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MEMS Multisensor Microsystem with Integrated Interface Circuits for Wireless Capsule and Biomedical Applications
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The overall goal of this research is to design and develop a wireless capsule having new MEMS sensor systems as well as low-power and high-resolution interface IC circuits with a wireless link and antenna for the real-time measurement and detection of physiological parameters in the GI tract. Along with system level design of wireless capsule, it is important to focus on system components, such as sensors, interface circuits, and antenna, to optimise the wireless capsule system for high sensitivity and resolution for sensing, low-power and low-noise interface circuits, and miniaturised antenna design. This research has produced a capacitive technique to measure pH within the stomach, a new analogue interface IC circuit to measure capacitive, inductive, and resistive sensor signals, a semi-digital integrated interface IC circuit to measure capacitive sensor signals, a meandered conformal antenna, and a wireless capsule system.
Since the pH of gastric acid of the stomach is in strong acidic ranges, the capacitive pH sensor designed in this study utilises permittivity of the sensor being a function of pH. It is designed and fabricated on both silicon and quartz substrates and also experimentally validated to measure strong acidic and basic ranges in this dissertation. The capacitance change from the pH sensor on silicon substrate is 29.57 pF over the range of pH 1.0 to 5.0. The pH sensor on quartz substrate produces capacitance change of 10.21 pF with reduced non-linear response over the range of pH 1.0 to 5.2.
This dissertation presents a frequency modulation based interface IC circuit that is designed and experimented for capacitive, inductive, and resistive sensors. The changes in capacitance of the pH sensor are converted to frequency shifts using a voltage controlled oscillator IC. Measurements confirmed a frequency shift of 30.96 MHz and 4.317 MHz for a change in pH of 1.0 to 5.0 and 10.0 to 12.0, respectively. A highly sensitive charge pump based circuit with negative feedback system is implemented to convert frequency changes to voltage changes. The circuit is reconfigurable to provide control over dynamic range, resolution, and nominal point of measurements. The feedback control system, which does not require another VCO in the feedback loop for converting frequency changes into voltage changes, minimises the overall power consumption. A voltage change of 419.7 mV is obtained using the frequency-to-voltage converter (FVC) circuit for a change in pH of 1.0 to 4.0. The interface IC circuit is fabricated in the UMC 0.18 µm CMOS process. It consumes 11.772 mW from a 1.8 V supply, of which 378 µW is consumed by the FVC circuit.
Another interface IC circuit using pulse-width modulation technique is also designed and experimentally evaluated to convert capacitance changes of a sensor into pulse-width variations. The circuit includes a differential structure of RC controlled pulse generator with high pass filter to reduce bandwidth of noise sources and a self-tuning inverter comparator to reduce threshold deviation due to supply, temperature, and process variations. The circuit is configurable to adjust sensitivity, dynamic range, and nominal point of measurements. The circuit, which is evaluated for a capacitive pressure sensor, provides sensitivity of 60 ns/kPa and 23 ns/kPa for pressure from 101 to 200 kPa and from 50 to 101 kPa, respectively. This circuit is also fabricated in the UMC 0.18 µm CMOS process and consumes 98 µW from a 1.8 V supply.
A new miniaturised antenna for wireless capsule operating at Industrial, Scientific and Medical (ISM) band with high gain and omnidirectional radiation patterns is designed. This dissertation presents a meandered conformal antenna which is fabricated on a flexible polyimide material and wrapped around the inner surface of a capsule to provide extra space for sensors and circuits inside a wireless capsule compared to an embedded antenna. The performance of the antenna is evaluated experimentally and shows centre frequency of 433 MHz with 124.4 MHz bandwidth and omnidirectional radiation patterns. The measured pathloss is 17.24 dB for in-body propagation distance of 140 mm. Due to the wide-band characteristics and omnidirectional radiation patterns, the antenna is suitable to integrate with wireless capsule system for wireless communication.
The last phase of the research is the integration of sensor systems and interface IC circuits with wireless transceiver systems and antenna for the design and implementation of a complete wireless capsule system platform to measure pH, pressure, and temperature of GI tract. This dissertation includes the integration of sensors, interface IC circuits, and transceiver circuit with meandered conformal antenna in a wireless capsule to measure physiological parameters in the GI tract and to address some design challenges associated with miniaturisation, power consumption, and wireless communication. A data receiver system is also designed to receive physiological data from wireless capsule and to send data to a computer for real-time display and recording. The wireless capsule, which is 27 mm in inner length and 12 mm in inner diameter, is packaged and evaluated experimentally in vitro to measure physiological parameters of the GI tract in real-time.