Williamson Hodge, Lucy A.
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The cross-sensitivity of microcantilever sensors presents a major obstacle in the development of a commercially viable microcantilever biosensor for point of care testing. This thesis concerns electrothermally actuated bi-material microcantilevers with piezoresistive read out, developed for use as a blood coagulometer. Thermal properties of the sensor environment including the heat capacity and thermal conductivity affect the ‘thermal profile’ onto which the higher frequency mechanical signal is superimposed. In addition, polymer microcantilevers are known to have cross-sensitivity to relative humidity due to moisture absorption in the beam. However it is not known whether any of these cross sensitivities have a significant impact on performance of the sensor during pulsed mode operation or following immersion into liquid. When analysing patient blood samples, any change in signal that is not caused by the change in blood viscosity during clotting could lead to a false result and consequently an incorrect dose of anticoagulants may be taken by the patient. In order to address these issues three aspects of the operation of polymer bi-material strip cantilevers has been researched and investigated: relative humidity; viscosity/density, and thermal conductivity of a liquid environment. The relative humidity was not found to affect the resonant frequency of a microcantilever operated in air, or to affect the ability of the cantilever to measure clot times. However, a decrease in deflection with increasing relative humidity of the SmartStrip microcantilever beams is observed at 1.1 ± 0.4 μm per 1% RH, and is constant with temperature over the range 10 – 37 °C, which is an issue that should be considered in quality control. In this study, the SmartStrip was shown to have viscosity sensitivity of 2 cP within the range 0.7 – 15.2 cP, and it was also shown that the influence of inertial effects is negligible in comparison to the viscosity. To investigate cross-sensitivity to the thermal properties of the environment, the first demonstration of a cantilever designed specifically to observe the thermal background is presented. Characterisation experiments showed that the piezoresistive component of the signal was minimised to -0.8% ± 0.2% of the total signal by repositioning the read out tracks onto the neutral axis of the beam. Characterisations of the signal in a range of silicone oils with different thermal conductivities gave a resolution to thermal conductivity of 0.3 Wm-1K-1 and resulted in a suggestion for design improvements in the sensor: the time taken for the thermal background signal to reach a maximum can be increased by increasing the distance between the heater and sensor, thus lessening the impact of the thermal crosstalk within the cantilever beam. A preliminary investigation into thermal properties of clotting blood plasma showed that the sensor can distinguish the change between fresh and clotted plasma.