Abstract
The Fiber Bragg Grating (FBG) technology has found wide application in optical sensing, communication, and biochemical areas. As FBG biosensors with good biocompatibility promise label-free, anti-electromagnetic interference, and non-destructive detection of stress monitoring, it holds the potential to rapidly identify the stress changes in microfluidics systems. In a microfluidics system, the rheological characteristics and deformable structures of the fluid significantly affect the flow behavior of fluids in the microchannel, which may be controlled by adjusting the flow rate. Despite extensive research on water shear stress within the microchannel, relatively few studies have investigated the induced pressure of fluid flow within soft microfluidic devices at varying flow rates. Therefore, in microfluidic applications that require highly accurate control of flow conditions, achieving high reliability of pressure and flow rate monitoring could play an important role. This thesis presents the following research efforts towards the biosensors based on FBG technique and microfluidic technique.Firstly, this thesis presents an embedded and miniaturized pressure sensor based on the FBG technique for the measurement of both flow rate and flow-induced pressure in a Polydimethylsiloxane (PDMS)-based microchannel. The analysis included theory, models, fabrication methods, experiments, and fluid simulation. Detailed pressure sensing measurements were obtained at different flow rates within the range from 11.8 ml/h to 118.0 ml/h with a step size of 11.8 ml/h. These and additional experimental sensing data were used to calibrate the fluid simulation. The experimental measurements indicate that the ratio between pressure induced by fluid flow and flow rate is 4.33 ~ 6.67 Pa⸱(ml/h)-1, which is consistent with the gradient of about 4.86 Pa⸱(ml/h)-1 obtained from simulation results. The sensitivity of the flow rate measurement within PDMS-based microchannels using this method was found to range from 0.26 to 0.40 Pa⸱(ml/h)-1. This corresponds to a level of detection (LoD) for the flow rate of 2.9 to 4.5 ml/h at 3σ/sensitivity, with a standard deviation of 0.39 pm. Good agreement was found between the predictions and experimentally in this range.
In addition, the analysis results of the microchannel within a broader flow dynamic range, which covers flow rates from 11.8ml to 212.4 ml/h, indicate that elastic deformation is the key parameter determining stress release. The random instability induced by the operation of the reconnection of the syringe back to the liquid flow system has been identified and the calibration for this instability is also presented in this paper. Based on signal processing, the relationship between FBG wavelength shift and flow rate is discussed.
Moreover, this thesis also presents experimental results using a hydrofluoric acid (HF)-etched optical fiber Bragg grating (EFBG) sensor embedded in a 5 cm *5 cm microchannel model to measure the refractive index change of solutions. In the experiments, the lowest NaCl solution concentration detected was 4%, the sensitivity of the sensor was around 1.25 nm/RIU, and the estimated minimum refractive index change was 0.01. The experimental results were analyzed based on theoretical data from COMSOL simulations, and the Bragg wavelength changes with the change in refractive index of the solution in the microchannel were estimated. The etching rate of the fiber in 40% solution was about 1 μm/min. For a cladding diameter of 40 μm, the effective refractive index increases with increasing refractive index of the surrounding medium. The sensitivity of the refractive index sensor is higher as the cladding diameter decreases. The problems of transient display and real-time measurement of flow rate change in the microfluidic system are solved, and a high-accuracy embedded and miniaturized biosensor based on FBGs is formed.
Compared with all the optical techniques reported for salinity and flow rate measurement, the proposed method has a simpler configuration and manufacturing process for the biosensor, and can achieve accurate measurement without complex information processing. This method also provides the possibility of introducing more FBGs to achieve distributed measurement and multi-point measurement. At the same time, it provides a reliable benchmark for further research. Therefore, the study of the bio-sensing system in this thesis provides a practical, non-invasive, real-time, and high-sensitivity detection method.
| Date of Award | Nov 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jing Wang (Supervisor) & Chengbo Wang (Supervisor) |