Over the past decade, the continuous development of soft multifunctional sensors capable of detecting important physical stimuli such as strain, pressure and temperature underpins the 4^th industrial revolution of society. In particular, tremendous research efforts have been directed towards improving the sensing performance of the soft multifunctional sensors in terms of their sensitivity, sensing range, linearity, response time and durability. Nevertheless, several key requirements essential to the commercialization of soft multifunctional sensors remain unresolved and preserve as great challenges to date. One requirement is the simultaneous detection of the magnitudes and the directions of different mechanical stimuli applied to the sensors. Another requirement is to realize the concu...[ Read more ]

Over the past decade, the continuous development of soft multifunctional sensors capable of detecting important physical stimuli such as strain, pressure and temperature underpins the 4^th industrial revolution of society. In particular, tremendous research efforts have been directed towards improving the sensing performance of the soft multifunctional sensors in terms of their sensitivity, sensing range, linearity, response time and durability. Nevertheless, several key requirements essential to the commercialization of soft multifunctional sensors remain unresolved and preserve as great challenges to date. One requirement is the simultaneous detection of the magnitudes and the directions of different mechanical stimuli applied to the sensors. Another requirement is to realize the concurrent detection and decoupling of multiple stimuli without interferences. In this thesis, novel multifunctional sensors based on one-dimensional (1D) carbon nanofibers (CNFs) are developed via a facile, low-cost, and scalable electrospinning technique to address the above challenges.

Among various materials used to fabricate the soft multifunctional sensors, carbon nanomaterials are superior to their metallic counterparts because of their abundance in Earth, good electrical and thermal conductivities, and ease to be functionalized. Electrospun CNFs stand out as the best amidst various carbon nanomaterials for fabricating soft multifunctional sensors owing to their distinct advantages of scalability, processability and low fabrication cost. In addition, CNFs of unique morphologies and thus of novel properties can be easily fabricated by adopting simple modifications to the electrospinning apparatus.

In this thesis, a novel multidirectional strain sensor is first developed using the aligned electrospun carbon nanofiber (ACNF) film/ polydimethylsiloxane (PDMS) composite. Compared to conventional soft strain sensors only capable of measuring strains in a uniaxial direction, the ACNF-based strain sensor demonstrates a unique ability to detect multiaxial strains. It distinguishes the directions and magnitudes of strains with a remarkable selectivity of 3.33. Further, its unconventional applications are demonstrated by detecting multi-degrees-of-freedom synovial joint movements of the human body and monitoring wrist movements for systematic improvement of golf performance. Second, we present a flexible temperature sensor based on the ACNF film. The ACNF-based temperature sensor exhibits outstanding sensitivity of 1.52% °C⁻¹, the highest sensitivity among carbon material-based soft temperature sensors. More importantly, it shows high discriminability towards temperature amidst other unwanted stimuli and maintains its original performance even after repeated stretch/release cycles because of the highly-aligned structure. Last, we present a soft all resistive multifunctional sensor with stimulus discriminability produced solely using electrospun CNFs as the sensing materials. Unlike other reported multifunctional sensors with stimulus discriminability, it accomplishes the pressure and temperature stimuli discriminability using a single type of output signal, namely the electrical resistance, which is the most convenient digital signal to monitor and process among others for device applications. The CNF-based soft multifunctional sensor’s ability to simultaneously detect and decouple temperature and pressure stimuli is also demonstrated for novel applications as a skin-mountable sensing device and a flexible game controller. Overall, the novel utilization of 1D electrospun CNFs to effectively solve the state-of-the-art challenges of the soft multifunctional sensors presented here will bring our society one step closer to realizing the 4^th industrial revolution.