Biomedical subjects

L E Scriven

Publications and source records attributed to L E Scriven.

At least 19 recordsLinked to original sources

PubMed2006

A model system for increasing the intensity of whole-cell biocatalysis: investigation of the rate of oxidation of D-sorbitol to L-sorbose by thin bi-layer latex coatings of non-growing Gluconobacter oxydans.

We developed a novel <50-microm thick nano-porous bi-layer latex coating for preserving Gluconobacter oxydans, a strict aerobe, as a whole cell biocatalyst. G. oxydans was entrapped in an acrylate/vinyl acetate co-polymer matrix (T (g) approximately 10 degrees C) and cast into 12.7-mm diameter patch coatings (cellcoat) containing approximately 10(9) CFU covered by a nano-porous topcoat. The oxidation of D-sorbitol to L-sorbose was used to investigate the coating catalytic properties. Intrinsic kinetics was studied in microbioreactors using a pH 6.0 D-sorbitol, phosphate, pyruvate (SPP) non-growth medium at 30 degrees C, and the Michaelis-Menten constants determined. By using a diffusion cell, cellcoat and topcoat diffusivities, optimized by arresting polymer particle coalescence by glycerol and/or sucrose addition, were determined. Cryo-FESEM images revealed a two-layer structure with G. oxydans surrounded by <40-nm pores. Viable cell density, cell leakage, and oxidation kinetics in SPP medium for >150 h were investigated. Even though the coatings were optimized for permeability, approximately 50% of G. oxydans viability was lost during cellcoat drying and further reduction was observed as the topcoat was added. High reaction rates per unit volume of coating (80-100 g/L x h) were observed which agreed with predictions of a diffusion-reaction model using parameters estimated by independent experiments. Cellcoat effectiveness factors of 0.22-0.49 were observed which are 20-fold greater than any previously reported for this G. oxydans oxidation. These nano-structured coatings and the possibility of improving their ability to preserve G. oxydans viability may be useful for engineering highly reactive adhesive coatings for multi-phase micro-channel and membrane bioreactors to dramatically increase the intensity of whole-cell oxidations.

Bioreactors↗

PubMed2006

Stress generation by solvent absorption and wrinkling of a cross-linked coating atop a viscous or elastic base.

An in-plane constrained cross-linked gel layer absorbs an equilibrium amount of solvent and experiences in-plane compressive stress. A stability analysis of such an elastic gel layer that is attached to either a viscous or an elastic bottom layer atop a rigid substrate is considered. The effects of the top and bottom layer moduli (E(t) and E(b)), the bottom-to-top layer thickness ratio (H/h), and the polymer solvent interaction parameter (chi) on the critical condition of wrinkling, wrinkle wavelength, and amplitude are examined. When the bottom layer is viscous, the compressed top layer is always unstable, and wrinkling is rate-controlled. The viscous flow of the bottom layer governs the rate and determines the fastest growing wavelength. As E(t) rises, the bending stiffness of the elastic layer does as well, and so the fastest growing wavelength (lambda(m)) rises and the equilibrium amplitude (A(e)) falls. As H/h rises, the constraint of the rigid substrate diminishes, and so lambda(m) and A(e) rise. As chi falls or as the solvent has higher affinity for the polymeric gel, lambda(m) falls and A(e) rises because better solvents create higher compressive strain that promote low-wavelength, high-amplitude wrinkles. When the bottom layer is elastic, a critical compressive stress exists. If the generated compressive stress by solvent absorption is greater than the critical stress, the top layer wrinkles. It was found that wrinkling is most likely at intermediate E(t), low E(b), high H/h, and low chi. Further, lower chi, higher H/h, and lower E(b) were found to promote higher equilibrium amplitude and higher wavelength wrinkles.

Journal Article↗

PubMed2006

Colloidal crystal layers of hexagonal nanoplates by convective assembly.

Particles of the zeolite ZSM-2 prepared as nearly hexagonal nanoplatelets were coated onto flat substrates by a convective assembly technique. On the submillimeter scale, coatings ranged in patterns from striped to continuous. Particles were preferentially oriented out-of-plane, as supported by X-ray diffractometry. The novel observation is that where the particle coating was only a monolayer thick, particles were locally close-packed and uniformly oriented both in and out of plane in a hexagonal colloidal crystalline arrangement that may be described as being tiled (observations by scanning electron microscopy). This is the first documented demonstration of convective assembly applied to anisometric nanoparticles that resulted in particulate coatings with locally ordered microstructure, i.e., colloidal crystallinity.

Journal Article↗

PubMed2006

Disordered network state in hydrated block-copolymer surfactants.

Hydration of poly(butadiene-b-ethylene oxide) diblock copolymers leads to various ordered and disordered phases, analogous to the aqueous phase behavior of surfactants and lipids. Small-angle x-ray scattering measurements corroborated by cryogenic scanning electron microscopy imaging reveal a random network (N) morphology at polymer compositions and water content intermediate to those associated with ordered cylinders (H1) and lamellae (L). This sequence of self-assembled structures is strikingly similar to the phase behavior of certain water-oil-surfactant microemulsions.

Butadienes↗

PubMed2006

The role of thickness transitions in convective assembly.

Here we examine the microscopic details of convective assembly, a process in which thin colloidal crystals are deposited on a substrate from suspensions of nearly monodisperse spheres. Previously, such crystals have been shown to exhibit a strong tendency toward the face-centered cubic structure, which is difficult to explain on thermodynamic grounds. Using real-time microscopic visualization, electron microscopy, and scanning confocal microscopy, we obtain clues about the crystallization mechanism. Our results indicate that the regions at which a growing crystal transitions from n to n + 1 layers can play an important and previously unrecognized role in the crystallization. For thin crystals, we show both from experiment and through simple modeling that these transition regions can generate specific crystal structures. In thicker crystals, the crystallization is more complicated, but the transition regions must still be considered before a complete understanding of convective assembly can be obtained.

Colloids↗

PubMed2005

Permeability and reactivity of Thermotoga maritima in latex bimodal blend coatings at 80 degrees C: a model high temperature biocatalytic coating.

Thermostable polymers cast as thin, porous coatings or membranes may be useful for concentrating and stabilizing hyperthermophilic microorganisms as biocatalysts. Hydrogel matrices can be unstable above 65 degrees C. Therefore a 55-microm thick, two layer (cell coat + polymer top coat) bimodal, adhesive latex coating of partially coalesced polystyrene particles was investigated at 80 degrees C using Thermotoga maritima as a model hyperthermophile. Coating permeability (pore structure) was critical for maintaining T. maritima viability. The permeability of bimodal coatings generated from 0.8 v/v of a suspension of non-film-forming 800 nm polystyrene particles with high glass transition temperature (T(g) = 94 degrees C, 26.9% total solids) blended with 0.2 v/v of a suspension of film-forming 158 nm polyacrylate/styrene particles (T(g) approximately -5 degrees C, 40.9% total solids) with 0.3 g sucrose/g latex was measured in a KNO3 diffusion cell. Diffusivity ratio remained above 0.04 (D(eff)/D) when incubated at 80 degrees C in artificial seawater (ASW) for 5 days. KNO3 permeability was corroborated by cryogenic-SEM images of the pore structure. In contrast, the permeability of a mono-dispersed acrylate/vinyl acetate latex Rovace SF091 (T(g) approximately 10 degrees C) rapidly decreased and became impermeable after 2 days incubation in ASW at 80 degrees C. Thermotoga maritima were entrapped in these coatings at a cell density of 49 g cell wet weight/liter of coating volume, 25-fold higher than the density in liquid culture. Viable T. maritima were released from single-layer coatings at 80 degrees C but accurate measurement of the percentage of viable entrapped cells by plate counting was not successful. Metabolic activity could be measured in bilayer coatings by utilization of glucose and maltose, which was identical for latex-entrapped and suspended cells. Starch was hydrolyzed for 200 h by latex-entrapped cells due to the slow diffusion of starch through the polymer top coat compared to only 24 h by suspended T. maritima. The observed reactivity and stability of these coatings was surprising since cryo-SEM images suggested that the smaller low T(g) polyacrylate/styrene particles preferentially bound to the T. maritima toga-sheath during coat formation. This model system may be useful for concentrating, entrapment and stabilization of metabolically active hyperthermophiles at 80 degrees C.

Biofilms↗

PubMed1999

A patch coating method for preparing biocatalytic films of Escherichia coli.

A method has been developed for immobilizing viable but nongrowing Escherichia coli in highly uniform patches. The patches consist of a thin layer of bacteria in acrylate vinyl acetate covered with a thin layer of the same polymer devoid of bacteria and sealed by the edges. This method permits study of immobilized cell physiology in biocatalytic films by the assay methods used for suspended cells. Large numbers of patches of immobilized E. coli can be generated on metal or polyester sheets. Those described here are 12.7 mm in diameter; in them the cell layer is 30 microm thick and contains more than 5 x 10(8) viable cells. The method allows the cell-plus-polymer layer and the polymer sealant to be varied in thickness from 5 to 60 microm and from 7 to 80 microm, respectively. No leakage of cells was detected from 87% of the patches during 15 days of rehydration. Culturability of the immobilized cells, released by shaking the cells out of the porous polymer layer, was 80% of pre coating culturability. E. coli beta-galactosidase activity and measurements of total RNA and DNA from immobilized and suspended cells indicated that cells immobilized in the thin polymer layer have higher specific beta-galactosidase activity and a slower total RNA degradation rate than suspended cells over 15 days.

Bacterial Proteins↗

PubMed1998

Paramagnetic tracer concentration evolution by NMR relaxation time mapping: application to Aris-Taylor dispersion.

A procedure to study tracer dispersion was proposed and tested for the case of tracer spreading in tube flow. Concentration maps of paramagnetic tracers Gd3+ were measured in time through direct measurements of spin lattice relaxation time T1 obtained by using a two-point stimulated echo pulse sequence. The procedure was used to test the linear dependence of Peclet number on inverse velocity in the range of flow rates 0.3-1.2 cc/min.

Contrast Media↗

PubMed1996

Dispersion of paramagnetic tracers in bead packs by T1 mapping: experiments and simulations.

NMR imaging was used to study dispersion in 6 mm bead pack. T1 maps were employed to measure the rate of axial spreading of paramagnetic tracers (GdCl3) inside the bead pack in the range of flow rate from 0.015 mL/s to 0.175 mL/s. From the T1 maps, tracer concentration profiles were obtained, which yielded dimensionless axial dispersion coefficient and mean transit time. Spatial variations in the dispersion coefficient were observed at flow rates above 0.08 mL/s. We hypothesized that the observed spatial oscillations in the dispersion coefficient arise from the spatial variations of the velocity distribution. To validate this mechanism we showed by simulation that similar dispersion coefficient variation occur in a layered network.

Contrast Media↗

PubMed1988

Controlled environment vitrification system: an improved sample preparation technique.

The controlled environment vitrification system (CEVS) permits cryofixation of hydrated biological and colloidal dispersions and aggregates from a temperature- and saturation-controlled environment. Otherwise, specimens prepared in an uncontrolled laboratory atmosphere are subject to evaporation and heat transfer, which may introduce artifacts caused by concentration, pH, ionic strength, and temperature changes. Moreover, it is difficult to fix and examine the microstructure of systems at temperatures other than ambient (e.g., biological systems at in vivo conditions and colloidal systems above room temperature). A system has been developed that ensures that a liquid or partially liquid specimen is maintained in its original state while it is being prepared before vitrification and, once prepared, is vitrified with little alteration of its microstructure. A controlled environment is provided within a chamber where temperature and chemical activity of volatile components can be controlled while the specimen is being prepared. The specimen grid is mounted on a plunger, and a synchronous shutter is opened almost simultaneously with the release of the plunger, so that the specimen is propelled abruptly through the shutter opening into a cryogenic bath. We describe the system and its use and illustrate the value of the technique with TEM micrographs of surfactant microstructures in which specimen preparation artifacts were avoided. We also discuss applications to other instruments like SEM, to other techniques like freeze-fracture, and to novel "on the grid" experiments that make it possible to freeze successive instants of dynamic processes such as membrane fusion, chemical reactions, and phase transitions.

Microscopy, Electron↗

PubMed1982

Imaging vesicular dispersions with cold-stage electron microscopy.

A fast-freeze, cold-stage transmission electron microscopy technique which can incorporate in situ freeze-drying of the sample is described. Its use in elucidating structure in unstained and stained, hydrated and freeze-dried, aqueous vesicular dispersions of biological and chemical interest is demonstrated with vesicles of L-alpha-phosphatidylcholine (bovine phosphatidylcholine) and of the synthetic surfactant sodium 4-(1'-heptylnonyl)benzenesulfonate (SHBS). The contrast features observed in transmission electron microscope images of frozen, hydrated samples are identified and explained with the dynamical theory of electron diffraction. Radiolysis by the electron beam is shown to increase contrast in vesicle images and to change their structure and size. Micrographs illustrate the freeze-drying of a dispersion in the microscope; the process causes vesicles to shrink and collapse.

Animals↗

PubMed1974