Recent advancements of fiber Bragg grating sensors in biomedical application: a review

Abstract

Due to attractive application in the medical field, fiber Bragg grating sensor has become increasing attractive from past few decades for various strain sensing applications. FBG sensor has been used in many applications such as different surgical devices, vital sign detection devices, invasive surgery, heart rate, dental applications and biosensing application as wearable sensing devices. This paper reviews the 55 recent research articles published on fiber Bragg grating sensor for biomedical application used the qualitative, quantitative and experimental method to identify the recent advancement and challenges. In this study, particular focus is placed on applications for biomechanical devices, temperature monitors, respiratory monitors, and biosensing applications. Critical things, demands, and emerging trends for these sensing devices are also discussed in order to determine what will be needed for the next generation.

Introduction to FBG sensor and working principle

Development of optical fiber Bragg grating technology has contributed to the development of intensification of studies and applications of FBG sensor for different medical applications. The development of optical fiber has opened up new possibilities for improving patient care, health outcomes, and quality of life. FBG sensors are employed as biosensors in addition to strain sensors. The sensor is also used to identify human breast cancer in its early stages. Investigations are made on the sensing system's sensitivity, Q factor, and transmission spectrum. Fiber Bragg grating sensors are integrated with disposable temperature sensors and intra-aortic Cather sensors. Different experimental conditions are set to analyze the performance of FBG sensors [1]. The basic working principle of FBG sensors are as follows.

When the fiber is stretched and compressed, FBG sensor will measure strain. Due to the mechanical deformation of the fiber, period of the fiber Bragg grating will change. Bragg wavelength is essentially defined by the period and refractive index of core (n_ef).

Strain dependence of fiber Bragg grating is obtained by differentiating wavelength

$$\frac{\mathrm{\Delta }\lambda }{{\lambda }_0}=\frac{\mathrm{\Delta }(n_{\mathit{ef}}\mathrm{\Lambda })}{n_{\mathit{ef}}\mathrm{\Lambda }}=\left(1+\frac{1}{n_{\mathit{ef}}}\frac{\partial n_{\mathit{ef}}}{\partial \epsilon }\right)\mathrm{\Delta }\epsilon =(1+p_e)\mathrm{\Delta }\epsilon \leftrightarrow .$$ 1

Temperature depended FBG sensor can be obtained by differentiating wavelength

$$\frac{\mathrm{\Delta }\lambda }{{\lambda }_0}=\frac{\mathrm{\Delta }(n_{\mathit{ef}}\mathrm{\Lambda })}{n_{\mathit{ef}}\mathrm{\Lambda }}=\left(\frac{1}{\mathrm{\Lambda }}\frac{\mathrm{\Lambda }}{\partial T}+\frac{1}{n_{\mathit{ef}}}\frac{\partial n_{\mathit{ef}}}{\partial T}\right)\mathrm{\Delta }T=(\alpha +\varsigma )\mathrm{\Delta }T.$$ 2

Begin a fiber optic sensor, fiber Bragg rating sensor has all the advantages such as immunity to electromagnetic interferences, compact in size and weight [1].

Different application of FBG sensor

FBG sensor for biomechanical sensing applications

Roriz et al. proposed a model to measure stress caused by stretching and elongation of a tendon also presented a model of a fiber optic-based buckled transducer to quantify it. This device is in E-shape and has central arm consisting of FBG sensors. These tendons change themselves to E-form when they are elongated central arm shifts to the Brag wavelength which is proportional to strain. This model also can act as conventional strain gauge buckle transducer [1]. Ren et al. discussed the study of biomechanical science has a method to measure the strain in tendons and ligaments, and also the unique and appropriate way of measuring this strain is through the use of FBG sensors. This is most friendly accurate and easy way of measuring the strain developed in tendons. This paper also published the new displacement sensor model which again based on fiber Bragg grating and shape memory alloy technology he also compared the FBG sensors against the displacement sensors and finally came to the conclusion that fiber brag sensors are highly accurate, more friendly and also cheaper method of measuring the strain in tendons [2]. Mishra et al. discussed that the drawbacks of the currently available technique to measure the strain in tissue that they cannot measure transverse compression and impractically large response times. So, in order to know the sudden response of the tissue such as spinal cord when they are stressed suddenly and also to measure spinal compression and the fast biomechanical processes involved. He developed a model, an optical pressure sensing scheme which again uses a fiber Bragg grating, and a narrow-band filter was designed to detect and demonstrate the transverse compression inside a spinal cord. In this we can also understand the sudden strain in internal soft tissue with high spatiotemporal resolution [3]. Conventional FBG sensing and calibration setup carried with FBG interrogator is shown in Fig. 1.

Fig. 1.

a Schematic representation of the longitudinal mid-cross-section of the instrumented spinal cord surrogate with the integrated fiber Bragg grating (FBG) sensor. b Transverse view of the mechanical and optical setup [3]

Socorro-Leranoz et al.’s paper discusses the use of parallel FBG sensors to sense the movement on wrist and fingers of hand. In order to detect this, it has reflective configuration made up of FBG’s. Fibers were embedded in polydimethylsiloxane for protection and to give flexibility to the optical detection setup, which has sensitivities up to 1.29 pm/με in strain and 64.23 pm/° in angle and came to the conclusion of single sensors which can monitor the movement of hand of the people who suffered from stroke [4]. Vakiel et al. discussed in this paper about the initiation and progression of the diseases such as Osteoarthritis, which was caused by the transfer of stress across the articular joints. This paper also discussed that management of stress in the menisci by FBG sensors which they inserted in menisci and measured the stress applied during various situation and also the development of fiber optic technology helped them to observe the measurement of stress in menisci [5]. Al-Mai et al. discuss the various characters of various modes multi-axis, compliant force and torque sensors based on fiber-optic sensing technology and introduced a new model called as nonlinear Hammerstein–Weiner model (NLHW) which has the special features to differentiate the nonlinear and linear behavior of prototypes. This model reduces the 40% of the root mean square error. This model was more accurate to measure the full ground reaction force and moment in real time with minimal gait disturbance. This model is so accurate, cheaper compared to other, and also flexible it can measure the large range of force and torque. This method has two estimation models which works as single calibrated system Least Square decision Trees to improve the sensors portability as well as sensors biocompatibility. There are so many advantages of this sensors that were discussed in this paper [6]. Kalinowski et al. discussed majorly how the bone fracture related problems can be analyzed using the FBG sensors; it talks about the high sensitivity of the FB sensors. The conventional method used in treatment of bone fracture involves the radiography which is very harmful and also requires a lot of clinical experience of analysis. Therefore, there was a need to develop another method of healing of the bone fracture; the author has used FBG sensors for measuring static and dynamic strain in an external fixation device. The distortion is reduced by 95% when four pins are fixed above the fracture, but the deformation is increased by about 112% when four pins are fixed above the fracture site. Through frequency analysis of FBG sensors, we can only measure this, according to Kalinowski et al. [7]. In this study, Tavares et al. discussed wheelchair pressure ulcers and designed a wheelchair. Based on FBG sensing technologies, pressure ulcers were created. Six FBG were used to the wheelchair monitor the bone areas namely scapula (right (SR) and left (SL)), ischiatic zone (right (IR) and left (IL)), and heels (right (HR) and left (HL)). The sensing technology was tested by female user to test its effectiveness. This resulted in a more compatible and more effective solution in preventing the wheelchair pressure sensors; this uses user detection software to alert the patient when the position has to be adapted to relieve the pressure and prevents the pressure ulcers [8]. Figure 2 shows the FBG embedded wheelchair.

Fig. 2.

FBG sensor mounted wheelchair [8]

Mec et al. discussed in the paper about the comparison change in the strain by conventional method and strain measured by sensors. The author has identified maximum structural deformation which causes failure of supporting structures called as bone or cartilage. Change in the strain in single concrete structure measured by FBG sensors and optical fiber-based sensors. In optical fiber-based sensor, change in the wavelength of deformed fiber is used to measure the strain [9]. Jha et al. discussed in this paper about the highly accurate sensor which can sense the finger flexure, with a resolution of 0.1° which is measured by mechanical setup. Here FBG is very sensitive to the lateral strain produced by the finger with the sensitivity of 180 pm/° which is more advantageous over other sensors. The results are evaluated using Inertial Measurement Unit (IMU), one more advantage of the sensor is dynamic response with IMU up to speed of 80°/s. Author came to the conclusion that the sensor is more accurate, and higher potential sensor is used to monitor the rehabilitation stroke patients [10]. Vilimek et al. discussed in the paper about the measurement about the measurement of tendon force by optical Fiber Bragg sensor (FBG Sensor). Because of generation of new tissue and muscles model validation, it is necessary to measure the mechanical properties of biological material. Here the usage of optical fiber in sensor made to measure the tendons [11].

Paulsen et al. discussed in this paper about the demerits of the foil-based strain sensors, i.e., limitation in the number of chordae that can be measured, so the FBG sensors can be used for this purpose. Optical strain gauze made up of silica fibers of diameter of 125 µm. Here force sensing neo chord (FSN) is created which take natural shape as native chord. Depending on spatial period of grating sensor reflects the specific wavelength of light. When the force is applied on the grating, it moves relative to one another, and wavelength of reflected light is get shifted which is directly proportion to the applied force. FSN was used to evaluate the 3D print of left heart simulator by using FBG sensor with a resolution of 1000 Hz, and came to the conclusion that this technology is more viable, long-life span compared to the other technology [12]. Presti et al. discussed in the paper such as pain in cervical spine region and also neck pain caused by the continuous seated work with inappropriate position of the body in order to overcome this the author discussed about wearable system based on sensing technologies; especially, FBG sensors is discussed in the paper. Here FBG sensors identify single neck movements which results in postural habits and which changes the frequency of respiration. Therefore, FBG sensors can be used to overcome this with good accuracy which detects flexion movements of the body [13]. Figure 3 shows FBG embedded with smart cervical system.

Fig. 3.

Smart cervical system with placement of FBG sensor [13]

Figure 4 shows the neck movement results for embedded FBG sensor. In this context, Kong et al. discussed in the paper about the motion of metatarsophalangeal join (MTPJ) of living being foot which is used in medical practice, but there was no exact method to measure it. The development of FBG sensors helped to measure the first MTPJ stiffness and motion in clinical trials in hospitals also with the good accuracy it was implemented to eight people and succeeded. By varying the MTPJ quasi-stiffness, both loading and unloading angular torque was observed, which provides to opportunity for future clinical trials also [13].

Fig. 4.

Neck movement results with FBG sensor [13]

Dennison et al. discussed about the use of FBG to measure the pressure in intervertebral pressure, intradiscal pressure response to compressive load in functional spinal cord is also measured by FBG sensor. It has the average sensitivity of e 5.770.085 pm/MPa, and sensitivity to compressive load varies from 0.70270.043 kPa/N (mean/SD) in a L1–L2 specimen. This measurement of FBG sensors also agrees with the studies in lumbar spines. To measure the intradiscal response of compressive load two strain gauge pressure sensors are used. The author also gave about the difference in the FBG sensors and strain gauge sensors and also their responses [14]. Kalinowski et al. discussed about Bragg grating and optical pressure sensor and its development and implementation to measure the mechanical deformation. The purposed model is used to measure the dislocation in the bovine mandible caused by food and contamination in food. This model is implemented in living being which identifies the bone deformation. The reflected signal is sent to pattern classification algorithm which finds out the chewing process of food. Author came to the conclusion about the effectiveness of the sensor and told the sensor is most effective and can evaluate the force during chewing process [15].

Vakiel et al. discussed on measuring internal stress in ovine meniscus by using fiber optic technology. Inside the menisci these tiny fiber Bragg grating sensors (FBG) are inserted during replicated gait cycles for measuring the in vivo stress in menisci. In vivo gait measurement, being an accurate device, measures mechanical stress with a high accuracy. The sensors being tiny in structure can be easily inserted; it is highly sensitive to orientation and variation in temperature which takes place in laboratory [5]. We can clearly see the change in stress magnitude at sensor position L1 and L2 are plotted over gait cycle (Fig. 1). In the next context, Lai et al. proposed on three axial force sensors based on fiber Bragg grating (FBG) for surgical robots. These three optical fibers along with fiber Bragg grating are placed in such a way that one occupies the center hole of sensing structure (1.4 mm) and other two are placed perpendicular to each other. On comparing these devices with commercial force sensor, a Nano17 was made also its error ranged from 4.50 to 6.18%. It has a sensitivity of 838.386 pm/N with a resolution of 1.19 mN [16]. In the next context, Chalani et al. discussed on fetal movement counting using optical fiber sensor. By observing the distortion in respiratory signal rhythm, fetal movement can be observed; by using independent component analysis (ICA) and high pass filter, this distortion can be viewed. On comparison with mother’s perception, it was found that system detected higher numbers of fetal movements (Fig. 3). Its flexibility, small size, multiplex capabilities make it a better choice for measuring fetal movement [17]. In the further context, we can see that Najafzadeh et al. proposed on Strain Monitoring and Fracture Recovery of Human Femur Bone by the application of Fiber Bragg Grating. Shifting of wavelength and observing strains femoral fracture can be visualized. The strains response varies from each other depending on different angles, size of fractures, etc. These strains are taken close to that of fracture area, linea aspera and popliteal surface areas, as well as at the proximal and distal ends of the synthetic femur. With Fiber Bragg Grating (FBG), arrays involving different number of gratings attached at specific critical bone surface locations at different angles were done. For any changes in strain level due to application of load resulted in displacement of bone which shows how relatively bone is recovering. This load bearing activity should be monitored very carefully to prevent overloading or damage [18]. In the next context, Abro et al. discussed on smart wearable belt for monitoring knee joint postures using Fiber Bragg Grating (FBG) sensor. This FBG-based smart wearable belt monitors knee position at various static and kinematic states of male subject, due to the presence of silica gel on belt which easily adheres the FBG sensor on its surface. Also, these special belts are adhered with polyvinyl chloride strips for protection. In static test for different angles at knee joint position, there was an increment in wavelength of sensor linearly. For jogging test, it was observed that measurement sensitivity was ranging between 0.018 and 0.021 nm/° for velocity of 2 and 3 km/h. These special belts are also useful for strain measurement for knee postures, and also it monitors stoke patients [19]. In the next context, Chethana et al. discussed on accessing real-time data of hand grip strength using Fiber Bragg Grating. These FBG sensor are of distributed Bragg reflector formed by periodic perturbation of refractive index of core, which reflects specific wavelength of light, and these reflections are noted as change in wavelength by the help of waveforms. By measuring different position of wrist like extension, flexion and normal real-time data has been recorded. Further obtained results are being converted to force in kilograms using load cell, which can be easily accessed by computer to obtain both real-time data and graph [20]. In another context, Cheng-Yu et al. proposed on FBG-based smart wearable ring using FDM for monitoring body joint motion at elbow and knee position. These FBG sensors were inserted into 3D printed ring. Polylactic acid because of being flexible and having ease of fabrication, they are used as raw material in smart ring fabrication process. The FBG sensors are inserted into hot printed polylactic acid during modeling process. The waveform obtained from the process clearly denotes that there was an increment in wavelength which linearly varied with change in bend angle of joints (Fig. 4). Its sensitivities for elbow joint were 0.0056 nm/°, and for knee joint, it was 0.0276 nm/°, with a maximum measurement angle of 90° and 100° [21]. In the next context, Ghosh et al. discussed on sensitivity of FBG sensors with application of four wave mixing (FWM) process. It can be visualized that there is a shift in wavelength by inducing a very small strain over FBG, and this can be magnified by using high order FWM process. Its sensitivity is found to be accurate and reliable even if concentration of strain is taken in less quantity (0.1 με). The Bragg wavelength difference between the FBG indicates the strain information as they are sensitive to strain applied on sensing FBG. Its compact design, low cost, high resolution (measure both positive and negative strain) makes its setup easily. Hence, it can be used in biomedical fields to measuring cardiac and respiratory activities [22]. In the next context, Mec et al. discussed on application of FBG for monitoring deformation of structures by using cement composites. Defect or failure of structures can be observed by determining large deformation. This experiment was held at Faculty of Civil engineering VŠB Technical University of Ostrava. Here a simple concrete beam this FBG sensor measures strain changes. By changing the wavelength of light, these sensor measures change in strain of fiber. The results obtained from this experiment were compared with measurements using traditional methods [23]. In another context, Jang et al. proposed on capturing finger motion using FBG Sensors. By measuring the strains induced on FBG, it can easily construct the entire shape of finger segments. There are two sensors being used here one is shape sensor and other is angle sensor. This shape sensor is used for reconstructing position and orientation of finger in 3D, whereas angle sensor calculates high curvature bends that occur on finger joints. The two sensors are very accurate and are having an error of 1.49 mm for shape sensor and an 0.21° error for an angle sensor [24]. In another context, Tavares et al. discussed on FBG-based sensing system for prevention of wheelchair pressure ulcer. Totally, there were six sensors which were well planned and kept in such a region on wheelchair to monitor certain prominent bones areas like scapula’s (R and L), ischiatic bones (R and L) and heels (R and L). Its high immunity to electromagnetic interferences, also insensitive to change in temperature (humid condition), small size, compact design, reliable, fast, etc., makes it far better than electronic sensors. This technique creates an alert so that the patients get to know when to relieve pressure in which specific area also when to take a new position. Hence, developing neuropathic ulcers will be controlled [25]. In another context, Kalinowski et al. discussed on evaluation of healing of bones using radiographs and manually examining mobility of fractured area. Here patients are exposed to very high radiation levels so proper techniques should be developed for good analysis. The patients have suffered fractures in lower limb; their analysis has been done by measuring strain (both static and dynamic) by using FBG sensor in an external fixation device. The result obtained is further compared with piezoelectric accelerometer to check if response of sensor is validated or not. This external fixator contained eight pins, four pins below the fracture and four above the fractured area. Based on result obtained, it was observed that four pins below the fracture showed an increase in deformation of 112.8%, whereas four pins above fracture had a decrease in deformation of 95%. Thus, by this analysis fractured bone could be easily differentiated from system and healing process could be done [26]. In another context, we can see that Mishra et al. proposed on Fiber Bragg Grating sensors for investigating in vitro spinal cord injuries. A fast biomechanical process is involved for investigating how much degree of spinal compression is occurred. The other techniques fail to measure transverse compression also for larger response of time they are not accurate. Hence, FBG sensor along with narrow band filter detects the transverse compression inside a spinal cord with a response time of 20 microseconds. On comparing with other schemes that are costly and analyzes slow optical spectra, this FBG sensors replacing spectral interrogation with power measurements results in a lesser loss in accuracy [3]. In the next context, Lee et al. discussed on utilizing hybrid high resolution Bragg Grating sensor to achieve fast response to dynamic, continuous motion and signal pattern monitoring instrument. By using Femtosecond laser Bragg grating process on an optical wave path, a wavelength shift pattern was obtained for measuring real time data in picometer units. Amount of change in wavelength can be adjusted by adjusting amount of light. This Bragg signal measurement without the need of continuous monitoring and sensing operations can improve its accuracy with one accurate frequency measurements. In reflection spectrum, the shift in Bragg wavelength is interpreted as an improvement in sensitivity to time. FBG reflection spectrum being a function of LPG spectrum position provides a resolution factor [27]. In the next context, Jha et al. demonstrated Fiber Bragg Grating (FBG) sensing glove for measuring finger flexure with an angular resolution of 0.1°. This sensing element is highly sensitive to axial strain induced by flexing of fingers. With varying the joint rotation angle, the reflection spectrum of FBG varies linearly. Its sensitivity is 18.45 pm/° with an angular resolution of 0.1°. Also, its maximum standard deviation were 0.30° and 0.79° on mechanical setup and human hand. This high accurate glove can be used to monitor progress in rehabilitation of stroke survivors due to its much better dynamic response compared to IM. Even this instrument can be used in Parkinson’s tremor (3–7 Hz) in stroke survivors [28] Fig. (5).

Fig. 5.

a Fiber Bragg grating (FBG) device attached to the foot to sense the first metatarsophalangeal (MTPJ) motion, with a load cell to measure the force applied by a clinician. b Schematic representation of FBG system setup [14]

FBG for biosensing applications

Samavati et al. discussed on Au or FBG-based designed special kit which is embellished by GO to examine COVID-19 virus by taking the sample of their saliva. Totally, six samples were collected of patients whose ages were between 34 and 72 years. By examining the changes in sensing element, also the coalition in COVID-19 virus density measured what exactly is the condition in saliva. After the probe is placed in saliva, light is passed through it which gets deviated. Deviation of intensity and wavelength clearly shows that virus is present (Fig. 1). From the analyses, it was found that the patient in his/her early stage of disease had an intensity of 1.32 dB and a shift in wavelength was found to be 0.98 nm, but if disease is in its hyperinflammatory stage, then the intensity is 2.01 dB and shift in wavelength is 1.12 nm. This probe being extremely sensitive and precise has made this detection to achieve quickly within a fraction of 10 s as soon as probe is placed in patients saliva [29]. In the next context, Triana et al. proposed on using Fiber Bragg Grating (FBG) sensors for diagnosing hyperthermia breast cancer and also finding its treatment. By passing a strong electric field, an intense heated pattern is being created surrounding the affected tissues through cancer. By using FBG sensors, we can find out the relationship between density of electric field applied and temperature inside the tissues. This dual comb electro optic fiber sensor generates a continuous wave of laser and a modulator of Mach–Zehnder at a particular voltage for creating additional modes around the laser wavelength. This modulated frequency of Mach–Zehnder is best for configuring temperature amplitude resolution [30]. In another context, Domingues et al. discussed developing eHealth solutions for monitoring bodily activity of cancer patients by using FBG sensors. Here patients need not go to hospital for continuous check-up, rather they could relax at home and do the exercise properly under the overseeing of a physiotherapist. This solution has been so useful that it has led to an improvement in patients' physical as well as emotional health. All the small movement in hand grip force is being analyzed by the sensor and being evaluated [31]. In another context, Sun et al. discussed an inline narrow interferometer spurted with FBG for detecting biomarkers of breast cancer (HER2). FBG being insensitive to changes in temperature can be used as temperature thermometer since this interferometer is sensitive to refractive index [RI] (Fig. 2). Its limit of detection is 2 ng/ml which is very low. This method is henceforth advantageous in early detection of disease [32]. In the next context Ran et al. discussed Fiber Bragg grating immunosensor for testing cardiac biomarker. This FBG organizes the reflection of the harmonic in such a way that they eliminate the temperature cross. These reflections are placed at 1 and 1.55 μm of wavebands. This helps us in understanding biomolecular simulation of the body at different temperatures. Hence, this enables an instant diagnosis of heart disease [33]. In the next context, D'Acquisto et al. discussed on comparison of firmness of clinical thermometers and the FBG probe which is prototypal after the sterilization process. Here, repeatedly using a used probe needs a sterilization process in order to be prevented from infection, so after sterilization temperature measurements were taken. It was found that accuracy of the FBG probe was high when compared to that of clinical thermometers. Even for repetitive sterilization, this FBG probe maintained its stability and provided an accurate measurement [34]. In another context, Liu et al. discussed on fiber optics-based immunosensor for detection of cardiac biomarkers. The immunosensor used here is troponin I (cTn-I) which is based upon a phase shift of microfiber probe. This phase shift induces a small reflective signal for improving the resolution of spectrum also making the sensor to perceive even a small change in refractive index due to capturing of troponin I antigens. Its limit of detection is 0.03 ng/ml and its sensing ranges from 0.1 to 10 ng/ml. This can also be used for collecting the samples of human serum [35]. In the next context, Dandan Sun discussed on FBG embedded with a taper interferometer for detection of breast cancer. This tapered interferometer optic sensor is highly sensitive to refractive index (RI), but the wavelength of FBG sensors are insensitive to changes in RI. These FBG used as temperature thermometer can monitor the unintended drifts due to variation in temperature, but this method is useful only during early stages of cancer, because of its lower limit of detection (LOD), i.e., 5 ng/ml [36]. In another context, Liu et al. discussed on special FBG sensors label free biosensors for diagnosing acute cardiac vascular disease. This can be highlighted to what extent biomarkers are found in the blood of patients. It is found that the observation limit is found to be 6 pg/ml, when compared to cardiac troponin I, which is also a kind of biomarker. This method has been very beneficial due to early detection of disease which can be cured early [37] (Fig. 6, 7).

Fig. 6.

Shifting of wavelength (a) and changing of intensity (b) of the light detected which is passed through the probe at various exposure timings

Fig. 7.

Wavelength response of FBG and tapered optic fiber interferometer a refractive Index [RI], b temperature

Sridevi et al. discussed about Biosensors and bioelectronics which is very sensitive and accurate which is based on finding C-Reactive protein using FBG sensors. This method is carried out by observing the wavelength surrounded by anti CRP antibody graphene oxide complex. This complex is based on Fourier transform infrared spectroscopy and atomic spectroscopy. This is highly specific to CRP even in the presence of glucose urea. This has a limit of detection of 0.01 mg/l which is achieved by 0.01 mg/l to 100 mg/l which includes clinical range of CRP [38]. Tait et al. author in this paper talks about one of the most prominent optic sensors which is based on FBG coated with polymer material to detect biological agent. The sensor element utilizes a sensitive transmission spectroscopy method and has receiver to minimize the laser intensity noise problem. Volumetric expansion of the polymer surface features strain to Brag grating and modify the grating period other than sensing index of refraction. This requires a continuous tunable laser; therefore, a compact rugged a solid type laser at 1550 nm is taken for tuning between discrete wavelength. This process effects discrete laser wavelength and sampled grating responses and creates artificial neural network. This sensor has been used in application where high sensitivity is required and where high speed and mobility is required and also in remote operability areas like in vibrational, electromagnetic, and explosive environments [39]. Srinivasan et al. author discussed in this paper about a highly sensitive and which can give a real-time response to measure pressure, gas, biomolecules, i.e., etched FBG sensor. These sensors are nonreproducible in efficiency and result and upscales for large production. Here the refractive index is increased by electrospinning the polyvinyl alcohol reduced graphene oxide; this uses a target and sandwiched structure. Increased RI caused the lowering increased sensibility and uniform, linear range for case study. This kind of sensor is really effective, portable, label free condition [40].

FBG sensors for body temperature monitoring applications

Ali et al. discussed in the paper about accurate temperature measurement of human body with high resolution using FBG Sensors. The advantage of using FBG sensor is it has short time grating and compact in size. This can measure the temperature ranging from 35 to 41 °C which is typical human body temperature range; this is the range from hypothermia to hyperthermia in medical terms. This has high resolution of 81.5%. This also has the ability to work in environment such as MRI (magnetic resonance imaging). This sensor involving measurement devices has application in clinical and medical fields [41]. Burunkaya et al. discussed in the paper about control of incubator of temperature using FBG-based sensors. Incubator is the biological and medical device that provide suitable and comfortable environment for the new born baby which is born before its date. The temperature inside the incubator is adjusted or provided by FBG sensor; for this six FBG sensors were used, and close result is obtained by relation R = 0.9989. Here user-friendly interface is used to monitor the FBG sensors, and measurement is made more accurate and economic. This system overcomes the errors and delimits of default conventional sensors, problems such as flow of air in incubator in due to confusion in sensors can be overcome by this method. Therefore, this sensor can be used effectively in maintaining the environments for new born infants [42]. Pant et al. discussed about the breast cancer treatment using the FBG sensors thermal feasibility sensor. Breast cancer is the most dangerous diseases among the women. The only way to cure these diseases is the early detection and regular breast screening. To detect the malignancy many advanced technologies have been used, but FBG-based sensors are the simple, effective, cost-efficient technique based on thermal sensor array which detects the breast cancer. In this method, the agar gel phatom is used to stimulate the tissue and inside this phatom heaters are present to detect the cancer cells or tumors. COMSOL Multiphysics software provides the 3D view of the prototype. Here variation of ≥ 0.3 °C is present between experimental and simulated result. Thus, we can conclude this technique has greatest accuracy in finding the tumors [43]. Ashokan et al. discussed in this paper about the high-pressure measurement for space application using temperature compensated diaphragm-based FBG sensors. This system can measure the pressure up to 700 bar. Here martensitic stainless steel covers the FBG sensors, and FBG is bounded in stain free region of the diaphragm. It has a pressure sensitivity up to 3.64 pm/bar and hysteresis error of 0.75% of full-scale pressure in the range of 0 to 700 bar, with correlative coefficient of 99.99%. Here COMSOL Multiphysics software is used to detect the structural and thermal stress analysis of diaphragm, and to evaluate this result, shock tubes and vibration tests have been carried out. This can measure the pneumatic pressure in launch vehicles and both static and dynamic pressure of cryogenic propellant [44]. Jiang et al. discussed in this paper about fiber optical malfunction human–machine interface for machine for motion capture, temperature, and constant force monitoring. This is based on FBG technology which is embedded in glove and provides capturing of motion of human hand finger tracking, wrist rotation and force constant detection. Here FBG sensor is presented as 3D shape sensing with edge FBG. Here the data is transmitted into smart glass in real time; the motions are captured effectively and accurately by this method [45].

FBG sensor for respiratory monitoring application

Sinha et al. discussed on fiber-optic rate respiratory sensor for measuring respiratory rate. By measuring the reflection of light also the spectrum reflection present at the sensing film of fiber, it is observed that there is shift in wavelength for every breath which in turn results in varying the intensity of reflection of spectrum. In order to check the accuracy of various technique, an investigation is been done. Firstly, by using thoracic impedance pneumography (TIP) and capnometry device, the respiration rate of participants was taken at 5, 12 and 30 per minute with continuous monitoring over the fiber system. Here the participants wore two varieties of face masks. In two parts, they were given to wear anesthetic face masks and normal and low tidal breathing rate was calculated. From the analysis, it was observed that fiber-optic respiratory sensors could easily specify between low tides and normal volumes with higher accuracy than capnometry devices [46]. In next context, Prata et al. discussed on FBG sensors for monitoring respiratory rate of a person while sitting in their office chair. For every position breathing rate was calculated. It consists of a sponge and pillow like two instrumented shapes, where application of force was been calculated by volunteers [47]. In another text, Issatayeva et al. discussed monitoring of respiratory rate continuously by wearing FBG-based wearables. It consists of two FBG arrays with five sensors in it each attached to two belts. These belts are placed at abdomen and chest since there is large expansion in those areas, which results in elongation of belts. Hence use of ten FBGs placed at different position results in high accuracy of output. The output obtained is then compared with reference rate via mobile application [48]. In next context, Presti et al. proposed on monitoring of respiratory rate of video terminals operator using fiber optic. It is observed that person's mental and physical stress is very much related to its respiratory rate. FBG due to its excellent properties like high sensitivity, light, etc., are making them useful for integrating into garments for monitoring appropriately. Hence, over video terminal workers 40 min of tracing of respiratory rate tracking was done, which when compared with standard instrument resulted in error of less than 1 bpm [49]. In another context, Tommasi et al. discussed smart mattress for monitoring respiratory rate using FBG. FBG sensors due to their high sensitivity, immune to electromagnetic fields, flexibility, etc., are used in monitoring cardiorespiratory. A smart mattress was developed with five FBGs array for recording breathing rate. For two different positions which is prone and supine, both tachypnea and normal respiratory rate were recorded. It was observed that error occurred is less than 0.26 bpm when compared with standard instrument [50].

Al-Halhouli et al. made a suggestion in the following context on the evaluation of a wearable inkjet strain gauge sensor for measuring respiratory rate while in various positions. On 15 males, the experiment was run twice. There wasn't a significant difference between any of the four positions (i.e., sitting, standing, flower's position at 45°, and supine) [51]. The FBG sensor for the respiratory system is depicted in Fig. 8. In another setting, Koyama et al. considered evaluating the strain placed on the diaphragm and lungs while breathing to verify respiratory rate. When comparing the rate of respiration in strain determined by FBG sensors with the data from temperature sensors fixed to masks, the results were perfectly consistent (Fig. 1). The strain was collected from chest, abdomen and shoulder, as moving away from abdomen resulted in decrease in amplitude of signal. Also, it was found that these strains couldn’t be found in elbows, wrists. Thus, concluding that these strains couldn’t be found in peripheral areas, i.e., propagating only from the abdomen region to the shoulder part [52]. In another context, Ali et al. discussed the high resolution of the human body and its accurate temperature measurement using FBG. These FBG measured from 35 to 41 °C temperature which is of human normal temperature. The FBG having high sensitivity and resolution (i.e., 85.9%) could be easily used for clinical purposes. This method worked effectively during COVID-19 crisis which led to high contribution in measuring patient’s case [53–58] (Table 1).

Fig. 8.

FBG sensor for respiratory system [59]

Table 1.

Different applications of FBG sensors

S. nos	References	Application
1	[2]	Biomechanical
2	[4]	Hand mechanics
3	[5]	Biomechanics
4	[7]	Bone healing
5	[10]	Grip force
6	[13]	Respiratory
7	[5]	Gait analysis

Conclusion

An overview of FBG-based sensing systems for biomedical applications has been taken into consideration in this article. Discussion has been had regarding the current state of biomechanical sensing, biosensing, respiratory monitoring, and temperature monitoring bioapplications. In-depth discussion is also given to potential and steps that must be taken to further technology. Different benefits over traditional sensors have been assessed. Because of a lack of understanding on technology transfer, the majority of FBG sensors integrated with produced devices have not reached the marketing level. Future considerations for sensing system include optimization in terms of sensitivity, mobility, and packaging.