Advanced
Application of a Textile-based Inductive Sensor for the Vital Sign Monitoring
Application of a Textile-based Inductive Sensor for the Vital Sign Monitoring
Journal of Electrical Engineering and Technology. 2015. Jan, 10(1): 364-371
Copyright © 2015, The Korean Institute of Electrical Engineers
This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/)which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
  • Received : January 22, 2014
  • Accepted : July 23, 2014
  • Published : January 01, 2015
Download
PDF
e-PUB
PubReader
PPT
Export by style
Article
Author
Metrics
Cited by
TagCloud
About the Authors
Sun Ok Gi
Research center for Textile & Fashion, Yonsei University, Korea.
Young Jae Lee
Dept. of Biomedical Engineering, College of Biomedical & Health Science, Konkuk University, Korea.
Hye Ran Koo
Dept. of Clothing and Textile, College of Human ecology, Yonsei University, Korea.
Seonah Khang
Dept. of Clothing and Textile, College of Human ecology, Yonsei University, Korea.
Kyung-Nam Kim
Dept. of Biomedical Engineering, College of Biomedical & Health Science, Konkuk University, Korea.
Seung-Jin Kang
Dept. of Biomedical Engineering, College of Biomedical & Health Science, Konkuk University, Korea.
Joo Hyeon Lee
Dept. of Clothing and Textile, College of Human ecology, Yonsei University, Korea.
Jeong-Whan Lee
Corresponding Author: Dept. of Biomedical Engineering, College of Biomedical & Health Science, Konkuk University, Korea (ljhyeon@yonsei.ac.kr)

Abstract
In this study, we developed a feasible structure of a textile-based inductive sensor using a machine embroidery method, and applied it to a non-contact type vital sign sensing device based on the principle of magnetic-induced conductivity. The mechanical heart activity signals acquired through the inductive sensor embroidered with conductive textile on fabric were compared with the Lead II ECG signals and with respiration signals, which were simultaneously measured in every case with five subjects. The analysis result showed that the locations of the R-peak in the ECG signal were highly associated with sharp peaks in the signals obtained through the textile-based inductive sensor (r=0.9681). Based on the results, we determined the feasibility of the developed textile-based inductive sensor as a measurement device for the heart rate and respiration characteristics.
Keywords
1. Introduction
Generally, physical health can be confirmed through medical instruments such as electrocardiogram (ECG), electroencephalogram (EEG), electrooculogram (EOG) and electromyogram (EMG) at a hospital [1 , 2] . Although these apparatuses obtain accurate vital signs from the human body, they still have some weaknesses, including their lack of inability to monitor health conditions continuously on a daily basis.
In recent years, the concepts of personal health care and well-being throughout life have attracted new attention. In this context, people require smart equipment that enables more convenient use of the health care systems [2 , 3] . Responding to these needs, there have been various studies of ubiquitous healthcare monitoring systems for daily use. Similar interests have also emerged in the field of wearable monitoring systems, in which revolutionary smart textile applications have been developed to assist with ubiquitous health care. Various wearable devices have been integrated with sensors to detect signals such as the heart rate, body temperature, respiration, blood pressure, motion, acceleration, and blood glucose level [4] .
In the field of wearable monitoring system, devices for the daily monitoring of vital signs developed thus far can be categorized into two types, contact-type and noncontact-type sensing systems [5 , 6] . While research on contact-type sensors has generally been conducted based on the principle of electrical resistance or piezoelectricity, investigations of noncontact-type sensors have mostly focused on the capacitive methods [5 - 8] . In research on noncontact type sensing, the magnetic-induced sensing method has recently emerged [9 - 12] .
In this study, we aim to examine preliminarily the feasibility of a noncontact sensor which use a conductive textile material for the measurement of vital signs and, whose functions work on the basis of the magnetic-induced conductivity sensing principle.
2. The Principles of Heart Activity Based on Magnetic Induction
The principles of the measurements of vital signs using a coil-shaped inductive sensor, for the purpose of this study, are explained below:
An AC current flow in a coil-shaped sensor, with a magnetic field
PPT Slide
Lager Image
along its axis can be produced to create magnetic flux ϕE . If a coil is placed near living tissue which is considered as diamagnetic water [13] , the ϕE flux inside the living tissue changes over time. According to Faraday’s law of induction [14 , 15] , ϕE creates a secondary circular current, i.e., an eddy current ieddy in the living tissue. This eddy current would create a secondary flux ϕI that would affect the loading on the net magnetic flux of the coil and, thus its impedance. Also, the eddy current density
PPT Slide
Lager Image
in living tissue is governed by a diffusion Equation [16] for the current density
PPT Slide
Lager Image
, as expressed by Eq. (1).
PPT Slide
Lager Image
Here, σ is the electrical conductivity, and μ is the permeability of a living tissue specimen. From (1), it is evident that the eddy current induced by an external time-varying magnetic field depends on the magnetic characteristics, especially on the electrical conductivity, of the living tissue ( Fig. 1 ) [17] .
PPT Slide
Lager Image
The basic principle of a magnetic-induced conductivity measurement system [18].
3. Development of a Textile-based Inductive Sensor for the Measurement of Vital Signs
- 3.1 Conductive materials for a textile-based inductive sensor
The conductive textile material used for the coil-shaped inductive sensor developed in this study requires optimization, including the appropriate conductivity, suitable flexibility, and proper durability.
To be specific, silver-plated nickel yarn can scarcely withstand friction and tension during the fabrication processes such as embroidery, due to its insufficient flexibility and durability. In contrast, due to the polymer structure of polyester, the polyester yarn has appropriate mechanical properties, allowing it to undergo the subsequent fabrication process. Hence, in this study, the unsuitable properties of metal yarn were offset by a yarn treatment which combined a metal ply with polyester ply. The resulting hybrid yarn created by this combination was found to have sufficient mechanical properties to withstand the fabrication processes used here.
Following the method of a previous study [12] , for the conductive material of the textile-based sensor, a type of polyester-metal hybrid thread was developed through a treatment process. A single strand of the conductive thread consists of nineteen units of yarn. Each unit of yarn in the conductive thread is a combination of two heterogeneous components fabricated by a twisting method, multiple filaments of polyester (75 denier per thickness in single filament) and eight lines of silver-plated nickel yarn, consisting of six lines of 30㎛ yarn and two plies of 50㎛ yarn( Fig. 2 ). ‘Denier’ is a unit that represents the fineness of a textile yarn. One denier indicates that one gram of the textile, material is required to realize a yarn length of 9000 meters [19] .
PPT Slide
Lager Image
SEM photo of the cross-section of the conductive yarn
- 3.2 The textile-based electrode for the inductive sensor
The basic shape of the textile-based electrode for the inductive sensor in this study is a flat spiral, as inspired from a coil-shaped inductor. Ten turns of a spiral line using the conductive thread were applied to the polyester fabric by means of an embroidery machine ( Fig. 3 ).
PPT Slide
Lager Image
The embroidered textile-based inductor and its electromagnetic characteristics
The electromagnetic characteristics of the textile inductor were measured by a network analyser (HP8735D, Agilent, U.S.A). Within the frequency bands of interest, 0.5~1.5MHz, its inductance was 2.6±0.3 μH ( Fig.3 ).
As the inductive coil sensor works as a transceiver, the size of the electrode plays a crucial role. Increasing the size of the textile electrode is an effective way to obtain stable heart activity signals. On the other hand, a large textile electrode embedded in a garment tends to have unwanted effects, such as reflecting the influence of motion artifacts during breathing. Taking into consideration the advantages and disadvantages of increasing the textile electrode size, the optimized size was set such that the diameter was 5.9 cm.
- 3.3 Computer simulation of the induced magnetic flux and eddy currents in biological tissues
In order to simulate a change in the magnetic field distribution and induced eddy current with changes in the size of the heart, the three-dimensional model shown in Fig. 4 , was used.
PPT Slide
Lager Image
A simulation of a three-dimensional model of the excitation coil of the sensor.
The magnetic field simulation was performed using the COMSOL Multiphysics (COMSOL AB, U.S.A), Magnetic field model. The electromagnetic characteristics, i.e., the conductivity, permittivity, and permeability of living tissues around 1MHz, are summarized in Table 1 [10] .
Electromagnetic characteristics of the Material contents of the simulation parts
PPT Slide
Lager Image
Electromagnetic characteristics of the Material contents of the simulation parts
The simulation consisted of two steps. The first step was to evaluate the induced eddy current by the excitation of an external coil, and the second step was to evaluate the induced magnetic fields by an eddy current. Given that the direction of an induced magnetic field caused by an eddy current is opposite to an excitation magnetic field according to Ampere’s law, the overall magnetic fluxes that were coupled with the external coil were assumed to vary according to the change of the induced magnetic field ( Fig. 5 ).
PPT Slide
Lager Image
The changes of the magnetic flux depending on the heart radius
Fig. 6 shows the induced magnetic fields at the point-of-interest according to a change in the size of the heart. As the size of the heart increased, the induced magnetic fields caused by the eddy current in the heart muscle also increase. Therefore, during cardiac cycles in which the atrium and ventricles of the heart are beating continuously, the induced magnetic fields sensed at a fixed location would be proportional to the movement of living tissues.
PPT Slide
Lager Image
Magnetic field strength of a point of interest
- 3.4. Experiment
Measurements were taken with five male subjects in their twenties, all of whom were in good health. Each subject was asked to sit in a chair while his heart activity signals were measured. The textile-based inductive sensor developed for this work, was attached onto the surface of the shirt the subject was wearing, over the left chest. In order to check whether the magnetic induction principle was applied, the heart activity signals with the proposed textile based inductive sensor were acquired and compared with the signals from a standard ECG signal (lead II) and respiration signals ( Biopac System. MP150 MODEL RSPEC-R Wireless Trans metre), which were simultaneously measured in each subject.
4. Results and Discussion
- 4.1 Detection of the heart beat using the textile-based inductive sensor
The signals from the textile-based inductive sensor were compared with the R-peaks in the standard lead II ECG signals ( Fig. 7 ).
PPT Slide
Lager Image
The relationship of the peak point between the inductive sensor signals measured on the anterior side and the standard Lead II ECG signals (Subjects 1-5).

* The asterisk on the graph means the peak point.

From the data obtained in the experiment ( Fig. 7 ), we conjectured that the magnetic-induced conductivity of the heart muscles was reflected in the inductance changes of the textile-based inductive sensor. Although the peaks from the inductive sensor are slightly lagged, the peak-to-peak intervals appear to be associated with the ventricular contraction activity of the heart shown in the ECG. Fig. 7 shows the relationship between the peak-to-peak intervals in the ECG and those in the signals from the inductive sensor.
The tendency of signal lagging from the textile-based sensor, in comparison with the QRS-complex in the ECG, stems from the measurements of the two different physical activities in the heart, in each sensing method. In other words, while ECG represents the electrical activity of the heart, the textile-based sensor in this study represents the mechanical activity, contraction and expansion, of the cardiac muscle, especially the ventricular muscle. Considering that the mechanical activity of the heart is triggered by the electrical activity of the SA node, the time lagging tendency between the ECG and the measured signals in this study could be interpreted to indicate the timely sequences of the heart conduction. This assumption could be verified by results which showed that the waves in the magnitudes of the heart activity signals mostly match the respiration cycle ( Fig. 9 ) [20] .
To verify the association between these sets, a regression equation(y=1.050x-0.039) was calculated and a correlation coefficient (r=0.9681) was derived ( Fig. 8 ). On the basis of the relationship between the peak-to-peak intervals above, we can infer that the peaks in the signals from the textile inductive sensor indicate the ventricular contractions in the cardiac cycles.
PPT Slide
Lager Image
The correlation between the peak-to-peak intervals in the obtained signals through the inductive sensor and those in the ECG (Subject1).
- 4.2 Respiratory effect on the measured inductive sensor signals
Although we examined the feasibility of the inductive sensor as a heart activity sensor, the results suggest another intriguing phenomenon pertaining to the respiratory effect on the signals from the textile inductive sensor.
Fig. 9 shows the association phenomena between the respiration signals and the enveloped signals from the inductive sensor, which were simultaneously measured on the anterior side of the torso. From Fig. 9 , it is inferred that when the subject draws a breath-in, the magnitudes of the heart activity signals tend to enlarge. In contrast, when the subject exhales the magnitudes in the heart activity signals tend to shrink. A similar phenomenon in which thoracic changes by respiration were found to be associated to the magnetic induced measurements of heart activity signals, have been reported in earlier research [11 , 21] .
PPT Slide
Lager Image
The association between the respiration signal and the enveloped signals from textile inductive sensor measured on the chest (Subject1).

*The asterisk on the graph denotes the breath-in point

The change of the distance between the textile-based inductive sensor and the VOI (i.e., volume of interest) in the heart area is inferred to be the key factor influencing on this phenomenon. The fluctuation in the magnitude of the obtained signal is inferred to have been caused by the change in the distance between the sensor and the heart, due to the following reactions to respiration inside the thorax.
During inhalation, as the diaphragm contracts it moves to the lower position, while the thorax expands to the triple direction - to the left, right and front. Consequently, the pressure level the inside thorax decreases causing air to rush into the lungs, which causes rapid enlargement of the volume of the lungs [21] . It is inferred that the enlarged lungs push the heart towards the front, which makes the distance between the textile-based inductive sensor and the VOI becomes closer when inhaling, and vice versa during exhaling. This change of the distance results in a change in the conductivity distribution in the VOI, which induces the subsequent fluctuation in the magnetic flux.
This inference enables to explain how and why the waves in the magnitudes of the heart activity signals mostly match the respiratory cycles ( Fig. 9 ).
In order to check the correlation between the respiration signals and the enveloped signals from the textile inductive sensor in this study, a correlation coefficient (r=0.8526) and a regression equation were derived ( Fig. 10 ). On the basis of the relationship above, we hold that the peaks in the signals from the textile inductive sensor indicate its respiratory effects.
PPT Slide
Lager Image
The correlation between the respiration signal and the envelope of the inductive textile-based coil sensor signals measured on the chest (Subject1).
Comparing the correlation coefficient between the peak-to-peak intervals in the signals obtained using the inductive sensor and those in the ECG shows a result of about 0.97, while that between the respiration signal and the enveloped signals from textile inductive sensor measured on the chest is about 0.85.
While the reference signals obtained from the electroresistive respiration sensor detect changes in the abdominal girth, the enveloped signals from the textile-based inductive sensor detect the changes in the thoracic cavity.
This slightly lower correlation coefficient between the respiration signals and the envelops of the inductive coil sensor signals compared to that of the heart activity signal, is likely related to the difference in the sensed parts in the body.
5. Conclusion
In this study, the feasibility of a textile-based inductive sensor for the sensing of vital signals was examined. We observed that the changes in a magnetic-induced eddy current due to the electrical changes of biological tissues and the motion of the ventricle walls in the heart were apparently reflected in the inductance change of the textile-based inductive sensor. Based on results indicating a high correlation between the measured signals using the proposed method and QRS-peaks in the standard lead II ECG., the feasibility of the device proposed in this study as a heart rate sensor was verified.
In addition, the magnitudes of the signals obtained from the inductive coil sensor, were also found to be influenced by pulmonary activity.
As mentioned previously, this is a preliminary study which sought to examine the feasibility of the textile-based inductive sensor for the measurement of the vital signs. The result of this study is expected to suggest some guidelines for further studies which adopt textile-based inductive sensors for vital sign measurements.
Acknowledgements
This research was supported by the Mid-Career Researcher Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2012R1A2A2A04045455).
BIO
Sun Ok Gi She received B.S degree in Clothing & Textile in 2000 from DongEui University and M.E degree in Design Education from Pukyung University in 2003, Busan, Korea. She received B.A degree in International Fashion business in 2006 and M.A degree in fashion Design from Nottingham Trent University at Nottingham, U.K in 2007. She received the Ph.D. degree in Clothing & Textile from Yonsei University, Seoul, Korea in 2014. She is currently taking Post-doctoral course in Yonsei University. Her research interests include smart clothing and wearable technology. She can be contacted at gisunok@hotmail.com
Young Jae Lee He received the B.S. and M.S. degrees in biomedical engineering in 2008 and 2010 from Konkuk University, Chungju, Korea, where he is pursuing the Ph.D. degree in biomedical engineering, respectively. His current research interests are in the area of biomedical instrument and development of ubiquitous healthcare device and sensor network, real-time digital signal processing, biomedical signal processing of electrocardiogram and motion sensing and pattern classification. He can be contacted at zetzlyj@gmail.com.
Hye Ran Koo She received B.S. degree in Clothing and Textiles from Ewha Womans University and M.A. degree in Fashion Design from Drexel University, Philadelphia, U.S.A. She is currently working toward her Ph.D in Yonsei University. Her research interests include smart clothing and wearable technology. She can be contacted at kuran@hotmail.com
Seonah Khang She received B.S. degree in Fashion Design in 1997 from Hansung University, Seoul, Korea and M.S. degree in Cognitive Science in 2000 from Yonsei University, Seoul, Korea. She worked as a researcher for 5 years at LG Chemical Design Research Institute, Seoul, Korea and worked as a research assistant for 2 years at the Center for Biomedical Engineering in Korea Institute of Science and Technology, Seoul, Korea. She is pursuing the Ph.D. degree in Clothing and Textiles at Yonsei University. Her research interests include smart clothing and wearable technology. She can be contacted at seonak@gmail.com
Kyung-Nam Kim He received the B.S degree in biomedical engineering in from Konkuk University in 2014. He’s studying M.S degree at Biomedical Engineering Instruments and Measurements Research Lab of Konkuk University. His research interests are firmware programming and signal processing. He can be contacted at kimkn27@naver.com.
Seung-Jin Kang He received the B.S. degree in biomedical engineering from Konkuk University, Chungju, South Korea, in 2014. He is currently studying in the M.S. degree at Biomedical Engineering Instruments and Measurements Research Lab of Konkuk University. His research interests include biomedical signal processing, ubiquitous healthcare and Rehabilitation. He can be contacted at riversjin@gmail.com.
Joo Hyeon Lee She received the B.S. and M.S. degrees in Clothing & Textile in 1983 and 1985, respectively, and the Ph.D degree in Clothing & Textile from the Yonsei University, Seoul, Korea, in 1990. She received the A.A.S from Parsons School of Design in 1992, New York, U.S.A. She worked as a Design Associate of He-Ro group in New York City, U.S.A. Since 1995, she has been a Faculty Member in Clothing & Textile of the School of Human ecology at Yonsei University, Seoul, Korea. Her research interests smart clothing and wearable technology. She can be contacted at ljhyeon@yonsei.ac.kr.
Jeong-Whan Lee He received the B.S. and M.S. degrees in electrical engineering in 1992 and 1994, respectively, and the Ph.D degree in electrical and computer engineering from the Yonsei University, Seoul, Korea, in 2000. He worked as a member of the technical staff, M_Application Project Team and a Project Manager, Ubiquitous-health Project Team at Samsung Advanced Institute of Technology, Kyounggi, Korea, from 2000 to 2004. Since September 2004, he has been a Faculty Member of the School of Biomedical Engineering, Konkuk University, Chungju, Korea. His research interests include biomedical signal processing and instrumentation, wearable/wireless devices for pervasive healthcare, sensing of vital signs/signatures, heart-rate variability, and biological signal measurements by electromagnetic waves and microwave radio-thermometry. He can be contacted at jwlee@kku.ac.kr
References
Teng X. F. , Zhang Y. T. , Poon C. , Bonato P. 2008 “Wearable medical systems for p-health” Biomedical Engineering, IEEE Reviews 1 (1) 62 - 74    DOI : 10.1109/RBME.2008.2008248
.Lymberis A. 2003 “Smart wearable systems for personalised health management: Current R and D and future challenges,” Proceedings of the 25th Annual International Conference of the IEEE, 2003 4 3716 - 3719
Rutherford J. J. 2012 ”Wearable Technology: Health-Care Solutions for a Growing Global Population” IEEE Engineering in Medicine and Biology Magazine 19 - 24
Doherty S.T. , Oh P. 2012 ”A Multi-Sensor Monitoring System of Human Physiology and Daily Activities” Telemedicine and e-Health 18 (3) 185 - 192    DOI : 10.1089/tmj.2011.0138
Oum J.H. , Lee S.E. , Kim D.-W. , Hong S. 2008 “Noncontact heartbeat and respiration detector using capacitive sensor with Colpitts oscillator” Electronics Letters 44 (2)
Linz T. , Gourmelon L. , angereis G. 2007 “Contactless EMG sensors embroidered onto textile” 4th International Workshop on Wearable and Implantable Body Sensor Networks , IFMBE Proceedings 13 29 - 34
Cho S.H. , Koo S.M. , Lee J.H. , Cho H.K. , Kang D.H. , Song H.Y. , Lee J.W. , Lee K.H. , Lee Y.J. 2011 “Heart Monitoring garments using textile electrodes for healthcare applications ” Journal of Medical Systems 35 (2) 189 - 201    DOI : 10.1007/s10916-009-9356-8
Lim Y. G. , Kim K. K. , Park K. S. , Jeong D. U. 2005 “ECG Measurement Method during Sleep with Array of Capacitive Electrodes Attached to Bed” IEEK Fall Conference 28 (2) 521 - 524
Teichmann D. , Foussier J. , Busche M. , Walter M. , Leonhardt S. 2013 “Textile Integration of a Magnetic Induction Sensor for Monitoring of Cardiorespiratory Activity” World Congress on Medical Physics and Biomedical Engineering, IFMBE Proceedings 39 1350 - 1353
Steffen M. , Aleksandrowicz A. , Leonhardt S. 2007 “Mobile Noncontact Monitoring of Heart and Lung Activity” IEEE, Transactions on biomedical circuits and systems 1 (4) 250 - 257    DOI : 10.1109/TBCAS.2008.915633
Koo H. R. , Lee Y.J , Gi S.O , Khang S.N , Lee J.H , Lee J.H , Lim M.G , Park H..J , Lee J.W 2014 “The Effect of Textile-Based Inductive Coil Sensor Positions for Heart Rate Monitoring” Journal of Medical Systems Published online 38 (2)
GI S.O , Lee Y.J , Koo H.R , Khang S.A. , Park H.J , Kim K. S. , Lee J.H , Lee J.W 2013 “An Analysis on the Effect of the Shape Features of the Textile Electrode on the Non-contact Type of Sensing of Cardiac Activity Based on the Magnetic-induced Conductivity Principle” The Transactions of the Korean Institute of Electrical Engineers 62 (6) 803 - 810    DOI : 10.5370/KIEE.2013.62.6.803
Varpula. T. 1983 “Magnetic-Susceptibility Plethysmography” Il Nuovo Cimento D 624 - 641
Hart W.H. , Buck J.A. 2012 “Engineering electromagnetics” 8th edition McGraw-Hill Science 277 - 296
Cheng D. K. 1989 “Field and wave electromagnetics” 4th edition Addison-Wesley 307 - 353
Kriezis E. E. , Tsiboukis T. D. , Panas S. M. , Tegopoulos J.H. 1992 “Eddy currents: Theory and application” Proceedings of the IEEE. Vol.41, pp. 2231-2249, 1996 80 (10) 1559 - 1589    DOI : 10.1109/5.168666
Gabriel C. , Gabriely S. , Corthout E. 1996 “The dielectric properties of biological tissues: I. Literature survey” Physics in Medicine and Biology 41 (11) 2231 - 2249    DOI : 10.1088/0031-9155/41/11/001
Gabriel C. , Gabriely S. , Corthout E. 1996 “The dielectric properties of biological tissues: I. Literature survey” Physics in Medicine and Biology 41 (11) 2231 - 2249    DOI : 10.1088/0031-9155/41/11/001
GI S.O , Lee Y.J , Lee J.H , Lee J.W 2013 “Inductive bio signal detection sensor based on conductive textile” Patent. No.10-2012-0092695 KOREA
Kim S.R 2011 “Material for fabric” 3th edition Kyo moon sa 245 -
Morhrmnan D. E. , Heller H.J. 1997 “cardiovascular physiology” 4th edition McGraw-Hill international companies 49 -
www.revisionworld.com/gcse-revision/biology/human-body/breathing-respiratory-system