The dielectric properties of tissues have been extracted from the literature of the past five decades and presented in a graphical format. The purpose is to assess the current state of knowledge, expose the gaps there are and provide a basis for the evaluation and analysis of corresponding data from an on-going measurement programme.

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The dielectric properties of biological tissues: I. Literature survey

: Knowledge of the dielectric properties of biological materials is of importance in solving electromagnetic interaction problems. There is, as yet, no consensus on such data among scientists dealing with these issues. This project is geared towards producing a database of dielectric data based on measurements using recently developed techniques. This has been achieved through measurement over a wide frequency range. The new data were evaluated by comparison with corresponding data from the literature where available. To facilitate the incorporation of the dielectric data in numerical solutions, their frequency dependence was modelled to a spectrum characterised by 4 dispersion regions. The conductivity of tissues below 100 Hz was estimated from the recent measurements mitigated by data from the literature and used to estimate the body and of various body parts.

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Compilation of the Dielectric Properties of Body Tissues at RF and Microwave Frequencies.

In an APT method, the surface-contact electrodes are located in a closed loop or rosette array on one planar, or nominally planar, skin surface of a body to be investigated, and electrically connected to data acquisition and processing equipment. The invention also includes apparatus for carrying out this method, comprising an array of surface-contact electrodes (8), adapted to be applied to the skin of a body to be investigated, the electrodes (8) being arranged on a flexible carrier (10) in a closed loop, or rosette formation on one common plane, or nominal plane.

https://iopscience.iop.org/article/10.1088/0022-3735/17/9/002/pdf

Applied potential tomography

A two-pronged approach, review and measurement, has been adopted to characterize the conductivity of tissues at frequencies below 1 MHz. The review covers data published in the last decade and earlier data not included in recent reviews. The measurements were carried out on pig tissue, in vivo, and pig body fluids in vitro. Conductivity data have been obtained for skeletal and myocardial muscle, liver, skull, fat, lung and body fluids (blood, bile, CSF and urine). A critical analysis of the data highlights their usefulness and limitations and enables suggestions to be made for measuring the electrical properties of tissues.

https://iopscience.iop.org/article/10.1088/0031-9155/54/16/002/pdf

Electrical conductivity of tissue at frequencies below 1 MHz.

For >20 years, noninvasive transcranial stimulation techniques like repetitive transcranial magnetic stimulation (rTMS) and direct current stimulation (tDCS) have been used to induce neuroplastic-like effects in the human cortex, leading to the activity-dependent modification of synaptic transmission. Here, we introduce a novel method of electrical stimulation: transcranial random noise stimulation (tRNS), whereby a random electrical oscillation spectrum is applied over the motor cortex. tRNS induces consistent excitability increases lasting 60 min after stimulation. These effects have been observed in 80 subjects through both physiological measures and behavioral tasks. Higher frequencies (100–640 Hz) appear to be responsible for generating this excitability increase, an effect that may be attributed to the repeated opening of Na+ channels. In terms of efficacy tRNS appears to possess at least the same therapeutic potential as rTMS/tDCS in diseases such as depression, while furthermore avoiding the constraint of current flow direction sensitivity characteristic of tDCS.

https://www.jneurosci.org/content/jneuro/28/52/14147.full.pdf

Increasing Human Brain Excitability by Transcranial High-Frequency Random Noise Stimulation

The electrical and dielectric properties of wet, bovine, compact bone were determined in three orthogonal planes, using a differential technique, for a frequency range of 1 kHz-l MHz. For axial specimens, at 10 kHz, the specific resistance, specific capacitance, and specific impedance were 16.6 k?/cm, 60.9 pF. cm-1 and 16.59/?3.65°k?·cm, respectively. Similarly, the dielectric properties, namely the relative dielectric constant, relative dielectric loss factor and dielectric dissipation factor were 688, 10.8 ×103, and 15.7, respectively. The electrical properties were found to be highly dependent on the frequency and the moisture content of bone. The bone was also found to be highly anisotropic in its electrical behavior, the impedance being lowest in the axial direction. It is shown that the differential method provides an accurate means of measuring the electrical properties of bone, avoiding stray capacitance errors.

G. N. Reddy

Papers

Electrical and Dielectric Properties of Wet Bone as a Function of Frequency