Showing papers by "Worcester Polytechnic Institute published in 1994"

Time-Dependent Diffusion of Water in a Biological Model System

Abstract: Packed erythrocytes are ideally suited as a model system for the study of water diffusion in biological tissue, because cell size, membrane permeability, and extracellular volume fraction can be varied independently. We used a pulsed-field-gradient spin echo NMR technique to measure the time-dependent diffusion coefficient D(t) in packed erythrocytes. The long-time diffusion constant, D(eff), depends sensitively on the extracellular volume fraction. This may explain the drop in D(eff) during the early stages of brain ischemia, where just minutes after an ischemic insult the extra-cellular volume in the affected region of the brain is significantly reduced. Using an effective medium formula, we estimate the erythrocyte membrane permeability, in good agreement with measurements on isolated cells. From the short-time behavior of D(t), we determine the surface-to-volume ratio of the cells, approximately (0.72 micron)-1.

Steps towards an ecology of infrastructure: complex problems in design and access for large-scale collaborative systems

Abstract: This paper analyzes the initial phases of a large-scale custom software effort, the Worm Community System (WCS), a collaborative system designed for a geographically dispersed community of geneticists. Despite high user satisfaction with the system and interface, and extensive user feedback and analysis, many users experienced difficulties in signing on and use, ranging from simple lack of resources to complex organizational and intellectual trade-offs. Using Bateson's levels of learning, we characterize these as levels of infrastructural complexity which challenge both users and developers. Usage problems may result from different perceptions of this complexity in different organizational contexts.

Rapid tissue oxygen tension mapping using 19F inversion-recovery echo-planar imaging of perfluoro-15-crown-5-ether.

Abstract: Fluorine-19 inversion-recovery, echo-planar imaging (IR-EPI) was used in conjunction with a new PFC emulsion, perfluoro-15-crown-5-ether, to map the spatial distribution of oxygen tension in murine liver, spleen and radiation induced fibrosarcoma (RIF-1) tumors. Intravenously administered PFC emulsions were allowed to sequester in the liver, spleen, and tumor 3 to 7 days prior to imaging experiments. Seven, 64 x 64 IR-EPIs were acquired with successively increasing inversion times (TI). A nonlinear least-squares regression algorithm was used to fit the seven two-dimensional matrices, on a pixel-by-pixel basis, to solve for the relaxation rate, R1, of the sequestered PFC. From in vitro calibration curves, the oxygen tension (pO2) was calculated from the measured R1. Oxygen tension maps were then murine liver and spleen were produced (in 2.5 min) to demonstrate the technique and changes in tissue oxygenation as a function of breathing gas (air and carbogen (95% O2-5% CO2)) are presented. Tissue pO2 maps from RIF-1 tumors (n = 5) were obtained in less than 10 min and changes in tumor pO2 were studied when the breathing gas was switched from air to carbogen. The results from tumor pO2 maps were compared with 19F MR spectroscopy measurements to check for consistency. Histogram analysis yielded an average liver and spleen pO2 of 43 torr and 26 torr for RIF-1 tumors when the animals were breathing air. Statistically significant changes in tumor oxygenation as a function of breathing gas were obtained from both pO2 maps (6 +/- 2 torr, P < 0.05) and 19F MR spectroscopy (13 +/- 3 torr, P < 0.01) as evaluated using the Student's paired t test.

Spreading waves of decreased diffusion coefficient after cortical stimulation in the rat brain.

Abstract: A method is demonstrated for the noninvasive detection and study of spreading cortical depression. Spreading depression (SD) was elicited in rats by topical application of potassium chloride to the exposed cortex. The apparent diffusion coefficient (Dapp) of water in a region of the cortex, measured using a PFG-NMR spin echo sequence with an observation time of 40 ms, declines 35% within 30 s and recovers to the normal value within the next 30 s. The region of decreased Dapp was shown to be 2 mm in size and to move in the cortex, away from the point of application, with a uniform velocity of 3.3 +/- 0.5 mm/min. The behavior of the affected region is consistent with other reports of the behavior of SD as monitored by electrophysiological means. The technique can be implemented on currently available MRI equipment and makes possible the noninvasive study of SD in animal models of neurological disorders, their therapeutic intervention, and possibly the study of SD in humans.

Importance-Weighted Marginal Bayesian Posterior Density Estimation

Abstract: Markov chain sampling schemes generate dependent observations {Θi, 0 ≤ i ≤ n} from a full joint posterior distribution π(θdata). Frequently, only certain marginals of this full posterior density are of interest; thus an interesting problem is how to estimate the marginal posterior densities based on the dependent observations {Θi, 0 ≤ i ≤ n} from π(θ data). We propose a new importance-weighted marginal density estimation (IWMDE) method. An IWMDE is obtained by averaging many dependent observations of the ratio of the full joint posterior densities multiplied by a weighting conditional density w. The asymptotic properties for the IWMDE and the guidelines for choosing a weighting conditional density w are also considered. A bivariate normal model and a constrained linear multiple regression model are used to illustrate how to derive the IWMDE's for the marginal posterior densities.

Artemisinin production by transformed roots of Artemisia annua

Abstract: Transformed root cultures of several strains of Artemisia annua were obtained by infection with Agrobacterium rhizogenes ATCC 15834 Production of artemisinin, measured by HPLC, ranged from 0–042 % of dry weight (DW) in 10 different clones Artemistene, artemisinic acid, and arteannuin B were also measured Comparisons to literature reports suggest that the commercial production of artemisinic compounds using transformed roots is feasible

Temperature dependent change of apparent diffusion coefficient of water in normal and ischemic brain of rats.

Abstract: To identify the temperature dependent change of the apparent diffusion coefficient (ADC) of water in brain tissue, the ADC values of normal rat brain were measured over a range of body temperatures...

Radioactive microsphere validation of a volume localized continuous saturation perfusion measurement.

Abstract: Detre et al. (Magn. Reson. Med. 23, 37-45 (1992)) and Zhang et al., (Magn. Reson. Med. 25, 362 (1992)) have recently demonstrated a technique for the measurement of regional cerebral blood flow (rCBF) based on the continuous saturation (or inversion) of the arterial blood supply to the brain in rats at 4.7 T. In the work reported here, we combined this technique with volume localized (PRESS) readouts to benefit from recording "perfusion" signals averaged over a larger volume, resulting in rapid acquisition of data with sufficient signal-to-noise ratio for application at 2.0 T. In 10 baseline flow measurements, the mean error between the NMR technique and the microsphere flow measurement was -1.5% with a standard deviation of 15.2%. For five measurements obtained with occlusion of the middle cerebral artery, the mean error was -32.4 +/- 20.2%. Perfusion measurements from a single animal under hypercapnic conditions indicated that the NMR technique could underestimate rCBF at high flow rates. An error analysis of the NMR perfusion model is also presented, along with results for typical parameters encountered at 2.0 T.

Strength and reliability of carbon-fiber-reinforced cement composites

Abstract: The effect of carbon fibers on the mechanical properties of cement paste composites is studied. The addition of polyacrylonitrite-based carbon fiber to a cementitious paste matrix results in a significant improvement in the tensile and flexural properties of the composites. The uniaxial tensile strength results are obtained using the novel cementitious composites axial tensile technique. The addition of 1,2 and 3 %vol. of carbon fiber to a cement matrix results in an increase in the uniaxial tensile strength of 32, 48 and 56%, respectively. The enhancement of the composite flexural strength was more significant, as compared to the uniaxial tensile strength. The flexural strength of cement matrix increased by 72, 95 and 138% with the addition of 1,2 and 3 %vol. of carbon fiber, respectively. Weibull statistics indicate that reliability in flexure was not enhanced by fiber addition and there was no correlation between the percent fiber loading and the reliability of the composites. However, the reliability of the carbon-fiber-reinforced composite in tension was greatly improved and there is a positive correlation between fiber loading and the Weibull modulus, m.

Axiomatic design and concurrent engineering

Abstract: The ability to utilize concurrent engineering fully as a strategy for achieving shorter time to market, reduced development costs, and higher-quality products depends on an ability to communicate design decisions and to coordinate the creative process among diverse disciplines. To date, the use of concurrent engineering has dealt mainly with redefining organizational structures. However, there is a lack of a systematic framework for conducting group design activities, and basic principles for decision making. In addition, the information necessary to design and manufacture a quality product is constantly evolving with new market opportunities, and a comprehensive database is too large for human designers to handle efficiently. The paper presents the concept of axiomatic design as a framework for concurrent engineering. It is based on two design axioms that provide basic principles for decision making. The axiomatic approach discussed in this paper also provides a theoretical framework for introducing the computer as a design assistant, and an architecture for a computational design environment is presented. The paper concludes with a brief discussion of the data model that is central to the computational environment.

Effects of substrate branching characteristics on kinetics of enzymatic depolymerizaion of mixed linear and branched polysaccharides: I. Amylose/amylopectin alpha-amylolysis.

Abstract: Hydrolysis reactions of homopolysaccharides, which differ in their degree of branching, and mixtures of linear and branched polymers were carried out with alpha-amylase. The branching structures of both the original amylopectin substrate and the cluster domains of amylopectin, obtained by ethanol precipitation of the products of the action of alpha-amylase, were characterized via enzymatic digestion with debranching enzyme (i.e., isoamylase), followed by the fractions of the resulting products using gel filtration chromatography. The structural properties (i.e., molecular weight, molecular weight distribution, and branching characteristics) of the resulting products during depolymerization of amylose, amylopectin and their mixtures via alpha-amylase were characterized by size exclusion chromatography coupled with a low angle laser right scattering (SEC/LALLS) technique. It was determined that substrate branching characteristics strongly influence both the observed enzymatic activity as well as the enzyme's action pattern. A simplified kinetic model that represents the hydrolysis reactions of amylose and amylopectin mixtures via endo-acting alpha-amylase is proposed. We found that that reaction kinetics (i.e., enzyme affinity) was also governed by the substrate's conformation in solution. The relationships between the mass fraction of branched polymers and the kinetic parameters during alpha-amylolysis were compared with those predicted by the kinetic model. Excellent agreement was found between the model predictions and the experimental observations. The results reported here imply and interrelationship between enzyme action and polymeric substrate structural properties. (c) 1994 John Wiley & Sons, Inc.

Quantitative Determination of Displacements and Strains from Holograms

Abstract: In this chapter, procedures for quantitative determination of displacements and strains directly from holograms are presented. This presentation begins with the definition of projection matrices and discussion of their application in holographic analysis. Then, system geometries used for recording and reconstruction of holograms are defined in terms of the illumination, observation, and sensitivity vectors. This is followed by a discussion of determination of displacements for the cases when the fringe order is known, when the fringe order is unknown, and when multiple holograms are used. Next, the fringe-vector method, relating the holographically determined strains and rotations to the object surface via the projection matrices based on the object’s local surface normals, is outlined. Finally, recent advances in electronic acquisition, storage, processing, and display of optical interference information is discussed and their implementation in quantitative measurements of displacements, due to static and dynamic loading conditions, is illustrated with representative examples.

Journal of Engineering Education Round Table: Reflections on the Grinter Report

Abstract: The Round Table is a new presentation format for the Journal of Engineering Education. The purpose of the Round Table is to present the comments of several distinguished indi­ viduals about a topic as well as their responses to the comments offered by their colleagues. For the initial Journal of Engineering Education Round Table we asked for reflections about the Grinter Report, published in September 1955. After a brief introduction to the topic of discussion, the invited participants present their views, and, then, respond to the remarks of their colleagues. The goal is two-fold: to pre­ sent a spectrum of views on the topic, and to provoke a discus­ sion of the topic by the community.

Observation of above-barrier transitions in superlattices with small magnetically induced band offsets

Abstract: Using magneto-optical absorption in ZnSe/${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se superlattices, we have observed a series of transitions involving above-barrier states of the conduction and valence bands, at the zone center and at the zone edge of the superlattice Brillouin zone. The Mn concentration in ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se layers (x\ensuremath{\sim}0.04) has been specifically chosen such that the bowing of the band gap with x and the strain in the superlattice together give rise to conduction- and valence-band offsets, which are nearly zero in the absence of a magnetic field. Owing to the large Zeeman splittings of band edges that occur in the ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se layers, the band alignment in these superlattices can be externally tuned by an applied magnetic field, providing a unique laboratory for investigating above-barrier subbands in a superlattice defined by very small periodic-potential variations (intermediate-dimensionality regime). The magnetic-field tuning, together with calculations based on an eight-band k\ensuremath{\cdot}p model, allow us to unambiguously identify the transitions, and to compare the observed transition energies with theoretical predictions.

Voltage distortion in distribution feeders with nonlinear loads

Abstract: The voltage of three real-life 13.8 kV feeders supplying customers with nonlinear loads was analyzed by means of computer simulations. Three classes of nonlinear loads were considered. Each class is characteristic for different types of AC to DC converters such as the input DC supply used for adjustable-speed-drives, battery chargers, PCs, TVs and electronically ballasted lights. The analysis is based on the determination of the most harmonic susceptible busses and their response to each harmonic frequency. A new expeditive method that takes into account the background harmonic voltage phasor, and an equivalent bus impedance was developed and used to compute the maximum nonlinear loads that yields VTHD=5%, (voltage total harmonic distortion). The main conclusion of this work is that when mitigation methods are not used, for a 15 kV class feeder with a maximum 10 MVA installed load, the total nonlinear residential load should not exceed 300 kW if the ITHD 100%. >

An efficient oxygen independent two-photon photosensitization mechanism.

Abstract: A novel oxygen-independent photosensitization mechanism from the upper triplet state (Tn) of rose bengal has been demonstrated by selectively populating Tn by sequential two-color laser excitation. Products formed from Tn inhibit red blood cell acetylcholinesterase and decrease viability of P388D1 mouse macrophage monocyte cells as measured by trypan blue exclusion assay. Laser flash photolysis studies indicate that Tn reacts efficiently, as evidenced by permanent photobleaching of T1 absorption, with chemical yields approaching unit efficiency. This mechanism may have application for oxygen deficient photosensitization under high intensity, pulsed laser irradiation.

Predicting voltage distortion in a system with multiple random harmonic sources

Abstract: A method for statistically describing power system harmonic voltages in terms of the parameters of the harmonic current sources is presented. It is shown that for a large enough number of sources, a complete probabilistic characterization of the harmonic voltages can be found in terms of the second order moments of each current phasor's rectangular components. The method is tested on an existing distribution system by using both simulation and field measurements. >

Impact Properties of A356-T6 Alloys

Abstract: The Charpy impact energy of A356 alloys has been measured. Instrumented Charpy impact tests have been conducted at 25,100,150, and 200 °C. The Charpy specimens were machined from plate castings ortapered cylindrical castings. The plates were produced in sand molds, and tapered cylinders were produced in water-cooled copper molds. Both unmodified and strontium-modified castings were tested. The results indicate that strontium modification improves the impact properties of sand and permanent mold castings. The impact energy increases with solution treatment time. Strontium modification reduces the solution treatment time for attaining a specific impact property level in the casting.

Hybrid approach to deformation analysis

Abstract: Current trends in development of components, structures, and systems place unprecedented requirements on their designers. To satisfy these demands, new, highly efficient materials and structural designs are being employed and integrated utilization of the most sophisticated technologies is being made. This paper addresses some of the pertinent issues relating to this integration and explores how to take advantage of the analytical, computational, and experimental solution methodologies in relation to a given problem. More specifically, a hybrid approach to deformation analysis is described, based on recent developments in merging, or unifying, of the finite element method with experimental methodologies, and especially with optical metrology. This approach emphasizes the analogy between the methodologies used and employs them to obtain solutions that may not have been otherwise obtainable, to ease the existing solution procedures, or to attain improvements in the results. The paper begins with an introduction to the methodologies used, continues with a discussion of the analytical fundamentals, and concludes with a presentation of representative results.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Fracture Intensity Distributions during Compression of Puffed Corn Meal Extrudates: Method for Quantifying Fracturability

Abstract: The distribution of fracture intensities occurring during compression of puffed corn meal extrudates was described using an exponential function, and parameters from that analysis were used as fracturability indices. Since “jagged” or oscillating stress-strain functions are typical for porous and brittle materials, fracture intensities were determined by measuring the abrupt, sequential reductions in stress produced during compression. Both distribution exponent and cumulative fracture stress correlated strongly with fracturability measured by other techniques, including fractal and Fourier analysis of stress-strain functions. Distribution parameters also indicated textural differences due to process parameters (structural modification through addition of different levels of sucrose) and storage conditions (equilibration at various relative humidities).

Polarized optical vortex solitons: Instabilities and dynamics in Kerr nonlinear media

Abstract: Optical vortex solitons are known to be stable in a scalar nonlinear system, and they have even been observed. We now explore the effects of polarization on the vortices, and find that they are unstable in Kerr defocusing media, except when the beam is circularly polarized. Numerical simulations verify this and show remarkable beam evolution, including vortex shedding and reversal of topological charge.

Rich and lean representations of information for knowledge work: the role of computing packages in the work of classical scholars

Abstract: Applying information systems to complex intellectual tasks requires the representation and codification of ambiguous and fragmentary forms of data. This application effects changes not only in representation of this data, but in the relationships between users and tools, techniques, or systems for data interpretation. It also affects the complex infrastructures that support this process. This article uses a package metaphor to examine the impact on one domain of knowledge work, classical scholarship, of the “computerization” of a key data source, the textual edition. The construction of one on-line textual databank, the Thesaurus Linguae Graecae (TLG), has altered the traditional relationships between text “owners” and “users” has changed the role of the text as a conduit for social and historical information, and has disrupted traditional patterns of transmitting domain expertise. A rich information resource has become lean in its electronic form.The TLG has standardized the corpus of Greek literature and eased access to a broad range of works, including rare and out-of-print materials. At the same time, its construction has decoupled often-contested textual sources from their accompanying critical notes and supplemental materials. The use of the TLG has also shifted notions of objectivity, accuracy, and requisite expertise within the community. The transmission of domain knowledge must now be coupled with the transmission of technical knowledge, a process for which no infrastructure is currently in place. These experiences parallel those of other knowledge workers. “Mechanistic” paradigms of information and knowledge cannot accommodate important components of computing packages, including the transmission of expertise and infrastructures for tool development and evaluation. Recent developments in information storage and dissemination, including gophers and ftp sites may indicate that despite technical advances that could be used to support rich representations (such as hypermedia and multimedia), leaner forms of data may prevail.

A saddle-point theorem with application to structural optimization

Abstract: The relaxation for optimal complicance design is independent of whether the underlying elastic problem is formulated in terms of displacements or strains. For the purposes of numerical experimentation and computation, it may be advantageous to formulate optimal design problems in terms of displacements as is done in Ref. 1. The relaxed problem delivered by the displacement-based formulation is of min-min-max type. Because of this, efficient numerical implementation is hampered by the order of the last two min-max operations. We show here that the last two min-max operations may be exchanged, facilitating an efficient numerical algorithm. We remark that the rigorous results given here corroborate the numerical methods and experiments given in Ref. 1.