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Insects are continually exposed to Radio-Frequency (RF) electromagnetic fields at different frequencies. The range of frequencies used for wireless telecommunication systems will increase in the near future from below 6 GHz (2 G, 3 G, 4 G, and WiFi) to frequencies up to 120 GHz (5 G). This paper is the first to report the absorbed RF electromagnetic power in four different types of insects as a function of frequency from 2 GHz to 120 GHz. A set of insect models was obtained using novel Micro-CT (computer tomography) imaging. These models were used for the first time in finite-difference time-domain electromagnetic simulations. All insects showed a dependence of the absorbed power on the frequency. All insects showed a general increase in absorbed RF power at and above 6 GHz, in comparison to the absorbed RF power below 6 GHz. Our simulations showed that a shift of 10% of the incident power density to frequencies above 6 GHz would lead to an increase in absorbed power between 3-370%.
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Exposure of Insects to Radio-
Frequency Electromagnetic Fields
from 2 to 120 GHz
Arno Thielens1,2, Duncan Bell3, David B. Mortimore4, Mark K. Greco5, Luc Martens1 & Wout
Joseph1
Insects are continually exposed to Radio-Frequency (RF) electromagnetic elds at dierent frequencies.
The range of frequencies used for wireless telecommunication systems will increase in the near future
from below 6 GHz (2 G, 3 G, 4 G, and WiFi) to frequencies up to 120 GHz (5 G). This paper is the rst
to report the absorbed RF electromagnetic power in four dierent types of insects as a function of
frequency from 2 GHz to 120 GHz. A set of insect models was obtained using novel Micro-CT (computer
tomography) imaging. These models were used for the rst time in nite-dierence time-domain
electromagnetic simulations. All insects showed a dependence of the absorbed power on the frequency.
All insects showed a general increase in absorbed RF power at and above 6 GHz, in comparison to the
absorbed RF power below 6 GHz. Our simulations showed that a shift of 10% of the incident power
density to frequencies above 6 GHz would lead to an increase in absorbed power between 3–370%.
Radio-Frequency (RF) electromagnetic elds (EMFs) enable wireless communication between billions of users
worldwide. Presently, this mainly occurs at RF frequencies located between 100 MHz and 6 GHz1. Wireless tel-
ecommunication base stations are the dominant sources of outdoor RF-EMFs1. Humans and animals alike are
exposed to these elds, which are partially absorbed by their bodies, e.g. reported for insects in2. e absorbed
dose depends on the frequency3,4, and can be strongly enhanced when a full-body or partial-body resonance
occurs3. is RF absorption has already been studied for particular insects at dierent individual frequencies:
27 MHz5,6, 900–915 MHz68, and 2450 MHz9.
is absorption may cause dielectric heating10. Heating aects insect behavior, physiology, and morphology11.
Reviews of studies that investigate RF heating of insects are presented in1214. Other authors focus on environ-
mental RF exposure of insects15,16 or expose insects to RF radiation in order to investigate potential biological
eects17,18. Studies on non-thermal eects of exposure to RF-EMF exist:19 presents a review of potential mecha-
nisms for non-thermal eects and a review of non-thermal eects of EMF exposure wildlife is presented in20. Most
existing studies focus on RF frequencies below 6 GHz. e same frequencies at which the current generations
of telecommunication operate1. However, due to an increased demand in bandwidth, the general expectation
is that the next generation of telecommunication frequencies will operate at so-called millimeter-wavelengths:
30–300 GHz21,22. erefore, future wavelengths of the electromagnetic elds used for the wireless telecommu-
nication systems will decrease and become comparable to the body size of insects and therefore, the absorption
of RF-EMFs in insects is expected to increase. Absorption of RF energy was demonstrated in insects between
10–50 GHz23, but no comparison was demonstrated with the RF absorption at frequencies below 10 GHz. e
radar cross section of insects has been determined above 10 GHz, but this quantity includes both scattering and
absorption24. It is currently unknown how the total absorbed RF power in insects depends on the frequency to
which they are exposed.
Most of the previously cited studies depend on measurements using RF equipment such as antennas, wave-
guides, and dielectric probes to determine the absorption of RF-EMFs in insects. An alternative approach would
1Department of Information Technology, Ghent University - imec, Ghent, B-9052, Belgium. 2Department of
Electrical Engineering and Computer Sciences, University of California Berkeley, Berkeley Wireless Research Center,
Berkeley, CA, 94704, USA. 3Department of Science and Technology, Faculty of Health and Science, University of
Suolk, Ipswitch, IP30AQ, United Kingdom. 4Newbourne Solutions Ltd, Newbourne, Woodbridge, IP12 4NR, United
Kingdom. 5Charles Sturt University, Medical Imaging, SDHS, Faculty of Science, Wagga Wagga, NSW 2678, Australia.
Correspondence and requests for materials should be addressed to A.T. (email: arno.thielens@berkeley.edu)
Received: 27 September 2017
Accepted: 20 February 2018
Published: xx xx xxxx
OPEN
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be to use numerical simulations. is approach was previously used to determine the absorption of RF-EMFs in
humans and requires numerical models or phantoms2528.
Techniques for creating heterogeneous, three-dimensional insect models with micrometer resolution have
previously been demonstrated in29.
However, up to now, insect phantoms have not been used in electromagnetic simulations.
e aims of this study were to, for the rst time, numerically evaluate RF-EMF absorption in real models of
insects and to determine a potential dierence in RF absorption in insects due to current and future telecom-
munication networks. To this aim, we studied the absorbed RF power in four dierent insect models obtained
using micro-CT imaging as a function of frequency in a broad band, 2 GHz up to 120 GHz, that covers both the
existing and the foreseen future wireless telecommunication bands. Voxelling precision in the order of 5–20 μm
is obtained, required for accurate electromagnetic simulations.
Methods
The Insects. Australian Stingless Bee (Tetragonula carbonaria). is bee (Tetragonula carbonaria) is native
to Australia. e scanned insect was approximately 4.5 mm long, 3.0 mm wide, and has a mass of 2.5 mg.
Western Honeybee (Apis mellifera). is bee (Apis mellifera) originated in Europe. It is the most common honey-
bee. e studied specimen was approximately 11.0 mm long, 5.0 mm wide, and has a mass of 900 mg.
Desert Locust (Schistocerca gregaria). e studied locust (Schistocerca gregaria) was approximately 55.0 mm
long, 18.0 mm wide, and has an approximate mass of 3.5 g.
Beetle (Geotrupes stercorarius). e studied beetle is a dor beetle (Geotrupes stercorarius). e beetle was found
and scanned (see below) at Aberdeen University in Scotland. e beetle’s length was 8.01 mm and its width is
4.5 mm. e insect’s mass was not measured at the time of scanning. e average mass of a dor beetle is 220 mg30.
Scanning Methods. Australian Stingless Bee. MicroCT scans were performed with a Skyscan 1172
high-resolution MicroCT system (Bruker MicroCT, Kontich, Belgium). is system has a sealed, microfocus
x-ray tube with a 5 μm focal spot size. e x-rays were produced by exposing the anode to 40 kV at 100 μA. Prior
to scanning, the sample containing the insect was placed on the pedestal between the x-ray source and the CCD
detector. Aer positioning the sample, 600 2D x-ray images over 180° were captured by exposing the sample and
then rotating it to the next exposure position with a slice-to-slice rotation distance of 2 μm, and a total acquisition
time of approximately 60 min: each 2D image represents one slice. e scanner soware then converted each slice
to axial orientation and created 998 bitmap images (16 bit grey scale) which were stored for 2D viewing and 3D
reconstruction as a 983 Mb dataset. e resulting isotropic voxel size was 5 μm.
Western Honeybee. A bench-top MicroCT scanner (Quantum GX MicroCT Imaging System, PerkinElmer,
Hopkinton, MA, USA) at the Western Sydney University National Imaging Facility (Sydney, Australia) was used
to scan the bee. e following parameters were used: 50 kVp, 80 μA, high resolution 2048 × 2048 pixels image
matrix, with 20 μm isotropic voxel size. Scanning time was 3.0 s for each of the 180 projections with 3.0 s rotation
in between each projection. e total scan time was approximately 18 min per whole bee. e Quantum GX,
bench-top MicroCT scanner’s soware was used to reconstruct the 180 projection images and then to convert
them into a 2D rendered image stack of 512, 16 bit bitmap images. Bee volume data were then acquired by loading
the image stack into BeeView volume rendering soware (DISECT Systems Ltd, Suolk, UK).
Desert Locust. e locust was suspended vertically in a 30-mm acrylic tube that was mounted tightly on the
micro-CT’s inclination stage. is stage was used to ensure that the rotation axis was at 90° to the x-ray source.
Exposure factors were: 50 kVp and 198 μA. e data were isotropic 16 bit 2000 × 2000 pixels with 1048 rows. Pixel
size was 10.469 μm. Skyscan NRecon soware version 1.5.1.4 (Bruker, Kontich, Belgium) was used to reconstruct
the projection data31. Having obtained the projection data in the form of an image stack of 2-D TIFF les the data
was viewed as a 3-D model using Disect soware, DISECT Systems29.
Beetle. e beetle was scanned at Aberdeen University on a Skyscan 1072 Micro-CT scanner (Bruker, Kontich,
Belgium) using 50 kV and 197 μA, at 10.46 μm pixels isotropically. e images were then converted to axial slices
using Skyscan’s NRECON soware (version 1.4). e produced axial image stack was further processed and ana-
lyzed using the Tomomask soware (www.tomomask.com) before viewing in disect.
Development of 3D models. 3D models of the insects were created using the soware TomoMask (www.
tomomask.com). e image stack for each insect was rstly imported into the soware together with details of
the pixel and slice spacing. Regions to be converted into a 3D model are dened in TomoMask by drawing a mask
of the wanted regions on each slice. is can be done automatically using the Luminance mask function which
creates a mask based on the grey level of the pixels. e threshold values for the mask are set to include all of the
insect tissue but will exclude air cavities and very ne structures, such as wings. e 3D model (generated by a
marching cubes algorithm32) is exported as an STL (STereo Lithography)33 format le. STL les describe only the
surface geometry of a three-dimensional object without any representation of colour or texture. Typically some
smoothing of the models is required and this is realized using the Taubin λ/μ smoothing scheme34 implemented
in MeshLab35. e Taubin method is good at removing noise whilst preserving shapes and features. Dimensions
of the models and mesh integrity are nally checked (and corrected if necessary) using Netfabb (Autodesk, San
Rafael, CA, USA).
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Dielectric Properties. e propagation of EMFs inside and around the obtained 3D insect phantoms will
depend on their dielectric properties: the relative permittivity (εr) and conductivity (σ). In this study, we have
executed and relied on a literature review of previous measurements of dielectric properties of insects, predom-
inantly using the coaxial-line probe method36. ere exist alternative methods. A toroidal resonator was used to
determine the dielectric properties of two insects at 2370 MHz37. Dielectric properties of Rice Weevils (Sitophilus
oryzae) are obtained using the coaxial probe method for frequencies from 5 × 104–2 × 1010 Hz2. e same tech-
nique was used on three other insects: the Red Flour Beetle (Tribolium castaneum), the Sawtooth Grain Beetle
(Oryzaephilus surinamensis), and the Lesser Grain Borer (Rhyzopertha dominica), from 0.2–20 GHz36. e same
method was also used to measure dielectric properties of four insects: the Codling Moth (Cydia pomonella), the
Indian Mealmoth (Plodia interpunctella), the Mexican fruit y (Anastrepha ludens), and the Navel Orange Worm
(Amyelois transitella) from 27–1800 MHz6. Coaxial measurements on a Colorado Beetle (Leptinotarsa decemline-
ata) were performed from 0.1–26.5 GHz and used to derive a t to the measurement data38.
We have pooled the data series, real and imaginary part of εr as a function of frequency, obtained by6,36,38 and
interpolated them from 2–120 GHz in steps of 0.1 GHz. We have then averaged over all available data at every
frequency steps considered in the simulations.
Numerical Simulations. e Finite-Dierence Time-Domain (FDTD) technique implemented in the com-
mercial simulation soware Sim4life (ZMT, Zurich, Switzerland) is used to evaluate absorption of RF-EMFs
inside the insects as a function of frequency. is technique is commonly used to determine absorption of
RF-EMF in heterogeneous human body models3. e FDTD method requires one to discretize the simulation
domain using a three-dimensional grid. e simulation domain is divided in a number of cubes (discretized) with
spatial extends that are dened by the spatial grid steps in the simulation domain. RF-EMFs can be incident from
any direction. erefore, we have chosen to work with 12 incident plane waves with a root-mean-squared electric
eld strength of 1 V/m, illustrated in Fig.1, along 6 directions dened by Cartesian axes, with two orthogonal
polarizations of the incident RF-EMFs along each axis.
e exposure was modeled using single frequency sinusoidal (harmonic) continuous plane waves. We did
not take into account a potential modulation of the waves, which might be present in real telecommunication
signals. is same technique has previously been used to evaluate the frequency dependence of RF absorption in
the human body3. Simulations were executed for sinusoidal plane waves at 7 harmonic (single) frequencies: 2, 3,
6, 12, 24, 60, and 120 GHz. is resulted in a dataset of 4 (insects) ×7 (frequencies) ×12 (plane waves: 6 angles of
incidence ×2 polarizations) = 336 simulations.
e Australian Stingless Bee, the Western Honey Bee, and the Beetle were discretized with steps of 0.05 mm in
each direction, while the larger Locust was discretized with steps of 0.2 mm in each direction at frequencies below
60 GHz and a step of 0.1 mm at 60 GHz and 120 GHz. ese spatial steps provided a balance between simulation
time (which depends on the number of grid steps and the relative grid step size in comparison to the wavelength)
and spatial resolution of the insects’ features. A stable FDTD simulation yields reproducible results that converge
over time. e quantities determined using the FDTD algorithm should converge to a constant value as the sim-
ulation progresses in time. Aer a certain simulation time, these values will remain constant, this is referred to as
a steady state. A grid step smaller than one tenth of the smallest wavelength in the simulation domain is necessary
for a stable FDTD simulation39. is is a requirement of the FDTD algorithm39 and remains valid in all our
Figure 1. Illustration of the RF-EMF exposure set up. e insect (Beetle shown here in pink) is exposed to
twelve RF plane waves incident from six directions along the positive and negative directions of the Cartesian
axes shown on the bottom le with two orthogonal polarizations for each direction. e twelve wave vectors
kij/
are indicated in blue (dashed arrows), while the polarization of the incident electric elds
Ei
are indicated in red.
i and j indicate the conguration number, from 1 to 12.
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simulations. e smallest wavelength in tissue
λε
(/)
r
is 1.1 mm at 120 GHz. At this frequency we used grid steps
of 0.05 mm
λε≤. ×(0045 /)
r
for all insects, except for the locust where we used 0.1 mm
(009 /)
r
λε≤. ×
.
We ensured that the grid steps were small enough to prevent disconnections in the models. All insects were
considered as consisting of homogeneous tissue with frequency-dependent dielectric parameters obtained as an
average of the values we found in literature (previous section). is is an approximation, since real insects have
heterogeneous tissue properties. Each simulation was executed until a steady state was reached. e number of
periods necessary to reach a steady state solution depended on the studied insect and frequency and was between
20–80. is was controlled by temporal monitoring of the electric eld strength along a line in the simulation
domain until it reached a steady state. Additionally, the chosen number of simulation periods allowed for propa-
gation of at least 3 times the length of the insects’ diagonal (see Table1).
Aer every simulation, the absorbed RF-EMF power (Pabs) in the insect was extracted. e Pabs is calculated
as the product of the conductivity and the squared electric eld strength integrated over the volume of the insect.
e whole-body averaged specic absorption rate can be obtained by dividing Pabs by the insects’ mass (assuming
a homogeneous mass density). Absorbed RF-EMF power is generally used as a proxy for dielectric tissue heat-
ing10. We have not executed full thermal simulations due to uncertainties on the specic heat capacities of the
insects and heat dissipation mechanisms.
Results
3D Models. Figure2 shows the used 3D models obtained using micro-CT scanning of four insects.
Dielectric Properties. Figure3 shows the imaginary and real parts of εr obtained by averaging those values
that were available in6,36,38. e real part is positive and decreases with frequency, while the imaginary part is
negative (lossy media) and shows a minimum between 10–20 GHz. ese are in line with the Debye dielectric
curves proposed in38. Figure3 adds further perspective by showing the corresponding conductivity in (S/m) and
the RF penetration depth.
Numerical Simulations. Figure4 illustrates the frequency dependence of the absorption of RF-EMFs in
the Western Honeybee in terms of the ratio of the electric eld strength inside the insect to the maximum electric
eld in the simulation domain. At the currently used frequencies for telecommunication (<6 GHz), the wave-
length is relatively large compared to the insects and the waves do not penetrate into the insects, which results in
lower Pabs values. At 12–24 GHz, the elds penetrate more and more into the insect as the wavelength becomes
comparable to the insects’ size and the conductivity increases as well. At the highest studied frequencies, the elds
penetrate less deep into the insect, but their amplitude is higher, resulting in a similar or slightly lower Pabs.
Figure5 shows the Pabs linearly averaged over all twelve plane waves as a function of frequency for all studied
insects. e absorbed power increases with increasing frequency from 2–6 GHz for all insects under exposure at
a constant incident power density or incident electric eld strength of 1 V/m. e absorbed power in the Locust,
the largest studied insect, decreases slightly at the studied frequencies >6 GHz, but remains higher than at 2 and
3 GHz. e Western Honeybee shows an increase up to 12 GHz, followed by a slight decrease up to 120 GHz (Pabs
remains more than 10× higher than <6 GHz). e smaller Australian Stingless Bee shows an increase of Pabs
with frequency up to 60 GHz and a slight decrease at 120 GHz. e Pabs in the Beetle increases until 24 GHz and
slightly decreases at higher frequencies.
Table1 lists the dimensions of the dierent studied insects, compared to the wavelength λ-range in which the
maximal Pabs will be located. e Pabs is simulated for discrete frequency steps. erefore, the λmax that corre-
sponds to the maximum Pabs is located in between the wavelength steps right below and above the wavelength
step that corresponds to the maximum simulated Pabs, see Fig.4. e main diagonal of the insects’ bounding box
is within the range in which the wavelength of maximal absorption λmax is located for three out of the four studied
insects. is indicates that the absorption is (partly) determined by the size of the insects.
Numerical simulations are never the same as reality and there are always uncertainties associated with any
EM simulation technique. We report the following sources of uncertainty: model variations and variation on
dielectric properties.
e insect models are scanned with a resolution of 20 μm, 10.5 μm, 10.5 μm, and 5 μm, for the Honey Bee,
the Locust, the Beetle, and the Australian Stingless Bee, respectively. ese are 40%, 5–10%, 21%, and 10% of
the spatial grid step used in the simulations of the Honey Bee (0.05 mm), the Locust (0.1–0.2 mm), the Beetle
(0.05 mm), and the Australian Stingless Bee (0.05 mm), respectively. is indicates that the grid step is dominant
Insect L (mm) W
(mm) H
(mm) D
(mm) Range λmax
(mm)
Beetle 8.01 4.5 4.29 10.14 5–25
Australian
Stingless Bee 4.89 3.39 3.99 7.16 2.5–12.5
Western
Honey B ee 11 4.154 4.044 12.43 12.5–50
Locust 54.99 18.49 17.55 60.61 25–100
Table 1. Dimensions of the studied insect models along the dierent axes shown in Fig.1. L, W, and H, are the
dimensions in the X, Y, and Z, directions, respectively. D is the size of the diagonal of the brick with dimensions
L × W × H. e nal column lists the range in wavelengths where the maximal Pabs(λmax) will be located.
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in determining the spatial extends of the used models and not the resolution of the scanning method. In order
to investigate the eect of the chosen grid step on the obtained Pabs values, we have executed the simulation with
conguration 9 (Fig.1) at 120 GHz with a maximal grid step thatis half of the grid step used in our simulations
using all four studied insects. We assume the largest eect of grid step size at the highest frequency. A 50%
reduction in grid step (more accurate modelling) resulted in deviations of 1.1%, 2.5%, 0.32%, and 0.24%, for the
Honey Bee, the Locust, the Beetle, and the Australian Stingless Bee, respectively. ese deviations are small in
comparison to the variations as a function of frequency, see Fig.5, and the uncertainty caused by the dielectric
parameters, see the next paragraph.
Deviations on εr will inuence Pabs: the real part of εr will (partly) determine the magnitude of the inter-
nal electric elds, while Pabs scales linearly with conductivity. e maximal relative deviations on the real and
imaginary part of εr are (13, +36)% and (40, +36)%, respectively, which occur between 2–3 GHz. We have
executed a simulation using conguration 1 at 2 GHz for the Beetle phantom, shown in Fig.1, using ve dier-
ent sets of dielectric properties accounting for the deviations mentioned above: [Re(εr), Im(εr)], [1.36 × Re(εr),
1.36 × Im(εr)], [1.36 × Re(εr), 0.6 × Im(εr)], [0.87 × Re(εr), 1.36 × Im(εr)], and [0.87 × Re(εr), 0.6 × Im(εr)], in
order to determine the eect of the uncertainty of dielectric properties on Pabs. We found maximal relative devia-
tions of [57, +59]% relative to the value obtained using [Re(εr), Im(εr)]. ese deviations are small in compari-
son to the variations as a function of frequency, see Fig.5.
Figure 2. Frontal, side, and Top view of the four studied insects. (a) Australian Stingless Bee, (b) Western
Honeybee, (c) Beetle, and (d) Locust.
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Previous studies have indicated that large dierences in dielectric properties might exist between adult insects
and larvae40. Worst-case deviations of [Re(εr)/7, Im(εr)/5] at 5 GHz and [Re(εr)/6, Im(εr)/8] at 15 GHz were
observed in40. We have executed simulations of conguration 1 using the beetle (shown in Fig.1) at 6 GHz and
12 GHz where we have applied these reduced dielectric parameters. We found an increase in Pabs of 4% at 6 GHz
and a decrease of 66% in Pabs at 12 GHz. Figure5 shows that these variations are smaller than the variations we
observed for varying angles of incidence at a xed frequency.
Discussion
In this study, we have evaluated the absorption of RF-EMFs in insects as a function of frequency. To this aim, we
obtained novel insect models using micro-CT imaging, which were used in FDTD simulations. In these simula-
tions they were exposed to plane waves incident from six directions and two polarizations.
e frequency of the incident harmonic plane waves was varied from 2–120 GHz and resulted in Pabs as a
function of frequency.
Figure 3. From top to bottom: Real part of the used dielectric permittivity, Imaginary part of the used dielectric
permittivity, and conductivity with RF-EMF penetration depth as an inset. Markers show measurements
obtained from literature. e black line with circular markers shows the average over the available data series at
those frequencies.
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Figure 4. Normalized Electric eld strength (dB) in a mid-transverse cross section of the Western Honey
Bee as a function of frequency for a single plane wave incident from below with polarization orthogonal to
the shown plane (No. 5 in Fig.1). Normalization was executed for each simulation separately, i.e. Emax can be
dierent in each subgure.
Figure 5. Pabs for an incident eld strength of 1 V/m as a function of frequency for all studied insects. e
markers indicate the average over all twelve plane waves at each of the simulated frequencies, while the whiskers
indicate the minimal and maximal Pabs values obtained during the simulations.
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Previous studies have shown that Micro-CT imaging can be successfully used as a non-invasive technique to
accurately scan insects and develop 3D models with micrometer resolution29,41. Models with micrometer resolu-
tion are necessary to obtain accurate results in FDTD simulations at 120 GHz (λ = 2.5 mm), since a discretization
of λ/10 in the simulation domain is recommended to obtain stable results39. It has been demonstrated for human
body models that real anatomical models generally result in more accurate and realistic results than approximate
models3,25,28. erefore, we also expect our real insect models to lead to more accurate results regarding absorbed
RF power than, for example, cylindrical phantoms with dierent diameters and heights, which were used in pre-
vious studies of RF exposure of insects42.
e dielectric properties that were assigned to the studied insects were obtained from an interpolation of data
found in literature. Ideally, the simulations should be executed with dielectric properties measured in the actual
insects that were used to create the models. Figure3 does show that most insects show a similar frequency behav-
ior, which we have averaged by using an interpolation over values listed in literature.
Our numerical simulations show that the absorption of RF-EMFs in the insect models is frequency depend-
ent. e Pabs is smallest at the lowest studied frequencies 2 GHz and 3 GHz, for all insects. Pabs shows a peak, which
depends on the size and/or mass of the insects. e three smaller insects show their maximum at a frequency
higher than 6 GHz: 60 GHz, 24 GHz, and 12 GHz for the Australian StinglessBee, the Beetle, and the Honey Bee,
respectively. e Locust shows a maximum at 6 GHz. We attribute this frequency behavior to two eects: rst, the
eciency of RF-EMFs coupling into the models is maximal at frequencies comparable to the length of the insects,
as Table1 illustrates. Second, the interpolation of the imaginary part of the dielectric constant shows a minimum
at 12 GHz, which means that RF-EMFs can cause the highest local SAR at these frequencies, see Fig.3.
e dierence between the maximal and minimal Pabs found at one frequency for dierent angles of incidence
is smaller at the frequencies >6 GHz, than at the frequencies <6 GHz, in particular for the three smaller insects.
is indicates that the angle of incidence is less important at these frequencies. is suggests that there is little
dierence in eciency when depositing RF power in the studied insects with a single plane wave compared to
depositing the same power using uncorrelated sources or reections coming from all directions. In this study, we
have only used single plane-wave simulations to determine Pabs. e averaging over Pabs does not include interfer-
ence eects, which might result in lower (destructive interference) or higher (constructive interference) bounds
on the Pabs values shown in Fig.5.
A similar frequency behavior (increase, peak, decrease, and dependency on body size) is observed in human
body models3,4. However, at frequencies which are roughly a factor 100–1000 times lower, because the human
body is approximately the same order of magnitude largerthan that of the studied insects. For example, the het-
erogeneous adult human body model Duke shows an increase in Pabs from 10 MHz–80 MHz, a peak between
80 MHz–90 MHz, followed by a decrease in Pabs (and a second peak at higher frequencies)3. e smaller child
phantom elonius shows an increase in Pabs from 10 MHz–100 MHz, a peak between 100 MHz–200 MHz, fol-
lowed by a decrease in Pabs3.
In order to quantify the eect of a shi to higher telecommunication frequencies on Pabs, one can use the data
presented in Fig.5. If we assume an incident Erms = 1 V/m which is uniformly distributed over 2, 3, and 6 GHz, we
nd average Pabs values of 0.71 nW, 2.6 nW, 5.7 nW, and 990 nW, for the Australian Stingless Bee, the Beetle, the
Honey Bee, and the Locust, respectively. If we assume that 10% of this incident eld would be evenly distributed
over the frequencies above 6 GHz, the Pabs increases to 2.6 nW, 7.7 nW, 14 nW, and 1.0 μW, for the Australian
Stingless Bee, the Beetle, the Honey Bee, and the Locust, respectively. ese are increases of 370%, 290%, 240%,
and 3%, respectively. Note that this is a conservative estimation of the increase in Pabs, since we assume a con-
stant incident eld and a uniform distribution of the currently used frequencies <6 GHz. Nowadays, most of the
incident power density used for telecommunication is located at frequencies 2 GHz1, where all insects show a
minimal Pabs. In an isolated approximation (no convection or conduction) and under the assumption of unchang-
ing mass and specic heat capacitance, the rate of temperature increase scales linearly with increasing Pabs. As an
example, for the Australian Stingless Bee (mass = 2.5 mg) a Pabs of 3 × 10–8 W is estimated for an incident eld
strength of 1 V/m at 60 GHz. Under the assumption that the insect has a specic heat capacity equal to that of
water (4179 J/K kg43), this RF-EMF exposure would result in a temperature increase of 3 × 10–6 K/s, in an isolated
approximation.
Strengths and Limitations
Our paper has several clear strengths and contributions to the state of the art in literature. To our knowledge,
this is the only paper in which real insects are used to create models for numerical simulations. Moreover, this
is the rst paper that investigates the exposure of electric elds with RF frequencies associated with 5 G wire-
less communication and that shows that the absorbed power in insects is expected to increase in unchanged
environmental conditions with respect to the one of current wireless communication systems (3 G and 4 G). A
disadvantage of our study is the use of homogeneous models in the simulations, whereas real insects will have
heterogeneous tissue parameters. Variations on dielectric parameters can exist on a scale that is smaller than the
spatial resolution that any scanning method can currently obtain44. e FDTD method requires a division of the
simulation domain in a number of voxels, which each have to be assigned homogeneous dielectric properties39.
Any numerical simulation will be an approximation of reality. To our knowledge, the FDTD method, although
faced with uncertainties3,39,44 is the best simulation method currently available to estimate the quantities studied
in this manuscript. is paper is limited to electromagnetic dosimetry, which is focused on determining absorbed
powers values. ese can be used as an input in thermal modelling of the insects. However, a full thermal analysis
was outside the scope of this paper. Finally, we have included variations in angles and polarizations of incident
waves. However, we have only looked at a limited number of plane waves, whereas real exposure is composed of
plane waves from any direction.
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9
Scientific REpoRTs | (2018) 8:3924 | DOI:10.1038/s41598-018-22271-3
Future Research
In our future research, we would like to model more insects to get a better understanding of the frequency
dependence of the absorbed RF-EMF power as a function of insect size. We would also like to develop heter-
ogeneous insect models with tissue-specic dielectric parameters. Finally, our goal is to determine the eect of
absorption of RF-EMFs on the core temperature of insects as a function of frequency. To this aim, we want to use
infrared temperature measurements of insects exposed to high electromagnetic elds as function of frequency.
Conclusions
We investigated the absorbed radio-frequency electromagnetic power in four dierent real insects as a function of
frequency from 2–120 GHz. Micro-CT imaging was used to obtain realistic models of real insects. ese models
were assigned dielectric parameters obtained from literature and used in nite-dierence time-domain simula-
tions. All insects show a dependence of the absorbed power on the frequency with a peak frequency that depends
on their size and dielectric properties. e insects show a maximum in absorbed radio frequency power at wave-
lengths that are comparable to their body size. ey show a general increase in absorbed radio-frequency power
above 6 GHz (until the frequencies where the wavelengths are comparable to their body size), which indicates that
if the used power densities do not decrease, but shi (partly) to higher frequencies, the absorption in the studied
insects will increase as well. A shi of 10% of the incident power density to frequencies above 6 GHz would lead to
an increase in absorbed power between 3–370%. is could lead to changes in insect behaviour, physiology, and
morphology over time due to an increase in body temperatures, from dielectric heating. e studied insects that
are smaller than 1 cm show a peak in absorption at frequencies (above 6 GHz), which are currently not oen used
for telecommunication, but are planned to be used in the next generation of wireless telecommunication systems.
At frequencies above the peak frequency (smaller wavelengths) the absorbed power decreases slightly.
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Acknowledgements
is project has received funding from the European Unions Horizon 2020 research and innovation programme
under the Marie Skłodowska-Curie grant agreement No 665501 with the research Foundation Flanders (FWO).
A.T. is an FWO [PEGASUS]2 Marie Skłodowska-Curie Fellow.e Eva Crane Trust (Charitable Incorporated
Organisation Number: 1175343) has partly funded the insect data collection, which was used for this study by
M.K.G.
Author Contributions
A.T. conducted the numerical simulations, analyzed the results, and draed the manuscript, M.K.G., D.B., and,
D.B.M. conducted the imaging and post processing of the imaging. W.J. and L.M. contributed to analyzing the
methodology and results. All authors reviewed the manuscript and provided input to the dierent sections.
Additional Information
Competing Interests: e authors declare no competing interests.
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... Now days, android phones have also entered in this category with advance cellular system i.e. 5G. The 5G utilizes frequencies up to 120 GHz (Thielens et al., 2018). Their adverse effects have already been seen on vertebrates (Humans, mice, birds) and up to limited extent on arthropods especially bees (Atwal 2018). ...
... According to Thielens et al., (2018) when insects are exposed to EM fields, it is partially absorbed by their body based on their frequency. If frequency is at or above 6 GHz, it will increase the general absorption of RF power. ...
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... These successful applications include microwave cooking, microwave drying, microwave-enhanced plasma chemical vapor deposition (MPCVD), microwave sintering, microwave-assisted metallurgy, microwaveassisted chemical synthesis, etc. 3,4 With the development of millimeter-wave technology, millimeter-wave power applications attract researchers' attention. [5][6][7][8][9][10] The higher frequency of millimeter waves compared to microwaves results in different physical processes in the processed materials, which opens up new opportunities for applications. The further development of millimeter-wave applications requires efficient and available radiation sources. ...
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Low operating voltage is highly attractive for medium-power millimeter-wave gyrotrons since it can reduce their size and cost, increase their safety, and, thus, improve usability for applications. However, at low voltages, the voltage depression caused by DC space-charge fields significantly limits the electron current and transverse power in the beam. Moreover, this current limitation is more pronounced for a beam with a higher pitch factor. As a result, for a given anode voltage, there is a pitch factor at which the transverse beam power in the gyrotron cavity is the maximum. This ultimate transverse power is found analytically in the non-relativistic approximation. Such a power is reached when the pitch factor calculated without taking into account voltage depression is only 0.82; voltage depression decreases the axial electron velocities, thus, increasing the actual pitch factor value in the cavity up to 1.4. As a result of this effect, high power and high efficiency cannot be obtained simultaneously in a low-voltage gyrotron. Using particle-in-cell simulations, two variants of low-voltage (5 kV) gyrotrons have been designed, namely, a device with higher power and an optimal pitch factor of 0.82 in the cavity and a device with a high pitch factor and high efficiency, but lower power.
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This report summarizes the effects of anthropogenic radiofrequency electromagnetic fields with frequencies above 100 MHz on flora and fauna presented at an international workshop held on 5–7 November 2019 in Munich, Germany. Anthropogenic radiofrequency electromagnetic fields at these frequencies are commonplace; e.g., originating from transmitters used for terrestrial radio and TV broadcasting, mobile communication, wireless internet networks, and radar technologies. The effects of these radiofrequency fields on flora, fauna, and ecosystems are not well studied. For high frequencies exceeding 100 MHz, the only scientifically established action mechanism in organisms is the conversion of electromagnetic into thermal energy. In accordance with that, no proven scientific evidence of adverse effects in animals or plants under realistic environmental conditions has yet been identified from exposure to low-level anthropogenic radiofrequency fields in this frequency range. Because appropriate field studies are scarce, further studies on plants and animals are recommended.
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There is enough evidence to indicate we may be damaging non-human species at ecosystem and biosphere levels across all taxa from rising background levels of anthropogenic non-ionizing electromagnetic fields (EMF) from 0 Hz to 300 GHz. The focus of this Perspective paper is on the unique physiology of non-human species, their extraordinary sensitivity to both natural and anthropogenic EMF, and the likelihood that artificial EMF in the static, extremely low frequency (ELF) and radiofrequency (RF) ranges of the non-ionizing electromagnetic spectrum are capable at very low intensities of adversely affecting both fauna and flora in all species studied. Any existing exposure standards are for humans only; wildlife is unprotected, including within the safety margins of existing guidelines, which are inappropriate for trans-species sensitivities and different non-human physiology. Mechanistic, genotoxic, and potential ecosystem effects are discussed.
Article
Insects are exposed to environmental radio-frequency electromagnetic fields (RF-EMFs), which are partially absorbed by their body. This absorption is currently unknown for most insect types. Therefore, numerical simulations were performed to study the far-field absorption of RF-EMFs by different insect types at frequencies between 2 and 120 GHz, which are (expected to be) used in (future) wireless communication. The simulations were done using anatomically accurate as well as spheroid models of the insects. The maximum absorbed power, which ranged from 7.55 nW to 389 nW for an incident electric field strength of 1 V/m for the studied insect types, was obtained at wavelengths comparable to the insects’ size. We created a log-linear model that can estimate absorbed power in insects with an average relative error < 43% between 6 and 120 GHz using only the insects’ volume and the frequency as an input using the simulation results. Additionally, our simulations showed a very high correlation ( r > 0.95) between the absorbed power predicted with anatomically accurate insect models and those predicted with spheroid models at frequencies between 6 and 24 GHz. This suggests that such models could be used to evaluate RF-EMF exposure of insects in future studies.
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Radio-frequency (RF) and microwave (MW) heating for phytosanitary treatment of green wood in compliance with international standards for phytosanitary measures no. 15 (ISPM-15) were evaluated and compared to assess treatment time, depth of electromagnetic wave (EMW) penetration and heating uniformity. White oak (Quercus alba) cants (48 cm long with cross-section dimensions ranging from 10 × 10 cm2 to 25 × 25 cm2) were heated in a 19 MHz RF or 2.45 GHz MW laboratory oven using an equivalent heating power (3.4 kW). In each specimen, temperature was measured at different depths (distance from the upper face). Specimens were held in the treatment chamber for 2 min after the target temperature of 60 °C was achieved through the profile of the specimen to ensure compliance with the ISPM-15 treatment schedule. Thermal image analyses of treated specimens as well as theoretical depth of penetration for dielectric energy were explored. Wood specimens were also heated using RF at high power (9–11 kW) and results were compared with RF heating at 3.4 kW. For wood with cross-section dimensions of 10 × 10 cm2 to 15 × 15 cm2, heating rates for RF and MW were relatively similar. However, above 15 × 15 cm2, RF heating was more than 40 % faster with greater heating uniformity than MW. The theoretical values derived for depth of penetration and thermal image analyses indicate that RF (19 MHz) penetrates wood more uniformly and is better suited than MW (2.45 GHz) for bulk volume treatments of wood.
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Rhynchophorus ferrugineus Oliv., known as the red palm weevil (RPW), has quickly spread in Southern Europe, infesting and destroying an increasing number of palms, particularly the Phoenix canariensis ones. Of the various techniques suggested for treating the palms, high power microwave applications are considered an attractive, eco-compatibile solution. However, in order to correctly design the exposure system, a knowledge of the electromagnetic properties of the materials involved is required. In this paper, we present a broad-band electromagnetic characterization in the 0.4-18 GHz frequency range of the tissues (both healthy and damaged) of the P. canariensis, with different moisture content, and of the R. ferrugineus in different stages (larva, pupa and adult). The palm tissues had a high water content and a dielectric model of the vegetation was applied to the experimental data in order to estimate the volume fraction of free water and of the bulk vegetation-bound water mixture as well as the ionic conductivity of the free-water solution.
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The capacity to explore soft tissue structures in detail is important in understanding animal physiology and how this determines features such as movement, behaviour and the impact of trauma on regular function. Here we use advances in micro-computed tomography (micro-CT) technology to explore the brain of an important insect pollinator and model organism, the bumblebee (Bombus terrestris). Here we present a method for accurate imaging and exploration of insect brains that keeps brain tissue free from trauma and in its natural stereo-geometry, and showcase our 3D reconstructions and analyses of 19 individual brains at high resolution. Development of this protocol allows relatively rapid and cost effective brain reconstructions, making it an accessible methodology to the wider scientific community. The protocol describes the necessary steps for sample preparation, tissue staining, micro-CT scanning and 3D reconstruction, followed by a method for image analysis using the freeware SPIERS. These image analysis methods describe how to virtually extract key composite structures from the insect brain, and we demonstrate the application and precision of this method by calculating structural volumes and investigating the allometric relationships between bumblebee brain structures.
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Microwave is an effective means to deliver energy to food through polymeric package materials, offering potential for developing short-time in-package sterilization and pasteurization processes. The complex physics related to microwave propagation and microwave heating require special attention to the design of process systems and development of thermal processes in compliance with regulatory requirements for food safety. This article describes the basic microwave properties relevant to heating uniformity and system design, and provides a historical overview on the development of microwave-assisted thermal sterilization (MATS) and pasteurization systems in research laboratories and used in food plants. It presents recent activities on the development of 915 MHz single-mode MATS technology, the procedures leading to regulatory acceptance, and sensory results of the processed products. The article discusses needs for further efforts to bridge remaining knowledge gaps and facilitate transfer of academic research to industrial implementation. © 2015 Institute of Food Technologists®
Article
Almost all mobile communication systems today use spectrum in the range of 300 MHz-3 GHz. In this article, we reason why the wireless community should start looking at the 3-300 GHz spectrum for mobile broadband applications. We discuss propagation and device technology challenges associated with this band as well as its unique advantages for mobile communication. We introduce a millimeter-wave mobile broadband (MMB) system as a candidate next-generation mobile communication system. We demonstrate the feasibility for MMB to achieve gigabit-per-second data rates at a distance up to 1 km in an urban mobile environment. A few key concepts in MMB network architecture such as the MMB base station grid, MMB inter-BS backhaul link, and a hybrid MMB + 4G system are described. We also discuss beamforming techniques and the frame structure of the MMB air interface.
Examining the effectiveness of microwave energy for control of the Colorado potato beetle (CPB) in potato crops begins with an investigation of the dielectric properties of the insect and plant. This paper presents the measured complex permittivity of the CPB and potato plant over the frequency range 100 MHz to 26.5 GHz obtained using coaxial probes and an automatic network analyzer. Specific measurements include the beetle prothorax, thorax, and abdomen as well as the plant leaflet, petiole and stem. An RMS error of less than 0.79 is obtained for both the real and imaginary permittivity over the band for measurements of 0.1 M saline, similar errors are expected for the biological samples.
Article
Radio frequency (RF) dielectric heating was tested to control Crypt testes ferrungineus S. in the bulk wheat samples (ca.152 g, dia. = 50 mm, ht. = 100 mm) at the MCs (%, w. b.) of 12, 15, and 18 using a pilot-scale RF heater (1.5 kW, 27.12 MHz) in the batch mode. When the temperature of the hottest spot (geometric center) of the sample, T-H was at 80 degrees C, all the adult insects were found dead at the cold spots, near bottom-wall, at 50.7 to 56.0 degrees C depending up on the wheat MCs. The temperatures of the insect-slurries higher than that of the bulk wheat by 0.8 to 15.1 degrees C indicated the selective heating of the insects. The mortalities of adult insects were almost constant within the quarantine period, QP1 (5 wk). The elapsed time during QP1 had a significant effect only on the insects' mortalities with the wheat at 12% MC. The wheat MC had only marginal significance on the absolute mortalities of insects. The larvae were completely destroyed at temperatures between 55 and 60 degrees C. The complete mortality of all life stages (eggs, larvae, pupae, and adults) of the insect was achieved at T-H = 80 degrees C without any emergence of the insects during QP2 (8 wk). The RF treatment enhanced the germination of the wheat kernels at 12% MC while it was decreased by 2 to 33% depending up on the wheat MC, and the treatment temperature. Temperature had no significant effect on the falling numbers, and the yields of flour, bran, and shorts, and the peak bandwidth and the MC of the wheat, and the flour protein values. The means of the mixing-development-time deferred from the controls mostly for the wheat at 15% MC and T-H = 70 degrees C, and 18% MC and T-H = 70 and 80 degrees C. The mean-peakheight and the color values varied between 4 and 16%, and 3 and 6% off the controls depending up on the temperatures. The uniform temperature of 60 degrees C should be enough to control all life stages of the insect completely with a little or no changes in the important product qualities and germination of the wheat at MCs safe for the storage. Future research mainly focused on better estimation of the insect-to-grain electric field intensities is essential.