Concentric Ring Tattoo Electrodes for Biosignal Recordings

Gema Prats-Boluda

, Eduardo Garcia-Breijo

, José L. Martinez-de-Juan

Javier Garcia- Casado

, Yiyao Ye-Lin

, Oleksandr Makeyev

and Piero Cossedu

Centro de Investigación e Innovación en Bioingenieria, Universitat Politècnica de València, 46022, Spain

Instituto Interuniversitario de Investigación de Reconocimiento Molecular y Desarrollo Tecnológico, 46022, Spain

School of STEM, Diné College, Tsaile, AZ 86556, U.S.A.

Department of Electrical and Electronic Engineering, University of Cagliari, via Marengo, Cagliari, 09123, Italy

piero.cosseddu@diee.unica.it

Keywords: Tattoo Electrodes, Concentric Ring Electrodes, Electrocardiogram, Inkjet.

Abstract: Non-invasive bioelectrical recordings utilize monopolar or bipolar disc electrodes. However, these electrodes

suffer from poor spatial resolution, leading to susceptibility to physiological interferences. Concentric ring

electrodes have been implemented on rigid and flexible substrates to enhance spatial resolution. The present

work aims to develop an ultra-flexible and ergonomic concentric ring tattoo electrode based on PEDOT: PSS

ink and check its feasibility of picking up surface bioelectric signals such as the electrocardiogram. Results

reveal that it is possible to capture good quality bioelectric signals with tattoo electrodes implemented through

inkjet techniques on tattoo paper substrate using PEDOT: PSS as ink. The main problem associated with this

option is the cost in time of the machine for manufacturing the electrodes.

1 INTRODUCTION

The recording of electrophysiological signals in its

simplest form, that is, through contact electrodes

attached to the skin, is subject to continuous studies

both to optimize these recordings and to search for

new technologies that improve the manufacturing

process.

Today, diagnosis, therapy, and health monitoring

are largely based on the recording of

encephalographic, cardiac, and muscle (myoelectric)

signals. Even so, most recording systems for these

signals continue to have a traditional approach, using

monopolar disk electrodes (mainly Ag or AgCl). The

use of conventional electrodes entails important

limitations. On the one hand, they have limited

ergonomics and comfort, and the costs of the

https://orcid.org/0000-0002-9362-5055

https://orcid.org/0000-0002-9745-8485

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https://orcid.org/0000-0003-1410-2721

https://orcid.org/0000-0003-2929-181X

https://orcid.org/0000-0003-2648-0500

manufacturing techniques used are also high. On the

other hand, the records obtained show a low spatial

resolution, which originates mainly from the

smearing effect due to the different conductivities of

the body volume conductor (Bradshaw et al., 2001).

This fact limits the ability to localize the sources of

bioelectric potential, which translates, for example,

into the difficulty of using conventional

electrocardiography (ECG) for the diagnosis of

pathologies associated with localized alterations in

the electrical conduction of the heart, as occurs in the

case of ventricular ischemia, atrial hypertrophy

(Macias et al., 2019), or difficulty in accurately

locating epileptic foci using conventional

electroencephalographic recordings (Aghaei-Lasboo

et al., 2020). Addressing these issues often

necessitates invasive electrophysiology, a procedure

Prats-Boluda, G., Garcia-Breijo, E., Martinez-de-Juan, J., Garcia-Casado, J., Ye-Lin, Y., Makeyev, O. and Cossedu, P.

Concentric Ring Tattoo Electrodes for Biosignal Recordings.

DOI: 10.5220/0012427800003657

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 17th Inter national Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2024) - Volume 1, pages 159-163

ISBN: 978-989-758-688-0; ISSN: 2184-4305

159

associated with considerable risks for the patient, and

an extended diagnosis time.

In recent years, an effort has been made to search

for alternative geometries and new technologies for

the manufacture of contact electrodes that allow

obtaining signals of better quality and/or with

improved spatial resolution. In addition, the

integration of the electrodes into clothing is sought,

which would lead to medical control beyond the

clinical work environment. To this end, the

development of electrodes on flexible biocompatible

substrates that provide characteristics such as

lightness, high flexibility, and adaptation to the

surface is sought out (Trung & Lee, 2016). Of these,

screen printing is the most widely used and mature

technology that has been used for decades in the

manufacturing of electronic systems and, more

recently, in the manufacturing of bioelectrodes.

To overcome the limited spatial resolution of

bioelectric recordings obtained from conventional

disk electrodes, the recording of the surface Laplacian

potential has been proposed (Lu & Tarjan, 2002)(Liu

et al., 2020) (Wang et al., 2023). The literature has

confirmed that Laplacian recordings are capable of

providing better spatial resolution of surface potential

recordings, that is, they can improve the detection of

bioelectric potential sources closest to the recording

electrodes, rejecting the contribution of sources of

bioelectric potential that are further away.

First, the surface Laplacian potential was

estimated using monopolar disc electrodes and

applying discretization techniques to estimate the

Laplacian potential from them (Hjorth,

1975)(Tandonnet et al., 2005)(Prats-Boluda et al.,

2007). Subsequently, concentric annular electrodes

were designed, whose configurations (bipolar, quasi-

bipolar, and tripolar) allowed obtaining a direct

estimate of the Laplacian of the potential captured on

the body surface. Subsequently, Laplacian potentials

of ECG, electrohysterographic, and

electroenterographic signals were obtained with

concentric ring electrodes (CRE) implemented on

rigid substrates, mainly using printed circuit boards

(Lu & Tarjan, 2002) (Prats-Boluda et al., 2011) (Mas-

Cabo et al., 2017)

The next step was the development of new

flexible CREs (fCREs). Different printing

technologies (screen printing, inkjet, gravure) and

materials such as Melinex or Ultem, MEMS) were

studied and compared for the development of fCREs

(Wang et al., 2023)(Trung & Lee, 2016),(Wei et al.,

2016). These fCREs allowed the capture of more

robust signals against respiratory interference than

conventional disc electrode recordings. Still, they

presented greater low-frequency interference, this last

observation being attributable to the fact that the

recordings with fCRE were carried out dry.

Therefore, despite the improvements introduced by

the implementation of CREs on flexible substrates,

the use of fCREs has not yet been transferred to the

clinical setting.

The objective of this work was to create an ultra-

flexible and ergonomic concentric ring tattoo

electrode using PEDOT: PSS ink assessing its

capability to capture surface bioelectric signals,

specifically the ECG. With this type of recording,

tools could be developed that would bring

electromyography closer to clinical practice in

different areas such as cardiology or obstetrics,

improving and/or complementing the information

provided by the non-invasive recording systems

common in clinical settings.

2 MATERIALS AND METHODS

A concentric ring electrode was designed using the

AutoCAD 2017 software (see Figure 1) and

considering the results of previous works (Prats-

Boluda et al., 2016).

Figure 1: Diagram of the concentric ring electrode design.

The technology tested in the manufacturing of

CRE tattoo prototypes was inkjet printing using the

Dimatix printer available at DEALAB (Figure 2).

Tattoo paper (tattoo 2.1) and commercial PEDOT:

PSS conductive ink GSD9011 (Heraeus®) have been

used as a substrate. Before printing the electrodes, the

tattoo paper is activated by plasma (30', 100W).

Likewise, the printing machine was programmed with

the configuration parameters shown in Figure 2.

To ensure correct deposition of the material, a 5-

layer print was made. The next step in the

manufacturing of these prototypes has been allowing

BIODEVICES 2024 - 17th International Conference on Biomedical Electronics and Devices

160

the extraction of signals from the tattoo electrodes. To

do this, two cables have been glued to the conductive

exit tracks of the electrodes prepared for this purpose,

using the CW 2400 conductive glue, and the joint has

been insulated and protected with Kapton adhesive.

In addition, snap connectors are glued and crimped on

the opposite end of the electrodes to allow their

connection to the biosignal capture system available

in the DEALAB, TMSI Porti 7®, see Figure 3. To

cure the conductive glue it is necessary to put it in the

electrode in the oven for 10 minutes at a temperature

of 50º.

Figure 2: Inkjet printer (top panel) and the setup screen

(bottom panel).

Figure 3: Output cables and connectors (at right) glued to

the CRE tattoo.

Both, concentric bipolar (CRE-ECG; outer ring

minus inner disc) and Lead II ECG (Lead II-ECG)

signals were simultaneously recorded with the TMSI

Porti7®, from DC to 500Hz, and acquired at 2048 Hz.

3 RESULTS

The capacity of this prototype of fCRE for the capture

of bioelectric signals was tested. Specifically, an ECG

signal was recorded. First, the skin surface where the

fCRE was positioned, comparable to precordial lead

V1 (CMV1), was shaved to minimize contact

impedance and minimally exfoliated using Nuprep

from Weaver and Company, USA. Following the

subjects' skin was cleaned with alcohol.

Subsequently, tattoo paper where the fCRE was

printed was placed over the area to be recorded,

CMV1, eliminating the “sacrificial layer”, a top

transparent film that covers the printed electrode. It

protects the electrode and helps transfer it onto the

skin. Figure 4 shows the arrangement of the

developed tattoo electrode on the chest to test its

ability to record the CRE-ECG signal. Together with

the CRE electrode, the Lead II-ECG was

simultaneously recorded.

Figure 4: Placement of the CRE tattoo of PEDOT: PSS, for

recording ECG signal.

Figure 5 corresponds to the simultaneous raw

recordings of the Lead II-ECG with conventional

commercial disc electrodes and with the PEDOT:

PSS tattoo electrode printed with the Dimatix on

tattoo paper. Recordings in Figure 5 demonstrate the

feasibility of capturing the ECG signal with the

PEDOT: PSS CRE tattoo electrodes on tattoo paper.

As expected, the amplitude of the signal captured

with the CRE (ECG-CRE) is much weaker than that

associated with Lead II-ECG recordings made with

conventional disc electrodes. In this regard, it should

be pointed out that one of the advantages of the

designed CRE is the possibility of picking up the

cardiac signal without using an external reference

electrode located on the hip or extremities. Still,

rather the tattoo electrode itself incorporates its own

reference. Also, the presence of powerline noise is

Concentric Ring Tattoo Electrodes for Biosignal Recordings

161

Figure 5: 40 seconds or raw signals corresponding to simultaneous recording of ECG-CRE bipolar signal captured with CRE

tattoo of PEDOT: PSS printed by inkjet using the Dimatix printer on plasma-activated tattoo paper (top panel) and Lead II-

ECG recording obtained with conventional disc electrodes (bottom panel).

noticeable in the recorded signals, both in the ECG-

CRE recording and in the Lead II-ECG signal.

Concerning this aspect, tests to check the ECG signal

capture capacity with the CRE tattoos printed with the

Dimatix were carried out in the laboratory, where a

large number of machines and wiring are located, that

is, in a very unfavorable environment for the

recording of bioelectric signals.

One of the main problems with this technology is

the slowness of printing, requiring 3.5 hours of

machine use per electrode. That is why it is proposed

in the future to carry out a study about developing

tattoo electrodes using screen printing or gravure

techniques, that enable the large-scale production of

this type of electrodes at an industrial scale.

Additionally, to enhance signal quality and facilitate

extensive use, it would be advisable to incorporate

additional elements such as electrolytic gel, adhesive,

and connectors commonly used in the biomedical

industry.

Another important aspect that needs to be

addressed to bring the use of CRE electrodes into the

clinical setting is to establish standard recording

positions on the torso and extract patterns of

normality/abnormality from the morphology of

cardiac waves. In this regard, preliminary studies

have been conducted using flexible concentric multi-

ring electrodes in precordial positions (Prats-Boluda

et al., 2016) or even mapping the torso (Besio &

Chen, 2007) (Prats-Boluda et al., 2018). However,

comprehensive studies are needed to validate the

results and, above all, to determine biomarkers

associated with pathological conditions.

As for the applicability of the developed tattoo

electrode, in the present work, we have focused on

ECG recording for the electrode design. Similar

electrodes could be used for picking up other

bioelectric signals (Estrada-Petrocelli et al., 2021)

(Ye-Lin et al., 2022). The optimal TCRE dimensions

will depend on the depth of the bioelectric source (the

deeper the higher the electrode diameter), the

required signal-to-noise ratio, or spatial resolution

(Makeyev et al., 2021).

4 CONCLUSIONS

It is possible to capture good quality bioelectric

signals with tattoo electrodes implemented through

inkjet techniques on tattoo paper substrate using

PEDOT: PSS as ink. The main problem associated

with this option is the cost in time of the machine for

manufacturing the electrodes. Future works will be

carried out to develop tattoo electrodes by screen-

printing or gravure techniques.

ACKNOWLEDGMENTS

This work was supported by the Generalitat

Valenciana (Spain) BEST/2019/168.

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