EXPERIMENTAL APPARATUS FOR FINGER ECG BIOMETRICS

Renato Lourenc¸o

, Paulo Leite

, Andr´e Lourenc¸o

1,2

, Hugo Silva

2,3

, Ana Fred

and David Coutinho

2,4

DEETC, ISEL-IPL, Lisbon, Portugal

Instituto de Telecomunicac¸˜oes, IST-UTL, Lisbon, Portugal

PLUX - Wireless Biosignals, S.A., Lisbon, Portugal

Centro de C´alculo, ISEL-IPL, Lisbon, Portugal

Keywords:

Dry Ag/AgCl, Electrolycra, ECG, Electrodes, Hands, Fingers.

Abstract:

Current Electrocardiographic (ECG) signal acquisition methods are generally highly intrusive, as they involve

the use of pre-gelled electrodes and cabled sensors placed directly on the person, at the chest or limbs level.

Moreover, systems that make use of alternative conductive materials to overcome this issue, only provide heart

rate information and not the detailed signal itself. We present a comparison and evaluation of two types of

dry electrodes as interface with the skin, targeting wearable and low intrusiveness applications, which enable

ECG measurement without the need for any apparatus permanently ﬁtted to the individual. In particular, our

approach is targeted at ECG biometrics using signals collected at the hand or ﬁnger level. A custom differential

circuit with virtual ground was also developed for enhanced usability. Our work builds upon the current state-

of-the-art in sensoring devices and processing tools, and enables novel data acquisition settings through the

use of dry electrodes. Experimental evaluation was performed for Ag/AgCl and Electrolycra materials, and

results show that both materials exhibit adequate performance for the intended application.

1 INTRODUCTION

Electrocardiography (ECG) is the recording of the

electrical activity of the heart over time, by means of

adequate signal acquisition and conditioning electron-

ics. These signals have found application in a variety

of ﬁelds, from healthcare and quality of life (Dubin,

2000) to security (Lourenc¸o et al., 2011b).

The acquisition of ECG signals is typically per-

formed on the chest with pre-gelled electrodes, which

is an intrusive placement as it requires the subjects

to expose an intimate part of their body. For clinical

applications, such a setup is accepted by the person

due to the added value that it introduces in assessing

the health conditions; however in applications such as

biometric recognition this is not the case.

For heart monitoring during leisure and sports

activities, state-of-the-art systems already use alter-

native conductive materials for increased usability.

Nonetheless, current devices only provide heart rate

information; furthermore, sensors are typically de-

signed to be in contact with the subjects chest, or

snapped to the limbs.

On the overall, for applications within the biomet-

ric recognition realm, conventional methods are either

unpractical and limiting for regular use in extended

periods, hindering the adoption in everyday life, or do

not provide sufﬁciently detailed information.

Hence, our work was motivated by the need of

ﬁnding alternative types of measurement settings, that

(a) do not require gel to perform an adequate inter-

face with the skin, (b) use a more convenient acqui-

sition location with limited intrusiveness, potentiat-

ing the use of ECG biometrics for authentication pur-

poses (Lourenc¸o et al., 2011c; Silva et al., 2007; Shen,

2005).

Our approach targets the reduction of the intru-

siveness that traditional systems introduce, by requir-

ing access to concealed parts of the body, or having

cables connected to the subject. The goal is to bring

ECG acquisition apparatuses to the level of accept-

ability found in commercial biometric systems, and

more traditional modalities (Jain et al., 2007; Ross

et al., 2006; Jain et al., 2005).

In this paper, we present a hardware setup and

comparison of dry Ag/AgCl electrodes and Electroly-

cras, a type of fabric with conductive properties. The

purpose is to evaluate the quality of ECG signals ac-

quired at the hand palms and ﬁngers, as well as the

possibility of using only two measurement leads to-

196

Lourenço R., Leite P., Lourenço A., Silva H., Fred A. and Coutinho D..

EXPERIMENTAL APPARATUS FOR FINGER ECG BIOMETRICS.

DOI: 10.5220/0003777701960200

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2012), pages 196-200

ISBN: 978-989-8425-91-1

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

gether with non-gelled interfaces with the skin, in a

setup with which the person can easily interact with.

The rest of the paper is organized as follows: Sec-

tion 2 brieﬂy describes the current approaches to ECG

signal acquisition; Section 3 introduce the proposed

approach; Section 4 describes the experimental evalu-

ation; Section 5 presents a brief discussion on the out-

comes of our work; and Section 6 outlines the main

ﬁndings and conclusions.

2 RELATED WORK

Clinical grade ECG signal measurement methods are

based on the acquisition of signals from 12 or more

leads mounted on the chest and limbs, with pre-gelled

electrodes or conductive paste to improve conductiv-

ity with the skin (Chung, 2000).

New developments in signal acquisition technolo-

gies have greatly improved usability, and several sys-

tems already exist that have sensors integrated in a

more convenient way, that enable the monitoring of

the cardiac function without the need for conductive

gel.

In (Gamboa et al., 2010), a device is described

which uses Ag/AgCl electrodes, and that can be

placed around the subjects neck for heart rate mea-

surement. (Chou et al., 2006) describe a capacitive

sensor that can be attached to a t-shirt to measure

the ECG. A smart t-shirt was introduced by (Cunha

et al., 2007), which integrates the standard measure-

ment leads in the fabric, as a way of achieving a more

practical acquisition setup.

Also in commercial systems for sports and leisure

activities, an effort has been seen to improve over

standard clinical setting practices. In the portfolio

of (Polar Electro, 2011), conductive rubber and con-

ductive fabric based chest strap options can be found;

more recently, (EA Sports Active, 2011) has launched

a wearable armband. Nonetheless, in these cases the

full ECG readings are not accessible.

Current techniques require the application of the

sensing apparatus to the subject’s body, which in

some cases involves the placement of multiple sen-

sor leads. The existing less intrusivehardware devices

only provide information about the subjects heart rate.

For biometric purposes, not all application scenar-

ios cope well with these requirements. Targeting a

real-world usage scenario, the signal acquisition must

be performed in more convenient ways, which led us

to the development of an alternative solution.

3 PROPOSED APPROACH

For biometric applications, subjects are already fa-

miliar with commonly used modalities, which require

some degree of contact and proximity to the sensoring

device. These include ﬁngerprint (Jain et al., 2004),

where the subject needs to place or scroll the ﬁn-

ger on the reader, and hand geometry (Ross et al.,

2006), where the subject needs to place the hand on

the reader.

We propose an experimental apparatus for ECG

biometrics, that matches the usability and intrusive-

ness levels of conventional biometric systems. Our

device is comprised of a surface with integrated elec-

trodes and signal conditioning circuitry, where the

subject rests his/her hand palms, enabling the signal

acquisition without the need for conductive gel nor

access to more intimate parts of the body.

Figure 1: Block diagram of the proposed solution.

EXPERIMENTAL APPARATUS FOR FINGER ECG BIOMETRICS

197

The proposed system can either be used with dry

Ag/AgCl electrodes or Electrolycras as interface be-

tween the sensor and the skin, for improved usability.

The former are based on conventional electrodes used

for clinical ECG acquisition; the later are generally

used in Faraday shields, and exhibit a surface resistiv-

ity < 0.5Ω/ when unstreched.

A custom signal conditioning circuit was devel-

oped, which only requires two contact points with

the skin, through the use of a virtual ground circuit.

This design improves upon existing sensors (Webster,

1998), as traditional ECG sensors require positive (+)

and negative (-) poles, together with a ground (GND)

lead. With our design, only the (+) and (-) leads are

required.

The virtual ground is generated using the positive

) and negative (V

) supply voltage of the circuit

to which the sensor is connected. A voltage divider

creates an intermediate voltage between these two,

which is taken as the common mode voltage of the

body in the remaining ampliﬁcation and ﬁltering cir-

cuitry; a unity gain buffer was added for increased

stability of the generated signal.

Figure 1 depicts the block diagram of the proposed

approach with: a) the electrodes that provide the in-

terface with the skin; b) the virtual ground generation

circuit; c) ﬁrst stage of signal ampliﬁcation with gain

10; d) analog band-pass ﬁltering in the 1-30Hz range;

and e) second stage of signal ampliﬁcation with gain

100.

Although the remaining steps fall outside the

scope of this paper, the analog signal is then fed to the

digital conversion and transmission chain, comprised

of: f) analog-to-digital conversion; g) digital ﬁltering;

h) heartbeat waveform segmentation; i) pattern evalu-

ation; and j) storage at the base station.

Figure 2, depicts the proposed setup. In the pro-

posed conﬁguration, and for experimental validation

purposes, the device allows the acquisition of ECG

signals at the hand palm, through dry Ag/AgCl elec-

trodes, and at the index and middle ﬁngers through

Electrolycras.

4 EXPERIMENTAL EVALUATION

To evaluate the experimental setup, we performed an

extensive data collection in 31 subjects (24 males and

7 females) with an average age of 31.1±9.46 years.

Subjects were only asked to rest their left/right hands

as indicated in the device.

Two custom ECG sensors were used for signal

acquisition, with a total gain of 1000, and analog

band pass ﬁltering between the 1-30Hz range. In

Figure 2: Experimental apparatus: at the top, Electrolycras

enable the acquisition of ECG using the index and middle

ﬁngers; at the bottom, dry Ag/AgCl electrodes acquire ECG

at the hand palms.

Figure 2 both sensors are visible, one connected to

the Ag/AgCl electrodes, and another connected to the

Electrolycra strips placed at the index and middle ﬁn-

ger level.

To guarantee electrical insulation of the sensors,

in order to avoid ground coupling, two independent

biosignal acquisition units were used, one per sensor.

We recurred to the bioPLUX research system (PLUX,

2011), which enables Bluetooth wireless transmission

of the collected signals to the base station. Data was

acquired at 1000Hz sampling rate and with 12-bit res-

olution.

Synchronization of the acquisition units was per-

formed optically using a syncPLUX kit and a light-

dependent resistor (LDR). To one of the systems a

triggering switch was connected, which simultane-

ously activated the digital input port of the system and

an LED.

To the other system, a LDR was connected to one

of the analog input channels, and placed in direct con-

tact with the LED of the ﬁrst system, in such way that

a synchronization signal was obtained whenever the

LED was lit.

This allowed us to have the data collected by each

system synchronized, without recurring to any elec-

trical connection between them. Signals were ac-

quired during a period of approximately 2min, in

which the supervisor in charge of the experimental

procedure would describe the experiment, goals and

related work.

We evaluated the signal-to-noise ratio (SNR) for

each individual signal; a 6th order butterworth digital

BIODEVICES 2012 - International Conference on Biomedical Electronics and Devices

198

Table 1: Experimental results for ECG sensor data collected

using the proposed approach.

Type of SNR [dB] MSE Raw/Filt.

Electrodes µ± σ µ± σ

Ag/AgCl −3.94± 6.24 0.60± 0.20

Electrolycra −3.15± 4.65 0.60± 0.18

ﬁlter was used, for which we considered the 2-30Hz

bandwidth as the signal, and the remainder as noise.

The mean squared error (MSE) was also computed to

provide additional insight regarding the morphologi-

cal difference between the raw and ﬁltered signals for

each type of electrode. This metric can be used as a

correlation indicator, as it is zero when both signals

possess an exact match.

Results are summarized in Table 1, and Figure 3

shows a segment of collected signal for one of the in-

dividuals, at the hand palms using dry Ag/AgCl elec-

trodes, and at the ﬁngers using Electrolycras. Al-

though high frequency noise is noticeable in the raw

data, digital ﬁltering is able to enhance the signals, as

shown in Figure 4.

As we can observe, numerical analysis revealed

that dry Ag/AgCl electrode and Electrolycras exhibit

a comparable performance; signals acquired with

both electrodes exhibit similar levels of detail. Al-

though the P wave is masqueraded by baseline noise,

the QRS-T complexes are clearly noticeable, and the

heartbeat waveforms morphology show identical be-

haviors. Our conﬁguration is the equivalent to a Lead-

I ECG placement.

2 3

Figure 3: Example of collected data: At the top, in blue, the

ECG signal acquired at the hand palm using dry Ag/AgCl

electrodes; at the bottom, in green, the ECG signal acquired

at the ﬁngers using Electrolycras.

2 3

Ag/AgCl

Elect roly cra

Figure 4: Example of ﬁltered data: At the top, in blue, the

ECG signal acquired at the hand palm using dry Ag/AgCl

electrodes; at the middle, in green, the ECG signal acquired

at the ﬁngers using Electrolycras.

5 DISCUSSION

Previous work from our group has shownthe potential

of ECG signals collected at the hands and ﬁngers for

biometric applications. In (Lourenc¸o et al., 2011b), a

single lead setup with a standard ECG triode sensor

was used to acquire signals at the hand and ﬁngers

level.

In a standard conﬁguration, the signal condition-

ing circuitry measures an electrical potential differ-

ence between two points, referred to the common

mode voltage of the body. The later is provided

through a ground lead directly connected to the per-

son. This setup is particularly advantageous since the

electronics is simpler, however, three contact points

are always required.

With our work, we were able to further improve

this setup, by introducing a virtual ground, which gen-

erates a reference voltage to the signal conditioning

circuitry from the sensor supply voltage. This conﬁg-

uration only requires 3 additional components in the

circuit, and allows the ECG signal acquisition to be

performed with only 2 contact points, by eliminating

the ground lead.

In the work by (Lourenc¸o et al., 2011a) and (Silva

et al., 2011) tests were performed with this experi-

EXPERIMENTAL APPARATUS FOR FINGER ECG BIOMETRICS

199

mental setup, with the purpose of evaluating the bio-

metric potential. Results have shown that the obtained

signals enable recognition rates within the conﬁdence

intervals of what is known in the ﬁeld.

6 CONCLUSIONS

The ﬁeld of application of electrocardiographic sig-

nals is expanding to new areas, which far extend the

medical and quality of life applications to which it

is typically associated with. Biometrics is currently

emerging as one of these novel application ﬁelds.

Within the scope of biometric recognition, con-

ventional acquisition apparatuses have speciﬁcities

which limit the acceptability by the potential end-

users. This arises from the fact that, in general,

devices require pre-gelled electrodes or conductive

paste to acquire the signals, but more importantly, be-

cause they need to be applied to the subjects body.

Furthermore, current methods require three con-

tact points with the subjects body, namely for positive

(+) and negative (-) poles, plus a ground (GND) lead.

In this paper we presented an experimental setup,

which allows ECG acquisition in a format that has us-

ability levels comparable to those found in readers of

other biometric modalities (e.g. ﬁngerprint, hand ge-

ometry, among others).

Our approach recurs to a custom two-lead ECG

sensor, that can use either dry Ag/AgCl electrodes or

Electrolycras as interface with the skin. For signal ac-

quisition, the user only needs to rest his/her hand and

ﬁngers over the reader without any other constraint.

Experimental results have revealed that the col-

lected signals provide adequate informative content.

In particular, the QRS-T segments are detectable with

high deﬁnition. Also, a good correlation was found

between signals acquired with each type of electrode,

material allowing the biometric system designer to se-

lect the type of material that improves the usability on

the intended application.

ACKNOWLEDGEMENTS

This work was partially funded by Fundac¸˜ao

para a Ciˆencia e Tecnologia (FCT) un-

der grants SFRH/BD/65248/2009 and

SFRH/PROTEC/49512/2009, and by the De-

partamento de Engenharia de Electr´onica e

Telecomunicac¸˜oes e de Computadores, Instituto

Superior de Engenharia de Lisboa, whose support the

authors gratefully acknowledge.

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