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Ruchi Warty
Advisor: Afshin Daryoush, Ph.D.
Abstract:
Research on microwave antennas in medical applications has been primarily focused on medical therapeutic and diagnostic applications such as hyperthermia treatment of cancer, tissue ablation, and microwave near field imaging. In recent years attention has been paid to implantable antennas used for biotelemetry. Telecommunication between the medical devices (e.g., pacemakers and defibrillators) implanted inside the human body and the receivers used externally necessitate, characterizing implantable antennas in terms of their radiation characteristics.
The focus of this thesis is on the forward and backward scattering
of an implantable 2.45 GHz Planar Inverted-F Antenna (PIFA), designed
for wireless interrogation of sensor that measures Intra-Cranial
Pressure (ICP). The PIFAs have been designed using a full wave
electromagnetic simulation; i.e. Method of Moments (MoM) in Ansoft
Designer. A measurement set-up was implemented to emulate the
implant environment using an 8-mm thick scalp phantom (dielectric
constant = 50 and conductivity = 2.2 S/m) and an absorbing chamber.
Characterization was performed in terms of the S11 of the PIFA,
the S21 for the radio link with the PIFA being connected to transmitting
port and a chip antenna at the receiving port. Several PIFA prototypes
were fabricated on an FR4 substrate (dielectric constant = 4.25
and loss tangent = 0.03) and tested with a biocompatible coating;
i.e., silicone (dielectric constant = 3.7 and loss tangent = 0.02)
of 0-25 mils (0-0.6 mm) thick. The performances of the silicone
loaded PIFA were studied in terms of shift on the resonant frequency
of the PIFA due to coating thickness. The resonant frequency for
different versions of the PIFAs was observed to be extremely sensitive
to the coating thickness with a typical sensitivity of 80-100
MHz/mil at a thickness of 15.5 mils. A thickness of 15.5 mils
(0.38 mm) was found to be the optimum coating thickness for the
selected PIFA in terms of resonance at 2.45 GHz. Overtime drift
in the S11 and S21 behaviors of the PIFA, when exposed to the
wet (biological) environment was also studied. Transmission measurements
were performed using both a Vector Network Analyzer (VNA) and
a spectrum analyzer in an absorbing chamber. Direct measurements
of S21 with the PIFA connected at port 1 and indirect measurements
using the received power from an ICP device mimic incorporating
the PIFA in cranial tissue environment.
Measurements over a period of two days indicated a 1.8 dB decrease in S11 and a 3 dB increase in S21 along with a 15 MHz shift in the resonant frequency. Both methods were used to estimate the Effective Isotropic Radiated Power (EIRP) through a scalp phantom by careful calibrations and characterization of an external probing antenna as receiver. An EIRP of 2-3 mW was estimated per 1 W of input power to the PIFA at 2.45 GHz using the VNA approach. There is a 4-15 % error in estimation between the EIRPs for the different versions of the PIFA for independent measurements performed at separations of 11" and 15" between the PIFA and the chip antenna. From the power measurements on spectrum analyzer, an EIRP of 3 mW at 2.45 GHz was estimated. This EIRP measurement helps to quantify the communication link loss between the implanted antenna and the receiver antenna, which will assist us to estimate the delivered power required to establish a reliable telemetry link at a certain distance. For a receiver with sensitivity of -110 dBm and a communication radio link distance of 10 m, the amount of power required to be delivered to an implantable antenna is calculated to be -22.8 dBm, based on a measured EIRP of 2 mW at 2.45 GHz.
Wednesday, December 19th, 2007 at 1 p.m.
Bossone 302
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