The development of stretchable fiber optic sensors promotes changing the operation of soft robots and smart systems. Recently, a team of scientists from the U.S. has developed a distributed fiber sensor that uses low-cost LEDs and dyes. The developed fiber optic sensors are stretchable and allow for finding different deformations (strain, pressure, bending).
To be more precise, these distributed sensors can be applied in soft robotic systems and augmented reality technology. The fiber optic sensors offer the opportunity to feel the same tactile sensations as animals use to navigate the natural world. The ability to determine deformations(for instance, strain, pressure, and bending) makes it possible to find exact locations.
The scientists claim that such a fiber optic technology will be useful for applications in sports medicine and physical therapy. The development of fiber sensors is based on previous technology, “in which light was sent through an optical waveguide, and a photodiode detected changes in the beam’s intensity to determine when the material was deformed.” Then numerous similar sensing systems have been created.
It should be noted that the operating principle is based on silica-based distributed fiber sensors. The distributed fiber optic sensors allow for recording the slightest wavelength shifts to determine, for example, changes in humidity, temperature, and strain. Nevertheless, silica optical fibers are not efficient with soft and stretchable devices.
Herewith, it is easy to deform soft materials in difficult-to-reach areas. Therefore, scientists develop a fiber sensor that can overcome these challenges. The new fiber optic system consists of a stretchable light guide for multimodal sensing. The two cores of optical fiber are made of polyurethane elastomeric. The first core is transparent, the second has dyed.
A pair of cores in distributed fiber optic sensors increase the number of outputs. They, in turn, help to find various deformations, their exact location, and magnitudes. Compared to standard distributed sensors, new sensing systems do not need high-resolution detection equipment.
Stretchable fiber optic sensors apply small optoelectronics with a lower resolution. Thus, they have a lower cost, simple manufacturing, and can be easily installed into small systems. Moreover, it is possible to wear this fiber optic technology. For example, scientists have installed these fiber sensors in a 3D-printed glove.
Finally, this fiber optic technology allows scientists to measure tactile interactions in real life. Additionally, the team finds the way how distributed fiber optic sensors can advances virtual and augmented reality experiences. The thing is that these systems’ operation is based on motion capture, and fiber sensors can add new experiences.
Recently a team of scientists from an American research center has presented a fiber optic technology used to create a battery management system applying embedded fiber optic sensors and machine learning. The thing is that the combination of fiber sensors and machine learning allows for developing more efficient and low-cost designs of smart battery charge management systems.
The important fact is that scientists pay careful attention to lithium-ion battery packs applied in hybrid and electric devices. Therefore, these fiber optic sensors allow for monitoring cell degradation and health data and predicting remaining battery life. Moreover, the fiber sensors have been already tested and demonstrated high precision across different use-cases at both the cell and module levels.
The operating principle of such a fiber optic system is based on the wavelength-shift detection technology that “measures the signals from fiber sensors installed within the active chemistry of the battery.” The fiber optic system has a resolution of 30fm and the KHz speed leading to obtaining the data about the charge’s state and failure warning.
Additionally, a read-out device records these data. Then machine learning algorithms evaluate it to offer real-time performance management of battery charge. Recently, the tests of fiber optic sensors have been finished at the module level of electric car batteries after tests on individual lithium-ion cells. According to the test results, these fiber sensors enable to decrease in the form factor of hybrid and electric car batteries by more than 25% leading to low-cost manufacturing at the same energy density.
Nevertheless, the team plans to perform additional testing and research to learn how to scale the fiber optic technology for larger batteries. Thus, scientists want to use original equipment manufacturers to examine the fiber optic system installed in hybrid and electric vehicles and to open new applications of fiber optic sensors for various energy and structural systems.
The thing is that the developed fiber sensors are regarded as a part of the energy technology program that aims at developing clean and abundant energy for a wide range of applications. Finally, the team paid attention to chemical energy storage for hybrid and electric systems, consumer electronics, and the electrical grid; advanced energy conversion devices; wireless fiber sensors; and advanced analytics before the novel fiber optic technology to increase energy utilization.
A team of scientists has produced novel compact fiber optic sensors that allow for detecting even slight force parameters affected by tiny objects. To be more precise, such a fiber sensor is ahead of standard force sensors based on micro-electro-mechanical technology.
The applications of new fiber optic sensors involve various fields beginning from medical devices to manufacturing. Even though such applications are numerous, however, the problem of compact and versatile sensing systems that enable to detect force om small objects yet remains.
This fiber sensor is considered to be the smallest one and most versatile force sensing system that meets all the requirements. The design of the fiber optic sensor includes silica-glass material of cylinder form with 800 microns long and 100 microns in diameter (similar to the diameter of a human hair).
Herewith, fiber optic technology has been already tested and allowed to detect “the force with a resolution better than a micronewton by applying it to measure the stiffness of a dandelion seed or the surface tension of a liquid.” Such benefits of fiber sensors as sensing of high-resolution and wide range are regarded as very useful for sensitive manipulation and machining of tiny objects, surface tension measurements, etc.
When it comes to micro-electro-mechanical technology, these sensing systems offer miniature force sensing capabilities but their applications are very limited because there are requirements in specific protective coatings and numerous electrical connections. It should be noted that the absence of a suitable coating leads to the absence of biocompatibility and such sensors can not be employed in water.
The new fiber optic sensors are made of glass. The scientists use a special etching process developed by them to produce complex optical fiber microstructures. The micromachining technique results in the appearance of a fiber sensor based on a Fabry-Perot interferometer.
Additionally, the coating and an air-sealed cavity of novel fiber sensors make them suitable for application in biochemical environmental conditions. It is possible to put these fiber optic sensors into different liquids, herewith, they detect positive and negative forces and does not require any additional coatings for numerous applications.
Finally, the fiber sensor measures the stiffness of a human hair and common dandelion seed. Moreover, such sensing systems measure the retraction force with a resolution of about 0.6 micronewtons and a range of about 0.6 millinewtons. Herewith, this tiny sensors can be a part of a complex fiber optic sensors, for instance, sensors that detect magnetic and electric fields.
A dramatic increase in the application of composite materials, involving aerospace technology, as well as other fields that need high reliability of structures, for instance, oil production, building industry, etc., makes the task of structural health monitoring very important.
One of the most promising techniques is considered to be fiber optic sensors as an integral part of the monitoring system. Fiber optic sensors have several essential benefits over conventional ones. Compared to standard sensing systems, fiber sensors offer:
– a small mass;
– a high level of sensitivity;
– an electromagnetic compatibility;
– the ability to be combined in a network, as well as multiplexing;
– the compatibility with other systems for structural health monitoring;
– the ability to measure a variety of quantities.
A great benefit provided by fiber sensors also includes the ability to measure a variety of parameters such as deformation, pressure and force, electric and magnetic fields, sound and vibration, pH and viscosity, the presence of molecules, bacteria, and so on.
The sensitive element of a point fiber sensor is a fiber Bragg grating (FBG). The fiber Bragg grating reflects emission with a certain wavelength and it is transparent to other wavelengths. This selective reflection is achieved by writing a periodic structure in the optical fiber core. The reflected signal is registered by the receiving device.
FBG sensors do not have electronic components, i.e. they are passive. Thus, this property of FBG sensors opens up a wide range of possibilities for the application of such sensors in hazardous areas and areas with strong electromagnetic interference.
Herewith, multiple fiber Bragg gratings can be created on a single optical fiber. Each of the FBGs will reflect laser beam emission at its wavelength. This allows for producing a distributed monitoring system with wavelength multiplexing. The use of fiber optic sensors for structural health monitoring is regarded as very promising.
The widespread use of composite materials in the aviation industry, as well as the creation and application in the future of “smart” materials and structures even with adaptive properties, will require continuous improvement of the structural health monitoring systems. Promising integrated sensing systems installed in optical fibers will significantly transform the way FBG sensors are employed in aviation materials and structures.
New fiber optic solutions provide high-definition and fast fiber optic systems for sensing that become very promising in several space applications. Such advantages as composites in design and lightweight materials make fiber optic sensors highly important for the aerospace industry leading to a great requirement in non-destructive testing.
To be more precise, the development of advanced materials results in the fact that fiber optic sensors are considered to be essential in the design, production of aerospace vehicles, as well as their non-destructive testing. Fiber sensors are regarded as systems with flexible, low-profile optical fibers that do not need electrical sources.
It is possible to use fiber optic systems at sharply curved areas installed within devices or mounted directly to electrical components. Even though fiber optic sensors are compact and lightweight, they allow for distributed sensing directly stress, strain, acoustic, or temperature.
Compared to conventional strain gauges, fiber optic applications offer critical information with high density and low additional cost for various measurement points. A company-manufacturer of fiber optic systems from the U.S. has presented fiber sensors for aerospace industries.
Such fiber optic sensors perform more than 1,000 strain or temperature sensing per meter of a conventional compact, lightweight sensors. Additionally, “the high definition data can fully map the contour of strain or temperature for a structure under test or during manufacturing.”
These fiber sensors are suitable for dynamic applications or where lower sensor density is needed offering high-speed multipoint sensing, with tens or hundreds of fiber optic sensors on versatile optical fibers that cover long distances. Herewith, they provide the opportunity to perform several measurement types, for example, strain, temperature, vibration, or displacement by a single optical fiber.
Fiber optic applications include the aerospace industry and offer a more detailed design validation at every stage of the structural integrity building block process. It should be noted that composites provide a high level of strength-to-weight ratios. Nevertheless, new devices for validating the performance of fiber sensors are needed for their unique properties.
The compact size and distributed sensing of fiber optic sensors enable in-situ characterization for coupon testing, curing process validation, components/module testing as well as full-scale structural health monitoring of complex structures. It is possible to apply these fiber sensors in the predictive maintenance of smart elements.
Researchers-manufacturers of fiber optic solutions from the U.S have presented new fiber Bragg grating sensors (FBG sensors) with copper and aluminum coating. Herewith, this fiber optic system has a compact size, it is hermetically sealed, and can maintain high temperatures leading to new opportunities for metal-coated fiber optic sensors.
To be more precise, FBG sensors and gold-coated sensors allow for developing new “inherently humidity-proof strain, temperature, displacement, acceleration, pressure, load, tilt, bio, and other useful fiber sensors and systems.” The researchers claim that FBG technology is considered to be very useful for numerous sensing applications in harsh environmental conditions because of its benefits provided.
The benefits of FBG sensors include the ability of absolute temperature measurement, rapid response, numerous sensing points on a single optical fiber strand with minimal mechanical burden and intrusion, as well as EMI immunity, spark-free, and chemical inertness.
Nonetheless, such conditions as a high level of humidity or temperatures, corrosive chemicals, or strong mechanical stress often presented in real environmental conditions create obstacles for fiber optic sensors with glass coating. New FBG sensors with copper, aluminum, and gold coatings enable researchers to enlarge current applications and develop new ones.
It should be noted that such processes as stripping and recoating are necessary for all laser writing methods included metals. The researchers demonstrate a robust technique to produce fiber sensors with acrylate, polyimide, aluminum, copper, and gold coatings installed into conventional high-temperature fiber Bragg gratings, which then are recoated with acrylate, polyimide, or gold coatings.
Thus, such FBG technology makes it possible to change lengths of window stripping and recoating as well as control material thickness and length. Different types of inscription and coating allow for employing FBG sensors in different conditions from the cryogenic temperature of -200℃ to the high temperatures of +1000℃.
These FBG sensors have a metal coating, and they are created by excimer and/or femtosecond laser writing methods. Additionally, the fiber optic system has been already tested, and the results show specific benefits in offering multipoint and multifunction sensing abilities in a constantly expanding range of applications not previously addressable by standard FBGs. The thing is that the coating of properly designed fiber optic sensors plays a crucial role in the integrity, survivability, functionality, and durability of FBG sensors.
A team of researchers presents a fiber optic technology based on fiber Bragg gratings (FBGs) for sensing to monitor the activity of an active volcano. The monitoring of volcanic activity plays a crucial role in better understanding and even prediction of important and potentially disruptive volcanic events, therefore, the fiber optic sensing system has to maintain harsh environmental conditions.
Nonetheless, the recording process of seismic activity now faces several difficulties concerning both discriminating between various sources of seismic wave, and the design of fiber optic sensing systems that can operate in active volcanic settings without any damages.
The team of researchers from France demonstrates the results obtained from the first high-resolution seismometer based on FBG sensors installed on an active volcano. It should be noted that the lifetime of modern fiber optic systems is quite short during their operation at high temperatures and the billowing, sulfurous, acidic gases near a fumarole.
Additionally, standard FBG sensors can fail in emergency deployment, or repair, even in pre‐eruptive phases. The operating principle of novel fiber optic sensing systems is based on interferometry forms that apply more sensitive fiber optic elements such as fiber Bragg grating resonators that enable to detect the acceleration of the ground as a change in the signal from the FBG sensor.
These fiber optic systems can be used for networking across long distances and monitoring these distance via optical fibers. The FBG sensor is considered to be “a purely optomechanical geophone that is interrogated through a 1.5-kilometer fiber optic cable by a remote, and thus it is a much safer fiber optic system down the volcano’s flank.”
Moreover, the fiber optic sensing system has been already tested and recorded tiny seismic events within the volcano for nine months. The development of new FBG sensors lasts almost a decade, the researchers use previous researches of a high-resolution optical seismometer prototype that includes a 3-kilometer fiber optic cable.
Finally, FBG sensors are regarded as highly reliable, fiber optic technology allows installing the sensors in locations that were not previously practical, providing more data about microseismic events under a volcano’s dome. The researchers claim that such fiber optic sensing systems offer more detailed information about “the fumarole signature, which helps to constrain the geometry and activity of the plumbing system of the dome”.
Fiber optic sensors of different physical quantities based on fiber Bragg gratings (FBGs) find its popularity and they are actively used in various fields of industry to solve a variety of engineering problems. The general operating principle of such fiber sensors is based on a change in the FBG wavelength under the action of external impacts. It should be noted that nowadays the population presents specific requirements to the application of assistive technology and, in particular, towards novel healthcare tools and fiber optic sensors. For instance, novel fiber sensors offer such benefits he electromagnetic field immunity, high flexibility, high sensitivity for mechanical parameters higher elastic limits, and impact resistance.
To be more precise, such benefits of fiber optic technology comply ideally with the instrumentation requirements of numerous healthcare tools and in movement analysis. Additionally, fiber optic sensors are considered to be lightweight, compact, stable to chemical substances, and they have also multiplexing capabilities. Therefore, fiber sensors are regarded as safe technology suitable for industrial, medical, and structural health monitoring applications.
The thing is that fiber optic sensors allow measuring various parameters, for example, angle, refractive index, temperature, humidity, acceleration, pressure, breathing rate, oxygen saturation, etc. It is even possible to install optical fibers in textiles for sensing applications, as well as incorporate them in composite metals, concrete and etc.
The development of fiber sensors based on fiber optic technology leads to high demands for healthcare systems, especially because of the population aging. The appearance of new medical systems results in higher demands on the fiber sensors’ performance because reliable control strategies require a robust fiber optic sensing system.
The modern fiber optic sensor should be tiny, herewith, saving its flexibility and compactness as possible. Herewith, intrusive sensing systems have higher requirements – they also need for biocompatibility, this is the reason why fiber optic technology continues developing to overcome the current challenges and provide high performance of novel healthcare systems and tools.
The increasing demands on fiber optic sensors and the fast development of the technology result in the appearance of numerous sensing systems for healthcare and medical tools. The fiber sensors enable to examine bones decalcification and strain distribution, evaluation of intervertebral disks, dental splints, cardiac monitoring, and pathologies detection.
Electrical sensing systems (strain sensors, string-based, potentiometric, etc.) have been the main method of measuring physical and mechanical phenomena for decades. Despite their widespread application, electric sensing systems have a number of disadvantages, such as loss of signal transmission, susceptibility to electromagnetic interference, the need to organize an intrinsically safe electrical circuit (if there is a danger of explosion).
These inherent limitations make electrical sensors unsuitable or difficult to use for a number of tasks. The application of fiber optic sensing solutions is an excellent way to overcome these problems. The signal in fiber optic sensors is light in the optical fiber used instead of electricity in the copper wire of standard electrical sensors.
Over the past twenty years, a huge number of innovations in optoelectronics and in the field of fiber optic telecommunications have led to a significant reduction in the price of fiber sensorcomponents and to a significant improvement in the quality of fiber optic systems. These improvements allow fiber optic sensors to move from the category of experimental laboratory devices to the category of widely used devices in such areas as monitoring of buildings and structures, etc.
The most widespread type of sensors
One of the most commonly used fiber optic sensors is considered to be fiber Bragg grating sensors (FBG). The fiber Bragg gratings in these sensors reflect a light signal whose spectral characteristic (wavelength) shifts along with changes in the measured parameter (temperature and/or deformation). During the manufacture of gratings, a region with a periodic change in the refractive index is created inside the optical fiber core, herewith, this region is directly called the FBG.
Optical fibersand fiber sensors are non-conductive, electrically passive, and immune to EM interference. The interrogation using a tunable high-power laser allows measurements to be made over long distances with virtually no signal loss. Additionally, in contrast to the electrical sensing system, each optical fiberchannel can interrogate a variety of FBG sensors, which significantly reduces the size and complexity of such afiber optic system.
Applications of fiber sensors
Fiber optic sensing solutions are ideal for applications where conventional electrical sensors (strain gauges, strings, thermoresistors, etc.) have proved difficult to use due to extreme conditions (long distances, EM fields, explosion protection, etc.). Since the installation and operation of fiber sensors are similar to conventional electrical sensors, it is easy to switch to fiber optic solutions. Understanding how such fiber optic systems work and the benefits of using them can greatly facilitate various measurement tasks (for example, structural health monitoring).
Benefits of FBG type
In short, the main advantages of FBG sensorsinclude:
high sensitivity and performance;
relatively large range of measured deformations;
the best weight and overall dimensions, small size;
high noise immunity, insensitivity to electromagnetic interference, such as microwave field, spark discharge, magnetic fields, electromagnetic pulses of various nature and any intensity;
absolute electrical safety due to the absence of electrical circuits between the fiber optic sensor and the recording module;
full electrical, explosion and fire safety, high chemical resistance of sensor elements.
Extreme environmental conditions
The conditions of the environment and controlled conditions in which one or more external factors — radiation, temperature, electromagnetic field, aggressiveness, humidity, pressure, and deformation — have the maximum possible constant values are regarded as extreme.
In such conditions, primary converters of control systems for dangerous technological processes (oil production, transportation, and processing of oil and gas, nuclear power generation, storage of radioactive waste), monitoring and diagnostics systems for complex construction and engineering structures (dams, bridges, mines, etc.), and military and emergency management systems operate.
Currently, fiber optic technologies are widely used in various fields of science and technology. One of the main applications of fiber optics is the creation of portable high-sensitivity sensors. Pressure, strain, vibration, tilt, linear motion, and temperature sensorsare widely applied in the industries of structural health monitoring pipelines, heating lines, power cables, mines, etc.
Application in radioactive conditions
Compared to fiber sensors, the lack of power supply at the location of electrical sensing systemsdoes not prevent continuous remote monitoring of dangerous objects, such as nuclear power plants, in an emergency beyond design situations. For instance, the well-known events at the Japanese nuclear power plant “Fukushima-1” in 2011 were characterized by the fact that during the two weeks when the nuclear power plant was completely de-energized, there was no information from electronic sensors, which was extremely important for monitoring the technical condition of the emergency station.
Application in extreme temperatures
Problems of standard sensing systems control of tightness of tanks with liquid hydrogen, which is the fuel of modern rocket engines, has a temperature of -253 °C and very high fluidity, due to the fact that at such temperatures, most materials become very fragile, and the sensitivity of palladium sensors quickly decreases.
It is problematic to measure the pressure and dryness of superheated steam in gas generators and superheated gas in jet engine nozzles at temperatures up to + 600 °C since piezoelectric sensors quickly degrade at temperatures above + 300 °C. Modern FBG sensorsof physical quantities are heat-resistant (up to +2300 °C) and cold-resistant (up to -270 °C). This provides reliable and long-term monitoring of the technical condition of high-temperature and cryogenic objects.
Operation during electromagnetic interference
Measurements of physical quantities using electrical sensing systems in conditions of high-power electromagnetic interference, including guidance on coaxial electrical cables and sensors from lightning discharges, in conditions of monitoring the patient’s pulse in a medical nuclear magnetic resonance facility, as well as measurements of high voltages and high currents in electrical engineering, are highly problematic.
Fiber Bragg grating sensors are completely immune to electromagnetic interference and are stable insulators. This makes it possible to measure high voltages up to 800 kV and high currents up to 200 kA with high accuracy (class 02s) by fiber optic sensing technology.
Application in an aggressive environment
Measurements of physical quantities of chemically aggressive media, long — term measurements of deformation of dynamically loaded objects and structures, as well as multi-sensor measurements-with the number of control points in several hundred and thousands, are also problematic for electrical sensing systems since the volume of measuring electrical cables is unacceptably increasing.
Distributed fiber optic sensorsare multi-sensors: up to 10 thousand consecutive intra-fiber sensors can be used in one optical fiber (fiber optic cable) to measure physical quantities (temperature, strain, seismoacoustics, pressure, radiation, etc.). Multimode fiber optic cablesallow performing remote measurements with high accuracy using borehole video cameras, and temperature fields — using pyrometers and thermal imagers.
Advantages of metrological calibration
A serious problem of electrical sensing systems embedded in objects (in the concrete of hydraulic dams and bridges, in the pylons and walls of high-rise buildings, etc.) presents the practical difficulty of their periodic calibration (metrological verification).
Modernfiber sensorshave the function of metrological self-monitoring (FMSM) due to the multimodality of the optical signal, which allows for self-calibration of fiber optic sensors in real-time without stopping the controlled processes and without verification standards.
Today’s situation
In the last decade, there were implemented many similar applications of modern fiber sensorsand systems in extreme environments of nuclear, oil and gas, and aerospace industries, shipbuilding, hydraulic engineering, energy, construction, military, and natural emergencies.
Moreover, the durability of FBG sensors in these extreme conditions creates an obvious advantage of their use in the energy, oil and gas, aerospace, construction, and transport industries in comparison with non-optical types of measuring systems.
Thus, the extreme operating conditions of fiber Bragg grating sensors, for example in wells (extreme parameters, flammable, aggressive and abrasive environments) or power plants (ultra-high currents and discharges, voltages and fields, significant ionizing radiation), actually belong to the usual operating conditions of fiber optic sensors.
Nowadays fiber Bragg gratings are actively applied in the aerospace industry. The thing is that fiber optic multiplexing abilities of sensors based on FBG technology allow performing structural health monitoring of airborne vehicles resulting in an increase of their lifetime. Thus, fiber Bragg grating sensors play a crucial role in the spacecraft industry where mistakes and damage can lead to death.
It should be noted that fiber Bragg gratings are considered to be a thin optical fiberdevice that includes a physical “grating” area at its core. Herewith, the FBG core is not homogeneous, and the fiber optic sensor has a periodic variation in the refractive index of the material. Also, there is a dependency between the wavelength of light (reflected vs transmitted) and the periodic spacing of the grating.
FBG sensors can block specific wavelengths and transmit others like in laser cavities during the mode choice. Additionally, such factors as pressure and strain also influence the qualities of FBGs and the wavelengths resulting in stretching or compressing the grating period while temperature leads to thermo-optic effects. These and some other effects (for instance, vibration and displacement) promote the application of fiber Bragg grating sensors to monitor various physical effects.
FBG sensors enable to determine ultrasonic and acoustic wave signals that are important in structural health monitoring of aerospace vehicles. For instance, acoustic-ultrasonic determination provided FBG technology helps to find out damage when the spacecraft is not mobile.
The detection offered by fiber optic sensors is regarded as highly accurate and quantitative because it is possible to monitor both the form function of the waves and the repetition of measurements. Nevertheless, the resolution and the bandwidth limitation of conventional tools employed with fiber Bragg gratings (for example, optical spectrum analyzers) do not enable accuracy in high-frequency determination.
The fact is that accurate determination of ultrasonic waves requires a demodulation method to interpret the detected signals. Four demodulation methods are distinguished in FBG technology both in practice and in laboratory testing: “a broadband light source (power detection), laser light source (edge-filter detection), Erbium-Doped Fiber Laser (EDFL), and modulated lasers.” Moreover, it is necessary to pay careful attention to the installation technique of the FBGs.
Finally, specialists apply several various techniques to employ fiber Bragg grating sensors into a vehicle or craft. The fiber optic sensors have been already tested at their installation into composite materials (inside of a fiber honeycomb sandwiches.) However, the technique can cause signal distortion, that is why an ideal wayfor spacecraft is gluing fiber Bragg gratings on with some adhesive.
The development of stretchable fiber optic sensors promotes changing the operation of soft robots and smart systems. Recently, a team of scientists from the U.S. has developed a distributed fiber sensor that uses low-cost LEDs and dyes. The developed fiber optic sensors are stretchable and allow for finding different deformations (strain, pressure, bending).
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