Young-Sik Seo

Graduation Semester and Year

2014

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering

Department

Electrical Engineering

First Advisor

Unknown

Abstract

This work proposed an integrated circuit (IC) design that provides the required functionality for a wireless, batteryless implant. For biomedical applications, the physiological signals can be detected if the parameter of interest can be converted to a change in electrical parameters such as capacitance, resistance or an amperometric signal. The implantable transponder is suitable for sensors that generate one of the aforementioned analog signals. In the proposed universal platform, a voltage-controlled oscillator (VCO) was used to detect variations of voltage or current. Similarly, a relaxation oscillator (RO) was used to detect changes in capacitance or resistance. Thus, the VCO and the RO were integrated in a single chip, thereby providing the capability to transduce all of the possible analog signals into frequencies. To minimize mutual interferences that can occur when multiple sensors are in operation, a multiplexer (MUX) was employed to isolate the operation of the oscillators. The MUX was switched by a local clock (CLK) to alternate the power between the two oscillators. As a result, at any given time, only one of the oscillators was in operation. The outputs of the oscillators were fed to the demultiplexer (DEMUX) sequentially to modulate the signals into a single carrier. The C5N (0.5-µm) technology by On Semiconductor was used to design the proposed platform. The chip was fabricated through MOSIS. The dimension of the die was 1.95 × 1.95 mm 2 . The design also took into account all considerations for electrostatic discharge (ESD) protection of the chips. Benchtop tests were performed to measure the performance of the fabricated chip. The modulated signals from both the VCO and the RO were measured successfully without any overlap. The miniaturization of implantable devices is limited by the size of the battery. Thus, for further reduction of size, a batteryless and wireless operation for the implant has been proposed. Inductive coupling and load modulation methods have been proposed to achieve wireless power transfer and wireless communications respectively. The batteryless solution was implemented by wireless power transfer at a carrier frequency of 1.3 MHz. This frequency was carefully chosen for its relatively high power transfer and low attenuation in tissues. The wireless communication was achieved via load modulation using a switched MOSFET in the implant circuitry. The wireless reader was composed of an envelope detector and frequency counter to demodulate the received signals from the implant. Furthermore, antenna optimization was investigated. Various size and number of coil turns in reader antennas were used to obtain power transfer and efficiency while the size of the implant antenna was fixed. The changes in power transfer and efficiency was also measured for various antenna separations between the two antennas. Antennas of 5-cm radius were determined as optimum in terms of power transfer, efficiency, current consumption, and size. Among the 5-cm antennas, the 17- and 18-turns of coil yielded relatively higher power transfer and efficiencies. The misalignment of the reader and the implant antenna due to motion artifacts was considered for practical biomedical applications. The changes in power transfer and efficiency due to mis-alignments were measured by changing the position of the implant antenna around the reader antenna. Divergence, intensity, and 3 dB (half power) of transferred power and efficiency were also obtained and analyzed. Using the fabricated chip and the optimum reader antenna, a wireless implantable device was fabricated on a printed circuit board (PCB). A 2-stage charge pump and an implant antenna (1 × 1.7 cm2 ) were integrated on the PCB with the chip. Wireless communication was successfully tested by analyzing the signals obtained by the reader. The two oscillators were operated with capacitive, resistance, and voltage inputs. To demonstrate the feasibility of the wireless, batteryless multi-sensing platform, the implantable device on PCB was integrated with an impedance sensor, and a pH sensor. The aforementioned dual sensor platform has applications in GERD detection. The impedance and pH sensors were connected as input sources to the relaxation oscillator and VCO, respectively. The universal platform was able to deliver distinguishable signals from the two oscillators when various fluids of different pH levels were brought into contact with the sensors. The frequency ranges and the operating conditions closely matched the design considerations. Thus, the wireless batteryless implant using the IC-based multi-sensing platform for biomedical applications has been successfully demonstrated.

Disciplines

Electrical and Computer Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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