Graduation Semester and Year

2009

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering

Department

Electrical Engineering

First Advisor

Jung-Chih Chiao

Abstract

Miniaturized devices in vivo for accurate diagnosis and prognosis in therapy to detect various physiological parameters inside human body have been a goal for efficient healthcare. In this perspective, microelectromechanical system (MEMS) devices providing small size features and new transduction functions benefit such biomedical applications. The features in MEMS devices also allow integration of CMOS circuitry and wireless communication for electronic control as well as actuation functionalities for driving mechanical parts for scanning, moving and probing. In this work, feasibility studies for two novel sensing systems: an in vivo optical fiber based spectral optical coherence tomography (OCT) imaging system and a passive wireless pressure sensor, having potential applications in diagnosis and therapy for gastrointestinal (GI) disorders have been proposed and demonstrated. Spectral OCT is a noninvasive imaging system and has several advantages such as high penetration depths, cross sectional imaging and micrometer resolution into the tissues. OCT can provide virtual biopsy into the depth of the tissues and 3-D visualization of the cells. Spectral OCT provides fast scanning and high resolution for the tissues. In our design, an improved scanner using electromagnetic actuation for fiber-based OCT systems has been demonstrated. The optical fiber provides small features allowing in vivo applications inside human body using conventional endoscopes. This system employs external electromagnetic actuation making it very small for in vivo uses without the requirement of electrical connection from outside to the scanner. Cantilevers coated with various non-magnetized ferromagnetic materials and actuated by external magnetic fields were designed and characterized to demonstrate the feasibility for remote scanning. Finite element analysis and analytic solutions were developed to design and characterize the designs of the scanner. Different magnetic materials such as cobalt, iron and nickel submicron-scale powders were used to demonstrate the actuation of the imaging systems. Magnetic materials characterized using the magnetic hysteresis curves, magnetic properties and mechanical properties were found to guide design principles for the scanner. Optical experiments were conducted for each device to verify the designs. Plastic cantilevers were coated with a mixture of 50% enamel paint and 50% various ferromagnetic materials. The dynamic measurements were performed under the external excitation of an electromagnet. Experiments with different ferromagnetic materials and different suspended cantilever lengths of 80mm, 70mm, and 60mm were performed to compare with theory. The dynamic displacements and resonant frequencies of the actuation were measured. The results presented could be used to guide designs of magnetically actuated cantilever scanners toward specific requirements in applications. Cobalt coated 80mm long cantilever had the highest scanning distance of 4.96mm while the iron coated 60mm long cantilever had the highest scanning frequency of 16.8Hz with 1mm thick coatings. Imaging feasibility on human tissue was demonstrated using a nickel coated 70mm long cantilever. A scan distance of 1.4mm was obtained with a maximum scanning frequency of 28Hz. For GI manometry, we proposed a novel miniature, passive wireless pressure sensing system on a flexible substrate. The sensor can be incorporated with thin-film metal or biodegradable esophageal stents for therapy and prognosis. Planar variable interdigitated capacitors (IDC) were designed to measure the variations in radial pressures and strains. Encapsulation by a layer of poly-dimethylsiloxane (PDMS) of the sensors made the device elastic, deformable and biocompatible. The flexible IDC was fabricated allowing changes in capacitance due to pressure variations. Capacitance characterization using linear pull test showed that the capacitance of the IDC for a given pull distance remained constant even after repeated cycles. Force characterization showed a sensitivity 0.5pF/N. Radial pressure measurement feasibility was demonstrated using an in vitro system. Pressure sensitivity was found to be 0.1pf/kPa. In an integrated wireless batteryless environment, the sensitivity of the variable IDC induced frequency change was 0.14kHz/kPa. This type of sensor can be easily incorporated in commercial metal stents clinically used or a biodegradable one providing in vivo remote pressure measurements for GI motility. Strain tests using the IDC showed that the sensor is suitable for axial pressure measurements. The sensor can be incorporated with thin-film metal on biodegradable esophageal stents for therapy and prognosis. This allows for monitoring pressure over fixed pre-determined periods of time after which the sensor passes through the digestive system. The use of biodegradable materials thus eliminates the need for additional procedures to remove the sensor and the stent.

Disciplines

Electrical and Computer Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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