Design of Flexible Micro/Nanostructures for Bio-Inspired Multifunctional Tactile Sensor

Abstract

The electronic skin (E-skin) has been intensively explored in advanced soft electronics such as wearable devices due to the polymer-based intrinsic high deformability (i.e., flexibility, stretchability) for facilitating the use of physiological sensing, which contains electrocardiogram (ECG), electromyography (EMG), electroencephalography (EEG), and electrooculography (EOG) based on physical, thermal, and electrophysiological signals, by attachment on curved human body. By closing the human-machine interface (HMI) the electronics should have evolved to be miniaturized and seamlessly integrated for improving the quality of human activities. Thus, the multifunctionalities (e.g., self-healing, self-powered, biocompatibility, and multimodal stimulations perception) are required to achieve high stability as well as high sensing performances in a device. To address these challenges, recently, bio-inspired tactile sensors that imitate the novel functions and structure of nature are employed for the enhancement of sensing performances and offer novel concepts of sensors with multifunctionalities. In particular, the geometrical inspiration of novel structures provides remarkable enhancement of sensing performance by manipulating the contact area and deformable behaviors. Moreover, distinctive functions enable to suggestion of the new paradigm of tactile sensors for achieving attachable, implantable E-skin that improves the human skin interactivity. The ionic skin (I-skin), inspired by the ion transportation mechanism within human skin, has great potential to substitute conventional E-skin owing to the unique characteristics of mobile ions that are directionally polarized under environment changes (e.g., temperature, humidity, and physical deformation). The charge imbalance caused by the ion polarization enables to emit and enhance the detectable electric output signals, utilizing the self-powered and high sensitivity of tactile sensors. Moreover, the charged ions can manipulate the mechanical properties of the composite by interacting with polymer chain segments, offering high stretchability and autonomous self-healing capabilities. These characteristics can address challenges inherent in conventional electronic devices, such as high modulus of mechanical properties, high power consumption, and limited compatibility with the human body. The advanced strategies in bio-inspired novel structure modification have been widely explored to optimize the tactile sensory system to prospective applications. This thesis covers our recent studies about bio-inspired tactile sensors for the advancement of challenges and limitations of conventional tactile sensors such as complicated fabrication processes, and inherent trade-offs in output signal under environment quantity variation. In chapter 1, bio- inspiration strategies including novel structure modification and multifunctionalities for advancement are highlighted as well as the primary introduction of E-skin and I-skin with fundamental operating mechanisms. In chapter 2, we demonstrated sprayable micro/nano-multilevel hierarchical structures with piezoresistive flexible pressure sensors. By employing a binary particle system composed of sea- urchin-shaped spiky surface particles and spherical flat surface particles, a hierarchically structured film is formed due to the manipulation of surface area to wet the polymer binder. Multilevel hierarchical surfaces and randomly distributed porous structures control the conductivity under pressure gradient by step-wised generation of electrical pathways. In this study, the sensor can deal with the linear response via a wide pressure range and mass productive capabilities. In chapter 3, we demonstrated piezoionic bilayer films with significantly improved output signals and response time. The ion-dipole interaction- based self-healing properties offer the high stability of remaining intact under mechanical deformation as well as an ion migration interface that facilitates the driving force to increase ion polarization through the electrostatic attraction of two species of ions. In chapter 4, we summarize this thesis along with future perspectives that should be considered for next-generation e-skin electronics. Our investigations into micro/nano structures for bio-inspired multifunctional tactile sensors have the potential to establish inventive approaches for developing innovative wearable electronic systems, characterized by outstanding multifunctionality and advanced sensory capabilities.