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With the advantages of high flexibility, strong real-time monitoring capabilities, and convenience, wearable devices have shown increasingly powerful application potential in medical rehabilitation, health monitoring, the Internet of Things, and human-computer interaction. In this paper, we propose a novel and wearable optical microfiber intelligent sensor based on a wavy-shaped polymer optical microfiber (WPOMF) for cardiorespiratory and behavioral monitoring of humans. The optical fibers based on polymer materials are prepared into optical microfibers, fully using the advantages of the polymer material and optical microfibers. The prepared polymer optical microfiber is designed into a flexible wave-shaped structure, which enables the WPOMF sensor to have higher tensile properties and detection sensitivity. Cardiorespiratory and behavioral detection experiments based on the WPOMF sensor are successfully performed, which demonstrates the high sensitivity and stability potential of the WPOMF sensor when performing wearable tasks. Further, the success of the AI-assisted medical keyword pronunciation recognition experiment fully demonstrates the feasibility of integrating AI technology with the WPOMF sensor, which can effectively improve the intelligence of the sensor as a wearable device. As an optical microfiber intelligent sensor, the WPOMF sensor offers broad application prospects in disease monitoring, rehabilitation medicine, the Internet of Things, and other fields.

Shape reconstruction is growing as an important real-time monitoring strategy for applications that require rigorous control. Polymer optical fiber sensors (POF) have mechanical properties that allow the measurement of large curvatures, making them appropriate for shape sensing. They are also lightweight, compact and chemically stable, meaning they are easy to install and safer in risky environments. This paper presents a sensor system to detect angles in multiple planes using a POF-intensity-variation-based sensor and a procedure to detect the angular position in different planes. Simulations are performed to demonstrate the correlation between the sensor's mechanical bending response and their optical response. Cyclic flexion experiments are performed at three test frequencies to obtain the sensitivities and the calibration curves of the sensor at different angular positions of the lateral section. A Fast Fourier Transform (FFT) analysis is tested as a method to estimate angular velocities using POF sensors. The experimental results show that the prototype had high repeatability since its sensitivity was similar using different test frequencies at the same lateral section position. The proposed approach proved itself feasible considering that all linear calibration curves presented a coefficient of determination (R2) higher than 0.9.

This Letter demonstrates the application of polymer optical fibers (POFs) damaged by the fiber fuse effect to curvature sensing and dynamic angular monitoring. The curvature sensing performance using the fused-POF is compared to POF without the fuse effect. Both POFs are submitted to angles of up to 90 deg in flexion/extension cycles with angular velocities ranging from 0.48 rad/s to 5.61 rad/s. The fused POF is found to show higher performance with respect to sensitivity, correlation coefficient with linear regression, and hysteresis. For instance, at the angular velocity of 0.48 rad/s, the fused POF shows >3 times higher sensitivity and significantly lower hysteresis than those of the non-fused POF. In addition, the fused POFs have lower cross-sensitivity and hysteresis variations on the tests with different angular velocities. These results indicate that the fused POFs are potential candidates to develop curvature sensors with various advantages over non-fused POFs, for applications such as gait analysis and wearable robotics.

This paper presented the force and displacement analyses of a diaphragm-embedded fiber Bragg grating (FBG) sensor. In the first step, a numerical analysis (via finite element method) was performed considering linear elastic materials, where there is a linear variation on the strain in the optical fiber for both displacement and force (or pressure). In the second step, the experimental analysis was performed using two approaches: (i) controlling the displacement applied in the diaphragm-embedded FBG (while the force is also measured). (ii) Controlling the force applied in the sensor (also with the measurement of the displacement). Results showed reflected optical power variations and wavelength shift following the application of displacement and force. The sensitivities of both wavelength shift and optical power were different (and non-proportional) when displacement and force were compared. However, a higher correlation, determination coefficient (R2) of 0.998, was obtained in the analysis of the wavelength shift as a function of the displacement, which indicated that the strain transmission in the optical fiber is directly related to the strain in the diaphragm, whereas the force has an indirect relation with the strain and depends on the material features. Then, the possibility of simultaneous estimation of force and displacement was investigated, where the linear relation of both parameters (displacement and force) with the wavelength shift and the optical power were obtained in a limited range of displacement and force. In this range, root mean squared errors of 0.37 N and 0.05 mm were obtained for force and displacement, respectively. In addition, the force variation with a step displacement input also shows the possibility of using the proposed FBG device for the characterization of the materials' viscoelastic features such as phase delay, creep, and stress relaxation, which can be employed for in situ characterization of different viscoelastic materials. [ABSTRACT FROM AUTHOR]

This letter presents the development and application of a multimaterial polymer optical fiber (POF) for temperature and force sensing. The POF is fabricated from the well-known polymethyl methacrylate (PMMA) through a conventional extrusion method combined with the Polycaprolactone (PCL), a biodegradable material with low melting point, which is thermally molded into the PMMA. In this approach, the high customizability enables the development of two configurations: (i) one input, one output (configuration 1); (ii) one input, two outputs (configuration 2). An advantageous feature of the PCL is its transmittance increase as a function of the temperature, which is confirmed via spectrophotometry at different temperatures. In the experimental analysis, the configuration 1 is used in the temperature tests and demonstrates a high linearity (coefficient of determination of 0.981) and resolution below 1°C, whereas its temperature limit is around 55°C. Configuration 2 is tested in simultaneous variations of temperature and transverse force, resulting in relative force insensitivity in temperatures below 40°C, whereas an optical power attenuation as a function of the applied force is found in temperatures around 55°C. Thus, the proposed approach shows a novel, flexible and customized technique for POF multimaterial fabrication, which can be employed not only on sensors applications, but also in the development of optical components based in POFs. [ABSTRACT FROM AUTHOR]

This paper reports the first microstructured solid-core fiber drawn from a 3D-printed preform and the first fiber Bragg gratings inscribed in a fiber of this type. The presented fiber is made of polycarbonate and displays single-mode behavior. The fiber attenuation was the lowest reported so far for a POF drawn from a 3D-printed preform across a broad range of wavelengths. In addition, extensive fiber characterization results are presented and discussed including: fiber attenuation, mode simulations, dynamic thermomechanical analysis and thermo-optic coefficient. Fiber Bragg gratings are successfully inscribed in the produced fiber using three different lasers: a continuous wave helium-cadmium laser, a pulsed femtosecond frequency doubled ytterbium laser and ultra-violet nanosecond krypton fluoride laser. Mechanical testing of the fiber showed that the 3D printing approach did not introduce any unexpected or undesirable characteristics. [ABSTRACT FROM AUTHOR]

This letter presents the development and thermal and mechanical considerations of sensing elements using different diaphragm materials which are embedded in optical fiber Bragg gratings (FBGs). Diaphragms made of polyurethane, polydimethylsiloxane (PDMS) and nitrile rubber were fabricated, and all of them with embedded FBGs. Tensile tests were performed in each material, showing the lower Young’s modulus of the PDMS (0.8 MPa), which is orders of magnitude lower than the ones of polyurethane (2.0 MPa) and nitrile rubber (362.2 MPa). Moreover, the highest strain limit was obtained in the PDMS diaphragm (24.5%). These results indicated the potential for higher pressure sensitivity of the PDMS, which was confirmed in the pressure characterization tests (pressure sensitivity of 1.4 nm/kPa). In addition, temperature tests indicated a higher sensitivity of the nitrile rubber, which presented a sensitivity of 49.8 pm/°C. Considering applications in which there are simultaneous variation of pressure and temperature, the nitrile presented a cross-sensitivity of 0.01 nm kPa−1°C−1 which is the lowest among the materials tested, indicating the possibility of obtaining the mitigation of temperature influence in the pressure response. It is also worth noting that the highest measurement range was obtained in the nitrile rubber diaphragm with linear responses in the range of pressures up to 3 kPa. [ABSTRACT FROM AUTHOR]

Interest in polymer optical fiber Bragg gratings (POFBGs) arises from the different material properties and sensing modalities brought by polymers relative to silica. Polymer fibers typically offer twice the sensitivity to temperature of conventional silica fiber and increased sensitivity to strain overall. In addition, polymer fibers have higher elastic limits and as a result a larger range of operation for physical constraints. While some polymers are effectively humidity insensitive, others present inherent humidity sensitivity. Their organic properties also allow a variety of chemical processes to create (bio)chemical sensors, with the consequences of fiber breakage in situ being less hazardous than silica. These attributes have led to the use of POFBGs for applications that remain complex using silica fibers. This review paper covers the progress toward commercialization and the increasing number of specific applications. [ABSTRACT FROM AUTHOR]

Highlights • Polymer optical fiber-embedded instrumented support for wearable devices. • Intensity variation-based embedded in 3D-printed structures for low cost systems. • Microclimate and human-robot interaction forces assessment in wearable robots. • Compensation of temperature and humidity effects on the POF sensor for force assessment. Abstract This paper presents the fabrication, characterization and applications of a 3D-printed instrumented support for wearable devices. Intensity variation-based polymer optical fiber (POF) sensors were employed in the support instrumentation, in order to obtain a low-cost system. In the proposed instrumented support, two POFs are embedded in a 3D-printed acrylonitrile butadiene styrene (ABS) rigid structure for microclimate assessment (without the influence of the applied force). Additionally, other two POFs are embedded in a thermoplastic polyurethane (TPU) flexible structure for the force assessment, where one sensor measures the applied force in flexion cycles and the other in extension cycles. All four sensors are characterized with respect to temperature and humidity. Both force sensors are also characterized with respect to the force and their cross-sensitivities with respect to the temperature and humidity. The results show the feasibility of the proposed sensor system embedded in the 3D-printed support, where the errors for the force assessment are below 2.8 N, and for temperature and humidity the errors are about 1.1 °C and 2.5%, respectively. The feasibility of the proposed instrumented support in measuring force and microclimate variations was also verified in the application as a shank support for a lower limb exoskeleton for knee rehabilitation and on a passive orthosis for gait assistance. [ABSTRACT FROM AUTHOR]

Highlights • FBG-embedded diaphragm for temperature and pressure measurements. • Simultaneous measurement of pressure and temperature with a single FBG. • The technique relies on differences between frequency responses of each parameter. • The technique leads to similar precision when compared with a system with two FBGs. Abstract This paper presents the development of a single polymer diaphragm-based fiber Bragg grating (FBG) sensor for simultaneous measurement of pressure and temperature. The FBG response is analyzed through a transient model for heat conduction and the pressure is estimated with the diaphragm and FBG strain, where the frequency difference between temperature and pressure variations were used to decouple both variables. Results show root mean squared error (RMSE) of 1.86 °C for the temperature analysis with constant pressure and 0.92 kPa for the pressure analysis with constant temperature. Experiments with variation of both temperature and pressure were conducted and the errors were higher than the ones with only temperature or pressure variations due to a residual cross-sensitivity. Therefore, the variation of both pressure and sensitivity leads to a RMSE of 3.88 °C and 5.13 kPa for temperature and pressure, respectively, which represent low errors of about 5% for both pressure and temperature. [ABSTRACT FROM AUTHOR]

Highlights • Multiplexing technique for intensity variation-based sensors. • Analytical approach for the analysis of maximum number of sensors and sensitivities. • Experimental validation for 3-DOF angle measurement using three-sensor array. • Technique validation for quasi-distributed array in multi-parameter applications. Abstract This paper presents a multiplexing technique for polymer optical fiber (POF) intensity variation-based sensors. The technique relies on the side-coupling between the light source and the POF lateral section. Thus, each sensor has its own light source which has its activation as well as its signal acquisition (made with two photodetectors, one at each end of the POF) controlled by a microcontroller. With this technique, a matrix with the number of columns equal to the number of photodetectors multiplied by the number of light sources (or sensors) is obtained. This enables the decoupling of the response of each sensor. The presented analytical approach shows a tradeoff between the number of sensors and their sensitivities (considering the dynamic range of each sensor). In addition, experimental results show the feasibility of the technique to measure angles in a 3°-of-freedom (DOF) systems with errors as low as 3°. Furthermore, the proposed approach was also tested in multi-parameter applications, where temperature, angle and force were estimated with errors up to 5% with an array with 3 POF sensors. The results presented in this paper can pave the way for novel applications of quasi-distributed sensors with intensity variation-based sensors both in multi-DOF systems and in multi-parameter applications with the additional advantages of lower cost than fiber Bragg gratings-based systems and lower spatial resolution than distributed sensors. [ABSTRACT FROM AUTHOR]

Highlights • POF-embedded smart textile for breath and heart rates monitoring. • Analytical approach for the sensor operation principle. • Technique for detecting heart and breath rates even when the subject is moving. • Signal processing to mitigate the influence of gait frequency in the sensor response. Abstract In this paper, we present the development of a polymer optical fiber (POF) sensor for simultaneous measurement of breath (BR) and heart rates (HR). The sensor is embedded as a smart textile solution that can be used within the user’s clothes. In addition, a signal processing technique is proposed for obtaining the HR and BR without the influence of body movements and in different positions of the user’s chest. Sensor signal processing and analysis are made in the frequency domain and different filters are applied. Results show errors below 4 beats per minute and 2 breaths per minute for the HR and BR, respectively, even when the user is performing periodic body movements such as the ones induced by the gait. Thus, the proposed POF-based smart textile is a low-cost solution with good accuracy that can be readily applied in the remote monitoring of patients at home without disturbing their daily activities. [ABSTRACT FROM AUTHOR]

The influence of annealing under different humidity conditions on temperature, strain, and transverse force performance of polymer optical fiber Bragg gratings (POFBG) inscribed in cyclic transparent amorphous fluoropolymers is presented. The POFBG are imprinted using direct-write, plane-by-plane femtosecond laser inscription method. Further improvement to the sensor's performance, with respect to sensitivity, linearity, and hysteresis, is obtained with a simple technique based on polymer viscoelastic response applied to the annealed POFBGs. The sensors were characterized for temperature, strain, and transverse force after annealing and results show that sensors annealed under water have the lowest hysteresis. Additionally, the temperature sensitivity increased when the sensor was annealed in low humidity conditions. Finally, for strain and transverse force, the highest sensitivity was obtained in POFBG sensors annealed under water. The compensation technique did not show a substantial decrease in hysteresis response for applied strain tests; however, for temperature and transverse force tests, the hysteresis decreased by more than two and four times, respectively. [ABSTRACT FROM AUTHOR]

This paper presents the application of polymer optical fiber (POF) sensors in wearable devices for gait assistance. Intensity variation-based POF curvature sensor for the acquisition of joint angles and a POF insole based to detect gait events were developed. Three wearable devices were employed for the practical validation of the developed sensors: an active orthosis with one degree of freedom, a robotic exoskeleton, and a functional electrical stimulation (FES) system for gait assistance. POF is a viscoelastic material, which does not have a constant response to stress or strain and also can present hysteresis in dynamic measurements. In order to mitigate these effects, a compensation technique based on the polymer viscoelastic features was applied on both the curvature sensor and the instrumented insole. The results obtained for the POF curvature sensor on the knee joint angle measurement of the orthosis and the exoskeleton show root mean squared error of 3.7° and 3.8°, respectively. Regarding the POF insole results, it was used to identify five gait events in the FES system application. The reported results pave the way for new instrumentation approaches in wearable devices with flexible sensors that can be especially desirable in the rigid robots presented but also in soft robotics applications. [ABSTRACT FROM AUTHOR]

Ground reaction force (GRF) measurement is usually made by means of force platforms, which can lead to errors when the foot is not precisely placed inside the platform boundaries. Although electronic in-shoe plantar pressure sensors solve this problem, they lack in stability, robustness to impact loads and also can present some errors and inconsistencies. Polymer optical fibers (POF) present high elastic strain limits, fracture toughness, flexibility in bend, and relative low cost and are not influenced by electromagnetic fields, which enable their application for in-shoe plantar pressure measurements. This paper presents an in-shoe system to measure the vertical GRF during the gait cycle. The system comprises of four POF sensors in an insole. Since the polymer does not have a constant response with stress or strain due to its viscoelasticity, a compensation technique for this effect is also proposed. The proposed insole is compared with a commercial force platform, and a root mean squared error of 5.30 N was obtained. Results also show the feasibility of the system to measure the GRF during the gait and the identification of all subdivisions of the stance phase with the additional advantages of full portability, lower cost, and higher flexibility than the one presented by commercial force platforms and fiber Bragg gratings-based insoles. [ABSTRACT FROM PUBLISHER]

Curvature sensors based on polymer optical fibers (POFs) present some advantages over the conventional technologies for joint angle assessment such as compactness, electromagnetic field immunity, and multiplexing capabilities. However, the polymer is a viscoelastic material, which does not have a constant response with stress or strain. In order to understand and model this effect, this paper presents the dynamic characterization of a POF. The effects of temperature, frequency, and loads on the fiber are analyzed for obtaining the influence of these parameters on the polymer dynamic Young modulus and time constant. Results show that a temperature on the range between 24 °C and 45 °C does not lead to considerable variations on the sensor output. Moreover, it is possible to estimate the storage modulus and loss factor from the frequency and temperature. The polymer time constant is defined on creep recovery experiments. Since the viscoelastic parameters are evaluated in different conditions of temperature, frequency, and load, a model for the stress behavior of the fiber is proposed. Such model leads to a root mean squared error between the modeled and measured results over 15 times lower than the one obtained with the model for bending stress without account the POF viscoelastic behavior. [ABSTRACT FROM PUBLISHER]

This paper presents a system capable of measuring temperature and relative humidity with polymer optical fiber (POF) sensors. The sensors are based on variations of the Young’s and shear moduli of the POF with variations in temperature and relative humidity. The system comprises two POFs, each with a predefined torsion stress that resulted in a variation in the fiber refractive index due to the stress-optic effect. Because there is a correlation between stress and material properties, the variation in temperature and humidity causes a variation in the fiber’s stress, which leads to variations in the fiber refractive index. Only two photodiodes comprise the sensor interrogation, resulting in a simple and low-cost system capable of measuring humidity in the range of 5–97% and temperature in the range of 21–46 °C. The root mean squared errors (RMSEs) between the proposed sensors and the reference were 1.12 °C and 1.36% for the measurements of temperature and relative humidity, respectively. In addition, fiber etching resulted in a sensor with a 2 s response time for a relative humidity variation of 10%, which is one of the lowest recorded response times for intrinsic POF humidity sensors. [ABSTRACT FROM AUTHOR]

• Polymer optical fiber with exceptionally low Young's modulus and high strain limit. • Thermal and mechanical characterizations for pressure sensors applications. • Dynamic response and viscoelastic modeling of the fiber in sensors applications. • Five times higher sensitivity when compared with systems employing commercial fibers. This letter reports on the development of a low-cost intensity variation-based pressure sensor using polymer optical fibers (POFs) with high flexibility fabricated through the light polymerization spinning process. The fibers were characterized by tensile tests that indicate a Young's modulus of 18.11 MPa. The pressure response of the proposed system shows a sensitivity five times higher when compared with systems employing commercial POFs. Furthermore, results also show low hysteresis (3.5%) and resolution potentially higher than the systems with spectral based sensors. [ABSTRACT FROM AUTHOR]

This paper demonstrates the use of polymer optical fiber Bragg gratings (POFBGs) for angle measurements over a range of different oscillatory frequencies. The POFBGs are inscribed in low-loss, cyclic transparent amorphous fluoropolymers (CYTOP) and are imprinted using the direct-write, plane-by-plane femtosecond laser inscription method. As the polymer has a viscoelastic response and given that the Young’s modulus depends on the oscillatory frequency, a compensation technique for sensor frequency cross-sensitivity and hysteresis is proposed and verified. Results show that the proposed compensation technique is able to provide a root mean squared error (RMSE) reduction of 44%, and a RMSE as low as 2.20° was obtained when compared with a reference potentiometer. The hysteresis reduction provided by the proposed technique is 55%, with hysteresis <0.01. The results presented in this paper can pave the way for movement analysis with POFBG providing higher sensitivity and low hysteresis over a large range of motion frequencies. [ABSTRACT FROM AUTHOR]

It is still a challenge to reduce the amount of plastic discarded in the environment. In this scenario, a low-cost method of producing polymer optical fibers (POFs) (mainly compared to powder extruders or pulling towers) from discarded plastics that have transparency is presented in this work using the extrusion method. As a result, this study reports an alternative to the disposal of transparent plastics: the manufacturing of POFs that can be used for different applications, including the fabrication of sensors. Temperature, curvature angle, refractive index and pH sensors were made with the recycled POFs. As part of the optical characterization of the fabricated POF, we used the cutback method to measure the attenuation and a spectrophotometer to determine the relationship between the transmittance and wavelength for Polymethyl Methacrylate (PMMA) and Polyethylene Terephthalate (PET) materials. The mechanical characterization was conducted with a strain–stress test. For temperature sensing, the temperature ranged from room temperature (T ≈ 25 ° C) to T ≈ 50 ° C using a furnace. In this test, PMMA POF had a linear behavior. The linear regression fits the experimental data for PMMA POF ( R 2 = 0.97), while PET POF had little variation in optical power at temperatures lower than T = 45 ° C , exhibiting nonlinearity in this case (determination coefficient lower than 0.9, R 2 = 0.815). In angle sensing, both POFs showed linear behavior: PMMA ( R 2 = 0.973) and PET ( R 2 = 0.914). Similarly, both fibers were able of measuring refractive index with a linear behavior: PMMA ( R 2 = 0.968) and PET ( R 2 = 0.944). Then, the PMMA and PET POFs were doped with Rhodamine B dye to obtain a pH sensor. The proposed approach resulted in pH sensors with sensitivities of 517.85 a.u./pH and 277.14 a.u./pH for the PMMA and PET, respectively. Thus, the results of this work show the feasibility of using recycled plastics on the biosensors production, where the plastics discarded to the environment can be recycled as sensors for environmental sensing. [ABSTRACT FROM AUTHOR]