Lun-Chen Hsu

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

A batteryless, wireless, sensing system, suitable for biomedical applications, was studied in this dissertation. The sensing system contains a reader and a transponder and the working principle is based on passive RFID systems. The transponder harvests the RF energy from the reader to power up the frequency generator circuit which is modulated by sensor signals. Sensor signals are encoded into frequency shifts to provide the immunity to motion artifacts and the robust performances of sensing system are demonstrated. The sensing platform can also be used for multiple sensor integration for different medical applications. In this dissertation, the physiological measurements are categorized into two groups - external and internal ones - depending on the deployment of the sensing devices. Two physiological sensing systems were studied to demonstrate the system architecture. An external application of pressure sensing for pressure sore monitoring and an internal application for gastroesophageal reflux diagnosis were investigated.For external physiological sensing application, a wireless batteryless piezoresistive pressure sensing system was presented for applications of monitoring pressure sore development in paralyzed patients who might be in beds or in wheelchairs. The transponder device included an energy harvesting circuit, a force sensing resistor, a resistance-to-frequency converter, and an antenna. The reader provided power to the device remotely and measures the sensor values in terms of frequency shift simultaneously. The experimental results showed good linearity, sensitivity, and resolution in the pressure range form 0 to 10 psi while the corresponding frequency shift detected by the reader was between 7.35 and 8.55 kHz depending on different testing scenarios. Stability tests of the sensor were carried out for long-term monitoring. The experimental results indicated that the drifting of reading was smaller than the readings of high-pressure indications so the sensors can be used for a long period of time. A pressure sensor array was arranged to detect pressure distribution in a certain area for sensing wound condition applications. The sensors were able to identify high pressure points dynamically.For internal physiological sensing applications, an integration of impedance and pH sensors was presented for diagnosis of gastroesophageal reflux disease that affects approximately 15% of adult population in the United States. Our system included an implantable transponder that had impedance- and pH- sensing electrodes, and a wearable, external reader that wirelessly powered the transponder and recorded the radio-frequency signals transmitted by the transponder. We tested our system by placing the sensor on the wall of the tube flushing with different liquids with known pH values, and by attaching the sensor to the esophagus wall of a live pig, after which those same liquids were flushed through the esophagus. In both cases, the external reader was positioned properly to record the impedance/pH data continuously from implanted capsule. The data were compared with those recorded discretely by an adjacently-placed Bravo capsule. The experiment results showed that the impedance sensor picked up every simulated reflux episode and the pH sensor indicated the pH value of each episode. Our dual sensors responded to the stimulated reflux episodes immediately while Bravo pH sensor remained the same reading during the short simulated reflux episodes. Our impedance sensor was able to pick up the episodes with the pH value of 7 and the Bravo pH sensor failed to detect those events. The Bravo pH sensor malfunctioned when pH values were greater than 10, and our pH sensor still can detect those episodes. Our implantable, batteryless, wireless sensing system can detect acid and nonacid reflux episodes providing both impedance and pH information simultaneously for GERD monitoring. The in vivo statistical data of our dual sensors also showed good reliability and repeatability, with the standard deviation less than 0.16 kHz for impedance sensor, and the pH values of 0.33 for pH sensor in most of the cases. In this dissertation, two medical sensing systems were designed, fabricated, and tested. The robustness of the sensing system were characterized and demonstrated with both in vitro and in vivo experiments. The capability of the sensing platform to integrate with multiple sensors was also demonstrated.

Disciplines

Electrical and Computer Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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