**6. Current research scenario**

The multiplexing, multi-parameter and minimally invasive sensing capabilities even in high electric/magnetic field environments of fiber grating sensors make them befitting for

Optical Fiber Gratings in Perspective of Their Applications in Biomedicine 133

and 3D sensing systems for utilization in "smart" fiber-based Human Machine Interfaces (HMI) employed in clinical beds, amputee sockets and wheelchair seating systems, targeting

Recently some studies on the possible use of FBG as a strain gage in bones have been undertaken. Although in-vivo strain measurement in humans is not very common, the researchers at Hadassah University Hospital, Israel have reported use of instrumented bone staples made of electrical strain gages in some volunteers (Milogram 2000, 2003). Talaia et al (2007) have first reported use of FBG sensor array to study strains in fracture fixation of synthetic femur. As strain measurement on bone plates using ESG is technically difficult and not feasible, FBGs are good alternative to assess the stiffness of callus formation of fractured bones. Use of FBG sensors in place of ESGs have been validated to measure deformation in human cadaver femur bone specimen under in-vitro loading condition (Fresvig et al 2008). The **e**ffect of **decalcification** on strain response of a goat tibia was investigated *in-vitro* using FBG sensors by our group (Mishra et al 2010). In the investigation, two tibia bone samples were taken from the same animal. The FBG sensor was directly bonded onto the surface at the midpoint of the bone shaft, which is the most vulnerable point, using standard cynoacrylate adhesive and cured properly. One sample was decalcified in gradual steps while the other was kept in saline solution for a comparative study. Both the bone samples were strained by using three-point bending technique and corresponding Bragg wavelength shifts were recorded. Strain response of the decalcified and untreated bones was studied concurrently to monitor the effects of calcium loss and that of degradation with time. The strain generated for the same stress increased with greater degree of decalcification e.g. calcium loss of even 0.3906 gm (treatment 1) resulted in 1.3 times/ 24% more strain for the same load while calcium loss of 1 gm resulted in 50% increase in strain and when calcium loss was 2.78 gm the increase in strain reached more than 2000% i.e. 22 times more strain as compared to that before decalcification. Figures 4a &4b show the schematic of the experimental setup developed and the results obtained respectively. The Dexa results of

It is possible to measure strain less than 5 micro-strains accurately using this sensing technique, which can be indicative of the onset of decalcification. The objective of this

pressure ulcer and wound treatment. **(Pleros et al 2009).** 

Fig. 3. Schematic of the FBG sensor array for smart bed

bone samples matched with our results for mineral loss.

various applications in healthcare as evident by past and ongoing research activities all over the world. Some areas of current activities are described briefly in subsequent sub-sections.

### **6.1 Biomechanics**

Biomechanics involves application of engineering mechanics to biological systems to better understand them and make use of conventional electrical strain gages (ESGs) as a tool. These ESGs, considered gold standard for strain measurement, consist of metallic parts and it is difficult to make them adhere on the surface of a bone or other biological tissues. Besides, working of an ESG depends upon measuring electric resistance that varies with applied strain and so it is not suitable in strong electric and magnetic field environment associated with medical appliances. Moreover, they cannot be made completely biocompatible. Fiber gratings have an edge over the conventional electrical gages even for *in-vivo* applications because they have smaller risk for infection and can also be used even on curved surfaces or in locations where use of a conventional gage is technically and medically not feasible.

A temperature independent array of FBGs with proper design can be used as pressure sensor to measure muscular strength of hands or weight profile of patients when used under foot. In a study started almost a decade earlier, FBG pressure sensors embedded in Carbon Fiber reinforced material were designed and investigated by a research group in Singapore for monitoring the foot pressure of diabetic patients (Hao et al 2003). These sensors give better results in terms of sensitivity, cost and compactness as compared to conventional foot pressure sensing systems. This concept along with development of multiple neural networks for continuous monitoring of various parameters simultaneously can be used for *human gait analysis*. Gait analysis is important for patients with cerebral palsy, neuromuscular disorders or diabetes and many hospitals worldwide now have gait labs both to design treatment plans, and for follow-up monitoring. Existing systems of gait analysis use large numbers of sensors or complex imaging systems, whereas several FBGs on a single strand of fiber with one interrogation unit can be embedded within materials forming a surface without loss of material strength. Recently, FBG sensors have been investigated for distributed tactile sensing (Cowie et al 2006, 2007) in a grid arrangement along with a neural network to detect the position and shape of a contacting load simultaneously in real- time.

Multiple FBG sensors in a single fiber can be bonded at strategic locations on the patients' bed to continuously monitor their movements from a remote station. The concept of a smart bed is under investigation in Singapore (Hao et al 2010). A series of 12 FBG sensors underneath the patient's mattress with suitable algorithm give pressure profile and respiratory rate of the patient while another set of gratings placed on top of the mattress detect heart rate. Existing methods require different techniques for each individual parameter while this single system shown schematically in figure 3, can monitor respiratory rate, heart rate, pressure points and occupancy of patient on bed in a continuous, nonintrusive and robust manner.

An ongoing project entitled**, "Intelligent adaptable surface with optical fiber sensing for pressure- tension relief" (IASIS)** of European Commission is incorporating FBG-based 2D

various applications in healthcare as evident by past and ongoing research activities all over the world. Some areas of current activities are described briefly in subsequent sub-sections.

Biomechanics involves application of engineering mechanics to biological systems to better understand them and make use of conventional electrical strain gages (ESGs) as a tool. These ESGs, considered gold standard for strain measurement, consist of metallic parts and it is difficult to make them adhere on the surface of a bone or other biological tissues. Besides, working of an ESG depends upon measuring electric resistance that varies with applied strain and so it is not suitable in strong electric and magnetic field environment associated with medical appliances. Moreover, they cannot be made completely biocompatible. Fiber gratings have an edge over the conventional electrical gages even for *in-vivo* applications because they have smaller risk for infection and can also be used even on curved surfaces or in locations where use of a conventional gage is technically and

A temperature independent array of FBGs with proper design can be used as pressure sensor to measure muscular strength of hands or weight profile of patients when used under foot. In a study started almost a decade earlier, FBG pressure sensors embedded in Carbon Fiber reinforced material were designed and investigated by a research group in Singapore for monitoring the foot pressure of diabetic patients (Hao et al 2003). These sensors give better results in terms of sensitivity, cost and compactness as compared to conventional foot pressure sensing systems. This concept along with development of multiple neural networks for continuous monitoring of various parameters simultaneously can be used for *human gait analysis*. Gait analysis is important for patients with cerebral palsy, neuromuscular disorders or diabetes and many hospitals worldwide now have gait labs both to design treatment plans, and for follow-up monitoring. Existing systems of gait analysis use large numbers of sensors or complex imaging systems, whereas several FBGs on a single strand of fiber with one interrogation unit can be embedded within materials forming a surface without loss of material strength. Recently, FBG sensors have been investigated for distributed tactile sensing (Cowie et al 2006, 2007) in a grid arrangement along with a neural network to detect the position and shape of a contacting load

Multiple FBG sensors in a single fiber can be bonded at strategic locations on the patients' bed to continuously monitor their movements from a remote station. The concept of a smart bed is under investigation in Singapore (Hao et al 2010). A series of 12 FBG sensors underneath the patient's mattress with suitable algorithm give pressure profile and respiratory rate of the patient while another set of gratings placed on top of the mattress detect heart rate. Existing methods require different techniques for each individual parameter while this single system shown schematically in figure 3, can monitor respiratory rate, heart rate, pressure points and occupancy of patient on bed in a continuous, non-

An ongoing project entitled**, "Intelligent adaptable surface with optical fiber sensing for pressure- tension relief" (IASIS)** of European Commission is incorporating FBG-based 2D

**6.1 Biomechanics** 

medically not feasible.

simultaneously in real- time.

intrusive and robust manner.

and 3D sensing systems for utilization in "smart" fiber-based Human Machine Interfaces (HMI) employed in clinical beds, amputee sockets and wheelchair seating systems, targeting pressure ulcer and wound treatment. **(Pleros et al 2009).** 

Fig. 3. Schematic of the FBG sensor array for smart bed

Recently some studies on the possible use of FBG as a strain gage in bones have been undertaken. Although in-vivo strain measurement in humans is not very common, the researchers at Hadassah University Hospital, Israel have reported use of instrumented bone staples made of electrical strain gages in some volunteers (Milogram 2000, 2003). Talaia et al (2007) have first reported use of FBG sensor array to study strains in fracture fixation of synthetic femur. As strain measurement on bone plates using ESG is technically difficult and not feasible, FBGs are good alternative to assess the stiffness of callus formation of fractured bones. Use of FBG sensors in place of ESGs have been validated to measure deformation in human cadaver femur bone specimen under in-vitro loading condition (Fresvig et al 2008). The **e**ffect of **decalcification** on strain response of a goat tibia was investigated *in-vitro* using FBG sensors by our group (Mishra et al 2010). In the investigation, two tibia bone samples were taken from the same animal. The FBG sensor was directly bonded onto the surface at the midpoint of the bone shaft, which is the most vulnerable point, using standard cynoacrylate adhesive and cured properly. One sample was decalcified in gradual steps while the other was kept in saline solution for a comparative study. Both the bone samples were strained by using three-point bending technique and corresponding Bragg wavelength shifts were recorded. Strain response of the decalcified and untreated bones was studied concurrently to monitor the effects of calcium loss and that of degradation with time. The strain generated for the same stress increased with greater degree of decalcification e.g. calcium loss of even 0.3906 gm (treatment 1) resulted in 1.3 times/ 24% more strain for the same load while calcium loss of 1 gm resulted in 50% increase in strain and when calcium loss was 2.78 gm the increase in strain reached more than 2000% i.e. 22 times more strain as compared to that before decalcification. Figures 4a &4b show the schematic of the experimental setup developed and the results obtained respectively. The Dexa results of bone samples matched with our results for mineral loss.

It is possible to measure strain less than 5 micro-strains accurately using this sensing technique, which can be indicative of the onset of decalcification. The objective of this

Optical Fiber Gratings in Perspective of Their Applications in Biomedicine 135

mandible showing the feasibility of using FBG to monitor dynamic strain in complex biomechanical systems. A research group in Portugal has applied FBG sensors to assess the performance of dental implant system by measuring static and dynamic bone strains around it. Conventional techniques can not be used to measure strains inside bones (Schiller et al

Now-a-days, the use of custom-made mouthguards as preventive measures for persons participating in sports activities is being encouraged as they have an injury-preventing ability. To evaluate the performance characteristics of such mouthguards, no standard technique exists till date. A unique experimental scheme utilizing fiber Bragg gratings (FBGs) as distributed strain sensors is proposed and investigated by our group to estimate impact absorption capability of custom-made mouthguards. In the scheme, a pendulum based fixture with interchangeable impact load e.g. cricket, hockey and steel balls, was custom made for the investigation. Two sets of FBGs were used; one at the mouthguard surface and the other at similar position on a jaw model. The fixture was used to simulate impact using different balls released from varying angles and Bragg wavelength shifts of FBGs at mouthguard and that at the jaw model were recorded. Figures 5a & 5b show photograph of the jaw model, mouthguard with pendulum setup and change in FBG spectrum due to impact**.** It **is** obvious from the FBG response shown in figure 5b that for the same impact there is no detectable change in the Bragg wavelength of the FBG bonded on

2006). This study can lead to significant improvement in the design of dental implants.

Fig. 5a. Photograph of the Experimental Set up for Mouthguard Experiment

Fig. 5b. Spectral shift due to Ball Impact of FBG Sensors fixed on Mouthguard and on Jaw

Model

investigation was to develop a different, efficient and safe method to estimate calcium levels in bones. The small size of fiber can be utilized to make strain staples much smaller then the existing ones so that it can be implanted using *minimally-invasive* surgical method, or, as this technology is still developing it may advance into *non-invasive* method in future.

Fig. 4a. Schematic Illustration of the Experimental Set up

Fig. 4b. The Results of Bone Decalcification Study

First application of embedded FBG sensors in dental biomechanics was reported (Tjin et.al 2001) to monitor the force and temperature as a function of time in dental splints used by patients with obstructive sleep apnoea. This monitoring gives a clear indication of the compliance of the patient with regard to the proper usage of splint which is necessary for its effectiveness. In another type of investigation FBGs written in polarization maintaining fibers were used to monitor the drying of dental cement and the corresponding stress build-up (Ottevaere et al 2005). To measure strain at a mandible surface caused by impact loads on dental implants, an FBG sensor was employed by JCC Silva et al (2004) on a dried cadaveric

investigation was to develop a different, efficient and safe method to estimate calcium levels in bones. The small size of fiber can be utilized to make strain staples much smaller then the existing ones so that it can be implanted using *minimally-invasive* surgical method, or, as this

First application of embedded FBG sensors in dental biomechanics was reported (Tjin et.al 2001) to monitor the force and temperature as a function of time in dental splints used by patients with obstructive sleep apnoea. This monitoring gives a clear indication of the compliance of the patient with regard to the proper usage of splint which is necessary for its effectiveness. In another type of investigation FBGs written in polarization maintaining fibers were used to monitor the drying of dental cement and the corresponding stress build-up (Ottevaere et al 2005). To measure strain at a mandible surface caused by impact loads on dental implants, an FBG sensor was employed by JCC Silva et al (2004) on a dried cadaveric

technology is still developing it may advance into *non-invasive* method in future.

Fig. 4a. Schematic Illustration of the Experimental Set up

Fig. 4b. The Results of Bone Decalcification Study

mandible showing the feasibility of using FBG to monitor dynamic strain in complex biomechanical systems. A research group in Portugal has applied FBG sensors to assess the performance of dental implant system by measuring static and dynamic bone strains around it. Conventional techniques can not be used to measure strains inside bones (Schiller et al 2006). This study can lead to significant improvement in the design of dental implants.

Now-a-days, the use of custom-made mouthguards as preventive measures for persons participating in sports activities is being encouraged as they have an injury-preventing ability. To evaluate the performance characteristics of such mouthguards, no standard technique exists till date. A unique experimental scheme utilizing fiber Bragg gratings (FBGs) as distributed strain sensors is proposed and investigated by our group to estimate impact absorption capability of custom-made mouthguards. In the scheme, a pendulum based fixture with interchangeable impact load e.g. cricket, hockey and steel balls, was custom made for the investigation. Two sets of FBGs were used; one at the mouthguard surface and the other at similar position on a jaw model. The fixture was used to simulate impact using different balls released from varying angles and Bragg wavelength shifts of FBGs at mouthguard and that at the jaw model were recorded. Figures 5a & 5b show photograph of the jaw model, mouthguard with pendulum setup and change in FBG spectrum due to impact**.** It **is** obvious from the FBG response shown in figure 5b that for the same impact there is no detectable change in the Bragg wavelength of the FBG bonded on

Fig. 5a. Photograph of the Experimental Set up for Mouthguard Experiment

Fig. 5b. Spectral shift due to Ball Impact of FBG Sensors fixed on Mouthguard and on Jaw Model

Optical Fiber Gratings in Perspective of Their Applications in Biomedicine 137

cells cultured in osteogenic-induced conditions over an optical fiber and in parallel, the sensing capability of FBG sensors throughout the culture time was assessed. The results showed that in addition to the excellent osteoblastic cytocompatibility, FBGs maintained the physical integrity and functionality, as their sensing capability was not affected throughout the cell culture period. Results suggest the possibility of in-vivo osseointegration of the optical fiber/FBGs anticipating a variety of applications in bone mechanical dynamics.

The ability of LPGs to detect refractive index variation in their vicinity has great potential for detection of clinical analytes and can be made to detect extremely low concentrations. An LPG with an immobilized antibody film on its surface is a very efficient device to detect target antigen bonding to this film by means of refractive index change associated with the process. The advantage of using LPG is that it is a direct and label free sensor which does not require any additional reagents to visualize binding. Figure 6 indicates the basic experimental set up for an LPG based biosensor. DeLisa et al (2000) have first reported use of LPG as biosensor for detection of human IgG by specific antibody-antigen binding with immobilized goat anti-human IgG antibody on the chemically treated surface of LPG. The system could work for antigen solution concentration between 2-100

> **Photograph of the glass cell with LPG fixed inside**

Luna Analytics Inc. (Blacksburg, VA) had recently started developing an LPG based biosensor system though the product is yet to be commercialized. The sensitivities of these LPG sensors were found to be comparable to those of ELISA techniques (Baird & Myszka 2001, Pennington et al, 2001)*.* LPG sensors have also been used for monitoring microbial activity (Carville, 2002]. Higher sensitivity in LPG sensors can be achieved by using gratings with smaller period or reduced- diameter cladding (Patrick & Kersey, 1998, Shu et al 2002). Chen et al (2007) have verified high sensitivity of smaller period LPGs and their reusability

Fig. 6. The Experimental set up for an LPG Based Biosensor

by detecting DNA hybridization.

**Liquid in**

**6.2 Biosensors** 

μg mL-1

the jaw model while FBG on the mouthguard has shown large wavelength shift with each impact. Relative Bragg wavelength shift with respect to each impact load determines the protection capability of the mouthguard. Due to multiplexing capability of FBGs, it is possible to fix multiple sensors in series at various points of mouthguard and denture to evaluate the effect of a single impact on different locations simultaneously. Impact tests on various locations can detect the vulnerable points where the mouthguard is less protective.

Through such studies it will be possible to quantify the level of protection and hence to predict the required modifications in the mouthguard. This research work thus, is important for the establishment of guidelines for design of safer mouthguards. (Tiwari et al 2011)

J Paul et al (2005) had suggested use of five FBGs written at different wavelengths to measure **handgrip strength** through a grip holder. Handgrip strength monitoring is rated as one of the top ten fitness tests to evaluate different physical and functional disorders related to healthcare. The conventional methods (dynamometer) are rough, uncomfortable and do not provide individual finger strengths; thus not suitable especially for rehabilitation programs.

Researchers at Nanyang Technological University, Singapore have reported an FBG based sensor in instrumented tibial spacer (ITS) to correct **misalignment during total knee replacement** surgery. The sensor, comprising of optical fibers with sampled chirped gratings inscribed on each fiber to generate a pressure profile, was embedded in a fiberreinforced composite. During a total knee joint replacement procedure, the ITS sensor can slide in place of the prosthetic spacer. The femur can be rolled over the ITS sensor and the alignment checked from the pressure map displayed. Any misalignment can be corrected with repeated checking. After the measurements are taken and the required alignment achieved, the ITS sensor can be replaced by the actual tibial prosthetic spacer and the knee joint can be sutured (Mohanty et al 2007).

Methods were developed by Dennison et al 2007 to measure **intervertebral disc pressure** response to compressive load in five lumbar functional spine units, using FBG in a patented configuration. The pressure measurement with FBGs is less disruptive than the existing techniques. In an improved configuration FBG sensor placed in silicone filled needle were applied to intervertebral disc pressure measurements in a cadaveric porcine functional spinal unit and the results were in agreement to those obtained with the standard strain gage sensor. (Dennison et al 2009 1 &2).

Investigative study of FBG sensor for in-vitro **biomechanical properties of porcine tendons** was reported by Miloslav Vilimek (2008). The **tendon force** was calibrated using Bragg wavelength measurement of the FBG bonded on the tendon with applied load. FBG was used as displacement sensor on cadaver Achilles tendon and knee ligament for **movement measurement of tendons and ligaments** (Ren et al 2007). Study of change in length of ligament under various strain conditions is important as ligaments experience much higher strain as compared to bone for same loading. The FBG sensors exhibited higher sensitivity, low noise and same accuracy as compared to stereo-optic measurement which are though non-invasive have limitations of poor accuracy and high noise level.

In a very recent development a research group from Portugal has investigated osteoblastic **biocompatibility** of optical fibers and stability of the properties of FBG sensors for their invivo usage. (Carvalho et al 2011) The study analysed the behaviour of human bone marrow cells cultured in osteogenic-induced conditions over an optical fiber and in parallel, the sensing capability of FBG sensors throughout the culture time was assessed. The results showed that in addition to the excellent osteoblastic cytocompatibility, FBGs maintained the physical integrity and functionality, as their sensing capability was not affected throughout the cell culture period. Results suggest the possibility of in-vivo osseointegration of the optical fiber/FBGs anticipating a variety of applications in bone mechanical dynamics.
