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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.
86 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ
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
88 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ
!
" !#
$$ ! %!
&
'
!
!
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).
90 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ
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
92 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ
[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.
94 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ
WG
setu
paW
Pce
llT
EM
cella
Res
onat
orN
onunifor
mity
ofSA
R[%
]M
onol
ayer
eval
uati
on20
2739
-Tem
per
ature
load
[C
/(W
/kg)]
Sing
lepo
int
mea
sure
men
t0.
020.
230.
040.
03b
Max
imum
acco
rdin
gto
sim
ulat
ionc
0.02
0.25
0.05
-Pow
ereffi
cien
cy[W
/kg
/W]
Mon
olay
erev
alua
tion
1.3
0.84
6.1
≈7
Vol
um
eeffi
cien
cy[c
m2/l
]d
Mon
olay
erev
alua
tion
80/
160
80/
210
0/
8010
0/
>20
0Expos
ure
/en
vir
onm
enta
lco
ntr
ol yes
/ye
sye
s/
noe
(yes
)f/
yes
yes
/no
g
aD
ata
for
835
MH
zis
giv
en.
bSta
bility
of±
0.3C
up
to10
W/kg
aver
age
SA
Ris
report
edin
[115].
cSin
gle
poin
tm
easu
red
valu
esw
ere
scale
dby
sim
ula
ted
ratios
bet
wee
nm
axim
um
tose
nso
rte
mper
atu
re.
dA
vailable
surf
ace
for
cell
monola
yer
[cm
2]ver
sus
setu
pvolu
me
[l].
eE
xposu
reand
sham
gro
up
are
not
insi
de
the
sam
ein
cubato
r.f E
xposu
reco
ntr
ol
with
apow
erm
easu
rem
ent
isonly
reliable
for
const
ant
reflec
tions
(refl
ecte
dand
abso
rbed
pow
erca
nnot
be
dis
tinguis
hed
).gSham
gro
up
isnot
expose
din
asi
milar
cham
ber
.
Tab
le7.
4:Per
form
ance
sum
mar
yfo
rce
llm
onol
ayer
expo
sure
sw
ith
WG
setu
p,W
Pce
ll,T
EM
cell
and
reso
nato
r.
7.5. RESULTS 95
WG
setu
paW
Pce
llT
EM
cella
Res
onat
orN
onunifor
mity
ofSA
R[%
]E
ntir
em
ediu
mev
alua
ted
135
141
46-
Med
ium
wit
hout
men
iscu
sev
alua
ted
5459
39-
Tem
per
ature
load
[C
/(W
/kg)]
b
Sing
lepo
int
mea
sure
men
t0.
090.
180.
050.
03c
Max
imum
acco
rdin
gto
sim
ulat
iond
0.12
0.20
0.05
-Pow
ereffi
cien
cy[W
/kg
/W]
Ent
ire
med
ium
eval
uate
d0.
741.
65.
7≈
7M
ediu
mw
itho
utm
enis
cus
eval
uate
d0.
491.
16.
1-
Onl
ym
enis
cus
eval
uate
d2.
74.
93.
7-
Vol
um
eeffi
cien
cy[m
l/l]
e
Susp
ensi
onev
alua
tion
25/
160
25/
220
/80
20/
>20
0Expos
ure
/en
vir
onm
enta
lco
ntr
olye
s/
yes
yes
/no
f(y
es)g
/ye
sye
s/
noh
aD
ata
for
835
MH
zis
giv
en.
bD
ata
isnorm
alize
dto
the
aver
age
SA
Rfo
rm
ediu
mev
alu
ation
wit
hout
men
iscu
s.cSta
bility
of±
0.3C
up
to10
W/kg
aver
age
SA
Ris
report
edin
[115].
dSin
gle
poin
tm
easu
red
valu
esw
ere
scale
dby
sim
ula
ted
ratios
bet
wee
nm
axim
um
tose
nso
rte
mper
atu
re.
eC
ellm
ediu
mvolu
me
[ml]
ver
sus
setu
pvolu
me
[l].
f Exposu
reand
sham
gro
up
are
not
insi
de
the
sam
ein
cubato
r.gE
xposu
reco
ntr
ol
with
apow
erm
easu
rem
ent
isonly
reliable
for
const
ant
reflec
tions
(refl
ecte
dand
abso
rbed
pow
erca
nnot
be
dis
tinguis
hed
).hSham
gro
up
isnot
expose
din
asi
milar
cham
ber
. Tab
le7.
5:
96 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ
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-
98 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ
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%.
100 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ
Ave
rage
SAR
Non
unifo
rmity
Ave
rage
impe
danc
eaT
herm
allo
ad[W
/kg
/(A
2/m
2) r
ef]b
[%]
[Ω]
[C
/(W
/kg)]
[MH
z]83
593
583
593
583
593
583
593
5C
ellm
onol
ayer
expos
ure
2.2
ml
0.17
0.20
1717
8.1
8.1
-0.
014
3.1
ml
0.31
0.39
2021
1111
0.01
70.
015
4.0
ml
0.52
0.67
2325
1415
-0.
016
4.9
ml
0.81
1.06
2829
1718
-0.
019
Cel
lsu
spen
sion
expos
ure
c
3.1
ml
0.17
0.15
5453
1214
0.09
0.08
aR
atio
bet
wee
naver
age
inte
rnalE
-fiel
dand
H-fi
eld.
bD
ata
isnorm
alize
dto
the
H-fi
eld
ofth
ere
fere
nce
posi
tion
(dis
hce
nte
rposi
tion
at
the
short
).cM
ediu
mw
ithout
men
iscu
sev
alu
ate
d.
Tab
le7.
6:D
etai
led
dosi
met
ryfo
rW
Gse
tup
“sX
c-90
0”.
7.5. RESULTS 101
Unce
rtai
nty
ofSA
RA
sses
smen
tW
G(M
l.)W
G(S
u.)
WP
cell
Fit
for
SAR
extr
apol
atio
nto
mon
olay
er3.
1%-
-Fit
for
vary
ing
med
ium
volu
me
0.3%
--
Ver
tica
llo
cati
onof
cells
(0.1
mm
laye
r)5.
5%-
-N
umer
ical
disc
reti
zati
on(0
.1m
m3
refe
renc
e)8.
6%4.
9%4.
9%D
eter
min
atio
nof
med
ium
volu
me
(±5µ
l)0.
3%<
0.1%
<0.
1%D
iele
ctri
cpa
ram
eter
s11
%15
%15
%E
-fiel
dpr
obe
7.6%
7.6%
7.6%
Pro
bepo
siti
onin
g2.
3%2.
3%0.
9%Se
nsor
calib
rati
onfo
rin
cide
ntfie
lds
11%
11%
11%
Com
bine
dre
lative
unce
rtai
nty
(RSS
)20
%21
%21
%D
evia
tion
bet
wee
nM
easu
rem
ent
and
Sim
ula
tion
E-fi
eld
met
hoda
15%
17%
25%
Tem
pera
ture
met
hod
(sin
gle-
poin
t)-
8%7%
aSA
Rw
as
evalu
ate
din
the
range
from
0-
4.5
mm
for
WG
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.
102 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ
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
104 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ
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
106 CHAPTER 7. IN VITRO SETUPS FOR 900 MHZ
(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.