in vitro exposure setups for rf exposures at 900 mhz

Chapter 7

In Vitro ExposureSetups for RFExposures at 900 MHz

83

84 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ

SUMMARY

In the context of the European research programs, a refer-ence setup based on waveguides has been developed for theconduct of standardized in vitro experiments at the mobilefrequency bands of 835MHz and 935 MHz. Its performanceis compared to those apparatus which are widely applied toexpose cells settled as a monolayer on the bottom of a Petridish as well as for cells in suspension. The key figures of meritare uniformity of SAR, temperature load, power and volumeefficiency as well as ability to ensure tight exposure and envi-ronmental control. For the exposure of cell monolayers, thewaveguide setup with Petri dishes in the H-field maximum issuperior in all respects. Reasonable performance is achievedwith the resonator and transverse electromagnetic (TEM)cell. The best performance for cell suspensions is providedby the TEM cell for flasks in k-polarization. Acceptable per-formance is also achieved with the waveguide system andresonator. The thermal load is an issue of concern for allsetups. Measures to suitably control the temperature loadand to maintain equal temperature profiles between expo-sure and sham have been implemented for the waveguideand TEM cell setups. Localized temperature “hot spots” inthe cell medium can be excluded for all investigated systems.

7.1 Introduction

The assessment of the health risk posed by radio frequency (RF) elec-tromagnetic field (EMF) exposures will be based mainly on epidemio-logical studies supported by in vivo results. In vitro studies are fasterand cheaper but of less direct relevance to human health. Neverthe-less, they are fundamental for detecting possible risk factors as wellas sites and mechanisms of interactions. Although in vitro experi-ments can be replicated with less effort than in vivo or epidemiolog-ical studies, unsatisfactory and conflicting results have emerged fromthese studies. In many cases the contradictions could not be resolved,since the exposure parameters regarding detailed exposure character-ization, dosimetry and thermal loads or hot spots were insufficiently

7.1. INTRODUCTION 85

described. It is obvious that well characterized exposure including acomprehensive analysis of possible artifacts is a prerequisite for theinterpretation and repeatability of experiments. The minimum re-quirements were described in [68].

The task to uniformly expose a few centiliters of cell medium atthe level of occupational exposure has always been underestimated.Some authors used the incident field levels as exposure merit and ne-glected that the coupling mechanism to Petri dishes is fundamentallydifferent than that for human exposure and strongly depends uponthe polarization of the incident fields, flask, amount of medium, loca-tion of the flask with respect to the exposure chamber and effects ofthe medium on the incident field [17], [39], [105]. Also underestimatedwas the meniscus [107] and the temperature load. Tight temperaturecontrol is not possible without active cooling, even for exposure levelsas low as 1 W/kg.

An exception is the exposure of cell monolayers inside Petri dishesby E-polarization [17], [106]. This setup results in the most uniformcell exposure and a low thermal load [109], enabling the exposure ofcells to levels above 2W/kg with air cooling only.

Schonborn et al. [105] presented a qualitative comparison betweenthe main concepts of setups, including TEM cell, HF chamber, RadialTransmission Line, Waveguide and Wire-patch cell. The purpose ofthe current study was to conduct a quantitative comparison betweenthe setups used in the European studies REFLEX and PERFORMB, which were also applied in other national programs. The refer-ence setup presented here is a newly developed exposure unit basedon waveguides (WG setup). The performance is compared to an en-hanced Wire-patch cell (WP cell) originally presented by Laval et al.[73], a TEM cell utilizing k-polarization [92] and the resonator withwater cooling developed by Toivo et al. [115] for E-polarization. Theperformance parameters of the first three setups have been analyzedby the methodology described hereafter. The values for the resonatorsetup were derived from the literature as far as available.


7.2 Performance Parameters

The setups were compared with respect to the following performanceparameters:

Uniformity of SAR: The SAR distribution over the entirevolume of cells should be as uniform as possible.

Temperature Load: Any in vitro experiments focussing onnonthermal effects should be performed without significant tem-perature differences (< 0.1C) between the exposed and shamgroups. Therefore, the maximum temperature increase togetherwith its spatial distribution was analyzed.

Power Efficiency: The power efficiency describes the ratiobetween the average SAR and the input power at the feed ofthe exposure system. Low efficiency is often a reason for highcosts, because a high power amplifier is required.

Volume Efficiency: The volume efficiency describes the ratiobetween the volume of medium versus space requirements forthe entire exposure unit. The volume is an issue since if thevolume efficiency is poor then the setup cannot be used withinan incubator, which would increase the effort for environmentalcontrol.

Provision of Exposure and Environmental Control: Thequality of experimental data depends on the documentation ofthe conditions during the experiment. Stable RF exposure ofcells depends on many details which are difficult to keep wellcontrolled, such as amplifier drift, frequency drift, changes of RFcable and connectors, slight mechanical change of the RF cou-pler, location of the flasks with respect to exposure unit, changesof the dielectric properties of the medium, amount of medium,changes in the meniscus profile and many more. Therefore, welldefined mechanical properties of the exposure chamber and con-tinuous monitoring of exposure and environmental conditions isa prerequisite for high quality experiments.

7.3. EXPOSURE SYSTEMS 87

7.3 Exposure Systems

7.3.1 Waveguide Setup sXc900

The WG setup “sXc900” (sXc = systems for eXposure of cells) is anadaptation of the sXc1800 system described in [109]. It was originallydeveloped for cell monolayer exposures and was later extended for theexposure of cells in suspension. The setup is based on two rectangularR9 waveguides in which only the fundamental TE01 mode can prop-agate for the frequency bands of interest, i.e., 800 MHz - 950 MHz.The dimensions are given in Figure 7.1. In order to guarantee thesame environmental conditions (temperature, humidity, CO2/O2) forthe exposed and sham cells, both waveguides are placed inside thesame incubator and a common location for the air inlet was realized.The latter is necessary since incubators can have a large temperatureprofile of several tenths of a degree.

In contrast to the narrow-band cavity of the sXc1800, broad bandcoax-to-waveguide adapters combined with 3-stub tuners (Maury Mi-crowave Cooperation, 1878B) were used to enable operation at 835and 935 MHz. Consequently, the power efficiency of the setup is con-siderably lower than for sXc1800, due to the losses in the adapter,cables and tuner.

The larger cross-section of the R9 waveguide allows the placementof eight Petri dishes per field maximum, compared to only two dishesin the R18 waveguide. Cell monolayers are exposed in the H-fieldmaximum and cell suspension in the E-field maximum. The Petridishes are arranged in two towers, each with four dishes (Figure 7.2).

The system was optimized and the dosimetry provided for thesame Petri dish arrangements as for sXc1800:

Cell Monolayers: 35 mm Petri dishes (effective inner diameter33 mm) filled with 2.2 - 4.9 ml of DMEM medium (Dulbecco’sModified Eagle’s Medium);

Cell Suspensions: Petri dishes exposed in E-polarization providehigher uniformity of exposure when placed in a large Petri dishfilled with water or medium, due to the strong coupling at thesides of the dishes [73]. The setup was optimized for 3.1 ml


!

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Figure 7.1: Geometry and functional parts of the sXc900 waveguidesetup. The total setup dimensions are 540 x 500 x 580 mm3 (height xwidth x depth). Distances of dishes to short cut and waveguide centerare given in Table 7.1.

DMEM in the inner 35 mm Petri dish and 4.9ml distilled waterin outer 60 mm Petri dish (effective inner diameter 54mm).

As for sXc1800, tight exposure and environmental control was re-alized using field sensors, temperature sensors for the air environment,and an optimized airflow system. The cooling effectivity was enhancedby two strong fans per waveguide (Papst, 4212NGN: air flow 165m3/h)combined with two side plates at the dish holder to channel airflow tothe area of the Petri dishes. The distances of the dishes to the shortand to the waveguide center are given in Table 7.1.

The field sensor is based on a loop antenna (diameter 4 mm), lo-cated in the center of the short (H-field maximum independent offrequency).

The signal unit of the WG setup is described in [109]. It providesflexible modulation schemes (GSM Talk/DTX, DAMPS, etc.), con-

7.3. EXPOSURE SYSTEMS 89

Figure 7.2: Dish holders for sXc900. 35 mm diameter Petri dishesare used for monolayer exposure. 35 mm dishes placed inside a 60mmdish filled with distilled water are applied for suspension exposure.

tinuous field control and regulation (update rate 0.1 Hz) and allowsblinded protocols with high data security.

7.3.2 Wire-Patch Cell

The Wire-Patch Cell (WP cell) is an open radiating setup operated at900 MHz. Its compact design is based on a 150mm x 150 mm parallelplate configuration (distance 29 mm), short-circuited at the edges byfour plots [73]. Eight 35 mm Petri dishes (placed inside a 60 mm dish

Distance Petri dishes [mm] 835 MHz 935 MHz

to short (monolayer) 260 211

to short (suspension) 119 96

to waveguide center (monolayer) 30 30

to waveguide center (suspension) 39 39

Table 7.1: Position of the Petri dishes inside the R9 waveguide withinner dimensions of 125 x 250 x 520mm3 (height x width x length).


with distilled water) are arranged symmetrically around the centralcoaxial feed. Since the WP cell is an open setup, exposure and shamgroups need to be placed in two different incubators. The systemin the investigated configuration has no fan nor other cooling. Forthe purpose of this study, the original system was enhanced by (1)distance keepers for the Petri dishes, (2) optimized Petri dish loading(to reduce the thermal load, distilled water instead of cell mediumis used for the 60mm dish), (3) E-field sensors for monitoring andregulation and (4) a computer-controlled signal unit, allowing blindedexposure protocols [109].

Research activities to overcome the low sham isolation and thehigh thermal load of the WP cell were recently presented by Pintoet al. [93]. Systems for the exposure of four Petri dishes at 900 and1800 MHz were developed. Shielding is realized with metallic gridsthat isolate two independent WP systems placed within the sameincubator. Contact cooling of the dish bottoms is applied by watercooling of the ground plate.

7.3.3 TEM Cell

Schonborn et al. [105] have shown that reasonable uniform exposures(SD: <40%) can also be achieved with k-polarization. An elaboratedsetup based on commercial TEM Cells (Instruments for Industry Inc.,IFI 110) and setup performance for exposure of cells in suspension at835 MHz was presented in [89]. The setup consists of two modifiedTEM cells that fit inside an incubator. T25 flasks (25 cm2) filled with5 ml of DMEM medium are used. An air flow system for cooling andfast atmospheric exchange is applied. A signal unit similar to thatof the sXc setups enables monitoring and blinded protocols. In thecontext of this study, the performance of the setup for cell monolayershas been evaluated in addition to the data presented in [89].

7.3.4 Resonator Setup

The resonator setup consists of a short-circuited waveguide chamberat 900 MHz [115]. Four 60 mm diameter Petri dishes are exposed ina standing wave E-field maximum in E-polarization. The dishes areplaced on a glass plate, which is water cooled from below. In this way

7.4. METHODS 91

835 MHz 935 MHZ 900 MHzεr σ εr σ εr σ c k ρ

Medium 72 2.1 72 2.1 72 2.3 4187 0.55 1000Water 73 0.11 73 0.15 74 0.14 4187 0.55 1000Plastic 2.5 0.0004 2.5 0.002 2.6 0.001 900 0.16 1200

Table 7.2: Material parameters used in the simulation (εr: relativepermittivity, σ: electric conductivity [S/m], c: specific heat capacity[J/kgK], k: thermal conductivity [W/Km], ρ: mass density [kg/m3]).

a temperature stability of ±0.3C over the range from 0 - 10W/kgaverage SAR is achieved [115]. The setup is operated outside of anincubator, whereby an air flow system connects the exposure areaand the incubator atmosphere. A monopole sensor is used to monitorthe incident fields. The setup does not allow for blinded exposureprotocols.

7.4 Methods

7.4.1 Optimization and Analysis Methods

The WG setup, the WP cell and the TEM cell were developed andcharacterized in compliance with the guidelines recommended by [68].Similar dosimetric methods as described in [109] have been appliedto characterize the exposure conditions. The performance parametersfor the resonator setup were derived from the literature but are notcomplete. A numerical electro-thermal FDTD analysis was performedwith the simulation platform SEMCAD. The numerical models, witha voxel resolution of 0.4 mm, contained meniscus profiles for all liquidsurfaces. Menisci were modelled according to the functions given in[107]. The material parameters used in the simulation are listed inTable 7.2. Several medium volumes in the range from 2.2ml - 4.9 ml(2, 3, 4, 5 mm liquid height) were simulated for the monolayer expo-sure of the WG setup. A vertical extrapolation of SAR to the cellmonolayer was applied [109]. For the suspension exposure in the WGsetup, for the WP cell and for the TEM cell, only one geometry wasanalyzed (WG setup & WP cell: 3 ± 0.2ml DMEM and 5 ± 0.2ml


[W/Km2] WG setup (Ml.) WG setup (Su.) WP cell

hdish−air 70 25 4.5

hmedium−air 7.1 4.5 4.5

hwater−air - 25 4.5

Table 7.3: Numerically applied heat transfer coefficients for modelingof thermal boundaries. Models for WG setup monolayer exposures(Ml.), suspension exposures (Su.) and for the WP cell were analyzed.

distilled water; TEM cell: 5ml DMEM). Monolayer SAR values forthe WP cell and TEM cell were derived by averaging all voxels fromthe lowest medium layers (z = 0.2mm). For suspension exposures,the SAR was evaluated (1) for the entire medium, (2) the mediumseparately from the meniscus, and (3) for only the meniscus. FDTDcorner voxels at the curved dish media interfaces of the Petri disheswere not considered for all SAR evaluations, because they are sensi-tive to numerical staircasing artifacts. The total amount of neglectedmedium is less than 0.1 ml.

The temperature distribution was analyzed for a numerical modelconsisting of only one Petri dish or flask. A standing wave field ex-posure at 900 MHz was applied by two incident plane waves resultingin a similar SAR distribution as for the WG and WP setups. Heattransfer due to conduction, radiation, natural convection and forcedconvection was considered with the same methods as described in[109]. The combined heat transfer coefficients for the thermal bound-ary modeling are given in Table 7.3. Note that due to the missing aircooling of the WP cell much less heat transfer via the dish and waterinterfaces is present. The simulations were validated by single-pointtemperature measurements (see below).

7.4.2 Experimental Verification Methods

The experimental evaluation of the system was performed with theDASY3 near-field scanner (SPEAG), equipped with free space anddosimetric probes. A 1 mm diameter E-field probe [95] was used tomap a vertical line in the center of the cell media (WG setup mono-

7.5. RESULTS 93

layer: 4.9ml DMEM, WG setup suspension & WP cell: 3.1± 0.1mlDMEM).

The temperature load was determined with resistive thermistorprobes [109]. The data for the temperature response of the mediumwas used for (1) determination of the local SAR by evaluation of thefirst 50 s of exposure, (2) determination of the maximum thermal loadat the steady state temperature and (3) validation and optimizationof the numerical simulations (heat transfer coefficients for convectionwere adjusted to fit the measurement data).

7.5 Results

7.5.1 Performance Summary

The results for the performance of the investigated exposure systemsare shown in Table 7.4 for cell monolayer and in Table 7.5 for cellsuspension exposures. Quantitative data for the nonuniformity of theSAR distribution, the temperature load, the power efficiency and thevolume efficiency are given. Experimental and environmental controlis qualitatively evaluated.


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7.5. RESULTS 95

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Table 7.5: Performance summary for cell suspension exposures with WG

setup, WP cell, TEM cell and resonator.

Figure 7.3: SAR distribution for cell monolayer exposure with theWG setup at 835MHz. (a) Vertical planes in the dish center for threemedium volumes are shown. (b) The lowest horizontal plane in themedium (z = 0.2mm) is shown for the two upper dishes.

Figure 7.3 shows the SAR distribution for the monolayer exposureof the WG setup. Vertical planes in the center of the Petri dishare shown for three liquid volumes (Figure 7.3a). The amount ofmedium inserted must be carefully controlled since the SAR stronglydepends upon the medium height. Most of the monolayer area iswithin a 2 dB radius from the maximum SAR value (Figure 7.3a). Cellmedium internal E-field and H-field are perpendicular to each otherand polarized in the plane of the monolayer. The field impedance islow.

The exposure of cells in suspension with the WG setup and theWP cell result in similar SAR distributions (Figure 7.4). SAR valuesare 4 − 5 times higher in the meniscus as for the rest the medium

7.5. RESULTS 97

Figure 7.4: SAR distribution for the WG setup for cell suspensionexposure (at 835 MHz) and for the WP cell (at 900 MHz). A verticalplane in the dish center is shown.

(Table 7.5). Uniformity of SAR is considerably enhanced if the cellsare not located in the meniscus area. Therefore, the sedimentationbehavior of the cells must be analyzed. Exposure should be startedafter a sufficient time period in which the cells have settled from themeniscus. In contrast to the other exposure the internal E-field ispolarized perpendicular to the medium surface or monolayer, whereasthe H-field is parallel. The field impedance is also low.

Figures 7.5 and 7.6 show the simulated SAR and steady statetemperature distributions for the single dish models. Temperaturebecomes distributed without areas of localized “hot spots” for all se-tups. It can be seen that the temperature probe was placed insidethe temperature maximum for the WG setup in monolayer exposure(Figure 7.5) but not for cell suspension configuration nor the WP cell(Figure 7.6). Therefore, the measured temperature load values in Ta-ble 7.5 were multiplied by the simulated ratio of maximum to sensorvalue, ratios 1.3 (WG setup), 1.1 (WP cell) and 1.1 (TEM cell). De-


Figure 7.5: Simulated SAR and steady state temperature distributionfor the single dish model at 900 MHz for monolayer exposure of theWG setup (start temperature: 37C). The same SAR values as forthe measurement are used in the simulation. The location of thetemperature probe used for the experimental assessment is indicatedin the figure.

tailed dosimetry data for the WG setup at 835 MHz and 935 MHz andfor different cell medium volumes are given in Table 7.6.

7.5.2 Uncertainties and SAR Verification

The uncertainties for the SAR assessment and SAR variabilities wereevaluated according to [114]. Estimated standard deviations wereused. Table 7.7 summarizes the investigated parameters for the un-certainty of the SAR assessment of the WG setup and WP cell (forthe TEM cell refer to [89]. Also given in the table are the deviations

7.5. RESULTS 99

Figure 7.6: Simulated SAR and steady state temperature distribu-tion for the single dish model at 900 MHz for the WG setup sus-pension exposure and for the WP cell. The same SAR values as forthe measurement are used in the simulation (WG setup: 7.8 W/kgaverage SAR (without meniscus evaluation), SARmax = 10W/kg;WP cell: 4.6 W/kg average SAR (without meniscus evaluation),SARmax = 5W/kg). The start temperature of thermal simulationwas 37C. The location of the temperature probe used for the exper-imental assessment is indicated in the figures.

between the SAR measurements and simulation. For the WG setupan average deviation of 15% for the cell monolayer configuration and17% / 8% for E-field / temperature evaluations in suspension werefound. The corresponding deviations for the WP cell are 25% / 7%.


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setu

pm

onola

yer

exposu

reand

from

1.2

-2.4

mm

for

WG

setu

psu

spen

sion

exposu

reand

for

WP

cell.

Tab

le7.

7:U

ncer

tain

tyan

alys

isfo

rW

Gse

tup

mon

olay

er(M

l.),s

uspe

nsio

n(S

u.)

and

WP

cell

expo

sure

s.U

ncer

tain

ties

for

the

aver

age

SAR

asse

ssm

ent

are

com

pare

dto

the

devi

atio

nsbe

twee

nm

easu

rem

ent

and

sim

ulat

ion.


Var

iability

Anal

ysi

sof

SA

RW

G(M

l.)W

G(S

u.)

WP

cell

Eva

pora

tion

ofdi

still

edw

ater

(max

.2m

l)a

-19

%26

%U

sing

aco

ver

for

60m

mPet

ridi

sh-

2.4%

-D

eter

min

atio

nof

med

ium

volu

me

(±5µ

l)0.

3%<

0.1%

<0.

1%D

ish

hold

erm

ispl

acem

ent

(±2m

m)

0.1%

1.6%

0.6%

Inci

dent

field

asse

ssm

ent

1.4%

1.4%

1.4%

Dri

ft0.

5%0.

5%0.

5%C

ombi

ned

rela

tive

vari

abili

ty(R

SS)

2.1%

19(3

.2)%

b26

(1.6

)%b

Non

unifor

mity

ofSA

Rc

17-

29%

54%

27/

59%

aC

an

be

elim

inate

dby

usi

ng

hig

hhum

idity

incu

bato

ren

vir

onm

ent

or

pla

cing

aco

ver

on

top

ofth

eoute

r60

mm

dis

h.

bValu

ein

bra

cket

sco

rres

ponds

toco

nfigura

tion

wit

hlid

on

top

ofth

e60

mm

dis

h.

cN

onuniform

ity

isdep

enden

tupon

med

ium

volu

me

for

sXc

(Ml.)

and

upon

monola

yer

or

susp

ensi

on

exposu

refo

rth

eW

Pce

ll.

Tab

le7.

8:SA

Rva

riab

ility

anal

ysis

for

WG

setu

pm

onol

ayer

(Ml.)

,su

spen

sion

(Su.

)an

dW

Pce

llex

posu

res.

7.6. DISCUSSION 103

Figure 7.7 shows the data for the E-field measured SAR values ofthe WG setup (SAR was evaluated in the two upper dishes). Whilegood agreement was achieved for the cell monolayer configuration,boundary and immersion depth errors are present for suspension andWP cell measurements. The difficulties arise from the geometricalconfiguration, whereby the field probe needs to be positioned parallelto the incident as well as the induced E-fields. This leads to (1) fieldenhancements when the probe tip comes close to the dish bottom, (2)immersion depth errors when the tip comes close to the medium topand (3) coupling and scattering at the holes that were milled in orderto position the probes. Nevertheless, measurement and simulationcome close to each other in the area of the medium center, which isfree of artifacts.

The deviations between simulation and measurement are in therange of the combined uncertainty for the SAR assessment (Table 7.7):WG setup (Ml.): 15% vs. 20%, WG setup (Su.): 17% vs. 21%, WPcell: 25% vs. 21%. Therefore, the measured results can be regarded asa verification of the numerical techniques and support the reliabilityof the dosimetry.

The variability of SAR is summarized in Table 7.8 and set intorelation to the nonuniformity of the SAR distribution.

7.6 Discussion

The performance of the different exposure systems are discussed inthis section:

7.6.1 Cell Monolayer Exposure

The WG setup provides the best performance for cell monolayers.High uniformity of SAR (deviations: 20%) together with a low thermalload (∆T = 0.02C/W/kg) is achieved (Table 7.4). Due to the SARdistribution with its maximum at the dish boundary, air cooling isvery efficient and allows exposures up to 5 W/kg without significanttemperature load (∆T < 0.1C). The efficiency could be greatlyimproved by internal tuning. Additionally, the WG setup fulfills allrequirements for exposure and environmental control and provides low


Figure 7.7: SAR along the center of the medium for the two up-per dishes (towers 1 and 2). E-field measured data is compared toFDTD simulations at 835 MHz for the monolayer configuration (4.9 mlmedium, 5 mm liquid height) and suspension configuration (3.1 mlmedium, 3 mm liquid height) of the WG setup. z = 0 correspondsto the dish/medium interface.

variability. Liquid cooling, based on an external medium at a differenttemperature is not required.

The same is true for the TEM cell, which also provides reasonablygood uniformity. Without active cooling, the WP cell can only beapplied for exposures with SAR values up to 0.4 W/kg (∆T < 0.1C).

Active cooling has been introduced for the resonator and WP cell,thus overcoming some of these disadvantages. However, the potentialengineering problems to guarantee the same cooling effectivity andthe same absolute temperature for the cells in sham and exposureshould not be underestimated. Passive systems are less susceptible topossible malfunctions.

All systems have a comparable exposure area of about 100 cm2.

7.7. CONCLUSIONS 105

7.6.2 Cell Suspension Exposure

Uniform exposure of cells in suspension is an issue for all investigatedsystems. None of them fulfills the requirement of less than 30% devia-tion from uniformity of SAR as formulated in [68]. However, the TEMcell provides good exposure performance: nonuniformity of SAR: 46%,thermal load: 0.05C/W/kg and power efficiency: 6 W/kg/W (Ta-ble 7.5). The necessary exposure and environmental control is alsoprovided.

Reasonable performance is achieved with the WG setup for thecase of cells not in the meniscus. Then a nonuniformity of 54% ispresent at a temperature load of 0.12C/W/kg (Table 7.5). Evapo-ration of distilled water in the 60 mm diameter dishes for WG andWP exposures represents a source of considerable SAR variation (Ta-ble 7.8: 19 - 26%). It can be compensated by using a high humidityincubator environment or using a cover for the 60 mm dishe.

High SAR values for cell suspensions can be achieved with theresonator, due to its water cooling system (up to 10 W/kg with a sta-bility of ±0.3C). However, to quantify the resonator performance indetail, an evaluation of the uniformity of the exposure for cell mediummodels with meniscus profiles is required. The resonator should beupgraded to allow blinded protocols.

Also for cell suspension, the WP cell without cooling cannot berecommended.

The exposure volume of about 20 ml is comparable for all systems.

7.7 Conclusions

In this report different in vitro exposure setups have been dosimet-rically analyzed for their performance with respect to uniformity ofSAR, temperature load, power efficiency, volume efficiency and expo-sure/environmental control. The investigated setups were the novellydeveloped sXc900 waveguide setup, Wire-Patch cell [73], TEM cellin k-polarization [92] and a resonator setup with water cooling [115].The analysis revealed that the waveguide system provides superiorperformance for the exposure of cell monolayers. The key parametersare the low nonuniformity of SAR (20%) and the low temperature load


(0.02C per W/kg), while providing full exposure and environmentalcontrol. Uniformity of SAR is a fundamental issue for cell suspen-sions. Here, the best performance is given by the TEM cell. Thenonuniformity of SAR of 46% and the temperature load of 0.05Cper W/kg still represents reasonably good exposure conditions. Therequired uniformity of less than 30% [68] was not achieved by any ofthe setups for cell suspensions. The Wire-Patch cell for the investi-gated configuration without cooling cannot be recommended, due toits dominant temperature load. Water cooled systems, TEM cells,WP cells and resonators must be carefully analyzed before utilizationfor in vitro experiments with high SAR exposures.

in vitro exposure setups for rf exposures at 900 mhz

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