**6.2 Biosensors**

136 Biomedicine

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

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

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

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

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

non-invasive have limitations of poor accuracy and high noise level.

programs.

joint can be sutured (Mohanty et al 2007).

gage sensor. (Dennison et al 2009 1 &2).

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 μg mL-1

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

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 by detecting DNA hybridization.

Optical Fiber Gratings in Perspective of Their Applications in Biomedicine 139

objective measure of tip-to-tissue contact force during the catheter ablation procedure. Force control is essential for delivering appropriate laser ablation pulses needed to produce lesions that are induced in the heart walls. If the contact force is too great, the catheter tip may perforate the heart wall and if it is too light, the procedure may be ineffective. Such force-sensing catheters with accompanying system have undergone extensive pre-clinical and clinical validation in the United States & Europe and are

Measurement of the pelvic muscles pressure was demonstrated using FBG based intravaginal probe in Portugal (Ferreira et al 2006). This measurement is essential for understanding pelvic floor disorders pathophysiology. The system was tested in a sample of patients with known pelvic floor disorders and the preliminary investigations indicated good sensitivity to radial pressure changes within the pelvic floor due to normal breathing

High-intensity ultrasonic fields are used in various medical applications like ultrasound surgery, hyperthermia, lithotripsy and even diagnostic ultrasound. Thus safety concerns demand their accurate level assessment. The conventional detection techniques utilizing piezoelectric devices are susceptible to electromagnetic interference and signal distortion. Fisher et al (1998) demonstrated that FBG sensors can be used for this purpose by implementing them to detect signals of frequencies in the range of 500 kHz and 4 MHz.

A temperature independent FBG or LPG can be designed for non-invasive measurement of the torso movement during respiration ventilatory movements to understand respiratory physiology and to monitor the lung function. In a preliminary work, Günther Wehrle et al in 2001 employed FBG sensor on the chest using an elastic belt to hold it in place to detect thorax movements during artificial ventilation, even in the presence of electrical bursts caused by electrodes situated on the chest. Expansion of the thorax cage during respiration was accordingly transmitted to the sensor grating and caused it to deform under the strain. First application of a multiplexed LPG array on curvature sensing garment used to monitor the thoracic and abdominal movements of a human during respiration was reported by T. Allsop et al 2003, 2007. They have shown that, it is possible to generate a geometric profile

Thermal therapy involves destruction of redundant tissue by heating or freezing without surgery. Examples of thermal therapies include treatment of benign prostatic diseases, ablation therapy of cardiac arrhythmia, microwave induced hyperthermia for radio therapy

of the chest and abdomen in three dimensions with an array of 20 sensors.

currently undergoing precertification trials5.

**6.5 High intensity ultrasonic field measurements** 

**6.6 Respiration monitoring system** 

**6.7 Temperature monitoring** 

5 http://www.endosense.com/home.html

**6.4 Gynecology** 

cycle of the patient.

In normal FBGs as the optical signals are confined to the fiber core regions they are insensitive to external refractive index variation, but they can be made sensitive either by writing tilted grating or etching the cladding part. A Bovine Serum Albumin (BSA) immobilized tilted FBG based immunosensor was reported recently to detect anti-BSA (Maguis et al 2008). In another approach, highly sensitive etched FBG sensors have been demonstrated by Chryssis et al (2005) to detect hybridization of single strand DNA. All the FBG based biosensing research is currently at the level of laboratory investigations only as it requires either specially designed or etched FBGs that are difficult if not impractical to fabricate and expensive as compared to LPGs.

FBG/LPG sensor systems can be incorporated with the microchips to detect chemical changes in nanolitre amounts of liquids and have the potential of being part of a **µ-TAS**  (micro total analysis system) or **lab-on-chip**. Miniaturization and integration of light sources, sensors, detectors, as well as the corresponding signal processing is required for implementation of these concepts in a practical analysis system.
