Evaluation of the effect of curvature on a fibre-optic sensor for temperature monitoring during mini-invasive body contouring treatments

Obesity has recognised as one of the main factors responsible for increasing the risk of various potentially deadly diseases. Obesity can be cured, but regardless of treatment, the rapid weight loss often results in certain irregularities in body contours. These imperfections can be corrected by body contouring techniques. Currently, the laser lipolysis technique has become the elective approach for this type of treatment, as it guarantees good results with reduced recovery times and risks. In laser lipolysis, light is delivered using an optical fibre inserted through the cannula under the skin. During the operation, the physician must move the cannula under the skin to properly heat the entire target area. This movement is crucial: if the physician moves the cannula too slowly, it may cause an overaccumulation of energy and burn the tissues; otherwise, if the cannula is moved too fast, the treatment may be ineffective. This thesis is part of a larger project aimed at developing a laser applicator for thermal treatments that integrates a temperature sensor system to optimize the outcomes and help reducing the risk of burns. Fibre optic sensors (FOSs) are the best solution for this application because they do not produce artefacts when exposed to the laser beam; moreover, they are biocompatible, and their small size minimizes the invasive impact. FOSs, however, suffer from cross-sensitivity between temperature and strain. This is particularly detrimental for the considered application because the movement of the cannula during lipolysis treatment causes an error in FOS temperature estimation. The aim of this thesis is to design an all-fibre sensor configuration to discriminate the strain and temperature effects, devise a compensation algorithm and validate its feasibility through experiments. The selected configuration is based on a combination of Fibre Bragg Grating (FBG) sensors positioned to produce different strain readings depending on their orientation but a common temperature reading. For an initial analysis, the sensitivity to a two-dimensional strain was assessed. For this purpose, an applicator embedding two FBGs respectively positioned on the top and on the bottom of a miniaturised cantilever was realised. In order to accurately characterise the sensor to strain, a set of grooves with constant curvature and custom design was fabricated using a 3D printer. A specific code was written in Matlab to analyse the data and build the relationship between FBG wavelength shift and the applicator curvature. Subsequently, the sensor was characterised in a climatic chamber to assess its behaviour with respect to temperature. The relationships obtained from the two characterisations were then used in a Matlab code, which allows the sensor to be used to measure temperature even when bent. The validity demonstrated by preliminary experimental tests for the proposed solution made it possible to extend the analysis to the three-dimensional case. For this purpose, the applicator design was modified to incorporate four FBGs and to simulate the appearance of a laser lipolysis cannula. Once the prototype was built, the new sensor was subjected to experimental tests to verify the correct discrimination of direction changes in three-dimensional space. Finally, a Matlab code was developed to estimate the sensor direction of movement. Through an accurate characterisation of the sensor, it will be possible to obtain a more detailed evaluation of the sensor motion.