Browsing by Author "O'Neill, Adrian Thomas"

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    Development of Closed Cell Metallic Foam Using Casting Techniques
    (2004-11-29) O'Neill, Adrian Thomas; Dr. Afsaneh Rabiei, Committee Chair; Dr. Jeffrey Eischen, Committee Member; Dr. William Roberts, Committee Member
    The research sited in this paper involves the development of a new metal foam composite material using casting techniques. This work included the design of the material and the development of a process to produce the metal foam. The materials used to produce the foam consisted of low carbon steel hollow spheres and an aluminum alloy. The foam is comprised of steel hollow spheres packed into a random dense arrangement, with the interstitial space between spheres infiltrated with a casting aluminum alloy. Using prefabricated hollow spheres assures a uniform pore size and cell wall thickness. Casting a metal into the interstitial space provides a solid media to add structural support to the foam. The goal of this research has been to develop metal foam that demonstrates improvements in product uniformity and mechanical properties over the currently available foams. To accomplish this goal, the study included the identification of the various technologies used to manufacture metal foams, the assessment of the improvements needed to augment the quality of foamed metals, and the design of a new product and processing technique that substantiates these goals. The experimental equipment was designed and procured, while the raw materials were obtained. Then the hollow sphere foam samples were successfully produced. Using these samples a series of characterization studies was done to qualify and quantify the results. These findings were then compared to presently published data to gauge the relative success of the work. The hollow sphere metal foam developed in this study displayed significant improvements in the measures of compressive strength and energy absorption capacity, all the while maintaining the characteristic properties of cellular metals. The improvements were measured against the next best existing technology. The newly developed foam averaged 67 MPa over a region of 10 – 50% strain, with densification beginning at approximately 50% strain. The value for energy absorption is 30 MJ/m3 at 50% strain. This foam also has a strength to density ratio on level with the best reported results to date. The combination of these properties gives opportunity for use in previously unidentified applications, such as an energy absorption media for buildings subject to seismic motion. This foam can also be designed in such applications as automobile crumple zones, as structural members in air and space craft, and in biomedical prosthesis. Several areas for improvement have been identified for this technology. The bonding strength between sphere and matrix needs improvement, and different material choices and processing changes have been identified in this research to achieve these improvements. The packing density of the spheres can be improved, and a new method of vibrating the sphere arrangement prior to molding may increase the packing density. The porosity of the aluminum matrix can be reduced, and the design of the casting mold and processing conditions can be modified to reduce undesirable porosity. Additional testing methods have been identified to further characterize the foam and reveal insights for further improvement. The iterative process of sampling, characterization, and analysis will continue to improve this product to satisfy the objectives of this research program.
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    A Microfluidic Platform for Human Epidermal Keratinocyte Cytotoxicity Assays
    (2009-04-23) O'Neill, Adrian Thomas; Henry Hsiao, Committee Member; Edward Grant, Committee Member; Nancy Monteiro-Riviere, Committee Member; Greg McCarty, Committee Member; Glenn Walker, Committee Chair
    Linear dilution is a method to create linearly varying concentrations of a solution. Linear dilutions are commonly used in biological studies where the threshold concentration at which a physiological reaction occurs is unknown, whether it be a minimum effective or a maximum tolerable dosage. In this dissertation we present a summary of the approaches used for creating dilutions with microfluidics followed by a detailed methodology for constructing a proportional mixing linear dilution microfluidic device. The microfluidic device presented here is made with a rapid and inexpensive microfabrication method, soft lithography. The device is capable of generating nine linearly varying dilution values with an R-squared value exceeding 0.999 and the linearity of the dilutions is independent of the input flowrate, making it a very robust approach to creating linear dilutions. With the model presented the device can be expanded to an arbitrary number of dilutions with commensurate savings in reagent usage and time. Human epidermal keratinocytes (HEK) are skin cells of primary importance in maintaining the body’s defensive barrier and are used in vitro to assess the irritation potential and toxicity of chemical compounds. Microfluidic systems hold promise for high throughput irritant and toxicity assays, but HEK growth kinetics have yet to be characterized within microscale culture chambers. This research demonstrates HEK patterning on microscale patches of Type I collagen within microfluidic channels and maintenance of these cells under constant medium perfusion for 72 h. HEK were shown to maintain 93.0% - 99.6% viability at 72 h under medium perfusion ranging from 0.025 – 0.4 microliters per minute. HEK maintained this viability while ~100% confluent a level not possible in 96 well plates. Microscale HEK cultures offer the ability to precisely examine the morphology, behavior and viability of individual cells which may open the door to new discoveries in toxicological screening methods and wound healing techniques. A microfabricated cell curtain is presented that facilitates cellular assays. The cell curtain is defined as a poly(dimethylsiloxane) (PDMS) wall that extends from the ceiling of a cell culture microchamber to within microns of the chamber floor. Curtain use is demonstrated by observing monolayer human epidermal keratinocyte (HEK) colonies for 48 hrs longer than possible with non-curtained microfluidic chambers. The curtains were further characterized by integrating them into a 96-chamber high-throughput microfluidic cell culture device. As proof of concept, this device was used to assay a range of ethanol dilutions spanning 0 – 22% in cell culture medium. Cells exposed to 12% ethanol or less for 30 min would recover to 85% viability at 24 hr, while cells exposed to higher concentrations had viabilities below 10%. The data also showed that cells exposed to 6% ethanol or less grew in population size, 8% ethanol exposure stunted growth, and higher concentrations led to population loss. Curtain use permitted high initial cell seeding densities and increased the amount of time cells can be cultured compared to multi-well plates. It has been established that dermal exposure to jet fuel and its aromatic / aliphatic hydrocarbon components cause cytotoxicity. Two markers of this are cell viability and cytokine release. Measuring cell viability detects failure of some mechanism in the cell indicating eventual death. Cytokine release from HEK is an indicator of inflammation. Specifically, the cytokine interleukin-8 (IL-8) is released by the cell and attracts neutrophils, lymphocytes, T-cells and basophils, all part of an immune response. Significant work has been done to establish the HEK in vitro cell culture model and protocols to predict cytotoxicity. However, these can be expensive and time consuming. In this research we design a microfluidic device capable of simplifying the procedures for detecting those two forms of cytotoxicity, viability and IL-8 release. This device has been used to determine a highest non-toxic dose (HNTD) value and 50% lethality dose (LD-50) of the aromatic hydrocarbon cyclohexyl benzene. Results indicate that the HNTD value is 0.01% CHB and the LD-50 value is greater than 10% CHB in cell culture medium. The device has also been used to show that microfluidic HEK cultures can generate enough cytokines to be detected using a Bio-Plex commercial assay. These two capabilities, combined with a linear or serial dilution microfluidic device, constitute a high throughput device capable of reducing the time, labor, reagent and tissue use over conventional methods.