Electrospun polymer fibers for triboelectric energy harvesting applications in smart textiles
Wearable biosensors embedded into smart textiles have the potential to revolutionise healthcare by enabling the measurement of vital signs in real-time. However, the deployment of these devices is hindered by their power requirements. Current batteries are too bulky and rigid, and require frequent charging. A promising solution to this issue involves energy harvesting devices capable of transforming surrounding waste energy into useful electricity, potentially reducing the battery size and frequency of charging. Amongst energy harvesters, triboelectric generators are excellent candidates for smart textile applications due to their potential to convert mechanical energy arising from body movements into electrical energy. Typically, triboelectric energy harvesters rely on contact-generated charges between pairs of materials situated at opposite ends of the triboelectric series, which is an empirical scale that ranks materials according to their charge donating/accepting tendencies. Such devices can be manufactured into yarns by coating a conductive core with a triboelectric material. However, current triboelectric yarns lack the power output, durability and washing resistance required for textile-based applications. This work addresses these issues by developing nanostructured functional polymer coatings, namely Nylon-11, polymethyl methacrylate (PMMA) and polyvinylidene difluoride (PVDF), using electrospinning, which is a widely used, scalable fiber-production method. These materials are selected because they respectively occupy the top, middle and bottom of the triboelectric series and are thus representative of a wide spectrum of triboelectric materials. Multiple electrospinning processing parameters including voltage polarity, humidity and polymer concentration are optimised to enhance energy harvesting performance and durability of triboelectric generators. The effects of processing on the resulting polymer crystal structure and surface properties, and subsequently triboelectric performance, are investigated in detail using a combination of characterisation techniques, including scanning probe microscopy, X-ray diffraction and infrared spectroscopy. Optimised Nylon-11 and PVDF coatings are subsequently used to develop tribopositive and tribonegative yarns. The triboelectric yarns are fabricated using a customised electrospinning process in which the polymer is directly spun onto a conductive carbon nanotube yarn, which serves as the conducting electrode. This method creates a uniform and stable core-shell structure with excellent adhesion between the polymer coating and the conducting core. The two triboelectric yarns exhibit remarkable triboelectric energy harvesting during fatigue testing with an average 35\% power output improvement after 200,000 fatigue cycles. Furthermore, the triboelectric yarns demonstrate high abrasion and water resistance by retaining their functionality following hundreds of rubbing and 10 washing cycles. Finally, the Nylon-11 and PVDF yarns are woven together creating a proof-of-concept triboelectric textile. The textile showed high peak power density output compared to previously reported devices (237 mW/m$^{2}$ across an impedance matched load resistance, in response to an applied mechanical force of 2 N and 2 Hz). Furthermore, the motion-sensing capabilities of the textile were demonstrated by building a textile-based touchpad and force sensor. In summary, the unique yarn fabrication process and the resulting high-performance triboelectric yarns are promising platform technologies that can accelerate the development of smart textiles beyond energy harvesting.
energy harvesting, smart textiles, triboelectric, electrospinning
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