Distributed Strain Sensor Based on Self-Powered, Stretchable Mechanoluminescent Optical Fiber

stretch-release


Introduction
With the advent of the artificial intelligence era, research has been conducted to develop flexible and stretchable strain sensors for health monitoring, [1,2] human motion detection, [3,4] soft robotics, [5,6] and human-machine interfaces (HMIs), [7,8] which have significant advantages over traditional rigid and fragile strain sensors due to their flexible, stretchable, and conformable properties. [9,10]Inspired by the development of new functional materials such as liquid metal, [11,12] photonic soft materials, [13,14] MXenes, [15,16] mechanoluminescent (ML) materials, [17][18][19] piezoelectric materials, [20,21] and carbon nanotubes, [22,23] researchers make efforts to integrate these smart materials into soft matrix materials such as polyurethane, silicone elastomer, and develop many different types of flexible strain sensors, in which optical or electrical parameters vary with the exerted strain.26][27][28] Therefore, from the perspective of a real-world application scenario, it is highly desirable and important to develop a flexible and stretchable sensor with location identification and large strain quantification capability together.Driven by these requirements, continuous efforts have been made to develop flexible and stretchable electronic distributed strain sensors by embedding grids of point sensors in soft substrates. [29]For example, Wang et al. develop a large intrinsically stretchable array of 6300 transistors over an area of around 4.4 Â 4.4 cm 2 for pressure sensing. [30]Sundaram et al. integrated 584 piezoresistive strain sensing units on gloves and used deep convolutional neural networks to process complex signals generated for HMIs. [31]o improve the resolution of the sensing array, bulky wiring and signal detection units will be introduced, which greatly increase the complexity and manufacturing cost of the sensor for the wearable device.
Mechanoluminescence (ML) as a new class of luminescence in which mechanical stimulation can be converted to luminescence has drawn significant attention for its practical significance in self-powered strain sensors. [17,18,32]Researchers have integrated ML materials into flexible substrates to prepare film-type self-powered sensors capable of monitoring pressure distribution without any wires and electrodes. [33,34]However, detecting the spatial distribution of ML signals requires the use of CCD in a fixed position in a dark environment.The susceptibility to environmental light interference, bulky signal detection equipment, and inconvenient sensing testing greatly limit the application of these sensors in the wearable field.
Alternatively, flexible and stretchable optical fiber sensors, where the optical signal is confined in the core medium by total internal reflection, have grasped considerable attention due to their attractive characteristics of conformability, biocompatibility, lightweight, small size, and resistance to environmental interference.Meanwhile, inspired by related optical fiber communication techniques such as time-division multiplexing (TDM) and wavelength-division multiplexing (WDM), researchers have further realized the ability to quantify strain magnitude and identify strain location together in optical fiber.Leal-Junior et al. demonstrated an instrumented insole comprised of a polymer optical fiber and thermoplastic polyurethane (TPU) substrate, in which 15 sensing segments based on optical signal intensity determined by pressure were independently demodulated using TDM technique, enabling to map the foot pressure distribution. [35]Based on the WDM technique, some researchers have integrated multiple silica fiber Bragg grating (FBG) sensors with different reflection wavelengths into silicone elastomer to realize distributed strain sensing. [28,36]In addition, Guo et al. embedded three dyes with different absorption spectra at different positions in the stretchable hydrogel optical fiber, in which the strain from different sensing areas corresponds to the loss of different color light intensities, verifying the feasibility of distributed sensing with absorption type wavelength coding method. [37]urthermore, Bai et al. combined the dual-core parallel structure and the wavelength coding method to prepare a multifunctional sensor that not only realized distributed sensing but also recognized multiple deformation patterns. [26]Although the aforementioned efforts in respect of stretchable distributed optical fiber strain sensors have shown promising concepts and applications, there are still shortcomings.For example, due to the large elastic modulus mismatch between the rigid silica FBG and the soft substrate, this WDM-based flexible FBG strain sensor network had poor stability and was limited to measuring small deformations.In addition, sensors based on the optical intensity loss principle were to measure the changes in transmitted light intensity as the fiber was deformed by strain, with sensitivity being mainly determined by the compromise between the attenuation coefficient and the transmission characteristics of the optical sensing fiber.Moreover, the multisensor crosstalk problem made it difficult to quantify the exact amount of strain and to identify the exact location of strain. [26]Requiring a bulky light source and a complicated demodulation system, these stretchable distributed strain sensors precluded the benefits of miniaturization, portability, and low power consumption.Therefore, the development of a self-powered distributed optical fiber strain sensor with stretchable and wearable properties for real-time quantitative strain measurement and precise location identification would be highly imperative.
In this article, we propose a self-powered distributed optical fiber strain sensing system, which is based on the integration of mechanoluminescence (ML) phosphor materials with an elastomer optical fiber.By a simple three-step process, a stretchable optical fiber is fabricated from two silicone-based elastomers with a high refractive index difference, where ML phosphors with different emission color lights are integrated onto the outer cladding of the elastomer optical fiber.This mechanoluminescent optical fiber is capable of emitting light just driven by the strain to be measured, without the need of an external light source or electric power.The strain-induced emitted light can be collected and guided along the stretchable optical fiber, where the strain applied on different sensing positions corresponds to the intensities of different emission color light.Benefiting from the synergy of stretchable optical fiber and superior ML phosphor materials, this self-powered sensor exhibits rapid and stable linear strain response up to axial strain over a range of 10-60% at all sensing positions.Based on the wavelength coding technique and time-domain filtering comparison method, we have further realized the capability of strain magnitude quantification and strain location identification together in a single stretchable optical fiber, even at multiple positions simultaneously in strainapplied situations.Moreover, this stretchable optical fiber strain sensor shows insensitivity to pressure and temperature disturbance, displays consistent signals over 8000 stretch-release motion cycles, and demonstrates good durability of the sensor.Due to the excellent light confinement of the elastomer optical fiber, this stretchable distributed strain sensor is demonstrated in bright-field measurement, saline water operation, and wearable glove application, thus demonstrating its potential as a promising technology for future self-powered distributed optical sensor systems.

Sensor Design and Fabrication
Figure 1 illustrates the concept and structure of self-powered stretchable distributed optical fiber strain sensor (SSDOFS).This device has a stretchable and flexible optical fiber structure where several predefined positions outside the fiber cladding are coated with different ML materials as sensing layers.When the sensing section of the SSDOFS is stretched, without the need of an external light source for any pre-or postirradiation, the sensing layer of the SSDOFS is capable of converting the mechanical stimuli into visible light emission.Benefiting from the structure of an optical fiber, the strain-induced ML light signal could be collected and guided along the fiber to the far end for detection.Similar to previously reported flexible strain sensing devices based on ML materials, [4,17,[38][39][40][41] the ML intensity of SSDOFS increased linearly with the applied strain in a certain range, and the applied strain could be accurately demodulated from the optical fiber collected light intensity.Along the stretchable optical fiber at different positions, we have already integrated different types of ML material sensing layers that emit light of different colors (emission wavelength) under strain.The ML color of SSDOFS changed directly with the specific sensing position, and the strain-applied position could be clearly discriminated from the characteristic spectra of the light collected by the optical fiber.Benefiting from the rapid development of ML field, various ML materials that could emit different color light under mechanical stimuli, [42][43][44][45][46] have already been developed to fully guarantee the mass positions locating requirement of SSDOFS.Moreover, even with only two independent ML materials (such as typical ML materials of ZnS:Mn 2þ and ZnS:Cu 2þ ), [32,[47][48][49] the color could also be controlled by regulating the mixing or weight ratio of the two phosphors in a soft organic matrix, which greatly reduces the number of ML material types required and increases the number of sensing positions along the optical fiber to theoretical infinity.
Silicone elastomers are generally used as the matrix material for fabricating the flexible strain sensors due to their excellent elasticity, low Young's modulus, and conformability with human skin. [50,51]In our case, a high refractive index (n 1 = 1.53) silicone elastomer optical encapsulant (OE-6550, Dow Corning) was selected as the core material and a low refractive index (n 2 = 1.41) silicone tube as the cladding material to fabricate the stretchable optical fiber.The transmittance and elongation at break of the OE-6550 and silicone tube materials were investigated, as shown in Figure S1 and S2, Supporting Information, respectively.The fabricated 1 mm thick OE-6550 film exhibited high transmittance (>85%) in the visible light region (Figure S1, Supporting Information).The elongation at break of the OE-6550 film and silicone tube were measured to be approximately 90% and 228%, respectively (Figure S2, Supporting Information), suggesting that these materials can withstand large strain and were suitable for most wearable applications.Previous studies have shown that ZnS:Cu 2þ and ZnS:Mn 2þ possess a series of remarkable properties including relatively low ML threshold, high luminescence intensity, reproducible luminescence, and no need of pre-or postirradiation, [4,32,47,48,52] making them the best candidates for this strain-responsive stretchable optical fiber.These two most classic and common metal-ion-doped zinc sulfides (ZnS:Cu 2þ and ZnS:Mn 2þ were purchased from Lonco Company Limited) were chosen to be dispersed and embedded in a polydimethylsiloxane (PDMS) substrate as the ML sensing layer.To verify the feasibility of the above-mentioned wavelength coding method, the spectra of ZnS:Cu 2þ and ZnS:Mn 2þ ML phosphors mixed at different ratios in PDMS film were investigated.As shown in Figure S3, Supporting Information, the pure ZnS:Cu 2þ embedded PDMS film exhibited a single peak luminescence with 517 nm central wavelength under applied strain, the corresponding color was green, while the ZnS:Mn 2þ exhibited a single peak luminescence with 587 nm central wavelength, the corresponding color was orange.When these two ML phosphor materials were mixed in a weight ratio of 1:1, a broadband luminescence spectrum could be observed.The ML spectra of other ZnS:Cu 2þ /ZnS:Mn 2þ ratios were shown in Figure S4, Supporting Information, and the Coordinates Internationale de L'eclairage (CIE) coordinates of these spectra were shown in Figure S5, Supporting Information, which intuitively described the color changed with the mixing ratio of the two ML phosphors.
As shown in Figure 2a, a three-step approach has been developed to fabricate the strain-responsive stretchable optical fiber.First, after stirring and degassing, the uniform mixture of OE-6550 precursor solution (1:1 mixing ratio) was aspirated into a silicone tube with an inner diameter of 1.2 mm using a syringe.Both ends of the silicone tube were then inserted into 1 cm deep PMMA optical fiber as pigtails for long-distance optical signal delivery.After thermal curing at 65 °C for 4 h, a stretchable optical fiber with OE core and silicone cladding structure was obtained.Second, several silicone tubes with 4 mm inner diameter and 8 mm outer diameter were sleeved on the stretchable optical fiber, filled with PDMS precursor solution (1:10 mixing ratio), and heated at 65 °C for 1 h to form the nodes.These nodes not only separated the stretchable optical fiber into several different sensing segments but also served as the handles for applying axial loads without crushing the fiber structure at the contact area.Finally, three PDMS þ ZnS mixture precursors were prepared as follows: three ML phosphor mixtures were prepared by uniform mixing with 0:10, 5:5, and 10:0 weight ratios of ZnS:Cu 2þ and ZnS:Mn 2þ , respectively.The ML phosphor mixtures were separately mixed with PDMS precursor (10:1 base to curing agent weight ratio) in a 1:3 weight ratio.Subsequently, these three PDMS þ ZnS mixture precursors were uniformly coated outside the cladding of the fabricated stretchable optical fiber at three different segments each and heated at 65 °C for 1 h.
The cross-section of the fabricated strain-responsive stretchable optical fiber was shown in Figure 2b, where the diameters of the core, cladding, and sensing layer were 1.2, 2.2, and 3.6 mm, respectively.Attributed to an all-in-one structure composed of silicone elastomer, this strain-responsive stretchable optical fiber could be substantially bent and stretched without any mechanical failure, with a minimum bending radius of 7 mm.As shown in Figure 2c, the sensing segments coated with different ML phosphor mixtures were able to emit bright orange, yellow, and green light, respectively, when subjected to axial stretch deformation, which was clearly visible to the naked eye in a darkened room.These uniform luminescence indicated that the sensing segments had a uniform stress distribution during stretching (Figure S6, Supporting Information).To characterize the light-guiding efficiency of the optical fiber, light from a 470 nm LED was coupled into the fiber to measure the propagation loss using the cutback method.As shown in Figure 2d, the (ii) Photographs of as-prepared strain-responsive stretchable optical fiber.Photographs of the optical fiber as it is bent (iii) and stretched (iv).c) ML emission images with different optical fiber sections stretched.d) Propagation loss of the optical fiber measured in the air using the cutback method.e) ML emission spectra collected and delivered by the optical fiber.f ) Stress-strain curve of the optical fiber.fabricated strain-responsive stretchable optical fiber, which benefits from the high transparency of the OE-6550 fiber core, had a low loss of %0.26 dB cm À1 .The intensity distribution of LED light propagating in optical fibers was shown in Figure S7, Supporting Information.Furthermore, as shown in Figure S8, Supporting Information, the other end of optical fiber retained 80% of the light intensity even after being stretched by 60% strain when 470 nm light was introduced at one end of the optical fiber.However, the sensing segments were uniformly covered by ML sensing layer, so most of the ML signals did not pass through the entire deformation area when the sensing segments were stretched.Therefore, the loss of ML signals propagation should be much less than 20%.Thus, the strain-induced ML light signals from the sensing layers could be effectively collected and guided by this low-loss stretchable optical fiber, and delivered to the spectrometer or photodetector for measurement.As shown in Figure S9, Supporting Information, and 2e, the ML emission collected by optical fiber could be clearly observed and distinguished when different sensing segments were involved, and the corresponding spectra were consistent with the spectra of the PDMS composite films mentioned above.For ease of description, we named the three segments O (orange), M (mixed light), and G (green) according to the color of the ML emissions.In addition to the axial strain, other deformation modes, such as bending and compression, could not excite the sensing segments to emit ML light (Video S1, Supporting Information).This only responsive to axial strain feature indicated that SSDOFS could focus on measuring only axial strain and completely ignore the strong interference from other common deformation modes, such as bending and compression.We further investigated the mechanical properties of this strainresponsive stretchable optical fiber using tensile testing method.The strain-stress curve of this stretchable optical fiber was shown in Figure 2f, which exhibited two failure points A and B. Here, point A corresponded to the failure of the optical fiber core at elongation of 118%, and point B was the complete failure of the entire fiber.Prior to the failure of the optical fiber core (point A), the stress had a good linear response to the strain (R 2 = 0.98696), which could fully satisfy the mechanical stretch requirement for most wearable and implantable applications.

Operation Mechanism and Strain Measurement Characteristics
Figure 3a shows a schematic diagram of the strain-sensing performance test setup based on the SSDOFS.The fabricated strainresponsive stretchable optical fiber was clamped to a tensile strain tester controlled by the preset program.Conventionally, the spectrometer has been widely used to detect optical signals in previously reported flexible strain sensing devices based on ML materials. [33,34,41,53]However, limited by the scanning speed and large volume, it is difficult for the spectrometer to respond quickly to the ML signal from SSDOFS during the stretching or releasing process, it is also difficult to develop wearable and portable photonic device based spectrometer demodulation.Here, we have developed a time-domain filtering comparison method that avoids the use of a spectrometer, greatly improving response speed and reducing device volume.The optical fiber outports were connected to two photomultiplier tubes (PMTs) with internal 585 nm AE 10 nm and 520 nm AE5 nm bandpass filters, respectively.The ML emission collected by the optical fiber could be detected in real-time and converted into a voltage signal by the PMTs with 20 kHz frequency bandwidth.The detailed time-domain filtering comparison method is shown schematically in Figure 3b.When the sensing section is stretched, the ML pulse signal is triggered and emitted, and a portion of the ML pulse signal collected and delivered to both ends of the optical fiber is filtered into two discrete narrowband spectral signals by two different bandpass filters and detected in real-time by PMTs.The integral intensity of the ML pulse signal is defined as the integral of the area of ML emission produced during stretching or releasing, which can be expressed by Where I is the time-integrated intensity, U(t) is time-intensity signal, t 1 and t 2 represent the beginning and end of the ML pulse signal (typically 5% height width of each ML emission pulse).Compared with the strain-induced ML spectra shown in Figure 1, the time-domain ML emission signal has better synchronization with the motion events; it also had a better ability to resolve the motion event details in the time domain.As shown in Figure 3b, for different ML spectra produced by stretching different sections along the optical fiber, the relative intensity of two narrowband spectra after filtering is completely different.Specifically, when the O section is stretched with 580 nm orange band ML emission, channel 1 (signal passing through 585 nm AE 10 nm bandpass filter) will detect an apparent optical signal, while at the same time, channel 2 (signal passing through 520 nm AE 5 nm bandpass filter) will detect no signal.Based on similar principle, both channel 1 and channel 2 will detect a certain amount of optical signal when M section is stretched with superposition of 520 nm green band and 580 nm orange band ML emission.Contrary to the case of O section, when the G section is stretched with 520 nm green band ML emission, channel 2 will detect an obvious optical signal while channel 1 will detect no signal.Therefore, the time-domain integral intensity ratio of dual-channel signals can be used to distinguish and locate the strain-applied position along the optical fiber.
A series of tensile testing experiments were carried out to investigate the location capability and quantitative strain-sensing characteristics of SSDOFS.First, the fiber was subjected to repeat stretching and releasing cycles with 40% peak strain applied to the fiber at a constant moving rate of 16 mm s À1 tensile speed at each sensing section.As shown in Figure 3c, the ML optical pulse signals were emitted twice within one cycle, corresponding to the stretching and releasing of the optical fiber, respectively, and all the ML optical pulse signal peaks emitted from the single segment were very uniform and stable (Figure S10, Supporting Information).Based on the aforementioned time-domain filtering comparison method, the spatial position could be easily distinguished from the time-domain combination of dual-channel ML optical pulse signals.Due to the difference in ML lifetime between ZnS:Mn 2þ and ZnS:Cu 2þ , ML optical pulse signal from O segment is slightly wider than G segment, showing a slight afterglow.In addition to single-segment stretching, synchronous stretching of two and three segments was also investigated.As shown in Figure S11, Supporting Information, the stress-strain curves of the three sensing segments in the stretch-release cycles were basically the same, ensuring a consistent response from each sensing segment with equal elastic modulus.Similar to single-segment stretching, two-and three-segment stretching still produce uniform and stable ML pulse signals were still observed (Figure S12, Supporting Information) and two signal pulses in one cycle (Figure 3d) reflected stretching and releasing of the optical fiber, respectively.Figure 3e shows that the time-domain integral intensity ratios between channel 1 and channel 2 ( ments were ∞, 2.17, 0, 13.7, 0.89, and 6.11 under 40% appliedstrain, respectively, proving that the six stretching patterns could be easily discriminated according to the time-domain filtering comparison method. To explore the quantitative relationship between the time integral intensity and strain, we further carried out strain-applying tests varying from 10% to 60% for the six stretch patterns above.As shown in Figure 4a,b, and S13, Supporting Information, the time-domain ML pulse signal peak increases with increasing strain.Excellent linearity as a function of strain was observed for all six stretch patterns (R 2 > 0.98 for all patterns).Therefore, a series of linear fitting equations were determined to describe the relationship between time integral intensity and applied strain (linear fitting equations are listed in Note S1, Supporting Information).Thus, based on the linear calibration, the quantitative relationship between the time integral intensity and the applied strain could be established.In terms of the ability to discriminate the segment under different applied strains, the time-domain integral intensity ratio I 1 /I 2 of six stretch patterns under 10-60% strain was calculated and illustrated in Figure 4c.Time-domain integral intensity ratio I 1 /I 2 remained almost constant under different applied strains for all six stretch patterns.Although slight variations could be observed for the O þ M and O þ M þ G segment cases, all the I 1 /I 2 values for six stretch patterns were in nonoverlapping areas, indicating that the detailed strain-applied position could be discriminated just from the I 1 /I 2 value alone under any applied strain.

Stability and Practicality
To explore the potential application of the SSDOFS in diverse environments, the stability of the SSDOFS was investigated for testing under bright field, temperature disturbance, saltwater, and repeatedly stretching, respectively.Previously reported ML material-based films for strain/stress sensing have only been observed and quantified in the dark field for fluorescence detection, [33,34,41,53] a limitation that has severely limited their practicality.Due to the excellent light confinement of our fabricated elastomer optical fiber, this SSDOFS is uniquely capable of collecting and delivering the ML light signal in both bright-and dark-field environments.Figure 5a shows the time-domain ML pulse signal of the SSDOFS with 40% applied strain in the dark and bright environment.When the dark environment was suddenly brightened by switching on a flashlight, the ML pulse signal curve obtained in the bright field was the same as in the dark, as shown in Figure 5b, except for the slightly elevated baseline due to the flashlight, which could be easily subtracted by signal processing.
To investigate temperature stability, the SSDOFS was subjected to stretch-release cycles with 40% applied strain over a temperature range of À10 to 60 °C (10 °C increment).Figure 5c shows the time integral intensity of ML pulse signals as a function of temperature, the variation amplitude of time integral intensity between 0 and 40 °C was less than 10%, indicating that the SSDOFS could operate stably in cold or hot environments.In many cases, wearable devices may be exposed to water, even salty environment, especially during exercise, which might dramatically reduce the performance and life of the device.In our case, the SSDOFS attached to the finger was subjected to the same amplitude of flexion and extension in the air and saline water which had a higher salinity than sweat, respectively.As shown in Figure 5d, the time-domain ML pulse signals showed no obvious change when immersed in saline water, which was attributed to the excellent light collection and transmission capabilities of our fabricated elastomer optical fiber.To further investigate the stability and repeatability of the SSDOFS, the sensor was subjected to a periodic stretch-release motion for 8000 cycles with a peak applied strain of 40%.Figure 5e showed that the time-domain ML pulse signals were reproducible throughout the overall cycles with little variation, benefiting from the all-in-one silicone elastomer optical fiber structure and the remarkable robustness of ZnS ML materials.
Human motion monitoring is highly desirable for personal healthcare and HMI.Encouraged by stretchable, conformable, highly linear, and distributed sensing performance of the SSDOFS, we demonstrated its potential application in biomonitoring, even the subtle movements of different finger joints.This wearable distributed strain sensor was demonstrated on a glove that integrated the SSDOFS on the index finger to monitor the movement of finger joints.PDMS precursor was used as an adhesive to attach the G, M, and O segments of the elastomer optical fiber to the distal interphalangeal (DIP), proximal interphalangeal (PIP), and metacarpophalangeal (MCP) joints of the index finger at the glove positions, respectively.Figure 5g shows the dual-channel ML pulse signals corresponding to the four-finger gesture patterns.The SSDOFS was stretched when the finger was bent, and the strain-induced ML pulse was emitted twice in response to the flexion and extension of the finger joints.The integral intensity of the ML pulse signals was recorded and calculated to quantify the exact applied strain induced by the finger joint movement, and the time-domain integral intensity ratio of dual-channel signals was observed to discriminate and identify which finger joint was moving.As shown in Figure 5g, the local strains induced by the bending finger joints could be quantified and located based on the aforementioned time-domain filtering comparison method, even in the cases of multiple bending joints such as Gestures 1 and 2. The signal processing details and flowcharts are shown in Figure S14, Supporting Information.These results confirm that we can acquire and discriminate strain information from multiple positions based on just one single elastomer optical fiber, which is significant for the development of an efficient wearable sensor network.

Conclusion
In conclusion, we have developed a self-powered and stretchable optical fiber strain sensor with distributed sensing capability.This stretchable, strain-responsive optical fiber is based on the segments, the distribution map of the time integral intensity I 1 and I 2 with respect to the strain from 10% to 60%; (ii) Time-domain integral intensity ratio I 1 /I 2 as a function of the applied stretching strain in the range from 10% to 60%.integration of ML material with an elastomer optical fiber, where ML materials with different color light emission have been integrated onto different positions of optical fiber outer cladding.This stretchable optical fiber strain sensor is capable of emitting light just driven by external strain to be measured, without any external power source, and the strain-induced emitted light is effectively collected and guided along the stretchable optical fiber for measurement.Thus, based on the linear strain-intensity feature and time-domain filtering comparison method, the functions of strain magnitude quantification and strain location identification could be realized together in a single stretchable optical fiber, even in the situation of simultaneous strain application at multiple positions.This stretchable optical fiber strain sensor exhibits a rapid and stable linear strain response up to axial strain over a range of 10%-60% at all sensing positions with position discrimination capability.In addition, this stretchable optical fiber strain sensor is insensitive to bending, compression, and temperature disturbances and displays consistent signals over 8,000 stretch-release motion cycles, demonstrating the good durability of the sensor.Due to the excellent light confinement of the elastomer optical fiber, this stretchable distributed strain sensor is demonstrated in bright-field measurement, saline water operation, and wearable glove application, thereby, demonstrating potential as a promising technology for future self-powered distributed optical sensor systems.

Figure 1 .
Figure 1.Schematic illustration of the self-powered stretchable distributed optical fiber sensor with strain magnitude quantification and strain location identification capability.

Figure 2 .
Figure 2. Fabrication and structural characteristics of the strain-responsive stretchable optical fiber.a) Schematic illustration of the fabrication process of the optical fiber.b) (i)The SEM image of the cross-section in the sensing section of the optical fiber.(ii) Photographs of as-prepared strain-responsive stretchable optical fiber.Photographs of the optical fiber as it is bent (iii) and stretched (iv).c) ML emission images with different optical fiber sections stretched.d) Propagation loss of the optical fiber measured in the air using the cutback method.e) ML emission spectra collected and delivered by the optical fiber.f ) Stress-strain curve of the optical fiber.

Figure 3 .
Figure 3. a) Schematic diagram of the SSDOFS.b) Comparison of spectral and time domain forms of signals from different sensing segments.c) Dualchannel time-domain signals of each individual segment subjected to a stretch-release cycle at 40% peak strain.d) Dual-channel time-domain signals of multisection subjected to a stretch-release cycle at 40% peak strain.e) Time-domain integral intensity ratio (I 1 /I 2 ) with respect to six location patterns (O, M, G, O þ M, G þ M, and O þ G þ M) under 40% peak strain.

Figure 4 .
Figure 4. a) (i) Time-domain ML pulse signal of O segment under a stretch-release cycle range of 10%-60% strain; (ii) Integral intensity of the ML pulse signal as a function of the applied strain during stretching (red line) and releasing (blue line).b) (i) Time-domain ML pulse signal of G section under a stretch-release cycle range of 10-60% strain; (ii) Integral intensity of the ML pulse signal as a function of the applied strain during stretching (red line) and releasing (blue line).c) (i) For the other four stretch patterns of M, O þ M, G þ M, and O þ M þ Gsegments, the distribution map of the time integral intensity I 1 and I 2 with respect to the strain from 10% to 60%; (ii) Time-domain integral intensity ratio I 1 /I 2 as a function of the applied stretching strain in the range from 10% to 60%.

Figure 5 .
Figure 5. a) Time-domain ML pulse signal of the SSDOFS in dark and bright field with 40% applied strain under stretch-release cycles.b) Comparison of time-domain ML pulse signals in dark and light fields.c) Normalized time-domain integral intensity of SSDOFS at temperatures from À10 to 60 °C.d) Time-domain ML pulse signal obtained from the SSDOFS worn on the index finger in air and saltwater at a concentration of 10 g L À1 .e) Time-domain ML pulse signal produced from SSDOFS during 8000 periodic stretch-release cycles with 40% applied peak strain.f ) Schematic of the SSDOFS worn on the hand to detect the movement of finger joints.Distal interphalangeal (DIP), proximal interphalangeal (PIP), and metacarpophalangeal (MCP) joint movements were recorded and detected from the attached G, M, and O segments, respectively.g) Dual-channel output signals acquired from different finger gestures and corresponding strain distributions of SSDOFS were detected and demodulated.(Gesture 1 corresponds to bending all joints.Gesture 2 corresponds to bending DIP and PIP joints together.Gesture 3 and Gesture 4 correspond to bending PIP joint and MCP joint, respectively).