Microcrack-based High Performance CNTs/elastomer Strain Sensor: Design, Synthesis and Mechanistic Study

Flexible and stretchable electronics have attracted extensive research interests for both materials science and practical application reasons. Synthesis of novel functional materials and/or applying innovative structures are two of the most adopted approaches in developing flexible electronics and devices. Flexible materials and structures are emerging as alternatives to traditional rigid, silicon-based electronics in various touching screens, foldable displays, flexible battery, wearable medical devices, soft robotics, energy harvester and sensors. However, achieving stretchability poses further challenges as it requires a wider range of ductility, compliance, and toughness stability/tunability. Development of stretchable conductive material is a critical first step in developing functional building block and its behavior under deformation dictate the overall device performance. Current stretchable electrodes and mechanical sensors have limited sensitivity, linearity, hysteresis, stability, and reproducibility. In addition to material synthesis, structure design is needed; engineering defect structures can be an effective and necessary approach in improving material physical properties, particularly those associated with mechanical responses. In this dissertation, we demonstrated the tuning and significant improvements of piezoresistive effect in a mechanically coupled 2D conductive network/elastomer bilayer through microcrack engineering. Different from thin films, patterned or composite based piezoresistive sensing materials; in this material system, microcrack generation, configuration and evolution in the network layer can be effectively controlled and is confined by the elastomer deformation. This structure allows seamless integration of the complimentary electro-mechanical advantages of carbon nanotube, 2D-network, tactile structure, and elastomer to result in appreciable enhancement in material electromechanical stretchability, sensitivity, linearity, stability, and repeatability. Furthermore, such coupled bilayers can be cost effectively produced using direct ink writing techniques with high throughput, reproducibility and tunability. This is a perfect example of extending the performance limits of existing materials and developing manufacturing routine through microstructural design, defect engineering and utilizing coupling effects among different types of material and practical application of modern additive material synthesis. In addition, we developed Monte Carlo simulation modeling and analyses approaches to study electrical properties of 2D conductive network, we included microcracks in the model and show how crack configuration and evolvement under strain can affect the mechano-electrical responses of the stretchable bilayers.