Porous polymer based wearable pressure monitoring system powered by triboelectric nanogenerators

Abstract

The electronics industry has grown rapidly for a few tens of years. Recently, there has been enormous interest in wearable electronic devices. The wearable devices have already made much improvement in both academic and industry field, and people are benefiting from this high-level technology. Wearable devices operate by giving or receiving information between a user and a device. Information from the user is collected through sensors, and information from the device is transmitted through the displays or speakers. In addition, the combination of energy sources for driving the above-mentioned devices allows the operation of wearable devices. The pressure sensor is receiving much attention these days due to its wide range of possible applications in daily life as well as sports or medical field. This dissertation provides reliable strategies for the fabrication of highly sensitive capacitive pressure sensors using porous structures and surface modification of the sensing layer. High deformability of the dielectric layer is required to realize the high sensitivity of the pressure sensor since the capacitance value varies with changes in the thickness of the dielectric layer under pressure. Electrospinning is a good candidate for producing the porous structures through randomly deposited nanofibers, resulting in enhanced deformability of a dielectric layer. The effect of spinning time on the stiffness of the electrospun dielectric layer and the sensitivity of the sensors was investigated. Besides, a simple analytical model for the relationship between the capacitance change ratio and the external pressure was established. Some confirmatory experiments were also conducted for practical applications. The deformability of the dielectric layer could also be improved with the proper selection of materials like polydimethylsiloxane (PDMS) whose Young’s modulus is 3 ~ 4 MPa. A fast and simple fabrication method was developed for the porous structure inside the PDMS dielectric layer by mixing with a sacrificial solvent and curing the mixture via microwave irradiation. Higher porosity and pore size could be obtained with the higher solvent ratio which means the layer becomes more deformable under the same applied external pressure. However, when the ratio of the solvent exceeds a critical point, the PDMS and solvent were not perfectly mixed, resulting in ununiform porous structures. With the modified surface morphology, the deformability of the layer could be further enhanced. Glass mold was used to form surface patterns, and the effect of pattern distance on the sensitivity of the pressure sensor was investigated. The sensitivity of the sensors could be remarkably enhanced with the optimized pattern distance as well as the properly selected solvent ratio. In addition to a pressure sensor, an energy source has attracted much attention to widen applications of wearable electronics. Recently, the triboelectric nanogenerator (TENG) has attracted much attention as a good green energy source. TENG shows high electrical performance, simple structure, eco-friendly nature, and cost-effective fabrication process. However, the power density of the TENG is still insufficient for practical applications due to its low output current. TENG’s power density is mostly affected by charge density on the surface of the dielectric layer. To improve the power density of TENG, the surface area of the dielectric layer is important because charges are induced on the surface. Electrospun nanofiber could greatly enlarge the surface area of the dielectric layer. Charges in the nanofibrous membranes are induced not only on the surface of the membrane but also on the surfaces of the inner fibers by electrostatic induction, resulting in great improvement of electrical performances of TENG. The effects of various fabrication parameters on the electrical performance of an electrospun nanofiber-based triboelectric nanogenerator (EN-TENG) are systematically investigated through the design of experiments. Four fabrication parameters were selected to examine, namely: (1) working distance (needle to collector distance), (2) needle gauge, (3) electrospinning time, and (4) counter materials. The optimized parameters were proposed, and the power density of the best case and worst case were compared. The performance of the best EN-TENG was demonstrated by charging capacitors and lighting 200 light-emitting diodes. Cyclic test and dynamic energy harvesting were also checked, and the proposed EN-TENG showed good capability as a wearable energy source. In the above-mentioned optimization process, polyimide (PI) was found as the best counter material. It was assumed that the electrospun PI nanofiber significantly improves the performance. PI nanofibrous membrane was successfully fabricated through the proper selection of a solvent and PI powder even if the PI has high solvent resistance. Nanofiber-based TENG was compared with film-based TENG, and the nanofiber-based TENG showed at least 7 times higher performance than film-based TENG. Effects of powder ratio were then investigated, and confirmatory experiments such as cyclic test, lit up the LEDs, body attached TENGs and powering small electronic devices were conducted. Enlarged surface area and proper selection of the counter material could extremely enhance the electrical performance of TENGs. The methodologies suggested in this dissertation for the pressure sensors and the TENGs would be effective in the fabrication of highly sensitive pressure sensors powered by TENGs.