sensors
ArticleNoise-Reducing Fabric Electrode for ECG Measurement
Takamasa Terada 1 , Masahiro Toyoura 1, * , Takahide Sato 2 and Xiaoyang Mao 1
1

2*

Department of Computer Science and Engineering, University of Yamanashi, Kofu, Yamanashi 400-8511,
Japan; g19dts03@yamanashi.ac.jp (T.T.); mao@yamanashi.ac.jp (X.M.)
Department of Electrical and Electronic Engineering, University of Yamanashi, Kofu, Yamanashi 400-8511,
Japan; takahides@yamanashi.ac.jp
Correspondence: mtoyoura@yamanashi.ac.jp

Abstract: In this work, we propose a fabric electrode with a special structure that can play the role ofa noise reduction filter. Fabric electrodes made of the conductive fabric have been used for long-term
ECG measurements because of their flexibility and non-invasiveness; however, due to the large
impedance between the skin and the fabric electrodes, noise is easily introduced into the ECG signal.
In contrast to conventional work, in which chip-type passive elements are glued to the electrodeto reduce noise, the proposed electrode can obtain a noise-reduced ECG by changing the structureof fabric. Specifically, the proposed electrode was folded multiple times to form a capacitor with acapacitance of about 3 nF. It is combined with the skin-electrode impedance to form a low-pass filter.
In the experiment, we made a prototype of the electrodes and measured ECG at rest and during
EMG-induced exercise. As a result, the SNR values at rest and during exercise were improved about
12.02 and 10.29 dB, respectively, compared with the fabric electrode without special structure. In
conclusion, we have shown that changing the fabric electrode structure effectively removes noise in
ECG measurement.



Keywords: textile; electrocardiography; denoising

Citation: Terada, T.; Toyoura, M.;
Sato, T.; Mao, X. Noise-Reducing
Fabric Electrode for ECG
Measurement. Sensors 2021, 21, 4305.
https://doi.org/10.3390/s21134305
Academic Editor: Ki H. Chon
Received: 20 May 2021
Accepted: 22 June 2021
Published: 23 June 2021

Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims inpublished maps and institutional affiliations.

Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms andconditions of the Creative Commons
Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

1. Introduction
In the field of e-textiles, conductive yarns, and conductive fabrics are used as sensors.
Conductive fabrics have the advantages of being flexible and lightweight compared toconventional electrodes such as metal plates. In addition, conductive fabrics have theadvantage of non-invasiveness, so they are also used as electrodes for measuring biologicalsignals, as in electrocardiogram (ECG). However, it is known that fabric electrodes cause
the introduction of a great deal of noise because the skin-electrode impedance is higher
than with a wet electrode, such as an Ag/AgCl electrode [1]. In general, noise reduction
techniques are required in ECG analysis such as the automatic diagnosis of heart disease.
If noise reduction is not sufficient, the accuracy of the automatic diagnosis may decrease
because the ECG feature extraction accuracy like R-peak detection will also decrease [2].
For noise reduction, various methods have been proposed, such as the improvement ofelectronic circuits, materials of electrodes, and noise reduction algorithms. In this work, we
propose a noise reduction method focusing on the internal structure of the electrode.
Conventional ECG measurement devices use an analog filter that combines flexible
materials and passive elements, such as chip resistors and capacitors. For example, one
proposed device involves a printed conductive pattern and a passive device attached to
the fabric to form an analog filter. This structure makes it possible to acquire a less noisy
signal than when using electrodes without a filter. However, there is a risk of damaging the
conductive pattern and the passive device by disturbance. To prevent this damage, a new
approach that omits the process of attaching the passive device to the fabric and makes the
fabric itself function as a circuit is required. Recently, a pressure sensor that can reduce thenumber of conductors connecting threads and circuits has been proposed [3]. This sensor

Sensors 2021, 21, 4305. https://doi.org/10.3390/s21134305

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is focused on the structure of thread intersections, and it has been shown that just changing
the structure of the fabric can provide a new function. This means that the passive elements
used in the circuits can be replaced by changing the internal fabric structure. In the fieldof ECG measurement device development, an electrode that changes only the structureof the conductive fabric itself to add function as a filter has not been proposed. Therefore,
we propose a new noise reduction method that allows band-limiting by changing only theinternal electrode structure without using passive elements, such as chip devices.
In this work, based on the idea in e-textiles of adding functions by changing thestructure of conductive yarns, we consider changing the structure of fabric electrodes to
add new functions. As a new function, the multi-layer structure of the electrode provides
a capacitor which has a sufficient capacitance. Then, the capacitor and the skin-electrode
impedance are combined to form a low-pass filter. The contributions of this work are
as follows:•••

We have discovered that elements made of fabric can play the same role as generalpassive elements;
We show that the impedance of the human body and passive devices made of conductive fabric can work as a filter;
We show that changing the internal structure of the conductive fabric is effective innoise reduction.

In this paper, we show that, by changing the structure of the electrode itself, it functions as a filter. In the experiment, the proposed electrode performs an ECG measurementwith little noise even when the electrode is dry. The paper is organized as follows: Section 2
describes related works; Section 3 describes the proposed electrode structure; in Section 4,
the ECG measurement system is used to evaluate the proposed electrode; Section 5 describes the discussion and future works; and finally, Section 6 summarizes this work.
2. Related Works
Monitoring systems for various activities, including early detection of heart disease,
have been proposed [4]. In such system, clean ECG data are required for activity identification. To realize continuous ECG measurements in daily life, various improvement of
measurement devices and electrodes have been proposed.
Komensky et al. [5] proposed a method for an active electrode in which a pre-amplifier
and an electrode were mounted on a polychlorinated biphenyl (PCB) substrate. This
device is a belt-type device that can perform an ECG by being wrapped around the chest.
However, it has been pointed out that electrodes using PCBs are hard and may cause
discomfort to the wearer. Using flexible instead of hard materials is one way to reduce suchdiscomfort. For example, measurement devices using conductive rubber materials [6–8]
and graphene [9–11] as electrodes have been proposed. Reyes et al. [12] proposeda carbon black powder/PDMS composite electrode, and showed that ECG waveforms
could be measured in dry and water immersion, as well as with the Ag/AgCl electrode.
Furthermore, flexible materials coated with conductive materials on non-conductive fabrics
and yarns coated with conductive materials [13,14] have also been proposed. Taji et al. [15]proposed a wristband device made of silver-coated nylon as a flexible material and showedthat ECGs could be performed correctly, even with the fabric electrode. Rattfalt et al. [16]found that differences in electrode materials can change the electrical properties of fabricelectrodes, such as impedance. Sneh et al. [17] proposed fabric electrode composited thegraphene and PEDOT:PSS which has lower impedance. They showed that it is possible tomeasure the ECG with high peak amplitude.
It has been reported that not only the material but also the structure of the electrodes
affects the quality of the ECG. Wu et al. [18] developed an electrode that provides both
comfort during measurement and quality of signal by changing the ratio of silver tocotton yarn. Xiao et al. [19] created a fabric electrode with a honeycomb structure madeof silver-coated nylon. This work showed that different proportions of conductive andnon-conductive yarns and different weaves affected the quality of ECGs and the wearer’s

Sensors 2021, 21, 4305

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comfort when the electrodes were attached. However, changing the structure to add
functions has not been discussed.
An improvement of the combination of the electrode and the device has also been
proposed. To reduce patient discomfort, Li et al. [20] proposed a wearable device in whicha band with a thick fabric electrode was wrapped around the chest to ensure sufficientskin-to-electrode contact. Meanwhile, Lee et al. [21] proposed a wearable device using a
conductive foam as a material instead of fabric, which allows the skin and the electrodeto be in closer contact and reduces low-frequency noise. These devices are designed toplace an insulator, such as clothing, between the skin and the electrodes, so there is a
capacitor between the electrodes and the skin. By inserting bias resistors from a few MΩ
to a few GΩ in parallel with the pre-amplifier, the low-frequency noise can be reduced.
However, it has been reported that other noises can be introduced easily depending on
the thickness of the clothing. On the other hand, Lee et al. [22] show an example of alow-pass filter that includes a capacitor in parallel before the pre-amplifier. In their device,
the capacitance generated between the electrodes and the device shield is small, a few
pF, and is treated as parasitic capacitance. Takano et al. [23] proposed an application to
bed sheets. They proposed a structure in which capacitance is generated by inserting ashield between the electrode and GND, but without changing the structure of the electrode
itself. Ozkan et al. [24] proposed a remote ECG monitoring system in which a conductive
fabric was used as an electrode for the device. Their fabric electrodes were removable
and washable, but electrodes in their system were just conductive fabric, meaning theelectrodes themselves do not include a filter like an active electrode. This implies that, if
the fabric itself has a function to reduce noise, the noise rejection performance of the system
can be improved.
Conventional fabric electrodes are made of conductive yarn or a non-conductive fabriccoated with conductive materials. There are methods to build a high-pass filter using thecapacitance between the skin and the electrode and a resistor before the pre-amplifier, butthey require a resistor of several MΩ to several GΩ in the electrode. Such a large resistanceis difficult to miniaturize for integrated circuits, and chip resistors must be mounted ona conductive fabric. In addition, electrodes using electronic elements such as resistors
and capacitors lose the advantages of fabric because they may be damaged by moistureor impact. If these elements are flexible like fabric and do not require adhesion or othertreatments to attach them to the fabric, we can make more robust electrodes. We, therefore,
propose an electrode with a capacitor by changing the structure of the fabric electrodeand show that the skin-electrode impedance forms a low-pass filter. Then we design afirst-order low-pass filter to show that the proposed electrode works as a filter.
3. An Analog Filter by the Conductive FabricIn this section, the structure of the proposed fabric filter and the actual fabric electrodeare presented.
3.1. Requirements for Analog Filter
As described in Section 2, this work aims to create a low-pass filter without anelectronic device by changing the structure of the electrodes. We will firstly describe therelationship between the proposed electrodes and the noise to be filtered, then the proposed
structure of the fabric. As shown in Figure 1, there are several types of noise, such as powerline interference (PLI), electrode motion (EM), and the signal from organs other than ECG,
such as electromyogram (EMG). It is known that PLI is 50 or 60 Hz, EM is low frequency,
and EMGs are high frequency [2]. Since EMGs are noise originating in the human body,they are known to affect ECG preprocessing and diagnosis classification accuracy adversely.
Therefore, they should be reduced before the circuit’s input, which amplifies the ECG. To
reduce their effect, we aim to create a filter that reduces the high-frequency noise like EMG.

Sensors 2021, 21, 4305

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PLI
R1

Rc

Rn

… +
EMG

C1

+

+
EM

Cn

ECG
measurement
circuit

Clean ECG

Cc

Rbody
Skin-Electrodeinterface

Human body

Figure 1. Relationship between the human body, skin-electrode interfaces, ECG measurement
system, and noises. PLI, EMG and EM mean power line interference, electromyogram, and electrodemotion, respectively.

We first consider implementing a low-pass filter between the skin and electrode toattenuate the EMG. Figure 2 shows an electronic circuit of the low-pass filter. Elements
constructed the low-pass filter in Figure 2 are Rbody , Rc , Cc , and Ce . Each element means
the internal resistance of human, the resistance, and the capacitor consist of skin-electrode
impedance, and the capacitor of the proposed electrode, respectively. Here, we consider
Rbody to be 500 Ω, which is the maximum internal resistance from the fingertip to the foot.
It is known that the skin-electrode impedance consists of the resistor Rc and capacitor
Cc , which have several MΩ and several hundred pF to several nF, respectively [25]; however, the actual values of Rc and Cc are uncertain. If we can estimate the skin-electrode
impedance, we can realize the low-pass filter by placing the appropriate values of Ce before
the circuit’s inputs. The transfer function combined with the Ce can express as follows:
Vout
(s) =
Vin
Rbody +

1
sCe
.
Rc
1
1+sRc Cc + sCe(1)

Because the range of EMG is from 100 Hz to a few kHz or more, we measured theskin-electrode impedance to determine the appropriate capacitance of Ce .

Rbody

Rc

Cc

Human bodyECG
measurement
circuitCe

Skin-Electrode Proposed fabricelectrodeinterface

Figure 2. An overview of the proposed analog filter.

3.2. Impedance Constituting the Textile Filter
Here, we consider the skin-electrode impedance to determine the capacitance of theelectrode to be designed. The skin-electrode impedance was measured by using one target
electrode and two reference electrodes in accordance with previous work [19,26]. As
shown in Figure 3, when a target electrode is attached between the reference electrodes,
the impedance Z1x between the reference electrode E1 and the target electrode Ex can bemeasured. Z1x is the sum of the impedances Z1 , Zbody1 , and Zx , which are the impedancefrom E1 to the skin, the impedance in the body, and the impedance from the skin to
the target electrode, respectively. Similarly, the impedance Z2x from Ex to E2 and theimpedance Z12 from E1 to E2 are also the sum of the impedance of each electrode, the skin,Sensors 2021, 21, 4305

5 of 17and the impedance in the body. The following equation expresses the impedances between
each electrode:
Target Ex

Reference E1

Reference E2

Zx

Z1

Z2
Skin

Zbody1

Zbody2

L1

L2

Body

Figure 3. The relationship of the impedance between each electrode.

Z1x = Z1 + Zbody1 + Zx ,Z2x = Z2 + Zbody2 + Zx ,(2)

Z12 = Z1 + Zbody1 + Zbody2 + Z2 .
Assuming that Zbody1 and Zbody2 are equal when the distances L1 and L2 from thetarget electrode Ex to the reference electrodes E1 and E2 are equal, the final impedance Zx
is expressed by the following equation:
if L1 = L2 :Zx = Z1x −Z12
Z= Z2x − 12 .
2
2

(3)

In this case, impedance Zx was estimated based on Equation (3) by measuring Z1x , Z2x ,and Z12 when the surface area is 2 × 2 cm2 . In the measurement, 3M Red Dot monitoring
electrode was used as the reference electrode. The measurement results are shown in
Figure 4. Z12 was 151.2 kΩ at 1 Hz and 89.8 kΩ at 20 Hz, then we assume to be ignored
when comparing with Z1x and Z2x . The impedance above 10 Hz shows that the resistanceand capacitance of the skin-electrode impedance are about 1.0 to 1.5 MΩ and about 1 to
3 nF, respectively. The measurement revealed the impedance between the skin and theelectrodes, but in the actual environment, the impedance may change due to various factors(e.g., sweating). In order to determine the impedance of the electrodes that can be used asa filter even if the impedance of the skin changes, we conducted the simulation using thecircuit shown in Figure 2 and LTspice.
10,000

Impedance (k )

ȍ

Z1x
Z1x
Z2x
Z2x

1,000
1000

Average
Average

100
1

10

100

Frequency (Hz)Figure 4. The impedance between the skin and the electrode.

The result of circuit simulation is shown in Figure 5. When Cc is assumed to be 3 nF
and Ce is set to 1 nF, the attenuation above 100 Hz is as low as −2.48 dB at 1.0 kHz. On theSensors 2021, 21, 4305

6 of 17other hand, an attenuation of about −6.02 dB can be expected by changing Ce to 3 nF. If
Cc is 1 nF, a maximum attenuation of about −12.73 dB can be expected when Ce is 3 nF.
If the capacitance of Ce is set to 10 nF or greater than Cc , it can be attenuated more thanwhen Ce is about 3 nF. However, the signal at about 10 Hz may be attenuated, and the ECG
waveform may not be obtained. If the skin surface is wet, the resistance may decrease.
If Rc is set to 0.5 MΩ, the attenuated frequency moved into the high-frequency domain.
Therefore, we assume that Rc and Cc are 1.5 MΩ and 3 nF, and we adopt 3 nF for Ce .
0

-2
Ϋʹ

Power [dB]

ΫͶ
-4
Ϋ͸
-6
Rc = 1.5M, Cc = 1n, Ce = 3nΫͺ
-8

Rc = 1.5M, Cc = 3n, Ce = 1n

-10
ΫͳͲ

Rc = 1.5M, Cc = 3n, Ce = 3n
Rc = 1.5M, Cc = 3n, Ce = 10n

-12
Ϋͳʹ

Rc = 0.5M, Cc = 3n, Ce = 3n

ΫͳͶ
-14
0.1

1.0

10.0

100.0

ͳͲͲͲǤͲ
1,000.0

Frequency [Hz]Figure 5. The simulation result changing Rc , Cc , and Ce .

3.3. Fabric Structure
In this section, the actual structure of the electrode is described. As shown in Figure 6a,in the case of touch sensors made of conductive fabric, conductive yarns covered with aninsulator, such as polyester, are crossed to create a capacitor. However, when the yarns arecrossed, as in a pressure sensor with conductive yarns, the capacitance value per crossing
point is at most several pF. To increase the capacitance value, we propose an electrode withthe structure shown in Figure 6b.
The proposed electrode is made by sandwiching a non-conductive material between
two pieces of conductive fabric and folding them. In focusing on the internal structure,
the proposed structure is similar to the touch sensor in principle, because the sandwichedfabric layers work like a large number of capacitors in a fabric touch sensor. As a materialof the proposed electrode, AGposs conductive wearable electrode tape was used. The tapeused silver-plated conductive fibers, and the resistance per cm2 was less than 1.0 Ω. In
addition, silver has been reported to be less harmful to human health, making it a suitablematerial for electrodes that are directly attached to the human body. A polyimide was usedas the non-conductive material to stack conductive tape. Although we have not evaluatedthe flexibility of the filter, we confirmed that it could be attached to the arm even if theground surface was curved, and that it was flexible enough not to impair its function as afilter in actual measurements. The capacitance per cm2 when the fabric tape stacked withthe polyimide was measured with a LCR Meater, and it was 20 pF. Therefore, the capacitor
must have a surface area of about 150 cm2 due to the capacitance per unit area. As shownin Figure 7, the area between the skin and electrode was 2 × 2 cm2 . The capacitor was madefrom the conductive tape with a length of 30 cm and a width of 5 cm. The electrode has
seven layers, and the capacitance of each layer can be regarded as a single capacitor thatis synthesized.
It should be noted that the capacitance of the fabric filter changes according to theapplied pressure due to the characteristics of folded fabric. We further investigated theSensors 2021, 21, 4305

7 of 17differences in the capacitance of two prototype electrodes when different external forces
are applied to them. As shown in Figure 8, a digital force gauge and LCR meter wereused to measure the capacitance by applying constant pressure to the electrodes. The
capacitance of each electrode was measured 600 times every 0.5 seconds for about fiveminutes. The average values of the capacitance when 10, 30, and 50 N were applied to oneof the electrodes were 1.66, 2.24, and 3.35 nF, respectively. Similarly, the average values
for the other electrode were 2.39, 2.96, and 3.34 nF, respectively. There was a difference incapacitance of about 0.75 nF when an external force of 10 N was applied to each electrode,but the capacitance became almost the same value when the external force was increased.
Therefore, once the pressure is applied, the capacitance value in the fabrics can be kept
almost constant.
:HIW\DUQ

D&RQGXFWLYHPDWHULDO:DUS\DUQ&RQGXFWLYHIDEULF
9HUWLFDO,QVXODWRU
9RXW&RQGXFWLYHIDEULF+RUL]RQWDO

«

&HQ

E

(OHFWURGH&H&H&H &H&H«&HQ Q 

6NLQ

Figure 6. The structure of the fabric capacitor. (a) The conventional fabric such as pressure sensor. (b)The proposed fabric.

Conductive fabric(Horizontal )Insulator

Connector
Conductive fabric(Vertical)(a) Top view
Figure 7. Cont.

5cm

Sensors 2021, 21, 4305

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Conductive fabric2cm

Non-conductive fabric(b) Bottom view
Figure 7. The prototype of the proposed electrode. (a) and (b) show the top view and bottom
view, respectively. The fabric capacitor consists of the vertical and the horizontal conductive fabric.
The area connecting with the skin is 2 × 2 cm2 . To prevent skin touching the fabric capacitor, weused a non-conductive fabric in (b). The total thickness of the fabric electrode including that of anon-conductive fabric was about 1 cm.

Digital Force Gauge

LCR Meter୭୳୲ ୧୬

ElectrodeFigure 8. The environment for measuring the capacitance of the electrode.

4. Evaluation of Textile Filter
In Section 3, it was confirmed that the proposed capacitor was determined to havethe desired capacitance. We evaluated whether it can be used as an electrode of an ECG
measurement device through an actual measurement. The signal quality obtained by our
proposed electrodes and ordinal Ag/AgCl ones was compared.
4.1. Settings
Because the fabric filter was connected before the pre-amplifier, it was necessary
to confirm the attenuation of the noise by paying attention to the noise mixed with the
movement of the body. Therefore, ECGs at rest and exercise were performed. During
exercise, the subject grasped the grip strength meter to mix the EMG; in order to confirm
the attenuation of the EMG mixed in when grasping an object, a wristband-type systemthat affects the EMG more was used. Figure 9 shows an overview of the system used
for the ECG measurement. As in the conventional method, this system acquires the ECG
using electrodes and electronic circuits attached to the body. The acquired ECG signals

Sensors 2021, 21, 4305

9 of 17are converted into a digital signal by a 12-bit A/D converter in ESP32 and are then stored
in a paired terminal through the Bluetooth module. The sampling frequency during themeasurement was set to 1.0 kHz. System power was supplied from a mobile battery tominimize noise generated from the GND of the measurement equipment.

Filters
INA
121

A/D

Vref
Vၱ

Amp

TX

V+
ElectrodeRX

BluetoothFigure 9. Overview of the ECG measurement system. The system has three inputs from the right arm
V− , the left arm V+ , and the reference electrode Vref . V− and V+ are our proposed fabric electrodes,
and Vref is a non-folded fabric electrode. The ECG signal is sent to laptop via Bluetooth.

Figure 10 shows the electronic circuit used in the ECG measurement system. The
signal obtained from the right-hand and left-hand electrodes is input to the instrumentation
amplifier INA121 (Texas Instruments, input impedance 1 TΩ). The first stage instrumentation amplifier and the second stage non-inverting amplification circuit amplified 20 dB
and 40 dB, respectively, resulting in a total amplification of 60 dB. Because the aim wasto confirm the performance of a low-pass filter using the textile filter, the filters between
the first-stage amplification and the second-stage amplification were combined as a twin-t
notch filter and a high-pass filter. The twin-t notch filter is composed of a resistor anda capacitor, and it reduces noise from a commercial power supply of 50 Hz. The cut-off
frequency of the high-pass filter was set to 0.5 Hz. To confirm the effect of noise reduction
on the high frequency of the proposed fabric electrode, we experimented with the proposed
electrode and fabric electrode without the proposed structure called non-folded fabric. The
non-folded fabric electrode also has a 2 × 2 cm2 surface area and was connected to the
input of the electronic circuit directly.

Sensors 2021, 21, 4305

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Twin-TV+
R1High pass filter

R6

R7

C1

C2

R8

C3

C4

V−

R2
Ref

Ce
R4
Vref

ʶ
−

INA121

R5

−
ʶ

R3

R9
R10

Vout

R11
Vcc : 3.3 V

ʶ
−

ʶ
−

R12

R13

R1 = R2 = 2.8 kȍ, R3 = 10 kȍ, R4 = R5 = 390 kȍ, R6 = R7 = 31.8 kȍ,
R8 = 15.9 kȍ, R9 = 680 kȍ, R10 = 1 kȍ, R11 = 100 kȍ, C1 = C2 = 0.1 ȝF, C3 = 0.2 ȝF, C4 = 1 ȝF

Figure 10. The circuit for the ECG measurement. The red line indicates the proposed electrode. The
circuit consists of the twin-t notch filter, the first order high-pass filter and the amplifiers.

For the evaluation of the acquired signal, signal-to-noise ratio (SNR) was used as aquantitative evaluation index. The SNR calculation is as follows:
N

∑ S2x

SNR = 10 log10

x =1
N

∑

,

(4)

(Sx − S0x )2x =1

where Sx and S0x denote the x-th data of the clean signal and the acquired signal, respectively.
To evaluate the difference of the high-frequency component, we applied the FIR filters
to Sx . These filters are the high-pass filter ( f c = 0.67 Hz), the bandwidth rejection filter( f c = 49 Hz, 51 Hz), and the low-pass filter ( f c = 100 Hz). Similarly, the same high-pass filterand bandwidth rejection filter was applied to S0x . In addition, power spectral density (PSD)
is also shown to indicate the frequency components contained in the measured signals.
The method of Welch et al. [27] was used to calculate the PSD. To further compare themorphology of the ECG, we used temporal and spectral measures of HRV, peak-to-peakamplitude, and cross-correlation index to compare with Ag/AgCl electrodes, similar to
the indices described in [12]. The ECG measurements using the proposed electrode andthe non-folded fabric electrode were taken at rest and during exercise for three minutes,
once for each. The subject was a university male student in his 20 s. In order to check thefeasibility of using proposed fabric electrodes, the experiment was conducted with only
one subject.
4.2. The Measurement Result during Resting
Figures 11 and 12 show the ECG measurement results of the non-folded fabric electrode and the proposed electrode, respectively. The signal using a non-folded fabricelectrode in Figure 11a is noisy, and it can be seen that the R-peak is buried in the noise. In
particular, the potential of the T wave after the R-peak is similar to the R-peak, making it
difficult to distinguish between the R wave and the T wave. The signal obtained from the
proposed electrode in Figure 12a has less noise than in Figure 11a, and it is easier to differentiate between R and T waves. Compared to Figure 12b, Figure 11b has more high-frequency
component other than ECG. SNR values using the non-folded fabric and proposed fabricbased on Equation (4) were 7.43 and 19.45 dB, respectively. It means that the SNR valueSensors 2021, 21, 4305

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was improved 12.02 dB by using the proposed electrode. Therefore, we can confirm theattenuation of the high-frequency noise by the filtering of the proposed electrode.
0.4

1s

(a)

Amplitude (V)

0.2

0.0

ΫͲǤʹ
-0.2
Noisy signal
Filtered signal

ΫͲǤͶ
-0.4

Time (s)0.4(b)

Amplitude (V)

0.2

0

ΫͲǤʹ
-0.2

ΫͲǤͶ
-0.4

Time (s)Figure 11. The ECG measurement result using the non-folded fabric electrode at rest. (a) The
measured signal and the signal filtered by FIR filters. (b) The high-frequency component above
100 Hz in (a). (a,b) are the common in Figures 12–14.
0.4

1s

(a)

Amplitude (V)

0.2

0

ΫͲǤʹ
-0.2
Noisy signal

ΫͲǤͶ
-0.4

Filtered signal

Time (s)0.4(b)

Amplitude (V)

0.2

0

ΫͲǤʹ
-0.2

ΫͲǤͶ
-0.4

Time (s)Figure 12. The ECG measurement result using the proposed fabric electrode at rest.

4.3. The Measurement Results during Exercise
The results of the measurements during exercise using the non-folded fabric electrodeand the proposed electrode are shown in Figures 13 and 14, respectively. As shown in
Figure 13a, the signal measured during exercise had more noticeable noise overall thanthat in Figure 11a. Similarly, when using the proposed electrode, the signal in Figure 14a
contains more noise than in Figure 12a. As shown in Figure 13b, the high-frequency noise
other than ECG contained in Figure 13a is greater than the noise in Figure 11b. Therefore,Sensors 2021, 21, 4305

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we can see that this signal is the noise introduced by the exercise. On the other hand,
as shown in Figure 14b, the signal using the proposed electrode has less noise than thatin Figure 12b. SNR values using the non-folded fabric and proposed fabric based on
Equation (4) were 7.59 and 17.88 dB, respectively. As well as during resting, the SNR value
in exercise was also improved 10.29 dB by using the proposed electrode. Therefore, it
can be said that high-frequency component noise can be reduced by incorporating a filter
structure in the electrodes, even during exercise.
0.4

1s

(a)

Amplitude (V)

0.2

0

ΫͲǤʹ
-0.2
Noisy signal
Filtered signal

ΫͲǤͶ
-0.4

Time (s)0.4(b)

Amplitude (V)

0.2

0

-0.2
ΫͲǤʹ

-0.4
ΫͲǤͶ

Time (s)Figure 13. The ECG measurement result using the non-folded fabric electrode in exercise.
0.4

1s

(a)

Amplitude (V)

0.2

0

ΫͲǤʹ
-0.2
Noisy signal

ΫͲǤͶ
-0.4

Filtered signal

Time (s)0.4(b)

Amplitude (V)

0.2

0

ΫͲǤʹ
-0.2

ΫͲǤͶ
-0.4

Time (s)Figure 14. The ECG measurement result using the proposed fabric electrode in exercise.

Finally, the power spectral density (PSD) of each measurement is shown in Figure 15.
The power of all signals was similar up to around 20 Hz, and the power of the signals
above 20 Hz differed depending on whether they were at rest or exercise, and whether the
proposed electrode or non-folded fabric electrode was used. Both electrodes showed anincrease in power above 100 Hz during exercise. An attenuation of −6.02 dB was expectedSensors 2021, 21, 4305

13 of 17in the simulation result, as shown in Figure 5, where Rc , Cc , and Ce were assumed tobe 1.5 MΩ, 3 nF, and 3 nF, respectively. The attenuation above 100 Hz by the proposed
electrode was −5.96 to −10.56 dB at rest and −6.67 to −10.91 dB during exercise comparedwith the ordinal electrodes. The average attenuation above 100 Hz were −9.06 dB at
rest and −9.41 dB during exercise compared with the ordinal electrodes. Therefore, theperformance of the low-pass filter with the proposed electrode was confirmed as desired.
Ͳ
Non-folded_restingΫͳͲ

Non-folded_exercise

Power Sperctral Density [dB]ΫʹͲProposed_restingΫ͵ͲProposed_exercise

ΫͶͲ
ΫͷͲ
Ϋ͸Ͳ
Ϋ͹Ͳ
ΫͺͲ
ΫͻͲ
ΫͳͲͲ
1

10

100

Frequency [Hz]Figure 15. The power spectral density.

4.4. Signal Quality Comparison with Ag/AgCl ElectrodeThe comparison between the proposed electrode and an Ag/AgCl electrode at rest andduring exercise is shown in Figure 16, Tables 1 and 2. In Figure 16, there is a difference inthe shape of T-wave between the fabric electrode and an Ag/AgCl electrode, but the othershapes are similar. In addition, it can be confirmed that the proposed electrode can reduce
the noise because distortions can be seen on the non-folded electrode compared with theproposed electrode. In Tables 1 and 2, the cross-correlation coefficients were slightly larger
for the proposed electrode than for the non-folded fabric, and the proposed electrodes was
closer to the Ag/AgCl results. All the temporal measures were lower for the proposed
electrode in both resting and exercise conditions. The larger NN50 values for the Ag/AgCland the non-folded electrodes were due to the fact that the function of low-pass filter was
not installed in both of them, and only the proposed electrode with a built-in low-passfilter was more stable. In spectral measures, the power of the Ag/AgCl electrode was thehighest, and that of the proposed electrode was the lowest. The peak-to-peak amplitudeof the proposed electrode was highest, and the differences from the Ag/AgCl electrodewere 0.081 V at rest and 0.086 V during exercise. From the experimental results above, we
confirmed that the proposed electrode can measure ECG, as well as an Ag/AgCl electrode.

Sensors 2021, 21, 4305

14 of 17ͲǤͶȀ
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

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ǦͲǤͶ
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(b)

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ǦͲǤʹ
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ǦͲǤ͵
ΫͲǤ͵
ΫͲǤͶ
ǦͲǤͶ
Ϋ͵ͲͲ
Ǧ͵ͲͲ

ΫʹͲͲ
ǦʹͲͲ

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Figure 16. ECG template for each electrode. (a) At rest, (b) during exercise.

Table 1. ECG signal quality indices at rest.
Parameter

Ag/AgCl

Non-Folded

Proposed

Cross-correlation index

ρ (unitless)

—

0.980

0.988

Temporal measuresof HRV

meanNN(ms)SDNN(ms)
RMSSD(ms)NN50(ms)

729.4750.35231.71225740.2933.43430.15120

701.8827.941
20.2562

Spectral measuresof HRVLF (ms2 )HF (ms2 )Total (ms2 )LF/HF (unitless)367.27527.881151.200.696

259.33188.06628.23
1.379

291.41124.63498.122.338

Peak-to-peak amplitude

Vpp (V)0.545 ± 0.040

0.587 ± 0.050

0.626 ± 0.042

Parameter

Ag/AgCl

Non-Folded

Proposed

Cross-correlation index

ρ (unitless)

—

0.974

0.988

Temporal measuresof HRV

meanNN(ms)SDNN(ms)
RMSSD(ms)NN50(ms)

716.4742.51432.56329

722.2233.08329.72224

684.0227.71926.17617

Spectral measuresof HRVLF (ms2 )HF (ms2 )Total (ms2 )LF/HF (unitless)384.62324.551010.30
1.185

193.73212.31518.330.913

123.57126.47298.14
0.977

Peak-to-peak amplitude

Vpp (V)

0.541 ± 0.043

0.625 ± 0.068

0.627 ± 0.047

Table 2. ECG signal quality indices during exercise.

Sensors 2021, 21, 4305

15 of 17

5. Discussion
The experimental results showed that the proposed fabric electrodes attenuated thehigh-frequency component both at rest and exercise. Due to the characteristics of the analog
filter embedded in the electrode, there was a possibility that the phase of each electrodewould be shifted and the ECG could not be observed, but the ECG was confirmed with theproposed electrode. It was shown that the fabric itself can be used as a passive device bychanging the internal structure of the fabric electrodes, and it can be applied as an analog
filter. The filter characteristics can be adjusted by the area and the number of layers, so it
can be used as an equalizer. Since the capacitance can be changed for each location on aplane, it may be used in conjunction with signal analysis to obtain signals with less noise,
and also to analyze signals that vary from location to location. For more efficient noise
attenuation, we believe there are improvements that can be made in terms of electronic
circuits and electrodes.
To confirm the performance of the fabric electrode filter, we compared the minimum
electronic circuits required to obtain the ECG. However, in the actual ECG measurement
system, it is also possible to combine an analog filter with the fabric electrode and a lowpass filter using more passive devices like chip capacitors and resistors. The improvementof noise reduction performance combining the fabric electrodes and the electronic circuit is
an issue to be addressed in the future. In addition, because the proposed fabric electrodeused a low-pass filter using the impedance of the human body, the attenuation of thehigh-frequency component did not decrease after a certain value. Therefore, we consider
it possible to extend it to a multi-stage filter using only conductive fabric by combining
fabric materials that can be used in place of resistors and capacitors. If a multi-stage
filter such as a band-pass filter could be created, it would be possible to design it witha more stable frequency response independent of the pressure applied to the skin andelectrode. In the future, we will focus on the fine structure at the thread level and study theelectrode structure that can be incorporated into a fabric electrode as a passive device. In
the experiment, the capacitance of fabric electrode Ce was stable when it was sufficiently
pressed with a band, although the layers of the proposed electrode were not bonded toeach other. It will be possible to make the electrode more stable by replacing the layerswith a woven structure. The structure of weaving coated conductive yarns is promising
for more capacity and reduced thickness and area. Capacitance is generated between theyarns, then capacitance is generated even within a single piece of fabric. Furthermore, by
weaving the fabric into layers, we can realize even more capacitance. Additionally, it ispossible to replace the material with a more flexible one, and it is desirable to discover sucha material.
6. Conclusions
In this paper, we proposed a fabric electrode in which the fabric itself plays the roleof a filter without the conventional passive devices, such as chip capacitors and resistors.
The proposed electrode has sufficient capacitance to form an analog filter by folding the
conductive fabric. By combining the capacitor with the skin-electrode impedance, theelectrode has the function of a low-pass filter, which attenuates high-frequency component
noise. During the evaluation, the ECG was performed while the subject was at rest andduring exercise while being intentionally influenced by EMG noises. It was confirmed that
the SNR values of the proposed electrode improved around 12.02 dB at rest and 10.29 dB
during exercise compared to when the non-folded fabric electrode was used. The results
indicate that the performance of the folded fabric capacitor is comparable to a typicalceramic capacitor and that fabric electrodes can be used for ECG measurements. Rc and Cc
in the skin-electrode interface can differ when the skin is sweating or the electrode is wet.
We have confirmed that the desired filter characteristics can be obtained within a certain
range, as shown in Figure 5. However, it is necessary to consider the fact that the filter
characteristics may not be within this range during intense exercise. We believe that there
are enough applications where the electrode can be used despite the limitations. In futureSensors 2021, 21, 4305

16 of 17

work, we will clarify the optimal capacitor parameters and the structure of the new fabricelectrode. Additionally, we would like to verify the versatility of the system by increasing
the number of subjects and conducting experiments over a long period of time as future
work.
Author Contributions: Conceptualization, T.T., M.T., T.S., and X.M.; methodology, T.T., M.T., T.S.,
and X.M.; software, T.T.; validation, T.T., M.T., T.S., and X.M.; formal analysis, T.T., M.T., T.S., andX.M.; investigation, T.T., M.T., T.S., and X.M.; resources, M.T., and T.S.; data curation, T.T.; writing—original draft preparation, T.T., and M.T.; writing—review and editing, T.T., M.T., T.S., and X.M.;
visualization, T.T.; supervision, M.T.; project administration, M.T.; funding acquisition, T.T., M.T.,
T.S., and X.M. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by JSPS KAKENHI Grant Numbers JP19J22555, JP20H04472
and JP20K04597.
Institutional Review Board Statement: Ethical review and approval were waived for this study,
because non-invasive measurements on an author him/herself were performed.
Data Availability Statement: The data presented in this study are available on request from thecorresponding author.
Conflicts of Interest: The authors declare no conflicts of interest.

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