Post-processing of electrospun fibres for wearable flexible sensing

Abstract

With advancement in nanofabrication and flexible electronics, wearable flexible sensors offer a feasible route towards non-invasive and continuous health monitoring. Nevertheless, several challenges remain to realise this potential including the need for improvements in device sensitivity, long-term stability, multi-analyte detection, scalability, and self-powering, while ensuring skin conformity. In an attempt to achieve this, several approaches have been implemented in research literature by utilising a combination of stretchable substrates and nanomaterials to attain desirable material properties. However, many of these nanomaterials suffer from high fabrication cost, poor reproducibility, low yields, and toxicity issues.

Electrospinning is a simple fabrication technique for producing polymer fibres, on the micro- and nanoscale, with comprehensively understood parameters used to control fibre morphology, distribution, and alignment. The wide range of processable materials via electrospinning can assist in producing porous, fibrous membranes with high surface-area-to-volume ratios.

Furthermore, these materials can be controlled in terms of elasticity, electrical conductivity, biocompatibility, and skin-conformity, which demonstrates high suitability for wearable sensing applications. As a consequence, electrospinning has been employed in many studies for the development of wearable flexible sensors. Despite this, there still exists many areas in the field of wearable sensing where electrospinning may be implemented or improved to enhance device performance and fabrication. With these points considered, the work demonstrated in this thesis focussed on the use of different electrospinning and post-processing techniques for the development of enhanced and novel wearable sensing applications.

In the initial study, electrospinning of elastomeric thermoplastic polyurethane (TPU) was investigated and optimised by varying processing parameters and utilising different precursor solution additives. The elastic fibrous membrane was then examined for its mechanical properties and suitability towards stability and skin conformity for wearable sensing. It was found that through addition of tetraethylammonium bromide salt and hydrochloric acid that smaller fibre diameters could be attained, thus enhancing the surface-area-to-volume ratio. However, by decreasing fibre diameter, the tensile strength and elongation at break of the resulting fibrous membranes was also decreased. Hence, by use of these additives, the TPU fibre morphology could be tuned to suit different wearable applications. The TPU fibres were also post-processed by coating with the conductive polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), and through sputter coating of gold (Au). The in situ chemical oxidative polymerisation technique used for PEDOT coating was found to be quick and simple but the use of the resulting materials for potential piezoresistive strain sensing applications demonstrated low sensitivities and slow recovery times due to high elastic hysteresis of the fibrous structure.

The subsequent study utilised the Au-coated TPU fibres for wearable surface-enhanced Raman spectroscopy (SERS) sweat pH sensing. Sensor fabrication was achieved through simple self-assembled monolayer chemisorption of the pH-sensitive molecules 4-mercaptobenzoic acid and 4-mercaptopyridine onto the fibre surface. Calibration with low volumes (1 µL) of buffer solutions, stability tests with cyclic strain cycles and ionic interferents, and measurements with real sweat samples demonstrated the potential of these electrospun Au/TPU SERS substrates for repeatable and continuous wearable sweat sensing of pH and beyond.

The final study investigated post-processing of electrospun piezoelectric poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) fibres, produced using a high-throughput nozzle-free setup, for wearable impact force sensing applications. PVDF-TrFE fibres were coated with a thin spontaneously polymerised polydopamine (PDA) layer to assist in anchoring zinc oxide (ZnO) nanoparticles for subsequent hydrothermal growth into ZnO nanorods. It was found, through the use of different impact force testing setups, that attachment of ZnO nanorods to the PVDF-TrFE fibres enhanced the piezoelectric output of the membranes, with a 5.8-fold increase in impact force sensitivity compared to fibres without ZnO. This self-powered piezoelectric sensor was then integrated into a wearable headband to measure the impact force of a falling soccer ball on the head of a human subject. From the results achieved in this thesis, it was found that combinations of electrospinning and post-processing techniques can be used to develop sensitive wearable sensors with a variety of applications and have great potential for further studies in the future.