ENERGY HARVESTING FOR SELF-FOLDING MICRO
DEVICES
Pedro Anacleto
1,2
, Evin Gultepe
2
, David H. Gracias
2,3
and Paulo M. Mendes
1
1
Centro Algoritmi, Universidade do Minho, Braga, Portugal
2
Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, U.S.A.
3
Department of Chemistry, Johns Hopkins University, Baltimore, U.S.A.
Keywords: Biomedical Device, Micro Antennas, Small Antennas, Energy Harvesting, Micro Fabrication.
Abstract: The miniaturization of medical devices allows numerous new solutions in medicine including implantable
devices that can diagnose, treat and monitor patients. Drug delivery systems have the potential to drastically
change the drug administration and hence to improve their therapeutic efficiency. These devices can be
fabricated by combining self-folding methods with the conventional multi-layer lithography. This combined
lithography technique allows precise patterning of two dimensional (2D) templates that can transform into
three dimensional (3D) structures with higher surface area to volume ratios. The same technique allows the
incorporation of small antennas with devices and enabling wireless capabilities. An efficient wireless link
between an external reader and the implanted device provides a remarkable advantage to both patients and
caregivers including greater patient ease of movement, continuous data feeds, higher quality and reliability
of data reporting. This paper proposes a system which is composed of a 500x500μm
2
square loop antenna
with 5GHz operating frequency, embedded on a SU-8 cubic container suitable for small implantable
medical devices.
1 INTRODUCTION
Implantable biomedical devices are becoming
smaller and smaller due to the availability of the
high levels of miniaturization. Millimeter scale
structures can work as medical tools (Randall, 2011)
(Rahimi, 2011) and perform tasks that are only
possible because of their miniaturized dimensions.
Several miniaturized tools such as drug delivery
systems (Yang, 2009), blood glucose (Ahmadi,
2009), blood pressure (Cong, 2009), ECG
monitoring systems (Fu, 2011) and neural probes
(Kensall, 2008) have been reported in the literature.
Implantable drug delivery systems have a variety
of applications in medicine due to their efficacy to
control the administration of drugs both in time and
space. They have the ability to control drug release
rate and to target specific organs or locations inside
the body. With local precision, effective dosage and
prompt drug release, these devices can reduce the
negative side-effects of traditional systemic
medication and significantly improve the therapeutic
efficiency (Barbé, 2004). Drug release systems can
be divided into two main categories: Passive and
active release. Passive devices depend on the rate of
diffusion for the drug release while active devices
can control release by an external trigger such as a
RF signal (Smith, 2007). These devices can be
fabricated using standard lithographic techniques
that can pattern on silicon wafer substrates at
micrometer and/or nanometer scale. The main
limitation of the standard lithographic fabrication
technique is its two dimensional (2D) restrictions.
2D nano or micro fabrication can result in bulky
devices when compared with 3D structures that have
greater surface area to volume ratio allowing more
usable surface and small form factors (Randall,
2007).
Self-folding of lithographically patterned structures
can overcome the inherent 2D limitations, thus
enabling 3D structures. Figure 1.A shows a 2D
template with liquefiable hinges that folds upon
heating and transforms into a 3D structure. This
process enables the fabrication of containers (Figure
1.B) that can be made out of metals, oxides and
polymers which are suitable for drug delivery
applications. The fabrication of optically transparent
polymeric containers has been reported (Azam,
364
Anacleto P., Gultepe E., H. Gracias D. and M. Mendes P..
ENERGY HARVESTING FOR SELF-FOLDING MICRO DEVICES.
DOI: 10.5220/0003793103640367
In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2012), pages 364-367
ISBN: 978-989-8425-91-1
Copyright
c
2012 SCITEPRESS (Science and Technology Publications, Lda.)
2010) where polymer panels were patterned using
conventional photolithography. The reported
lithographic fabrication process is also compatible
with electronic components integration which
represents a promising new approach in the design
and fabrication of implantable devices with
integrated antennas.
Figure 1: Fabrication of 3D structures using planar
lithography. (A) SEM image of a 2D template of a cubic
container. Bar scale: 10μm. (B) SEM image of a container
after self-folding. Bar scale: 100μm.
Implanted medical devices with wireless
capability allow efficient communications between
the device and an external reader, providing a higher
level of autonomy for either inpatients or outpatients
and thus improving their quality of life (Fang, 2011).
A small polymeric box with an embedded efficient
antenna system can unveil a wireless link suitable
for such purposes. As an implanted device, the
described system has energy requirements that are
still challenging to fulfill especially for miniaturized
implants. While significant achievements in battery
technology reached higher levels of energy density
e.g. lithium-ion batteries, the bulky size of the
batteries considerably increases the implant
dimensions. Also, batteries eventually need to be
replaced (Olivo, 2011) which is not desired or
possible in some applications.
Transmitting power through electromagnetic
waves (EM) has been proposed as a promising
technology to resolve the dependency on the wired
energy transfer or storage (Jabbar, 2010)
(Nishimoto, 2010). However, harvesting energy
from an external power source is a challenge for
miniaturized devices because it requires an efficient
small antenna that has to fulfill the power
requirements of the device. The amount of power
harvested through EM radiation depends on antenna
parameters such as size, geometry, gain and
efficiency plus the power loss due to tissue
attenuation. The antenna plays a major role in the
wireless capability, and its performance gets
severely constrained due to the high degrees of
miniaturization (Huang, 2011).
2 PROPOSED SYSTEM AND
ANALYSIS
This paper proposes the integration of a square loop
antenna with a polymeric container. Figure 2 shows
the schematic of a 2D polymeric template where the
antenna metallic structure is patterned as a common
wire antenna. After the self-folding, the 2D template
transforms into a cubic container with an embedded
square loop antenna.
Figure 2: The schematic of the proposed system. A) 2D
template of the container with an antenna (red) patterned
onto polymeric panels. B) The polymeric cube and
embedded antenna after folding.
The proposed system was simulated and
analyzed with AnSoft HFSS v.12 which utilizes
finite element technique for electromagnetic
computation. The model is composed of a cubic box
layer (5x5x5mm
3
) whose dielectric properties are
similar to those of real human tissues and the
proposed system is in the centre of the box. In order
to study the antenna properties when implanted in
human tissue, cubic tissue samples of skin, fat and
muscle were individually tested. Since the human
body can be considered as a non-uniform dielectric
with frequency dependent conductivity and
permittivity all tissues were programmed with their
specific dielectric properties for the tested
frequencies (values taken from (Gabriel, 1996)).
The simulated antenna design (Figure 3) has a
total length of 1.98mm (with a lumped port of
20x20μm
2
), with a 0.2x10
2
μm cross section (200nm
thick and 10μm wide). Gold and SU-8 were chosen
as the antenna and the cube material respectively.
Among the possible antenna geometries, a loop
antenna was chosen since its square geometry is
suitable for this specific application. The loop
antenna efficiently utilizes the container's cubic
shape while maximizing the antenna electrical size
and it is compatible with the container’s fabrication
process.
As a result of the physical size of the antenna
which is inversely proportional to the frequency, the
proposed antenna has a resonant operating central
ENERGY HARVESTING FOR SELF-FOLDING MICRO DEVICES
365
frequency of 163 GHz. At this frequency severe
tissue attenuation occurs and virtually no power can
be transmitted to the implant. Thus, the antenna was
matched to operate at lower frequencies between 5
to 20 GHz by changing its feeding source resistance
and capacitance.
Figure 3: HFSS design of proposed antenna and SU8 cube.
The HFSS radiation efficiency was the chosen
antenna parameter to evaluate the efficiency of the
wireless link since it reflects not only the antenna's
efficiency but also the tissue attenuation losses. The
simulations results reveal that each tissue has an
optimal frequency that maximizes the antenna
radiation efficiency. As it can be seen in Table.1,
muscle tissue and skin tissue share the same
optimum 5 GHz because of their similar dielectric
properties with radiation efficiencies of -24.8 dB and
-25.45 dB respectively. For fat tissue, the optimal
frequency centers around 20 GHz with -23.81 dB
radiation efficiency.
Table 1: Antenna radiation efficiency of tested tissues at
their optimum frequencies.
Tissue
Optimum Frequency
(GHz)
Radiation
Efficiency (dB)
Fat 20
-23.81
Muscle 5
-24.8
Skin 5
-25.45
3 ANTENNA PERFORMANCE
FOR ENERGY HARVESTING
The device should be fed by a RF signal provided by
an external source. Assuming 1μW as the device's
power requirement, one can calculate the required
incoming power by using the radiation efficiency
(Table 2). For example, considering a muscle tissue
box of 5x5x5 mm
3
and the referred 1μW power
requirement, the external RF signal generator should
provide 264μW to fulfill 1 μW requirement.
Increasing the size to a 10x10x10 mm
3
box
inevitably decreases the radiation efficiency due to
tissues losses and a signal of 776μW would be
needed to power the device.
Table 2: Antenna and radiation efficiency at 5GHz for
different tissues types and thicknesses.
Tissue Tissue Box mm
3
Radiation Efficiency
(dB)
Fat
5x5x5 -25.98
10x10x10 -31.73
Muscle
5x5x5 -24.23
10x10x10 -28.90
Skin
5x5x5 -25.29
10x10x10 -30.82
The simulations and the power calculations
suggest that, for the chosen operating frequency, an
energy harvesting application is possible for the
proposed system even though the antenna efficiency
is severely constrained due to tissue losses.
Nevertheless, several improvements can be
achieved, using self-folding techniques, in order to
maximize the antenna efficiency by exploring
different 3D antenna profiles that would increase the
antenna electrical size and thus its efficiency.
4 CONCLUSIONS
Incorporating an electrically small antenna into a
miniaturized polymeric cube using self-folding
fabrication techniques could unveil new applications
such as implanted active drug delivery systems. In
this paper we showed that wireless energy
harvesting can be explored using relatively low
frequencies (5GHz) and thus diminishing tissue
losses that occur at higher frequencies. We showed
by computational simulations that it is possible to
supply 1μW power to a miniaturized device
integrated with a square loop antenna through an
exterior RF source of several hundreds of
microwatts.
ACKNOWLEDGEMENTS
This work was supported by Portuguese Foundation
for Science and Technology (SFRH/BD/63737/2009
and FCT - PTDC/EEA-TEL/65286/2006).
BIODEVICES 2012 - International Conference on Biomedical Electronics and Devices
366
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