Negative curvature hollow core fiber sensor for the measurement of strain and temperature.

Three different types of strain and temperature sensors based on negative curvature hollow core fiber (NCHCF) are proposed. Each sensor is produced by splicing a small section of the NCHCF between two sections of single mode fiber. Different types of interferometers are obtained simply by changing the splicing conditions. The first sensor consists on a single Fabry-Perot interferometer (FPI). The remaining two configurations are attained with the same sensing structure, depending on its position in relation to the interrogation setup. Thus, a double FPI or a hybrid sensor, the latter being composed by an FPI and a Michelson interferometer, are formed. The inline sensors are of submillimeter size, thus enabling nearly punctual measurements.

Embedded Fiber Sensors to Monitor Temperature and Strain of Polymeric Parts Fabricated by Additive Manufacturing and Reinforced with NiTi Wires.

This paper focuses on three main issues regarding Material Extrusion (MEX) Additive Manufacturing (AM) of thermoplastic composites reinforced by pre-functionalized continuous Nickel-Titanium (NiTi) wires: (i) Evaluation of the effect of the MEX process on the properties of the pre-functionalized NiTi, (ii) evaluation of the mechanical and thermal behavior of the composite material during usage, (iii) the inspection of the parts by Non-Destructive Testing (NDT). For this purpose, an optical fiber sensing network, based on fiber Bragg grating and a cascaded optical fiber sensor, was successfully embedded during the 3D printing of a polylactic acid (PLA) matrix reinforced by NiTi wires. Thermal and mechanical perturbations were successfully registered as a consequence of thermal and mechanical stimuli. During a heating/cooling cycle, a maximum contraction of ≈100 µm was detected by the cascaded sensor in the PLA material at the end of the heating step (induced by Joule effect) of NiTi wires and a thermal perturbation associated with the structural transformation of austenite to R-phase was observed during the natural cooling step, near 33.0 °C. Regarding tensile cycling tests, higher increases in temperature arose when the applied force ranged between 0.7 and 1.1 kN, reaching a maximum temperature variation of 9.5 ± 0.1 °C. During the unload step, a slope change in the temperature behavior was detected, which is associated with the material transformation of the NiTi wire (martensite to austenite). The embedded optical sensing methodology presented here proved to be an effective and precise tool to identify structural transformations regarding the specific application as a Non-Destructive Testing for AM.

Polymer optical fiber Bragg grating inscription with a single Nd:YAG laser pulse.

We experimentally demonstrate the first polymer optical fiber Bragg gratings inscribed with only one Nd:YAG laser (266 nm) pulse. The gratings have been inscribed in a single-mode poly (methyl methacrylate) optical fiber, with a core doped with benzyl dimethyl ketal for photosensitivity enhancement. One laser pulse with a duration of 8 ns and energy of 72 µJ is adequate to introduce a refractive index change of 0.5 × 10-4 in the fiber core. The stability of the gratings has been confirmed and the strain and temperature sensitivity measurements demonstrate their tunable properties.

Optical sensors based on plastic fibers.

The recent advances of polymer technology allowed the introduction of plastic optical fiber in sensor design. The advantages of optical metrology with plastic optical fiber have attracted the attention of the scientific community, as they allow the development of low-cost or cost competitive systems compared with conventional technologies. In this paper, the current state of the art of plastic optical fiber technology will be reviewed, namely its main characteristics and sensing advantages. Several measurement techniques will be described, with a strong focus on interrogation approaches based on intensity variation in transmission and reflection. The potential applications involving structural health monitoring, medicine, environment and the biological and chemical area are also presented.

Cortisol AuPd plasmonic unclad POF biosensor.

This paper presents the development and feasibility tests of a cortisol immunosensor. The sensor is based on surface plasmon resonance (SPR) using an unclad plastic optical fiber (POF) in which the SPR is used as sensitivity enhancer, promoted by a gold/palladium (AuPd) alloy coating. The AuPd coated fibers were functionalized with an anti-cortisol antibody and passivated with bovine serum albumin (BSA) to be tested in the presence of cortisol as target analyte. The antibody-antigen binding reaction caused a variation of the refractive index on the surface of the AuPd coating, which leads to a shift of the SPR signature wavelength. The sensor was tested for different cortisol concentrations, ranging from 0.005 to 10 ng/mL. The reported biosensor presented a total wavelength shift of 15 nm for the testing range, putting in evidence a high sensitivity. Control tests for selectivity assessment were also performed. Concentrations as high as 10 ng/mL of cortisol, in a sensor functionalized with anti-hCG antibodies, only resulted in 1 nm variation of the resonance wavelength, 15 times lower than the one functionalized with the anti-cortisol antibodies, which indicates a high selectivity for the proposed approach. For this sensing approach the limit of detection (LOD) was determined to be 1 pg/mL. The proposed SPR based POF sensor has a low-cost interrogation method, high sensitivity and low LOD, straightforward signal processing and find important applications in different biological fields.

Clinical evaluation of an optical fiber-based probe for the assessment of central arterial pulse waves.

The aim of this study, enrolling 118 patients, was to clinically evaluate the accuracy of carotid pulse waveform acquisition with a new non-invasive optical fiber probe using invasive and non-invasive pressure readings as references. Pulse waves were acquired simultaneously in the ascending aorta and right common carotid; for the non-invasive study, the pulses were sequentially acquired using the optical fiber device and the Complior Analyse® (Alam Medical, France) device in the right carotid artery. For all subjects, the pulse waveforms assessed using the optical fiber sensor and the references were superimposed to analyze the deviation and point-by-point correlation. Augmentation index and central pressure were compared using intraclass correlation and Bland-Altman analyses with a confidence interval of 95%. For the invasive study, the acquired waves presented a mean deviation of 11 ± 3% and a mean intraclass correlation of 0.97 ± 0.02. Concerning the augmentation index and central systolic pressure, correlations of 0.79 (p < 0.001) and 0.94 (p < 0.001) were found, respectively. In the non-invasive comparison, the assessed mean deviation between the morphologies of the waves was 13 ± 5%, with correlation coefficients of 0.91 (p < 0.001) for the augmentation index and 0.98 (p < 0.001) for central systolic pressure. The results show that the optical fiber probe results were highly correlated with those obtained using the reference techniques in terms of the pulse waveforms, central systolic pressure (cSP), and augmentation index assessment.

Central arterial pulse waveform acquisition with a portable pen-like optical fiber sensor.

OBJECTIVE: Pulse waveform features related to cardiovascular pathologies and arterial stiffness have been extensively studied, and optical fiber sensors have been studied with an aim to simplify the pulse waveform acquisition in the carotid artery. In this paper, a novel optical fiber sensor to record pulse waveform in the carotid artery has been proposed. METHODS: The pulse waveform optical fiber sensor design, based on fiber Bragg gratings, is presented. The probe was characterized, and its response to controlled waveforms was studied. Finally, tests were performed on human subjects. RESULTS: The developed sensor has a displacement sensitivity of 21.2 pm/µm, with ability to detect the carotid pulse wave in the neck surface, with a resolution of 1.3 mmHg. CONCLUSION: This study revealed a new technological approach for acquisition of the central pulse waveform.

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.

Characterization of different water/powder ratios of dental gypsum using fiber Bragg grating sensors.

The impact of five different water/powder (w/p) ratios in the characterization of high strength dental stone was evaluated, since the recommendations of the gypsum' manufacturers are not always correctly followed by the dental prosthesis technicians. Fiber Bragg grating (FBG) sensors were used to measure the setting expansion and temperature variation which occurred during the setting reaction for each w/p ratio, as well as the thermal expansion coefficient. Thick mixtures with low w/p ratios had more crystals impinging upon each other during crystal growth, resulting in more expansion and more heat released. This thermal behavior was only achieved to w/p ratios within the manufacturer-recommended mixing ratio range. The results also revealed the existence of boundary condition; this corresponding to the limit of the mixing ratio recommended by the gypsum' manufacturer. Data provided in this study are particularly important for dental technicians with a view to attaining the best results in accuracy of fit for their prosthetic works.

In the trail of a new bio-sensor for measuring strain in bone: osteoblastic biocompatibility.

Fibre Bragg Grating (FBG) is an optical sensor recorded within the core of a standard optical fibre, which responds faithfully to strain and temperature. FBG sensors are a promising alternative to other sensing methodologies to assess bone mechanics in vivo. However, response of bone cells/bone tissue to FBGs and its sensing capability in this environment have not been recorded yet. The present study addressed these issues in long-term human osteoblastic cell cultures. Results showed that osteoblastic cells were able to adhere and proliferate over the fibre and, also, the protective polymer coating. RT-PCR analysis showed the expression of Col I, ALP, BMP-2, M-CSF, RANKL and OPG. In addition, cultures presented high ALP activity and the formation of a calcium phosphate mineralized extracellular matrix. Cell behavior over the fibre without and with the coating polymer was similar to that found in cultures grown in standard tissue culture plates (control). In addition to the excellent osteoblastic cytocompatibility, FBGs maintained the physical integrity and functionality, as its sensing capability was not affected through the culture period. Results suggest the possibility of in vivo osseointegration of the optical fibre/FBGs anticipating a variety of applications in bone mechanical dynamics.