitable tool for HPV detection and genotyping; it also proved to be superior for establishing HPV genotyping methods.
Journal of clinical microbiology 04/2009; 47(5):1428-35. · 4.16 Impact Factor
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ABSTRACT: Peptide-polymer hybrid molecules are being introduced, where one part of the molecule (i.e., the peptide) promotes the adhesion of living cells, whereas the other part of the molecule (i.e., the synthetic polymer) is known to prevent cell adhesion. The hybrid copolymer, poly(dimethylacrylamide) (PDMAA)-glycine-arginine-glycine-aspartic acid-serine-proline (GRGDSP) was synthesized by first preparing an initiator-modified peptide and in a second step growing the PDMAA block directly off the peptide through atom transfer radical polymerization (ATRP). The PDMAA block length can be varied by adjusting appropriate polymerization conditions, thereby changing progressively the amount of the cell-repelling part of the molecule. The hybrid copolymer was further used to prepare surface-attached peptide-polymer monolayers at planar solid glass substrates through a photochemical immobilization process. By blending of the hybrid copolymer with PDMAA homopolymer (i.e., without peptide), the apparent peptide film concentration can be varied in a very simple manner. The adhesion of human skin fibroblast cells in serum-free medium was investigated as a function of the amount of peptide-polymer in the solution used for film preparation. Cells do not adhere to a pure PDMAA monolayer; however, already 0.02 wt % of peptide in the film is enough to induce cell adhesion, and 0.1 wt % promotes stress-fiber formation within adherent cells. Using lithographical means, chemically micropatterned peptide-polymer films were prepared that allow for a spatial control of the adhesion of living cells and thus they constitute a simple platform for the design of live-cell biochips.
Biomacromolecules 03/2008; 9(2):543-52. · 5.48 Impact Factor
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L Riegger,
M Grumann,
J Steigert,
S Lutz,
C P Steinert,
C Mueller,
J Viertel,
O Prucker, J Rühe,
R Zengerle,
J Ducrée
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ABSTRACT: We present a novel concept to process human blood on a spinning polymer disk for the determination of the hematocrit level by simple visual inspection. The microfluidic disk which is spun by a macroscopic drive unit features an upstream metering structure and a downstream blind channel where the centrifugally enforced sedimentation of the blood is performed. The bubble-free priming of the blind channel is governed by centrifugally assisted capillary filling along the sloped hydrophilic side-wall and the lid as well as the special shape of the dead end of the two-layer channel. The hematocrit is indicated at the sharp phase boundary between the plasma and the segregated cellular pellet on a disk-imprinted calibrated scale. This way, we conduct the hematocrit determination of human blood within 5 min at a high degree of linearity (R(2) = 0.999) and at a high accuracy (CV = 4.7%) spanning over the physiological to pathological working range. As all processing steps including the priming, the metering to a defined volume as well as the centrifugation are executed automatically during rotation, the concept is successfully demonstrated in a conventional PC-CDROM drive while delivering the same performance (R(2) = 0.999, CV = 4.3%).
Biomedical Microdevices 01/2008; 9(6):795-9. · 3.03 Impact Factor
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European cells & materials 01/2005; 10(SUPPL.5). · 3.03 Impact Factor
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ABSTRACT: 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.
The Journal of Chemical Physics 06/2004; 120(18):8807-14. · 3.33 Impact Factor
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ABSTRACT: The preparation and swelling behavior of surface-attached charged statistical copolymer brushes are described. The brushes are prepared by free radical chain polymerization of monomer mixtures of 4-vinylpyridine (VP) and dimethylacrylamide (DMAA) using a surface bound azo-initiator (surface-initiated polymerization). So-prepared neutral PVP-stat-PDMAA copolymer brushes are then transferred into polyelectrolyte brushes through a polymer-analogous quaternization reaction of the respective PVP segments. By adjusting the ratio of the monomers added to the polymerization feed, the concentration of the corresponding polymer segments generated in the final surface-attached layer can be controlled, and thus, the charge density of the resulting polyelectrolyte copolymer brush can be determined. Finally, the swelling behavior of such polyelectrolyte brushes in aqueous solutions containing low molecular weight salts is investigated by using multiple-angle null ellipsometry. Depending on the charge density of the copolymers, the brushes exhibit a swelling behavior between that of a polyelectrolyte and a neutral polymer brush. Thus, the sensitivity of the layers against salt present in the surrounding medium can be precisely controlled.
02/2004;
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01/2004;
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ABSTRACT: 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.
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.
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.
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.
The Annals of Thoracic Surgery 06/2001; 71(5 Suppl):S437-40. · 3.74 Impact Factor
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ABSTRACT: Recent surface rheology and film balance experiments on monolayers of PEG lipopolymers and phospholipid/PEG lipopolymer mixtures at the air−water interface have revealed a new class of quasi two-dimensional physical networks. Two different kinds of associative interactions are necessary to form the network: microcondensation of alkyl chains of lipopolymers to form small clusters and water molecule mediation of the interaction between adjacent PEG clusters via hydrogen bonding. In the experiments presented here, we are interested to learn whether the physical gelation is PEG specific or whether it is a more general characteristic of lipopolymers at the air−water interface. To address this topic, we have expanded our surface rheology and film balance experiments to poly(oxazoline) lipopolymers. Our experiments indicate the occurrence of a rheological transition if the poly(oxazoline) lipopolymers consist of a dioctadecylglycerol anchor. This shows that the physical gelation among lipopolymers is not a PEG-specific phenomenon. No physical gelation is found, however, if the dioctadecylglycerol anchor of the lipopolymer is replaced by a dioctadecylamine anchor. The observed importance of the hydrophobic anchor supports our previous findings that the alkyl chain condensation should be seen as one of two kinds of physical junctions necessary for the formation of the physical network.
04/2001;
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ABSTRACT: 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.
Journal of Biotechnology 10/2000; 74(3):137-58. · 3.05 Impact Factor
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ABSTRACT: We present an experimental study on the swelling of polyelectrolyte brushes covalently attached to planar solid surfaces in contact with humid air. Monolayers of poly-N-methyl-[4-vinylpyridinium]iodide (MePVP) with film thicknesses of several hundred nanometers were used in this study. The MePVP brushes were attached to the surfaces of silicon wafers as well as to evaporated silicon oxide films on solid substrates by using self-assembled monolayers of an azo initiator and radical chain polymerization in situ. The film thicknesses of the surface-bound monolayers were measured by optical waveguide spectroscopy (OWS) as a function of the humidity of the environment. The MePVP brushes show strong increases in thickness as well as a strong decrease of the refractive index of the surface-attached layer due to water incorporation caused by the exposure to the humid environment.
11/1999;
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ABSTRACT: A novel concept for the synthesis of molecularly thin polyelectrolyte layers covalently attached to planar solid surfaces is introduced. The polymer monolayers are generated directly at the surface by using self-assembled monolayers of an azo initiator and radical chain polymerization in situ. This “grafting from” approach yields surface-bound polymer molecules with a high molecular weight and with high graft densities of the attached chains (“brush”-like state). As an example for the preparation of a positively charged polyelectrolyte “brush” at first a monolayer of poly(4-vinylpyridine) (PVP) was generated at the surface, which was subsequently quarternized to yield a poly[4-vinyl-N-n-butylpyridinium] bromide (BuPVP) monolayer. The thickness of this resulting cationic monolayer can be controlled in a wide range, starting from 2 nm to more than 1000 nm in the solvent-free state.
03/1999;
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ABSTRACT: 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.
Journal of Biomaterials Science Polymer Edition 02/1999; 10(8):859-74. · 1.69 Impact Factor
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ABSTRACT: A novel concept for the generation of molecularly thin polymer layers attached to a solid surface is described. In contrast to commonly used techniques, where functional groups of the polymers are reacted with appropriate sites on the surfaces of a substrate the polymer layers are formed in situ by using self-assembled monolayers of an initiator. As an example, the formation of polystyrene monolayers terminally attached to silicon oxide surfaces through radical chain polymerization which has been started by a self-assembled monolayer of an azo initiator is described. The thickness of the attached polymer films can be adjusted over a wide range up to values of several hundred nanometers by variation of polymerization parameters such as temperature and azo conversion. When suitable conditions are chosen, monolayers with thicknesses inaccessible by other techniques of preparation can be obtained.
10/1998;
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ABSTRACT: We report on the radical-chain polymerization of styrene using self-assembled monolayers of azo initiators covalently bound to high surface area silica gels. In this process monolayers of poly(styrene) molecules terminally attached to the surface of the inorganic substrate are obtained. As the initiator molecules are immobilized at the surfaces in a one-step reaction, well-reproducible layers can be prepared and the surface concentration of the initiator can be adjusted in a wide range between the limit of detection and full surface coverage. In the subsequent polymerization reactions polymer monolayers with high, controlled graft density can be obtained. The synthesis of the attached layers and the characterization by X-ray photoelectron spectroscopy, diffuse reflectance infrared spectroscopy (DRIFT), and elemental analysis are described. After cleavage of an ester group that connects the polymers to the surface, the molecular weights of the polymers were determined. The results of the study show that this “grafting from” technique can be used for the preparation of polymer layers with controlled, high graft densities.
01/1998;
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ABSTRACT: The kinetics and mechanism of a radical chain polymerization reaction initiated from a self-assembled monolayer of an azo initiator attached to the surfaces of silica particles are investigated. The rate of the decomposition of the surface-attached initiator is followed by differential scanning calorimetry and volumetry. The kinetics of formation of the terminally attached polymer is studied by dilatometry. After polymerization the polymer is removed from the surfaces and the molecular weight averages and molecular weight distribution of the degrafted polymer are studied as a function of reaction parameters during polymerization. From the molecular weight, the mass of the attached polymer, and the specific surface area, the number of polymer molecules per area (or the distance between the anchoring sites) can be calculated and compared to the corresponding values of the initiator monolayers. The mechanism of a polymerization with surface-attached radicals is compared to that of conventional radical chain polymerization in solution.
01/1998;
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ABSTRACT: To improve the properties of materials in biomedical applications and to allow a better interaction of the medical device with the biological system surrounding it, frequently polymeric coatings are applied. However, the adhesion of the coating to the substrate usually poses a problem as the materials involved have either rather inert surfaces or strongly varying surface chemistries. We describe a new approach which allows to attach a wide variety of polymer films to organic substrates either of polymeric or biological origin. The technique is based on photochemical processes occurring in benzophenone group containing polymers, which lead to simultaneous crosslinking of the polymer in the coating and surface-attachment of the forming polymer network. The synthesis and characterization of monolayers and surface-attached polymer networks through this route are described and possible applications of this approach in the biomedical area are discussed.
Surface Science. 570:111-118.
