Head Equivalent Liquids: A Review on Composing, Recipes and Standards
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paper_title: Dielectric properties of tissue-equivalent liquids and their effects on electromagnetic power absorption

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Mobile phone safety has been evaluated by the specific absorption rate (SAR). The measurement method, including recipes for tissue-equivalent dielectric liquids, have been discussed among international organisations. Standard documents currently recommend glycol-type liquids as tissue-equivalent liquids for frequencies above 1 GHz. Although the relative amounts of the ingredients are specified in the recipes provided, possible difficulties relating to the stability of dielectric properties are not clearly described. In this work we measured the time dependence of the dielectric properties of glycol-based dielectric liquids, and evaluated their effects on SAR. The experimental results suggest that relative permittivities decrease with time and that conductivities remain nearly constant. SAR, however, was not affected significantly by the change in dielectric properties.
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paper_title: Dielectric properties of brain tissue between 0.01 and 10 GHz

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Dielectric permittivity and conductivity are reported for grey and white matter from dog brain tissue between 0.01 and 10 GHz. Between 0.01 and approximately 1 GHz, the permittivity decreases and conductivity increases as a power law of frequency. Above 1 GHz, the conductivity increases quadratically with frequency due to dipolar reorientation of free water molecules in tissue; the apparent rotational relaxation frequency at 37 degrees C is 21-24 GHz, slightly below the 25 GHz characteristic frequency of pure water at that temperature. The microwave data are analysed using the Maxwell mixture theory applicable for a suspension of nonconducting, low permittivity spheres in bulk water. From the increase in conductivity above 1 GHz, and the tissue permittivity at 2-4 GHz, the apparent volume fraction of water is approximately 0.70 and 0.55 for grey and white matter, respectively, about 10-15% lower than respective values from the literature.
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paper_title: Dielectric properties of tissues and biological materials: a critical review

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Abstract We critically review bulk electrical properties of tissues and other biological materials, from DC to 20 GHz, with emphasis on the underlying mechanisms responsible for the properties. We summarize the classical principles behind dielectric relaxation and critically review recent developments in this field. Special topics include a summary of the significant recent advances in theories of counterion polarization effects, dielectric properties of cancer vs. normal tissues, properties of low-water-content tissues, and macroscopic field-coupling considerations. Finally, the dielectric properties of tissues are summarized as empirical correlations with tissue water content in other compositional variables; in addition, a comprehensive table is presented of dielectric properties. The bulk electrical properties of tissues are needed for many bioengineering applications of electric fields or currents, and they provide insight into the basic mechanisms that govern the interaction of electric fields with tissue.
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paper_title: The dielectric behaviour of some types of human tissues at microwave frequencies

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The dielectric properties of four types of human tissues have been investigated by a coaxial line method at frequencies corresponding to free space wavelengths in the range 6 to 17 cm. The results have been examined in relation to the Debye dispersion equations and it has been found that the dielectric behaviour of some types of tissues can be described approximately by them if an additional term for ionic conductivity is included. The results have been further discussed with reference to ion and protein hydration and to the ionic conductivities of the intracellular and extracellular fluids.
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paper_title: Formulas for preparing phantom muscle tissue at various radiofrequencies.

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The dielectric properties of various test samples of phantom tissue were measured using an automated and temperature-controlled slotted line. The ingredients for phantom materials were determined for simulating high-water content tissue at 13.56, 27.12, 40.68, 70, 100, 200, 300, 433, 750, 915, and 2,450 MHz. The ingredients consisted of water, TX-150 (a gelling agent), sodium chloride, and polyethylene powder (200-2,450 MHz) or aluminum powder (13.56-100 MHz). The dielectric constant and conductivity of these materials at different temperatures (15, 22, 30 degrees C) were characterized.
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paper_title: The dependence of electromagnetic energy absorption upon human head tissue composition in the frequency range of 300-3000 MHz

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The requirements for testing compliance of cellular phones with electromagnetic safety limits demand evaluation of the maximum exposure that may occur in the user group under normal operational conditions. Under these conditions, the tissues of the ear region are most exposed, the tissue composition of which is complex and varies considerably from user to user. The objective of this paper is to derive head tissue equivalent dielectric parameters that enable the utilization of one generic homogeneous head for testing compliance for the entire user group, i.e., granting no underestimation, but also not greatly overestimating the actual maximum user exposure. As a primary study, a simple analytical model of an infinite half-space layered tissue model exposed to a plane wave was utilized to investigate the impact of impedance matching standing waves, etc. On the spatial-peak specific absorption rate. The tissue layers were varied in composition and thickness, representing the anatomical variation of the exposed head region covering the user group including adults and children ( 90% percentile). Based on the worst-case tissue layer compositions with respect to absorption at each frequency, head tissue equivalent dielectric parameters for homogeneous modeling were derived, which result in the same spatial-peak absorption. The validity of this approach for near-field exposures was demonstrated by replacing the plane wave by different near-field sources (dipoles and generic phones) and the layered structure with magnetic-resonance-image-based nonhomogeneous human head models.
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paper_title: Human Brain Phantom Modeling Based on Relative Permittivity Dielectric Properties

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This study aims to investigate the clinical outcomes of human brain dielectric properties in order to build a human-like brain phantom using simple material. This artificial intelligence human brain was developed according to the dielectric properties of a real human brain. The regions cover grey matter and white matter tissues. Elements used for phantom building demonstrate adequate internal consistency of er (relative permittivity) within 60-80 for a range of frequency from 1-6 GHz. A surprisingly finding, quantity of element sugar is the main reason of affecting the permittivity. The findings give a big step closer for building an artificial human brain without defect.
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paper_title: Simulated biological materials for electromagnetic radiation absorption studies.

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For the study of electromagnetic dosimetry and hyperthermia, it is necessary to simulate human biological materials. This can be done by chemical mixtures that are described in this paper. Formulas are presented for simulating bone, lung, brain, and muscle tissue in the frequency range of 100 MHz to 1 GHz. By using these preparations a realistic equivalent to the human body can be constructed.
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paper_title: Comparison of numerical and experimental methods for determination of SAR and radiation patterns of handheld wireless telephones

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Some recent developments in both the numerical and experimental methods for determination of SARs and radiation patterns of handheld wireless telephones are described, with emphasis on comparison of results using the two methods. For numerical calculations, it was possible to use the Pro-Engineer CAD Files of cellular telephones for a realistic description of the device. Also, we used the expanding grid formulation of the finite-difference time-domain (FDTD) method for finer-resolution representation of the coupled region, including the antenna, and an increasingly coarser representation of the more-distant, less-coupled region. Together with the truncation of the model of the head, this procedure led to a saving of computer memory needed for SAR calculations by a factor of over 20. Automated SAR and radiation pattern measurement systems were used to validate both the calculated 1-g SARs and radiation patterns for several telephones, including some research test samples, using a variety of antennas. Even though widely different peak 1-g SARs were obtained, ranging from 0.13 to 5.41 W/kg, agreement between the calculated and the measured data for these telephones, five each at 835 and 1900 MHz, was excellent and generally within ±20% (±1 dB). An important observation was that for a maximum radiated power of 600 mW at 800/900 MHz, which may be used for telephones using AMPS technology, the peak 1-g SARs can be higher than 1.6 W/kg unless antennas are carefully designed and placed further away from the head. Bioelectromagnetics 20:93–101, 1999. © 1999 Wiley-Liss, Inc.
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paper_title: The Human Brain in Numbers: A Linearly Scaled-up Primate Brain

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The human brain has often been viewed as outstanding among mammalian brains: the most cognitively able, the largest-than-expected from body size, endowed with an overdeveloped cerebral cortex that represents over 80% of brain mass, and purportedly containing 100 billion neurons and 10x more glial cells. Such uniqueness was seemingly necessary to justify the superior cognitive abilities of humans over larger-brained mammals such as elephants and whales. However, our recent studies using a novel method to determine the cellular composition of the brain of humans and other primates as well as of rodents and insectivores show that, since different cellular scaling rules apply to the brains within these orders, brain size can no longer be considered a proxy for the number of neurons in the brain. These studies also showed that the human brain is not exceptional in its cellular composition, as it was found to contain as many neuronal and non-neuronal cells as would be expected of a primate brain of its size. Additionally, the so-called overdeveloped human cerebral cortex holds only 19% of all brain neurons, a fraction that is similar to that found in other mammals. In what regards absolute numbers of neurons, however, the human brain does have two advantages compared to other mammalian brains: compared to rodents, and probably to whales and elephants as well, it is built according to the very economical, space-saving scaling rules that apply to other primates; and, among economically built primate brains, it is the largest, hence containing the most neurons. These findings argue in favor of a view of cognitive abilities that is centered on absolute numbers of neurons, rather than on body size or encephalization, and call for a re-examination of several concepts related to the exceptionality of the human brain.
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paper_title: A brain-equivalent solid phantom and its application to SAR estimation by the thermographic method

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Recently, with the rapid proliferation of mobile communication devices, the effects of electromagnetic waves on the human body have drawn attention. Although these effects depend significantly on frequency, the thermal effects due to absorption of the electromagnetic energy are considered predominant at microwave frequencies. In general, as the guideline for the thermal effects, the power per unit mass absorbed in the biological body, namely, the specific absorption rate (SAR [W/kg]), is used. Cellular phones are employed in close proximity to the head where vital functions of the body are concentrated. Even if the output is weak, there is the possibility that the power absorption may become locally excessive. Therefore, it is extremely important from a safety point of view to establish an experimental evaluation procedure regarding the SAR for the human head. In this paper, the characteristics of thermography are recognized by means of which the surface SAR can be measured, and thus the SAR distribution in an arbitrarily shaped medium. Using the biologically equivalent phantom developed in the authors' laboratory for improving the conventional phantom used for the same purpose, a fundamental investigation is carried out as to how accurately the SAR distribution can be measured. © 2000 Scripta Technica, Electron Comm Jpn Pt 2, 83(12): 24–34, 2000
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paper_title: Tissue water content in rats measured by desiccation.

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Tissue water content was determined by desiccation to constant weight at 40 degrees-50 degrees C in 14 tissues from two groups of rats weighing 200-250 and 270-430 g, respectively. The water content (mean +/- SE; ml/g) was highest in testes (0.861 +/- 0.002) and lowest in adipose (0.183 +/- 0.017) followed by bone (0.446 +/- 0.017) and skin (0.651 +/- 0.007). The average water content in the remaining tissues was 0.763 (+/- 0.003). Upon correction for the water content of residual tissue blood, significant difference between the uncorrected and corrected tissue water was observed for spleen, lungs, kidneys, heart, liver, and brain. Tissue water was independent of body weight, and was the same for right and left kidneys as well as testes and bone. Whereas the position of the muscle (back, abdomen, hindlimb) and adipose tissue (perirenal and subcutaneous) had no influence on water content, for skin, a slight difference was found between back and abdomen. In general, the current results are in agreement with composite literature values, but provide in one study data for all tissues used in the development of physiologically based pharmacokinetic models of rat.
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Title: Head Equivalent Liquids: A Review on Composing, Recipes and Standards

Section 1: Introduction
Description 1: Introduce the significance of electromagnetic field studies, the necessity of head equivalent liquids, and the historical context and challenges in preparing HELs.

Section 2: Composing Head Equivalent Liquids (HELs)
Description 2: Detail the electrical characteristics of HELs, the influence of various chemicals, preparation methods for different frequency domains, and provide examples from previous studies.

Section 3: Measurement of Electrical Values of HELs
Description 3: Explain the methods for measuring the electrical properties of HELs, including the use of vector network analysers, dielectric test cells, and other recommended methods by IEEE.

Section 4: Conclusion
Description 4: Summarize the importance of HELs, the challenges and solutions in their preparation, and the implications for future research.