Paper Ref: S0212_P0338 3rd International Conference on Integrity, Reliability and Failure, Porto/Portugal, 20-24 July 2009

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A NOVEL ELECTRODE FOR PASTELESS ECG MONITORING D. Vasconcelos*, **, A.C. Alves*, G. Barreto*, P. Pedrosa**,***, D. Freitas*, F. Vaz*** and C. Fonseca*,** *FEUP – Faculdade de Engenharia, Universidade do Porto, R. Roberto Frias 4200-465, Porto, Portugal

** INEB – Instituto de Engenharia Biomédica, Divisão de Biomateriais; Universidade do Porto, Rua do Campo

Alegre, 823, 4150-180 Porto, Portugal

*** Dept. de Física, Universidade do Minho, Campus de Azurém, 4800-058 Guimarães, Portugal Emails: bio06007 fe.up.pt, bio06024 fe.up.pt, bio06031 fe.up.pt, paulomat35253@gmail.com,

dfreitas@fe.up.pt, fvaz@fisica.uminho.pt, cfonseca@fe.up.pt

ABSTRACT

A dry bracelet electrode for electrocardiographic (ECG) signal monitoring was developed and

successfully tested in human volunteers. The new electrode dispenses with the usual gel

application and the previous skin preparation to monitor the signal. It was fabricated from a

polyethylene tereftalate (PET) polymer sheet that was coated with a thin conductive titanium

nitride (TiN) layer. As the direct sputtering of the TiN layer on the polymeric surface results

in a poor adherence of the layer, a specific polymer activation treatment was developed,

consisting of the polymer surface bombardment with alternate beams of argon and titanium

ions. The adhesion tests and the SEM analysis of the coatings proved the success of the

treatment. The TiN coating displays the fcc crystalline structure of the δ-TiN bulk material

with a rough, pyramidal-like morphology. The electrochemical analysis that was carried out

in a saline solution (to mimic the skin sweat contact) showed delamination of the film after

prolonged contact with the saline solution..

The new electrode was tested against the classic gelled silver/silver chloride electrode

(Ag/AgCl) and the ECG records proved to be very similar with both devices. The new

bracelet electrode is ideal for monitoring in ambulatory conditions, as the usual abrasive skin

preparation and the gel application may be skipped. It may be particularly suitable for long-

term monitoring, where the prolonged contact with the gel paste often induces strong allergic

reactions.

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1. INTRODUCTION

The life expectancy in the industrialized countries has risen in the last 20 years from 72 to 80

years old, bringing a strong increase of the health associated costs. One of the strategies that

are being adopted to reduce such costs is the adoption of preventive medicine, involving the

monitoring of important health parameters in ambulatory conditions. The ECG is definitively

one of such parameters, as the cardiovascular diseases are the main cause of death and

morbidity in the developed countries. However, the ambulatory long-term ECG monitoring is

still difficult, due to the characteristics of the actual electrodes, which either demand the

presence of a gel (that dehydrates with air exposure) or, in the case of the disposable

electrodes, have a limited shelf-life and loose their performance after a few days in service

[McAdams, 2006]. Finally, the classic silver/silver chloride (Ag/AgCl) electrodes, either

reusable or disposable, may cause strong allergic reaction, especially if they have to be in

contact with skin for a long time (McAdams, 2006).

Dry electrodes allow skipping the previous skin preparation and gel application, what makes

them “plug-and-play” devices, ideal for ECG monitoring in ambulatory conditions. Presently,

there are no marketed ECG dry electrodes even if this is a very active research topic.

Gruetzmann (Gruetzmann, 2007) tested a conductive foam as a candidate to an ECG sensor

and Baek (Baek, 2008) fabricated and tested a bracelet ECG electrode from a PDMS flexible

polymer. Both authors explored the possibility of using deformable sensors in order to

achieve an improved skin/electrode coupling. Even if the principle was demonstrated,

Gruetzmann stated that his device still had to be improved and Baek’s electrode follows a

complex fabrication process. Yu (Yu, 2009) proposed an electrode based in a micro-needles

array that was deposited on silicon. Besides the infection risk and discomfort due to the

invasive nature of the device, there is the possibility that the needles will become

encapsulated by scar tissue at the long-term, with a dramatic increase of the contact

impedance.

We developed a PET (polyethylene tereftalate) based bracelet device, where the polymeric

substrate was coated with a TiN conductive layer, involving a simple procedure and cheap

materials. TiN films are well known for their excellent chemical and mechanical properties,

which led to a very broad range of applications, as diverse as the mechanical protection of

machine parts and cutting tools (Malik, 2004), coatings for orthopedic and dental prosthesis

(Piscanec, 2004), or even as diffusion barriers for electronic devices (Gao, 2004).

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Despite the rigid nature of the electrode a good skin contact could be achieved due to

electrode design and its small size. This new electrode proved to be able to withstand daily

handling and cleaning.

2. EXPERIMENTAL PROCEDURES

2.1. Surface activation and sputtering of the TiN layer

The electrodes were prepared by depositing a TiN thin film on a 3 mm thick PET sheet

surface (Goodfellow Metals Inc.), by reactive DC magnetron sputtering. The deposition

system is a “home-made” laboratory-sized deposition system, composed of two vertically

opposed rectangular magnetrons (unbalanced of type II) in a closed field configuration. The

film was prepared with the substrate holder positioned at 70 mm from the Ti target (99.6 at.

%), coupled with a DC current density of 100 A·m-2 during 1200 s. A gas atmosphere

composed of argon (60 sccm) and nitrogen (5.5 sccm, corresponding to a partial pressure of

4.1×10-2 Pa) was used. The working pressure was approximately 0.4 Pa. The substrates were

grounded, and no external heating was used. In order to improve the adhesion of the TiN film

to the PET substrate, usually very weak, a three-fold procedure was used. The procedure

started with a first plasma treatment in an Ar atmosphere (80 sccm - 4.8×10-1 Pa), using a

pulsed DC power supply (200 kHz, Ton = 1536 ns) with 0.5 A for 600 s. This Ar

flow/pressure was selected from a series of three different treatments (40 sccm - 2.7×10-1 Pa,

60 sccm - 3.9×10-1 Pa and; 80 sccm - 4.8×10-1 Pa), in which the last revealed to be the one

that promoted the most favourable changes in the polymer substrates surface, both in terms of

roughness and contact angle (CA) variations. These optimization studies were carried out in

the group by Pedrosa et al. and A. Ferreira et al. (articles in preparation). After this plasma

activation/treatment, a very thin layer of TiN was deposited in a plasma atmosphere

composed of Ar (flow of 60 sccm) and nitrogen (5.5 sccm), using a DC current density of 100

A m-2, during 120 s. This set of parameters corresponds to those that would be used for the

deposition of the film itself. A third step comprised a second plasma treatment, similar to the

first one. This second plasma treatment was carried out in order to promote some collisions of

ions with the thin layer deposited in the previous step. In this way, some atoms of the thin

layer were “pushed” deeper inside the polymer’s surface, which was expected to act as

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bonding centres and thus improving even further the adhesion of the main layer to be

deposited after.

2.2. Characterization of the prepared film

The surfaces of the TiN films were observed with a Jeol JSM 6301F microscope operating at

10 KeV. The crystalline structure of the film was scanned by X-ray diffraction in a Philips

PW1710 equipment, operating with the Cu Kα radiation in a Bragg-Brentano configuration.

The adhesion tests in the deposited layer were carried out following the ASTM D3359-08 X-

cut tape test standard. It consists on doing 2 cuts on the surface of the TiN films with a 45º

angle between them, in order to form the shape of an X. The surface is controlled before and

after this X-cut by optical microscopy. Then, a pressure sensitive tape is placed on top of the

cut and pulled-out, leading to a certain degree of delamination of the interface between the X-

cut and the film itself. Ten, the effect of the test is assessed by optical microscopy, comparing

with the images of the cut before application of the tape. The adhesion will then be rated

according with the scale present in the above-referred standard.

Electrochemical impedance spectra were acquired daily during one week, using a Solartron

1250 frequency response analyzer connected to a EG&G PAR 273A potentiostat, driven by

the Zplot software from Solartron. The testing frequencies ranged from 20 kHz to 2 mHz, and

the amplitude of the AC signal was 7 mV (rms). The Zview software was used for the

simulation of the experimental spectra. The saturated calomel electrode (SCE) was used as the

reference electrode and a graphite rod as the counter electrode. The electrolyte was a 0.1M

NaCl solution. The AFM and SEM microscopies were performed with a Pico Scan controller

atomic force microscope using the tapping mode, and a Jeol JSM 6301F microscope operating

at 20 KeV respectively. The ECG was acquired with the Biopac@ system (hardware and

software) in a three electrode configuration, with two electrodes placed in the left and right

wrists and the reference electrode in the right leg. The Ag/AgCl electrodes used as controls

were hydrogel electrodes of the disposable type, also from Biopac.

3. RESULTS AND DISCUSSION

3.1. Surface activation treatment – Roughness and Contact angle

In the study developed by Pedrosa et al. and A. Ferreira et al., a set of polymeric substrates

(polypropylene, PP, polycarbonate, PC and PET) were plasma treated with three different Ar

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plasma conditions: 40 sccm (2.7×10-1 Pa), 60 sccm (3.9×10-1 Pa) and 80 sccm (4.8×10-1 Pa).

The conditions that were found to give the best adhesion in the case of PET were those

corresponding to high surface roughness (5 nm) and low CA (varying from about 75º to 83º),

see Fig.1 for the AFM analysis. It is clear that the surface irregularities of the PET completely

disappeared after the plasma etching treatment, giving rise to a rough and protruded surface.

In addition to these results, it was proved that the three-fold plasma activation procedure

conditions were successful in promoting a good level of adhesion between PET and TiN

films, as we will see later on (chapter 3.3.).

Fig.1. AFM images of the PET surface (a) before and (b) after the plasma activation

treatment

The main mechanism responsible for these morphological and chemical changes is the ion

bombardment suffered by the polymer’s surface, which induces roughness through plasma

cleaning and plasma etching, and also changes at the surface chemical environment, as a

consequence of this ionic bombardment.

3.2 Morphological and structural characterization of the TiN film

After the three-fold activation procedure, the PET substrates were coated with a TiN film,

using 5.5 sccm of N2 (4.1×10-2 Pa). They were, then, characterized by scanning electron

microscopy (SEM). Fig. 2 shows an image of the obtained morphology at the film’s surface.

It is characterized by a rough and porous aspect, with a clear tendency to a pyramidal-like

shape at the columns top (coating surface). Furthermore, the cross-section observation

revealed a film morphology characterized by the existence of extensive intercolumnar

spacing, which are most probably due to the columnar disaggregation that goes deep into the

film thickness.

(a) (b)

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Fig. 2. Morphology of the TiN film deposited with 5.5 sccm N2.

Fig, 3 shows the XRD spectrum of the TiN film.

Fig. 3. XRD diffraction patterns of the TiN film.

The first conclusion that can be drawn form Fig.3 is related with the poor crystallinity of the

prepared film, which is in fact consistent with the low mobility of the arriving species at the

10 20 30 40 50 60 70 800

100

200

300

400

500

Inte

nsity

(a.u

.)

Angle 2θ (º)

δ-Ti

N (2

00)

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growing film, resulting form the absence of any external heating of the PET substrate.

Moreover, and in spite of this low crystallinity (major diffraction peaks are those from the

PET substrate itself), it is possible to see the (200) reflection form a cubic-type structure of

the films, corresponding to the face centered cubic δ TiN-like. Anyway, only this reflection is

visible, and thus not many considerations about the structure can be drawn. Nevertheless, the

low intensity and the apparent large FWHM of such a diffraction peak (a simulation attempt

shows that this peak is probably very broad) shows that the structure is probably quasi-

amorphous and thus the stress levels shouldn’t be very high, which in fact may be a positive

point in the good adhesion that was observed (3.3).

3.3. Adhesion tests

With the objective of understanding if the plasma treatments were effective in promoting the

desired level of adhesion between the TiN film and the PET substrate, a simple qualitative X-

cut tape test was performed. Fig. 4 shows the TiN coated PET electrode surface (a) before the

X-cut, (b) after the X-cut and (c).

Fig. 4. Surface aspect of the TiN coated PET electrodes; (a) before the X-cut, (b) after the X-cut and (c) after

removal of the tape.

a

b c

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From these figures, it is possible to see that only very small or simply no delamination of the

film was observed after the X-cut, Fig.4c. This is a good indication of the excellent adhesion

of the TiN film to the PET substrate..

According to the standard used in this test, it is possible to rate the adhesion based on a scale

that goes from 0A (removal beyond the area of the X) to 5A (no peeling or removal).

Analyzing the interface of the X-cut, it is fair to say that the adhesion is between 4A (trace

peeling or removal along incisions or at their intersection) and 5A (no peeling or removal). In

the presence of such good adhesion between the PET substrate and the TiN film, it is clear

that the three-fold plasma activation treatment is very effective in promoting the adhesion of

such kind of film and the particular polymer that was used to fabricate the sensor.

3.3. Electrochemical analysis

The TiN-coated PET samples were immersed in a saline solution (0.1M), in order to mimic

the behaviour in contact with the skin sweat layer that builds-up during the exam. The

behaviour of the samples was monitored by electrochemical impedance spectroscopy (EIS)

during 9 days. The Bode spectra that were obtained in these conditions are reported in Fig.5,

at the second and ninth day.

Fig 5. (a) EIS spectra as a function of immersion time for a TiN coated PET sample, in a

saline solution.

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The most remarking feature corresponds to the constant impedance part of the spectra

(10.000-100 Hz), related with the pure resistive behaviour of the electrolyte and the film (null

phase angle), that increases with the immersion time. This is an indication of the progressive

degradation of the film, first with the formation of cracks and then with delamination from the

substrate, as it was confirmed by microscopic and visual analysis.

3.4. Testing of the device for ECG acquisition in human volunteers A photo of the new electrode is reported in Fig. 6. The central part is coated with a TiN thin

film. The electric contact was made with conductive epoxidic glue on the side of the electrode

and then coated with an insulating varnish.

Fig. 6. Image of the dry PET ECG electrode (left) and ECG records with the dry and

Ag/AgCl electrodes

The ECG records displayed in the right part of the figure were obtained in rest conditions,

both with the classic Ag/AgCl and the new dry electrodes. It is apparent that the records show

very similar signals, that allowed to obtain the same cardiac rate. The same test was

performed in exercise conditions but neither of the electrodes allowed monitoring reliable

signals in such conditions.

The electrodes were also intensively used in a science fair, for four days, and no degradation

of the recording performance was noticed.

Dry Ag/AgCl

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4.CONCLUSIONS A dry ECG electrode with the shape of a bracelet was developed and tested for ECG signal

acquisition. This electrode dispenses with the use of any previous skin preparation or gel

application, making it ideal for ambulatory utilization. The electrode substrate was a 3 mm

thick PET polymer, whose surface was activated through an argon plasma treatment, in order

to promote the adhesion of the TiN conductive layer that was afterwards deposited by reactive

magnetron sputtering.

A simple X-cut adhesion test proved the excellent adhesion of the TiN film to the polymeric

substrate. The X-rays analysis showed the poor cristallinity of the TiN layer and the SEM

analysis showed a rough, porous and highly textured surface.

On the other hand, the electrochemical tests performed in saline solution showed that the film

progressively degradates with prolonged immersion in saline solution (9 days), what may be a

drawback of the developed coating if long-term exams are envisaged. At present, TiN films

with lower amounts of nitrogen are being produced, in order to achieve compact films that are

expected to display a higher corrosion resistance than the porous layers of the present work.

Concerning the performance of the new electrode, the ECG data showed that the signals are

very similar to the signal obtained with the classic Ag/AgCl electrodes, opening the

possibility for the future utilization of these devices in ambulatory conditions.

ACKNOWLEDGEMENTS D. Vasconcelos, A.C. Alves and G. Barreto are grateful to the MIB (Mestrado Integrado em Bioengenharia) teachers for having given them the opportunity to perform this work in the framework of the Laboratorios Integrados de Engenharia Biomédica I discipline.

C. Fonseca and F. Vaz would like to thank to the CRUP Germany-Portugal Bilateral program, through the A12/08 fund.

F. Vaz acknowledges the funding from the Portuguese Science Foundation “Fundação para a Ciência e Tecnologia”, project PTDC/CTM/69362/2006.

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REFERENCES McAdams in “Encyclopedia of Medical Devices and Instrumentation”, Second Edition, edited

by John G. Webster (2006). 

- Gruetzmann, A., Stefan Hansen, S., Muller, J. Novel dry electrodes for ECG monitoring,

Physiol. Measur. 28; 2007; p.1375-90.

- Baek, J., An, J., Choi, J. Flexible polymeric dry electrodes for the long-term monitoring of

ECG, Sensors and Actuators, A143; 2008; 423-429.

- Vaz, F., Cerqueira P., Rebouta L. et al Structural, Optical and Mechanical Properties of

Coloured TiNxOy Thin Films”, Thin Solid Films, 447-448; 2004; p.449-454.

- H. Malik, R.Mgaloblishvili, B.Mills, J. Mat. Science Letters 19 (19) (2004) 1779.

- S. Piscanec, L. Ciacchi, E. Vesselli, G. Comelli, O. Sbaizero, S. Meriani, A. De Vita, Acta

Materialia 52 (2004) 1237.

- L. Gao, J. Gstöttner, R. Emling, Ch. Linsmeier, A. Wiltner, W. Hansch, D.Schmitt-

Landsiedel, Microelectronic Engineering 76 (1-4) (2004) 76.