Dielectric Properties Of Human Skin Research Articles

A Resonant Coupler for Subcutaneous Implant.

A resonator coupler for subcutaneous implants has been developed with a new impedance matching pattern added to the conventional loop antenna. The tuning element of a concentric metal pad contributes distributed capacitance and inductance to the planar inductive loop and improves resonance significantly. It provides a better qualify factor for resonant coupling and a much lower reflection coefficient for the implant electronics. Practical constraints are taken into account for designs including the requirement of operation within a regulated frequency band and the limited thickness for a monolithic implant. In this work, two designs targeting to operate in the two industrial, scientific, and medical (ISM) bands at 903 MHz and 2.45 GHz are considered. The tuning metal pad improves their resonances significantly, compared to the conventional loop designs. Since it is difficult to tune the implant antenna after implantation, the effects of tissue depth variations due to the individual’s surgery and the appropriate implant depths are investigated. Simulations conducted with the dielectric properties of human skin documented in the literature are compared to measurements done with hydrated ground pork as phantoms. Experiments and simulations are conducted to explain the discrepancies in frequency shifts due to the uses of pork phantoms. The design method is thus validated for uses on human skin. A noninvasive localization method to identify the implant under the skin has been examined and demonstrated by both simulations and measurements. It can efficiently locate the subcutaneous implant based on the high quality-factor resonance owing to the tuning elements in both implant and transmitter couplers. The planar resonant coupler for wireless power transfer shows good performance and promise in subcutaneous applications for implants.

Bio Phantoms Mimicking the Dielectric and Mechanical Properties of Human Skin Tissue at Low-Frequency Ranges

Tissue phantoms are widely used as substitute materials for real tissue validation of various newly emerging biomedical technologies such as ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI). However, there is no specific recipe for fabricating skin-mimicking phantoms which can mimic both the mechanical and dielectric properties of human skin at lower frequency ranges. 
 
 The objective of this paper is to present a variety of tissue-mimicking materials for filling this research gap in the lower frequency range from 20 Hz to 300 kHz. The starting point of our experiments is based on the oil-in-gelatin based tissue-mimicking materials (TMMs) that have shown to mimic the dielectric properties of human skin in higher frequency ranges. This paper examines the mechanical and dielectric performance of five major classes of tissue-mimicking materials (1) Oil-in-gelatin, (2) lignin and graphene nanopowder in gelatin, (3) gelatin and distilled water, (4) mixed oil in gelatin and distilled water, and (5) lignin in gelatin and distilled water. 
 
 Mechanical and electrical testing was performed using compression testing and parallel plate method respectively. The effect of electrode polarization was considered in the measured data and the intrinsic impedance values were found to be following the Cole-Cole equation. The Young's modulus range of all tissue-mimicking materials was within the range of skin.

On the computation and comparison of specific absorption rate (SAR) in a skin tissue using analytical and numerical methods

AbstractSpecific absorption rate (SAR) is a measure of safety and a requirement for portable radio frequency devices which are regulated mainly by the standards recommended by the Institute of Electrical and Electronics Engineers (IEEE) and International Commission on Non‐Ionizing Radiation Protection (ICNIRP). Accurate approximation of SAR is therefore important for safety as well as for compliance purposes. This article presents the analytical representation and derivation of the SAR from the basics of electromagnetic theory, Maxwell Equations, and law of conservation of energy in electromagnetic theory. It is shown, analytically, that for a time harmonic electromagnetic wave propagating in a homogeneous lossy medium having permittivity ϵ, permeability µ, and conductivity σ, the time rate change of electric and magnetic energy densities at steady state inside a given volume is zero. It is also shown that at the steady state, the net power flowing through a volume, bounded by a surface is equal to the power dissipated in that volume. Furthermore, this work demonstrates that at the steady state, SAR can be represented in terms of power density or in terms of power dissipated in the given volume. Moreover, calculation of SAR for a 1 mm × 1 mm × 0.01 m box having the dielectric properties of human skin was performed using the derived formulas and then compared with the numerical results, obtained using a full wave 3D simulation tool, HFSS. A good agreement was found between the analytical and numerical results.

Measuring the Dielectric Properties of Human Skin in Order to Understand How Tumor Treating Fields Distribute Within the Body

Tumor Treating Fields (TTFields) are low intensity alternating electric fields in the 100-500 KHz frequency range that are known to have an anti-mitotic effect on cancerous cells, and are for the treatment of Glioblastoma Multiforme (GBM). Clinical trials testing the feasibility, safety and/or efficacy of TTFields therapy for various thoracic and abdominal tumors are currently underway. TTFields therapy is delivered to the patient through two pairs of transducer arrays placed on the patient's skin. The transducer arrays are made of ceramic disks with a high dielectric constant, which adhere to the body through a thin conductive medical gel. Computational studies performed by our group have shown that field intensity within the body is highly dependent on the conductivity values assigned to skin and sub-cutaneous adipose tissue (SAT). This leads to the conclusion that an accurate understanding about how TTFields distribute within the body, requires determining the dielectric properties of the skin and SAT with a high degree of accuracy. To accurately determine the properties of the skin and SAT, we measured impedance of skin folds around the waists of human volunteers in the frequency range of 100 kHz -1 MHz. The impedance measurements were performed using electrodes made of the same ceramic disks and medical gels present in the transducer arrays used to deliver TTFields to the patients. The measurements were performed by placing the electrodes on both sides of a skin fold, at 5-6 different locations around the waist of each volunteer. The thickness of the fold at each location was measured, and the impedance was plotted against the thickness of the folds. Analysis of these plots showed that in the 0.1-1 MHz frequency range, the dielectric properties of the skin can be described using a three-layer model that includes epidermis, dermis and a layer of SAT. The epidermis is modelled as a thin dielectric layer, that acts as a capacitor, which is attached in series to the dermis and SAT layers which act as resistors in series. Our analysis showed that the relative permittivity of the epidermis was 448-1190 (851 @200KHz) and the conductivities at 200 kHz of the dermis and SAT were 0.19-0.24 S/m and 0.34-0.37 S/m respectively. When assigning these conductivity values to the skin and SAT of realistic computational phantoms, and simulating application of TTFields to the torso, the total resistance measured in the model was about 30 Ohm, which is in good agreement with the total resistance measured in pancreatic cancer patients treated with TTFields. The results of this study provide new information on the dielectric properties of skin and SAT in the frequency range relevant to TTFields simulations. These measurements will lead to a better understanding of how TTFields interact with the human body.

In vivo spectroscopy of healthy skin and pathology in terahertz frequency range

Biomedical applications of terahertz (THz) technology and, in particular, THz pulsed spectroscopy have attracted considerable interest in the scientific community. A lot of papers have been dedicated to studying the ability for human disease diagnosis, including the diagnosis of human skin cancers. In this paper we have studied the THz material parameters and THz dielectric properties of human skin and pathology in vivo, and THz pulsed spectroscopy has been utilized for this purpose. We have found a contrast between material parameters of basal cell carcinoma and healthy skin, and we have also compared the THz material parameters of dysplastic and non-dysplastic pigmentary nevi in order to study the ability for early melanoma diagnosis. Significant differences between the THz material parameters of healthy skin and pathology have been detected, thus, THz pulsed spectroscopy promises to be become an effective tool for non-invasive diagnosis of skin neoplasms.

Millimeter wave silicon micromachined waveguide probe as an aid for skin diagnosis – results of measurements on phantom material with varied water content

More than 2 million cases of skin cancer are diagnosed annually in the United States, which makes it the most common form of cancer in that country. Early detection of cancer usually results in less extensive treatment and better outcome for the patient. Millimeter wave silicon micromachined waveguide probe is foreseen as an aid for skin diagnosis, which is currently based on visual inspection followed by biopsy, in cases where the macroscopical picture raises suspicion of malignancy. Demonstration of the discrimination potential of tissues of different water content using a novel micromachined silicon waveguide probe. Secondarily, the silicon probe miniaturization till an inspection area of 600 × 200 μm2, representing a drastic reduction by 96.3% of the probing area, in comparison with a conventional WR-10 waveguide. The high planar resolution is required for histology and early-state skin-cancer detection. To evaluate the probe three phantoms with different water contents, i.e. 50%, 75% and 95%, mimicking dielectric properties of human skin were characterized in the frequency range of 95-105 GHz. The complex permittivity values of the skin are obtained from the variation in frequency and amplitude of the reflection coefficient (S11), measured with a Vector Network Analyzer (VNA), by comparison with finite elements simulations of the measurement set-up, using the commercially available software, HFSS. The expected frequency variation is calculated with HFSS and is based on extrapolated complex permittivities, using one relaxation Debye model from permittivity measurements obtained using the Agilent probe. Millimeter wave reflection measurements were performed using the probe in the frequency range of 95-105 GHz with three phantoms materials and air. Intermediate measurement results are in good agreement with HFSS simulations, based on the extrapolated complex permittivity. The resonance frequency lowers, from the idle situation when it is probing air, respectively by 0.7, 1.2 and 4.26 GHz when a phantom material of 50%, 75% and 95% water content is measured. The results of the measurements in our laboratory set-up with three different phantoms indicate that the probe may be able to discriminate between normal and pathological skin tissue, improving the spatial resolution in histology and on skin measurements, due to the highly reduced area of probing. The probe has the potential to discriminate between normal and pathological skin tissue. Further, improved information, compared to the optical histological inspection can be obtained, i.e. the complex permittivity characterization is obtained with a high resolution, due to the highly reduced measurement area of the probe tip.

Broadband Tissue-Equivalent Phantom for BAN Applications at Millimeter Waves

The extension of body area networks from microwaves to millimeter waves requires to develop experimental phantoms emulating the dielectric properties of human skin for the accurate, reproducible, and well-controlled characterization of wearable antennas, on-body propagation channel, and absorption of the electromagnetic power by the human body. Here we introduce a broadband skin-equivalent semisolid phantom whose composition is optimized to coincide with measured values of the human skin permittivity in the 55-65-GHz range. To confirm the accuracy of this phantom, specific absorption rate measurements are performed at 60 GHz using a temperature-based approach. An excellent agreement between the experimental and numerical results is demonstrated.

Human skin-equivalent phantom for on-body antenna measurements in 60 GHz band

The accurate characterisation of the on-body performance of wearable antennas upcoming for future high-data-rate 60 GHz body area networks requires the development of experimental phantoms reproducing the dielectric properties of human skin. In this reported work, a low-cost and easy-to-fabricate semi-solid human skin phantom for body-centric antenna measurements has been manufactured and validated by measurements. It is demonstrated that the complex permittivity of the phantom equals that of human skin within the maximum error of 10% in the 57–64 GHz range. It is believed that this is the first skin-equivalent experimental model developed for on-body antenna characterisation at millimetre waves.

Modelling and validation of dielectric properties of human skin in the MHz region focusing on skin layer morphology and material composition

Human skin consists of several layers with distinct dielectric properties. Physiological processes leading to changes in dielectric properties of the specific layers can potentially be non-invasively monitored employing dielectric spectroscopy. So far no comprehensive skin and underlying tissue model is available for this purpose in the frequency range between 1 and 100 MHz. Focusing on this dispersion-dominated frequency region, different multilayer skin models are investigated. First, with sublayers obtained from two-phase mixtures, second, three-phase mixtures of shelled cell-like ellipsoids and finally, multiphase mixtures obtained from numerical models of single cells generated using a flexible surface parametrization method. All models are numerically evaluated using the finite-element method and a fringing field sensor on the top of the multilayer system serving as a probe. Furthermore, measurements with the sensor probing skin in vivo were performed. The validity of the models was tested by removing the uppermost skin layer, the stratum corneum (SC). It was found that only a three-phase mixture (extracellular medium, cell membrane and cytoplasm) at least can qualitatively reproduce the measured dispersion still occurring without the SC if the model is set up without a priori knowledge of the dispersive behaviour as e.g. a Cole–Cole fit to measured data. Consequently, microstructural features of tissue have to be part of any accurate skin model in the MHz region.

A dielectric inverse problem applied to human skin measurements during glucose excursions

A fringing field capacitive sensor has been used to measure the dielectric properties of human skin and underlying tissue in the MHz frequency range. It has recently been shown in clinical experimental studies that these dielectric properties can be related to the effects of in vivo glucose variations of the test subject. Previously, the relationship between electrical impedance and the glucose level has been established via statistical methods, such as the regression method. In this work, we explored a different approach, namely the resolution of the so-called inverse problem. First we applied the method on an artificial two-layer lossy system in order to test the sensitivity of the solution to forced changes in the layer properties and its stability to a constant setting. After validation of this method on artificial systems, a similar inverse problem was set and solved for dielectric measurements on human skin during an induced glucose excursion, where the skin is also modelled as a double-layer system. The changes of the measured permittivity and conductivity of the second layer versus the glucose changes are calculated for 22 study days. The statistical distribution shows that the median slopes of both dielectric properties are negative. These results can be used to test our hypothesis and to continue building potential explanations for the phenomena induced by the glucose changes on the skin layer dielectric parameters.

Dielectric properties of human skin at an acupuncture point in the 50-75 GHz frequency range: A pilot study

The reason for using acupuncture points as exposure sites in some applications of millimeter wave therapy has been unclear. Acupuncture points have been suspected to exhibit particular direct current (DC), low-frequency electrical and optical properties compared to surrounding skin. To assess if such a biophysical correlation could exist at millimeter wave frequencies used in the therapy, we investigated the dielectric properties of an acupuncture point on the forearm skin within the 50-75 GHz range. These properties were compared with those of a neighboring ipsilateral control area and a corresponding contralateral control area. The complex reflection coefficient at the end of an open-ended rectangular waveguide loaded with a Teflon plug was measured with a vector network analyzer. A suitable model of the aperture admittance was used to calculate the dielectric properties of the skin at the measured spots. Statistical analyses were conducted with an ANOVA to compare the three sites. From these analyses, the dielectric properties of the acupuncture site were found to be somewhat different from those of surrounding non-acupuncture sites from 50 to about 61 GHz, in the case of the real part of the complex permittivity.

Free water content and monitoring of healing processes of skin burns studied by microwave dielectric spectroscopy in vivo

We have investigated the dielectric properties of human skin in vivo at frequencies up to 10 GHz using a time-domain reflectometry method with open-ended coaxial probes. Since γ-dispersion results from the reorientation of free water molecules, the free water content of skin is quantitatively determined by dielectric measurements. The free water content of finger skin increased by about 10% after soaking in 37 °C water for 30 min, and it systematically decreased again through the drying process, as expected. Thus this analytical method has been applied to the study of skin burns. The free water content of burned human cheek skin due to hydrofluoric acid was significantly lower than that of normal skin, and the burned skin recovered through the healing process. In the case of a human hand skin burn due to heat, although the free water content was almost the same as that of normal skin at the beginning, it decreased during the healing process for the first 10 days, then began to increase. Although the number of test subjects was one for each experiment, it was shown that free water content is a good indicator for evaluating skin health and can be well monitored by dielectric spectroscopy.

Modelling of the dielectric properties of normal and irradiated skin

We have shown that irradiation changes the dielectric properties of human skin at radiofrequencies. Both the dielectric constant and the conductivity of the irradiated skin decrease, especially at low frequencies. The experimental data were analysed using two bioelectric models. Relevant dielectric parameters were determined by curve fitting. The dielectric relaxation of the radiation-induced acute or late reaction of the skin occurred at higher frequencies than with normal or non-irradiated skin, while the static conductivity and static dielectric constant of the irradiated skin decreased. We conclude that bioelectric modelling provides a useful tool in the evaluation of cellular changes in irradiated skin.

In vivo dielectric properties of human skin from 50 MHz to 2.0 GHz

The authors report in vivo complex permittivity measurements of human skin using the open-ended coaxial line sensor technique. The results indicate that considerable differences exist between the skin complex permittivities obtained from different regions of the body. The in vivo dielectric properties of skin are found to depend for reproducibility upon the pressure and length of time of sensor application. Variations between individuals could also be significant. The potential biomedical uses of this technique are briefly discussed.