“Head Only”-Exposure of Continuously Growing Rats to 900 MHz GSM Signals Oliver Spathmann, Volkert Hansen, Joachim Streckert, Yi Zhou, Markus Clemens University of Wuppertal Chair of Electromagnetic Theory Wuppertal, Germany
[email protected] Abstract— A relevant number of mobile phone users are children and adolescents. An in vivo study of continuously growing rats investigates the effect of electromagnetic exposure to a GSMmodulated 900 MHz field in consideration of e.g. the cognitive development. During lifetime, the body weight of the animals increases significantly while the brain-averaged specific absorption rate (SARbrain) should be kept as constant as possible. A novel electro-magnetic exposure design was developed to expose up to 48 animals at 4 different excitation levels simultaneously. Keywords-Exposure; GSM; in-vivo; RF; SAR; Numerical dosimetry
I.
INTRODUCTION
In its Agenda 2010 [1], the WHO substantiates the research needs for further animal studies with the “paucity of information concerning the effects of (…) early life exposure to RF EMF on subsequent development and behavior.” Children and young adolescents represent a significant part of the population using mobile communication systems. Therefore, it was decided for the present study to use the widespread GSM900 mobile communication signal in order to investigate the suspected higher sensitivity of young subjects to electromagnetic fields in specific view of development parameters, cognition and behaviour. Due to the necessary long-term (quote WHO:) “partial (head only) exposure (…) at relatively high specific absorption rate (SAR) levels” (unquote WHO) only an in vivo test with a suitable animal model came into consideration. A concept for an experiment under the above given criteria was designed by the Jacobs University Bremen. For the implementation of this concept an exposure system was developed at the University of Wuppertal. This system is able to expose 48 restrained female Wistar rats during growth simultaneously to electromagnetic fields at a frequency of 900 MHz such that a concentrated absorption of energy occurs in the brain of the animals. In the literature, set-ups for the RF head exposure of rodents have been presented since the mid-nineties. An approved concept is the arrangement of a number of restrained subjects on a circle around a common source, e.g. using a carousel [2] or a radial waveguide [3] set-up, and to align the snouts towards the feed. In case of intended brain exposure, such a concept has been criticized by experts, however, arguing
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Karen Grote, Melanie Klose, Alexander Lerchl Jacobs University Bremen gGmbH School of Engineering and Science MoLife Research Center Bremen, Germany
that the sensitive sensors around the rodents’ snouts could be overexposed, thus masking the expected small effects on the brain [4]. Moreover, the specific absorption rate within the brain which lies further ahead in the wave propagation direction would be substantially smaller. Although the arrangement could be shielded against environmental influences by an absorber-lined cover, a further disadvantage is that the different animals are electromagnetically coupled leading to variations of the exposure fields by even slight movements of an animal or due to a vacancy of one or more exposure sites. In order to avoid the frontal field impact onto the animal another concept is to mount a loop antenna onto the restrainer tube directly above the head of the subject [5] [6] [7]. Nevertheless, the use of such an open structure is also disadvantageous, especially if multiple animals are to be exposed, because of field coupling between adjacent antennas and feedback from the environment. In fact, the concept must essentially be regarded as a 1-animal exposure set-up, and the simultaneous application of several of such devices in the same room did require large-area absorber panels to separate the exposure sites from each other in actual experiments [7]. Such effort would make it difficult and expensive to fulfill the demand of a sufficiently high number of exposed animals needed for a statistically profound outcome. Moreover, the antennas’ radiation properties and thereby the exposure fields depend rather critical on the position of the subject [5], i.e. the animals must rigorously be fixed within the tubes what is experimentally impracticable since immobilization contradicts the rules of many ethics commissions worldwide. After a thorough examination of existing concepts for selective head exposure of rodents, developed both in-house [3] [8] and by other groups, the decision was made in favor of a new system.
II.
EXPOSURE CONDITIONS
For the experimental series, the number of 48 animals is split into four groups characterized by specific absorption rates of levels 0 W/kg (sham exposure), 0.4 W/kg, 2 W/kg and 10 W/kg, which are defined as averages over the rats’ brains
(SARbrain) for each group. Regarding SARbrain, the exposure must be arranged in a blinded manner, so that the personnel handling the animals cannot distinguish between the different groups. The exposure system must provide identical environmental conditions, adequate ventilation and easy access to the animals. Due to the appropriate experimental time sequence (e.g. for insertion of the animals), it is suitable to have eight identical exposure devices, each accommodating six animals. Two devices each are assigned to the same exposure group. Exposure is intended to be carried out for up to 2 hours a day for rats from day 14 postpartum up to senescence. Taking this into account, the dimensions of the restrainer tubes must be adjusted repeatedly to the continuously growing animals during the course of the experiment. This involves also the suitable matching of the immission field to the animals, both structurally as well as dosimetrically. III.
DEVELOPMENT OF THE EXPOSURE SYSTEM
Especially with regard to the improvement of the electromagnetic decoupling of the individual exposure units from each other, the new system provides a separate waveguide for each of the 48 animals. Any six waveguides form eight exposure devices and any two exposure devices each are assigned to an exposure group. The underlying idea was to position the aperture of the lower end of a suitably dimensioned upright-standing rectangular waveguide closely above the restrainer tube and to expose the part of the animal’s head containing the brain selectively by using the radiated field from the waveguide. To achieve an optimal coupling of the electromagnetic field with the animal, a flat waveguide design was established and optimized by many numerical test calculations for various waveguide structures using both an FDTD in-house code [9] and the commercial simulation environment CST Studio Suite™ [10]. a)
b)
c)
d)
The photographs in Fig. 1 illustrate the practical implementation of this idea. Fig. 1a shows the vertically oriented flat waveguide made of aluminum with a SMA input connector for the 900-MHz GSM signal. By means of the internal cone antenna (see Fig. 1b) and the shortcut at the waveguide’s upper end the field of the feeding coaxial cable is transformed into the downward propagating fundamental waveguide mode. The inner cross section measures 200 x 10 mm2. The waveguide is extended by an adapter with the same inner cross section and with an appropriate opening at the lower end for the tight transition to a restrainer tube made of Makrolon® which is orientated perpendicular to the waveguide axis. For each waveguide, a set of 5 exchangeable adapters with the respective tubes is available (Fig. 1c) to reach an optimum fit for the age-dependent size of the animal to be exposed. In order to enhance the electrical field strength in the cross-sectional center, a double ridge is formed out parallel to the waveguide axis (Fig. 1b). It continues within the adapter and ends at the tunnel-shaped opening for the docking of the tubes. By this, part of the resulting reflection losses due to the aperture which terminates the adapter (Fig. 1d) and due to the animal’s body can be compensated for. Below the aperture, the guided wave transforms into the immission field: Thus, a fraction of the input power of the waveguide, which is determinable by measurements and numerical dosimetry, penetrates the head of the animal fixed in the tube and exposes the target organ, the brain. 48 waveguides have been produced to build up the total exposure system. As shown in Fig. 1e, any six individual waveguides forming an exposure device are arranged in the shape of a hexagon with the tubes for the animals located at the outside. A base made of PVC material contains ramps as support for the tubes. These ramps were constructed with a slight downward slope in order to allow urine from the animals drain off during exposure time. Each complete exposure device with six waveguides, adapters, tubes and ramps is mounted onto a table with a central opening, letting air stream upwards through the “chimney” formed by the six waveguides with a perforated plate on top. The necessary feed cables for the eight exposure devices (Fig. 1e) are powered from a single source via a distribution network with decoupled outputs which has been installed under the ceiling of the laboratory. The source consists of a signal generator (Rohde & Schwarz SME 03) for the 900 MHz carrier, a modulator (BUW BS825F) for a GSM-typical generic signal [11], and a power amplifier (SSB Electronic PA 9100).
e)
The “chimney” of an exposure device also contains a subdistribution network for the feeding signals of the six waveguides. IV.
Figure 1. Constituent parts of the exposure system: a) Waveguide as exposure unit with adapter and restrainer, b) view into the waveguide’s lower opening, c) five differently sized adapters and restrainers for optimally matching the rats during growth, d) view onto the aperture terminating the adapter, e) built-up exposure system with eight exposure devices and control desk in the center
978-1-4673-0717-8/12/$31.00 ©2012 IEEEt The authors gratefully acknowledge initialization and funding of this project by the German Federal Office for Radiation Protection.
NUMERICAL SIMULATIONS
Electromagnetic field and SAR distributions of the exposure system to be developed, especially the performance of the waveguide-tube-animal sequence, have been simulated prior to construction on the basis of detailed numerical 3D computer simulations. The electric field of the undisturbed eigenmode of the waveguide structure is shown in Fig. 2a in a vertical central plane. It is very similar to the distribution of the TE10 mode in a
a)
b)
|E| / |Emax|
|E| / |Emax|
a)
0.034
0.044
|E| / |Emax|
0.024
wave propagation
0.09
0.1 y in m
c)
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Figure 2. Numerical results of the electric field distribution in a vertical central plane of the waveguide a) for the eigenmode of the structure with a field elevation due to the ridges, b) for the realistic arrangement with the exciting cone-antenna and the shortcut at the upper end
The simulation scenario in Fig. 3a additionally contains the adapter with the aperture at its lower end and a high-resolution dielectric 3D voxel model of a 56-day-old rat which is restrained in a tube and inserted into the adapter. The electric field distribution along the adapter axis shows the continuation of the waveguide field of Fig. 2b. The concentration of the field in the vicinity of the tunnel-shaped opening is obvious, but due to the sudden drop of the field at the boundary to the biological object the field within the rat is not recognizable. Furthermore, Fig. 3b shows the electric field across a longitudinal cut of the rat model. Within the rat’s body a concentration of the field occurs in the area of the head while the environmental field decays rapidly with increasing distance from the aperture due to geometrical attenuation. In addition to the desired selective exposure of the brain this guarantees the electromagnetic decoupling of neighboring animals that are exposed simultaneously. |E| / |Emax|
a)
0
b)
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Figure 3. Numerical results of the electric field distribution in the exposure unit with adapted restrainer tube and 56-day rat model a) in a vertical central plane, b) in a longitudinal cut through the rat’s body
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Brain weight [g]
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Model 3
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b)
10 log (SAR/SARmax) [dB]
In Fig. 2b the field excitation with the cone-antenna and with the shortcut is simulated showing the transition from the disturbed electric field at the rf input to the characteristic fundamental mode’s transverse field distribution near the connection plane to the adapter. The field is superimposed here by the reflected wave from the still open end of the waveguide.
In Figs. 4a and 4b results are presented for computed SAR distributions in the 56-day rat model with a voxel resolution of 0.74 mm. The SAR values refer to the mass of local tissue elements of voxel size, which have been related to the maximum SARmax occurring in the head. They are displayed in a logarithmic scale. Fig. 4a shows a cross-sectional view through the head. When averaged over approx. 2 g of tissue the highest SAR lies in the upper half of the head where the brain is located. Likewise, the SAR distribution in the longitudinal cut of the model (Fig. 4b) demonstrates that the goal of an exposure aiming mainly at the brain region is obtainable with the novel design of the exposure device. 10 log (SAR/SARmax) [dB]
standard waveguide, but with an elevation of the field strength along the axis due to the narrow region between the ridges, see upper part of Fig. 2a.
Model 2
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Literature
Model 1 0.5 0.0 0
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Age [days]
Figure 4. Numerical results for the SAR distribution a) in a cross-section through the 56-day rat’s head, b) in a longitudinal cut through the rat’s body. c) Brain weights of the 11 voxel models compared to the literature weights
For the numerical dosimetry of 14-day-old rats up to adult animals, the use of a single scaled voxel model is inadequate. Therefore, a total of four dielectric voxel models derived from MRI-scans of rats at the age of 14, 28, 49 and 140 days have been generated as base models [12]. Furthermore, these basic models were extrapolated with respect to the brain weight development described in the literature [13] providing a total of 11 voxel models for the numerical dosimetry simulations. A comparison of the literature brain weights and the ones of the used models is shown in Fig. 4c. V.
QUALITY CONTROL
During the fabrication of the experimental set-up numerous measurements were carried out as a production control in a comprehensive test program in order to ensure a constant quality of the finished waveguides and adapters. When manufacturing the 48 aluminum waveguides, first of all 24 waveguides of double length were milled and equipped with two coaxial connectors and cone antennas representing an input and an output port. A prototype is shown in Fig. 5a. The aluminum foil on the top side was used to close holes in the plates which had been drilled to measure the electric field distribution inside the waveguide with a monopole field probe during the process of development. For the 24 waveguides a quality control was established following a rigorous test pattern
including full S-parameter measurements. A typical measurement report logged by a network analyzer (Agilent E8363A) for the S11 parameter is given in Fig. 5b showing an example for the frequency dependance of the input reflection with a value of approx. –20 dB around 900 MHz. Only waveguides with reflectivities smaller than –12 dB were accepted for further consideration.
rare cases, deviations from the expected result appeared. Then, data were taken at a number of points or even for the full matrix in order to analyze the problem. Lastly, 5 waveguides could be overhauled to deliver the standard results while 2 waveguides had to be exchanged due to material defects.
b)
a)
a) waveguide
adapter
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z = -10 mm
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Figure 5. a) Prototype of a symmetric waveguide of double length with input and output plugs, b) exemplary protocol of an S11 measurement
Subsequently, the waveguides were cut in the middle at a right angle to the wave propagation direction, thereby creating in each case two waveguides of the kind as already shown in Fig. 1a. Before being approved as exposure units, any of the 48 waveguides had to undergo also electric field strength measurements in order to check the compatibility with the field patterns aimed at. Fig. 6a shows the test setup with a calibrated field probe (SPEAG ER3DV6) connected to a measuring system (SPEAG Easy4) and with a single waveguide with adapter mounted to an adjustable holder. The whole arrangement is placed on an absorber panel to reduce the reflections from the laboratory bench. The electric field strength was measured at precisely defined locations below the adapter for an input power of 0.5 W. The measuring matrix covered 73 positions indicated as dots in Fig. 6b. In Fig. 6c measured (blue line) and numerically calculated (red stars) results are compared for the electric field strength along a line 10 mm below the adapter (indicated by the red colored dots in Fig. 6b. Since simulation and measurement showed an excellent fit for the first tested waveguides, only a central measuring point below the waveguide was routinely used for the next tests yielding a standard deviation of 5.8%. In some
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Figure 6. a) Test arrangement with device under test, measuring probe and absorber panel at the bottom, b) schematic measuring matrix indicated by black and red dots, c) comparison of measured and calculated results for the electric field strength at an input power of 0.5 W
In order to reach most constant exposure conditions for the group of animals with the highest applied SARbrain of 10 W/kg, 12 waveguides have been selected under the consideration of a uniform relation between input power and electric field strength. The selected waveguides show a standard deviation of only 0.95%. VI.
ADJUSTMENT OF INPUT POWER
In order to achieve a constant SARbrain during the exposure of the continuously growing rats a number of adjustments were necessary for the input power at the input of the waveguides under consideration of the previously mentioned numerical simulations. In Tab. I the brain weights and the calculated input powers for achieving the required SARbrain = 10 W/kg are summarized. Along with the change of the input power the waveguide adapters had to be changed.
TABLE I. CALCULATED INPUT POWER AT THE WAVEGUIDE PORTS FOR ACHIEVING SARBRAIN = 10 W/KG FOR RATS OF DIFFERENT AGE UP TO 140 DAYS AND CORRESPONDING ADJUSTED POWER
Rat models Age [days] Model 1: 14 Model 2: 21 28 35 Model 3: 42 45 49 56 70 Model 4: 90 140
Brainweight [g]
Calculated input power for 10 W/kg [W]
Adjusted average input power [W]
1.09
0.4
0.4
1.51 1.68 1.79
0.42 0.36 0.37
1.74 1.80 1.85 1.88 1.91
0.39 0.43 0.44 0.43 0.42
0.42
1.86 2.05
0.48 0.48
0.48
0.38
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[3] [4]
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VII. BIOLOGICAL RESULTS Since the exposure (start on December 04, 2010) and the regular behavioral tests with the animals are still running and the blinding is not yet broken, final biological results cannot be given so far with regard to the development parameters mentioned at the beginning. Out of the 96 exposed and shamexposed rats (the exposure system is used twice a day for 2x 48 rats) and of the 24 cage controls all animals have survived the first year of exposure. The differences of the average body weight between (sham-)exposed animals and the cage controls show a maximum of 7% at their actual age of about 400 days, shown in Fig. 7. This difference, albeit small, is considered to be a consequence of restraining the animals for 2 hours per day [14]. Whether the restraint has an effect on the animals’ performance in the different behavioral tasks (memory, spatial orientation, open field, and Morris water maze) will be seen after completion of the study.
Figure 7. Measured mean body weight of experimental rats, blue: cage controls, red: exposed / sham exposed
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[13]
[14]
WHO Research Agenda for Radiofrequency Fields, 2010, p. 17, http://whqlibdoc.who.int/publications/2010/9789241599948_eng.pdf. M. Burkhardt, Y. Spinelli, N. Kuster: Exposure setup to test effects of wireless communications systems on the CNS. Health Phys 73: 770– 778, 1997. V. Hansen, A. Bitz, J. Streckert: RF exposure of biological systems in radial waveguides. IEEE Trans EMC 41: 487–493, 1999. Workshop FGF: The Blood-Brain-Barrier - Can it be influenced by RF field interactions? Reisenburg, Germany. Forschungsgemeinschaft Funk/COST 281, 2003. C. Chou, K. Chan, J. McDougall, A. Guy: Development of a rat head exposure system for simulating human exposure to RF fields from handheld wireless telephones. Bioelectromagnetics 20: 75-92, 1999. P. Leveque, C. Dale, B. Veyre, J. Wiart: Dosimetric analysis of a 900MHz rat head exposure system. IEEE Trans MTT 52: 2076-2083, 2004. D. Dubreuil, T. Jay, J. Edeline: Head-only exposure to GSM 900-MHz electromagnetic fields does not alter rat's memory in spatial and nonspatial tasks. Behav. Brain Res., 145, 51-61, 2003. N. Prochnow, T. Gebing, K. Ladage, D. Krause-Finkeldey, A. El Ouardi, A. Bitz, J. Streckert, V. Hansen, R. Dermietzel: Electromagnetic field effect or simply stress? Effects of UMTS exposure on hippocampal longterm plasticity in the context of procedure related stress hormone release" PLoS ONE 6(5): e19437, 1-13, 2011. A. Bitz.: FDTD program code, University of Wuppertal, 2002. CST Suite 2010TM, CST AG, Bad Nauheimer Straße. 19, 64289 Darmstadt, Germany, www.cst.com M. Schüller, J. Streckert, A. Bitz, K. Menzel, B. Eicher: Proposal for generic GSM test signal. Proc. 22st BEMS Annual Meeting, Munich, Germany, 2000, 122-123. E. Hahn Institute for Magnetic Resonance Imaging, UNESCO World Cultural Heritage Zollverein, Arendahls Wiese 199, 45141 Essen, Germany, www.hahn-institute.de. D. Morgan, H. Price, R. Fernando, S. Chanda, R. O’Connor, S. Barone, D. Herr, R. Beliles: Gestational mercury vapor exposure and diet contribute to mercury accumulation in neonatal rats, Environ. Health Perspect. 114: 735-739, 2006. A. Lerchl: To restrain or not to restrain animals in RF_EMF exposure settings. Frequenz 63: 144-146, 2009.
