Optical fibre sensor in medical application

Abstract

Cochlear implantation is a treatment of choice for the habilitation of severe profound sensorineural hearing loss. During the cochlear implant surgery, the electrode will pass the ear structure, of which the tissues are very soft. The insertion force of implantation was felt by the surgeon, without any assistance of equipment. The limiting force was from 70mN to 100mN, which is very little for human to sense. High insertion force will result in penetrating or tearing of the basilar membrane and therefore a real-time insertion force monitoring is necessary. Implementation of FPI force sensor in cochlear implant surgery for real time force sensing is feasible because of its compact size, electromagnetic immunity and high force sensitivity. An extrinsic optical fibre sensor, Fabry-Perot interferometer (FPI), was developed for real-time force sensing in cochlear implant application. For FPI-based force sensor, the structure of the diaphragm of the sensor was developed in spherical structure, so that the object can touch it tangentially and then the diaphragm was deformed accordingly. Polydimethylsiloxane (PDMS) was used to fabricate diaphragm material because of its biocompatibility, high sensitivity to low loads. Several attempts of fabricating a PDMS spherical diaphragm on the tip of the capillary were made. However, most of the attempts were not successful and the fabrication cannot be controlled. The spherical PDMS diaphragm was fabricated at the tip of silica capillary by using blockage and transferring method. Single Mode Fibre (SMF) was then inserted into the capillary until the interference pattern was shown on the reflected optical spectrum. The interference pattern varied and shifted when the reflection caused by PDMS diaphragm changed. Based on the wavelength shift of the minima, contact force can be sensed by the fabricated FPI sensor. The force sensitivity of the proposed sensor was around 0.5nm/mN in 0-100mN range, which was high and suitable for insertion force measurement during cochlear implant, however, the hysteresis of the sensor was high either, which means the releasing force sensitivity of the sensor was not same as that of compression force. PDMS is sensitive to temperature and humidity. In cochlear implant application, body temperature will affect the performance of the force sensor so temperature calibration was performed on the sensor to investigate how body temperature affect the sensor. The temperature sensitivity of the sensor was around 0.13nm/℃. The force-temperature cross-sensitivity was around -0.2642mN/℃ and -0.83mN/℃ for sample A and Sample B. Humidity calibration were also performed on the developed sensor. and the humidity sensitivity was 0.13nm/%. In order to lower the humidity sensitivity, the FPI force sensor was gold-coated. The humidity sensitivity was decreased by around 50%, became 0.07nm/%, while the temperature sensitivity was increased by 10 times, which was not desirable. Further research will be performed on sensing the temperature simultaneously. One way is to inscribe a fibre-Bragg grating (FBG) into end of the inserted SMF, which is close to the tip of capillary. Temperature sensitivity of FBG can be recorded by performing temperature calibration on it. By developing a mathematical model, the contact force and temperature can be distinguished.