Development of Robust Fiber Optic Sensors Suitable for Incorporation into Textiles, and a Mechanical Analysis of Electronic Textile Circuits
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2008-04-30
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Electronic and optoelectronic textiles are an emerging area with many potential applications including the distribution of sensors within fabrics. One of the principle challenges of textile based electronics is to understand how the conductors and fiber optics behave under the mechanical forces of construction and use. This first part of this thesis focuses on the development of fiber optic sensors that are sensitive, yet robust enough to be incorporated into textiles. The second part of the thesis focuses on the problem of understanding the bending stiffness of fabric-based electrical circuits and presents a model to quantify the effect of incorporating rigid elements, such as conducting threads into woven fabrics.
Several novel fiber optic sensors are developed in this thesis, with the main focus on optical affinity sensors that employ surface plasmon resonance effects that are sensitive to local changes in the surrounding refractive index. These sensors can be employed to monitor the change of concentration of a chemical or adsorption of chemical or biological molecules on the surface of the sensor. A variety of techniques to control the surface plasmon are investigated, including annealing of gold thin films to form discontinuous and islanded thin films with nanoscale size dimensions, colloidal nanoparticles of gold, and surface plasmon engineered thin films where the extraordinary transmission of arrays of sub wavelength apertures can be exploited. In general these sensors are tested by changing the surrounding refractive index medium and monitoring the optical transmission. To demonstrate the detection of molecules, the detection of Streptavidin binding to Biotin is demonstrated. A variety of novel in-line temperature sensors are also developed, by incorporating temperature sensitive nanoparticles within the core of the optical fiber, or by constructing an inline Fabry-Perot cavity within the structure of the fiber.
The first major result is the development of in-line fiber optical systems that are constructed by fusing different fiber optic elements together into a continuous fiber with uniform diameter. Using this approach, the light can be transmitted to the sensor efficiently by a single mode optical fiber, a coreless optical fiber can be used as a beam expansion region or as a sensor region, and short segments of graded index fibers can be used as lenses to collimate or focus the light as appropriate. In addition to being substantially more robust than tapered fiber optic sensors or fiber optic sensors where the cladding is removed by etching, the interaction region of the sensor with the environment can be substantially increased.
The second major result is the incorporation of metallic and semiconducting nanoparticles into the core of fiber optic in-line sensors. These sensors are fabricated by coating the tip of the optical fiber with the desired metal or material such as vanadium oxide, and over coating the tip with a protective layer of silicon dioxide. The fiber can then be annealed, and fused to another optical fiber to form the in-line sensor. Since the size of the metallic nanoparticles can be determined by the annealing and fusion process, the optical properties of the sensor can be optimized. This approach is used to fabricate temperature sensors, where for gold nanoparticles, the shift and broadening of the plasmon resonance is used, and for vanadium oxide compounds where shifts in the absorption edge are monitored. Using thicker metallic films, partial mirrors can also be constructed using this assembly method to form a Fabry-Perot cavity where positions of the resonances can be monitored to track changes in temperature.
In addition to in-line sensors, fiber tip sensors are constructed, where nanoparticles or engineered nanostructures of gold are placed on the fiber tips and used to monitor the refractive index changes of a solution. The use of arrays of subwavelength apertures constructed on the ends of etched and tapered fiber tip using focused ion beam milling are shown to be especially promising for future optical affinity sensors.
In the second part of the thesis, the problem of bending stiffness of fabric-based electrical circuits is addressed. This is approached by assessing the effect of incorporating rigid elements such as conducting threads into woven fabrics. Although the models described in this thesis are employed to understand the bending behavior of fabric circuits, these models can also be applied for determining the bending rigidity of other fabrics with rigid metallic threads woven into the fabric structure. The key elements of a model employed to determine fabric bending rigidity are described and a procedure to determine fabric bending rigidity, based on an energy method that incorporates fabric structural parameters is developed. A theoretical model that explains the effect of incorporating finite number of rigid threads in a fabric, in the warp and weft directions, along with flexible threads on the bending rigidity of the fabric is presented. The theoretical models are compared with experimental measurements. An elastica-based theoretical model is developed and applied to fabrics containing multiple welded, fused, or soldered interconnects at the crossover point.
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Keywords
electronic textiles, optical fiber sensors, robust, nanoparticles, temperature sensors, chemical and biological sensing, surface plasmons
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Degree
PhD
Discipline
Fiber and Polymer Science
Electrical Engineering
Electrical Engineering
