Doctor of Philosophy
School of Chemistry
Due to an increasing demand for development of cost-effective portable microfluidics using textile substrates, a foundation study on the effect of fibre surface chemistry on the performance of textiles was undertaken to elucidate its applicability to textile-based microfluidics (Chapter 3). Composite fibres consisting of low-density polyethylene (LDPE) fibres with liquid crystalline graphene oxide (LCGO) fillers, at a range of loadings, were successfully prepared by a melt spinning process and then incorporated in parallel with commercial polyester yarns (PET), via a tubular knitting process, to produce 3D textile-based microfluidic structures. It was shown that the LCGO filler increased the surface polarity of fibres, as a result of accumulation of oxygen on the polymer surface, and the increase in O/C ratio amplified the surface and inter-fibre capillary fluid driving force in textile structure. Fluid was shown to move up to 6x faster in 3D knitted structures comprised of 5w/w% LCGO/LDPE fibre compared to the knitted structure without any composite fibre. It was demonstrated that the ion rejection and/or absorption phenomenon which occur between fluid ions and fibre surface functional groups played the most important role in determination of fluid flow rate. The flow rate achievable was found to be proportional to the LCGO loading, providing the potential to control flow through fibre composition. Significantly, using this approach fluid pumping of fluid against a gravity feed head height (anti-gravitational) was observed as a consequence of the LCGO filler interactions at the surface of the LDPE/LCGO composite fibres.
Recently, electric fields have been used to move or separate analytes in textile-based microfluidics to achieve a precise control over the fluid flow. However, applying electric fields to move or separate solutes within fluids typically results in Joule-heating which adversely affects the efficiency of the separations. In this thesis, the idea of preparing thermally conducting fibres and assembling them into 3D textile structures to facilitate dissipating the Joule-heating was investigated (Chapter 4) using LDPE/LCGO composite fibres, where LCGO was partially reduced to impart improved thermal conductivity. LCGO/LDPE composite fibres were successfully prepared and incorporated into a 3D PET knitted structures and their capability to dissipate the Joule-heating in electrofluidic experiments probed. Monitoring the temperature change during electrofluidic experiments showed that incorporation of reduced LCGO/LDPE fibres into 3D knitted structures resulted in lower temperature rise during the experiments and more importantly, final temperature decreased by an increase in the LCGO loading. However, loading more than 5 w/w% LCGO into LDPE fibres, utilising a powder coating and melt spinning approach, proved to be impractical due to agglomeration of LCGO within composite fibres resulting in poor mechanical properties and therefore limited knittability.
To eliminate the issue of poor filler distribution, a solvent-based wet-spinning technique was adopted (Chapter 5). A solvent processable non cross-linked biocompatible grade polyurethane (PU) elastomer was filled with LCGO to produce LCGO/PU fibres. These fibres were successfully incorporated into 3D knitted structures in parallel to the PET yarns and then chemically reduced to improve thermal conductivity. The ability of the reduced LCGO/PU composite fibres as heat dissipators was shown to be limited by their electrical conductivity. Fibres were shown to become effective in Joule-heating dissipation at the point that they became electrically conductive resulting in potential short-circuits which should be avoided in high voltage electrofluidic experiments. As a consequence, boron nitride nanopowder (BNNP) filler was chosen to make BNNP/PU composite fibres as it was thermally conducting but electrically insulating (band gap of ~ 5 eV). It was shown that incorporating BNNP/PU composite fibres into 3D textile structures effectively dissipated the heat generated by Joule-heating and kept the textile structure at low temperature during electrofluidic experiments. This novel idea of utilizing thermally conducting fibres into textile-based microfluidics could be an advantageous for fibre based capillary electrophoresis studies specifically when proteins, living cells and thermosensitive analytes are being used.
Textile substrates have been widely used to make wearable electrochemical sensors. Therefore, as a proof-of-concept study, two different 3D textile designs (utilizing knitting and braiding techniques) with integrated electrodes as potential wearable electrochemical sensors were investigated (Chapter 6). The electrochemical behaviour of stainless steel (SS) filament working electrodes were shown to be far from ideal (or reversible). These filaments were surface modificated by the electrodeposition of polypyrrole and gold nanoparticles to give improved electrode surface responses. These modified electrodes were successfully incorporated into 3D braided structures, whereby all electrodes were not in direct electrical contact, consisting of two parallel SS (counter and working) electrodes with the addition of a silver-coated nylon yarn as pseudo reference electrode. This braided 3 electrode system was shown to be a functional 3D textile platform capable of electrochemical detection in a similar manner as a classical 3-electrode electrochemical system. In an alternative approach, a 3D knitted structure with 3 separate conductive strips, i.e. two SS yarn and a silver-plated nylon in the middle separated by insulating yarns, was successfully created to perform amperometric detection under a gravity assisted electrolyte flow system.
In summary, this thesis demonstrates the feasibility of the approaches investigated and their incorporation into textile structures. Significantly the approaches shown are relatively simple to fabricate, cheap, flexible and easily incorporated into textile systems to provide real time sensing and monitoring, fluid transportation and heat dissipation, all of which are critical for the implementation of textiles into active and functional devices.
Farajikhah, Syamak, Electroactive Fibre Sensor Systems for Fluidics, Doctor of Philosophy thesis, School of Chemistry, University of Wollongong, 2018. https://ro.uow.edu.au/theses1/333