WEARABLE TECHNOLOGY

Development of Polypyrrole Textile Electrodes for Electromyography

S. Rodrigues, R. Miguel, J. Lucas, C. Gaiolas

Textile Science and Technology Department, Textile and Paper Materials R&D Unit, University of Beira Interior

Rua Marquês d’Ávila e Bolama, Covilhã, Portugal

P. Araújo

Computer Science Department, University of Beira Interior, Covilhã, Portugal

N. Reis

Biosurfit S.A., Lisbon, Portugal

Keywords: Wearable Technologies, Electromyography, Smart Structures Monitoring, Textile Electrodes, Polypyrrole

Conducting Polymer.

Abstract: Following the work already done, in particular the “textile arm" by the authors, incorporating metallic

filaments in the fabric during the weaving process with the aim of capturing surface electromyography

signals, we concluded that there was a need to maximise the area of contact with skin to improve the

obtained electrodes signals. The main objective of this work is the development of electrodes embedded into

textile materials able to capture electromyography signals, using the polypyrrole conducting polymer, both

in coating of electrodes directly into the textile fabric and preparing a conductor yarn with this polymer to

obtain a knit structure. This work allowed developing wearable technology textile structures for muscle

activity monitoring, through the measurement of electromyography signals, keeping simultaneously comfort

and easy-care textile characteristics. The signals obtained with this approach had a good definition

compared with commercial electrodes and with the great advantage of this procedure can be applied in

industrial production.

1 INTRODUCTION

Through times, many have been the fields of science

and technology that progressed in separate ways.

During the last years, a considerable convergence

appeared among some of these fields, with

surprising results. The smart technology for

materials and structures is one of these results

(Lucas, 2007).

1.1 Wearable Technology

The smart textiles concept was firstly defined in

Japan in 1989 (Langenhove, 2007). With the

discovery of shape memory materials in 19600 and

intelligent polymeric gels in the 1970 decade, it was

accepted the birth of really smart materials. These

have been introduced in textiles only in the 90´s

(Langenhove, et al., 2006). Smart textiles can be

described as textiles that can feel stimuli from

environment, react and adapt themselves to them by

integrating functionalities in the textile structure

(Langenhove, et al., 2006).

The stimulus, as well as the response may be

electrical, thermal, chemical, magnetic or of other

kind. The degree of intelligence can be divided into

three subgroups:

• Passive smart textiles that only feel, being

sensors;

• Active smart textiles that perceive external

stimuli and react to them, besides being

sensors, they act also as actuators;

402

Rodrigues S., Miguel R., Lucas J., Gaiolas C., Araújo P. and Reis N. (2009).

WEARABLE TECHNOLOGY - Development of Polypyrrole Textile Electrodes for Electromyography.

In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 402-409

DOI: 10.5220/0001548904020409

 SciTePress

• Truly smart textiles, being one step ahead

and able to adapt its behavior to the

circumstances (Zhang, 2001).

It would be wonderful to have clothes like our

skin, which is a smart material layer. Our skin has

sensors to detect pressure, pain, heat, etc. and,

together with the brain, can work in a smart manner

with environmental stimuli, generating large

amounts of perspiration to cool our body when in a

hot surrounding and stimulating blood circulation

when it is cool. It changes colour when exposed to a

high sun radiation level, in order to protect the body,

It is breathable and allows moisture penetration,

avoids the entry of undesired species, it can be

damaged, repaired and regenerate itself. Thus, to

study and develop a smart material like our skin is a

big challenge (Boczkowska, Leonowics, 2006).

To Bonato (2005), when looking for a better way

of life, more and more solutions have been

investigated to contribute to a comfort improvement

and well-being. The research field is going on and

several research groups have already demonstrated a

relevant application and the potential impact of this

technology is remarkable in clinic practices

concerning physical and rehabilitation treatments.

Presently, the main use of clinic techniques resides

on methods that embed devices into smart clothes on

in the patient’s body. The wearable technology has

a large potential to redirect these techniques into

functioning during the normal day life (Bonato,

2005).

The development of small size sensors that can

be connected to the body or can be a part of clothes,

embed elements into cloth fabrics, open a large

number of possibilities to monitor patients during

long periods of time. This is of particular relevance

to the practice of physical and rehabilitation

treatment. The wearable technology allows to

clinical staff the treatment of data, acquired at home

or by direct observations relating to the impact of

clinic interventions in mobility, thus reaching a

higher independence level and a better quality of

(Bonato, 2005).

1.2 Conductive Polymers

The firs conductive polymer was discovered

accidentally by Hideki Shirakawa in 1971

(Shirakawa, Ikeda, 1971) during a reaction with

acetylene. From this, it began research to understand

the charge transfer mechanisms in these (Neto,

2007).

The key to get a conductive polymer is the

presence of conjugate double bonds in the polymer

main chain. During conjugation, links between

carbon atoms are alternately single and double

bonds. Each double bond has a localized bond that

gives a strong attraction (σ bond) and a weaker one

(π bond). The electrical conductivity in polymers is

an effect due to π electron displacement in alternate

sequence of single and double bonds (Lucas, 2007),

as seen in Figure 1.

Figure 1: Illustration of π electrons displacement in

alternate sequence – electrical conduction (Lucas, 2007).

Conjugation is not enough to assure conduction

in a polymer. The doping function is now of an

agent seen as polymer charge carrier, as extra or

“missing” electrons (hole). One of these holes can be

filled by an electron jumping from a neighbor

position, creating a new hole and, thus, successively,

allowing a moving charge through a long (Lucas,

2007).

Polypyrrole is one of the most studied

conductive polymers due to its high conductivity,

good environmental stability, harmless and easy to

synthesize. Consequently, polypyrrole has

advantages in real applications in microelectronics,

informatics industry and biomedical information.

However, polypyrrole is not soluble in water and in

organic solvents, due to the strong intermolecular

and intramolecular interaction and crossed polymer

chains. For this reason, to overcome the non

solubility issue of polypyrrole, many research works

have been done (Pires, 2007).

Recently, soluble polypyrrole was synthesized

with large dope agents, such as dodecyl-benzene-

sulphonate acid (DBSA) and naphtale-sulphonate

(Lim, et al., 2005).

According to Dall’Acqua, L. et al. (2006), the

affinity of different types of fibers, yarns and fabrics

with doped polymers allows production of textile

composites having improved electrical properties.

Following the study of Lim., H. K. et al. (2005),

a new soluble and conductive polypyrrole, doped

with DBSA and combined with a polymeric agent,

the poly (ethylene glycol) (PEG).

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403

1.3 Electromyography Signal

Acquisition

According to J. Basmajian e C. De Luca (1985)

electromyography measures the electric activity

resulting from skeletal muscle activation, these

being responsible by the support and movement of

skeleton.

According to the Hospital Sant Pere Claver

Electromyography Unit at Barcelona,

electromyography is an electrophysiological study of

neuromuscular system; it is not a complementary

proof but an extension of a neurological study. This

basis of all electrophysiological research is the

registration of excitable cells potentials.

Electromyography deals with registering of such

potentials, self acting in the muscle and

electroneurography of evoked potentials, both on the

muscle and on nerve branches, by generally

electrical stimulation of nerves having anatomic

connection with the registering area. The electrical

properties of excitable nervous and muscle fibers

derive from a semi permeable membrane that

separates intracellular and extracellular fluids having

different ionic concentration origination a

transmembrane potential.

When measuring electromyography signals

(EMG’s) electrodes can be used placed inside the

muscle (deep electromyography) or electrodes

placed on the skin (surface electromyography)

(Sousa et al., 2007).

According to the type of acquisition, electrodes

may be invasive or not. When an electrode becomes

into contact with the body fluid is called invasive,

which can be of the yarn or needle type and presents

characteristics such as low electrical impedance, this

is, it does not need of conductive gel, it acquires

larger amplitudes and its power spectrum goes up to

10KHz (Ricciotti, 2006). They are most indicated

for clinical analysis, where it is possible to detect the

acting potential of a moving unit, as well as

exploring an isolated of a deep muscle. The invasive

electrodes have several drawbacks, being among

them excellent sterilization, the danger of breakage

of yarns inside the muscle and, after all, the patient´s

(Ricciotti, 2006).

According to Jacquelin Perry (1998), from the

28 main muscles that control each lower member,

most of them are surface muscles, being possible to

monitor their activity using surface

electromyography. During the last years, surface

electromyography (sEMG) has become very

frequent when studying and treating several muscle

dysfunctions, due to the fact that it is a secure

technique, easy to use and non-invasive. Most of

knowledge fields use EMG signals as starting point

to several human muscle system function and

dysfunction analysis. Recent studies relate the

muscle fatigue process to a displacement of signal

spectral contents. Other investigations emphasized

the importance of these signals in neuromuscular

reeducation of patients having hyperactive muscles

by electromyography biofeedback (Sartori et al.,

2004).

There several ways to configure EMG signal

acquisition through electrodes, the most common

being those requiring one (monopolar) and two

(bipolar) capturing points. The monopolar

configuration deals with the potential difference

between two points (a capturing and a reference

point). The bipolar configuration is characterized by

the potential difference between to acquisition

points, measured respecting to a third point

(reference). Electrodes are normally made of silver

coated with silver chloride (Ag-AgCl), since it is a

non polarizable noble metal.

The surface electrodes allow capturing signals

representing the activity of a muscle as a whole or a

group of muscles in a global manner and can be

subdivided into passive and active electrodes

(Barros, 2005 in: Ricciotti, 2006). The passive

electrode is constituted by a metal disk (in general,

Ag-AgCl), that must be placed over the skin (Figure

2). To decrease the contact impedance between

electrode and skin, it may be required a tricotomy

(pile removal from the region where the electrode

will be placed), the use of conductive abrasives and

gel or paste (Ricciotti, 2006).

a) Adhesive for electrode fixation;

b) Reusable electrodes;

c) Reusable electrodes detail;

d) Reusable electrodes rear view;

e) Reusable electrodes front view.

Figure 2: Examples of commercial passive electrodes

(Ricciotti, 2006).

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The active electrodes have a magnifying circuit

encapsulated near the electrode acquisition place.

Generally, these are bipolar electrodes, this meaning

that the amplifier used is of differential type. The

active electrodes, since they are made of a

differential amplifier, require a reference electrode

placed ion an inactive region to avoid measurement

interference (Basmajian, De Luca, 1985). Such

electrodes are also called dry electrodes, since

generally do not need to use conducting gel, paste,

abrasives or pile (Ricciotti, 2006).

The textile support represents the first

functionality level of a wearable technology

architectures system, since it comprises the infra-

structure to embed the basic functions as energy and

data transmission or the sensorial (Rodrigues et al.,

2008).

Following work already done, namely the

“textile arm” (Rodrigues et al., 2008), where

conductive metallic yarns are incorporated into the

fabric during weaving with the aim to acquire

surface electromyography signals, it was observed

the need to optimise the contact region with the skin

to improve electrode signal acquisition. Aiming to

contribute to problem solving, it is studied now the

applicability of polypyrrole as a conductive polymer.

2 MATERIALS AND METHODS

2.1 Printed Polypyrrole Electrodes

2.1.1 Textile Substrate

As textile substrate the dobby fabric developed in

the “textile arm” research work was used (Rodrigues

et al., 2008), having a compound weave based on

heavy twill 3 (Figure 3).

The main fabric characteristics are:

 Cover Factor: 95.3%;

 Weft density (DT): 19 yarns/cm;

 Warp density (DB): 24 yarns/cm;

 Warp yarn – 2/36Nm (45WO/53PES/2EA)

 Weft yarn – 2/50Nm (45WO/55PES)

2.1.2 Reagent Preparation

The pyrrole solution was prepared with: 9% of

DBSA – doping agent that reduces inter and intra

interactions in polypyrrole chains, improving pyrrole

solubility in organic solvents; 9% of PEG –

polymeric additive and 82% distilled water.

These reagents are mixed at room temperature

with slow stirring (10-15minutes). After, pyrrole is

added on a 0.11ml/g ration of the prepared solution.

After dissolving completely, the resulting solution

was isolated from air. Due to the need of a viscous

solution, for coating, a printing paste with only a

thickener (6% Sodium Alginate) in water was

prepared.

Figure 3: Compound weave module based on 2/1 twill 3

with metallic yarn (Rodrigues et al., 2008).

Since the pyrrole solution is quite unstable, the

oxidizing solution must be prepared before, which is

more stable. The oxidizing solution was prepared

with 20% of APS – oxidizing agent and 80%

distilled water.

2.1.3 Electrode Coating Methods

Following, the sodium alginate thickener was added

to the pyrrole solution on a 1/1 proportion. After a

complete mixing, the resulting paste was isolated

from air and kept at low temperature.

The fabric was pre-scoured in a bath containing

1g/l of Invadine LU (CIBA) – wetting agent, at 40ºC

during 20 minutes.

For electrode coating the respective area was

delimited and, to achieve a good electrode area

definition, the liquid oxidising solution was applied

first in excess followed by the pyrrole solution

thickened with sodium alginate (Figure 4). The

coated fabric was kept at room temperature for 15 to

30 minutes and then placed in a woven during 2

WEARABLE TECHNOLOGY - Development of Polypyrrole Textile Electrodes for Electromyography

405

Figure 4: Electrode coating device.

minutes at 100ºC.

To take out polypyrrole in excess from fabric

surface and, thus, to improve washing fastness, a

1g/l Reoklen detergent scouring at 50ºC and during

20 minutes was carried out.

2.2 Embroidered Polypyrrole

Electrodes

2.2.1 Reagent Preparation

A pyrrole solution was prepared having a ratio of

9ml of pyrrole to 100ml of DBSA and PEG in

distilled water, in the same way as described before

in 2.1.2. The oxidising solution used was also the

same as that employed in electrode coating.

2.2.2 Conductive Yarn Preparation

The chemical polymerization was achieved by

making the yarn go through the pyrrole solution and

after, through the oxidising solution, leaving

exposed at room temperature during 15 minutes and

after in a woven at 100ºC during 4 hours. To

eliminate excess of polypyrrole at the knit surface

and improve washing fastness, a detergent scouring

was performed.

2.2.3 Textile Substrate

As textile substrate a jacquard knit was used, being

the polypyrrole conductive yarn inserted into the

knit structure as shown in Figure 5. For knitting, a 3

ply 100% cotton yarn of 2/32Nm count was used. At

the electrode region, a polypyrrole treated 3 ply

100% wool yarn of 2/36Nm count was employed.

Figure 5: Jacquard knit structure showing electrode

positioning.

In Figures 6 and 7 knit images are shown, being

emphasized conductive yarn and electrodes.

Figure 6: Knit right face with electrodes.

Figure 7: Knit back face with conductive yarns.

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3 RESULTS AND DISCUSSION

3.1 Electromyography Signal

Acquisition

To acquire surface electromyography signals

(EMG), an experimental system was developed

made by a lap top computer, a differential amplifier

and software for measuring physiological signals

(Elektor Electronics, 2007), as shown in Figure 8.

Figure 8: Experimental system for sEMG signal

acquisition.

To acquire surface electromyography signals

(aEMG), electrodes were placed on the arm and,

when hand squeezing a ball, the stimulated muscle

contracted. At the beginning, commercial electrodes

were used, these being placed on the arm to test the

equipment (Figure 9).

Figure 9: sEMG captured signal using commercial

sensors.

Figure 10 shows the signal captured in the case

of “textile arm” produced with metallic filament and

embroidered electrodes using conductive yarn

(Rodrigues et al., 2008). In fact, the signal obtained

has a good definition and shows that this method is

adequate and can be used to measure the muscle

Figure 10: sEMG captured signal using hand embroidered

electrodes with metallic yarns (Rodrigues et al., 2008).

activity, for example, in rehabilitation.

However, this method doesn’t show a good

industrial feasibility, mainly because electrodes have

to be hand embroidered. For this reason, it became

necessary to develop embedded electrodes into

textile materials able to capture electromyography

signals and, at the same time, maximise the area of

contact with skin to improve the acquired signals by

electrodes. These signals captured by the “textile

arm” made with printed polypyrrole electrodes are

shown in figure 11. Also, the obtained signals with

the electrodes of the knit structure made with

polypyrrole treated yarns are shown in figure 12.

Figure 11: sEMG captured signal using polypyrrole coated

electrodes.

Figure 12: sEMG captured signal using polypyrrole

knitted electrodes.

WEARABLE TECHNOLOGY - Development of Polypyrrole Textile Electrodes for Electromyography

407

This data shows that we can obtain good definition

signals with the advantage that may have industrial

applicability.

4 CONCLUSIONS

In this research work electrodes for wearable

technology were developed, enabling simultaneously

a high level of integration, to keep textile properties,

the conductive polymer properties and the signal

capturing through substrate functionality. Analysing

the clarity of acquired signals it can be concluded

that they are similar, the solutions found presenting,

compared to commercial sensors, the same

advantages than the hand embroidered electrodes in

the “textile arm” (Rodrigues et al., 2008), this is,

textile comfort, mobility, cleanness and

maintenance, etc. Besides these, they also exhibit

advantages at industrial feasibility level.

The acquired results are satisfactory, considering

the possible advantages of these may present sport

and clinical rehabilitation level and even considering

research work on neuromuscular development. Thus,

this work presents the development of textile

solutions that, being improved, both from the textile

point of view, through incorporating electronic

devices, and counting on the informatics

contribution, gather potentialities to perform in

future an important role in people’s quality of life.

ACKNOWLEDGEMENTS

The Authors wish to tank to Eduardo Jorge de Jesus,

technician at the Knitting Laboratory of Textile

Science and Technology Department of University

of Beira Interior, for his collaboration concerning

weaving knit substrates.

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