Miniaturized Implantable Coplanar Waveguide Antenna for Biomedical Applications

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Miniaturized Implantable Coplanar Waveguide
Antenna for Biomedical Applications
Adel Damaj, Hilal M. El Misilmani, and Soubhi Abou Chahine
Electrical and Computer Engineering Department
Beirut Arab University
Beirut, Lebanon
adelwd@gmail.com, hilal.elmisilmani@ieee.org, achahine@bau.edu.lb
Abstract— Implanting electronic devices in the human body is a
delicate operation that can be hazardous and of negative
consequences. Such implantable devices must be safe and small
without sacrificing their performance characteristics, namely their
efficiency and accuracy. This paper presents, the design,
simulation, fabrication, and validation of small antenna
implementation suitable for implanting in the human body. The
development comprises small CPW feed antenna that can operate
around 1.4 GHz standard Wireless Medical Telemetry Service
(WMTS) band, with an ability to tune the frequency by merely
varying the length of the monopole antenna. The behavior of the
designed antenna thoroughly analyzed under a state-of-the-art
simulator. The effectiveness of the fabricated antenna ensured
through extensive in vitro tests and comparisons with the response
obtained through simulations. The results confirm the successful
development of small and implantable antenna with appealing
performance characteristics.
Keywords—Implantable antennas, biomedical applications,
biomedical telemetry, CPW feed antennas.
I. INTRODUCTION
The importance of implantable devices can quickly be
figured, a tremendous number of people use such tools, by which
they are used to improve the conditions and the quality of their
lives [1]. Implantable devices are currently taking a considerable
interest in biomedical applications, ranging from human’s
treatment to medical diagnosis and prevention, such as sugar
level check, blood pressure measurements, pacemaker
connection, insulin push out, real-time pressure measurements,
and dental system for health care applications [2]. In such
applications, sensors can also be implemented to collect and
transmit information needed into a nearby receiver connected to
a computer, which simulates the required data by doctors [3].
The implanted sensors usually consist of bio-actuators, antenna,
and power supply [4]. A typical implanted biomedical system is
shown in Fig. 1. Several frequencies bands are recommended to
be used in medical implant communications; they are listed in
Table 1.
Due to the impressive features of the coplanar waveguide
(CPW), such as proper impedance matching, wide bandwidth,
lower radiation losses and less dispersion than microstrip
antennas, easy integration with active devices and its simplicity
upon fabrication on a single metallic layer, CPW has been an
attractive solution for many applications. An additional feature
that can also be used in this work is the antenna size
TABLE I. FREQUENCY BANDS OF MEDICAL IMPLANT COMMUNICATION
Frequency Band
Frequency range
(ISM): industrial, scientific and
medical radio bands
clustered around 2.4 GHz
(MICS): Medical Implant
Communications Services
401–406 MHz
(WMTS): Wireless Medical
Telemetry Service
608-614 MHz, 1395-1400 MHz,
and 1427-1432 MHz
WMTS
pc
In Door
Data Transfer
Via Internet
Medical Center
WMTS
Out Door
GSM Link
Long distance wirless data transfer
Blue Tooth
Figure 1. Wireless implantable system
reduction [5]. For this, CPW is used as the method of feeding in
the proposed system.
Many efforts have been made to design CPW-fed antennas
capable of single-band operation or capable of dual or multiband
operation but with more complex geometrical structure. This
paper aims to design a compact implanted CPW antenna, to be
used in biomedical applications. These antennas had
encountered fast growth due to their use as an essential part in
the overall implanted communication system for bidirectional
communication with the outside monitoring/ control equipment
[6].
A miniaturized -slot PIFA antenna for implanted biomedical
applications is presented in [7]. The antenna has the following
dimensions: 19×30×1.6 mm3, and can operate in the MICS band
at 433MHz, WMTS band at 1430 MHz and ISM band at 2.4
GHz. In [8], a dual folded dipole antenna of a 25×34×2.5 mm3
size is presented, operating in the ISM band and WMTS. A
compact and low-profile monopole antenna with a simple
structure operating in the 2.6 – 2.73 GHz frequency band is
presented in [9].
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TABLE II. COMPARISON BETWEEN DIFFERENT ANTENNA STRUCTURES
AND FREQUENCY BANDS USED IN REFERENCE PAPERS
Ref.
Dimensions
(mm)
Design
Type
Frequency
Band
Operating
frequency
[10]
15×15×1.92
L- Shaped
fed spiral
ISM, MICS
868 MHz
[7]
19 ×30 ×1.6
Slot PIFA
ISM,
MICS,WMTS
433 MHz,
1430 MHz,
2400 MHz
[11]
8.5×25.9×3.2
Folded slot
dipole
ISM
2450 MHz
[12]
15×15
PIFA
ISM
2450 MHz
[8]
25×34×2.53
Dual folded
ISM, WMTS
1400 MHz,
2400 MHz
[13]
11.5×8×8
Fractal Arc.
spirals
MICS
400 MHz
[14]
14×14
Magnetic-
type loop
MICS
403 MHz
[15]
18 × 24 ×
0.65
L- slot,
U- slot
ISM
2400 MHz
[16]
65×40
PIFA
MICS
441 MHz
[17]
18.5×25.9
Slot Dipole
ISM
2430 MHz
[18]
14×14×15.
Spiral
MICS
402 MHz
[19]
30×30×1.6
Dual -
spiral
ISM
2410 MHz
A rectangular patch of CPW monopole implanted antenna
operating at a frequency of 2.45 GHz, with a bandwidth of 320
MHz, covering the ISM band, is presented in [15]. The antenna
dimensions are 18×24×0.65 mm3. A helical antenna attached to
a wireless sensor is presented in [4], with a size of 15×4.5 mm2,
operating at 868 MHz A compact L-shaped fed spiral antenna
with dimensions 15×15×1.92 mm3, designed for operation in
MICS and ISM band, is presented in [10]. An antenna operating
in the ISM band at 2.45 GHz, is shown in [11], with dimensions
of 8.5×25.9×3.2 mm3. A combination of Hilbert-type fractal and
Archimedean Spirals has presented in [19] with a size of
8×11.5×8 mm3. A summary of the comparison of different
biomedical antenna structures and frequency bands used in the
referenced papers listed in Table II.
In this paper, a compact coplanar waveguide (CPW) antenna,
suitable for implantable biomedical application, operating at 1.4
GHz (WMTS), with a capability to be tuned from around 1.2 –
2 GHz is proposed. The main advantage of the proposed antenna
is its compact size, reaching 10×18×1.59 mm3, with an
important size reduction compared to the smallest investigated
antennas operating at this frequency and suitable for biomedical
applications.
The paper is organized as follows. Section II presents the design
of the proposed antenna, along with the used model of three
layers, muscle, fat, and skin, that is used in simulations and to
mimic the human body. Section III presents the simulations and
results of the proposed antenna, reaching to a design operating
in the desired frequency range. The simulations in this section
are done using CST, with the antenna inserted in the body model.
Section IV presents the fabricated antenna, and the measurement
procedure in vitro using pork, meat slice. A good analogy has
seen when comparing the simulated reflection coefficients
results to those measured in vitro.
Lp
Lf
W1
L2
Wf
gg
Wp
d
w
L
L1
W1
Lp
Wp
H
Lf
x
y
z
Figure 2. Antenna Dimensions.
SKIN
FAT
MUSCLE
Figure 3. Design of the three layers’ model
II. ANTENNA DESIGN
Designing a compact antenna suitable for implementation is
a target for many types of research. Due to the fundamental
limitations in size and performance, achieving miniaturization
with good antenna performance is challenging. Electrically
small antennas are limited in bandwidth and radiating efficiency.
In general, miniaturization techniques might include choosing a
suitable dielectric material, so a high dielectric substrate leads to
smaller resonance frequencies, and optimizing the antenna
structure itself, e.g., stretching the current way of the radiator or
using meander line antenna. A thorough investigation of
different designs suitable for implantable biomedical application
has been done in [2]. It is seen that all the investigated antennas
possess overall dimensions ranging between 30 to 55 mm in
length, and 5 to 10 mm in width. The aim of this paper is to
design an antenna working in the lower frequency band, with
overall dimensions that outperform the miniaturization done in
the investigated papers.
The proposed coplanar waveguide antenna is shown in Fig. 2.
The different antenna parameters indicated have been
investigated, with an aim to reach a resonance in the WMTS
band, with a compact size. This was achieved with the
dimensions listed in Table IV, for which a resonance at 1.4 GHz
has been attained. Fig. 4 shows the reflection coefficient results
of simulating the proposed antenna for different lengths (L1). As
the length increases, the resonance frequency decreases between
1.2 – 2 GHz, by simply varying the length of the monopole
antenna. Table V lists the different resonance frequencies
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TABLE III. DIMENSIONS OF THE FABRICATED ANTENNA.
Parameters
Values (mm)
Wf
1.4
Lf
7.25
W1
3.6
L1
7
Wp
4.1
Lp
5.6
g
0.2
d
1
W
10
L
18
H
1.56
Figure 4. Reflection coefficient with different length (L1) sizes
TABLE IV. RESONANT FREQUENCIES FOR DIFFERENT LENGTHS
L1 (mm)
Resonant frequency (GHz)
2
1.971
3
1.851
4
1.731
5
1.61
6
1.5
7
1.41
8
1.34
9
1.27
10
1.21
attained in each case. As can be seen, with a length L1 of 7 mm
a resonance frequency of 1.41 GHz is achieved, which is suitable
for biomedical applications (WMTS) band. The gain pattern of
the proposed antenna at 1.41 GHz is shown in Fig. 5, with main
lobe gain of 1.81 dB.
The reached size of the antenna is 10×18×1.59 mm3, which
is smaller than all antennas studied in the literature and designed
to be used in implantable biomedical application, and more
specifically in the single band antennas operating around 1.4
GHz. This makes this miniaturized proposed antenna of high
advantage.
Figure 5. Gain pattern
III. FABRICATION AND MEASUREMENTS
The proposed antenna with a length L1 of 7 mm has been
fabricated and tested. Fig. 6 shows the fabricated antenna. Fig. 7
shows the in vitro measurement procedure, for which the
antenna is inserted into pork meat, which has very close electric
properties to that of the human body. Fig. 8 shows a comparison
of the reflection coefficient results, simulated and measured,
where a good agreement between the simulated and measured
results is seen. It can be inspected that the measured bandwidth
is significantly larger than the simulated one, which could be due
to soldering, fabrication, and tissues effect.
Fig 6. Fabricated antenna
Fig 7. Fabricated antenna implanted
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Fig 8. Reflection coefficient for L=7 mm
IV. CONCLUSION
An electrically small coplanar waveguide antenna suitable
for biomedical applications has been presented. With a compact
size of 10×18.5×1.56 mm3, the proposed antenna has a simple
structure and easy to be printed on an FR4 substrate. A
parametric study on the length of the antenna has been done to
reach a resonance at 1.41 GHz (WMTS), suitable for biomedical
applications. The antenna has been further fabricated and tested
in vitro using pork meat, where a good analogy has been seen
between the simulated and measured reflection coefficient
results.
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