Harvesting mechanical energy with super-elastic fibers and textiles

Current fiber based triboelectric nanogenerators suffer from complex fabrication schemes and poor performance. We used the scalable thermal drawing technique to make soft and resilient microstructured triboelectric fibers and textiles with excellent performance both as sensors and energy harvesters.

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Flexible electronics and particularly soft fibers and textiles are becoming key implantable and wearable components in future devices for health and personal care, neural probes and prosthesis, soft robotics and human-machine interaction. However, the proper operation of these devices is always impeded by outdated, heavy and rigid 2D power systems, requiring frequent charging. It is therefore particularly important to realize high performance, reliable, deformable and durable fiber and textile based self-powered systems.

Thus far however, as is the case for most functionalities, the performance of reported energy harvesting fibers and textiles remains well below their 2D counterparts. In particular, triboelectricity is one of the most promising forms of energy conversion, yet fiber and textile based triboelectric systems exhibit relatively poor performance compared to planar configurations, making them unpractical. Moreover, their fabrication entails multi-step processes, is labor-intensive, and has not yet demonstrated a strong potential for scalability.

In our lab at EPFL, we employ a unique technique  ̶  preform-based thermal drawing  ̶  the same process used in industry to fabricate optical fibers, for the realization of multi-functional fibers that integrate materials with particular electronic, optical and mechanical properties in exquisite and precise micro- and nano-structured arrangements. As an interdisciplinary group, while we study the fundamental aspects of materials processing at the heart of our innovative fabrication techniques, we also work on developing novel devices with applications in a variety of fields such as sensing and smart fabrics, health care and food engineering, drug delivery, or neural stimulation and recording. For more information, please refer to our recent review paper in Advanced Materials.

In our work just published in Nature Communications, we demonstrate the design and scalable manufacture of novel super-elastic fiber and textile based triboelectric nanogenerators. We identified a thermoplastic elastomer that combines desirable mechanical softness and stretchability, better triboelectric performance than commonly used materials, with rheological properties compatible with our processing technique. We selected a liquid metal conductor as the electrode material to be integrated within the thermoplastic elastomer, because it is inherently conductive, soft, stretchable, and compatible with our processing strategy. We further optimized the energy harvesting performance by the structural designs of integrating multiple electrodes and introducing micro-textured pattern to the elastomer fiber surface.

Fiber structures and breathing monitoring applications. a A super-elastic fiber with a single liquid metal electrode. b-c A fiber features six embedded electrodes and a microtextured surface. d The microtextured fiber is fixed on a stretchable belt, which is worn around the torso. e The generated waveforms allow the quantitative assessment of breathing patterns.

The produced fibers exhibit high stretchability with the capability of sustaining up to 560% elongation. Such superior deformability of the fibers participates to their high performance, and enables their integration into large-area, elastic and machine-washable energy-harvesting textiles, with an open-circuit voltage of 490 V, and a short-circuit transferred charge of 175 nC, which is much higher than existing fiber technologies, and comparable to, or better than, state-of-the-art planar configurations. We also demonstrate that the fibers can work as highly efficient and self-powered embedded deformation sensors capable of monitoring low-force stimuli such as breathing and gesture sensing. Combined with well-developed industrial weaving strategies, the proposed soft thermally drawn microstructured fibers are expected to address the challenges associated with the integration of low profile and efficient powering units, as well as self-powered devices, within wearable systems and advanced textiles.

We are always looking for outstanding and highly motivated PhD or postdoc candidates willing to work on challenging scientific projects, in a serious but also friendly and very cooperative and team-work oriented environment. Please contact with Prof. Fabien Sorin (fabien.sorin@epfl.ch) if you are interested.

Chaoqun Dong

PhD student, École polytechnique fédérale de Lausanne

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