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Abstract
It is significant to monitor respiration conveniently and in real time for people suffering from respiratory diseases. Polymer optical fibers (POFs) have the advantages of flexibility and light weight, which is highly desirable for wearable respiratory monitoring. However, in most current applications, the POFs are stitched on the textile substrates in the form of macro-bending. This method is complex to fix the bending with certain curvatures and uncomfortable compared with the POF sensors woven into the textile. In this paper, a respiratory fabric sensor based on the side luminescence and photosensitivity mechanism of POF is proposed and demonstrated. The 750µm-diameter POFs were woven into a fabric as warp and laser marking was performed at their designed positions to make them release or couple light. The spacing change between the POFs caused by the respiratory movement accordingly makes the light intensity change in the photosensitive fiber. We chose four fabric widths (10cm, 8cm, 6cm and 4cm) and four fabric weaves (plain weave, honeycomb weave, 1/3 right twill weave and 8/3 warp satin weave) to implement the full-factor experiment for exploring the measurement effect of the respiratory fabric sensor. The result is that the fabric with width of 4cm and weave of 8/3 warp satin is optimal. The calm and deep respiratory tests of the human chest and abdomen in sitting and standing posture were carried out and the test performance of the fabric sensor is almost comparable to that of the medical monitor. The proposed respiratory fabric sensor is comfortable, easily woven and high in precision, which is expected to realize industrialized scale production.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Respiration is an important vital sign, which can offer useful information for the diagnosis and treatment of respiratory diseases, such as anesthetic sensitivity, sudden infant death syndrome, obstructive sleep apnea syndrome, et.al [1–4]. Therefore, it is necessary to develop a convenient, wearable and effective sensor for monitoring respiratory movement.
The traditional electronic respiratory sensors are stiff and not suitable for daily wearing. Long-term contact with the electronic setups will cause skin irritation and discomfort [5–7]. In addition, the sensitivity to electromagnetic interference also limits the application of the electronic respiratory sensors in the strong electromagnetic environment, such as magnetic resonance imaging (MRI) [8,9].
The optical fiber sensors offer advantages such as lightweight, low cost, and miniaturization [10]. In the textile field, the polymer optical fiber (POF) sensors are adopted because of their higher flexibility, lower Young’s modulus, higher strain limits, fracture toughness, impact toughness [11–14], textile weaving resistance and safe operation [15–17]. In Ref. [15], the POF fabric mainly measured pulse and blood oxygen of the fingertip. The ends of POFs bent into 90 degree were used for luminescence and photosensitivity. The comfort levels of the fabrics should be considerable. Compared with the fiber Bragg grating (FBG) [18–20], the intensity-modulated POF sensors have simpler demodulated method and lower cost [21,22].
Most current applications of wearable POF sensors are based on the macro or micro bending principles where the deformation causes the intensity changes of the light in the optical fiber. J. Witt et al. and M. Krehel et al. presented the U-shaped POF strain sensor stitched on the elastic fabric for monitoring of respiratory movement [23,24]. A. Grillet et al. proposed a macro-bending sensor based on bending effects of POF which was sewed on an elastic fabric to monitoring the respiratory rate of the human abdomen [10]. A corset consisting of a sensor with the POF of a spiral configuration has been described in Ref. [25] for monitoring the various body parts’ respiratory movement. In order to increase the attenuation of the internal light, W. Zheng et al. and A. G. Leal et al. presented that the POF with notches made either on the outer surface or inner surface was stitched on the elastic belt in the form of macro-bending to measure the respiratory rate of the subject's chest [26,27]. D. Ahn et al. proposed a respiratory sensor with seven in-fiber micro-holes spaced at 1500µm-diameter POF stitched in a waistband to measure the respiratory of the abdomen, whose bending radius was based on the curvature of the body's waist [28]. However, it is typically difficult that these POF sensors are stitched on the textile substrates with the certain bending curvatures [29]. Additionally, they are incorporated into the textile substrate by stitching, which is less comfortable and tedious than the POF directly woven into the fabric.
This paper demonstrates a respiratory fabric sensor (RFS) based on the side luminescence and photosensitivity mechanism, which was realized by the laser marking and weaving with the POFs as warp yarns into a fabric. The spacing change between the POFs caused by respiratory movement makes the light signal coupled from the side-luminous fiber into the side-photosensitive fiber increase or decrease. We studied the influence of fabric width (fabric length in weft direction) and fabric weave on POF-RFS. To evaluate the performance of the POF-RFS, a commercial medical monitor was performed for test and comparison. The proposed POF-RFS has the characteristics of simple weaving technology with one-time molding, low-cost and excellent comfort (compared with the POF sensors stitched on the textile substrates), and has the possibility of large-scale production.
2. Sensor principle
Figure 1 shows the structure and principle of the proposed POF-RFS in which the POFs are woven into the fabric as warp yarns and the side-luminous and side-photosensitive fiber sections suspend on the fabric surface as float yarns. The light emitted from the side-luminous fibers (f1, f2, f3) can be coupled into the adjacent side-photosensitive fibers (J1, J2) and the coupled light intensity depends on the spacing between optical fibers.
The initial spacing between the POFs is di (i = 1, 2, 3, 4) as shown in Fig. 1. Assume that the side light intensities from each luminous fiber are the same. The expanding or contracting of the chest and abdomen caused by the respiration of the wearer makes the spacing between the POFs increase or decrease (Fig. 1(A) and (B)), which leads to relevant changes of the light intensities in the photosensitive fibers. The light intensities (PJ1 and PJ2) received by the photosensitive fibers (J1 and J2) can be expressed as:
Fig. 1. Schematic diagram of POF-RFS (the X axis is the weft direction of the fabric, the Y axis is the warp direction of the fabric, yellow is weft yarn, green is warp yarn, red is luminous fiber, blue is photosensitive fiber, and purple is light coupled by photosensitive fiber)
Owing to having the same structure and material for these side-luminous and side-photosensitive units, the side-light intensities and the optical coupling coefficients mentioned above are assumed to be the same, namely Pf12 = Pf21 = Pf22 = Pf and CJ11 = CJ12 = CJ21 = CJ22 = C. The total output light intensity (Pout) of the photosensitive fiber is expressed as:
The optical space transmission loss coefficient g(di) has a negative exponential relationship with di as shown in Eq. (3), where a is the absorption coefficient and L0 is the initial light intensity. However, the spacing variation (Δdi) between the two POFs caused by respiratory movement is very small, with the result that the expression of g(di) and di can be approximated as a negative linear relationship, where m is the optical space transmission loss rate and n is the original optical space transmission loss (di =0).
When respiration causes a change of the optical fiber stretching amount (Δd, Δd=Δd1+Δd2+Δd3+Δd4), the variation of the total output light intensity (ΔPout) can be expressed as:
It can be seen that there is an inverse linear relationship between the Δd and the ΔPout. Therefore, the respiration can be monitored by detecting the ΔPout.
3. Sensor material, design and fabrication
The POF-RFS is composed of liner fabric and optical fiber fabric (Fig. 2(A)). The X axis is the weft direction of the fabric, and the Y axis is the warp direction of the fabric. Along the warp direction the liner fabric is at both ends of the POF-RFS. The POF is floating above the fabric. When this part of fabric is stitched together with the clothing, the POF can be protected from being touched by the sewing machine. The POF fabric is a key part of POF-RFS and lies between two of liner fabric. It consists of two distinct parts based on POF’s interlacing and function. In the middle of the POF-RFS, there is POF floating part. The POFs of the side-luminous structure and side-photosensitive structure adopt the floating weave and don’t interlace with the fabric. POF interlacing parts are on both sides of POF floating part along the warp direction. The optical fibers are interlaced and woven into the fabric in the POF interlacing part.
Fig. 2. Design and fabrication of the POF-RFS: A. The fabric structure of the POF-RFS B1. Plain weave B2. Honeycomb weave B3. 1/3 right twill weave B4. 8/3 warp satin weave (yellow is viscose/nylon warp yarn, green is nylon-spandex weft yarn, red is the luminous fiber, blue is the photosensitive fiber) C. Fabric weaving C1. Loom and the enlarged view of the fabric C2. The looming drafting of liner fabric and POF floating part C3. The looming drafting of POF interlacing part (Upper-left is drafting plan, middle-left is denting plan, bottom-left is weave diagram and bottom-right is lifting plan, Gray is warp weave point, white is weft weave point, and black is POF-warp weave point.) D. POF bundling E1. The side-luminous and side-photosensitive structure of POF making E2. The appearance and microscope diagrams of the side-luminous and side-photosensitive structure after marking E3. The side-luminous and side-photosensitive structure and the output light at the end of the bundled POFs after injecting the red light
3.1 Materials
The warp yarns of POF-RFS applied the 750µm-diameter PMMA polymer optical fibers (Jiangxi Dasheng Plastic Optical Fiber Co., Ltd., China) and 30S/2 viscose/nylon with low elasticity and high melting point (Dongguan Zhengyu Textile Co., Ltd., China). The weft yarns employed 140D nylon-spandex core-spun yarns with high elasticity and high melting point (Zibo Tailin Textile Co., Ltd., China). The melting points of the warp and weft yarns (except POFs) are comparably higher than that of POFs. The high temperature resistance of the yarn can well prevent it from being damaged by laser marking. The high elasticity of the weft yarns would remarkably benefit fabric corresponding displacement in the weft direction due to human’s respiration, which is also highly good for the spacing change between the warp POFs, resulting in some difference in the coupling light intensity of the photosensitive fiber.
3.2 Design
Although using elastic weft yarns to realize fabric elasticity, four kinds of fabric weaves were designed to better strengthen fabric elasticity in Fig. 2((B1) to (B4)), such as plain weave, honeycomb weave, 1/3 right twill weave and 8/3 warp satin weave. Additionally, fabric width along the weft direction was still a key point to be considered in order to promote the spacing change more obvious between the adjacent POFs. Therefore, there were four kinds of fabric widths devised, for example 10cm, 8cm, 6cm and 4cm.
3.3 Fabrication
The fabrication of POF-RFS included fabric weaving (Fig. 2(C1) to (C3)), POF bundling (Fig. 2(D)), and the side-luminous and side-photosensitive structure making (Fig. 2(E1)).
The POF-RFS was woven by dobby loom (Tianjin Jiacheng Electromechanical Equipment Co., Ltd., DWL5016) (Fig. 2(C1)). For example, the POF-RFS adopting 8/3 warp stain weave would be realized. The warp density of 20 ends/cm was designed. A weave repeat unit was devised to keep the spacing between two adjacent POFs. Firstly, the liner fabric based on Fig. 2(C2) was woven. Then the POF interlacing part based on Fig. 2(C3) was successively woven. The fabric weave of POF floating part with a length of 1 cm was identical with that of the liner fabric. The POF floating part was contributed to the side-luminous and side-photosensitive effect. Finally, the POF interlacing part and liner fabric were woven symmetrically. The whole fabric sensor was completed.
To better protect the luminous/photosensitive fibers and facilitate their connection with photoelectric setups, the pigtail fibers of the luminous/photosensitive fibers were bundled with thermoplastic tubes by a hot air gun with 100°C, respectively (Fig. 2(D)).
A carbon dioxide laser with a wavelength of 10.6µm (Shenzhen Han's Laser Technology Co., Ltd., China) was used to mark the PMMA POFs of 1cm-length in the POF floating part to form the side-luminous and side-photosensitive structure (Fig. 2(E1)). In the fabrication process, the laser was firstly set to focus on the surface of the POFs, and then the laser repeatedly marked the surface of the POFs according to the S-shaped track, and finally the continuous grooves were formed. The laser output power was 6W, the marking speed was 350mm/s, and the marking repetition times were 5 times. The appearance and microscope diagrams of the POF with the side-luminous and side-photosensitive structure after marking were shown in Fig. 2(E2). Each optical fiber in the POF floating part was marked 9 grooves, each of which had a length of about 1mm and a depth of about 130µm. One end of the bundled side-luminous fibers was injected the red light of 660nm. The partial enlarged view of the side-luminous and side-photosensitive fibers in the fabric and the light output at the other end were shown in Fig. 2(E3), which shows that the side-photosensitive fibers can couple to the output light by the side-luminous fibers.
4. Experiment and results
4.1 POF-RFS performance test
4.1.1 Experimental setup
The experimental setup for evaluating the performances of the POF-RFS was shown in Fig. 3(A). It was composed of a fixed station, an electric translation station and its controller, a signal detector and a data acquisition software module. The two ends of the POF-RFS were respectively fixed on the fixed station and the electric translation station. The luminous fibers and the photosensitive fibers were connected to a light-emitting diode (LED) and a photodiode (PD) in the signal detecting module, respectively. The LED (L660905-07AU, 4.8mm×4.7mm×1.3mm, Shenzhen maisi electronic technology co., ltd., China) was used to inject red light of 660nm to the luminous fibers. The measured spectral range of the PD (PIN-8.0-CSL, 5mm×4mm×1.3mm, Shenzhen maisi electronic technology co., ltd., China) was 350nm-1100nm. The signal processing of the photosensitive fibers included photoelectric conversion, signal acquisition and data transmission, which was performed by the signal detection module. The transmitted data was received, recorded and shown by the PC.
Fig. 3. A. The experimental setup for the performance of the POF-RFS B. The circuit diagram of photoelectric conversion
Figure 3(B) is the circuit diagram of photoelectric conversion, where the PD was performed in the reverse bias mode. Therefore, the relationship between the photocurrent (I) and the output voltage of the circuit is as follows:
As shown in Eq. (5), the output light intensity of the photosensitive fibers can be expressed by the photocurrent (I).
4.1.2 Experiment of POF-RFS stretch response
Stretch response of POF-RFS was tested using the experimental setup in Fig. 3(A). The fabric width (length in weft direction) of this sample was 4 cm and the fabric weave was 8/3 warp satin weave. The research team found that human respiratory movement can stretch the fabric within 2 cm in the weft direction. Therefore, an electric translation station was performed to stretch the tested sample from 0 to 2 cm in steps of 0.2 cm. The output voltages corresponding to different stretching amounts were recorded and converted into corresponding photocurrents.
Figure 4(A) shows the relationship between the fabric stretching amount (the moving distance of the electric translation station is within 2 cm) and the POF stretching amount ($\Delta \textrm{d}$.) and the linear fitting curve of its mean values for three repeated experiments. It can be seen that there is a positive linear relationship between the whole fabric stretching amount and the POF stretching amount.
Fig. 4. Experimental of the POF-RFS's fabric stretch response: A. The relationship between fabric stretching amount and POF stretching amount ($\Delta \textrm{d}$) B. Test results of three stretching/restoring experiments C. The linear fitting between POF stretching amount ($\Delta \textrm{d}$) and photocurrent (I)
The fabric was stretched and restored repeatedly in the POF stretching range of 0.36 cm. The photocurrent (I) values during the stretching/restoring processes were recorded for three times as shown in Fig. 4(B). It can be seen that the light intensity changes of the fabric is basically the same during the stretching and restoring process, so it has a good stability for repeated measurement. Figure 4(C) shows the fitting curve of photocurrents’ (I) mean values obtained during repeating the stretching/restoring experiment process. It suggests that there is a good linear relationship between I and Δd.
4.1.3 Influence of the fabric width and fabric weave on POF-RFS’s sensing effect
Preferable measurement result of POF-RFS is mainly related to the ability of efficient conversion into POF spacing change originating from fabric overall stretching amount. The fabric width and fabric weave are the primary consideration. Four kinds of fabric widths (10cm, 8cm, 6cm and 4cm) and fabric weaves (plain weave, honeycomb weave, 1/3 right twill weave and 8/3 warp satin weave) were devised to explore its influence on the measurement result making use of the full-factor experiment.
In order to characterize the test effect of the POF-RFS, the parameter of light intensity loss ($\mathrm{\gamma }$) is introduced (Eq. (6)). It indicates the degree of optical attenuation in the photosensitive fiber caused by the POF spacing change. The greater the optical fiber spacing change, the greater the light intensity loss.
where Imax is the maximum of the photocurrent signal in one period, Imin is the minimum of the photocurrent signal in one period.The fabric samples were stretched using the electric translation station. Stretching distance was set to 2cm. The light intensity loss ($\mathrm{\gamma }$) was calculated according to the photocurrent signal obtained during the stretching process. The experimental results are recorded in Fig. 5.
When the fabric samples with the same weave are stretched to the same distance, the light intensity loss (γ) tends to become greater for the fabrics with smaller width. The four kinds of fabric weaves have the same variation trend for the intensity loss. In our design, the number of POFs and the width of the POFs area are the same for each fabric sample with the same weave. The fabric with smaller whole width has larger ratio of the POFs area to the whole width. Thus, in the case of the same stretching length, the fabric with smaller width can get greater variation in POFs spacing and the intensity loss is greater.
In addition, Fig. 5 also shows that the fabric with fewer interweaving points has greater intensity loss for the same fabric width. This is mainly caused by two reasons. On the one hand, the less interweaving points make the whole fabric possess better elasticity, which is beneficial to the fabric deformation between the POFs. On the other hand, for the different weaves, the number of interweaving points on the POFs is the same (shown in Fig. 2(C3)), but number of interweaving points on the other warps depends on its weave type. The interweaving point ratio of POFs to other warps increases in the order of plain, honeycomb, 1/3 right twill and 8/3 warp satin weave. The larger the ratio, the greater the influence of weft tension on the POF compared with other warps, which results in the more obvious change between the POFs spacing. Thus, when the fabrics are stretched the same distance, the 8/3 weave fabric has the largest change in the POFs spacing, while the plain weave fabric has the smallest change in the POFs spacing, which led to the result shown in Fig. 5.
In conclusion, in order to achieve the obvious light intensity loss (γ) of the POF-RFS, the fabric with less width and interweaving point number should be preferred. In this paper, the 4cm fabric width and 8/3 fabric weave were highly recommended. In the following a respiratory belt for actual measurement of body respiration was made basing on these parameters.
4.2 Measurement of human respiratory movement
4.2.1 Experimental setup of human respiratory movement
On the basis of the experimental setup shown in Fig. 3(A), a respiratory belt, the two ends of POF-RFS efficiently integrated with inelastic fabric via a conventional sewing method, was worn on the human body to monitor respiratory movement instead of the setup for simulating respiration (a fixed station, an electric translation and its controller). The tested data was recorded by the PC and compared with that of the commercial Mindray medical monitor IPM10 which required the subject to wear five electrodes.
4.2.2 Respiratory test in different body parts, postures and respiratory amplitudes
The proposed POF-RFS was tested in the assessment of respiratory movement at different body parts, postures and respiratory amplitudes. A healthy volunteer (male, 23 years) wore the respiratory belt below his chest or around abdomen while standing and sitting. The calm respiratory (calm-R) signals was measured. As shown from Fig. 6((A1) to (D1)), there are calm-R of the chest while standing, calm-R of the chest while sitting, calm-R of the abdomen while standing, calm-R of the abdomen while sitting. And the corresponding respiratory waveform and respiratory rates (R-R) were acquired in Fig. 6((A2) to (D2)).
Fig. 6. The measurement and analysis of respiratory movement in different body parts, postures and respiratory amplitudes: A1. Calm-R of the chest while standing B1. Calm-R of the chest while sitting C1. Calm-R of the abdomen while standing D1. Calm-R of the abdomen while sitting A2. Calm-R waveform of the chest while standing B2. Calm-R waveform of the chest while sitting C2. Calm-R waveform of the abdomen while standing D2. Calm-R waveform of the abdomen while sitting A3. Light-R waveform of the chest while standing A4. Deep-R waveform of the chest while standing E. Light intensity loss analysis of calm-R in different postures and parts F. Light intensity loss analysis of different respiratory amplitudes
As shown from Fig. 6((A2) to (D2)), each respiratory waveform obtained by the POF-RFS is quite visible. This characteristic is in agreement with that of the commercial medical monitor. Additionally, a red circle in the figure represents the completion of an exhalation and inhalation. The number of the red circle defines the R-R. Obviously, it can be found that the subject's calm R-R is 17–18 rpm, which is within the scope of normal adults’ R-R (16–20rpm) [30]. These results reveal the POF-RFS has good waveform response and stretch sensitivity to monitor the human respiratory signals under different body parts and postures.
Figure 6(E) shows that the light intensity loss of the POF-RFS is different between chest and abdominal respiration under the same postures. The abdominal respiration has higher stretch sensitivity, which reveals that its greater fluctuations can cause greater spacing change of the POFs. Additionally, the sitting posture can also arise higher stretch sensitivity than the standing posture under the same body parts. It is attributed that the sitting posture can cause greater fluctuations of human body cavity, and therefore more obvious spacing change of the POFs is produced.
Due to the lower stretch sensitivity of calm-R for the chest while standing from Fig. 6(E), different respiratory amplitudes of body were tested as shown in Fig. 6((A2) to (A4)). Obviously, the contour of each respiratory waveform is clearly visible and the R-R is consistent with a literature [30]. It highly verifies that R-R the POF-RFS produced can judge whether the respiratory movement is abnormal. Moreover, Fig. 6(F) shows that the light intensity loss ($\mathrm{\gamma }$) increases with the enhancement of body respiratory amplitudes. These results reveal that the POF-RFS can be of excellent respiratory waveform response, stretch sensitivity and wide respiratory monitoring range.
In conclusion, the POF-RFS is capable of effectively monitoring and obtaining the human respiratory signals of different body parts, postures and respiratory amplitudes.
4.2.3 Accuracy test for respiratory rate
In order to evaluate the repeatability and accuracy of POF-RFS, 10 healthy subjects whose average age was 23 years old worn the respiratory belt under the chest to test the calm-R signals while sitting and standing. The R-R value of each subject was recorded three times by the PC and the commercial medical monitor respectively.
Table 1 shows the tested data and errors for 10 subjects’ calm R-R measured by the proposed POF-RFS and the commercial medical monitor. The maximum error between them is 2 rpm. It can be found that the error-free for sitting and standing accounts for 66.7% and 60.0%, respectively. Figure 7 shows the mean error and error standard deviation of multiple respiration rate measurement results of the 10 subjects. The mean and standard deviation of error for standing are 0.53 rpm and 0.72 rpm, respectively. The mean and standard deviation of error for sitting are 0.37 rpm and 0.55 rpm, respectively. These indicate that the accuracy of the proposed POF-RFS is comparable to that of the medical monitor. Moreover, the measurement accuracy of sitting posture is slightly better than that for standing posture. Combining the test results in Fig. 6(E), respiration causes a greater change in circumference of chest for sitting posture. Thus, the output signal amplitude of POF-RFS is larger and the signal-to-noise ratio is improved, which facilitate subsequent signal processing and improve the accuracy of respiration rate measurement.
Table 1. Tested data and error of 10 subjects’ calm R-R by the POF-RFS and the commercial medical monitor
5. Conclusions
In this paper, a POF-RFS based on the side luminescence and photosensitivity mechanism of POF was designed and fabricated, which was realized by weaving the POFs as warp yarns into a fabric. To evaluate the performance of the POF-RFS, a fabric with 8/3 warp satin weave and 4cm-width was optimized. It had the excellent performance to monitor and capture the respiratory signals from different body parts (chest and abdomen), postures (sitting and standing), and respiratory amplitudes (light-R, calm-R and deep-R). The results show that the POF-RFS can be applied to help distinguish between healthy individuals and those with related respiratory diseases. Additionally, the POF-RFS features a high accuracy to obtain the R-R, which is comparable to a commercial medical monitor. This study provides the merits with simple weaving process, low-cost, good comfort, wearable monitoring and future industrialization, representing an advancement in the development of wearable textiles.
Funding
Tianjin Municipal Special Foundation for Key Cultivation of China (No.XB202007); Enterprise Entrusted Projects (2019-1200-24-001150); Tianjin Science and Technology Program (20YDTPJC01380).
Disclosures
All authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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