| Summary: | 博士 === 國立成功大學 === 工程科學系碩博士班 === 97 === Flexible electronics have developed rapidly in recent years for sensors and actuators due to their large area applications and low cost manufacturing. These flexible devices can be bent, expanded, and manipulated during use. In addition, flexible electronics made with organic materials, carbon-based materials, or transistors could be patterned onto thin films or bendable surfaces using printing technologies. However, there are few practical applications of flexible electronics in passive sensors; most studies have focused on small scale devices or imbedded bulk devices on flexible substrates. Furthermore, the material characteristics of flexible materials have yet to be completely investigated for applications in fabrication processes. Therefore, this study proposes analysis methods for flexible materials, investigates the material characteristics of flexible substrates, and designs and fabricates the flexible sensors.
The material characteristics of flexible substrates, PVDF and PI films, include phase transformation, surface morphologies, optical spectra, thermomechanical, behavior, nanoindentation phenomena, and mechanical properties. Experimental results show that thermomechanical characteristics of PVDF film are greatly influenced at stretching ratios of over 4 in the stretching direction. The hardness is almost uniform and Young’s modulus is about 0.25 ± 0.01 and 3.44 ± 0.14 GPa, respectively. Unstretched PVDF films have a higher absorbance in the UV light range than stretched films do. PVDF with a stretching ratio of over 3 has above 90% transmittance at near infrared light. For PI film, UV light is not transmitted into the films and the transmittance of IR light is about 86%. Nanoindentation experiments show an almost uniform hardness and a reduced Young’s modulus of about 0.181 ± 0.03 and 3.21 ± 0.06 GPa, respectively. Thermomechanical characteristics are greatly influenced for specimens with thicknesses of 8.3 and 25 μm due to the higher relaxation of thin PI films. Thus, the material characteristics analysis provides useful information for the design and fabrication of flexible substrates.
A flexible tactile sensor and a flexible physiological sensor are investigated in this study. Flexible tactile sensors were designed and fabricated using printing technologies for applications in multi-touching and large area sensing. The sensors are based on polyimide substrates, with thixotropy materials used to print novel organic resistance and a bump on the top polyimide. The gap between the bottom electrode layer and the resistance layer provides a buffer distance to reduce erroneous contact during extreme bending. Experimental results show that the top membrane with a bump protrusion and the resistance layer have a large deflection and a quick sensitive response. The bump and resistance layer provide a concentrated force of von Mises stress and inertial force on the top membrane center. Linear algorithm matrixes with Gaussian elimination are used for multi-touching detection.
The flexible physiological sensor is based on a PI substrate for a printed circuit and uses on a non-woven material to package the module with a hot-press. The module is sufficiently thin and light to be pasted on human wrists for monitoring body temperature and heart beat. The thickness of the flexible physiological sensor is about 2 mm and the minimum radius of curvature is about 2.5 cm. The sensor can detect a temperature and heart rate of 25 - 45℃ and 50 - 200 bpm, respectively. The feasibility studies show that printing technology is appropriate for large area applications and that it can be used for the low-cost fabrication of flexible electronics.
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