Heterogeneity design for stretchable electronics

• Main Author: Wang, Ting
• Other Authors: Alfred Tok Iing Yoong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/151917
• Institution: Nanyang Technological University

Wearable electronics is a promising candidate for edging devices that can direct interact with users and collect data from them and process information locally close to users, which are the fundamental level for cyber-physical systems in the applications such as personalized healthcare, human-machine interfaces, and smart manufacturing. Conventional silicon-based electronics are rigid and fragile that are not comfortable with mechanical deformations from daily activities and are incompatible with human arbitrary body/skin/organs. This make a huge demanded for developing stretchable electronics with good electrical performance under various mechanical forces. Stretchable memristor, as a vital component in wearable electronics system, is accountable for data processing, information storage, neuromorphic and in-memory computing. Conventional memristor is a sandwich structure with a continuous bulk active layer between two electrodes. The insulative materials are metal oxide which is rigid and easily fracture under the forces from deformations or from accidently mechanical damages. The fracture of insulative materials threaten the electrical performance of memristor and cause the data loss. Thus, to develop a memristor with good mechanical adaptable to the various deformations and mechanical resistance against the damages is challenge. Although few papers have developed organic-based memristors with electrical performance under deformation and keep function under the mechanical damage, the organic materials are sensitive to the solvent and UV light that make it incompatible with CMOS techniques. The inorganic memristors that can work under mechanical deformations as well as mechanical damage have not been reported to the best of our knowledge. Herein, a flexible inorganic-based memristor with mechanical adaptability and mechanical damage endurance is developed via a novel structure in active layer by combining the discrete rigid structure with soft materials. The results demonstrate that the memristors based discrete structure possesses excellent stretchability (~40%) and flexibility (half-folded). Importantly, it performs an outstanding tolerance against the different level of mechanical damages such as puncture and tear, while remaining a good memory function. Meanwhile, even if undergoing serious mechanical damage, device can keep information from the lost by some extent of self-healing property. This approach offers a new strategy for the next-generation stretchable memristor with mechanical damage resistance, which is promising and significant to wearable electronics in exploring more possibilities for applying flexible electronics in real-world environments. Stretchable strain sensor is an essential component in wearable electronic system for data detection and collection, which transduces mechanical stimuli into electrical signals and is promising to apply for personal healthcare monitoring and motion detection system. The conventional crack-based stretchable strain sensors are fabricated through as homogenous thin layer on elastomer. The cracks are generated and propagated within the thin film when undergoing mechanical forces and cause the electrical conductivity change. However, the mechanical properties and electrical properties are a dilemma as large stretchability and high sensitivity cannot be obtained both. Inspired by biological materials from nature, the heterogenous structure with gradient change has a gradually force change rather than abrupt change, which reduce the strain concentration and can achieve a balance with two contradictory properties. Thus, we employ a heterogenous structure thin film with gradually change in thickness (thickness-gradient) on elastomer and investigate the electrical properties and mechanical properties of stretchable strain sensors. The results show that variation of the thickness-gradient affect the electromechanical properties of strain sensors. The anisotropic electrical property was discovered when force applied in a transverse and longitudinal manner corresponding to the thickness-gradient direction. A high sensitivity of strain sensor (GF~1665 at large strain ~30%) can be realized by this heterogenous structure. This thesis has demonstrated how heterogenous structures being employed into stretchable memristors and strain sensors and endow them with a good electrical property and mechanical property, which provides promising and universal strategies for other stretchable electronics.