Design, development and characterisation of piezoresistive and capacitive polymeric pressure sensors for use in compression hosiery
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The work in this thesis was focused in developing a flexible and cost-effective pressure sensor capable of detecting pressure variations within the low working range (0-6kPa) of compression hosiery. For this cause, both piezoresistive and capacitive pressure sensors were developed and characterised, utilising conductive and non-conductive polymeric elements to sense compressive loads. In the first case, the developed piezoresistive sensor is composed of a conductive filler - polymer composite, with a force-dependent conductivity, encapsulated in between a structured and unstructured configuration of electrodes. Initially, as the sensing element of the sensor a multi-walled carbon nanotubes-polydimethylsiloxane (MWCNT-PDMS) composite was tested. A fabrication process is also proposed for developing the MWCNT-PDMS composite which involves a series of successive direct ultrasonications and shear mixing in order to disperse the two constituents of the composite, with the use of an organic solvent. Developing the composite over a range of different filler concentrations revealed a sharp step-like conductivity behaviour, typical amongst percolating composites. The MWCNT-PDMS sensor exhibited a positive piezoresistive response when subjected to compression, which was substantially enhanced when structured electrode layers were utilised. A Quantum Tunnelling Composite (QTC) material was also tested as the sensing material, which displays a large negative piezoresistive response when deformed. The QTC pressure sensor exhibited an improved performance, which was similarly significantly increased when a structured electrode was employed. In the second case, a parallel-plate capacitive pressure sensor was developed and characterised, which successfully provided a pressure sensitivity within the working range of compression hosiery. The sensor employs an ultra-thin PDMS blend film, with tuneable Young’s modulus, as the dielectric medium of the capacitor, bonded in between two rigid copper-coated glass layers. A casting process is also presented, involving the use of a sacrificial mould, in order to pattern the polymeric film with a micro-pillar structure to assist the deformation of the medium under compressive loads. The performance of the sensor with regards to the polymeric film thickness, structure and mechanical softness was explored. Overall, the combination of an ultra-thin dielectric medium with a very low Young’s modulus and a microstructured surface resulted in a capacitive pressure sensor with a good performance within the desired pressure regime.