ENERGY HARVESTING FOR SELF-FOLDING MICRO

DEVICES

Pedro Anacleto

1,2

, Evin Gultepe

, David H. Gracias

2,3

and Paulo M. Mendes

Centro Algoritmi, Universidade do Minho, Braga, Portugal

Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, U.S.A.

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

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

 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

) 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

), with a 0.2x10

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

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

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

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).

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366

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