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Stretchable Multi-functional Fiber for Energy Harvesting and Strain Sensing
(from left: Professor Steve Park, Jeongjae Ryu and Professor Seungbum Hong) Fiber-based electronics are expected to play a vital role in next-generation wearable electronics. Woven into textiles, they can provide higher durability, comfort, and integrated multi-functionality. A KAIST team has developed a stretchable multi-functional fiber (SMF) that can harvest energy and detect strain, which can be applied to future wearable electronics. With wearable electronics, health and physical conditions can be assessed by analyzing biological signals from the human body, such as pulse and muscle movements. Fibers are highly suitable for future wearable electronics because they can be easily integrated into textiles, which are designed to be conformable to curvilinear surfaces and comfortable to wear. Moreover, their weave structures offer support that makes them resistant to fatigue. Many research groups have developed fiber-based strain sensors to sense external biological signals. However, their sensitivities were relatively low. The applicability of wearable devices is currently limited by their power source, as the size, weight, and lifetime of the battery lessens their versatility. Harvesting mechanical energy from the human body is a promising solution to overcome such limitations by utilizing various types of motions like bending, stretching, and pressing. However, previously reported, fiber-based energy harvesters were not stretchable and could not fully harvest the available mechanical energy. Professor Seungbum Hong and Professor Steve Park from the Department of Materials Science and Engineering and their team fabricated a stretchable fiber by using a ferroelectric layer composed of P(VDF-TrFE)/PDMS sandwiched between stretchable electrodes composed of a composite of multi-walled carbon nanotubes (MWCNT) and poly 3,4-ethylenedioxythiophene polystyrenesulfonate (PEDOT:PSS). Cracks formed in MWCNT/PEDOT:PSS layer help the fiber show high sensitivity compared to the previously reported fiber strain sensors. Furthermore, the new fiber can harvest mechanical energy under various mechanical stimuli such as stretching, tapping, and injecting water into the fiber using the piezoelectric effect of the P(VDF-TrFE)/PDMS layer. Professor Hong said, “This new fiber has various functionalities and makes the device simple and compact. It is a core technology for developing wearable devices with energy harvesting and strain sensing capabilities.” This article, led by PhD candidate Jeongjae Ryu, was published in the January 2019 issue of Nano Energy. Figure 1.Schematic illustration of an SMF fiber and its piezoelectric voltage output and response to strain. Figure 2. Photographs of a stretchable multi-functional fiber being stretched by 100%, bent, and twisted.
Flexible Nanogenerator Technology
KAIST research team successfully developed the foundation technology that will enable to fabrication of low cost, large area nanogenerator. Professor Lee Gun Jae’s team (Department of Materials Science and Engineering) published a dissertation on a nanogenerator using nanocomplexes as the cover dissertation of the June edition of Advanced Materials. The developed technology is receiving rave reviews for having overcome the complex and size limitations of the nanogenerator fabrication process. A nanogenerator is an electricity generator that uses materials in the nanoscale and uses piezoelectricity that creates electricity with the application of physical force. The generation technology using piezoelectricity was appointed as one of top 10 promising technologies by MIT in 2009 and was included in the 45 innovative technologies that will shake the world by Popular Science Magazine in 2010. The only nanogenerator thus far was the ZnO model suggested by Georgia Tech’s Professor Zhong Lin Wang in 2005. Professor Lee’s team used ceramic thin film material BaTiO3 which has 15~20 times greater piezoelectric capacity than ZnO and thus improved the overall performance of the device. The use of a nanocomplex allows large scale production and the simplification of the fabrication process itself. The team created a mixture of PDMS (polydimethylsiloxane) with BaTiO3 and either of CNT (Carbon Nanotube) or RGO (Reduced Graphene Oxide) which has high electrical conductivity and applied this mixture to create a large scale nanogenerator.
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