Post-processing of electrospun fibres for wearable flexible sensing
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Date
12/08/2022Item status
Restricted AccessEmbargo end date
06/09/2023Author
Chung, Michael
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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.