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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.sub.11]), resonance frequency ([f.sub.r]), and transmission coefficient through the phantom ([S.sub.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 [f.sub.r], drift of [f.sub.r], [S.sub.11]. and [S.sub.21] over time, and the effective radiated power (ERP) from the transmission ([S.sub.21]) measurements were evaluated through careful measurements. A decrease in [S.sub.11] of 1.2-2.3 dB and an increase in [S.sub.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 [f.sub.r] 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 [f.sub.r] increase of approximately 14.5 MHz was also observed by raising the temperature from 20[degrees] C to 37[degrees] C, mainly because of the negative temperature coefficient of the phantom permittivity. Transmission measurements performed using both [S.sub.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. Index Terms--Effective radiated power (ERP), implantable antennas, intracranial pressure (ICP), planar inverted-F antenna (PIFA), scalp phantom.

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 x Hg obtained for a sub-dural device with a capacitive microelectromechanical system sensor compared to 2 mm x Fig obtained for an epidural device with a piezoresistive sensor. Index Terms--Intracranial pressure, microelectromechanical systems (MEMS) capacitive sensors, 2.4-GHz industrial-scientific-medical (ISM) band, wireless implants.

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. Index Terms--Biological tissue, complex-permittivity measurement, millimeter wave, test fixture, TRL calibration.

Our analysis and measurements of a custom-designed two-port microstrip test fixture for biological tissue characterization at microwave and millimeter-wave frequencies demonstrated that the transmission parameter S21 would provide a better sensitivity to the complex permittivity change than the reflection coefficient S11. However, the standard through-reflect-line (TRL) calibration method employed for the extraction of the tissue complex permittivity did not fully remove the coaxial-to-microstrip adaptors' induced errors, which were manifested by ripple artifacts on the measured two-port S parameters. A simple deconvolution method was demonstrated wherein these errors were removed by postcalibration correction of the measured S21 of the tissue under test (TUT) by using water as a reference material. This paper provides a theoretical analysis of this method based on a model presented for postcalibration adaptors. Our detailed analysis shows that the error for S21 using the deconvolution method linearly depends on the difference between the S11 of the TUT and the reference material. Measurement and error estimation are also provided for various biological tissues and are consistent with analytical expectations. Our analysis provides support that systematic errors of numerically modeled S21 utilized for complex permittivity extraction can significantly be reduced by the deconvolution method. On the other hand, the analysis also shows that the S21 numerical modeling errors and the postcalibration adaptors' error terms have a similar impact on the extracted complex permittivity using the standard time-gating technique and are irreducible, unless the deconvolution method is used. Our analysis also identifies water as a better reference sample than methanol for accurate extraction of the complex permittivity of tissues in the range of ϵ' > 9 and ϵ" > 7 at 30 GHz. [ABSTRACT FROM AUTHOR]

Discusses correction methods to remove residual errors due to the lack of repeatability of coaxial-to-microstrip launchers as part of the through reflect line calibration procedure. Analytical basis for a two-tier de-embedding process; Difference between the simulated and measured insertion loss for distilled water as a correction transfer function to be applied to the measured tissue insertion loss; Mechanical structure of each test fixture cavity.