ISM band antenna scattering from scalp phantom for intracranial pressure monitoring implants
Authors:   , Warty Ruchi Vivek , Daryoush Afshin S. 
Published: July 2021

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 imaging. In recent years attention has been directed to implantable antennas for biotelemetry. Communication between the medical devices (e.g., pacemakers and medical sensors) implanted inside the human body and the receiver used externally necessitates 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 a sensor that measures Intracranial 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 ([epsilon]r = 50 and [sigma] = 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 ([epsilon]r = 4.25 and tan[delta] = 0.03) and tested with a biocompatible coating, i.e., silicone ([epsilon]r = 3.7 and tan[delta] = 0.02) of 0-25 mils (0-0.6 mm) thickness. The performance of the silicone loaded PIFA was studied in terms of the shift in the resonant frequency of the PIFA due to the 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 1 GHz/ 0.1 mm 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 a selected PIFA in terms of achieving resonance at 2.45 GHz. Overtime drift in the S11 and S21 behaviors of the PIFA, when exposed to the 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 to port 1 and measurements of the received power from an ICP device mimic incorporating the PIFA in cranial tissue environment were conducted as part of the transscalp transmission characterization of the PIFAs. A decrease in S11 of 1.2-2.3 dB and an increase in S21 of 2.2-2.4 dB, over a period of two days, were observed at 2.45 GHz. A decrease in the PIFAs' resonant frequency of 8-18 MHz was also observed over the same period of time. Both methods were used to estimate the Effective Radiated Power (ERP) achievable through the scalp phantom by careful calibrations and characterization of the external probing chip antenna as the receiver. A maximum ERP of 2-3 mW was estimated per 1 W of the input power to the PIFA at 2.45 GHz. There was a 4-15 % error in estimation between the ERPs for the different versions of the PIFA for independent measurements performed at separations of 11" and 15" between the PIFA and the chip antenna. This ERP 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 free space 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 an ERP of 2 mW at 2.45 GHz.