Quasi-static Finite-difference Time-domain Research Articles

DNA sequencing by Förster resonant energy transfer.

We propose a new DNA sequencing concept based on nonradiative Förster resonant energy transfer (FRET) from a donor quantum dot (QD) to an acceptor molecule. The FRET mechanism combined with the nanopore-based DNA translocation is suggested as a novel concept for sequencing DNA molecules. A recently-developed hybrid quantum/classical method is employed, which uses time-dependent density functional theory and quasistatic finite difference time domain calculations. Due to the significant absorbance of DNA bases for photon energies higher than 4 eV, biocompatibility, and stability, we use Zinc-Oxide (ZnO) QD as a donor in the FRET mechanism. The most sensitivity for the proposed method to DNA is achieved for the Hoechst fluorescent-dye acceptor and 1 nm ZnO-QD. Results show that the insertion of each type of DNA nucleobases between the donor and acceptor changes the frequency of the emitted light from the acceptor molecule between 0.25 to 1.6 eV. The noise analysis shows that the method can determine any unknown DNA nucleobases if the signal-to-noise ratio is larger than 5 dB. The proposed concept and excellent results shed light on a new promising class of DNA sequencers.

중간 주파수 해석을 위한 수정된 준정적 FDTD 기법과 이론 해의 비교

The Finite-Difference Time-Domain (FDTD) is widely used in electromagnetic exposure analysis in high-resolution models such as the human body. The required analysis time, however, extremely increases when a standard FDTD technique is used in a band below the intermediate frequency. In this paper, the difference between the modified quasi-static FDTD method and the theoretical solution was analyzed by calculating the induced electric field in the dielectric sphere from the incident electric field and magnetic field, respectively.

Interband plasmon-enhanced optical absorption of DNA nucleobases through the graphene nanopore.

We propose a novel, to the best of our knowledge, plasmonic-based methodology for the purpose of fast DNA sequencing. The interband surface plasmon resonance and field-enhancement properties of graphene nanopore in the presence of the DNA nucleobases are investigated using a hybrid quantum/classical method (HQCM), which employs time-dependent density functional theory and a quasistatic finite difference time domain approach. In the strong plasmonic-molecular coupling regime where the plasmon and DNA absorption frequencies are degenerated, the optical response of DNA molecule in the vicinity of the nanopore is enhanced. In contrast, when the plasmon and nucleobases resonances are detuned the distinct peaks and broadening of the molecular resonances represent the inherent properties of the nucleobase. Due to the different optical properties of DNA nucleobases in the ultraviolet (UV) region of light, the signal corresponding to the replacement of nucleobases in a DNA block can be determined by considering the differential absorbance. Results show the promising capability of the present mechanism for practical DNA sequencing.

A potential sensing mechanism for DNA nucleobases by optical properties of GO and MoS2 Nanopores

We propose a new DNA sensing mechanism based on optical properties of graphene oxide (GO) and molybdenum disulphide (MoS2) nanopores. In this method, GO and MoS2 is utilized as quantum dot (QD) nanopore and DNA molecule translocate through the nanopore. A recently-developed hybrid quantum/classical method (HQCM) is employed which uses time-dependent density functional theory and quasi-static finite difference time domain approach. Due to good biocompatibility, stability and excitation wavelength dependent emission behavior of GO and MoS2 we use them as nanopore materials. The absorption and emission peaks wavelengths of GO and MoS2 nanopores are investigated in the presence of DNA nucleobases. The maximum sensitivity of the proposed method to DNA is achieved for the 2-nm GO nanopore. Results show that insertion of DNA nucleobases in the nanopore shifts the wavelength of the emitted light from GO or MoS2 nanopore up to 130 nm. The maximum value of the relative shift between two different nucleobases is achieved by the shift between cytosine (C) and thymine (T) nucleobases, ~111 nm for 2-nm GO nanopore. Results show that the proposed mechanism has a superior capability to be used in future DNA sequencers.

An advanced numerical technique for a quasi-static electromagnetic field simulation based on the finite-difference time-domain method

In this paper, an advanced numerical technique is proposed for a low-frequency electromagnetic field simulation. The finite difference time domain (FDTD) method is one of the best methods for analyzing electromagnetic field problems, which require high levels of precision, such as a heterogeneous whole-body voxel human model. However, directly using the standard FDTD method in the quasi-static frequency band requires an extra-large number of iterations. We overcome this drawback using the hybrid of surface equivalence theorem and quasi-static FDTD (QS-FDTD) method. The surface equivalence theorem is used to consider arbitrary shape of source and the QS-FDTD is applied to obtain electromagnetic response quickly in the low-frequency region. The results of the proposed method are in good agreement with those from a commercial electromagnetic simulator. The proposed method is valid where the quasi-static approximation is satisfied.

Induced Current Calculation in Detailed 3-D Adult and Child Model for the Wireless Power Transfer Frequency Range

Induced current density in the human body is the basic restriction of the electromagnetic field protection guidelines for frequencies . Due to the difficulty of measurement, numerical methods are often used for assessment of basic restrictions. In this paper, induced current distributions are calculated in the high-resolution 3-D adult and child models exposed to magnetic fields between 100 kHz and 10 MHz, which is the frequency range of the resonant wireless power transfer (WPT) systems. For this frequency range, it is difficult to apply the conventional finite-difference time-domain (FDTD) method for bioelectric field computation. Thus, the quasi-static FDTD method is used to reduce the number of time steps. The results are analyzed according to different human models, grounding conditions, organs, frequencies, and orientations of the incident magnetic field. In addition, the plane wave exposure is compared with exposure to magnetic field of transmit coil of WPT system. Using the calculation results, the feasibility of the magnetic field reference level in the International Commission on Non-Ionizing Radiation Protection guideline is analyzed.

Computational dosimetry for grounded and ungrounded human models due to contact current

This study presents the computational dosimetry of contact currents for grounded and ungrounded human models. The uncertainty of the quasi-static (QS) approximation of the in situ electric field induced in a grounded/ungrounded human body due to the contact current is first estimated. Different scenarios of cylindrical and anatomical human body models are considered, and the results are compared with the full-wave analysis. In the QS analysis, the induced field in the grounded cylindrical model is calculated by the QS finite-difference time-domain (QS-FDTD) method, and compared with the analytical solution. Because no analytical solution is available for the grounded/ungrounded anatomical human body model, the results of the QS-FDTD method are then compared with those of the conventional FDTD method. The upper frequency limit for the QS approximation in the contact current dosimetry is found to be 3 MHz, with a relative local error of less than 10%. The error increases above this frequency, which can be attributed to the neglect of the displacement current. The QS or conventional FDTD method is used for the dosimetry of induced electric field and/or specific absorption rate (SAR) for a contact current injected into the index finger of a human body model in the frequency range from 10 Hz to 100 MHz. The in situ electric fields or SAR are compared with the basic restrictions in the international guidelines/standards. The maximum electric field or the 99th percentile value of the electric fields appear not only in the fat and muscle tissues of the finger, but also around the wrist, forearm, and the upper arm. Some discrepancies are observed between the basic restrictions for the electric field and SAR and the reference levels for the contact current, especially in the extremities. These discrepancies are shown by an equation that relates the current density, tissue conductivity, and induced electric field in the finger with a cross-sectional area of 1 cm2.

공진형 무선전력전송 대역의 100kHz~10MHz 자기장에 의한 인체유도전류계산과 전자기장 인체보호기준 분석

As the technologies such as middle-range resonant WPT (wireless power transfer) advance that utilizes medium and low-frequency magnetic field, the importance of safety of such magnetic field is growing. The research on the effect of electromagnetic field on the human body has been mainly done on the GHz range of mobile phones, or 50～60Hz range of electric power systems. However, there has been relatively few works on the 100kHz～10MHz range used in the resonant wireless power transfer. Since there is a difference in the limiting value of magnetic field between widely used ICNIRP EMF guideline and IEEE C95.1 standard, there can be possible confusion when establishing EMF (Electromagnetic Field) standard on the wireless power transfer device in the future. In this paper, the induced current in the human body, which is the basic restriction of the EMF guideline, is calculated using Quasi-static FDTD method when 3D high-resolution human model is exposed to the 100kHz～10MHz magnetic field. Using this result, the feasibility of the magnetic field reference level in the ICNIRP guideline is analyzed.

An electric field induced in the retina and brain at threshold magnetic flux density causing magnetophosphenes

For magnetic field exposures at extremely low frequencies, the electrostimulatory response with the lowest threshold is the magnetophosphene, a response that corresponds to an adult exposed to a 20 Hz magnetic field of nominally 8.14 mT. In the IEEE standard C95.6 (2002), the corresponding in situ field in the retinal locus of an adult-sized ellipsoidal was calculated to be 53 mV m–1. However, the associated dose in the retina and brain at a high level of resolution in anatomically correct human models is incompletely characterized. Furthermore, the dose maxima in tissue computed with voxel human models are prone to staircasing errors, particularly for the low-frequency dosimetry. In the analyses presented in this paper, analytical and quasi-static finite-difference time-domain (FDTD) solutions were first compared for a three-layer sphere exposed to a uniform 50 Hz magnetic field. Staircasing errors in the FDTD results were observed at the tissue interface, and were greatest at the skin–air boundary. The 99th percentile value was within 3% of the analytic maximum, depending on model resolution, and thus may be considered a close approximation of the analytic maximum. For the adult anatomical model, TARO, exposed to a uniform magnetic field, the differences in the 99th percentile value of in situ electric fields for 2 mm and 1 mm voxel models were at most several per cent. For various human models exposed at the magnetophosphene threshold at three orthogonal field orientations, the in situ electric field in the brain was between 10% and 70% greater than the analytical IEEE threshold of 53 mV m–1, and in the retina was lower by roughly 50% for two horizontal orientations (anterior–posterior and lateral), and greater by about 15% for a vertically oriented field. Considering a reduction factor or safety factors of several folds applied to electrostimulatory thresholds, the 99th percentile dose to a tissue calculated with voxel human models may be used as an estimate of the tissue's maximum dose.

Numerical Investigation of the Effect of a Longitudinally Layered Armature on Coilgun Performance

The effect of a longitudinally layered armature on coilgun performance is investigated by using a 2-D axially symmetric cylindrical quasi-static finite-difference time domain method. The singularity extraction and Mur-type absorbing boundary condition are adopted with the numerical solution. The results obtained show that the best coilgun performance in the sense of the induced propulsive armature force is observed when the conductivity of the outer layer of the armature is smaller than that of the inner layer. This phenomenon can be explained in terms of impedance matching based on skin depth evaluation.

Effect of the averaging volume and algorithm on the in situ electric field for uniform electric- and magnetic-field exposures

The present study quantified the volume-averaged in situ electric field in nerve tissues of anatomically based numeric Japanese male and female models for exposure to extremely low-frequency electric and magnetic fields. A quasi-static finite-difference time-domain method was applied to analyze this problem. The motivation of our investigation is that the dependence of the electric field induced in nerve tissue on the averaging volume/distance is not clear, while a cubical volume of 5 × 5 × 5 mm3 or a straight-line segment of 5 mm is suggested in some documents. The influence of non-nerve tissue surrounding nerve tissue is also discussed by considering three algorithms for calculating the averaged in situ electric field in nerve tissue. The computational results obtained herein reveal that the volume-averaged electric field in the nerve tissue decreases with the averaging volume. In addition, the 99th percentile value of the volume-averaged in situ electric field in nerve tissue is more stable than that of the maximal value for different averaging volume. When including non-nerve tissue surrounding nerve tissue in the averaging volume, the resultant in situ electric fields were not so dependent on the averaging volume as compared to the case excluding non-nerve tissue. In situ electric fields averaged over a distance of 5 mm were comparable or larger than that for a 5 × 5 × 5 mm3 cube depending on the algorithm, nerve tissue considered and exposure scenarios.

Basic Restriction and Reference Level in Anatomically-based Japanese Models for Low-Frequency Electric and Magnetic Field Exposures

Human exposed to electric and/or magnetic fields at low frequencies may cause direct effect such as nerve stimulation and excitation. Therefore, basic restriction is regulated in terms of induced current density in the ICNIRP guidelines and in-situ electric field in the IEEE standard. External electric or magnetic field which does not produce induced quantities exceeding the basic restriction is used as a reference level. The relationship between the basic restriction and reference level for low-frequency electric and magnetic fields has been investigated using European anatomic models, while limited for Japanese model, especially for electric field exposures. In addition, that relationship has not well been discussed. In the present study, we calculated the induced quantities in anatomic Japanese male and female models exposed to electric and magnetic fields at reference level. A quasi static finite-difference time-domain (FDTD) method was applied to analyze this problem. As a result, spatially averaged induced current density was found to be more sensitive to averaging algorithms than that of in-situ electric field. For electric and magnetic field exposure at the ICNIRP reference level, the maximum values of the induced current density for different averaging algorithm were smaller than the basic restriction for most cases. For exposures at the reference level in the IEEE standard, the maximum electric fields in the brain were larger than the basic restriction in the brain while smaller for the spinal cord and heart.

Quasi-Static FDTD Method for Dosimetry in Human due to Contact Current

The present study proposes a quasi-static finite-difference time-domain (FDTD) method for dosimetry in humans due to contact current at low frequencies (∼10kHz). Our attention focused on wave sources which can reduce computational time. The computational time was found to be reduced using a voltage source of a step function with smooth start. The computational time required for the proposed method was smaller than a quasi-static FDTD method proposed in a previous study. Comparison between our computational results and those in a previous study suggested the effectiveness of our proposal. The difference in in-situ electric field due to different human models was a factor of 2 or so.

In-situ electric field and current density in Japanese male and female models for uniform magnetic field exposures

The present study quantified the in situ electric field and induced current density in anatomically based numeric Japanese male and female models for exposure to extremely low-frequency magnetic fields. A quasi-static FDTD method was applied to analyse this problem. The computational results obtained herein reveal that the 99 th percentile value of the in situ electric field in the nerve tissue and the current density averaged over an area of 1 cm(2) of the nerve tissue (excluding non-nerve tissues in the averaging region) in the female models were less than 35 and 25 %, respectively. These induced quantities in the Japanese models were smaller than those for European models reported in a previous study, which is mainly due to the difference in cross-sectional area of the body.

중간주파수 대역에서 준정적(Quasi-Static) FDTD 기법을 이용한 인체 유도전류 분석

본 논문에서는 준정적 FDTD 기법을 FORTRAN 프로그래밍을 통해 직접 구현하고 이를 이용해 중간주파수 대역의 인체유도전류 분포를 해석하였다. 제작된 프로그램의 타당성을 검증하기 위하여 기존 FDTD 기법을 적용하기 어려운 테스트 모델에 대한 계산결과를 이론적 해와 비교하고, 타임스텝이 크게(5.68×10?배) 단축되는 것을 확인하였다. 검증된 수치해석 기법을 이용하여 20[㎑] 자기장과 1[㎒] 전기장에 노출된 3차원 고해상도 인체모델에 유기되는 유도전류의 분포를 계산하고 양 발의 접지조건이 유도전류의 분포와 크기에 미치는 영향을 분석하였다. 본 연구는 인체유도전류의 안전성 평가 생체전기 응용 진단장치 개발 등을 포함한 다양한 분야에서 활용될 것으로 기대된다.

Dosimetry in Japanese male and female models for a low-frequency electric field

The present study quantified induced current in anatomically based Japanese male and female models for exposure to low-frequency electric fields. A quasi-static finite-difference time-domain (FDTD) method was applied to analyze this problem. For our computational results, the difference of the induced current density averaged over an area of 1 cm2 between Japanese male and female models was less than 30% for each nerve tissue. The difference of induced current density between the present study and earlier works was less than 50% for the same conductivities, despite the different morphology. Particularly, maximum current density in central nerve tissues appeared in the retina of Japanese models, the same as in the earlier works.

Computation of electromagnetic field inside a tissue at mobile communications frequencies

The increasing diffusion of mobile communications is stimulating the study of the interaction mechanisms between electromagnetic (EM) fields and biological systems at radio frequencies. This paper is devoted to the modeling of the interaction between EM fields and a tissue, represented with spherical cells. Different EM approaches have been used to analyze the problem and, in particular, the lumped-element finite-difference time-domain (FDTD) technique has been used to model the cell's membrane represented with the Hodgkin-Huxley (HH) model, and the Floquet theorem to study a tissue by analyzing only few cells. The EM problem has been solved by using the FDTD technique to be independent from the geometry. Computations have been performed at GSM900 and GSM1800 frequencies overcoming the problem relevant to the computation time by using the quasi-static FDTD technique. The results show the field distribution inside the tissue at GSM frequencies; the effect of the application of the HH model to the membrane is also shown.

A quasistatic FDTD source model for coaxially driven monopole antennas

A quasistatic FDTD source model for coaxially driven monopole antennas

High-resolution organ dosimetry for human exposure to low-frequency electric fields

Evaluation of the fields induced in the human body, provides a means of quantification of various exposures to low-frequency electric and magnetic fields. A new hybrid numerical method is used to compute these quantities, generated in an anatomically-based high-resolution human body model, by low-frequency uniform electric fields. The hybrid method combines scalar-potential finite-difference and quasistatic finite-difference time-domain methods, and is capable of accurately computing the induced fields in a full-body model composed of 1 736 872 cubic voxels with 3.6-mm edges. Results specific to various organs are given for exposure to 60-Hz fields, with the body model in three positions relative to a ground-plane. The effect of tissue conductivity is investigated. The data presented can be linearly scaled to frequencies up to 100 kHz without appreciable error; provided that changes of tissue conductivity with frequency are taken into account. Evaluations such as the present one should prove useful in the development of protection standards, and are also expected to aid in the understanding of results from various animal and tissue culture studies.

A comparison of 60 Hz uniform magnetic and electric induction in the human body

High-resolution computations of induced fields are used to assess equivalent source levels for human exposure to uniform low-frequency electric and magnetic fields. These results pertain to 60 Hz foot-to-head electric excitation of the body in three positions with respect to a ground plane, and to magnetic excitation by three orthogonal source orientations. All computations are based on an anatomically derived human body model composed of 1736 873 cubic voxels with 3.6 mm edges. The data for magnetic excitation are computed using a scalar potential finite difference (SPFD) method, while those for electric excitation are computed using a hybrid method based on the SPFD method coupled with a quasistatic finite difference time domain code. The data are analysed in two ways, using an induced current density threshold of . Firstly, the various field strengths required to produce a whole-body average current density magnitude equal to the threshold are derived for each configuration, and the associated current density levels in various organs and tissues are presented. It is found that the average current density magnitude values in at least one tissue group can be up to 3 (5) times greater than the whole-body average under electric (magnetic) excitation, and that the associated maximum values can be up to 46 (28) times greater than the whole-body average under electric (magnetic) excitation, for at least one source/body configuration. Secondly, the data are analysed from the opposite point of view, in which the source levels required to induce average or maximum induced current density magnitudes at the threshold level in specific tissue groups are determined. Evaluations such as the present one should prove useful in the development of protection standards, and are also expected to aid in the understanding of results from various animal and tissue culture studies.