materials
Article
Preparation of a Carbon Doped Tissue-Mimicking
Material with High Dielectric Properties for
Microwave Imaging Application
Siang-Wen Lan 1 , Min-Hang Weng 2 , Ru-Yuan Yang 3, *, Shoou-Jinn Chang 1 , Yaoh-Sien Chung 3 ,
Tsung-Chih Yu 2 and Chun-Sen Wu 2
1
2
3
*
Department of Electrical Engineering and Advanced Optoelectronic Technology Center,
Institute of Microelectronics, National Cheng Kung University, Tainan 701, Taiwan;
[email protected] (S.-W.L.); [email protected] (S.-J.C.)
Medical Devices and Opto-Electronics Equipment Department,
Metal Industries Research and Development Center, Kaohsiung City 811, Taiwan;
[email protected] (M.-H.W.); [email protected] (T.-C.Y.); [email protected] (C.-S.W.)
Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology,
Pingtung County 912, Taiwan; [email protected]
Correspondence: [email protected]; Tel.: +886-8-7703202
Academic Editor: Frank A. Müller
Received: 30 April 2016; Accepted: 7 July 2016; Published: 9 July 2016
Abstract: In this paper, the oil-in-gelatin based tissue-mimicking materials (TMMs) doped with
carbon based materials including carbon nanotube, graphene ink or lignin were prepared. The volume
percent for gelatin based mixtures and oil based mixtures were both around 50%, and the doping
amounts were 2 wt %, 4 wt %, and 6 wt %. The effect of doping material and amount on the
microwave dielectric properties including dielectric constant and conductivity were investigated over
an ultra-wide frequency range from 2 GHz to 20 GHz. The coaxial open-ended reflection technology
was used to evaluate the microwave dielectric properties. Six measured values in different locations
of each sample were averaged and the standard deviations of all the measured dielectric properties,
including dielectric constant and conductivity, were less than one, indicating a good uniformity of
the prepared samples. Without doping, the dielectric constant was equal to 23 ˘ 2 approximately.
Results showed with doping of carbon based materials that the dielectric constant and conductivity
both increased about 5% to 20%, and the increment was dependent on the doping amount. By proper
selection of doping amount of the carbon based materials, the prepared material could map the
required dielectric properties of special tissues. The proposed materials were suitable for the phantom
used in the microwave medical imaging system.
Keywords: dielectric property; oil-in-gelatin; microwave image; tissue-mimicking material
(TMM); carbon
1. Introduction
Microwave imaging (MWI) technology attracts much attention in medical use and seems
an alternative imaging modality for the well developed modalities such as X-ray, ultrasound,
or magnetic resonance imaging (MRI) due to some advantages including light and portability, low-cost,
non-ionizing radiation, and imaging without the need of contrast agents [1–5]. The tissue-mimicking
materials (TMMs) with desired microwave dielectric properties including dielectric constant and
conductivity are thus developed for the microwave medical imaging techniques in the application of
mimicking the human tissue and calibrating the imaging systems. Many different materials have been
presented to prepare the TMMs, such as polyacrylamide gel (PAG), oil-in-gelatin, and carbon-based
Materials 2016, 9, 559; doi:10.3390/ma9070559
www.mdpi.com/journal/materials
Materials 2016, 9, 559
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synthetics [6–12]. PAG is typically comprised of Acrylamide (C3 H5 NO) polymerized in liquid solvent
and used to simulate the behavior of biological tissues in applicators for microwave hyperthermia or
in scaled-phantom experiments, with the advantages including excellent optical transparency and
high elasticity [6,7]. However, the fabricated phantoms using PAG showed a short usage life with only
several hours as exposed to air or several weeks as held in a tight-covered container.
Compared with the above materials, gelatin-based materials are popular because of the simple
fabrication, stable mechanical properties and a long usage time [8]. The gelatin-based materials mixed
with oil were typically used especially to imitate the dielectric properties of the human soft tissues,
since they could provide a high competitiveness for the controllable and tunable characteristics of
dielectric properties by varying the concentration of oil [9,10]. Moreover, the oil-in-gelatin based TMMs
have long-term stability in heterogeneous configurations without change in geometry or dielectric
properties. However, the microwave dielectric properties of the oil-in-gelatin based TMMs could not
be tuned significantly by varying a small quantity of oil. Therefore, the dielectric properties of the
prepared TMMs usually provided a small tuning range through varying the volume percent of oil
in general.
In another carbon-based material, using urethane as the matrix material mixed with graphite
and carbon black, was newly adopted to fabricate the microwave phantom of mimicking fatty tissues,
providing the characteristics of stable and flexible dielectric properties [11,12]. Carbon nanotubes
are known as electronic conductive materials and can be used to vary the dielectric properties in
composition. They exhibit a high dielectric constant at microwave frequency due to their fibrous
shapes [13]. Similarly, graphene is also an electronic conductive material and possesses good electrical
and mechanical properties due to the unique structure of two-dimensional aromatic sheets [14].
The lignin, extracted from the renewable resource material, is suggested as an ionic conductive
material and typically has high electrical resistance due to the absence of unsaturated bonds. However,
the components of lignin are highly hydrophilic and might have high polarization. Lignin has been
proposed as a low cost carbon material with good mechanical properties [15,16]. Moreover, the carbon
based materials typically have a general characteristic of electromagnetic wave absorption at high
frequency spectrums, which leads to a slow wave and influences the dielectric properties [13,14].
Based on the above information, it is desired to know the doping effect of the carbon based materials,
such as carbon nanotube, graphene ink, and lignin, on the preparation of oil-in-gelatin based materials.
To develop a new tissue-mimicking material for phantom, many issues shall be known, including
the preparation method, uniformity, measurement accuracy, measured dielectric properties, mechanical
properties and stability. In this paper, we focused on the doping effect of the carbon-based synthetic
materials on the microwave dielectric properties of the oil-in-gelatin based TMMs. To clear the doping
effect, the gelatin and oil were both fixed at the volume percent of 50%. The prepared samples were
measured over an ultra-wide frequency range from 2 GHz to 20 GHz. It was found that the microwave
dielectric properties of the studied samples were tuned significantly by varying small doping amounts
of the carbon based materials. The preparation and measurement were presented in detail as following,
and the microwave dielectric properties of the prepared samples were investigated and discussed in
this paper.
2. Methodology
2.1. Preparation of the Tissue-Mimicking Materials
In general, the investigated materials were mainly a mixture of gelatin and oil, thus still regarded
as the oil-in-gelatin based TMMs. In this study, the volume percent for gelatin based mixtures and oil
based mixtures were both around 50%. To investigate the doping effect on the microwave dielectric
properties, three carbon based materials, comprising carbon nanotubes, graphene ink, or lignin,
were added individually into the oil-in-gelatin based TMMs during the preparation procedure.
The weight percent of the added carbon based materials were decided as the variation factors to
Materials 2016, 9, 559
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variation factors to tune the dielectric properties of the prepared samples, and chosen as 2, 4, and 6
tune
properties
of the
and
chosendoping
as 2, 4,carbon
and 6 wt
%. For comparison
wt %.the
Fordielectric
comparison
purposes,
weprepared
preparedsamples,
a sample
without
nanotubes,
graphene
Materials
2016,
9, 559
3 of 15 was
purposes,
we
prepared
a
sample
without
doping
carbon
nanotubes,
graphene
ink,
or
which
ink, or lignin, which was expressed as 0 wt % in the plotted diagram. The mixed oillignin,
was prepared
by
expressed
0 wtkerosene
% in the plotted
diagram.
The mixed
was prepared
by combining
50% kerosene
combiningas50%
and 50%
safflower
oil. Tooilillustrate
the preparing
procedure
of the
variation factors to tune the dielectric properties of the prepared samples, and chosen as 2, 4, and 6
and
50% safflower
oil.
To illustrate
the
preparing procedure
of the proposed
samples
simply,
main
proposed
simply,
the mainwe
preparing
arewithout
summarized
shown
in Figure
1. Itthe
is noted
wt %. samples
For comparison
purposes,
prepared asteps
sample
dopingas
carbon
nanotubes,
graphene
preparing
steps
are
summarized
as
shown
in
Figure
1.
It
is
noted
that
the
four
main
steps
are
that the
main
steps
areexpressed
generalized
based
on plotted
the fabrication
ofwas
oil-in-gelatin
ink, four
or lignin,
which
was
as 0 wt
% in the
diagram. procedure
The mixed oil
prepared bybased
generalized
based
onkerosene
the
fabrication
procedure
ofoil.
oil-in-gelatin
TMMs
[9].
However,
after
doing
TMMs
[9]. However,
after
doing
many
experiments
find based
thethemethod
of doping
carbon
combining
50%
and 50%
safflower
To toillustrate
preparing
procedure
of
thebased
many
experiments
to
find
the
method
of
doping
carbon
based
materials
into
the
oil-in-gelatin,
it
proposed
simply, the main
preparing
as shownshould
in Figure
It is noted
materials
intosamples
the oil-in-gelatin,
it was
found steps
that are
thesummarized
doping materials
be1.added
intowas
the
found
that
materials
should before
be
added
intofabrication
the mixed
solution
under
vigorous
stirring
that
thethe
fourdoping
main steps
are generalized
based
on the
oil-in-gelatin
based
mixed
solution
under
vigorous
stirring
adding
gelatin
inprocedure
order
to of
spread
the
carbon
based
TMMs
[9].gelatin
However,
after
doing
many
toin
find
the1.method
of doping
carbonare
based
before
adding
inthe
order
to
spread
theexperiments
carbon
based
materials
uniformly
in the prepared
samples,
materials
uniformly
in
prepared
samples,
as shown
Step
Thus,
additional
steps
adopted
materials
into
the
oil-in-gelatin,
it
was
found
that
the
doping
materials
should
be
added
into
the
as
shown
in
Step
1.
Thus,
additional
steps
are
adopted
because
of
the
doping
of
carbon
based
materials.
because of the doping of carbon based materials.
mixed
solution
under
vigorous
stirring
before
adding
gelatin
in
order
to
spread
the
carbon
based
The
thethe
proposed
samples,
as extended
fromfrom
the main
four steps
The detailed
detailedprocedure
procedureofofpreparing
preparing
proposed
samples,
as extended
the main
four
materials uniformly in the prepared samples, as shown in Step 1. Thus, additional steps are adopted
in
Figure
1,
is
illustrated
as
below
[9].
Herein,
the
variation
of
typical
prepared
and
studied
samples
is
stepsbecause
in Figure
1, is illustrated as below [9]. Herein, the variation of typical prepared and studied
of the doping of carbon based materials.
shown
in
Figure
2.
samples is shown in Figure 2.
The detailed procedure of preparing the proposed samples, as extended from the main four
steps in Figure 1, is illustrated as below [9]. Herein, the variation of typical prepared and studied
samples is shown in Figure 2.
Figure 1.
1. The
The main
main procedure
procedure of
of preparing
preparing the
the proposed
proposed oil-in-gelatin
oil-in-gelatin based
based materials
materials doped
doped with
with
Figure
Figure 1. The main procedure of preparing the proposed oil-in-gelatin based materials doped with
carbon based
based materials.
materials.
carbon
carbon based materials.
(a)
(a)
(b)
Figure(b)
2. Cont.
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(c)
(d)
Figure 2. The variation of typical prepared and studied samples (a) mixed solutions; (b)
Figure 2. The variation of typical prepared and studied samples (a) mixed solutions; (b) water-gelatin
water-gelatin mixtures; (c) oil-gelatin mixtures and (d) the solidified samples. The doping
mixtures;
(c) oil-gelatin mixtures and (d) the solidified samples. The doping concentrations of carbon
concentrations of carbon based materials are schematically chosen as 6 wt %.
based materials are schematically chosen as 6 wt %.

‚
During step 1:
During
step 1:
Prepare 0.8 g of n-propanol in a beaker, mix the n-propanol with 0.2 g of p-toluic acid (powder)
and heat the solution until the p-toluic is completely dissolved.
Prepare 0.8 g of n-propanol in a beaker, mix the n-propanol with 0.2 g of p-toluic acid (powder)
Mix the solution with 19 g of deionized (DI) water (18 Mohms∙cm of resistivity).
and heat the
until
the p-toluic
is including
completely
dissolved.
Addsolution
the carbon
based
materials,
carbon
nanotube (95% concentration, CDW-181,
Mix
the
solution
with
19
g
of
deionized
(DI)
water
(18
Mohms¨cm
of resistivity).
Taiwan Carbon Nano Technology Corporation, Miaoli County,
Taiwan),
graphene ink (0.5%
Add
the
carbon
based
materials,
including
carbon
nanotube
(95%
concentration,
CDW-181,
Taiwan
concentration, G1208, Legend Star International Co. Ltd., New Taipei City, Taiwan),
or lignin
(371017,
Shanghai, China),
individually
into the solution,
andink
stir(0.5%
the solution
by
Carbon
NanoSigma-Aldrich,
Technology Corporation,
Miaoli
County, Taiwan),
graphene
concentration,
using
a magnetic
stirrer at about
rpmNew
at room
temperature
for 10or
minutes
solutions
are
G1208,
Legend
Star International
Co.600
Ltd.,
Taipei
City, Taiwan),
lignin (mixed
(371017,
Sigma-Aldrich,
shownChina),
in Figure
2a).
Shanghai,
individually
into the solution, and stir the solution by using a magnetic stirrer at
about 600During
rpm atstep
room
2: temperature for 10 minutes (mixed solutions are shown in Figure 2a).
‚
Add 3.5 g of gelatin (G9391, Sigma-Aldrich, Shanghai, China), which was extracted from bovine
During
step 2:
skin Type B, into the solution to obtain a water-gelatin mixture.
thegelatin
beaker,(G9391,
containing
water-gelatin mixture,
with
a plastic
film,was
and extracted
heat the beaker
AddCover
3.5 g of
Sigma-Aldrich,
Shanghai,
China),
which
from by
bovine
double-boiling at about a temperature of 90 °C.
skin Type B, into the solution to obtain a water-gelatin mixture.
Remove the beaker from the double-boiling system after the gelatin is completely dissolved,
Cover the beaker, containing water-gelatin mixture, with a plastic film, and heat the beaker by
and take off the plastic film. In this stage, hold
the temperature of the beaker at 50 °C by a hot plate
double-boiling
about a temperature
of 90 ˝ C.(water-gelatin mixtures are shown in Figure 2b).
to avoid theatwater-gelatin
mixture solidifying
Remove the beaker from the double-boiling system after the gelatin is completely dissolved,

During step 3:
and take off the plastic film. In this stage, hold the temperature of the beaker at 50 ˝ C by a hot plate to
15.5 g ofmixture
mixed oil
(50% kerosene
and 50% safflower
oil)are
in another
and2b).
preheat
avoid thePrepare
water-gelatin
solidifying
(water-gelatin
mixtures
shown beaker
in Figure
‚
it to 50 °C.
Pour
the3:mixed oil into the beaker, containing water-gelatin mixture, to obtain an oil-gelatin
During
step
mixture (oil-gelatin mixtures are shown in Figure 2c).
Prepare 15.5 g of mixed oil (50% kerosene and 50% safflower oil) in another beaker and preheat it

During the step 4:
to 50 ˝ C.
g of oil
liquid
(Ultra
Ivory, Procter
& Gamblemixture,
Professional,
Cincinnati,
OH,
PourAdd
the 8.6
mixed
intosurfactant
the beaker,
containing
water-gelatin
to obtain
an oil-gelatin
USA) and 0.08 g of formaldehyde into the oil-gelatin mixture, and stir mixture violently by using
mixture (oil-gelatin mixtures are shown in Figure 2c).
magnetic stirrer at about 800 rpm at temperature of 50 °C.
‚
During the step 4:
Add 8.6 g of liquid surfactant (Ultra Ivory, Procter & Gamble Professional, Cincinnati, OH, USA)
and 0.08 g of formaldehyde into the oil-gelatin mixture, and stir mixture violently by using magnetic
stirrer at about 800 rpm at temperature of 50 ˝ C.
Materials 2016, 9, 559
5 of 15
Pour the oil-gelatin mixture into a container for solidifying and shaping after the oil-gelatin
mixture becomes uniform and there are no bubbles remaining beneath the surface. It is noted that the
container was in a sealed condition to avoid particles attaching on the surface of the oil-gelatin mixture
for at least five days at room temperature (the solidified samples are shown in Figure 2d).
In this study, to increase usage lifetime of the prepared samples, the formaldehyde is used to
preserve the sample, and the weight of the formaldehyde is chosen as 0.22 g per gram of gelatin.
The surfactant is used to reduce the interfacial tension between the oil and water, and the weight of the
surfactant is chosen as 0.55 g per gram of oil. For different doping materials, the weight percentages of
all component materials of the prepared TMMs are list on Table 1.
Table 1. Weight percentages of the each component material of the presented tissue-mimicking
phantom. (Unit: wt %).
Doping
Materials
N-Propanol
P-Toluic
Acid
Di Water
Gelatin
Mixed Oil
Surfactant
Formaldehyde
2
4
6
1.651
1.617
1.583
0.041
0.04
0.04
39.2
38.4
37.6
7.221
7.074
6.926
31.979
31.326
30.674
17.743
17.381
17.019
0.165
0.162
0.158
2.2. Measurement of the Tissue-Mimicking Material
The microwave dielectric properties of the prepared samples were measured and determined
using the coaxial open-ended reflection technology. Figure 3 shows (a) the equivalent structure and
(b) practical structure of the coaxial open-ended reflection probe. The dielectric properties of the
samples can be expressed as Equation (1) [17]:
εd “
?
?
?
j2π f L ε d
´jc ε t 1 ´ Γm e2γt D j2π f L ε d
p
qcothp
q,
2π f L 1 ` Γm e2γt D
c
c
(1)
and the unknown length (L) can be expressed as Equation (2) [18]:
L“
c
?
j2π f ε water
tanh´1 p
?
εt
1 ´ Γwater e2γt D
q,
?
1 ` Γwater e2γt D ε water
(2)
where some constants are known first including: c is the light speed in vacuum (c is 3 ˆ 108 m/s), εt is
the dielectric constant of the probe kit (εt = 2.03), D is the length of the probe kit (D = 112 mm), εwater is
the dielectric constant of water (εwater = 78.3), and some symbols are the measured results including:
f is the operated frequency, Γm is the measured reflection coefficient of the probe kit, Γwater is the
reflection coefficient of water, and γt is the propagation constant. To extract the real value of the
unknown length (L), an iterative calculation was done by substituting the known dielectric constant of
air and water [18].
The measurement system for microwave dielectric properties of the TMMs in this study were set
by using an Agilent 8722ES Vector Network Analyzer (VNA) (Santa Clara, CA, USA) combined with
the open-ended probe kit, as shown in Figure 4. One side of the open-ended probe kit was connected to
the port of VNA through a coaxial cable, and the other side touches the surface of the prepared TMMs
to take the S-parameter. Before connecting the open-ended probe kit to the end of the coaxial cable,
the coaxial cable was connected to three calibration kits (open kit, short kit, and match kit) individually
for the calibration. Three calibrated results were extracted and calculated by VNA to ensure the
accuracy during the measuring period. Herein, electronic calibration kits 85056D (Agilent, Santa Clara,
CA, USA) were used to calibrate the coaxial cable. The expected accuracy of the dielectric probe
measurement technique is with maximum differences of 10% or less between measured dielectric
properties and values derived from published theoretical Debye models, as discussed in [18].
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(a)
(a)
(b)
(b)
Figure 3. (a) The equivalent structure and (b) practical structure of the coaxial open-ended
Figure 3. (a) The equivalent structure and (b) practical structure of the coaxial open-ended
Figure
3. probe.
(a) The equivalent structure and (b) practical structure of the coaxial open-ended
reflection
reflection probe.
reflection probe.
Figure 4. Schematic structure of the experimental setup for the measurement of dielectric properties.
Figure
Figure 4.
4. Schematic
Schematicstructure
structure of
of the
the experimental
experimental setup
setup for
for the
the measurement
measurement of
of dielectric
dielectric properties.
properties.
It shall be noted that the probe end was pressed to contact the surface of the prepared samples
It shall
be noted
that the it
probe
end
was pressed
to To
contact
the surface
of the prepared
samples
slightly
without
puncturing
when
measuring
[18].
enhance
the accuracy
of the measured
It shall
be noted
that the probe
end
was pressed
to contact
the surface
of the prepared
samples
slightly
without
puncturing
itperiod,
wheneach
measuring
[18].
To enhance
thedifferent
accuracy
of the measured
results,
during
the
measuring
sample
was
measured
at
six
positions
under
the
slightly without puncturing it when measuring [18]. To enhance the accuracy of the measured results,
results,
during the measuring
period,
each
sample was
measured
atthen
six different
positions
under
the
same
environment
condition
at
room
temperature
of
26
°C,
and
all
the
six
measured
values
during the measuring period, each sample was measured at six different positions under the same
same
environment
at
room temperature
of 26 °C, and then
all the
six
measured
values
were
averaged
and condition
recorded.
Moreover,
the standard
(SDs)
dielectric
properties
environment
condition
at room
temperature
of 26 ˝ C,deviations
and then all
the of
sixthe
measured
values
were
were
averaged
and
recorded.
Moreover,
the
standard
deviations
(SDs)
of
the
dielectric
properties
were
calculated
to
evaluate
the
uniformity
for
each
sample.
The
formula
of
SD
for
each
sample
can
averaged and recorded. Moreover, the standard deviations (SDs) of the dielectric properties were
were
calculated
to evaluate the uniformity for each sample. The formula of SD for each sample can
be
expressed
as:
calculated to evaluate the uniformity for each sample. The formula of SD for each sample can be
be expressed as:
expressed as:
g1 6
2
66 ( x i  x )
SD f
(3)
f 11 ÿ
2
2
e
6
SD“
( xi i´xqx ), ,
i 1 px
SD
(3)
(3)
66 i 1
i “1
,
where SD is the value of standard deviation, xi is the measured result of the dielectric properties, and
where SD
SD isisthe
thevalue
valueofofstandard
standard
deviation,
xi the
is the
measured
result
of dielectric
the dielectric
properties,
where
deviation,
xi is
measured
result
of the
properties,
x is the averaged
result of six recording
dielectric
properties.
Figures
5 and 6 show
the SDand
of
and
x the
is the
averaged
result
of
six
recordingdielectric
dielectricproperties.
properties.Figures
Figures5 5and
and66show
showthe
the SD
SD of
of
x
is
averaged
result
of
six
recording
dielectric constant and conductivity versus the frequency for the prepared samples with different
dielectric
constant
and
conductivity
versus
the
frequency
for
the
prepared
samples
with
different
dielectric
constant Obviously,
and conductivity
versus the
frequency
forallthe
samples
with
different
doping materials.
the calculated
values
of SD of
theprepared
measured
dielectric
properties,
doping materials.
materials. Obviously,
Obviously, the
the calculated
calculated values
values of
of SD
SD of
of all
all the
the measured
measured dielectric
dielectric properties,
properties,
doping
including dielectric constant and conductivity, are less than 1, indicating a good uniformity of the
including dielectric
dielectric constant
constant and
and conductivity,
conductivity, are
are less
less than
than 1,
1, indicating
indicating aa good
good uniformity
uniformity of
of the
the
including
prepared samples.
prepared
samples.
prepared samples.


Materials 2016, 9, 559
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(a)
(a)
(b)
(b)
(c)
(c)
Figure 5. The calculated standard deviation of the measured dielectric constant for the prepared
Figure 5. The calculated standard deviation of the measured dielectric constant for the prepared
Figure
5. The
standard
deviation
of the (a)
measured
dielectric (b)
constant
for the
samples
withcalculated
different doping
materials,
including
carbon nanotube;
graphene
ink prepared
and (c)
samples with different doping materials, including (a) carbon nanotube; (b) graphene ink and (c)
samples
with
different
doping
materials,
including
(a)
carbon
nanotube;
(b)
graphene
ink
and
(c) lignin.
lignin.
lignin.
(a)
(a)
Figure 6. Cont.
Materials
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9, 559
559
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15
8 of 15
(b)
(b)
(c)
(c)
Figure 6. The calculated standard deviation of the measured conductivity for the prepared samples
Figure
Figure 6.
6. The
The calculated
calculated standard
standard deviation
deviation of
of the
the measured
measured conductivity
conductivity for
for the
the prepared
prepared samples
samples
with different doping materials, including (a) carbon nanotube; (b) graphene ink and (c) lignin.
with
with different
differentdoping
dopingmaterials,
materials,including
including(a)
(a)carbon
carbonnanotube;
nanotube;(b)
(b)graphene
grapheneink
inkand
and(c)
(c)lignin.
lignin.
3. Results and Discussion
3. Results
3.
Resultsand
andDiscussion
Discussion
Figure 7 shows the measured microwave dielectric properties at a frequency range from 2 GHz
Figure
shows
the measured
measured
microwave
dielectric
properties
at aa frequency
frequency
range from
from
GHz
shows
the
microwave
dielectric
at
range
22 GHz
to Figure
20 GHz77 as
functions
of the doping
concentrations
ofproperties
carbon nanotube.
It is observed
that with
to
20
GHz
as
functions
of
the
doping
concentrations
of
carbon
nanotube.
It
is
observed
that
with
to increasing
20 GHz asthe
functions
of the doping
concentrations
is observed
thatand
with
carbon nanotube
concentration
from 0 of
wtcarbon
% to 6 nanotube.
wt %, the dielectric
constant
increasing
from
wt % in
to Figure
6 wt
wt %,
%,7a,b.
the dielectric
dielectric
constant
and
increasing
the carbon
nanotube
the
and
conductivity
both increase
at aconcentration
fixed frequency
as 0shown
This
resultconstant
might be
conductivity
both
increase
a fixed
frequency
asby
shown
in7a,b.
Figure
This
result
might
contributedboth
by the
parasitic
capacitor
effect,
formed
between
the7a,b.
carbon
nanotubes
due to be
conductivity
increase
at aatfixed
frequency
as
shown
ingaps
Figure
This
result
might
be contributed
the
frequency
at microwave
spectrum,
as discussed
in [19].nanotubes
Asthe
doping
of nanotubes
0 to
wtthe
%, operating
2 wtdue
%, to
contributed
by the
parasitic
capacitor
formed
by gaps
between
carbon
by
theoperating
parasitic
capacitor
effect,
formedeffect,
by gaps
between
the carbon
due
wt % at
and
6 wt %, the
dielectricasconstants
are
26.2,
27.8
and
respectively,
and
the
the4 operating
frequency
at
microwave
spectrum,
as21.8,
discussed
in [19].
doping
of 0%,
wt4 %,
%,
frequency
microwave
spectrum,
discussed
in
[19].
As doping
ofAs
029.1,
wt
%,
2 wt
wt2%wt
and
conductivities
(S/m)
are
3.9,
4.9,
5.4
and
6.0,
respectively,
at
10
GHz.
The
increment
variation
of
the
4
wt
%
and
6
wt
%,
the
dielectric
constants
are
21.8,
26.2,
27.8
and
29.1,
respectively,
and
the
6 wt %, the dielectric constants are 21.8, 26.2, 27.8 and 29.1, respectively, and the conductivities (S/m)
dielectric
constant
and
conductivity
are6.0,
summarized
in at
Figure
7c,d.The
Itofisthe
clearly
found
that of
the
conductivities
(S/m)
are
3.9,
4.9, 5.4 at
and
respectively,
10variation
GHz.
increment
variation
the
are
3.9, 4.9, 5.4
and
6.0,
respectively,
10
GHz.
The increment
dielectric
constant
and
increments
of
the
dielectric
constant
become
small
but
the
increments
of
the
conductivity
become
dielectric constant
and conductivity
are
summarized
Figure
7c,d.
is clearly of
found
that the
conductivity
are summarized
in Figure
7c,d.
It is clearlyinfound
that
the It
increments
the dielectric
large while
the
from
5 of
GHz
20
GHz.
increments
ofincreasing
thesmall
dielectric
constant
become
small
but
the increments
the while
conductivity
become
constant
become
butfrequency
the
increments
thetoconductivity
becomeof
large
increasing
the
large whilefrom
increasing
from 5 GHz to 20 GHz.
frequency
5 GHz the
to 20frequency
GHz.
(a)
(a)
Figure 7. Cont.
Materials 2016, 9, 559
Materials 2016, 9, 559
9 of 15
9 of 15
(b)
(c)
(d)
Figure 7. (a) Dielectric constant; (b) conductivity; (c) increment variation of the dielectric constant,
Figure 7. (a) Dielectric constant; (b) conductivity; (c) increment variation of the dielectric constant,
and (d) increment variation of the conductivity of the prepared samples as functions of the doping
and (d) increment variation of the conductivity of the prepared samples as functions of the doping
concentrations of carbon nanotube.
concentrations of carbon nanotube.
Figure 8 shows the measured microwave dielectric properties at a frequency range from 2 GHz
Figure
8 shows
theof
measured
microwave
dielectric
propertiesink.
at aSimilarly,
frequencyit range
from
2 GHz
to 20 GHz as
functions
the doping
concentrations
of graphene
is found
that
with
to
20
GHz
as
functions
of
the
doping
concentrations
of
graphene
ink.
Similarly,
it
is
found
that
increasing of the graphene ink concentration from 0 wt % to 6 wt %, the dielectric constantwith
and
increasing
of
the
graphene
ink
concentration
from
0
wt
%
to
6
wt
%,
the
dielectric
constant
and
conductivity both increase at a fixed frequency as shown in Figure 8a,b. This result might also be
conductivity
increase
at acapacitor
fixed frequency
shownwith
in Figure
8a,b. nanotubes
This result[19].
might
be
contributed toboth
by the
parasitic
effect, asasdoping
the carbon
As also
doping
contributed
to
by
the
parasitic
capacitor
effect,
as
doping
with
the
carbon
nanotubes
[19].
As
doping
of
of 0 wt %, 2 wt %, 4 wt % and 6 wt %, the dielectric constants are 21.8, 25.6, 26.5 and 27.5,
0respectively,
wt %, 2 wt %,and
4 wtthe
% and
6 wt %, the dielectric
and 27.5, respectively,
conductivities
(S/m) areconstants
3.9, 4.7, are
4.9 21.8,
and 25.6,
5.1, 26.5
respectively,
at 10 GHz. and
The
the
conductivities
(S/m)
are
3.9,
4.7,
4.9
and
5.1,
respectively,
at
10
GHz.
The
increment
variation
increment variation of the dielectric constant and conductivity are summarized in Figure
8c,d. Itof
is
the
dielectric
and conductivity
summarized
in Figure
8c,d. It but
is shown
that the increments
shown
that constant
the increments
of the are
dielectric
constant
are small,
the increments
of the
of
the dielectricare
constant
are small,
but thethe
increments
the conductivities
are large when increasing
conductivities
large when
increasing
frequencyoffrom
5 GHz to 20 GHz.
the frequency from 5 GHz to 20 GHz.
Materials 2016, 9, 559
Materials 2016, 9, 559
10 of 15
10 of 15
(a)
(b)
(c)
(d)
Figure 8.
8. (a)
(a) Dielectric
Dielectric constant;
constant; (b)
(b) conductivity;
conductivity; (c)
(c) increment
increment variation
variation of
of the
the dielectric
dielectric constant,
constant,
Figure
and
(d)
increment
variation
of
the
conductivity
of
the
prepared
samples
as
functions
of
the
doping
and (d) increment variation of the conductivity of the prepared samples as functions of the doping
concentrations of
of graphene
graphene ink.
ink.
concentrations
Figure 9 shows the measured microwave dielectric properties at a frequency range from 2 GHz
to 20 GHz as functions of the doping concentrations of lignin. It is shown that with increase of the
Materials 2016, 9, 559
11 of 15
Figure
the measured microwave dielectric properties at a frequency range from 2 GHz
Materials
2016,99,shows
559
11 of to
15
20 GHz as functions of the doping concentrations of lignin. It is shown that with increase of the lignin
concentration
from 0 from
wt %0towt
4 wt
%,4the
constant
and conductivity
both increase
at a fixed
lignin concentration
% to
wt dielectric
%, the dielectric
constant
and conductivity
both increase
at
frequency,
as shown
Figurein9a,b.
As 9a,b.
doping
0 wt %,
6 wt
dielectric
a fixed frequency,
asin
shown
Figure
As of
doping
of 20 wt
wt %,
%,42wt
wt%%,and
4 wt
% %,
andthe
6 wt
%, the
constants
21.8, 22.9,
and the conductivities
(S/m) are 3.9,
4.2, are
5.8 and
5.0,
dielectric are
constants
are 28.4
21.8,and
22.9,25.7,
28.4respectively,
and 25.7, respectively,
and the conductivities
(S/m)
3.9, 4.2,
respectively,
at 10 GHz. Although
the
lignin is the
notlignin
an electronic
thematerial,
lignin with
5.8 and 5.0, respectively,
at 10 GHz.
Although
is not anconductive
electronic material,
conductive
the
polysaccharide
components
is
highly
hydrophilic
and
might
have
high
polarization
to
increase
the
lignin with polysaccharide components is highly hydrophilic and might have high polarization to
dielectric
properties.
suggested
needs further
study.
The increment
of
increase the
dielectricThe
properties.
Themechanism
suggested still
mechanism
still needs
further
study. Thevariation
increment
the
dielectric
constant
andconstant
conductivity
are summarized
in Figure 9c,d.
to theSimilar
dopingtowith
variation
of the
dielectric
and conductivity
are summarized
in Similar
Figure 9c,d.
the
carbon
andnanotube
grapheneand
ink,graphene
the increments
of increments
the dielectric
become
small, become
but the
doping nanotube
with carbon
ink, the
of constant
the dielectric
constant
increments
of the
conductivities
large with
increase
of the
to 20 GHz.
small, but the
increments
of thebecome
conductivities
become
large
withfrequency
increase from
of the5 GHz
frequency
from
However,
theGHz.
doping
effect ofthe
lignin
reaches
a maximum
increment
at 4 wt % in
this study.
5 GHz to 20
However,
doping
effect
of lignin reaches
a maximum
increment
at The
4 wtresult
% in
is
not
clear
now
and
might
be
due
to
the
saturation
of
the
doping
effect
since
too
much
lignin
dispersed
this study. The result is not clear now and might be due to the saturation of the doping effect since
in
oil in
gelatin
based matrices
reduces
the based
polarization
ability.
toothe
much
lignin
dispersed
in the oil
in gelatin
matrices
reduces the polarization ability.
(a)
(b)
(c)
Figure 9. Cont.
Materials 2016, 9, 559
12 of 15
Materials 2016, 9, 559
12 of 15
(d)
Figure
Figure9.9.(a)
(a)Dielectric
Dielectricconstant;
constant;(b)
(b)conductivity;
conductivity; (c)
(c) increment
increment variation
variation of
of the
the dielectric
dielectric constant,
constant,
and
and(d)
(d)increment
incrementvariation
variationof
ofthe
theconductivity
conductivityof
ofthe
theprepared
preparedsamples
samplesas
asfunctions
functionsof
ofthe
thedoping
doping
concentrations
concentrationsof
oflignin.
lignin.
Dielectric constant and conductivity of the prepared oil-in-gelatin based materials doped with
Dielectric
constant and
conductivity
the prepared
oil-in-gelatin
materials
doped
with
carbon based materials
at the
microwave of
frequency
are a function
of thebased
frequency,
doping
material
carbon
based
materials
at
the
microwave
frequency
are
a
function
of
the
frequency,
doping
material
and doping amount. For the samples without doping, the dielectric constant is equal to 23 ± 2 and
and
amount.
For the
samples
without
the dielectric
constant
is equal
23 ˘the
2 and
the
the doping
conductivity
is from
2 to
12 (S/m)
over doping,
the measured
frequency.
In order
to to
clear
doping
conductivity
from 2 toconstant
12 (S/m)
over
measured frequency.
In to
order
clear
the doping
effect on
effect on the is
dielectric
and
thethe
conductivity,
it is desired
plotto
the
increment
variations,
as
the
dielectric
constant
and
the
conductivity,
it
is
desired
to
plot
the
increment
variations,
as
shown
in
shown in Figures 7–9.
Figures
7–9.
It is
clearly observed that the increment of dielectric constant is larger at a lower frequency and
It
is
clearly
observedInthat
the increment
of dielectric
largerabout
at a lower
higher doping amount.
general,
the dielectric
constantconstant
increasesis from
5% tofrequency
20% after
and
higher
doping
amount.
In
general,
the
dielectric
constant
increases
from
about
5%carbon
to 20%based
after
increasing the doping amount from 0 to 6 wt %. Comparison between these added
increasing
doping amount
from
0 toprepared
6 wt %. samples
Comparison
between
these added
carbon based
materials, the
the dielectric
constant
of the
are varied
significantly
by doping
a little
materials,
the
dielectric
constant
of
the
prepared
samples
are
varied
significantly
by
doping
a little
bit of lignin, but slightly by doping the same amount of graphene ink. For example, at 10 GHz,
the
bit
of lignin,
but slightly
by about
doping26.6%
the same
of graphene
ink. For
10 GHz,
dielectric
constant
increases
(fromamount
21.8 to 27.6)
after doping
4 wtexample,
% carbonatnanotube,
the
constant
increases
about about
26.6% 20.2%
(from 21.8
to 21.8
27.6)to
after
doping
4 wt % carbon
anddielectric
the dielectric
constant
increases
(from
26.2)
after doping
6 wt %nanotube,
graphene
and
the
dielectric
constant
increases
about
20.2%
(from
21.8
to
26.2)
after
doping
6
wt % graphene
ink. However, the dielectric constant increases about 30.3% (from 21.8 to 28.4) after merely
doping 4
ink.
wt %However,
lignin. the dielectric constant increases about 30.3% (from 21.8 to 28.4) after merely doping
4 wt %
It lignin.
is also observed that the increment of conductivity is larger at higher frequency and higher
It
also observed
that the
conductivity
is larger
at higher
and
higher
dopingisamount.
However,
the increment
increment of
ratio
of conductivity
is lower
thanfrequency
that of the
dielectric
doping
amount.
the increment
of conductivity
is lower
that of the
dielectric
constant.
DopingHowever,
of the lignin
still hasratio
more
obvious effect
on thethan
dielectric
constant
and
constant.
Doping
of
the
lignin
still
has
more
obvious
effect
on
the
dielectric
constant
and
conductivity
conductivity than the other two carbon based materials. The reason is now not clear and needs
than
the investigation.
other two carbon based materials. The reason is now not clear and needs further investigation.
further
Figures
Figures10
10and
and11
11show
showthe
thecomparison
comparisonfor
forthe
themicrowave
microwavedielectric
dielectricproperties
propertiesof
ofthe
theprepared
prepared
TMMs
and
skin
tissue.
To
validate
the
proposed
materials,
we
compared
the
dielectric
properties
of the
TMMs and skin tissue. To validate the proposed materials, we compared the dielectric properties
of
prepared
samples
and
the
biological
tissues
measured
using
a
four-pole
Cole–Cole
model
summarized
the prepared samples and the biological tissues measured using a four-pole Cole–Cole model
and
parameterized
by Gabriel et al.
summarized
and parameterized
by[20]:
Gabriel et al [20]:
44
ÿ
εpωq
) ε 8 `
 ( “
 n
∆ε
i
n
` i ,

1´α  jωε
0
1
`
pjωτ
q
j
n
)
n“1 1  ( j
n 1
σ
1n
n
n
(4)
(4)
0
,
where
thethe
infinite
frequency,
ω isωthe
frequency,
τ is relaxation
time,
whereε8
ε∞isisdielectric
dielectricconstant
constantinin
infinite
frequency,
is angular
the angular
frequency,
τ is relaxation
and
σ and
is theσ ionic
By mapping
the required
microwave
dielectric
properties
of theof
skin
time,
is the conductivity.
ionic conductivity.
By mapping
the required
microwave
dielectric
properties
the
tissue,
the
doping
quantities
of
carbon
nanotube,
graphene
ink,
and
lignin
were
chosen
optimally
as
skin tissue, the doping quantities of carbon nanotube, graphene ink, and lignin were chosen
6optimally
wt %, 6 wtas%,
and
4
wt
%,
respectively.
The
results
exhibit
that
all
of
the
prepared
TMMs,
doped
by
6 wt %, 6 wt %, and 4 wt %, respectively. The results exhibit that all of the prepared
carbon
nanotube,
ink, andgraphene
lignin, are
suitable
to mimic
the skintotissue
controlling
TMMs, doped by graphene
carbon nanotube,
ink,
and lignin,
are suitable
mimicbythe
skin tissuethe
by
microwave
dielectric
properties
effectively.
controlling the microwave dielectric properties effectively.
Materials 2016, 9, 559
Materials 2016, 9, 559
Materials 2016, 9, 559
13 of 15
13 of 15
13 of 15
(a)
(a)
(b)
(b)
(c)
(c)
Figure 10. Comparison of dielectric constants between the skin tissue and the prepared samples
Figure 10. Comparison of dielectric constants between the skin tissue and the prepared samples with
Figure
10. Comparison
of dielectric of
constants
between
the (b)
skin
tissue and
with
different
doping concentrations
(a) carbon
nanotube;
graphene
ink the
and prepared
(c) lignin. samples
different doping concentrations of (a) carbon nanotube; (b) graphene ink and (c) lignin.
with different doping concentrations of (a) carbon nanotube; (b) graphene ink and (c) lignin.
(a)
(a)
Figure 11. Cont.
Materials 2016, 9, 559
14 of 15
Materials 2016, 9, 559
14 of 15
(b)
(c)
Figure 11.
11. Comparison
Comparison of
of conductivities
conductivities between
between the
the skin
skin tissue
tissue and
and the
the prepared
prepared samples
samples with
with
Figure
different
doping
concentrations
of
(a)
carbon
nanotube;
(b)
graphene
ink
and
(c)
lignin.
different doping concentrations of (a) carbon nanotube; (b) graphene ink and (c) lignin.
In our sample, the dielectric properties are stable and unchanged within two weeks after
In our sample, the dielectric properties are stable and unchanged within two weeks after
preparation. It was reported that the oil-in-gelatin materials obtained long-term stability [9]. Since
preparation. It was reported that the oil-in-gelatin materials obtained long-term stability [9]. Since the
the basic matrix of the tissue-mimicking material is oil-in-gelatin, it is expected that the proposed
basic matrix of the tissue-mimicking material is oil-in-gelatin, it is expected that the proposed material
material can have a good stability. We are now still studying comprehensively the effect of the
can have a good stability. We are now still studying comprehensively the effect of the carbon based
carbon based materials on the mechanical properties and long time stability with different
materials on the mechanical properties and long time stability with different environment conditions
environment conditions such as temperature and humidity.
such as temperature and humidity.
4. Conclusions
4.
Conclusions
In this
thispaper,
paper,
have
successfully
synthesized
oil-in-gelatin
based materials
doped
with
In
wewe
have
successfully
synthesized
oil-in-gelatin
based materials
doped with
different
different of
amounts
carbon
based materials,
carbon nanotube,
andfor
lignin
amounts
carbonofbased
materials,
includingincluding
carbon nanotube,
graphenegraphene
ink and ink
lignin
the
for
the
tissue-mimicking
use.
The
dielectric
constant
and
conductivity
of
the
prepared
materials
both
tissue-mimicking use. The dielectric constant and conductivity of the prepared materials both increased
increased
by adding
more
amounts
of carbon
graphene
ink due capacitor
to the parasitic
by
adding more
amounts
of carbon
nanotube
andnanotube
grapheneand
ink due
to the parasitic
effect.
capacitor with
effect.doping
However,
with
lignin,
the dielectric
constant and
thefirst
conductivity
first
However,
lignin,
thedoping
dielectric
constant
and the conductivity
both
increased both
and then
increased
and
then
slightly
decreased,
indicating
a
saturation
of
the
doping
effect.
The
results
slightly decreased, indicating a saturation of the doping effect. The results showed that the dielectric
showed that the dielectric properties of the oil-in-gelatin based material can be further tuned by
properties of the oil-in-gelatin based material can be further tuned by adding a small amount of the
adding a small amount of the carbon based material and mapping it to the dielectric properties of
carbon based material and mapping it to the dielectric properties of the special tissue.
the special tissue.
Acknowledgments: This study was funded by the Metal Industries Research and Development Center,
and
Ministry of Economic
Taiwan.
Theby
authors
would
also like Research
to thank Eric
Tsai Center,
of National
Acknowledgments:
This Affairs,
study was
funded
the Metal
Industries
andChih-Ming
Development
and
Cheng
Kung
University,Affairs,
TaiwanTaiwan.
for supplying
microwave
measurement
technology.
Ministry
of Economic
The authors
would
also like to thank
Eric Chih-Ming Tsai of National
Cheng Kung
University,Min-Hang
Taiwan forWeng,
supplying
microwave
measurement
technology.
Author
Contributions:
Ru-Yuan
Yang, Shoou-Jinn
Chang
and Tsung-Chih Yu conceived and
designed the experiments; Siang-Wen Lan and Yaoh-Sien Chung performed the experiments; Siang-Wen Lan
Author
Contributions:
Min-Hangthe
Weng,
Chang
andChun-Sen
Tsung-Chih
conceived
and
Yaoh-Sien
Chung analyzed
data;Ru-Yuan
Ru-YuanYang,
Yang,Shoou-Jinn
Tsung-Chih
Yu and
WuYu
contributed
and designed the experiments;
Siang-Wen
LanLan
andand
Yaoh-Sien
Chung
performed
experiments;
reagents/materials/analysis
tools;
Siang-Wen
Min-Hang
Weng
wrote thethe
paper;
Ru-YuanSiang-Wen
Yang and
Shoou-Jinn
Chang advised
experiments.
Lan and Yaoh-Sien
Chungthe
analyzed
the data; Ru-Yuan Yang, Tsung-Chih Yu and Chun-Sen Wu contributed
reagents/materials/analysis
tools;
Siang-Wen
Lan and
Min-Hang Weng wrote the paper; Ru-Yuan Yang and
Conflicts of Interest: The authors declare no conflict
of interest.
Shoou-Jinn Chang advised the experiments.
Materials 2016, 9, 559
15 of 15
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