Fiber Bragg Grating Sensors for Reinforcing Bar Slippage Detection and Bond-Slip Gradient Characterization.

The detection of bond-slip between the reinforcing bar (RB) and concrete is of great importance to ensure the safety of reinforced concrete (RC) structures. The techniques to monitor the connection between the RB and concrete are in constant development, with special focus on the ones with straightforward operation and simple non-intrusive implementation. In this work, a simple configuration is developed using 10 optical fiber sensors, allowing different sections of the same RC structure to be monitored. Since the RB may suffer different strains along its length, the location of the sensors is critical to provide an early warning about any displacement. Bragg gratings were inscribed in both silica and polymer optical fibers and these devices worked as displacement sensors by monitoring the strain variations on the fibers. The results showed that these sensors can be easily implemented in a civil construction environment, and due to the small dimensions, they can be a non-intrusive technique when multiple sensors are implemented in the same RC structure.

Sensing System Based on FBG for Corrosion Monitoring in Metallic Structures.

As corrosion has slow development, its detection at an early age could be an alternative for reducing costs of structural rehabilitation. Therefore, the employment of structural health monitoring (SHM) systems, sensing configurations collecting data over time allowing for observing changes in the properties of the materials and damage emergence, for monitoring corrosion can be a good strategy to measure the damage and to decide the better moment for intervention. Nonetheless, the current corrosion sensor technology and the high costs of the sensing system implementation are limiting this application in the field. In this work, an optical fiber Bragg grating (FBG)-based sensing system is proposed for monitoring the thickness loss of a 1020 carbon steel metal plate subjected to controlled corrosion. The natural frequency of the plate was collected as a function of the corrosion time over 3744 h. To validate the experimental results, ultrasound measures and electrochemical tests were also carried out under similar conditions. The experimental results show adequate reliability, indicating the suitable functionality of the proposed system for monitoring the thickness loss caused by corrosion in metallic structures, in comparison with traditional methods, as ultrasonic and electrochemical measures.

Respiratory and heart rate monitoring using an FBG 3D-printed wearable system.

This work proposes a 3D-printed sensor based on fiber Bragg grating (FBG) technology for respiratory rate (RR) and heart rate (HR) monitoring. Each sensor is composed of a single FBG fully encapsulated into a 3D-printable Flexible, during the printing process. Sensors with different material thicknesses and infill densities were tested. The sensor with the best metrological properties was selected and preliminary assessed in terms of capability of monitoring RR and HR on three users. Preliminary results proved that the developed sensor can be a valuable easy-to-fabricate solution, with high reproducibility and high strain sensitivity to chest wall deformations due to breathing and heart beating.

Sensor Cell Network for Pressure, Temperature and Position Detection on Wheelchair Users.

This work proposes an optical sensing network to monitor pressure and temperature in specific areas of a wheelchair to prevent pressure ulcers and to monitor the position of the wheelchair user by analyzing its pressure distribution. The sensing network is composed of six optical fiber Bragg grating (FBG)-based sensor cells. Each sensor cell is built from a polylactic acid (PLA) base and has two FBGs, one embedded in epoxy resin to monitor pressure variations (FBGP) and another without resin to monitor temperature (FBGT). Once produced, all sensor cells were experimentally characterized for pressure and temperature variations, resulting in an average pressure sensitivity of 81 ± 5 pm/kPa (FBGP) and -5.0 ± 0.4 pm/kPa (FBGT), and an average temperature sensitivity of 25 ± 1 pm/°C (FBGP) and 47.7 ± 0.7 pm/°C (FBGT). The sensor cells were then placed in six specific areas of a wheelchair (four in the seat area and two in the shoulder blade area) to carry out experimental tests, wherein the response of the sensors to a specific sequence of relief positions was tested. During the execution of the test, the optical signal of all sensors was monitored, in real time, with the pressure and temperature values detected in each zone of the wheelchair. In addition, random position changes were performed in order to evaluate the precision of the proposed sensing network in the identification of such positions.

High Rate Dynamic Monitoring with Fabry-Perot Interferometric Sensors: An Alternative Interrogation Technique Targeting Biomedical Applications.

Fabry-Perot interferometric (FPI) sensors are an accurate and well-established sensing technology that are used to monitor a wide range of parameters such as strain, temperature, and refractive index, among many others. Nevertheless, due to the limited number and high cost of existing interrogation techniques for FPIs, its use is often restricted to discrete measurements, not being so explored for dynamic applications. The development of an alternative interrogation technique for a high rate of acquisition may propel this type of sensor into less explored fields such as dynamic biomedical applications. In this work, we present the theoretical and experimental analyses of an FPI sensing architecture by using an alternative high rate dynamic acquisition methodology, based on frequency to amplitude conversion, where the FPI spectral shift is detuned by the convolution of the optical light source with the FPI interference pattern. The good agreement between the theoretical and experimental results verified the reliability of the proposed methodology. Moreover, preliminary results show that the developed sensing architecture can be a suitable solution to monitor biomedical parameters such as the carotid pulse wave.

Polymerization Shrinkage Evaluation of Restorative Resin-Based Composites Using Fiber Bragg Grating Sensors.

The purpose of this study was to compare the linear polymerization shrinkage of different restorative resin-based composites (RBCs) using fiber Bragg grating (FBG) sensors. Five RBCs were evaluated: Zirconfill® (ZFL); Aura Bulk-Fill (ABF); Tetric® N-Ceram Bulk-Fill (TBF); FiltekTM Bulk-Fill (FBF); and Admira Fusion-Ormocer® (ADF). Ten samples per resin were produced in standardized custom-made half-gutter silicone molds. Two optical FBG sensors were used to assess temperature and polymerization shrinkage. Light curing was performed for 40 s and polymerization shrinkage was evaluated at 5, 10, 40, 60, 150, and 300 s. Statistical analysis was accomplished for normal distribution (Shapiro-Wilk, p > 0.05). Two-way repeated measures ANOVA with Greenhouse-Geisser correction followed by Bonferroni's post-hoc test was used to analyze the linear shrinkage data (p < 0.05). ZFL showed the highest linear shrinkage and ADF the lowest. Shrinkage increased for all RBCs until 300 s, where significant differences were found between ADF and all other resins (p < 0.05). Among bulk-fill RBCs, TBF showed the lowest shrinkage value, but not statistically different from FBF. The ADF presented lower linear shrinkage than all other RBCs, and restorative bulk-fill composites exhibited an intermediate behavior.

Pulp Temperature Rise Induced by Light-Emitting Diode Light-Curing Units Using an Ex Vivo Model.

The aim of this research was to compare the pulp temperature (PT) rise induced by four light-emitting diode light-curing units (LED LCUs) (Bluephase 20i, Demi Ultra, SPEC 3, and Valo) in different curing modes. Immediately after extraction, the pulp chamber of 11 premolars was accessed from the palatal cervical third of the crown for insertion of fiber Bragg grating (FBG) sensors for temperature measurement and kept in a 37.0° water bath. The teeth were then submitted to a random sequence of curing modes with four irradiations at 30 s intervals. Care was taken to ensure complete pulp temperature reset between curing modes. The curing modes were classified as high-energy (above 80 J/cm²) or low-energy (below 40 J/cm²) according to the total energy density delivered. Statistical analysis was performed with repeated ANOVA measures and Pearson's correlation for the association between energy density and temperature variation. The significance level was set to 0.05. All curing units promoted a statistically significant PT rise (p < 0.01). After four emissions, the PT rise was higher than 5.0 °C for the high-energy curing modes. The low-energy modes induced approximately a 2.5 °C rise. A strong positive correlation was found between energy density and PT increase (R = 0.715; p = 0.01). Exposure of intact premolars to LED LCUs induced significant and cumulative PT rise. Curing modes emitting high energy densities produced higher PT variations. Radiant exposure was positively correlated to PT variation.

Wheelchair Pressure Ulcer Prevention Using FBG Based Sensing Devices.

In this work, a fiber Bragg grating (FBG) based sensing system for wheelchair pressure ulcer prevention was developed. Six FBGs were strategically positioned in a wheelchair to monitor the more prominent bone areas, namely scapulas (right (SR) and left (SL)), ischiatic zone (right (IR) and left (IL)), and heels (right (HR) and left (HL)). The sensing architecture was tested by a female user during pressure relief exercises, to verify its effectiveness on pressure monitoring. The proposed system proves to be a compact and reliable solution for wheelchair pressure ulcer prevention, making it a suitable alternative to existing conventional electronic sensors, with the advantage of being immune to electromagnetic interferences and usable in humid environments. In addition to the pressure, the breathing rate was also monitored. By combining the proposed sensing architecture with a wheelchair user detection software, it is possible to create alerts for the user to know when a new position should be adopted, in order to relieve the pressure in a specific area, thus avoiding one of the biggest problems for such patients, pressure ulcers.

Optical Fiber Magnetic Field Sensors Based on Magnetic Fluid: A Review.

Magnetic field sensing is an important issue for many application areas, such as in the military, industry and navigation. The current sensors used to monitor this parameter can be susceptible to electromagnetic interferences, however due to their advantages over the traditional sensors, the optical fiber devices could be an excellent alternative. Furthermore, magnetic fluid (MF) is a new type of functional material which possesses outstanding properties, including Faraday effect, birefringence, tunable refractive index and field dependent transmission. In this paper, the optical fiber magnetic field sensors using MF as sensing element are reviewed. Due to the extensive literature, only the most used sensing configurations are addressed and discussed, which include optical fiber grating, interferometry, surface plasmon resonance (SPR) and other schemes involving tailored (etched, tapered and U-shaped) fibers.

Design and characterization of a curvature sensor using fused polymer optical fibers.

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.

Strain, temperature, moisture, and transverse force sensing using fused polymer optical fibers.

This paper presents the characterization of polymer optical fibers (POFs) submitted to the catastrophic fuse effect towards intensity-variation-based sensing of strain, transverse force, temperature, and moisture. In the experiments, POFs with and without the fuse effect are tested and the results are compared with respect to the sensitivity, linearity, and root mean squared error (RMSE). The fused POFs have higher linearity and lower RMSE than non-fused POFs in strain and transverse force sensing. Also, the sensitivity of the fused POFs is higher in transverse force and temperature sensing, which can be related to the higher sensitivity to the curvature that the transverse force creates on the POF and to the more significant variations of the refractive index with temperature increase. Additionally, the fused POFs present lower moisture absorption than the non-fused POFs. The presented results indicate a great potential of the fused POFs intensity-variation-based sensing applications of various physical parameters.

Dynamic mechanical analysis on fused polymer optical fibers: towards sensor applications.

This Letter presents, for the first time, to the best of our knowledge, the dynamic mechanical analysis of a polymer optical fiber (POF) that was previously damaged by the catastrophic fuse effect. The variation of the fiber Young's modulus was evaluated with respect to the increase of temperature, humidity, and frequency of strain cycles. The obtained data for the fused POF are compared with the ones for the same POF without the fuse effect. The results show the feasibility of the fused POF for sensor applications, such as strain and acceleration measurement, since it presents temperature sensitivity almost two times lower in temperatures between 26°C and 90°C and Young's modulus 2.3 times lower than those obtained with the bare fiber. The Young's modulus variation with the humidity is 1.5 MPa/%RH in a humidity range of 66-96%. In addition, the fused POF presented a variation of its dynamic modulus with the frequency increase four times lower than non-fused POFs on the range of 0.01-100.00 Hz. These results pave the way for future applications of fused POFs as sensing elements.

Gait Shear and Plantar Pressure Monitoring: A Non-Invasive OFS Based Solution for e-Health Architectures.

In an era of unprecedented progress in sensing technology and communication, health services are now able to closely monitor patients and elderly citizens without jeopardizing their daily routines through health applications on their mobile devices in what is known as e-Health. Within this field, we propose an optical fiber sensor (OFS) based system for the simultaneous monitoring of shear and plantar pressure during gait movement. These parameters are considered to be two key factors in gait analysis that can help in the early diagnosis of multiple anomalies, such as diabetic foot ulcerations or in physical rehabilitation scenarios. The proposed solution is a biaxial OFS based on two in-line fiber Bragg gratings (FBGs), which were inscribed in the same optical fiber and placed individually in two adjacent cavities, forming a small sensing cell. Such design presents a more compact and resilient solution with higher accuracy when compared to the existing electronic systems. The implementation of the proposed elements into an insole is also described, showcasing the compactness of the sensing cells, which can easily be integrated into a non-invasive mobile e-Health solution for continuous remote gait monitoring of patients and elder citizens. The reported results show that the proposed system outperforms existing solutions, in the sense that it is able to dynamically discriminate shear and plantar pressure during gait.

Incorporation of Fiber Bragg Sensors for Shape Memory Polyurethanes Characterization.

Shape memory polyurethanes (SMPUs) are thermally activated shape memory materials, which can be used as actuators or sensors in applications including aerospace, aeronautics, automobiles or the biomedical industry. The accurate characterization of the memory effect of these materials is therefore mandatory for the technology's success. The shape memory characterization is normally accomplished using mechanical testing coupled with a heat source, where a detailed knowledge of the heat cycle and its influence on the material properties is paramount but difficult to monitor. In this work, fiber Bragg grating (FBG) sensors were embedded into SMPU samples aiming to study and characterize its shape memory effect. The samples were obtained by injection molding, and the entire processing cycle was successfully monitored, providing a process global quality signature. Moreover, the integrity and functionality of the FBG sensors were maintained during and after the embedding process, demonstrating the feasibility of the technology chosen for the purpose envisaged. The results of the shape memory effect characterization demonstrate a good correlation between the reflected FBG peak with the temperature and induced strain, proving that this technology is suitable for this particular application.

Low-Cost Interrogation Technique for Dynamic Measurements with FBG-Based Devices.

Fiber Bragg gratings are widely used optical fiber sensors for measuring temperature and/or mechanical strain. Nevertheless, the high cost of the interrogation systems is the most important drawback for their large commercial application. In this work, an in-line Fabry-Perot interferometer based edge filter is explored in the interrogation of fiber Bragg grating dynamic measurements up to 5 kHz. Two devices an accelerometer and an arterial pulse wave probe were interrogated with the developed approach and the results were compared with a commercial interrogation monitor. The data obtained with the edge filter are in agreement with the commercial device, with a maximum RMSE of 0.05 being able to meet the requirements of the measurements. Resolutions of 3.6 pm and 2.4 pm were obtained, using the optical accelerometer and the arterial pulse wave probe, respectively.

Insole optical fiber Bragg grating sensors network for dynamic vertical force monitoring.

In an era of unprecedented progress in technology and increase in population age, continuous and close monitoring of elder citizens and patients is becoming more of a necessity than a luxury. Contributing toward this field and enhancing the life quality of elder citizens and patients with disabilities, this work presents the design and implementation of a noninvasive platform and insole fiber Bragg grating sensors network to monitor the vertical ground reaction forces distribution induced in the foot plantar surface during gait and body center of mass displacements. The acquired measurements are a reliable indication of the accuracy and consistency of the proposed solution in monitoring and mapping the vertical forces active on the foot plantar sole, with a sensitivity up to 11.06 ?? pm / N . The acquired measurements can be used to infer the foot structure and health condition, in addition to anomalies related to spine function and other pathologies (e.g., related to diabetes); also its application in rehabilitation robotics field can dramatically reduce the computational burden of exoskeletons' control strategy. The proposed technology has the advantages of optical fiber sensing (robustness, noninvasiveness, accuracy, and electromagnetic insensitivity) to surpass all drawbacks verified in traditionally used sensing systems (fragility, instability, and inconsistent feedback).

Diamond-coated fiber Bragg grating through the hot filament chemical vapor process for chemical durability improvement.

In recent years, the coating of fiber Bragg gratings (FBGs) with a specific material has opened up the possibility to broaden the limits of applicability of this technology. Diamond has a set of properties that makes it an attractive candidate to protect the optical fiber against chemically harsh environments, whose sensing is also a great challenge. One of the most used techniques to obtain these coatings is through the hot filament chemical vapor deposition (HFCVD); in this process, the temperature reaches, typically, around 850°C-900°C. In this work, the regeneration of a seed FBG during its coating with a nanocrystalline diamond thin film through the HFCVD process is presented. Simultaneously, the thermal monitoring of the process was also performed using the same grating. The resistance test in a corrosive medium reveals an improvement on the durability of the sensing properties of the diamond-coated FBG compared with an uncoated FBG, foreseeing a vast range of applications.

Internal and External Temperature Monitoring of a Li-Ion Battery with Fiber Bragg Grating Sensors.

The integration of fiber Bragg grating (FBG) sensors in lithium-ion cells for in-situ and in-operando temperature monitoring is presented herein. The measuring of internal and external temperature variations was performed through four FBG sensors during galvanostatic cycling at C-rates ranging from 1C to 8C. The FBG sensors were placed both outside and inside the cell, located in the center of the electrochemically active area and at the tab-electrode connection. The internal sensors recorded temperature variations of 4.0 ± 0.1 °C at 5C and 4.7 ± 0.1 °C at 8C at the center of the active area, and 3.9 ± 0.1 °C at 5C and 4.0 ± 0.1 °C at 8C at the tab-electrode connection, respectively. This study is intended to contribute to detection of a temperature gradient in real time inside a cell, which can determine possible damage in the battery performance when it operates under normal and abnormal operating conditions, as well as to demonstrate the technical feasibility of the integration of in-operando microsensors inside Li-ion cells.

Cuspal Displacement Induced by Bulk Fill Resin Composite Polymerization: Biomechanical Evaluation Using Fiber Bragg Grating Sensors.

Polymerization shrinkage is a major concern to the clinical success of direct composite resin restorations. The aim of this study was to compare the effect of polymerization shrinkage strain of two resin composites on cuspal movement based on the use of fiber Bragg grating (FBG) sensors. Twenty standardized Class II cavities prepared in upper third molars were allocated into two groups (n = 10). Restorations involved the bulk fill placement of conventional microhybrid resin composite (Esthet•X® HD, Dentsply DeTrey) (Group 1) or flowable "low-shrinkage" resin composite (SDR™, Dentsply DeTrey) (Group 2). Two FBG sensors were used per restoration for real-time measurement of cuspal linear deformation and temperature variation. Group comparisons were determined using ANCOVA (α = 0.05) considering temperature as the covariate. A statistically significant correlation between cuspal deflection, time, and material was observed (p < 0.01). Cuspal deflection reached 8.8 µm (0.23%) and 7.8 µm (0.20%) in Groups 1 and 2, respectively. When used with bulk fill technique, flowable resin composite SDR™ induced significantly less cuspal deflection than the conventional resin composite Esthet•X® HD (p = 0.015) and presented a smoother curve slope during the polymerization. FBG sensors appear to be a valid tool for accurate real-time monitoring of cuspal deformation.

Feasibility studies of Bragg probe for noninvasive carotid pulse waveform assessment.

The arterial stiffness evaluation is largely reported as an independent predictor of cardiovascular diseases. The central pulse waveform can provide important data about arterial health and has been studied in patients with several pathologies, such as diabetes mellitus, coronary artery disease and hypertension. The implementation and feasibility studies of a fiber Bragg grating probe for noninvasive monitoring of the carotid pulse are described based on fiber Bragg grating technology. Assessment tests were carried out in carotids of different volunteers and it was possible to detect the carotid pulse waveform in all subjects. In one of the subjects, the sensor was also tested in terms of repeatability. Although further tests will be required for clinical investigation, the first studies suggest that the developed sensor can be a valid alternative to electromechanical tonometers.