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IEEE SENSORS JOURNAL, VOL. 22, NO. 15, 1 AUGUST 2022
Development and Evaluation of a Respiratory
Monitoring Smart Garment Based
on Notched Optical Fiber
Sensing Fabric
Jun Xu , Yucong Zhou , Cheng Zhang , Yitong Li, Xuehui Ma, Dali Ma, and Changyun Miao
Abstract —Clothing-based respiratory monitoring can provide real-time non-invasive respiratory monitoring services
for subjects, and can be used as a reference for daily health
observation. In this study, an optical fiber sensing section
woven into plain fabric is produced by a hand-weaving
machine, and the produced sensing fabric is then made
into a respiratory monitoring garment. This paper explored
the luminescence and coupling principle of notched optical fiber through theoretical analysis and simulation, and
demonstrated the feasibility of making it into respiratory
sensing fabric. We used this structure to make a fabric and
then sew the fabric with the yoke to make a smart garment.
In testing, we obtained the static respiratory waveform and
the respiratory rate (R-R) during daily common movements,
which were also compared with two types of commercial
sensors. The results showed that the smart garment based on
optical fiber sensing fabric maintains good consistency with
the monitoring waveform of commercial sensors and good
R-R obtained in static motion (error not more than 2 rpm).
Monitoring tests showed that the garment has high monitoring accuracy and motion monitoring stability. Moreover,
it was also very comfortable and can be used as daily respiratory monitoring equipment.
Index Terms — Polymer optical fiber, fabric sensor, respiratory monitoring, wearables.
I. I NTRODUCTION
ITH the continuous demands of consumers for health
monitoring, smart garments with physiological signmonitoring functionality has attracted significantly more attention [1]. Among them, the development of new sensors
W
Manuscript received 16 May 2022; revised 19 June 2022;
accepted 19 June 2022. Date of publication 30 June 2022; date of
current version 1 August 2022. This work was supported in part by
the Tianjin Municipal Special Foundation for Key Cultivation of China
under Grant XB202007. The associate editor coordinating the review of
this article and approving it for publication was Prof. Carlos Marques.
(Corresponding author: Cheng Zhang.)
Jun Xu, Yucong Zhou, Yitong Li, and Dali Ma are with the School of
Textile Science and Engineering, Tiangong University, Tianjin 300387,
China (e-mail: msdrxujun@163.com).
Cheng Zhang and Changyun Miao are with the Tianjin Key Laboratory of Optoelectronic Detection Technology and System, Tianjin
300387, China, and also with the School of Electronic and Information Engineering, Tiangong University, Tianjin 300387, China (e-mail:
zhangcheng@tiangong.edu.cn).
Xuehui Ma is with the School of Electronic and Information
Engineering, Tiangong University, Tianjin 300387, China.
Digital Object Identifier 10.1109/JSEN.2022.3186022
applied to clothing is almost a necessity [2]. In addition
to monitoring accuracy, they should have little impact on
clothing comfort [3]. Sensors directly woven into fabric are
easily combined with clothing and can ensure wrinkle-free
clothing after suturing. They have become an important form
of monitoring sensor.
As a basic vital sign, respiration can indirectly reflect the
life and health status of the human body through non-invasive
monitoring methods [4]. Respiratory rate (R-R) is an important
indicator of respiratory status. The R-R of normal adults is
generally 12–20 times per minute. Abnormal R-Rs mean that
the monitored person may have cardiopulmonary arrest, heart
failure, a pulmonary embolism, or other potentially deadly
medical conditions [5], [6]. Therefore, the development of
smart garments with respiratory monitoring functionality has
become quite important, which helps wearers understand their
health in real-time.
Optical fibers are not subject to electromagnetic interference
and crosstalk, and they have good flexibility and small size
[7], [8]. Optical fiber sensors are widely used in the fields
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XU et al.: DEVELOPMENT AND EVALUATION OF A RESPIRATORY MONITORING SMART GARMENT
of physics [9], chemistry and biology [10]–[12], physiological
signal monitoring [13], [14]. And the combination of optical fiber sensors and textiles has become an important part
in the development of wearable devices [15], [16]. Among
them, the combination of optical fiber sensor for respiratory
monitoring and textiles has attracted much attention. In recent
years, many different types of optical fiber sensors have been
used in respiratory monitoring [17]–[25]. Among the three
common types of sensors, fiber Bragg grating sensors are
usually combined with textiles through packaging [26], [27],
bonding [29]–[31],or embroidery [32], [33]. This is because it
is difficult to control the effect of strain sensing when directly
weaving textiles. Regarding the optical fiber macro bending
sensor, researchers have tried to sew “U” [34] optical fibers
and 8-shaped [35] optical fibers on the elastic belt, or directly
weaving them into the fabric [36], [37]. The bending shape
of optical fibers makes the combination of this kind of sensor
and fabric difficulties with respect to fabric flatness, shape
stability, etc. These micro-bend optical fiber sensors are often
placed in the seat back [38] or bed [39] and usually need the
aid of an optical fiber micro-bender [38] (a lining material for
micro bending of optical fiber) to achieve micro bending. This
increases the number of steps needed in weaving the optical
fiber sensor into the fabric. To better solve the combination
problem of the optical fiber sensor and textile, an optical fiber
sensor structure that can be used as a warp woven into woven
fabric is proposed. The woven optical fiber sensing fabric is
flat, the yarn is uniform, and the fabric substrate with high
elasticity can ensure better wearing comfort.
In this study, a smart garment based on notched optical fibers is developed, which can be used for real-time
respiratory monitoring. This work introduces the principle
of luminescence and coupling, the principle of fabric sensing, a respiratory monitoring smart garment, and respiratory
monitoring tests. Via verification through theoretical analysis
and simulation, the notched optical fiber can luminesce and
couple. The fabric made by notched optical fiber can generate
light between optical fibers and respond to tensile stimuli.
With the help of 3D human body scanning, we found that
a high waist circumference is more suitable for torso-based
respiratory monitoring. Therefore, we designed the yoke at
the waist height of the front garment, where the garment can
make trunk undulations caused by breathing movements to
stretch the fabric in a comfortable way. The actual test results
of the garment show that the respiratory waveform obtained
under different breathing modes is basically consistent with
the reference sensor, and the R-R error of static or dynamic
monitoring will not exceed 2 rpm. So the proposed smart
garment has great potential as daily respiratory monitoring
equipment.
II. W ORKING P RINCIPLE
A. Luminescence and Coupling Principle of Notched
Optical Fiber
1) Luminescence Principle: For the notched optical fiber
connected to the light source (Figure 1(a)), the light propagating in the optical fiber is divided into two categories: the
L1 reaching the interface between the core and the cladding
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Fig. 1. Working principle of the sensor. (a) Schematic diagram of the
luminescent optical fiber; (b) Schematic diagram of the receiving optical
fiber coupling; (c) Human expiratory mechanism and corresponding
changes in a pair of sensing units; (d) Human inspiratory mechanism
and corresponding changes in a pair of sensing units.
and the L2 reaching the notch. For L1 , when its incident angle
is greater than the critical angle of total internal reflection,
the light only reflects and does not refract. Therefore, it can
continuously propagate inside the optical fiber without being
emitted from the optical fiber, and the propagation law is the
same as that of general optical fibers. The critical angle of
total internal reflection ψc (◦ ) is given by the formula (1)
n cl
ϕc = arcsin
(1)
n co
where ψc is the critical angle of total internal reflection, nco
is the refractive index of optical fiber core material, and ncl is
the refractive index of the optical fiber cladding material.
For L2 , the light will experience refraction and reflection at
the notch. After being refracted by the notch, L2 follows the
orange propagation path in Figure 1(a), where it is emitted
into the air from side a. The incident angle of L2 reflected
by the notch when the light reaches the interface between the
core and the cladding is θ , and given by
θ = ϕc − 2θ1
(2)
where θ1 is the incident angle when L2 reaches the notch.
According to Eq. (2), when most of the light is reflected
through the notch at the interface between the core and
the cladding, the next incident angle is less than ψc . The
total internal reflection conditions of the optical fiber are not
satisfied, and light is emitted from side b of the optical fiber.
The luminous effect on the side of the notched optical fiber
is mainly due to the L2 emitted from the optical fiber through
refraction and reflection. The refracted light is primarily emitted from side a, and the reflected light is primarily emitted
from side b. Therefore, when the end face of the notched
optical fiber is connected to the light source, there is light
on both sides a and b of the optical fiber notch.
2) Coupling Principle: For a notched optical fiber to receive
light (Figure 1(b)), L3 and L4 irradiation is received to the
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IEEE SENSORS JOURNAL, VOL. 22, NO. 15, 1 AUGUST 2022
TABLE I
S UMMARY OF S IMULATION R ESULTS
Fig. 3. Optical fiber position change during breathing. L1 -L3 are lightemitting fibers, R1 and R2 receiving fibers, and d1 and d2 the distances
between fibers during expiration and inspiration, respectively.
Fig. 2. Simulation results. (a) Irradiance map of side a of the luminescent
fiber detection surface; (b) Irradiance map of side b of the luminescent
fiber detection surface; (c) Irradiance map of the receiving fiber section
when the light source is on side a; (d) Irradiance map of the receiving
fiber section when the light source is on side b.
notch surface from side b of the optical fiber (the incident
angle is θ2 , θ3 ). Due to the reflection at the notch, the
incident angle of the reflected light is equal to ψc . For light
with an incident angle between L3 and L4 (incident angle
greater than θ2 but less than θ3 ), the incident angle after
reflection by the notch is greater than ψc . The light meeting
this condition can be successfully coupled into the optical
fiber (i.e., the light is continuously reflected to the end of the
optical fiber). Regarding L5 incidence light from side a of the
optical fiber, the next incident angle θ4 must be greater than ψc
after reflection and refraction. It can also can be successfully
coupled into the optical fiber. Therefore, in addition to the
luminescent fiber, the notched optical fiber can also be used
to couple the light on both sides into the optical fiber as the
receiving fiber.
3) Simulation Experiment: With the help of simulation software, the luminescence and coupling principle of the notched
fiber were re-verified. Figure 2(a), and (b) show the irradiance
diagram of different light detection planes of luminescent fiber.
Figure 2(c), and (d) show the irradiance diagram of the optical
fiber section when the light source is on both sides of the
receiving fiber, a and b, respectively. We have summarized the
simulation results in Table I. Therefore, for the luminescent
fiber, both side a and side b emit light. For the receiving light,
the outside light on side a and side b of the optical fiber can
be successfully coupled.
B. Fabric Sensing Principle
The notched optical fiber can be combined into a luminescent fibers and receiving fibers group, which can be combined
with the fabric, and the light intensity induced by the optical
fiber distance can be used for sensing. Figure 1 shows the
state change of a pair of luminescent and receiving fibers in
the sensor following the respiratory movement. When exhaling
(Figure 1(c)), the chest cavity collapses, the fabric shrinks, the
distance between the luminescent fiber and the receiving fiber
is small, and the light intensity received in the receiving fiber is
stronger. When inhaling (Figure 1(d)), the chest expands, the
fabric stretches, the distance between the luminescent fiber and
the receiving fiber increases, and the light intensity received
in the receiving fiber decreases.
When the luminous intensity of the luminescent fiber and
the coupling ability of the receiving fiber are fixed, the change
in distance between the optical fibers will lead to a change in
light intensity at the receiving fiber, and the light intensity at
the receiving fiber meets the attenuation formula of light in the
air, where I0 is the initial light intensity, a is the absorption
coefficient, and d is the light penetration distance. In this study,
the optical fiber changes by a small distance, so the light
intensity attenuation coefficient can be approximated as linear.
I R1 = kb · I L1a · (−m · di + n) + ka · I L2b · (−m · di + n)
I R2 = kb · I L2a · (−m · di + n) + ka · I L3b · (−m · di + n)
(3)
Figure 3 shows the positional relationship diagram of
five optical fibers, of which three are luminescent optical
fibers (L1 , L2 , and L3 ) and two are receiving optical fibers
(R1 and R2 ). It is assumed that the distance between optical
fibers is the same as di . The coupling light intensities IR1 and
IR2 in the receiving fiber are, respectively:
I R1 = kb · I L1a · (−m · di + n) + ka · I L2b · (−m · di + n)
I R2 = kb · I L2a · (−m · di + n) + ka · I L3b · (−m · di + n)
(4)
where IL1a and IL2a are the side light intensities of the side
a of the notch surface of the luminescent optical fibers,
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XU et al.: DEVELOPMENT AND EVALUATION OF A RESPIRATORY MONITORING SMART GARMENT
14895
L1 and L2 , respectively; IL2b and IL3b are the side light
intensities on the opposite side of the notch. Ka and Kb are
the optical coupling coefficients on the side a and side b of
the receiving fiber, respectively.
Since the optical fiber is composed of the same material and
the notch properties are the same, it can be considered that the
light intensities IL1a and IL2a on side a of the optical fiber are
ILa , and the light intensities IL2b and IL3b on side b are ILb .
The total light intensity (I) output by the whole sensor can be
expressed as:
I = 2(kb · I La + ka · I Lb ) · (−m · di + n)
(5)
When the distance between optical fibers caused by breathing
changes by d, the light intensity change I is:
I = |I2 − I1 | = 2m · Ic · d
Ic = kb · I La + ka · I Lb
d = d2 − d1
(6)
Fig. 4. Number of notches - fiber loss.
where IC is the ideal coupling light intensity, which is the light
intensity emitted by the light-emitting fiber directly coupled
to the receiving fiber without attenuation in the ideal state.
Therefore, the distance change between optical fibers caused
by respiratory movement can be detected by the light intensity
change received in the receiving optical fiber. In addition,
when the distance change between optical fibers caused by
respiratory movement is determined, increasing the ideal coupling light intensity can increase the amount of light intensity
change.
III. M ATERIALS AND M ETHODS
A. Manufacturing Method and Sensing Function Tests
of Optical Fiber Sensing Fabric
1) Determination of the Number of Fiber Notches: In order
to study the side luminescence of notched fiber, we used
650nm red light source and an optical power meter (Boyuan
Photoelectric Technology Co., Ltd.) to explore the fiber loss of
fiber with 1-15 notches respectively. The plastic optical fiber
(Jiangxi Dasheng Plastic Optical Fiber Co., Ltd.) is the D750
optical fiber with a diameter of 750μm. The core material of
the optical fiber is PMMA (nco = 1.49), and the cladding
material is fluororesin (ncl = 1.417). The notch on the optical
fiber is made by carbon dioxide laser system (Shenzhen Han’s
Laser Technology Co.,Ltd., China). The fiber loss (L) can be
calculated according to
L=
Pi − P0
P0
Fig. 5. (a) Sensing fabric and structural diagram of sensing fabric;
(b) Photo graph of notched optical fiber, cross section, and longitudinal
section of the optical fiber (the notch shape is approximated as an arc
with a radius of 400µm).
(7)
where Pi is the optical power detected when the optical fiber
has i notches, P0 is the optical power detected when the optical
fiber has no notches.
The fiber loss corresponding to the number of notches is
shown in Figure 4. With the increase of the number of notches,
the fiber loss also increases, but the increasing speed of fiber
loss slows down. The fiber with 10 notches was finally selected
for two reasons: the length of sensitive area of optical fiber
with 10 notches is suitable for the size of fabric, and too many
fiber notches will cause the fiber to break easily.
2) Fabric Weaving Method: The optical fiber sensing fabric
(Figure 5(a)) developed by our research group is a respiratory
sensing fabric woven with plastic optical fiber and ordinary
yarn. Different from ordinary fabrics, five notched plastic
optical fibers are used in the center of the optical fiber sensing
fabric to replace some warp yarns. The warp yarn of the
fabric is 30S/2 viscose/nylon with low elasticity (Dongguan
Zhengyu Textile Co., Ltd., China) and the weft yarns employed
are 140D nylon-spandex core-spun yarns with high elasticity
(Zibo Tailin Textile Co., Ltd.), so that the fabric sensor has
transverse tensile properties.
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The sensing fabric was woven by dobby loom (Tianjin
Jiacheng Electromechanical Equipment Co., Ltd., DWL5016),
and the size of the fabric is 6cm ∗ 12cm. The entire sensing
fabric length is divided into five parts as shown in Figure 5(a),
according to whether the optical fiber and weft are interleaved.
The optical fiber in the sensing center and the weft yarn are not
intertwined to ensure the reflection of light between the optical
fibers. Plain weave is adopted in the fixed area to ensure that
the optical fiber and weft have the most interleaving points and
are more closely interleaved, which plays an important role in
fixing the optical fiber. In the free zone between the fixed
areas, the optical fibers and weft yarn are not intertwined, and
a seam allowance is prepared for sewing fabrics and clothing.
The rest of the yarn adopts an 8/3 warp satin weave to ensure
that the fabric has good elasticity.
The central sensing area of the optical fiber sensing fabric is
processed by laser grooving to realize the side luminescence
and coupling effect of optical fibers, which is an important step
in generating the sensing functions in the fabric. The notched
fiber is connected to red light with a 650nm wavelength. The
photo after amplification is shown in Figure 5(b). The notch
properties of the optical fiber after grooving are shown in
Figure 5(b), and the longitudinal section shape of the notch
can be approximated by a circular arc. It is worth mentioning
that the laser grooving processing must be carried out after the
fabric is made. This is because after the optical fiber is woven
into the fabric, the position of the optical fiber must be adjusted
to leave the optical fiber light transmission part outside of the
fabric (the heat shrink tubes in Figure 5(a), three light-emitting
optical fibers as a group and two receiving optical fibers as
a group for bunching). At the same time, the grooved optical
fiber must not be fractured in the process of making the fabric
due to the reduced intensity.
3) Fabric Fabrication Repeatability Verification: The fabrication process of the sensor mainly includes two parts: the
making of fiber notch and the weaving of fabric sensor. Both
parts are mainly completed by machine, so the repeatability
of the sensor sample can be guaranteed. In order to verify the
repeatability of sensor fabrication, we verified the similarity of
notches’ shape, spacing of optical fibers in fabrics and fabrics’
sensing performance.
4) Fabric Tensile Repeatability Test: In order to verify
the feasibility of the fabric undergoing repeated stretching,
we recorded the stretching amount current drop ratio of the
1st stretching, the 50th stretching and the 100th stretching.
B. Respiratory Monitoring Garment
1) Selection of Monitoring Parts: Because the respiratory
monitoring garment uses the fluctuation change of the respiratory movement monitoring part for monitoring, the greater
the fluctuation change of the monitoring part and the less
interference from other actions, the more favorable it is to
obtain good monitoring effects. This refers to the breathing difference (inspiratory circumference - expiratory circumference)
and expiratory difference of other movements (|expiratory
circumference of other movements - expiratory circumference
of standing posture|) of four alternative parts: chest circumference, high waist circumference (lower edge of the seventh rib),
IEEE SENSORS JOURNAL, VOL. 22, NO. 15, 1 AUGUST 2022
Fig. 6. (a) 3D human body scanner; (b) Schematic diagram of scanning
results and measuring parts; (c) Three test postures (from left to right
standing, left turn and sitting).
Fig. 7. (a) Boxplot of respiratory difference data; (b) Boxplot of expiratory
difference data.
waist circumference, and abdominal circumference. Twenty
men aged 20 to 25 were scanned with a 3D human body
scanner (Figure 6(a), Anthroscan M4Plus, Litai Technology
Co., Ltd., China).
The experiment requires the subjects to scan 3D images
of their body when inhaling and exhaling in the standing
posture and 3D images of the body when exhaling in left
turn and sitting postures. The three body postures of the
subjects are shown in Figure 6(c). After the body scanning
of 20 subjects was completed, the chest circumference (B),
high waist circumference (HW), waist circumference (W) and
abdominal circumference (A) were measured using the scanner
supporting software. Figure 6(b) is a schematic diagram of the
girth measurement using the software.
As shown in Figure 7(a), a boxplot of the respiratory
differences of each of the four parts is shown. The mean value
of respiratory difference at the middle chest circumference
and high waist circumference of the four parts is close,
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XU et al.: DEVELOPMENT AND EVALUATION OF A RESPIRATORY MONITORING SMART GARMENT
Fig. 8. Smart garment respiratory monitoring system and schematic
diagram of the yoke.
which is greater than the waist circumference and abdominal
circumference. Data showing poor breathing at the abdominal
circumference are more concentrated, but the mean and overall
data are smaller, and a considerable portion of the data shows
that the poor breathing at the abdominal circumference is
negative, which indicates that the circumference at inhalation
is smaller than that at exhalation, which may be the difference
between individuals caused by different breathing patterns.
Therefore, chest circumference and high waist circumference
are more suitable for respiratory monitoring. Figure 7(b)
shows a boxplot of chest circumference and high waist expiratory difference in left turn and sitting postures. It can be seen
from Figure that under the left turn posture, the mean value
of expiratory difference at chest circumference and high waist
circumference is basically the same, but the data of high waist
circumference is relatively concentrated. In the sitting posture,
the mean expiratory difference of chest circumference is small,
but compared with the expiratory difference of higher waist
circumference, the data floating range is wide.
Through the 3D human body scanning data of 20 samples,
it can be seen that among the four alternative parts of chest
circumference, high waist circumference, waist circumference, and abdominal circumference, the respiratory difference
between chest circumference and high waist circumference
is larger, which is basically concentrated between 3-6cm.
By comparing the expiratory differences between the left turn
and sitting postures, the mean difference is small, but the
data at the high waist circumference is relatively concentrated,
and the difference between individuals is small. Therefore,
high waist circumference was selected for placement of the
respiratory monitoring sensor in this study.
2) Respiratory Monitoring Smart Garment: The design of
the smart garment respiratory monitoring system is shown in
Figure 8. The smart garment uses changes in the chest contour
caused by breathing for monitoring, so the garment selects the
elastic and tight T-shirt (We have made a size garment, which
is suitable for people who are 170-175cm tall and 78-86cm
high waist), so that the sensing fabric can stretch and contract
with breathing. The garment fabric is a blended knitted fabric
of nylon (83%) and spandex (17%) (Boying Textile Co.,
Ltd, China, T6114). The yoke design diagram of stitching
sensing fabric is shown in the yellow dashed box of Figure 8.
The outer layer is a black fabric made of the same material
as the garment, and the inner layer is a non-elastic fabric
(Fuyulai Textile Technology Co., Ltd., China, 207-12).
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The back part of the high waist of the garment was
made of elastic fabric, which can ensure that the stretching
amount of the sensing fabric is within its effective stretching
range (0-2cm) when the wearer with different girths wears
the garment. The inelastic fabric in the inner layer of the
sensing yoke makes the change of chest circumference caused
by breathing to mainly focus on the breathing fabric, so as
to improve the sensing effect of the fabric. In addition to the
sensing fabric for monitoring, the garment also integrates a
circuit board for respiratory signal processing, a power supply,
and a power cord for the circuit board. The circuit board is
placed in the interlayer of the yoke, and a power pocket is
designed in the dotted line part of the front body to provide
an independent power device for the sensor. The power cord
used to connect the power supply and circuit board shall be
placed according to the position shown by the red line, and the
power cord shall be hidden in the side seam and yoke of the
garment to not affect the comfort and aesthetics of the garment.
In addition to the smart garment integrating power supply,
optical fiber sensing fabric, and signal demodulation and
transmission circuit, the garment respiratory monitoring system also includes a Bluetooth receiving module and personal
computer. During testing, the volunteers participating in the
test wore a smart garment to ensure that they were within
the transmission range of Bluetooth. The output signal of the
optical fiber sensing fabric is collected by the circuit module in
the garment, and the signal is wirelessly transmitted to the PC
through Bluetooth for data acquisition, so as to realize a signal
comparison with the reference sensor. The whole respiratory
monitoring system is portable and wearable, which can realize
the real-time monitoring of users’ breathing.
C. Accuracy Evaluation of Respiratory Monitoring
1) Monitoring Method: The accuracy of breath measurement
when the human body is still was evaluated using a reference sensor. During testing, volunteer A wear the reference sensor, a Huake respiratory wave sensor (Hefei Huake
Electronic Technology Research Institute, China, HKH-11C),
while wearing the smart garment developed in this study,
as shown in Figure 9. Respiratory monitoring was carried
out simultaneously in sitting and standing states to verify
the monitoring accuracy of the smart garment monitoring
system. In addition, to verify the distinction of the garment
with respect to different breathing states, normal breathing,
deep breathing, rapid breathing, and stop breathing were
monitored during static conditions. To ensure the monitoring
effect of the reference respiratory belt, the subjects remained
stationary during the monitoring process. After filtering the
data collected by the smart garment and normalizing the data
collected by the reference sensor, the respiratory waveform
is drawn. By comparing the two respiratory waveforms and
calculating their Pearson correlation coefficient, the accuracy
of the monitoring system during a static state of the subject
is verified.
To comprehensively evaluate the monitoring accuracy of the
monitoring system on the R-R during long-term daily monitoring, test No. 6 obtained the R-R of seven postures (standing,
sitting, bending, turning, repeatedly bending, walking,
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IEEE SENSORS JOURNAL, VOL. 22, NO. 15, 1 AUGUST 2022
Fig. 9. Six testing conditions and their corresponding reference sensors,
the Huake HKH-11C and SNORE CIRCLE, respectively.
Fig. 10. Width and depth of notches on 3 optical fiber samples.
TABLE II
N OTCH S IZE S TATISTICS
and running) that often occur during daily life. The SNORE
CIRCLE mask breathing sensor (Shenzhen Yunzhongfei Electronics Co., Ltd, China, YS20) for monitoring respiratory
airflow was selected as a reference for R-R monitoring. Three
groups of R-R data were recorded under each action, and the
R-R obtained by the two monitoring methods were compared.
Test No. 6 was conducted by two volunteers of different
genders wearing the same size: volunteer A (gender: male,
height: 170cm, high waist: 86cm) and volunteer B (gender:
female, height: 172cm, high waist: 78cm).
2) Data Analysis: To evaluate the monitoring accuracy of the
respiratory monitoring smart garment developed in this study,
the Pearson correlation coefficient and Bland-Altman diagram
are introduced to evaluate the consistency of respiratory monitoring results between smart garment and reference sensors.
Since the respiratory signals obtained by the smart garment
and reference sensor have different dimensions, to facilitate
appropriate comparisons, formula (8) is used to normalize the
two groups of data:
x =
x − x min
x max − x min
(8)
where x’ is the processed dimensionless data, x is the original
data, xmin is the minimum value in the original data, and
xmax is the maximum value in the original data.
To numerically analyze the performance of the sensor,
the Pearson Correlation Coefficient (PCC) is introduced to
calculate the degree of linear correlation between these measurements, which is defined by:
yi
N
x i yi − x i
(9)
r= N
x i2 − ( x i )2 N
yi2 − ( yi )2
where xi and yi are the sample values of the two groups of data
obtained from the smart garment and the commercial sensor,
respectively, and N is the sample size of the data. Generally,
a PCC exceeding 0.8 indicates a strong correlation between
the two groups of data.
With respect to the Bland-Altman diagram, the R-R monitoring difference between the smart garment and reference
sensor is used for analysis [40]. If the distribution of the
difference follows a normal distribution, 95% of the difference
should be between d-1.96SD and d+1.96SD, which is called
a 95% consistency limit. If most of the differences between
the measurement results of the two methods are within this
interval, it is considered that the two measurement methods
have a good consistency.
IV. R ESULTS
A. Fabric Repeatability Test Results
1) Experimental Results of Notch Making Repeatability: The
experimental results of notches’ difference comparison are
shown in Figure 10. The width and depth of the first notch of
the three fibers were smaller than those of other notches, which
may be caused by the laser processing equipment itself. Except
for the first notch, the average width, depth and standard
deviation of the three optical fibers were shown in Table II.
The average notch size of the sample can be controlled within
10μm, it can be considered that the optical fiber manufacturing
process is repeatable.
2) Experimental Results of Fabric Weaving Repeatability: The
comparison of the spacing between the two fabrics is shown
in Figure 11. The average optical fiber spacing between two
fabrics will not exceed 50μm. The slight difference in fiber
spacing may be caused by the different warp tension in the
fabric weaving process.
3) Experimental Results of Fabrics’ Sensing Performance
Comparison: The fitting curve of stretching amount current
drop ratio during the stretching and recovery process of two
fabrics is shown in Figure 12. The current drop ratio of fabric
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XU et al.: DEVELOPMENT AND EVALUATION OF A RESPIRATORY MONITORING SMART GARMENT
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Fig. 11. The statistics of the spacing between the two fabrics.
Fig. 13. Comparison of fitting curve between stretching amount current
drop ratio during the 1st , 50th and 100th stretching of fabric sample B.
Fig. 12. Comparison of fitting curve between stretching amount current
drop ratio during stretching and recovery of two sensor samples.
during stretching and recovery is basically the same. With the
increase of stretching amount, both fabrics have the response
characteristics that the current decreases, and have a good
linearity.
4) Fabric Stretching Repeatability Test Results: The fitting
curve of stretching amount current drop ratio of fabric sample
B in the 1st , 50th and 100th stretching processes is shown in
Figure 13. Repeated stretching will not have a great impact
on the sensing performance, the sensing fabric has good
repeatability.
B. Respiratory Monitoring Test Results
The comparison of respiratory signals synchronously collected by the volunteers wearing a smart garment and the
reference sensor in test Nos. 1-5 is shown in Figure 14,
and the corresponding respiratory signal correlation coefficient
analysis results are shown in Table III.
The monitoring signals of the two monitoring methods show
good consistency. Comparisons of respiratory signals collected
in tests No.1 and No. 2 are shown in Figure 14(b), and (c),
and the Pearson correlation coefficients are 0.8046 and 0.8205,
Fig. 14. (a) Photo graph of the breath monitoring smart garment;
(b) Waveform comparison of normal breathing for one minute in
test No. 1 in the sitting position; (c) Waveform comparison of normal breathing for one minute in test No. 2 in the standing position; (d) Waveform comparison of 5 s normal breathing-10s deep
breathing-5s normal breathing in test No. 3 in the sitting position; (e) Waveform comparison of 5 s normal breathing-10s rapid
breathing-5s normal breathing in test No. 4 in the sitting position;
(f) Waveform comparison of 5 s normal breathing-10s stopped
breathing-5s normal breathing in test No. 5 in the sitting position.
TABLE III
R ESULTS F ROM THE C ORRELATION A NALYSIS OF R ESPIRATORY
S IGNALS B ETWEEN G ARMENT AND C OMMERCIAL S ENSORS
respectively. It can be considered that there is a strong correlation between the respiratory signal monitoring results of
the smart garment and the reference sensor during the two
static states of sitting and standing. When special breathing
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IEEE SENSORS JOURNAL, VOL. 22, NO. 15, 1 AUGUST 2022
TABLE IV
R ESULTS F ROM THE C ORRELATION A NALYSIS OF R ESPIRATORY
S IGNALS B ETWEEN G ARMENT AND C OMMERCIAL S ENSORS
Fig. 15. (a) Comparative experiment of R-R monitoring (volunteer A);
(b) The Bland Altman plot (difference: garment-SNORE CIRCLE; average: mean of garment and SNORE CIRCLE); (c) Comparative experiment of R-R monitoring (volunteer B); (d) The Bland Altman plot
(difference: garment-SNORE CIRCLE; average: mean of garment and
SNORE CIRCLE).
types are added in tests Nos. 3-5, a signal comparison of the
two monitoring methods is shown in Figure 14(d), (e), and (f).
It can be seen that the waveform amplitude changes significantly during deep breathing, but during rapid breathing, the
amplitude spacing decreases, and ultimately, when breathing is
stopped, the respiratory wave disappears. It can be considered
that the smart garment has a good distinguishing function
for special breathing types. The two comparison waveforms
obtained through tests Nos. 3-5 also have good consistency,
and the Pearson correlation coefficients are 0.8588, 0.8245,
and 0.8287, respectively. Arguably, the monitoring results of
the smart garment can still maintain good consistency with
the reference sensor when deep breathing, rapid breathing,
or stopped breathing are monitored.
Figure 15(a), (c) shows the experimental process monitored
by volunteers under test No. 6 conditions. The R-R results
are shown in Table IV. Bland-Altman analysis was performed
on R-R with a maximum acceptable error of 2 rpm, and an
analysis diagram was drawn, as shown in Figure 15(b), (d).
Figure shows the difference between the measured R-R of
smart garment and respiratory mask. The x-axis and y-axis
are the average and difference of the R-R of the two sensors,
respectively (there are coincidence points in Figure because the
average R-R of multiple groups of data is the same). In the
42 pairs of R-R results collected, most of the results are within
the 95% consistency limit. The maximum error of respiratory
rate monitoring of the two volunteers was not more than 2rpm.
The data collected by the smart garment and the reference
sensor have good consistency. Therefore, it can be considered
that the smart garment of one size is suitable for the wearers of
different genders and different body girths. Smart garment has
the potential to meet the testing needs of wearers of different
body types through different sizes.
TABLE V
C OMPARISON TABLE OF O PTICAL F IBER S ENSORS
FOR R ESPIRATORY M ONITORING
Table V compares some optical fiber sensors used to monitor respiration. From the table it is seen that the fabric sensor in
this work has a small size and flexible base. So it has obvious
advantages in flexibility and softness, and is more suitable for
wearable devices.
V. C ONCLUSION
This paper describes the principle of luminescence and
coupling, the principle of fabric sensing, garment integration,
and monitoring effect tests. At first, the principle of luminescence and coupling of notched optical fiber is studied, and
the luminescence and coupling of notched optical fiber are
verified via simulation. The feasibility of using the notched
optical fiber group to make the fabric to realize tensile sensing
is discussed theoretically, and the manufacturing methods
of fabric and notched optical fiber are briefly introduced.
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XU et al.: DEVELOPMENT AND EVALUATION OF A RESPIRATORY MONITORING SMART GARMENT
Through experiments, we have verified the repeatability of the
fabric sensor in fabrication and use. And then, the breathing
difference and expiratory difference of 20 models in different
positions and postures were counted by a 3D human body
scanner, and the high waist circumference was selected as the
breathing monitoring part. Therefore, an inelastic yoke was
designed in the front body of the high waist circumference,
and the wearability of the monitoring system was preliminarily
realized by integrating the sensing fabric and elastic T-shirt.
At last, commercial respiratory monitoring equipment was
used to monitor the garment synchronously, and the monitoring function of garment was tested. Test results show that
the optical fiber smart garment maintains good consistency
with the reference sensor in different static posture (sitting
and standing) and special breathing state (deep breathing, rapid
breathing, and stop breathing). Compared with the reference
sensor, the R-R obtained when simulating daily movements
(standing, sitting, bending, turning, repeated bending, walking,
and running) for a long time has good consistency, and
maximum error not more than 2 rpm. It is considered that
the garment made of notched optical fiber sensing fabric has
good respiratory monitoring accuracy and stability in static and
daily activities and exhibits certain discriminatory ability for
special respiratory states. The smart garment is comfortable
and fits into the development trend of intelligent clothing.
Therefore, the development of respiratory monitoring clothing
with optical fibers has large potential.
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Jun Xu received the B.S. degree from Tiangong
University, Tianjin, China, the M.Sc. degree
from the Niederrhein University of Applied Sciences, Germany, and the Ph.D. degree in textile mechanics engineering from the Univeristé
de Haute-Alsace, France. She is currently an
Associate Professor with the School of Textile
Science and Engineering, Tiangong University.
Her research interests include key technologies
for multifunctional clothing and smart clothing,
garment processing, and process monitoring.
Yitong Li received the B.S. degree from the
Zhongyuan University of Technology, China. She
is currently pursuing the M.Sc. degree with
Tiangong University, Tianjin, China. Her research
interest is smart garment for physiological signal
monitoring.
Xuehui Ma received the B.S. degree from
Dezhou University, China. She is currently pursuing the M.Sc. degree with Tiangong University,
Tianjin, China. Her research interest is optical
fiber sensing technology.
Dali Ma received the B.S. degree from the Tianjin Institute of Textile Technology and the master’s degree. He is currently a Professor with
the School of Textile Science and Engineering,
Tiangong University, Tianjin, China. His research
interests include clothing brand and smart garment for physiological signal monitoring.
Yucong Zhou received the B.S. degree from
the Taiyuan University of Technology, China.
She is currently pursuing the M.Sc. degree with
Tiangong University, Tianjin, China. Her research
interest is smart garment for physiological signal
monitoring.
Cheng Zhang received the B.S., M.Sc., and
Ph.D. degrees from Tiangong University, Tianjin,
China. Since 2007, he has been engaged
in scientific research and teaching with the
School of Electronic and Information Engineering, Tiangong University. His research interests
include research on wearable human information
detection technology, artificial intelligence algorithm, and research on new generation integrated
optical fiber sensing technology.
Changyun Miao received the M.Sc. degree
from Liaoning Technical University, China, and
the Ph.D. degree from Tianjin University, China.
Since 1997, he has been engaged in scientific
research and teaching with the School of Electronic and Information Engineering, Tiangong
University, Tianjin, China. His research interests include modern communication network and
systems, and photoelectric detection technology
mechanical electronic information technology.
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