J. P. Carmo, L. M. Goncalves, R. P. Rocha and J. H. Correia
University of Minho, Dept. Industrial Electronics, Campus Azurem, 4800-058 Guimaraes, Portugal
Keywords: Wireless EEG, IrO
electrodes, Thermoelectric microsystems, Energy scavenging.
Abstract: This paper presents a wireless EEG acquisition system powered by a thermoelectric energy scavenger,
which was optimised to convert the small thermal power available in the human-body. The wireless EEG
system is composed with up to four EEG electrodes. A radio-frequency (RF) transceiver for operation in the
2.4 GHz ISM band, was optimised and fabricated in the UMC RF 0.18 μm CMOS process. The receiver has
a sensitivity of -60 dBm and consumes 6.3 mW from a 1.8 V supply. The transmitter delivers an output
power of 0 dBm with a power consumption of 11.2 mW. Because listening and emitting are power-intensive
activities, innovative topics concerning efficient power management was taken into account during the
design of the RF CMOS transceiver.
Energy scavengers are currently emerging for a
number of applications from automotive to
medicine. Micro energy scavengers are small
electromechanical devices which harvest ambient
energy and convert it into electricity. Energy
scavengers could harvest different types of energies.
Solar energy can be harvested with photovoltaic
solar cells, thermal energy can be harvested with
thermoelectric generators, mechanical energy can be
harvested with piezoelectric, electromagnetic or
electrostatic converters, and finally electromagnetic
energy can be harvested through RF resonators. It
exists two types of energy scavenging systems:
macro energy scavengers, typically in the cm
and micro energy scavengers, typically in the mm
range and manufactured using micromachining
techniques. Micro energy scavengers are still in the
R&D phase. Direct thermal-to-electric energy
conversion without moving mechanical parts is
attractive for a wide range of applications because it
provides compact and distributed power, quiet
operation, and is usually environmentally friendly.
Thus, worldwide efforts are undertaken to expand
the technology of thermoelectric devices into the
field of micro-systems technologies (MEMS).
Previsions made by the specialists of the
microsystems area, shows that the most expected
growth of these devices, will be with medical
An emerging technology for ultra-low power
communication platforms triggered renewed interest
in power sources for wireless-sensor, in special
wireless-wearable-sensors, with power consumption
nodes of few mW. Today, almost all of these
platforms are designed to run on batteries which not
only have a very limited lifetime, but are also in
many areas a cost-prohibitive solution. An attractive
alternative is powering the sensors with energy
harvested from the environment. The developed
project aims at a solution for energy microgeneration
through energy harvesting by taking advantage of
temperature differences. A viable energy source for
low-powered devices such as micro sensor systems,
ZigBee chipsets, wearable electronics, implantable
medical devices, active RFID tags and many other
applications is proposed, provided a temperature
difference exists, between the two surfaces of a
thermoelectric microgenerator (in a wearable device,
the difference between the body and environment
can be tens of degree, depending on the environment
temperature). This temperature difference can be
converted into electrical energy using the Seebeck
principle. Since many of wireless sensors are
powered in a peak basis (e.g., the transmission of
data needs much more current than standby or
receiving mode) and the temperature gradient could
not always be present, the energy is stored in a
Carmo J., Goncalves L., Rocha R. and Correia J. (2009).
In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 380-383
DOI: 10.5220/0001544303800383
rechargeable thin-film battery of the Li-ion type
(integrated in the system). Ultra-low power
electronics performs DC-DC rectification with a
variable conversion factor and recharge the battery
on optimal conditions. Since a small volume is
required, integration into an IC is desirable. A
single-chip regulated thermoelectric power source is
the final goal to be achieved.
Future investigations, must prove if the operation
from low temperature gradients (a minimum
temperature difference of 3 ºC between ambient and
target thermo-source must provide an IC-compatible
voltage) and the power of 2 mW/cm
are possible to
be obtained. The fabrication of macro-scale
thermoelectric devices is based on standard
technologies for decades. Bismuth and antimony
tellurides was be used as thermoelectric materials
since these materials have the highest performance
figure-of-merit (ZT) at room temperature
(Gonçalves et al, 2006). The co-deposition method
was used to fabricate these thermoelectric thin films.
A very stable evaporation rate of each element
(Bi/Te and Sb/Te for the bismuth telluride and
antimony telluride, respectively) allows the
deposition of polycrystalline n-type and p-type
materials, when the substrate is heated in the range
200-300 ºC. The design of a thermoelectric
microdevice, with vertical microcolumns, connected
in series by metal contact areas, requires the
application of microsystem technologies
(Gonçalves et al, 2007). Reactive ion etching, lift-of
and wet-etching (hydrochloridric acid / nitric acid
solution) techniques were tested to create the vertical
columns. A thin (50 nm) layer of nickel (deposited
by e-beam) interfaces between the thermoelectric
material and the metal contact areas (1 µm of
aluminum), preventing the diffusion of the metal
from the contacts into the thermoelectric film. The
contact resistance plays a major role in the
performance of the device, and a value smaller then
was achieved. A silicon substrate
was used for integration with microelectronics,
while at same time providing good thermal contact
with heat source and sink. The fabricated battery, as
well as, all the electronic circuitry to receive the
energy and to recharge the thin-film integrated
Li-ion battery (open circuit voltages between 1.5 V
and 4.2V, maximum current of few mA/cm
with a
charge-storage capacity around 100 µAh/cm
), was
placed on the bottom side of the generator. Thin-film
solid-state batteries show a very high life cycle and
are intrinsically safe (Bates at al, 2000).
The Figure 1 shows an artist impression of a
thermoelectric microdevice, with vertical
microcolumns, connected in series by metal contact
areas. On this thermoelectric generator will be
placed the thin-film integrated battery and all the
electronic circuitry to receive the energy and to
recharge the battery. The application of this
thermoelectric scavenging energy systems, is in the
powering of a wireless EEG acquisition system.
Hot side junctions
Cold side junctions
Figure 1: An artist impression of a thermoelectric
The standard wireless EEG solutions use a braincap
with wires running from the electrodes position to a
bulky central unity (amplification, signal filtering
and analog-to-digital conversion) (IMEC 2003). A
more interesting solution is to use compact wireless
EEG modules, where the electronics, the antenna
and each electrode are mounted together. The power
supply for these modules is obtained locally from
the thermoelectric generator. Also, it is possible to
integrate additional electronics (amplification,
filtering and high-resolution digital conversion), for
local signal processing in these small-size individual
wireless EEG modules.
Bipolar or unipolar electrodes can be used in the
EEG measurement. In the first method the potential
difference between a pair of electrodes is measured.
In the second method, the potential of each electrode
is compared, either to a neutral electrode or to the
average of all electrodes. Figure 2 shows the full
block diagram of the wireless EEG module, where it
can be seen the electrode connected to an amplifier,
followed by an analog-to-digital converter (ADC).
In order to meet the EEG specifications, the
amplifier was designed to have enough gain, to
amplify signals with amplitudes of only 70 μV. The
ADC must have at least a resolution of 22 bits and a
minimum sampling frequency of 2000 Hz.
Analog-to-digital converter
Microchip 1
Microchip 2
Figure 2: Wireless EEG module. Note that the neutral
electrode, which is connected to the grounds of the module
it is not shown.
This wireless acquisition system uses a RF link to
communicate with an external base-station. It was
used the UMC RF 0.18 μm CMOS process in the
design and fabrication of the RF transceiver.
Figure 3 shows the block schematic of this RF
transceiver, which consists of a receiver, a
transmitter, an antenna-switch and a
Phase-locked Loop (PLL) as a frequency
syntheziser. The RF transceiver was built to operate
at the 2.4 GHz ISM band. The receiver adopts a
direct demodulation, by means of envelope detection
and has a sensibility of -60 dBm for power
consumption of 6.3 mW from a 1.8 V supply. The
transmitter delivers an output power of 0 dBm with a
power consumption of 11.2 mW. These
characteristics fullfill the requirements for
communications up to ten meters, with a bit error
probability less than 10
Moreover, without proper design, communication
tasks may increase network power consumption
significantly because listening and emitting are
power-intensive activities (Enz et al, 2004). Thus, in
order to optimise the power consumption, the RF
transceiver design predicted the use of control
signals. With these control signals it is possible to
enable and disable all the transceiver subsystems.
These signals allows, e.g., to switch off the receiver
when a RF signal is being transmitted, to switch off
the transmitter when a RF signal is being received,
and allows the transceiver to enter to sleep when RF
signals are neither being transmitted, nor being
received. The Figure 4 shows a die photograph of
the RF transceiver.
Local oscilator
2.4 GHz
Enable receiver
Enable transmitter
Figure 3: The block schematic of the transceiver.
Figure 4: A die photograph of the RF transceiver.
The electrodes to acquire the EEG signals, with the
proposed wireless acquisition system, are of
sputtered iridium oxide (IrO
) type. The
experimental results shown a better performance of
the sputtered IrO
electrodes compared with the
standard sintered Ag/AgCl ring electrodes
(Slavcheva et al., 2004). These results promise a
new opportunity for the application of a dry IrO
electrodes in our wireless EEG modules, without the
need to use conductive gel in the interface between
the electrode and the skin. This will allow patients to
wear a brain cap with the electrodes and maintain
their mobility, while simultaneously having their
electrical brain activity monitored. The Figure 5
shows an artist impression of the thermoelectric
scavenging system and an wireless EEG module,
both attached to a cap (the zoomed part in that
Figure). The temperature gradient between forehead
and the environment will generate energy in the
thermoelectric microdevice and charge the solid-
state thin-film battery.
BIODEVICES 2009 - International Conference on Biomedical Electronics and Devices
The modules must offer the plug-and-play feature, in
order to mount distributed networks in the patient’s
head. Moreover, as the EEG data is periodically
acquired in all the modules, thus the latencies of data
transmissions are not allowed. The proposed EEG
modules uses a communication protocol that
overcomes these problems (Afonso et al, 2006). This
protocol combines the distributed and coordination
modes, e.g., when a new module is putted in the
head of patient, a contention based time interval is
used to make the registration request in the network.
A contention-less time interval, constituted by
time-slots, is granted to the new EEG module if the
registration is successful completed on the network.
The maximum number of simultaneous modules is
limited to the number of time-slots in the contention-
free interval.
Figure 5: An artist impression of the thermoelectric
scavenging system and an wireless EEG module.
This paper presented a thermoelectric scavenging
energy system, whose final goal is to supply a
wireless EEG system. The wireless EEG system is
composed by plug-and-play modules, one for each
electrode. Each wireless module is composed by an
electrode, processing electronics, a radio-frequency
transceiver and an associated antenna.
The RF transceiver was fabricated in the UMC RF
0.18 μm CMOS process, for the operation in the
2.4 GHz ISM band, in order to optimise the
consumed power.
The use of microsystems techniques, makes possible
the integration of the whole thermoelectric
scavenging system, in an wireless EEG system.
The authors would like to thanks the Portuguese
Foundation for Science and Technology
(FCT/PTDC/EEA-ENE/66855/2006 project).
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