Polymeric Optical Fibres for Biomedical Sensing

In this thesis we focus on the integration of optical fibres in textiles to create wearable sensing systems. In the introduction (Chapter 3) we describe the basics of light guiding in optical fibres and methods of lateral light coupling. Subsequently, a literature review and motivation are presented. Afterwards two main parts can be differentiated. In the first one (Chapters 4, 5) a new method of continuous extrusion of optical fibres and their medical sensing application using photoplethysmography (PPG) is described. In the second part (Chapters 6 and 7) we present an optical fibre based force sensor, and demonstrate its application in respiratory monitoring. Commercially available optical fibres are not suitable for integration into textiles for two main reasons. These are: low flexibility in bending the fiber and relatively high price. Thus, a new type of Polymeric Optical Fibres (POFs) was developed in Empa St. Gallen. These fibres have enhanced strength and flexibility (0.027 GPa for in-house produced POF vs 2.6-3.5 GPa for commercial POF). additionally, these optical fibres where produced by means of melt spinning, which increased significantly the production speed (400 m/min) and thus otentially reduce the price of these POFs. Cyclo-olefin polymer (COP) (Zeonor n=1.5) was used as a core material and THV fluoropolymer (THVP, n=1.35) as a cladding. These produced POFs features propagation loss at the level of 9 dB/m at 652 nm. These optical fibres were embroidered into textiles such that out-coupling and in-coupling of light at narrow bends in the fiber was possible and the efficiency was improved by one order of magnitude (0.07 % vs formerly achieved 0.008 %). These flexible POFs served to manufacture a textile-based PPG monitoring system. The sample featuring the best coupling efficiency was used to measure heart rate and oxygen saturation (SpO2). Obtained results were compared with a hospital standard device and showed very good correlation. Moreover, the system was adapted to work in reflection mode which makes the sensor more versatile. In the second part of the thesis we present a new type of polymeric optical fibre for applications in force sensing systems in textile fabrics. The detectable force ranges from 0.05 N to 40 N (applied on 3 cm of fibre length) and can be adjusted beyond these limits. The fibres have attenuation parameters between 0.16 - 0.25 dB/cm at 652 nm and the yield strength ranging from 3.9 to 5.4 MPa. Additionally, these optical fibres where employed to develop a textile-based respiratory sensing system. Fibres with different sensitivity together with different setups were evaluated. The results obtained from the textiles based system were compared with commercial standards and showed good correlation. Furthermore, we showed that such a wearable system is able to differentiate the type of breathing (diaphragmatic, upper costal and mixed) when the sensor is placed at different torso positions. Overall, we demonstrated that the new types of POFs exhibit great potential in the sensing textile applications. Moreover, the proposed production technique of POFs is fast, and thus the price of the POFs can be lowered, and therefore accepted by the textile industry