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Biomedical subjects

Christopher K Ober

Publications and source records attributed to Christopher K Ober.

16 recordsLinked to original sources
PubMed2006

Surfaces of fluorinated pyridinium block copolymers with enhanced antibacterial activity.

Polystyrene-b-poly(4-vinylpyridine) copolymers were quaternized with 1-bromohexane and 6-perfluorooctyl-1-bromohexane. Surfaces prepared from these polymers were characterized by contact angle measurements, near-edge X-ray absorption fine structure spectroscopy and X-ray photoelectron spectroscopy. The fluorinated pyridinium surfaces showed enhanced antibacterial activity compared to their nonfluorinated counterparts. Even a polymer with a relatively low molecular weight pyridinium block showed high antimicrobial activity. The bactericidal effect was found to be related to the molecular composition and organization in the top 2-3 nm of the surface and increased with increasing hydrophilicity and pyridinium concentration of the surface.

Anti-Bacterial Agents↗
PubMed2006

Anti-biofouling properties of comblike block copolymers with amphiphilic side chains.

Surfaces of novel block copolymers with amphiphilic side chains were studied for their ability to influence the adhesion of marine organisms. The surface-active polymer, obtained by grafting fluorinated molecules with hydrophobic and hydrophilic blocks to a block copolymer precursor, showed interesting bioadhesion properties. Two different algal species, one of which adhered strongly to hydrophobic surfaces, and the other, to hydrophilic surfaces, showed notably weak adhesion to the amphiphilic surfaces. Both organisms are known to secrete adhesive macromolecules, with apparently different wetting characteristics, to attach to underwater surfaces. The ability of the amphiphilic surface to undergo an environment-dependent transformation in surface chemistry when in contact with the extracellular polymeric substances is a possible reason for its antifouling nature. Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) was used, in a new approach based on angle-resolved X-ray photoelectron spectroscopy (XPS), to determine the variation in chemical composition within the top few nanometers of the surface and also to study the surface segregation of the amphiphilic block. A mathematical model to extract depth-profile information from the normalized NEXAFS partial electron yield is developed.

Geologic Sediments↗
PubMed2006

Functionalized surface arrays for spatial targeting of immune cell signaling.

The effect of surface topography and chemistry on cellular response is of fundamental importance, especially where living systems encounter device surfaces as in medical implants, tissue engineering, and cell-based sensors. To understand these biological processes on surfaces, there is a widespread interest in tailored surface-active materials produced by a combination of surface chemistry coupled to advanced patterning processes. We utilize self-assembled monolayers (SAMs) as molecular templates with submicrometer-scale spatial resolution to engage and cluster IgE receptors on rat basophilic leukemia (RBL) mast cells. Bioactive templates consisted of gold arrays on silicon with patterns from 1 mum down to 45 nm. These gold arrays served as molecular tethering sites, enabling covalent binding of functionalized self-assembled monolayers of alkanethiols. The free ends of the monolayers were functionalized with 2,4-dinitrophenyl(DNP)-caproate-based ligands which interact specifically with anti-DNP IgE bound to its high affinity cell surface receptor, FcepsilonRI on RBL mast cells. Present results on structures 1 mum down to 600 nm in size indicate that these ligand-immobilized patterned arrays can function as a powerful tool for visualization and systematic characterization of cell membrane involvement in IgE receptor-mediated immune cell signaling.

Microscopy, Confocal↗
PubMed2006

Real-time analysis of enzymatic surface-initiated polymerization using surface plasmon resonance (SPR).

The kinetics of enzymatic surface-initiated polymerization of PHB on gold surface has been examined by SPR and the resultant polymer layers characterized by AFM and FT-IR spectrometry. The immobilized enzyme catalyzed surface-initiated polymerization of 3HB-CoA, resulting in the formation of a polymer brush on the surface. The rate of polymer growth from the surface was monitored by SPR in real-time. Polymer growth as measured by the increase in the resonance angle showed no apparent lag phase during the polymerization reaction. SPR analysis also revealed that the thickness of the polymer film could be controlled by varying the initial enzyme density on the surface. The average thicknesses of the PHB film after polymerization reaction were 95, 45 and 15 nm for the surfaces that were treated with 0.5, 0.3 and 0.1*10(-6) M of enzyme, respectively. The binding of PHA synthase at different concentration to the mixed SAMs and subsequent polymerization.

Acyltransferases↗
PubMed2006

Dinitrophenyl ligand substrates and their application to immunosensors.

We present an approach for the development of highly specific and sensitive antibody based biosensors by chemically tailoring the sensor surface with materials that control specific and nonspecific binding of biologically relevant molecules. As a model system we employed surface immobilized 2,4-dinitrophenyl (DNP)-ligands that bind specifically to anti-DNP antibodies. Self-assembling characteristics and minimization of the nonspecific interactions were used in the ligand design. The redox activity of the DNP-head group was used to calculate the surface density (coverage) of these assemblies using cyclic voltammetry. Quartz crystal microbalance (QCM) and impedance analysis were used to assess the ligand-antibody interaction and estimate the quantity of antibodies bound to the surface. The ligand surface density and the QCM data were useful in determining the sensitivity of our model system. A simple two-step kinetic model was shown to fit the experimental data.

Antibodies↗
PubMed2006

Comparison of the fouling release properties of hydrophobic fluorinated and hydrophilic PEGylated block copolymer surfaces: attachment strength of the diatom Navicula and the green alga Ulva.

To understand the role of surface wettability in adhesion of cells, the attachment of two different marine algae was studied on hydrophobic and hydrophilic polymer surfaces. Adhesion of cells of the diatom Navicula and sporelings (young plants) of the green macroalga Ulva to an underwater surface is mainly by interactions between the surface and the adhesive exopolymers, which the cells secrete upon settlement and during subsequent colonization and growth. Two types of block copolymers, one with poly(ethylene glycol) side-chains and the other with liquid crystalline, fluorinated side-chains, were used to prepare the hydrophilic and hydrophobic surfaces, respectively. The formation of a liquid crystalline smectic phase in the latter inhibited molecular reorganization at the surface, which is generally an issue when a highly hydrophobic surface is in contact with water. The adhesion strength was assessed by the fraction of settled cells (Navicula) or biomass (Ulva) that detached from the surface in a water flow channel with a wall shear stress of 53 Pa. The two species exhibited opposite adhesion behavior on the same sets of surfaces. While Navicula cells released more easily from hydrophilic surfaces, Ulva sporelings showed higher removal from hydrophobic surfaces. This highlights the importance of differences in cell-surface interactions in determining the strength of adhesion of cells to substrates.

Biopolymers↗
PubMed2005

Oligo(ethylene glycol) containing polymer brushes as bioselective surfaces.

The nitroxide-mediated polymerization of styrenic monomers containing oligo(ethylene glycol) (OEGn) moieties was chosen for the preparation of biocompatible polymer brushes tethered to silicon oxide surfaces due to the broad range of monomer structures available and the use of a nonmetallic initiator. These surfaces were characterized by near-edge X-ray absorption fine structure and water contact angle measurements. The biocompatibility of these grown polymer brushes was studied and compared with deposited assemblies of surface-bound OEGn-terminated silanes with selected chain lengths. Grown polymer brushes with short OEGn side chains suppressed protein adsorption significantly more than the deposited assemblies of short OEGn chains, and this was attributed to higher surface coverage by the brushes. Cell adhesion studies confirmed that OEGn-containing polymer brushes are particularly effective in preventing nonspecific adhesion. Studies of protein adsorption and cell localization carried out with specific ligands on surfaces patterned demonstrated the potential of these surface-tethered polymer brushes for the formation of micro- and nanoscale devices.

Adsorption↗
PubMed2004

Control of surface properties using fluorinated polymer brushes produced by surface-initiated controlled radical polymerization.

Surface-grafted styrene-based homopolymer and diblock copolymer brushes bearing semifluorinated alkyl side groups were synthesized by nitroxide-mediated controlled radical polymerization on planar silicon oxide surfaces. The polymer brushes were characterized by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and time-dependent water contact angle measurements. Angle-resolved XPS studies and water contact angle measurements showed that, in the case of the diblock copolymer brushes, the second block to be added was always exposed at the polymer-air interface regardless of its surface energy. Values of z*/Rg were estimated based on the radius of gyration, Rg, of the grafted homopolymer or block copolymer chains for the grafted brushes and thickness of the brush, z*. The fact that z*/Rg > 1 suggests that all these brushes are stretched. These results support the idea that after grafting the first block onto the surface the nitroxide-end capped polymer chains were able to polymerize the second block in a "living" fashion and the stretched brush so formed was dense enough that the outermost block in all cases completely covers the surface. NEXAFS analysis showed a relationship between the surface orientation of the fluorinated side chains and brush thickness with thicker brushes having more oriented side chains. Time-dependent water contact angle measurements revealed that the orientation of the side chains of the brush improved the surface stability toward reconstruction upon prolonged exposure to water.

Absorption↗
PubMed2004

Fluorinated polymers: liquid crystalline properties and applications in lithography.

Fluorinated polymers form an interesting class of materials with a wealth of unique properties including self-assembly, remarkably low surface energies, low absorbance to 157 nm UV light, and solubility in supercritical carbon dioxide. As a result many fluorinated polymers are of use in advanced technology applications. We review some of our work on the synthesis and characterization of block copolymers with fluorinated side chains, with special emphasis on surfaces formed using these polymers. The use of fluorinated polymers as photoresists for 157 nm lithography, with the possibility for processing in environmentally friendly supercritical carbon dioxide is also discussed.

Journal Article↗
PubMed2003

Coatings based on side-chain ether-linked poly(ethylene glycol) and fluorocarbon polymers for the control of marine biofouling.

The preparation of side group modified polystyrene-based surface-active block copolymers (SABC) for use as marine fouling resistance/release applications is described. Modifying moieties such as poly(ethylene glycol) (PEG) and semifluorinated segments were used. A novel bilayer methodology has been employed that provides both suitable mechanical properties through the use of an elastomeric primer layer of styrene-ethylene/butylene-styrene (SEBS) and control of surface-chemistry through use of the SABCs. This approach has potential as a cost-effective technology for environmentally benign coatings that resist and release marine biofouling. Initial testing of these materials included determination of captive bubble contact angles and protein adsorption. Testing against marine fouling organisms was performed using settlement and adhesion bioassays with zoospores of the green alga Enteromorpha. The results showed that all surfaces had markedly reduced levels of zoospore settlement compared with glass controls and that adhesion strength was strongly affected by the semifluorinated SABC. The results are discussed in terms of surface properties.

Adhesiveness↗
PubMed2002

An efficient two-photon-generated photoacid applied to positive-tone 3D microfabrication.

A two-photon-activatable photoacid generator, based on a bis[(diarylamino) styryl]benzene core with covalently attached sulfonium moieties, has been synthesized. The photoacid generator has both a large two-photon absorption cross section (delta = 690 x 10(-50) centimeter(4) second per photon) and a high quantum yield for the photochemical generation of acid (phiH+ = 0.5). Under near-infrared laser irradiation, the molecule produces acid after two-photon excitation and initiates the polymerization of epoxides at an incident intensity that is one to two orders of magnitude lower than that needed for conventional ultraviolet-sensitive initiators. This photoacid generator was used in conjunction with a positive-tone chemically amplified resist for the fabrication of a three-dimensional (3D) microchannel structure.

Journal Article↗
PubMedDate not supplied

Methods for the topographical patterning and patterned surface modification of hydrogels based on hydroxyethyl methacrylate.

Hydrogels have gained broad acceptance as a class of biocompatible materials. In this paper, we report the topographic patterning and regiospecific functionalization of hydrogel surfaces. Both photolithography and soft lithography are combined in a hybrid process to form these topographic features. By functionalization of a base layer surface followed by lithographic patterning steps, it is possible to introduce chemical functions to specific regions of the patterned surface. The model systems investigated were based on 2-hydroxyethyl methacrylate (HEMA), which is well-known for its low toxicity and widespread use in biomedical applications. Tests of Ni-NTA modified hydrogel surfaces showed successful binding of fluorescently labeled proteins to selected regions of the patterned hydrogel surface. These processes can be expanded to a wide range of monomer systems.

Biocompatible Materials↗
PubMedDate not supplied

Enzymatic surface-initiated polymerization: a novel approach for the in situ solid-phase synthesis of biocompatible polymer poly(3-hydroxybutyrate).

A novel system for surface-initiated enzymatic polymerization of a film of polyhydroxyalkanoate (PHA) on solid surfaces has been developed and characterized. PHAs are aliphatic polyesters produced by a variety of microorganisms as a reserve of carbon and energy, and their properties range from elastomers to thermoplastics, depending on their monomeric composition. The PHA synthase from Ralstonia eutropha H16 was expressed as a poly-histidine fusion in Escherichia coli and immobilized onto several solid substrates through a transition-metal complex, Ni(2+)-nitrilotriacetic acid. The immobilized PHA synthase catalyzed the surface-initiated polymerization of 3-(R)-hydroxybutyryl-CoA, forming a polymer film with a uniform thickness on the surface. In this work, we describe the patterned immobilization of the intact enzyme on silicon and subsequent enzymatic polymerization. The immobilized enzyme had a lower specific activity and did not exhibit a lag phase as compared to the soluble enzyme.

Acyltransferases↗
PubMedDate not supplied

Self-assembled monolayers and polymer brushes in biotechnology: current applications and future perspectives.

The chemistry and topography of a surface affect biological response and are of fundamental importance, especially when living systems encounter synthetic surfaces. Most biomolecules have immense recognition power (specific binding) and simultaneously have a tendency to physically adsorb onto a solid substrate without specific receptor recognition (nonspecific adsorption). Therefore, to create useful materials for many biotechnology applications, interfaces are required that have both enhanced specific binding and reduced nonspecific binding. Thus, in applications such as sensors, the tailoring of surface chemistry and the use of micro or nanofabrication techniques becomes an important avenue for the production of surfaces with specific binding properties and minimal background interference. Both self-assembled monolayers (SAMs) and polymer brushes have attracted considerable attention as surface-active materials. In this review, we discuss both of these materials with their potential applications in biotechnology. We also summarize lithographic methods for pattern formation using combined top-down and bottom-up approaches and briefly discuss the future of these materials by describing emerging new applications.

Adsorption↗