Mohammad-Reza Tofighi

Characterization of Implantable Antennas for Intracranial Pressure Monitoring: Reflection by and Transmission Through a Scalp Phantom

Characterization of implantable planar inverted-F antennas, designed for intracranial pressure (ICP) monitoring at 2.45 GHz, is presented. A setup, incorporating a scalp phantom emulating the implant environment and an absorbing chamber, was implemented for characterizing the antennas, in terms of their reflection coefficient (S 11), resonance frequency (fr), and transmission coefficient through the phantom (S 21) , and is reported for the first time. As a result of our observations that even a very slight change of the biocompatible (silicone) thickness can drastically change the characteristics of such antennas, several antenna prototypes with various silicone thicknesses were tested for a better understanding of the change in their performance with thickness. The main contributions of this paper rest in the evaluation of the antenna characteristics with respect to time, temperature, and far-field radiation, in an emulated biological environment. In this regard, the impact of the coating thickness on fr, drift of fr, S 11, and S 21 over time, and the effective radiated power (ERP) from the transmission (S 21) measurements were evaluated through careful measurements. A decrease in S 11 of 1.2-2.3 dB and an increase in S 21 of 2.2-2.4 dB, over a period of two days, were observed at 2.45 GHz. A decrease of 8-18 MHz for fr was also observed over the same period of time. This drift was due to the absorption of saline by the silicone, leading to a change in its effective dielectric property. An fr increase of approximately 14.5 MHz was also observed by raising the temperature from 20 degC to 37 degC, mainly because of the negative temperature coefficient of the phantom permittivity. Transmission measurements performed using both S 21 and the received power measurement (for an ICP device mimic) yielded a maximum ERP of approximately 2 mW per 1 W of power delivered to the antennas at 2.45 GHz.

In-Vitro and In-Vivo Trans-Scalp Evaluation of an Intracranial Pressure Implant at 2.4 GHz

Elevation of intracranial pressure is one of the most important issues in neurosurgery and neurology in clinical practice. The prevalent techniques for measuring intracranial pressure require equipments that are wired, restricted to a hospital environment, and cause patient discomfort. A novel method for measuring the intracranial pressure is described. A wireless completely implantable device, operating at an industrial-scientific-medical band of 2.4 GHz, has been developed and tested. In-vitro and in-vivo evaluations are described to demonstrate the feasibility of microwave pressure monitoring through scalp, device integrity over a long period of time, and repeatability of pressure measurements. A distinction between an epidural and sub-dural pressure monitoring techniques is also described. Histo-pathological results obtained upon a long-term device implantation favor the utilization of the sub-dural pressure monitoring method. On the other hand, in-vivo studies illustrate a maximum pressure reading error of 0.8 mm middot Hg obtained for a sub-dural device with a capacitive microelectromechanical system sensor compared to 2 mm middot Hg obtained for an epidural device with a piezoresistive sensor.

A 2.5-GHz InGaP/GaAs differential cross-coupled self-oscillating mixer (SOM) IC

This letter presents a monolithic differential cross-coupled self-oscillating mixer (SOM). The SOM chip is fabricated using an InGaP/GaAs heterojunction bipolar transistor (HBT) foundry process and operates at 2.5 GHz. The chip provides voltage controlled oscillator (VCO) operation, up- and down-conversion mixing, and injection locking functionalities. The voltage down-conversion gain and the power up-conversion gain of up to 15 and 11.5 dB, respectively, are measured for the circuit. There is a compromise between obtaining a high conversion gain, and the oscillator power (-0.3 dBm for a 5-V supply) and phase noise (-84 dBc/Hz at 100 kHz). However, phase noise improvement of 32dB is observed by injection of a -30-dBm stable reference.

Characterization of the complex permittivity of brain tissues up to 50 GHz utilizing a two-port microstrip test fixture

Broad-band complex-permittivity values of biological tissues above 20 GHz obtained from direct measurements have not been reported in the literature. This paper presents for the first time the measurement results of complex permittivity of brain grey and white matters from 15 to 50 GHz utilizing a two-port microstrip test fixture. Test fixture S-parameters are simulated employing the finite-element method. To apply the data obtained from the simulation in complex-permittivity extraction, an efficient procedure, using the linear least square technique, is introduced to fit the modeling results to a rational function of complex permittivity, which is similar to the transfer function for a linear system. This fitting procedure is computationally more efficient than the previously developed fitting methods. Measurements are performed on slices of brain sample using a calibrated network analyzer utilizing custom designed through-reflect-line (TRL) calibration standards. The measurements are corrected for the residual errors observed in the measurement results due to the lack of performance repeatability of coaxial-to-microstrip launchers utilized in the TRL calibration standards. Finally, the measured results for brain matters are fitted to a single term Cole-Cole relation representing the dispersion characteristics of white and grey matters up to 50 GHz.

Characterization of biological tissues up to millimeter wave: test fixture design

Various design aspects of a two-port test fixture are presented to measure permittivity of biological tissues. Dimensions of this fixture are optimized using a commercial finite element method package. High measurement sensitivity to the tissue parameters is obtained up to 45 GHz by careful design of the microstrip feed line and aperture dimensions. Material inhomogeneity with millimeter spatial resolution is predicted using this optimized fixture.

Study of the activity of neurological cell solutions using complex permittivity measurement

Experimental results of the exposure of neurological cells to radio frequency are presented. The exposure is quantified by the mean of the complex permittivity of cell solutions as a function of frequency. A set-up is used for measurement of complex permittivity of live and dead neurological cell cultures from 20 to 40 GHz. Differences are observed between the two cultures and are compared against the expected measurement error. The statistical significance of these differences is also studied and reported in this paper.

IC based broadband digital receiver for 4G wireless communications

Digital receivers are pursued as part of future civilian and military communication applications. Design and implementation of a broadband receiver is presented that operates for 4G wireless communications. Performance of the realized hardware is evaluated in terms of system parameters.

PLL and injection locked PLL (ILPLL) operations of a push-pull self oscillating mixer (SOM)

This paper presents the experimental results of a push-pull Self-Oscillating Mixer (SOM) tunable in a range of 421-463 MHz operating in PLL and Injection Locked PLL (ILPLL) regimes. By careful selection of the oscillator feedback resistor, an excellent down-conversion gain of up to 24.3 dB is observed. As a result, for the first time, the phase detection is performed as part of the SOM without the need for an external phase detector and gain stages. Issues such as tuning voltage-frequency variation, SOM phase-frequency variation, tracking range, pull in range, phase noise, and SOM phase controllability are discussed in the paper.

An IC based self-oscillating mixer for telecommunications

This work presents the measurement results of a 2.5 GHz push-pull self-oscillating mixer (SOM) circuit with internal and external LC resonators. This SOM is used for injection locked phase locked loop (ILPLL) applications, where a separate phase detector (PD) circuit is not required. This SOM design was fabricated using an InGaP/GaAs HBT foundry process. Output power of about 0 dBm is attained over a large frequency tuning range. The power consumption of the SOM is typically about 60 mW using a 5 V supply. The voltage down-conversion gain and power up-conversion gain of up to 15 and 11.5 dB are measured for the circuit. It is also observed that there is a compromise between obtaining a high conversion gain and low close-in to carrier phase noise. It is demonstrated that a phase noise improvement of up to 6 dB can be achieved by implementing an external LC resonator.