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Abstract
Flexibly wearable sensors are widely applied in health monitoring and personalized therapy. Multiple-node sensing is essential for mastering the health condition holistically. In this work, we report a multi-node wearable optical sensor (MNWOS) based on the cascade of microfiber Bragg gratings (µFBG), which features the reflective operation mode and ultra-compact size, facilitating the functional integration in a flexible substrate pad. The MNWOS can realize multipoint monitoring on physical variables, such as temperature and pressure, in both static and dynamic modes. Furthermore, the eccentric package configuration endows the MNWOS with the discernibility of bending direction in addition to the bending angle sensing. The multi-parameter sensing is realized by solving the sensing matrix that represents different sensitivity regarding the bending and temperature between FBGs. The MNWOS offers great prospect for the development of human-machine interfaces and medical and health detection.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Wearable sensors have great potential for human health monitoring [1,2] and human-computer interaction [3,4]. They could be used for rehabilitation therapy, for example. to check the recovery function of human muscles and limb functions [5]. Owing to that the human limb activities are often driven by the multiple joints, and the patient's movement range is very subtle at the beginning of rehabilitation training, it is essential to develop a wearable sensor that holds high sensitivity and capability of simultaneous multi-node monitoring to comprehensively assess human health.
At present, the work on wearable electrical sensors based on resistive [6], piezoelectric [7], and capacitive [8] has been extensively studied and the related products were put into practical applications. More flexible and scalable sensors based on conductive nanomaterials (e.g., metal nanowires [9,10], carbon nanotubes [11,12], and graphene [13]) have also been proposed, making a great development in this field. However, wearable electrical sensors undergo electromagnetic interference (EMI), and inadequate insulation, which hinder their application in real medical environments [14]. Optical sensors can be an excellent alternative to address those issues. In recent years, optical fiber sensor has been employed for human health monitoring [14–17] due to its intrinsic advantages of small size, biocompatibility, anti-EMI, and capability of multi-parameters distribute sensing, etc [18–20]. Despite this, the standard silica fibers sensor is rigid and has poor detection sensitivity, which is incompatible with joint flexion monitoring and difficult to sense minute motions of the human body [21]. To address those problems, polymer optical fibers [22] and micro-nano fibers [23–25] were utilized to devise the wearable sensors. Although these sensors offer notable improvements in flexibility and sensitivity, the multiplexing remains challenging to realize simultaneous monitoring of multiple nodes.
Here, in order to overcome the aforementioned problems, we reported a multi-node wearable optical sensor (MNWOS) based on microfiber Bragg grating (Fig. 1(a)) for multi-joint monitoring of the human body. The MNWOS consists of the core sensing element µFBG and PDMS packaging film. The µFBG has the advantages of strong ability of multiplexing, excellent wavelength selectivity, small size (µFBG length of a few millimeters), stable structure, anti-electromagnetic interference and the ability to work in both transmission and reflection modes. Due to the of µFBG, the multi-node sensor based on µFBG can continuously sense the spatial distribution, and change information of physical parameters of multiple nodes, just through only one optical fiber transmission path excellent multiplexing capabilities, and the sensor can obtain sensing data from each node. The inherent reflective working mode of µFBG endows the sensor still work even if the fiber is broken. In contrast to conventional fiber, combining optical microfiber with FBG sensors endowed the sensor with the advantages of reduced bending loss, strain effect amplification, and exceptional flexibility. According to the magnified strain effect, it can be seen that the Bragg wavelength of the µFBG is susceptible to the axial strain alteration, which effectively improves the sensitivity of the FBG sensor. The encapsulation of the sensor devices is PDMS, an extremely flexible and biocompatible low refractive index (n = 1.40) polymer material, increases the ductility of the sensor devices and also protects them from environmental perturbations and fiber optic device damage. In addition, the eccentric configuration of the embedded FBG enables the sensor to discern the bending direction. The MNWOS can measure a variety of physical parameters such as pressure, bending angle, and temperature in both static and dynamic modes. This study employs wavelength division multiplexing to concurrently broadcast various Bragg wavelength signals in the fiber in order to maximize the multiplexing potential of the sensor. The MNWOS has broad applications in fields such as human-machine interface and human joint rehabilitation monitoring.
Fig. 1. (a) The structure of MNWOS. (b) Three different inscription methods. (i) Different masks with the same diameter. (ii) The same mask with different diameters. (iii) Different masks with different diameters. (c) Fabrication process of the MNWOS.
2. Principle and concept of the MNWOS
The mathematical theoretical study model of fiber Bragg grating sensing comes from the correlation of its Bragg wavelength and the effective refractive index. The fiber Bragg grating is formed by periodically modulating the refractive index of the fiber core [26]. The Bragg wavelength (${\lambda _B}$) of the fiber Bragg grating is given from (eq.1):
The resonant wavelength of the fiber grating depends on the grating period $\mathrm{\Lambda }$ and the effective refractive index ${n_{eff}}$. Temperature, stress, and other physical quantities applied to the FBG are what cause the drift of the Bragg wavelength by causing small changes in refractive index or grating period.
As shown in Eq. (1), the change of grating period $\mathrm{\Lambda }$ and its effective refractive index will cause the FBG Bragg wavelength to fluctuate. Among all the external environmental influences, the effect of temperature and stress changes can be directly reflected by the two parameters of grating period and effective reflectivity, causing the FBG Bragg wavelength to drift. The FBG Bragg wavelength drift can be calculated by Eq. (2):
Based on the strong wavelength selectivity of FBG, to explore the multiplexing capabilities of µFBG sensors with the sensing network configuration, we have used the wavelength division multiplexing (WDM). In fiber grating sensing system, broadband light source covers a wide bandwidth. If the wavelength could be divided into sub-regions, the Bragg wavelength of the chosen fiber grating sensor is at the Bragg wavelength of the corresponding sub-region, and the sub-region range is greater than the sensor by using FBGs with different central wavelengths. That is, the wavelength difference between two FBGs should be large enough to avoid crosstalk in sensing process.
3. Experimental section
The method of µFBG fabrication can be found in the Supplement 1. Due to the distinct and stable Bragg wavelengths of FBG, and occupy a narrow bandwidth, cascade multiplexing is possible. When inscribing µFBGs, it needs to interleave the Bragg wavelengths of each grating to produce multiplexed sensing of nodes to implement wavelength division multiplexing in FBG. In order to achieve FBG multiplexing, three different inscription methods were used in the work. (These three inscription methods have no effect on the sensing performance of the MNWOS.) The first method was using different phase masks to inscribe FBGs on the same fiber's tapered area (Fig. 1(b)(i)). The second method involved using the same phase mask to inscribe FBGs on the same fiber in different tapered areas (Fig. 1(b)(ii)). The third method was taken both different phase masks and fiber diameters into consideration (Fig. 1(b)(iii)).
The MNWOS is composed of multiple microfibers that have different FBGs engraved on them and are encapsulated in PDMS elastomers. Depending on the requirements of the specific application, the number of FBGs and the distance between them can be changed. To bestow the MNWOS stretchability, we created a set of procedures to construct the MNWOS sensor utilizing an S-shaped mold with curved bumps on the bottom of the mold design, as illustrated in Fig.1c. The procedures mainly involved into three parts: 1) PDMS substrate preparation. PDMS and hardener were mixed in a 10:1 ratio and then 0.4 ml of degassed PDMS was evenly applied to the mold to create a flat PDMS substrate with curved surface recesses. The substrate was cured at 80 °C for 20 min to form PDMS film. After curing, the PDMS substrate is peeled off from the mold. 2) Deposition of the µFBG, the µFBG is deposited in the substrate S-shaped. 3) Sealing of the µFBG. The embedded µFBG was fixed using 0.6 ml of degassed PDMS and cured at 80°C for 20 min. 4) Acquisition of the flexible MNWOS. The 500-micrometer-thick PDMS-µFBG-PDMS sensing element was detached from the glass slides. (The thickness of PDMS film is adjusted based on its dosage.) Due to the creeping effect, the Bragg wavelength of FBG will be red-shifted after PDMS encapsulation with a heating and cooling process. However, the creeping effect becomes low once the MNWOS is fabricated.
4. Results and discussion
4.1 Pressure sensing
In the distributed pressure sensing experiment, using the inscription method of Fig. 1(b) (i). Gratings 1 and 2 with Bragg wavelengths of 1565 nm and 1575 nm, respectively, were written using different phase masks (Λ1:1067, Λ2:1074) in the same fiber diameter (d = 29 µm) region. On the measurement grating, weights of different grams were placed for quantitative testing and analysis. A plastic disc (1.45 cm in diameter) was inserted at the measurement grating to account for the various bottom areas of weights of various grams and prevent the force area from influencing the experimental findings. Using OSA to record the reflectance spectra of MNWOS at different pressures. As the pressure increases, as seen in Fig. 2(a), (c), the Bragg wavelength of the grating under pressure changes toward red shift, whereas the grating without pressure applied nearly has no wavelength change. The sensor's effective distributed pressure sensing performance could be inferred from this. Additionally, the fluctuation of the Bragg wavelength varies linearly with pressure (0–25 kPa), and the pressure sensitivity of the 29 µm diameter sensor is about 19 pm/kPa, which is nearly a 3-fold improvement compared to conventional diameter-type fiber optic sensors [21](Fig. 2(b),(d)). Due to the amplifying effect of the local strain, the fiber diameter has a substantial impact on the sensor's sensitivity [27]. Additionally, PDMS has a low elastic modulus but a high Poisson's ratio, which enhances the sensor's responsiveness to strain.
Fig. 2. Characterization of MNWOS for pressure sensing. (a) (c) The Bragg wavelengths of grating 1 and 2 are redshift with increasing pressure, respectively. (b) (d) The peak wavelength of grating 1 and 2 is linear with the increase of pressure, respectively. The pressure sensitivity for a diameter of 29 µm MNWOS is almost 19 pm/kPa. (e) The press platform. (f) (g) Spectrograms of the dynamic responses of the four FBGs in the Spectrograms of the dynamic responses of the three FBGs in the Visualization 1 are shown.
Then, a series of dynamic response tests were performed. To investigate the responsiveness of MNWOS arrays under various single-point touch, and multi-touch spatial and temporal situations. A microfiber grating with two FBGs written in two cones of different diameters (d = 32 µm and 23 µm) was selected and placed on the platform to create a 2*2 sensing array. They have respective Bragg wavelengths are 1558 nm, 1572 nm, 1577 nm, and 1583 nm. The sensing array (shown in Fig. 2(e)) is set up with four points, labeled 1, 2, 3, and 4. Figure 2(f) and (g) show the spectral changes when one point is pressed sequentially, when both points 1 and 3, 2 and 4 are pressed simultaneously, and when all four points are pressed simultaneously. The sensor has high repeatability and real-time responsiveness, and it can precisely detect and discriminate the response of each grating under various pressing postures (shown in Visualization 1). The peak of the FBG may become distorted during pressing due to the phase mask approach for FBG inscription [28], however, this has no bearing on the sensor's ability to detect objects. According to the aforementioned experimental findings, the MNWOS array can be applied to wearable, medical, and interfaces for human-computer interaction.
4.2 Bend sensing
Additionally, MNWOS may be utilized to detect scattered stresses related to bending deformation. Here we use using the inscription method of Fig. 1(b) (iii), by employing a different phase mask (Λ1:1063, Λ2:1072) inscription method in various diameter cone (d1 = 15µm, d2 = 35µm) sections inscribed FBGs. The Bragg wavelengths of the two FBGs inscribed were 1550 nm and 1572 nm, respectively. To change the bending angle during the experiment, MNWOS was mounted to a mechanical rotating platform. The spectrograms of the two FBGs in the bending angles of 0 to 90° are shown in Fig. 3(a), b. As the bending angle increases, the Bragg wavelength of the bent FBG drifts toward the long wavelength band, and the drift of the Bragg wavelength of the unbent FBG is almost zero, thus verifying that the sensor has distributed bending sensing performance. The recording and analysis of the sensor's Bragg wavelength are displayed in the insets of Fig. 3(a), b. Between 0° and 90°, the Bragg wavelength and bending angle are linearly linked, the 15 µm diameter fiber grating has a bending sensitivity of around 78 pm/°, which is almost two times higher than the bending sensitivity of a 35 µm diameter fiber grating, which has a bending sensitivity of about 19 pm/° (Fig. 3(a), b).
Fig. 3. Characterization of MNWOS for bend sensing. (a)(b) Reflection spectra of grating 1 and 2 for bend response, respectively. Inset: Bending response of MNWOS with microfiber diameters of 15µm and 35µm, respectively. (c)(d) Spectrograms of the dynamic responses of the three FBGs in the Visualization 2 are shown. (e) Schematic illustration of the MNWOS sensor for measurements of bending directions. (f) Wavelength shifts of the sensor versus the positive/negative bending direction.
To investigate the sensor's distributed bending dynamic response, three FBGs were inscribed on the same microfiber that has three diameter cone regions (20 µm, 22 µm, and 15 µm, respectively). Three FBGs were fixed on the knuckles of the index, middle, and ring fingers in sequence to detect the motion of the three finger joints simultaneously, respectively. Their respective Bragg wavelengths are 1578 nm, 1558 nm, and 1553 nm. Bend each of the three fingers individually, then simultaneously bend the index and middle fingers, the middle finger and the ring finger, and the index and ring fingers, respectively, and finally bend all three fingers at the same time (shown in Visualization 2). As illustrated in Fig. 3(c), d, which displays the spatiotemporal response of the three FBGs with finger flexion, MNWOS is able to recognize and discriminate each finger's motion with high levels of accuracy, consistency, and responsiveness. These demonstrations highlighted the enormous potential of sensors as wearable devices for applications such as motion monitoring, rehabilitation, robotics, and hand control in virtual reality.
Moreover, the eccentric embedding structure of the FBG offers a method of differentiating the bending direction in this case. Given that no matter how the bend is curved, the length of the PDMS centerline stays constant (Fig. 3(e)). As a result, positive bending extends the eccentrically packed FBG, causing $\; {\lambda _B}$ to drift in the direction of the long wave. In contrast, negative bending compresses the FBG and causes the $\; {\lambda _B}$ to drift in the other direction. Figure 3(f) demonstrates that the sensor exhibits opposing drift patterns when bent in the opposite direction, demonstrating the capability of directional recognition. In both positive and negative bending orientations, the bending curvature and the drift of the Bragg wavelength are linearly linked.
4.3 Temperature sensing
In temperature sensing tests, a method for inscribing FBGs is selected to separate the Bragg wavelengths of several FBGs using the same mask version on the same fiber, but with varied diameter cone zones (the inscription method of Fig. 1(b) (ii)). The distributed temperature sensing performance of the sensor is then examined. The same mask (period 1055) was used in the experiment to inscribe grating 1 on a fiber cone with a 125 µm diameter and grating 2 on a fiber cone with a 10 µm diameter. The Bragg wavelengths were 1554 nm and 1537 nm, respectively. The distributed temperature sensing performance of the sensor is then examined. The entire temperature experiment was carried out at 26°C room temperature to assure the precision of the sensor results. The spectrum was recorded at various temperatures (30∼110°C) while the measured grating is mounted on a temperature-controlling platform. When seen in Fig. 4(a), (c), the temperature responses of the two FBGs are independent, with only the heated grating exhibiting a wavelength shift. It indicates that the sensor has a well-distributed capacity for temperature sensing. The wavelength drift varies linearly with temperature change in the range of temperature fluctuation from 30°C to 110°C. as shown in Fig. 4(b), d. Defining the temperature sensitivity as $S = \frac{{\mathrm{\Delta }\lambda }}{{\mathrm{\Delta }T}}$, where $\Delta \lambda $ is the displacement of the Bragg peak wavelength and $\Delta T$ is the temperature change. The microfiber component of the MNWOS achieves a sensitivity of up to 47.58 pm/°C, which is an improvement of more than three times over the 125µm component of the MNWOS (14.28 pm/°C). Since our prepared microfiber was tapered from commercial multimode fiber that had a considerably larger diameter (≥10 µm, resulting in a much weaker evanescent field), the refractive index change of PDMS film induced by heating would not significantly affect the temperature sensitivity of µFBG. As a result, the elongation of PDMS film mediated by the thermal expansion coefficient (9.6×$10{^{ - 4}}$ /°C), which is three orders of magnitude higher than the silica fiber (5.5×$10^{ - 7}$ /°C), took the main role in the temperature sensitivity improvement by conferring an extra strain to the µFBG.
Fig. 4. Characterization of a MNWOS for temperature sensing. (a) The reflectance spectrum of grating 1 temperature response. (b) Bragg wavelength drift of grating 1 towards longwave with temperature from 30 °C to 110 °C. The temperature sensitivity for a diameter of 125 µm MNWOS is 14.48 pm/°C. (c) The reflectance spectrum of grating 2 temperature response. (d) Bragg wavelength drift of grating 2 towards longwave with temperature from 30 °C to 110 °C. The temperature sensitivity for a diameter of 10 µm MNWOS is 47.58 pm/°C.
In conclusion, different MNWOS diameters respond differently to pressure, temperature, and bending. As a result, we can engrave various FBGs into the two tapered regions with various diameters on the same fiber, and by analyzing the wavelength shifts of the two FBGs and figuring out the sensitivity matrix, we can simultaneously determine changes in two parameters (such as temperature and curvature). For instance, we want to keep track of both the index finger's bending angle and the temperature change. We installed two different diameters of MNWOS (35 µm and 125 µm) at the joints, each with different sensitivity to temperature and bending (0.0235 nm/°C and 0.0311 nm/° for 35 µm, and 0.0144 nm/°C and 0.007 nm/° for 125 µm). By resolving the matrix equation in the following equation, it is possible to determine the variation of these two parameters simultaneously:
We can set up multiple MNWOS with various diameter cone areas if it needs to measure more parameters in time (different sensitivities). Additionally, a multimode fiber with tapered fibers was used to prepare MNWOS. Multiple measurements can also be found using multimode coupled Bragg reflection. This offers more dimensional demodulation of the measurement solution.
5. Conclusion
In summary, we reported a wearable sensor based on a cascade of µFBGs with high sensitivity, excellent multiplexing capabilities, and can be used for real-time multi-node on-body measurements. The µFBG cascade was packaged in a flexible PDMS substrate to form a MNWOS, which could sensitively respond to the change of temperature, pressure, and bending both statically and dynamically. The PDMS combined with the µFBG not only realized the ability of simultaneous monitoring of multiple nodes of the MNWOS but also improve the sensitivity to temperature, pressure, and bending angle. In the experiment, the spectral frequency of the spectrometer was set to once every 1 second, and the MNWOS can complete the response within 1 second. Moreover, the eccentric packaging approach used by the µFBG enables the sensor to recognize the bending's direction. Based on the excellent multiplexing capability and structural stability of FBG itself, a single fiber can be used to monitor numerous nodes under varying spatial and temporal pressures, as well as several finger knuckles in various bending orientations. Finally, the sensitivity difference of optical fibers with different diameters is combined with the sensitivity matrix to realize simultaneous sensing of multiple parameters (temperature and bending angle) on one MNWOS. Future research could be focused on the better demodulation methods to achieve decoupling of more parameters (such as polarization demodulation [29–31] or sensing matrix demodulation [32]), and combining AI algorithms and technology to achieve integrated monitoring and analysis. Such wearable sensor device can find in a wide range of application, such as in human-computer interaction, robotics, and medical protection.
Funding
National Natural Science Foundation of China (62335010); Fundamental Research Funds for the Central Universities (21623203); Natural Science Foundation of Guangdong Province.
Disclosures
The authors declare no conflicts of interest related to this article.
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.
Supplemental document
See Supplement 1 for supporting content.
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