Design of Micro/Nanostructures for Multifunctional Electronic Skins

Abstract

Recently, a large number of electronic skins (e-skins) and tactile sensors with capabilities of detecting physical/chemical stimuli have been reported for applications in robotics, wearable electronics, and healthcare monitoring devices. For accurate and reliable detection and monitoring of received signals, e-skins and tactile sensors with enhanced sensitivity, selectivity, response time, and mechanical durability are required. In accordance with these demands, e-skins and tactile sensors with various bio-inspired micro/nanostructures (i.e., interlocking, hierarchical, crack, whisker, and fingerprint) and 2D or 3D structures (i.e., serpentine, wrinkle, pyramid, dome, and porous) have been reported aiming to obtain high-performance and multifunctional e-skins. Owing to their effective geometrical advantages, various microstructures have been used to improve the performance of e-skins and tactile sensors based on different operation modes (e.g., resistive, capacitive, piezoelectric, and triboelectric as well as optical tactile sensors) to induce a large change in contact area, localized stress concentration and directional structure deformation. In addition, the synthesis of multi-functional composites, which are made up of various conductive fillers (carbon nanotube, MXene, upconversion nanocrystal, quantum dot and silver nanowire) and polymer matrix (i.e., ferroelectric polymer, elastomer), can suggest a variety of advanced sensory functions and potential applications such as wearable electronics, healthcare devices, and thermoacoustic loudspeaker. This thesis covers recent studies about 2D/3D microstructured e-skins for advanced sensory functions and versatile tactile transduction signals. First, chapter 1 highlights recent trend of e-skins based on electrical/visualization transduction modes and their applications. In chapter 2, we demonstrate that multidirectional force sensitive e-skins are shown to possess customizable force sensitivity and selectivity by controlling the specific microstructure geometry. To investigate the underlying relationship between microstructure geometry and force sensing capability, three kinds of piezoresistive sensors were fabricated based on carbon nanotube/elastomer composites having different surface microstructures (e.g., dome, pyramid, and pillar). To facilitate comparison of the geometrical shape effect on the stress sensing properties, an interlocked geometry of the different microstructure arrays was employed. This has been reported to enhance the variation in contact area and localized stress in the microstructures, thereby improving the stress sensitivity and multidirectional force sensing capabilities of e-skins. In chapter 3, 3D porous structure is used to enhance piezoelectric sensing performance because it can increase the variation in the contact area and localized stress concentration in response to applied pressure. We demonstrated MXene (Ti3C2Tx) -based piezoelectric e-skins with high sensitivity and broad sensing range. For the fabrication of 3D porous structure of MXene/PVDF e-skins, we utilized a vapor-induced phase separation method with precisely controlled parameters such as vapor temperature, mass fraction and exposure time. Moreover, MXene was used as heterogenous nucleation agent to increase ferroelectric properties of PVDF, because abundant surface functional groups of negative charged MXene induce the crystallization of ferroelectric PVDF by the intermolecular hydrogen bonding between surface functional groups of MXene and PVDF. In chapter 4, we demonstrated the use of upconversion nanocrystals to create a dynamic force tactile optical sensor that intuitively visualizes the static and shear forces without complicated signal conversion process, reliable detecting results from non-interference to electrical signals. We utilized the efficient force transfer mechanism of human skin and mimicked the sensory microarchitectures to create a dynamic force amplifying pad. In chapter 5, we fabricated transparent conductive films consisting of conductive MXene nanosheet and ultrathin substrate, which is utilized as thermoacoustic loudspeaker providing high mechanical properties as low bending stiffness, resulting in no cracks or voids under external strain. It offers constant electrical and acoustic outputs due to a high degree of bendability. Finally, in chapter 6, we summarize this thesis and present future perspective for next-generation e-skin electronics. Our e-skins, based on 2D/3D microstructured e-skins, can provide a new-concept of wearable devices for advanced sensory functions and versatile tactile transduction signals.