Design and development of embroidered textile electrodes for continuous measurement of electrocardiogram signals

Abstract

Textile electrodes are a new and potential choice for long term and continuous monitoring of electrocardiogram measurements. In this research, textile electrodes have been designed and developed by embroidering conductive yarn on the polyester fabric to measure electrocardiogram biosignals of patients. Silver-coated nylon thread is embroidered on 100% polyester fabric in square form 11 mm × 11 mm with satin stitches in order to form three heart vectors (V3, V4, and V5). These electrodes are characterized based on electrode impedance and electrocardiogram measurement and the results are compared with the commercially available disposable Ag/AgCl electrodes. Further, additional tests have been performed using an electrode designed with 50% of the conductive thread used in the original sample. Real-time electrocardiogram signals, QRS-complex, P-wave, and T-wave were obtained for 10 subjects using existing hospital electrocardiogram equipment. The performance and repeatability of the electrode designed with 50% less conductive thread and effect of laundering were also studied. The results showed that the impedance of electrode has an acceptable value of 1.45 MΩ/cm². Ageing tests showed that there is only a negligible deviation in the performance of the electrode. The results after laundering showed that the performance of the electrode is not affected by the laundering process.

Introduction

Over the past few years, a number of wearable physiological monitoring systems have been developed for health monitoring of patients in hospitals and real-life situations [1]. The wearable sensing systems aid the daily acquisition and processing of multi-parametric health data, providing an early detection of pathological signs and improving the curative rate of disease without intervening in the patient’s daily life [2].

The electrocardiogram (ECG) is a widely studied biosignal and it describes the electric action of the heart. The ECG consists of three main parts: QRS-complex, P-wave, and T-wave that are used to measure biopotential. Biopotentials are electrical potentials in the human body, where Cl⁻, Ca⁺, and Na⁺ ions transport charges in an organic system, in contrast to electrons in the leads of a sensing device. In order to measure these biopotentials, the ion currents in the body have to be changed to electron currents in the electrode.

Today disposable Ag/AgCl electrodes are most commonly used in ECG measurements. As the outer layer of the skin has a dry dielectric layer, called the stratum corneum, this will cause reduction of the transfer mechanism from ions to electrons, the Ag/AgCl electrode cannot be used directly, and hence is used as a wet electrode with the help of a conduction gel that moisturizes the skin outer layer and makes it highly ion conductive. However, this procedure is very cumbersome and also presents difficulty for the patient. Also, there is a possibility that the gel may leave a residue on the skin. Further, there is a possibility of short circuit of the electrodes when excess gel is applied. Another disadvantage is the short operating time and one time use of the electrode [3]. In addition, these aforementioned preparation procedures are time consuming, uncomfortable, and even painful for patients, as the skin preparation usually involves the abrasion of the outer skin layer. Repeated skin preparations and gel applications may also cause allergic reactions or infections. In addition, the ECG signal quality may degrade over an extended time period as the skin regenerates and/or the conduction gel dries.

These inherent weaknesses of the present Ag/AgCl electrode can be overcome with the help of textile electrodes. This paper details the design and development of a embroidered textile electrode for continuous measurement of ECG signals. Also a comparison of the developed electrode with conventional electrode is presented.

Recently, integrated electronics of textile electrodes in clothing have been developed for monitoring physiological signals in healthcare services. These intelligent textile electrodes make use of conductive yarn, integrated into the fabric to sense biosignal [4]. In contrast to the conventional wet electrode, the conductive yarn embroidered electrode exhibits both polarization and conductivity due to the partly polarizable electric characteristic of electrically conductive yarn and provides a strong capacitive behavior at the sensor–skin interface. Moreover, sweat and skin humidity may also aid the conductive path.

Long-term ECG recording is a standard procedure in monitoring cardiac patients. In case of ECG recording during life activity, washable and comfortable textile electrodes can be woven at precise target location in the wearable garments and hence make the ECG recording less hectic concerning electrode handling and adjustments on the body unlike conventional gelled electrodes [5,6].

Song et al. designed textile electrodes woven with conductive yarn in the jacquard woven structure and studied the biosignal of the ECG measurement [7]. They have designed electrodes with double-faced weave and woven with a silver covering yarn in the weft direction. Their findings demonstrate that the woven structure has less strain properties and is more uniform than knit structure. Pola et al. reported that the textile electrodes are well suited for measuring ECG and the best results were achieved with the embroidered electrodes, which have a large contact area with the skin [8]. Mankodiya et al. studied the comparison of textile electrode with the conventional electrode in terms of their ability to perform long-term recording and reported that the textile electrode shows a promising outcome as it exhibited low skin electrode impedance and a high signal to noise ratio (SNR) for long-term monitoring [5]. Beak et al. produced a polydimethylsiloxane (PDMS)-based surface electrode for the long-term measurement of ECG signals and reported that in special applications such as long-term unsupervised monitoring dry electrodes offer benefits over wet electrode [9]. All of the sensors above are fabric-based sensors, that is, the sensors are in a two-dimensional (2D) textile. The fabric sensors are popularly used in biomonitor applications due to its wearable and comfortable properties. However, the repeatability of the electrode, electrode performance after washing, and effect of conductive thread consumption are not studied in the previous work.

In this paper, the textile electrodes designed by embroidering the conductive yarn on polyester fabric was used for ECG biosignal measurement. The electrode was made with silver-coated nylon thread using satin 2/1 stitch type. Wilcom embroidery software was used to design the embroidery stitches. The performance of the developed electrode based on reduction of conductive thread by 50%, laundering and repeatability was evaluated and compared with commercially available Ag/AgCl electrode. The purpose of this work was to design a conductive yarn embroidered textile electrode to evaluate its feasibility as a biosignal electrode in ECG measurements for continuous physiological monitoring and for mass production.

Materials and methods

The textile electrodes were fabricated by embroidering conductive yarns on polyester fabric. 100% polyester woven fabric with 75D warp and 150D weft was used as base fabric for embroidering the electrode. This polyester fabric acts as an insulator, endures abrasion very well, absorbs very little moisture, and dries fast. Multifilament silver-coated nylon yarn of 110D/24f was purchased for the construction of electrodes. These yarns are highly conductive, more flexible, soft, and pliable. Metal snap fasteners made up of stainless steel having thickness of 0.6 mm and size No. 3 were used in the textile electrodes, for making connection with lead wires during ECG measurement.

Electrode design

The electrode was designed in the form of 11 mm × 11 mm square using the silver-coated nylon yarn as shown in Figure 1(a) and filled with under layered satin stitches. The design was carried out with the help of Wilcom embroidery software. Three sensing electrodes were designed on the base fabric using hand embroidery technique. Hand stitching was used for fixing the snap fastener on the surface of the electrode. The electrode embroidered polyester fabric was attached to two chest belts joined using Velcro. This facilitates adjustment and removal of the electrodes from the belt. The photographs of the designed electrode and elastic Velcro attachment are shown in Figure 1(b).

Figure 1.

(a) Design of silver yarn embroidered textile electrode. (b) Images of the fabricated electrode.

Electrical characteristics of the embroidered textile electrode

In order to measure the skin–electrode impedance of the proposed textile electrodes, the skin-textile electrode equivalent circuit model was used as shown in Figure 2. The commercial Ag/AgCl ECG electrode was used as a reference electrode [10].

Figure 2.

Textile electrode impedance measurement set up (top) and skin-textile electrode model (bottom).

The proposed textile electrode, in this case an unknown electrode (Z_X), is located at the center between two reference electrodes (Z_e1 and Z_e2) such that d₁ ≡ d₂. The endodermic impedance components between a reference and an unknown electrode were assumed to be Z_b1 and Z_b2, respectively [7]. If the impedances of the test points (a, b, and c) are measured, the following impedances are obtained:

$\begin{array}{c}\multicolumn{1}{c}{Z_{\text{ab}}=Z_{\text{b}1}+Z_{\text{e}1}+Z_{\text{x}}}\\ \multicolumn{1}{c}{Z_{\text{bc}}=Z_{\text{b}2}+Z_{\text{e}2}+Z_{\text{x}}}\\ \multicolumn{1}{c}{Z_{\text{ac}}=Z_{\text{b}1}+Z_{\text{b}2}+(Z_{\text{e}1}+Z_{\text{e}2})}\end{array}$

(1)

Since the same reference electrode was used, it is assumed that Z_b1 ≡ Z_b2 = Z_b and Z_e1 ≡ Z_e2 = Z_e and a more simplified impedance of unknown electrode can be obtained:

$Z_{\text{x}}=Z_{\text{ab}}-Z_{\text{ac}}/2(\text{or})Z_{\text{x}}=Z_{\text{bc}}-Z_{\text{ac}}/2$

(2)

Digital multimeter with 20 Hz frequency range was used to measure the impedance between the test points a and b, b and c, and a and c and electrode impedance Z_x was calculated using the equations (1) and (2). When measuring the impedance of a textile electrode, the impedance tends to decrease because of the increasing contact area and the compression of the silver yarn under a certain level of pressure. Thus, elastic Velcro attachment was used to apply constant level of clothing pressure of 1000 N/m².

The ECG was measured using Philips ECG monitoring equipment with a recording speed of 25 mm/s. The SNR value of the ECG signal was calculated from the root mean square (RMS) measurement of the isoelectric region.

ECG measurement

The standard 12-lead system is universally used for performing ECG measurement in the hospitals, of which 6 are used for chest leads and 4 for limb or peripheral leads. The chest electrodes are numbered V1 to V6 as shown in Figure 3. Of the six chest leads, V3, V4, and V5 are considered to be the most important. Hence, these three leads were taken for the study with the help of the developed conductive yarn embroidered electrode.

Figure 3.

Position of the silver yarn embroidered electrode by the elastic Velcro band.

The measuring positions for the ECG are shown in Figure 3. For the ECG measurement, the embroidered electrodes were distributed inside the elastic Velcro band and wrapped around the chest. The silver yarn embroidered electrodes were connected to the biosignal amplifier for the ECG. ECG wave forms were recorded using Philips ECG monitoring equipment with 25 mm/s speed. The ECG signals of the three subjects in a resting condition were recorded and the physical characteristics of the subjects are listed in Table 1. The experiments were performed using a constant pressure of 1000 N/m² [7]. This was achieved with the help of the adjustable Velcro strips fixed in the belt.

Table 1.

Physical characteristic of subjects.

Subject	Age (years)	Height (cm)	Weight (kg)	Chest (cm)	BMI (kg/m2)
A	22	175	61	85	19.92
B	22	172	72	96	24.32
C	22	169	55	80	19.23

BMI: body mass index.

Placement of the silver yarn embroidered electrode on the body surface

The electrode positioning methodology was derived from the previous research [7], as it takes into account the modified limb electrode positions considering the relationship between the heart vector and the body surface points. This relationship indicates that as the distance between two electrodes increases, a larger potential difference can be obtained. However, even if the distance between the electrodes is reduced, the body surface potential originating from the heart dipole could be theoretically obtained.

Based on the above statements the chest leads V3, V4, and V5 were taken as the test electrode configuration. The distance of the electrode configuration was reduced by decreasing the distance between the two bipolar electrodes. The above lead configuration was found to be less sensitive to the motion artifacts due to the shortest distance between the electrodes. Another reason for the selection of the leads V3, V4, and V5 was the lead points V3 and V4 face the heart in the same direction and it monitors the heart’s electrical activity from the vantage point of the anterior surface of the heart.

Evaluation of electrode quality

The investigated properties of the ECG signal and basis of evaluation are shown in Table 2. The QRS-complex, P-wave, and T-wave are the main parts of ECG and describe the action of heart. If these parts are clearly visible, 2 points were allotted and 0 points if it is otherwise.

Table 2.

Evaluation basis for analysis of designed electrodes performance.

S No.	Parameters	Basis of evaluation
		2 point	1 point	0 point
1	QRS-complex	Was found	–	Not found
2	P-wave	Was found	–	Not found
3	T-wave	Was found	–	Not found
4	QT-interval	No deviation	1 ∼ 2 ms	>2 ms
5	R-wave amplitude	>0.35 V	0.30–0.35 V	<0.30 V
6	Stabilization time	≤10 s	11–60 s	>60 s

The baseline level describes how well the signal baseline settled at the level of 1.6 V, which is determined in the software. The stabilization time was measured from 0 s until the baseline settled to the level of 1.6 V. If time was under 10 s, 2 points, between 11 and 60 s 1 point, and over 60 s 0 points were given. The R-spike peak amplitude refers to the signal strength. If it was over 0.35 V, 2 points were allotted and if it was under 0.01 V, 0 points [8].

Impact of thread consumption

The study was performed to find out the optimum value of thread consumed for a single electrode in order to reduce the cost of the textile electrodes. Initially, 2.6 m of yarn was utilized for a single electrode. The signal offered from this electrode was taken as the base value. From the base value, longer and shorter lengths of yarn were used to make textile electrodes and the corresponding signal strength was measured. It was found that 1.31 m yarn itself was suitable and provided the required signal quality without any loss in performance as compared to the original 2.6 m yarn. Hence a 50% savings in yarn was achieved.

Impact of laundering

The electrode should have washability character in order to perform over extended periods and also for long-term monitoring. The developed electrode was washed using the method ISO105-C06:1994 (color fastness to domestic and commercial laundering) to analyze whether the electrodes have the required washability character and whether the laundering process affects the performance of the electrode [11]. The impact of laundering is discussed in results and discussion section.

Study of repeatability

Repeatability refers to measure of the number of times the electrode provides same results without any deterioration in the performance with respect to ageing. This repeatability character was used as a base for the long-term monitoring. For this study, six subjects were tested using the developed electrode for the period of 2-week intervals. In between 2nd and 4th week, the electrode was washed once.

Results and discussions

Electrode characterization

The electrical resistance of the silver-coated multifilament nylon yarn of 110D/24f was measured. Figure 4 shows the resistance values of the silver-coated nylon yarn with respect to its length. The test was carried out between the lengths of 0.25 m to 1.5 m. The change in resistance was found to be directly proportional to the length.

Figure 4.

Effect of yarn length on its resistance value.

The impedance values of the designed textile electrodes were measured as per the methodology given earlier [7,10] and the results are tabulated in Table 3. It is observed that the impedance values of the silver yarn embroidered electrodes fall in the acceptable range of 1 ∼ 5 MΩ. The impedance values are similar to the results observed by Song et al. [7].

Table 3.

Impedance value of the silver yarn embroidered electrode.

S No.	Impedance between reference electrode (a) and silver yarn embriodered electrode (b) (Zab) in MΩ	Impedance between reference electrodes a and c (Zac) in MΩ	Distance (d1 or d2) in cm	Impedance value of silver yarn embriodered electrode (ZX) in MΩ
1	1.440	0.060	5	1.41
2	1.412	0.065	8	1.38
3	1.588	0.056	10	1.56
Average				1.45

The quality of the signal was measured as per the methodology in terms of SNR values [7]. The results are shown in Table 4. The silver yarn embroidered electrode has an SNR value of 31.46 dB, which shows that the background noise is less noticeable in the signal and the gain in the signal is more. Song et al. have also achieved an SNR value of 33.67 dB for the convex jacquard electrodes of the half removed 50% warps with conductive paste [7].

Table 4.

SNR value of silver yarn embroidered electrodes.

S No.	Electrode	Measurement	Value 1	Value 2	Average Value	SNR (dB)
1	Silver yarn embriodered electrode	Signal	0.168	0.356	0.262	31.46
		Noise	0.003	0.011	0.007

SNR: signal to noise ratio.

Comparison of ECG signals characteristics

The characteristics of the measured ECG signals of the original textile electrode, electrode made with 50% reduction in conductive thread, electrode after washing, and commercially available Ag/AgCl electrode are shown in Table 5 and Figure 5. In general, the wave form of the ECG signal from the textile electrode is similar to that of the Ag/AgCl electrode. Specifically QRS-complex, P-wave, and T-wave appear clearly and are similar to commercial electrode. Similar wave forms were also observed for the electrodes made of 50% reduction in conductive yarn consumption. After washing, it is seen that there is no difference in the performance. The reason for the good results is that the conductive yarn is on the surface of embroidered electrode and makes good contact with the skin.

Table 5.

Comparison of ECG signal characteristics of Ag/AgCl electrode and textile electrodes.

ECG signal characteristics	Ag/AgCl electrode	Textile electrode	Textile electrode with 50% reduction in conductive thread	Textile electrode after washing
Rate	71	78	84	71
PR (ms)	136	140	140	144
QRSD (ms)	94	94	96	96
QT (ms)	356	364	372	364
QTC (ms)	387	415	440	395

ECG: electrocardiogram.

Figure 5.

Comparison of ECG signal characteristics (a to d).

Comparison of ECG results of embroidered textile electrode with commercial electrode

Comparison of quality analysis of commercially available electrode and developed electrode is shown in Table 6. The results of the embroidered textile electrode are good and are similar when comparison with commercial electrodes. The reason for the good results is that the conductive yarn is on the surface of the electrode and hence it has a good contact with the skin. Pola et al. [8] have compared four structurally different textile electrodes in ECG measurement (three of them industrially manufactured electrodes and one hand made by embroidering conductive yarn on the fabric) and they have obtained very good results with embroidered electrode for dry skin.

Table 6.

Measurement and result analysis of electrode performance.

Particulars	Commercial electrode		Silver-coated nylon embroidered textile electrode
	Actual	Ratings	Actual	Ratings
QRS complex	Found	2	Found	2
P wave	Found	2	Found	2
T wave	Found	2	Found	2
QT interval	7 ms	2	8 ms	2
R wave amp.	0.42 V	2	0.32 V	1
Stabilization time	10 s	2	10 s	2
Total		12		11

Effect of reduction in conductive thread consumption

Initially 2.6 m of yarn was used for a single electrode of 11 mm × 11 mm size. A 50% reduction in conductive thread consumption was done in the electrode fabrication in order to study the performance of the electrode. Comparison of performance ratings of the original electrode, electrode with 50% less conductive yarn, and commercial electrode (Ag/AgCl) at the V3, V4, and V5 leads are shown in Figure 6.

Figure 6.

Effect of thread consumption on textile electrode performance.

From Figure 6, it is observed that the textile electrode made with 50% reduction in the conductive thread (1.31 m) and the textile electrode with 100% thread consumption (2.6 m) show an equivalent rating of 34. The overall performance rating percentage of the electrode with 50% and 100% thread consumption are the same and is 94%, whereas the commercial disposable electrode has 91.6%. It shows that there is no change in the electrode performance due to the reduction in the conductive thread.

Effect of laundering

The developed electrode was washed 15 times to study the effect of laundering on the electrode performance [11]. Figure 7 shows the effect of laundering on the performance of the developed textile electrode. The performance rating percentage for the developed electrodes after and before laundering is 97.22% and 94.4%, respectively. From the results, it is confirmed that laundering process does not affect the performance but only enhances the performance. It is in line with the findings of Pola et al. [8]. However, the enhancement in the performance may be due to the closing of threads in the embroidery structure due to washing and also some structural changes in the base fabric. Because of this, the contact area of the conducting surface might have increased, which resulted in better performance. Hence, the electrode can be reused and also can be used for long-term monitoring.

Figure 7.

Effect of laundering on the textile electrode performance.

Study of repeatability

Performance ratings of the electrode after 2 weeks and 4 weeks for the six subjects were measured at the V3, V4, and V5 leads and ratings are shown in Table 7.

Table 7.

Repeatability of the textile electrode for six subjects.

Subject	V3		V4		V5
	Performance		Performance		Performance
	After 2 weeks	After 4 weeks	After 2 weeks	After 4 weeks	After 2 weeks	After 4 weeks
A	12	12	11	9	12	11
B	12	12	11	8	12	12
C	10	10	10	8	11	9
D	10	10	10	10	10	10
E	10	10	11	10	11	10
F	10	10	11	10	11	10

It is observed from Table 7 that the performance of the developed textile electrode with respect to ageing is negligible. So, the developed electrode can be used for long-term monitoring.

Conclusions

A textile electrode using conductive yarn embroidered on polyester textile fabric was designed and developed for ECG measurement, and the results were compared with commercial disposable electrode in terms of their performance. The impedance of the developed electrode has an acceptable value of 1.45 MΩ. The ECG signals of the developed electrode were similar to Ag/AgCl electrode.

Performance ratings evaluation analysis shows similar results between the developed electrodes and commercially available electrodes. Based on the results of the repeatability study and the electrode performance with respect to 50% less conductive thread consumption and laundering, we conclude that the conductive yarn embroidered textile electrodes are well suited for continuous measurement of ECG signals.

The next step of this research is to develop a wearable monitoring garment that can work in continuous real-time monitoring of heart activity during daily activities with wireless transmission to computers for remote diagnosis of the wearer.