<p>Efforts to create a reliable, long–term implantable glucose sensor have been stymied by the effects of the foreign body response and wound healing that introduce delayed response times as well as unpredictable sensor performance. Loss of vascularization from fibrotic encapsulation around implanted sensors is purported as a key contributor to sensor failure, as glucose and oxygen transport to the sensor becomes impeded. Improving sensor performance by increasing angiogenesis and/or reducing capsule thickness using tissue-modifying textured coatings is attractive because texturing is not dependent upon a depletable drug reservoir. A significant range of materials and pore sizes are capable of promoting angiogenesis and reducing capsule thickness, provided pores have open-architecture with dimensions sufficiently large enough to allow inflammatory cell infiltration. </p><p><br></p><p>Poly–L–lactic acid was gas foamed/salt leached with ammonium bicarbonate to produce porous coatings for Medtronic MiniMed SOF–sensor glucose sensors. Coating properties included 30μm pore diameters, 90% porosity, and 50μm wall thickness. Cytotoxicity, degradation, and sensor response time studies were performed to ensure the porous coatings were non–toxic and negligibly retarded glucose diffusion prior to <italic>in vivo</italic> testing. Histology was used to evaluate angiogenesis and collagen deposition adjacent to porous coated and bare (i.e. smooth, uncoated) non–functional sensor strips after three weeks in the rat dorsal subcutis. Functional Medtronic glucose sensors, with and without porous coatings, were percutaneously implanted in the rat dorsum to assess if the angiogenic–inducing properties observed around the non–functional porous coated sensor strips translated into stable, non–attenuated sensor signals over two and three weeks. MiniLink<super>TM </super>transmitters were attached to the rats, permitting continuous glucose monitoring. Vessel counts and collagen deposition adjacent to sensors were determined from histological analysis. A one–sided dorsal window model was developed to further evaluate the interplay between vascularization and sensor performance Sensors were inserted beneath the windows, allowing visualization of microvascular changes adjacent to sensor surfaces, with simultaneous evaluation of how vascular changes impacted interstitial glucose monitoring. </p><p><br></p><p>Porous coating did have angiogenic–inducing effects on the surrounding tissue. When fully implanted in the rat dorsum, sensor strips with porous coatings induced three–fold more vessels within 100μm<super>2</super> of the sensor strip surface after three weeks and two-fold more cumulative vessel lengths within 1mm<super>2</super> after two weeks, compared to bare surfaces. In contrast, when percutaneously implanted in the rat dorsum, porous coated and bare sensors were equally highly vascularized, with two–fold more vessels than fully implanted bare sensors. </p><p><br></p><p>Despite increased angiogenesis adjacent to percutaneous sensors, sensor signal attenuation occurred over 14 days, suggesting that angiogenesis plays a secondary role in maintaining sensor function. Percutaneously implanted porous coated sensors had greater reductions in baseline current (20 to 50+%) over two weeks than bare sensors (10 to 30%). Mechanical stresses imposed by percutaneous tethering may override the beneficial effects of porous coatings. Furthermore, integration of the porous coating with the surrounding tissue may have increased tissue tearing at the porous coating–tissue, increasing inflammation and collagen deposition resulting in greater signal attenuation compared with bare sensors. Future investigations of the role mechanical irritation has on wound healing around percutaneous glucose sensors are warranted.</p> / Dissertation

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A unique approach for lightweight multifunctional composites was developed using Fe3O4 nanoparticles and polypyrrole-coated Fe3O4 particles as fillers. Fe3O4 particles are a good candidate for filler in a multifunctional composite system because they can reinforce mechanical properties of a polymer matrix and impart magnetic properties into a composite. Polypyrrole coating on Fe3O4 particles was utilized to incorporate electrical conductivity to the properties of composites. The effects of filler size and filler content were studied on both the mechanical and electromagnetic properties. Fe3O4 nanoparticles improved fracture toughness, but they compromised strength and modulus. Polypyrrole-coated Fe3O4 has potential for multifunctional material applications because the coating allows for concurrent increase in magnetic permeability and electrical conductivity in a composite. The polypyrrole coating also improved the strength of the composite. Fe3O4 nanoparticles were a major part of this work from their synthesis to their application in composites. The surface effect on magnetic properties was analyzed for Fe3O4 nanoparticles, resulting in a more accurate calculation of the magnetically dead layer thickness than previously reported. The results from this work contributed to further understanding of synthesis and characterization of magnetic nanoparticles, fabrication and characterization of nanocomposites, and design and development of lightweight multifunctional materials. Although the properties of the fabricated composites require further improvement, the methodology and approach provide a basis for future work in development of lightweight multifunctional composites.

The detection of volatile organic compounds in liquid is of interest for applications in public health, workplace safety and environmental monitoring. Traditionally, water samples were taken and analyzed in the laboratory using classical laboratory instrumentation. Current trends target real-time measurements using e.g. chemical microsensors built with microfabrication technologies. Among these, mass-sensitive chemical sensors, based on cantilever beams or surface acoustic devices, have shown substantial promise in gas-phase applications. In a liquid environment, the resonant microstructures typically suffer from high damping, which negatively affects the sensor resolution. In this work, a novel disk-type resonator developed at Georgia Tech was investigated as chemical microsensor for liquid-phase applications. The micromachined resonator vibrates in a rotational in-plane mode shape, reducing damping in a liquid environment. As part of the present research, a measurement setup with a custom-made flow cell for liquid-phase chemical measurements and a coating system to locally deposit polymer sensitive films onto the resonators were developed. To improve the film adhesion on the resonator surface in liquid, physical and chemical binding techniques were developed and tested on wafer samples. Polymers such as poly(4-vinylpyrrolidone), poly(ethylene-co-propylene) and poly(styrene-co-butadiene) were deposited using the custom-designed coating system onto the disk-type resonators. Liquid-phase measurements using tetrachloroethylene as the chemical analyte were performed. The experimental results are discussed, sources of problems are identified and recommendations for future research are made.

The purpose of the thesis is to investigate the improvement of single-crystal Si solar cell efficiency using porous Indium tin oxide (ITO)/ITO double layer antireflection(AR) coating. The resistivity, transmittance and refraction index of the porous ITO films prepared by supercritical CO2 treatment were investigated. At a 2000 psi pressure and 60¡CC, the resistivity of porous ITO films is 15 £[-cm, the average transmittance is better than 95 %, and the refraction index is 1.54. In addition, the resistivity of ITO thin films fabricated by reactive ratio-frequency magnetron sputtering is 7¡Ñ10-4 £[-cm, the average transmittance are 85 %, and the refraction index is 2.0. For the single crystal-Si solar cell with porous ITO/ITO double layer AR coating, the open circuit voltage, short circuit current, fill factor and efficiency are measured.

This paper aims to create a hard modification layer of WC/Co/Fe on the surface of SKD11 work steel by using a new closed-type method of surface electrical discharge coating with the self-made tool electrode and CNC electrical discharge machine. The tool electrode is the composition of a semi-sintered powder compacted electrode and a cooper rod. The sintered powder compacted electrode making process is first to mix the WC/Co and Fe powders uniformly at 8:1, 4:1, 2:1 and 1:1 in weight ratio. Continually, it will form the cylinder of 8mm in diameter and 18mm in length approximately by compacting in different pressure (50~200 MPa) and sintering temperature (300~900 ¢J). The EDM condition is 1~12A discharge current, 25~500£gs pulse time, and 50% duty factor. The electrical discharge machining is proceeding in kerosene with tool electrode as cathode and workpiece as anode. The result is as following. It can be concluded that the best condition to fabricate the sintered powder compacted electrode is 1:1 in weight ratio between WC/Co and Fe powders; with 200Mpa compacting pressure and sintering temperature at 900¢J, which results in lowest electrical resistivity. Under such condition, the area covered ratio can reach 100% at best EDM condition, which is pulse time £non = 50 £gs, rest time £noff = 50 £gs, 8A in current and 1.5 min in machining time. The surface hardness of workpiece increases with machining time. The surface hardness dramatically increases to Hv1500 as machining time over 1.5 min. The hardness of modification layer equals to the WC/Co particle itself which brings to the best wear ability. Moreover, the hardness of under surface in between 48 £gm is much higher than it of the SKD11 work steel. The hardness in between 30 £gm can reach up to Hv1200 in particular. The hardness of surface modification layer increases linearly with machining time. However, the limit of surface modification layer is about 30 £gm, and the needed time is below 5 min.

The spherical inverted-F antenna (SIFA) is a relatively new conformal antenna design that consists of a microstrip patch resonator on a spherical ground. The SIFA resembles a planar inverted-F antenna (PIFA) that has been conformally recessed onto a sphere. The basic design, simulation, and fabrication of a SIFA were recently reported. The aim of this thesis is to provide a three-fold improvement to the study of the SIFA: the fabrication of a dielectric-coated SIFA, a new analytical model based on the cavity method, and the analysis of a randomly oriented SIFA’s operation in a remote networking scenario. A key improvement to the basic SIFA design is the addition of a lossy dielectric coating to the outside of the sphere for purposes of impedance stability, bandwidth control, and physical ruggedization. The first contribution of this thesis is the fabrication of such a dielectric-coated SIFA. Two antennas are fabricated: a coated SIFA operating at 400 MHz, and an uncoated SIFA operating at 1 GHz for comparison. Both SIFAs are constructed of foam and copper tape; the coating is comprised of silicone rubber and carbon fiber. The fabricated designs perform with reasonable agreement to corresponding simulations, providing a basic proof of concept for the coated SIFA. The SIFA was previously studied analytically using a transmission line model. The second task of this thesis is to present a new model using the cavity method, as employed in microstrip patches. The SIFA cavity model uses a curvilinear coordinate system appropriate to the antenna’s unique geometry and is able to predict the antenna’s performance more accurately than the transmission line model. The final portion of this thesis examines the performance of the SIFA in a remote network scenario. Specifically, a line-of-sight link between two SIFAs operating in the presence of a lossy dielectric ground is simulated assuming that each SIFA is randomly oriented above the ground. This analysis is performed for both uncoated and coated SIFAs. A statistical analysis of the impedance match, efficiency, and power transfer between these antennas for all possible orientations is presented that demonstrates a design tradeoff between efficiency and predictability.

Organic polymer materials have been broadly applied in optical storage, optical communication and optical signal process. It has been revealed that these organic materials have some superior characteristics such as larger electro-optical (EO) coefficients, broader bandwidth and shorter response time, which make it good for EO modulator application. In our study, the goal is to study the EO coefficient of novel polymer material to be used in low driving voltage EO modulator. During the experiment, the dependence of the second harmonic generation (SHG) intensity on doping concentration of DR1/PMMA was observed and a reasonable explanation of the nonlinear dependency was given. We measured the EO coefficient of a new material, ASF/PMMA, using a Mach-Zehnder interferometer. We also observed the relaxation process of SHG intensity of this new material, which was compared with that of DR1/PMMA. Under the same measurement condition, we found that the EO coefficient of ASF/PMMA (13.1 pm/V) is significantly larger than that of DR1/PMMA (3 pm/V). It is also found that relaxation time of ASF/PMMA and DR1/PMMA are 22 and 8.5 seconds, respectively. Because of the superior characteristics of this material, it is suitable to be used in EO modulator.

Due to the fast expansion and development in the optical communication industry, the demand for the broad-band laser light source as used to the optical transmission network system has correspondingly increased. Cr4+ doped YAG crystal fibers, with its broad-band spectral property, is thus becoming more and more indispensable to the growth of the industry. However, the Cr4+:YAG crystal fiber in its own repetitive growth process brings with itself one problem: after each re-growth, the concentration of Cr2O3 and Cr4+ ions would reduce appreciatively. Hence, finding sound solutions for effectively raising the concentration of Cr4+ is now becoming an essential issue in the field. The thesis mainly focuses on the development of using Cr4+:YAG as the laser gain medium. Thin layers of CaO, MgO, and Cr2O3 was coated on the circumference of the Cr:YAG crystal fiber. The LHPG method was then employed to re-grow the coated samples, during which the doped concentration of CaO, MgO and Cr2O3 can in-diffuse. And the effect of charge compensation would go further to simultaneously raise the concentration of Cr4+ ions. Now we have successfully enhance the concentration of Cr4+ ions to 4.86x10-3 wt.%. This study, with the use of the E-Gun coating machine and the IAD (ion-beam assisted deposition) system, also probes the technical side of how to better improve the quality of the crystal fiber laser. Both end faces of the Cr4+:YAG crystal fiber were coated with optical thin films by TiO2 and SiO2 targets. In addition to raise the quality of the thin films, the IAD system also functions to create a laser cavity in which both the anti-reflectance (AR) effect (for pumping the light source) and the high reflectance (HR) effect (for stimulating the light source) are achieved. The accompanied benefits would be the reduction of transmission loss, the increased laser efficiency, and thus a more successful and more stable crystal fiber laser.