Microwave artificially structured periodic media microfluidic sensor

Abstract

In this paper, microstrip-based spiral structured artificial magnetic media (metamaterial) coupled with microfluidic channel is experimentally demonstrated for sensing applications. It is found that the resonant frequency and the amplitude changes due to dielectric loading from the introduction of chemical substances in the microfluidic channels. Different concentrations of water - methanol and water - isopropanol samples are used in the characterization of the sensor. For water - methanol mixtures, the resonant frequency shifts from 2.15 GHz to 2.0 GHz with change in dielectric constant from 25 to 75. Results show that the wave propagation in LH-media can be used for interrogation of minute volumes of samples with high sensitivity.

Full text

Page 1

Microwave Artificially Structured Periodic Media Microfluidic Sensor

Nophadon Wiwatcharagoses, Kyoung Youl Park, Jose A. Hejase, Lanea Williamson, and Prem Chahal
Terahertz Systems Lab (TeSLa)
Department of Electrical and Computer Engineering, Michigan State University
East Lansing, MI 48824, USA

Abstract
In this paper, microstrip-based spiral structured artificial
magnetic media (metamaterial) coupled with microfluidic
channel is experimentally demonstrated for sensing
applications. It is found that the resonant frequency and the
amplitude changes due to dielectric loading from the
introduction of chemical substances in the microfluidic
channels. Different concentrations of water - methanol and
water - isopropanol samples are used in the characterization of
the sensor. For water - methanol mixtures, the resonant
frequency shifts from 2.15 GHz to 2.0 GHz with change in
dielectric constant from 25 to 75. Results show that the wave
propagation in LH-media can be used for interrogation of
minute volumes of samples with high sensitivity.

Introduction
Rapid characterization of chemical and biological samples
is increasingly important in clinical, security, safety, drug
discovery and industrial applications. Sensing approaches are
needed that does not require tagging, (e.g., using fluorescent
markers) in order to maintain the samples in their original
form while under study. Along with rapid label-free
characterization, interrogation of small sample volumes is
critically needed in the areas of clinical diagnosis and drug
discovery. In this paper, periodic media co-integrated with
microfluidic leading to a novel RF near-field sensor is
implemented to tackle these challenges. The proposed sensor
is simple, cost effective, and can be used for label-free sensing
and detection
Spiral structured artificial magnetic media (metamaterial)
designs have been widely used in the design of compact
coplanar waveguides (CPW) and microstrip-based circuit
topologies. Recently, split-ring based metamaterial structures
that are edge-coupled to a microstrip line have been used in
the sensing of biomolecules [1]. In this structure, the
interrogation signal (RF) edge couples from a microstrip
transmission line to a ring resonator. The biomolecules are
made to bind onto the ring resonator. A direct approach of
interrogation will be desirable which is more compact and
provides improved sensitivity and yet still simple to fabricate
and implement. To meet this goal, in this paper, metamaterial
structure that is integral part of the microstrip line is employed
for sensing application. A spiral based metamaterial
transmission was recently introduced, [2 – 3], and this design
is implemented here for sensing applications.
In spiral based metamaterial transmission lines, the
periodic arrays of the spiral structure employ left-handed (LH)
propagation properties and support backward waves at their
fundamental resonance [2]. Motivated by the wave
propagation phenomenon of this medium, a microfluidic
sensor that interrogates samples in the near-field region is
attractive to achieve high sensitivity using low-volumes of
samples. This sensor is designed and implemented for
different resonant frequencies and results from one of the
design are presented here. Microfluidic channels and the
metamaterial based transmission lines are co-integrated on a
single substrate to achieve high sensitivity. The proposed
sensor consists of spiral resonators positioned in series
forming a double and triple spiral LHMs transmission line
structure fabricated on a commercially available printed
wiring board. The micromachined fluidic channel is designed
and fabricated on the top layer of the spiral structure signal
strips. The proposed method can provide the benefit of faster,
easier, low-cost, high-throughput, and more convenient
analysis of chemical substances and biological specimens at
RF frequencies.

Design and Fabrication
Fig. 1 shows the layout of an artificially structured
periodic media to be used for the design of a sensor. It is
composed of two single spirals connected in series on the
front side and a solid ground plane on the back side of the
board. This unit cell forms a double spiral resonator with
dimensions dx = 6.4 mm and dy = 3.2 mm. The periodic
analysis of the double spiral unit cell shows that the structure
supports backward waves as LH media at their fundamental
resonance [3]. Fig. 2 shows a fabricated metamaterial
transmission line based on triple cells of double spiral
components. It was fabricated on RT/Duroid® 5880 substrate
having dielectric constant εr = 2.2 and thickness of 1.58 mm
using conventional microfabrication
measured and simulated, using the Ansoft HFSS®, frequency
responses are shown in Fig. 3. These results are before the
integration of microfluidic layer. The simulated and measured
responses match very closely and the measured insertion loss
is approximately -1 dB for this structure. Upon fabrication of
microfluidic layer on top of this structure, the resonance
frequency shifts to lower frequency as discussed ahead.
Microfluidic channels were fabricated from elastomer
polydimethylsiloxane (PDMS) using a SU8-2000 mold. A
100 µm thick layer of SU8-200 was spin coated on a thin
polyimide (PI) film (50 µm). The polyimide film in turn was
attached to a Si wafer during spin coating. SU-8-2000 has
better adhesion with PI than bare Si-wafer. A prebake process,
UV lithography, postbake, and etching are performed to
fabricate the mold (master). To form the microfluidics
channels, Polydimethyl Siloxane (PDMS) elastomer is used
that is a two-part resin system containing vinyl groups and
hydrosiloxane groups. PDMS is a soft material and it can
easily be separated from the SU-8 master, leaving the master
intact for the fabrication of additional microfluidic channels.
The PDMS microfluidic channel is designed in S-shape
following the spiral line to couple with locations having
approaches. The

Page 2

stronger electric and magnetic fields on the spiral structures.
An example fabricated microfluidics channel made from
PDMS is shown in Fig. 4. The depth of the microfluidic
channel is 100 µm.


Fig. 1 Model of the artificially structured periodic media based double spiral.


Fig. 2 Fabricated Metamaterial microstrip transmission line based double
spiral structure.

Scattering Parameters
Fig. 3 Simulated and measured frequency responses of the metamaterial
microstrip technology based spiral structure. The simulated structure model is
depicted in the inset.

Instead of directly attaching the mold to the circuit board,
a thin film of PI (50 µm) was introduced between the PDMS
channels and the copper traces. This film was attached to
PDMS using a siloxane adhesive. Polyimide acts as a
chemical barrier between the copper traces and the liquid
sample in the channels. Although this reduces the sensitivity,
but it helps prevent any damage to the copper traces if harsh
chemicals are to be interrogated. Mounting of the PDMS onto
the metamaterial microstrip structure loads the circuit and
changes the S-parameters of Fig. 3. Simulated and measured
S-parameters of this loaded structure are shown in Fig. 5 and
they match very closely. Slight difference may be due to the
dielectric properties of the PDMS used in the simulation of
the structure.


Fig. 4 PDMS microfluidic channels (a) photograph showing the close up (b)
optical photograph of the channel before PI film attachment.

Scattering Parameters
Fig. 5 Simulated and measured frequency responses of the fabricated
metamaterial RF device attached with PDMS microfluidic channel on the top.
The simulated structure model is depicted in the inset.

RF Measurements and Discussions
Preliminary tests were carried out with the microfluidic
channel filled with different concentrations of 2-propanol-
water and methanol-water sample mixtures. The solutions
were prepared by molar fraction from Mallinckrodt Chemicals
and de-ionized water at the concentration index, X = 0, 0.2,
0.4, 0.6, 0.8, and 1.0 at the room temperature. During the
measurements, the sample was allowed to continuously flow
using a syringe pump. Teflon tubing (inner diameter = 0.8
mm, outer diameter = 1.6 mm) was attached to the PDMS
using epoxy. A syringe-30mL was used for sample handling.
A Vector Network Analyzer was used to measure the
scattering parameters of the loaded transmission line.
Sufficient amount of liquid was allowed to flow through
between samples to remove any residue in the microfluidics
channels between the measurements. The transmission (S21)
and reflection (S11) scattering parameters were measured for
all the sample solutions. A close-up view of the measurement
setup is shown in Fig. 6.
Fig. 7 and Fig 8 show the measured scattering-parameters
of different concentrations of water - methanol and water - 2-
propanol liquid solutions. The transmission properties and the
resonant frequency (fr) shifts depending upon the dielectric
properties of the sample in the microfluidic channel. The shift
11.522.533.54
-60
-50
-40
-30
-20
-10
0
Frequency (GHz)
S11 and S21 (dB)


s11-simulation
s11-experiment
s21-simulation
s21-experiment
11.522.533.54
-60
-50
-40
-30
-20
-10
0
Frequency (GHz)
S11 and S21 (dB)


s11-simulation
s11-experiment
s21-simulation
s21-experiment

Page 3

in frequency indicates the effective dielectric constant and the
change in amplitude indicates the loss-tangent of the liquid
sample. Minimum value of the S11 parameter was used as the
reference point of frequency measurement. This frequency
can easily be measured from the phase plot of S11.



Fig. 6 Fabricated Metamaterial RF device with PDMS microfluidic channels
attached.

Methanol+water mixtures
(a)
Measured insertion losses
(b) Measured return losses
(c)
Measured phase insertion losses
Fig. 7. Measured scattering parameters under test corresponding different
concentrations of water - methanol mixtures
The mixtures in various molar fractions can be translated
approximately to the dielectric constant, ε´ by the Davidson-
Cole equation that is expressed by [4].
?∗(?) = ??+ ∆?/(1 + ???)?
where ε* is the complex permittivity at an angular frequency
ω, ε is the permittivity at ? → ∞, Δε is the relaxation
increment, and τ is the relaxation time. Referring to the data
from T. Sato et al. [3], the dielectric constants of water-
methanol mixtures in various frequencies can be investigated
by Eq. (1). Fig 9 shows the dielectric constant (ε´) and loss
tangent (tan-δ) of water-methanol solutions for various
concentrations based on the above equation. The results show
that the dielectric constant and loss-tangent of methanol are
approximately 27 and 0.65 at 2 GHz, respectively. Also for
DI-water, these values are approximately 80 and 0.12,
respectively. These values match very closely with those
reported by J. Barthel et al. [5] and C. Oliver Kappe et al. [6].
These were used in the translation of concentration to
dielectric constant values.

IPA+water mixtures
(1)
(a)
Measured insertion losses
(b) Measured return losses
(c)
Measured phase insertion losses
Fig. 8. Measured scattering parameters under test corresponding different
concentrations of water - 2-propanol mixtures
11.21.41.61.822.12.22.42.62.83
-30
-25
-20
-15
-10
-5
0
Freq (GHz)
s11 (dB)


X=0.0
X=0.2
X=0.4
X=0.6
X=0.8
X=1.0
11.21.41.61.822.12.22.42.62.83
-80
-70
-60
-50
-40
-30
-20
-10
0
Freq. (GHz)
s21 (dB)
Methanol+water mixtures


X=0.0
X=0.2
X=0.4
X=0.6
X=0.8
X=1.0
11.21.41.61.822.12.22.42.62.83
-135
-90
-45
0
45
90
135
Freq (GHz)
s11-Phase (Degree)
Methanol+water mixtures


X=0.0
X=0.2
X=0.4
X=0.6
X=0.8
X=1.0
11.21.41.61.822.12.22.42.62.83
-35
-30
-25
-20
-15
-10
-5
0
Freq. (GHz)
s11 (dB)


X=0.0
X=0.2
X=0.4
X=0.6
X=0.8
X=1.0
11.21.41.61.822.12.22.42.6 2.83
-80
-70
-60
-50
-40
-30
-20
-10
0
Freq. (GHz)
s21 (dB)
IPA+water mixtures


X=0.0
X=0.2
X=0.4
X=0.6
X=0.8
X=1.0
11.21.41.61.81.9
Freq. (GHz)
22.12.22.42.62.83
-135
-90
-45
0
45
90
135
s11-Phase (Degree)
IPA+water mixtures


X=0.0
X=0.2
X=0.4
X=0.6
X=0.8
X=1.0

Page 4

(a)
(b)
Fig. 9. Dielectric constant and loss tangent of water-methanol mixtures at
various frequencies based on the data from [3].

Water and Methanol Mixtures
(a)
(b)
Fig. 10. Dielectric constant of water-methanol (a) and water-2-propanol (b)
mixtures by cubic curve fitting based on the data from [3].

Referring from the study by J. Barthel et al. [5], dielectric
relaxation parameters of 1-propanol and 2-propanol are
almost the same and are readily available. For first order
approximation, the dielectric relaxation parameters of water -
1-propanol in [4] are used in Eq. (1) for the dielectric
relaxation parameters of water-2-propanol samples used in
this paper. As a result, the correlation between dielectric
constants of water –methanol and water-2-propanol mixtures
with different concentration index can is plotted in Fig. 10.
After converting the unit from the concentration index to
dielectric constant, the variation of the frequency, fr with
approximate dielectric constant (ε´) of water-methanol and -2-
propanol mixtures can be expressed in Fig. 11. This shows a
significant improvement in sensitivity as compared to data
presented in ref. [7] for similar liquid mixtures. Furthermore,
the circuit is simple to design, is compact and easy to integrate
with microfluidic channels.

Water- Methanol and -2-Propanol Mixtures
Fig. 11. Change in the resonance frequency, S11 (fr), with approximate
dielectric constant of water –methanol and water-2-propanol mixtures.

Conclusions
A novel microstrip-based metamaterial transmission line
coupled with microfluidic channel is demonstrated for sensing
of micro-liter volume of samples. The fluidic channels were
routed directly above the locations on the spiral where the
fields are strong in order to achieve high sensitivity. For
methanol-water mixture, the resonant frequency shifts from
2.15 GHz to 2.0 GHz with change in dielectric constant from
25 to 75. For water-2-propanol mixture, the resonant
frequency shift from 2.25 to 2.0 with dielectric constant
change from 4 to 80. Further improvements in sensitivity can
be made by using a very thin chemical barrier PI layer
between the copper traces and the microfluidic channels.
Results of this paper show that LH-media can be used in
making a sensor having very high sensitivity. Further
improvements can lead to sensing of nano- to pico-liter
sample volumes. The sensor can be used for sensing of
various types of chemical and biological agents both in liquid
and solid form. Furthermore, the frequency band of
interrogation can be designed to sense specific samples in
order to achieve high specificity along with high sensitivity.

Acknowledgments
The authors would like to thank Rogers Corporation for
providing the substrate materials, Mr. Xianbo Yang and Mr.
Brian Wright for helpful discussions.

References
1. H. Lee, and J. Yook, “Biosensing using split-ring
resonators at microwave regime”, Applied Physics
Letters, 92, pp. 254103-1 – 3, 2008.
2. Y. Guo, G. Goussetis, A. P. Feresidis, and J. C.
Vardaxoglon, “Efficient modeling of novel uniplanar left-
00.1 0.20.30.4
Concentration index (X)
0.50.60.70.80.91
20
30
40
50
60
70
80
Dielectric constant
Water and Methanol Mixtures


f1 = 1.9 GHz
f2 = 2.0 GHz
f3 = 2.1 GHz
f4 = 2.2 GHz
f5 = 2.3 GHz
00.10.20.30.4
Concentration index (X)
0.50.60.70.80.91
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Loss tangent
Water-Methanol Mixtures


f1 = 1.9GHz
f2 = 2.0 GHz
f3 = 2.1 GHz
f4 = 2.2 GHz
f5 = 2.3 GHz
00.10.20.3 0.4
Concentration index (X)
0.5 0.60.70.80.91
20
30
40
50
60
70
80
Dielectric constant


0 0.10.20.30.4
Concentration index (X)
0.50.60.70.80.91
0
10
20
30
40
50
60
70
80
90
Dielectric constant
Water and 2-Propanol Mixtures


0 102030405060 70 8090
1.95
2
2.05
2.1
2.15
2.2
2.25
Dielectric Constant
Frequency (GHz)


water-methanol mixtures
water-2-propanol mixtures

Page 5

handed metamaterials”, IEEE Trans. Microwave Theory
Tech., vol. 53, pp. 1462-1468, Apr. 2005.
3. T. Kokkinos, A. P. Feresidis, and J. C. Vardaxoglou,
“Analysis and application of metamaterial spiral-based
transmission line”, IEEE IWAT ’07 International
Workshop on (2007), pp. 233-236, Mar. 2007.
4. S. Mashimo, and T. Umehara, “Structure of water and
primary alcohol studied by microwave dielectric
analyses”, J. Chem. Phys., vol. 95, pp.6257-6260, Nov.
1991.
5. J. Barthel, K. Bachhuber, R. Buchner, and H. Hetzenauer,
“Dielectric spectra of some common solvents in the
microwave region water and lower alcohols”, Chem. Phys.
Lett., vol. 165, pp.369-373, Jan. 1990.
6. C. Oliver Kappe, and A. Stadler, Microwave in Organic
and Medicinal Chemistry, Wiley-VCH (Weinheim, 2005),
pp. 13.
7. C. Chunrong, and P. Wang, “A radio frequency device for
measurement of minute dielectric property changes in
microfluidic channels”, Appl. Phys. Lett., vol. 94, 023901,
Jan. 2009.