Dark-induced Secondary Dormancy in Silene
Caldwell, Alex, Lauran Halpin, Adah Walker, Will White, and Pat Peroni
Exposure of physiologically nondormant seeds to unfavorable environmental conditions may induce a state of secondary dormancy, which prevents seeds from germinating in response to normal environmental cues. Preliminary observations suggested that darkness may induce secondary dormancy in white campion (Silene latifolia), a model organism for metapopulation studies that maintains a dormant soil seed bank. In this study, we examined the effect of dark pre-incubation on the ability of white campion seeds to germinate when subsequently exposed to optimal germination conditions. Seeds pre-incubated in darkness for 7 d displayed significantly lower mean fraction germination after 6 d in the light than seeds germinated in the light for 6 d without a dark pre-incubation. Seeds collected from two Virginia white campion populations exhibited significantly different responses to dark pre-incubation. These results indicate that dark incubation induces secondary dormancy in white campion. In the field, buried seeds may remain dormant and ungerminated even after exhumation, and levels of secondary dormancy may differ among populations.
Using Automated Recording Systems to
Monitor Anuran Calling Behavior: A Comparative Survey
Cocklin, Ross R., M. Jason Todd, and Michael E. Dorcas
Amphibian decline has been extensively documented over the past 10 years. A number of monitoring programs have been established to study amphibian populations. Many of these programs rely on calling surveys; therefore, information on temporally and environmentally induced calling variation can be used to optimize such surveys. We extensively surveyed two ponds, Thompson Pond and Davis Pond, using data loggers to record environmental conditions and automated recording systems to monitor frog calling activity. Our objective was to compare daily and seasonal calling variation between the two ponds and determine how environmental variation affects calling behavior. We recorded four species of anurans: Western Chorus frog, Spring Peeper, Northern Leopard Frog, and Pickerel Frog. Considerable seasonal differences among the species were documented. For example, Spring Peepers began calling 5 days earlier at Thompson Pond when compared to Spring Peepers at Davis Pond. We also documented considerable daily variation in calling behavior. For example, Western Chorus frogs called more often and more intensely at Davis Pond than at Thompson Pond. Environmental variation also affected calling behavior. For example, Western Chorus frogs called 30% of the time (at Davis Pond) when shallow water temperature ranged from 12 to 14 ° C. However, Western Chorus frogs called only 8% of the time when shallow water temperature was above 20° C. Data such as these provide a better understanding of factors that affect anuran behavior and can be used to optimize amphibian monitoring programs. We are working on a model that uses our data to predict the likelihood of anuran calling based on time of day, season, and environmental conditions. Preliminary results reveal accurate predictions about 66% of the time.
Development of Micro-Scale Assays for
Characterization of Liquid and Solid Media Growth of Chlamydomonas
reinhardtii: The basis for characterization of resistance
to protoporphyrinogen oxidase inhibiting herbicides in tip
Cook, Nathaniel and Karen K. Bernd
The global need for increased crop production has created a demand for effective and efficient herbicides to control unwanted plants. Detailed knowledge of the molecular mechanisms that underlie the chemical/organism interaction can aid in the design of safer and more effective products. Chlamydomonas reinhardtii, a photosynthetic alga, is particularly useful as a model for herbicide resistance. Unlike higher plants, non-photosynthetic Chlamydomonas mutants are viable with the provision of a carbon source, and are amenable to morphological, genetic, and biochemical analyses. In this study, micro-scale assays to characterize Chlamydomonas growth in microtiter plates were developed and validated. These micro-scale assays produce results equivalent to traditional large-scale assays, but save time, materials, and space by allowing simultaneous characterization of a number of strains under a variety of conditions. The assays detailed were used to establish baseline growth curves for a subset of tip strains isolated by Bernd and Kohorn (1998) and to characterize these strains for resistance to the protoporphyrinogen oxidase inhibitor herbicide, flumiclorac pentyl ester. The data show that the tip1-1 strain exhibits significant resistance to the herbicide. Currently, the link between the relaxed translocation requirements of the tip strains and herbicide resistance is not known. This work provides the foundation for further studies that will investigate the underlying mechanism of resistance.
developing bioinformatics tools to elucidate the evolution of
Karnik, Rahul and A. Malcolm Campbell
The rapid growth of protein sequence databases has presented an opportunity to find evolutionary relationships and common functional domains between proteins. We present a systematic method of investigation using publicly available software tools to search for such relationships. Also presented is a new program, Divide-and-BLAST, which uses a divide-and-conquer algorithm in conjunction with the Basic Alignment and Search Tool (BLAST) program to detect remote similarities that are usually not seen in normal BLAST output. The system was tested using two proteins, isocitrate dehydrogenase 1 (IDH1) and caspase 3 (apopain), and yielded results that could be used to direct further study.
Geographic variation in turtle populations
in the Davidson, NC area
Lindsay, Shane D., Jennifer L. Roberts, and Michael E. Dorcas
Geographic variation in species over small spatial scales can often be attributed to habitat differences and can sometimes be linked to Ahabitat quality.@ We examined small-scale geographic variation in aquatic turtle populations among three ponds with varying degrees of cattle impact in the vicinity of Davidson, NC. We conducted a mark-recapture study to compare turtles among the ponds at community and population levels in addition to testing for morphological variation and differences in reproduction. We found considerable variation in species present and in relative abundances among species. Bottom dwelling turtles, mud turtles (Kinosternon subrubum) and snapping turtles (Chelydra serpentina), were absent from the pond with the highest cattle impact and mud turtles were most abundant at the pond with no cattle impact. Likewise, mud turtles were larger in all dimensions and their eggs were bigger at the pond with no cattle impact. We suspect that observed geographic variation may be due to habitat differences among the ponds, possibly varying levels of cattle use.
Development of Undergraduate DNA Sequencing.
Nugent, Liz and Shannon Riedley
Rapid advances in biotechnology make it imperative that interested undergraduate students become familiar with the most-up-to-date techniques and methodologies in DNA sequencing. Students at Davidson have not had the chance to become familiar with these techniques. To address this need among the students of Davidson, we have been developing rapid DNA sequencing and non-radioactive detection protocols that may be completed by undergraduate students during a normal lab time. DNA was isolated from E. coli. A biotinylated M13 oligonucleotide served as the primer for sequencing reactions. Reactions were electrophoresed using the Novex sequencing system and the products were transferred to a nylon membrane. The products were detected by a streptavidin-alkaline phosphatase detection system. This protocol has been developed to the point that DNA has been detected, but the results are not yet consistent. Consistency in results should be possible with some of the proposed changes.
An Apparent Lack of Physiological Thermoregulation
During Heating and Cooling in the Eastern Diamondback Rattlesnake,
Rice, Aaron N., T. Luther Roberts, IV, and Michael E. Dorcas
Temperature affects almost every aspect of the biology of ectothermic animals. Many ectotherms regulate their body temperatures to optimize important physiological functions, usually via behavioral thermoregulation. Additionally, physiological mechanisms can be employed by large ectotherms to alter their heating and cooling rates. We used temperature-sensitive radiotransmitters to monitor the heating and cooling rates of four eastern diamondback rattlesnakes (Crotalus adamanteus) over two temperature ranges, 10-25 °C and 5-35°C. Surprisingly, we found no difference between the rates of heating and cooling in C. adamanteus. However, snakes stayed coiled more frequently when cooling then when heating. As expected, large snakes heated and cooled more slowly than smaller individuals. Finally, the rates of heating and cooling closely matched those of water-filled copper pipes, which served as controls, suggesting a lack of physiological thermoregulation in C. adamanteus. Our findings contradict previous research conducted on reptiles of similar size and raise many questions regarding thermoregulation in C. adamanteus and related species.
Historical Comparisons of Reptile and
Amphibian Distribution and Diversity in Iredell and Mecklenburg
Roberts, T. Luther IV, Justin G. Pritchard, Aaron N. Rice, and Michael E. Dorcas
A decline in reptile and amphibian populations can often be indicative of the effects of habitat destruction and general deterioration. The Charlotte-Metro area has been developed extensively over recent years, specifically Northern Mecklenburg and Southern Iredell in the vicinity of Lake Norman. Our objectives in this study were to analyze historical and current amphibian and reptile distributions and diversity in Iredell and Mecklenburg Counties over the last 50 years, and to evaluate the effectiveness of multiple-technique herpetological sampling versus traditional methods. We found that until 1999-2000 (when our lab began sampling), the number of species recorded per year decreased steadily beginning in the early 1980's. Of the 58 species of amphibians and reptiles historically recorded in Iredell and Mecklenburg counties, 41 have been found in the past two years (70.7%), 30 of which occur on the Davidson Campus (51.7% of the total, and 73.2% of the 41 current species). We found two species which had not been recorded in this area (Green Treefrog and Yellowbelly Slider) .Our lab has found a higher diversity and more individuals than was recorded in any single year before 1998. We believe this difference is due to our use of multi-technique sampling, which is more effective than traditional methods for observing reptiles and amphibians. Current records show a similar geographic distribution to historical data. There are insufficient historical data from Iredell and Mecklenburg Counties to confirm amphibian and reptile decline. Long-term, intensive sampling is needed to elucidate the status of these herpetofaunal populations.
The Effects of Seed Density on the Rate
and Percent Germination of White Campion (Silene latifolia)
Siefert, Megan, Kelly Carter, Rahul Karnik, Alexis Joiner, Elizabeth Shafer, and Patricia A. Peroni
We tested the effects of seed density, independent of resource delpletion, on the rate and percent germination of white campion (Silene latifolia) seeds. We used a "focal plant" design that involved exposing focal seeds to four different densities of surrounding white campion seeds (0, 5, 10, and 15 surrounding seeds). Each replicate was supplied with an excess of water to reduce the possibilty of seed competition for this resource. We measured percent germination of the focal and the surrounding seeds, and recorded the timing of focal seed germination. Percent germination of focal seeds decreased significantly as the density of surrounding seeds increased. However, the density of surrounding seeds did not signifcantly affect the timing of focal seed germination. The fraction of surrounding seeds that germinated also decreased significantly as seedling denstiy increased. Our results suggest that chemicals released by imbibing seeds influence the likelihood of seed germination, but not the timing of germination for those seeds that germinated.
The Effect of Inbreeding on Seed Germination
in White Campion (Silene latifolia).
White, J. Wilson, III and Patricia A. Peroni
Dormant seed banks could influence the effects of inbreeding depression on a small population by selectively retaining inbred seeds in the soil. However, the impacts of seed banks on plant population dynamics remain unknown. White campion (Silene latifolia) is a weedy plant that maintains dormant seed banks. Previous research indicates that white campion seeds display increased germination in the light relative to the dark and that inbred seeds exhibit less germination than outbred seeds in the greenhouse. In order to determine if inbreeding influences seeds' responses to light and dark environments, I germinated outbred, single-generation inbred, and second-generation inbred S. latifolia seeds in both light and dark conditions in laboratory growth chambers. The interaction of light treatment * cross type had a significant effect on germination; outbred seeds displayed higher germination rates than inbred seeds in dark conditions, but in the light all three cross types exhibited similar germination rates. Maternal family also significantly affected germination rate and response to the light treatments. These results indicate that germination behavior of S. latifolia seeds is subject to environment-dependent inbreeding depression that is expressed only in sub-optimal conditions. Such inbreeding depression could lead to selective retention of inbred seeds in the seed bank and consequently a reduction in the proportion of highly inbred individuals that enter the above ground cohort. Germination behavior is also influenced by a strong maternal effect that could be either heritable or environmental in nature. Due to the existence of both spatial genetic structure and environmental heterogeneity among white campion populations, this maternal effect, whatever its source, could cause spatial differences in the behavior of seed banks among populations.
Rabbits: A simulation of a simple food
In nature there exists a delicate balance between the members of a food chain. Producers (primarily plants) are eaten by primary consumers (herbivores) which are in turn eaten upon by predators. Population changes at any of these three levels can be disastrous for the other members of the food chain and can lead to overpopulation or even extinction of species. This program is a model of a simple food chain in which grass is eaten by rabbits that are, in turn, preyed upon by foxes. The user can change several parameters of in the world such as amount of grass, number of rabbits or foxes, world size, etc. and observe the affect that this has on the population. With this model it is possible to construct both populations that exhibit fairly stable populations over time and populations that fluctuate at regular intervals. By observing this basic model it is possible to learn a lot about food chains and predator/prey relationships as well as the impact that human interference can have on an ecosystem.
Top of Page
Variations of the Piloty pyrrole synthesis.
Boyce, Ross and Erland Stevens
The Piloty pyrrole synthesis, first discovered in 1896, provides a means of synthesizing a variety of pyrrole products from various ketones and hydrazines. The synthesis is, however, limited in that a wide range of ketone and almost aldehyde intermediates cannot be prepared using the traditional Piloty mechanism. We believe this limitation is due to the difficult formation of the bis-(ene)-hydrazine intermediate, necessary for the ultimate synthesis of the pyrrole. Over the course of our research, we tried many methods of facilitating this step, including the addition of heat, acid catalysis, and the isolation of the azine intermediate, none of which demonstrated great promise. Yet, we were able to produce some products, never before synthesized by the Piloty pyrrole reaction, using phthaloyl dichloride to essentially force the formation of the bis-(ene)-hydrazine intermediate. This reaction mechanism merits further investigating with a wide range of ketones and aldehydes, which ultimately may be useful in the synthesis of a large variety of pyrroles.
Synthesis and characterization of an
ionic liquid and applications as a solvent for organic synthesis.
Clary, Richard S. and David M. Brown
Ionic liquids are a novel class of compounds composed of an organic cation and an inorganic anion. They possess a number of unique and interesting properties and are relatively cheap and easy to prepare. Considered "green" (environmentally-friendly) due to the fact that they are easily recycled, ionic liquids are very effective solvents for a wide variety of solutes, ranging from organic to inorganic and from small molecules to polymers. Because ionic liquids remain in the liquid phase from well below -96 °C to in excess of 200 °C, they possess a larger thermal "window" in which reactions can be performed. Finally, they are less hazardous than conventional organic solvents because they exert no vapor pressure. This property also allows for the removal of volatile organic compounds via vacuum. This project involves the synthesis, purification, and characterization of an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIm+][PF6-] and one potential application of this ionic liquid to organic synthesis--alkylation reactions of polyaromatic heterocycles.
Derivatization and characterization of
Cody, David Mark, David M. Brown, and David N. Blauch
Corn starch is a readily available, cheap, "green" (renewable) polysaccharide useful in many industrial and commercial applications. Unfortunately, many of the potential applications are undermined by the starch's excellent property as a feedstock for bacteria. If starch could be rendered antibacterial, many applications currently unsuitable would suddenly become suitable, even desirable. This problem has been historically addressed by spiking starch mixtures with dangerous biocides that may leach into the environment in an uncontrolled and unmonitored fashion, perhaps causing unintended environmental damage. Conceivably, a better way to inhibit bacterial growth in starch would be to modify the starch polymer structure by chemically (covalently) attaching a group onto the polymer backbone, which would inhibit bacterial growth without leaching into the environment. Such derivatizations of polysaccharides are known, but to our knowledge none have been designed specifically to inhibit the attack on the starch by bacteria. The chemical derivatization and characterization of corn starch is reported in this research with the goal of rendering starch unattractive as a feedstock for bacteria. Ultimately, samples will be submitted and tested in bacterial cultures to elucidate the effectiveness of this approach versus virgin starch and starch spiked with conventional biocides.
Comparison of poly(epsilon-caprolactone)
and poly(dimethylsiloxane) in long-acting, implantable contraceptive
Dainty, Erin E.
Many present contraceptive methods do not meet the needs of potential users, including low cost, accessibility, and low maintainence in remote areas. Because of these needs the use of polymeric materials in long-acing drug delivery systems, most notably Norplant and Capronor, have become significant. Both Capronor and Norplant are polymeric implantable systems that administer a dosage of levonorgestrel over an extended period of time. Capronor is a single-capsule, biodegradable polymer that releases hormone over a 12-18 month period utilizing a poly(epsilon-caprolactone) matrix, while Norplant is a six-capsule, nonbiodegradable device made of poly(dimethylsiloxane) that lasts for approximately five years. Norplant implants are much more widely used, due to a great deal of clinical trials in third world coutnries that have had favorable results. Norplant capsules function as a reservoir drug-delivery system, whereby levonorgestrel diffuses through the poly(dimethylsiloxane) matrix at a rate that is proportional to the surface area of the capsule, while maintaining a constant delivery rate over time. Capronor releases levonorgestrel at a rate ten times faster than Norplant, due to the hydrolytic degradation of the polymeric capsule in vivo. Degradation of the poly(epsilon-caprolactone) matrix occurs through bulk hydrolysis of ester linkages, which is autocatalyzed by the carboxylic acid end groups of the polymer, eventually forming carbon dioxide and water. Capronor has become favorable over Norplant in recent years because it requires fewer implants, it is biodegradable, easy to insert and remove, less obvious under the skin, and can be formed via anion, cationic, radical, or coordination polymerization.
Building a light-absorbing chromophore.
Davis, Karin and Durwin Striplin
Mother nature, through photosynthesis, converts light energy into electrical energy by the splitting of water molecules. If the water splitting abilities of photosynthesis can be mimicked using a synthetic catalyst, then chemists can produced a reusable, clean and abundant energy source. Once split by the catalyst, water can easily be recombined using well-known fuel cell technology to produce a powerful and clean energy source. To split water molecules, the catalyst must contain three major components; a light absorbing species, an electron donor species and an electron acceptor species. A ruthenium complex acts as the light absorbing species, a phenothiazine molecule works as an electron donor and an anthraquinone molecule fills the role of the electron acceptor. All three of the components are linked together via an oligoproline chain. When the ruthenium complex absorbs light, the ruthenium becomes oxidized when it promotes an electron to the ligand that links it to the oligoproline backbone in a metal to ligand charge transfer. The purpose of this research project was to build a chemical model able to predict the electronic properties of different linking ligands. To construct the chemical model, a set of rhenium complexes containing different linking ligands were synthesized. Rhenium was used for two reasons, it does not cost as much as ruthenium and its complexes are easier to synthesize. However, the properties of the rhenium complexes should closely mimic those of the ruthenium complexes. The following ligands were complexed to the rhenium metal: 2,2-bipyridine; 2,2-bipyridine-4,4-dicarboxylic acid; 4,4-dimethyl-2,2-bipyridine; 4-methyl-2,2-bipyridine-4-carboxcylic acid; 2,9-dimethyl-1,10-phenathroline (neocuproine monohydrate) and 2,2:6,2-terpyridine. Cyclic voltammetry was employed to obtain the reduction and oxidation potentials for each of the complexes. The initial data from the cyclic voltammograms does prove to be consistent with an existing theoretical model. Future studies of the electronic properties of additional linking ligands will allow the chemical model to be refined.
Identification of accelerants in fire
Dobrin, Becky A. and David Blauch
Arson investigations involve identifying the presence or absence of an accelerant(s) in burned matrices (i.e., furnishings, carpeting, flooring) based on the gas chromatograms and mass spectra of the fire debris analyzed. Because accelerants are classified according to the compounds they contain, a unique pattern of peaks is observed in the total ion chromatograms as well as in the single ion chromatograms. Background chromatograms were obtained from the vapors of unleaded gasoline, kerosene, diesel fuel, charcoal lighter fluid, and lacquer thinner in order to compare them to the fire debris sample chromatograms. After the matrices (carpeting and plywood) were burned with the accelerants and stored in quart-size paint cans, charcoal strips were placed inside the paint cans to absorb any accelerant vapors present. The charcoal strips were eventually removed and added to small test tubes containing carbon disulfide, allowing the release of the vapors into the solvent. Chromatograms were was obtained for neat accelerant vapors absorbed by charcoal strips. It was possible to detect the presence of gasoline and of kerosene in the residue of materials burned using these liquids as accelerants.
Investigation of pigment effects on drying
oils: a gas chromatographic analysis.
Fuller, Amelia A. and Ruth Beeston
Drying oils bind paint to a surface and consist of triglyceride mixtures. These triglycerides contain different fatty acid chains that may undergo a free-radical polymerization process during drying. It is significant to note that only the unsaturated fatty acid chains of the oil are involved in this reaction. Certain pigments may facilitate or inhibit this polymerization reaction and thus change the rate at which the oils dry. This study investigates the extent to which cobalt blue and Vandyke brown pigments facilitate or inhibit drying in linseed and walnut oils due to their interactions with radicals. Gas chromatography was used to separate the saturated and unsaturated components in the oils and to observe how their relative peak areas changed as the unsaturated components were consumed in the polymerization reaction. The ratio of the peak area for unsaturated components to the peak area for the palmitate species (a saturated compound) decreased with drying time most dramatically in walnut oil with cobalt blue. In general, it was observed that the unsaturated:palmitate peak area ratio decreased with drying time, as expected. Additionally, the ratios obtained from the neat oils and the oils with Vandyke brown were approximately the same and greater than those from cobalt blue samples. Such results confirm that the Vandyke brown pigment does inhibit drying while cobalt blue facilitates the drying.
GORE-TEX: A Multifunctional Biomedical
Product Comprised Of Microporous Expanded Polytetrafluorethane
Harper, Scott A.
Polytetrafluoroethane, which has the following general polymer structure:
is a commonly-used clinical material from the exciting and relatively new field of biomedical polymers. Due to PTFE's hydrophobicity, strength, chemical inertness, and overall tissue biocompatibility, it has a number of applications in various parts of the body. GORE-TEX, the most common PTFE product, is comprised of solid nodes of polytetrafluorethane from which numerous longitudinally-oriented fibers extend; GORE-TEX is currently being used as cardiovascular patches, vascular grafts, sutures, soft tissue repair patches, cast liners, synthetic ligaments, grafts for nasal reconstruction, and many other medical applications. In my poster, I will focus on describing the physical properties of polytetrafluorethane, discussing the composition and preparation of GORE-TEX, and highlighting some of its most interesting current medical uses.
Investigations into the metalation of
heteroaryls and subsequent boronic ester synthesis.
Hirsekorn, Kurt and Erland Stevens
Heterobiaryls have proven to be useful medicinal and agricultural products. The Suzuki cross-coupling reaction offers high yields and low waste generation for biaryl synthesis. While typical Suzuki coupling requires aryl boronic acids as starting reagents, heteroaryl coupling requires stable heteroaryl boronic acids. Because the number of stable heteroaryl boronic acids is small, the scope of heteroaryl Suzuki coupling is limited. To solve this problem and widen the range of Suzuki products, this project explored the metalation of heterocyclic systems adjacent to imine-type nitrogens, and subsequent production of stable heteroaryl boronic esters to serve as precursors for Suzuki couplings. While our results support previous findings that metalation of the 2 position of an imine-type-containing heterocycle is difficult, there was evidence to suggest that synthesis of a stable heteroaryl boronic ester is possible.
Diffusion Controlled Model of a Chemical
The dynamics of a chemical reaction have long been a subject of study. Using a computer it is possible to simulate a reaction for a small number of molecules. This applet will do precisely that, simulating a reaction containing no more than a few hundred molecules. This simulation creates a two-dimensional grid partially filled with molecules of two given types. Each molecule moves in a random walk pattern, whereby each step it moves a distance of one grid point in a completely random direction. When two molecules of different types come into contact a reaction may occur and a third type of molecule may be formed. At the user's discretion this new molecule may be inert or it may react with the original molecules. Furthermore, the user can dictate the initial distribution of molecules to be either random or follow predetermined pattern. As time progresses, details of the reaction (i.e. reaction rate, etc) can be determined based on the concentrations of the various molecules.
A New Application of Tissue Engineering:
Developing scaffold materials with biopolymers.
Tissue engineering has recently become a hot topic across the sciences due to the relative paucity of transplant tissue and organs available for donation and the huge amount of individuals awaiting transplants. In general, tissue engineering is the use of a combination of competent cells, polymer scaffold material, and sometimes bioactive peptides to repair or regenerate tissues. My poster specifically focuses on the biodegradable scaffold material that provides the surface for the tissue cells to grow on in the early stages of forming a viable extracellular matrix, and also provides an organization scheme for the generating tissue. Finally, my poster explores future uses of biopolymers in scaffold materials, such as polymer membranes for immunoisolation and prevention of immune rejection, incorporation of biologically active proteins into the polymer to control cell-cell interaction, and alteration of the polymer surface structure to change cell-polymer adhesion.
Ethylene Glycol-Containing, pH-Sensitive
Hydrogels for Insulin Delivery.
Poly(methacrylic acid-g-ethylene glycol) (P(MAA-g-EG) graft copolymers hydrogels swell with changes in pH. At low pH, the gels will be relatively compact; at high pH, the gels will be of a much larger volume. These gels can be designed to swell with respect to glucose concentration by including in the matrix of the gel the enzyme glucose oxidase, which would oxidize the glucose to gluconic acid, lowering the pH in the area around the gel, and causing it to shrink. These glucose-sensitive hydrogels could be included in a devise engineered to release certain amounts of insulin at various blood glucose levels.
Lifetime of phosphorescence of cyclodextrin-bromonapthol
complexes and its applications in molecular thermometry.
Kidd, Matt and Merle Schuh
Bromonapthol forms an inclusion complex with two a-cyclodextrins (6-membered cyclic oligomers of glucose) in aqueous solution. These complexes exhibit relatively long-lived phosphorescence, the lifetime of which changes tenfold over a range of 0 to 60 oC. This variation in lifetime could be used to construct a molecular thermometer that relies on that variation in lifetime of phosphorescence with respect to temperature. This project was designed to test the thermal stability of the cyclodextrin-bromonaphthol and its suitability for use in a molecular thermometer. There is evidence that the complex is stable at temperatures of 60 oC in darkness, but is unstable at the same temperature when exposed to light. Light appears to cause photolysis at elevated temperatures, but not at room temperature.
Immune responses to polymeric implants
and cell-containing microcapsules.
Lee, Christopher D.
Implantation of polymer biomaterials has many potential applications, including a protective barrier for cells engineered to produce enzymes in individuals with specific genetic deficiencies. An advantage of polymer over tissue transplantation is that polymers are typically more stabile in the presence of an unsuppressed immune system. Microcapsules are small polymeric spheres around a group of cells, with sufficient permeability in the polymer to allow nutrients to enter and therapeutic products out. Cells are protected from the immune system, because the polymer prevents contact between cells and mediators of the immune system. Upon implantation, polymeric biomaterials cause an acute inflammatory response, which can become chronic depending on biomaterial composition, porosity, roughness, and other factors. Mast cells release histamine to initiate this response, which recruits other leukocytes, whose binding to polymer surfaces can compromise implant function, particularly in the case of microcapsule permeability. Polymers for microcapsules can act as adjuvants in combination with foreign cellular components (including xenografts), which can activate an immune response against the cells. Depending on the permeability of the polymer, antibodies and components of the complement system can penetrate the microcapsule and kill transplanted cells. Various polymers are discussed in terms of studies conducted on their immunocompatibility.
Poly (ortho esters) as microcapsulating
agents used for drug delivery.
Lindenberg, Robert S.
Currently research is being conducted in an attempt to develop a controllable, non-carcinogenic, non-irritating, bioabsorbable and biocompatible drug delivery system. Poly (ortho esters) (POEs) is a class of polymers that have been studied as potentially useful for drug delivery through controlled polymer erosion. Ortho esters have desirable hydrolytically unstable properties, while at same time being hydrophobic. Their release rate can be controlled by using different molecular weights or by the addition of acids, such as lactic and glycolic acid, built into the polymer backbone. Thus, the focus of this study will be on the development of three families of poly (ortho esters) (POEs), describing their chemical properties and drug release capabilities.
A comparison of polycationic polymers
employed in in vivo gene therapy.
Monroe, William G., IV
Gene therapy promises the ability to treat and cure all inherited diseases and those acquired diseases with a genetic component. Many different approaches have been explored to inhibit the mechanisms of genetic diseases such as hindering oncogenes, down-regulating over-expressed disease-producing genes, and interfering with diseased proteins before they interfere with normal cellular activities. Antisense oligonucleotides have shown the ability to specifically down-regulate and even turn off individual genes of diseases. Antisense mechanisms are very effective in in vivo studies, but due to delivery problems results have not been comparable in in vitro. Polycationic polymers such as polylysine, polyamidoamine, polyethyleneimine, and the polysaccharide chitosan are leading polymer-based delivery systems because of their ability to effectively bind nucleotides under physiologic conditions. All four polymers contain either primary, secondary, tertiary, or quaternary amines, which along with their binding characteristics act as a endosomal buffering system preventing degradation and rejection of the gene therapy agent. Studies have shown the benefits and detriments of each polymer delivery system, requiring scientists to customize a polymer delivery system to their gene therapy agent. Experience and knowledge of polymer delivery systems is continually increasing, allowing scientists better results in delivering gene therapy agents that hope to cure disease.
Diversity of Polyurethane Structure and
In addition to functioning effectively, polymeric materials fabricated for biomedical use must satisfy the stringent constraints of in vivo application. It is often required that polymer chemists accept sacrifices of certain polymer properties in order to achieve other desired characteristics. In addition to being extremely biocompatible, polyurethanes are tremendously versatile substances due to the diverse nature of their structures (and therefore properties, as well). Altering either the basic chemical groups of polyurethane structure, isocyanate and polyoyl, or the method of polymer synthesis allows for this high degree of adaptability. The exceptional versatility of polyurethanes is manifest in its extensive biomedical applications.
Synthesis of two monomers based on N,N'-bis(p-carbo-methoxybenzoyl)ethanediamine
Pfeiffer, Matthew W. and David M. Brown
The objective of this project is to synthesize two novel bisesteramide monomers based upon N,N'-bis(p-carbo-methoxybenzoyl)ethanediamine (T2T-dimethyl) which will subsequently be copolymerized with poly(ethylene terephthalate) to yield modified PET polymers. The two structures will substitute a phenyl group for the ethylene group in the T2T-dimethyl monomer through an amide linkage and ester linkage respectively. The resulting polymers are expected to be more rigid and to exhibit higher glass transition temperatures and melting points than PET alone and PET copolymerized with T2T-dimethyl monomer. This product would result in high performance versions of PET for end uses such as reinforcing auto tires and conveyor belts. The two monomers were synthesized and characterized by NMR and melting point. They will be sent to KoSa in Spartanburg, SC for polymerization and further testing.
Investigation of Hemodialysis Membranes
in Treatment of End-stage Renal Failure
Pritchard, Justin Gregg
End-stage renal disease is currently treated with hemodialysis allowing patients to extend their life and, therefore, extend their ability to find kidney donor prospects. The primary element of hemodialysis is the membrane used to filter the blood. Membrane properties must allow for the removal of waste particles while avoiding excess water loss. When searching for a membrane to achieve these goals, blood compatibility becomes a key factor. All membranes create some sort of surface reaction with blood. These reaction fall into the categories of complement activation, platelet adhesion, and coagulation. Membranes must reduce the effect these reactions have on the filtration properties. Both synthetic and natural membranes are used to attempt to solve the equation for the perfect substance. Cellulose membranes were first commercially available in 1943 and continue to be a primary polymer for dialysis. Synthetic membranes such as polyacrylonitril (PAN), polymethylmethacrylate (PMMA), and ethylvinylalcohol (EVAL) have all been used with similar success to regenerated cellulose. Each of these materials has trade-offs involving hemocompatibility, trade-offs related to both polymer material and synthesis technique.
The synthesis and characterization of
hydroxymethyl crown ether.
Ragheb, Ragy and Susan Hendrickson
Several strategies for constructing ion-selective electrodes are based upon ion-selective polymer films. One strategy for imparting metal-ion selectivity to a polymer is to incorporate a crown ether into the polymer. The selectivity of the crown ether is controlled by the inner diameter of the crown ether ring and the diameter of the metal ion. For polymers possessing cationic sites, a crown-ether bearing an anionic pendant group may by incorporated into the polymer by ion exchange. The crown ether, 15-crown-5, is selective for sodium ions. This crown ether was synthesized with a hydroxymethyl functional group, which may then oxidized to yield a carboxyl group. The crown ether was synthesized from tetraethylene glycol and allyl glycidyl ether following a procedure published by M. Okahara. The side chain on the 15-crown-5 ether was subsequently modified to produce the desired hydroxymethyl substituent. This synthetic approach is a multi-step process, and the yields for the individual steps were low.
Biocompatibility in the Polymers for
Hemodialysis: Current biomaterials and future directions.
Riedley, Shannon E.
For hemodialysis patients, receiving life-saving treatment requires repeated venous access which implies the contact of blood with foreign materials. It is important to understand what biomaterials are used for these patients and why certain materials are more common than others. There are seven major biocompatibility issues involved in vascular access that are examined in detail: 1) coagulation and clotting, 2) complement and leukocyte activation, 3) susceptibility to infection, 4) leaching, 5) surface alterations, 6) allergic reactions, and 7) shear. The ways to prevent complications through chemical modifications of the polymers and future perspectives are discussed. It is obvious that contact with the blood for disease treatment requires new devises to be developed and older polymers to be improved to limit the biocompatibility problems. These biocompatibility problems and changes in the polymers will be discussed within this research.
The Use of Polyanhydrides in Biodegradable
Drug Delivery Systems
Aleksandra I. Smidowicz
The polyanhydrides are biodegradable polymers composed of monomer units connected by anhydride bonds. These polymers, synthesized from the dicarboxylic molecules in a process of melt polycondensation, have been engineered specifically for applications in controlled release drug delivery systems. In such systems, the drug is incorporated into a bioerodible polyanhydride which is then implanted into the body. As the polymer comes in contact with body fluids it begins to erode releasing the drugs into the surrounding tissues. The rate at which the drug is released from the polymer matrix is determined by the rate of polymer's surface erosion. This rate can be controlled by adjusting the composition of the copolymer - the rate of drug release can be increased by incorporation of more aliphatic monomers into the polymer chain, and slowed down by increasing the relative ratio of aromatic monomers. The best linear release rates have been obtained for polyanhydrides composed of carboxyphenoxy alkanoic acids. One such polymer, poly[1, 3-bis(p-carboxyphenoxy)propane-co-sebacic acid], is used in the delivery of a chemotherapeutic drug, carmustine, to the brain for treatment of glioblastoma multiformae.
Temperature effects on the helix-to-coil
transition of k-carageenan.
Talbird, Victor M. and David Blauch
k-Carageenan is a polysaccharide that undergoes a conformational helix-to-coil transition upon increasing the temperature from ca. 10 to 30 oC. Since the distance between adjacent charged groups depends on the k-carageenan's conformation, the spatial distribution of the sodium counter-ions around the polymer also depends upon the polymer conformation. The changes in conformation therefore result in a change in the conductivity of polymer solutions, making conductivity measurements an attractive tool for studying the helix-to-coil transformation. The conformation of the k-carageenan may be characterized by the ratio of the diffusion coefficients of sodium ions in the polymer solution and in a solution containing no polymer. The sodium salt of k-carageenan was used in the study, and reference measurements were made with solutions of sodium chloride. Conductivities of the k-carageenan and NaCl solutions were measured at several concentrations over the temperature range of 0 to 50 oC. The diffusion coefficient ratio, obtained from the experimental data, plotted against the temperature reveals the conformational transition.
Polymeric Alternatives for Vascular Grafts
Several medical conditions, most notably atherosclerosis and vascular aneurysms, require the patching or replacement of an artery or vein. Vascular grafts composed of polymeric fibers have been developed as an alternative to conventional materials such as transplanted veins. However, a major constraint to developing these polymeric grafts has been matching their chemical and physical properties to those of the host tissue. This poster focuses on evaluating various polymeric materials and determining their viability for use as vascular graft alternatives.
Biomedical material used for bone grafting
Bone grafts are used in orthopedic surgery when a bone fracture is accompanied by bone loss, to help a bone that is not healing, and to fuse joints to prevent movement. The procedure involves exposing the defected bone, then shaping the bone graft (supplemental bone), and then the insertion of the bone graft into and around the defective area. The bone graft is held in place by plates, pins, and screws and then is immobilized with a cast. There are currently four sources of bone graft material: autograft, allograft, xenograft, and alloplast. Autograft and allograft are both human bone tissue, with the only difference being that allograft is tissue from a different member of the same species and autograft is from the same member of the species. Xenograft is also a naturally occurring tissue; however, it is obtained from a different species, usually bovine. Finally, alloplasts are synthetic polymers, ceramics, or composites, which mimic natural tissue. Most current research on synthetic grafts involves the use of polylactide (PLA), polyglcolide (PGA), hydroxyapatite (HA), a combination of two of these, or all three. Herein will be presented the benefits and shortcomings of current matrices available.
Polymer alternatives to latex in medical
gloves and condoms.
White, J. Wilson, III
Natural rubber latex membranes are inexpensive, thin, flexible, and impermeable to most viruses and bodily fluids. These properties make latex an ideal material for many biomedical applications, including medical gloves and condoms. Unfortunately, approximately 3% of the general population and approximately 17% of health care workers mount Type I or Type IV allergic reactions to protein and chemical allergens in latex. Allergic reactions range in severity from mild (skin rash) to lethal (anaphylactic shock). Consequently, several manufacturers are beginning to produce a variety of non-allergenic, non-latex alternatives, using materials including polyethylene (PE), polyvinylchloride (PVC) and thermoplastic elastomers (TPEs) such as styrene ethylene butadiene copolymer and polyesterurethane. Polyethylene and PVC are generally less durable and allow more viral leakage than latex, but TPEs have higher tensile strengths and deteriorate less rapidly than latex. However, several investigations have produced conflicting data regarding the relative viral protection afforded by TPEs. Due to their higher tensile strengths and thermoplastic properties, TPE condoms provide several other advantages over latex competitors, including reduced thickness and tighter fit for greater sensation and the possibility of novel manufacturing processes that afford greater quality control.
Polymers in Targeted Drug Delivery
Wolf, Rachel W.
Often drugs intended for specific sites of action are administered systemically but lack mechanisms by which to accumulate at their intended site of action. Targeting a drug to its site of action is advantageous. One approach to targeted delivery is implantation of the drug at the target site. However, polymeric systems are also being designed for localized delivery of systemically administered drugs. Polymeric targeting takes advantage of the enhanced permeability and retention effect (EPR) as well as the macrostructure's route of cell internalization via endocytosis followed by liposomotropic uptake. Polymeric approaches to targeted delivery are currently being explored for uses in chemotherapy and gene therapy as well as for diagnostic procedures. These approaches may involve passive "stealth" targeting, the goal of which is to prolong half-life in circulation. Other carrier systems utilize both passive and active targeting, via physical or chemical homing mechanisms such as through antibodies and sugar moieties and as thermo-sensitive micelles. While early attempts to utilize polymers in site specific drug delivery failed to meet expectations, current research suggests that specific, site tailored polymeric systems will prove a viable option for medical diagnosis and select therapies in the future.
The Chemistry of Dental Impression Materials
Polymer chemistry has played a significant role in the advances made in the field of dental materials. One facet of dental materials is the area of dental impressions; they are used to make a mold of a patient's mouth. The mold can then be used to create a protective mouth guard, a bridge, a crown, etc. The type of polymer chosen to create the impression material will define the properties of the dental impression. I studied four polymers commonly used to make dental impressions; the polymers were polysulfide, silicone, polyether, and polyvinyl siloxane. I looked at the mechanism of polymerization as well as the differences in physical properties for each polymer. I found that each polymer has its own pros and cons; thus concluding that one polymer is not exceedingly superior to another polymer.
Top of Page
Basins of Roots and Periodicity in Newton's
Method for Cubic Polynomials
Smith, Amy (Richard Neidinger, Advisor)
Newton's method is a useful tool for finding roots of functions when analytical methods fail. The goal of our research was to understand the dynamics of Newton's method on cubic polynomials with real coefficients. Usually iterations will converge quickly to the root. However, there are more interesting things that can happen. When we allow initial values to be chosen from the complex plane, we find that the points that converge are bounded by fractals. For some polynomials we found interesting phenomena including chaos and attracting periodic cycles. We classified which polynomials could have attracting periodic cycles.
Top of Page
An Analysis of Numerical Methods
Using Projectile Motion.
Campbell, J. Peter
Computers are excellent learning tools for the education of concepts that are difficult to teach in the classroom. My applet investigates several different algorithms for the calculation of projectile motion. The user has the option of employing the Euler, Runge-Kutta second order, or Runge-Kutta fourth order algorithms; the user can also vary the time step for the calculation. The benefits of the higher order algorithms quickly become apparent. At small time steps, due to the relative simplicity of two-dimensional projectile motion, the error is small. At larger time steps, however, the higher order algorithms perform much better than the basic Euler method. The applet also has direct benefit to the teaching of projectile motion. Due to the mathematical complexity of realistic effects, most introductory physics students are only introduced to idealized dynamical concepts. My applet visualizes the effects of air resistance and wind velocity on the motion of a projectile, with the user being able to vary the initial velocity and angle of the trajectory, as well as the wind velocity. The physical relationships are both displayed in an animation of the projectile and in graphs representing the position, velocity and kinetic and potential energy of the projectile at every point.
Use of Artificial Intelligence
in a Simple Pong Game.
Artificial Intelligence (AI) has been studied for several decades by many people. It is the goal of my applet to also develop an AI algorithm to be used in a simple pong game. In this game, the user is able to control one of the paddles, while the computer controls the other. The ball is controlled by a Java Thread. The user will be able to specify the difficulty of the game (a different AI algorithm will be used for each difficulty level), as well as the game speed and the maximum score.
Two Balls Bouncing in One Dimension
Under the Effects of Gravity
This study uses a Java applet to analyze the complex chaotic behavior that occurs in a simple mechanical system: two balls bouncing on top of each other under the influence of gravity. An elastic collision occurs when a ball collides with the other ball or the ground. Both energy and momentum are conserved during this process. When a ball collides with a stationary surface, the velocity of the ball reverses itself. When a ball collides with a moving surface, the resultant velocities of the two objects do not reverse themselves, but change following the laws of conservation and momentum. The final velocities differ depending on the mass ratio of the two objects. In this study, two balls are moving in one-dimensional space allowing both types of collision occur. If the mass ratio of the top ball to the bottom ball is less than unity, the motion is chaotic almost everywhere. If the mass ratio is greater than unity, the motion shows a parabolic behavior with chaotic features. By means of animation and graphical demonstration, the trajectories of the balls can be observed.
Modeling the Motion of One-dimensional
Cupples, Laura M.
This program was written using the Java programming language as a culmination of the Physics 200 curriculum at Davidson College. By modeling the motion of two masses in a one dimensional coupled oscillator system with varying masses and spring constants, it gives users the opportunity to visualize the behavior of such systems. The model of the program solves the differential equations for the motion of the masses using Euler's method. The displacements of the two masses are plotted with respect to one another parametrically as time progresses using an Sgraph object. This parametric plot shows the eigenvector axes of the system, as they vary with changing masses and spring constants. Different initial conditions set by the user will result in different eigenvalues and eigenvectors for the system and subsequently different characteristic frequencies of oscillation.
Diffusion Controlled Model
of a Chemical Reaction
The dynamics of a chemical reaction have long been a subject of study. Using a computer it is possible to simulate a reaction for a small number of molecules. This applet will simulates a reaction containing as many as a few hundred molecules. The simulation takes place on a two-dimensional grid partially filled with molecules of two given types. Each molecule moves in a random walk pattern, whereby each step it moves a distance of one grid point in a completely random direction. When two molecules of different types come into contact, a reaction may occur and a third type of molecule may be formed. At the user's discretion, this new molecule may be inert or it may react with the original molecules. Furthermore, the user can dictate the initial distribution of molecules to be either random or follow predetermined pattern. As time progresses, details of the reaction (i.e. reaction rate, etc) can be determined based on the concentrations of the various molecules.
A Percolation Model for Galaxies
While the mechanism for the configuration of galactic structures has largely been attributed to dynamic effects, propagating star formation provides many of the observed properties of galaxies and may be the dominant mechanism for converting gas into stars. This computer model presents star formation through percolation as the main mechanism for the development of galactic structure. A region of a galaxy containing the necessary ingredients for star formation (gas, temperature), will alone form nothing. The explosion of a massive star into a supernova provides energy to compress interstellar gas into a dense molecular cloud from which stars form. Because the supernova itself is the result of earlier nearby star formation, it is a source of percolation; some of the stars formed may become supernovas themselves and repeat the process. The uncertainty and variability for star formation is contained within a single parameter p, the probability that a supernova explosion in one region gives rise to star formation in a neighboring region, which, in turn, results in the formation of another supernova. The effect of changes in this and other parameters will be shown for the simulation of a galaxy.
Chemical Composition during
the Life Cycle of a Star
A star is created when a large body of gas begins to collapse on itself due to its own gravitational pull. As the star contracts the temperature and pressure begin to rise. Once a critical temperature and pressure have been met, fusion begins to power the star. The power generated outward by this fusion process, of turning hydrogen into helium, counteracts the inward gravitational pull due to the star's mass. The initial percentages of hydrogen and helium are seventy-one and twenty-seven percent respectively. The rates at which hydrogen is consumed and helium is used depend on the initial temperature and density of the body. This applet models the evolution of a star starting at a point in time when the star becomes a zero age main sequence star. After all the fuel is consumed then the star's gravitational pull takes over and the death sequence of the star begins.
Slope Fields and Differential
Slope fields are an important tool to investigate the behavior of first order differential equations. This applet allows the user to visualize the slope fields for two coupled first order differential equations in two unknowns. Solutions to the differential equation are shown wherever the user chooses to click the mouse as well as the isoclines of the differential equation. The applet is designed to teach students exactly what a slope field looks like and what isoclines are. It could be used to see the behavior of y(x) of an unsolvable differential equation. Solutions to differential equations that model predator and prey problems, harmonic oscillations problems, growth rate of bacteria problems, and other applications will be shown.
Schoewe, Andrew R.
The Ising applet uses the Monte Carlo method to model the internal energy of magnetic materials. In addition to thermal energy, the Ising Model can also show us how the magnetic material responds to an external magnetic field. Ising uses a 2-dimensional model that contains thousands of "domains." These domains can have one of two spin directions, north or south. The difference between domains pointing north and domains pointing south gives the net magnetization of the material. As seen in real experiments, my magnetic material loses its magnetism as temperature increases, and at low temperatures the material forms large clusters of domains pointing in the same direction. This phenomenon is due to the energy provided by the outside heat source. By measuring the standard deviation of the magnetic field at different temperatures, we can also find the phase change of the magnetic material. The phase change occurs when the magnetic field fluctuates the most (ie. large standard deviations).
Rabbits: Population dynamics
simulation of a simple food chain.
Willson, J. D.
In nature there exists a delicate balance between the members of a food chain. Producers (primarily plants) are eaten by primary consumers (herbivores) that are in turn eaten upon by predators. Population changes at any of these three levels can be disastrous for the other members of the food chain and can lead to overpopulation or even extinction of species. This program is a model of a simple food chain in which grass is eaten by rabbits that are, in turn, preyed upon by foxes. The user can change several parameters of in the world such as amount of grass, number of rabbits or foxes, world size, etc. and observe the affect that this has on the population. With this model it is possible to construct both populations that exhibit fairly stable populations over time and populations that fluctuate at regular intervals. By observing this basic model it is possible to learn a lot about food chains and predator/prey relationships as well as the impact that human interference can have on an ecosystem.
Heat Pipe Spectroscopy: Raman
Scattering and Four-wave Mixing in Sodium and Cesium.
A high-power Nd/YAG pumped dye laser was used to study two-photon excitation in Na and Cs atoms. The laser excites the atom to real or virtual energy states followed by either Raman scattering or four-wave mixing. Emission spectra were used to determine which process occurred. Elemental samples of cesium and sodium were heated to several hundred Kelvin at approximately 2 Torr in heat pipes. Light from a Nd:YAG pumped, tunable dye laser was passed through the heated sample. The emitted light was then passed through a spectrometer and intensity as a function of wavelength was measured using a photo-diode or an IR detector. Data interpretation was handled through graphical analysis on Microsoft Excel.
Chaos in Oscillatory Systems.
Stewart, Phillip M.
The properties and behaviors of oscillatory systems are often studied in physics, but rarely are the systems fully described with the physical constants when nonlinear effects are involved. This study presents experimental results from three different oscillating systems: an LRC circuit, a "chaotic pendulum", and a torsion pendulum. Period doubling, quadrupling, octupling, and finally, chaotic behavior were observed. The chaotic pendulum produced nice Poincaré sections, a special way to view the chaos phenomenon. The torsion pendulum made it easy to determine all associated physical constants using analytical experiments. After finding the spring constant, moment of inertia, and damping coefficient, the system was fully described using it's nonlinear differential equation. Each pendulous system produced potential energy wells which were studied. Additionally, the effects of damping and driving were explored using the chaotic and torsion pendulums.
The study of Fourier transform interferometry has been of interest since the invention of the interferometer by Dr. Albert A. Michelson in 1880. Since then, the principles of interferometry have been utilized in devices such as a scanning Fabry-Perot interferometer in order to determine the spectrum of a light source, for example. This experiment investigated the spectrum of a Helium-Neon laser and a sodium lamp using a normal Michelson interferometer. Through a Fourier transform, the resulting fringe pattern yields the spectral data, a single peak for the HeNe laser and a doublet peak for the sodium lamp.
Temperature Dependence of Photoluminescence
From Semiconductor Heterostructures
Weindruch, F. Emmett
Photoluminescence is the process by which a material emits light due to being struck with incident laser light. By examining this emitted light with a spectrometer, one can learn about the electronic structure of the sample. In this lab, I used a He-Ne laser to study a GaAs/AlGaAs heterostructure with three quantum wells of varying thickness. By placing the sample in a cryostat, I was able to conduct this experiment at temperatures varying from 77 K to room temperature. Consequently, I was able to investigate how the peak energy and emission intensity of each well varied as a function of temperature. As expected, these peak energies increased as the sample temperature decreased. Finally, by looking at the Boltzmann "tails" of the quantum well emissions, I could study how the energy distribution within each well changed as a function of temperature. These "tails" reflect the fact that at higher temperatures, confined electrons will be able to occupy a slightly wider range of energies than at near absolute zero. Using Boltzmann statistics I could then experimentally calculate the temperature at which the data was taken.
Supersymmetric Quantum Mechanics
Valdés, Tim (Faculty Advisor: Mario Belloni)
Quantum mechanics lies at the heart of our understanding of matter and energy. Unfortunately relatively few exactly solvable systems are presently known. However, using the ideas and techniques of supersymmetry (SUSY)---a method that factors the one-dimensional Schrodinger equation---many systems have been recently discovered and successfully solved. This investigation into SUSY quantum mechanics looked at potentials of known systems with exact solutions and, by applying SUSY techniques, found new systems and their exact energy states. In addition, "superpotentials" have been created and examined to discover what types of systems are represented. Examples of these procedures will be shown and future work will be discussed.
Top of Page
Matching Objecs to Emotions: Impairments
in young children with autism.
Berry, Debra L.
This study explored the differences between children with autism and typically developing children in their understanding of people and emotions. It employed a new procedure that allowed testing of younger children (than previously tested) and children with differing communication abilities because the task was nonverbal and relatively simple. Children with autism (n = 22) and typically developing children (n = 6) matched pictures of objects to corresponding pictures in three conditions: object-object, object-emotion, and object-person. Children with autism only matched 56% correctly on the object-emotion task and 61% on the object-person task whereas typically developing children matched 90% correctly on the object-emotion task and 97% correctly on the object-person task. Another finding was a high negative correlation between the diagnostic measure of autism, the Childhood Autism Rating Scale (CARS), and children with autism's ability to match objects to people, which means that the CARS is a good predictor of children with autism's understanding of other people. The relationship between the CARS and the object-object and object-emotions tasks approached significance. This research offers a new perspective to the specific delays in children with autism and suggests that a core cognitive deficit leads to the diverse patterns of behavior in autism.
The Mechanisms Underlying the Word Length
Effect: Decay or interference?
Dobey, Kimberly A., and Kristi S. Multhaup
In a typical memory span task, the participant receives a list of words and recalls the words in the order in which they were presented. This study examined the mechanisms responsible for the word length effect (WLE; short words recalled better than long words). Some researchers (e.g., Baddeley, Thompson, & Buchanan, 1975; Multhaup, Balota, & Cowan, 1996) have argued that decay underlies the WLE. In this argument, the maintenance of words depends on refreshing the traces of those words through rehearsal. If the traces are not rehearsed, the items will decay in about 2 s. Since short words can be rehearsed faster than long words, more short words can be remembered than long words, which leads to the WLE. However, recent research (Neath, Nairne, & Serra, 1997) suggests that interference from previous lists leads to the finding of a WLE in memory span tasks. In the current study, memory span was studied under conditions of reduced interference. Interference was reduced by (a) changing semantic categories between trials, (b) changing word length between trials, and (c) using a 5-min interval with a non-verbal filler task between every four trials (block). Fourteen older and 11 younger adults saw 20 lists of words and completed a serial order recall task for each list. The results showed significant main effects of word length, age, and block. The finding of a WLE suggests that the effect is due to decay rather than to interference because after multiple reductions to interference the WLE was still present.
Effects of Schemas on Reprocessing
This study examined the influence of the memorial representation of a text on later reading of a different text. Twelve 5th graders and twelve 2nd graders read aloud pairs of passages that either had a context or no context. The first passage of each pair was followed by a word stem test with half of the words being from the passage. Participants then read another version, a paraphrase or word overlap, of the first passage. Second and 5th grade children had more savings in reading times between the first and second passages in the paraphrase conditions compared to word overlap conditions. Paraphrase passages were only read faster than original passages in the context condition. For word overlap passages without a context, fifth graders read the second passage slower than the first passage. Both grades completed more word stems with words from the previous passage in the context condition than in the no context condition. Although the findings support and extend the episodic view of transfer, the results also suggest that further research should differentiate between levels of fluent reading in children and adults.
Differences Across Age in the Antinociceptive
Effects of Mu Opioids
Gray, Jason D.
This study evaluated the antinociceptive effects of high- (morphine, levorphanol, and buprenorphine), and low- (butorphanol, nalbuphine, and nalorphine) efficacy mu opioids in young (3 months) and aged (24 months) Fisher 344 male rats. Antinociception was assessed by measuring the latency of tail withdraws from 50° C (low stimulus intensity) and 55° C (high stimulus intensity) water. Morphine, levorphanol, and burprenorphine produced maximal effects across both groups and stimulus intensities in the tail withdrawal procedure. Butorphanol produced high levels of antinociception at the low stimulus intensity, but only low levels of antinociception at the high stimulus intensity across groups. Nalbuphine and nalorphine produced little effect at the high stimulus intensity in each group. However, at the low stimulus intensity both drugs produced high levels of antinociception in the aged rats, but only low levels of antinociception in the young rats. Across conditions, the young rats were significantly less sensitive than the aged rats, and this effect was most pronounced when lower efficacy opioids were tested at the high nociceptive intensity. At the low stimulus intensity, nalorphine produced a combination of agonist and antagonist activity when combined with morphine in the aged rats, but produced only antagonist effects in young rats. These data expand the behavioral literature on the antinociceptive effects of opioids in young and aged rats by showing that the agonist-antagoinst profile of low efficacy opioids can differ across subject populations.
The Relationship Between Distortions
in Memory Tasks and Extrapolation Tasks
Michew, Joy H. (Faculty mentor: Margaret P. Munger)
The relationship between performance on a memory-for-position task and an extrapolation-of-motion task was measured. Each subject's level of dream imagery was measured, as well. Past research shows positive distortion of the final position in a memory task (Freyd & Finke, 1984), which could be interpreted as automatic anticipation of future motion. Research shows undershooting in an extrapolation task (Cooper & Munger, 1993), suggesting an inability to anticipate future motion under certain conditions. These conflicting distortions make it unclear whether performance on these two tasks is linked. The comparison was designed to investigate whether memory tasks and extrapolation tasks are performed by visual imagery mechanisms. It was hypothesized that students with higher dream imagery levels would show more representational momentum in the memory task, and extrapolate further in the extrapolation task. Twenty-four Davidson College students participated in the experiment. Participants performed a memory task and an extrapolation task, and completed a dream imagery questionnaire. When sorted into two groups based on dream imagery level, the subjects with higher imagery scores showed more representational momentum in the memory task and undershot less on the extrapolation task than those with low imagery scores. There were significant velocity effects in the two tasks. In the fast conditions of the memory task, downward motion caused more positive distortion than upward motion, suggesting a gravity effect. In the fast conditions of the extrapolation task, motion through the vertical axis was negatively distorted, and motion toward the vertical axis was positively distorted, suggesting a vertical anchoring effect.
The Moderating Effect of Gender on Changes
in Personality in Home and Work Situations.
Changes in personality profiles between home and work situations were examined to find whether gender had a moderating effect on personality flexibility. Participants completed two NEO FFIs, one at work and one and home, and then had NEO FFIs completed on them by their spouse and a coworker. The results found that personality did change significantly across situation for the Big Five factors of negative emotionality and conscientiousness for both males and females. Additionally, gender was a moderating variable across situations only for the factor of openness. Males reported higher openness scores at work than at home while females reported higher openness scores at home compared to work. These findings support the Conditional Theory of Personality as proposed by Mischel and suggest that personality is better conceptualized as situationally variable than as a transsituational entity.
The Role of Object Parts in Memory for
Object Position: Evidence for a privileged object-part relationship?
Starr, Jennifer A. (Faculty mentor: Margaret P. Munger)
Representational momentum (RM) occurs when observers view a sequence of static images that imply an object in motion and misremember the object as further along the implied event. Previous work examined the role of the object's underlying structural description, specifically its geons, and found that RM is sensitive to changes in this description, with no RM occurring when depicted events lead to changes in the geon structure. The current experiment examines whether this sensitivity is specific to the structural description of an object per se or is general to the structural description of any array. The overall pattern of the data suggests that RM is somewhat sensitive to the structural description of an array but may be more sensitive to the structural description of an object. The extent to which the results can be interpreted to comment upon any privileged object-parts relationship is limited by the unexpected specific directions of the memory distortions found in the current experiment. These unexpected directions could be due to the combination of a weakened RM effect, due to a decrease in event complexity within an experimental block between previous work and the current experiment, and a memory averaging effect.
Sensitivity to the Effects of Central
Nervous System Depressants on Motor Coordination: Influence of
Task Difficulty and Chronic Phenobarbital Administration
Stoops, William W. and Mark A. Smith
The present investigation examined sensitivity to the motor-impairing effects of CNS depressants in rats treated chronically with phenobarbital. Eight rats were trained to walk on a rotorod treadmill at low (8 rpm), and high (24 rpm) speeds. Prior to the chronic regimen, pentobarbital, amobarbital, phenobarbital, diazepam and clonazepam produced dose-dependent decreases in motor coordination at both speeds. During chronic treatment with phenobarbital (100 mg/kg/day), tolerance was conferred to the effects of all the drugs examined as evidenced by rightward shifts in their dose-effect curves. For all drugs, the magnitude of this tolerance was generally consistent across the two speeds. Following a six-week washout period during which no drugs were administered, dose-effect curves for each drug were shifted back toward their original (i.e., pre-chronic) positions. Under all conditions, the doses required for each drug to impair motor coordination at the low speed were higher than those required to impair motor coordination at the high speed. These data suggest that sensitivity to the motor impairing effects of CNS depressants is influenced by the difficulty of the behavioral task, and that this effect is consistent before, during and after a regimen of chronic phenobarbital administration.
Top of Page
Biology Department Home Page
Chemistry Home Page
Mathematics Home Page
Physics Home Page
Psychology Home Page
© Copyright 2000 Department of Biology, Davidson College, Davidson, NC 28036
Send comments, questions, and suggestions to: email@example.com