A WIRELESS EEG ACQUISITION SYSTEM WITH

THERMOELECTRIC SCAVENGING MICRODEVICE

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.

1 INTRODUCTION

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

range,

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

applications.

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

380

Carmo J., Goncalves L., Rocha R. and Correia J. (2009).

A WIRELESS EEG ACQUISITION SYSTEM WITH THERMOELECTRIC SCAVENGING MICRODEVICE.

In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 380-383

DOI: 10.5220/0001544303800383

 SciTePress

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.

2 THERMOELECTRIC

GENERATOR

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

1×10

-6

ohm.cm

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

Microbattery

Cold side junctions

Figure 1: An artist impression of a thermoelectric

microdevice.

3 EEG ACQUISITION SYSTEM

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

A WIRELESS EEG ACQUISITION SYSTEM WITH THERMOELECTRIC SCAVENGING MICRODEVICE

381

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.

S/H ADC

control

memory

transceiver

IrO

electrode

Analog-to-digital converter

Microchip 1

Impexa

gigaAnt

antenna

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

-6

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.

Antenna

Filter

LNA

Post-amplifier

Envelope

detector

On-chip

switch

RXD

TXD

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.

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382

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.

4 CONCLUSIONS

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.

ACKNOWLEDGEMENTS

The authors would like to thanks the Portuguese

Foundation for Science and Technology

(FCT/PTDC/EEA-ENE/66855/2006 project).

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